Ken Kanazawa

Optical doping: active control of metal-insulator transition in nanowire.

The reversible control of metal-insulator transition (MIT) in In/Si(111) nanowires is demonstrated by tuning the band filling of the one-dimensional surface state by optical doping. The control of MIT is carried out by regulating the Fermi level in the surface state around the half-filled position, depending on the carrier density introduced at the interface. We successfully achieved the reversible and active control of MIT via the charge doping by regulating the intensity of photoexcitation. This method is widely applicable to other low-dimensional systems and makes MIT more controllable and suitable for use in nanowires as an active element in future architectures of nanosized functional devices as well as nanoscale interdevice wiring.

Formation of homochiral glycine/Cu(111) quantum corral array realized using alanine nuclei

Glycine has enantiomeric isomers on a Cu(111) surface through the dissociation of hydrogen from the carboxyl group and forms an array of quantum corrals of ~1.3 nm diameter. Stable homo-chiral glycinate trimers are formed in the first step, which subsequently form a network with a hexagonal arrangement. However, domains with R- or S-chirality coexist with the same probability. On the other hand, α-alanine has D- and L-chirality in nature and forms a similar quantum corral array on Cu(111) with R- and S-chirality, respectively. Here, by using α-alanine molecules as nuclei, the chirality of glycine molecules was controlled and a homochiral quantum corral array was successfully formed, which indicates the possibility that the optical isomers can be separated through a method such as preferential crystallization.

Anomalous Light Emission from Metal Phthalocyanine Films on Au(111) Activated by Tunneling-Current-Induced Surface Plasmon

We performed a scanning-tunneling-microscopy-induced light emission (STM-LE) study on Cu- and Zn-phthalocyanine films formed on a Au(111) surface. Optical emission with transition between the highest occupied and lowest unoccupied molecular orbitals (HOMO–LUMO) was observed at bias voltages lower than the energy corresponding to the molecular HOMO–LUMO gap. The voltage and current dependences of STM-LE intensities show an inconsistency for both samples, i.e., although the former shows the existence of onsets below the HOMO–LUMO gap energies, suggesting excitations by a two-electron process via an intermediate state, the latter shows a linear relationship expected from one-electron excitation processes.

Scanning Tunneling Microscopy and Spectroscopy Studies of Glycine on Cu(100): Inelastic-Tunneling Manipulation of Single Glycine Molecule

The chemical reaction of a glycine molecule on a Cu(100) surface from the neutral (NH2CH2COOH) to the glycinate (NH2CH2COO-) form via deprotonation, which had been analyzed on the basis of the results obtained by, for example, vibrational spectroscopy and diffraction methods, was studied at the single-molecule level by scanning tunneling microscopy (STM) and spectroscopy. The chemical reaction was successfully reproduced by the inelastic tunneling process thorough deprotonation. The threshold bias voltage for the reaction process could be attributed to the energy of the O–H stretch vibrational mode, and the asymmetric STM image after the reaction was in good agreement with the structure based on the predicted model.

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

Abstract 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.

Cr impurity-induced electronic states in ZnTe(110) surface

The impurity states of Cr atoms, which substituted Zn sites in the topmost layer of a p-type ZnTe(110) surface, were investigated by scanning tunneling microscopy/spectroscopy (STM/STS) and we firstly observed Cr-induced impurity states in the energy gap region of the host ZnTe including the unoccupied states by STS. Furthermore, we compared the observed energy levels and spatial distributions of the local density of states with those in the previous theoretical study [Katayama-Yoshida et al., Phys. Status Solidi A 204, 15 (2007)] and successfully identified the impurity states as the respective spin-polarized impurity states predicted by the theoretical study.

Probing of electronic structures of La@C82 superatoms upon clustering realized using glycine nanocavities

We have succeeded in the first direct probe of the change in the electronic structures of La@C82 superatoms upon clustering by scanning tunneling microscopy/spectroscopy (STM/STS). An array of ~1.3-nm-diameter glycine nanocavities self-assembled on a Cu(111) surface was used as a template. Isolated La@C82 superatoms were stably observed on terraces without diffusion to step edges, which enabled us to observe the change in the electronic structures associated with single, dimer, and clustered La@C82. A cluster with four La@C82 superatoms showed electronic structures similar to those obtained for thin films in previous works.

Strutural and Magnetic Properties of Ternary Transition-metal Chalcogenide CrFeTe Grown by MBE

We fabricated thin films of a ternary compound CrFeTe by molecular beam epitaxy (MBE). By investigating the magnetic properties of the films grown under various flux ratios of Fe to Cr (Fe/Cr), we found that the film grown at Fe/Cr~1 exhibited room-temperature ferromagnetic properties. This result suggests that the coexistence of Fe and Cr enhances the ferromagnetic interaction between magnetic atoms.

Cross-sectional STM study of impurity states in diluted magnetic semiconductor (Zn,Cr)Te

We performed cross-sectional scanning tunneling microscopy / spectroscopy (STM/STS) measurement on multi-layered structure (Zn0.97,Cr0.03)Te / buffer undoped ZnTe / substrate p-ZnTe. The result of STS measurements on (Zn,Cr)Te clearly showed the existence of Cr impurity states at a deep position within the band-gap of the host ZnTe.

Structural analysis of Cr aggregation in ferromagnetic semiconductor (Zn,Cr)Te

The Cr aggregation in a ferromagnetic semiconductor (Zn,Cr)Te was studied by performing precise analyses using TEM and XRD of microscopic structure of the Cr-aggregated regions formed in iodine-doped Zn1−xCrxTe films with a relatively high Cr composition x ∼ 0.2. It was found that the Cr-aggregated regions are composed of Cr1−δTe nanocrystals of the hexagonal structure and these hexagonal precipitates are stacked preferentially on the (111)A plane of the zinc-blende (ZB) structure of the host ZnTe crystal with its c-axis nearly parallel to the {111}ZB plane.