Physics / Fizik
Permanent URI for this collectionhttps://hdl.handle.net/11147/6
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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.Article Citation - WoS: 4Citation - Scopus: 4Indirect Exchange Interaction in Two-Dimensional Materials With Quartic Dispersion(American Physical Society, 2022) Canbolat, Ahmet Utku; Sevinçli, Haldun; Çakır, ÖzgürWe investigate the indirect magnetic exchange interaction between two magnetic moments in a two-dimensional semiconductor with quartic dispersion, featuring a singularity at the band edge. We obtain the Green's functions analytically to calculate the magnetic exchange interaction at zero temperature. We show that the singularity in the density of states (DOS) for quartic dispersion gives rise to an enhancement in the amplitude of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction as the Fermi energy is swept toward the band edge. Furthermore, a region of finite exchange interaction arises, with a range increasing as the Fermi energy approaches the band edge. The results lay the possibility of an electrical/chemical control over the exchange interactions.Article Citation - WoS: 7Citation - Scopus: 6Anisotropic Etching of Cvd Grown Graphene for Ammonia Sensing(Institute of Electrical and Electronics Engineers Inc., 2022) Yağmurcukardeş, Nesli; Bayram, Abdullah; Aydın, Hasan; Yağmurcukardeş, Mehmet; Açıkbaş, Yaser; Peeters, François M.; Çelebi, CemBare chemical vapor deposition (CVD) grown graphene (GRP) was anisotropically etched with various etching parameters. The morphological and structural characterizations were carried out by optical microscopy and the vibrational properties substrates were obtained by Raman spectroscopy. The ammonia adsorption and desorption behavior of graphene-based sensors were recorded via quartz crystal microbalance (QCM) measurements at room temperature. The etched samples for ambient NH3 exhibited nearly 35% improvement and showed high resistance to humidity molecules when compared to bare graphene. Besides exhibiting promising sensitivity to NH3 molecules, the etched graphene-based sensors were less affected by humidity. The experimental results were collaborated by Density Functional Theory (DFT) calculations and it was shown that while water molecules fragmented into H and O, NH3 interacts weakly with EGPR2 sample which reveals the enhanced sensing ability of EGPR2. Apparently, it would be more suitable to use EGRP2 in sensing applications due to its sensitivity to NH3 molecules, its stability, and its resistance to H2O molecules in humid ambient.Article Citation - Scopus: 1Oxidizer Gases Effects on the Diameter-Controlled Synthesis of Carbon Nanotubes(MIM Research Group, 2021) İnce Yardımcı, Atike; Öğütlü, Ahmet Sabri; Öğütlü, DenizIn this study, the influence of the oxidizers on the synthesis of carbon nanotubes by C2H4 decomposition over Fe catalyst has been investigated. CO2, O2, and H2O have been used as oxidizers, and to control catalyst particle formation and their sizes in the pretreatment stage. The same oxidizers have also been used in the growth stage to maintain the catalyst particle size, remove amorphous carbon formation to keep catalyst particle active. The results of scanning electron microscopy indicated that the average diameters of nanotubes decreased from 13.4±1.2 nm to 6.2±0.5 nm and extremely dense nanotubes were obtained when we added a small amount of CO2. Adding O2 extremely decreased the areal carbon nanotube density while widens the diameter distribution. H2O addition resulted in larger average diameters and made the growth strongly pretreatment dependent. Within the parameters tried for catalyst pretreatment and CNT growth processes, CO2 seemed the best choice for a weak oxidizing assistant. The strong dependency of the average diameter on pretreatment conditions indicated that pretreatment is a very important step in deciding the final diameters and their distribution.Article Citation - WoS: 14Citation - Scopus: 17Enhancement of Thermoelectric Efficiency of T-Hfse2 Via Nanostructuring(American Physical Society, 2021) Ünsal, Elif; Senger, Ramazan Tuğrul; Sevinçli, HaldunIn this work, ab initio calculations based on density functional theory and the Landauer formalism are carried out to investigate ballistic thermoelectric properties of T-HfSe2 nanoribbons (NRs). The zigzag-edged NRs are metallic, and they are not included in this study. The armchair NRs possess two types of edge symmetries depending on the number of atoms present in a row; odd-numbered NRs have mirror symmetry, whereas the even-numbered NRs have glide reflection symmetry. The armchair-edged NRs are dynamically stable and show semiconducting properties with varying band gap values in the infrared and visible regions. Detailed transport analyses show that the n-type Seebeck coefficient and the power factor differ because of the structural symmetry, whereas the p-type thermoelectric coefficients are not significantly influenced. It is shown that the phonon thermal conductance is reduced to a third of its two-dimensional value via nanostructuring. The p-type Seebeck coefficient and the power factor for T-phase HfSe(2 )are enhanced in NRs. We report that the p-type ZT value of HfSe2 NRs at 300 and 800 K are enhanced by factors of 4 and 3, respectively.Article Citation - WoS: 32Citation - Scopus: 33Ballistic Thermoelectric Transport Properties of Two-Dimensional Group Iii-Vi Monolayers(American Physical Society, 2021) Çınar, Mustafa Neşet; Özbal Sargın, Gözde; Sevim, Koray; Özdamar, Burak; Kurt, Gizem; Sevinçli, HaldunBallistic transport and thermoelectric properties of group III-VI compounds (XY: X = B, Al, Ga, In, Tl; Y = O, S, Se, Te, Po) are investigated based on first-principles calculations and Landauer formalism. This large family is composed of 25 compounds which stands out with their unique electronic band structures. Mexican hat shaped valence band, which exhibits quartic energy-momentum relation gives rise to a sharp peak in the density of states as well as a steplike electronic transmission spectrum near the valence band edge. The intriguing electronic band structure and transport properties motivate us to explore thermoelectric properties of group III-VI monolayers. We find that, in addition to the stepwise transmission at the band edge, flat bands, valley degeneracy, and band degeneracy are the factors that enhance thermoelectric efficiencies. For heavier compounds, better thermoelectric efficiencies are possible for both n-type and p-type carriers.Article Citation - WoS: 12Citation - Scopus: 14Laser Assisted Synthesis of Anisotropic Metal Nanocrystals and Strong Light-Matter Coupling in Decahedral Bimetallic Nanocrystals(Royal Society of Chemistry, 2021) Mert Balcı, Fadime; Sarısözen, Sema; Polat, Nahit; Güvenç, Çetin Meriç; Karadeniz, Uğur; Tertemiz, Necip Ayhan; Balcı, SinanThe advances in colloid chemistry and nanofabrication allowed us to synthesize noble monometallic and bimetallic nanocrystals with tunable optical properties in the visible and near infrared region of the electromagnetic spectrum. In the strong coupling regime, surface plasmon polaritons (SPPs) of metal nanoparticles interact with excitons of quantum dots or organic dyes and plasmon-exciton hybrid states called plexcitons are formed. Until now, various shaped metal nanoparticles such as nanorods, core-shell nanoparticles, hollow nanoparticles, nanoprisms, nanodisks, nanorings, and nanobipyramids have been synthesized to generate plasmon-exciton mixed states. However, in order to boost plasmon-exciton interaction at nanoscale dimensions and expand the application of plexcitonic nanocrystals in a variety of fields such as solar cells, light emitting diodes, and nanolasers, new plexcitonic nanocrystals with outstanding optical and chemical properties remain a key goal and challenge. Here we report laser-assisted synthesis of decahedral shaped noble metal nanocrystals, tuning optical properties of the decahedral shaped nanocrystals by galvanic replacement reactions, colloidal synthesis of bimetallic decahedral shaped plexcitonic nanocrystals, and strong plasmon-plasmon interaction in bimetallic decahedral shaped noble metal nanocrystals near a metal film. We photochemically synthesize decahedral Ag nanoparticles from spherical silver nanoparticles by using a 488 nm laser. The laser assisted synthesis of silver nanoparticles yields decahedral (bicolored) and prism (monocolored) shaped silver nanocrystals. The decahedral shaped nanoparticles were selectively separated from prism shaped nanoparticles by centrifugation. The optical properties of decahedral nanocrystals were tuned by the galvanic replacement reaction between gold ions and silver atoms. Excitons of J-aggregate dyes and SPPs of decahedral bimetallic nanoparticles strongly couple and hence decahedral shaped plexcitonic nanoparticles are prepared. In addition, localized SPPs of decahedral shaped bimetallic nanocrystals interact strongly with the propagating SPPs of a flat silver film and hence new hybrid plasmonic modes (plasmonic nanocavities) are generated. The experimental results are further fully corroborated by theoretical calculations including decahedral shaped plexcitonic nanoparticles and decahedral nanoparticles coupled to flat metal films.Article Citation - WoS: 11Citation - Scopus: 13Effect of Cnt Incorporation on Pan/Ppy Nanofibers Synthesized by Electrospinning Method(TÜBİTAK, 2020) İnce Yardımcı, Atike; Tanoğlu, Metin; Yılmaz, Selahattin; Selamet, YusufIn this study, carbon nanotubes (CNTs) added polyacrylonitrile/polypyrrole (PAN/PPy) electrospun nanofibers were produced. Average diameters of the nanofibers were measured as 268 and 153 nm for 10 and 25 wt% of PPy contents, respectively. A relatively higher strain to failure values (23.3%) were observed for the low PPy content. When as-grown CNTs (1 and 4 wt%) were added into the PAN/PPy blends, disordered nanofibers were observed to form within the microstructure. To improve the interfacial properties of CNTs/PAN/PPy composites, CNTs were functionalized with H2SO4/HNO3/HCl solution. The functionalized CNTs were well dispersed within the nanofibers and aligned along the direction of nanofibers. Therefore, beads formation on nanofibers decreased. The impedance of the nanofibers was found to decrease with the PPy content and CNT addition. These nanofibers had a great potential to be used as an electrochemical actuator or a tissue engineering scaffold.Article Citation - WoS: 9Structural, Electronic, and Magnetic Properties of Point Defects in Polyaniline (c3n) and Graphene Monolayers: a Comparative Study(American Institute of Physics, 2020) Sevim, Koray; Sevinçli, HaldunThe newly synthesized two-dimensional polyaniline (C3N) is structurally similar to graphene and has interesting electronic, magnetic, optical, and thermal properties. Motivated by the fact that point defects in graphene give rise to interesting features, like magnetization in an all carbon material, we perform density functional theory calculations to investigate vacancy and Stone-Wales type point defects in monolayer C3N. We compare and contrast the structural, electronic, and magnetic properties of these defects with those in graphene. While monovacancies and Stone-Wales defects of C3N result in reconstructions similar to those in graphene, divacancies display dissimilar geometrical features. Different from graphene, all vacancies in C3N have metallic character because of altered stoichiometry; those that have low-coordinated atoms have finite magnetic moments. We further investigate the robustness of the reconstructed structures and the changes in the magnetic moments by applying tensile and compressive biaxial strain. We find that, with the advantage of finite bandgap, point defects in C3N are qualified as good candidates for future spintronics applications.Article Citation - WoS: 12Citation - Scopus: 15Magnetic Mechanism for the Biological Functioning of Hemoglobin(Nature Publishing Group, 2020) Mayda, Selma; Kandemir, Zafer; Bulut, Nejat; Maekawa, SadamichiThe role of magnetism in the biological functioning of hemoglobin has been debated since its discovery by Pauling and Coryell in 1936. The hemoglobin molecule contains four heme groups each having a porphyrin layer with a Fe ion at the center. Here, we present combined density-functional theory and quantum Monte Carlo calculations for an effective model of Fe in a heme cluster. In comparison with these calculations, we analyze the experimental data on human adult hemoglobin (HbA) from the magnetic susceptibility, Mossbauer and magnetic circular dichroism (MCD) measurements. In both the deoxygenated (deoxy) and the oxygenated (oxy) cases, we show that local magnetic moments develop in the porphyrin layer with antiferromagnetic coupling to the Fe moment. Our calculations reproduce the magnetic susceptibility measurements on deoxy and oxy-HbA. For deoxy-HbA, we show that the anomalous MCD signal in the UV region is an experimental evidence for the presence of antiferromagnetic Fe-porphyrin correlations. The functional properties of hemoglobin such as the binding of O-2, the Bohr effect and the cooperativity are explained based on the magnetic correlations. This analysis suggests that magnetism could be involved in the functioning of hemoglobin.