Self-Oriented Single Crystalline Silicon Nanorod Arrays through a

Jan 22, 2010 - resulted in a triangular orthographic projection network on the Si (111) surface. A modified vapor-liquid-solid. (VLS) mechanism for th...
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J. Phys. Chem. C 2010, 114, 2471–2475

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Self-Oriented Single Crystalline Silicon Nanorod Arrays through a Chemical Vapor Reaction Route Ke-Ji Wang,†,‡ Kai-Xue Wang,*,† He Zhang,†,‡ Guo-Dong Li,‡ and Jie-Sheng Chen*,† †

School of Chemistry and Chemical Engineering, Shanghai Jiao Tong UniVersity, Shanghai 200240, P. R. China, and ‡ State Key Laboratory of Inorganic Synthesis and PreparatiVe Chemistry, College of Chemistry, Jilin UniVersity, Changchun 130012, P. R. China ReceiVed: September 25, 2009; ReVised Manuscript ReceiVed: January 4, 2010

Self-oriented single crystalline Si nanorod (SiNR) arrays have been successfully prepared on silicon (111) substrate via a vacuum chemical vapor reaction (CVR) process in a hermetic quartz tube. Monoclinic Ag2Te obtained by prior chemical reaction of evaporated tellurium with silver film on Si substrate was used as the catalyst for the formation of SiNR arrays. The preferred growth of the nanorods along the [100] direction resulted in a triangular orthographic projection network on the Si (111) surface. A modified vapor-liquid-solid (VLS) mechanism for the growth of SiNR arrays has been proposed on the basis of experimental results. 1. Introduction Oriented arrays of one-dimensional (1D) semiconductor nanostructures (nanowires, nanorods, and nanotubes) have attracted considerable attention due to their potential applications in the next generation electronic and optoelectronic devices, such as sensors, laser diodes, field-effect transistors, and light-emitting diodes.1-5 Semiconductor nanowires have been prepared directly on solid substrates through various approaches, including thermolysis, molecular beam epitaxy, solution preparation, and microwave-assisted and vapor synthesis.6-9 Among the commonly employed techniques, chemical vapor deposition (CVD) and physical vapor deposition (PVD) are efficient in the preparation of high-density one-dimensional nanowires on large area substrates. However, both CVD and PVD processes involve the use of a special reaction apparatus with a carrier gas. In addition, the majority of nanowires obtained by CVD and PVD tend to be randomly oriented.10-13 A few exceptional cases of nanowire columns grown on substrates resulted from the particular anisotropic habit of the substance.14 Because a cubic crystal has not the intrinsic tendency to grow along a particular direction, it is even difficult to obtain regular arrays of crystals with a cubic structure through CVD and PVD routes. One-dimensional crystals of silicon, which has a cubic structure, are of particular importance due to their crucial role in both fundamental research and practical applications. Silicon nanowires have been used as building blocks for the fabrication of electronic devices,15,16 thermoelectric devices,17,18 and labelfree cancer markers.19 A well-accepted mechanism of silicon nanowire (SiNW) formation through a gas-phase reaction is the so-called vapor-liquid-solid (VLS)20-22 process with a catalyst particle at the nanowire tip as its symbol.23-25 In the traditional VLS process, the vapor precursor of silicon decomposes at a metal catalyst droplet, diffuses through the catalyst, and then gets solidified at the solid-liquid interface when the liquid droplet becomes supersaturated. In the case of silicon, gold is the most commonly used catalyst. When the gold nanoparticles are larger than 30 nm in diameter, the silicon nanowires preferentially grow along the [111] direction, leading to verti* E-mail: [email protected], Tel: (+86) 21 54743266, Fax: (+86) 21 54741297.

cally aligned nanowires on the Si (111) substrate.26 To the best of our knowledge, only a few reports have demonstrated the feasibility for VLS growth of Si nanowires mediated by metals other than Au, such as Ga,27 Cu,28 Al, and In.29 The catalytic efficiency of these non-Au metals were low and the corresponding silicon nanowire huddles are nonoriented in contrast with those catalyzed by Au. In this paper, we describe the preparation of well-aligned SiNR arrays on silicon substrate through a catalyst-assisted vacuum chemical vapor reaction (CVR) route, in which Ag2Te plays an important role as an effective catalyst for the array growth. On the basis of the experimental results, it is presumable that the growth of the SiNRs follows a modified VLS mechanism, in which the Si source is provided by etching of the Si wafer with molten Ag2Te at elevated temperatures. The morphology and crystal structure of the SiNR products have been elucidated in detail. 2. Experimental Details 2.1. Sample Preparation. Prior to the CVR process, Si (111) wafers were cleaned and degreased successively with a 10:1 (v:v) H2SO4:H2O2 solution (at 120 °C) and deionized water, followed by blowing to dry with hot ultrapure nitrogen. Subsequently, the wafers were immersed in dilute HF acid (1 wt %) for 1 min to remove native oxides, such as SiO2 from the surface, followed by rinsing with deionized water and drying in hot N2 stream. A silver film with a thickness of approximately 200 nm was deposited on the HF-treated Si wafers via the thermal evaporation of Ag (99.99%) foil under high vacuum (6.0 × 10-4 Pa). 2.2. Growth of Si Nanorod Arrays. After Ag film deposition, the Si wafer (about 5 × 5 mm2) was placed in an endclosed quartz tube with a 6 cm distance from the tellurium powder (about 70 mg, 99.99%) which was located at the closed end of the tube. With an exact length of 19 cm, the quartz tube was then sealed after evacuation. The sealed tube was placed in a tube furnace (Figure 1) with a length of ∼37 cm and heated to 750 °C with a ramp rate of 5 °C min-1. The Te-containing end was placed in the middle region of the furnace, and a temperature gradient of the tube from the Te powder to the Si substrate was achieved. The reaction system was cooled to room

10.1021/jp909244q  2010 American Chemical Society Published on Web 01/22/2010

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Figure 1. Experimental setup for the preparation of Si nanorod arrays.

Figure 3. Typical EDX spectra for the individual SiNR: catalyst tip on the SiNR (A) and body of the SiNR (B).

Figure 2. Low- (a) and high-magnification (b) SEM images of the SiNR arrays grown on a Si (111) substrate; enlarged SEM image (c) for the SiNRs with the inset showing the rectangular-shaped cross section; and side view of the SiNR arrays (d).

temperature naturally after being maintained at 750 °C for about 7 h. The brushy state of the substrate surface indicates the formation of SiNR arrays. 2.3. Characterization. The thickness of the silver film on Si wafer was measured to be ∼200 nm by a Dektak 150 Surface Profiler. The surface morphology was observed by a JSM-6700F scanning electron microscope (SEM) equipped with an energy dispersive X-ray (EDX) spectrometer. Prior carbon coating on the samples was conducted before the SEM observation and EDX analysis. The cross-section SEM image was taken on a JSM-7401F scanning electron microscope. The high-resolution transmission electron microscopy (HRTEM) images and the selected area electron diffraction (SAED) patterns were obtained with a transmission electron microscope (TEM, JEM-3010) operated at 300 kV. Samples were scraped off the Si wafer and dispersed into anhydrous ethanol then filtrated on a holey carbon TEM copper grid. Information about the crystal phase and crystallinity was acquired using an X-ray diffractometer (XRD, Rigaku) with Cu KR (λ ) 1.5418 Å) radiation over the 2θ range from 10° to 80°. 3. Results and Discussion As shown in the top-view SEM images in Figure 2a,b, welloriented three-dimensional (3D) nanorod arrays are obtained on a silicon (111) substrate. All nanorods grow along a direction

that has a certain angle θ with the substrate surface. As marked with white arrows in Figure 2a, the projected directions of nanorod arrays are preferentially along the three directions of [112j], [12j1], and [2j11] with a 120° angle between each two directions. Examination of the crystal geometry of a cubic system readily reveals that the growth of Si nanorods on the Si (111) substrate is along the identical [100] directions. Theoretically, the angle θ between the nanorods and the substrate surface should be 35.26°. By rotating the substrates around their surface normal and tilting about suitably chosen axes, we can observe the nanorods from different perspectives. Figure 2d shows the cross-section SEM image of the as-prepared samples. As marked with the black arrow, nanorods grow in a direction with an angle of approximately 38 ( 2° with the surface of the Si wafer, in good agreement with the theoretical angle 35.26°. The orthographic projections of nanorods grown in the other two directions lead to an angle of nearly 90° with the former direction marked with a black arrow. Close inspection reveals that almost all of the nanorods have a catalyst particle at their tips. The gradual tapering toward the crystal growth direction makes the nanorod needle sharp (Figure 2b,c). As shown in Figure 2c inset, the bodies of these nanorods are rectangularly shaped. The size distribution of the nanorods can be obtained through frequency statistics and a gauss fit (see details in the Supporting Information, Figure S1). The average length of the nanorods is approximately 17.5 µm, and the mean diameter measured at the half-maximum is about 356 nm, with standard deviations of 4.5 µm and 159 nm, respectively. A layer of nanowires grown underneath the nanorod arrays is observed (Figure S2, Supporting Information). The nanowires randomly coated on the substrate have a length of 2.5-6.5 µm and a diameter of 190-450 nm at the root and 50-150 nm at the tip, respectively. Elemental analyses of the nanorods were conducted on the EDX instrument attached to the SEM microscope. The corresponding spectra recorded at the center of the marked circle in the inset are presented in Figure 3. The EDX spectrum of the nanorod shown in Figure 3B confirms that the obtained nanostructures are formed by Si. The spectrum A in Figure 3 shows that the Ag/Te molar ratio is very close to 2, indicating that the catalyst particles on the tip of SiNRs are composed of Ag2Te. Because the samples were prepared under sealed vacuum conditions, the signal of oxygen is attributed to the presence of ethanol used as a solvent, whereas the signals for carbon and copper arise from the copper grid. Similar analysis has been conducted on the nanowires grown beneath the SiNR arrays. The corresponding spectra in Figure S3 (Supporting Information)

Single Crystalline Silicon Nanorod Arrays

Figure 4. XRD pattern of the as-prepared sample on the Si wafer (a) compared with the standard diffraction peaks of cubic Si (b) (JCPDS file No.77-2107). The asterisks represent the diffractions of monoclinic Ag2Te (JCPDS No.81-1820).

Figure 5. TEM image of a single Si nanorod (a); HRTEM images of the nanorod in region b (b) (the top-left inset shows the enlarged TEM image of the marked area in (a) and the bottom-right inset displays the corresponding SAED pattern) and in region c (c) with the inset showing the corresponding SAED pattern.

demonstrate that both the thick- and the fine-parts of a nanowire are pure silicon. The as-prepared SiNRs on silicon wafers were characterized through X-ray diffraction. In the XRD pattern of the products (Figure 4a), the peaks located at 28.54°, 47.38°, 56.18°, 69.30°, and 76.44° can be assigned to the (111), (220), (311), (400), and (331) diffractions of silicon with a cubic crystal structure (JCPDS 77-2107, a ) 5.419 Å), respectively. Those marked with an asterisk result from the monoclinic Ag2Te on the tips of the nanorods (JCPDS 81-1820). The calculated lattice constants of the monoclinic Ag2Te are a ) 8.16 Å, b ) 4.46 Å, and c ) 8.98 Å. Therefore, the formation of crystalline silicon nanorods catalyzed by Ag2Te via our CVR process is also confirmed by X-ray diffraction. In addition to the peaks ascribed to silicon and Ag2Te, a broad hump centered at around 24° is also observed in the XRD pattern. This hump is caused by the glass slide used as the sample support for the XRD measurement. The TEM studies provide further information on the nanorod growth. As shown in the TEM images in Figure 5a,b, the SiNRs have a smooth surface and a catalyst Ag2Te particle at their tips. The HRTEM images (Figure 5b,c) taken in the region

J. Phys. Chem. C, Vol. 114, No. 6, 2010 2473 marked in Figure 5a show that both the nanorod and the Ag2Te particle are highly crystalline. In Figure 5b, the marked interplanar d spacing is of approximately 0.68 nm, corresponding to the (100) lattice planes of monoclinic Ag2Te; and the corresponding fast Fourier transform (FFT) along the [02j1] zone axis is shown as an inset. The diffraction spots with a lattice spacing of 0.68, 0.32, and 0.29 nm are associated with the (100), (11j2j) and (21j2j) planes of the monoclinic Ag2Te crystal. The dihedral angle of the (100) and (21j2j) lattice planes is measured to be about 58.8°, in good agreement with the theoretical value, 58.7°. In Figure 5c, the marked interplanar d spacing is 0.19 nm (Figure 5c), corresponding to that of the (220) lattice planes of cubic Si. Further inspection reveals that the nanorods grow along the [100] direction. The corresponding electron diffraction along the [001] zone axis (inset in Figure 5c) can also be indexed to the cubic structure of silicon. The diffraction spots with a lattice spacing of 0.19 and 0.13 nm arise from the (220) and (400) Bragg reflections, respectively. Given the presence of Ag2Te tips and the tapering phenomenon, the epitaxial growth of the SiNRs is believed to have started from the surface of Ag2Te grains, via a growth mechanism analogous with the VLS mechanism for the growth of various one-dimensional materials from surrounding vapor. The main difference between the CVR and VLS processes is that the vaporized silicon precursor in our CVR process arises from chemical reaction of solid Si wafer with evaporated tellurium in the sealed quartz tube. The detailed formation process of the SiNRs via a CVR method is described as follows. In the sealed quartz tube, the Te powder in the hotter region is evaporated and transported to the cooler region, where vaporous Te reacts with silver coated on the surface of silicon wafer to form Ag2Te (eq 1, where s and g represent the solid and the vapor phases, respectively). The temperature at which the reaction takes place is approximately 450 °C, which is slightly lower than that (470 °C) reported in the literature. The high reactivity of nanosized silver film may take responsibility for the lowered reaction temperature.

Ag(s) + Te2(g) f Ag2Te(s)

(1)

Increasing the reaction temperature leads to the dissolution of the silicon substrate to form a low-melting-point Ag2Te-Si eutectic droplet (eq 2, where l represents the liquid phase). Meanwhile, the etched silicon substrate also reacts with Te vapor to form solid Si2Te3. However, the subsequent decomposition of Si2Te3 generates Siw(s), Te2(g), and SiTe(g) (eq 3, where Siw represents the finally solidified Si nanowires), as proposed in the literature.30,31

Ag2Te(l) + Si(s) f Ag2Te-Si(l)

(2)

Si(s) + Te(g) f Si2Te3(s) f Siw(s) + SiTe(g) + Te2(g)

(3)

The solid Siw, one of the decomposed products of Si2Te3, forms the silicon nanowires coating on the silicon wafer underneath the SiNR arrays. The gaseous silicon resource, that is, the vaporous SiTe, is responsible for the SiNRs epitaxial growth. The SiTe gas is condensed continuously into Ag2Te-Si liquid droplet, followed by decomposition to Si and Te2. When the concentration of Si in the Ag2Te-Si liquid droplet reaches supersaturation, Si nucleates at the interface between the liquid

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Figure 6. Schematic illustration of the epitaxial growth of Si nanorod on Si (111) substrate (A) and the formation process of SiNR arrays (B).

droplet and the substrate, and grow along a certain lattice direction, the [100] direction in our system. This process can be described by the following equation (eq 4, where SiR represents the SiNRs).

Ag2Te-Si(l) + SiTe(g) f Ag2Te(l) + SiR(s) + Te2(g) (4) The corresponding schematic of the SiNRs growth mechanism is presented in Figure 6. The cross-section view (Figure 6A) in the scheme shows the epitaxial relationship between the nanorods and the substrate, whereas Figure 6B shows the formation process of the SiNR arrays. After the generation of the supersaturated eutectic liquid droplet (part I in Figure 6B), the growth along the three equivalent [100], [010], and [001] directions in a cubic crystal system begins simultaneously (part II in Figure 6B). As a result, the regularly aligned SiNR arrays are obtained on the silicon surface (part III in Figure 6B). The etching of SiO2 layer on Si wafer before silver deposition is essential for the SiNR formation. If no SiO2 etching is conducted, the reaction between the Si wafer and Te does not take place due to the high chemical stability of SiO2, and consequently, there is no SiTe vapor that acts as Si source to support the growth of the SiNRs. Under this circumstance, only spheres on the silicon wafer are obtained (see details in the Supporting Information, the inset of Figure S4B). As determined by EDX analysis (Figure S4B), the molar ratio of Ag/Te in the spheres is approximately 2, indicating the formation of Ag2Te catalyst without growth of silicon nanorods in this case. The Si signal should be caused by the Si substrate, and the C signal arises from the pretreatment of the sample for EDX measurement. The catalyst Ag2Te plays an important role in the formation of the SiNRs. Employing Ag or Te alone as the catalyst, no SiNRs are formed under identical experiment conditions. The inset of Figure S4A (Supporting Information) presents the products obtained by using Ag as the catalyst. The EDX analysis indicates that the particles are silver. In another experiment, Si wafers are transferred into the quartz tube immediately after HF etching. Without catalyst, no SiNRs are found either on the Si wafer after treated under the same reaction conditions. The reaction temperature is also an important factor in the prepara-

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Figure 7. SEM images of the Si nanorods grown at 750 °C for about 1 h (a), 3 h (b), 7 h (c), and 10 h (d), respectively.

tion of the SiNRs. As shown in the SEM images, irregular spheres with an Ag/Te molar ratio of approximately 2 are obtained after reaction at about 450 °C (Figure S5a, Supporting Information). There is no apparent change in morphology until the temperature is increased to about 730 °C. At 730 °C, the morphology of the product appears varied, and the Ag/Te molar ratio of the spheres obtained becomes distinctly smaller (Figure S5b, Supporting Information), suggesting that the composition in these spheres is different from Ag2Te because the Si starts to react with Te to form Si2Te3. When the temperature reaches 750 °C, the formed Si2Te3 decomposes into gaseous SiTe, which facilitates the SiNR growth. The effect of the reaction time on the SiNR growth is shown in Figure 7. The nanorods have an average length of approximately 1, 10, and 17 µm after reaction for 1, 3, and 7 h (Figure 7). However, there is no obvious increase in the length of nanorods even if the reaction time is prolonged to over 10 h, probably because there is no Te at the hot end of the quartz tube (Figure 7d). No more Te reagent reacts with Si to supply SiTe vapor, and as a result, the growth of the SiNRs is terminated. 4. Conclusions Regularly oriented 3D cubic silicon nanorod arrays have been fabricated on a silicon substrate via a CVR growth process. The in situ formation of Ag2Te plays an important role in the Si nanorod growth, and it is the first time that Ag2Te is found to be efficient for catalyzing one-dimensional Si formation. The epitaxial growth of the SiNRs, which start from the surface of Ag2Te grains, is facilitated by the gaseous SiTe formed by chemical reaction of a solid Si wafer with tellurium in a sealed reactor. The highly crystalline nanorods grow along the [100] directions to form a self-oriented SiNR array. These SiNR arrays may find applications in various fields, and especially they are promising for use as the anode material32,33 of lithium batteries. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (20731003) and the National Basic Research Program (2007CB613303). Supporting Information Available: Quantitative analysis of size distributions for the Si nanorods; SEM images and EDX

Single Crystalline Silicon Nanorod Arrays patterns of the Si nanowires grown under the SiNR array, and further SEM images and EDX patterns that demonstrate the effects of reaction temperature and different catalysts on the formation of Si nanorods. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Arnold, M. S.; Avouris, P.; Pan, Z. W.; Wang, Z. L. J. Phys. Chem. B 2003, 107, 659. (2) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Webber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. (3) Nguyen, P.; Ng, H. T.; Yamada, T.; Smith, M. K.; Li, J.; Han, J.; Meyyappan, M. Nano Lett. 2004, 4, 651. (4) Kim, H. M.; Cho, Y. H.; Lee, H.; Kim, S. II; Ryu, S. R.; Kim, D. Y.; Kang, T. W.; Chung, K. S. Nano Lett. 2004, 4, 1059. (5) Wang, X. D.; Summers, C. J.; Wang, Z. L. Nano Lett. 2004, 4, 423. (6) Casagrande, L. G.; Juang, A.; Lewis, N. S. J. Phys. Chem. B 2000, 104, 5436. (7) Alivisatos, A. P.; Harris, A. L.; Levinos, N. J.; Steigerwald, M. L.; Brus, L. E. J. Chem. Phys. 1988, 89, 4001. (8) Terukov, E. I.; Khuzhakulov, E´. S. Semiconductors 2005, 39, 1371. (9) Tonkikh, A. A.; Cirlin, G. E.; DubrovskiI¨, V. G.; Ustinov, V. M.; Werner, P. Semiconductors 2004, 38, 1202. (10) Hu, J. Q.; Bando, Y.; Liu, Z. W.; Sekiguchi, T.; Golberg, D.; Zhan, J. H. J. Am. Chem. Soc. 2003, 125, 11306. (11) Huang, B. R.; Hsu, J. F.; Huang, C. S.; Shih, Y. T.; Lu, K. S. Mater. Sci. Eng. C 2007, 27, 1197. (12) Du, J.; Du, P. Y.; Hao, P.; Huang, Y. F.; Ren, Z. D.; Han, G. R.; Weng, W. J.; Zhao, G. L. J. Phys. Chem. C 2007, 111, 10814. (13) Kang, J.; Keem, K.; Jeong, D. Y.; Park, M.; Whang, D.; Kim, S. J. Mater. Sci. 2008, 43, 3424. (14) Zhou, J.; Gong, L.; Deng, S. Z.; Chen, J.; She, J. C.; Xu, N. S. Appl. Phys. Lett. 2005, 87, 223108.

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