Synthesis and Characterization of Silicon Nanowires on Mesophase

Francis S. Galasso,‡ and Joe F. DiCarlo§. Institute of ... obtained SiNWs are homogeneous with average diameters below 50 nm and lengths up to micr...
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J. Phys. Chem. B 2005, 109, 3291-3297

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Synthesis and Characterization of Silicon Nanowires on Mesophase Carbon Microbead Substrates by Chemical Vapor Deposition Wei-Na Li,† Yun-Shuang Ding,† Jikang Yuan,† Sinue Gomez,† Steven L. Suib,*,†,‡ Francis S. Galasso,‡ and Joe F. DiCarlo§ Institute of Materials Science, U-3136, UniVersity of Connecticut, Storrs, Connecticut 06269, Department of Chemistry, U-3060, UniVersity of Connecticut, Storrs, Connecticut 06269, and Yardney Technical Products, Inc., Pawcatuck, Connecticut 06379 ReceiVed: NoVember 5, 2004; In Final Form: December 2, 2004

Silicon nanowires (SiNWs) have been fabricated by chemical vapor deposition at ambient pressure using SiCl4 as a silicon source and mesophase carbon microbead powder as a substrate without any templates and/or metal catalysts. The SiNWs have a crystalline core with a very thin amorphous SiOx sheath. The obtained SiNWs are homogeneous with average diameters below 50 nm and lengths up to micrometers. Temperature and time effects on the growth of SiNWs were systematically studied. Higher reaction temperatures and longer reaction times resulted in larger diameters and higher yields of SiNWs. SiNWs with a better crystallinity can be obtained at higher temperatures and longer reaction times. The obtained SiNWs were characterized by field-emission scanning electron microscopy, X-ray diffraction, Raman spectroscopy, and transmission electron microscopy.

1. Introduction Nanoscale one-dimensional materials, such as nanotubes, nanowires, and nanorods, have stimulated a great deal of research because of their potential for testing and understanding the fundamental concepts about the roles of dimensionality and size on material properties and for optical, electrical, and mechanical applications.1 Semiconductor nanostructures, nanoagglomerates, and nanowires have been widely investigated due to their considerable technological promise for developing devices and circuits on micro- and nanometer scales.2-5 One of the most important semiconductors, silicon, has been grown as nanowires (SiNWs) by various methods.5-14 Besides laser ablation, stress limited oxidation, simple evaporation, and oxideassisted growth (OAG), the most widely used process to fabricate SiNWs is chemical vapor deposition (CVD) based on a vapor-liquid-solid (VLS) growth mechanism with the assistance of templates and/or metal catalysts. Recently Wu et al. reported a template-free and catalyst-free growth of SiNWs with an average diameter of 80 nm under a vacuum of 1 Torr.7 Generally, SiH4 or SiCl4 is used as the silicon source;5-12 the use of SiH4 yields much thinner whiskers than SiCl4.15-17 Researchers found that the average diameter of SiNWs lies in the range of 50-100 nm when SiCl4 is used, while SiNWs as thin as 10 nm in diameter have been obtained with SiH4.15-17 Graphite is the most popular anode material for lithium ion batteries due to its many advantages such as good cycle performances and a high charge and discharge efficiency in the first cycle.18-20 However, the drawbacks, such as a relatively low capacity (theoretical value is 372 mA h/g) and sensitivity to some electrolytes, limit its applications.18-20As an anode material, silicon has a very high theoretical capacity of 4000 * To whom correspondence may be addressed. University of Connecticut, Department of Chemistry, Unit 3060, 55 North Eagleville Road, Storrs, CT 06269. Tel: (860) 486-2797. Fax: (860) 486-2981. E-mail: [email protected]. † Institute of Materials Science, University of Connecticut. ‡ Department of Chemistry, University of Connecticut. § Yardney Technical Products, Inc.

mA h/g. However, the relatively low capacity for the first charge and discharge efficiency in the first cycle and the poor reversibility of silicon retard its widespread use.18-20 Recently studies found that nanodispersed silicon in carbon electrodes exhibit specific capacity near 600 mA h/g and excellent reversibility on multiple cycles,21-23 which are superior to these for pure graphite and silicon. Hence, it is of great interest to grow SiNWs on graphite. To our best knowledge, no reports mentioned the growth of SiNWs with an average diameter smaller than 50 nm without the use of a template and/or a metal catalyst under ambient pressure using SiCl4 as a silicon source with a CVD method. Here we report the fabrication of SiNWs under such conditions on a mesophase carbon microbead (MCMB) substrate, which is a type of pure graphite pretreated at very high temperature (2800 °C). The effects of some factors, such as temperature and reaction time, on the growth of SiNWs have been studied systematically. A preliminary discussion about the growth mechanism is included. 2. Experimental Section 2.1. Synthesis of Silicon Nanowires. The synthesis equipment is depicted in Figure 1. A reactor was fabricated consisting of a gas inlet, a bubbler to produce a silicon vapor source, SiCl4, a furnace containing a quartz tube, and an exhaust into a basic solution trap. An 8.5-cm long quartz holder covered by MCMB powder was placed in the center of the quartz tube, which was located in a horizontal furnace. The mixture of 2.5% SiCl4 carried by H2 in pure H2 was introduced into the furnace, with the substrate temperature varying from 950 °C to 1100 °C. Before SiCl4 was passed through the furnace, a pure H2 flow was introduced in the furnace for 30 min in order to remove oxygen and water in the tube. The MCMB used in this study was MCMB-10-28 (Osaka Gas Co., Japan), which means that the MCMB was partially graphitized at 2800 °C with a median particle size of 6 µm. The morphology and structure of the MCMB were investigated by field-emission scanning electron miscroscopy (FESEM) and X-ray diffraction (XRD) as shown in parts a and b of Figure 2, respectively.

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Figure 1. Synthesis system for SiNWs.

Figure 2. (a) FESEM image and (b) XRD pattern of MCMB powder.

In the first studies, the effect of temperature on the growth of SiNWs was investigated. SiCl4/H2 was passed over the substrate in a 200-mL atmosphere of H2 with a temperature from 950 °C to 1100 °C for 60 min of deposition. The morphology and microstructure of the obtained nanowires on MCMB powder at different temperatures were examined, as shown in Figure 3. Figures 4 and 5 show the XRD patterns and Raman spectra respectively, which demonstrate the structure of the obtained SiNWs. Further information of the as-grown SiNWs was obtained from transmission electron microscopy (TEM) as shown in Figure 6. Figure 7 summarizes the temperature effects on the diameter of the nanowires. Experiments were also conducted on MCMB substrates for different deposition periods with a stable temperature at 1100 °C. Figure 8 shows the morphology of as-grown nanowires formed at different deposition times. Figure 9 shows how reaction time affects the diameter of the nanowires. The effects of time on the structures of the nanowires, shown in Figures 10 and 11, were indicated in the XRD patterns and Raman spectra. 2.2. Characterization. 2.2.1. FESEM. The morphologies of the SiNWs on MCMB substrates were studied using a Zeiss DSM 982 Gemini fieldemission scanning electron microscope with a Schottky emitter. Samples were sonicated and dispersed on a silicon wafer under one day of vacuum before using. 2.2.2. XRD. Structural analysis was conducted using powder XRD. The data were collected on a Scintag XDS 2000 diffractometer with Cu KR radiation. Samples were placed on glass slides after they were cooled to room temperature. 2.2.3. Raman Spectrum. The Raman spectra were obtained on a Reinshaw Ramascope System 2000 using an Ar+ laser (514.5 nm) source.

2.2.4. TEM. High-resolution TEM (HRTEM) studies were carried out using a JEOL 2010 at accelerating voltages of 200 kV with an energy-dispersive system analyzer. The samples were prepared by dispersing the material in 2-propanol. Then a drop of the dispersion was placed on a carbon-coated copper grid and allowed to dry. 3. Results 3.1. Temperature Effects on the Growth of SiNWs. 3.1.1. Morphology. The morphology of MCMB particles was observed by FESEM, which showed that MCMB powder has a spherical shape consisting of many small graphite particles (Figure 2a). A substantial amount of small nonspherical graphite particles are also present as observed in Figure 2a. A typical low-magnification FESEM image of asgrown nanowires, illustrated in Figure 3a, reveals that a high density of nanowires with a length of several micrometers were uniformly formed over the entire substrate at reaction temperatures from 1000 to 1100 °C after a 60-min deposition time. A small proportion of SiNWs was deposited over MCMB powder when the reaction was carried out at 950 °C. The high-magnification FESEM studies demonstrated that the SiNWs generated at each temperature are very homogeneous and have a diameter of less than 50 nm, which is unexpected by other researchers and experiments under a template and metal catalyst-free condition at 1 atm. The diameters and the yields of the nanowires become larger with increasing temperature. A round tip at the end of the nanowires is observed instead of a metal alloy appearing at the top of the nanowires as occurs in the VLS process. Figure 7 clearly indicates how the diameter of the nanowires varies with reaction temperature.

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Figure 3. FESEM images of SiNWs on MCMB powder at (a) 1050 °C at low magnification and (b) 950 °C, (c) 1000 °C, (d) 1050 °C, and (e) 1100 °C at high magnification.

3.1.2. Structure. The XRD pattern of MCMB is shown in Figure 2b. The MCMB is a very well graphitized carbon with a strong (002) diffraction line (JCPDS 41-1487). The d spacing of d(002) equals 0.337 nm, which is the same as perfectly well ordered graphite (d(002) ) 0.338 nm). Figure 4 shows XRD patterns of the nanowires grown on MCMB powder at different temperatures. The peaks present are identified as the diffraction peaks of Si, confirming the presence of Si crystals in the nanowires. The Si peaks become higher when the substrate temperature was increased, which indicates better crystallinity. At 950 °C, no crystalline Si peaks were observed probably due to the poor yields of SiNWs and relatively low crystallinity at this temperature. However, three major Si peaks are apparent in the XRD patterns of the nanowires produced at the other three temperatures, and the graphite background is very strong. We see no reflections due to a carbide or other impurity phases.

Raman is another nondestructive and important tool to characterize SiNWs and a direct probe for quantum confinement effects. A central Raman band at 518 cm-1, as shown in Figure 5, demonstrates that the presence of the crystalline Si in SiNWs is produced in the temperature range of 950 to 1100 °C. At higher temperatures, a stronger intense Raman peak was seen. The peak at 960 cm-1 corresponds to the second-order optical phonon mode of Si.24 TEM gives further information on the structure of the nanowires. Figure 6 corresponds to TEM micrographs of the uniform and straight nanowires generated from a SiCl4/H2 mixture heated to a temperature of 1050 °C at ambient pressure for 1 h. As seen in Figure 6a, TEM analysis of the obtained SiNWs demonstrated the high uniformity of the nanowires on the substrate with a diameter of about 40 nm, which is consistent with the FESEM images. From HRTEM, illustrated in Figure

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Li et al. 20 min. Neither crystalline (518 cm-1) nor amorphous (centered at 480 cm-1) Si peaks were shown in the Raman spectrum of SiNWs produced in a 10-min reaction. A major peak centered at 518 cm-1 with a shoulder at 496 cm-1 was shown in the Raman spectra of SiNWs generated at 20-40 min. After a 60min reaction, the shoulder disappeared in the Raman spectra. The downshift and peak broadening are also observed in Figure 11.

Figure 4. XRD patterns of SiNWs produced on MCMB powder at different temperatures: (a) 950 °C; (b) 1000 °C; (c) 1050 °C; (d) 1100 °C.

Figure 5. Raman spectra of SiNWs produced on MCMB powder at different temperatures: (a) 950 °C; (b) 1000 °C; (c) 1050 °C; (d) 1100 °C.

6b, we could observe a very thin amorphous sheath about 2-3 nm in thickness around the core of crystalline SiNWs. EDAX data demonstrate the presence of Si and O in the nanowires. The inset shows the corresponding selected area diffraction (SAD) pattern characteristic of a face-centered cubic material. 3.2. Reaction Time Effects on the Growth of SiNWs. 3.2.1. Morphology. Figure 8 shows SEM images of samples processed at 1100 °C under atmospheric pressure for different reaction times. A longer reaction time gives a higher yield and a larger diameter for SiNWs. Parts a-d of Figure 8 reveal how reaction time affects the growth of SiNWs on MCMB powder. A wormlike structure was observed after a 10-min reaction, which is very different from those formed for a reaction time longer than 10 min. With a variation of reaction time from 20 to 60 min, the obtained SiNWs grows longer and straighter with the diameter increasing from 26 to 46 nm. The as-grown SiNWs are randomly oriented when deposition temperature and reaction time were varied. Time effects on the diameter of the SiNWs are summarized in Figure 9. 3.2.2. Structure. The XRD patterns obtained from the nanowires formed for different reaction periods are depicted in Figure 10. The XRD peaks are indexed as pure graphite and bulk silicon (c-Si). No other crystalline forms were detected. The intensity of crystalline Si diffraction peaks increases with increasing reaction time. In Figure 10, no crystalline Si peaks were observed when the reaction was carried out for 10 and

4. Discussion 4.1. Temperature Effects on the Growth of the SiNWs. Temperature is a very important factor for diameter control of SiNWs as well as yields. As the preparation temperature increased, the average diameters of the obtained SiNWs grew larger. At 950 °C, the average diameters of the grown SiNWs are 34 nm, and they increased to 46 nm when the substrate temperature increased to 1100 °C. The growth rate of SiNWs from 950 to 1000 °C is 0.02 nm/°C, which is much lower than that of 0.11 nm/°C from 1000 to 1100 °C. Increasing temperature is very helpful to obtain SiNWs with a relative large diameter up to 46 nm at 1100 °C as shown in Figure 7. Temperature affects the crystallinity of the SiNWs grown on MCMB powder, which can be observed from Figures 4 and 5. XRD patterns show that crystalline silicon peaks increased in intensity with the temperature varying from 950 °C to 1100 °C. At 950 °C, no crystalline peaks of silicon were observed in the XRD pattern, which may be due to the strong background of crystalline graphite peaks and a relatively low yield of SiNWs. As the preparation temperature increased, the Si peaks became stronger as shown in Figure 4. The position of each silicon peak is consistent with that of bulk silicon (JCPDS card 27-1402). Unlike XRD, Raman experiments show a sharp peak centered at 518 cm-1 for samples prepared at 950-1100 °C, which indicates the existence of crystalline silicon in the samples as shown in Figure 5. Even at 950 °C, a crystalline silicon peak with a shoulder at 496 cm-1 was observed. The shoulder may be attributed to the presence of a silicon oxide (SiOx) shell, which causes Raman scattering between 400 and 550 cm-1.25,26 No amorphous silicon Raman peak (located at 480 cm-1) is observed in the Raman spectrum of the nanowires. The higher the preparation temperature, the higher the peak intensities, which means that silicon with a better crystallinity was produced at a higher temperature. This confirms the results from XRD data. Compared with the c-Si pattern, the Raman spectra of SiNWs show a downshift of the Si peak and asymmetric broadening, which may be attributable to the small diameter and internal structural defects in nanowires. Many researchers investigated the Raman spectrum of SiNWs and proposed that the Raman peak of SiNWs change as compared to that for c-Si in position, full-with at half maximum, symmetry, and the Raman peak changes with the wavelength of an exciting laser, etc.25-31 Generally, SiNWs give a downshifted, broader, and asymmetric Raman peak compared with c-Si. According to S. T. Lee et al., the position and width of the Raman peak of Si vary with the diameter of the nanowires.27 The nanowires with a diameter larger than 15 nm give a similar peak position and width for bulk crystalline silicon.27 The size confinement effect will not significantly affect the electron and photon properties of crystals unless their size is less than the Bohr radius of silicon which is less than 10 nm at room temperature. Because the average diameter of SiNWs in our samples is larger than 10 nm,27 no significant frequency downshift caused by the size confinement effect was observed.

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Figure 6. (a) TEM low magnification image with a SAD pattern with the beam along the [111h] direction for SiNWs prepared at 1050 °C. (b) HRTEM image of SiNWs grown on MCMB powder.

Figure 7. Temperature effect on the growth of SiNWs.

TEM analysis provides the structure of the SiNWs in detail. The incident electron beam is along the [111h] direction, which is perpendicular to the growth direction of the nanowires. The d spacing of the nanocrystals calculated from the HRTEM image are consistent with d(111) for silicon, thus corroborating the Raman and XRD data, which indicate the existence of silicon nanocrystals growing perpendicular to the [111] direction. On the basis of the electron diffraction (ED), the obtained nanowires grow along the 〈110〉 direction, which means that the (111) planes are parallel to the growth of the SiNWs. Similar results have also been obtained by other methods, and an OAG mechanism developed by Lee et al. is likely.8,13,32-36 The researchers proposed that SiNWs prepared by OAG techniques are primarily oriented in the 〈112〉 and 〈110〉 directions and rarely in the 〈100〉 or 〈111〉 directions, directly based on cross-sectional TEM images. However, the 〈111〉 direction is the preferred growth direction of SiNWs produced with conventional VLS methods. The ED pattern, shown in Figure 6a, indicates that the nanowire is essentially a single crystal. The core of the nanowires is well-crystallized, while an amorphous layer of 2-3 nm covers the surface. The absence of the characteristic Raman band of amorphous silicon (Figure 4) demonstrates that the amorphous phase could be composed of more elements than only silicon. EDAX data (not shown here) reveal that only Si and O are present in the nanowires, which may indicate that the amorphous sheath is composed of SiOx. In our case, the oxygen source could either be O2 existing in the quartz tube or air since SiNWs were oxidized upon exposure to air.

4.2. Reaction Time Effects on the Growth of the SiNWs. Reaction time also plays an important role in the growth of SiNWs. After a 10-min reaction time, a wormlike product was observed instead of a nanowire. The wormlike morphology was also observed by other researchers when the thicknesses of Au catalysts coated on the substrates were changed.32 As the reaction time increased, abundant SiNWs with a length up to micrometers were observed over the whole substrate. At 20-30 min, the diameter of the SiNWs grows from 26 to 33 nm with an increasing rate of 0.7 nm/min, while the increasing rate decreases to 0.2 nm/min from 30 to 40 min. When the reaction time was lengthened to 60 min, the rate increases to 0.55 nm/min as shown in Figure 9. We assume that the wormlike structure rearranged to straight nanowires with a random direction when the deposition periods was lengthened. The crystallinity of asgrown SiNWs increases for longer reaction times as observed in Figures 10 and 11. The three main peaks of c-Si become apparent with a variation of reaction time from 20 to 60 min, which indicates the existence of crystalline Si particles in the nanowires. Raman spectra confirmed the results from XRD. Since the shoulder of 496 cm-1 corresponds to the Raman peak from SiOx,26,27 the amount of silicon oxide sheath covering the surface of Si core decreased with an increasing reaction time. However, almost no crystalline peaks of Si produced from a 20-min reaction were seen in the XRD patterns, which may be due to the strong background of the pure graphite and a relatively poor crystallinity of the obtained nanowires. An apparent Raman peak with a shoulder of the nanowires obtained at 20 min was shown in Raman spectra, which demonstrates that the crystalline Si exists in the nanowires. Increasing deposition temperatures and reaction times causes the growth of SiNWs with better crystallinity and larger diameters, which gives a guide in controlling the growth of the SiNWs. The possible mechanism of the production of SiNWs is briefly discussed below

SiCl4 + 2H2 f Si + 4HCl

(1)

Si + x/2 O2 f SiOx

(2)

SiOx + (x - 1) C f SiO + (x - 1) CO

(3)

2SiO f Si + SiO2

(4)

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Figure 8. FESEM images of SiNWs on MCMB powder with a stable temperature of 1100 °C at different reaction times: (a) 10 min; (b) 20 min; (c) 30 min; (d) 40 min.

Figure 9. Time effect on the growth of SiNWs.

Crystalline silicon vapor, first obtained from decomposition of SiCl4, reacted with O2 left in the reactor to produce silicon oxides. The formed silicon oxides were reduced to silicon monoxide by carbon according to carbothermal reduction.12 Crystalline silicon, formed in eq 4, nucleates and grows perpendicular to the (111) plane to form the nanowires. Here O2 may come from either the physical adsorption of O2 on MCMB powder or the O2 or H2O left in the reactor and around the wall. Similar reactions have been proposed for the OAG synthesis of SiNWs, although the monoxide type species is generated by other means.13,14,33,37,38 In the OAG technique, oxide plays an important role in inducing the nucleation and growth of nanowires with a preferential growth direction, uniform size, and long length instead of metals in the conventional metal-catalytic VLS process. A silicon suboxide cluster is first deposited on the substrate, and some of its highly reactive

Figure 10. XRD patterns of SiNWs produced on MCMB powder at different reaction times: (a) 10 min; (b) 20 min; (c) 30 min; (d) 40 min; (e) 60 min.

Si atoms are strongly bound to the substrate (silicon) atoms limiting the cluster motion on the substrate. Then the nonactive Si atoms in the same cluster are exposed to vapor with their available dangling bonds directed outward from the surface. They act as nuclei to absorb additional reactive silicon oxide clusters and facilitate the formation of SiNWs with a certain orientation. The OAG mechanism also applies to our synthesis. The major difference is that Si suboxide is transformed as a result of conversion of SiCl4 to SiOx. MCMB powder here acts as both a reducing agent and a substrate. Moreover, the fact that no nanowires were formed below 950 °C is also consistent with SiO disproportionation as Lee et al. already reported that no SiNWs were formed below 900 °C.

Silicon Nanowires on MCMB Substrates

Figure 11. Raman spectra of SiNWs produced on MCMB powder at different reaction times: (a) 10 min; (b) 20 min; (c) 30 min; (d) 40 min; (e) 60 min.

5. Conclusions SiNWs were homogeneously formed on a MCMB substrate using a mixture of 2.5% SiCl4 in H2 under atmospheric pressure without any templates or metal catalysts. The obtained SiNWs have an average diameter less than 50 nm, which is unique for a metal catalyst-free and template-free method to prepare SiNWs using SiCl4 as the silicon source at 1 atm. Temperature and reaction time can affect the growth of the SiNWs. When the deposition temperature was increased from 950 to 1000 °C, the average diameters of the SiNWs grew from 34 to 35 nm with an average increasing rate of 0.02 nm/°C, while the growth speed increased to 0.11 nm/°C from 1000 to 1100 °C. With an increase in reaction time from 20 to 60 min with a stable temperature of 1100 °C, the average diameter of the obtained SiNWs grows from 26 to 46 nm, while the morphology of the as-grown materials is wormlike instead of nanowires at the first 10-min deposition. SiNWs with a better crystallinity can be obtained at higher temperatures and longer reaction times. XRD, Raman, and TEM studies show that the obtained SiNWs have a crystalline silicon core with a very thin oxide sheath (for example, 2-3 nm SiOx in thickness at 1050 °C for 1 h deposition) on the surface. The OAG mechanism plays an important role in the growth of SiNWs. The electrochemical performances of the potential anode materials for lithium ion batteries consisting of SiNWs and MCMB are under investigation. Acknowledgment. We would like to thank the Army under contract number DAAD 17-01-C-0044 for financial support of this work. We acknowledge the support from Yardney Technical Products. Thanks to Dr. Bill Willis for XPS tests, Santo Iaconetti for electrochemistry tests, and Dr. Lichun Zhang, Laura Espinal, Chen Li, Jun Nable, and Xiongfei Shen for helpful discussions. References and Notes (1) Lieber, C. M. Solid State Commun. 1998, 107, 607.

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