Chiral self-organized single 2D-layers of tetramers from a functional donor-acceptor molecule by the surface template effect.

The ability to control the structural properties of molecular layers is a key for the design and preparation of organic electronic devices. While microscopic growth studies of planar, rigid and symmetric π-conjugated molecules have been performed to a larger extent, this is less the case for elongated donor-acceptor molecules with flexible functional groups, which are particularly interesting due to their high dipole moments. Prototypical molecules of this type are merocyanines (MCs), which have been widely studied for the use as efficient absorbers in organic photodetectors. For maximized light absorption and optimized electronic properties the molecular arrangement which is affected by the initial assembly of the films at the supporting substrate interface is decisive. The situation deserves special attention, when the surface nucleation leads to so far not known and bulk-unlike aggregates. Here, we report on the growth of a typical MC (HB238) on the Ag(100) surface, serving as the substrate. In the energetically preferred phase, the molecules adsorb in a face-on geometry and organize in tetramers with a circular dipole arrangement. The tetramers further self-order in large, enantiopure domains with a periodicity that is commensurate to the Ag(100) surface, likely due to a specific bonding of the thiophene and thiazol rings to the Ag surface. Using scanning tunneling microscopy (STM) in combination with low energy electron diffraction we derive the detailed structure of the tetramers. The center of the tetramer, which is most prominent in STM images, consists of four upward pointing tert-butyl groups from four molecules. It is encircled by a ring of four hydrogen bonds between terminal CN-groups and thiophene rings on neighboring molecules. In parallel, the surface interaction modifies the intramolecular dipole, which is revealed from photoemission spectroscopy. Hence, this example shows how the surface template effect leads to an unforeseen molecular organization which is considerably more complex compared to that in the bulk phases of HB238, which feature paired dipoles.

Probing the Synergistic Effects of Mg2+ on CO2 Reduction Reaction on CoPc by In Situ Electrochemical Scanning Tunneling Microscopy.

We report herein the in situ electrochemical scanning tunneling microscopy (ECSTM) study on the synergistic effect of Mg2+ in CO2 reduction reaction (CO2RR) catalyzed by cobalt phthalocyanine (CoPc). ECSTM measurement molecularly resolves the self-assembled CoPc monolayer on the Au(111) substrate. In the CO2 environment, high-contrast species are observed in the adlayer and assigned to the CO2 adsorption on CoPc. Furthermore, the contrast of the CO2-bound complex is higher in Mg2+-containing electrolytes than in Mg2+-free electrolytes, indicating the formation of the CoPc-CO2-Mg2+ complex. The surface coverage of adsorbed CO2 is positively correlated with the Mg2+ concentration as the additive in electrolytes up to a plateau of 30.8 ± 2.7% when c(Mg2+) > 30 mM. The potential step experiment indicates the higher CO2 adsorption dynamics in Mg2+-containing electrolytes than without Mg2+. The rate constants of CO2 adsorption and dissociation in different electrolytes are extracted from the data fitting of statistical results from in situ ECSTM experiments.

Mixed-Sandwich Titanium(III) Qubits on Au(111): Electron Delocalization Ruled by Molecular Packing.

Organometallic sandwich complexes are versatile molecular systems that have been recently employed for single-molecule manipulation and spin sensing experiments. Among related organometallic compounds, the mixed-sandwich S = 1/2 complex (η8-cyclooctatetraene)(η5-cyclopentadienyl)titanium, here [CpTi(cot)], has attracted interest as a spin qubit because of the long coherence time. Here the structural and chemical properties of [CpTi(cot)] on Au(111) are investigated at the monolayer level by experimental and computational methods. Scanning tunneling microscopy suggests that adsorption occurs in two molecular orientations, lying and standing, with a 3:1 ratio. XPS data evidence that a fraction of the molecules undergo partial electron transfer to gold, while our computational analysis suggests that only the standing molecules experience charge delocalization toward the surface. Such a phenomenon depends on intermolecular interactions that stabilize the molecular packing in the monolayer. This orientation-dependent molecule-surface hybridization opens exciting perspectives for selective control of the molecule-substrate spin delocalization in hybrid interfaces.

Atomic-Scale Rectification and Inelastic Electron Tunneling Spectromicroscopy.

The phenomenon of rectification describes the emergence of a DC current from the application of an oscillating voltage. Although the origin of this effect has been associated with the nonlinearity in the current-voltage I(V) relation, a rigorous understanding of the microscopic mechanisms for this phenomenon remains challenging. Here, we show the close connection between rectification and inelastic electron tunneling spectroscopy and microscopy for single molecules with a scanning tunneling microscope. While both techniques are based on nonlinear features in the I(V) curve, comprehensive line shape analyses reveal notable differences that highlight the two complementary techniques of nonlinear conductivity spectromicroscopy for probing nanoscale systems.

Towards π-wires on a semiconductor surface: Benzyne on Si(001).

Towards the goal of covalently bound molecular wires on silicon, the adsorption of benzyne on Si(001) was studied by means of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and density functional calculations (DFT). The benzyne molecule is found to adsorb preferentially via the strained triple bond on one dimer of the Si(001) surface which results in an intact π system covalently bound to the surface. With increasing coverage, the molecules primarily adsorb along the dimer rows; on stepped surfaces, these molecular wires are all oriented in the same direction.

Observing Electrocatalytic Processes via In Situ Electrochemical Scanning Tunneling Microscopy: Latest Advances.

Electrocatalysis is the foundation of many techniques currently used to address environmental and energy problems. Therefore, understanding the electrocatalytic processes is essential to guide the rational design of electrocatalysts. Scanning tunneling microscopy (STM), developed in the 1980s, remains one of the few techniques that allow surface imaging at the atomic level, making it incredibly useful in electrocatalytic research. In this review, we introduced the basic concept and latest applications of the STM technique for in situ studies of electrocatalytic processes, particularly its capability in active site identification, species adsorption/desorption analysis, surface reconstruction imaging, and electrocatalyst dissolution detection, as well as its advantages and limitations.

Strain-Relief Patterns and Kagome Lattice in Self-Assembled C60 Thin Films Grown on Cd(0001).

We report an ultra-high vacuum low-temperature scanning tunneling microscopy (STM) study of the C60 monolayer grown on Cd(0001). Individual C60 molecules adsorbed on Cd(0001) may exhibit a bright or dim contrast in STM images. When deposited at low temperatures close to 100 K, C60 thin films present a curved structure to release strain due to dominant molecule-substrate interactions. Moreover, edge dislocation appears when two different wavy structures encounter each other, which has seldomly been observed in molecular self-assembly. When growth temperature rose, we found two forms of symmetric kagome lattice superstructures, 2 × 2 and 4 × 4, at room temperature (RT) and 310 K, respectively. The results provide new insight into the growth behavior of C60 films.

Comparison Between Submucosal Tunneling Endoscopic Resection (STER) and Other Resection Modules for Esophageal Muscularis Propria Tumors: A Retrospective Study.

BACKGROUND Surgery has been considered to be the primary approach for resection of esophageal muscularis propria tumors. With the development of endoscopic technology, new techniques such as endoscopic submucosal dissection (ESD) and submucosal tunneling endoscopic resection (STER) have emerged for resecting these lesions. Previous studies have demonstrated that STER might be an intriguing alternative. This study aimed to evaluate the clinical value of STER by comparing it to other resecting modules. MATERIAL AND METHODS Clinical outcome and data were retrospectively collected from patients with esophageal muscularis propria tumors who underwent resection either endoscopically or thoracoscopically. The clinical data were statistically analyzed. RESULTS A total of 137 patients were enrolled. They were divided into 3 groups: a STER group (27 patients), an ESD group (42 patients), and a thoracoscopic enucleation (TE) group (68 patients). There were no significant differences among the 3 groups in gender, age, pathological type of tumors, or major adverse events (P>0.05). However, the STER group had the shortest duration of hospitalization and the lowest cost (P<0.05). Furthermore, the STER group was superior in operation time and the location of tumors to the TE group. Moreover, STER outperformed ESD in the resection of large tumors (P<0.05). Although STER had the lowest en bloc resection rate, no recurrence or metastasis was noted during a mean follow-up of 22.14 months (range 3 to 60 months). CONCLUSIONS STER is a feasible, safe, and effective approach for the resection of esophageal muscularis propria tumors ≤40 mm. We recommend STER as a potent alternative for these tumors.

Magnetic and defect probes of the SmB6 surface state.

The impact of nonmagnetic and magnetic impurities on topological insulators is a central focus concerning their fundamental physics and possible spintronics and quantum computing applications. Combining scanning tunneling spectroscopy with transport measurements, we investigate, both locally and globally, the effect of nonmagnetic and magnetic substituents in SmB6, a predicted topological Kondo insulator. Around the so-introduced substitutents and in accord with theoretical predictions, the surface states are locally suppressed with different length scales depending on the substituent's magnetic properties. For sufficiently high substituent concentrations, these states are globally destroyed. Similarly, using a magnetic tip in tunneling spectroscopy also resulted in largely suppressed surface states. Hence, a destruction of the surface states is always observed close to atoms with substantial magnetic moment. This points to the topological nature of the surface states in SmB6 and illustrates how magnetic impurities destroy the surface states from microscopic to macroscopic length scales.

Probing the Structure and Dynamics of Interfacial Water with Scanning Tunneling Microscopy and Spectroscopy.

Water/solid interfaces are ubiquitous and play a key role in many environmental, biophysical, and technological processes. Resolving the internal structure and probing the hydrogen-bond (H-bond) dynamics of the water molecules adsorbed on solid surfaces are fundamental issues of water science, which remains a great challenge owing to the light mass and small size of hydrogen. Scanning tunneling microscopy (STM) is a promising tool for attacking these problems, thanks to its capabilities of sub-Ångström spatial resolution, single-bond vibrational sensitivity, and atomic/molecular manipulation. The designed experimental system consists of a Cl-terminated tip and a sample fabricated by dosing water molecules in situ onto the Au(111)-supported NaCl(001) surfaces. The insulating NaCl films electronically decouple the water from the metal substrates, so the intrinsic frontier orbitals of water molecules are preserved. The Cl-tip facilitates the manipulation of the single water molecules, as well as gating the orbitals of water to the proximity of Fermi level (EF) via tip-water coupling. This paper outlines the detailed methods of submolecular resolution imaging, molecular/atomic manipulation, and single-bond vibrational spectroscopy of interfacial water. These studies open up a new route for investigating the H-bonded systems at the atomic scale.

All-electronic Nanosecond-resolved Scanning Tunneling Microscopy: Facilitating the Investigation of Single Dopant Charge Dynamics.

The miniaturization of semiconductor devices to scales where small numbers of dopants can control device properties requires the development of new techniques capable of characterizing their dynamics. Investigating single dopants requires sub-nanometer spatial resolution, which motivates the use of scanning tunneling microscopy (STM). However, conventional STM is limited to millisecond temporal resolution. Several methods have been developed to overcome this shortcoming, including all-electronic time-resolved STM, which is used in this study to examine dopant dynamics in silicon with nanosecond resolution. The methods presented here are widely accessible and allow for local measurement of a wide variety of dynamics at the atomic scale. A novel time-resolved scanning tunneling spectroscopy technique is presented and used to efficiently search for dynamics.

Probing Charge Transport through Peptide Bonds.

We measure the conductance of unmodified peptides at the single-molecule level using the scanning tunneling microscope-based break-junction method, utilizing the N-terminal amine group and the C-terminal carboxyl group as gold metal-binding linkers. Our conductance measurements of oligoglycine and oligoalanine backbones do not rely on peptide side-chain linkers. We compare our results with alkanes terminated asymmetrically with an amine group on one end and a carboxyl group on the other to show that peptide bonds decrease the conductance of an otherwise saturated carbon chain. Using a newly developed first-principles approach, we attribute the decrease in conductance to charge localization at the peptide bond, which reduces the energy of the frontier orbitals relative to the Fermi energy and the electronic coupling to the leads, lowering the tunneling probability. Crucially, this manifests as an increase in conductance decay of peptide backbones with increasing length when compared with alkanes.

Manipulation of Origin of Life Molecules: Recognizing Single-Molecule Conformations in ß-Carotene and Chlorophyll-a/ß-Carotene Clusters.

Carotenoids and chlorophyll are essential parts of plant leaves and are involved in photosynthesis, a vital biological process responsible for the origin of life on Earth. Here, we investigate how ß-carotene and chlorophyll-a form mixed molecular phases on a Au(111) surface using low-temperature scanning tunneling microscopy and molecular manipulation at the single-molecule level supported by density functional theory calculations. By isolating individual molecules from nanoscale molecular clusters with a scanning tunneling microscope tip, we are able to identify five ß-carotene conformations including a structure exhibiting a three-dimensional conformation. Furthermore, molecular resolution images enable direct visualization of ß-carotene/chlorophyll-a clsuters, with intimate structural details highlighting how they pair: ß-carotene preferentially positions next to chlorophyll-a and induces switching of chlorophyll-a from straight to several bent tail conformations in the molecular clusters.

Approaches to single-molecule studies of metalloprotein electron transfer using scanning probe-based techniques.

The single-molecule properties of metalloproteins have provided an intensely active research area in recent years. This brief review covers some of the techniques used to prepare, measure and analyse the electron transfer properties of metalloproteins, concentrating on scanning tunnelling microscopy-based techniques and advances in attachment of proteins to electrodes.

Probing Single Vacancies in Black Phosphorus at the Atomic Level.

Utilizing a combination of low-temperature scanning tunneling microscopy/spectroscopy (STM/STS) and electronic structure calculations, we characterize the structural and electronic properties of single atomic vacancies within several monolayers of the surface of black phosphorus. We illustrate, with experimental analysis and tight-binding calculations, that we can depth profile these vacancies and assign them to specific sublattices within the unit cell. Measurements reveal that the single vacancies exhibit strongly anisotropic and highly delocalized charge density, laterally extended up to 20 atomic unit cells. The vacancies are then studied with STS, which reveals in-gap resonance states near the valence band edge and a strong p-doping of the bulk black phosphorus crystal. Finally, quasiparticle interference generated near these vacancies enables the direct visualization of the anisotropic band structure of black phosphorus.

Fabrication of Gate-tunable Graphene Devices for Scanning Tunneling Microscopy Studies with Coulomb Impurities.

Owing to its relativistic low-energy charge carriers, the interaction between graphene and various impurities leads to a wealth of new physics and degrees of freedom to control electronic devices. In particular, the behavior of graphene's charge carriers in response to potentials from charged Coulomb impurities is predicted to differ significantly from that of most materials. Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) can provide detailed information on both the spatial and energy dependence of graphene's electronic structure in the presence of a charged impurity. The design of a hybrid impurity-graphene device, fabricated using controlled deposition of impurities onto a back-gated graphene surface, has enabled several novel methods for controllably tuning graphene's electronic properties. Electrostatic gating enables control of the charge carrier density in graphene and the ability to reversibly tune the charge and/or molecular states of an impurity. This paper outlines the process of fabricating a gate-tunable graphene device decorated with individual Coulomb impurities for combined STM/STS studies. These studies provide valuable insights into the underlying physics, as well as signposts for designing hybrid graphene devices.

Atomically Traceable Nanostructure Fabrication.

Reducing the scale of etched nanostructures below the 10 nm range eventually will require an atomic scale understanding of the entire fabrication process being used in order to maintain exquisite control over both feature size and feature density. Here, we demonstrate a method for tracking atomically resolved and controlled structures from initial template definition through final nanostructure metrology, opening up a pathway for top-down atomic control over nanofabrication. Hydrogen depassivation lithography is the first step of the nanoscale fabrication process followed by selective atomic layer deposition of up to 2.8 nm of titania to make a nanoscale etch mask. Contrast with the background is shown, indicating different mechanisms for growth on the desired patterns and on the H passivated background. The patterns are then transferred into the bulk using reactive ion etching to form 20 nm tall nanostructures with linewidths down to ~6 nm. To illustrate the limitations of this process, arrays of holes and lines are fabricated. The various nanofabrication process steps are performed at disparate locations, so process integration is discussed. Related issues are discussed including using fiducial marks for finding nanostructures on a macroscopic sample and protecting the chemically reactive patterned Si(100)-H surface against degradation due to atmospheric exposure.

Spectroscopic study of Gd nanostructures quantum confined in Fe corrals.

Low dimensional nanostructures have attracted attention due to their rich physical properties and potential applications. The essential factor for their functionality is their electronic properties, which can be modified by quantum confinement. Here the electronic states of Gd atom trapped in open Fe corrals on Ag(111) were studied via scanning tunneling spectroscopy. A single spectroscopic peak above the Fermi level is observed after Gd adatoms are trapped inside Fe corrals, while two peaks appear in empty corrals. The single peak position is close to the higher energy peak of the empty corrals. These findings, attributed to quantum confinement of the corrals and Gd structures trapped inside, are supported by tight-binding calculations. This demonstrates and provides insights into atom trapping in open corrals of various diameters, giving an alternative approach to modify the properties of nano-objects.

Rotational Spectromicroscopy: Imaging the Orbital Interaction between Molecular Hydrogen and an Adsorbed Molecule.

A hydrogen molecule can diffuse freely on the surface and be trapped above an adsorbed molecule within the junction of a scanning tunneling microscope. The trapped dihydrogen exhibits the properties of a free rotor. Here we show that the intermolecular interaction between dihydrogen and Mg-porphyrin (MgP) can be visualized by imaging j=0 to 2 rotational excitation of dihydrogen. The interaction leads to a weakened H-H bond and modest electron donation from the dihydrogen to the lowest unoccupied molecular orbital of MgP, a process similarly observed for the interaction between dihydrogen and an adsorbed Au atom.