Thin Film Coating Thin Film Coating through Sol-Gel ...

Research Journal of Chemical Sciences _______________________________________ ___________ E-ISSN 2231-606X Vol. 6(7), 65-72, July (2016) Res. J. Chem. Sci.

Review Paper

Thin Film Coating through Sol-Gel Gel Technique Gaurav Bahuguna1,2, Neeraj Kumar Mishra1, Pratibha Chaudhary3, Amit Kumar1,4 and Rajeev Singh1* 1

Material/ Organometallics Laboratory, Department of Chemistry, ARSD College, University of Delhi-110021, Delhi India 2 AIAS, Amity University, Noida, Uttar Pradesh 201313, India 3 Maitreyi College, University of Delhi, Bapudham Complex, Chanakyapuri, New Delhi, 110021, India 4 Department of Polymer Science, Bhaskaracharya College of Applied Sciences, University of Delhi, Dwarka, New Delhi, 110075, India In [email protected]

Available online at: www.isca.in, www.isca.me Received 31st March 2016, revised 2nd May 2016, accepted 30th June 2016

Abstract Thin film coatings are a much explored domain, since films are well suited for the studies of physical, particularly optical properties and have numerous scientific, technological, and commercial applications. For depositing thin films, a large number of techniques have been used which involves different types of precursors. This review will highlight coating technology utilizing the sol-gel gel mediated precursor, which delivers high homogeneity, low temperature processing and various other advantages. Besidess this, thin film depositing techniques like spin coating, dip coating have also been discussed with their various applications. Keywords: Sol-gel, Thin film, Coatings, A Applications.

Introduction Sol gel: Sol-gel process is an amalgamation of a number of methods for synthesizing materials from solutions, which involves gel formation as one of its stages of processing. Most popular type of the sol-gel gel synthesis is based on controlled hydrolysis of compounds, usually alkoxides1 M (OR) x (M = Ti, Si, V, Zr, V, Mo, Al, Sn, W, Zn, Ge, etc.) or corresponding

chlorides2, in an organic or aqueous medium. This method of synthesizing materials is based on the phase conversion of a sol obtained from metallic organometallic or alkoxides precursors. The obtained sol which is a solution of particles in suspension gets polymerized at low temperature, eventually forming wet gel3. The solvent is eliminated by exposure to standard heating to form the dry gel4.

Figure-1 Steps involved in controlling the absolute morphology of the desired material5

International Science Community Association

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Research Journal of Chemical Sciences __________________________________________________________E-ISSN 2231-606X Vol. 6(7), 65-72, July (2016) Res. J. Chem. Sci. At the initial stage of this process the polycondensation and hydrolysis reactions directs to the development of a colloidal solution, commonly known as sol, of hydroxide particles of size in nano regime. Modulating, primarily increase in the mass of the desired material or any significant changes in the outer surroundings like that of substitution of solvent, potential of hydrogen (pH), solvent substitution, etc. results in excessive contact formation between dispersed material and thus monolithic gel is formed6, which have a flexible and very stable three-dimensional grid of hydroxides which encloses the solvent. Purification of sols thus formed after gelation can be achieved by extraction, dialysis (electrodialysis), ultrafiltration and evaporation of the corresponding sols at relatively lower temperatures. One of the crucial roles in this method is ruled by different techniques of solvent elimination after gel formation. Various products (xerogels7, ambigels8, cryogels9, aerogels10) can be obtained depending on the route of synthesis, whose applications can be done in accordance with their differing range of properties. The preservation of the size in nano regime of the product and exceptionally high surface to volume ratio (hundreds of m2/g) is the common regularity that has been obtained from all these processes, even though bulk mass of the precursor can be different by thousands of times. A wide range of products of this method are brought into play as the starting material for obtaining ceramics and nanopowder oxides. With a pronounced quasi-one-dimensional arrangement xerogels can also be synthesized via this process11. For example, vanadium oxide nanotubes can be synthesized using V2O5·nH2O xerogel as the12. Polymer to gel process is also included in this method, which involves gel development by the Pechini method (citrate gel)13 or by evaporation of the solution of the water soluble in the initial solution. Methods like supercritical drying or freezedrying gels via treatment of consequent heat in an inert ambience is used for the fabrication of carbon aerogels and cryogels. One of the importance of these coatings is that these can be deposited at a low temperature, and thus allowing the coating of substrates in polymeric form. Furthermore, low temperature coating has very less impact on the concluding properties of the surface so, specific functional properties can be obtained that were previously not a part of the conventional processes. Sol-gel technique has wide range of applications. One such application is the formation of thin film coatings through sol-gel which involves metal-organic or inorganic compounds as precursors14. Mostly metal alkoxides are used as metal-organic precursors because they lead to a solid amorphous metal by reacting with water through condensation and hydrolysis reactions. For either metal-organic or inorganic precursors, the modification in the structure of polymers or oligomeros relates directly to the hydrolysis extent and the functionality of the metal or preferred coordination number. In the scheme


including metal organic precursor, the rate of hydrolysis is controlled by the hydrolysis ratio, whereas in inorganic precursors, this rate is often controlled by the pH15. The formation of thin films is done by centrifugal of gravitational exhausting, escorted with vigorous stirring and then drying which leads to the establishment of the shape of fluid outline, magnitude of the forces exerted on the solid phase and timescale of the deposition process. In late 1980s and 1990s, various technical fields also started exploiting this technique, such as biomedical applications17. In 1989, sol-gel expertise for coating and powder purposes came into existence in Australia. This development is credited to the scientists from University of NSW, ANSTO, University of Technology, Monash University, DSTO and Silicon Technologies Pt. Ltd. A number of research papers were published covering wide a range of ceramics from ceramic superconductors, titanates, calcium phosphate, single and mixed oxides18. History: The sol-gel process through a suspension of colloidal particles came into existence with the origin of chemistry. This technique in 1846 was applied leading to the emergence of a new technology, when a scientist, Ebelmen observed the polycondensation and hydrolysis of tetraethoxysilane which is also known as tetraethylorthosilicate or TEOS. In 1939, the first patent for sol-gel was published which inculcates the synthesis and characteristics of SiO2 and TiO2 for optical applications19. The possibility of this technique in the production of high purity glasses which was unfeasible with established ceramic processing route was acknowledged in 195520. Consequently, the first details on the relevance of this condensation technology to produce consistent multi-component glasses came into existence. Seventy-six years ago Berger and Geffcken reported the solgel19 and synthesized single oxide coatings. Later, Schroeder21 formulized thin film coatings by using layers of many mixed oxide and single oxide. In 1953, the appearance of first thin film product took place in the market and by the end of 1959 considerable making of rear-view mirrors of automobiles (titanium and silicon oxides) started. With the progress in research works all around the world, antireflection coatings in 196422 and sunshield in 1969 made their way into the blooming market23. It was in 1969 when Hinz and Dislich24 explained the chemistry of the synthesis of multi-component oxides, process technology and the fundamental standard of the chemical reactions was known. In 1969, applications for patent were filed25 which was granted in the countries undergoing industrial development, with the details being published. Concurrently and autonomously, a comparable type of scheme was applied taken by Thomas and Levene and was filed as well26. From then it has been identified that this sol-gel process can be worn to synthesize any kind of multi-component oxide using various elements like any crystalline substance such as a glass or a ceramic of glass in their alkoxide form. For the same time it has

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Research Journal of Chemical Sciences _________________________________ _________________________________________________ _________________________E-ISSN 2231-606X Vol. 6(7), 65-72, July (2016) Res. J. Chem. Sci. also been known that the synthesis process following these routes leads to the formation of exceedingly pure and extremely homogeneous modified multicomponent oxide. During that time there was a very modest knowledge about the particulars of the steps involved and mainly about the central sol sol-gel reaction. Major changes have taken place till now in the reaction process. A substantial number of parameters should be distinctively known and ought too be regulated within close limits to certify uniformity in the desired material. After all this chronological observation, we must give some deliberation to the contemporary feature. Glass layer being one such example and it is the first invention in the market27. Advantages of sol-gel technique: For the preparation of thin films a number of methods have been employed including chemical vapour deposition, e beam evaporation, sol sol-gel process and sputtering. The conventional sol-gel gel scheme involves continuous us condensation of the particles formed initially due to the hydrolysis of the alkoxide precursor and finally forming gel which is commonly known as hydrolytic route. For two main reasons, the sol-gel gel technique is predominantly advantageous

for the development ment of thin coatings of oxides. Firstly, coating of the complex shapes became significantly easier utilizing the dipping which is thought to be the major drawback of the coatings via vapour deposition. Secondly, very small quantities of precursor are required ired and making the process economic, thus, the price of metal organic raw material is not of much significance. Regardless of large shrinkages in the drying process, thin oxide coatings actually do not fracture when the surface arrangements are fine, as rather ather than shrinking laterally the gel film would shrink in the width direction28. Room temperature is the appropriate temperature for carrying out this procedure and the desirable morphology can be monitored by controlling the reaction conditions. The liquid li raw material can be easily functionalized on a base by spraying or spinning, heat treated at lower temperatures and dipping therefore, sol-gel gel process route is very much attractive scalingscaling up of synthesized oxide thin film. In case of non planar structures tures and on semiconductor chips, spray-coating spray practice offers the benefit of even film deposition.

Figure-2 Steps for preparation of thin films and powder by the sol sol–gel process16


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Research Journal of Chemical Sciences __________________________________________________________E-ISSN 2231-606X Vol. 6(7), 65-72, July (2016) Res. J. Chem. Sci. Moreover, the sol-gel process maintains the option of synthesizing a large area and small area coating of thin oxide films at an econmical technological applications29. Role of the thin film coating may be to provide one or more of a number of functions: (i) Chemical protection (e.g. corrosion protection)30 (ii) Mechanical protection (e.g. abrasion resistance)31 (iii) Optical properties (e.g. anti-reflective, optoelectronic)32 (iv) Electronic properties (e.g. ferroelectric, conductive)7,23 (v) Catalytic activity (usually associated with high surface area)33. Many different industries are gaining profit from adopting solgel because of its flexibility in fabricating a wide range of materials with desirable characteristics. Today, a lot of examples are found in the automotive, biomedical sectors, construction, communications, and electronics. Glasses and ceramics fabricated by sol-gel have identical material properties as that of those synthesized by the other conventional routes. The advantages of using sol-gel processing route as an alternative of high temperature processing methods are low synthesis temperature34,35, high purity36, high homogeneity37, better yield38, novel materials39, low capital cost34,35. Disadvantages: Apart from being a great synthesis route, the sol-gel technique also has some serious drawbacks. The foremost ones being the drying of gel and the extreme volume shrinkage at the time of gelation, the elimination of the unwanted residuals (hydroxyls and organics) and the occurrence of large amount of pores. Perhaps, deficient technical and precise knowledge about the sol-gel process is the main disadvantage that creates wide range of complexities in the process 40.

Sol-gel thin film Coatings Thin coatings are remarkably thin layers of raw materials (SiO2, TiO2, MgF2, Au, Al etc.), that can be functionalized to the substrate like metal, glass or crystals, ceramics to effect a modification in its opto-electrical properties. Properties such as refraction, conductivity, reflection and anti-reflection can be shaped and tailored by specialist calibration of material selection, the relevant thicknesses of each thin layer and the number of layers. All these parameters are in relation to the incidental wavelengths of light. Application of thin films over different materials is done to improve and achieve brilliant properties that give superior advantages and applications of the materials which was not possible otherwise. Thin films of variable thicknesses ranging from micrometre to nanometre can be synthesized according to our need. The utility of thin oxide films is greatly increased as it shows different applications and properties which involves light emitting diodes (electrical devices), computer memory applications, interference and electrochromic filter, magnetic films (data storage), solar cells, anti-reflective liquid crystal devices films, compact discs (optical storage devices) and electroluminescent devices42. The function of an optical coating is to alter the reflectance and transmittance of the material (substrate) on which they are functionalized. Classifications of the coatings on the basis of their application are as follows: Antireflection coating: It is used for the inhibition of unnecessary reflections, usually occurring at the boundary between the two different substrates. Example: boundary between glass and air. High reflection coating: In contrary to the antireflection coatings, reflectivity of a surface can be increased by mirror coatings. Partial reflection coating: Beam splitters transmit a particular ratio of the entering light and return the rest of it. They are frequently used at nonzero angles of incidence. Polarization coating: Using polarisers, the incident light can be separated into components that are orthogonally, characteristically reflecting one and transmitting the other. Filter coating: These can be used to of the preceding functions and the selectivity is based on wavelength; for example, reflecting one wavelength and highly transmitting the other.

Figure-3 Scratch resistant coating (left half of slide coated, right half uncoated)41


Dip Coating: It is the easiest method of thin film synthesis from chemical solutions at a fast rate with the maximum degree of control, making it appropriate for Research & Development and productions at small scale. In specific high technology cases this method can be used for deposition of coatings of large surfaces. The principle being a simple one, simply withdrawing the substrate at a constant speed which is initially dipped in a

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Research Journal of Chemical Sciences _________________________________ _________________________________________________ _________________________E-ISSN 2231-606X Vol. 6(7), 65-72, July (2016) Res. J. Chem. Sci. solution. At this stage of dipping process, solution homogeneously and naturally stretch on the plane of the material (substrate) by the mutual effect of capillary rise and viscous pull. Then solidification of the final coat coating takes place by natural or forced evaporation. A fine adjustment of the evaporation conditions (relative vapour pressure and temperature) and withdrawal speed is necessary to perfectly control the characteristics of the film which includes the thickness of te film and the inner structure. Amid all accessible methods used for this function, dip coating provides the exceptional prospect to precisely direct the vital parameters. In comparison to the other conservative thin film formation processes such as forced or air evaporation, sputtering, chemical vapour deposition (CVD), sol-gel gel dip coating is potentially economical and requires less equipments. Nevertheless, the most imperative benefit of sol-gel gel technique over usual thin film coating processes is thee capability to mould the micro microarrangement of the films to be formed. Spin coating: The main difference between spin and dip coating is that in spin coating the film being deposited thins by evaporation and centrifugal draining. Spin coating can be divided ed into 4 stages: deposition, evaporation, spin off and spin spinup, even though in case of sol-gel gel technique for thin film coating, usually evaporation overshadows the former stages of stages. During the deposition stage, surplus amount fluid is expelled on the he surface of substrate. Now, in the spin spin-up stage, due to the centrifugal force, the outward radial flow of liquid start. Further in the spin-off off stage, excess liquid flows to the edge and leaves as droplets. There is a greater resistance to flow as the film ilm thins so there is a decrease in the rate of removal of excess liquid by the spin-off off process, which can further be justified by the increase in viscosity due to the increase in non nonvolatile component. In the concluding stage, the primary mechanism of thinning hinning is taken over by evaporation44. The plus

point of spin-coating coating is that during the spin-off spin process, the uniformity that is attained tends to remain that way until there is any variation in the viscosity because of different substrate and is independent dent on shear force. This affinity is because of the resonance between the two main forces: and viscous force (acts radially inward) and centrifugal force (acts radially outward).

Applications Optical Coating: Thin films are known for their distinctive optical properties that can be applied in electronic devices, data communication, ultra fast optical data storage, and so forth. Nanoscale platinum, gold, silver, etc bipyramids, triangular shapes, cubes, and spheres res all display diverse colours because they absorb or scatter light of different frequencies as the electrons in the conduction band oscillates collectively at different resonance frequencies for different shapes. The intensity and frequency of Surface Plasmon P Resonance (SPR) are dependent on the charge distribution at the nanoscale, which in turn is shape dependent45. Thin films offers an important number of applications such as blue laser diodes, white light emitting diodes (LEDs) and optically controlled controll switches46. Electronic films: As we move from bulk to nano scale, the surface to volume ratio changes drastically, inculcating some exceptional properties including quantum confinement (Q-dots, (Q observed in semiconductors), confined surface plasmon resonance, catalytic (electrocatalytic properties) and supersuper paramagnetism (observed in some nanocomposites and metal oxide nanoparticles). All these new and exceptional properties have been productively applied to bio-imaging bio probes, biosensors, memory devices evices and optoelectronic devices. The production of these nanomaterials can be done through toptop down or bottom-up up approach. Once the nanomaterial has been synthesized, it can be deposited as coatings, or can be used as any other form by manipulation of the th colloidal solution47.

Figure-4 Schematic of dip coating technique43


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Research Journal of Chemical Sciences __________________________________________________________E-ISSN 2231-606X Vol. 6(7), 65-72, July (2016) Res. J. Chem. Sci. Protective Films: To protect metals from corrosion organic coating are being used in wide range of industries. Ceramic coatings with good thermal and electrical properties and higher resistance to oxidation are attracting the mass as these can be worn at high temperature environments. In a wide range of field, TiO2 coatings based on photo-induced charge transfer are being applied such as, pollutant degradation, solar cells or super hydrophilic materials. Its application as cathodic protection of metals (under UV illumination) is also being applied. Pure TiO2 coating cannot be used under dark conditions (charge recombination problem), that can be overcome by using Fe doped TiO2 layer under a TiO2 coating by coupling another semiconductor having different energy level which could reduce the charge recombination48.

3.

Rogojan R., Andronescu E., Gghiţulica C. and Vasile B.S. (2011). Analysis of structure and morphologyof hydroxyapatite nanopowder obtained by sol-gel and pirosol methods. Advanced material research, 590(1), 63-67.

4.

Murali P. and Sarma G.V.S. (2014). Studies on HEBM and Sintering Process via Sol-Gel Method on Alumina and Copper Alumina Nanocomposites. International Journal of Engineering Research and Reviews, 2(4), (133-143).

5.

Niederberger M. and Pinna N. (2009). Metal oxide nanoparticles in organic solvents synthesis, formation, assembly and application. 217, ISBN: 978-1-84882-670-0.

6.

Hench L.L. (1997). Sol-gel material for bioceramic applications. Curr. Opin. Solid State Mater. Sci., 2, 604-10.

Porous Films: Porous films are nowadays uses in various fields because of their unexpectedly high surface area. They are used in solar cells, many surface reactions, sensors, etc. For gas and chemical sensing, metal oxide materials have been used, which include gases like H2, CO, Cl2, CO2, NOx, CH4, NH3, SO2, etc.49. In a solar cell, dye-sensitized mesoporous semiconductor film electrode is used as a sensor. For the metal oxide sensors, the performance is regulated with the particle size, shape, necking structure, porosity and film thickness50. For the application of surface reactions, porous TiO2 thin films are desirable because of ultra high surface and thus many scientists have reported on its synthesis51.

7.

Guzman G., Beteille F., Morineau R. and Livage J. (1996). Electrical switching in VO2 sol–gel films. J. Mat. Chem., 6, 505-506.

8.

Hench L.L. (1986). Use of drying control chemical additives (DCCAs) in controlling sol-gel processing. In L. L. Hench and D. R. Ulrich eds. Science of Ceramic Chemical Processing. John Wiley & Sons, Inc. 1986, 52-64.

9.

Babic B., Djokic D. and Krstaji N. (2005). Characterization of carbon cryogels synthesized by sol-gel polycondensation. J.Serb.Chem.Soc., 70(1), 21-31.

Conclusion The sol-gel technique is one of the easiest techniques for the preparation of thin films, powder, xerogel, aerogels and glasses. Various parameters involved in the sol-gel technique i.e. concentration and composition of the corresponding metal alkoxide or chloride, synthesis temperature, solvents, sequence of the added precursors, can be modulated desirably to get the type of material required and hence, thin films having desirable properties and applications can be deposited. Different depositing techniques like dip coating and spin coating can be easily exploited to obtain the desired thickness of the film. A great variety of sol-gel derived films like optical, porous, protective and electronic have been synthesized for different applications.

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20. Roy D.M. and Roy R. (1995). An experimental study of the formation and properties of synthetic sepentines and related layer silicates. Am. Mineral, 39, 957-975. 21. Schroeder H. (1965). Water-dispersed industrial and architectural coatings. Paint Varnish Prod., 55, 31-46. 22. Coradin T. and Livage J. (2006). Sol-gel synthesis of of solids. Encyclopedia of inorganic chemistry. 23. Alhamed M. and W. Abdullah W. (2010). Structural and optical properties of ZnO:Al films prepared by the sol–gel method. Journal of Electron Devices, 7, 246- 252.

37. Schmidt H.K., Geiter E., Mennig M., Krug H., Becker C. and Winkler R.P. (1998). The sol-gel process for nano technologies: nanocomposites with interesting optical and mechanical properties. Journal of Sol-Gel Science and Tech., 13, 397-404. 38. Attia S.M., Wang J., Wu G., Shen J. and Ma J. (2002). Review on Sol—Gel Derived Coatings: Process, Techniques and Optical Applications. J. Mater. Sci. Technol., 18(3), 211-218.

24. Dislich H. (1971). New Routes to Multicomponent Oxide Glasses. Angew.Chem. (Engl), 10, 363-370.

39. Dimitriev Y., Ivanova Y. and Iordanova R. (2008). History of sol-gel science and technology. Journal of the University of Chemical Technology and Metallurgy, 43(2), 181-192.

25. Sakka S. (2003). Yogyo- Kyokai- Shi. J. Sol -Gel and Tech., 85, 299.

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26. Schroeder WC, et. all (1961). Apparatus for feeding finely divided solids. United State Patent, 3001652.

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27. Dislich H. and Hinz P. (1982). History and principles of sol-gel process, and some new multicomponent oxides coatings. Journal of Non-Crystalline Solids, 48, 11-16. 28. Mackenzie J.D. (1988). Applications of the Sol-Gel Process. Journal of Non-Crystalline Solids, 100, 162-168. 29. Schmidt H. (1988). Chemistry of material preparation by sol-gel process. Journal of Non-Crystalline Solids, 100, 5164. 30. Wang D. and Bierwagen G.P. (2009). Sol–gel coatings on metals for corrosion protection. Progress in Organic Coatings, 64, 327-338. 31. Miyazawa K., Suzuki K. and Wey M.Y. (1995). Microstructure and Oxidation-Resistant Property of SolGel-Derived ZrO2-Y2O3 Films Prepared on Austenitic Stainless Steel Substrates. J. Am. Ceram. Soc., 78, 347-355. 32. Dislich H. and Hussamann E. (1981). Amorphous and crystalline dip coatings obtained from organometallic solutions: Procedures, chemical processes and products. Thin Solid films., 77, 129-139. 33. Aegerter M.A. and Mennig M. (2004). Sol–Gel Technologies for Glass Producers and Users. Kluwer Academic Publishers, Norwell, Mass., 37-48. 34. Lou X.B., Shen H.L. and Zhang H. (2007). Optical Properties of Nanosized ZnO Films Prepared by Sol-Gel Process. Transactions of Nonferrous Metals Society of China, 17, 814-817.


42. Cruz M.R.A., Zarzoza G.O., Castanon G.A.M. and Martínez J.R. (2012). Thin films from different materials obtained by the Sol-Gel method: study of the morphology through Atomic Force Microscopy (AFM). Current Microscopy Contributions to Advances in Science and Technology, 2, 1370-1376. 43. Brinker C.J., Hurd A.J., Frye G.C., Ward K.J. and Ashley C.S. (1990). Sol-gel thin-film formation. Journal of NonCrystalline Solids, 121, 294-302. 44. Bornside D.E., Macosko C.W. and Scriven L.E. (1989). Spin coating: Onedimensional model. Journal of Applies Physics, 66, 5185-5193. 45. Wani I.A., Khatoon S., Ganguly A., Ahmed J., Ganguli A.K. and Ahmad T. (2010). Silver nanoparticles: Large scale solvothermal synthesis and optical properties. Materials Research Bulletin, 45(8), 1033-1038. 46. Sadekar H.K., Ghule A.V. and Sharma R. (2013). Nanocrystalline ZnSe thin films prepared by solution growth technique for photosensor application. Composites: Part B, 44(1), 553–557. 47. Benlarbi M., Farre C., Chaix C., Lawrence M.F., Blum L.J., Lysenko V. and Marquette C.A. (2010). Semiconducting properties of thin films with embedded nanoparticles. Synthetic Metals, 163(23-24), 2675–2680. 48. Shen G.X., Chen Y.C. and Lin C.J. (2005). Corrosion protection of 316 L stainless steel by a TiO2 nanoparticle

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Thin Solid Films, 90 (1982) 1-14 PREPARATION

AND CHARACTERIZATION

HETEROGENEOUS

NUCLEATION

AND GROWTH

OF THIN FILMS*

M. HARSDORFF Institutfir

Angewandte Physik, UniversitCt Hamburg, Jungiusstrafi

(Received August

18,198l;

accepted

September

II, 2000 Hamburg 36 (F.R.G.)

21,198l)

A review is given of the current discussion concerning heterogeneous nucleation limited to the Volmer-Weber mechanism. We consider the theoretical but stress the experimental aspects. Various coating techniques, e.g. evaporation under ultrahigh vacuum conditions and r.f. sputtering, along with modern analysis and data-processing methods, such as transmission electron microscopy, the scanning electron microscopy combined with X-ray fluorescence analysis and digital quantitative image analysis, are described. The efficiency of existing theoretical models is discussed in terms of selected examples of experimentally investigated systems such as noble metals on alkali halides, mica and carbon. Furthermore, the extension of the kinetic nucleation model to the condensation of alloys, condensation on disturbed surfaces and oriented growth will be considered.

1. INTRODUCTION

Recently some progress has been made towards a better understanding of the heterogeneous nucleation and growth of thin films. For extremely high supersaturations the thermodynamic theory of condensation is not applicable, but a simple atomic model of two-dimensional polymerization has been found to be valid in several cases. This model was first proposed by Frenkel’ and was extended by Zinsmeister’ and other researchers 3-5. This theoretical background will be used in the discussion of experimental work in this paper. Three different types of condensation mechanisms can be distinguished, depending on the strength of interaction between the deposited atoms and between the atoms and the substrate material: (1) the van der Merwe mechanism, leading to a layer-by-layer growth; (2) the Volmer-Weber mechanism, characterized by threedimensional nucleation and island growth; (3) the Stranski-Krastanov mechanism, starting with the adsorption of a monolayer of the deposited material with subsequent nucleation on top of this layer. In this paper only the Volmer-Weber mechanism will be discussed. * Paper presented 21-251981.

at the Fifth International

004Ct-6090/82/0000-OOOO/ooo/%o2.75

Thin Films Congress,

Herzlia-on-Sea,

0 Elsevier Sequoia/Printed

Israel,

September

in The Netherlands

2

M. HARSDORFF

The specimens can be prepared in various ways: (1) electrodeposition; (2) condensation of evaporated materials onto clean substrates in an ultrahigh vacuum; (3) condensation by dc. or r.f. sputtering techniques. In this paper, only techniques (2) and (3) will be considered. 2.

EXPERIMENTS

2.1. Preparation of specimens The most direct way to obtain information about the heterogeneous nucleation process is by in situ observation with an electron microscope. The substrate has to be an electron-transparent thin film and the material under investigation must be evaporated inside the object chamber of the instrument. These conditions lead to serious restrictions on the choice of material combinations. Possible alteration of the physical properties of the substrate material caused by electron bombardment is an additional problem. In spite of these difficulties this method has been used by many physicists. The first investigation of this type was reported by Bassett6, and later other workers7S8 used this technique. The difficulties in obtaining ultrahigh vacuum conditions inside the object chamber of an electron microscope, the restrictions on the substrate materials and the possible alterations to the specimens caused by electron bombardment have led other groups to prepare the specimens under ultrahigh vacuum conditions in separate coating plants or by d.c. or r.f. sputtering in a noble gas atmosphere. Island films must be stabilized before they are floated off the substrates by the condensation of carbon onto them to avoid artefacts in the film after exposure to air. Timedependent phenomena are observed by a “snapshot” technique. Before the deposition the substrate surface is prepared by cleaving a single crystal in an ultrahigh vacuum environment or for sputtering experiments in a noble gas atmosphere, to avoid the adsorption of vapours. Another way of preparing clean substrate surfaces is the evaporation of the substrate material onto such a cleavage plane just before deposition of the material under investigation. These clean substrate surfaces are exposed to the vapour beam for a predetermined time. The substrate temperature and the density of the vapour beam are held constant. After the condensation is stopped, the specimen is stored and a new experiment is started with the same deposition parameters but a longer exposure time. The disadvantage of this procedure is that the growth of individual crystallites cannot be observed. The growth process can only be observed statistically with an interval between successive observations. The great advantage of this method is the clear definition of the experimental parameters and the possibility of using a large number of material combinations. This method of studying nucleation processes is preferred by many groups. Typical evaporation plants for the preparation of specimens for nucleation experiments in an ultrahigh vacuum are well known. The vapour beam is usually generated in a Knudsen cell. The temperature of the vapour source is regulated by the feedback signal of a mass spectrometer tuned to the mass of the evaporated atoms or molecules. The stability of this arrangement is about 2% over several hours’. The substrate crystal is heated in a crystal oven, the temperature of which is regulated to _+1 “C. Crystals can be cleaved and exposed to the vapour beam for a

HETEROGENEOUS

NUCLEATION

AND GROWTH

OF THIN FILMS

3

predetermined time. After the deposition the layers are stabilized by the evaporation of a carbon film and are stored. The same procedure is repeated to obtain thicker film specimens. When using the r.f. sputtering technique, the crystal is cleaved in a noble gas atmosphere and exposed to the sputtered material for a predetermined time. To prevent bombardment of the substrate surface by charged particles, a magnetic trap is installed. Hence only neutral particles can reach the surface and these have energies which are definitely higher than those of particles obtained by thermal evaporation. In both cases about ten specimens can be prepared in one run, i.e. before breaking the vacuum. The specimens, which contain the deposited material covered by a stabilizing carbon film, are stripped off the substrates and examined in an electron microscope. 2.2. Investigations by imaging the specimens Information about the specimens can be obtained by imaging or by “averaging” investigation procedures (Section 2.3). Firstly the data obtained from imaging methods, using an electron microscope, will be discussed. In Fig. 1 the information obtained as well as the method of obtaining this information is illustrated schematically. The first step is the production of bright field and dark field micrographs. In the related electron diffraction patterns the reflection used for the image formation is indicated by an aperture symbol; for a bright field image the (000) reflection is used while that for the dark field image is the selected (hkl) reflection. In the related pictures in the first and second columns the data obtained by the use of these images are shown. Firstly, the total number of particles can be used to obtain the number density of crystallites as a function of the deposition time for the bright and the dark field modes separately. From these number densities the fraction of oriented crystallites can be calculated, which yields important information for the study of epitaxial growth. All this information is sketched in the third column of Fig. 1. The slopes of the N(t) curves in their initial phases are the so-called nucleation rates for all the particles and the oriented particles. Another piece of information is the maximum in the number density, caused by a balance of nucleation and coalescence. In the next two pictures of the first column the counting of particles under certain conditions is shown. Only particles with diameters greater than a given value are counted. By varying this threshold value and calculating the difference of the counting results the diameter distribution of the particles can be obtained. The result is shown schematically in the third column. The diameter distributions obtained in different condensation stages lead to information about elementary processes, e.g. the occurrence of heavy coalescence. By measuring all the particle coordinates the spatial distribution function of the particles can be calculated, leading to information about the reactions and forces between adjacent crystallites. Further details obtained by evaluation of the micrographs are the relative coverage of the surface with crystallites and the mean particle diameter as functions of the deposition parameters. To obtain reliable information on the nucleation and growth processes, a large number of specimens must be investigated, so that the data should have a high

4

M. HARSDORFF

brightfield

darkf ield

information

N d,O

1

N2 d+d2

Gl

xiaYt

S S t a

t Fig. 1. Information about the nucleation and growth processes obtained from electron micrographs. In the first and second columns the measurements using bright and dark field images are sketched, and in the third column the information obtainable is indicated (N, number density; d, diameter ofcrystallites; P(r), spatial distribution function; S, area covered; xi, ye coordinates).

statistical significance and because of the large number of experimental parameters, e.g. the vapour flux density, the substrate temperature, the deposition time and the material combination. This can be done manually if the counting of particles only is required. Observations required for the computation of diameter distributions or for the measurement of the area covered with crystallites are more difficult. The measurement of the spatial distribution and the correlation function is nearly impossible by hand and modern pattern recognition techniques must be used. The block diagram of an image-analysing apparatus as used by our group is sketched in Fig. 2. The micrographs are imaged by a video camera and the resultant data are fed into the detector system which classifies the picture points into 64 grey levels. If a scanning microscope is used, the video signal of this instrument is fed directly into the grey level discriminator. The required data are calculated in the computer, e.g. the total number of particles with a particular grey level, the number of particles with a diameter greater than a given value, the perimeters and projected diameters,

HETEROGENEOUS NUCLEATION AND GROWTH OF THIN FILMS

5

the x-y coordinates and all combinations of these data. Complex data such as form factors can also be calculated for all the particles in this way. The results are displayed on a monitor and printed out by a teletype. If the scanning electron microscope is used, the image analyser and the microscope must be synchronized. This is done by the use of a digital scan generator for both instruments.

TEM

SEM X-RAY DETECTOR

II

DETEKTOR

7,

4,

DETEKTOR

SCAN-

,I

COMPUTER

+

IMAGE

ANALYSER

X-RAY

*

MULTI-

c

CHANNEL

t

l

MICROANALYSER

Fig. 2. Electron image analysis and X-ray microanalysis for obtaining data automatically by means of electron microscopes and computers.

2.3. Averaging investigation methodr By the use of a scanning electron microscope, information about the chemical composition of the specimen can be obtained in addition to the micrograph. The Xrays generated by the electron bombardment of the specimen are converted in the Xray detector into pulses whose heights are correlated with the energy of the radiation. The spectrum is displayed in a multichannel analyser. To determine the mass and the composition of the deposit, the data are ZAF corrected (correction for the absorption of the electron beam, for the absorption of X-rays and for the fluorescence excitation) by means of a computer. The results are displayed on a monitor and are printed out by a teletype. The block diagram of this set-up is also shown in Fig. 2. Not only the composition of alloys but also the mass of the deposited material can be computed with this technique if a very thick (i.e. 100 urn at an electron energy of 25 kV) specimen of the same material is used as a standard. In this way the condensation coefficient can be measured as a function of the substrate temperature, the vapour flux density and the deposition time. In nucleation experiments a material is deposited onto a substrate which is usually of a different material. In most cases, the condensation coefficient depends strongly on the deposition time. At the start of the nucleation experiment material A

6

M. HARSDORFF

is deposited onto substrate material B with a well-defined condensation coefficient for that particular combination. Later part of the surface becomes covered with crystallites, so that an increasing fraction of the depositing material A is condensing onto material A with a condensation coefficient that is specific for material A. This is why it is necessary to measure the true condensation coefficients at a very early stage of the condensation. Anton and Harsdorff” have calibrated an Auger electron spectrometer to measure condensation coefficients at very early stages when no crystallites are visible in an electron microscope. Unfortunately, no in situ experiments are possible because the substrate damage caused by electron irradiation influences the condensation coefficients. Krause” has shown that the X-ray excitation of Auger electron processes has little or no influence on the surface, so the deposition can occur step by step. A disadvantage of this method is the very long time of measurement needed for the data obtained to be statistically significant. 3.

THEORY AND COMPARISON

WITH EXPERIMENTAL

RESULTS

In Section 1 attention was drawn to the fact that nucleation of films takes place under very high supersaturation. Under these conditions the so-called critical nucleus should be very small and in extreme cases should contain only one or two atoms. It is possible to consider the condensation process by regarding it as a twodimensional polymerization. In Fig. 3 the condensation process is shown schematically. Atoms are condensed and some are re-evaporated either after or without accommodation. Others form dimers, trimers etc. Coalescence occurs by diffusion and growth of crystallites and finally the continuous layer is produced during the filling-in stage. The first stages of the process can be described by the rate equation system

dN, = R-~-Z&N,‘-N1

-

F KiNi+25ts

dt

dN, -=

dt

z I

i=2

K,N,‘-K,N,N,-$+?

where Ni is the density of aggregates containing i atoms, R is the deposition rate, Ki is the reaction constant for impact of atoms on molecules containing i atoms and ri is the residence time or lifetime for an aggregate containing i atoms. vapour

beam

coalescence

Fig. 3. Processes

occurring

in the nucleation

and growth

of crystals

on substrates.

thin film

HETEROGENEOUS

NUCLEATION

AND GROWTH

OF THIN FILMS

For a stable diatomic molecule and a defect concentration

dN, -= dt

L

R-~-K,N,L-2K,N,2-KNIN 71

dN = KINI + K,N,L = J dt where N is the total number of aggregates with i > 1 and K is the average reaction constant. For a condensation coefficient u of much less than unity -

Nl

-

b K,N,L-2K1N12-KNIN

71

K,=Aexp

-$ (

J

1

= C,R2 exp(2E;~Ep)+C2LRexp(~)

(1)

where El is the adsorption energy, E, is the activation energy for surface migration of atoms and Cr and C, are pre-exponential factors. The variation in the density of atoms with time is dominated by the deposition rate R and the re-evaporation rate and by the formation of molecules. When the binding energy of the diatomic molecule is high, these molecules can be assumed to be stable, so that the atom itself is the critical nucleus. In the presence of preferred adsorption sites, the atoms can be trapped and become stable nuclei. 3.1. Nucleation

rate

In all these cases, on the assumption of a condensation coefficient of less than unity, the rate equations can be simplified and the nucleation rate can be computed. Random nucleation will result in a quadratic dependence on the deposition rate while for nucleation at surface defects the dependence will be linear. In Fig. 4 some experimental results for gold on rock salt, carbon and mica are shown (Fig. 5(a) for ultrahigh vacuum evaporation and Fig. 5(b) for r.f. sputtering). For Au/C and Au/NaCl a quadratic dependence is observed with ultrahigh vacuum evaporation, showing that random nucleation is taking place. For Au/mica a linear dependence is observed, indicating that the nucleation takes place at preferred sites. A similar behaviour is observed for Au/NaCl at higher deposition rates using r.f. sputtering (see Fig. 5(b)). High energy particles will produce surface defects of high density, which will trap the deposited atoms. The data obtained by Robinson and Robins” and SchmeiDer et a1.13 are presented as examples of random nucleation, and those due to Bohnr4 and Grothe” as examples of preferred surface defects induced by electron bombardment. For the random processes the slope is 2 and for defect-controlled processes the slope is 1. In Fig. 5 the nucleation rates are plotted as functions of the substrate temperature. From the slopes of the plots we can calculate the energy parameters, i.e. 2E1 -E, for random nucleation and E, -E, for nucleation in the presence of

M. HARSDORFT

10’2

(4

10’5

10’4

10”

R

cm-2$1

Fig. 4. Nucleation rate VS.deposition rate (see eqn. (1)): (a) ultrahigh vacuum results; (b) r.f. sputtering results (for the Au/NaCl system) compared with ultrahigh vacuum results of various workers’*-“.


9

C

;’ 106 10

R=13 10’3cti2s-’

,L ‘2

,,,I,

‘4

‘6

18 20

22

24

26

V-4 (a) Fig. 5. Nucleation rate us. substrate temperature (see eqn. (1)): (a) ultrahigh vacuum results; (b) r.f. sputtering results (for the Au/NaCI system) compared with ultrahigh vacuum results of SchmeiBer and HarsdorfP3 and Robinson and Robins”.

trapping centres where E, is the adsorption energy and E, the activation energy for surface diffusion for the related processes. Figure 5(a) gives the results for ultrahigh vacuum evaporation and Fig. 5(b) gives those for r.f. sputtering. In Fig. 5(b) data of SchmeiBer et al.’ 3 and Robinson and Robins” are plotted as examples of random nucleation, together with the data obtained by Harsdorff and Jark16 and Lane and Anderson”. The use of r.f. sputtering clearly leads to higher energy parameters, as expected. 3.2. The condensation coeficient The condensation coefficient is the fraction of the deposited atoms which remain on the surface. The dependence of the condensation coefficient on the deposition rate and the substrate temperature can be calculated using the rate equations. In Fig. 6 the linear dependence of the condensation coefficient on the deposition rate is shown for substrates of rock salt and carbon. Data for substrates with high defect concentrations have not previously been measured, although this information would be very interesting because for these cases the nucleation theory predicts that the condensation coefficient would be independent of the deposition rate. In Fig. 7 the dependence of the condensation coefficient on the substrate temperature for various combinations is shown. These results permit an independent calculation of the energy parameter 2E1 -E, for random nucleation. The data are nearly the same as those calculated from the nucleation rate data. Deviations from theoretical predictions are observed, as expected, for cases in which the condensation coefficient is not considerably less than unity, e.g. gold on carbon.

M. HARSDORFF

10

3.3. The growth of crystallites After the nucleation the aggregates will grow by the addition of atoms, in an early phase from the condensed phase by surface migration of atoms and later by the direct impingement of atoms. The theory allows a prediction of the dependence of

1P

I

I

1012

to”

(0’4

Fig. 6. Condensation

‘0’5

coefficient

ci = C,Rexp{v]

Fig. 7. Condensation

1

1 III

1.2

,*

,*

IS

20

09 T

c&

VS. deposition

rate for various

substrates:

+C,exp{T}

coefficient

VS.substrate

temperature

for various substrates:

a = C,Rexp{v}+C,exp{v}

lO(

623K/

9d3c~-2$

II

lo10 Fig. 8. Mean diameter Au/mica (A) :

I

I

100

1000

of crystallites

us. deposition

time for the systems

I lOoal

Au/NaCl

f set

(x),Au/C

(0)

and


11

particle diameters on the deposition time. Because of the limitations in the experimental technique only the growth in the mean diameter of crystallites has been observed. In Fig. 8 the experimental results are plotted. The predicted square root dependence on time is confirmed for various material combinations. Deviations are observed if very large crystallites are formed with a high number density; these conditions violate the assumptions of the theory. The growth of aggregates can also be followed by observing the variation in the fraction of the surface which is covered, i.e. the surface coverage. The condensation model predicts a quadratic rise in the surface coverage with the deposition time. In Fig. 9 the experimental results show the validity of this prediction, at least for small coverages.

,

0

10

20

30

40

(a)

50

,*

60 ld2 ($$

Fig. 9. Dependence of the covered area on the deposition time for (a) Au/mica (T = 785 K) and (b) Au/C (T = 916 K): S(t) = C.R’I’eltp{~}+C,~R’t’eap~~}

3.4. Extensions of the model In all the processes discussed up to now, the motion of single atoms was the driving force. Brownian motion of larger aggregates was excluded. The influence of brownian motion of crystallites can be studied.by observing the maximum number

12

M. HARSDORFF

density of the crystallites, or the diameter and spatial distribution of the crystallites. These processes are governed by coalescence caused by growth and mobility of crystallites. A hint of the mobility of larger crystallites is given in Fig. 10 in which the P(r) function plotted is the number density in an annular area around a particular crystallite divided by the average number density. The experimental result shows that P(r) drops below unity at small distances between adjacent crystallites. The diffusing crystallites are clearly kept a certain distance apart by a repulsive force. However, P(r) reaches a higher value at a certain distance, showing the range of the repulsive force, which is perhaps caused by a distortion of the substrate lattice. Kern et al.‘*, Harsdorff and Reiners” and Takeuchi et al. ‘O have made careful investigations of the mobility effects. A comparison with the theoretical work of Stoop and van der Merwe”, Kappus” and Kashchievz3 still leads to unsatisfactory results. The dependence of the maximum of the curve on the diameters of adjacent crystals, as predicted by the theories, was not observed experimentally. Venablesz4 has extended the nucleation theory to take the mobility of small particles into account. The correlation functions calculated by use of this model and fitted to experimental data lead to the conclusion that the mobilities of atoms, causing a local adatom density variation, and of larger particles can account for the experimental results25. Physical arg uments exclude the interpretation based on adatom effects because unrealistic values for the diffusion constant must be assumed for this interpretation to be valid. In conclusion, the mobility of small clusters is considered to be responsible for the observed effects. The theoretical correlation functions are compared with experimental results for Au/NaCl in Fig. 11.

Fig. 10. Radial distribution function of crystallites for the system Au/KBr (N = 4.5 x 10” cm- 2). Fig. 11. Comparison of theoretical correlation function? with experimental data” (N = 3 x 10” cm-‘): -- -, according to the mobility of adatoms, i.e. 1 _&k/225 A) ’ Kcl(O.2) I 1 -,

according to the mobility of the crystallites, i.e. 1

_

WWO 4 Km 1)


13

3.5. Further extensions of the kinetic nucleation model Anton and Harsdorff4 have extended the nucleation model to binary alloys. The comparison with experimental results, carried out by Anton et a1.26shows that the model agrees closely with experimental results even in this complicated situation. It has also been shown5 that the oriented overgrowth of crystals can be deduced on the basis of this model, if the investigations are restricted to the nucleation process. The latter processes are complicated by the mobility of small crystallites with different diffusion constants. It is beyond the scope of this paper to give a detailed discussion of these extensions of the model. 4. INVESTIGATIONS REQUIRED IN THE FUTURE The experimental and theoretical results reported up to now might suggest that all the problems of heterogeneous nucleation have been solved. However, this would be a great misunderstanding. In recent experimental work Robins2’ was able to show the marked influence of charged particles in the nucleation process. Charged particles are present in all ultrahigh vacuum deposition experiments. Their sources may for instance be the evaporation crucibles or the ionization gauges for pressure measurement. A further problem is the deviation of the energy parameters, which were measured directly by Harsdorff and Knabbe2* using the time-of-flight technique, from those computed by use of the dependence of the nucleation rate and the condensation coefficients on the substrate temperature. The deviations are not severe but are remarkable. Recent measurements of Behling29, also using the timeof-flight technique, show that not only single atoms but also diatomic molecules reevaporate from the crystal surfaces, so that the assumptions of the theory are invalid. All these observations show that an improvement in the theory is required to account for all the experimental results. REFERENCES 1 J. Frenkel, Z. Whys., 26 (1924) 117. 2 G. Zinsmeister, Thin Solid Films, 2 (1968) 497. 3 D. Walton, J. Chem. Phys., 37(1962) 2182. 4 R. Anton and M. Harsdorff, Thin SolidFilms, 44 (1977) 341. 5 A. Puskeppel and M. Harsdorff, Thin Solid Films, 35 (1976) 99. 6 G. A. Bassett, in E. Rutner, P. Goldfinger and J. P. Hirth (eds.), Proc. Int. Symp. on Condensation and Evaporation of Soliris, Gordon and Breach, New York, 1964, p. 599. 7 G. Honjo, S. Schinozaki and H. Sato, Appl. Phys. Lett., 9 (1966) 23. H. Poppa, R. D. Moorhead and K. Heinemann, Nucl. Instrum. Methodr, 102 (1972) 521. 8 A. Barna, P. B. Bama and J. F. Pocza, Vacuum, 17 (1976) 219. 9 H. SchmeiEer, Vak.-Tech., 21(1972) 165. 10 R. Anton and M. Harsdorff, Thin SolidFilms, 22 (1974) S23. 11 A. Krause, Thesis, University of Hamburg, 1979. 12 V. N. E. Robinson and J. L. Robins, Thin SolidFilms, 20(1974) 155. 13 H. SchmeiEer and M. Harsdorlf, Z. Naturforsch., 25a (1970) 1896. 14 H. Bohn, to be published. 15 M. Grothe, to be published.

14

16 17 18 19 20 21 22

M. HARSDORFF

M. Harsdorff and W. Jark, to be published. C. E. Lane and J. C. Anderson, Thin SolidFilms, 26 (1975) 5. R. Kern, A. Masson and J. J. MCtois, Surf: Sci., 27 (1971) 483. M. Harsdorff and G. Reiners, J. Cryst. Growfh, 45 (1978) 17. H. Takeuchi, H. Fujimoto, K. Ishii and K. Kinosita, Thin Solid Films, 57 (1979) 291. L. C. A. Stoop and J. H. van der Metwe, J. Cryst. Growth, 24 (1974) 289. W. Kappus, 2. Phys. B, 29 (1978) 239. 23 D. Kashchiev, Surf. Sci., 55 (1976) 477. 24 J. A. Venables, Philos. Mug., 27 (1973) 697. 25 H. SchmeiBer, Thin Solid Films, 22 (1974) 99. 26 R. Anton, M. Harsdorff and Th. Martens, Thin SolidFilms, 57 (1979) 233. 27 J. L. Robins, personal communication, 1981. 28 M. Harsdorff and E. A. Knabbe, Surf. Sci., 86 (1979) 36. 29 R. Behling. to be published.

]OURNA

Journal of Non-CrystallineSolids 147&148(1992)424-436 North-Holland

i OF

NON-CRYSTALLINE SOLIDS

Review of sol-gel thin film formation C.J. Brinker, A.J. H u r d , P.R. Schunk, G.C. F r y e and C.S. A s h l e y Sandia National Laboratories, Albuquerque, NM 87185-5800, USA

Sol-gel thin films are formed by gravitational or centrifugal draining accompanied by vigorous drying. Drying largely establishes the shape of the fluid profile, the timescale of the deposition process, and the magnitude of the forces exerted on the solid phase. The combination of coating theory and experiment should define coating protocols to tailor the deposition process to specific applications.

1. Introduction Despite significant advances in technologies based on sol-gel thin film processing (e.g refs. [1-29]) there has been relatively little effort directed toward understanding the fundamentals of sol-gel coating processes themselves (see for example refs. [30-39]). This paper reviews recent studies that address the underlying physics and chemistry of sol-gel thin film formation by dip(or spin-) coating. We first discuss the salient features of dip- and spin-coating with consideration of single component fluids and binary fluid mixtures. We then address the deposition of inorganic sols with regard to timescales, drying theory, tendency toward cracking, and development of microstructure. We conclude with a discussion of topics for future study.

2. Dip-coating In dip-coating, the substrate is normally withdrawn vertically from the coating bath at a constant speed, U0 (see fig. 1) [40]. The moving substrate entrains the liquid in a fluid mechanical boundary layer that splits in two above the liquid bath surface, returning the outer layer to the bath [38]. Since the solvent is evaporating and drain-

ing, the fluid film acquires an approximate wedge-like shape that terminates in a well-defined drying line (x = 0 in fig. 1). When the receding drying line velocity equals the withdrawal speed, U0, the process is steady state with respect to the liquid bath surface [39]. For alcohol-rich fluids common to sol-gel dip-coating, steady state conditions are attained in several seconds. The hydrodynamic factors in dip-coating (pure liquids, ignoring evaporation) were first calculated correctly by Landau and Levich [41] and recently generalized by Wilson [42]. In an excellent review of this topic, Scriven [38] states that the thickness of the deposited film is related to the position of the streamline dividing the upward and downward moving layers. A competition between as many as six forces in the film deposition region governs the film thickness and position of the streamline: (1) viscous drag upward on the liquid by the moving substrate; (2) force of gravity; (3) resultant force of surface tension in the concavely shaped meniscus; (4) inertial force of the boundary layer liquid arriving at the deposition region; (5) surface tension gradient; and (6) the disjoining (or conjoining) pressure (important for films less than 1 Ixm thick). When the liquid viscosity, ~7, and substrate speed are high enough to lower the curvature of

0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

425

C.J. Brinker et al. / Sol-gel thin film formation

SOL-GEL DIP-COATING /

/

DEPOSITED ~ FILM ~ . ~

/ / j

x =u

/

.......... FILM COLLAPSE AND/ OR PORE FORMATION

Capillary pressure exerted at final stage of drying as menisci recede into

I

.

....

get mtertor

Cp = 2YLVcos(0)

GELATION

ALC( EVI

AGGREGATION GRAVITATIONAL DRAINING + EVAPORATION

i ~ i ~ :~:~'~

PORE SIZE CONTROLLED BY:

~.~ , = (T~Uo)2/3/TLV1/6(p g)1/2 ~/~ ~ ~: ' ~ f ENTRAINED DILUTE SOL V/J ~ ~ ~ RESERVOIR J ~J SURFACE [/J DILUTE SOL

- SIZE, STRUCTURE, COMPOSITION - RATESOF CONDENSATION/ EVAPORATION -

CAPILLARYPRESSURE

Fig. 1. Schematic of the steady state dip-coating process, showing the sequential stages of structural development that result from draining accompanied by solvent evaporation, continued condensation reactions, and capillary collapse.

the gravitational meniscus, the deposited film thickness, h, is that which balances the viscous drag ( c~ ~TUo/h)and gravity force (pgh) [38]: h = C l ( ' r l U o / p g ) 1/2,

(1)

where the constant c I is about 0.8 for Newtonian liquids. When the substrate speed and viscosity are low (often the case for sol-gel film deposition), this balance is modulated by the ratio of viscous drag to liquid-vapor surface tension, 7LV, according to the relationship derived by Landau and Levich [41]: h = 0.94(~TUo) 2 / 3 / 7 1 / 6 ( p g )1/2

structures ranged from rather weakly branched polymers characterized by a mass fractal dimension to highly condensed particles [40]. The slopes are quite close to 0.66 in keeping with the expectations from eq. (2). This reasonable correspondence between the thickness of the deposited

'

i

r_~ 1000~

(2)

Figure 2 plots the logarithm of the product of thickness and refractive index minus 1 * versus the logarithm o f U 0 for films prepared from a variety of silicate sols in which the precursor

03

O "J

Slope

/ / y

• Strongly Branched (0.61) [] Weakly Branched (0.62) O Particulate95 nm (0.67)

.~

• 100

* Since the quantity ( n - 1 ) is proportional to the volume fraction solids, ~h, the product h ( n - 1) is proportional to the mass per unit area of film and takes into account the film porosity.

J

,

,

,

Particulate 72 nm (0.66)

, , ,I 10

,

,

,

Log Coating Rate (in/min)

Fig. 2. Product of film thickness and refractive index minus 1 (proportional to film mass/unit area) versus withdrawal rate plotted according to eqs. (1) or (2).


426

films and a theory developed for gravitational draining of pure fluids suggests that the entrainment of the inorganic species has little effect on the hydrodynamics of dip-coating, at least in the early stages of deposition where the entrained sol is quite dilute. Thus, some insight into sol-gel film deposition may be gained by closer examination of the details of gravitational draining (and evaporation) of pure (and binary) fluids.

2.1. Film thickness profiles during dip-coating." pure fluid Previous theories of gravitational draining of pure fluids have not taken into account simultaneous evaporation. Although the thickness of the fluid entrained at the bath surface is apparently not sensitive to evaporation, the film is progressively thinned by evaporation as it is transported by the substrate away from the coating bath. For depositing sols, thinning by evaporation causes a corresponding increase in sol concentration, hence an understanding of simultaneous draining and evaporation is essential to the underlying physics of sol-gel film deposition. In order to address this problem, Hurd and Brinker [39] developed an imaging ellipsometer

that allows aquisition of spatially resolved thickness and refractive index data over the entire area of the depositing film. A thickness profile of an ethanol film obtained by imaging ellipsometry is shown in.fig. 3 [43]. Instead of the wedge expected for a constant evaporation rate, the film profile is distinctly blunt near the drying line (x = 0 in figs. 1 and 3), indicative of more rapid thinning and, hence, a greater evaporation rate there. This position sensitive evaporation rate is a consequence of the film geometry: the blade-like shape of the depositing film in the vicinity of the drying line enhances the rate of diffusion of vapor away from the film surface [43] (at large x, the strongest concentration gradients of vapor are normal to the surface, while near x = 0, stronger gradients are established parallel to the surface). Near any sharp boundaries, the evaporation rate, E, diverges, but the vaporized mass must remain integrable. For the knife blade geometry (infinite sheet), E varies with x as follows [43]:

E ( x ) = -Dva~X -1/2,

(3)

where D v is the diffusion coefficient of the vapor ( ~ 0.1 cmZ/s) and a I is a constant. Since thickness varies inversely with evaporation rate, the

2,0

I

I

I

b 1.5

1.5

I

i

I

E ,,¢:

(/) LU Z

1.0

1.0

o_ "r

e-

* ('0 ~ ~

0.5

0.5

0,0 0

I

I

[

1

2

3

x (mm)

o.o 0

- x'O.62

J

I

I

500

1000

1500

2000

POSITION x ( g m )

Fig. 3. (a) Thickness profile of dip-coated ethanol film (solid dots). T h e profile is quite well fit by the form h ~ x" with u = 0.5 _+0.01 (solid line). From H u r d and Brinker [43]. (b) Thickness profile of a titanate sol during dip-coating as determined by imaging ellipsometry. Position x is defined in fig. 1. From Brinker et al. [37].

427

C.J. Brinker et al. / Sol-gel thin film .formation

propanol/water (50:50)

divergence in the evaporation rate as x---> 0 accounts for the blunt profile shown in fig. 3, where the data are fit to the form

0 0

h(x)~l/E(x)~x

~,

u=0.50_+0.01,

0

(4)

0 0

according to the expectations from eq. (3). The singularity strength (exponent) in eq. (4) is sensitive to the geometry of the film. For coating a fiber, we would expect a logarithmic singularity [44], and as a coated cylinder decreases in radius, the profile should pass smoothly from x t/2 toward ln x. Although some .experimental evidence exists for this behavior by extrapolating to small cylinder diameters [44], it is a difficult proposition to prove experimentally. Surface tension effects make it difficult to coat a fiber fast enough for the drying line to be well-separated from the gravitational meniscus at the reservoir surface. 2.2. Film thickness profiles during dip-coating." binary fluid

The most common coating sols are composed of two or more miscible liquids, e.g., e t h a n o l water. Differences in their evaporation rates and surface tensions alter the shape of the fluid profile in the vicinity of the drying line and, in some cases, create rib-like instabilities in a region near the liquid bath surface. Ellipsometric images [44] of depositing alcohol-water films show two roughly parabolic features (fig. 4), that correspond to successive drying of the alcohol- and water-rich regions, according to the non-constant evaporation model (eq. (4)). This suggests that each component has an independent evaporation singularity. If the water-rich phase is denoted as phase 1 and ethanol is phase 2, then the independent profiles are additive: h 1 = al X1/2,

hz=a2(x

h 2 = 0,

--X ~1/2 21

x > O,

(5)

x>x2,

(6)

x x2)

(8)

where the surface tension is assumed to follow a simple mixing law, 3' = &t3'1 + 623'2 where bi is the volume fraction of component i. ** Since at the liquid-vapor boundary the viscous shear force must balance the force imposed by surface tension gradients, ~7 d u / d z = d 3 " / d x ( z = h), liquid flows into the water-rich foot with velocity, u: u = 1 / r l [ d 3 " / d x ] z - Uo,

(9)

the so-called 'Marangoni effect'. The foot slowly ,

where h is the total thickness, h 1 4- h2, and x 2 is the position of the 'false' drying line created by the substantial depletion of ethanol.

** For ethanol-water mixtures, the surface tension does not obey a simple linear mixing law. The surface tension can

be approximated by 1//('y27) = q~H/(TI2i7)q- 'OE/--(TE27) where the subscripts H and E refer to water and ethanol, respectively [69].

428

C.J. Brinker et al. / Sol-gel thin fihn formation

grows until this flux is balanced by that of evaporation from the expanding free surface. The surface tension gradient driven flow of liquid through the thin neck created by the preferential evaporation of alcohol can create quite high shear rates during dip-coating. A striking example is that of toluene and methanol [44]. The surface tension gradient driven flows are strong enough to greatly distort t h e double parabolic profile. The film thins then thickens, creating a 'pile-up' of toluene near the drying line. A crude estimate of the surface tension gradient, zXy/Ax = (10 d y n / c m ) / 1 0 -1 cm, leads to a shear rate, d u / d z = 104 s -1, in the thin region, from eq. (9), assuming ~7 = 0.01 P. Conceivably these shear fields may be sufficiently strong to align or order the entrained inorganic species.

inward. The thickness of an initially uniform film during spin-off is described by

h(t) = h 0 / ( 1 + 4pw2h2t/3~7) 1/2

(10)

where h 0 is .the initial thickness, t is time, p is the density, and o) is the angular velocity. Even films that are not initially uniform tend monotonically toward uniformity, sooner or later following eq. (10). Equation (10) pertains to Newtonian liquids that do not exhibit a shear rate dependence of the viscosity during the spin-off stage. If the liquid is shear-thinning (often the case for aggregating sols), the lower shear rate experienced near the center of the substrate causes the viscosity to be higher there and the film to be thicker. This problem might be avoided by metering the liquid from a radially moving arm during the deposition/spin-up stage.

3. Spin-coating Spin-coating differs from dip-coating in that the depositing film thins by centrifugal draining and evaporation. Bornside et al. [45] divide spincoating into four stages: deposition, spin-up, spinoff and evaporation, although for sol-gel coating, evaporation normally overlaps the other stages. An excess of liquid is dispensed on the surface during the deposition stage. In the spin-up stage, the liquid flows radially outward, driven by centrifugal force. In the spin-off stage, excess liquid flows to the perimeter and leaves as droplets. As the film thins, the rate of removal of excess liquid by spin-off slows down, because the thinner the film, the greater resistance to flow, and because the concentration of the non-volatile components increases, raising the viscosity. In the final stage, evaporation takes over as the primary mechanism of thinning. According to Scriven [38], an advantage of spin-coating is that a film of liquid tends to become uniform in thickness during spin-off and, once uniform, tends to remain so, provided that the viscosity is not shear-dependent and does not vary over the substrate. This tendency is due to the balance between the two main forces: centrifugal force, which drives flow radially outward, and viscous force (friction), which acts radially

4. Effects of entrained condensed phases The preceding discussion has ignored the effects of the entrained inorganic species, either polymers or particles, on the film deposition process. During dip-coating, these species are initially concentrated by evaporation of solvent as they are transported from the coating bath toward the drying line within the thinning fluid film (see fig. 1). Steady-state conditions in this region require conservation of non-volatile mass; thus, the solids mass in any horizontal slice of the thinning film must be constant [43]: h ( x ) &s( x ) = constant,

(11)

where &s is the volume fraction solids. From eq. (11), we see that &s varies inversely with h. Since for a planar substrate we expect a parabolic thickness profile, ,;bs should vary as 1/h = x-i/2 in the thinning film. When coating a fiber, we expect &(x) ~ (lnx) -1. The rapid concentration of the entrained inorganic species by evaporation is more evident from consideration of the mean particle (polymer) separation distance, ( r ) , which varies as the inverse cube root of &, ( r ) ~ X 1/6. This is a very precipitous function: half the distance between particle


(polymer) neighbors is traveled in the last 2% of the deposition process ( ~ 0.1 s). The centrifugal acceleration needed to cause an equivalent rate of crowding is as much as 106 G's! § The increasing concentration can lead to aggregation, network formation, or a colloidal crystalline state, altering the sol rheology from Newtonian (dilute conditions) to shear-thinning (aggregated systems) or thixotropic (ordered systems). For polymeric sols, the reduced viscosity shows a strong concentration dependence [46], and, in general, the viscosity increases abruptly at high concentrations. Bornside et al. [45], in their studies of spin-coating, assumed the following relationship: r7 = */0(1 - X A ) 4 + rl s,

(12)

where r/ is the viscosity of the sol, rls is the viscosity of the solvent, T0 is the viscosity of the polymer, and XA is the mass fraction of solvent. In dip-coating, the thickness of the entrained film (c~ U ff/3) and the evaporation rate establish the timescale of the deposition process, which is typically several seconds. The forced convection created during spin-coating increases the evaporation rate, establishing an even shorter timescale. These short timescales significantly reduce the time available for aggregation, gclation, and aging compared with bulk gel formation. We anticipate several consequences of the short timescale of the film deposition processes. (1) There is little time available for reacting species to 'find' low energy configurations. Thus (for reactive systems) the dominant aggregative process responsible for network formation may change from reaction-limited (near the reservoir surface) to transport-limited near the drying line. (2) For sols composed of repulsive particles, there is little time available for the particles to order as they are concentrated in the thinning film. (3) There is little time available for condensation reactions to occur. Thus gelation may actually occur by a physical process, through the

§ This assumes that there exists no steric barriers to concentration; often aggregation/network formation will interupt this dramatic compaction process.

429

concentration dependence of the viscosity (e.g., eq. (12)), rather than a chemical process. (In some systems this is evident by the fact that the deposited film is quickly re-solubilized when immersed in solvent.) (4) Since the gels are most likely more weakly condensed and hence more compliant than bulk gels, they are more easily compacted, first by evaporation and then by the capillary pressure exerted at the final stage of the deposition process (see fig. 1). In such compliant materials the effects of capillary forces are enhanced, because greater shrinkage precedes the critical point, causing the pore size to be smaller and the maximum capillary pressure to be greater.

5. Drying of films 5.1. Capillary pressure

As stated above, drying accompanies both the dip- and spin-coating processes and largely establishes the shape of the fluid film profile. The increasing concentration that results from drying often leads to the formation of an elastic or viscoelastic gel-like state. Further evaporation gives rise to capillary tension in the liquid, P, and that tension is balanced by compressive stresses on the solid phase, causing it to contract further [47]. The maximum capillary tension occurs at the critical point, when the menisci enter the pores, and the radius of curvature of the meniscus, rm, is related to the pore radius, rp, by r m = rp/COS 0, where 0 is the contact angle of the receding meniscus within the emptying pore [47]. Then the tension at the drying surface is given by Laplace's equation: Pmax = 2TLV/rm = 2TLV Cos(O)/rp.

(13)

Since rp can be of molecular dimensions, the magnitude of Pmax can be very large. Using values of minimum menisci radii, rmin, determined from desorption isotherms and assuming complete wetting (cos0 = 1), it is possible to estimate from eq. (13) the maximum capillary tension of the liquid prior to tensile failure [48]. For ethanol (Yi.v = 22.75 d y n / c m at 20°C), rmin is estimated to be

430


about 1.3 nm, and Pmax--~348 bar. For water

(YLv/= 72.8 d y n / c m at 20°C), rmin is estimated to range from 1.1 to 1.55 nm, so Pmax ranges from 940 to 1320 bar! These large tensile pressures drive the solvent into metastable states analogous to superheating. Burgess and Everett suggest that the liquid does not boil, because nucleation cannot occur in such small pores [49]. Very little shrinkage occurs after the menisci recede into the pores, so the pore size in a dry gel is largely established by the forces exerted at the critical point [47]. For very compliant materials, the network cannot resist the capillary forces (which increase continuously as rp decreases toward rmin), so there is no critical point, and the pores collapse completely [37,50]. Conversely, for stiffer materials, shrinkage ceases at an earlier stage of drying, causing rp to be larger and Pm~x to be smaller. This situation leads to porous films.

5.2. Stages of drying Scherer [40,47] divides the drying of gels into two stages: a constant rate period (CRP) and a falling rate period. During the constant rate period, mass transfer is limited by convection away from the gel surface, whereas during the falling rate period, mass transfer is limited by the permeability of the gel. Extending these ideas to dip-coating, we might expect that a CRP would obtain throughout most of the deposition process, since the liquid-vapor interface remains located at the exterior surface of the thinning film except at the final stage of drying (see fig. 1). A constant evaporation rate implies a wedge-shaped film profile. This is not observed for pure fluids, nor is it observed for inorganic sols. Figure 3(b) shows the film profile of a titanate sol prepared in ethanol. Thickness varies with distance from the drying line as h(x)~x °'62, which indicates that the evaporation rate increases as x -+ 0 (see eqs. (3) and (4)) although not as rapidly as for pure ethanol (h(x)~x°5). Thus, even for the deposition of inorganic sols, the film profile, and hence the concentration profile, are largely established by the dependence of the evaporation rate on the geometry of the depositing film. For sols containing fluid mixtures of differing volatilities, the fluid

composition changes with distance, x, contributing to further changes in the evaporation rate. In the CRP, the rate of drying is usually calculated from an external mass transfer correlation such as [51] mass flux/unit area = kmt ( Ps - P~),

(14)

where Ps is the theoretical density of solvent in equilibrium with the surface of the coating, p= is the theoretical density of solvent vapor far removed from the coating surface, and kmt is the mass transfer coefficient (m/s). For modeling sol-gel dip-coating, kmt must be position-dependent. The critical point should mark the beginning of the falling rate period. Depending on the distribution of the liquid in the pores, for example funicular or pendular, the drying rate is limited by flow (funicular state) or diffusion (pendular state) [40]. For compliant molecular networks that are collapsed prior to the critical point, drying occurs by Fickian diffusion if the temperature is above the glass transition temperature of the mixture [52]. The onset of a falling rate period near the drying line may account for the differences in the exponents that describe the shape of the pure fluid and the titanate sol film profiles (compare figs. 3(a) and 3(b)).

5.3. Drying stress and cracking As the film dries, it shrinks in volume. Once the film is attached to the substrate and unable to shrink in that direction, the reduction in volume is accommodated completely by a reduction in thickness. When the film has solidified and stresses can no longer be relieved by flow, tensile stresses develop in the plane of the substrate. Croll [53] estimated the stress, o-, as o-= [ E / ( 1 - u)] [(fs - f r ) / 3 ]

(15)

where E is Young's modulus (Pa), ~, is Poisson's ratio, fs is the volume fraction solvent at the solidification point, and fr is the volume fraction of residual solvent in the 'dry' film. The solidification point was defined for a polymer film as the concentration where the glass transition temperature has risen to the experimental temperature.


Thus stress is proportional to Young's modulus and the difference between the fraction solvent at the solidification point and the dried coating. Scherer [40,47] states that the stress in the film is very nearly equal to the tension in the liquid (~ = P ) . Despite such a large stress, it is commonly observed that cracking of films does not occur if the film thickness is below a certain critical thickness h c -~ 0.5-1 p~m [40]. For films that a d h e r e well to the substrate, the critical thickness for crack propagation or the growth of pinholes is given by [54,55]

431

linearly with a reduction in particle size. Since, for unaggregated particulate films, the pore size scales with the particle size, this may be due to an increase in the stress caused by the capillary pressure (or-~ P ) a n d / o r an increase in the volume fraction solvent at the solidification point resulting from the manner in which the electrostatic double layer thickness (estimated by the D e b y e - H u c k e l screening length) varies with particle size [40].

6. Control of microstructure

h c = (Kic/O-~Q) 2

(16)

where KIc is the critical stress intensity and ~2 is a function that depends on the ratio of the elastic modulus of the film and substrate (for gel films s2--1). For films thinner than h c, the energy required to extend the crack is greater than the energy gained from relief of stresses near the crack, so cracking is not observed [40]. When the film thickness exceeds hc, cracking occurs, and the crack patterns observed experimentally are qualitatively consistent with fractal patterns predicted by computer simulation [56]. Atkinson and Guppy [57] observed that the crack spacing increased with film thickness and attributed this behavior to a mechanism in which partial delamination accompanies crack propagation. Such delamination was observed directly by Garino [58] during the cracking of sol-gel silicate films. Based on eqs. (15) and (16) above, strategies to avoid cracking include: (1) increasing the fracture toughness, Kit , of the film, (2) reducing the modulus of the film, (3) reducing the volume fraction of solvent at the solidification point, and (4) reducing the film thickness. In organic polymer films, plasticizers are often added to reduce the stiffness of the film and thus avoid cracking [51]. For sol-gel systems, analogous results are obtained by organic modification of alkoxide precursors [32], chelation by multidentate ligands such as B-diketonates [59], or a reduction in the extent of hydrolysis of alkoxide precursors [158]. It should be noted that for particulate films Garino [60] observed that the maximum film thickness obtainable without cracks decreased

The final film microstructure depends on the structure of the entrained inorganic species in the original sol (for example, size and fractal dimension), the reactivity of these species (for example, condensation or aggregation rates), the timescale of the deposition process (related to evaporation rate and film thickness), and the magnitude of shear forces and capillary forces that accompany film deposition (related to surface tension of the solvent or carrier and surface tension gradients). The most common means of controlling the film microstructure is to control the particle size. For unaggregated monosized particulate sols, the pore size decreases and the surface area increases with decreasing particle size. Asymmetric, gupported membranes have been prepared successfully from particulate sols for use in ultrafiltration [18,19]. As noted above, difficulties arise when trying to prepare microporous membranes due to an increased tendency for cracking. Partic~atate sols may be intentionally aggregated prior to film formation to create very porous films (e.g., > 65 vol.% porosity) [40]. For electrostatically stabilized silica sols, a transition from random close packing to ordered packing is observed with increasing substrate withdrawal rates, U0. This may be due to a longer timescale of the deposition process (providing more time for ordering) or an increase in the shear rate accompanying deposition for higher U0 [37]. A second strategy for controlling porosity is based on the scaling of mass, Mr, and size, rf, Of a mass fractal object [40]: Mf ~ rfD,

(17)

C.J. Brinker et al. / Sol-gel th#l film formation

432

Table 1 Refractive index, % porosity, pore size, and surface area of multicomponent silicate films versus sol aging times prior to film deposition Sample aging times a)

Refractive index

unaged

1.45

Porosity b) (%)

Median pore radius (nm) < 0.2

0-3 days

Surface area (m2/g)

Applications

1.2-1.9

Dense protective, electronic and optical films

)

Microporous films for sensors and membranes

3 days

1.31

16

1.5

146]

1 week

1.25

24

1.6

220~

2 week 3 week ~)

1.21 1.18

33 52

1.9 3.0

263]

!

(

Mesoporous films for sensors, membranes, catalysts, optics

245}

a~ Aging of dilute sol at 50°C and pH3 prior to film deposition. b) Determined from N 2 adsorption isotherm. c) The 3-week sample gelled. It was re-liquified at high shear rates and diluted with ethanol prior to film deposition.

where D is the mass fractal dimension (in threedimensional space, 0 < D < 3). Since density equals mass/volume, the density, Of, of a mass fractal object varies in three dimensional space as p f ~ ry/r~, and the porosity varies a s 1 / p f r~3-z)). Thus, the porosity of a mass fractal object increases with its size. Providing that such fractals do not completely interpenetrate during film formation (i.e., they are mutually opaque, requiring D > 1.5 [40]), the porosity may be controlled by the size of the entrained fractal species prior to film formation. The efficacy of this approach is illustrated in table 1 [37] where the refractive index, % porosity, pore size, and surface area are seen to vary monotonically with aging time employed to grow the fractal species prior to film deposition. §~ The extent of interpenetration of colliding fractals depends on their respective mass fractal dimensions and the condensation rate or 'sticking probability' at points of intersection. A reduction of either D or the condensation rate increases the interpentration and decreases the porosity [37,40]. From eq. (17) and surrounding §§ The film porosity, pore size, and surface area were measured in situ using a surface acoustic wave technique developed by Frye et al. [14].

discussion, it follows that, to generate porosity using this fractal scheme, rf should be large, 1.5

and ejection processes

melting temperatures and initiate significant evaporation. Material ejection is accompanied by an easily recognisable snapping sound as the velocity of some of the species exceeds the speed of sound in the immediate environment, as a bright-coloured plasma (or plume) of emitted particles is formed just above the target surface. Although a very large number of thin films have been produced by PLD, it is the multi-component systems that can benefit from this technique, such as the new families of high temperature superconductors and various ferroelectric, magnetic and sensor layers. The underlying non-equilibrium mechanisms involved in this removal process are extremely difficult to model. Several recent papers have nevertheless addressed this complicated subject.‘7,‘8 It is instructive to model the energy balance of the system as a conventional laser heating problem. By taking temperature averaged values of the usual optical and thermal constants one can obtain reasonable values of the fluences required to melt a range of materials. For example, for 248nm and 1064 nm light, these are calculated to be at least 70 mJ/cm* and 260 mJ/cm*, respectively, for YBaCuO, and fluences of 140 and 560 mJ/cm* are found to be the minimum values to induce thermal removal,i9 in good agreement with the reported observations.20-23 A more complete model clearly must take into account the non-linear behaviour of the optical and thermal properties with temperature, as well as the spatial and temporal extent of the beam but nonetheless these are already good estimates for the process. Interactions between the photons and the ablated plasma must also be considered. For example,

and (right) pressure

effects of PLD.

electrons present near the surface will strongly couple the electromagnetic radiation, accelerate and collide with any plasma ions or any nearby solid or gas phase atoms (inverse bremsstrahlung). The electron population can then be increased and excited further, causing extremely rapid cascade ionisation. The collective properties of the plasma can thus strongly determine what happens to the remainder of the incident light. The energy absorbed in the plasma will be rapidly shared amongst the individual particles, raising their kinetic energy, i.e. temperature, and shielding the surface from further exposure to the laser beam. If the optical absorption of the plasma falls, its increased transparency will enable further surface exposure to re-populate the plasma and again increase absorption and surface shielding. Thus, it could be deduced24 that an instantaneous equilibrium plasma density, once formed, would try to self-regulate against any changes in density, temperature or irradiance. The plasma temperature can be as much as 20,000 K, whilst the surface temperature of the underlying solid will only reach a modest amount above the evaporation point (-2000-3000 K). Clearly, the mechanisms by which material leaves the surface, and their subsequent interactions and behaviour, are much more complex than can be predicted using thermal evaporation. For laser ablation using visible or UV wavelengths it has been shown25 that within a cone of acceptance the angle of incidence of the incident laser beam does not affect the trajectory of the ejected material, which is virtually always perpendicular to the irradiated surface. The spatial density of the plume is affected by many factors,

Thin$lm growth by PLD the most important of which include beam fluence and spot-size, and ambient gas content and pressure. Analysis of the deposits obtained by short wavelength laser ablation often reveals two components to the thickness contour.25,26 The expected broad cos 4 profile due to thermal evaporation is usually present, but superimposed upon this is a highly peaked deposit whose profile can be fitted by a cos” 4 curve, where 4 < n < 15 (depending especially upon gas pressure), is superimposed. 19*2628 For shorter wavelengths at fluences near the threshold for material removal only the pure cos $I form is found, while a cos”$ deposition is found only at higher fluences.25 The irradiation geometry can be altered to modify the deposited profile. One way, amongst many, to homogenise the deposit is to tilt the rotating target during ablation in order to “spray” the deposit across the substrate. An angle of wobble of about 30” can produce a uniform film over a few square centimetres.29 Other tilting approaches can also be applied, and positioning the substrate edge-on inside the plume instead of directly perpendicular to it has also been shown to produce more homogeneous films. However, Rutherford back-scattering studies have shown that while the centres of the peaked areas grown by PLD are essentially stoichiometric, the outer edges (containing the cos 4 component only) are not,25 an observation which may limit some approaches to homogenisation. The ablation mechanism, as well as the confinement of the ejectant, has been described in terms of hydrodynamic sputtering,” whilst the effects of shock waves, surface superheating, and sub-surface explosions3@33 have also been proposed. Singh et al. ‘7,33 have modelled a plasma expanding isotropically into the vacuum and have been able to obtain similar atomic profiles to those mentioned above. The most spectacular accompaniment to the ablation growth of films is the colourful plume that forms between target and substrate. It occupies a volume containing high densities of electrons, excited and ionised atoms, and molecular clusters, all moving at very high velocities away from the surface. Its formation and content are subject to intense investigation using, for example, Laser Ion Mass Spectroscopy34 and optical emission spectroscopy. 35 The phenomena are reminiscent of dielectric breakdown, caused by the highly localised electric field strengths at the surface. Many neutral species are also known to form, apparently more so when longer wavelength lasers are used (i.e. 1064 nm rather than 193 nm).22 This is most likely due to the restricted variety of interactions available to the less energetic photons, thus

431 eliminating, for example, the possibility of photoionisation of the species in the plasma. Electron impact ionisation or cascade ionisation will also be less and so generally the plasmas associated with the shorter wavelength ablation will be the most energetic and reactive. With CO2 laser ablation, very efficient inverse bremsstrahlung photoncoupling can be achieved in the plasma. Particle velocities can thus be much higher than for 248 nm ablation,36 though it is not clear why stoichiometric ablation can only be achieved at 10.6 pm for low and not high fluences.37 Because of the usual irradiation geometries used, the plasma, consisting of ions, electrons, atoms, neutrals, molecules, and clusters and particulates of varying sizes, will initially be thin and flat, spreading above the surface exposed to the laser beam. Soon after its formation and until the end of the laser pulse it can be considered isothermal, with temperatures exceeding 1O3K ( z 7.5 eV). Immediately after irradiation, the amount of material augmenting the plasma will drop considerably. It will then expand preferentially away from the target, this being the direction of the greatest density gradients, into the surrounding gas or vacuum, accelerating the species to speeds approaching cm/ps (up to lo3 times the usual velocities encountered during thermal evaporation). This expansion can push any existing gases away from the target, setting up a pressure wave. As the particles slow down and lose energy, they give rise to characteristic UV and visible emission patterns that can be studied in situ. Such spectroscopic studies have revealed the predominance of excited elemental and monoxide species, agreeing with the mass spectrometry results, although collisions within the plasma may lead to the formation of more intricate structures. Time-resolved photography has recently been applied to the expanding plume in vacua and moderate pressures,38 and evidence of particulate back-reflection leading to redeposition on the target at high pressures has been found.39 Figure 1 summarises the general scheme of the ablation processes. The explosive velocity distributions associated with the particles leaving the ablated surface have been measured directly using time-of-flight (TOF) measurements40 to be about 2 km/s in 100 mTorr pressures,40-42 while speeds up to 2 x lo6 cm/s for particles some 7 cm away from the target have been measured using an ion probe.4’ The velocity distributions compare well with the formation of supersonic molecular beams in chemical studies, where the velocities can be described by:14 f(v) = Av3exp(m(v

- ~~)~/2krs)

(1)

I. W. Boyd

432 A major problem that often arises during the application of PLD is the presence of particulates (from 100s of nm to 10s of pm in size) on the film surface. Many of these, known also as droplets or blobs, are associated with particles ejected from the target, which travel behind the plume at velocities of typically 5&200 m/s.‘* In general, more droplets are formed at higher fluences, for picosecond rather than nanosecond pulses, and where less dense or porous targets are used. Several techniques can minimise droplet formation. The most successful involves target rotation. This ensures that the remnants of previous ablation events, often resolidified off-stoichiometric mixtures, are not re-irradiated and ejected from the surface. Barr43 and others have used a 10,000 rpm rotating deflector that selectively deflects slow particles and transmits faster moving species. Gaponov et al4 used two colliding beams to generate a high pressure zone which expanded towards the substrate, encouraging heavier lumps to miss the surface altogether. A negative substrate bias of several hundred volts can also considerably reduce heavy clusters that are similarly charged, and this significantly improved the growth of Ge by PLD.45 Koren et a1.46 and Chiba et a1.47 have also used a second laser beam to excite and dissociate the larger particles within the plasma. Sankur et al. 48 found that using molten target sources virtually eliminated droplet formation with Ge, and for limited fluences, B2O3. Alternative geometries, such as the edge-on placement of substrates inside the plume, have also been shown to be varyingly successful. In summary, the ablation process (see Fig. 2) may be considered as follows. Under intense absorption of optical energy, all the component atoms are ejected in a violent rupture of an ultrathin surface layer. This ensures stoichiometric transfer. Once formed, the plasma, containing ions, electrons, neutrals, and clusters, may absorb the remaining light in the incident laser pulse and attain a very high temperature. The species speed away from the surface, the dynamics of the plasma expansion and subsequent emission being delicately related to the background gas type and pressure. After collisions with themselves and with any background gas they lose kinetic energy, and particles may form in the gas phase. If they are intercepted by a solid, a thin film can grow. Thus, the substrate position with respect to the plume is critical to film quality. This is because, in some cases, the parameters that minimise droplet formation on the surface may not necessarily be identical to those necessary for optimum film quality.

1.6 1.4 3

1.2

I

1

I

I

JO -::

I

I

I

7059

Si(lp0) Si(ll1)

i

E s 1.0

0

E 0.8 @ 0.6

-. 8 B

sc 0.4

’

0.2 0 0

0

a

I

I

1

20

40

60

o I

80

B I

100

I

Q --I I

120 140

Thickness (nm) Fig. 2. Evolution of the FWHM values of the 111 peaks of CeOl films grown on Si and glass.

3 EXPERIMENTAL

APPROACH

In this section, specific examples of PLD will be presented. In fact, a vast number of layers can be grown by this method. I9 A typical experimental set-up, as already shown in Fig. 1, basically consists of an evaporation chamber, with a laser beam and directing optics positioned outside the cell. The target pellet is commonly irradiated at an angle of around 45-60”. The substrate upon which the film grows is usually positioned parallel to the target, although this is not essential, and is often heated to high temperatures to help promote epitaxy. To assist in optimising stoichiometric transfer during irradiation, the target is either rotated, or the impinging beam is scanned across it. Although working pressures around 10e6 Torr are common, precise amounts of 0 2, N20, N2, or other gases may be pumped through the chamber during deposition. The beam fluences used for ceramic ablation are around l-4 J/cm* (higher values may occasionally be employed), while most of the lasers operate at 10 Hz, although in principle higher rates might be desirable. Typical deposition rates are around 0.5-5 A/pulse, and in 10-20 min films up to 1 pm in thickness can be grown. Depending on the nature of the target irradiated, and more importantly the thermal and chemical properties of the host substrate and its temperature, the grown films may be amorphous or crystalline. Mismatch in lattice and thermal expansion is important as in any heterogeneous layer growth. Any chemical reactivity at the interface should not be detrimental to the growing film. Substrates of strontium titanate (SrTi03), magnesium oxide (MgO), and yttrium stabilised zirconia (YSZ) have been host to amongst the best thin film superconductors reported to date.

Thin jilm growth by PLD

433

There is presently a great deal of investigation into the possible use of buffer layers that might in future enable successful combination of superconducting films with c-Si devices. Crucial amongst the criteria to be observed are: Temperature

(a) Intermixing and chemical reaction at the superconductor-semiconductor interface. (b) Thermal mismatch between the two components. (c) Strain introduced by mismatch in the lattice constants of the two materials. Buffer layer growth on Si, especially Ce02 because of its good lattice match and dielectric properties, has been studied in recent years. Both substrate temperature and 02 pressure are important in determining the crystalline quality of the films. Figure 2 shows that the substrate properties are not crucial for ordered growth, as a narrowing of the FWHM of the (Ill) peak of the XRD pattern with film thickness occurs independently of whether the substrate was Si( 11 l), Si( loo), or 7059 glass. The dense packing of the Ce atoms on the glass substrate or on the native oxide present on the Si, due to their strong affinity for 0, leads to preferred growth in this direction. Differently doped buffers can also be grown by PLD to engineer the lattice constant. For example, CeOz doped with 40% La203 can change the lattice constant of PLD-grown buffers by some 2.5% .49 The growth of superconducting Ba2YCus07 films on crystal substrates by PLD is well established. Conditions used by the vast majority of groups are based on the work of Inman et ~1.~~Although modifications are possible, typical conditions involve the use of fluences of 1.5 J/cm* at 248 nm, about 100 mTorr of 02, and substrate temperatures from 700 to 750°C. High quality films can also be grown on buffer layers, such as Ce02 or YSZ, deposited by PLD on a variety of substrates, such as c-Si. The BiSrCaCuO system is much more difficult to prepare and process. This homologous series Bi$rzCa,Cu, + I0, contains three members at n = 0, 1 and 2, with superconducting transitions of 20 K, 80 K and 1 IO K. These are structurally interrelated and differ only by the insertion of the CaCu02 accommodated by extra layers and are often described by the number of Cu-0 planes, i.e. 1 (n = 0), 2 (n = l), or 3 (n = 2). The chemistry of these phases is very complex and isolation of pure single phase ceramic samples, particularly of the n = 3 member, has proven to be a major challenge. This 2223 (n = 2, r, = 110 K) phase, for example, is only stable over a 10 K range near its melting point. In general, the thermodynamically stable 2212 (n = 1, T, = 80 K) phase is formed

5

10

15

(K)

20

25

30

3.5

40

2 Theta

Fig. 3. XRD

and

resistivity of multilayer-grown SrCaCuO films.

Bi(Pb)-

preferentially and the 110 K phase is then produced by decomposition. Many attempts to grow films of BiSrCaCuO have been reported, although the level of activity is substantially lower than that for Ba2YCu307, The incorporation of lead (Pb) on to the bismuth site promotes the formation of the 2223 phase over 2212, and nearly single phase material can be produced by very long annealing times (- 100 h) in the presence of a Pb vapour. Alternate PLD of BiSrCaCuO and PbO layers in sandwich structures has been shown to promote the 2223 phase.5’ Figure 3 shows the X-ray diffraction pattern and electrical resistivity of such a multilayered film, annealed for 15 h at 854°C. This method has the advantage that it enables independent control over the Pb content in the film and the temperature cycling. A similar approach has been adopted to grow the first films of PbSrYCaCuO by making sandwiches of PbO and semi-conducting SrYCaCUO.~~ The ability to fabricate novel materials by PLD thus opens up a vast new area of exciting research. The methods of sandwiching and reactive growth in O2 shown here are only a few of the myriad of possibilities available with PLD. A further example of reactive PLD is the growth of TIN, obtained by ablating pure Ti metal in N2. Approaches using less dense and more porous TIN targets often report undesirable incorporation of oxygen impurities. The pure Ti method produces crystalline films at 450°C with a much reduced 0 content and have smooth mirror-like surfaces.53 In terms of simple oxide layers, optimised PLD can compare extremely favourably with conventional deposition techniques. For example, ZnO films have been grown at 350°C by ablating ZnO targets in O2 pressures of 14 x 1O-3 Torr, with a refractive index of 1.97, a bandgap of 3.26 eV, and optical transmission of 85% in the visible, which are c-axis oriented with the FWHM value of their

I. W. Boyd

434 (002) XRD reflection being less than 0.15”. These are amongst the best properties ever reported for thin ZnO films.54 In summary, the process of PLD has been described. An outline of the controlling mechanisms has been presented in terms of absorption, particle ejection, and plume dynamics. PLD has very exciting prospects and offers advantages over some traditional deposition technologies. It is clear, however, that PLD will not displace any of these, but will introduce increased scope for new film growth. PLD still retains disadvantages; film uniformity is not optimum and particle formation can be difficult to completely eliminate. These most likely remain a mischief rather than a fundamentally insurmountable problem. Probably the biggest challenge to PLD is the film area that can be currently produced, limited by the beam sizes available with present-day laser technology. REFERENCES 1. BOYD, I. W., Laser Processing of Thin Films and Microstructures. Springer Verlag, Germany, 1987. 2. SMITH, H. M. & TURNER, A. F., Appl. Optics, 4 (1965) 147. 3. ZAVITSANOS, P. D. & BREWER, L., Proc. Nat. Electron Conf.. 24 (1968) 864. 4. BAN, V. S. & KRAMER, D. A., J. Mater. Sci.. 5 (1970) 978. 5. SANKUR, H. & CHEUNG, J. T., J. Vat. Sci. Technoi., Al (1983) 1806. 6. BOYD, I. W., Laser Processing of Thin Films and Microstructures. Springer Verlag, Berhn, 1987, pp. 28&l. 7. CHEUNG. J. T. & MAGEE. T.. J. Vat. Sci. Technol. Al (1983) i604. 8. CALI, C., DANEU, V., ORIOLI, A. & RIVASANSEVERINO, S., Appl. Optics, 15 (1976) 1327. 9. BALEVA, M. I., MAKSIMOV, M. H. & METEV, S. M., J. Mater. Sci. Left., 5 (1986) 533. 10. CHEUNG, J. T. & MADDEN, J., J. Vat. Sci. Technol., B5 (1987) 705. 11. KELLY, R. & ROTHENBERG, J. E., Nucl. Inst. and Methods in Phys. Res., B7/8 (1985) 755. 12. SRINIVASON, R. & MAYNE-BANTON, V., Appl. Phys. Lett., 41 (1982) 576. 13. DIJKKAMP, D., VENKATESAN, T., WU, X. D., SHAHEEN, S. H., JISRAWI, N., MIN-LEE, Y. H., MCLEAN, W. L. & CROFT, M., Appl. Phys. Left., 51 (1987) 619. 14. ANDREW, J. E., DYER, P. E. & FORSTER, D., Appl. Phys. Left., 43 (1983) 717. 15. DUPENDANT, H., GAVIGAN, J. P., GIVORD, D., LIENARD, A., REBOUILLAT, J. P. & SOUCHE, Y., Appl. Surf. Sci., 43 (1989) 36. 16. KWOK, H. S., SHAW, D. T., YING, Q. Y., ZHENG, J. P., WITANACHCHI, S., PETROU, E. & KIM, H. S., Proc. SPZE, 1187 (1989) 161. 17. SINGH, R. K. & NARAYAN, J., Phys. Rev. B, 41 (1990) 8843. 18. KOOLS, J. C. S., BALLER, T. S. & DIELEMAN, J., J. Appl. Phys., 71 (1992) 4547. 19. BEECH, F. & BOYD, I. W., Laser ablation of electronic materials. In Photochemical Processing of Electronic Materials, ed. I. W. Boyd & R. Jackman. Academic, UK, 1992. 20. DIJKKAMP, D., VENKATESAN, T., WU, X. D., SHAHEEN, S. A., JISRAWA, N., MIN-LEE, Y. H., MCLEAN, W. L. & CROFT, M., Appi. Phys. Left., 51 (1987) 619. I

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BIUNNO, N., SINGH, R., NARAYAN, J., HOLLAND, 0. W. & AUCHIELLO, O., Appi. Phys. Left., 51 (1987) 1845. P., SHI, L., WANG, X. 22. KWOK, H. S., MATTOCKS, W., WITANACHCHI, S., YING, Q. Y., ZHENG, J. P. & SHAW, D. T., Appl. Phys. Lett., 52 (1988) 1825. K. F. & GOWER, M. C., 23. SAVVA, N., WILLIAMS, IEEE J. Quant. Elect., 25 (1989) 2399. A. & GRATTON, R., Plasma Phys., 10 24. CARUSO, (1968) 867. T., WU, X. D. & WACHTMAN, J. 25. VENKATESAN, B., Appl. Phys. Lett., 52 (1988) 1193. M., SHILOH, M., JACKMAN, R. B. & 26. BROWN, BOYD, I. W., Appl. Surf. Sci., 43 (1989) 382. B. R. & KRASINSKI, H. 27. LYNDS, L., WEINBERGER, A., Appl. Phys. Left., 52 (1988) 320. 28. MARINE. W.. PERAY. M. & PAILHAREY. 1D.. , ADDS. 1. Surf. Sci., 43 (1989) 377.’ K., SABA, F. & BOYD, I. 29. SAJJADI, A., KUEN-LAU, W., Appl. Surf. Sci., 46 (1990) 84. A. M. & IMAS, Y. A., Sov. 30. BONCH-BRUEVICH, Phys. Tech., 12 (1968) 1407. 31. GAGLIANO, F. P. & PAEK, U. C., Appl. Opt., 13 (1974) 274. 32. DYER, P. E., ISSA, A. & KEY, P. H., Appl. Phys. Left., 57 (1990) 186. J., SPIE, 33. SINGH, R. K., TIWARI, P. & NARAYAN, 1187 (1989) 182. 34. MELE, A., CONSALVO, D., GIARDINI, A. & TEGHIL, R., Appl. Surf. Sci., 43 (1989) 398. O., ATHAVALE, S., HANKINS, 0. E., 35. AUCIELLO, SITO, M., SCHREINER, A. F. & BIUNNO, N., Appl. Phys. Lett., 53 (1988) 72. 36. DYER, P. E., GREENOUGH, RD., ISSA, A., KEY, P. H.. ADD/. Surf: Sci.. 43 (1989) 387. S:, Y&HiTAKE,T., SATOH, T. & 37. MI&A, SHOHATA, N., Appl. Phys. Lett., 52 (1988) 1008. 38. GEOHEGAN, D. B., Appl. Phys. Lett., 89 (1993) 6103. 39. GEOHEGAN, D. B., PURETZKY, A. A., HETTICH, R. L., ZHENG, X. Y., HAUFLER, R. E. & COMPTON, R. N., In Laser and Ion Beam Modification of Materials, ed. I. Yamada, H. Ishiwara, E. Kamijo, T. Kawai, C. W. Allen & C. W. White. Elsevier, Tokyo, 1994, pp. 349-354. 40. ZHENG, J. P., HUANG, Z. Q., SHAW, D. T. & KWOK, H. S., Appl. Phys. Lett., 54 (1989) 280. 41. MASHBURN, D. N. & GEOHEGAN, D. B., SPIE, 1187 (1989) 172. 42. BYKOVSKII, YU.A., SIL’NOV, S. & SHESTAKOV, B., Sov. Phvs. JETP, 66 (1987) 285. 43. BARR, W.-P., J. Phys. E, 2 (1969) 1024. 44. GAPONOV, S. V., GUDKOV, A. & FREEMAN, A., Sov. Phvs. Tech. Phvs.. 27 (1982) 1130. 45. LUBBEN, D., BARNETT,’ s., SUZUKI, K., GORBATIN, S. & GREENE, J. E., J. Vat. Sci. Technol., B3 (1985) 968. 46. KOREN, G., BASEMAN, R. J., GUPTA, A., LUTWYCHE, M. I. & LAIBOWITZ, R. B., Appl. Phys. Lett., 56 (1990) 2144. 47. CHIBA, H.; MURAKAMI, K., ERYU, O., SHIHOYAMA. K.. MOCHIZUKI. T. & MASUDA. K., Jpn J. Appi.‘Phys. Left., 30 (1991) 4B. 48. SANKUR, H., GUNNING, W. J., DENATALE, J. & FLINTOFF, J., Appl. Phys. Left., 65 (1989) 2475. 49. AMIRHAGHI. S.. LI. Y.. KILNER. J. & BOYD. I. W.. J. Mater. Sci. Engng, b34’( 1995) 192: 50. INMAN, A., HEDGE, M. S., WU, X. D., VENKATESAN, T., ENGLAND, P., TARASCON, J. M., MICELLI, P. F., CHANG, C. C. & WACHTMAN, J. B., Appl. Phys. Left., 53 (1988) 908. 51. SAJJADI, A. & BOYD, I. W., Appl. Phys. Left., 63 (1991) 3373. 52. NAQVI, S. H. H., BEECH, F. & BOYD, I. W., Electronics Lett., 27 (1991) 430. 53. CRACIUN, V. & BOYD, I. W., Mater. Sci. Engng (B), 18 (1992) 178. 54. CdACIuN, V., ELDERS, J., GARDINIERS, J. G. E. & BOYD, I. W., Appl. Surf. Sci., 86 (1995) 99.

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ISSN: 0976-8610 CODEN (USA): AASRFC

Sol-gel technique: A veritable tool for crystal growth Don Okpala V. Uche. Department of Industrial Physics, Anambra State University, Uli, Anambra State, Nigeria _____________________________________________________________________________________________ ABSTRACT This paper looks at the applications of sol-gel deposition technique to crystals growth. Sol gel technique which is one of the oldest methods of crystal growth is defined as a wet-chemical technique widely used in the fields of materials science and ceramic engineering. It is used primarily for the fabrication of materials starting from a colloidal solution (sol) that acts as the precursor for an integrated network or gel of either discrete particles or network polymers. It makes use of cheap and available materials. It can be used in ceramics processing and manufacturing as an investment casting material, or as a means of producing very thin films of metal oxides for various purposes. Sol-gel derived materials have diverse applications in optics, electronics, energy, space, (bio)sensors, medicine e.g., controlled drug release, reactive material and separation e.g., chromatography technology. ___________________________________________________________________________________________ INTRODUCTION 1.1 SOL-GELL The methods of crystal growth can generally be grouped into two, thus; Vapour Phased Deposition which includes, Evaporation, Molecular Beam Epitaxy (MBE), Sputtering, Chemical Vapour Deposition(CVD) and Atomic Layer Deposition (ALD) and Liquid Based Growth which includes Chemical Solution Deposition, Electrochemical Deposition, Chemical Bath Deposition(CBD), Successive Ionic Layer Adsorption and Reaction (SILAR), Langmuir- blodgett films and Self Assembled Monolayers (SAM) and Sol gel method [1,2]. Crystals can be grown through any of the aforementioned methods. Our interest is on sol gel method. Sol-gel process is a wet-chemical technique widely used in the fields of materials science and ceramic engineering. It is used primarily for the fabrication of materials (typically metal oxides) starting from a colloidal solution (sol) that acts as the precursor for an integrated network (or gel) of either discrete particles or network polymers. Typical precursors are metal alkoxides and metal salts (such as chlorides, nitrates and acetates), which undergo various forms of hydrolysis and polycondensation reactions. In this chemical procedure, the 'sol' (or solution) gradually evolves towards the formation of a gel-like diphasic system containing both liquid phase and solid phase whose morphologies range from discrete particles to continuous polymer networks. In the case of the colloid, the volume fraction of particles (or particle density) may be so low that a significant amount of fluid may need to be removed initially for the gel-like properties to be recognized. This can be accomplished in any number of ways. The simplest method is to allow time for sedimentation to occur, and then pour off the remaining liquid or by the use of centrifugation to accelerate the process of phase separation. Removal of the remaining liquid (solvent) phase requires a drying process, which is typically accompanied by a significant amount of shrinkage and densification. The rate at which the solvent can be removed is ultimately determined by the distribution of porosity in the gel. The ultimate microstructure of the final component will clearly be strongly influenced by changes imposed upon the structural template during this phase of processing. Afterwards, a thermal treatment, or firing process, is often necessary in order to favor further polycondensation and enhance mechanical properties and structural stability via final sintering, densification and grain growth. One of the

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Don Okpala.V. Uche. Adv. Appl. Sci. Res., 2013, 4(1):506-510 _____________________________________________________________________________ distinct advantages of using this methodology as opposed to the more traditional processing techniques is that densification is often achieved at a much lower temperature. The precursor sol can either be deposited on a substrate to form a film (e.g., by dip coating or spin coating), cast into a suitable container with the desired shape (e.g., to obtain monolithic ceramics, glasses, fibers, membranes, aerogels), or used to synthesize powders (e.g., microspheres, nanospheres). The sol-gel approach is a cheap and lowtemperature technique that allows for the fine control of the product’s chemical composition. Even small quantities of dopants, such as organic dyes and rare earth elements, can be introduced in the sol and end up uniformly dispersed in the final product. It can be used in ceramics processing and manufacturing as an investment casting material, or as a means of producing very thin films of metal oxides for various purposes. Sol-gel derived materials have diverse applications in optics, electronics, energy, space, (bio)sensors, medicine (e.g., controlled drug release), reactive material and separation (e.g., chromatography) technology. The interest in sol-gel processing started in the mid-1800s with the observation that the hydrolysis of tetraethyl orthosilicate (TEOS) under acidic conditions led to the formation of SiO2 in the form of fibers and monoliths. Solgel research grew to be so important that in the 1990s more than 35,000 papers were published worldwide on the process [3-5, 6-8]. The discrete particles or continuous polymer network form the sol which evolves towards the formation of an inorganic network containing a liquid phase (gel). Formation of a metal oxide involves connecting the metal centers with oxo (M-O-M) or hydroxo (M-OH-M) bridges, therefore generating metal-oxo or metalhydroxo polymers in solution. Silicon tetraethoxide, or tetraethyl orthosilicate (TEOS) is a well studied alkoxide. The chemical formula for TEOS is given by: Si(OC2H5)4, or Si(OR)4 where the alkyl group R = C2H5. Alkoxides react readily with water and as such is seen as ideal chemical precursors for sol-gel synthesis. It is called hydrolysis, because a hydroxyl ion becomes attached to the silicon atom as follows: Si(OR)4 + H2O → HO-Si(OR)3 + R-OH The amount of water and catalyst present determines whether hydrolysis may continue to completion, so that all of the OR groups are replaced by OH groups, as follows: Si(OR)4 + 4H2O → Si(OH)4 + 4R-OH[ ] Complete hydrolysis of organometallics often requires a significant excess of water and/or the presence of hydrolysis catalysts such as acetic acid or hydrochloric acid. Intermediate species [(OR)2–Si-(OH)2] or [(OR)3–Si(OH)] may result from partial hydrolysis reactions [9]. The two partially hydrolyzed molecules can link together in a condensation reaction to form a siloxane [Si–O–Si] bond: (OR)3–Si-OH + HO–Si-(OR)3 → [(OR)3Si–O–Si(OR)3] + H-O-H or (OR)3–Si-OR + HO–Si-(OR)3 → [(OR)3Si–O–Si(OR)3] + R-OH Polymerization is associated with the formation of a 1, 2, or 3- dimensional network of siloxane [Si–O–Si] bonds accompanied by the production of H-O-H and R-O-H species. Condensation liberates a small molecule, such as water or alcohol. This type of reaction can continue to build larger and larger silicon-containing molecules by the process of polymerization. A polymer is a huge molecule (or macromolecule) formed from hundreds or thousands of units called monomers. The number of bonds that a monomer can form is called its functionality. Polymerization of silicon alkoxide, for instance, can lead to complex branching of the polymer, because a fully hydrolyzed monomer Si(OH)4 is tetrafunctional (can branch or bond in 4 different directions). At low water concentration, under certain conditions, fewer than 4 of the OR or OH groups (ligands) will be capable of condensation, so relatively little branching will occur. The mechanisms of hydrolysis and condensation, and the factors that bias the structure towards linear or branched structures are the most critical issues of sol-gel science and technology. This reaction is favored in both basic and acidic conditions [10-17]. 1.2 METHODS OF GEL PRODUCTION There are four methods for the production of gels(i) Flocculation of lyophilic- colloids by salts or precipitating liquids. (ii) Evaporation of certain colloidal solutions (iii) Chemical reactions that lead to change in shape of lyophilic molecules (e.g. the denaturation of albumen on heating involves some uncoiling of the protein molecules and a gel structure results).


Don Okpala.V. Uche. Adv. Appl. Sci. Res., 2013, 4(1):506-510 _____________________________________________________________________________ (iv) Swelling of a dry colloid (xerogel) when placed in contact with a suitable liquid (e.g. starch granules added to water). The presence of a network formed by the interlocking of particles of the gelling agent gives rise to the rigidity of a gel. The nature of the particles and the type of form that is responsible for the linkages determine the structure of the network and the property of the gel. Aerogels are sol-gel derived solid materials with porosities from about 80-98%. The high porosity is achieved through supercritical drying of wet gel in an autoclave [18, 19]. 2.1 PROPERTIES OF GEL There are about four properties of gel; swelling, syneresis, ageing and rheological properties. (i) Swelling occurs if a xerogel is placed in contact with a liquid that solvates it, then an appreciable amount of the liquid is often taken up and the volume of the xerogel increases. (ii) Syneresis occur when gels contract spontaneously and exude some of the fluid medium and the degree to which it occurs usually decreases as the concentration of the gelling agent increases. (iii) Ageing occurs when colloidal system exhibit slow spontaneous aggregation. Ageing results in the gradual formation of a denser network of gelling agent. (iv) Rheological properties occur when the gel exhibits mechanical properties of rigidity, tensile strength and elasticity that are characteristic of solids. 2.2 Applications of sol gel 2.2.1 Protective coatings The applications for sol gel-derived products are numerous [20-25]. For example, scientists have used it to produce the world’s lightest materials and also some of its toughest ceramics. One of the largest application areas is thin films, which can be produced on a piece of substrate by spin coating or dip coating. Protective and decorative coatings and electro-optic components can be applied to glass, metal and other types of substrates with these methods. Cast into a mold, and with further drying and heat-treatment, dense ceramic or glass articles with novel properties can be formed that cannot be created by any other method. Other coating methods include spraying, electrophoresis, inkjet printing or roll coating. 2.2.2 Thin films and fibers If the viscosity of a sol is adjusted into a proper range, both optical and refractory ceramic fibers can be drawn which can be used for fiber optic sensors and thermal insulation, respectively. Many ceramic materials, both glassy and crystalline, have found use in various forms from bulk solid-state components to high surface area forms such as thin films, coatings and fibers [26, 27]. 2.2.3 Nanoscale powders In the process of precipitation, ultra-fine and uniform ceramic powders can be formed which are powders of single and multiple component compositions of nanoscale particle size for dental and biomedical applications. Composite powders have been patented for use as agrochemicals and herbicides. Powder abrasives, used in a variety of finishing operations, are made using a sol-gel type process. One of the more important applications of sol-gel processing is to carry out zeolite synthesis. Other elements (metals, metal oxides) can be easily incorporated into the final product and the silicate sol formed by this method is very stable. Sol gel is applied in research to entrap biomolecules for sensory (biosensors) or catalytic purposes, by physically or chemically preventing them from leaching out and, in the case of protein or chemically-linked small molecules, by shielding them from the external environment yet allowing small molecules to be monitored. The major disadvantages are that the change in local environment may alter the functionality of the protein or small molecule entrapped and that the synthesis step may damage the protein. To avoid this, various strategies have been explored, such as monomers with protein friendly groups (e.g. glycerol) and the inclusion of polymers which stabilize protein [28]. Other products fabricated with this process include various ceramic membranes for microfiltration, ultrafiltration, nanofiltration, pervaporation and reverse osmosis. When the liquid in a wet gel is removed under a supercritical condition, a highly porous and extremely low density material called aerogel is obtained. On drying the gel by means of low temperature treatments (25-100 °C), it is possible to obtain porous solid matrices called xerogels. In 1950s, a sol-gel process was developed for the production of radioactive powders of UO2 and ThO2 for nuclear fuels, without generation of large quantities of dust.


Don Okpala.V. Uche. Adv. Appl. Sci. Res., 2013, 4(1):506-510 _____________________________________________________________________________ 2.2.4 Opto-mechanical Sol gel route can be used to make macroscopic optical elements and active optical components as well as large area hot mirrors, cold mirrors, lenses and beam splitters all with optimal geometry at low cost. In the processing of high performance ceramic nanomaterials with superior opto-mechanical properties under adverse conditions, the size of the crystalline grains is determined largely by the size of the crystalline particles present in the raw material during the synthesis or formation of the object. Thus a reduction of the original particle size well below the wavelength of visible light (~ 0.5 µm or 500 nm) eliminates much of the light scattering, resulting in a translucent or even transparent material. Also, results indicated that microscopic pores in sintered ceramic nanomaterials, mainly trapped at the junctions of microcrystalline grains, cause light to scatter and prevented true transparency. It was also observed that the total volume fraction of these nanoscale pores (both intergranular and intragranular porosity) must be less than 1% for high-quality optical transmission. The density has to be 99.99% of the theoretical crystalline density [14, 15]. CONCLUSION In this work, sol gel deposition technique has been studied. It is used primarily for the fabrication of materials (typically metal oxides) starting from a colloidal solution (sol) that acts as the precursor for an integrated network (or gel) of either discrete particles or network of polymers. It makes use of cheap and available materials. It can be used in ceramics processing and manufacturing as an investment casting material, or as a means of producing very thin films of metal oxides for various purposes. Sol-gel derived materials have diverse applications in optics, electronics, energy, space, (bio)sensors, medicine (e.g., controlled drug release), reactive material and separation (e.g., chromatography) technology. It is recommended that researchers develop more interest in the use of sol gel method in the growth of various thin films and crystals for enhanced output. REFERENCES [1] Milton, O, Elsevier Inc, 2003, 3, 5. [2] Jaeger, R.C, Film Deposition: Introduction to Microelectronic Fabrication, Upper Saddle Rivers, Prentice Hall, Rivers, 2002, 83. [3] Brinker, C.J, G.W. Scherer, Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing, Academic Press, ISBN 0-12-134970-5, 1990. [4] Hench, L.L, J.K. West, Chemical Reviews 1990, 90, 33. [5] Klein, L, Sol-Gel Optics: Processing and Applications, Springer, Verlag, 1994 ISBN 0-7923-9424-0. [6] Okpala U. V, Ezema F.I, Osuji R.U, Advances in Applied Science Research, 2012, 3(1): 103-109. [7] Okpala U. V, Ezema F.I, Osuji R.U, Advances in Applied Science Research, 2012, 3 (2): 1175-1184. [9] Hanaor D.A, Chironi I, Karatchevtseva I, Triani G, Sorrell C.C, Single and Mixed Phase TiO2 Powders Prepared by Excess Hydrolisis of Titanium Alkoxide, 2012. [10] Dislich H, Angewandte Chemie International Edition in English, 1971 10 (6): 363. [11] Matijevic Egon, Langmuir 1986, 2: 12. [12] Brinker C. J, Mukherjee S. P, Journal of Materials Science, 1981, 16 (7): Bibcode 1981JMatS..16.1980B. doi:10.1007/BF00540646. [13] Sakka S, Kamiya K, Journal of Non-Crystalline Solids, 1980, 42: 403. Bibcode 1980JNCS...42..403S. doi:10.1016/0022-3093(80)90040-X. [14] Yoldas B. E, Journal of Materials Science, 1979, 14 (8): 1843. Bibcode 1979JMatS..14.1843Y. doi:10.1007/BF00551023. [15] Prochazka S, Klug F. J, Journal of the American Ceramic Society 1983, 66 (12): 874. [16] Ikesue Akio, Kinoshita T, Kamata K, Yoshida K, Journal of the American Ceramic Society, 1995, 78 (4): 1033. [17] Ikesue, A, Optical Materials, 2002, 19: 183. Bibcode 2002OptMa..19..183I. doi:10.1016/S09253467(01)00217-8 [18] Roy R, Proceedings of World Congress on Superconductivity, Singapore, 1999, 29. [19] Ulrich D.R, Chem. Eng. News, 2004, 29. [20] Wright J.D, Sommerdijk N.A, Sol-Gel Materials: Chemistry and Applications, 2000, 10. [21] Aegerter M.A, Mennig M, Sol-Gel Technologies for Glass Producers and Users, 1999, 20. [22] Phalippou J, Solgel.com, 2000, 2. [23] Brinker, C.J. and Scherer, G.W., Sol-Gel Science, The Physics and Chemistry of Sol-Gel Processing, Academic Press, 1990, 35. [24] German Patent 736411 (Granted 6 May 1943) Anti-Reflective Coating, W. Geffcken and E. Berger, Jenaer Glasswerk, Schott, 1943. [25] Klein L.C, Sol-Gel Optics: Processing and Applications, Springer Verlag, 1994, 21.


Don Okpala.V. Uche. Adv. Appl. Sci. Res., 2013, 4(1):506-510 _____________________________________________________________________________ [26] Sakka S, et al., J. Non-Crystalline Solids, 1992, 48, 31. [27] Patel, P.J., et al.,Inorganic Optical Materials II, Marker, A.J. and Arthurs, E.G, 2000, 1. [28] Gupta R, Chaudhury N.K, Biosens Bioelectron, 2007, 22 (11): 2387–99.
