Electrochromic photonic crystal displays with versatile color tunability Lijun Liu a , Siva Krishna Karuturi a , Liap Tat Su a , Qing Wang b , Alfred Iing Yoong Tok a, ⁎ a School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore, Singapore b Department of Materials Science and Engineering, NUSNNI-Nanocore, National University of Singapore, 5 Engineering Drive 2, 117576 Singapore, Singapore abstract article info Article history: Received 25 August 2011 Received in revised form 8 September 2011 Accepted 8 September 2011 Available online 16 September 2011 Keywords: Electrochromic Photonic crystals Color tunability Display devices An electrochromic photonic crystal (EPC) display device that combines chemical (electrochromic) and phys- ical (photonic) coloring mechanisms is reported for the first time. This EPC exhibits superior and versatile color tunability. The TiO 2 inverse opals fabricated by atomic layer deposition are adopted as EPC material. Results show that the photonic band gaps selectively modified the optical properties of the EPC and enabled facile tuning of electrochromic colors. In addition, the reversible photonic and photonic modified electrochro- mic coloring states with insertion/extraction of lithium ions enable novel and promising approaches for future display applications. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Electrochromism (EC) is a phenomenon in which a material changes color when an electric potential is applied to it. Several appli- cations have emerged from electrochromism, such as self-darkening rear view mirrors [1,2] and EC windows [1,3–5]. The current main challenges to advance a viable EC device are the lack of primary colors and the slow response time. These are strongly determined by the optical properties of the EC materials and the kinetics of electrochem- ical reactions. In recent years, photonic crystals (PCs) have attracted great interest due to their ability to display a full range of colors [6–8]. The color of PC displays can be tuned simply by controlling the position of photonic band gap (PBG) through changing either the effective refractive index or the lattice constant of the periodic media. V 2 O 5 inverse opal has re- cently been reported for its significantly fast switching times and im- proved coloration contrasts. This is attributed to the interconnected and fully accessible macroporous structure, which greatly facilitates the penetration of electrolyte into the structure, thereby shortening the diffusion length and improving the response time [9]. In this paper, high quality TiO 2 inverse opals (TIOs) of different lat- tice sizes were prepared by atomic layer deposition (ALD). These in- verse opal PCs were further used as electrochromic materials to facilitate facile color tunability through selective overlapping of pho- tonic stop bands with their optical absorptions. This is the first time that an EPC display that combines chemical and physical mechanisms of coloring was adopted to accomplish the color tunability. This novel EPC is simple because it does not rely on either the change of the elec- tronic structure of EC materials, or the use of color filters to realize dif- ferent colors. It also eliminates the requirement to reversibly change either the refractive index or the lattice size of the photonic crystals for displaying reversible color states. 2. Experimental Opals prepared by 288 nm and 424 nm polystyrene spheres were used as template to fabricate TIOs (TIO288 and TIO424). ALD was used for TiO 2 deposition in stop-flow process at 70 °C [10,11]. The 10 μm layer of nanocrystalline TiO 2 (Nano-TiO 2 ) electrode was depos- ited on fluorine-doped tin-oxide (FTO) coated glass by using TiO 2 paste (Dyesol 18NR-T) [12]. Field-emission scanning electron micro- scope (JEOL, JSM-6340F) was used to investigate the surface morphol- ogy of TIOs. Cyclic voltammetric measurements of the electrodes were carried out using an electrochemical workstation (Ivium Compactstat) at a scan rate of 1 mV/s, with lithium foil as both the counter and ref- erence electrodes, and 1 M LiPF 6 dissolved in ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio of 1:1 as the elec- trolyte. Spectroelectrochemical measurements were conducted by discharging TiO 2 electrodes to a targeted potential at a constant cur- rent density of ~ 0.1 mA/cm 2 and holding at that potential for 1 h. The absorption and reflectance spectra were collected by using a spec- trophotometer (Varian, Cary 5000). 3. Results and discussion FESEM images of TIOs are shown in Fig. 1. These clearly show that TIOs have spherical pores arranged in ordered hexagonal arrays. The Electrochemistry Communications 13 (2011) 1163–1165 ⁎ Corresponding author. Tel./fax: + 65 67904935. E-mail address: miytok@ntu.edu.sg (A.I.Y. Tok). 1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2011.09.007 Contents lists available at SciVerse ScienceDirect Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom