arXiv subjects

Tugrul Guner

Publications and source records attributed to Tugrul Guner.

3 recordsLinked to original sources

arXiv2019

Green Fabrication of Lanthanide doped Hydroxide-based Phosphors: Y(OH)3:Eu3+ Nanoparticles for White Light Generation

Phosphors serve as color conversion layers to generate white light with varying optical features including CRI, CCT, and luminous efficacy. However, they have been produced in harsh synthesis conditions such as high temperature, high pressure, and/or employing a huge amount of solvents. In this work, facile, water-based and a rapid method has been proposed to fabricate lanthanide doped hydroxide-based phosphors. In this sense, submicrometer-sized Y(OH)3:Eu3+ particles, as red phosphor, were synthesized in water at ambient conditions in <60 min reaction time. The doping ratio is controlled from 2.5 to 20% in terms of mole. Meanwhile, first principle calculations were also performed on Y(OH)3:Eu3+ to understand the preferable doping scenario and its optoelectronic properties. As an application, these fabricated red phosphors were integrated into PDMS/YAG:Ce3+ composite to generate white light. The resulting white light showed a remarkable improvement (~24%) for LER, a slight reduction of CCT (from 3900K to 3600K), and an unchanged CRI (~60) as the amount of Y(OH)3:Eu3+ increases.

cond-mat.mtrl-sci↗

arXiv2018

CsPbBr3 Perovskites: Theoretical and Experimental Investigation on Water-Assisted Transition From Nanowire Formation to Degradation

Recent advances in colloidal synthesis methods have led to increased research focus on halide perovskites. Due to highly ionic crystal structure of perovskite materials, stability issue pops up especially against polar solvents such as water. In this study, we investigate water-driven structural evolution of CsPbBr3 by performing experiments and state-of-the-art first-principles calculations. It is seen that while optical image shows the gradual degradation of yellowish-colored CsPbBr3 structure under daylight, UV illumination reveals that the degradation of crystals takes place in two steps; transition from blue-emitting to green-emitting structure and and then transition from green-emitting phase to complete degradation. We found that as-synthesized CsPbBr3 NWs emit blue light under 254 nm UV source and before the degradation, first CsPbBr3 NWs undergoes a water-driven structural transition to form large bundles. It is also seen that formation of such bundles provide longer-term environmental stability. In addition theoretical calculations revealed how strong is the interaction of water molecules with ligands and surfaces of CsPbBr3 and provide atomistic-level explanation to transition from ligand-covered nanowires to bundle formation. Further interaction of green-light-emitting bundles with water causes complete degradation of CsPbBr3 and photoluminescence signal is entirely quenched. Moreover, Raman and XRD measurements revealed that completely degraded regions are decomposed to PbBr2 and CsBr precursors. We believe that findings of this study may provide further insight into the degradation mechanism of CsPbBr3 perovskite by water.

cond-mat.mtrl-sci↗

arXiv2017

Statistical Approach to Tunneling Time in Attosecond Experiments

Tunneling, transport of particles through classically forbidden regions, is a pure quantum phenomenon. It governs numerous phenomena ranging from single-molecule electronics to donor-acceptor transition reactions. The main problem is the absence of a universal method to compute tunneling time. This problem has been attacked in various ways in the literature. Here, in the present work, we show that a statistical approach to the problem, motivated by the imaginary nature of time in the forbidden regions, lead to a novel tunneling time formula which is real and subluminal (in contrast to various known time definitions implying superluminal tunneling). This entropic tunneling time, as we call it, shows good agreement with the tunneling time measurements in laser-driven He ionization. Moreover, it sets an accurate range for long-range electron transfer reactions. The entropic tunneling time is general enough to extend to the photon and phonon tunneling phenomena.

quant-ph↗