Phd Degree / Doktora
Permanent URI for this collectionhttps://hdl.handle.net/11147/2869
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Doctoral Thesis Two Dimensional Material Based Field Effect Transistor for Biosensing Applications(01. Izmir Institute of Technology, 2023) İnanç, Dilce; Yıldız, Ümit Hakan; Çelebi, CemThis thesis presents research on the use of two-dimensional material graphene as an area-effective transistor and its application in biological fields. The formation of wrinkled and flat structures on the surface of a single-layer graphene area-effective transistor, epitaxially grown for determining the bio-membrane dynamics of graphene, was examined using two different methods of deposition (thermal evaporation and pulsed electron accumulation) of a silicon dioxide (SiO2) layer. The investigation aimed to evaluate the pH and lipid bilayer formation performance of both wrinkled and flat GFETs. Increased sensitivity was determined through electrical measurements, as the oxide layer becomes thinner due to the existence of wrinkles, thus providing electrostatic coating on graphene. A sensor platform of chemiresistor type was developed for the differential determination of volatile organic compounds (VOCs) by synthesizing single-layer, bilayer, and multilayer graphene, enabling the analysis of ethanol (EtOH) and methanol (MetOH). Sensors produced using three different graphene morphologies demonstrated differential MeOH-EtOH responses attributed to the differential intercalation phenomenon in multilayer graphene morphologies when compared to ethanol. For the detection of VOCs such as acetone, ethanol, and hexane in human breath, a polymer nanofiber/multi-walled carbon nanotube or poly (3,4-ethylenedioxythiophene)/gold (Au) and iron oxide (Fe) hybrid bioelectronic interface was developed. Sensitivity studies were conducted by applying pure VOCs at different concentrations to the sensor platforms, and the behavior of the sensor platforms against interfering elements was evaluated by recharacterizing them under CO2 and humidity conditions. Considering the responses of MWCNT-PLLCL-Fe-based sensors to acetone, ethanol, and hexane, the tendency of water molecules to adhere to the Fe surface was shown to decrease water condensation on the conductive layer compared to other sensor configurations, indicating that the humidity effect was minimized in MWCNT-PLLCL-Fe-based sensors.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.Doctoral Thesis Electronic, Magnetic and Transport Properties of Graphene Quantum Dots With Charged Impurities(Izmir Institute of Technology, 2020) Polat, Mustafa; Güçlü, Alev DevrimIn this thesis, electronic, magnetic, and transport properties of armchair edged hexagonal and zigzag edged triangular graphene quantum dots (GQDs) are investigated in the presence of charged impurities. In this manner, a special attention has been paid to the Coulomb impurity problem in these structures. The collapse of the wave functions starting from the 1S$_{1/2}$ state is studied in the presence of not only the Coulomb impurity but also in the presence of a Coulomb charged vacancy with the help of tight-binding and extended mean-field Hubbard (MFH) models. Here, we report an interaction induced renormalization of the critical coupling constant ($\beta_{c}$). In addition, our results suggest that the induced charge for the interacting fermions is smaller than that of the non-interacting fermions. Furthermore, the transport coefficients reveal two different characteristics of the subcritical ($\beta$ $<$ $\beta_{c}$) and supercritical ($\beta$ $>$ $\beta_{c}$) regimes. As for the charged vacancy, the bare carbon vacancy induces a local magnetic moment in the hexagonal GQDs, but it is suppressed when the vacancy is charged with the subcritical Coulomb potential. Except the pristine cases of the GQDs, we numerically study a Coulomb impurity problem for the interacting fermions restricted in disordered hexagonal GQDs. In the presence of randomly distributed lattice defects and spatial potential fluctuations induced by Gaussian impurities, the response of $\beta_{c}$ for atomic collapse is mainly investigated by local density of states (LDOS) calculations within the MFH model. We find that both types of disorder cause an amplification of the critical threshold. As for the zigzag edged triangular GQDs, in the presence of the bare vacancy, we exactly obtain the spin splitting with the help of LDOS calculations in the energy spectrums, which are dominated by the edge states around the Fermi level. Similar to the hexagonal GQDs, if the vacancy is charged, the local magnetic moment disappears in these GQDs.Doctoral Thesis Single-Photon Generation From Defects and Manipulation With Nanostructures(Izmir Institute of Technology, 2019) Özçeri İyikanat, Elif; Aygün, Gülnur; Tarhan, Enver; Tarhan, Enver; Aygün Özyüzer, GülnurSingle-photon sources are essential components for several applications in the field of quantum information technologies, such as quantum cryptology and quantum computation. To this aim, efficient generation and detection of single-photons are the crucial to be achieved. Among single-photon sources that are extensively studied in the literature, defect centers in solid are very promising due to their room temperature operation and their stability. The aim of this thesis is to generate single photons at room temperature and control their optical properties by nanostructures. Single-photon emission from TMDCs originates from localized weakly bound excitons at cryogenic temperatures due to their small exciton binding energies. However, room temperature SP emission from WS2 can be obtained by creatingWO3 defects. In our study, room temperature emission from defects in WO3 was investigated. Density functional theory calculations showed that the source of the emission can be oxygen defects. Additionally, the emission was brightened by plasmonic gold nanoparticles. Furthermore, defects in two-dimensional (2D) hexagonal boron nitride (hBN) is offered as an efficient room temperature SPS. HBN is a wide bandgap 2D material, in which defect centers create discrete energy level to generate single photons. In our study, reversible single-photon emission control from defects in hBN was demonstrated by Förster-like resonance energy transfer between the single-photon emitter and a graphene layer. To this aim an ionic liquid based device structure was used.Doctoral Thesis Developing Graphene-Organic Hybrid Electrodes for Silicon Based Schottky Devices(Izmir Institute of Technology, 2018) Aydın, Hasan; Çelebi, Cem; Varlıklı, CananThis thesis focused on developing graphene-organic hybrid electrodes for silicon based Schottky devices. Two different sets of carboxylic acid based SAMs were used to improve the rectification character of the Schottky junction formed at graphene/Si interface. While the first set of SAMs consists of MePIFA and DPIFA, the second set of SAMs contains TPA and CAR. In addition to this, P3HT, which is known to be an electron donor and absorb light in the visible spectrum, was utilized to form P3HT-graphene bilayer electrode. Current-voltage characteristics of bare and SAMs modified devices showed rectification behavior confirming a Schottky junction formation at the graphene/Si interface. The DPIFA SAMs device exhibited better diode performance compare to MePIFA SAMs due to the absence of methyl group which hinders π-π interaction between SAMs molecule and graphene. Furthermore, the CAR-based device indicates better diode characteristic with respect to the TPA-based device due to smaller energy differences between graphene and CAR. The effect of P3HT-graphene bilayer electrode on the photoresponsivity characteristics of Silicon based Schottky photodetectors have been also investigated. Current-voltage measurements of graphene/Si and P3HT-graphene/Si revealed rectification behavior confirming Schottky junction formation at the graphene/Si interface. Time-resolved photocurrent measurements exhibited excellent durability and fast response speed. Moreover, the maximum photoresponsivity of P3HT-graphene/Si photodetector increased compared to that of bare graphene/Si photodetector. The observed increment in the photoresponsivity of P3HT-graphene/Si devices was attributed to the charge transfer doping from P3HT to graphene within the spectral range between near-ultraviolet and near-infrared. Finally, P3HT-graphene electrode was found to improve the specific detectivity and noise equivalent power of graphene/Si photodetectors.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.Doctoral Thesis Modelling Electronic and Structural Properties of Graphene and Transition Metal Chacogenide Nanostructures(Izmir Institute of Technology, 2016) Özaydın, Hediye Duygu; Senger, Ramazan TuğrulThe purpose of this thesis is to investigate the electronic and structural properties of one- and two-dimensional materials such as graphene, graphene-like transition metal chalcogenides by using density functional theory. The single-atom thickness of graphene sheet is a novel material and attracts great interest due to its unique features. In recent years, theoretical and experimental studies on graphene provide quick knowledge and have opened up possibilities for many other two-dimensional new materials. Among these materials, especially transition metal chalcogenides have recently been the focus of studies of condensed matter physics. Unlike many superior properties of graphene, lack of band gap in electronic structure have highlighted the necessity of such transition metal chalcogenides materials for electronic applications. As compared to graphene, transition metal chalcogenides have various physical properties and possess sizable band gaps, for this reason they are promising candidate for many applications. Many experiments have revealed that the surfaces of graphene and graphene-like structures can play an active role as a host surface for clusterization of metal atoms. Motivated by these observations, we investigate characteristic properties of Pt atoms on graphene, MoS2 and TaS2. Similarly, TiSe2 is very recently synthesized two-dimensional transition metal dichalcogenide material and stable in 1T phase. Two-dimensional TiSe2 has a metallic electronic property and widely studied material. We analyze how to change the structural and electronic properties of TiSe2 by functionalization with hydrogen atom. Again to the effects of hydrogenation on two-dimensional TiSe2 monolayer we also study the structural and electronic properties of this material in nanoribbon form. At the same time, PtSe2 which is also very recently synthesized two-dimensional transition metal dichalcogenide and stable in 1T phase like TiSe2, its nanoribbon structural and electronic properties have also been investigated and compared with TiSe2 nanoribbons. Finally, TiS3 which is also transition metal chalcogenide but entirely different crystal structure, is recently widely studied materials. The structural and electronic properties as well as carrier mobility and strain response of TiS3 nanoribbons have been investigated. Besides many comprehensive theoretical studies, a lot of experimental studies are avaibale about the synthesis of these materials. In brief, these materials which tackles a contemporary and rapidly developing field, the nanoribbon form and functionalization of them that hold promise for many other applications.