Thiol-Free Synthesized Copper Indium Sulfide Nanocrystals as

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Thiol-Free Synthesized Copper Indium Sulfide Nanocrystals as Optoelectronic Quantum Dot Solids David So† and Gerasimos Konstantatos*,†,‡ †

ICFO, Institute of Photonic Sciences, The Barcelona Institute of Science and Technology, Castelldefels 08860, Spain Institució Catalana de Recerca i Estudis Avançats (ICREA), Passeig Lluís Companys 23, 08010 Barcelona, Spain

‡

S Supporting Information *

ABSTRACT: Colloidal semiconductor nanocrystals are a platform for developing applications in optoelectronics but are mostly based on materials with toxic elements. Copper indium sulfide nanocrystals (CIS NCs) are an alternative but have been investigated in only a few reports as a functional optoelectronic film. We synthesized various sizes of emitting copper-poor CIS NCs in the absence of dodecanethiol. We employed a Zn-shelling as a surface stabilizing agent that yielded high quantum yield and stable colloids in both organic and aqueous solvents. We found that these nanocrystals possess high exciton binding energies and exhibit a decrease in photoluminescence lifetimes with distance under the Förster relationship. We then deposited CIS QD films by solid-state ligand exchange and characterized their field effect transistor and photoconductive characteristics.

1. INTRODUCTION Colloidal semiconductor nanocrystals, exhibiting size- and surface-tunable optical and electrical properties, are a versatile toolkit for the development of various applications.1−3 For example, efficient light-emitting diodes from highly emissive nanocrystals have been fabricated.4,5 Furthermore, solar cells from this class of materials,6 commonly based on lead sulfide (PbS), have reached impressive power conversion efficiencies of 9.2%.7 Being dispersed in solution, these nanocrystals can be readily processed into films and allow easy access to a variety of architectures, such as a monolayer of sensitizers, as a heterojunction with other materials, or as a multicomponentblended solid. Advances in performance can be attributed to the synthesis of high quality nanocrystals6 that can be deposited as functional optoelectronic films.7−13 Typically, these materials are synthesized by injecting a decomposable chalcogenide source into ligand-stabilized metal complexes dissolved in a hot organic solvent, yielding monodisperse nanocrystals with narrow singlepeak emissions.6 For PbX (X = S, Se) nanocrystals, the surface chemistry has been well-studied, largely following a hard−soft acid−base rule (HSAB) for ligand replacement.14 This ligand exchange protocol enables its deposition and control on the quantum dot solid’s properties: enhancing carrier mobilities,9,10 shifting band edge levels,11 and managing the degree of surface passivation.12,13 Indeed, initially, this has facilitated its use as photoactive films in photodetectors15 and recently has led to the record photovoltaic device of this class7 as the chargetransporting absorber layer. The use of toxic materials however can limit their widespread deployment. Copper indium sulfide CuInS2 (CIS) is a candidate materialunique in being one of the few materials © 2015 American Chemical Society

with tunable optical properties and being free from Cd and Pb, elements that are restricted because they pose a hazard to health and safety. CIS is already showing promise with efficiencies of 7.04%16 as a sensitizer,16−18 whereas as a quantum dot solid-state unannealed film, it has shown more modest efficiencies of 0.2%,19,20 reaching 1.5% for annealed films.21 The synthesis of CuInS2 nanocrystals has been extensively reviewed,22−24 mainly being derived from the decomposition of dodecanethiol, which results in highly monodisperse, broadly emitting, high quantum yield materials.25−27 Despite this, complete removal of dodecanethiol (Figure S1) by ligand exchange proves to be difficult,16,25 hindering film formation via a low-temperature ligand exchange procedure typical of CQD optoelectronics, thus highlighting the performance gap between devices with CIS as sensitizers and those with CIS as a quantum dot solid. For CuInS2 nanocrystals to be added to the repertoire of nanomaterials in solar cell technology based on colloidal quantum dot solids, it is important to first fabricate conductive and photoconductive solution-processable films. Here, we report a thiol-free synthesis of CuInS2 nanocrystals with tunable emission, describing its reaction, optical properties, structure and composition, and surface chemistry. We deposit the nanocrystals as conducting films, looking into the effect of ligand lengths and charge transport in the film. Finally, we demonstrate its photoresponse as a photodetector and underlying material properties that need to be addressed for its introduction as a viable material for quantum dot photovoltaics. Received: October 12, 2015 Revised: November 30, 2015 Published: November 30, 2015 8424

DOI: 10.1021/acs.chemmater.5b03943 Chem. Mater. 2015, 27, 8424−8432

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Chemistry of Materials

series spin-coater system. Then, while spinning, a drop of 2% vol formic acid in methanol was reacted with the film for another 30 s. The layer ended with 5 drops of methanol and 5 drops of toluene, resulting in 30 nm thick films. Thinner films were achieved by reducing the nanocrystal concentrations. Solutions can be transferred from a nonpolar solvent such as toluene to a polar solvent such as water by exchanging the oleic acid on the surface with MPA. In a typical process, 1 mL of 1 mg/mL of CISZn-OA NCs was mixed with 0.1−1 mL MPA and shaken for 10 min, leading to partial flocculation. To this was added 1 mL of basic water (0.4 g NaOH in 50 mL water), and the solution was again shaken vigorously. It was then centrifuged, which led to a clear, colored polar phase. This was cleaned by two cycles of precipitation with 4 mL of acetone, centrifugation, and redispersion in 1 mL of deionized water. Materials Characterization. The optical properties of materials were determined by absorption (Cary 5000 UV−vis−NIR spectrophotometer) and steady-state and time-resolved photoluminescence spectroscopy (Horiba FL1057) of dilute solutions (1 mg/mL). Structural properties were elucidated by powder X-ray diffraction from a table top source (PANalytical X’Pert PRO) or from a synchrotron source (ALBA, Spain) by Raman spectroscopy (Renishaw InVia) of drop-casted films and by high-resolution transmission electron microscopy (JEOL JEM 2011) of dilute solutions deposited on carbon-coated Cu grids (Ted Pella Inc.). A chemical description of the material was obtained by combining information from the chemical composition of CISZn-OA of various sizes through energy-dispersive X-ray spectroscopy (FEI SEM FEG Inspect), the oxidation states of constituent elements through X-ray photoelectron spectroscopy (ICN2, Spain) analyzed using XPSPeak, and the signatures of organic groups at various stages of material processing through Fourier transform infrared spectroscopy (Agilent Cary 600 series). Device Fabrication and Characterization. Field effect transistors (FETs) were fabricated by depositing 30 nm of the material via the LbL process onto 285 nm thermally grown silica on p-doped silicon. The 50 nm gold source and drain contacts were thermally evaporated (Kurt J Lesker Nano 36) through masks with 1 mm channel widths and 30 μm channel lengths (Ossila evaporation mask). The gate was contacted by scratching the silica layer to reach the pdoped silicon. Individual transistors were isolated from neighboring transistors by scratching away the deposited material. Mobility and majority carrier type of the NC solid were determined from the transfer characteristics (Agilent B1500A) of the FETs. Photoconductive photodetectors were made by depositing 60 nm of the material onto interdigitated gold electrodes. The photodetector was illuminated by a pulsing 635 nm laser light of various intensities between 15 and 520 μW/cm2 (Thorlabs 4-Channel Fiber-Coupled Laser Source, Kiethley 3390 function generator), and the transient rise and decay was recorded (Agilent B1500A).

2. EXPERIMENTAL SECTION Chemicals. Copper(I) iodide (CuI) 99.999%, indium(III) chloride (InCl3) 99.999%, zinc(II) acetate dihydrate (Zn(OAc)2·2H2O) 98%, bis(trimethylsilyl) sulfide (TMS) synthesis grade, trioctylphosphine (TOP) 97%, oleylamine (OlAm) 70%, oleic acid (OA) 90%, 1octadecene (ODE) 90%, formic acid 90%, mercaptopropionic acid (MPA) ≥99%, propionic acid 99.5%, pentanoic acid 99%, and tetradecanoic acid 99% were purchased from Sigma-Aldrich Co., LLC. Apart from OlAm and ODE, all chemicals were used without further purification. OlAm and ODE were dried by heating the chemicals under vacuum at 80 °C for more than 6 h and afterwards kept in a drybox along with TMS for future use. Reagent grade sodium hydroxide, ethanol, acetone, methanol, and toluene were purchased from Panreac Quimica SLU. Synthesis of CIS-Zn-OA 3.2 nm Diameter Nanocrystals. CuI (1.5 mmol, 0.285g) and InCl3 (1.5 mmol, 0.330g) were weighed inside a glovebox and placed in a sealed flask. To this were added 7.5 mL of TOP, 15 mL of ODE, and 9 mL of OlAm, and the flask was connected to a Schlenk line outside the glovebox. This was allowed to react for 30 min at 95 °C under vacuum to form a clear solution. The flask environment was switched to that of argon and raised to 170 °C. At this temperature, 1.5 mmol TMS (310 μL) in 7.5 mL of ODE was injected. Upon injection of the sulfur precursor, the color changed from clear, to yellow, to orange, to red, and finally to black. The flask temperature naturally lowered to 150 °C, and the reaction was continued for 15 min at that temperature. For the product to be separated, CIS-OlAmTOP from the reaction, 5 mL of toluene, and then 70 mL of ethanol were injected into the flask. This mixture was centrifuged at 3500 rpm for 5 min. The supernatant was decanted, and the pellet was redispersed in 6 mL of toluene. In another flask, 3 mmol of Zn(OAc)2·2H2O (0.659g), 6 mL of ODE, and 3 mL of OA were heated to 100 °C for 30 min under vacuum to form a clear solution. The flask environment was switched to that of argon, and 6 mL of the isolated CIS-OlAmTOP NC solution was injected. The environment was returned to a vacuum, and the solution was reacted for 30 min at 100 °C. For the product to be separated, CISZn-OA NCs from the reaction, 5 mL of toluene, and then 40 mL of acetone were injected. The mixture was centrifuged at 3500 rpm for 5 min. The supernatant was decanted, and the pellet was redispersed in 4 mL of toluene. This solution was again centrifuged after which the supernatant was instead collected and the pellet discarded. To this solution was added 10 mL of methanol, and the mixture was centrifuged. Next, the supernatant was discarded, and the pellet was redispersed in 2 mL of toluene and then precipitated with 5 mL of methanol. After centrifugation, the supernatant was discarded, and the pellet was dried with a stream of nitrogen, redispersed in 2 mL of toluene, and centrifuged. Afterwards, the supernatant was kept. This usually results in solutions at 85 mg/mL concentrations. Synthesis of CIS-Zn-OA Nanocrystals with Diameters Smaller than 3.2 nm. Keeping the same quantities of reagents and reaction times, we reduced the particle size by lowering the injection temperature of the sulfur precursor. These injection temperatures ranged from 50 to 150 °C at 20 °C intervals and resulted in final solutions that were light yellow to dark red. At lower injection temperatures, the nanocrystals were grown at the temperature where the solution naturally cools at injection and were treated with zinc oleate at their growth temperature if this temperature was less than 100 °C. Being smaller particles, the products were isolated by a higher nonsolvent to solvent ratio. The quantities used are tabulated in Table S1. Ligand Exchange and Phase Transfer. Films of this material were deposited via a layer-by-layer (LbL) solid state ligand exchange process on various substrates, such as glass, 285 nm thermally grown silica on degenerately p-doped silicon (Si-Mat), or interdigitated gold electrodes on glass cleaned by sequential 15 min sonication in soapy water, acetone, and isopropanol and dried with a stream of nitrogen. The nanocrystal solutions were diluted to a concentration of 25 mg/ mL. The layer started by spin-coating the material on selected substrates at 2000 rpm for 30 s using a Specialty Coating Systems 6800

3. RESULTS AND DISCUSSION Effect of Ligands and Zn Treatment. Dodecanethiol (DDT) has been used in CIS synthesis acting both as a sulfur precursor and as a stabilizing ligand.25−27 However, we have found it difficult to remove the long DDT ligand during solidstate post processing (Figure S1) as the thiol may be participating in a crystal-bound or X-type ligand coordination mode.28,29 We are therefore interested in developing an NC synthesis that does not employ thiols as a ligand or sulfur source. Concomitantly, Allen and Bawendi30 had reported a synthesis for CuInSe2 that did not use DDT, instead using the chalcogen precursor bis(trimethylsilyl) selenide and the stabilizing ligands OlAm and TOP. We found through control synthetic experiments that TOP inhibits the reactivity of copper, whereas OlAm inhibits the reactivity of indium and uncontrolled NC growth (Figure S2). It has been reported that copper-rich CIS is plasmonic31 and 8425


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Chemistry of Materials that the incorporation of In atoms dampens the plasmon peak in Cu2S nanoparticles.32 That TOP affects the reactivity of Cu is corroborated by the presence of Cu2−xS signals in the Raman spectrum of particles synthesized in the absence of TOP. The appearance of Cu2−xS is accompanied by the emergence of a plasmonic peak in the absorption spectra that reduces with increasing TOP. Without OlAm, however, red indium-rich particles, which form 200−400 nm aggregates, precipitate out of solution. The combination of TOP and OlAm provide a balance in the reactivities of Cu and In to form CIS nanoparticles. The as-synthesized product, CIS-OlAmTOP, however, is chemically and colloidally unstable. Over time, it reduced in photoluminescence quantum yield and increased in free carrier absorption (Figures S3 and S4), probably due to the etching of NCs by OlAm30 or the catalytic oxidation of nanocrystals in the presence of amines.25 These particles become insoluble after multiple precipitations and redispersions and as such cannot be cleaned to form a stable colloid. To make these chemically stable and dispersible, we treated the surface of these particles with zinc oleate. These particles, kept in a glovebox, remain soluble and maintain their PLQY even after a year (Table S2). Zinc treatment of CIS NCs has been reported to blue-shift the absorption spectrum due to the formation of shells,33 to increase PLQY and lifetime,27 or to form alloys of copper indium zinc sulfide as seen in shifts in the X-ray diffractograms.34 In trying to replace DDT, five molecules were introduced to substitute the functions it performed: as solvent (ODE), to control reactivities (TOP, OlAm), as a sulfur source (TMS), and as a stabilizing ligand (ZnOA-subsequent after Zn treatment). Size and Crystallinity. CuInS2 occurs in three crystal structures: hexagonal wurtzite, cubic zincblende, and tetragonal chalcopyrite with the last being the thermodynamically stable phase at room temperature in the bulk. Even so, these unstable phases have been reported to exist in nanocrystalline form.35,36 The X-ray diffractograms (Figure 1a, in Q (Å−1), where Q = 4π/λsin θ) indicate a crystal structure that is either cubic or tetragonal. Although it is difficult to distinguish between these two phases, we believe that these particles are in the cubic phase. The diffractograms lack the reflection lower than 15 Å−1 (Figure S5),36 which is a characteristic peak for chalcopyrite (JCPDS 85-1575). Further, as is shown later, these particles exhibit off-stoichiometry, which is more allowed in a cubic structure.35 These diffractograms exclude the possibility of the formation of Cu2−xS and β-In2S3 in the product, as was also confirmed in Raman spectroscopy (Figure 1b) by the absence of peaks referring to these sulfides. The Raman spectra show main overlapped peaks at 305 and 347 cm−1 modes, which signify a disordered CIS with a copper deficient CuAu structure, which is downshifted from the usual 290 cm −1 for chalcopyrites.37 It is, however, difficult to distinguish the presence of ZnS as its peaks overlap with that of CIS, though no distinguishable change is observed between CIS-OlAmTOP and CIS-ZnOA Raman spectra. In our Zn-treatment, blue-shifts in the absorption spectra (Figure S4) and shifts to higher Q values in XRD peak position (Figure 1a) occur more prominently with materials synthesized and Zn-treated at lower temperatures. The PLQY for shelled materials is also increased (Table S2). We believe that this indicates that for larger particles zinc binds to the surface, allowing for passivation as seen through higher PLQY and

Figure 1. (a) XRD data from a synchrotron source and from a tabletop equipment (bottom-most) showing cubic peaks and peak broadening. Calculated diameters are placed in-figure and colors correspond to synthesis temperatures, from blue, 50°C to black, 170°C. (b) Raman spectra of as-synthesized and zinc-treated CIS nanocrystals. (c) TEM micrograph of CIS-ZnOA nanocrystals synthesized at 170C, (111) planes shown. (d) SAXS diffractograms for materials synthesized at different temperatures.

reduction of short lifetime component, for the binding of oleic acid for dispersibility, and that for smaller sizes this leads to partial alloying of the material with zinc. The line widths of the parent peak at (111) in wide angle Xray diffractograms using a synchrotron source (Figure 1a) increase with decreasing synthesis temperature. Using the Scherrer equation, we obtain diameters ranging from 2.0 to 3.3 nm. This calculated diameter of 3.3 nm agrees well with the size calculation of 3.2 nm from XRD data using a table top source and as well with TEM sizing (Figure S6), providing an average value of 3.2 nm (Figure 1c), showing clear (111) planes. XRD sizing by the Schrerrer equation however can lead to an underestimation of sizes due to material strain and is more prone to error for much smaller sizes (as in Figure 2d). The changes in size can also be observed in defined peaks in the SAXS data, which shift according to synthesis temperature (Figure 1d). Absorption and Photoluminescence. The absorption and photoluminescence of CISZn-OA correlate with changes in size, shifting to longer wavelengths with higher synthesis temperatures. If the bandgap of these materials is calculated from the Tauc plot25,26 (Figure S7), it can be tuned from a confined value of 2.51 eV (synthesized at 50 °C) to its bulk 8426


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Further, the photoluminescence excitation spectra of samples probed at short and long wavelengths of the emission spectrum (Figure S8) overlap well with the absorption spectra and suggest the presence of an emitting species with a single modal size distribution. The relationship of NC diameters and bandgap for selected ternary sulfides has been theoretically calculated using an effective mass approximation (EMA) with finite-depth wells by Omata et al.43 and Zhong et al.,26 demonstrating good agreement with their experimental data from synthesized CIS NCs using the Tauc plot onset as the bandgap with exciton binding energies in the range of 32−95 meV.26 Figure 2d shows the dependence of CISZn-OA diameters with the Tauc plot onset and how it deviates from the theoretical calculation. In fact, although the exciton Bohr diameter of CIS has been calculated to be 8 nm,22,44 we see that the bandgap of the materials presented in this work to be at their bulk value even at 3.3 nm. Although it seems that there would be a better fit to the EMA prediction using the excitonic peak as the definition for the bandgap, these particles are Cu-poor and Zn-alloyed and a much higher bandgap is expected; these particles possess much narrower bandgaps for their corresponding size. This discrepancy suggests large exciton binding energies given that the exciton binding energy is the difference between the electronic bandgap with the optical bandgap.26 Composition and Oxidation States. Figure 3a shows the high resolution X-ray photoelectron spectra for Cu, In, Zn, and S. Here, the metals and sulfur are in their correct oxidation states and agree well with the reported binding energies of CIS (NIST standard). A better fit for the high resolution XPS spectrum of In and Zn is obtained if two species are introduced and suggests that the surfaces are In- and Zn-rich. Though it has been reported that the bandgap, emission peaks, and PLQY of ternary nanocrystals can be tuned by changing the NC stoichiometry,45 we observe that these particles maintain a near constant Cu/In replacement ratio throughout different synthesis temperatures (Table 1, EDX spectra Figure S9) and that it is their size that affects optical properties. With a model wherein a copper-deficient core is decorated by surface indium ions (calculations in Table S3), we see little variation in the calculated stoichiometry change through copper replacement by indium (18−24%) in the range of 90−170 °C injection temperatures. The apparent change in stoichiometry with bandgap is an effect of the size with indiumrich, indium-decorated materials decreasing in Cu/In ratio with decreasing size by the influence of a greater surface to volume ratio. This trend is also seen for copper-rich particles26 (increasing Cu/In ratio with decreasing size). This model is a logical assertion: as 2 Vcu′ + InCu●● is a common defect formation in chalcopyrite materials, as there is little change in the copper content after zinc treatment and as zinc replaces more indium for smaller particles (from 16 to 35%). Although the total amount of zinc increases as the particles become smaller, as an effect of the incorporation of zinc through the surface, the amount of zinc on the surface is lowered for smaller particles. This agrees with the supposition that alloying occurs for smaller particles, as pointed out in the parent peak shifts in Figure 1a. This is shown in Scheme 1, wherein a copperdeficient core decorated with indium atoms is treated with zinc oleate. Ligand Exchange and Phase Transfer. Prior to Zn treatment, these particles are coordinated with OlAm (Figure 3b), showing a characteristic N−H peak at 3360 cm−1. TOP

Figure 2. The optical properties of CIS quantum dots can be tuned. (a) Photograph of CIS-ZnOA synthesized at various temperatures showing changes in color. (b) The absorption spectra of CIS and their corresponding photoluminescence spectra overlaid. The spectra are shifted for clarity. (c) Calculated bandgap, PL peak, and excitonic peak positions. (d) The difference between theoretical bandgap and experimental bandgap with varying size.

value of 1.54 eV (170 °C) (Figure 2a, b). Furthermore, measures of the bandgap using other definitions, such as the excitonic peak27 or the sharpest onset, were calculated (Figure 2c, d and Figure S7). The absorption profiles are characterized by a broad excitonic peak (whose position is calculated from the first derivative zero value or first derivative minimum if this does not exist) that appears as a shoulder, similar to previous reports of CIS NCs, despite having narrow size distributions.25−27 The origin of this broad absorption peak has been attributed to slight variations in nanocrystal shapes, differences in nanocrystal to nanocrystal composition, or a distinct electronic feature inherent to CIS NCs due to an overlap of near edge transitions.25 Shabaev et al. calculated from theoretical models that hole levels near the band edge in CIS contribute to long absorption tails that lessen for smaller NCs.38 We observe a distinguishable excitonic peak for materials synthesized at lower temperatures. This supports the prediction by Shabaev or marks a size regime of stronger quantum confinement. These materials are also luminescent with fwhm >100 nm and large Stokes shifts in the range of 130−160 nm following a reduction in emission redshifted with decreasing bandgaps (Figure 2c). Photoluminescence quantum yields (PLQY) can reach as high as 20% with average PL lifetimes of 0.27 μs (Figure S3 and Table S2). These NCs have quantum yields lower than the highest reported value of 80%27 but exhibit one of the higher PL lifetimes among those previously reported (14026−190 ns27). Broad emissions, large Stokes shifts, and long PL lifetimes have been attributed to the emission of CIS NCs via a donor−acceptor pair.25,39,40 However, because of the extent that the emission peak position can be tuned (>1 eV), it is likely that these photoluminescence properties instead originate from donor-level to quantized-valence band,27 quantized-conduction band to acceptor-level transitions,41,42 or from the inherent band structure of chalcopyrites.38 8427


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disappearance of the C−H vibration modes at 2700−3100 cm−1. Furthermore, the peak at ν > 3000 cm−1, which is characteristic of CC−H vibrations present in oleic acid, is reduced with treatments. These nanocrystals have mostly zinc and indium at the surface, which are classified as hard metals according to the HSAB theory.14 Hard ligands remove most of the native OA on the surface with a 94% peak height reduction after a 3 min immersion in formic acid (Figure 3c). In contrast, ethylenedithiol, being a soft base thiolate source, results in a much weaker ligand replacement, whereas ethylenediamine, being a harder ligand, leads to almost complete removal of native ligands (Figure S10). With this, it is possible to transfer these nanocrystals into a polar phase using a bifunctional molecule, such as MPA, for different NC sizes (Figure S10). Through control experiments, using a dithiol, a dicarboxylic, acid and a thiol-carboxylic acid, and monitoring the precipitation, we found that it is the acid group that binds to the surface of the nanocrystals upon phase transfer with the deprotonated thiol group exposed to water. Ethanolic solutions maintain their luminescence properties (Figure S5) and may cater to biolabeling applications as a nonrestricted alternative material containing no toxic elements. Photoluminescence Decay with Decreasing Ligand Length. The effect of nanocrystal to nanocrystal distance on the photoluminescence properties was probed by looking at the steady-state and time-resolved photoluminescence spectra of CIS, synthesized at 100 °C, in dilute solution and in a dried drop-casted film. There is a photoluminescence redshift of 50 nm from solution to solid state (Figure 4a), which has been noted in PbS films as an indication of good electronic coupling between NCs due to an overlap of wave functions between adjacent nanocrystals.46 The time-resolved photoluminescence spectra of CIS in solution are invariant when probed at different wavelengths in the emission spectrum, whereas for CIS in film, photoluminescence decays are faster for shorter wavelengths (Figure 4b). This points to a possible energy transfer from smaller to larger sized crystals within a size distribution of a single sample47 or from the ensemble of higher energy distribution to lower ones in a small band of degenerate excited states. Further, we saw a characteristic rise time in the PL decay (Figure S11) when the emission was probed at longer wavelengths of the emission, supporting resonant energy transfer from within size distributions within the NC solid assembly.47 To further elaborate on this, we looked at the photoluminescence of films treated with carboxylic acids of various ligand lengths, keeping the same solution concentration for spin coating. Figure 4c shows the steady-state photoluminescence of CIS films exchanged with tetradecanoic, pentanoic, propionic, and formic acid. A redshift in emission appears and photoluminescence decreases with the decrease in molecule length and, consequently, NC to NC distance. Photoluminescence lifetimes decrease with ligand length as also

Figure 3. Surface properties of CIS nanocrystals. (a) X-ray photoelectron spectra of Zn-treated CIS nanocrystals demonstrating the constituent elements in their correct oxidation states. Zn and In have two species present in the material, which can be interior and surface atoms. (b) FTIR spectra of as-synthesized and Zn-treated nanocrystals. (c) The removal of oleic acid as seen through the reduction of the C−H and CC−H peaks.

does not have any unique peak that could be assigned in the spectra apart from weak signals at 2663 cm−1. However, we do see the presence of phosphorus in the EDX spectra of CISOlAmTOP (Figure S9). After zinc oleate treatment, N−H and TOP peaks vanish (P vanishes from the XRD, Figure S9), whereas the symmetric and assymmetric C−O vibration modes appear, suggesting their replacement with a deprotonated OA and that OA binds in an ionic binding mode, inferred by comparing νsym-νas to its salt. By spin-coating the material on double-sided polished silicon and exposing the film to various ligands, we monitored the extent of ligand removal through the

Table 1. Composition of CIS for Various Synthesis Temperatures and Zn Treatments 90 °C Cu Zn In S

130 °C

170 °C

Zn treatment

as synthesized

Zn treatment

as synthesized

Zn treatment

as synthesized

0.20 0.94 0.75 2.11

0.23

0.33 0.56 0.9 2.2

0.30

0.48 0.43 0.98 2.12

0.50

1.92 1.85

8428

2.07 1.63

1.36 2.14 DOI: 10.1021/acs.chemmater.5b03943 Chem. Mater. 2015, 27, 8424−8432

Article

Chemistry of Materials Scheme 1. Indium-Decorated Copper-Poor Cores Stabilized with TOP and OlAm Treated with Zinc Oleate

Lingley et al, nonradiative resonant energy transfer dominates with low transfer rates, and that for short distances, coherent tunneling dominates through the overlap of wave functions leading to higher transfer rates.47 Apart from a reduction in NC spacing, coherent tunneling in NC assemblies is favored when a high coupling energy is matched with a low exciton binding energy arising from high dielectric constants. We do not see any transition between these two regimes in the range of distances studied. The transition to a coherent charge transfer mechanism by tunneling is perhaps limited by high exciton binding energies, which is a deterrence to dissociated carriers. Evidence of high exciton binding energies has been seen in the discrepancy in optical bandgap and theoretical (Figure 2c). Conductivity Type and Mobility. Nonetheless, these films conduct, though they have low carrier mobilities and moderate carrier concentrations. We were able to fabricate field effect transistors (FETs) (Figure 5a) to extract the carrier mobility. Figure 5b shows the transfer characteristics of a CIS synthesized at 170, 130, and 90 °C for NC-FETs with formic acid as the ligand. Modulation occurs at negative gate voltages and indicates that the film has p-type conductivity, which is expected for CIS semiconducting films. We obtained a hole carrier mobility of 1.9 × 10−5 cm2 V−1 s−1 for CIS synthesized at 170 °C. This mobility is 2 orders of magnitude lower than that for EDT-treated PbS films9 and 3 orders lower than that made from chalcopyrite films containing Se.51 These films show low carrier concentrations of 5.7 × 1016 cm−3 despite the large off-stoichiometry of the films. Earlier reports have shown that mobility decreases with nanocrystal size for the same nanocrystal to nanocrystal distance, which has been ascribed to increased hopping phenomena.9 However, Yazdani et al.52 demonstrated that, for small enough internanoparticle spacing, significant overlap of wave functions will lead to higher mobilities for assemblies from smaller particles and that a decrease in overall mobility might arise from trapping. As a Photodetector. We further evaluated the optoelectronic properties of the CIS film by constructing photoconductors whose structure is shown in Figure 5c. We subjected the devices to 200 s laser light pulses at varying light intensities. The measured photocurrent I is linear with light intensity P and follows power law I ∝ Pb with a sublinear trend b = 0.7 (Figure 5d). For 0.5 > b > 1, the electron−hole generation in the semiconductor is dominated by trapping and recombination;53 furthermore, the responsivity tends to saturate at higher intensity, wherein the available number of trap states is lowered due to trap filling.54 Indeed, we see in Figure 5d that the responsivity decreases with higher intensities and levels off at 250 μW/cm2. We normalized the photocurrent for different light intensities to show the asymmetry in rise and

Figure 4. The effect of interparticle spacing on the photoluminescence of CIS assemblies. (a) A significant redshift between CIS nanocrystals synthesized at 110 °C in solution and as a dropcast film. (b) Timeresolved photoluminescence probed at various points (short to long wavelength) in the emission spectra; data shown are taken from solid circles in (a). A reconstruction of the steady state emission was achieved by integration of (b) with time. (c) Photoluminescence decreases with ligand length and (d) follows the Förster relationship.

observed in PbS assemblies.48 We found that the transfer rates (kFRET = 1/τfilm − 1/τsolution) of these CIS films decrease with ligand length following a linear trend with sixth power of distance (Figure 4d), agreeing with the Förster relationship

⎛ R ⎞6 kFRET = k isolated⎜ 0 ⎟ ⎝ d ⎠ where kisolated = 1/τsolution, d is the center to center distance between two species, and R0 is the Förster radius. A Förster radius of 5.8 nm was calculated, which is comparable to that reported for PbS (5 nm49) and CdSe (8 nm50). Choi et al. has proposed a model for transport in two distance regimes in PbS assembliesthat for large distances, such as observed by 8429


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Figure 5. Transport and photoconductive properties of CIS NC films. (a) Device structure for bottom-gated top-contacted field effect transistors with 10 μm channel width and 1 mm channel length. (b) Transfer characteristics of CIS NC FET. (c) Device structure for CIS photodetectors with 3 mm length and 50 μm width. (d) The material shows a sublinear increase in photocurrent and a decrease in responsivity with light intensity. (e) Longer rise and decay times are observed for decreasing light intensity.

■

decay response and the differences in rise and decay times at different intensities (Figure 5e). For materials that do not have traps, the rise and decay times should be invariant with light intensity. The presence of traps increases the rise and decay times observed as when light intensity is decreased because trapped carrier density outweighs free carrier density when traps remain unfilled.55 The sublinearity of photocurrent with light intensity, saturation of responsivity, and long response times that increase with decreasing light intensity describe an NC solid film having a high trap density.

ASSOCIATED CONTENT

S Supporting Information *

Additional phase transfer details. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b03943. Additional experimental details, photoluminescence lifetimes, excitation spectra, Raman and absorption spectra, and composition of synthesis variations, images of phase transfers, and rise and decay times; LbL processes and phase transfer on DDT-based nanocrystals; size distribution by TEM sizing; Tauc plot, first derivative of the absorption curve, and XRD sizing by secondary peaks; and EDX spectra (PDF)

4. CONCLUSIONS In summary, we reported the synthesis of copper-poor CIS NCs with controllable absorption and emission, which can be deposited as films through a solid-state ligand exchange with shorter carboxylic acids. However, these particles show bulk bandgap values below the exciton Bohr radius and may indicate a high exciton binding energy. We showed that shorter interparticle distances imposed a redshift in the photoluminescence and a decrease in photoluminescence lifetimes following the Förster relationship for energy transfer. We proposed that no coherent charge transfer such as tunneling occurs and that the high exciton binding energies inhibit the dissociation of carriers. To further investigate the transport in the film, we fabricated field effect transistors from the CIS NC film and saw p-type conductivity with low carrier mobilities and moderate carrier concentrations. Mobility was seen to decrease with NC size, which had been described as an indicator of trapping. This trapping was evidenced as well in the behavior of photoconductors made from the NC film: sublinearity of photocurrent with light intensity, saturation of responsivity, and long response times. Copper indium sulfide is an alternative to existing Cd- and Pb-containing nanocrystals used in optoelectronics. Although we have reported its synthesis, its formation into films and its fabrication into field effect transistors and photodetectors, the quantum dot solid still suffers from low carrier mobilities, high exciton binding energies, and high trap density, which remain as challenges for its use as a competitive environmentally friendly CQD alternative.

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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 The research leading to these results has received funding from Fundació Privada Cellex, and European Community's Seventh Framework program (FP7-ENERGY.2012.10.2.1) under grant agreement 308997. We also acknowledge Financial support from the Spanish Ministry of Economy and Competitiveness (MINECO) and the “Fondo Europeo de Desarrollo Regional” (FEDER) through grant MAT2014-56210-R. This work was also supported by AGAUR under the SGR grant (2014SGR1548). G.K. acknowledges the Ramon y Cajal Fellowship. Diffraction experiments were performed at BL11NCD beamline at ALBA Synchrotron with the collaboration of ALBA staff. We are also thankful to Maria Bernechea for fruitful discussions.

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REFERENCES

(1) Kovalenko, M. V.; Manna, L.; Cabot, A.; Hens, Z.; Talapin, D. V.; Kagan, C. R.; Klimov, V. I.; Rogach, A. L.; Reiss, P.; Milliron, D. J.; Guyot-Sionnnest, P.; Konstantatos, G.; Parak, W. J.; Hyeon, T.;

8430


Article


Processed Nanocrystalline CuInS2 Solar Cells. ACS Nano 2015, 9, 5857−5867. (22) Kolny-Olesiak, J.; Weller, H. Synthesis and Application of Colloidal CuInS2 Semiconductor Nanocrystals. ACS Appl. Mater. Interfaces 2013, 5, 12221−12237. (23) Aldakov, D.; Lefrançois, A.; Reiss, P. Ternary and Quaternary Metal Chalcogenide Nanocrystals: Synthesis, Properties and Applications. J. Mater. Chem. C 2013, 1, 3756−3776. (24) Liu, S.; Su, X. The Synthesis and Application of I−III−VI Type Quantum Dots. RSC Adv. 2014, 4, 43415−43428. (25) Xie, R.; Rutherford, M.; Peng, X. Formation of High-Quality I− III−VI Semiconductor Nanocrystals by Tuning Relative Reactivity of Cationic Precursors. J. Am. Chem. Soc. 2009, 131, 5691−5697. (26) Zhong, H.; Lo, S. S.; Mirkovic, T.; Li, Y.; Ding, Y.; Li, Y.; Scholes, G. D. Noninjection Gram-Scale Synthesis of Monodisperse Pyramidal CuInS2 Nanocrystals and Their Size-Dependent Properties. ACS Nano 2010, 4, 5253−5262. (27) Li, L.; Pandey, A.; Werder, D. J.; Khanal, B. P.; Pietryga, J. M.; Klimov, V. I. Efficient Synthesis of Highly Luminescent Copper Indium Sulfide-Based Core/Shell Nanocrystals with Surprisingly Long-Lived Emission. J. Am. Chem. Soc. 2011, 133, 1176−1179. (28) Turo, M. J.; Macdonald, J. E. Crystal-Bound vs Surface-Bound Thiols on Nanocrystals. ACS Nano 2014, 8, 10205−10213. (29) Dierick, R.; Van den Broeck, F.; De Nolf, K.; Zhao, Q.; Vantomme, A.; Martins, J. C.; Hens, Z. Surface Chemistry of CuInS2 Colloidal Nanocrystals, Tight Binding of L-Type Ligands. Chem. Mater. 2014, 26, 5950−5957. (30) Allen, P. M.; Bawendi, M. G. Ternary I−III−VI Quantum Dots Luminescent in the Red to Near-Infrared. J. Am. Chem. Soc. 2008, 130, 9240−9241. (31) Niezgoda, J. S.; Yap, E.; Keene, J. D.; McBride, J. R.; Rosenthal, S. J. Plasmonic CuxInyS2 Quantum Dots Make Better Photovoltaics Than Their Nonplasmonic Counterparts. Nano Lett. 2014, 14, 3262− 3269. (32) Wang, X.; Swihart, M. T. Controlling the Size, Shape, Phase, Band Gap, and Localized Surface Plasmon Resonance of Cu2−xS and CuxInyS Nanocrystals. Chem. Mater. 2015, 27, 1786−1791. (33) Park, J.; Kim, S.-W. CuInS2/ZnS Core/Shell Quantum Dots by Cation Exchange and Their Blue-Shifted Photoluminescence. J. Mater. Chem. 2011, 21, 3745−3750. (34) De Trizio, L.; Prato, M.; Genovese, A.; Casu, A.; Povia, M.; Simonutti, R.; Alcocer, M. J.; D’Andrea, C.; Tassone, F.; Manna, L. Strongly Fluorescent Quaternary Cu−In−Zn−S Nanocrystals Prepared from Cu1‑xInS2 Nanocrystals by Partial Cation Exchange. Chem. Mater. 2012, 24, 2400−2406. (35) Lei, S.; Wang, C.; Liu, L.; Guo, D.; Wang, C.; Tang, Q.; Cheng, B.; Xiao, Y.; Zhou, L. Spinel Indium Sulfide Precursor for the PhaseSelective Synthesis of Cu−In−S Nanocrystals with Zinc-Blende, Wurtzite, and Spinel Structures. Chem. Mater. 2013, 25, 2991−2997. (36) Pan, D.; An, L.; Sun, Z.; Hou, W.; Yang, Y.; Yang, Z.; Lu, Y. Synthesis of Cu−In−S Ternary Nanocrystals with Tunable Structure and Composition. J. Am. Chem. Soc. 2008, 130, 5620−5621. (37) Oja, I.; Nanu, M.; Katerski, A.; Krunks, M.; Mere, A.; Raudoja, J.; Goossens, A. Crystal Quality Studies of CuInS2 Films Prepared by Spray Pyrolysis. Thin Solid Films 2005, 480, 82−86. (38) Shabaev, A.; Mehl, M. J.; Efros, A. L. Energy Band Structure of CuInS2 and Optical Spectra of CuInS2 Nanocrystals. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 92, 035431. (39) Zhong, H.; Zhou, Y.; Ye, M.; He, Y.; Ye, J.; He, C.; Yang, C.; Li, Y. Controlled Synthesis and Optical Properties of Colloidal Ternary Chalcogenide CuInS2 Nanocrystals. Chem. Mater. 2008, 20, 6434− 6443. (40) Castro, S. L.; Bailey, S. G.; Raffaelle, R. P.; Banger, K. K.; Hepp, A. F. Synthesis and Characterization of Colloidal CuInS2 Nanoparticles from a Molecular Single-Source Precursor. J. Phys. Chem. B 2004, 108, 12429−12435. (41) Omata, T.; Nose, K.; Kurimoto, K.; Kita, M. Electronic Transition Responsible For Size-Dependent Photoluminescence of

Korgel, B. A.; Murray, C. B.; Heiss, W. Prospects of Nanoscience with Nanocrystals. ACS Nano 2015, 9, 1012−1057. (2) Talapin, D. V.; Lee, J.-S.; Kovalenko, M. V.; Shevchenko, E. V. Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications. Chem. Rev. 2010, 110, 389−458. (3) Kairdolf, B. A.; Smith, A. M.; Stokes, T. H.; Wang, M. D.; Young, A. N.; Nie, S. Semiconductor Quantum Dots for Bioimaging and Biodiagnostic Applications. Annu. Rev. Anal. Chem. 2013, 6, 143−62. (4) Wood, V.; Bulović, V. Colloidal Quantum Dot Light-Emitting Devices. Nano Rev. 2010, 1, 1. (5) Yang, Y.; Zheng, Y.; Cao, W.; Titov, A.; Hyvonen, J.; Manders, J. R.; Xue, J.; Holloway, P. H.; Qian, L. High-Efficiency Light-Emitting Devices Based on Quantum Dots with Tailored Nanostructures. Nat. Photonics 2015, 9, 259−266. (6) Hines, M. A.; Scholes, G. D. Colloidal PbS Nanocrystals with Size-Tunable Near-Infrared Emission: Observation of Post-Synthesis Self-Narrowing of the Particle Size Distribution. Adv. Mater. 2003, 15, 1844−1849. (7) Carey, G. H.; Abdelhady, A. L.; Ning, Z.; Thon, S. M.; Bakr, O. M.; Sargent, E. H. Colloidal Quantum Dot Solar Cells. Chem. Rev. 2015, 150624090720003. (8) Chuang, C.-H. M.; Brown, P. R.; Bulović, V.; Bawendi, M. G. Improved Performance and Stability in Quantum Dot Solar Cells through Band Alignment Engineering. Nat. Mater. 2014, 13, 796−801. (9) Liu, Y.; Gibbs, M.; Puthussery, J.; Gaik, S.; Ihly, R.; Hillhouse, H. W.; Law, M. Dependence of Carrier Mobility on Nanocrystal Size and Ligand Length in PbSe Nanocrystal Solids. Nano Lett. 2010, 10, 1960−1969. (10) Zhang, J.; Tolentino, J.; Smith, E. R.; Zhang, J.; Beard, M. C.; Nozik, A. J.; Law, M.; Johnson, J. C. Carrier Transport in PbS and PbSe QD Films Measured by Photoluminescence Quenching. J. Phys. Chem. C 2014, 118, 16228−16235. (11) Brown, P. R.; Kim, D.; Lunt, R. R.; Zhao, N.; Bawendi, M. G.; Grossman, J. C.; Bulović, V. Energy Level Modification in Lead Sulfide Quantum Dot Thin Films through Ligand Exchange. ACS Nano 2014, 8, 5863−5872. (12) Ip, A. H.; Thon, S. M.; Hoogland, S.; Voznyy, O.; Zhitomirsky, D.; Debnath, R.; Levina, L.; Rollny, L. R.; Carey, G. H.; Fischer, A. Hybrid Passivated Colloidal Quantum Dot Solids. Nat. Nanotechnol. 2012, 7, 577−582. (13) Jeong, K. S.; Tang, J.; Liu, H.; Kim, J.; Schaefer, A. W.; Kemp, K.; Levina, L.; Wang, X.; Hoogland, S.; Debnath, R. Enhanced Mobility-Lifetime Products in PbS Colloidal Quantum Dot Photovoltaics. ACS Nano 2012, 6, 89−99. (14) Nag, A.; Kovalenko, M. V.; Lee, J.-S.; Liu, W.; Spokoyny, B.; Talapin, D. V. Metal-free Inorganic Ligands for Colloidal Nanocrystals: S2−, HS−, Se2−, HSe−, Te2−, HTe−, TeS32−, OH−, and NH2−as Surface Ligands. J. Am. Chem. Soc. 2011, 133, 10612−10620. (15) Konstantatos, G.; Howard, I.; Fischer, A.; Hoogland, S.; Clifford, J.; Klem, E.; Levina, L.; Sargent, E. H. Ultrasensitive SolutionCast Quantum Dot Photodetectors. Nature 2006, 442, 180−183. (16) Pan, Z.; Mora-Seró, I.; Shen, Q.; Zhang, H.; Li, Y.; Zhao, K.; Wang, J.; Zhong, X.; Bisquert, J. High-Efficiency “Green” Quantum Dot Solar Cells. J. Am. Chem. Soc. 2014, 136, 9203−9210. (17) Jara, D. H.; Yoon, S. J.; Stamplecoskie, K. G.; Kamat, P. V. SizeDependent Photovoltaic Performance of CuInS2 Quantum DotSensitized Solar Cells. Chem. Mater. 2014, 26, 7221−7228. (18) McDaniel, H.; Fuke, N.; Pietryga, J. M.; Klimov, V. I. Engineered CuInSexS2−x Quantum Dots for Sensitized Solar Cells. J. Phys. Chem. Lett. 2013, 4, 355−361. (19) Scheunemann, D.; Wilken, S.; Parisi, J.; Borchert, H. Towards Depleted Heterojunction Solar Cells with CuInS2 and ZnO Nanocrystals. Appl. Phys. Lett. 2013, 103, 133902. (20) Borchert, H.; Scheunemann, D.; Frevert, K.; Witt, F.; Klein, A.; Parisi, J. Schottky Solar Cells with CuInS2 Nanocrystals as Absorber Material. Z. Phys. Chem. 2015, 229, 191−203. (21) Halpert, J. E.; Morgenstern, F. S.; Ehrler, B.; Vaynzof, Y.; Credgington, D.; Greenham, N. C. Charge Dynamics in Solution8431


Article

Chemistry of Materials Colloidal CuInS2 Quantum Dots. J. Mater. Chem. C 2014, 2, 6867− 6872. (42) Liu, W.; Zhang, Y.; Zhai, W.; Wang, Y.; Zhang, T.; Gu, P.; Chu, H.; Zhang, H.; Cui, T.; Wang, Y. Temperature-Dependent Photoluminescence of ZnCuInS/ZnSe/ZnS Quantum Dots. J. Phys. Chem. C 2013, 117, 19288−19294. (43) Omata, T.; Nose, K.; Otsuka-Yao-Matsuo, S. Size Dependent Optical Band Gap of Ternary I-III-VI2 Semiconductor Nanocrystals. J. Appl. Phys. 2009, 105, 073106. (44) Czekelius, C.; Hilgendorff, M.; Spanhel, L.; Bedja, I.; Lerch, M.; Müller, G.; Bloeck, U.; Su, D.-S.; Giersig, M. A Simple Colloidal Route to Nanocrystalline ZnO/CuInS2 Bilayers. Adv. Mater. 1999, 11, 643− 646. (45) Chen, B.; Zhong, H.; Zhang, W.; Tan, Z.; Li, Y.; Yu, C.; Zhai, T.; Bando, Y.; Yang, S.; Zou, B. Highly Emissive and Color-Tunable CuInS2-Based Colloidal Semiconductor Nanocrystals: Off-Stoichiometry Effects and Improved Electroluminescence Performance. Adv. Funct. Mater. 2012, 22, 2081−2088. (46) Zarghami, M. H.; Liu, Y.; Gibbs, M.; Gebremichael, E.; Webster, C.; Law, M. p-Type PbSe and PbS Quantum Dot Solids Prepared with Short-Chain Acids and Diacids. ACS Nano 2010, 4, 2475−2485. (47) Clark, S. W.; Harbold, J. M.; Wise, F. W. Resonant Energy Transfer in PbS Quantum Dots. J. Phys. Chem. C 2007, 111, 7302− 7305. (48) Lingley, Z.; Lu, S.; Madhukar, A. The Dynamics of Energy and Charge Transfer in Lead Sulfide Quantum Dot Solids. J. Appl. Phys. 2014, 115, 084302. (49) Choi, J. J.; Luria, J.; Hyun, B.-R.; Bartnik, A. C.; Sun, L.; Lim, Y.F.; Marohn, J. A.; Wise, F. W.; Hanrath, T. Photogenerated Exciton Dissociation in Highly Coupled Lead Salt Nanocrystal Assemblies. Nano Lett. 2010, 10, 1805−1811. (50) Mork, A. J.; Weidman, M. C.; Prins, F.; Tisdale, W. A. Magnitude of the Förster Radius in Colloidal Quantum Dot Solids. J. Phys. Chem. C 2014, 118, 13920−13928. (51) Draguta, S.; McDaniel, H.; Klimov, V. I. Tuning Carrier Mobilities and Polarity of Charge Transport in Films of CuInSexS2−x Quantum Dots. Adv. Mater. 2015, 27, 1701−1705. (52) Yazdani, N.; Bozyigit, D.; Yarema, O.; Yarema, M.; Wood, V. Hole Mobility in Nanocrystal Solids as a Function of Constituent Nanocrystal Size. J. Phys. Chem. Lett. 2014, 5, 3522−3527. (53) Li, L.; Auer, E.; Liao, M.; Fang, X.; Zhai, T.; Gautam, U. K.; Lugstein, A.; Koide, Y.; Bando, Y.; Golberg, D. Deep-Ultraviolet SolarBlind Photoconductivity of Individual Gallium Oxide Nanobelts. Nanoscale 2011, 3, 1120−1126. (54) Konstantatos, G.; Clifford, J.; Levina, L.; Sargent, E. H. Sensitive Solution-Processed Visible-Wavelength Photodetectors. Nat. Photonics 2007, 1, 531−534. (55) Park, S.; Lim, B. T.; Kim, B.; Son, H. J.; Chung, D. S. High Mobility Polymer Based on a π-Extended Benzodithiophene and its Application for Fast Switching Transistor and High Gain Photoconductor. Sci. Rep. 2014, 4, 5482.

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Thiol-Free Synthesized Copper Indium Sulfide Nanocrystals as

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