ARTICLE pubs.acs.org/JPCC
Spectroscopic Investigation of Oxygen Sensitivity in CdTe and CdTe/CdS Nanocrystals Yueran Yan,† Lei Wang,† Cheryl B. Vaughn,†,§ Gang Chen,‡ and P. Gregory Van Patten*,† † ‡
Department of Chemistry and Biochemistry, Ohio University, Athens, Ohio 45701, United States Department of Physics and Astronomy, Ohio University, Athens, Ohio 45701, United States ABSTRACT: Excited-state dynamics of CdTe nanocrystals and CdTe/CdS core/shell nanocrystals have been studied on the picosecond time scale using pump probe transient absorption spectroscopy. The results show that exposure of CdTe nanocrystals to air can produce profound changes in the ultrafast excited-state dynamics. These changes occur even after a brief air exposure, but they are not observed in CdTe nanocrystals that are protected by a thin CdS shell. The quenching is caused by reaction of molecular oxygen with Te atoms on the nanocrystal surface, which produces traps for one or both excited carriers. The trapping event occurs on the single picosecond time scale and is sufficiently fast to compete with hot carrier cooling. This phenomenon has important implications for interpretation of excited-state dynamics in CdTe-containing nanoheterostructures as well as ultrafast charge separation in photoexcited nanostructures.
’ INTRODUCTION Colloidal semiconductor nanocrystals (NCs) possess interesting and unique electronic and optical properties, which have identified them as candidate materials and interesting model systems for a wide range of technologies.1 6 CdTe has become an interesting material for colloidal nanostructures because of its electronic properties and the existence of facile routes for preparation of high-quality CdTe nanocrystals.7 12 These characteristics, combined with its electronic properties, have made CdTe a favorite for inclusion in nanoheterostructures with other II VI semiconductors.12 22 CdTe can be combined with CdSe or CdS to produce type-II or quasi-type-II nanoheterostructures,22,23 which hold interesting possibilities for optical gain media,24 bioimaging applications,16,21,22 and solar energy conversion.13,14,18,25,26 The behavior of photoexcited charge carriers in these type-II nanoheterostructures is an area of intense current interest. Several groups, including ours, have used femtosecond transient absorption (TA) spectroscopy to show evidence for ultrafast intraparticle charge separation across semiconductor/semiconductor interfaces in colloidal NCs.14,27 30 This intraparticle charge separation phenomenon has important implications for regulating hot carrier relaxation to the band edge, for enhancing carrier extraction efficiencies in solar energy conversion schemes, and for inhibiting carrier recombination. To fully understand the role of NC heterojunctions in assisting sub-picosecond charge separation, it is necessary to be able to measure and interpret changes in the carrier dynamics that accompany growth of a new semiconductor phase (e.g., a conformal shell) onto an NC core. Unfortunately, this objective is complicated by the fact that shell growth can contribute to changes in ultrafast dynamics through a r 2011 American Chemical Society
variety of other mechanisms in addition to intraparticle charge separation. The first few picoseconds after photoexcitation is an eventful time in semiconductor NCs, as carriers relax to the band edge through multiple pathways and may be trapped at the surface.31,32 Epitaxial growth of a wide-band-gap inorganic shell onto a semiconductor NC is generally expected to alter carrier trapping probabilities and rates at the NC surface. In some cases, the shell might also affect intraband relaxation rates, but this effect can be effectively removed from femtosecond TA data by pumping near the band edge. In contrast, surface trapping events would be rather difficult to exclude from the experiments. For this reason, it is important to consider any possible shell-related changes in surface reactivity when comparing ultrafast dynamics between core and core/shell systems. Surface reactivity and carrier trapping tendencies in the core NC may significantly affect the ultrafast dynamics and thus confound comparison of the core and core/shell systems. This point is illustrated in the present paper using CdTe, a common component in nanoheterostructure systems. We show that the surface reactivity of CdTe cores toward oxygen requires that special care be exercised in drawing comparisons between ultrafast dynamics in core and core/shell systems. CdTe NCs undergo surface reactions and luminescence quenching in the presence of air. The sensitivity of CdTe NCs to oxidation and quenching in air has been noted and investigated Received: July 15, 2011 Revised: October 21, 2011 Published: October 26, 2011 24521
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previously by other research groups.18,33 However, the dynamics of the air-induced quenching and its effect on the ultrafast photophysical response of CdTe NCs is unexplored. Furthermore, little is known about the extent to which the air sensitivity can be controlled through the synthetic reaction route, through choice of surface capping ligands, or through encapsulation within an ultrathin shell made of another semiconductor material. We report here a study on the air sensitivity of CdTe nanocrystals with an emphasis on the immediate and significant photophysical consequences of air exposure and the remarkable protection offered by even a very thin CdS shell layer. For the present study, CdTe NCs were produced using several different synthetic methods,9,11,12,23 all of which involved rapid injection of precursors at high temperature into high-boiling organic solvents. Various methods were also used for deposition of thin CdS shells onto the CdTe NCs.12,23,30,34 The resulting core and core/shell NCs were subjected to a series of spectroscopic measurements to probe the universality of the CdTe NCs’ sensitivity to air. Femtosecond TA measurements were used to probe the changes in ultrafast exciton dynamics that accompany air-induced quenching. The results show that even brief exposure of CdTe NCs to air can significantly alter their ultrafast dynamics relative to samples coated with even a monolayer-thin CdS shell.
This process was repeated four more times in rapid succession. After the air was bubbled through the sample, the cuvette was resealed to minimize evaporation of chloroform, and the spectroscopic measurements were repeated on the air-exposed sample. Absorption spectra were measured using an HP 8453 diode array spectrophotometer. PL spectra were obtained using a spectrofluorometer (model C-60) from Photon Technology International (PTI). A 75 W xenon arc lamp was used for excitation. A monochromator was used to select the excitation wavelength. Luminescence from the sample was collected and focused through a double grating monochromator and then detected with a PMT (Hamamatsu R928) in photon counting mode. Emission spectra were corrected for the wavelength-dependent instrument response. PL decay measurements were made using the time-correlated single-photon counting method as described previously.30 Transient absorption (TA) measurements were performed using equipment that has been described in detail previously.30 Neutral density filters were used to limit the pump beam intensity and thus avoid biexciton production in the nanocrystals. The pump wavelength was 400 nm, and the pump fluence at the sample was typically 6 μJ/cm2 (1 mm pump beam spot size). The sample path length was 2 mm.
’ EXPERIMENTAL SECTION
’ RESULTS AND DISCUSSION Figure 1a shows the UV visible absorption spectra of the CdTe NCs before (solid black curve) and after (dashed red curve) air exposure. The lowest-energy absorption peak appeared at 2.00 eV (620 nm) for the argon-protected sample, and this peak was observed to blue shift approximately 0.01 eV after exposure to air. Other than the small blue shift, the spectra under argon and air are very similar. The extinction coefficient at the lowest-energy peak is almost unchanged. Additional experiments in which much more air was bubbled through the sample over a period of 3 min showed increased blue shifts. The mechanism for the observed blue shift is not certain. Oxidative etching of the nanocrystal surface is one possible explanation, but previous studies have also shown that oxygen can bind to the surface of CdTe nanocrystals, oxidizing exposed Te ions to form surface-bound TeOx species.33 This surface oxidation could effectively reduce the size of the CdTe NC and thus produce a confinement-induced blue shift. Either way, as will be shown below, exposure to air produces irreversible photophysical changes that are consistent with permanent chemical modification of the CdTe nanocrystal surface. Figure 1b illustrates the dramatic PL quenching of the CdTe NCs that occurs within approximately 15 min after bubbling 5 mL of air through the sample. For this particular sample, the air exposure quenched the PL intensity approximately 70% within 15 min. With additional time and/or with additional exposure to air, the quenching continues to increase, effectively reaching 100% within several hours. We attribute the quenching to the formation of TeOx species on the surfaces of the NCs, as described above, which has been previously correlated with reduced quantum yield in CdTe NC samples.33 The CdTe core NCs prepared by different methods and with different capping ligands showed slightly different behavior in our experiments. These differences were noted only in the kinetics of the surface reaction, not in the ultimate fate of the CdTe cores. All the samples showed significant quenching within a day of being exposed to air. Certain ligands, such as HDA and
Chemicals. n-Tetradecylphosphonic acid (TDPA) was pur-
chased from PCI Synthesis. Oleic acid (OA, 99%) and hexadecylamine (HDA, technical grade, 90%) were purchased from Alfa Aesar. Cadmium oxide (CdO, 99%) was purchased from Fluka. Stearic acid (97%) was purchased from Acros. Elemental sulfur (99.999%) and tri-n-octylphosphine (TOP, 97%) were obtained from Strem Chemicals. 1-Octadecene (ODE, tech. 90%), tributylphosphine (TBP, 97%), tellurium powder (200 mesh, 99.8%), and oleylamine (70%) were purchased from Aldrich. Chloroform (CHCl3) was purchased from VWR, and methanol (MeOH) was obtained from J.T. Baker. All reagents were used as received. CdTe cores were prepared by several different methods, all of which have been previously reported.9,11,12,23,30 In some cases, CdS shells were deposited onto the CdTe cores. The shells were likewise grown by multiple, previously reported methods. These included SILAR methods12,34 as well as gradual, simultaneous injection of Cd and S precursors.23,30 The shell growth procedures were matched to the respective core synthesis methods. After synthesis, the nanocrystals were transferred via a syringe to a sealed, argon-filled container and placed in an argon-filled glovebox for storage. Spectroscopic Measurements. Samples for air-free spectroscopic measurements were prepared inside the glovebox. To prepare these samples, the crude nanocrystal suspension was diluted with argon-saturated chloroform, loaded into a cuvette, and sealed with a rubber septum to avoid air exposure. The sealed cuvette was then removed from the glovebox for spectroscopic measurements. Steady-state absorption and photoluminescence (PL) spectra and time-resolved luminescence decays were collected using 10 mm path length quartz cuvettes. Transient absorption spectra were collected using 2 mm path length quartz cuvettes. Once data collection on the argon-protected samples was completed, the seals were removed from the cuvettes and the samples were exposed to air. A 1 mL syringe was filled with air, and this air was bubbled into the sample over a period of 5 10 s.
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Figure 2. Normalized time-resolved emission decay of the CdTe NCs under argon and the air.
Figure 1. (a) UV vis spectra of the CdTe NCs under argon (solid black curve) and after a brief air exposure (red dashed curve). (b) Photoluminescence emission spectra of the CdTe NCs under argon (solid black curve) and after a brief air exposure (red dashed curve). PL emission continued to decrease further with continued exposure to air, until the quenching reached 100%.
oleic acid, were able to reduce the rate of PL degradation upon exposure to air. When HDA was included in the CdTe synthetic reaction mixture, the time scale for degradation was shifted from a few minutes to over an hour. Oleic acid, when used in place of TDPA or stearic acid, was effective at preserving the PL intensity for up to 1 day. Unfortunately, substitution of oleic acid for TDPA significantly alters the NC growth kinetics, generally resulting in broader size distributions and lower-quality NC samples. For samples prepared without OA and HDA, it was observed that brief exposure of the CdTe NCs to air caused rapid (within minutes) PL quenching; however, quickly washing the NCs via standard precipitation/resuspension procedures could partially restore the PL intensity. After the washing, however, further rapid quenching was observed, and this quenching became irreversible within an hour. In some cases, the decline in intensity was noticeably accelerated after washing the nanocrystals. The partial recovery of intensity after washing the nanocrystals was not universal, but it implies the existence—at least in some cases—of a fast, reversible step in the reaction with air that involves one or more ligands bound to the surface. The removal of the affected ligands via washing caused a partial recovery in the PL intensity. Despite the differences noted above, the various ligands employed
all failed to preserve the luminescence intensity after continued air exposure. In all cases, air exposure ultimately led to quantitative, irreversible PL quenching in the CdTe cores. The quenching phenomenon was observed on different sizes of CdTe NCs. The data presented in Figure 1 were collected from 3.9 nm CdTe cores, but the quenching phenomenon was observed in a wide range of sizes. The results were qualitatively the same, and comparing the extent of quenching in these samples is not particularly informative since all effectively reached 100% quenching with prolonged air exposure. We did, however, notice differences in the ultrafast quenching kinetics in different-sized NCs. These data will be discussed in further detail below. Experiments were performed to differentiate the effects of oxygen and water vapor on the CdTe NCs. First, a flask of deionized water was bubbled with argon for 30 min to remove other dissolved gases. The stream of [water-saturated] argon exiting the water flask was then directed into a vessel containing the argon-protected CdTe NCs. After 5 min of bubbling watersaturated argon through the NC mixture, the PL quantum yield was unchanged. Another sample of argon-protected NCs was bubbled with pure, dry oxygen for 5 min, and the PL was quenched by more than 80%. The quenching by molecular oxygen is consistent with the aforementioned idea that the quenching may be due to the reaction of O2 with Te atoms on the NC surface. Figure 2 shows normalized PL emission decays of the CdTe NCs under argon (black curve) and after air exposure (magenta curve). The emission decays of the two samples are nearly identical. Average lifetimes for these two samples, as determined from a multiexponential fit using three lifetimes, were 47 ns for the argon-protected sample and 43 ns for the air-exposed sample. The slight difference in these two values is likely due to the very slight variance between the two curves at long time delays, and this slight variance is within the uncertainty of the background subtraction that was used to correct for the dark counts accumulated during the measurements. Taking this uncertainty into consideration, the decays are essentially indistinguishable, and the air exposure appears to have no impact on the observed luminescence lifetime of the NCs. The invariance of the PL decay rate after air exposure implies that the excited state is quenched on an ultrafast time scale, yielding essentially binary toggling between unquenched and completely quenched states. Following excitation, the NCs return to the 24523
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Table 1. Multiexponential Fit Parameters (Amplitudes and Lifetimes) for Argon-Protected and Air-Exposed CdTe Nanocrystals arise τrise (ps)
Figure 3. TA kinetic traces of (a) 4 nm CdTe NCs and (b) 3 nm CdTe NCs under argon (black triangles) and after exposure to air (red circles). The inset shows the same data over a longer pump probe delay time (up to 150 ps).
ground state (or proceed to a nonemissive intermediate state) on a time scale that is too fast to observe with our PL decay measurements. To monitor the ultrafast dynamics of the quenched nanocrystals, we employed femtosecond pump probe transient absorption (TA) spectroscopy. Figure 3 shows TA kinetic traces of CdTe NCs under argon (black triangles) and after exposure to air (red circles). Panel (a) depicts data collected for CdTe cores of approximately 4 nm in diameter while panel (b) shows data collected on smaller cores, approximately 3 nm in diameter. In both cases, the kinetic traces were recorded at the wavelength corresponding to the 1S3/2(h) 1S(e) bleach, so they depict the population dynamics of carriers near the band edge. Two distinct phenomena contribute to the rising edge of the kinetic traces in Figure 3. These two distinct phenomena can most clearly be seen in the larger core data shown in Figure 3a. Absorption of a 3.1 eV photon excites the NCs to a high-lying excited state, and transitions between this excitonic state and the biexcitonic state are shifted relative to the ground-state absorption features due to Coulombic interactions between the excited carriers.31,35 These shifts in the transition energies appear instantaneously and thus produce a fast rising bleach component at the probe wavelength used in this experiment. This fast rise component matches the temporal characteristics of the instrument response function (approximately 190 fs fwhm). Immediately after excitation, the highly excited electron and hole begin to relax to the lowest exciton state, and as they reach the lowest-energy exciton state,
a1
τ1 (ps)
a2
τ2 (ps)
argon-protected 0.428 0.43
0.098 32
0.172 1100
air-exposed
0.181
0.139
0.345 0.33
2.6
50
a3
τ3 (ps)
0.163 1000
further bleaching is observed due to state-filling.31,36 This is the process represented by the slower rising component in the curves (most pronounced in the larger cores under argon). The time scale for this carrier cooling process is dependent on the material, and 300 900 fs is typical for CdTe NCs in the strong confinement regime.30 In Figure 3a, it is clear that the fast-rising components for the argon-protected and air-exposed NCs are nearly identical. Note that the data have not been normalized or rescaled in this figure. Because the ground-state absorption of the sample is the same before and after air exposure, and the excitation conditions are identical, the two curves can be directly compared using the same vertical axis scaling. It was confirmed that the maximum amplitude of the Stark effect peak (observed as a photoinduced absorption on the long wavelength side of the 1S3/2(h) 1S(e) bleach) was the same before and after air exposure. In contrast, the peak intensity of the 1S3/2(h) 1S(e) bleach is reduced by approximately 25% after exposure to air. Because the groundstate absorption is nearly unchanged after air exposure (Figure 1), this decrease in bleach amplitude measured at the wavelength corresponding to the emitting state implies that a fraction of the excited NCs fail to reach the emitting state due to some ultrafast quenching process. In addition to the reduced bleach intensity, the air-exposed sample exhibits a fast decay component that is not present in the argon-protected sample. The data for the smaller cores, shown in Figure 3b, show changes upon air exposure that are qualitatively similar to those observed in the larger cores; however, the faster dynamics seen in the smaller cores under argon makes the air-induced change less obvious. To better understand the air-induced changes in the TA kinetics, the data from the larger cores (Figure 3a) were fit to multiexponential functions, including rising edge components convolved with a 190 fs instrument response function. The fit parameters are summarized in Table 1. As shown in the table, the dynamics of the argon-protected samples could be described by a function with one rise time and two decay lifetimes. The rise time was approximately 430 fs, which is similar to values previously recorded for high-quality CdTe and CdTe/CdS NCs.30 The decay was composed of two major components, one of which was approximately 50 ps, and one of which was approximately 1 ns. The latter lifetime includes significant relative uncertainty because of the limited time window used in the fits. After exposure to air, additional fast decay pathways were introduced into the kinetics. The first of these can be observed qualitatively in Figure 3a as a reduction in the maximum bleach amplitude. In the fit parameters, this change can be observed as a decrease in the amplitude of the rising component as well as a decrease in the measured lifetime of the rise component. This decrease in the lifetime of the rising edge component is likely due to a competing, ultrafast process (such as carrier trapping at the nanocrystal surface) that quenches the excited state before the carriers can relax to the lowest excitonic energy level. In addition, the data 24524
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The Journal of Physical Chemistry C from the air-exposed sample exhibit an extra decay component with a very short (2.6 ps) lifetime. The significant effect of these additional decay pathways on the excited-state dynamics is revealed in the inset of Figure 3. Within 150 ps, the bleach intensity of the air-exposed NCs falls to approximately half that of the argon-protected nanocrystals. The kinetics of the two samples on longer time scales was observed to be very similar, in accordance with the data from the time-resolved PL measurements. As mentioned above, the TA data in Figure 3 showed that, after the first 100 ps, the bleach intensity of the air-exposed sample was reduced as compared with the argon-protected sample at the same delay time. However, the decrease in the bleach intensity was less than 50%. In contrast, the PL quenching was much greater than 50% in the air-exposed sample. This apparent discrepancy between the TA and PL results may be rationalized by considering that the quenching mechanism may be due to a process other than direct ground-state recovery. For example, surface trapping of an individual carrier in a given nanocrystal could quantitatively quench photoluminescence in that nanocrystal, but the trapped carrier state could still produce some bleach intensity in the TA measurements. Thus, the fast processes that lead to rapid decay of the TA signal in the airexposed sample may not be a direct ground-state relaxation, but probably instead involve an intermediate state, such as a surfacetrapped carrier. We have previously shown evidence for sizedependent hole trapping in CdTe cores, even under argon.30 In that work, fast hole trapping on the 1 ps time scale was reported in CdTe cores that were smaller than about 3.2 nm in diameter. The CdTe cores investigated here are large enough (4.0 nm) that the intrinsic hole-trapping tendency is largely suppressed. Similar luminescence quenching experiments were performed on core/shell CdTe/CdS nanocrystals. Like the CdTe cores, the CdS shells were synthesized by several different methods drawn from the literature.12,23,30,34 In some cases, CdS shells were grown via slow, simultaneous injection of Cd and S precursors using a syringe pump, whereas in other cases, they were grown via the SILAR method. An interesting and important aspect of the SILAR approach is that the thicknesses of the resulting shells can be finely controlled since the reagents are delivered one shell layer at a time. The CdS shells provided remarkable protection from quenching for the CdTe nanocrystals. Quantum yields of the core/shell samples routinely reached 65 70%, and the quantum yields were typically decreased by no more than a few percent after long-term (several days) air exposure. Figure 4 shows the TA kinetics of CdTe/CdS NCs under argon (black triangles) and under air (red circles). These core/shell NCs were grown from the same cores represented in Figure 3a. The kinetic traces are very similar, with only a slight decrease in amplitude for the airexposed nanocrystals. Multiexponential fits to these samples both yielded three components with very similar lifetimes. The inset shows that the difference between the two TA traces remains small even after a 3 ns pump probe delay. SILAR and slow injection shell growth experiments demonstrated that even a single monolayer of CdS is sufficient to provide full protection from air-induced quenching. In fact, as we have reported elsewhere,30 the shell growth seems to terminate after a very thin shell is deposited unless the system is driven hard with high temperature or highly reactive monomers. Several avenues of characterization were used to confirm the ultrathin nature of the shell deposited in these experiments. The measurements and typical results on these ultrathin shell measurements
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Figure 4. TA kinetic traces of CdTe/CdS core/shell nanocrystals under argon (black triangles) and after exposure to air (red circles). The inset shows the same data over a longer pump probe delay time (up to 3 ns). These core/shell NCs were prepared from 4 nm CdTe cores similar to those represented in Figure 3a.
have been reported elsewhere.30 CdTe cores and CdTe/CdS core/shell NCs were measured directly from transmission electron micrographs. The average diameter of the NCs typically grew by 6 8 Å during shell growth, indicating a shell thickness of no more than 3 4 Å. The shell thicknesses could have been a bit less than that estimated from the change in diameter since Ostwald ripening may also have contributed, in part, to the increase in NC size during shell growth. The particle size distribution was also measured via small-angle X-ray scattering (SAXS). The size distribution estimated from SAXS data was very similar to that determined from TEM. The peak locations in the particle size distribution of the core/shell NCs showed a diameter increase of 6.5 Å during the shell growth. Elemental analysis by ICP-OES indicated that the quantity of sulfur in the core/shell nanocrystals was insufficient even to provide a full monolayer of coverage. We note that these results are consistent with other results published in the literature. In particular, Wang et al. showed strong increases in luminescence quantum yield (up to 80%) after addition of only 1 ML (monolayer) of CdS to CdTe NCs.37 In that report, the quantum yield reached its optimum level (92%) after only 2 MLs. Growth of additional CdS layers beyond the second monolayer led to significant reduction of the quantum yield. The effectiveness of a single CdS monolayer in protecting the CdTe core implies that direct binding of oxygen to the CdTe surface is a requirement for quenching, in agreement with the Te oxidation model discussed above. The effectiveness of a single CdS monolayer in protecting the CdTe cores is remarkable since the SILAR reaction yield is expected to be less than 100%, and the shell uniformity is likely to be imperfect. From these considerations, it follows that some bare CdTe probably remains exposed after the first monolayer of CdS shell is added. That the remaining exposed CdTe does not lead to quenching suggests that the Te surface atoms most susceptible to oxidation react preferentially with the CdS shell precursors. This idea opens the possibility that factors, such as shape control, surface annealing during particle synthesis, and/or surface stoichiometry,38 may allow some control over the air sensitivity of CdTe NCs. However, our experience with numerous synthetic procedures failed to yield any CdTe NC batches that were truly resistant to air oxidation. 24525
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CdTe nanocrystals and CdTe-containing heterostructures14,19,30 must take into account these differences in oxygen sensitivity.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Present Addresses §
Department of Physical Sciences, Columbus State Community College, Columbus, OH 43215. Figure 5. Comparisons of TA kinetic traces from CdTe cores (black triangles) and CdTe/CdS core/shell NCs (red circles) under (a) air and (b) argon.
The effects of CdS shell growth on the ultrafast exciton dynamics of CdTe NCs can be appreciated by directly comparing the data shown in Figures 3a and 4. At the same time, we can examine the importance of making this comparison under an inert atmosphere. Figure 5 shows comparisons of TA kinetic traces from CdTe cores and CdTe/CdS core/shell NCs (a) under air and (b) under argon. The experiment run under air shows that the shell strongly modifies the ultrafast exciton dynamics, whereas the experiment run under argon shows only subtle differences between the two samples. When the experiment is run in the presence of air, the data seem to suggest that the core/shell sample possesses additional relaxation processes on the 1 ps time scale; such changes could be attributed to a reduced rate of intraband carrier cooling or to an intraparticle charge separation. However, the comparison under argon demonstrates that the observed changes are really due to protection of the NC surface against oxygen by the CdS shell. Furthermore, these results show that even brief exposure to air is sufficient to significantly affect the ultrafast TA dynamics in CdTe NCs. For this reason, great care must be exercised in the interpretation of TA data collected from air-exposed CdTe NCs and in comparisons between these and related (e.g., nanoheterostructure) systems.
’ CONCLUSION In summary, unprotected CdTe NCs are highly sensitive to quenching upon exposure to air. The quenching is caused by oxygen binding to the NC surface, and it is irreversible. The rate of the reaction with oxygen depends on the details of the procedure used to synthesize the CdTe nanocrystals; however, in all cases, significant quenching is observed within a period of minutes to hours. TA measurements showed that the quenching was caused by ultrafast relaxation processes in the air-exposed samples that were not observed in argon-protected samples. These relaxation processes were sufficiently fast to compete with intraband carrier cooling and thus prevented many of the nanocrystals from reaching the emitting state. Even those that did reach the lowest exitonic level showed an ultrafast (2.6 ps) decay to a dark state. Because the quenching occurs on such a fast time scale, the measured PL decay is unchanged; only the unquenched population of NCs is observed in the PL decay measurement. On the other hand, the TA rising edge and ultrafast decay dynamics were both strongly altered by the quenching. From these results, it is clear that meticulous exclusion of oxygen is required in order to reliably measure and interpret ultrafast exciton dynamics in bare CdTe NCs. In addition, comparisons between the excited-state dynamics of
’ ACKNOWLEDGMENT This material is based upon work supported by the National Science Foundation under Grant nos. CHE-0947031 (P.G.V.) and DMR-0906825 (G.C.). Y.Y. was supported by funding from the Condensed Matter & Surface Sciences Program at Ohio University. ’ REFERENCES (1) Milliron, D. J.; Gur, I.; Alivisatos, A. P. Hybrid organic-nanocrystal solar cells. MRS Bull. 2005, 30, 41–44. (2) Parak, W. J.; Gerion, D.; Pellegrino, T.; Zanchet, D.; Micheel, C.; Williams, S. C.; Boudreau, R.; Le Gros, M. A.; Larabell, C. A.; Alivisatos, A. P. Biological applications of colloidal nanocrystals. Nanotechnology 2003, 14, R15–R27. (3) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Semiconductor nanocrystals as fluorescent biological labels. Science 1998, 281, 2013–2016. (4) Nozik, A. J. Exciton multiplication and relaxation dynamics in quantum dots: Applications to ultrahigh-efficiency solar photon conversion. Inorg. Chem. 2005, 44, 6893–6899. (5) Nozik, A. J. Quantum dot solar cells. Physica E 2002, 14, 115–120. (6) He, J.; Zhong, H. Z.; Scholes, G. D. Electron-hole overlap dictates the hole spin relaxation rate in nanocrystal heterostructures. Phys. Rev. Lett. 2010, 105, 046601. (7) 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. (8) Rogach, A. L.; Katsikas, L.; Kornowski, A.; Su, D. S.; Eychm€uller, A.; Weller, H. Synthesis and characterization of thiol-stabilized CdTe nanocrystals. Phys. Chem. Chem. Phys. 1996, 100, 1772–1778. (9) Yu, W. W.; Wang, Y. A.; Peng, X. G. Formation and stability of size-, shape-, and structure-controlled CdTe nanocrystals: Ligand effects on monomers and nanocrystals. Chem. Mater. 2003, 15, 4300–4308. (10) 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. (11) Kloper, V.; Osovsky, R.; Kolny-Olesiak, J.; Sashchiuk, A.; Lifshitz, E. The growth of colloidal cadmium telluride nanocrystal quantum dots in the presence of Cd0 nanoparticles. J. Phys. Chem. C 2007, 111, 10336–10341. (12) 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. (13) Lo, S. S.; Mirkovic, T.; Chuang, C. H.; Burda, C.; Scholes, G. D. Emergent properties resulting from type-II band alignment in semiconductor nanoheterostructures. Adv. Mater. 2011, 23, 180–197. (14) Chuang, C. H.; Lo, S. S.; Scholes, G. D.; Burda, C. Charge separation and recombination in CdTe/CdSe core/shell nanocrystals as a function of shell coverage: Probing the onset of the quasi type-II regime. J. Phys. Chem. Lett. 2010, 1, 2530–2535. (15) Zhong, H. Z.; Scholes, G. D. Shape tuning of type II CdTeCdSe colloidal nanocrystal heterostructures through seeded growth. J. Am. Chem. Soc. 2009, 131, 9170–9171. 24526
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