Materials Science and Engineering / Malzeme Bilimi ve Mühendisliği
Permanent URI for this collectionhttps://hdl.handle.net/11147/4719
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Article Citation - WoS: 2Citation - Scopus: 2Tuning Thermal Transport in Graphene Via Combinations of Molecular Antiresonances(Elsevier Ltd., 2018) Sevim, Koray; Sevinçli, Haldun; Sevim, Koray; Sevinçli, Haldun; 03.09. Department of Materials Science and Engineering; 04.05. Department of Pyhsics; 03. Faculty of Engineering; 04. Faculty of Science; 01. Izmir Institute of TechnologyWe propose a method to engineer the phonon thermal transport properties of low dimensional systems. The method relies on introducing a predetermined combination of molecular adsorbates, which give rise to antiresonances at frequencies specific to the molecular species. Despite their dissimilar transmission spectra, thermal resistances due to individual molecules remain almost the same for all species. On the other hand, thermal resistance due to combinations of different species are not additive and show large differences depending on the species. Using a toy model, the physics underlying the violation of resistance summation rule is investigated. It is demonstrated that equivalent resistance of two scatterers having the same resistances can be close to the sum of the constituents or ∼ 70% of it depending on the relative positions of the antiresonances. The relative positions of the antiresonances determine the net change in transmission, therefore the equivalent resistance. Since the entire spectrum is involved in phonon spectrum changes in different parts of the spectrum become important. Performing extensive first-principles based computations, we show that these distinctive attributes of phonon transport can be useful to tailor the thermal transport through low dimensional materials, especially for thermoelectric and thermal management applications.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; Sevinçli, Haldun; Sevim, Koray; Kurt, Gizem; Sevim, Koray; Sevinçli, Haldun; 03.09. Department of Materials Science and Engineering; 04.05. Department of Pyhsics; 03. Faculty of Engineering; 04. Faculty of Science; 01. Izmir Institute of TechnologyUsing 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: 8Citation - Scopus: 10Experimental and Computational Investigation of Graphene/Sams Schottky Diodes(Elsevier Ltd., 2018) Aydın, Hasan; Bacaksız, Cihan; Senger, Ramazan Tuğrul; Karakaya, Caner; Mermer, Ömer; Selamet, Yusuf; Senger, Ramazan Tuğrul; Şahin, Hasan; Şahin, Hasan; 04.04. Department of Photonics; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologyWe have investigated the effect of two different self-assembled monolayers (SAMs) on electrical characteristics of bilayer graphene (BLG)/n-Si Schottky diodes. Novel 4″bis(diphenylamino)-1, 1′:3″-terphenyl-5′ carboxylic acids (TPA) and 4,4-di-9H-carbazol-9-yl-1,1′:3′1′-terphenyl-5′ carboxylic acid (CAR) aromatic SAMs have been used to modify n-Si surfaces. Cyclic voltammetry (CV) and Kelvin probe force microscopy (KPFM) results have been evaluated to verify the modification of n-Si surface. The current–voltage (I–V) characteristics of bare and SAMs modified devices show rectification behaviour verifying a Schottky junction at the interface. The ideality factors (n) from ln(I)–V dependences were determined as 2.13, 1.96 and 2.07 for BLG/n-Si, BLG/TPA/n-Si and BLG/CAR/n-Si Schottky diodes, respectively. In addition, Schottky barrier height (SBH) and series resistance (R s ) of SAMs modified diodes were decreased compared to bare diode due to the formation of a compatible interface between graphene and Si as well as π–π interaction between aromatic SAMs and graphene. The CAR-based device exhibits better diode characteristic compared to the TPA-based device. Computational simulations show that the BLG/CAR system exhibits smaller energy-level-differences than the BLG/TPA, which supports the experimental findings of a lower Schottky barrier and series resistance in BLG/CAR diode.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, Hasan; Çelebi, Cem; 03.01. Department of Bioengineering; 04.05. Department of Pyhsics; 04.04. Department of Photonics; 03. Faculty of Engineering; 04. Faculty of Science; 01. Izmir Institute of TechnologyWe 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: 54Citation - Scopus: 53Nitrogen Doping for Facile and Effective Modification of Graphene Surfaces(Royal Society of Chemistry, 2017) Yanılmaz, Alper; Tomak, Aysel; Selamet, Yusuf; Bacaksız, Cihan; Özçeri, Elif; Arı, Ozan; Senger, Ramazan Tuğrul; Selamet, Yusuf; Tomak, Aysel; Senger, Ramazan Tuğrul; 03.01. Department of Bioengineering; 01. Izmir Institute of Technology; 04.05. Department of Pyhsics; 03. Faculty of Engineering; 04. Faculty of ScienceWe report experimental and theoretical investigations of nitrogen doped graphene. A low-pressure Chemical Vapor Deposition (CVD) system was used to grow large-area graphene on copper foil, using ethylene as the carbon source. Nitrogen-doped graphene (N-graphene) was prepared by exposing the graphene transferred to different substrates to atomic nitrogen plasma. The effect of varying nitrogen flow rates on doping of graphene was investigated while keeping the power and time constant during the process. The N-graphene was characterized via Raman Spectroscopy, X-ray Photoelectron Spectroscopy (XPS), Scanning Tunneling Microscopy and Spectroscopy (STM and STS), and Fourier Transform Infrared spectroscopy (FTIR). Raman mapping of N-graphene was also performed to show homogeneity of nitrogen on the graphitic lattice. XPS results have revealed the presence of different nitrogen configurations in the graphitic lattice with similar doping concentrations. Density functional theory (DFT) based calculations showed that the periodic adsorption of N atoms predominantly occurs on top of the C atoms rather than through substitution of C in our N-graphene samples. Our results indicate a feasible procedure for producing N-graphene with homogenous and effective doping which would be valuable in electronic and optical applications.Article Citation - WoS: 4Citation - Scopus: 4Effect of Aromatic Sams Molecules on Graphene/Silicon Schottky Diode Performance(Electrochemical Society, Inc., 2016) Yağmurcukardeş, Nesli; Selamet, Yusuf; Can, Mustafa; Yanılmaz, Alper; Mermer, Ömer; Okur, Salih; Selamet, Yusuf; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologyAu/n-Si/Graphene/Au Schottky diodes were fabricated by transferring atmospheric pressure chemical vapor deposited (APCVD) graphene on silicon substrates. Graphene/n-Si interface properties were improved by using 5-[(3-methylphenyl)(phenyl) amino]isophthalic acid (MePIFA) and 5-(diphenyl)amino]isophthalic acid (DPIFA) aromatic self-assembled monolayer (SAM) molecules. The surface morphologies of modified and non-modified films were investigated by atomic force microscopy and scanning electron microscopy. The surface potential characteristics were obtained by Kelvin-probe force microscopy and found as 0.158 V, 0.188 V and 0,383 V as a result of SAMs modification. The ideality factors of n-Si/Graphene, n-Si/MePIFA/Graphene and n-Si/DPIFA/Graphene diodes were found as 1.07, 1.13 and 1.15, respectively. Due to the chain length of aromatic organic MePIFA and DPIFA molecules, also the barrier height φB values of the devices were decreased. While the barrier height of n-Si/Graphene diode was obtained as 0.931 eV, n-Si/MePIFA/Graphene and n-Si/DPIFA/Graphene diodes have barrier height of 0.820 and 0.720 eV, respectively.Article Citation - WoS: 6Citation - Scopus: 6Influence of Buffer Layers on Ni Thin Film Structure and Graphene Growth by Cvd(IOP Publishing Ltd., 2015) Özçeri, Elif; Selamet, Yusuf; Selamet, Yusuf; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of TechnologyBuffer and/or adhesive layers were used to decrease the dewetting of Ni thin film at graphene growth temperatures of around 900 °C. Depositing a thin buffer (Al2O3) layer onto SiO2/Si substrate significantly reduced the dewetting effect and surface roughness of Ni catalyst film. Thin adhesive (Cr) layers with or without Al2O3 buffer layers increased the texturing in (1 1 1) orientation, which was promoted by growing at an elevated temperature (450 °C). The effects of pretreatment and growth temperature on crystal orientation, grain size and surface roughness of Ni film were analyzed. Our results indicated a large positive correlation coefficient between the film thickness and surface roughness for thinner and non-buffered films, and a negative correlation coefficient between the thickness and 900 °C -annealed film roughness for thicker and buffered films. The graphene coverage was greatly improved over the films grown with Al2O3 and/or Cr layers. In summary, we suggest that growing high quality, large area, 1- or 2-layer graphene on polycrystalline Ni transition metal thin film is optimized by using Al2O3 and/or Cr layers to reduce Ni dewetting, surface roughness, and groove depth while controlling grain size and texturing in (1 1 1) orientation by annealing at 900 °C.