Designing Shape Evolution of SnSe 2 Nanosheets and their Optoelectronic Property

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Introduction
As complements to graphene, two-dimensional (2D) LCMs, especially MX 2 (M=Mo, W, Sn, Ti, Zr, Hf, Nb, Ta, and X = S, Se), have drawn tremendous attention in recent years. Compared to graphene, these MX 2 materials own band gaps in the range of 1-2 eV, which are suitable for digital electronic applications. [1][2][3] Besides, the weak van der Waals interaction between adjacent layers renders monolayer or few multilayer LCMs available. Based on these features, plenty of gorgeous progresses have been achieved for synthesis and applications of LCMs. For instance, monolayer MoS 2 have been realized by mechanical exfoliation 4 . Chemical vapor deposition (CVD) method could synthesize monolayer MoS 2 with wafer scale. 5 The measured carrier mobility of MoS 2 in extremely low temperature can be as high as 34,000 cm 2 V -1 s -1 . 6 Atomic p-n diodes at the ultimate thickness limit were successfully realized on monolayer WSe 2 . 7 Meanwhile, other members of LCMs, such as SnS 2 , exhibits superior properties as ultrafast phototransistors with response time as short as 5 μs. 8 The responsivity of phototransistors based on GaTe could be as high as 800 A/W. 9 High performance field effect transistors based on WSe 2 and MoTe 2 could go through a transfer from hole conduction to electron conduction by gate voltage regulation. 10,11 During the deep excavation of LCMs, fancy devices with high performances emerge, which significantly put forward the innovation and development of semiconductor industry. As an emerging member of LCMs, SnSe 2 is of great value to explore. It is an n-type semiconductor with a band gap ~1.0 eV, which normally crystallizes in a hexagonal structure with two layers of Se atoms sandwiching a Sn layer. 12  To the best of our knowledge, owing to the relatively thick thickness, the SnSe 2 synthesized by CVD method reported do not exhibit their superiorities as layer materials to fabricate electrical or optoelectronic devices. 19,20 Ultrathin SnSe 2 NSs synthesized by CVD with thickness lower than 10 nm and their potential application as photodetector have not been reported yet.
Herein we present a recipe to successfully synthesize SnSe 2 NSs on SiO 2 /Si substrates. Scanning electron microscope (SEM), Raman spectroscopy, electron energy dispersive X-ray spectroscopy (EDX) and transmission electron microscope (TEM) were employed to characterize the composition and crystal structures of the NSs. The thickness of as-grown SnSe 2 NSs, which is measured with an atomic force microscope (AFM), could be lower than 10 nm with the lateral size ~ 5 μm.
In particular, the products could evolve from square SnSe NSs to varied shapes of SnSe 2 , successively shaping as hexagon, truncated triangles and triangle along the decreasing direction of temperature gradient. Unlike SnSe 2 , layered SnSe has an orthorhombic crystal structure. 21 Grown models are developed to figure out the mechanism of NSs evolution. We also fabricate photodectors based on as-grown SnSe 2 NSs and study their optoelectronic properties. Under illumination of 800 nm laser, the on/off ratio of photo switch could be ~100.
Our study may provide a method to synthesize LCMs with designed shape and broaden the candidates for high performance photodetectors.

Results and discussion
The experimental setup and morphology of as grown nanostructures are depicted in Fig. 1. In contrast to the previous work, 20 cleaned SiO 2 /Si substrates were placed downstream from the second heating zone ~14 -18cm. Fig. 1b shows the temperature gradient of the furnace. As the temperature of substrates declines, the shape of the deposited NSs resembles square, hexagonal, truncated triangular and triangular in sequence, which are displayed in Fig. 1c-j. Along the flow direction, we first observed square NSs on substrates at ~ 510 o C. The NSs' size could augment as the dosage of SnSe powder increases. Furthermore, by adjusting the temperature and precursor dose for experimental settings, the dominating shape of NSs can be controlled. For example, hexagonal or half-hexagonal SnSe 2 NSs would be the majority when use the mixed powder as the precursor only. The reason for this trend will be discussed later.
To confirm the crystallinity and composition of NSs imaged by OM and SEM in Fig. 1, further characterization was made by Raman spectroscopy and EDX. Accordingly, Fig. 2a and 2b show the Raman and EDX spectrums corresponding to the square NSs. As illustrated in Fig. 2a, the three intense Raman peaks at 110.3 cm -1 (B 3g ), 131.9 cm -1 (A g ), and 149.8 cm can be attributed to the characteristics features of SnSe. [21][22][23] Moreover, the high crystallinity of SnSe NSs is apparent from the uniform color contrast and high intensity of the Raman mapping (B 3g phonon mode) in inset of Fig. 2a. We further demonstrate the Sn: Se atomic ratio of the NSs close to 1:1 by EDX, which are illustrated in Fig. 2b. The hexagonal, truncated triangular and triangular NSs are characterized in similar ways. Fig.s 2c, e and g respectively exhibit their Raman spectra. The intense peaks at ~118.5 cm -1 and ~185.2 cm -1 correspond to A g1 and E g mode of SnSe 2 . 17 The Raman mappings of A g1 mode in the insets are in homogeneous intensity, revealing the good crystalline quality of the NSs. As it is evident from their respective EDX, the atomic ratios close to 1:2 agree with their expected stoichiometric composition. Based on the above critical observations, we believe that we successfully synthesized square SnSe and various shaped SnSe 2 NSs on SiO 2 /Si substrates. Transmission electron microscope (TEM) and atomic force microscope (AFM) are used to further evaluate the crystal structure and thickness of NSs we synthesized. Fig. 3a presents a TEM image of SnSe dispersed onto a holey carbon film. The regular diffraction fringes (HRTEM) and corresponding selected electron diffraction pattern (SAED) shown in Fig. 3b and c, indicating the single crystal nature and high crystallinity of as grown SnSe NSs. The lattice spacing ~ 4.25 Å and corresponding orthogonally symmetric SAED pattern are in great agreement with the orthorhombic crystal structure of SnSe NS.  a half-hexagonal SnSe 2 NS. The corresponding HRTEM and SAED are depicted in Fig. 3f, g, j, and k, with the lattice spacing of 3.29 Å and 3.31 Å indexed to (100) plane, and the zone axis of hexagonally symmetric SAED spots assigned to [001] direction. These results demonstrate the synthesized SnSe 2 NSs are single crystalline with high crystal quality. The AFM image displayed in Fig. 3d, h and l demonstrate that the thickness of as grown smooth NSs is ~ 153.3 nm, 73.8 nm and 9.4 nm. The thin SnSe 2 NSs are mainly found at the downstream locations of the substrates, where nucleate sites are fewer than the edge of upstream. 24 More AFM data of the thin NSs are shown in Fig. S1. The X-Ray Diffraction (XRD) results of as-grown NSs are depicted in Fig. S2. To the best of our knowledge, this is the first report to come up with ultrathin SnSe 2 NSs with thickness bellow 10 nm. Our CVD approach, therefore, resulted in the thinnest ever reported SnSe 2 NSs endowing immense application in electronic and optoelectronic devices. Generally, only one kind of material other than multiple ones would be obtained under a certain growth condition. However, during our experimental process, materials with two kinds of component in various shapes are synthesized. It is of great value to understand the growth mechanism during this process, which may help us controllably synthesize LCMs in the future. As shown in Fig. 4a, the growth process may experience three steps: (1) the sublimation of source materials at proper temperature and their carriage by carrier gas, (2) adsorption and desorption of source materials atoms onto substrates, (3) nucleation and reaction of adsorbed atoms on substrates. It is easy to understand that the deposition rate differs between different atoms. On substrates of higher temperatures, more active atoms would quickly desorb into the carrier gas. Accordingly, the ratio of deposited atoms would vary as the temperature of substrates changes.
According to the results of our experiment, on substrates of higher temperature, adsorbed selenium atoms incline to quickly desorb into the carrier gas, thus SnSe nucleates and grows into square NS under proper gas flow and pressure. When the temperature of substrates decreases, the amount of desorbed selenium atoms declines and there are enough adsorbed ones to react with Sn. As a result, SnSe 2 NSs are synthesized. This trend is in agreement with a former report, in which SnSe would be deposited by selenization Sn film from 530 o C to 570 o C, while SnSe 2 would be deposited from 430 o C to 470 o C. 25 The phenomenon of shape evolution may also confirm this trend. It has been established that the most commonly and energetically favored edge structures of MoS 2 are Mo zigzag (Mo-zz) terminations and S zigzag (S-zz) terminations, 26,27 which alternatively constructs the six edges of a hexagonal shaped MoS 2 NS. Since SnSe 2 own the same hexagonal crystal structure with MoS 2 , in which Sn (Se) atoms replace the position of Mo (S) atoms, it should follow a similar principle. For Sn-zz terminations, each Sn atom has two dangling bonds, while for Se-zz terminations, the dangling bonds become one. This difference may result in the distinct chemical activity of those terminations, which may further lead to their diverse growth rates. In consequence, the shape of crystals evolves, as shown in Fig. 4b. During the growth process of our experiment, the concentration of Se atoms deposited on the substrates plays a significant role in the shape evolution. As the temperature of substrates decreases, more and more Se atoms deposited. At surface of substrates where Se atoms are sufficient, the Sn-zz terminations become more active to meet and bond with Se atoms around, thus grow faster than Se-zz terminations. In consequence, the Sn-zz terminations become shorter and shorter (shaping like truncated triangles) and eventually disappear (shaping like triangles) as the temperature of substrates decrease. The effect of precursor amounts on the shape evolution attest the assumption, too. The reduction of Se powder (Se precursor) and SnSe powder (Sn precursor) leads to the formation of hexagonal and triangular shapes respectively. This phenomenon is also observed for other LCMs, such as MoS 2 , 28, 29 which further corroborates our assumption. Moreover, the pressure during the gas flow also plays significant role in affecting the shape of SnSe 2 NSs. The regular NSs is achieved only under pressure 220~230 Pa in our experimental settings. Otherwise, the products tend to form into amorphous SnSe and SnSe 2 domains. We further explore the optoelectronic properties of SnSe 2 NSs. A photo detector device based on as-grown SnSe 2 NSs is fabricated through a standard EBL process, following by thermal deposition of Cr/Au (5 nm/80 nm) as electrodes. Fig.  5a displays an AFM image of the detector with a SnSe 2 NS ~21.1 nm. To examine the performance of the device, we put it under illumination of 800 nm laser and measure its optoelectronic properties at 140 K to diminish the effect of laser heating. As depicted in Fig. 5b, the detector exhibits a monotonically increasing photocurrent as the incident power of illumination increases from 0.227 W/mm 2 to 4.380 W/mm 2 . We also calculate the responsivity (R λ ) and photogain (G) from following equations: 9, 30 ܴ ఒ ൌ ‫ܫ‬ /ܲܵ, G ൌ ‫ܫ‬ ‫ܵܲݍ/ߥ݄‬ where I ph is the photocurrent, ݄ is the Planck constant, ߥ and P is the incident light frequency and power respectively, and S is the effective illuminated area. Fig. 5c shows the relationship between them and the incident beam power. Accordingly, the R λ can reach ~1900 mA/W and the on/off ratio of this device as photos witches could be 100. To evaluate the performance of our fabricated SnSe 2 photodetectors, we list the figure-ofmerit of some devices based on other 2D materials, as shown in Table 1. [31][32][33][34][35][36] The measured responsivity of SnSe 2 Fig. 4 The schematic of the growth mechanism. (a) A schematic illustrates the major three steps during the vapor deposition process. (b) A simple model to expound the shape evolution.
photodetector is acceptable among 2D materials, and the on/off ratio of photo switch is among the largest ones in the table, indicating the potential of SnSe 2 NSs as high performance photo switches.

Conclusions
In conclusion, we develop a one-step method to synthesize SnSe and SnSe 2 NSs on SiO 2 /Si substrates. Along the decreasing direction of temperature gradient, the shape of as-grown NSs changes from square SnSe, hexagonal and truncated triangular SnSe 2 to triangular SnSe 2 by controlling the amount of source materials and the pressure during the growth process. Models are proposed to explain the shape evolution under our experimental setups. Photodetectors based on as-grown SnSe 2 NSs exhibit a high photo switch on/off ratio ~100 under illumination of 800 nm laser. Our work will open up a new way in the synthesis of LCMs and broaden the building blocks for high performance optoelectronic devices.

Experimental
Synthesis of SnSe and SnSe 2 NSs on SiO 2 /Si substrates. A horizontal vacuum tube furnace with two heating zones is used for the synthesis process. The quartz tube is in diameter of 2 in. and length of 40 in. Se powder (99.999%, Alfa Aesar) and SnSe powder grinded from bulk (99.999%, Alfa Aesar) served as the precursors. Prior to the growth, 300 nm SiO 2 /Si substrates (~1×1 cm 2 ) were cleaned by a piranha solution at 120 o C for 2 h followed by rinsing in deionized water and drying in fluid nitrogen. As shown in Fig. 1a, during a typical growth process, 0.1 g Se powder were placed in a quartz boat in the middle of the first heating zone. 0.01 g SnSe powder mixed with 0.3 g Se powder were placed in another quartz boat in the middle of the second heating zone. The cleaned SiO 2 /Si substrates were placed downstream from the second heating zone ~14 -18cm. After flushing with Ar gas three times and pumping to a vacuum lower than 1 Pa, the gas flow of Ar was controlled at 50 sccm. Then, the Se powder were heated up to 250 o C in 10 minutes to provide an atmosphere of selenide before the mixed powder were heated up to 750 o C  in 15 minutes. The growth process was sustained for 30minutes. After that, the furnace was naturally cooled to room temperature. Square SnSe and varied shapes of SnSe 2 NSs were deposited on SiO 2 /Si in different location. Characterizations for SnSe and SnSe 2 NSs. The morphology of SnSe and SnSe 2 NSs were characterized an SEM (FESEM, Hitach S-4800) and an AFM (Veeco Multimode). Raman spectroscopy (Renishaw InVia, 532 nm excitation laser) and EDX attached to the SEM were used to further characterize the composition of experimental products. The crystal structure information were obtained by a TEM (Tecnai F20). Device fabrication and electrical measurement. The as-grown thin SnSe 2 NSs were transferred onto heavily p-doped 300 nm SiO 2 /Si substrates. A standard e-beam lithography (EBL) process was used to define the electrical metal electrodes. After that, Cr/Au (5 nm/ 80 nm) metal electrodes were deposited by thermal evaporation. A probe station (Lakeshore, TTP4), a semiconductor characterization system (Keithley 4200) and an 800 nm laser were utilized for optoelectronic measurements.