Characterization and formation mechanism of black Au films deposited under varied Ar gas pressures

Nanoporous black Au (B-Au) films were fabricated under varied Ar gas pressures (2, 60, and 100 Pa) via thermal evaporation. Film structures and properties were assessed with a consistent Au atom quantity of approximately 1.6 × 10 −2 mol m −2. Irrespective of the deposition pressure, all samples displayed a black appearance devoid of metallic luster, resulting in almost zero specular and diffuse reflections. The highest optical absorption, reaching 99.8%, occurred at 2 Pa deposition pressure. The film density decreased notably with increasing pressure, enabling the creation of films with a density below 1.0 g/cm 3 — equivalent to 5% of the filling factor of bulk Au — by carefully controlling the deposition pressure. FESEM observation and XRD analyses revealed gas-phase metal particle formation in the preparation of B-Au films, similar to our previous study on BAg films. In addition, when deposited on a quartz crystal microbalance (QCM), the B-Au film exhibited enhanced response of the QCM sensor to ethanol vapor. The applicability of this technology to gas sensors has been confirmed.