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Protected and spin-polarized transport channels are the hallmark of topological insulators, coming along with an intrinsic strong spin-orbit coupling. Here we identified such corresponding chiral states in epitaxially grown zigzag graphene nanoribbons (zz-GNRs), albeit with an extremely weak spin-orbit interaction. While the bulk of the monolayer zz-GNR is fully suspended across a SiC facet, the lower edge merges into the SiC(0001) substrate and reveals a surface state at the Fermi energy, which is extended along the edge and splits in energy toward the bulk. All of the spectroscopic details are precisely described within a tight binding model incorporating a Haldane term and strain effects. The concomitant breaking of time-reversal symmetry without the application of external magnetic fields is supported by ballistic transport revealing a conduction of G = e2/h.

We report on the fabrication of Polyacrylonitrile (PAN) based electrospun carbon nanofiber (ECNF) reinforced with multi-walled carbon nanotubes (MWCNTs) and characterization of the graphitic structure as well as electrical transport properties for four different ECNF systems. The different systems were obtained by the combination of two different ways of cyclization for pristine PAN and PAN with CNTs addition (PAN/CNT), typically by the application of constant creep stress and a constraint fixing. The addition of CNTs improved the graphitic structure of PAN. For the detailed analysis of different systems, carbonization was performed at 1700 °C and the graphitic structures were characterized. The CNTs improve the alignment of graphitic domains even for non-creep cyclized systems. The application of creep stress during cyclization results in highly aligned graphene planes. However, the creep stress cyclized PAN/CNT system exhibited a reduction in the alignment of graphene planes coming along with the observation of discontinuous, randomly orientated domains and a reduced conductivity. The electrical anisotropy increased for creep cyclized PAN and PAN/CNT compared to non-creep stress cyclized system. Our findings reveal that electrical transport in carbon nanofibers need to be considered as a cumulative effect of hopping and band transport along chemically and structurally inhomogeneous ECNFs.

Graphene was shown to reveal intriguing properties of its relativistic two-dimensional electron gas; however, its implementation to microelectronic applications is missing to date. In this work, we present a comprehensive study of epitaxial graphene on technologically relevant and in a standard CMOS process achievable Ge(100) epilayers grown on Si(100) substrates. Crystalline graphene monolayer structures were grown by means of chemical vapor deposition (CVD). Using angle-resolved photoemission spectroscopy and in situ surface transport measurements, we demonstrate their metallic character both in momentum and real space. Despite numerous crystalline imperfections, e.g., grain boundaries and strong corrugation, as compared to epitaxial graphene on SiC(0001), charge carrier mobilities of 1 × 104 cm2/Vs were obtained at room temperature, which is a result of the quasi-charge neutrality within the graphene monolayers on germanium and not dependent on the presence of an interface oxide. The interface roughness due to the facet structure of the Ge(100) epilayer, formed during the CVD growth of graphene, can be reduced via subsequent in situ annealing up to 850 °C coming along with an increase in the mobility by 30%. The formation of a Ge(100)-(2 × 1) structure demonstrates the weak interaction and effective delamination of graphene from the Ge/Si(100) substrate.

Even though there have been many experimental attempts and theoretical approaches to understand the process of electromigration (EM), it has not been quantitatively understood for ultrathin structures and at grain boundaries. Nevertheless, we showed recently that it can be used reliably for the formation of single atomic point contacts after careful pre-structuring of the initial Ag nanostructures. The process of formation of nanocontacts by EM down to a single-atom point contact was investigated for ultrathin (5 nm) Ag structures at 100 K by measuring the conductance as a function of the time during EM. In this paper, we compare the process of thinning by EM of structures with constrictions below the average grain size of Ag layers (15 nm) with that of structures with much larger initial constrictions of around 150 nm having multiple grains at the centre constriction prior to the formation of a point contact. Even though clear morphological differences exist between both types of structures, quantized conductance plateaus showing the formation of single point contacts have been observed for both. Here we put emphasis on the thinning process by EM, just before a point contact is formed. To understand this thinning process, the semi-classical regime before the contact reaches the quantum regime was analyzed in detail. For this purpose, we used experimental conductance histograms in the range between 2G0 and 15G0 and their corresponding Fourier transforms (FTs). The FT analysis of the conductance histograms exhibits a clear preference for thinning along the [100] direction. Using well-established models, both atom-by-atom steps and ranges of stability, presumably caused by electronic shell effects, can be discriminated. Although the directional motion of atoms during EM leads to specific properties such as the instabilities mentioned, similarities to mechanically opened contacts with respect to cross-sectional stability were found.

The synthesis of [Au{S=C(NH2)2}2][SO3Me] (1) (a) by the anodic oxidation of gold metal in an anolyte of thiourea and methansulfonic acid and (b) by the reaction of Au(OH)3 with an aqueous solution of methanesulfonic acid in the presence of thiourea is reported. The structure of 1 in the solid state has been determined by single-crystal X-ray diffraction showing a linear S–Au–S unit with the thiourea ligands in a leaflet structure folded by 113.2(3)°. The cation of complex 1 is a dimer, based on short S · · · C interactions between two adjacent mononuclear cations. The thermal decomposition behavior of 1 was studied by TG and TG-MS confirming that it decomposes under inert gas or oxygen atmosphere in four steps in the temperature range of 200–650°C. Initial decomposition starts with the release and fragmentation of one of the thiourea ligands, followed by the anion degradation. Powder X-ray diffraction studies specified the formation of gold metal. Based on this observation, complex 1 was used as precursor for the formation of gold nanoparticles (Au NPs) in 1-hexadecylamine (c = 4.0 mol L−1) at T = 330°C without any addition of reducing agents. TEM, electron diffraction, and UV/Vis spectroscopy studies were carried out. Au NPs of size 15 ± 4 nm were formed, showing the characteristic surface plasmon resonance at 528 nm.

The strong sensitivity of plasmonic excitations on nanostructures to their environment is studied, going to the ultimate limit of single atomic chains. As a first step, we investigated how metallic...

Proximity coupling is discussed as a promising approach to tune further graphene’s properties. Organic π-conjugated molecules couple per se weakly to surfaces and the class of phthalocyanines provides manifold possibilities for molecular functionalization. We have investigated the adsorption of lead phthalocyanine (PbPc) on epitaxial graphene by means of scanning tunneling microscopy and in-situ electronic transport measurements in order to quantify both a molecular charge transfer and modifications of the carrier mobility in graphene. Compared to other metallic surfaces, monolayer structures of PbPc adsorbed on graphene solely pointing upwards, apparently, due to the antibonding character of the Pb-states with the pz-orbitals of graphene. The squared lattice of the molecular layer is not commensurate with the honeycomb lattice of graphene and the PbPc molecules are slightly tilted, thus forming a chiral monolayer structure. Despite this interaction with graphene, the charge state of the molecule as well as the mobility of the charge carriers in graphene are not altered. Therefore, monolayer structures of Pb molecules are promising candidates for proximity studies in graphene.

Graphene nanoribbons (GNRs) are considered as major building blocks in future carbon-based electronics. The electronic performance of graphene nanostructures is essentially influenced and determined by their edge termination and their supporting substrate. In particular, semi-conducting, as well as metallic GNRs, can be fabricated by choosing the proper template which is favorable for device architecture designs. This study highlights the impact of microscopic details of the environment of the GNRs on the charge transport in GNRs. By means of lateral force, conductive atomic force and nanoprobe measurements, we explore the charge propagation in both zig-zag and armchair GNRs epitaxially grown on SiC templates. We directly image transport channels on the nanoscale and identify SiC substrate steps and nano-instabilities of SiC facets as dominant charge scattering centers. [Figure not available: see fulltext.].

This article shows a new way to produce hierarchical microsieves by layering three types of float-cast microsieves, differing from each other in their pore diameters (approximately 68 μm, 7 μm, and 0.24 μm) on top of each other. The unsupported microsieves with 7 and 0.24 μm pore sizes are mechanically fragile. The complete hierarchical sieve composed of all three layers, however, can be handled manually without special precaution. This article further investigates the flow through the hierarchical sieve and filtration via experiment, theory (Hagen-Poiseuille's and Sampson-Roscoe's law), and simulation (numerically solving the Navier-Stokes equations for a predefined set of discrete volumetric elements). The experimental, theoretical, and simulated permeances of the microsieves and the hierarchical sieves are in reasonable agreement with each other and are significantly higher than the permeances of conventional filtration media. In filtration experiments, the hierarchical sieves show the expected sharp size cut-off, retaining particles of diameters exceeding the pore diameter.

Self-organized gold chains on vicinal Si(111) surfaces represent prototype examples of quasi-one-dimensional objects that are stabilized by hybridization with Si surface states. Their plasmons contain important information about the unoccupied bandstructure close to the Fermi level. Using Si(775)–Au as an example, we report here the modifications of the plasmon dispersion by the simple atomic adatom species H and O. Using a combination of low energy electron diffraction and high-resolution electron energy loss spectroscopy, we study the interconnection between plasmonic excitation and the corresponding local surface structure. Both adsorbates do not destroy metallicity, but, similar to Si(553)–Au, atomic hydrogen enhances dimerization of the Au chains, which at small concentrations counteracts the disorder introduced by random adsorption. This effect, most likely caused by electron donation of H to the surface states, is missing in case of adsorbed oxygen, so that only the effect of disorder is observed. For both adsorbates increasing disorder as a function of adsorbate concentration finally results in plasmon localization and opening of a band gap.

Helical molecules have recently attracted interest due to their capability for robust spin polarization of transmitted electrons. By means of mechanically controlled break Au junctions, we analyze the transport properties of single lysine-doped and cysteine-terminated polyalanine (PA) molecules of various lengths (2.4--5.4 nm). The conductance varies exponentially with the (effective) length of the molecules and does not depend on the temperature (90--300 K), thus electron tunneling is the dominant transport mechanism. The decay constant for the PA molecule is found to be $3.5\phantom{\rule{0.28em}{0ex}}{\mathrm{nm}}^{\ensuremath{-}1}$, significantly smaller compared to those of other organic molecules, emphasizing the high conductivity along the helical polypeptides and reflecting a low tunneling barrier, which decreases further for fields exceeding $5\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{0.28em}{0ex}}\mathrm{V}/\mathrm{cm}$. The conductance histograms of all PA molecules investigated reveal characteristic satellite peaks, which correlate with the apparent molecule length in multiples of characteristic peptide sequences. We attribute this effect to a racheting of interdigitated molecules adsorbed on each side of the electrodes during the opening/closing cycles.

In bismuth quantum films, a fascinating interplay between (bulk) quantum well and interface (edge) states is observed in this comprehensive study of DC magnetoconductance and Hall resistivity as a function of film thickness. While for thin layers (up to 40 bilayers, BL) conductance is governed by the surface states, strong contributions of a quantum well state dominate above 70 BL. Quantum corrections in a magnetic field normal to the surface, however, are dominated by the interface states for all thicknesses investigated for reasons discussed in the paper.

The low-temperature spin-orbit density phase of Pb quantum wires on Si(557) was investigated by scanning tunneling microscopy. High resolution images of this phase allow the proposal of an atomistic model for the densely-packed Pb coverages on the (223) facets. A metal-insulator phase transition was confirmed by local spectroscopy and the BCS-like gap feature of $2\mathrm{\ensuremath{\Delta}}\ensuremath{\approx}30\phantom{\rule{0.28em}{0ex}}\text{meV}$ at 10 K correlates with the transition temperature of ${T}_{c}=78\phantom{\rule{0.28em}{0ex}}\text{K}$ seen in surface transport experiments. Moreover, Pb excess coverage adds an extra row of Pb atoms on the mini-(111) terraces. These atoms induce a metallic state located at the step edge at low temperature and quench the charge and spin correlations of the spin-orbit density phase.

The ability to define an off state in logic electronics is the key ingredient that is impossible to fulfill using a conventional pristine graphene layer, due to the absence of an electronic bandgap. For years, this property has been the missing element for incorporating graphene into next-generation field effect transistors. In this work, we grow high-quality armchair graphene nanoribbons on the sidewalls of 6H-SiC mesa structures. Angle-resolved photoelectron spectroscopy (ARPES) and scanning tunneling spectroscopy measurements reveal the development of a width-dependent semiconducting gap driven by quantum confinement effects. Furthermore, ARPES demonstrates an ideal one-dimensional electronic behavior that is realized in a graphene-based environment, consisting of well-resolved subbands, dispersing and non-dispersing along and across the ribbons respectively. Our experimental findings, coupled with theoretical tight-binding calculations, set the grounds for a deeper exploration of quantum confinement phenomena and may open intriguing avenues for new low-power electronics., Here, the authors investigate armchair graphene nanoribbons by angle-resolved photoelectron spectroscopy, and show the development of a width-dependent semiconducting gap driven by quantum confinement effects, and an ideal one-dimensional electronic behaviour.

Highly efficient magnetic spin memory devices operating at ambient temperatures were recently realized via self-assembled molecular layers of an enantiomeric phase of polyalanine (PA). However, the repeatability and yield of such molecular devices rely sensitively on the layer quality. In this work, we grew molecular layers of α-l-PA (36 monomers of α-amino acid, 5.4 nm) on highly oriented pyrolytic graphite surfaces and investigated the structure by scanning tunneling microscopy under ambient conditions at the liquid–solid interface, complemented by X-ray photoelectron and Raman spectroscopies. Our results suggest that interdigitation of adjacent molecules takes place, which leads to a packing density of 0.8 PA/nm2, which is more than a factor of 2 higher than that of the closed-packed structure. The interdigitation not only stabilizes the PA film by additional H bonds but also ensures chiral alignment between adjacent molecules, which may be a prerequisite for a coherent electron transmission.

The photocatalytic performance of MoO3 is limited due to its weak visible light absorption ability and quick recombination of charge carriers. In the present work, we report the facile synthesis of Fe(iii)-grafted MoO3 nanorods using a hydrothermal method followed by an impregnation technique with the aim of enhancing the light harvesting ability and photocatalytic efficiency of MoO3. The prepared samples were characterized through the standard analytical techniques of XRD, SEM-EDS, TEM, XPS, UV-Vis-DRS, FT-IR, TG-DTA and PL spectrophotometry. XPS and TEM analyses reveal that Fe(iii) ions are successfully grafted onto the surface of the MoO3 nanorod with intimate interfacial contact. The photocatalytic performances of the prepared samples were investigated by studying the degradation of methylene blue (MB), rhodamine B (RhB) and 4-nitrophenol (4-NP) under visible light irradiation. The surface-modified MoO3 with Fe(iii) ions showed excellent photocatalytic activity towards the degradation of the above-mentioned pollutants, where Fe(iii) ions act as effective cocatalytic sites to produce hydroxyl radicals through multi-electron reduction of oxygen molecules. The improved photocatalytic activity could be ascribed to the effective separation of charge carriers and efficient production of hydroxyl radicals via the rapid capture of electrons by Fe(iii) through a well-known photoinduced interfacial charge transfer mechanism. Based on scavenger analysis study, a mechanism for the enhanced photocatalytic activity has been discussed and proposed. The concept of surface grafting onto large bandgap semiconductors with ubiquitous elements opens up a new avenue for the development of visible-light-responsive photocatalysts with excellent photocatalytic activity.

Twin grain boundaries in MoSe2 are metallic and undergo a metal to insulator Peierls transition at low temperature. Growth of MoSe2 by molecular beam epitaxy results in the spontaneous formation of a high density of these twin grain boundaries, likely as a mechanism to incorporate Se deficiency in the film. Using scanning tunneling microscopy, we study the grain boundary network that is formed in homoepitaxially grown MoSe2 and for MoSe2 grown heteroepitaxially on MoS2 and HOPG substrates. No statistically relevant variation of the grain boundary concentration has been found for the different substrates, indicating that the grain boundary formation is substrate independent and depends mainly on the growth conditions. Twin grain boundaries exhibit three crystallographically identical orientations, and thus they form an intersecting network. Different intersection geometries are identified that imply varying defect configurations. These intersection points act as preferential nucleation sites for vapor-depo...

Permeation experiments on Surface Modified Graphite (SMG) membranes showed an improved mixed gas separation factor α (H 2 /CO 2 ) of 9.2 (+80%) and α (H 2 /H 2 O) of 10.2 (+70%) compared with the pristine graphite membranes with α (H 2 /CO 2 ) of 5.1 and α (H 2 /H 2 O) of 6.0. The surface modification was obtained by the treatment with different silanes at 140 °C, for 2 h under reflux and continuous stirring in an argon atmosphere. The dried powders were pressed to discs and evaluated for the ternary feed mixture (41 vol.‐% H 2 /41 vol.‐% CO 2 /18 vol.‐% H 2 O). XPS analysis of the membrane surface revealed sp 2 and sp 3 hybridisation states, carbonyl, carboxyl groups and CF x bonds for the pristine and SMG membranes. Spatially resolved micro-focused Raman spectroscopy showed that the improved separation performance correlates with a specific modification of the G and D1 modes of the pristine graphite surface for each functionalisation carried out. Therefore, the improved performances result from the specific chemical and physical interactions of H 2 , CO 2 and H 2 O with the applied defect modifications of the SMG membranes.

The continuous improvement of the sublimation process of SiC allows using epitaxial graphene nowadays for quantum metrology. While it is known that the interface between graphene and the SiC surface is crucial for graphene's transport properties, almost no information about the composition of the SiC substrate after the sublimation process is available. In this study we present high resolution ${\mathrm{c}}_{\text{s}}$-corrected scanning transmission electron microscopy (STEM) experiments on $6H$-SiC(0001) samples after growth of graphene. A Si deficiency within the first three SiC bilayers was found by atomically resolved energy dispersive x-ray spectroscopy (EDX). The Si concentration within the first bilayer can be reduced up to 50%. In addition, as probed by electron energy loss spectroscopy, the hybridization state of C within the first five bilayers revealed a ${\mathrm{sp}}^{2}$ contribution, which we refer to as the precipitation of small carbon clusters. Our analysis clearly shows that the electronic interface of epitaxial graphene on $6H$-SiC(0001) is not atomically sharp.

The Si(553)-Au system resembles a heteroatomic chain ensemble with a delicate spin-charge interplay. The ordering of the $\ifmmode\times\else\texttimes\fi{}3$ reconstruction vanishes via a phase transition taking place at ${T}_{c}=100$ K. Our directional-dependent surface transport measurements showed that this order-disorder phase transition is not driven by the formation of a charge-density wave, as previously suggested. Instead, at 65 K there is a pronounced increase of the surface-state conductivity along the wires. We attribute this to activated charge transfer between the localized Si dangling bond states and the proximate Au bands revealing a $\ifmmode\times\else\texttimes\fi{}2$ periodicity. Apparently, a quasiorthogonality between the wave functions of the two proximal reconstructions is also responsible for a missing $\ifmmode\times\else\texttimes\fi{}6$ periodicity along the wires. The electronic charge transfer is in agreement with recent band-structure calculations.

Electronic properties of low dimensional structures on surfaces can be comprehensively explored by surface transport experiments. However, the surface sensitivity of this technique to atomic structures comes along with the control of bulk related electron paths and internal interfaces. Here we analyzed the role of Schottky-barriers and space charge layers for Si-surfaces. By means of a metal submonolayer coverage deposited on vicinal Si(1 1 1), we reliably accessed subsurface transport channels via angle- and temperature-dependent in situ transport measurements. In particular, high temperature treatments performed under ultra high vacuum conditions led to the formation of surface-near bulk defects, e.g. SiC-interstitials. Obviously, these defects act as p-type dopants and easily overcompensate lightly n-doped Si substrates.

One of the ways to use graphene in field effect transistors is to introduce a band gap by quantum confinement effect. That is why narrow graphene nanoribbons (GNRs) with width less than 50 nm are considered to be essential components in future graphene electronics. The growth of graphene on sidewalls of SiC(0001) mesa structures using scalable photolithography was shown to produce high quality GNRs with excellent transport properties. Such epitaxial graphene nanoribbons are very important in fundamental science but if GNRs are supposed to be used in advanced nanoelectronics, high quality thin (

We studied the adsorption of atomic hydrogen on monolayer graphene and quasi free monolayer graphene, epitaxially grown on SiC(0001). By means of in-situ surface transport measurements, a metal-insulator transition was found on both n- and p-type doped two dimensional electron systems. The detailed analysis of the temperature dependent resistivity revealed that even ultra-low concentrations ( n H ≈ 10 12 cm − 2 ) of locally chemisorbed H-clusters act as effective scattering centers for the propagating electrons and limit the mean-free path L 0 ∝ 1 / n H . Despite the weak disorder due to adsorption, strong localization was found. The activation energy for destroying the phase coherence within the system is around 30 meV. Our analysis rules out the formation of a band insulator or even a”bad metal” due to adsorption of hydrogen.

Hydrogen production by catalytic steam reforming of renewable hydrocarbons like bio-methane or bio-ethanol has become an attractive goal of sustainable chemistry. Side reactions as in ethanol steam reforming decrease the hydrogen selectivity. A low-temperature catalytic membrane reactor with a hydrogen-selective membrane is expected to solve this problem. Three different carbon membranes are investigated with respect to their performance to extract hydrogen selectively from the binary and ternary reaction mixtures (H2/CO2), (H2/CO2/H2O), and (H2/ethanol) as model systems for bio-ethanol steam reforming. The three carbon membranes under study are (i) an amorphous carbon layer prepared by physical vapour deposition (PVD) of carbon on an porous alumina support using a carbon fibre yarn, (ii) a turbostratic carbon layer obtained by pyrolysis of a supported organic polymer blend as precursor, and (iii) a crystalline carbon prepared by pressing of graphite flakes into a self-supporting disc. For the equimolar binary feed mixture (H2/CO2) all carbon membranes were found to be hydrogen selective. For the ternary feed mixture (41vol.-% H2/41vol.-% CO2/18vol.-% H2O), in the case of the amorphous and crystalline carbon membrane, the hydrogen selectivity remains also in the presence of steam. The turbostratic carbon membrane separates preferentially steam (H2O) from the ternary feed mixture (H2/CO2/H2O).

Surfaces of insulating single crystals (like Si at and below room temperature) provide perfect templates for ordered arrays of atomic wires, which in many cases exhibit one-dimensional (1D) properties. The physics in these quasi-one-dimensional systems is quite intriguing and gives rise to the observation of new phenomena such as spin and charge order. Here we present a short overview, concentrating on Pb on Si(557) close to monolayer coverage, which a priori is an anisotropic 2D system, as shown, e.g., by magnetoconductance or by plasmonic excitations. At certain critical conditions, however, it turns into a 1D system, as most obvious from dc conductance and angular resolved photoemission. Conditions and stability of its occurrence will be discussed.

Generally, it is supposed that the Fermi level in epitaxial graphene is controlled by two effects: p-type polarization doping induced by the bulk of the hexagonal silicon carbide (SiC)(0001) substrate and overcompensation by donor-like states related to the buffer layer. The presented work is evidence that this effect is also related to the specific underlying SiC terrace. Here a periodic sequence of non-identical SiC terraces is fabricated, which are unambiguously attributed to specific SiC surface terminations. A clear correlation between the SiC termination and the electronic graphene properties is experimentally observed and confirmed by various complementary surface-sensitive methods. This correlation is attributed to a proximity effect of the SiC termination-dependent polarization doping on the overlying graphene layer. These findings open a new approach for a nano-scale doping-engineering by the self-patterning of epitaxial graphene and other 2D layers on dielectric polar substrates. (Less)

The multiprobe technique based on scanning tunneling microcopy (STM) has become very popular over the past few years and has demonstrated its capability to reveal fundamental nanoscale charge transport properties . Compared to conventional, i.e. lithographically-driven concepts, the four-tip STM technique can directly contact delicate nanostructures with upmost precision.

We present here a combined experimental Raman spectroscopy and ab initio theoretical study of the vibrational modes of the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})$ reconstructed SIC phase of Pb on Si(111) and discuss their relation to the atomic surface structure. The Raman response of the surface localized vibrational modes, in particular, is identified in the low-frequency spectral range (down to $15\phantom{\rule{3.33333pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1})$. We demonstrate that Raman spectroscopy is a very powerful approach to test atomic structures of surfaces and a valuable complement to standard surface analytics. While the calculated spectra of H3 and T4 are too similar to allow a discrimination of these phases, the good overall agreement to the measured Raman spectra enables a classification of the observed vibrational modes.

The formation and structure of Ge2Pt clusters was studied in order to understand their germanene termination layer. The Ge2Pt clusters are formed by depositing a few layers of Pt on a Ge(110) surface. Annealing at temperatures above 1043 K results in eutectic Ge-Pt droplets that etch grooves on the surface in the [1 1 ¯0] direction. Upon cooling down, they solidify and decompose into a Ge2Pt phase and a pure Ge phase. Electron diffraction reveals that the hut-shaped clusters have their (001) plane oriented parallel to the Ge(110) surface and their (100) plane facing in the Ge[1 1 ¯0] direction. The facets of the Ge2Pt hut clusters have been determined to be the {101} and {011} planes. The germanene layers which cover these facets are commensurate with the {101} and {011} facets of the Ge2Pt substrate.

We report on electronic transport measurements in rotational square probe configuration in combination with scanning tunneling potentiometry of epitaxial graphene monolayers which were fabricated by polymer-assisted sublimation growth on SiC substrates. The absence of bilayer graphene on the ultralow step edges of below 0.75 nm scrutinized by atomic force microscopy and scanning tunneling microscopy result in a not yet observed resistance isotropy of graphene on 4H- and 6H-SiC(0001) substrates as low as 2%. We combine microscopic electronic properties with nanoscale transport experiments and thereby disentangle the underlying microscopic scattering mechanism to explain the remaining resistance anisotropy. Eventually, this can be entirely attributed to the resistance and the number of substrate steps which induce local scattering. Thereby, our data represent the ultimate limit for resistance isotropy of epitaxial graphene on SiC for the given miscut of the substrate.

Epitaxial Bi(111) films were subject to many and partly even controversial studies on the semimetal-semiconductor transition triggered by a robust quantum confinement. The residual conductance was ascribed to conducting surface channels. We investigated ultrathin crystalline Bi films on Si(111) as a function of film thickness $d$ between 20 and 100 bilayers by means of electric transport measurements. Varying temperature and magnetic field, we disentangled two transport channels. One remains indeed metallic at all thicknesses investigated and exhibits a slightly increasing conductance as a function of $d$, whereas the second is activated with a ${d}^{\ensuremath{-}1}$ dependence of the activation energy, indicating a quasiharmonic confining potential. Both channels reflect the electronic properties of the entire film and do not allow us to strictly separate surface and bulk states. While there is clearly no bulk conductivity, the activated channel is consistently described as electronic excitation into the partly occupied quantum well states, which are also responsible for the metallic conductance and preferentially located close to both interfaces of the film.

High quality graphene nanoribbons epitaxially grown on the sidewalls of silicon carbide (SiC) mesa structures stand as key building blocks for graphene-based nanoelectronics. Such ribbons display 1D single-channel ballistic transport at room temperature with exceptionally long mean free paths. Here, using spatially-resolved two-point probe (2PP) measurements, we selectively access and directly image a range of individual transport modes in sidewall ribbons. The signature of the independently contacted channels is a sequence of quantised conductance plateaus for different probe positions. These result from an interplay between edge magnetism and asymmetric terminations at opposite ribbon edges due to the underlying SiC structure morphology. Our findings demonstrate a precise control of transport through multiple, independent, ballistic tracks in graphene-based devices, opening intriguing pathways for quantum information device concepts., Financial support by the Deutsche Forschungsgemeinschaft (Te386/12-1 and Te 386/13-1 (FlagEra Tailspin project)) is gratefully acknowledged by J.A. and C.T. P.B. and H.J.W.Z. thank the Stichting voor Fundamenteel Onderzoek der Materie (FOM, FV157 14TWDO07) for financial support. We acknowledge N. Vinogradov and Thi Thuy Nhung Nguyen for STM experiments and J. Schommartz for technical support. S.R.P. acknowledges funding from the European Unions Horizon 2020 research and innovation programme under the Marie Skodowska-Curie grant agreement No 665919 and from the Irish Research Council under the laureate awards programme. S.R. acknowledges funding from the Spanish Ministry of Economy and Competitiveness and the European Regional Development Fund (project no. FIS2015-67767-P MINECO/FEDER, FIS2015-64886-C5-3-P) and the European Union Seventh Framework Programme under grant agreement no. 785219 (Graphene Flagship). ICN2 is funded by the CERCA Programme/Generalitat de Catalunya and supported by the Severo Ochoa programme (MINECO, Grant. No. SEV-2013-0295). Research at DTU is supported by the Danish National Research Foundation, Project No. DNRF103. A.Z. acknowledges the Swedish Research Council (Vetenskapsrådet) for the Tailspin project support.

Monolayer structures of Pb on Si(111) attracted recently considerable interest as superconductivity was found in these truly two-dimensional (2D) structures. In this study, we analyzed the electronic surface band structure of the so-called striped incommensurate Pb phase with $\frac{4}{3}$ ML coverage by means of spin-resolved photoemission spectroscopy. Our results fully agree with density functional theory calculations done by Ren et al. [Phys. Rev. B 94, 075436 (2016)]. We observe a local Zeeman-type splitting of a fully occupied and spin-polarized surface band at the ${\overline{\text{K}}}_{\sqrt{3}}$ points. The growth of this densely packed Pb structure results in the formation of imbalanced rotational domains, which triggered the detection of ${C}_{3v}$ symmetry forbidden spin components for surface states around the Fermi energy. Moreover, the Fermi surface of the metallic surface state of this phase is Rashba spin split and revealed a pronounced warping. However, the 2D nesting vectors are incommensurate with the atomic structure, thus keeping this system rather immune against charge density wave formation and possibly enabling a superconducting behavior.

Adsorption of Au on vicinal Si(111) surfaces results in growth of long-range ordered metallic quantum wires. In this paper, we utilized site-specific and selective adsorption of oxygen to modify chemically the transport via different channels in the systems Si(553)-Au and Si(557)-Au. They were analyzed by electron diffraction and four-tip STM-based transport experiments. Modeling of the adsorption process by density functional theory shows that the adatoms and rest atoms on Si(557)-Au provide energetically favored adsorption sites, which predominantly alter the transport along the wire direction. Since this structural motif is missing on Si(553)-Au, the transport channels remain almost unaffected by oxidation. © 2017 American Physical Society. DFG/FOR/1700

The low-energy plasmonic excitations of the Ge(0 0 1)-Au close to one monolayer coverage of Au were investigated by momentum-resolved high resolution electron energy loss spectroscopy. A very weak plasmonic loss was identified dispersing along the chain direction of the [Formula: see text] formed at these Au coverages. The measured dispersion was compared with the Tomonaga-Luttinger-liquid (TLL) model and with a model for an anisotropic Fermi liquid. Using the TLL model both for single and arrays of wires, no consistent picture turned up that could describe all available data. On the contrary, a quasi-one-dimensional model of a confined 2D electron gas gave a satisfactorily consistent description of the data. From these results for the collective low-energy excitations we conclude that the Ge(0 0 1)-Au system is reasonably well described by a strongly anisotropic 2D Fermi liquid, but is incompatible with a TLL.

We present a new fabrication method for epitaxial graphene on SiC which enables the growth of ultra-smooth defect- and bilayer-free graphene sheets with an unprecedented reproducibility, a necessary prerequisite for wafer-scale fabrication of high quality graphene-based electronic devices. The inherent but unfavorable formation of high SiC surface terrace steps during high temperature sublimation growth is suppressed by rapid formation of the graphene buffer layer which stabilizes the SiC surface. The enhanced nucleation is enforced by decomposition of polymer adsorbates which act as a carbon source. With most of the steps well below 0.75 nm pure monolayer graphene without bilayer inclusions is formed with lateral dimensions only limited by the size of the substrate. This makes the polymer assisted sublimation growth technique the most promising method for commercial wafer scale epitaxial graphene fabrication. The extraordinary electronic quality is evidenced by quantum resistance metrology at 4.2 K with until now unreached precision and high electron mobilities on mm scale devices., 20 pages, 6 Figures

The charge transport through ferrocene 1,1'-diamine (FDA) molecules between gold electrodes is investigated using the mechanically controllable break junction technique combined with a theoretical framework of density functional theory simulations to understand the physics of these molecular junctions. The characteristic conductances of the molecule are measured at low bias as well as current-voltage (IV) characteristics. By fitting the IV characteristics to the single-level model, the values for the position of the molecular level, mainly responsible for the transport, and its coupling to the leads, are obtained. The influence of the binding sites, molecular conformation, and electrode distance are systematically studied from a theoretical perspective. While a strong dependence of conductance on the adsorption geometry is found, the decrease of conductance as a function of electrode distance arises mainly from a decrease of coupling strength of the molecular electronic orbitals through a reduced overlap and, to a lesser extent, from a shift of their alignment with respect to the Fermi energy. published

Ultrathin metallic silicide nanowires with extremely high aspect ratios can be easily grown, e.g., by deposition of rare earth elements on semiconducting surfaces. These wires play a pivotal role in fundamental research and open intriguing perspectives for CMOS applications. However, the electronic properties of these one-dimensional systems are extremely sensitive to atomic-sized defects, which easily alter the transport characteristics. In this study, we characterized comprehensively TbSi2 wires grown on Si(100) and correlated details of the atomic structure with their electrical resistivities. Scanning tunneling microscopy (STM) as well as all transport experiments were performed in situ using a four-tip STM system. The measurements are complemented by local spectroscopy and density functional theory revealing that the silicide wires are electronically decoupled from the Si template. On the basis of a quasiclassical transport model, the size effect found for the resistivity is quantitatively explained in terms of bulk and surface transport channels considering details of atomic-scale roughness. Regarding future applications the full wealth of these robust nanostructures will emerge only if wires with truly atomically sharp interfaces can be reliably grown. © 2016 American Physical Society. DFG/FOR/1700

We have realized nanometer size constrictions in ballistic graphene nanoribbons grown on sidewalls of SiC mesa structures. The high quality of our devices allows the observation of a number of electronic quantum interference phenomena. The transmissions of Fabry-Perot like resonances were probed by in-situ transport measurements at various temperatures. The energies of the resonances are determined by the size of the constrictions which can be controlled precisely using STM lithography. The temperature and size dependence of the measured conductances are in quantitative agreement with tight-binding calculations. The fact that these interference effects are visible even at room temperature makes the reported devices attractive as building blocks for future carbon based electronics., Comment: to appear in Phys. Rev. Lett

The collective electronic excitations of arrays of Au chains on regularly stepped Si(553) and Si(775) surfaces were studied using electron loss spectroscopy with simultaneous high energy and momentum resolution (ELS-LEED) in combination with low energy electron diffraction (SPA-LEED) and tunneling microscopy. Both surfaces contain a double chain of gold atoms per terrace. Although one-dimensional metallicity and plasmon dispersion is observed only along the wires, two-dimensional effects are important, since plasmon dispersion explicitly depends both on the structural motif of the wires and the terrace width. The electron density on each terrace turns out to be modulated, as seen by tunneling spectroscopy (STS). The effective wire width of 7.5\,\AA\ for Si(553)-Au -- 10.2\,\AA\ for Si(775)-Au -- , determined from plasmon dispersion is in good agreement with STS data. Clear evidence for coupling between wires is seen beyond nearest neighbor coupling., Comment: 5 pages, 4 figures

Epitaxially grown sidewall graphene nanoribbons show a robust quantum conductance of e2/h. By means of in-situ transport measurements with a nanoprobe system, we realized invasive and non-invasive 4-point-probe configurations. The invasiveness correlates with the contact resistance of the voltage probes. In particular, we achieved now non-invasive voltage probes revealing an almost zero resistance in a collinear 4 point-probe measurement. This proofs the ballistic nature of our epitaxially grown sidewall nanoribbons on SiC(0001) mesa structures.

By characterizing and manipulating the conductive properties of single Ag nano-grain boundaries with electromigration—originally considered only as a detrimental effect for metallic nanostructures—we show that atomic point contacts can be generated at well-defined locations with extreme reliability in ultra-thin (5 nm) and ultra-small (minimum width 16 nm) Ag nanostructures, deposited on hydrogen terminated low-doped Si(100) samples. Single contacts were always obtained once the smallest constriction of the structures was below the average grain size of the Ag films and competing thermal migration was suppressed. These ultra-thin and laterally open structures on Si provide complete accessibility for local characterisation of the molecular junction.

Electromigration (EM) as a structuring tool was investigated in Ag nanowires (width 300 nm, thickness 25 nm) and partly in notched and bow-tie Ag structures on a Si(1 0 0) substrate in ultra-high vacuum using a four-tip scanning tunneling microscope in combination with a scanning electron microscope. From simulations of Ag nanowires we got estimates of temperature profiles, current density profiles, EM and thermal migration (TM) mass flux distributions within the nanowire induced by critical current densities of 108 A cm-2. At room temperature, the electron wind force at these current densities by far dominates over thermal diffusion, and is responsible for formation of voids at the cathode and hillocks at the anode side. For current densities that exceed the critical current densities necessary for EM, a new type of wire-like structure formation was found both at room temperature and at 100 K for notched and bow-tie structures. This suggests that the simultaneous action of EM and TM is structure forming, but with a very small influence of TM at low temperature.

Quantum wires with spin-orbit coupling provide a unique opportunity to simultaneously control the coupling strength and the screened Coulomb interactions where new exotic phases of matter can be explored. Here we report on the observation of an exotic spin-orbit density wave in Pb-atomic wires on Si(557) surfaces by mapping out the evolution of the modulated spin-texture at various conditions with spin-and angle-resolved photoelectron spectroscopy. The results are independently quantified by surface transport measurements. The spin polarization, coherence length, spin dephasing rate and the associated quasiparticle gap decrease simultaneously as the screened Coulomb interaction decreases with increasing excess coverage, providing a new mechanism for generating and manipulating a spin-orbit entanglement effect via electronic interaction. Despite clear evidence of spontaneous spin-rotation symmetry breaking and modulation of spin-momentum structure as a function of excess coverage, the average spin polarization over the Brillouin zone vanishes, indicating that time-reversal symmetry is intact as theoretically predicted. DFG/FOR/1700 Swiss National Science Foundation

In addition to the chemical and physical properties of nanostructures their successful utilization for applications is strongly triggered by economic aspects. Electrospinning of nanowires from solution followed by subsequent annealing steps is a comparably cheap technique to fabricate conductive carbon nanofibers (CNF) made from polyacrylonitrile (PAN) molecules in large quantities. In this work, we investigated the microscopic properties of the CNFs with diameters of 100-300 nm by means of Raman and x-ray photoelectron spectroscopy and correlated these results with transport measurements done with a 4-tip STM. In particular, we investigated the effect of fiber alignment and knot densities, which can be controlled by applying constant creep due to stress during the stabilization process. The comparison of the conductivity obtained from single CNFs revealed further that the fiber crossings within the ensemble structure act as scattering centers and proofs that the transport is along the surfaces of the CNFs.

Structured Silicon Carbide was proposed to be an ideal template for the production of arrays of edge specific graphene nanoribbons (GNRs), which could be used as a base material for graphene transistors. We prepared periodic arrays of nanoscaled stripe-mesas on SiC surfaces using electron beam lithography and reactive ion etching. Subsequent epitaxial graphene growth by annealing is differentiated between the basal-plane mesas and the faceting stripe walls as monitored by means of atomic force microscopy (AFM). Microscopic low energy electron diffraction (μ-LEED) revealed that the graphene ribbons on the facetted mesa side walls grow in epitaxial relation to the basal-plane graphene with an armchair orientation at the facet edges. The ¼- band system of the ribbons exhibits linear bands with a Dirac like shape corresponding to monolayer graphene as identified by angle-resolved photoemission spectroscopy (ARPES).