Rodrigo G. Amorim

Electronic structure and quantum transport properties of trilayers formed from graphene and boron nitride

We report the results of a theoretical study of graphene/BN/graphene and BN/graphene/BN trilayers using the van-der-Waals-corrected density functional theory in conjunction with the non-equilibrium Greens Function method. These trilayer systems formed from graphene and BN exhibit distinct stacking-dependent features in their ground state electronic structure and response to an applied electric field perpendicular to the trilayer planes. The graphene/BN/graphene system shows a negligible gap in the electronic band structure that increases for the AAA and ABA stackings under an external electric field, while the zero-field band gap of BN/graphene/BN remains unaffected by the electric field. When both types of trilayer systems are contacted with gold electrodes, a metal-like conduction is predicted in the low-field regime, which changes to a p-type conduction with an increase in the applied perpendicular bias field.

Silicene as a new potential DNA sequencing device

Silicene, a hexagonal buckled 2D allotrope of silicon, shows potential as a platform for numerous new applications, and may allow for easier integration with existing silicon-based microelectronics than graphene. Here, we show that silicene could function as an electrical DNA sequencing device. We investigated the stability of this novel nano-bio system, its electronic properties and the pronounced effects on the transverse electronic transport, i.e., changes in the transmission and the conductance caused by adsorption of each nucleobase, explored by us through the non-equilibrium Greens function method. Intriguingly, despite the relatively weak interaction between nucleobases and silicene, significant changes in the transmittance at zero bias are predicted by us, in particular for the two nucleobases cytosine and guanine. Our findings suggest that silicene could be utilized as an integrated-circuit biosensor as part of a lab-on-a-chip device for DNA sequencing.

Strain- and electric field-induced band gap modulation in nitride nanomembranes

The hexagonal nanomembranes of the group III-nitrides are a subject of interest due to their novel technological applications. In this paper, we investigate the strain- and electric field-induced modulation of their band gaps in the framework of density functional theory. For AlN, the field-dependent modulation of the bandgap is found to be significant whereas the strain-induced semiconductor-metal transition is predicted for GaN. A relatively flat conduction band in AlN and GaN nanomembranes leads to an enhancement of their electronic mobility compared to that of their bulk counterparts.

Energetic stability, STM fingerprints and electronic transport properties of defects in graphene and silicene

Novel two-dimensional materials such as graphene and silicene have been heralded as possibly revolutionary in future nanoelectronics. High mobilities, and in the case of silicene, its seemingly natural integration with current electronics could make them the materials of next-generation devices. Defects in these systems, however, are unavoidable particularly in large-scale fabrication. Here we combine density functional theory and the non-equilibrium Greens function method to simulate the structural, electronic and transport properties of different defects in graphene and silicene. We show that defects are much more easily formed in silicene, compared to graphene. We also show that, although qualitatively similar, the effects of different defects occur closer to the Dirac point in silicene, and identifying them using scanning tunneling microscopy is more difficult particularly due to buckling. This could be overcome by performing direct source/drain measurements. Finally we show that the presence of defects leads to an increase in local current from which it follows that they not only contribute to scattering, but are also a source of heating.

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.

Switching Behaviors of Graphene-Boron Nitride Nanotube Heterojunctions

High electron mobility of graphene has enabled their application in high-frequency analogue devices but their gapless nature has hindered their use in digital switches. In contrast, the structural analogous, h-BN sheets and BN nanotubes (BNNTs) are wide band gap insulators. Here we show that the growth of electrically insulating BNNTs on graphene can enable the use of graphene as effective digital switches. These graphene-BNNT heterojunctions were characterized at room temperature by four-probe scanning tunneling microscopy (4-probe STM) under real-time monitoring of scanning electron microscopy (SEM). A switching ratio as high as 105 at a turn-on voltage as low as 0.5 V were recorded. Simulation by density functional theory (DFT) suggests that mismatch of the density of states (DOS) is responsible for these novel switching behaviors.

Commensurability effect on the electronic structure of carbon nanostructures: Impact on supercell calculations in nanotubes

Friedel oscillations are ubiquitous features seen in all impurity-doped metallic structures but in the case of graphene-like materials they are not so evident because the relevant wavelengths are perfectly commensurate with the lattice parameter. Here we demonstrate that this commensurability effect leads to a slow convergence of supercell-based total energy calculations in impurity-doped carbon nanotubes. We derive a mathematically transparent expression for the formation energy and identify a very distinctive dependence on the size of the supercell unit. We make use of this dependence through a simple extrapolation scheme to obtain density functional theory results with accuracy levels that would otherwise require enormously large unit cells.

Benchmark investigation of diamondoid-functionalized electrodes for nanopore DNA sequencing

Small diamond-like particles, diamondoids, have been shown to effectively functionalize gold electrodes in order to sense DNA units passing between the nanopore-embedded electrodes. In this work, we present a comparative study of Au(111) electrodes functionalized with different derivatives of lower diamondoids. Focus is put on the electronic and transport properties of such electrodes for different DNA nucleotides placed within the electrode gap. The functionalization promotes a specific binding to DNA leading to different properties for the system, which provides a tool set to systematically improve the signal-to-noise ratio of the electronic measurements across the electrodes. Using quantum transport calculations, we compare the effectiveness of the different functionalized electrodes in distinguishing the four DNA nucleotides. Our results point to the most effective diamondoid functionalization of gold electrodes in view of biosensing applications.

Insights into the detection of mutations and epigenetic markers using diamondoid-functionalized sensors

Nanogaps functionalized with small diamond-like particles, diamondoids, have been shown to effectively distinguish between different DNA nucleotides. Here, we focus on the detection of mutations and epigenetic markers using such devices. Based on quantum mechanical simulations within the density functional theory approach coupled with the non-equilibrium Green’s function scheme, we provide deeper insight into the inherent differences in detecting modified nucleotides. Our results strongly underline the influence of the type of functionalization molecule of the nanogap, as well its conformational details within the nanogap, on the sensing efficiency of the device. The electronic features for the mutations and epigenetic markers are compared to those for the respective canonical nucleotides that are detected by different devices. The calculations directly correlate the structural and electronic properties of the different nucleotides with the electronic transmission across the diamondoid-based device. The latter was found to be controlled by the functionalizing molecule and its binding to the nucleotides. We report on the direct connection of these characteristics to the sensitivity of the diamondoid-functionalized nanogaps, which could eventually be embedded in a nanopore device, and discuss the implications for DNA sensing.

Hybridization effects on the out-of-plane electron tunneling properties of monolayers: is h-BN more conductive than graphene?

Electron transport properties through multilayers of hexagonal boron nitride (h-BN) sandwiched between gold electrodes is investigated by density functional theory together with the non-equilibrium Greens function method. The calculated results find that despite graphene being a gapless semimetal and h-BN two-dimensional layer being an insulator, the transmission function perpendicular to the atomic layer plane in both systems is nearly identical. The out-of-plane tunnel current is found to be strongly dependent on the interaction at the interface of the device. As a consequence, single layer h-BN coupled with atomically flat weakly interacting metals such as gold may not work as a good dielectric material, but the absence of sharp resonances would probably lead to more stable out-of-plane electronic transport properties compared to graphene.