Structural and vibrational properties of Mg doped ZnO ...

Structural and vibrational properties of Mg doped ZnO alloy nanostructures B. Karthikeyan, T.Pandiyarajan Department of Physics, National Institute of Technology, Tiruchirapalli-620015 Email:[email protected] ABSTRACT We report structural and vibrational properties of Mg doped ZnO nanoparticles. Structural studies are performed by X-ray diffraction technique, confirms that the prepared particles are in hexagonal wurtzite structure and the lattice parameters changes considerably due to doping. Vibrational properties done with Fourier Transform Infrared red spectroscopy (FTIR) show a band centered at 427cm-1 corresponds to E1(TO) mode. It is also observed that the intensity decreases with the increase of Mg concentration, apart from that the surface phonon modes are appeared at 460 and 521cm-1. Compare to the undoped sample all the normal modes show red shift. Keywords: Semiconductor nanoparticles, II-VI semiconductor, Structural studies, FTIR

INTRODUCTION In the past few decades’ development of nanoscience and nanotechnology gained momentum due to its new kind of properties [1]. Oxide nanomaterials have been received much attention due to their properties and potential application such as biomedical, luminesencent materials and fuel cells [2] etc. Among the oxide semiconductors, ZnO is an II-IV semiconductor with wide direct band gap of 3.37eV and it has the exciton binding energy of 60meV, which makes high efficiency UV light emitters and exciton related optoelectronic devices [3]. The physical and chemical properties of ZnO nanoparticles can be altered by introducing dopants. Among the dopants Mg2+ doping makes wide range of applications like hydrogen storage material [4], field effect transistor [5], sensors [6], optoelectronic devices [7] and piezoelectric devices [8]. There are several reports on Mg doped ZnO nanostructures and films having different properties and different applications. Chia et al have prepared Mg doped ZnO nanostructures and study their Biexciton emission [9], Li et al have reported the tunning the ferromagnetic properties by adding Mg ion in the ZnO structures [10], Wei et al have reported the fabrication of Mg doped ZnO films and study their band gap alterations [11] and Trunk et al reported deep level emission from Mg doped ZnO nanostructures [12]. Similarly enhancements of the electrical properties were found on Mg doped ZnO. Ku et al have reported the Effects of Mg on the electrical characteristics and thermal stability of ZnO thin films [13], Ma et al have reported the Electrical properties of ZnO co doped with Mg and Na ions [14] and Kilinc et al have studied the structural and electrical properties of Mg doped ZnO nanoparticles [15]. Owing to the importance of metal doping in ZnO, we have prepared Mg doped ZnO alloy nanostructures and studied the structural and vibrational properties.

EXPERIMENTAL SECTION

Nanophotonic Materials VIII, edited by Stefano Cabrini, Taleb Mokari, Proc. of SPIE Vol. 8094, 80940D · © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.889928

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All chemicals used d in the experiiment are of hiigh purity gradde purchased frrom Sigma- Alldrich. Brieflyy, 0.05M Zinc nitrate hexahydrate h was w dissolved in 100 ml doubble distilled water, 0.1M NaaOH was dissolved in 100 ml m double distilled wateer and then thiss solution was added drop wise in the abovve solution, yieeld a white gel. Similarly, Mgg doping on ZnO was done as follow ws: 0.001M, 0.002M and 0.0003M, of Mg (N NO3)2.6H2O waas added into the t zinc nitrate solution 0 respecctively. Then NaOH solutioon was added drop wise in the above mentioned of 0.049M, 0.048M and 0.047M solutions, a white w gel was obtained. Thee formed whitte gel was keppt at room tem mperature overr 12 hours periiod. The precipitates were w collected and dried in hot h air oven at 60˚ C for fourr hours. The prepared sampples are code named n as ZnMg 0, ZnM Mg 1, ZnMg 2 and ZnMg 3. Thee crystalline naature of the puure and Mg doped ZnO aree investigated by X-ray diffrraction (Rigakku Dmax 2000). The power p of the XRD (Cu -Kα raadiation) is fixxed at 40 KV and a 30 mA andd the diffractionn is measured between angles (2θ) from f 10◦ to 80 0◦. FTIR studiees are performeed using RXI model Perklinn Elmer spectroometer in the range of 4000 to 400 cm c -1.

R RESULTS A AND DISCU USSION STRUCTURAL STUD DIES: XRD patterns p of puree and Mg dopeed ZnO are shoown in Figure 1. 1 Indexed difffraction patternns show particlles are in hexagonal wurtzite w structurre. No impuritties peaks are detected d such as MgO. It is confirmed thaat dopant Mg, does not alter the hexagonal wurtzitte structure, whhile it causes the lattice parrameters to chaange slightly as a observed byy shift in 2 (100) peak shhown in Figuree 1 b. This shifft is mainly duee to substitutionn of Mg2+ in thhe place of Zn2+ .

Figure.1 (a) XRD paatterns of undoped and Mg dopeed ZnO. (b) Dopant induced peakk shift due to inccorporation of Mg M 2+

Lattice param meters of a sem miconductor ussually depend on o the concentrration of foreiggn atoms, defeects, external sttrain and their differennce of ionic raadii with respeect to the substtituted matrix ion [16]. Sincce there is a diifference in ionic radii between Zn2++ (0.74 Å) and Mg2+ (0.66 Å)) which leads too the changes in i the lattice paarameters. Thee lattice parameeters ‘a’, ‘c’ and the unit cell vo olume ‘v’ is calculated byy using the fformulae[17] a =


λ

3sinθ

λ (h + hk+ h k ), c = sinn θ 2

2

and

3a 2 c . Evolution of lattice parameters ‘a’, ‘c’ annd the unit cell volume ‘v’ is shown in Tablle1. Calculatioons show 2 a’, ‘c’ and ‘v’’ are increase for 0.001M of o Mg2+ concenntration in ZnO O then it decrrease for that the latticce constants ‘a 2+ 2+ 0.003M of Mg M concentraation in ZnO. It indicates thhat the Mg ioons reside at leeast partially inn tetrahedral (pprobably Zn) sites. Morphology of th he prepared paarticles having rod like naturee was shown inn Figure 2. v=

Table 1. Variiation of lattice parameter p along with Mg concenntrations

Samplee ZnMg00 ZnMg11 ZnMg33

‘a’ axis (Å) 3.265 3.267 3.236

‘c’ axis (Å Å) 5.223 5.226 5.179

Volume ‘v’ (Å)3 48.2255 48.3122 46.9488

Figure.2 Sccanning electronn microscopic im mage of Mg dopeed ZnO

VIBRATIIONAL STU UDIES Vibrrational modess of pure and Mg M doped ZnO O nanostructurees are investigated by FTIR spectroscopy. Figure.3 a. shows FTIIR spectra of pure p and Mg doped d ZnO nannoparticles. Forr clarity, the sppectrum whichh has the rangee of 400600cm-1 has been shown. All the samplles show IR acctive optical phonon p modes of ZnO show w a characteristtic broad restrahlen baand in the spectral range off 300-600 cm-11[18].Gaussian fitted band shhow each band is compose of three different bannds. These fitted d bands are nam med as B1, B2 and B3. (See Fiig. 3b)


Figgure.3 (a) FTIR R spectra of undooped and Mg dopped ZnO (b) Gauussain decompossed band named as B1, B2 and B3.

The bandd at 427 cm-1 iss assigned to E1 (TO) for un doped ZnO, foor Mg doped ZnO Z shows redd shift. This shiift is due to difference in the ionic diifference between Zn2+ and Mg M 2+. The bandd centered at 460 4 cm-1 (B2) and a at 521 cm-1(B3) are corresponds to t the surface phonon p modess (SPM) namedd as SPM [A1 (TO)] ( and SPM M [E1 (TO)] resspectively. Varriation in the IR modess of Zn1-xMgxO nano particlees are shown inn Figure 4 a. Bond B length off the doped andd Mg doped ZnnO nano particles can be determined d from the bannd position of E1 mode usingg the relation [[19] ν =

1 ⎛k⎞ ⎜ ⎟ 2πc ⎜⎝ μ ⎟⎠

( 12 )

Where ν is the

wave numberr, c is the velocity of light, k average force constant of Znn (Mg) – O bonnd, and µ is thhe effective maass of the m + (1 − x)Mzn] where M , M and M are the atomic weeights of the O, Zn and bond which is i given by μ = Mo × [xMm o z zn m Mo + [xMm m + (1 − x )Mzn] Mg respectivvely. Force con nstant k can be related to the average a Zn (M Mg)-O bond lenngth [20] by thee equation k = the calculatedd values of effe fective mass, thhe force constannt and the bondd length are lissted in the Tablle 2. Table.2. The T FTIR band centre and bonnd length calcuulation Sam mple codee

Wave number c -1 cm

Efffective mass

F Force coonstant N/cm

Bond length (Å)

×110-26 kg ZnM Mg0

4 427

2.1338

1.3809

2.3090

ZnM Mg1

4 476

2.1335

1.717

2.1473

ZnM Mg2

4 452

2.1332

1.5469

2.2230

ZnM Mg3

4 455

2.1331

1.5670

2.2137


17 , r3

Figure. 4(a) Variation of phonon modees with the Mg concentration c (b)) Variation of unnit cell volume with w Mg concenttration

Effective maass of the bond is decreased with w increase inn Mg concentraation, while foorce constant inncreases whichh leads to the decrease in bond lengtth. There is a strong correlaation between the unit cell volume v and the bond length. In the present casee Mg doped ZnO nanoparrticles show decrease in unit u cell voluume and decrrease of bondd length correspondinngly. ONCLUSION CO ` We have investigated structurall and vibraionaal properties of o Mg doped ZnO Z alloy nannostructures. Structural studies confirrmed that the prepared p particcles are in wurttzite structure and Mg ion dooping did not leead to the form mation of Mg related seecondary phasees. With increaase in Mg conccentration latticce constants ’aa’, ’c’ and ‘v’ decreases. d FTIR R studies show the shiift in the E1 (LO) mode is atttributed to the incorporation of Mg ions. Also A doping with Mg ions makes m the decrease in thhe bond length h of ZnO REFERENCES M M. J, “Synthesis and appplications of nanocrystalline n e ceramics”, Mater. M Des 14, 323-329 (19933) [1] Mayo [2] Schoonman S J, “Nanoionics”, Solid. State Ioonics 157 (20033) 319– 326 [3] Djurisic D A. B, Leung Y. H, “Optical Properrties of ZnO Naanostructures”, Small 2, 944 – 961 (2006) [4] Pan P H, Luo J, Sun H, Feng Y, Poh C , Liin J, “Hydrogeen storage of ZnO Z and Mg doped d ZnO nannowires” Nannotechnology 17, 2963-2967 (2006) [5] Ju J S, Li J, Pim mparkar N, Alaam M. A, Chaang R. P. H, Jaanes D. B, “N--Type Field-Efffect Transistors Using Mulltiple Mg-Dopeed ZnO Nanoroods”, IEEE Traansactions on Nanotechnolog N gy, 6, 390-395 (2007) [6] Chang R.C, Chu C S.Y, Yeh P.W P , Hong C.S, C Kao P.C, Huang H Y.J “The influence of o Mg doped ZnO Z thin ms on the propeerties of Love wave w sensors” Sens. Actuat. B 132, 290–295 (2008) film [7] Sonawane B. K, K Bhole M. P, P Patil D. S, “Structural, “ optical and electrrical properties of post anneealed Mg f optoelectronics applicatioons” Opt. Quannt. Electron 41,, 17–26(2009) doped ZnO films for


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