We have investigated the atomic structure, electronic states, and stress tensor for a Ge/Si(113)-2×2 surface by using first-principles total energy calculations. We have found that the model made up of tilted pentamers with an interstitial atom and rebonded atoms has the lowest surface energy of the models employed. The local density of states calculated for this surface provides a satisfactory description of recent scanning tunneling microscope images. Furthermore, it has been found that the surface stress is quite anisotropic, resulting in anisotropic growth of Ge films on Si(113).
Carbon nanowires and flat single-crystal graphite (0001) terraces have been competitively formed on a carbon-doped Ni(111) substrate, for the first time, only by a heat treatment in ultrahigh vacuum. The growth mechanism is interpreted in terms of a bulk-to-surface precipitation process of internal carbon atoms that were doped in a high-purity Ni(111) substrate in advance. The observed carbon nanowires are single wires or bundles. Nanometer-scale morphology and chemical properties of the wires have been clarified by low-energy electron diffraction, Auger electron spectroscopy and scanning tunneling microscopy (STM). A simple manipulation technique of a single carbon nanowire is demonstrated by STM.
We have successfully grown Be-doped p-type GaN on an Al2O3(0001) substrate by RF-MBE by conserving the surface polarity during growth. It is found that the surface polarity of undoped GaN grown on Al2O3(0001) substrate is N-terminated, but the polarity changes from N to Ga during growth when Be-doping is performed. This polarity change results in extremely high resistivity of the grown layer and the defect level centered at approximately 2.0 eV appears prominently. By using the AlN buffer layer, GaN layer can be grown under the Ga-terminated condition from the beginning. By growing Be-doped GaN on the Ga-terminated GaN, we have successfully grown Be-doped p-type GaN without inducing polarity change. In addition, the optical property of the sample is dramatically improved. We have confirmed that the Be acceptor level is shallower than that of the Mg by approximately 100 meV.
The adsorption of naphthalene on a Si(100)-2×1 surface at room temperature was investigated using infrared absorption spectroscopy (IRAS) in the multiple internal reflection geometry (MIR). To determine the adsorption configurations on the surface in detail, IRAS spectra in the C-H stretching vibration region were analyzed in comparison with calculations based on the density functional theory. As a result, it was found that naphthalene adsorbs on the surface in different manners depending on the surface coverage of the molecules. At low coverage, a single configuration is favored, in which the 1, 4, 6, 9 carbon atoms of a naphthalene molecule are bound to the dangling bonds of two adjacent Si dimers to form sp3 bonds. At high coverage, on the other hand, the molecules adsorb in several energetically preferred configurations. Furthermore, the effect of coupling of vibration modes between two adjacent molecules adsorbed on the surface was observed at high coverage.
The energy barrier and penetration depth of a hole are studied for ultrathin SiO2/Si interfaces. Layer thickness dependence and injected carrier concentration dependence are calculated for the superlattice structure, by using a first-principles pseudo-potential approach based on the density-functional theory. A new method to evaluate the energy barrier is proposed, which is derived from calculation of both the energy band structure and behavior of the Bloch function. The energy barrier for a hole is reduced with a decrease in Si layer thickness by confinement effect and is reduced also with the monolayer thickness of SiO2. The energy barrier increases by electron injection and decreases by hole injection.
Si(111)√3×√3-Ag surfaces, prepared under various annealing conditions, was systematically investigated by scanning tunneling microscopy (STM) and reflection-high-energy electron diffraction (RHEED). Although the RHEED patterns were seemingly the same for all cases, the STM images showed clear difference in surface morphology and local structures at steps; step edges were roundish and decorated by random adsorption of clusters when annealed below 600oC. However, the steps were straight with periodic protrusions along them by higher-temperature and prolonged annealing, due to a formation of very narrow domains of 6×1-Ag structure at step edges. Such distinctions may be the origin for different results in core-level photoemission spectroscopy (CLPES). Since it is known that two-dimensional adatom gas (2 DAG) phase of excess Ag atoms exits on terraces, which changes the band bending in the substrate, the step edges (and clusters adsorbed there) may play as reservoirs of the 2 DAG. For the samples without sufficient annealing, the concentration of 2 DAG and the resulting band bending are inhomogenous over the sample surfaces, which make the CLPES spectra broaden. Sufficient annealing removes the reservoirs and 2 DAG, resulting in a homogenous band bending over the surfaces to get very sharp CLPES spectra.
Oxidation of 2-propanol to acetone on a Pt/TiO2 photocatalyst by UV pulse irradiation was observed by time-resolved IR spectroscopy (TR-IR). The hole transfer to the 2-propanol, which triggers the oxidation, was completed within 0.5 μs. Transient vibrational spectra showed a new band at 1640 cm−1, which appeared immediately after the irradiation. This band gradually transformed to a band at 1700 cm−1, which was assigned to the C=O stretching mode of acetone, the product of 2-propanol oxidation. Thus, the band at 1640 cm−1 was suggested to be the reaction intermediate that transforms to acetone. We discussed the origin of this band using density functional calculation, and an acceptable agreement was obtained with the anion radical of acetone adsorbed on the catalyst.
We have investigated the chemical reaction of benzene molecule adsorbed on Cu(110) surface induced by the injection of tunneling electrons. The bonding site of the benzene molecule is identified at the hollow site of the Cu(110). The height of the benzene molecule increased by 40% in case tunneling electrons with the energy of 2−5 eV (sample bias is positive) were dosed, which indicates chemical reaction occurs on the benzene molecule in this process. STM-IETS measurement on the benzene molecule before and after the chemical reaction shows a clear difference between these two; ν(C-H) mode is observed only for the molecules after the reaction together with the confirmation of the isotope effect for the vibrational energy. This is interpreted that the dehydrogenation occurs in the benzene molecule by the injection of the tunneling electrons, which changes the bonding configuration of the benzene molecule from flat to non-flat to the surface. The reaction probability shows a sharp rise at the sample bias voltage of 2.4 V that saturates at 3.0 V. The rise is followed by another sharp rise at the voltage of 4.4 V. The current dependence on the reaction probability indicates that the reaction is the one-electron process. We propose a model that the dehydrogenation and chemical reaction of the benzene molecule is induced by a temporal trapping of the tunneling electron at the unoccupied state formed by the π orbitals of the C atoms of the benzene ring, which is not directly related to the C-H bonding broken in the process.
A new method for simultaneous measurement of topographic images and current-voltage (I-V) characteristics, based on atomic force microscopy (AFM), has been demonstrated with nano-scale resolution. Point-contact current-imaging atomic force microscopy (PCI-AFM) was developed to solve the incompatibility of electrically stable contact with nano-scale resolution in conductive probe AFM. We herein describe the detail of the PCI-AFM apparatus. Furthermore, the apparatus has been evaluated by the measurement of I-V characteristics of single-walled carbon nanotubes and deoxyribonucleic acid indicating high performance of PCI-AFM.
We studied the atomic structure of InSb(001)-c(8×2) surface using high-resolution transmission electron microscopy in the profile-imaging geometry (HR-profile TEM). A structure model was proposed, in which (i) In dimers are located in the third layer and (ii) In atoms with a reduced site occupancy exist in both the first and second layers. Most of features in HR-profile TEM images were well reproduced by the multi-slice simulations based on the present structure model. Also, Auger electron spectroscopy measurements provided satisfactory surface composition for the InSb(001)-c(8×2) surface.
Detection of pA order minute current by scanning probe microscopy encounters some problems under high humidity atmospheres, such as the increased background level and the poor reproducibility, because of the surface leak current. We were able to overcome these problems by using hermetic sealing and keeping the whole electronics equipment in a humidity-controlled atomic force microscope system. Detection of minute current of sub-pA level has been achieved even under high humidity atmospheres.