Search

Highlights Dopant-tunable transparent conductive oxide (≤ 50 nm) fabricated via electric-field-driven metal implantation (m-TCOs; m= Ni, Ag, and Cu) is demonstrated.The m-TCOs exhibit ultrahigh transparency, low sheet resistance, and broad work function tunability, leading to outstanding performance in various optoelectronic devices.The work function change is attributed to the interstitial metal atoms that provide the empty d-orbital, resulting in the shift of the Fermi level. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-021-00735-y., Ultrathin film-based transparent conductive oxides (TCOs) with a broad work function (WF) tunability are highly demanded for efficient energy conversion devices. However, reducing the film thickness below 50 nm is limited due to rapidly increasing resistance; furthermore, introducing dopants into TCOs such as indium tin oxide (ITO) to reduce the resistance decreases the transparency due to a trade-off between the two quantities. Herein, we demonstrate dopant-tunable ultrathin (≤ 50 nm) TCOs fabricated via electric field-driven metal implantation (m-TCOs; m = Ni, Ag, and Cu) without compromising their innate electrical and optical properties. The m-TCOs exhibit a broad WF variation (0.97 eV), high transmittance in the UV to visible range (89–93% at 365 nm), and low sheet resistance (30–60 Ω cm−2). Experimental and theoretical analyses show that interstitial metal atoms mainly affect the change in the WF without substantial losses in optical transparency. The m-ITOs are employed as anode or cathode electrodes for organic light-emitting diodes (LEDs), inorganic UV LEDs, and organic photovoltaics for their universal use, leading to outstanding performances, even without hole injection layer for OLED through the WF-tailored Ni-ITO. These results verify the proposed m-TCOs enable effective carrier transport and light extraction beyond the limits of traditional TCOs. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-021-00735-y.

Silicon is a semiconductor and widely used as the key element for modern electronic devices. Various metallic superconducting phases have been reported, but most retain their crystal structure at high pressures. Thus, it remains a challenge to search for potential superconducting Si allotropes. In this article, the authors propose novel metallic Si allotropes that meet the conditions for dynamic, mechanical, and thermal stability at ambient pressure through machine learning and first-principles electronic structure calculations. The new allotropes are superconductors and even exhibit a nontrivial band topology, providing a promising platform for realizing a topological superconducting state in all-Si systems.

Silicene has a two-dimensional buckled honeycomb lattice and is chemically reactive because of its mixed sp2–sp3 bonding character unlike graphene. Despite recent advances in epitaxial growth, it r...

Herein, we report a novel strategy to promote electrochemical oxygen evolution reaction (OER) on cobalt (Co) surface by coupling Co to molybdenum carbide (Mo2C). Chemically coupled Co and Mo2C nanoparticles were synthesized through a simple heat treatment of the mixture containing Co and Mo precursors and graphitic carbon nitride (g-C3N4). Transmission electron microscopy (TEM) images obviously showed that Co and Mo2C nanoparticles were coupled at Co/Mo2C interfaces. X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculation results revealed that electrons were transferred from Co to Mo2C nanoparticles across the interfaces. The electron transfer makes the Co surface more electrophilic by d-band center of Co upshift, leading to an increase in OH− affinity. As a result, the Co nanoparticles coupled with Mo2C have OER-favorable Co-oxo and Co-hydroxo configuration within their oxidized surfaces, and hence, can accelerate the overall OER than bare Co nanoparticles. This work demonstrates that the Co nanoparticles chemically coupled to Mo2C exhibited excellent OER activity and stability in an alkaline electrolyte and suggests a promising way to design an active OER catalyst.

Lithium was substituted on the alkali site of an O3-type layered structure as cathode material for sodium-ion batteries (SIBs). Role of Li in Na intercalation and high performance is reported for the first time. The behavior of Na during intercalation is a critical factor that determines electrochemical performance. Although transition-metal oxides with layered structures have been studied extensively for SIBs, the focus has been on substitution at transition metal sites rather than at the alkali site. Here, substitution of Li at the alkali site of Nax[FeyMn1−y]O2 (0 ≤ x, y ≤ 1) layered materials is reported for the first time. The substituted element at the alkali site can directly interfere with the behavior of Na during intercalation. In situ XRD, synchrotron powder XRD, neutron powder diffraction, ex situ XANES, and DFT calculation revealed that Li at alkali sites stabilized the layered structure during the electrochemical reaction and assisted Na intercalation by lowering the energy barrier for Na-hopping. Contrary to common understanding, Li substituted material showed an improved cyclic and rate performance in spite of the smaller interlayer spacing of the O3-type layered structure. The result provides new insight into the role of a substituted element at the alkali site and shows that the Li at the alkali site is a key factor for achieving high performance of a layered cathode material.

Doping two-dimensional (2D) semiconductors beyond their degenerate levels provides the opportunity to investigate extreme carrier density-driven superconductivity and phase transition in 2D systems. Chemical functionalization and the ionic gating have achieved the high doping density, but their effective ranges have been limited to ∼1 nm, which restricts the use of highly doped 2D semiconductors. Here, we report on electron diffusion from the 2D electride [Ca2N]+·e– to MoTe2 over a distance of 100 nm from the contact interface, generating an electron doping density higher than 1.6 × 1014 cm–2 and a lattice symmetry change of MoTe2 as a consequence of the extreme doping. The long-range lattice symmetry change, suggesting a length scale surpassing the depletion width of conventional metal–semiconductor junctions, was a consequence of the low work function (2.6 eV) with highly mobile anionic electron layers of [Ca2N]+·e–. The combination of 2D electrides and layered materials yields a novel material design i...

Here we report a method of finding multiple crystal structures similar to the known crystal structures of materials on database through machine learning. The radial distribution function is used to represent the general characteristics of the known crystal structures, and then the variational autoencoder is employed to generate a set of representative crystal replicas defined in a two-dimensional optimal continuous space. For given chemical compositions and crystal volume, we generate random crystal structures using constraints for crystal symmetry and atomic positions and directly compare their radial distribution functions with those of the known and/or replicated crystals. For selected crystal structures, energy minimization is subsequently performed through first-principles electronic structure calculations. This approach enables us to predict a set of new low-energy crystal structures using only the information on the radial distribution functions of the known structures.

Transition metal compounds are known to be tricky for ab initio calculations mainly because of the strongly localized nature of transition metal d electrons. Nonetheless, the bulk of current theoretical studies of MXenes (transition metal carbide or nitride) relies on the density functional theory using a semilocal PBE functional, whose notorious self-interaction error grossly misrepresents electronic and magnetic properties of many well-known transition metal compounds. Although several studies have adopted Hubbard-U corrections to MXenes, the lack of systematic guidelines on how to determine the U parameters has led to a cornucopia of different results. To shed some light on the reliability of different methods (different functionals or different U parameters), we performed ab initio calculations for 22 carbide MXenes (M2CT2 with M= Sc, Y, La, Ti, Zr, Hf, V, Nb, Ta, Mo, W, and T= O, F) using density functional theory and four different methods: PBE, SCAN, HSE06, and PBE+U. In addition to trivial improvement of electronic structures of MXenes by SCAN, HSE06, and PBE+U, we found new ground-state structures for two MXenes (Hf2CF2 and Ta2CF2) and magnetic states for three MXenes (V2CO2, V2CF2, and Mo2CF2), which have not previously been reported. In addition, we found that SCAN, HSE06, and PBE+U dramatically improve the dynamical stability of V2CO2 and Mo2CF2 compared with PBE. This paper offers an overview of a broad range of MXenes with a systematic verification of various methods.

Owing to the indirect band gap nature, Ge exhibits poor optical properties, limiting its usage for optical devices. However, since the direct band gap of Ge is only higher by 0.14 eV than the indirect band gap, band gap engineering has drawn much attention to realize the direct band gap. Here, we report a strategy to design the direct band gap in Ge/Sn core–shell nanowires (NWs), based on first-principles calculations. For [111]-oriented NWs, we show that the direct band gaps can be tuned by controlling the diameter and the core-to-shell ratio. We find that the intrinsic strain induced by the lattice mismatch between Ge and Sn drives an indirect-to-direct band gap transition. Even for Ge/Sn core–shell NWs with intrinsically indirect band gaps, the direct band gaps can be achieved by applying an external tensile strain lower than the critical values for pure Ge NWs and bulk Ge. The optical transitions of the direct band gaps are all dipole-allowed, suggesting that [111]-oriented Ge/Sn core–shell NWs are pr...

Lately, the so-called inverse method of materials design has drawn much attention, where specific material properties are initially assigned and target materials are subsequently searched for. Although this method has been successful for some problems, the success of designing complex crystal structures containing many atoms is often limited by the efficiency of the search method utilized. Here we combine the global optimization method of conformational space annealing (CSA) with first-principles quantum calculations and report a new scheme named AMADEUS (Ab initio MAterials DEsign Using cSa). We demonstrate the utility of AMADEUS through the discovery of direct band gap Si crystals. The newly-designed direct gap Si allotropes show excellent optical properties and the spectroscopic limited maximum efficiencies comparable to those of best-known non-silicon photovoltaic materials. Our scheme not only provides a new perspective for the inverse problem of materials design but also may serve as a new tool for the computational design of a wide range of materials.

For practical applications of two-dimensional topological insulators, large band gaps and Dirac states within the band gap are desirable because they allow for device operation at room temperature and quantum transport without dissipation. Based on first-principles density functional calculations, we report the tunability of the electronic structure by strain engineering in quasi-one-dimensional nanoribbons of transition metal dichalcogenides with a 1T' structure, MoX2 with X = (S, Se, Te). We find that both the band gaps and Dirac points in 1T'-MoX2 can be engineered by applying an external strain, thereby leading to a single Dirac cone within the bulk band gap. Considering the gap size and the location of the Dirac point, we suggest that, among 1T'-MoX2 nanoribbons, MoSe2 is the most suitable candidate for quantum spin Hall (QSH) devices.

Monoclinic transition metal dichalcogenides offer the possibility of topological quantum devices, but they are difficult to realize. One route may be through switching from the common hexagonal phase, for which a method is now shown. Layered transition metal dichalcogenides (TMDs) have attracted renewed interest owing to their potential use as two-dimensional components in next-generation devices1,2. Although group 6 TMDs, such as MX2 with M = (Mo, W) and X = (S, Se, Te), can exist in several polymorphs3, most studies have been conducted with the semiconducting hexagonal (2H) phase as other polymorphs often exhibit inhomogeneous formation1,4,5,6. Here, we report a reversible structural phase transition between the hexagonal and stable monoclinic (distorted octahedral or 1T′) phases in bulk single-crystalline MoTe2. Furthermore, an electronic phase transition from semimetallic to semiconducting is shown as 1T′-MoTe2 crystals go from bulk to few-layered. Bulk 1T′-MoTe2 crystals exhibit a maximum carrier mobility of 4,000 cm2 V−1 s−1 and a giant magnetoresistance of 16,000% in a magnetic field of 14 T at 1.8 K. In the few-layered form, 1T′-MoTe2 exhibits a bandgap opening of up to 60 meV, which our density functional theory calculations identify as arising from strong interband spin–orbit coupling. We further clarify that the Peierls distortion is a key mechanism to stabilize the monoclinic structure. This class of semiconducting MoTe2 unlocks the possibility of topological quantum devices based on non-trivial Z2-band-topology quantum spin Hall insulators in monoclinic TMDs (ref. 7).

Black phosphorus is an emerging material in nanoelectronics and nanophotonics due to its high carrier mobility and anisotropic in-plane properties. In addition, the polymorphism of phosphorus leads to numerous searches for new allotropes that are more attractive than black phosphorus in a variety of applications. On the basis of ab initio evolutionary crystal structure search computation, we report the prediction of a phosphorus allotrope called green phosphorus (λ-P), which exhibits direct band gaps ranging from 0.7 to 2.4 eV and strong anisotropy in optical and transport properties. Free-energy calculations show that a single-layer form, termed green phosphorene, is energetically more stable than blue phosphorene, and a phase transition from black to green phosphorene can occur at temperatures above 87 K. We suggest that green phosphorene can be synthesized on corrugated metal surfaces rather than clean surfaces due to its buckled structure, providing guidance to achieving epitaxial growth.

Diamond Si is a semiconductor with an indirect band gap that is the basis of modern semiconductor technology. Although many metastable forms of Si were observed using diamond anvil cells for compression and chemical precursors for synthesis, no metallic phase at ambient conditions has been reported thus far. Here we report the prediction of pure metallic Si allotropes with open channels at ambient pressure, unlike a cubic diamond structure in covalent bonding networks. The metallic phase termed P6/m-Si_{6} can be obtained by removing Na after pressure release from a novel Na-Si clathrate called P6/m-NaSi_{6}, which is predicted through first-principles study at high pressure. We identify that both P6/m-NaSi_{6} and P6/m-Si_{6} are stable and superconducting with the critical temperatures of about 13 and 12 K at ambient pressure, respectively. The prediction of new Na-Si and Si clathrate structures presents the possibility of exploring new exotic allotropes useful for Si-based devices.

Based on the first-principles evolutionary materials design, we report a stable boron Kagome lattice composed of triangles in triangles on a two-dimensional sheet. The Kagome lattice can be synthesized on a silver substrate, with selecting Mg atoms as guest atoms. While the isolated Kagome lattice is slightly twisted without strain, it turns into an ideal triangular Kagome lattice under tensile strain. In the triangular Kagome lattice, we find the exotic electronic properties, such as topologically non-trivial flat band near the Fermi energy and half-metallic ferromagnetism, and predict the quantum anomalous Hall effect in the presence of spin-orbit coupling.

We investigate the migration pathway and barrier for B diffusion at SiGe/SiO 2 interface through first-principles density functional calculations. Similar to the diffusion mechanism reported for Si/SiO 2 interface, a substitutional B, which initially forms a B-self-interstitial complex in SiGe, diffuses to the interface and then to the oxide in form of an interstitial B. At the defect-free interface, where bridging O atoms are inserted to remove interface dangling bonds, it is energetically more favorable for the interstitial B to intervene in the Ge–O bridge bond rather than the Si–O bridge bond at the interface. As a result of the B intervention, interface Ge atoms significantly enhance the stability of B-related defects in the interface region and thereby act as traps for B dopants. At the interface with the Ge–O bridge bond, the overall migration barrier for B diffusion from SiGe to SiO 2 is estimated to be about 3.7 eV, much higher than the reported value of about 2.1 eV at Si/SiO 2 interface. Our results provide a clue to understanding the experimental observation that B segregation toward the oxide is suppressed in SiGe/SiO 2 interface.

Amorphous oxide semiconductors are promising channel materials for transparent, flexible thin-film transistors, but actual devices suffer from instability in threshold voltage, due to structural defects. First-principles calculations reveal that undercoordinated cations in nonstoichiometric samples lead to localized subgap states that are the root of the instability. These results establish a physical understanding of the precise origin of these states, and offer guidance to control such defects for successful applications.

Despite considerable interest in layered transition metal dichalcogenides (TMDs), such as $M{X}_{2}$ with $M=(\mathrm{Mo},\mathrm{W})$ and $X=(\mathrm{S},\mathrm{Se},\mathrm{Te})$, the physical origin of their topological nature is still poorly understood. In the conventional view of topological phase transition (TPT), the nontrivial topology of electron bands in TMDs is caused by the band inversion between metal $d$- and chalcogen $p$-orbital bands where the former is pulled down below the latter. Here, we show that, in TMDs, the TPT is entirely different from the conventional speculation. In particular, $M{\mathrm{S}}_{2}$ and $M\mathrm{S}{\mathrm{e}}_{2}$ exhibits the opposite behavior of TPT such that the chalcogen $p$-orbital band moves down below the metal $d$-orbital band. More interestingly, in $M\mathrm{T}{\mathrm{e}}_{2}$, the band inversion occurs between the metal $d$-orbital bands. Our findings cast doubts on the common view of TPT and provide clear guidelines for understanding the topological nature in new topological materials to be discovered.

We report pure carbon-based superlattices that exhibit direct band gaps and excellent optical absorption and emission properties at the threshold energy. The structures are nearly identical to that of cubic diamond except that defective layers characterized by five- and seven-membered rings are intercalated in the diamond lattice. The direct band gaps lie in the range of 5.6~5.9 eV, corresponding to wavelengths of 210~221 nm. The dipole matrix elements of direct optical transition are comparable to that of GaN, suggesting that the superlattices are promising materials as an efficient deep ultraviolet light emitter. Molecular dynamics simulations show that the superlattices are thermally stable even at a high temperature of 2000 K. We provide a possible route to the synthesis of superlattices through wafer bonding of diamond (100) surfaces., Comment: 9 pages, 11 figures

We perform first-principles density functional calculations to study the electronic structure of Ni/HfO 2 and Ni/SiO 2 interfaces and the effect of O-vacancy (V O ) defects on the Schottky barrier height and the effective work function. We generate two interface models in which Ni is placed on O-terminated HfO 2 (1 0 0) and α-quartz (1 0 0) surfaces. As the concentration of V O defects at the interface increases, the p-type Schottky barrier height tends to increase in the Ni/HfO 2 interface, due to the reduction of interface dipoles, whereas it is less affected in the Ni/SiO 2 interface.

We investigate the stability of boron dopants near the interface between crystalline Si and amorphous SiO2 through first-principles density functional calculations. An interstitial B is found to be more stable in amorphous SiO2 than in Si, so that B dopants tend to segregate to the interface. When defects exist in amorphous SiO2, the stability of B is greatly enhanced, especially around Si floating bond defects, while it is not significantly affected near Si–Si dimers, which are formed by O-vacancy defects.

We perform first-principles density functional calculations to investigate the stability of various B-related defects near Si/SiO"2 interface, and propose a mechanism for boron segregation to the interface. In Si, a substitutional B is energetically very stable and does not diffuse into the oxide in the absence of Si self-interstitials. Under nonequilibrium conditions, where self-interstitials are abundant, B dopants diffuse via the formation of a defect pair which consists of a B dopant and a self-interstitial. It is found that diffusing B dopants further segregate toward the oxide near the interface in form of positively charged interstitials, resulting in the suppression of activated dopants.

We investigate the effects of vacancy defects on the electronic and transport properties of carbon nanotubes through density functional calculations. In both cases, where vacancies aggregate into larger clusters and are disordered, conductance changes from metallic to insulating regime, while their origins are different. For small vacancy clusters, the suppression of conductance is led by the defect states associated with π-topological and σ-dangling bond defects, while the local gap opening plays a role for large vacancy clusters. In disordered tubes with various types of vacancy defects, conductance decreases exponentially due to the Anderson localization. The localization length not only depends on the type of vacancy defects but also the tube chirality.

We perform density-functional calculations to investigate the defect properties of group-V elements (P, As, Sb) in Si, Ge, and Si1–xGex alloy nanowires. In all nanowires, P dopants have a tendency to form donor-pair defects, which consist of two dopants at the first nearest distance, when the wire diameter decreases below a critical value. The quantum confinement and chemical bonding effects play a role in stabilizing donor-pair defects against isolated substitutional donors. As the donor-pair defect has a deep level in the band gap, which is electrically inactive, the doping efficiency is reduced in small-diameter nanowires. As the Ge concentration increases, the formation of the donor-pair defect becomes more favorable, lowering further the doping efficiency. On the other hand, with As and Sb dopants, which have the larger atomic radii, the formation of donor-pair defects is suppressed due to large strain energies. Uniaxial compressive strain also reduces the stability of donor-pair defects and thereby ...

We investigate the magnetic properties of Mn-doped ZnO nanowires (NWs) using the local spin density approximation (LSDA) and the LSDA+U approach, where U represents the on-site Coulomb interaction. In carrier-free (Zn,Mn)O NWs, the majority Mn ta states are fully occupied, leading to an antiferromagnetic ground state. We examine the effect of additional p-type doping on the ferromagnetism by considering surface O dangling bonds, Zn vacancies, and N impurities. For all cases, localized hole carriers are generated in the majority ta states and promote a ferromagnetic ordering via double exchange interactions, similar to the trend of bulk (Zn,Mn)O. The ferromagnetic coupling tends to increase with increasing of the hole carrier density.

The ability to create graphene nanoribbons with atomically sharp edges is important for various graphene applications because these edges significantly influence the overall electronic properties and support unique magnetic edge states. The discovery of graphene self-folding induced by traveling wave excitation through atomic force microscope scanning under a normal force of less than 15 nN is reported. Most remarkably, the crystallographic direction of self-folding may be either along a chosen direction defined by the scan line or along the zigzag or armchair direction in the presence of a pre-existing crack in the vicinity. The crystalline direction of the atomically sharp edge is confirmed via careful lateral force microscopy measurements. Multilayer nanoribbons with lateral dimensions of a few tens of nanometers are realized on the same graphene sheet with different folding types (e.g., z-type or double parallel). Molecular dynamics simulations reveal the folding dynamics and suggest a monotonic increase of the folded area with the applied normal force. This method may be extended to other 2D van der Waals materials and lead to nanostructures that exhibit novel edge properties without the chemical instability that typically hinders applications of etched or patterned graphene nanostructures.

The origin of the ballistic hole gas recently observed in Ge/Si core-shell nanowires has not been clearly resolved yet, although it is thought to be the result of the band offset at the radial interface. Here we perform spin-polarized density-functional calculations to investigate the defect levels of surface dangling bonds and Au impurities in the Si shell. Without any doping strategy, we find that Si dangling bond and substitutional Au defects behave as charge traps, generating hole carriers in the Ge core, while their defect levels are very deep in one-component Si nanowires. The defect levels lie to within 10 meV from or below the valence band edge for nanowires with diameters larger than 33 A and the Ge fractions above 30%. As carriers are spatially separated from charge traps, scattering is greatly suppressed, leading to the ballistic conduction, in good agreement with experiments.

We investigate the atomic structure of the interface between crystalline ZnO and amorphous HfO2 (a-HfO2) and the electronic properties of oxygen vacancy (VO) near the interface through first-principles density-functional calculations. From the band alignment of ZnO/a-HfO2 interfaces, the conduction band offset is estimated to be 2.14–2.39 eV, while the potential barrier for hole conduction is nearly zero. The defect level of VO is higher in gate oxide than in ZnO. As VO behaves as a charge trap center in gate oxide, this defect can cause threshold voltage instability, while it does not in the ZnO region.

One-dimensional carbon nanotubes are considered as promising materials for nanoscale devices. A review is given here on the electrical transport properties of single-walled, double-walled, and telescoping carbon nanotubes studied by first-principles calculations. We first investigate the effect of carbon vacancies on the electrical properties of single-walled nanotubes, considering reconstruction around vacancies and randomness in their distribution. The band structure is severely modified by increasing the number of vacancies, which results in the gap opening. Next, we consider the band structure of double-walled nanotubes and the transport properties of telescoping nanotubes, and discuss the effect of intertube interactions on the electronic and transport properties and the accuracy of tight-binding calculations.

Doping control is an important issue in wideband gap semiconductors such as nitrides and oxides that can be characterized by doping asymmetry, indicating that it is di cult to achieve both lowresistivity pand n-type semiconductors. Despite theoretical predictions that group-V acceptors have high activation energies, p-type ZnO doped with N, P, As and Sb has been experimentally realized with a maximum hole concentration reaching 1019 cm 3. Recently, p-type conduction was also reported in ZnO doped with Li impurities. Based on the results of rst-principles theoretical calculations, the electronic structure of various defects related to group-V and group-I dopants, the compensation mechanism of acceptors and the origin of p-type conduction in ZnO are discussed. Finally, control of p-type doping is examined with a focus on the activation of acceptors by co-doping with two di erent dopant sources and by hydrogenation followed by annealing.

We investigate the stability of the zinc-blende (ZB) phase in GaN doped with transition metal ions using generalized-gradient approximation (GGA) calculations. The GGA calculations show that the ZB phase can be stabilized when the concentrations of the Cr and Mn ions are greater than 21% and 27% in GaCrN and GaMnN alloys, respectively. In GaFeN alloys, the stabilization of the ZB phase occurs at a much higher concentration of the Fe ions. If strong on-site Coulomb repulsion (U) is included, the ZB phase cannot be stabilized for all Fe concentrations. We also study the effects of electron and hole doping on the stability of the ZB phase in GaMnN alloys, and find that the critical concentration is reduced to 13% by doping one electron per Mn ion, while hole doping with a hole per Mn ion does not much affect the critical concentration.

We investigate the effect of Ge on the retardation of B diffusion in SiGe alloys through first-principle calculations, and find that the Ge bonding effect is most significant in the nearest-neighborhood of B. The B dopant diffuses from a self-interstitial–B pair via an interstitialcy mechanism for neutral charge state, while a kick-out mechanism is also possible for 1+ charge state. The migration and activation energies depend on the number and positions of the Ge atoms and are generally enhanced by the presence of Ge, reducing the B diffusivity.

We use a first-principles computational scheme to study the transport properties of devices based on telescoping carbon nanotubes. The transmission function is calculated through the matrix Green's function method using a Gaussian basis set. Varying the overlap region of the two nanotubes, we compare the effect of interwall interactions on the transport characteristics with that obtained from a simple tight-binding model. The leakage current through ultrathin gate oxides is also studied for various Si/SiO2 interface models, which are manipulated by varying oxide thickness and crystal phase.

We report the results of first-principles theoretical calculations for the electronic structure of aluminate nanotubes. A tubular structure in the form of AlO2 is energetically stable and exhibits metallic conduction. Due to weak interactions between Li atoms and nanotubes, Li doping does not alter the stability of AlO2 nanotubes and only increases the Fermi level. On the other hand, stable AlO nanotubes can be obtained by hole doping with Be and Mg impurities.

Despite considerable interest in two-dimensional (2D) topological insulators (TIs), a fundamental question still remains open how mesoscopic conductance fluctuations in 2D TIs are affected by spin-orbit interaction (SOI). Here, we investigate the effect of SOI on the universal conductance fluctuation (UCF) in disordered 2D TIs. Although 2D TI exhibits UCF like any metallic systems, the amplitude of these fluctuations is distinguished from that of conventional spin-orbit coupled 2D materials. Especially, in 2D systems with mirror symmetry, spin-flip scattering is forbidden even in the presence of strong intrinsic SOI, hence increasing the amplitude of the UCF by a factor of "Equation missing" compared with extrinsic SOI that breaks mirror symmetry. We propose an easy way to experimentally observe the existence of such spin-flip scattering in 2D materials. Our findings provide a key to understanding the emergence of a new universal behavior in 2D TIs.

Silicon is the most popular material used in electronic devices. However, its poor optical properties owing to its indirect band gap nature limit its usage in optoelectronic devices. Here we present the discovery of super-stable pure-silicon superlattice structures that can serve as promising materials for solar cell applications and can lead to the realization of pure Si-based optoelectronic devices. The structures are almost identical to that of bulk Si except that defective layers are intercalated in the diamond lattice. The superlattices exhibit dipole-allowed direct band gaps as well as indirect band gaps, providing ideal conditions for the investigation of a direct-to-indirect band gap transition. The transition can be understood in terms of a novel conduction band originating from defective layers, an overlap between the valence- and conduction-band edge states at the interface layers, and zone folding with quantum confinement effects on the conduction band of non-defective bulk-like Si. The fact that almost all structural portions of the superlattices originate from bulk Si warrants their stability and good lattice matching with bulk Si. Through first-principles molecular dynamics simulations, we confirmed their thermal stability and propose a possible method to synthesize the defective layer through wafer bonding.

Controllable bandgap widening from 1.8 to 2.6 eV is reported from oxidized MoS2 sheets that are composed of quilted phases of various MoSxOy flakes. The exfoliated flakes have large size (≥100 μm × 100 μm) sheets with average thickness of 1.7 nm. Remarkably, fine reversible tuning of the bandgap is achieved by postprocessing sulfurization of the MoSxOy sheets.

We study the defect properties of P dopants in ZnO through first-principles pseudopotential calculations. Because of the large size-mismatch between the P and O atoms, the acceptor level of a substitutional P (P O ) at an O lattice site is deeper than for N acceptors. A substitutional P (P Zn ) at a Zn antisite is found to be the dominant donor. Under Zn-rich condition, the P Zn donors are abundant, leading to n-type ZnO. As going to O-rich condition, Zn vacancies ( V Zn ) are energetically more favorable than the P O acceptors. Energy-lowering interactions between the P Zn defect and two Zn vacancies stabilizes the formation of a P Zn –2 V Zn complex, with a shallow acceptor level. This defect complex is suggested to be an important species in giving p -type ZnO together with the Zn vacancy.

We investigate the defect properties of group-I elements such as Li and Na in ZnO through first-principles calculations. Compared with group-V elements such as N, P, and As, Li and Na dopants at substitutional sites have shallower acceptor levels, but, these acceptors are mostly compensated by coexisting interstitial donors. Our calculations show that a codoping technique with hydrogen severely suppresses the concentration of interstitial donors, and greatly enhances the solubility of group-I dopants via the formation of hydrogen–acceptor complexes. The hydrogen-passivated acceptors easily recover the electrical activity by post-annealing, and thus low-resistivity p-type ZnO is achievable with dopants different from group-V elements.

We investigate electronic and magnetic properties of carbon nanotubes interacting with Fe atoms through first-principles theoretical calculations. For a single Fe atom on the tube surface, due to the curvature effect, there exists a difference in effective coordination number between inside and outside of the nanotube, and a complete promotion of 4s electrons into 3d orbitals occurs inside the nanotube. When Fe atoms are encapsulated in the form of nanowires inside the nanotube, their magnetic properties strongly depend on wire thickness. For thin nanowires with very weak interactions between Fe and C atoms, magnetic moments are similar to those for their free-standing nanowires, and electron conduction mostly occurs through the wires well protected from oxidation. On the other hand, the magnetic moments of thicker nanowires are greatly reduced.

Based on first-principles spin-density functional calculations, we investigate the magnetic properties of Mn-doped GaN. We find that the magnetic interaction between two Mn ions has a short-range nature, effective for Mn–Mn distances up to about 7 A, and it favors the ferromagnetic coupling via the double exchange mechanism. As the Mn atom interacts with more Mn atoms by introducing additional Mn atoms, the superexchange coupling is enhanced, stabilizing the antiferromagnetic state. In Mn-doped GaN, we find that Ga vacancies are energetically more stable near the Mn layer than in the bulk region due to the charge transfer from the Mn to the Ga vacancy, which may give rise to p-type conductivity.

We propose a novel epitaxial growth method, named as bridged mask growth (BMG), where the epitaxial growth characteristics under the bridged mask is different from that outside the BMG area. The growth rate under the bridged mask can be controlled by changing the bridge width, the opening gap width between neighboring bridges, and the thickness of spacer layer. The transition of growth characteristics between the BMG area and its outside region is confined in a very short distance. We find the relation of bridge pattern dimensions with spacer thickness for good surface morphology.