Browsing by Author "Bakan, Gökhan"

Now showing 1 - 12 of 12

Citation Count: 10
Activation energy of metastable amorphous Ge₂Sb₂Te₅ from room temperature to melt
(Amer inst Physics, 2018) Muneer, Sadid; Scoggin, Jake; Dirisaglik, Faruk; Adnane, Lhacene; Cywar, Adam; Bakan, Gokhan; Gokirmak, Ali; Department of Electrical & Electronics Engineering
Resistivity of metastable amorphous Ge2Sb2Te5 (GST) measured at device level show an exponential decline with temperature matching with the steady-state thin-film resistivity measured at 858 K (melting temperature). This suggests that the free carrier activation mechanisms form a continuum in a large temperature scale (300 K - 858 K) and the metastable amorphous phase can be treated as a supercooled liquid. The effective activation energy calculated using the resistivity versus temperature data follow a parabolic behavior, with a room temperature value of 333 meV, peaking to similar to 377 meV at similar to 465 K and reaching zero at similar to 930 K, using a reference activation energy of 111 meV (3k(B)T/2) at melt. Amorphous GST is expected to behave as a p-type semiconductor at T-melt similar to 858 K and transitions from the semiconducting-liquid phase to the metallic-liquid phase at similar to 930 K at equilibrium. The simultaneous Seebeck (S) and resistivity versus temperature measurements of amorphous-fcc mixed-phase GST thin-films show linear S-T trends that meet S = 0 at 0 K, consistent with degenerate semiconductors, and the dS/dT and room temperature activation energy show a linear correlation. The single-crystal fcc is calculated to have dS/dT = 0.153 mu V/K-2 for an activation energy of zero and a Fermi level 0.16 eV below the valance band edge. (C) 2018 Author(s).
Citation Count: 44
Biocompatible Electroactive Tetra(aniline)-Conjugated Peptide Nanofibers for Neural Differentiation
(Amer Chemical Soc, 2018) Arioz, Idil; Erol, Ozlem; Bakan, Gokhan; Dikecoglu, F. Begum; Topal, Ahmet E.; Urel, Mustafa; Guler, Mustafa O.; Department of Electrical & Electronics Engineering
Peripheral nerve injuries cause devastating problems for the quality of patients' lives, and regeneration following damage to the peripheral nervous system is limited depending on the degree of the damage. Use of nanobiomaterials can provide therapeutic approaches for the treatment of peripheral nerve injuries. Electroactive biomaterials, in particular, can provide a promising cure for the regeneration of nerve defects. Here, a supramolecular electroactive nanosystem with tetra(aniline) (TA)-containing peptide nanofibers was developed and utilized for nerve regeneration. Self-assembled TA conjugated peptide nanofibers demonstrated electroactive behavior. The electroactive self-assembled peptide nanofibers formed a well-defined three-dimensional nanofiber network mimicking the extracellular matrix of the neuronal cells. Neurite outgrowth was improved on the electroactive TA nanofiber gels. The neural differentiation of PC-12 cells was more advanced on electroactive peptide nanofiber gels, and these biomaterials are promising for further use in therapeutic neural regeneration applications.
Citation Count: 8
Colorimetric detection of ultrathin dielectrics on strong interference coatings
(Optical Soc Amer, 2018) Ayas, Sencer; Bakan, Gokhan; Ozgur, Erol; Celebi, Kemal; Torunoglu, Gamze; Dana, Aykutlu; Department of Electrical & Electronics Engineering
Metal films covered with ultrathin lossy dielectrics can exhibit strong interference effects manifested as the broad absorption of the incident light resulting in distinct surface colors. Despite their simple bilayer structures, such surfaces have only recently been scrutinized and applied mainly to color printing. Here, we report the use of such surfaces for colorimetric detection of ultrathin dielectrics. Upon deposition of a nanometer-thick dielectric on the surface, the absorption peak red shifts, changing the surface color. The color contrast between the bare and dielectric- coated surfaces can be detected by the naked eye. The optical responses of the surfaces are characterized for nanometerthick SiO2, Al2O3, and bovine serum albumin molecules. The results suggest that strong interference surfaces can be employed as biosensors. (C) 2018 Optical Society of America
Citation Count: 18
The design and fabrication of supramolecular semiconductor nanowires formed by benzothienobenzothiophene (BTBT)-conjugated peptides
(Royal Soc Chemistry, 2018) Khalily, Mohammad Aref; Usta, Hakan; Ozdemir, Mehmet; Bakan, Gokhan; Dikecoglu, F. Begum; Edwards-Gayle, Charlotte; Guler, Mustafa O.; Department of Electrical & Electronics Engineering
pi-Conjugated small molecules based on a [1]benzothieno[3,2-b]benzothiophene (BTBT) unit are of great research interest in the development of solution-processable semiconducting materials owing to their excellent charge-transport characteristics. However, the BTBT -core has yet to be demonstrated in the form of electro-active one-dimensional (1D) nanowires that are self-assembled in aqueous media for potential use in bioelectronics and tissue engineering. Here we report the design, synthesis, and self-assembly of benzothienobenzothiophene (BTBT)-peptide conjugates, the BTBT-peptide (BTBT-C-3-COHN-Ahx-VVAGKK-Am) and the C-8-BTBT-peptide (C-8-BTBT-C-3-COHN-Ahx-VVAGKK-Am), as -sheet forming amphiphilic molecules, which self-assemble into highly uniform nanofibers in water with diameters of 11-13(+/- 1) nm and micron-size lengths. Spectroscopic characterization studies demonstrate the J-type - interactions among the BTBT molecules within the hydrophobic core of the self-assembled nanofibers yielding an electrical conductivity as high as 6.0 x 10(-6) S cm(-1). The BTBT -core is demonstrated, for the first time, in the formation of self-assembled peptide 1D nanostructures in aqueous media for potential use in tissue engineering, bioelectronics and (opto)electronics. The conductivity achieved here is one of the highest reported to date in a non-doped state.
Citation Count: 57
Fabrication of Supramolecular n/p-Nanowires via Coassembly of Oppositely Charged Peptide-Chromophore Systems in Aqueous Media
(Amer Chemical Soc, 2017) Khalily, Mohammad Aref; Bakan, Gokhan; Kucukoz, Betul; Topal, Ahmet Emin; Karatay, Ahmet; Yaglioglu, H. Gul; Guler, Mustafa O.; Department of Electrical & Electronics Engineering
Fabrication of supramolecular electroactive materials at the nanoscale with well-defined size, shape, composition, and organization in aqueous medium is a current challenge. Herein we report construction of supramolecular charge-transfer complex one-dimensional (1D) nanowires consisting of highly ordered mixed-stack pi-electron donor-acceptor (D-A) domains. We synthesized n-type and p-type beta-sheet forming short peptide-chromophore conjugates, which assemble separately into well-ordered nanofibers in aqueous media. These complementary p-type and n-type nanofibers coassemble via hydrogen bonding, charge-transfer complex, and electrostatic interactions to generate highly uniform supramolecular n/p-coassembled 1D nanowires. This molecular design ensures highly ordered arrangement of D-A stacks within n/p-coassembled supramolecular nanowires. The supramolecular n/p-coassembled nanowires were found to be formed by A D-A unit cells having an association constant (K-A) of 5.18 x 10(5) M-1. In addition, electrical measurements revealed that supramolecular n/p-coassembled nanowires are approximately 2400 and 10 times more conductive than individual n-type and p-type nanofibers, respectively. This facile strategy allows fabrication of well-defined supramolecular electroactive nanomaterials in aqueous media, which can find a variety of applications in optoelectronics, photovoltaics, organic chromophore arrays, and bioelectronics.
Citation Count: 0
Infrared absorption spectroscopy of monolayers with thin film interference coatings
(Optica Publishing Group (formerly OSA), 2017) Ayas,S.; Bakan,G.; Ozgur,E.; Celebi,K.; Dana,A.; Department of Electrical & Electronics Engineering
We report high performance Infrared spectroscopy platforms based on interference coatings on metal using CaF2 dielectric films and Ge2Sb2Te5 (GST) phase-change films. IR vibrational bands of proteins and organic monolayers are also detected. © 2017 OSA.
Citation Count: 0
Infrared absorption spectroscopy of monolayers with thin film interference coatings
(Institute of Electrical and Electronics Engineers Inc., 2017) Ayas,S.; Bakan,G.; Ozgur,E.; Celebi,K.; Dana,A.; Department of Electrical & Electronics Engineering
We report high performance Infrared spectroscopy platforms based on interference coatings on metal using CaF2 dielectric films and Ge2Sb2Te5 (GST) phase-change films. IR vibrational bands of proteins and organic monolayers are also detected. © 2017 IEEE.
Citation Count: 0
Infrared Absorption Spectroscopy of Monolayers with Thin Film Interference Coatings
(Ieee, 2017) Ayas, Sencer; Bakan, Gokhan; Ozgur, Erol; Celebi, Kemal; dana, Aykutlu; Department of Electrical & Electronics Engineering
We report high performance Infrared spectroscopy platforms based on interference coatings on metal using CaF2 dielectric films and Ge2Sb2Te5 (GST) phase-change films. IR vibrational bands of proteins and organic monolayers are also detected.
Citation Count: 0
Interference coatings for infrared spectroscopy and colorimetric sensing
(Optica Publishing Group (formerly OSA), 2018) Bakan,G.; Ayas,S.; Ozgur,E.; Celebi,K.; Dana,A.; Department of Electrical & Electronics Engineering
Constructive interference and strong interference surfaces are created to sense ultrathin probe materials such as monolayer protein molecules using enhanced infrared absorption spectroscopy and colorimetric detection, respectively. © 2018 The Author(s).
Citation Count: 28
Invisible Thin-Film Patterns with Strong Infrared Emission as an Optical Security Feature
(Wiley-v C H verlag Gmbh, 2018) Bakan, Gokhan; Ayas, Sencer; Serhatlioglu, Murat; Elbuken, Caglar; Dana, Aykutlu; Department of Electrical & Electronics Engineering
Spectrally selective thermal emission is in high demand for thermophotovoltaics, radiative cooling, and infrared sensing applications. Spectral control of the emissivity is historically achieved by choosing the material with suitable infrared properties. The recent advancements in nanofabrication techniques that lead to enhanced light-matter interactions enable optical properties like infrared emissivity that are not naturally available. In this study, thermal emitters based on nanometer-thick dielectrics on field-enhancement surfaces as optical security features are proposed. Such a function is achieved by generating patterns by ultrathin dielectrics that are transparent in the visible and exhibit strong infrared absorption in the spectral range of thermal cameras. The invisible patterns are then revealed by thermal imaging. The field-enhancement surfaces enhance the emissivity of the patterns, in turn reduce the minimum temperature to detect the thermal emission down to approximate to 30 degrees C from >150 degrees C to exploit ubiquitous heat sources like the human body. The study provides a framework for the use of thermal emitters as optical security features and demonstrates applications on rigid and flexible substrates.
Citation Count: 0
Multispectral plasmonic structures using native aluminum oxide and aluminum
(Optica Publishing Group (formerly OSA), 2017) Ayas,S.; Bakan,G.; Dana,A.; Department of Electrical & Electronics Engineering
We report the use of native aluminum oxide to fabricate periodic metal-insulator-metal resonators with simultaneous resonances in the visible and IR wavelengths. The cavity size is in the order of λ3/25000 in the NIR. © 2017 OSA.
Citation Count: 7
Tunable enhanced infrared absorption spectroscopy surfaces based on thin VO₂ films
(Optical Soc Amer, 2018) Bakan, Gokhan; Ayas, Sencer; Dana, Aykutlu; Department of Electrical & Electronics Engineering
Infrared absorption spectroscopy takes advantage of the electric field enhancement to detect low amounts of materials such as monolayer biomolecules. While the plasmonic field enhancement is the popular approach, it has been demonstrated that the interference-based uniform field enhancement using a simple dielectric/metal structure exhibits higher sensitivity and larger spectral bandwidth for ultrathin materials. Here, we numerically demonstrate that the enhancement bandwidth of such interference coatings can be further increased by inserting a VO2 thin film between the dielectric and metal layers. The field enhancement spectrum blueshifts upon thermally-induced insulator-to-metal transition in the VO2 layer. The structure that maximizes the enhancement bandwidth is determined as 880-nm-thick CaF2 on 350-nm-thick VO2 on optically thick Al. The study is completed with the investigation of using a bottom metal layer as an internal heater to electrothermally induce the phase change. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement