Citrate-stabilized palladium nanoparticles as catalysts for sub-20 nm

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Jul 13, 2010 - which small Pd NPs with a narrow diameter distribution were produced. Subsequently, we use these Pd NPs for the low temperature chemical ...
APPLIED PHYSICS LETTERS 97, 023105 共2010兲

Citrate-stabilized palladium nanoparticles as catalysts for sub-20 nm epitaxial silicon nanowires J. V. Wittemann,1,a兲 A. Kipke,2 E. Pippel,1 S. Senz,1 A. T. Vogel,1 J. de Boor,1 D. S. Kim,1 T. Hyeon,3 and V. Schmidt1 1

Max Planck Institute of Microstructure Physics, Weinberg 2, D-06120 Halle, Germany Center of Materials Science, Heinrich-Damerow-Str. 4, D-06120 Halle, Germany 3 School of Chemical and Biological Engineering, Seoul National University, Seoul 151-744, Republic of Korea 2

共Received 10 June 2010; accepted 16 June 2010; published online 13 July 2010兲 Citrate-stabilized palladium nanoparticles with a mean diameter of 13 nm were synthesized in aqueous solution. These particles were utilized as catalysts to grow epitaxial silicon nanowires by chemical vapor deposition at temperatures below 500 ° C. The resulting nanowires have a mean diameter of 15 nm. It is found that during the growth process the palladium particles transform into dipalladium silicide. © 2010 American Institute of Physics. 关doi:10.1063/1.3460918兴 Silicon nanowires 共Si NWs兲 are mostly catalyzed by liquid Au–Si alloy droplets, implying growth via the vaporliquid-solid 共VLS兲 mechanism. For this aim, often a Au thin film is deposited onto the substrate. Upon annealing at elevated temperatures this Au film breaks up into separate Au–Si alloy droplets. These may then serve as catalyst for Si NW growth. The procedure as described above, however, provides only limited control over the diameter distribution of the catalyst droplets. By adjusting metal film thickness, annealing temperature, and annealing time, the mean diameter of the Au–Si alloy droplets can be adjusted to some degree. This does not hold for the width of the diameter distribution. The process of droplet agglomeration during annealing 共Ostwald ripening兲 typically leads to a size distribution function with a full width at half maximum on the order of the mean. Since the length of the NWs, their preferred growth direction as well as their electronic properties may depend on the NW diameter, a sharper diameter distribution would be highly desirable from application point of view.1–3 For an eventual application of Si NWs in nanoelectronics the Si NWs should additionally be grown gold-free with processing temperatures not exceeding 500 ° C.3 Regarding the Au-problem, a lot of effort was invested into identifying alternative catalyst materials. Yet, since most alternative catalyst metals 共or metal Si alloys兲 exhibit high temperature melting points, VLS growth at moderate temperatures seems not to be an option. Alternatively, one may also consider growth via the vapor-solid-solid 共VSS兲 growth mechanism, i.e., employing a solid catalyst particle. Except for Al and Ag,4–6 the potential non-Au catalyst materials for VSS growth will form different silicide phases. Due to its well positioned defect level in Si and its chemical stability, palladium 共Pd兲 the most attractive silicide forming alternative catalyst. As for the narrow diameter distribution of catalysts and Si NWs, respectively, different methods were employed to achieve that aim. The most prominent approach is to predefine the size of the catalyst particles by, e.g., nanosphere lithography, electron beam lithography or use of colloid particles.7–13 Using standard chemical equipment, colloid a兲

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particles of well-defined mean diameter can be synthesized with rather sharp size distributions. In the following, we show how Pd nanoparticles 共NPs兲 can be synthesized and used as catalysts for Si NW growth. First we introduce the wet chemical synthesis method by which small Pd NPs with a narrow diameter distribution were produced. Subsequently, we use these Pd NPs for the low temperature chemical vapor deposition 共CVD兲 growth of ultrathin, epitaxial Si NWs. Finally, morphology and composition of both Si NWs and Pd silicide catalyst particles and their epitaxial relation with respect to each other will be discussed in detail. To prevent aggregation, NPs are stabilized by an organic shell.14 One possible stabilizer for Pd NPs is citrate,15 which, in contrast to other ligands, is advantageous as the stabilizing shell can be easily removed by acidification. The method we employed for Pd NP synthesis utilizes hydroxylamine as reducing agent for the Pd salt; Pd共NH3兲2Cl4 was obtained by cosolution of Pd共II兲 chloride and NH4Cl in purified water/ ethanol solution, followed by recrystallization. 2.5 ml of Pd共NH3兲2Cl4-solution containing 1.0 mg Pd/ml 共9.4 ⫻ 10−3 mmol/ ml兲 were diluted in 50 ml of ultrapure water in a 250 ml beaker; to this mixture 3.0 ml of 1.0 wt% trisodium-citrate solution were added. This mixture was then slowly heated to 90 ° C under stirring. After careful addition of 0.1 mmol 共NH3OH兲Cl 共hydroxylammonium chloride兲, dissolved in 30 ml ultrapure water, the temperature was kept constant for 1 h until the characteristic brownish color was noticeable, while stirring the solution continuously. This gave the standard solution of citrate-stabilized Pd NPs with a diameter of 共13.0⫾ 1.3兲 nm. In Fig. 1共a兲 a TEM micrograph of the as-produced colloid NPs is shown. This sol was stable for several months. The Pd NP diameter distribution can be affected by changing the relative quantities as well as the intervals between adding the Pd compound and 共NH3OH兲Cl as the reducing agent. The mean diameter can be varied from 5 to 26 nm. With larger average diameters, the size distributions broaden, which is attended by a loss of the rounded sphericity of the NPs. For the growth of Si NWs 3 ml of the aqueous Pd colloid solution was dropped onto a low p-doped 共111兲 4 inch Si wafer cleaned with acetone, ethanol, and purified water in

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Pd 25

40

[at.%] 60

Si

10 nm

100

80

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EDX analysis [at.%] 9

VLS 892 ºC VSS

(b)

Pd3Si Pd2Si

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20 nm

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Pd NP Pd SiNW

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Diameter [nm]

FIG. 1. 共Color online兲 共a兲 TEM micrograph of Pd NPs taken with Philips CM20T. 共b兲 Schematic of Pd-Si binary phase diagram with temperature regimes of VSS and VLS Si NW growth. 共c兲 45° tilted view SEM micrograph 共JEOL JSM 66701F兲 of Si NWs grown by Pd colloids. 共d兲 Diameter distributions of colloids 쏗 = 共13⫾ 1兲 nm 共count 308兲 and Si NW 쏗 = 共15⫾ 1兲 nm 共count 155兲.

advance. Then 4 ml diluted 共5%兲 hydrofluoric acid 共HF兲 was added, triggering following three reactions: firstly, the native silicon oxide layer is removed and the 共111兲 silicon surface becomes hydrogen terminated, as required for epitaxial NW growth; secondly, the HF removes the citrate shell from the NP by protonation; and thirdly, the now uncovered Pd NP adhere to the substrate surface. Afterwards the wafer was rinsed with purified water and blown dry with nitrogen gas. Following the deposition, the wafer was immediately loaded into the ultrahigh-vacuum CVD system 共base pressure ⬍5 ⫻ 10−10 mbar兲 for Si NW synthesis. Once in the system, the Pd NP covered Si substrates were annealed for 45 min at temperatures not exceeding 400 ° C. Already this annealing step presumably transforms Pd colloid NPs into Pd2Si NPs.16,17 For VSS Si NW growth using a silicide forming metal as catalyst, we expect the catalyst particle to consist of the most Si-rich silicide phase. According to the Si-Pd binary phase diagram 关cf. Fig. 1共b兲兴 is the most Si-rich stable silicide at temperatures below 800 ° C. Afterwards, the temperature was raised to 490 ° C and diluted monosilane 关5% in Ar, flux 20 SCCM 共SCCM denotes standard cubic centimeter per minute at STP兲兴 was introduced into the system for Si NW synthesis. The wires shown in Fig. 1共c兲 were grown for 3 h at a silane partial pressure of 0.2 Torr. The diameter distribution of the initial Pd NPs and of the resulting Si NWs grown from these NPs is depicted in Fig. 1共d兲. The mean diameter of the Si NWs was 共15.3⫾ 1.4兲 nm, which is about 15% larger than the initial mean diameter of the colloid particles. Some increase in the particle diameter could be expected as a transformation from Pd to Pd2Si is accompanied by a volumetric expansion of about 40%. Considering a spherical particle, such an expansion would imply a diameter increase in about 12%.18,19 It seems that the volu-

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O 37

Si Pd

EDX scan

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Counts [at.%]

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FIG. 2. 共Color online兲 共a兲 TEM micrograph taken with Philips CM20T microscope of Si NW grown by Pd2Si catalyst 共in dotted circle兲. 共b兲 STEM micrograph of the Pd2Si catalyst from 共a兲 in HAADF mode taken with FEI TITAN 80–300 microscope. The arrow indicates an EDX scan along the catalyst particle exhibiting the amounts of Pd, Si, O in atomic percent. Solid circle indicates EDX point measurement 共probe size 0.2 nm兲 in the center of the catalyst. 共c兲 Resulting content in atomic percent. 共d兲 Scheme of the Si NW with an oxide shell covering the wire and the silicide catalyst.

metric expansion during the phase transformation from pure Pd to Pd-silicide is the main cause for the diameter increase. What can also be seen in Fig. 1共d兲 is that both distributions 共Pd NPs and Si NWs兲 have a standard deviation of about 3 nm, thus the diameter distribution did not broaden significantly. One may therefore conclude that the Pd NPs did not coalesce and that the resulting Si NWs indeed grew from a single Pd NP each. The growth rate of the wires was relatively low 共⬍0.6 nm/ min兲, an observation frequently made for VSS grown Si NWs. About 60%–80% of the NPs catalyzed a nanowire and the yield of vertical SiNWs was over 70% as can be seen in Fig. 1共c兲. In the following the morphology of both the Si NW and the catalyst particle is investigated: In order to verify the composition of the catalyst particle 关see Fig. 2共a兲兴 we used energy dispersive x-ray spectroscopy 共EDX兲 instrument of a FEI Titan 80–300 scanning transmission electron microscope 共STEM兲. Figure 2共b兲 shows a high angle annular dark field 共HAADF兲 image of the catalyst as indicated in by the circled area in Fig. 2共a兲. An EDX line scan 共each recording 1 s; lateral resolution 0.2 nm兲— indicated by the arrow in Fig. 2共b兲—was taken across the catalyst particle. Silicon, Pd, and oxygen were detected and the relative proportion of these three elements 共normalized to 100 at.%兲 is shown in Fig. 2共b兲. On the left and the right edge of the particle only Si and O were found, which indicates the presence of a Si-oxide layer covering the catalyst particle. The shell presumably originated from silicon that segregated from the silicide catalyst particle during cooldown and that later on oxidized in air. The solid circle in Fig. 2共b兲 marks the spot at which a high resolution EDX measurement was taken 共recording for 6 s; uncertainty ⫾2 at.%兲. The resulting portions of Pd, Si, and O in atomic percent are given in Fig. 2共c兲. At first view, the ratio between Pd and Si would suggest for a PdxSi with x around 1.5; but, according to the phase diagram 关cf. Fig. 1共b兲兴 is a stable Pd-silicide with such a high Si content does not exist. However, considering that a Si-oxide shell which is presumably present, which is also unintentionally measured, the actual Si content of the catalyst particle can be expected to be lower than the 37 at. % displayed in Fig. 2共c兲. This speaks in favor of Pd2Si, the most Si-rich Pd-silicide present at this temperature. One advantage of Pd2Si 共Refs. 1, 18, and 20–23兲 as

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(b) Pd2Si 5 nm

Si NW (c) [011]

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Si & Pd2Si {10.0} Pd2Si {111} Si {200} Si {01.1},{11.1} Pd2Si {220} Si {20.0} Pd2Si

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FIG. 3. 共Color online兲 HRTEM micrographs in 共a兲, 共b兲, and 共d兲 taken with a ¯ 1兴 zone axis. 共a兲 Si NW with catalyst JEOL JEM 4010 microscope with a 关01 particle 共dark gray, top region兲 on a 共111兲 Si substrate with 关111兴 growth direction. 共b兲 Close up of catalyst/Si NW interface indicating hetero-epitaxy between crystalline silicon and dipalladium silicide. 共c兲 FFT of the interface of Pd2Si and Si NW of 共b兲 showing parallel alignment of Si 共111兲 and Pd2Si 兵10.0其. 共d兲 Micrograph of the substrate Si NW interface showing epitaxial alignment with emerging stacking faults.

catalyst, is that after cool-down the NW has a well defined electrical contact at its tip. In Fig. 2共d兲 the proposed geometry of the Pd2Si catalyst, the Si NW, and the Si-oxide shell is schematically depicted. Further investigations of the catalyst particle and its crystal structure were conducted by high resolution transmission electron microscopy 共HRTEM兲. Figure 3共a兲 shows a HRTEM micrograph 共taken with a JEOL JEM 4010 TEM兲 of a Si NW, grown on a 共111兲 Si substrate and the corresponding catalyst particle. The growth direction of the Si NW is 关111兴, ¯ 1兴. A close-up of the Si NW with catathe zone axis was 关01 lyst particle is shown in Fig. 3共b兲. The corresponding fast Fourier transform of the interface between catalyst and Si NW is shown in Fig. 3共c兲. By using silicon as the internal standard the Pd2Si lattice could be identified. The Si 共111兲 plane was found to be aligned in parallel to the hexagonal silicide Pd2Si planes 兵00.1其 and 兵00.2其 leading to a heteroepitaxial alignment of Pd2Si and Si NW with a lattice mismatch of about 2%.24 The finding that Si NWs were grown by the VSS mechanism is supported by Hofmann et al.18 who studied the Pd-silicide catalyzed growth of Si NWs by using an environmental TEM at growth temperatures slightly higher 共50 K兲 than in our experiments. In Fig. 3共d兲 the epitaxial alignment of the wire with the substrate can be seen. It exhibits that stacking faults and twin boundaries of 共111兲 planes are present in the wire. Twins are known to be the most dominant defect in silicon grown at comparable temperatures via Pd2Si-mediated solid-phase-epitaxy.16

In conclusion, we have shown the application of citrate stabilized Pd NPs for VSS synthesis of Si NWs. The Pd NPs undergo a chemical reaction to Pd2Si NPs, which then act as catalysts. We have identified low temperature growth conditions 共temperature ⬍500 ° C兲 at which Si NWs grew from individual colloid particle. The resulting Si NWs had an average diameter of 15 nm and were mostly vertical and epitaxially aligned with respect to the substrate. The NWs furthermore exhibited a high density of planar defects which presumably can be linked to the VSS growth mechanism. This work was partially funded by the joint MPG and Fraunhofer-Gesellschaft project nanoSTRESS, and the German Israel Project, DFG 共Grant No. DIP-K.6.1兲. The authors thank S. Hopfe for TEM sample preparation and Y. Piao for supporting discussions about nanoparticle synthesis. Above all we would like to express our deep gratitude to Professor U. Gösele for his support and encouragement. E. I. Givargizov, J. Cryst. Growth 31, 20 共1975兲. V. Schmidt, S. Senz, and U. Gösele, Nano Lett. 5, 931 共2005兲. 3 V. Schmidt, J. V. Wittemann, S. Senz, and U. Gösele, Adv. Mater. 共Weinheim, Ger.兲 21, 2681 共2009兲. 4 Y. W. Wang, V. Schmidt, S. Senz, and U. Gösele, Nat. Nanotechnol. 1, 186 共2006兲. 5 V. Schmidt, J. V. Wittemann, and U. Gösele, Chem. Rev. 共Washington, D.C.兲 110, 361 共2010兲. 6 J. V. Wittemann, W. Münchgesang, S. Senz, and V. Schmidt, J. Appl. Phys. 107, 096105 共2010兲. 7 Y. Cui, L. J. Lauhon, M. S. Gudiksen, J. F. Wang, and C. M. Lieber, Appl. Phys. Lett. 78, 2214 共2001兲. 8 X. Y. Zhang, L. D. Zhang, G. W. Meng, G. H. Li, N. Y. Jin-Phillipp, and F. Phillipp, Adv. Mater. 共Weinheim, Ger.兲 13, 1238 共2001兲. 9 K. K. Lew, C. Reuther, A. H. Carim, J. M. Redwing, and B. R. Martin, J. Vac. Sci. Technol. B 20, 389 共2002兲. 10 B. M. Kayes, M. A. Filler, M. C. Putnam, M. D. Kelzenberg, N. S. Lewis, and H. A. Atwater, Appl. Phys. Lett. 91, 103110 共2007兲. 11 H. Schmid, M. T. Björk, J. Knoch, H. Riel, W. Riess, P. Rice, and T. Topuria, J. Appl. Phys. 103, 024304 共2008兲. 12 B. Fuhrmann, H. S. Leipner, H. R. Hoche, L. Schubert, P. Werner, and U. Gösele, Nano Lett. 5, 2524 共2005兲. 13 S. J. Han, T. K. Yu, J. Park, B. Koo, J. Joo, T. Hyeon, S. Hong, and J. Im, J. Phys. Chem. B 108, 8091 共2004兲. 14 J. Park, J. Joo, S. G. Kwon, Y. Jang, and T. Hyeon, Angew. Chem., Int. Ed. 46, 4630 共2007兲. 15 J. Turkevic and G. Kim, Science 169, 873 共1970兲. 16 W. F. Tseng, Z. L. Liau, S. S. Lau, M. A. Nicolet, and J. W. Mayer, Thin Solid Films 46, 99 共1977兲. 17 C. Canali, S. U. Campisano, S. S. Lau, Z. L. Liau, and J. W. Mayer, J. Appl. Phys. 46, 2831 共1975兲. 18 S. Hofmann, R. Sharma, C. T. Wirth, F. Cervantes-Sodi, C. Ducati, T. Kasama, R. E. Dunin-Borkowski, J. Drucker, P. Bennett, and J. Robertson, Nature Mater. 7, 372 共2008兲. 19 Properties of Metal Silicides, edited by K. Maex and M. van Rossum 共INSPEC, London, UK, 1995兲. 20 R. S. Wagner, W. C. Ellis, S. M. Arnold, and K. A. Jackson, J. Appl. Phys. 35, 2993 共1964兲. 21 R. S. Wagner and W. C. Ellis, Appl. Phys. Lett. 4, 89 共1964兲. 22 G. A. Bootsma and H. J. Gassen, J. Cryst. Growth 10, 223 共1971兲. 23 V. A. Nebol’sin and A. A. Shchetinin, Inorg. Mater. 39, 899 共2003兲. 24 D. Cherns, D. A. Smith, W. Krakow, and P. E. Batson, Philos. Mag. A 45, 107 共1982兲. 1 2

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