Amorphous Carbon Thin Films for Optical Sensing of Humidity

Abstract: Amorphous carbon (a-C) films consist of a large amount of disordered graphite-like clusters. Edge and point-type defects of the clusters are chemical active and easily interact with water molecules from the environment. Raman spectroscopy is the powerful tool for characterizing defects and dopants in carbonaceous materials. However, the interpretation of Raman spectra of a-C is still not a trivial task. In this work, we study the chemical functionalization by investigation of the temperature-dependent behavior of Raman scattering of original, dried and water-enriched a-C films. In Raman spectra of water-enriched film the new Raman bands at approx. 1600 and 1720 cm–1 are observed that are assigned to carboxyl and carbonyl groups, respectively. The Raman spectra of the a-C films is deconvoluted into several elementary components that related to COH, COOH, C–O–C, C=O groups. The water content in the a-C films is determined by the relative contribution of the intensity of defective Raman bands to the spectrum. These results are confirmed by measurements of electrical resistance of the a-C films. The change in the electrical resistance of the a-C films is observed after multiple heating and cooling in ambient air. Water-enriched film reveal an about 20% increase in magnitude of electrical resistivity after heating and cooling cycle. That is related to the irreversible chemical doping of edge or point-type effects with intercalated water during mild annealing. The observed possibility of the a-C films to absorb water molecules can be used for developing the flexible humidity sensors and next-generation adsorbents. © 2022, Allerton Press, Inc.