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2005, 109, 3731-3733 Published on Web 02/12/2005
Silicon Nanoclusters Selectively Generated by Laser Ablation in Supercritical Fluid Ken-ichi Saitow* Material Science Center, Natural Science Center for Basic Research and DeVelopment (N-BARD), Hiroshima UniVersity, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan ReceiVed: December 17, 2004; In Final Form: January 28, 2005
A novel method for generating nanomaterials is developed by performing laser ablation of silicon crystals in supercritical fluid. The method is shown to successfully generate silicon nanoclusters, and to allow for the selective generation of clusters having different electronic structures. This selective fabrication enables us to obtain the nanoclusters via a dry process, by changing only the fluid density and/or temperature when ablation is performed. The experimental procedures are not highly specialized, and a variety of researchers unfamiliar with wet chemical processes are able to obtain nanoclusters with different properties in only a few minutes. This new method can also be used to selectively obtain other semiconductor or metal nanomaterials with different properties.
Decreasing the size of condensed matters to nanoscale or fabricating nanostructures often generates novel functionalities that have not yet appeared in the bulk.1 Silicon is a typical example: its optical property in the nanometer region is considerably changed by changing its size and structure.2-6 Some silicon nanomaterials show photoluminescence ranging from the ultraviolet to visible regions, despite the negligible luminescence of bulk silicon in the infrared red region. For example, it has been shown that silicon nanoclusters having chain, branch, ladder, and cubic shapes emit ultraviolet, blue, green, and red light, respectively.4,7 Despite these differences in photoluminescence, each of these clusters consists of 8 silicon atoms. In addition, remarkable advances have been made in the use of silicon in electrodevices over the last forty years. Thus the possibility of controlling the optical properties of silicon offers a great opportunity; i.e., light-emitting nanodevices of silicon on the silicon IC-tip having electoro-nanodevies could lead to advances in telecommunications and/or information storage.8-10 In the present study we introduce a novel method for selective fabrication of nanomaterials using supercritical fluid and pulsed laser ablation. The supercritical fluid used resembles a compressed gas in high density, and its thermodynamic state exceeds the gas-liquid critical point. Supercritical fluids have been utilized for green chemistry, e.g., for extraction, organic synthesis, decomposition of endocrine disrupting chemicals, and cleaning of electrodevices.11-13 These functionalities are the result of the following intrinsic properties: (i) the density is changed continuously over a wide range under the absence of the phase transition from gas to liquid; (ii) solute molecules in the supercritical fluid are often adsorbed by the fluid molecules; (iii) the degree of adsorption is controlled by the fluid pressure and temperature. With regard to pulsed laser ablation,14 when the bulk matter is irradiated by an intense pulse laser, the irradiated surface is significantly heated by thermal dissipations * Corresponding author. E-mail:
[email protected].
10.1021/jp0442551 CCC: $30.25
via relaxation processes of excited atoms and/or molecules. The ablation is accomplished as a result of the ejection of very hot (e.g., 1500 K) atoms and/or molecules15 from the heated and expanded surface. These ejected products can generate various advanced materials, e.g., thin films, clusters, and nanoparticles of metal or semiconductor. The pulsed laser ablation enables us to obtain these materials from a substance that has a high melting point. The ablation has been also applied to solid silicon.6,15,16 We here report the first attempt to combine supercritical fluid with pulsed laser ablation for the fabrication of nanomaterials. That is, laser ablation of silicon crystal was carried out in supercritical CO2, as shown in Figure 1. As a result, silicon nanoclusters were generated, and clusters having different electronic structures could be selectively generated within a few minutes. This method allowed us to obtain the nanoclusters via a dry process, by changing only the fluid density and/or temperature when ablation is performed. The system used in the present study was developed in our laboratory and consists of a high-pressure cell, an absorption spectrometer, and a Q-switched frequency-doubled Nd:YAG laser. The high-pressure cell was made of stainless steel (SUS316) and was designed in our laboratory. The pressure was adjusted with an HPLC pump, and the temperature was controlled by a set of heaters, a PID controller, and a thermocouple. Fluctuations of pressure and temperature were suppressed to within (0.1% during measurement. This stability was sufficient for the supercritical fluid, whose structure is very sensitive to changes of temperature and pressure in the vicinity of the critical point. The critical temperature, pressure, and density of CO2 are reported to be Tc ) 304.21 K, Pc ) 7.383 MPa, and Fc ) 466 mg cm-3, respectively.17 The absorption spectrometer was designed to measure the spectrum in situ under a high-pressure condition. The light source and detector were a halogen lamp and a CCD camera equipped with a monochromator, respectively. The light was © 2005 American Chemical Society
3732 J. Phys. Chem. B, Vol. 109, No. 9, 2005
Figure 1. Schematic diagram of laser ablation in supercritical fluid. (a) Typical phase diagram around the gas-liquid critical point (CP). Blue spheres in the diagram represent molecules in each phase. SCF means supercritical fluid. (b) Laser ablation at two supercritical states, (i) and (ii) in the diagram. The adsorption of fluid molecules to the nanocluster is displayed to be changed significantly by pressure and temperature.
introduced into an optical fiber equipped with a collimation lens, and the collimated light was passed through the cell. The light output from the cell was focused into another optical fiber, and was detected with the CCD camera. By using a pair of optical fibers, we were able to realize not only high flexibility for in situ measurement but also minimization of stray light. The detection limit of absorbance was confirmed to be 10-3 under the high-pressure condition. In the experiments, the difference absorption spectrum was measured. According to the Lambert-Beer law, the difference absorption spectrum is obtained from the equation of log(I0(ω)/I(ω)), where I0(ω) and I(ω) represent the transmission spectra before and after laser irradiation, respectively. Because the difference between these transmission spectra is ascribed to the laser irradiation of silicon, we are able to extract only the change of spectral components between before and after the ablation, and thereby obtain the absorbance change as ∆absorbance. The laser was operated upon excitation at 532 nm of 30 mJ/pulse at a repetition ratio of 20 Hz. The bulk silicon immersed in supercritical CO2 was irradiated by the laser for 5 min. Thus, the difference absorption spectra were measured by changing the pressure of CO2 from 0.1 to 13 MP. The temperature was set at 322.5 K, which corresponds to the reduced temperature Tr ) T/Tc ) 1.06. Under these conditions, both fabrication and measurement of nanoclusters were performed at a density change of a factor of 400 under an isothermal condition of 6% above the critical temperature. The chemical purities of silicon crystal and CO2 were commercially guaranteed to be 99.9999% and 99.99%, respectively.
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Figure 2. Difference absorption spectra of silicon clusters in supercritical CO2. The pressures and densities when the ablations are performed are represented.
As shown in Figure 2, the difference absorption spectra of products were observed at all densities and/or pressures within the present range. These spectra show broad absorption from the visible to the near-infrared region, and their electronic transitions have several structures. Note that the spectral profiles significantly change according to the density when the ablation is performed. Next, the size of the clusters was investigated by dynamic light scattering measurement under the high-pressure condition.18-20 By analyzing the obtained time correlation function, which indicates the diffusion dynamics of clusters in the fluid, using the Stokes-Einstein equation,21 the hydrodynamic radius of generated clusters was estimated as less than 3 nm. Accordingly, it is revealed that the silicon nanoclusters are fabricated by the laser ablation of bulk silicon in supercritical CO2, and that these spectral shapes are considerably changed by the ablation condition in the supercritical fluid. With regard to other significant aspects of the present method, the experimental procedure is relatively simplified. That is, nanoclusters with different properties are obtained via a dry process, and by changing only the fluid pressure and/or temperature. A variety of researchers and scientists unfamiliar with wet chemical processes can use this method to selectively obtain nanomaterials with different properties in a very short time, such as a few minutes. The obtained spectra were well characterized by fitting several Gaussian bands, as shown in Figure 2. As density increases, the band at around 1.7 eV decreases, whereas the band at around 2.5 eV increases. On the other hand, the former increases and the latter decreases as the density further increases. In addition, a new band appears. Figure 3 shows the relative ratios of each band component as a function of density, whose values are
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J. Phys. Chem. B, Vol. 109, No. 9, 2005 3733 Acknowledgment. This work was partially supported by a Nanohana Venture award, a Saneyoshi Scholarship grant, and a Grant-in-Aid for Science and Research from the Ministry of Education, Science, and Culture of Japan (WAKATE A: No. 16685001). These grants helped K.S. to develop the instrument. References and Notes
Figure 3. Spectral components in the difference absorption spectra as a function of density of supercritical CO2.
obtained from the integrated absorption intensities of decomposed bands. It is clearly seen that each ratio significantly depends on the density at which the ablation is performed. The ratio is particularly changed around the density Fr ) 0.6-0.8; i.e., maximum and minimum ratios and a new band at 1.4 eV appear around Fr ) 0.6-0.8, as shown in Figure 3. We will briefly discuss this density. A number of physical and chemical properties of the supercritical fluids are commonly intermediate between gas and liquid states.11-13 The results of our previous investigations using Raman scattering,22 terahertz absorption spectroscopy,23 and molecular polarizability24 measurements all show that the fluid local structure in the vicinity of a molecule changes from a gaslike to a liquidlike structure at around a density of Fr ) 0.7. The local structure of the supercritical fluid changes the degree of aggregation or the cooling rate of hot atoms in the fluid, which plays an important role in determining the generation processes of nanoclusters. Thus, it is most likely that the influential nature of nanoclusters generated by laser ablation in supercritical fluid is responsible for the local structure in the vicinity of ejected hot atoms. As for the molecular structures and electronic structures of the generated silicon clusters, plans are underway to investigate the former by Raman spectroscopy with a newly developed instrument capable of operating under a high-pressure condition, and to investigate the latter by a computer simulation.
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