Two Distinct Transitions in CuxInS2 Quantum Dots. Bandgap versus

Letter pubs.acs.org/JPCL

Two Distinct Transitions in CuxInS2 Quantum Dots. Bandgap versus Sub-Bandgap Excitations in Copper-Deficient Structures Danilo H. Jara, Kevin G. Stamplecoskie,† and Prashant V. Kamat* Radiation Laboratory, Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: Cu-deficient CuInS2 quantum dots (QDs) synthesized by varying the [Cu]:[In] ratio allow modulation of optical properties as well as identification of the radiative emission pathways. Absorption and emission spectral features showed a strong dependence on the [Cu]:[In] ratio of CuxInS2 QDs, indicating two independent optical transitions. These effects are pronounced in transient absorption spectra. The bleaching of band edge absorption and broad tail absorption bands in the subpicosecond− nanosecond time scale provide further evidence for the dual optical transitions. The recombination process as monitored by photoemission decay indicated the involvement of surface traps in addition to the bandgap and sub-bandgap transitions. Better understanding of the origin of the optical transitions and their influence on the photodynamics will enable utilization of ternary semiconductor quantum dots in display and photovoltaic devices.

C

sub-bandgap states relative to II−IV semiconductor nanocrystals (NCs).41,42 Recently, another mechanism has been invoked to explain the emission and absorption properties.43,44 In these works, the magneto-optical properties of CuInS2 QDs were compared with other Cu-doped semiconductors. Excited-state behavior including broad absorption band stretching into the visible, broad luminescence band, large Stokes shift, magnetic-exchange splitting between singlet and triplet excited states, and similar zero-field splitting of the triplet excited state were found to be similar for all samples.44 Furthermore, it was concluded in this work that CuInS2 QD luminescence was the same as that proposed for the Cu+-doped semiconductor NCs. In this mechanism, the radiative recombination process occurss via electron relaxation from the CB to a localized hole, assigned to a lattice copper. Mechanistic quandaries are still far from being settled and are the subject of an ongoing discussion in this field of research. Because CuInS2 properties are strongly dependent on defects states, we have now systematically conducted a study to elucidate the charge carrier dynamics of CuInS2 QDs by intentionally introducing defects into the crystal structure. CuInS2 QDs were synthesized by varying the copper and indium precursor ratio, and the photophysical properties were examined using time-resolved absorption and emission spectroscopies. The relationship between two optical transitions in the absorption spectrum and emission behavior of CuxInS2 QDs is discussed.

uInS2 quantum dots (QDs) have recently emerged as a new class of semiconductor materials with potential applications in light-emitting diodes (LED),1−9 solar cells,10−18 and bioimaging19−28 because of their visible photoluminescence, well-matched bandgap to the solar spectrum (1.5 eV bulk material), and exclusion of heavy metals. Despite the extensive interest in CuInS2 QDs, their optical properties are yet to be understood fully. A few recent efforts have focused on exploring excited-state behavior using time-resolved spectroscopy techniques.17,29−38 For example, some of these studies consider charge carrier recombination from the conduction band (CB) to acceptor defect states near the valence band (VB).34,36,38 Other studies have proposed recombination mechanisms based on transitions involving a donor−acceptor pair (DAP), transitions from a donor defect state near the CB to the VB, as well as relaxation from the CB to a Cu1+/2+ doped acceptor state. The difficulty in proposing a universal physical model of the charge carrier dynamics in CuInS2 QDs is due to the uncertainties associated with the internal defect states located within the bandgap. Low-temperature photoluminescence of single-crystal CuInS2 studies also indicate that its photophysical properties are complex. They are dominated by donor and acceptor sub-bandgap states originating from defects such as Cu and In vacancies (VCu and VIn) and Cu (In) replacing In (Cu) (CuIn and InCu) defects.39,40 Generally, CuInS2 QDs exhibit a broad luminescence and absorption band with no clear exciton properties. Although the reason behind such complexity is not clear, it has been ascribed to unique electronic properties of I−III−IV semiconductors, namely, irregular composition and distribution of elements, size and shape inhomogeneity, and © 2016 American Chemical Society

Received: March 11, 2016 Accepted: April 4, 2016 Published: April 4, 2016 1452

DOI: 10.1021/acs.jpclett.6b00571 J. Phys. Chem. Lett. 2016, 7, 1452−1459

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The Journal of Physical Chemistry Letters Synthesis and Morphological Characterization. CuInS2 QDs were synthesized according to the literature procedure34,46 with some modification, the details of which are provided in the Experimental Section. The pyramidal-shaped NCs obtained from this procedure show a high degree of crystallinity, narrow size distribution, and relatively high emission quantum yield. The precursor composition was varied to achieve different [Cu]:[In] ratios in the QDs. A white solid appears when the synthesis of QDs was conducted with a nonstoichiometric [Cu]:[In] ratio. This white solid becomes more pronounced in QD synthesis with lower [Cu]:[In] ratio. Because elemental analysis of these QDs by inductively coupled plasma (ICP) indicates In content lower than the one employed in the precursor mixture, we attribute this white solid to the unreacted indium precursor. Previous reports have attributed white powder to the formation of polymeric materials formed as a result of reaction between the indium precursor and 1dodecanthiol.46 Before the first washing procedure, the white solid was removed by filtration. To test the validity of the procedure in yielding QDs with reproducible values of [Cu]: [In], each preparation was carried out in triplicate. The [Cu]: [In] ratio was determined by ICP, and the average was calculated from the values obtained for the three samples (Table 1). The experimentally determined [Cu]:[In] ratios Table 1. Particle Size and Emission Quantum Yield of CuxInS2 QDs [Cu]:[In] precursors

[Cu]:[In] ICP

size by STEM (nm)

Φ (%)

1.0

1.26 ± 0.06

2.6 ± 0.4

0.9 ± 0.2

1.0

1.14 ± 0.12

4.0 ± 0.6

3.3 ± 1.5

0.7 0.5 0.25

0.77 ± 0.03 0.61 ± 0.02 0.48 ± 0.02

2.5 ± 0.3 2.6 ± 0.4 2.5 ± 0.2

7.7 ± 2.5 18 ± 2.7 16 ± 2.5

sample name Cu1.2InS2 (2.6 nm) CuInS2 (4.0 nm) Cu0.8InS2 Cu0.6InS2 Cu0.5InS2

Figure 1. STEM image showing the pyramidal shape and size of the CuxInS2: (A) Cu1.2InS2, (B) CuInS2, (C) Cu0.8InS2, (D) Cu0.6InS2, and (E) Cu0.5InS2. (F) XRD patterns for all samples showing a tetragonal chalcopyrite-like structure.

the reproducibility. (The absorption spectra in Figure S2 confirm the similar spectral features in samples with the same composition.) A broad absorption feature is observed for the for the Cu1.2InS2 and CuInS2 samples. However, as the [Cu]: [In] ratio decreases, a sharper transition appears around 500 nm, which is characteristic of an excitonic peak. In particular, the peak becomes more prominent in the Cu0.5InS2 QDs. The excitonic peak emerges in CuInS2 QDs prepared with a precursor ratio of [CuI]:[In(Ac)3] = 1:4 or 1:8. Both of these samples attain similar [Cu]:[In] ratio in QD samples as confirmed from the ICP measurements (Figure S2 and Table S1). In our earlier studies, we have shown size-dependent absorption properties of CuInS2 QDs with 1:1 ratio.17 The absorption edge was dependent on the QD size as it shifted from 680 to 850 nm with QD size increase from 2.9 to 5.3 nm. None of these 1:1 [Cu]:[In] ratio QDs showed any prominent peaks that could be attributed to excitonic transition. The presence of sub-bandgap transitions were found to contribute to tail absorption. In the present study, we focused on the QDs with varying [Cu]:[In] ratio. Except for one CuInS2 sample of 4 nm, all other samples employed in the present study were of similar particle size (∼2.5 nm). Thus, reduction in tail absorption with decreasing ratio of [Cu]:[In] arises from the disappearance of sub-bandgap transitions and not from the size quantization effect. Concurrently, we also see the appearance of

confirm the yield of nonstoichiometric QD samples. From the three samples, we selected the one with the highest emission quantum yield to analyze and quantify the QD samples as Cu1.2InS2, CuInS2, Cu0.8InS2, Cu0.6InS2, and Cu0.5InS2 (taking indium as one mole and assuming two moles of sulfide). Morphological characterization was conducted by employing transmission electron microscopy (TEM) and powder X-ray diffraction (XRD). The XRD patterns scanned in a range of 20−85° 2θ (Figure 1F) show five diffraction peaks at 28, 47, 55, 67, and 76° corresponding to (112), (204/220), (312), (400), and (316/322) planes of a tetragonal chalcopyrite-like structure (JCPDS entry 75-0106), respectively.47 It is noted that a change in the [Cu]:[In] ratio does not affect the crystal structure of the CuxInS2 QDs. Scanning TEM (STEM) was used to determine the size distribution of the CuInS2 QDs by measuring the average edge length of each nanoparticle (Figure S1). When the reaction was set at 30 min, pyramidal particles with an average size of 2.5 nm were obtained for all Cu-deficient samples except for stoichiometric [Cu]:[In] where the average size was around 4 nm. However, efforts to prepare QDs with smaller size (2.6 nm) from a stoichiometric precursor by reducing the reaction time to 20 min led to greater Cu concentration (Cu1.2InS2). Absorption Properties. Figure 2A shows the absorption spectra of CuInS2 QDs of different [Cu]:[In] ratios in chloroform solution. Each sample was synthesized in triplicate to confirm 1453

DOI: 10.1021/acs.jpclett.6b00571 J. Phys. Chem. Lett. 2016, 7, 1452−1459

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Figure 2. (A) Absorption spectra, (B) emission spectra (λex = 490 nm), and (C) emission quantum yield of CuxInS2 QDs with different [Cu]:[In] ratio, dispersed in chloroform. The circled numbers 1 and 2 in panel A indicate the excitonic and Cu-related sub-bandgap transitions, respectively.

Figure 3. Transient absorption spectra of CuxInS2 QDs deareated chloroform solutions excited at (A−C) 387 nm, (D−F) 530 nm, or (G−I) 580 nm pump wavelength with different delay time and 20 mW/cm2. The circled numbers 1 and 2 indicate the excitonic and Cu-related sub-bandgap transitions, respectively.

Cu1.2InS2, Cu0.8InS2, and Cu0.5InS2, are shown in Figure 3. The spectra corresponding to the other two samples, CuInS2 and Cu0.6InS2, are included in Figure S3. When the QD samples are subjected to excitation with a 387 nm laser pulse, we see collective transitions as represented by the bleaching of the absorption in Figures 3A−C. The transient absorption spectra recorded with Cu1.2InS2 (2.6 nm) show a broader bleach with spectral features similar to the CuInS2 (4 nm) sample, prepared with stoichiometric amount of copper and indium precursors. These results are in agreement with earlier reports showing broad transient bleach following the excitation of CuInS2 QDs.17,33−38 It is important to note that the majority of earlier ultrafast studies of carrier dynamics were carried out using stoichiometric CuInS2 and CuInS2−ZnS structures.33−38,48 As discussed in the present study, non-

an excitonic peak at 500 nm. These results confirm that variance in the [Cu]:[In] ratio has a marked effect on the optical transitions of CuxInS2 QDs. To obtain more information about the optical properties and the contribution of transitions responsible for absorption at 500 nm and tail absorption at longer wavelengths, we carried out pump−probe experiments in the femtosecond−nanosecond time domain. By selectively exciting CuxInS2 QD samples at 387, 530, and 580 nm, it was possible to induce optical transitions. Laser pulses with 150 fs pulse width and an energy density of 40 μJ/cm2 were employed for excitation. Deaerated CuxInS2 QDs solutions in CHCl3 were placed in a 2 mm path length cuvette, and the difference absorption spectra were recorded at various delay times following laser pulse excitation. The time-resolved spectra of three representative samples, 1454

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Figure 4. Excitation spectra (dotted colored lines) of (A) Cu1.2InS2 (2.6 nm), (B) Cu0.8InS2, and (C) Cu0.5InS2 QD samples recorded at different monitoring wavelength (λmo). Absorption and emission spectra of the corresponding samples are included for comparison. The λmo are marked on the corresponding emission spectrum.

band (670 nm) in Cu1.2InS2 and CuInS2 becomes predominant in Cu0.5InS2 QDs. On the basis of these spectral features, we can conclude that Cu-rich QDs exhibit an emission peak in the red region (∼670 nm), while QDs with lower [Cu]:[In] ratio have a predominant peak closer to the band edge (∼630 nm). These results further support our assertion that there are at least two emissive states involved in the radiative charge recombination processes. Although the low-energy emission is representative of a sub-bandgap state,45 its origin is not yet fully understood. The relative emission quantum yield (Φ) of the CuxInS2 QDs samples was measured using tris(bipyridine)ruthenium(II) chloride as a standard (Table 1 and Figure 2C). In comparison to the samples prepared with a stoichiometric amount of copper and indium precursors, the Φ of Cu-deficient QDs showed higher values. The Cu0.6InS2 sample yielded a maximum Φ of 18%. The increased crystal defects arising from Cu deficiency are known to increase the overall emission yield of Cu-deficient CuxInS2 QDs.2,4,31,45,49−51 It is interesting to note that the QD size also influences emission quantum yield. CuInS2 QDs of 2.6 and 4 nm prepared using the 1:1 molar ratio of the precursors were 0.9 and 3.3%, respectively. Although emission yield increases with increasing particle size for smaller QDs (4 nm or less), they exhibit a decrease in emission yield for larger (>5 nm) QDs.17 These results further indicate that the [Cu]:[In] ratio has more influence on the emission yield than the particle size of CuxInS2 QDs. As shown earlier by Castro et al.42 and Zhong et al.,52 evaluation of excitation spectra of CuInS2 provides insight into the mechanism of radiative processes. The dependence of the excitation spectrum on the monitoring emission wavelength (λmo) was ascribed to different donor−acceptor transitions. To further explore the origin of the two emission bands in the present study, we recorded the excitation spectra by monitoring the emission at different λmo. The excitation spectra for QDs with varying [Cu]:[In] ratio are presented in Figure 4A−C and Figure S4. The excitation spectra recorded for each sample were multiplied by a weight factor to account for the difference in intensity of the λmo. The multiplication factor was derived by normalizing the emission intensity at different λmo. It is evident from these excitation spectra that the spectral feature is strongly dependent on the λmo. When monitored at higher-energy emission (λmo 700 nm), the 510 nm peak in the excitation spectra disappears. Clearly, the longer-wavelength emission is more responsive to low-energy optical transitions, specifically those contributing to the broad tail region of the absorption spectrum. Thus, the excitation spectra recorded in Figure 4 enable us to differentiate these two transitions. On the basis of these results, we can conclude that the broad emission of CuxInS2 QDs originates from two separate transitions, viz., high-energy (or band edge) excitation and low-energy (subbandgap) excitation. Time-correlated single-photon counting was used to analyze the emission lifetimes. An earlier study of CuInS2 nanocrystals revealed a wavelength-dependent multiexponential emission decay consisting of a short and long lifetime component.52 For example, Li et al. described the luminescence decay as the sum of two exponentials, where the short- and long-lived components were ascribed to surface and internal defects, respectively.33 On the other hand a triexponential fitting have also been attempted in other reports.29,30,34 The three lifetimes τ1, τ2, and τ3 were assigned to an initially populated core state, surface defects, and donor−acceptor transitions, respectively. In the present study, we considered a triexponential kinetic fit to analyze the luminescence decay curves. To better understand the luminescence decay kinetics of CuxInS2 QDs, emission lifetimes were determined at wavelengths within the emission band (580−780 nm) for CuxInS2 QDs (Figure S5). Except for Cu-deficient samples, all decay traces exhibited a good fit to triexponential decay fit. For Cudeficient QDs, the decay traces were fitted to biexponential functions when λmo ≥ 680 nm. It is interesting to note that the lifetime values were dependent on the λmo (Figure S6). For the sample Cu1.2InS2, which exhibits a low Φ value, the lifetime and relative amplitude (in parentheses) are τ1 = 1.8 (15%) and 3 (4%) ns, τ2 = 13 (34%) and 22 (17%) ns, and τ3 = 88 (51%) and 164 (79%) ns, corresponding to λmo = 580 and 780 nm, respectively. Cu0.6InS2, the sample with the highest Φ value, exhibits τ1 = 2.3 (3.4%) and 3.8 (0.7%) ns for 580 and 680 nm, respectively, and τ2 = 20 (17%) and 37 (2.6%) ns and τ3 = 139 (79%) and 164 (97%) ns corresponding to λmo = 580 and 780 nm, respectively. Therefore, a higher Φ in CuxInS2 QDs arises from the greater contribution of the third emissive channel with longer lifetime. In contrast, τ1 and τ2 have greater contributions in the least radiative samples. For Cu-deficient samples, the τ1 and τ2 contributions became negligible and very small, respectively, as λmo is changed from the higher-energy to lower-energy region of the emission band. Optical Transitions and Radiative Recombinations Dictating the Photodynamics of CuxInS2 QDs. Cu-deficient CuInS2 QDs were obtained in the present study by changing the [Cu]:[In] ratio of the initial precursors and using an excess of 1-dodecanethiol. STEM and powder XRD analysis indicated that all of these CuInS2 QDs with different compositions had similar particle size, shape, and crystal structure. Hence, the observed differences in the optical properties of CuxInS2 QDs originates from the concentration of copper into the crystal structure. Previously reported absorption spectra of stoichiometric CuInS2 QDs showed only a broad absorption feature with a tail absorption extending into the red spectral region. However, a

Scheme 1. Schematic Diagram Illustrating Optical Transitionsa

a

(1) Excitonic/bandgap transition, (2) sub-bandgap transition, and radiative recombination pathways identified with three lifetimes (τ1,τ2, and τ3).

spectral profiles recorded for CuxInS2 QDs samples (Figure 3) show that the optical transition at lower energy is strongly dependent on the [Cu]:[In] ratio, and its contribution diminishes upon lowering the concentration of copper. This behavior also causes the absorption peak near the band edge to become well-defined in the absorption spectrum of the Cu0.5InS2 sample. Because the valence band of CuInS2 exhibits high Cu d character,53 we can anticipate a strong dependence of electronic properties of CuxInS2 QDs on the [Cu]:[In] ratio. We assign the high-energy band to a band-edge transition which is similar to excitonic transitions in metal chalcogenide QDs. We also verified the excitonic transition near the band edge from its dependence on the particle size. The excitation spectra of three different size CuxInS2 QDs were also separately recorded by monitoring the emission band’s blue spectral edge (Figure S7). With increasing particle size we see a red-shift of the peak position, a phenomenon consistent with a quantum confinement effect. The transition from Cu-induced state to conduction band can be seen from the longer wavelength response in the excitation spectra during monitoring in the low-energy emission region. As shown previously with Cu+-doped semiconductors, the optical transition originating from Cu+ sub-bandgap states appears at the low-energy region of the absorption spectrum.44,54 Furthermore, magneto-optical experiments demonstrated that the ground state of CuInS2 NCs is diamagnetic, consistent with the presence of Cu+ ions. However, the presence of Cu2+ is evidenced in the excited 1456

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The Journal of Physical Chemistry Letters state as a result of transfer of one electron from the Cu+ state to the conduction band.44 Both bandgap and sub-bandgap optical transitions involve transition to conduction band because both absorption and excitation spectra exhibit similar spectral features at shorter wavelengths (Figures 2A and 4). These two transitions occur independently as evidenced from the transient bleaching response to bandgap and sub-bandgap excitations. Scheme 1 illustrates the various transitions contributing to the excitation of CuxInS2 QDs and deactivation of the excited state. The excited state primarily involving conduction band electrons decays via radiative and nonradiative routes. As discussed earlier, the emission of CuxInS2 QDs samples can be resolved into three components with characteristic relaxation lifetimes τ1, τ2, and τ3 (Scheme 1). The emission in the red spectral region is dominated by the component having a longer lifetime (τ3). This component correlates well with the subbandgap transition. In contrast, the other two components corresponding to τ1 and τ2 contribute to the higher-energy region (or blue spectral region) of the emission band. We attribute these two radiative components to band edge and defect-induced charge recombination processes. Indeed, the excitation spectra recorded using emission wavelengths corresponding to the blue region of the spectrum show a peak at 500 nm in correspondence with the band edge transition. Finally, the results presented in this Letter will provide valuable information for understanding the photophysical properties in analogous ternary chalcogenides NCs such as AgInS2. It has been stated that the charge carrier recombination mechanism in this material occurs via subbandgap states originated by crystal and surface defects.55−57 In summary, we present here detailed excited-state charge carrier dynamics of CuInS2 QDs that are dependent on the [Cu]:[In] ratio. The QD properties are influenced by the compositional changes, and they can be tuned by varying the [Cu]:[In] ratio. Spectroscopic evidence points to two optical transitions contributing to the absorption properties of CuInS2 QDs. The origins of the first and second transitions were assigned to excitonic and Cu-related sub-bandgap state absorption, respectively. The photoluminescence lifetime measurements exhibit a wavelength-dependent decay, indicating a multistate relaxation pathway. Understanding these photophysical mechanisms will help to control CuInS2 QDs properties and aid in improving performance of photovoltaic and light-emitting devices.

3−5 times with dichloromethane/(Methanol:Acetone) solvents. Characterization. The [Cu]:[In] ratio were measured using an inductively coupled plasma optically emitting spectroscopy (ICP-OES) instrument (PerkinElmer Optima 8000). The samples were prepared by dissolving a small amount of sample in HNO3, and the sample was diluted in deionized water. To determine the concentration of the samples, a set of standards from 0.1 to 50 ppm were prepared using a RICCA 1000 ppm of copper and indium in 3% HNO3 standard stock solutions. Ultraviolet−visible absorption spectra were collected using a Varian Cary-50 Bio spectrometer. Steady-state photoluminescence spectra were recorded using a Horiba Fluorolog spectrometer with a 500 nm long-pass filter to exclude scattering from the excitation source. Emission lifetime measurements were recorded using a Jobin Yvon single-photon counting system with a 371 nm LED excitation source incident on a 1 cm quartz cuvette and using a 400 nm long-pass filter before the detector. The emission lifetime decays were reordered over a 200 ns window. Transient absorption measurements were performed using a Clark MXR-2010 laser system (775 nm fundamental, 1 mJ/pulse, fwhm = 130 fs, 1 kHz repetition rate) using Helios software provided by Ultrafast system. The pump beam, corresponding to 95% of the fundamental frequency, doubled to 387 nm, and the probe beam, corresponding to the remaining 5%, were focused through a Ti:sapphire crystal to generate a white light continuum are incident on CuInS2 solution (CHCl3) purged with N2 contained in a 2 mm quartz cuvette. Pump wavelengths of 530 and 580 nm were generated by a traveling-wave optical parametric amplifier of white-light continuum (TOPAS-C) light conversion. Transmission electron microscopy images were collected using a Titan 80-300 electron microscope at an accelerating voltage of 300 kV. The samples for TEM were prepared by dropping a diluted solution of CuInS2 QDs into a carbon-coated nickel grid, and the samples were dried under vacuum at room temperature overnight. X-ray diffraction (XRD) measurements were performed by using a Bruker D8 X-ray diffractometer with a scan rate of 2° min−1 from 2θ values of 20−85° employing Cu Kα radiation (λ = 1.5406 Å).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b00571. TEM images, QD size distribution histograms, emission spectra, absorption spectra, ICP and quantum yield of emission data for CuxInS2 samples, and photoluminescence lifetime decay analysis (PDF)

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EXPERIMENTAL SECTION Materials. Copper(I) iodide (Aldrich, 99.998%), indium(III) acetate (Aldrich, 99.99%), 1-dodecanethiol (Aldrich, ≤ 98%), toluene (Fisher Scientific, certified ACS grade), methanol (Fisher Scientific, certified ACS grade), methylene chloride (Fisher Scientific, GS/MS grade, not stabilized), chloroform (AMRESCO, biotechnology grade), and nitric acid (Macron Fine Chemicals, AR select) were used without purification. Synthesis of CuxInS2 QDs. In a typical synthetic procedure, predetermined amounts of CuI and In(Ac)3 were placed in a three-neck round-bottom flask containing 10 mL of 1dodecanethiol (DDT). DDT when used in excess as a solvent serves as a sulfur source as well as a capping ligand. The mixture was first heated to 120 °C and purged with N2 for 15 min. The temperature of the reaction mixture was elevated to 200−210 °C, and the mixture was maintained at this temperature for 30 min to stabilize the CuInS2 QDs. The QDs were then washed

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Website: kamatlab.com. Present Address †

K.G.S.: Department of Chemistry, Queen’s University, Canada.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research described herein was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of 1457

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Basic Energy Sciences of the U.S. Department of Energy through Award DE-FC02-04ER15533. This is document no. NDRL 5112 from Notre Dame Radiation Laboratory. We thank Center for Environmental Science and Technology (CEST), University of Notre Dame, for ICP facility and Jon Loftus for his assistance. D.H.J. thanks Comisión Nacional de Investigación Cientı ́f ica y Tecnológica (CONICYT) for the Becas Chile Scholarship, code 72110038.

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