Nanoscale laser processing and diagnostics

The article summarizes research activities of the Laser Thermal Laboratory on pulsed nanosecond and femtosecond laser-based processing of materials and diagnostics at the nanoscale using optical-near-field processing. Both apertureless and apertured near-field probes can deliver highly confined irradiation at sufficiently high intensities to impart morphological and structural changes in materials at the nanometric level. Processing examples include nanoscale selective subtractive (ablation), additive (chemical vapor deposition), crystallization, and electric, magnetic activation. In the context of nanoscale diagnostics, optical-near-field-ablation-induced plasma emission was utilized for chemical species analysis by laser-induced breakdown spectroscopy. Furthermore, optical-near-field irradiation greatly improved sensitivity and reliability of electrical conductance atomic force microscopy enabling characterization of electron tunneling through the oxide shell on silicon nanowires. Efficient in-situ monitoring greatly benefits optical-near-field processing. Due to close proximity of the probe tip with respect to the sample under processing, frequent degradation of the probe end occurs leading to unstable processing conditions. Optical-fiber-based probes have been coupled to a dual-beam (scanning electron microscopy and focused ion beam) system in order to achieve in-situ monitoring and probe repair.     

Three-dimensional opto-fluidic devices fabricated by ultrashort laser pulses for high throughput single cell detection and processing

Three-dimensional flow-through microchannels were fabricated inside bulk fused silica glass via ultrashort pulsed laser direct writing. The device fabrication sequence takes advantage of the nonlinear volumetric absorption in glass and the subsequent preferential chemical etching process. Optical waveguides were also written into the glass specimen and integrated with the fluidic conduits. Flow tests using both fluorescent particles and red blood cells (RBCs) were conducted on various three-dimensional channel configurations. Experiments showed the possibility for laser-induced cell processing inside the microchannels. To evaluate cytometer functionality, RBCs were detected inside the manufactured microchannel via both transmission and fluorescence probing.     

Characterisation of cryopreserved cells freshly isolated from human bone marrow

Osteoblast progenitor cells (OBPCs) isolated from bone marrow have the ability to differentiate into osteoblasts and thus potential therapeutic use to tissue-engineer bone. In order for OBPCs to be available for clinical use a means of storing viable cells is necessary. The aim of this study was to determine whether a simple method of cryopreservation had an effect on osteogenic differentiation or growth of OBPCs isolated from fresh human bone marrow. Stro-1 was used to identify the isolated OBPCs. The osteoblastic potential of the marrow cells was confirmed as culture with osteogenic supplements (OS) significantly increased osteoblastic protein production (alkaline phosphatase (ALP), osteopontin and osteocalcin) compared with standard conditions (P less than 0.05). Ten further marrow aspirates were harvested; each was halved for either cryopreservation or control culture. Primary cultures from both populations formed colonies with recognised OBPC morphology. OS stimulated both cryopreserved and control populations to produce significantly more osteoblastic proteins (P less than 0.05) and there was no significant difference between the increase in osteogenic proteins when cultured with OS (P great than 0.2). The proliferation rate after 5 days in culture was not significantly affected by cryopreservation (P greater than 0.05). It has been suggested that OBPCs are immuno-privileged; so allogenic cells could be implanted into patients for tissue engineering bone without causing a hypersensitivity reaction. Our study demonstrates a method of storage, which allows OBPCs to be available for use without affecting osteoblastic potential or viability.     

Flexible Organic Photovoltaic Cells with In Situ Nonthermal Photoreduction of Spin‐Coated Graphene Oxide Electrodes

The first reduction methodology, compatible with flexible, temperature‐sensitive substrates, for the production of reduced spin‐coated graphene oxide (GO) electrodes is reported. It is based on the use of a laser beam for the in situ, non‐thermal, reduction of spin‐coated GO films on flexible substrates over a large area. The photoreduction process is one‐step, facile, and is rapidly carried out at room temperature in air without affecting the integrity of the graphene lattice or the flexibility of the underlying substrate. Conductive graphene films with a sheet resistance of as low as 700 Ω sq^?1 and transmittance of 44% can be obtained, much higher than can be achieved for flexible layers reduced by chemical means. As a proof of concept of our technique, laser‐reduced GO (LrGO) films are utilized as transparent electrodes in flexible, bulk heterojunction, organic photovoltaic (OPV) devices, replacing the traditional ITO. The devices displayed a power‐conversion efficiency of 1.1%, which is the highest reported so far for OPV device incorporating reduced GO as the transparent electrode. The in situ non‐thermal photoreduction of spin‐coated GO films creates a new way to produce flexible functional graphene electrodes for a variety of electronic applications in a process that carries substantial promise for industrial implementation.     

Square-Root-Domain Realization of Single-Cell Architecture of Complex TDCNN

Square-root-domain (SRD) CMOS analog realization of a single cell architecture of the complex Temporal Derivative Cellular Neural Networks (TDCNNs) is introduced in this paper. TDCNN initiates time derivative ‘diffusion’ between CNN cells for non-separable spatiotemporal filtering applications, where the input to the CNN is an image that changes over time. The evaluation of the performance of the complex SRD TDCNN cell has been done using the Cadence Orcad software with TSMC 0.18-μm CMOS process model parameters. The provided simulated results confirm the validity of the theory.     

Stabilizing an uncertain production system

Consider a production system that consists ofm machines each of which can produce parts ofn types. When machinek is used, it produces a part of typei with probabilityp _ki . Requests arrive for parts, one at a time. With probability _i an arriving request is for a part of typei. The requests must be served without waiting. Thus, if a requested part is not available, it must be produced. We find necessary and sufficient conditions for the existence of a strategy (a choice of the machines to be used) which makes the inventory of parts stable and we provide such a strategy.Two variations of this model are also considered: the case of batch arrivals of requests, and that of a system where the requests can be queued.     

Finite automata based algorithms on subsequences and supersequences of degenerate strings

In this paper, we present linear-time algorithms for the construction two novel types of finite automata and show how they can be used to efficiently solve the Longest Common Subsequence (LCS), Shortest Common Supersequence (SCS) and Constrained Longest Common Subsequence (CLCS) problems for degenerate strings.     

Global optimization for molecular conformation problems

A primal-relaxed dual global optimization algorithm is presented along with an extensive review for finding the global minimum energy configurations of microclusters composed by particles interacting with any type of two-body central forces. First, the original nonconvex expression for the total potential energy is transformed to the difference of two convex functions (DC transformation) via an eigenvalue analysis performed for each pair potential that constitutes the total potential energy function. Then, a decomposition strategy based on the GOP algorithm [1–4] is designed to provide tight upper and lower bounds on the global minimum through the solutions of a sequence of relaxed dual subproblems. A number of theoretical results are included which expedite the computational effort by exploiting the special mathematical structure of the problem. The proposed approach attains-convergence to the global minimum in a finite number of iterations. Based on this procedure global optimum solutions are generated for small Lennard-Jones and Morse (a=3) microclustersn7. For larger clusters (8N24 for Lennard-Jones and 8N30 for Morse), tight lower and upper bounds on the global solution are provided which serve as excellent initial points for local optimization approaches.     

Neck propagation in polyethylene

Intrinsic true stress–true strain response was evaluated at room temperature for three linear polyethylene samples deformed in conventional tensile tests. It was observed that high crystallinity is associated with a low rate of strain hardening that results in a sharp neck and a large drop in nominal stress. The maximum and minimum deformation loads are accounted for by Considère's analysis of neck initiation and stabilization, respectively. Following stabilization, neck propagation occurs at a load or nominal stress that is lower than the yield stress. The jump analysis of Ericksen and Hutchinson/Neale predicts steady state neck propagation stresses that are in very good (ca. 10%) agreement with experiment. Although the jump analysis is done in terms of uniaxial stress, the actual value of the propagation stress is established by the triaxial stress state in the neck shoulders. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2081–2091, 2004     

Equation of state molecular parameters for a theory of pure r-mer fluids in the liquid phase

Costas, M. and Sanctuary, B.C., 1984. Equation of state molecular parameters for a theory of pure r-mer fluids in the liquid phase. Fluid Phase Equilibria, 18: 47–60.Equation of state parameters for a theory of pure r-mers (Costas and Sanctuary, 1981) are presented for sixty common substances. The equation of state is tested for its ability to reproduce and predict thermodynamic data in the liquid phase. The theory has only two adjustable parameters: ν^*, the close-packed volume of the r-mer, and ?^* = z?/2, where ? is the nonbonded mer-mer attractive interaction energy and z is the lattice coordination number, set to be a constant equal to 12. The number of r-mers per molecule, r, is also fixed for a given liquid to be equal to the number of atoms in the molecule other than hydrogen. The fitting proceduce is fast and simple. The adjustment only involves the use of widely available density data. Comparisons between experimental and calculated first- and second-order thermodynamic properties are shown for several substances. A comparison with the equation of state due to Jain and Simha (1981) for PVT data for n-dodecane is presented. The theory qualifies as a useful tool, especially in engineering applications, for estimating thermodynamic properties in the liquid phase.