Tailoring optical and electrical characteristics of layered materials through van der Waals heterojunctions

Abstract

The feasibility of isolation of layered materials and arbitrary stacking of different materials provide plenty of opportunities to realize van der Waals heterostructures (vdWhs) with desired characteristics. In this thesis, we experimentally demonstrate the tunability of optical and electrical characteristics of transition metal dichalcogenides (TMDs), a class of layered materials, using their vdWhs. Monolayer (1L) TMDs exhibit remarkable light-matter interaction by hosting direct bandgap, strongly bound excitonic complexes, ultra-fast radiative decay, many-body states, and coupled spin-valley degrees of freedom. However, their sub-nm thickness limits light absorption, impairing their viability in photonic and optoelectronic applications. The physical proximity of layers in vdWhs drives strong interlayer dipole-dipole coupling resulting in nonradiative energy transfer (NRET) from one layer (donor) to another (acceptor) under spectral resonance. Motivated by the high efficiency of NRET in vdWhs, we study the prospect of enhancement of optical properties of a 1L-TMD stacked on top of strongly absorbing, non-luminescent, multilayer SnSe2 whose direct bandgap is close to exciton emission of 1L-TMDs MoS2 and WS2. We show that NRET enhances both single-photon and two-photon luminescence by one order of magnitude in such vdWhs. We also demonstrate a new technique of Raman enhancement driven by NRET in vdWhs. We achieve a 10-fold enhancement in the Raman intensity, enabling the observation of the otherwise invisible weak Raman modes. We establish the evidence for NRET-aided photoluminescence (PL) and Raman enhancement by modulating the degree of enhancement by systematically varying multiple parameters - donor material, acceptor material, their thickness, physical separation between donor and acceptor by insertion of spacer layer (hBN), sample temperature, and excitation wavelength...