Kumam et al. Nanoscale Research Letters (2015) 10:347 DOI 10.1186/s11671-015-1053-6 NANO IDEA Open Access Enhancement of luminescence in white emitting strontium fluoride core @ calcium fluoride shell nanoparticles Nandini Kumam, Ningthoujam Premananda Singh, Laishram Priyobarta Singh and Sri Krishna Srivastava* Abstract Synthesis of lanthanide-doped fluoride SrF2:3Dy and SrF2:3Dy@CaF2 nanoparticles with different ratios of core to shell (1:0.5, 1:1 and 1:2) has been carried out by employing ethylene glycol route. X-ray diffraction (XRD) patterns reveal that the structure of the prepared nanoparticles was of cubical shape, which is also evident in TEM images. The size of the nanoparticles for core (SrF2:3Dy) is found to increase when core is covered by shell (CaF2). It is also evident from Fourier transform infrared spectroscopy (FTIR) that ethylene glycol successfully controls the growth and acts as a shape modifier by regulating growth rate. In the photoluminescence investigation, emission spectra of SrF2:3Dy is found to be highly enhanced when SrF2:3Dy is covered by CaF2 due to the decrease of cross relaxation amongst the Dy3+-Dy3+ ions. Such type of enhancement of luminescence in homonanostructure SrF2:3Dy@CaF2 (core@shell) has not been studied so far, to the best of the authors’ knowledge. This luminescent material exhibits prominently white light emitting properties as shown by the Commission Internationale d’Eclairage (CIE) chromaticity diagram. The calculated correlate colour temperature (CCT) values for SrF2:3Dy, SrF2:3Dy@CaF2 (1:0.05), SrF2:3Dy@CaF2 (1:1) and SrF2:3Dy@CaF2 (1:2) are 5475, 5476, 5384 and 5525 K, respectively, which lie in the cold white region. Keywords: Core shell fluoride nanomaterials; Luminescence; White emitter Background transitions of trivalent dysprosium (Dy+3 ion) having 4f9 4 6 Fluorides have wide applications in optics as windows, electronic configuration, which are F9/2 → H15/2 (blue) 4 6 lenses and scintillation crystals and also exhibit interesting and F9/2 → H13/2 (yellow-orange wavelength region) 4 6 properties in optoelectronics such as lasing [1, 2], light [15, 16]. F9/2 → H13/2 is hypersensitive transition and amplification, and upconversion [3–5] as host crystals for strongly depend on crystal-field environment. White lanthanide ions. The solid fluoride materials have drawn light emission can also achieved by doping with lanthan- attention due to their unique properties, like low-energy ide ions (Ln3+) which emit primary colour (red, blue and phonons, high ionicity, electron-acceptor behaviour, high green). White emitter characteristics are also exhibited resistivity and anionic conductivity [6–8] and also have by nanomaterials prepared by surfactant route with wide range of potential optical applications in optics, multiple lanthanide doping [17]. However, Dy3+ ion biological labels and lenses [9, 10]. Fluoride base system emits white light at a suitable yellow-to-blue intensity possesses very low vibrational energy comparing oxide- ratio [18, 19]. base system therefore quenching of excite states of the lan- Enhancement of luminescence intensity of the lan thanide ions is minimal in fluoride-based system [11, 12]. thanide-doped nanoparticles has potential applications in Strontium fluoride (SrF2) has immense applications in optoelectronic devices. A core-shell formation in lant microelectric and optoelectric devices because of its hanide-doped nanoparticles is one of the useful methods dielectric properties and is used as an attractive host for for enhancing luminescence. phosphors [13, 14]. There are two main fluorescence In this present work, we have reported dysprosium ion (3 at.% )-doped SrF2 (SrF2:3Dy) nanoparticles and studied * Correspondence: [email protected] its photoluminescence properties. In order to enhance Department of Chemistry, Manipur University, Imphal 795003, India © 2015 Kumam et al. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Kumam et al. Nanoscale Research Letters (2015) 10:347 Page 2 of 8 luminescence intensity, SrF2:3Dy (core) is covered by CaF2 All the luminescence spectra were recorded using LS-55 (shell) at different ratios of shells (1:0.5, 1:1, 1:2). The Photoluminescence Spectrometer. Powder samples were Commission Internationale d’Eclairage (CIE) colour coor- dispersed in methanol and spread over the quartz slide dinates are found to be very close to ideal white values. and dried at room temperature. Methods Results and discussion Chemicals and reagents X-ray analysis Strontium nitrate (Sr(NO3)2, 99.9 %), dysprosium (III) Figure 1 shows the XRD pattern of SrF2,SrF2:3Dy and nitrate pentahydrade (Dy(NO3)3·5H2O, 99.99 %), from SrF2:3Dy@CaF2 at two different ratios of shell (1:1 and 1:2). Sigma-Aldrich, ethylene glycol, ammonium fluoride All the peaks pattern for pure and SrF2:3Dy can be index NH4F and calcium chloride, CaCl2 from Merck, and with the Joint Committee on Powder Diffraction Standards methanol were used as received without further purifica- (JCPDS) card number 06-0262 having cubical structure tion. Double distilled water was used throughout the with space group Fm3m (225). In case of SrF2:3Dy@CaF2 experiment. core-shell, the peak patterns are found coexisting for both SrF2 and CaF2 phases, indicating the CaF2 coated on the Preparation of SrF2:3Dy and SrF2:3Dy core @CaF2 shell SrF2.The star mark in the Fig. 1 indicates the SrF2 nanoparticles phase,and the hutch mark indicates the CaF2 phases. For 3+ Pure SrF2 and Dy ion (3 at.%)-doped SrF2 nanoparticles CaF2, the peaks patterns are found to be face centre cubical were synthesised by ethylene glycol route. In a typical structure with space group Fm3m (225) (JCPDS No. synthesis procedure, stoichiometric amount of precursor 35-0816). All the prepared samples are found to be of Dy3+ ions and strontium nitrate were heated at 40 °C crystalline structure, and there is no extra impurity for 30 min and ammonium fluoride was added. The peaks from the precursors of core SrF2 and shell CaF2. resulting precipitate solution was allowed to mix with The lattice parameter and unit cell volume of the prepared 50 ml of ethylene glycol and refluxed at 120 °C for 2 h. The resulting white precipitate was collected by centrifu- gation at 10,000 rpm after washing with methanol. In case of core-shell, i.e., SrF2:3Dy@CaF2 (1:0.5), SrF2:3Dy@CaF2 (1:1) and SrF2:3Dy@CaF2 (1:2), once the above procedure of lanthanide doping is done, requisite amount of calcium chloride solution is added and refluxed at 40 °C for about 30 min. Further, ammonium fluoride solution is added, and whole content is refluxed again at 120 °C about 2 h. Characterisation of nanoparticles Philips x-ray diffractometer (PW 1071) with Cukα (1.5 Å) radiation having Ni filter was used for x-ray diffraction (XRD) study. All patterns were recorded over the angular range 10 ≤ 2θ/deg ≤ 70 with a step size of Δ2θ =0.02.The powder samples were ground and dispersed in methanol on a glass slide and allowed to dry. The average crystallite size and strain were calculated using Williamson-Hall model. The lattice parameters were calculated from the least square fitting of the diffraction peaks. For the characterisation of the nanoparticles, Fourier transform infrared spectroscopy (FTIR) spectra were recorded on a Perkin Elmer Spectrum 400 FT-IR spectrometer. Powder samples were studied by making thin pellets with KBr. JEOL 2000 FX transmission electron microscope was used for recording TEM images. For the TEM measure- ment, the powder samples were ground and dispersed in methanol. A drop of the dispersed particles was put over 3+ the carbon coated copper grid and evaporated to dryness Fig. 1 XRD pattern of pure, doped (3 at.% Dy ion) SrF2 core and CaF shell at different ratios of shell at room temperature. 2 Kumam et al. Nanoscale Research Letters (2015) 10:347 Page 3 of 8 samples are given in the Table 1. Williamson-Hall model Table 2 The strain and crystallite size for of SrF2, SrF2:3Dy and [20] has been used to calculate the microstrain (ε) for the SrF2:3Dy@CaF2 nanoparticles system under study. Sl no. Samples Strain Size (nm) 1 SrF2 0.00397 30 βcosθ ¼ 0:9λ=D þ 4ε sinθ ð1Þ 2 SrF2:3Dy 0.00432 37 3 SrF :3Dy@CaF (1:0.5) 3.6888E-4 32 Here, β is the full width at half maxima (FWHM) at 2 2 Bragg’s angle (2θ), with λ being the x-ray wavelength 4 SrF2:3Dy@CaF2 (1:1) 2.6888E-4 31 5 SrF :3Dy@CaF (1:2) 2.4534E-4 25 (CuKα = 1.54 Å). The calculated crystallite sizes are in 2 2 the ranges of 20–40 nm (Table 2). From the Table 1, we 3+ observed that the unit cell volumes of the SrF2:3Dy is CaF2 which can be explained as, some of Dy ion located larger than SrF2 this can be explained as follows. The near the surface gets distributed into CaF2 leading to 2+ 2+ — ionic radii of the Sr (rSr = 1.27 Å) [21] is larger than decrease in electronic repulsion between F ions and unit 3+ 3+ the size of Dy (rDy = 1.03 Å) [22], and according to cell volume decreases. Moreover, with increased coating 3+ Vegard’s law, incorporation of Dy ion in SrF2 results in of CaF2 over SrF2 particle, there is decrease of penetration shrinking of unit cell volume as compared to the standard depth of x-ray passing through the core.