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Van Der Waals heterostructures: Fabrication and nanoscale and thermal characterisation via SPM methods

Research output: ThesisDoctoral Thesis

Publication date11/2021
Number of pages251
Awarding Institution
Thesis sponsors
  • National Physical Laboratory, Teddington
Award date6/01/2022
  • Lancaster University
<mark>Original language</mark>English


Since the dawn of two-dimensional (2D) era, graphene and the plethora of other atomically thin layered materials cousins, including their combinations in heterostructures, have revolutionized material science and established themselves as promising candidates for the next-generation of nanoscale devices. However, the landscape of opportunities and possible combinations is so vast, that many promising research topics remain widely unexplored. The aim of this work is to address some of these questions, contributing to the progress and understanding in three different areas: fabrication of novel 2D materials and their heterostructures; electronic properties and doping mechanisms in these at the nanoscale; and characterization of the nanoscale thermal and thermoelectric properties of 2D materials.
Starting with the fabrication procedures, different techniques for mechanical exfoliation and polymeric dry transfer of 2D materials and heterostructures were explored. This research route led to the establishment of a new fabrication facility at the National Physical laboratory (NPL), and to the production of a wide variety of samples, from exfoliated flakes to complex heterostructures and devices. A selection of the produced samples was employed during this thesis to investigate the electronic and thermal properties of 2D materials via spectroscopic and advanced scanning probe microscopy (SPM) techniques. Regarding the electronic response of graphene, two routes were undertaken: first, the electronic response of different types of graphene towards humidity acting as a p-dopant was studied for the first time using Raman spectroscopy. Second, a procedure improving current methods of quantitative probe calibration in Kelvin probe force microscopy (KPFM) was developed, establishing the determination of reliable and consistent work-function values with high-resolution. This method was then employed to study the electronic properties and doping of encapsulated graphene heterostructures, providing quantitative values of the work-function of the system, as well as demonstrating the capability of KPFM as an excellent visualisation and characterisation technique for buried layers otherwise inaccessible by other methods. Finally, various thermal properties of 2D materials were studied via advanced SPM techniques: Scanning thermal microscopy (SThM) that was used for the determination of the thickness dependence of the thermal conductance of exfoliated InSe, and scanning thermal gate microscopy (STGM) that was employed to explore the thermovoltage, and thus Seebeck coefficient, variation in encapsulated graphene heterostructures with patterned constrictions.
The main highlights of the work developed during this thesis are: (1) the formulated need and subsequent realisation of various approaches towards the fabrication of 2D materials and heterostructures. For this, shared expertise with other researchers and institutions, and access to different fabrication facilities were essential; and (2) the exploitation of the potential of spectroscopic and advanced SPM methods in providing reliable characterisation of the 2D material and heterostructure’s properties with nanoscale resolution. The findings of this thesis have provided new insights in a varied number of areas, and hold promise for different future applications: from single material thermocouples to graphene-gas sensors, including improved fabrication procedures for 2D materials, or even, probe-calibration and characterisation methods.