Article pubs.acs.org/JPCC
Optical Properties of Silicon and Germanium Nanowire Fabric Vincent C. Holmberg, Timothy D. Bogart, Aaron M. Chockla, Colin M. Hessel, and Brian A. Korgel* Department of Chemical Engineering, Texas Materials Institute, Center for Nano and Molecular Science and Technology, The University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *
ABSTRACT: The optical properties of free-standing nonwoven silicon (Si) and germanium (Ge) nanowire fabrics, including diffuse and specular reflectance, diffuse and direct transmittance, and absorptance spectra, were measured using an integrating sphere to account fully for all incident photons. Very thin, 50 μm thick, sheets with ∼90% void volume have extremely high optical densities. They are optically opaque across nearly all wavelengths from ultraviolet to near-infrared, and only minimal light penetrates when photon energies are below the band gap. The high optical density arises from a combination of scattering and absorption due to their high aspect ratio and narrow diameter as well as surface effects due to their very high surface area to volume ratio.
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growth.3,15,16,20−28 Here we report the optical properties of Si and Ge nanowire fabric composed of SFLS-grown nanowires. Transmittance, absorptance, and reflectance spectra were measured using an integrating sphere to account fully for all incident photons. The nanowire ensembles are extremely opaque due to multiple light scattering and light-trapping effects.29 The resulting optical properties have strongly enhanced broadband absorption and very high optical densities compared with bulk Si and Ge.
INTRODUCTION Semiconductor nanowires possess a unique combination of electronic, optical, and mechanical properties and are being studied for a wide range of applications, ranging from photovoltaic devices to printed high mobility field effect transistor materials to structural reinforcement additives in polymer composites.1 Nanowires are crystalline solids yet are also mechanically flexible and extremely strong due to their nanoscale dimensions.2,3 They have electrical conductivity that can be modulated with applied electric field and optical excitation like typical semiconductors but also act as nanoscale optical cavities that propagate light over long distances like waveguides.4−14 One way to use semiconductor nanowires might be as a nonwoven fabric or paper recently made from silicon (Si) and germanium (Ge) nanowires.3,15,16 These nanowire meshes do not need an underlying mechanical support (or added binder in battery applications)16 and offer the combination of electrical and optical properties characteristic of crystalline semiconductors with flexible mechanical properties and light weight. Carbon nanotubes have also been fashioned into self-supported films, sheets, and paper17−19 but have a relatively limited range of properties because they are electrically conductive and black in color,17 although they can be made transparent by making them thin enough.18,19 Semiconductor nanowires can have a wide range of band gap energies by varying composition and size, making them suitable for optoelectronic applications requiring light harvesting and conversion. To make fabric, nanowires must be relatively thin with diameters less than about 100 nm and aspect ratios (i.e., the length/width ratio) higher than ∼1000. The small diameter imparts mechanical flexibility, and the high aspect ratio ensures tangling of the nanowires for mechanical stability.3,15,16 Nanowires with the appropriate dimensions can be made in large enough amounts using either supercritical fluid−liquid− solid (SFLS) or solution−liquid−solid (SLS) © 2012 American Chemical Society
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EXPERIMENTAL DETAILS Materials. Anhydrous toluene (99.8%) was purchased from Sigma-Aldrich. Diphenylgermane (DPG; >95%) and monophenylsilane (MPS; >95%) were purchased from Gelest. Gold (Au) nanocrystals with 2 nm diameters capped with 1dodecanethiol (Aldrich; ≥98%) were prepared using the method of Brust et al.30 Hydrogen tetrachloroaurate(III) trihydrate (≥99.9%), tetraoctylammonium bromide (98%), and sodium borohydride (≥98%) were purchased from Aldrich. Anhydrous ethanol was purchased from Pharmco-AAPER, and hydrofluoric acid (48%) was provided by EMD Chemicals. Hydrochloric acid (37.2%) and nitric acid (69.3%) were purchased from Fisher Scientific. Trisilane (TS; 100%) was obtained from Voltaix. (Caution: Trisilane is pyrophoric and must be stored and handled under an inert atmosphere!) Ge Nanowire Synthesis. Ge nanowires were synthesized from DPG using a Au nanocrystal-seeded, SFLS growth process.21−25 A 30 mL solution of anhydrous toluene containing 15 mg/L Au nanocrystals and 35 mM DPG was injected into a toluene-filled 10 mL titanium reactor maintained at 380 °C and 6.5 MPa at a rate of 0.5 mL/min for 40 min. After synthesis, a 33 vol % solution of 1-dodecene in anhydrous toluene was added to the reactor at 220 °C and held for 2 h to Received: August 18, 2012 Revised: September 28, 2012 Published: October 1, 2012 22486
dx.doi.org/10.1021/jp308231w | J. Phys. Chem. C 2012, 116, 22486−22491
The Journal of Physical Chemistry C
Article
sonicating, placing the dispersion in a 2.5 × 2.5 cm Teflon trough, and allowing the solvent to evaporate.15,16 A mat of nanowires forms on the surface of the trough as the solvent evaporates. After the nanowires dry, a free-standing sheet of nanowire fabric is peeled from the Teflon substrate. About 200−300 mg of nanowires makes enough semiconductor fabric to cover 100 cm2. Optical Measurements. Optical properties of nanowire fabric were measured using a Labsphere DRA-CA-5500 integrating sphere attached to a Cary 500 UV−vis−NIR spectrophotometer. The nanowire fabric was immobilized in a semirigid frame made from card stock, aluminum foil, and double-sided tape. Frames were formed by cutting 3.0 × 3.4 cm rectangles out of card stock, which were covered on one side by double-sided tape. The adhesive-covered side of the card stock was then adhered to the polished side of a sheet of aluminum foil, leaving plenty of excess aluminum foil around the perimeter of the frame. A 1.4 × 2.4 cm window was cut in the center of the frame. Excess aluminum foil was cut and folded over the top side of the frame to ensure that all frame surfaces were covered in highly reflective aluminum foil. The aluminum foil layer decreases the likelihood that secondary reflections within the integrating sphere will be absorbed by the frame. The folded aluminum flaps were opened, and small pieces of double-sided tape were placed on opposing sides of the window edges. The frames were then lowered (adhesive side down) and adhered to precut 2.0 × 2.5 cm sheets of nanowire fabric, and the aluminum foil flaps were folded back down to hold the entire structure in place. Total optical transmittance (diffuse + direct) measurements were made by placing the nanowire fabric sample over the entrance port of the integrating sphere and moving the focal point of the incident beam to the front surface of the sphere. This measurement detects all of the light transmitted through the nanowire fabric, as illustrated schematically in Figure 1. The directly transmitted light was measured from a normal transmittance measurement without the integrating sphere to determine the amount of diffusely transmitted light.
passivate the nanowire surface.23,24 Crystalline Ge nanowires were produced with average diameter of 45 ± 15 nm. Si Nanowire Synthesis. Si nanowires were synthesized using either MPS or TS. Nanowires were grown with MPS using a reactant solution of 30 mL anhydrous toluene containing 40 mg/L Au nanocrystals and 135 mM MPS prepared in a nitrogen-filled glovebox. The reactant solution was injected into a toluene-filled 10 mL titanium tubular reactor maintained at 490 °C and 10.3 MPa at a rate of 0.5 mL/ min for 40 min.25−27 Crystalline Si nanowires were produced with average diameter of 25 ± 10 nm and a coating of amorphous polyphenylsilane that is 15 ± 10 nm thick.16 Si nanowires grown with TS used a reactant solution prepared by combining 0.3 mL of anhydrous toluene with 0.55 mL of a gold stock solution (50 mg/mL) and 0.25 mL of TS (1.8 M). Nanowire growth was carried out in a reactor in a nitrogen-filled glovebox.28 With a closed outlet, the precursor solution was injected into a toluene-filled 10 mL titanium reactor heated to 450 °C and pressurized to 6.9 MPa at a rate of 3.0 mL/min for 1 min.28 Over the course of the injection, the reactor pressure built from 6.9 to 15.2 MPa. In a single reaction, roughly 100 mg of crystalline Si nanowires with an average diameter of 50 ± 20 nm were produced. Polyphenylsilane Shell Etching. MPS-grown Si nanowires were dispersed in chloroform at a concentration of 0.3 mg/mL. Aqua regia (3:1 volume ratio of HCl/HNO3) was added to make a 3:2 chloroform to aqua regia volume ratio. This mixture was stirred as an emulsion for 3.5 h to oxidize the shell. The aqueous phase was discarded. Residual aqua regia was removed from the chloroform phase by repeated solvent extraction with deionized water. Nanowires were pelleted by centrifugation at 8000 rpm for 5 min and washed three times by repeated dispersion in chloroform, centrifugation, and decanting of the supernatant. After purification, nanowires were dispersed in chloroform at a concentration of 0.3 mg/mL and divided into 40 mL batches. Twenty mL of a 1:1:1 volume ratio HF/H2O/EtOH solution was added to each 40 mL batch and stirred into an emulsion for 15 min to etch the oxidized shell. After stirring, the aqueous layer was removed, and the nanowires were cleaned and dispersed in chloroform following the centrifugation procedure described above. Au Removal from Trisilane-Grown Nanowires. TSgrown Si nanowires were dispersed in chloroform at a concentration of 0.5 mg/mL and divided into 40 mL batches. Twenty mL of a 1:1:1 volume ratio HF/H2O/EtOH solution was added to each batch and stirred into an emulsion for 30 min to remove surface oxide and ensure that the Au nanocrystals on the surface were exposed to the solution. After stirring, the aqueous layer was discarded, and the nanowires were purified using the centrifugation procedure described for the shell-etched nanowires. After purification, the nanowires were dispersed in chloroform, and aqua regia (3:1 volume ratio of HCl/HNO3) was added to form a mixture with a 3:2 chloroform to aqua regia volume ratio. This mixture was stirred into an emulsion for 1.5 h to etch the Au. After stirring, the chloroform layer was removed, and the remaining aqueous layer was centrifuged at 8000 rpm for 5 min to pellet the nanowires. The nanowires were washed by dispersion and centrifugation in deionized water, followed by washing two more times in ethanol. Nanowire Fabric Formation. Sheets of Ge and Si nanowires were prepared by dispersing nanowires in toluene (Au-free TS-grown Si nanowires were dispersed in ethanol),
Figure 1. Schematic diagram of three integrating sphere configurations used to measure the total transmittance (left), absorptance (center), and reflectance (right) of nanowire fabric.
Total reflectance measurements were made by placing the nanowire fabric in a reflectance port on the back side of the integrating sphere and moving the focal point of the incident beam to the back surface of the integrating sphere, making sure the beam spot was entirely encompassed by the sample surface. This measurement detects all of the diffusely and specularly reflected light, as illustrated in Figure 1. The amount of diffusely reflected light was determined by inserting a light trap into the integrating sphere to trap the specularly reflected beam. Absorptance measurements were made on nanowire fabric suspended in the middle of the integrating sphere with the incident beam focused in the center of the sphere, such that the beam was encompassed entirely by the suspended sample. As 22487
dx.doi.org/10.1021/jp308231w | J. Phys. Chem. C 2012, 116, 22486−22491
The Journal of Physical Chemistry C
Article
illustrated in Figure 1, this configuration detects all of the transmitted and reflected light (known as a transflectance measurement). The remaining incident light is absorbed by the sample. These five measurements allow a full accounting of photons incident on the sample (diffuse reflectance + specular reflectance + diffuse transmittance + direct transmittance + absorptance = 1). For each measurement configuration, it was necessary to move the focal point of the incident beam, and for each of the five measurements, a new zero and baseline were collected to account for changes in the measurement configuration.
Figure 3. UV−vis-NIR transmittance spectra of a 725 μm thick Si wafer compared with a 50 μm thick fabric of (Au-removed) TS-grown Si nanowires. A photograph of the Si nanowire fabric (with yellow color) is shown in the inset.
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RESULTS AND DISCUSSION Figure 2 shows scanning electron microscopy (SEM) images of Si and Ge nanowire fabric. Figure 3 shows a comparison of the
has ∼90% void volume, it is opaque across nearly the entire spectral range from 250 to 2500 nm. MPS-Grown Si Nanowire Fabric. Si nanowire fabric has a pale-yellow color, as shown in Figure 4, and is completely
Figure 4. (a) Interactions of an incident light beam with nanowire fabric: direct transmission (blue), diffuse transmission (light blue), specular reflection (orange), diffuse reflection (pink), and absorption (black). (b−e) Optical characterization of a 50 μm thick sheet of freestanding MPS-grown Si nanowire fabric: (b) with polyphenylsilane shell and (c) after etching away the shell. (d) Absorptance spectra of MPS-grown Si nanowire fabric with (black) and without (tan) the polyphenylsilane shell. (e) Photographs of framed sheets of MPSgrown Si nanowire fabric (left) with and (right) without the shell.
Figure 2. SEM images of (a) MPS-grown Si and (b) Ge nanowire fabric. Images were acquired using a Zeiss Supra 40 SEM operated at 5 keV.
transmittance spectra of a thin sheet of TS-grown Si nanowire fabric to a much thicker Si wafer. Both the wafer and the fabric absorb most of the light with energy greater than the Si band gap (1.1 eV), but the fabric allows
