A protected annealing strategy to enhanced light emission and photostability of YAG:Ce nanoparticle-based films

Abstract

A significant obstacle in the development of YAG:Ce nanoparticles as light converters in white LEDs and as biological labels is associated with the difficulty of finding preparative conditions that allow simultaneous control of structure, particle size and size distribution, while maintaining the optical properties of bulk samples. Preparation conditions frequently involve high-temperature treatments of precursors (up to 1400 °C), which result in increased particle size and aggregation, and lead to oxidation of Ce(iii) to Ce(iv). We report here a process that we term protected annealing, that allows the thermal treatment of preformed precursor particles at temperatures up to 1000 °C while preserving their small size and state of dispersion. In a first step, pristine nanoparticles are prepared by a glycothermal reaction, leading to a mixture of YAG and boehmite crystalline phases. The preformed nanoparticles are then dispersed in a porous silica. Annealing of the composite material at 1000 °C is followed by dissolution of the amorphous silica by hydrofluoric acid to recover the annealed particles as a colloidal dispersion. This simple process allows completion of YAG crystallization while preserving their small size. The redox state of Ce ions can be controlled through the annealing atmosphere. The obtained particles of YAG:Ce (60 ± 10 nm in size) can be dispersed as nearly transparent aqueous suspensions, with a luminescence quantum yield of 60%. Transparent YAG:Ce nanoparticle-based films of micron thickness can be deposited on glass substrates using aerosol spraying. Films formed from particles prepared by the protected annealing strategy display significantly improved photostability over particles that have not been subject to such annealing.

Facile synthesis of YAG:Ce3+ thick films for phosphor converted white light emitting diodes

“…This is due to their excellent optical properties, such as broad emission spectrum, long lifetime, high reliability, good chemical stability and low consumption of energy [1][2][3]. However, commercial YAG:Ce 3+ phosphors based pc-WLEDs exhibit limited conversion efficiency due to their high scattering and reflective loss of the emission from irregular micro-size phosphors/epoxy resin layers [3][4][5][6]. In addition, the organic binder with low thermal conductivity in commercial pc-WLEDs may age easily and turn yellow due to the accumulating heat emitted from the blue light chip, which adversely affects the pc-WLED properties such as luminous efficacy and colour coordination, and therefore reduces its long-term life-span and reliability [7,8].…”

“…Recently, new types of pc-WLEDs have been developed by incorporating solid-state converted phosphor layers without any organic binder. It has been proposed that transparent polycrystalline YAG:Ce 3+ ceramic plate phosphors (CPP) [7,9], YAG:Ce 3+ glass ceramic phosphors (GCP) [8,10] and YAG:Ce 3+ films phosphors (FP) [3][4][5][6] can solve the above problems. Transparent YAG:Ce 3+ phosphor films with nanometer particle size, superior thermal conductivity and better adhesion, are recognized as good candidates in pc-WLEDs applications [3][4][5][6]11].…”

“…Several technologies were employed to prepare transparent YAG:Ce 3+ phosphor films, such as RF-sputtering [12], sol-gel approaches [3,6,11], liquid phase epitaxy [13,14], and nano YAG:Ce 3+ powders fabricated thick films [4,5,15]. However, in order to be compatible with the pc-WLEDS package, the YAG:Ce 3+ film must be thick enough and the size distribution must be narrow enough to maintain considerable optical performance to match the optical properties of the blue light chip [4].…”

Nanophosphor Coatings: Technology and Applications, Opportunities and Challenges

“…Nyman et al (2009) reported an increase of quantum efficiency of YAG:Ce nanopowder to 57% and its successful epoxy-encapsulation. The quantum yield of the phosphor was further increased to 60% by a "protected annealing" in porous silica by Revaux et al (2011c). The same group investigated the incorporation of nanophosphors into a TiO 2 sol-gel coating in order to match the refractive index and thus further decrease scattering (Revaux et al, 2011a;Revaux et al, 2011b).…”

Electrospinning Preparation and Photoluminescence Properties of Y3Al5O12:Eu3+ Nanobelts

“…Recently, rare-earth-doped nanocrystals have been widely used as high performance luminescent devices, magnets and other functional materials. YAG has received considerable attention when doped with rare earth ions owing to their interesting properties [16][17][18][19][20][21] . YAG doped with Eu 3+ ions is an important phosphor with a variety of applications in many luminescent and optical devices.…”