J. Phys. Chem. 1996, 100, 6381-6384
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Picosecond Electronic Relaxation in CdS/HgS/CdS Quantum Dot Quantum Well Semiconductor Nanoparticles Valey F. Kamalov, Reginald Little, Stephan L. Logunov, and Mostafa A. El-Sayed* School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400 ReceiVed: December 13, 1995; In Final Form: February 22, 1996X
Subpicosecond photoexcitation of CdS/HgS/CdS quantum dot quantum well nanoparticles at wavelengths shorter than their interband absorption (390 nm) leads to a photobleach spectrum at longer wavelengths (440740 nm). The photobleach spectrum changes and its maximum red-shifts with delay time. These results are explained by the rapid quenching of the initially formed laser-excited excitons by two types of energy acceptors (traps); one is proposed to be due to CdS molecules at the CdS/HgS interface, and the other trap is that present in the CdS/HgS/CdS well. The results of the excitation at longer wavelengths as well as the formation and decay of the bleach spectrum at different wavelengths support this description.
Introduction Multilayered quantum dots were prepared and characterized recently by Mews and coauthors.1 They are composed of two semiconductor materials, the material with the smaller bulk bandgap is embedded between a core and an outer shell of the material with the larger bandgap. This nanostructure is a new direction in the research of low-dimensional semiconductor particles. These particles combine features of quantum dots and quantum wells named quantum dot quantum well (QDQW).1 Scientifically, the understanding of the spectroscopy and the excitation transport in quantum dots2,3 and quantum wells4,5 is important due to quantum size effects and their low-dimensionality. In addition, they are also important to their potential applications in microelectronics. Electronic relaxation in CdS quantum dots was studied extensively.6-10 Fast exciton dynamics in semiconductor quantum wells was the subject of recent reports.4,5,11 It was shown12 that the linear absorption of the QDQW composite particles, having diameters of between 5 and 12 nm, differs considerably from the sum of the linear absorptions of the respective subunits. Schooss et al.13 developed an extended theoretical approach for calculating the 1s-1s electronic transition in spherical semiconductor quantum dots. They proposed a model of effective-mass approximation with the Coulomb interaction of electron and hole and finite potential wells at the particle boundaries. They were able to describe the absorption spectra of QDQW in a reasonable manner. Energies and wave functions of the 1s-1s transition were determined.13 The wave functions for both electrons and holes are found to change with the size of the particle. The maxima of amplitudes of wave functions of both electrons and holes were found in the region of the intermediate layer (HgS), so that a quantum well is formed between the core and the outer shell of the semiconductor with larger bandgap (CdS). Eychmuller et al.14 had shown the transient photobleaching of CdS/HgS/CdS particles using nanosecond flash photolysis apparatus. They observed the transient bleaching between 600 and 700 nm with UV excitation. In the present paper we report the picosecond relaxation dynamics of the CdS/HgS/CdS semiconductor particles. Initial transient photobleaching was found to depend on the excitation wavelength. The decay rate of the transient photobleaching was found to be wavelength X
Abstract published in AdVance ACS Abstracts, April 1, 1996.
0022-3654/96/20100-6381$12.00/0
dependent with the faster decay at shorter wavelength. The observed fast dynamics is explained in terms of exciton relaxation in the quantum well formed by the thin intermediate layer. Experimental Section CdS/HgS/CdS nanoparticles were prepared following the procedure given by Mews et al.1 The first step in the preparation of the QDQW nanoheterostructure involved the formation of the CdS core. 0.25 mL of 0.1 M Cd(ClO4)2 and 0.25 mL of 0.1 M sodium polyphosphate were mixed with 125 mL of water in a three-necked round-bottom flask. This mixture was purged with argon for 20 min followed by the injection of 0.6 cm3 of H2S gas at atmospheric pressure. The enclosed mixture was allowed to stand for 10 min. The absorption spectrum of the resulting CdS colloid is presented in Figure 1 as curve 1. The shoulder in the spectrum is due to the exciton transition, which is revealed by the pronounced minimum at 465 nm in the second derivative spectrum. The dependence of exciton transition on particle size was investigated extensively by Vossmeyer et al.8 We used their correlation of exciton energy vs particle size to estimate an average particle size. The energy gap of bulk CdS is 2.5 eV and the bandgap increases to 5 eV as the particle size decreases below 2 nm. We observed excitonic transition at 465 nm (2.7 eV), which corresponds to an average particle diameter of 6 nm with about 10% deviation.8 The second step in the heterostructural synthesis involved the formation of the HgS shell around the CdS core. The HgS shell was formed by substituting the outer surface Cd2+ with Hg2+. 10 mL of 0.001 M Hg(ClO4)2 was added to the CdS colloid. This stoichiometric amount of Hg(ClO4)2 resulted in monolayer HgS formation about each CdS core. Monolayer formation of HgS is ensured by the slow solid-state diffusion and surface passivation of HgS. However, the HgS/CdS heterostructure is very unstable with respect to solid-state mixing and must be rapidly protected by enclosure with an outer CdS shell. (Indeed, this instability makes accurate absorbance and emission measurements difficult.) Formation of the outer CdS shell produces the final QDQW nanoheterostructure. This outer CdS shell is formed by slow addition of 25 mL of H2S/H2O saturated (where the amount of H2S is 25% in excess of the stoichiometric amount) solution to 125 mL of colloidal solution. The absorption spectrum of the resulting CdS/HgS/CdS colloidal solution is presented in Figure © 1996 American Chemical Society
6382 J. Phys. Chem., Vol. 100, No. 16, 1996
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Figure 2. Transient bleaching band of CdS/HgS/CdS in glycerol/water glass at 9 K at zero delay (a) and 8.5 ps delay (b) after excitation at 390 nm (photon energy 3.18 eV). Figure 1. Absorption spectra of CdS (1), CdS/HgS (2), and CdS/HgS/ CdS (3) colloids in aqueous solution. T ) 300 K.
1 as curve 3. In comparison to the CdS spectrum, the absorption extends to 700 nm. The absorption of both CdS and CdS/HgS/ CdS are consistent with the spectra of Mews et al.1 This spectra contains a contribution of inhomogeneous broadening due to size and shape distributions. We cooled particles down to 10 K and burned spectral holes by subpicosecond laser pulse. The nature of this absorption in the range of 450-700 nm and the associated spectral dynamics for this range is the subject of this letter. To observe the absorption and spectral dynamics at low temperature, the CdS/HgS/CdS particles were resuspended in a glycerol/water mixture (70/30 volume ratio). Changing the solvent involved the addition of 10 mL of glycerol followed by rotoevaporation of water. The resulting mixture was transferred to a 2 mm cell and mounted in a cryostate (Model CCS-150 Janis Research Co., Inc.). This concentrated mixture formed a glass at low temperatures. Comparison of the absorbance and emission spectra before and after the solvent change ensured that no change in the integrity of the nanoheterostructure occurred. The spectrometer used for time-resolved measurements was described in detail in ref 15. Briefly, the laser system consists of a commercial Coherent Satori dye laser pumped by an Antares mode-locked YAG laser. As a result, 250 fs pulses with a repetition rate of 76 MHz at wavelengths between 595 and 615 nm were generated. The output of the dye laser was amplified by a regenerative amplifier (Quantel, RGA 60) in a dye amplifier (Quantel, PTA 60) at 10 Hz. Amplified pulses with an energy of about 0.5 mJ at 605 nm, 350 fs pulse duration were obtained. The amplified 595-615 nm pulse was mixed with residual fundamental radiation (1064 nm) of regenerative amplifier in KDP crystal in order to obtain radiation at 380390 nm. The output energy in 380 nm typically is about 0.2 mJ. Amplified pulse was split into two beams, one of them was used as an excitation pulse, the residual 600 nm pulse was focused in a 1 cm cell containing a mixture of D2O and H2O to generate white light continuum. The continuum was split into two beams which were used as probe and reference. The pump beam was overlapped with the probe beam in the sample after
traveling through a variable delay line (up to l ns). The reference beam was passed through a nonexcited part of the sample. The diameters of the probe and the reference beams were about 0.1 mm, and that of pump beam was about 0.3 mm. The reference beam had an energy of less than 50 nJ. The reference and probe beams were passed through a monochromator or polychromator and were detected by two photodiodes or CCD detector (Princeton Instruments, EUV-1024, controller ST-130), respectively. Kinetics were analyzed by the leastsquares method. Results and Discussion Excitation at 390 nm. Upon excitation at 390 nm, it is not possible to create a hole at this wavelength at zero time. The bleaching was rather observed at longer wavelengths between 440 and 740 nm (Figure 2a). This suggests that the initial excited system at 390 nm rapidly relaxes to their ground state giving its excitation energy to systems (traps) absorbing in the 440-740 nm region. The 8 ps delay spectrum (Figure 2b) shows a decrease in the bleach intensity at 550 nm relative to that at long wavelength as well as a red-shift of the maximum to 665 nm. This suggests that there are at least two different trap distributions, one absorbing with a maximum around 550 nm and the other with a maximum of 665 nm. Furthermore, both of them are excited as a result of the rapid quenching of the initial excitation at 390 nm. The change in the band shape of the bleached spectrum on changing the delay from 0 to 8 ps suggests that the lower energy trap distribution(s) are also excited by energy transfer from the higher energy trap distribution(s). The above conclusions are further supported from the observed bleach formation and recovery curves in Figure 3. Probing the bleach formation and recovery for the different traps shows that traps absorbing at 495 and 565 nm have rapid excitation time (occurring within the laser pulse width) while the lowest energy trap absorbing at 650 nm has a slightly longer formation (bleaching) time. The formation at 650 nm (open circles in Figure 3a) can be best fitted with two-exponential curve, where the first time (fast component) is due to instantaneous population of low-energy traps but slow component is due to energy transfer between high- and low-energy traps. We found the decay time of the high-energy bleach spectrum correlates with the slow component of formation of the lower
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J. Phys. Chem., Vol. 100, No. 16, 1996 6383
a
Figure 4. (a) Steady-state absorption spectrum at 9 K; the arrow indicates the position of excitation. Transient bleaching band of CdS/ HgS/CdS in glycerol/water glass at 9 K at zero delay (b) and 8.5 ps delay (c) after excitation at 650 nm.
b
Figure 3. (a, top) Formation of bleaching at 565 (closed circles) and 650 nm (open circles) together with the response function (dotted line). The solid lines are the best fit obtained by deconvolution of laser profile with instantaneous formation (565 nm) and two-exponential rise of 65% instantaneous component and 35% of the 3 ps component (650 nm). (b, bottom) Temporal behavior of transient bleaching of CdS/HgS/CdS in glycerol/water glass at 9 K at 495 (1), 565 (2), and 650 nm (3). Excitation 390 nm.
energy bleach spectrum. This suggests that rapid energy transfer populates both types of traps while some contribution of slower energy-transfer processes (due to trap-to-trap hopping) populates the lowest energy trap. The above conclusion also explains the observed decay of trap excitation as shown from the decay of their absorption bleach in Figure 3. The highest energy traps (absorbing at 390 nm) decay fastest (Figure 3b.1) with a single component of decay constant of 2.5 ps. The rapid decay is due to the higher density of acceptors that can quench the excited system at this energy. The decay of the excited trap in the middle energy (Figure 3b.2) is slightly longer (4 ps). The decay of the lowest energy traps (curve 3, with the lowest density of energy acceptors) shows two components; one with a decay time of 5 ps (relative amplitude of 0.4) and another component of longer lifetime than 50 ps and having a relative amplitude of 0.6. This
is most likely the component that was observed by Weller et al. in their nanosecond experiments.14 The above results suggest that the absorption at 390 nm leads to excitation of the CdS core (the most absorbing system). At least two different distributions rapidly quench the initial CdS excitation at 390 nm. The high-energy distribution could be the core CdS surface molecules modified by the presence of the HgS shell. The lower energy distribution could be the exciton in the quantum well. Rapid energy transfer from the CdS core exciton leads to the excitation of both these trap distributions. Furthermore, energy transfer between the high energy to the lower energy traps leads to the observed change in the bleach spectrum with time. Excitation at 605 nm. Excitation at 605 nm (Figure 4) creates a hole not far from the exciting wavelength at zero time (Figure 4b). However, the observed bleaching at shorter wavelength than 605 nm suggests either a large homogeneous width or the presence of two-photon excitation (a careful intensity dependence of the width is now under examination in order to distinguish between these possibilities).16 The most interesting result is that most of the traps formed by this excitation are the lowest energy traps. Smaller shape changes are observed by delaying the monitoring pulse after the excitation pulse (Figure 4c). This results from 8 ps energy transfer between the high-energy donors to low-energy acceptors within the trap distribution present in the quantum well. Absorption of the CdS/HgS/CdS nanoparticle was assigned to 1s-1s transition of electron and hole previously.13 Spectral dynamics observed in the present study indicates a complicated character of the band. The lowest energy bleaching band, 1.9 eV, formed a few picoseconds after excitation (curve b in Figure 2 and curve c in Figure 4) can be assigned to 1s-1s transition. Higher transitions such as 1s-2p, 1s-2s may contribute to the absorption at a higher energy, located below the interband
6384 J. Phys. Chem., Vol. 100, No. 16, 1996 transition for CdS along (in the well of HgS). Thus the electronic relaxation between these higher states can be also the reason for the picosecond spectral dynamics. Picosecond energy trapping by the quantum well is observed on rather slower time scales as compared to the femtoseconds electronic relaxation observed in bulk semiconductors and quantum dots. This is probably due to the low density of the final states present in the well. Density of states increasing toward shorter wavelengths, with a continuum of states in the UV region (with CdS interband absorption at wavelength shorter 450 nm). Our bleaching band starts below the interband transition of CdS (Figure 2a) that is due to faster relaxation in the CdS band followed by slower relaxation to the CdS interfacial CdS molecules and finally to the molecules in the well. Thus our time-resolved measurements also can be treated as exciton localization dynamics at the intermediate HgS layer. Acknowledgment. We wish to thank Professor A. Henglein and Mr. T. Ahmadi for helpful discussions. The support of the Office of Naval Research (Grant No. N00014-95-1-0306) is greatly appreciated. References and Notes (1) Mews, A.; Eychmuller, A.; Giersig, M.; Schooss, D.; Weller, H. J. Phys. Chem. 1994, 98, 934-941.
Letters (2) Brus, L. E. J. Phys.Chem. 1986, 86, 2555. (3) Henglein, A. Chem. ReV. 1989, 89, 1861. (4) Becker, P. C.; Lee, D.; Barros, M. R. X.; Johnson, A. M.; Prosser, A. G.; Feldman, R. D.; Austin, R. F.; Behringer, R. D. J. Quantum Electron. 1992, 28, 2535-42. (5) Deveaud, B.; Morris, D.; Regreny, A.; Planet, R.; Gerard, J. M.; Barron, M. R. X. Semiconduct. Sci. Technol. 1994, 9, 722-6. (6) Katsikas, L. E.; Giersig, M.; Weller, H. Chem. Phys. Lett. 1990, 172, 201-204. (7) Shum, K.; Wang, W. B.; Jones, K. M. Phys. ReV. Lett. 1992, 68, 3904-7. (8) Vossmeyer, T.; Katsikas, L.; Giersig, M.; Popovic, I. G.; Giesner, K.; Chemseddine, A.; Eychmuller, A.; Weller, H. J. Phys. Chem. 1994, 98, 9665-7673. (9) Weller, H.; Eychmuller, A. AdV. Photochem. 1995, 20, 165-216. (10) Woggon, U.; Portune, M. Phys. ReV. B 1995, 51, 4719-22. (11) Wegener, M.; Bar-Joseph, I.; Sucha, G.; Islam, M. N.; Sauer, N.; Chang, T. Y.; Chemla Phys. ReV. B 1989, 39, 12794-801. (12) Eychmuller, A.; Mews, A.; Weller, H. Chem. Phys. Lett. 1993, 208, 59-62. (13) Schooss, D. M. A.; Eychmuller, A.; Weller, H. Phys. ReV. B 1994, 49, 17072-78. (14) Eychmuller, A.; Vobmeyer, T.; Mews, A.; Weller, H. J. Lumin. 1994, 58, 223-226. (15) Logunov, S.; El-Sayed, M. A.; Song, L.; Lanyi, J. J. Phys. Chem., in press. (16) Kamalov, V. F.; Little, R.; Logunov, S. L.; El-Sayed, M. A., unpublished.
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