Browsing by Subject "Surfaces"

Now showing 1 - 20 of 26

Open Access
Adhesive energy, force and barrier height between simple metal surfaces
(1992) Çıracı, Salim; Tekman, E.; Gökçedag, M.; Batra, I. P.; Baratoff, A.
Using the self-consistent field pseudopotential method we calculated the adhesive energy, perpendicular and lateral forces and barrier height between two rigid A1(001) slabs modeling the sample and a blunt tip. We found that the adhesive energy and forces are site specific, and can lead to a significant corrugation in the constant force mode with negative force gradient. Lateral forces, which determine friction on the atomic scale are not simply proportional to the perpendicular force, and are typically one order of magnitude smaller. Our results confirm that perpendicular tip force and barrier height are interrelated for separations where the force gradient is positive. © 1992.
Open Access
Adsorption site of alkali metal overlayers on Si(001) 2 × 1
(1992) Batra, I. P.; Çıracı, Salim
The alkali metal semiconductor interfaces are currently being investigated by a variety of tools. Most studies to date at half a monolayer coverage have shown preference for either a quasi-hexagonal (H) site or a long-bridge (B) site. At this coverage one-dimensional chain structure for K on Si(001) 2 × 1 have now been confirmed by scanning tunneling microscopy (STM). The data, however, is consistent with either of the two sites. STM investigations at low coverages suggested that alkali metals like K and Cs occupy a novel site, Y, which is a bridge site between two Si atoms belonging to different dimers along the dimer row [110] direction. The total energy calculations for this new Y site, discovered by STM, have shown that it is indeed a site of (local) energy minimum. The ability of the surface silicon atoms, which are not adjacent to the alkali metal atom, to buckle makes the Y site a competitive adsorption site. We deduce the nature of bonding between alkali metals and Si using the STM data. It is concluded that the bond is substantially ionic in nature. © 1992.
Open Access
Analysis of an arbitrary profile reflector antenna having resistive-type surface-H-polarization case
(IEEE, 2008-06-07) Oǧuzer, T.; Altıntaş, Ayhan; Nosich, A. I.
A regularization-based numerical solution is obtained for arbitrary-shape conic section profile reflector antenna in 2-D, for the H-polarization case. New point is that the reflector surface is assumed a resistive-type material. The problem is treated by reducing the singular integral equation obtained from the boundary condition to the dual series equations and application of the Riemann Hilbert Problem (RHP) technique. The resulting matrix equation has regularized form. Sample numerical results are obtained for various values of the eccentricity of the conic section contour of reflector and the resistivity of its surface. © 2008 IEEE.
Open Access
Analysis of double-negative materials with surface integral equations and the multilevel fast multipole algorithm
(IEEE, 2011) Ergül O.; Gürel, Levent
We present a fast and accurate analysis of double-negative materials (DNMs) with surface integral equations and the multilevel fast multipole algorithm (MLFMA). DNMs are commonly used as simplified models of metamaterials at resonance frequencies and are suitable to be formulated with surface integral equations. However, realistic metamaterials and their models are usually very large with respect to wavelength and their accurate solutions require fast algorithms, such as MLFMA. We consider iterative solutions of DNMs with MLFMA and we investigate the accuracy and efficiency of solutions when DNMs are formulated with two recently developed formulations, namely, the combined tangential formulation (CTF) and the electric and magnetic current combined-field integral equation (JMCFIE). Numerical results on canonical objects are consistent with previous results in the literature on ordinary objects. © 2011 IEEE.
Open Access
Calculation of the scalar diffraction field from curved surfaces by decomposing the three-dimensional field into a sum of Gaussian beams
(Optical Society of America, 2013) Şahin, E.; Onural, L.
We present a local Gaussian beam decomposition method for calculating the scalar diffraction field due to a twodimensional field specified on a curved surface. We write the three-dimensional field as a sum of Gaussian beams that propagate toward different directions and whose waist positions are taken at discrete points on the curved surface. The discrete positions of the beam waists are obtained by sampling the curved surface such that transversal components of the positions form a regular grid. The modulated Gaussian window functions corresponding to Gaussian beams are placed on the transversal planes that pass through the discrete beam-waist position. The coefficients of the Gaussian beams are found by solving the linear system of equations where the columns of the system matrix represent the field patterns that the Gaussian beams produce on the given curved surface. As a result of using local beams in the expansion, we end up with sparse system matrices. The sparsity of the system matrices provides important advantages in terms of computational complexity and memory allocation while solving the system of linear equations.
Open Access
Characterization and imaging with lamb wave lens at gigahertz frequencies
(IEEE, 1994-10-11) Bozkurt, Ayhan; Yaralıoğlu, Göksenin; Atalar, Abdullah; Köymen, Hayrettin; Kramer, K.
Lamb wave lenses with conical refracting surfaces are fabricated for use at 400 MHz and 1 GHz. The conical surfaces are ground and polished with mechanical means and they are sufficiently smooth for the frequencies of interest. The wide bandwidth of transducers allow frequency tuning necessary for Lamb wave lenses. The fabricated lenses show the expected V(Z) performance. At high frequencies the attenuation in the coupling medium can be very high, but due to the smaller wavelength the resolution is better and defocus distance can be reduced. Inherently higher leaky wave sensitivity of Lamb wave lens enables a good V(Z) characterization ability at higher frequencies as compared to the conventional spherical lens. Subsurface imaging with these Lamb wave lenses gives satisfactory results for layered structures. Chosen object has leaky wave modes within the angular coverage of the lens. The images exhibit a resolution close to the diffraction limit. Experimental V(Z) curves obtained with these lenses along with images are presented.
Open Access
Electrostatic force spectroscopy of near surface localized states
(Institute of Physics Publishing Ltd., 2005) Dâna, A.; Yamamoto, Y.
Electrostatic force microscopy at cryogenic temperatures is used to probe the electrostatic interaction of a conductive atomic force microscopy tip and electronic charges trapped in localized states in an insulating layer on a semiconductor. Measurement of the frequency shift of the cantilever as a function of tip-sample bias voltage shows discrete peaks at certain voltages when the tip is located near trap centres. These discrete changes in frequency are attributed to one by one filling of individual electronic states when the quantized energies traverse the substrate conduction band Fermi energy as the tip-sample voltage is increased. Theoretical analysis of the experiment suggests that such a measurement of the cantilever frequency shift as a function of bias voltage can be interpreted as an AC force measurement, from which spectroscopic information about the location and energy of localized states can be deduced. Experimental results from the study of a sample with InAs quantum dots as trap centres are presented.
Open Access
Fabrication of 100 nm pMOSFETS With Hybrid AFW / STM lithography
(IEEE, 1997-06) Soh, H. T.; Wilder, K.; Atalar, Abdullah; Quate, C. F.
Scanning probe lithography (SPL) is an emerging area of research in which the scanning tunneling microscope (STM) or atomic force microscope (AFM) is used to pattern nanometer-scale features. Four factors will dictate the viability of SPL as a patterning technology for the semiconductor industry: 1) resolution, 2) alignment accuracy, 3) reliability, and 4) throughput. We present a new SPL technique-a hybrid between the AFM and STMto address these issues. We demonstrate its capabilities and its compatibility with semiconductor processing by fabricating a pMOSFET with an effective channel length (L,ff) of 100 nm and report the device characteristics.
Open Access
Fe promoted NOx storage materials: structural properties and NOx uptake
(American Chemical Society, 2010) Kayhan, E.; Andonova, S. M.; Şentürk, G. S.; Chusuei, C. C.; Ozensoy, E.
Fe promoted NOx storage materials were synthesized in the form of FeOx/BaO/Al2O3 ternary oxides with varying BaO (8 and 20 wt %) and Fe (5 and 10 wt %) contents. Synthesized NOx storage materials were investigated via TEM, EELS, BET, FTIR, TPD, XRD, XPS, and Raman spectroscopy, and the results were compared with the conventional BaO/Al2O3 NOx storage system. Our results suggest that the introduction of Fe in the BaO/Al2O3 system leads to the formation of additional NOx storage sites which store NOx mostly in the form of bidentate nitrates. NO2 adsorption experiments at 323 K via FTIR indicate that, particularly in the early stages of the NOx uptake, the NOx storage mechanism is significantly altered in the presence of Fe sites where a set of new surface nitrosyl and nitrite groups were detected on the Fe sites and the surface oxidation of nitrites to nitrates is significantly hindered with respect to the BaO/Al2O3 system. Evidence for the existence of both Fe3+ as well as reduced Fe2+/(3-x)+ sites on the freshly pretreated materials was detected via EELS, FTIR, Raman, and XRD experiments. The influence of the Fe sites on the structural properties of the synthesized materials was also studied by performing ex situ annealing protocols within 323-1273 K followed by XRD and Raman experiments where the temperature dependent changes in the morphology and the composition of the surface domains were analyzed in detail. On the basis of the TPD data, it was found that the relative stability of the stored NOx species is influenced by the morphology of the Ba and Fe containing NOx-storage domains. The relative stabilities of the investigated NOx species were found to increase in the following order: N2O3/NO+ < nitrates on γ-Al2O3 < surface nitrates on BaO < bidentate nitrates on FeOx sites < bulk nitrates on BaO.
Open Access
Fine-tuning the dispersion and the mobility of BaO domains on NO x storage materials via TiO2 anchoring sites
(American Chemical Society, 2010) Andonova, S. M.; Şentürk, G. S.; Ozensoy, E.
In an attempt to control the surface dispersion and the mobility of BaO domains on NOx storage materials, TiO2/TiOx anchoring sites were introduced on/inside the conventional γ-Al 2O3 support matrix. BaO/TiO2/Al 2O3 ternary oxide materials were synthesized via two different sol-gel preparation techniques, with varying surface compositions and morphologies. The synthesized NOx storage materials were studied via XRD, Raman spectroscopy, BET surface area analysis, TPD, XPS, SEM, EDX-mapping, and in situ FTIR spectroscopy of adsorbed NO2. NOx uptake properties of the BaO/TiO2/Al2O3 materials were found to be strongly influenced by the morphology and the surface structure of the TiO2/TiOx domains. An improved Ba surface dispersion was observed for the BaO/TiO2/Al2O3 materials synthesized via the coprecipitation of alkoxide precursors, which was found to originate mostly from the increased fraction of accessible TiO 2/TiOx sites on the surface. These TiO2/ TiOx sites function as strong anchoring sites for surface BaO domains and can be tailored to enhance surface dispersion of BaO. TPD experiments suggested the presence of at least two different types of NOx species adsorbed on the TiO2/TiOx sites, with distinctively different thermal stabilities. The relative stability of the NOx species adsorbed on the BaO/TiO2/Al2O3 system was found to increase in the following order: NO+/N2O 3 on alumina ≪ nitrates on alumina < surface nitrates on BaO < bridged/bidentate nitrates on large/isolated TiO2 clusters < bulk nitrates on BaO on alumina surface and bridged/bidentate nitrates on TiO2 crystallites homogenously distributed on the surface < bulk nitrates on the BaO sites located on the TiO2 domains. © 2010 American Chemical Society.
Open Access
Fractional Fourier transform as a tool for analyzing beam propagation and spherical mirror resonators
(Optical Society of America, 1994) Özaktaş, Haldun M.; Mendlovic, D.
The complex amplitude distributions on two spherical reference surfaces of given curvature and spacing are simply related by a fractional Fourier transform. The order of the fractional Fourier transform is proportional to the Gouy phase shift between the two surfaces. This result provides new insight into wave propagation and spherical mirror resonators as well as the possibility of exploiting the fractional Fourier transform as a mathematical tool in analyzing such systems.
Open Access
Fresnel lamb wave and V-groove lenses with tunable mode selectivity
(IEEE, 1995) Yaralıoğlu, Göksen Göksenin; Atalar, Abdullah; Köymen, Hayrettin
The Lamb wave and V-groove lenses are distinguished by their high surface wave excitation efficiencies. However, due to the fixed incidence angle, a particular lens can only be used for materials having surface wave velocities within a limited range. Hence, it is desirable to have lenses with adjustable incidence angle. Conventional spherical lenses implemented in Fresnel planar lens form have been demonstrated earlier. In this work, Lamb wave and V-groove lenses constructed as Fresnel lenses are presented. We also discuss the feasibility of Fresnel lenses with air as the coupling medium. It is shown that it is possible to build air coupled Fresnel lenses with a reasonable conversion efficiency into subsurface waves.
Open Access
Growth of high crystalline quality semi-insulating GaN layers for high electron mobility transistor applications
(2006) Yu H.; Caliskan, D.; Özbay, Ekmel
Semi-insulating character (sheet resistivity of 3.26 × 10 11 Ω/sq) of thick GaN layers was developed for AlGaN/GaN high electron mobility transistor (HEMT) applications on an AlN buffer layer. Electrical and structural properties were characterized by a dark current-voltage transmission line model, x-ray diffraction, and atomic force microscope measurements. The experimental results showed that compared to semi-insulating GaN grown on low temperature GaN nucleation, the crystal quality as well as surface morphology were remarkably improved. It was ascribed to the utilization of a high quality insulating AlN buffer layer and the GaN initial coalescence growth mode. Moreover, the significant increase of electron mobility in a HEMT structure suggests that this is a very promising method to obtain high performance AlGaN/GaN HEMT structures on sapphire substrates. © 2006 American Institute of Physics.
Open Access
Low-amplitude, force gradient imaging of Cu(100) surface using tunnel current feedback
(Institute of Physics Publishing, 2004) Özer, H. Ö.; Norris, A.; Oral, A.; Hoffmann, P. M.; Pethica, J. B.
The large corrugation amplitudes in scanning tunnelling microscope (STM) images of metal surfaces have been commonly attributed to the action of forces between the tip and the sample. We have investigated the Cu(100) surface using a high-resolution non-contact atomic force microscope/scanning tunnelling microscope (nc-AFM/STM) in UHV. Force gradient and STM topography images were acquired simultaneously using constant tunnelling current feedback. Force gradient images showed atomic resolution whereas STM scans exhibited almost no contrast, corresponding to a flat tip trajectory during scans. The corrugation height in force gradient images was found to increase as the set tunnelling current was increased. Force gradient and tunnel current were directly measured as a function of separation, to determine the operating conditions during imaging. The STM operation regime is found to lie between the minimum of the stiffness curve and the start of repulsive force.
Open Access
Magnetic field effects on an electron near an impenetrable dielectric surface
(1996) Saqqa, B.; Senger, R. T.; Erçelebi, A.
The interaction of an extrinsic electron with the surface optical modes of a semi-infinite medium is retrieved under the effect of a weak magnetic field. It is observed that for an electron in a bound state near the surface, the magnetic field enhances the effective phonon coupling rather prominently and thus leads to an increased degree of localisation of the electron towards the surface. This feature is seen to be more marked for larger coupling strengths. © Tübi̇tak.
Open Access
MaterialVis: material visualization tool using direct volume and surface rendering techniques
(Elsevier Inc., 2014) Okuyan, E.; Güdükbay, Uğur; Bulutay, C.; Heinig, Karl-Heinz
Visualization of the materials is an indispensable part of their structural analysis. We developed a visualization tool for amorphous as well as crystalline structures, called MaterialVis. Unlike the existing tools, MaterialVis represents material structures as a volume and a surface manifold, in addition to plain atomic coordinates. Both amorphous and crystalline structures exhibit topological features as well as various defects. MaterialVis provides a wide range of functionality to visualize such topological structures and crystal defects interactively. Direct volume rendering techniques are used to visualize the volumetric features of materials, such as crystal defects, which are responsible for the distinct fingerprints of a specific sample. In addition, the tool provides surface visualization to extract hidden topological features within the material. Together with the rich set of parameters and options to control the visualization, MaterialVis allows users to visualize various aspects of materials very efficiently as generated by modern analytical techniques such as the Atom Probe Tomography.
Open Access
Nano-structured organically modified silica thin films for functional surfaces
(2011) Bayındır, Mehmet; Yıldırım, Adem; Budunoglu, Hülya; Yaman, Mecit; Deniz, Hakan; Güler, Mustafa O.
We report a template-free sol-gel method for preparation of nanoporous ormosil thin films at ambient conditions. The thin films are coated to the surfaces by using colloidal suspensions of ormosil gels. Gels are synthesized by using a trifunctional organosilane monomer, methyltrimethoxysilane (MTMS), with a two-step acid base reaction. We prepared several ormosil thin films on glass, metal, plastic and paper surfaces with different functionalities like superhydrophobic, antireflective, antifogging and ice retarding properties, from gels prepared in different conditions. Also films on flexible substrates exhibits durable surface properties after several bending cycles. In addition, we also demonstrate that these thin films can be used for fluorescent sensing of explosives by doping them with fluorescent dyes.
Open Access
Nature of the Ti-Ba interactions on the BaO/TiO2/Al 2O3 NOx storage system
(2009) Andonova, S. M.; Şentürk, G. S.; Kayhan, E.; Ozensoy, E.
A ternary oxide-based NO* storage material in the form of BaOZTiO2Zy-Al2O3 was synthesized and characterized. Thermally induced structural changes occurring on the surfaces of the TiO2Zy-Al2O3 and BaOZ TiO 2Zy-Al2O3 systems were studied in a comparative manner within 300-1273 K via X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy, and BET surface area analysis. The surface acidity of the studied oxide systems was also investigated via pyridine adsorption monitored by in-situ Fourier transform infrared (FTIR) spectroscopy. BaO/TiO2γ-Al 2O3 ternary oxide was synthesized by incorporating different loadings of (8-20 wt %) BaO onto the TiO2/γ Al 2O3 support material, which was originally prepared using the sol-gel method. In the TiO2Zy-Al2O3 binary oxide support material, anatase phase exhibited a relatively high thermal stability at T < 1073 K. The presence of TiO2 domains on the surface of the alumina particles was found to alter the surface acidity of alumina by providing new medium-strength Lewis acid sites. SEMZEDX results indicate that in the BaO/TiO2γ-Al2O3 system, TiO2 domains present a significant affinity toward BaO and/or Ba(NO3) 2 resulting in a strong Ti-Ba interaction and the formation of overlapping domains on the surface. The presence of TiO2 also leads to a decrease in the decomposition temperature of the Ba(N03) 2 phase with respect to the Ti-free Ba(N03) 2ZyAl2O3 system. Such a destabilization is likely to occur due to a weaker interaction between Ba(N03) 2 and y-Al203 domains in the ternary oxide as well as due to the change in the surface acidity in the presence of TiO 2. At relatively high temperatures (e.g., 873-1273 K) formation of complex structures in the form of BaTiO3, Ba1.23Al 2.46Ti5.54O16, BaTiO5, andor Ba x:AlyTizOn., were also observed. © 2009 American Chemical Society.
Open Access
Position-invariant surface recognition and localization using infrared sensors
(SPIE, 2003) Barshan, B.; Aytaç, T.
Low-cost infrared emitters and detectors are used for the recognition of surfaces with different properties in a location-invariant manner. The intensity readings obtained with such devices are highly dependent on the location and properties of the surface in a way that cannot be represented in a simple manner, complicating the recognition and localization process. We propose the use of angular intensity scans and present an algorithm to process them. This approach can distinguish different surfaces independently of their positions. Once the surface is identified, its position can also be estimated. The method is verified experimentally with the surfaces aluminum, white painted wall, brown kraft paper, and polystyrene foam packaging material. A correct differentiation rate of 87% is achieved, and the surfaces are localized within absolute range and azimuth errors of 1.2 cm and 1.0 deg, respectively. The method demonstrated shows that simple infrared sensors, when coupled with appropriate processing, can be used to extract a significantly greater amount of information than they are commonly employed for. © 2003 Society of Photo-Optical Instrumentation Engineers.
Open Access
Possibility of superconductivity of two-dimensional electrons on the surface of liquid heliuM, films
(Pergamon Press, 1993) Tanatar, Bilal; Hakioǧlu T.
We consider the possibility of superconductivity in a system of two-dimensional electrons on the surface of liquid helium films. Taking into account of the interaction between electrons and the surface excitations of liquid helium films-ripplons, within the weak coupling BCS theory, we estimate the superconducting transition temperature for various interaction strengths, film thicknesses, and electron densities. The superconducting transition temperature Tc, under experimentally realizable conditions, is calculated to be a few mK's. © 1993.