Resolution Scanning Tunneling Microscopy Research Articles

Surface-Confined Dynamic Covalent System Driven by Olefin Metathesis.

Understanding how the constitutional dynamics of a dynamic combinatorial library (DCL) adapts to surfaces (compared to bulk solution) is of fundamental importance to the design of adaptive materials. Submolecular resolved scanning tunneling microscopy (STM) can provide detailed insights into olefin metathesis at the interface. Analysis of the distribution of products has revealed the important role of environmental pressure, reaction temperature, and substituent effects in surface-confined olefin metathesis. We also report an unprecedented preferred deposition and assembly of linear polymers, and some specific oligomers, on the surface that are hard to obtain otherwise.

Light-Induced Contraction/Expansion of 1D Photoswitchable Metallopolymer Monitored at the Solid-Liquid Interface.

The use of a bottom-up approach to the fabrication of nanopatterned functional surfaces, which are capable to respond to external stimuli, is of great current interest. Herein, the preparation of light-responsive, linear supramolecular metallopolymers constituted by the ideally infinite repetition of a ditopic ligand bearing an azoaryl moiety and Co(II) coordination nodes is described. The supramolecular polymerization process is followed by optical spectroscopy in dimethylformamide solution. Noteworthy, a submolecularly resolved scanning tunneling microscopy (STM) study of the in situ reversible trans-to-cis photoisomerization of a photoswitchable metallopolymer that self-assembles into 2D crystalline patterns onto a highly oriented pyrolytic graphite surface is achieved for the first time. The STM analysis of the nanopatterned surfaces is corroborated by modeling the physisorbed species onto a graphene slab before and after irradiation by means of density functional theory calculation. Significantly, switching of the monolayers consisting of supramolecular Co(II) metallopolymer bearing trans-azoaryl units to a novel pattern based on cis isomers can be triggered by UV light and reversed back to the trans conformer by using visible light, thereby restoring the trans-based supramolecular 2D packing. These findings represent a step forward toward the design and preparation of photoresponsive "smart" surfaces organized with an atomic precision.

Electronic Properties of a 1D Intrinsic/p-Doped Heterojunction in a 2D Transition Metal Dichalcogenide Semiconductor.

Two-dimensional (2D) semiconductors offer a convenient platform to study 2D physics, for example, to understand doping in an atomically thin semiconductor. Here, we demonstrate the fabrication and unravel the electronic properties of a lateral doped/intrinsic heterojunction in a single-layer (SL) tungsten diselenide (WSe2), a prototype semiconducting transition metal dichalcogenide (TMD), partially covered with a molecular acceptor layer, on a graphite substrate. With combined experiments and theoretical modeling, we reveal the fundamental acceptor-induced p-doping mechanism for SL-WSe2. At the 1D border between the doped and undoped SL-WSe2 regions, we observe band bending and explain it by Thomas-Fermi screening. Using atomically resolved scanning tunneling microscopy and spectroscopy, the screening length is determined to be in the few nanometer range, and we assess the carrier density of intrinsic SL-WSe2. These findings are of fundamental and technological importance for understanding and employing surface doping, for example, in designing lateral organic TMD heterostructures for future devices.

Crystal Structure Induced Preferential Surface Alloying of Sb on Wurtzite/Zinc Blende GaAs Nanowires.

We study the surface diffusion and alloying of Sb into GaAs nanowires (NWs) with controlled axial stacking of wurtzite (Wz) and zinc blende (Zb) crystal phases. Using atomically resolved scanning tunneling microscopy, we find that Sb preferentially incorporates into the surface layer of the {110}-terminated Zb segments rather than the {112̅0}-terminated Wz segments. Density functional theory calculations verify the higher surface incorporation rate into the Zb phase and find that it is related to differences in the energy barrier of the Sb-for-As exchange reaction on the two surfaces. These findings demonstrate a simple processing-free route to compositional engineering at the monolayer level along NWs.

Electrochemical Kinetics: a Surface Science-Supported Picture of Hydrogen Electrochemistry on Ru(0001) and Pt/Ru(0001)

In this short review, we compare the kinetics of hydrogen desorption in vacuum to those involved in the electrochemical hydrogen evolution/oxidation reactions (HER/HOR) at two types of atomically smooth model surfaces: bare Ru(0001) and the same surface covered by a 1.1 atomic layer thick Pt film. Low/high H2 (D2) desorption rates at room temperature in vacuum quantitatively correspond to low/high exchange current densities for the HOR/HER in electrochemistry. In view of the “volcano plot” concept, these represent two surfaces that adsorb hydrogen atoms, Had, too strongly and too weakly, respectively. Atomically smooth, vacuum annealed model surfaces are the closest approximation to the idealized slab geometries typically studied by density functional theory (DFT). A predictive volcano plot based on DFT-based adsorption energies for the Had intermediates agrees well with the experiments if two things are considered: (i) the steady-state coverage of Had intermediates and (ii) local variations in film thickness. The sluggish HER/HOR kinetics of Ru(0001) allows for excellent visibility of cyclic voltammetry (CV) features even in H2-saturated solution. The CV switches between a Had- and a OHad-/Oad-dominated regime, but the presence of H2 in the electrolyte increases the Had-dominated potential window by a factor of two. Whereas in plain electrolyte two electrochemical adsorption processes compete in forming adlayers, it is one electrochemical and one chemical one in the case of H2-saturated electrolyte. We demonstrate and quantitatively explain that dissociative H2 adsorption is more important than H+ discharge for Had formation in the low potential regime on Ru(0001).Graphical Left: Cyclic voltammograms of Ru(0001) in 0.1 M HClO4, with and without H2 present in solution. Left inset: atomic resolution scanning tunnelling microscope (STM) images of Ru(0001). Centre: volcano plot showing the theoretically predicted hydrogen evolution/oxidation (HER/HOR) current densities on Ru(0001), with variable Had coverage in the adlayer. These values are compared with the exchange current densities for the HER/HOR on Pt(111) and pseudomorphic overlayers of Pt on Ru(0001), where the Pt overlayer thickness is indicated. Temperature programmed desorption (TPD) spectra of D2 on Ru(0001) (upper left) and Pt/Ru(0001) (upper right) are shown along with the respective adsorbate coverage obtained at 300 K. Right: STM image of Pt/Ru(0001). The line indicated how the Pt overlayer thickness varies across the surface, as illustrated at the bottom.

Correlating electronic and magnetic coupling in large magnetic molecules via scanning tunneling microscopy

In an effort to improve the spin coupling in single-molecule magnets, we rationally designed a new building-block molecule with significantly enhanced spin coupling compared to a previously established molecule. We relate this to a stabilization of aromaticity in the central connecting carbon ring, promoting the spin-polarization mechanism. This correlation between magnetic and electronic properties is supported by bulk measurements as well as submolecularly resolved scanning tunneling microscopy and spectroscopy experiments, where we found distinct differences in the local density of states distribution of the two molecules, especially at the central carbon ring. While the established molecule exhibits localized, spatially decoupled and even switchable states, the improved building block exhibits symmetric local density of states delocalized over the entire molecule, also revealing that this main characteristic electronic property is preserved upon adsorption on a metal surface. Due to their planar geometry, these molecules can serve as model systems for scanning-probe based studies of molecular magnetism.

Thickness-Dependent Adsorption of Melamine on Cu/Au(111) Films

Supported metal films represent a model binary metallic system wherein the surface properties can be finely tuned with the composition/thickness of the film. Thus, it has raised increasing research interests from various aspects. In this work, the adsorption and assembly behavior of melamine on the Cu films grown on the (22 × √3) reconstructed Au(111) substrate were investigated as a function of Cu thickness. The atomically resolved scanning tunneling microscopy (STM) images in combination with density functional theory (DFT) calculations reveal that the ultrathin submonolayer copper film forms a pseudomorphic (1 × 1) lattice by agglomerating at the subsurface, while the true Cu adlayers start to appear on the three-layer thick films and hold the bulk Cu(111) lattice. Correspondingly, the adsorption and assembly of the melamine molecules significantly differ from those on pure metals, experiencing a gradual transformation from physisorption to chemisorption assemblies with the Cu film thickening. These fi...

Imaging structural transitions in organometallic molecules on Ag(100) for solar thermal energy storage

The use of opto-thermal molecular energy storage at the nanoscale creates new opportunities for powering future microdevices with flexible synthetic tailorability. Practical application of these molecular materials, however, requires a deeper microscopic understanding of how their behavior is altered by the presence of different types of substrates. Here we present single-molecule resolved scanning tunneling microscopy imaging of thermally- and optically-induced structural transitions in (fulvalene)tetracarbonyldiruthenium molecules adsorbed onto a Ag(100) surface as a prototype system. Both the parent complex and the photoisomer display distinct thermally-driven phase transformations when they are in contact with a Ag(100) surface. This behavior is consistent with the loss of carbonyl ligands due to strong molecule-surface coupling. Ultraviolet radiation induces marked structural changes only in the intact parent complex, thus indicating a photoisomerization reaction. These results demonstrate how stimuli-induced structural transitions in this class of molecule depend on the nature of the underlying substrate.

Fabrication of graphene–silicon layered heterostructures by carbon penetration of silicon film

A new, easy, in situ technique for fabricating a two-dimensional graphene–silicon layered heterostructure has been developed to meet the demand for integration between graphene and silicon-based microelectronic technology. First, carbon atoms are stored in bulk iridium, and then silicon atoms are deposited onto the Ir(111) surface and annealed. With longer annealing times, the carbon atoms penetrate from the bulk iridium to the top of the silicon and eventually coalesce there into graphene islands. Atomically resolved scanning tunneling microscopy images, high-pass fast Fourier transform treatment and Raman spectroscopy demonstrate that the top graphene layer is intact and continuous, and beneath it is the silicon layer.

Role of the Pinning Points in epitaxial Graphene Moiré Superstructures on the Pt(111) Surface.

The intrinsic atomic mechanisms responsible for electronic doping of epitaxial graphene Moirés on transition metal surfaces is still an open issue. To better understand this process we have carried out a first-principles full characterization of the most representative Moiré superstructures observed on the Gr/Pt(111) system and confronted the results with atomically resolved scanning tunneling microscopy experiments. We find that for all reported Moirés the system relaxes inducing a non-negligible atomic corrugation both, at the graphene and at the outermost platinum layer. Interestingly, a mirror “anti-Moiré” reconstruction appears at the substrate, giving rise to the appearance of pinning-points. We show that these points are responsible for the development of the superstructure, while charge from the Pt substrate is injected into the graphene, inducing a local n-doping, mostly localized at these specific pinning-point positions.

Interaction of a Self-Assembled Ionic Liquid Layer with Graphite(0001): A Combined Experimental and Theoretical Study.

The interaction between (sub)monolayers of the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide [BMP](+)[TFSA](-) and graphite(0001), which serves as a model for the anode|electolyte interface in Li-ion batteries, was investigated under ultrahigh vacuum conditions in a combined experimental and theoretical approach. High-resolution scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and dispersion-corrected density functional theory (DFT-D) calculations were employed. After vapor deposition at 300 K, XPS indicates molecular adsorbates with a 1:1 ratio of cations/anions. Cool down to ∼100 K leads to the formation of an ordered (2D) crystalline phase, which coexists with a mobile (2D) liquid. DFT-D calculations reveal that adsorbed [BMP](+) and [TFSA](-) species are arranged alternately in a row-like adsorption structure (cation-anion-cation-anion) and that adsorption is dominated by dispersion interactions between adlayer and substrate, on the one hand, and electrostatic interactions between the ions in a row, on the other hand. Simulated STM images of that structure closely resemble the experimental molecular resolved STM images and show that the resolved features mostly stem from the cations.

Charge sensing of a few-donor double quantum dot in silicon

We demonstrate the charge sensing of a few-donor double quantum dot precision placed with atomic resolution scanning tunnelling microscope lithography. We show that a tunnel-coupled single electron transistor (SET) can be used to detect electron transitions on both dots as well as inter-dot transitions. We demonstrate that we can control the tunnel times of the second dot to the SET island by ∼4 orders of magnitude by detuning its energy with respect to the first dot.

Mn doped InSb studied at the atomic scale by cross-sectional scanning tunneling microscopy

We present an atomically resolved study of metal-organic vapor epitaxy grown Mn doped InSb. Both topographic and spectroscopic measurements have been performed by cross-sectional scanning tunneling microscopy (STM). The measurements on the Mn doped InSb samples show a perfect crystal structure without any precipitates and reveal that Mn acts as a shallow acceptor. The Mn concentration of the order of ∼1020 cm−3 obtained from the cross-sectional STM data compare well with the intended doping concentration. While the pair correlation function of the Mn atoms showed that their local distribution is uncorrelated beyond the STM resolution for observing individual dopants, disorder in the Mn ion location giving rise to percolation pathways is clearly noted. The amount of clustering that we see is thus as expected for a fully randomly disordered distribution of the Mn atoms and no enhanced clustering or second phase material was observed.

Atomic resolution scanning tunneling microscope imaging up to 27 T in a water-cooled magnet

We report the first atomically resolved scanning tunneling microscope (STM) imaging in a water-cooled magnet (WM), for which extremely harsh vibrations and noise have been the major challenge. This custom WM-STM features an ultra-rigid and compact scan head in which the coarse approach is driven by our newly designed TunaDrive piezoelectric motor. A three-level spring hanging system is used for vibration isolation. Room-temperature raw-data images of graphite with quality atomic resolution were acquired in the presence of very strong magnetic fields, with a field strength up to 27 T, in a 32-mm-diameter bore WM with a maximum field strength of 27.5 T at a power rating of 10 MW, calibrated by nuclear magnetic resonance (NMR). This record field strength of 27 T exceeds the maximal field strength achieved by the conventional superconducting magnets. Besides, our WM-STM has paved the way to STM imaging using a 45 T, 32-mm-diameter bore hybrid magnet, which is the world’s flagship magnet, producing the strongest steady magnetic field.

Spatial resolution of scanning tunneling microscopy

Spatial resolution of scanning tunneling microscopy

Investigation of √7 × √3 structures grown on In/Si(111) surfaces at room temperature

In/Si(111) superstructures formed by the deposition of indium on a √3 × √3-In surface at room temperature were investigated by using a scanning tunneling microscope (STM). The 2×2, ‘striped’, hexagonal √7 × √3 (√7 × √3-hex), and rectangular √7 × √3 (√7 × √3-rec) structures were identified. We demonstrated that the ‘striped’ and the √7 × √3-hex structures were falsely identified as √7 × √3-hex and √7 × √3-rec in a previous report [A. A. Saranin et al., Phys. Rev. B 74, 035436 (2006)]. As in the √7 × √3-hex and √7 × √3-rec structures, a √7 × √3 periodicity was observed in the resolved STM features of the ‘striped’ structure. These three √7 × √3 structures formed at room temperature were shown to be identical to the corresponding In-induced phases formed at high temperature. The apparent height difference between the ‘striped’ and the √7 × √3-hex structures in the topographic STM image suggests that the √7 × √3-hex structure consists of double l√ayers of In. This possibility contrasts with recent theoretical predictions of single-layer In for the √7 × √3-hex structure.

Electronic structure of reconstructed Au(111) studied with density functional theory

The interplay between the electronic structure and reconstruction of the geometry on a surface is an intriguing and exciting investigation. One classic example that has been one of the first systems to be identified using the angularly resolved photo-emission spectroscopy and scanning tunnelling microscopy is the Herringbone reconstruction on the Au(111) surface. Here, we report on the results derived from electronic structure calculations employing the density functional theory to investigate both the atomistic geometry and the electronic states near the Fermi energy, in particular the Shockley surface state. We find that despite the reconstruction of the electronic structure at the surface, it is actually relatively little modified from its unreconstructed counterpart. We further discuss the consequences in systems of weakly adsorbed species on this surface.

Visualizing Majorana fermions in a chain of magnetic atoms on a superconductor

A chain of magnetic atoms on the surface of a superconductor provides a versatile platform for realizing a one-dimensional topological superconductivity phase with edge-bounded Majorana fermions zero modes. This platform lends itself to spatial resolved measurements with scanning tunneling microscope (STM) that enables direct visualization of the presence of a localized Majorana zero mode. Experiments on self-assembled chains of Fe atoms on the surface of Pb show that such a system can be experimentally fabricated and studied using various high-resolution STM measurement techniques. Spatial and energy resolved STM experiments provide strong evidence for Majorana bound states that emerge due to the combination of Fe’s ferromagnetism and spin–orbit coupling of the superconducting Pb substrate. These studies provide a roadmap for optimizing topological superconductivity in this one-dimensional platform and its extension to realize chiral two-dimensional superconductors.

Graphene–Silicon Heterostructures at the Two-Dimensional Limit

The integration of heterogeneous two-dimensional materials has the potential to yield electronic behavior approaching theoretical limits and facilitate the exploration of new fundamental physical phenomena. Here, we report the integration of graphene with two-dimensional, semiconducting crystalline silicon. Sequential deposition of carbon and silicon on Ag(111) in ultrahigh vacuum results in the synthesis of both lateral and vertical graphene–silicon heterostructures. The one-dimensional in-plane interfaces demonstrate atomically precise material transitions both structurally and electronically. The vertical heterostructures show noninteracting van der Waals behavior as shown by energetically resolved scanning tunneling microscopy coupled with ex-situ Raman analysis. The pristine and direct integration of graphene with two-dimensional, semiconducting crystalline silicon couples two of the most studied electronic materials into a hybrid structure with high potential for next-generation nanoelectronics.

Magnification of signatures of a topological phase transition by quantum zero point motion

In this letter we show that the zero-point motion of the vortex in superconducting doped topological insulators leads to significant changes in the electronic spectrum at the topological phase transition in this system. This topological phase transition is tuned by the doping level and the corresponding effects manifest in the density of states at energies which are of the order of the fluctuations frequency. This frequency might be much larger than the electronic energy gap in the spectrum generated by a stationary vortex. As a result the quantum zero-point motion can move the spectral signature of the topological vortex phase transition to energies which are well within the resolution of scanning tunneling microscopy. Moreover, the phenomena studied in this letter present novel effects of Magnus force on the vortex spectrum which are not present in the ordinary s-wave superconductors. Our results show that quantum zero point fluctuations can bring the fingerprints of topological phase transitions to more experimentally accessible grounds and point to the importance of study of quantum fluctuations in different candidates for realizing topological phase transitions.