On the surface: Effects of functionalization and defects in two-dimensional MXenes

MXenes are two-dimensional (2D) transition metal carbides and nitrides. Due to their extraordinary properties, MXenes have been utilized for various applications in energy storage and conversion, electronics, biomedicine, sensors, and catalysis. Their structure is composed of transition metals (M) and carbon or nitrogen (X). During synthesis (etching in acid) the surface of MXenes is covered by functional groups, such as F, O, and OH. Theoretical calculations often showed that different surface functional groups give rise to a dramatic change in their electronic properties, work function, and ability to possess semiconducting and magnetic behaviors. However, these computational studies were performed on idealized pure functionalizations and they contradict the experimental studies, which reported a formation of F, O, and OH mixtures on the surface of MXenes. Moreover, the point defects can be created during synthesis and characterization due to an interaction of the surface atoms with an acid and electron beam. Nevertheless, very few prior theoretical studies have been conducted regarding the defect formation on the surface of MXenes. Thus, to realize the aforementioned applications and control the properties, the surface functionalization and defects in MXenes need to be studied, taking into account the etching and characterization conditions. In this dissertation, we first develop a computational multi-scale scheme to accurately determine the distribution and composition of functional groups, carefully accounting for the interactions of the surface with a solution through a proper treatment of chemical potentials as well as including the role of experimental factors, such as pH and the electrode potential. For that, we employ first principles calculations in combination with alloy theoretical approaches, such as cluster expansion and Monte Carlo. We find realistic distributions of functional groups on the surface of several MXenes. Strong mixing of the functional groups was observed for all studied MXenes regardless of the type of metal, carbon, or nitrogen species and/or number of atomic layers. Furthermore, we demonstrate the equilibrium compositions of the functional groups for each system as a function of external conditions and determine a range of composition tunability. With the help of the obtained structures, we evaluate the electronic properties for the whole range of functional group compositions. Then, we evaluate the formation of metal and carbon/nitrogen vacancies for the mixed functionalizations as a function of etching conditions. We find that formation of metal vacancies largely depends on the surface functionalization and external conditions, such as pH and electrode potential. Finally, we evaluate the sputtering threshold energies of O, H, F, and metal atoms under electron beam for many different MXenes and estimate the sputtering cross sections and rates for these atoms in Ti3C2. Knowing the sputtering threshold energies and sputtering rates helps to identify optimal measurement parameters and consequently reduce damage to the sample.