Spectroscopic analysis of tungsten oxide thin films

Abstract

We present a detailed study of the morphology and composition of tungsten oxide (WO3) thin films, grown by radio frequency magnetron reactive sputtering at substrate temperatures varied from room temperature (RT) to 500 °C, using infrared (IR) absorption, Raman spectroscopy, and x-ray photoelectron spectroscopy (XPS). This work includes valuable new far-IR results about structural changes in microcrystalline WO3. Both IR absorption and Raman techniques reveal an amorphous sample grown at RT and initial crystallization into monoclinic structures for samples grown at temperatures between 100 and 300 °C. The Raman spectra of the samples grown at high temperatures indicate, apart from the monoclinic structure, a strain effect, with a distribution revealed by confocal Raman mapping. XPS indicates that the film surface maintains the stoichiometry WOx, with a value of x slightly greater than 3 at RT due to oxygen contamination, which decreases with increasing temperature.

Comparative microscopic and spectroscopic analysis of temperature-dependent growth of WO3 and W0.95Ti0.05O3 thin films

“…These results reveal, for the WO 3 samples, main O1s peaks at 529.6 eV for the sample grown at a substrate temperature of 300°C, and at 529.3 eV for the sample grown at a substrate temperature of 500°C, plus a weak feature at 532.4 eV, which is attributed to surface oxygen contamination. With the temperature varying between RT and 500°C, there is also a slight shift of about 0.5 eV towards lower binding energies in the position of the O1s feature for WO 3 at 530.1 eV. While this binding energy falls between those of the main O1s peaks of the WO 3 samples and that observed in the literature for TiO 2 , it is worth pointing out that its position is closer to the one observed for TiO 2 [24].…”

“…These confocal Raman, infrared absorption, and XPS results indicate structural changes of WO 3 films from an amorphous phase to a monoclinic structure [3]. Since an important question to consider is the degree to which doping can affect the porosity of the WO 3 film structure and/or its chemistry, which can have an influence on the effective surface area and ultimately can determine the number of surface active sites for oxygen adsorption (our previously reported SEM and XRD measurements on the 5 % doped WO 3 [1,2] revealed explicit structural changes for W 0.95 Ti 0.05 O 3 thin films), an analysis of these samples at a molecular level could bring additional valuable insights.…”

“…The outcomes of the current work, together with our previously reported results obtained by scanning electron microscopy (SEM) and X-ray diffraction (XRD) [1][2][3], provide a detailed and complete characterization of the materials under study and, consequently, of their future potential technical applicability. In addition, since the effect of crystallite size on the physical and chemical properties of solids plays a crucial role in the ongoing quest to improve and optimize material properties, and in view of the fact that there is increased interest in W-Ti-O thin films as new sensing materials, this study demonstrates the value of applying sensitive techniques for observing small structural modifications and for providing useful information on the inherent complexity of Ti-doped WO 3 samples, which have structural properties that are far from being completely clarified in the literature.…”

“…The optical and surface spectroscopic measurements that we performed on WO 3 samples made essential contributions to confirming morphological changes for different substrate temperatures, enabling us to optimize the growth conditions [3]. These confocal Raman, infrared absorption, and XPS results indicate structural changes of WO 3 films from an amorphous phase to a monoclinic structure [3].…”

“…Although pure WO 3 is a recognized candidate for gasochromic, electrochromic, and photochromic devices, its morphological characteristics are strongly dependent on the conditions and methods used in its fabrication, as revealed in the literature [1][2][3][4][5][6][7][8][9][10]. An improvement in its gasochromic detection sensitivity and in its increased temperature range of operation as a sensor, which for undoped thin films of WO 3 grown by radio-frequency (RF) magnetron sputtering are achieved at a deposition temperature of *200°C [9,10], as well as in its electrochromic and photocatalytic properties, has been achieved by doping with appropriate metals or by addition of metal/metal oxide additives [7][8][9][10][11][12][13][14][15][16][17][18].…”