FABRICATION AND OPTICAL STUDIES OF AIR-SENSITIVE TWO-DIMENSIONAL MATERIALS

Abstract

The isolation and study of graphene has led to research into a large number of other atomically thin, two-dimensional (2D) materials. These materials have various fascinating properties that complement those in graphene; allowing for the building of designer van der Waals heterostructures. However, many of these 2D materials are unstable in ambient conditions, restricting their study. In this work, methods to isolate and protect these materials in an inert argon environment are given; revealing novel insights into the optical properties of few-layer, air-sensitive 2D semiconducting materials and their heterostructures. Specifically, the lack of degradation due to encapsulation enable the observation of Raman spectra and second harmonic generation in monolayer InSe as well as Raman and photoluminescence spectra for few-layer GaSe crystals for the first time. These isolated 2D semiconductors are then stacked adjacent to one another, forming heterojunctions with a type-II band alignment. These heterojunctions reveal new interlayer excitonic states with an energy that may be tuned by select- ing the layer thicknesses. The recombination of these exciton states is suggested to be direct or quasi-direct in momentum-space, in contrast to previous attempts that demonstrated momentum-indirect interlayer excitonic states in 2D semicon- ductors. These momentum-direct interlayer excitons show bright luminescence and, as their energy is tunable, increase the spectral coverage available for van der Waals heterostructures. Lastly, the high quality fabrication methods presented also enable the first observation of resonantly hybridised excitons between two closely aligned TMDC monolayers. The close crystallographic alignment promotes the hybridisation of interlayer and intralayer excitonic states, revealing new states that inherit the properties of each excitonic component.

Date of Award	6 Jan 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Konstantin Novoselov (Supervisor) & Roman Gorbachev (Supervisor)