Quantum size effects in layered VX2 (X=S, Se, Te) materials: Manifestation of metal to semimetal or semiconductor transition

Most of the 2D transition metal dichalcogenides (TMDC) are nonmagnetic in pristine form. However, 2D pristine VX2 (X=S, Se, Te) materials are found to be ferromagnetic. Using spin polarized density functional theory (DFT) calculations, we have studied the electronic, magnetic and surface properties of this class of materials in both trigonal prismatic 2H- and octahedral 1T-phase. Our calculations reveal that they exhibit materially different properties in those two polymorphs. Most importantly, detailed investigation of electronic structure explored the quantum size effect in 2H-phase of these materials thereby leading to metal to semimetal (2H-VS2) or semiconductor (2H-VSe2, 2H-VTe2) transition when downsizing from bilayer to corresponding monolayer.Comment: 18 pages, 12 figures, 3 table

First principles design of divacancy defected graphene nanoribbon based rectifying and negative differential resistance device

We have elaborately studied the electronic structure of 555-777 divacancy (DV) defected armchair edged graphene nanoribbon (AGNR) and transport properties of AGNR based two-terminal device constructed with one defected electrode and one N doped electrode, by using density functional theory and non-equilibrium Green's function based approach. The introduction of 555-777 DV defect into AGNRs, results in a shifting of the {\pi} and {\pi}* bands towards the higher energy value which indicates a shifting of the Fermi level towards the lower energy. Formation of a potential barrier, very similar to that of conventional p-n junction, has been observed across the junction of defected and N doped AGNR. The prominent asymmetric feature of the current in the positive and negative bias indicates the diode like property of the device with high rectifying efficiency within wide range of bias voltages. The device also shows robust negative differential resistance (NDR) with very high peak-to-valley ratio. The analysis of the shifting of the energy states of the electrodes and the modification of the transmission function with applied bias provides an insight into the nonlinearity and asymmetry observed in the I-V characteristics. Variation of the transport properties on the width of the ribbon has also been discussed.Comment: 28 Pages, 12 Figures and 1 tabl

Tuning the electronic and magnetic properties of graphene/h-BN hetero nanoribbon: A first-principles investigation

Inspired by the successful synthesis of phase separated in-plane graphene/h-BN heterostructures, we have explored the design of one dimensional graphene/h-BN hetero nanoribbon (G/BNNR). Using first-principles density functional based approach, the electronic and magnetic properties of the hybrid nanoribbons with mono-hydrogenated edges have been investigated for different configurations with alternative composition of C-C and B-N units in a ribbon of fixed width. Our results suggest that the electronic as well as magnetic properties of the ribbons can be regulated by varying the number of C-C (or B-N) units present in the structure. Both the hetero nanoribbons, either with N or B terminated edges, undergo a semiconductor-to-semimetal-to-metal transition with the increase in the number of C-C units for a fixed ribbon width. The spin density distribution indicates significant localization of the magnetic moments at the edge carbon atoms, that gets manifested when the number of C-C units is greater than 2 for most of the structures

Substrate induced modulation of electronic, magnetic and chemical properties of MoSe2 monolayer

Monolayer of MoSe2, having a typical direct band gap of ∼1.5 eV, is a promising material for optoelectronic and solar cell applications. When this 2D semiconductor is supported on transition metal substrates, such as Ni(111) and Cu(111), its electronic structure gets modulated. First principles density functional investigation shows the appearance of de-localized mid-gap states in the density of states. The work function of the semiconductor overlayer gets modified considerably, indicating n-type doping caused by the metal contacts. The charge transfer across the metal-semiconductor junction also significantly enhances the chemical reactivity of the MoSe2 overlayer, as observed by Hydrogen absorption. Furthermore, for Ni contact, there is a signature of induced magnetism in MoSe2 monolayer