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Abstract MoS2 has attracted intense interest in many applications. Natural MoS2 and field-effect transistors made of it generally exhibit n-type characteristics, but its origin is unknown. Herein, we show that C is the origin of the universal n-type doping of natural MoS2. Photoemission spectroscopies reveal that while many MoS2 samples with C detected are n-type, some without C exhibit p-type characteristics. The C-free, p-type MoS2 changes to n-type over time with the concurrent appearance of C that is out-diffused from bulk, indicating that C induces the n-type doping. The C-origin is verified by C-deposition and supported by theoretical calculations. This carbon appears as nanometer-scale defects frequently observed in scanning tunneling microscopy. In addition, we propose, based on the calculations, that S vacancies are responsible for the p-type characteristics, which contrasts with the widespread belief. This work provides new perspectives on MoS2 doping and presents a new direction for fabricating reliable MoS2 devices.

Vanadium ditelluride (VTe2) has been intensively explored to understand the charge density wave (CDW) phase and its connection to magnetic properties. Here, we conduct a systematic study to understand the fine structure of CDW via scanning tunneling microscopy (STM) combined with density functional theory (DFT) calculations. STM topograph at 79 K shows that a CDW phase in VTe2 has a stripe modulation with 3 × 1 periodicity, following the double zigzag chain of distorted Te lattices. Interestingly, the 3 × 1 CDW modulation undergoes contrast inversion between filled and empty state topographs. Atomistic features and contrast changes of CDW observed in STM are clearly reproduced in our DFT simulation images. Charge distribution calculation indicates that the spatial extension and density of Te 5p orbitals have strong variations with filled and empty states, explaining the fine structure of 3 × 1 CDW in VTe2. Our finding provides an inspiring insight to further research on the less explored electronic structure of VTe2.

Using a Green function method based on an atomic vibration model, herein we report the results from our investigation of phonon transport through multiple Si/PtSi layered structures. In contrast with values predicted using elastic wave theory and an impedance mismatch method, we find that a detailed atomic-vibration approach exhibits significantly suppressed phonon transport and leads to a 30-times reduction of the thermal conductance, compared to that of Si bulk. We attribute the origin of the suppression to the lack of PtSi phonon modes in the energy range of 20-30 meV, and to the effects of interface scattering between Si and PtSi layers. [ABSTRACT FROM AUTHOR]

Impurities interact with a charge density wave (CDW) and affect the phase transitions in low-dimensional systems. By using scanning tunneling microscopy, we visualize the interaction between oxygen impurities and the CDW in indium atomic wires on Si(111), a prototypical one-dimensional electronic system, and unveil the microscopic mechanism of the intriguing O-induced increase of the transition temperature ( T _c ). Driven by the fluctuating CDW, the O atoms adopt an asymmetric structure. By adjusting the asymmetry, a pair of O impurities in close distance can pin the one-dimensional CDW, which develops into the two-dimensional domains. First-principles calculations showed that the asymmetric interstitially-incorporated O defects induce shear strains, which assists the formation of hexagon structure of the CDW phase. The cooperative interplay between the O impurities and the CDW is responsible for the enhancement of the CDW condensation and the consequent increase in T _c .

The structural properties and magnetic phase stability of [Fe.sub.(x)][Pt.sub.(1-x)] alloys in [L1.sub.2] crystal structure in bulk as well as thin film on Pt (110) substrate are analyzed by using highly precise full-potential linearized augmented plane-wave method within generalized gradient approximation. The ferromagnetic (FM) preference of thin film is predicted to fast suppress as the film thickness has increased and the antiferromagnetic (AFM) preference is recovered for the film thickness of five atomic layers.

We investigate the effects of dielectric environments on the transition levels of Re and Nb dopants in monolayer MoS2 through density functional theory calculations. The inherently weakly screened Coulomb interaction in free-standing monolayer MoS2 makes the dopant electrons and holes strongly bound, and the Re and Nb impurities are found to produce deep levels. It is shown that when the monolayer MoS2 is placed near high permittivity dielectrics the screened Coulomb interaction induces carrier derealization with generating shallow levels. [ABSTRACT FROM AUTHOR]

The atomic and electronic structures and stability of native defects in a single-layer MoS2 are investigated, based on density-functional theory calculations. Native defects such as a S vacancy (Vs), a S interstitial (Si), a Mo vacancy (VMo)> and a Mo interstitial (Moi) are considered. The Si is found to have S-adatom configuration on top of a host S atom, and the Moi has Mo-Moi split interstitial configuration along the c direction. The formation energies of the native defects in neutral and charged states are calculated. For the charged states, the artificial electrostatic interactions between image charges in supercells are eliminated by a supercell size scaling scheme and a correction scheme that uses a Gaussian model charge. It is found that the Vs has a low formation energy of 1.3-1.5 eV in the Mo-rich limit condition, and the Si has 1.0 eV in the S-rich limit condition. The Vs is found to be a deep single acceptor with the (0/-) transition level at 1.7 eV above the valence-band maximum (VBM). The Si is found to be an electrically neutral defect. The Mo-related native defects of VMo and Moi are found to be high in formation energy above 4 eV. The VMo is a deep single acceptor and the Moi is a deep single donor, of which the (0/-) acceptor and (+/0) donor transition levels are found at 1.1 and 0.3 eV above the VBM, respectively. [ABSTRACT FROM AUTHOR]

Abstract The energetics and the electronic structure of the Si/Mo(110) surface have been investigated using density functional theory calculations based on the generalized gradient approximation. The calculated potential energy surface for a single Si adatom reveals that a hollow site is favored for the adsorption of Si on Mo(110). The energy barrier for hopping between the hollow sites is located at the bridge site and is found to be 0.64 eV. The electron density plot indicates that four Mo-Si covalent bonds were formed around the Si atom at the hollow site. According to the surface formation energy for different Si coverage, 1 ML Si/Mo(110)-p(1× 1) is energetically favorable for a Si-rich environment. For the Si-poor case, the clean Mo(110) surface is the most stable structure. [ABSTRACT FROM AUTHOR]

The electron conduction mechanism in indium oxide (In2O3) and its implications for amorphous transport have been investigated from an orbital overlap perspective. Combined density functional theory and empirical tight binding modeling reveal that the electron transport is facilitated by the neighboring metal atomic s orbital overlap "without" oxygen's p-orbital involvement. In other words, the electron transport pathway in oxides is only due to the metal-metal medium range connection. This electron conduction mechanism is extended to amorphous In2O3 which unveils that the amorphous disorder influences the electron transport through impacting the metal-metal medium range order including metal-metal coordination number and metal-metal separation. Our results provide an insight into the current theoretical understanding of electron transport in amorphous oxide semiconductors. [ABSTRACT FROM AUTHOR]

MoS2 has attracted intense interest in many applications. Natural MoS2 and field-effect transistors made of it generally exhibit n-type characteristics, but its origin is unknown. Herein, we show that C is the origin of the universal n-type doping of natural MoS2. Photoemission spectroscopies reveal that while many MoS2 samples with C detected are n-type, some without C exhibit p-type characteristics. The C-free, p-type MoS2 changes to n-type over time with the concurrent appearance of C that is out-diffused from bulk, indicating that C induces the n-type doping. The C-origin is verified by C-deposition and supported by theoretical calculations. This carbon appears as nanometer-scale defects frequently observed in scanning tunneling microscopy. In addition, we propose, based on the calculations, that S vacancies are responsible for the p-type characteristics, which contrasts with the widespread belief. This work provides new perspectives on MoS2 doping and presents a new direction for fabricating reliable MoS2 devices. [ABSTRACT FROM AUTHOR]

Vanadium ditelluride (VTe2) has been intensively explored to understand the charge density wave (CDW) phase and its connection to magnetic properties. Here, we conduct a systematic study to understand the fine structure of CDW via scanning tunneling microscopy (STM) combined with density functional theory (DFT) calculations. STM topograph at 79 K shows that a CDW phase in VTe2 has a stripe modulation with 3 × 1 periodicity, following the double zigzag chain of distorted Te lattices. Interestingly, the 3 × 1 CDW modulation undergoes contrast inversion between filled and empty state topographs. Atomistic features and contrast changes of CDW observed in STM are clearly reproduced in our DFT simulation images. Charge distribution calculation indicates that the spatial extension and density of Te 5p orbitals have strong variations with filled and empty states, explaining the fine structure of 3 × 1 CDW in VTe2. Our finding provides an inspiring insight to further research on the less explored electronic structure of VTe2. [ABSTRACT FROM AUTHOR]

Magnesium alloys have a wide range of application values. To design and develop magnesium alloys with excellent mechanical properties, it is necessary to study the deformation process. In this paper, the uniaxial compression (UC) process of AZ31 magnesium alloy with different solute atom content is simulated by the molecular dynamics method. The effect of the solute atom on the uniaxial compression of magnesium alloy is investigated. It is found that solute atoms can inhibit the grain refinement of magnesium, can effectively improve the plastic strength of the alloy, can change the lattice distortion during uniaxial compression of magnesium alloy, can inhibit the generation of BCC structure, and can slow down the increase of FCC structure and dislocation density. The direction of the FCC structure diffusion is 90° to the grain boundary direction. Shockley partial dislocations are generated around the FCC structure. The direction in which the FCC structure spreads is consistent with the direction in which Shockley partial dislocations move. [ABSTRACT FROM AUTHOR]

Substitutional doping in 2D semiconductor MoS2 was investigated by charge transition level (CTL) calculations for Nitrogen group (N, P, As, Sb) and Halogen group (F, Cl, Br, I) dopants at the S site of monolayer MoS2. Both n-type and p-type dopant levels are calculated to be deep mid-gap states (~1 eV from band edges) from DFT total energy-based CTL and separate DFT + GW calculations. The deep dopant levels result from the giant renormalization of hydrogen-like defect states by reduced dielectric screening in ultrathin 2D films. Theoretical analysis based on Keldysh formulation provides a consistent impurity binding energy of ~1 eV for dielectric thin films. These findings of intrinsic deep impurity levels in 2D semiconductors MoS2 may be applicable to diverse novel emerging device applications. [ABSTRACT FROM AUTHOR]

The adsorption structures and thermal desorption behavior of C[SUB2]H[SUB2] on Ge(100) were studied in ultrahigh vacuum by scanning tunneling microscopy (STM) and temperature programmed desorption (TPD). The STM investigation revealed that, at low coverage, C[SUB2]H[SUB2] initially adsorbs onto the Ge(100) surface with two types of configurations: (i) a di-σ configuration on top of a single Ge-Ge dimer (on-top) and (ii) a tetra-σ configuration parallel to the dimer axes, bridging two neighboring Ge dimers (p-bridge). TPD measurements show that chemisorbed C[SUB2]H[SUB2] desorbs from Ge(100) nondissociatively with two different desorption features, denoted as α (520 K) and β (560 K). In addition, it was found that the desorption of C[SUB2]H[SUB2] follows first order kinetics for both states and that the desorption energies of the α (520 K) and β (560 K) states are 1.3 and 1.4 eV, respectively. STM studies of the adsorption of C[SUB2]H[SUB2] at various Ge surface temperatures indicate that the α and β features correspond to the on-top and p-bridge configurations, respectively. [ABSTRACT FROM AUTHOR]

A correction to the acknowledgement section of the article “Magnetic Transition to Antiferromagnetic Phase in Gadolinium Substituted Topological Insulator Bi2Te3" that was published in the May 14, 2015 issue is presented.

First principle calculation based on density functional theory (DFT) was used to evaluate some physical properties of Uranium Nitrides. Adsorption of oxygen O atom and O2 molecule on/in (001) surfaces of both Uranium monoNitride (UN) and diNitride (UN2) was then studied and compared mutually. To treat the strong correlation effects caused by 5f Uranium valence electrons, Hubbard-U advanced (DFT + U) approach was employed to correct the exchange correlation functional GGA and PBE which are based on generalized gradient approximation. The functional are developed for the Vienna Abinitio Simulation Package (VASP) and were used with the projector-augmented wave (PAW) pseudo potentials. The structural and elastic-mechanical UN and UN2 properties were calculated within DFT and DFT + U methods. Then, Potential Energy Surfaces (PES) concepts which correspond to the interaction between O atom (respectively O2 molecule) and (001) on-surfaces / sub-surfaces uranium nitrides for several positions were determined to identify favorable adsorption sites. Physical properties calculation results of UN or UN2 are in order of magnitude of other theoretical values and show an acceptable precision compared to experiments. Hubbard U value of the DFT + U formalism was optimized to achieve Antiferromagnetic (AFM) UN configuration and was effective at U = 1.625 eV. Optimization of UN2 was accomplished to attain experimental cell parameter of 5.31 A° and was reached for U = 2.6 eV. According to our calculation results, O2 diffusion through UN(001) and UN2(001) clean surfaces have demonstrated dissociation of the molecule from a distance of approximately d = 1.5 Å. Favored on surface modes for O atom adsorption were found to be near the bridge site for UN(001) and UN2(001). The O incorporation through UN(001) surface was at the bridge site, nevertheless, for UN2, merging of O atom in the (001) surface bridge site was not allowed. [ABSTRACT FROM AUTHOR]

Large-area GaAs nanowires are fabricated using SiO2 nanoparticles as the etching mask. SiO2 nanoparticle monolayer is spin coated on the GaAs substrate. To obtain a uniform monolayer of SiO2 nanoparticles across the substrate, raised temperature, adequate solution concentration, and the substrate treated with a solvent for interface activation are required. With the monolayer of SiO2 nanoparticles as the etching mask, the GaAs substrate is etched by induced-coupled plasma reactive ion etcher (ICP-RIE) to form GaAs nanowires with a high aspect ratio. The diameter and length of GaAs nanowires are 70 nm and 1.2 μm, respectively. The diameter and length of GaAs nanowires can be controlled by the size of SiO2 nanoparticles and etching time of ICP-RIE. [ABSTRACT FROM AUTHOR]

Since 1986, most of the interest in superconductivity became focused on high-Tc cuprates. The discovery of the superconducting quaternary borocarbide system Y–Ni–B–C with Tc as high as ∼⃒12 K inspired research into intermetallic superconductors (IMS) once again. Several reasons can be attributed to this revival of interest in IMS: (i) In the tetragonal quaternary magnetic superconductors RNi2B2C, superconductivity and magnetism occur with Tc and TN ∼⃒ 10 K, thereby allowing studies of exotic phenomena associated with, and arising from, the interplay of superconductivity and magnetism. (ii) High TN's and a variety of commensurate and incommensurate magnetic structures in RNi2B2C (Fermi surface nesting playing a central role) strongly suggest that R-spins are coupled via the RKKY-exchange interaction. Hence, unlike in most other magnetic superconductors known so far, conduction electrons take part in superconductivity and magnetism. (iii) Quaternary borocarbides open up new pathways to try and synthesize multicomponent intermetallic superconductors. Their remarkable intrinsic superconducting and magnetic properties and the availability of high quality samples (bulk polycrystalline, large single crystals and thin films) make RNi2B2C particularly special to investigate. Several unusual phenomena have been reported, such as, to name a few, dramatic phonon mode softening at Tc, Hc2(T) exhibiting a positive curvature near Tc and a four-fold anisotropy in the basal plane; a variety of exceptional and fascinating flux line lattice (FLL) related effects — FLL-symmetry transformations and alignments with the underlying crystal lattice as a function of applied field (manifestation of nonlocal electrodynamics despite high κ ∼⃒ 10, and thermal fluctuation effects even though Tc, ∼⃒ 16 K, is not too high) and a four-fold symmetric star-shaped (in real space) vortex core. RNi2B2C are strong coupling s-wave BCS superconductors and, remarkably, have a superconducting gap with extreme anisotropy. Strong experimental evidence shows that the four-fold symmetric superconducting gap has point nodes along the 100- and 10-directions, a feature that has been shown consistent with (s + g)-Cooper pairing. An energy gap with such strong anisotropy is unusual for an s-wave superconductor and, hence, calls for a pairing mechanism different from conventional electron–phonon coupling. Antiferromagnetic fluctuations possibly play an important role in the mechanism. Magnetic superconductors RNi2B2C (R = Dy, Ho, Er, Tm) reveal several phenomena, not observed earlier, associated with the interplay of superconductivity and magnetism. Microscopic evidence (via square FLL interacting with magnetism) of the coexistence of magnetism and superconductivity; intrinsic FLL-pinning by magnetic ions; weak ferromagnetism (local moment) coexisting with superconductivity (down to the lowest temperature) and the spontaneous vortex phase (ErNi2B2C); superconductivity setting in an already magnetically ordered lattice (DyNi2B2C) and pair-breaking by nonmagnetic ions in such materials; rich and complex magnetic structures and double (nearly) re-entrant superconductivity (HoNi2B2C) and changes in the FLL-symmetry in the vicinity of magnetic transition (TmNi2B2C) and 4f-quadrupole ordering (TmNi2B2C) are several exciting phenomena that magnetic superconductors RNi2B2C exhibit.At the end of this review are indicated some possible further studies in quaternary borocarbide superconductors. These studies may turn out to be important not only with respect to borocarbides themselves but also from the standpoint of superconductivity in general. [ABSTRACT FROM AUTHOR]

The effect of interlayer interactions on the electronic specific heat of high- Tc layered superconductors has been studied within the framework of the Hirsch model. In the present paper we extend our previous work in order to evaluate the expression for specific heat. It is shown that the inclusion of interlayer interactions suppresses the height of the jump in the specific heat at Tc. [ABSTRACT FROM AUTHOR]

Microscopic mechanism of autocatalytic structural transformations of fullerenes is investigated by the action-derived molecular dynamics. Dynamic pathways and the corresponding activation energies are obtained for the Stone-Wales transformation in fullerene and the fullerene coalescence, under the presence of extra carbon atoms. The adatom-assisted Stone-Wales transformation is proved to be a highly probable process unit for the structural transformations and annealing treatments of carbon-based graphitic networks. The complex processes of adatom-assisted fullerene coalescence, yielding very low activation energies, are presented. [ABSTRACT FROM AUTHOR]

We demonstrate that the controlled distribution of nanoparticles can be achieved by employing the spin-coating method. The Co and Ag nanoparticles were uniformly distributed on the Si and SiO[sub 2] substrates with this method. The particle density was controllable by varying the concentration of colloids. The spatial distribution of the nanoparticles within the patterned area was also shown to be uniform with small boundary effect, which is favorable for current microelectronics technology. We propose that the spin-coating method can be utilized in developing mass production processes for future nanodevices. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Byline: In-Ho Lee, Sukky Jun, Hanchul Kim, Seung-Yeon Kim, Jooyoung Lee Action-derived molecular dynamics for the simulation of rare event and slow mode systems is reviewed. Theoretical background, implementation details, and comparison with other methods are presented. Numerical examples demonstrated include the structural formation and transformation of carbon fullerenes, and molecular reconfiguration of alanine dipeptide and valine dipeptide. It is shown that the action-derived molecular dynamics is efficient to explore the dynamic pathways of various chemical reactions.

The introduction of ferromagnetic order in topological insulators in general breaks the time-reversal symmetry and a gap is opened in topological surface bands. Various studies have focused on gap-opened magnetic topological insulators, because such modified band structures provide a promising platform for observing exotic quantum physics. However, the role of antiferromagnetic order in topological insulators is still controversial. In this report, we demonstrate that it is possible to restore the topological surface states by effectively reducing the antiferromagnetic ordering in Gd-substituted Bi2Te3. We successfully control the magnetic impurities via thermal treatments in ultra-high vacuum condition and observe apparent restoration of topological surface band dispersions. The microscopic mechanism of atomic rearrangements and the restoration process of topological surface states are unraveled by the combination of scanning tunneling microscopy measurements and density functional theory calculations. This work provides an effective way to control the magnetic impurities which is strongly correlated with topological surface states. [ABSTRACT FROM AUTHOR]

Recent studies on a quasi-one-dimensional (quasi-1D) charge-density-wave (CDW) system, In atomic wires on Si(111), have brought intriguing issues on topological solitons in quasi-1D systems: the existence of few-atom-sized solitons, chirality of solitons, and the realization of logic exploiting the chirality switching. Using scanning tunneling microscopy and first-principles calculations, we show that the previously reported "short" phase-flip defects are In adatoms and thus have nonsolitonic nature, resolving the controversy over the existence of highly localized solitons. The observed "long" phase-flip and phase-slip defects are genuine solitons with and without chirality, respectively. While achiral solitons (phase-slip defects) can exist on the pristine CDW (8×2) surface, chiral solitons (phase-flip defects) cannot due to their breakage of 8×2 ordering. The chiral solitons can exist only when they are trapped by In adatoms and constitute a part of a closed-loop domain wall. The intertwinement of chiral solitons and In adatoms implies the limitations of the previously proposed logic utilizing soliton chirality, but it provides an opportunity to realize this by controlling the In-adatom defect. [ABSTRACT FROM AUTHOR]