Sandeep Sohal

Synthesis and photoluminescence properties of hierarchical architectures of YBO3:Eu3+

We present a general hydrothermal method without any organic solvent or surfactant for the synthesis of YBO3:Eu3+ nano- and micro-structures. The samples were of excellent quality with uniform, well dispersed, self-assembled and self-purified YBO3:Eu3+. Single crystal nanoflakes gathered together to evolve into a hierarchical architecture with eight different types of three dimensional morphologies. Strong photoluminescence spectra were obtained at 592, 611 and 627 nm at excitations wavelengths 254 and 363 nm. The emission spectra are associated with the transitions from the excited 5D0 level to the 7FJ (J = 1, 2, 3, 4) levels of Eu3+ activators. YBO3:Eu3+ prepared using ethanol solvent exhibited the highest chromaticity values yet reported.

Effect of Tb3+ concentration on the optical and vibrational properties of YBO3 tri-doped with Eu3+, Ce3+, and Tb3+

Structural and optical studies are reported of yttrium orthoborate YBO3 when tri-doped with Eu3+, Ce3+, and Tb3+, focusing on the role of terbium concentration. Incorporation of Tb3+ affects emission properties for photoluminescence (PL) excited by near ultraviolet light. For constant cerium and europium concentrations, increasing the Tb3+ results in diminished PL from the Ce3+ and Tb3+ color centers. Simultaneously, the PL excitation bands related to both Ce3+ and Tb3+ increase in intensity for red emission from the Eu3+. Results are consistent with a Ce3+ → (Tb3+)n → Eu3+ energy transfer scheme, where (Tb3+)n denotes a chain incorporating n terbium ions. A high red to orange PL intensity ratio is obtained, ranging from 1.34 to 2.09. Raman vibrational bands show a systematic change, with Tb3+ concentration, in the B3O9 ring terminal oxygen bending mode coordinated with the yttrium site where dopant ions substitute. The structural changes are interpreted as variations in the local neighborhood of these sites in the YBO3:Ce3+,Tb3+,Eu3+ crystal structure.Structural and optical studies are reported of yttrium orthoborate YBO3 when tri-doped with Eu3+, Ce3+, and Tb3+, focusing on the role of terbium concentration. Incorporation of Tb3+ affects emission properties for photoluminescence (PL) excited by near ultraviolet light. For constant cerium and europium concentrations, increasing the Tb3+ results in diminished PL from the Ce3+ and Tb3+ color centers. Simultaneously, the PL excitation bands related to both Ce3+ and Tb3+ increase in intensity for red emission from the Eu3+. Results are consistent with a Ce3+ → (Tb3+)n → Eu3+ energy transfer scheme, where (Tb3+)n denotes a chain incorporating n terbium ions. A high red to orange PL intensity ratio is obtained, ranging from 1.34 to 2.09. Raman vibrational bands show a systematic change, with Tb3+ concentration, in the B3O9 ring terminal oxygen bending mode coordinated with the yttrium site where dopant ions substitute. The structural changes are interpreted as variations in the local neighborhood of these si...

Role of phonons in the optical properties of magnetron sputtered ZnO studied by resonance Raman and photoluminescence

The temperature dependence of phonons in ZnO has been studied using resonance Raman and photoluminescence (PL) emission measurements. Excitation with wavelength 363.8 nm (photon energy 3.409 eV) is used to establish incoming resonance near room temperature. Broad PL emission is seen at room temperature with peak position at 3.25 eV. This coincides with the overtone of the longitudinal optic (LO) band. Up to six LO phonon orders are observed. Temperature dependence of the LO phonon energy is described by a two-phonon decay mechanism with energies 100 and 496 cm−1. The temperature dependence of the PL shift is interpreted based on electron-phonon interactions. A two-phonon description is sufficient to describe the temperature shift in the band gap through occupation at average acoustic and optic phonon energies 125 and 500 cm−1, respectively. LO phonon sidebands (PSBs) are also observed at low temperature (23 to 100 K). The temperature shift in the PSB energies is interpreted based on the band gap shift com...

Temperature and excitation intensity dependence of photoluminescence in AlGaN quantum wells with mixed two-dimensional and three-dimensional morphology

The temperature dependences of the time-integrated and time-resolved photoluminescence (PL) properties for AlGaN multiple quantum wells are examined. The wells are grown so that a range of mixed two-dimensional (2D) and three-dimensional (3D) morphology is obtained, ranging from primarily 2D to primarily 3D. The temperature dependence of the band edge is studied using absorption spectroscopy and found to be described by electron-phonon interactions. The temperature dependence of the PL emission intensities and lifetimes are described by a model incorporating the effects of thermal activation and hopping, both of which compete with the radiative process. These factors are smaller than the observed Stokes shift between absorption and emission, and attributed to intra-well electron transfer that varies according to 2D and 3D character. High excitation intensity reveals two PL bands in the sample with intermediate 2D/3D well morphology. The intensity dependence is described based on screening and bandgap reno...

Influence of phonons on the temperature dependence of the band gap of AlN and AlxGa1−xN alloys with high AlN mole fraction

The temperature dependence of the optical band gap of AlN and AlxGa1−xN alloys, with x ∼ 0.6, has been studied using optical absorption spectroscopy. The band gap shrinkage is interpreted based on electron-phonon interactions using a two-phonon oscillator model. The two-oscillator model includes average acoustic and optic phonon energies and thermal occupation factor described by the Bose function. The temperature dependence of the transition width, from transparent to opaque, is also described based on electron-phonon interactions.