Impact of Chalcogenide Ligands on Excited State Dynamics in CdSe

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Impact of Chalcogenide Ligands on Excited State Dynamics in CdSe Quantum Dots Kyle J. Schnitzenbaumer, Tais Labrador, and Gordana Dukovic J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b02880 • Publication Date (Web): 12 May 2015 Downloaded from http://pubs.acs.org on May 16, 2015

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The Journal of Physical Chemistry

Impact of Chalcogenide Ligands on Excited State Dynamics in CdSe Quantum Dots Kyle J. Schnitzenbaumer, Tais Labrador, and Gordana Dukovic* Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, United States

ABSTRACT The ligands that passivate the surfaces of semiconductor nanocrystals play an important role in excited state relaxation and charge transfer. Replacement of native long-chain organic ligands with chalcogenides has been shown to improve charge transfer in nanocrystal-based devices. In this report, we examine how surface-capping with S2-, Se2-, and Te2- impacts the excited state relaxation in CdSe quantum dots (QDs). We use transient absorption spectroscopy with state-specific pumping to reveal the kinetics of electron and hole cooling, band edge electron relaxation, hole trapping, and trapped hole relaxation, all as a function of surface-capping ligand. We find that carrier cooling is not strongly dependent on the ligand. In contrast, band edge relaxation exhibits strong ligand dependence, with enhanced electron trapping in chalcogenide-capped QDs. This effect is the weakest with the S2- ligand, but is very strong with and Se2- and Te2-, such that the average band edge electron lifetimes for QDs capped with those ligands are under 100 ps. We conclude that, unlike the case of S2-, improvements in electron transfer rates with Se2- and Te2- ligands may be overshadowed by the extreme electron lifetime shortening that may lead to low quantum yields of electron transfer.

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INTRODUCTION The surfaces of semiconductor nanocrystals are typically passivated with surface-capping ligands that provide colloidal stability in solution. These ligands play an important role in excited state dynamics of nanocrystals because they influence relaxation pathways of photoexcited carriers and their binding to the surface can govern the rates of carrier trapping at surface sites.1-3 For applications that require efficient transfer and collection of photogenerated charge carriers, such as nanocrystal-based photovoltaics, the native long-chain aliphatic ligands used in synthesis serve as insulating barriers that hinder charge transfer and transport.4-6 Recent years have seen remarkable progress in the design and implementation of new surface-bound species that can replace the native ligands and significantly improve performance of optoelectronic devices.7-25 Given the importance of surface-capping ligands in excited state relaxation, it is important to understand how the new functionalities on the nanocrystal surface impact the dynamics of photoexcited electrons and holes. Single-atom chalcogenide ligands S2-, Se2-, and Te2- stand in contrast to insulating long-chain ligands in that they create ultra-thin barriers for charge transfer and transport.13, 16, 26-27 A number of reports have described the use of chalcogenide-ligand capped nanocrystals in devices with strong performance. CdSe quantum dots (QDs) capped with S2- ligands have been employed in field effect transistors yielding high electron mobilities with little hysteresis.13, 15-16 Solar cells sensitized with CdSe QDs demonstrated improved photovoltaic performance upon exchange of aliphatic ligands with S2attributed to increased rates of electron transfer from the QDs to TiO 2 nanoparticles enabled by the thin ligand shell.24, 28 While these reports indicate that chalcogenide ligands can increase charge transfer rates, little is known about how ligand exchange from native organic ligands to chalcogenide ligands impacts the excited state decay pathways in the nanocrystals. Understanding the impact of chalcogenide ligands on nanocrystal photophysics is important because the transfer of a photoexcited carrier out of the


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nanocrystal is in direct competition with the relaxation pathways such as carrier trapping. The use of S2ligands has been shown to lead to photoluminescence (PL) quenching of approximately an order of magnitude in comparison to aliphatic ligands.13, 16 PL quenching is normally a consequence of enhanced trapping rates for electrons or holes, or both, as PL signal involves both carriers. To understand which structural parameters are ultimately important for charge transfer and device design, it is desirable to deconvolve and understand the relaxation pathways of electrons and holes in chalcogenide-capped nanocrystals. In this article, we describe the excited state relaxation dynamics of CdSe QDs with a radius of 2.8 nm functionalized with chalcogenide ligands S2-, Se2-, and Te2-. We denote these systems as CdSe-S, CdSe-Se, and CdSe-Te, respectively, and compare them with CdSe QDs capped with the native organic ligand (CdSe-ODA) and 3-mercaptopropionic acid (CdSe-MPA). We monitor several features in the transient absorption (TA) spectra to follow carrier cooling, trapping of photoexcited holes, relaxation of trapped holes, and relaxation of band edge electrons. We find that time constants for both electron and hole cooling remain relatively ligand-independent (97%), sodium sulfide nonahydrate (>98%), sodium selenide (99.9%, Materion), sodium telluride (99.9%, Materion), octylamine (99%), nonanoic acid (97%), anhydrous toluene (>99.8%), anhydrous methanol (>99.8%), and formamide (>99.5%) were purchased from Sigma-Aldrich, unless otherwise noted, and used as received. Ligand Exchange. Ligand exchanges were performed as previously described27 based upon literature procedures.13, 29-31 In order to avoid the complication of oxidative effects, all ligand exchanges were performed in an oxygen- and water-free Ar atmosphere glovebox. 3-Mercaptopropionic acid (MPA) ligand exchanges were performed following a previously described procedure.29-31 Briefly, 1 mL of a ligand solution of 3-mercaptopropionic acid (MPA) in methanol, with pH adjusted to deprotonate MPA, was added to approximately 0.5 mL of the ODA-capped CdSe QD solution. The CdSe-MPA QDs were precipitated by addition of toluene and centrifugation, and then redissolved in formamide (FA). Sulfide, selenide, and telluride ligand exchanges were also performed following a previously reported procedure.13 Briefly, a solution of ODA-capped CdSe QDs in toluene was added to a solution of 0.045 M Na 2 X (X = S, Se, Te) in FA and agitated until the QDs migrated from the toluene to the FA. The CdSe-X QDs, precipitated by addition of acetonitrile and centrifugation, were redissolved in FA.


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Transient Absorption Spectroscopy. The transient absorption (TA) setup has previously been described in detail.30 Experiments were performed using a Helios transient absorption spectrometer (Ultrafast Systems). Both pump and probe pulses were derived from a Solstice Ti:sapphire regenerative amplifier (Spectra-Physics) with output pulses ~100 fs in duration at 800nm. Pump pulses of various wavelengths were generated using a TOPAS-C (Light Conversion) and white light probe pulses were generated using a sapphire crystal. Samples were stirred vigorously in air-tight 2mm path length quartz cuvettes sealed under an Ar atmosphere. Excitation fluences were controlled such that QDs would not experience more than one excitation per pulse. TA kinetics of CdSe-ODA were measured at various pump powers to identify a regime in which the kinetics were independent of pump pulse energy. Pulse energies were then adjusted to generate equivalent excitation profiles when exciting different excitonic states in the various samples. A probe wavelength dependent instrument response function (IRF) for each sample was determined using a cuvette filled with solvent.32-34 The data collected on the solvent allows for correcting the chirp of the white light probe and accurate knowledge of the instrumental response time with minimal adjustments to the experimental setup. Further details are available in the Supporting Information (Section 1). IRF timescales for the experiments reported here fall in the range of 65 – 200 fs, depending on the pulse characteristics and the extent of pump/probe detuning for each measurement. All measurements discussed in this work were repeated in triplicate. Reported values and error margins represent the average ± one standard deviation of the three measurements.

RESULTS Because of the extensive literature precedent for TA spectroscopy of CdSe QDs capped with the native aliphatic ligands, CdSe QDs are an attractive system for studying the impact of surface ligand modification on QD photophysics.2, 35 A schematic representation of CdSe QD energy levels is shown in


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Figure 1a. The first three exciton absorption peaks in the typical room temperature CdSe QD absorption spectrum are assigned to the 1S(e) – 1S 3/2 (h), 1S(e) – 2S 3/2 (h), and 1P(e) – 1P 3/2 (h) excitons. Following previous work, we refer to these as the X1, X2, and X3 excitons, respectively.36 To examine the impact of surface-capping ligands on excited state dynamics of CdSe QDs, we perform post-synthetic ligand exchanges on a sample of CdSe-ODA QDs with radii of 2.8 nm. The surface ligands were modified to the chalcogenide ligands S2-, Se2-, and Te2- following ligand exchange procedures developed by Talapin et al.,13 allowing for solubility in the polar solvent formamide (FA). This work has shown that chalcogenide ligands completely displace aliphatic ligands and allow for stable colloidal nanocrystal solutions.13 For comparison with an aliphatic ligand that also makes QDs soluble in FA, a ligand exchange to 3-mercaptopropionic acid (MPA) was performed following previously published procedures.29-31 These ligand-exchanged samples are referred to as CdSe-S, CdSe-Se, CdSe-Te, and CdSe-MPA, respectively. Steady state UV-Vis absorption spectra of CdSe-ODA and the ligand-exchanged samples are shown in Figure 1b. In previous work, we showed that chalcogenide ligands can dramatically shift band gap transitions of CdTe QDs (up to 250 meV for the smallest QDs) due to carrier delocalization into the ligand shell.27 This effect is considerably smaller for CdSe QDs in Figure 1b, with the largest red-shift being 16 meV between CdSe-ODA and CdSe-Se. This contrast is due to the larger carrier effective masses in CdSe and the relatively large size of CdSe QDs used here. These two differences decrease the extent of carrier confinement and decrease the perturbation of the QD energy levels induced by the different ligands. This work therefore focuses on the impact of ligand identity on excited state dynamics rather than their impact on electronic structure.


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Figure 1. (a) Schematic representation of the energy levels in the CdSe QDs. The vertical arrows denote the states pumped in this work. (b) Absorption spectra of r = 2.8 nm CdSe-ODA (grey), CdSe-MPA (black), CdSe-S (red), CdSe-Se (blue), and CdSe-Te (green). Dashed vertical lines indicate the positions of the X1, X2, and X3 transitions in the CdSe-ODA sample.

Relevant features in TA spectra of CdSe QDs. In this section, we describe the features in the transient absorption (TA) spectra of CdSe QD that we use to probe the dynamics of five processes: (i) cooling of the electron, (ii) cooling of the hole, (iii) relaxation of the electron from the 1S(e) band edge state, (iv) trapping of the hole from the 1S 3/2 (h) band edge state, and (v) relaxation of the trapped hole. It is important to note the distinction between the pump pulses that excite transitions X1, X2, and X3 and the features in the TA spectrum that will be monitored. Figure 2a shows the UV-Vis spectrum of CdSeODA sample along with the spectra of laser pulses used to pump the X1, X2, and X3 transitions. Figure 2b shows a representative TA spectrum of CdSe-ODA 280 fs after X3 photoexcitation. There are three features, denoted as B1, A1, and PA, which we will use to extract carrier dynamics in these samples. TA spectra of all ligand-exchanged samples exhibit similar features, as shown in Figure S2 of the Supporting Information. Following convention, we refer to the bleach of the band gap transition (~615 nm) in the TA spectra as B1.37 This feature is well established in cadmium chalcogenide nanocrystals to be


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representative of electron population in the 1S(e) band edge state.35, 38-39 The feature referred to in the literature as A1 (occurring here at ~640 nm),37 arises due to the attractive interaction of the biexciton formed upon band edge absorption of the probe pulse in the presence of a hot exciton.35 This feature is therefore indicative of a carrier, either electron or hole, which has not yet relaxed (cooled) to the band edge state. The feature labeled PA (observable at all wavelengths red of A1) is a broadband photoinduced absorption that has been addressed in the literature for CdSe QDs on multiple occasions.4043

Through the use of electron (benzoquinone) and hole (phenothiazine) acceptors, we confirm the

assignment of the PA feature to holes (Figures S3 and S4).42 In the Discussion section, we further elaborate on this feature by showing that the signal can be assigned specifically to trapped holes. Because TA spectra are insensitive to holes in the 1S 3/2 (h) state, we do not have information about nontrapping relaxation of 1S 3/2 (h) holes.

Figure 2. (a) Representative pump pulse spectra for X1 (red), X2 (green), and X3 (blue) excitations, plotted with the steady state UV-Vis absorption spectrum of CdSe-ODA QDs. Similar pump spectra are


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used in all TA experiments described in this work. (b) Transient absorption spectrum of CdSe-ODA, excited with X3 pump, at a time delay of 280 fs. Some notable features are labeled: B1, which is assigned to photoexcited electrons occupying the 1S(e) state; A1, which is assigned to the absorption of an X1 exciton in the presence of a hot photoexcited carrier; and PA, which is assigned to trapped holes.

Determination of electron and hole cooling rates using state-specific pumping. In order to examine the ligand dependence of photoexcited carrier cooling, we employ the exciton-selective, stateresolved experimental approach pioneered by Kambhampati et al.36,

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This technique allows for

determination of the 1P(e)  1S(e) and 2S 3/2 (h)  1S 3/2 (h) cooling rates through the comparison of TA kinetics following carefully chosen excitation energies. Excitation with a pump pulse of energy X1, signified by the red arrow in Figure 1a and the red laser spectrum in Figure 2a, generates an exciton with both photoexcited carriers in their band edge 1S(e) and 1S 3/2 (h) states. Excitation with pump pulse energy of X2, however, signified by the green arrow in Figure 1a and green laser spectrum in Figure 2b, generates an exciton with the electron in the 1S(e) state and the hole in the 2S 3/2 (h) state. The only difference between X2 and X1 excitation is that X2 produces a hot hole. Since the A1 feature corresponds to the presence of a hot carrier, the difference in the decays of the A1 feature following X1 and X2 excitations reveals the decay of the hot 2S 3/2 (h) hole. Similarly, excitation with pump pulse energy of X3, signified by the blue arrow in Figure 1a and blue excitation spectrum in Figure 2b, generates an exciton with the electron in the 1P(e) state and the hole in the 1P 3/2 (h) state. By subtracting the kinetics of the B1 feature following X1 and X3 excitations, we reveal the kinetics of electron arrival from the hot 1P(e) state to the band edge 1S(e) state. Below, we use these methods to determine electron and hole cooling rates in CdSe QDs as a function of surface-capping ligand.


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Figures 3a – 3e show the B1 (613-617 nm, depending on the sample) kinetics for CdSe-ODA, CdSe-MPA, CdSe-S, CdSe-Se, and CdSe-Te following excitation of X1 and X3 transitions. The differences in the decay kinetics following X1 and X3 excitation occur only at the very early times and are identical after 2 ps. Thus, the kinetics are normalized between 3 and 4 ps. Following the notation of Kambhampati et al, we refer to the difference of these normalized kinetics as ∆∆A. The ∆∆A kinetics for the five samples are shown as the black markers in Figures 3f – 3j. These ∆∆A traces can be fit with a single exponential decay convolved with the instrument response function (IRF) (Section 1 of the SI): (1) The time constant τ for the single exponential decay corresponds to the 1P(e)  1S(e) relaxation. The 1P(e)  1S(e) cooling time constants for CdSe QDs with each ligand species are summarized in the first row of Table 1. The values in the table reflect the average (weighted by the errors of the fits) and standard deviation of three measurements on each sample. With the exception of CdSe-Te, the 1P(e)  1S(e) cooling time constants are in the 130-200 fs range for all the ligands. For CdSe-Te, the B1 kinetics following X1 and X3 excitation exhibit virtually no difference, leading to a ∆∆A trace that cannot be fit to a single exponential decay. Rather, this ∆∆A trace can be fit to the IRF. We therefore conclude that the 1P(e)  1S(e) time constant for CdSe-Te is faster than our instrumental resolution (100-120 fs for the experiments on this sample).


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Figure 3. (a – e) Normalized B1 (~615 nm) kinetic traces of CdSe-ODA (a), CdSe-MPA (b), CdSe-S (c), CdSe-Se (d), and CdSe-Te (e) following X3 (blue) and X1 (red) excitation. (f – j) ∆∆A traces generated by subtracting the normalized kinetics in the panel above. The red traces are fits of the data to Equation 1 used to extract the 1P(e)  1S(e) relaxation time constant. Horizontal dashed lines denote ∆A and ∆∆A of zero.

Table 1. Summary of the time constants measured in this work. CdSe-ODA

CdSe -MPA

CdSe -S

CdSe -Se

CdSe -Te

1P(e)1S(e)a

200 ± 50 fs

200 ± 100 fs

200 ± 60 fs

130 ± 70 fs

< 100 fs

1S(e) lifetimeb

11 ± 4 ns

3.0 ± 0.2 ns

1.8 ± 0.8 ns

70 ± 10 ps

9 ± 3 ps

2S 3/2 (h) 1S 3/2 (h)c

400 ± 100 fs

240 ± 90 fs

300 ± 80 fs

280 ± 50 fs

130 ± 30 fs

1S 3/2 (h)T(h)d

520 ± 80 fs

700 ± 100 fs

600 ± 400 fs

17 ± 4 ps

180 ± 50 fs

T(h) lifetime e

6 ± 3 ns

3.0 ± 0.4 ns

0.9 ± 0.3 ns

20 ± 4 ns

3 ± 3 ns

(a) Measured by state-specific pumping of X1 and X3 transitions. (b) Average lifetime of the B1 feature calculated using Equation 2.


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(c) Measured by state-specific pumping of X1 and X2 transitions. (d) 1S 3/2 (h)T(h) time constants come from the X1-pumped PA rise time constant. (e) T(h) lifetimes come from the decay of the X1-pumped PA feature.

Figures 4a – 4e show the kinetics of the A1 feature (640-650 nm, depending on the sample) for CdSe-ODA, CdSe-MPA, CdSe-S, CdSe-Se, and CdSe-Te following excitation of X1 and X2 transitions. As is done for the 1P(e)  1S(e) rate above, the 2S 3/2 (h)  1S 3/2 (h) cooling rate can be obtained by fitting the difference of these kinetic traces to a single exponential convolved with the IRF (Equation 1). As in the electron cooling case above, these traces exhibit the same kinetics after 2 ps and are normalized between delays of 3 and 4 ps. The ∆∆A traces, arising from subtraction of the A1 kinetics following X1 and X2 excitation, and the fits to Equation 1 are shown in Figures 4f – 4j. The 2S 3/2 (h)  1S 3/2 (h) cooling lifetimes are summarized in the third row of Table 1. As in the electron cooling case, the values in the table reflect the average, weighted by the fit error, and standard deviation of three measurements on each sample. The hole cooling lifetimes are somewhat longer than ones measured for electron cooling, and range from 130 to 400 fs, with CdSe-Te again exhibiting the fastest cooling.


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Figure 4. (a – e) Normalized A1 kinetic traces of CdSe-ODA (a), CdSe-MPA (b), CdSe-S (c), CdSe-Se (d), and CdSe-Te (e) following X2 (green) and X1 (red) excitation. (f – j) ∆∆A traces generated by subtracting the normalized kinetics in the panel above. The red traces are fits of the data to Equation 1 used to extract the 2S 3/2 (h)  1S 3/2 (h) relaxation time constant. Horizontal dashed lines denote ∆A and ∆∆A of zero.

Decay kinetics of band edge 1S(e) electrons. While electron cooling in CdSe QDs occurs on a sub-ps timescale, relaxation from the band edge 1S(e) state can occur over a range of timescales, encompassing processes such as radiative and non-radiative electron-hole recombination and electron trapping.1-3 The decay of the B1 feature (~615 nm, Figure 2b) corresponds to the decay of photoexcited electrons in the 1S(e) state.35, 38-39 By directly exciting the band edge X1 transition, we remove the earlytime contributions to the B1 signal from carrier cooling. Figure 5 shows the B1 decay kinetics of CdSeODA, CdSe-MPA, CdSe-S, CdSe-Se, and CdSe-Te following X1 excitation. The kinetics shown in Figure 5 exhibit such a strong dependence on ligand identity that we need to use a logarithmic time axis to visualize all the decays in one plot. The native ODA-capped CdSe QDs have the slowest 1S(e) decay. Ligand exchange to MPA makes the decay faster. Ligand exchange to chalcogenides results in even faster electron decay, with increasingly shorter decays going from CdSe-S to CdSe-Te. The simplest way to compare the ligand-dependence of 1S(e) band edge electron decay is by comparing the average lifetimes, i.e., the average times the electron spends in the 1S(e) state, defined as:45

(2) To estimate ∆A(t), we fit each kinetic trace to a triple exponential decay convolved with the IRF and calculated

for three separate measurements on each sample. Average lifetimes progressively


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decrease as a function of ligand from CdSe-ODA (11 ns), CdSe-MPA (3.0 ns), CdSe-S (1.8 ns), CdSeSe (70 ps) to CdSe-Te (9 ps) (Table 1). The value of

for CdSe-ODA has particularly large error

associated with it because it is larger than the experimental window (3 ns). The average 1S(e) lifetimes for CdSe-ODA and CdSe-MPA measured here agree with the 8.0 ns and 2.6 ns previously reported for phosphonate and MPA-capped CdSe QDs respectively.46 Similarly, the average 1S(e) lifetime of CdSeS is similar to the 4.2 ns lifetime measured for a film of CdSe-S QDs of the same diameter drop cast from solution at 60 °C.47 The large differences in 1S(e) lifetimes in Table 1 suggest drastic differences in electron decay pathways among the samples, with CdSe-ODA, CdSe-MPA, and CdSe-S having relatively long lifetimes and CdSe-Se and CdSe-Te having decays that are two orders of magnitude faster.

Figure 5. B1 kinetics of CdSe-ODA (grey), CdSe-MPA (black), CdSe-S (red), CdSe-Se (blue), and CdSe-Te (green). The x-axis is split into a linear scale for time delays less than 10 ps and a logarithmic scale thereafter. The dashed vertical line marks this split. These traces, which monitor the depopulation of the 1S(e) state, demonstrate the drastically different ligand-dependent kinetics.


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Trapped hole kinetics. The spectrally broad positive signal in the TA spectra, denoted as PA in Figure 2b corresponds to trapped holes, which we denote as T(h), and thus provides insights into the ligand dependence of hole behavior. The assignment to trapped holes will be detailed in the Discussion section. Since the PA feature is weak, spectrally broad, and its kinetics are the same across the feature, we increase the signal to noise ratio of its kinetics by averaging the spectra between 700 and 750 nm at each delay time. Figure 6 shows these averaged PA feature kinetics for CdSe-ODA, CdSe-MPA, CdSeS, CdSe-Se, and CdSe-Te following X1 excitation. Using the X1 excitation allows us to avoid complications due the cooling of either carrier. The PA kinetics generally rise quickly, on the sub-ps timescale. The notable exception to this is CdSe-Se, which has a significantly slower rise time of 17 ps. Except in the case of CdSe-Te, the rise times are longer than the IRF of the experiment (~200 fs at wavelengths associated with the PA feature). The PA signal then decays relatively slowly, on the nanosecond timescale, as has been observed for the PA feature in phosphonate-capped CdSe QDs.42 With the exception of CdSe-S, the PA signal does not reach zero inside the 3ns time window of the experiment. To quantitatively describe these kinetics, we fit the data with a sum of two exponentials, a rise (τ rise ) and a decay (τ decay ), convolved with the IRF. (3) The fits of the data are indicated by the solid traces in Figure 6. The associated time constants of the rise and subsequent decay are shown in Table 1, where the values represent the average and standard deviation of three measurements. The time constant for the rise corresponds to hole trapping, while the decay corresponds to the removal of trapped holes.


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Figure 6. PA feature kinetics for CdSe QDs passivated with ODA (grey), MPA (black), S2- (red), Se2(blue), and Te2- (green). A horizontal dashed line indicates ∆A of zero in each panel. The x-axis is split into a linear scale for time delays less than 10 ps and a logarithmic scale thereafter. The solid lines in each panel indicate fits of the data to Equation 3 as described in the text.


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DISCUSSION The values reported in Table 1 represent all the time constants measured in this work. As described above, the 1S(e) average lifetime, electron and hole cooling, and hole trapping time constants for CdSe-ODA agree with values previously reported in the literature.42, 46, 48-49 Similarly, we find good agreement between our measured 1S(e) lifetime in CdSe-MPA and CdSe-S and prior reports.46-47 The remaining 19 time constants in Table 1, including those for the chalcogenide capped CdSe QDs, have not been previously reported. In the following discussion, we compare and contrast these values, and in so doing, highlight the impacts of chalcogenide ligands on CdSe QD excited state deactivation pathways. Ligand dependence of carrier cooling. In CdSe QDs, electron cooling occurs primarily through an Auger relaxation mechanism, where the hot electron relaxes by transferring energy to the band-edge hole, creating a hot hole and a band edge electron.2, 49 This process requires spatial overlap between the electron and the hole. The removal of the hole from the valence band state and the subsequent decoupling from the electron via spatial separation of the carriers can result in significantly lengthened electron cooling times, up to hundreds of picoseconds.50-51 The weighted averages of the 1P(e)  1S(e) cooling time constants for CdSe-ODA, CdSe-MPA, CdSe-S, CdSe-Se, and CdSe-Te, determined by the method shown in Figure 3, are plotted in Figure 7a. The error bars indicate the standard deviation of the three measurements. CdSe-ODA (200 ± 50 fs), CdSe-MPA (200 ± 100 fs), CdSe-S (200 ± 60 fs), and CdSe-Se (130 ± 70 fs) all have similar 1P(e)  1S(e) relaxation time constants. The notable exception, CdSe-Te, is depicted in Figure 7a as shorter than 100 fs, because any time constant longer than 100 fs would have been measureable with our time resolution. We note that our measured time constant for CdSe-ODA is in agreement with that previously reported for CdSe QDs of the same size (r = 2.8 nm).49 The observation here that electron cooling is largely independent of ligand identity (Figure 7a) indicates


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that both carriers retain significant spatial overlap on the subpicosecond timescale. The unusually fast cooling observed in CdSe-Te will be discussed later in the text.

Figure 7. (a) 1P(e)  1S(e) cooling time constants plotted as a function of surface-capping ligand for r = 2.8 nm CdSe QD. The values are determined by taking a weighted average of three separate experiments. Error bars indicate the standard deviation of these measurements. The error bar for CdSeTe is as high as 100 fs, as this is the IRF of our measurement. A slower time constant would have been detected. The grey X indicates the value reported for CdSe-ODA in reference 49. (b) 2S 3/2 (h)  1S 3/2 (h) relaxation time constants plotted as a function of ligand for r = 2.8 nm CdSe QD. The values are determined by taking a weighted average of three separate experiments. Error bars indicate the standard deviation of these measurements. The grey X indicates the value reported for CdSe-ODA in reference 49.

It is thought that hole cooling in CdSe QDs occurs through a combination of energy transfer to ligand vibrational modes, through phonon modes of the QD crystal lattice, and via processes mediated by surface states.52-56 The Auger energy loss channel is not accessible to the hole because the energy gap between the 2S 3/2 (h) and 1S 3/2 (h) levels is smaller than the energetic spacing between the 1P(e) and 1S(e) levels so the 2S 3/2 (h) hole cannot cool by exciting a hot electron. The average hole cooling time constants extracted from the data in Figure 4 and associated standard deviations for CdSe-ODA (400 ± 100 fs), CdSe-MPA (240 ± 90 fs), CdSe-S (300 ± 80 fs), CdSe-Se (280 ± 50 fs), and CdSe-Te (130 ± 30


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fs) are shown in Figure 7b. A previously reported time constant for hole cooling in CdSe-ODA, 250±15 fs, is slightly faster than the range we observe here.49 For all the samples, the hole relaxation time constants are 1.5 – 2 times slower than those of the electron (Figure 7a). This is due to the absence of the Auger energy loss channel for hole cooling. Ligand identity does not drastically alter the hole relaxation time constant, with the fast (130 ± 30 fs) time constant of CdSe-Te being the one exception, which will be addressed below. Replacement of organic surface-capping ligands with chalcogenides certainly alters the ligand vibrational modes available to mediate hole cooling. The fact that we do not observe slowing of hole cooling upon ligand exchange suggests that sufficient vibrational modes remain accessible for hole cooling and/or that the impact of surface-state mediated processes offsets the lack of aliphatic vibrations. Ligand dependence of decay kinetics of band edge 1S(e) electrons. While carrier cooling in CdSe QDs is largely unaffected by ligand identity, relaxation from the band edge states is strongly dependent on which ligand passivates the surface. As shown in Figure 5, the lifetimes of electrons in the 1S(e) state for the chalcogenide ligands are drastically different than that of the native, aliphatic CdSeODA. Strikingly, the average lifetimes of 1S(e) electrons in CdSe-Se and CdSe-Te are under 100 ps, compared to several ns for organic-capped CdSe QDs. For such drastic differences in the 1S(e) lifetime to be observed, additional deactivation channels must be present. One possibility is an increased number of electron trap sites on the surface. To evaluate this possibility, we analyze the B1 decay using a kinetic model that has been previously used to describe surface trapping in nanocrystalline systems.57-59 The premise of this model is that each nanocrystal has the same rate constant for electron-hole recombination and electron trapping, but the ensemble of nanocrystals has a distribution in the number of traps per particle, which can be described with Poisson statistics. Thus the trapping rate, the product of the rate constant and the number of traps, varies among the nanocrystals in the ensemble. To adequately fit the


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CdSe-S, CdSe-Se, and CdSe-Te B1 kinetic traces using this model, two distinct types of trap states, which we denote “type A” and “type B,” are necessary. The decay of the B1 signal in Figure 5 can then be described with:57-58

(4) whereτ QD represents the lifetime for electron-trap independent electron-hole recombination in the CdSe QD, which we treat as ligand-independent to minimize the number of adjustable parameters. τ trA and τ trB represent the time constants for electron trapping to trap states A and B, respectively, and m trA and m trB represent the average number of trap states A and B, respectively, per QD in the ensemble sample. We suppose that ligand exchange to chalcogenide ligands introduces an additional type of electron trap, which has a different trapping rate constant, but that the trapping rate constants do not vary between samples. Rather, the quantity that varies among the samples is the number of traps. Thus, we perform global fitting of the B1 decay of CdSe-MPA, CdSe-S, CdSe-Se, and CdSe-Te simultaneously and require that the time constants for τ QD , τ trA , and τ trB are the same for each sample. Figure S5 shows the global fits to the decay of the B1 kinetics following X1 excitation using Equation 4. The results of this analysis are presented in Table 2. The value of m trA is small (

Impact of Chalcogenide Ligands on Excited State Dynamics in CdSe

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