Structure and electrochromic properties of tungsten oxide films

Abstract

The aim of this project is to investigate the compositions, structure, mechanical and electronchromic properties, and their correlations of tungsten oxide (WO3) films deposited by magnetron sputtering and thermal evaporation at various conditions. Tungsten oxide films with a board spectrum of structures, ranging from porous, amorphous and polycrystalline states were fabricated using the two techniques. For thermally evaporated films deposited at high oxygen partial pressure (Po2), the film structure is amorphous and highly porous (40% porosity). The density is as low as 4.5g cm-3 which is just 60% of that of the crystalline WO3. The oxygen-to-tungsten ration is close to three. This indicates that the atoms in the films are arranged to form a porous of WO3-like network. When Po2 drops, the film structure becomes less porous, and the density rises to 6.9 g cm-3. For magnetron-sputtered samples, the deposition process proceeded in atomic scale, so that the film was less porous and the density is high. Without any substrate heating, the film density was drastically risen to 7.7 g cm-3. Under the observation of atomic force microscope, the topological image of the film surface was found to be smoother compared to the thermally evaporated film. When substrate temperature was further increased to 300 C, X-ray diffraction data showed that the film starts to crystallize, and the grain size grew with increasing substrate temperature. With the structure change mentioned above, it was found that after 10000 coloring/bleaching cycles, the magnetron-sputtered film worked very well. However, the electrochromic properties of the thermally evaporated film degraded quickly after 1000 cycles. The coloration efficiency of the former was at least two times larger than that of the later. Since the structure of the thermally evaporated sample contained more pores, such that it was less stable under the actions of intercalation and deintercalation of ions. As a result, that the durability of the performance of electrochromism of the thermally evaporated film was inferior to that of magnetron sputtered film. Associated with the structural change, the hardness of the thermally evaporated film deposited at Po2 of 10 mTorr is as low as 0.96 GPa. When Po2 decreases to the high vacuum background pressure, the hardness rises to 2.94 GPa, where the film structure still remains to be amorphous, but contains fewer pores. For the sputtered film on unheated substrate with no crystallization, a great portion of defects are removed, and the hardness finally reaches 5.57 GPa. For further increase in substrate temperature, the film is crystallized. However, the crystallized structure does not contribute to the hardness of the film. Another important finding is that ion sputtering in XPS depth profile analysis leads to preferential sputtering of oxygen from tungsten oxide films. This effect alters the original film composition, such that the O/W ratio drops significantly. Meanwhile, due to the knocking out of oxygen atoms the WV, WIV, WIII, WII states and metallic tungsten state appear in the W4f photoelectron spectrum. Results of angle-resolved(45 and normal) XPS analysis also show that after experiencing ion irradiation the oxygen content near the sputtered surface is small, further supporting the conjecture that some oxygen is preferentially sputtered by the ion beam.