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Using density functional theory, we have analyzed the ways and means of improving the minority carrier lifetime (MCL) by calculating the band structure dependent quantities contributing to the MCL. We have computationally modeled silicon doped with different elements like B, C, N, O, P, Ti, Fe, Ga, Ge, As, In, Sn, Sb, and Pt and looked at the effect of doping on MCL. In co-doping, the systems Si-B-Ga, Si-B-Ge, Si-B-2Ge, Si-B-Pt, Si-Ga-Ge, Si-Ga-2Ge, and Si-Ga-Pt are investigated. From our calculation, it is found that by doping and co-doping of Si with suitable elements having “s” and “p” electrons, there is a decrease in the recombination activity. The predicted effective minority carrier lifetime indicates the possibility of significant improvements. Based on the above studies, it is now maybe possible, with suitable choice of dopant and co-dopant material, to arrive at part of a standard production process for solar grade Si material. [ABSTRACT FROM AUTHOR]

The electron transport properties with the relativistic intrinsic and Rashba spin-orbit (SO) couplings in a graphene nanoribbon are investigated by using the third-nearest-neighbor tight-binding model and the nonequilibrium Green function method. The numerical results display a perfect quantum spin Hall state (QSH) surrounded by a simple insulator state, presented in Rashba SO coupling vs sublattice potential plane. The QSH state is enhanced by the intrinsic SO coupling, indicating a quantum topological property of graphene nanoribbons. [ABSTRACT FROM AUTHOR]

Metallic melts above the liquidus temperature exhibit nearly Arrhenius-type temperature dependence of viscosity. On cooling below the equilibrium liquidus temperature metallic melts exhibit a non-Arrhenius temperature dependence of viscosity characterized by liquid fragility phenomenon which origin is still not well understood. Structural changes and vitrification of the Pd42.5Cu30Ni7.5P20 liquid alloy on cooling from above the equilibrium liquidus temperature are studied by synchrotron radiation X-ray diffraction and compared with the results of first-principles calculations. Subsequent analysis of the atomic and electronic structure of the alloy in liquid and glassy states reveals formation of chemical short-range order in the temperature range corresponding to such a non-Arrhenius behavior. The first-principles calculations were applied to confirm the experimental findings. [ABSTRACT FROM AUTHOR]

We discover a method of opening the conductive channels of zigzag graphene nanoribbons (ZGNRs) by using hydrogenation chains to separate the nanoribbon into two strips with a ∼0.7 Å distance, although the overall hydrogenation on graphene transforms the highly conductive semimetal sheet into an insulator. Two edge-like states emerge around each hydrogenation chain. The conductance enhancement, made by the hydrogenation chain, is found in nanoribbons with 5 to 7 chains, e.g. 5ZGNRH, 6ZGNRH, 7ZGNRH, and 7ZGNR2H (7ZGNR with two hydrogenation chains). The ZGNRs with hydrogenation chains illustrate their potential in nanoelectronics and carbon electronics as electronic leads and nonlinear devices. [ABSTRACT FROM AUTHOR]

The structural stability of coinage metal nanotubes with a square cross-section has been investigated by the first-principles numerical simulations. In addition to the reported (4, 4) silver tube, it is found that the hollow (4, 4) copper and gold nanotubes can also be formed by applying an appropriate stress to an 8A/8B fcc wire. The stability of these coinage metal (4, 4) nanotubes, formed by tip-stretching the wires, has been explained by a local minimum in the string tension variation with their tube lengths. Interestingly, we have explained why a low-stress stretching is needed to obtain the (4, 4) Cu tube in contrast to a higher one for both the (4, 4) Ag and Au tubes due to the larger stiffness coefficient of copper than those of silver and gold, which could be proved by future experiments. [ABSTRACT FROM AUTHOR]

We use density functional theory based nonequilibrium Green's function to self-consistently study the current through the 1,4-benzenedithiol (BDT). The elastic and inelastic tunneling properties through this Au-BDT-Au molecular junction are simulated, respectively. For the elastic tunneling case, it is found that the current through the tilted molecule can be modulated effectively by the external gate field, which is perpendicular to the phenyl ring. The gate voltage amplification comes from the modulation of the interaction between the electrodes and the molecules in the junctions. For the inelastic case, the electron tunneling scattered by the molecular vibrational modes is considered within the self-consistent Born approximation scheme, and the inelastic electron tunneling spectrum is calculated. [ABSTRACT FROM AUTHOR]

Group-IV element clathrates have attracted considerable interest in recent years. Here, we report an ab initio study on the structural stability of carbon clathrates at high pressure and identify fcc-C136 clathrate as the third most stable carbon phase after cubic diamond and hexagonal graphite. A pressure-induced phase transition is predicted to occur around 17 GPa from hexagonal graphite to fcc-C136, which is more stable than other carbon clathrates such as hex-C40 and sc-C46, and the recently predicted metastable M-carbon up to 26 GPa. Phonon dispersion calculations confirm the dynamic stability of fcc-C136 as well as diamond. [ABSTRACT FROM AUTHOR]

With nonequilibrium Green’s function approach combined with density functional theory, we perform an ab initio calculation to investigate transport properties of graphene nanoribbon (GNR) junctions self-consistently. Tight-binding approximation is applied to model the zigzag (ZGNR) electrodes, and its validity is confirmed in comparison to the GAUSSIAN03 periodic boundary condition calculation result of the same system. The origin of abnormal jump points usually appearing in the transmission spectrum is explained with the detailed tight-binding ZGNR band structure. Transport property of an edge-defect ZGNR junction is investigated, and the tunable tunneling current can be sensitively controlled by transverse electric fields. [ABSTRACT FROM AUTHOR]

The structure and vibrational properties of high- and low-density amorphous (HDA and LDA, respectively) ices have been determined using reverse Monte Carlo, molecular dynamics, and lattice dynamics simulations. This combined approach leads to a more accurate and detailed structural description of HDA and LDA ices when compared to experiment than was previously possible. The water molecules in these ices form well connected hydrogen-bond networks that exhibit modes of vibration that extend throughout the solid and can involve up to 70% of all molecules. However, the networks display significant differences in their dynamical behavior. In HDA, the extended low-frequency vibrational modes occur in dense parallel two dimensional layers of water that are approximately 10 nm thick. In contrast, the extended modes in LDA resemble a holey structure that encapsulates many small pockets of nonparticipating water molecules. [ABSTRACT FROM AUTHOR]

Using first-principles calculations based on gradient corrected density functional theory, we have studied the interaction of NH3, H2, and O2 with Ti-benzene complexes [Ti(Bz)2 and Ti2(Bz)2]. The energy barriers as the gas molecules approach the Ti-benzene complexes as well as the geometries of the ground state of these interacting complexes were obtained by starting with several initial configurations. While NH3 and H2 were found to physisorb on the Ti(Bz)2 complex, the O2 reacts with it strongly leading to dissociative chemisorption of the oxygen molecule. In contrast all the gas molecules react with the Ti2(Bz)2 complex. These studies indicate that the reaction of certain, but not all, gas molecules can be used to probe the equilibrium geometries of organometallic complexes. Under special conditions, such as high pressure, the Ti atom intercalated between benzene molecules in Ti(Bz)2 and the Ti2(Bz)2 complexes could store hydrogen in chemisorbed states. The results are compared to available experimental data. [ABSTRACT FROM AUTHOR]

We use density functional theory based nonequilibrium Green’s function to calculate the current through the different rodlike molecules at the finite temperatures self-consistently, which was compared to the experimental measurements presented by Reichert et al. [Phys. Rev. Lett. 88, 176804 (2002)] and by Mayor et al. [Angew. Chem. Int. Ed. 42, 5834 (2003)], respectively. Our results agree with the measurements very well, especially for the bias around ±1.0 V. The investigation of the topological effect for the symmetrical molecule reveals the fact that the para position compound provides a considerably larger conductance than the meta one. [ABSTRACT FROM AUTHOR]

Electronic transport properties of alkanedithiols are calculated by a first-principles method based on density functional theory and nonequilibrium Green’s function formalism. At small bias, the I-V characteristics are linear and the resistances conform to the Magoga’s exponential law. The calculated length-dependent decay constant γ which reflects the effect of internal molecular structure is in accordance with most experiments quantitatively. Also, the calculated effective contact resistance R0 is in good agreement with the results of repeatedly measuring molecule-electrode junctions [B. Xu and N. Tao, Science 301, 1221 (2003)]. [ABSTRACT FROM AUTHOR]

The bistable molecular switches have been studied theoretically based on the first-principles calculation. The geometry structures of the switches studied in this paper can be triggered between two symmetrical structures by using an external applied electric field. I-V characteristic curves of the different molecule configurations have been calculated, and distinguishability of these characteristic curves indicates a switching behavior, the performance of which can be improved significantly by some suitable donors and acceptors. [ABSTRACT FROM AUTHOR]

The quantum transport of a gated polythiophene nanodevice is analyzed using density functional theory and nonequilibrium Green’s function approach. For this typical molecular field effect transistor, we prove the existence of two main features of electronic components, i.e., negative differential resistance and good switching. Ab initio based explanations of these features are provided by distinguishing fixed and shifting conducting states, which are shown to arise from the interface and functional molecule, respectively. The results show that proper functional molecules can be used in conjunction with metallic electrodes to achieve basic electronics functionality at molecular length scales. [ABSTRACT FROM AUTHOR]

Using the first-principles method with the generalized gradient approximation, the authors have studied the structural and electronic properties of Al12X+ (X=C, Si, Ge, Sn, and Pb) clusters in detail. The ground state of Al12C+ is a low symmetry Cs structure instead of an icosahedron. However, the Si, Ge, Sn, and Pb atom doped cationic clusters favor icosahedral structures. The ground states for Al12Si+ and Al12Ge+ are icosahedra, while the C5ν structures optimized from an icosahedron with a vertex capped by a tetravalent atom have the highest binding energy for Al12Sn+ and Al12Pb+ clusters. The Ih structure and the C5ν structure are almost degenerate for Al12Ge+, whose binding energy difference is only 0.03 eV. The electronic properties are altered much by removing an electron from the neutral cluster. The binding strength of a valence electron is enhanced, while the binding energy of the cluster is reduced much. Due to the open electronic shell, the band gaps between the highest occupied molecular orbital and the lowest unoccupied molecular orbital are ∼0.3 eV for the studied cationic clusters. [ABSTRACT FROM AUTHOR]

We use a self-consistent method to study the current of the single molecular transistor modulated by the transverse field in the level of the density functional theory and the nonequilibrium Green function method. The numerical results show that both the polyacene-dithiol molecules and the fused-ring thiophene molecules are the potential high-frequency molecular transistors controlled by the transverse field. The longer molecules of the polyacene-dithiol or the fused-ring thiophene are in favor of realizing the gate-bias controlled molecular transistor. The theoretical results suggest the related experiments. [ABSTRACT FROM AUTHOR]

We study theoretically the vertical electron transport in semiconductor multiple-quantum-well structures, where sequential tunneling between neighboring wells takes place. The nonuniformity of electric field along the growth axis and charge redistribution among the quantum wells, as well as between the inner wells and contacts, are taken into account. A simple and efficient model of charged contact layers is proposed. The calculated I-V curves exhibit regions of conventional N-shaped negative differential conductivity and Z-shaped portions of intrinsic bistability, both arising due to the tunneling resonances. A general explanation of their formation mechanism is given, which is valid for any form of interwell transitions of resonant nature. The conditions of N- and Z-shaped curve observation and controllable transition between them are discussed. [ABSTRACT FROM AUTHOR]

Using gradient-corrected density functional theory and supercell technique, we have calculated total energies, electronic structure, and magnetic properties of Cr-doped ZnTe in both bulk and thin-film configurations. Calculations with full geometry optimization for a Zn1-xCrxTe supercell were carried out for different Cr concentrations (x=0.095, 0.143, and 0.19) and by varying the sites Cr atoms occupy. We show that the ferromagnetic phase of Zn1-xCrxTe in both bulk and thin film is energetically the most preferable state irrespective of the concentration and/or site occupation of the Cr atom. The strong hybridization between Cr 3d and Te 5p states is found to be responsible for the ferromagnetic coupling, in agreement with recent experiments. [ABSTRACT FROM AUTHOR]

Using density functional theory with generalized gradient approximation for exchange and correlation, we show that otherwise antiferromagnetically coupled chromium atoms in very small chromium clusters couple ferromagnetically when doped with a nitrogen atom, thus leading to giant magnetic moments. For example, the magnetic moment of Cr[sub 2]N is found to be 9μ[sub B] while that of Cr[sub 2] is 0μ[sub B]. Strong bonding between Cr and N atoms brings about this magnetic transition. The Cr atoms nearest neighbor to N couple ferromagnetically with each other and antiferromagnetically with nitrogen. The significance of these results in understanding the ferromagnetic order in Cr-doped GaN is discussed. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Investigates a body-centered-cubic lattice model with realistic many-body interactions to describe both crystalline and molten states of silicon. Energy of the atoms surrounded by tetrahedral first-neighbors; First-order transition between a solid with diamond structure and a melt; Nucleation and domain growth in certain range of supercooling.

Inspired by the successful synthesis of phase separated in-plane graphene/h-BN heterostructures, we have explored the design of one dimensional graphene/h-BN hetero nanoribbon (G/BNNR). Using first-principles density functional based approach, the electronic and magnetic properties of the hybrid nanoribbons with mono-hydrogenated edges have been investigated for different configurations with alternative composition of C-C and B-N units in a ribbon of fixed width. Our results suggest that the electronic as well as magnetic properties of the ribbons can be regulated by varying the number of C-C (or B-N) units present in the structure. Both the hetero nanoribbons, either with N or B terminated edges, undergo a semiconductor-to-semimetal-to-metal transition with the increase in the number of C-C units for a fixed ribbon width. The spin density distribution indicates significant localization of the magnetic moments at the edge carbon atoms, that gets manifested when the number of C-C units is greater than 2 for most of the structures. [ABSTRACT FROM AUTHOR]

The electronic and magnetic properties of double-impurities-doped TiO2 (rutile) are explored using first-principles calculations within the generalized gradient approximation to examine their potential use as spintronic system. Calculations are performed on all possible sets of double impurities (M1 and M2) from M1=Cr, Mn, Fe, Co, and Ni, and M2=Mo, W, and Re. The results show the overlapping of the highest occupied impurity states with the bottom of the conduction band of the host system and the half-metallic nature in Fe- and W-doped TiO2, making them suitable as spintronic systems. These impurities preferentially substitute adjacent Ti sites and form a stable magnetic complex with oxygen atoms. [ABSTRACT FROM AUTHOR]

Blueshifts in the ultraviolet absorption spectra have been observed in cerium oxide nanocrystallites. The mechanism of the absorption is a charge-transfer optical transition. The relationships among the blueshift, valence state of cerium ions, and the particle size are formulated. The blueshifts are well explained for diameters down to less than a few nanometers by the change in the electronic band structure. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The properties of binary H2+ CH4 clathrate hydrates have been estimated using the extended van der Waals and Platteeuw statistical thermodynamic model that takes into account the lattice relaxation, host-guest, and guest-guest interactions as well as the quantum nature of guest behavior in the clathrate cavities. It has been found that at a small methane concentration in the gas phase the stable hydrate phase has cubic structure II (CS-II) and at a methane concentration of 6% stabilizes cubic structure I, which is metastable in the case of the pure hydrogen hydrate. This is in agreement with recent experimental data. The amount of hydrogen storage depends on the methane concentration in the gas phase as well as the thermodynamic conditions of hydrate formation. Hydrogen storage up to 2.6 wt. % can be achieved in the binary H2+ CH4 CS-II hydrate at T = 250K [ABSTRACT FROM AUTHOR]

Unexpectedly strong magnetic coupling is observed in boride insulators containing the B12 icosahedra as a structural unit. We have successfully synthesized Dy0.74B22C2N, which takes the RB22C2N-type crystal structure. The Dy phase was previously thought to not be possible to form. Spin glass-like behavior was observed with a broad peak in the zero field cooled (ZFC) magnetic susceptibility at T ∼ 44 K, while a large divergence for ZFC and field cooled (FC) curves is observed. The Curie-Weiss constant is determined to be θ=-43.9 K, and this system is indicated to have the strongest magnetic coupling ever observed for any rare earth boride insulator. Electronic structure and chemical bonding calculations were also made. Conventional mechanisms for f-electron magnetism cannot explain the strong coupling observed, which further indicates that the B12 icosahedra is functioning as a novel mediator of magnetic interaction. [ABSTRACT FROM AUTHOR]

The geometry, stability, and electronic properties of B@Co12@Bn and Co13@Bn clusters with a wide range n=55 up to 92 are studied by ab initio calculations. We find that B@Co12@B80 and Co13@B80 with closed B80 shell are two stable magic clusters with nearly perfect icosahedral symmetry, and B@Co12@B80 is more stable than Co13@B80 energetically. The strong core-shell bonding yields a very large energy gain of ∼30 eV. This high stability is attributed to the favorable closed-shell atomic and electronic structures. The B@Co12@B80 exhibits a large highest occupied and lowest unoccupied energy gap (0.96 eV) that is close to the value for isolated B80 fullerene. [ABSTRACT FROM AUTHOR]

A comprehensive theoretical study of electronic and magnetic properties of V-doped ZnO in bulk as well as [formula] thin films has been performed using density functional theory. Vanadium atoms substituted at Zn sites show very little selectivity of site occupancy. More importantly, different geometries with ferromagnetic, ferrimagnetic, and antiferromagnetic configurations are found to be energetically nearly degenerate both in Zn1-xVxO bulk and subsurface layers of the thin film. On the other hand, V atoms couple ferromagnetically when they occupy surface sites of the thin film. The diverse magnetic behaviors in V-doped ZnO account for the many reported conflicting experimental results. [ABSTRACT FROM AUTHOR]

The authors report ab initio total energy calculations on the first-order structural transition of the ferromagnetic MnCo1-xGe(0.00≤=x≤=0.25) intermetallic compound. They show that increasing Co vacancies induce a transition from an orthorhombic structure at 0≤=x≤=0.08 to a hexagonal structure at x>0.08. A concomitant high-to-low moment magnetic transition and a large magnetovolume effect occur due to the change of the symmetry and the resulting coupling distance between the magnetic atoms. These results provide an excellent account for the experimental results and reveal the crucial role of the Co vacancies in determining the relative structural stability and the magnetic properties of MnCo1-xGe [ABSTRACT FROM AUTHOR]

Based on ab initio Monte Carlo approach, finite-temperature magnetism of tetragonal Fe is studied. It is shown that the ground state magnetic structure changes from ferromagnetic to antiferromagnetic as the structure transforms from bcc to fcc. The Curie temperature (TC) or Néel temperatures (TN) are very sensitive to the lattice distortion, which decreases from bcc to fcc and then increases over the fcc range. There are a maximum of TC at the bcc and a minimum of TN around the fcc limit due to the change of the coupling distance and symmetry consideration. [ABSTRACT FROM AUTHOR]

Contrary to theoretical prediction that Cr-doped bulk ZnO is ferromagnetic, recent experiments on Cr-doped ZnO thin film reveal the coupling to be antiferromagnetic. Using first-principles calculations based on gradient corrected density functional theory, we show that a possible origin of this disagreement may be associated with the site preference of Cr. In bulk, when Cr substitutes Zn, bond contraction occurs and Cr atoms prefer to cluster around O atoms. The ferromagnetic coupling among Cr atoms is driven by Cr 3d and O 2p exchange interactions as in Cr2O cluster. However, when Cr atoms replace Zn on the surface, due to the different bonding environment, bonds expand preventing Cr atoms from clustering around O atoms. Consequently, the coupling between Cr atoms becomes antiferromagnetic. [ABSTRACT FROM AUTHOR]

Transport properties have been studied for a perovskite heterojunction consisting of SrRuO3 (SRO) film epitaxially grown on SrTi0.99Nb0.01O3 (Nb:STO) substrate. The SRO/Nb:STO interface exhibits rectifying current–voltage (I–V) characteristics agreeing with those of a Schottky junction composed of a deep work-function metal (SRO) and an n-type semiconductor (Nb:STO). A hysteresis appears in the I–V characteristics, where high resistance and low resistance states are induced by reverse and forward bias stresses, respectively. The resistance switching is also triggered by applying short voltage pulses of 1 μs–10 ms duration. [ABSTRACT FROM AUTHOR]

Cerium oxide nanocrystalline particles are synthesized and monodispersed in the size range from 2 to 8 nm in diameter. The dependence of the lattice parameters on particle size is obtained by x-ray and electron diffraction analyses. The size dependence well coincides with the estimation based on the assumption that the surface is composed of one layer of Ce2O3 and the inside consists of CeO2. The effect of particle size on lattice parameters is discussed from the differences in the fabrication method and the surface structure. [ABSTRACT FROM AUTHOR]

We have performed a detailed density functional theory study on the structural and electronic properties of NanC60- (n = 1-12) clusters. The calculated vertical detachment energies show good agreement with the experimental data, which confirms the 3p (n = 3p) oscillation rule. The oscillation can be attributed to the combination of the charge depletion distribution induced by removing electrons and the number of the sodium atoms in direct contact with the fullerene. Based on the structural and electronic properties, the Na atoms can be categorized into two groups, one is for the metal atoms directly bonded to the fullerene surface, and the other one is for those without bonding to the fullerene. The Na atoms in group one would donate electrons to both the fullerene and the Na atoms in group two. As the total number of the sodium atoms increases, the number of Na atoms in group one would continue increasing till the size n = 3p - 1 to meet a shoulder from n = 3p - 1 to n = 3p, which accounts for the maximum vertical detachment energy at the size of n = 3p as drawn from the detailed electronic property studies. [ABSTRACT FROM AUTHOR]

A two-dimensional artificial crystal, which is made up of artificial atoms—identical Al clusters with nanometer size and spacing, was fabricated by taking advantage of surface-mediated clustering on a growth template. In situ scanning tunneling microscopy analysis and first-principles total energy calculations were used to determine the atomic structure of the Al nanoclusters. The Al clusters exhibit more remarkable thermal stability than the In clusters we reported previously. Based on our systematic observations and calculations, the formation mechanism and the high stability of these magic clusters are discussed. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Positive and negative bias-voltages are applied to single-walled carbon nanotubes (SWNTs) in magnetized alkali-metal and alkali-fullerene plasmas. When accelerated ions are irradiated to the SWNTs through plasma sheaths, drastic structural deformations such as deflection and tube cutting of the SWNTs are observed to take place. Furthermore, this phenomenon is found to be accompanied by the fullerene encapsulation inside the SWNTs in the case of the positive-bias application in the alkali-fullerene plasma, giving the possibility that various kinds of atoms and molecules can effectively be intercalated by our plasma method. © 2001 American Institute of Physics. [DOI: 10.1063/1.1427744]. [ABSTRACT FROM AUTHOR]

Using the local-density approximation, and calculating the Hellmann-Feynman forces, the stability of the perovskite-like structures of CaTiO[sub 3] have been studied by applying the direct method and deriving the phonon dispersion relations. The cubic Pm3¯m phase shows dispersionless soft phonon branch spreading from R to M points of the cubic Brillouin zone. Low-symmetry phase I4/mcm can be considered as a result of the soft mode condensation at R point. The experimentally observed Pmnb phase is a consequence of the intersection of Imma and P4/mbm space groups, being the result of soft mode condensation at R and M points, respectively. Thus, the Pmnb can be regarded as simultaneous condensations of two soft modes. The phonon dispersion relations of Pmnb suggest that this phase is stable. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The formation of atom-doped fullerenes has been investigated by using several types of radionuclides produced by nuclear reactions. It was found that the endohedral fullerenes (Kr@C[sub 60], Xe@C[sub 60]) and their dimers, furthermore, heterofullerenes, such as AsC[sub 59], GeC[sub 59] and their dimers, are produced by a recoil process following nuclear reactions. Other nuclear reaction products (Na, Ca, Sc, etc.) may destroy most of the fullerene cage in the same process. Carrying out ab initio molecular-dynamics simulations based on an all-electron mixed-basis approach, we confirmed that the formation of Kr- (or Xe-) atom-doped endohedral fullerenes and of substitutional heterofullerenes doped with an As atom is really possible. The experimental and theoretical results indicate that the chemical nature of doping atoms is important in the formation of foreign-atom-doped fullerenes. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]