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Various forms of periodic, lateral, and vertical heterostructures constructed from magnetic transition metal dichalcogenide monolayers, FeTe2 and NiTe2, have been investigated by using hybrid density functional calculations. The lateral heterostructures formed from the stripes of these constituents joined along their armchair edges are found to be half-metallic within both constituent stripes with integer number of total magnetic moment per cell; they are semiconductor for spin-down electrons but metallic for spin-up ones. Additionally, the indirect band gap of these half-metallic structures in the energy range of visible light as well as their normal band lineup exhibit features that can be tunable with the width of stripes. Normal band lineup can lead to the confinement of excess spin-down electrons and eventually to the change of dimensionality from two to one. Strong variations of the band gap can be attained by δ-doping. Half-metallicity spreads from the FeTe2 side of the junction to the adjacent NiTe2 side, which is metallic when stand-alone, through strong Fe–Te–Ni bonds at the boundary. In contrast, in the thin, vertical heterostructures, where the interlayer coupling is rather weak, the half-metallic character is retained only in the FeTe2 side; the NiTe2 side remains a spin-polarized metal. Hence, these vertical heterostructures form a semiconductor/metal junction with a Schottky barrier for one spin direction to function as a diode but a metal/metal junction for the opposite spin direction to function as a conductor. These lateral and vertical heterostructures have inhomogeneous magnetic moment configurations due to p–d hybridization; in both sides of the junction, chalcogen atoms have magnetic moments antiparallel to those at transition metal atoms. Functionalities revealed here are rare and can lead to crucial applications.

The magnetic anisotropy of single semiconductor (ZnO and GaN) nanowires incorporating both a transition metal (Co and Mn, respectively) as a substitutional surface dopant and a heavy metal (Au, Bi, or Pt) adatom is studied by performing density-functional supercell calculations with the Hubbard U correction. It is found that a substantial enhancement in the magnetic anisotropy energy is obtained through the deposition of Bi; the deposition of Au and Pt leads to significant variation in other magnetic properties, but not in the magnetic anisotropy energy. An analysis within a band description shows that the coexistence of Bi adatom and a surface dopant with large spin moment activates a mechanism involving reorientation and readjustment of the spin moments of electrons in occupied bands in response to the change of magnetization direction, which promotes giant magnetic anisotropy. Our results for adsorption energetics indicate that the accommodation of Bi in the neighborhood of the surface dopant is more likely in GaN nanowires, because the Bi adatom does (not) tend to be closer to the Mn (Co) dopant on the surface of GaN (ZnO) nanowire. The stability of GaN nanowire with giant magnetic anisotropy owing to the incorporation of both Mn and Bi is demonstrated by performing ab initio molecular dynamics simulations at temperatures considerably higher than room temperature. These results suggest that adatom deposition and surface doping can be used complementarily to develop single nanowire-based spintronic devices.

BiTeI kristalinin elektronik bant yapısında, güçlü spin-yörünge etkileşmesi Rashba tipi spin ayrışmasına yol açmaktadır. Bu nedenle, dar bant aralıklı bir yarıiletken olan BiTeI potansiyel bir spintronik malzeme olarak ilgi çekmektedir. Bu makalede, BiTeI’deki Rashba tipi spin yarılmaların gerinim ile nasıl değiştiği incelenmiştir. Bu amaçla, yoğunluk fonksiyonel teorisi çerçevesinde kristal yapısı optimizasyonları ve bant yapısı hesaplamaları gerçekleştirilmiştir. Bu hesaplamaların sonuçları, BiTeI’deki Rashba tipi spin yarılmasının gerinim ile kontrol edilebileceğini göstermektedir. Bu, BiTeI kristalinde gerinimin spin-yörünge etkileşimini arttırdığı (kristal sıkıştırıldığında) ya da azalttığı (kristal genleştiğinde) bulgusu ile açıklanmıştır.

The segregation tendencies, defect energetics and electrical behavior of transition-metal (Mn and Co) dopants in wide band gap semiconductor (GaN and ZnO) nanowires are investigated by performing density-functional supercell calculations with the Hubbard U correction. Defect calculations and ab initio molecular dynamics simulations are carried out for a comparative exploration of various doping configurations where the dopant resides on interior, subsurface or surface sites. Mn and Co dopants in GaN and ZnO nanowires, respectively, are found to have different segregation tendencies: whereas a uniform distribution of Co dopants throughout ZnO nanowires takes place, indicating no segregation behavior, GaN nanowires can accommodate the majority of Mn dopants in the interior or surface sites, depending on the position of the Fermi level, which indicates not only segregation, but also that the direction of segregation can be reversed by shifting the Fermi level. Due to the latter, the Mn dopants can homogeneously be incorporated into the GaN nanowires only if the Fermi level remains in a certain range. A theoretically justified generalization of the segregation energy is imperative for obtaining these results, which are substantiated by comparison to experimental characterizations. Our findings demonstrate that the segregation tendency of an impurity in a semiconductor nanowire can be tuned by adjusting the position of the Fermi level (as in the case of Mn in GaN nanowires), which is, however, not always possible (as in the case of Co in ZnO nanowires). The analysis of the defect transition energies reveals that substitutional Mn and Co defects in GaN and ZnO nanowires form deep acceptor and deep donor levels, regardless of the doping site. This means that some other means such as codoping, stoichiometry control or gating must be used, if and as required, to shift the Fermi level of Mn-doped GaN or Co-doped ZnO nanowires in order to alter the type of electrical conduction and/or the segregation direction.

Our analysis based on the results of hybrid and semilocal density-functional calculations with and without Hubbard $U$ correction for on-site Coulomb interactions reveals the true magnetic ground states of three transition-metal dichalcogenide monolayers, viz., ${\mathrm{FeTe}}_{2},{\mathrm{VS}}_{2}$, and ${\mathrm{NiTe}}_{2}$, which comprise inhomogeneous magnetic moment configurations. In contrast to earlier studies considering only the magnetic moments of transition-metal atoms, the chalcogen atoms by themselves have significant, antiparallel magnetic moments owing to the spin polarization through $p\text{\ensuremath{-}}d$ hybridization. The latter is found to be true for both $H$ and $T$ phases of ${\mathrm{FeTe}}_{2},{\mathrm{VS}}_{2}$, and ${\mathrm{NiTe}}_{2}$ monolayers. Our predictions show that the ${\mathrm{FeTe}}_{2}$ monolayer in its lowest-energy structure is a half metal, which prevails under both compressive and tensile strains. Half metallicity occurs also in the ${\mathrm{FeTe}}_{2}$ bilayer but disappears in thicker multilayers. The ${\mathrm{VS}}_{2}$ monolayer is a magnetic semiconductor; it has two different band gaps of different character and widths for different spin polarization. The ${\mathrm{NiTe}}_{2}$ monolayer, which used to be known as a nonmagnetic metal, is indeed a magnetic metal with a small magnetic moment. These monolayers with intriguing electronic and magnetic properties can attain new functionalities for spintronic applications.

In this paper we investigate periodic lateral and vertical heterostructures of transition metal dichalcogenides (TMDs). Lateral heterostructures are constructed by the alternating metallic and semiconducting single-layer stripes of TMDs joined commensurately along their armchair edges and attain different states depending on the widths of constituents. While these heterostructures acquire a composite character with metallic state for narrow stripes, large stripes lead to the confinement of electronic states and function as metal-semiconductor junctions with a tunable Schottky barrier. The interface or boundary between constituent stripes has finite extension and allows charge transfer between them. On the other hand, the weak van der Waals interaction between layers sets the features of vertical heterostructures. Their interfaces are sharp: metal-semiconductor junction and Schottky barrier developed thereof can be induced even within a few layers. In the absence of dopants, we find minute charge transfer across the interface with negligible band bending in vertical heterostructures. The δ-doping of the semiconducting constituent by the metallic one forms strictly two-dimensional metallic electrons in a three-dimensional layered semiconductor and leads to crucial directionality effects and quantization of conductance. Our work unveils significant differences between lateral and vertical heterostructures.

Defect calculations using the density and hybrid functionals in combination with the supercell approach are employed to characterize the electrical properties of a number of ZnO nanowires of various thicknesses doped with Bi atoms occupying surface sites. The variation of the differences between the total energies of charged and neutral supercells with the supercell size is studied, which led the authors to devise an extrapolation procedure to obtain reliable defect energetics in the dilute defect limit. The calculated defect formation energies indicate that although the substitution of Bi into Zn or O sites can take place spontaneously under suitable thermodynamic conditions, the substitution into Zn sites is generally more likely. The defect (charge-state) transition energies are computed and parameterized as a function of the nanowire thickness. It is revealed that the substitution of Bi into O (Zn) sites on the surface of ZnO nanowires yields deep acceptor (shallow donor) levels (except for extremely thin nanowires). It is therefore concluded that the incorporation of Bi into the surface of ZnO nanowires results in $n$-type doping.

The effect of applying an external electric field on doping-induced spin-orbit splitting of the lowest conduction-band states in a bismuth-doped zinc oxide nanowire is studied by performing electronic structure calculations within the framework of density functional theory. It is demonstrated that spin splitting in Bi-doped ZnO nanowires could be tuned and enhanced electrically via control of the strength and direction of the applied electric field, thanks to the nonuniform and anisotropic response of the ZnO:Bi nanowire to external electric fields. The results reported here indicate that a single ZnO nanowire doped with a low concentration of Bi could function as a spintronic device, the operation of which is controlled by applied lateral electric fields.

Our predictions, based on density-functional calculations, reveal that surface doping of ZnO nanowires with Bi leads to a linear-in-$k$ splitting of the conduction-band states, through spin-orbit interaction, due to the lowering of the symmetry in the presence of the dopant. This finding implies that spin polarization of the conduction electrons in Bi-doped ZnO nanowires could be controlled with applied electric (as opposed to magnetic) fields, making them candidate materials for spin-orbitronic applications. Our findings also show that the degree of spin splitting could be tuned by adjusting the dopant concentration. Defect calculations and ab initio molecular dynamics simulations indicate that stable doping configurations exhibiting the foregoing linear-in-$k$ splitting could be realized under reasonable thermodynamic conditions.

Planar composite structures formed from the stripes of transition metal dichalcogenides joined commensurately along their zigzag or armchair edges can attain different states in a two-dimensional (2D), single-layer, such as a half metal, 2D or one-dimensional (1D) nonmagnetic metal and semiconductor. Widening of stripes induces metal-insulator transition through the confinements of electronic states to adjacent stripes, that results in the metal-semiconductor junction with a well-defined band lineup. Linear bending of the band edges of the semiconductor to form a Schottky barrier at the boundary between the metal and semiconductor is revealed. Unexpectedly, strictly 1D metallic states develop in a 2D system along the boundaries between stripes, which pins the Fermi level. Through the δ doping of a narrow metallic stripe one attains a nanowire in the 2D semiconducting sheet or narrow band semiconductor. A diverse combination of constituent stripes in either periodically repeating or finite-size heterostructures can acquire critical fundamental features and offer device capacities, such as Schottky junctions, nanocapacitors, resonant tunneling double barriers, and spin valves. These predictions are obtained from first-principles calculations performed in the framework of density functional theory.

The evolution of the electronic structure of BiTeI, a layered semiconductor with a van der Waals gap, under compression is studied by employing semilocal and dispersion-corrected density-functional calculations. Comparative analysis of the results of these calculations shows that the band-gap energy of BiTeI decreases till it attains a minimum value of zero at a critical pressure, after which it increases again. The critical pressure corresponding to the closure of the band gap is calculated, at which BiTeI becomes a topological insulator. Comparison of the critical pressure to the pressure at which BiTeI undergoes a structural phase transition indicates that the closure of the band gap would not be hindered by a structural transformation. Moreover, the band-gap pressure coefficients of BiTeI are computed, and an expression of the critical pressure is devised in terms of these coefficients. Our findings indicate that the semilocal and dispersion-corrected approaches are in conflict about the compressibility of BiTeI, which result in overestimation and underestimation, respectively. Nevertheless, the effect of pressure on the atomic structure of BiTeI is found to be manifested primarily as the reduction of the width of the van der Waals gap according to both approaches, which also yield consistent predictions concerning the interlayer metallic bonding in BiTeI under compression. It is consequently shown that the calculated band-gap energies follow qualitatively and quantitatively the same trend within the two approximations employed here, and the transition to the zero-gap state occurs at the same critical width of the van der Waals gap.

A polythiophene derivative bearing TEMPO radical was synthesized by oxidative chemical polymerization of its monomer. The polymer had a high spin density (2.05 × 10 21 spins/g of polymer). CV studies of the polymer showed that the electrochemical redox reaction of the TEMPO radicals were completely reversible. We demonstrated, for the first time, construction and charge/discharge characteristics of an organic radical battery utilizing a TEMPO bearing polythiophene based cathode material. The battery had an initial specific discharge capacity of 79 A h/kg (87% of the theoretical capacity) and an average output voltage of 3.6 V. The specific energy capacity initially discharged was 268 W h/kg.

First-principles calculations have been carried out to obtain the size- and shape-dependent energetics of the nanocrystals made of transition-metal elements Ni, Cu, Rh, Pd, Ag, Ir, Pt, and Au. A structural parameter characterizing the nanocrystal size and shape has been devised, which is a combination of variables such as the total and surface number of atoms. With the aid of the latter, structure–property relationships have been formulated, interlinking the (first-principles) energetics to (atomistic) structure.

A computational modeling methodology is described, which is based on the total-energy and defect calculations using the density and hybrid functionals in combination with the supercell approach. This methodology is employed to study the structure and energetics of zinc oxide (ZnO) nanowires, and the electrical properties of a bismuth-doped zinc oxide (ZnO:Bi) nanowire. A simple model is developed, where the nanowire formation energy is expressed as a function of the nanowire diameter and cross-sectional morphology (hexagonal or triangular). Defect calculations are employed to characterize the ZnO:Bi nanowire, in regard to the location and charge-state of the dopant. The latter indicates that the dopant atoms are mostly substituted into the Zn sites on the nanowire surface, and therefore Bi in ZnO electrically acts as a donor.

Aim: In this study, we aimed to determine the incidence of gestational trophoblastic disease by using data of patients who were treated for molar pregnancies. Material and methods: . The patients diagnosed as gestational trophoblastic disease between January 2009- January 2014 was evaluated retrospectively. Patients’ age, gravidity, parity and aborts were noted. Patients’ history, ultrasonographic findings, β HCG and laboratory results, hystopathology findings, treatment modalities and β HCG for following-up were recorded. Results: The incidence of molar pregnancy was found as 8 per 1000 deliveries. The mean age of patients was 28.72 ±7,5 and mean gestastional week was 10±2,4. β HCG values was found between 240 and 145100. Out of all patients, 20 (33.3%) had ultrasonographic signs of molar pregnancy. Pathology results revealed partial mole in 45 (75%), complete mole in 12 (20%) patients and choriocarcinoma in 3 (5%) patients. No mortality related to disease and complications was observed, Amaç: Bu çalışmada kliniğimizde molar gebeliktanısıyla tedavi edilen hastaların verilerikullanılarak gestasyonel trofoblastik hastalıklarıninsidansını belirlemeyi amaçladık.Gereç ve yöntem: Ocak 2009 ve Ocak 2014 yıllarıarasında kliniğimizde molar gebelik tanısı almışhastaların dosyaları retrospektif olarak incelendi.Hastaların yaş, gravida, parite ve abortus bilgilerikaydedildi. Olguların hikayesi, ultrasonografibulguları, β HCG ile birlikte laboratuar değerleri,histopatoloji sonuçları, tedavi yöntemleri ve takipteβ HCG değerleri kayıt edildi.Bulgular: Kliniğimizdeki molar gebelik insidansı1000 doğumda 5 olarak bulundu. Hastalarınortalama yaşı 28.72 ±7,5 olup ortalama gestasyonelyaş 10±2,4 haftaydı. β HCG değerleri 240 ile145100 arasında değişmekteydi. Hastaların20’sinde (%33.3) ultrason bulgusu molar gebelik ileuyumlu izlenmişti. Patoloji sonuçları 45 (%75)hastada parsiyel mol, 12 (%20) hastada kompletmol ve 3 (%5) hastada koryokarsinom gelmiştir.Olgularda hastalık ve komplikasyonlarına ait ölümolgusuna rastlanmamıştır.Sonuç: Kliniğimizde gestasyonel trofoblastikhastalık insidansı 1000 doğumda 5 olup daha kesinbir insidansa ulaşmak için ileri düzeyde araştırmalaryapılması gerekmektedir.Anahtar Kelimeler: Gestasyonel trofoblastikhastalık, insidans

We first develop a descriptor-based representation of atomic environments by devising two local similarity indices defined from an atom-partitioned quantum-chemical descriptor. Then, we employ this representation to explore the size-, shape- and composition-dependent nanocrystal energetics. For this purpose, we utilize an energy difference {\mu} that is related to the atomic chemical potential, which enables one to characterize energetic heterogeneities. Employing first-principles calculations based on the density functional theory for a set of database systems, namely unary atomic clusters in the shape of regular polyhedra and the bulk solids of C, Si, Pd and Pt, we explore the correlations between the energy difference {\mu} and similarity indices. We find that there exists an interconnection between nanocrystal energetics and quantum similarity measures. Accordingly, we develop a means for computing total energy differences from the similarity indices via interpolation, and utilize a test set comprising a variety of unary nanocrystals and binary nanoalloys/nanocompounds for validation. Our findings indicate that the similarity-based energies could be utilized in computer-aided design of nanoparticles.

We performed ab initio zero temperature and finite temperature molecular dynamics calculations to investigate the homoepitaxial growth on the Si(0 0 1) surface. How do the deposited atoms (adatoms) form addimers and how do the addimers reach their favorable positions at the nucleation site of the growth process are presented. Once two epitaxial addimers, one over the dimer row and oriented perpendicular to the surface dimer bonds and the other over the adjacent trough, are aligned at high temperature, the nucleation site of the growth process is formed. The concerted bond exchange between these addimers and the reconstructed surface dimers is found to be the atomistic mechanism that leads to the homoepitaxial growth. © 2001 Elsevier Science B.V.

We predict new forms of carbon consisting of one and two dimensional networks of interlinked single wall carbon nanotubes, some of which are energetically more stable than van der Waals packing of the nanotubes on a hexagonal lattice. These interlinked nanotubes are further transformed with higher applied external pressures to more dense and complicated stable structures, in which curvature-induced carbon sp$^{3}$ re-hybridizations are formed. We also discuss the energetics of the bond formation between nanotubes and the electronic properties of these predicted novel structures., Comment: 4 pages, 4 postscript figures; To be appear in PRB

We present a first-principles study to elucidate the nature of the bonding, stability, energetics, and dynamics of individual X8H8 molecules (X = C, Si, Ge). The results obtained from both “local basis” and “pseudopotential” ab initio methods are in good agreement with the experimental data that exists for cubane (C8H8). The trends among these molecules are reminiscent of those prevailing in the bulk solids of C, Si, and Ge. High-temperature dynamics and fragmentation of X8H8 were studied by the quantum molecular dynamics method which shows that at high temperatures cubane is transformed to the 8-fold ring structure of cyclooctotetraene.

In the density-functional studies of materials with localized electronic states, the local/semilocal exchange-correlation functionals are often either combined with a Hubbard parameter $U$ as in the LDA+$U$ method or mixed with a fraction of exactly computed (Fock) exchange energy yielding a hybrid functional. Although some inaccuracies of the semilocal density approximations are thus fixed to a certain extent, the improvements are not sufficient to make the predictions agree with the experimental data. Here we put forward the perspective that the hybrid functional scheme and the LDA+$U$ method should be treated as complementary, and propose to combine the range-separated (HSE) hybrid functional with the Hubbard $U$. We thus present a variety of HSE+$U$ calculations for a set of II-VI semiconductors, consisting of zinc and cadmium monochalcogenides, along with comparison to the experimental data. Our findings imply that an optimal value $U^\ast$ of the Hubbard parameter could be determined, which ensures that the HSE+$U^\ast$ calculation reproduces the experimental band gap. It is shown that an improved description not only of the electronic structure but also of the crystal structure and energetics is obtained by adding the $U^\ast$ term to the HSE functional, proving the utility of HSE+$U^\ast$ approach in modeling semiconductors with localized electronic states.

First-principles total-energy calculations for hydrogen impurities in CuInSe2 (CIS) and CuGaSe2 (CGS) show that H+ takes up the Cu–Se bond center position, whereas H0 and H− take up tetrahedral interstitial site next to In (in CIS) or Ga (in CGS). Hydrogen creates a negative-U center (i.e., H0 is never stable), with a (+/−) transition level at Ec−0.39 eV in CIS, and Ec−0.57 eV in CGS. However, once combined with the 2VCu−+IIICu2+ complex, hydrogen forms shallower centers with transition levels at Ec−0.15 eV in CIS, and Ec−0.39 eV in CGS. We conclude that hydrogen could convert CIS to n type, but not CGS.

First-principles total-energy calculations suggest that interstitial hydrogen impurity forms a shallow donor in SnO2, CdO, and ZnO, but a deep donor in MgO. We generalize this result to other oxides by recognizing that there exist a “hydrogen pinning level” at about 3.0±0.4 eV below vacuum. Materials such as Ag2O, HgO, CuO, PbO, PtO, IrO2, RuO2, PbO2, TiO2, WO3, Bi2O3, Cr2O3, Fe2O3, Sb2O3, Nb2O5, Ta2O5, FeTiO3, and PbTiO3, whose conduction band minimum (CBM) lie below this level (i.e., electron affinity>3.0±0.4 eV) will become conductive once hydrogen is incorporated into the lattice, without reducing the host. Conversely, materials such as BaO, NiO, SrO, HfO2, and Al2O3, whose CBM lie above this level (i.e., electron affinity

Electrodes made of organic polymers bearing redox-active radical pendant groups have attractive features for use in rechargeable batteries. Electronic structure and electrochemical properties of cathode-and anode-active organic polymers are investigated here by means of first-principles calculations performed in the framework of the density functional theory. We consider organic radical polymers (ORPs) that consist of trans-polyacetylene derivatives bearing a variety of nitroxide radicals. A number of neutral and charged supercells are utilized to compute the ionization potentials and electron affinities as well as the one-electron states of these ORPs. By revealing the polyacetylene-derived highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) as well as the radical-derived singly occupied molecular orbital (SOMO), the variation of the SOMO energy within the HOMO-LUMO gap is determined in the course of the oxidization or reduction of ORPs. Our results indicate that the ionization potential I and electron affinity A of polyacetylene would act as a lower or upper bound in the variation of the electrochemical potential of cathode- or anode-active ORPs in the course of battery discharge or charge owing to pinning of the radical-derived SOMO to the polyacetylene-derived HOMO or LUMO. Accordingly, it is anticipated that the electrochemical "window" [-I,-A] of the polymeric backbone of ORPs will impose certain limitations in accomplishing a high charge/discharge voltage range in a totally organic rechargeable battery with positive and negative electrodes made of cathode-and anode-active ORPs, respectively. On the other hand, our findings suggest that one could, in principle, take advantage of using two different (conducting) polymeric backbones in the anode and cathode with adjusted HOMO and LUMO offsets once the electron transfer is accomplished to take place through the conducting backbones.

I will review the basic physical principles underlying the formation energy of various intrinsic defects in common photovoltaic materials. I then use the above principles to explain why doping of semiconductors is in general, limited and which design principles can be used to circumvent such limits. This work can help design strategies of doping absorber materials as well as explain how TCOs work. Recent results on the surprising stability of polar [112] + (1~1~2~) surfaces of CIS will also be described in this context.

An impurity in a semiconductor can have either amphoteric behavior (no net production of electron or holes), or be an energetically deep center (carriers produced only at high temperature), or a shallow center (carriers produced even at low temperature). In most semiconductors (e.g., Si, GaAs, GaP, InP, and ZnSe) hydrogen impurities do not produce free carriers, being instead an amphoretic center; yet hydrogen does dope n-type some oxides such as ${\mathrm{SnO}}_{2}$ and ZnO. We studied theoretically whether or not H could dope chalcopyrite ${\mathrm{I}\ensuremath{-}\mathrm{I}\mathrm{I}\mathrm{I}\ensuremath{-}\mathrm{V}\mathrm{I}}_{2}$ compounds, ${\mathrm{CuInSe}}_{2}$ and ${\mathrm{CuGaSe}}_{2}.$ Based on the first-principles calculations, we find that nonsubstitutionally incorporated hydrogen forms a deep donor in ${\mathrm{CuGaSe}}_{2},$ but a relatively shallow donor in ${\mathrm{CuInSe}}_{2}.$ The interaction of hydrogen with the abundant defect complex ${(2V}_{\mathrm{Cu}}+{\mathrm{In}}_{\mathrm{Cu}}{)}^{0}$ yields an even shallower donor, making ${\mathrm{CuInSe}}_{2}$ n type. In addition, our results show that hydrogen passivates the acceptorlike copper vacancies in both ${\mathrm{CuInSe}}_{2}$ and ${\mathrm{CuGaSe}}_{2},$ thus eliminating p type behavior. These findings, in conjunction with typical conditions under which ${\mathrm{CuInSe}}_{2}$ and ${\mathrm{CuGaSe}}_{2}$ are grown, indicate that ${\mathrm{CuInSe}}_{2}$ could be doped n type via hydrogen incorporation, whereas ${\mathrm{CuGaSe}}_{2}$ could not. The reason for the different behavior of ${\mathrm{CuInSe}}_{2}$ and ${\mathrm{CuGaSe}}_{2}$ towards hydrogen is that in the latter case the conduction-band minimum is at a considerably higher energy than in the former case. Despite this difference in electrical properties, it is predicted that hydrogen can be stored in both ${\mathrm{CuInSe}}_{2}$ and ${\mathrm{CuGaSe}}_{2}$ via implantation since the implanted hydrogens decorate copper atoms as well as preexisting copper vacancies.

SnO2 is a prototype "transparent conductor," exhibiting the contradictory properties of high metallic conductivity due to massive structural nonstoichiometry with nearly complete, insulator-like transparency in the visible range. We found, via first-principles calculations, that the tin interstitial and oxygen vacancy have surprisingly low formation energies and strong mutual attraction, explaining the natural nonstoichiometry of this system. The stability of these intrinsic defects is traced back to the multivalence of tin. These defects donate electrons to the conduction band without increasing optical interband absorption, explaining coexistence of conductivity with transparency.

We present a systematic analysis of the effect of radial deformation on the atomic and electronic structure of zigzag and armchair single wall carbon nanotubes using the first principle plane wave method. The nanotubes were deformed by applying a radial strain, which distorts the circular cross section to an elliptical one. The atomic structure of the nanotubes under this strain are fully optimized, and the electronic structure is calculated self-consistently to determine the response of individual bands to the radial deformation. The band gap of the insulating tube is closed and eventually an insulator-metal transition sets in by the radial strain which is in the elastic range. Using this property a multiple quantum well structure with tunable and reversible electronic structure is formed on an individual nanotube and its band-lineup is determined from first-principles. The elastic energy due to the radial deformation and elastic constants are calculated and compared with classical theories., Comment: To be appear in Phys. Rev. B, Apr 15, 2002

The band gap of a semiconducting single wall carbon nanotube decreases and eventually vanishes leading to metalization as a result of increasing radial deformation. This sets in a band offset between the undeformed and deformed regions of a single nanotube. Based on the superlattice calculations, we show that these features can be exploited to realize various quantum well structures on a single nanotube with variable and reversible electronic properties. These quantum structures and nanodevices incorporate mechanics and electronics., 7 pages, 4 figures, To be appear in PRB

The electronic and structural properties of molecular and solid cubane have been studied by first-principles, self-consistent field total energy calculations. Calculated molecular properties such as equilibrium geometry and electronic and vibrational spectra are found to be in good agreement with experimental data. Structural parameters and the energetics of both the low-temperature, orientationally ordered and high-temperature, orientationally disordered or plastic phases of solid cubane are determined. The valence band of solid cubane is derived from the molecular states; the energy gap between the lowest unoccupied and highest occupied molecular orbital bands is rather large due to the saturated carbon atoms. The effect of alkali-metal-atom doping on the electronic energy bands is investigated. It is found that the metallic band of doped cubane is derived from the undoped solid cubane's lowest conduction band with a significant contribution from the alkali-metal atom.

Cubane (C8H8) is an immensely strained molecule whose C-C-C bond angle is 90 degrees rather than 109.5 degrees as expected for sp(3) bonding of carbon. We have measured the intramolecular vibrational spectrum of cubane using inelastic neutron scattering. The neutron data are used to test the transferability of various phenomenological potentials and tight-binding models to this highly strained molecule. Unlike these models, first-principles calculations of the INS spectrum (both energy and intensity) agree well with the experimental data.

The conductance through a quantum point contact created by a sharp and hard metal tip on the graphite surface has features which to our knowledge have not been encountered so far in metal contacts or in nanowires. In this paper we first investigate these features which emerge from the strongly directional bonding and electronic structure of graphite, and provide a theoretical understanding for the electronic conduction through quantum point contacts. Our study involves the molecular-dynamics simulations to reveal the variation of interlayer distances and atomic structure at the proximity of the contact that evolves by the tip pressing toward the surface. The effects of the elastic deformation on the electronic structure, state density at the Fermi level, and crystal potential are analyzed by performing self-consistent-field pseudopotential calculations within the local-density approximation. It is found that the metallicity of graphite increases under the uniaxial compressive strain perpendicular to the basal plane. The quantum point contact is modeled by a constriction with a realistic potential. The conductance is calculated by representing the current transporting states in Laue representation, and the variation of conductance with the evolution of contact is explained by taking the characteristic features of graphite into account. It is shown that the sequential puncturing of the layers characterizes the conductance., LaTeX, 11 pages, 9 figures (included), to be published in Phys. Rev. B, tentatively scheduled for 15 September 1998 (Volume 58, Number 12)

First-principles calculations have been performed to obtain the size- and shape-dependent energetics of the unary nanocrystals made of transition metal elements Ni, Cu, Rh, Pd, Ag, Ir, Pt, or Au. The variation of the nanocrystal chemical potential μ with the interatomic distance d has been studied, leading to derivation of an energy-distance (μ-d) relation. A variety of nanocrystal morphologies have been employed to explore general features of this relation. It is found that the curves representing the μ-d relationship for the entire set of nanocrystals could all be represented by a single universal function, regardless of the atomic species or the nanocrystal morphology or magnetic ground-state.