Thesis Title : "Engineering Light-Matter Coupling and Thermal Transport in Freestanding 
                Two-dimensional Materials"

Name of the Student : Mr. Manavendra Pratap Singh

Degree Registered : Ph D Engineering 

Advisor : Prof. Akshay Naik, CeNSE

Date : 25th August 2026, (Tuesday)

Time : 11:00 AM

Venue : CeNSE Seminar hall

Abstract:

Two-dimensional (2D) materials, especially semiconducting transition-metal dichalcogenides (TMDCs), 
are widely studied due to their strong interactions with light and their unique optical and thermal 
properties. They hold promise for future optoelectronic, sensing, and nanoscale thermal management 
devices. The surrounding dielectric environment significantly influences the properties of these 
atomically thin materials. The optical and thermal properties of 2D materials can be modulated by 
varying the underlying substrate material, thickness, or trench depth in suspended samples, thereby 
altering interference, absorption, and heat dissipation. In this thesis, we study the fabrication 
and characterization of various trench geometries to tune Raman and photoluminescence (PL) signals 
and to investigate thermal transport in 2D TMDCs.

In the first part of the thesis, we fabricated trenches of varying depths on the substrate via 
grayscale electron-beam lithography (g-EBL). Previous studies have used multiple samples to optimize 
substrate thickness for enhanced optical properties, making it difficult to separate sample-to-sample 
variations. To overcome this issue, it would be ideal to have a single flake on/ suspended over varying 
substrate thicknesses/trench depths in a single sample. Our strategy enables us to fabricate trenches of 
different depths with nanometre-scale control on the same SiO2/Si substrate, followed by reactive-ion etching (RIE). 
Finally, we introduce two types of grayscale structures: (i) staircase structure, and (ii) holey structure in 
the SiO2 for systematic investigation of Raman and PL of the 2D TMDCs.

The second part presents the Raman and PL enhancements of 2D TMDCs transferred onto these grayscale structures. 
We have observed ∼90-fold Raman and ∼150-fold PL intensity enhancements in monolayer MoS2 for a specific air 
gap and SiO2 thickness combination. The multilayer reflection model (MRM) captures the observed intensity 
variations with SiO2 thickness in our experiments and allows us to predict changes in Raman peak positions as a 
function of absorbed power. By using a single flake across the array of structures, these structures offer a simple 
solution to reduce sample-to-sample data variation. This method can also be applied to other 2D materials to enhance 
their emitted optical intensity.

The final part of the thesis investigates the anisotropic in-plane thermal conductivity (κ) of freestanding few-layer 
rhenium disulfide (ReS2). Literature suggests strong anisotropic behaviour in the thermal studies of bulk ReS2 
(thickness > 60 nm). Whether this anisotropy is maintained in few-layer ReS2 has not yet been explored. We have 
fabricated freestanding few-layer ReS2 samples on SiO2/Si holey substrates for optothermal Raman characterization. 
Polarization-dependent Raman measurements revealed variation in thermal conductivity along the high symmetry axes. 
Our results demonstrate clear in-plane anisotropy in κ for both AA and AB stacking orders. AA stacking shows higher 
κ than AB stacking along both directions, and κ increases with thickness, more predominant in AB stacking. The 
experimental results are further supported by density functional theory (DFT) calculations. The in-plane thermal 
conductivity of few-layer ReS2 devices, which varies with thickness, holds significant potential for nanoscale 
thermoelectric applications.