Akira Ikeuchi

Broadband, polarization-sensitive photodetector based on optically-thick films of macroscopically long, dense, and aligned carbon nanotubes.

Increasing performance demands on photodetectors and solar cells require the development of entirely new materials and technological approaches. We report on the fabrication and optoelectronic characterization of a photodetector based on optically-thick films of dense, aligned, and macroscopically long single-wall carbon nanotubes. The photodetector exhibits broadband response from the visible to the mid-infrared under global illumination, with a response time less than 32 μs. Scanning photocurrent microscopy indicates that the signal originates at the contact edges, with an amplitude and width that can be tailored by choosing different contact metals. A theoretical model demonstrates the photothermoelectric origin of the photoresponse due to gradients in the nanotube Seebeck coefficient near the contacts. The experimental and theoretical results open a new path for the realization of optoelectronic devices based on three-dimensionally organized nanotubes.

Holders for in situ treatments of scanning tunneling microscopy tips

We have developed holders for scanning tunneling microscopy tips that can be used for in situ treatments of the tips, such as electron bombardment (EB) heating, ion sputtering, and the coating of magnetic materials. The holders can be readily installed into the transfer paths and do not require any special type of base stages. Scanning electron microscopy is used to characterize the tip apex after EB heating. Also, spin-polarized scanning tunneling spectroscopy using an Fe coated W tip on the Cr(001) single crystal surface is performed in order to confirm both the capability of heating a tip up to about 2200 K and the spin sensitivity of the magnetically coated tip.

Correlation between OH density and Fe electronic states of H/Fe3O4(001) film surfaces studied by scanning tunneling microscopy/spectroscopy

We report two types of adsorption structures in H/Fe3O4(001) film surfaces and the correlation between OH density and Fe electronic states, which have been studied by scanning tunneling microscopy/spectroscopy (STM/STS). Two types of bright protrusions (BPs), whose lengths along the atomic rows are different, are observed in the STM images. The shorter and longer BPs consist of Fe atoms with one and with two OH groups neighbor, respectively. In addition, STS measurements show the higher local density of states (LDOS) just below the Fermi level of Fe atoms with increasing neighboring OH groups. The variation can be attributed to the difference in the gain of electrons from H atoms, which is due to the difference in the number of neighboring OH groups. These results reveal that surface OH density is a factor for determining the LDOS just below the Fermi level of surface Fe atoms.

Atomically Resolved Observations of Antiphase Domain Boundaries in Epitaxial Fe3O4 Films on MgO(001) by Scanning Tunneling Microscopy

We have studied the surface atomic configurations around antiphase domain boundaries (APBs) in epitaxial magnetite (Fe3O4) thin films on MgO(001) by scanning tunneling microscopy (STM). The observed surface of the Fe3O4 films is the B-plane terminating surface with the (√2×√2)R45° reconstruction. Several variations of APBs are observed by STM at atomic resolution. The observed APBs are categorized into a APBs labeled by three different phase shift vectors: in-plane 1/4[110], in-plane 1/2[100], and out-of-plane 1/4[101]. We discussed how these APBs appear on the surface. The proportions of the APBs with 1/4[110], 1/2[100], and 1/4[101] shifts are about 38, 1, and 61%, respectively, in our experiment.

Direct observation of subsurface charge ordering in Fe3O4(001) by scanning tunneling microscopy/spectroscopy

In this study, we characterized Fe3O4(001) films using scanning tunneling microscopy/spectroscopy (STM/STS). On the film surfaces, individual iron atoms and ()R45° reconstructed structures were observed by STM. The STS results showed that the local density of states just below the Fermi level was higher in narrow sections than in wide sections of the surface reconstruction perpendicular to the iron rows. Periodic density of states modulations reproducing this electronic structure were clearly observed in the differential tunneling conductance map. These experimental results revealed the presence of subsurface charge ordering of Fe2+–Fe2+ and Fe3+–Fe3+ dimers, as proposed in previous density functional theory studies.

Noncontact Atomic Force Microscopy Observation of Fe3O4(001) Surface

Fe3O4 is one of the important oxide materials and its surface structure should be well understood to enable application of this material. We report the first noncontact atomic force microscopy (NC-AFM) results for Fe3O4(001) thin films. The observed films were grown homoepitaxially on magnetite thin films substrate. A low-energy electron diffraction pattern shows the well-known (√2×√2)R45° reconstructed structure. The observed minimum step height is 0.21 nm, corresponding to the distance between the same planes. We obtain two types of atomic-scale NC-AFM images. One image shows bright protrusions along the [100] and [010] directions at intervals of 0.84 nm corresponding to a unit cell of the (√2×√2)R45° reconstructed structure. The other image shows a more detailed atomic structure with 0.6 and 0.3 nm corrugations.