J. Phys. Chem. C 2009, 113, 19077–19081
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Ultrafast Transient Absorption Measurements of Charge Carrier Dynamics in Single II-VI Nanowires Christopher R. Carey, Yanghai Yu, Masaru Kuno, and Gregory V. Hartland* Department of Chemistry and Biochemistry, 251 Nieuwland Science Hall, UniVersity of Notre Dame, Notre Dame, Indiana 46556-5670 ReceiVed: July 27, 2009; ReVised Manuscript ReceiVed: September 16, 2009
Ultrafast transient absorption experiments have been performed on single CdTe and CdSe nanowires with close to diffraction-limited spatial resolution. The traces for the CdTe nanowires show fast picosecond time scale dynamics, which is assigned to charge carrier trapping at surface states. The time constants vary for different nanowires. This is attributed to differences in the energy and/or density of the trap states, presumably due to variations in surface chemistry. The fast decay component is absent in experiments on CdSe nanowires performed under identical conditions, which is consistent with the much higher emission quantum yield of CdSe nanowires compared to CdTe nanowires. Experiments were also performed with the pump and probe positioned at different points along a single CdTe nanowire. The traces show different trapping times, due to spatial variation in the energy and/or density of trap states. These results highlight the importance of single particle measurements for these systems. Introduction Single particle spectroscopy is a powerful tool for studying nanomaterials. It can reveal features hidden by ensemble averaging and illustrate how differences in size, shape, composition, and environment affect optical properties.1-4 For example, experiments on single semiconductor quantum dots (QDs) under cw illumination show a phenomenon called fluorescence intermittency (“blinking”), where the emission turns on and off at seemingly random times.5-7 This effect cannot be seen in ensemble measurements due to averaging over many particles. Fluctuations in the blinking rate can arise from changes in either radiative or nonradiative relaxation, and the relative contributions from these two processes can be unraveled using time-resolved measurements based on time-correlated single photon counting (TCSPC).8-10 However, many of the important photophysical processes in semiconductor nanostructures, such as Auger recombination and surface trapping, occur on times scales 0.1 pJ pulse-1) cause optical damage over a time scale of a few minutes, which decreases the transient absorption signal. The damage is observed in the scattered light images of the nanowires as a change in contrast. This could be due to changes in the nanowires, but it more likely an effect from damaging the medium surrounding the wires (the wires are rarely observed to completely break in the scattered light images). The time-resolved traces also contain a background signal, presumably due to thermal lensing,38,39 that has a similar magnitude to the transient absorption signal. The background has been subtracted from the traces in Figure 2 and in the figures presented below. The majority of the traces recorded correspond to transient absorption signals (negative ∆I/I). Several nanowires gave transient bleach signals (positive ∆I/I, see Figure 2c, for example), and in some cases the signal changed from an absorption to a bleach (Figures 2a and 2b). For a given wire, the form of the signal does not appear to depend on the probe wavelengthsfor the limited tuning range of our laser system. The bleach signals are attributed to state filling near the band edge of the nanowires,40 whereas the absorption signals are assigned to intraband (free carrier)
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Figure 2. (a-c) Transient absorption traces from three different CdTe nanowires. The lines show a fit to the data using a double exponential function. The time constants for the fast decay extracted from the fits are given in the figure. Note that for each trace we have subtracted a background signal that has a similar magnitude to the transient absorption signal.
transitions of the excited charge carriers.12,41 An alternative assignment for the absorption signal is to transitions arising from trapped charge carriers.42 However, in this case we would expect to resolve a growth in the signal corresponding to the trapping time. All the CdTe nanowires examined in this work showed an instrument response limited rise followed by a fast decay. This is consistent with an absorption signal that is due to free carriers, which disappears as the free carriers relax. The general form of the traces measured here (fast picosecond decay in bleach or absorption followed by a longer time scale component) are similar to that observed in ensemble measurements on solution-liquid-solid grown CdTe nanowires.34 Transient absorption spectra recorded in the ensemble experiments show a bleach at the band edge, with a weaker absorption signal to the red.34 For our single particle measurements, whether a bleach or absorption occurs for a given nanowire depends on the band edge/absorption onset of the wire compared to the probe laser wavelength. Wires with a band edge near the probe wavelength are expected to give bleach signals, whereas wires with blue-shifted spectra should give absorptions.34 The wires in this study are too large for shifts in the absorption onset to arise from quantum confinement. One possibility is that the absorption spectra of the nanowires are perturbed by Stark effects: trapped carriers at the nanowire surface provide an electric field that shifts the onset of the absorption band.43 Longlived surface charges have been observed for solution grown nanowires,44 and it is likely that such charges exist for CVD nanowires as well. Different amounts of surface charge would cause different Stark shifts, changing the transient signal of the nanowires in our experiments. The solid lines in Figure 2 are fits to the data using a double exponential function plus an offset that has been convoluted with our instrument response function. The dominant time constants extracted from the data are given in the figure. Note that the time constants are different for the different wires. A
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Figure 4. (a) and (b) Transient absorption traces from two different CdSe nanowires, recorded under the same conditions as the data in Figure 2. The traces show almost no dynamics over the 10 ps time scale of these measurements.
Figure 3. (a) Transient absorption traces for a single nanowire taken with different pump laser powers (given in the figure). The time constant of the fast decay measured for the different traces is τ ) 1.4 ( 0.1 ps. (b) Relative amplitude of the fast decay component compared to the long time signal for a different nanowire, plotted as a function of pump power.
variety of processes, such as trapping at defects, Auger recombination, or electron-phonon coupling, could be responsible for the dynamics observed in Figure 2.11,12,45 To understand the photophysics of these materials in more detail, we performed intensity dependent measurements. If Auger recombination is a significant effect, we would expect that both the time constant and the relative amplitude of the fast decay depend on pump intensity.11,12 Figure 3a shows transient absorption traces recorded at different pump intensities from a single nanowire. The fast time constant does not depend on the pump intensity, for the range of intensities used. Figure 3b shows data from a different wire where we have plotted the amplitude of the fast decay divided by the amplitude of the longer time signal. Again, there is no change with pump power. These results are not consistent with Auger recombination.11 The above discussion implies that the fast decay for the CdTe NWs arises from either trapping of charge carriers into defect states, presumable at the surface of the nanowires, or from electron-phonon coupling.45 The nanowires in these experiments have relatively large widths, much greater than the Bohr radius of the exciton in CdTe.46 This means that the intrinsic properties of the nanowires should be similar to the bulk material.45 The electron-phonon coupling time for bulk CdTe is ∼80 fs,47 which is much faster than our instrument response time. Thus, the electron-phonon coupling process should be complete within the time of the pump pulse in our experiments and cannot be responsible for the picosecond time scale decay observed in the transient absorption traces. From these considerations, the fast decay in the data in Figures 2 and 3 is assigned to charge carrier trapping at surface states in the nanowires. Note that the change in sign observed in some of the transient absorption traces (see Figure 2b, for example) implies that the trapping process changes the form of the transient absorption spectrum of the nanowire. This is not too surprising, as the transient absorption spectrum for trapped charge carriers is expected to be different from that for free carriers.12
It is interesting to compare the transient absorption results for CdTe to data from CdSe nanowires. The Auger kinetics for CdSe NWs have been characterized by ensemble transient absorption experiments,36 and the emission quantum yields for CdSe NWs have also been shown to be larger than those for CdTe NWs.34 This last point implies that rapid charge carrier trapping should not occur for CdSe NWs. Figure 4 presents transient absorption data for two CdSe nanowires, recorded under the same excitation conditions as that for Figure 2 (pump laser power of 0.1 pJ pulse-1). All the CdSe nanowires examined in this work gave a transient absorption signal, which is assigned to intraband absorption from excited charge carriers. Only absorption signals are observed for the CdSe nanowires because their absorption onset is further to the blue than that for CdTe.35,34 Thus, the transient bleach from state filling at the band edge does not affect the probe laser in these experiments.11,36 Note that the magnitude of the signal for the CdSe NWs is similar to that for the CdTe NWs. However, the form of the traces for CdSe is very different to that for CdTe: the signal shows almost no decay over the 10 ps range of these measurements. The traces in Figure 4 are consistent with the ensemble transient absorption measurements for CdSe nanowires, which showed Auger recombination times in excess of 100 ps.36 The data in Figure 2 show that the time constant for charge carrier trapping in CdTe NWs varies from instrument response limited (
