Search

Decagonal quasiperiodic ordering of Sn thin film on $d$-Al-Ni-Co, is shown based on scanning tunneling microscopy (STM), low-energy electron diffraction and density functional theory (DFT). Interestingly, the decagonal structural correlations are partially retained up to a large film thickness of 10 nm grown at a 165$\pm$10 K. The nucleation centers called 'Sn white flowers' identified by STM at submonolayer thickness are recognized as valid patches of the decagonal clathrate structure with low adsorption energies. Due to the excellent lattice matching (to within 1%) between columns of Sn dodecahedra in the clathrate structure and pentagonal motifs at the $d$-Al-Ni-Co surface, the interfacial energy favors clathrate over the competing Sn crystalline forms. DFT study of the Sn/Al-Ni-Co composite model shows good mechanical stability, as shown by the work of separation of Sn from Al-Ni-Co slab that is comparable to clathrate self-separation energy. The relaxed surface terminations of the R$_2$T$_4$ clathrate approximant are in self-similarity correspondence with the motifs observed in the STM images from monolayer to thickest Sn film., Phys. Rev. B (in press) with 17 figures and 2 Tables in the main text. The supplementary material contains 9 figures, 3 Tables, one note, VASP CONTCAR files, and LEED video files

Mn2NiGa is a potential magnetic shape memory alloy with austenite to the martensite phase transition. Here, we have investigated the bulk electronic structure of Mn2NiGa in the austenite and martensite phases studying its valence band using hard x-ray photoelectron spectroscopy (HAXPES). In the austenite phase, we observe a wide (≈10 eV) valence band (VB) spectrum with several prominent features. In order to explain the HAXPES VB spectra, we have compared our experimental VB spectra with the theoretical VB calculated using the partial density of states from our existing density functional theory (DFT) calculations. The shape of the experimental VB and energy positions of all features are in excellent agreement with the calculated VB and we find that the former is dominated by Ni 3d as well as 4s states of Mn, Ni, and Ga. An important observation is that experimental VB combined with the DFT-based VB calculation establishes the prevalence of the anti-site disorder in Mn2NiGa. Compared to the austenite phase, in the martensite phase, the VB shows a marginal decrease in the density of states around −0.5 eV below the Fermi level (E F ), and the main peak is slightly shifted towards E F . These experimental observations have been explained by considering the tetragonally distorted structure with the anti-site disorder in the martensite phase.

Single-crystal to single-crystal transformation is an elegant example of altering a crystalline solid-state of matter to a solid-state with a different chemical identity. Herein, we present an allu...

Colloidal quantum dots (QDs) are promising materials for optoelectronic devices. In this paper, monodispersed and environment stable cobalt (Co)-doped CdS QDs have been synthesized and characterize...

Bimetallic Ni–Fe catalysts (Ni/Fe, 1 : 1, 1 : 3, and 3 : 1) are synthesized and explored for their catalytic activity in semihydrogenation of internal alkynes using H2 gas in water–ethanol solution. Our findings revealed that over the Ni1Fe3 catalyst a high diastereoselectivity for Z-alkenes with a high conversion for a wide range of internal alkynes can be achieved at moderate reaction temperature (40 °C). Notably, the selectivity for the Z-alkenes is enhanced in the presence of n-butyl amine as an additive. Deuterium labeling experiments evidenced that H2 gas becomes dissociated homolytically over the catalyst surface to hydrogenate alkynes to alkenes. Synthesized catalysts were successfully characterized by HR-TEM, SEM, XPS, EDS, P-XRD and H2-TPD.

When Fe is doped in ZnO, a situation of charge imbalance is created due to the higher charge of Fe3+. A charge balance may be obtained by co-doping Li+0.5Fe3+0.5 combinations. A solid solution of Zn1-x(Fe0.5Li0.5)xO (0 ≤ x ≤ 0.03125) is synthesized with this viewpoint. The crystallites belong to a wurtzite P63mc space group, with lattice parameters a, b and c increasing nominally for x = 0.0156 and thereafter remaining invariant. The size varies in the range ~ 60–142 nm. Interstitials of Li and Zn ions are formed. Along with Fe3+ substitution these defects are reasons for O interstitials. These oxygen interstitials increase the red emission while reduction of oxygen vacancies reduces the green emission. These point defects create structural distortion and strain which can generate Zn vacancies. Bandgap reduces due to shallow defects. Mid-bandgap states due to oxygen interstitials and Fe 3d-O 2p hybridization result in NIR emission. On the other hand the crystal surface deforms due to Li addition which hardens the materials. A weak ferromagnetism appears at very low temperature which is enhanced by Li+ addition. Long range exchange mechanism between Fe3+ ions appears in the samples, mediated by magnetic polarons due to point defects.

Circular and rectangular Cr islands of nanometre size and monoatomic height exhibit an impromptu long range order on locally stepped Au(111) surface with narrow terraces. The circular islands are detached from the {100} microfaceted Au(111) step edges and nucleate between a pair of discommensuration lines of the herringbone reconstruction on ≈ 9.5 nm wide terraces. In contrast, the rectangular islands are attached to the {111} microfaceted step edges and are formed on even narrower terraces of ≈ 6 nm width. Both types of islands prefer the hcp region of the substrate as the nucleation centre. Cr deposition at elevated temperatures such as 470 K results in formation of triangular islands and spiral dislocations on wide substrate terraces. Interestingly however, at higher coverages of 3–4 ML, huge elongated ridges that are 90–260 nm in length, 10–20 nm in width and up to almost 3 nm height are observed. These ridges occur primarily along 1 1 ¯ 0 > direction, with the rest forming at an angle of 60 or 120° with respect to this direction.

Physical review research 3(1), 013151 (2021). doi:10.1103/PhysRevResearch.3.013151, The bulk electronic structures of two high-order quaternary approximants of F-type icosahedral (i)-Al-Pd-TM quasicrystal: Al-Pd-Cr-Fe and Al-Pd-Mo-Fe, having similar electron to atom (e/a) ratio as i-Al-Pd-Mn quasicrystal, have been investigated by hard x-ray photoelectron spectroscopy. We establish the presence of a well-formed pseudogap at the Fermi level in both the approximants. The pseudogap turns out to be deeper in the approximants compared to i���Al���Pd���Mn, and this is supported by specific heat data. Modifications in the line shape of Al 2s core-level main peak as well as the plasmon loss peaks indicate enhanced hybridization of Al sp and transition metal d states in the approximants, which could be one of the possible reasons for their pseudogap. The absence of magnetic exchange splitting in the Fe 2p core-level spectra establishes the nonmagnetic nature of the approximants., Published by APS, College Park, MD

Physical review / B 103(24), L241106 (2021). doi:10.1103/PhysRevB.103.L241106, The influence of disorder on the critical electron states in a quasiperiodic lattice is a subject of intense research. In this work, we report the occurrence of Anderson localization in an icosahedral (i) polygrain quasicrystal Al-Pd-Re due to site disorder using hard x-ray photoemission spectroscopy, resistivity, and density functional theory (DFT). Photoemission spectroscopy shows that the density of states is enhanced at the Fermi level in polygrain i-Al-Pd-Re compared to single-grain i-Al-Pd-Re. In contrast, the conductivity of the former is an order of magnitude reduced compared to the latter, indicating that these electron states are localized. DFT shows that these states originate primarily from Re 5d-Pd 4d hybridization and are enhanced in polygrain i-Al-Pd-Re due to compositional difference, but are broadened because of disorder that brings about Anderson localization. This is established by the Mott variable range hopping behavior of conductivity, and the estimated localization length is 23 Å., Published by Inst., Woodbury, NY

Magnetic properties of N-ion implanted GaN films (150 nm) have been reported. It is found that GaN films grown by the MOCVD technique show strong room temperature ferromagnetic behavior, which can be tuned by implanting N-ions at different fluences (1 × 1015 to 5 × 1016 ions-cm−2). Presence of implanted N at interstitial sites of the GaN host matrix is indicated from the strain observed in GaN by analysis of XRD data. PL spectra show presence of different types of defects in the as deposited film and engineering of defects after N-ion implantation. XPS spectra of Ga 3d-core level and valence band reveal the bonding of implanted N with the host Ga and/or N. The origin of ferromagnetic behavior is ascribed to unpaired electrons created at N sites due to Ga vacancies. First principle-based calculations also confirm ferromagnetism due to Ga vacancies and the reduction of magnetic behavior in Ga deficient GaN with N-ion implantation at interstitial site. The systematic reduction in the saturation magnetic moment value after N-ion implantation is explained on the basis of pairing of the unpaired electrons due to the bond formation of interstitial N with Ga and N present in the host matrix.

We present a photoemission study of desorption of a monolayer of argon (Ar) physisorbed on the Au(111) surface by monitoring the Ar 3p core-level spectral intensity and shape as a function of temperature. The variation of the Ar 3p signal indicates a difference of 7 K between the start and finish of the desorption process, the desorption temperature being 46 K. From an analysis of the Ar 3p core-level line shape, we have related this to two types of hybridization of the Ar atoms with different regions of the herringbone reconstructed Au(111) surface. The interaction between the ArA and Au atoms in the discommensurate region is relatively weaker compared to that between the ArB and Au atoms in the fcc and hcp regions of the herringbone reconstructed surface resulting in the desorption of the former at a lower temperature (41 K).

Abstarct. Two dimensional (2D) derivatives of tin (Sn) have obtained special deliberations recently due to practical realization of planar, as well as, buckled hexagonal lattice of Sn called stanene. However, it has been observed that proper choice of substrate is very important for growth of stanene like films owing to large core size of Sn that prefers sp 3 hybridization over sp 2. Transition metal dichalcogenides (TMDs) like MoS2 or WS2 with honey comb lattice structure seem to be promising substrate candidates for 2D growth of Sn. In the present work, we report mechanical exfoliation of few layers of WS2 under ultra-high vacuum (UHV) conditions and investigations of growth and local electronic structure by in-situ scanning tunneling microscopy (STM) and spectroscopy (STS) studies. Flat WS2 surface with honeycomb lattice structure in the atomic scale with a lattice constant of 0.34 nm is evident in the STM investigations, whereas, STS measurements reveal local density of states (LDOS) of WS2 with a bandgap of approximately 1.34 eV. Density functional theory (DFT) calculations performed by considering bulk WS2 reveal conduction and valence band states comprised of S p and W d at both sides of the Fermi energy (EF) and an indirect bandgap of 1.38 eV. Experimental observations upon Sn adsorption, reveal commensurate growth of Sn atoms on the sulfur `S’ sites with a buckling height of 40 ±10 pm. STS measurements exhibit local electronic structure of the Sn adsorbed surface with clear evidence of in-gap states. DFT calculations quantify the experimental results demonstrating `S’ sites as the most stable sites for the atomic adsorption of Sn with a buckling height of around 80 pm and reveal signature of in-gap hybridized states comprised of Sn p and W d orbitals.

We report here on room temperature (RT) adsorption of Sn atoms on mechanically cleaved WS2 hexagonal crystal surface under ultra-high vacuum (UHV) conditions. Our in-situ low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) studies clearly indicate atomic configurations of bare WS2 and Sn adsorbed surfaces. Bare WS2 displays an atomic periodicity with lattice constant, a, as 0.34 nm. RT adsorption of Sn indicates a nearly commensurate atomic adsorption on W or S sites with a lateral distance between two adsorbed Sn atoms as ∼0.325 nm and bond length of Sn with W/S as ∼40 pm. We do not find any evidence of Sn adsorption on the hollow sites of the hexagonal surface mesh of WS2. Analyses of the LEED spots and the FFT of the STM images reveal a stretched hexagonal surface after Sn adsorption.

Thin films of β-W are the most interesting for manipulating magnetic moments using spin–orbit torques, and a clear understanding of α to β phase transition in W by doping impurity, especially oxygen, is needed. Here we present a combined experimental and theoretical study using grazing incidence X-ray diffraction, photoelectron spectroscopy, electron microscopy, and ab initio calculations to explore atomic structure, bonding, and oxygen content for understanding the formation of β-W. It is found that the W films on SiO2/Si have 13–22 at.% oxygen in A15 β structure. Ab initio calculations show higher solution energy of oxygen in β-W, and a tendency to transform locally from α to β phase with increasing oxygen concentration. X-ray absorption spectroscopy also revealed local geometry of oxygen in β-W, in agreement with the simulated one. These results offer an opportunity for a fundamental understanding of the structural transition in α-W and further development of β-W phase for device applications.

By employing x-ray photoelectron spectroscopy, we present a detailed study of the core level spectra of LaTe3 in its charge density wave phase at room temperature. The analysis of the Te 3d spectrum by curve fitting using least square error minimization reveals that the Te atoms exist in two different charge states: the two Te atoms in the Te-Te layer has a valency of −0.5, whereas the Te atom in La-Te layer has a valency of −2. The La 3d spectrum shows three peaks for each spin orbit component, where the main peak and the satellite peaks appear due to different final states related to charge transfer from different ligand Te states.

We study the effect of different surface preparation conditions on 10-fold surface of decagonal (d)-Al-Ni-Coquasicrystal using low energy electron diffraction (LEED), scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES). It is observed that after the rare gas ion sputtering process, the annealing time period (tA) at a temperature of 1043K plays an important role in obtaining a clean, stoichiometric quasicrystalline surface with flat terraces at annealing temperature of 1043 K.

Ferromagnetic shape memory alloys have drawn immense attention in recent years owing to their important functional properties. We have performed low energy electron diffraction (LEED) and x-ray photoelectron spectroscopy (XPS) study of the growth of Sn on Ni2MnGa(100) single crystal surface at room temperature in ultra-high vacuum. We find that Sn growth is not ordered at room temperature. However, as we anneal the Sn layer to higher temperatures up to 773 K, it becomes ordered exhibiting sharp LEED pattern similar to the substrate indicating pseudomorphic growth. XPS shows absence of adsorbate-substrate intermixing as well as desorption of the adsorbate due to annealing.

Cr adlayers on magnetic and non-magnetic substrates have attracted attention in the recent years due to the interesting magnetic properties of these systems. In this work, we explore the magnetic properties of Cr adlayers on the (001) surface of a ferromagnetic Heusler alloy, namely Ni2MnGa, using density functional theory based electronic structure calculations. The magnitudes and orientation of the magnetic moments of the Cr adlayers on both the Mn-Ga and Ni terminated Ni2MnGa (001) surfaces have been investigated and we find that the magnetic ordering is sensitive to the substrate atoms and the adlayer thickness as well. For one monolayer of Cr adlayer on Mn-Ga (Ni) surface, the Cr atoms are ferromagnetically (anti-ferromagnetically) coupled within the layer. However, for two monolayers of Cr, the Cr adatoms are ferromagnetically coupled, irrespective of the surface termination. The ferromagnetic ordering of the Cr atoms has been explained from the Stoner criterion. The origin of different magnetic orderings of the Cr adatoms with the substrate has been understood by analyzing the Heisenberg exchange coupling parameters between the adatoms and the substrate atoms beneath the adlayer. Further, to obtain a chemical understanding of the observed magnetic behavior, we have employed the crystal orbital hamilton population bonding analyses. The results corroborate well with the results of Heisenberg exchange coupling parameters as well as spin-polarized atom-derived density of states.

${\mathrm{CeMnNi}}_{4}$ exhibits an unusually large spin polarization, but its origin has baffled researchers for more than a decade. We use bulk sensitive hard x-ray photoelectron spectroscopy (HAXPES) and density functional theory based on the Green's function technique to demonstrate the importance of electron-electron correlations of both the Ni $3d$ (${U}_{\text{Ni}}$) and Mn $3d$ (${U}_{\text{Mn}}$) electrons in explaining the valence band of this multiply correlated material. We show that Mn-Ni antisite disorder as well as ${U}_{\text{Ni}}$ play a crucial role in enhancing its spin polarization: Antisite disorder broadens a Ni $3d$ minority-spin peak close to the Fermi level (${E}_{F}$), while an increase in ${U}_{\text{Ni}}$ shifts it toward ${E}_{F}$, both leading to a significant increase of minority-spin states at ${E}_{F}$. Furthermore, the rare occurrence of a valence state transition between the bulk and the surface is demonstrated highlighting the importance of HAXPES in resolving the electronic structure of materials unhindered by surface effects.

Noble metal surfaces (Au, Ag and Cu etc.) have been extensively studied for the Shockley type surface states (SSs). Very recently, some of these Shockley SSs have been understood from the topological consideration, with the knowledge of global properties of electronic structure. In this letter, we show the existence of Dirac like excitations in the elemental noble metal Ru, Re and Os based on symmetry analysis and first principle calculations. The unique SSs driven Fermi arcs have been investigated in details for these metals. Our calculated SSs and Fermi arcs are consistent with the previous transport and photo-emission results. We attribute these Dirac excitation mediated Fermi arc topology to be the possible reasons behind several existing transport anomalies, such as large non-saturating magneto resistance, anomalous Nernst electromotive force and its giant oscillations, magnetic breakdown etc. We further show that the Dirac like excitations in these elemental metal can further be tuned to three component Fermionic excitations, using symmetry allowed alloy mechanism.

The electronic structure of Au-Sn intermetallic layers of different compositions grown on Au(111) to the thickness of several nanometers has been studied in this work. The layer, interface and the substrate related components in the Au 4$f$ and Sn 4$d$ core-level spectra obtained using x-ray photoelectron spectroscopy (XPS) vary with deposition parameters to reveal the details of the Au-Sn formation. While AuSn is grown by deposition at room temperature, Au rich compounds form as a result of heat treatment through inter diffusion of Au and Sn. Deposition at high temperature forms more Au rich compositions compared to post annealing at the same temperature due to the kinetic energy of the impinging Sn atoms in the former case. Post annealing, on the other hand, stabilizes the bulk phases such as AuSn and Au$_5$Sn and exhibits an activated behavior for transition from the former to the latter with increasing temperature. The XPS valence band spectra of AuSn and Au$_{5}$Sn layers show good agreement with the density functional theory calculation, indicating that these have the bulk structure reported in literature. However, the influence of anti-site defects is observed in Au$_5$Sn. Low energy electron diffraction study reveals that although the AuSn layer is ordered, its top surface is disordered at room temperature. Surface order is obtained by annealing or deposition at elevated temperatures and dispersing bands are observed by angle resolved photoemission spectroscopy. Both electron-like and hole-like bands are evident for the ($\sqrt{3}$$\times$$\sqrt{3}$)R30$^{\circ}$ phase, while a nearly free electron-like parabolic surface state is observed for the p(3$\times$3)R15$^{\circ}$ phase., Comment: Phys. Rev. B (in press); 38 pages, 14 figures and 1 table in main text and 3 figures and 5 tables in supplementary material

Transition metal dichalcogenides (TMDs) have drawn immense interest recently owing to their exotic electronic and optoelectronic properties. We report here our work on highly pure WS2 crystal which was cleaved under ultra-high vacuum (UHV) conditions and was investigated by in-situ low energy electron diffraction (LEED), scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) measurements. LEED and STM measurements clearly indicate atomic configuration of the surface and the hexagonal arrangement of the WS2 crystal, whereas, STS measurements reveal the local electronic structure of the surface via tunneling of electrons through conduction and valence bands. From the STS studies performed on WS2 surface at different points, we estimate the band gap which is around 2.0 eV.

Oxidation and reduction reactions are of central importance in chemistry as well as vital to the basic functions of life and such chemical processes are generally brought about by oxidizing and reducing agents, respectively. Herein, we report the discovery of an interfacial reduction reaction (IRR) - without the use of any external reducing agent. In course of metal-ligand coordination, spontaneous reduction of Cu(II) to Cu(I) at a solid-liquid interface was observed-unlike in a liquid-phase reaction where no reduction of Cu(II) to Cu(I) was occurred. High-quality thin films of a new coordination network compound bearing a Fe(II)-CN-Cu(I) link were fabricated by IRR and employed for efficient electro-catalysis in the form of oxygen reduction reaction. Also, thermally activated reversible structural phase transition modulated the electron transport property in thin film. This work unveils the importance of chemical reactions at solid-liquid interfaces that can lead to the development of new functional thin film materials.

The interaction of oxygen with noble metals such as silver has been an important topic of research for many decades. Here, we show occurrence of a peak in the density of states (DOS) at the Fermi level ($E_F$) when oxygen atoms occupy disordered substitutional positions in noble metals such as Ag, Au or Ag-Au alloy. This results in large enhancement of DOS at $E_F$ with respect to Ag or Au metal. Its origin is attributed to O 2$p$ related disorder broadened flat band that straddles almost all the high symmetry directions of the Brillouin zone. Our work suggests that if a large concentration of disordered oxygen can be realized in nano-structures of noble metals, it may lead to interesting phenomenon., 7 figures

The intriguing physical properties of SmFeO3 such as spin-phonon coupling and spin reorientation transition (~480 K) make it interesting from fundamental point of view and a suitable candidate for oxide-based spintronic applications. Here, we have studied the temperature dependent structural, vibrational, and magnetic properties of polycrystalline Sm0.9Bi0.1Fe0.9Mn0.1O3 prepared by conventional solid-state reaction method. The compound stabilizes in orthorhombic structure with space group “Pnma” and exhibits no structural phase transitions in the investigated temperature range (300–500 K). Magnetic measurements reveal the weak ferromagnetic–paramagnetic transition at 620 K. Thermal evolution of phonon modes investigated using Raman spectroscopy in the temperature range 300–800 K reveal that Ag(3) phonon mode related to FeO6 vibrations exhibits anomalous behaviour below magnetic transition temperature, which we attribute to spin-phonon coupling. The optical band gap value of ∼ 5.17 eV has been estimated from the analysis of UV–Vis diffuse reflectance spectroscopy using the Tauc relation. The value of ΔEt2u→t2g is estimated to be ~ 2.6 eV for p-d charge transfer transitions in Fe/MnO6 octahedra. The obtained valence states from X-ray photoelectron spectroscopy analysis of all the elements of the sample are in excellent agreement with the expected values.

The electrical device characteristics of Ni/β-Ga2O3 vertical Schottky barrier diodes (SBDs) were measured in situ during the irradiation of 120 MeV Ag7+ swift heavy ions (SHIs). These devices exhibit SHI irradiation-induced degradation with 120 MeV Ag7+ ions in the ion fluence ranges of 1 × 1010 to 1 × 1012 ions/cm2. The height of the Schottky barrier is found to decrease from 1.11 to 0.93 eV, and the ideality factor increases from 1.16 to 2.06. These changes indicate the degradation of the device with SHI irradiation. A significant four orders increase is observed in the leakage current density from 4.04 × 10−8 to 1.98 × 10−4 A/cm2 at −1 V, and the series resistance also increases from 3.38 × 103 to 1.15 × 104 Ω. X-ray photoelectron spectroscopy measurements show that the Ga ions are present in divalent and trivalent states with the spectral features having the binding energies centered at 20.2 eV and 19.9 eV (Ga 3d core-levels) before and after ion irradiation. The O 2s peak shifts to 23.7 eV, and there is an increase in intensity and peak broadening due to the change in the trivalent to divalent state of Ga due to the irradiation. The O(I) peak appears at 530.7 eV in the pristine sample with the Ga–O bonding with the Ga3+ state in pure Ga2O3. Moreover, there is a significant change in the intensity and the peak width of O(II) centered at 533.0 eV after ion irradiation at the fluence of 1 × 1012 ions/cm2. This indicates that there is an increase in the surface adsorbed/lattice oxygen, resulting in GaO.

Inverse photoemission spectroscopic (IPES) measurements along with LDA based band structure calculations have been used to investigate the unoccupied electronic structure of FeSe1−xTex system. The observed doping and temperature dependent pseudogap in this system is found to be linked to the change in the chalcogen height in their geometric structure. The depletion in spectral weight from the near EF states at low temperature in IPES has been correlated with the enhancement of the 3z2–r2 orbitals in the photoemission spectroscopy (PES). The Coulomb correlation energy U, estimated from the combined PES and IPES spectra, signifies the enhancement in electron correlations in FeSe1−xTex, with doping. The formation of pseudogap in PES and IPES confirms the importance of correlations in the 11 family of Fe superconductors.

We demonstrate formation of thin AuSn and Au2Sn surface alloy layers at room and elevated substrate temperatures ( T S = 413–493 K), respectively by the deposition of Sn on Au(1 1 1) using core-level X-ray photoelectron spectroscopy and scanning tunneling microscopy. Low energy electron diffraction patterns show different surface structures: p(3 × 3) R15 ° at room temperature for AuSn, 2 1 1 3 at T S = 413 K and ( 3 × 3 ) R30 ° at T S = 493 K for Au2Sn. Angle resolved photoemission spectroscopy reveals evolution of bands from an electron-like surface state to appearance of a hole-like band structure corresponding to AuSn and Au2Sn. The 2 1 1 3 phase of Au2Sn, which has a unit cell of oblique symmetry, exhibits an interesting linear band that meets at the Fermi level in the zone-center and have Fermi velocity comparable to graphene. Thus, our study establishes that Au-Sn bimetallic surface alloys have interesting electronic properties with potential for future applications.

The structure and electrochemical properties of layer-by-layer-assembled gold nanoparticles (AuNPs)-decorated first-generation (G1) poly(amidoamine) dendrimer (PD) with reduced graphene oxide (rGO) core as a highly sensitive and label-free biosensing platform with a controllable three-dimensional (3D) nanoarchitecture for the rapid voltammetric analysis of DNA hybridization at ultratrace levels were characterized. Mercaptopropinoic acid (MPA) was self-assembled onto Au substrate, then GG1PD formed by the covalent functionalization between the amino terminals of G1PD and carboxyl terminals of rGO was covalently linked onto MPA, and finally AuNPs were decorated onto GG1PD by strong physicochemical interaction between AuNPs and −OH of rGO in GG1PD, which was characterized through different techniques and confirmed by computational calculation. This 3D controllable thin-film electrode was optimized and evaluated using [Fe(CN)6]3–/4– as the redox probe and employed to covalently immobilize thiol-functionalized...

The flexibility of amorphous anodized alumina (AAO) in developing radiation dosimeter for hadron therapy is reported by controlled carbon ion implantation, followed by thermoluminescence (TL) measurements. The efficacy of amorphous AAO in controlling TL sensitivity is found to be governed by an increase in F+ defect centers as a function of carbon concentration, as revealed from the close resemblance of the trend in photoluminescence intensity. Moreover, its nanoporous structure is demonstrated to be advantageous for defect engineering due to the increase in the surface-to-volume ratio. Detailed X-ray photoelectron spectroscopy analysis suggests the formation of F+ centers by substituting Al3+ ions with C2+ in the vicinity of oxygen vacancies, where depth-dependent study showed the evolution of conducting channels owing to sp2 hybridized C–C bonding, leading to a differential charging effect. This work provides a direction to tune nanoporous AAO in its amorphous form for future ion beam dosimetry.

On the basis of x-ray photoelectron spectroscopy, we show that when Sn is deposited on Au(111) single crystal surface at a substrate temperature TS=373 K, surface alloying occurs with the formation of AuSn phase. The evolution of the surface structure and the surface morphology has been studied by low energy electron diffraction and scanning tunneling microscopy, respectively as a function of Sn coverage and substrate temperatures.On the basis of x-ray photoelectron spectroscopy, we show that when Sn is deposited on Au(111) single crystal surface at a substrate temperature TS=373 K, surface alloying occurs with the formation of AuSn phase. The evolution of the surface structure and the surface morphology has been studied by low energy electron diffraction and scanning tunneling microscopy, respectively as a function of Sn coverage and substrate temperatures.

Nano-structures comprising of a pit surrounded by a circular ridge that resemble nano-donuts have been observed on flat terraces of both Au(1 1 1) and Al(1 0 0) surfaces after low energy (1.5–2 keV) rare gas ( rg ) ion implantation. From time lapse scanning tunneling microscopy, we demonstrate that these donuts originate from the rg bubbles that migrate out from the sub-surface region. The circular shape of the donuts is observed for both Ar and Ne bubbles. The donuts and the related nano-structures represent different stages of large time scale co-operative relaxation of Au atoms by long range elastic interaction after the rg bubbles leave the metal.

Surface states consisting of helical Dirac fermions have been extensively studied in three-dimensional topological insulators. Yet, experiments to date have only investigated fully formed topological surface states (TSS) and it is not known whether preformed or partially formed surface states can exist or what properties they could potentially host. Here, by decorating thin films of Bi

Surface states consisting of helical Dirac fermions have been extensively studied in threedimensional topological insulators. Yet, experiments to date have only investigated fully formed topological surface states (TSS) and it is not known whether preformed or partially formed surface states can exist or what properties they could potentially host. Here, by decorating thin films of Bi2Se3 with nanosized islands of the same material, we show for the first time that not only can surface states exist in various intermediate stages of formation but they exhibit unique properties not accessible in fully formed TSS. These include tunability of the Dirac cone mass, vertical migration of the surface state wave-function and the appearance of mid-gap Rashba-like states as exemplified by our theoretical model for decorated TIs. Our experiments show that an interplay of Rashba and Dirac fermions on the surface leads to an intriguing multi-channel weak anti-localization effect concomitant with an unprecedented tuning of the topological protection to transport. Our work offers a new route to engineer topological surface states involving Dirac-Rashba coupling by nano-scale decoration of TI thin films, at the same time shedding light on the real-space mechanism of surface state formation in general.

Realization of an elemental solid-state quasicrystal has remained a distant dream so far in spite of extensive work in this direction for almost two decades. Here, we report the discovery of quasiperiodic ordering in a thick layer of elemental Sn grown on icosahedral ($i$)-Al-Pd-Mn. The STM images and the LEED patterns of the Sn layer show specific structural signatures that portray quasiperiodicity but are distinct from the substrate. Photoemission spectroscopy reveals the existence of the pseudogap around the Fermi energy up to the maximal Sn thickness. The structure of the Sn layer is modeled as a novel form of quasicrystalline clathrate on the basis of the following: Firstly, from ab-initio theory, the energy of bulk Sn clathrate quasicrystal is lower than the high temperature metallic $\beta$-Sn phase, but higher than the low temperature $\alpha$-Sn phase. A comparative study of the free slab energetics shows that surface energy favors clathrate over $\alpha$-Sn up to about 4 nm layer thickness, and matches $\beta$-Sn for narrow window of slab thickness of 2-3 nm. Secondly, the bulk clathrate exhibits gap opening near Fermi energy, while the free slab form exhibits a pronouced pseudogap, which explains the pseudogap observed in photoemission. Thirdly, the STM images exhibit good agreement with clathrate model. We establish the adlayer-substrate compatibility based on very similar (within 1%) the cage-cage separation in the Sn clathrate and the pseudo-Mackay cluster-cluster separation on the $i$-Al-Pd-Mn surface. Furthermore, the nucleation centers of the Sn adlayer on the substrate are identified and these are shown to be a valid part of the Sn clathrate structure. Thus, based on both experiment and theory, we propose that 4 nm thick Sn adlayer deposited on 5-fold surface of $i$-Al-Pd-Mn substrate is in fact a metastable realization of elemental, clathrate family quasicrystal., Comment: 10 figures in the Manuscript and the 8 figures in the Supplementary material

We have studied the nature of nanostructures and their time evolution on Au(111) created by argon ion irradiation followed by either subsequent or simultaneous annealing at different temperatures. For subsequently annealed surface, an initial linear decrease in the areas of the islands and pits is obtained with time. We show that the mass flow of the surface atoms is governed by the rate of detachment, rather than the rate of diffusion. The small islands and pits are found to disappear. In contrast, the areas of the large islands and pits initially decrease and become nearly constant as they attain a regular hexagonal shape. On the other hand, the simultaneously irradiated and annealed Au surface is much smoother with wide terraces and mostly pits of irregular hexagonal shape are observed. We also show that local stepped areas that resemble vicinal surfaces can form on Au(111) due to Ar ion irradiation.

The strong magnetoelastic interaction in ternary X2YZ Heusler alloys is reponsible for the appearance of magnetostructural phase transitions and related functional properties such as the magnetocaloric and magnetic shape-memory effects. Here, X and Y are transition metal elements and Z is usually an element from the III–V group. In order to discuss possibilities to optimize the multifunctional effects, we use density functional theory calculations from which the martensitic driving forces of the magnetic materials can be derived. We find that the electronic contribution arising from the band Jahn–Teller effect is one of the major driving forces. The ab initio calculations also give a hint of how to design new intermetallics with higher martensitic transformation temperatures compared to the prototype alloy system Ni–Mn–Ga. As an example, we discuss quarternary PtxNi2−xMnGa alloys which have properties very similar to Ni–Mn–Ga but exhibit a higher maximal eigenstrain of 14%.

By imaging the spatial intensity distribution of the electrons from a Stoffel-Johnson (SJ) type low energy electron source for inverse photoemission spectroscopy (IPES), we find that the focus is distorted when the beam current exceeds the limiting value due to space charge effect. The space charge effect and the contact potential difference suppress the beam current at low energies (

Influence of disorder, antisite defects, martensite transition and compositional variation on the magnetic properties and electronic structure of Mn$_2$NiGa and Mn$_{1+x}$Ni$_{2-x}$Ga magnetic shape memory alloys have been studied by using full potential spin-polarized scalar relativistic Korringa-Kohn-Rostocker (FP-SPRKKR) method. Mn$_2$NiGa is ferrimagnetic and its total spin moment increases when disorder in the occupancy of Mn$_{\rm Ni}$ (Mn atom in Ni position) is considered. The moment further increases when Mn-Ga antisite defect[1] is included in the calculation. A reasonable estimate of $T_C$ for Mn$_2$NiGa is obtained from the exchange parameters for the disordered structure. Disorder influences the electronic structure of Mn$_2$NiGa through overall broadening of the density of states and a decrease in the exchange splitting. Inclusion of antisite defects marginally broaden the minority spin partial DOS (PDOS), while the majority spin PDOS is hardly affected. For Mn$_{1+x}$Ni$_{2-x}$Ga where 1$\geq$$x$$\geq$0, as $x$ decreases, Mn$_{\rm Mn}$ moment increases while Mn$_{\rm Ni}$ moment decreases in both austenite and martensite phases. For $x$$\geq$ 0.25, the total moment of the martensite phase is smaller compared to the austenite phase, which indicates possible occurrence of inverse magnetocaloric effect. We find that the redistribution of Ni 3$d$- Mn$_{\rm Ni}$ 3$d$ minority spin electron states close to the Fermi level is primarily responsible for the stability of the martensite phase in Mn-Ni-Ga., Comment: 10 pages, 5 figures