Scanning Tunneling Microscopy Experiments Research Articles

Spin-flip excitations induced by dehydrogenation in a magnetic single-molecule junction.

Recent scanning tunneling microscopy experiments on electron transport through iron(ii) phthalocyanine (FePc) molecules adsorbed on Au(111) surfaces have revealed that the measured differential conductance signals can be modulated through a selective dehydrogenation process [R. Li et al., Chem. Commun. 54, 9135 (2018)]. To understand the physical origin of the variation of line shapes in the measured dI/dV spectra, we employ a first-principles-based quantum transport simulation to calculate the electronic structures and transport properties in the dehydrogenated FePc/Au(111) composite system. Theoretical results indicate that the removal of eight outermost hydrogen atoms in the FePc molecule distorts the planar molecular conformation by increasing the distance between the Fe center and substrate and thus breaks the ligand field exerted on the Fe 3d orbitals. Such variations lead to a weaker coupling with the substrate along with a different local electronic configuration of the Fe center compared with that of the intact case, which is responsible for the suppression of Kondo resonance and the appearance of spin-flip excitation in the system. The simulated dI/dV plots are consistent with the experimental observations, showing the typical step features at finite bias associated with spin-flip excitations of a spin-1 quantum magnet. These findings provide important insights into the electron correlation effects modulated by the structural and chemical environment of the molecular spin center.

Electrical-Pulse-Induced Mixture and Separation in Surface Supramolecular Hybrids: STM Experiments and Theoretical Approaches

Using scanning tunneling microscopy (STM) we investigated the effect of electrical pulses upon a supramolecular hybrid comprised of pentacenes and 1,3,5-tris(4-carboxyphenyl)benzenes at the octanoic acid/graphite interface. STM results revealed that positive electrical pulses can promote the formation of such hybrids, whereas negative pulses selectively remove pentacene molecules from the complex. We attribute such results to the voltage-pulse-induced instant electrical attraction and repulsion between pentacene and substrate, thereby inducing the hybrid to form or to deform, respectively. Furthermore, these observed phenomena were elucidated via a coarse-grained molecular dynamic model using the continuum approximation.

Surface Majorana flat bands in j = 3/2 superconductors with singlet-quintet mixing

Recent experiments [Science Advances 4 eaao4513 (2018)] have revealed the evidence of nodal-line superconductivity in half-Heusler superconductors, e.g., YPtBi. Theories have suggested the topological nature of such nodal-line supercon-ductivity and proposed the existence of surface Majorana flat bands on the (111) surface of half-Heusler superconductors. Due to the divergent density of states of the surface Majorana flat bands, the surface order parameter and the surface impurity play essential roles in determining the surface properties. We study the effect of the surface order parameter and the surface impurity on the surface Majorana flat bands of half-Heusler superconductors based on the Luttinger model. To be specific, we consider the topological nodal-line superconducting phase induced by the singlet-quintet pairing mixing, classify all the possible translationally invariant order parameters for the surface states according to irreducible representations of C3v point group, and demonstrate that any energetically favorable order parameter needs to break the time-reversal symmetry. We further discuss the energy splitting in the energy spectrum of surface Majorana flat bands induced by different order parameters and non-magnetic or magnetic impurities. We propose that the splitting in the energy spectrum can serve as the fingerprint of the pairing symmetry and mean-field order parameters. Our theoretical prediction can be examined in the future scanning tunneling microscopy experiments.

Effect of anomalous diffusion of fluctuating Cooper pairs on the density of states of superconducting NbN thin films

Recent scanning tunnelling microscopy experiments in NbN thin disordered superconducting films found an emergent inhomogeneity at the scale of tens of nanometers. This inhomogeneity is mirrored by an apparent dimensional crossover in the paraconductivity measured in transport above the superconducting critical temperature $T_c$. This behavior was interpreted in terms of an anomalous diffusion of fluctuating Cooper pairs, that display a {\em quasi-confinement} (i.e., a slowing down of their diffusive dynamics) on length scales shorter than the inhomogeneity identified by tunnelling experiments. Here we assume this anomalous diffusive behavior of fluctuating Cooper pairs and calculate the effect of these fluctuations on the electron density of states above $T_c$. We find that the density of states is substantially suppressed up to temperatures well above $T_c$. This behavior, which is closely reminiscent of a pseudogap, only arises from the anomalous diffusion of fluctuating Cooper pairs in the absence of stable preformed pairs, setting the stage for an intermediate behavior between the two common paradigms in the superconducting-insulator transition, namely the localisation of Cooper pairs (the so-called bosonic scenario) and the breaking of Cooper pairs into unpaired electrons due to strong disorder (the so-called fermionic scenario).

Enantiospecific equilibrium adsorption and chemistry of d-/l-proline mixtures on chiral and achiral Cu surfaces.

A fundamental understanding of the enantiospecific interactions between chiral adsorbates and understanding of their interactions with chiral surfaces is key to unlocking the origins of enantiospecific surface chemistry. Herein, the adsorption and decomposition of the amino acid proline (Pro) have been studied on the achiral Cu(110) and Cu(111) surfaces and on the chiral Cu(643)R&S surfaces. Isotopically labelled 1-13 C-l-Pro has been used to probe the Pro decomposition mechanism and to allow mass spectrometric discrimination of d-Pro and 1-13 C-l-Pro when adsorbed as mixtures. On the Cu(111) surface, X-ray photoelectron spectroscopy reveals that Pro adsorbs as an anionic species in the monolayer. On the chiral Cu(643)R&S surface, adsorbed Pro enantiomers decompose with non-enantiospecific kinetics. However, the decomposition kinetics were found to be different on the terraces versus the kinked steps. Exposure of the chiral Cu(643)R&S surfaces to a racemic gas phase mixture of d-Pro and 1-13 C-l-Pro resulted in the adsorption of a racemic mixture; i.e., adsorption is not enantiospecific. However, exposure to non-racemic mixtures of d-Pro and 1-13 C-l-Pro resulted in amplification of enantiomeric excess on the surface, indicative of homochiral aggregation of adsorbed Pro. During co-adsorption, this amplification is observed even at very low coverages, quite distinct from the behavior of other amino acids, which begin to exhibit homochiral aggregation only after reaching monolayer coverages. The equilibrium adsorption of d-Pro and 1-13 C-l-Pro mixtures on achiral Cu(110) did not display any aggregation, consistent with prior scanning tunneling microscopy (STM) observations of dl-Pro/Cu(110). This demonstrates convergence between findings from equilibrium adsorption methods and STM experiments and corroborates formation of a 2D random solid solution.

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.

High graphene permeability for room temperature silicon deposition: The role of defects

Graphene (Gr) is known to be an excellent barrier preventing atoms and molecules to diffuse through it. This is due to the carbon atom arrangement in a two-dimensional (2D) honeycomb structure with a very small lattice parameter forming an electron cloud that prevents atoms and molecules crossing. Nonetheless at high annealing temperatures, intercalation of atoms through graphene occurs, opening the path for formation of vertical heterojunctions constituted of two-dimensional layers. In this paper, we report on the ability of silicon atoms to penetrate the graphene network, fully epitaxially grown on a Ni(111) surface, even at room temperature. Our scanning tunneling microscopy (STM) experiments show that the presence of defects like vacancies and dislocations in the graphene lattice favor the Si atoms intercalation, forming two-dimensional, flat and disordered islands below the Gr layer. Ab-initio molecular dynamics calculations confirm that Gr defects are necessary for Si intercalation at room temperature and show that: i) a hypothetical intercalated silicene layer cannot be stable for more than 8 ps and ii) the corresponding Si atoms completely lose their in-plane order, resulting in a random planar distribution, and form strong covalent bonds with Ni atoms.

Extended α-phase Bi atomic layer on Si(1 1 1) fabricated by thermal desorption

Two-dimensional materials with sizable spin-orbit coupling are not only interesting to fundamental studies, but also useful candidates for technological applications. Here, we report on the fabrication of extended α-phase Bi atomic layer on Si(1 1 1) surface by means of thermal desorption. The atomic and electronic structures have been investigated by combining scanning tunneling microscopy (STM) experiments with density functional theory (DFT) calculations. While β-phase Bi trimer with 3×3R30° surface reconstruction can be prepared by room-temperature deposition with subsequent post annealing at 700 K, the α-phase Bi monomer only appears at 800 K due to thermal desorption with the Bi desorption rate of 0.26 ML/min. From our DFT calculations, the α-phase Bi shows Rashba energies ER = 15 meV and 3 meV at high symmetry points of M¯ and K¯, respectively. Since α-phase Bi monomer represents isotropic lattice symmetry with a dilute Bi density, it may serve as a potential template to tune Rashba effect by incorporating with multiple elements.

Directional massless Dirac fermions in a layered van der Waals material with one-dimensional long-range order

One or a few layers of van der Waals (vdW) materials are promising for applications in nanoscale electronics. Established properties include high mobility in graphene, a large direct gap in monolayer MoS2, the quantum spin Hall effect in monolayer WTe2 and so on. These exciting properties arise from electron quantum confinement in the two-dimensional limit. Here, we use angle-resolved photoemission spectroscopy to reveal directional massless Dirac fermions due to one-dimensional confinement of carriers in the layered vdW material NbSi0.45Te2. The one-dimensional directional massless Dirac fermions are protected by non-symmorphic symmetry, and emerge from a stripe-like structural modulation with long-range translational symmetry only along the stripe direction as we show using scanning tunnelling microscopy. Our work not only provides a playground for investigating further the properties of directional massless Dirac fermions, but also introduces a unique component with one-dimensional long-range order for engineering nano-electronic devices based on heterostructures of vdW materials.

Quantum phase diagram of Shiba impurities from bosonization

A characteristic feature of Shiba impurities is the existence of a spin- and parity-changing quantum phase transition (known as "0-$\pi$" transition) which has been observed in scanning tunneling microscopy (STM) and transport experiments. Using the Abelian bosonization technique, here we analyze the ground-state properties and the quantum phase diagram of a classical (i.e., Ising-like) Shiba impurity. In particular, we analyze the cases of an impurity in a three- and a one-dimensional superconductor. Within the bosonization framework, the ground-state properties are determined by simple soliton-like solutions of the classical equations of motion of the bosonic fields, whose topological charge is related to the spin and parity quantum numbers. Our results indicate that the quantum phase diagram of the superconductor can be strongly affected by geometrical and dimensional effects. Exploiting this fact, in the one-dimensional case we propose an experimental superconducting nanodevice in which a novel parity-preserving, spin-changing "0-0" transition is predicted.

Topology- and symmetry-protected domain wall conduction in quantum Hall nematics

We consider domain walls in nematic quantum Hall ferromagnets predicted to form in multivalley semiconductors, recently probed by scanning tunneling microscopy experiments on Bi(111) surfaces. We show that the domain wall properties depend sensitively on the filling factor $\ensuremath{\nu}$ of the underlying (integer) quantum Hall states. For $\ensuremath{\nu}=1$ and in the absence of impurity scattering we argue that the wall hosts a single-channel Luttinger liquid whose gaplessness is a consequence of valley and charge conservation. For $\ensuremath{\nu}=2$, it supports a two-channel Luttinger liquid, which for sufficiently strong interactions enters a symmetry-preserving thermal metal phase with a charge gap coexisting with gapless neutral intervalley modes. The domain wall physics in this state is identical to that of a bosonic topological insulator protected by $U(1)\ifmmode\times\else\texttimes\fi{}U(1)$ symmetry, and we provide a formal mapping between these problems. We discuss other unusual properties and experimental signatures of these anomalous one-dimensional systems.

STM Imaging of Electron Migration in Real Space and Time: A Simulation Study.

Using a simulation protocol that mimics ultrafast scanning tunneling microscopy (STM) experiments, we demonstrate how pump-probe ultrafast STM may be used to image electron migration in molecules. Two pulses are applied to a model system, and the time-integrated current through the tip is calculated versus the delay time and tip position to generate STM images. With suitable pump and probe parameters, the images can track charge migration with atomistic spatial and femtosecond temporal resolutions.

Topological Ising pairing states in monolayer and trilayer TaS2

We study the possibility of topological superconductivity in the noncentrosymmetric monolayer and trilayer ${\mathrm{TaS}}_{2}$ with out-of-plane mirror symmetry. A time-reversal-invariant nodal $(s+f)$-wave pairing state with even mirror parity is found to be a promising candidate. This mixed state holds 12 (36) nodes at the Fermi pockets around $\mathrm{\ensuremath{\Gamma}}$ for the monolayer (trilayer) case and its unconventional superconductivity is consistent with the scanning tunneling microscopy experiments observed in $2H\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$ thin flakes. Furthermore, with doping or under uniaxial pressure for trilayer $2H\text{\ensuremath{-}}{\mathrm{TaS}}_{2}$, large-Chern-number time-reversal symmetry-breaking mixed states between $(d+id)$- and $(p\ensuremath{-}ip)$-wave pairings can be realized in the phase diagram.

Strongly Hole-Doped and Highly Decoupled Graphene on Platinum by Water Intercalation.

Scanning tunneling microscopy and spectroscopy experiments under ultrahigh vacuum and low-temperature conditions have been performed on water-intercalated graphene on Pt(111). We find that the confined water layer, with a thickness around 0.35 nm, induces a strong hole doping in graphene, i.e., the Dirac point locates at round 0.64 eV above the Fermi level. This can be explained by the presence of a single "puckered bilayer" of ice-Ih, which has not been experimentally found on bare Pt(111), being confined in between graphene and Pt(111) surface. Moreover, the water intercalation makes graphene highly decoupled from the substrate, allowing us to reveal the intrinsic graphene phonons and double Rydberg series of even and odd symmetry image-potential states. Our work not only demonstrates that the electronic properties of graphene can be tuned by the confined water layer between graphene and the substrate, but also provides a generally applicable method to study the intrinsic properties of graphene as well as of other supported two-dimensional materials.

STM Images of Anionic Defects at CeO2(111)-A Theoretical Perspective.

We present a theoretically oriented analysis of the appearance and properties of plausible candidates for the anionic defects observed in scanning tunneling microscopy (STM) experiments on CeO2(111). The simulations are based on density functional theory (DFT) and cover oxygen vacancies, fluorine impurities and hydroxyl groups in the surface and sub-surface layers. In the surface layer, all three appear as missing spots in the oxygen sublattice in filled state simulated STM images, but they are distinguishable in empty state images, where surface oxygen vacancies and hydroxyls appear as, respectively, diffuse and sharp bright features at oxygen sites, while fluorine defects appear as triangles of darkened Ce ions. In the sub-surface layer, all three defects present more complex patterns, with different combinations of brightened oxygen ion triangles and/or darkened Ce ion triangles, so we provide image maps to support experimental identification. We also discuss other properties that could be used to distinguish the defects, namely their diffusion rates and distributions.

Low-Temperature Heterolytic Adsorption of H2 on ZnO(101̅0) Surface

The interaction of molecular hydrogen with a ZnO(1010) surface at cryogenic conditions was investigated by in situ scanning tunneling microscopy (STM) experiments combined with density functional ...

Coinage Metal–Sulfur Complexes: Stability on Metal(111) Surfaces and in the Gas Phase

We provide a comprehensive theoretical assessment at the level of density functional theory (DFT) of the stability of various coinage metal–sulfur complexes, both in the gas phase and also for the complexes adsorbed on the (111) surface of the same coinage metal. Our primary interest lies in the latter where earlier scanning tunneling microscopy (STM) experiments were interpreted to suggest the existence of adsorbed S-decorated metal trimers, sometimes as a component of more complex adlayer structures. Recent STM studies at 5 K directly observed other isolated adsorbed metal–sulfur complexes. For these adsorbed species, we calculate various aspects of their energetics including a natural measure of stability corresponding to their formation energy from sulfur adsorbed on terraces and from metal atoms that are in thermal equilibrium with the substrate. From this perspective, our DFT analysis shows that all of Ag2S3, Ag3S3, and many larger complexes on Ag(111) are strongly stable, Cu2S3 is stable, and some larger complexes are marginally stable on Cu(111), but only Au4S4 on Au(111) is stable. Results are consistent with STM observations for Cu(111) and Ag(111) surfaces but appear to deviate slightly for Au(111). A systematic analysis relating stability in the gas phase with that of adsorbed species is achieved within the framework of Hess’s law. This analysis also unambiguously elucidates various energetic contributions to stability.

Nitrogen-induced reconstructions on the Cr(001) surface

We use spin-polarized density functional theory to describe the surface reconstructions formed when the Cr(001) surface is exposed to nitrogen. Our primary goal is to clarify the atomic structure of the N induced c(2 × 2) Cr(001) reconstruction and to study its properties. After analyzing the relative stability of several models with a 1/2 monolayer of N atoms on the surface, it was found that only two models were stable for different growth conditions. The first stable model shows two N atoms (per cell) adsorbed on hollow sites following a diagonal pattern. In the second, N atoms replace Cr atoms of the first monolayer, and after optimization, they end up in almost second layer positions. To compare with scanning tunneling microscopy experiments, simulated STM images, using the Tersoff-Hamann approximation, were obtained. Our two models can explain the two different c(2 × 2) Cr(001) reconstructions observed experimentally. The magnetic properties, such as magnetic moments and alignments, are discussed compared with experimental values.

Anomalously Low Barrier for Water Dimer Diffusion on Cu(111).

A molecular-scale description of water and ice is important in fields as diverse as atmospheric chemistry, astrophysics, and biology. Despite a detailed understanding of water and ice structures on a multitude of surfaces, relatively little is known about the kinetics of water motion on surfaces. Here, we report a detailed study on the diffusion of water monomers and the formation and diffusion of water dimers through a combination of time-lapse low-temperature scanning tunnelling microscopy experiments and first-principles electronic structure calculations on the atomically flat Cu(111) surface. On the basis of an unprecedented long-time study of individual water monomers and dimers over days, we establish rates and mechanisms of water monomer and dimer diffusion. Interestingly, we find that the monomer and the dimer diffusion barriers are similar, despite the significantly larger adsorption energy of the dimer. This is thus a violation of the rule of thumb that relates diffusion barriers to adsorption energies, an effect that arises because of the directional and flexible hydrogen bond within the dimer. This flexibility during diffusion should also be relevant for larger water clusters and other hydrogen-bonded adsorbates. Our study stresses that a molecular-scale understanding of the initial stages of ice nanocluster formation is not possible on the basis of static structure investigations alone.

Multi-electron Reduction Capacity and Multiple Binding Pockets in Metal-Organic Redox Assembly at Surfaces.

Metal-ligand complexation at surfaces utilizing redox-active ligands has been demonstrated to produce uniform single-site metals centers in regular coordination networks. Two key design considerations are the electron storage capacity of the ligand and the metal-coordinating pockets on the ligand. In an effort to move toward greater complexity in the systems, particularly dinuclear metal centers, we designed and synthesized tetraethyltetra-aza-anthraquinone, TAAQ, which has superior electron storage capabilities and four ligating pockets in a diverging geometry. Cyclic voltammetry studies of the free ligand demonstrate its ability to undergo up to a four-electron reduction. Solution-based studies with an analogous ligand, diethyldi-aza-anthraquinone, demonstrate these redox capabilities in a molecular environment. Surface studies conducted on the Au(111) surface demonstrate TAAQ's ability to complex with Fe. This complexation can be observed at different stoichiometric ratios of Fe:TAAQ as Fe 2p core level shifts in X-ray photoelectron spectroscopy. Scanning tunneling microscopy experiments confirmed the formation of metal-organic coordination structures. The striking feature of these structures is their irregularity, which indicates the presence of multiple local binding motifs. Density functional theory calculations confirm several energetically accessible Fe:TAAQ isomers, which accounts for the non-uniformity of the chains.