Externally-triggerable optical pump-probe scanning tunneling microscopy with a time resoloution of tens-picosecond.

Photoinduced carrier dynamics of nanostructures play a crucial role in developing novel functionalities in advanced materials. Optical pump-probe scanning tunneling microscopy (OPP-STM) represents distinctive capabilities of real-space imaging of such carrier dynamics with nanoscale spatial resolution. However, combining the advanced technology of ultrafast pulsed lasers with STM for stable time-resolved measurements has remained challenging. The recent OPP-STM system, whose laser-pulse timing is electrically controlled by external triggers, has significantly simplified this combination but limited its application due to nanosecond temporal resolution. Here we report an externally-triggerable OPP-STM system with a temporal resolution in the tens-picosecond range. We also realize the stable laser illumination of the tip-sample junction by placing a position-movable aspheric lens driven by piezo actuators directly on the STM stage and by employing an optical beam stabilization system. We demonstrate the OPP-STM measurements on GaAs(110) surfaces, observing carrier dynamics with a decay time of [Formula: see text] ps and revealing local carrier dynamics at features including a step edge and a nanoscale defect. The stable OPP-STM measurements with the tens-picosecond resolution by the electrical control of laser pulses highlight the potential capabilities of this system for investigating nanoscale carrier dynamics of a wide range of functional materials.

Hydrogen Bonding Propagated Phase Separation in Quasi-Epitaxial Single Crystals: A Pd-Br Molecular Insulator.

In condensed matter, phase separation is strongly related to ferroelasticity, ferroelectricity, ferromagnetism, electron correlation, and crystallography. These ferroics are important for nano-electronic devices such as non-volatile memory. However, the quantitative information regarding the lattice (atomic) structure at the border of phase separation is unclear in many cases. Thus, to design electronic devices at the molecular level, a quantitative electron-lattice relationship must be established. Herein, we elucidated a PdII-PdIV/PdIII-PdIII phase transition and phase separation mechanism for [Pd(cptn)2Br]Br2 (cptn = 1R,2R-diaminocyclopentane), propagated through a hydrogen-bonding network. Although the Pd···Pd distance was used to determine the electronic state, the differences in the Pd···Pd distance and the optical gap between Mott-Hubbard (MH) and charge-density-wave (CDW) states were only 0.012 Å and 0.17 eV, respectively. The N-H···Br···H-N hydrogen-bonding network functioned as a jack, adjusting the structural difference dynamically, and allowing visible ferroelastic phase transition/separation in a fluctuating N2 gas flow. Additionally, the effect of the phase separation on the spin susceptibility and electrical conductivity were clarified to represent the quasi-epitaxial crystals among CDW-MH states. These results indicate that the phase transitions and separations could be controlled via atomic and molecular level modifications, such as the addition of hydrogen bonding.

Versatile Post-Doping toward Two-Dimensional Semiconductors.

We have developed a simple and straightforward way to realize controlled postdoping toward 2D transition metal dichalcogenides (TMDs). The key idea is to use low-kinetic-energy dopant beams and a high-flux chalcogen beam simultaneously, leading to substitutional doping with controlled dopant densities. Atomic-resolution transmission electron microscopy has revealed that dopant atoms injected toward TMDs are incorporated substitutionally into the hexagonal framework of TMDs. The electronic properties of doped TMDs (Nb-doped WSe2) have shown drastic change and p-type action with more than 2 orders of magnitude increase in current. Position-selective doping has also been demonstrated by the postdoping toward TMDs with a patterned mask on the surface. The postdoping method developed in this work can be a versatile tool for 2D-based next-generation electronics in the future.

Subcycle mid-infrared coherent transients at 4 MHz repetition rate applicable to light-wave-driven scanning tunneling microscopy.

We produce subcycle mid-infrared (MIR) pulses at a 4 MHz repetition rate via the optical rectification (OR) of sub-10 fs near-infrared pulses delivered by an optical parametric chirped pulse amplifier. The coherent MIR pulses generated in a GaSe crystal under an ultrabroadband phase-matching condition contain only 0.58-0.85 oscillation cycles within the full width at half-maximum of the intensity envelope. The use of OR enables excellent phase stability of 56 mrad over 5.6 h, which is confirmed by field-resolved detection using electro-optic sampling. An electromagnetic simulation using a finite integration technique reveals that the peak field strength can easily exceed 10 V/nm owing to the field enhancement resulting from focusing MIR pulses onto a tunnel junction.

Continuous Heteroepitaxy of Two-Dimensional Heterostructures Based on Layered Chalcogenides.

The in-plane connection and layer-by-layer stacking of atomically thin layered materials are expected to allow the fabrication of two-dimensional (2D) heterostructures with exotic physical properties and future engineering applications. However, it is currently necessary to develop a continuous growth process that allows the assembly of a wide variety of atomic layers without interface degradation, contamination, and/or alloying. Herein, we report the continuous heteroepitaxial growth of 2D multiheterostructures and nanoribbons based on layered transition metal dichalcogenide (TMDC) monolayers, employing metal organic liquid precursors with high supply controllability. This versatile process can avoid air exposure during growth process and enables the formation of in-plane heterostructures with ultraclean atomically sharp and zigzag-edge straight junctions without defects or alloy formation around the interface. For the samples grown directly on graphite, we have investigated the local electronic density of states of atomically sharp heterointerface by scanning tunneling microscopy and spectroscopy, together with first-principles calculations. These results demonstrate an approach to realizing diverse nanostructures such as atomic layer-based quantum wires and superlattices and suggest advanced applications in the fields of electronics and optoelectronics.

Evolution of local conductance pathways in a single-molecule junction studied using the three-dimensional dynamic probe method.

Understanding of the dynamics of the bonding states of molecules with electrodes while the molecular conformation is changed is particularly important for elucidating the details of electrochemical devices as well as molecular devices in which the reaction dynamics of the electrodes and molecules plays an important role, such as in fuel cells, catalysis and bioelectrochemical devices. However, it has been difficult to make measurements when the distance between counter electrodes is short, namely, the molecule is raised from a lying form, almost parallel and close to the electrodes, toward a standing form and vice versa. We previously have developed a method called the three-dimensional (3D) dynamic probe method, which enables conductance measurement while the conformation of a single-molecule junction is precisely controlled by scanning tunneling microscopy (STM) techniques. Here, by combining this method with density functional theory (DFT) calculations, it has become possible to simultaneously consider the effects of the dynamics of molecular structures and the bonding states at the electrodes on the local transmission pathways, local-bond contributions to conductance. Here, by performing an analysis on 1,4-benzenediamine (BDA) and 1,4-benzenedithiol (BDT) single molecule junctions, we have observed, for the first time, the effect of a change in the molecular conformations and bonding states on the local transmission pathways for a short Au electrode distance condition.

Surface-mediated spin dynamics probed by optical-pump-probe scanning tunneling microscopy.

In current materials science and technologies, surface effects on carrier and spin dynamics in functional materials and devices are of great importance. In this paper, we present the surface-sensitive probing of electron spin dynamics, performed by optical-pump-probe scanning tunneling microscopy (OPP-STM). Time-resolved spin lifetime information on a manganese (Mn)-deposited GaAs(110) surface was successfully obtained for the first time. With increasing Mn density via in situ evaporation, a nonlinear change in the spin lifetime in the picosecond range was clearly observed, while directly confirming the Mn density by STM. In comparison with the results obtained by the conventional OPP method, we have also demonstrated that the observed nonlinear spin lifetime behavior was surface-mediated, which can be characterized using only the surface-sensitive OPP-STM technique.

The effect of nitrogen lone-pair interaction on the conduction in a single-molecule junction with amine-Au bonding.

We have applied our previously developed three-dimensional dynamic probe method to analyze the conductance in a Au-/1,4-benzenediamine (BDA)/Au single molecule junction. This structure is a typically used example to demonstrate the high performance of the break junction (BJ) method for measuring conductance with small variations, however, details of the interaction of the nitrogen (N) lone-pair in the amine group with a Au electrode, which is considered to have a fundamental role in determining the conductance of the single molecule junction with the amine, have not yet been clarified and still remain an important issue to be resolved. In this study, we have succeeded, for the first time, in observing the site-dependent change in conductance of this system while the molecular conformation was accurately controlled, and the results were well reproduced by a simulation taking account of the effect of the N lone-pair in an amine bonding with a Au electrode.

Revealing the Conformational Dynamics in a Single-Molecule Junction by Site- and Angle-Resolved Dynamic Probe Method.

Single-molecule junctions have been extensively studied because of their high potential for future nanoscale device applications as well as their importance in basic studies for molecular science and technology. However, since the bonding sites at an electrode and the molecular tilt angles, for example, cannot be determined experimentally, analyses have been performed assuming the structures of such interactive key factors, with uncertainties and inconsistencies remaining in the proposed mechanisms. We have developed a methodology that enables the probing of conformational dynamics in single-molecule junctions simultaneously with the direct characterization of molecular bonding sites and tilt angles. This technique has revealed the elemental processes in single-molecule junctions, which have not been clarified using conventional methods. The mechanisms of the molecular dynamics in 1,4-benzenedithiol and 4,4'-bipyridine single-molecule junctions, which, for example, produce binary conductance switching of different types, were clearly discriminated and comprehensively explained.

Modulation of electrical potential and conductivity in an atomic-layer semiconductor heterojunction.

Semiconductor heterojunction interfaces have been an important topic, both in modern solid state physics and in electronics and optoelectronics applications. Recently, the heterojunctions of atomically-thin transition metal dichalcogenides (TMDCs) are expected to realize one-dimensional (1D) electronic systems at their heterointerfaces due to their tunable electronic properties. Herein, we report unique conductivity enhancement and electrical potential modulation of heterojunction interfaces based on TMDC bilayers consisted of MoS2 and WS2. Scanning tunneling microscopy/spectroscopy analyses showed the formation of 1D confining potential (potential barrier) in the valence (conduction) band, as well as bandgap narrowing around the heterointerface. The modulation of electronic properties were also probed as the increase of current in conducting atomic force microscopy. Notably, the observed band bending can be explained by the presence of 1D fixed charges around the heterointerface. The present findings indicate that the atomic layer heterojunctions provide a novel approach to realizing tunable 1D electrical potential for embedded quantum wires and ultrashort barriers of electrical transport.

Attractive interaction between Mn atoms on the GaAs(110) surface observed by scanning tunneling microscopy.

Scanning tunneling microscopy/spectroscopy (STM/STS) was carried out to investigate the structures of Mn atoms deposited on a GaAs(110) surface at room temperature to directly observe the characteristics of interactions between Mn atoms in GaAs. Mn atoms were paired with a probability higher than the random distribution, indicating an attractive interaction between them. In fact, re-pairing of unpaired Mn atoms was observed during STS measurement. The pair initially had a new structure, which was transformed during STS measurement into one of those formed by atom manipulation at 4 K. Mn atoms in pairs and trimers were aligned in the <110> direction, which is theoretically predicted to produce a high Curie temperature.

Microscopic basis for the band engineering of Mo1-xWxS2-based heterojunction.

Transition-metal dichalcogenide layered materials, consisting of a transition-metal atomic layer sandwiched by two chalcogen atomic layers, have been attracting considerable attention because of their desirable physical properties for semiconductor devices, and a wide variety of pn junctions, which are essential building blocks for electronic and optoelectronic devices, have been realized using these atomically thin structures. Engineering the electronic/optical properties of semiconductors by using such heterojunctions has been a central concept in semiconductor science and technology. Here, we report the first scanning tunneling microscopy/spectroscopy (STM/STS) study on the electronic structures of a monolayer WS2/Mo1-xWxS2 heterojunction that provides a tunable band alignment. The atomically modulated spatial variation in such electronic structures, i.e., a microscopic basis for the band structure of a WS2/Mo1-xWxS2 heterojunction, was directly observed. The macroscopic band structure of Mo1-xWxS2 alloy was well reproduced by the STS spectra averaged over the surface. An electric field of as high as 80 × 10(6) Vm(-1) was observed at the interface for the alloy with x = 0.3, verifying the efficient separation of photoexcited carriers at the interface.

Mechanically activated switching of Si-based single-molecule junction as imaged with three-dimensional dynamic probe.

Understanding and extracting the full functions of single-molecule characteristics are key factors in the development of future device technologies, as well as in basic research on molecular electronics. Here we report a new methodology for realizing a three-dimensional (3D) dynamic probe of single-molecule conductance, which enables the elaborate 3D analysis of the conformational effect on molecular electronics, by the formation of a Si/single molecule/Si structure using scanning tunnelling microscopy (STM). The formation of robust covalent bonds between a molecule and Si electrodes, together with STM-related techniques, enables the stable and repeated control of the conformational modulation of the molecule. By 3D imaging of the conformational effect on a 1,4-diethynylbenzene molecule, a binary change in conductance with hysteresis is observed for the first time, which is considered to originate from a mechanically activated conformational change.

Dynamic probe of ZnTe(110) surface by scanning tunneling microscopy.

The reconstructed surface structure of the II-VI semiconductor ZnTe (110), which is a promising material in the research field of semiconductor spintronics, was studied by scanning tunneling microscopy/spectroscopy (STM/STS). First, the surface states formed by reconstruction by the charge transfer of dangling bond electrons from cationic Zn to anionic Te atoms, which are similar to those of IV and III-V semiconductors, were confirmed in real space. Secondly, oscillation in tunneling current between binary states, which is considered to reflect a conformational change in the topmost Zn-Te structure between the reconstructed and bulk-like ideal structures, was directly observed by STM. Third, using the technique of charge injection, a surface atomic structure was successfully fabricated, suggesting the possibility of atomic-scale manipulation of this widely applicable surface of ZnTe.

Real space probe of short-range interaction between Cr in a ferromagnetic semiconductor ZnCrTe.

The short-range interaction between Cr atoms was directly examined by scanning tunneling microscopy measurements on a Zn(0.95)Cr(0.05)Te film. Our measurements revealed that a Cr atom formed a localized state within the bandgap of ZnTe and this state was broadened for a pair of Cr atoms within a distance of ∼ 1 nm.

Probing ultrafast spin dynamics with optical pump-probe scanning tunnelling microscopy.

Studies of spin dynamics in low-dimensional systems are important from both fundamental and practical points of view. Spin-polarized scanning tunnelling microscopy allows localized spin dynamics to be characterized and plays important roles in nanoscale science and technology. However, nanoscale analysis of the ultrafast dynamics of itinerant magnetism, as well as its localized characteristics, should be pursued to advance further the investigation of quantum dynamics in functional structures of small systems. Here, we demonstrate the optical pump-probe scanning tunnelling microscopy technique, which enables the nanoscale probing of spin dynamics with the temporal resolution corresponding, in principle, to the optical pulse width. Spins are optically oriented using circularly polarized light, and their dynamics are probed by scanning tunnelling microscopy based on the optical pump-probe method. Spin relaxation in a single quantum well with a width of 6 nm was observed with a spatial resolution of ∼ 1 nm. In addition to spin relaxation dynamics, spin precession, which provides an estimation of the Landé g factor, was observed successfully.

Bases for time-resolved probing of transient carrier dynamics by optical pump-probe scanning tunneling microscopy.

The tangled mechanism that produces optical pump-probe scanning tunneling microscopy spectra from semiconductors was analyzed by comparing model simulation data with experimental data. The nonlinearities reflected in the spectra, namely, the excitations generated by paired laser pulses with a delay time, the logarithmic relationship between carrier density and surface photovoltage (SPV), and the effect of the change in tunneling barrier height depending on SPV, were examined along with the delay-time-dependent integration process used in measurement. The optimum conditions required to realize reliable measurement, as well as the validity of the microscopy technique, were demonstrated for the first time.

Nanoscale probing of transient carrier dynamics modulated in a GaAs-PIN junction by laser-combined scanning tunneling microscopy.

The modulation of carrier dynamics in a GaAs-PIN junction after photoexcitation by an ultrashort-pulse laser was probed by shaken-pulse-pair-excited scanning tunneling microscopy (SPPX-STM), which enables nanoscale mapping of time-resolved STM images. The effect of the built-in potential on the carrier dynamics, diffusion and drift, which cannot be probed by the optical pump-probe technique, was successfully visualized in real space.

Physical performance and knee osteoarthritis among community-dwelling women in Japan: the Hizen-Oshima Study, cross-sectional study.

Osteoarthritis (OA) is the most common chronic joint disorder. Relationships between knee OA and physical performance have been examined, but mainly in patients with knee OA. Clarifying the relationship between knee OA and physical performance among community-dwelling individuals is thus important. Subjects comprised 563 community-dwelling Japanese women. Radiographic knee OA was defined as Kellgren-Lawrence criteria grade 2 or higher. Painful knee OA was defined as radiographic OA combined with knee pain. We evaluated performance-based measures of physical functioning. Student's t tests were used to compare continuous variables. Adjusted means of performance-based measures were compared between groups using general linear modeling methods. Mean age was 64.3 years. Women with radiographic OA were older than those without OA (P < 0.0001). BMI was greater in women with radiographic OA than in women without OA (P < 0.0001). In univariate analysis, women with radiographic OA displayed worse physical functioning than women without OA, with longer chair stand time, longer walking time, and shorter functional reach. Performance-based measurements with painful OA resembled those with radiographic OA. Age- and BMI-adjusted means of chair stand time and walking time were longer in women with radiographic or painful knee OA than in women without OA (P < 0.0001 each). Furthermore, chair stand and walking took longer for women with painful knee OA than for women with radiographic knee OA. Women with knee OA showed deteriorated performance of chair stand and walking. Painful knee OA was associated with poorer performance than radiographic knee OA.

Scanning tunneling microscopy/spectroscopy on self-assembly of a glycine/Cu(111) nanocavity array.

The step-by-step analysis of a hierarchical self-assembly revealed the incorporation of nanocavity blocks in a metastable orientation to stabilize the organized array. The confinement of 2D electrons by a quantum corral was verified. Furthermore, manipulation of an isolated C(60) molecule was realized using nanocavities of ∼1.3 nm diameter as a template.