A Novel Strong Cyan Luminescence Emission of Tb2O3 Particles

Terbium oxide (Tb 2 O 3 ) particles (NPs) were synthesized by precipitation method using ammonium carbonate as precipitation agent. Effects of precursor molarity (0.1, 0.15 and 0.2 M) on photoluminescence (PL) behaviour of the NPs were investigated. The presence of the Tb 2 O 3 phase was conrmed by X-Ray Diffraction (XRD) and Fourier-transform infrared spectroscopy (FT-IR) analyses. Morphological investigations of the produced powders were made by Field Emission Gun-Scanning Electron Microscopy (FEG-SEM). It showed that the morphology of Tb 2 O 3 particles transformed from the nanograin chain to bundles morphology of rod-like as the amount of precursor molarity increased. Emission spectrum were investigated by Photoluminescence (PL) Spectroscopy. All the Tb 2 O 3 particles exhibited the strongest peak at 493 nm ascribed to 5 D 4 - 7 F 6 (magnetic dipole (MD), C 2 ) transition. The increase in the number of C 2 sites released from the MD transition with the increase of the precursor molarity caused a negative increase in the b* (yellowness/blueness of the emission) value in the CIE diagram, indicating that the colour shifted to the blue region. The Tb 2 O 3 particles produced by the precipitation method exhibited novel strong cyan colour and the PL emission intensity increased with increasing molarity.


FT-IR Analyses
The FT-IR analyses of the particles were given in Fig. 2

FE -SEM Analysis
It is clearly seen from the scanning electron microscopy (FE-SEM) images in Fig. 3

PL Measurements
In PL emission spectra of all the particles, only two emission peaks which were sharp peak at 493 and broad peak at around 537 nm were observed (Fig. 4). Although the peak at 493 nm increased and sharpened with increasing the precursor molarity, no change was observed in the peak at around 537 nm.
The peaks at 493 and around 537 nm were associated to 5 D 4 -7 F 6 (magnetic-dipole (MD) transition, C 2 ) and 5 D 4 -7 F 5 (electron-dipole (ED) transition, C 2 ) transitions, respectively [17]. Tb 2 O 3 particles were exhibited a novel cyan emission. Since it was not possible to understand actual colour released from the PL optical spectra, these data were converted to the CIEXYZ fundamental chromaticity diagram. The CIEXYZ colour system was converted to the CIELab colour space and L*, a* and b* parameters provided information about lightness, redness/greenness and yellowness/blueness of the emission, respectively. CIEXYZ diagram and colour space coordinate values of all the particles were given in Fig. 4b and Table 2, respectively. MD transitions are responsible for "blueness" while ED transitions are responsible for "greenness". As the precursor molarity increased, the number of C 2 sites released from MD transition increased and b* value increased negatively; this showed that the colour had shifted to blueness. There are a limited number of studies in the literature examining the PL emissions of Tb 2 O 3 particles, and there are many peaks in the PL spectra obtained in these studies, with the strongest emission peak around 545 nm [13] [6] [1] [16]. However, it was found for the rst time that Tb 2 O 3 powder exhibited the strongest PL emission at 493 nm in this study. The PL intensity was also increased with both the increase in the precursor molarity and the change of morphology to bundles.

Conclusions
The effect of precursor molarity (0.10, 0.15 and 0.2 M) of particles produced by precipitation method on the PL properties was investigated in this study. It was determined by XRD analysis that the produced powder had a body-centred cubic crystal structure and the CI value of the 0.10M-Tb 2 O 3 coded sample was 78%. In FE-SEM analysis, it was observed that the particles displayed a homogeneous structure and transformed into bundles morphology as the precursor molarity increased. In the PL measurements, all the particles were exhibited extremely strong cyan emission at 493 nm. It was observed that the precursor molarity, grain size and morphology were effective on the PL intensity of the particles. The increase in molarity increased the particle size and transformed the morphology into the bundles, and increased in the PL intensities were observed. There was an increase of 208% in the PL intensity of the peak at 493 nm of the particle with bundles morphology according to the PL intensity of the particle with an agglomerate structure of regular nanograins. As the precursor molarity increased, there was a negative increase in the b* value, indicating that the PL emission colour shifts to blueness. Figure 2 FT-IR analysis of all the particles.    PL intensity and grain size as a function of the precursor molarity for em = 493 nm.