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Control of Thickness and Orientation of Solution-Grown Silicon Nanowires

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Science  25 Feb 2000:
Vol. 287, Issue 5457, pp. 1471-1473
DOI: 10.1126/science.287.5457.1471

Abstract

Bulk quantities of defect-free silicon (Si) nanowires with nearly uniform diameters ranging from 40 to 50 angstroms were grown to a length of several micrometers with a supercritical fluid solution-phase approach. Alkanethiol-coated gold nanocrystals (25 angstroms in diameter) were used as uniform seeds to direct one-dimensional Si crystallization in a solvent heated and pressurized above its critical point. The orientation of the Si nanowires produced with this method could be controlled with reaction pressure. Visible photoluminescence due to quantum confinement effects was observed, as were discrete optical transitions in the ultraviolet-visible absorbance spectra.

One-dimensional quantum wires are expected to play a vital role as both interconnects and functional components in future mesoscopic electronic and optical devices and also to provide an opportunity to test fundamental quantum mechanical concepts (1,2). As the wire diameter approaches the carrier de Broglie wavelength, quantum confinement effects shift band gap energies and, in Si, induce visible photoluminescence (3). The electronic and optical properties of the nanowires strongly depend on size; therefore, size control and tunability are key to the success of any method of synthesizing quantum wires. Dimensionality also affects the material properties of nanowires. The absence of translational symmetry in Si could profoundly affect the electronic properties: Bulk Si is an indirect semiconductor with a band gap of 1.1 eV, whereas linear polysilane chains exhibit a 3.89-eV direct gap (4). The lattice orientation in a wire can provide a tuning parameter, unavailable in quantum dots, to adjust material properties to suit particular applications. In carbon nanotubes, for example, the bonding geometry and orientation profoundly affect the electronic structure and can lead to either metallic or insulating behavior (5). Calculations for Si nanowires have shown that bond orientational order will influence their physical properties as well (6). These effects, however, have not been experimentally observed in Si nanowires, largely because of inability to obtain highly crystalline samples with narrow wire diameter distributions and tunable crystallographic orientation.

The high-aspect-ratio quantum wires (those with a high ratio of length to diameter) must have diameters less than 10 nm. Self-assembly strategies are best suited for directing materials synthesis on this mesoscopic length scale. Many successful strategies have been developed for semiconductor quantum dots (7). These methods rely on the ability to isolate nanometer-scale reaction compartments, control crystallization kinetics, provide a growth template, or some combination of these. Strategies have also been developed for semiconductor nanowire self-assembly in solution and in the gas phase; however, greater control over the wire diameter distributions and crystallinity is desired (1, 2, 8). Certainly, the production of single-walled carbon nanotubes in the gas phase demonstrates that self-assembly methods can yield very high-quality one-dimensional (1D) materials (2). Because solution-phase synthetic methods can enable great chemical flexibility and synthetic tunability, as has been demonstrated for quantum dots (7), it is desirable to extend the highly controlled solution-phase self-assembly methods to semiconductor nanowires. The work presented here demonstrates the ability to self-assemble Si nanowires in solution with a very high degree of control, producing wires with highly crystalline cores, narrow diameter size distributions, aspect ratios greater than 1000, and tunable crystallographic orientation.

We used solvent-dispersed, size-monodisperse, alkanethiol-capped gold (Au) nanocrystals to direct Si nanowire growth with narrow wire diameter distributions. Sterically stabilized Au nanocrystals were dispersed in supercritical hexane with a silicon precursor, diphenylsilane, at temperatures of 500°C and 270 bar (or 200 bar in some cases). At these temperatures, the diphenylsilane decomposes to Si atoms. The phase diagram for Si and Au indicates that at temperatures above 363°C, Si and Au form an alloy in equilibrium with pure solid Si when the Si concentration with respect to Au is greater than 18.6% (2). Under the reaction conditions used, the Si atoms most likely dissolve into the sterically stabilized Au nanocrystals until reaching supersaturation, at which point they are expelled from the particle as a thin nanometer-scale wire. Figure 1 shows a schematic of the nanocrystal-directed nanowire self-assembly process. This fluid medium provides the high temperatures necessary to promote Si crystallization. More than 30 years ago, this phase behavior was exploited to grow micrometer-diameter Si wires on a substrate by chemical vapor deposition (9). Au droplets placed on a Si substrate directed Si “whisker” formation in the vertical direction by a process called vapor-liquid-solid (VLS) growth (10). Recently, VLS growth was applied to the gas phase to form nanometer-scale Si wires (11). Gas-phase liquid Au droplets were produced by laser ablation in the presence of silane at high temperatures. Si wires spontaneously formed. Although this method produces relatively large quantities of nanometer-scale Si wires, the liquid droplets naturally have a broad size distribution, and therefore, the nanowires also have relatively broad size distributions (10). In the supercritical (sc) fluid environment, relatively size-monodisperse Au nanocrystals can be maintained to seed nanowire growth at the high temperatures necessary to achieve wire growth.

Figure 1

Schematic of the proposed nanowire growth process. Thermal degradation of diphenylsilane results in free Si atoms that dissolve in the Au nanocrystal until reaching a Si:Au alloy supersaturation, when Si is expelled from the nanocrystal as a crystalline nanowire. This wire is depicted with a preferred <111> orientation.

Figure 2 shows typical transmission electron microscopy (TEM) images of the wires formed with this process. The 40 to 50 Å diameter nanowires are 1D “single crystals” with extremely narrow diameter size distributions (less than ±10% SD about the mean wire diameter) and aspect ratios greater than 1000. These nanowires photoluminesce in the blue because of quantum confinement effects (Fig. 3), and the optical properties were found to strongly depend on the wire crystallographic orientation measured by TEM and electron diffraction: <100> oriented wires exhibit higher exciton energies than the <110> oriented wires with a diameter of 40 to 50 Å.

Figure 2

TEM images of Si nanowires synthesized at 500°C in hexane at pressures of 200 bar (A andB) and 270 bar (C and D). The nanowires are highly crystalline. In both (B) and (D), the lattice fringes are the Si (111) planes, separated by 3.14 Å. The nanowire samples were prepared for imaging by dispersing them in chloroform and evaporating a drop onto a carbon-coated copper TEM grid. A JEOL 2010 transmission electron microscope with 1.7 Å point-to-point resolution operating with a 200-kV accelerating voltage with a GATAN digital photography system was used for TEM. In situ EDS (Oxford Instruments) confirmed that the samples were high in Si content with residual C and O.

Figure 3

Room-temperature optical properties of the Si nanowires imaged in Fig. 2. (A) Absorbance spectra (from a Varian Cary 500 UV-Vis-NIR spectrophotometer) for a dilute nanowire suspension in hexane. The spectra labeled <100> are for nanowires formed at 200 bar (Fig. 2, A and B) and <110> at 270 bar (Fig. 2, C and D). (B) Photoluminescence spectra (from a SPEX Fluorolog-3 spectrophotometer) of nanowires dispersed in hexane with excitation energies of 4.46 eV (277 nm, <100>) and 4.12 eV (300 nm, <110>).

The Si nanowires were prepared by thermally degrading diphenylsilane in sc-hexane [supercritical temperature and pressure (T c = 235°C;P c = 30 bar)] at 500°C and 200 or 270 bar in an inconnell high-pressure cell (12). The nanowire synthesis relies on dodecanethiol-capped Au nanocrystals, ranging between 20 and 30 Å in diameter, formed using standard arrested precipitation procedures to direct wire formation (13). A light brown precipitate consisting of Si nanowires formed during the course of the reaction (TEM images in Fig. 2). No color change was observed in the absence of diphenylsilane. In the absence of Au nanocrystals, the solution quickly turned from orange to brown and then to clear as polydisperse black micrometer-sized Si particles formed and settled on the walls of the reaction vessel. Chloroform was used to extract the nanowires from the cell upon cooling and depressurization. The sc solvent provides a high diffusion coefficient, on the order of 10−3 to 10−4 cm2 s−1(14), which is necessary for rapid reactant diffusion to help avoid Si nanocrystal growth in the bulk solvent.

The Si nanowires were characterized with a variety of techniques. Energy dispersive x-ray spectroscopy (EDS) and x-ray photoelectron spectroscopy (XPS) revealed that the nanowires consisted of a high concentration of Si, with residual amounts of C, O, and Au (15). As shown in Fig. 2, TEM revealed Si (111) lattice planes with 3.14 Å spacing in all of the samples, confirming that the nanowire cores did indeed consist of pure crystalline Si. The wires exhibited diameters approximately 10 Å larger than the nanocrystal diameter, and the thickness appeared to fluctuate along the length of the wire by ±1 lattice plane; however, these variations may result from the resolution limitations of the microscope itself. The wires formed at 200 bar exhibited a preponderance of <100> oriented wires, whereas the samples synthesized at 270 bar contained wires oriented almost exclusively along the <110> direction. Pressure appears to be the determining factor in tuning the lattice orientation in the wire: At higher pressures, the brown color appeared sooner than at lower pressures, indicating that diphenylsilane degrades faster at higher pressures, and therefore it appears that the nanowire growth rate affects the nanowire orientation. A semitransparent coating of oxide or hydrocarbon on the nanowires was apparent in the TEM images of all of the wires (15). It is unclear whether this coating forms after removal from the reaction vessel or if it forms on the wire surface during nanowire growth. However, the thickness of this coating increased over time if the wires were exposed to air.

The absorption edge of the Si nanowires was strongly blue-shifted from the bulk indirect band gap of 1.1 eV and showed sharp discrete absorbance features and relatively strong “band edge” photoluminescence (PL). These optical properties likely result from quantum confinement effects, although we cannot rule out the possibility of additional surface states as well (2,16). The <110> oriented nanowires exhibited distinctly molecular-type transitions, whereas the <100> oriented wires exhibited a strong feature reminiscent of the L→L critical point in the Si band structure with a slowly rising phonon-assisted optical transition. The <100> oriented wires exhibited a significantly higher exciton energy than the <110> oriented wires, as predicted by Yorikawa and co-workers (6).

Thus, the tunability of the lattice orientation in the silicon nanowires leads to different optical properties that could be exploited in applications. The <100> nanowires exhibited a single sharp PL feature at 3.75 eV with a weak shoulder shifted to lower energy at 1.9 eV. The weak low-energy peak likely results from the surrounding oxide (16). The <110> oriented wires exhibited three distinct peaks at 3.35, 2.9, and 2.55 eV. The relatively robust PL exhibited initially after synthesis became completely quenched over the course of about 1 week, but the PL energy did not appear to shift to lower energy upon exposure to air. Certainly, the lattice orientation is expected to affect electron transport in future mesoscopic interconnects employing Si and must be considered in the design and manufacture of such devices.

  • * To whom correspondence should be addressed. E-mail: korgel{at}mail.che.utexas.edu

  • Present address: Department of Chemistry, University College Cork, Ireland.

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