Influence of Hole-Sequestering Ligands on the Photostability of CdSe

Dec 14, 2012 - Using photoluminescence and density functional theory (DFT) calculations, we show that the enhanced stability results from hole transfe...
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Influence of Hole-Sequestering Ligands on the Photostability of CdSe Quantum Dots Yizheng Tan, Song Jin, and Robert J. Hamers* Department of Chemistry, University of WisconsinMadison, Madison, Wisconsin 53706, United States S Supporting Information *

ABSTRACT: Chalcogenide nanocrystals or quantum dots (QDs) such as CdSe and PbSe have great potential as absorbers for QD-sensitized solar cells, but their practical utility is limited by fast degradation when exposed to ambient environments. Here we present results showing that small organic molecules acting as hole-accepting ligands can be very effective in reducing photooxidation of CdSe QDs. The aromatic amine, 4dimethylaminothiophenol (DMATP), is shown to be especially effective in enhancing stability of CdSe QDs when illuminated in air or in aqueous environments. Using photoluminescence and density functional theory (DFT) calculations, we show that the enhanced stability results from hole transfer from the QD to the ligand and delocalization of the resulting positive charge on the aromatic ring and amino group instead of the sulfur atom that links the molecule to the CdSe.

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INTRODUCTION Inorganic semiconductor quantum dots (QDs) are promising alternatives to organic dyes as visible-light absorbers in sensitized solar cells.1−3 Inorganic QDs are advantageous because their optical and electronic properties are size dependent and therefore can be tuned to optimize solar absorption as well as the energy alignment between the QD and metal oxide acceptor for favorable electron transfer.4−6 In addition, recent studies have demonstrated that some QDs have the ability to support hot electrons7,8 or multiple excitons per incident photon.9−12 Various semiconductor QDs such as CdSe,4,13 PbSe,5,6,8,14 and PbS11,15,16 have been studied as sensitizers for quantum dot sensitized solar cells (QDSSCs). While QDs have many outstanding properties, the practical utilization of chalcogenide QDs is hindered by their propensity to undergo oxidation,17−21 requiring strictly air- and water-free conditions to remain stable. Protective inorganic shells such as ZnS13 and Al2O322 have been used to passivate the surface and control their photooxidation, but these wide-bandgap shells may introduce potential barriers to charge transfer if their thickness is not properly controlled.23,24 Compounds containing aromatic amino groups are widely used as hole conductors, but in general these are larger molecules or polymers with more extensive conjugation.25,26 These groups are also widely used in donor-π-acceptor type structures for dye-sensitized solar cells, with the arylamine acting as an electron donor.27−29 A similar approach can be used to remove oxidizing holes from a QD, by functionalizing the QD with hole-accepting ligands. Here, we explore the influence of hole-accepting ligands on the stability of CdSe QDs and on CdSe-sensitized TiO2 (referred to as CdSe/TiO2). Figure 1 shows the ligands investigated here. Our results show that small conjugated ligands slow photocorrosion in comparison with long alkyl ligands. In particular, an electron-donating amino group in the © XXXX American Chemical Society

Figure 1. Ligand molecules used in this study to cap CdSe QDs.

conjugated ligand, such as in 4-dimethylaminothiophenol (DMATP, see Figure 1), provides remarkable stabilization of CdSe QDs. We combine photocorrosion, photoluminescence, and density functional calculations to understand how the molecular structure of the ligand affects QD stability. Our results show that removing localized charge from the sulfur with an electron-donating amino group stabilizes the ligand and minimizes photocorrosion of the QD.

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EXPERIMENTAL SECTION Chemicals. Trioctylphosphine oxide (TOPO) 99+%, CdO 99.99% metal basis, oleic acid (OA) 90%, trioctylphosphine (TOP) 90%, selenium 99.99% metal basis, 3-mercaptopropionic acid (MPA) 99+%, 1-dodecanethiol (DT) 98+%, and thiophenol (TP) ≥99% were purchased from Sigma-Aldrich. 4Dimethylaminothiophenol (DMATP) was purchased from Oakwood Products.

Received: September 27, 2012 Revised: December 9, 2012

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Fourier Transform Infrared (FTIR) Spectroscopy. All measurements were taken with a Vertex 70 (Bruker) spectrometer with a resolution of 4 cm−1 and constant dry-air purging. The L-CdSe/TiO2 surfaces were measured in singlebounce reflection absorption mode (VeeMax II accessory, Pike Technologies) with p-polarized light at an incident angle of 50° from the sample normal. The spectra were referenced against a clean TiO2 film prepared using identical procedures. To minimize the effects of atmospheric water and CO2, each sample and the clean TiO2 reference sample were measured as closely together in time as possible. This method of referencing produced the most reproducible results. X-ray Photoelectron Spectroscopy (XPS). Measurements were done on a custom-built XPS system (Physical Electronics) with an Al−Kα source (model 10-610, 1486.6 eV photon energy), torroidal monochromator (model 10-420), and hemispherical analyzer with a 16 channel detector array (model 10-360). We used an electron takeoff angle of 45° and measured at a resolution of 0.05 or 0.1 eV. Peak areas were obtained by fitting the spectra to a Voigt function. Shirley baseline corrections were used as needed. Photodegradation Studies. For studies in H2O, the samples were sandwiched into a cell with another piece of FTO glass and a 127 μm spacer; the open region was filled with 18 MΩ cm H2O (Barnstead Nanopure). The cell has windows to allow light penetration and time-dependent absorption studies without disassembling the cell. The sample was illuminated through the plain FTO piece and water. Light from a solar simulator (Newport 91160, equipped with AM1.5G filters and set to 100 mW/cm2 as measured by a Scientech calorimeter) was passed through a filter transmitting only light with wavelengths longer than 475 nm. This filter was used to ensure that light is only absorbed by the CdSe QD and not by the TiO2. With the filter, the irradiance was measured to be 81 mW/cm2. This optical setup was used for all of our degradation studies. Transmission UV−visible absorbance spectra (Shimadzu, UV-2401PC) were obtained at up to 10 min of exposure time. For studies in air, samples were left under the light under ambient conditions without assembly into a cell. Absorption spectra were taken at exposure times up to 15 min. Photoluminescence (PL). The QDs were precipitated with methanol, centrifuged, and resuspended with chloroform to give ∼2 μM concentration. In all of the experiments, they were excited with 450 nm light, close to the second absorption peak that corresponds to the second excitonic transition. Ligands, at a concentration of ∼70 μM, were added and mixed immediately before experiments. Steady state experiments were performed using an ISS K2 fluorometer. For transient PL measurements, 3 ns pulses from a laser (Ekspla NT340, Nd:YAG with an optical parametric oscillator, 250 μJ/pulse, 20 pulses/s) were used to excite the QDs. The transient fluorescence was collected with a photomultiplier tube (Hamamatsu R6357, rise time 1.4 ns) and recorded using an oscilloscope (Agilent, DSO5054A, 500 MHz). Density Functional Theory (DFT) Calculations. To better understand the nature of charge separation in these compounds, we performed DFT calculations of the relevant molecules, using a Cd6Se6 cluster to model the CdSe surface. Calculations were performed using the Gaussian09 program using the B3LYP hybrid density functional and the LANL2DZ basis set for all atoms.36 Calculations on the free molecules were also performed using the Dunning-Hay D95 basis set;37 since these results were nearly identical to those using the

Preparation of Nanocrystalline TiO2 Films. Fluorinedoped tin oxide (FTO) coated glass (Hartford Glass) was precleaned with detergent, acetone, and ethanol. Anatase TiO2 nanoparticles in the form of a paste (Ti-Nanoxide, T20/SP, Solartonix) were screen-printed onto the FTO glass to give ∼2 μm thick films. The screen-printing process produces multiple films with the same thickness and nanoporous structure. Each experimental set described here was performed using films prepared from the same batch of films. These films of 0.5 cm in diameter were then sintered and annealed in air following a procedure adapted from the literature30 at 325 °C for 5 min, at 375 °C for 5 min, at 450 °C for 15 min, and finally at 500 °C for 30 min. Before use, the films were given an additional annealing step at 500 °C for 15 min to remove any adsorbed water and organic contaminants. Synthesis of CdSe QDs. The synthesis was adapted from Peng and Peng.31 Briefly, 3.1 g of TOPO, 0.23 g of CdO, and 1.8 g of OA are combined into a 3-necked flask. The mixture was then heated under Ar flow until it turned optically clear (∼290 °C). The mixture was then allowed to cool to 250 °C. A solution of TOPSe made by combining 0.04 g of Se and 1.2 mL of TOP was injected into the flask. The reaction was cooled to ∼80 °C and was then quenched with toluene. The CdSe QD solution was purified four times by precipitation and centrifugation with methanol. The size and concentration of the CdSe QDs were determined from the wavelength and absorbance of the first exciton peak, using empirical relationships established by Peng and co-workers.32 This analysis yielded particle diameters that were typically 3.0−3.2 nm. The QDs were kept in toluene and in the dark until further use. Because both OA and TOPO are present during the initial synthesis, both ligands are present on the starting QD samples. For convenience, these as-synthesized QDs will be referred to as OA−CdSe. CdSe−TiO2 Adduct Formation and Subsequent Ligand Exchange. After synthesis, the CdSe nanoparticles were linked to TiO2 by one of two methods: (1) using a bifunctional ligand to provide a covalent CdSe−TiO2 linkage or (2) by direct physical adsorption. For the first case, after the CdSe nanoparticles were attached to TiO2, we performed ligand exchange on the CdSe QDs to investigate the effect of the ligands on physical properties of the CdSe nanoparticles. Attachment of CdSe to TiO2 via covalent linkage was performed using 3-mercaptopropionic acid (MPA). This approach obtains good control over CdSe coverage.33−35 The TiO2 films were immersed in 0.1 M MPA in anhydrous acetonitrile (ACN) for 6−8 h in the dark. They were rinsed with anhydrous ACN and toluene. Samples were then immersed in 20−50 μM CdSe QDs in toluene (QD molarities in this work are defined by number of moles of QDs per liter of solution) for 16−18 h in the dark and rinsed with toluene. Functionalization with ligands depicted in Figure 1 was then performed by soaking the samples in a solution of the respective ligand (0.1 M in toluene or just pure toluene for the OA sample) for ∼24 h in the dark. They were rinsed with toluene, then heptane, and dried with N2. Experiments were also conducted using direct absorption of CdSe QDs to TiO2, with no linker; in this case the CdSe QDs (also 20−50 μM) were precipitated with methanol, centrifuged, and resuspended in dichloromethane (DCM). The TiO2 films were then immersed in the QD/DCM solution33 for 24 h, rinsed with DCM, and dried with N2. B

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The asymmetric CH2 stretch of OA-CdSe/TiO2 and DTCdSe/TiO2 occurs at 2927 and 2926 cm−1, respectively, slightly larger than the value of 2924 cm−1 for neat OA and DT. In contrast, prior studies have found that the asymmetric mode decreases by ∼6−8 cm −1 when forming a crystalline monolayer.43 Thus, our FTIR data indicate that the OA and DT layers formed on CdSe are in a very fluidic local environment. Figure 3a shows visible absorption spectra of OA-CdSe QDs that were linked to TiO2 films and were then exposed to water

LANL2DZ basis set, all results reported here used the latter. The CdSe cluster was constrained to maintain the proper symmetry while leaving all bond distances unconstrained. A Natural Bond Orbital (NBO) analysis38 was used to determine the charges on the individual atoms.

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RESULTS Figure 2 shows FTIR spectra of CdSe/TiO2 functionalized with the different ligands used in this work. Spectra of the neat

Figure 2. IR spectra of L-CdSe/TiO2 indicating binding of ligands to the CdSe QD surface. The 3160−2980 cm−1 region is enlarged 8× and overlaid directly above each spectrum. Ar = Aromatic.

ligands are shown in the Supporting Information (SI). The CdSe QDs, as functionalized on TiO2, exhibit C−H peaks at 2856 and 2928 cm−1, close to the 2854 and 2924 cm−1 of neat OA (see SI) and a C−H peak at 3005 cm−1 from the CCH group of oleic acid. These nanoparticles show two peaks at 1551 and 1404 cm−1 that are characteristic of the carboxylate group, while neat OA shows a single large peak at 1710 cm−1 in agreement with previous studies.39,40 The peaks at 1551 and 1404 cm−1 have previously been attributed to the asymmetric and symmetric stretching vibration modes (respectively) of carboxylate groups bonded to CdS surfaces, while 1710 cm−1 is typical of the CO stretch of a free carboxylic acid.39 Both OA on CdSe and MPA bound to TiO2 contribute to these carboxylate peaks. Therefore, they are still present even after the displacement of OA molecules with other ligands. The DT functionalized sample shows similar features but is marked by the absence of any significant intensity near 3005 cm−1, thereby indicating removal of the oleic acid groups. The TP-modified samples show a C−H feature at 3061 cm−1, slightly lower than the ∼3070 cm−1 observed for the parent compound, while the DMATP-CdSe/TiO2 samples show a peak at 3076 cm−1. These features are nearly identical to those observed previously for thiophenol on gold and are attributed to the aromatic C−H stretching modes.41 The DMATP samples show a more complex spectrum in the 2700−3000 cm−1 region where the C−H modes of the −N(CH3)2 groups would be expected; this region is similar to that of the pure parent compound (see SI) and to N,N-dimethylaniline.42 Our FTIR data establish that ligand exchange from the initially functionalized samples is successful, although some small amounts of OA may remain.

Figure 3. (a) Visible absorption of OA-CdSe/TiO2 exposed to light and H2O at 0, 3, 5, and 10 min. (b) The dark control for L = OA. Spectra stacked for clarity. (b) Baseline subtraction procedure used to obtain the peak. (c) Resulting peaks after baseline subtraction for L = OA. (d) Fraction of original peak amplitude, A/A0, versus exposure time for all ligands. Error bars are standard deviations obtained from four separate samples for each ligand.

and light. After illumination, the CdSe exciton peak broadened, the amplitude decreased, and the peak position slightly shifted. The dark control shown in Figure 3b, on the other hand, shows very little change in the excitonic peak, indicating that light is required to induce degradation. A shift and broadening indicate that the size and distribution of the QDs have changed as a result of photocorrosion. While loss of the exciton features could also be attributed to desorption of whole QDs from the TiO2 film, control experiments performed under dark conditions show that the nanoparticles are stable in the dark; C

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thus, loss of the exciton feature indicates photodegradation of the QD/TiO2 structure. After light exposure, significant changes were observed in the first exciton peak, but this peak was riding on a large rising background. For subsequent analyses, we isolated the exciton peak by subtracting the rising background as depicted in Figure 3c to quantify this degradation. The background was fitted to a line in the region around the peak. Figure 3d shows the results of the baseline subtraction. To systematically track the degradation, we calculated the fraction of the original peak amplitude, A/A0, where A0 is the amplitude at the peak of the baselined curve at time = 0 min and A is the amplitude at time >0 min, obtained at the same wavelength as A0. This ratio was calculated for each sample first and then averaged over multiple samples (prepared identically) for each ligand to evaluate the statistical variation between samples. We compared the photodegradation of CdSe/TiO2 adducts before and after substitution of the native CdSe ligands with three different ligands (L) depicted in Figure 1: 4dimethylaminothiophenol (DMATP), thiophenol (TP), and 1-dodecanethiol (DT). Figure 3e shows the calculated ratio A/ A0 plotted versus the time of illumination of the L-CdSe/TiO2 adducts in water. These data show that thiol-substituted ligands on CdSe are more stable than those of the starting OA-CdSe/ TiO2 adducts. A comparison of all four ligands reveals that the order of stability is: DMATP > TP ≈ DT > OA. When comparing the thiol ligands used here, somewhat surprisingly, our results show that the shorter, phenylterminated ligands provide comparable (in the case of TP) or better (in the case of DMATP) stability than the long-chain dodecanethiol. In contrast, Aldana et al. reported that for thiols terminated with carboxylic acids, the aromatic thiol-coated CdSe QDs were less stable than aliphatic thiol-coated ones.44 Conjugation in the molecule can provide protection against degradation, with DMATP outperforming the rest, as measured by the noticeably smaller change in the A/A0 values. Notably, the dimethylamino group at the distal end of the DMATP molecule provides better protection than the similar molecule lacking this group (i.e., thiophenol). To investigate whether there were significant differences in packing of ligand molecules on the CdSe QDs, XPS measurements were performed on freshly prepared samples to obtain the molecular coverages on CdSe QDs. Raw data of Cd(3d), Se(3d), N(1s), and C(1s) of the L-CdSe/TiO2 samples are shown in the SI. Quantitative analysis of the S and Se regions is complicated by the fact that the S(2s) and S(2p) peaks have significant overlap with the Se(3s) and Se(3p) peaks. Figure 4 shows the sulfur 2s region; here, the peak at 230 eV is assigned to Se(3s), while the peak at 227 eV is assigned to S(2s). This assignment was verified by the fact that only the peak at 230 eV was observed when CdSe was functionalized directly on TiO2 without using the MPA linker. Quantitative analysis of molecular packing densities on nanoparticulate samples must take into account the geometric shape of the nanoparticles and inelastic scattering taking place within the nanoparticle core and the surface ligands.20 To properly account for electron scattering effects, we used direct numerical integration to determine the ratio of the C to Cd signal expected from QDs of 1.6 nm radius surrounded by organic layers, including full scattering corrections. Details of the numerical integration and the parameters used are described in the SI. Figure 4b shows the resulting molecular packing densities determined from the XPS data. The data

Figure 4. (a) XPS spectra of S(2s) regions of L-CdSe/TiO2. Spectra of CdSe functionalized on TiO2 without the MPA linker are also shown in the S(2s) region to confirm that the lower binding energy peak is from Se(3s). (b) Molecular coverage for each ligand calculated from the Cd(3d5/2) and C(1s) intensities.

show that DT, TP, and DMATP molecules have similar packing densities; that of OA is somewhat smaller, likely due to the labile nature of the carboxylic acid ligands and the unsaturated nature of the ligand, which disrupts packing crystallinity.45 These data show that the enhanced photostability of DMATP cannot be explained simply on the basis of molecular packing densities. The enhanced stability of DMATP-functionalized CdSe/ TiO2 films is also evident in air. Figure 5 shows A/A0 values of

Figure 5. Photostability under light and ambient air. Fraction of original peak amplitude, A/A0, plotted versus exposure time for all ligands. Error bars are standard deviations obtained from 4 or 5 separate samples for each ligand. Inset: photograph of samples after ∼2 weeks exposure to ambient laboratory lighting conditions.

L-CdSe/TiO2 plotted versus exposure time. The degradation in air is slightly slower than that in water. While the OA sample had ∼60% of the original peak amplitude after 10 min exposure in air, the same OA ligand had only ∼40% of the original amplitude after the same amount of exposure time in water. In this case, desorption of whole QDs should not occur, as samples were not immersed in liquid; therefore, the photodegradation measured was from corrosion of the QD itself. The photodegradation effects can also be seen visually in a photograph of the CdSe/TiO2 films that have been functionalized with the different ligands and exposed to ambient lighting for two weeks (Figure 5c, inset). After several days, only the DMATP-capped CdSe/TiO2 has retained its dark orange color. D

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changed after ligand modification. The short time constant τ1 decreased slightly with DT and more with TP, and then DMATP had the shortest time constant of ∼4 ns. We interpret these changes as reflecting the dynamics of hole transfer from the QD to the ligands. Fast hole transfer from CdSe QDs has also been observed with similar nitrogen and sulfur containing aromatic molecules.54,55 Burda et al.54 found that hole transfer from CdSe QDs to 4-aminothiophenol occurs in 3 ps, while Huang et al.55 used phenothiazine and measured hole transfer of 300 ps to 40 ns depending on the ligand concentration. Due to the resolution limit of our instrument, we cannot precisely determine the rate of transfer in our DMATP-CdSe system except that it must be faster than 3−4 ns. The above results demonstrate that all thiol groups strongly quench the luminescence from the CdSe QDs, suggesting that hole transfer from CdSe to the thiol group of the molecule is facile for all three thiols investigated. However, DMATP clearly excels in its ability to reduce photodegradation. To help understand this phenomenon, we used density functional calculations to help characterize the system. Using DFT calculations on the free ligands and using Koopmans’ Theorem56,57 to relate the ionization potential to the energy of the highest-occupied molecular orbital, we estimate ionization potentials of 6.3 eV for butanethiol, 6.1 eV for thiophenol, and 5.3 eV for DMATP; this shows that DMATP has the largest driving force for injection of electrons into the excited CdSe QD. Because prior work indicated that the resulting holes trapped on the interfacial S atoms induce disulfide formation and subsequent desorption of the ligands,44 we also performed calculations on a Cd6Se6 cluster with the thiol ligands attached. In these calculations the Cd and Se atoms were terminated with H atoms except for one exposed Cd−Se pair whose local geometry mimicked that of the nonpolar CdSe(112̅0) surface, the lowest-energy face of bulk CdSe.58 Energies were calculated for the molecule−surface cluster in neutral form, the cation in the neutral-optimized geometry, and for the fully relaxed cation. While these clusters are too small to adequately represent the electronic structure of the CdSe QDs, previous studies have shown that clusters of similar size adequately represent trends in ligand binding energies.59 Using the Natural Bond Order (NBO) analysis we determined the natural charges associated with the individual atoms, which allows us to determine how much of the charge was localized on the molecule. Figure 7 shows the optimized molecular structure for a DMATP molecule on the Cd6Se6 cluster. It is notable that the geometry around the N atom is locally planar instead of pyramidal, in agreement with previous studies.60,61 We calculated the charge distribution on the molecule− cluster adduct in the neutral state and then for the cation, in both the sudden limit (i.e., using the neutralized geometry) and the adiabatic limit (after full relaxation of the cation) for butanethiol, thiophenol, and DMATP. Calculations in the

The films capped with other ligands became lighter in color, turning from orange-red to pale-orange. The enhanced stability of QDs that are capped with DMATP compared to other ligands is readily visible. To understand the origins of this stability trend, we performed photoluminescence (PL) experiments on the CdSe QDs in solution functionalized with the different capping ligands. Figure 6a shows the steady-state emission of the CdSe

Figure 6. (a) Steady-state and (b) transient PL of L-CdSe in chloroform. Ligand concentration was 35× of QD.

QDs, while Figure 6b shows the transient luminescence. Figure 6a shows that the thiol-capped CdSe QDs have much smaller fluorescence (∼100-fold reduction in intensity) compared with the OA-terminated QDs (note also the scale change, as the signal from the OA-capped QDs has been reduced 10-fold for presentation on the same graph). Thiol-capped CdSe QDs quench the PL, as seen and investigated by many others,46−50 and is thought to be due to hole transfer to the thiol end group.47,51 Trapping of holes in the ligand is a nonradiative pathway; therefore, as the propensity for hole transfer increases, the photoluminescence is quenched more effectively. The extent of quenching by the capping ligand also agrees with the photostability trend; ligands that trap holes more efficiently lead to increased CdSe stability. When the holes are pulled away from the CdSe into the ligand, oxidation of the CdSe itself is prevented. With the DMATP ligand, we observed a >3000fold reduction in PL signal as compared to the OA-capped CdSe QDs. To better understand the PL dynamics, we also performed time-resolved PL on the ligand-functionalized CdSe QDs in solution using 450 nm excitation from a pulsed laser (∼3 ns pulses, 20 Hz). The PL decay from the CdSe QDs was observed to be multiexponential, consistent with prior studies.52,53 Our data were fit best to a biexponential function I(t) = A1e−t/τ1 + A2e−t/τ2, consistent with prior reports of the nanosecond dynamics being controlled by two primary populations of trap states.50,52,53 For the OA-capped QDs, we found a short time constant of τ1 ∼ 8 ns and a longer time constant of τ2 ∼ 40 ns. Results of the fits are shown in Table 1. Both the amplitude and the time constant of the PL transients

Table 1. Biexponential Fit Parameters from Time-Resolved Photoluminescence (TR-PL) Studiesa

a

τ1 (ns)

A1

ligand OA DT TP DMATP

0.580 0.661 0.756 0.977

± ± ± ±

0.005 0.004 0.007 0.005

9.4 7.7 6.5 3.9

± ± ± ±

τ2 (ns)

A2

0.1 0.1 0.1 0.1

0.462 0.392 0.290 0.098

± ± ± ±

0.006 0.004 0.007 0.004

41.3 48.8 39.8 32.2

± ± ± ±

0.4 0.5 0.1 1.5

(ns) 34.2 40.2 29.9 16.7

± ± ± ±

4.5 5.6 2.2 4.5

The last column shows the effective lifetime = (A1τ21 + A2τ22)/(A1τ1 + A2τ2). E

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electrons to fill holes in the excited QD state, shutting off the pathway for radiative photoluminescence46 and eliminating the driving force for photocorrosion of the QD core. However, even in this case oxidation of the thiol group or other components of the ligand may lead to their loss from the surface. Aldana et al.44 reported that photooxidation of ligandmodified QDs is initiated by diffusion of oxygen through the molecular layer to the QD core, where they oxidize the thiol head groups to disulfides that are then released from the surface. On the basis of this and other studies, it can be inferred that stabilization of the CdSe QDs requires four criteria: (1) stable binding of the ligand to the QD, (2) tight packing of the molecular chains to prevent diffusion of oxygen and water to the QD core, (3) the ability to inject electrons from the ligand into the QD, and (4) the ability to stabilize positive charge on the ligand in a manner that does not lead to subsequent oxidation of the ligands. Our experiments demonstrate that the first criterion is met by using thiols. This conclusion is in agreement with previous investigations of molecular layers bound to CdSe via amines,47,58,59,66 carboxylic acids,58,59 phosphine oxides,58,59,66 phosphonic acids, 58,67,68 dithiocarbamates, 69 and thiolates.44,50,66 These prior studies have generally found amines and carboxylic acids are among the weakest-binding, while phosphonic acids and thiols are the strongest-binding ligands on CdSe QDs. The most significant result of our work lies in our demonstration that short aromatic thiols such as DMATP can be highly effective at reducing photodegradation of CdSe. This result is surprising in light of studies by Aldana et al.,44 who concluded that CdSe QDs capped with aromatic thiols were less stable than those capped with aliphatic thiols. Our studies show that the nature of the substituent groups on the aromatic ring plays a very important role, evidenced by the fact that dimethylaminothiophenol (DMATP) is significantly more effective than the parent thiophenol molecule. The apparent discrepancy between our results and those of Aldana et al. can be resolved by noting that in Aldana’s studies 44 4mercaptobenzoic acid (MBA) was the only aromatic thiol investigated. However, MBA contains an electron-withdrawing carboxylic acid group in the para position, which makes MBA a poorer electron donor than thiophenol.70 Thus, while MBA may be poorer than alkyl thiols at reducing photocorrosion, replacing the electron-withdrawing carboxylate group with an electron-donating group such as the dimethylamino group of DMATP greatly enhances the photostability and achieves nearly complete quenching of the luminescence. Our data show that while thiophenol reduces the CdSe fluorescence intensity by a factor of 100, DMATP reduces the fluorescence intensity by a factor of >3000, to below our detection limit. Thus, we conclude that DMATP is a significantly more effective electron donor than thiophenol. In addition to the electron-donating ability of the ligand, the location of the residual positive charge can also play an important role in the resulting photostability because oxidation of S atoms leads to formation of disulfide linkages and desorption of the molecular layers, thereby leaving the NP particle core exposed to subsequent degradation.44 Our computational results in Figure 7 show that in addition to DMATP being a highly effective electron donor only a small fraction of the resulting positive charge remains localized on the oxidation-sensitive S atom. The ability to delocalize the resulting positive charge after electron donation is likely an

Figure 7. (a) Energy-minimized structure of DMATP on the Cd6Se6 cluster. (b) Results of NBO analysis of charge distribution on the molecule and on the linking S atom.

adiabatic and sudden limit yielded similar values. Using the NBO analysis on the neutral and cation for each molecule− Cd6Se6 complex, we determined the charge on the molecule (including the S atom linker) and also the amount of charge on the S atom alone. Figure 7b summarizes these calculations. For butanethiol (a mimic for dodecanethiol), only ∼0.22 of the total +1 charge is on the molecule. More importantly, however, is that most of that charge is localized on the S atom of the thiol linker. In contrast, TP and especially DMATP both have a larger fraction of the charge localized on the molecule and yet have a smaller total charge on the S atom. Thus, in addition to being a more effective electron donor to the CdSe, the conjugated linker DMATP is also more effective at removing the charge away from the oxidation-sensitive thiol group.

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DISCUSSION While surface-bound ligands are widely known to play an important role in QD photostability, the links between ligand structure, photocorrosion, and optical properties are complex and not yet fully understood.44,62−64 When CdSe is optically excited, the holes can oxidize surface atoms from Se2− to elemental Se0 in the presence of water65 or SeO2 in the presence of air.20 While photooxidation can be problematic for QDs in applications such as fluorescence imaging, the problems are particularly acute in structures such as QD-sensitized solar cells1−3 because in solar cells the excited electron is transferred from the QD into an electron acceptor, leaving the QD overall positively charged and therefore particularly susceptible to oxidation. Previous studies have shown that densely packed ligands can passively stabilize QDs against photooxidation by preventing diffusion of oxygen and/or water to the nanoparticle surface, thereby helping to prevent formation of higher oxidation products.44,64 Ligands can also play an active role by donating F

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important factor impacting the photostability of ligandmodified CdSe QDs. The relative importance of the electron donation vs electron delocalization can also be qualitatively estimated using Hammett constants σp and σp+, the former reflecting substituent-induced changes in electron donation in the absence (σp) and presence (σp+) of resonance stabilization. For −N(CH3)2 they are quite different (σp = −0.83, σp+ = −1.70).70,71 The large negative value of σp+ for −N(CH3)2 shows that it is a strong electron donor, while the large difference between σ p and σ p + shows that −N(CH 3 ) 2 substantially enhances resonance stabilization of the DMATP cation. The ability to achieve corrosion protection using short, conjugated ligands is important because long alkyl chains are expected to prevent facile electron transfer and are therefore not likely to be suitable for applications such as nanocrystal- or QD-based thin film optoelectronic devices72,73 that require electronic communication between QDs and adjacent moieties. Since short, conjugated ligands provide the strongest electron transfer,74−78 the ability to stabilize the QDs against photocorrosion by using electron-donating aromatic ligands may enhance the overall functionality of QD-based devices.

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CONCLUSIONS Our results show that aromatic ligands bearing electrondonating substituents can provide excellent protection of CdSe QDs against water and air oxidation during illumination when functionalized on a QD-sensitized TiO2. By stabilizing the sulfur end of the molecule with electron-donating groups, these small molecules can shuttle holes quickly and away from the thiol and the QD surface, thus inhibiting oxidation of the QD and also providing protection for the interfacial thiol linker. We expect this method of protection can be extended to other metal chalcogenide-sensitized systems, especially the ones that are even more water and oxygen sensitive, such as PbSe and PbS.

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ASSOCIATED CONTENT

S Supporting Information *

FTIR spectra of the neat ligands used to functionalize CdSe QDs, raw XPS data of the Cd(3d), Se(3d), N(1s), and C(1s) regions of the L-CdSe/TiO2 samples, and details of the numerical integration and the parameters used for quantitative XPS analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Dr. Ryan Franking for his help in the TiO2 film preparation and to Prof. John Wright for helpful discussions. This work was supported by the Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award DE-FG02-09ER46664.

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