Nobuyuki Ishida

Characterization of two-dimensional hexagonal boron nitride using scanning electron and scanning helium ion microscopy

Characterization of the structural and physical properties of two-dimensional (2D) materials, such as layer number and inelastic mean free path measurements, is very important to optimize their synthesis and application. In this study, we characterize the layer number and morphology of hexagonal boron nitride (h-BN) nanosheets on a metallic substrate using field emission scanning electron microscopy (FE-SEM) and scanning helium ion microscopy (HIM). Using scanning beams of various energies, we could analyze the dependence of the intensities of secondary electrons on the thickness of the h-BN nanosheets. Based on the interaction between the scanning particles (electrons and helium ions) and h-BN nanosheets, we deduced an exponential relationship between the intensities of secondary electrons and number of layers of h-BN. With the attenuation factor of the exponential formula, we calculate the inelastic mean free path of electrons and helium ions in the h-BN nanosheets. Our results show that HIM is more sensitive and consistent than FE-SEM for characterizing the number of layers and morphology of 2D materials.

Superhydrophilic TiO2 surfaces generated by reactive oxygen treatment

The authors show that superhydrophilic TiO2 can be obtained without irradiation of the surface with ultraviolet (UV) light and concomitant excitation of electron-hole pairs. The authors demonstrate that the treatment of TiO2 surfaces with reactive oxygen species generated by air plasma removes the surface organic contaminants, leading to almost 0° contact-angle wetting of the surface. The superhydrophilicity can be explained by the positive spreading coefficient calculated using the effective surface tensions. Our results point toward UV-light irradiation as an indirect cause of the superhydrophilicity of TiO2 and support the hypothesis that this property arises from a self-cleaning effect based on the photo-oxidation and decomposition of organic contaminants at the surface.

Nazca Lines by La ordering in La2/3−xLi3xTiO3 ion-conductive perovskite

We report an unique planar defect that draws the Nazca Lines in epitaxially grown La2/3−xLi3xTiO3 (LLTO) (x ≈ 0.11) ion-conductive perovskite. Transmission electron microscopy and electron energy loss spectroscopy reveal that the lines are produced by changing the regular arrangement of alternate stacks of La-rich and La-poor layers along the c-axis into all La-rich layers near the defect. The first-principle calculation suggests that the La-rich layers should act as a barrier to Li conductivity and are, therefore, important for the application of epitaxially grown LLTO thin films.

Active voltage contrast imaging of cross-sectional surface of multilayer ceramic capacitor using helium ion microscopy

We studied active voltage contrast (AVC) imaging using helium ion microscopy (HIM). We observed secondary electron (SE) images of the cross-sectional surface of multilayer ceramic capacitors (MLCCs) with and without a voltage applied to the internal electrodes. When no voltage was applied, we obtained an image reflecting the material contrast between the Ni internal electrode region and the BaTiO3 dielectric region of the cross-sectional surface of the MLCC. When a voltage was applied, the electrical potential difference between the grounded and the positively biased internal electrodes affected the contrast (voltage contrast). Moreover, attenuation of the SE intensity from the grounded to the positively biased internal electrodes was observed in the dielectric region. Kelvin probe force microscopy (KPFM) measurements of the contact potential difference (CPD) were performed on the same sample. By using the AVC image from the HIM observation and the CPD image from the KPFM measurement, we could quantitativ...

Isotropic photo-decomposition of spherical organic polymers on rutile TiO2(110) surfaces

We observed the photo-decomposition process of polystyrene latex (PSL) spheres on a rutile TiO₂(110) single crystal surface by using atomic force microscopy. During the decomposition process, both the height and width of the PSL spheres linearly decreased with the irradiation time in a similar way from the beginning, suggesting that the PSL spheres are isotropically decomposed. This indicates that the interface between the PSL spheres and the TiO₂ surface is not a dominant reaction site, as expected from normal photocatalytic reactions.

In situ voltage-application system for active voltage contrast imaging in helium ion microscope

In this study, the authors present a new method for electrical potential observation in which helium ion microscope (HIM) observations can be performed while applying a selected voltage (up to approximately +5 V) to a sample. The in situ voltage-application system is operated in a high vacuum HIM chamber and consists of a transfer rod, tilt joint, x-, y-, and z-axis mechanisms, and two probes at the end of the transfer rod. The new system was used to acquire a secondary electron (SE) images of the cross-sectional surfaces of a multilayer ceramic capacitor (MLCC) with voltages (from 0.5 to 5 V) applied to the internal electrodes of the MLCC. The active voltage contrast corresponding to the electrical potential in the SE images could be observed when the applied voltage was less than or equal to 1.5 V. This technique enables any voltage to be applied to the samples from outside the chamber and can be used to measure the nanometer-scale electrical potential distribution of Li-ion rechargeable batteries, solar cells, etc.In this study, the authors present a new method for electrical potential observation in which helium ion microscope (HIM) observations can be performed while applying a selected voltage (up to approximately +5 V) to a sample. The in situ voltage-application system is operated in a high vacuum HIM chamber and consists of a transfer rod, tilt joint, x-, y-, and z-axis mechanisms, and two probes at the end of the transfer rod. The new system was used to acquire a secondary electron (SE) images of the cross-sectional surfaces of a multilayer ceramic capacitor (MLCC) with voltages (from 0.5 to 5 V) applied to the internal electrodes of the MLCC. The active voltage contrast corresponding to the electrical potential in the SE images could be observed when the applied voltage was less than or equal to 1.5 V. This technique enables any voltage to be applied to the samples from outside the chamber and can be used to measure the nanometer-scale electrical potential distribution of Li-ion rechargeable batteries, sola...

SPM characterization of next generation solar cells under light irradiation: Optoelectronic study from nano to macroscopic scale

Solar cells (SCs) that contain elaborate nanostructures, such as quantum dots and quantum wells, have been rigorously investigated as a way to harvest a wide range of the solar spectrum [1]. However, the energy conversion efficiency of those SCs still remains low. For the further improvement of the device performance, a much deeper understanding of the role of nanostructures in the photovoltaic conversion process is essential to gain the effective design criteria. To achieve this, local electronic properties including electrical potential, energy states, and charge distribution around the excitation centers have to be characterized under light irradiation since they govern the behavior of excited carriers. These properties have so far been indirectly deduced from macroscopic characterization such as current-voltage (I-V) measurement; however, it is not sufficient to clarify rather complicated roles of the nanostructures [2]. Thus, a direct measurement of those properties with high spatial resolution is required to understand the detailed mechanisms of the photovoltaic conversion process. To this end, we have been developing a platform for performing scanning tunneling microscopy/spectroscopy (STM/STS), atomic force microscopy (AFM), and Kelvin probe force microscopy (KPFM) working under light irradiation conditions.Here, we outline the characterization of a multiple quantum well (QW) SC based on III-V compounds that is expected to be a potential candidate of intermediate band type SC. First, we show the electrical potential measurements along the p-i-n junction of the SC using KPFM in air. Measurements were performed in open and short circuit configurations under light irradiation conditions [Fig.1]. We demonstrate that the dependence of the open circuit voltage on the intensity of light can be successfully measured by careful interpretation of the KPFM data. Second, we introduce some examples of the atomic scale characterization of the multiple QW using ultrahigh vacuum STM including the atomic arrangement, electronic states, and band profile. Also, charge accumulation at the QW is discussed based on the topographic measurement under light irradiation.jmicro;63/suppl_1/i12/DFU042F1F1DFU042F1Fig. 1.(a) Schematic illustration of measurement system of KPFM in air. (b) Effect of light irradiation on potential profile in open circuit configuration.