Çelebi, Cem
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Çelebi, C
Celebi, Cem
Çelebi, C. C.
Celebi, C. C.
Celebi, C
Celebi, C.
Çelebi, C.
Celebi, Cem
Çelebi, C. C.
Celebi, C. C.
Celebi, C
Celebi, C.
Çelebi, C.
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cemcelebi@iyte.edu.tr
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04.05. Department of Pyhsics
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48
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51
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Supervised PhD Theses
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367
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400
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7.20
Scopus Citations per Publication
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33
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19
| Journal | Count |
|---|---|
| Sensors and Actuators, A: Physical | 4 |
| Nanotechnology | 3 |
| Applied Physics Letters | 2 |
| Journal of Alloys and Compounds | 2 |
| IEEE Sensors Journal | 2 |
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Doctoral Thesis Functionalized Cvd Grown Graphene for Gas Sensing Applications(Izmir Institute of Technology, 2017) Yağmurcukardeş, Nesli; Çelebi, Cem; Çelebi, Cem; Ünverdi, ÖzhanGraphene is a two dimensional one-atom thick sheet of sp2 bonded carbon atoms arranged in a honeycomb lattice structure. It has high electron mobility and it is the material with the lowest resistivity at room temperature. By changing the edge properties with chemical modification, few-layer graphene may gain new magnetic properties. Besides having unusual electronic properties, single-layer graphene has important gas sensing capability. With the adsorption of the gas molecules, the local carrier concentration of graphene is modified and its resistance is altered. The high mobility, large area ohmic contact and metallic conductivity of graphene help to reduce the background noise and thus make it highly sensitive device even small molecular changes at atomic ranges. In this dissertation, Chemical Vapor Deposition (CVD) grown graphene layers were functionalized by self-assembled monolayers (SAMs) and etched anisotropically by H2 for the first time to improve sensor characteristics for toxic gas sensing. CO, CO2, NH3 gases were used as target molecules. Characterization techniques such as Optical Microscopy, Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Kelvin Probe Force Microscopy (KPFM), Raman Spectroscopy, Quartz Crystal Microbalance (QCM) and amperometric measurements were used for the investigation of the metal thin film, graphene layers and gas adsorbed film structures. Results indicate that the SAM modification enhanced CO and NH3 absorbing capability of graphene films and also improved their periodic reversible response characteristics. The resistivity results are consistent with frequency change results. Humidity sensitivity of sensors are also decreased significantly due to the applied etching process.Master Thesis The Growth of Epitaxial Graphene for Two-Dimensional Electronics(Izmir Institute of Technology, 2015) İbrahim, Alnazir; Çelebi, Cem; Çelebi, CemGraphene is a fundamentally new type of 2D electronic material exhibits extraordinary properties. In this work, we used the thermal decomposition of SiC in vacuum principle to grow epitaxial graphene on Both C-face and Si-face SiC, that because epitaxial graphene is a reliable candidate for all kind of applications in 2D electronics and it has similar properties to carbon nanotube and graphene grown by exfoliation method, but it is more appropriate for the design of electronic device as it can be grown on wafer-sized scale. We review the physical and electronic properties of graphene and what’s makes it different when compare to the ordinary semiconductors. Since the interest in graphene increases as a perfect candidate for future electronics applications, many methods are used to synthesis graphene. We give a brief review to several methods used to produce graphene. For electronic device applications, graphene should be grown in a high homogeneity, uniformity and with low growth rate. Therefore the growth rate graphene should be controlled to get high homogeneity and uniformity. Raman spectroscopy considers a quick way to detect the presence of graphene, determine the number of the layers and to check the defects in the grown layer. At the wavelength of 514 nm , Raman spectroscopy is used to investigate epitaxial graphene grown on both C-face and Si-face SiC with different parameters and to study the evolution and the proportionality of the grown layers with the time . Finally we used AFM to study the morphology of epitaxial graphene grown on SiC substrate. The morphology of graphene on C-face of SiC is compared with the one on Si-face of SiC.Article Citation - WoS: 12Citation - Scopus: 17Junction Area Dependent Performance of Graphene/Silicon Based Self-Powered Schottky Photodiodes(Elsevier, 2021) Fidan, Mehmet; Ünverdi, Özhan; Çelebi, CemThis work reports the impact of junction area on the device performance parameters of Graphene/n-Silicon (Gr/n-Si) based Schottky photodiodes. Herein, three batches of Gr/n-Si photodiode samples were produced based on various sized CVD grown monolayer graphene layers transferred on individual n-Si substrates. The fabricated devices exhibited strong Schottky diode character and had high spectral sensi-tivity at 905 nm peak wavelength. The optoelectronic measurements showed that the spectral response of Gr/n-Si Schottky photodiodes has a linear dependence on the active junction area. The sample with 20 mm(2) junction area reached a spectral response of 0.76 AW(-1), which is the highest value reported in the literature for self-powered Gr/n-Si Schottky photodiodes without the modification of graphene electrode. In contrast to their spectral responsivities, the response speed of the samples were found to be lowered as a function of the junction area. The experimental results demonstrated that the device performance of Gr/n-Si Schottky photodiodes can be modified simply by changing the size of the graphene electrode on n-Si without need of external doping of graphene layer or engineering Gr/n-Si interface. This study may serve towards the standardization of junction area for the development of high performance Gr/Si based optoelectronic devices such as solar cells and photodetectors operating in between the ultraviolet and near-infrared spectral region. (C) 2021 Elsevier B.V. All rights reserved.Master Thesis Graphene Transfer Approaches With Different Support Materials on the Substrates With Cavities(Izmir Institute of Technology, 2019) Duman, Sinem; Balantekin, Müjdat; Çelebi, CemA micro capacitive sensor characteristically embraces a thin conductive membrane which is freely suspended above an immovable counter electrode in a parallel plate geometry. Such capacitive structures are found in broad range of applications as a transducer like capacitive micro-machined ultrasonic transducer (CMUT), pressure sensor, resonator and biological or chemical material sensing element. The input can be an ultrasound wave, pressure, chemical or biological mass attachment which result in the deflection of the membrane. Emerging nano materials have shown great potential as candidates for generation of nano and micro electromechanical systems (NEMS, MEMS). Among these nano materials, graphene is regarded as a promising material because of its ultra low mass, thickness, high surface to volume ratio, flexibility, and extraordinary electrical and mechanical properties. However, the transfer of graphene on substrates with micro scale cavities is challenging since the fabrication of large area membranes with a smaller air gap often results in membrane tearing or collapse driven by capillary or electrostatic forces. This study presents a research on the fabrication and the characterization of graphene membranes to be used in micro capacitive sensor applications. Substrates which span a large array of circular and hexagonal micro cavities between 2-100 μm in diameter are fabricated. Graphene transfer with different support materials are studied to fabricate graphene micro membranes. Up to 5 μm diameter membranes on 300 nm deep cavities are demonstrated via scanning electron microscope (SEM) and atomic force microscope (AFM) tools.Article Citation - WoS: 3Citation - Scopus: 3Amperometric Detection of Nh3 by Aromatic Sam-Modified Graphene(IEEE, 2023) Yağmurcukardeş, Nesli; Bayram, Abdullah; Aydın, Hasan; Can, Mustafa; Demiç, Şerafettin; Açıkbaş, Yaşar; Çelebi, CemAmmonia (NH3) is a toxic substance resulting in various acute and chronic effects on individuals. NH3 detection, monitoring methods, and detection tools are desperately needed. In this work, we improved the NH3 sensing capabilities of grapheme (GP) films deposited by chemical vapor deposition (CVD) by modifying aromatic self-assembled monolayer (SAM) molecules such as 5-[(3-methylphenyl) (phenyl) amino] isophthalic acid (MeIPA) and 5-(diphenyl)amino] isophthalic acid (PhIPA) on amperometric detection method. Morphological investigations of the films were carried out by optical and scanning electron microscopy (SEM). Surface potential was characterized with Kelvin probe force microscopy (KPFM), and vibrational properties were characterized with Raman spectroscopy. MeIPA modification increased NH3 uptake by two times compared to unmodified GP. The results indicated that the SAM modification enhanced NH3 molecule adsorption and improved its periodic reversible and reproducible response using the amperometric detection system, indicating that SAM molecules might be a feasible probe for NH3. © 2001-2012 IEEE.Doctoral Thesis Performance Enhancement of Graphene/Silicon Based Near-Infrared Schottky Photodiodes(Izmir Institute of Technology, 2022) Fidan, Mehmet; Çelebi, CemThis thesis presents an experimental investigation on the performance enhancement of graphene/silicon based near-infrared Schottky photodiodes. The photodiode devices were fabricated by transferring CVD graphene layers onto n-type silicon (n-Si) substrates. The samples exhibited strong Schottky diode character and had high spectral sensitivity at 905 nm peak wavelength. The Schottky contact characteristics of the samples (e.g., barrier height, ideality factor and sheet resistance) were determined by analyzing the current-voltage measurement data. All the samples demonstrated a clear photovoltaic activity under light illumination. The Schottky barrier height (SBH) in Gr/n-Si photodiodes was tuned as a function of light power density. Light power density driven modification of the SBH was correlated with the variation in the measured open-circuit voltage. The impact of junction area and number of graphene layers on the spectral responsivity and response speed of Gr/n-Si based Schottky photodiodes were also investigated. Firstly, three batches of Gr/n-Si photodiode samples with junction area of 4 mm2, 12 mm2 and 20 mm2 were produced by transferring monolayer CVD graphene on individual n-Si substrates. The sample with 20 mm2 junction area reached a spectral response of 0.76 AW-1, which is the highest value reported in the literature for self-powered Gr/n-Si Schottky photodiodes without the modification of graphene electrode. In contrast to their spectral responsivities, the response speed of the samples was found to be lowered as a function of the junction area. After that, we increased the number of graphene layers on n-Si. Wavelength-resolved and time-dependent photocurrent measurements demonstrated that both spectral responsivity and response speed are enhanced as the number of graphene layers is increased from 1 to 3 on n-Si substrates. This thesis showed that the device performance of Gr/n-Si Schottky photodiodes can be modified simply by changing the size of graphene electrode and/or as well as the number of graphene layers on n-Si without need of external doping of graphene layer or engineering Gr/n-Si interface.Master Thesis Field Emission Electron Source Based on Silicon Carbide Nanopillars(Izmir Institute of Technology, 2017) Yeşilpınar, Damla; Çelebi, Cem; Demir, Mustafa MuammerIn this thesis work, I studied the fabrication and the field emission characteristics of SiC nanopillar based electron field emitters. The first objective of this thesis was to fabricate a large area nanopillar array on bulk 6H-SiC substrate. Accordingly, a nanosphere assisted technique was developed to create a conventional Cr/Ni hard mask to acquire desired etch mask pattern on the C-terminated face of 6H-SiC. The nanopillars were then fabricated by ICP-RIE. Two sets of nanopillars with different aspect ratios and geometries were fabricated for two different ICP-RIE durations. 1 min long etch resulted in nanopillar arrays with blunt tip apex and an aspect ratio of 3.4, where 2 min long etch produced nanopillar arrays with an aspect ratio of 4.9 and a sharp tip apex with an estimated radius of curvature of about 18 nm. As the second objective; the electron field emission characteristics of the produced nanopillars with two different aspect ratios and geometries were investigated and the obtained results were compared with each other. We found that the nanopillars with sharp tip apex produced field emission currents up to 240 μA/cm2 under 17.4 V/μm applied electric field, as the nanopillars with blunt tip apex produced an emission current of 70 μA/cm2. The threshold electric fields were found to be 9.1 V/μm and 7.2 V/μm for the nanopillars with blunt and sharp tip apex, respectively. Time dependent stability measurements yielded stable electron emission without any abrupt change in the respective current levels of both samples.Article Citation - WoS: 9Citation - Scopus: 9High Voltage Response of Graphene/4h-sic Uv Photodetector With Low Level Detection(Elsevier, 2023) Jehad, Ala K.; Ünverdi, Özhan; Çelebi, CemA self-powered graphene/silicon carbide (G/4H-SiC) ultraviolet photodetector of a p-i-n like-structure with high voltage response has been fabricated to detect and measure low intensity ultraviolet (UV) radiation. Bilayer graphene sheet grown by chemical vapor deposition (CVD) method was transferred on the top of an epilayer structure of n-/n+ 4 H-SiC. In this structure, two Schottky contacts were formed: one at G/ n- 4H-SiC interface and the other at bulk-4 H-SiC/Cr/Au interface. The photodetector's characteristic measurements revealed low dark current of - 0.58 nA and spectral voltage responsivity of - 0.75 V/W at 300 nm wavelength. Under low level UV illumination of 300 nm wavelength, the photodetector exhibited a leakage current and a photogenerated response voltage of 1.1 nA and 10 mV, respectively. The time-dependent photovoltage measurements displayed a rapid photovoltage response with rise and decay times of -74 ns and - 580 ns, respectively. This novel device holds promise for applications requiring sensitive and self-powered UV detection.Article Citation - WoS: 2Citation - Scopus: 2Impact of Encapsulation Method on the Adsorbate Induced Electrical Instability of Monolayer Graphene(AVS Science and Technology Society, 2019) Kalkan, Sırrı Batuhan; Yanılmaz, Alper; Çelebi, CemMonolayer graphene transferred onto a set of silicon carbide (SiC) substrates was encapsulated with a thin SiO2 film in order to prevent its interaction with atmospheric adsorbates. The encapsulation of graphene samples was realized by using two different thin film growth methods such as thermal evaporation (TE) and state-of-the-art pulsed electron deposition (PED). The encapsulation efficiency of these two techniques on the structural and electrical characteristics of graphene was compared with each other. Scanning electron microscopy (SEM) analysis showed that unlike the SiO2 thin film grown with PED, structural defects like cracks were readily formed on TE grown films due to the lack of surface wettability. The electronic transport measurements revealed that the electrical resistivity of graphene has been increased by two orders of magnitude, and the carrier mobility has been subsequently decreased upon the encapsulation process with the PED method. However, in-vacuum transient photocurrent spectroscopy (TPS) measurements conducted for short periods and a few cycles showed that the graphene layer encapsulated with the PED grown SiO2 film is electrically far more stable than the one encapsulated with TE grown SiO2 film. The results of TPS measurements were related to the SEM images to unravel the mechanism behind the improved electrical stability of graphene samples encapsulated with the PED grown SiO2 film.Master Thesis The Impact of Adsorbates on the Optoelectronic Properties of Graphene/Silicon Based Schottky Barrier Photodiodes(01. Izmir Institute of Technology, 2020) Şahan, Nusret; Çelebi, CemThe aim of this study is to investigate the effect of atmospheric adsorbates on the electronic and optoelectronic properties of graphene/n-type Silicon (Gr/n-Si) based Schottky barrier photodiodes. Wavelength resolved photocurrent spectroscopy and transient photocurrent spectroscopy measurements conducted under high-vacuum conditions revealed that the adsorbates cause hole doping in graphene and hence increase the zero-bias Schottky barrier height of the Gr/n-Si heterojunction from 0.71 to 0.78 eV. Adsorbate induced increment in the barrier height promotes the separation of photo-excited charge carriers at the depletion region of the heterojunction and leads to an improvement in the maximum spectral response (e.g., from 0.39 to 0.46 A W^-1) and response speed of the Gr/n-Si photodiode in the near-infrared region. The experimentally obtained results are expected to give an insight into the adsorbate induced variations in the rectification and photo-response characters of the heterojunctions of graphene and other 2D materials with different semiconductors.