Jae Su Yu

A novel strategy for controllable emissions from Eu3+ or Sm3+ ions co-doped SrY2O4:Tb3+ phosphors

Trivalent rare-earth (RE) ions (Eu(3+), Tb(3+) and Sm(3+)) activated multicolor emitting SrY(2)O(4) phosphors were synthesized by a sol-gel process. The structural and morphological studies were performed by the measurements of X-ray diffraction profiles and scanning electron microscope (SEM) images. The pure phase of SrY(2)O(4) appeared after annealing at 1300 °C and the doping of RE ions did not show any effect on the structural properties. From the SEM images, the closely packed particles were observed due to the roughness of each particle tip. The photoluminescence (PL) analysis of individual RE ions activated SrY(2)O(4) phosphors exhibits excellent emission properties in their respective regions. The Eu(3+) co-activated SrY(2)O(4):Tb(3+) phosphor creates different emissions by controlling the energy transfer from Tb(3+) to Eu(3+) ions. Based on the excitation wavelengths, multiple (green, orange and white) emissions were obtained by Sm(3+) ions co-activated with SrY(2)O(4):Tb(3+) phosphors. The decay measurements were carried out for analyzing the energy transfer efficiency and the possible ways of energy transfer from donor to acceptor. The cathodoluminescence properties of these phosphors show similar behavior as PL properties except the energy transfer process. The obtained results indicated that the energy transfer process was quite opposite to the PL properties. The calculated CIE chromaticity coordinates of RE ions activated SrY(2)O(4) phosphors confirmed the red, green, orange and white emissions.

A facile and efficient strategy for the preparation of stable CaMoO4 spherulites using ammonium molybdate as a molybdenum source and their excitation induced tunable luminescent properties for optical applications

Stable CaMoO4 spherulites were synthesized by a facile hydrothermal method using (NH4)6Mo7O24·4H2O as a Mo source and these spherulites were formed according to the theoretical predictions of the crystal splitting theory. Rietveld refinement and photoluminescence studies confirmed that the CaMoO4 spherulites are defect-free. The CaMoO4 spherulites showed greenish-blue emission and the single emitting component of CaMoO4:Eu3+ spherulites led to a novel excitation induced efficient emission property like organic light emitting diodes. Cathodoluminescent properties of the CaMoO4:Eu3+ exhibited individual emissions from MoO42− clusters and Eu3+ ions. The white color emissions were clearly explained using Gaussian fitting curves. The corresponding CIE chromaticity coordinates provided their emission potentiality in the green, red and white regions for optical and biological applications.

Concentration and penetration depth dependent tunable emissions from Eu3+ co-doped SrY2O4:Dy3+ nanocrystalline phosphor

A series of Dy3+ ion single-doped and Dy3+/Eu3+ ion co-doped white-light emitting SrY2O4 nanocrystalline phosphors were synthesized by a citrate sol–gel method. After the samples were annealed at 1300 °C, the X-ray diffraction patterns confirmed their orthorhombic structure. The particles were closely-packed and their optical properties were monitored by photoluminescence spectroscopy. The Dy3+ ions acted as luminescent centers and substituted Y3+ ions in the SrY2O4 host lattice, where they are located in Cs sites, and the characteristic emission of Dy3+ ions (4F9/2 → 6H15/2 and 4F9/2 → 6H13/2 transitions) with intense yellow emission band at 578 nm occurred. When the Eu3+ ions were co-doped into the SrY2O4:Dy3+ (1 mol%) nanocrystalline phosphor, white-light emission was observed under excitation at 381 nm. The energy transfer between Eu3+ and Dy3+ ions was calculated and the chromaticity coordinates were also presented. The cathodoluminescence (CL) spectra confirmed that the penetration depth is inversely proportional to the total atomic weight and atomic number of the compound. From CL analysis, the emission intensities of Dy3+ ion single-doped and Dy3+/Eu3+ co-doped SrY2O4 nanocrystalline phosphors increased continuously upon increasing both the accelerating voltage from 1 to 5 kV and the filament current from 30 to 47 μA.

UV-A and UV-B excitation region broadened novel green color-emitting CaGd2ZnO5:Tb3+ nanophosphors

Green color-emitting novel CaGd2ZnO5:Tb3+ (CGZO:Tb3+) nanophosphors were synthesized by a citrate sol–gel method. The structural and morphological properties were elucidated by X-ray diffraction and transmission electron microscope measurements. The photoluminescence properties of orthorhombic-phased CGZO:Tb3+ nanophosphors were studied as a function of Tb3+ ion concentration. The CGZO:Tb3+ nanophosphors revealed the enhanced broadband excitation between ultraviolet (UV)-B and UV-A regions. Under 317 nm excitation, even at dilute Tb3+ ion concentrations, only the emission transitions from 5D4 energy level were exhibited. This unusual behavior is due to the occurrence of nonradiative energy transfer via f–d transition rather than cross-relaxation process. The cathodoluminescence also showed similar behavior at low accelerating voltages. These luminescent powders are expected to find potential applications such as white light-emitting diodes and optical display systems.

Novel orange and reddish-orange color emitting BaGd2O4:Sm3+ nanophosphors by solvothermal reaction for LED and FED applications.

BaGd2O4 (BG):Sm(3+) nanophosphors were synthesized by a solvothermal reaction method. The powder X-ray diffraction pattern confirmed their orthorhombic structure, and the morphological studies were carried out by taking the scanning and transmission electron microscopy images. The photoluminescence (PL) emission and PL excitation (PLE) spectra were investigated as a function of Sm(3+) ion concentration. The PLE spectra revealed both Gd(3+) and Sm(3+) excitation bands in the shorter and longer wavelength regions, indicating that the efficient energy transfer occurred from the Gd(3+) to Sm(3+) ions in the BG host lattice. The PL spectra exhibited an intense orange emission due to ((4)G5/2→(6)H7/2) transition along with two other moderate intense emission peaks due to the ((4)G5/2→(6)H5/2) and ((4)G5/2→(6)H9/2) transitions. Based on the emission performance related to ((4)G5/2→(6)H7/2) transition, the Sm(3+) ion concentration was optimized to be at 1 mol%. The low-voltage cathodoluminescent (CL) measurements were also performed for BG:1 mol% Sm(3+) nanophosphors as a function of accelerating voltage and filament current. From the CL spectra, the reddish-orange emission was observed. The Commission International De I-Eclairage chromaticity coordinates of BG:Sm(3+) nanophosphors were found to be very close to the chromaticity coordinates of Nichia Corporation developed amber light-emitting diodes.

Pump power induced tunable upconversion emissions from Er3+/Tm3+/Yb3+ ions tri-doped SrY2O4 nanocrystalline phosphors

RE3+ (Er3+/Tm3+/Yb3+) ions tri-doped SrY2O4 nanocrystalline phosphors were synthesized by a citrate sol–gel method. After annealing at 1300 °C, the structure was identified to be pure orthorhombic and the particles were found to be nearly spherical in shape. The up-conversion emission spectra of the SrY2O4:1Er3+ phosphor revealed a bright green emission (4S3/2 → 4I15/2) around 551 nm. To enhance the red emission of the Er3+ ions, Yb3+ ions were co-doped with Er3+ ions, leading to a greenish yellow emission under 980 nm excitation. In the case of the 1Er3+/1Tm3+/3Yb3+ ions tri-doped phosphor, white-light emission was observed corresponding to the 1G4 → 3H6 (Tm3+), 4S3/2 → 4I15/2 (Er3+) and 4F9/2 → 4I15/2 (Er3+) transitions at 487, 551 and 664 nm, respectively. It was interestingly noticed that, when the exciting pump power was changed from 100 to 900 mW, the blue emission band of the Tm3+ ions was enhanced by the suppression of the red and green emission bands of the Er3+ ions so that the chromaticity coordinates were also moved to the cyan emission region. This feature suggests that the Er3+ ion may also act as a sensitizer for the Tm3+ and Yb3+ ions and a possible energy transfer mechanism was also discussed.

Artificial inverted compound eye structured polymer films with light-harvesting and self-cleaning functions for encapsulated III–V solar cell applications

We report the artificial inverted compound eye structured (ICESs) polydimethylsiloxane (PDMS) films with light-harvesting and self-cleaning functions for the enhancement of solar power generation in encapsulated III–V gallium arsenide (GaAs) single-junction solar cells. The ICESs PDMS films are fabricated by facile, simple, and cost-effective soft lithography using sapphire master molds with the CESs consisting of hierarchical nanotextures/periodic microgratings prepared by thermally dewetted gold nanopatterning and subsequent dry etching processes. By attaching the ICESs PDMS film with a hydrophobic surface (i.e., water contact angle (θCA) of ∼121°) to the coverglass, the total and diffuse transmittances of the coverglass are simultaneously increased over a wide wavelength range of 350–900 nm, exhibiting higher solar weighted transmittance (SWT) of ∼94.3% and average haze ratio (Havg) of ∼67.6% than those of the bare coverglass (i.e., θCA ≈ 32°, SWT ≈ 90.2%, and Havg ≈ 4.1%, respectively). The resulting encapsulated III–V GaAs single-junction solar cells with the ICESs PDMS/coverglass show an enhanced power conversion efficiency (PCE) of 24.3% compared to the encapsulated solar cell with the bare coverglass (i.e., PCE = 22.96%) due to the increased short circuit current density from 26 to 27.64 mA cm−2, indicating the PCE increment percentage of ∼5.8%. Moreover, it also exhibits superior device performance at varying angles of incident light. For the long-term self-cleaning effect on the device efficiency, there is no significant variation in the PCE after approximately one month, indicating a low PCE drop percentage of ∼1.4%.

Improvement in light harvesting of dye-sensitized solar cells with antireflective and hydrophobic textile PDMS coating by facile soft imprint lithography.

We demonstrated the improved conversion efficiency (η) of dye-sensitized solar cells (DSSCs) using the textile-patterned polydimethylsiloxane (PDMS) antireflection layers prepared by metal-coated textile master molds by a simple soft imprint lithography. When light propagates through the textile-patterned surface of PDMS (i.e., textile PDMS) laminated on the outer glass surface deposited with fluorine-doped tin oxide (i.e., FTO/glass), both the transmitted and diffused lights into the photo-anode of DSSCs were simultaneously enhanced. Compared to the bare FTO/glass, the textile PDMS increased the total transmittance from 82.3 to 85.1% and its diffuse transmittance was significantly increased from 5.9 to 78.1% at 550 nm of wavelength. The optical property of textile PDMS was also theoretically analyzed by the finite-difference time-domain simulation. By laminating the textile PDMS onto the outer glass surface of DSSCs, the η was enhanced from 6.04 to 6.51%. Additionally, the fabricated textile PDMS exhibited a hydrophobic surface with water contact angle of ~123.15°.

Antireflective gradient-refractive-index material-distributed microstructures with high haze and superhydrophilicity for silicon-based optoelectronic applications

We fabricate gradient-index (n) material-based microstructures, i.e., magnesium fluoride (MgF2, n ∼ 1.37) film-coated SU8 ultraviolet curable polymer (n ∼ 1.59) microcones (MCs) with tapered architectures, on silicon (Si, n ∼ 3.9) substrates using a soft imprint lithography method for high-performance Si-based optoelectronic applications. The effects of various geometry parameters (i.e., height, filling ratio, and period) of conical MCs on the SU8 film/Si structure including different thicknesses of MgF2 films on the reflectance properties are investigated by a theoretical analysis using a rigorous coupled-wave analysis simulation. For the fabricated samples, their optical characteristics and surface wetting behaviors are also explored. The MgF2 film-coated SU8 MCs on Si substrates (i.e., MgF2/SU8 MCs/Si) exhibit a superhydrophilic surface with very low water contact angles of <10° and reduce the surface reflection of the bare Si over a wide wavelength of 350–1100 nm, showing the lower average reflectance (Ravg) and solar weighted reflectance (Rsw) values of ∼14% and ∼12.1%, respectively, (i.e., Ravg ∼ 15.1% and Rsw ∼ 13% for the SU8 MCs/Si and Ravg ∼ 38.5% and Rsw ∼ 38% for the bare Si). Furthermore, the MgF2/SU8 MCs on glasses also show strong light scattering in transmissions at wavelengths of 350–1100 nm, which indicates an average haze ratio of ∼89.8%, while maintaining high total transmissions. Both the measured and calculated reflectance spectra showed similar results.

Solar power generation enhancement of dye-sensitized solar cells using hydrophobic and antireflective polymers with nanoholes

We improve the power conversion efficiency (PCE) of dye-sensitized solar cells (DSSCs) using polydimethylsiloxane (PDMS) patterned with negatively tapered nanoholes (NHs) as a protective antireflection layer of the external glass surface. The NHs PDMS layers are prepared by a soft lithography via silicon molds with conical nanopillars. The NHs PDMS with a NH depth of ∼320 nm decreases the surface reflection of fluorine doped tin oxide (FTO)-coated glass over a wide wavelength range of 350–800 nm at incident angles (θin) of 0–70°, exhibiting a lower solar weighted reflectance (RSW) value of ∼7.1% at θin = 0° and a lower average RSW value of ∼8.5% at θin = 20–70° than those (i.e., RSW ≈ 10.1% at θin = 0° and average RSW ≈ 15.6% at θin = 20–70°) of the FTO glass. In DSSC device applications, it increases the short-circuit current density (JSC) from 15.69 to 16.52 mA cm−2, thus resulting in an enhanced PCE value of 7.56% compared to the reference DSSC (i.e., PCE = 7.15%). For different NH depths, the optical reflectance characteristics of the NHs PDMS/FTO glass are theoretically investigated using a rigorous coupled-wave analysis method, showing similar trends between the calculated and measured results. For solar spectrum angle-dependent photocurrents, it also shows a remarkable device performance at θin = 20–70°. Besides, the NHs PDMS exhibits a hydrophobic surface with a water contact angle of ∼115°.