Energy-saving transparent heat mirrors based on tungsten oxide-gold WO ₃/Au/WO ₃ multilayer structures

Transparent heat mirrors are multilayer structures that transmit visible light while reflecting infrared heat. Heat mirrors based on tungsten oxide and gold multilayers WO ₃/Au/WO ₃ were fabricated by thermal evaporation, and their performance was investigated as a function of the thickness of the gold layer. First, the properties of individual layers were investigated. Atomic force microscopy revealed that all layers possessed smooth surfaces that were suitable for optical applications. The transmittance of the gold layers was found to decrease as the thickness is increased, with an opposite trend followed by infrared reflectance. In the multilayers, the thickness of the WO ₃ was fixed at 34nm, whereas the thickness of the gold layers was varied in the range 20-44nm. X-ray photoelectron spectroscopy was used to investigate the elemental diffusion among the various layers, and it revealed the presence of inter-diffusion of elements throughout the layers. The performance of the heat mirrors was evaluated on the basis of their optical behavior. The optimum thickness of the gold layer was found to be 36nm, with a peak spectral transmittance of 84%.