A goal in the characterization of supported metal catalysts is to achieve particle-by-particle analysis of the charge state strongly correlated with the catalytic activity. Here, we demonstrate the direct identification of the charge state of individual platinum nanoparticles (NPs) supported on titanium dioxide using ultrahigh sensitivity and precision electron holography. Sophisticated phase-shift analysis for the part of the NPs protruding into the vacuum visualized slight potential changes around individual platinum NPs. The analysis revealed the number (only one to six electrons) and sense (positive or negative) of the charge per platinum NP. The underlying mechanism of platinum charging is explained by the work function differences between platinum and titanium dioxide (depending on the orientation relationship and lattice distortion) and by first-principles calculations in terms of the charge transfer processes.

We have successfully developed a transmission-type GaAs/GaAsP strained superlattice (SL) photocathode, and a high spin-polarization (SP) (90%) with a super-high brightness (~10 7 A⋅cm −2 ⋅sr −1 ) of electron beam was achieved [1]. In this study, we report the design and fabrication of an optimized transmission-type photocathode with strain-compensated SL for higher quantum efficiency (QE).In the GaAs/GaAsP strained SL, a compressive strain was introduced in the GaAs well layers to obtain a large band-splitting between heavy-hole and light-hole mini-bands. The increasing SL pair-number causes strain relaxation with resultant SP degradation. A smaller SL layer thickness is one reason behind the limited value of the QE. To overcome this problem by increasing the SL layer thickness without degradation, the use of strain-compensated SL was proposed [2]. In this structure, a strain is introduced in the SL barrier layers to the opposite direction to compensate the strain in the SL well layers. Figure 1 shows the GaAs/GaAsP strain-compensated SL structure. The maximum pair of the prepared SL is 90.X-ray diffraction revealed that the strain relaxation by thickness increase was effectively controlled. Figure 2 shows the change of maximal spin-polarization with the SL pair number. The superlattice photocathodes up to 36-pair maintain high SP of about 90%. Then, the SP obviously decreased. During the transport, the spin-polarized electrons should flip by scattering with holes. The scattering effect becomes stronger in the thicker SL photocathodes. The thickness effect on the QE and transport time will be investigated.

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For understanding of carrier behavior in semiconductors, it is important to measure the carrier relaxation time. In the present study, the relaxation times of inter-valley transition from the Γ valley to the X valley in GaP were evaluated by near-band-gap photoemission spectroscopy of electrons emitted from a surface with a negative electron affinity state. In the energy distribution curves, two peaks, which originate from the electron population accumulated in the Γ valley and the X valley, were observed. From the temperature dependence of the energy of these two peaks, we could successfully evaluate the temperature dependence of the energies of the Γ valley and the X valley. Furthermore, the relaxation times of the inter-valley transition from the Γ valley to the X valley were estimated from the ratio of the electron concentration of the Γ valley and the X valley. The values of the relaxation time are good agreement with the previous studies. These results indicate that the near-band-gap photoemission spectroscopy can directly investigate conduction electrons and also evaluate the carrier dynamics in semiconductor.

We developed an angle-resolved photoemission spectroscopy system for the analysis of conduction-band electrons. By forming a negative electron affinity surface on a semiconductor surface, electrons in conduction bands are emitted into a vacuum and measured by using an analyzer. This method enables us to determine the energy and momentum of the conduction electrons. Furthermore, it can be used to determine unoccupied conduction band structures. The main challenges of this method are that the energies of the emitted electrons are extremely low and the trajectories of the electrons change due to various influences. We overcame these problems by placing the shielding mesh close to the sample and parallel to the sample surface. The entire chambers, including the shielding mesh, were grounded, and a negative bias voltage was applied only to the sample. This configuration realizes the acceleration of electrons while preserving the momentum component parallel to the sample surface. Another problem is the establishment of a method for converting a detected angle into the corresponding wavevector. We focused on the emission angle of electrons emitted from a sample and their minimum energy and then established an analytical method for converting detected angles into corresponding wavevectors on the basis of the minimum energy.