Shigeyuki Nemoto

Excitation-wavelength-dependent photoluminescence evolution of CdSe∕ZnS nanoparticles

The wavelength-dependent photoluminescence (PL) evolution of CdSe∕ZnS (core/shell) nanoparticles (NPs) was investigated from the viewpoint of excitation wavelength dependence. In addition, the mechanisms involved in the photoinduced changes of the NP PL spectrum are discussed. The NPs of CdSe∕ZnS placed on a thin film of an insulator (GaAsOx∕GaAs or SiO2∕Si) exhibited PL decay and blueshifts on a time scale of minutes depending on the excitation wavelength, i.e., higher photon energy induces a faster PL decay with a larger and faster blueshift. The PL decay with 435nm excitation was a single exponential in temporal evolution, while the delayed acceleration of decay was observed with 365nm excitation. The excitation-wavelength dependence of the blueshift was much larger than that of the PL decay, indicating that they originate from independent photoinduced reactions. This was confirmed by an ultrahigh vacuum measurement where only PL decay was observed without any blueshift. This result clearly reveals tha...

Photoinduced reduction and pattern preservation of giant surface potential on tris(8-hydroxyquinolinato) aluminum(III) thin films

The characteristics and mechanisms of photoinduced reduction in giant surface potential (gSP) on tris(8-hydroxyquinolinato) aluminum(III) (Alq3) thin films were investigated and discussed from the results of the reduction dependences on exposure time/intensity and the preservation of photopatterned gSP. The reduction dependences are explained well with numerical-model calculation of surface potential reduction due to the drift of photoexcited carriers assuming the Poole–Frenkel formula for electron mobility. The preservation of patterned gSPs suggests the existence of deep traps involved in the carrier-drift mechanism.

Direct Measurement of Transfer Functions in Kelvin Probe Force Microscopy Using Artificially Patterned Surface Potentials

We report a direct measurement of two-dimensional (2D) transfer functions of conductive probes used in Kelvin probe force microscopy (KFM). The 2D transfer functions are obtained by measuring a well-defined step pattern of surface potentials, prepared on thin films of tris(8-hydroxyquinolinato)aluminum(III) (Alq3) by contact-mask exposure. The experimentally obtained 2D transfer functions are highly asymmetric and are spread over 50 µm. A new finding is the observation of negative values in the KFM transfer function, which cannot be explained by conventional KFM formula. The reconstruction of true surface-potential profiles by model-fitting calculation is demonstrated. The technique presented in this study, i.e., the preparation of surface-potential patterns on Alq3 thin films, is ideal for determining the KFM transfer functions experimentally.

Kelvin probe force microscope with near-field photoexcitation

We developed a combined probe microscope—a scanning probe near-field optical microscope (SNOM) combined with Kelvin probe force microscope (KFM) that uses a slim and bent optical fiber probe (S/B fiber probe). The developed SNOM-KFM system enables near-field photoexcitation through an apex of the S/B fiber probe during KFM measurement, so that the photoexcitation effects on surface potential (SP) can be measured with submicron spatial resolution. By measuring the SP of tris(8-hydroxyquinolinato) aluminum(III) (Alq3) thin films, we found that the S/B fiber probes have large negative values in the KFM transfer function. Near-field photoexcitation was performed on Alq3 thin films through the S/B fiber probes, and the spatial pattern of photoinduced reduction in SP was visualized by KFM measurement with the same probe.

Evaluation of insulator thickness through excitation-wavelength dependence of photoluminescence of CdSe∕ZnS nanocrystals

We have observed that the photoluminescence (PL) intensity for CdSe∕ZnS nanocrystals (NCs) placed on insulator films depends on excitation wavelength and insulator thickness, and thus the visualization and thickness evaluation of insulator patterns can be achieved by the PL measurement of CdSe∕ZnS NCs. On the basis of the multireflection/interference calculation, the change of the PL intensity according to the thickness/wavelength is attributed to the interference of excitation light. The evaluation of the thickness profile of the insulator pattern is demonstrated for GaAs-oxide ring patterns formed by solution etching of GaAs. The advantages of the thickness evaluation through the PL of CdSe∕ZnS NCs are simplicity in measurement and horizontal resolution up to 10 nm.