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White-Light-Emitting Silicon Nanocrystal Generated by Pulsed Laser Ablation in Supercritical Fluid: Investigation of Spectral Components As a Function of Excitation Wavelengths and Aging Time Shaoyu Wei,† Tomoharu Yamamura,† Daisuke Kajiya,‡ and Ken-ichi Saitow*,†,‡ †

Department of Chemistry, Graduate School of Science, and ‡Natural Science Center for Basic Research and Development (N-BARD), Hiroshima University, 1-3-1 Kagamiyama, Higashi-hiroshima, Hiroshima 739-8526, Japan

ABSTRACT: White-light-emitting silicon nanocrystals (Si-NCs) ranging from the near UV to the red region were fabricated by pulsed laser ablation (PLA) of a bulk silicon crystal in a supercritical fluid. The broad photoluminescence (PL) spectra, white light continuum, were investigated by measuring time evolution against aging in the atmosphere or oxygen ambience. The results show that the PL intensity of the higher-energy component increases, whereas that of the lower-energy component decreases as aging time increases. According to rate constants of PL intensity enhancement, the increase in the PL intensity was ascribed to the oxidation of the Si-NCs. This enhancement became significant when the sample was generated at the thermodynamic state, showing a critical anomaly of supercritical CO2. That is, rapid cooling of the hot Si-NC in supercritical CO2 immediately after PLA produces a luminescent Si-NC in the blue-green wavelength region. On the basis of PL spectral measurements at five excitation wavelengths, the lower- and higher-energy PL components were assigned to electronic structures arising from the quantum confinement effect of the Si-NC and the electron−hole recombination at the radiative centers at the surface of the SiNC, respectively.

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INTRODUCTION Since the discovery of visible photoluminescence (PL) from porous silicon,1 silicon photonics has attracted much attention over the recent decades. Research and production technologies for silicon nanomaterials have proceeded rapidly not only for innovative applications in optoelectronics, such as LEDs2 and lasers,3,4 but also for quantum dots with nontoxicity and high tissue compatibility within the human body.5−7 In particular, silicon nanocrystals (Si-NCs) have been synthesized by various methods, e.g., chemical synthesis,8,9 plasma process,10−12 chemical vapor deposition (CVD),13−15 electrochemical reduction,16 ion implantation,17−19 magnetron sputtering,20,21 and so on. The physical properties, as well as the structure of the resulting Si-NCs, have been investigated by spectroscopy, electron microscopy, and diffraction methods. Recently, the pulsed laser ablation (PLA) technique has become popular for generation of both nanoparticles and other nanostructures, and recent progress in this field has been well summarized by Barcikowski et al.22 Utilizing PLA on most solid targets will easily yield a particle with a diameter of a few nanometers. © 2012 American Chemical Society

The studies on PLA of bulk Si in the gas phase started in the 1980s. To the best of our knowledge, Werwa et al.23 first reported the generation of Si-NCs in 1994, showing visible PL, by PLA in the gas phase. After this study, other studies of PLA were developed for inert gas such as He24−30 or Ar,25,31 to reactive gas of H2,32−35 and O2,25,31,36 with an ambient pressure of 10−3−10 Torr24,26−28,30−33,36 to 10−103 Torr.25,29,35 The generated Si-NCs have a spherical shape in their primary structure (2−6 nm)26,28,29,32,33 and a long-range column-,33 cauliflower-,31,33 fiber-,33 or web-like25 or spherical24,26 shape in their secondary structure (10−20 nm).24,25,27,31 The PL spectra show a red emission centered at 560−730 nm24,28,31 as a principle band and a higher-energy band centered at 420−470 nm25,37 as a minor component. These bands are attributed to the quantum confinement effect in Si-NCs26,28,31 or to a surface effect of the Si/SiO2.24,27,32,33 Received: October 20, 2011 Revised: December 21, 2011 Published: January 6, 2012 3928

dx.doi.org/10.1021/jp210080k | J. Phys. Chem. C 2012, 116, 3928−3934

The Journal of Physical Chemistry C

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optics. The fluid pressure is adjusted by an HPLC pump, and the fluid temperature is controlled by a set of heaters, a thermocouple, and a proportional-integral-derivative (PID) controller. The chemical purities of CO2 and single-crystal silicon were commercially guaranteed to be 99.99% and 99.9999%, respectively. The critical pressure, temperature, and density of CO2 are reported as Tc = 304.13 K, Pc = 7.377 MPa, and ρc = 0.465 g cm−3.55 The second harmonic of the Nd:YAG laser served as a light source for PLA and was operated at an excitation wavelength of 532 nm, a pulse width of 9 ns, an energy of 19 mJ pulse−1, and a repetition rate of 20 Hz. The fluence of the light source was set as 0.8 J cm−2. The single-crystal silicon ⟨111⟩ immersed in supercritical CO2 was irradiated by the laser pulse for 20 min to obtain sufficient nanocrystals for PL measurements. PLA was conducted at isothermal conditions of T = 322.4 K (49.2 °C) and 352.8 K (79.6 °C), corresponding to the reduced temperatures Tr = T/Tc = 1.06 and 1.16, respectively. Si-NCs were generated at pressure series from low to high P = 4.6− 17.1 MPa, corresponding to the reduced densities ρr = ρ/ρc = 0.2−1.6, respectively. The product generated in the cell was collected by a microspatula and was deposited on a stainless steel plate. For the aging process, the sample was exposed to a clean air environment or to pure oxygen (99.99%). The PL spectra of the aged samples were measured by two commercial PL microscopes (HORIBA-JY, LabRAM HR-800, and T64000) using various excitation wavelengths, i.e., λex = 325 nm (He−Cd laser), λex = 458, 488, and 515 nm (Ar+ laser), and λex = 633 nm (He−Ne laser). The laser power was commonly set as 10 μW at the sample surface. All PL spectra were calibrated by an instrument function, which was obtained from an NIST standard halogen lamp in the longer wavelength (400−730 nm) and 2-aminopyridine56 and quinine sulfate57 in the shorter wavelength (340−480 nm).

PLA has also been conducted in the liquid phase and is a very simple method. In the liquid phase, PLA is performed with a cuvette filled with liquid, whereas the gas phase or vacuum condition requires expensive equipment, such as large vacuum chambers and vacuum pumps. On the other hand, the product and reaction mechanism for PLA in liquids are relatively complex owing to many solvent molecules in the vicinity of the irradiated target. Thus, various liquids38,39 are used to produce Si-NCs, e.g., water,37,38,40−46 hexane,37,38 octane,38 toluene,37,38 ethanol,38,44,47,48 SDS aqueous solution,49 or DMSO,50 and there are Si-NCs with spherical morphology either with a 3−6 nm diameter in their primary structure37,40−43,49 or with a 10− 40 nm diameter in secondary structure.40,43,49 In addition, the PL band of the red-wavelength region becomes minor, whereas that in the blue region, centered around 410−430 nm,37,40,41,43,49 is found to be a principle band. Thus, the red and blue PL spectra of Si-NCs are obtained by PLA in both the gas and liquid phases. However, full color bands have not yet been obtained by any one of the above methods, and the luminescence mechanism at higher energy regions has not been clearly interpreted. We recently developed a unique method for generating nanomaterials, by taking advantage of PLA in both the gas and liquid phases.51−54 That is, PLA was performed in a supercritical fluid that has properties of both gas and liquid. This method enabled us to obtain gold nanoparticles with specific morphology52,53 and the red, blue, green (RGB) lightemitting Si-NCs.53,54 PLA in a supercritical fluid has several distinct properties: (a) PLA ambience is changed continuously from a gas-like to a liquid-like density by varying the fluid pressure, (b) thermophysical properties, such as thermal conductivity and heat capacity, are controlled by a factor of a few hundred, and (c) ablation plasma is confined near the irradiated target under a high-pressure condition. In the previous study, PLA of bulk Si was performed in supercritical CO2, and Si-NCs were fabricated as a function of the fluid pressure.54 The product was analyzed by the electron microscope, small-angle X-ray scattering, Raman spectra, photoluminescence spectra, and elemental analysis. As a result, the product was found to be surface-oxidized Si-NCs having a diameter of 6 nm, and the color of RGB light-emitting Si-NCs was controlled by the fluid pressure during PLA. In the present study, we found the broad PL of Si-NCs, i.e., a white light continuum ranging from near-ultraviolet to red wavelength regions, by UV excitation. The electronic structures were investigated by both changing the aging time in the atmosphere and in an oxygen ambience and tuning the excitation wavelengths, as well as by utilizing the thermal properties of supercritical fluid. As a result, the lower- and higher-energy components of the broad PL spectrum were attributed to the quantum confinement effect of Si-NC and the electronic structures relating to the radiative centers of the oxidized crystal surface, respectively. To the best of our knowledge, this is the first time that a Si-NC showing a white light continuum has been fabricated by PLA, as well as the first time the luminescence mechanism in white light generation has been investigated.

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RESULTS AND DISCUSSION Figure 1a shows typical PL spectra of Si-NCs generated in ρr = 1.1 of supercritical CO2 at the excitation wavelength of λex = 325 nm (3.82 eV). The spectra are observed from 1.7 to 3.5 eV. Figure 1b displays PL images of the white-light-emitting powdered Si-NCs, whose picture was captured by a fluorescence microscope at the excitation wavelength of λex = 325 nm. Because the change in the PL spectra by aging was first observed in our previous study,54 we exposed the Si-NCs to the atmosphere from 1 h after the laser irradiation to 15 days to investigate the broad spectra. The results are shown in Figure 1a; the spectral shape is changed by aging. That is, the PL intensity near the blue-ultraviolet region is increased by aging, whereas that in the red region is slightly decreased. To analyze the aging effect, the spectra were decomposed into three Gaussian functions using best fittings to the experimental data (see Figure 2a and b), i.e., 1.8 eV (688 nm), 2.3 eV (539 nm), and 2.9 eV (427 nm). From the integrated intensity of their components, we evaluated the ratio of each PL component as a function of the aging time, as shown in Figure 2c. The higher-energy component increases, whereas the lower-energy components decrease with increase in the aging time. These experimental data clearly reveal that the electronic structure of each component has different properties. Using the other Si-NCs generated by different densities, we also measured the spectral change caused by aging. As a result, similar time profiles were obtained. The time profiles were

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EXPERIMENTAL SECTION Si-NCs were fabricated with an instrument developed in our lab and described elsewhere.51 Briefly, the instrument is composed of a high-pressure sample cell made of stainless steel (SUS316) and a Q-switched frequency-doubled Nd:YAG laser and related 3929



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Figure 2. Photoluminescence spectra at the excitation wavelength of 325 nm. The spectra are decomposed by best-fitted Gaussian functions peaked at 1.8 eV (red PL), 2.3 eV (green PL), and 2.9 eV (blue PL). The time evolution of aging in air displayed at (a) 1 h and (b) 8 days. (c) Ratios of PL components are obtained from the integrated intensities of three PL components as a function of aging time. The sample used was fabricated at temperature Tr = 1.06 and pressure P = 11.0 MPa in supercritical CO2.

Table 1. Reciprocal of Time Constants of Exponential Function Used to Characterize Time Profiles of Gaussian Components Observed in Figure 2ca

Figure 1. (a) Typical photoluminescence spectra of Si-NCs at the excitation wavelength of 325 nm. The sample was fabricated at temperature Tr = 1.06 and pressure P = 11.0 MPa in supercritical CO2. (b) Typical image of white-light-emitting powdered Si-NCs measured by a fluorescence microscope at the excitation wavelength of 325 nm.

ρr −1 k1.8 −1 k2.3 −1 k2.9

(day) (day) (day)

0.2

1.1

1.6

1.9 (decay) 7.7 (decay) 4.2 (rise)



a

analyzed by an exponential function, and the resulting time constants are listed in Table 1. To further examine aging, we studied the spectral changes by the pure oxygen gas at P = 0.1 MPa. The PL spectra at the excitation wavelength λex = 458 nm were measured by changing the aging time in the pure oxygen ambience, as shown in Figure 3. The spectra were then fitted by two Gaussian functions. Figures 3a−d and 3e show the PL spectra and the ratio of the two components, respectively, as a function of the aging time in the oxygen ambience. Note that a similar feature shown in Figure 2 is observed in Figure 3. That is, the higher-energy component increases, whereas the lower-energy component decreases as the aging time increases. Here we interpret the higher-energy component to be increased by oxidation and the lower-energy component to be reduced by oxidation. The time profiles observed in Figure 3e were analyzed by exponential −1 functions and reciprocal values, giving time constants of k1.9 = −1 1.9 days and k2.3 = 1.9 days. By comparing these values to those −1 −1 of the Si-NC exposed to ambient air, i.e., k1.8 = 4.3 days and k2.3 = 7.1 days listed in Table 1, we found that the kinetics of lowerand higher-energy bands of aging in the pure oxygen ambience is approximately twice and four times faster than those of aging in the atmosphere, respectively: the band of 1.8 eV in Figure 2 corresponds to that of 1.9 eV in Figure 3 within fitting error. Since both samples are generated under the same supercritical fluid conditions, we determined that aging by oxygen

The subscript in k represents the centered energy (eV) of each component. The sample used was fabricated at temperature Tr = 1.06 and densities ρr = ρ/ρc = 0.2, 1.1, and 1.6 in supercritical CO2.

significantly enhances the PL intensity in the higher-energy region.59 Thus, it was revealed that the oxidation becomes very important to consider the PL component of Si-NC. Here, we need to describe another issue, briefly. According to Figures 2c and 3e, it was shown that behaviors of the PL component at 2.3 eV change by the excitation wavelengths. The time profile of PL intensity decreases at λex = 325 nm but increases at λex = 458 nm. This difference is due to the lifetime that changes on three PL components centered at 1.8, 2.3, and 3.0 eV, on the basis of our preliminary measurements. That is, we have measured the PL lifetimes of Si-NC for the luminescence energy from 1.9 to 3.0 eV and have made sure that the lifetime becomes faster by the order of the PL components centered at 3.0, 2.3, and 1.8 eV. This reveals that the intensity of the PL component centered at 3.0 eV becomes higher than those of the other two at λex = 325 nm (3.82 eV). Similarly, the intensity of 2.3 eV is higher than that of 1.8 eV at λex = 458 nm (2.71 eV). Such a feature was observed in Figures 2c and 3e. To examine the relationship between PL components and oxidation, we measured the PL spectra by changing the excitation wavelength, i.e., λex = 458 nm (2.71 eV), 488 nm (2.54 eV), 515 nm (2.41 eV), and 633 nm (1.96 eV), and 3930



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633 nm) are not affected. Namely, the components higher than 2.0 eV are increased by aging. Consequently, we concluded that the higher-energy component is ascribed to electronic structures corresponding to the radiative centers that are related to the oxidation. That is, electron−hole recombination occurs at the radiative centers at the oxidized surface of the SiNC.10,27,39,58 We also concluded that the lower energy arises from the silicon itself and not from the oxidation. The lowerenergy component is understood as the interband transition of the Si-NC owing to the quantum confinement effect, according to the current size of the Si-NC with the diameter of 6 nm and the published data.12,26,28,60 To ensure the electronic structures of higher- and lowerenergy components of the PL spectra, we show the relationships among the PL spectra, the aging effect, and the properties of the supercritical fluid. As mentioned in the Introduction, the thermal properties of the supercritical fluid vary significantly around the critical point. The heat capacity and thermal conductivity of supercritical CO2 are obtained from the equation of state61 and displayed in Figure 5a and b.

Figure 3. Photoluminescence spectra at the excitation wavelength of 458 nm. The spectra are decomposed by best-fitted Gaussian functions peaked at 1.9 and 2.3 eV. The time evolution of aging in oxygen displayed at (a) 4 h, (b) 3 days, (c) 11 days, and (d) 39 days. (e) Ratios of PL components are obtained from the integrated intensities of two PL components as a function of aging time. The sample used was fabricated at temperature Tr = 1.06 and pressure P = 14.8 MPa in supercritical CO2.

changing the aging time. The excitation to the different electronic structures allows us to analyze whether the properties of the spectral components are different. Figure 4 shows the PL

Figure 5. Heat capacities (a) and thermal conductivity (b) of supercritical CO2 calculated from the equation of state at two isotherms of Tr = 1.06 (solid line) and 1.16 (dashed line) as a function of density. The data show a critical anomaly at Tr = 1.06 and ρr = 1.0. The solid circles correspond to data points, where the Si-NCs are generated by laser ablation in supercritical CO2. (c) Cooling time of hot Si-NCs was calculated from heat capacities (a) and thermal conductivity (b) immediately after laser ablation. Rapid cooling is observed at Tr = 1.06.

Note that their values change significantly around the critical density ρr = 1.0. In addition, the critical anomaly does not appear at Tr = 1.16 but is observed at Tr = 1.06. Here we calculate the cooling time τ of hot Si-NCs immediately after PLA in supercritical CO2 by the formula τ = RSi2ρSi2CSi2/ 9ρCO2CCO2λCO2,52,54,62,63 where RSi is the radius of the Si-NC, ρSi the density of Si, CSi the specific heat capacity of Si, ρCO2 the density of CO2, CCO2 the specific heat capacity of CO2, and λCO2 the thermal conductivity of supercritical CO2. Thus, calculated τ values at two fluid temperatures are shown in Figure 5c as a function of the density of supercritical CO2. The cooling times at Tr = 1.06 and 1.16 are 16 and 47 ps,

Figure 4. Photoluminescence spectra at the excitation wavelengths of (a) 458 nm, (b) 488 nm, (c) 515 nm, and (d) 633 nm. All spectra are measured as a function of aging time in the air. The sample was fabricated at temperature Tr = 1.06 and pressure P = 9.8 MPa in supercritical CO2.

spectra measured by four different excitation wavelengths. From these results, we found that the spectra measured by higher-energy excitations, i.e., λex = 458, 488, and 515 nm, are changed by aging, whereas those by the lower-energy one (λex = 3931



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respectively, where the density of CO2 is ρr = 1.0. That is, the former τ is about three times faster than that at Tr = 1.16, and the heated Si-NC is cooled rapidly at Tr = 1.06 immediately after PLA. This clarifies the significant difference that exists in generation processes at two temperatures with the same fluid density. Finally, to clarify the relation between the cooling time and PL intensity, we investigate the aging effects of two Si-NC samples generated in supercritical CO2 at Tr = 1.06 and 1.16. The results are shown in Figure 6. The PL intensity of the Si-

Figure 7. Schematic diagram of the Si-NC lattice before and after rapid cooling in a supercritical fluid. The rapid cooling produces the defect-rich Si-NC consisting of many dangling bonds that are oxidized by the aging, i.e., increase of radiative center. The capping of the dangling bonds of the Si-NC by oxygen reduces the dangling bond, i.e., decrease of nonradiative center.

cooling is not readily influenced by oxidization. Thus, the PL intensity of the Si-NC with smaller numbers of the nonradiative centers is not affected by aging, as shown in Figures 6c and d. Consequently, the broad PL spectra and their electronic structures in the lower- and higher-energy regions of the SiNCs were revealed by both the aging effect and measurements at various excitation wavelengths, as well as by utilizing the prominent thermal properties of the supercritical fluid.

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CONCLUSIONS Silicon nanocrystals were generated by pulsed laser ablation of bulk silicon in supercritical CO2. The products were analyzed by photoluminescence, and a white-light continuum was observed. The broad photoluminescence spectra were investigated by changing the aging time, excitation wavelengths, and the thermal properties of supercritical CO2. The lower- and higher-energy components were attributed to the electronic structures owing to the quantum confinement effect of the SiNC and electron−hole recombination at the radiative centers relating to oxidation, respectively. The significant PL intensity enhancement was due to the capping of dangling bonds during oxidation. By measuring the PL spectra at different temperatures, but at the same fluid density, we confirmed that rapid cooling enhances the PL intensity in the higher-energy region. The reason rapid cooling in the supercritical fluid, in which the thermodynamic state shows the critical anomaly of thermal properties, generates a luminescence Si-NC is to produce many dangling bonds, which turns nonradiative into radiative centers, at blue and green PL colors, by the capping with oxygen.

Figure 6. Photoluminescence spectra measured at the excitation wavelength of 458 nm. The spectra show the time evolution of aging (a), (c) 40 min and (b), (d) 2 days in the air. The sample was fabricated at temperatures (a), (b) Tr = 1.06 and (c), (d) Tr = 1.16. Significant enhancement of the PL intensity caused by aging is observed for the Si-NC produced by PLA at Tr = 1.06.

NCs generated at Tr = 1.06 is enhanced significantly by aging and becomes five times greater than that before aging. However, the PL intensity of the Si-NCs generated at Tr = 1.16 does not change, although except for the temperature the generation conditions are exactly the same as those of the sample prepared at Tr = 1.06. Namely, the Si-NC produced by rapid cooling after PLA is highly sensitive to the oxidation process, and its PL intensity increases significantly. However, the PL intensity of the Si-NC produced by slow cooling is not enhanced by aging. Accordingly, we concluded that the Si-NC generated by rapid cooling enhances the PL intensity in the higher-energy region after oxidation. The rapid cooling produces a defect-rich Si-NC consisting of many dangling bonds that are oxidized by the aging process. Such a feature is schematically illustrated in Figure 7. According to recent reports for Si-NC by various groups, a similar feature has been observed.39,58,64 Namely, it was described that the capping of the dangling bonds of the Si-NC by oxygen reduces the nonradiative centers in the Si-NC, corresponding to the dangling bond. This interpretation is consistent with the current experimental results. That is, many dangling bonds, known as a nonradiative centers,27,65 are generated by rapid cooling and are capped by oxidation. Then, the decrease in the nonradiative centers results in the enhancement of the PL intensity. These features are observed in Figures 6a and b. We also observed that the defect-poor Si-NC produced by slow

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AUTHOR INFORMATION

Corresponding Author

*Telephone & Fax: +81-82-424-7487. E-mail: saitow@ hiroshima-u.ac.jp.

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ACKNOWLEDGMENTS We acknowledge the Hamamatsu photonics K.K. for the preliminary experiments on the lifetime measurements. K.S. kindly acknowledges Professor Yoshio Okamoto of Nagoya University, a research supervisor for “Structure Control and Function” of PRESTO of Japan Science and Technology agency (JST). PRESTO substantially supported this research. This work was also supported by a Grant-in-Aid for Young Scientists (A) (16685001) and a Grant-in-Aid for Scientific 3932



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Research (B) (21350015) from the Ministry of Education, Science and Culture of Japan.

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