Structural lubricity under ambient conditions.

Despite its fundamental importance, physical mechanisms that govern friction are poorly understood. While a state of ultra-low friction, termed structural lubricity, is expected for any clean, atomically flat interface consisting of two different materials with incommensurate structures, some associated predictions could only be quantitatively confirmed under ultra-high vacuum (UHV) conditions so far. Here, we report structurally lubric sliding under ambient conditions at mesoscopic (∼4,000-130,000 nm(2)) interfaces formed by gold islands on graphite. Ab initio calculations reveal that the gold-graphite interface is expected to remain largely free from contaminant molecules, leading to structurally lubric sliding. The experiments reported here demonstrate the potential for practical lubrication schemes for micro- and nano-electromechanical systems, which would mainly rely on an atomic-scale structural mismatch between the slider and substrate components, via the utilization of material systems featuring clean, atomically flat interfaces under ambient conditions

Spintronic properties of carbon and silicon based nanostructures

Ankara : The Department of Physics and the Institute of Engineering and Science of Bilkent University, 2007.Thesis (Ph.D.) -- Bilkent University, 2007.Includes bibliographical references leaves 101-109.In this thesis, nanostructures which may display novel spintronic behaviors are revealed and their properties are investigated by using first-principles methods. We have concentrated on three different systems, namely carbon linear chains, singe-wall carbon nanotubes and silicon nanowires. First of all, an extensive study of the electronic, magnetic and transport properties of finite and infiniteperiodic atomic chains composed of carbon atoms and 3d transition metal (TM) atoms are carried out. Finite-size, linear molecules made of carbon atomic chains caped with TM atoms, i.e. TM-Cn-TM structures are found to be stable and exhibit interesting magnetoresistive properties. The indirect exchange interaction of the two TM atoms through a spacer of n carbon atoms determines the type of the magnetic ground state of these structures. The n-dependent variations of the ground state between ferromagnetic (F) and antiferromagnetic (AF) spin configurations exhibit several distinct features, including regular alternations and irregular forms. We present a simple analytical model that can successfully simulate these variations, and the induced magnetic moments on the carbon atoms. The periodically repeated TM-Cn atomic chains exhibit half-metallic properties with perfect spin polarization at the Fermi level (EF ). When connected to appropriate electrodes the TM-Cn-TM atomic chains act as molecular spin-valves in their F states due to the large ratios of the conductance values for each spin type. Secondly, a systematic study of the electronic and magnetic properties of TM atomic chains adsorbed on the zigzag single-wall carbon nanotubes (SWNTs) is presented. The adsorption on the external and internal wall of SWNT is considered and the effect of the TM coverage and geometry on the binding energy and the spin polarization at EF is examined. All those adsorbed chains studied have F ground state, but only their specific types and geometries demonstrated high spin polarization near EF . Their magnetic moment and binding energy in the ground state display interesting variation with the number of d−electrons of the TM atom. Spin-dependent electronic structure becomes discretized when TM atoms are adsorbed on finite segments of SWNTs. Once coupled with non-magnetic metal electrodes, these magnetic needles or nanomagnets can perform as spindependent resonant tunnelling devices. The electronic and magnetic properties of these nanomagnets can be engineered depending on the type and decoration of adsorbed TM atom as well as the size and symmetry of the tube. Finally, bare, hydrogen terminated and TM adsorbed Silicon nanowires (SiNW) oriented along [001] direction are investigated. An extensive analysis on the atomic structure, stability, elastic and electronic properties of bare and hydrogen terminated SiNWs is performed. It is then predicted that specific TM adsorbed SiNWs have a half-metallic ground state even above room temperature. At high coverage of TM atoms, ferromagnetic SiNWs become metallic for both spin-directions with high magnetic moment and may have also significant spin-polarization at EF . The spin-dependent electronic properties can be engineered by changing the type of adsorbed TM atoms, as well as the diameter of the nanowire. Most of these systems studied in this thesis appear to be stable at room temperature and promising for spintronic devices which can operate at ambient conditions. Therefore, we believe that present results are not only of academic interest, but also can initiate new research on spintronic applications of nanostructures.Durgun, EnginPh.D

A study of adsorption of single atoms on carbon nanotubes

Cataloged from PDF version of article.The adsorption of individual atoms on the semiconducting and metallic singlewall carbon nanotubes (SWNT) have been investigated by using first-principles pseudopotential plane wave method within Density Functional Theory. The stable adsorption geometry and binding energy have been determined for a large number of foreign atoms ranging from alkali and simple metals to the transition metals and group IV elements. We have found that the character of the bonding and associated physical properties strongly depend on the type of adsorbed atoms, in particular on their valence electron structure. Our results indicate that the properties of SWNTs can be modified by the adsorbed foreign atoms. While the atoms of good conducting metals, such as Zn, Cu, Ag and Au, form very weak bonds, transition metal atoms, such as Ti, Sc, Nb and Ta, and group IV elements C and Si are adsorbed with relatively high binding energy. Owing to the curvature effect, these binding energies are larger than the binding energies of the same atoms on the graphite surface. We have showed that the adatom carbon can form strong and directional bonds between two SWNTs so that the tubes are connected. These connects can be used to produce nanotube networks or grids. Most of the adsorbed transition metal atoms excluding Ni, Pd and Pt have a magnetic ground state with a significant magnetic moment. Our results suggest that carbon nanotubes can be functionalized in different ways by their coverage with different atoms, showing interesting applications such as one-dimensional nanomagnets or nanoconductors and conducting connects etc.Durgun, EnginM.S

Functionalization of Carbon-based Nanostructures with Light Transition-metal Atoms

In a recent letter [T. Yildirim and S. Çiraci, Phys. Rev. Lett. 94, 175501 (2005)], the unusual hydrogen storage capacity of Ti decorated carbon nanotubes has been revealed. The present paper extends this study further to investigate the hydrogen uptake by light transition-metal atoms decorating various carbon-based nanostructures in different types of geometry and dimensionality, such as carbon linear chain, graphene, and nanotubes. Using first-principles plane-wave method we show that not only outer but also inner surface of a large carbon nanotube can be utilized to bind more transition-metal atoms and hence to increase the storage capacity. We also found that scandium and vanadium atoms adsorbed on a carbon nanotube can bind up to five hydrogen molecules. Similarly, light transition-metal atoms can be adsorbed on both sides of graphene and each adsorbate can hold up to four hydrogen molecules yielding again a high-storage capacity. Interestingly, our results suggest that graphene can be considered as a potential high-capacity H2 storage medium. We also performed transition state analysis on the possible dimerization of Ti atoms adsorbed on the graphene and single-wall carbon nanotube

Gallic acid treatment protects intestinal tissue against ischemia-reperfusion

Background: This study aimed to investigate the protective effects of gallic acid (GA) in the rat intestine against ischemia-reperfusion (IR) injury. Materials and methods: Thirty male Wistar albino rats with a mean weight of 200–250 g were used. Animals were categorized into the sham, IR, and IR+GA groups. Ischemia of the intestine was induced for 3 h by occluding the superior mesenteric artery (SMA) and then left for 3 h of reperfusion. In the IR+GA group, after ischemia induction, 50 mg/kg GA was orally administered to the animals. Blood samples were collected for biochemical assays. Intestinal tissues were excised for histopathologic and immunohistochemical processing. Results: Malondialdehyde (MDA) levels were increased, and catalase (CAT) and glutathione (GSH) levels were decreased in the IR group compared to the sham group. After GA treatment, MDA levels decreased and CAT and GSH levels increased in the GA-treated group compared to the IR group. In the sham group, normal intestinal histology was observed. In the IR group, the villi structures were completely degenerated. In the IR+GA group, histology was improved after GA treatment. In the sham group, the Caspase-3 reaction was generally negative in the epithelium and glands. In the IR group, the Caspase-3 reaction increased in apoptotic bodies and inflammatory cells. The Caspase-3 reaction was negative in goblet cells and the epithelium. A moderate Caspase-3 reaction was observed in the IR+GA group. The Beclin-1 reaction was negative in epithelial cells and goblet cells in villi in the sham group. In the IR group, the Beclin-1 reaction was positive in the degenerated villi. An intense Beclin-1 reaction was also observed in some inflammatory cells. After GA treatment, the Beclin-1 reaction was positive in a few cells. In general, moderate Beclin-1 positivity was observed. Conclusions: GA, with its antioxidative effect, inhibited the apoptotic pathway (Caspase-3) through Beclin-1 regulation

Single- and multi-layer arsenene as an anode material for Li, Na, and K-ion battery applications

Revealing ideal electrode materials with required functionalities is a crucial step to develop high-performance alkali-ion batteries. In this study, we investigate the potential of single- (SL) and multi-layer (ML) arsenene phases (buckled and symmetric washboard) to be used as an anode material by means of abinitio calculations. The interaction of alkali metal atoms (M: Li, Na, and K) with arsenene is examined to reveal strong adatom-electrode binding and low diffusion barriers. Provided that the initial crystalline patterns are maintained, the possible M orderings (MxAs) are investigated for varying ion concentrations (x). The structural deformations and the decrease in formation energy with increasing x indicate probable structural transformations. The abinitio molecular dynamics simulations confirm that the ordered patterns are prone to instability and crystalline to amorphous transition is induced at ambient temperature. The calculated (average) open-circuit voltages are between 0.65–0.98 V with the specific capacity range of 358–715 mAhg−1 for SL- and ML-MxAs. Strong metal-electrode interaction, fast diffusion, and desired voltage range suggest arsenene as a promising anode material for alkali-ion batteries to be utilized in low charging voltage applications. © 2020 Elsevier B.V

Bulk and surface electronic structure of Bi4_4Te3_3 from GWGW calculations and photoemission experiments

We present a combined theoretical and experimental study of the electronic structure of stoichiometric Bi$_4$Te$_3$, a natural superlattice of alternating Bi$_2$Te$_3$ quintuple layers and Bi bilayers. In contrast to the related semiconducting compounds Bi$_2$Te$_3$ and Bi$_1$Te$_1$, density functional theory predicts Bi$_4$Te$_3$ to be a semimetal. In this work, we compute the quasiparticle electronic structure of Bi$_4$Te$_3$ in the framework of the $GW$ approximation within many-body perturbation theory. The quasiparticle corrections are found to modify the dispersion of the valence and conduction bands in the vicinity of the Fermi energy, leading to the opening of a small indirect band gap. Based on the analysis of the eigenstates, Bi$_4$Te$_3$ is classified as a dual topological insulator with bulk topological invariants $\mathbb{Z}_2$ (1;111) and magnetic mirror Chern number $n_M=1$. The bulk $GW$ results are used to build a Wannier-functions based tight-binding Hamiltonian that is further applied to study the electronic properties of the (111) surface. The comparison with our angle-resolved photoemission measurements shows excellent agreement between the computed and measured surface states and indicates the dual topological nature of Bi$_4$Te$_3$

Unraveling Molecular Fingerprints of Catalytic Sulfur Poisoning at the Nanometer Scale with Near-Field Infrared Spectroscopy

Fundamental understanding of catalytic deactivation phenomena such as sulfur poisoning occurring on metal/metal-oxide interfaces is essential for the development of high-performance heterogeneous catalysts with extended lifetimes. Unambiguous identification of catalytic poisoning species requires experimental methods simultaneously delivering accurate information regarding adsorption sites and adsorption geometries of adsorbates with nanometer-scale spatial resolution, as well as their detailed chemical structure and surface functional groups. However, to date, it has not been possible to study catalytic sulfur poisoning of metal/metal-oxide interfaces at the nanometer scale without sacrificing chemical definition. Here, we demonstrate that near-field nano-infrared spectroscopy can effectively identify the chemical nature, adsorption sites, and adsorption geometries of sulfur-based catalytic poisons on a Pd(nanodisk)/Al2O3 (thin-film) planar model catalyst surface at the nanometer scale. The current results reveal striking variations in the nature of sulfate species from one nanoparticle to another, vast alterations of sulfur poisoning on a single Pd nanoparticle as well as at the assortment of sulfate species at the active metal-metal-oxide support interfacial sites. These findings provide critical molecular-level insights crucial for the development of long-lifetime precious metal catalysts resistant toward deactivation by sulfur