Field enhancement of semiconductors is highly dependent on the wavelength of light. The wavelength dependence is particularly promising for light− matter interaction applications, such as sensors, photocatalysts, and solar cells, as well as to achieve an understanding of plasmon-free field enhancement. However, there are few reports on the field enhancement of two-dimensional (2D) semiconductors and no reports on the wavelength dependence to date. Here, we present the field enhancement of tungsten disulfide (WS 2 ) for a 2D layered semiconductor. The enhancement factor (EF) of a single WS 2 flake was evaluated by using confocal fluorescence microspectroscopy. The wavelength dependence was investigated both by experiment and with theoretical calculations for the range from the visible to the near-infrared (NIR) region (400−2000 nm). As a result, the EFs for a single WS 2 flake were visualized as maps upon six different excitation wavelengths. The maps were compared with maps of electric fields from finite-difference time-domain calculations, based on the x−y−z position of a single WS 2 flake at the same position measured by using atomic force microscopy. The EF in the NIR region showed the largest enhancement, and the maximum EF was demonstrated at a specific number of layers (150 layers) as EF = 100.

Titanium dioxide (TiO 2 ) is a crucial semiconductor for photocatalysts, solar cells, hydrogen evolution reactions, and antivirus agents. The properties and performances of these applications can improve significantly if the integrated TiO 2 acts as a light harvester through a large field enhancement. This study investigates the electromagnetic field enhancement of a nanogap-rich TiO 2 film with a large area, prepared by a facile dry process at room temperature. Herein, the loading pressure is applied to the TiO 2 particles for closely packing them in the film. The field enhancement, as a function of the loading pressure, is explored from the fluorescence intensity enhancement of a dye molecule. An average enhancement factor >2000 is achieved, which is a remarkable record for semiconductors. Furthermore, the reproducibility is significant; the relative standard deviation value is small (∼4%). Calculations were performed using the finite-difference-time-domain method. A nanogap of 5 nm yields the highest EF for triangular-prism TiO 2 particles.

Electron-stimulated desorption (ESD) of positive ions from weakly physisorbed molecules has been investigated. From methanol adsorbed on a solid Ar substrate, the protonated cluster ions of the type H+(CH3OH) n(n = 1 - 4) are emitted, together with the fragment ions such as [Formula: see text](n = 0 - 3), H3O+, CHO+, CH3O+, etc. The yields of these ions are markedly enhanced at the smallest coverage and decay steeply with increasing coverage. Coulomb explosion between valence holes confined in adsorbed nanoclusters is responsible for the enhanced ion yields. Very few ions except for H+ are emitted from a thick layer as well as nanoclusters adsorbed directly on a metal substrate due to the delocalization of valence holes.

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