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In-plane anisotropic faceting of ultralarge and thin single-crystalline colloidal SnS nanosheets

The colloidal synthesis of large thin two-dimensional (2D) nanosheets is fascinating but challenging, since the growth along the lateral and vertical dimensions need to be controlled independently. In-plane anisotropy in 2D nanosheets is attracting more attention as well. We present a new synthesis for large colloidal single-crystalline SnS nanosheets with the thicknesses down to 7 nm and lateral sizes up to 8 um. The synthesis uses trioctylphosphine-S (TOP-S) as sulfur source and oleic acid (with or without TOP) as ligands. Upon adjusting the capping ligand amount, the growth direction can be switched between anisotropic directions (armchair and zigzag) and isotropic directions ("ladder"directions), leading to an edge-morphology anisotropy. This is the first report on solution-phase synthesis of large thin SnS NSs with tunable edge faceting. Furthermore, electronic transport measurements show strong dependency on the crystallographic directions confirming structural anisotropy.

Recently, we investigated SnS NSs synthesized using thioacetamide (TAA) as sulfur source and tuning the oleic acid/trioctylphosphine (OA/TOP) ligand ratio to get NSs in different shapes (between squared and hexagonal shapes) and various thicknesses (~20-50 nm). 16 In this new approach, we adopt TOP-S as sulfur source as an improved recipe.  Figure  1c) reveals that the thickness of a single sheet is around 14-15 nm, and the calculated average thickness with XRD data using the Scherrer equation is 10 nm. Powder X-ray diffraction (XRD, Figure 1d) for the capillary sample matches well the peaks from the reference card (00-039-0354) for OR SnS. The XRD pattern of a drop-casted sample shows prominent preferred orientation due to the texture effect (the large lateral size compared to the small vertical thickness makes crystallographic orientations not random), with significantly pronounced (040), (080) reflexes. This indicates that the thickness direction is [010], showing highly textured orientation for drop-casted samples. This is consistent with the SAED suggesting that the surface is oriented along  Recently, in-plane electronic anisotropy of 2D materials has gained great attentions and it has been widely investigated for potential electronic devices. 5,[17][18] Therefore, insolution tuning of edge faceting plays an important role in the application of electronic anisotropy for 2D colloidal nanomaterials. Generally, after adjusting reaction parameters, we achieved two different types of SnS NSs in respect to edge faceting: 1.
Rectangular NSs with exposed edges parallel to the (100), (001) planes (parallel to zigzag and armchair direction, respectively); 2. Rectangular (nearly square-shaped) NSs with edges parallel respectively to the {101} planes, which are along four isotropic "ladder" directions ( Figure 2). We define them as anisotropic-SnS (a-SnS) and isotropic-SnS (i-SnS) NSs respectively. The crystal structure of SnS is shown in Figure   - (Figure 3e, g, h) confirmed that the four main edges are parallel to the exposed {101} planes and the two truncated corner edges correspond to the exposed (100) facets ( Figure S3b).
In the previous work, we have already investigated the adsorption energies of capping ligands and their role in anisotropic growth of SnS square and hexagonal shaped NSs. 16 Here, we demonstrate the anisotropic growth of nanocrystals under different reaction conditions to form two types of sheets (or anisotropic faceting). After taking aliquots to check the growth process ( Figure S4), we confirmed a similar growth mechanism as the NSs reported recently. 16  The thicknesses are 10, 12, 36 nm determined from XRD data and each corresponding AFM thickness is about 5~7 nm larger compared to the XRD thickness (partly shown in Figure S6). Interestingly, if TOP only serves as solvent for dissolving S (TOP-S), with 0.64 mmol OA, only undefined-shaped NSs are formed ( Figure S5d). Increasing amount of TOP-S, a-SnS NSs start to appear composed of four explicit rectangular edges (even no truncated corners formed, Figure S5e). The lateral size decreases from nearly 8 to 3.5 µm, whereas the thickness fluctuates from 8.9 to 9.3 nm according to the XRD data ( Figure S5f). A similar tendency can also be observed when we raised the amount of OA without any TOP in the flask before injection ( Figure S7). The square NSs were produced with a higher amount of OA (from 0.64 to 1.5 and 2.0 mmol) in the reaction. Nevertheless, edge faceting switches to exposed facets parallel to isotropic directions, forming i-SnS NSs with 1.5 and 2.0 mmol OA (Figure S7b, c). In terms of morphology, the lateral size becomes smaller upon the increase of the OA amount (respectively 7900 to 1500 and 1400 nm) and the thickness becomes larger (respectively 8.9 to 17.6 and 27 nm, defined by XRD, Figure S7d (Table 1). Compared to our previous work using TAA for the i-SnS NS synthesis, TOP-S is considered as the key factor for obtaining a-SnS NSs. In the case of TAA for the SnS NS synthesis, i-SnS NSs can be prepared with the co-passivation of applied ligands (oleate, TOP and OA), due to the high adsorption energy of TOP and oleate on {101} and (100) facets. 16 Here, the formation of a-SnS is mainly due to the high adsorption energy of TOP-S on (100), (001) facets (DFT, Table 1). Higher adsorption energy of TOP-S may sterically hinder the approaching of SnS monomers to the surface, subsequently slowing the growth speed of the facets. In detail, the presence of TOP-S in the reaction solution obviously exert a strong influence on the SnS crystal surface, preferentially stabilizing the (100), (001) facets. This finally impacts on the growth rate of these facets during growth, so it's rational to propose the (100) and (001) facets turn to be more stable and present lower growth rate, which in the end serve as the exposed facets for the final sheets. It finally results in specific faceting with edges along anisotropic directions. 21 The simulations of TOP-S on the facets using simplified molecules with different chain lengths also show similar tendencies (Table S1). What is more fascinating is that the inplane faceting can be switched from a-SnS to i-SnS by adjusting the amount of OA in the reaction even in the presence of TOP-S. This is also supported by the DFT results in our previous work 16 , exhibiting high absorption energy of oleate on (100) and {101} facets, leading to square-shaped sheets with (100) truncated corner edges in the end. The zigzag and armchair directions have been investigated to demonstrate anisotropy in electronic behavior based on the difference in atomic ordering along these two directions. 6 To investigate the electrical properties of these NSs along different crystallographic directions, we contacted them individually and measured their room temperature conductivity. In one group of the devices, four contacts were placed parallel to the edges of the NSs in order to investigate the conductivity along the anisotropic directions (Figure 4a). For the other group, the contacts were placed from corner to corner to probe the isotropic directions (Figure 4b). Figure 4c shows the IV characteristics of these sheets along the anisotropic directions. The conductivity along the (100) direction (zigzag) reaches 65 S/m while for the (001) direction (armchair), a significantly lower conductivity of 39 S/m can be observed. The ratio of the conductivity along these directions (zigzag/armchair) is equal to 1.67, which is in very good agreement with the literature. 6 The effective mass of the carriers, which are mainly holes 6,22 , differs considerably along these two directions. 6,[22][23] As a result of this difference, the carriers have a higher mobility along the zigzag direction, which results in a higher conductivity. 6 On the other hand, the conductivity was measured along the (101), (101 ̅ ) directions (ladder directions). The conductivity along these directions is isotropic (2.2 S/m and 2.3 S/m, Figure 4d). The arrangement of atoms along these directions is exactly identical which results in a comparable electrical conductivity.
In summary, we investigated the crystal-structure orientation switching in respect to the edge morphology of rectangular-shaped SnS NSs. We report the synthesis conditions when the edge faceting can be switched between faceting along two anisotropic directions and along four isotropic directions. The tuning of the thickness (down to 7 nm) and lateral size (up to 8 µm) for SnS NSs manifests a promising advantage of these materials. The synthesis opens the possibility to contact individual nanosheet on the substrate, with predefined orientations, promoting the applications of colloidal 2D nanomaterials for new devices with anisotropic properties. We exemplify this by electrical measurements. They confirm different conductivities for crystallographically distinct direction. It displays an in-plane electronic anisotropy in conductivity along the zigzag and armchair directions, but isotropic behavior from corner to corner (ladder directions).

Experimental Section
Preparation and Characterization of 2D SnS Nanosheets: The synthesis and characterization methods are detailly described in the supporting information.
Device preparation and transport measurements: the individual SnS nanosheet was contacted by e-beam lithography and measured by a semiconductor parameter analyzer (detail in the supporting information).

Associated Content
Supporting Information is available free of charge on the ACS Publications website at DOI: