Confinement-Induced Isosymmetric Metal-Insulator Transition in Ultrathin Epitaxial VO Films.

Dimensional confinement has shown to be an effective strategy to tune competing degrees of freedom in complex oxides. Here, we achieved atomic layered growth of trigonal vanadium sesquioxide (VO) by means of oxygen-assisted molecular beam epitaxy. This led to a series of high-quality epitaxial ultrathin VO films down to unit cell thickness, enabling the study of the intrinsic electron correlations upon confinement.

Two-dimensional tellurium superstructures on Au(111) surfaces.

Two-dimensional (2D) allotropes of tellurium (Te), recently coined as tellurene, are currently an emerging topic of materials research due to the theoretically predicted exotic properties of Te in its ultrathin form and at the single atomic layer limit. However, a prerequisite for the production of such new and single elemental 2D materials is the development of simple and robust fabrication methods. In the present work, we report three different 2D superstructures of Te on Au(111) surfaces by following an alternative experimental deposition approach.

Growth and Characterization of Ultrathin Vanadium Oxide Films on HOPG.

Integration of graphene into various electronic devices requires an ultrathin oxide layer on top of graphene. However, direct thin film growth of oxide on graphene is not evident because of the low surface energy of graphene promoting three-dimensional island growth. In this study, we demonstrate the growth of ultrathin vanadium oxide films on a highly oriented pyrolytic graphite (HOPG) surface, which mimics the graphene surface, using (oxygen-assisted) molecular beam epitaxy, followed by a post-annealing.

Spin excitations of individual magnetic dopants in an ionic thin film.

Individual magnetic transition metal dopants in a solid host usually exhibit relatively small spin excitation energies of a few meV. Using scanning tunneling microscopy and inelastic electron tunneling spectroscopy (IETS) techniques, we have observed a high spin excitation energy around 36 meV for an individual Co substitutional dopant in ultrathin NaCl films. In contrast, the Cr dopant in the NaCl film shows much lower spin excitation energy around 2.

Identifying Native Point Defects in the Topological Insulator BiTe.

We successfully identified native point defects that occur in BiTe crystals by combining high-resolution bias-dependent scanning tunneling microscopy and density functional theory based calculations. As-grown BiTe crystals contain vacancies, antisites, and interstitial defects that may result in bulk conductivity and therefore may change the insulating bulk character. Here, we demonstrate the interplay between the growth conditions and the density of different types of native near-surface defects.

Unraveling the atomic structure, ripening behavior, and electronic structure of supported Au clusters.

The free-standing Au cluster has a unique tetrahedral shape and a large HOMO-LUMO (highest occupied molecular orbital-lowest unoccupied molecular orbital) gap of around 1.8 electron volts. The "magic" Au has been intensively used as a model system for understanding the catalytic and optical properties of gold nanoclusters.

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.

Controlling Water Intercalation Is Key to a Direct Graphene Transfer.

The key steps of a transfer of two-dimensional (2D) materials are the delamination of the as-grown material from a growth substrate and the lamination of the 2D material on a target substrate. In state-of-the-art transfer experiments, these steps remain very challenging, and transfer variations often result in unreliable 2D material properties. Here, it is demonstrated that interfacial water can insert between graphene and its growth substrate despite the hydrophobic behavior of graphene.

Scanning probe microscopy induced surface modifications of the topological insulator BiTe in different environments.

We investigated the topological insulator (TI) BiTe in four different environments (ambient, ultra-high vacuum (UHV), nitrogen gas and organic solvent environment) using scanning probe microscopy (SPM) techniques. Upon prolonged exposure to ambient conditions and organic solvent environments the cleaved surface of the pristine BiTe is observed to be strongly modified during SPM measurements, while imaging of freshly cleaved BiTe in UHV and nitrogen gas shows considerably less changes of the BiTe surface. We conclude that the reduced surface stability upon exposure to ambient conditions is triggered by adsorption of molecular species from ambient, including HO, CO, etc which is verified by Auger electron spectroscopy.

Adsorption of Te atoms on Au(1 1 1) and the emergence of an adatom-induced bound state.

We report on the adsorption of Te adatoms on Au(1 1 1), which are identified and investigated relying on scanning tunnelling microscopy, Auger electron spectroscopy, and density functional theory. The Te adatoms lift the 23  ×  √3 surface reconstruction of the Au(1 1 1) support and their organization is similar to that of previously reported chalcogen adatoms on Au(1 1 1), which are also known to lift the herringbone reconstruction and can adopt a (√3  ×  √3)R30° structure. The adatoms show strong interaction with the Au(1 1 1) surface, resulting in scattering and confinement of the Au surface state (SS) electrons near the Fermi level.

High Electronic Conductance through Double-Helix DNA Molecules with Fullerene Anchoring Groups.

Determining the mechanism of charge transport through native DNA remains a challenge as different factors such as measuring conditions, molecule conformations, and choice of technique can significantly affect the final results. In this contribution, we have used a new approach to measure current flowing through isolated double-stranded DNA molecules, using fullerene groups to anchor the DNA to a gold substrate. Measurements were performed at room temperature in an inert environment using a conductive AFM technique.

Electronically decoupled stacking fault tetrahedra embedded in Au(111) films.

Stacking faults are known as defective structures in crystalline materials that typically lower the structural quality of the material. Here, we show that a particular type of defect, that is, stacking fault tetrahedra (SFTs), exhibits pronounced quantized electronic behaviour, revealing a potential synthetic route to decoupled nanoparticles in metal films. We report on the electronic properties of SFTs that exist in Au(111) films, as evidenced by scanning tunnelling microscopy and confirmed by transmission electron microscopy.

Electron-phonon coupling in engineered magnetic molecules.

We probe the electron-phonon coupling for in situ engineered porphyrin-based magnetic molecular layers supported on weakly reactive surfaces. Using high-resolution scanning tunneling microscopy and spectroscopy at 4.5 K we show that the electronic and magnetic properties of the engineered molecules are the result of interplay between many-body spin-flip excitations and electron-phonon interactions.

Annealing-Induced Bi Bilayer on Bi2Te3 Investigated via Quasi-Particle-Interference Mapping.

Topological insulators (TIs) are renowned for their exotic topological surface states (TSSs) that reside in the top atomic layers, and hence, detailed knowledge of the surface top atomic layers is of utmost importance. Here we present the remarkable morphology changes of Bi2Te3 surfaces, which have been freshly cleaved in air, upon subsequent systematic annealing in ultrahigh vacuum and the resulting effects on the local and area-averaging electronic properties of the surface states, which are investigated by combining scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and Auger electron spectroscopy (AES) experiments with density functional theory (DFT) calculations. Our findings demonstrate that the annealing induces the formation of a Bi bilayer atop the Bi2Te3 surface.

Atomic resolution of nitrogen-doped graphene on Cu foils.

Atomic-level substitutional doping can significantly tune the electronic properties of graphene. Using low-temperature scanning tunneling microscopy and spectroscopy, the atomic-scale crystalline structure of graphene grown on polycrystalline Cu, the distribution of nitrogen dopants and their effect on the electronic properties of graphene were investigated. Both the graphene sheet growth and nitrogen doping were performed using microwave plasma-enhanced chemical vapor deposition.

Size-Dependent Penetration of Gold Nanoclusters through a Defect-Free, Nonporous NaCl Membrane.

Membranes and their size-selective filtering properties are universal in nature and their behavior is exploited to design artificial membranes suited for, e.g., molecule or nanoparticle filtering and separation.

Moiré superlattices at the topological insulator Bi2Te3.

We report on the observation of complex superlattices at the surface of the topological insulator Bi2Te3. Scanning tunneling microscopy reveals the existence of two different periodic structures in addition to the Bi2Te3 atomic lattice, which is found to strongly affect the local electronic structure. These three different periodicities are interpreted to result from a single small in-plane rotation of the topmost quintuple layer only.

Probing Magnetism in 2D Molecular Networks after in Situ Metalation by Transition Metal Atoms.

Metalated molecules are the ideal building blocks for the bottom-up fabrication of, e.g., two-dimensional arrays of magnetic particles for spintronics applications.

Alkoxylated dehydrobenzo[12]annulene on Au(111): from single molecules to quantum dot molecular networks.

We demonstrate the effective confinement of surface electrons in the pores of molecular networks formed by dehydrobenzo[12]annulene (DBA) molecules with butoxy groups (DBA-OC4) on Au(111). Investigation of the network formation starting from single molecules reveals a considerable interaction of the molecules with the substrate, which is at the origin of the observed confinement.

Electronic Band Structures and Native Point Defects of Ultrafine ZnO Nanocrystals.

Ultrafine ZnO nanocrystals with a thickness down to 0.25 nm are grown by a metalorganic chemical vapor deposition method. Electronic band structures and native point defects of ZnO nanocrystals are studied by a combination of scanning tunneling microscopy/spectroscopy and first-principles density functional theory calculations.

Lateral manipulation of atomic vacancies in ultrathin insulating films.

During the last 20 years, using scanning tunneling microscopy (STM) and atomic force microscopy, scientists have successfully achieved vertical and lateral repositioning of individual atoms on and in different types of surfaces. Such atom manipulation allows the bottom-up assembly of novel nanostructures that can otherwise not be fabricated. It is therefore surprising that controlled repositioning of virtual atoms, i.

Spontaneous doping of two-dimensional NaCl films with Cr atoms: aggregation and electronic structure.

Scanning tunneling microscopy (STM) experiments combined with density functional theory (DFT) calculations reveal that deposited Cr atoms replace either Na or Cl ions, forming substituting dopants in ultrathin NaCl/Au(111) films. The Cr dopants exchange electrons with the support thus changing the electronic properties of the film and in particular the work function. The Cr atoms spontaneously aggregate near the edges of the bilayer (2L) NaCl islands, forming a new phase in the insulator with a remarkably dense population of Cr dopants.

Topological transport and atomic tunnelling-clustering dynamics for aged Cu-doped Bi2Te3 crystals.

Enhancing the transport contribution of surface states in topological insulators is vital if they are to be incorporated into practical devices. Such efforts have been limited by the defect behaviour of Bi2Te3 (Se3) topological materials, where the subtle bulk carrier from intrinsic defects is dominant over the surface electrons. Compensating such defect carriers is unexpectedly achieved in (Cu0.

Inducing magnetism in pure organic molecules by single magnetic atom doping.

We report on in situ chemical reactions between an organic trimesic acid (TMA) ligand and a Co atom center. By varying the substrate temperature, we are able to explore the Co-TMA interactions and create novel magnetic complexes that preserve the chemical structure of the ligands. Using scanning tunneling microscopy and spectroscopy combined with density functional theory calculations, we elucidate the structure and the properties of the newly synthesized complex at atomic or molecular size level.

Self-doping of ultrathin insulating films by transition metal atoms.

Single magnetic Co atoms are deposited on atomically thin NaCl films on Au(111). Two different adsorption sites are revealed by high-resolution scanning tunneling microscopy (STM), i.e.