Akira Hida

Creation of nanodiamonds by single impacts of highly charged ions upon graphite

The local modification of the electronic states of highly oriented pyrolytic graphite (HOPG) surfaces using highly charged ion (HCI) irradiation has been demonstrated as a promising technique in the design of nanoscale materials. The high potential energy of HCI and subsequent surface treatment by either electron injection from a scanning tunneling microscopy (STM) tip or by He–Cd laser irradiation provide a localized transition from sp2 to sp3 hybridization in HOPG, resulting in the formation of nanoscale diamond-like structures (nanodiamond). It is of interest that a single impact of HCI creates one nanodiamond structure without inducing any defects in the area surrounding the impact region, suggesting potential applications of HCI in nanoscale material processing.

Electric field modulation spectroscopy by scanning tunneling microscopy with a nanometer-scale resolution

Instrumentation has been devised by coupling electric field modulation spectroscopy (EFMS) with scanning tunneling microscopy (STM) that enables the investigation of electronic band structures in semiconductors with a nanometer-scale spatial resolution. Model experiments using low-temperature-grown GaAs (LT–GaAs) epifilms show that a difference as small as 0.01 eV in electronic energy gaps between the GaAs substrate and the LT–GaAs epilayers can be distinguished in the EFMS spectra, demonstrating the high energy, as well as spatial, resolution of the STM–EFMS.

Nanoscale transformation of sp2 to sp3 of graphite by slow highly charged ion irradiation

Abstract Nanoscale transformation of electronic states by highly charged ion (HCI) impact on graphite surfaces is described. The high potential energy of slow HCI, which induces multiple emission of electrons from the surface, provides a strong modification of the electronic states of the local area upon graphite surfaces. The HCI impact and the subsequent surface treatment either by electron injection from a scanning tunneling microscopy tip or by He–Cd laser irradiation induce a localized transition from sp2 to sp3 hybridization in graphite, resulting in the formation of nanoscale diamond-like structures (nanodiamond) at the impact region. From Raman spectroscopic measurements on sp2 related peaks, it is found that the HCI irradiation creates vacancy complexes in contrast to ions having a lower charge state, which generate single vacancies. It is of interest that a single impact of HCI creates one nanodiamond structure, suggesting potential applications of HCI in nanoscale material processing.

Analysis of surface modifications on graphite induced by slow highly charged ion impact

Abstract Modifications of the highly oriented pyrolytic graphite surfaces induced by the single impacts of slow Ar+ and Ar8+ were investigated by Raman spectroscopy. The difference in the irradiation-induced disorder between Ar+ and Ar8+ was clearly revealed by comparing two kinds of changes in Raman features against ion fluences: one is the peak intensity ratio of the disorder-induced peak with respect to the E2g-mode peak in the first-order Raman spectra, and the other is the full width at half maximum of the E2g-mode peak. Judging from the peculiar dependence of them on the fluence of Ar8+, it was assumed that the defects introduced by Ar8+ impacts is not simple vacancies as is the case of Ar+ impacts but vacancy clusters. The formation mechanism of vacancy clusters under Ar8+ irradiation was also discussed from the change in the second-order Raman spectra.

Nanometer-scale measurements of photoabsorption spectra of individual defects in semiconductors

Photoabsorption measurements using scanning tunneling microscopy, in which a modulated component of the tunneling current induced by a wavelength-variable chopped light is detected, were conducted for a cleaved surface of GaAs to demonstrate that a simple scheme enables nanometer-scale imaging of individual subsurface defects isolated in the crystal with fingerprints of photoabsorption spectra associated with them. The origin of the signal modulation and the image contrast formation mechanism are discussed in terms of photothermal expansion caused by nonradiative recombinations at the defect, photoinduced defect transformation, and an electrostatic surface potential change due to a charge alteration of the defect.

Nanoscale modification of electronic states of graphite by highly charged Ar-ion irradiation

This article describes a new nanomodification technique of electronic states. This technique involves highly charged ion (HCI) irradiation on the solid surface. The high potential energy of slow HCI, which induces multiple emission of electrons from the surface, provides a strong modification of the electronic states of the local area of surfaces. This unique process, induced by approaching slow HCI, converts metallic highly oriented pyrolytic graphite to a nonconductive structure, indicating the transformation of sp2 graphite to an sp3 nanoscale diamond-like structure.

STM study of ion tracks created in GaAs by GeV Xe ion irradiation

Defect structures in p-type GaAs bulk crystals irradiated with 3.54 GeV Xe 31þ ions were studied by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). STM images of the cleavage (1 1 0) surfaces revealed the fine structure of a single ion track that consists of a long straight linear defect of � 5 nm in diameter sheathed with a thin fringe in bright contrast. Unexpectedly, the surrounding region was observed to be irregular in contrast to atomically resolved images in non-irradiated regions. The local density of states measured by STS indicates that the tracks are amorphous whereas the sheath and the surrounding sites retain crystalline structures. Some of the defect contrasts can be explained as due to the shift of the Fermi level and the variation of the LDOS in the energy bands. Though the tracks and the surrounding regions were unaffected by STM observations and STS measurements under mild imaging conditions (tunneling current < 1 nA), we found that the contrasts of the tracks are erased when we scan the tracks with the STM tip at tunneling current greater than 2 nA. These facts strongly suggest that the defects introduced by GeV Xe ion irradiation are recovered by atomic motion enhanced by injection of tunneling current. 2003 Elsevier B.V. All rights reserved.

Electronic structure around an As antisite near the (110) surface of GaAs

The electronic structure around a single As antisite in GaAs is investigated in bulk and near the surface both in the stable and the metastable atomic configurations. The most characteristic electronic structures of As antisite is the existence of the localized p-orbitals extending from the As antisite. The major component of the highest occupied state on As antisite in the stable configuration is s-orbital connecting with neighboring As atoms with nodes whereas that in the metastable configuration is p-orbital connecting without nodes. Localized p-orbitals on the surrounding As atoms around the As antisite exist in every configuration of As antisite. Such features are retained except the case of the As antisite located just in the surface layer in which the midgap level is smeared into the conduction band and no localized states exist near the top of the valence band. Scanning tunneling microscopic images of defects observed in low-temperature grown GaAs, possibly assigned as As antisite, the origin of the metastability, and the peculiarity of the defects in the surface layer are discussed.

Modification of highly oriented pyrolytic graphite (HOPG) surfaces with highly charged ion (HCI) irradiation

Abstract Slow Ar 8+ irradiation and the following electron injection creates a non-conductive area to a depth of a few nanometers on a highly oriented pyrolytic graphite surface (HOPG). The annealing of the sample in a hydrogen atmosphere stabilizes the Ar 8+ impact area, which results in the field emission from the impact region. The property of the field emission is similar to that of diamond-like carbon (DLC), suggesting the possibility that sp 3 hybridization occurs at the Ar 8+ impact region. The work function estimated from the Fowler–Nordheim plot is much lower than non-treated HOPG and similar to a hydrogen-terminated DLC film.

The strain field around a single point defect in semiconductors spatially resolved by electric field modulation scanning tunneling spectro-microscopy

We have developed a novel instrumentation that allows us to image with a nano-scale resolution the strain field localized around single point defects in semiconducting crystals. The instrument that coupled the electric field modulation spectroscopy with the scanning tunneling microscopy was applied to point defects in low-temperature-grown GaAs (LT-GaAs) epi-films. The strain field image constructed by the present method exhibited features consistent with other experimental results.