ABSTRACT
Heterostructures of two-dimensional (2D) atomic crystals provide fascinating molecular-scale design elements for emergent physical phenomena and functional materials, as integrating distinct monolayers into vertical heterostructures can afford coupling between disparate properties. However, the available examples have been limited to either van der Waals (vdW) or electrostatic (ES) heterostructures that are solely composed of noncharged and charged monolayers, respectively. Here, we propose a "vdW-ES heterostructure" chemical design in which charge-neutral and charged monolayer-building blocks with highly disparate chemical and physical properties are conjugated vertically through asymmetrically charged interfaces. We demonstrate vdW-ES heteroassembly of semiconducting MoS2 and dielectric Ca2Nb3O10- (CNO) monolayers using an amphipathic molecular starch, resulting in the emergence of trion luminescence observed at the lowest energy among MoS2-related materials, probably due to interfacial confinement effects given by vdW-ES dual interactions. In addition, interface engineering leads to tailored exciton of the vdW/ES heterostructures owing to the pronounced dielectric proximity effects, bringing an intriguing interlayer chemistry to modify 2D materials. Furthermore, the current approach was successfully extended to create a graphene/CNO heterostructure, which verifies the versatility of the preparative method.
ABSTRACT
This study investigated the intrinsic optical properties of MoS2 monolayers and MoS2/WS2 van der Waals (vdW) heterostructures, grown using chemical vapor deposition. To understand the effect of the growth substrate, samples grown on a SiO2/Si surface were transferred and suspended onto a porous substrate. This transfer resulted in a blue shift of the excitonic photoluminescence (PL) peak generated by MoS2 monolayers, together with an intensity increase. The blue shift and the intensity increase are attributed to the release of lattice strain and the elimination of substrate-induced non-radiative relaxation, respectively. This suspension technique also allowed the observation of PL resulting from interlayer excitons in the MoS2/WS2 vdW heterostructures. These results indicate that the suppression of lattice strain and non-radiative relaxation is essential for the formation of interlayer excitons, which in turn is crucial for understanding the intrinsic physical properties of vdW heterostructures.
ABSTRACT
A van der Waals (vdW) heterostructure composed of multivalley systems can show excitonic optical responses from interlayer excitons that originate from several valleys in the electronic structure. In this work, we studied photoluminescence (PL) from a vdW heterostructure, WS2/MoS2, deposited on hexagonal boron nitride (hBN) flakes. PL spectra from the fabricated heterostructures observed at room temperature show PL peaks at 1.3-1.7 eV, which are absent in the PL spectra of WS2 or MoS2 monolayers alone. The low-energy PL peaks we observed can be decomposed into three distinct peaks. Through detailed PL measurements and theoretical analysis, including PL imaging, time-resolved PL measurements, and calculation of dielectric function ε(ω) by solving the Bethe-Salpeter equation with G0 W0, we concluded that the three PL peaks originate from direct K-K interlayer excitons, indirect Q-Γ interlayer excitons, and indirect K-Γ interlayer excitons.
ABSTRACT
Semiconductor heterojunction interfaces have been an important topic, both in modern solid state physics and in electronics and optoelectronics applications. Recently, the heterojunctions of atomically-thin transition metal dichalcogenides (TMDCs) are expected to realize one-dimensional (1D) electronic systems at their heterointerfaces due to their tunable electronic properties. Herein, we report unique conductivity enhancement and electrical potential modulation of heterojunction interfaces based on TMDC bilayers consisted of MoS2 and WS2. Scanning tunneling microscopy/spectroscopy analyses showed the formation of 1D confining potential (potential barrier) in the valence (conduction) band, as well as bandgap narrowing around the heterointerface. The modulation of electronic properties were also probed as the increase of current in conducting atomic force microscopy. Notably, the observed band bending can be explained by the presence of 1D fixed charges around the heterointerface. The present findings indicate that the atomic layer heterojunctions provide a novel approach to realizing tunable 1D electrical potential for embedded quantum wires and ultrashort barriers of electrical transport.
