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The realization of spin gapless semiconductor (SGS) and half-metal (HM) behavior in two-dimensional (2D) transition metal (TM) dichalcogenides is highly desirable for their applications in spintronic devices. Here, using density functional theory calculations, we demonstrate that Fe, Co, Ni substitutional impurities can not only induce magnetism in MoSe2 monolayer, but also convert the semiconducting MoSe2 to SGS/HM system. We also study the effects of mechanical strain on the electronic and magnetic properties of the doped monolayer. We show that for all TM impurities we considered, the system exhibits the robust SGS/HM behavior regardless of biaxial strain values. Moreover, it is found that the magnetic properties of TM-MoSe2 can effectively be tuned under biaxial strain by controlling the spin polarization of the 3d orbitals of Fe, Co, Ni atoms. Our findings offer a new route to designing the SGS/HM properties and modulating magnetic characteristics of the TM-MoSe2 system and may also facilitate the implementation of SGS/HM behavior and realization of spintronic devices based on other 2D materials.

According to recent studies, gas sensors based on MoSe2 have better detection performance than graphene-based sensors, especially for N-based gas molecules, but the reason for that is not fully understood at the microscopic level. Here, we investigate the adsorption of CO, CO2, NH3, NO and NO2 gas molecules on MoSe2 monolayer by the density functional theory calculations. Our results reveal that indeed MoSe2 monolayer is more sensitive to adsorption of N-containing gas molecules than C-containing, which can be attributed to the distinct charge transfer between the gas molecules and MoSe2. The conductance was further calculated using the nonequilibrium Green's function (NEGF) formalism. The reduced conductance was found for NH3 and NO2 adsorbed MoSe2, consistent with the high sensitivity of MoSe2 for NH3 and NO2 molecules in the recent experiments. In addition, the adsorption sensitivity can significantly be improved by an external electric field, which implies the controllable gas detection by MoSe2. The magnetic moments of adsorbed NO and NO2 molecules can also be effectively modulated by the field-sensitive charge transfer. Our results not only give microscopic explanations to the recent experiments, but also suggest using MoSe2 as a promising material for controlled gas sensing.

Recently, the fabricated MoS2 field effect transistors (FETs) with 1T-MoS2 electrodes exhibit excellent performance with rather low contact resistance, as compared with those with metals deposited directly on 2H-MoS2 (Kappera et al 2014 Nat. Mater. 13 1128), but the reason for that remains elusive. By means of density functional theory calculations, we investigated the carrier injection at the 1T/2H MoS2 interface and found that although the Schottky barrier height (SBH) values of 1T/2H MoS2 interfaces can be tuned by controlling the stacking patterns, the p-type SBH values of 1T/2H MoS2 interfaces with different stackings are lower than their corresponding n-type SBH values, which demonstrated that the metallic 1T phase can be used as an efficient hole injection layer for 2H-MoS2. In addition, as compared to the n-type Au/MoS2 and Pd/MoS2 contacts, the p-type SBH values of 1T/2H MoS2 interfaces are much lower, which stem from the efficient hole injection between 1T-MoS2 and 2H-MoS2. This can explain the low contact resistance in the MoS2 FETs with 1T-MoS2 electrodes. Notably, the SBH values can be effectively modulated by an external electric field, and a significantly low p-type SBH value can be achieved under an appropriate electric field. We also demonstrated that this approach is also valid for WS2, WSe2 and MoSe2 systems, which indicates that the method can most likely be extended to other TMDs, and thus may open new promising avenues of contact engineering in these materials.

Herein we report the successful doping of tellurium (Te) into molybdenum disulfide (MoS2) monolayers to form MoS2x Te2(1-x) alloy with variable compositions via a hydrogen-assisted post-growth chemical vapor deposition process. It is confirmed that H2 plays an indispensable role in the Te substitution into as-grown MoS2 monolayers. Atomic-resolution transmission electron microscopy allows us to determine the lattice sites and the concentration of introduced Te atoms. At a relatively low concentration, tellurium is only substituted in the sulfur sublattice to form monolayer MoS2(1-x)Te2x alloy, while with increasing Te concentration (up to ∼27.6% achieved in this study), local regions with enriched tellurium, large structural distortions, and obvious sulfur deficiency are observed. Statistical analysis of the Te distribution indicates the random substitution. Density functional theory calculations are used to investigate the stability of the alloy structures and their electronic properties. Comparison with experimental results indicate that the samples are unstrained and the Te atoms are predominantly substituted in the top S sublattice. Importantly, such ultimately thin Janus structure of MoS2(1-x)Te2x exhibits properties that are distinct from their constituents. We believe our results will inspire further exploration of the versatile properties of asymmetric 2D TMD alloys.

Non-molecular solid nitrogen, metastable even at zero pressure and low temperature, was recently manufactured by Eremets et al. (Nature 411 (2001) 173) and investigated with respect to its pressure stability. In this study we present analytical potential molecular-dynamics simulations that allow reproducing all the stages of the experiment. The bonding structure and energetics of the N polymeric state obtained from the dynamical simulations are verified to be reasonable using a plane-wave ab initio method. We predict that the metastable low-pressure phase has predominantly 3-folded bonded atoms in a disordered configuration. The energy accumulated in the metastable phase is about 1 eV/atom.

The development of spatially homogeneous mixed structures with boron (B), nitrogen (N) and carbon (C) atoms arranged in a honeycomb lattice is highly desirable, as they open the possibility of creating stable two-dimensional materials with tunable band gaps. However, at least in the free-standing form, the mixed BCN system is energetically driven towards phase segregation to graphene and hexagonal BN. It is possible to overcome the segregation when BCN material is grown on a particular metal substrate, for example Ru(0 0 0 1), but the stabilization mechanism is still unknown. With the use of density-functional theory we study the energetics of BN/Ru slabs, with different types of configurations of C substitutional defects introduced to the h-BN overlayer. The results are compared to the energetics of free-standing BCN materials. We found that the substrate facilitates the C substitution process in the h-BN overlayer. Thus, more homogeneous BCN material can be grown, overcoming the segregation into graphene and h-BN. In addition, we investigate the electronic and transport gaps in free-standing BCN structures, and assess their mechanical properties and stability. The band gap in mixed BCN free-standing material depends on the concentration of the constituent elements and ranges from zero in pristine graphene to nearly 5 eV in free-standing h-BN. This makes BCN attractive for application in modern electronics.

Using atomistic computer simulations, we study how ion irradiation can be used to alter the morphology of a graphene monolayer by introducing defects of specific type, and to cut graphene sheets. Based on the results of our analytical potential molecular dynamics simulations, a kinetic Monte Carlo code is developed for modelling morphological changes in a graphene monolayer under irradiation at macroscopic time scales. Impacts of He, Ne, Ar, Kr, Xe and Ga ions with kinetic energies ranging from tens of eV to 10 MeV and angles of incidence between 0\circ and 88\circ are studied. Our results provide microscopic insights into the response of graphene to ion irradiation and can directly be used for the optimization of graphene cutting and patterning with focused ion beams.

The weak ferromagnetism reported for graphite and related carbon nanostructures is frequently related to the magnetic coupling of point defects such as vacancies or hydrogen adatoms that interact with only one sublattice in the bipartite graphene lattice. In this paper, using density functional theory calculations we study the magnetic coupling between point defects, such as carbon adatoms, which form bonds with atoms located on both sublattices. We show that there is ferromagnetic coupling for small separations between the adatoms. Further, we demonstrate that it is energetically favorable for C adatoms to agglomerate. Our results indicate that the magnetism induced in graphite by irradiation can be explained as being due to the existence of adatom agglomerations and small graphene flakes formed from isolated adatoms rather than uniformly distributed point defects.

We combine electron irradiation experiments in a transmission electron microscope with kinetic Monte Carlo simulations to determine the mobility of interstitial carbon atoms in single-walled carbon nanotubes. We measure the irradiation dose necessary to cut nanotubes repeatedly with a focused electron beam as a function of the separation between the cuts and at different temperatures. As the cutting speed is related to the migration of displaced carbon atoms trapped inside the tube and to their recombination with vacancies, we obtain information about the mobility of the trapped atoms and estimate their migration barrier to be about 0.25 eV. This is an experimental confirmation of the remarkably high mobility of interstitial atoms inside carbon nanotubes, which shows that nanotubes have potential applications as pipelines for the transport of carbon atoms. We combine electron irradiation experiments in a transmission electron microscope with kinetic Monte Carlo simulations to determine the mobility of interstitial carbon atoms in single-walled carbon nanotubes. We measure the irradiation dose necessary to cut nanotubes repeatedly with a focused electron beam as a function of the separation between the cuts and at different temperatures. As the cutting speed is related to the migration of displaced carbon atoms trapped inside the tube and to their recombination with vacancies, we obtain information about the mobility of the trapped atoms and estimate their migration barrier to be about 0.25 eV. This is an experimental confirmation of the remarkably high mobility of interstitial atoms inside carbon nanotubes, which shows that nanotubes have potential applications as pipelines for the transport of carbon atoms. We combine electron irradiation experiments in a transmission electron microscope with kinetic Monte Carlo simulations to determine the mobility of interstitial carbon atoms in single-walled carbon nanotubes. We measure the irradiation dose necessary to cut nanotubes repeatedly with a focused electron beam as a function of the separation between the cuts and at different temperatures. As the cutting speed is related to the migration of displaced carbon atoms trapped inside the tube and to their recombination with vacancies, we obtain information about the mobility of the trapped atoms and estimate their migration barrier to be about 0.25 eV. This is an experimental confirmation of the remarkably high mobility of interstitial atoms inside carbon nanotubes, which shows that nanotubes have potential applications as pipelines for the transport of carbon atoms.

The specific, local modification of the electronic structure of carbon nanomaterials is as important for novel electronic device fabrication as the doping in the case of silicon-based electronics. Here, we report low temperature scanning tunneling microscopy and spectroscopy study of semiconducting carbon nanotubes subjected to hydrogen-plasma treatment. We show that plasma treatment mostly results in the creation of paired electronic states in the nanotube band gap. Combined with extensive first-principle simulations, our results provide direct evidence that these states originate from correlated chemisorption of hydrogen adatoms on the tube surface. The energy splitting of the paired states is governed by the adatom–adatom interaction, so that controlled hydrogenation can be used for engineering the local electronic structure of nanotubes and other sp2-bonded nanocarbon systems.