Electronic and optical properties of tungsten oxide related materials and first-principles theory of electrochromism
Tungsten trioxide WO 3 is an interesting semiconductor with a wide-range of potential applications. One important property of WO 3 is its electrochromic behavior, which has generated significant research interest. Electrochromic materials exhibit reversible and persistent changes of the optical properties, hence their color, upon applying an electrical pulse. The applications of the electrochromic WO 3 range from information display, light shutters, to energy efficient smart windows. Although there are many materials that exhibit electrochromic behavior, tungsten trioxide is one of the most extensively studied ones due to its superior coloration efficiency, short response time and reversibility. Enhanced electrochromic properties in WO 3 nanowires have been reported recently. Despite much research effort, a first-principles theory for the coloration mechanism in this material has not emerged. In this work, we establish a first-principles theory for the coloration mechanism in Na x WO x , which is also able to explain the electrochromism in WO 3 . Chapter 1 gives a brief introduction to electrochromism in WO 3 and related materials. In Chapter 2, we summarize the theories and computational methods used in this work including the local density approximation (LDA) within density functional theory (DFT), pseudopotential planewave formalism and the GW approximation. We study the crystal and electronic structures of WO 3 in Chapter 3. WO 3 has a basic octahedron structure. From -140 ∼ 830°C, the crystal structure changes from monoclinic to triclinic, again monoclicnic, then successively orthorhombic, tetragonal, and again tetragonal. Several groups have investigated the electronic structure of WO 3 within DFT, but the band gap is severely underestimated compared with experiment. We have carried out quasiparticle calculations within the GW approximation. The calculated band gap is much closer to experimental results. Chapter 4 and Chapter 5 discuss the optical properties and coloration mechanism of WO 3 upon charge insertion. The calculated dielectric functions, reflectance, transmission and absorption coefficient agree very well with experiments. Our results explain the systematic change in color of Na 3 WO 3 from blue to golden-yellow with increasing sodium concentration x . We find that proper accounts for the free-carriers contribution to the optical response are critical for a quantitative understanding of the coloration mechanism in this system. Besides WO 3 , we have studied another "smart material", VO 2 . The results are reported in chapter 6. The most interesting property of VO 2 is its metal-insulator transition (MIT) at T c =340 K. The crystal structure changes from a high-temperature rutile phase to a low-temperature monoclinic phase at T c . The MIT in VO 2 has led to many practical applications such as thermocoatings, optical switching devices etc. However, it has long been a controversial issue regarding the mechanism behind the MIT. It is still not clear whether the insulating behavior is driven by the electron correlation or structural distortions. In this work, we perform first-principles electronic structure calculation using both LDA and LDAU method. It is found that the correlation effect is very important to explain the insulating phase of VO 2 . However, correlation effects alone cannot help open a band gap for the insulating phase of VO 2 . Structural distortion also plays an important role. It seems that it is the subtle interplay between the electron-electron correlation and electron-lattice interaction that ultimately drives the development of an insulating gap.