Article pubs.acs.org/cm
Shape-Controlled Synthesis of SnE (E = S, Se) Semiconductor Nanocrystals for Optoelectronics Xin Liu,† Yue Li,† Bin Zhou,‡ Xianliang Wang,† Alexander N. Cartwright,‡ and Mark T. Swihart*,† †
Department of Chemical and Biological Engineering and ‡Department of Electrical Engineering, The University at Buffalo (SUNY), Buffalo, New York 14260-4200, United States S Supporting Information *
ABSTRACT: We report new methods of synthesizing colloidal SnE (E = S, Se) nanocrystals (NCs) with morphologies including quantum dots (QDs), nanoplates, single-crystalline nanosheets, nanoflowers, and nanopolyhedra. Both the selection of chalcogenide precursors and the combination of ligands play essential roles in determining the morphology of the SnS and SnSe NCs. In contrast with previous methods for synthesis of tin chalcogenide NCs, which generally used a relatively expensive and flammable organic tin precursor, bis[bis(trimethylsilyl)amino]tin(II), we employ inexpensive SnCl2 and identify chalcogenide precursors with appropriate reactivity to enable production of nearly monodisperse SnSe NCs and SnS nanostructures. A metal− semiconductor−metal (MSM) device was fabricated to study the optoelectronic properties of SnSe NC thin films, spin-cast from colloidal SnSe NC dispersions. The photoresponse of the SnSe thin film to AM 1.5 simulated solar illumination demonstrates the potential of SnSe NCs for use in optoelectronics and photovoltaics.
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INTRODUCTION Over the past decade, synthesis of IV−VI colloidal semiconductor nanocrystals (NCs) and their potential applications in the field of optoelectronics have been widely studied1−4 The most heavily studied materials in this family (PbS, PbSe) have a narrow band gap that can be tuned across the infrared and visible spectrum by varying the NC size, through quantum confinement.5,6 This allows creation of NCs with appropriate band gap for different components of photovoltaic devices and enables creation of tandem solar cells from a single material.7 Nanocrystals of tin-based semiconductors including SnS,8−16 SnSe,17−22 SnTe,23,24 and their alloy NCs25,26 are also of interest because their band gap can be tuned from near-infrared to visible wavelengths. Moreover, the relatively high abundance of their constituent elements and low toxicity of tin, compared with lead and cadmium, make tin chalcolgenide NCs more promising than most other NC materials as a basis for longterm, low-cost, high-volume, environmentally benign photovoltaic and optoelectronic technologies.27−29 However, previous studies have had little success in preparing high-quality SnS, SnSe, and SnTe NCs using costeffective tin precursors such as tin-oleate, tin chloride, or tin © 2014 American Chemical Society
acetate, primarily due to the low reactivity of these precursors.17,23 The conventional strategy for synthesizing SnE NCs requires a more reactive tin precursor, typically bis[bis(trimethylsilyl)amino]tin(II).8,17,23 The high reactivity of this precursor supports reaction with common organochalcogenide precursors to form colloidal SnE (E = S, Se, Te) NCs. However, the instability and high cost of this reactive organo-metallic complex may limit the large-scale production of these materials, and therefore their viability for use in practical optoelectronic devices. To solve the problem, here we demonstrate a facile method for controllable synthesis of SnE (E = S, Se) NCs with various morphologies including quasi-spherical dots, nanoplates, singlecrystalline nanosheets, nanoflowers, and nanopolyhedra using SnCl2, an inexpensive tin salt, as the metal precursor. In contrast to previously reported methods, our synthetic strategy focuses on studying and preparing chalcogenide precursors that are compatible with a low-cost tin precursor. We found that the Received: March 24, 2014 Revised: May 7, 2014 Published: May 8, 2014 3515
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0.1 M OAm−Se solution. The solution was held at 146 °C for 30 s for nanocrystal growth. For synthesis of orthorhombic SnSe nanoplates, 3.5 g of dodecylamine (DAm), in place of OAm, was used with 1 mL of OA to dissolve the SnCl2 powder. Other reaction conditions were kept the same as those used for synthesis of SnSe NCs with dotlike morphology. Synthesis of SnS Nanosheets. In a typical synthesis, 0.25 mmol of SnCl2 powder was mixed with 2 mL of OAm, 1 mL of OA, and 10 mL of ODE in a flask. The solution was degassed at 120 °C for 30 min under nitrogen protection and then heated to 220 °C. Two milliliters of 1-dodecanethiol was injected into the solution and the solution was held at 216 °C for 22 min. Synthesis of SnS Polyhedral NCs. In a typical synthesis, 0.25 mmol of SnCl2 powder was mixed with 1 mL of OAm, 0.5 mL of OA, and 10 mL of ODE in a flask. The solution was degassed at 120 °C for 30 min under nitrogen protection and then heated to 200 °C. Two milliliters of octanethiol was injected, and then the mixture was held at 193 °C for 45 min. Synthesis of Flowerlike SnS NCs. In a typical synthesis, 0.5 mmol of SnCl2 powder was mixed with 2 mL of OAm and 10 mL of ODE in a flask. The solution was degassed at 120 °C for 30 min under nitrogen protection and then heated to 150 °C. Two milliliters of octanethiol was injected, and then the mixture was held at 146 °C for 6 min. Separation and Purification of SnSe and SnS NCs. To separate NCs from the reaction product, ethanol was added to the solution followed by centrifugation. SnSe and SnS NCs can be well-dispersed in common organic solvents including chloroform and toluene. The centrifugation and redispersion procedure was repeated twice to remove residual ligands and unreacted precursors. Characterization. Transmission Electron Microscopy (TEM). The size and morphology of SnSe and SnS NCs were characterized using a JEOL JEM-2010 microscope at a working voltage of 200 kV. Samples were prepared for analysis by drop-casting from a dilute dispersion onto a carbon-coated copper TEM grid. Powder X-ray Diffraction (XRD). The crystal phases in SnSe and SnS NCs were determined using powder XRD (Bruker Ultima IV with Cu Kα X-rays). Samples were prepared by drop-casting highconcentration SnSe and SnS NC dispersions onto glass. UV−Vis−NIR Spectroscopy. Optical absorption spectra of SnSe and SnS NC dispersions were measured using a Shimadzu 3600 UV− visible−NIR scanning spectrophotometer. Atomic Force Microscopy (AFM). Surface topology was measured using an Asylum Research MFP-3D microscope with Asylum Research model AC160TS probe. The spring constant (k) of the cantilever was 42 N/m with a resonance frequency (f) of 300 kHz. The images were obtained using a silicon tip. The measurements were carried out in tapping mode. The scan rate was varied between 0.6 and 1 Hz. Scan points and scan lines were varied between 256 and 512. Samples for AFM imaging were prepared by drop-casting from a chloroform dispersion onto a mica substrate. Device Fabrication. The fabrication procedure for metal− semiconductor−metal (MSM) photoconductors was as follows. First, a glass substrate was cleaned using acetone, methanol, and deionized water. Then a primer and a photoresist (Shipley 1818) were spincoated onto the glass substrate at 4000 rpm for 30 s. The film was then heated at 120 °C for 5 min. Subsequently, the sample was exposed to UV illumination through a photomask for 15 s and developed using MF319 developer for 50 s. Metal was deposited using an electron beam evaporator. A titanium layer (10 nm), to promote adhesion, and a gold layer (50 nm) were deposited at a rate of 0.5 Å/s under a background pressure of 5 × 10−8 Torr. Finally, a lift-off process using acetone produced the patterned electrodes. A schematic of the fabrication of the MSM interdigitated structure is provided in Figure S3 of the Supporting Information. The ∼7.5 nm dotlike SnSe NCs dispersed in chloroform were drop-cast on the MSM structure to produce a film spanning the gap between the metal electrodes. Dark current and photocurrent of the MSM device were measured using a
well-developed Se-alkylphosphine (trioctylphosphine (TOP) or tri-tert-butylphosphine (TBP)) precursors were not effective for synthesis of small SnSe NCs using SnCl2. Instead, large SnSe nanosheets are always formed. Previously, selenourea was shown to be an effective precursor for synthesis of SnSe NCs, in combination with SnCl2 as tin precursor.21 On the basis of our previous work on Cu2−xSe NCs,30 in this work we studied the effectiveness of two organo-Se precursors, oleic acid (OA)− Se and oleylamine (OAm)−Se, on the SnSe NC synthesis. We found that monodisperse SnSe NCs with dotlike morphology could be easily prepared using these precursors. To the best of our knowledge, this is the first report of synthesis of highquality dotlike and platelike SnSe QDs using SnCl2, rather than bis[bis(trimethylsilyl)amino]tin(II), as the tin precursor. Moreover, SnSe and SnS single-crystalline nanosheets, SnS nanoflowers, and SnS nanoparticles can be obtained by varying the chalcogenide precursor.
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EXPERIMENTAL SECTION
Chemicals. Tin chloride (SnCl2, reagent grade 98%), oleylamine (OAm, technical grade 70%), dodecylamine (DAm, 98%), oleic acid (OA, technical grade 90%), octadecene (ODE, technical grade 90%), trioctylphosphine (TOP, technical grade 90%), tri-tert-butylphosphine (TBP, technical grade 90%), selenium powder (Se, 99.99%), octanethiol (≥98.5%), and dodecanethiol (≥98%) were purchased from Sigma-Aldrich and were used as-received. Preparation of OA−Se Precursor. One millimole of Se powder was dissolved in 10 mL of OA. The mixture was degassed at 120 °C for 30 min under nitrogen protection. Then the solution was heated to 320−330 °C and kept at this temperature until a clear solution formed. Upon cooling to room temperature, the solution became a gel with a light yellow color. The gel was easily melted by gentle heating for subsequent use as a liquid. Preparation of OAm−Se Precursor. One millimole of Se powder was dissolved in 10 mL of OAm. The mixture was degassed at 120 °C for 30 min under nitrogen protection. Then the solution was rapidly heated to 300−310 °C and kept at this temperature until a clear solution formed. The solution was cooled to room temperature and was stored in an inert atmosphere glovebox for further use. Synthesis of SnSe Nanosheets Using TBP−Se and TOP−Se Precursors. In a typical synthesis, 0.25 mmol of SnCl2 powder was mixed with 2.5 mL of OAm, 2.5 mL of OA, and 10 mL of ODE in a flask. The solution was degassed at 120 °C for 30 min to remove dissolved oxygen and water. Then the solution was heated to 230 °C followed by injection of 1 mL of 1 M TBP−Se solution. After injection, the solution was heated to 218 °C and kept at this temperature for 7 min. If TOP−Se was used for the synthesis, 1 mL of 1 M TOP−Se was injected into the Sn precursor solution at 230 °C. The solution was then kept at 225 °C for 5 min. Synthesis of Dotlike SnSe NCs Using OA−Se. In a typical synthesis, 0.25 mmol of SnCl2 powder was mixed with 4 mL of OAm, 1 mL of OA, and 10 mL of ODE in a flask. The solution was degassed at 120 °C for 30 min to remove dissolved oxygen and water. In parallel, the OA−Se precursor was warmed using a heat gun or water bath to melt the gel. The Sn precursor solution was heated to 156 °C followed by injecting 2.5 mL of the 0.1 M OA−Se solution. The solution color immediately turned to dark red, indicating formation of SnSe NCs. The solution was held at 150 °C for 30 s for nanocrystal growth to produce SnSe with a mean diameter of 7.5 nm. The same procedure produced SnSe NCs with a mean diameter of 9.2 nm when the injection temperature and growth temperature were increased to 170 and 162 °C, respectively. Synthesis of 7.2 nm Dotlike SnSe NCs and Orthorhombic Nanoplates Using OAm−Se. In a typical synthesis, 0.25 mmol of SnCl2 powder was mixed with 4 mL of OAm, 1 mL of OA, and 10 mL of ODE in a flask. The solution was degassed at 120 °C for 30 min. Then the solution was heated to 150 °C followed by injecting 1 mL of 3516
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Labview-controlled voltage/source meter (Keithley 2400) in the dark and under AM 1.5G illumination (100 mW/cm 2), respectively.
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RESULTS AND DISCUSSION Role of TBP and TOP in the Synthesis of SingleCrystal SnSe Nanosheets. We began the investigation using tributylphosphine-selenium (TBP−Se) and trioctylphosphineselenium (TOP−Se) as the precursors for the synthesis. In a typical synthesis, SnCl2 powder was dissolved in a mixture of noncoordinating (octadecene) and coordinating (oleylamine and oleic acid) solvents to form the organo-Sn precursor. We found that SnSe NCs with sheetlike morphology were always obtained if an alkyl-phosphine Se precursor (TBP−Se or TOP−Se) was used (Figure 1A). Selected area electron
Figure 2. AFM height images of SnSe nanosheets (top) and corresponding section analysis (topological profile taken along the solid line drawn on the height images, bottom) show that the SnSe nanosheets are typically 25−30 nm thick.
Synthesis of Nearly Monodisperse Dotlike and Platelike SnSe NCs Using OA−Se and OAm−Se Precursors. Previous investigations showed that cost-effective tin precursors such as tin chloride, tin acetate, and tin oleate were ineffective for synthesizing small, monodisperse tin chalcogenide NCs due to their low reactivity. Thus, a highly reactive precursor bis[bis(trimethylsilyl)amino]tin(II) was developed to synthesize high-quality SnS, SnSe, and SnTe NCs in previous studies. Thus, previous methods for synthesis of dotlike SnSe NCs were based on modifying the reactivity of the tin precursor to facilitate nucleation and crystal growth at temperatures compatible with the alkylphosphine−Se precursors. Here, we take the alternative approach of developing a Se precursor with appropriate reactivity for SnSe NC synthesis using a low-cost, less reactive Sn precursor. Here, OA and OAm were employed as coordinating solvents to dissolve Se powder as reported previously (Experimental Section). The organo-Sn precursor was formed by dissolving SnCl2 in an ODE/OAm/OA mixture at 120 °C and the solution was degassed at this temperature for 30 min. Note that the mixture should be slowly heated from room temperature to 120 °C. Rapid heating may induce formation of a cloudy solution, which we attribute to reaction with residual dissolved oxygen. The reaction was executed by injecting presynthesized OA−Se precursor at the appropriate temperature, followed by aging the solution for nanocrystal growth at lower temperature (Experimental Section). TEM showed that nearly monodisperse dotlike SnSe NCs were synthesized using the OA−Se precursor (Figure 3). The NC diameter could be tuned using reaction temperature, with 9.2 nm SnSe NCs produced at higher activation temperature (∼170 °C) and 7.5 nm NCs obtained at lower temperature (∼156 °C). Compared with TOP−Se, OA−Se may have higher reactivity, which allows high nucleation density, leading to formation of small NCs by subsequent growth on the relatively large number of nuclei. Consistent with this scenario, rapid color change was observed after injection of OA−Se into a hot solution containing the Sn precursor, indicating formation of SnSe nuclei, while slow color evolution was observed in the case of TOP−Se. Moreover, higher synthesis temperature was
Figure 1. (A) TEM image and (B) HRTEM image and SAED of SnSe nanosheets prepared using TBP, showing that they are singlecrystalline. (C) Powder XRD pattern of SnSe nanosheets.
diffraction (SAED) combined with high-resolution TEM (HRTEM) show that the SnSe nanosheets are single crystals. The thickness of the SnSe nanosheets was measured by atomic force microscopy (AFM), showing that SnSe nanosheets were typically 25−30 nm in thickness. The representative one for which a height profile is provided in Figure 2 was 28 nm thick. Other nearby sheets measured 20, 22, 30, 40, and 60 nm in thickness, where the 60 nm structure appears to be two stacked sheets. No dotlike SnSe NCs were obtained using TBP or TOP. This is consistent with previous reports stating that SnSe NCs could mainly be produced using bis[bis(trimethylsilyl)amino]tin(II) as the Sn precursor in combination with TOP or TBP. The powder XRD patterns from the SnSe nanosheets (Figure 1c) were consistent with PDF Card 00-48-1224 for orthorhombic SnSe. 3517
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Figure 3. TEM images of SnSe NCs synthesized using OA−Se precursors. (A) 7.5 ± 1.0 nm SnSe NCs and (B) 9.2 ± 1.1 nm SnSe NCs.
Figure 4. (A) TEM of 7.2 ± 1.2 nm SnSe NCs, (B) TEM, and (C) HRTEM of SnSe NCs with orthorhombic morphology and mean projected area of ∼56 nm2, and (D) crystalline core−amorphous shell SnSe NCs.
required when using TOP−Se rather than OA−Se, further confirming the lower reactivity of TOP−Se compared to OA− Se under these synthesis conditions. We also found that the combination of OAm and OA ligands could influence the size distribution of SnSe NCs. The optimal volume ratio of OAm/ OA used to dissolve SnCl2 was determined to be 4:1, which resulted in formation of nearly monodisperse SnSe NCs. An OAm−Se precursor was also employed to synthesize SnSe NCs based on the same reaction model. An OAm/OA/ ODE mixture at a 4:1:10 volume ratio was used to dissolve SnCl2 and create the organo-Sn precursor for these experiments. Lower reaction temperature was used for this synthesis using OAm−Se precursor, while keeping the rest of the reaction procedure the same as the synthesis using OA−Se (Experimental Section). TEM revealed that the SnSe NCs synthesized using OAm−Se had a dotlike shape and 7.2 ± 1.2 nm size (Figure 4A). We found that the size and monodispersity of the SnSe NCs was affected by the molar ratio between the Sn and Se precursors. Excess Sn precursor compared to Se precursor used in the synthesis could effectively control the size of SnSe NCs. We also investigated a shorter alkylamine, dodecylamine (DAm) for formation of SnSe NCs. DAm was used in place of OAm to dissolve SnCl2, forming a modified organo-Sn precursor (DAm/OA/ODE). TEM showed that the introduction of DAm led to an obvious increase in the size of SnSe NCs (Figure 4B). This may be attributed to the short hydrocarbon chain in DAm that reduces its ability to suppress particle growth compared with OAm. HRTEM (Figure 4C) revealed that the introduction of DAm resulted in formation of crystals with a well-defined orthorhombic shape with ∼6 nm side length and ∼4 nm height. Interestingly, particles with a crystalline core/ amorphous shell structure (Figure 4D) were obtained if the reaction time was increased to 6 min, although the mechanism of their formation is not yet clear, and remains the subject of ongoing investigations. Powder X-ray diffraction (XRD) patterns (Figure 5) showed that the SnSe NCs synthesized using different OA−Se and
Figure 5. XRD pattern of SnSe NCs synthesized by (1) injecting OA− Se into SnCl2 dissolved in OAm/OA/ODE, (B) injecting OAm−Se into SnCl2 dissolved in OAm/OA/ODE, and (C) injecting OAm−Se into SnCl2 dissolved in DAm/OA/ODE solution.
OAm−Se precursors had the same orthorhombic crystal phase, which corresponds to the thermodynamically stable crystal phase for bulk SnSe. The optical absorption of the SnSe NCs was measured by UV−vis-NIR spectrometry. The absence of absorbance at NIR wavelengths (Figure 6A) shows that the SnSe NCs were subjected to quantum confinement; bulk SnSe has an indirect band gap of ∼0.9 eV. We estimated the band gaps of the SnSe NCs using the Tauc method (Figure 6C) and found that SnSe NCs synthesized using OAm−Se precursors in an OAm/OA/ODE mixture had a wider band gap than those synthesized in DAm/OA/ODE. This is consistent with the TEM-based size measurements showing that SnSe NCs synthesized in an OAm/OA/ODE mixture had smaller size than those prepared using DAm/OA/ODE. Figure 6B and 6D show the absorbance and estimated band gap of the SnSe NCs 3518
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S2 in the Supporting Information). Herein, we studied using alkyl-thiols as the sulfur precursors because they simultaneously serve as sulfur donor and ligands to suppress growth of NCs. Alkyl-thiols have previously been used as both sulfur precursors and ligands in the synthesis of other metal sulfide NCs.31 However, we are aware of no prior reports of the use of alkylthiols to synthesize SnS NCs. We show here that alkyl-thiols can be used to controllably vary the morphology of SnS NCs, based on the process we developed to synthesize SnSe NCs. The organo-Sn precursor was prepared using OAm and OA to dissolve SnCl2, just as for the SnSe synthesis. To synthesize SnS nanosheets, 1-dodecanethiol was injected into the Sn precursor solution at 220 °C and the solution was kept at 216 °C for 22 min. TEM revealed the sheetlike morphology of SnS NCs and SAED showed that SnS nanosheets synthesized using this method were single crystals (Figure 7A,B). AFM showed
Figure 6. (A) Absorbance of samples 1 and 2, which were synthesized by injecting OAm−Se precursor into SnCl2/OAm/OA/ODE solution and SnCl2/DAm/OA/ODE solution, respectively, (B) absorbance of samples 3 and 4, which were synthesized using OA−Se precursor at 170 and 156 °C, respectively, and (C) and (D) are Tauc plots used to estimate the indirect band gaps of samples 1, 2, 3, and 4 as indicated.
synthesized using OA−Se at different reaction temperatures. Band gaps higher than that of the bulk were also observed in these SnSe NCs, indicating that these SnSe NCs were subject to quantum confinement. Moreover, SnSe NCs synthesized at lower temperature had a wider band gap, corresponding to the smaller NC size observed using TEM. In summary, we developed and demonstrated cost-effective methods to synthesize high-quality SnSe NCs without relying on the pyrophoric organo-tin precursor commonly used in previous studies of these materials. We investigated the roles of different selenium precursors in controlling the morphology and size of SnSe NCs. We found that use of an alkylphosphine−selenium precursor in our reaction system always induced formation of single-crystalline SnSe nanosheets. Growth of these sheets can be attributed to the low reactivity of the TOP−Se or TBP−Se precursors, which would result in low nucleation density (and therefore fewer and larger NCs) and growth only on the most reactive facets. To our knowledge, very few reports have demonstrated controllable synthesis of high-quality SnSe NCs without using bis[bis(trimethylsilyl)amino]tin(II).21 This provides a new pathway for producing SnSe NCs, which may be extensible to other IV−VI semiconductor NCs. Because of the relatively low cost of the precursors and facile synthetic process, this significantly enhances the potential for application of this emerging IV− VI semiconductor nanocrystal in solution-processed optoelectronic and photovoltaic devices. Synthesis of SnS NCs with Tunable Morphologies. To extend this reaction model to synthesize SnS NCs, we tested the feasibility of using OA−S and OAm−S as sulfur precursor for synthesis of SnS NCs. However, little product can be obtained if the reaction temperature is lower than 200 °C when using OA−S and/or OAm−S. This indicates that nucleation does not occur below 200 °C when using these precursors in the present reaction model. Highly polydisperse SnS NCs were formed at higher temperature using OAm−S precursor (Figure
Figure 7. (A) TEM and (B) HRTEM of SnS nanosheets. SAED (inset of B) shows that the SnS nanosheets are single crystalline. TEM images of (C) polyhedral SnS NPs and (D) SnS nanoflowers.
that typical SnS nanosheets had a thickness of 20−25 nm (Figure 8). Powder XRD showed that these NCs have the herzenbergite crystal phase, which is the thermodynamically favored bulk phase for SnS (Figure 9). When the synthesis was carried out at lower temperature, the sheet morphology was largely replaced by a polyhedral morphology. When octanethiol, rather than dodecanethiol, was used as the sulfur precursor, the fraction of polyhedral particles increased (Figure 7C). Powder XRD showed that these SnS NCs with mainly polyhedral shape (mixed with some nanosheets) have the same herzenbergite crystal structure as the SnS nanosheets (Figure 9). However, the diffraction peak corresponding to the (013) crystal planes was much weaker for the polyhedral SnS NC than for the nanosheets. This suggests that the single-crystalline SnS nanosheets grow anisotropically along the direction perpendicular to the (013) crystal plane. These two-dimensionally oriented SnS nanosheets may be of great interest for nanoelectronic and optoelectronic applications because higher lateral charge mobility and fewer defects to trap charge carriers can be expected in the single-crystalline nanosheets than in thin 3519
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Figure 10. Optical absorption of SnS nanosheets, nanopolyhedra, and nanoflowers. Figure 8. AFM height images of SnS nanosheets (top) and corresponding section analysis (topological profile taken along the solid line drawn on the height image, bottom) reveal typical SnS nanosheets are approximately 20 nm thick.
Figure 11. (A) Interdigitated microstructure of metal−semiconductor−metal (MSM) device. (B) Photoresponse of SnSe NC thin film to AM 1.5 illumination.
from the colloidal SnSe NC dispersion by drop-casting it on top of the electrodes, such that the film filled the gap between the electrodes. Current−voltage (I−V) curves were measured in the dark and under AM 1.5 simulated solar illumination. The film showed significant photoresponse (Figure 11B), suggesting that SnSe NCs could potentially be used for optoelectronic devices. However, both the dark current and photocurrent observed in the MSM device were relatively low given the high applied bias voltage. We attribute this to the following factors: (1) the SnSe NC thin film was directly fabricated from a colloidal SnSe NC dispersion in which SnSe NCs were stabilized by insulating long hydrocarbon ligands. No further chemical treatment (for example, solid-state ligand exchange) was applied to the SnSe NC thin film; (2) the 5 μm gap between the Au fingers is much larger than the charge-transport distance (thickness of the thin films) in vertical device structures that are often used in solution-processed optoelectronics and photovoltaics.
Figure 9. XRD patterns of SnS nanosheets and polyhedral SnS NPs.
films assembled from small NCs. We also observed that SnS NCs with flowerlike morphology could be obtained if only OAm was employed as the ligand to dissolve SnCl2 to form the organo-Sn precursor (Figure 7D). This demonstrates that the selection of ligands plays an important role in controlling crystal growth and morphology of SnS NCs. The optical absorption spectra of SnS nanosheets, polyhedral NPs, and nanoflowers are shown in Figure 10. The broad absorbance of the SnS nanosheets and polyhedral SnS NPs, spanning the visible to NIR spectral range, matches the solar spectrum very well, supporting their promise as absorbers in photovoltaic devices. The blue shift of absorbance in SnS nanoflowers compared with SnS nanosheets may be attributed to quantum confinement due to the small NC size. Photoresponse of SnSe NC Thin Film. To investigate the photoresponse of a SnSe NC thin film, we fabricated a metal− semiconductor−metal (MSM) device. The interdigitated microstructure of the MSM electrodes is shown in the optical micrograph of Figure 11A. A SnSe NC thin film was fabricated
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CONCLUSION In this work, we demonstrated facile methods for synthesizing nearly monodisperse colloidal SnSe NCs with tunable sizes. In contrast to previous methods for synthesis of monodisperse high-quality colloidal SnSe NCs, we do not employ the expensive and flammable bis[bis(trimethylsilyl)amino]tin(II) precursor. We show that the common alkylphosphine−Se precursors, including TBP−Se and TOP−Se, generally induce 3520
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the formation of large SnSe nanosheets when used in combination with SnCl2. Alkyl-thiols were investigated as the sulfur precursors for synthesis of SnS NCs. The NC morphology could be adjusted among sheetlike, polyhedral, and flowerlike by varying the ligands employed in the synthesis. The optoelectronic properties of colloidal SnSe NCs were studied by fabricating a SnSe NC thin film between interdigitated gold electrodes. The photoresponse of the SnSe NC thin film to simulated solar light indicates that the materials have potential for use in solution-processed photovoltaic and optoelectronic devices.
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ASSOCIATED CONTENT
S Supporting Information *
Size distributions of SnSe NCs obtained by counting 120 particles, TEM image of polydisperse SnS NCs produced using OAm−S at 200 °C, and schematic illustration of the process for fabricating MSM devices. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: swihart@buffalo.edu. Telephone: 716-645-1181. Notes
The authors declare no competing financial interests.
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ACKNOWLEDGMENTS We thank the Professor Marina Tsianou and Dr. Biswa Das for help with AFM imaging and for valuable discussions of the results.
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REFERENCES
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