Tip–surface interactions in noncontact atomic force microscopy on reactive surfaces

The imaging process in noncontact atomic force microscopy (AFM) is studied on a number of reactive surfaces, namely, the Takayanagi reconstructed Si(111), InP(110), and GaAs(110). We show that on these surfaces, the short-range dangling-bond type of interaction between the tip and the surface is decisive in achieving atomic resolution. The short-range tip–surface interaction is modeled in the density functional theory within the GGA approximation. We show that we can achieve quantitative agreement with the experimental data in the commonly used frequency modulation technique for AFM surface corrugation with a very simple model for the tip geometry treating the tip–surface interaction in the perturbation theory. The nature of the short-range tip–surface interaction on the three surfaces is considered and the consequences thereof for the experiments is discussed.     

4D scanning transmission ultrafast electron microscopy: Single-particle imaging and spectroscopy

We report the development of 4D scanning transmission ultrafast electron microscopy (ST-UEM). The method was demonstrated in the imaging of silver nanowires and gold nanoparticles. For the wire, the mechanical motion and shape morphological dynamics were imaged, and from the images we obtained the resonance frequency and the dephasing time of the motion. Moreover, we demonstrate here the simultaneous acquisition of dark-field images and electron energy loss spectra from a single gold nanoparticle, which is not possible with conventional methods. The local probing capabilities of ST-UEM open new avenues for probing dynamic processes, from single isolated to embedded nanostructures, without being affected by the heterogeneous processes of ensemble-averaged dynamics. Such methodology promises to have wide-ranging applications in materials science and in single-particle biological imaging.     

Heterogeneous electron transfer processes in self-assembled monolayers of amine terminated conjugated molecular wires

A versatile synthesis of triarylamine and phenothiazine end-capped oligo(phenyleneacetylene) molecular wires which are terminated by thiol functions is described. The repetitive synthesis allows the preparation of molecular wires with different chain length and different substituents attached to the wire backbone. These molecular wires were used to form dense self-assembled monolayers (SAM) on gold substrates as proved by cyclic voltammetry and quartz crystal microbalance measurements. The heterogeneous electron transfer rate constant of these SAMs was measured by impedance spectroscopy between 1 MHz and 0.1 Hz. The rate constants are somewhat larger for the triarylamine terminated systems than for the phenothiazine compound, due to the higher reorganization energy in the latter. While the molecular wires with electron withdrawing substituents display an electron transfer which is slow enough to be measurable with our impedance setup, we were unable to determine the rate of molecular wires with electron donating substituents.     

Electron spin resonance studies of plasma-induced polystyrene radicals

Plasma-induced polystyrene radicals were first studied by electron spin resonance (ESR). The room temperature ESR spectrum was compared with those obtained by γ-irradiation, UV-irradiation, and mechanical fracture. It was found that even less than a few seconds of plasma-irradiation gave rise to a large amount of polystyrene radicals and the ESR spectrum consisted of two types of spectra, a triplet and a single broad line. The spectral feature of the triplet was nearly identical with that of γ-irradiated polystyrene. Thus, it was assigned to the structure of a cyclohexadienyl-type radical formed by a nearly random addition of a hydrogen atom to the aromatic ring. The single broad line, thought to be an outline of multicomponent spectrum, was assigned to an immobilized dangling-bond sites at the plasma-induced crosslinked portion of the polystyrene surface.     

Electron transport in a pi-stacking molecular chain

We have investigated the electronic structure and transport properties of a pi-stacking molecular chain which is covalently bonded to a H/Si(100) surface, using the first-principles density functional theory approach combined with Green's function method. The highest occupied molecular orbital (HOMO) dispersion is remarkably reduced, but remains noticeable ( approximately 0.1 eV), when a short pi-stacking styrene wire is cut from an infinitely long wire and sandwiched between metal electrodes. We find that the styrene chain's HOMO and lowest unoccupied molecular orbital (LUMO) states are not separated from Si, indicating that it does not work as a wire. By substituting -NO2 or -NH2 for the top -H of styrene, we are able to shift the position of the HOMO and LUMO with respect to the Fermi level. More importantly, we find that the HOMO of styrene-NH2 falls into the band gap of the substrate and is localized in the pi-stacking chain, which is what we need for a wire to be electrically separated from the substrate. The conductance of such an assembly is comparable to that of Au/benzene dithiolate/Au wire based on chemical bonding, and its tunability makes it a promising system for a molecular device.     

Effects of High Salt Concentration and Residue on Copper and Aluminum Corrosion

Traditional researches on metal corrosion under salt solutions deposit conditions are usually carried out by visual, electron microscopic observations and simple electrochemical measurement via a traditional one-piece electrode. These techniques have difficulties in measuring localized corrosion that frequently occur in inhomogeneous media. This paper reports the results from the experiments using specially shaped coupons and a relatively new method of measuring heterogeneous electrochemical processes, namely, the wire beam electrode(WBE). Preliminary results from copper and aluminum corrosion in highly concentrated sodium chloride solutions with and without solid deposits show that the method is useful in simulating and studying corrosion especially localized corrosion in pipelines.     

Single crystalline ordered silicon wire/Pt nanoparticle hybrids for solar energy harvesting

In this work, we developed a method combining lithography and metal-catalyzed Si etching to fabricate highly ordered Si wire arrays with uniform coverage over an entire 4 in. wafer. The wire periodicity and dimensions depend on the lithography and the etching conditions. Photoelectrochemical measurements show that the as-prepared Si wire samples are photoactive owing to surface enhancement and become especially effective in solar-to-electrical conversion and water splitting after surface modification with Pt nanoparticles.     

A DFT Study of Alkenes and Alkynes Reacting with H-GaN （0001） Surface

The addition reactions of alkenes and alkynes to the H-terminated GaN （0001） surface with a Ga dangling-bond have been studied employing periodic density functional theory （PDFT） calculations. Detailed information on the reaction pathways of these alkenes and alkynes with H-GaN （0001） surface is provided, which indicates that the reactions contain two steps separated by the metastable intermediates： elementary addition reaction and H-abstraction process. From the energy curves, the reactions are clearly viable in the cases of ethene, styrene and phenylacetylene; while for ethyne, the H-abstraction barrier is higher than the desorption barrier of the intermediate, so the adsorbed C2H2 in intermediate is more likely to be desorbed back into the gas phase than to form a stable adsorbed species. Furthermore, it is obvious that for either alkenes or alkynes, the systems substituted by phenyl have more stable intermediates because π conjugation could improve their stabilities.     

Effect of pressure on spin–orbit interaction in a quantum wire with V-shaped cross section

In the present work, we have considered a GaAs/Ga_1−xAl_xAs V-shaped quantum wire with a hydrogenic donor impurity at the center. First, the Schrödinger equation is analytically solved without the impurity. Second, we have used variational approximation to obtain the ground state binding energy. Third, the spin–orbit interaction (SOI) is studied by the perturbation theory. We also have investigated the effect of pressure on the binding energy and SOI in this quantum wire. According to the obtained results, it is found that i) the binding energy increases with increasing pressure, ii) the level splitting increases by increasing pressure, iii) the splitting decreases with increasing wire size.     

Simple template-based method to produce bradbury-nielsen gates

A Bradbury-Nielsen gate (BNG) consists of two interleaved and electrically isolated sets of wires and can transmit or deflect charged particles by applying a varying voltage difference across the two wire sets. We present a simple template-based method to fabricate BNGs with wire spacings as small as 50 microm with minimal use of a microscope. The small wire spacing allows modulation rates at tens of megahertz. Using this method, we have fabricated four BNGs with wire spacings of 500, 200, 100, and 50 microm using 10 microm gold-coated tungsten wires. The performance of the four BNGs is characterized using an imaging detector and compared with theoretical predictions.     

Selective chain reaction of acetone leading to the successive growth of mutually perpendicular molecular lines on the si(100)-(2 x 1)-H surface

The successive growth of mutually perpendicular molecular lines from one dangling-bond (DB) site on the Si(100)-(2 x 1)-H surface has been realized through a substrate-mediated chain reaction at 300 K. Among various molecules, acetone molecules undergo the most facile chain reaction with a DB site, which proceeds selectively on the Si(100)-(2 x 1)-H surface, resulting in only single molecular lines in the parallel-row (parallel to the dimer row) direction. The smaller size and higher reactivity of acetone molecules enable us to successively grow a parallel-row acetone line from the end of a cross-row (perpendicular to the dimer row) allylmercaptan line simply by changing the feed of gas molecules into the reaction chamber. Since the length of a molecular line is controlled by the number of gas molecules impinged, it is possible to turn a chain reaction from the cross-row direction to the parallel-row direction at any desired point on the surface. The reaction path of the adsorbing molecules is discussed. The present study provides a new means of fabricating mutually perpendicular molecular lines through a chain reaction initiating at a preselected DB site on the Si(100)-(2 x 1) surface.     

Role of Ser65, His148 and Thr203 in the Organic Solvent‐dependent Spectral Shift in Green Fluorescent Protein

The photophysics of green fluorescent protein (GFP) is remarkable because of its exceptional property of excited state proton transfer (ESPT) and the presence of a functional proton wire. Another interesting property of wild‐type GFP is that its absorption and fluorescence excitation spectra are sensitive to the presence of polar organic solvents even at very low concentrations. Here, we use a combination of methodologies including site‐specific mutagenesis, absorption spectroscopy, steady‐state and time‐resolved fluorescence measurements and all‐atom molecular dynamics simulations in explicit solvent, to uncover the mechanism behind the unique spectral sensitivity of GFP toward organic solvents. Based on the evidences provided herein, we suggest that organic solvent‐induced changes in the proton wire prevent ground state movement of a proton through the wire and thus bring about the spectral changes observed. The present study can not only help to understand the mechanism of proton transfer by further dissecting the intricate steps in GFP photophysics but also encourages to develop GFP‐based organic solvent biosensors.     

Electrostatic potential profile and nonlinear current in an interacting one-dimensional molecular wire

We consider an interacting one-dimensional molecular wire attached to two metal electrodes on either side of it. The electrostatic potential profile across the wire-electrode interface has been deduced solving the Schrodinger and Poisson equations self-consistently. Since the Poisson distribution crucially depends on charge densities, we have considered different Hamiltonian parameters to model the nano-scale wire. We find that for very weak electron correlations, the potential gradient is almost zero in the middle of the wire but are large near the chain ends. However, for strong correlations, the potential is essentially a ramp function. The nonlinear current, obtained from the scattering formalism, is found to be less with the ramp potential than for weak correlations. Some of the interesting features in current-voltage characteristics have been explained using one-electron formalism and instabilities in the system. Dedicated to Professor C N R Rao on his 70th birthday     

Self-excited drop oscillations in electrowetting

We studied millimeter-sized aqueous sessile drops in an ambient oil environment in a classical electrowetting configuration with a wire-shaped electrode placed at a variable height above the substrate. Within a certain range of height and above a certain threshold voltage, the drop oscillates periodically between two morphologies where it is either attached to the wire or detached from it. We determine the range of control parameters, wire height, and voltage in which oscillations occur and explain it by a simple capillary model. Furthermore, we analyze the dynamics of the oscillations using high-speed video microscopy and numerical fluid dynamics modeling. We develop a one-dimensional harmonic oscillator model that describes the dependence of the drop oscillations on the relevant intrinsic (surface tension, viscosity, density) and extrinsic (wire height, voltage) parameters.     

Thermo-mechanical properties of snake-like NiTi wires and their use in miniature devices

In this study, we present the thermo-mechanical characterization of snake-like shape memory alloy (SMA) NiTi wires. Several samples were formed at different thermal treatments and analyzed through calorimetry and thermo-mechanical tests. It was found that for small deformations the snake-like SMA wire behaves as a quasi-elastic material, presenting the flag-shape pseudoelastic behavior at high thermal treatment temperatures. The mechanical performance seems to depend basically on the snake geometry instead on the thermal treatment; only for the 773?K treatment sample the performance rapidly gets worse. Stabilization is reached in few cycles. After the thermo-mechanical characterization, we studied the performance of the snake-like SMA wire mounted in a miniature rotational actuator, suggesting an innovative application in the actuator field.     

Effect of electron-phonon coupling on the conductance of a one-dimensional molecular wire

The effect of inelastic scattering, particularly that of the electron-phonon interactions, on the current-voltage characteristics of a one-dimensional tight-binding molecular wire has been investigated. The wire has been modeled using the Su-Schreiffer-Heeger Hamiltonian and we compute the current using the Landauer's scattering formalism. Our calculations show that the presence of strong electron-lattice coupling in the wire can induce regions of negative differential resistance (NDR) in the I-V curves. The reasons for this can be traced back to the quasidegeneracy in few of the low-energy molecular levels in the presence of electron-phonon coupling and an external applied bias. The molecular levels become highly delocalized at the critical bias at which the NDR is seen, corresponding to the vanishing of the electron-phonon coupling with equal bond lengths.     

Electrical properties of polypyrrole wires measured by scanning probe microscope

Hybrid wires composed of metal and conducting polymer (polypyrrole, Ppy) were electrochemically synthesized using a template synthesis method. We showed that the dimensions of the hybrid wires can be tailored by controlling the time of electrolysis. The electrical properties of Ppy having different lengths were measured using a scanning probe microscope (SPM) tip as an electrode without aligning the hybrid wires on a substrate. Especially, temperature-dependent characteristics of the I-V curve were determined. As we expected, the shorter the Ppy wire is, the better the current response produced. The activation energy of each Ppy wire is determined. It is confirmed that the electrical properties of a single wire of Ppy can be measured by SPM.     

Conduction properties of bipyridinium-functionalized molecular wires

We present a detailed analysis of the coherent electron transport through a redox-active, 4,4'-bipyridinium (viologen)-functionalized molecular wire, which was studied in several recent experiments. Our calculations for the bare viologen predict conductances differing by 2 orders of magnitude depending on the contact geometry. For the alkyl-wired viologen unit, we obtain an exponential decay of the conductance with the wire length. Because this exponent also governs the conductance in the incoherent transport regime, comparison with experiments is legitimate and we find a good agreement. Furthermore, our calculations indicate that the experimentally observed conductance switching behavior is not amenable to an explanation inside a coherent transport picture. A possible incoherent mechanism is being discussed.     

Mass‐selective axial ejection from a linear ion trap with a direct current extraction field

We have developed a new mass‐selective axial ejection method from a linear ion trap (LIT). In this method, a set consisting of a trap wire lens and an extraction wire lens positioned orthogonally to each other was placed between quadrupole rods. The trap wire lens confines the ions inside the trap, and the extraction wire axially extracts ions from the trap. Ions introduced into the LIT are trapped between the inlet lens and the trap wire lens. In addition to the wire lenses, a set of excitation lenses, which are aligned orthogonally to the trap wire lens, are inserted between rods. The ions are resonantly excited in the direction perpendicular to the trap wire lens by applying a supplemental alternating current (AC) to the excitation lenses. Excited ions with a large motion pass over the trap wire lens, while unexcited ions remain trapped inside. Ions that have passed over the trap wire lens are then extracted by the extraction wire lens. The characteristics of the mass‐selective ejection with a direct current (DC) extraction field were investigated by both simulation and experiment. A mass resolving power of m/Δm = 1300 was achieved at a scan rate of 500 Th/s. The dependence of the ejection efficiency on trap wire lens bias was measured, and an ejection efficiency of 20% at a scan rate of 500 Th/s was achieved by optimizing the DC bias on the trap wire lens. Copyright © 2009 John Wiley & Sons, Ltd.     

Exploring the effects of intramolecular vibrational energy redistribution on the operation of the proton wire in green fluorescent protein

The operation of the proton wire in Green Fluorescent Protein has been simulated by quantum dynamics and considering the coupling to the protein environment by means of a bath of harmonic oscillators. The simulation consists of 36 explicit and fully quantum degrees of freedom: 6 degrees of freedom represent the configuration of the proton wire, which are coupled to 30 bath coordinates. Regimes of weak and strong coupling have been studied. It is found that presence of the bath induces a fast energy transfer from the proton wire to the bath, with characteristic times under 400 fs. This internal vibrational redistribution happens at the expense of the potential energy content of the proton wire, deformed through the interaction to the bath from its uncoupled state. Strong coupling induces a slowing-down of the operation of the wire because it hinders to some extent the approaching of donor and acceptor atoms to distances in which proton transfer can occur. Internal vibrational energy redistribution affects the dynamics, but from our simulations we conclude that it cannot be the only cause responsible for the experimentally reported fluorescence rise times.