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Cellulose disassembly is an important issue in designing nanostructures using cellulose-based materials. In this work, we present a combination of experimental and theoretical study addressing the disassembly of cellulose nanofibrils. Through 2,2,6,6-tetramethylpiperidine-1-oxyl-mediated oxidation processes, combined with atomic force microscopy results, we show the formation of nanofibers with diameters corresponding to that of a single-cellulose polymer chain. The formation of these polymer chains is controlled by repulsive electrostatic interactions between the oxidized chains. Further, first-principles calculations have been performed in order to provide an atomistic understanding of the cellulose disassembling processes, focusing on the balance between the interchain (IC) and intersheet (IS) interactions upon oxidation. First, we analyze these interactions in pristine systems, where we found the IS interaction to be stronger than the IC interaction. In the oxidized systems, we have considered the formation of (charged) carboxylate groups along the inner sites of elementary fibrils. We show a net charge concentration on the carboxylate groups, supporting the emergence of repulsive electrostatic interactions between the cellulose nanofibers. Indeed, our total energy results show that the weakening of the binding strength between the fibrils is proportional to the concentration and net charge density of the carboxylate group. Moreover, by comparing the IC and IS binding energies, we found that most of the disassembly processes should take place by breaking the IC O-H···O hydrogen bond interactions and thus supporting the experimental observation of single- and double-cellulose polymer chains.

Low-dimensionality materials are highly susceptible to interfaces. Indeed, intercalation of different chemical species in between epitaxial graphene and silicon carbide (SiC), for instance, may decouple the graphene with respect to the substrate due to the conversion of the buffer layer into a graphene layer. O-intercalation is known to release the strain of such 2D material and to lead to the formation of high structural quality AB-stacked bilayer graphene. Nonetheless, this interface transformation concomitantly degrades graphene electronic transport properties. In this work we employed different techniques in order to better understand the structure of the graphene/SiC interface generated by O-intercalation and to elucidate the origin of the poor electronic properties of graphene. Experimental results revealed the formation of a SiO2 rich layer with a defective transition layer in between it and the SiC, which is characterized by the existence of silicon oxycarbide structures. Scanning tunneling spectroscopy measurements revealed an extensive presence of electronic states just around the Fermi level all over the sample surface, which may suppress the charge carriers mobility around this region. According to theoretical calculations, such states are mainly due to the formation of silicon oxicarbides within the interfacial layer.

Based on first-principles calculations, we show that the oxidation of ultrathin films of Ca2N electrides (electrenes) drives a hexagonal → tetragonal structural transition. The ground-state configu...

Understanding the role of microscopic attributes in nanocomposites allows for a controlled and, therefore, acceleration in experimental system designs. In this work, we extracted the relevant parameters controlling the graphene oxide binding strength to cellulose by combining first-principles calculations and machine learning algorithms. We were able to classify the systems among two classes with higher and lower binding energies, which are well defined based on the isolated graphene oxide features. By a theoretical X-ray photoelectron spectroscopy analysis, we show the extraction of these relevant features. Additionally, we demonstrate the possibilities of a refined control within a machine learning regression between the binding energy values and the system's characteristics. Our work presents a guiding map to the control graphene oxide/cellulose interaction.

The electronic transport properties in molecular heterojunctions are intimately connected with the molecular conformation between the electrodes, and the electronic structure of the molecule/electrode interface. In this work, we perform an ab initio density-functional-theory investigation of the structural and transport properties through self-assembled CuPc molecules sandwiched between gold contacts with (111) surfaces. We demonstrated (i) a tunneling regime ruled by the π orbitals of the aromatic rings of CuPc molecules; and (ii) a high variation (up to two orders of magnitude) of the current density with the orientation of the CuPc molecules relative to the gold surface. The source of this variation is the geometrical dependence of the energy of the highest-occupied-molecular-orbital with respect to the chemical potential of the metal and the generation of intra-molecular transport channels for a configuration with CuPc molecules tilted with respect to the gold surface.

Intercalation of foreign atoms in two-dimensional (2D) hosts has been considered a quite promising route to engineer the electronic and magnetic properties in 2D platforms. In the present study, we performed a first-principles theoretical investigation of the energetic stability and the magnetic/electronic properties of 2D ${\mathrm{GeP}}_{3}$ doped by Cr atoms. Our total energy results reveal the formation of thermodynamically stable Cr doped ${\mathrm{GeP}}_{3}$ bilayer and quadrilayer, characterized by interstitial Cr atoms lying in the van der Waals gap between the stacked ${\mathrm{GeP}}_{3}$ layers. The Cr-doped systems become magnetic, and the magnetic ordering can be tuned through the application of compressive mechanical strain. Moreover, the systems are metallic, characterized by the emergence of strain-induced spin-polarized channels at the Fermi level. These findings reveal that the atomic intercalation offers a set of degree of freedom not only to design but also to control the magnetic/electronic properties by mechanical strain in 2D systems.

Jacutingate, a recently discovered Brazilian naturally occurring mineral, has been shown to be an experimental realization of the Kane-Mele topological model. In this paper we present a class of materials, ${M}_{2}N{X}_{3}$ ($M=\mathrm{Ni}$, Pt, and Pd; $N=\mathrm{Zn}$, Cd, and Hg; and $X=\mathrm{S}$, Se, and Te), sharing jacutingaite's key features, i.e., high stability and a topological phase. By employing first-principles calculations we extensively characterize the energetic stability of this class while showing a common occurrence of the Kane-Mele topological phase. Here we present Pt-based materials surpassing jacutingaite's impressive topological gap and lower exfoliation barrier while retaining its stability.

The ability to construct 2D systems, beyond materials' natural formation, enriches the search and control capability of new phenomena, for instance, the synthesis of topological lattices of vacancies on metal surfaces through scanning tunneling microscopy. In the present study, we demonstrate that metal atoms encaged in a silicate adlayer on silicon carbide is an interesting platform for lattice design, providing a ground to experimentally construct tight-binding models on an insulating substrate. Based on the density functional theory, we have characterized the energetic and electronic properties of 2D metal lattices embedded in the silica adlayer. We show that the characteristic band structures of those lattices are ruled by surface states induced by the metal-s orbitals coupled by the host-p

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.

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) we present a comprehensive picture connecting the atomic arrangement of borophene and the X-ray absorption spectra. Once we have 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 those BSLs were examined based on total energy DFT calculations. We find a highly anisotropic electronic structure, characterized by the electronic confinement effects, and the formation of metallic channels along the superlattices. Combining DFT and the Landauer-B\"uttiker formalism, we investigate the electronic transport properties in the BSLs. Our results of the transmission probability reveal that the electronic transport is ruled by {\pi} or a combination of {\pi} and {\sigma} transmission channels, depending on the atomic arrangement and periodicity of the superlattices. Finally we show that there is huge magnification on the directional dependence of the electronic transport properties in BSLs, in comparision with the pristine borophene phase. Those findings indicate that BSLs are quite interesting systems in order to design conductive nanoribbons in a 2D platform., Comment: Nanoscale 2019

Very recently two dimensional layers of boron atoms, so called borophene, have been successfully synthesized. It presents a metallic band structure, with a strong anisotropic character. Upon further hydrogen adsorption a new material is obtained, borophane; giving rise to a Dirac cone structure like the one in graphene. We have performed a first-principles study of the electronic and transport properties of borophene and borophane through the Landauer–Buttiker formalism. We find that borophene presents an electronic current two orders of magnitude larger than borophane. In addition we verified the direction dependence of the electronic current in two perpendicular directions, namely, Ix and Iy; where for both systems, we found a current ratio, η = Ix/Iy, of around 2. Aiming to control such a current anisotropy, η, we performed a study of its dependence with respect to an external strain. Where, by stretching the borophane sheet, η increases by 11% for a bias voltage of 50 mV.

${\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ is a semiconductor material that can be stabilized in different crystal structures (at least one 3D and several 2D layered structures have been reported) with diverse electrical and optical properties. This feature has plagued its characterization over the years, with reported band gaps varying in an unacceptable range of 1 eV. Using first-principles calculations based on density functional theory and the HSE06 hybrid functional, we investigate the structural and electronic properties of four layered phases of ${\mathrm{In}}_{2}{\mathrm{Se}}_{3}$, addressing their relative stability and the nature of their fundamental band gaps, i.e., direct versus indirect. Our results show large disparities between fundamental and optical gaps. The absorption coefficients are found to be as high as those in direct-gap III-V semiconductors. The band alignment with respect to conventional semiconductors indicate a tendency to $n$-type conductivity, explaining recent experimental observations.

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.

The structural and electronic properties of few layers of blue phosphorus and their van der Waals heterostructures with graphene were investigated by means of first-principles electronic structure calculations. We study the four energetically most stable stacking configurations for multilayers of blue phosphorus. For all of them, the indirect band-gap semiconductor character, are preserved. We show that the properties of monolayer graphene and single-layer (bilayer) blue phosphorus are preserved in the van der Waals heterostructures. Further, our results reveal that under a perpendicular applied electric field, the position of the band structure of blue phosphorus with respect to that of graphene is tunable, enabling the effective control of the Schottky barrier height. Indeed, for the bilayer blue phosphorene on top of graphene, it is possible to even move the system into an Ohmic contact and induce a doping level of the blue phosphorene. All of these features are fundamental for the design of new nanodevices based on van der Waals heterostructures.

We use first-principles simulations to investigate the magnetic and the electronic transport properties of functionalized graphene layers upon the presence of extended linear defects (ELDs). We have considered electron-donor molecules, tetrathiafulvalene (TTF), lying on the graphene sites neighboring the ELD (TTF/ELD). Those molecules bond to the graphene sheet mediated by van der Waals interactions, giving rise to a net (molecule$\ensuremath{\rightarrow}$graphene) charge transfer. The $n$-type doping of graphene, as well as the molecule-graphene interaction, are strengthened by the presence of the ELD. There is a charge density accumulation on the C atoms along the defect sites, promoting the formation of magnetic states in TTF/ELD. We show that such a net magnetization can be modified through an external electric field (${E}_{\text{field}}$). Further electronic transport calculations reveal that the transmission coefficients exhibit a spin anisotropy which can be controlled by the ${E}_{\text{field}}$, and thus showing that TTF/ELD is a quite interesting system to realize tunable spin-polarized electronic currents on graphene.

The structural and electronic properties of pristine and H-passivated C-terminated β-SiC(0 0 1)-c(2 × 2) surface are addressed by ab initio calculations. Here, we verify the formation of C chains composed by double-bonded dimers rows (C C), separated by triple-bonded bridged dimers (C C). The surface states near the bandgap are confined along the C C dimer rows, with no electronic contribution from the C C bridged dimers. After hydrogenation, the C-chains are strongly modified, forming subsurface voids or nanotunnel (NT) structures. By considering a plausible set of energy release steps for increasing hydrogenation, we obtain a C-rich NT ruled by the C C dimer rows. Somewhat similar to that recently reported on the Si-rich termination, but 0.8 eV lower in energy. The electronic band structures of both Si-rich and C-rich NTs have been examined within the hybrid HSE06 functional, which are compared with those previously reported using a semilocal functional.

The growth and self-assembling properties of nickel-tetraphenyl porphyrins (NiTPP) on the Cu(111) surface are analysed via scanning tunnelling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT). For low coverage, STM results show that NiTPP molecules diffuse on the terrace until they reach the step edge of the copper surface forming a 1D system with disordered orientation along the step edges. The nucleation process into a 2D superstructure was observed to occur via the interaction of molecules attached to the already nucleated 1D structure, reorienting molecules. For monolayer range coverage a 2D nearly squared self-assembled array with the emergence of chiral domains was observed. The XPS results of the Ni 2p(3/2) core levels exhibit a 2.6 eV chemical shift between the mono- and multilayer configuration of NiTPP. DFT calculations show that the observed chemical shifts of Ni 2p(3/2) occur due to the interaction of 3d orbitals of Ni with the Cu(111) substrate.

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.

We predict a new class of large band gap quantum spin Hall insulators, the fluorinated PbX (X = C, Si, Ge and Sn) compounds, that are mechanically stable two-dimensional materials. Based on first principles calculations we find that, while the PbX systems are not topological insulators, all fluorinated PbX (PbXF2) compounds are 2D topological insulators. The quantum spin Hall insulating phase was confirmed by the explicitly calculation of the Z2 invariant. In addition we performed a thorough investigation of the role played by the (i) fluorine saturation, (ii) crystal field and (iii) spin-orbital coupling in PbXF2. By considering nanoribbon structures, we verify the appearance of a pair of topologically protected Dirac-like edge states connecting the conduction and valence bands. The insulating phase which is a result of the spin orbit interaction, reveals that this new class of two dimensional materials present exceptional nontrivial band gaps, reaching values up to 0.99 eV at the Γ point and an indirect band gap of 0.77 eV. The topological phase is arisen without any external field, making this system promising for nanoscale applications, using topological properties.

We report first-principles calculations of the energetic stability and electronic properties of metal-phthalocyanine (MPc) molecules (M = Cr, Mn, Fe, Co, Ni, Cu, and Zn) adsorbed on the $\ensuremath{\delta}$-doped Si(111)-$\mathrm{B}(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})$ reconstructed surface. (i) It can be seen that CrPc, MnPc, FePc, and CoPc are chemically anchored to the topmost Si atom. (ii) Contrastingly, the binding of the NiPc, CuPc, and ZnPc molecules to the $\mathrm{Si}(111)\text{\ensuremath{-}}\mathrm{B}(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})$ surface is exclusively ruled by van der Waals interactions, the main implication being that these molecules may diffuse and rearrange to form clusters and/or self-organized structures on this surface. The electronic structure calculations reveal that in point (i), owing to the formation of the metal-Si covalent bond, the net magnetic moment of the molecule is quenched by $1{\ensuremath{\mu}}_{\mathrm{B}}$, remaining unchanged in point (ii). In particular, the magnetic moment of CuPc $(1{\ensuremath{\mu}}_{\mathrm{B}})$ is preserved after adsorption. Finally, we verify that the formation of ZnPc, CuPc, and NiPc molecular (self-assembled) arrangements on the Si(111)-$\mathrm{B}(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$) surface is energetically favorable, in good agreement with recent experimental findings.

Based on first-principles calculations, we investigate the energetic stability and the electronic properties of (i) a single layer phosphorene (SLP) adsorbed on the 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 bonds to a-sio2 through van der Waals interactions, even upon the presence of oxygen vacancy on the surface. The \slp/a-\sio\ 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-\sio\ substrate. The structural, and the surface-potential corrugations promote the formation of electron-rich and -poor regions on the phosphorene sheet and at the SLP/a-sio2 interface. Such charge density puddles have been strengthened by the presence of oxygen vacancies in a-sio2. In (ii), due to the amorphous structure of the surface, we have considered a number of plausible geometries of water embedded in the SLP/a-sio2 interface. There is an energetic preference to the formation of hydroxyl (OH) groups on the a-sio2 surface. Meanwhile, upon the presence of oxygenated water or interstitial oxygen in the phosphorene sheet, we find 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 (XAS) simulations have been done, aiming to provide additional structural/electronic informations of the oxygen atoms forming hydroxyl groups or P--O--Si chemical bonds at the interface region.

We use first-principles simulations to investigate the topological properties of Bi$_2$Se$_3$ thin films deposited on amorphous SiO2, Bi$_2$Se$_3$/a-SiO$_2$, which is a promising substrate for topological insulator (TI) based device applications. The Bi$_2$Se$_3$ films are bonded to a-SiO$_2$ mediated by van der Waals interactions. Upon interaction with the substrate, the Bi$_2$Se$_3$ 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 Bi$_2$Se$_3$ 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 Bi$_2$Se$_3$ QLs, and (ii) by deformation of the QL in contact with the a-SiO$_2$ substrate. We also investigate the role played by oxygen vacancies (V$_O$) on the a-SiO$_2$, which increases the energy splitting between the two Dirac-like cones. Finally, by mapping the electronic structure of Bi$_2$Se$_3$/a-SiO$_2$, we found that the a-SiO$_2$ surface states, even upon the presence of V$_O$, play a minor role on gating the electronic transport properties of Bi$_2$Se$_3$.

We have performed an ab initio theoretical investigation of substitutional Mn atoms in planar structures of ZnO, viz., monolayer [(ZnO)1] and bilayer [(ZnO)2] systems. Due to the 2-D quantum confinement effects, in those Mn-doped (ZnO)1 and (ZnO)2 structures, the antiferromagnetic (AFM) coupling between (nearest neighbor) MnZn impurities have been strengthened when compared with the one in ZnO bulk systems. On the other hand, we find that the magnetic state of these systems can be tuned from AFM to FM by adding holes, which can be supplied by a p-type doping or even photoionization processes. Whereas, upon addition of electrons (n-type doping), the system keeps its AFM configuration.

The adsorption of thiophene on Si(111)–7×7 has been studied with scanning tunneling microscopy (STM) and kinetic Monte Carlo (kMC) modelling. Previous experimental studies of this system clearly demonstrated that thiophene prefers to chemisorb on the faulted half of the 7×7 unit cell. The STM studies reported here concur with this and provide further information about thiophene site preference as a function of coverage. Additionally, an ab initio theoretical investigation of this system demonstrated that the occupancy of available adsorption sites could not be explained using equilibrium binding energies, as these were calculated to be the same for all experimentally identified adsorption sites (≈1.0 eV). To investigate the possibility that site selection is kinetically controlled, a kMC model was developed. This model places the molecule in a mobile precursor state, allowing the molecule to traverse the surface before chemisorbing. The kMC model was found to reproduce the STM data, providing compelling evidence that site occupancy in this system is indeed kinetically controlled at room temperature. Activation energy differences, for each the four unique chemisorption geometries, could be extracted from a fit of the kMC model predictions to the experimental data (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The interface between the (0001) surface of the hexagonal boron nitride (hBN) and the (111) surface of the cubic boron nitride (cBN) is studied through first principles plane-wave pseudopotentials within the density functional theory. Four different structural models for the pseudomorphic growth of the cBN on hBN have been investigated, two with tetrahedral and two with hexagonal arrangements of the atoms at the interface. The interfaces with N-terminated cBN(111) surface are seen to have the lowest formation energies. The studied interface models present a metallic character, with the levels at Fermi energy spatially confined in the interface region. The band offsets show type I band lineups, with large valence and conduction band discontinuities.

We provide a mini review of recent theoretical investigations of nanostructure formation aided by self-organised Bi nanolines on the Si(0 0 1) surface. It is suggested that hydrogen-passivated single-domain Si(0 0 1) produced by the formation of defect-free, hundreds of nm long, and 1.2–1.5 nm wide Bi nanolines provides an appealing template with preferential sites for adsorption of other elements. Based on ab initio pseudopotential calculations it is suggested that using the Bi nanoline template it should be possible to grow the following structures: mixed Ge–Si dimer structures on the Si(0 0 1) terrace between two neighbouring Bi nanolines; small In nanoclusters along the nanoline; and line and cluster structures of Fe atoms with novel electronic and magnetic properties.

First principles density functional calculations have been used to investigate the structural conformations and the electronic properties of biphenyl molecule adsorbed on the clean and on the partially hydrogenated Si(1 0 0) surfaces. The results show that the biphenyl can be adsorbed on several stable conformations on the clean Si(1 0 0) surface and on the Si(1 0 0) surface modified with hydrogens. The biphenyl is chemisorbed on the Si surface with adsorption energies very similar to each other for all conformations. The electronic structure results reveal that the occupied states are quite different for each conformation, while the empty states are very similar to each other, making possible to identify the different conformations only throw occupied states topography. The biphenyl in any conformation on the Si surface does not induce surface conductivity, even when the biphenyls on the Si surface are permitted to bond to each other forming a row of biphenyl molecules.

We have theoretically investigated the atomic structure, magnetic behaviour, and electronic properties of Mn adsorbates on hydrogen passivated self-organised Bi nanolines on the Si(0 0 1)surface. It is found that the most stable geometry for 1 6 monolayer (ML) coverage of Mn is just underneath the first Si(0 0 1) surface layer. The Mn atoms in the optimised configuration are seven-fold coordinated with their neighbouring Si atoms. Total energy calculations suggest that the Mn adsorbates form a degenerate state of ferromagnetic and anti-ferromagnetic lines parallel and adjacent to the self-assembled Bi lines. The density functional band structure calculation within the local-spin density approximation shows that the ferromagnetic system behaves like a metal in both spin channels. On the other side, the anti-ferromagnetic phase exhibits a half-metallic phenomenon with semiconducting character for the majority spin channel and semi-metallic character for the minority spin channel.

The energetic stability and the equilibrium geometry of Ge adsorption on the Si(0 0 1) surface covered with Bi nanolines were examined by ab initio total energy calculations. We find that there is an energetic preference of Ge atoms lying on the Si(0 0 1) terraces, forming Si down –Ge up mixed dimers. Further investigations reveal a repulsive interaction between the mixed dimers and the Bi nanolines, suggesting that the formation of Si down –Ge up dimers can be tailored by the presence of the Bi nanolines.

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 .

We have performed an \it {ab initio} investigation of transition metals (TMs = Mo, Ru, Co, and Pt) embedded in twisted bilayer graphene (tBG) layers. Our total energy results reveal that, triggered by the misalignment between the graphene layers, Mo and Ru atoms may form a quasi-periodic (triangular) array of intercalated atoms. In contrast, the formation of those structures is not expected for the other TMs, Co and Pt atoms. The net magnetic moment (m) of Mo and Ru atoms may be quenched upon intercalation, depending on the stacking region (AA or AB). For instance, we find a magnetic moment of 0.3 $\mu_{\rm B}$ (1.8 $\mu_{\rm B}$) for Ru atoms intercalated between the AA (AB) regions of the stacked twisted layers. Through simulated scanning tunneling microscopy (STM) images, we verify that the presence of intercalated TMs can be identified by the formation of bright (hexagonal) spots lying on the graphene surface., Comment: 8 pages 6 Figures. To appear on PRB

We have performed an ab initio theoretical study of the energetic stability and the electronic properties of pristine and hydrogen-adsorbed grain boundaries (GBs) in silicene. We find that GBs in silicene present lower formation energy when compared with their counterparts in graphene. Removing the inversion symmetry, by applying an external electric field perpendicular to the silicene sheet, we verify the formation of valley-indexed metallic states lying along the GBs, characterizing the quantum valley Hall effect (QVHE). Here, we find the maintenance of the QVHE upon the presence of disordered and asymmetric geometries along the GBs. Those metallic states are suppressed upon the adsorption of H adatoms along the GBs. The H adatoms promote an unbalance on the electronic occupation of the unsaturated $\ensuremath{\pi}$ electrons beside the hydrogenated GB rows, giving rise to (i) a net magnetic moment on the Si atoms along the edge sites of the hydrogenated GBs and (ii) an electronic band structure characterized by spin-polarized valley states protected against backscattering processes.

In this work we have performed an ab initio total energy investigation of the Ge adsorption process on the Si-terminated SiC(0 0 0 1)- ( 3 × 3 ) R 30 ° and (3 × 3) surfaces. We find that Ge adatoms lying on the topmost sites of the ( 3 × 3 ) R 30 ° and (3 × 3) surfaces represent the energetically more stable configurations at the initial stage of the Ge adsorption on the SiC(0 0 0 1) surface. The Si → Ge substitutional adsorption processes have been examined as a function of the Si and Ge chemical potentials. Increasing the Ge coverage, we verify that the formation of Ge wetting layer on the ( 3 × 3 ) R 30 ° surface, and Ge nanocluster on the (3 × 3) surface are the energetically more stable configurations, in accordance with recent experimental findings.

The structural and electronic properties of a metallic single-walled carbon nanotube (CNT), adsorbed on hydrogenated Si(0 0 1) surfaces, have been investigated by first-principles calculations. We find that the electronic properties of the adsorbed CNT can be ruled by the H concentration along the CNT–H/Si(0 0 1) contact region. (i) On the fully hydrogenated Si(0 0 1), the CNT is physisorbed, preserving almost unchanged its metallic character. (ii) Removing half the H atoms along the adsorption site, we find an enhancement on the metallicity of the adsorbed CNT. (iii) When all the H atoms along the adsorption site are removed, the adsorbed CNT becomes semiconducting, exhibiting an energy gap. These results suggest that metallic CNTs adsorbed on H/Si(0 0 1) could be transformed into metal–semiconductor junctions by grading the H concentration along the CNT–surface interface.

A study of the Bi nanoline geometry on Si(0 0 1) has been performed using a combination of ab initio theoretical technique and scanning tunnelling microscopy (STM). Our calculations demonstrate decisively that the recently proposed Haiku geometry is a lower energy configuration than any of the previously proposed line geometries. Furthermore, we have made comparisons between STM constant-current topographs of the lines and Tersoff–Haman STM simulations. Although the Haiku and the Miki geometries both reproduce the main features of the constant-current topographs, the simulated STM images of the Miki geometry have a dark stripe between the dimer rows that does not correspond well with experiment.

Self-organized Bi lines that are only 1.5 nm wide can be grown without kinks or breaks on Si(0 0 1) surfaces to lengths of up to 500 nm. Constant-current topographical images of the lines, obtained with the scanning tunneling microscope, have a striking bias dependence. Although the lines appear darker than the Si terraces at biases below ≈∣1.2∣ V, the contrast reverses at biases above ≈∣1.5∣ V. Between these two ranges the lines and terraces are of comparable brightness. It has been suggested that this bias dependence may be due to the presence of a semiconductor-like energy gap within the line. Using ab initio calculations it is demonstrated that the energy gap is too small to explain the experimentally observed bias dependence. Consequently, at this time, there is no compelling explanation for this phenomenon. An alternative explanation is proposed that arises naturally from calculations of the tunneling current, using the Tersoff–Hamann approximation, and an examination of the electronic structure of the line.

We report first-principles calculations on the adsorption of an armchair (6,6) single-walled carbon nanotube (CNT) on the Si(0 0 1) surface. We study several well-ordered adsorption configurations for the nanotube on the Si surface. Our results show stable geometries between two consecutive Si-dimer rows (the surface trench). The binding energy per tube length for the CNT in the most stable geometry is calculated to be 0.2 eV/� In this geometry, we observe the formation of C–Si chemical bonds. The density of states along the dimer rows for the lowest-energy adsorbed configuration shows an increase in the number of states at the Fermi level. This suggests an enhancement of the nanotube metallic character throughout the contact with the Si surface due to the formation of the C–Si bonds. These properties may lead to consider metallic CNTs as one-dimensional wires on the silicon surface with promising applications for contact and interconnections of future nanoscale electronic devices. � 2004 Elsevier B.V. All rights reserved.

In this work we have performed a theoretical study of the atomic geometry and scanning tunelling microscopy simulation of the GaAs(111)A-Se(2√3x 2√3) surface. The calculated geometry with Se trimers on H 3 sites agrees well with results reported recently from STM and RHEED experiments. Simulated STM images, corresponding to orbital localisation near the fundamental gap, support the experimental observations of bright spots on Se trimers.

In this work we have performed an ab initio investigation of the Bi-covered InAs(1 1 0) surface. We have considered two different structural models viz. ECLS and (1 × 2) missing row model. Our total energy results indicate that the (1 × 2) model is energetically more favourable than the ECLS model, thus, supporting the recent experimental results by Betti et al. [Phys. Rev. B 59 (1999) 15760]. However, the calculated equilibrium atomic geometry indicates some disagreement with respect to the experimental measurements. The “surface electronic topology” of the Bi/InAs(1 1 0) surface was investigated through simulations of scanning tunneling microscopy (STM) images, where we inferred a semimetallic character along the Bi-chains of the (1 × 2) model.

We present an overview of self-organised formation of lines of (semi)metal elements on semiconductor surfaces, together with an ab initio theoretical modelling of such quasi-one-dimensional systems. Results of calculations are presented for the atomic geometry, electronic states, and dispersion of the most tightly bound image state (IS) for a self-organised thin indium chain on the silicon surface forming the Si(1 1 1)–In(4 × 1) nanowire system. It is pointed out that strong anisotropic behaviour of the image state observed in inverse photoemission measurements originates from the anisotropy in the surface corrugation potential. Results are also presented for theoretically simulated STM images of self-organised Bi-lines on the Si(0 0 1) surface, which suggest a low density of states close to the valence band maximum localized on the Bi-lines, supporting a proposed model of a quantum antiwire system.

Based on a first-principles pseudopotential calculation, within the local density approximation, we have studied a series of different structural models for the In-rich InAs(0 0 1) surface, including a very recent model proposed by Kumpf et al. based in X-ray diffraction experiments. Our data suggests that the ζ(4 × 2) model corresponds to the most probable structure, from the energetic point of view. The stability of the ζ(4 × 2) structure is mainly attributed to electrostatic and structural characteristics of the surface. The atomic and electronic structure of this system is analysed in detail. Experimental scanning tunnelling microscopy images are reinterpreted in the lights of our theoretical simulated images for the proposed structure, with a view to explain contradicting experimental and theoretical results.

We have performed an ab initio study of the stability, atomic geometry and electronic structure of the Bi-covered (√3 ×√3) reconstructed Si(111) surface. We find that the energetically stable structure changes from the milkstool model (for 1 monolayer (ML) coverage) to the T4 model (for 1/3 ML coverage), without going through a stable structure for the honeycomb model (2/3 ML coverage). Our theoretical scanning tunnelling microscopy (STM) simulation for the 1 ML coverage reveals the formation of Bi trimers for occupied states, and a honeycomb image for empty states. This result, together with the energetically unstable structure for 2/3 ML coverage, suggests that the experimentally observed STM image in the form of the honeycomb structure does not mean that the minimum energy configuration corresponds to Bi coverage of 2/3 ML, but rather represents current tunnelling into the empty states localized between Bi trimers for the milkstool model with 1 ML coverage.

We have perfomed an ab initio theoretical study of the stability, atomic geometry and electronic structure of the self-organised Bi-lines on the Si(0 0 1) surface. Our results show that the Bi-lines are formed by Bi-dimers parallel to the surrounding Si-dimers, with a missing dimer row between the Bi-dimers. In contrast to a recently proposed model of symmetrically disposed surface Si-dimers (i.e. with no buckling), our total energy calculations indicate that the buckling of the Si-dimers is an exothermic process, reducing the surface total energy by 0.11 eV/dimer. Our theoretically simulated STM results suggest a low density of states close to the valence band maximum, localized on the Bi-lines, supporting a recently proposed model of quantum antiwire systems for Bi-lines on the Si(0 0 1) surface.

Van der Waals (vdW) metal/semiconductor heterostructures have been investigated through first-principles calculations. We have considered the recently synthesized borophene (Mannix et al 2015 Science 350 1513), and the planar boron sheets (S1 and S2) (Feng et al 2016 Nat. Chem. 8 563) as the 2D metal layer, and the transition metal dichalcogenides (TMDCs) MoSe2, and WSe2 as the semiconductor monolayer. We find that the energetic stability of those 2D metal/semiconductor heterojunctions is mostly ruled by the vdW interactions; however, chemical interactions also take place in borophene/TMDC. The electronic charge transfer at the metal/semiconductor interface has been mapped, where we find a a net charge transfer from the TMDCs to the boron sheets. Further electronic structure calculations reveal that the metal/semiconductor interfaces, composed by planar boron sheets S1 and S2, present a p-type Schottky barrier which can be tuned to a p-type ohmic contact by an external electric field.