Atomic-scale view of the photoinduced structural transition to form sp3-like bonded order phase in graphite.

Photoexcitation of solids often induces structural phase transitions between different ordered phases, some of which are unprecedented and thermodynamically inaccessible. The phenomenon, known as photoinduced structural phase transition (PSPT), is of significant interest to the technological progress of advanced materials processing and the fundamental understanding of material physics. Here, we applied scanning tunnelling microscopy (STM) to directly characterise the primary processes of the PSPT in graphite to form a sp3-like carbon nano-phase called diaphite. The primary challenge was to provide microscopic views of the graphite-to-diaphite transition. On an atomic scale, STM imaging of the photoexcited surface revealed the nucleation and proliferation processes of the diaphite phase; these were governed by the formation of sp3-like interlayer bonds. The growth mode of the diaphite phase depends strongly on the photon energy of excitation laser light. Different dynamical pathways were proposed to explain the formation of a sp3-like interlayer bonding. Potential mechanisms for photon-energy-dependent growth were examined based on the experimental and calculated results. The present results provide insight towards realising optical control of sp2-to-sp3 conversions and the organisation of nanoscale structures in graphene-related materials.

Oxidation of Anatase TiO2(001) Surface Using Supersonic Seeded Oxygen Molecular Beam.

We investigated the oxidation of oxygen vacancies at the surface of anatase TiO2(001) using a supersonic seeded molecular beam (SSMB) of oxygen. The oxygen vacancies at the top surface and subsurface could be eliminated by the supply of oxygen using an SSMB. Oxygen vacancies are present on the surface of anatase TiO2(001) when it is untreated before transfer to a vacuum chamber. These vacancies, which are stable in the as-grown condition, could also be effectively eliminated by using the oxygen SSMB.

Atomic-resolution imaging of rutile TiO2(110)-(1 × 2) reconstructed surface by non-contact atomic force microscopy.

The structure of the rutile TiO2(110)-(1 × 2) reconstructed surface is a phase induced by oxygen reduction. There is ongoing debate about the (1 × 2) reconstruction, because it cannot be clarified whether the (1 × 2) structure is formed over a wide area or only locally using macroscopic analysis methods such as diffraction. We used non-contact atomic force microscopy, scanning tunneling microscopy, and low-energy electron diffraction at room temperature to characterize the surface. Ti2O3 rows appeared as bright spots in both NC-AFM and STM images observed in the same area. High-resolution NC-AFM images revealed that the rutile TiO2(110)-(1 × 2) reconstructed surface is composed of two domains with different types of asymmetric rows.

Fabrication of tungsten tip probes within 3 s by using flame etching.

A tungsten (W) tip has been used as a standard tip probe because of its robustness at the highest boiling temperature; the use cases include a field emission (FE) electron source for scanning electron microscopy (SEM) and a scanning probe microscopy tip. The W tip probe has generally been fabricated through a chemical etching process with aqueous solutions. In this study, we propose a new method-flame etching. Without using aqueous solutions, a W tip probe was successfully fabricated within 3 s in air, which is very fast and convenient, and beneficial for mass production (additionally, no expensive setup is necessary). A W tip probe was obtained simply by putting a W wire into an oxygen-liquefied petroleum (O2+LP) gas flame (giving the highest temperature of ∼2300 K) through a microtorch for a few seconds. The obtained W tip provided atomically resolved scanning tunneling microscopic images. Also, since FE electrons were detected by applying ∼106 V/m, the tip can be used as an FE-SEM source. Generation and vaporization of WO3 on the W surface are important processes to form a tip shape.

Controlled Deposition Number of Organic Molecules Using Quartz Crystal Microbalance Evaluated by Scanning Tunneling Microscopy Single-Molecule-Counting.

Precise control of organic molecule deposition on a substrate is quite important for fabricating single-molecule-based devices. In this study, we demonstrate whether a quartz-crystal microbalance (QCM) widely used for a film growth calibration has the ability to precisely measure the number of organic molecules adsorbed on a substrate. The well-known Sauerbrey's equation is extended to formulate the relation between QCM resonant frequency shift and the number of adsorbed molecules onto the QCM surface. The formula is examined by QCM measurements of sublimation of π-conjugated organic molecules and direct counting of the deposited molecules one by one onto metal substrates, using ultrahigh vacuum low-temperature scanning tunneling microscopy (STM). It is revealed that the number of adsorbed molecules evaluated by QCM ( NQCM) show good agreement with those counted from the STM images ( NSTM) within the error of ±25%. The results ensure the QCM capability for controlling the deposition number of organic molecules with high accuracy, that is, if one needs to deposit 100 molecules on the substrate, QCM control promises deposition of 100 ± 25 molecules.

Room temperature stable film formation of π-conjugated organic molecules on 3d magnetic substrate.

An important step toward molecule-based electronics is to realize a robust and well-ordered molecular network at room temperature. To this end, one key challenge is tuning the molecule-substrate electronic interactions that influence not only the molecular selfassembly but also the stability of the resulting structures. In this study, we investigate the film formation of π-conjugated metal-free phthalocyanine molecules on a 3d-bcc-Fe(001) whisker substrate at 300 K by using ultra-high-vacuum scanning tunneling microscopy. On bare Fe(001), hybridization between the molecular π and the Fe(001) d-states prevents the molecular assembly, resulting in the disordered patchy structures. The second- and third-layer molecules form densely packed films, while the morphologies show clear difference. The second-layer molecules partially form p(5 × 5)-ordered films with the rectangular edges aligned along the [100] and [010] directions, while the edges of the third-layer films are rounded. Remarkably, such film morphologies are stable even at 300 K. These findings suggest that the molecular self-assembly and the resulting morphologies in the second and third layers are affected by the substrate bcc(001), despite that the Fe-d states hybridize only with the first-layer molecules. The possible mechanism is discussed with the kinetic Monte Carlo simulation.

Accurate Extraction of Electrostatic Force by a Voltage-Pulse Force Spectroscopy.

The classification of interaction forces between two approaching bodies is important in a wide range of research fields. Here, we propose a method to unambiguously extract the electrostatic force (F(ele)), which is one of the most significant forces. This method is based on the measurement of the energy dissipation under applied voltage pulse between an atomic force microscopy (AFM) tip and sample. It allowed us to obtain F(ele) as a function of the tip-sample distance and voltage including the distance-independent part, to which conventional AFM is insensitive. The obtained F(ele) curves nicely fit the analytical model, enabling estimation of the geometry of the tip. The distance-dependent contact potential difference could also be correctly obtained by the measured F(ele), opening an alternative route to quantitative Kelvin probe force microscopy.

Room-temperature-concerted switch made of a binary atom cluster.

Single-atom/molecule manipulation for fabricating an atomic-scale switching device is a promising technology for nanoelectronics. So far, scanning probe microscopy studies have demonstrated several atomic-scale switches, mostly in cryogenic environments. Although a high-performance switch at room temperature is essential for practical applications, this remains a challenging obstacle to overcome. Here we report a room-temperature switch composed of a binary atom cluster on the semiconductor surface. Distinctly different types of manipulation techniques enable the construction of an atomically defined binary cluster and the electronic switching of the conformations, either unidirectionally or bidirectionally. The switching process involves a complex rearrangement of multiple atoms in concerted manner. Such a feature is strikingly different from any switches mediated by single-atom/molecule processes that have been previously reported.