J. Phys. Chem. C 2010, 114, 17519–17528
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Role of Surface States in the Exciton Dynamics in CdSe Core and Core/Shell Nanorods Zhong-Jie Jiang and David F. Kelley* UniVersity of California, Merced, Merced, California 95343 ReceiVed: June 29, 2010; ReVised Manuscript ReceiVed: August 24, 2010
Static and time-resolved photoluminescence (PL) along with transient absorption (TA) measurements have been used to elucidate the relaxation and recombination dynamics of single excitons and multiexcitons in CdSe nanorods with organic ligands and CdSe/CdS core/shell nanorods. The core/shell particles are synthesized from the same 3.5 × 17 nm nanorods as used in the studies on ligated particles. The core/shell particles are studied with and without adsorbed hole acceptors, specifically, hexadecanethiol (HDT) and phenothiazine (PZT). The time-resolved PL and quantum yield results show that following single-exciton photoexcitation, the core nanorods undergo exciton recombination dynamics on several different time scales. The sample is inhomogeneous, and most of these particles are not observed at all. Of the particles that are observed, most undergo exciton quenching by hole transfer to surface states, leaving the electron in the conduction band. The extent of hole trapping depends on the adsorbed ligands, with alkyl amines partially passivating the hole traps. High fluence excitation produces a significant concentration of biexcitons, which undergo Auger recombination on the approximately 100 ps time scale. Very different dynamics are observed in the CdSe/ CdS core/shell nanorods. These nanorods exhibit high PL quantum yields (>50%), indicating that most of the surface hole traps have been passivated. High-fluence TA measurements indicate extensive conduction band state filling by multiexcitons. The Auger rate depends on the number of electron-hole pairs and is therefore state dependent. Auger recombination times are approximately 700 ps for the lowest conduction band level, 100 ps for the next level at 780 cm-1, and 13 ps for the π conduction band level at 3650 cm-1. The presence of adsorbed hexadecanethiol or phenothiazine changes the single- and multiexciton dynamics. Hole transfer quenches excitons, resulting in much faster PL decays, with little change in the X0 TA kinetics. These dynamical changes are accompanied by the appearance of a broad absorption of the PTZ+• radical cation, which persists for >1 ns. In the case of adsorbed HDT, the loss of valence band holes results in slower Auger recombination of electrons in the π conduction band level. Introduction The photophysics of CdSe and other types of nanocrystals (NCs) is of great interest because of their potential use in solar energy and many other applications.1,2 Combined with the high absorption cross section and the low-cost solution-based synthesis and processing, these materials hold great promise for efficient solar energy conversion in inexpensive, thin film photovoltaic devices. In comparison to bulk materials, nanoparticles have large surface to volume ratios. Surface properties are therefore important in determining the nanoparticle spectroscopy and photophysics. The photophysics can be dramatically altered by surface passivation with organic or inorganic compounds or adsorption of active species, such as electron or hole acceptors.3-14 Attaching organic ligands to the NC surface can provide solubility and in certain cases can improve high photoluminescence (PL) quantum efficiency,15 and coating a thin shell of a wide band gap semiconductor allows substantial improvement of their stability and PL quantum yield.3,16 The adsorption of active species (molecular electron or hole acceptors) on the nanoparticle surface can cause charge transfer reactions, thereby altering the electron and hole dynamics. We have recently shown that adsorbed hole acceptors can quench excitons by either static or dynamic mechanisms. These ZnSand CdS-coated particles with adsorbed hole acceptors show differing extents of static and dynamic quenching. The dynamics * To whom correspondence should be addressed. E-mail: dfkelley@ ucmerced.edu.
may be understood in terms of the different shell morphologies, valence band offsets, and the differences in the hole acceptor energetics.13 Recently, a great deal of research interest has been devoted to the dynamics of biexcitons and multiexcitons, which has been stimulated by studies of NC lasing, as well as potential applications of carrier multiplication (CM) in solar energy conversion.17 CM, a process in which multiple electron-hole pairs (excitons) are produced by absorption of a single photon,17 has the potential of permitting the development of nanoparticlebased photovoltaics with very high efficiencies. However, due to strong confinement of the electron and hole wave functions in NCs, a significant enhancement in carrier-carrier Coulomb interactions occurs, resulting in ultrafast decay of multiple electron-hole pairs (multiexcitons) by Auger recombination. Auger recombination of an electron-hole pair results in the energy being transferred to a third carrier. In NCs, Auger recombination leads to a reduction of optical gain lifetime and is often the main process that quenches multiexcitons produced by CM. It also limits the available time to extract multiple electrons and holes produced via CM. It is generally believed that the rate of Auger recombination depends on the strength of carrier-carrier Coulomb coupling and the degree of spatial overlap between electronic wave functions. The investigation of Auger dynamics in spherical NCs has shown that the Auger recombination rate is significant greater than the radiative decay rate and is dependent upon the size of the particles.18,19,23 In
10.1021/jp1060045 2010 American Chemical Society Published on Web 09/24/2010
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rod-like particles, interacting excitons are separated along the rod axis, which can lead to reduced exciton-exciton Coulomb coupling and suppress Auger recombination. For example, pump-probe transient absorption (TA) spectra spectroscopy has been used to investigate hexylphosphonic acid/trioctylphosphine oxide capped CdSe nanorods.24 On the basis of the spectral evolution, it was concluded that intraband electron relaxation in rods with diameter ≈ 4.2 nm occurs in 2-3 ps, which is much slower than in spheres.24 These authors make a density of states argument to explain these results. In similar studies, Creti et al.25 measured femtosecond TA on bare CdSe nanorods and on CdSe/ZnS core/shell nanorods. These results are analyzed in terms of state filling only, and Stark shifts are not considered to be important after electron relaxation. The positive absorptions are assigned to absorption from carriers trapped in surface states, which is at odds with the usual interpretation of nanoparticle TA results.19 Creti et al.26 also studied the role of shell thickness in CdSe/ZnS core/shell nanorod dynamics. The “thick” shell is 0.8 ML previously studied, and the “thin” shell is 0.5 ML. However, it appears that these studies do not use the same core particles, making direct comparison somewhat problematic. Results for the two types of particles are similar, except that there is a more prominent bleach of a higher energy band and slower bleach decays for particles having thicker shells. The adsorption of hole acceptors onto the surface of NCs can have large effects on the dynamics of electron and holes. Electron relaxation in the conduction band is dominated by an Auger cooling mechanism in which electrons relax through the relatively sparse conduction band states by giving energy to holes through Coulombic coupling. The effects of adsorbed hole acceptors on Auger processes have been studied by Klimov et al.27 They find that if holes are trapped on the surface, this mechanism is far less efficient and electron relaxation is slowed. This has also been shown to be a large effect in studies involving intraband relaxation of charged particles.28 The literature makes clear that many factors, including particle morphology and surface states, can have large effects on the dynamics following multiphoton excitation. In this paper we use static and time-resolved photoluminescence (PL) and transient absorption (TA) spectroscopy to examine the roles of surface states and the core/shell morphology in nanoparticle multiexciton dynamics. Specifically, we examine the exciton dynamics in 3.5 × 17 nm CdSe nanorods with hexadecylamine (HDA) and with octadecylphosphonic acid (ODPA) surfaceadsorbed ligands. We also examine the exciton dynamics in core/shell particles derived from these nanorods. Specifically, we examine different thicknesses of CdS and ZnS shells on these cores. These studies elucidate the effects of surface states on Auger dynamics in core and core/shell nanorods. Finally, we examine the role of hole acceptor ligands (hexadecanethiol, HDT and phenothiazine, PTZ) on the exciton dynamics. Results and Discussion Static Spectra and Spectral Assignments. The CdSe core particles used in these studies have dimensions of 3.5 × 17 nm. Deposition of a ZnS or CdS shell increases both the axial and the radial dimensions. The deposition rate along the axial direction is faster than that in the radial direction, maintaining an approximately constant particle aspect ratio as shell growth proceeds.13 In the present case, the final CdSe/CdS nanorods following 15 injections of CdS precursors have dimensions of about 5.0 × 22 nm. The absorption and PL spectra of CdSe/ CdS core/shell nanorods having 15 CdS injections are shown
Jiang and Kelley
Figure 1. Absorption and PL spectra of the CdSe/CdS core/shell nanorods. Also shown are the assignments of the absorption features, based on calculations of Hu et al.29
in Figure 1. Core nanorods have an absorption onset and emission maximum at 633 nm. After shell deposition, the absorption onset and emission maximum shift to the red and the CdSe/CdS core/shell nanorods absorb and emit at 648 nm. These spectra, along with TEM images of both types of particles, indicate that these particles are essentially identical to those used in a previous study of emission quenching by adsorbed hole acceptors.13 In the present studies we focus on core particles with TOPO/ODPA ligands, core particles with HDA ligands, and CdSe/CdS core/shell particles with the equivalent of 5 monolayers (15 CdS injections) in the shell. Some results on nanorods with a thinner (1.6 monolayer) CdS shell and with the equivalent of a 9 monolayer ZnS shell will also be discussed. The absorption spectra of these nanorods have several poorly resolved features, shown in Figure 1. The core/shell spectra show a well-resolved onset and diffuse shoulders at about 600 and 520 nm. The electronic structure and transition oscillator strengths of CdSe nanorods having several different sizes and aspect ratios have been calculated using different methods.29-32 The observed spectrum may be assigned based on the semiempirical pseudopotential calculations by Hu et al.29 The largest particles for which energy levels are calculated have dimensions of 3.0 × 15 nm and 3.8 × 8 nm, so some extrapolation is needed to estimate the relative energies of the present 3.5 × 17 nm nanorods. However, based on these calculations, transitions to the 1σv-1σc, 2σv-2σc and 1,2πv-1,2πc states in the nanorods can be assigned as indicated in Figure 1. In previous studies, transitions from the ground state to these states were denoted the X0, X1, and X2 bands, respectively,25,26 and we will adopt the same notation. Furthermore, based on these calculations,29 it seems that the 3σv-3σc state may be at approximately the same energy as the X2 band. Core Nanorods. Core nanorods having either ODPA or HDA surface ligands have been studied. In both cases these nanorods exhibit a sharp absorption onset at about 622 nm and comparatively weak PL peaked at 633 nm. There is no red-shifted deep trap PL in either case. The PL decays are strongly nonexponential and depend on which surface ligands are present. Somewhat faster overall decays are observed from nanorods having ODPA on the surface, compared to HDA, as shown in Figure 2. The average lifetimes33 are 620 and 860 ps for the particles ligated with ODPA and HDA, respectively. There is a much larger difference in the PL quantum yields. The ODPA and HDA quantum yields are 0.2% and 2.3%, respectively; ligand exchange increases the PL quantum yield by about an order of magnitude. From the PL decay kinetics and the quantum yields,
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Figure 2. Comparison of the PL decays with the high- and low-power X0 TA decays for core nanorods with HDA ligands (A) and ODPA ligands (B). The results are normalized to match the long-time (>600 ps) decays. Also shown are fits to the PL decays, calculated from the convolution of the instrument response function and a triexponential. In both cases, the fast component corresponds to a 50 ps decay.
TABLE 1: Multiexponential Fitting of Emission Decay particles
τ1/ps
τ2/ps
τ3/ns
τave/ ns33
Φ
f
core (ODPA) core (HDA) CdS 1.6 ML CdS 5 ML ZnS 9 ML
43 (81%) 52 (75%)
800 (12%) 800 (15%) 150 (34%) 160 (34%) 150 (38%)
7.0 (7%) 7.0 (10%) 12.1 (66%) 13.5 (66%) 12.9 (62%)
0.62 0.86 8.0 9.0 8.0
0.0023 0.022 0.62 0.66 0.54
0.046 0.32 0.94 1.0 0.87
the fraction of particles that are observed by the time-resolved PL experiment can be determined. This fraction, f, is given by f ) Φτrad/τave, where τrad and τave are the radiative and average emission decay times, respectively, and Φ is the PL quantum yield. The radiative lifetimes are taken to be 12.5 ns for the core nanorods13 and the longest observed decay time for the core/shell nanorods, respectively. These values along with the kinetic parameters and quantum yields are collected in Table 1. When fast decay components are present, the observed average decay time depends on the time resolution of the experiment. In this case, time-correlated photon counting gives a time resolution of about 40 ps. Particles that undergo radiationless decay significantly faster than about 40 ps are not observed. This analysis shows that the fraction of particles observed is much larger for HDA-ligated particles: 0.046 for ODPA and 0.32 for HDA. The conclusion is that the vast majority of the ODPA-ligated particles have surface states that cause rapid radiationless decay. Similar effects of “bright” and “dark” particles have been observed in single-particle studies.34 Exchanging the original ODPA ligands with HDA passivates a significant fraction of these surface states, and a larger fraction of the particles are bright. The nature of the surface states and the emission quenching mechanism are elucidated by comparison of the PL and TA kinetics. The high-power (about 9 photons per particle) TA difference spectra of particles with ODPA and with HDA ligands are shown in Figure 3. TA spectra show features resulting from Coulombic (Stark shift) effects and state filling. The roles of these two processes have been recently discussed in a review by Klimov.19 Coulombic interactions between excitons lower the energy of the biexciton state and thereby shift the transient absorption
Figure 3. TA difference spectra of core particles with HDA ligands (red curve) and ODPA ligands (black curve) obtained 4 ps following excitation. Also shown are the t < 0 baseline curves. The wavelengths corresponding to the X0, X1, and X2 bands are also indicated.
spectrum to the red. The resulting TA (difference) spectrum resembles a derivative curve of the static absorption spectrum. State filling gives a net negative absorbance change (bleach) from the combination of stimulated emission and loss of absorption. Both types of features are seen in the spectra of these particles. HDA-ligated particles show a strong negative absorption peaked at 622 nm, a weaker negative absorption at 520 nm, and a positive absorption at 570 nm. The 622 nm negative absorption (the X0 band) is due to bleaching of the lowest energy absorption, that is, state filling of the bottom of the conduction band. The negative and positive features at 520 and 570 nm, respectively, are assigned to the Stark shift of the 520 nm absorption band. It is of interest to note that significant state filling occurs only for the X0 band. Similar spectra are obtained for the ODPA-ligated particles. However, in the ODPA case, there is a short-lived (few picoseconds to tens of picoseconds) negative absorption in the 500-610 nm region. This bleach is assigned to state filling on the X2 and X1 bands. The kinetics of the high- and low-power X0 TA bands are shown in Figure 2. The kinetics of the X0 bleach are strongly nonexponential, exhibiting fast (less than 200 ps) and much slower components. The relative amplitudes of the fast and slow components are excitation power dependent, with the relative amplitude of the fast component increasing with increasing
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Figure 4. High-power X0 TA decay curves for core nanorods with HDA ligands (solid) and ODPA ligands (open). Also shown are fit curves corresponding to triexponential decays. The decay times of fast components are indicated in the figure, and the slow component is taken to be 7 ns in both cases.
excitation power. The high power results are obtained with a power density corresponding to an average of 9 photons absorbed per particle. The low-power results are obtained with power densities about an order of magnitude lower. These considerations indicate that the fast component is associated with multiphoton absorption. The high- and low-power X0 TA kinetics can be compared to the PL decay kinetics, also shown in Figure 2. The PL kinetics are taken at extremely low power, precluding the possibility of multiexciton or particle charging effects. The PL decays show an approximately 50 ps decay component that is absent in the low-power X0 TA decays. The differences between the lowpower TA and the PL decays give considerable insight into the electron and hole dynamical processes. These differences can be understood in terms of what is measured in each type of experiment. PL quenching occurs upon electron-hole recombination or when any dynamical process greatly diminishes the overlap of the electron and hole, specifically, trapping of either carrier. Radiative recombination occurs on the 10 ns time scale. Thus, the fast decays in the PL decay kinetics are almost completely indicative of the sum of the electron and hole trapping rates. In contrast, TA kinetics do not simply measure exciton lifetimes. The degeneracy of the CdSe valence band states makes the state filling signal much more sensitive to the presence of conduction band electrons, and the bleach in the TA spectrum is primarily due to electron state filling of the conduction band. The TA kinetics of the X0 transition give the lifetimes of conduction band electrons. The lack of a large fast decay component in the TA kinetics therefore indicates that the PL quenching process results in little loss of conduction band state filling. We conclude that excitons are quenched by loss of valence band holes, that is, PL is quenched by hole trapping at surface states. The above conclusion allows us to understand the comparison of the high-power HDA and ODPA X0 TA decay kinetics, shown in Figure 4. The fast decay component is a result of multiexciton Auger processes, requiring both electrons and holes, and is faster in the case of the HDA-ligated particles. This is in contrast with the PL decays, where a slightly larger and faster decay component is observed for the ODPA-ligated particles. Both types of kinetics have the same simple qualitative explanation: the ODPA-ligated particles have more hole traps on the surface and very rapidly lose a larger fraction of the valence band holes. This results in a larger fast PL decay component and a lower
Jiang and Kelley
Figure 5. Kinetics of the X2 and X0 bands for the ODPA-ligated core particles. Also shown are curves fit to these data and the fitting times.
TABLE 2: Auger Recombination Times (ps) core (ODPA) core (HDA) CdSe/CdS core/shell (5 ML)
X0
X1
X2
176 110 700
20 7.5 100
2.2 50%, and the fraction of particles observed approaches unity, see Table 1. This suggests that the surface states which result in hole trapping in the core particles are
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Figure 6. (A) Transient absorption difference spectra of CdSe/CdS core/shell particles taken at 5 ps increments. (B) TA kinetics at the indicated wavelengths for CdSe/CdS (solid circles) and CdSe/ZnS (open circles) core/shell particles. Also indicated are the Auger decay times for the CdSe/ CdS particles.
effectively passivated by the deposition of the shell. We find that with the core/shell particles, the low-fluence TA decays match the PL decay, also indicating that hole traps are effectively passivated by either the CdS or the ZnS shell. This observation also suggests that the small extent of exciton quenching that occurs is due to electron trapping at defects. These defects may be at the particle surfaces or at the core/shell interface. However, we observe that the PL quantum yields tend to level out as the shell thickness is increased, as indicated in the comparison of 1.6 and 5 monolayer CdS shell particles in Table 1. This trend suggests that at least some of the defects are at the core-shell interface. The high-power TA spectra and the multiexciton dynamics in the core/shell particles are very different than in the core particles. The TA spectra show several prominent features, as seen in Figure 6A. Strong X0, X1, and X2 band bleaches are observed at 632, 606, and 517 nm, respectively. There is also a weak and shortlived absorption at wavelengths greater than 660 nm, which is assigned to Stark-shifted absorption onset. The X0, X1, and X2 bleach bands are assigned to state filling of successive conduction band levels. These spectral features decay at different rates, with the result being that the spectrum evolves over the first nanosecond. A positive absorption at λ > 660 nm is observed, which decays with an approximately 2 ps time constant. This reflects the time necessary for electron cooling into the lowest conduction band level and is consistent with previously reported electron cooling times in CdSe nanorods.24 The decays of the X0, X1, and X2 bands take place on longer time scales. The X1 and X2 bands decay by Auger recombination with time constants of approximately 100 and 13 ps, respectively. Radiative and other nonradiative decay processes are relatively slow, and these decay times directly give the Auger recombination time. The decay time of the X0 band is considerably longer, and decay from other nonradiative processes cannot be neglected in determining the Auger rate. The Auger time can be determined by a subtraction procedure, similar to that described by Klimov.18 In the procedure used here, the high- and lowpower excitation curves are scaled to the same long-time asymptote and the low-power curve subtracted from the highpower curve. The Auger time is obtained from the low-power and difference curves. To extract the Auger rate, the low-power curve is fit to a triexponential
-∆Al(t) ) A1 exp(-t/τ1) + A2 exp(-t/τ2) + A3 exp(-t/τ3)
where τ3 is the long-time asymptote. The X0 Auger rate, ka, is then obtained by fitting to an equation that follows directly from consideration of parallel Auger and other nonradiative processes. Specifically -∆Ah-l(t) ) A1exp[-(kat + 2t/τ1)] + A2exp[-(kat + 2t/τ2)]
When this procedure is applied to the CdSe/CdS 5 ML particles, X0 Auger times of about 700 ps are obtained. In bulk semiconductors, the Auger rates scale as the cube of the exciton density.19 If the same scaling is assumed here, the Auger rates for the X1 and X2 bands can be calculated from the numbers of carriers associated with each of these bands and the X0 ka value. The calculated values can be compared with measured values. Two electrons (and hence two excitons) are required to fill the 1σ conduction band level, X0. The X1 band corresponds to the next excited sigma state,29 and it will be partially filled by three excitons and completely filled by four. Maximum bleaching of the X1 band occurs with four excitons, compared with two excitons for the X0 band. Thus, the exciton density will be twice as great and the X1 Auger recombination rate is predicted to be 8 () 23) times as large as in the X0 case. The measured Auger recombination times for the X1 and X0 bands are about 100 and 700 ps, respectively, and this prediction is close to quantitatively borne out. The next conduction band level to be filled (X2) is the doubly degenerate 1,2πc level.29 These degenerate levels are filled with an additional 4 electrons, corresponding to a total of 8 conduction band electrons or 4 times the number needed to fill the X0 band. Thus, the Auger decay time for the X2 band is predicted to be a factor of 64 () 43) times as fast as the X0 band. The decay time is therefore predicted to be (700 ps)/64 ) 11 ps, in good agreement with the observed 13 ps decay time. These values are collected in Table 2. Figure 6 shows that roughly comparable decay times are observed for the CdSe/ZnS core/shell particles. These results are roughly independent of the nature of the shell. The rapid increase in Auger rate with number of excitons is also consistent with the trend reported by Klimov.18 The TA spectra shown in Figure 6 are in contrast to the case of the core particles with HDA ligands, where little or no state
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filling is observed above the X0 band, compare Figures 3 and 6A. Even at very high powers, not enough excitons are accumulated in the core particles to show well-resolved bleaching of the X1 band and bleaching of the X2 bands is at most very short lived. The spectral differences result from the differences in the decay kinetics. The lack of X2 state filling in the case of the HDA-ligated particles is because rapid Auger recombination depletes these states on the same time scale that they are populated by electron cooling from the initially excited level. Electron cooling takes place on the 2 ps time scale in the case of the core/shell particles. To the extent that intraband electron cooling involves Auger processes, surface hole trapping is expected to make electron cooling slower in the case of the core particles.28,35 The lack of a resolved X2 feature in the HDAligated core particle TA spectrum is because of rapid depletion from compared to relaxation into the X2 level. The presence of resolved, long-lived state filling of the X1 and X2 bands in the core/shell particles is the result of much slower Auger recombination. It is tempting to try to apply the above analysis to the X0, X1, and X2 Auger recombination times obtained for the core particles, particularly the ODPA-ligated particles, for which accurate values for all three bands are determined. However, determining the carrier density for such an analysis is problematic for two reasons. First, a large fraction of the holes is trapped on the surface very rapidly, in less than the electron cooling time. While this fraction can be determined for single excitons from PL measurements, it is unknown for multiexcitons. Second, Figure 2 shows that hole trapping continues on the 50 ps time scale, that is, hole trapping occurs over a wide range of time scales. Thus, the carrier density is changing as Auger recombination occurs as a result of both trapping and the Auger recombination process. This combination of different processes makes it difficult to extract how the Auger rates depend on carrier density for the core particles. We note that although these nanoparticles are very uniform and have (for the core/shell particles) very high PL quantum yields, dynamical processes such as hole trapping occur on a wide range of time scales. This type of inhomogeneity is very common in semiconductor nanoparticles, leading to the distinction between static and dynamic quenching13 and to “bright” and “dark” nanoparticles in the same ensemble.34 The slow Auger recombination times obtained for the core/ shell particles are in contrast with those obtained for core particles, which show much faster Auger recombination. There are several factors that can contribute to this difference. The Auger rate depends on the carrier density and hence the particle volume. The core/shell particles are derived from the same core particles as used in these studies, so they have exactly the same volumes. However, the presence of the shell further delocalizes the wave functions, effectively increasing the particle volume. This is why the absorption onset of the core/shell particles is further to the red than that of the bare core particles. This effect can be quantified by the extent of the red shift, which is about 15 nm. In spherical particles having comparable absorption onsets,36 this corresponds to a diameter increase from 6.4 to 7.6 nm, which corresponds to a radius increase of 18% and a volume increase of 65%. Thus, the Auger rate, scaling as the carrier density cubed, would be expected to be a factor of 4.5 slower in the core/shell particles. Compared to the 700 ps Auger time in the core/shell particles, a core particle X0 Auger recombination time of 150 ps is predicted. This is significantly greater than the observed time of 110 ps for the HDA-ligated core particles, see Table 2. Furthermore, in the core particles,
Jiang and Kelley a large fraction of the holes (70% for single photon absorption) has been surface trapped. Thus, the actual carrier density is considerably below what it would be in the absence of hole trapping and the X0 Auger recombination would be faster than the observed 110 ps if the number of carriers were as high as it is in the core/shell particles. We conclude that the faster Auger rate seen in the core particles can only be partially explained in terms of particle volume effects. We speculate that much of the differences in these rates is due to differences in the core versus core/shell particle interfaces. Specifically, recent theoretical results indicate that the sharpness of the interface plays a major role in determining Auger rates.37 These ideas are used to explain the lack of Auger processes in giant CdSe/CdS nanoparticles23,38,39 and the absence of blinking in graded-shell CdSe/ZnSe nanoparticles.40 In the present case, the core/shell wave functions can tunnel into the shell to some extent, which essentially results in a more diffuse interface than in the core particles. Indeed, this is the reason the absorption and PL spectra red shift in the core/shell particles. Further studies examining how the sharpness of the interface in core/shell particles affects multiexciton Auger rates are currently in progress and will be reported in future papers. Exciton Dynamics in the Presence of Hole Acceptors. The presence of the hole acceptors phenothiazine (PTZ) or hexadecanethiol (HDT) adsorbed on the CdSe/CdS 5 ML core/shell particles has several effects on the PL and TA dynamics. At low excitation powers, PTZ and HDT quench PL due to hole trapping. PL quenching by adsorbed hole acceptors can be loosely divided into two categories: dynamic and static quenching.13 In the former case, the extent to which the PL QY is diminished is given by the ratio of observed PL lifetimes with and without the adsorbed quencher. In the latter case, the PL QY is diminished with no observed effect on the PL kinetics; quenching occurs much faster than the time resolution used to measure the PL decay. Typically, both types of quenching are observed to varying degrees. Of course, the distinction between static and dynamic quenching depends on the time resolution of the PL kinetics. Despite this point, the distinction is often a useful one. In this case, the kinetics are obtained by timecorrelated photon counting with a time resolution of about 40 ps, which is faster than the Auger X0 and X1 decay times but slower than the X2 Auger decay times in the core/shell particles. Thus, static quenching occurs prior to Auger recombination for the X0 and X1 bands and may or may not occur faster than Auger recombination of the X2 band. The extent and time scale of dynamic quenching may be assessed from comparison of the PL decays with and without adsorbed hole acceptors, shown in Figure 7. The decay curves in Figure 7 are normalized such that the areas under the curves are proportional to their relative PL quantum yields. Both the HDT and the PTZ decays are considerably faster than the bare particles, indicating dynamic quenching. In addition, the HDT curve starts out at a lower value than the PTZ and bare particle curves, indicating static quenching. Thus, this comparison shows that dynamic quenching occurs on the hundreds of picoseconds time scale and is therefore comparable to or slower than the X0 and X1 Auger processes. In the case of CdSe/CdS core/shell particles, PTZ exhibits almost exclusively dynamic quenching while HDT exhibits both dynamic and static PL quenching. It is of interest to compare the PL and TA kinetics, and the TA kinetics obtained at several wavelengths for the same samples are shown in Figure 8A.
Exciton Dynamics in CdSe Core and Core/Shell Nanorods
Figure 7. Normalized PL decays for CdSe/CdS core/shell particles with no hole acceptors (black), with adsorbed phenothiazine (blue), and with adsorbed hexadecylamine (red).
In the case of PTZ absorbed on the particles, the TA kinetics are affected by the formation of the PTZ radical cation, PTZ+•. This species exhibits a broad absorption peaking at about 515 nm.41 As a result, the PTZ and HDT TA kinetics show significant differences from each other and from the bare (HDAligated) core/shell particle kinetics. The most obvious difference is that the PTZ kinetics show a smaller bleach at long times compared to the HDT or bare particle kinetics, particularly in the 515 nm kinetics. This is due to the positive absorption of the PTZ+• and is also seen at 571 nm, where the PTZ kinetics show a positive absorption. This absorption difference persists for >1 ns, indicating that a significant fraction of the PTZs remains oxidized for that long, that is, transfer of conduction band electrons to the oxidized PTZs has components that are on the nanosecond time scale or longer. Despite the PTZ+• absorption, the presence of the adsorbed hole acceptors has a relatively small effect on the X0 (637 nm) kinetics, which is in contrast with the PL kinetics in Figure 7. As discussed above, this is because the TA experiment primarily probes the conduction band electron population which is not directly affected by hole transfer. Hole transfer does, however, indirectly affect the electron population by lowering the Auger recombination rate. The differences in the Auger rates are seen in the TA kinetics but are far less dramatic than the effects of
J. Phys. Chem. C, Vol. 114, No. 41, 2010 17525 hole trapping on the PL kinetics. Higher time resolution X2 band kinetics for the bare (HDA-ligated) and HDT-ligated core/ shell particles are compared in Figure 8B. These kinetics show a somewhat slower decay in the case of HDT. This indicates that hole transfer diminishes the number of valence band holes and therefore reduces the Auger rate. With a reduced Auger rate, the X2 conduction band electron population lives significantly longer. A significant component of rapid (
