A two-dimensional electron gas (2DEG) was formed at the interface of an ultrathin Al 2 O 3 /TiO 2 heterostructure that was fabricated using atomic layer deposition (ALD) at a low temperature (<300 °C) on a thermally oxidized SiO 2 /Si substrate. A high electron density (∼10 14 cm −2 ) and mobility (∼4 cm 2 V −1 s −1 ) were achieved, which are comparable to those of the epitaxial LaAlO 3 /SrTiO 3 heterostructure. An in situ resistance measurement directly demonstrated that the resistance of the heterostructure interface dropped significantly with the injection of trimethylaluminum (TMA), indicating that oxygen vacancies were formed on the TiO 2 surface during the TMA pulse in the ALD of Al 2 O 3 films, such that they provide electron donor states to generate free electrons at the interface of the ultrathin Al 2 O 3 /TiO 2 heterostructure. The activation energy of the electron donor states to move to the Ti 3d conduction band plays an essential role in the electrical characteristics of the 2DEG. Interestingly, the donor state level can be tailored by the control of TiO 2 crystallinity, which eventually adjusts the electron density. The activation energy was decreased to less than 20 meV to generate ultrashallow donor states while improving the TiO 2 crystallinity, such that the 2D electrons become readily delocalized, even at room temperature, to create a 2DEG.

Novel concepts for developing a surface-enhanced Raman scattering (SERS) sensor based on biocompatible materials offer great potential in versatile applications, including wearable and in vivo monitoring of target analytes. Here, we report a highly sensitive SERS sensor consisting of a biocompatible silk fibroin substrate with a high porosity and gold nanocracks. Our silk-based SERS detection takes advantage of strong local field enhancement in the nanoscale crack regions induced by gold nanostructures evaporated on a porous silk substrate. The SERS performance of the proposed sensor is evaluated in terms of detection limit, sensitivity, and linearity. Compared to the performance of a counterpart SERS sensor with a thin gold film, SERS results using 4-ABT analytes present that a significant improvement in the detection limit and sensitivity by more than 4 times, and a good linearity and a wide dynamic range is achieved. More interestingly, overlap is integral, and a quantitative measure of the local field enhancement is highly consistent with the experimental SERS enhancement.

In contrast to conventional surface-enhanced Raman scattering (SERS) platforms implemented on non-biological substrates, silk fibroin has the unique advantages of long-term biosafety and controllable biodegradability for in vitro and in vivo biomedical applications, as well as flexibility and process-compatibility. In this study, a silk fibroin film was developed to fabricate a flexible SERS sensor template with nanogap-rich gold nanoislands. The proposed biological SERS platform presents fairly good enhancements in detection performance such as detection limit, sensitivity, and signal-to-noise ratio. In particular, the sensitivity improvement was by more than 10 times compared to that of the counterpart sample, and an excellent spatial reproducibility of 2.8% was achieved. In addition, the near-field calculation results were consistent with the experimental results, and the effect of surface roughness of the silk substrate was investigated in a quantitative way. It is believed that biological SERS-active sensors could provide the potential for highly sensitive, cost-effective, and easily customizable nanophotonic platforms that include new capabilities for future healthcare devices.

The electronic structure and the electron-phonon couplings in a novel massproduction-compatible Al 2 O 3 /TiO 2 2D electron system (2DES) are investigated using resonant inelastic soft X-ray scattering. The experimental data from the samples of various TiO 2 thicknesses unequivocally show that the Ti 3+ state indeed exists at the deep interface to serve as an n-type dopant for the 2DES. The electronic structure of Ti 3+ species is scrutinized as entirely separated from that of the Ti 4+ host lattice. Furthermore, features of sub-eV energy loss phonon modes are clearly observed, indicating substantial electron-phonon coupling effects. Such low energy loss features are enhanced in thinner TiO 2 samples, implying that polaronic local lattice deformation is enhanced due to the presence of Ti 3+ . These findings suggest that the 2DES properties can be controlled via well-established TiO 2 engineering, thereby enthroning the binary oxide heterostructure as a promising candidate for 2DES device applications.

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