Assembling molecular electronic junctions one molecule at a time

Diffusion of metal atoms onto a molecular monolayer attached to a conducting surface permits electronic contact to the molecules with minimal heat transfer or structural disturbance. Surface-mediated metal deposition (SDMD) involves contact between "cold" diffusing metal atoms and molecules, due to shielding of the molecules from direct exposure to metal vapor. Measurement of the current through the molecular layer during metal diffusion permits observation of molecular conductance for junctions containing as few as one molecule. Discrete conductance steps were observed for 1-10 molecules within a monolayer during a single deposition run, corresponding to "recruitment" of additional molecules as the contact area between the diffusing Au layer and molecules increases. For alkane monolayers, the molecular conductance measured with SDMD exhibited an exponential dependence on molecular length with a decay constant (β) of 0.90 per CH(2) group, comparable to that observed by other techniques. Molecular conductance values were determined for three azobenzene molecules, and correlated with the offset between the molecular HOMO and the contact Fermi level, as expected for hole-mediated tunneling. Current-voltage curves were obtained during metal deposition showed no change in shape for junctions containing 1, 2, and 10 molecules, implying minimal intermolecular interactions as single molecule devices transitioned into several molecules devices. SDMD represents a "soft" metal deposition method capable of providing single molecule conductance values, then providing quantitative comparisons to molecular junctions containing 10(6) to 10(10) molecules.     

Books: One molecule at a time

A review of Single-Molecule Optical Detection, Imaging, and Spectroscopy.     

Triangulation of simple arbitrarily shaped polyhedra by cutting off one vertex at a time

We describe a heuristic method of triangulating arbitrarily shaped polyhedra without the addition of Steiner points. The polyhedra are simple, with each vertex connected to at least 3 other vertices (ie, coplanarity and colinearity are not considered). They may, however, be convex or concave and consist of dozens or even hundreds of facets. This makes the treatment universal enough to well meet the requirements of models used to simulate fractured rock masses. Certain concepts are defined in the work, eg, adjacent vertices, polygon of adjacent vertices, and closed cone of a vertex. A polygon of adjacent vertices of an apex can be subdivided into a set of nonoverlapping triangles without adding any vertices. These triangles, together with the apex, form tetrahedra whose union is the closed cone of the apex. The polyhedron is thus the union of the closed cones. Subsequently, we triangulate the polyhedron by gradually removing the closed cones of its vertices. The number of vertices of the polyhedron decreases by one each time a closed cone is removed. A block with n vertices can produce no more than n?3 tetrahedra. We present the analysis procedure and discuss the core issues of the method proposed.     

Electromagneto-hydrodynamic instability in a horizontal viscoelastic fluid layer with one relaxation time

Summary The stability of viscoelastic conducting liquid (Walters's liquid B) heated from below in the presence of a magnetic field is considered. Linear stability theory is used to derive an eigenvalue system of sixth order, and an exact eigenvalue equation for a neutral instability is obtained. Under somewhat artificial boundary conditions, this equation can be solved exactly to yield the required eigenvalue relationship from which various critical values are determined in detail. Critical Rayleigh number and wavenumber for the onset of instability are presented graphically as functions of the Chandrasekhar number at a Prandtl numberP _r=100 and for various values of the one relaxation time and the elastic parameters.     

Electron microscopy reveals structure and morphology of one molecule thin organic films

Transmission electron microscopy was used to determine the structure of molecular films of self-assembled monolayers of pentathiophene derivatives supported on various electron transparent substrates. Despite the extreme beam sensitivity of the monolayers, structural crystallographic maps were obtained that revealed the nanoscale structure of the film. The image resolution is determined by the minimum beam diameter that the radiation hardness of the monolayer can support, which in our case is about 90 nm for a beam current of 5 × 10(6) e(-)/s. Electron diffraction patterns were collected while scanning a parallel electron beam over the film. These maps contain uncompromised information of the size, symmetry and orientation of the unit cell, orientation and structure of the domains, degree of crystallinity, and their variation on the micrometer scale, which are crucial to understand the electrical transport properties of the organic films. This information allowed us to track small changes in the unit cell size driven by the chemical modification of the support film.     

Using tool life models to minimize processing time on a flexible machine

Economic tool life models are presented for machines with finite capacity tool magazines and variable processing speed capability. Single and multiple part models for minimizing the total throughput time are formulated as nonlinear, integer programs (NLIP). An algorithm is presented for the NLP relaxation and a marginal analysis approach for solving the NLIP is detailed, giving an optimal tool loading policy as well as the processing speeds for each of the part types so as to minimize the makespan. A numerical example illustrates the procedures.     

The up and down time distribution in a 1 out of n system with general life time and repair time distributions

In repairable systems with redundancy, failed units can be replaced by spare units in order to reduce system down time. The failed units are sent to a repair shop or manufacturer for maintenance and are subsequently returned for re-use.In this paper we consider a 1 out of n system with cold standby and we assume that repaired machines are as ‘good-as-new’. We derive an approximation method for the distribution of the up time and the down time of the system in case of a general failure distribution and a general repair time distribution. This allows the model to be used in a wide variety of applications.     

Probabilistic analysis of a continuous review perishable inventory system with Markovian demand,Erlangian life and non-instantaneous lead time

Summary This article obtains an explicit cost expression for a continuous review (S, s) ordering policy inventory system of perishable items, in the stationary case. The cost expression is closely related to the stationary distribution of the stochastic processL(t), representing the inventory at any timet. The stochastic behaviour of the process {L(t),t-0} is characterised by identifying an imbedded MRP. The demands of the system are governed by a Poisson process. The items in the inventory have a useful life time characterised by an Erlangian distribution. The lead time is a random variable with specified probability distribution. It is assumed that the demands that arrive when the inventory is zero are lost.

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Zusammenfassung Der vorliegende Artikel behandelt einen expliziten Kostenausdruck für eine stetige Überprüfung der (S, s) Bestellpolitik eines Lagerhaltungssystems verderblicher Güter im stationären Fall. Der Kostenausdruck ist eng verbunden mit der stationären Verteilung des stochastischen ProzessesL(t), der den Lagerbestand zu jeder beliebigen Zeitt darstellt. Das stochastische Verhalten des Prozesses {L(t),t-0} wird characterisiert durch die Identifikation eines eingebetteten MRP. Die Nachfragewerte des Systems werden durch einen Poisson-Prozeß bestimmt. Die gelagerten Güter haben eine nutzbare Lebensdauer, die durch eine Erlang-Verteilung beschrieben wird. Die Durchlaufzeit ist eine Zufallsvariable mit spezifierter Wahrscheinlichkeitsverteilung. Es wird zudem unterstellt, daß nicht befriedigte Nachfrage bei leerem Lager verloren geht.

     

Proposal for a one GeV plasma wakefield acceleration experiment at SLAC

A plasma-based wakefield acceleration experiment E-157 has been approved at SLAC to study acceleration of parts of an SLC bunch by up to 1 GeV/m over a length of 1 m. A single SLC bunch is used to both induce wakefields in the 1 m long plasma and to witness the resulting beam acceleration. The experiment will explore and further develop the techniques that are needed to apply high-gradient plasma wakefield acceleration to large-scale accelerators. The 1 m length of the experiment is about two orders of magnitude larger than for other high gradient plasma wakefield acceleration experiments and the 1 GeV/m accelerating gradient is roughly ten times larger than that achieved with conventional metallic structures. Using existing SLAC facilities, the experiment will study high gradient acceleration at the forefront of advanced accelerator research.     

Entropy-driven single molecule tug-of-war of DNA at micro-nanofluidic interfaces

Entropy-driven polymer dynamics at the nanoscale is fundamentally important in biological systems but the dependence of the entropic force on the nanoconfinement remains elusive. Here, we established an entropy-driven single molecule tug-of-war (TOW) at two micro-nanofluidic interfaces bridged by a nanoslit, performed the force analysis from a modified wormlike chain in the TOW scenario and the entropic recoiling process, and determined the associated scalings on the nanoconfinement. Our results provide a direct experimental evidence that the entropic forces in these two regimes, though unequal, are essentially constant at defined slit heights, irrespective of the slit lengths and the DNA segments within. Our findings have the implications to polymer transport at the nanoscale, device design for single molecule analysis, and biotechnological applications.     

Influence of the transformation of a non-proportional multiaxial loading to a proportional one on the high cycle fatigue life

The work here presented intends to study the influence of the shape of loading on the fatigue life in case of multiaxial loading. The practical undergoing problem is the fatigue life of high speed railways wheels. They are here considered as loaded in tension (or compression) and in torsion.

Several multiaxial fatigue life criteria exist, but the most of them use in their formulation a proportional signal. However, actual loading are non-proportional, and the problem consists in transformation of a non-proportional signal to a proportional one. The Palin-Luc’s criterion will be the basis of this work and extension to variable amplitude multiaxial alternated loading will be proposed.

After presenting some multiaxial criteria, we propose a method for transforming the signals. Then, experiments and results are presented and discussed in view of a fatigue life prediction of the wheel.     

Fatigue life prediction at a ship deck/superstructure intersection

Difficulties arise in identifying an appropriate method to predict fatigue life for the weld joint at the intersection of a ship's deck and superstructure. The problem relates to the gross stress concentration and rapidly changing stress at this location. Variations on the 'hot spot' stress approach to fatigue life analysis are reviewed, and applied to results from an experimental programme of fatigue tests on welded joints. Finally tentative recommendations are made for a procedure to predict fatigue life of the structure.     

Measuring gas diffusion coefficients by means of a capillary open at one end

An apparatus and improved diffusion cell are described along with the method of use; the results are compared with published ones.Translated from Izmeritel'naya Tekhnika, No. 5, pp. 61–63, May, 1994.     

Progress in life time modeling of APS-TBC Part II: critical strains,macro-cracking,and thermal fatigue

Abstract

Here TBC top coat failure and life time are considered as a two-step process, where step 1 is time to delamination macro-cracking and step 2 is time to through macro-cracking. This two-step failure mechanism comes directly from critical strain measurement data and traditional coating failure theory. The critical strain data was measured by using a four point bend test on preoxidized samples. Included in the model is damage accumulation due to bond coat oxidation and thermal fatigue. The damage terms in the model have their origins in acoustic emission data from cyclic oxidation samples. The life time model is then used in concert with measured thermally grown oxide micro-crack lengths from isothermal oxidation samples. Finally a full model is presented for isothermal and cyclic oxidation of APS–TBC.     

Ordering policies under one time only discount and price sensitive demand

Purchase price reductions, even on a one-time-only basis, allow retailers the opportunity to lower their own selling prices, albeit temporarily, in an effort to increase sales. This results in not only additional revenues but also in extra inventory costs. The model presented in this paper examines such a cost/revenue tradeoff, through simultaneous determination of the most profitable: (i) stock level to be purchased by the retailer; (ii) discount level to be passed on to the customers; (iii) stock level qualifying for the retailer's discount; and (iv) time to initiate the discount. Numerical examples are included throughout to illustrate the main features of the model.