We compare the results of surface enhanced Raman scattering (SERS) and infrared reflection absorption spectroscopy (IRRAS) of carbon dioxide (CO2) on cold-deposited copper films. The SERS spectra of CO2 on copper films deposited at 40 K display neutral species at SERS active sites with bands not observed by Raman spectroscopy of CO2 gas, but identical to the loss bands of gaseous CO2 in electron energy loss spectroscopy. The absence of one component of the Fermi doublet of CO2 in SERS proves that the local electromagnetic field enhancement at SERS active sites cannot deliver signals above the noise level. The activated anionic CO2 − is observed by transient electron transfer from the anionic molecule to the copper metal at a subgroup of SERS active sites, which are annealed below 200 K. The IRRAS spectra show only the expected infrared (IR) active modes of neutral CO2 representing the “majority species” of adsorbed CO2.

Surface enhanced Raman spectroscopy (SERS) of NO on cold-deposited Cu yields only bands of the dissociation products of NO, but not the stretching bands of NO. In contrast, infrared reflection-absorption spectroscopy (IRRAS) on similarly prepared samples displays strong NO stretching bands and only a very weak band of N 2 O from multilayer NO. The differences can be explained neither by thermal or photodesorption nor by photochemical effects in the laser focus. Whereas the SERS results may be explained by the action of special sites, it is still unclear why the IRRAS signal from the dissociation products of NO at the 'catalytically active sites' is below the noise level.

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Metals are known to grow three-dimensionally on clean ionic crystal surfaces with low surface energies. As a consequence, the naturally grown films consist of islands and are non-conductive up to the percolation threshold, which is at ∼1 nm for iron films grown on MgO(001) at room temperature and is much larger for noble metals and higher temperatures, respectively. In this paper we show that CO exposure at ∼10 −8 mbar during deposition modifies the growth on MgO(001) for iron and copper. The development of infrared transmission spectra versus metal film thickness clearly is changed by CO exposure. The spectral differences correspond to a significant percolation threshold decrease induced by CO. Infrared conductivity parameters calculated from transmittance spectra indicate that the surface relaxation of the free charge carriers is decreased due to CO exposure during growth but the plasma frequency is unchanged.

We present vanadium dioxide (VO2) thin films having high resistivity contrast with silicon substrates through use of crystallized alumina (Al2O3) buffer layers, engineered for this purpose. We first optimized the process by depositing VO2 onto C-plane sapphire substrates prior to alumina thin films. The latter of which were grown via atomic layer deposition on silicon substrates. We then applied rapid thermal annealing (RTA) to crystallize the alumina films. Scanning electron microscopy results indicated a thickness of 107 nm for each VO2 film, which yielded hot–cold resistivity contrast ratios of 9.76 × 104, 1.46 × 104, and 3.66 × 103, when deposited on the C-plane sapphire, the annealed buffers, and the as-deposited alumina buffers, respectively. Atomic force microscopy of the film surface roughness of the VO2 films indicated root mean squared roughness (Rq) of 4.56 nm, 6.79 nm, and 3.30 nm, respectively, for the films grown on the C-plane sapphire, annealed buffers, and as-deposited buffers. Finally, x-ray diffraction (XRD) of the VO2 films indicated the desired composition and strong (0h0)/(00h) texturing, when deposited on both the C-plane sapphire and the annealed alumina buffer layers. XRD results indicated a series of peaks corresponding to the α-Al2O3/C-plane sapphire, and an XRD analysis of the buffers alone confirmed crystallization of the buffer layer via RTA. The process defined in this paper produced a series of highly textured VO2 films making them most valuable for the integration of VO2 with silicon-based devices.

Infrared (IR) and surface-enhanced Raman spectroscopy (SERS) of similarly prepared samples show different results that cannot be explained by different selection Tules. One main difference is the missing of a signal ofNO in the SERS spectra, only reaction products like N, and N,O could be seen, whereas in IR clearly NO vibrations were detected. IR spectra were hken to investigate the influence of additional heat and additional photons during the measurement. Besides desorption, no influence like increase of the reaction products N2 or N20 could be found. It seems to us that a dishlrbance of the investigated system due to Laser irradiation during SERS is not the explanation for the different findings.

This paper presents an antenna-coupled non-linear vanadium dioxide (VO2) microbolometer operating in the non-linear metal–insulator transition (MIT) region with an ultra-high responsivity of 6.55 × 104 V/W. Sputtered VO2 films used in this device exhibit 104 times change in resistivity between the dielectric and conductive states. The VO2 microbolometer is coupled to a wideband dipole antenna operating at 31–55 GHz and a coplanar waveguide for probed measurement. To enhance the sensitivity, the sensor is suspended in air by micro-electro-mechanical systems process. The large thermal coefficient of resistance of VO2 is utilized by DC biasing the device in the MIT region. Measurements for the fabricated sensor were performed, and a high responsivity was demonstrated, owing to non-linear conductivity change in the transition region. The measured sensitivity is >102 times higher than the state-of-the-art sensors. In addition, the concept of utilizing the proposed VO2 sensor in a mmWave imager was demonstrated by the radiation pattern measurement of a 4 × 4 (16 elements) antenna-coupled VO2 sensor array. The results presented in this work reveal the initial step to employ VO2's MIT for a hyper-sensitive sensor in future mmWave sensing and imaging applications.

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