Lateral Heterostructures of Multilayer GeS and SnS van der Waals Crystals

Abstract

Engineered heterostructures derive distinct properties from materials integration and interface formation. Twodimensional crystals have been combined to form vertical stacks and lateral heterostuctures with covalent line interfaces. While thicker vertical stacks have been realized, lateral heterostructures from multilayer van der Waals crystals, which could bring the benefits of high-quality interfaces to bulk-like layered materials, have remained much less explored. Here, we demonstrate the integration of anisotropic layered Sn and Ge monosulfides into complex heterostructures with seamless lateral interfaces and tunable vertical design using a two-step growth process. The anisotropic lattice mismatch at the lateral interfaces between GeS and SnS is relaxed via dislocations and interfacial alloying. Nanoscale optoelectronic measurements by cathodoluminescence spectroscopy show the characteristic light emission of joined high-quality van der Waals crystals. Spectroscopy across the lateral interface indicates valley-selective luminescence in the bulk SnS component that arises due to anisotropic electron transfer across the interface. The results demonstrate the ability to realize high-quality lateral heterostructures of multilayer van der Waals crystals for diverse applications, e.g., in optoelectronics or valleytronics.

Multilayer Lateral Heterostructures of Van Der Waals Crystals with Sharp, Carrier–Transparent Interfaces

“…Multilayer GeS–SnS heterostructures ( Figure 1 a ) were prepared via a two‐step growth process (see Experimental Section). [ 9 ] Large SnS seeds were grown on mica under conditions favoring thick (>25 nm) flakes bounded by long straight {110} side facets (Figure 1b ). [ 11 , 12 ] Occasional thinner flakes show rounded shapes, consistent with previous results.…”

“…[ 17 ] Nanobeam electron diffraction (Figure 2c,d ) identifies the peripheral band as single‐crystalline GeS [ 18 ] imaged along the [001] zone axis, while the center shows a superposition of single‐crystal diffraction patterns of vertically stacked SnS [ 19 ] and GeS with aligned lattices. [ 9 ] GeS and SnS have the same orthorhombic structure. In the plane, the lattice mismatch is negligible (≈0.3%) along [010] ( b ), with b SnS = 0.4443 nm and b GeS = 0.4455 nm, but is large (≈8.9%) along [100] ( a ), with a SnS = 0.4024 nm and a GeS = 0.3666 nm (Figure S2 , Supporting Information).…”

“…Growth processes for lateral integration have been developed for different 2D crystals, including graphene‐hBN [ 5 ] and several transition metal dichalcogenides. [ 6 , 7 ] Limited so far to monolayers, the concept of lateral heterostructures could be extended to few‐layer [ 8 ] and even multilayer van der Waals crystals, [ 9 ] e.g., to combine intralayer carrier manipulation with an enhanced optical thickness and emerging photonic properties. [ 10 ] Here, we discuss multilayer lateral heterostructures integrating SnS and GeS, two anisotropic van der Waals semiconductors.…”

Unconventional van der Waals heterostructures beyond stacking

“… (G) Schematic and STEM image of a lateral heterostructure between multilayer SnS and GeS (top); hyperspectral CL linescan and corresponding STEM intensity profile across the lateral GeS-SnS interface (bottom). Adapted from Sutter et al. (2020a) .…”

“…On the SnS side far from the interface, CL spectra show a characteristic pair of luminescence peaks corresponding to the bandgaps along the x and y -valleys in SnS ( Lin et al, 2018 ). Close to the interface the lower energy (1.35 eV) x -valley emission is strongly quenched while the (1.55 eV) y -valley peak retains a constant intensity, indicating a valley-selective carrier separation at the lateral GeS-SnS interface ( Sutter et al., 2020a ). Besides optoelectronics, CL can also be used to probe nanophotonics in 2D/layered crystals and heterostructures, based on the ability of the focused high-energy electron beam to induce propagating photonic modes in layered semiconductors down to the ultrathin limit.…”