Browsing by Author "Sahin, M."

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Open Access
Improved selectivity from a wavelength addressable device for wireless stimulation of neural tissue
(Frontiers Research Foundation, 2014) Seymour, E. Ç.; Freedman, D. S.; Gökkavas, M.; Özbay, Ekmel; Sahin, M.; Ünlü, M. S.
Electrical neural stimulation with micro electrodes is a promising technique for restoring lost functions in the central nervous system as a result of injury or disease. One of the problems related to current neural stimulators is the tissue response due to the connecting wires and the presence of a rigid electrode inside soft neural tissue. We have developed a novel, optically activated, microscale photovoltaic neurostimulator based on a custom layered compound semiconductor heterostructure that is both wireless and has a comparatively small volume (<0.01 mm3). Optical activation provides a wireless means of energy transfer to the neurostimulator, eliminating wires and the associated complications. This neurostimulator was shown to evoke action potentials and a functional motor response in the rat spinal cord. In this work, we extend our design to include wavelength selectivity and thus allowing independent activation of devices. As a proof of concept, we fabricated two different microscale devices with different spectral responsivities in the near-infrared region. We assessed the improved addressability of individual devices via wavelength selectivity as compared to spatial selectivity alone through on-bench optical measurements of the devices in combination with an in vivo light intensity profile in the rat cortex obtained in a previous study. We show that wavelength selectivity improves the individual addressability of the floating stimulators, thus increasing the number of devices that can be implanted in close proximity to each other. © 2014 Seymour, Freedman, Gökkavas, Özbay, Sahinand Ünlü.
Open Access
Reordering orbitals of semiconductor multi-shell quantum dot-quantum well heteronanocrystals
(American Institute of Physics, 2012-01-27) Sahin, M.; Nizamoglu, S.; Yerli, O.; Demir, Hilmi Volkan
Based on self-consistent computational modeling of quantum dot-quantum well (QDQW) heteronanocrystals, we propose and demonstrate that conduction-electron and valence-hole orbitals can be reordered by controlling shell thicknesses, unlike widely known core/shell quantum dots (QDs). Multi-shell nanocrystals of CdSe/ZnS/CdSe, which exhibit an electronic structure of 1s-1p-2s-2p-1d-1f for electrons and 1s-1p-2s-2p-1d-2d for holes using thin ZnS and CdSe shells (each with two monolayers), lead to 1s-2s-1p-1d-1f-2p electron-orbitals and 1s-2s-1p-1d-2p-1f hole orbitals upon increasing the shell thicknesses while keeping the same core. This is characteristically different from the only CdSe core and CdSe/ZnS core/shell QDs, both exhibiting only 1s-1p-1d-2s-1f-2p ordering for electrons and holes.
Open Access
Self-consistent computation of electronic and optical properties of a single exciton in a spherical quantum dot via matrix diagonalization method
(American Institute of Physics, 2009-08-21) Sahin, M.; Nizamoglu, S.; Kavruk, A. E.; Demir, Hilmi Volkan
In this study, we develop and demonstrate an efficient self-consistent calculation schema that computes the electronic structure and optical properties of a single exciton in a spherical quantum dot (QD) with an interacting pair of electron and hole wave functions. To observe modifications on bands, wave functions, and energies due to the attractive Coulomb potential, the full numeric matrix diagonalization technique is employed to determine sublevel energy eigenvalues and their wave functions in effective mass approximation. This treatment allows to observe that the conduction and valance band edges bend, that the electron and hole wave functions strongly localize in the QD, and that the excitonic energy level exhibits redshift. In our approach for the Coulomb term between electron and hole, the Poisson-Schrodinger equations are solved self-consistently in the Hartree approximation. Subsequently, exciton binding energies and associated optical properties are computed. The results are presented as a function of QD radii and photon energies. We conclude that all of these numerical results are in agreement with the experimental studies.