Optical Response of Aged Doped and Undoped GaAs Samples.

We studied epitaxial GaAs samples doped with Ge and Sn up to 1×1019 cm -3, which were stored in a dry and dark environment for 26 years. The optical response of the GaAs samples was determined through the photoluminescence and photoreflectance techniques, taken at different times: just after their fabrication in 1995, 2001 and 2021. The evolution of defects formed by the action of O 2 in the samples and their correlation with doping with Ge and Sn impurities were studied. We obtained the result that aging formed defects of type vacancies, mainly As, which produced energy levels of deep traps linked to the L band. The concentration of vacancies over the 26 years could be as large as 1017 cm -3, and these vacancies form complexes with doping impurities.

Nonlinear optical response of graphene oxide quantum dots fabricated from electrospun polyacrylonitrile fibers.

The nonlinear optical response of graphene oxide quantum dots (GOQDs) fabricated by the carbonization and exfoliation of electrospun polyacrylonitrile (PAN) fibers is reported. Electrospun and carbonized fibers were characterized by SEM and XPS. SEM micrograph confirmed the formation of PAN fibers of 153.44 ± 6.44 nm, while by XPS the binding energies associated with sp2 and sp3 carbon hybridizations were found, after the carbonization process. On the other hand, the GOQDs obtained were characterized by photoluminescence (PL), UV-Vis, Raman spectroscopy, and High-Resolution Transmission Electron Microscopy (HRTEM). The GOQDs size of 10 nm was estimated by HRTEM. Raman spectroscopy showed the D and G bands associated with the sp2 and sp3 hybridizations of the GOQDs, by PL two energy values of 2.67 and 2.97 eV were calculated. The UV-Vis spectrum showed two absorption bands confirming the presence of GOQDs. The nonlinear characterization was carried out using the P-scan technique, previously photodepositing GOQDs onto an optical fiber, using a coherent radiation source at a wavelength of 1550 nm. The results obtained showed a saturable absorption behavior with a value of ß = - 2.474 × 10 - 4 m / W and a nonlinear susceptibility of χ ( 3 ) ≈ - 7.749 × 10 - 4 ( e s u ) . The results of this work showed that GOQDs obtained can be used for optical switching applications.

Optical properties of anatase TiO2: synergy between transition metal doping and oxygen vacancies.

Charge carriers (electrons and holes) are generated on the TiO2 using UV radiation; this excitation energy can be reduced by modifying the material electronic structure, for example, by doping or creating oxygen vacancies. Here, the electronic structure of a transition metal-doped anatase, bulk and surface, and their interaction with oxygen vacancies are studied using density functional theory. The visible light response of metal-doped TiO2 (101) is also determined. Transition metals generate intra-band gap states, which reduce the excitation energy but may also act as charge recombination sites. Dopants Fe, Co, and Ni remarkably enhance the visible light response due to the states in the middle of the gap. However, Co and Ni create heavier charge carriers. Our results show that Pd and Pt-doped TiO2 generate states near the valence and conduction band with a "clean" band gap (without states in the middle of the gap). Moreover, Pt-doped TiO2 maintains the charge mobility because it presents a small charge carriers mass. Hence, Pt-doped TiO2 represents the best alternative to activate TiO2 under visible light. The optical response of transition metal-doped TiO2 follows the order 3d > 4d > 5d. The oxygen vacancies reduce the response of metal-doped TiO2 to visible light because the unpaired electrons generated occupy the empty states of metal-doping. Graphical Abstract Density of states of TiO2 (101) surface doped with transition metals and oxygen vacancies.