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Transition-metal dichalcogenide (TMDC) homo- and heterostacks hold tantalizing prospects for being integrated as active components in future van der Waals (vdW) electronics and optoelectronics. How...

The adsorptions of CO molecule on various fcc(111) surfaces (Rh, Ir, Pt, and Cu) have been studied by diffusion quantum Monte Carlo (DMC) calculations, and the results show that the top site is the...

Motivated by the synthesis of the layered structure CoO2via Li atom deintercalation from LixCoO2, herein, we investigated the electronic structure, lattice dynamics, electron–phonon interaction, and superconductivity of monolayer CoO2 using first-principles calculations. This 2D material was predicted to have a ferromagnetic ground state with a metallic band structure and the total magnetization of 0.83μB. Remarkably, the non-spin polarized calculations show that the monolayer CoO2 possesses phonon-mediated superconductivity at 25–28 K owing to its intermediate to strong electron–phonon coupling (EPC). The rather strong EPC in this compound is mainly driven by the acoustic phonons, making this compound one of the highest-temperature superconductors among the existing 2D materials. Moreover, the CoO2 sheets could be synthesized via exfoliation from bulk CoO2 owing to the relatively small interlayer binding energy while maintaining its stability under normal experimental conditions. Compared to its bulk and bilayer counterparts, monolayer CoO2 was found to have highest EPC.

We report on the formation and comprehensive study of atomic and electronic structures of a new Tl-Mg two-dimensional compound reconstruction with $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ periodicity on a Si(111) substrate. Owing to properties of its constituent elements and crystal structure with a breathing kagome lattice of Mg and a dense Tl layer (4/3 monolayer), this double-layer system displays promising features, including spin-split metallic bands, nearly flat bands, and a saddle point with corresponding divergences in the local density of states. These features indicate the possibility that strongly correlated electronic states exist in this structure after tuning the Fermi level position. In accordance with the ${C}_{3}$ symmetry, we observe spin-orbit-coupling-driven Rashba-type spin splitting with spin degeneracy at the $\overline{M}$ point and Zeeman-like splitting at the $\overline{K}$ point. In contrast to the previously reported cases, splitting at the $\overline{K}$ point is significant to produce fully out-of-plane spin-polarized cones. Due to opposite Berry curvatures at the $\overline{K}$ and ${\overline{K}}^{\ensuremath{'}}$ points of the hexagonal surface Brillouin zone, spins at these valleys have opposite signs, which ensure effective spin-pseudospin coupling.

Metastable double-atomic layer of Tl on Si(111) has recently been found to display interesting electric properties, namely superconductivity below 0.96 K and magnetic-field-induced transition into an insulating phase intermediated by a quantum metal state. In the present work, using a set of experimental techniques, including low-energy electron diffraction, scanning tunneling microscopy, angle-resolved photoelectron spectroscopy, in a combination with density-functional-theory calculations, we have characterized atomic and electronic properties of the Tl double layer on Si(111). The double Tl layer has been concluded to contain ∼ 2.4 monolayer of Tl. A top Tl layer has a ‘1 × 1’ basic structure and displays 6 × 6 moire pattern which originates from various residence sites of Tl atoms. Upon cooling below ∼ 140 K, the 6 × 6 moire pattern changes to that having a 6 3 × 6 3 periodicity. However, the experimentally determined electron band dispersions show a 1 × 1 periodicity. The calculated band structure unfolded into the 1 × 1 surface Brillouin zone reproduces well the main features of the photoelectron spectra.

This report presents a series of low-temperature (4.5 K) scanning tunneling microscopy and spectroscopy experimental results on monolayer MoS2 deposited on highly oriented pyrolytic graphite using chemical vapor deposition. To reveal the detailed connection between atomic morphology and conductivity in twisted MoS2/graphite heterojunctions, we employ high-sensitivity tunneling spectroscopy measurements by choosing a reduced tip-sample distance. We discern previously unobserved conductance peaks within the band gap range of MoS2, and by comparing the tunneling spectra from MoS2 grains of varying rotation with respect to the substrate, show that these features have small but non-negligible dependence on the moire superstructure. Furthermore, within a single moire supercell, atomically resolved tunneling spectroscopy measurements show that the spectra between the moire high and low areas are also distinct. These in-gap states are shown to have an energy shift attributed to their local lattice strain, matching corresponding behavior of the conduction band edge, and we therefore infer that these features are intrinsic to the density of states, rather than experimental artifacts, and attribute them to the twisted stacking and local strain energy of the MoS2/graphite heterointerface. New moire-related states emerge in the bandgap of molybdenum disulfide when interfaced with highly oriented pyrolytic graphite. Two-dimensional materials enable designer heterostructures to be created layer-by-layer, but greater understanding of how different interfaces affect the electronic structure is required if they are to be fully exploited for applications. An international team of researchers led by Minn-Tsong Lin from National Taiwan University use low-temperature scanning tunneling microscopy and spectroscopy to show that in-gap states can form at molybdenum disulfide/graphite heterointerfaces, which are sensitive to both the stacking angle and local strain, and have a small dependence on the moire superstructure. These tunable in-gap states, which are attributed to interlayer charges, show the complexity, but also potential of using stacking and strain for engineering the electronic structure of heterostructures.

An ordered 4×4-periodicity 2D compound has been found in the (Tl, Pb)/Si(111) system and its composition, structure and electronic properties have been characterized using low-energy electron diffraction, scanning tunneling microscopy observations and density-functional-theory calculations. The compound has been concluded to contain 9 Tl atoms and 12 Pb atoms per 4×4 unit cell, i.e., 0.56 ML of Tl and 0.75 ML of Pb. Structural model was proposed for the 4×4-(Tl, Pb) compound where building blocks are a hexagonal array of 12 Pb atoms, a triangular array of 6 Tl atoms and a Tl trimer. The proposed structure has a C3 symmetry and occurs in the two equivalent orientations. The electron band structure of the compound contains two metallic spin-split surface-state bands. Bearing in mind the advanced properties of the known 3 × 3 2D compound in the same (Tl, Pb)/Si(111) system (i.e., combination of giant Rashba effect and superconductivity), the found 4×4-(Tl, Pb) compound is believed to be a promising object for exploration of its superconductive properties.

In this work, a high thermoelectric figure of merit, zT of 1.9 at 740 K is achieved in Ge1−xBixTe crystals through the concurrent of Seebeck coefficient enhancement and thermal conductivity reduction with Bi dopants. The substitution of Bi for Ge not only compensates the superfluous hole carriers in pristine GeTe but also shifts the Fermi level (EF) to an eligible region. Experimentally, with moderate 6–10% Bi dopants, the carrier concentration is drastically decreased from 8.7 × 1020 cm−3 to 3–5 × 1020 cm−3 and the Seebeck coefficient is boosted three times to 75 μVK−1. In the meantime, based on the density functional theory (DFT) calculation, the Fermi level EF starts to intersect with the pudding mold band at L point, where the band effective mass is enhanced. The enhanced Seebeck coefficient effectively compensates the decrease of electrical conductivity and thus successfully maintain the power factor as large as or even superior than that of the pristine GeTe. In addition, the Bi doping significantly reduces both thermal conductivities of carriers and lattices to an extremely low limit of 1.57 W m−1K−1 at 740 K with 10% Bi dopants, which is an about 63% reduction as compared with that of pristine GeTe. The elevated figure of merit observed in Ge1−xBixTe specimens is therefore realized by synergistically optimizing the power factor and downgrading the thermal conductivity of alloying effect and lattice anharmonicity caused by Bi doping.

We have carried out first-principles calculations for Al(100), Al(110), and Al(111) films up to 30 monolayers to study the oscillatory quantum size effects (QSEs) exhibited in the surface energy, work function, electron-phonon coupling constants, and superconductivity transition temperature ${T}_{c}$. Significant oscillatory QSEs on these physical properties are reported. These oscillations are correlated with the thickness dependence of the energies of confined electrons, which can be properly modeled by an energy-dependent phase shift of the electronic wave function upon reflection at the interface. The oscillations in the surface energy and work function for Al(111) and Al(110) films as a function of film thickness can be well fitted by a damped sinusoidal function with the periodicity determined by one Fermi wave vector along the [111] direction or a combination of three Fermi wave vectors along the [110] direction. A quantitative description of these QSE requires the full consideration of the crystal band structure.

Ultrathin Pb and Ge films deposited on Ag(111) surfaces have been investigated and compared. We found that at 1/3 ML, both films formed surface alloys, ${\mathrm{Ag}}_{2}\mathrm{Pb}$ and ${\mathrm{Ag}}_{2}\mathrm{Ge}$, with $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}R{30}^{\ensuremath{\circ}}$ and $\frac{19}{20}\sqrt{3}\ifmmode\times\else\texttimes\fi{}\frac{19}{20}\sqrt{3}R{30}^{\ensuremath{\circ}}$ structures on Ag(111) but the surface electronic structures exhibit a most evident difference at the Ag(111) surface zone boundary ${\overline{M}}_{\mathrm{Ag}(111)}$, where the single band and the splitting ones were observed, respectively. Up to 1 ML, ${\mathrm{Ag}}_{2}\mathrm{Ge}$ subsequently develops into germanene with a striped phase and then a quasifreestanding phase, as previously reported [Lin et al., Phys. Rev. Mater. 2, 024003 (2018)], while ${\mathrm{Ag}}_{2}\mathrm{Pb}$ evolves to a dense Pb(111) phase that also reveals splitting bands at ${\overline{M}}_{\mathrm{Ag}(111)}$. We discover that the larger (smaller) atomic size of a Pb (Ge) atom with respect to an Ag atom causes the commensurate (incommensurate) interfaces and further demonstrate that the splitting bands of ${\mathrm{Ag}}_{2}\mathrm{Ge}$ surface alloy and 1-ML Pb film originated from the commonly incommensurate interface with Ag(111), which mediates umklapp scattering by inducing the mirror image of the pristine ${\mathrm{Ag}}_{2}\mathrm{Ge}$ and Pb(111) bands relative to ${\overline{M}}_{\mathrm{Ag}(111)}.$

Photodriven dipole reordering of the intercalated organic molecules in halide perovskites has been suggested to be a critical degree of freedom, potentially affecting physical properties, device performance, and stability of hybrid perovskite-based optoelectronic devices. However, thus far a direct atomically resolved dipole mapping under device operation condition, that is, illumination, is lacking. Here, we map simultaneously the molecule dipole orientation pattern and the electrostatic potential with atomic resolution using photoexcited cross-sectional scanning tunneling microscopy and spectroscopy. Our experimental observations demonstrate that a photodriven molecule dipole reordering, initiated by a photoexcited separation of electron–hole pairs in spatially displaced orbitals, leads to a fundamental reshaping of the potential landscape in halide perovskites, creating separate one-dimensional transport channels for holes and electrons. We anticipate that analogous light-induced polarization order transitions occur in bulk and are at the origin of the extraordinary efficiencies of organometal halide perovskite-based solar cells as well as could reconcile apparently contradictory materials’ properties.

In this work, a high thermoelectric figure of merit, zT of 1.9 at 740 K is achieved in Ge1−xBixTe crystals through the concurrent of Seebeck coefficient enhancement and thermal conductivity reduction with Bi dopants. The substitution of Bi for Ge not only compensates the superfluous hole carriers in pristine GeTe but also shifts the Fermi level (EF) to an eligible region. Experimentally, with moderate 6–10% Bi dopants, the carrier concentration is drastically decreased from 8.7 × 1020 cm−3 to 3–5 × 1020 cm−3 and the Seebeck coefficient is boosted three times to 75 μVK−1. In the meantime, based on the density functional theory (DFT) calculation, the Fermi level EF starts to intersect with the pudding mold band at L point, where the band effective mass is enhanced. The enhanced Seebeck coefficient effectively compensates the decrease of electrical conductivity and thus successfully maintain the power factor as large as or even superior than that of the pristine GeTe. In addition, the Bi doping significantly reduces both thermal conductivities of carriers and lattices to an extremely low limit of 1.57 W m−1K−1 at 740 K with 10% Bi dopants, which is an about 63% reduction as compared with that of pristine GeTe. The elevated figure of merit observed in Ge1−xBixTe specimens is therefore realized by synergistically optimizing the power factor and downgrading the thermal conductivity of alloying effect and lattice anharmonicity caused by Bi doping.

We report on the low-temperature scanning tunneling microscopy/spectroscopy (STM/STS) study of the (Bi, Na)/Si(1 1 1)[Formula: see text] reconstruction that is known to possess Fermi surface with apparently good nesting. We found that defects on this surface produce a one-dimensional-like pattern with the periodicity of 8.2 [Formula: see text] 0.4 [Formula: see text] that is incommensurate with the [Formula: see text] lattice period. The [Formula: see text] mapping analysis reveals an occurrence of the k-dependent branch associated with quasi-particle interference and the k-independent branch associated with the nesting vector connecting the parallel segments of the (Bi,Na)/Si(1 1 1)[Formula: see text] Fermi surface, the fingerprint of the charge-density-wave (CDW) phase. The STS data demonstrates that development of the CDW phase leads to reducing electron density of states at the Fermi level.

Using first-principles density functional theory calculations, we made an accurate structural characterization of the C60 superstructures self-assembled on the Tl-adsorbed Si(111) and Ge(111) surfaces, which finds a good agreement with the recent scanning tunneling microscopy observations. Our band structure calculations revealed the semi-metallic character of the C60/Tl/Si(111) system, while the C60/Tl/Ge(111) system was found to show up the pronounced metallic character due to the cascade of the flat bands lying in the vicinity of the Fermi level. The latter is a fingerprint for strong correlation effects in the C60/Tl/Ge(111) system, which makes it a promising object for studying electrical transport phenomena and opens the prospects for its application in the molecular-based electronic devices. We elucidated the details of the molecule-substrate and intermolecular interactions and discussed the character of a charge transfer in both systems.

Phase transition in thermoelectric (TE) material is a double-edged sword-it is undesired for device operation in applications, but the fluctuations near an electronic instability are favorable. Here, Sb doping is used to elicit a spontaneous composition fluctuation showing uphill diffusion in GeTe that is otherwise suspended by diffusionless athermal cubic-to-rhombohedral phase transition at around 700 K. The interplay between these two phase transitions yields exquisite composition fluctuations and a coexistence of cubic and rhombohedral phases in favor of exceptional figures-of-merit zT. Specifically, alloying GeTe by Sb

Observations of quasiparticle interference have been used in recent years to examine exotic carrier behavior at the surfaces of emergent materials, connecting carrier dispersion and scattering dynamics to real-space features with atomic resolution. We observe quasiparticle interference in the strongly Rashba split 2DEG-like surface band found at the tellurium termination of BiTeBr and examine two mechanisms governing quasiparticle scattering: We confirm the suppression of spin-flip scattering by comparing measured quasiparticle interference with a spin-dependent elastic scattering model applied to the calculated spectral function. We also use atomically resolved STM maps to identify point defect lattice sites and spectro-microscopy imaging to discern their varying scattering strengths, which we understand in terms of the calculated orbital characteristics of the surface band. Defects on the Bi sublattice cause the strongest scattering of the predominantly Bi 6p derived surface band, with other defects causing nearly no scattering near the conduction band minimum.

Liquid methanol and methanol clusters have been comprehensively studied to reveal their local structure and hydrogen bond networks. However, our understanding of the crystal forms of methanol is rather limited. The known crystal structures of solid methanol, α, β, and γ, are composed of infinite hydrogen bond chains in their unit cell. The structural diversity of solid methanol is much less than that of liquid methanol, in which both chain and ring structures exist and have been confirmed by experiments. In this study, we employed ab initio random structure searching (AIRSS) to study possible solid methanol structures. AIRSS predicted known solid methanol phases as well as various ring structures that have not been considered. A new possible candidate structure for the δ phase was also discovered. The relative stability of known solid methanol phases and our newly discovered structures were also investigated through dispersion corrected density functional theory. The density functional calculation provides reliable phase transition pressures between the known phases and the searched structures compared with experimental suggestions. In addition, the simulation result indicated that CH⋯O hydrogen bonds play a major role in stabilizing the methanol crystals under high pressures.

The electronic structures of Fe-based and Ru-based full Heusler compounds have been investigated systematically by density functional theory (DFT) with PBE, PBE + U, and HSE06 exchange-correlation (XC) functionals. In order to have a better systematic and quantitative comparison between the results of different approximations, the average deviation of eigenvalues (ADE) between any two electronic band structures were calculated. From quantitative analysis of the ADEs, we have shown that different XC functionals used in the DFT calculations will result in very different and inconsistent electronic band structures. However, the discrepancies are dramatically reduced and get more consistent band structures after the GW calculations. Furthermore, comparing the experimental and calculated Seebeck coefficients and band-gap values of Fe2VAl, it implies that the GW methods including dynamically screened Coulomb interactions are more reliable than DFT with PBE or HSE06 functionals. Conclusively, contrast to the fact that DFT methods give inconsistent band structures when using different XC functionals, the GW methods have better predictive power for the band structures of Fe-based and Ru-based full Heusler compounds.

We report a study of the temperature-dependent electrical resistivity, Seebeck coefficient, thermal conductivity, specific heat, and $^{27}\mathrm{Al}$ nuclear magnetic resonance (NMR) in Heusler-type ${\mathrm{Ru}}_{2}\mathrm{TaAl}$, to shed light on its semimetallic behavior. While the temperature dependence of the electrical resistivity exhibits semiconductorlike behavior, the analysis of low-temperature specific heat reveals a residual Fermi-level density of states (DOS). Both observations can be realized by means of a semimetallic scenario with the Fermi energy located in the pseudogap of the electronic DOS. The NMR Knight shift and spin-lattice relaxation rate show activated behavior at higher temperatures, attributing to the thermally excited carriers across a pseudogap in ${\mathrm{Ru}}_{2}\mathrm{TaAl}$. From the first-principles band structure calculations, we further provide a clear picture that an indirect overlap between electron and hole pockets is responsible for the formation of a pseudogap in the vicinity of the Fermi level of ${\mathrm{Ru}}_{2}\mathrm{TaAl}$. In addition, an effort for improving the thermoelectric performance of ${\mathrm{Ru}}_{2}\mathrm{TaAl}$ has been made by investigating the thermoelectric properties of ${\mathrm{Ru}}_{1.95}{\mathrm{Ta}}_{1.05}\mathrm{Al}$. We found significant enhancements in the electrical conductivity and Seebeck coefficient and marked reduction in the thermal conductivity via the off-stoichiometric approach. This leads to an increase in the figure-of-merit $ZT$ value from $6.1\ifmmode\times\else\texttimes\fi{}\phantom{\rule{0.16em}{0ex}}{10}^{\ensuremath{-}4}$ in ${\mathrm{Ru}}_{2}\mathrm{TaAl}$ to $3.4\ifmmode\times\else\texttimes\fi{}\phantom{\rule{0.16em}{0ex}}{10}^{\ensuremath{-}3}$ in ${\mathrm{Ru}}_{1.95}{\mathrm{Ta}}_{1.05}\mathrm{Al}$ at room temperature. In this respect, a further improvement of thermoelectric performance based on ${\mathrm{Ru}}_{2}\mathrm{TaAl}$ through other off-stoichiometric attempts is highly probable.

The Dirac semimetal phase found in Cd$_{3}$As$_{2}$ is protected by a $C_{4}$ rotational symmetry derived from a corkscrew arrangement of systematic Cd vacancies in its complicated crystal structure. It is therefore surprising that no microscopic observation, direct or indirect, of these systematic vacancies has so far been described. To this end, we revisit the cleaved (112) surface of Cd$_{3}$As$_{2}$ using a combined approach of scanning tunneling microscopy and \textit{ab initio} calculations. We determine the exact position of the (112) plane at which Cd$_{3}$As$_{2}$ naturally cleaves, and describe in detail a structural periodicity found at the reconstructed surface, consistent with that expected to arise from the systematic Cd vacancies. This reconciles the current state of microscopic surface observations with those of crystallographic and theoretical models, and demonstrates that this vacancy superstructure, central to the preservation of the Dirac semimetal phase, survives the cleavage process and retains order at the surface.

Scanning tunneling microscopy visualizations of quasiparticle interference (QPI) enable powerful insights into the k-space properties of superconducting, topological, Rashba and other exotic electronic phases, but their reliance on impurities acting as scattering centers is rarely scrutinized. Here we investigate QPI at the vacuum-cleaved (001) surface of the Dirac semimetal ZrSiS. We find that interference patterns around impurities located on the Zr and S lattice sites appear very different, and can be ascribed to selective scattering of different sub-sets of the predominantly Zr 4d-derived band structure, namely the m = 0 and m = +/-1 components. We show that the selectivity of scattering channels requires an explanation beyond the different bands' orbital characteristics and their respective charge density distributions over Zr and S lattices sites. Importantly, this result shows that the usual assumption of generic scattering centers allowing observations of quasiparticle interference to shed light indiscriminately and isotropically upon the \textit{q}-space of scattering events does not hold, and that the scope and interpretation of QPI observations can therefore be be strongly contingent on the material defect chemistry. This finding promises to spur new investigations into the quasiparticle scattering process itself, to inform future interpretations of quasiparticle interference observations, and ultimately to aid the understanding and engineering of quantum electronic transport properties., Comment: This version is as accepted for PRB

The Heusler-type compound of ${\mathrm{Ru}}_{2}\mathrm{NbGa}$ has been successfully synthesized. X-ray analysis confirms that ${\mathrm{Ru}}_{2}\mathrm{NbGa}$ crystallizes in a cubic $L{2}_{1}$ structure. The electronic properties of ${\mathrm{Ru}}_{2}\mathrm{NbGa}$ have been characterized by means of the transport and nuclear magnetic resonance (NMR) measurements. The temperature dependence of the electrical resistivity exhibits a typical semimetallic behavior. The NMR spin-lattice relaxation rate shows activated behavior at higher temperatures, attributing to the thermally excited carriers across a pseudogap. We have also deduced a low Fermi-level density of states (DOS), being consistent with the semimetallic characteristic for ${\mathrm{Ru}}_{2}\mathrm{NbGa}$. In addition, we have performed first-principles total-energy calculations including ${G}_{0}{W}_{0}$ and $G{W}_{0}$ corrections for band gaps to investigate the electronic band structure of ${\mathrm{Ru}}_{2}\mathrm{NbGa}$. The calculated result reveals an indirect overlap between electron and hole pockets that leads to a residual DOS at the Fermi level, providing a consistent explanation for the experimental observations.

The adsorption of Na atoms on the Si(111)-7 × 7 surface is studied using first-principles calculations. Compared to the Triangle-Trimer model reported previously, we propose a more stable and robust Na 6 Si 3 Hexagon-Trimer model which has six Na atoms with hexagon shape and three Si edge adatoms moving inward to form a trimer. The total energy of Hexagon-Trimer model is 0.252 eV lower than that of Triangle-Trimer model and 0.552 eV lower than that of Hexagon model. The simulated STM images of Hexagon-Trimer model are in good agreement with experimental STM observations. The most probable formation process of Hexagon-Trimer model is analyzed. The reaction is catalyzed by a Na atom and the energy barrier is reduced from 0.89 to 0.44 eV. These results have provided a complete picture for the formation mechanism of Na 6 Si 3 surface magic clusters on the Si(111)-7 × 7 surface.

Scanning tunneling microscopy (STM) observations of the close-packed C60 fullerene arrays on Si(111) 3 × 3 - Ag surface have revealed the presence of dim C60 molecules which constitute 9–12% of all fullerenes. The dim C60 fullerenes reside ~ 1.6 A lower than the bright (“normal”) C60. While the bright C60 are in continuous rotation, the dim C60 are fixed in one of the single orientations, indicating a more tight bonding to the surface. At room temperature (RT), the dynamic switching from bright to dim C60 and vice versa has been detected. Switching slows down with decreasing temperature and becomes completely frozen at 110 K, which implies that the switching is a thermally driven process. RT deposition of ~ 0.1 monolayer of Ag onto C60 array eliminates completely the dim C60 molecules. Experimental results can be understood if one assumes that formation of the dim C60 is associated with disintegration of Ag trimer on Si(111) 3 × 3 - Ag surface under a given C60 fullerene.

We acquired the Raman spectra of adenine in powder and aqueous phase using excitation lasers with 532, 633, and 785 nm wavelengths for the region between 300 and 1500 cm–1. In comparison to the most distinct peak at 722 cm–1, the peaks between 1200 and 1500 cm–1 exhibited a characteristic increase in cross-section with decreasing excitation wavelength in both phases. This trend can be reproduced by different density functional theory (DFT) calculations for the adenine molecule in the gas phase as well as in the aqueous phase. Furthermore, from the calculation on the π-stacked dimer, hydrogen-bonded dimer, and trimer, we find that this trend toward excitation laser wavelength is not sensitive to the packing. When comparing the Raman spectra given by different excitation wavelength, one should take care in analyzing the cross-section, and present day DFT calculations are able to capture general trends in the excitation laser wavelength dependence of the Raman activity.

Crystalline atomic layers on solid surfaces are composed of a single building block, unit cell, that is copied and stacked together to form the entire two-dimensional crystal structure. However, it appears that this is not an unique possibility. We report here on synthesis and characterization of the one-atomic-layer-thick TlxBi1−x compounds which display quite a different arrangement. It represents a quasi-periodic tiling structures that are built by a set of tiling elements as building blocks. Though the layer is lacking strict periodicity, it shows up as an ideally-packed tiling of basic elements without any skips or halting. The two-dimensional TlxBi1−x compounds were formed by depositing Bi onto the Tl-covered Si(111) surface where Bi atoms substitute appropriate amount of Tl atoms. Atomic structure of each tiling element as well as arrangement of TlxBi1−x compounds were established in a detail. Electronic properties and spin texture of the selected compounds having periodic structures were characterized. The shown example demonstrates possibility for the formation of the exotic low-dimensional materials via unusual growth mechanisms.

Using statistical analysis of the scanning tunneling microscopy images of the 3 × 4-In surface magic cluster (SMC) arrays on Si(100)1 × 2 substrate at low In coverages, the main regularities of the space distribution of the nucleated SMCs have been established. It has been found that a nucleated SMC perturbs the surface potential relief within a limited zone around itself in a way that some of the sites in the zone demonstrate a great preference for nucleating a new cluster, while in the other sites the nucleation is greatly suppressed. Outside the zone, the nucleation probability is close to that of the random nucleation simulated using Monte-Carlo technique. Energetic background of the observed phenomena has been proved using density-functional theory calculations, which clearly demonstrate that the lower the formation energy the higher the occurrence probability of a given cluster-pair configuration.

Random structure searching has been proved to be a powerful approach to search and find the global minimum and the metastable structures. A true random sampling is in principle needed yet it would be highly time-consuming and/or practically impossible to find the global minimum for the complicated systems in their high-dimensional configuration space. Thus the implementations of reasonable constraints, such as adopting system symmetries to reduce the independent dimension in structural space and/or imposing chemical information to reach and relax into low-energy regions, are the most essential issues in the approach. In this paper, we propose the concept of "object" which is either an atom or composed of a set of atoms (such as molecules or carbonates) carrying a symmetry defined by one of the Wyckoff positions of space group and through this process it allows the searching of global minimum for a complicated system to be confined in a greatly reduced structural space and becomes accessible in practice. We examined several representative materials, including Cd

Basing on the results of the scanning tunneling microscopy (STM) observations and density functional theory (DFT) calculations, the structural model for the Cu magic clusters formed on Si(1 1 1)7 × 7 surface has been proposed. Using STM, composition of the Cu magic clusters has been evaluated from the quantitative analysis of the Cu and Si mass transport occurring during magic cluster converting into the Si(1 1 1)‘5.5 × 5.5’-Cu reconstruction upon annealing. Evaluation yields that Cu magic cluster accommodates ∼20 Cu atoms with ∼20 Si atoms being expelled from the corresponding 7 × 7 half unit cell (HUC). In order to fit these values, it has been suggested that the Cu magic clusters resemble fragments of the Cu 2 Si-silicide monolayer incorporated into the rest-atom layer of the Si(1 1 1)7 × 7 HUCs. Using DFT calculations, stability of the nineteen models has been tested of which five models appeared to have formation energies lower than that of the original Si(1 1 1)7 × 7 surface. The three of five models having the lowest formation energies have been concluded to be the most plausible ones. They resemble well the evaluated composition and their counterparts are found in the experimental STM images.

The clean-off reaction of AgO added rows by CO on Ag(110) and Au/Ag(110) bimetallic surfaces was studied by scanning tunneling microscopy (STM) and compared with density functional theory (DFT). This combined study of a model system illustrated the complexity of catalytic enhancement in bimetallic systems. By analyzing in situ time-lapsed STM image series, we found that CO oxidation on a Au-enriched Ag(110) surface leads to an exponential depletion of oxygen with time and a reaction rate that is synergistically enhanced by the presence of Au. First principles calculations indicate that the local atomic configuration around the active reaction sites at the chain ends and the preference of Au atom substitution into the subsurface second Ag layer are of critical importance. By calculating CO adsorption energies and reaction barriers for plausible reaction pathways, a detailed description of the CO oxidation reaction emerges. For the optimal reaction pathway, a large (∼0.09 eV) barrier reduction and a small b...

The vibrational features of self-organized submonolayer trans-stilbene on the Ag/Ge(111)−(√3 × √3)R30° surface have been investigated by Raman spectroscopy in ultrahigh vacuum at 100 K. A red shift in the C═C stretching mode of the olefinic group at 1625 cm−1 and enhanced Raman activity of the peak at 1568 cm−1 were observed, as compared with those corresponding to a multilayer trans-stilbene. Electronic interaction between trans-stilbene and the underneath surface and Raman spectra were calculated based on density functional theory. The calculated Raman spectra are in good agreement with the experimental results, supporting the calculated molecular geometry of the adsorbed trans-stilbene, which exhibits a predominant bond elongation in the olefinic group and a torsional angle between the phenyl ring and the olefinic plane. Analysis of the partial density of states shows that the lowest unoccupied molecular orbital is broadened and lowered to cross the surface Fermi level by the interaction with the surfa...

The adsorption and desorption of stilbene on Ag/Ge(111)-(√3 × √3)R30° (Ag/Ge(111)-√3) were investigated using low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), temperature-programmed desorption (TPD), and density functional theory (DFT). Both trans- and cis-stilbenes form a (2 × 1) overlayer structure on Ag/Ge(111)-√3 at a coverage of ∼1 ML. The STM images show parallel strips with three equivalent directions, indicating a self-ordered molecular structure. At a coverage of less than 1 ML, the TPD of cis-stilbene shows only one peak, attributed to submonolayer desorption. The TPD peaks are indistinguishable for desorption of trans-stilbene from the surface submonolayer and multilayer. This is due to the simultaneous desorption and/or thinning of adsorbed multilayers during the TPD process, as determined from the STM analysis of adsorbed trans-stilbene structures before and after annealing. The TPD traces fit the half-order kinetics for molecular desorption of stilbene from Ag/Ge(...

A model to evaluate the color break-up (CBU) of field-sequential color liquid crystal display (FSC-LCD) had been successfully established. In order to quantify the CBU, “Color Break-Up Angle (CBUA)” was proposed as the evaluation index. From the psychophysical experiments, the CBUA=0.22° was reported as the averaged threshold value for indistinguished color break-up. Consequently, the minimum frame rate could be derived from the model to suppress CBU in the various FSC-LCDs.

The addition of copper chlorophyll and its derivatives (Cu-Chl) to vegetable oils to disguise them as more expensive oils, such as virgin olive oils, would not only create public confusion, but also disturb the olive oil market. Given that existing detection methods of Ch-Chl in oils, such as LC-MS are costly and time consuming, it is imperative to develop economical and fast analytical techniques to provide information quickly. This paper demonstrates a rapid analytical method based on surface-enhanced Raman spectroscopy (SERS) to detect Cu-Chl in vegetable oils; the spectroscopic markers of Cu-Chl are presented and a detection limit of 5 mg kg(-1) is demonstrated. The analysis of a series of commercial vegetable oils is undertaken with this method and the results verified by a government agency. This study shows that a SERS-based assessment method holds high potential for quickly pinpointing the addition of minute amounts of Cu-Chl in vegetable oils.

A two-dimensional compound made of one monolayer of Tl and one monolayer of Sn on Si(111) has been found to have a sandwichlike structure in which the Sn layer (having the milk-stool arrangement) resides on the bulklike terminated Si(111) surface and the Tl layer (having the honeycomb-chained-trimer arrangement) is located above the Sn layer. The electronic band structure of the compound contains two spin-split surface-state bands, of which one is nonmetallic and the other is metallic. Near the Fermi level the metallic band is split with the momentum splitting $\ensuremath{\Delta}{k}_{\ensuremath{\parallel}}=0.037$ \AA{}${}^{\ensuremath{-}1}$ and energy splitting $\ensuremath{\Delta}{E}_{\mathrm{F}}=167$ meV. The steep dispersion of the band when crossing the Fermi level corresponds to an electron velocity of $\ensuremath{\approx}8.5\ifmmode\times\else\texttimes\fi{}{10}^{5}$ m/s, which is comparable to the value reported for graphene. The 2D Fermi contours have almost circular shape with spin texture typical for hexagonal surfaces.

We have used ab initio methods to study the possible transition between icosahedral (ico) and cuboctahedral (fcc) structures in lead nanoclusters of sizes up to 309 atoms. Spontaneous fcc-to-ico transition in Pb(13) was observed in the ab initio molecular dynamics (MD) simulations at various temperatures. The transition path can be described predominantly by an angular variable s, which can, generally be applied to the similar transitions in clusters of larger sizes and was observed to follow the Mackay model. We have calculated the two-dimensional energy surface that describes the transition in Pb(13) and found a barrierless fcc-to-ico transition path, which is consistent with the observed spontaneous transition in the ab initio MD simulations. The atomic displacements in the transition were identified as one of the vibrational eigenmodes of these two Pb(13) clusters. For clusters of larger sizes (Pb(n), where n = 55, 147, and 309), the possible transitions following similar paths were determined not to be barrierless and the sizes of the barriers were determined by the ab initio elastic band method.

The Si(111)2$\ifmmode\times\else\texttimes\fi{}$2-In reconstruction can be considered as a precursor phase for the formation of the metallic $\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ phases of In overlayers on a Si(111) surface. Using the ab initio random structure searching method, comparison of simulated and experimental scanning tunneling microscopy images, and resemblance of the calculated band structure to the experimental angle-resolved photoelectron spectra, we examined various 2$\phantom{\rule{0.16em}{0ex}}\ifmmode\times\else\texttimes\fi{}\phantom{\rule{0.16em}{0ex}}$2 structure models with 0.5, 0.75, 1.0, and 1.25 monolayer In coverage. The only model which fits well all the requirements is the one-monolayer model, where three In atoms in the ${T}_{4}$ sites form a trimer centered in the ${H}_{3}$ site and the fourth In atom occupies the on-top (${T}_{1}$) site.

This work is licensed under a Creative Commons Attribution-NonCommercialNoDerivs 3.0 Unported License.-- et al., To exploit Rashba effect in a 2D electron gas on silicon surface for spin transport, it is necessary to have surface reconstruction with spin-split metallic surface-state bands. However, metals with strong spin-orbit coupling (e.g., Bi, Tl, Sb, Pt) induce reconstructions on silicon with almost exclusively spin-split insulating bands. We propose a strategy to create spin-split metallic bands using a dense 2D alloy layer containing a metal with strong spin-orbit coupling and another metal to modify the surface reconstruction. Here we report two examples, i.e., alloying Bi/Si(111) √3 × √3 reconstruction with Na and Tl/Si(111)1 × 1 reconstruction with Pb. The strategy provides a new paradigm for creating metallic surface state bands with various spin textures on silicon and therefore enhances the possibility to integrate fascinating and promising capabilities of spintronics with current semiconductor technology., Part of this work was supported by the Russian Foundation for Basic Research (Grant Nos 12-02-00416, 13-02-00837, 13-02-12110, 14-02-31070) and the NSh-167.2014.2.

We find that electron diffraction patterns can be directly inverted to provide three-dimensional atomic structures for the system studied. Depending on the scattering process, either holography or a Patterson inversion scheme is used. For diffraction patterns which were generated from a localized emitter source or predominantly by an inelastic-scattering feature like low-energy Kikuchi electrons, holography inversion is needed. The information obtained from Kikuchi electron holography includes the building blocks on the surface and their relative position to the atoms below the surface layer. On the other hand, for diffraction patterns generated predominantly by an elastic-scattering feature like low-energy electron diffraction (LEED), a Patterson inversion is needed. The information obtained from the Patterson transform of the LEED I(E) curves is the relative positions of surface atoms to the atoms in underlying layers; no intra-layer information can be extracted with this method. High-fidelity and artifact-free three-dimensional atomic structures obtained by inversion of low-energy Kikuchi electron patterns and low-energy electron diffraction curves are presented. The results from the two inversion methods are complementary and can be used to construct or to discriminate the surface atomic structural models. The future of these direct methods by inverting diffraction patterns is discussed.

We propose all the diffraction patterns can be directly transformed to provide three-dimensional atomic structures for the system studied. Depending on the scattering process, either the holography or Patterson transform scheme is used. For diffraction patterns which are generated from a localized emitter source or dominated by an inelastic-scattering feature like core-level photoelectron or low-energy Kikuchi electron, holography transform is needed. On the other hand, for diffraction patterns which were dominated by elastic-scattering, like grazing-incidence X-ray diffraction, electron correlated thermal diffuse scattering or low-energy electron diffraction curves, Patterson transform is needed. To prove our point, high-fidelity and artifact-free three-dimensional atomic structures obtained by transform of low-energy Kikuchi electron patterns and low-energy electron diffraction curves are presented. The future of these direct methods by transforming diffraction patterns will be discussed.