No description available.

No description available.

No description available.

No description available.

No description available.

Rare-earth ions holmium (Ho+3), Thulium (Tm+3), and Samarium (Sm+3) were investigated for infrared emission and their possible biomedical applications by a photoluminescence (PL) system. Holmium’s (Ho+3) emission peaks were the result of transitions 5 S2 → 5 I7, and 5 S2 → 5 I5 respectively. Samarium’s (Sm+3) emission peaks were 936 nm and 1863 nm. Thulium’s (Tm+3) emission peaks were the a result of transitions 3 H4 → 3 H6, 3 H5 → 3 H6 , and 3 F4 → 3 H6 respectively. Erbium Oxide nanoparticles (Er2O3) mixed with water by a photoluminescence (PL) system. Erbium Oxide’ (Er2O3) nanoparticle’s emission peaks were the a result of transitions 4 I15/2 → 4 S3/2 , 4 I15/2 → 4 I13/2 respectively. The process was also repeated in vacuum and it was found that the green emission enhances tremendously when the nanoparticles are excited in vacuum. This enhanced luminescence from the Erbium Oxide nanoparticles shows their potential importance in the optical devices and Biomedical applications. / Department of Physics and Astronomy

No description available.

Graduation date: 1997

In this thesis we develop a new first-principles method for the calculation of optical absorption spectra and band structures in semiconductors and insulators at finite tem- peratures. The theoretical framework of our methodology originates back to 1950s in two pivotal research papers by F. Williams and M. Lax. Here, we expand the scope of the pioneering works by Williams and Lax, and we present a new theory of phonon- assisted optical absorption and temperature-dependent band structures. We demon- strate that our technique is highly efficient and simple to the point that a single calcu- lation is sufficient to capture temperature-dependent absorption coefficients including the effect of quantum zero-point motion. We report calculations of optical absorption spectra and of direct and indirect band gaps of Si, C, GaAs and MAPbI₃ . We obtain good agreement with experiment and with previous calculations. The approach pro- posed in this thesis is highly versatile, and can straightforwardly be combined with improved descriptions of the dielectric function by including electron-hole effects via the Bethe-Salpeter and GW equations. In this thesis we also investigate the underly- ing mechanisms leading to the "inverse Varshni effect" in materials that belong to the family of metal halide perovskites. We show, using the example of MAPbI 3 , that the vibrational modes with metal-halide-metal bending or rocking character are the major cause of the band gap opening with increasing temperature. To this aim we present an approach for elucidating the physics underpinning the changes of the band gap with vibrational motion. Our methodologies developed in this thesis are simple to imple- ment in any electronic structure package as a post-processing step, having the potential to find broad applications in the ab-initio community. We anticipate that our work will open the way to predictive calculations, as well as will contribute to the better under- standing of the optoelectronic properties of solids at finite temperatures.

It is of fundamental significance to differentiate an enantiomer from its mirror image (i.e., enantiodifferentiation), through monitoring optical activity (OA) of enantiomers that is typically characterized by electronic circular dichroism (ECD or CD) in the UV-visible region. However, sub-wavelength molecular dimensions substantially prevent enantiomers from effectively perceiving the different circular polarization states, leading to low enantiomeric OA and weak enantiodifferentiation. Some approaches have been developed to amplify the enantiomeric OA; alternatively, on the basis of the emerging chiral metamaterials of metallic helical nanoparticles (HNPs) I devise two methods to enhance the enantiodifferentiation. First, I employ glancing angle deposition (GLAD) to deposit Ag HNPs with a helical pitch (P) larger than wire diameter (d) of the helical, i.e., Ag nanohelices (AgNHs). AgNHs exhibit strong plasmonic CD composed of a broadband longitudinal mode (i.e., L-mode) in the visible region, a transverse mode (i.e., T-mode) at a wavelength of ~370 nm, and a dielectric mode in the deep UV region (at a wavelength shorter than 320 nm). Adsorption of alkyl ligands on the AgNHs markedly weakens the two plasmonic CD modes, and the T-mode is weakened more seriously than the L-mode. The deterioration of the plasmonic CD is exacerbated with increasing the bonding energy of the Ag-alkyl ligand contacts, attributed to the increase of the dielectric constant of the medium of the AgNHs (εr) and the electron withdrawal from the AgNHs towards the alkyl ligands. Derived from the ligand-induced weakening of the plasmonic CD, enantiodifferentiation of L-Glutathione (L-GSH) from D-GSH is dramatically enhanced. The chiroptical weakening sensitively varies with the absolute configuration of GSH, resulting in an enantiodifferentiation anisotropic g factor of ~0.5 that is independent on the AgNH helicity. The AgNH-induced anisotropy g factor is superior to those obtained by other methods, by 2 - 4 orders of magnitude. It is the largest achieved up-to-date, as high as one-fourth of the theoretical maximum. Second, I operate GLAD with fast substrate rotation to reduce P less than d, to generate AgHNPs that exhibit negligible dielectric CD in the deep UV region, offering a helical substrate to directly amplify the OA of enantiomers grafted on the AgHNPs. The anchoring of enantiomers on AgHNPs with the sub-5 nm P leads to the enantioselective amplification of the enantiomeric OA in roughly ten folds; the LH- and RH-AgHNPs give rise to amplify the OA of (S)- and (R)-enantiomers, respectively. It is ascribed to the change of the dihedral angle of an enantiomer adsorbed on AgHNPs. Such the enantioselective amplification tends not to occur as long as P > 5 nm. Moreover, given the enantiodifferentiation of biomolecules that are typically dissolved in an aqueous solution, the effect of water on the plasmonic CD of AgHNPs is investigated and compared with that of AgNHs. Hydrophobic AgNHs with high structural porosity give rise to the irreversible water effect on the plasmonic CD; and hydrophilic AgHNPs with low structural porosity lead to the reversible water effect. At the end, I devise a new methodology to generate plasmonic CD through chirality transfer from chiral host to achiral guest, owing to the helicity duplication of the achiral guest from the chiral host. It leads to inducing chiroptical activity of the achiral guest made of some plasmonic materials that aren't facilely sculptured in the helical. The new methodology effectively broadens the range of materials made from the chiral nanostructures, which is on demand to develop diverse chirality-related bioapplications.