Electrophoretic Deposition of Nano- and Micron-Sized Ba₂SiO₄:Eu²⁺Phosphor Particles

Abstract

Electrophoretic deposition (EPD) is a technique to deposit charged particles from a stable suspension under the force of an applied electric field. Although nanoparticles of a variety of materials have been coated by EPD, there have been few direct comparisons of EPD of nano-and micron-sized particles of the same material. The objective of this study is to compare EPD of nano-, nano core/SiO 2 shell and micron-sized (Ba 0.97 Eu 0.03 ) 2 SiO 4 phosphor particles for application in a near-UV LED-based light source. EPD from an amyl alcohol bath was able to produce uniform films for all particle sizes, whereas uniform films were produced only of micron-sized particles in an isopropyl alcohol bath. Electrophoretic deposition (EPD) is a useful method to fabricate particulate films.1 The process is simple, scalable and often costeffective. First, the particles are charged in a suspension, subsequently they are transported to an electrode under an electric field, and finally the particles adhere to the substrate. Each step has been systematically investigated in previous work in a bath of isopropyl alcohol (IPA) with nitrate salts, by studying the effects of the suspension medium chemistry on the zeta potential of the particles and the formation of the adhesive agents. 2-4For producing white solid-state lighting devices, EPD has been used to deposit micron-sized phosphors placed above an LED. Y 3 Al 5 O 12 :Ce 3+ phosphor was deposited onto a flexible indium tin oxide (ITO)-coated polymer substrate on top of blue light-emitting diodes (LEDs).5 In other work, both layered and blended phosphor films using Eu-activated Ca-α-SiAlON, β-SiAlON and CaAlSiN 3 (yellow-, green-and red-emitting, respectively) were deposited on an ITO-coated substrate that was placed on top of a blue-emitting LED. 6 Recently, EPD was used to deposit both layered and blended phosphor films that generated white light using Eu 2+ -activated Sr 2-x Ca x Si 5 N 8 , Ba 2 SiO 4 , LiCaPO 4 , (Sr 0.75 Ba 0.25 ) 2 SiO 4 and (Sr 0.5 Ba 0.5 ) 3 SiO 5 (red-, green-, blue-, yellow-and orange-emitting, respectively) with near-UV LEDs. 7,8 The phosphor films must be ∼10 to 40 μm thick for full conversion of the UV emission from the LED. Also the film thickness must be very uniform. It was determined that the deposited film quality was poor when a higher applied voltage was used. 9 This has led to the development of the EPD processes using lower voltages (∼80 V) and longer deposition times up to 30 min 7,8 than previous EPD processes. [1][2][3][4] Typically, micron-sized phosphors are being used in solid-state lighting. However, the micron-sized phosphors have higher optical scattering losses due to an increased optical path length and increased number of reflections or transmissions at interfaces. 10 The nano-phosphors (<300 nm) with comparable quantum efficiency to typical micron-sized phosphors may be a mean to reduce the losses and improve efficiency of the white UV-based LEDs.11 But it is well known that the emission intensity of the phosphor decreases as the...

Review—Electrophoretic Deposition of Phosphors for Solid-State Lighting

“…54,55 It was found that the luminescence intensity, chemical stability and thermal stability were improved by the presence of inert shells. [56][57][58] It has been found that EPD from suspension of nano-sized particles of (Ba 0.97 Eu 0.03 ) 2 SiO 4 in an IPA bath yielded a non-uniform and porous film, 28 as shown in Figure 15, for both photographs (under excitation of 365 nm light) and SEM micrographs. The increased surface area of nano-sized particles adsorbs a larger quantity of water than for micron-sized particles.…”

“…28 Micron-sized (5 μm) and nano-sized (320 nm) (Ba 0.97 Eu 0.03 ) 2 SiO 4 was compared using bath a consisting of amyl alcohol, 5 g/L of the phosphor and Mg(NO 3 ) 2 (ζ = 22 mV). However, due to the limited availability of the nano-sized phosphors, a very small bath volume of 60 mL was used.…”

“…62 These R-T instabilities are unusual for EPD and developed in part due to the small bath volume used in these experiments and because the bath was not stirred. 28 The R-T instabilities should not occur if the suspension is well mixed. Therefore, EPD was performed with gentle stirring of the bath by a small magnetic stirrer placed in the bottom of the bath under the same deposition conditions.…”

“…For near-UV LEDs, the deposit thickness must be optimized so that it is thick enough to ensure full conversion (no unwanted near-UV light escapes), but not so thick as to reduce light output; these films must be between 10-40 μm thickness for full conversion. 28 The creation of thick films creates processing problems with EPD: thick films typically require longer deposition times. At long times if the solution is not stirred, settling of the powders has to be taken into account as the concentration of particles in the bath decreases with time.…”

A novel efficient mesoporous silica assisted green emitting phosphors-an exotic remote phosphor with high quantum yield

“…Several reports are available in literature on the preparation of silicate phosphors using a variety of silicate source and by adopting various strategies. [16][17][18][19][20][21] Most of the research utilizes the conventional silica or the organic tetra ethyl orthosilicate (TEOS) as silicate sources. But, here as a novel input, we have made use of the versatile mesoporous silica (MPS) as the silicate source for the making of phosphor.…”