J. Phys. Chem. B 2001, 105, 59-66
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Decay Dynamics and Quenching of Photoluminescence from Silicon Nanocrystals by Aromatic Nitro Compounds Igor N. Germanenko, Shoutian Li,† and M. Samy El-Shall* Department of Chemistry, Virginia Commonwealth UniVersity, Richmond, Virginia 23284-2006 ReceiVed: June 30, 2000; In Final Form: October 22, 2000
The decay dynamics and the quenching of the photoluminescence (PL) from Si nanocrystals are investigated. Electron acceptors whose reduction potentials lie below the conduction band (CB) edge of the Si nanocrystals quench the red emission from the Si nanocrystals. The quenching rate constants obtained from Stern-Volmer analyses for 3,5-dinitrobenzonitrile, 4-nitrophthalonitrile, 1,4-dinitrobenzene, 4-nitrobenzonitrile, 2,3-dinitrotoluene, 3,4-dinitrotoluene, 2,4-dinitrotoluene, and 2,6-dinitrotoluene are in the range of 106-107 M-1 s-1. The quenching mechanism occurs via an electron transfer from the CB band of the Si nanocrystals to the vacant orbitals of the quenchers. The PL decay profiles of the Si nanocrystals, in the presence and absence of the quencher, are well described by the stretched exponential decay law. The band gap of the Si nanocrystals estimated from the present study is larger than the PL peak energy. The results are consistent with a quantumconfinement model, where recombination of electrons and holes occurs in a surface state. The ability of nitrotoluenes to quench the PL from Si nanocrystals could be used to develop a sensor based on Si nanostructures for the detection of explosives.
Introduction Silicon nanostructures have stimulated much interest because of their unique properties such as single-electron tunneling, nonlinear optical properties, and visible photoluminescence (PL).1-7 Studies of Si nanostructures have grown extensively since the discovery of efficient PL from porous Si (p-Si) with the ultimate goals of achieving a complete fundamental understanding of the phenomenon as well as developing potential display devices and chemical-sensor applications.1-10 Si nanocrystals have optical and PL properties very similar to those of p-Si, and it is generally accepted that they are the emitting chromophores in p-Si. Kinetic quantum confinement appears to be the most reliable model to explain the origin and properties of the PL from Si nanocrystals. The higher energy shift in the PL of Si nanocrystals is attributed to the three-dimensional quantum size effect. The nonradiative recombination process is decreased significantly in the nanocrystal as the electronhole pairs in separate nanocrystals are electrically isolated. However, in both p-Si and Si nanocrystals, the PL properties are related not only to the respective nanocrystalline size but also to the structure and properties of the surrounding medium. The study of these effects is essential for utilizing Si nanostructures in potential device applications. Many organic and inorganic molecules have been shown to efficiently quench the PL from p-Si, and both energy- and electron-transfer mechanisms have been proposed to explain this PL quenching.11-30 Sailor and co-workers found a number of polar solvent molecules that reversibly quenched the PL from p-Si.11-15 They also studied the quenching by aromatic molecules and concluded that energy transfer from the p-Si excited state to the triplet levels of the quencher molecules predominantly drives the quenching process.15 Bocarsly and co-workers found that Brønsted bases such as Na2CO3 and NaOH reversibly quench the PL from p-Si and that the PL is restored by * To whom correspondence should be addressed. † Current address: Ethyl Petroleum Additives, Inc., Richmond, VA 23218-2158.
subsequent exposure to Brønsted acids.16-18 They explained the quenching process in terms of a proton-transfer abstraction mechanism. Coffer and co-workers investigated both steric and electronic effects in the PL quenching of p-Si by Lewis bases such as alkylamines and compared the PL quenching from p-Si and Si nanocrystals.19-21 They found an appreciable difference in the behavior of monodentate and bidentate quenchers in quenching the PL from Si nanocrystals, which was not the case for p-Si.21 Fauchet and co-workers investigated the electronand hole-transfer quenching channels using several conductionband (CB) and valence-band (VB) quenchers.22 They provided evidence for a larger band gap than that of the PL peak energy in p-Si and suggested that the recombination of electrons and holes occurs in a surface state. Other studies have reported the PL quenching from p-Si by metal ions in solutions of Cu2+, Ag+, and Au3+;23,24 surfactants;25and polycyclic organic and dye molecules.26-28 In relation to the quenching studies, reduction of the PL intensity from p-Si upon exposure to methanol and quenching of the H-terminated p-Si have been reported.29,30 Brus applied an electron-kinetics model to study the PL in wet and dry p-Si.31 This model suggested that carrier relaxation rates in Si nanostructures would be faster in polar environments. With the exception of the study of Coffer and co-workers,21 who studied the PL quenching from Si nanocrystals extracted from p-Si, all of the quenching studies have focused on p-Si. Although significant progress has been made in optimizing the preparation conditions of p-Si, it still remains a very complex system to characterize, and several factors could interfere and contribute to the different origins of the PL quenching. A more fundamental understanding of the mechanism can be obtained by investigating the electron- and hole-transfer pathways accessible to Si nanocrystals. In this paper, we study the PL quenching of Si nanocrystals by several aromatic nitro compounds. The results provide evidence for an electron-transfer pathway from the CB of the Si to the vacant orbitals of the quenchers and support a PL model, which involves surface states in quantum-confined Si nanocrystals.
10.1021/jp002340c CCC: $20.00 © 2001 American Chemical Society Published on Web 12/05/2000
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Figure 1. (a) TEM, (b) HRTEM images, and (c) electron-diffraction pattern from Si nanocrystals.
The organization of the paper is as follows: First, we provide a brief summary of the characterization of the Si nanocrystals used in this study. Second, we study the decay dynamics of the PL from the Si nanocrystals and compare different models to describe the time-resolved PL data. Next, we present the PL quenching data and discuss the results in terms of an electrontransfer mechanism. Finally, we conclude this paper with a brief summary of the results and an outlook of future experiments. Experimental Section The Si nanoparticles were prepared by the laser vaporization with controlled condensation (LVCC) method, which has been described in several publications.36,38,39 The method is based on coupling the laser vaporization of semiconductors or metals with controlled condensation from the vapor. Here, we offer only a brief description. The silicon atomic vapor was generated by pulsed laser vaporization using the second harmonic (532 nm) of a Nd:YAG laser (15-30 mJ/pulse). The Si target, with a stated purity of 99.99%, was obtained from Dow Corning. 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, whereas the temperature of the top plate, where the nanoparticles were deposited, was kept between -80 and -90 °C. For the quenching measurements, stock solutions of suspended Si nanocrystals in methanol and the quencher in methanol with concentrations of 0.9 mg/mL and 0.042 M, 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 were added successive amounts of the quencher stock solution. 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. Results and Discussion In a recent study, we investigated the physical, optical, and PL properties of Si nanocrystals prepared by the LVCC technique.37-39 Detailed characterizations showed that our Si nanocrystals have a 1-2 nm surface oxide layer over a crystalline Si core, whose lattice constant is similar to that of bulk crystalline Si. A size-distribution analysis of over 500
particles showed an average particle size of 5-6 nm, with a relatively broad size distribution.38,39 Selected-area electrondiffraction patterns of the Si nanocrystals confirm the expected lattice spacings for Si in the diamond cubic phase. Figure 1 shows typical transmission electron microscopy (TEM) images of the Si nanocrystals and the associated electron-diffraction pattern. The PL quantum yield is about 4% at room temperature, with lifetimes of 20-70 µs, depending on the emission energy. Dispersed PL spectra obtained with 355 nm pulsed laser excitations are shown in Figure 2. The spectra differ by the position of the boxcar gate, which ranges from a 0.25 µs delay with respect to the laser excitation pulse to a 8 µs delay. The short delay spectrum enhances the blue emission component (arising from the surface oxide layer of the Si nanoparticles),33.36 because the lifetime associated with it is short (less than 20 ns) compared to the lifetimes of the red emission. The PL decays of the red emission are multiexponential, and the PL lifetimes are emission wavelength dependent, increasing from a short emission wavelength to a long emission wavelength. To establish a consistent model to analyze the timeresolved PL quenching data, we measured the decay profiles for the Si nanocrystals suspended in methanol. The decay profiles were analyzed using three different fitting models as summarized below. (a) Biexponential Function.
I(t) ) I1 exp(-t/τ1) + I2 exp(-t/τ2)
(1)
This is a simple form to fit the data, but it does not describe a specific physical process. It simply treats the decay as a sum of two independent processes, each having its own characteristic lifetime τ1 and τ2.36 (b) Micellar Kinetics Model.40
I(t) ) I0 exp{-t/τ - 〈n〉[1 - exp(-ktrt)]}
(2)
This model has been used to describe the PL decay of aged p-Si samples.41-43 The model implies that two processes are involved: radiative decay with a rate constant of 1/τ (τ is the radiative lifetime) and nonradiative deactivation by quenching centers (traps) with a quenching probability of ktr. The traps are assumed to obey a Poissonian distribution where the average number is 〈n〉. The total probability of the deactivation is then given by 1/τ + 〈n〉ktr. (c) Stretched Exponential Function.44
I(t) ) I0(τ/t)1-β exp[-(t/τ)β]
(3)
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TABLE 1: Fitting Parameters for the PL Decay Profiles from Si Nanocrystals Suspended in Methanol stretched exponential
biexponential
micellar
λem, nm
τ, µs
χ2
β
τ2, µs
χ2
τ1, µs
τ, µs
χ2
ktr
〈n〉
590 600 610 620 630 640 650 660 670 680 690 700 710 720
18.7 22.1 26.6 24.4 29.2 35.9 36.4 38.3 44.5 46.9 51.1 56.7 62.7 65.7
0.263 0.238 0.182 0.163 0.134 0.100 0.086 0.083 0.073 0.073 0.085 0.089 0.107 0.140
0.500 0.562 0.601 0.658 0.677 0.692 0.710 0.725 0.733 0.740 0.753 0.752 0.753 0.760
24.4 29.0 33.6 31.5 35.5 40.2 39.9 41.7 46.2 47.7 50.1 53.9 56.6 58.2
0.467 0.488 0.481 0.454 0.454 0.430 0.387 0.424 0.395 0.397 0.421 0.433 0.459 0.453
0.78 2.06 3.16 3.32 3.91 4.30 4.13 4.54 4.96 5.05 5.08 5.40 5.24 5.26
25.3 29.3 34.3 32.1 36.3 41.1 40.8 42.5 47.2 48.7 51.0 55.1 57.8 59.3
0.370 0.402 0.401 0.379 0.378 0.353 0.313 0.346 0.322 0.322 0.347 0.354 0.381 0.383
0.68 0.33 0.21 0.20 0.17 0.15 0.16 0.15 0.14 0.14 0.14 0.13 0.13 0.14
2.94 1.98 1.65 1.46 1.35 1.26 1.19 1.12 1.07 1.04 0.98 0.98 0.95 0.92
TABLE 2: PL Lifetimes from Si Nanocrystaline Powder Obtained from Stretched Exponential Fits to the Decay Profiles at 92 and 300 K τ, µs
Figure 2. Dispersed emission from Si nanocrystals (solid) at different gate delays following the 355 nm pulsed laser excitation.
where τ is an effective decay time, β is a dispersion or a distribution factor between 0 and 1 (the lower the value of β, the broader the lifetime distribution), and I0 is the amplitude at t ) 0. This is a well-known function describing “long-tailed” relaxations in many disordered systems controlled by random walks of reacting components in space with the dimensionality equal to 2β.45-47 In the application of this decay law to p-Si and Si nanocrystals, it is considered that because of the different nanocrystal sizes the quantum-confined levels are spread in energy, leading to the formation of a band of localized states. Carrier diffusion among different sites can be due to the excitation of carriers from localized to extended states or to hopping among localized states.46 The slow and fast luminescence decays are due to the recombination of electron-hole pairs generated from different or the same spatial regions, respectively. We have found that the stretched exponential function gives the best fits to the experimental data across the entire spectral range. Figure 3a shows a comparison of the three functions used to fit the PL decays monitored at an emission wavelength of 640 nm, and Figure 3b shows the stretched exponential fits to the decay profiles across the entire spectral range. The full lines through the experimental data are least-squares fits using the models discussed above. Table 1 lists the calculated lifetimes using the three functions at different emission wavelengths, along with the statistical deviations χ2. The effective decay time τ depends on the temperature. Figure 4 compares the PL decay profiles at 640 nm and at 92 and 300
λem, nm
T ) 92 K
T ) 300 K
575 600 625 650 675 700
163 187 193 204 249 264
16 28 36 40 42 50
K of the Si nanocrystals solid sample. The stretched exponential function still provides a good fit for the data even at the low temperature of 92 K. The PL lifetime of the Si nanocrystals increases significantly at lower temperatures. The lifetimes calculated from these decay curves at 92 and 300 K are 204 and 40 µs, respectively (see Table 2). It is important to indicate that the temperature dependence of τ in Si nanocrystals is very similar to that in p-Si.48-50 This again indicates that the decay dynamics of the red PL is not sensitive to the nature of the oxide- or hydrogen-terminated surface passivation layer. The dependence of the PL intensity (monitored at 640 nm) on the temperature is shown in Figure 4b. The PL intensity increases with a lowering of the temperature down to about 150 K and then decreases at lower temperatures. This behavior is very similar to that reported by Kanemitsu for dry particles48,49 and somewhat different from the behavior of colloidal suspension particles reported by Brus and co-workers,4 where the PL intensity increased monotonically down to 50 K and stayed essentially constant below 50 K. At higher temperatures, the thermally activated nonradiative recombination results in a decrease of the PL intensity, and at very low temperatures, the nonradiative recombination can be enhanced by tunneling.50 Figures 5-8 display the steady-state and the time-resolved PL spectra of the Si nanocrystals upon the addition of successive concentrations of the quencher for some of the compounds used in this study. The data shown in Figures 5-8 indicate a systematic loss of PL intensity with an increase in the quencher concentration. However, this decrease in the PL intensity may not be used as evidence of quenching because the quenchers may absorb the 355 nm laser light. Therefore, we use the timeresolved data to determine the decrease in the PL lifetime as a function of the concentration of the quencher. This decrease in the lifetime is taken as evidence that PL quenching has occurred. The time-resolved PL decays show a clear decrease in the lifetime of the red emission with the addition of the quencher, as shown in Figures 5b, 6b, 7b, and 8b. The stretched exponential function (3) was used to fit the quenching decay profiles shown in Figures 5b, 6b, 7b, and 8b.
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Figure 3. (a) Comparison of the PL decay fittings using biexponential, micellar kinetics, and stretched exponential functions. (b) PL decay profiles at different emission wavelengths fitted using the stretched exponential function.
Figure 4. (a) Comparison of PL decay curves (640 nm) at 92 and 300 K. (b) Temperature dependence of the PL intensity at 640 nm.
The quenching data follows Stern-Volmer kinetics51 according to
1/τ ) 1/τ0 + kq[Q]
(4)
where τ is the effective lifetime with quenchers and τ0 is the lifetime without quenchers. The Stern-Volmer plots for several quenchers used in this study are shown in Figure 9a,b. From the slopes of these plots, the effective quenching rate constants are calculated, and the results are shown in Table 3. We also
include in Table 3 the other compounds used in this study that showed no evidence for PL quenching from the Si nanocrystals suspended in methanol. The quenching of the PL from the Si nanoparticles by the aromatic nitro compounds can be understood as an electrontransfer process from Si to the quencher. If electron transfer occurs from the CB of the Si nanocrystals to the vacant orbitals of the quencher molecule, the PL intensity and the lifetime will decrease with an increase in the concentration of the quencher. If the quencher reduction-potential level lies below the CB edge
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Figure 5. (a) Steady-state and (b) time-resolved PL spectra of Si nanocrystals suspended in methanol upon the addition of successive aliquots of 3,5-dinitrobenzonitrile in methanol.
Figure 6. (a) Steady-state and (b) time-resolved PL spectra of Si nanocrystals suspended in methanol upon the addition of successive aliquots of 4-nitrophthalonitrile in methanol.
Figure 7. (a) Steady-state and (b) time-resolved PL spectra of Si nanocrystals suspended in methanol upon the addition of successive aliquots of 1,4-dinitrobenzene in methanol.
of the Si nanocrystals, then no barrier for electron transfer from the Si to the quencher exists and oxidation of the Si occurs. The most efficient quenchers among the molecules listed in Table 3 are 3,5-dinitrobenzonitrile, 4-nitrophthalonitrile, 1,4dinitrobenzene, and 4-nitrobenzonitrile with effective quenching rate constants of 2.3 × 107, 9.3 × 106, 7.7 × 106, and 3.0 × 106 M-1 s-1, respectively. The reduction potentials of 3,5dinitrobenzonitrile, 1,4-dinitrobenzene, and 4-nitrobenzonitrile (vs a normal hydrogen electrode (NHE) in acetonitrile) are -0.64, -0.80, and -1.00 V, respectively.22 It is clear that the quenching rate constant correlates with the reduction potential: 3,5-dinitrobenzonitrile, with a potential of -0.64 V, decreases
the PL lifetime significantly, whereas 1,4-dinitrobenzene, with a potential of -1.00 V, has a relatively smaller effect on the PL lifetime. It is also interesting to note that 4-nitrotoluene, with a redox potential of -1.19 V, has almost no effect on the PL lifetime, and no quenching effect was detected, as shown in Figure 10a. These results suggest that the CB of our Si nanocrystals is between -1.00 and -1.19 V (vs NHE in acetonitrile). If we use the electrode potentials for bulk crystalline silicon reported by Fauchet22 (ECB ) -0.70 V and EVB ) 0.40 V vs NHE), then we can estimate the shift in the CB of our Si nanocrystals because of the quantum size effect as 0.4 V. Experiments and calculations suggest that, as the band
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Figure 8. (a) Steady-state and (b) time-resolved PL spectra of Si nanocrystals suspended in methanol upon the addition of successive aliquots of 4-nitrobenzonitrile in methanol.
Figure 9. Stern-Volmer plots of the PL quenching from Si nanocrystals by aromatic nitro compounds.
TABLE 3: Effective Quenching Rate Constants Derived from the Stern-Volmer Analyses for the Series of Molecules Shown To Quench the PL from Si Nanocrystals and a List of the Molecules Showing No Quenching in the Present Study (A) Quenching Effect molecule
kq, M-1 s-1
3,5-dinitrobenzonitrile 4-nitrophthalonitrile 1,4-dinitrobenzene 4-nitrobenzonitrile 2,3-dinitrotoluene 3,4-dinitrotoluene 2,4-dinitrotoluene 2,6-dinitrotoluene
22.95 × 106 9.25 × 106 7.65 × 106 3.03 × 106 1.31 × 106 1.29 × 106 1.27 × 106 0.46 × 106
(B) No Quenching Effect nitrobenzene benzonitrile 4-nitrotoluenea 1,2,4-trimethoxybenzene benzeneb,c toluene c 1,3,5-trimethylbenzene p-xyleneb styrene hexamethylbenzeneb a This molecule showed quenching of p-Si (ref 22). b These molecules showed no quenching of p-Si (ref 22). c These molecules showed quenching of p-Si (ref 11).
gap opens, the shift in the VB should be twice as large as that of the CB.52-55 This means that the shift in the VB of our Si nanocrystals should be 0.8 V. Thus, the location of the VB of
our Si nanocrystals is near 1.2 V (vs a NHE). With the CB at -1.1 V, the band gap of our Si nanocrystals is estimated as 2.3 eV. We note that the values of the electrode potentials used in our estimate were measured in acetonitrile; however, methanol was the solvent used in our quenching experiments. Therefore, a small correction due to the solvation difference between methanol and acetonitrile could effectively increase the band gap, and for this reason, our estimate of 2.3 eV for the band gap of Si nanocrystals is a lower bound value. The data shown in Table 3 agrees well with the PL quenching study from p-Si reported by Fauchet and co-workers.22 For example, our results indicate that 1,4-dinitrobenzene quenches the PL from the Si nanocrystals more effectively than 4-nitrobenzonitrile. The same trend was observed in the p-Si study.22 Also, in our work, p-xylene and hexamethylbenzene with oxidation potentials of 2.20 and 1.80 V, respectively, did not show VB quenching from the Si nanocrystals, which is in full agreement with the p-Si study.22 Furthermore, 4-nitrotoluene and 1,2,4-trimethoxybenzene (with electrode potentials of -1.19 and 1.22 V, respectively, vs NHE in acetonitrile) did not show PL quenching from the Si nanocrystals but showed very little quenching of the PL from p-Si. In fact, a drop-off in the quenching ability of the adsorbate was observed when the redox potential reached a value of -1.2 ( 0.2 V for the CB quenchers and 1.4 ( 0.2 V for the VB quenchers of p-Si. We note that both benzene and toluene did not quench the PL from the Si nanocrystals, which is in agreement with Fauchet’s study of p-Si.22 Figure 10b shows that the addition of benzene to the Si nanocrystals suspended in methanol has no quenching effect on the PL lifetime, as shown from the decay profile measured
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J. Phys. Chem. B, Vol. 105, No. 1, 2001 65
Figure 10. PL decays from Si nanocrystals suspended in methanol as a function of the concentrations of (a) 4-nitrotoluene and (b) benzene showing no quenching effect in both cases.
Figure 11. (a) Plots of ln I0/I vs quencher concentration, [Q], according to the Perrin model. (b) Correlation between the effective quenching rate constants (kq) obtained from the Stern-Volmer model and the radius of the effective quenching sphere (R) obtained from the Perrin model.
at 640 nm. However, this result differs from the p-Si quenching study by Sailor and co-workers, who reported PL quenching by o-xylene, toluene, and benzene.11 The difference between the two p-Si quenching studies could be a result of different sample preparations.11,22 To check the consistency of the relative quenching efficiency of the studied quenchers, we considered the application of the Perrin model to our data.51,56 This model assumes that an excited state is instantaneously deactivated without emitting light if a quencher molecule is within an effective “quenching sphere” of radius R, whereas an acceptor molecule located outside this sphere has no influence on the emission from the Si nanocrystal. The Perrin model can be expressed as
ln(I/I0) ) VN[Q]
(5)
where I and I0 are the emission intensity in the absence and presence of the quencher, respectively, V is the volume of the active sphere of quenching, and N is Avogadro’s number. It should be noted that, according to the Perrin model, the PL lifetime is independent of the presence of the quencher. This is obviously not the case in the PL quenching from Si nanocrystals, as illustrated by the data and the Stern-Volmer analysis. However, the application of the Perrin model could be useful in obtaining an estimate of the effective quenching volume around the Si nanocrystals. The plots of ln(I/I0) vs [Q] are shown in Figure 11a for three quenchers. From the slopes of these lines, we find that the radii R of the effective quenching sphere for 3,5-dinitrobenzonitrile, 4-nitrophthalonitrile, and 1,4-dinitrobenzene are 60, 54, and 48 Å, respectively. This trend indicates
that the quenching efficiency of these molecules varies in the order of 3,5-dinitrobenzonitrile > 4-nitrophthalonitrile > 1,4dinitrobenzene. It is interesting to note that this order correlates well with the effective quenching rate constants calculated from the Stern-Volmer model. This correlation, shown in Figure 11b, supports the application of the Stern-Volmer model to obtain the relative efficiency of the studied molecules in quenching the PL from Si nanocrystals. The band-gap energy (2.3 eV) of the Si nanocrystals as estimated from the present quenching study is larger than the PL peak energy observed experimentally (ca. 1.9 eV; see Figure 2). This is also the same trend observed by Fauchet and coworkers, where the sum of the VB and CB shifts of p-Si (g2.6 eV) is larger than the measured peak PL (ca. 2.0 eV).22 These results are also in agreement with the work of Kux and Chorin, who used PL excitation spectroscopy to show that the average band gap of p-Si lies 0.2 eV above the luminescence line.57 Similar results have been reported by Van Buuren and coworkers.58 Therefore, the result that the band gap in Si nanocrystals is larger than the PL peak energy is consistent with the same trend observed in p-Si. This has important implications in the mechanism of PL from Si nanocrystals. It suggests that some relaxation mechanism is operating between absorption and emission, and during this relaxation, a significant energy loss (≈0.4 eV) occurs. This interpretation supports a model which includes surface states in quantum-confined Si nanocrystals.59,60 According to this model, absorption takes place in confined Si nanocrystals, where quantum confinement is responsible for widening the band gap. Following the absorption, a relaxation process to localized surface states within the nanocrystals takes
66 J. Phys. Chem. B, Vol. 105, No. 1, 2001 place where the electron-hole pair is generated. The luminescence then results from radiative recombination of the trapped carriers. This mechanism provides a natural explanation for the shift between the band gap and the PL peak energy. Conclusions and Outlook In this paper, we demonstrated that electron acceptors whose reduction potentials lie below the CB edge of the Si nanocrystals quench the red emission from the Si nanocrystals. The quenching rate constants obtained from Stern-Volmer analyses are in the range of 106-107 M-1 s-1. The quenching mechanism occurs via an electron transfer from the CB band of the Si nanocrystals to the vacant orbitals of the quenchers. The PL decay profiles of the Si nanocrystals, in the presence and absence of the quencher, are well-described by the stretched exponential decay law. The band gap of the Si nanocrystals estimated from the present study is larger than the PL peak energy. The results are consistent with a quantum-confinement model, where a recombination of electrons and holes occurs in a surface state. Finally, we comment on two opportunities created by the results of this work. First, the ability of nitrotoluenes to quench the PL from Si nanocrystals could be used to develop a sensor based on Si nanostructures for the detection of explosives particularly in mines. Several applications based on p-Si have been described,12,13,17 with the most recent application being the detection of chemical warfare agents.61 Second, it would be interesting to use ultrafast spectroscopy to study the electrontransfer process and the formation of the radical anions of the quencher molecules following the excitation of the Si nanocrystals in the presence of the quencher. This might provide a more fundamental understanding of the quenching mechanism. Acknowledgment. We thank Professor Jin Z. Zhang (University of California at Santa Cruz) for the HRTEM of the Si nanocrystals and Dr. Eugene P. Petrov (Academy of Sciences of Belarus) for helpful discussions. Financial support from the NASA Microgravity Materials Science Program (Grant NAG81484) and the National Science Foundation (Grant CHE 9816536) is gratefully acknowledged. References and Notes (1) Canham, L. T. Appl. Phys. Lett. 1990, 57, 1046. (2) Cullis, A. G.; Canham, L. T. Nature 1991, 353, 335. (3) Brus, L. E. Nature 1991, 353, 301. (4) Wilson, W. L.; Szajowski, P. F.; Brus, L. E. Science 1993, 262, 1242. (5) Collins, R. T.; Fauchet, P. M.; Tischler, M. A. Phys. Today 1997, 50, 24-31. (6) Vijayalakshmi, S.; Shen, F.; Grebel, H. Appl. Phys. Lett. 1997, 71, 3332. (7) Choi, B. H.; Hwong, S. W.; Kim, I. G.; Shin, H. C.; Kim, Y.; Kim, E. K. Appl. Phys. Lett. 1998, 73, 3129. (8) Dowd, A.; Elliman, R. G.; Samoc, M.; Luther-Davies, B. Appl. Phys. Lett. 1999, 74, 239. (9) Brus, L. J.; Szajowski, P. F.; Wilson, W. L.; Harris, T. D.; Schuppler, S.; Citrin, P. H. J. Am. Chem. Soc. 1995, 117, 2915. (10) Prokes, S. M. In Nanomaterials: Synthesis, Properties and Applications; Edelstein, A. S., Cammarata, R. C., Eds.; Institute of Physics Publishing: Bristol, U.K., 1996; Chapter 17, pp 439-457. (11) Lauerhaas, J. M.; Grace, M. C.; Heinrich, J. L.; Sailor, M. J. J. Am. Chem. Soc. 1992, 114, 1911-1912. (12) Harper, J.; Sailor, M. J. Anal. Chem. 1996, 68, 3713-3717. (13) Lauerhaas, J. M.; Sailor, M. J. Science 1993, 261, 1567-1568. (14) Fisher, D. L.; Harper, J.; Sailor, M. J. J. Am. Chem. Soc. 1995, 117, 7846-7847. (15) Song, J. H.; Sailor, M. J. J. Am. Chem. Soc. 1997, 119, 7381-7385. (16) Chun, J. K. M.; Bocarsly, A. B.; Cottrell, T. R.; Benziger, J. B.; Yee, J. C. J. Am. Chem. Soc. 1993, 115, 3024-3025. (17) Kelly, M. T.; Chun, J. K. M.; Bocarsly, A. B. Nature 1996, 382, 214-215. (18) Kelly, M. T.; Chun, J. K. M.; Bocarsly, A. B. J. Phys. Chem. B 1997, 101, 2702-2708.
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