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Hot and Relaxed Electron Transfer from the CdSe Core and Core/Shell Nanorods Zhong-Jie Jiang and David F. Kelley* University of California, Merced, Merced, California 95343, United States
bS Supporting Information ABSTRACT: Transient absorption spectroscopy has been used to study the rates of electron transfer (ET) from CdSe and CdSe/ZnS core/shell nanorods to adsorbed methyl viologen, MV2þ. The nanorods are excited with 387 nm light, producing electrons 7700 cm-1 above the conduction band edge. Kinetics are measured in particles without adsorbed MV2þ, giving electron cooling and electron-hole recombination times. The kinetics obtained with and without adsorbed MV2þ are compared to infer the ET rates. The results indicate that electron cooling occurs on the 0.7-1.8 ps time scale, with the fastest cooling occurring from the highest energy states. Hot electron transfer from the highest energy levels competes with electron cooling, occurring on the 0.5 ps time scale. Bare particle (relaxed) electron transfer occurs on the time scale of less than or about 4 ps. This is faster than biexciton Auger recombination which occurs on the 50 ps time scale. The energy dependence of the ET times can be semiquantitatively understood in terms of penetration of the conduction band wave function past the particle surface and overlap with the adsorbed MV2þ. In CdSe/ZnS particles, ET to adsorbed MV2þ is slower than electron cooling, and hot electron transfer does not occur. For a 1.0 nm thick ZnS shell, the ET from the bottom of the conduction band occurs on a range of time scales, with the fastest component of about 45 ps.
’ INTRODUCTION Interfacial electron transfer (ET) involving semiconductor nanocrystals is central to the use of these particles in electronic and photovoltaic devices. ET from a photoexcited nanoparticle to adsorbed electron acceptors and/or semiconductors is a wellstudied phenomenon. Nanoparticle to acceptor ET results in charge separation, which is the crucial step in the operation of nanoparticle-based photovoltaics. When a photon having an energy matching that of the semiconductor band gap is absorbed, an electron-hole pair having little or no excess energy is formed. However, if the photon has energy in excess of the bandgap, then the excess energy is in the nascent electron-hole pair. In cases where the effective mass of the electron is less than that of the hole, momentum conservation requires that most of the excess energy is in the electron. Following photoexcitation, the electron usually relaxes to the bottom of the conduction band prior to interfacial electron transfer. Thus, the excess energy of the photon is dissipated into phonons and wasted. However, if electron transfer from the initially populated high-energy states is faster than electron cooling, then transfer of the hot electron can efficiently occur. This allows ET to acceptors that would not be energetically accessible following bandgap excitation. This is important because it provides a mechanism for increasing the output voltage of a photovoltaic based on these donor and acceptor components. r 2011 American Chemical Society
The extent to which hot electron transfer occurs depends on the competition between electron cooling (EC) and ET from highly excited conduction band levels. Because of this competition, the dynamics of both EC and ET processes are crucial to efficient hot ET. Electron cooling has been studied extensively in semiconductor nanoparticles. In most cases, EC has been found to occur rapidly, within a few picoseconds.1-7 This was originally somewhat surprising because the electron energy levels in semiconductor nanoparticles are relatively widely spaced compared to the phonon energies. Thus, dissipation of the electronic energy would require the generation of several phonons. The Franck-Condon factors for this process are very small which results in a “phonon bottleneck”, limiting the rate of EC.8 Alternatively, the electron can cool through an “Auger” mechanism, in which the electron energy is dissipated through electrodynamic interactions to the accompanying hole.1,9-11 Consistent with this mechanism, much slower EC is observed when the hole is not present.12,13 Because EC rates depend on the magnitude of electron-hole coupling, these rates also depend on the particle morphology.7 The electron-hole separation is greater in nanorods than in spheres having comparable radii, resulting in slower Received: December 31, 2010 Revised: February 7, 2011 Published: February 28, 2011 4594
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The Journal of Physical Chemistry C EC in the rods. In the usual situation where both the electron and hole are present, ET competes with EC only when it occurs on the time scale of a few picoseconds or less. Fast ET requires large donor-acceptor coupling and a small energetic barrier and is known to occur in many different types of chemical systems. It should therefore be possible to achieve efficient hot electron transfer in a variety of nanoparticle-electron acceptor systems. In this paper, we show that these requirements can be met in a specific nanoparticle-electron acceptor system. This paper focuses on CdSe nanorods with an adsorbed electron acceptor, methyl viologen (MV2þ). The CdSe nanorods are either bare (ligated with organic molecules and having no semiconductor shell) or CdSe/ZnS core/shell nanorods. We examine the ET rates from excited and relaxed conduction band levels of the bare and core/shell particles. Following photoexcitation above the bandgap, the electron has considerable excess energy and can undergo relaxation, recombination with holes at surface recombination centers, Auger recombination if other electron-hole pairs are present, and interfacial electron transfer if electron acceptors are present. It is the hot electron transfer dynamics that are elucidated here. Our approach in this paper is the following. We first determine the rates at which excited and lowest conduction band levels are populated and depopulated in bare particles having only organic ligands on the surface. These dynamics are controlled by EC, electron-hole recombination at surface defects, and Auger recombination processes. Analogous rates are measured in CdSe/ZnS core/shell particles, in which the surface defects that cause rapid nonradiative electronhole recombination have been passivated. Comparison of the core and core/shell dynamics gives the electron cooling rates. We then measure the electron dynamics in the same particles with adsorbed MV2þ. ET rates are extracted from the differences in the kinetics obtained with and without the adsorbed MV2þ. The relative ET rates from different conduction band levels can be understood in terms of the extent to which calculated electron wave functions extend beyond the particle surface and overlap the adsorbed acceptor.
’ EXCITON DYNAMICS OF BARE AND ZNS-COATED CDSE NANORODS IN THE ABSENCE OF MV2þ The nanorods used in the present work are synthesized with methods similar to those used previously.14 In the present case, we synthesize nanorods with slightly smaller dimensions (3.2 15.5 nm). Results are reported here on the bare particles (ligated with hexadecyl amine, HDA) and CdSe/ZnS core/shell particles derived from the same bare nanorods. The details of the syntheses are described in the Supporting Information. The spectroscopy and photophysics of both types of particles are very similar to previously reported results.14,15 The absorption and photoluminescence (PL) spectra of the bare CdSe nanorods are shown in Figure 1. These nanorods have a sharp absorption onset and a narrow emission peak with no deep trap luminescence, indicating good monodispersity and surface quality. The absorption spectra show several resolved features, specifically a sharp absorption onset with a peak at 605 nm and diffuse shoulders at about 575 and 500 nm. On the basis of previously reported calculations,16 these features are assigned to the 1σv1σc, 2σv-2σc, and 1πv-1πc transitions, respectively. The CdSe/ZnS core/shell particles show similar features that are slightly shifted to the red, also shown in Figure 1. The bare
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Figure 1. Absorption and PL spectra of bare (black curves) and ZnScoated CdSe nanorods (red curves).
particles exhibit moderately intense PL (quantum yield of 15%) with a peak at 617 nm. Much more intense PL is observed in the case of the CdSe/ZnS particles, with a quantum yield of 72%. Time-resolved PL measurements give average lifetimes for bare CdSe and CdSe/ZnS core/shell nanorods of 7.7 and 10.8 ns, respectively. From the PL decay kinetics and quantum yields, the fraction of particles that is observed by the time-resolved PL experiment can be determined. Consistent with literature reports, we find that higher PL quantum yields are usually associated with a larger fraction of luminescent particles.14,17 These values along with the PL kinetic parameters and quantum yields are summarized in Table 1. Single and multiexciton dynamics of the bare nanorods have been previously studied using transient absorption (TA) spectroscopy.15 The transient spectra show the effects of state filling and exciton-exciton coulomb interactions. State filling leads to bleaching of the allowed optical transition and gives a diminished absorbance. Coulombic interactions between excitons result in a decrease in the energy of the biexciton state, thereby red shifting the absorption.18,19 The TA difference (transient minus static) spectra are typically reported (and are what are reported here) and simply referred to as “TA spectra”. These spectra usually exhibit a derivative-like feature due to the combination of state filling and coulomb interactions. In the case where the lowest conduction band level is only partially filled (having one, rather than two, electrons), then the bleach and the red-shifted positive absorbance band are both observed. Due to overlap of these positive and negative spectral features, the bleach will appear shifted to the blue of the static absorption peak. Complete filling of this level results in only the bleach feature in the TA spectra. The kinetics of these spectral features give the time-dependent electron populations in different conduction band levels. Thus, the analysis of TA kinetics at different wavelengths can be used to extract the dynamics of excitons in the corresponding conduction band states. The degeneracy of the valence band is such that the kinetics are insensitive to the hole dynamics.19 The present HDA-ligated nanorods exhibit several distinct features in the TA spectra, as shown in Figure 2. Similar features have previously been observed in the TA spectra of CdSe nanorods.15,20,21 Specifically, a strong negative absorption (bleach) peaked at 595 nm, weak negative absorptions at 575 4595
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Table 1. Multiexponential Fitting of Emission Decay of Bare and ZnS-Coated Nanorods type of nanorods
τ1/ns
τ2/ns
τ3/ns
HDA-ligated CdSe
0.16 (15%)
1.29 (31%) 0.78 (22%)
CdSe/ZnS
τave/ns
Φ
fraction observeda
13.5 (54%)
7.7
0.15
0.26
13.6 (78%)
10.8
0.72
0.91
The fraction observed is given by Φ τrad/τave, where τrad and τave are the radiative and average PL decay times, respectively, and Φ is the PL quantum yield. The radiative lifetimes are taken to be the longest observed decay times.
a
and 496 nm, and positive absorptions at 540 nm and at wavelengths >615 nm. The bleaches at 595, 575, and 496 nm are referred to as the X0, X1, and X2 bleaches, respectively.20 The negative absorption peaking at 595 nm is due to state filling and hence bleaching of the lowest energy (1σv-1σc) transition. The further red absorption is assigned to partial filling of the 1σc level and hence a Stark-shifted 1σv-1σc transition. Because these features in the TA spectra are primarily signatures of conduction band electron populations, the X0 bleach is assigned to state filling of the bottom of the conduction band.19 The poorly resolved X1 bleach at 575 nm is due to state filling of the second conduction band sigma state (the 2σv-2σc transition), and the negative and positive features at 496 and 540 nm are due to bleaching and Stark shift of the 1πv-1πc transition. Similar features are seen in the transient absorption spectrum of the CdSe/ZnS core/shell particles. The kinetics of these bands for both core and core/shell nanorods following high fluence, 387 nm excitation are shown in Figure 3. In the CdSe/ZnS core/shell particles at low excitation fluence, where all the nanorods have no more than one exciton, the decay of the X0 bleach is a relatively slow process, hundreds of picoseconds to nanoseconds. At high fluence corresponding to the production of multiple excitons per nanorod, the X2, X1, and X0 bleach kinetics have very fast rise and decay components due to electron relaxation and slower decay components due to Auger decay of multiexcitons. There is a separation of the EC and Auger recombination time scales in the core/shell particles. Auger recombination is much faster in bare particles, and this clean separation of time scales is no longer present, particularly for the X2 band. Although the physics underlying Auger recombination in nanocrystals is still poorly understood, it is known that Auger recombination rates depend on the magnitude of electron-hole Coulombic coupling and the extent of spatial overlap of the electronic wave functions. A reduction in particle dimensions leads to the increase of Auger recombination rate due to the enhancement of Coulombic coupling and spatial overlap. Smaller particles may also give faster Auger recombination rates due to the relaxation of momentum conservation constraints in the Auger process.22 In semiconductor nanocrystals, because of quantization of nanocrystal electron and hole populations, Auger lifetimes are not continuous but are also quantized.23 It has been demonstrated that the Auger recombination rates depend on the number of electron-hole pairs and are therefore state dependent. The 1σc, 2σc, and 1πc states fill with 2, 4, and 8 electrons, respectively. Auger rates are expected to scale like the exciton density cubed, and the ratio of the Auger rates for the X0, X1, and X2 bands is expected to be kX0:kX1:kX2 = 1:8:64. For CdSe/CdS core/shell nanorods with an aspect ratio of 4.8, we previously reported the ratio of Auger recombination rates for X0, X1, and X2 bands to have approximately this 1:8:64 ratio.15,19 For the CdSe/ZnS nanorods presented here, the Auger recombination times for the X0, X1, and X2 states can be extracted from bleaching kinetics shown in Figure 3A and longer time kinetics. Specifically, decay components having time constants of 500, 50,
Figure 2. TA spectra of bare CdSe nanorods taken at the indicated times for the X0 band following 387 nm excitation. The spectrometer chirp results in the bluer and redder wavelengths corresponding to slightly later and earlier times, respectively.
and 8 ps are observed for the X0, X1, and X2 bands, respectively. These decay components are assigned to Auger recombination times, and we conclude that the Auger recombination rates for those bands are also close to having 1:8:64 ratios. The electron energies of the initially excited 1πc and 1σc conduction band levels can be determined from a combination of calculated energetics and effective mass considerations. The electron energy in the initially excited state depends on the excitation wavelength and the electron and hole effective masses. The nanorods are excited with 387 nm light, which is 9600 cm-1 in excess of the (617 nm) bandgap energy. Momentum conservation requires that the ratio of electron and hole energies be equal to the ratio of hole and electron effective masses, Eelec/Ehole = m*hole/m*elec. This ratio is 4.0 (the effective masses are 0.11m0 and 0.44m0 for the electron and the hole, respectively24), indicating that the electron energy of the initially excited state is 80% of the excess energy, or about 7700 cm-1. Pseudopotential calculations16 put the 1πc level about 2900 cm-1 above the band edge, which is in agreement with the static spectra in Figure 1. Thus, cooling from the initially excited level to the 1πc level requires the dissipation of about 4800 cm-1 of excess energy. The results discussed below permit the determination of two EC times: from the initially excited level to the 1πc level (4800 cm-1) and from the 1πc level to the bottom of the conduction band (2900 cm-1). The TA spectra also show a partially resolved X1 bleach, and in principle, EC rates into and out of this state could be determined. However, the close energetic proximity of the 1σc and 2σc levels probably results in rapid X1 to X0 relaxation. In any case, the large extent of spectral overlap with the X0 band makes obtaining X1 kinetics impractical, and we will not try to determine these rates. The CdSe/ZnS nanorods have well-passivated surfaces, minimizing the effects of electron-hole recombination at surface 4596
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Figure 3. (A) Chirp-corrected TA kinetics for the bands indicated in Figure 2 for CdSe/ZnS nanorods. Also shown are curves calculated from an exponential rise and biexponential decay, convolved with the known instrument response function (1.2 ps, fwhm). The calculated X2 curve corresponds to a 0.7 ps rise time and 1.8 and 8 ps decay components. The calculated X0 curve corresponds to a 1.8 ps rise and long (>100 ps) decay components. (B) Same as (A) except for bare CdSe nanorods. The calculated X2 curve corresponds to a 0.5 ps rise time and 1.2 and (very small) 8 ps decay components. The calculated X0 curve corresponds to a 1.2 ps rise and 1.4 and 50 ps decay components.
defects. From the comparison of the bare and core/shell particle kinetics, EC and electron-hole recombination times may also be extracted. Both processes compete with hot electron transfer, and are of interest here. One caveat regarding this comparison needs to be mentioned. If EC takes place by interaction with the holes, then hole trapping can affect the cooling time. Electron cooling may therefore be slower in the bare particles. This is probably not a large effect because it is unlikely that all the holes are trapped; that is, following multiphoton excitation of the bare particles, there will be valence band holes needed to facilitate EC. The calculated fitting curves in Figure 3A show that following 387 nm excitation of the CdSe/ZnS core/shell nanorods the X2 band shows a 0.7 ps rise time. This is a direct measure of the rate of EC from the initially excited state to the 1πc level. The rise of the X2 band is followed by a nonexponential decay having mainly 1.8 and 8 ps components. These components are assigned to subsequent electron cooling to lower conduction band levels and Auger recombination, respectively. There is also a longer component due to spectral overlap with lower-energy transitions. The X0 band has a 1.8 ps rise time which matches the fast component of the X2 decay. This is assigned to EC to the bottom of the conduction band. Bare particles show faster and more complicated kinetics due to the additional electron-hole recombination processes (see Figure 3B). We have previously shown that holes are rapidly trapped at the surfaces of CdSe nanorods and that the Auger recombination processes are much faster in bare particles.15 We therefore assign the difference in bare and ZnS-coated particle kinetics to these additional depletion mechanisms. In the case of the bare particles, the X2 band exhibits a somewhat faster rise time, 0.5 ps. This is a small, but resolvable and reproducible, difference compared to the ZnScoated particles. If the EC and recombination rates are taken to be additive, ktot = krec þ kEC, where ktot = (0.5 ps)-1 and kEC = (0.7 ps)-1, we get that krec = 0.57 ps-1. Otherwise stated, electron-hole recombination from the initially excited and the subsequently populated states having energies between 2900 and 7700 cm-1 occurs with a time constant of about 1.75 ps. There is considerable uncertainty in this value because it is obtained from the difference of two risetimes, both of which are comparable to the instrument temporal response function. We estimate that it is good to within a factor of 2. A similar analysis can be applied to
Scheme 1. Electron Cooling and Recombination and Transfer Times for HDA-Ligated CdSe Particlesa
a
Also indicated are hot and relaxed electron transfer times to adsorbed methyl viologen.
the decay of the X2 band and the rise time of the X0 band. This analysis is valid if the decay times are much longer than the rise times, which in this case is a pretty good approximation. The bare particle X2 decay is measured to be 1.2 ps, which matches the X0 bleach rise time. It is significantly shorter than the 1.8 ps time observed for the ZnS-coated particles. The difference is assigned to nonradiative electron-hole recombination from the 1πc level. As before, the electron-hole recombination rate is given by the difference of rates obtained from bare and ZnS-coated particles. Specifically, (1.2 ps)-1 = (1.8 ps)-1 þ krec. We get that krec = 0.28 ps-1, or the electron-hole recombination time from the 1πc conduction band level (2900 cm-1 above the band edge) is about 3.6 ps. The presence of recombination centers on the bare particle surfaces also causes electron-hole recombination from the bottom of the conduction band. This rate is determined from the decay of the X0 band and is about 1.4 ps. These results may be 4597
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summarized very succinctly: The EC rate increases with excess energy and occurs with times of about 0.7 ps from the initially excited level to about 2 ps at the bottom of the conduction band. Nonradiative electron-hole recombination at the surface defects of these particles occurs on the time scale of 2-4 ps, which is roughly independent of the electron excess energy. These kinetics are summarized in Scheme 1. The EC times observed here are comparable to those previously reported for CdSe nanorods7,25 and longer than what is typically observed for CdSe nanospheres.2,7,8,26 Since efficient hot electron transfer requires that ET compete with EC, the slower EC suggests that hot electron transfer may be more efficient in nanorods, as discussed below.
’ EXCITON DYNAMICS OF BARE CDSE NANORODS IN THE PRESENCE OF MV2þ Methyl viologen, MV2þ, is widely used as an electron acceptor in molecular and nanoparticle systems.27-32 The MV2þ reduction potential is E0 = -0.445 V vs NHE,33 and the conduction band of bulk CdSe is at -0.30 V vs NHE.34 From the absorption onset and the ratio of effective masses we calculate that in the present case quantum confinement moves the conduction band 1740 cm-1 further negative, to -0.52 V vs NHE, which is only slightly negative of the MV2þ reduction potential. These considerations suggest that there is less than 0.1 V of driving force for electron transfer for particles of this size. We find that adsorption of MV2þ almost completely quenches the PL of bare CdSe nanorods and conclude that electron transfer from this size of CdSe nanorods is energetically favorable, but only slightly so. TA spectroscopy can be used to determine the rates of ET to MV2þ from different levels of the bare particle conduction band. The TA spectra of bare particles with and without adsorbed MV2þ are compared in Figure 4, and the kinetics of the X0, X1, X2, and 625 nm bands obtained with MV2þ are shown in Figure 5. The kinetics shown in Figure 5 can be compared with those obtained in the absence of MV2þ (Figure 3B). The spectra and kinetics obtained with and without MV2þ show several distinct differences. The most obvious differences are the diminution of the X0, X1, and X2 bleaches and the increased absorbance in the 625 nm region. These differences are due to depletion of the conduction band populations from ET to the MV2þ and absorption of the reduced methyl viologen, the MVþ• radical. The MVþ• radical has a broad absorption with the maximum at 600 nm.27,35 This positive absorption overlaps the X0 band, further reducing the X0 bleach, and gives rise to a strong absorption to the red of the X0 band. Comparisons of the X2 and X0 intensities and kinetics in Figure 5 with those in Figure 3B give the ET rates. The rise time of the X2 band in Figure 5 is too fast to be resolved with the present apparatus,
