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.

Performance Degradation in Graphene-ZnO Barristors Due to Graphene Edge Contact.

The physical and chemical characteristics of the edge states of graphene have been studied extensively as they affect the electrical properties of graphene significantly. Likewise, the edge states of graphene in contact with semiconductors or transition-metal dichalcogenides (TMDs) are expected to have a strong influence on the electrical properties of the resulting Schottky junction devices. We found that the edge states of graphene form chemical bonds with the ZnO layer, which limits the modulation of the Fermi level at the graphene-semiconductor junction, in a manner similar to Fermi level pinning in silicon devices. Therefore, we propose that graphene-based Schottky contact should be accomplished with minimal edge contact to reduce the limits imposed on the Fermi level modulation; this hypothesis has been experimentally verified, and its microscopic mechanism is further theoretically examined.

Magic Clusters of MoS2 by Edge S2 Interdimer Spacing Modulation.

Edge atomic and electronic structures of S-saturated Mo-edge triangular MoS2 nanoclusters are investigated using density functional theory calculations. The edge electrons described by the S2-p xp xπ* (S2-Π x) and Mo-d xy orbitals are found to interplay to pin the S2-Π x Fermi wavenumber at kF = 2/5 as the nanocluster size increases, and correspondingly, the ×5 Peierls edge S2 interdimer spacing modulation is induced. For the particular sizes of N = 5 n - 2 and 5 n, where N is the number of Mo atoms at one edge representing the nanocluster size and n is a positive integer, the effective ×5 interdimer spacing modulation stabilizes the nanoclusters, which are identified here to be the magic S-saturated Mo-edge triangular MoS2 nanoclusters. With the S2-Π x Peierls gap, the MoS2 nanoclusters become far-edge S2-Π x semiconducting and subedge Mo-d xy metallic as N → ∞.

Investigations of Vacancy Structures Related to Their Growth in h-BN Sheet.

The atomic, electronic, and magnetic properties of vacancy structures with triangular shape related to their growth in single hexagonal boron nitride (h-BN) sheet are investigated using density functional theory calculations. We find that the optimized structures of triangular vacancies depend on the vacancy sizes with N-terminated zigzag edge. Then, vacancy structures obtained during the vacancy evolution in h-BN sheet are considered by removing a boron-nitrogen pair (BN pair) from edges of triangular vacancies. The magnetic properties of those vacancy structures are investigated by local density of states and spin densities. It is found that the stability of the optimized structures with a BN missing pair depends on the BN-pair missing position: the most stable structure is a BN-pair missing structure at the edge face region with the smallest magnetic moment.

Edge-functionalization of armchair graphene nanoribbons with pentagonal-hexagonal edge structures.

Using density functional theory calculations, we have studied the edge-functionalization of armchair graphene nanoribbons (AGNRs) with pentagonal-hexagonal edge structures. While the AGNRs with pentagonal-hexagonal edge structures (labeled (5,6)-AGNRs) are metallic, the edge-functionalized (5,6)-AGNRs with substitutional atoms opens a band gap. We find that the band structures of edge-functionalized (5,6)-N-AGNRs by substitution resemble those of defect-free (N-1)-AGNR at the Γ point, whereas those at the X point show the original ones of the defect-free N-AGNR. The overall electronic structures of edge-functionalized (5,6)-AGNRs depend on the number of electrons, supplied by substitutional atoms, at the edges of functionalized (5,6)-AGNRs.

Monolayer MoS2 Bandgap Modulation by Dielectric Environments and Tunable Bandgap Transistors.

Semiconductors with a moderate bandgap have enabled modern electronic device technology, and the current scaling trends down to nanometer scale have introduced two-dimensional (2D) semiconductors. The bandgap of a semiconductor has been an intrinsic property independent of the environments and determined fundamental semiconductor device characteristics. In contrast to bulk semiconductors, we demonstrate that an atomically thin two-dimensional semiconductor has a bandgap with strong dependence on dielectric environments. Specifically, monolayer MoS2 bandgap is shown to change from 2.8 eV to 1.9 eV by dielectric environment. Utilizing the bandgap modulation property, a tunable bandgap transistor, which can be in general made of a two-dimensional semiconductor, is proposed.

Electronic Properties of Bilayer Graphene Strongly Coupled to Interlayer Stacking and an External Electric Field.

Bilayer graphene (BLG) with a tunable band gap appears interesting as an alternative to graphene for practical applications; thus, its transport properties are being actively pursued. Using density functional theory and perturbation analysis, we investigated, under an external electric field, the electronic properties of BLG in various stackings relevant to recently observed complex structures. We established the first phase diagram summarizing the stacking-dependent gap openings of BLG for a given field. We further identified high-density midgap states, localized on grain boundaries, even under a strong field, which can considerably reduce the overall transport gap.

Atomic-scale dynamics of triangular hole growth in monolayer hexagonal boron nitride under electron irradiation.

The production of holes by electron beam irradiation in hexagonal boron nitride (hBN), which has a lattice similar to that of graphene, is monitored over time using atomic resolution transmission electron microscopy. The holes appear to be initiated by the formation of a vacancy of boron and grow in a manner that retains an overall triangular shape. The hole growth process involves the formation of single chains of B and N atoms and is accompanied by the ejection of atoms and bundles of atoms along the hole edges, as well as atom migration. These observations are compared to density functional theory calculations and molecular dynamics simulations.

Origins of dihydrogen binding to metal-inserted porphyrins: electric polarization and Kubas interaction.

Using density functional theory calculations, we have investigated the interactions between hydrogen molecules and metalloporphyrins. A metal atom, such as Ca or Ti, is introduced for incorporation in the central N(4) cavity. Within local density approximation (generalized gradient approximation), we find that the average binding energy of H(2) to the Ca atom is about 0.25 (0.1) eV/H(2) up to four H(2) molecules, whereas that to the Ti atom is about 0.6 (0.3) eV per H(2) up to two H(2) molecules. Our analysis of orbital hybridization between the inserted metal atom and molecular hydrogen shows that H(2) binds weakly to Ca-porphyrin through a weak electric polarization in dihydrogen, but is strongly hybridized with Ti-porphyrin through the Kubas interaction. The presence of d orbitals in Ti may explain the difference in the interaction types.