Since the isolation and characterization of graphene, there has been a growing interest in 2D materials owing to their unique properties compared to their 3D counterparts. Recently, a family of 2D materials of early transition metal carbides and nitrides, labelled MXenes, has been discovered (Ti₂CT_z, Ti₃C₂T_z, Mo₂TiC₂T_z, Ti₃CNT_z, Ta₄C₃T_z, Ti₄N₃T_z among many others), where T stands for surface-terminating groups (O, OH, and F). MXenes are mostly produced by selectively etching A layers (where A stands for group A elements, mostly groups 13 and 14) from the MAX phases. The latter are a family of layered ternary carbides and/or nitrides and have a general formula of Mn+1AXn (n = 1-3), where M is a transition metal and X is carbon and/or nitrogen. The produced MXenes have a conductive carbide core and a non-conductive O-, OH- and/or F-terminated surface, which allows them to work as electrodes for energy storage applications, such as Li-ion batteries and supercapacitors.

Prior to this work, MXenes were produced in the form of flakes of lateral dimension of about 1 to 2 microns; such dimensions and form are not suitable for electronic characterization and applications. I have synthesized various MXenes (Ti₃C₂T_z, Ti₂CT_z and Nb₂CT_z) as epitaxial thin films, a more suitable form for electronic and photonic applications. These films were produced by HF, NH4HF2 or LiF + HCl etching of magnetron sputtered epitaxial Ti3AlC2, Ti2AlC, and Nb2AlC thin films. For transport properties of the Ti-based MXenes, Ti₂CT_z and Ti₃C₂T_z, changing n from 1 to 2 resulted in an increase in conductivity but had no effect on the transport mechanism (i.e. both Ti₃C₂T_x and Ti₂CT_x were metallic). In order to examine whether the electronic properties of MXenes differ when going from a few layers to a single flake, similar to graphene, the electrical characterization of a single Ti₃C₂T_z flake with a lateral size of about 10 μm was performed. These measurements, the first for MXene, demonstrated its metallic nature, along with determining the nature of the charge carriers and their mobility. This indicates that Ti₃C₂T_z is inherently of 2D nature independent of the number of stacked layers, unlike graphene, where the electronic properties change based on the number of stacked layers.

Changing the transition metal from Ti to Nb, viz. comparing Ti₂CT_z and Nb₂CT_z thin films, the electronic properties and electronic conduction mechanism differ. Ti₂CT_z showed metallic-like behavior (resistivity increases with increasing temperature) unlike Nb₂CT_z where the conduction occurs via variable range hopping mechanism (VRH) - where resistivity decreases with increasing temperature.

Furthermore, these studies show the synthesis of pure Mo₂CT_z in the form of single flakes and freestanding films made by filtering Mo₂CT_z colloidal suspensions. Electronic characterization of free-standing films made from delaminated Mo₂CT_z flakes was investigated, showing that a VRH mechanism prevails at low temperatures (7 to ≈ 60 K). Upon vacuum annealing, the room temperature, RT, conductivity of Mo₂CT_x increased by two orders of magnitude. The conduction mechanism was concluded to be VRH most likely dominated by hopping within each flake.

Other Mo-based MXenes, Mo₂TiC₂T_z and Mo₂Ti₂C₃T_z, showed VRH mechanism at low temperature. However, at higher temperatures up to RT, the transport mechanism was not clearly understood. Therefore, a part of this thesis was dedicated to further investigating the transport properties of Mo-based MXenes. This includes Mo₂CT_z, out-of-plane ordered Mo₂TiC₂T_z and Mo₂Ti₂C₃T_z, and vacancy ordered Mo_1.33CT_z. Magneto-transport of free-standing thin films of the Mo-based MXenes were studied, showing that all Mo-based MXenes have two transport regimes: a VRH mechanism at lower temperatures and a thermally activated process at higher temperatures. All Mo-based MXenes except Mo_1.33CT_z show that the electrical transport is dominated by inter-flake transfer. As for Mo_1.33CT_z, the primary electrical transport mechanism is more likely to be intra-flake.

The synthesis of vacancy ordered MXenes (Mo_1.33CT_z and W_1.33CT_z) raised the question of possible introduction of vacancies in all MXenes. Vacancy ordered MXenes are produced by selective etching of Al and (Sc or Y) atoms from the parent 3D MAX phases, such as (Mo_2/3Sc_1/3)₂AlC, with in-plane chemical ordering of Mo and Sc. However, not all quaternary parent MAX phases form the in-plane chemical ordering of the two M metals; thus the synthesis of the vacancy-ordered MXenes is restricted to a very limited number of MAX phases. I present a new method to obtain MXene flakes with disordered vacancies that may be generalized to all quaternary MAX phases. As proof of concept, I chose Nb-C MXene, as this 2D material has shown promise in several applications, including energy storage, photothermal cell ablation and photocatalysts for hydrogen evolution. Starting from synthetizing (Nb_2/3Sc_1/3)₂AlC quaternary solid solution and etching both the Sc and Al atoms resulted in Nb_1.33C material with a large number of vacancies and vacancy clusters. This method may be applicable to other quaternary or higher MAX phases wherein one of the transition metals is more reactive than the other, and it could be of vital importance in applications such as catalysis and energy storage.