Electrochromic Behavior of W x Si y O z Thin Films Prepared by Reactive Magnetron Sputtering at Normal and Glancing Angles Jorge Gil-Rostra, † Manuel Cano, ‡ Jose ́ M. Pedrosa, ‡ Francisco Javier Ferrer, § Francisco García-García, Francisco Yubero, † and Agustín R. Gonza ́ lez-Elipe* ,† † Instituto de Ciencia de Materiales de Sevilla (CSIC-USE), Avenida Ame ́ rico Vespucio 49, E-41092 Sevilla, Spain ‡ Departamento de Sistemas Físicos, Químicos y Naturales, Universidad Pablo de Olavide, Ctra. Utrera km. 1, E-41013 Sevilla, Spain § Centro Nacional de Aceleradores (CSIC-USE), Thomas A. Edison 7, E-41092 Sevilla, Spain * S Supporting Information ABSTRACT: This work reports the synthesis at room temperature of transparent and colored W x Si y O z thin films by magnetron sputtering (MS) from a single cathode. The films were characterized by a large set of techniques including X-ray photoelectron spectroscopy (XPS), Rutherford backscattering spectrometry (RBS), Fourier transform infrared (FT-IR), and Raman spectroscopies. Their optical properties were determined by the analysis of the transmission and reflection spectra. It was found that both the relative amount of tungsten in the W-Si MS target and the ratio O 2 /Ar in the plasma gas were critical parameters to control the blue coloration of the films. The long-term stability of the color, attributed to the formation of a high concentration of W 5+ and W 4+ species, has been related with the formation of W-O-Si bond linkages in an amorphous network. At normal geometry (i.e., substrate surface parallel to the target) the films were rather compact, whereas they were very porous and had less tungsten content when deposited in a glancing angle configuration. In this case, they presented outstanding electrochromic properties characterized by a fast response, a high coloration, a complete reversibility after more than one thousand cycles and a relatively very low refractive index in the bleached state. KEYWORDS: electrochromic films, GLAD, W x Si y O z , reactive magnetron sputtering, colored thin films, tungsten oxide, optical properties ■ INTRODUCTION Even if tungsten oxide electrochromic thin films have been studied and manufactured for more than 30 years, 1-4 new developments are still required to improve the behavior of these materials when used for smart windows and other related applications, 5-8 both in the laboratory and at industrial scale. 9 For example, improvements are still required to properly nanostructuring the films to enhance their coloration capacity, to decrease the response time, 10-13 to apply the electrochromic devices on flexible plastic substrates 14-16 or to effectively control their optical properties. 17-19 To cope with all these requirements, new manufacturing processes and novel nano- structures have been proposed to both simplify the film processing protocols and to design more reliable final devices. A commonly admitted requisite in this regard is that the electrochromic films, prepared by sol-gel, electrochemical methods, thermal evaporation, magnetron sputtering or other related procedures, 12,13,20-23 possess a significant portion of empty and accessible space to favor the exchange of charge and the incorporation of foreign cations. An electrochromic film device based on tungsten oxide consists of a reducible WO 3 layer, another thin film electrode, and an electrolyte where there is a cation M + that becomes incorporated in the film during the reduction cycle. Typically, this cation is H + or Li + , although other alkaline cations can be also used. 24,25 For an optimal performance of the device, a fast incorporation of M + within the film and its reversible release to the electrolyte during the reduction and oxidation cycles are required, respectively (i.e., WO x +e - +M + → WO x (M) for the reduction cycle). Optimizing the electrochromic behavior implies to increase the incorporation capacity and to maximize the diffusion rate of the M + cation within the film structure. Compared to sol-gel and other chemical- or solvent-based synthetic routes, MS is a one-step process yielding directly the final formulation of the oxide that has the additional advantage of avoiding the production of undesirable byproducts. The easy scalability of this method and the possibility to work at low temperature open de way for the fabrication of electrochromic films onto polymers. However, a possible drawback of MS thin films is that, because of their generally high compactness, the diffusion rate of M + cations are slow and the switching times too long. 26-28 An alternative to the conventional MS Received: June 6, 2011 Accepted: December 30, 2011 Research Article www.acsami.org © XXXX American Chemical Society A dx.doi.org/10.1021/am2014629 | ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX