Power Dissipation and Thermal Transport of Two-Dimensional (2D) Electronic Devices

Abstract

The preceding decade have witnessed incredible advances in the research of two-dimensional (2D) materials such as graphene, transition metal carbides and nitrides (MXenes), and transition metal dichalcogenides (TMDCs). Since their discovery, 2D materials have enabled the design of nano-scale devices with unique functionalities that are otherwise unavailable in conventional 3D systems. This dissertation focus on the electrical an thermal properties of these materials and presents the study of (1) contribution of the encapsulating layers and (2) surface parameters to the thermal transport at the van der Waals interfaces, (3) synthesis of quasi-binary TMDC alloys through computationally predicted stability maps, and (4) the phase‐dependent band gap engineering in alloys induced by charge density wave (CDW) phases. In the first and second project, power dissipation and thermal management in the nanoscale structures are investigated which is of great importance for the design and operation of energy-efficient 2D nano-devices. Energy transport is heavily dependent of the thermal boundary conductance (TBC) at the van der Waals interfaces, particularly coupling at the interface of 2D channels with their underlying 3D substrates. A low TBC with underlying substrates puts an extrinsic limitation on the ability of 2D materials to conduct heat and dissipate the applied power. In this project a novel self-heating/self-sensing electrical thermometry platform based on atomically thin Ti3C2Tz MXene sheets is developed, which enables experimental investigation of the thermal transport at a Ti3C2Tz/SiO2 interface, with and without an encapsulating layer. Furthermore, the hydrophilic nature and variability of MXene surface terminations together with their metallic nature, provide a new platform to study the effect of the surface parameters on the thermal transport through and along the 2D flakes. In the third project, at theory-guided synthesis approach is employed to achieve 25 unexplored quasi-binary TMDC alloys through computationally predicted stability maps and equilibrium temperature-composition phase diagrams. Compared to other 2D materials, TMDCs exhibit diverse, exciting physical properties, including topological insulator behavior, superconductivity, valley polarization, and superior electrocatalytic activity compared to noble metals. Their properties can be further tuned — or even new properties engineered — by alloying two different elements at either the transition metal or the chalcogen site to form quasi-binary alloys, or by simultaneous alloying at both the sites to form quaternary alloys. The synthesized alloys can be exfoliated into 2D structures, and some of them exhibit: (i) outstanding thermal stability tested up to 1230K, (ii) exceptionally high electrochemical activity for CO2 reduction reaction, (iii) excellent energy efficiency in a high rate Li-air battery, and (iv) high break-down current density for interconnect applications. In the last project, a novel form of bandgap engineering involving alloying non‐isovalent cations in a 2D transition metal dichalcogenide (TMDC) is presented. By alloying semiconducting MoSe2 with metallic NbSe2, two structural phases of Mo0.5Nb0.5Se2, the 1T and 2H phases, are produced each with emergent electronic structure. At room temperature, it is observed that the 1T and 2H phases are semiconducting and metallic, respectively. For the 1T structure, scanning tunneling microscopy/spectroscopy (STM/STS) is used to measure band gaps. Electron diffraction patterns of the 1T structure obtained at room temperature show the presence of a nearly commensurate charge density wave (NCCDW) phase with periodic lattice distortions.