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Unraveling the Growth Mechanism Forming Stable #-In2S3 and #-In2S3 Colloidal Nanoplatelets Faris Horani, and Efrat Lifshitz Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00013 • Publication Date (Web): 15 Feb 2019 Downloaded from http://pubs.acs.org on February 17, 2019

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Chemistry of Materials

Unraveling the Growth Mechanism Forming Stable -In2S3 and -In2S3 Colloidal Nanoplatelets Faris Horani, Efrat Lifshitz* Schulich Faculty of Chemistry, Solid State Institute, Russell Berrie Nanotechnology Institute, Nancy and Stephen Grand Technion Energy Program, Technion, Haifa 3200003, Israel ABSTRACT: Inorganic two-dimensional semiconductor nanostructures intrigue the scientific community, due to their tunable and sparse electronic states and their high conductivity. The current work deals with colloidal nanoplatelets based on In 2S3 compound, focusing on the growth mechanism that leads to the formation of two different phases, trigonal γ-In2S3 and defect-spinel β-In2S3, both stabilized at room temperature and characterized by ordered metal voids. In particular, we substantiate the experimental factors (e.g., temperature, reaction duration, and surface ligands) that control the growth progress. The results indicated the formation of hexagonal NPLs of the γ-phase at an elevated temperature and dodecagon NPLs of the γ-phase at a lower temperature. A long reaction duration time transformed the hexagons/dodecagons into truncated triangular shapes. Furthermore, the analysis of thermodynamic and kinetic factors indicated a phase transformation from the γ-phase to the β-phase. All phases were produced by a new colloidal procedure based on a single precursor. The structures created were verified by X-ray diffraction, HR-TEM analyses and Raman measurements. Elementary optical properties were identified by absorption and emission measurements. The nanoplatelets discussed offer lowtoxicity, optical activity in the UV and visible spectral regimes and an option for electrical or magnetic doping, enabled by the existing voids.

Introduction Chalcogenide semiconductors have attracted considerable scientific and technological interest, due to their combination of optical and electrical properties. The most commonly explored chalcogenides are based on the II-VI and IV-VI groups, such as CdS, CdTe, PbSe, PbS. However, the introduction of these materials in modern industrial applications, including photonics, optoelectronics and biological imaging, is limited due to toxicity impact on the environment.1,2 Therefore, there has been an increased demand to develop indium based chalcogenides materials from the III-VI group of elements.3 Indium sulfide (In2S3) is a promising candidate, due to its low toxicity, high absorption coefficient,4 photo-electrical properties,5–7 long term stability and wide band gap energy in its bulk form.8 An In2S3 bulk semiconductor has a large exciton Bohr radius of 33.8nm and exists in three crystalline polymorphs: α-In2S3 (defect-cubic), β-In2S3 (defect-spinel) and γ-In2S3 (layered hexagonal).9 Among the three phases, β-In2S3 is the most chemically stable (up to 420°C) and the most studied phase with n-type conductivity and a wide band gap of 1.9–2.4 eV.10,11 The γIn2S3 was found to be thermally unstable, existing only at elevated temperatures (775°C-1045°C) or high pressures (35 kbar), and therefore its isolation at ambient conditions has been an obstacle since 1976.9,12–14 The α-In2S3 phase was prepared at 420°C– 700°C and showed a partial chemical stability.15,16 Nevertheless, In2S3 bulk or thin-film forms have already demonstrated practical use as a buffer layer material replacing the CdS thin films in solar cells,17–19 and showed improvement of the open circuit voltage and current collection in the blue wavelength region.18,20 Moreover, In2S3 has been used as efficient photocatalysts,21–23 photodetectors,24,25 electrochemical storage cells and display panels.26–29

New efforts have been demonstrated in recent years for the development of In2S3 two-dimensional (2D) nanostructures, e.g., thin films or nanoplatelets (NPLs), using either chemical vapor deposition or colloidal chemical procedures.30–35 It is anticipated that these 2D materials should possess tunable electronic band structure, high conductivity and efficient optical transitions.36,37 Moreover, the unique defect-cubic and defect-spinel structures of In2S3 compound include voids which can accommodate functional electrical or magnetic dopants, useful attribute for practical applications. Indeed, recent studies used Cu or Mn doped β-In2S3 nanostructures in prototype solar cells and light emitting diodes.38–41 The structure of shape stability was discussed only in a single case, showing transformation of wrinkled nanostructures into nanodisk shapes.41 Despite the efforts mentioned in preparation and characterization of nanoscale In2S3 phases, the fundamental growth mechanism and the factor inducing structural stabilization or transformation, are inadequately understood. The current work focuses on colloidal 2D In2S3 NPLs, having merits beyond the corresponding thin films, when prepared as free-standing species in solutions, covered by organic ligands, produced in variable sizes and suitable for incorporating in practical applications. The current work deals with colloidal nanoplatelets based on an In2S3 compound, focusing on the growth mechanism that leads to the formation of two different phases trigonal γ-In2S3 and defect-spinel β-In2S3 - both stabilized at room temperature. We discuss the influence of the experimental factors (temperature and reaction duration) on the phases' growth mechanism and we show, for the first time, a structural transformation from the γ-In2S3 to the β-In2S3 phase. The results revealed variable shape for the different phases: (a) a dodeca-

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gon shape in the γ-In2S3 phase when using a relatively low temperature and short reaction duration; (b) a γ-In2S3 hexagon shape which was prepared at an elevated temperature and short reaction duration; (c) a β-In2S3 truncated triangular shape using low/high temperatures after long reaction duration. All phases were produced by a new colloidal procedure based on a single precursor, which lead to creation of structural stability at room temperature. The phases produced were verified by X-ray diffraction, high-resolution transmission electron microscopy and Raman measurements. The elementary optical properties were identified by absorption and emission measurements. The knowledge accumulated indicates future potential use of the In2S3 phases in various optoelectronic devices.

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beyond 64 minutes at 230°C (or after 32 minutes at 250°C) induced a γ → β phase transition accompanied with a gradual color change from yellow to dark orange. The purification of the product is similar to the procedure mentioned above for γIn2S3 NPLs. X-ray Diffraction (XRD). Thin films of In2S3 NPLs were prepared by concentrated suspension drop-casting onto a clean glass microscope slides and drying for further characterization. The X-ray diffraction patterns were then acquired on a Rigaku SmartLab 9.0 kW diffractometer with the X-ray source operating at 45 kV and 150 mA. The instrument was equipped with a Cu Kα radiation (λ = 1.5418Å) and was used in the 2θ scan geometry for data acquisition using parallel beam (PB) geometry. Transmission Electron Microscopy (TEM) and Elemental Analysis. Transmission electron microscopy (TEM) images, high resolution TEM (HR-TEM) images, and energy dispersive X-ray spectroscopy (EDS) spectra were taken by using a FEI Tecnai T20 operated at 200 keV. High-Angle Annular DarkField (HAADF) imaging in STEM mode (HAADF-STEM) was obtained by FEI Titan 300keV S-TEM system. The samples for TEM measurement were prepared by dropping the solution containing NPLs on an ultrathin carbon-coated copper grid at room temperature. Absorbance and Photoluminescence Characterization. Optical absorbance spectra of diluted hexane dispersions of NPLs were recorded in quartz cuvettes of 1 cm path-length at different reaction stages and times using JASCO V-570 UV−VIS-NIR spectrometer and Shimadzu (UV-1800) Spectrophotometer. Photoluminescence spectra were obtained by JobinYvon (Fluorolog-3) Fluorometer. Both absorbance and photoluminescence spectra were collected in air and at room temperature. Raman Spectroscopy. The sample was prepared by drop-casting a concentrated suspension onto a clean silicon/glass substrate. The measurements were performed with a Horiba JobinYvon (LabRAM HR Evolution®) Confocal Raman Microscope, using visible laser excitation source (532nm). The spectra were recorded by using 50x and 100x objective lenses and a grating of 1800 gr/mm. Crystal Structure Design. γ-In2S3 and β-In2S3 NPL's crystal structures were designed by Crystal Maker software. Several parameters were calculated for each unit cell structure model, e.g., filled space, void space, unit cell volume, density, atoms per unit cell and bond lengths. The calculated parameters are corrected for first-nearest-neighbor sphere overlap and site visibility.

Experimental Section Chemicals and Materials. Indium (III) acetate (In(OAc)3, 99.99%), 1-Dodecanthiol (DDT, ≥98%), Oleylamine (OLAm; tech. grade, 70%). Methanol (absolute), toluene (analytical), hexane (analytical), and isopropanol (cp) were purchased from Bio-Lab Ltd. These chemicals were used without further purification. Synthesis of Hexagonal and Dodecagonal γ-In2S3 NPLs. In2S3 NPLs were prepared using a colloidal procedure, involving either injecting or dissolving precursors in a mother solution, including stabilizing ligands. All synthesis procedures were undertaken using standard Schlenk line techniques assisted by a nitrogen-filled glovebox. The growth of In2S3 NPLs was initiated by preparation of a precursor consisting of both the indium and chalcogenide elements, noted here as a single precursor source. The precursor was prepared by mixing In(OAc)3 (0.292gr, 1 mmol), OLAm (5.0 mL) and DDT (5.0 mL) in a three-neck round-bottom flask, equipped with a thermocouple and a magnetic stirrer. Then the mixture was degassed at 100°C for 1 hour, leading to the formation of a colorless In-thiolate [In(DDT)3] solution, where In(DDT)3 is considered hereon as a single precursor. Thereafter, the mixture was further heated under dry inert nitrogen gas flow until a yellowish color was developed (≥215°C), accompanied by decomposition of the single precursor into In 2S3 monomers. The synthesis stages are shown schematically in Figure 1a. The growth process which was continued up to 32 minutes at 250°C produced hexagon shapes, while reaction duration up to 64 minutes at 230°C produced NPLs with dodecagon shapes. The reaction was terminated by removal of the heating mantle and prompt cooling. Then toluene, isopropanol and methanol were added to the reaction solution and the NPLs were separated by centrifugation via two cycles. Afterward, the NPLs were dried and then redissolved in organic solvent such as hexane or toluene for further characterization. Extra centrifugation and drying rounds are responsible for significant desorption of surface-bound ligands and aggregation of the NPLs. Structural characterizations as discussed below, confirmed the formation of hexagon and dodecagon NPLs of the γ-phase. Synthesis of Truncated Triangular β-In2S3 NPLs. The same chemical procedures used for the growth of the γ-phase, was implemented to the growth of the -phase, however, the reaction time was extended up to 128 minutes at 230°C, or 64 minutes at 250°C. The X-ray and electron microscopy measurements (see results and discussion section) proved the formation of NPLs with a truncated triangular shape with a -In2S3 crystallographic structure. Hence, increase of the reaction duration

Result and discussion In2S3 NPLs with Different Morphologies. In2S3 NPLs were prepared using a colloidal procedure as described in the experimental section above, using In(DDT)3 precursors as the source of In2S3 monomers for the growth of platelets (see Figure 1a). The nucleation process initiated only after increase of the temperature up to 230°C or 250°C. Figure 1b demonstrates a set of TEM images associated with the produced NPLs, extracted at various stages of the reaction. The differences among the images reflect the morphology's evolution with temperature and time. Figure 1b shows that a reaction at T=230°C for half an hour induces crystalline growth of a rounded twelve-sided polygon, called a dodecagon. Then, extension of the reaction to one hour at the same temperature produced a dodecagon with clearly visible sides. A reaction at


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T=250°C for half an hour led to the formation of hexagon shaped In2S3 NPLs. Both hexagon and dodecagons NPLs have a thickness value of ~1.8nm (see supporting information (Figure S1)). However, extension of the reaction duration at each temperature; i.e., 230°C and 250°C, beyond 64 minutes and 32 minutes, respectively, brought about a transformation from hexagon or dodecagon crystallites to truncated triangular shaped NPLs. The lateral dimensions of the different shapes are shown in Figure 1b. Figure 1c displays schemes of the produced NPLs transformations, compatible with the TEM images given in Figure 1b. Figure 1d exhibits a snapshot photo of aliquots withdrawn from the reaction solution after 32, 64 and 128 minutes. This figure reveals a change in a color from pale yellow through bright-orange to a dark orange. The yellow solution is associated with the formation of both hexagon and dodecagon morphologies, while the dark orange solution corresponds to the truncated triangular morphology. In2S3 Crystallographic Structures. The previous section discussed the correlation of the reaction conditions with the visual appearance, viz., crystalline morphology and apparent color of the reaction solutions. This section supplies a detailed description of the different In2S3 crystallographic structures. Figure 2a depicts a set of X-ray diffraction patterns of different aliquots taken at intermediate stages during the NPLs' growth. The observed diffraction angles of each aliquot, and the corresponding

crystallographic planes and inter-planar d-spacing, are summarized in Table 1. The bottom pattern in Figure 2a (black line) corresponds to an aliquot taken at 32 minutes, characterized by a yellow color and a hexagon or dodecagon morphology. This X-ray pattern contains two intense peaks at diffraction angles as listed in Table 1. The top pattern in Figure 2a (red line) corresponds to an aliquot taken after 128 minutes, characterized by a dark-orange color and a truncated triangular morphology. The top pattern exhibits eight different diffraction peaks, as given in Table 1. Beside the experimental observations, Figure 2a contains two sets of stick diagrams at the bottom and the top of the frame, associated with the database diffraction angles of the γIn2S3 and β-In2S3 phases, respectively (see also Table 1). A comparison between the experimental patterns with the database sets designates that the hexagon and dodecagon morphologies have a γ-In2S3 crystal structure, while the truncated triangular shape has a β-In2S3 crystal structure. An additional diffraction pattern is illustrated in Figure 2a (green curve) and related to an aliquot taken from the reaction solution after 64 minutes at 230°C (or time interval between 32 – 64 minutes at 250°C). The green pattern nominally includes two reflection peaks at 27.27o and 47.96o with the corresponding d-spacing of 3.26Å and 1.90Å (see table 1). A blow up of a diffraction regime between 25o-32o (highlighted by a yellow background in Figure 2a) is illustrated in Figure 2b, enabling a

Figure 1. (a) Schematic illustration of a colloidal synthesis setup (top) and the chemical reactions used for the formation of a single molecule precursor and its decomposition product, the In2S3 compound (bottom). (b) Representative TEM images of In2S3 NPLs, produced at various temperatures and reaction durations, revealing the formation of hexagon, dodecagon and truncated triangular shaped NPLs. The lateral length values of each shape are given on the diagram. (c) Scheme reproducing the NPLs' morphologies in correlation with the TEM observations shown in (b), illustrating the morphology dependence on reaction temperature and on evolution of the reaction st ages (nucleation, growth and morphology change) with progress in time. (d) Photographs of aliquots withdrawn from a reaction produced at 230°C, exhibiting color change as reaction time extended.



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Table 1. Summary of diffraction angles, inter-planar dspacing and corresponding crystallographic planes of the X-ray diffractions shown in Figure 2.

Figure 2. (a) X-ray diffraction patterns of three aliquots withdrawn from the reaction solution as different time intervals. The bottom black line is attributed to γ-phase and corresponds to an aliquot taken after 32 minutes. The top red line is attributed to β-phase and corresponds to an aliquot taken after 128 minutes. The middle pattern (green line) is attributed to an intermediate phase and corresponds to an aliquot taken after 64 minutes at 230oC (or time interval between 32 - 64 minutes at 250oC). Two sets of stick diagrams at the bottom (black) and the top (red) of the frame are associated with the database diffraction angles of the γ-In2S3 and β-In2S3 phases, respectively. (b) Expanded scale of a diffraction regime between 25o–32o (yellow area) from panel (a), displaying a comparison between the intermediate pattern (middle) with those of the γ- and -phases. The orange Gaussian fit depicts a deconvolution of the green pattern into two peaks indexed by (I) and (II). The blue vertical lines are drawn to guide the eye.

six S-2 ions, and each S-2 anion is surrounded by four In+3 cations. Furthermore, two adjacent octahedra are arranged with an anti-prismatic triangular face-sharing along a [001] direction, and edge sharing with three neighboring octahedra along a N[001] direction, as drawn in Figure 3b. A face and an edge sharing induce structural distortions, which are particularly pronounced in materials with relatively large cations and valenceelectron volume. Such structural distortions stimulate displacement of cations from the octahedral centers or migration to vacant positions, all in order to minimize the cation-cation electrostatic repulsion. A displacement of the cation center, a JahnTeller distortion, creates unequal In-S bond lengths (2.746Å and 2.513Å) within a unitary octahedron, as displays in Figure 3c. Considering the local distortions, a cut view of a unit cell along [112̅0] direction was derived, as illustrated by a ball-filling presentation in Figure 3d. The unit cell structure is composed of six layers, each consisting of close-packed S2- anions (yellow balls). The In3+ cations (purple balls) occupy two-thirds of the octahedral hole sites between the adjacent S2- layers. The layers are stacked in a sequence along the [0001] direction, when 1 and 3 correspond to the S-2 layers; layer 2 represents the In+3 cations at interstitial positions. The S-2 anions' layers are flat and overlap each other. In contrast, individual In+3 layers have a non-flat appearance, where part of the cations lie closer to atop anion layer and other cations lie closer to the bottom anion layer (see small framed section). Furthermore, adjacent In+3 layers have a lateral displacement from one another. They are indexed by A, B and C. The filled-ball display also reveals a mirror image arrangement, with a mirror plane at half way, and while each fraction has a dipole polarity, the overall unit cell as a zerodipole moment.

comparison between the green pattern (middle) with those of the γ- and -phases. Figure 2b depicts a deconvolution of the green pattern by a Gaussian fit, revealing the existence of two diffraction composites at that stage, emphasized by the dashed orange curves and labeled as (I) and (II). The comparison shown in Figure 2b emphasizes the similarity of the green pattern to those of the identified phases (black and red patterns) and the diffractions angles laying between those of the phases. Furthermore, the green diffraction peaks show pronounced broadening. The observations designate the occurrence of a structural phase transformation. The broadening can be related to the existence of structural domains at an intermediary stage of phase transformation, with an inhomogeneous domain strain and structural reconstruction via creation of vacancies, dislocations or layer faults.42 Overall, the X-ray observations discussed reveal unequivocally, for the first time, the occurrence of structural phase transition from a γ-In2S3 to β-In2S3 phase. γ-In2S3 Crystal Structure. The γ-phase of In2S3 accommodates a corundum-type structure of crystalline alumina (α-Al2O3), crystallizing with trigonal symmetry of D 3d, and can be described by a primitive right-rhombic prism with lattice parameters of a=6.56Å and c=17.57Å (c/a=2.67). A polyhedral representation of the γ-In2S3 structure is displayed in Figure 3a, showing that a single unit cell is built from octahedral connectivity. Each octahedron is composed of an In+3 cation binds to

The correlation between the γ-In2S3 crystal structure and the morphology of the NPLs is discussed below. It might be easier to follow such a discussion by viewing the γ-phase crystal structure as three right-primitive rhombohedral unit cells, enfolded within a primitive hexagonal close packed (HCP) frame. An HCP prism is enclosed with two c-planes {0001}, six first-order


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Figure 3. (a) Right-rhombic prism of In2S3 unit cell, represented by polyhedral model. (b) Blow up of a segment of a unit cell, showing face-sharing (red trigonal plane) and edge-sharing (red line) between neighboring octahedral units. (c) Single distorted octahedral unit ̅ 0) composed of two different In-S bond lengths as marked on the figure. (d) Cut view of the polyhedron shown in (a) exposing the (112 plane and representing the indium (In+3) and sulfur (S-2) layers sequence along the [0001] direction. The S-2 layers are given by the indexes 1 and 3, and In+3 layers are depicted by the symbols A, B and C. Pink lines represent the unit cell frame, and blue dashed lines exhibit the non-flat indium planes within two S-2 layers. (e) Hexagonal closed packed (HCP) crystal structure form of the In 2S3 γ-phase, ̅ 0) consisting of three right-primitive rhombohedral unit cells. The HCP presentation reveals three types of planes: the m-plane (101 ̅ which is marked by the orange area, a-plane (1120) marked by the green area, and the c-plane (on top). The a- and m-planes are rotated by an angle of 30° (pink arrow). (f) A stereographic projection representing twelve crystallographic directions; six growth directions of the m-planes (orange vectors), and another six growth directions of the a-planes (green vectors). The three-fold axis symmetry element of γ-phase with D3d point group symmetry is shown at the center (black triangle).

m-planes {101̅0} and six second-order a-planes {112̅0}, as illustrated in Figure 3e. The angle between the m- and a-planes is 30°, and overall, the m- and a-planes are normal to those of the c-planes. Figure 3f illustrates a stereographic projection of the m- and a-planes shown by orange and green arrows, respectively, and the direction of the planes is marked on the circumference of the projection. β-In2S3 Crystal Structure. The β-In2S3 crystallizes with a tetragonal crystal symmetry, described as a defect-spinel structure with lattice parameters of a=7.62Å and c=32.36Å (c/a ratio= 4.25),43 as illustrated by a polyhedron representation in Figure 4a. The latter presentation reveals that a defect-spinel structure is composed of two kinds of octahedra and one type of tetrahedron unit, as indicated in the inset of Figure 4a. Unlike the case of γ-In2S3, In3+ centers are surrounded either by six or by four S2- anions in the octahedral (InS 6) and tetrahedral (InS 4) units, respectively. Figure 4b illustrates a balls-and-sticks presentation of the β-In2S3 phase, designating the elements as given in the inset. It is seen from Figure 4b that S2- anions form a distorted cubic closed-packing of a sub-lattice, within the octahedra are fully occupied, however one third of tetrahedral sites remain unoccupied, forming negatively charged vacant sites (VIn), which is marked by a blue cloud. The VIn centers are ordered along the 41 screw axis, parallel to the z axis, giving rise

to a small distortion from the conventional cubic symmetry of the regular spinel structure, called a defect-spinel. The distortions lead to the formation of a tetragonal structure of the βIn2S3 with point group D4h19 and space group I41/amd. The discussed distortions also induce variability in the In-S bond lengths. Figure 4c presents the three different types of coordination, displayed with a color code to match the polyhedrons in Figure 4a, illustrating from left-to-right: an octahedron with two different In-S bond lengths of 2.660Å and 2.537Å; a tetrahedron with two In-S bond lengths of 2.465Å and 2.467Å; and an additional octahedron with four different In-S bond lengths of 2.540Å, 2.629Å, 2.643Å and 2.695Å. Figure 4d shows connectivity among the octahedron/tetrahedron units of a certain fraction in the unit cell, when neighboring units are connected either through a corner bridge along [001] direction, or through edgesharing along the normal direction N[001], without incidence of a face sharing connectivity. It is worth noting that the β-In2S3 is characterized by their intrinsic VIn centers; however, some VS can occur accidentally, and both defects induce charge trapping of opposing types, which can eventually lead to the donor-acceptor type of recombination.44 Correlation between the Morphology and Crystal Structure of the In2S3 Phases. The previous sections discussed separately the polymorphs and the fundamental crystal structures of In2S3



NPLs, while this section discusses the correlation between them, by following the various observations which are illustrated in Figure 5. Figure 5a presents a TEM image of a hexagon with lateral dimension of ~ 120nm. Figure 5b shows the Fast Fourier Transform (FFT) pattern of a single hexagon similar to that shown in Figure 5a, exhibiting three rings, each consisting of six diffraction spots. These rings are ascribed to the {110} planes with d=0.33nm, {300} planes with d=0.19nm and {220} planes with d=0.16nm. The observed diffraction planes in the FFT pattern are compatible with those shown in the X-ray diffraction as shown in Figure 2. Figure 5c depicts the same FFT as in Figure 5b, but it includes additional analysis done by stretching bars between adjacent diffraction points, creating a six-fold symmetry frames (color coded with the FFT rings marked in Figure 5b). It is seen from the frames that the diffraction points of {110} and {220} (green and blue frames) overlie and both are twisted with respect to the {300} points (yellow frame) by α=30o. An angle of 30° is consistent with the tilt angle between the m- and a-planes, as shown schematically in Figure 3d and Figure 3e for the γ-In2S3. It is important to note that FFT patterns of the dodecagonal NPLs exhibit identical crystallographic diffractions as shown in Figure 5c and Figure 5d; hence, the dodecagon morphology is also identified with the γ-In2S3 phase, in agreement with the X-ray observations. Figure 5d displays an FFT pattern of two superimposed NPL hexagons with a 5o rotation between them. Figure 5e shows the derived image

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of the FFT shown in Figure 5d, indicating an alternating arrangement of [ABAB]. Figure 5f displays an image of two or more NPL hexagons overlapping along the [0001] direction, experiencing crystalline mismatch, leading to the formation of a Moiré pattern (see scheme in inset). A Moiré pattern becomes feasible only in regions in which the NPLs' surfaces accommodated a low concentration of surfactants because of the sample preparation procedure (see methods section), allowing a mutual orientation between adjacent platelets. Figure 5g presents the FFT of a Moiré pattern zone in Figure 5f, showing a 2o-3o rotation angle along the {110} planes. The hexagonal morphology of the γ-In2S3 NPLs and their overlapping phenomenon are further displayed by HAADF-STEM measurements (see supporting information (Figure S2)). An acquired HAADF line which is stretched over a thin γ-In2S3 NPL shows a staircase line profile which indicates the existence of several adjacent overlapping NPLs (see Figure S2). Figure 5h displays a TEM image of the β-In2S3 exposing the truncated triangular shape as discussed in Figure 1 and the corresponding high-resolution image is shown in Figure 5i. The FFT image is seen in the inset of Figure 5i, while a close-up of a small region of the image is shown in Figure 5j. A few diffraction planes, {204}, {408} and {219}, and their d-spacing are marked on Figure 5j, showing compatibility with the diffractions measured by an X-ray method. Figure 5k exemplifies a TEM image of a superposition between two or more β-In2S3

Figure 4. (a) Tetragonal defect-spinel unit cell structure of the -In2S3 phase, represented by a polyhedral model, including two types of octahedra, tetrahedron and vacancies. (b) Ball-and-stick presentation of the -In2S3 phase, emphasizing the existence of vacancies, when sulfur atoms appear as yellow balls, indium atoms as green/red/black balls, and electron-rich vacancy sites (VIn) are shown by the blue clouds, while the hole-rich centers (VS) are displayed by black circles. (c) Three types of coordination centers consisting of two distorted octahedra (left and right) and one tetrahedron (middle); each coordination geometry is composed of different In-S bond lengths. (d) Schematic display of the connectivity among the octahedron/tetrahedron units of a certain fraction in the unit cell, revealing a corner-bridging connection (pink dashed circle) along the [001] direction, and an edge-sharing bond (pink lines) along the N[001] direction.


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Figure 5. (a) TEM image of hexagon of γ-In2S3. (b) FFT pattern of hexagonal γ-In2S3 NPL, taken from [001] zone axis (ZA) from the image in (a). Diffraction spots are gathered into three circumferences emphasized by different colors, attributed to {110} (green), {300} (yellow), {220} (blue) planes, and the corresponding d-spacing values are shown on the pattern. (c) FFT pattern of the image in (a), marked by 6-fold symmetry frames, showing α=30° rotation angle between two adjacent frames. (d) FFT pattern of two overlapping NPLs, exhibiting a relative rotation angle of 5°. (e) Derived image based on the FFT pattern in (d), representing the superimposition of two or more rotated NPLs forming [ABAB] alternating atomic arrangement. (f) TEM image of two overlapping NPLs, generating a mutual Moiré structure (see scheme at the inset). (g) FFT image of a Moiré pattern zone exhibits a 2o-3o rotation along the {110} planes. (h) TEM image of truncated triangular morphology of the β-In2S3 phase. (i) HR-TEM image of a representative β-In2S3 NPL, and the sharp FFT diffraction pattern (inset). (j) Blow up image of figure (i) represents three planes {204}, {408} and {219} as marked in the panel. (k) TEM image including several overlapping with a β-structure, forming a moiré pattern (see dashed frame). (l) FFT image taken from the Moiré pattern area in panel (k), showing two rings of the planes {109} and {116} with the corresponding d-spacing of 0.32nm (pink) and 0.38nm (yellow), respectively.

NPLs, which formed a Moiré pattern. The corresponding FFT pattern is shown in Figure 5l, displaying two rings, the inner ring belongs to {116} and the outermost ring belongs to {109} planes with d-spacing of 0.38nm and 0.32nm, respectively. The diffractions observed in Figure 5l confirm the first and secondorder reflection peaks in the X-ray diffraction pattern of β-In2S3 phase. Kinetic and Thermodynamic Effects in the Growth Mechanism of In2S3 NPLs. Keeping in mind the correlation between the observed polymorphs and the crystal structures of the In2S3 phases, the discussion in this section reviews a plausible growth mechanism of each polymorph provided with a qualitative analysis. Figure 6a displays from left to right the NPLs formation stages, starting from the preliminary materials, generating the single precursor, formation of monomers and the growth into two different polymorph shapes. The bar below designates symbols of chemical constituents. The growth process after nucleation is analyzed in depth. While the monomers' formation and the nucleation stage are determined solely by thermodynamic

considerations, the growth stage is dictated by a kinetically controlled reaction, combined with thermodynamic effects. The nucleation stage generated polyhedron shaped seeds with some defined faceting. Anisotropy of a polyhedron induces growth along specific crystallographic directions and eventually leads to the formation of an anisotropic morphology. The growth on top of a seed is dictated by the chemical potential of the bulk solution, which is dependent on the concentration of [In2S3] monomers, and consequently, the kinetic diffusion rate of the monomers toward nucleus faceting can be affected. A facet's surface energy depends on the atomic compactness and chemical reactivity of surface atoms. Figure 6b depicts a scheme presenting a few different growth directions. An exemplary polyhedron with HCP crystal structure is shown at the left side of the scheme; the {0001} facets have the highest atomic density per unit area compared to the normal directions, when the c-plane is terminated by In+3 cations (In-rich), as also shown in the scheme of Figure 3f. Hence, the In2S3 NPL's surface contains a large number of under-coordinated cations, bounded to



three anions only, rather than six, as in a bulk position. The under-coordinated surface can probably be passivated by organic ligands, such as thiols and amines (see low bar) which bind to an In-rich surface via a single or a multiple bonding,45 and accordingly, a stabilization of the exterior surface energy. The capping ligands screen the surface from a flux of diffusing monomers, reducing the rate of the deposition, and avoiding any further growth along the direction. Rate of deposition is defined in following texts as the rate of diffusion per unit area of a facet and indicated by Ri (i=1,2,3), so the growth along the direction is defined as R1 (see Figure 6b). Alternatively, the growth develops along the lateral dimensions, when monomer units [In2S3] are deposited either on the {101̅0} m-planes or on the {112̅0} a-planes, with deposition rates R2 and R3, respectively. Under non-equilibrium kinetic growth with a high flux of monomers, preferential growth takes place along the kinetically most favorable direction with low activation energy barriers. The diffusion of the free monomer units is driven by the potential gradient (∆P) between the bulk potential (Pbulk) and a facet potential (Pa,m). Schematic scaling of a potential gradient is shown at the right side of Figure 6b. Increasing the growth temperature to 250°C brings about an initial monomer-rich solution with a high chemical potential, increasing the gradient between the bulk solution and the NPLs' a- and m-planes; viz., ∆Pbulk→a,m>>0. However, as defined above, the deposition rate is governed by the diffusion rate per unit area of a facet and since m-planes have smaller area than that of the a-planes, then under ∆Pbulk→a,m>>0 a preference growth normal to the m-

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planes occurs, in order to compensate for their larger surface strain. This preference leads to the formation of hexagonally shaped γ-In2S3 NPLs. At a temperature of 230°C, there is a relatively low [In2S3] monomers' concentration in the bulk solution and although ∆Pbulk→a > ∆Pbulk→m > 0, the overall deposition rate is nearly equivalent, due to the difference in the planes' surface area. Such a uniform deposition rate advances for the formation of dodecagon shape of the NPLs. Thus, Figure 6b elucidates three routes: Option (I) considers R1>> R2, R3, a case which is not relevant to our study, due to passivation of the cplanes; Option (II) considers R1

Unraveling the Growth Mechanism Forming Stable ... - ACS Publications

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