Scanning Tunneling Microscopy Investigation Research Articles

Inducing Transient Charge State of a Single Water Cluster on Cu(111) Surface.

The hydrated electron on solid surface is a crucial species to interfacial chemistry. We present a joint low-temperature scanning tunneling microscopy and density functional theory investigation to explore the existence of a transient hydrated electron state induced by injecting tunneling electrons into a single water nonamer cluster on Cu(111) surface. The directional diffusion of water cluster under the Coulomb repulsive potential has been observed as evidence for the emergence of the transient hydrated electron. A critical structure transformation in water cluster for the emergence of hydrated electron has been identified. A charging mechanism has been proposed based on density functional theory calculation and scanning tunneling microscope results.

Study of the electrode tool wear and the probe tip sharpening phenomena during the nanoscale STM electric discharge lithography of the bulk HOPG surface

Scanning tunneling microscope (STM)-based electric discharge lithography or machining is capable of fabricating nanostructures or nano-devices out of the carbon-based materials with low cost and high accuracy. As a tip-based nanofabrication method, the evolution of the STM probe tip, which functionalized as the electrode tool, is of great importance to the machining capability and performance of the STM electric discharge lithography. In this work, both the electrode tool wear (ETW) and the probe tip sharpening phenomena were investigated during the SPM electric discharge lithography of the bulk highly oriented pyrolytic graphite (HOPG) sample surface. The influence of the electrode tool wear evolution on the machining results was experimentally studied. The geometric parameters of the STM probe tip were measured from SEM imaging. Moreover, the probe tip sharpening phenomenon was specifically observed by applying the negative polarity machining. The evidences of the underlying mechanisms for the tip sharpening effect were revealed experimentally in this study, which have never been reported in the previous work. It is further proved that this effect could be utilized to the in-situ sharpening of the STM probe tip. Finally, the effective methods to improve the machining consistency and the tool lifetime were proposed.

Competition between Hexagonal and Tetragonal Hexabromobenzene Packing on Au(111)

Low-temperature scanning tunneling microscope investigations reveal that hexabromobenzene (HBB) molecules arrange in either hexagonally closely packed (hcp) [Formula: see text] or tetragonal [Formula: see text] structure on Au(111) dependent on a small substrate temperature difference around 300 K. The underlying mechanism is investigated by density functional theory calculations, which reveal that substrate-mediated intermolecular noncovalent C-Br···Br-C attractions induce hcp HBB islands, keeping the well-known Au(111)-22×√3 reconstruction intact. Upon deposition at 330 K, HBB molecules trap freely diffusing Au adatoms to form tetragonal islands. This enhances the attraction between HBB and Au(111) but partially reduces the intermolecular C-Br···Br-C attractions, altering the Au(111)-22×√3 reconstruction. In both cases, the HBB molecule adsorbs on a bridge site, forming a ∼15° angle between the C-Br direction and [112̅]Au, indicating the site-specific molecule-substrate interactions. We show that the competition between intermolecular and molecule-substrate interactions determines molecule packing at the subnanometer scale, which will be helpful for crystal engineering, functional materials, and organic electronics.

Ripples near edge terminals in MoS2 few layers and pyramid nanostructures

Atomically thin transition-metal dichalcogenides are of great interest due to their intriguing physical properties and potential applications. Here, we report our findings from scanning tunneling microscopy and spectroscopy investigations on molybdenum disulfide (MoS2) mono- to few-layers and pyramid nanostructures synthesized through chemical vapor deposition. On the few-layered MoS2 nanoplatelets grown on gallium nitride (GaN) and pyramid nanostructures on highly oriented pyrolytic graphite, we observed an intriguing curved region near the edge terminals. The measured band gap on these curved regions is 1.96 ± 0.10 eV, consistent with the value of the direct band gap in MoS2 monolayers. The curved features near the edge terminals and the associated electronic properties may contribute to the catalytic behaviors of MoS2 nanostructures and have potential applications in future electronic devices and energy-related products based on MoS2 nanostructures.

Direct observation of condensate and vortex confinement in nanostructured superconductors

In this work we report a scanning tunneling microscopy investigation of lithographically defined superconducting nanosquares. The obtained spectroscopic maps reveal the spatial evolution of both the superconducting condensate and the screening currents as a function of the applied magnetic field. The symmetry of the nanostructure is imposed on the condensate and it controls the distribution of the vortices inside the nanosquare. Our local study allows exploring the impact of small structural defects, omnipresent in these kind of structures, on both the supercurrent and vortex distribution. As a result, direct experimental evidence of vortex pinning and current crowding at the nanoscale has been obtained.

Scanning tunneling microscopy and density functional theory investigations on molecular self-assembly of graphene on Ru(0 0 0 1)

Investigations on the bottom-up fabrication of graphene with 1,3,5-triphenylbenzene as precursor on Ru(0001) was carried out using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. Upon annealing 1,3,5-triphenylbenzene overlayer on Ru(0001) at 550°C, the precursors dehydrogenated and coalesced into graphitized flakes, and subsequent annealing up to 600°C results in complete graphene conversion. The migration behavior and close-packing morphology of precursors were captured during STM measurements, and DFT calculations indicated that the inter-molecular interaction is responsible for the accumulation and close-packing of the precursors. The noticeable increment in the dehydrogenation barrier from 1.27eV for monomer adsorption to 1.62eV for dimer adsorption is well consistent with the observed drastic reduction of the graphitization temperature at lower precursor coverage, suggesting the crucial influence of inter-molecular vdW interaction on the dehydrogenation process.

A Suggestion of the Graphene/Ge(111) Structure Based on Ultra-High Vacuum Scanning Tunneling Microscopy Investigation

We report on the 5.5√3 × 5.5√3 − R30 ∘ overlayer superstructure observed by the scanning tunneling microscopy on the Ge(111) surface. It shows pronounced effects of the local density of states leading to the strong dependence of STM images on the bias voltage and some dynamic changes of images at 300 K. This overlayer is tentatively interpreted as graphene formed in small submonolayer amounts due to the pyrolysis of hydrocarbon constituents of the residual atmosphere of the vacuum chamber during the annealing of a Ge(111) sample at 900 K. We suggest a model of the graphene/Ge(111)-5.5√3 × 5.5√3 − R30 ∘ heteroepitaxial interface, featuring the reconstructed Ge(111) substrate with no long-range order under the graphene layer, the latter being corrugated due to spatial variations of the interatomic geometry of the Ge(111) and graphene(0001) atomic lattices with extremely large mismatch.

Orbital-Resolved Imaging of the Adsorbed State of Pyridine on GaP(110) Identifies Sites Susceptible to Nucleophilic Attack

Artificial photosynthesis by photoelectrocatalytic CO2 reduction is dependent, as is natural photosynthesis, on interfacial electron transfer to couple light excitation energy to reaction centers. For heterogeneous systems, in the context of frontier orbital theory artificial reaction centers are defined through the interactions of filled and empty orbitals within a few electronvolts of the Fermi energy of the adsorbate complex. Here we report a scanning tunneling microscopy (STM) and density functional theory investigation of the orbital-resolved adsorption state defining the dative bonding interaction between a III–V semiconductor surface [GaP(110)] and a N-containing heteroaromatic (pyridine). This system was selected for its relevance to photoelectrocatalysis utilizing heteroaromatic cocatalysts, which has been reported to yield highly selective CO2 reduction to fuels. By examining the distribution of unoccupied molecular orbitals, we show that STM images can be used to positively identify the sites o...

Ordered heteromolecular overlayers formed by metal phthalocyanines and porphyrins on rutile titanium dioxide surface studied at room temperature.

Molecular heterostructures are formed from meso-tetraphenyl porphyrins-Zn(II) (ZnTPP) and Cu(II)-phthalocyanines (CuPc) on the rutile TiO2(011) surface. We demonstrate that ZnTPP molecules form a quasi-ordered wetting layer with flat-lying molecules, which provides the support for growth of islands comprised of upright CuPc molecules. The incorporation of the ZnTPP layer and the growth of heterostructures increase the stability of the system and allow for room temperature scanning tunneling microscopy (STM) measurements, which is contrasted with unstable STM probing of only CuPc species on TiO2. We demonstrate that within the CuPc layer the molecules arrange in two phases and we identify molecular dimers as basic building blocks of the dominant structural phase.

Cleaved thin-film probes for scanning tunneling microscopy

We introduce an alternative type of probe for scanning tunneling microscopy (STM). Instead of using a needle-like tip made from a piece of metallic wire, a sharp-edged cleaved insulating substrate, which is initially covered by a thin conductive film, is used. The sharp tip is formed at the intersection of the two cleaved sides. Using this approach a variety of materials for STM probes can be used, and functionalization of STM probes is possible. The working principle of different probes made of metallic (Pt, Co, and CoB), indium-tin oxide, as well as Cu/Pt and Co/Pt multilayer films are demonstrated by STM imaging of clean Cu(001) and Cu(111) surfaces as well as the epitaxial Co clusters on Cu(111).

Atomically Precise Prediction of 2D Self-Assembly of Weakly Bonded Nanostructures: STM Insight into Concentration-Dependent Architectures.

A joint experimental and computational study is reported on the concentration-dependant self-assembly of a flat C3 -symmetric molecule on a graphite surface. As a model system a tripodal molecule, 1,3,5-tris(pyridin-3-ylethynyl)benzene, has been chosen, which can adopt either C3h or Cs symmetry when planar, as a result of pyridyl rotation along the alkynyl spacers. Density functional theory (DFT) simulations of 2D nanopatterns with different surface coverage reveal that the molecule can generate different types of self-assembled motifs. The stability of fourteen 2D patterns and the influence of concentration are analyzed. It is found that ordered, densely packed monolayers and 2D porous networks are obtained at high and low concentrations, respectively. A concentration-dependent scanning tunneling microscopy (STM) investigation of this molecular self-assembly system at a solution/graphite interface reveals four supramolecular motifs, which are in perfect agreement with those predicted by simulations. Therefore, this DFT method represents a key step forward toward the atomically precise prediction of molecular self-assembly on surfaces and at interfaces.

Two-dimensional TiOx nanostructures on Au(111): a scanning tunneling microscopy and spectroscopy investigation

We investigated the growth of titanium oxide two-dimensional nanostructures on Au(111), produced by Ti evaporation and post-deposition oxidation. Scanning tunneling microscopy and spectroscopy (STM and STS) and low-energy electron diffraction measurements characterized the morphological, structural and electronic properties of the observed structures. Five distinct TiOx phases were identified: the honeycomb and pinwheel phases appear as monolayer films wetting the gold surface, while nanocrystallites of the triangular, row and needle phases grow mainly over the honeycomb or pinwheel layers. Density Functional Theory investigation of the honeycomb structure supports a structural model based on a Ti-O bilayer having Ti2O3 stoichiometry. The pinwheel phase was observed to evolve, for increasing coverage, from single triangular crystallites to a well-ordered film forming a superstructure, which can be interpreted within a moiré-like model. Structural characteristics of the other three phases were disclosed from the analysis of high-resolution STM measurements. STS measurements revealed a partial metallization of honeycomb and pinwheel and a semiconducting character of row and triangular phases.

Opto-mechano-electrical tripling in ZnO nanowires probed by photocurrent spectroscopy in a high-resolution transmission electron microscope

Photocurrent spectroscopy of individual free-standing ZnO nanowires inside a high-resolution transmission electron microscope (TEM) is reported. By using specially designed optical in situ TEM system capable of scanning tunneling microscopy probing paired with light illumination, opto-mechano-electrical tripling phenomenon in ZnO nanowires is demonstrated. Splitting of photocurrent spectra at around 3.3 eV under in situ TEM bending of ZnO nanowires directly corresponds to nanowire deformation and appearance of expanded and compressed nanowire sides. Theoretical simulation of a bent ZnO nanowire has an excellent agreement with the experimental data. The splitting effect could be explained by a change in the valence band structure of ZnO nanowires due to a lattice strain. The strain-induced splitting provides important clues for future flexible piezo-phototronics.

On-surface reaction of tetraphenylporphyrins with Os3(CO)12 precursors and Os clusters: A scanning tunnelling microscopy investigation

The ability of porphyrin molecules to incorporate metal atoms into the cavity of the macrocycle is the primary factor that enables the plethora of their applications. The fabrication and characterisation of surface confined metal-organic architectures by employing porphyrins promise unique technical applications in the field of nanotechnology. Here we report on the efforts to use triosmium dodecacarbonyl as a metal precursor for the on-surface Os functionalisation of porphyrins under ultra-high vacuum conditions. We address the effects of the temperature treatment of mixtures of tetraphenylporphyrins and the Os precursor molecules, which can decompose to yield Os clusters, on Ag(111) via scanning tunnelling microscopy, a technique that provides real-space visualisation of the reaction products formed. It is shown that free base porphyrins can be metallated to osmium porphyrins. Furthermore the presence of Os on the Ag(111) surface catalyses intramolecular cyclodehydrogenations in tetraphenylporphyrins, as well as intermolecular tetraphenylporphyrin polymerisation.

Surface−Enhanced Polymerization via Schiff-Base Coupling at the Solid–Water Interface under pH Control

On-surface polymerization realized at the solid–liquid interface represents a promising route to obtain stable and conductive organic layers with tunable properties. We present here spectroscopic evidence of π-conjugated polymer formation at the interface between an iodine-modified Au(111) and an aqueous solution. Schiff-base coupling has been used to drive the reaction by changing the pH. Scanning tunneling microscopy (STM) investigations show that the substrate acts as a template driving the formation of 1D ordered nanostructures. All the chemical states of the molecules on the surface have been identified and their evolution as a function of the pH has been monitored by synchrotron radiation X-ray photoelectron spectroscopy (XPS), demonstrating that two polymeric phases, undistinguishable by STM, exist on the surface: intermediate state and π-conjugated final product. The I/Au(111) substrate enhances the formation of π-conjugated polymers, as established comparing their production on the surface and in...

Chiral Transition of the Supramolecular Assembly by Concentration Modulation at the Liquid/Solid Interface

Global hetero- and homochiral polymorphous assemblies from an achiral fluorenone derivative were successfully constructed with multiple intermolecular hydrogen bonds by concentration modulation. Scanning tunneling microscopy investigations reveal that a heterochiral supramolecular double rosette-like structure was fabricated for the first time via hydrogen bond interactions with achiral 1-octanoic acid under low concentrations. When the solution concentration was increased, the structural transition from a heterochiral double rosette-like structure to a homochiral windmill-like pattern was observed. Interestingly, these two metastable structures ultimately could transform into a stable zigzag pattern at a bias voltage prompted by the STM tip. At high concentrations, only an achiral octamer arrangement could be obtained, owing to the changes of intermolecular hydrogen bonding, van der Waals force, and dipole–dipole interactions. The present results provided an important impetus for the induction and contro...

Pushing the Potential of Probe Microscopy

Musing over the most important scientific “sound bite” to pass onto future civilizations, Richard Feynman opted for: “All things are made of atoms—little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another.” [1]. It is perhaps no wonder then that Feynman said that it was akin to a “religious experience” when he saw atoms for the first time with a scanning tunneling microscope (STM) [2]. We can only guess, but he would probably have been just as impressed with STM’s sibling technique, atomic force microscopy, which allows us to probe the atomic forces he found so elemental to our scientific understanding. Now, Christian Wagner from the Julich Research Center (Forschungszentrum Julich) in Germany and his coworkers have pushed the capabilities of force microscopy to a new level by attaching a single molecule at the sharp end of an STM probe and exploiting it as a quantum dot capable of sensing electrostatic fields [3]. The molecular quantum dot terminating the tip allows the electrostatic potential to be mapped out around a target atom or molecule, thus giving even deeper insights into the atomic-scale forces which so fascinated Feynman. The increased sensitivity offered by this technique could be used to characterize rough surfaces, adsorbed materials on semiconductors, or artificial nanostructures.

Synthesis and Molecular Structures of BINOL Complexes: An STM Investigation of 2D Self-Assembly

Three new (R)-BINOL complexes have been prepared. 2D self-assembly has been observed in the system consisting of crystalline (R)-BINOL derivatives (guest) and 1,3,5-tris(10-carboxydecyloxy)-benzene (host) by means of the scanning tunneling microscopy (STM) technique. Density functional theory (DFT) calculations help to reveal the assembly on the HOPG surface.

Atom-resolved scanning tunneling microscopy investigations of molecular adsorption on MoS2 and CoMoS hydrodesulfurization catalysts

We present an atomic-scale investigation of the molecular interactions relevant to hydrodesulfurization (HDS) catalysis at the catalytically active edge sites of single-layer MoS2 and cobalt-promoted MoS2 (CoMoS) nanoparticles synthesized as model catalysts. Atom-resolved scanning tunneling microscopy observations of the adsorption of thiophene, pyridine, and various refractory alkyl-substituted dibenzothiophenes have allowed us to reveal their predominant adsorption modes, study possible active sites, and provide explanations for the observed selectivity in HDS reactions, which are relevant to the two identified HDS pathways – the direct desulfurization route and the hydrogenation route. Hydrogenation is proposed to occur through interaction with edge S–H groups when the molecules are adsorbed in a flat-lying configuration near the nanoparticle edges of single-layer MoS2 and CoMoS. This adsorption mode is directly observed with STM at low temperatures and is realized due to the interaction of the aromatic π-system of the adsorbing molecule with metallic edge states (brim states) in MoS2 and CoMoS which change the adsorption properties of the edges compared to the inert basal plane. The model studies also demonstrate the strong effect of sterical hindrance on the adsorption onto sulfur vacancies on MoS2 edges in direct desulfurization of the most refractory compounds, but it is also established that certain corner sites may exist for MoS2 and the promoted CoMoS structures, which directly facilitate strong adsorption.

Study of the resistive switching of vertically aligned carbon nanotubes by scanning tunneling microscopy

The effect of an external electric field on the electromechanical properties and regularities of the resistive switching of a vertically aligned carbon nanotube (VA CNT) has been studied experimentally using scanning tunneling microscopy. It has been shown that the VA CNT resistivity ratio in the high- and low-resistance states is higher than 25 as the distance between the scanning tunneling microscope (STM) probe and the VA CNT is 1 nm at a voltage of 8 V and depends on the voltage applied between the probe and the VA CNT. The proposed mechanism of resistive switching of VA CNTs is based on an instantaneous deformation and induction of a VA CNT internal electric field as a result of the sharp change in the time derivative of the external electric field strength. The obtained results can be used for the design and fabrication of resistive energy-efficient memory elements with a high density of storage cells on the basis of vertically aligned carbon nanotubes.