We have investigated the adsorption and dissociation of water and its co-adsorption with CO on atomically defined cobalt oxide nanoislands on Pt(111). The CoO islands were prepared under ultrahigh vacuum (UHV) conditions by reactive deposition of Co metal in oxygen atmosphere. The island structure was characterized by scanning tunneling microscopy (STM), showing that the nanoislands consist of a CoO bilayer and are regularly shaped with island edges that are mainly terminated by Co2+ ions. D2O was dosed in UHV onto the CoO islands on Pt(111) after pre-saturation with CO. D2O dissociation was monitored in situ by isothermal and temperature programmed infrared reflection absorption spectroscopy (IRAS). Isotopic exchange experiments were performed with H2O, D2O, and D218O to elucidate the nature of the hydroxyl groups. Three principal types of OD species are identified: (i) isolated OD at the edges of the CoO islands (Co-OeD), (ii) OD groups within larger hydroxylated areas on the CoO islands (Co-OcD), and (iii) isolated OD groups on the CoO terraces (Co-OtD). At 400 K, water adsorbs dissociatively on the CoO islands and forms isolated hydroxyl species (Co-OeD) at the island edges only. At room temperature (300 K), the coverage of hydroxyl groups increases rapidly, in line with the water-assisted hydroxylation reaction suggested previously. Adsorption experiments with D218O suggest that two equivalent groups are formed from one water molecule after dissociation at island edges, leading to the formation of larger hydroxylated areas on the CoO islands (Co-OcD) and, in addition, isolated OD species on the CoO terraces (Co-OtD). While the initial step of D2O dissociation is facile, the formation of larger hydroxylated areas is a slow and irreversible process. At 200 K, the formation of hydroxylated areas is accompanied by the co-adsorption of molecular water. The hydroxyl groups on the CoO islands are shown to interact with the CO preadsorbed on the CoO/Pt(111) model system. In particular, we observe a new CO species, stabilized by OD groups on the CoO islands, which adsorbs much stronger than CO on the OD-free CoO surface.

Scanning tunneling microscopy and spectroscopy (STM/STS) are used to electronically switch atomically-thin memristors, referred to as “atomristors”, based on a graphene/molybdenum disulfide (MoS2)/Au heterostructure. A gold-assisted exfoliation method was used to produce near-millimeter (mm) scale MoS2 on Au thin-film substrates, followed by transfer of a separately exfoliated graphene top layer. Our results reveal that it is possible to switch the conductivity of a graphene/MoS2/Au memristor stack using an STM tip. These results provide a path to further studies of atomically-thin memristors fabricated from heterostructures of two-dimensional materials such as graphene and transition metal dichalcogenides (TMDs).

The scanning tunneling microscope is essential to nanoscience and nanotechnology because it can provide images of surfaces at the atomic level with sub-nanometer resolution. Now a LabVIEW-based virtual instrument is available as a free-download at our company website (www.newpathresearch.com) for simulating the full operation of a scanning tunneling microscope. This is an executable version to be run on a Windows operating system without requiring other software.

Scanning tunneling microscopy (STM) is an excellent technique to image the surfaces of materials with extreme spatial resolution. However, it is difficult to maintain its imaging quality when applying the technique under the conditions used in many practical processes, such as chemical vapor deposition and catalysis. In this article, we describe two special classes of STM instruments that are capable of maintaining good imaging quality under “difficult” conditions, namely, one for high and variable temperatures and the other for the combination of high temperatures and high gas pressures. In both cases, we discuss the special design features that make these instruments robust with respect to the challenging imaging conditions and provide examples to illustrate how they are applied.

The recent spotlight on supported nanoparticles (NPs) has attracted attention in the field of catalysis and fuel cell technology. Supported NPs can be used as model catalysts to gain a fundamental understanding of the catalytic properties at the interface. Here, especially the wet-chemical preparation of platinum NPs in alkaline ethylene glycol is a powerful approach to synthesize stable particles with a narrow size distribution in the nanometer regime. We combine high resolution imaging by scanning tunneling microscopy with electrochemical characterization by cyclic voltammetry to gain insights into the underlying degradation mechanism of supported platinum NPs, paving the way toward a rational design of supported catalysts with controlled activity and stability.

Electric-field noise from the surfaces of ion-trap electrodes couples to the ion’s charge causing heating of the ion’s motional modes. This heating limits the fidelity of quantum gates implemented in quantum information processing experiments. The exact mechanism that gives rise to electric-field noise from surfaces is not well-understood and remains an active area of research. In this work, we detail experiments intended to measure ion motional heating rates with exchangeable surfaces positioned in close proximity to the ion, as a sensor to electric-field noise. We have prepared samples with various surface conditions, characterized in situ with scanned probe microscopy and electron spectroscopy, ranging in degrees of cleanliness and structural order. The heating-rate data, however, show no significant differences between the disparate surfaces that were probed. These results suggest that the driving mechanism for electric-field noise from surfaces is due to more than just thermal excitations alone.

Growth of unexpected phases from a composite target of BiFeO3:BiMnO3 and/or BiFeO3:BiCrO3 has been explored using pulsed laser deposition. The Bi2FeMnO6 tetragonal phase can be grown directly on SrTiO3 (STO) substrate, while two phases (S1 and S2) were found to grow on LaAlO3 (LAO) substrates with narrow growth windows. However, introducing a thin CeO2 buffer layer effectively broadens the growth window for the pure S1 phase, regardless of the substrate. Moreover, we discovered two new phases (X1 and X2) when growing on STO substrates using a BiFeO3:BiCrO3 target. Pure X2 phase can be obtained on CeO2-buffered STO and LAO substrates. This work demonstrates that some unexpected phases can be stabilized in a thin film form by using composite perovskite BiRO3 (R = Cr, Mn, Fe, Co, Ni) targets. Furthermore, it also indicates that CeO2 can serve as a general template for the growth of bismuth compounds with potential room-temperature multiferroicity.

Using scanning tunneling microscopy (STM) we observed atomic scale interference patterns on quasi-freestanding WSe2 islands grown on top of graphene. The bias-independent double atomic size periodicity of these patterns and the sharp Brillouin zone edge revealed by 2D STM Fourier analysis indicate formation of optical phonon standing waves due to scattering on intercalating defects supporting these islands. Standing wave patterns of both synchronized and non-synchronized optical phonons, corresponding to resonant and non-resonant phonon scattering regimes, were experimentally observed. We also found the symmetry breaking effect for individual phonon wave packets, one of the unique features distinguishing phonon standing waves. We show that vibrational and electronic anharmonicities are responsible for STM detection of these patterns. A significant contribution to the interference contrast arises from quantum zero-point oscillations.

X-ray beam-induced damage in nanoscale metal islands was investigated. Monolayer-high Ni islands were prepared on a Cu(111) substrate. High brilliance X-rays with photon energies between 8.45 and 8.85 keV illuminated the sample for about 11 hours. In order to track changes in the morphology of the islands, the synchrotron X-ray scanning tunneling microscopy (SX-STM) technique was utilized. The result shows that X-ray illumination onto Ni islands does not induce noticeable damage. The study demonstrates that local beam-induced changes can be studied using SX-STM.

Organic materials are fascinating and promising candidates for nanoscale spintronic devices and may open viable routes toward quantum computing. Previous experiments on spin transport in organic devices, through break junctions or spin valves, unveiled exciting new frontiers of molecular magnetism. However, much more effort is needed to understand the properties of organic/magnetic interfaces at a microscopic level. In this article, we show how spin-polarized scanning tunneling microscopy and spectroscopy (SP-STM/STS) can provide unprecedented insights into organic/magnetic interfaces as an initial step toward favorably tailoring such interfaces in order to increase device efficiency. Based on the unique combination of spin-sensitivity, atomic-scale spatial resolution, and high-energy resolution, SP-STM/STS has proven to be an invaluable method for exploring spatial and bias dependences of spin-polarized currents through individual molecules as well as for revealing individual spin-split molecular orbitals interacting with ferromagnetic substrates.

Molecules such as dithiols are of significant interest for potential molecular electronics applications. To investigate their properties, an efficient method for measuring their electrical conductance is crucial. This research focuses on the time domain measurement, a novel technique capable of measuring hundreds of molecules within a matter of seconds. Measurements were conducted using STM with the tip positioned within tunneling distance over a SAM of 1,8-octanedithiol on Au(111)-mica substrate submerged in toluene. Bonding/debonding events between the tops of molecules and the tip were observed through jumps in the time domain current waveform. A new time-spent histogram data analysis technique was developed to extract conductance values from complex waveforms. Conductance of 2.6 nS was obtained for a single 1,8-octanedithiol molecule, consistent with results obtained from wellestablished but time consuming break junction technique, validating the new STM based time domain technique for fast measurement of molecular conductance.

A statistically sound procedure for the unambiguous identification of the underlying Bravais lattice of an image of a 2D periodic array of objects is described. Our Bravais lattice detection procedure is independent of which type of microscope has been utilized for the recording of the image data. It is particularly useful for the correction of Scanning Tunneling Microscope (STM) images that suffer from a blunt scanning probe tip artifact, i.e. simultaneously recording multiple mini-tips. The unambiguous detection of the type of translation symmetry presents a first step towards making objective decisions about which plane symmetry a 2D periodic image is best modeled by. Such decisions are important for the application of Crystallographic Image Processing (CIP) techniques to images from Scanning Probe Microscopes (SPMs).

Graphene, a single atomic sheet of sp2-bonded carbon atoms arranged in a honeycomb lattice, exhibits extraordinary electrical and mechanical properties, attracting much attention in both academia and industry. The preparation of high quality large-area graphene and the tuning of graphene electronic properties are important topics in this field. In this feature paper, we review our recent work on epitaxial graphene (EG) on SiC(0001). First, we introduce the bottom-up growth mechanism of the first few EG layers on SiC(0001), and the modification of graphene electronic properties by means of surface transfer doping with electron withdrawing materials (F4-TCNQ and MoO3). Next, we summarize the adsorption behaviors of organic (PTCDA, ClAlPc, and C60F48) and inorganic (bismuth) materials on EG/SiC(0001). Finally, as an example of tuning the electronic properties of graphene by reducing its dimensionality, we demonstrate the molecular self-assembly of atomically precise armchair graphene nanoribbons with varying widths and electronic structures.

Solar water splitting has shown promise as a source of environmentally friendly hydrogen fuel. Understanding the interactions between semiconductor surfaces and water is essential to improve conversion efficiencies of water splitting systems. TiO2 has been widely adopted as a reference material and rutile surfaces have been studied experimentally and theoretically. Scanning Tunneling Microscopy (STM) is commonly used to study surfaces, as it probes the atomic and electronic structure of the surface layer. A systematic and transferable method to simulate constant current STM images using local atomic basis set methods is reported. This consists of adding more diffuse p and d functions to the basis sets of surface O and Ti atoms, in order to describe the long range tails of the conduction and valence bands (and, thus, the vacuum above the surface). The rutile TiO2 (110) surface is considered as a case study.

Atomistic lattice-gas models for thermodynamically and kinetically directed assembly are applied to Ru nanocluster formation on a monolayer of graphene supported on Ru(0001) at 309 K. Nanocluster density, mean size, height distribution, and spatial ordering are analyzed by kinetic Monte Carlo simulations. Both models can reproduce the experimental data, but additional density functional theory analysis favors the former.

Few techniques are available to systematically synthesize and characterize metal particles below 1nm in size. We build nanoparticles in an atomically defined manner through the use of a high-fidelity molecular container we call an atomic metron, which is used to select and count the metallic ions that will make up the resultant nanoparticle. After a defined number of ions are selected, the metron may be spatially isolated and the metallic ions reduced to an isolated nanoparticle. Each step in the process is characterized via analytical methods. AFM is used to demonstrate the formation of sub-nanometer particles. The counting of atoms, isolation, and formation of nanoparticles, shows high potential for easy synthesis of sub-nanometer particles with fine control over the number of atoms in each particle.

Various nanostructures with high-aspect-ratio formed in a low-resistivity silicon wafer by the nano-processing using a carbon nanotube (CNT) probe of a scanning tunneling microscope (STM) have been investigated. The multi-wall CNT probes were obtained with our original pulling-method from CNT dispersion liquid. Nanostructures of point configurations (pit and mound) and line configurations were obtained at the constant tunneling current of 0.1 nA by controlling the bias voltages up to 10 V, processing times up to 300 s and scanning speeds of probe up to 480 nm/s for a line configuration. The aspect-ratio of the pit configuration fabricated at the bias voltage of 3 V increased about 6 times in proportion to the increase in processing time. Remarkable influence of the bias voltage on the configurations indicated that there exists a threshold bias voltage for the transition from the pit configuration to the mound one between 3 V and 5 V, and the aspect ratio of all nanostructures fabricated by the CNT probe were larger than those by a conventional tungsten probe. Finally, cross-sectional TEM observations were also applied to clarify the difference in the formation mechanisms between the pit configuration and the mound configuration. The TEM image of the pit configuration showed neither dislocations nor remarkable strains existed, but in the case of the mound shape TEM analysis indicated the existence of single crystalline silicon region solidified with atomic defects under the mound configuration. Therefore the drastic change of the configurations was attributed to the changes of the atomic-scale microstructures by applying the bias voltages.

Scanning Tunneling Microscope (STM) was used to examine the morphologies of selfassembled InGaAs quantum dots (QDs). In order to induce the self-assembly, unlike the conventional Stranski-Krastanov (S-K) growth method, spatial thermal modulations in nanoscale were created in-situ on strained-but-flat InGaAs surfaces in a Molecular Beam Epitaxy (MBE) growth reactor by applying interferential irradiations of laser pulses (IILP). As-irradiated surfaces were examined using an attached ultra-high vacuum (UHV) STM. STM images indicate that the irradiation of 7 nano second laser pulse induces self-assembly of QDs. The average size of laser-induced QDs is smaller while their density is larger than that of QDs formed by annealing strained but flat epilayers conventionally. Furthermore, the dot density is modulated sinusoidally with a periodicity commensurate with that of the interference, which suggests that the placement of QDs can be controlled on the scale of the optical wavelength used. QD volume analysis suggests that dots grow faster laterally than vertically so that dots become flattened as they get larger.

The structure of the (100) surface of the orthorhombic Al13Co4quasicrystalline approximant is described based on a combined scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and density functional theory (DFT) study. We discuss the interplay between the surface structure and the cluster substructure of this approximant as well as possible off-stoichiometry effects that could explain the discrepancies found in the literature on possible surface models.

Based on X-ray photoelectron spectroscopy, Gd5Ge4(010) does not show evidence of surface segregation. Scanning tunneling microscopy reveals two types of terraces which alternate laterally on the surface. From the step heights, these two surface terminations are assigned as dense, Gd-pure layers in the bulk structure. There is evidence of reconstruction on one type of terrace.