We have elucidated the electronic states for Ru/SiON/Si(100) and Pt/HfO2/Pt structures under device operation by means of bias-application in hard x-ray photoelectron spectroscopy (BA-HXPES). For the Ru/SiON/Si(100) structure, interface electronic states are increased and new states near the conduction band minimum and valence band maximum are formed compared to the case of the Ru/SiO2/Si(100) structure. This is because nitrogen atoms at the SiO2/Si interface cause bond breaking and weakened the Si-O and Si-Si bonds at the interface. For the Pt/HfO2/Pt structure, oxygen atoms migrate to the top electrode under forward bias, forming Pt-O bond at the Pt/HfO2 interface. Under reverse bias, on the other hand, oxygen and Hf atoms are migrated to the top electrode, resulting in the formation of Pt-O and Pt-Hf bonds at the interface. The migration and bond formation at the interface should form vacancy in the HfO2 film, which might induce resistance switching behavior in this system. Here, it is emphasized that we present our approach using BA-HXPES, which successfully detects the electronic states under device operation. Therefore BA-HXPES could be applicable to various kinds of advanced materials and will be indispensable for evaluating their physical properties in detail.
Reflection high-energy electron diffraction (RHEED) method is one of the powerful techniques for a surface structural analysis and is conventionally used for an atomically flat crystal surface. In the case of nano crystal islands such as nano clusters or nano dots on a flat substrate, transmission pattern is observed especially on a grazing incident condition. Morphology of such nano clusters influences the spot shape of the transmission electron diffraction due to the effects of refraction and Laue function. It is shown that the chevron shape of the diffraction spots observed from Ge hut clusters grown on Si(001) substrate can be reproduced well by kinematic calculation. Relation between the spot shape and the cluster's morphology is summarized. It has been confirmed that the diffraction spots are shifted to downward normal to the facets where electrons enter or exit and their intensity distributions are prolonged by the effect of Laue function for the cluster's edge region where electrons can transmit.
The atomic structure and its electric conductance of the Pt nano contact in a benzene atmosphere were investigated. The conductance, shot noise, and conductance fluctuation measurements revealed the number of the molecules bridging between Pt electrodes. The vibration spectroscopy of single molecule junction and isotopic shift in vibration spectroscopy confirmed the bridging of the benzene molecule. We could determine the atomic structure of the single benzene molecule junction and its conductance around 1 G0(2e2/h). Highly conductive single molecule junction could be fabricated by direct biding of π conjugated benzene molecule to metal electrodes. In addition, we could fabricate single molecule junctions showing high and fixed conductance values, by using Ag as electrodes or C60 as molecules.
Interpretation of fragment ions of complex samples such as bio samples and polymer mixed samples obtained with time-of-flight secondary ion mass spectrometry (TOF-SIMS) is often difficult because of overlapping of fragment ions. Molecular ions or large fragment ions in TOF-SIMS spectra are often hidden among strong peaks of secondary ions. Since G-SIMS, which has been developed to investigate secondary ions in detail, enhances molecular ions and more intact fragment ions, and therefore it is useful to interpret intricate sample spectra. In this study, G-SIMS was applied to a thin film of polyethylene (average molecular weight : 540-640) on Si substrate in order to investigate effectiveness of G-SIMS on polymer samples. As a result, G-SIMS spectra enhancing molecular ions and more intact fragment ions were obtained and indicate relationship between particular fragment ions, which is useful to interpret TOF-SIMS spectra.
We report a study of dynamics and kinetics in physisorption of CH3Cl on an highly-oriented pyrolytic graphite (HOPG). The oriented molecular beam scattering was used to monitor the dynamical interaction of CH3Cl with HOPG in the zero coverage limit, demonstrating that the CH3Cl scattering intensity depends on the molecular orientation of the incident CH3Cl. The observed steric preference is not sensitive to the surface temperature. These results suggest that the moderate anisotropy in the interaction potential induces the molecular-orientation dependence of energy dissipation during the transient trapping into the physisorption well. On the other hand, the thermal energy atom scattering (TEAS) was used to probe the kinetics of thermal CH3Cl adsorption on HOPG during the coverage evolution. The desorption energy of CH3Cl on HOPG changes from 0.25 to 0.30 eV with increasing surface coverage, suggesting the attractive interaction between CH3Cl molecules on the surface.
We investigated the molecular adsorption and desorption on single-walled carbon nanotubes (SWCNTs) by thermal-desorption spectroscopy (TDS) and Fourier transform infrared spectroscopy (FTIR). By examining the as-purified and air-oxidized SWCNTs, the adsorption of H2, N2 and CO on the groove, inside and interstitial channel were clearly identified. In the case of N2 adsorption at low temperature, occupation of the inside site was found to be diffusion-limited. Co-adsorption of H2 and N2 at different temperatures revealed that the groove and inside sites are selectively occupied by either of the two molecules. FTIR spectra of CO on SWCNTs exhibited two distinct features for the CO stretching mode depending on the adsorption site.
Ultralow power consumption is an important requirement for thin film transistors (TFTs) used in system liquid crystal displays (LCDs). The nitric acid oxidation of Si (NAOS) method can form an ultrathin SiO2 layer with excellent interface characteristics, leading to vast decrease in the power consumption. The total thickness of the gate oxide with an ultrathin NAOS SiO2 layer of poly-Si-based TFT can be decreased from 80 nm to 20 nm. The thin gate oxide lowers the threshold voltage from 12 V to 1.5 V, resulting in a reduction of power consumption to 1/64. The gate leakage current is below the noise level, and the on/off ratio is more than 108. The S value is 80-100 mV/dec, and the channel mobility is 130-100 cm2/V·s for P-ch TFTs and 200-160 cm2/V·s for N-ch TFTs.
Magnetoelectric coupling at metal surfaces opens up a new possibility for metallic non-volatile magnetic data-storage devices, in which the magnetic bits are controlled by an electric field. We studied the atomic and magnetic order in bilayer Fe nano-islands grown on a Cu(111) substrate with a scanning tunneling microscopy setup in ultra-high vacuum at 4.5 K. Electric field pulses (108-109 V/m) were found to be able to cause a displacement of the Fe atoms, switching simultaneously the crystalline and the magnetic order, which is the prove of magnetoelectric coupling at the metallic Fe surface. We also succeeded in controlling the direction of the switching by the polarity of the electric field.
Anion concentrations at the air/water interface of saline droplets are important in atmospheric and environmental chemistry, because gaseous halogens emitted from the droplet surface mediate various key tropospheric chemical processes. In this study, the adsorption of water on alkali halide nanocrystals (KBr, KCl, KF, NaCl) on SiO2 and their deliquescence was in-situ investigated by noncontact atomic force microscopy (AFM) in an amplitude-modulation mode based on electrostatic forces. We also used ambient-pressure X-ray photoelectron spectroscopy in a synchrotron facility to complement our AFM results. For KBr, KCl and NaCl, deliquesced droplets show negative surface potentials relative to the surrounding region, indicating the preferential segregation of Br- and Cl- anions to the air/solution interface, even in the presence of a liquid/solid interface located a few nanometers away. This enhancement of anion concentration at the thin-droplet surface is more drastic for larger anions, meaning that heterogeneous reactions of gas-phase molecules with saline droplets to emit gaseous halogens can be more significant with larger anions.
We have investigated the inelastic process for alkanethiol self-assembled monolayers by using the isotope labeling and unambiguously determine the vibrational modes active in the inelastic electron tunneling spectroscopy (IETS). By the selective deuteration of the hydrogen, we can show that (1) the vibrational modes containing substantial amount of C-C displacement are active in IETS and (2) the different parts of the molecule contribute approximately equally to the IETS. These findings are qualitatively explained by the Fermi's Golden rule of IETS and well reproduced by our calculations based on the density functional theory and nonequilibrium Green's function.
Graphene is epitaxially grown on vicinal SiC(0001) surfaces via high temperature annealing in vacuum. To obtain higher quality graphene films and to investigate growth mechanisms it is beneficial to use well-defined SiC surface structures, which can be achieved by high temperature hydrogen etching before graphitization. The SiC surfaces with the periodic nanofacet structure resulted from the etching are thermally decomposed and graphitized to form epitaxial graphene layers. Temperature and time dependences of graphene growth are mainly investigated by means of low energy electron microscopy (LEEM). The graphene growth is accomplished by the layer-by-layer mode with strong anisotropy in the direction parallel to the steps and is limited by Si desorption from the surface/interface, where the 0th layer (buffer layer) is nucleated.
Calcium phosphate hydroxyapatite, Ca10(PO4)6(OH)2, is a typical apatite compound which has a high thermal stability and an affinity for organic compounds of biomass derivatives. It has a capacity for substitutions of Ca2+ ions and PO43- ions by other cations and anions. Here in, we will report the selective conversions of biomass derivatives, such as ethanol and lactic acid, into useful chemicals over the apatite compounds, such as Ca10(PO4)6(OH)2, Sr10(PO4)6(OH)2, Pb10(PO4)6(OH)2, and Ca10(VO4)6(OH)2. The apatite compounds were powders prepared by the hydrothermal method. In the ethanol conversion, the Sr-P apatite catalyst exhibited high 1-butanol selectivity (sel. > 80 C-%), which was significantly higher than that over the Ca-P apatite catalyst. In contrast, the Pb-P apatite catalyst selectively gave acetaldehyde, and the Ca-V apatite catalyst accelerated the dehydration into ethylene.