Tunneling Density Research Articles

Defects in Highly Anisotropic Transition-Metal Dichalcogenide PdSe2.

The atomic and electronic structures of pristine PdSe2 as well as various intrinsic vacancy defects in PdSe2 are studied comprehensively by combining scanning tunneling microscopy, spectroscopy, and density functional theory calculations. Other than the topmost Se atoms, sublayer Pd atoms and the intrinsic Pd and Se vacancy defects are identified. Both VSe and VPd defects induce defect states near the Fermi level. As a result, the vacancy defects can be negatively charged by a tip gating effect. At negative sample bias, the screened Coulomb interaction between the scanning tunneling microscopy (STM) tip and the charged vacancies creates a disk-like protrusion around the VPd and crater-like features around VSe. The magnification effect of the long-range charge localization at the charged defect site makes sublayer defects as deep as 1 nm visible even in STM images. This result proves that by gating the probe, scanning probe microscopy can be used as an easy tool for characterizing sublayer defects in a nondestructive way.

Growth and characterization of Ca-Mo mixed oxide films on Mo(001).

Calcium-molybdate ultrathin films were prepared on a Mo(001) crystal and characterized by means of scanning tunneling microscopy (STM), electron diffraction, photoelectron spectroscopy, and density functional theory (DFT). The films were grown via reactive Ca deposition, followed by a vacuum annealing step to trigger Mo diffusion from the support into the Ca-O ad-layer. A series of crystalline oxide configurations was revealed that evolves from a (3 × 3) to a (4 × 4) and (6 × 6) superstructure with increasing annealing temperature and finally decays to a binary MoOx phase. The stoichiometry of the initial (3 × 3) phase was estimated to CaMo3O6, yet with decreasing Ca concentration at higher temperature. In the search for a suitable structure model for DFT calculations, we have started with a bulk CaMo5O8 configuration that was iteratively modified to match the experimental data. The optimized structure is made of regular sequences of flat-lying and upright standing Mo octahedrons, being separated from each other by Ca2+ ion rows. With decreasing Ca content, the central Mo units grow in size, which explains the observed transition from (3 × 3) to (6 × 6) superstructures upon annealing. The proposed structure model rationalizes the periodicity and corrugation of the regular oxide surface as well as the characteristic domain patterns in the film. Its electronic properties, as deduced from STM conductance spectroscopy, can be correlated with an increasing metallicity of the ad-layer upon annealing. Our work presents a facile pathway to produce high-quality ternary oxide films via interdiffusion of atoms from a suitable metal support into a binary oxide layer.

Single-Phase Borophene on Ir(111): Formation, Structure, and Decoupling from the Support.

Artificial two-dimensional (2D) materials, which host electronic or spatial structure and properties not typical for their bulk allotropes, can be grown epitaxially on atomically flat surfaces; the design and investigation of these materials are thus at the forefront of current research. Here we report the formation of borophene, a planar boron allotrope, on the surface of Ir(111) by exposing it to the flux of elemental boron and consequent annealing. By means of scanning tunneling microscopy and density functional theory calculations, we reveal the complex structure of this borophene, different from all planar boron allotropes reported earlier. This structure forms as a single phase on iridium substrate in a wide range of experimental conditions and may be then decoupled from the substrate via intercalation. These findings allow for production of large, defect-free borophene sheets and advance theoretical understanding of polymorphism in borophene.

Effect of Additive DCTA on Electrochemical Tunnel Etching of Aluminum Foil

The effect of additive DCTA (1,2-diaminocyclohexane-tetraacetic acid) to HCl + H2SO4 solution on electrochemical tunnel etching behavior of aluminum foil was investigated. The results show that DCTA can activate Al foil surface by decreasing the self-corrosion potential and breakdown potential, which may be attributed to that DCTA forms chelate with dissolved Al3+, thus suppressing the oxide film formation. The tunnel density and effective tunnel number as well as uniformity of tunnel length of etching foil can be increased after DCTA addition. Furthermore, the specific capacitance of etching foil can be enhanced with trace addition of DCTA.

On-Surface Intramolecular Dehalogenation of Vicinal Dibromides for the Direct Formation of C–C Double Bonds

The intramolecular dehalogenation reaction as an essential step is generally involved in the deprotection of carbon–carbon double bonds in solution chemistry. However, harsh reaction conditions often limit the functional group compatibility and cause the selectivity problems. In comparison with traditional solution reactions, on-surface chemical reactions always occur on metal substrates, which could serve as catalysts to effectively reduce the reaction barriers in many cases. On the other hand, on-surface intermolecular dehalogenative homocoupling reactions have been extensively demonstrated, while, to our knowledge, intramolecular dehalogenation reactions are less discussed on surfaces. Herein, by the combination of high-resolution scanning tunneling microscopy imaging and density functional theory calculations, we introduce the vicinal dibromo group on chemically different surfaces, such as Cu(110), Ag(110), and Au(111), respectively, to investigate the generality of intramolecular dehalogenation reactions. As a result, we successfully achieved the direct formation of C–C double-bonded products in a facile manner on all three surfaces.

Bottom-Up Fabrication of a Metal-Supported Oxo–Metal Porphyrin

In situ preparation of oxotitanium tetraphenylporphyrin (TiO-TPP) on Ag(111) under ultrahigh vacuum conditions was achieved in a multistep procedure starting from adsorbed free-base tetraphenylporphyrin (2H-TPP). The final product as well as the intermediate titanium tetraphenylporphyrin (Ti-TPP) was characterized by a suite of surface-sensitive spectroscopic tools combined with scanning tunneling microscopy and density functional theory (DFT), and compared against the parent 2H-TPP species. Facile oxidation of Ti-TPP with molecular oxygen was observed at 300 K, with X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) from the Ti 2p core levels supporting a change in the oxidation state from Ti2+ to Ti4+. N K-edge and Ti L-edge NEXAFS suggest that the tetrapyrrole macrocycle conformation is modified upon binding to oxygen, in agreement with DFT calculations that predict a marked change of the local environment of the Ti centers upon oxygen attachment. O K-edge NEXAFS and O 1s energy-scanned photoelectron diffraction from the resulting TiO-TPP monolayer provide strong evidence for the presence of a titanium–oxygen double bond, with the latter technique yielding a bond length of 1.56 ± 0.02 Å. The majority of adsorbed TiO-TPP species have the oxo group pointing away from the surface rather than toward it, and thus the oxygen atom can potentially interact with external species. Both the highly reactive, intermediate Ti-TPP species and the final product TiO-TPP are of great interest for catalytic applications.

Jammed hard-sphere hcp crystals permeated with trivacancy tunnels

Supported by simple table-top experiments involving stackings of ball bearings and theoretical analysis, we have discovered crystal packings of identical hard spheres that are permeated by a high concentration of large tunnels and yet are jammed (mechanically stable). We show that starting with a strictly jammed hexagonal close-packed (hcp) crystal of identical hard spheres, removal of certain subsets of those spheres can produce mechanically stable vacancy arrangements involving compact (equilateral triangle) trivacancies such that they produce linear trivacancy tunnels. These tunnels can extend over the entire macroscopic length of the hcp medium, and their width is sufficient to allow contained “test” hard spheres with diameters less than 5−1=1.23606… to migrate over that entire length without contacting the static tunnel-wall spheres. A search for the stable (strictly jammed) periodic framework that hosts the highest density of parallel trivacancy tunnels has identified a structure exhibiting a packing fraction ϕ=π/32=0.55536…, which is equal to 3/4 of the maximum monovalent sphere packing fraction ϕmax=π/18=0.74048…. In that periodic arrangement, filling the interior of the contained tunnels with movable unit-diameter spheres may approach the greatest possible “rattler” density within jammed monovalent sphere systems subject to periodic boundary conditions. It will be of interest to study the physical and chemical properties of these anisotropic porous crystal structures. Our findings may have practical implications for engineered separation and catalytic processes.

Epitaxial growth and air-stability of monolayer Cu2Te**Project supported by the National Key Research & Development Program of China (Grant Nos. 2016YFA0202300 and 2018YF A0305800), the National Natural Science Foundation of China (Grant Nos. 61888102, 11604373, 61622116, and 51872284), the CAS Pioneer Hundred Talents Program, China, the Strategic Priority Research Program of Chinese Academy of Sciences (Grant Nos. XDB30000000 and XDB28000000),

A new two-dimensional atomic crystal, monolayer cuprous telluride (Cu2Te) has been fabricated on a graphene-SiC(0001) substrate by molecular beam epitaxy (MBE). The low-energy electron diffraction (LEED) characterization shows that the monolayer Cu2Te forms a superstructure with respect to the graphene substrate. The atomic structure of the monolayer Cu2Te is investigated through a combination of scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations. The stoichiometry of the Cu2Te sample is verified by x-ray photoelectron spectroscopy (XPS) measurement. The angle-resolved photoemission spectroscopy (ARPES) data present the electronic band structure of the sample, which is in good agreement with the calculated results. Furthermore, air-exposure experiments reveal the chemical stability of the monolayer Cu2Te. The fabrication of this new 2D material with a particular structure may bring new physical properties for future applications.

Erratum: Tunneling density of states, correlation energy, and spin polarization in the fractional quantum Hall regime [Phys. Rev. B 100 , 085107 (2019)

Received 1 November 2019DOI:https://doi.org/10.1103/PhysRevB.100.209903©2019 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasFractional quantum Hall effectSpin polarizationPhysical SystemsStrongly correlated systemsTwo-dimensional electron systemCondensed Matter, Materials & Applied Physics

High-Density Isolated Fe1O3 Sites on a Single-Crystal Cu2O(100) Surface.

Single-atom catalysts have recently been subject to considerable attention within applied catalysis. However, complications in the preparation of well-defined single-atom model systems have hampered efforts to determine the reaction mechanisms underpinning the reported activity. By means of an atomic layer deposition method utilizing the steric hindrance of the ligands, isolated Fe1O3 motifs were grown on a single-crystal Cu2O(100) surface at densities up to 0.21 sites per surface unit cell. Ambient pressure X-ray photoelectron spectroscopy shows a strong metal-support interaction with Fe in a chemical state close to 3+. Results from scanning tunneling microscopy and density functional calculations demonstrate that isolated Fe1O3 is exclusively formed and occupies a single site per surface unit cell, coordinating to two oxygen atoms from the Cu2O lattice and another through abstraction from O2. The isolated Fe1O3 motif is active for CO oxidation at 473 K. The growth method holds promise for extension to other catalytic systems.

Surface resonance of the (2×1) reconstructed lanthanum hexaboride (001)-cleavage plane: A combined STM and DFT study

We performed a combined study of the (001)-cleavage plane of lanthanum hexaboride (LaB$_\text{6}$) using scanning tunneling microscopy (STM) and density functional theory (DFT). Experimentally, we found a (2$\times$1) reconstructed surface on a local scale. The reconstruction is only short-range ordered and tends to order perpendicularly to step edges. At larger distances from surface steps, the reconstruction evolves to a labyrinth-like pattern. These findings are supported by low-energy electron diffraction (LEED) experiments. Slab calculations within the framework of DFT shows that the atomic structure consists of parallel lanthanum chains on top of boron octahedra. Scanning tunneling spectroscopy (STS) shows a prominent spectral feature at -0.6 eV. Using DFT, we identify this structure as a surface resonance of the (2$\times$1) reconstructed LaB$_\text{6}$ (100)-surface which is dominated by boron dangling bond-states and lanthanum d-states.

Template-controlled on-surface synthesis of a lanthanide supernaphthalocyanine and its open-chain polycyanine counterpart

Phthalocyanines possess unique optical and electronic properties and thus are widely used in (opto)electronic devices, coatings, photodynamic therapy, etc. Extension of their π-electron systems could produce molecular materials with red-shifted absorption for a broader range of applications. However, access to expanded phthalocyanine analogues with more than four isoindoline units is challenging due to the limited synthetic possibilities. Here, we report the controlled on-surface synthesis of a gadolinium-supernaphthalocyanine macrocycle and its open-chain counterpart poly(benzodiiminoisoindoline) on a silver surface from a naphthalene dicarbonitrile precursor. Their formation is controlled by the on-surface high-dilution principle and steered by different metal templates, i.e., gadolinium atoms and the bare silver surface, which also act as oligomerization catalysts. By using scanning tunneling microscopy, photoemission spectroscopy, and density functional theory calculations, the chemical structures along with the mechanical and electronic properties of these phthalocyanine analogues with extended π-conjugation are investigated in detail.

Dynamic band structure and capacitance effects in scanning tunneling spectroscopy of bilayer graphene

We develop a fully self-consistent model to describe scanning tunneling spectroscopy (STS) measurements of Bernal-stacked bilayer graphene (BLG), and we compare the results of our model with experimental measurements. Our results show that the STS tip acts as a top gate that changes the BLG band structure and Fermi level, while simultaneously probing the voltage-dependent tunneling density of states (TDOS). These effects lead to differences between the TDOS and the local density of states; in particular, we show that the bandgap of the BLG appears larger than expected in STS measurements, that an additional feature appears in the TDOS that is an artifact of the STS measurement, and that asymmetric charge distribution effects between the individual graphene layers are observable via STS.

Single and multi domain buckled germanene phases on Al(111) surface

The simultaneous formation of single domain (3×3) and multi domain (√7×√7)R(±19.1°) germanene phases on Al(111) surface in the sub-monolayer range was studied using scanning tunneling microscopy (STM) and density functional theory (DFT) based simulations. Experimental results revealed that both germanene phases nucleate and grow independently from each other and regardless of Al substrate temperature within significantly expanded range Ts = 27–200 °C. Our results unambiguously showed that STM images with hexagonal contrast yield correct-resolved structure for both germanene phases, while honeycomb contrast is a result of an artificial tip-induced STM resolution. First-principles calculations suggested atomic models with strongly buckled germanene (2×2)/Al(111)(3×3) and (√3×√3)R30°/Al(111)(√7×√7)R(±19.1°) with one of eight and one of six Ge atoms protruding upward respectively, that consistently describe the experimentally observed STM images both for single and multi domain surface phases. According to the DFT based simulations both germanene (2×2) and (√3×√3)R30° superstructures have a stretched lattice strain with respect to the ideal free-standing germanene by 6.2% and 13.9%, respectively. Hence, numerous small domains separated by domain boundaries in the (√3×√3)R307Al(111)(√7×√7)R(±19.1°) germanene phase tend to reduce the surface energy and prevent the formation of extended single domains, in contrast to the (2×2)/Al(111)(3×3) phase. However, our experimental results showed that the nucleation and growth of germanene on Al(111) surface yield strong modifications of Al surface even at room temperature (RT), which may be contributed to the formation of Al-Ge alloy due to Ge surface solid-states reactivity that was ignored in recent studies. It is already evident from our present findings that the role of Al atoms in the formation of (3×3) and (√7×√7)R(±19.1°) germanene phases is worthy to be carefully studied in the future, which could be an important knowledge for large-quantity fabrication of germanene on aluminum.

Atomic‐Scale Visualization of the Stepwise Metal‐Mediated Dehalogenative Cycloaddition Reaction Pathways: Competition between Radicals and Organometallic Intermediates

AbstractDehalogenative cycloaddition reaction is a powerful strategy to generate new ring scaffolds with π‐conjugated features on a surface, and thus holds great promise toward atomically precise electronic devices or nanomaterials. The ortho‐dihalo substitution provides a good strategy to realize cycloaddition. However, the limited understanding of intermediate states involved hinders mechanistic exploration for further precise design and optimization of reaction products. Now, the evolutions of competing surface‐stabilized radicals and organometallic intermediates in real space were visualized toward the formation of dominant conjugated four‐membered ring connections. From the interplay of scanning tunneling microscopy and density functional theory calculations, the stepwise metal‐mediated dehalogenative cycloaddition pathway is elucidated both experimentally and theoretically. The results provide fundamental insights into the intermediate states involved in on‐surface synthesis.

Empirical Study on the Driving Force of Urban Utility Tunnel Development

Urban utility tunnel has been regarded as an important symbol of urban modernization, so it is important to understand the quantized relationship between city development and urban utility tunnel for city management and policy making. In this study, the internal relationship between the development amount of urban utility tunnel and urban population density was discussed, as well as urban GDP per capita, urban construction land area per capita and urban land price index, based on the IBM SPSS platform, through adopting the development amount of urban utility tunnel and the urban development data of the past 46 years of Japan. It was shown that the correlation relationship of the development amount of urban utility tunnel with the density of urban population was comparatively strong negative, and relatively strong positive, comparatively strong positive, relatively strong negative for GDP per capita, urban construction land area per capita and urban land price index respectively, which explained the driving effect that urban development imposed on urban utility tunnel construction in essence. Furthermore, a quantitative model was proposed for the relationship between the development amount of urban utility tunnel and urban development indexes, and the model could be a reference for decision making of urban utility tunnel development in China and other countries.

On chip carbon nanotube tunneling spectroscopy

We report an experimental study of the band structure of individual carbon nanotubes (SCNTs) based on investigation of the tunneling density of states, i.e. tunneling spectroscopy. A common approach to this task is to use a scanning tunneling microscope (STM). However, this approach has a number of drawbacks, to overcome which, we propose another method – tunneling spectroscopy of SCNTs on a chip using a tunneling contact. This method is simpler, cheaper and technologically advanced than the STM. Fabrication of a tunnel contact can be easily integrated into any technological route, therefore, a tunnel contact can be used, for example, as an additional tool in characterizing any devices based on individual CNTs. In this paper we demonstrate a simple technological procedure that results in fabrication of good-quality tunneling contacts to carbon nanotubes.

Interface Effects on Epilayer Surface Density of States by Scanning Tunneling Spectroscopy and Density Functional Theory

AbstractSub‐nanometric Ni‐silicide films are self‐assembled by solid phase epitaxy of Ni on a vicinal Si(111) surface. The combination of in situ scanning tunneling microscopy and surface electron diffraction with ex situ X‐ray photoelectron spectroscopy allows us to identify θ‐Ni2Si as the major constituent crystal phase at the overgrown surface. Then scanning tunneling spectroscopy is employed with ab initio electronic structure calculations, to identify the origins of the surface local density of states and use these as a fingerprint for in situ phase identification. The above approach enhances the chemical contrast capability of scanning tunneling microscopy, and offers a useful local probe with atomic resolution for in situ crystal phase identification of ultra‐thin surface structures.

On-Surface Reactive Planarization of Pt(II) Complexes.

A series of Pt(II) complexes with tetradentate luminophores has been designed, synthesized, and deposited on coinage metal surfaces with the aim to produce highly planar self‐assembled monolayers. Low‐temperature scanning tunneling microscopy (STM) and density functional theory (DFT) calculations reveal a significant initial nonplanarity for all complexes. A subsequent metal‐catalyzed separation of the nonplanar moiety at the bridging unit via the scission of a C−N bond is observed, leaving behind a largely planar core complex. The activation barrier of this bond scission process is found to depend strongly on the chemical nature of both bridging group and coordination plane, and to increase from Cu(111) through Ag(111) to Au(111).

Oxidation Mechanism on One-Dimensional Pt-Induced Nanowires on Ge(001)

The oxidation mechanism on one-dimensional Pt-induced nanowires on Ge(001) (NWs/Ge(001)) was investigated on the atomic scale, by scanning tunneling microscopy and density functional theory calcula...