Thin graphite membranes for laser photoacoustic spectroscopy

The mechanical behavior of thin suspended graphite membranes (thickness of about 100 nm) has been investigated by a method utilizing Laser Photoacoustic Absorption Spectroscopy (PAS). Membranes prepared by multiple mechanical cleavage of highly ordered pyrolytic graphite (HOPG) were attached to a glass window with circular opening (diameter 4 mm). The movement of the membranes was induced by pressure changes inside a photoacoustic cell and detected via a laser beam reflected from the membranes onto a position sensitive detector. The movement of graphite membranes has been compared to the movement of a membrane of a condenser microphone. Several membranes have been tested, differing in method of preparation and/or in design. The pressure induced movement of the membranes is influenced by mechanical properties of the membranes itself, as well as by the acoustic properties of the photoacoustic cell. The free volume on the sides of the membranes had the most significant influence on its movement. On the other hand, metallization by thin layer of gold (70 nm) had negligible effect. The sensitivity of membranes utilized as an optical microphone employed in PAS method has been tested. Using methanol vapor as testing gas, we have found that the sensitivity of the membranes is comparable with the sensitivity of a top class microphone and is expected to be further enhanced by reducing the thickness of the membrane, by increasing its diameter or different (e.g. cantilever like) geometry.