Silicene Like Domains on IrSi3 Crystallites

Recently, silicene, the graphene equivalent of silicon, has attracted a lot of attention due to its compatibility with Si-based electronics. So far, silicene has been epitaxy grown on various crystalline surfaces such as Ag(110), Ag(111), Ir(111), ZrB2(0001) and Au(110) substrates. Here, we present a new method to grow silicene via high temperature surface reconstruction of hexagonal IrSi3 nanocrystals. The h-IrSi3 nanocrystals are formed by annealing thin Ir layers on Si(111) surface. A detailed analysis of the STM images shows the formation of silicene like domains on the surface of some of the IrSi3 crystallites. We studied both morphology and electronic properties of these domains by using both scanning tunneling microscopy/spectroscopy and first-principles calculation methods

From spin-polarized interfaces to giant magnetoresistance in organic spin valves

We calculate the spin-polarized electronic transport through a molecular bilayer spin valve from first principles, and establish the link between the magnetoresistance and the spin-dependent inter- actions at the metal-molecule interfaces. The magnetoresistance of a Fe|bilayer-C70|Fe spin valve attains a high value of 70% in the linear response regime, but it drops sharply as a function of the applied bias. The current polarization has a value of 80% in linear response, and also decreases as a function of bias. Both these trends can be modelled in terms of prominent spin-dependent Fe|C70 interface states close to the Fermi level, unfolding the potential of spinterface science to control and optimize spin currents.Comment: 13 pages, 5 figure

A Workstation for microassembly

In this paper, an open-architecture, reconfigurable microassembly workstation for efficient and reliable assembly of micromachined parts is presented. The workstation is designed to be used as a research tool for investigation of the problems in microassembly. The development of such a workstation includes the design of: (i) a manipulation system consisting of motion stages providing necessary travel range and precision for the realization of assembly tasks, (ii) a vision system to visualize the microworld and the determination of the position and orientation of micro components to be assembled, (iii) a robust control system and necessary mounts for the end effectors in such a way that according to the task to be realized, the manipulation tools can be easily changed and the system will be ready for the predefined task. In addition tele-operated and semi-automated assembly concepts are implemented. The design is verified by implementing the range of the tasks in micro-parts manipulation. The versatility of the workstation is demonstrated and high accuracy of positioning is sho

Analysis of nomofobic behaviors of adolescents regarding various factors

Although the developments in technology have made our lives and daily activities easier, it is believed that problematic and excessive use of technology could have some negative effects on people. One of these negative effects is the prevalence of nomophobia, which is considered as a new phobia in recent years. Nomophobia is defined as the fear and anxiety when an individual cannot access his or her mobile/smart phone or cannot have the chance to communicate online and offline via mobile devices. Nomophobic tendencies can change individuals’ daily habits. Negative emotions due to nomophobic tendencies like fear and anxiety especially in young people is thought to affect their school lives and academic achievements. The purpose of this research is to analyze the prevalence of nomophobia among high school students regarding various factors. The Nomophobia Scale (NMP-Q), which was used in this study, was developed by Yildirim and Correia (2015), and adapted into Turkish by Yildirim, Sumuer, Adnan and Yildirim (2015). The study group consists of 475 high school students attending six different schools situated in diverse socio-economic areas in Izmir and Edirne in 2015-2016 education year. The study was conducted in survey model and descriptive statistics. T-test for independent samples and one-way analysis of variance (ANOVA) techniques were used at the analysis stage. According to the findings, mean scores obtained from the scale demonstrated that the levels of nomophobic behaviors of high school students were above the average. Besides, female students have higher nomophobia levels compared to male students in terms of gender variable and a significant difference was found in terms of the duration of mobile internet usage. A significant difference could not be found in terms of the variables such as grade, parents’ education levels, and the duration of smartphone usage. Furthermore, the study has revealed that high school students use their smartphones and mobile internet largely for social networks, music, communication, photographs, education, research, games and videos.

Titanium dioxide nanostructures for photocatalytic and photovoltaic applications

Ankara : The Department of Physics and the Institute of Engineering and Science of Bilkent University, 2008.Thesis (Ph.D.) -- Bilkent University, 2008.Includes bibliographical references leaves 120-128.In this thesis, TiO2 nanostructures and their photocatalytic and photovoltaic ap- plications have been investigated by using the ¯rst-principles calculations based on density functional theory. We have concentrated on three di®erent systems, namely TiO2 clusters, nanowires and surfaces. TiO2 is widely used in various applications, since it is chemically stable in di®erent conditions, ¯rm under il- lumination, non toxic, and relatively easy and cheap to produce. Most of the technological applications such as photovoltaic and photocatalytic of TiO2 are mainly related to its optical properties. First of all, structural, electronic, and magnetic properties of small (TiO2)n (n=1{10) clusters have been studied. Various initial geometries for each n have been searched to ¯nd out the ground state geometries. In general, it has been found that the ground state structures (for n=1{9) have at least one dangling or pendant O atom. Only the lowest lying structure of n=10 cluster does not have any pendant O atom. In the ground state structures, Ti atoms are at least 4{fold coordinated for n ¸ 4. Clusters prefer to form three{dimensional and compact structures. All clusters have singlet ground state. The formation energy and HOMO{LUMO gap have also been calculated as a function of the number of TiO2 unit to study the stability and electronic properties. The formation energy increases with increasing size of the cluster. This means that clusters become stronger as their size grows. The interaction of the ground state structure of each (TiO2)n cluster with H2O has been investigated. The binding energy Eb of H2O molecule decreases and oscillates as the cluster size increases. The interaction of the ground state structure of n=3, 4, 10 clusters with more than one H2O molecule has also been studied. We have calculated Eb per adsorbed molecule and we have shown that it decreases with increasing number of adsorbed H2Omolecule (N). When N ¸ 2 for n=3 and N ¸3 for n=4 clusters, H2O molecules bind more strongly to n=10 cluster. The adsorption of transition metal (TM) atoms such as V, Co, and Pt on n=10 cluster has been studied as well. All these elements interact with the cluster forming strong chemisorption bonds, and the permanent magnetic moment is induced upon the adsorption of Co or V atoms. Second of all, structural, electronic and magnetic properties of very thin TiOx (x=1,2) nanowires have been presented. All stoichiometric TiO2 nanowires ex- hibit semiconducting behavior and have non{magnetic ground state. There is a correlation between binding energy (Eb) and the energy band gap (Eg) of these TiO2 nanowires. In general, Eb increases with Eg. In non-stoichiometric TiO nanowires, we have both metallic and semiconducting nanowires. In addition to non{magnetic TiO nanowires, we have also ferromagnetic nanowires. Three{ dimensional (3D) structures are more energetic than planar ones for both stoi- chiometries. The stability of TiOx nanowires is enhanced by the increase of the size and coordination number of Ti and O atoms which tend to possess at least four and two nearest neighbors, respectively. We have also studied the structural and electronic properties of rutile (110) nanowires obtained by cutting bulk ru- tile along the [110] direction with a certain cross section. The bulk nanowires are more energetic than the thin nanowires after a certain diameter. Like thin TiO2 nanowires, all bulk wires are semiconducting and Eg oscillates with the cross section of these (110) nanowires. Third of all, we have studied the interaction of perylenediimide (PDI){based dye compounds (BrPDI, BrGly, and BrAsp) with both the unreconstructed (UR) and reconstructed (RC) anatase TiO2 (001) surfaces. All dye molecules form strong chemical bonds with the surface in the most favorable adsorption struc- tures. The lowest binding energy is 2.60 eV which has been obtained in the adsorption of BrPDI dye on the UR surface. In UR{BrGly, RC{BrGly and RC{ BrAsp cases, we have observed that HOMO and LUMO levels of the adsorbed molecules appear within the band gap and conduction band regions, respectively. Moreover, we have obtained a gap narrowing upon adsorption of BrPDI on the RC surface. Because of the reduction in the e®ective band gap of the surface{dye system and possibly achieved the visible light activity, these results are valuable for photovoltaic and photocatalytic applications. We have also considered the e®ects of the hydration of surface on the binding of BrPDI. It has been found that the binding energy drops signi¯cantly for the completely hydrated surfaces.Fourth of all, we have considered the interaction of BrPDI, BrGly, and BrAsp dye molecules with defect free rutile TiO2 (110) surfaces. All dye molecules form moderate chemical bonds with surface in the most stable adsorption structures. The average binding energy of dye molecules is about 1 eV. Regardless of the type of dye molecules, HOMO and LUMO levels of the adsorbed dye appear within the gap and the conduction band region of defect free surface, respectively. The e®ect of the slab thickness on the interaction strength between the dye and the surface has also been examined. Unlike the four layers slab, BrGly and BrAsp molecules are dissociatively adsorbed on the three layers slab. The interaction between BrPDI and partially reduced rutile (110) as well as platinized surface has been also considered in order to ¯gure out the e®ects of O vacancy and preadsorbed small Ptn (n=1, 3 and 5 ) clusters on the binding, electronic, and structural properties of the dye{surface system. It has been found that BrPDI dye prefers to bind to the O vacancy site for the partially reduced surface case. Transition metal deposition on metal oxides plays a crucial role in various industrial areas such as catalysts and photovoltaic cells. Finally, an extensive study of the adsorption of small Ptn (n=1{8) and bimetallic Pt2Aum (m=1{5) clusters on partially reduced rutile TiO2 (110) has been presented. The e®ect of surface O vacancies on the adsorption and growth of Pt and bimetallic Pt{Au clusters over the defective site of the 4£2 rutile surface has been studied. Struc- tures, energetics and electronic properties of adsorbed Ptn and Pt2Aum clusters have been analyzed. The surface O vacancy site has been found to be the most active site for a single Pt monomer. Other Pt clusters, namely dimer, trimer and so on, tend to grow around this monomer. As a result, O vacancy site behaves as a nucleation center for the clustering of Pt atoms. Small Pt clusters interact strongly with the partially reduced surface. Eb per adsorbed Pt atom is 3.38 eV for Pt1 case and Eb increases as the cluster size grows due to the formation of strong Pt{Pt bonds. Pt clusters prefer to form planar structures for n = 1{6 cases. The calculated partial density of states of Ptn{TiO2 surface has revealed that the surface becomes metallic when n ¸ 3. In the case of bimetallic Pt-Au clusters, Aum clusters have been grown on the Pt2{TiO2 surface. Previously ad- sorbed Pt dimer at the vacancy site of the reduced surface acts as a clustering center for Au atoms. This Pt2 cluster also inhibits sintering of the Au clusters on the surface. The interaction between the adsorbed Au atoms and titania surface as well as previously adsorbed Pt dimer is weak compared to Pt{TiO2 surface interactions. Since charge state of the clusters adsorbed on the oxide surfaces iscrucial for catalysis applications of these clusters, total charge on each atom of the metal clusters has also been calculated. Charge transfer among the cluster atoms and underlying oxide surface is more pronounced for Ptn clusters. Furthermore, the absolute value of total charge on the clusters is greater for Ptn than that of bimetallic Pt{Au case.Çakır, DenizPh.D

Mechanical and electronic properties of metal chain nanowires

Cataloged from PDF version of article.The fabrication of stable gold monoatomic chains suspended between two gold electrodes is one of the milestones in nanoscience and technology, since miniaturization of the electronic components is one of the great importance in development and improvement of new devices in nanoelectronic. Monoatomic chain nanowires show unusual mechanical and electronic properties such as quantized conductance and much stiff bonds compare to the ones in bulk. Ohnishi et al. [1], has visualized the monoatomic chains by using transmission electron microscopy (TEM). At the same time, Yanson et al [2], have produced the monoatomic chains and they measured its conductance. In the bond length measurement of monoatomic chains, unusually long interatomic lengths have been observed compare to interatomic distances in the bulk and dimer. In order to understand the nature of bonding and unusual structural properties, in this thesis, mechanical and electronic properties of metal chain nanowires are investigated from first principles by using pseudopotential plane wave calculations. Six different metals (Au, Ag, Al, Cu, Pt and Na) are studied in detail. All metals under study show two wire structure which are linear and zigzag structure. Au, Al and Pt show two different zigzag structure. All the wires are metallic. Relative stabilities are investigated by calculating the tension corresponding to apply force to keep the wire at a specific length. Au and Pt have bigger breaking force at breaking point relative to other metallic wires. In this thesis, effect of H, H2 and C impurities on mechanical and electronic properties of Au monoatomic chains are also studied. In wires with H and C impurities, wire under tension break from Au-Au bond away from the impurity. However, wire break from Au-H bond in H2 system. Except from Au-H system, wire become insulator when it contain C or H2 impurities. Before breaking, Au-impurity-Au bond length is in the range of long interatomic distance observed in experiment. So, the presence of an impurity can explain the observed long interatomic distances. However, changing of bond lengths and breaking bond during the stretching of wire depends on the type of impurity. If one stretch the Au-H system, all bond lengths increase in the same amount before breaking. However in Au-C system, Au-Au bond length away from the C impurity increase much more than other bonds. It is shown that absorption of impurity atoms modify the stiffness of the bonds in the wire. This related to the charge transfer from Au to impurity (for H and C). In H and C systems, wire break from Au-Au bond away from the impurity. However in H2 system, wire break from Au-H bond.Çakır, DenizM.S

Tuning of the electronic and optical properties of single layer black phosphorus by strain

Using first principles calculations we showed that the electronic and optical properties of single layer black phosphorus (BP) depend strongly on the applied strain. Due to the strong anisotropic atomic structure of BP, its electronic conductivity and optical response are sensitive to the magnitude and the orientation of the applied strain. We found that the inclusion of many body effects is essential for the correct description of the electronic properties of monolayer BP; for example while the electronic gap of strainless BP is found to be 0.90 eV by using semilocal functionals, it becomes 2.31 eV when many-body effects are taken into account within the G0W0 scheme. Applied tensile strain was shown to significantly enhances electron transport along zigzag direction of BP. Furthermore, biaxial strain is able to tune the optical band gap of monolayer BP from 0.38 eV (at -8% strain) to 2.07 eV (at 5.5%). The exciton binding energy is also sensitive to the magnitude of the applied strain. It is found to be 0.40 eV for compressive biaxial strain of -8%, and it becomes 0.83 eV for tensile strain of 4%. Our calculations demonstrate that the optical response of BP can be significantly tuned using strain engineering which appears as a promising way to design novel photovoltaic devices that capture a broad range of solar spectrum

A versatile and reconfigurable microassembly workstation

In this paper, a versatile and reconfigurable microassembly workstation designed and realized as a research tool for investigation of the problems in microassembly and micromanipulation processes and recent developments on mechanical and control structure of the system with respect to the previous workstation are presented. These developments include: (i) addition of a manipulator system to realize more complicated assembly and manipulation tasks, (ii) addition of extra DOF for the vision system and sample holder stages in order to make the system more versatile (iii) a new optical microscope as the vision system in order to visualize the microworld and determine the position and orientation of micro components to be assembled or manipulated, (iv) a modular control system hardware which allows handling more DOF. In addition several experiments using the workstation are presented in different modes of operation like tele-operated, semiautomated and fully automated by means of visual based schemes

Evidence of association of vitamin D receptor Apa I gene polymorphism with bone mineral density in postmenopausal women with osteoporosis

The vitamin D receptor (VDR) was the firstcandidate gene to be studied in relation to osteoporosis, andmost attention has focused on polymorphisms situated nearthe 3' flank of VDR. The aim of this study was toinvestigate the association about VDR gene Apa I polymorphismwith bone mineral density (BMD) in postmenopausalwomen with osteoporosis. We studied a total of 136postmenopausal women with a mean age of 56.36±10.29 years. Among them, a total of 75 had osteoporosis,37 had osteopenia, and 24 had normal BMD. Venous bloodsamples were obtained for evaluation of bone metabolismand genotyping. The VDR Apa I genotype was determinedby polymerase chain reaction-restriction fragment lengthpolymorphism. BMDs at the lumbar spine and hip weremeasured by dual-energy X-ray absorptiometry. Postmenopausalwomen with aa genotype had significantly lowerBMD values (grams per centimeter square) at lumbar spinescompared to persons with AA genotype. Also, postmenopausalwomen with AA genotype had significantly higherserum Ca level than the subjects with aa genotype. Inconclusion, our result may indicate that VDR Apa I genepolymorphism may be responsible for a important part ofthe heritable component of lumbar spine BMD in postmenopausalwomen, possibly related to impaired calciumabsorption from the bowel