J. Phys. Chem. B 1998, 102, 7319-7322
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Semiconductor Nanoparticles in Contact: Quenching of the Photoluminescence from Silicon Nanocrystals by WO3 Nanoparticles Suspended in Solution Shoutian Li, Igor N. Germanenko, and M. Samy El-Shall* Department of Chemistry, Virginia Commonwealth UniVersity, Richmond, Virginia 23284-2006 ReceiVed: June 2, 1998
Weblike aggregates of Si nanocrystals and tungsten oxide nanoparticles are produced by a laser vaporization controlled condensation technique. XRD, FTIR, and Raman results confirm that the Si nanoparticles have a diamond-like crystalline structure and WO3 nanoparticles exhibit a monoclinic crystalline structure. Due to the quantum size effect, the band gaps of Si nanocrystals and WO3 nanoparticles shift to higher energies by 0.68 and 0.55 eV, respectively, from the bulk values. The red emission of Si nanocrystals is quenched by adding successive amounts of the WO3 nanoparticles to Si nanoparticles suspended in a methanol solution. The quenching follows Stern-Volmer kinetics and the quenching rate constants are 14.1 × 106 and 3.2 × 106 (Ms)-1 for WO3 and W2O5, respectively. The quenching mechanism is explained by an electron transfer from the CB of the Si to the CB of the WO3 nanoparticles.
Introduction Since the remarkable discovery of photoluminescence (PL) from porous Si, great interest has grown in the synthesis and properties of Si nanocrystals because they are believed to be the luminescent centers in porous Si.1 Research in this area is motivated by the possibility of designing Si-based nanostructured materials that possess novel electronic and optical properties that can be used in new devices for optical communication. In a recent study, we investigated the physical, optical, and PL properties of Si nanocrystals prepared by a laser vaporization controlled condensation (LVCC) technique.2 The weblike aggregates of coalesced Si nanocrystals, passivated by a SiOx barrier layer, exhibit red PL upon excitation with visible or UV light. This emission is characterized by a multiexponential decay having a long component that increases in lifetime (80130 µs at 300 K) and intensity with emission wavelength (630740 nm). Although many different mechanisms have been proposed to explain the visible PL from porous Si and Si nanocrystals,1,3 most of the PL properties of Si nanocrystals can be explained by the quantum confinement mechanism.1-3 According to an extended version of this mechanism, the larger band gap in Si nanocrystals is attributed to quantum-confined Si structures where recombination of electrons and holes occurs in a surface state.4 Therefore, a more fundamental understanding of the mechanism can be obtained by investigating the electron- and hole-transfer pathways accessible to Si nanocrystals. Recent studies have examined the PL quenching from porous Si by small solvent molecules and organic electron donors and acceptors.5,6 Both energy- and electron-transfer mechanisms have been proposed to explain the PL quenching of porous Si. The ultimate goal of such studies is to probe the absolute magnitude of valence-band (VB) and conduction-band (CB) shifts from the bulk value rather than probing only the magnitude of the band gap. No studies have been reported on the PL quenching from Si nanocrystals by other nanoparticles. In this letter, we report on the quenching of PL from Si nanocrystals by WO3 and W2O5 nanoparticles. The results
provide evidence for an electron-transfer pathway from the CB of the Si to the CB of the WO3 nanocrystals and support a model which involves surface states in quantum-confined Si nanocrystals for the PL mechanism. Experimental Section The Si and WO3 nanoparticles were prepared by the LVCC method which has been described in several publications.2,7 The method is based on coupling laser vaporization of metals with controlled condensation from the vapor phase, which eliminates the need for high temperatures and for chemical precursors and leads to good control over the particles size and aggregate state. A detailed description of the method and the reaction chamber can be found elsewhere;2,7 here, we offer only a brief description. The metal atomic vapor was generated by pulsed laser vaporization using the second harmonic (532 nm) of a Nd:YAG laser (15-30 mJ/pulse). The tungsten and Si targets, with a stated purity of 99.99%, were obtained from Aldrich and Dow Corning, respectively. For the synthesis of WO3, the chamber was filled with O2 at a pressure of 800 Torr, which acted as a carrier as well as a reactive gas. For the synthesis of Si nanocrystals, the chamber was filled with He (99.999%) at a pressure of 800 Torr. The temperature of the bottom plate was kept at room temperature, while the temperature of the top plate, where the nanoparticles were deposited, was kept between -80 and -90 °C. X-ray diffraction spectra were obtained on a Rigaku diffractrometer equipped with a Cu KR source. The Raman spectra were obtained using an Ar-ion laser (514.5 nm) for excitation coupled with a SPEX-1403, 0.85 m double-arm spectrometer. A steel cone with an inner diameter of 0.15 cm is used to mount the nanoparticle sample at ambient temperature. The resolution of the spectrometer is better than 1 cm-1 in the 100 to 1000 cm-1 range. FTIR spectra were obtained using a Perkin-Elmer 1600 FTIR in the range from 400 to 4000 cm-1 with 4 cm-1 resolution. The sample (about 0.2 mg) was pressed into KBr pellets using a Fisher IR-pellet maker. The UV-vis spectra of the nanoparticles ultrasonically suspended in methanol were obtained using a Shimadzu UV-265 spectrometer. For the
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7320 J. Phys. Chem. B, Vol. 102, No. 38, 1998
Letters
(a)
(b)
Figure 2. UV-vis absorption and dispersed luminescence spectra obtained with 355 nm pulsed laser excitation of the suspended Si nanocrystals.
(c) Figure 3. PL spectra of the suspended Si nanoparticles in methanol upon addition of successive aliquots of suspended WO3 in methanol.
Figure 1. (a) FTIR and (b) Raman spectra of the white form of WO3 nanoparticles before and after irradiation with the 532 nm laser beam. (c) Absorption spectra of the WO3 nanoparticles in methanol before and after the 355 nm illumination.
quenching measurements, colloid stock solutions of Si and WO3 nanoparticles in methanol with concentrations of 0.13 and 0.9 mg/mL, respectively, were used. The excitation wavelength was the third harmonic of a Nd:YAG laser (355 nm). To 1 mL of the Si stock solution in a quartz cuvette, successive amounts of WO3 stock solution were added. WO3 nanoparticles do not emit light upon excitation with 355 nm. Suspended Si nanoparticles in methanol emit red light, and the luminescence was dispersed by a SPEX 1 m spectrometer equipped with an EMI 9558 photomultiplier (s20 photocathode). An OG590 filter was used to block the laser light and the blue emission from the surface oxide layer of the Si nanoparticles. For dispersed luminescence spectra, the photomultiplier output was processed by a PAR model 162 boxcar averager and recorded by a computer. Timeresolved decays were averaged by a LeCroy 9350 oscilloscope and fit by a computer.
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J. Phys. Chem. B, Vol. 102, No. 38, 1998 7321
Figure 4. PL decays of Si nanocrystals at an emission wavelength of 660 nm as a function of the concentration of (a) white WO3 nanoparticles and (b) blue W2O5 nanoparticles.
Results and Discussion Tungsten oxide is known as a photochromic and electrochromic material since it changes color upon absorption of light and in response to an electrically induced change in oxidation state.8 The X-ray diffraction pattern of WO3 nanoparticles shows a monoclinic crystalline structure similar to the bulk material. The color of bulk WO3 is yellow, while the WO3 nanoparticles prepared by the LVCC method exhibit a white color. This white color changes to blue upon irradiation of the nanoparticles with the second harmonic of the Nd:YAG laser (532 nm) in air. We measured the IR and the Raman spectra of the particles before and after irradiation with the 532 nm laser beam, and the results are shown in Figure 1 (parts a and b). The spectrum measured after irradiation clearly shows all the features associated with the reduction of WO3 to W2O5. These features include the decrease in the IR absorption at 800 cm-1 and the disappearance of the Raman bands at 808 and 714 cm-1, which are the characteristic IR and Raman features of WO3. The photoreduction of the WO3 nanoparticles can also occur by irradiation of the suspended particles in methanol solution with the third harmonic of the Nd:YAG laser at 355 nm. The absorption spectra of the WO3 nanoparticles in methanol before and after the 355 nm illumination are shown in Figure 1c. The UV-vis absorption and the dispersed luminescence spectra obtained with 355 nm pulsed laser excitation of the suspended Si nanocrystals in methanol are shown in Figure 2. The suspended WO3 nanoparticles in methanol show no emission upon excitation with the 355 nm laser beam. Figure 3 shows a series of PL spectra of the suspended Si nanoparticles in methanol obtained following the addition of successive aliquots of the suspended WO3. A systematic decrease in the PL intensity with increasing WO3 concentration is observed. However, this decrease in the PL intensity may not be used as evidence of quenching because the WO3 nanoparticles absorb the 355 nm laser light. The time-resolved PL decay shows a clear decrease in the lifetime of the red emission from the Si nanocrystals with the addition of WO3. Figure 4a shows the PL decays of Si nanoparticles at an emission wavelength of 660 nm as a function of the concentration of WO3 nanoparticles.
It is interesting that the blue W2O5 nanoparticles in methanol can also quench the red emission from the Si nanocrystals, although to a lesser extent, as shown in Figure 4b. The lifetime is obtained by fitting the PL decays with a stretching exponential model according to9,10
I(t) ) I0
(τt )
1-β
β
( (τt ) )
exp -
(1)
This decay law is characteristic for systems in which the emitting centers with the decay time τ undergo quenching associated with random walks in the fractal space, and the scaling factor β is used to approximate the kinetics of the process.9,10 We have found that this function gives good fits to the experimental data within the entire spectral range. The quenching data follows the Stern-Volmer kinetics according to11
1/τ ) 1/τ0 + kq[Q]
(2)
where τ is the lifetime with quenchers and τ0 is the lifetime without quenchers. The Stern-Volmer plots for the white form WO3 and blue form W2O5 are shown in Figure 5. From the slopes of these plots, the quenching rate constant for the white WO3 is kq ) 14.1 × 106 (mol/L)-1 s-1 and for the blue form W2O5 is kq ) 3.2 × 106 (mol/L)-1 s-1. The quenching of the PL from the Si nanoparticles by WO3 nanoparticles can be understood by an electron-transfer process from Si to WO3. The electron transfer is only possible when the electrode potential of the CB of Si nanoparticles is above that of the CB of WO3 nanoparticles so that no energy barrier in this process exists. The CB position of bulk Si is -0.7 V against NHE.6 Due to the quantum-size effect, the band gap of our Si nanoparticles increases from 1.1 to 1.78 eV.2 Experiments supported the calculation that as the band gap opens, the shift in the VB should be twice as large as the shift in the CB.6,12 Thus, the CB position of our Si nanoparticles should be -0.93 V against NHE. The CB position of bulk WO3 is -0.17 V against NHE.13 The band gap of bulk WO3 is 2.8 eV.14 From the UV-vis absorption spectrum, we obtained a band gap for WO3 nanoparticles of 3.35 eV, which is larger by
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Letters Experiments designed to investigate this point are currently in progress in our laboratory. Acknowledgment. The authors gratefully acknowledge financial support from the NASA Microgravity Materials Science Program (Grant NAG8-1276). We thank Prof. Jim Terner (VCU) for the Raman measurements. References and Notes
Figure 5. Stern-Volmer plots for the white form WO3 and blue form W2O5.
SCHEME 1
0.55 eV than the bulk value. Then the CB position of the WO3 nanoparticle would be -0.35 V against NHE. The relative energy positions of the CB and VB of Si and WO3 nanoparticles are show in the Scheme 1. We note that experiments using a bulk powder of WO3 suspended in methanol did not show any quenching of the PL from the Si nanocrystals. This seems surprising since the CB of bulk WO3 lies below the CB of the WO3 nanoparticles. However, in the bulk powder, the average particle size is on the order of several microns, which leads to poor contact and inefficient adsorption on the surface of the Si nanocrystals suspended in the methanol solution. It is important to note that the quenching ability of W2O5 nanoparticles is significantly lower than that of WO3. This is consistent with the photoreduction of WO3 into W2O5 under UV irradiation. One interesting possibility could arise if the electron transfer from the CB of Si to the CB of WO3 can enhance the reduction of WO3. In this case, the Si nanocrystals would act as a photocatalyst for the reduction of WO3.
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