A proposal of chemical sensor based on polycrystalline Cu2O nanofilm

This talk is addressed to describe the use of thermal evaporation of transition metals to fabricate metal thin films. Film thickness control is precisely achieved through modulation of the Z-position (metal source-to-substrate distance). Then, the metal grain size ) is strongly correlated with the film thickness (ω), being described by a power law (), where n is found to be approximately 0.4 for copper films. Measurements reveal a resistivity range from 3.3 to 4.6 µΩ´cm for the as-fabricated copper films, aligning closely with expectations for copper films composed of crystallites sized between 20 and 26 nm, which is consistent with the grain sizes obtained from x-ray diffraction (XRD) data. In order to produce cuprite (Cu2O) thin films, thermal annealing process was carried out at 200°C in air for 20 hours. XRD data analysis indicates mean crystallite sizes varying from ~9 nm up to ~24 nm as the film thickness increases. The film thicknesses ranging from ~ 77 nm to ~ 434 nm were estimated from optical transmittance data analysis. The optical bandgap showed a film thickness dependence, which varies monotonically from 2.31 eV to 2.17 eV as the film thickness increases. The bandgap opens with the decrease of the film thickness, this behavior being assigned to quantum confinement effect, due to the grain size reduction. Moreover, the refractive index and extinction coefficient also showed a film-thickness dependence. The latter was evidenced by the linear dependence between the refractive index and the charge carrier density. Electrical measurements indicate p-type semiconductors with carrier concentration of ~1014 cm-3, which shows a slight decrease as the film thickness increases. In addition, it was observed a decreasing trend of the resistivity with reduction of the cuprite film thickness. Finally, testing on gas sensors demonstrated a response to ethanol gas at room temperature, with the thinner films being more sensitive to the gas detection than the thicker ones. This finding was related to the reduced mean grain size in the former. It is expected that this talk can stimulate the interest in gas sensors with high response and able to operate at room temperature in portable devices, in particular for breath ethanol testing.