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The filling process of nanoconduits is an active research topic. In this study, we use molecular dynamics simulations to identify the filling process of water molecules in a multiply connected carbon nanotube (MCCNT). For water permeation, a local change in the channel cross-section affects the water filling of MCCNTs because it may lead to irregularities in the permeation profile. A decrease in hydrogen bonds at the junctions of the structure characterizes the permeability of MCCNTs. In contrast to pristine CNTs, the complex nanochannel exhibits a different imbibition profile due to the energy changes at the junction. Next, we examine the local water density and velocity patterns in MCCNT channels to understand how junction regions affect steady-state water transport. We find that there is congestion and irregularities in steady water flow density and velocity profiles. Through this study, we expect to develop effective channels with more complex geometries for water purification and drug delivery. ISSN:1567-1739 ISSN:1878-1675

Combining chemical reaction and low-temperature annealing allows to form perfectly stoichiometric 1T-VS2, where multiple charge density wave orders are observed due to strong electron–phonon coupling.

Pristine graphene has been considered one of the most promising materials because of its excellent physical and chemical properties. However, various defects in graphene produced during synthesis or fabrication hinder its performance for applications such as electronic devices, transparent electrodes, and spintronic devices. Due to its intrinsic bandgap and nonmagnetic nature, it cannot be used in nanoelectronics or spintronics. Intrinsic and extrinsic defects are ultimately introduced to tailor electronic and magnetic properties and take advantage of their hidden potential. This article emphasizes the current advancement of intrinsic and extrinsic defects in graphene for potential applications. We also discuss the limitations and outlook for such defects in graphene.

We report on the electroluminescent (EL) and electrical characteristics of graphene-inserted ZnO nanorods (NRs)/p-GaN heterojunction diode. In a comparative study, ZnO NRs/p-GaN and ZnO NRs/graphene/p-GaN heterojunctions exhibit white and yellow EL emissions, respectively, at reverse bias (rb) voltages. The different EL colors are results of different dichromatic EL peak intensity ratios between 2.25 and 2.8 eV light emissions which are originated from ZnO and p-GaN sides, respectively. The 2.25 eV EL is predominant in both the heterojunctions, because of recombination by numerous electrons tunneled from p-GaN to ZnO across the thin barriers of the staggered broken gap with a large band offset in ZnO/p-GaN and the van der Waals (vdW) gap formed by graphene insertion at ZnO NRs/p-GaN. However, as for the 2.8 eV EL intensity, ZnO NRs/graphene/p-GaN hardly shows the EL emission, whereas ZnO NRs/p-GaN exhibits the substantially strong EL peak. We discuss that the significantly reduced 2.8 eV EL emission of ZnO NRs/graphene/p-GaN is a result of decreased depletion layer thickness at p-GaN side where the recombination events occur for 2.8 eV EL before the reverse bias-driven tunneling because the insertion of graphene (or vdW gap barrier) inhibits the carrier diffusion whose amount determines the depletion thickness when forming the heterojunctions. This study opens a way of suppressing (or enhancing) the specific EL wavelength for the dichromatic EL-emitting heterojunctions simply by inserting atom-thick vdW layer.

Electrides are ionic compounds in which electrons behave as anions in the interior of a positively charged framework. As a layered electride, ${\mathrm{Gd}}_{2}\mathrm{C}$ receives attention because of its ferromagnetism. Although previous research has focused on the bulk properties of ${\mathrm{Gd}}_{2}\mathrm{C}$, few studies have focused on ultrathin layers or surfaces for two-dimensional (2D) characteristics. Here, we report a first-principles study of the electronic properties of few-layer ${\mathrm{Gd}}_{2}\mathrm{C}$ structures. ${\mathrm{Gd}}_{2}\mathrm{C}$ has a work function of 3.35 eV. When a layered electride is exfoliated, the interstitial layer becomes a surface and may be exposed to the outside. Because the interlayer region has changed to the surface, the properties of the electron gases once located in the interlayer in the past will also change. We found that the surface anionic electrons accounted for about 25% of the number of electrons in the interlayer region in the absence of an external electric field. When we applied an external electric field, the number of surface electrons increased, and the increase was proportional to the square of the field intensity. Since the electronic properties of 2D materials can be understood through scanning tunneling spectroscopy (STS), we also performed the STS simulations. At $\ensuremath{-}0.9\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$, the STS image was blurred because of surface anionic electrons. In contrast to the spin-up electron, an interlayer band of the spin-down electron crossed the Fermi level in the ultrathin ${\mathrm{Gd}}_{2}\mathrm{C}$ layers. Our findings open a possibility that the spin-polarized electronic gas in the few-layer electride could be used for spintronics.

We perform total-energy electronic-structure calculations of a water molecule inside a (7, 7) carbon nanotube/boron nitride nanotube (CNT/BNNT) heterojunction. The van der Waals interaction is also considered in this study. We find that the equilibrium distance between the water molecule and the wall of the CNT (BNNT) is ≈ 3.3 A, and the encapsulation energy is 0.22 eV (0.25 eV). The energy profile along the tube axis exhibits a dramatic change in the vicinity of the heterojunction. A speed change of water flow is expected to occur near the heterojunction. Such information would provide valuable insight in nanostructure design for nanofluidics.

Polyphenylene superhoneycomb network (PSN) and covalent triazine framework (CTF) are experimentally realized periodically porous graphene derivatives. Such ultrathin layers with homogeneously distributed pores of controllable sizes are highly desirable for applications in molecular separations such as water purification. The permeation energy barrier is expected to be a function of not only the pore size, but also the specific permeation trajectory as determined by hydrogen bonding interactions at the water-pore interface. Here, we report a detailed first-principles study of permeation of a single H2O molecule through a monolayer PSN and CTF-0, as well as its diffusion behavior inside a bilayer PSN. The calculated energy barrier of 1.44 eV indicates the infeasibility of using PSN as a water permeation membrane. However, the barrier decreases considerably to 0.94 eV when three C-H pairs at the pore are replaced with N atoms into CTF-0. Inside a bilayer PSN, we find facile interlayer sliding as well as interlayer expansion owing to out-of-plane reorientation of the H2O molecule. In all cases, the functional groups at the pore significantly alter the orientation of the H2O molecule and the corresponding barriers. Such atomistic insights at the porous interface would provide a valuable guidance in advancing rational pore design principles.

Silver-based nanomaterials have been versatile building blocks of various photoassisted energy applications; however, they have demonstrated poor electrochemical catalytic performance and stability, in particular, in acidic environments. Here we report a stable and high-performance electrochemical catalyst of silver telluride (AgTe) for the hydrogen evolution reaction (HER), which was synthesized with a nanoporous structure by an electrochemical synthesis method. X-ray spectroscopy techniques on the nanometer scale and high-resolution transmission electron microscopy revealed an orthorhombic structure of nanoporous AgTe with precise lattice constants. First-principles calculations show that the AgTe surface possesses highly active catalytic sites for the HER with an optimized Gibbs free energy change of hydrogen adsorption (-0.005 eV). Our nanoporous AgTe demonstrates exceptional stability and performance for the HER, an overpotential of 27 mV, and a Tafel slope of 33 mV/dec. As a stable catalyst for hydrogen production, AgTe is comparable to platinum-based catalysts and provides a breakthrough for high-performance electrochemical catalysts.

Using density functional theory calculations, we have studied the edge-functionalization of armchair graphene nanoribbons (AGNRs) with pentagonal-hexagonal edge structures. While the AGNRs with pentagonal-hexagonal edge structures (labeled (5,6)-AGNRs) are metallic, the edge-functionalized (5,6)-AGNRs with substitutional atoms opens a band gap. We find that the band structures of edge-functionalized (5,6)-N-AGNRs by substitution resemble those of defect-free (N-1)-AGNR at the {\Gamma} point, whereas those at the X point show the original ones of the defect-free N-AGNR. The overall electronic structures of edge-functionalized (5,6)-AGNRs depend on the number of electrons, supplied by substitutional atoms, at the edges of functionalized (5,6)-AGNRs., Comment: 12 pages, 5 figures

Detecting dopamine is of great biological importance because the molecule plays many roles in the human body. For instance, the lack of dopamine release is the cause of Parkinson's disease. Although many researchers have carried out experiments on dopamine detection using carbon nanotubes (CNTs), there are only a few theoretical studies on this topic. We study the adsorption properties of dopamine and its derivatives, L-DOPA and dopamine o-quinone, adsorbed on a semiconducting (10, 0) CNT, using density functional theory calculations. Our computational simulations reveal that localized states originating from dopamine o-quinone appear in the bandgap of the (10, 0) CNT, but those originating from dopamine and L-DOPA do not appear in the gap. Therefore, dopamine o-quinone is expected to be detectable using an external electric field but dopamine and L-DOPA should be difficult to detect., Comment: 13 pages, 5 figures

Two-dimensional (2D) heterostructures are interesting candidates for efficient energy storage devices due to their high carrier capacity by reversible intercalation. We employ here density function...

Using density functional calculations, we study atomic and electronic structures of monolayer hexagonal boron nitride (h-BN) and vertical graphene/h-BN heterostructures with substitutional carbon defects. For monolayer h-BN, a carbon atom substituted for a boron or nitrogen atom causes spin polarization. In the presence of an adjacent graphene sheet, however, the spin magnetic moment disappears owing to interlayer charge transfer for a CB defect. Interestingly, two carbon defects in a hexagon of monolayer h-BN do not induce magnetic properties. In contrast, two CB defects (two CN defects) can have spin polarization in the vertical graphene/h-BN heterostructures because of interlayer charge transfer.

We investigated the atomic and electronic structures of metal-functionalized graphene using density functional theory calculations. As dopants, Fe, Co, Ni, and Cu atoms were considered in the form of a single transition metal (TM) atom and a two-atom pair. Our results show that a single Co atom and a Co-Co pair are the most promising for graphene functionalization, compared with Fe, Ni, and Cu atoms. Among the heteronuclear pair structures, the Co-Ni pair is energetically favorable. The density of states analysis shows hybridization between the TM 3d and C or N 2p orbitals. The spin magnetic moments of the heteronuclear TM pairs may disappear or decrease, depending on which type of TM impurityis paired. As in metalloporphyrins, the structure has a lower formation energy when four nitrogen atoms are replaced as neighbors of the transition metal atom. The spin magnetic moment is found to be in the order Fe > Co > Ni > Cu, which correlates with the d-orbital filling of the metal atom. We believe that our work will shed light on the design of spintronic devices based on TM-functionalized graphene.

Recently, two-dimensional van der Waals materials such as graphene and hexagonal boron nitride have attracted interest as research topics. The two-dimensional material of polyphenylene superhoneycomb network (PSN) is similarly interesting because it is a type of periodic porous graphene. In this paper, we report a first-principles study of the geometric and electronic properties of vertical heterostructures comprising graphene and PSN. AA, AA′, and AB stacking configurations of a graphene sheet on a PSN sheet produce band gaps of 63, 16, and 3 meV, respectively. We also determine the relationships between the band gap and the interlayer distance between the graphene and PSN sheets. Finally, we present computationally simulated scanning tunneling microscopy images, which indicate the local electronic structures of the surfaces of the graphene and PSN sides.

Direct observation of grains and boundaries is a vital factor in altering the electrical and optoelectronic properties of transition metal dichalcogenides (TMDs), that is, MoSe2 and WSe2. Here, we report visualization of grains and boundaries of chemical vapor deposition grown MoSe2 and WSe2 on silicon, using optical birefringence of two-dimensional layer covered with nematic liquid crystal (LC). An in-depth study was performed to determine the alignment orientation of LC molecules and their correlation with other grains. Interestingly, we found that alignment of liquid crystal has discrete preferential orientations. From computational simulations, higher adsorption energy for the armchair direction was found to force LC molecules to align on it, compared to that of the zigzag direction. We believe that these TMDs with three-fold symmetric alignment could be utilized for display applications.

Water confinement inside a carbon nanotube (CNT) has been one of the most exciting subjects of both experimental and theoretical interest. Most of the previous studies, however, considered CNT structures with simple cylindrical shapes. In this paper, we report a classical molecular dynamics study of the equilibrium structural arrangement of water molecules confined in a multiply connected carbon nanotube (MCCNT) containing two Y-junctions. We investigate the structural arrangement of the water molecules in the MCCNT in terms of the density of water molecules and the average number of hydrogen bonds per water molecule. Our results show that the structural rearrangement of the H2O molecules takes place several angstroms ahead of the Y-junction, rather than only at the CNT junction itself. This phenomenon arises because it is difficult to match the boundary condition for hydrogen bonding in the region where two different hydrogen-bonded structures are interconnected with each other., Comment: 7 pages, 5 figures, 2 tables, and supporting information

Much research has been done on reliable and low-cost electrocatalysts for hydrogen generation by water splitting. In this study, we synthesized thin films of silver selenide (Ag2Se) using a simple thermal evaporation route and demonstrated their electrocatalytic hydrogen evolution reaction (HER) activity. The Ag2Se catalysts show improved electrochemical surface area and good HER electrocatalytic behavior (367 mV overpotential @ 10 mA&middot, cm&minus, 2, exchange current density: ~1.02 &times, 10&minus, 3 mA&middot, 2, and Tafel slope: 53 mV&middot, dec&minus, 1) in an acidic medium). The reliability was checked in 0.5 M sulfuric acid over 20 h. Our first-principles calculations show the optimal energy of hydrogen adsorption, which is consistent with experimental results. The works could be further extended for finding a new catalyst by associating the selenide, sulfide or telluride-based materials without complex catalyst synthesis procedures.

Using first-principles calculations, we investigate an atomic impurity at the interface of a van der Waals heterostructure (vdW heterostructure) consisting of a zigzag graphene nanoribbon (ZGNR) and a hexagonal boron nitride (h-BN) sheet. To find effects of atomic intercalation on geometrical and electronic properties of the ZGNR on the h-BN sheet, various types of impurity atoms are considered. The embedded atoms are initially placed at the edge or the middle of the ZGNR located on the h-BN sheet. Our results demonstrate that most of the impurity atoms are more stable at the edge than at the middle in all cases we consider. Especially, a nickel atom has the smallest energy difference (~0.15 eV) between the two embedding positions, which means that the Ni atom is relatively easy to intercalate in the structure. Finally, we discuss magnetic properties for the vdW heterostructure with an intercalated atom.

First-principle calculations were conducted on porous graphene structures to compare the properties of a polyphenylene super-honeycomb network (PSN) with those of an oxygen-terminated PSN (O-PSN) obtained from oxygen substitution. The band structures, densities of states, and simulated STM images were presented along with the infrared and Raman spectra, which could distinguish the properties of O-PSN from PSN. Moreover, the work functions of PSN and O-PSN were found to be ~1 eV higher than that of perfect graphene. Furthermore, our results showed that the two oxygen atoms replacing the hydrogen atoms tend to be separated as far as possible at the PSN pores.

The graphene grain boundary (GGB), line defect, has a marked influence on an electrical properties and mechanical strength of graphene. Observation of the grain size and defect distribution is important for tuning the properties of graphene. Here we show imaging GGBs with rhodamine B (RB) by fluorescence microscopy (FM), and demonstrate the selective adsorption of RB molecules on the GGBs after a pre-annealing process and RB treatment. The hill structure of oxidized Cu below GGB is formed by a pre-annealing process, and induces RB molecules to selectively physisorb on GGBs due to the interplay between graphene and oxidized Cu, which is directly confirmed by kelvin probe force microscopy (KPFM) and density function theory (DFT) calculation. The selectively adsorbed RB was completely removed by acetone rinsing without disrupting the graphene sheets. This simple observation method provides an efficient and convenient means of evaluating the quality and reliability of graphene as well as other two-dimensional materials, such as WS2, WSe2, MoS2, MoSe2 and BN.

Currently, the catalysts performance for the oxygen reduction reaction (ORR) is limited by two major factors, their price and the scaling relationship, i.e. constant offset, between the binding energy of reactants and reaction intermediates. Here we show that both limitations may be eased by using nanoparticle catalysts composed of a core of earth-abundant Mn and Pt shell of two atomic layers. The unusual atomic-structure of Mn core renders the distribution of coordination numbers and related atomic-level stresses on the Pt shell very broad in comparison to pure Pt particles. This increases the chances of nearby sites on the shell to affect each other’s activity such that the dissociative absorption of molecular oxygen is facilitated without correspondingly increasing the binding strength of reaction intermediates, thereby promoting the ORR kinetics. Our findings introduce the concept of structural flexibility of the 3d-metal core and 5d-Pt shell as a strategy for catalysis performance.

Defect engineering in wide-gap semiconductors is important in controlling the performance of single-photon emitter devices. The effective incorporation of defects depends strongly on the ability to control their formation and location, as well as to mitigate attendant damage to the material. In this study, we combine density functional theory, molecular dyamics (MD), and kinetic Monte Carlo (KMC) simulations to study the energetics and kinetics of the silicon monovacancy ${V}_{\mathrm{Si}}$ and related defects in $4H$-SiC. We obtain the defect formation energy for ${V}_{\mathrm{Si}}$ in various charge states and use MD simulations to model the ion implantation process for creating defects. We also study the effects of high-temperature annealing on defect position and stability using KMC and analytical models. Using a larger (480-atom) supercell than previous studies, we obtain the temperature-dependent diffusivity of ${V}_{\mathrm{Si}}$ in various charge states and find significantly lower barriers to diffusion than previous estimates. In addition, we examine the recombination with interstitial Si and conversion of ${V}_{\mathrm{Si}}$ into ${\mathrm{C}}_{\mathrm{Si}}{V}_{\mathrm{C}}$ during annealing and propose methods for using strain to reduce changes in defect concentrations. Our results provide guidance for experimental efforts to control the position and density of ${V}_{\mathrm{Si}}$ defects within devices, helping to realize their potential as solid-state qubits.

Nematic liquid crystal (LC) molecules adsorbed on two dimensional materials are aligned along the crystal directions of the hexagonal lattice. It was demonstrated that short electric pulses can reorient the aligned LC molecules in the preferred armchair direction of hexagonal boron nitride (h-BN). Several states with a variety of colors were obtained by changing the direction and strength of the electric pulses. The ab initio calculations based on density functional theory was carried out to determine the favorable adsorption configurations of the LC molecules on the h-BN surface. A non-volatile display, in which pixel resolution can be determined by grains of hexagonal surface, is proposed, which can offer a pathway towards dynamic high-quality pixels with low power consumption, and could define a new paradigm for all non-volatile display applications.

Phenol, phenyl azide, and phenylnitrene are hazardous organic molecules; therefore, the fabrication of sensors or filters with high sorption capabilities for the chemicals is necessary. Considering van der Waals interaction, we perform first-principles density functional theory calculations to investigate the adsorption properties of the hazardous molecules on graphene. For parallel stacking configurations, AB stacking is slightly more favorable than AA stacking for all the adsorbates that we considered. We find that phenyl azide has a higher adsorption energy than phenol. Phenylnitrene forms covalent bonds with graphene in oblique stacking structures, resulting in a bandgap opening in graphene.

Graphene, when deposited on a supporting substrate with a step edge, may be deformed in the presence of the step edges of the substrate. In this study, we have investigated a spatial variation in the local electronic structure near the step region, by performing first-principles calculations for carpetlike armchair graphene nanoribbons (C-AGNR) and two-dimensional periodic carpetlike graphene sheets (PCGS). Our results indicate no practical difference in the local density of states (LDOS) between those of flat and step regions. Interestingly, however, the PCGS shows a remarkable variation in the LDOS with an external electric field (E-field). Furthermore, we also discuss the dependence of the direction and the magnitude of the applied E-field on the spatial variation in the LDOS.

We report a first-principles study on tuning the electronic band gap of graphyne, consisting of two-dimensional sp–sp2 hybrid carbon atoms, by chemical functionalization. Halogen atoms form a sp2 h...

Density functional theory is used to study dimerization of metal-encapsulated gold nanoclusters M@Au12 (M = W, Mo) with Ih or Oh symmetry, and their structural and electronic properties. To determine the most stable dimer structure in each case, various configurations are considered. We find that during dimerization, gold atoms near the interface tend to form inter-cluster triangular bonds, which stabilize two monomer clusters by about 3.3–3.5 eV. The dimerization along a specific axis selected as the z axis causes the symmetry reduction of each M@Au12 cluster resulting in the modification of electronic structures. It is found that all the stable dimers exhibit a much smaller HOMO–LUMO gap than those of their comprising monomers. Such a gap decrease is mainly attributed to the dz2 orbital splitting of the central atoms owing to dimerization. We also calculate the vibrational modes and the corresponding IR-active spectra, which are distinguishable for different dimer configurations. In addition, we find that the IR-active modes of the Oh-based dimer structures appear to be red-shifted in comparison to those of Ih-based ones. Thus, the IR spectra may be utilized experimentally to discriminate dimer configurations with different central metal atoms and/or dissimilar structural symmetries.

Using first-principles calculations, we investigate the electronic structures and binding properties of nicotine and caffeine adsorbed on single-walled carbon nanotubes to determine whether CNTs are appropriate for filtering or sensing nicotine and caffeine molecules. We find that caffeine adsorbs more strongly than nicotine. The different binding characteristics are discussed by analyzing the modification of the electronic structure of the molecule-adsorbed CNTs. We also calculate the quantum conductance of the CNTs in the presence of nicotine or caffeine adsorbates and demonstrate that the influence of caffeine is stronger than nicotine on the conductance of the host CNT.

Unzipping carbon nanotubes has recently attracted interest as a promising route to synthesizing semiconducting graphene nanoribbons. Here, the band structures of O- and F-driven partially unzipped armchair carbon nanotubes (PUCNTs) are computed using the ab initio pseudopotential method. Although the model structures exhibit a similar pinhole with a length of ∼1 nm along the tube axis, the band structures differ distinctly. The O-driven PUCNT has many localized states in the valence band arising mainly from the O 2p orbitals, and preserves metallic properties. In contrast, the F-driven PUCNT shows properties of a semiconductor with a bandgap of ∼0:15 eV, and its localized states occur in the conduction band. When the unzipping process continues further, the O-driven PUCNT also shows the semiconducting behavior. & 2013 Published by Elsevier Ltd.

Securing a semiconducting bandgap is essential for applying graphene layers in switching devices. Theoretical studies have suggested a created bulk bandgap in a graphene layer by introducing an asymmetry between the A and B sub-lattice sites. A recent transport measurement demonstrated the presence of a bandgap in a graphene layer where the asymmetry was introduced by placing a graphene layer on a hexagonal boron nitride (h-BN) substrate. Similar bandgap has been observed in graphene layers on metal substrates by local probe measurements; however, this phenomenon has not been observed in graphene layers on a near-insulating substrate. Here, we present bulk bandgap-like features in a graphene layer epitaxially grown on an h-BN substrate using scanning tunneling spectroscopy. We observed edge states at zigzag edges, edge resonances at armchair edges, and bandgap-like features in the bulk.

Using ab initio density functional calculations, we predict subatomic-resolution atomic force microscopy (AFM) and scanning tunneling microscopy (STM) images of vertical heterostructures of graphene/hexagonal boron nitride (h-BN) with an intercalated metal atom (Li, K, Cr, Mn, Co, or Cu), and study the effects of the extrinsic metal defect on the interfacial coupling. We find that the structural deformation of the graphene/h-BN layer caused by the metal defect strongly affects the AFM images, whereas orbital hybridization between the metal defect and the graphene/h-BN layer characterizes the STM images.

Rhombohedral multilayer graphene is a physical realization of the chiral two-dimensional electron gas that can host zero-line modes (ZLMs), also known as kink states, when the local ap opened by inversion symmetry breaking potential changes sign in real space. Here we study how the variations in the local stacking coordination of multilayer graphene affects the formation of the ZLMs. Our analysis indicates that the valley Hall effect develops whenever an interlayer potential difference is able to open up a band gap in stacking faulted multilayer graphene, and that ZLMs can appear at the domain walls separating two distinct regions with imperfect rhombohedral stacking configurations. Based on a tight-binding formulation with distant hopping terms between carbon atoms, we first show that topologically distinct domains characterized by the valley Chern number are separated by a metallic region connecting AA and AA$'$ stacking line in the layer translation vector space. We find that gapless states appear at the interface between the two stacking faulted domains with different layer translation or with opposite perpendicular electric field if their valley Chern numbers are different., 10 pages, 11 figures