The Electronic Anisotropic of SnSe2 Nanoflakes
Authors: Tsai Liang-Feng , Wang Chiu-yen 
Published: February 2020

The 2D layered materials have drawn much attention in recent years due to their large specific surface area, large bonding anisotropy and tunable bandgaps. Nowadays, multiple 2D materials had been investigated, which can classify as several groups. For instance, transition metal dichalcogenides (TMDs) (MoS2
[1]), IIIA-VIA group (InSe[2]), IVA–VIA group (SnSe2
[3]) and pure element (black phosphorus[4]). As a member of IVA–VIA group 2D layered material, SnSe2 is a hexagonal crystal with ~1 eV indirect bandgap[3], which has a three-layer structure like Se-Sn-Se, the Sn layer is sandwiched between two Se layers. It has been proposed that SnSe2 has outstanding electronics characteristic[5], optoelectronics characteristic[3] and which is earth abundant. In addition, due to the layer structure the large anisotropy of strong in-plane bonding and weak layer-layer van der waals force, which is appropriate to probe anisotropy.
In this work, we investigate the electrical anisotropy for SnSe2 nanoflakes. Single crystal SnSe2 nanoflakes were synthesized by chemical vapor deposition (CVD) method in the two-zone heating furnace. Sn and Se powder as the precursor of CVD method, which was placed at the center of first heating zone and upstream of first heating zone respectively, and 80 nm SiO2/Si substrate was placed at the center of second heating zone. The temperature of first heating zone and second heating zone were heating up to 600 °C and 350 °C respectively, and hold 15 min at the setting temperature. The scanning electron microscope (SEM), energy dispersive X-ray spectrometry (EDS), X-ray diffraction (XRD), transmission electron microscope (TEM) and selected area electron diffraction (SAED) were used to detect quality of SnSe2 nanoflakes, such as component, grain size, thickness, length, morphology and crystalline. The upright SnSe2 nanoflakes with half hexagon shape was observed in SEM image that the edge length and thickness is about 20 μm and 100 nm, respectively. According to XRD analysis, only (001), (002), (003) and (004) planes can observe apparently on spectra indicate that the priority orientation is along [001] direction. The full-width-at-half-maximum (FWHM) for this four diffraction peaks are all approximately 0.05° which demonstrate the high crystallinity for SnSe2 nanoflakes. Scherrer equation is not applicable to calculate the grain size for SnSe2 nanoflakes due to FWHM for the diffraction peaks are too narrow. The grain size increase is not a major reason about expand for diffraction peaks FWHM when diffraction peaks FWHM is narrow, which cause a large error. So the SAED was future certified the SnSe2 nanoflakes whether a signal crystal.
To explore the electrical anisotropy properties of the SnSe2 nanoflakes. Two different devices were used to investigate the electronic characteristic of SnSe2 nanoflakes along the direction which parallel to the layer and perpendicular to the layer. The device for carrier transport parallel to the layer was made by placing SnSe2 nanoflakes on SiO2/Si substrate, and carrier transport perpendicular to the layer was made by placing SnSe2 nanoflakes on Pt/SiO2/Si substrate. The electronic characteristic parallel to the layer was probed by top-contact, and the electronic characteristic perpendicular to the layer was probed by top-bottom-contact, while the top contact of SnSe2 without any electrode and Pt electrode was used to be bottom contact. Rather than previous report employ E-beam lithography or sputter the electrodes on SnSe2 nanoflakes. The detector was direct contact on the top of SnSe2 nanoflakes to reduce the effect which caused by electrode such as electrode resistance. The observe anisotropy of conductivity parallel to the layer is 5 order than perpendicular one, which is attributed to the large difference of bond energy between covalent bond within the layer and Van der waals force perpendicular to the layer. This electrical anisotropy property may understand deeper about the effect of crystal structure on electronic component, which may cause more rational design than nowadays electronic component.
Reference:
[1] Y. Zhan, Z. Liu, S. Najmaei, P.M. Ajayan and J. Lou. Small. 2012, 8, 966.
[2] S. Sucharitakul, N.J. Goble, U.R. Kumar, R. Sankar, Z.A. Bogorad, F.C. Chou, Y.T. Chen and X.P.A. Gao. Nano Letters. 2015, 15, 3815.
[3] Y. Huang, K. Xu, Z. Wang, T.A. Shifa, Q. Wang, F. Wang, C. Jiang and J. He. Nanoscale. 2013, 00, 1.
[4] L. Li, Y. Yu, G.J. Ye, Q. Ge, X. Ou, H. Wu, D. Feng, X.H. Chen and Y. Zhang. Nature Nanotechnology. 2014, 9, 372.
[5] T. Pei, L. Bao, G. Wang, R. Ma, H. Yang, J. Li, C. Gu, S. Pantelides, S. Du and H.J. Gao. Applied Physics Letters. 2016, 108, 053506.