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ARTICLE pubs.acs.org/JPCC

Ultrafast Exciton Dynamics in CdTe Nanocrystals and Core/Shell CdTe/ CdS Nanocrystals Yueran Yan,† Gang Chen,‡ and P. Gregory Van Patten*,† †

Department of Chemistry and Biochemistry, and ‡Department of Physics and Astronomy, Ohio University, Athens, Ohio 45701, United States

bS Supporting Information ABSTRACT: CdTe and CdTe/CdS core/shell nanocrystals (NCs) have been synthesized, structurally characterized, and studied using steady-state and transient optical methods. Timeresolved photoluminescence (PL) and transient absorption (TA) decays both showed exciton lifetimes in the tens of nanoseconds. The TA spectra of the CdTe NCs showed multiple bleaches, which have been assigned to the 1S3/2(h) 1S(e), 2S3/2(h)1S(e), and 1P3/2(h)1P(e) transitions. The spectral shifts of these bleaches with NC size and after shell deposition have been analyzed in the context of a quasi-type-II carrier distribution in the core/shell samples. The measured energy level spacings show clear evidence for electron delocalization and strong core-confinement of the hole in the core/shell NCs. In addition, the ultrafast evolution of these bleach features has been examined to extract electron cooling rates. For core and core/shell NCs of similar band gap, the arrival time of the electron in the 1S(e) level is approximately 40% longer for the core/shell than for the core NCs. The difference is attributed to reduced Auger rates in the core/shell NCs, where the electron and hole are spatially separated. Analysis of TA dynamics of CdTe cores also provides evidence for ultrafast hole trapping with a size-dependent rate constant. An additional bleach is observed in the core/shell samples at the blue end of the visible spectrum and is tentatively assigned to state filling of the lowest energy CdS transition due to penetration of the electron into the CdS conduction band. These observations provide a means to study carrier-resolved dynamics in these NCs using only the visible TA data.

’ INTRODUCTION Colloidal CdTe nanocrystals (NCs) have been the subject of intense research interest because of the relatively narrow bulk band gap, the facile synthetic routes, and the compatibility with other readily synthesized, colloidal IIVI semiconductor materials.16 NCs and nanoheterostructures containing CdTe appear to hold promise for multiple applications, including NIR lumophores and solar energy conversion materials.710 The valence and conduction bands of CdTe are offset from those of CdSe and CdS, so that it is possible to prepare type-II core/shell NCs from combinations of these materials.4,7,11 The type-II excitonic structure has important consequences for the electronic and optical properties of the resulting heterostructures, including intraparticle charge separation along the radial coordinate, dramatic reduction of the radiative recombination rate, and lowering of the optical band gap energy, sometimes below that which is observed in the component bulk materials. These effects could be harnessed to enable or improve charge carrier separation prior to carrier cooling, to improve the rates and efficiencies of photoinduced charge separation, and to tailor the optical response of these materials in the near-infrared. Pumpprobe transient absorption (TA) spectroscopy is a powerful tool for studying the ultrafast photophysics of molecules and colloidal nanostructures.1230 It has been used with tremendous success in detailed studies of CdSe NCs to r 2011 American Chemical Society

understand ultrafast carrier dynamics.12,13,15,16,2732 Unlike the CdSe system, the CdTe system has not been fully characterized on ultrafast time scales. While several groups have reported ultrafast TA measurements on nanoheterostructures containing CdTe,14,20,22,3341 major gaps remain in our understanding of this system. Studying the spectral and dynamic characteristics of these nanoheterostructures will provide insight into their photophysical response, including charge carrier cooling and localization (i.e., intraparticle charge transfer) dynamics. A strong understanding of the behavior of pure CdTe NCs will be required before it is really possible to understand and interpret measurements on CdTe-containing nanoheterostructures. As mentioned above, the ultrafast photophysics of CdSe NCs has already been extensively explored, and a complex picture of the excitonic behavior of those NCs is beginning to emerge. Interactions between charge carriers in specific excitonic and biexcitonic states induce transient shifts in energy levels and in the associated optical transitions observed in TA spectra. Furthermore, the intraband carrier relaxation (a phenomenon that has primary importance in the development of solar cells and lasers) is apparently governed by a complex interplay of Received: May 12, 2011 Revised: October 11, 2011 Published: October 13, 2011 22717

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The Journal of Physical Chemistry C excitonphonon interactions, electronhole interactions, carrier interactions with surface states, and carrier interactions with vibrational energy levels of the organic molecules (i.e., ligands and solvent) just outside the NC surface.13,28,30,31,42,43 Many of the previous studies on CdSe NCs have examined carrier cooling and trapping kinetics as a function of NC size and/or surface functionalization. By altering NC composition, surface reactivity, and crystal structure, it should be possible to learn more about these mechanisms and to ascertain whether the behavior of CdSe is universal and representative of a wide range of NC systems. There are several differences between CdSe and CdTe that might affect ultrafast carrier dynamics. For example, electron cooling rates in CdSe NCs are mediated in large part by Auger interactions in which hot conduction band electrons undergo intraband transitions by transferring excess energy to the valence band hole, which has a higher density of states than the conduction band electron. CdTe has a larger dielectric constant than does CdSe (11 in CdTe versus approximately 6 in CdSe),44,45 and the additional dielectric screening could reduce the Coulombic coupling that governs Auger processes in NCs. In addition, the density of hole states near the valence band edge should be reduced in CdTe relative to CdSe due to the very large (0.9 eV) spinorbit interaction. The strong spinorbit interaction shifts the J = 1/2 hole states far from the valence band edge, and this reduced density of low energy hole states could further reduce the rate of electron cooling via Auger interactions. The reduced density of low-energy valence band states in CdTe may not only affect the relaxation of conduction band electrons, but may also reduce the relaxation rate of valence band holes in comparison with CdSe. It has been shown that hole relaxation in CdSe NCs occurs faster than would be expected for phonon-mediated processes and that the relaxation rates are nearly independent of particle size.28,31 Those results have been rationalized on the basis of nonadiabatic processes in which the intraband transitions of valence band holes are coupled to vibrational modes of surface-bound ligands. Depending on the mechanism that predominates in CdTe NCs, the size-dependent density of valence band states may play an important role in determining the rate of cooling for photoexcited holes. If relaxation of holes bound within the NC core is retarded in CdTe relative to CdSe, it may enhance the importance of hot hole trapping at the NC surface or on surface-bound hole acceptors. The role of surface trapping may be further altered in CdTe by differences in the surface reactivity patterns of tellurides as compared to selenides. In this Article, we report and analyze size-dependent, visible transient absorption spectra and dynamics from CdTe NCs and CdTe/CdS core/shell NCs. Assignments of the most prominent features, including the three lowest energy bleaches, are made in accordance with previous experimental and theoretical work. The effect of the shell on the energy level spacings in the conduction band and valence band is examined. The results are consistent with a delocalization of the conduction band electron into the CdS shell and strong confinement of the valence band hole within the CdTe core. The TA bleach assignments, in combination with kinetic analysis of the individual peaks, provide evidence for size-dependent changes in hole relaxation pathways using information from the visible spectral region. In the present experiments, samples were excited well above the band gap energy with a single pump wavelength (400 nm). Carrier cooling dynamics observed in these experiments arise from a combination of processes involving several excitonic

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states, so the observed cooling rates may differ from those observed when near-band-edge excitation or state-selective pumping is used. On the other hand, these measurements provide a look at the ultrafast carrier dynamics that prevail in CdTe NCs under conditions of nonspecific photoexcitation with a large excess photon energy. The results can be directly compared to earlier TA results obtained by Klimov and co-workers42 on CdSe NCs to gauge the differences between these two material systems. In the core/shell systems, a new, high energy bleach is reported that is believed to be associated with the penetration of the conduction band electron into the shell material. The spectral position of this bleach is near the ground-state absorption of the CdS shell. The signal associated with this bleach feature should be exclusively dependent on electron occupation of the conduction band and should not be strongly influenced by hole dynamics. As mentioned above, the analysis of ultrafast time evolution of multiple peaks in the TA data can be used to glean information about hole dynamics. Similarly, the analysis of this high energy bleach together with the lower energy bleaches should provide a means to isolate and follow electron dynamics in the excited core/shell NCs. Distinguishing contributions of electrons and holes to statefilling-induced bleaches in TA spectral data has not been routinely achieved. In the past, analysis of single carrier dynamics in colloidal semiconductor NCs has usually relied on TA measurements in the mid-infrared spectral region.13,15,42,43,4649 In recent years, Kambhampati and co-workers have extracted electron and hole dynamics in low-lying excitonic levels through analysis of carefully designed experiments that selectively pump specific initial excitonic states.2729,31 Indeed, they were the first to emphasize the importance of the differences between the two lowest energy absorption features. Their experiments typically involve measurements using two different pump wavelengths, allowing extraction of the individual carrier dynamics by subtraction of kinetic traces at specific probe wavelengths. An advantage of those experiments is that specific state-to-state transition rates can often be determined, allowing for estimation of individual carrier cooling rates through the lowest few excitonic levels. Weiss and co-workers have isolated and followed individual carrier dynamics by a different method. They have found that analysis of TA measurements in the near-infrared spectral range can be used to directly follow the individual carrier dynamics in CdSe NCs.50,51

’ EXPERIMENTAL SECTION The synthesis of the CdTe and CdTe/CdS NCs was based on modifications of methods that have been previously reported.11,52,53 Further details regarding the synthesis of the NCs, as well as their characterization via transmission electron microscopy, X-ray diffraction, time-resolved optical spectroscopy, small-angle X-ray scattering, and inductively coupled plasmaoptical emission spectroscopy, are provided in the Supporting Information. After synthesis, NC samples were stored in the crude reaction mixture in an Ar-filled glovebox until ready for use. All NCs were protected from air exposure before and during optical spectroscopic measurements unless otherwise noted. ’ RESULTS Basic Characterization. CdTe and CdTe/CdS NCs of various sizes were prepared and studied. The trends described in this 22718

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Figure 2. TEM images of CdTe core (left panel) and CdTe/CdS core/ shell (right panel) NCs.

Figure 1. UVvisible absorption and photoluminescence emission spectra of (A) small CdTe core NCs, (B) small CdTe/CdS core/shell NCs, (C) large CdTe core NCs, and (D) large CdTe/CdS core/shell NCs. Photoluminescence quantum yield (ΦPL) is also reported for each sample.

Article apply to all of the NC samples studied, but for simplicity and clarity, the results reported here focus on a selected group of these samples. Most of the data presented here come from four particular samples, designated “small core”, “small core/shell”, “large core”, and “large core/shell”. Figure 1 shows absorption () and emission (- - -) spectra of these four NC samples. The sizes of CdTe and CdTe/CdS NCs used in this work were determined using UVvisible absorption, TEM, and small-angle X-ray scattering (SAXS) data. Absorption measurements provide a reliable measure of particle size for CdTe cores using the empirical formula reported by Yu et al.52 On the basis of the absorption spectra shown in Figure 1, the particle sizes for the small and large cores are estimated to be 3.2 and 3.9 nm, respectively. Unfortunately, absorption measurements cannot be directly used to estimate the size or shell thickness of CdTe/ CdS core/shell particles. For this reason, TEM and SAXS were also used to estimate average particle size. TEM images of the CdTe core NCs (Figure 2a) reveal uniform, monodisperse particles with a narrow size distribution. The particles self-organize on the TEM grid into a close-packed monolayer, and local hexagonal symmetry is evident in the packing arrangement. As compared to the cores, the core/shell NCs (Figure 2b) exhibit a broader size distribution and a variety of shapes. The size and shape variation is probably due to nonuniform CdS growth caused by the 11.5% lattice mismatch54 and the difference in preferred crystal symmetry in the core and shell materials. Diameters of a few hundred particles were measured directly from TEM images of both core and core/shell samples. For the CdTe cores, the mean size was always very close to the value estimated from absorption spectra. Core/shell particles had mean diameters that were slightly larger than the cores from which they were grown. Although the TEM images always showed that core/shell samples had slightly larger mean sizes than their parent cores, the distribution of core/shell sizes was broadened significantly. Through extensive experimentation, we were able to confirm that the smaller particles in the distribution did not arise from nucleation of CdS particles but rather originated from the parent CdTe cores. Nucleation of CdS particles was observed

if temperature and the rate of precursor addition were not carefully controlled, but CdS nucleation was avoided in the samples discussed here. In addition, attempts to grow thick shells generally resulted in gross morphological changes (for example, tetrapod or rod formation). To corroborate the particle size information obtained from TEM, SAXS measurements were performed on colloidal suspensions of the NCs in toluene. The SAXS data were largely consistent with the TEM data (see the Supporting Information), but, as compared to TEM measurements, SAXS measurements provide better statistical averaging over the entire NC ensemble. SAXS measurements are also less subject to difficulties in the measurement of nonspherical particles or in selecting a representative sample of particles for imaging and measurement. For these reasons, SAXS data have been used here as the primary measure of particle size for the purpose of identifying core sizes and shell thicknesses. To calculate mean particle size from the SAXS data, the volume-weighted size distribution obtained from SAXS was converted to a number-weighted distribution,55 and the mean value of the resulting number-weighted distribution was computed. The particle sizes as determined by SAXS were 3.2 and 4.2 nm for the small and large core particles, and 3.8 and 4.9 nm for the small and large core/shell particles. Like the absorption and TEM data, the SAXS measurements suggest the growth of a thin CdS shell. Shell thicknesses for these samples measured by all methods were approximately 3.5 Å. The SAXS measurements were not in perfect agreement with the size estimates based on absorption data. For example, the SAXS and absorbance measurements of the large cores differed by 3 Å. Nevertheless, the values obtained by all three methods were in reasonable agreement, and, most importantly, all three methods showed that the core/shell particles were, on average, larger than the parent cores. To confirm that sulfur was incorporated into the NCs during the shell growth process, the NCs were analyzed via ICP-OES. These measurements showed definitive incorporation of sulfur in the NCs. For example, in large core/shell samples (similar to sample D in Figure 1), the Cd:Te:S ratio was found to be 19:9:7, which corresponds to a Te:S ratio of 56:44. Powder XRD was used to further probe the structural changes that accompanied the shell growth process. For the thin-shelled particles used in many of the optical studies, the XRD pattern failed to show clear evidence for a separate CdS phase; however, by growing thicker shells, it was possible to demonstrate that a separate CdS phase was forming and to rule out the possibility of alloying. The XRD pattern of large core NCs (Figure 3, black line) matched the zincblende pattern for CdTe (PDF #01070-8041). Addition of the sulfur precursors caused negligible 22719

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Figure 4. Photoluminescence emission decays of CdTe cores () and CdTe/CdS core/shell NCs (red 9). The left panel depicts decay traces from small core and core/shell NCs, and the right panel depicts decay traces from large core and core/shell NCs.

Figure 3. Measured powder diffraction patterns from CdTe core (bottom) and CdTe/CdS core/shell (top) NCs. Vertical bars at the top and bottom of the figure show the patterns for zincblende CdTe (blue dotted lines, PDF 01-070-8041) and wurtzite CdS (red solid lines, PDF 01-074-9663), respectively.

changes in these CdTe reflexes, but produced an entirely new set of reflexes that could be readily identified with wurtzite phase CdS (Figure 3). The appearance of two distinct diffraction patterns demonstrates that the material is not alloyed, but is instead composed of two distinct phases. In contrast to some previous studies on the CdTe/CdS system,41,56 these XRD patterns provide convincing evidence for growth of a CdS shell on the outside of the CdTe cores. Photophysics. During shell growth, a significant red shift was observed in the lowest energy absorption peak (Figure 1). In the case of the small cores, the absorption peak shifted from 2.26 to 1.97 eV during shell growth. For the large cores, the peak shifted from 2.00 to 1.77 eV. Similar large shifts have been previously reported for CdTe/CdS core/shell NCs,7,11,57 and the origin of these shifts is believed to be due to a type-II band alignment in CdTe and CdS. The conduction bands of CdTe and CdS are close in energy,58 and this close alignment results in delocalization of the conduction band electron through the core and shell, with subsequent reduction of that carrier’s confinement energy. Figure 1 reports the photoluminescence quantum yield (ΦPL) of the four samples along with their respective emission spectra. The results show that the quantum yield of the CdTe cores increases during the growth process. The quantum yield of the small cores was 30%, while the large cores showed a quantum yield of 40%. Experience with numerous core-only samples shows that the quantum yield increases immediately after nucleation but then passes through a maximum before beginning to decline again as the particles continue to grow. The size at which ΦPL reaches its maximum depends in a complex manner on the reaction conditions, as has previously been reported for CdSe.59 Deposition of a CdS shell on the CdTe cores increased both the quantum yield and the resistance to quenching upon exposure to air. These effects are often associated with the deposition of a wide band gap shell on a narrow band gap NC core.60,61 Together with the strong absorption red-shifts, the XRD data, and the size measurements, these quantum yield increases demonstrate deposition of a CdS shell over the CdTe NC. For the small NCs, addition of the CdS shell increased ΦPL (measured under argon) from 30% to 68%, while ΦPL for the large NCs showed a more modest increase, from 40% to 45% upon addition of the shell.

Figure 4 shows emission decay traces for the core NC samples. None of the decays were well fit by single exponential decay functions, but all could be fit well either by a sum of three exponentials or a sum of a single exponential with a stretched exponential. After fitting, the average lifetime, Æτæ, was calculated from the decay fit functions according to Æτæ ¼

∑i ai 3 τ2i

ð1Þ

where the ai are the coefficients of the exponential terms in the fit function (amplitudes), and the τi are the lifetimes of the respective components. The larger particles generally showed slower decays than the smaller particles. It is clear from the figure that the addition of the shell led to a modest increase in the measured luminescence lifetime. In the case of the small particles, the average lifetime increased from 29 to 39 ns upon deposition of the CdS shell. This 35% increase in average lifetime is significantly less than the 125% increase in quantum yield that accompanied shell deposition. This rather modest increase in observed lifetime coupled with a much larger increase in quantum yield might appear to indicate that the radiative rate constant increased in these samples upon shell deposition. Ordinarily, the quantum yield, Φ, can be related to the radiative rate constant, krad, and to the rate constant for nonradiative decay, knrad, according to: Φ = krad/(krad + knrad) = krad/kobs = kradτobs. In this treatment, it is assumed that the PL decay lifetime observed experimentally, τobs, is simply the reciprocal of the sum of the radiative and nonradiative rate constants, krad + knrad = kobs. Because, in the present case, the relative increase in Φ is greater than the relative increase in τobs, it might appear that krad must also have increased. However, such an increase in krad would be inconsistent with formation of a quasi-type-II core/shell structure because the separation of excited carriers into separate NC domains should reduce the rate of radiative recombination as a consequence of the decreased spatial overlap of the carrier wave functions. It will be shown later that for the core NCs there are ultrafast nonradiative processes that are too fast to observe in these PL decay experiments. This means that the average exciton lifetime in the small cores is actually much shorter than estimated from the PL decay shown in Figure 4. The same argument applies to the large core and core/shell samples. These samples showed a slight increase in both quantum yield (from 40% to 45%) and average lifetime (from 45 to 50 ns) upon shell deposition. However, the presence of some ultrafast nonradiative decay in the cores prevents extraction of the radiative rate constant from the quantum yield and PL decay data. We expect that radiative 22720

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Figure 5. Transient absorption spectra at three different pumpprobe delays in (A) small CdTe core, (B) small CdTe/CdS core/shell, (C) large CdTe core, and (D) large CdTe/CdS core/shell NCs. Pumpprobe delays were 0.2 ps (red 0), 0.5 ps (green 1), and 2.0 ps (blue O).

recombination is slower in all of the core/shell samples discussed here than in the core-only NCs from which they were grown. Figure 5 shows chirp-corrected transient absorption (TA) spectra of the four NC samples at three different probe delay times (0.2, 0.5, and 2 ps). The spectra show multiple photoinduced absorption and bleach features, which have been labeled with “A” and “B” designations, respectively, according to the nomenclature of Klimov et al.42 The three main bleach features, labeled B1, B2, and B3, will be assigned in the next section to the 1S3/2(h)1S(e), 2S3/2(h)1S(e), and 1P3/2(h)1P(e) exciton states, respectively. The spectra of all of these samples are very similar to those that have been previously reported for highquality CdSe NCs.15,25,42 At very short delay time (e0.2 ps), a derivative-like feature is observed near the lowest energy peak in the ground-state absorption spectrum. This feature, which is perhaps easiest to discern in the large core spectrum at 0.2 ps delay (Figure 5c, red 0), is due to absorption of the excited nanocrystals to a biexcitonic state. The biexciton absorption is red-shifted relative to the lowest energy ground-state absorption feature due to Coulombic interactions between the excited carriers in the biexcitonic state (biexciton binding). At slightly longer times (Figure 5c, 0.5 ps, green 1), the biexciton absorption becomes more pronounced, and its effect on higher-energy transitions in the spectrum becomes evident as well. Ultrafast dynamics of the biexciton absorption in CdSe has been examined by Sewall et al., who noted that the magnitude of these biexcitonic interactions is specific to each particular excitonic state.27 As the hot carriers relax to the band edge within the first 13 ps, the derivative

feature observed near 1.9 eV gives way to a prominent bleach at 2.0 eV that results from state-filling of the lowest exciton state. At higher energy, the spectrum contains additional bleach features that will be discussed further in the next section. After the first few picoseconds, the spectral shape ceases to evolve further; the TA signal at longer times is simply characterized by a monotonic decay that lasts much longer than the 3 ns time window afforded by our instrument. Fits to the longest decay component give an estimated lifetime of a few tens of nanoseconds, similar to the time scale observed in the PL decays. TA kinetic traces obtained at the position of the lowest energy bleach show interesting behavior at early delay times. The rising edge of the bleach is characterized by two distinct components. The first component is a steep increase in bleach intensity that is observed to occur on a time scale comparable to the instrument response function. The second component is usually somewhat slower and varies from sample to sample. A similar behavior has been observed in CdSe NCs, and the slower component of the rising edge in those samples has been attributed to intraband relaxation of the excited electrons to the lowest conduction band level.42 As explained later, additional processes are involved in these CdTe NC samples. Figure 6 compares the rising edge TA kinetics of four core and core/shell NCs. (Note that the vertical axis has been inverted relative to those in Figure 5, and the traces have been normalized to unit peak height to facilitate direct comparison.) Both core NC samples show the initial, fast rising edge, which accounts for between one-third and one-half of the total peak amplitude. The large core sample also shows a much slower component, 22721

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Figure 6. Transient absorption kinetics at the B1 bleach (1S3/2(h)1S(e)) position of four CdTe and CdTe/CdS NC samples. Inset shows the extended decay, out to 3 ns delay. Black, red, green, and blue curves represent small core, large core, small core/shell, and large core/shell, respectively.

accounting for at least one-half the total amplitude, which continues to rise until a delay time of between 2 and 3 ps. In contrast, the rising edge of the small core sample is so fast throughout that a slower component is difficult to even discern. The rising edges were fit to biexponential functions convolved with a fixed pulse width of 190 fs (the full-width-at-halfmaximum of the instrument response function). These fits provided a quantitative and objective estimate of the time constant for the slower rise component. From the data shown in Figure 6, rise times of 150 and 430 fs were obtained for the small and large CdTe cores, respectively. The kinetic traces for small and large core/shell NCs look very similar over the first 6 ps. Both show relatively slow rises, and both reach their peak amplitude after 3 or 4 ps. Careful examination shows that the small core/shell trace has a slightly slower second rise component. Biexponential fits confirm this and yield a slow component rise time of 610 fs for the small core/shell and 530 fs for the large core/shell sample. It is interesting to compare the kinetics between large core and small core/shell NCs because these two samples have similar band gap energies and similar absorption spectra. This means that the excess energy photon with 400 nm pumping is similar for these two samples. As shown in Figure 6, the rising edge of the bleach signal is faster for the large cores than for the small core/shell NCs (430 fs vs 610 fs). It can also be seen that the subsequent decay of the bleach signal is faster for the large cores than for the small core/shell particles. The inset of Figure 6 shows the same kinetic traces over a much longer, 3 ns time window. These data demonstrate that both samples exhibit a fast decay component of approximately 100 ps and much longer components of 10 ns or more. In addition, the small core sample has a faster decay component of 1.7 ps, which can even be observed in the mainframe of the figure. Figure 7 compares the early kinetics of the three main bleach peaks, B1, B2, and B3, in the spectrum of the small CdTe cores. Unlike the B1 peak, the B2 peak from this sample shows clear evidence for a slow rise component like that which is seen in the B1 peak of the other three samples (see Figure 6). The time constant for this slow rise of the B2 feature is 490 fs, which is close to the time constant for the decay of the B3 feature, 550 fs. Additionally, the B2 peak does not show the rapid, 1.7 ps decay that is evident in the B1 peak. As will be discussed later, these data

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Figure 7. Early TA dynamics of the three principal bleaches in small CdTe cores.

Figure 8. (A) Transient absorption spectra of large core/shell NCs in PBS at three different pumpprobe delays. (B) Kinetic traces at the position of the B1 bleach for large core/shell NCs in chloroform (black curve, 2) and in PBS (red curve, b).

provide evidence that surface trapping of valence band holes is an important process in the early TA dynamics of the small cores. Figure 8 shows TA spectra at 0.2, 0.5, and 2 ps delay for a sample of large core/shell particles in phosphate-buffered saline (PBS) at pH 7.4. These particles were prepared from a batch of cores different from the large core/shell particles depicted in Figures 1, 5, and 6. These particles were capped with TGA using established ligand exchange procedures, and the measurements were conducted in air-saturated solution. These particles are highly luminescent, with 50% quantum yield, similar to that measured prior to ligand exchange. TA spectra and dynamics of 22722

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these particles are also similar to those observed for the large core/shell particles prior to ligand exchange. Like the large core/ shell particle TA spectra shown in Figure 5, this sample shows a strong bleach at photon energies above 2.6 eV. This bleach develops relatively late, as it is completely absent at 0.5 ps probe delay time.

’ DISCUSSION Uncoated CdTe core NCs are air sensitive.20,53,62,63 The luminescence intensity of our core-only NCs degrades significantly over a period of a few minutes to hours after exposure to air. The degradation is attributed to oxidation at the surface as has been previously reported and studied by Borchert et al.62 To avoid complexities related to the surface oxidation processes, the measurements presented here have been made under argon atmosphere except where otherwise noted. Deposition of a thin CdS layer dramatically improves the stability of the particles in air with respect to exciton quenching, so that PL and TA measurements on CdTe/CdS core/shell particles in air are essentially indistinguishable from those under argon. Great care was taken during the TA measurements to limit pump intensity so that the system remained safely within the single excitation regime and so that photoinduced sample degradation could be avoided. The value of ÆNæ, the number of excitons per nanocrystal in the sample, was held below 0.1 in all cases. This required low pump fluence, especially for some of the large core/shell samples, which had very high extinction coefficient at the pump wavelength due to the presence of the CdS shell, which absorbs strongly in the blue and UV. To avoid sample photodegradation on the time scale of the experiment, the sample was stirred rapidly during data collection. Absence of photodegradation during the experiment was demonstrated by collecting three full sequential scans for each experiment. The data reported are the average of these three scans. After completion of each experiment, the three scans were compared to verify that the ratio between the peak ΔOD value at short delay time and the ΔOD value at the end of the scan (150 ps or 3 ns, depending on the experiment) was the same within the experimental noise for all three scans. No evidence for photodegradation of the samples was observed under the experimental conditions employed. Assignments of TA Spectral Features. Recently, Zhong et al. have mapped out the size-dependent excitonic transition energies for CdTe NCs using photoluminescence excitation spectroscopy.64 Their data provide an excellent foundation for understanding the transient spectra of the CdTe cores reported here. Figure 5a and c depicts transient absorption spectra for the small and large core samples. At delay times longer than 1 ps, the spectra are dominated by bleach features labeled B1, B2, and B3. In each case, the position of B1 corresponds to the lowest energy peak in the linear absorption spectra (Figure 1). This feature is due to state-filling of the 1S3/21Se exciton state. Using the data plotted in Figure 3 of Zhong et al.,64 it is possible to assign the features labeled as B2 and B3. The B2 feature corresponds to the 2S3/21Se state, and the B3 feature is the 1P3/21Pe state. The positions of these two features in both the small core and the large core spectra are in perfect agreement with the previous report. The bleach at B3 vanishes quickly due to the rapid cooling of carriers to the band edge. This behavior is consistent with the assignment as the 1P3/2(h)1P(e) state. In less than 2 ps, the

Figure 9. Energy level diagram showing the lowest two levels for conduction band electrons and lowest three levels for valence band holes in CdTe (left) and CdTe/CdS core/shell NCs (right). Dashed lines represent the carrier potential energies of the conduction and valence band due to the CdTe and CdS lattices. When the CdS shell is added, the confinement energy of conduction band electrons decreases as these carriers delocalize throughout the core and shell. Valence band holes remain confined to the core due to the large valence band offset between the two materials. Vertical arrows represent the transitions, B1 (blue), B2 (red), and B3 (green), that are discussed in the transient absorption spectra.

conduction band electron and valence band hole fall to the 1S(e) and 1S3/2(h) energy levels, respectively. This gives rise to the dominant bleach feature at B1. The feature at B2 is shifted to higher energy by 0.10.2 eV, corresponding to the energy required to excite the hole from the 1S3/2(h) to the 2S3/2(h) level. The partial bleach at this energy does not imply that excited holes are present in the system; rather it is due to the filling of the 1S(e) state by the conduction band electron. Because 2S3/2(h)1S(e) is an allowed transition, the filling of the 1S(e) state leads to partial bleaching of this transition even though the 2S3/2(h) state is empty. In the core/shell NCs, the energy levels of the conduction band are modified by the presence of the CdS shell (Figure 9). The conduction band of bulk CdS is approximately 0.35 eV lower in energy than that of bulk CdTe,58 allowing for delocalization of the excited electron throughout the core and shell. With a sufficiently thick shell layer, the electron could completely localize in the CdS shell, but the shells here are too thin for this to happen in these samples. Instead, the conduction band electrons are delocalized throughout the NC. In contrast, the valence band offset of 1.31 eV effectively contains the valence band holes within the CdTe cores. The offset bands produce a quasi-type-II carrier distribution with partial charge separation along the radial coordinate. The quasi-type-II carrier distribution alters the apportionment of confinement energy between the electrons and holes in the core/shell particles. For this reason, the exciton energy levels in the core/shell particles cannot be assigned exclusively on the basis of the data from Zhong et al.64 As the CdS shell is added, the spectral red shift that is observed is due primarily to reduction in the confinement energy of conduction band carriers. 22723

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Figure 10. Energy spacing, ΔE, between 1S3/2 and 2S3/2 valence band hole levels plotted versus band gap (measured as 1S3/2(h)1S(e) energy) for CdTe cores (blue b) and CdTe/CdS core/shell NCs (magenta f). The black line shows the relationship between ΔE and lowest-energy PL excitation peak determined experimentally by Zhong et al. for CdTe core NCs.64

The energies of the holes, which remain confined to the core, are only weakly affected. This slightly complicates assignment of the B2 bleach in the TA spectra of the core/shell particles (Figure 5b and d. For example, in Figure 5d, the lowest energy bleach, B1, appears at 1.79 eV. Referring to Figure 3 of Zhong et al.,64 we find that the B2 [2S3/2(h)1S(e)] transition in such a sample should appear no higher than 1.85 eV, which is measurably below the feature labeled B2 in Figure 5d. In fact, at 1.85 eV, the B2 feature would be difficult to resolve from the main bleach feature. In our core/shell sample, the feature labeled B2 is nearer to 1.93 eV, which is very close to the predicted position of the B3 [1P3/2(h)1P(e)] transition. However, this latter assignment can be ruled out on the basis of the kinetic evolution of the spectrum. If the 1.93 eV bleach in Figure 5d corresponded to the 1P transition, it would disappear in less than 2 ps. Instead, that bleach feature persists for nanoseconds. On this basis, the bleach feature labeled B2 in the core/shell spectra (Figure 5b and d) can be safely assigned as the same 2S3/2(h)1S(e) transition that was assigned in the core-only spectra. Comparison of the hole energy level spacings in the core and core/shell NCs provides interesting information. Figure 10 shows a graph of ΔE, the difference between the energies of the 1S3/2 and 2S3/2 hole levels, versus optical band gap (as measured by the position of the 1S3/2(h)1S(e) bleach). The ΔE values were measured from the TA spectra as the energy spacing between the two lowest energy bleach features (i.e., energy difference between the 1S3/2(h)1S(e) and 2S3/2(h) 1S(e) bleach positions). Symbols in Figure 10 represent the ΔE values for several sizes of CdTe cores (blue b) and CdTe/CdS core/shell particles (magenta f). Spectral positions of the individual bleach features were determined by a least-squares fit of multiple Gaussian peaks to the 2 ps TA spectra. The solid black line represents the hole energy level spacings for CdTe NCs as determined experimentally by Zhong et al.64 It is clear from the figure that the ΔE values obtained from TA spectra of our CdTe core samples are in close agreement with the energy spacings measured via PL excitation measurements. Furthermore, the figure shows that for the core/shell particles, the hole confinement energies are systematically larger than those of CdTe cores with the same optical band gap (i.e., all magenta f

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are well above the solid line). The reason is that the electron confinement energy in the core/shell particles has been decreased due to delocalization of the electron throughout the core and shell (see Figure 9), whereas the hole confinement energy has remained almost unchanged. This comparison of hole energy levels provides direct evidence for the differential localization of electrons and holes in the CdTe/CdS core/shell system. Charge Transfer Bleach. The large core/shell sample (Figure 5d) exhibits a significant transient bleach feature at the high energy end of the spectrum. This feature, appearing at photon energies above 2.6 eV, develops at long probe delay times, greater than 500 fs. The feature has been observed in spectra from multiple, large core/shell samples and is absent in all CdTe core samples examined so far. It is also evident in the TA spectra of thiol-capped, aqueous CdTe/CdS NCs, as shown in Figure 8, which were produced from a different batch of core/ shell NCs. The spectral position of the high energy feature suggests that it is associated with the absorption of the CdS shell. As the CdS shell is added, the ground-state absorption increases significantly in the short wavelength region due to the deposition of the wide band gap CdS. Control experiments were performed to determine whether this high energy bleach is observed when longer wavelength excitation is used. Even when the NCs are pumped well to the red of the high energy bleach (2.2 eV), this transient bleach is observed. This observation excludes the possibility that the feature could be due to pure CdS nuclei that are present in the system. If the feature is due to bleaching of the ground-state CdS absorption, then its presence in the TA spectrum and its late appearance (after Δt = 1 ps) could be associated with charge redistribution that occurs as the conduction band electron wave function spreads into the shell. This intraparticle charge transfer process is not yet well understood, but other groups have also noted spectral dynamics occurring on the 1 ps time scale in nanoheterostructures and attributed these observations to intraparticle charge transfer events.14,20,34,6567 A curious characteristic of the high-energy bleach is that its evolution does not seem to be associated with noticeable changes in the lowest energy bleach. If the high-energy bleach is related to charge redistribution, then some red shift of the lowest energy bleach might be expected as the conduction band electron relaxes to its fully delocalized state; however, no spectral shift is evident in the spectra shown here. Further investigation and experimentation is needed to definitively assign and fully understand the nature of the high-energy bleach. Intraband Relaxation and Ultrafast Hole Trapping. Figure 6 shows the rising edge of the 1S3/2(h)1S(e) bleach for the CdTe and CdTe/CdS samples. The small core sample is noteworthy because of its very fast rise (τ = 150 fs) and decay (τ = 1.7 ps) as compared to the other samples. The rising edge of the lowest energy bleach is often identified with the intraband relaxation of the conduction band electron from the 1P(e) to the 1S(e) energy level.27,42 This 1S3/2(h)1S(e) bleach feature contains information about both the conduction band electron and the valence band hole. At early delay times, there is also some contribution from absorption to the biexciton state, but this contribution decays rapidly. The electron is regarded as the main contributor to this lowest energy bleach signal because the electron degeneracy is lower than that of the hole, and the lowest energy hole levels are mixed with higher-lying states that are close in energy.15 Furthermore, the electron usually dominates the rising edge kinetics of the 1S bleach because its effective mass is lower than that of the hole, thus giving a low density of states near the 22724

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The Journal of Physical Chemistry C conduction band edge. This low density of states produces a “phonon bottleneck”,68 which leads to slow electron kinetics. For these reasons, the electron cooling rate is often the rate-limiting factor in the development of the 1S bleach. This phonon bottleneck may be broken if other processes, such as Auger-mediated energy transfer between carriers,25,68 facilitate carrier cooling; however, even in these cases, the electron cooling is ordinarily rate-limiting. Klimov and co-workers demonstrated that in high-quality CdSe NC samples the rising edge kinetics of the 1S3/2(h)1S(e) bleach matched the decay kinetics of the 1P3/2(h)1P(e) bleach.25,42 This cross-verification provided strong evidence that the observed rising edge was indeed due to the arrival of the conduction band electron in the 1S(e) energy level and that the electron was arriving directly from the 1P(e) level without being trapped in an intermediate state. Taken together, these results seemed to be most consistent with the Auger-dependent model for intraband relaxation that had previously been proposed by Efros et al.68 Examination of the 1P kinetics of the small CdTe cores (B3 bleach, Figure 7) shows that the 1P decay (B3) approximately matches the rise of the 2S3/2(h)1S(e) bleach (B2) for those particles. For those small core particles, fits to the B2 and B3 kinetic traces yielded transition times of 490 and 550 fs. Similar behavior was observed in all of the CdTe core samples. For example, for the large cores, the 1P(e)-to-1S(e) transition times extracted from fits to the B2 and B3 features were 670 and 740 fs, respectively. For all samples, the B2 bleach recovery and B3 rise gave satisfactory agreement with each other given the uncertainties in the fits. Surprisingly, the 1S3/2(h)1S(e) rise time in CdTe cores did not typically match either the 1P decay time or the 2S3/2(h) 1S(e) rise. This behavior is in contrast to that which has been observed in numerous CdS and CdSe NC samples.46 The difference was particularly striking for smaller core samples, as shown in Figure 7. That figure shows the obvious difference between the B1 rise time, which is 150 fs, and the B2 rise time and B3 decay time, which are both approximately 500 fs. In addition, the B1 feature shows a rapid decay component of 1.7 ps that is not observed in the B2 feature. The close match between the B3 decay and the B2 rising edge in every sample suggests that these dynamics represent the same process, the cooling of the electron to the 1S(e) level. However, this conclusion implies that the 1S3/2(h)1S(e) rising edge does not accurately yield the electron cooling and raises a question regarding the origin of the observed kinetics in the B1 feature. Because the 1S3/2(h)1S(e) transition contains a bleach contribution from both carriers, but 2S3/2(h)1S(e) contains a bleach contribution only from the conduction band electron (see Figure 9), the difference in the dynamics between these two bleach features can be understood in terms of the fate of the valence band hole. Specifically, the absence of a slower rising edge component on the order of 500 fs is evidence for ultrafast hole trapping that competes with relaxation to the band edge. In addition, the fast 1.7 ps decay observed in the 1S3/2(h)1S(e) bleach is indicative of surface trapping of the fully relaxed hole. Because the 2S3/2(h)1S(e) kinetic essentially contains only information about the occupation of the 1S(e) level, that bleach feature must give the correct information about the 1P(e)-to-1S(e) relaxation process. The close agreement between the B3 decay time and the B2 rise time strongly support sthis argument.

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Analysis of the B2 and B3 dynamics in these small core NCs would appear to suggest much slower electron cooling rates for CdTe core NCs than for CdSe NCs of similar size. For example, Klimov et al. using similar experimental conditions and pump wavelength measured 1P(e) decay times and 1S(e) build-up times of approximately 120 fs in CdSe NCs that were similar in both size (3.4 nm diameter) and in band gap (∼2.3 eV) to our small CdTe cores.42 This represents a 4-fold difference in observed electron cooling times between the small CdTe cores and similar CdSe NCs. However, it is important to remember that the ultrafast hole trapping discussed above would be expected to slow electron relaxation by turning off the Auger mechanism for electron cooling. Thus, the observation of slow dynamics in the B2 and B3 features is consistent with the notion of subpicosecond hole trapping that removes the primary pathway for electron cooling. Hole trapping on the single picosecond time scale is consistent with previous results from CdSe NCs.13,29,32,43,46,69 Some of those reports have employed hole-trapping ligands to predictably alter the NC response; however, in recent work, McArthur et al.51 observed hole trapping with a 1.6 ps time constant in unmodified CdSe NCs using NIR TA spectroscopy. The small amplitude of the fast decay component in the B1 feature and the moderate PL quantum yield of the small core sample would seem to suggest that ultrafast hole trapping occurs in a relatively small fraction of the small core NCs. However, it may be premature to draw such a conclusion because (i) the holes are generally expected to make only a minority contribution to the amplitude of the B1 bleach feature, and (ii) very recent work by Knowles et al.50 has shown that, at least in CdSe NCs, trapped holes can still participate in radiative recombination that is spectrally indistinguishable from the band edge emission. In the small cores, the ultrafast hole trapping results from high wave function amplitude at the surface in the smaller particles and from the abundance of defects that arise due to the very short growth and annealing period (2 min) used in the synthesis of these NCs. Hole trapping undoubtedly occurs to some extent in the large core sample, too; indeed, surface carrier trapping is probably the primary process that limits the PL quantum yield in the core samples. In the large cores, however, the rate of hole trapping is significantly diminished in comparison with the small core sample: there is no evidence that the process competes with carrier cooling in the large cores. Accordingly, it seems reasonable to expect that the Auger mechanism for electron cooling is fully active in the larger core NCs throughout the cooling process. The large cores and all core/shell samples exhibited B1 dynamics that reflected the electron cooling process. Like the small cores, the large cores, whose band gap is 2.0 eV, can be compared to literature reports on CdSe NCs of similar band gap. Klimov et al. reported a 1S build-up time of 390 fs for 5.6 nm CdSe NCs with a band gap of approximately 2.0 eV.42 This is quite comparable to the 430 fs measured for the large cores here (see Figure 6). Although further work is needed to fully characterize the size dependence of the cooling rate in CdTe NCs, these initial results appear to show close agreement with the values in CdSe NCs. An interesting question in the present system is whether the differential distribution of electron and hole wave functions within the quasi-type-II core/shell NCs has a significant effect on the electron cooling rates. Because electron cooling in NCs is believed to depend on Auger-type energy transfer from electron to hole, the electron cooling rate would be expected to decrease 22725

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The Journal of Physical Chemistry C as the Coulomb interaction between the carriers decreases. As the carriers become spatially separated in the core/shell NCs due to the quasi-type-II exciton structure, this Coulomb interaction should decrease. The results presented in Figure 6 show that the electron cooling rate does indeed decrease as the CdS shell is added. For example, the B3 1P(e)-to-1S(e) decay time increased from approximately 500 to 700 fs as the shell was deposited on the small cores. A similar increase was observed in the large NCs. Perhaps the best measure of the effect of carrier separation in the core/shell system, however, comes from a comparison of the large core and small core/shell NCs. These have similar band gap energies, so the excess photon energy is nearly the same, 1.1 eV, in both cases. The B1 rise time measured in the small core/shell NCs is approximately 40% longer (610 fs vs 430 fs) than that measured in the large core NCs. This is despite the fact that TEM and SAXS data both indicate that the diameter of the large cores (4.2 nm) is more than 10% larger than that of the small core/shells (3.7 nm). On the basis of these results, it might be possible to use type-II systems to slow carrier cooling and thus enhance efficiencies of hot carrier extraction. Using their state-selective pumpprobe experiments, Sewall et al. have previously shown that the B1 bleach in CdSe NCs arises almost exclusively from signals attributable to the conduction band electron in that material.27 Our results suggest that the situation may be different in CdTe; differences in crystal symmetry and spinorbit interactions may alter the effective degeneracies of the valence band levels near the band edge and may thus increase the contribution of valence band holes to bleaching near the band edge. Additional experiments are underway to answer this question. Because the electron and hole overlap depends in a complex manner on the core size and shell thickness, it would be interesting to analyze the dependence of the carrier cooling rates in terms of the detailed shell structure. Unfortunately, we have not yet established sufficient control over the shell thickness to enable systematic study of this question in the CdTe/CdS. Growth of very thin shells, on the order of 0.5 nm, has been achieved routinely using both SILAR and slow injection methods. The shell growth typically saturates after the deposition of 12 monolayers of CdS on the surface of the core. Additional CdS can be deposited with the continued addition of Cd and S precursors in large excess, but the particle shapes become irregular and nonuniform. Estimating the spatial overlap of electron and hole in such samples would be problematic because of the ill-defined structure of the NCs and the sample heterogeneity. When this shell growth is driven to extreme, tetrapods are often formed, which presumably comprise zincblende CdTe cores decorated with wurtzite CdS arms. Effects of Air, Ligand Exchange, and Aqueous Solvent on CdTe/CdS NCs. Recently, Ghosh and co-workers38,39 reported transient absorption spectra from colloidal CdTe and CdTe/ CdS quantum dots. Their samples were prepared in aqueous media using hydrophilic thiols as capping ligands, according to a widely used synthetic procedure. We note that the TA spectra and excited-state dynamics reported by those researchers are qualitatively very different from those presented here. Samples prepared in aqueous media typically have broader size distributions, and the excitation densities used by Ghosh et al. are much higher than those used in our experiments. To determine if hydrophilic thiol capping ligands or the aqueous environment had a significant impact on CdTe/CdS TA spectra,

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we transferred some CdTe/CdS NCs into phosphate buffered saline at pH 7.2 after replacing the native capping ligands with thioglycolic acid. Figure 8 shows that the TA spectra were not greatly altered by transfer into aqueous solvent or capping with hydrophilic thiols. (Note that this is a different batch of core/ shell particles from those shown in Figure 5. The absence of the B2 bleach is a feature of this sample that stems from the larger core size, which results in collapse of the spacing between the hole states near the band edge. It is not a consequence of the transfer into water.) Furthermore, the decay kinetics of the main B1 bleach after ligand exchange and solvent transfer are similar to the kinetics in chloroform prior to ligand exchange. It appears, then, that the differences noted in the TA spectra of Ghosh and co-workers stem from something other than the aqueous environment and the thiol capping ligands. Their particles may be strongly affected by structural defects that may be present in samples prepared at low temperature, their results may be affected by high pump fluence, or there may be interactions between the particles and some of the reagents, such as excess borohydride, which is used to prepare the NaHTe precursor.

’ CONCLUSION Ultrafast carrier dynamics have been explored in CdTe and CdTe/CdS core/shell NCs. The bleaches in the TA spectra have been assigned, and analysis of the energy spacings between the 1S3/2(h) and 2S3/2(h) levels confirms the prevailing picture of quasi-type-II carrier distribution in the core/shell particles, with delocalized electrons and core-localized holes. In addition, a new understanding of the rising edge kinetics of the lowest energy bleach (B1) has emerged for these systems. While the rising edge kinetics of the lowest energy bleach in semiconductor NCs are often associated with intraband relaxation of conduction band electrons, it is shown here that at least in the case of the small CdTe cores, the fast rise of the B1 feature also reflects trapping of the valence band holes. Electron cooling rates in the CdTe cores have been measured and compared to rates measured in CdSe cores under similar conditions. The large CdTe cores show electron cooling rates that are very similar to those reported previously for CdSe cores of similar band gap energy. The small CdTe cores did not lend themselves to such comparison because the ultrafast hole trapping observed in these NCs can dramatically retard electron relaxation in the conduction band. The effect of the CdS shell on the electron cooling dynamics was also investigated by comparing the subpicosecond bleach dynamics in CdTe cores and CdTe/CdS core/shell NCs. The results show that arrival times for conduction band electrons in the 1S(e) level after excitation high (1.1 eV) above the band gap are approximately 40% longer for core/shell NCs than for the core-only NCs. This difference is likely due to reduction of the Auger coupling between electron and hole due to the quasi-typeII excitonic structure, which leads to some spatial separation of the carriers in the core/shell structure. Finally, a new, high-energy bleach feature has been identified in the large CdTe/CdS core/shell samples, and this feature has tentatively been assigned as a bleach of the CdS absorption. This feature could, in principle, be used to isolate and study the dynamics of the conduction band electron individually in certain types of nanoheterostructures, but further study of the nature of this feature is necessary first. 22726

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’ ASSOCIATED CONTENT

bS

Supporting Information. Detailed explanation of NC synthesis and experimental procedures as well as particle size distributions obtained from TEM and SAXS measurements. This material is available free of charge via the Internet at http://pubs. acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]

’ ACKNOWLEDGMENT This material is based upon work supported by the National Science Foundation under Grant No. CHE-0947031. Y.Y. was supported by funding from the Condensed Matter & Surface Sciences Program at Ohio University. We thank Prof. M. E. Kordesch for use of the transmission electron microscope. ’ REFERENCES (1) Halpert, J. E.; Porter, V. J.; Zimmer, J. P.; Bawendi, M. G. Synthesis of CdSe/CdTe Nanobarbells. J. Am. Chem. Soc. 2006, 128, 12590–12591. (2) Bailey, R. E.; Strausburg, J. B.; Nie, S. M. A new class of far-red and near-infrared biological labels based on alloyed semiconductor quantum dots. J. Nanosci. Nanotechnol. 2004, 4, 569–574. (3) Manna, L.; Milliron, D. J.; Meisel, A.; Scher, E. C.; Alivisatos, A. P. Controlled growth of tetrapod-branched inorganic nanocrystals. Nat. Mater. 2003, 2, 382–385. (4) Kim, S.; Fisher, B.; Eisler, H. J.; Bawendi, M. Type-II quantum dots: CdTe/CdSe(core/shell) and CdSe/ZnTe(core/shell) heterostructures. J. Am. Chem. Soc. 2003, 125, 11466–11467. (5) Gaponik, N.; Talapin, D. V.; Rogach, A. L.; Hoppe, K.; Shevchenko, E. V.; Kornowski, A.; Eychm€uller, A.; Weller, H. Thiol-capping of CdTe nanocrystals: An alternative to organometallic synthetic routes. J. Phys. Chem. B 2002, 106, 7177–7185. (6) Peng, Z. A.; Peng, X. G. Formation of high-quality CdTe, CdSe, and CdS nanocrystals using CdO as precursor. J. Am. Chem. Soc. 2001, 123, 183–184. (7) Smith, A. M.; Mohs, A. M.; Nie, S. Tuning the optical and electronic properties of colloidal nanocrystals by lattice strain. Nat. Nanotechnol. 2009, 4, 56–63. (8) Bang, J. H.; Kamat, P. V. Quantum dot sensitized solar cells. A tale of two semiconductor nanocrystals: CdSe and CdTe. ACS Nano 2009, 3, 1467–1476. (9) Blackman, B.; Battaglia, D.; Peng, X. G. Bright and water-soluble near IR-emitting CdSe/CdTe/ZnSe Type-II/Type-I nanocrystals, tuning the efficiency and stability by growth. Chem. Mater. 2008, 20, 4847–4853. (10) Zhong, H. Z.; Zhou, Y.; Yang, Y.; Yang, C. H.; Li, Y. F. Synthesis of type II CdTeCdSe nanocrystal heterostructured multiple-branched rods and their photovoltaic applications. J. Phys. Chem. C 2007, 111, 6538–6543. (11) Chang, J. Y.; Wang, S. R.; Yang, C. H. Synthesis and characterization of CdTe/CdS and CdTe/CdSe core/shell type-II quantum dots in a noncoordinating solvent. Nanotechnology 2007, 18, 345602. (12) Burda, C.; Green, T. C.; Link, S.; El-Sayed, M. A. Electron shuttling across the interface of CdSe nanoparticles monitored by femtosecond laser spectroscopy. J. Phys. Chem. B 1999, 103, 1783–1788. (13) Burda, C.; Link, S.; Mohamed, M.; El-Sayed, M. The relaxation pathways of CdSe nanoparticles monitored with femtosecond timeresolution from the visible to the IR: Assignment of the transient features by carrier quenching. J. Phys. Chem. B 2001, 105, 12286–12292.

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