Shell Coaxial Nanowire Heterostructures by

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2008, 112, 20138–20142 Published on Web 12/03/2008

Synthesis of Ag/Si Core/Shell Coaxial Nanowire Heterostructures by the Vapor-Liquid-Solid Technique Tandra Ghoshal, Subhajit Biswas,‡ and Soumitra Kar*,‡ Department of Materials Science, Indian Association for the CultiVation of Science, Kolkata 700 032, India ReceiVed: October 20, 2008; ReVised Manuscript ReceiVed: NoVember 18, 2008

Metal/semiconductor core/shell radial heterostructured coaxial nanowires are reported for the first time. The radial coaxial nanowires consisted of a crystalline metallic Ag core and semiconducting amorphous Si shell. The Ag core nanowire growth was guided by the gold-catalyst-assisted vapor-liquid-solid process, whereas the amorphous Si shell formation could be attributed to the surface diffusion of the Si species through the liquid Au droplets to the nanowire surface. Introduction One-dimensional nanowires have been the focus of the research in nanotechnology due to their unusual properties and potential applications as building blocks in nanoscale electronics, optoelectronics, and sensing devices.1-14 For the development of a wide selection of nanoscale building blocks for nanometersized electrical and optical lines with new properties, it is important that the potential barrier between adjacent constituents has the appropriate current-voltage characteristics. This can be realized by the creation of various heterostructures including p-n junctions, metal-oxide-semiconductor junctions, or metal-semiconductor contacts that allow reliable signal processing. The formation of one-dimensional heterojunctions has led to materials with unique properties and multiple functionalities not realized in single-component structures that are useful for a wide range of applications. Diverse heterostructures assembled in either radial or axial directions within a single nanometer-scale building block (examples include group IV (Si/ Ge) semiconductor nanowires, heterojunctions of carbon nanotubes and silicon nanowires, and NiSi/Si nanowire heterostructures) have been fabricated.11,12,15-19 The idea that the functions of a nanowire could be improved by fabricating a sheath of different material around it has provided the impetus for the synthesis of nanowires such as Si/SiO2, Si/SiO2/C,19,20 and SiC/SiO2.17,21,22 As the metal contact to an individual device is the first step toward nanodevice integration and as such electrodes formed by a lithography-based method would define a much larger size scale than the nanometer-size building block itself,4 it is always advantageous to form a metal-semiconductor heterojunction by the bottom-up approach. Among all metals, silver nanowires are of particular interest because bulk silver exhibits highest electrical and thermal conductivity,23 and performance of silver in many applications could be potentially enhanced by fabricating one-dimensional nanostructures. Axially assembled heterojunctions of Ag and Si nanowires were fabricated by a * To whom correspondence should be addressed. Telephone: 1-407-8822848. Fax: 1-407-882-2819. E-mail: address: [email protected]. ‡ Present address: NanoScience Technology Center, University of Central Florida, Orlando, Florida 32826.

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combination of electrochemical deposition and chemical vapor deposition with AAO templates.16 Also, the Ohomic contact between Ag nanowires and a-CNT12 has universal significance and offers a wide opportunity for fabricating large-scale interconnects. In this letter, we report a simple thermal evaporation route to synthesize silver/silicon core/shell nanowire radial heterostructures. During our attempts to fabricate Ag2S nanowires, we accidentally observed the growth of silver/silicon core/shell nanowires. In order to understand the unexpected results, we have done systematic studies using different growth conditions. The results are presented here along with the corresponding analyses to demonstrate the formation of metal/semiconductor core/shell nanowires. The growth mechanism is discussed here. To the best of our knowledge, the metal/semiconductor Ag/Si core/shell nanowire heterostructure is synthesized for the first time. Experimental Procedure A conventional horizontal quartz tube furnace fitted with gas flow systems and a rotary vacuum pump was used for the synthesis. Si wafers coated with a thin film of Au (30 nm) were used as the substrates. Silver chloride (AgCl) was used as the precursor and was placed in the center of the quartz tube. Sulfur powder was placed at the upstream end of the quartz tube. The central zone of the quartz tube was maintained at 1073 K, and the upstream end where the S source was placed was maintained at 673 K. The substrates were placed at the top of the quartz boat containing AgCl, maintaining a ∼5 mm distance between the source and substrate. The Au-coated surfaces of the substrates were placed downward, facing the AgCl source. After evacuating the quartz tube up to ∼10-3 Torr, Ar gas was passed through it with a constant flow rate of 100 cm3/min for the entire deposition period. The quartz tube with all of the sources and substrates kept in their proper position was placed directly inside of the preheated furnace. After the desired deposition time, the quartz tube was taken out of the furnace to allow rapid cooling to room temperature. Black depositions were found in all of the substrates. Silver chloride was chosen as a metal source because it has a good thermal stability and does not decompose  2008 American Chemical Society

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Figure 1. XRD pattern of the sample deposited on the Au-coated Si substrate in the (a) presence and (b) absence of a sulfur source. The pattern depicted in part (c) was recorded on the residual inside of the quartz boat containing the AgCl source when the synthesis was carried out in the presence of a sulfur source. The XRD pattern in part (d) represented the sample deposited on the bare Si substrate in the experiment conducted using a sulfur source.

in the temperature regime used it easily reacts at the desired temperature, and is often has a sufficiently high vapor pressure, which allows easy transportation by the argon flow. The products were characterized using a Seifert 3000P X-ray diffractometer with Cu KR radiation. Microstructures of the nanoforms were obtained by scanning electron microscopy (SEM, Hitachi S-2300) and transmission electron microscopy (TEM, JEM 2010). Crystal structure and compositional analysis was studied through the high-resolution transmission electron microscopy (HRTEM, JEM 2010). Results and Discussion The as-prepared samples were characterized by XRD in order to identify the crystalline phase of the products. Figure 1a shows the XRD pattern of the sample deposited on the Si substrate. The XRD pattern shows a sharp peak at 2θ ) 38.11° along with a broad hump at ∼2θ ) 29°. The sharp peak was assigned to the (111) peak of metallic silver. The appearance of the broad peak indicated the formation of certain kind of amorphous or very poorly crystalline material. However, on the contrary, the sharp and dominant nature of the (111) peak indicated the formation of very crystalline Ag with preferred orientation along the (111) direction. The broad peak at ∼ 2θ ) 29° could be attributed to amorphous Si. Surprisingly, no peaks corresponding to the Ag2S phases were identified. In order to understand the unexpected formation of metallic Ag, we have also carried out the synthesis in the absence of the sulfur powder, keeping all of the remaining experimental parameters unchanged. Figure 1b shows the XRD pattern of the sample deposited on the Si substrate in the absence of the sulfur source. The XRD pattern

shows quite a few sharp peaks. Thorough indexing of the peaks reveals the formation of two crystalline phases, which are AgCl and Ag. Thus, these two XRD patterns indicated that presence of sulfur in the synthesis environment helps in complete decomposition/conversion of the AgCl. In order to investigate the mechanism of this sulfur-assisted decomposition of AgCl, we have studied the residual powder (of the first experiment, i.e., in presence of S) collected from the quartz boat containing the AgCl powder. The XRD pattern of the powder shown in Figure 1c reveals the formation of the Ag2S phase. These results indicated that AgCl might have reacted with the S to form Ag2S, which decomposes rather easily compared to AgCl to produce the metallic Ag phase. Thus, two different types of crystalline materials were formed at two different positions under the same experimental conditions, that is, when the synthesis was carried out in presence of the S source, Ag2S was formed on the quartz boat containing AgCl, and phase-pure Ag was formed on the gold-coated Si substrate placed 5 mm above the AgCl source. This showed that gold might have some role in the decomposition of the Ag2S phase. In order to understand the formation process of Ag, we have performed another experiment with a bare Si substrate, that is, without using gold and keeping all of the remaining experimental parameters identical to that of the first experiment. The XRD pattern of the sample prepared in the absence of gold (Figure 1d) reveals the formation of the Ag2S phase on the Si substrate. This shows that the gold film had a definite role in the decomposition of the Ag2S phase. The reasons will be discussed after the SEM and TEM results. It was observed that the deposition was not uniform throughout the Au-coated Si substrate. The deposition was more uniform

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Figure 3. TEM image (a) of the Ag nanorods deposited after 30 min, exhibiting the core/shell heterostructure and HRTEM image (b) of the highlighted part depicted in part (a). TEM (c) and HRTEM (d) images of the core/shell nanowire, revealing the crystalline core amorphous shell configuration. The inset of (d) depicts the SAED pattern of the corresponding nanowire.

Figure 2. SEM images exhibited in parts a and b show the formation of Ag nanorod arrays on the Au-coated Si substrate directly facing the AgCl source, whereas the SEM images depicted in parts c and d show the formation of sparsely distributed Ag nanorods on the portion of the Au-coated Si substrate extended outside of the AgCl-containing chamber after a 30 min deposition period. The SEM images depicted in parts e and f show the formation long Ag nanowires on the Aucoated Si substrate after 1 and 2 h of deposition, respectively.

on the portion which was directly facing the AgCl source, whereas the deposition was nonuniform at the extended portion of the substrates. Morphologies of the products were identified by the SEM studies. Figure 2a and b shows the SEM image of the uniform portion deposited after 30 min. Figure 2a reveals the formation of a uniform nanorod array. Figure 2b depicts a magnified view of the aligned nanorods. The diameter of the nanorods was ∼50 nm. Figure 2c and d shows the distribution of the nanostructures on the extended nonuniform portion. The density of the nanorods was much less at the extended portion of the substrate. A closer observation of the SEM images shown in Figure 2c and d reveals the nucleation stage of the nanorods. In addition to the nanorods, a porous thin-film-like feature was also observed on the surface of the substrate as depicted in the magnified SEM image (Figure 2d). In order to identify the composition of the nanorods and the porous film, an EDAX study was carried out on a selected area. The EDAX study revealed the presence of Si as the major component in the nanorods along with the presence of Ag as minor components. However, in the film-like portions, only Si was detected. No signal corresponding to sulfur was detected in the selected as well as total area EDAX scan performed on the sample. In order to discard the effect of the Si substrate on the EDAX signal, the elementary analyses were also performed with an EDAX spectrometer coupled with the TEM, and the results are discussed after the TEM results. Syntheses were also done with longer periods of time to monitor the nanowire growth pattern. Figure 2e and f depicts the morphology of the products deposited on the Au-coated Si substrate after 1 and 2 h, respectively. It was observed that the nanowire length increased consistently with time. The SEM studies also revealed the formation of

undefined features when the syntheses were performed in the absence of S and Au. The nanostructures were also studied through TEM. Figure 3a shows the morphology of the products synthesized after 30 min of time. The TEM image reveals a core/shell-type radial heterostructure with a high-contrast inner core and an amorphous shell surrounding the core nanowire. The core diameter was ∼10-12 nm, and the shell thickness was ∼20 nm. Figure 3b shows the high-resolution TEM images of the highlighted part in Figure 3a. The figure shows that the inner core part was crystalline and the outer part was amorphous in nature. The inner crystalline part possessed one spherical-cap-like portion at one side of the nanowire. The compositions of the crystalline nanowire, the spherical cap portion, and outer amorphous shell part were determined from the EDAX analysis, and the figure is indexed accordingly. Figure 3c shows the TEM image of the product obtained after 2 h. The core/shell structure is much more prominent from this image. The image also shows the presence of a hollow channel inside of the shell part, that is, the inner core part is not continuous throughout the length. Three different zones are marked in this image from where the EDAX signals were recorded to further confirm the core and shell composition. The details are discussed below. Figure 3d shows the HRTEM images of one nanowire. The crystalline lattice fringe is not very clear due to the presence of a very thick shell. The measured lattice spacing was 0.235 nm, which represents the (111) lattice plane of the crystalline cubic phase of Ag (JCPDS 04-0783). This indicates that the growth direction of the Ag nanowires was (111), which matches well with the XRD studies. The image in the inset of Figure 3d depicts the selected area electron diffraction (SAED) pattern of the core/shell nanowires. The SAED pattern also indicated that the growth direction of the nanowires is (111). Appearance of a diffused ring in the SAED pattern could be attributed to the amorphous Si shell. The SEM and TEM studies show the formation of a core/ shell heterostructured nanowire. The XRD study reveals the formation of cubic Ag along the (111) preferred orientation. The EDAX spectrometer coupled with the SEM reveals the presence of Si as the major component. From the SEM-based EDAX data, it is difficult to make any conclusion since the samples were grown on the Si wafer. Thus, in order to avoid the substrate effect on EDAX, the compositional analysis was performed with the EDAX spectrometer coupled with the TEM. The EDAX signal was recorded from different parts of the core/ shell nanowires as marked by drawing white circles in Figure

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Figure 4. EDAX spectra recorded from different parts of the core/ shell nanowires. The position of the nanowire from where the EDAX signals (a, b, and c) were recorded are indicated in the TEM image shown in Figure 3c by 1, 2, and 3, respectively.

3c. Figure 4a shows the EDAX spectrum obtained from the position “1” marked in Figure 3c. The compositional analysis reveals the presence of 80% Si, 17% Ag, and only 3% O. A minute quantity of the oxygen could be due to the surface oxidation of the amorphous Si shell. Figure 4b shows the EDAX spectrum recorded from the shell part “2” revealing the presence of only Si. Figure 4c shows the EDAX spectrum recorded on the exposed portion “3” of the core Ag nanowire revealing the presence of 98% Ag and 2% Si. Thus, the EDAX study coupled with the TEM images proved beyond doubt that the synthesized nanowires were Ag/Si core/shell nanowires. The core is the crystalline cubic silver, and the shell is the amorphous silicon. The morphological studies indicated that the Ag nanowire growth was definitely guided by the vapor-liquid-solid (VLS) mechanism. The SEM images shown in Figure 2 indicated the presence of spherical particles at the tip of the nanorods and nanowires. The TEM image presented in Figure 3a and b also shows the presence of the spherical particle at the tip of the nanowire. The EDAX study reveals the presence of Au and Ag together at the nanoparticle portion. Extensive SEM studies also reveal the presence of spherical particles on the relatively longer nanowires (Figure 5a). Thus, the presence of the spherical particles at the tip of the nanowires indicated the role of the

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Figure 5. (a) The SEM image shows the presence of spherical particles at the tip of the nanowires. (b) The EDAX signal recorded on the spherical particles shown in part (a) reveals the presence of Au, Ag, and Si. The SEM image depicted in part (c) shows the cross-sectional image of the Si substrates after nanowire growth, revealing surface corrosion.

vapor-liquid-solid (VLS) process for nanowire growth. The composition of the spherical particles was also studied by EDAX spectroscopy. The EDAX spectrum shown in Figure 5b reveals the presence of Au along with Ag and Si in the sphericalparticle-like features. This further confirms the role of Au as a catalyst. There are two main concerns associated with the formation of the core/shell nanowires, (i) the role of Au in decomposition of the Ag2S phase and (ii) the formation of the amorphous Si shell. It was reported earlier that phase-pure Ag could be obtained from the molten phase of Ag2S upon vaporization.24 Thus, we propose that AgCl reacted with S to produce Ag2S vapor through the following reaction (eq 1)

2AgCl + 3S ) Ag2S + S2Cl2

(1)

1 Ag2S(l) f 2Ag(s) + S2(g) 2

(2)

The Ag2S then dissolves in the liquid Au droplet to form the Au-Ag2S alloy. It is well-known that in the VLS process, the solute part precipitates as a solid upon supersaturation. It was reported earlier24 that at ∼1073 K, Ag2S melts and subsequent thermal decomposition produces Ag following the reaction in eq 2. Thus, Ag2S decomposes in the molten state to produce phase-pure Ag. As discussed earlier during the discussion on XRD data, Ag2S products were identified on a Si substrate when the synthesis was carried out in the absence of Au. Therefore, it seems that when the Ag2S vapor phase directly condenses to

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Figure 6. Schematic diagram of the formation mechanism of the core/ shell nanowire.

the solid state, it does not decompose at ∼1073 K. Subsequently, we can say that the nucleation of the Ag nanowires is somewhat different from the conventional VLS technique. In the conventional VLS technique, the constituent element/molecule vapor of the resultant nanowires gets absorbed in the liquid Au droplets and upon supersaturation gets precipitated as solid nanowires. However, in the present case, Ag2S vapors gets absorbed in the liquid Au droplets followed by thermal decomposition producing solid Ag species. These solid Ag species gets deposited uniaxially as Ag nanowires. This might have guided the nucleation of the Ag nanowire from Au droplets, although the melting point of Au (1337.4 K) is little bit higher than that of Ag (1234.9 K). The gaseous byproduct S2Cl2 is a toxic material and also acts as an erosion reagent of Si. As a precaution, the gas outlet was passed through a NaOH solution to convert the toxic gas into nontoxic byproducts through the following reaction

2S2Cl2 + 6NaOH ) 4NaCl + 3S + Na2SO3 + 3H2O (3) The toxic S2Cl2 gas etches the Si wafer, producing the Si vapor surrounding the Au nanodroplets. The Si surface etching was evident from the SEM studies. One representative cross-sectional SEM image is shown in Figure 5c to exhibit the Si surface corrosion. The formation mechanism of the amorphous shell is represented schematically in Figure 6. As soon as the Ag nanowires originated from the Au droplets, the etched Si species accumulated around the substrate. These Si vapor species traveled around liquid droplets and was deposited on the surface of the Ag nanowires. Due to the low eutectic point of Au-Si (637 K), the Si vapor species were also absorbed in the Au droplets followed by precipitation surrounding the already growing Ag nanowires. These dual deposition routes of Si surrounding the Ag core could be the reason behind the formation of the thick Si shell as revealed by the TEM images shown in Figure 3. Once the growth stopped, the Si species could not travel further, and they covered the Au tips at the end. When the reaction was stopped after just 30 min, the prematurely grown Ag nanowires or the seeds were also covered by the Si layer. As the synthesis temperature was lower than the eutectic point of Ag-Si (1103 K), Ag and Si did not form an alloy; instead, they coexisted as a core/shell without altering their own entity. The Ag/Si core/shell nanowires were also characterized by the Raman spectroscopy, and one representative Raman spectrum of core-shell nanowires is shown in Figure 7. The spectrum shows a Raman band at 517 cm-1 corresponding to the first-order transverse optical (TO) phonon mode of silicon.25 A red shift (3 cm-1) and asymmetrical broadening of the optical phonon peak (full width at the half-maximum ) 17 cm-1) resulting from the spatial confinement effect confirms the presence of Si nanostructures as a shell.25

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Figure 7. Raman spectrum of the core/shell nanowire.

Conclusion These results demonstrated a convenient bottom-up approach to synthesize silicon-coated Ag nanowires on a gold-coated Si substrate. VLS growth along with shell formation is responsible for the growth of coaxial nanowires. The results presented here demonstrate the capability of the formation of the metal/ semiconductor coaxial heterostructures. These metal/semiconductor coaxial core/shell nanowires could be useful for microelectromechanical and nanoelectromechanical systems. Acknowledgment. One of the authors (T.G.) thanks CSIR (Council of Scientific and Industrial Research, Govt. of India) for financial assistance. References and Notes (1) Acharya, S.; Panda, A. B.; Efrima, S.; Golan, Y. AdV. Mater. 2007, 19, 1105. (2) Acharya, S.; Patla, I.; Kost, J.; Efrima, S.; Golan, Y. J. Am. Chem. Soc. 2006, 128, 9294. (3) Chung, S. W.; Yu, J. Y.; Heath, J. R. Appl. Phys. Lett. 2000, 76, 2068. (4) Duan, X. F.; Huang, Y.; Agarwal, R.; Lieber, C. M. Nature 2003, 421, 241. (5) Kar, S.; Chaudhuri, S. J. Phys. Chem. B 2005, 109, 3298. (6) Kar, S.; Chaudhuri, S. J. Phys. Chem. B 2006, 110, 4542. (7) Kar, S.; Dev, A.; Chaudhuri, S. J. Phys. Chem. B 2006, 110, 17848. (8) Kar, S.; Ghoshal, T.; Chaudhuri, S. Chem. Phy. Lett. 2006, 419, 174. (9) Kar, S.; Pal, B. N.; Chaudhuri, S.; Chakravorty, D. J. Phys. Chem. B 2006, 110, 4605. (10) Kar, S.; Santra, S.; Chaudhuri, S. Cryst. Growth Des. 2008, 8, 344. (11) Kar, S.; Santra, S.; Heinrich, H. J. Phys. Chem. C 2008, 112, 4036. (12) Luo, J.; Zhu, J. Nanotechnology 2006, 17, S262. (13) Rao, C. N. R.; Deepak, F. L.; Gundiah, G.; Govindaraj, A. Prog. Solid State Chem. 2003, 31, 5. (14) Rao, C. N. R.; Vivekchand, S. R. C.; Biswasa, K.; Govindaraja, A. Dalton Trans. 2007, 3728. (15) Lauhon, L. J.; Gudiksen, M. S.; Wang, C. L.; Lieber, C. M. Nature 2002, 420, 57. (16) Luo, J.; Zhang, L.; Zhang, Y. J.; Zhu, J. AdV. Mater. 2002, 14, 1413. (17) Meng, G. W.; Zhang, L. D.; Mo, C. M.; Zhang, S. Y.; Qin, Y.; Feng, S. P.; Li, H. J. J. Mater. Res. 1998, 13, 2533. (18) Moore, D.; Morber, J. R.; Snyder, R. L.; Wang, Z. L. J. Phys. Chem. C 2008, 112, 2895. (19) Shi, W. S.; Peng, H. Y.; Xu, L.; Wang, N.; Tang, Y. H.; Lee, S. T. AdV. Mater. 2000, 12, 1927. (20) Morales, A. M.; Lieber, C. M. Science 1998, 279, 208. (21) Zhang, H. X.; Ge, J. P.; Li, Y. D. Chem. Vap. Deposition 2005, 11, 147. (22) Zhang, L. D.; Meng, G. W.; Phillipp, F. Mater. Sci. Eng., A 2000, 286, 34. (23) Wang, Z. H.; Liu, J. W.; Chen, X. Y.; Wan, J. X.; Qian, Y. T. Chem.sEur. J. 2005, 11, 160. (24) Piacente, V.; Scardala, P.; Ferro, D. J. Mater. Sci. Lett. 1990, 9, 365. (25) Li, C.; Fang, G. J.; Sheng, S.; Chen, Z. Q.; Wang, J. B.; Ma, S.; Zhao, X. Z. Physica E 2005, 30, 169.

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