Origin of Dual Photoluminescence States in ZnS–CuInS2 Alloy

Article pubs.acs.org/JPCC

Origin of Dual Photoluminescence States in ZnS−CuInS2 Alloy Nanostructures Gary Zaiats,† Sachin Kinge,‡ and Prashant V. Kamat*,† †

Notre Dame Radiation Laboratory, Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States ‡ Advanced Technology Division, Toyota Motor Europe, Hoge Wei 33, B-1930 Zaventem, Belgium S Supporting Information *

ABSTRACT: ZnS−CuInS2 (ZCIS) alloy nanostructures are becoming increasingly important materials because of their photoluminescence properties. Here we explore the emission properties of ZCIS quantum dots (QDs) capped with dodecanethiol, which exhibit Zn:Cu-dependent emission properties. Absorption and photoluminescence excitation spectra indicate a single, composition-independent light absorbing state. The emission spectra point out the existence of two emissive states with lifetimes of ∼10 ns and ∼100 ns. The photoluminescence and time-resolved emission analysis provide insight into the synergy between the two intraband states and the possibility of modulating the emission through variation in the Zn/Cu ratio. Better understanding of light absorbing and emission mechanisms in alloyed nanostructures is essential for future development of photoelectric and display devices.

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INTRODUCTION Development of heavy-metal free semiconductor nanoparticles as light absorbing and emitting material is quickly gaining momentum in energy and display applications.1−5 Pioneering exploration of optical properties with semiconductor nanocrystals such as PbSe and CdSe has led to their continued role as a major driver in the field.6−14 However, the presence of heavy metal elements and growing environmental restrictions on these elements have necessitated socially and environmentally acceptable materials.5,15−17 One of the attractive heavy-metal free alternative groups is based on I−III−VI multinary semiconductor nanoparticles (I = Cu, Ag; III = Al, In, Ga; VI = Se, S, Te).18−20 Their structural flexibility21,22 allows engineering of different heterostructures, varying between doped nanoparticles,23−26 core/shell heterostructures,27−29 or solid solutions.20,21,30−32 The resulting optical properties are the direct outcome of the selected size, geometry, composition and type of the heterostructures.18,19,33 Among possible combinations of the above-mentioned heterostructures, Cu−In−S (CIS) based materials have been widely studied for photovoltaic applications16,34,35 and lightemitting or display devices.3,26,36,37 The tuning of energy bandgap (Eg) of these materials allows light absorption in the entire visible spectrum.17,38 CIS nanoparticles exhibit broad absorption and emission peaks and weak Eg dependence on the composition, but strong dependence of photoluminescence (energy and emission yield) on the elemental composition.3,23,32,33,39 Such unusual optical properties are explained by considering distribution of vibrational states and the dominance of the donor−acceptor recombination mechanism.3,28,34 The structural tolerance of the CIS framework © 2016 American Chemical Society

allows convenient control over optical properties by synthesis of nonstoichiometric compounds.19,33 Moreover, it allows incorporation of a wide range of other elements (for example, Zn2+, Ga2+) in the structure as a part of a homogeneous alloy or as a building block for core\shell structure.3,30,32 Incorporation of Zn into CIS structures induces higher emission energy and longer emission lifetime.24,28,32 Moreover, subsequent addition of the ZnS shell further increases the quantum yield because of surface passivation.3,28,32 Understanding the excited state properties of multinary nanoparticles is essential for engineering materials for light emitting or absorbing devices based on these materials. Although the effect of alloying of I−III−VI compounds with Zn on the optical properties of ZnS−CuInS2 (ZCIS) quantum dots (QDs) has been explored,3,28,40 the emission dynamics is yet to be understood fully. Here we investigate the origin of emission and the mechanism of donor−acceptor photoluminescence in ZCIS nanoparticles using steady state and time-resolved photoluminescence measurements.

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EXPERIMENTAL SECTION Synthesis and Characterization of Zn−CuInS2 (ZCIS) Alloyed Quantum Dots (QDs). The synthesis of the ZCIS QDs was performed using a modified version of an earlier reported heat up method.3 The precursors copper iodide (CuI), indium-acetate (In(ac)), and zinc-acetate dihydrate Received: February 24, 2016 Revised: April 26, 2016 Published: April 26, 2016 10641

DOI: 10.1021/acs.jpcc.6b01905 J. Phys. Chem. C 2016, 120, 10641−10646

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Figure 1. (A) Absorption and (B) emission spectra of Zn−Cu−In−S nanoparticles with different compositions. All samples were measured in hexane as solvent. The photoluminescence data were collected following excitation at a wavelength of 450 nm.

corresponding spectrum at 300 nm. These absorption spectra which exhibit broad absorption features are characteristic of multinary nanoparticles arising from a distribution of vibrational states, variation in the size\shape inhomogeneity, and the dominance of internal and surface efects.18,19,34,39 Figure 1(B) shows the corresponding photoluminescence spectra that were obtained by excitation at 450 nm (2.76 eV). At a high Zn/Cu ratio of 16 (curve (a)), a nonsymmetric photoluminescence spectrum is obtained, with maximum emission at 540 nm (2.25 eV) and a long tail toward higher wavelengths. As the Zn/Cu ratio decreased to 9, the emission band broadens, but the position of the emission maximum remains unchanged. When we decrease the Zn/Cu ratio to 1, we see the appearance of a new emission band with maximum at 700 nm (∼1.8 eV). The photoluminescence analysis suggests two emission bands representing dual charge recombination pathways. The possibility also exists for the formation of a CIS/ZnS core/ shell structure as a result of the cation exchange or epitaxial growth of the ZnS layer.28 We checked this possibility of CIS/ ZnS core/shell structure formation in the present experiments by XPS analysis. The composition dependence of the binding energy of Cu and Zn cations supports the alloy structure,32,41 and hence, we rule out the involvement of core/shell structure (see Figure S2 in Supporting Information for XPS spectra). Furthermore, the absorption spectrum of nanocrystals (Figure 1A) remains mostly unchanged. On the basis of these observations we conclude that the CIS−ZnS alloy structure is formed in all these four compositions. Origin of the Emitting States. The obvious question is whether the broad emission band(s) seen in Figure 1B is the result of direct bandgap excitation or intrabandgap excitation. Since it was not possible to observe the excitonic peak in the absorption spectra, we recorded photoluminescence excitation (PLE) spectra to trace the origin of the emitting states (Figure 2A and B). Despite the change in the composition (Zn/Cu ratio from16 to 1 in Figure 2A, traces a−d) we observe the peak at 360 nm in the excitation spectra, in good agreement with a previously observed PLE feature in ZCIS alloyed nanorods.39 In addition we also varied the monitoring wavelength for recording the excitation spectra. For three different monitoring wavelengths in Figure 2B we observe similar excitation spectra with maximum at 360 nm. This characteristic spectral peak is close to the shoulder seen in the absorption spectra. This analysis supports the fact that the origin of the emission is independent of the Zn:Cu composition and arises following the bandgap excitation. This measured bandgap is smaller than the bandgap of ZnS (340 nm, ∼3.65 eV).39 Therefore, it does not originate from conulceation of ZnS nanoparticles. Although the

(Zn(ac)·2H2O) were mixed in 10 mL of octadecene, and the solution was heated to 120 °C under vacuum. After achieving the requested temperature, 2 mL of dodecanethiol (DDT) was injected, and the solution was further mixed for 20 min. Sequentially, 1 mL of oleic acid was added, and the solution was degassed for an additional 20 min, until formation of transparent solution (dependent on the composition it was colorless or had a slight yellow color). Following the degassing stage, the solution was heated to 230 °C for 30 min. The reaction was quenched by pouring flask content into cold toluene, and the particles were precipitated and washed of excess ligands by addition of ethanol (ethanol:toluene, 2:1). Optical Characterization. Absorption spectra were recorded using a Cary 50 Bio UV−vis dual beam spectrophotometer. Photoluminescence and photoluminescence-excitation spectra were recorded using a Horiba Jobin Yvon Fluorolog-3 spectrofluorometer. Unless otherwise specified the excitation wavelength for photoluminescence measurement was at 450 nm. The photoluminescence decay curves were measured using a Horiba Jobin Yvon single-photon counting system, with nanoLED laser and emitting wavelength at 458 nm. Composition Characterization. XPS analysis of nanocrystals was preformed using a PHI VersaProbe II, equipped with monochromatic X-ray beam. Samples for XPS were prepared by drop casting colloids onto a microscope glass slide. The measurements were conducted under ion neutralization with excitation voltage of 20 kV, and the binding energy was calibrated relative to the binding energy of carbon (284.8 eV).

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RESULTS AND DISCUSSION Optical Characterization. In order to establish the influence of Zn and Cu cations on the optical properties, we prepared samples that contain a low amount of Cu (in a flask with Cu cation concentration of 2.5%). By keeping the Cu concentration constant and changing Zn concentration in the precursor solution we obtained ZCIS nanoparticles with varying Zn:Cu ratio. The precise composition of each element incorporated in the nanocrystals was determined from the XPS analysis, and the stoichiometric values are normalized to the content of In (see Supporting Information, SI for XPS analysis). It is important to note that the actual ratio of Zn:Cu as measured by the XPS is different from the one employed in the precursor solution. The dependence of the actual elemental composition in ZCIS on the precursor concentration is shown in Figure S1 (Supporting Information). Figure 1(A) shows the absorption spectra of Zn−Cu−In−S (ZCIS) nanoparticle suspension with varying composition of cations (viz., Zn:Cu ratio), normalized to the absorption of the 10642

DOI: 10.1021/acs.jpcc.6b01905 J. Phys. Chem. C 2016, 120, 10641−10646

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Information). The emission monitored at 545 nm (high energy band) exhibits faster decay (τ1 ∼ 10 ns) than the emission recorded at 700 nm (low energy band with a lifetime of τ2 ∼ 100 ns). Indeed previously published works34,42 have discussed two emitting states in CuInS2 nanoparticles, but in these cases the emission was not spectrally resolved. Moreover composition dependence of emission rate from these states (Figure 3B) implies a different cation as the origin for each one of them. We also monitored the emission decay at high and low energy regions for samples that did not exhibit spectrally resolved emission peaks (Figure S3), and they showed similar two decay component behavior. Figure 3B shows the dependence of two lifetime values on the composition of ZCIS with different Zn/Cu ratio. The lifetimes of both components increase with increasing Zn concentration. Previously published28,40 reports also suggest stabilization of the excited state with addition of Zn. It is interesting to note that both components seem to exhibit a logarithmic dependence on the composition with similar slope. We also determined the contribution of each of these lifetime components to overall photoluminescence decay based on α1 and α2 values. The relative contribution of each of these two states exhibits complementary behavior (Figure 3C). For the Zn/Cu = 1 ratio, the fast decay component arising from the high energy state dominates, while for the ZCIS sample with Zn/Cu ratio 16, the slower decay component arising from the lower energy state dominates. This observation indicates the fact that state 2 becomes more dominant at the expense of state 1 as we increase Zn/Cu ratio. The dependence of emission decay components on the composition of ZCIS further ascertains our earlier observation that both Cu and Zn play an important role in dictating the emissive states. Photoluminescence Mechanism in ZCIS Nanoparticles. On the basis of the absorption and emission behavior of ZCIS nanoparticles we can further elucidate the mechanism of photoluminescence. The mechanism proposed in Scheme 1 shows the possible decay pathways that follow the bandgap excitation. As established through excitation spectra, the low and high energy emissive states arise from the bandgap excitation. The photogenerated electrons are transferred nonradiatively to the intrabandgap sites (1) and (2), respectively. The electrons from both of these states undergo radiative and nonradiative recombination with holes. As seen from the lifetime analysis, site (1) exhibits a faster decay (τ1 ∼ 10 ns) than that arising from site (2) with a lifetime τ2 ∼ 100 ns. This implies that there is an additional decay pathway for state (1) transferring the charge to state (2). This inter communication between the two states is supported from the

Figure 2. (A) Photoluminescence excitation spectra of ZCIS nanoparticles in hexane with different chemical compositions as referred to in the legend. Excitation spectra were monitored at the corresponding emission peak of each sample from Figure 1B. (B) Photoluminescence excitation spectra for a selected batch of nanoparticles Cu0.05InZn0.47 (Zn/Cu ratio ∼10:1) recorded at different monitoring wavelengths (510, 580, and 670 nm). Observation of similar excitation profile with peak corresponding to Eg indicates the origin of emission to bandgap excitation, and it is independent of the chemical composition.

change in composition of Cu and Zn cations in ZCIS influences the emission,32 the properties of the bandgap remain unchanged. Photoluminescence Dynamics. Figure 3(A) shows photoluminescence decay curves for one of the ZCIS nanoparticle samples with Zn/Cu = 1 (with two spectrally resolved emission peaks) at two different monitoring wavelengths (700 and 545 nm). A corresponding photoluminescence curve is shown in the inset of Figure 3A. Other photoluminescence decay traces are included in the Supporting Information (Figure S3). The emission decay is multiexponential and can be fitted to biexponential decay function: I = I0 + α1e−t/τ1 + α2e−t/τ2, where I is the emission intensity, τ1,τ2 are the characteristic lifetime values, and α1, α2 express the relative contribution of each one of the components to the total emission. The values (α1, α2, τ1, and τ2) analyzed through the biexponential fit are given in Table S1 (Supporting

Figure 3. (A) Photoluminescence decay curves for ZCIS quantum dots with Zn/Cu = 1, monitored at different wavelengths as specified in the legend and on the corresponding photoluminescence curve in the inset. (B) Fast (blue) and slow (red) photoluminescence lifetime coefficients as a function of Zn/Cu ratio. (C) Relative contribution of the fast decay (blue) and the slow decay (red) to the emission. 10643

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increasing Zn/Cu ratio. This blue-shift in turn implies that the position of the emissive state (2) in ZCIS nanocrystals is strongly dependent on Zn/Cu ratio. With increasing Zn/Cu ratio the probability of states responsible for low energy emission decreases, thus favoring direct radiative decay from state (1). As a result we see mainly high energy emission from ZCIS QDs with high Zn/Cu ratio. In summary, the optical properties of ZCIS alloyed nanoparticles are governed by the intrabandgap states arising from Zn and Cu cations. By varying the ratio of Zn/Cu, it is possible to modulate the photoluminescence properties of ZCIS QDs. Interestingly the bandgaps of all the ZCIS nanocrystals (Cu < 10% atomic) tested in the present investigation do not show any dependence on the composition. The presence of intrabandgap emissive states and their role in the deactivation of the excited state dictate the overall performance of these quantum dots in photovoltaic and display devices. Ultrafast transient absorption spectroscopy measurements are underway to shed more light on the intraband transitions that influence the Zn- and Cu-induced emissive states.

Scheme 1. Photoluminescence Process in ZCIS Nanoparticles

complementary dependence of lifetime and α values on the Cu and Zn composition. As presented in Figure 1B with increasing Zn/Cu ratio we observe the emission band to become narrower along with blue-shifting of the low energy emission tail. On the basis of the dependence of optical properties of ZCIS on the composition we can attribute the two emitting states, viz., states (1) and (2), to be dictated by Zn and Cu, respectively. At a Zn/ Cu ratio of 1, we see spectrally resolvable emission from both states. With increasing Zn/Cu ratio we see higher emission from state (1). Deconvolution of Photoluminescence Spectra. In order to examine our proposed photoluminescence mechanism involving two emissive states we analyzed the PL spectra. A deconvolution of the emission spectrum for ZCIS nanoparticles, with Zn/Cu = 1, using two Gaussian functions is shown in Figure 4A. The two convoluted spectra are

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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.jpcc.6b01905. Results of XPS analysis, time-resolved photoluminescence decay traces, summary of fitting parameter for photoluminescence decay traces, and deconvolution of photoluminescence peaks and TEM images are presented (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Website: www3.nd.edu/~kamatlab/. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research described here was supported by a grant from Toyota Motor Europe. Authors thank Alexander Robinson for his help in preparation of some of the samples. We thank Dr. Ian Lightcap and the ND Energy Materials Characterization Facility for the use of the XPS instrument. This is document no. NDRL 5109 from the Notre Dame Radiation Laboratory which is supported by the by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through award DE-FC0204ER15533.

Figure 4. (A) Deconvolution of PL spectra of ZCIS nanoparticles with different Zn/Cu ratios, as specified in the legend. (B) Emission energy of each one of the deconvoluted peaks as a function of Zn/Cu ratio (verified by XPS).

represented by A1 and A2 representing two emissive states. The peak position of each one of the states as a function of Zn/ Cu ratio is exhibited in Figure 4B. The result of deconvolution of other samples is shown in Figure S4 (SI). The analysis of emission peak position (Figure 4B) shows that the high energy emission band arising from the Zn remains constant irrespective of different Zn/Cu ratios. This further indicates that the emissive state lies near the conduction band, capturing most of the photogenerated electrons. The captured electrons can either undergo radiative recombination or get transferred to state (2). The radiative decay of state (1) is about 10 times faster than the decay of state (2) indicating a decay pathway between states (1) and (2), the latter being a more stabilized state. The emission peak arising from Cu on the other hand (Figure 4B) reveals a shift to higher energy with

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DOI: 10.1021/acs.jpcc.6b01905 J. Phys. Chem. C 2016, 120, 10641−10646