Wanderlã L. Scopel

Lithium incorporation at the MoS2/graphene interface: an ab initio investigation

Based on ab initio calculations, we examine the incorporation of Li atoms in the MoS2/graphene interface. We find that the intercalated Li atoms are energetically more stable than Li atoms adsorbed on the MoS2 surface. The intercalated atoms interact with both graphene sheet and MoS2 layer, increasing the Li binding energies. However, the equilibrium geometries are ruled by the MoS2 layer, where the intercalated Li atoms lie on the top (LiT) and hollow (LiH) sites of the MoS2 layer. We calculate the Li diffusion barriers, along the LiT → LiH diffusion path, where we find similar energy barriers compared with that obtained for Li adatoms on the MoS2 surface. Our results allow us to infer that the Li storage capacity increases at MoS2/G interfaces, in comparison with Li adatoms on the MoS2 surface-however, with no reduction on the mobility of the intercalated Li atoms. Those properties are interesting/useful to the development of Li batteries based on MoS2.

On the connection between structural distortion and magnetism in graphene with a single vacancy

Abstract The correlation between structural distortion and emergence of magnetism in graphene containing a single vacancy was investigated using first-principles calculations based on density functional theory (DFT). Our results have shown that a local distortion is formed around the vacancy, with reconstruction of two atomic bonds and with a dangling bond remaining at the third atom adjacent to the vacancy. A systematic investigation of the possible out-of-plane displacement of this third atom was then carried out, in order to ascertain its effects on the magnetic features of the system. The ground state was definitely found to be magnetic and planar, with spin-resolved σ and π bands contributing to the total magnetic moment. However, we have also found that metastable solutions can be achieved if an initial shift of the third atom above a minimum threshold from the graphene plane is provided, which leads to a nonplanar geometry and a nonmagnetic state.

Determination of the hyperfine magnetic field in magnetic carbon-based materials: DFT calculations and NMR experiments.

The prospect of carbon-based magnetic materials is of immense fundamental and practical importance, and information on atomic-scale features is required for a better understanding of the mechanisms leading to carbon magnetism. Here we report the first direct detection of the microscopic magnetic field produced at 13C nuclei in a ferromagnetic carbon material by zero-field nuclear magnetic resonance (NMR). Electronic structure calculations carried out in nanosized model systems with different classes of structural defects show a similar range of magnetic field values (18–21 T) for all investigated systems, in agreement with the NMR experiments. Our results are strong evidence of the intrinsic nature of defect-induced magnetism in magnetic carbons and establish the magnitude of the hyperfine magnetic field created in the neighbourhood of the defects that lead to magnetic order in these materials.Information on atomic-scale features is required for a better understanding of the mechanisms leading to magnetism in non-metallic, carbon-based materials. This work reports a direct evaluation of the hyperfine magnetic field produced at 13C nuclei in ferromagnetic graphite by nuclear magnetic resonance (NMR). The experimental investigation was made possible by the results of first-principles calculations carried out in model systems, including graphene sheets with atomic vacancies and graphite nanoribbons with edge sites partially passivated by oxygen. A similar range of maximum hyperfine magnetic field values (18-21T) was found for all systems, setting the frequency span to be investigated in the NMR experiments; accordingly, a significant 13C NMR signal was detected close to this range without any external applied magnetic field in ferromagnetic graphite.

Nano-structured interface of graphene and h-BN for sensing applications.

The atomically-precise controlled synthesis of graphene stripes embedded in hexagonal boron nitride opens up new possibilities for the construction of nanodevices with applications in sensing. Here, we explore properties related to the electronic structure and quantum transport of a graphene nanoroad embedded in hexagonal boron nitride, using a combination of density functional theory and the non-equilibrium Greens functions method to calculate the electric conductance. We find that the graphene nanoribbon signature is preserved in the transmission spectra and that the local current is mainly confined to the graphene domain. When a properly sized nanopore is created in the graphene part of the system, the electronic current becomes restricted to a carbon chain running along the border with hexagonal boron nitride. This circumstance could allow the hypothetical nanodevice to become highly sensitive to the electronic nature of molecules passing through the nanopore, thus opening up ways to detect gas molecules, amino acids, or even DNA sequences based on a measurement of the real-time conductance modulation in the graphene nanoroad.The atomically-precise controlled synthesis of graphene stripes embedded in hexagonal boron nitride opens up new possibilities for the construction of nanodevices with applications in sensing. Here, we explore properties related to electronic structure and quantum transport of a graphene nanoroad embedded in hexagonal boron nitride, using a combination of density functional theory and the non-equilibrium Green’s functions method to calculate the electric conductance. We find that the graphene nanoribbon signature is preserved in the transmission spectra and that the local current is mainly confined to the graphene domain. When a properly sized nanopore is created in the graphene part of the system, the electronic current becomes restricted to a carbon chain running along the border with hexagonal boron nitride. This circumstance could allow the hypothetical nanodevice to become highly sensitive to the electronic nature of molecules passing through the nanopore, thus opening up ways for the detection of gas molecules, amino acids, or even DNA sequences based on a measurement of the real-time conductance modulation in the graphene nanoroad.

Interaction between single vacancies in graphene sheet: An ab initio calculation

Abstract In order to investigate the interaction between single vacancies in a graphene sheet, we have used spin-polarized density functional theory (DFT). Two distinct configurations were considered, either with the two vacancies located in the same sublattice or in different sublattices, and the effect of changing the separation between the vacancies was also studied. Our results show that the ground state of the system is indeed magnetic, but the presence of the vacancies in the same sublattice or in different sublattices and the possible topological configurations can lead to different contributions from the π and σ orbitals to magnetism. On the other hand, our findings reveal that the net magnetic moment of the system with the two vacancies in the same sublattice move towards the value of the magnetic moment per isolated vacancy with the increase of the distance between the vacancies, which is ascribed to the different contributions due to π electrons. Moreover, it is also found that the local magnetic moments for vacancies in the same sublattice are in parallel configuration, while they have different orientations when the vacancies are created in different sublattices. So, our findings have clearly evidenced how difficult it would be to observe experimentally the emergence of magnetic order in graphene-based systems containing randomly created atomic vacancies, since the energy difference between cases of antiferromagnetic and ferromagnetic order decreases quickly with the increase in the distance separating each vacancy pair.

H2O incorporation in the phosphorene/a-SiO2 interface: a first-principles study

Based on first-principles calculations, we investigate (i) the energetic stability and electronic properties of single-layer phosphorene (SLP) adsorbed on an amorphous SiO2 surface (SLP/a-SiO2), and (ii) the further incorporation of water molecules at the phosphorene/a-SiO2 interface. In (i), we find that the phosphorene sheet binds to a-SiO2 through van der Waals interactions, even in the presence of oxygen vacancies on the surface. The SLP/a-SiO2 system presents a type-I band alignment, with the valence (conduction) band maximum (minimum) of the phosphorene lying within the energy gap of the a-SiO2 substrate. The structure and the surface-potential corrugations promote the formation of electron-rich and electron-poor regions on the phosphorene sheet and at the SLP/a-SiO2 interface. Such charge density puddles are strengthened by the presence of oxygen vacancies in a-SiO2. In (ii), because of the amorphous structure of the surface, we consider a number of plausible geometries for H2O embedded in the SLP/a-SiO2 interface. There is an energetic preference for the formation of hydroxyl (OH) groups on the a-SiO2 surface. Meanwhile, in the presence of oxygenated water or interstitial oxygen in the phosphorene sheet, we observe the formation of metastable OH bonded to the phosphorene, and the formation of energetically stable P-O-Si chemical bonds at the SLP/a-SiO2 interface. Further x-ray absorption spectra simulations are performed, which aim to provide additional structural/electronic information on the oxygen atoms forming hydroxyl groups or P-O-Si chemical bonds at the interface region.

An ab initio investigation of Bi2Se3 topological insulator deposited on amorphous SiO2.

We use first-principles simulations to investigate the topological properties of Bi2Se3 thin films deposited on amorphous SiO2, Bi2Se3/a-SiO2, which is a promising substrate for topological insulator (TI) based device applications. The Bi2Se3 films are bonded to a-SiO2 mediated by van der Waals interactions. Upon interaction with the substrate, the Bi2Se3 topological surface and interface states remain present, however the degeneracy between the Dirac-like cones is broken. The energy separation between the two Dirac-like cones increases with the number of Bi2Se3 quintuple layers (QLs) deposited on the substrate. Such a degeneracy breaking is caused by (i) charge transfer from the TI to the substrate and charge redistribution along the Bi2Se3 QLs, and (ii) by deformation of the QL in contact with the a-SiO2 substrate. We also investigate the role played by oxygen vacancies ([Formula: see text]) on the a-SiO2, which increases the energy splitting between the two Dirac-like cones. Finally, by mapping the electronic structure of Bi2Se3/a-SiO2, we found that the a-SiO2 surface states, even upon the presence of [Formula: see text], play a minor role on gating the electronic transport properties of Bi2Se3.