MoS2 on Gold

: Achieving extremely high exfoliation yield and optical contrast for Van der Waals monolayers

Abstract

Experimental work was carried out to improve the yield and optical contrast of exfoliated two-dimensional (2D) Van der Waals (VdWs) materials. Molybdenum disulphide (MoS2) on gold (Au) is investigated in detail as it provides an extremely large exfoliation yield and a high optical contrast. These techniques are further explored with other VdWs and substrate materials. Further experimental work in electrically excited plasmonic tunnel junctions with a scanning tunnelling microscope (STM) was carried out to work towards integrating 2D materials as tunnelling barriers in various tunnel junction architectures.

Extremely large coverage of exfoliated monolayer MoS2 was observed on freshly produced Au substrates. The observed large coverage was quantified as a function of Au film thickness and roughness. How this coverage changes with the time the film is exposed to air before exfoliation is carried out was investigated. Large coverage is observed to drop off across a period of ~ 15 minutes once the Au surface is exposed to air. Using water contact angle (WCA) measurements, the effect this contamination has on the surface adhesive energy was investigated. The WCA investigation is used to compare the effect of roughness and surface contamination on the resulting observed coverage as a function of time. Across the 15 minute period investigated, the water contact angle is seen to gradually increase with the time Au samples are exposed to air. Atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM) measurements were carried out to characterize layer thickness and surface potential of the large coverage of MoS2 on Au substrates. The dominant monolayer coverage of these films is attributed to strain-induced stacking displacements and layer dependent doping between the two layers of MoS2 closest to the Au film, caused by the tunnelling nature of charge transfer between layers. This differential doping is suggested to create a small repulsive electrostatic force between the two closest layers to the Au. This investigation was further extended to other metal substrates and a range of 2D materials. These initial results appear promising but highlight the need for in vacuum techniques to prevent metals used to enhance adhesion reacting with air, lowering the exfoliated yield.

Experimental work was carried out to characterize the optical contrast of 2D materials. Heterostructures were used to maximize the optical visibility of 2D materials, making it easier to identify monolayer flakes. The effects of Au film thickness on the optical contrast were investigated. The peak contrast is observed to increase by ~4 times compared to bulk Au substrates. The effect of the illumination angle on contrast is also investigated. It is found that the peak contrast increases by an additional ~4 times with optimized illumination angles, compared to situations of no angle control. This section finishes with work carried out to increase the contrast of hexagonal Boron Nitride (h-BN) on these heterostructures. Variations in the contrast between samples were also seen to be enhanced with contrast. Contributions from possible sources of these variations are identified to understand the errors present in the system. The effect of indentation on the optical contrast of thin Au films has been directly measured, and this is attributed to a change in interband absorption energies resulting from a change in the Fermi energy of the Au’s surface. By incorporating 2D materials onto Au and other metal-covered contrast-enhancing substrates, it opens up a range of possible applications from data storage to light emission devices.

Finally, light emission was investigated from a homebuilt STM light emission (LE) system designed to investigate both the tip and prism coupled plasmonic components of light emission from the STM system. It was investigated spectroscopically, by measuring the emission spectra and in local intensity investigations, through a method known as photon mapping. These were carried out simultaneously for both tip and prism coupled emission. Experiments were carried out on bare Au surfaces of varying thickness, in order to demonstrate the application of the tip-sample plasmonic tunnel junction as an electrical antenna for exciting propagating surface plasmon polaritons (SPPs) on Au films. These plasmonic tunnel junctions may act as electronic sources of SPPs and are of interest for possible applications in plasmonic circuitry.

Date of Award	Dec 2020
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Engineering & Physical Sciences Research Council
Supervisor	Solveig Felton (Supervisor) & Fumin Huang (Supervisor)