The Properties of Evaporated Gold and Tungsten Oxides

Abstract

Gold blacks, prepared by evaporating gold from a tungsten filament in tank nitrogen at several mm pressure, were found to contain tungsten oxides which greatly affected the optical and electrical properties. Deposits containing more than 0.5 mole percent of tungsten oxides were black and showed resistivities 103 to 108 times that of bulk gold. The resistivities of gold blacks increased on exposure to oxygen. On the other hand, oxide-free deposits were yellow and possessed a low stable resistivity, unaffected by exposure to oxygen. Exposure to air or oxygen caused an increase in the electrical resistance, and optical transmission of films of tungsten dioxide and tungsten trioxide evaporated in high vacuum. It was concluded that evaporated films of pure gold are unaffected by exposure to air and that the changes in optical and electrical characteristics, often observed in evaporated gold films exposed to air, may be attributed to contamination by oxides of tungsten. It was also concluded that so-called gold blacks are black because of the presence of tungsten oxides, since gold evaporated in pressures of several millimeters of oxygen-free nitrogen formed bright reflecting films rather than black absorbing deposits.