Effective Cooling Generates Efficient Emission: Blue, Green, and Red

J. Phys. Chem. C 2009, 113, 8465–8470

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Effective Cooling Generates Efficient Emission: Blue, Green, and Red Light-Emitting Si Nanocrystals Ken-ichi Saitow*,†,‡,§ and Tomoharu Yamamura‡ N-BARD (Natural Science Center for Basic Research and DeVelopment) and Graduate School of Science, Hiroshima UniVersity, 1-3-1 Kagamiyama, Higashi-hiroshima 739-8526, Japan, and PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan ReceiVed: January 4, 2009; ReVised Manuscript ReceiVed: February 24, 2009

Nanosecond pulsed laser ablation of bulk silicon crystal upon the excitation of 532 nm was conducted in supercritical CO2 to generate silicon nanocrystals, whose properties were studied by seven experimental methods. According to the photoluminescence spectra and fluorescence microscope images, emissions of near-ultraviolet, violet, blue, green, and red were observed in air, at room temperature, and without cooling in liquid nitrogen or a helium cryogenic system. A preferable emission channel of carriers, generated by photoexcitation of Si/SiO2 of core/shell structure, was responsible for interface states with defect sites. This luminescence process caused color changes and intensity increase, enhanced by a factor of 100, where thermal properties of supercritical CO2 were maximized, due to critical anomaly. It was found that colors and intensities of photoluminescence of silicon nanocrystals are controlled by a cooling rate during ablation, whose quantity is manipulated by the supercritical fluid pressure. Introduction Since the discovery of a light-emitting porous silicon, visible light-emitting silicon with a zero- or one-dimensional structure has become a material of interest that promises not only innovations in optoelectrical devices of integrated circuits, but also production of labeling agents of low toxicity and high compatibility with human tissue.1-4 There have been numerous reports for red light-emitting silicon nanomaterials; however, the number of studies on higher energy emission, i.e., the ultraviolet to green region, are limited due to technical difficulties. During the past 3 years, efficient light-emitting silicon nanocrystals (Si-nc) were generated by combining plasma synthesis with chemical passivation of nanocrystal surface.5,6 The generation of Si-nc in the high-density-plasma ambient enhanced luminescence intensity. On the other hand, the laser ablation generates plasma near a target by irradiating intense optical pulses, and successively fabricates the target nanomaterials.7 Several groups reported the photoluminescence spectra in the green8,9 and blue10-13 wavelength regions of silicon nanomaterials, fabricated by the pulsed laser ablation in a vacuum and a low-pressure gas. We developed a novel method of fabricating nanomaterials by conducting pulsed laser ablation in a high-pressure supercritical fluid.14,15 This method has three distinct properties: (i) The plasma density is locally enhanced, because the plasma generated near the target is spatially confined by the highpressure fluid, e.g., 20 MPa; (ii) the cooling rate for the thermal relaxation processes is controlled during the generation of nanoparticles, because thermal conductivity and heat capacity of a supercritical fluid have infinite values at the vapor-liquid critical point, and vary easily by changing the pressure (Figure * To whom correspondence should be addressed. Tel. and fax (Japan): +81-82-424-7487, E-mail: [email protected]. † N-BARD (Natural Science Center for Basic Research and Development), Hiroshima University. ‡ Graduate School of Science, Hiroshima University. § PRESTO, Japan Science and Technology Agency.

1); and (iii) a variable density is achieved, from gas-like to liquid-like, by changing the fluid pressure in the absence of a phase transition. It was found that the local structure of supercritical fluid changes from a gas-like to a liquid-like near the extension of vapor pressure curve, such as a boundary line, on the pressure-temperature (P-T) phase diagram (Figure 1).16 In our previous study, the pulsed laser ablation for solid materials was conducted17,18 and was recently performed for the first time in a supercritical fluid.14,15 We irradiated a nanosecond pulsed laser to a silicon single crystal14 and a gold plate15 immersed in a supercritical CO2. Changing the pressure or density of the fluid during laser ablation generated silicon nanoclusters with a variety of absorption bands, and gold nanoparticles having morphology of a necklace structure. Here we show near-UV, violet, blue, green, and red light-emitting Si-nc, generated by laser ablation of bulk silicon in a supercritical fluid. Note that changing the pressure varies the luminescence colors and increases the luminescence intensity by a factor of 100 at a point where the value of the thermal properties of supercritical fluid becomes maximal due to critical anomaly. Experimental Methods We developed an instrument to fabricate nanoparticles,14,15 consisting of a homemade high-pressure cell made of stainless steel (SUS316) and a Q-switched frequency doubled Nd:YAG laser (Litron Optical, LPU4000). The cell windows were made of sapphire and sealed with Teflon gaskets. The fluid pressure was increased with an HPLC pump (Nihonseimitu Kagaku, NPS-323). The temperature was controlled by a set of heaters, a proportional-integral-derivative (PID) controller (Chino, DB1000), and a thermocouple. Nd:YAG laser served as a light source for pulsed laser ablation and was operated with an excitation wavelength of 532 nm, energy of 19 mJ/pulse, a repetition rate of 20 Hz, a fluence of 0.8 J/cm2, and a pulse width of 9 ns. A single silicon crystal immersed in supercritical CO2 was irradiated by the laser for 10 min under isothermal

10.1021/jp900067s CCC: $40.75  2009 American Chemical Society Published on Web 04/16/2009

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Figure 1. Properties of supercritical CO2. Thermodynamic states conducted by the silicon nanocrystal generation are plotted as solid circles. (a) Pressure vs temperature phase diagram of CO2 in the vicinity of the vapor-liquid critical point. CP represents the vapor-liquid critical point and is shown by an open circle. The solid line is the vapor-liquid coexistence curve. Dashed line is an extrapolated line of the vapor-liquid coexistence curve such as a boundary between gaslike and liquid-like structures in the supercritical region, according to our previous studies in ref 16. The CO2 molecules are schematically displayed, and the density in the supercritical region is continuously changed in the absence of a vapor-liquid phase transition. (b) Thermal conductivity, (c) heat capacity, and (d) density as functions of pressure at an isotherm Tr ) T/Tc ) 1.06. Critical anomalies of supercritical CO2 are observed as peaks at a pressure value of 10 MPa.

condition of T ) 322.4 K, corresponding to the reduced temperature Tr ) T/Tc ) 1.06. The crystal surface 〈111〉 of silicon was irradiated by the laser, and the flesh surface was irradiated in every laser irradiation. Thermodynamic states of a supercritical CO2 are shown in Figure 1, where thermodynamic states used for the generation of Si-nc are plotted as solid circles. The thermal conductivity, heat capacity, and density vs pressure shown in Figure 1b-d, respectively, are obtained from the empirical data and equation of state of supercritical CO2,19,20 using measured values of P and T. The chemical purities of CO2 (Taiyo Nippon Sanso Co. Ltd.) and the silicon crystal (Peer Optics) were both commercially guaranteed to be 99.99 and 99.9999%, respectively. The critical constants of CO2 are reported to be Tc ) 304.13 K, Pc ) 7.377 MPa, and Fc ) 0.465 g cm-3.19 The density of CO2 is represented by using the reduced density Fr ) F/Fc. The products generated by the laser ablation in the supercritical CO2 were analyzed by seven experimental methods: i.e., photoluminescence microscopy, Raman microscopy, fluorescence microscopy, field-emission scanning electron microscope (FE-SEM), small-angle X-ray scattering (SAXS), electron probe microanalyser (EPMA), and combustion elemental analysis. The photoluminescence microscope measurements were performed

Saitow and Yamamura at an excitation wavelength of 457.9 nm with an Ar+ laser and at 325 nm with a He-Cd laser, using Si-nc deposited on a SUS plate. The Raman microscope measurements were performed using a He-Ne laser at an excitation wavelength of 632.8 nm. These spectra were measured with an excitation power of 10 µW for 3 s using a monochromator having a focal length of 800 mm and equipped with a CCD camera. The measured spectra were corrected by an instrumental function that was obtained by measuring the power spectrum of a halogen lamp, calibrated by the National Institute of Standards and Technology (NIST). The luminescence image of the products deposited on the SUS plate was captured using a fluorescence microscope (Keyence, VB-7000) equipped with a color CCD camera and suitable band-pass filters. The luminescence images were captured at an excitation of 375 nm using a diode-pumped solidstate laser (Showa Optronics, D375C-16). The product size was analyzed by SAXS measurements (Rigaku, RINT-TTR III). The product deposited on a Kapton film was placed in a sample holder at a position normal to the incident X-rays. The X-rays scattered from the sample were detected with a CCD camera, and the scattering intensity was recorded as a function of the scattering angle θ. X-rays scattered from a blank Kapton film were recorded to compensate for background signals. The size and structure of the product were also observed by the FE-SEM (Hitachi, S-5200), whose sample was prepared as Si-nc deposited on a carbon disk. The compositional elements of the products were analyzed by EPMA (JEOL, JXA-8200) measurements of the Si-nc deposited on a carbon disk. The element composition of carbon in the product was also confirmed by conventional combustion analysis. Results and Discussion Figure 2a shows a typical SEM image of the products from the pulsed laser ablation of silicon in supercritical CO2. The morphology exhibits a stringlike structure, with lengths of up to 10 µm. The inset shows a magnified image of a lowdimensional nanostructure consisting of nanoparticles 5 nm in diameter. A similar morphology of silicon nanoparticles was observed by a nanosecond21 and femtosecond22 laser ablation in helium gas ambient. The structure consisted of amorphous silicon nanoparticles with a diameter of D ) 10-20 nm. To evaluate the size in the present study, we performed the SAXS measurements. Figure 2b shows that the size distribution of products was obtained from the analysis of SAXS data. A typical profile of the scattered X-rays is displayed in the inset as a function of the scattering vector. The profile was analyzed using the scattering curve in the transmission configuration that incorporates the Γ function to obtain the size distribution function, shown as follows.23,24

I(θ) ∝ |F(s, R0, M)| 2 )

Ω(q, R) )

( )∫ ( )

1 M M ∞ -MR/R0 -1+M e R × 0 Γ(M) R0 R0 3 |reFf(s) Ω(s, R)| 2 dR (1) R

4πR3 [sin(sR) - (sR) cos(sR)] (sR)3

(2)

where I(θ) is the scattering intensity as a function of the scattering angle θ, s the scattering vector, R0 the averaged diameter of particles, M the shape parameter, Γ the gamma function, R the radius of a particle, re the radius of an electron,

Effective Cooling Generates Efficient Emission

Figure 2. Silicon nanocrystals generated in supercritical CO2. (a) Images captured by the FE-SEM. The inset is a magnified image. The silicon nanocrystals observed in SEM images were generated by laser ablation at a pressure value of 4.56 MPa, density Fr ) F/Fc ) 0.2, and temperature Tr ) T/Tc ) 1.06. (b) Size distributions are obtained from the analysis of SAXS measurements. Blue and green solid lines are data of the size distributions of silicon nanocrystals generated at pressure values of 4.56 and 10.4 MPa, respectively. The inset is a typical profile of scattered X-rays (black curve) and the fitting function (red curve) as a function of the scattering vector s. (c) The Raman spectra of silicon nanocrystals generated at three different pressures or densities. The band at around 520 cm-1 has been assigned to the band of crystal silicon in ref 26, whose reproduced data are shown in the left inset. In contrast, the band of amorphous silicon is shown in the right inset reproduced from the data of ref 27.

F the electron density, f(s) the average atomic scattering factor at s, and Ω(s,R) the form factor of the particle with radius R at s. s ) 4π(sinθ)/λ, where λ is the wavelength of the X-rays (Cu KR, λ ) 1.5 Å). The black and red curves in the inset represent the experimental data and the fitting function, respectively. By using the best-fitted data, we obtained the distribution function of the product, having two features. First, the peak position at 6 nm is in good agreement with the diameter of the nanoparticle consisting of stringlike products, observed in SEM measurements. The average diameter of the particles resulted in 6 nm.25 Second, the distribution functions for low and high pressures are similar, indicating that the size is independent of pressure. The size independence of the fluid was also observed during

J. Phys. Chem. C, Vol. 113, No. 19, 2009 8467 the generation of gold nanoparticles by laser ablation in supercritical CO2.15 Figure 2c shows the Raman bands of products at three typical pressure values. The spectra show a single sharp band located at 520 cm-1, and also the spectral shapes are independent of the pressure of supercritical CO2. A band of silicon at 520 cm-1 has been assigned to an optical phonon mode of Si crystal shown in the left inset,26 whereas a band of amorphous silicon differs completely from the crystal shown in the right inset27 in Figure 2c. Thus, we ensured that the product is not amorphous, but crystalline silicon. Figure 1 shows the physical properties of the supercritical CO2 at the same temperature and pressure ranges as the present experimental conditions.19,20 All values changed considerably by changing the pressure; however, the size and the degree of crystallinity are independent of the conditions of supercritical CO2, as shown in Figure 2. Thus, the size and crystallinity were not affected by the properties of supercritical CO2 within the present experimental ranges. Next, we conducted elemental analysis of the products with both EPMA measurements and the combustion method. Their results showed that the products are composed of silicon (38 ( 1.6%), oxygen (62 ( 1.6%), and negligible carbon (e1%). These composition ratios were independent of the samples within experimental errors. As a result, the products generated by different pressures have the same ratios, and the compositional elements are the same for all Si-nc. Here, we summarize the experimental results, briefly. According to the SAXS and EPMA measurements, the sizes and compositional elements were independent of the thermodynamic states of supercritical CO2. The Raman spectra showed that products consisted of silicon crystals. These results were consistent with the model that the surface of the product is oxidized, and the core is a silicon crystal. By using the values of D ) 6 nm of the products and the element composition ratios, we can estimate the core size and shell thicknesssa Si crystal core of 2 nm and SiO2 shell of 2.1 nm in thickness account for the SAXS, EPMA, and Raman data. Since the experimental results are independent of the conditions of supercritical CO2, it is considered that Si-nc having a core-shell is a common structure within the present experimental ranges. We measured the photoluminescence spectra of Si-nc with excitation wavelengths of 457.9 nm (2.71 eV) and 325 nm (3.82 eV). Figure 3 shows the photoluminescence spectra and luminescence images captured using the fluorescence microscope. We observe that Si-nc emits blue, green, and red light. The spectra and luminescence images were measured in air, at room temperature, and without cooling in liquid N2 or a He cryogenic system. In contrast, Figure 3d shows the photoluminescence of bulk silicon, emitting light near the infrared region (near 1100 nm). This wavelength has been assigned to a band gap of bulk silicon (1.1 eV).28 The photoluminescence spectra of Si-nc, however, differ significantly from that of bulk silicon. Namely, the laser ablation of silicon in supercritical CO2 generates the Si-nc having different electronic structure against the bulk Si. As another distinct property, the spectra were changed by variation in the pressure. Under low pressure, the product is a red light-emitting Si-nc, whereas high pressure selectively generates a green light-emitting Si-nc. This indicates that variation in the fluid pressure fabricates multicolored Sinc. To further investigate the pressure dependence of photoluminescence, we measured the luminescence intensity of Si-nc as a function of pressure during the laser ablation. To obtain an accurate pressure dependence of the intensity, the data were

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Figure 3. Photoluminescence spectra and luminescence images of silicon nanocrystals. (a) Red light-emitting silicon nanocrystals generated at a low pressure at 4.56 MPa of supercritical CO2, measured by the excitation at 457.9 nm of an Ar+ laser. (b) Green light-emitting silicon nanocrystals generated at a high pressure at 14.8 MPa of supercritical CO2, measured by excitation at 457.9 nm using the Ar+ laser. (c) Blue light-emitting silicon nanocrystals generated at a pressure of 11 MPa of supercritical CO2, measured by excitation at 325 nm of a He-Cd laser. The blue emission was observed in the nanocrystals generated in all pressures. (d) Near-IR light-emitting bulk silicon measured by excitation at 632.8 nm of a He-Ne laser. In near-IR measurements, the detector was used as a photodiode alley of InGaAs. (e) Photoluminescence images measured by a fluorescence microscope at an excitation wavelength of 375 nm.

collected by 100 spectral measurements of Si-nc at each pressure value and were averaged in a computer. Figure 4a shows the obtained photoluminescence spectra, and a considerable change in luminescence intensity is observed by changing the pressure during the laser ablation. Figure 4b shows the integrated intensities of the photoluminescence as a function of pressure. The intensity at high pressure (15 MPa) is 50 times greater than that generated at a low pressure (4 MPa). Thus, the photoluminescence intensity is increased by increasing the pressure during the laser ablation. However, it should be noted that the maximum intensity, i.e., 100 times greater than that at a low pressure, does not appear at the highest pressure but at a medium pressure; i.e., the intensity is maximized at the pressure, where the heat capacity of the supercritical CO2 is maximal, due to the critical anomaly. To evaluate the thermal properties of supercritical CO2 and heat dissipations of Si-nc, we calculate a cooling rate of heated Si-nc as a function of pressure in the following equation.28,29

τ)

RSi2FSi2CSi 9FCO2CCO2λCO2

(3)

where RSi, FSi, and CSi are radius, density,31 and heat capacity31 of Si-nc, respectively, and FCO2, CCO2, and λCO2 are density, heat

Figure 4. Photoluminescence spectra and integrated intensity of silicon nanocrystals as a function of pressure during laser ablation. (a) Spectra measured at varying pressure during nanocrystal generation with laser ablation at an excitation of 457.9 nm of an Ar+ laser. The intensity increases as the pressure of supercritical CO2 increases during laser ablation. (b) Integrated intensity of photoluminescence spectra as a function of pressure. The symbols and solid curves represent data points and fitting curves for guides to the eye, respectively. The black, blue, and red solid circles represent the data of aging for 40 min, 2 days, and 2 months, respectively.

capacity, and thermal conductivity of supercritical CO2, respectively. The cooling rate is expressed by τ-1 and shown in Figure 5 as a function of pressure of supercritical CO2. The inset represents the cooling time τ. As shown in Figures 4b and 5, the profile of pressure dependence of photoluminescence intensity is in good agreement with that of the cooling rate of Si-nc in a supercritical CO2. This indicates that a rapid cooling (∼picosecond) of Si-nc brings an efficient photoluminescence emission of Si-nc. As another distinct feature, the aging of Sinc in the atmosphere significantly enhances the photoluminescence intensity, as shown in Figure 4b. The intensity is the greatest at around the highest cooling rate. The intensity of Sinc of aging for 2 months is 50 times greater than that of aging for 40 min. The intensity was not so changed after the aging of 2 months. To discuss the intensity and emission color changes by the change in pressure values, we introduce the photoluminescence studies on silicon nanomaterials briefly. As for the photoluminescence of Si-nc in the visible region, two kinds of emission processes have been recognized by various researchers:1-4,7-13,32,33 i.e., (i) pure quantum confinement model (quantum confinement effects in Si nanocrystal without surface states effects) and (ii) interface localization model (quantum confinement effects in

Effective Cooling Generates Efficient Emission

Figure 5. Cooling rate of silicon nanocrystal in supercritical CO2 obtained from thermodynamic calculation as a function of pressure. Inset shows the cooling time of silicon nanocrystal, indicating the heat dissipation time. ps represents picoseconds.

Si nanocrystal with surface effect).3,32 The former model represents an interband transition in pure Si-nc, having no lattice defects and no surface oxidations, in a wider band gap than that of bulk Si (1.1 eV). The latter model is characterized by luminescence from carriers and/or exciton at localized interfaces between core Si nanocrystal and SiO2 shell, whose process is initiated by photoabsortion of core Si-nc upon excitation of visible light. In the current system, the second model plays an important role in the luminescence process, because elemental analyses show the Si-nc composing of Si (38%) and oxygen (62%), and then the luminescence intensity increases by aging in the atmosphere. As shown in Figure 4, the emission intensity at around 530 nm (2.3 eV) significantly increases, while the emission intensity at around 670 nm (1.9 eV) does not change. Namely, the green band (2.3 eV) is sensitive to oxidation, whereas red band (1.9 eV) is insensitive. In fact, it was observed in the current system that the peak position of photoluminescence changes from 1.9 to 2.3 eV by the aging. According to studies on oxidation for photoluminescence of Si-nc, the intensity of the band located at around 2.3 eV increases significantly by depositing the Si-nc in oxygen atmosphere.9,33 The band at around 2.3 eV was assigned to the oxygen-related defects in disordered nanostructures. As another study, a band at around 2.3 eV was analyzed using controlled films, and an increase in ratio of oxygen in the film causes a shift in the peak position from 1.9 to 2.3 eV.33 These results and the spectral shapes reported are in good agreement with the current results. The above-mentioned mechanism is consistent with the results of color change with pressure and the anomaly of luminescence intensity. An increase in pressure during the laser ablation caused the shift in peak energy from 1.9 to 2.3 eV, as shown in Figure 5. The thermal conductivity and the heat capacity of the supercritical CO2 increased by the fluid pressure (Figure 1), and the cooling rate increased (Figure 5). Rapid cooling generates many defects in Si-nc, whose band is assigned to be located at green (2.3 eV).9,33 The greatest intensity was observed at a pressure value that gave the fastest cooling time. The surface oxidation of Si-nc enhanced the intensity of green luminescence. Here we summarize the current photoluminescence processes, on the basis of present results and previous reported papers. The photoabsorption of the core Si-nc upon the excitation of 457.9 nm (2.71 eV) generates the electron and hole exiting in the conduction and valence bands, respectively. An electron-hole pair is recombined by the interband transition between conduction and valence bands of the Si core, and the red emission is

J. Phys. Chem. C, Vol. 113, No. 19, 2009 8469 observed. On the other hand, an electron-hole pair in the Sinc having defects and oxidized surfaces is recombined at the interface states between the Si core and SiO2 shell, giving the green luminescence. Namely, the luminescence colors and intensity in the Si-nc are determined by a blanching ratio whether the electron-hole pairs are relaxed to the interband transition at the Si core or the interface states between the Si core and SiO2 shell. The intensity of green luminescence increases when the Si-nc has many defects and oxidation sites. Such a situation is caused by the rapid cooling and the aging of the Si-nc, as shown in Figure 4. From the viewpoint of relaxation dynamics, it is also considered that the transition rate in the green luminescence is faster than that of the red one. The lifetime measurements of photoluminescence of Si-nc show that the bands at around 550 and 700 nm are faster and slower transition rates, respectively.34 Taking into account the carriers localized at the interface of Si-core/Si-shell, e.g., ∆l ≈ 0.5 nm, the transition is considered to become the preferred emission channel, according to relaxation of momentum conservation, opening pseudodirect transition. This mechanism is consistent with the result of blue emission. The photoabsorption of the core Si upon the excitation of 325 nm (3.82 eV) generates the electron-hole pair. Figure 3c shows that the emission color is not red (1.9 eV) but blue (3.0 eV). That is, the luminescence does not occur at the interband transition (1.9 eV) but an interface state (3.0 eV), whose rate can be also enhanced by the momentum conservation, opening pseudo-direct transition.35 Finally, we introduce the other distinct properties of the present Si-nc in brief. First, the generated Si-nc does show not only Red/Green/Blue (RGB) primary colors of light but also violet and near-UV light with higher energy, as shown in Figure 4c, whose photoluminescence was obtained at an excitation wavelength of 325 nm. The shortest emission wavelength approaches 350 nm with a photon energy corresponding to 3.5 eV. Note that the higher energy photoluminescence is observed at room temperature without any cooling. Second, the Si-nc was generated by the pulsed laser ablation in high-density-plasma ambient, confined by a high-pressure fluid. It was recently reported that Si-nc synthesized by the plasma method brings an efficient photoluminescence with a high quantum efficiency φ after the surface passivation of Si-nc; i.e., φ ) 30% at 420 nm5 and φ ) 70% at 800 nm.6 We started passivation of the Si-nc, which seems to bring an increase in luminescence intensity. Conclusions Pulsed laser ablation of bulk silicon crystal performed in a supercritical CO2 generated silicon nanocrystals. According to the experiments of SEM, SAXS, EPMA, photoluminescence, Raman, luminescence microscope, and combustion elemental analysis, the generated particles had a diameter of 6 nm. The particles consisted of a core of Si-nc with a diameter of 2 nm and a shell SiO2 with a of thickness of 2 nm. The core silicon nanocrystal absorbed the excitation laser and generated carriers, which were relaxed by emitting the light. The dominant channel of photoluminescence was an interface state of Si/SiO2 with defects (530 nm, 2.3 eV), which was enhanced by effective thermal dissipation of Si-nc in supercritical CO2. It was found that rapid cooling (∼picosecond) of Si-nc changes the emission color and intensity. The critical anomaly of supercritical fluid caused significant changes for photoluminescence. Acknowledgment. K.S. greatly acknowledges Mr. Koichi Udo and Mrs. Yayoi Taniguchi at the Rigaku Corp. for their

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assistance in the small-angle X-ray scattering experiments. K.S. greatly acknowledges Prof. Dr. Yohko F. Yano of Ritsumeikan University for the discussion of the SAXS data analysis. The authors greatly acknowledge Mr. Kazuhisa Omi of Showa Optronics Co. Ltd, who helped in using the light source of a fluorescence microscope. This work was partially supported by a Grant-in-Aid for Young Scientists (A) (Grant 16685001) from the Ministry of Education, Science and Culture of Japan. References and Notes (1) Canham, L. T. Appl. Phys. Lett. 1990, 57, 1046. (2) Iyer, S. S.; Xie, Y.-H. Science 1993, 260, 40. (3) Lockwood, D. J, Ed. Light emission in silicon: From physics to deVices; Academic: San Diego, 1998. (4) Khriachtchev, L, Ed. Silicon nanophotonics: Basic principles, present status and perspectiVes; World Scientific: Singapore, 2008. (5) Sankaran, R. M.; Holunga, D.; Flagan, R. C.; Giapis, K. P. Nano Lett. 2005, 5, 537. (6) Jurbergs, D.; Rogojina, E.; Mangolini, L.; Kortshagen, U. Appl. Phys. Lett. 2006, 88, 233116. (7) Perriere, J., Millon, E., Fogarassy, E., Eds. Recent adVances in laser processing of materials; Elsevier: Oxford, U.K., 2006. (8) Yamada, Y.; Orii, T.; Umezu, I.; Takeyama, S.; Yoshida, T. Jpn. J. Appl. Phys., Part 1 1996, 35, 1361. (9) Yang, D.-Q.; Ethier, V.; Sacher, E.; Meunier, M. J. Appl. Phys. 2005, 98, 024310. (10) Makino, T.; Yamada, Y.; Suzuki, N.; Yoshida, T.; Onari, S. J. Appl. Phys. 2001, 90, 5075. (11) Kim, J. H.; Jeon, K. A.; Lee, S. Y. J. Appl. Phys. 2005, 98, 014303. (12) Chen, X. Y.; Lu, Y. F.; Wu, Y. H.; Cho, B. J.; Liu, M. H.; Dai, D. Y.; Song, W. D. J. Appl. Phys. 2003, 93, 6311. (13) Svrcek, V.; Sasaki, T.; Shimizu, Y.; Koshizaki, N. Appl. Phys. Lett. 2006, 89, 213113. (14) Saitow, K. J. Phys. Chem. B 2005, 109, 3731. (15) Saitow, K.; Yamamura, T.; Minami, T. J. Phys. Chem. C 2008, 112, 18340. (16) (a) Saitow, K.; Kajiya, D.; Nishikawa, K. J. Am. Chem. Soc. 2004, 126, 422. (b) Saitow, K.; Otake, K.; Nakayama, H.; Ishii, K.; Nishikawa, K. Chem. Phys. Lett. 2002, 368, 209. (c) Nakayama, H.; Saitow, K.; Sakashita, M.; Ishii, K.; Nishikawa, K. Chem. Phys. Lett. 2000, 320, 323.

Saitow and Yamamura (d) Saitow, K.; Nakayama, H.; Ishii, K.; Nishikawa, K. J. Phys. Chem. A 2004, 108, 5770. (e) Saitow, K.; Sasaki, J. J. Chem. Phys. 2005, 122, 104502. (17) Saitow, K.; Ichinose, N.; Kawanishi, S.; Fukumura, H. Chem. Phys. Lett. 1998, 291, 433. (18) Saitow, K.; Banjo, H.; Ichinose, N.; Kawanishi, S.; Masuhara, H.; Fukumura, H. J. Photochem. Photobiol. A 2001, 145, 159. (19) Span, R.; Wagner, W. J. Phys. Chem. Ref. Data 1996, 25, 1509. (20) Vesovic, V.; Wakeham, W. A.; Olchowy, G. A.; Sengers, J. V.; Watson, J. T. R.; Millat, J. J. Phys. Chem. Ref. Data 1990, 19, 763. (21) Li, S.; Silvers, S. J.; El-Shall, M. S. J. Phys. Chem. B 1997, 101, 1794. (22) Tull, B. R.; Carey, J. E.; Sheehy, M. A.; Friend, C.; Mazur, E. Appl. Phys. A 2006, 83, 341. (23) Nagao, O.; Harada, G.; Sugawara, T.; Sasaki, A.; Ito, Y. Jpn. J. Appl. Phys 2004, 43, 7742. (24) Omote, K.; Ito, Y.; Kawamura, S. Appl. Phys. Lett. 2003, 82, 544. (25) As shown in Figure 2b, a weak peak is seen at 25 nm. In the analysis of SAXS data, the abundance obtained does not correspond with the number of particles but the volume of particles. As a result, the quantity of particles with the size of 25 nm is so less that we can consider it negligible. (26) Parker, J. H.; Feldman, D. W.; Ashkin, Phys. ReV. 1967, 155, 712. (27) Smith, J. E.; Brodsky, M. H.; Crowder, M. B. L.; Nathan, M. I.; Pinczuk, A. Phys. ReV. Lett. 1971, 266, 642. (28) Haynes, J. R.; Westphal, W. C. Phys. ReV. 1956, 101, 1676. (29) Hartland, G. V. Phys. Chem. Chem. Phys. 2004, 6, 5263. (30) Wilson, O. M.; Hu, X. Y.; Cahill, D. G.; Braun, P. V. Phys. ReV. B 2002, 66, 224301. (31) CRC Handbook of Chemistry and Physics, 77th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1996. (32) Kanemitsu, Y.; Ogawa, T.; Shiraishi, K.; Takeda, K. Phys. ReV. B 1993, 48, 4883. (33) Bineva, I.; Nesheva, D.; Aneva, Z.; Levi, Z. J. Lumin. 2007, 126, 497. (34) Trojanek, F.; Neudert, K.; Bittner, M.; Maly, P. Phys. ReV. B 2005, 72, 075365. (35) When ∆l of the Si core is characterized by 2 nm, the momentum uncertainty ∆p of the core becomes smaller than that of the interface ∆l ≈ 0.5 nm. In this case, the enhancement of transition rate in the core results in smaller than that in the interface. To ensure the transition rates, we will measure lifetimes of each emission in the near future.

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