WoS İndeksli Yayınlar Koleksiyonu / WoS Indexed Publications Collection
Permanent URI for this collectionhttps://hdl.handle.net/11147/7150
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Article Passive Matrix Schottky Barrier 2d Photodiode Array on Graphene/Soi Platform(Springer Science and Business Media Deutschland GmbH, 2025) Yanilmaz, A.; Ünverdi, Ö.; Çelebi, C.We fabricated 4 × 4 pixel two-dimensional (2D) photodiode array (PDA) out of monolayer graphene and n-type silicon (n-Si) electrodes on a silicon-on-insulator (SOI) substrate. Our device design is based on passive matrix sensor array architecture consisting of individual graphene and silicon electrodes aligned perpendicular to each other. I-V measurements conducted at room temperature to reveal the electronic characteristics of graphene and Si junction in the device structure. The spectral responsivity, respond speed and the optical crosstalk of each G/Si pixels in the array have been determined by wavelength resolved and time dependent photocurrent spectroscopy measurements. Micro-Raman mapping measurements were conducted to examine the surface coverage of graphene electrode on each pixel. The results of Micro-Raman mapping measurements were correlated with the corresponding photocurrent data acquired under light illumination. We believe that this work constitutes a significant potential in integrating variety of 2D materials and SOI technology into next generation image sensing and multiple pixel light detection applications. © The Author(s) 2025.Article Citation - WoS: 3Citation - Scopus: 3Stable Single Layer Structures of Aluminum Oxide: Vibrational and Electronic Characterization of Magnetic Phases(Elsevier, 2022) Özyurt, A. Kutay; Molavali, Deniz; Şahin, HasanThe structural, magnetic, vibrational and electronic properties of single layer aluminum oxide (AlO2) are investigated by performing state-of-the-art first-principles calculations. Total energy optimization and phonon calculations reveal that aluminum oxide forms a distorted octahedral structure (1T′-AlO2) in its single layer limit. It is also shown that surfaces of 1T′-AlO2 display magnetic behavior originating from the O atoms. While the ferromagnetic (FM) state is the most favorable magnetic order for 1T′-AlO2, transformation to a dynamically stable antiferromagnetic (AFM) state upon a slight distortion in the crystal structure is also possible. It is also shown that Raman activities (350–400 cm−1) obtained from the vibrational spectrum can be utilized to distinguish the possible magnetic phases of the crystal structure. Electronically, both FM and the AFM phases are semiconductors with an indirect band gap and they can form a type-III vdW heterojunction with graphene-like ultra-thin materials. Moreover, it is predicted that presence of oxygen defects that inevitably occur during synthesis and production do not alter the magnetic state, even at high vacancy density. Apparently, ultra-thin 1T′-AlO2 with its stable crystal structure, semiconducting nature and robust magnetic state is a quite promising material for nanoscale device applications.Article Citation - WoS: 6Citation - Scopus: 6Adsorbate-Induced Enhancement of the Spectral Response in Graphene/Silicon-based Schottky Barrier Photodetectors(Springer Verlag, 2020) Sahan, N.; Fidan, Mehmet; Çelebi, CemThe impact of atmospheric adsorbates on the spectral response and response speed of p-type graphene/n-type Silicon (p-Gr/n-Si) based Schottky barrier photodetectors are investigated. Wavelength resolved photocurrent and transient photocurrent spectroscopy measurements conducted under high-vacuum conditions revealed that the atmospheric adsorbates such as O-2 and H2O stuck on graphene electrode lead to hole doping in graphene and therefore shift its Fermi level towards higher energy states below its Dirac point. Such a shift in graphene's Fermi level due to adsorbates increases the zero-bias Schottky barrier height of the p-Gr/n-Si heterojunction from 0.71 to 0.78 eV. Adsorbate induced increment in the barrier height promotes the separation of photo-generated charge carriers at the depletion region and leads to an improvement in the maximum spectral response (e.g., from 0.39 to 0.46 AW(-1)) and response speed of the p-Gr/n-Si photodetector in the near-infrared region. The experimentally obtained results are expected to give an insight into the adsorbate related variations in the rectification and photo-response characters of the heterojunctions of graphene and other 2D materials with different semiconductors.Article Citation - WoS: 5Citation - Scopus: 6Contribution of O-2 Plasma Treatment and Amine Modified Gos on Film Properties of Conductive Pedot:pss: Application in Indium Tin Oxide Free Solution Processed Blue Oled(Elsevier, 2019) Diker, Halide; Yeşil, Fatih; Varlıklı, CananPrimary (n-propyl amine, n-PRYLA), secondary (dipropyl amine, DPRYLA) and alcohol (propanol amine, PRPOHA) amine derivatives were used as amine sources in graphene oxide (GO) modification and obtained samples were named as nPRYLA-GO, DPRYLA-GO and PRPOHA-GO, respectively. Modified graphene oxide (mGO) derivatives were doped in poly (3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PH1000) and O-2 plasma treatment (70W, 3 min) was applied on the spin casted films. PH1000:mGO films presented high optical transparency values (> 90%) and low resistivity (177-183 Q/sq). The roughness values were increased especially when the hydrophobic alkyl chain containing DPRYLA-GO and nPRYLA-GO were doped in PH1000. Prepared films were utilized as anode in solution processed blue organic light emitting diode. PH1000:PRPOHA-GO anode presented more than 30 nm of decrement in full with at half maximum and 1.6, 1.5 and 1.9 fold enhancements in current, power and external quantum efficiency values, compared to those of ITO anode, respectively.Article Citation - WoS: 7Citation - Scopus: 7Effects of Random Atomic Disorder on the Magnetic Stability of Graphene Nanoribbons With Zigzag Edges(American Physical Society, 2018) Çakmak, Korhan Ertan; Altıntaş, Abdulmenaf; Güçlü, Alev DevrimWe investigate the effects of randomly distributed atomic defects on the magnetic properties of graphene nanoribbons with zigzag edges using an extended mean-field Hubbard model. For a balanced defect distribution among the sublattices of the honeycomb lattice in the bulk region of the ribbon, the ground-state antiferromagnetism of the edge states remains unaffected. By analyzing the excitation spectrum, we show that while the antiferromagnetic ground state is susceptible to single spin-flip excitations from edge states to magnetic defect states at low defect concentrations, its overall stability is enhanced with respect to the ferromagnetic phase.Article Citation - WoS: 134Citation - Scopus: 137Structural, Vibrational, and Electronic Properties of Single-Layer Hexagonal Crystals of Group Iv and V Elements(American Physical Society, 2018) Özdamar, Burak; Özbal, Gözde; Çınar, Mustafa Neşet; Sevim, Koray; Kurt, Gizem; Kaya, Birnur; Sevinçli, HaldunUsing first-principles density functional theory calculations, we investigate a family of stable two-dimensional crystals with chemical formula A2B2, where A and B belong to groups IV and V, respectively (A=C, Si, Ge, Sn, Pb; B=N, P, As, Sb, Bi). Two structural symmetries of hexagonal lattices P6m2 and P3m1 are shown to be dynamically stable, named as α- and β -phases correspondingly. Both phases have similar cohesive energies, and the α phase is found to be energetically favorable for structures except CP, CAs, CSb, and CBi, for which the β phase is favored. The effects of spin-orbit coupling and Hartree-Fock corrections to exchange correlation are included to elucidate the electronic structures. All structures are semiconductors except CBi and PbN, which have metallic character. SiBi, GeBi, and SnBi have direct band gaps, whereas the remaining semiconductor structures have indirect band gaps. All structures have quartic dispersion in their valence bands, some of which make the valence band maximum and resemble a mexican-hat shape. SnAs and PbAs have purely quartic valence band edges, i.e., E-αk4, a property reported for the first time. The predicted materials are candidates for a variety of applications. Owing to their wide band gaps, CP, SiN, SiP, SiAs, GeN, GeP can find their applications in optoelectronics. The relative band positions qualify a number of the structures as suitable for water splitting, where CN and SiAs are favorable at all pH values. Structures with quartic band edges are expected to be efficient for thermoelectric applications.Article Citation - WoS: 28Citation - Scopus: 30P3HT-graphene bilayer electrode for Schottky junction photodetectors(IOP Publishing Ltd., 2018) Aydın, Hasan; Kalkan, Sırrı Batuhan; Varlıklı, Canan; Çelebi, CemWe have investigated the effect of a poly (3-hexylthiophene-2.5-diyl)(P3HT)-graphene bilayer electrode on the photoresponsivity characteristics of Si-based Schottky photodetectors. P3HT, which is known to be an electron donor and absorb light in the visible spectrum, was placed on CVD grown graphene by dip-coating method. The results of the UV-vis and Raman spectroscopy measurements have been evaluated to confirm the optical and electronic modification of graphene by the P3HT thin film. Current-voltage measurements of graphene/Si and P3HT-graphene/Si revealed rectification behavior confirming a Schottky junction formation at the graphene/Si interface. Time-resolved photocurrent spectroscopy measurements showed the devices had excellent durability and a fast response speed. We found that the maximum spectral photoresponsivity of the P3HT-graphene/Si photodetector increased more than three orders of magnitude compared to that of the bare graphene/Si photodetector. The observed increment in the photoresponsivity of the P3HT-graphene/Si samples was attributed to the charge transfer doping from P3HT to graphene within the spectral range between near-ultraviolet and near-infrared. Furthermore, the P3HT-graphene electrode was found to improve the specific detectivity and noise equivalent power of graphene/Si photodetectors. The obtained results showed that the P3HT-graphene bilayer electrodes significantly improved the photoresponsivity characteristics of our samples and thus can be used as a functional component in Si-based optoelectronic device applications.Article Citation - WoS: 44Citation - Scopus: 47Electric Field Controlled Transport of Water in Graphene Nano-Channels(American Institute of Physics, 2017) Çelebi, Alper Tunga; Barışık, Murat; Beşkök, AliMotivated by electrowetting-based flow control in nano-systems, water transport in graphene nano-channels is investigated as a function of the applied electric field. Molecular dynamics simulations are performed for deionized water confined in graphene nano-channels subjected to opposing surface charges, creating an electric field across the channel. Water molecules respond to the electric field by reorientation of their dipoles. Oxygen and hydrogen atoms in water face the anode and cathode, respectively, and hydrogen atoms get closer to the cathode compared to the oxygen atoms near the anode. These effects create asymmetric density distributions that increase with the applied electric field. Force-driven water flows under electric fields exhibit asymmetric velocity profiles and unequal slip lengths. Apparent viscosity of water increases and the slip length decreases with increased electric field, reducing the flow rate. Increasing the electric field above a threshold value freezes water at room temperature.Article Citation - WoS: 6Citation - Scopus: 6Few-Layer Mos2 as Nitrogen Protective Barrier(IOP Publishing Ltd., 2017) Akbalı, Barış; Yanılmaz, Alper; Tomak, Aysel; Tongay, Sefaattin; Çelebi, Cem; Şahin, HasanWe report experimental and theoretical investigations of the observed barrier behavior of few-layer MoS2 against nitrogenation. Owing to its low-strength shearing, low friction coefficient, and high lubricity, MoS2 exhibits the demeanor of a natural N-resistant coating material. Raman spectroscopy is done to determine the coating capability of MoS2 on graphene. Surface morphology of our MoS2/graphene heterostructure is characterized by using optical microscopy, scanning electron microscopy, and atomic force microscopy. In addition, density functional theory-based calculations are performed to understand the energy barrier performance of MoS2 against nitrogenation. The penetration of nitrogen atoms through a defect-free MoS2 layer is prevented by a very high vertical diffusion barrier, indicating that MoS2 can serve as a protective layer for the nitrogenation of graphene. Our experimental and theoretical results show that MoS2 material can be used both as an efficient nanocoating material and as a nanoscale mask for selective nitrogenation of graphene layer.Article Citation - WoS: 7Citation - Scopus: 7Effects of Interedge Scattering on the Wigner Crystallization in Graphene Nanoribbons(American Physical Society, 2017) Modarresi, Mohsen; Güçlü, Alev DevrimWe investigate the effects of coupling between the two zigzag edges of graphene nanoribbons on the Wigner crystallization of electrons and holes using a combination of tight-binding, mean-field Hubbard and many-body configuration interaction methods. We show that the thickness of the nanoribbon plays a crucial role in the formation of Wigner crystal. For ribbon widths smaller than 16 Å, increased kinetic energy overcomes the long-range Coulomb repulsion and suppresses the Wigner crystallization. For wider ribbons up to 38 Å wide, strong Wigner localization is observed for an even number of electrons, revealing an even-odd effect also found in the Coulomb-blockade addition spectrum. Interedge correlations are found to be strong enough to allow simultaneous crystallization on both edges, although an applied electric field can decouple the two edges. Finally, we show that Wigner crystallization can also occur for holes, albeit weaker than for electrons.
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