Temperature-Sensitive Photoluminescence of CdSe Quantum Dot

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J. Phys. Chem. B 2005, 109, 13899-13905

13899

Temperature-Sensitive Photoluminescence of CdSe Quantum Dot Clusters Vasudevanpillai Biju,*,† Yoji Makita,‡ Akinari Sonoda,‡ Hiroshi Yokoyama,§ Yoshinobu Baba,†,| and Mitsuru Ishikawa†,⊥ Nano-Bioanalysis Team, Health Technology Research Center, National Institute of AdVanced Industrial Science and Technology (AIST), 2217-14 Hayashi-cho, Takamatsu, Kagawa 761-0395, Japan, Health Hazard Reduction Team, AIST, Takamatsu, Kagawa 761-0395, Japan, Nanotechnology Research Institute, AIST, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan, and Department of Applied Chemistry, Graduate School of Engineering, Nagoya UniVersity, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan ReceiVed: January 25, 2005; In Final Form: May 9, 2005

The formation of narrow size dispersed and nanometer size aggregates (clusters) of cadmium selenide (CdSe) quantum dots (QDs) and their temperature-sensitive photoluminescence (PL) spectral properties close to room temperature (298 K) are discussed. CdSe QDs formed stable clusters with an average diameter of ∼27 nm in the absence of coordinating solvents. Using transmission electron microscopy (TEM) imaging, we identified the association of individual QDs with 2-5 nm diameters into clusters of uniform size. A suspension of these clusters in different solvents exhibited reversible PL intensity changes and PL spectral shifts which were correlated with temperature. Although the PL intensity of CdSe QDs encapsulated in host matrixes and the solid state showed a response to temperature under cryogenic conditions, the current work identified for the first time QD clusters showing temperature-sensitive PL intensity variations and spectral shifts at moderate temperatures above room temperature. Temperature-sensitive reversible PL changes of clusters are discussed with respect to reversible thermal trapping of electrons at inter-QD interfaces and dipole-dipole interactions in clusters. Reversible luminescence intensity variations and spectral shifts of QD clusters show the potential for developing sensors based on QD nanoscale assemblies.

Introduction Nanometer size aggregates (clusters) of organic and inorganic materials are promising building units for artificial solids with desired properties. The electronic, magnetic, mechanical, and optical properties of substances change radically as they evolve from the atomic or molecular scale into bulk materials. Likewise, the properties of nanoscale organic materials, metals, and semiconductors are different from those of their bulk solids, constituent atoms, or molecules. Detailed descriptions of the preparation, properties, and applications of different nanoscale materials are included in recent review articles.1-7 Among different nanoscale materials, cadmium selenide (CdSe) quantum dots (QDs) attracted considerable attention for their unique optical and electronic properties including extended optical absorption in the UV-vis region, bright photoluminescence (PL), narrow emission band, size tunable PL, and photostability.1,2,5,8-16 These superior properties make CdSe QDs a promising building material for device applications.17-19 Temperature effects on the PL properties of QDs are identified in single QDs, homogeneous solution phases containing different capping agents, thin films, and polymer matrixes under cryogenic conditions.19-25 Thermal quenching of the PL intensity and red-shifted PL spectral maximum are generally observed in investigations correlating temperature and PL properties; * Corresponding author. E-mail: [email protected]. Phone: (81) 87-8693558. Fax: (81) 87-869-4113. † Nano-Bioanalysis Team, AIST. ‡ Health Hazard Reduction Team, AIST. § Nanotechnology Research Institute, AIST. | Nagoya University. ⊥ E-mail: [email protected].

controversial results of PL enhancement during the increase of temperature have recently been reported, though.22,23 Steady state and time resolved spectroscopic investigations supported the contribution of carrier quenching involving surface related states to the transient decay of photoexcited QDs, and thermal quenching of PL is substantiated in terms of thermally activated nonradiative carrier quenching.20,21,25-37 PL enhancement with increasing temperature is likely due to thermal rectification of surface trap states by specific capping agents, an effect similar to photoinduced PL efficiency enhancement of QDs during surface passivation.38-40 These investigations supported the contribution of surface related emission and surface trap states to the PL efficiency of QDs, and temperature effects on the PL properties of QDs depend on the host matrix/capping agent and surface electronic properties. To date, investigations of the temperature-dependent PL properties of CdSe QDs have focused on single particles and the ensemble level in the solution phase and in the solid state.19-25 Although nanocrystals of QDs are known to get together and form superstructures by preparing films or by limiting coordinating solvent with nonsolvents,5,19,41-50 the temperature-sensitive PL properties of different nanoscale assemblies of QDs are not investigated in detail. One of the difficulties associated with the preparation of nanoscale assemblies without a supporting matrix was phase separation of QDs of different sizes.44,48 The difficulty of phase separation was surmounted by incorporating QDs of different sizes in glassy films. The superstructures of QDs showed characteristic optical and electronic properties including energy transfer,49 optical anisotropy,51 and environment-dependent spectral shifts45,47,48,50,51 and photoconductivity,43 different from those

10.1021/jp050424l CCC: $30.25 © 2005 American Chemical Society Published on Web 06/16/2005

13900 J. Phys. Chem. B, Vol. 109, No. 29, 2005 of constituent QDs. The distinctive properties of these QD assemblies are explained in terms of inter-QD dipole-dipole interactions and charge separation within a superstructure from investigations correlating optical/electronic properties with external parameters such as electric field, temperature, and pressure. A detailed understanding of the properties of QD assemblies and inter-QD interactions is highly desirable for constructing QD based materials for electronic, nonlinear optical, and sensing applications. For modeling nanoscale assemblies of QDs, we prepared clusters of CdSe QDs with an average diameter of 27 nm and investigated PL properties close to room temperature. Clusters associating individual QDs with 2-5 nm diameters were formed by either limiting the concentration of a coordinating solvent, tri-n-octylphosphine oxide (TOPO), in a solution of CdSe QDs or preparing CdSe QDs excluding standard coordinating solvents. We identified temperaturesensitive reversible PL intensity variations and PL spectral shifts of clusters. The current work is different from a previous report19 that (1) we used clusters of CdSe QDs, (2) reversible PL intensity changes are identified for clusters at moderate temperatures above room temperature, (3) reversible PL spectral shifts are identified for clusters at moderate temperatures, and (4) temperature sensitivity of PL was attained for QD clusters without using supporting host matrixes. Our findings of temperature-sensitive reversible PL intensity variations and PL spectral shifts of CdSe QD clusters show potential for developing cluster based sensor devices. Materials and Methods CdSe QDs were prepared following a method developed by Qu and Peng,52 by reacting cadmium acetate dihydrate [Cd(Ac)2‚2H2O, Aldrich] with tri-n-octylphosphine selenide (TOPSe) at 298 K. By selecting this temperature, we simply examined the possibility of QD preparation at room temperature. Coordinating solvent was not used during the preparation of CdSe QDs. TOPSe was prepared following a literature method,13 by reacting 1:2 equivalents of selenium shots (Aldrich) and tri-noctylphosphine (TOP, Aldrich). The amount of TOP was calculated on the basis of a 90% purity of commercial TOP. For a typical CdSe QD preparation, a round-bottom flask was charged with Cd(Ac)2‚2H2O (0.262 g) and degassed by continuous nitrogen purging. Nitrogen purged TOPSe (0.72 g) was added dropwise to Cd(Ac)2‚2H2O over 20 min. The reaction mixture was stirred at 298 K for 20 h. Aliquots of samples (25 µL) were withdrawn at different time intervals from the reaction mixture, and absorption and PL spectra were recorded. The formation of CdSe QDs was identified from a gradual yellow coloration in the reaction mixture and also an increase of absorbance in the UV-Vis region. Slow growth of CdSe QDs is presented in Figure 1, a function of absorbance at 400 nm and time under reaction. Typical absorption spectrum of CdSe QDs, likely clusters, is provided in the inset of Figure 1. After 20 h, the reaction was quenched by the addition of 10 mL of n-butanol (Wako, Tokyo), and the Cd(Ac)2‚2H2O left unreacted was removed by centrifugation. CdSe QDs were separated as a yellow residue by the addition of methanol (50 mL, Wako, Tokyo) and were isolated by centrifugation (13 000 rpm). The yellow residue of CdSe QDs was purified by repeated (five times) precipitation from a n-butanol/methanol mixture (1:5 v/v). Control samples of CdSe QDs were prepared in the presence of TOPO at 75 °C. We prepared a control sample in the presence of TOPO for comparing cluster formation without TOPO and in the presence of a small amount of TOPO. Considering the melting point of TOPO is >60 °C, the control sample was not

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Figure 1. Change of optical density (measured at 400 nm) with time under reaction at 25 °C of a mixture of Cd(Ac)2‚2H2O and TOPSe. Inset: absorption (a) and PL (b) spectra of CdSe QD clusters in n-butanol. The PL spectrum was recorded by exciting at 450 nm.

prepared at 298 K. The sample prepared at 75 °C and in the presence of TOPO contained essentially no clusters, likely due to surface passivation of individual QDs with TOPO. From transmission electron microscopy (TEM) and PL spectral analyzes of nonclustered QDs, we identified QDs of 2.8-4.0 nm in size as the major components in our samples. A stock solution of CdSe QD clusters was prepared by dissolving QDs (sample prepared without TOPO) in n-butanol to a final concentration of 0.5 µM. For this, the required amount of QDs was weighed into a sample tube followed by making a paste with n-butanol. An additional amount of n-butanol was added dropwise to the CdSe paste with continuous stirring to the final concentration (0.5 µM). The presence of TOP, n-butanol, or other impurity molecules on the surface of CdSe QDs was not considered during calculation of concentration. Indeed, TOP or n-butanol was essentially not detected during 1H NMR spectral analysis of the purified and vacuum-dried CdSe sample. The control CdSe QD cluster solution in a TOPO/ n-butanol mixture was prepared by making a paste of QDs (CdSe prepared at 75 °C in the presence of TOPO) by mixing 5 µmol of CdSe QDs and 500 mg of molten TOPO, followed by adding 9.5 mL of n-butanol. The nonclustered control sample was prepared in a 1:1 (v/v) TOPO/n-butanol mixture. We observed reduced PL quantum efficiency (Φf ∼ 0.1) for clusters compared to nonclustered CdSe QDs (Φf ∼ 0.25). The reduced quantum yield of clusters is likely due to energy transfer from smaller to bigger QDs within individual clusters. Absorption and emission spectra were recorded using a Hitachi-4100 spectrophotometer and a Hitachi-4500 spectrofluorometer, respectively. 31P and 1H NMR spectra were recorded using a Varian Innova 400 MHz (162 MHz for 31P) spectrometer. TEM images were recorded using a 300 kV JEOL JEM 3010 microscope. Results and Discussion Absorption and PL spectra of a CdSe QD cluster solution are shown in the inset of Figure 1. A structureless absorption band in the UV-Vis region was observed for the cluster solution. The absence of absorption features characteristic of discrete electronic transitions in the present case is likely due to the aggregation of QDs and the presence of CdSe QDs of different sizes. Indeed, well-resolved absorption features are standard for narrow size dispersed QD solutions.9,13,52 The presence of CdSe QDs of a size range was further supported by a wide [full width at half-maximum (fwhm) of 59 nm] PL spectrum.13 The clustering effect likely contributed to the wide PL band. Indeed, isolation of narrow size dispersed QDs from a cluster solution was difficult following literature methods such as size selective precipitation from solvents of varying composi-

Temperature-Sensitive PL of CdSe QD Clusters

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Figure 2. TEM images of cluster samples prepared from n-butanol solutions of different concentrations: (A) 10 nM; (B) 500 nM; (C) 0.05 nM. (D) TEM image of a sample prepared from a 10 nM cluster solution in a 1:19 (v/v) TOPO/n-butanol mixture. Clusters were not formed in a solution containing >15% TOPO. (E) Histogram of the size distribution of clusters. Size of all the images (A-D) are 500 nm × 500 nm.

tion. The difficulty associated with size selection was likely due to a strong association of QDs within individual aggregate structures, those we identified as clusters using TEM. Typical TEM images of clusters, prepared by dispersing purified CdSe QDs in n-butanol, are shown in Figure 2A-C. Samples for TEM measurements were prepared by dispersing n-butanol solutions of CdSe QDs (10 nM, Figure 2A) on carbon coated copper grids followed by drying under vacuum for 24 h. Interestingly, clusters of uniform size (average diameter ∼27 nm) are identified in the TEM images. On the other hand, QD samples prepared in the presence of 20% (v/v) or more TOPO showed no characteristic clusters in the TEM image (figure not shown), consistent with previous observations.5,50 We examined the possible origin of clusters, preformed in the solution phase or formed during drying of TEM samples, by preparing TEM samples from solutions of different QD concentrations. Interestingly, TEM images of samples prepared using 500, 10, and 0.05 nM CdSe solutions contained clusters of uniform size except for concentration-dependent density of clusters. TEM images of samples prepared using 500 and 0.05 nM CdSe solutions are shown in parts B and C of Figure 2, respectively. All dilutions were made from a 500 nM stock solution of clusters, described in the previous section. The size distribution of QD clusters is presented in Figure 2E. The formation of aggregated CdSe QDs was observed previously by adding nonsolvent to homogeneous QD solutions.5,50,53,54 Clusters were also observed for QD samples prepared in the presence of TOPO by limiting the concentration of TOPO with n-butanol [Figure 2D, the TEM sample was prepared in a TOPO/n-butanol mixture (1:19 v/v)].

Figure 3. TEM images of a single cluster (A) and individual CdSe QDs of different sizes observed within clusters (B-E). The scale bars in parts A-E are all 3 nm.

If clusters were formed during the drying of TEM samples, the concentration was likely to have an effect on the size of the clusters. The presence of uniform size clusters independent of CdSe QD concentration suggested clusters likely existed in a stock solution and did not form during the drying of TEM samples. A detailed investigation is underway to identify the origin of uniform size clusters. Using TEM imaging, we identified the origin of the structureless absorption band and wide PL spectrum of CdSe QDs. A TEM image of a single cluster is shown in Figure 3A. Close examination of the structure of individual clusters identified the association of QDs of a size range in clusters. TEM images of QDs of 5, 3.5, 3, and 2 nm in size observed within a cluster are shown in parts B-E of Figure 3. It is not surprising that QDs of different sizes aggregated into clusters in the absence of coordinating solvents. Coordinating solvents such as TOPO,

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Figure 5. (A) PL spectra of a cluster solution in n-butanol recorded at 298 K: (e) initial spectrum before heating and cooling; (d-a) spectra after one to four heating-cooling cycles. Spectra a-d are normalized for wavelength with respect to the initial PL spectrum (e). (B) PL spectra of a cluster solution recorded at (a) 298 K before heating-cooling cycles and (b-e) at 353 K after one to four heating cycles. Spectra b-e are normalized for intensity with respect to spectrum before heating (a).

Figure 4. (A) PL spectra of a CdSe cluster solution in n-butanol recorded at different temperatures: (a) 298; (b) 313; (C) 323; (D) 333; (E) 343; (F) 353 K. The PL spectra are normalized for wavelength shift with respect to the initial room temperature spectrum. Inset: Arrhenius plot showing linearity of thermal quenching of PL. (B) PL intensity variations of a cluster solution during heating-cooling cycles. PL intensities were recorded at 298 K during cooling and 353 K during heating. (C) PL spectral shifts (∆λ) of a cluster solution during heatingcooling cycles. PL maxima were recorded at 298 K during cooling and 353 K during heating. All PL measurements were carried out by exciting cluster solutions at 450 nm. Reversibility of PL intensity variations and PL shift was attained after four heating-cooling cycles.

fatty acids, aliphatic and aromatic mercaptanes, and aliphatic amines are known to stabilize individual QDs within solvent headgroups due to either surface functionalization of QDs or nonbonding interactions. Indeed, we identified the addition of a coordinating solvent (TOPO) prevented the formation of clusters. TOPO likely separates and stabilizes individual QDs due to hydrophobic interactions between nonpolar octyl chains. In the absence of coordinating solvents, QDs of different sizes aggregated into clusters, as observed in the TEM images (Figure 2A-C), and the presence of QDs of different sizes contributed structureless absorption and wide PL spectral bands to the cluster solutions. Characterization of the exact contribution of individual size to each cluster was tedious work considering the close association of QDs within clusters. Correlating the PL spectrum with previous reports on size-dependent PL of CdSe QDs, it is

reasonable that QDs of 2.5-4 nm in size are the major components of clusters.15,55 Cluster solutions of CdSe QDs showed a reversible response to temperature between 298 and 353 K. The PL intensity of a cluster solution decreased and increased with increasing and decreasing temperature, respectively. Likewise, the PL spectral maximum showed reversible response to temperature, that is, the PL band red-shifted and recovered during the heating and cooling of a cluster solution, respectively. Cluster solutions were prepared adopting the procedure used for TEM samples, by dissolving the required amount of QDs in n-butanol to a final concentration of 250 nM. Photoluminescence spectra of this solution recorded at 298, 313, 323, 333, 343, and 353 K during a heating cycle are shown in Figure 4A. The spectra in Figure 4A were recorded during a continuous heating process within a reversible cycle and are normalized for wavelength shifts with respect to the room temperature spectrum (trace a in Figure 4A). Interestingly, we noticed temperature has a reversible effect on the PL intensity changes and PL spectral shifts. Linearity of an inverse relation between temperature and PL intensity is identified by constructing an Arrhenius plot. For this, ratios of PL intensities at different temperatures and room temperature PL intensity are plotted against 1/T (inset of Figure 4A). Gradual recovery of PL intensity and PL blue-shift was observed during the cooling of a hot cluster solution. The reversibility of PL intensity changes and PL spectral shifts during the heating and cooling of a cluster solution are displayed in parts B and C of Figure 4, respectively. However, we noticed during one to four heating and cooling cycles that the PL intensity changes were not completely reversible (one to four cycles in Figure 4B). Indeed, heating to 353 K followed by cooling back to 298 K of a cluster solution during the first four cycles resulted in a 50, 85, 95, and 97% increase of luminescence intensity beyond the original room temperature PL intensity. An increase of PL intensity after each heating-cooling cycle is shown in Figure 5A (normalized for spectral shift). After four to five heatingcooling cycles, the PL intensity changes are reversible (93 ( 3%). Likewise, the PL spectral shifts showed anomalous

Temperature-Sensitive PL of CdSe QD Clusters behavior, not completely reversible, during one to four heatingcooling cycles (Figure 4C). The PL spectra recorded at 353 K showed 8, 14, 16, and 18 nm red-shifts during the first four heating-cooling cycles between 298 and 353 K. PL shifts are shown in Figure 5B (normalized for intensity changes with respect to the initial room temperature PL maximum). When measured at room temperature, the red-shifts were partly recovered by 3, 6, 7, and 8 nm after one to four heating-cooling cycles, leaving behind a net red-shift (∆λ) of 10 nm. From the fourth heating-cooling cycle onward, >95% reversibility of spectral shifts was identified (Figure 4C). The cluster solutions were stable during >10 heating-cooling cycles; however, we noticed degradation of clusters into smaller particles and the formation of slightly bigger aggregates of ∼50 nm size over 3 weeks. We have investigated the origin of the PL intensity changes and the PL spectral shifts by examining the effect of solvents, UV-Vis irradiation, and temperature on the PL properties of clustered and nonclustered CdSe QD solutions. Although n-butanol is an ideal solvent for clusters, other cluster solutions were also prepared in chloroform, 2-propanol, and n-hexane. Decrease and increase of PL intensity and red-shift and recovery of PL spectra during the heating and cooling of cluster solutions were observed in these solvents, indicating solvent-independent temperature-sensitive PL. It was difficult to follow the PL properties of clusters in these solvents for a long time due to precipitation of larger particles, identified from relatively high background scattering during spectral measurements. Furthermore, we examined the effect of UV-Vis light irradiation on the PL properties of clusters and compared it with the PL enhancement of a CdSe QD control sample free from clusters. For this, a n-butanol solution of clusters and a TOPO capped nonclustered QD solution were separately irradiated with 400 nm light. The luminescence of the cluster solution was gradually increased with irradiation time, and the PL intensity reached 220% of the original value after 300 min. No further enhancement was observed under irradiation. On the other hand, the luminescence of TOPO capped QD solution, free from clusters, showed ∼4-fold enhancement of luminescence. The absorption spectrum showed essentially no change after irradiation. These observations are consistent with previous reports on the photoinduced PL enhancement of QDs, likely due to surface passivation and quenching of deep trap emission by dissolved oxygen or other capping agents.31,37,39,40,56-58 However, the PL enhancement (∼2-fold) of a cluster solution was not comparable to the PL enhancement of a homogeneous QD solution. This is understandable in terms of the small effective surface area of clusters available for photoinduced passivation compared to isolated QDs. Furthermore, a cluster solution under irradiation at constant temperature showed essentially no reversible luminescence intensity variations or spectral shifts. These observations indicate a contribution of the photoinduced effect to the initial PL enhancement of clusters beyond the original intensity, and after continuous irradiation likely an equilibrium stage with maximum photoinduced PL intensity was attained. All of the above observations point toward two aspects of the temperaturedependent luminescence intensity variations of clusters: (1) initial less-reversible PL variations are partly contributed by photoinduced effects on the surface electronic properties, and (2) the thermal effect contributes to the reversible nature of PL intensity and PL spectral shift. Contribution of a thermal annealing effect on the initial less-reversible nature of intensity changes and spectral shifts is not completely ruled out. An annealing effect possibly contributed to the PL properties by

J. Phys. Chem. B, Vol. 109, No. 29, 2005 13903 slightly better connections between particles during one to four heating-cooling cycles due to partial removal of internal grain boundaries. We investigated a possible relation between temperaturesensitive PL properties of QD clusters and dynamic variations of surface-ligand interactions by recording and analyzing 31P and 1H NMR spectra at room temperature and 323 K. Samples for NMR spectral analyses were prepared by passivating cluster surfaces with TOPO or n-butanol considering the temperaturedependent PL investigations of cluster solutions containing TOPO or n-butanol. Indeed, from proton NMR analyses, we identified a negligible contribution of surface capping agents such as TOP, TOPO, and n-butanol to purified and vacuumdried cluster samples. For this reason, 5 µL of TOPO was added to 10 mg of a dry CdSe QD sample and thoroughly mixed. This mixture was dissolved in chloroform d, and proton decoupled 31P NMR spectra were recorded at room temperature and 323 K. At room temperature, we noticed two resonances at δ ) 51.95 ppm and δ ) 59.66 ppm. It has been known that the 31P chemical shift of free TOPO in CDCl3 is δ ) ∼50 ppm.31 Therefore, in the present case, the NMR sample likely contained two types of TOPO, surface passivating and free. We attribute the resonance at δ ) 51.95 ppm to free TOPO in CDCl3 and the resonance at δ ) 59.66 ppm to surface passivating TOPO. The deshielding effect (1241 Hz) observed in the present case is expected for phosphorus due to interaction with CdSe QDs; however, a shielding effect was identified in a previous report.31 Interestingly, both of the resonances disappeared as we increased the temperature to 323 K, and a new band appeared at δ ) 53.75 ppm. This new resonance is attributed to an equilibrium condition between loosely bound TOPO and free TOPO. Disappearance of the band at δ ) 59.66 ppm or upfield shift to 53.75 ppm is likely due to less interaction between CdSe QDs and TOPO at higher temperature. Furthermore, the spectral full width at half-maximum (fwhm) increased from 2.95 Hz at room temperature to 17.5 Hz at 323 K. We attribute the new resonance at δ ) 53.75 ppm and the wide spectral band to the presence of a dynamic equilibrium between free TOPO and TOPO loosely interacting with the QD surface. A 31P NMR spectrum was recorded after cooling the solution back to room temperature; however, the resonance ∼60 ppm was not recovered. Indeed, the spectral width decreased to 5.53 Hz after cooling. After first heating and cooling essentially no upfield or downfield shifts were observed due to temperature change. These observations suggested that initial heating and cooling produced an annealing effect leading to an equilibrium condition that was not affected considerably by changing the temperature. In the case of QD clusters passivated with n-butanol, essentially no effect of temperature on the proton chemical shift or H-H coupling was observed. 31P and 1H NMR spectral analyses suggested that surface passivating solvents likely have a negligible effect on the reversible PL properties of QD clusters. The temperature-sensitive PL quenching is attributed to a thermal activation of surface trap states and an increased nonradiative Augur exciton recombination. Once the stress due to higher temperature is relieved by cooling the clusters, the trap state population is likely reduced, resulting in recovery of PL intensity. The current work identified for the first time (1) reversible temperature-sensitive PL properties of QD clusters and (2) temperature-PL correlations of QD clusters at moderate temperatures above the room temperature. Considering the involvement of clusters of QDs in the current work, temperaturesensitive reversible luminescence properties, and initial luminescence enhancement, we propose a modified model (Figure

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Biju et al. changes and spectral shifts are due to dynamic variations of surface passivating solvent shell during heating and cooling is not significant in the present case, identified from 31P and 1H NMR spectral analyses at different temperatures. Nonetheless, reduced PL quantum efficiency of cluster solutions compared to nonclustered CdSe QDs supported an energy transfer path, from smaller to bigger QDs within individual clusters, is also included in the temperature-sensitive PL quenching. Time resolved measurements are necessary for obtaining further insight into the details of excited state interactions and PL quenching. Construction of a cluster system from uniform size QDs is also necessary for validating the contribution of carrier transfer between particles of different sizes to the temperaturesensitive PL properties. Conclusions

Figure 6. Schematic presentation of photoinduced processes in CdSe QD clusters: (a) surface passivation of photoexcited clusters by solvent molecules or dissolved oxygen; (b) thermal activation followed by the formation of stabilized clusters due to the inner surface (IS) trap state population and inter-dot dipole-dipole interactions (OS and IS are marked on cross section of a cluster); (c) formation of deep trap states; (d) nonradiative carrier recombination of deep trap states (relatively low energy deep trap emission was not observed).

6) of the thermal population of trap states. In the present case, individual QDs of different sizes are embedded within clusters, considerably reducing the effective surface area (area exposed to the solvent environment) compared to an equal number of isolated QDs. Moreover, novel interfaces among QDs are formed within each cluster. This simply means that two types of surfaces/interfaces are pertinent for cluster systems: (1) an oxygen/solvent accessible outer surface (OS, Figure 6) that can be passivated with oxygen or solvent during irradiation and (2) oxygen/solvent inaccessible interparticle interfaces (inner surfaces, IS, Figure 6) embedded within a cluster. Considering photoinduced passivation of OS and thermal annealing are not reversible38-40,56 and the thermal population of IS is likely reversible, the PL intensity variations observed in the present investigation are attributed to the fact that (1) initial lessreversible PL enhancement is a OS passivation effect and (2) reversible variations of PL intensity with temperature are due to thermal trapping and detrapping of electrons in the IS states. However, deep trap emission of relatively low energy was absent in the present case, thus indicating effective nonradiative relaxation of trap states. Temperature induced reversible PL shifts (Figure 4C) are attributed to reversible changes of interdot dipole-dipole interactions in the excited state within individual clusters. Short interparticle distance in the clusters possibly facilitated the dissociation of excitons into electrons and holes in neighboring particles or at interparticle interfaces and increased dipole-dipole interactions between particles.27 This effect is opposite to the blue-shift of the PL spectrum of nonclustered CdSe QDs with an increase of temperature. Redshifted PL characteristics were observed for QD superstructures and accounted for a quantum mechanical tunneling through interdot barriers, or thermal excitation followed by hopping above the energy barrier due to interdot exciton coupling.50 In the present case, contribution of the thermal population of the IS is reasonable for dipole-dipole induced stabilization of the excited state of the clusters. Also, the absorption band of the clusters was not shifted considerably during heating and cooling unlike in the case of PL band. Therefore, excited state dipole interactions likely contributed a major part of the reversible luminescence spectral shifts. Yet another possibility, PL intensity

Clusters of CdSe QDs are formed in the absence of coordinating solvents and by limiting the concentration a coordinating solvent with a nonsolvent. Using TEM imaging, we identified the association of QDs of different sizes into clusters. Cluster solutions showed reversible PL intensity variations and PL spectral shifts which are different from photoinduced and thermal effects observed for homogeneous QD solutions. PL intensity variations and PL spectral shifts with temperature are attributed to the reversible thermal trapping of electrons in trap states at inter-QD interfaces within individual clusters. Rather than specifically investigating the temperaturesensitive luminescence properties of CdSe QD clusters, our observations identified a difference between thermal and photoinduced PL intensity variations in clusters. To date, all literature reports account the luminescence quenching of QDs in terms of enhanced nonradiative decay for which surface trap states are proposed candidates. By identifying a reversible effect of temperature on the PL properties of QD clusters containing solvent-particle interfaces and interparticle interfaces, the possibility of an interface related trap state model in cluster systems is proposed. The present observation of temperaturesensitive PL properties of CdSe QD clusters would be useful during the construction of optical and electronic materials for sensor applications. Acknowledgment. This work was partly supported by a regional research consortium project of the Shikoku Bureau of Economy, Trade, and Industry, Japan. V.B. and M.I. are grateful to the Grant-in-Aid for Scientific Research (KAKENHI17034068) in Priority Area ‘Molecular Nano Dynamics’ from Ministry of Education, Science, and Culture, Japan. Supporting Information Available: 31P NMR spectra of CdSe QD clusters and TEM image of clusters recorded after long time heating and cooling. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226-13239. (2) Alivisatos, A. P. Science 1996, 271, 933-937. (3) Collier, C. P.; Vossmeyer, T.; Heath, J. R. Annu. ReV. Phys. Chem. 1998, 49, 371-404. (4) Klabunde, K. J., Ed.; Nanoscale Materials in Chemistry; John Wiley & Sons: New York, 2001. (5) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. ReV. Mater. Sci. 2000, 30, 545-610. (6) Yoffe, A. D. AdV. Phys. 2001, 50, 1-208. (7) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293-346. (8) Banfi, G.; Degiorgio, V.; Ricard, D. AdV. Phys. 1998, 47, 447510.

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