Solution-Mediated Growth of Two-Dimensional SnSe@GeSe

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Solution-Mediated Growth of Two-Dimensional SnSe@GeSe Nanosheet Heterostructures Du Sun, and Raymond E. Schaak Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 16 Dec 2016 Downloaded from http://pubs.acs.org on December 17, 2016

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Chemistry of Materials

Solution-Mediated Growth of Two-Dimensional SnSe@GeSe Nanosheet Heterostructures Du Sun and Raymond E. Schaak* Department of Chemistry and Materials Research Institute, The Pennsylvania State University, University Park, PA 16802 ABSTRACT: Nanosheet heterostructures that directly interface multiple distinct metal chalcogenides are important twodimensional materials with unique synergistic properties. Solution routes to these materials offer potential advantages over existing vapor deposition and exfoliation/restacking methods, but they remain difficult to implement and control. Here, we show that SnSe can be grown in solution on micron-sized GeSe nanosheets and establish guidelines for achieving controllable deposition. Heating GeSe nanosheets in oleylamine with various amounts of SnCl2 and TOP-Se reveals distinct regimes that favor growth of SnSe on the edge sites vs. basal planes of GeSe. Using continuous injection to better control concentrations and reagent delivery, SnSe can be grown on GeSe to produce two distinct types of crystallographically aligned SnSe@GeSe nanosheet heterostructures.

INTRODUCTION Two-dimensional (2-D) metal chalcogenides exhibit a diverse range of properties that enable applications in catalysis, optoelectronics, thermoelectric power generation, and energy conversion and storage.1 As nanoscale materials, the properties of metal chalcogenides can change as a function of size and dimensionality due to quantum confinement effects, as is well known for systems such as CdS and CdSe.2 In some cases, entirely new properties can emerge from dimensionally confining metal chalcogenides. Transition metal dichalcogenides (TMDs) such as MoS2 change from indirect to direct band gap semiconductors as single-layer nanosheets, which leads to significantly enhanced photoluminescence.1 The creation of nanoscale heterostructures that directly interface multiple nanosheets has recently emerged as an exciting approach for further expanding the scope of new properties that can be achieved in 2-D materials systems.3 For example, by vertically integrating TMD nanosheets in superlattices having any desired stacking sequences and thicknesses, an almost unlimited library of new 2-D materials can be envisioned.1 Various twocomponent combinations of MoS2, MoSe2, WS2, and WSe2 have been achieved experimentally,1,3,4 including bandengineered WSe2/MoS2 heterostructures.5 Most 2-D metal chalcogenide heterostructures studied to date have been constructed using chemical6 or physical7 vapor deposition, molecular beam epitaxy,8 or individually restacking chemically or mechanically exfoliated nanosheets.9 Controllable lateral vs. vertical growth modes can sometimes be achieved on planar surfaces via selective deposition on edge vs. basal plane sites,10 which is useful for applications in many electronic and optical devices. However, these methods are not scalable, as they require nanosheet and thin film templates that are anchored to planar substrates. Solution routes offer an interesting and potentially scalable alternative that is capable of producing the types of highly conformal heterojunction interfaces that are needed to achieve targeted synergistic functions.11 For example, conformal shells of CdS and CdZnS

were grown around CdSe nanoplatelets,12 vertical heteroepitaxial growth of metal sulfides (CuS, ZnS, Ni3S2) on TiS2 crystals was achieved in a battery cell,13 and multishell Bi2Se3/Bi2Te3 nanostructures with dominant lateral growth were synthesized through sequential epitaxial deposition steps.14 However, despite these and other significant advances, solution-phase heterogrowth of 2-D metal chalcogenides is still poorly understood and remains difficult to achieve. Here, we provide an important step toward understanding how solution-phase heterogrowth of layered metal chalcogenides can be controlled to yield 2-D nanosheet heterostructures. We show that SnSe can be grown in solution with crystallographic alignment on micron-sized GeSe nanosheets and establish guidelines for achieving deposition on edges vs basal planes. GeSe and SnSe were chosen as model systems because they both adopt the GeS structure type and they are important narrow band gap semiconductors of interest for applications in catalysis and energy conversion and storage.15 Furthermore, GeSe and SnSe form nanosheets in solution and the pathway by which they form and grow has been well characterized.16 For example, during the formation of uniform square-shaped SnSe nanosheets in solution using a one-pot heat-up process, SnSe first grows laterally to generate uniform nanosheets and then vertically to increase in thickness without significantly changing lateral dimensions.16 This reaction pathway therefore is comprised of distinct lateral and vertical homogrowth steps, with SnSe depositing and growing predominantly on SnSe edge sites first and then basal planes, as shown schematically in Figure 1. Given the reaction pathway by which SnSe nanosheets form, we surmised that nanosheet formation could be arrested at various stages of these dominant lateral or vertical growth stages, and then reagents capable of growing a different material could be added to facilitate heterogrowth, ultimately creating nanosheet heterostructures such as those shown in Figure 1. To study and understand such heterogrowth processes, GeSe nanosheets having an elongated hexagon shape were used as substrates because they form in solu-

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tion at higher temperatures (260 – 320 °C)17,18 than SnSe nanosheets (240 °C)16 and therefore remain stable during SnSe deposition. SnSe initially deposited preferentially along the edges of the GeSe nanosheet templates, followed by more extensive growth that coated the GeSe basal plane with crystallographically aligned SnSe sheets. Given the expanding scope of chalcogenide materials that can be synthesized in solution as two-dimensional particles, we anticipate that the synthetic insights that lead to SnSe@GeSe heterostructures will be broadly applicable to other systems where heterogrowth is desired.

Figure 1. Stepwise formation of (left) SnSe nanosheets with increasing widths and thicknesses through distinct edge vs. basal plane homogrowth stages involving deposition of SnSe on SnSe (based on ref. 16) and (right) formation of SnSe@GeSe nanosheet heterostructures through related heterogrowth processes involving deposition of SnSe on GeSe.

EXPERIMENTAL SECTION Materials. Germanium (IV) iodide (GeI4, 99.99+%, Aldrich), hexamethyldisilazane (HMDS, >99%, Aldrich), diphenyl diselenide (Ph2Se2, >99%, Aldrich), hexadecylamine (98%, Aldrich), tin(II) chloride (SnCl2, 99%, Alfa Aesar), selenium power (Se, >99%, Alfa Aesar), oleylamine (OLAM, technical, 70%, Aldrich), and tri-noctylphosphine (TOP, >95%, TCI) were used as received without purification. All syntheses were carried out under Ar(g) using standard Schlenk techniques and work-up procedures were performed in air. Preparation of TOP-Se Stock Solution. A 1 M TOP-Se stock solution was prepared by adding selenium powder (790 mg, 10 mmol) into a 20 mL vial containing 10 mL of TOP and sonicating until a clear colorless solution was obtained. Synthesis of GeSe Nanosheet Templates. The GeSe nanosheets templates were synthesized using a literature method,18 with minor modifications as detailed below. In a typical synthesis, 10 mL of hexadecylamine was added to a 25-mL three-neck flask in air and then degassed at ∼120 °C for 30 minutes by pulling vacuum while stirring. GeI4 (29 mg) and diphenyl diselenide (7.8 mg) with ~1 mL oleylamine was dissolved in another vial and then the solution was transferred into the flask. The mixture was heated to 268 °C and aged for 30 min. Over this time, the reaction mixture turned black, indicating

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the formation of the GeSe product. Once the temperature dropped to 100 °C, 10 mL of toluene was injected into the reaction mixture to prevent the hexadecylamine from solidifying. The flask was then cooled to room temperature, and excess ethanol was added to precipitate the NCs, which can be collected through centrifugation at 9000 rpm for 3 min. The precipitates were washed three times with a 1:1 mixture of ethanol and toluene. The final products were dispersed in oleylamine for further use as templates for the deposition of SnSe, as described below. Synthesis of SnSe@GeSe Nanosheet Heterostructures. Several distinct types of SnSe@GeSe nanosheet heterostructures were synthesized; experimental details for each of these is provided below. Type #1: Substoichiometric amount of SnSe relative to GeSe, onepot heat-up. (These correspond to the SnSe@GeSe heterostructures highlighted in Figure 2a and 2b.) In a typical synthesis, 1 mL of an oleylamine suspension of GeSe nanosheet templates (~2 mg GeSe), 0.4 mg of SnCl2 (2 µmol), 2 µL of TOP-Se stock solution (1 M, 2 µmol), and 20 mL of oleylamine were added into a 100-mL 3-neck round-bottom flask with a condenser, thermometer adapter, thermometer, and rubber septum. Magnetic stirring was started and the solution was degassed under vacuum at 120 °C for ~5-10 minutes. The flask was backfilled with Ar and then heated to ~280 °C at ~10 °C/min. The solution was allowed to age for ~30 mins and then cooled rapidly by removing the flask from the heating mantle. The GeSe/SnSe heterostructures were precipitated by adding 20 mL of a 1:1 ethanol/toluene mixture and then centrifuged at 12 000 rpm for 5 min. The precipitate was washed three times using the same solvent mixture (with centrifugation in between washes) and could then be suspended in hexane, toluene, or ethanol to form a colloidal suspension for further characterization. Type #2: Comparable amounts of SnSe and GeSe, one-pot heat-up. (These correspond to the SnSe@GeSe heterostructures highlighted in Figure 2c and 2d.) The experimental setup and procedure are identical to that of sample #1 detailed above, except that the amount of SnCl2 and TOP-Se were increased to 2 mg and 10 µL, respectively. Type #3: Significant excess of SnSe relative to GeSe, one-pot heatup. (These correspond to the SnSe/GeSe sample highlighted in Figure S3 of the Supporting Information.) The experimental setup and procedure are identical to that of sample #1 detailed above, except that the amount of SnCl2 and TOP-Se were increased to 4 mg and 20 µL, respectively. Type #4: Conformal, predominant lateral growth by continuously injecting SnCl2 into GeSe nanosheet suspension. (These correspond to the SnSe@GeSe heterostructures highlighted in Figure 3.) In a typical synthesis, 1 mL of an oleylamine suspension of GeSe nanosheet templates (~2 mg GeSe), 0.4 mg of SnCl2 (2 µmol), 0.16 mL of the TOPSe stock solution (1 M, 0.16 mmol), and 10 mL of oleylamine were added into a 100-mL 3-neck round-bottom flask with a condenser, thermometer adapter, thermometer, and rubber septum. Magnetic stirring was started and the solution was degassed under vacuum at 120 °C for ~5-10 minutes. The flask was backfilled with Ar and then heated to ~280 at ~10 °C/min. Once the temperature reached 200 °C, a syringe pump was used to inject 10 mL of oleylamine solution containing 29.6 mg of SnCl2 at a rate of 0.5 mL/min. The reaction was cooled rapidly by removing the flask from the heating mantle after the injection was completed. The precipitates were washed following the same procedure as mentioned above for the other samples. Type #5: Predominant vertical growth by continuously injecting a GeSe nanosheet suspension into SnCl2. (These correspond to the SnSe@GeSe heterostructures highlighted in Figures 4 and 5.) In a typical synthesis, 30 mg of SnCl2 (0.16 mmol), 0.16 mL of the TOPSe stock solution (1 M, 0.16 mmol), and 20 mL of oleylamine were added into a 100 mL 3-neck round-bottom flask with a condenser, thermometer adapter, thermometer, and rubber septum. Magnetic stirring was started and the solution was degassed under vacuum at 120 °C for ~5-10 minutes. The flask was backfilled with Ar and then heated to ~280 °C at ~10 °C/min. Once the temperature reached 200 °C, a syringe pump was used to inject 1 mL of an oleylamine suspension containing ~2 mg GeSe nanosheet templates at a rate of 0.1 mL/min. The reaction was cooled rapidly by removing the flask from


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the heating mantle after the injection was completed. The precipitates were washed following the same procedure as mentioned above for the other samples. Characterization. Powder XRD data were collected using a Bruker D8 Advance X-ray diffractometer equipped with Cu Kα radiation. TEM images were obtained using a JEOL 1200 EX II TEM operating at 80 kV and a FEI Talos F200X operating at 200 kV. HRTEM imaging and acquisition of EDS spectra and STEM-EDS element maps were performed on a FEI Talos F200X operating at 200 kV.

RESULTS AND DISCUSSION GeSe nanosheets were synthesized by heating GeI4, diphenyl diselenide, oleylamine, and hexadecylamine to 268 °C for 30 min.18 Figure S1 of the Supporting Information shows transmission electron microscopy (TEM) images, scanning electron microscopy (SEM) images, element maps generated by scanning transmission electron microscopy coupled with energy dispersive spectroscopy (STEM-EDS), and powder Xray diffraction (XRD) data for the GeSe nanosheets. Consistent with previous reports by us17 and others18 and as evidenced in Figure S1, the GeSe nanosheets span a range of sizes and 2-D morphologies. However, the majority of the GeSe nanosheets can be described as elongated single-crystal hexagons having lateral dimensions of approx. 3 × 6 µm and thicknesses of approx. 5-25 nm, and these elongated hexagon nanosheets are the focus of the studies that follow.

Figure 2. Electron microscopy data characterizing the deposition of (a,b) substoichiometric and (c,d) approximately stoichiometric amounts of SnCl2 and TOP-Se on GeSe nanosheets. TEM images are shown in (a) and (c) and superimposed Se, Ge, and Sn STEMEDS element maps are shown in (b) and (d). (The GeSe nanosheets appear purple due to co-localization of Ge and Se, which are blue and red, respectively.) EDS point scans corresponding to each of the colored and numbered boxes in (b) and (d) are shown in (e) as indicated. The yellow boxes highlight regions where SnSe has deposited onto the GeSe nanosheets and the cyan boxes highlight basal plane regions with minimal SnSe.

Figure 2 shows the products obtained upon heating the GeSe nanosheets in oleylamine with various amounts of SnCl2 and a trioctylphosphine–selenium complex (TOP-Se) at 280

°C for 30 min, which are reagents and conditions known to form colloidal SnSe nanosheets through a process similar to that of GeSe.16 For small amounts of SnCl2 and TOP-Se relative to GeSe (e.g. substoichiometric and approximately stoichiometric), the TEM images in Figures 2a and 2c reveal that the basal planes of the GeSe nanosheets, which remain intact and unchanged morphologically, are decorated with small islands. STEM-EDS element maps of Ge, Se, and Sn are shown in Figure S2 of the Supporting Information for representative nanosheets. The superimposed element maps in Figures 2b and 2d show that the islands decorating the GeSe nanosheets contain Sn. Corresponding EDS spectra obtained from the points indicated in each STEM-EDS element map (Figure 2e) confirm that the islands contain a significant amount of Sn, while the remainder of the GeSe basal plane does not. The EDS spectrum for point #1, as a representative example, indicates a Ge:Sn:Se ratio of 26:23:51, which corresponds to a ~1:1 ratio of both Ge:Sn and (Ge+Sn):Se and therefore suggests that the island regions contain SnSe on GeSe. The STEM-EDS element maps and corresponding EDS spectra in Figures 2a and 2b also reveal that the GeSe nanosheet edges are densely coated with SnSe, more prominently in the sample containing a larger amount of SnCl2 and TOP-Se. This suggests that during the early stages of the reaction of GeSe nanosheets with SnCl2 and TOP-Se, where the formation of SnSe is reagent-limited, growth of SnSe occurs predominantly on the GeSe edge sites, with small islands beginning to grow on the GeSe basal plane. This observation indicates that while vertical heterogrowth of SnSe on GeSe is beginning to occur, lateral heterogrowth is preferred, in analogy to the related homogrowth processes by which SnSe nanosheets form.16 Figure S3 shows a TEM image and corresponding STEMEDS element map and EDS point scan of the product formed upon reacting GeSe nanosheets with a significant excess of SnCl2 and TOP-Se under conditions identical to those used to generate the products in Figure 2. While a small amount of Sn is present, there is no evidence that significant amounts of SnSe decorate either the GeSe edge sites or basal planes. Comparing the TEM images in Figures 2, S2, and S3 also reveals that as the concentrations of SnCl2 and TOP-Se increase, so does the formation of material not attached to the GeSe nanosheets. This corresponds to increasing homogeneous nucleation and growth of SnSe, which competes with the desired heterogeneous nucleation and growth of SnSe on GeSe. Taken together, the data presented in Figures 2, S2, and S3 suggest that to achieve optimal deposition of SnSe on GeSe, reagent concentrations must remain low in order to suppress unwanted and competing homogeneous nucleation processes, in analogy to the synthesis of core-shell nanoparticles,19 but also that it may be possible to tune between dominant edge vs. basal plane growth by controlling reagent delivery and reaction kinetics. Accordingly, we switched to a continuous injection setup that was better able to modulate reagent delivery (relative to the one-pot heat-up approach) in order to further investigate and control the growth of SnSe on GeSe. Heterogrowth of SnSe on GeSe was facilitated by slowly and continuously injecting an oleylamine solution of SnCl2 at a rate of 0.5 mL/min into a reaction mixture at 280 °C that contained GeSe nanosheets along with oleylamine, TOP-Se,



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and a small (substoichiometric) amount of SnCl2, which served to initiate SnSe deposition on the GeSe edge sites. By continuously injecting SnCl2, the concentration of SnCl2 always remained low and within the regime that the one-pot heat-up experiments suggested favored predominant edge deposition of SnSe on GeSe with only minimal island growth on the basal planes. The TEM images, STEM-EDS element maps, corresponding EDS line scan, and SEM image in Figure 3 confirm that under these conditions, SnSe deposits onto the GeSe nanosheet with dominant edge-site deposition. SnSe deposition on the GeSe nanosheet basal plane is also observed, but it is minimal relative to the amount of SnSe decorating the edges. Significant growth of SnSe on the GeSe basal planes has been largely suppressed in favor of edge deposition such that the SnSe islands that are present on the GeSe nanosheets are scattered and small.

Figure 3. Characterization of nanosheet heterostructures exhibiting predominant edge-site deposition of SnSe on GeSe. (a) TEM image of a SnSe@GeSe nanosheet, with the region outlined in yellow expanded in (b). (c) HAADF-STEM image (inset) and corresponding STEM-EDS elements maps for Se, Ge, Sn, and Se+Ge+Sn. (d) EDS line scan (normalized signals) as indicated by the arrow in (c). (e) SEM image and corresponding Ge+Se and Sn element maps for a SnSe@GeSe nanosheet.

To facilitate more extensive basal plane growth, where the scattered SnSe islands on the flat GeSe surface expand in size and coverage, an oleylamine suspension of GeSe nanosheets was slowly and continuously injected at a rate of 0.1 mL/min into a reaction mixture containing SnCl2, TOP-Se, and oleylamine. This reaction sequence, which is opposite of that used to produce the nanosheet heterostructures shown in Figure 3, ensures that the concentrations of SnCl2 and TOP-Se remain high relative to the amount of GeSe nanosheets that are present, but at the same time remain sufficiently low (facilitat-

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ed by the continuous injection process) to minimize undesirable homogeneous nucleation of SnSe. As such, vertical growth (e.g. more significant growth of the islands that decorate the basal plane) should be achievable. Figure 4 shows a TEM image, an SEM image, and STEM-EDS element maps for the product, which consists of large SnSe nanosheets that cover the GeSe nanosheet substrate. Significant basal plane growth of SnSe on GeSe, along with some continued edge growth, is clearly favored under these reaction conditions. Collectively, the data confirm that controlling the reaction kinetics by modulating the concentrations and delivery of reagents allows different types of SnSe@GeSe nanosheet heterostructures to be accessed.

Figure 4. Characterization of nanosheet heterostructures exhibiting predominant basal plane deposition of SnSe on GeSe. (a) TEM image of a SnSe@GeSe nanosheet. (b) SEM image of a tilted SnSe@GeSe nanosheet showing small SnSe plates protruding from the GeSe nanosheet edges and larger SnSe sheets decorating the GeSe basal plane. (c) HAADF-STEM image and corresponding STEM-EDS element map for (d) Ge, (e) Se, and (f) Sn. (g) EDS line scan (normalized signals) as indicated by the arrow in the inset (corresponding to the superimposed Se+Ge+Sn element maps from panels d, e, and f).

Taking a closer look at the TEM and SEM images in Figure 4, as well as the enlarged TEM image in Figure 5a, the SnSe nanosheets that decorate both the edges and basal planes appear to be aligned relative to one another and also relative to the GeSe nanosheet substrate, which suggests that there is crystallographic correlation between the growing SnSe


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nanosheets and the underlying GeSe basal plane. Figure 5b, which shows a SAED pattern for a representative SnSe@GeSe nanosheet heterostructure such as that shown in Figure 4, matches the pattern expected for the [100] view direction of the GeS structure type that both SnSe and GeSe adopt (Figure 5c). The observed single-crystal spot pattern, obtained for a single nanosheet heterostructure, is consistent with crystallographic alignment between SnSe and GeSe.

Figure 5. (a) TEM image of a SnSe@GeSe nanosheet heterostructure similar to that shown in Figure 4. The contrast was enhanced to facilitate visualization of the multiple nanosheet layers. The yellow dashed line shows the GeSe nanosheet border where SnSe was deposited through initial edge-site deposition, as is evident in the SEM image in Figure 4b. Subsequent basal plane deposition resulted in the formation of a high density of SnSe nanosheets on the GeSe nanosheet substrate. The blue lines highlight some of the SnSe nanosheet edges, which are parallel to the GeSe nanosheet edges and therefore suggest crystallographic alignment of SnSe on GeSe. (b) SAED pattern for a SnSe@GeSe nanosheet heterostructure, which matches well with the simulated [100] pattern in (c) and therefore also suggests crystallographic alignment of SnSe on GeSe.

shown in Figure 6 and also analogous to the XRD pattern for the GeSe nanosheets shown in Figure S1, the intensities of the (h00) peaks are significantly enhanced. Both SnSe and GeSe are present and exhibit the same preferred orientation direction, which provides further evidence for the crystallographically aligned deposition of SnSe on GeSe. CONCLUSIONS We have shown that the pathway by which SnSe and GeSe nanosheets form in solution, involving distinct lateral and vertical homogrowth steps,16 can be modified to facilitate crystallographically aligned heterogrowth of SnSe on GeSe. By modulating the reaction kinetics through control of concentrations and reagent delivery, SnSe can be grown on the edge sites and basal planes of GeSe nanosheet substrates to produce two types of SnSe@GeSe nanosheet heterostructures. SnSe@GeSe serves as an instructive model system that provides useful insights into the pathway by which solutionmediated heterogrowth occurs and can be controlled in layered metal chalcogenide systems. We anticipate that these fundamental insights and synthetic guidelines will be portable to other high-value targets including the 2-D transition metal dichalcogenides for which related superlattice structures are being shown to yield unique, synergistic, and device-relevant properties.1,11

ASSOCIAT ED CONT ENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional XRD and microscopy data (PDF)

AUTHOR INFORMATION Corresponding Author E-mail: [email protected]

ACKNOWLEDGMENT This work was supported by the U.S. National Science Foundation under grant DMR-1607135. TEM imaging was performed in the Penn State Microscopy and Cytometry facility and HRTEM imaging was performed at the Materials Characterization Lab of the Penn State Materials Research Institute.

REFERENCES

Figure 6. Powder XRD pattern for a sample of SnSe@GeSe nanosheet heterostructures, showing significant [100] preferred orientation for both SnSe and GeSe.

Powder XRD data, taken for a SnSe@GeSe nanosheet sample deposited onto the XRD sample holder by drop casting to preferentially orient the sheets, is also consistent with the crystallographically aligned deposition of SnSe on GeSe. As

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Solution-Mediated Growth of Two-Dimensional SnSe@GeSe

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