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    Adsorption of Ethylene and 1-methylcyclopropene (1-MCP) on Al-doped Graphene Structure: A DFT Study for Gas Sensing Application
    (Turkish Chemical Society, 2024) Aydin, Fatma; Sel, Kivanc
    Ethylene is the ripening hormone of fruits and vegetables. 1-Methylcyclopropene (1-MCP) is used as the inhibitor of the ethylene actions for extending the postharvest shelf life of the plants. To control the ripening and extending the shelf life of the plants, the adsorption characteristics of ethylene and 1-MCP on Al-doped graphene structure (AlG) were investigated as a gas sensing application by density functional theory (DFT) calculations. The geometric structures were optimized, HOMO and LUMO, energy gap, adsorption energies, the density of states (DOS), electrostatic potential (ESP) and the global reactivities were calculated for different distances between the adsorbed ethylene or 1-MCP and the adsorbent AlG. Chemisorption and physisorption interactions were analyzed. For the chemisorption process of ethylene and 1-MCP on AlG, the adsorption energies were 19.34 kJ/mol and 56.53 kJ/mol, respectively. Whereas for the physisorption process, the adsorption energies of ethylene and 1-MCP were-60.16 kJ/mol and-7.32 kJ/mol, respectively. As a result, it was presented that the AlG structure has sufficient characteristics to be a good adsorbent and a gas sensor of ethylene and 1-MCP. © 2024, Turkish Chemical Society. All rights reserved.
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    Detection of relative [Na+] and [K+] levels in sweat with optical measurements
    (Amer Inst Physics, 2014) Al-Omari, Mahmoud; Sel, Kivanc; Mueller, Anja; Edwards, Jeffery; Kaya, Tolga
    We describe the use of 2-hydroxy-1,4-naphthoquinone (HNQ, Lawsone) as a potential sweat electrolyte measurement marker. We use ultraviolet-visible absorbance measurements to determine the absorbance energy in a particular wavelength range (400 nm-500 nm). This novel approach allows us to eliminate the importance of the exact wavelength of the absorbance peak but find the integral of the range of interest. Although we numerically calculate the absorbance energy, it is imperative to use photodiodes to measure the intensity of the transmitted light that is fabricated particularly for the range of interest for future device implementations. We explored various mixing ratios of water and acetone to find the optimum solvent that would give the most sensitive and accurate relative electrolyte sensing. The pH value was also modified to see the effect on the absorbance energy and intensity. A representative group of subjects were used to collect sweat from the dehydration and hyperhydration cases. The results are convincing that HNQ solutions can be used as a wearable, continuous sweat sensor. (C) 2014 AIP Publishing LLC.
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    Determination of the refractive index of a dielectric film continuously by the generalized S-transform
    (Optical Soc Amer, 2010) Coskun, Emre; Sel, Kivanc; Ozder, Serhat
    The generalized S-transform was improved as a method to determine the refractive index of a dielectric film continuously by using the transmittance spectrum, and the applicability of the method was demonstrated on mica. The result determined from the generalized S-transform method was compared with the results determined from the S-transform and the fringe counting methods and published values. The advantage of the proposed method was explained, and the absolute error of the presented method was also calculated. (c) 2010 Optical Society of America
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    Effects of carbon content and plasma power on room temperature photoluminescence characteristics of hydrogenated amorphous silicon carbide thin films deposited by PECVD
    (Elsevier Science Sa, 2017) Gunes, Ibrahim; Sel, Kivanc
    The room temperature photoluminescence characteristics of a-SiCx:H thin films, deposited by plasma enhanced chemical vapor deposition technique with various carbon contents (x), at lower (30 mW/cm(2)) (LP) and higher (90 mW/cm(2)) (HP) power densities, were analyzed. The carbon content of the a-SiCx: H thin films was determined by X-ray photoelectron spectroscopy. The peak energies and the full width half maxima of the PL spectra were compared with the optical energies, determined by ultraviolet-visible transmittance measurements and a linearly correlated increase was observed as a function of x. PL peak energies shifted from 1.99 eV to 2.60 eV for LP and from 1.89 to 2.91 eV for HP films. The PL mechanisms were investigated in the frame of the static disorder model and the stokes shift model. Our analyses indicated that for LP films the stokes shift model and for HP films the static disorder model are the dominant mechanisms of PL; moreover it was determined that both of these independent models have contributions to PL mechanism. Additionally, PL spectra were analyzed by the joint density of tail states (JDOS) PL model. The increase in the PL peak energies and spectrum widths by the increase in the plasma power and carbon content considerably fitted to JDOS PL model, which could exclusively represent the PL mechanism by itself. (C) 2017 Elsevier B.V. All rights reserved.
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    Effects of phosphorus on the electrical characteristics of plasma deposited hydrogenated amorphous silicon carbide thin films
    (Pergamon-Elsevier Science Ltd, 2018) Alcinkaya, Burak; Sel, Kivanc
    The properties of phosphorus doped hydrogenated amorphous silicon carbide (a-SiCx:H) thin films, that were deposited by plasma enhanced chemical vapor deposition technique with four different carbon contents (x), were analyzed and compared with those of the intrinsic a-SiCx:H thin films. The carbon contents of the films were determined by X-ray photoelectron spectroscopy. The thickness and optical energies, such as Tauc, E-04 and Urbach energies, of the thin films were determined by UV-Visible transmittance spectroscopy. The electrical properties of the films, such as conductivities and activation energies were analyzed by temperature dependent current-voltage measurements. Finally, the conduction mechanisms of the films were investigated by numerical analysis, in which the standard transport mechanism in the extended states and the nearest neighbor hopping mechanism in the band tail states were taken into consideration. It was determined that, by the effect of phosphorus doping the dominant conduction mechanism was the standard transport mechanism for all carbon contents.
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    Effects of tail states on the conduction mechanisms in silicon carbide thin films with high carbon content
    (Pergamon-Elsevier Science Ltd, 2011) Sel, Kivanc; Akaoglu, Baris; Atilgan, Ismail; Katircioglu, Bayram
    Hydrogenated amorphous silicon carbide (a-SiCx:H) films of different carbon content (x) were deposited by radio frequency plasma enhanced chemical vapor deposition (PECVD) system. Apart from the X-ray photoelectron spectroscopy (XPS) and UV-Visible transmission analyses, the resistivity measurements between 293 K and 450 K were emphasized to assess the eventual transport mechanisms. The film resistivities are unexpectedly found relatively low, especially for high carbon content. In the frame of exclusive band conduction, the apparent thermal activation energies, evaluated from Arrhenius type plot remain too low compared to half values of the optical gaps. Numerical analyses were undertaken by extending conduction from the band conduction about the mobility edge inside the band gap by including the nearest neighbor hopping (NNH) conduction across the localized tail states. By successfully fitting the formulated conductivity expression to the experimental results, parameters such as tail states distributions, true activation energies to the mobility edge have been retrieved. (C) 2010 Elsevier Ltd. All rights reserved.
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    Electroluminescence properties of a PIN structure made by nearly stoichiometric a-SiCx:H active layer
    (Pergamon-Elsevier Science Ltd, 2009) Sel, Kivanc; Akaoglu, Baris; Ozdemir, Orhan; Atilgan, Ismail
    a-SiCx:H PIN diode has been fabricated within a single pump-down process under the same deposition conditions used for doped and undoped PECVD grown thin films, whose optical and electrical properties are determined and compared with a-Si:H. Current-voltage characteristics of PIN diode are evaluated and concluded to be limited by tunnelling of holes at p-i interface into valence band tail states. Electroluminescence measurements revealed radiative monomolecular recombinations. Deconvolution of the luminescence spectra is utilized to analyse recombination mechanism to be dominated by the transitions between band tails and deep states, which are created by the large density of both silicon and carbon dangling bonds, probable in the stoichiometric a-SiCx:H film. Finally, a small luminescence peak around 1.9 eV would be an evidence of reduced probability of tail to tail transitions, than that of the transitions between tail and deep states. (C) 2008 Elsevier Ltd. All rights reserved.
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    Refractive index and extinction coefficient determination of an absorbing thin film by using the continuous wavelet transform method
    (Optical Soc Amer, 2008) Coskun, Emre; Sel, Kivanc; Ozder, Serhat; Kurt, Mustafa
    We present the continuous wavelet transform (CWT) method for determining the dispersion curves of the refractive index and extinction coefficient of absorbing thin films by using the transmittance spectrum in the visible and near infrared regions at room temperature. The CWT method is performed on the transmittance spectrum of an a - Si1-xCx:H film, and the refractive index and extinction coefficient of the film are continuously determined and compared with the results of the envelope and fringe counting methods. Also the noise filter property of the method is depicted on a theoretically generated noisy signal. Finally, the error analyses of the CWT, envelope, and fringe counting methods are performed. (C) 2008 Optical Society of America
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    Room temperature photoluminescence spectrum modeling of hydrogenated amorphous silicon carbide thin films by a joint density of tail states approach and its application to plasma deposited hydrogenated amorphous silicon carbide thin films
    (Elsevier Science Sa, 2012) Sel, Kivanc; Gunes, Ibrahim
    Room temperature photoluminescence (PL) spectrum of hydrogenated amorphous silicon carbide (a-SiCx:H) thin films was modeled by a joint density of tail states approach. In the frame of these analyses, the density of tail states was defined in terms of empirical Gaussian functions for conduction and valance bands. The PL spectrum was represented in terms of an integral of joint density of states functions and Fermi distribution function. The analyses were performed for various values of energy band gap, Fermi energy and disorder parameter, which is a parameter that represents the width of the energy band tails. Finally, the model was applied to the measured room temperature PL spectra of a-SiCx:H thin films deposited by plasma enhanced chemical vapor deposition system, with various carbon contents, which were determined by X-ray photoelectron spectroscopy measurements. The energy band gap and disorder parameters of the conduction and valance band tails were determined and compared with the optical energies and Urbach energies, obtained by UV-Visible transmittance measurements. As a result of the analyses, it was observed that the proposed model sufficiently represents the room temperature PL spectra of a-SiCx:H thin films. (C) 2012 Elsevier B.V. All rights reserved.
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    Study of minority carrier injection phenomenon on Schottky and plasma deposited p-n junction diodes
    (Elsevier Sci Ltd, 2009) Ozdemir, Orhan; Sel, Kivanc
    Both temperature dependent ac capacitance (C) together with ac conductance (G) under various frequencies (omega) and dc current (I) measurements as functions of bias (V) were employed to investigate majority as well as minority carrier injection phenomenon on chromium-p-type crystalline silicon (Cr/p-cryst. Si/ohmic contact) Schottky and aluminum/plasma deposited n-type hydrogenated amorphous silicon/p-c-Si (Al/n-aSi:H/p-c-Si/ohmic contact) p-n structures. Ambipolar transport process was eventual for both diodes and behaved differently according to the mentioned techniques: for a well defined forward bias interval, there was an increase in capacitance towards maximum that interpreted as majority carrier injection from the back electrode. On the other side, minority carrier injection from front electrode begun when the bias went beyond the critical voltage (i.e., built-in voltage) that corresponded to the peak position and caused sharp decrease in measured capacitance, leading to observation of a hump in C-V measurement. Moreover, shape and peak position of the hump were frequency/temperature dependent. Remarkably, in the bias range where capacitance pronounced, space charge limited current (SCLC) was discerned as enrolled carrier flow mechanism according to I-V measurement, confirming further majority carrier injection. In addition, both dc conductivity and ac conductance followed the same dependence with bias voltage in forward direction. On the other side, reverse current seemed proportional to square root of reverse bias, implying generation current in the depletion region. These experimental identifications convinced us of the existence of conductivity modulation issue for the structures at hand. (C) 2009 Elsevier Ltd. All rights reserved.
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    The effect of Na+ and K+ doping on the properties of sol-gel deposited 2-hydroxy-1,4-naphthoquinone thin films
    (Amer Inst Physics, 2013) Al-Omari, Mahmoud; Sel, Kivanc; Mueller, Anja; Mellinger, Axel; Kaya, Tolga
    We describe the use of 2-hydroxy-1,4-naphthoquinone (HNQ) thin films as a potential water vapor and electrolyte sensing material towards the goal of non-invasive relative humidity and sweat detection. We have successfully made HNQ sol-gel thin films and studied the effects of sodium and potassium ions on their optical and electrical characteristics. Ultraviolet-visible absorbance and Fourier transform infrared spectroscopy measurements along with scanning electron microscopy demonstrated that we were able to dope HNQ thin films with Na+ and K+ ions, which are the main electrolyte contents in sweat. While the conductivity of thin films increased by at least an order of magnitude, energy band gaps decreased by doping HNQ with Na+ and K+ ions. Relative humidity test results showed that HNQ-based thin-films can be used as a sensing material for water vapor. Room temperature current-voltage measurements were also performed to determine the surface conductivity. (C) 2013 AIP Publishing LLC.
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    The optical properties of plasma polymerized polyaniline thin films
    (Elsevier Science Sa, 2013) Goktas, Hilal; Demircioglu, Zahide; Sel, Kivanc; Gunes, Taylan; Kaya, İsmet
    We report herein the characterizations of polyaniline thin films synthesized using double discharge plasma system. Quartz glass substrates were coated at a pressure of 80 Pa, 19.0 kV pulsed and 1.5 kV dc potential. The substrates were located at different regions in the reactor to evaluate the influence of the position on the morphological and molecular structure of the obtained thin films. The molecular structure of the thin films was investigated by Fourier transform infrared (FTIR) and UV-visible photospectrometers (UV-vis), and the morphological studies were carried out by scanning electron microscope. The FTIR and UV-vis data revealed that the molecular structures of the synthesized thin films were in the form of leuocoemeraldine and exhibited similar structures with the films produced via chemical or electrochemical methods. The optical energy band gap values of the as-grown samples ranged from 2.5 to 3.1 eV, which indicated that these materials have potential applications in semiconductor devices. The refractive index in the transparent region (from 650 to 1000 nm) steadily decreased from 1.9 to 1.4 and the extinction coefficient was found to be on order of 10(-4). The synthesized thin films showed various degrees of granular morphologies depending on the location of the substrate in the reactor. (C) 2013 Elsevier B.V. All rights reserved.