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Spectral Properties of AOT-Protected CdS Nanoparticles: Quantum Yield Enhancement by Photolysis Barbara A. Harruff and Christopher E. Bunker* Air Force Research Laboratory, Propulsion Directorate, Wright-Patterson Air Force Base, Ohio 45433 Received August 27, 2002. In Final Form: November 18, 2002 Absorption and steady-state emission spectra and time-resolved luminescence decays have been recorded for CdS nanoparticles prepared using the reverse-micelle method. Spectra were collected in hexane solution, with the surfactant AOT as stabilizer. The emission spectra of these particles are characteristic of trapstate emissions, being broad and red-shifted from the absorption-edge onset. Quantum yields for CdS particles prepared with w0’s of 3-7 (w0 ) [H2O]/[AOT]) are low (∼1-4%) and are not size dependent. CdS nanoparticles with w0 ) 4 were irradiated at the maximum of the first absorption peak. Irradiation resulted in a red shift and overall decrease in intensity of the absorption spectra, a red shift and increase in intensity of the emission spectra, an increase in the apparent lifetimes, and an increase in the quantum efficiency. Final quantum yields range from ∼9 to 13%.
Introduction Nanoparticles or quantum dots have received significant attention from the scientific community because of their unusual optical and electronic properties.1-6 Of the many materials investigated, CdS can be considered a model compound.7-11 CdS nanoparticles have been prepared using a number of methods.12-18 The preparation method can play an important role in determining the chemical, photophysical, and electronic properties of the nanoparticles through its ability to control particle size, size distribution, and the chemistry of the defect sites that form at the particle surface. The basic photophysical properties of colloidal CdS are well-understood.19-23 Absorption of a photon results in the formation of an electron-hole pair. The presence of defect sites that are (1) Henglein, A. Chem. Rev. 1989, 89, 1861. (2) Steigerwald, M. L.; Brus, L. E. Acc. Chem. Res. 1990, 23, 183. (3) Nirmal, M.; Brus, L. E. Acc. Chem. Res. 1999, 32, 407. (4) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226. (5) Sun, Y.-P., Ed.; Supercritical Fluid Technology in Materials Science and Engineering; Marcel Dekker: New York, 2002. (6) Murphy, C. J.; Coffer, J. L. Appl. Spectrosc. 2002, 56, 16. (7) Klimov, V. I.; Mikhailovasky, A. A.; Xu, S.; Malko, A.; Hollingsworth, J. A.; Leatherdale, C. A.; Eisler, H.-J.; Bawendi, M. G. Science 2000, 290, 314. (8) Tittel, J.; Go¨hde, W.; Koberling, F.; Basche´, Th.; Kornowski, A.; Weller, H.; Eychmu¨ller, A. J. Phys. Chem. B 1997, 101, 3013. (9) Lemon, B. I.; Crooks, R. M. J. Am. Chem. Soc. 2000, 122, 12886. (10) Vogel, W.; Urban, J. Langmuir 1997, 13, 827. (11) Tian, Y.; Fendler, J. H. Chem. Mater. 1996, 8, 969. (12) Kuczynski, J. P.; Milosavljevic, B. H.; Thomas, J. K. J. Phys. Chem. 1983 87, 3368. (b) Kuczynski, J. P.; Milosavljevic, B. H.; Thomas, J. K. J. Phys. Chem. 1984, 88, 980. (c) Lianos, P.; Thomas, J. K. Chem. Phys. Lett. 1986, 125, 299. (13) Tricot, Y.-M.; Fendler, J. H. J. Phys. Chem. 1986, 90, 3369. (14) Swayambunathan, V.; Hayes, D.; Schmidt, K. H.; Liao, Y. X.; Meisel, D. J. Am. Chem. Soc. 1990, 112, 3831. (15) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (16) Sun, Y.-P.; Rollins, H. W. Chem. Phys. Lett. 1998, 288, 585. (17) Gorer, S.; Penner, R. M. J. Phys. Chem. B 1999, 103, 5750. (18) Ohde, H.; Ohde, M.; Bailey, F.; Kim, H.; Wai, C. M. Nano Lett. 2002, 2, 721. (19) Rossetti, R.; Brus, L. J. Phys. Chem. 1982, 86, 4470. (b) Brus, L. J. Phys. Chem. 1986, 90, 2555. (c) Chestnoy, N.; Harris, T. D.; Hull, R.; Brus, L. E. J. Phys. Chem. 1986, 90, 3393. (20) Ramsden, J. J.; Webber, S. E.; Gra¨tzel, M. J. Phys. Chem. 1985, 89, 2740. (21) Dannhauser, T.; O’Neil, M.; Johansson, K.; Whitten, D.; McLendon, G. J. Phys. Chem. 1986, 90, 6074. (b) O’Neil, M.; Marohn, J.; McLendon, G. J. Phys. Chem. 1990, 94, 4356.
primarily associated with the surface of the particles and, thus, are of increasing importance as the particle size decreases and the surface-to-volume ratio increasess results in rapid (on the order of several tens of picoseconds)23 trapping of either the electron or the hole, which localizes the charge at surface-trap or deep-trap sites. Luminescence can be observed from either recombination of the electron-hole pair (excitonic emission) or radiative relaxation of the trap states. Excitonic emission is characterized by narrow, near-absorption edge luminescence, while trap-state emission is characterized by broad, highly red-shifted luminescence that sometimes exhibits multiple bands. CdS nanoparticles that are prepared with a presumed high density of defect sites generally possess low quantum efficiencies (i.e., on the order of 1-2% or less). Highly luminescent CdS nanoparticles have been obtained through chemical passivation.3,4,21,22a,24-27 Passivation prevents defect sites from competing with the radiative recombination of the exciton, thus enhancing the yield of the excitonic emission. Because the defect sites are blocked, the trap-state emissions decrease. The result is generally an increase in the photoluminescence yield, with a blue shift in the spectral profile. In this paper, we report the ability to increase the photoluminescence yield of CdS nanoparticles whose spectral profiles are those of trap-state emissions. The process does not involve chemical passivation but employs photoirradiation to induce the changes. The yields are increased by an order of magnitude, producing particles with ΦL values of ∼12%. The increase in luminescence efficiency is also accompanied by red shifts in the absorption and emission spectral profiles, a decrease in the absorption intensity, and an increase in the apparent lifetimes of the particles. The irradiation process has demonstrated a high degree of repeatability, offering the ability to investigate radiative and nonradiative trap-state processes with samples that can be easily obtained and are reproducible. (22) Spanhel, L.; Haase, M.; Weller, H.; Henglein, A. J. Am. Chem. Soc. 1987, 109, 5649. (b) Weller, H. Philos. Trans. R. Soc. London A 1996, 354, 757. (23) Logunov, S.; Green, T.; Marguet, S.; El-Sayed, M. A. J. Phys. Chem. A 1998, 102, 5652. (b) Burda, C.; El-Sayed, M. A. Pure Appl. Chem. 2000, 72, 165.
10.1021/la026483x CCC: $25.00 © 2003 American Chemical Society Published on Web 01/07/2003
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Experimental Section Materials. Cd(NO3), Na2S, dioctyl sulfosuccinate sodium salt (AOT), and 9-cyanoanthracene (97%) were purchased from Aldrich and used as received. Hexane and heptane (Optima grade) were obtained from Fisher Scientific, anhydrous ammonia was obtained from Matheson Gas Products, and deionized water was produced using a Barnstead NANOpure II water-filtration system. CdS nanoparticles were produced using the reverse-micelle method, with procedures being similar to those described in the literature.12c,28 In a typical experiment two separate solutions of CdNO3 and Na2S (both at 0.2 M) are prepared in 1 mL of water. These solutions are then employed to prepare two additional solutions: (1) 36 µL of the CdNO3 solution is injected into 5 mL of 0.1 M AOT solution in heptane, and (2) 36 µL of the Na2S solution is injected into 5 mL of 0.1 M AOT solution in heptane. In each case the final solution, which is allowed to equilibrate for 1 h prior to further use, is characterized by a [H2O]/[AOT] ratio of 4 (w0) and a bulk Cd2+ or S2- concentration of 2 × 10-4 M. The two solutions are then mixed and allowed to stand for ∼5 min. A 1 mL portion of the CdS/AOT/heptane solution is added to 10 mL of hexane and used in the investigations. For obtaining CdS nanoparticles at different w0 values, different amounts of the 0.2 M CdNO3 and Na2S aqueous solutions were added to the 0.1 M AOT solution in heptane; i.e., 27, 45, 54, 63, 73, 82, and 91 µL for w0 values of 3, 5, 6, 7, 8, 9, and 10, respectively. Measurements. UV-vis absorption spectra were obtained using a Perkin-Elmer Lambda 900 spectrophotometer. Luminescence spectra were recorded on a Spex Flurolog-3 photoncounting emission spectrometer that is equipped with a 450 W xenon source, double monochromators for excitation and emission, and a Hamamatsu R928P photomultiplier tube operated at -950 V as the detector. Observed luminescence spectra were corrected for the nonlinear response of the instrument using predetermined correction factors. Photoirradiations were conducted within the sample chamber of the emission spectrometer. The excitation wavelength was selected to correspond to the absorption-band maximum, and the samples were irradiated with a band-pass of 5 nm for time intervals of 15 or 60 min. Absorption and luminescence spectra collected before and after exposure were employed to measure the effects of photolysis on the CdS nanoparticles. Luminescence quantum yields ΦL were determined using a degassed solution of 9-cyanoanthracene (ΦF ) 1.0)29 in hexane as standard. Luminescence decays were obtained using a home-built system.30 Excitation was performed using 337 nm light generated by an LSI model VSL-337ND nitrogen laser, and emission was monitored at various wavelengths employing an American Holographic double-subtractive monochromator for wavelength selection, a fast-wired R928p photomultiplier as the detector, and a Tektronix TDS 640A oscilloscope for data acquisition.
Results CdS nanoparticles were prepared using the reverse micelle-method at several w0 values. Since w0 affects the size of the micelle formed, it also affects the size of the particle that grows within the micelle.12c,28 The absorption spectra of the CdS nanoparticles prepared with w0 values (24) Sooklal, K.; Hanus, L. H.; Ploehn, H. J.; Murphy, C. J. Adv. Mater. 1998, 10, 1083. (b) Lakowicz, J. R.; Gryczynski, I.; Gryczynski, Z.; Murphy, C. J. J. Phys. Chem. B 1999, 103, 7613. (25) Wu, F.; Zhang, J. Z.; Kho, R.; Mehra, R. K. Chem. Phys. Lett. 2000, 330, 237. (26) Moore, D. E.; Patel, K. Langmuir 2001, 17, 2541. (27) Nagesha, D. K.; Liang, X.; Mamedov, A. A.; Gainer, G.; Eastman, M. A.; Giersig, M.; Song, J.-J.; Ni, T.; Kotov, N. A. J. Phys. Chem. B 2001, 105, 7490. (28) Pileni, M. P. Langmuir 1997, 13, 3266. (b) Taleb, A.; Petit, C.; Pileni, M. P. Chem. Mater. 1997, 9, 950. (29) Hirayama, S.; Shobatake, K.; Tabayashi, K. Chem. Phys. Lett. 1985, 121, 228. (b) Hirayama, S.; Yasuda, H.; Okamoto, M.; Tanaka, F. J. Phys. Chem. 1991, 95, 2971. (c) Hirayama, S.; Iuchi, Y.; Tanaka, F.; Shobatake, K. Chem. Phys. 1990, 144, 401. (30) Gord, J. R.; Buckner, S. W.; Weaver, W. L.; Grinstead, K. D., Jr. Optical Method for Quantitating Dissolved Oxygen. U.S. Patent 5,919,710, July 6, 1999.
Figure 1. Normalized absorption and emission spectra of CdS nanoparticles prepared at w0 values of 10, 9, 8, 7, 6, 5, 4, and 3. Arrow indicates spectral-shift trend with decreasing w0.
ranging from 3 to 10 are shown in Figure 1. The spectra are very similar to those found in the literature.9,10,13-16,18,19,21,22,24,25 The spectra exhibit a well-defined peak that ranges from 310 to 380 nm, correlating with w0; the larger the w0, the larger the micelle core and the farther red-shifted the absorption peak for the CdS nanoparticle. A strong correlation has been demonstrated between the size of the CdS nanoparticles and the position of the absorption peak (or absorption-band onset).1,15,19b,22b On the basis of these correlations, our particles are estimated to range from ∼2.5 nm for the smallest w0 (3) to >10 nm for the largest (10). Emission spectra were also recorded as a function of w0 (Figure 1). Unlike the absorption spectra, our luminescence spectra do not exactly match other published CdS emission spectra.9,13-16,18,19,21,22,24,25 This discrepancy can be attributed to the inhomogeneous nature of nanoparticles and the effect of different preparation methods on the defect sites. However, our emission spectra do display well-defined, broad Gaussian-like emission bands that are red-shifted from the absorption-band onset (Figure 1). These features are characteristic of trap-state emissions. Again, the position of the emission-band maximum correlates with w0, shifting to higher energies with decreasing particle size. When examined on the wavenumber scale, both the fwhm and the energy gap between the absorption- and emission-peak maxima are fairly constant (5800 ( 310 and 10400 ( 570 cm-1, respectively). These results indicate that the nature of the emitting sites is constant for different size particles; i.e., all of the particles display emission bands that originate from trap states. Quantum yields for the CdS nanoparticles at w0’s ranging from 3 to 7 were not size dependent (Figure 2). The average yield was 2.8% and varied from ∼1 to 4%. During the course of our analyses, we observed that replicate scans of the same CdS solution resulted in an increase in the observed luminescence intensity. Quantitative study of this phenomenon was performed for CdS nanoparticles prepared at w0 ) 4. In the analysis a 1 mL aliquot of a freshly prepared solution of CdS nanoparticles in heptane was added to 10 mL of hexane. Absorption and emission spectra were then obtained. For minimizing the effects of photoirradition during collection of the quantumyield spectra, monochromator slits were set as narrow as possible (2 nm band-pass). To achieve photoirradiation, the monochromator was set at the absorption-peak
AOT-Protected CdS Nanoparticles
Figure 2. Luminescence quantum yields ΦL for CdS nanoparticles prepared as a function of w0. Yields were determined against 9-cyanoanthracene (ΦF ) 1.0) as standard.
Figure 3. Absorption spectra of CdS nanoparticles with w0 ) 4 measured as a function of irradiation time. Arrow indicates spectral-shift trend with irradiation time. Inset: normalized emission spectra for same CdS nanoparticles.
maximum, the slits were adjusted for a 5 nm band-pass, and the sample was irradiated for time periods of either 15 or 60 min. Spectra were again collected, and the irradiation procedure was repeated. Absorption and emission spectra collected at 15 min intervals for a typical irradiation sequence are shown in Figure 3. The change in the absorption spectra with irradiation time occurred in two steps: (1) a bathochromic shift, with little or no change in absorption intensity, followed by (2) a decrease in absorption intensity, with no further spectral shift. These behaviors are very reproducible. The magnitude of the shift is on the order of 30-40 nm, depending somewhat on the initial position of the absorption-peak maximum and the apparent quality of the particles, as assessed by the absorption spectral profile.31 In most of the experiments, the final position of the absorption-band maximum was in the 360-370 nm (31) For a freshly prepared solution of CdS nanoparticles, the absence of a well-defined absorption peak correlates with nonreproducible behavior; i.e., the overall shift and the final plateau quantum yield will vary in an unpredictable manner.
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Figure 4. Luminescence quantum yields ΦL for CdS nanoparticles with w0 ) 4 recorded as a function of irradiation time. Different symbols indicate separate experimental trials (symbols to note: 0 with cross, excess Cd2+; 4 with cross, excess S2-). Arrow indicates approximate point at which no further spectral shifting of the absorption spectra is observed.
region, and the decrease in the absorption intensity was on the order of 20%. Since the concentration of the CdS nanoparticles was constant throughout the experiment (no indication of precipitation was observed with irradiation), the decrease in absorbance (unless due to decomposition, see oxygen discussion) may be caused by a decrease in the molar absorptivities of the CdS particles. The emission spectra also display a shift toward the red with irradiation time (typically 50 nm); however, unlike the absorption spectra, which decrease in intensity, the emission spectra undergo a significant increase in intensity. This can best be observed by examining the photoluminescence yields vs irradiation time (Figure 4). The data in Figure 4 were obtained from numerous experimental trials, including several that targeted key parameters (oxygen, excess Cd2+, and excess S2-). For these data the same characteristic irradiation-time dependence is observed: a linear increase in quantum efficiency ranging from ∼2% to a plateau at ∼12%. To ensure that the observed trend was not due to aging effects, an experiment was performed in which the sample was split; one aliquot was taken for irradiation, and the remainder was placed in the dark to be sampled at various times during the irradiation sequence. While the irradiated aliquot exhibited the same trend as that displayed in Figure 4, the solution kept in the dark and sampled as a function of time exhibited no change in quantum efficiency. Thus, the behavior observed in Figure 4 must be attributed to a photochemical process.32 Decay profiles for fresh and plateau CdS nanoparticles obtained from the blue side, the band maximum, and the red side of the emission spectrum are shown in Figure 5. All decays are multiexponential. The decay profiles of both the fresh and the plateau CdS particles exhibit emissionwavelength dependence, yielding an increase in the apparent lifetime as the emission wavelength moves toward the red. These results are an indication of a broad distribution of emitting sites for the CdS nanoparticles and agree well with the mechanistic details of colloidal (32) Preliminary measurements using TEM of the fresh and plateau CdS nanoparticles appear to indicate no change in particle size with both samples having diameters of 2-3 nm.
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suggest the possibility of dynamic quenching where the long-lived states are effectively quenched. Discussion
Figure 5. Emission decays for CdS nanoparticles with w0 ) 4 that were freshly prepared (no offset) and irradiated to reach the plateau (offset). Decays were obtained at emission wavelengths of 425 (s), 500 (- - -), and 575 nm (- ‚ ‚ -); IRF is shown for comparison (‚ ‚ ‚).
Figure 6. Emission decays for plateau CdS nanoparticles with w0 ) 4 particles that were exposed to ammonia (‚ ‚ ‚, IRF; s, 450 nm; - - -, 525 nm; - ‚ ‚ -, 600 nm). Inset: absorption and emission spectra for ammonia-treated CdS nanoparticles.
CdS.19 At each emission wavelength the plateau particles display longer apparent lifetimes than the fresh particles. Such observations are in agreement with the red shift experienced during irradiation; lower energy emissions are predicted to have longer lifetimes.19 Plateau particles were bubbled with ammonia to observe the effect of a known passivating agent on the spectral properties.16,33 The absorption and emission spectra are show in Figure 6, along with the emission decays (inset). Interestingly, exposure to ammonia results in a further red shift in both the absorption and the emission spectral profiles and in a significant decrease in the quantum efficiency (ΦL,NH3 ) 0.8%). The decays reveal a significant change as well, being shorter even than those of the freshly prepared CdS nanoparticles at all three emission wavelengths. These results are in contrast to the known behavior of colloidal CdS; the shorter decays do not correlate with the shift to longer wavelengths. These data (33) Wang, Y.; Suna, A.; McHugh, J.; Hilinski, E. F.; Lucas, P. A.; Johnson, R. D. J. Chem. Phys. 1990, 92, 6927.
Only a few papers in the literature discuss the effects of illumination on the spectral properties of CdS nanoparticles.12a,19a,22a,24b Rossetti and Brus reported that, in the presence of excess S2- and under acidic conditions, CdS colloids undergo photochemical aging in room light.19a Their observed increase in photoluminescence was also 10-fold and accompanied by a red shift; however, their initial and final quantum efficiencies were much lower (2 × 10-4 and 3 × 10-3, respectively), and the spectral profiles were characteristic of excitonic emissions (much narrower and close to the absorption-edge onset). CdS particles whose emission characteristics are closer to our own have recently been prepared with high quantum efficiencies (9.7%) by preparation and encapsulation within organic dendrimers.24b Using a photolysis setup similar to ours, these authors found that these particles show no dependence on irradiation. Other methods for enhancing the quantum efficiency of CdS nanoparticles have been reported;9,11,19a,21a,22a,24-26 these methods involve chemical passivation or antiquenching, as it was first termed.19a,21 The most impressive is that of Henglein and co-workers, who reported quantum yields for CdS nanoparticles treated with excess Cd2+ under basic conditions (ΦL > 50%).22a In most cases, however, the emission characteristics of the passivated CdS particles have been indicative of excitonic emissions and, thus, different from ours. To test our procedure under some of the conditions reported to influence the properties of CdS luminescence, we performed the irradiation experiment with 30% excess Cd2+ and S2-. Both experiments produced data with the same characteristic irradiation time dependence (Figure 4). The data fall within our experimental uncertainty; however, qualitatively, excess Cd2+ did provide a somewhat higher plateau yield and excess S2-, a somewhat lower yield. It has been reported that photoillumination of CdS nanoparticles in the presence of dissolved oxygen can result in decomposition of the particles.22a,34 To study the effect of oxygen on our system, we prepared particles for irradiation by the standard procedure and then put the solution of particles through 10 freeze-pump-thaw cycles to remove dissolved oxygen. Again, the data obtained show the same trend with irradiation. Dissolved oxygen seems to play no role in the formation of the plateau particles; however, once the plateau is reached, oxygen may decompose the CdS nanoparticles since further irradiation tends to decrease the absorption intensity but has no impact on the emission spectral profile or quantum yield. With regard to understanding the mechanism governing the photoenhanced luminescence yields, it appears that the observed spectral shifts and increased quantum efficiencies may not be dependent on each other. Numerous reports have associated the position of the absorption spectrum of CdS nanoparticles with particle size; the phenomenon is due to the quantum-confinement effect.1,15,19b,22b The red shift in the absorption and emission spectra with irradiation seems to indicate an increase in particle size.35 However, if we correlate the shift of the absorption spectra and the irradiation time and plot the (34) Henglein, A. J. Phys. Chem. 1982, 86, 2291. (b) Baral, S.; Fojtik, A.; Weller, H.; Henglein, A. J. Am. Chem. Soc. 1986, 108, 375. (35) Particles prepared at other w0 values display similar quantum yield enhancement and spectral shift trends; however, particle size does appear to impact the final plateau yield with larger particles displaying lower yields.
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point at which the spectra cease to red shift with the data in Figure 4, we find that this occurs only partway through the quantum-yield enhancement trend (arrow in Figure 4). It is also important to recall that we observed no dependence of the CdS nanoparticle quantum efficiencies on particle size (Figure 2). Therefore, it seems unlikely that an increase in particle size is responsible for the large increase in quantum efficiency. It also seems unlikely that the increased quantum efficiency (or the process that is driving the increase) could produce the observed spectral shifts. Experimentally, the plateau yields are easily lost with exposure to ammonia, a known passivation agent; yet, the red-shifted spectra do not revert to the preirradiation positions. In fact, the spectra shift further to the red. These observations would seem to indicate that the spectral shifts and the increased quantum efficiencies are decoupled. A more thorough investigationsincluding analysis of the effect of w0 on the irradiation phenomenon and quantum efficiencies, thin-film or 77 K studies to prevent micelle exchange, and TEM and X-ray crystallography to determine size and structuresmay aid in elucidation of the photoenhancement mechanism.
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Conclusion Highly luminescent CdS nanoparticles have been prepared through a photoirradiation procedure. The emission characteristics of these particles are those of trap states. While the emission characteristics can be adequately explained using the known mechanistic details of colloidal CdS, the effect of irradiation is unusual and complex. Clearly, this observed phenomenon warrants further investigation since well-understood particles exhibiting enhanced trap-state emissions offer the opportunity to explore the photophysics of trap states in greater detail. Acknowledgment. The authors thank Drs. J. R. Gord, D. K. Phelps, and S. W. Buckner and Prof. Y.-P. Sun for helpful discussions and Ms. M. M. Whitaker for editorial support. We also thank P. Pathak, Y. Lin, and Prof. Y.-P. Sun for providing assistance with the TEM. We acknowledge Dr. Julian Tishkoff and the Air Force Office of Scientific Research (AFOSR) for continuing support of fuels and combustion research. LA026483X
