Green Color Purification in Tb 3+ Ions through Silica Inverse Opal Heterostructure Vishnu Prasad Shrivastava, † Sri Sivakumar,* ,†,‡,§,⊥ and Jitendra Kumar* ,† † Materials Science Programme, ‡ Department of Chemical Engineering, § Centre for Environmental Science and Engineering, ⊥ Thematic Unit of Excellence on Soft Nanofabrication, Indian Institute of Technology Kanpur, Kanpur 208016, India * S Supporting Information ABSTRACT: The ordered SiO 2 :Tb 3+ inverse opal heterostructure films are fabricated through polystyrene spheres hetero-opal template using the convective self-assembly method to examine their potential for color purification. Their optical properties and photoluminescence have been investigated and compared with individual single inverse opals and reference (SiO 2 :Tb 3+ powder). The heterostructures are shown to possess two broad photonic stop bands separated by an effective pass band, causing suppression of blue, orange, and red emission bands corresponding to 5 D 4 → 7 F j ; j = 6, 4, 3 transitions, respectively and an enhancement of green emission (i.e., 5 D 4 → 7 F 5 ). Although the suppression of various emission occurs because of its overlap with the photonic band gaps (PSBs), the enhancement of green radiation is observed because of its location matching with the pass band region. The Commission International de l’Eclairage (CIE) chromaticity coordinates of the emission spectrum of the heterostructure based on polystyrene sphere of 390 and 500 nm diameter are x = 0.2936, y = 0.6512 and lie closest to those of standard green color (wavelength 545 nm). In addition, a significant increase observed in luminescence lifetime for 5 D 4 level of terbium in inverse opal heterostructures vis-a ̀ -vis reference (SiO 2 :Tb 3+ powder) is attributed to the change in the effective refractive index. KEYWORDS: photonic crystal, heterostructure, inverse opal, lanthanides, spontaneous emission, color 1. INTRODUCTION Photonic crystals (PhCs) have attracted attention over the past decade because of their potential applications in waveguides, single-photon generation, low-threshold lasers, solar cells, photoelectrochemical water splitting, chemical and biosensors, etc. 1-7 Basically, their refractive index as well as dielectric constant vary periodically with spacing in the range of optical wavelengths. As a result, a forbidden/stop band is formed that prohibits certain light frequencies to pass through. 3,8 This feature can be exploited to control the spontaneous emission of an embedded light emitter in PhCs by manipulating the local photonic density of state. Several studies have been undertaken on the nature of spontaneous emission of semiconductor quantum dots, 9-11 organic dyes, 12-14 and rare-earth ions 15-20 containing PhCs. In recent years, immense interest has arisen in (i) complex PhCs, produced by introduction of crystal defects (such as point, 8,21,22 line, 23-25 or planar 26,27 ) and (ii) photonic crystal heterostructures 28-30 (termed as PhCHs) because of their potential in improving ultrahigh quality nanocavity and light-harvesting efficiency besides developing quality optical filters. 31-34 A PhCH comprises two or more photonic crystals of different lattice parameters and/or refractive index materials and characterized by two or more photonic stop bands (PSBs). 29 Furthermore, a pass band (PB) can exist in between the PSBs of adjoining PhCs. 33 The combination of PB and PSBs may tune the spontaneous emission of embedded emitter (e.g., lanthanide ions/dyes/quantum dots) at different wavelengths. 35-37 PhCHs can be fabricated as opal heterostructures (OH) and inverse opal heterostructures (IOH) but the later ones have certain advantages, e.g., wide photonic stop band, significant difference in refractive index, possibility of stop bands in all directions with a high refractive index matrix, and uniform distribution of infiltrated emitter species. 38,39 Nevertheless, opal heterostructures have been fabricated with embedded dye or CdTe crystals to study spontaneous emission characteristics. 35,40 For example, Beart et al. 35 have observed emission enhancement in pass band and suppression at stop bands region in a fluorophore (disodium fluorescein molecules) infiltrated opal heterostructure of silica spheres. Similarly, Gaponik et al. 40 found nonlinearity in emission in pass band region from CdTe nanocrystals when placed in a silica sphere-based opal heterostructure. A few reports available on the fabrication of inverse opal heterostructures include sequential vertical deposition, 28 con- vective self-assembly, 41 and layer transfer approach method. 42 But, the investigations were focused only in describing techniques for preparation of the IOHs without embedding the luminescent species. Obviously, the emission characteristics could not be studied in them. Therefore, the current focus is to report the tuning of spontaneous emission from embedded lanthanide ions in inverse opal heterostructures fabricated by an established convective self-assembly method. 41 Lanthanide ions exhibit unique properties such as sharp emission, large Stokes shift, high resistance to optical blinking, and photobleaching Received: February 19, 2015 Accepted: May 19, 2015 Published: May 19, 2015 Research Article www.acsami.org © 2015 American Chemical Society 11890 DOI: 10.1021/acsami.5b01615 ACS Appl. Mater. Interfaces 2015, 7, 11890-11899