Plasmon resonance and enhanced near-field of anisotropic nanoparticle systems: unified analysis by factorization of light-excited dipole distribution.

We develop a simple factorization scheme to analyze the mechanism of dipole-plasmon resonance, which is controlled by the particle shape or the gap distance of neighboring particles. The method focuses on extracting the motion of local induced dipoles based on the discrete dipole approximation (DDA) and is applied to silver nanoparticles. Our analysis clarifies that the particle shape effect is characterized quantitatively by the oscillation of a small number of collective dipoles when the inhomogeneity of the distribution of induced dipoles is weak. Our factorization scheme is also applicable to a system consisting of neighboring nanoparticles and explains the relationship between the gap distance of neighboring nanoparticles and near-field enhancement. Our theoretical approach is useful for understanding the optical response of anisotropic- and multi-nanoparticle systems in a unified manner, and it provides a convenient view for the design of optical materials of nanoparticles.

Water Splitting Induced by Visible Light at a Copper-Based Single-Molecule Junction.

Water splitting is an essential process for converting light energy into easily storable energy in the form of hydrogen. As environmentally preferable catalysts, Cu-based materials have attracted attention as water-splitting catalysts. To enhance the efficiency of water splitting, a reaction process should be developed. Single-molecule junctions (SMJs) are attractive structures for developing these reactions because the molecule electronic state is significantly modulated, and characteristic electromagnetic effects can be expected. Here, water splitting is induced at Cu-based SMJ and the produced hydrogen is characterized at a single-molecule scale by employing electron transport measurements. After visible light irradiation, the conductance states originate from Cu/hydrogen molecule/Cu junctions, while before irradiation, only Cu/water molecule/Cu junctions were observed. The vibration spectra obtained from inelastic electron tunneling spectroscopy combined with the first-principles calculations reveal that the water molecule trapped between the Cu electrodes is decomposed and that hydrogen is produced. Time-dependent and wavelength-dependent measurements show that localized-surface plasmon decomposes the water molecule in the vicinity of the junction. These findings indicate the potential ability of Cu-based materials for photocatalysis.

Deep-Learning Approach to First-Principles Transport Simulations.

Large-scale first-principles transport calculations, while essential for device modeling, remain computationally demanding. To overcome this bottle neck, we combine first-principles transport calculations with machine learning-based nonlinear regression. We calculate the electronic conductance through first-principles based nonequilibrium Green's function techniques for small systems and map the transport properties onto local properties using local descriptors. We show that using the local descriptor as input features for deep learning-based nonlinear regression allows us to build a robust neural network that can predict the conductance of large systems beyond that of the current state-of-the-art first-principles calculation algorithms. Our protocol is applied to alkali metal nanowires, i.e., potassium, which have unique geometrical and electronic properties and hence nontrivial transport properties. We demonstrate that within our approach we can achieve qualitative agreement with experiment at a fraction of the computational effort as compared to the direct calculation of the transport properties using conventional first-principles methods.

A case of an atypical teratoid/rhabdoid tumor with distinctive histology in the pineal region in an adult patient.

A 31-year-old man suffered from headaches and presented at a hospital after the symptom worsened. Obstructive hydrocephalus and a pineal tumor were identified, and he was transferred to our hospital for further investigation and treatment. Cranial computed tomography revealed a hypodense mass lesion on the right of the pineal region, and calcifications and enlargement of the lateral and third cerebral ventricles were also evident. Blood tests were negative for all tumor markers. Laparoscopic biopsy and third-ventricle fenestration were performed that day as an emergency surgery to treat the obstructive hydrocephalus. Postoperative cranial magnetic resonance imaging revealed a solid tumor that was hypointense on T1-weighted imaging, hyperintense on T2-weighted imaging, and heterogeneously enhanced by Gd. Subsequently, the tumor increased in size, and craniotomy and tumorectomy were performed. Histologically, the tumor proliferated as round or short spindle-shaped cells in a myxoid matrix, forming arrays that surrounded the blood vessels. As a few cells with eosinophilic cytoplasm were also present and immunostaining for INI-1 was negative, the patient was diagnosed with atypical teratoid/rhabdoid tumor (AT/RT). AT/RT of the pineal region in adults is rare, and herein, we report the morphological characteristics of this case and reviewed the relevant literature.

Structural transition pathway and bipolar switching of the GeTe-Sb2Te3 superlattice as interfacial phase-change memory.

We investigated the resistive switching mechanism between the high-resistance state (HRS) and the low-resistance state (LRS) of the GeTe-Sb2Te3 (GST) superlattice. First-principles calculations were performed to identify the structural transition pathway and to evaluate the current-voltage (I-V) characteristics of the GST device cell. After determining the atomistic structures of the stable structural phases of the GST superlattice, we found the structural transition pathways and the transition states of possible elementary processes in the device, which consisted of a thin film of GST superlattice and semi-infinite electrodes. The calculations of the I-V characteristics were examined to identify the HRS and the LRS, and the results reasonably agreed with those of our previous study (H. Nakamura, et al., Nanoscale, 2017, 9, 9286). The calculated HRS/LRS and analysis of the transition states of the pathways suggest that a bipolar switching mode dominated by the electric-field effect is possible.

Intraplaque Microvascular Flow Signal in Superb Microvascular Imaging and Magnetic Resonance Imaging Carotid Plaque Imaging in Patients with Atheromatous Carotid Artery Stenosis.

Carotid artery atherosclerosis is one of the major risk factors for ischemic stroke. Intraplaque neovascularization (IPN) is one of the steps toward the development of vulnerable plaque. Superb microvascular imaging (SMI) is a new ultrasonographic technique for visualizing low-velocity and microvascular flow by clutter suppression to extract flow signals from large to small vessels and enables visualization of intraplaque microvascular flow (IMVF) without echo contrast media. We aimed to investigate the association between IMVF signal in SMI and MRI plaque imaging among patients with atherosclerotic carotid stenosis. We prospectively enrolled patients (>18 years old) with mild to severe carotid stenosis (more than 50% in cross-sectional area) diagnosed by carotid ultrasonography between August 2017 and April 2018, irrespective of sex and history of stroke. A total of 40 patients (31 men, 9 women; mean age, 75.1 ± 10.0 years) were enrolled. SMI revealed IPN findings in 21 patients. SMI clearly visualized the direction of pulsatile flow movement in microvessels and IPN was easily classified into the two types of Type V (n=2) and Type E (n=19). Multivariate logistic regression analysis presented that microvascular flow signal in carotid plaque on SMI was identified as a significant predictor of intraplaque hemorrhage as evaluated by MRI (OR, 8.46; 95%CI, 1.44-49.9; p=0.018). This study demonstrated a significant association between the presence of IMVF signal in SMI and intraplaque hemorrhage characterized by high-intensity lesions on MRI T1-FFE images.

Site-Selection in Single-Molecule Junction for Highly Reproducible Molecular Electronics.

Adsorption sites of molecules critically determine the electric/photonic properties and the stability of heterogeneous molecule-metal interfaces. Then, selectivity of adsorption site is essential for development of the fields including organic electronics, catalysis, and biology. However, due to current technical limitations, site-selectivity, i.e., precise determination of the molecular adsorption site, remains a major challenge because of difficulty in precise selection of meaningful one among the sites. We have succeeded the single site-selection at a single-molecule junction by performing newly developed hybrid technique: simultaneous characterization of surface enhanced Raman scattering (SERS) and current-voltage (I-V) measurements. The I-V response of 1,4-benzenedithiol junctions reveals the existence of three metastable states arising from different adsorption sites. Notably, correlated SERS measurements show selectivity toward one of the adsorption sites: "bridge sites". This site-selectivity represents an essential step toward the reliable integration of individual molecules on metallic surfaces. Furthermore, the hybrid spectro-electric technique reveals the dependence of the SERS intensity on the strength of the molecule-metal interaction, showing the interdependence between the optical and electronic properties in single-molecule junctions.

Competitive effects of oxygen vacancy formation and interfacial oxidation on an ultra-thin HfO2-based resistive switching memory: beyond filament and charge hopping models.

We studied the quantum transport mechanism of an ultra-thin HfO2-based resistive random access memory (ReRAM) cell with TiN electrodes and proposed the design of a sub-10 nm scale device. It is believed that formation and rupture of the conduction path in the local filament causes the switching between high and low resistive states. However, the validity of this simple filament model is not obvious in the sub-10 nm scale device because the redox processes occur mainly in a few nm range at the interface. Furthermore, the intrinsic transport mechanism of the device, in particular, quantum coherence, depends on device materials and length-scale. The relationship between the redox states and the transport mechanism like ballistic or hopping is still under debate when the device length scale is less than 10 nm. In the present study, we performed first-principles calculations of the non-equilibrium Green's function including electron-phonon interactions. We examined several characteristic structures of the HfO(x) wire (nano-scale conduction path) and the interfaces between the resistive switching layer and electrodes. We found that the metal buffer layer induced a change in the oxygen-reduction site from the interface of HfO(x)/TiN to the buffer layer. Even when the inserted buffer layer is a few atomic layers, this effect plays an important role in the enhancement of the performance of ON/OFF resistive switching and in the reduction of the inelastic electric current by electron-phonon scattering. The latter suppresses the hopping mechanism, which makes the ballistic conduction the dominant mechanism. We evaluated the activation energy in the high temperature limit by using the first-principles results of inelastic current. Our theoretical model explains the observed crossover of the temperature dependence of ReRAM cells and gives a new insight into the principle of operation on a sub-10 nm scale ReRAM device.

The effect of a Ta oxygen scavenger layer on HfO2-based resistive switching behavior: thermodynamic stability, electronic structure, and low-bias transport.

Reversible resistive switching between high-resistance and low-resistance states in metal-oxide-metal heterostructures makes them very interesting for applications in random access memories. While recent experimental work has shown that inserting a metallic "oxygen scavenger layer" between the positive electrode and oxide improves device performance, the fundamental understanding of how the scavenger layer modifies the heterostructure properties is lacking. We use density functional theory to calculate thermodynamic properties and conductance of TiN/HfO2/TiN heterostructures with and without a Ta scavenger layer. First, we show that Ta insertion lowers the formation energy of low-resistance states. Second, while the Ta scavenger layer reduces the Schottky barrier height in the high-resistance state by modifying the interface charge at the oxide-electrode interface, the heterostructure maintains a high resistance ratio between high- and low-resistance states. Finally, we show that the low-bias conductance of device on-states becomes much less sensitive to the spatial distribution of oxygen removed from the HfO2 in the presence of the Ta layer. By providing a fundamental understanding of the observed improvements with scavenger layers, we open a path to engineer interfaces with oxygen scavenger layers to control and enhance device performance. In turn, this may enable the realization of a non-volatile low-power memory technology with concomitant reduction in energy consumption by consumer electronics and offering significant benefits to society.

Temperature dependence of the thermopower and its variation of the Au atomic contact.

We performed simultaneous measurements of the thermopower, and conductance of Au atomic contacts during the self-breaking process under temperature control. During the whole measurement temperature regime (290-330 K), the thermopower randomly fluctuated from positive to negative in sign, and the average thermopower was negligibly small with respect to the variation of the thermopower of the contact. Meanwhile, the standard deviation of the thermoelectric voltage increased linearly with the temperature difference across the contacts. Above 320 K, we observed a decrease in the standard deviation of thermopower, which suggested a decrease in the density of defects near the contacts. The linear increase in the standard deviation of the thermoelectric voltage, and the decrease in the standard deviation of the thermopower above 320 K, indicate that the standard deviation of thermopower provides insight into the thermopower of an individual Au atomic contact and the atomic structure of Au atomic contacts, such as crystallinity and the distribution of defects near the contacts.

A supercell approach to the doping effect on the thermoelectric properties of SnSe.

We study the thermoelectric properties of tin selenide (SnSe) by using first-principles calculations coupled with the Boltzmann transport theory. A recent experimental study showed that SnSe gives an unprecedented thermoelectric figure of merit ZT of 2.6 ± 0.3 in the high-temperature (>750 K) phase, while ZT in the low-temperature phase (<750 K) is much smaller than that of the high-temperature phase. Here we explore the possibility of increasing ZT in the low-temperature regime by carrier doping. For this purpose, we adopt a supercell approach to model the doped systems. We first examine the validity of the conventional rigid-band approximation (RBA), and then investigate the thermoelectric properties of Ag or Bi doped SnSe as p- or n-type doped materials using our supercell method. We found that both types of doping improve ZT and/or the power factor of the low-temperature phase SnSe, but only after the adjustment of the appropriate doping level is achieved.

Single molecular resistive switch obtained via sliding multiple anchoring points and varying effective wire length.

A single molecular resistive (conductance) switch via control of anchoring positions was examined by using a molecule consisting of more than two same anchors. For this purpose, we adopted the covered quaterthiophene (QT)-based molecular wire junction. The QT-based wire consisted of two thiophene ring anchors on each side; thus, shift of anchors was potentially possible without a change in the binding modes and distortion of the intramolecular structure. We observed three distinct conductance states by using scanning tunneling microscope-based break junction technique. A detailed analysis of the experimental data and first-principles calculations revealed that the mechanism of the resistive switch could be explained by standard length dependence (exponential decay) of conductance. Here, the length is the distance between the anchoring points, i.e., length of the bridged π-conjugated backbone. Most importantly, this effective tunneling length was variable via only controlling the anchoring positions in the same molecule. Furthermore, we experimentally showed the possibility of a dynamic switch of anchoring positions by mechanical control. The results suggested a distinct strategy to design functional devices via contact engineering.

Thermoelectric efficiency of organometallic complex wires via quantum resonance effect and long-range electric transport property.

Superior long-range electric transport has been observed in several organometallic wires. Here, we discuss the role of the metal center in the electric transport and examine the possibility of high thermoelectric figure of merit (ZT) by controlling the quantum resonance effects. We examined a few metal center (and metal-free) terpyridine-based complexes by first-principles calculations and clarified the role of the metals in determining the transport properties. Quasi-resonant tunneling is mediated by organic compounds, and narrow overlapping resonance states are formed when d-electron metal centers are incorporated. Distinct length (L) and temperature (T) dependencies of thermopower from semiconductor materials or organic molecular junctions are presented in terms of atomistic calculations of ZT with and without considering the phonon thermal conductance. We present an alternative approach to obtain high ZT for molecular junctions by quantum effect.

Controlling formation of single-molecule junctions by electrochemical reduction of diazonium terminal groups.

We report controlling the formation of single-molecule junctions by means of electrochemically reducing two axialdiazonium terminal groups on a molecule, thereby producing direct Au-C covalent bonds in situ between the molecule and gold electrodes. We report a yield enhancement in molecular junction formation as the electrochemical potential of both junction electrodes approach the reduction potential of the diazonium terminal groups. Step length analysis shows that the molecular junction is significantly more stable, and can be pulled over a longer distance than a comparable junction created with amine anchoring bonds. The stability of the junction is explained by the calculated lower binding energy associated with the direct Au-C bond compared with the Au-N bond.

Universal medium-range order of amorphous metal oxides.

We propose that the structure of amorphous metal oxides can be regarded as a dual-dense-random-packing structure, which is a superposition of the dense random packing of metal atoms and that of oxygen atoms. Our ab initio molecular dynamics simulations show that the medium-range order of amorphous HfO2, ZrO2, TiO2, In2O3, Ga2O3, Al2O3, and Cu2O is characterized by the pentagonal-bipyramid arrangement of metal atoms and that of oxygen atoms, and prove the validity of our dual-random-sphere-packing model. In other words, we find that the pentagonal medium-range order is universal independent of type of metal oxide.

Collective bending motion of a two-dimensionally correlated bowl-stacked columnar liquid crystalline assembly under a shear force.

Stacked teacups inspired the idea that columnar assemblies of stacked bowl-shaped molecules may exhibit a unique dynamic behavior, unlike usual assemblies of planar disc- and rod-shaped molecules. On the basis of the molecular design concept for creating higher-order discotic liquid crystals, found in our group, we synthesized a sumanene derivative with octyloxycarbonyl side chains. This molecule forms an ordered hexagonal columnar mesophase, but unexpectedly, the columnar assembly is very soft, similar to sugar syrup. It displays, upon application of a shear force on solid substrates, a flexible bending motion with continuous angle variations of bowl-stacked columns while preserving the two-dimensional hexagonal order. In general, alignment control of higher-order liquid crystals is difficult to achieve due to their high viscosity. The present system that brings together higher structural order and mechanical softness will spark interest in bowl-shaped molecules as a component for developing higher-order liquid crystals with unique mechanical and stimuli-responsive properties.

Multi-scale electronics transport properties in non-ideal CVD graphene sheet.

In this work, we benchmark non-idealities and variations in the two-dimensional graphene sheet. We have simulated more than two hundred graphene-based devices structure. We have simulated distorted graphene sheets and have included random, inhomogeneous, asymmetric out-of-plane surface corrugation and in-plane deformation corrugation in the sheet through autocorrelation function in the non-equilibrium Green's function (NEGF) framework to introduce random distortion in flat graphene. These corrugation effects inevitably appear in the graphene sheet due to background substrate roughness or the passivation encapsulation material morphology in the transfer step. We have examined the variation in density of state, propagating density of transmission modes, electronic band structure, electronic density, and hole density in those device structures. We have observed that the surface corrugation increases the electronic and hole density distribution variation across the device and creates electron-hole charge puddles in the sheet. This redistribution of microscopic charge in the sheet is due to the lattice fields' quantum fluctuation and symmetry breaking. Furthermore, to understand the impact of scattered charge distribution on the sheet, we simulated various impurity effects within the NEGF framework. The study's objective is to numerically simulate and benchmark numerous device design morphology with different background materials compositions to elucidate the electrical property of the sheet device.

Nature of electron transport by pyridine-based tripodal anchors: potential for robust and conductive single-molecule junctions with gold electrodes.

We have designed and synthesized a pyridine-based tripodal anchor unit to construct a single-molecule junction with a gold electrode. The advantage of tripodal anchoring to a gold surface was unambiguously demonstrated by cyclic voltammetry measurements. X-ray photoelectron spectroscopy measurements indicated that the π orbital of pyridine contributes to the physical adsorption of the tripodal anchor unit to the gold surface. The conductance of a single-molecule junction that consists of the tripodal anchor and diphenyl acetylene was measured by modified scanning tunneling microscope techniques and successfully determined to be 5 ± 1 × 10(-4)G(0). Finally, by analyzing the transport mechanism based on ab initio calculations, the participation of the π orbital of the anchor moieties was predicted. The tripodal structure is expected to form a robust junction, and pyridine is predicted to achieve π-channel electric transport.

Probing embedded topological modes in bulk-like GeTe-Sb2Te3 heterostructures.

The interface between topological and normal insulators hosts metallic states that appear due to the change in band topology. While topological states at a surface, i.e., a topological insulator-air/vacuum interface, have been studied intensely, topological states at a solid-solid interface have been less explored. Here we combine experiment and theory to study such embedded topological states (ETSs) in heterostructures of GeTe (normal insulator) and [Formula: see text] [Formula: see text] (topological insulator). We analyse their dependence on the interface and their confinement characteristics. First, to characterise the heterostructures, we evaluate the GeTe-Sb[Formula: see text]Te[Formula: see text] band offset using X-ray photoemission spectroscopy, and chart the elemental composition using atom probe tomography. We then use first-principles to independently calculate the band offset and also parametrise the band structure within a four-band continuum model. Our analysis reveals, strikingly, that under realistic conditions, the interfacial topological modes are delocalised over many lattice spacings. In addition, the first-principles calculations indicate that the ETSs are relatively robust to disorder and this may have practical ramifications. Our study provides insights into how to manipulate topological modes in heterostructures and also provides a basis for recent experimental findings [Nguyen et al. Sci. Rep. 6, 27716 (2016)] where ETSs were seen to couple over thick layers.