W:Al2O3 Nanocomposite Thin Films with Tunable Optical Properties Prepared by Atomic Layer Deposition

A systematic alteration in the optical properties of W:Al2O3 nanocomposite films is demonstrated by precisely varying the W cycle percentage (W%) from 0 to 100% in Al2O3 during atomic layer deposition. The direct and indirect band energies of the nanocomposite materials decrease from 5.2 to 4.2 eV and from 3.3 to 1.8 eV, respectively, by increasing the W% from 10 to 40. X-ray absorption spectroscopy reveals that, for W% < 50, W is present in both metallic and suboxide states, whereas, for W% >= 50, only metallic W is seen. This transition from dielectric to metallic character at W% similar to 50 is accompanied by an increase in the electrical and thermal conductivity and the disappearance of a dear band gap in the absorption spectrum. The density of the films increases monotonically from 3.1 g/cm(3) for pure Al2O3 to 17.1 g/cm(3) for pure W, whereas the surface roughness is greatest for the W% = 50 films. The W:Al2O3 nanocomposite films are thermally stable and show little change in optical properties upon annealing in air at 500 degrees C. These W:Al2O3 nanocomposite films show promise as selective solar absorption coatings for concentrated solar power applications.