Boron-doped graphene topological defects: unveiling high sensitivity to NO molecule for gas sensing applications.

Global air quality has deteriorated significantly in recent years due to large emissions from the transformation industry and combustion vehicles. This issue requires the development of portable, highly sensitive, and selective gas sensors. Nanostructured materials, including defective graphene, have emerged as promising candidates for such applications. In this work, we investigated the B-doped topological line defect in graphene as a sensing material for various gas molecules (CO, CO2, NO, and NH3) based on a combination of density functional theory and the non-equilibrium Green's function method. The electronic transport calculations reveal that the electric current can be confined to the line defect region by gate voltage control, revealing highly reactive sites. The B-doped topological line defect is metallic, favoring the adsorption of NO and NH3 over CO and CO2 molecules. We notice changes in the conductance after gas molecule adsorption, producing a sensitivity of 50% (16%) for NO (NH3). In addition, the recovery time for nitride gases was calculated for different temperatures and radiation frequencies. At 300 K the ultraviolet (UV) has a fast recovery time compared to the visible (VIS) one by about two orders of magnitude. This study gives an understanding of how engineering transport properties at the microscopic level (by topological line defect and chemical B-doping) leads to promising nanosensors for detecting nitride gas.

Combined computational and experimental study about the incorporation of phosphorus into the structure of graphene oxide.

Phosphorus-containing graphene-based hybrids are materials with outstanding properties for diverse applications. In this work, an easy route to produce phosphorus-graphene oxide hybrid materials is described, involving the use of variable amounts of H3PO4 and H2SO4 during the reaction of oxidation of a graphitic precursor. The physical and chemical features of the hybrids change significantly with the variation in the acid amounts used in the syntheses. XPS and solid-state 13C and 31P NMR results show that the hybrids contain large amounts of oxygen functional groups, with the phosphorus incorporation proceeding mostly through the formation of phosphate-like linkages and other functions with C-O-P bonds. The experimental findings are supported by DFT calculations, which allow the assessment of the energetics and the geometry of the interaction between phosphate groups and graphene-based models; these calculations are also used to predict the chemical shifts in the 31P and 13C NMR spectra of the models, which show good agreement with the experimentally observed solid-state NMR spectra.

C-doping anisotropy effects on borophene electronic transport.

The electronic transport anisotropy for different C-doped borophene polymorphs (ß12andχ3) was investigated theoretically combining density functional theory and non-equilibrium Green's function. The energetic stability analysis reveals that B atoms replaced by C is more energetically favorable forχ3phase. We also verify a directional character of the electronic band structure on C-doped borophene for both phases. Simulated scanning tunneling microscopy and also total density of charge confirm the directional character of the bonds. The zero bias transmission forß12phase atE-EF= 0 shows that C-doping induces a local current confinement along the lines of doped sites. TheI-Vcurves show that C-doping leads to an anisotropy amplification in theß12than in theχ3. The possibility of confining the electronic current at an specific region of the C-doped systems, along with the different adsorption features of the doped sites, poses them as promising candidates to highly sensitive and selective gas sensors.

Electrically sensing Hachimoji DNA nucleotides through a hybrid graphene/h-BN nanopore.

The feasibility of synthesizing unnatural DNA/RNA has recently been demonstrated, giving rise to new perspectives and challenges in the emerging field of synthetic biology, DNA data storage, and even the search for extraterrestrial life in the universe. In line with this outstanding potential, solid-state nanopores have been extensively explored as promising candidates to pave the way for the next generation of label-free, fast, and low-cost DNA sequencing. In this work, we explore the sensitivity and selectivity of a graphene/h-BN based nanopore architecture towards detection and distinction of synthetic Hachimoji nucleobases. The study is based on a combination of density functional theory and the non-equilibrium Green's function formalism. Our findings show that the artificial nucleobases are weakly binding to the device, indicating a short residence time in the nanopore during translocation. Significant changes in the electron transmission properties of the device are noted depending on which artificial nucleobase resides in the nanopore, leading to a sensitivity in distinction of up to 80%. Our results thus indicate that the proposed nanopore device setup can qualitatively discriminate synthetic nucleobases, thereby opening up the feasibility of sequencing even unnatural DNA/RNA.

Embedded carbon nanowire in black phosphorene and C-doping: the rule to control electronic properties.

Tuning the properties of black phosphorene such as structural, electronic and transport are explored via substitutional C-doping. We employed density functional theory calculations in combination with the non-equilibrium Green's function for modeling the systems. Our results revealed that substitutional C-doped phosphorene is energetically favorable and ruled by the exothermic process. We also found that C-doping induces a change of the electric properties, such as a semiconductor-to-metal transition for the most lower concentration and zig-zag C-wire. Furthermore, for an armchair C-wire at the highest concentration, the semiconductor character is kept, meanwhile direct-to-indirect transitions are observed in the band gap nature. The band structures show that there exists a dependence of the electronic charge transport with the directional character of the C-doped configuration. The findings demonstrate that the directional doping could play a key role for the conductance of a 2D platform.

Tuning the photocatalytic water-splitting capability of two-dimensional α-In2Se3 by strain-driven band gap engineering.

In this work, we have investigated the effects of in-plane mechanical strains on the electronic properties of single-layer α-In2Se3 by means of density functional theory (DFT) calculations. Our findings reveal that this system exhibits a semiconductor character with an indirect band gap in the ground state, with a compressive biaxial strain leading to an indirect to direct band gap transition. Remarkably, along with the band gap transition, the system displays promising capability to produce hydrogen gas from a visible light photocatalytic water splitting process.

Controlled current confinement in interfaced 2D nanosensor for electrical identification of DNA.

The controlled synthesis of hybrid two-dimensional (2D) materials and the development of atomically precise nanopore fabrication techniques have opened up entirely new possibilities for sensing applications via nanoelectronics. Here, we investigate the electronic transport properties of an in-plane hybrid graphene/h-BN device, containing a graphene nanopore, to assess its feasibility to act as a molecular sensor. The results from our calculations based on density functional theory and the non-equilibrium Green's function formalism reveal the capability to confine the electric current pathways to the two carbon wires lining either edge of the nanopore, thereby creating conditions in which the conductance is highly sensitive to any changes in the electrical potential inside the nanopore. We apply this setup to assess whether it is possible to electrically determine the base sequence in a DNA molecule. Indeed, the modulation of the device conductance reveals a characteristic fingerprint of each nucleotide, which manifests itself in a pronounced difference in the sensitivity of the four different nucleotides, thereby allowing electrical discrimination. These findings lead us to propose this device architecture as a promising nanobiosensor. While fabrication in the lab may represent a profound experimental challenge, it should nevertheless in principle be feasible with existing contemporary techniques of hybrid 2D material synthesis, in conjunction with approaches for highly controlled nanopore creation.

Electronic stripes and transport properties in borophene heterostructures.

We performed a theoretical investigation of the structural and electronic properties of (i) pristine and (ii) superlattice structures of borophene. In (i), by combining first-principles calculations, based on the density functional theory (DFT), and simulations of the X-ray Absorption Near-Edge Structure (XANES) spectra we present a comprehensive picture connecting the atomic arrangement of borophene and the X-ray absorption spectra. Once we characterized the electronic properties of the pristine systems, we next examined the electronic confinement effects in 2D borophene superlattices (BSLs) [(ii)]. Here, the BSL structures were made by attaching laterally two different structural phases of borophene. The energetic stability and the electronic properties of these BSLs were examined based on total energy DFT calculations. We find a highly anisotropic electronic structure, characterized by the electronic confinement effects, giving rise to "electronic stripes", and metallic channels ruled by the superlattices. Combining DFT and the Landauer-Büttiker formalism, we investigated the electronic transport properties in BSLs. Our results of the transmission probability reveal that the electronic transport is ruled by π or a combination of π and σ transmission channels, depending on the atomic arrangement and periodicity of the superlattices. Finally, we show that there is a huge magnification of the directional dependence of the electronic transport properties in BSLs, in comparison with the pristine borophene phase. These findings indicate that BSLs are quite interesting systems in order to design conductive nanoribbons on a 2D platform.

Probing the local interface properties at a graphene-MoSe2 in-plane lateral heterostructure: an ab initio study.

We report a theoretical study of the local interface properties at a graphene-MoSe2 (G-MoSe2) in-plane lateral heterostructure. Using a combination of first-principles density functional theory (DFT) calculations and simulations of X-ray Absorption Near-Edge Structure (XANES) spectroscopy at the C K-edge, we examined different local interface arrangements. The simulated XANES signal from interface carbon atoms showed new features compared to the pristine graphene region, which provides a way of identifying different chemical environments and/or geometries of the local interface in the G-MoSe2 lateral hybrid system. Our results also revealed that the local electronic and magnetic properties are dependent on the interface atomic structure, where metallic, semiconductor or half-metallic character was achieved at the G-MoSe2 interface. These findings indicate the great potential of 2D lateral heterojunctions for nanoelectronic and spintronic applications.

Nanodots of transition metal dichalcogenides embedded in MoS2 and MoSe2: first-principles calculations.

The energetic stability and the electronic properties of nanodots (NDs) composed of transition metal dichalcogenides, XS2 and XSe2 (with X = Mo, W and Nb) embedded in single layer MoS2 and MoSe2 hosts, were investigated based on first-principles calculations. We find that through a suitable combination of the ND and host materials it is possible to control the electron-hole localization. For instance, in NDs of WS2 in the MoS2 host we find the highest occupied (hole) states localized in the ND region, while the lowest unoccupied (electron) states spread out in the MoS2 host. On the other hand, by changing the ND and host materials, the electron states become localized in the MoS2 ND in the WS2 host. Further electronic structure calculations show that the NDs of NbS2 and NbSe2 give rise to a set of spin degenerate empty states within the energy gap of the MoS2 and MoSe2 hosts. The spin degeneracy can be removed by negatively charging the ND system. Such n-type doping was examined by considering a van der Waals (vdW) heterostructure composed of a graphene layer lying on the NbS2 and NbSe2 NDs. Indeed we found a net magnetic moment localized in the ND region.

Electrical detection of nucleotides via nanopores in a hybrid graphene/h-BN sheet.

Designing the next generation of solid-state biosensors requires developing detectors which can operate with high precision at the single-molecule level. Nano-scaled architectures created in two-dimensional hybrid materials offer unprecedented advantages in this regard. Here, we propose and explore a novel system comprising a nanopore formed within a hybrid sheet composed of a graphene nanoroad embedded in a sheet of hexagonal boron nitride (h-BN). The sensitive element of this setup is comprised of an electrically conducting carbon chain forming one edge of the nanopore. This design allows detection of DNA nucleotides translocating through the nanopore based on the current modulation signatures induced in the carbon chain. In order to assess whether this approach is feasible to distinguish the four different nucleotides electrically, we have employed density functional theory combined with the non-equilibrium Green's function method. Our findings show that the current localized in the carbon chain running between the nanopore and h-BN is characteristically modulated by the unique dipole moment of each molecule upon insertion into the pore. Through the analysis of a simple model based on the dipole properties of the hydrogen fluoride molecule we are able to explain the obtained findings.

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.

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 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 to detect gas molecules, amino acids, or even DNA sequences based on a measurement of the real-time conductance modulation in the graphene nanoroad.

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 (13)C 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.

Theoretical investigation of Hf and Zr defects in c-Ge.

The introduction of high-permittivity gate dielectric materials into complementary metal oxide semiconductor technology has reopened the interest in Ge as a channel material mainly due to its high hole mobility. Since HfO(2) and ZrO(2) are two of the most promising dielectric candidates, it is important to investigate if Hf and Zr may diffuse into the Ge channel. Therefore, using ab initio density functional theory calculations, we have studied substitutional and interstitial Hf and Zr impurities in c-Ge,looking for neutral defects. We find that (i) substitutional Zr and Hf defects are energetically more favorable than interstitial defects; (ii) under oxygen-rich conditions, neither Zr nor Hf migration towards the channel is likely to occur; (iii) either under Hf- or Zr-rich conditions it is very likely, particularly for Zr, that defects will be incorporated in the channel.