J. Phys. Chem. C 2008, 112, 15943–15947
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ARTICLES Millimeter-Long and Uniform Silicon Nanocables Ming-Liang Zhang,† Xia Fan,‡ Jian-Sheng Jie,‡ Jyh-Ping Hsu,§ and Ning-Bew Wong*,† Department of Biology and Chemistry and Department of Physics and Materials Science, COSDA, City UniVersity of Hong Kong, Hong Kong, China, and Department of Chemical Engineering, National Taiwan UniVersity, Taipei, Taiwan 10617 ReceiVed: March 11, 2008; ReVised Manuscript ReceiVed: August 06, 2008
An ultralong Si nanocable has been prepared by simple thermal evaporation of SiO powder mixed with a small amount of Sn. The nanocable has a uniform diameter of about 680 nm and millimeters in length. The core of the nanocable is single-crystal Si with an average diameter of 160 nm, and the shell is composed of compact amorphous SiOx with a uniform thickness of 260 nm. The cladding SiOx emits strong light in a wide spectral range from UV to visible. In particular, each nanocable has a droplet head of Si-Sn eutectic and an uncovered Si core at the tail. The nanocables can be directly fabricated into metal-oxide-semiconductor field effect transistors (MOSFETs) via simple thermal deposition of Au electrodes. The performance of the MOSFETs reveals that the Si nanocables are p-type semiconductors with 1.1 × 1017 cm-3 hole concentration and 3.69 cm2 V-1 s-1 hole mobility. 1. Introduction Up to date, fabrication technology for nanomaterials to desirable nanostructures has been well developed for almost all kinds of substances. This work focuses on the application of these nanomaterials. Si nanostructures, such as nanowires, nanoribbons, nanowhiskers, and nanodots, have potential for application in semiconductor industry because of their compatibility with existing semiconductor technology. Silicon nanowires (SiNWs) are particularly attractive and being explored for miniaturization purpose in Si-based integrated circuits.1 A number of nanodevices based on SiNWs has been demonstrated, such as biological sensors,2,3 field-effect transistors (FETs),4,5 and integrated logic circuits.6 Each nanodevice was demonstrated with a single SiNW, or several dispersed SiNWs by means of photolithography, or electron beam lithography. Various growth mechanisms of SiNWs, including the vapor-liquid-solid (VLS),7 solid-liquid-solid,8 supercritical fluid-liquid-solid,9 and oxide-assisted growth (OAG)10,11 have been proposed. Also, SiNWs have been produced by numerous methods including laser ablation,12 thermal evaporation,13-15 chemical vapor deposition,16,17 molecular beam epitaxy,18 chemical etching,19 and solution growth.20 In general, metal particles, such as Au, Co, Fe, Ni, Sn, and Ti are required as catalysts in the VLS growth. The diameters and distributions of the SiNWs can be controlled by the size of metal catalyst particles. On the other hand, the OAG growth is a self-catalytic process and can produce a large amount of SiNWs without metal contamination. It is advantageous to combine these two ap* To whom correspondence should be addressed. Email: bhnbwong@ cityu.edu.hk. † Department of Biology and Chemistry and COSDAF, City University of Hong Kong. ‡ Department of Physics and Materials Science and COSDAF, City University of Hong Kong. § National Taiwan University.
proaches in the growth of SiNWs.21,22 Among many metal catalysts, Au is most commonly utilized in SiNWs synthesis. Nonetheless, Sn is a suitable catalyst to guide the growth of SiNWs or SiOx nanowires since Sn has a low melting point and Si-Sn alloy forms at a relatively low temperature.23-27 2. Experimental Section The Si nanocables were fabricated in a high-temperature tube furnace. The mixture containing 0.015 g of Sn and 2.985 g of SiO in an alumina boat was placed at the center of an alumina tube. After being evacuated to 1.0 × 10-2 Torr and flushed with high-purity Ar gas three times, the source was heated to 1100 at 30 °C/min and kept at this temperature for 8 h under continuous purging of 60 sccm Ar gas. After cooling naturally to room temperature, a light-brown wool-like product was collected at the downstream of the gas flow. The morphology and components of the prepared nanostructures were characterized by X-ray diffraction (XRD, Philips X’Pert), scanning electrom microscopy (SEM, Philips XL30 FEG) equipped with EDS and cathodoluminescence (CL) system (Oxford MonoCL2), transmission electron microscopy (TEM, Philips Technican 12), and high-resolution (HR) TEM (Philips CM200 operating at 200 kV). 3. Results and Discussion 3.1. Morphology and Components. Figure 1a shows the photo images of the as-prepared product. The XRD pattern of this product shown in Figure 1b reveals that it contains crystal Si (cubic) and Sn (body-centered). Figure 1c shows the SEM image of Si nanocables, which are hundreds of micrometers long and uniform in a large scale, and the inset reveals the clear cores and claddings of the cables. The top view of the product (Figure 1d) shows all nanocables with catalyst heads, and the inset clearly shows the dropletlike
10.1021/jp802110y CCC: $40.75 2008 American Chemical Society Published on Web 09/23/2008
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Figure 1. (a) SEM images and (b) XRD pattern of the as-prepared Si nanocables. (c) Large-scale view and high-magnification in inset. (d) Top view of Si nanocables and droplet tips are shown in the inset. (e) Droplet tip in high magnification. The inset shows a nanocable with a broken cladding layer. (f) Special features of the tails. (g and h) Energy-dispersive X-ray spectroscopy (EDX) of the body and tip of the nanocable, respectively.
shape of the heads. Details of the catalyst head are shown in Figure 1e, which reveals tight contact between the head and core and that the core and cladding are in same diameter. The core is linked to the droplet at the oblique side of droplet head, which is in agreement with the VLS growth mechanism. The diameter of the core is about 160 nm. Figure 1h is the EDX of the tip, showing that the head contains Sn, Si, and O. But only Si and O are detected in the body of the nanocable (Figure 1g). The inset of Figure 1e shows the part of Si nanocable with a broken cladding layer, in which the SiOx layer has no particular structural feature. In the previously reported morphology,23-27 the SiOx nanowires were sparse and distinguishable. So this nanocable is distinctively different in structure from the previous Si nanostructures. The possible reason due to different growth processes will be discussed below. Figure 1f and the inset of Figure 1d show the tails or other ends of the nanocables. The as-grown nanocables were dispersed in ethanol with ultrasonic treatment because they were very long and tend to entangle with each other. From the TEM images in Figure 2a, the nanocables are uniform and about 680 nm in diameter. Two bare tails observed in TEM agree with the SEM results. The upper inset shows the head of a Si nanocable in which the SiOx cladding tightly attaches to the catalyst droplet with same diameter. Consequently, as visualized in Figure 2a, during the entire growth process of the Si nanocable, the cladding and core grow synchronously, and the growth direction of the nanocable
Figure 2. (a) TEM images of Si nanocables. The upper inset shows the head and the bottom inset shows a nanocable with a broken SiOx layer; the arrows show the growth direction of the nanocable. (b-d) HRTEM images of Si nanocables, and the insets are FFT patterns.
are indicated with white arrows. The inset at the right bottom of Figure 2a is a nanocable with the cladding partially broken.
Millimeter-Long and Uniform Silicon Nanocables
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Figure 3. The CL images and spectrum. (a-c) SEM images. (a-c-CL) Corresponding CL images. (d) CL spectrum of the nanocables.
The broken shape shows that the SiOx cladding has a compact morphology. To study the growth directions of the crystal Si core of nanocable, HRTEM images are shown in parts b-d of Figure 2. Normally, thick cladding does not give atomic lattice images because it contains mainly amorphous SiOx. Accordingly, all the clear HRTEM images are obtained from the parts of nanocables with broken cladding layer. Parts b-d of Figure 2 show the Si cores with good crystal quality. The insets show the reciprocal lattice peaks obtained from fast Fourier transform (FFT) of the corresponding lattice-resolved HRTEM images. The growth direction of the Si nanocable can be determined by combining the FFT pattern, HRTEM image, and XRD data. From our experiments, three growth directions, [11-1], [100], and [31-1], have been found, and the [11-1] direction is the most dominant one. This result agrees with the previous reports7,26 and the VLS growth mechanism. Because these nanocables are very long, it is possible that some defects, such as dislocations and twists, may be produced during the growing process, which will change the growth direction. Actually, in parts a and b of Figure 1 and parts a and c of Figure 3, some twisted Si nanocables are observed. Thus, the existence of other growth directions is confirmed. 3.2. CL. The emission of Si nanowires, SiOx, and SiO2 has been extensively investigated.24-30 The emission wavelength ranges from 350 to 900 nm depending on the Si:O ratio, morphology, and structural feature. The as-prepared Si nanocables displayed strong CL emission (recorded with a CL
system, OXFORD MonoCL2, in a PHILIPS XL30 FEG SEM). All CL analyses were performed in the SEM chamber with 3.7 × 10-6 Torr at room temperature. The CL spectrum, images, and corresponding SEM images of the as-synthesized Si nanocables are shown in Figure 3. Parts a-CL and b-CL of Figure 3 show the luminescence of the whole nanocable, where the core is less distinguishable in the CL image. A possible reason is that the SiOx cladding layer emits CL signal, while the Si core does not. The CL images including the tips of the nanocable are shown in parts c and c-CL of Figure 3. The dark spots in 3c-CL are the tips of the Sn catalyst because they do not emit. The CL spectrum in Figure 3d is relatively broad, and four peaks at 256, 354, 425, and 475 nm can be differentiated. The observed UV emission below 300 nm from SiOx may originate from the very small SiOx nanoparticles with quantum size effect so that the emission wavelengths are largely blueshifted. Emission at longer wavelength regions can be tentatively assigned with reference to previous works.24-30 The 2.65-eV (470-nm) band is ascribed to the neutral oxygen vacancies (tSisSit), while the 3.0-eV (420-nm) band corresponds to some intrinsic diamagnetic defect centers, such as the 2-fold coordinated silicon lone pair center (OsSisO). Emission from green to red is possibly caused by unsaturated oxygen atomic centers. 3.3. MOSFET Devices. Since the head of nanocable is a eutectic droplet and the single-crystal Si core is exposed at the tail of nanocable, ohmic contact at both sides can readily be formed when they are covered with Au film. The Si core is
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Figure 5. Schematic illustration of the growth process of a nanocable. Both VLS and OAG growth mechanisms are involved. Figure 4. Typical Isd-Vsd curve of the nanocable MOSFET. The insets are schemes of the mask and device.
uniformly wrapped with a compact SiOx layer that serves as a dielectric barrier between the semiconductor and the metal. On the basis of this structural feature, MOSFETs are fabricated with nanocables. The nanocables were dispersed on a glass slide. They could be easily observed with an ordinary microscope; and the location for every individual nanocable was marked. The scheme of a self-prepared mask is shown in Figure 4. Two tungsten wires of 50-µm diameter are aligned closely in parallel, and a close triangle is formed at the other end, which is fixed into a frame. The parallel section of a tungsten wire was in contact with the middle of the nanocable, while the mask was attached to the glass substrate. An Au film of 100 nm thick was coated by thermal evaporation under high vacuum (
