Synthesis and optical characterization of sub-5 nm Terbium oxalate nanocrystals: A novel intense green emitting phosphor

Highlights

• A novel fully concentrated intense green emitting nanophosphor is identified.

• First time report on the synthesis of Terbium oxalate nanocrystals.

• Extremely low particle size (2-4 nm) suggests applications in bio-medical field.

• The CIE coordinates suggest enhanced colour purity for green emission.

Abstract

Green emitting novel Terbium oxalate decahydrate nanophosphor (Tb₂(C₂O₄)₃.10H₂O) with particle size in the range 2–4 nm, was synthesized by microwave assisted co-precipitation method. The structure and particle size of the sample were elucidated by X-ray powder diffraction and TEM analysis. The photoluminescence excitation spectra of the terbium oxalate nanophosphor exhibited several peaks in the 200-500 nm range of DUV to visible region with the strongest one at 369 nm in n-UV region and these excitation peaks matches well with the emission peaks of commercially available LED sources. The emission spectra recorded at the representative excitations exhibit intense and sharp green emission (⁵D₄→⁷F₅, at 543 nm) owing to the heavy population of ⁵D₄ level at the expense of ⁵D₃ level of Terbium ion. The decay curve is fitted to single exponential function with a decay time of 0.82 ms. The chromaticity co-ordinate is found to be (0.25, 0.54) and is located in green region with a colour purity of 68%. The experimental observations revealed that the synthesized ultrafine terbium oxalate nanocrystal is a suitable candidate as green emitting phosphor.

Introduction

Nowadays, Rare earth doped inorganic luminescent nanoparticles have been the research interest of scientists because of their intriguing applications in luminescent devices such as fluorescent lamps, WLED, solid-state laser, scintillators, plasma display panels etc. More importantly, Rare earth based nanofluorescent materials with smaller particles (quantum dots) is the most suitable alternative of semiconductor quantum dots in bio imaging and medical diagnostics [1], [2], [3], [4]. Among the various solid-state lighting applications, WLED's with tricolour phosphors (RGB) coated on UV- LED chips that can emit intense red, green and blue under n-UV excitation have attracted significant attention owing to its better colour purity and CRI [5], [6], [7], [8]. Especially, the green component of tri-colour centre plays a significant role, as the maximum of human eye sensibility falls in this region and the green colour constitutes more to the pixel ratio 3:6:1 (red: green: blue) for precise display applications [9]. Thus it is highly desirable to develop new green emitting phosphors with excitation wavelengths coinciding with the emission of commercially available UV LED's.

Terbium is used as an activator in various host lattices for green luminescence owing to its sharp emission originating from the intraconfigurational f-f transitions [9]. Most of the reports on green emitting luminescent materials are based on the doping of Tb³⁺ ion in different host lattices. Generally low doping of Tb³⁺ yields only weak luminescence while heavy doping results in quenching of luminescence. The most essential prerequisite for a favourable phosphor material is its intense and sharp luminescence “without concentration quenching” - but quenching free materials are less attempted despite of its assured intense luminescence. The high active ion concentration of the order of 10²¹ ions cm⁻³ in rare earth oxalate crystals facilitates high optical gain and hence it can find potential optical applications. Moreover, as an important bridging ligand, oxalate anion makes intriguing structures among the rare earth compounds and they exhibit superior optical properties [10]. Single crystals of Cerium oxalate, Neodymium praseodymium oxalate, Dysprosium praseodymium oxalate and Samarium oxalate were studied in recent years but the detailed spectroscopic investigations were not attempted [11], [12], [13], [14].

In our previous work on the synthesis and photo luminescent properties of Terbium oxalate decahydrate single crystals, we reported intense green luminescence in the oxalate matrix without concentration quenching [15]. It is of scientific interest to investigate the spectroscopic properties of nanostructured terbium oxalate as the nano counterpart exhibits promising properties compared to the bulk. Hence in the present work attention has been paid on the synthesis of Tb³⁺ activated fully concentrated Terbium oxalate decahydrate nanocrystals and to explore the photoluminescence properties. To the best of our knowledge, no work has been reported so far on terbium oxalate nanocrystals. As the particle size of nanophosphors becomes very low better resolution in flat panel displays can be achieved and also it exhibits reduced loading effects in fluorescent lamps [16], [17]. In this context, the synthesis and luminescence studies on terbium oxalate decahydrate nanocrystals with size as much low deserves special attention. In the present work, Terbium oxalate nanocrystals with extremely low particle size in the sub 5 nm range without concentration quenching was synthesized by a simple microwave assisted co-precipitation method and a detailed structural and spectroscopic characterization have been undertaken.