Modulation of the Electronic Properties of MXene (Ti3C2Tx) via Surface-Covalent Functionalization with Diazonium.

The physical and chemical properties of MXenes are strongly dependent on surface terminations; thus, the tailoring of surface functional groups in two-dimensional transition-metal carbides (MXenes) may extend the applicability of these compelling materials to a wider set of fields. In this work, we demonstrate the chemical modification of Ti3C2Tx MXene via diazonium covalent chemistry and the subsequent effects on the electrical properties of MXene. The 4-nitrophenyl group was grafted onto the surface of MXene through a solid-liquid reaction, which was confirmed by various characterization methods, including X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, electron energy loss spectroscopy, atomic force microscopy, and transmission electron microscopy. The degree of modification of MXene is expediently tunable by adjusting the concentration of the diazonium salt solution. The work function of functionalized MXene is modifiable by regulating the quantity of grafted diazonium surface groups, with an adjustable range of around 0.6 eV. Further, in this study, the electrical properties of modified MXene are investigated through the fabrication of field-effect-transistor devices that utilize modified MXene as a channel material. It was demonstrated that with increasing concentration of 4-nitrophenyl groups grafted onto the surface the on/off current ratio of the modified MXene was improved to as much as 3.56, with a corresponding decrease in conductivity and mobility. The proposed approach of controlled modification of surface groups in Ti3C2Tx may imbue Ti3C2Tx with favorable electronic behaviors and demonstrate prospects for use in electronic field applications.

Quasi-Fermi level splitting in nanoscale junctions from ab initio.

The splitting of quasi-Fermi levels (QFLs) represents a key concept utilized to describe finite-bias operations of semiconductor devices, but its atomic-scale characterization remains a significant challenge. Herein, the nonequilibrium QFL or electrochemical potential profiles within single-molecule junctions obtained from the first-principles multispace constrained-search density-functional formalism are presented. Benchmarking the standard nonequilibrium Green's function calculation results, it is first established that algorithmically the notion of separate electrode-originated nonlocal QFLs should be maintained within the channel region during self-consistent finite-bias electronic structure calculations. For the insulating hexandithiolate junction, the QFL profiles exhibit discontinuities at the left and right electrode interfaces and across the molecule the accompanying electrostatic potential drops linearly and Landauer residual-resistivity dipoles are uniformly distributed. For the conducting hexatrienedithiolate junction, on the other hand, the electrode QFLs penetrate into the channel region and produce split QFLs. With the highest occupied molecular orbital entering the bias window and becoming a good transport channel, the split QFLs are accompanied by the nonlinear electrostatic potential drop and asymmetric Landauer residual-resistivity dipole formation. Our findings underscore the importance of the first-principles extraction of QFLs in nanoscale junctions and point to a future direction for the computational design of next-generation semiconductor devices.