Significant Enhancement in Visible Light Absorption of TiO₂ Nanotube Arrays by Surface Band Gap Tuning

Band gap tuning of the wide band gap semiconductor, TiO₂, has great importance due to its versatile properties in solar cell applications. Visible light activity of TiO₂ can enhance the efficiency of the third-generation solar cells by acting itself as light absorbing material. In this study, we demonstrate a surface structure modification and band gap tuning of TiO₂ nanotube arrays (TNTA) by anodization accompanied by a short-term, quick annealing process. This TNTA shows absorption both in the UV and entire visible range (400–700 nm, an increase by 350%). The effective band gap is found to be 1.7 eV. Through a detailed analysis we show that the significantly enhanced visible light absorption in the entire visible range is due to the substitutional and interstitial carbon atoms on the surface which introduces a structural disorder and localized states at the surface compared to the bulk. Based on the results from photoemission spectra, the probable band gap modeling shows a band bending at the surface. This behavior points to the excessive electronic conduction at the surface which has both merits and demerits in the applications of TNTAs in photocatalytic and photovoltaics in terms of surface recombination. This is confirmed by a solar cell device fabrication and testing

Characteristics of layered tin disulfide deposited by atomic layer deposition with H2S annealing

Tin disulfide (SnS2) has attracted much attention as a two-dimensional (2D) material. A high-quality, low-temperature process for producing 2D materials is required for future electronic devices. Here, we investigate tin disulfide (SnS2) layers deposited via atomic layer deposition (ALD) using tetrakis(dimethylamino)tin (TDMASn) as a Sn precursor and H2S gas as a sulfur source at low temperature (150° C). The crystallinity of SnS2 was improved by H2S gas annealing. We carried out H2S gas annealing at various conditions (250° C, 300° C, 350° C, and using a three-step method). Angle-resolved X-ray photoelectron spectroscopy (ARXPS) results revealed the valence state corresponding to Sn4+ and S2- in the SnS2 annealed with H2S gas. The SnS2 annealed with H2S gas had a hexagonal structure, as measured via X-ray diffraction (XRD) and the clearly out-of-plane (A1g) mode in Raman spectroscopy. The crystallinity of SnS2 was improved after H2S annealing and was confirmed using the XRD full-width at half-maximum (FWHM). In addition, high-resolution transmission electron microscopy (HR-TEM) images indicated a clear layered structure

Ultimate terahertz field enhancement of single nanoslits

A single metallic slit is the simplest plasmonic structure for basic physical understanding of electromagnetic field confinement. By reducing the gap size, the field enhancement is expected to first go up and then go down when the gap width becomes subnanometer because of the quantum tunneling effects. A fundamental question is whether we reach the classical limit of field enhancement before entering the quantum regime, i.e., whether the quantum effects undercut the highest field enhancement classically possible. Here, by performing terahertz time domain spectroscopy on single slits of widths varying from 1.5 nm to 50 mu m, we show that ultimate field enhancement determined by the wavelength of light and film thickness can be reached before we hit the quantum regime. Our paper paves way toward designing a quantum plasmonic system with maximum control yet without sacrificing the classical field enhancements

Resonance tuning of electric field enhancement of nanogaps

We study the electric near-field enhancement of a metallic nanogap by far-field transmission measurement in the 0.6-2.3 mu m wavelength range. The electric field is resonantly enhanced at the gap and the enhancement factor is quantified experimentally. The resonance condition of field enhancement can be controlled to various wavelengths by changing the gap size, which is confirmed by theoretical calculation using a mode expansion method