Thin superconducting NbTiN and NbN films with a few nm thickness are used for various device applications including hot electron bolometer mixers. Such thin films have lower critical temperature (T c) and higher resistivity than corresponding bulk materials. To improve them, we have investigated an effect of the AlN buffer layer between the film and the substrate (quartz or soda lime glass). The AlN film is deposited by DC magnetron sputtering, and the process condition is optimized so as that the X-ray diffraction intensity from the 002 surface of Wurtzite AlN becomes highest. By use of this well-characterized buffer layer, T c and resistivity of the NbTiN film with a few nm thickness are remarkably increased and decreased, respectively, in comparison with those without the buffer layer. More importantly, the AlN buffer layer is found to be effective for NbN. With the AlN buffer layer, T c is increased from 7.3 K to 10.5 K for the 8 nm NbN film. The improvement of T c and resistivity originates Improvement of T c of NbTiN and NbN Thin Films Using the AlN Buffer Layer 2 from the good lattice matching between the 002 surface of AlN and the 111 surface of NbTiN or NbN, which makes better crystallization of the NbTiN or NbN film. This is further confirmed by the X-ray diffraction measurement.
Label-free, real-time, and in-situ measurement on cell apoptosis is highly desirable in cell biology. We propose here a design of terahertz (THz) metamaterial-based biosensor for meeting this requirement. This metamaterial consists of a planar array of five concentric subwavelength gold ring resonators on a 10 μm-thick polyimide substrate, which can sense the change of dielectric environment above the metamaterial. We employ this sensor to an oral cancer cell (SCC4) with and without cisplatin, a chemotherapy drug for cancer treatment, and find a linear relation between cell apoptosis measured by Flow Cytometry and the relative change of resonant frequencies of the metamaterial measured by THz time-domain spectroscopy. This implies that we can determine the cell apoptosis in a label-free manner. We believe that this metamaterial-based biosensor can be developed into a cheap, label-free, real-time, and in-situ detection tool, which is of significant impact on the study of cell biology.
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