Realization of Large Scale, 2D van der Waals Heterojunction of SnS₂/SnS by Reversible Sulfurization

Abstract

were prepared by direct exfoliation and transfer method, which is not scalable, and inefficient for device productions. Molecular beam epitaxy (MBE), in contrast, is a powerful technique to grow wafer-scale thin films with high quality, and it is also an attractive method for the growth of 2D heterojunctions in a large scale. [21] However, the possibility of growing one layer of 2D material on the other is restricted by the growth dynamics as well as the lattice mismatch between them. So far, successful MBE growth of 2D vdW heterojunctions is still rare, and usually limited to materials with similar structure and compositions, such as MoSe 2 /HfSe 2 , graphene/h-BN, bismuthene/BP-Bi. [22][23][24] Tin sulfides are known to have different chemically stable layered phases, i.e., SnS and SnS 2 . SnS has an orthorhombic crystal structure that possesses puckered honeycomb lattice with lattice constants of a = 4.04 Å, b = 4.31 Å, as shown in Figure 1a. [25][26][27][28][29] Such low-symmetry structure brings in strongly anisotropic electronic properties, and possible applications related with its in-plane ferroelectricity, piezoelectric, and nonlinear optical properties. [25,26,[30][31][32][33][34][35] Furthermore, SnS is a p-type semiconductor with indirect bandgap dependent on the film thickness, from 1.96 eV in the monolayer to 1.44 eV in six-layer films. [28] In contrast to SnS, the layer of SnS 2 exhibits hexagonal symmetry with lattice constants a = b = 3.65 Å, as shown in Figure 1b. [36][37][38][39] SnS 2 is a n-type semiconductor with larger indirect bandgap 2.18 eV in bulk and 2.41 eV in monolayer. [40] Although SnS and SnS 2 have completely different crystal structures including lattice constants and symmetry, they are both 2D vdW materials comprised by the same elements, with only different composition ratio. Thus, we consider it possible and attractive to fabricate 2D vdW heterojunctions of SnS 2 /SnS by direct sulfurization on SnS film which can be grown by MBE (Figure 1c).In this letter, we reported that few-layer SnS films with high quality and large scale can be synthesized on mica and Nbdoped SrTiO 3 (100) substrates by MBE growth. Moreover, we confirmed by the reflection high-energy electron diffraction (RHEED) and X-ray photoelectron spectroscopy (XPS) measurements that the top layer of SnS can be sulfurized to SnS 2 monolayer and the SnS 2 layer can recover to SnS by annealing SnS 2 /SnS without sulfur supply, suggesting the reversible formation of 2D vdW layered heterojunction of SnS 2 /SnS. Scanning tunneling microscope (STM) studies confirmed the surface atomic structure as well as the existence of moiré pattern, which can be attributed to the stacking between hexagonal SnS 2 monolayer and rhombohedral SnS layer. The scanning 2D van der Waals heterojunction provides an attractive opportunity for realizing novel electronic or optoelectronic devices. It remains challenging to realize high-quality heterostructures through scalable methods such as molecular epitaxy growth (MBE). Here, growth of few-lay...

Morphotaxy of Layered van der Waals Materials

“…Finally, morphotaxial chalcogenization is not restricted to TMDCs and TMOs. In particular, Li et al synthesized a 2D vdW lateral heterojunction of SnS grown by molecular beam epitaxy and a monolayer of SnS 2 formed by sulfurization . Interestingly, this process was found to be thermally reversible as annealing of SnS 2 resulted in the formation of the parent SnS compound.…”

“…Interestingly, this process was found to be thermally reversible as annealing of SnS 2 resulted in the formation of the parent SnS compound. In addition, the heterostructure between SnS and SnS 2 possessed a twist angle of ∼15° as revealed by STM while scanning tunneling spectroscopy showed a type II band alignment . In another example, Du et al investigated MAX structures (M = early transition metal, A = group 13–16 element, X = B, C, N, Si) as a scaffold to obtain TMDCs via reactions with chalcogen-containing vapors (H y Z, where Z = S, Se, or Te) such as the conversion of Mo 2 GeC to MoS 2 following reaction with H 2 S (Figure C) .…”

“…In particular, Li et al synthesized a 2D vdW lateral heterojunction of SnS grown by molecular beam epitaxy and a monolayer of SnS 2 formed by sulfurization. 111 Interestingly, this process was found to be thermally reversible as annealing of SnS 2 resulted in the formation of the parent SnS compound. In addition, the heterostructure between SnS and SnS 2 possessed a twist angle of ∼15°as revealed by STM while scanning tunneling spectroscopy showed a type II band alignment.…”

“…In addition, the heterostructure between SnS and SnS 2 possessed a twist angle of ∼15°as revealed by STM while scanning tunneling spectroscopy showed a type II band alignment. 111 In another example, Du et al investigated MAX structures (M = early transition metal, A = group 13−16 element, X = B, C, N, Si) as a scaffold to obtain TMDCs via reactions with chalcogen-containing vapors (H y Z, where Z = S, Se, or Te) such as the conversion of Mo 2 GeC to MoS 2 following reaction with H 2 S (Figure 3C). 112 This approach showed that a non-vdW solid can be converted completely into a vdW solid, resulting in high-quality layered structures.…”

“…117 Finally, it is important to note that experimental work on Janus structures is not confined to MoSSe and WSSe, with more recent research being focused on different cation/anion combinations. For instance, Sant et al reported the synthesis of PtSSe by sulfurizing PtSe 2 grown on Pt (111), employing morphotaxial techniques in both the synthesis of the PtSe 2 (selenization of Pt) and conversion to PtSSe (sulfurization of PtSe 2 by heating in H 2 S). 118 To confirm the formation of the Janus structure, the authors performed angle-resolved XPS, demonstrating the preferential substitution of S in the top layer of PtSe 2 .…”