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Liquid Exfoliation Few-Layer SnSe Nanosheets with Tunable Band Gap Yajie Huang, Liangliang Li, Yuanhua Lin, and Ce-Wen Nan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06096 • Publication Date (Web): 20 Jul 2017 Downloaded from http://pubs.acs.org on August 1, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

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Figure 1. (a) Photograph of single crystal SnSe flakes as initial material for liquid exfoliation. (b) Four solutions with different SnSe nanosheets. (c) SEM image of an individual few-layer SnSe nanosheet. (d) SEM image of densely packed SnSe nanosheets. 72x64mm (300 x 300 DPI)


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Figure 2. TEM analysis of SnSe nanosheets. (a) A thin SnSe nanosheet. (b and c) HRTEM image and corresponding FFT image of the nanosheet in (a), respectively. (d) TEM image of a typical SnSe nanosheet. (e and f) Elementary mapping of Sn and Se within the selected region in (d), respectively. 100x67mm (300 x 300 DPI)



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Figure 3. Raman spectra of SnSe nanosheets and self-made SnSe flakes with 532 nm laser excitation wavelength. 82x82mm (300 x 300 DPI)


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Figure 4. AFM characterization of SnSe nanosheets. (a−c) Height profiles of SnSe nanosheets in Sol B, Sol C, and Sol D, respectively. (d−f) Thickness histograms of the nanosheets in (a−c), respectively. (g) Plot of cumulative frequency versus thickness for three samples. 109x71mm (300 x 300 DPI)



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Figure 5. UV-Vis-NIR absorption spectra of SnSe nanosheets with different thicknesses. 67x55mm (300 x 300 DPI)


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Figure 6. Tauc’s plots of (αhν)1/2 as a function of photon energy for SnSe nanosheets with different thicknesses. 126x192mm (600 x 600 DPI)



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Figure 7. Calculated band structures for SnSe monolayer (a), 7-layer (b), 15-layer (c), and bulk (d). The Fermi energies are fixed on the top of the valence bands. 105x74mm (600 x 600 DPI)


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Figure 8. Dependence of the band gap of SnSe on the layer number. The measured and calculated band gap values are represented by black squares and red circles, respectively. The horizontal dash lines show the band gaps of SnSe bulk determined by our experiment (black line) and calculations (red line). 58x41mm (300 x 300 DPI)



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Liquid Exfoliation Few-Layer SnSe Nanosheets with Tunable Band Gap Yajie Huang, Liangliang Li*, Yuan-Hua Lin, Ce-Wen Nan State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China

*Corresponding author email: [email protected] Phone: +86-10-62797162 Fax: +86-10-62771160 Postal address: State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China

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Abstract Two-dimensional (2D) materials have recently drawn tremendous attention because of their novel properties and potential applications in high-speed transistors, solar cells, and catalysts. Few-layer SnSe is a new member of the 2D family with excellent performance in optoelectronic and thermoelectric devices. It is necessary to synthesize few-layer SnSe nanosheets in large scale for various applications. In this work, we develop a scalable liquid-phase exfoliation method to synthesize high-quality crystalline SnSe nanosheets. The morphology and microstructure of SnSe nanosheets are systematically investigated with high resolution transmission electron microscopy, atomic force microscopy, and Raman spectroscopy. The thinnest nanosheets are bilayered. The optical absorption properties of SnSe nanosheets from near infrared to ultraviolet light are studied. It is worth noting that the band gap of the nanosheets monotonically increases with the reduction of the nanosheet thickness. The electronic structure of SnSe nanosheets with various thicknesses is calculated by first principles calculations, the evolution of the band gap as a function of the nanosheet thickness is confirmed, and the mechanism of the band gap evolution is discussed. Our work paves the way for the scalable synthesis of 2D SnSe with tunable optical absorption and band gap, which has potential for use in photoelectronic and photocatalytic applications.

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Introduction Two-dimensional (2D) materials such as graphene1-3, transition metal dichalcogenides (TMDs)4,5, BN6,7, and phosphorene8-10 have novel properties compared with their bulk counterparts. Therefore, they can be used in various applications including field-effect transistors11-13, optoelectronic devices14,15, catalysts16,17, and thermal management6. For example, 2D MoS2 with high mobility and flexibility is used to fabricate flexible transistors with a large on/off current ratio11,18. Recently, 2D IV-VI materials such as SnS19, SnSe20-22, and GeSe23 have attracted much research interest. Among them, SnSe is a narrow band gap semiconductor comprised of environmental-friendly and earth abundant elements20-22,24-27. SnSe has an indirect band gap of 0.9 eV and a direct band gap of 1.3 eV, which leads to a large absorption in the major sunlight spectrum; therefore, it is a promising candidate for solar cells and photodetectors20,25,26. It is also reported that single crystal SnSe has an ultralow thermal conductivity and shows a remarkable thermoelectric performance28,29. Additionally, rock-salt SnSe is a native topological insulator30,31. SnSe nanosheets have been synthesized through wet chemistry20,21, Li-intercalation exfoliation22, and vaportransport deposition27. Shuang Yuan et al. synthesized SnSe nanosheets with wet chemistry, used them as the anode material of Na-ion batteries, and achieved a high capacity of 738 mAh g−1 and a high energy density of 141 Wh kg-1 21. Hyun Ju et al. produced SnSe nanosheets with a thickness of a few nanometers by Li-intercalation exfoliation and prepared SnSe nanosheet/PEDOT:PSS thermoelectric composite materials whose maximum figure of merit ZT was 0.32 at 300 K22. In addition, field3 / 24


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effect transistors based on vapor-transport deposited SnSe nanoplates were successfully fabricated27. The synthetic methods mentioned above, such as Li-intercalation exfoliation and vapor-transport deposition, have their own disadvantages. Li-intercalation exfoliation may damage the crystallinity of SnSe nanosheets22. Wet chemistry20,21 and vapor transport deposition methods27 are not suitable for mass production. Therefore, it is necessary to develop a facile synthetic method suitable for the mass production of highquality SnSe nanosheets. Liquid exfoliation method has been reported as a simple and feasible approach in high yield production of 2D materials. Various 2D nanosheets including graphene32, TMDs33, and phosphorene34,35 have been successfully synthesized with the liquid exfoliation method. In this method, powders of layered materials are sonicated with solvents including N-methylpyrrolidone (NMP), dimethylformamide (DMF), and isopropyl alcohol (IPA) for several hours to obtain the dispersions that contain nanosheets with different thicknesses and sizes36. Then, the nanosheets with controlled thickness and size can be extracted from the dispersions by further centrifugation37. However, there is no report in the literature on the synthesis and properties of SnSe nanosheets prepared by sonication liquid exfoliation, as far as we know. In this work, we synthesized few-layer SnSe nanosheets with various thicknesses by a facile sonication liquid exfoliation method. To obtain high-quality crystalline nanosheets, self-made SnSe flakes with a diameter of 23 mm were used as the precursor in liquid exfoliation. The morphology and microstructure of SnSe nanosheets were 4 / 24



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systematically investigated. The optical properties of SnSe nanosheets were measured by ultraviolet-visible-near infrared (UV-Vis-NIR) spectrophotometry. An easy way to tune the band gap, by adjusting the nanosheet thickness, was found. First principles calculations were carried out to study the influence of the nanosheet thickness on the band gap, and the calculation results were consistent with the experimental data.

Experimental and Theoretical Methods Preparation of SnSe flakes Self-made SnSe single crystal flakes were used as initial material (Figure 1a). Asreceived Sn and Se powders (99.99% purity, Aladdin, USA) and IPA (99.8%, Acros, Belgium) were used. Sn and Se powders were mixed and ball milled for 3 h to form SnSe compound powder. The SnSe powder was annealed at 800 °C in a quartz tube furnace (Lindberg/Blue M 1100 °C, Thermo Scientific, USA) under Ar protection for 12 h. Single crystal SnSe flakes with a size of 2–3 mm were produced after annealing (Figure 1a).

Preparation of SnSe nanosheets SnSe nanosheets were produced with typical bath sonication exfoliation procedures33,36-40. Self-made SnSe flakes (300 mg) were added to IPA (5 ml) in small reagent bottles sealed with Parafilm and sonicated in a sonic bath (KQ3200DE, Kunshan Shumei, China) for 20 h. The frequency of ultrasonication was 40 kHz and the power was set to 60W. The bath was cooled with ice bags. The temperature was 5 / 24


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below 40 °C during sonication. As-prepared dispersion was centrifuged at 3000 rpm for 30 min. The sediment was dispersed with fresh IPA to obtain Solution A, and the supernatant was collected as Solution B, as shown in Figure 1b. Next, Solution B was centrifuged at 8000 rpm and 12000 rpm for 30 min, and the supernatants were collected as Solution C and Solution D, respectively. The color of these solutions transformed from grey to light yellow with the increase of the centrifugation speed (Figure 1b). These four solutions were designated as Sol A, Sol B, Sol C and Sol D, respectively.

Characterization The morphology of SnSe nanosheets was observed by scanning electron microscopy (SEM, Merlin, Zeiss, Germany). The crystal structure was analyzed by Raman microscopy (LabRAM HR Evolution, HORIBA Scientific, France) with a 532 nm excitation wavelength at a power of 5 mW. The thickness of the nanosheets was measured by atomic force microscopy (AFM, AR Cypher, Oxford, UK) using AC160 probes with an AC tapping mode at a scan rate of 2 Hz. The solutions containing SnSe nanosheets were spin-coated on SiO2/Si substrate for AFM measurement. Spin coating was adopted to avoid sample aggregation19. The nanosheet thickness was obtained by measuring the maximum height difference between the nanosheet and the substrate. The microstructure was performed by transmission electron microscopy (TEM, JEOL 2100, JEM, Japan) with LaB6 gun. The sample composition was analyzed by field-emission transmission electron microscopy (FE-TEM, JEOL 2100F, JEM, Japan) 6 / 24



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equipped with energy dispersive X-ray spectroscopy (EDX, INCA, Oxford, UK). The solutions with SnSe nanosheets were dropped onto Cu grids with ultrathin carbon film for TEM. Optical properties were measured with UV-Vis-NIR spectrophotometry (Lambda 950, PerkinElmer, USA). An integrating sphere was employed in the experiments to eliminate the influence of scattering. Featureless absorption spectrum of IPA above 240 nm allowed an accurate measurement of SnSe optical absorption in the range of 250– 1300 nm41. The absorption coefficient  was calculated from the formula Abs=Cl, where Abs was the absorbance measured by UV-Vis-NIR spectrophotometry, C was the concentration of SnSe nanosheets in the solutions measured by inductive coupled plasma-atomic emission spectrometry (ICP-AES, VISTA-MPX, Varian Inc., USA), and l was the path length of the incident light. In our experiments, all samples were settled on standard quartz cuvettes with a path length of 1 cm, and hence l was 1cm.

First principles calculation SnSe is a typical IV-VI semiconductor with a layered orthorhombic crystal structure24-26. The space group of SnSe is Pnma at room temperature28. SnSe has a hinge-like structure with strong covalent bonding within a two-atom-thick layer and with weak intermolecular interaction between layers, similarly to black phosphorus12,20. All band structure calculations of SnSe in this work were conducted based on the first principles density functional theory (DFT) with the Cambridge Sequential Total Energy Package (CASTEP) code42,43. The Perdew-Burke-Ernzerhof (PBE) function of the 7 / 24


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generalized gradient approximation was adopted for the exchange correlation of electrons44. The semi-empirical correction of Grimme was adopted to describe the van der waals interaction between layers45. The kinetic energy cutoff for plane wave functions was set as 500 eV using ultrasoft pseudopotentials and the energy convergence threshold was set as 10-6 eV/atom. We employed the experimental lattice constants (a = 11.501 Å, b = 4.153 Å, and c = 4.445 Å) as initial parameters in the calculation46. The Monkhorst-Pack k-meshes of 4×11×11 and 1×11×11 were employed for the bulk and few-layer SnSe, respectively. The energy, max.force, max.stress, and max.displacement were 10-5 eV/atom, 0.03 eV/Å, 0.05 Gpa, and 0.001 Å, respectively, in order to optimize the bulk cell. The lattice constants after optimization were a = 11.613 Å, b = 4.187 Å, and c = 4.240 Å, which were consistent with the experimental data46. The lattice constants and atomic positions of few-layer and monolayer were derived from bulk without optimization. A 20 Å vacuum slab was used in the calculations of the SnSe monolayer and few-layer to avoid the interaction between periodically repeated supercells. Band gaps calculated by DFT method were usually underestimated, and thus a scissor of 0.5 eV was adopted for all structures in our calculations.

Results and Discussion Morphology and microstructure Figures 1c and 1d, respectively, show the SEM images of an individual SnSe nanosheet and densely-packed nanosheets. The lateral size of SnSe nanosheets varies 8 / 24



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from 50 to 500 nm, which is much smaller than that of the initial SnSe flakes (23 mm), indicating that sonication causes significant fragmentation of the initial flakes47. TEM and EDX were performed to further analyze the microstructure and composition of exfoliated SnSe nanosheets. Figure 2a shows a typical SnSe nanosheet. Figures 2b and 2c are a high resolution TEM (HRTEM) image of the nanosheet in Figure 2a and the corresponding fast Fourier transform (FFT) image, respectively. The lattice spacing in Figure 2b is determined to be 0.303 nm, which is consistent with the interplanar spacing of SnSe (011) plane (JCPDS No. 48-1224)20. The FFT shows a single crystal pattern with a [100] zone axis20. In addition, the elementary distribution was studied by EDX mapping. Figures 2e and 2f, respectively, show the distribution of Sn and Se in the nanosheet within the selected area in Figure 2d. Sn and Se are uniformly distributed and the atomic ratio of Sn to Se is 49.9 : 50.1. The TEM analysis demonstrates that we synthesized high-quality crystalline SnSe nanosheets. Figure 3 shows the Raman spectra of a SnSe nanosheet and self-made flake. Theoretically, orthorhombic SnSe is predicted to have 12 vibration modes (4 Ag, 2 B1g, 4 B2g, and 2 B3g) according to the crystal symmetry48. A previous study on single crystal SnSe has shown that different Raman vibration modes are observed when the incident laser beam is along the a, b or c axis of the crystal48. The 4 Ag (33, 71, 130, and 151 cm) and 2 B3g (37 and 108 cm-1) modes that correspond to rigid shear vibration are

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observed in Raman spectra when the incident laser beam is along the a axis48. 2 B1g (57 and 133 cm-1) as well as 4 Ag modes are observed when the incident laser beam is along the c axis48. Meanwhile, only 4 Ag modes are observed if the incident laser beam is 9 / 24


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along the b axis, because the 4 B2g modes are too weak to be detected48. For our SnSe nanosheet, there are 5 peaks in its Raman spectrum. Ag(1) and its neighboring B3g(1) modes merge into one peak at 32 cm-1. Other four peaks at 69, 107, 127, and 149 cm-1 correspond to Ag(2), B3g(2), Ag(3), and Ag(4) modes, respectively. The observation of 4 Ag and 2 B3g vibration modes proves that the sample is a highly crystalized SnSe nanosheet whose normal direction is the a axis. A shift of the Raman peak position often exists for 2D materials such as graphene and phosphorene49-51. Similarly, a slight red shift is observed for our SnSe nanosheets in comparison with self-made SnSe bulk flakes whose five Raman peaks are at 34, 71, 109, 131, and 149 cm-1, respectively. Figures 4a–c show the AFM images of the SnSe nanosheets dispersed in Sol B, Sol C, and Sol D, respectively. Figures 4d–f show the corresponding thickness histograms of the nanosheets obtained in Sol B, Sol C, and Sol D, respectively. Over 100 nanosheets were counted to yield the histogram for each sample. An obvious reduction in thickness was observed as the centrifugation speed increased. We plotted the cumulative frequency as a function of the nanosheet thickness (Figure 4g), and determined the median value of thickness at a cumulative frequency of 50%. The median thickness values are 8.9, 5.9, and 4.3 nm for Sol B, Sol C, and Sol D, respectively. These median thickness values correspond to 15, 10, and 7 layers, respectively, since the thickness of a single layer of SnSe is 0.58 nm along the a axis22. The nanosheets in Sol B produced after 3000 rmp centrifugation demonstrate a wide distribution of thickness from 2 to 44 nm. The majority of the nanosheets have a thickness of 610 nm. A typical thickness is shown in Figure S1 in the supporting 10 / 24



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information. As the centrifugation speed increases to 12000 rmp, the histogram shows that the thickness of more than 90% SnSe nanosheets is less than 10 nm. The nanosheets with a thickness of ~1 nm and a lateral size of ~50 nm were sometimes observed among the nanosheets from Sol C and Sol D, indicating the presence of exfoliated SnSe bilayers in these solutions (Figure S2 in the supporting information). The centrifugation technique is not only thickness-selecting, but also size-selecting. The lateral size of SnSe nanosheets decreases as the centrifugation speed increases (Figures 4ac), which is usual for 2D materials synthesized by liquid exfoliation47. Additionally, the thickness distribution of SnSe nanosheets can be further narrowed. Several methods such as density gradient ultracentrifugation 37,52 and liquid cascade centrifugation38 have been used to obtain nanosheets with narrow thickness distribution. Density gradient ultracentrifugation is proved to be effective in obtaining nanosheets with a thickness distribution of several nanometers, but the process procedure is relatively complicated and specific dispersants need to be used for different materials37,52. Liquid cascade centrifugation is a fast method to separate nanosheets with controlled thickness38. We adopted the liquid cascade centrifugation method to obtain SnSe nanosheets with narrower thickness distribution. An example is shown in Figure S3.

Optical properties and band gap The optical properties of SnSe nanosheets with various layers were studied with UV-Vis-NIR spectrophotometry. Figure 5 shows the absorption coefficient as a 11 / 24


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function of light wavelength for four samples. The absorption coefficient increases in the UV region sequentially as the nanosheet thickness decreases, whereas it decreases in the visible and IR regions. Interestingly, a significant blue shift for the optical absorbance edge is found when the nanosheet thickness is reduced. Sol A, mainly compromised of bulk SnSe, shows a broad absorption band from NIR to UV light. On the other hand, Sol D, which was produced after 12000 rmp centrifugation, is almost transparent to NIR light and its absorption coefficient rises rapidly from the visible region to the UV region. The absorption coefficient of Sol D at 250 nm is ~3 times larger than that of Sol A. For an intrinsic semiconductor, photon absorption occurs when the electrons near the top of the valence bands are excited to the conduction band by the incident photons whose energy is larger than the band gap. Therefore, the evolution of the absorption spectra indicates a change of SnSe band structure. We investigated the optical band gaps of four samples from the absorption spectra. The Tauc’s plots are shown in Figure 6. All samples exhibit indirect electron excitation. The indirect bandgaps of Sol A, Sol B, Sol C, and Sol D are 0.91, 1.13, 1.27, and 1.35 eV, respectively. The measured band gap of SnSe in Sol A (0.91 eV) is very close to that of bulk SnSe (0.90 eV)20. It is worth noting that the band gap of Sol D is significantly higher than that of SnSe in Sol A. The band gap increases; therefore, photons with higher energy are required to excite the electrons from the valence bands to conduction bands, causing a blue shift of the optical absorption edge. Therefore, the experimental data on the band gap of SnSe are in good agreement with the change of the optical absorption edge in Figure 5, together showing 12 / 24



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that the nanosheet thickness has an important influence on the electronic structure of SnSe. In addition, Sol D contains the SnSe nanosheets with a median thickness of 4.3 nm. These nanosheets have the largest specific surface area, which leads to the significant enhancement of the optical absorption in the UV region as shown in Figure 5. To confirm the variation of the band gap of SnSe nanosheets, first principles calculations were performed. Figure 7 shows the calculated band structure of a monolayer, 7-layer, 15-layer, and bulk SnSe. The overall band structure of bulk and finite layers is similar. The valence band maximum is along Γ−Y and the conduction band minimum is along Γ−X. The fundamental band gap of SnSe shows an obvious increase as the thickness decreases. The calculated band gap increases dramatically from 0.93 eV to 1.79 eV when the structure changes from bulk to monolayer. The band gap of SnSe monolayer is 1.79 eV, which is consistent with the previous calculation (1.63 eV)53. Figure 8 summarizes the band gap data obtained by theoretical calculations and experiments. Both calculations and experiments reveal that the band gap increases as the thickness of SnSe decreases. As the nanosheet thickness increases from monolayer to bulk, the wavefunction overlap will cause the reduction of band gap for various 2D material systems54,55. When the layer number is more than eight, the measured band gap of SnSe shows a strong dependence on the thickness, which is not observed from the calculation. A similar phenomenon has also been observed for black phosphorus, whose crystalline structure is close to that of SnSe56. This finding indicates that the wavefunction overlap is not the 13 / 24


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only reason for the change of the SnSe band gap and the subtle variation of the crystalline structure with the increase of thickness could be another cause57. In summary, we can tune the band gap and optical absorption performance of SnSe nanosheets by adjusting the nanosheet thickness. The tunable band gap and optical properties of SnSe nanosheets are desired for electrical devices and solar cells. For example, SnSe nanosheets with different band gaps can be used to fabricate a solar cell absorption layer with a graded band gap to maximize the utilization of solar energy58.

Conclusions Few-layer SnSe nanosheets were produced by scalable liquid-phase exfoliation. HRTEM and Raman spectrum analysis verified that the as-synthesized SnSe nanosheets were well crystalized. The thinnest nanosheets were bilayered. The thickness of SnSe nanosheets was adjusted by centrifugation. The optical properties of SnSe nanosheets with various thicknesses were examined. A blue shift of the optical absorption edge was observed when SnSe transformed from bulk to few-layer, and a strong absorption in the UV region was found for the nanosheets. The band gap of SnSe was studied experimentally and theoretically. Experiments and first principles calculations both revealed that the band gap increased as the thicknesses decreased. The tunable band gap of SnSe can be utilized to design functionally graded materials used for electronic and photovoltaic applications. The method established in our work also provides an insight to synthesize other 2D materials with tunable band structures and optical properties.

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Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant No. 51572149), National Key Research Program of China (Grant No. 2016YFA0201003), and National Basic Research Program of China (Grant No. 2013CB632504). We thank Mr. Jun Pei and Xiaoqing Xi at Tsinghua University for their help on material synthesis and optical measurement.

Figure captions Figure 1. (a) Photograph of single crystal SnSe flakes as initial material for liquid exfoliation. (b) Four solutions with different SnSe nanosheets. (c) SEM image of an individual few-layer SnSe nanosheet. (d) SEM image of densely packed SnSe nanosheets. Figure 2. TEM analysis of SnSe nanosheets. (a) A thin SnSe nanosheet. (b and c) HRTEM image and corresponding FFT image of the nanosheet in (a), respectively. (d) TEM image of a typical SnSe nanosheet. (e and f) Elementary mapping of Sn and Se within the selected region in (d), respectively. Figure 3. Raman spectra of a SnSe nanosheet and self-made SnSe flake with 532 nm laser excitation wavelength. Figure 4. AFM characterization of SnSe nanosheets. (ac) Height profiles of SnSe nanosheets in Sol B, Sol C, and Sol D, respectively. (df) Thickness histograms of the nanosheets in (ac), respectively. (g) Plot of cumulative frequency versus thickness for 15 / 24


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three samples. Figure 5. UV-Vis-NIR absorption spectra of SnSe nanosheets with different thicknesses. Figure 6. Tauc’s plots of (αhν)1/2 as a function of photon energy for SnSe nanosheets with different thicknesses. Figure 7. Calculated band structures for SnSe monolayer (a), 7-layer (b), 15-layer (c), and bulk (d). The Fermi energies are fixed on the top of the valence bands. Figure 8. Dependence of the band gap of SnSe on the layer number. The measured and calculated band gap values are represented by black squares and red circles, respectively. The horizontal dash lines show the band gaps of SnSe bulk determined by our experiments (black line) and calculations (red line).

Supporting Information Details of AFM profiles of the SnSe nanosheets in Sol B and Sol D, and the comparison of the thickness distribution between the nanosheets in Sol B and Sol B’. Sol B’ was produced after 1000, 2000, and 3000 rmp cascade centrifugation.

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