Highly Textured Tin(II) Sulfide Thin Films Formed from Sheetlike

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Highly Textured Tin(II) Sulfide Thin Films Formed from Sheetlike Nanocrystal Inks Steven M. Herron,† Jukka T. Tanskanen,‡,§ Katherine E. Roelofs,∇ and Stacey F. Bent*,§ †

Department of Chemistry, §Department of Chemical Engineering, and ∇Department of Materials Science and Engineering, Stanford University, Stanford, California 94305-4125, United States ‡ University of Eastern Finland, Department of Chemistry, P.O. Box 111, FI-80101 Joensuu, Finland S Supporting Information *

ABSTRACT: Highly textured tin(II) sulfide thin films are prepared from a nanocrystal ink comprised of high-aspect-ratio nanosheets. The orthorhombic nanosheets are synthesized colloidally to isolate lateral growth and minimize the presence of alternate crystal phases. The tin sulfide films deposited from the nanosheets exhibit pure elemental composition, micrometersized grains, and a remarkable degree of texturing. The films consist of lamellar stacking of nanosheets with some intercalation, and the average sheet thickness is ∼30 nm. The SnS films have an indirect band gap of 1.23 eV, and density functional theory calculations indicate minimal quantum confinement contributions. The anisotropic electronic properties of tin sulfide are greatly intensified in films formed by this process, yielding an in-plane mobility of 5.7 cm2/(V s) but an out-of-plane resistivity as high as 30 kΩ cm. This work represents a new strategy for nanocrystal inks in which the nanocrystal morphology is tailored to direct film orientation, grain size, and transport properties. The method provides a route for the deposition of high-quality, layered semiconductor thin films with applications in photovoltaics and two-dimensional (2-D) electronics.

I. INTRODUCTION High-quality semiconductor thin films form the basis of numerous electronic technologies today, made possible by a wide variety of thin-film deposition techniques that rely overwhelmingly on vacuum methods.1 Large-area electronics (sensors, displays, photovoltaics, and other optoelectronics) have tested the limits of these vacuum technologies, where the advantages of high purity and crystallinity can begin to be countered by deficiencies in large-area nonuniformity, high cost, low throughput and low materials utilization.2 As the demands for clean renewable energy and large-area electronics increase, the development of scalable film deposition technologies is paramount. Solution deposition methods, in which thin films are deposited on substrates via a chemical solution, offer a potential solution to scalability limits, and techniques such as chemical bath deposition (CBD), successive ion layer adsorption reaction (SILAR), electrodeposition (ED), spray pyrolysis (SP), and chemical inks have been employed in efficient photovoltaic devices.3−7 Among solution methods, chemical © 2014 American Chemical Society

inks boast high deposition rates and the potential for minimal liquid waste generation. Such inks can be comprised of both molecular and nanocrystal (NC) precursors, and nanocrystal inks herein refer to methods in which colloidal nanocrystals are applied to a surface and treated, either leaving the NCs intact or promoting coalescence to form larger grains. Unlike other methods, NC inks provide the unique ability to form the desired crystalline phase prior to actual film formation. This temporal decoupling of initial crystal nucleation and film deposition has several important implications. First, it promotes high deposition rates. While the deposition rates of CBD, SILAR, ED, and SP are kinetically limited by surface reactions, NC inks can quickly deposit thicker films, as they must only connect the surfaces of existing crystallites. Second, especially in multielement material systems, the preformation of nanocrystals promotes compositional homogeneity throughout the film Received: October 6, 2014 Revised: November 20, 2014 Published: November 20, 2014 7106

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II. METHODS

and is therefore less limited by selective reactivity of precursors.8 Furthermore, this separation allows a hightemperature synthesis of nanocrystals to promote crystallinity, while still permitting deposition at relatively low temperatures. In contrast, other low-temperature solution deposition methods can yield films suffering from small grain size or even an amorphous structure.9,10 Finally, the decoupling of nanocrystal formation from film deposition provides the opportunity to tailor nanocrystal properties including size and shape to elicit unique properties in thin films and superlattices.11,12 In this work, we extend this concept to controlling crystallographic orientation in thin films, specifically in the tin(II) sulfide system. In many semiconductor thin films, control over crystallographic orientation is key in attaining desirable transport properties.13,14 Tin(II) sulfide is a p-type semiconductor comprised of nontoxic and abundant elements, and it is a good candidate for a photovoltaic absorber, with an indirect band gap of 1.1 eV, a direct band gap of 1.4 eV, and a high absorption coefficient.15,16 The stable crystal structure at room temperature (α-SnS) is a layered compound with an orthorhombic unit cell comprised of two-atom-thick SnS layers parallel to the (010) plane, which are collectively held together by van der Waals forces.17 The unit cell contains two such layers of SnS along the b-axis, giving the cell dimensions [a = 4.329 Å; b = 11.192 Å; c = 3.984 Å].18 The consequences of the layered structure are manifested in anisotropic electrical properties, with hole mobilities in bulk SnS of 500 cm2/(V s) along the sheets, but only 90 cm2/(V s) in the perpendicular direction.19 Several other structures have been reported experimentally for SnS, including a pseudo-tetragonal (PT) modification of the α-SnS phase, previously called zinc-blende, which can take on a pyramidal morphology.20−22 Tin sulfide is poised to make efficient use of the terrestrial solar spectrum with a Shockley−Queisser efficiency limit of ∼24%, based on its indirect band gap. Although significant technical hurdles remain, recent strides have boosted the record efficiency to 4.4%.23−25 Recently, thin films of SnS have been prepared from chemical inks consisting of either spherical nanocrystals26 or a molecularly dissolved SnS powder,27 the latter demonstrating promising current collection and photovoltage in photoelectrochemical measurements.27 Generally, SnS photovoltaic devices still suffer from low open circuit voltage and fill factor, which may be the result of intrinsic defects, interface recombination, or the lack of appropriate heterojunction partners.24,28 In addition, a recent study has suggested that nonuniformity in crystallographic orientation at the surface of SnS can lead to variations in the band-edge energies of up to 0.9 eV, which may be a large contributor to low photovoltages.29 Therefore, gaining more fundamental control over the crystal structure, orientation, and deposition of these films remains an important task.30 We utilize nanocrystal ink deposition to gain some of this control in the tin sulfide system. Nanocrystal inks typically employ very small, spherical nanoparticles under 10 nm in diameter which, upon sintering together, form a polycrystalline film.5,26,31 Grain boundaries inevitably arise isotropically as this sintering occurs, giving rise to a randomly oriented crystal structure. Here, we present a novel synthesis of SnS nanosheets, ∼30 nm in thickness and 3 μm in width on average, and demonstrate that the high aspect ratio of these crystals provides a method for control over orientation, grain size, and grain boundary orientation in tin sulfide thin films.

Synthesis of SnS Nanocrystals. Tin sulfide nanosheets were prepared in a one-step synthesis. Oleylamine (Sigma−Aldrich, 98%) was degassed at 150 °C and 10 mTorr for 2 h. A sulfur precursor solution was prepared by dissolving elemental sulfur (17.7 mg, Sigma− Aldrich, 99.99%) in oleylamine, which was sonicated at room temperature for 2 h, forming a clear red solution. Separately, tin acetylacetonate [Sn(acac)2] (109 mg, Sigma−Aldrich, 99.99%) was weighed in a nitrogen glovebox, and 1 mL oleylamine was added and stirred briefly to form a clear yellow solution. Using standard Schlenk line techniques, 10 mL oleylamine and the sulfur solution were combined in a 3-neck round-bottom flask and heated to 50 °C. The Sn(acac)2 solution was added, changing the mixture from orange to very light yellow, and the reaction mixture was heated at 10 °C/min to 190 °C, where it remained for 16 h, and an aliquot was removed. The temperature was subsequently raised to 300 °C for 2 h, and finally cooled to room temperature. The reflective, gray product was isolated by three cycles of centrifugation for 10 min at 5000 rpm with uptake in hexane. The density of the nanoparticles precluded the need for a countersolvent in this purification scheme. They were finally redissolved in hexane to create a stock solution of ∼15 mg/mL. Preparation of SnS Ink and Film Deposition. Inks of the SnS nanocrystalline sheets were prepared by centrifuging 500-μL aliquots of the SnS stock suspension in hexane for 5 min at 5000 rpm and redissolving them in 50 μL of a 4:1 mixture of octane and 1hexanethiol for deposition. The solvent and concentration were optimized to prevent aggregation of plates upon drying. Films were deposited by drop casting or knife-coating onto silicon and glass substrates at room temperature. They were subsequently annealed in a tube furnace at 500 °C for 2 h under a nitrogen flow of ∼60 cm3/min. A separate polycrystalline SnS thin film was prepared for comparative measurements on glass by vapor transport deposition.16 Characterization Methods. Ultraviolet−visible light (UV-vis) spectroscopy measurements were performed on a spectrophotometer (Cary, Model 6000i) with an external diffuse reflectance accessory in transmission, diffuse reflection, and total reflection modes. Scanning electron microscopy (SEM) was performed on an FEI Magellan 400 XHR SEM microscope with a beam voltage of 5 kV and current of 50 pA. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) imaging were carried out on an FEI Tecnai G2 F20 X-TWIN system with a beam voltage of 200 kV. The SnS nanocrystals were dispersed onto an ultrathin carbon coating on a holey carbon film supported by a 300 mesh copper grid (Ted Pella, Inc., No. 01824), and image analysis was performed using ImageJ software (v. 1.48). Auger electron spectroscopy (AES) measurements were recorded with a PHI Model 700 scanning Auger nanoprobe. Compositions were normalized using commercial tin(II) sulfide powder (Alfa Aesar, 99.5%). X-ray diffraction (XRD) was performed with a PANalytical Model X’Pert PRO X-ray diffraction system using programmable divergence slits and a Pixel detector. Scans were performed over a 2θ range of 10°−70°. The polycrystalline tin sulfide sample prepared by vapor transport deposition was used as a nontextured reference.16 Four-point probe and Hall measurement were performed using a temperature microprobe in the Van der Pauw configuration (MMR Technologies) at room temperature. Grazingincidence small-angle X-ray scattering (GI-SAXS) was conducted at the Stanford Synchrotron Radiation Lightsource on Beamline 1-4. Computational Methods. Density functional theory (DFT) calculations utilizing Generalized Gradient Approximation (GGA) PBE-D32 and hybrid B3LYP-D33,34 functionals of orthorhombic SnS bulk and slab structures were performed using the CRYSTAL0935 program. Dispersion-corrected DFT36 was necessary because of van der Waals interactions, which are not properly described by standard DFT functionals, between noncovalently bound layers in orthorhombic SnS. Utilization of the two DFT functionals enabled evaluation of the sensitivity of the results, with respect to the utilized functional. A standard split-valence plus polarization (SVP)37,38 basis set was used for S and a modified SVP basis for Sn.39 In the slab calculations, tight tolerance factors of 8, 8, 8, 8, and 16 were utilized in 7107

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the evaluation of the Coulomb and exchange integrals (TOLINTEG in CRYSTAL09 input). Default optimization convergence threshold, extra-large integration grid, and 25 k-points in the irreducible Brillouin Zone generated by the Monkhorst−Pack method40 were adopted in the calculations. This computational approach provided converged energetics and band gaps for bulk SnS, as verified by performing calculations with 1 × 1 × 1 and 2 × 2 × 2 bulk unit cells, and the computed structural parameters for orthorhombic SnS were within 4% of the experimental values with both PBE-D and B3LYP-D. Expectedly, since weak interactions are generally challenging for DFT, the largest deviation from the experimental lattice vector was observed for the [010] direction, along which the noncovalently bound SnS layers are stacked. Periodic SnS slabs up to ∼9 nm in thickness were produced from the SnS 1 × 1 × 1 bulk by systematically repeating a SnS layer along the c-axis, followed by removing periodicity along the [010] direction. This approach produced slab unit cells with compositions ranging from Sn2S2 (single-layer SnS) to Sn32S32 (16 SnS layers) and all the slabs were fully optimized. Convergence of the energetics and band gaps of the slabs, with respect to unit cell size, was verified by comparing the results for single- and double-layer SnS using both 1 × 1 and 2 × 2 slab unit cells. The total energies per structural unit (SnS) and the band gaps were independent of the size of the unit cell.

1000, are among the highest reported for tin sulfide, and the sheets exhibit single facets, large size, and good size uniformity, in comparison with previous syntheses.22,42−44 The disappearance of pyramidal nanocrystals upon the heat treatment is consistent with previously reported conversion of pseudotetragonal particles to the orthorhombic structure.45 We obtained only limited evidence by SEM of such a transition occurring: in one micrograph, a sheet protrudes from a degraded pyramid, herein termed a subhedral particle (see Figures S2 and S3 in the Supporting Information). Other images show that the sharp edges characteristic of the pyramidal nanocrystals at 190 °C become blunted at 300 °C, which is consistent with diffusion of atoms at the high-energy edge and corner sites to form a more-stable sheet structure. However, there was no clear morphological evidence of whether pyramidal or sheetlike particles nucleated first, because the earliest precipitate in the reaction resembled neither (see Figure S4 in the Supporting Information). Although this may be a mechanism for initial sheet formation, it is evident that once nucleated, sheets continue to grow, in both thickness and width. The number density of the pyramidal particles significantly decreases upon 300 °C heat treatment (Figure 1), although some are still detected by XRD (see Figure 5,

III. RESULTS AND DISCUSSION The synthetic method for tin sulfide was developed in this work to create SnS sheets in sufficient quantity and purity for employment as a nanocrystal ink. The use of Sn(acac)2 as a precursor was chosen to reduce oxychloride contamination found in other Sn precursors, such as SnCl2, as well as to enable high purity.41 Oleylamine was chosen as a solvent and ligand for its flexibility in accommodating the growth of a variety of SnS morphologies, thus allowing temperature control to govern nanocrystal shape.20,21 Shortening the alkylamine chain from oleylamine to decylamine had little effect on the resultant SnS nanocrystal size and shape at 190 °C; thus, the longer chain was chosen for stability at 300 °C. The growth of sheetlike particles was found to rely on a relatively low reaction temperature, catering to a presumably lower activation barrier for growth along the layers of tin sulfide, i.e., the [200], [002], and [101] directions, rather than isotropic growth, which would add layers along the [020] direction (confirmed, vide infra, by TEM). In particular, by injecting precursors at 50 °C and slowly ramping to 190 °C, growth was largely restricted to the lateral directions, as evident by SEM. The resultant aliquot after this stage (Figure 1a) contained thin, bladed SnS sheets with dimensions of ∼0.5 μm × 2 μm, many of which had thickness below 5 nm, and some as large as 20 nm thick (see Figure S1a in the Supporting Information). This aliquot also had a small but significant number of pyramidal nanocrystals 100−300 nm in diameter. The presence of these pyramids is consistent with crystals previously reported to be zincblende or, more recently, attributed to a pseudo-tetragonal phase.22 In the context of this work, these crystals are considered to be impurities, because their three-dimensional morphology is a potential hindrance to the lamellar stacking of the nanocrystals. After the temperature of the reaction mixture was increased to 300 °C, these pyramidal impurities were almost entirely removed, leaving a solution of pure nanosheets (Figure 1b) with a reaction yield of 70%. The average size of the 300 °C product nanosheets measured by SEM was greater than 1 μm × 2 μm, and the average thickness measured from the cross section was ∼30 nm, with a maximum of ∼40 nm and a small minority below 5 nm (see Figure S1b in the Supporting Information). The resulting aspect ratios, ranging from 100 to

Figure 1. SEM images of the (a) 190 °C aliquot and (b) 300 °C product of the SnS nanosheet synthesis.

presented later in this paper). It is believed that a portion of them completely dissolve upon the 300 °C heat treatment in an Ostwald ripening process, both decreasing their concentration and contributing to further growth of the plates or other subhedral particles (see Figure S3 in the Supporting Information). The microstructures of SnS nanoparticles synthesized at 190 and 300 °C were further analyzed by TEM (see Figure 2). Typical nanocrystal ensembles of the 190 and 300 °C products are shown in Figures 2a and 2e, respectively, with morphology consistent with their appearance in Figure 1. Isolated sheets selected for analysis (Figures 2b and 2f) were identified as single crystalline, exhibiting the stable orthorhombic α-SnS structure, and all planes were indexed to the [010] zone axis. The (101) plane spacing was measured from HRTEM images to be 2.93 and 2.92 Å in each sample, respectively, in excellent agreement with the bulk value of 2.93 Å. The crystallographic [200] and [002] directions are annotated in Figures 2b and 2f, identifying the long edge of these crystals as the [200] surface. While the (200) and (002) planes were not as readily apparent in Figures 2c and 2g, these planes could be clearly seen in other HRTEM images (see Figure S6 in the Supporting Information). The four shorter edges of the six-sided crystals comprise the {101} surfaces, and preferential growth along these directions likely creates the elongated morphology of the 7108

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Figure 2. TEM images of (a) representative SnS nanosheets from the 190 °C aliquot and (b) a single sheet from the 190 °C aliquot used for imaging; (c) the associated HRTEM image (taken from an aligned region within the yellow area) and (d) fast Fourier transform. Panels (e−h) show the corresponding TEM images of SnS nanosheets from the 300 °C product.

quickly, the plates tend to aggregate, leaving large voids as well as a morphology more similar to a “house of cards”, with many sheets lying on their end.46 When dried too slowly, the sheets precipitate out of solution, after which they are swept into nonuniform clumps of material, because of uneven drying across the substrate. The hexane/octanethiol mixture yields an intermediate drying rate, allowing for optimal uniformity, both in knife coating and drop casting techniques. After annealing in nitrogen at 500 °C, the resulting films are a uniform shiny gray color. SEM reveals that the resultant thin films exhibit lamellar stacking to a large extent (see Figure 4). The films contain extremely flat domains that stretch for tens of micrometers (Figure 4a), and there is a degree of overlap in the sheets, causing bending, especially in the top layers (Figure 4b). The average film thickness, measured by a series of SEM cross sections, is 650 nm (Figure 4c). In some areas, the small isotropic nonsheet particles become intercalated between the sheets, spreading them apart (see Figure S5 in the Supporting Information). The structural properties were studied using X-ray diffraction on both annealed (to 500 °C) and unannealed SnS thin films on glass, and compared to a polycrystalline SnS standard film deposited by vapor transport (Figure 5).16 Both films exhibit texturing in the [010] direction, which is consistent with the tin sulfide layers lying in plane with the substrate. The texturing was so complete that peaks that were not associated with the {010} family were barely detected (see Figure S7 in the Supporting Information), and, furthermore, the normally invisible (020) peak was detected (Figure 5, inset). An approximate estimate of the relative texturing is given by the int int ratio T = Iint (040)/I(002), where I is the integrated peak intensity of each peak, giving the fraction of crystals oriented in the bdirection, relative to those oriented the c-direction. For a theoretical orthorhombic SnS powder sample,18 Ttheory = 1, and the value calculated from the polycrystalline SnS standard film is Tstandard = 0.58. By comparison, for the as-deposited and annealed SnS nanosheet films, T values of 210 and 410, respectively, were obtained. Thus, compared to the polycrystalline SnS standard, these thin films exhibit a relative texturing in the [010] direction of 360x and 710x, respectively. This is an unusually high degree of texturing for a solution-processed

low-temperature product in the oleylamine−sulfur−Sn(acac)2 system. The major in-plane growth that occurs between 190 and 300 °C is in the [200] direction, as evidenced by significant widening of the crystals in this dimension, seen in both Figures 1 and 2. The composition of the resultant nanosheets was obtained using AES, which was chosen for its high sensitivity to sulfur (see Figure 3). AES is a surface-sensitive technique, probing the

Figure 3. AES spectrum of SnS nanosheets imaged in Figure 1b.

outermost several nanometers of a film. The peak-to-peak height in the dN(E)/dE plot correlates with the abundance of each element, and each is scaled by a sensitivity factor to determine the atomic concentration values. The nanosheets were found to be oxygen-free and to contain an approximately stoichiometric Sn:S ratio of 49:42. The film contains 9% carbon at the surface, which is consistent with the presence of pure SnS with some oleylamine ligands or carbonaceous thermal decomposition product still present on the surface. Thin films of SnS were prepared by either drop-casting or knife-coating the high-temperature (300 °C) product nanosheets from a hexane/octanethiol mixture, which was found to offer an optimal solubility and drying rate. When dried too 7109

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Figure 4. SEM photomicrographs of annealed SnS thin films (a,b) normal to the substrate surface and (c) in cross-section.

Figure 5. X-ray diffraction of SnS thin films before and after annealing, compared to polycrystalline and calculated standards (JCPDS File Card No. 00-039-0354). The inset shows a magnified view of the region over the (020) peak. The (020), (040), and (080) reflections are due to orthorhombic SnS sheets, and the (201) pseudo-tetragonal SnS (formerly (111) zincblende) come from pyramidal impurity SnS particles.

nanocrystal thin film, and is attributable to the settling dynamics of the synthesized high-aspect-ratio SnS nanosheets. An additional peak present in both nanocrystal films is assigned to the (201) peak of the pseudo-tetragonal SnS phase.20,22 This assignment is supported by the lingering presence of pyramidal and subhedral nanocrystals seen in the SEM images. The relative fraction of these particles is small enough that only the cubic (111) (i.e., the pseudo-tetragonal distorted (201)) peak is detected. The Scherrer formula is utilized to estimate the average grain size of both nanosheets, using the (040) and (080) XRD reflections, as well as the subhedral and pyramidal particles, using the (201) reflection (see Table 1). The average grain size along the [010] direction, which, in the case of single-crystal nanosheets, equates to the nanosheet thickness, is ∼30 nm, and remains constant upon annealing (note that the (040) calculation is taken as being more precise, because of the more intense signal; see Figure 5). Thicknesses were also measured by SEM cross section on a random sampling of vertically oriented sheets (see Figure S1b in the Supporting Information), and found to be 27 ± 8 nm, corroborating the Scherrer analysis. Although the average sheet thickness is established to be ∼30 nm, the actual distribution evidently includes sheets too thin for analysis by either of these

Table 1. Grain Sizes of Nanosheets and Pyramidal/ Subhedral Particles in SnS Films from Scherrer Analysis of XRD Data in Figure 6 Nanosheet Thickness (nm) reflection unannealed film (040), orthorhombic 31 nm (080), orthorhombic 27 nm Subhedral Particle Size (nm) (111/201), cubic/PTD 115 nm

annealed film 30 nm 21 nm 68 nm

techniques (see Figure S1 in the Supporting Information). The thickness uniformity throughout heat treatment indicates that the nanosheets are not sintering together upon annealing, as would be desired to form high-quality thin films. Two possible inhibiting factors include the physical voids created by intercalated particles, as well as the presence of remaining oleylamine ligands, which may act as an insulating barrier to any sintering process. On the other hand, according to the Scherrer analysis on the (201) reflection, the average pyramidal/subhedral particle size decreases markedly from ∼115 nm to 70 nm, which is indicative of some extent of thermal decomposition of these particles, as observed previously.20,45 7110

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Figure 6. Plots depicting optical properties: (a) transmittance, total reflectance, and absorbance; (b) diffuse reflectance and absorbance; and (c) Tauc plot for indirect allowed transition.

To study the optical properties of the films, the optical response of SnS films on glass substrates was measured. An integrating sphere was employed to account for scattering and reflection. The absorbance (Figure 6a) shows a strong onset at ∼1000 nm. Of note, the total reflectance (including specular) at energies above the band gap remained at 40%, a high value for an absorber material, which can be ascribed to the extremely flat surfaces presented by the nanosheets. Surface roughening techniques would be expected to greatly reduce this reflectance. Furthermore, the absorbance was calculated from the diffuse reflectance using the Kubelka−Munk approximation for films thick enough to obscure the substrate (eq 1): f (R ∞) =

(1 − R ∞)2 α = S 2R ∞

(1)

(see Figure 6b). The Kubelka−Munk function f(R∞) is directly proportional to the absorbance (α) divided by the scattering coefficient (S), where R∞ is the diffuse reflectance of the film.47 From the absorbance data of Figure 6b, a Tauc plot can be generated (Figure 6c). The Tauc plot reveals an indirect band gap of 1.23 eV. A direct transition was not observed, likely because the absorbance had already saturated at wavelengths below 900 nm, rendering the film too optically thick in this range for quantitative absorbance values. Of interest, the indirect band gap of 1.23 eV is higher than the bulk value of 1.08 eV for SnS.19 With no observed oxygen contamination (which would increase Egap) and lateral grains on the micrometer length scale, one might expect the band gap to approach bulk values. Therefore, the possibility of quantum confinement along the [010] direction, normal to the layered SnS plane, was investigated. Quantum confinement in SnS nanostructures has been observed, typically on length scales lower than 10 nm, although claimed in structures as large as 50 nm in diameter.48,49 Density functional theory (DFT) calculations with PBE-D and B3LYP-D functionals were employed in a two-dimensional (2-D) slab model of SnS to probe the effect on band gap of size confinement along the [010] direction. The relationship of band gap versus thickness (Figure 7) reveals significant (>10%) exponential increases in band-gap values with decreasing thickness at slab thicknesses of 4 unit cells (2.0 nm) and less, compared to bulk values. Although the two functionals yielded different band gaps (lower band gap with PBE-D than with B3LYP-D, as expected), the relationship between band gap and thickness was functionalindependent, and the conclusion is drawn that there are no significant quantum confinement effects in slab thicknessess

Figure 7. DFT calculations of (a) SnS band gap versus thickness (in nanometers) and (b) formation energies (relative to bulk) of SnS slabs at varying thicknesses, calculated using the PBE-D functional.

above 8 unit cells (or 4.3 nm). Therefore, it is expected that quantum confinement effects are insignificant in the 30-nm SnS sheet distribution. While a few of the 300 °C product SnS nanosheets had diameters near or below 5 nm, these sheets are in the vast minority by both number and volume (Figure S1b in the Supporting Information gives a representative sampling), and their contribution to absorbance within the film, as a wholeand, therefore, the observed bandgapis expected to be negligible. One possible explanation for the positive deviation in measured bandgap from the bulk is that the reflectance and transmission measurements may be too insensitive to measure the low absorption values near a 1.1 eV band edge, instead, revealing higher-energy intraband transitions.15 However, we believe it is more likely that the deviation is attributed to orientation, because variations in the optical band gap of SnS along different crystallographic axes have been reported in the literature. For incident light traveling in the [010] direction, indirect bandgaps of 1.10 and 1.42 eV have been reported from absorption measurements with polarization parallel to the a- and c-axes, respectively, at 300 K.50 Similarly, the direct energy gaps in the same direction have 7111

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Table 2. Electrical Properties of Annealed SnS Nanosheet Films In-Plane Electrical Properties average standard deviation

Out-of-Plane Electrical Properties

resistivity (Ω cm)

mobility (cm2/(V s))

density (× 1018 cm−3)

ρshunt (× 104 Ω cm)

mobility (est.) (× 10−4 cm2/(V s))

1.07 0.12

5.7 0.30

1.0 0.15

3.0 0.33

2.1 0.11

confinement was predicted to have negligible influence on this value, based on DFT calculations. X-ray diffraction corroborated SEM observations of a typical sheet thickness of ∼30 nm and demonstrated an exceptionally high degree of texturing along the [010] direction, normal to the plane. Annealing did not induce coalescence of plates in that same direction, although the degree of texturing increased and the subhedral particle size decreased upon heat treatment. Finally, the deposited SnS films showed an extreme amplification of the anisotropy between in-plane and out-of-plane electrical properties, attributed primarily to the partially intercalated morphology. Future improvements in electrical mobility in all three directions hinge on the ability to limit non-sheetlike intercalations, as well as on modifying the surface chemistry of the nanocrystals to foment grain coalescence upon annealing and increase electronic coupling between nanosheets.

exhibited band gaps of 1.30 and 1.60 eV by electroreflectance measurements parallel to the a- and c-axes, and separate polarized absorption spectroscopy measurements confirmed this trend.51 More recently, ab initio DFT calculations have estimated large optical transitions of ∼1.5 eV for light traveling in the [010] direction, while a minimum transition energy of ∼1.2 eV occurred in the [001] direction.52 These elevated band gaps in the [010] direction are relevant to our system where optical measurements were performed normal to the [010] textured SnS plates. Finally, the electronic properties of the SnS thin films were tested using the Hall measurement in the four-point Van der Pauw configuration, and shunt resistivity was measured on a planar glass/ITO/SnS/Au device stack (see Table 2). Out-ofplane mobilities were estimated by ratioing the shunt and inplane resistivities. In bulk single-crystal SnS, the mobility in the [010] direction is 90 cm2/(V s), while mobilities in the plane are 500 cm2/(V s) at 300 K, which is a factor of 5.5 higher, because of the layered structure.19 The experimental SnS nanosheet films in this work achieve an in-plane mobility of 5.7 cm2/(V s), well below the bulk value, but comparable to other thin-film methods.15 Atomic-layer-deposited SnS has reported mobilites between 0.82 and 15.3 cm2/(V s), although these remain well below the in-plane mobilities achieved in epitaxial vapor transport deposition of 385 cm2/(V s).15,16 The p-type doping density for the highly textured SnS films of 1018 cm−3 is equivalent to the bulk value. However, the anisotropy of in-plane versus out-of-plane conductivity is greatly amplified in our films, a factor of 2.8 × 104, versus 5.5 in the bulk and 4.8 in epitaxial thin films.16,19 This magnified ratio is understood by considering the film morphology, where physical gaps in the out-of-plane layers (caused by intercalated particles) may severely limit conduction, while the lateral connectivity between plates preserves transport; controlling this anisotropy may be beneficial in twodimensional (2-D) electronics applications.53,54 It is further expected that, by replacing the insulating oleylamine ligands with shorter organic or inorganic moieties, increases in both lateral and out-of-plane mobility and conductivity could be obtained, both by increasing wave function overlap between adjacent nanosheets and by promoting grain coalescence during the annealing phase.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information document shows additional SEM images of the 190 and 300 °C products (Figures S1−S3) as well as SEM images of the earliest synthesis products (Figure S4) and the annealed films (Figure S5). Also included are TEM images and analysis (Figure S6) in which the (002) and (200) planes are seen in the HRTEM image. Finally, a logarithmic plot of the XRD data from Figure 5 is shown (Figure S7) for ease of observing the smaller XRD peaks. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 650-723-0385. Fax: 650-723-7980. E-mail: sbent@ stanford.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Studies were carried out as part of the Center on Nanostructuring for Efficient Energy Conversion, an EFRC funded by the U.S. Department of Energy, Office of Basic Energy Sciences under Award No. DE-SC0001060. S.H. acknowledges support from the National Science Foundation (NSF) for a Graduate Research Fellowship. J.T.T. gratefully acknowledges the Academy of Finland (Grant No. 256800/2012) and the Finnish Cultural Foundation for financial support.

IV. CONCLUSIONS Tin sulfide nanocrystals were synthesized in a route designed to maximize the yield of extremely high-aspect-ratio sheets. The synthesis also produced a small fraction of pyramidal particles that were partially removed upon prolonged heat treatment, both in the colloidal and thin film phases. Nanocrystal inks were then optimized through selection of solvent and concentration of the nanocrystal suspension, applied to substrates, and annealed in nitrogen to form oxygen-free thin films of the desired composition. The thin films were shown to exhibit lamellar stacking with some intercalation of subhedral particles, and the optical properties demonstrated high reflectivity and an indirect band gap of 1.23 eV. Quantum

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Article

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