NANO LETTERS
Enhanced Photothermal Effect in Si Nanowires
2003 Vol. 3, No. 4 475-477
N. Wang,* B. D. Yao, Y. F. Chan, and X. Y. Zhang Department of Physics and Institute of Nano Science and Technology, the Hong Kong UniVersity of Science and Technology, Clear Water Bay, Hong Kong, China Received January 13, 2003
ABSTRACT Si nanowires synthesized by the thermal evaporation of a silicon monoxide powder or a mixture of silicon and silica powders burned fiercely in air and exhibited a large photoacoustic effect when exposed to a conventional photographic flash. The energy required to ignite the Si nanowires was 0.1−0.2 J/cm2. Si nanowires showed a strong ability to confine energy from visible light. The remaining material obtained after burning Si nanowires consisted of various forms of nanostructures (e.g. nanoparticles, amorphous wires, and nanotubes).
Nanomaterials have shown unusual and often unexpected properties because of their unique structures and small dimensions. Here, we report that Si nanowires ignited in air and exhibited a large optoacoustic effect when exposed to a conventional photographic flash. Because of the enhanced photothermal effect, the microstructure of the Si nanowires was changed. We believe that Si nanowires have the strong ability to confine photoenergy from visible light. Si nanowires have attracted much attention in recent years for their potential applications in mesoscopic research and electronic nanodevices. Considerable effort has been devoted to synthesizing Si nanowires by employing different techniques such as laser ablation of metal-containing Si targets and oxide-assisted chemical vapor deposition.1,2 All of these techniques required complicated experimental setups and precisely controlled growth conditions. Metallic catalysts employed widely in the growth of Si nanowires were generally prepared by tedious processes. They always contaminated Si nanowires since eutectic alloy drops with a low melting point formed during growth. Our recent studies indicate that semiconductor oxides play a dominant role in the nucleation and growth of semiconductor 1D nanostructures.2,3 Thus, a novel mechanism called oxide-assisted growth has been established and proven by synthesizing various semiconductor nanowires in bulk quantities (e.g., Ge, GaAs, and GaP4-6). For the formation of Si nanowires, previous studies3 showed that temperature, vacuum, and carrier gases were critical parameters. In this work, we demonstrate a simple method of growing bulk-quantity Si nanowires. The experimental procedure is so simple that no special equipment or skill is needed. The synthesis of Si nanowires was carried out by sealing a silicon monoxide powder or a mixture of silicon and silica * Corresponding author. E-mail:
[email protected]. Fax: 852-2351652. Tel: 852-23587489. 10.1021/nl034019m CCC: $25.00 Published on Web 03/06/2003
© 2003 American Chemical Society
(1:1) in an evacuated (vacuum < 10 Torr) quartz tube and then inserting the tube into a preheated furnace (1200-1300 °C) for 20-30 min. The impurity of the source material was unimportant for the formation of Si nanowires in this process. No catalyst or special ambient gas was needed. We put one end of the tube outside the furnace to generate a temperature gradient between the source material and the nanowire formation zone. Spongelike Si nanowire product formed on the cooler parts of the tube where the temperature was about 800-1000 °C. A similar experiment was performed 50 years ago.7,8 Unfortunately, Si nanowires labeled as “light brown loose material” in ref 7 were overlooked at the time. The as-prepared Si nanowire samples were stable in air. No further oxidization was observed on the Si nanowires stored in air for years. A gas flame could not ignite the Si nanowires. We observed that they were slowly oxidized in the flame, forming silica nanowires. However, when exposed to a camera flash at short range (∼3 cm), Si nanowires ignited and fiercely burned in air. (See Figure 1a.) The light power needed for the ignition was 0.1-0.2 J/cm2. The pulse duration was about 5 ms. When the flashing experiment was performed in inert gases, no ignition was observed. We believed that Si nanowires absorbed and confined the energy from the flash, resulting in sufficiently high temperature and hence leading to instantaneously fierce oxidation. The ignition was found to start at certain areas of the sample, and then the fierce burning propagated throughout the entire sample. The remaining materials consisted of various forms of SiO2 nanostructures (e.g., particles, wires, and tubes). When flashing the Si nanowires with low light power, we observed a large audible acoustic wave (i.e., an optoacoustic effect). Large optoacoustic effects resulting from the absorption of the incident light also occurred in Ge and B nanowires. However, TiO2, ZnO, and SiO2 nanowires (all transparent to visible light) did not show obvious opto-
Figure 2. TEM image showing the Si nanowire converted into a silica tube in which the tip of the crystalline Si core (marked by the arrow) was molten in the silica tube during flashing.
Figure 1. (a) Burning of Si nanowires soon after flashing. The inset shows the original sample. (b) TEM images showing the morphology of both the original Si nanowires. (c) Morphology of the remaining materials after flashing the Si nanowires in Ar. Si nanoparticles formed in the nanowires. (d) Silicon oxide nanotubes formed by flashing the Si nanowires in vacuum.
acoustic effects. In addition, a high-powered flashlight was needed to ignite thick Si nanowires. Most Si nanowires prepared in this work consisted of Si single-crystalline cores and thin silicon oxide shells that were the same as those synthesized previously.3 Upon flashing, the ignition might start initially at the ends (or the parts containing very thin silicon oxide shells) of the nanowires. The remaining material obtained after burning the Si nanowires in air consisted of SiO2 nanoparticles and a few SiO2 nanowires and short nanotubes. It was understandable that the high temperature generated from burning the Si cores in air produced SiO2 particles. Since thicker silicon oxide shells protected Si cores from oxidation, some Si nanowires might ignite like blast fuses. For those Si nanowires with thick oxide shells, the Si cores might not be fully oxidized. They should react with the shells as described by the following reaction: Si + SiO2 f SiO (or + SiOx)
(1)
The temperature needed for this reaction should be higher than 1200 °C, and the resulting materials could be Si monoxide or suboxides that have much greater volatility than silica.7 If the resulting Si suboxides are volatilized during burning, then silica nanotubes are formed. Flashing Si nanowires in inert gases (e.g., Ar and He) resulted in a structural transformation from Si nanowires to Si nanoparticles. As illustrated in Figure 1b, Si nanoparticles (identified by electron diffraction) were embedded in the silicon oxide nanowires. It was believed that the Si cores suddenly became a melting state after flashing and subse476
quently segregated into crystalline Si nanoparticles in milliseconds. Therefore, the temperature within the Si cores must be at least ∼1500 °C. (The Si melting point is 1414 °C.) Since no oxygen was present, neither ignition nor oxidation occurred. In the remaining materials, we also observed a few silica nanotubes, which should form in the same way as discussed above. Since the flashing time was only milliseconds, most of the molten Si cores or the newly formed Si monoxide remained in the nanowires. However, when the flashing was done in vacuum (∼10-3 Torr), the molten Si cores or Si monoxide easily evaporated. This may be the reason that the remaining materials, after flashing Si nanowires in vacuum, contained many long silica nanotubes. (See Figure 1d.) Figure 2 is a TEM image of the Si nanowire converted into a silica tube. The crystalline Si core has a round tip, indicating that the temperature reached a maximum at the Si core, and the tip was molten in the silica tube during flashing. This structure is consistent with the formation mechanism of silica tubes discussed above. Recently, a similar photoeffect has been observed in singlewalled carbon nanotubes (SWNTs).9 SWNTs burned away in air after flashing, forming mainly CO2 gas. The associated large photoacoustic effect caused by the absorption of the flashlight on the SWNTs was suggested to be due to the expansion and contraction of the trapped gases in the tubes.9 Si nanowires, however, showed a distinct photoeffect because no gas was trapped in Si or other semiconductor nanowires (e.g., Ge and B (no oxide shell) nanowires). It was believed that Si oxides (e.g., SiO or SiOx) were not responsible for the large photoeffect, though they always formed on the surfaces of Si nanowires. Their optical absorption coefficients are very low for the wavelength of the incident light ranging from 400 nm to 7 µm. Since Si absorbed the energy from the flashlight, the absorption spectrum of Si nanowires was measured and compared with that of pure Si single crystals. No significant difference was found. We believe that the large photoeffect is another unusual phenomenon in Si nanowires. The flashlight emulates sunlight and mainly consists of visible light. The question raised is, why did Si nanowires capture so much energy from light? It is known that the photothermal effect refers to the heating of a material due to the absorption of light. Optical absorption by materials can result in the production of several forms of energy, for example, luminescence, electrical energy, and heat.10 Photothermal heating of a sample Nano Lett., Vol. 3, No. 4, 2003
generally produces a temperature rise, photoacoustic waves, and so forth at the same time. The optical absorption in solid materials can be described by I ) I0 exp(-Rd)
(2)
where I0 is the intensity of the incident light, R is the absorption coefficient of the sample, and d is the sample thickness. The absorption coefficient of crystalline Si is about 0.1-1 µm-1 at a wavelength of light ranging from 500 to 800 nm.11 For Si nanowires with diameters of 10-20 nm, there is no heat spread in the nanowires during flashing because every location of the Si nanowires is irradiated almost equally by the light. The temperature rise ∆T of a Si nanowire can be estimated as ∆T )
AηI0(1 - exp(-Rd)) FCV
(3)
where A is the cross-sectional area of the Si nanowire, η is the efficiency of the conversion of the absorbed energy into heat, F is the density of Si, C is the specific heat of Si, and V is the volume of the Si nanowire. Assuming that the photoenergy absorbed from the incident beam is totally converted into heat, ∆T is about 200 K for a flashing light power of 0.2 J/cm2 and R ) 0.1 µm-1. Obviously, such an increase in temperature is not able to ignite the Si nanowires. A similar conclusion has been obtained from the experiment of carbon nanotube ignition reported by Ajayan et al.,9 where the minimum light power needed for the ignition of SWNTs was 100 mW/cm2. The mechanism of the ignition is unclear. For carbon nanotubes, it has been hypothesized that the ignition arises from photophysical effects associated with a metal catalyst and carbon in chemical contact.12,13 We believed that the optical absorption in Si nanowires was enhanced by their special structure, and the following two factors may play important roles in the photoenergy absorption process: (1) the absorption coefficient of Si nanowires may largely increase compared to that of pure crystalline Si; (2) the Si cores may capture even more energy. As shown in Figure 3 a, ∆T increases to about 2000 K if R ) 10 µm-1. ∆T also increases when the nanowire diameter decreases according to eq 3 (see Figure 3b). This can explain why a higher-power flashlight is needed to ignite thick Si nanowires. However, it was found that Si nanowires with diameters larger that 40 nm could not be easily ignited by the conventional photographic flash. The increase in the absorption coefficient of a material means that the dielectric constant of the material may increase. Because of the quantum confinement effect in Si nanowires, Nishio et al.14 reported that the imaginary part of the dielectric constant of Si in the core of a Si nanowire was larger than that of the rest Si atoms in the nanowires. Their theoretical calculation may provide a hint toward the understanding of the enhanced photothermal effect in Si nanowires. The large photoenergy confinement effect in Si nanowires was unusual. This effect also indicated applications such as smart ignition systems, self-destructing systems for electronic devices, nanosensors, and nanostructural or nanophase reformation. Nano Lett., Vol. 3, No. 4, 2003
Figure 3. (a) Temperature increase ∆T of the Si nanowires (diameter 20 nm) estimated by assuming that the photoenergy absorbed from the flash was totally converted to heat. (b) ∆T increases when the Si nanowire diameter decreases.
Acknowledgment. N. Wang acknowledges the Chow Yei Ching Foundation for financial support. This work was partially supported by the Research Grant Council of Hong Kong (project no. HKUST1043/00P/6151/01P). References (1) Morales, M.; Lieber, C. M. Science (Washington, D.C.) 1998, 279, 208. (2) Wang, N.; Zhang, Y. F.; Tang, Y. H.; Lee, C. S.; Lee, S. T. Appl. Phys. Lett. 1998, 73, 3902. (3) Lee, S. T.; Wang, N.; Zhang, Y. F.; Tang, Y. H. MRS Bull. 1999, 24, 36. (4) Zhang, Y. F.; Tang, Y. H.; Wang, N.; Lee, C. S.; Bello, I.; Lee, S. T. Phys. ReV. B 2000, 61, 4518. (5) Shi, W. S.; Zheng, Y. F.; Wang, N.; Lee, C. S.; Lee, S. T. AdV. Mater. 2001, 13, 591. (6) Shi, W. S.; Zheng, Y. F.; Wang, N.; Lee, C. S.; Lee, S. T. J. Vac. Sci. Technol., B 2001, 19, 1115. (7) Hass, G. J. Am. Ceram. Soc. 1950, 33, 353. (8) Holland, L. Vacuum Deposition of Thin Films; Chapman and Hall Ltd.: London, 1966; p 485. (9) Ajayan, P. M.; Terrones, M.; Guardia, A. de la; Huc, V.; Grobert, N.; Wei, B. Q.; Lezec, H.; Ramanath, G.; Ebbesen, T. W. Science (Washington, D.C.) 2002, 296, 705. (10) Almond, D.; Patel, P. Photothermal Science and Techniques; Chapman and Hall: London, 1996; p 35. (11) Green, M. A.; Keevers, M. J. Prog. PhotoVoltaics 1995, 3, 189. (12) Bockarth, B.; Johson, J. K.; Sholl, D. S.; Howard, B.; Matranga, C.; Shi, W.; Sorescu, D. Science (Washington, D.C.) 2002, 297, 192. (13) Braidy, N.; Botton, G. A.; Adronov, A. Nano Lett. 2002, 2, 1277. (14) Nishio, K.; Koga, J.; Ohtani, H.; Yamaguchi, T.; Yonezawa, F. J. Non-Cryst. Solids 2001, 293-295, 705.
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