The diffusive motion of metal nanoparticles Au and Ag on monolayer and between bilayer heterostructures of transition metal dichalcogenides and graphene are investigated in the framework of density functional theory. We found that the minimum energy
barriers for diffusion and the possibility of cluster formation depend strongly on both the type of nanoparticle and the type of monolayers and bilayers. Moreover, the tendency to form clusters of Ag and Au can be tuned by creating various bilayers. Tunability of the diffusion characteristics of adatoms in van der Waals heterostructures holds promise for controllable growth of nanostructures.
The structural, electronic, and magnetic properties of pristine, defective, and oxidized monolayer TiS3 are investigated using first-principles calculations in the framework of density functional theory. We found that a single layer of TiS3 is a dire
ct band gap semiconductor, and the bonding nature of the crystal is fundamentally different from other transition metal chalcogenides. The negatively charged surfaces of single layer TiS3 makes this crystal a promising material for lubrication applications. The formation energies of possible vacancies, i.e. S, Ti, TiS, and double S, are investigated via total energy optimization calculations. We found that the formation of a single S vacancy was the most likely one among the considered vacancy types. While a single S vacancy results in a nonmagnetic, semiconducting character with an enhanced band gap, other vacancy types induce metallic behavior with spin polarization of 0.3-0.8 {mu}B. The reactivity of pristine and defective TiS3 crystals against oxidation was investigated using conjugate gradient calculations where we considered the interaction with atomic O, O2, and O3. While O2 has the lowest binding energy with 0.05-0.07 eV, O3 forms strong bonds stable even at moderate temperatures. The strong interaction (3.9-4.0 eV) between atomic O and TiS3 results in dissociative adsorption of some O-containing molecules. In addition, the presence of S-vacancies enhances the reactivity of the surface with atomic O, whereas it had a negative effect on the reactivity with O2 and O3 molecules.
Motivated by a recent experiment that reported the successful synthesis of hexagonal (h) AlN [Tsipas et al. Appl. Phys. Lett. 103, 251605 (2013)] we investigate structural, electronic and vibrational properties of bulk, bilayer and monolayer structur
es of h-AlN by using first-principles calculations. We show that the hexagonal phase of the bulk h-AlN is a stable direct-bandgap semiconductor. Calculated phonon spectrum displays a rigid-layer shear mode at 274 cm-1 and an Eg mode at 703 cm-1 which are observable by Raman measurements. In addition, single layer h-AlN is an indirect-bandgap semiconductor with a nonmagnetic ground state. For the bilayer structure, AA type stacking is found to be the most favorable one and interlayer interaction is strong. While N-layered h-AlN is an indirect bandgap semiconductor for N=1-10, we predict that thicker structures (N>10) have a direct-bandgap at the Gamma-point. The number-of-layer-dependent bandgap transitions in h-AlN is interesting in that it is significantly different from the indirect-to- direct crossover obtained in the transition metal dichalcogenides.
Atomically thin crystals have recently been the focus of attention in particular after the synthesis of graphene, a monolayer hexagonal crystal structure of carbon. In this novel material class the chemically derived graphenes have attracted tremendo
us interest. It was shown that although bulk graphite is a chemically inert material, the surface of single layer graphene is rather reactive against individual atoms. So far, synthesis of several graphene derivatives have been reported such as hydrogenated graphene graphane (CH), fluorographene (CF) and chlorographene (CCl). Moreover, the stability of bromine and iodine covered graphene were predicted using computational tools. Among these derivatives, easy synthesis, insulating electronic behavior and reversibly tunable crystal structure of graphane make this material special for future ultra-thin device applications. This overview, surveys structural, electronic, magnetic, vibrational and mechanical properties of graphane. We also present a detailed overview of research efforts devoted to the computational modeling of graphane and its derivatives. Furthermore recent progress in synthesis techniques and possible applications of graphane are reviewed as well.
Typical Raman spectra of transition metal dichalcogenides (TMDs) display two prominent peaks, E2g and A1g, that are well separated from each other. We find that these modes are degenerate in bulk WSe2 yielding one single Raman peak. As the dimensiona
lity is lowered, the observed peak splits in two as a result of broken degeneracy. In contrast to our experimental findings, our phonon dispersion calculations reveal that these modes remain degenerate independent of the number of layers. Interestingly, for minuscule biaxial strain the degeneracy is preserved but once the crystal symmetry is broken by uniaxial strain, the degeneracy is lifted. Our calculated phonon dispersion for uniaxially strained WSe2 shows a perfect match to the measured Raman spectrum which suggests that uniaxial strain exists in WSe2 flakes possibly induced during the sample preparation and/or as a result of interaction between WSe2 and the substrate. Furthermore, we find that WSe2 undergoes an indirect to direct bandgap transition from bulk to monolayers which is ubiquitous for semiconducting TMDs. These results not only allow us to understand the vibrational properties of WSe2 but also provides detailed insight to their physical properties.
Motivated by recent experimental observations of Tongay et al. [Tongay et al., Nano Letters, 12(11), 5576 (2012)] we show how the electronic properties and Raman characteristics of single layer MoSe2 are affected by elastic biaxial strain. We found t
hat with increasing strain: (1) the E and E Raman peaks (E1g and E2g in bulk) exhibit significant red shifts (up to 30 cm-1), (2) the position of the A1 peak remains at 180 cm-1 (A1g in bulk) and does not change considerably with further strain, (3) the dispersion of low energy flexural phonons crosses over from quadratic to linear and (4) the electronic band structure undergoes a direct to indirect bandgap crossover under 3% biaxial tensile strain. Thus the application of strain appears to be a promising approach for a rapid and reversible tuning of the electronic, vibrational and optical properties of single layer MoSe2 and similar MX2 dichalcogenides.
