Superstructural Ordering in Hexagonal CuInSe2 Nanoparticles

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Superstructural Ordering in Hexagonal CuInSe2 Nanoparticles Viviana Sousa, Bruna F. Gonçalves, Miguel Franco, Yasmine Ziouani, Noelia González-Ballesteros, M. Fátima Cerqueira, Vincent Yannello, Kirill Kovnir, Oleg I. Lebedev, and Yury V. Kolen'ko Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b04368 • Publication Date (Web): 07 Dec 2018 Downloaded from http://pubs.acs.org on December 8, 2018

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Chemistry of Materials

Superstructural Ordering in Hexagonal CuInSe2 Nanoparticles Viviana Sousa,a Bruna F. Gonçalves,a,b Miguel Franco,a Yasmine Ziouani,a Noelia González-Ballesteros,b M. Fátima Cerqueira,a,c Vincent Yannello,d Kirill Kovnir,e,f Oleg I. Lebedev,*,g and Yury V. Kolen’ko*,a a b c d

International Iberian Nanotechnology Laboratory, Braga 4715-330, Portugal Inorganic Chemistry Department, Biomedical Research Centre (CINBIO), Universidade de Vigo, Vigo 36210, Spain Center of Physics, University of Minho, Braga 4710-057, Portugal Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, USA

e

Department of Chemistry, Iowa State University, Ames, Iowa 50011, USA Ames Laboratory, U.S. Department of Energy, Ames, Iowa 50011, USA g Laboratoire CRISMAT, UMR 6508, CNRS-ENSICAEN, Caen 14050, France f

ABSTRACT: Chalcogenide semiconducting nanoparticles are promising building blocks for solution-processed fabrication of optoelectronic devices. In this work, we report a new large-scale colloidal synthesis of metastable CuInSe2 nanoparticles with hexagonal plate-like morphology. Powder X-ray diffraction analysis of the nanoparticles showed that the structure of the nanoparticles is not simple hexagonal wurtzite-type CuInSe2 (space group P63mc), indicating the formation of an ordered superstructure. Detailed insight into this structural aspect was explored by high-resolution electron microscopy, and the results evidence an unreported chemical ordering within the synthesized CuInSe2 nanoparticles. Specifically, while the Se sublattice is arranged in perfect wurtzite subcell, Cu and In are segregated over distinct framework positions, forming domains with lower symmetry. The arrangement of these domains within the hexagonal Se substructure proceeds through the formation of a number of planar defects, mainly twins and antiphase boundaries. As a semiconductor, the synthesized material exhibits a direct optical transition at 0.95 eV, which correlate well with its electronic structure assessed by density functional theory calculations. Overall, these findings may inspire the design and synthesis of other nanoparticles featuring unique chemical ordering; thus, providing an additional possibility of tuning intrinsic transport properties.

INTRODUCTION Nowadays Si photovoltaics (PVs) are established on the market. Si is an indirect band gap semiconductor with a low absorption coefficient, and to ensure a high efficiency of the final solar cell, 100 µm-thick layer of high purity Si is required. The fabrication of such a layer is energy demanding, resulting in increases of the final cost of the PV device.1,2 In contrast, thin-film solar cells rely on direct band gap semiconductors having a high absorption coefficient, which allows to decrease the thickness of the photoabsorber layer down to 1 µm.1 Bulk ternary CuInSe2 semiconductor with chalcopyrite structure has a direct band gap of 1.04 eV and a high absorption coefficient of 10–5 cm–1.3,4 This renders CuInSe2 outstanding for several important optoelectronic applications, such as PVs,5–8 light-emitting diodes (LEDs),3 and photocatalytic H2 generation.9 For instance, chalcopyrite Cu(In,Ga)Se2 thin-film solar cell recently reached the record efficiency of 22.8%.10 Nevertheless, this PV cell was fabricated using high-temperature evaporation/sputtering techniques, which are vacuum- and energy-demanding processes often requiring clean room operation. In sharp contrast, solution-based printing technologies, such as screen printing, spray printing, roll-to-roll processing,

offer a more cost effective thin-film PV device fabrication. Therefore, we recently embarked on the synthesis of CuInSe2 nanoparticles (NPs) for ink formulation to be used in the screen printing of CuInSe2-based PVs. One of the most attractive features of these NPs is that they can be solution-processed to obtain the photoabsorber layer,5,6,11 and furthermore, their optical and electronic properties can be tuned by varying the size and structure of the NPs.12,13 In particular, it was initially shown that CuInSe2 NPs can be used in drop-cast processing of the photoabsorber layer with 2.8% efficiency of the resultant solar cell.5 Subsequently, solar cell efficiency reached 8.2% with solutionprocessed CuInSe2 NPs.7,8 Colloidal synthesis of semiconducting I–III–VI NPs has been a subject of intense research efforts over the past three decades. Accordingly, several synthesis protocols have been established for CuInSe2 NPs, such as hot-injection,3,5,11,12,14–21 heatingup,6,22,23 solvothermal,24–29 and microwave-assisted8,30 procedures. Notably, most of the end products exhibit tetragonal chalcopyrite structure, common for CuInSe2. Hence, the influence of other crystallographic modifications with zincblende or wurtzite structures on the performance of CuInSe2-based PVs has not been widely studied. This is mainly due to limited reports on synthesis of CuInSe2 NPs with these structural types.

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For example, only three reports exists on the synthesis of hexagonal wurtzite CuInSe2 NPs, initial one by Norako and Brutchey,17 later by Wang and co-workers,16 and very recently by Brutchey group31. Other synthetic routes to hexagonal CuInSe2 NPs typically result in a mixture of different phases.9,32 Importantly, the few existing reports of hexagonal CuInSe2 NPs describe them as crystallizing in the wurtzite hexagonal cell with only one metal atomic site which implies a mixing of Cu and In in the same atomic site. Herein, we present a gram-scale synthesis of high quality CuInSe2 nanoparticles with well-defined hexagonal plate-like morphology. Further, the structural studies of the resultant NPs by means of high-resolution transmission electron microscopy show for the first time Cu/In ordering in the crystal structure giving rise to supercell. Finally, experimental and computational analyses of the optical properties of this new material show direct optical transition at 0.95 eV.

EXPERIMENTAL SECTION Reagents. In the current study, the following chemicals were used: hexadecylamine (HDA, 95%, TCI), diphenyl diselenide (Ph2Se2, 97%, TCI), indium(III) acetate (In(ac)3, 99.99%, Sigma-Aldrich), and tetrakis(acetonitrile)copper(I) tetrafluoroborate (TACT, 98%, TCI). Analytical reagent grade absolute ethanol and toluene were purchased from Fisher Scientific. Synthesis. In a typical experiment, HDA (60 g, 248.5 mmol), Ph2Se2 (2.516 g, 8.06 mmol), In(ac)3 (1.81 g, 6.2 mmol), and TACT (1.554 g, 4.94 mmol) were loaded under air into a 250 mL three-neck round-bottom flask equipped with magnetic stirring bar, thermocouple, condenser and vacuum adapter. The flask was attached to the Schlenk line, and the mixture was heated to 90 °C while stirring to melt HDA and to homogenize the reagents, resulting in a clear green reaction mixture. The low boiling liquids, such as possible water and acetic acid admixtures, were removed by degassing the mixture at 90 C for 30 min. Then, the flask was rapidly heated to 300 C under Ar and stirred at this temperature for 1 h. Next, the resultant brownblack reaction mixture was cooled to 70 C and diluted with 100 mL of toluene under stirring. After cooling to room temperature, the NPs were precipitated by the addition of a solvent mixture consisting of toluene and ethanol (3:1) and collected by centrifugation at 9000 rpm for 5 min. The resultant solid was washed twice with the same solvent mixture and again collected by centrifugation. After drying in vacuo, the NPs were homogenized with agate mortar and pestle to provide a powder. The yield of the as-synthesized CuInSe2 NPs was estimated to be ca. 90% (1.8 g). Powder X-ray Diffraction. Phase composition of the products was determined using powder X-ray diffraction (XRD). The data were collected using an X’Pert PRO diffractometer (PANalytical) with Ni filtered Cu Kα radiation and a PIXcel detector. The XRD patterns were matched to International Centre for Diffraction Data (ICDD) PDF-4 database using HighScore software package (PANalytical). Raman Spectroscopy. To inspect the local structure of the synthesized NPs, Raman spectroscopy measurements were performed on an alpha300 R confocal Raman microscope (WITec) using a 532 nm Nd:YAG laser for excitation. The laser beam

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with power of 0.5 mW was focused on the flattened powder sample by a ×50 lens (Zeiss). Afterwards, the spectra were collected with a 600 groove mm–1 grating using 100 acquisitions with a 2 s acquisition time. Infrared Spectroscopy. To analyze organic capping ligands present at the surface of the synthesized NPs, the room temperature Fourier-transform infrared spectroscopy (FTIR) in attenuated total reflectance (ATR) mode was employed using a VERTEX 80v spectrometer (Bruker). All spectra were recorded by averaging 64 scans with a resolution of 4 cm–1, and are background corrected. Thermal analysis. The organic content of the end product was investigated by means of thermogravimetric analysis (TGA) using TGA/DSC 1 STARe system (Mettler-Toledo). The samples were heated from 30 to 500 C at 10 C min–1 under a continuous Ar flow of 50 mL min–1. Electron Microscopy. The morphology of the synthesized samples was observed by scanning electron microscopy (SEM) using a Quanta 650 FEG ESEM microscope (FEI). To investigate fine microstructure and the chemical composition of the synthesised NPs, high-angle annular dark-field scanning transmission electron microscopy (HAADF–STEM), selected area electron diffraction (SAED) and energy-dispersive X-ray spectroscopy in STEM mode (STEM–EDX) were performed using JEM-ARM200F cold FEG probe and image aberration corrected microscope, operated at 200 kV and equipped with large angle CENTURIO EDX detector and QUANTUM GIF. DFT calculations. Band structure and density of state diagrams for the ordered monoclinic cell were calculated using the Vienna ab Initio Simulation Package (VASP).33,34 The calculations were run in the high-precision mode using the projector augmented wave (PAW) potentials provided with the package.35,36 To accurately determine the band gap, the Modified Becke-Johnson exchange potential37 was used with the default settings, resulting in an energy cut-off of 369.3 eV. A Γ-centered 12×10×11 k-point grid was utilized. Optical Properties. The optical band gap of as-synthesized NPs was determined using absorption UV−Vis−NIR spectroscopy, which was performed on the screen-printed thin film of the CuInSe2 NPs. Room temperature data were collected using a LAMBDA 950 UV/Vis/NIR spectrophotometer (PerkinElmer) equipped with 60 mm integrating sphere and InGaAs detector.

RESULTS High-Yield Protocol Providing Uncommon CuInSe2 on a Gram Scale. We selected to employ fatty amine HAD in our synthesis protocol to function, simultaneously, as a capping agent and as a high-boiling reaction solvent. Our first attempt of reacting In(ac)3 and TACT with Ph2Se2 in HDA showed that when the reaction temperature reaches ca. 200 C, the clear reaction solution turns turbid and becomes brown-black. According to the XRD analysis (not shown), 200 C is not a sufficient temperature to obtain desired phase-pure CuInSe2 in high yield. Hence, we increased the reaction temperature to 250 C and found that the resultant moderate-yield product is a mixture of common chalcopyrite CuInSe2 with tetragonal structure, small admixture of Cu2Se and, more interestingly, rare wurtzite


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Chemistry of Materials CuInSe2 with hexagonal structure. Encouraged by this surprising result, we next conducted the reaction at 300 C. Remarkably, this temperature was sufficient for driving the reaction to completion with a reasonably high yield of ca. 90%, thus affording hexagonal CuInSe2 NPs on a gram scale (1.8 g).

Figure 1. Top: Two models of CuInSe2, a disordered wurtzite model, where In (purple) and Cu (orange) share one atomic site, and ordered orthorhombic model. Se substructures (yellow) are identical in both models. Unit cells are shown as black lines. Bottom: Experimental (black) XRD pattern of the CuInSe2 NPs synthesized under optimized temperature of 300 C. Calculated patterns for the disordered wurtzite P63mc subcell with one Cu/In position (blue) and ordered Pmc21 supercell with doubled volume (red) are shown below. Dashed lines indicate the positions of the most intense superstructural reflections.

According to XRD, the end product synthesized at 300 C is phase-pure CuInSe2 crystallized in wurtzite-type hexagonal structure uncommon for CuInSe2 (Figure 1).16,17,31 All intense reflections on the powder XRD pattern can be described by wurtzite hexagonal model (P63mc, a = 4.08 Å, b = 6.69 Å). However, the careful examination of the pattern reveals several low-intensity reflections which cannot be assigned to the tetragonal chalcopyrite CuInSe2 or any other possible admixture phases. The reflections aligned well with the superstructure discovered by ED and HAADF-STEM studies, wherein the unit cell volume doubles and Cu and In are segregated by distinct atomic sites (vide infra). The lower peak intensity as compared

to the calculated ones might be explained by incomplete ordering and nanocrystalline nature of the sample resulting in overall peak broadening. Raman spectroscopy was used to examine the local structure of the synthesized NPs (Figure 2). A major sharp peak at 175 cm– 1 with full width at half maximum (FWHM) of 14.5 cm–1 corresponds to the A1 vibrational mode of CuInSe2.38 This mode results from the motion of the Se atom with the Cu and In atoms remaining at rest. The broader peaks at 125 and 213 cm–1 are in good agreement with the respective B1 and B2/E modes of CuInSe2, respectively.38,39 Interestingly, we observed two different zones in the sample, namely bright and dark ones showing a slight different Raman spectra: a shift of prominent A1 mode for these two zones was detected. Specifically, the peak position of the A1 mode of the dark zones was shifted 2 cm–1 to higher wavenumber with respect to the spectra from bright zones (Figure S1, SI). The observed Raman shift suggests that there is either strain or domains with different degrees of ordering in the synthesized CuInSe2 NPs.

Figure 2. Lorentzian fit (magenta) of experimental Raman data (black) for the synthesized CuInSe2 NPs. The position/FWHM (in cm–1) are provided for each component.

Analysis of the organic surface coating of the synthesized CuInSe2 NPs was performed by FTIR (Figure S2, SI). Comparison of the FTIR spectrum recorded for the CuInSe2 NPs with that of pure HDA demonstrates that the surface of the NPs is capped by the HDA ligand, indicating the interaction of amine groups of HDA with the CuInSe2 surface. To complement the FTIR observation, we verified the amount of organic matter present in the sample using TGA (Figure S3, SI). The total weight loss in the range 30–500 C in Ar was 6.4%, corresponding to the amount of HDA capping ligand on the surface of the CuInSe2 NPs. Chemically Uniform Hexagonal NPs. To investigate the morphology, microstructure, and the chemical composition of the NPs, electron microscopy studies were conducted. SEM images (Figure S4, SI) show that the CuInSe2 NPs are hexagonal platelike particles with size ranging from 10 nm to 130 nm. HAADF–STEM further confirms the hexagonal appearance of the resultant NPs, while the corresponding STEM–EDX element maps evidence the uniform distribution of key constituting



elements In, Cu, and Se through the NPs (Figure 3). EDX analysis also reveals that the Cu/In/Se atomic ratio in the obtained nanoparticles is 15(2)/18(2)/67(2) at.-%. The metals ratio agrees with the expected composition for CuInSe2.

Figure 3. Top: SAED patterns along main zone axis indexed as a mixture orthorhombic (o) and hexagonal (h) phases. Middle and bottom: representative low-magnification HAADF−STEM images of the synthesized CuInSe2 NPs, together with the simultaneously collected EDX maps of In L, Cu K, Se L, and their superposition.

Figure 4. Low-magnification (a, c) and high-resolution (b, d) HAADF−STEM images together with the respective SAED and FT patterns along [001]o and [100]o zone axes corresponding to the basal- and prism-faceted views of the synthesized hexagonal-shaped CuInSe2 NPs. In the overlaid structural images, In and Cu are

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shown as large purple and orange spheres, respectively, while Se is shown as small yellow spheres.

Figure 5. High-resolution [001]o HAADF−STEM image and the respective FT patterns of basal-faceted surface of the synthesized CuInSe2 particle. A, B, and C FT patterns correspond to selected in image area and evidence presence of (A) orthorhombic, (B) hexagonal, and (C) tweed orthorhombic structures within one single nanoparticle.

Figure 4 shows representative HAADF–STEM images of basaland prism-faceted views of CuInSe2 NPs. The particles are highly crystalline and regular shaped. No segregation or secondary phases were detected, indicating the phase-purity of the product. The high resolution HAADF–STEM image in Figure 4d evidences an amorphous and partially crystallize layers on the surface of the CuInSe2 NPs, which is most likely originates from partial surface oxidation and/or surface deficiency in cations, which typical for non-oxide NPs. Chemical Cu/In Ordering. The HAADF–STEM images along different zone axes reveal important information about the atomic level arrangements in the CuInSe2 NPs, which cannot be deducted solely from conventional XRD data. First, electron diffraction (Figure 3, top) clearly indicates the doubling of the at least one of the unit cell dimensions. The pattern still has hexagonal character and can be indexed in the cell with a = 2awurzite = 8.16 Å and c = cwurzite = 6.69 Å. Moreover, careful inspection of the high-resolution images in Figure 4b reveals a sharp contrast variation with lines of two bright and two dark spots. This variation is inconsistent with hexagonal or trigonal symmetry. We hypothesize that (i) the Se sublattice is arranged in perfect hexagonal manner identical to those in wurtzite subcell preserving hexagonal framework and (ii) Cu and In atoms are segregated over distinct framework positions providing the observed contrast.40 High similarity of the ionic radii for Cu+ and In3+ in tetrahedral coordination, 0.60 Å and 0.62 Å, respectively, allows for undisturbed Se framework


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Chemistry of Materials and overall hexagonal appearance of the particles and diffraction patterns. When only mirror planes and rotation axes are considered a model corresponding to ordered Cu/In distribution by reducing the structure symmetry to Pm, a = 6.74 Å, b = 8.17 Å, c = 7.08 Å, =  =  = 90 (CIF for the model is provided in the SI). A higher symmetry orthorhombic model in the Pmc21 space group with additional glide plane and screw axis was suggested by computational work of Lau et al. tackling different Cu/In ordered models of wurtzite CuInSe2.41 Calculated powder XRD patterns for Pm and Pmc21 models are similar but not identical. Unit cell parameters were refined from room temperature powder XRD data with Ge internal standard: space group Pmc21, a = 4.0863 Å, b = 7.0819 Å, c = 6.7405 Å, =  =  = 90 (CIF for the model is provided in the SI). The calculated XRD pattern for orthorhombic structural model is shown as a red line in Figure 1. The presence of weak superstructural reflections in the experimental pattern confirms that the Cu/In ordering takes place in the bulk sample rather than in a selected particle. In the ED patterns (Figure 3, top), there are two types of pseudo hexagonal [001] patterns, namely, full and partial ordering of In and Cu. FT patterns agree well with the measured ED patterns in Figure 4. Our work confirmed the computational predictions made 6 years ago by Lau et al.41

The partial nature of the ordering and intergrowth of the differently ordered 600 twinning domains is visible in the HAADF−STEM images (Figures 5–7). Analogous chemical ordering resulting in twinning has been reported previously in perovskite system.42 Hexagonal and orthorhombic domains can intergrow within the same nanoparticle (Figure 5), which explains the weak intensity of the superstructural peaks in the experimental XRD pattern (Figure 1). Chemically ordered Cu/In domains themselves are twinned due to the hexagonal nature of the Se substructure (Figure 6). This explains the overall hexagonal appearance of the particles and the ED diffraction patterns. Another mechanism of joining the orthorhombic domains is via the formation of antiphase boundaries (Figure 7).

Figure 7. (a) [001]o HAADF−STEM images of antiphase boundaries region together with corresponding FT pattern. Notice the presence of rows superstructure spots similar to ED in Figure 3. (b) Enlargement image antiphase boundaries together with the corresponding structural model (c). Se atoms are omitted for clarity in the model.

Figure 6. High resolution [001]o HAADF-STEM image demonstrating the presence of multiple twinning in different areas of the as-synthesized CuInSe2 hexagonal NPs. Enlargement and corresponding structural model are provided below. Se atoms are omitted for clarity in the model.

Direct Optical Transition at 0.95 eV. An ordered arrangement of Cu and In atoms allows for a precise band-structure examination by means of computational methods. For the ordered Pmc21 model the calculations with hybrid functionals predicted a direct bandgap at -point with a band gap value of 0.79 eV. We performed the computational analysis of the monoclinic model in Pm space group. To obtain an accurate estimate of the bandgap by DFT calculations MBJ XC-functional was employed.37 This potential is known to provide high accuracy, and at the same time, it is computationally cheaper than hybrid functionals. Pm band structure was calculated to be a direct bandgap semiconductor at the Γ point, with a band gap value of 0.95 eV (Figure 8). Full band structure (Figure S5) as well as density of states (DOS) plot (Figure S6) together with the orbital projections (PDOS), which were calculated for the ordered CuInSe2



monoclinic cell, are represented in the SI. The Pm band structure is similar to one calculated for Pmc21 orthorhombic model.41 Experimental investigations of the optical properties using solid-state electronic absorption spectroscopy confirm the computational predictions. The UV–Vis–NIR spectrum shows that the sample absorbs strongly through the visible and into the near-infrared region (Figure 9). To calculate the direct bandgap (𝐸𝑔 ), the following equation was used: 𝐸𝑔 = ℎ × 𝑐 ⁄𝜆, where ℎ is the Planck’s constant, 𝑐 is the speed of light and 𝜆 is the absorption cutoff wavelength on the absorption edge, obtained from the absorption spectra.43 The optical absorption edge of CuInSe2 NPs was estimated to be 0.95 eV, which is consistent with the brown-black color of the powder. The calculations using Tauc plots resulted in a similar bandgap of 0.91(4) eV. The obtained value is slightly lower as compared to the reported values for bulk chalcopyrite CuInSe2 (1.04–1.10 eV).4,17

Figure 8. Calculated electronic band structure for the ordered monoclinic Pm model of pseudo-hexagonal CuInSe2.

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As noted in introduction, there are only three reports available in the literature, where phase-pure wurtzite CuInSe2 NPs were obtained. Notably, all works employ a hot-injection method. In contrast, we have synthesized hexagonal-shaped plate-like CuInSe2 NPs using a convenient heating-up procedure, which takes advantage of reacting affordable and readily available metal precursors in fatty amine, at 300 C for 1 h. The success of the newly developed protocol was confirmed by synthesizing 1.8 g of high-quality CuInSe2 NPs in one run. To the best of our knowledge, gram-scale and high-yield synthesis route to metastable CuInSe2 NPs has not been reported. High quality of produced samples allows for the establishment of previously unreported superstructural chemical Cu/In ordering within hexagonal Se sublattice in the structure of CuInSe2 NPs. Detailed electron microscopy studies highlight that the Cu/In atomic arrangement cannot be described using hexagonal crystal symmetry, suggesting that our gram-scale synthesis protocol generates an unprecedented CuInSe2 phase with pseudohexagonal symmetry and ordered arrangement of Cu and In atoms. The driving force for the observed metal segregation is the different nature of metal–selenium chemical bonds. Atomic scale ordering promoted by preferences in Cu/Zn and Cu/P chemical bonds were previously reported. 44–46 Twinning and antiphase boundaries are the main mechanisms of joining ordering lower-symmetry fragments into pseudo-hexagonal arrangements which is promoted by the hexagonal nature of the Se substructure in the ordered compound. Two ordered models were considered, in the space groups Pm and Pmc21. Both models describe Cu and In atomic arrangement and optical spectra. Notably, hexagonal polymorph of CuInSe2 is considered to be thermodynamically metastable in comparison with stable chalcopyrite modification, and the phase transformation of hexagonal nanoparticles into chalcopyrite ones has been reported to occur at about 420 C.31 At the same time, our thermal stability results suggest that the as-synthesized CuInSe2 NPs remains stable after annealing at 500 C for 1 h in Ar, without any phase change, as revealed by XRD. More detailed studies of the thermal stability of the as-synthesized NPs will be reported in due course.

CONCLUSIONS Nanoparticles with chalcopyrite-type crystal structures are the common product of the colloidal syntheses of CuInSe2. In sharp contrast, we have developed a novel synthesis of uncommon plate-like CuInSe2 nanoparticles with nearly perfect hexagonal appearance. This preparation protocol offers phase-pure and highly crystalline product in large quantity. A superstructural ordering of Cu and In was detected by HAADF–STEM within a hexagonal selenium sublattice. This structural arrangement gives rise to a number of packing defects as a consequence of the adjustment of lower-symmetry domains within the hexagonal Se scaffold. Screen-printed fabrication of a photovoltaic device using the synthesized CuInSe2 nanoparticles is the subject of our ongoing research efforts.

ASSOCIATED CONTENT Figure 9. UV–Vis–NIR absorption spectrum of screen-printed thin film of the as-synthesized CuInSe2 NPs.

DISCUSSION

The Supporting Information is available free of charge on the ACS Publications website. Additional Raman, FTIR, TGA, SEM, DFT, and XRD data (PDF); CIF files for the ordered models.

AUTHOR INFORMATION


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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors thank Dr. D. Aldakov, Dr. J.L. Lado, Y.S. Rosen, Dr. L.M. Salonen, Prof. M. Shatruk and Prof. S. Magdassi for insightful discussions. This work was supported by ERDF COMPETE 2020 and Portuguese FCT funds under the PrintPV project (PTDC/CTM-ENE/5387/2014, Grant Agreement No. 016663). B.F.G. is grateful to the FCT for the SFRH/BD/121780/2016 grant, while Yu.V.K. is grateful to Portuguese NORTE 2020 programme (FROnTHERA project, NORTE-01-0145-FEDER-000023) for support of this research.

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