Silica nanoparticles boost bluish-green phosphor for white LEDs

Researchers have developed a bluish-green emitting barium orthosilicate phosphor doped with divalent europium using silica nanoparticles as the silicate precursor, and they report a 48 percent enhancement of emission intensity compared with a conventionally prepared counterpart. The work, published in Frontiers of Optoelectronics, also demonstrates a flexible remote phosphor film built from the optimized material, designed to sit away from the LED chip and thereby reduce the thermal stress that degrades conventional phosphor-in-resin white LEDs.

Those earlier materials showed increased luminescence efficiency relative to phosphors made with conventional silica. In the new work, the team prepared a series of silica-nanoparticle-assisted Ba2SiO4:Eu2+ phosphors with varying europium concentrations using a dry-phase solid-state reaction method, and compared their performance directly with equivalent phosphors made from conventional silica with a mesh size of 100.

Reactant materials of barium carbonate, europium oxide of 99.99 percent purity and the silica nanoparticles were ground together for one hour and calcined at 1000 degrees Celsius for four hours, yielding a Ba2SiO4:Eu3+ intermediate. Samples doped at 0.007, 0.008, 0.009, 0.01, 0.02 and 0.03 mol were labeled BSSP, while the conventional-silica equivalents were labeled BSCP. The silica nanoparticles themselves were produced through Stober’s process, in which tetraethoxy silane is added to an ethanol-water mixture and precipitated with ammonium hydroxide.

X-ray diffraction confirmed that the phosphors crystallized as single-phase Ba2SiO4 with an orthorhombic structure and space group Pmcn, matching the reference pattern ICSD-6246. No diffraction peaks corresponding to a hetero-phase were detected, even at the higher europium doping levels. The diffraction peaks of all the prepared phosphors shifted to larger angles, which the authors attribute to the substitution of larger barium ions by smaller divalent europium ions. The average crystallite size, calculated with the Scherrer formula, was 36.4 nanometers, a nanoscale dimension the researchers link to a high surface area and enhanced luminescence efficiency.

Excitation spectra recorded at an emission wavelength of 520 nanometers showed a broad band from 250 to 450 nanometers, assigned to the 4f7 to 4f65d1 transition of Eu2+, indicating strong absorption of both ultraviolet and blue light and potential suitability for UV- and blue-driven LEDs. Emission spectra excited at 360 nanometers produced a broad band from 450 to 650 nanometers with two peaks at 490 and 525 nanometers, which the authors ascribe to dopant ions sitting in two different environments. Emission intensity rose as the europium concentration increased from 0.007 to 0.01 mol, peaked at 0.01 mol for the nanoparticle-derived phosphor and 0.009 mol for the conventional-silica version, and then declined at higher dopant levels through concentration quenching. In a direct comparison of the optimized samples, BSSP-0.01 delivered 48 percent higher emission intensity than BSCP-0.009.

Field-emission scanning electron microscopy showed the silica nanoparticles were roughly 300 nanometers in size, while the conventional silica consisted of larger, irregularly shaped particles.

The team also probed why the emission splits into two bands. Deconvolution of the broad emission under 460-nanometer excitation yielded two Gaussian profiles peaking at 487 and 514 nanometers, supporting the idea that Eu2+ ions substitute at two different barium sites. Given the large ionic radius difference between silicon (0.4 angstroms) and divalent europium (1.30 angstroms), europium preferentially occupies the barium sites, where it is surrounded by either 9 or 10 oxygen neighbors, creating two emission centers that produce blue and green light. A concentration quenching analysis of the optimized phosphor yielded a critical distance of 28 angstroms between activators, and a Van Uitert analysis gave a theta value of 7.5, close to 8, indicating dipole-quadrupole energy transfer as the dominant quenching interaction.

In conventional white LEDs, a yellow Y3Al5O12:Ce3+ phosphor is embedded in epoxy resin directly on the InGaN chip, and prolonged heat exposure can discolor the resin, producing a yellowish tint and reduced brightness. Remote phosphor technology instead places the phosphor at a considerable distance from the chip, which may increase phosphor efficiency by lowering the amount of light backscattered onto the chip.

To translate the powder into a device-compatible form, the researchers mixed the optimized phosphor with silicon resin and cast thin films by drop casting, testing phosphor loadings of 30, 40, 50, 60 and 70 milligrams against a fixed 60 milligrams of silicon binder. Emission intensity climbed as the loading rose from 30 to 50 milligrams and then fell beyond that point, an effect attributed to scattering by the phosphor particles, so 50 milligrams was selected as the optimum. Under 360-nanometer excitation the powder itself emits bluish-green light at those same coordinates, consistent with the color point highlighted on the CIE chromaticity diagram.

In an eight-hour real-time test on the UV LED prototypes, the team observed little change in intensity or emission shift over time, with CIE coordinates remaining constant. X-ray photoelectron spectroscopy confirmed the presence of silicon, oxygen, barium and europium, with the europium 3d signal at 1137 electronvolts indicating the plus-three oxidation state, a consequence the authors attribute to X-ray-induced oxidation of the thermodynamically unstable divalent europium during measurement.

Subject of Research: Development of a silica-nanoparticle-assisted Ba2SiO4:Eu2+ bluish-green phosphor and flexible remote phosphor film for white light applications

Article Title: Silica nanoparticles assisted Ba2SiO4:Eu2+—a bluish-green emitting remote phosphor for white light application

Article References: Mayavan, A., Kannan, A., & Gandhi, S. (2025). Silica nanoparticles assisted Ba2SiO4:Eu2+—a bluish-green emitting remote phosphor for white light application. Frontiers of Optoelectronics, 18(1), Article 8. https://doi.org/10.1007/s12200-025-00150-w

Image Credits: AI Generated

DOI: 10.1007/s12200-025-00150-w

Keywords: Ba2SiO4:Eu2+, silica nanoparticles, remote phosphor, white LEDs, photoluminescence, thermal stability, concentration quenching, CIE coordinates, solid-state reaction, europium doping, optical band gap, UV LEDs