Tuning the Schottky barrier height in graphene/monolayer-GeI2 van der Waals heterostructure.

We use first-principles simulations to investigate the structural and electronic properties of a heterostructure formed by graphene and monolayer GeI2 (m-GeI2). While graphene has been extensively studied in the last 15 years, m-GeI2 has been recently proposed to be a stable 2D semiconductor with a wide-band gap, Liu et al (2018 J. Phys. Chem. C 122 22137). By staking both structures we obtain a metal-semiconductor junction, with great potential for applications in the designing of new (opto)electronic devices. The results show that the graphene Dirac cone is preserved in the graphene/m-GeI2 heterostructure. We find that there are no chemical bonds at the graphene and m-GeI2 interface, thus the heterostructure interactions are ruled by van der Waals (vdW) forces. The interface between graphene and m-GeI2 results in a n-type Schottky contact. Furthermore, we show that a transition from n-type to p-type Schottky contact can be obtained by decreasing the interlayer distance. We also modulated the Schottky barrier heights by applying a perpendicular external electric field through the vdW heterostructure. In particular, positive values resulted in an increase of the n-type Schottky barrier height, while negative electric field values induced a transition from n-type to p-type Schottky contact. From our results, we show that m-GeI2 is an interesting material to design new electronic Schottky devices based on graphene vdW heterostructures.

Investigating the preservation of π-conjugation in covalently functionalized carbon nanotubes through first principles simulations.

We performed a theoretical investigation of single-walled carbon nanotubes (CNTs) functionalized with triazine molecules. Upon adsorption, the influence of the molecule orientation on the CNTs' electronic properties is examined by combining first-principles density functional theory calculations and simulations of X-ray Absorption Near-Edge Structure (XANES) at the C K-edge. Our calculations show that the electronic properties of functionalized CNTs can preserve the same features of pristine CNTs, for both semiconductor and metallic CNTs, depending on the orientation of the covalently bonded molecule. For that configuration, we observe a breakage of the CNT C-C bond at the molecule adsorption site. Moreover, the XANES spectra reveal that sp2 bonding hybridization is preserved along the CNT network. On the other hand, the electronic properties of pristine CNTs are no longer preserved for adsorbed molecule orientations resulting in intact C-C bond at the adsorption site. In this case, the XANES spectra indicate that the molecule-CNT interactions result in sp3 hybridization. Our findings help to elucidate whether π-conjugation is preserved in functionalized CNTs, demonstrating that calculations of XANES spectra are a powerful tool to resolve such systems.

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.

Pyridine intercalated Bi2Se3 heterostructures: controlling the topologically protected states.

We use ab initio simulations to investigate the incorporation of pyridine molecules (C5H5N) in the van der Waals (vdW) gaps of Bi2Se3. The intercalated pyridine molecules increase the separation distance between the Bi2Se3 quintuple layers (QLs), suppressing the parity inversion of the electronic states at the Γ-point. We find that (i) the intercalated region becomes a trivial insulator. By combining the pristine Bi2Se3 region with the one intercalated by the molecules (py-Bi2Se3), we have a trivial/topological heterojunction (py-Bi2Se3/Bi2Se3) characterized by the presence of topologically protected metallic states at the interfacial region. Next, (ii) we apply an external compressive pressure to the system, and the results are a decrease of the separation distance between the QLs intercalated by pyridine molecules, and the metallic states are shifted toward the bulk region, turning the system back to the insulator. Our findings indicate that, through the intercalation of pyridine molecules in Bi2Se3 [(i)], we may have a number of topologically protected metallic channels embedded in (py-Bi2Se3) m /(Bi2Se3) n heterostructures/superlattices, in addition, through suitable tuning of the external pressure [(ii)], we can control its topological properties, turning on and off the topologically protected metallic states in (py-Bi2Se3)m /(Bi2Se3)n.

Organic molecules deposited on graphene: a computational investigation of self-assembly and electronic structure.

We use ab initio simulations to investigate the adsorption and the self-assembly processes of tetracyanoquinodimethane (TCNQ), tetrafluoro-tetracyanoquinodimethane (F4-TCNQ), and tetrasodium 1,3,6,8-pyrenetetrasulfonic acid (TPA) on the graphene surface. We find that there are no chemical bonds at the molecule-graphene interface, even at the presence of grain boundaries on the graphene surface. The molecules bond to graphene through van der Waals interactions. In addition to the molecule-graphene interaction, we performed a detailed study of the role played by the (lateral) molecule-molecule interaction in the formation of the, experimentally verified, self-assembled layers of TCNQ and TPA on graphene. Regarding the electronic properties, we calculate the electronic charge transfer from the graphene sheet to the TCNQ and F4-TCNQ molecules, leading to a p-doping of graphene. Meanwhile, such charge transfer is reduced by an order of magnitude for TPA molecules on graphene. In this case, it is not expected a significant doping process upon the formation of self-assembled layer of TPA molecules on the graphene sheet.