Master Degree / Yüksek Lisans Tezleri
Permanent URI for this collectionhttps://hdl.handle.net/11147/3008
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Master Thesis Utilization of Graphene and Mos2 for Volatile Organic Compound Sensor Applications(01. Izmir Institute of Technology, 2023) Duran, Tuna; Büyükçakır, Onur; Şahin, HasanThe novel 2D materials such as graphene and transition-metal dichalcogenides have already shown impressive volatile organic compound (VOC) gas monitoring performances as in sensitivity, limit of detection and response time. This thesis discusses the experimental-theoretical examination of optical, electronic and morphological properties of novel 2D materials and their utilization in VOC gas sensor field, by means of several characterization techniques and density functional theory (DFT). Aside from the basic familiarization with the experimental and theoretical methodology in Chapter 2, examination of the functionalization of exfoliated MoS2 using DDT (1-Dodecanethiol) in Chapter 3, which eventually led to a research paper. The DDT treatment is incorporated into the NMP (N-methyl pyrrolidone) exfoliation procedure, resulting in successful functionalization as confirmed by optical, morphological, and theoretical analysis. Raman spectroscopy showed the formation of graphitic species on MoS2 sheets, with decreased sulfur-vacant sites as the DDT ratio increased. STEM and AFM data confirmed the presence of graphitic quantum dots (GQDs) on MoS2 nanosheets, while PL intensities demonstrated significant improvements in photoluminescent properties. This study enhances our understanding of surface and edge chemistry in exfoliated MoS2 and expands the possibilities for broader applications of MoS2 and GQD particles. Moreover, the attention was drawn to the investigation of the contrasting responses of graphene gas sensors fabricated using different synthesis methods, in the submitted paper explained in Chapter 4. Exfoliated graphene sensors decrease in current when exposed to methanol, while CVD graphene sensors increase in current. The differences in edge site population and electrical properties contribute to these responses. The study provided theoretical and experimental findings for an understanding of the reasons behind the inverse sensor responses of CVD and exfoliated graphene on an atomic scale.Master Thesis Electron Optics in Graphene(01. Izmir Institute of Technology, 2022) Coşgel, Gürcan; Çakır, ÖzgürNegative refraction, also known as Veselago lensing, was first predicted by Victor Veselago in 1968 (Veselago (1968)). Its unique effect has a great potential for both scientific and technological applications such as superlenses. Unlike the conventional positive refractive index, focusing effect can be observed by negative refraction. In this thesis, the focusing effect was investigated theoretically through on n-p junction in graphene. The opposite chirality of electrons and holes enable the negative refraction where electrons( holes) have their momentum parallel(anti-parallel) to the group velocity. The case when potential barrier is directed perpendicular to KK direction, where K and K are the Dirac points were considered. The Green’s functions were calculated analytically and derived the susceptibility using the Green’s functions for various positions of the sources and the receiver at various Fermi energies. The spatial Green’s functions were calculated analytically and derived the static susceptibility (response function).Master Thesis Graphene-Like Materials for Electronic Applications(01. Izmir Institute of Technology, 2020) Başkurt, Mehmet; Şahin, Hasan; Balcı, SinanTwo-dimensional (2D) materials have gained vast interest in nanotechnology since these materials exhibit extraordinary properties due to electron confinement. Starting with graphene, many other 2D materials with characteristics of metals, semiconductors, insulators, and their magnetic analogues have been studied over the years. Insulators show importance as dielectric layers. Low dimensional metallic materials are used in electrical conduction. Ultra-thin semiconductors have variety of potential applications due to their characteristic band gap. Magnetic analogues of low dimensional materials are used in spintronics, offering high frequency, controllable switching. In addition, defects in these materials alter their physical properties and the concept can be adopted in order to use in different practices. Therefore it is important to study array of such materials and consider the alteration in their lattice theoretically and experimentally. In this thesis, first-principles calculations are used to predict insulating calcium halide single-layers are predicted, determine the effects of strain and V dopant in recently synthesized magnetic semiconducting VI3 single-layers, propose synthesis of magnetic, semiconducting manganese fluorides from manganese dichalcogenides, investigate the affects of defects and simulate scanning tunneling microscopy images in order to compare with experimental results, and finally to determine rather the detection of volatile organic compounds (VOC) such as methanol and ethanol by graphene-based sensors is feasible or not. Experiments are carried out to construct and further investigate the mechanism of VOC detection and working, highly sensitive alcohol sensors.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.Master Thesis Numerical and Experimental Investigation of Thermal Performance of Graphene Reinforced Aluminium(01. Izmir Institute of Technology, 2020) Yılmaz, Ahmet Berk; Toprak, Kasım; Kandemir, SinanGraphene is a material with superior properties such as high thermal conductivity and mechanical strength. These exceptional properties make graphene a good candidate for being used as a reinforcement agent in other materials. Aluminium is a widely used material in industry for thermal applications for being cheap, lightweight and having high thermal conductivity. In the literature, there are many examples of graphene reinforced aluminium production. Also, the effects of graphene on thermal conductivity and mechanical properties of aluminium are also investigated experimentally. However, there are limited molecular dynamics studies for graphene-aluminium composites. In this work, aluminium, graphene and graphene coated aluminium are modeled and simulated with non-equilibrium molecular dynamics method. Length, width, height, temperature dependence of thermal conductivity of these models are investigated. In addition, effects of graphene layer number, defect size and defect locations are also reported. Additionally, an experimental setup is designed and produced for a comparative study. Thermal performances of aluminium alloy and graphene nanoplatelet reinforced aluminium are investigated with a convection heat transfer test.Master Thesis Flexible Transparent Conducting Electrodes Based on Silver Nanowire, Graphene, and Two-Dimensional Transition Metal Dichalgogenide(01. Izmir Institute of Technology, 2020) Tertemiz, Necip Ayhan; Balcı, SinanIn recent years, transparent conductive electrodes have attracted great interests owing to their critical applications in various optoelectronic devices, such as light emitting diodes, solar cells, liquid crystal displays, optical modulators, and touch screens. In this thesis, graphene-silver nanowires-transition metal dichalcogenide based hybrid transparent and conductive electrodes have been fabricated. In order to reach this goal; (1) single layer graphene on copper foil has been synthesized in large area in a CVD furnace, (2) ultrathin and very long silver nanowires have been synthesized by using wet chemistry methods, (3) MoS2 and WS2 single layer flakes and multilayer thin films have been synthesized in a CVD furnace, (4) electrodes of graphene, silver nanowires, and transition metal dichalcogenides have been fabricated on rigid and flexible substrates.Master Thesis Investigation of Photodetectors Using Graphene Field Effect Transistors Incombination With Functional Dyematerials(Izmir Institute of Technology, 2020) Yakar, Ozan; Balcı, Sinan; Şahin, Hasan; Balcı, Sinan; Şahin, HasanA J-aggregate dye is a type of water-soluble, functional dye, which has a sharp and narrow absorption peak after it self-assembles into a brick-wall structure at high concentrations. The absorption peak of the J-aggregates is sharp, narrow and shifted to longer wavelengths compared to their monomer form and it is in the visible or near infrared spectrum. Due to its very sharp and narrow absorption, it has been used in silver halide photography, non-linear optics, lasing and sensing applications. On the other hand, graphene is one atom layer thick, honeycomb lattice of carbon atoms. In the pure, freestanding form, the bands of its electronic structure touch at one point, making it a gapless semimetal. Due to this characteristic, it is possible to manipulate its optical and electronic properties by changing the Fermi energy of graphene. Therefore, graphene found applications in many fields such as light emitting diodes, photodetectors, Hall sensors, optical modulators and flexible optoelectronics. The functional dye materials have not been combined with graphene photodetectors even though they are highly sensitive to light, less toxic than their competitors and stable at room temperature. In this thesis, using a J-aggregate dye, which has a sharp absorption peak around 585 nm wavelength, a graphene phototransistor has been demonstrated. By changing the charge concentration on graphene, using the charge carriers that arise from the excitation of J-aggregate dye, reversible modulation of graphene Dirac point has been demonstrated. In addition, a novel thin film formation technique has been developed in this study. Porous polyethylene membrane has been used to create thin films of water-soluble materials, such as J-aggregates, on hydrophobic surfaces.Master Thesis Fabrication and Characterization of Graphene/Silicon Based Schottky Photodiode(Izmir Institute of Technology, 2019) Dönmez, Gülçin; Çelebi, Cem; Sarı, EmreThis thesis focused on fabrication and characterization of CVD grown p-type graphene and n-type Si Schottky junction photodiode with rectification behavior. The device operated at wavelength range between 390 and 1100 nm at self-powered mode. The device was encapsulated with Epoxy Resin to prevent graphene from atmospheric adsorbates. The electronic and optoelectronic characterizations of the devices were done before and after coating the devices with ER. By encapsulation stability of the device was enhanced in terms of photoresponsivity. The maximum obtained photoresponsivity value of the bare device was 0.56 A/W. Also, time-resolved photocurrent spectroscopy measurements showed that the devices exhibited enhanced photodetector performance in terms of photo-switching characteristics. Furthermore, electrical characteristics of Gr/n-Si Schottky photodiode under various illumination power densities with 850 nm wavelength were investigated. The short circuit current showed linear response to power density. However, open circuit voltage exhibited two phased slow and fast increment with increased power density. Hall effect measurements were conducted in order to investigate hole carrier concentration and mobility of the graphene on n-Si. With increasing the power density the carrier concentration increased and the mobility decreased. Besides, light induced manipulation of the Schottky barrier height of Gr/n-Si photodiode was studied. Schottky barrier height of the graphene measured by KPFM method as 0.4 eV. With increasing power density we found that Schottky barrier height of the device increased from 0.4 eV to 0.5 eV and showed similar trend with the change in open circuit voltage.Master Thesis Modification of Graphene Surfaces for Detection of Biomicroparticles(Izmir Institute of Technology, 2019) Yeşiltaş, Gözde; Bulmuş, VolgaPathogens present in the food we consume and the water we drink pose a major threat to human health. Another major health concern is the metastasis of cancer in which cancer cells spread to new areas of the body, often by way of the lymph system or bloodstream. To minimize the burden on health and economy, the detection of biomicroparticles such as pathogens or circulating cancer cells in a highly sensitive and practical manner is higly desirable. This thesis aims to develop a method to create graphene-based biosensor substrate for detection of biomicroparticles such as bacteria, viruses or mammalian cells. For this aim, graphene surface was first functionalized using a linker molecule. The effect of solvent type on functionalization was investigated via Raman spectroscopy and X-Ray spectroscopy (XPS). AntiCD2 antibodies (Ab), as a model antibody, were then conjugated to the functionalized graphene via NHS/EDC chemistry. The Ab conjugation was verified by Raman spectroscopy and XPS analyses. Finally, Jurkat cells, as model biomicroparticles, were recognized and captured by Abconjugated graphene surface, as evidenced by optical microscopy. The temperature, medium, and method for interaction of cells with graphene surfaces as well as the specificity of the Ab- functionalized graphene surface were investigated. The results overall showed the specific and efficient recognition of model cell line by Abconjugated graphene surfaces.Master Thesis Rkky Interaction and Its Control in Graphene and Related Materials(Izmir Institute of Technology, 2019) Canbolat, Ahmet Utku; Çakır, ÖzgürGraphene got dramatic attention and lead the two-dimensional material physics after its first successful synthesis in 2004. Its unique electronic properties contain great potential for both scientific and technological applications. RKKY (Ruderman-Kittel-Kasuya Yosida) is an indirect exchange interaction mediated by conduction electrons. In graphene, the interaction strength decay as 1/R³ where R is the distance between the magnetic moments. In the first part of this work, we calculated that applying circular potential on a graphene sheet forms quasi-bound states in the potential region. Via these states, the RKKY interaction is enhanced between magnetic moments on the edge of the potential well. This can be thought of an electronic analog of the Purcell effect. We showed that the interaction strength is even more enhanced if the Fermi level is in resonance with the energies of the quasi-bound states. In the second part, we considered zigzag edged hexagonal nanoflakes. It is known that zigzag edged flakes have zero-energy edge-states. It is also known that the states with closer energies contribute more to RKKY interaction. Thus, we calculated that there is an enhancement between these edge-states. In the third part, we investigated the behavior of RKKY interaction for two dimensional materials with quartic dispersion. An energy dispersion is said to be quartic if it is of the form E = α(k² - kc² )². Here, α and kc are material dependent constants. There are many materials exhibiting the quartic dispersion such as nitrogene, phosphorene, and arsenene. These materials are also sharing two-dimensional hexagonal lattice structure with graphene. What makes quartic dispersion special is that it has van-Hove singularity in its density of states near the band-edge. RKKY interaction is sensitive to the density of states because it depends on the number of electrons contributing spin exchange. Thus, the larger the number of electrons, the stronger the coupling. In this part, we tuned the Fermi level so that it lies on the DOS singularity and then we calculated the interaction strength as a function of R. We found a slowly decaying RKKY interaction for quartic dispersion. If the energy dispersion is pure quartic (i.e. E = ak4), we found the interaction strength depends on 1/(kf R) instead of 1/R which makes the RKKY interaction long range for arbitrarily small Fermi level.
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