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Cite this: Phys. Chem. Chem. Phys., 2013, 15, 21051
Temperature dependence of photoluminescence dynamics of self-assembled monolayers of CdSe quantum dots: the influence of the bound-exciton state DaeGwi Kim,* Hiroki Yokota, Kunio Shimura and Masaaki Nakayama We have investigated the temperature dependence of photoluminescence (PL) dynamics of selfassembled monolayers (SAMs) of CdSe quantum dots (QDs). The PL decay profiles become slower with an increase in temperature up to 160 K, contrary to an ordinary behavior due to thermal quenching.
Received 23rd September 2013, Accepted 21st October 2013
Such anomalous temperature dependence of the PL-decay profile is explained using a four-state model which introduces a bound-exciton state into a conventional three-state model consisting of a ground
DOI: 10.1039/c3cp54012j
state and two excited states: a lower-lying dark-exciton state and a higher-lying bright-exciton state. Furthermore, it is proposed that the radiative decay time of QDs is strongly influenced by the presence
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or absence of the bound-exciton state.
I. Introduction Semiconductor quantum dots (QDs) have attracted considerable attention for understanding the intrinsic nature of physical/ chemical properties of QDs.1–4 Furthermore, the breakthrough in synthesizing colloidal QDs with a high photoluminescence (PL) yield has led to an explosive increase of QD studies and opened up possibilities for various applications such as biomolecular imaging,5 QD lasing,4,6 QD solar cells,7,8 and so on. One of the most characteristic PL properties of QDs is the observation of an optically passive state, the so-called dark-exciton state.5–12 The origin of the dark-exciton state is basically a spin triplet state that is forbidden for optical transitions by the spin-selection rule. In a bulk crystal, thermal effects inhibit the observation of the triplet-exciton state since the spin-exchange-splitting energy is usually smaller than 1 meV. In contrast, the quantum confinement effect in a QD system enhances the spin-exchange interaction, which results in a large splitting energy between the spin singlet and triplet states. The spin-exchange-splitting energy is not only dependent on the QD size but also strongly dependent on semiconductor materials. Since the chemical synthesis of size-controlled CdSe QDs with a high photoluminescence (PL) yield has been well established,9–11 CdSe QDs have become a model material for studies of optical properties of QDs.12–18 The splitting energy in Department of Applied Physics, Graduate School of Engineering, Osaka City University, 3-3-138, Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan. E-mail: [email protected]
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CdSe QDs changes from 2 to 20 meV with a decrease in the QD size.12 The temperature dependence of PL dynamics of CdSe and/or CdSe/ZnS QDs can be understood using a conventional three-state model consisting of a ground state and two excited states: a lower-lying dark-exciton state and a higher-lying bright-exciton state.19–21 On the basis of this model, the darkexciton state mainly contributes to PL processes at low temperatures. With an increase in temperature, the population in the higher-lying bright-exciton state with a fast decay component is increased by the thermal excitation from the lower-lying dark-exciton state with a slow-decay component, which leads to shortening the decay profiles. On the other hand, it was reported theoretically and experimentally that the spin-exchange-splitting energy can be of the order of 20 to 60 meV in CdS QDs, depending on the QD size.22,23 We reported that the PL decay profiles in CdS QDs exhibit a long decay time of hundreds of nanoseconds even at room temperature, which demonstrates the large contribution of the dark-exciton state to the PL processes.24 This is owing to the large splitting energy between the bright- and dark-exciton states in CdS QDs compared with other semiconductor QDs such as CdSe, CdTe, and Si QDs. The existence of the slow component even at room temperature is an intrinsic feature peculiar to CdS QDs. Furthermore, it is noted that the decay profiles of the band-edge PL exhibit characteristic temperature dependence; they become slower with an increase in temperature.24 An important point explaining the temperature dependence is the existence of a bound-exciton state on the low energy side of the dark-exciton state. The population in the upper-lying dark-exciton state with a slow decay component is increased with an increase in
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temperature by the thermal excitation from the lower-lying bound-exciton state with a fast-decay component, leading to prolonging the decay profiles. This temperature dependence is just contrary to that in the above-mentioned conventional three-state model. The surface-related shallow-bound state has been discussed for CdSe,25–27 CdTe,28,29 and InP QDs30 so far. Nirmal et al. reported the surface localization of photo-excited excitons in CdSe QDs by measuring size-selective PL spectra and PL-decay profiles at low temperatures below 10 K.25 Califano et al. suggested theoretically that that surface states greatly influence the PL-decay times in CdSe QDs27 Wang et al. reported the contribution of surface states to the PL process by measuring time-resolved PL in highly luminescent CdSe QDs at room temperature.26 Furthermore, Wang et al. provided a proof of the existence of surface states from the results of up-converted PL with an ultrashort time resolution in CdTe QDs at room temperature.28 However, little has been known about the systematic temperature dependence of the PL dynamics in the wide range from low temperature to room temperature from a viewpoint of the contribution to the PL processes of the bound-exciton state. It is well known that the thiol group influences PL properties of CdSe QDs.31,32 In ref. 32, the strong quenching of PL of CdSe QDs by thiol ligands is reported. It is demonstrated that the photogenerated holes are trapped by the thiol ligands on the surface of the QDs, which quenches the PL of the QDs and initiates the photooxidation of the surface ligands. Thus, it is expected that the bound-exciton state is formed intentionally by the thiol group. This is connected with our motivation in the present work. A self-assembled monolayer (SAM) of CdSe QDs can be used for this purpose since the QDs are usually attached to substrates by the thiol group in the SAM samples. Furthermore, since the QDs are uniformly adsorbed on the substrate, the SAM samples are very suitable for measurements of the temperature dependence of optical properties.33 In the present work, we have investigated PL properties of the SAM of the CdSe QDs from a viewpoint of the influence of the bound exciton state on the PL processes by measuring the temperature dependence of the absorption, PL, and PL-decay profiles. The PL-decay profiles become slower with an increase in temperature up to 160 K, which is contrary to ordinary behavior due to thermal quenching. This anomalous temperature dependence can be explained in terms of a four-state model including the bound-exciton state. Furthermore, it is demonstrated that the radiative decay time of the QDs can be controlled by the presence of the bound-exciton state.
II. Experiments CdSe QDs with B3.3 nm in diameter were purchased from Sigma-Aldrich. The substrates of quartz were cleaned by immersion in fresh piranha solution [a 1/3 (v/v) mixture of 30% H2O2 and 98% H2SO4] for 20 min. (Caution: Piranha solution reacts violently with organic materials.) Then the substrates were rinsed with water and used immediately after cleaning. In the beginning of the sample
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Fig. 1
The AFM image of the SAM of the CdSe QDs.
preparation, SAMs that consist of (3-mercaptopropyl) trimethoxysilane (MPTMS) molecules were deposited to anchor the QDs. The SAM formation was achieved by immersing the precleaned substrate in a MPTMS–toluene solution for 1 h. Next, the substrate was rinsed thoroughly with toluene. The SAM of the QDs linked by thiol molecules was prepared in a QD–toluene solution for 1 h. Finally, the samples were thoroughly rinsed with toluene and dried with a stream of nitrogen gas. The atomic force microscopy (AFM) image of the SAM of the CdSe QDs is shown in Fig. 1. The image shows the coverage of QDs on the substrate. Furthermore, the rootmean-square roughness was confirmed to be B1.5 nm. After the preparation, the sample was introduced into a closed-cycle helium-gas cryostat, and the temperature dependence of absorption, PL, and PL-decay profiles under vacuum was observed. The absorption spectra were measured using a double-beam spectrometer with a resolution of 0.2 nm. For PL measurements, the 325 nm line of a He–Cd laser was used as the excitation-light source, and the emitted PL was analyzed using a single monochromator with a spectral resolution of 0.5 nm. For measurements of PL-decay profiles, a laser-diode (405 nm, Hamamatsu PLP10-040) with a pulse duration of 100 ps and a repetition of 500 kHz was used as the excitation light. The pump fluence was 50 nJ cm2. The PL-decay profiles were obtained by a time-correlated single-photon counting method. The sample temperature was controlled using a closed-cycle helium-gas cryostat. We used the CdSe QDs covered with the ligands of longchain amine: the ligand length is estimated to be 2.6 nm. This long-chain ligand prevents the coupling between CdSe QDs. In fact, the absorption energy of the SAM structure is comparable to that of the solution sample in which the QDs are isolated, and no spectral shift is observed. Thus, it is noted that the PL properties of an ‘‘ensemble’’ of the non-interacting CdSe QDs are observed in the present work.
III. Results and discussion Fig. 2 shows the temperature dependence of the absorption and PL spectra of the SAM of the CdSe QDs. The absorption- and PL-peak energies shift to the higher energy side with a decrease in temperature. Fig. 3 shows the temperature dependence of the lowest absorption peak (exciton) energy. It is well known that the temperature dependence of the exciton energy in a direct-gap semiconductor can be described using Varshni’s law;34 E(T) = E(0) aT 2/(T + b), where E(0) is the exciton energy
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Fig. 5
Temporal profiles of the band-edge PL band at 10, 80, 120, and 160 K.
Fig. 2 Temperature dependence of the absorption and PL spectra of the SAM of the CdSe QDs.
Fig. 3
Temperature dependence of the lowest absorption peak (exciton) energy.
at T = 0 K, a is the temperature coefficient, and b is a parameter related to the Debye temperature of the crystal. The solid curve indicates the calculated result for the temperature dependence of the absorption energy, where the parameter values of E(0) = 2.41 eV, a = 3.7104 eV K1 and b = 150 K for a CdSe bulk crystal35 were used. The calculated results using the parameters for the bulk crystal quantitatively explain the experimental results, which indicates that the observed temperature dependence of the absorption-peak energy results from the intrinsic property of the CdSe crystal. As shown in Fig. 2, the band-edge PL is clearly observed at every temperature. Although a defect-related PL band with a large Stokes shift of B0.65 eV is dominant at low temperatures, the relative intensity of the band-edge PL band to that of the defect-related PL band is increased with an increase in temperature by the thermal activation of trapped carriers. Fig. 4 shows the temperature dependence of the integrated intensity of the bandedge PL band. The intensity of the band-edge PL band at room temperature is almost 10% of that at 10 K.
Fig. 4 Temperature dependence of the integrated intensity of the band-edge PL band.
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Next, in order to investigate the influence of the thiol group on the band-edge exciton state, we measured the temperature dependence of PL-decay profiles. Fig. 5 shows the temporal profiles of the band-edge PL band at 10, 80, 120, and 160 K. In this temperature range, temporal profiles become slower with an increase in temperature. In the usual case, as the temperature is increased, the nonradiative-decay rate related to the thermal quenching becomes larger, so that the PL intensity and the PL-decay time are decreased. In fact, as shown in Fig. 4, the band-edge PL intensity gradually decreases with an increase in temperature. Nevertheless, the observed decay profiles become longer, which is contrary to the above scenario. So far, the temperature dependence of PL dynamics of CdSe QDs has been explained using the conventional three-state model consisting of a ground state and two excited states: a lower-lying darkexciton state and an upper-lying bright-exciton state.19–21 On the basis of this model, we can expect that the population in the upper state with a fast decay component is increased with an increase in temperature by the thermal excitation from the lower state with a slow decay component, leading to shortening the decay profiles. However, the temperature dependence of the PL-decay profile shown in Fig. 5 is completely contrary to the previously reported results. We also measured the temperature dependence of PL properties for a drop cast film of the CdSe QDs. It is noted that the drop-cast-film samples are free from the thiol group. The PL-decay profiles become faster with an increase in temperature as in previous reports.19–21 The temperature dependence of the PL-decay profile for the dropcast film is fully contrary to that for the present SAM structure, in which the decay profiles become slower with an increase in temperature. Thus, these results clearly demonstrate that the temperature dependence of the PL-decay profile for the SAM structure is related to the thiol anchoring. In order to explain the anomalous temperature dependence of the decay profiles, we have to assume that at least two radiative states contribute to PL processes: a lower-lying emitting state with a relatively fast decay component and an upper-lying emitting state with a relatively slow decay component. On the basis of this model, we can expect that the population in the upper state with a slow decay component is increased with an increase in temperature by the thermal excitation from the lower state with a fast decay component, leading to prolongation of the decay profiles. The PL-decay
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profile exhibits a slow decay over hundreds of nanoseconds. This long decay profile suggests that the optically passive state, the so-called dark-exciton state, contributes to the PL process. Thus, we attribute the upper-lying emitting state with a slow decay component to the dark-exciton state. Since the darkexciton state is a ‘‘free-exciton’’ state, we have to consider a ‘‘bound-exciton’’ state related to a bright exciton as the origin of the lower-lying emitting state. Similar temperature dependence of PL decay profiles was previously observed in CdS QDs,24 and it was quantitatively explained using a three-state model consisting of a ground state and two excited states; a lower-lying bound-exciton state and a higher-lying dark-exciton state.24 An important point is the introduction of the bound-exciton state into the model. With an increase in temperature, the thermal population of the higher-lying dark-exciton state is significant, and the decay time becomes longer. Thus, in order to explain the temperature dependence of the PL-decay profiles in Fig. 5, we introduce the shallow-bound-exciton state that lies below the dark-exciton state and propose the four-state model shown in Fig. 6(a): a ground state |gi, a bound-exciton state |Bxi, and two freeexciton states of a lower-lying dark-exciton state |Dxi and a higher-lying bright-exciton state |Brxi. Here, the bound-exciton state |Bxi lies below the dark-exciton state by an energy spacing of DE. It is noted that the thiol group would influence the bandedge exciton state and give rise to the bound-exciton state in the preparation process of the SAM samples. We assume a Boltzmann distribution of excitons between three excited states of |Brxi, |Dxi, and |Bxi on the basis of a statistical ensemble of QDs.19 Under such a condition, the rate equation for the total number of the excitons N is given as dN 1 1 1 nBrx nDx nBx ; ¼ dt tBrx tDx tBx
(1)
where ni and 1/ti (i = Brx, Dx, and Bx) denote the exciton number and the radiative-decay rate of |Brxi, |Dxi, and |Bxi, respectively. The ratio of the exciton number of the three excited states is given as DE DE þ DST nBx : nDx : nBrx ¼ 1 : d exp : d exp kB T kB T (2) where d represents the ratio of the density of state of the ‘‘free’’exciton states (|Brxi and |Dxi) to that of the bound-exciton state (|Bxi). From eqn (1) and (2), we obtain the decay rate, 1/t, as 1 1 DE 1 DE þ DST þd þd exp exp 1 tBx tDx kB T tBrx kB T ¼ : (3) DE DE þ DST t þ d exp 1 þ d exp kB T kB T Fig. 6(b) shows the calculated result for the temperature dependence of the decay time using eqn (3) with tBrx = 42 ns, tDx = 1000 ns, tBx = 12 ns, DST = 14 meV, DE = 17 meV, and d = 38. We note that these parameter values are related to the experimental results described below. The calculated result qualitatively explains the experimental one; namely, the PL decay becomes slower with an increase in temperature up to 160 K. Further quantitative analysis is discussed below. The observed decay profiles consist of multiple-exponential components as shown in Fig. 5. The analysis of multipleexponential-decay profiles in colloidal QDs is still controversial. In order to discuss the PL-decay profiles quantitatively, we analyzed them using a Kohlraush–Williams–Watts-type function.36 The best fits were obtained with a combination of one monoexponential and one stretched exponential functions: A1exp(t/t1) + A2exp[(t/t2)b],
(4)
where the parameters A1 and A2 represent the relative weights of the exponential component and stretched exponential decay one. Two examples of the analysis are shown in Fig. 7. It is evident that the PL-decay profiles are reasonably explained by the fitting model in the whole time range. From the stretchedexponential PL-decay component, we can obtain an average decay time ht2i = (t2/b) G(1/b) as introduced, e.g., in ref. 36. In order to discuss the temperature dependence of the PL decay
Fig. 6 (a) Schematic energy levels of the four-state model in CdSe QDs. (b) Calculated results for the temperature dependence of the decay time using eqn (3).
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Fig. 7 Temporal profiles of the band-edge PL at 20 and 180 K (open circles). The solid curves indicate the best fits with a combination of one monoexponential and one stretched exponential functions.
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profiles, we adopt the decay time of ht2i as an intrinsic decay time because the fast components (t1) would reflect relaxation processes of excitons and carriers. We consider the inhomogeneity of d, which corresponds to the ratio of the density of state of the bound-exciton (|Bxi) to those of the ‘‘free’’-exciton states (|Brxi and |Dxi), as the origin of the distribution of the decay time of ht2i. The CdSe QDs are attached to the substrate by the ligand exchange of the long-chain amine and the thiol group of MPTMS molecules. The influence of the thiol binding on the PL properties is spatially inhomogeneous because of a random distribution of the QDs in the SAM structure. The spatial inhomogeneity seems to be the origin of the stretched exponential decay. Fig. 8 shows the temperature dependence of ht2i and the fast decay time of t1. The values of b and t2 used to evaluate the average decay time ht2i shown Fig. 8 are as follows: t2 = 12, 25, 35, and 33 ns at 60, 100, 160, and 200 K, respectively, and b = 0.45 for all temperatures. The slow decay time of ht2i becomes slower with an increase in temperature up to 160 K, while the value of t1 gradually increases. As discussed in Fig. 2, the relative intensity of the band-edge PL band to that of the defect-related PL band is increased with an increase in temperature by the thermal activation of trapped carriers. The suppression of the relaxation process to the defect state would cause an increase in t1 corresponding to the relaxation and/or scattering time with an increase in temperature. The tendency of ht2i to increase with temperature in the temperature range up to 160 K well corresponds to the temperature dependence of t calculated using eqn (3) shown in Fig. 6(b). Thus, we analyze the temperature dependence of the slow decay time of ht2i on the basis of the four-state model described above. The dashed curve in Fig. 8 represents the calculated result of t on the basis of eqn (3). The calculated result quantitatively explains the temperature dependence of ht2i up to 180 K. At temperatures higher than 180 K, the calculated result hardly depends on temperature because of the saturation of the thermal excitation from the bound-exciton state to the dark-exciton state, while ht2i begins to decrease. The discrepancy between the calculated and the experimental results can be attributed to the influence of the non-radiative recombination process because eqn (3) represents only the ‘‘radiative’’ decay rate in the four-state model.
Fig. 8 Temperature dependence of the decay time of t1 (closed triangles) and ht2i (closed circles). The dashed and solid curves indicate calculated results for the temperature dependence of the decay time using eqn (3) without and with consideration of the nonradiative decay rate, respectively.
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Thus, we take into account the temperature dependence of the nonradiative decay rate, 1/tnr(T), in the form of a well-known thermal-activation type, which is given as 1 1 Ea ¼ exp ; (5) tnr ðT Þ tnr ðT ¼ 0Þ kB T where Ea represents the thermal-activation energy for the nonradiative process. We calculated the temperature dependence of t(T ), which is defined as 1 1 1 ¼ þ ; tðT Þ tr ðT Þ tnr ðT Þ
(6)
where 1/tr(T ) is given by eqn (3). The solid curve in Fig. 8 represents the calculated result with use of the fitting parameters of tnr(0) = 0.1 ns and Ea = 150 meV. The calculated result quantitatively explains the temperature dependence of ht2i. Thus, the considerable decrease in ht2i at temperatures higher than 180 K is consistently explained by taking into account the nonradiative process. The activation energy of 150 meV for a nonradiative process is phenomenological. A plausible nonradiative process is an energy transfer from the excitons to defect states. The defect-related PL band with a Stokes shift of B0.65 eV is observed in Fig. 2, though the relative intensity of the defect-related PL band to the band-edge PL band decreases with an increase in temperature. This fact suggests that the energy transfer is suppressed with an increase in temperature. Thus, the defects related to the PL band are excluded. It should be noted that various types of defect would exist in the QDs. The energy transfer from the excitons to silent defects seems to be the origin of the activation energy. In general, the decay time (t) and PL efficiency (Z) for the pure ‘‘two-level system’’ are given as 1/t = kr + knr and Z = kr/(kr + knr), where kr (knr) represents a radiative (nonradiative) decay rate. In this case, the temperature dependence of the intensity and the decay time would be in agreement; namely, a decrease of the PL intensity leads to a decrease of the decay time. A noteworthy observation in the present work is that the PL-decay profiles become slower with an increase in temperature although the PL intensity is decreased. These results cannot be explained by the simple two-level system; therefore, we proposed the four-state model. With use of the four-state model, the extraordinary temperature dependence of the decay time observed in the present work is reasonably explained as shown by Fig. 8. As mentioned above, the temperature dependence of PL dynamics of CdSe QDs has been explained using the conventional three-state model19–21,37 without the bound-exciton state. Labeau et al.37 studied the temperature dependence of the PL decay of single CdSe–ZnS QDs between 2 and 140 K. Crooker et al.19 measured the temperature dependence of PL decay profiles of CdSe QDs from 380 mK to 300 K. Those experimental results are explained using the three-state model. The characteristic feature in the present study originates from the existence of the boundexciton state, which was intentionally formed by preparing the SAM samples. As a result, the completely different temperature dependence from that based on the conventional three-state model is observed.
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Fig. 9 Temperature dependence of the decay time based on the conventional three-state model (TSM; eqn (7)) and the present four-state model (eqn (3)) with tBrx = 42 ns, tDx = 1000 ns, tBx = 12 ns, DST = 14 meV, D E = 17 meV, and d = 38, where the non-radiative decay rate is not taken into account.
Next, we discuss the difference in the temperature dependence of the decay time owing to the existence of the bound-exciton state. Fig. 9 shows the temperature dependence of the decay time based on the conventional three-state model19,21 and the present fourstate model (eqn (3)), where the non-radiative decay rate is neglected. The decay rate, 1/tTSM, on the basis of the conventional three state model is given as 1 tTSM
¼
1 1 kDET þ e B tDx tBrx DE k T B
:
(7)
1þe
In the case of the conventional three-state model, which does not contain the bound-exciton state, it has a long decay time reflecting the population of the dark-exciton state at low temperatures. The population of the upper-lying bright-exciton state with a fast decay component is increased with an increase in temperature by the thermal excitation, leading to shortening of the decay time. On the other hand, if the bound-exciton state exists, the decay time becomes very fast even at low temperatures reflecting the decay time of the bound-exciton state with a bright exciton nature. With an increase in temperature, the thermal population from the bound-exciton state to the darkexciton state is increased, and the decay time becomes longer. At higher temperatures, the results of both the models approach each other. These results indicate that the radiative decay time in QDs is changeable by the presence or absence of the bound-exciton state. The surface modification of QDs, adsorption of the thiol group as reported in this paper, and impurity doping are possible methods of introducing the bound-exciton state into QDs.
IV. Conclusion We have investigated the temperature dependence of the absorption, PL, and PL dynamics of the SAM of the CdSe QDs. The temperature dependence of the exciton energy is quantitatively explained on the basis of the Varshni’s law. In the PL spectra, the band-edge PL is clearly observed even at a low temperature of 10 K. The observation of the band-edge PL enabled us to investigate the PL dynamics in detail. The PL-decay profile was fitted by a combination of monoexponential
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and stretched exponential functions corresponding to the fast and slow components, respectively. The slow component is attributed to the exciton decay process because its intensity is dominant. The temperature dependence of the decay time of the slow component exhibits the following anomalous behaviour: the decay time increases with an increase in temperature up to 160 K, then a further increase of temperature leads to a decrease of the decay time. The increase of the decay time is reasonably explained by the four-state model consisting of the ground state, bound-exciton state, and two free-exciton states; lower-lying dark-exciton and higher-lying bright-exciton states. It is demonstrated that the radiative decay time is strongly influenced by the presence or absence of the bound-exciton state.
References 1 U. Woggon, Optical Properties of Semiconductor Quantum Dots, Springer, 1996. 2 A. D. Yoffe, Adv. Phys., 2001, 50, 1. 3 Y. Masumoto and T. Takagahara, Semiconductor Quantums Dots, Springer, 2002. 4 V. I. Klimov, Semiconductor and Metal Nanocrystals, Marcel Dekker Inc., 2004. 5 I. L. Medintz, H. T. Uyeda, E. R. Goldman and H. Mattoussi, Nat. Mater., 2005, 4, 435 and references there in. 6 V. I. Klimov, A. A. Mikhailovsky, S. Xu, A. Malko, J. A. Hollingsworth, C. A. Leatherdale, H.-J. Eisler and M. G. Bawendi, Science, 2000, 290, 314. 7 A. J. Nozik, Physica E, 2002, 14, 115. 8 I. Gur, N. A. Fromer, M. L. Geier and A. P. Alivisatos, Science, 2005, 310, 462. 9 C. B. Murray, D. J. Norris and M. G. Bawendi, J. Am. Chem. Soc., 1993, 115, 8706. 10 X. Peng, L. Manna, W. D. Yang, J. Wickham, E. Scher, A. Kadavanish and A. P. Alivisatos, Nature, 2000, 404, 59. 11 J. J. Li, A. Wang, W. Guo, J. C. Keay, T. D. Mishima, M. B. Johnson and X. Peng, J. Am. Chem. Soc., 2003, 125, 12567. 12 Al. L. Efros, M. Rosen, M. Kuno, M. Nirmal, D. J. Norris and M. G. Bawendi, Phys. Rev. B: Condens. Matter Mater. Phys., 1993, 54, 4843. 13 V. I. Klimov, J. Phys. Chem. B, 2000, 104, 6112. ¨ler, U. Woggon, M. V. Artemyev and R. Wannemachev, 14 B. M. Mo Phys. Rev. B: Condens. Matter Mater. Phys., 2004, 70, 115323. 15 I. Chung and M. G. Bawendi, Phys. Rev. B: Condens. Matter Mater. Phys., 2004, 70, 165304. 16 D. Yu, B. L. Wehrenberg, P. Jha, J. Ma and P. GuyotSionnest, J. Appl. Phys., 2006, 99, 104315. ´, M. Bode and A. Meijerink, Phys. Rev. B: 17 C. M. Donega Condens. Matter Mater. Phys., 2006, 74, 085320. 18 V. I. Klimov, S. A. Ivanov, J. Nanda, M. Achermann, I. Bezel, J. A. Mcguire and A. Piryatinski, Nature, 2007, 447, 441. 19 S. A. Crooker, T. Barrick, J. A. Hollingsworth and V. I. Klimov, Appl. Phys. Lett., 2003, 82, 2793. ˙a, M. Bode and A. Meijerink, Phys. Rev. B: 20 C. M. Doneg Condens. Matter Mater. Phys., 2006, 74, 085320.
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21 D. Kim, K. Okazaki and M. Nakayama, Phys. Rev. B: Condens. Matter Mater. Phys., 2009, 80, 045322. 22 Z. Yu, J. Li, D. B. O’Connor, L. W. Wang and P. F. Barbara, J. Phys. Chem. B, 2003, 107, 5670. 23 D. O. Demchenko and L. W. Wang, Phys. Rev. B: Condens. Matter Mater. Phys., 2006, 73, 155326. 24 D. Kim, T. Mishima, K. Tomihira and M. Nakayama, J. Phys. Chem. C, 2008, 112, 10668. 25 M. Nirmal, C. B. Murray and M. G. Bawendi, Phys. Rev. B: Condens. Matter Mater. Phys., 1994, 50, 2293. 26 X. Wang, L. Qu, J. Zhang, X. Peng and M. Xiao, Nano Lett., 2003, 3, 1103. 27 M. Califano, A. Franceschetti and A. Zunger, Nano Lett., 2005, 5, 2360. 28 X. Wang, W. W. Yu, J. Zhang, J. Aldana, X. Peng and M. Xiao, Phys. Rev. B: Condens. Matter Mater. Phys., 2003, 68, 125318.
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29 A. G. Joly, W. Chen, D. E. McCready, J. O. Malm and J. O. Bovin, Phys. Rev. B: Condens. Matter Mater. Phys., 2005, 71, 165304. ´ and A. J. Nozik, Appl. 30 E. Poles, D. C. Selmarten, O. I. Mic´ic Phys. Lett., 1999, 75, 971. 31 N. Pradhan, D. M. Battaglia, Y. Liu and X. Peng, Nano Lett., 2007, 7, 312. 32 J. Aldana, Y. A. Wang and X. J. Peng, J. Am. Chem. Soc., 2001, 123, 8844. 33 H. Yokota, K. Okazaki, K. Shimura, M. Nakayama and D. Kim, J. Phys. Chem. C, 2012, 116, 5456. 34 Y. P. Varshni, Physica, 1967, 34, 149. ¨tz, in Properties of Wide Band Gap 35 W. Gebhart and G. Scho Semiconductors, ed. R. Bhargava, INSPEC, London, 1997, p. 113. ¨ps, N. L. Thomas, U. Woggon and M. V. Artemyev, 36 O. Scho J. Phys. Chem., 2006, 110, 2074. 37 O. Labeau, P. Tamarat and B. Lounis, Phys. Rev. Lett., 2003, 90, 257404.
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Temperature dependence of photoluminescence dynamics of self-assembled monolayers of CdSe quantum dots: the influence of the bound-exciton state DaeGwi Kim,* Hiroki Yokota, Kunio Shimura and Masaaki Nakayama We have investigated the temperature dependence of photoluminescence (PL) dynamics of selfassembled monolayers (SAMs) of CdSe quantum dots (QDs). The PL decay profiles become slower with an increase in temperature up to 160 K, contrary to an ordinary behavior due to thermal quenching.
Received 23rd September 2013, Accepted 21st October 2013
Such anomalous temperature dependence of the PL-decay profile is explained using a four-state model which introduces a bound-exciton state into a conventional three-state model consisting of a ground
DOI: 10.1039/c3cp54012j
state and two excited states: a lower-lying dark-exciton state and a higher-lying bright-exciton state. Furthermore, it is proposed that the radiative decay time of QDs is strongly influenced by the presence
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or absence of the bound-exciton state.
I. Introduction Semiconductor quantum dots (QDs) have attracted considerable attention for understanding the intrinsic nature of physical/ chemical properties of QDs.1–4 Furthermore, the breakthrough in synthesizing colloidal QDs with a high photoluminescence (PL) yield has led to an explosive increase of QD studies and opened up possibilities for various applications such as biomolecular imaging,5 QD lasing,4,6 QD solar cells,7,8 and so on. One of the most characteristic PL properties of QDs is the observation of an optically passive state, the so-called dark-exciton state.5–12 The origin of the dark-exciton state is basically a spin triplet state that is forbidden for optical transitions by the spin-selection rule. In a bulk crystal, thermal effects inhibit the observation of the triplet-exciton state since the spin-exchange-splitting energy is usually smaller than 1 meV. In contrast, the quantum confinement effect in a QD system enhances the spin-exchange interaction, which results in a large splitting energy between the spin singlet and triplet states. The spin-exchange-splitting energy is not only dependent on the QD size but also strongly dependent on semiconductor materials. Since the chemical synthesis of size-controlled CdSe QDs with a high photoluminescence (PL) yield has been well established,9–11 CdSe QDs have become a model material for studies of optical properties of QDs.12–18 The splitting energy in Department of Applied Physics, Graduate School of Engineering, Osaka City University, 3-3-138, Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan. E-mail: [email protected]
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CdSe QDs changes from 2 to 20 meV with a decrease in the QD size.12 The temperature dependence of PL dynamics of CdSe and/or CdSe/ZnS QDs can be understood using a conventional three-state model consisting of a ground state and two excited states: a lower-lying dark-exciton state and a higher-lying bright-exciton state.19–21 On the basis of this model, the darkexciton state mainly contributes to PL processes at low temperatures. With an increase in temperature, the population in the higher-lying bright-exciton state with a fast decay component is increased by the thermal excitation from the lower-lying dark-exciton state with a slow-decay component, which leads to shortening the decay profiles. On the other hand, it was reported theoretically and experimentally that the spin-exchange-splitting energy can be of the order of 20 to 60 meV in CdS QDs, depending on the QD size.22,23 We reported that the PL decay profiles in CdS QDs exhibit a long decay time of hundreds of nanoseconds even at room temperature, which demonstrates the large contribution of the dark-exciton state to the PL processes.24 This is owing to the large splitting energy between the bright- and dark-exciton states in CdS QDs compared with other semiconductor QDs such as CdSe, CdTe, and Si QDs. The existence of the slow component even at room temperature is an intrinsic feature peculiar to CdS QDs. Furthermore, it is noted that the decay profiles of the band-edge PL exhibit characteristic temperature dependence; they become slower with an increase in temperature.24 An important point explaining the temperature dependence is the existence of a bound-exciton state on the low energy side of the dark-exciton state. The population in the upper-lying dark-exciton state with a slow decay component is increased with an increase in
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temperature by the thermal excitation from the lower-lying bound-exciton state with a fast-decay component, leading to prolonging the decay profiles. This temperature dependence is just contrary to that in the above-mentioned conventional three-state model. The surface-related shallow-bound state has been discussed for CdSe,25–27 CdTe,28,29 and InP QDs30 so far. Nirmal et al. reported the surface localization of photo-excited excitons in CdSe QDs by measuring size-selective PL spectra and PL-decay profiles at low temperatures below 10 K.25 Califano et al. suggested theoretically that that surface states greatly influence the PL-decay times in CdSe QDs27 Wang et al. reported the contribution of surface states to the PL process by measuring time-resolved PL in highly luminescent CdSe QDs at room temperature.26 Furthermore, Wang et al. provided a proof of the existence of surface states from the results of up-converted PL with an ultrashort time resolution in CdTe QDs at room temperature.28 However, little has been known about the systematic temperature dependence of the PL dynamics in the wide range from low temperature to room temperature from a viewpoint of the contribution to the PL processes of the bound-exciton state. It is well known that the thiol group influences PL properties of CdSe QDs.31,32 In ref. 32, the strong quenching of PL of CdSe QDs by thiol ligands is reported. It is demonstrated that the photogenerated holes are trapped by the thiol ligands on the surface of the QDs, which quenches the PL of the QDs and initiates the photooxidation of the surface ligands. Thus, it is expected that the bound-exciton state is formed intentionally by the thiol group. This is connected with our motivation in the present work. A self-assembled monolayer (SAM) of CdSe QDs can be used for this purpose since the QDs are usually attached to substrates by the thiol group in the SAM samples. Furthermore, since the QDs are uniformly adsorbed on the substrate, the SAM samples are very suitable for measurements of the temperature dependence of optical properties.33 In the present work, we have investigated PL properties of the SAM of the CdSe QDs from a viewpoint of the influence of the bound exciton state on the PL processes by measuring the temperature dependence of the absorption, PL, and PL-decay profiles. The PL-decay profiles become slower with an increase in temperature up to 160 K, which is contrary to ordinary behavior due to thermal quenching. This anomalous temperature dependence can be explained in terms of a four-state model including the bound-exciton state. Furthermore, it is demonstrated that the radiative decay time of the QDs can be controlled by the presence of the bound-exciton state.
II. Experiments CdSe QDs with B3.3 nm in diameter were purchased from Sigma-Aldrich. The substrates of quartz were cleaned by immersion in fresh piranha solution [a 1/3 (v/v) mixture of 30% H2O2 and 98% H2SO4] for 20 min. (Caution: Piranha solution reacts violently with organic materials.) Then the substrates were rinsed with water and used immediately after cleaning. In the beginning of the sample
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Fig. 1
The AFM image of the SAM of the CdSe QDs.
preparation, SAMs that consist of (3-mercaptopropyl) trimethoxysilane (MPTMS) molecules were deposited to anchor the QDs. The SAM formation was achieved by immersing the precleaned substrate in a MPTMS–toluene solution for 1 h. Next, the substrate was rinsed thoroughly with toluene. The SAM of the QDs linked by thiol molecules was prepared in a QD–toluene solution for 1 h. Finally, the samples were thoroughly rinsed with toluene and dried with a stream of nitrogen gas. The atomic force microscopy (AFM) image of the SAM of the CdSe QDs is shown in Fig. 1. The image shows the coverage of QDs on the substrate. Furthermore, the rootmean-square roughness was confirmed to be B1.5 nm. After the preparation, the sample was introduced into a closed-cycle helium-gas cryostat, and the temperature dependence of absorption, PL, and PL-decay profiles under vacuum was observed. The absorption spectra were measured using a double-beam spectrometer with a resolution of 0.2 nm. For PL measurements, the 325 nm line of a He–Cd laser was used as the excitation-light source, and the emitted PL was analyzed using a single monochromator with a spectral resolution of 0.5 nm. For measurements of PL-decay profiles, a laser-diode (405 nm, Hamamatsu PLP10-040) with a pulse duration of 100 ps and a repetition of 500 kHz was used as the excitation light. The pump fluence was 50 nJ cm2. The PL-decay profiles were obtained by a time-correlated single-photon counting method. The sample temperature was controlled using a closed-cycle helium-gas cryostat. We used the CdSe QDs covered with the ligands of longchain amine: the ligand length is estimated to be 2.6 nm. This long-chain ligand prevents the coupling between CdSe QDs. In fact, the absorption energy of the SAM structure is comparable to that of the solution sample in which the QDs are isolated, and no spectral shift is observed. Thus, it is noted that the PL properties of an ‘‘ensemble’’ of the non-interacting CdSe QDs are observed in the present work.
III. Results and discussion Fig. 2 shows the temperature dependence of the absorption and PL spectra of the SAM of the CdSe QDs. The absorption- and PL-peak energies shift to the higher energy side with a decrease in temperature. Fig. 3 shows the temperature dependence of the lowest absorption peak (exciton) energy. It is well known that the temperature dependence of the exciton energy in a direct-gap semiconductor can be described using Varshni’s law;34 E(T) = E(0) aT 2/(T + b), where E(0) is the exciton energy
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Fig. 5
Temporal profiles of the band-edge PL band at 10, 80, 120, and 160 K.
Fig. 2 Temperature dependence of the absorption and PL spectra of the SAM of the CdSe QDs.
Fig. 3
Temperature dependence of the lowest absorption peak (exciton) energy.
at T = 0 K, a is the temperature coefficient, and b is a parameter related to the Debye temperature of the crystal. The solid curve indicates the calculated result for the temperature dependence of the absorption energy, where the parameter values of E(0) = 2.41 eV, a = 3.7104 eV K1 and b = 150 K for a CdSe bulk crystal35 were used. The calculated results using the parameters for the bulk crystal quantitatively explain the experimental results, which indicates that the observed temperature dependence of the absorption-peak energy results from the intrinsic property of the CdSe crystal. As shown in Fig. 2, the band-edge PL is clearly observed at every temperature. Although a defect-related PL band with a large Stokes shift of B0.65 eV is dominant at low temperatures, the relative intensity of the band-edge PL band to that of the defect-related PL band is increased with an increase in temperature by the thermal activation of trapped carriers. Fig. 4 shows the temperature dependence of the integrated intensity of the bandedge PL band. The intensity of the band-edge PL band at room temperature is almost 10% of that at 10 K.
Fig. 4 Temperature dependence of the integrated intensity of the band-edge PL band.
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Next, in order to investigate the influence of the thiol group on the band-edge exciton state, we measured the temperature dependence of PL-decay profiles. Fig. 5 shows the temporal profiles of the band-edge PL band at 10, 80, 120, and 160 K. In this temperature range, temporal profiles become slower with an increase in temperature. In the usual case, as the temperature is increased, the nonradiative-decay rate related to the thermal quenching becomes larger, so that the PL intensity and the PL-decay time are decreased. In fact, as shown in Fig. 4, the band-edge PL intensity gradually decreases with an increase in temperature. Nevertheless, the observed decay profiles become longer, which is contrary to the above scenario. So far, the temperature dependence of PL dynamics of CdSe QDs has been explained using the conventional three-state model consisting of a ground state and two excited states: a lower-lying darkexciton state and an upper-lying bright-exciton state.19–21 On the basis of this model, we can expect that the population in the upper state with a fast decay component is increased with an increase in temperature by the thermal excitation from the lower state with a slow decay component, leading to shortening the decay profiles. However, the temperature dependence of the PL-decay profile shown in Fig. 5 is completely contrary to the previously reported results. We also measured the temperature dependence of PL properties for a drop cast film of the CdSe QDs. It is noted that the drop-cast-film samples are free from the thiol group. The PL-decay profiles become faster with an increase in temperature as in previous reports.19–21 The temperature dependence of the PL-decay profile for the dropcast film is fully contrary to that for the present SAM structure, in which the decay profiles become slower with an increase in temperature. Thus, these results clearly demonstrate that the temperature dependence of the PL-decay profile for the SAM structure is related to the thiol anchoring. In order to explain the anomalous temperature dependence of the decay profiles, we have to assume that at least two radiative states contribute to PL processes: a lower-lying emitting state with a relatively fast decay component and an upper-lying emitting state with a relatively slow decay component. On the basis of this model, we can expect that the population in the upper state with a slow decay component is increased with an increase in temperature by the thermal excitation from the lower state with a fast decay component, leading to prolongation of the decay profiles. The PL-decay
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profile exhibits a slow decay over hundreds of nanoseconds. This long decay profile suggests that the optically passive state, the so-called dark-exciton state, contributes to the PL process. Thus, we attribute the upper-lying emitting state with a slow decay component to the dark-exciton state. Since the darkexciton state is a ‘‘free-exciton’’ state, we have to consider a ‘‘bound-exciton’’ state related to a bright exciton as the origin of the lower-lying emitting state. Similar temperature dependence of PL decay profiles was previously observed in CdS QDs,24 and it was quantitatively explained using a three-state model consisting of a ground state and two excited states; a lower-lying bound-exciton state and a higher-lying dark-exciton state.24 An important point is the introduction of the bound-exciton state into the model. With an increase in temperature, the thermal population of the higher-lying dark-exciton state is significant, and the decay time becomes longer. Thus, in order to explain the temperature dependence of the PL-decay profiles in Fig. 5, we introduce the shallow-bound-exciton state that lies below the dark-exciton state and propose the four-state model shown in Fig. 6(a): a ground state |gi, a bound-exciton state |Bxi, and two freeexciton states of a lower-lying dark-exciton state |Dxi and a higher-lying bright-exciton state |Brxi. Here, the bound-exciton state |Bxi lies below the dark-exciton state by an energy spacing of DE. It is noted that the thiol group would influence the bandedge exciton state and give rise to the bound-exciton state in the preparation process of the SAM samples. We assume a Boltzmann distribution of excitons between three excited states of |Brxi, |Dxi, and |Bxi on the basis of a statistical ensemble of QDs.19 Under such a condition, the rate equation for the total number of the excitons N is given as dN 1 1 1 nBrx nDx nBx ; ¼ dt tBrx tDx tBx
(1)
where ni and 1/ti (i = Brx, Dx, and Bx) denote the exciton number and the radiative-decay rate of |Brxi, |Dxi, and |Bxi, respectively. The ratio of the exciton number of the three excited states is given as DE DE þ DST nBx : nDx : nBrx ¼ 1 : d exp : d exp kB T kB T (2) where d represents the ratio of the density of state of the ‘‘free’’exciton states (|Brxi and |Dxi) to that of the bound-exciton state (|Bxi). From eqn (1) and (2), we obtain the decay rate, 1/t, as 1 1 DE 1 DE þ DST þd þd exp exp 1 tBx tDx kB T tBrx kB T ¼ : (3) DE DE þ DST t þ d exp 1 þ d exp kB T kB T Fig. 6(b) shows the calculated result for the temperature dependence of the decay time using eqn (3) with tBrx = 42 ns, tDx = 1000 ns, tBx = 12 ns, DST = 14 meV, DE = 17 meV, and d = 38. We note that these parameter values are related to the experimental results described below. The calculated result qualitatively explains the experimental one; namely, the PL decay becomes slower with an increase in temperature up to 160 K. Further quantitative analysis is discussed below. The observed decay profiles consist of multiple-exponential components as shown in Fig. 5. The analysis of multipleexponential-decay profiles in colloidal QDs is still controversial. In order to discuss the PL-decay profiles quantitatively, we analyzed them using a Kohlraush–Williams–Watts-type function.36 The best fits were obtained with a combination of one monoexponential and one stretched exponential functions: A1exp(t/t1) + A2exp[(t/t2)b],
(4)
where the parameters A1 and A2 represent the relative weights of the exponential component and stretched exponential decay one. Two examples of the analysis are shown in Fig. 7. It is evident that the PL-decay profiles are reasonably explained by the fitting model in the whole time range. From the stretchedexponential PL-decay component, we can obtain an average decay time ht2i = (t2/b) G(1/b) as introduced, e.g., in ref. 36. In order to discuss the temperature dependence of the PL decay
Fig. 6 (a) Schematic energy levels of the four-state model in CdSe QDs. (b) Calculated results for the temperature dependence of the decay time using eqn (3).
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Fig. 7 Temporal profiles of the band-edge PL at 20 and 180 K (open circles). The solid curves indicate the best fits with a combination of one monoexponential and one stretched exponential functions.
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profiles, we adopt the decay time of ht2i as an intrinsic decay time because the fast components (t1) would reflect relaxation processes of excitons and carriers. We consider the inhomogeneity of d, which corresponds to the ratio of the density of state of the bound-exciton (|Bxi) to those of the ‘‘free’’-exciton states (|Brxi and |Dxi), as the origin of the distribution of the decay time of ht2i. The CdSe QDs are attached to the substrate by the ligand exchange of the long-chain amine and the thiol group of MPTMS molecules. The influence of the thiol binding on the PL properties is spatially inhomogeneous because of a random distribution of the QDs in the SAM structure. The spatial inhomogeneity seems to be the origin of the stretched exponential decay. Fig. 8 shows the temperature dependence of ht2i and the fast decay time of t1. The values of b and t2 used to evaluate the average decay time ht2i shown Fig. 8 are as follows: t2 = 12, 25, 35, and 33 ns at 60, 100, 160, and 200 K, respectively, and b = 0.45 for all temperatures. The slow decay time of ht2i becomes slower with an increase in temperature up to 160 K, while the value of t1 gradually increases. As discussed in Fig. 2, the relative intensity of the band-edge PL band to that of the defect-related PL band is increased with an increase in temperature by the thermal activation of trapped carriers. The suppression of the relaxation process to the defect state would cause an increase in t1 corresponding to the relaxation and/or scattering time with an increase in temperature. The tendency of ht2i to increase with temperature in the temperature range up to 160 K well corresponds to the temperature dependence of t calculated using eqn (3) shown in Fig. 6(b). Thus, we analyze the temperature dependence of the slow decay time of ht2i on the basis of the four-state model described above. The dashed curve in Fig. 8 represents the calculated result of t on the basis of eqn (3). The calculated result quantitatively explains the temperature dependence of ht2i up to 180 K. At temperatures higher than 180 K, the calculated result hardly depends on temperature because of the saturation of the thermal excitation from the bound-exciton state to the dark-exciton state, while ht2i begins to decrease. The discrepancy between the calculated and the experimental results can be attributed to the influence of the non-radiative recombination process because eqn (3) represents only the ‘‘radiative’’ decay rate in the four-state model.
Fig. 8 Temperature dependence of the decay time of t1 (closed triangles) and ht2i (closed circles). The dashed and solid curves indicate calculated results for the temperature dependence of the decay time using eqn (3) without and with consideration of the nonradiative decay rate, respectively.
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Thus, we take into account the temperature dependence of the nonradiative decay rate, 1/tnr(T), in the form of a well-known thermal-activation type, which is given as 1 1 Ea ¼ exp ; (5) tnr ðT Þ tnr ðT ¼ 0Þ kB T where Ea represents the thermal-activation energy for the nonradiative process. We calculated the temperature dependence of t(T ), which is defined as 1 1 1 ¼ þ ; tðT Þ tr ðT Þ tnr ðT Þ
(6)
where 1/tr(T ) is given by eqn (3). The solid curve in Fig. 8 represents the calculated result with use of the fitting parameters of tnr(0) = 0.1 ns and Ea = 150 meV. The calculated result quantitatively explains the temperature dependence of ht2i. Thus, the considerable decrease in ht2i at temperatures higher than 180 K is consistently explained by taking into account the nonradiative process. The activation energy of 150 meV for a nonradiative process is phenomenological. A plausible nonradiative process is an energy transfer from the excitons to defect states. The defect-related PL band with a Stokes shift of B0.65 eV is observed in Fig. 2, though the relative intensity of the defect-related PL band to the band-edge PL band decreases with an increase in temperature. This fact suggests that the energy transfer is suppressed with an increase in temperature. Thus, the defects related to the PL band are excluded. It should be noted that various types of defect would exist in the QDs. The energy transfer from the excitons to silent defects seems to be the origin of the activation energy. In general, the decay time (t) and PL efficiency (Z) for the pure ‘‘two-level system’’ are given as 1/t = kr + knr and Z = kr/(kr + knr), where kr (knr) represents a radiative (nonradiative) decay rate. In this case, the temperature dependence of the intensity and the decay time would be in agreement; namely, a decrease of the PL intensity leads to a decrease of the decay time. A noteworthy observation in the present work is that the PL-decay profiles become slower with an increase in temperature although the PL intensity is decreased. These results cannot be explained by the simple two-level system; therefore, we proposed the four-state model. With use of the four-state model, the extraordinary temperature dependence of the decay time observed in the present work is reasonably explained as shown by Fig. 8. As mentioned above, the temperature dependence of PL dynamics of CdSe QDs has been explained using the conventional three-state model19–21,37 without the bound-exciton state. Labeau et al.37 studied the temperature dependence of the PL decay of single CdSe–ZnS QDs between 2 and 140 K. Crooker et al.19 measured the temperature dependence of PL decay profiles of CdSe QDs from 380 mK to 300 K. Those experimental results are explained using the three-state model. The characteristic feature in the present study originates from the existence of the boundexciton state, which was intentionally formed by preparing the SAM samples. As a result, the completely different temperature dependence from that based on the conventional three-state model is observed.
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Fig. 9 Temperature dependence of the decay time based on the conventional three-state model (TSM; eqn (7)) and the present four-state model (eqn (3)) with tBrx = 42 ns, tDx = 1000 ns, tBx = 12 ns, DST = 14 meV, D E = 17 meV, and d = 38, where the non-radiative decay rate is not taken into account.
Next, we discuss the difference in the temperature dependence of the decay time owing to the existence of the bound-exciton state. Fig. 9 shows the temperature dependence of the decay time based on the conventional three-state model19,21 and the present fourstate model (eqn (3)), where the non-radiative decay rate is neglected. The decay rate, 1/tTSM, on the basis of the conventional three state model is given as 1 tTSM
¼
1 1 kDET þ e B tDx tBrx DE k T B
:
(7)
1þe
In the case of the conventional three-state model, which does not contain the bound-exciton state, it has a long decay time reflecting the population of the dark-exciton state at low temperatures. The population of the upper-lying bright-exciton state with a fast decay component is increased with an increase in temperature by the thermal excitation, leading to shortening of the decay time. On the other hand, if the bound-exciton state exists, the decay time becomes very fast even at low temperatures reflecting the decay time of the bound-exciton state with a bright exciton nature. With an increase in temperature, the thermal population from the bound-exciton state to the darkexciton state is increased, and the decay time becomes longer. At higher temperatures, the results of both the models approach each other. These results indicate that the radiative decay time in QDs is changeable by the presence or absence of the bound-exciton state. The surface modification of QDs, adsorption of the thiol group as reported in this paper, and impurity doping are possible methods of introducing the bound-exciton state into QDs.
IV. Conclusion We have investigated the temperature dependence of the absorption, PL, and PL dynamics of the SAM of the CdSe QDs. The temperature dependence of the exciton energy is quantitatively explained on the basis of the Varshni’s law. In the PL spectra, the band-edge PL is clearly observed even at a low temperature of 10 K. The observation of the band-edge PL enabled us to investigate the PL dynamics in detail. The PL-decay profile was fitted by a combination of monoexponential
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and stretched exponential functions corresponding to the fast and slow components, respectively. The slow component is attributed to the exciton decay process because its intensity is dominant. The temperature dependence of the decay time of the slow component exhibits the following anomalous behaviour: the decay time increases with an increase in temperature up to 160 K, then a further increase of temperature leads to a decrease of the decay time. The increase of the decay time is reasonably explained by the four-state model consisting of the ground state, bound-exciton state, and two free-exciton states; lower-lying dark-exciton and higher-lying bright-exciton states. It is demonstrated that the radiative decay time is strongly influenced by the presence or absence of the bound-exciton state.
References 1 U. Woggon, Optical Properties of Semiconductor Quantum Dots, Springer, 1996. 2 A. D. Yoffe, Adv. Phys., 2001, 50, 1. 3 Y. Masumoto and T. Takagahara, Semiconductor Quantums Dots, Springer, 2002. 4 V. I. Klimov, Semiconductor and Metal Nanocrystals, Marcel Dekker Inc., 2004. 5 I. L. Medintz, H. T. Uyeda, E. R. Goldman and H. Mattoussi, Nat. Mater., 2005, 4, 435 and references there in. 6 V. I. Klimov, A. A. Mikhailovsky, S. Xu, A. Malko, J. A. Hollingsworth, C. A. Leatherdale, H.-J. Eisler and M. G. Bawendi, Science, 2000, 290, 314. 7 A. J. Nozik, Physica E, 2002, 14, 115. 8 I. Gur, N. A. Fromer, M. L. Geier and A. P. Alivisatos, Science, 2005, 310, 462. 9 C. B. Murray, D. J. Norris and M. G. Bawendi, J. Am. Chem. Soc., 1993, 115, 8706. 10 X. Peng, L. Manna, W. D. Yang, J. Wickham, E. Scher, A. Kadavanish and A. P. Alivisatos, Nature, 2000, 404, 59. 11 J. J. Li, A. Wang, W. Guo, J. C. Keay, T. D. Mishima, M. B. Johnson and X. Peng, J. Am. Chem. Soc., 2003, 125, 12567. 12 Al. L. Efros, M. Rosen, M. Kuno, M. Nirmal, D. J. Norris and M. G. Bawendi, Phys. Rev. B: Condens. Matter Mater. Phys., 1993, 54, 4843. 13 V. I. Klimov, J. Phys. Chem. B, 2000, 104, 6112. ¨ler, U. Woggon, M. V. Artemyev and R. Wannemachev, 14 B. M. Mo Phys. Rev. B: Condens. Matter Mater. Phys., 2004, 70, 115323. 15 I. Chung and M. G. Bawendi, Phys. Rev. B: Condens. Matter Mater. Phys., 2004, 70, 165304. 16 D. Yu, B. L. Wehrenberg, P. Jha, J. Ma and P. GuyotSionnest, J. Appl. Phys., 2006, 99, 104315. ´, M. Bode and A. Meijerink, Phys. Rev. B: 17 C. M. Donega Condens. Matter Mater. Phys., 2006, 74, 085320. 18 V. I. Klimov, S. A. Ivanov, J. Nanda, M. Achermann, I. Bezel, J. A. Mcguire and A. Piryatinski, Nature, 2007, 447, 441. 19 S. A. Crooker, T. Barrick, J. A. Hollingsworth and V. I. Klimov, Appl. Phys. Lett., 2003, 82, 2793. ˙a, M. Bode and A. Meijerink, Phys. Rev. B: 20 C. M. Doneg Condens. Matter Mater. Phys., 2006, 74, 085320.
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21 D. Kim, K. Okazaki and M. Nakayama, Phys. Rev. B: Condens. Matter Mater. Phys., 2009, 80, 045322. 22 Z. Yu, J. Li, D. B. O’Connor, L. W. Wang and P. F. Barbara, J. Phys. Chem. B, 2003, 107, 5670. 23 D. O. Demchenko and L. W. Wang, Phys. Rev. B: Condens. Matter Mater. Phys., 2006, 73, 155326. 24 D. Kim, T. Mishima, K. Tomihira and M. Nakayama, J. Phys. Chem. C, 2008, 112, 10668. 25 M. Nirmal, C. B. Murray and M. G. Bawendi, Phys. Rev. B: Condens. Matter Mater. Phys., 1994, 50, 2293. 26 X. Wang, L. Qu, J. Zhang, X. Peng and M. Xiao, Nano Lett., 2003, 3, 1103. 27 M. Califano, A. Franceschetti and A. Zunger, Nano Lett., 2005, 5, 2360. 28 X. Wang, W. W. Yu, J. Zhang, J. Aldana, X. Peng and M. Xiao, Phys. Rev. B: Condens. Matter Mater. Phys., 2003, 68, 125318.
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29 A. G. Joly, W. Chen, D. E. McCready, J. O. Malm and J. O. Bovin, Phys. Rev. B: Condens. Matter Mater. Phys., 2005, 71, 165304. ´ and A. J. Nozik, Appl. 30 E. Poles, D. C. Selmarten, O. I. Mic´ic Phys. Lett., 1999, 75, 971. 31 N. Pradhan, D. M. Battaglia, Y. Liu and X. Peng, Nano Lett., 2007, 7, 312. 32 J. Aldana, Y. A. Wang and X. J. Peng, J. Am. Chem. Soc., 2001, 123, 8844. 33 H. Yokota, K. Okazaki, K. Shimura, M. Nakayama and D. Kim, J. Phys. Chem. C, 2012, 116, 5456. 34 Y. P. Varshni, Physica, 1967, 34, 149. ¨tz, in Properties of Wide Band Gap 35 W. Gebhart and G. Scho Semiconductors, ed. R. Bhargava, INSPEC, London, 1997, p. 113. ¨ps, N. L. Thomas, U. Woggon and M. V. Artemyev, 36 O. Scho J. Phys. Chem., 2006, 110, 2074. 37 O. Labeau, P. Tamarat and B. Lounis, Phys. Rev. Lett., 2003, 90, 257404.
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