In 2004, a single atomic layer of graphite, called graphene, was isolated and its remarkable properties were characterized. Consequently, interest in two-dimensional, 2D, solids, where the lateral dimensions are much greater than the thickness has exploded. The awarding of the Nobel prize for graphene in 2010 fueled further interest. In general, 2D solids are characterized by very large surface areas. Furthermore, by confining the electrons, viz. the electric charge carriers, to 2D can also result in new physics. The most recent family of 2D solid discovered is called MXenes where M is a transition metal and X is carbon and/or nitrogen. In this project, single layers of select MXenes are being isolated on a substrate and characterized in order to better understand the effects of chemistry, temperature, surface terminations, order, and magnetic fields on electrical transport. These results are being used to better understand how charge flows in MXenes which, in turn, is of vital importance to designing better lithium batteries, supercapacitors, catalysts, and transparent conductive electrodes to name only a few potential applications of this new and exciting family of 2D solids. Graduate students involved in this project obtain international experience through participation in research activities in collaborating laboratories in Europe and Asia. Undergraduate students are involved in research every summer through Drexel University's STAR program (which supports students who completed their first year of studies, have high GPAs and are interested in research). Many of these students subsequently complete graduate degrees.
Quite recently, two new classes of quaternary ordered MAX phases (M',M")n+1AXn, have been discovered: o- and i-MAX for out-of-plane and in-plane order, respectively. The o-MAX phases are ones where one transition metal sandwiches the other. The i-MAX phases are 211 (n = 1) phases in which the M-planes are themselves ordered. With the judicious choice of etchants, neither, or only one, of the two M elements can be etched - together with the A-layers - to yield MXenes with ordered divacancies in the latter case. The overarching scientific goal of the proposed work is to understand, at a fundamental level, the effects of ordering, both of the M layers and point defects, on the magneto-transport and optical properties of single 2D flakes and spincast films made of the same flakes. Some of the questions posed involve the main electron scattering mechanisms in the ordered/disordered MXenes; what factors influence carrier densities; the effect of terminations and their order/disorder on these values; and whether a band gap can be induced in these materials. In terms of impact, MXenes will likely play a critical/enabling role in the "lego" approach to heterojunctions 'simply' as ohmic contacts. Furthermore, and despite their relatively young age, MXenes have already shown great promise as materials in energy storage - both in supercapacitors and ion batteries/capacitors of lithium and beyond - electro-magnetic shielding, water purification and desalination, catalysis, transparent conductive electrodes, reinforcements in polymer composites among many others. This work facilitates choosing the best MXene chemistry for any given application.
