Showing papers by "National Physical Laboratory published in 2011"

Luminscent Graphene Quantum Dots for Organic Photovoltaic Devices

Abstract: Recent research in organic photovoltaic (OPV) is largely focused on developing low cost OPV materials such as graphene. However, graphene sheets (GSs) blended conjugated polymers are known to show inferior OPV characteristics as compared to fullerene adduct blended with conjugated polymer. Here, we demonstrate that graphene quantum dots blended with regioregular poly(3-hexylthiophene-2,5-diyl) or poly(2-methoxy-5-(2-ethylhexyloxy)-1,4phenylenevinylene) polymer results in a significant improvement in the OPV characteristics as compared to GSs blended conjugated polymers. This work has implications for inexpensive and efficient solar cells as well as organic light emitting diodes.

The rational development of molecularly imprinted polymer-based sensors for protein detection.

Abstract: The detection of specific proteins as biomarkers of disease, health status, environmental monitoring, food quality, control of fermenters and civil defence purposes means that biosensors for these targets will become increasingly more important. Among the technologies used for building specific recognition properties, molecularly imprinted polymers (MIPs) are attracting much attention. In this critical review we describe many methods used for imprinting recognition for protein targets in polymers and their incorporation with a number of transducer platforms with the aim of identifying the most promising approaches for the preparation of MIP-based protein sensors (277 references).

Nanostructured metal oxide-based biosensors

Abstract: Nanostructured metal oxides (NMOs) have recently become important as materials that provide an effective surface for biomolecule immobilization with desired orientation, better conformation and high biological activity resulting in enhanced sensing characteristics. Nanostructured metal oxides with unique optical, electrical and molecular properties along with desired functionalities and surface charge properties provide interesting platforms for interfacing biorecognition elements with transducers for signal amplification. In this review, we discuss the various approaches that have been adopted for improving the performance of NMO-based biosensors for clinical and non-clinical applications. The performance of an NMO-based biosensor can be improved by tailoring the properties of the metal oxide–biomolecule interface through engineering of morphology, particle size, effective surface area, functionality, adsorption capability and electron-transfer properties. These interesting NMOs are expected to find applications in a new generation of miniaturized, smart biosensing devices.

The nature of in-plane skeleton Raman modes of P3HT and their correlation to the degree of molecular order in P3HT:PCBM blend thin films.

Abstract: The nature of main in-plane skeleton Raman modes (C═C and C–C stretch) of poly(3-hexylthiophene) (P3HT) in pristine and its blend thin films with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is studied by resonant and nonresonant Raman spectroscopy and Raman simulations. Under resonant conditions, the ordered phase of P3HT with respect to its disordered phase is identified by (a) a large shift in the C═C mode peak position to lower wavenumber (∼21 cm–1 shift), (b) a narrower fwhm of the C═C mode (∼9 cm–1 narrower), (c) a larger intensity of the C–C mode relative to the C═C mode (∼56% larger), and (d) a very small Raman dispersion (∼5 cm–1) of the C═C mode. The behavior of the C═C and C–C modes of the ordered and disordered phases of P3HT can be explained in terms of different molecular conformations. The C═C mode of P3HT in P3HT:PCBM blend films can be reproduced by simple superposition of the two peaks observed in different phases of P3HT (ordered and disordered). We quantify the molecular order of ...

New considerations for PM, Black Carbon and particle number concentration for air quality monitoring across different European cities

Abstract: In many large cities of Europe standard air quality limit values of particulate matter (PM) are exceeded. Emis- sions from road traffic and biomass burning are frequently reported to be the major causes. As a consequence of these exceedances a large number of air quality plans, most of them focusing on traffic emissions reductions, have been imple- mented in the last decade. In spite of this implementation, a number of cities did not record a decrease of PM levels. Thus, is the efficiency of air quality plans overestimated? Do the road traffic emissions contribute less than expected to am- bient air PM levels in urban areas? Or do we need a more specific metric to evaluate the impact of the above emissions on the levels of urban aerosols? This study shows the results of the interpretation of the 2009 variability of levels of PM, Black Carbon (BC), aerosol number concentration (N) and a number of gaseous pollu- tants in seven selected urban areas covering road traffic, ur- ban background, urban-industrial, and urban-shipping envi- ronments from southern, central and northern Europe. The results showed that variations of PM and N levels do not always reflect the variation of the impact of road traf-

The use of the PeakForceTM quantitative nanomechanical mapping AFM-based method for high-resolution Young's modulus measurement of polymers

Abstract: PeakForceTM quantitative nanomechanical mapping (QNMTM) is a new atomic force microscopy technique for measuring Young's modulus of materials with high spatial resolution and surface sensitivity by probing at the nanoscale. In this work, modulus results from PeakForce™ QNM™ using three different probes are presented for a number of different polymers with a range of Young's moduli that were measured independently by instrumented (nano) indentation testing (IIT). The results from the diamond and silicon AFM probes were consistent and in reasonable agreement with IIT values for the majority of samples. It is concluded that the technique is complementary to IIT; calibration requirements and potential improvements to the technique are discussed.

On-demand single-electron transfer between distant quantum dots

Abstract: Electrons strongly interact with other electrons and their environment, making it extremely difficult to isolate and detect a single moving electron in a similar way to single photons in quantum optics experiments. But now, in two unrelated reports, Hermelin et al. and McNeil et al. demonstrate that it is possible to emit a single electron from one quantum dot and detect it again with high efficiency after longevity propagation over several micrometres to another quantum dot. The single electron is isolated from other electrons as it is sent into a one-dimensional channel, where it is carried along on a surface acoustic wave induced by microwave excitation. McNeil et al. also show that the same electron can be transferred back and forth up to 60 times, a total distance of 0.25 millimetres. This work demonstrates a new way of transporting a single quantum particle over a long distance in nanostructures, and could pave the way for a range of quantum optics experiments and for quantum information circuits based on single electrons. Single-electron circuits of the future, consisting of a network of quantum dots, will require a mechanism to transport electrons from one functional part of the circuit to another. For example, in a quantum computer1 decoherence and circuit complexity can be reduced by separating quantum bit (qubit) manipulation from measurement and by providing a means of transporting electrons between the corresponding parts of the circuit2. Highly controlled tunnelling between neighbouring dots has been demonstrated3,4, and our ability to manipulate electrons in single- and double-dot systems is improving rapidly5,6,7,8. For distances greater than a few hundred nanometres, neither free propagation nor tunnelling is viable while maintaining confinement of single electrons. Here we show how a single electron may be captured in a surface acoustic wave minimum and transferred from one quantum dot to a second, unoccupied, dot along a long, empty channel. The transfer direction may be reversed and the same electron moved back and forth more than sixty times—a cumulative distance of 0.25 mm—without error. Such on-chip transfer extends communication between quantum dots to a range that may allow the integration of discrete quantum information processing components and devices.

Multifunctional nanoprobes for nanoscale chemical imaging and localized chemical delivery at surfaces and interfaces.

Abstract: Double take: Double-barrel carbon nanoprobes with integrated distance control for simultaneous nanoscale electrochemical and ion conductance microscopy can be fabricated with a wide range of probe sizes in less than two minutes. The nanoprobes allow simultaneous noncontact topographical (left image) and electrochemical imaging (right) of living neurons, as well as localized K+ delivery and simultaneous neurotransmitter detection.

Effect of Crystallization on the Electronic Energy Levels and Thin Film Morphology of P3HT:PCBM Blends

Abstract: We study thin films of semi-crystalline regioregular poly(3-hexylthiophene) (RR-P3HT) and amorphous regiorandom P3HT (RRa-P3HT) in blends with [6,6]-phenyl C61 butyric acid methyl ester (PCBM). Ultraviolet and (angle-resolved) X-ray photoelectron spectroscopy techniques together with absorption, photoluminescence and optical microscopy were used to measure electronic energy levels, vertical chemical compositions and optical properties of these films. We find that ordering the P3HT chains raises the highest occupied molecular orbital (HOMO) energy level of P3HT thin films (reducing ionization potential) and that the ordering of P3HT chains occurs preferentially at the film−air interface in RR-P3HT:PCBM thin films. This leads to a vertical phase separation between P3HT and PCBM molecules, which may be undesirable for conventional P3HT:PCBM solar cells.

Contrasting effect of gold nanoparticles and nanorods with different surface modifications on the structure and activity of bovine serum albumin

Abstract: Nanoparticles exposed to biofluids become coated with proteins, thus making protein–nanoparticle interactions of particular interest. The consequence on protein conformation and activity depends upon the extent of protein adsorption on the nanoparticle surface. We report the interaction of bovine serum albumin (BSA) with gold nanostructures, particularly gold nanoparticles (GNP) and gold nanorods (GNR). The difference in the geometry and surface properties of nanoparticles is manifested during complexation in terms of different binding modes, structural changes, thermodynamic parameters, and the activity of proteins. BSA is found to retain native-like structure and properties upon enthalpy-driven BSA–GNP complexation. On the contrary, the entropically favored BSA–GNR complexation leads to substantial loss in protein secondary and tertiary structures with the release of a large amount of bound water, as indicated by isothermal calorimetry (ITC), circular dichroism (CD), and Fourier transform infrared (FTIR...

Review of meta-heuristics and generalised evolutionary walk algorithm

Abstract: Meta-heuristic algorithms are often nature-inspired, and they are becoming very powerful in solving global optimisation problems. More than a dozen major meta-heuristic algorithms have been developed over the last three decades, and there exist even more variants and hybrids of meta-heuristics. This paper intends to provide an overview of nature-inspired meta-heuristic algorithms, from a brief history to their applications. We try to analyse the main components of these algorithms and how and why they work. Then, we intend to provide a unified view of meta-heuristics by proposing a generalised evolutionary walk algorithm (GEWA). Finally, we discuss some of the important open questions.

Dispersion of linear and nonlinear optical susceptibilities and the hyperpolarizability of 3-methyl-4-phenyl-5-(2-pyridyl)-1,2,4-triazole

Abstract: As a starting point for our calculation of 3-methyl-4-phenyl-5-(2-pyridyl)-1,2,4-triazole we used the XRD data obtained by C. Liu, Z. Wang, H. Xiao, Y. Lan, X. Li, S. Wang, Jie Tang, Z. Chen, J. Chem. Crystallogr., 2009 39 881. The structure was optimized by minimization of the forces acting on the atoms keeping the lattice parameters fixed with the experimental values. Using the relaxed geometry we have performed a comprehensive theoretical investigation of dispersion of the linear and nonlinear optical susceptibilities of 3-methyl-4-phenyl-5-(2-pyridyl)-1,2,4-triazole using the full potential linear augmented plane wave method. The local density approximation by Ceperley–Alder (CA) exchange–correlation potential was applied. The full potential calculations show that this material possesses a direct energy gap of 3.4 eV for the original experimental structure and 3.2 eV for the optimized structure. We have calculated the complex’s dielectric susceptibility e(ω) dispersion, its zero-frequency limit e1(0) and the birefringence. We find that a 3-methyl-4-phenyl-5-(2-pyridyl)-1,2,4-triazole crystal possesses a negative birefringence at the low-frequency limit Δn(0) which is equal to about −0.182 (−0.192) and at λ = 1064 nm is −0.193 (−0.21) for the non-optimized structure (optimized one), respectively. We also report calculations of the complex second-order optical susceptibility dispersions for the principal tensor components: χ(2)123(ω), χ(2)231(ω) and χ(2)312(ω). The intra- and inter-band contributions to these susceptibilities are evaluated. The calculated total second order susceptibility tensor components at the low-frequency limit |χ(2)ijk(0)| and |χ(2)ijk(ω)| at λ = 1064 nm for all the three tensor components are evaluated. We established that the calculated microscopic second order hyperpolarizability, βijk, the vector component along the dipole moment direction, at the low-frequency limit and at λ = 1064 nm, for the dominant component |χ(2)123(ω)| is 4.99 × 10−30 esu (3.4 × 10−30 esu) and 7.72 × 10−30 esu (5.1 × 10−30 esu), respectively for the non-optimized structure (optimized structure).

Non-volatile photochemical gating of an epitaxial graphene/polymer heterostructure.

Abstract: A novel heterostructure based on epitaxial graphene grown on silicon carbide combined with two polymers is demonstrated, with a neutral spacer and a photoactive layer that provides potent electron acceptors under UV light exposure. UV exposure of this heterostructure enables control of the electrical parameters of graphene in a non-invasive, non-volatile, and reversible way.

FTIR and circular dichroism spectroscopic study of interaction of 5-fluorouracil with DNA.

Abstract: 5-Fluorouracil (5FU) is an anticancer chemotherapeutic drug which exerts cytotoxic effect by inhibiting cellular DNA replication. In the present study, we explore the binding of 5FU with DNA and resulting structural and conformational changes on DNA duplex. UV-visible, Fourier transform infrared (FTIR) and circular dichroism (CD) spectroscopic techniques were employed to explore these interactions. A constant concentration of calf thymus DNA was incubated with varying concentrations of 5FU. UV-visible and FTIR spectroscopic results revealed that intercalation is the primary mode of interaction between 5FU and nitrogenous bases of the nucleic acid. The binding constant was found to be 9.7×10(4); which is indicative of moderate type of interaction between 5FU and DNA duplex. It was also observed that 5FU intercalates slightly more between AT base pairs compared to GC pairs. FTIR and circular dichroism spectroscopic results revealed that 5FU disturbs native B-conformation of DNA though, DNA remains in its B conformation even at higher concentrations of 5FU.

Study of size and mass distribution of particulate matter due to crop residue burning with seasonal variation in rural area of Punjab, India

Abstract: Emission from field burning of agricultural crop residue is a common environmental hazard observed in northern India. It has a significant potential health risk for the rural population due to respirable suspended particulate matter (RSPM). A study on eight stage size segregated mass distribution of RSPM was done for 2 wheat and 3 rice crop seasons. The study was undertaken at rural and agricultural sites of Patiala (India) where the RSPM levels remained close to the National Ambient Air quality standards (NAAQS). Fine particulate matter (PM2.5) contributed almost 55% to 64% of the RSPM, showing that, in general, the smaller particles dominated during the whole study period with more contribution during the rice crop as compared to that of wheat crop residue burning. Fine particulate matter content in the total RSPM increased with decrease in temperature. Concentration levels of PM10 and PM2.5 were higher during the winter months as compared to that in the summer months. Background concentration levels of PM10, PM2.5 and PM10−2.5 were found to be around 97±21, 57±15 and 40±6 μg m−3, respectively. The levels increased up to 66, 78 and 71% during rice season and 51, 43 and 61% during wheat crop residue burning, respectively. Extensive statistical analysis of the data was done by using pair t-test. Overall results show that the concentration levels of different size particulate matter are greatly affected by agricultural crop residue burning but the total distribution of the particulate matter remains almost constant.

A review of techniques for the determination of polycyclic aromatic hydrocarbons in air

Abstract: We provide an extensive review of the common methodologies employed in the analysis of airborne polycyclic aromatic hydrocarbons (PAHs). The review focuses on gas-chromatography-based approaches, in the light of their universal application with excellent separation, resolution, and sensitivity. We first describe collection methods for airborne PAHs in the gas and particle phases. We then evaluate the efficiency of extraction techniques employed for separating target PAHs from sampling media, using conventional solvent-based and emerging thermal-desorption approaches. We also describe commonly employed analytical methods with respect to their applicability to PAHs in gas and particle phases, collected from diverse environmental settings. As an essential part of basic quality assurance, we examine each method with special emphasis on key parameters (e.g., limit of detection and reproducibility). Finally, we address the likely directions of methodological developments, their limitations, and the future prospects for PAH analysis.

When should we change the definition of the second

Abstract: The microwave caesium (Cs) atomic clock has formed an enduring basis for the second in the International System of Units (SI) over the last few decades. The advent of laser cooling has underpinned the development of cold Cs fountain clocks, which now achieve frequency uncertainties of approximately 5×10(-16). Since 2000, optical atomic clock research has quickened considerably, and now challenges Cs fountain clock performance. This has been suitably shown by recent results for the aluminium Al(+) quantum logic clock, where a fractional frequency inaccuracy below 10(-17) has been reported. A number of optical clock systems now achieve or exceed the performance of the Cs fountain primary standards used to realize the SI second, raising the issues of whether, how and when to redefine it. Optical clocks comprise frequency-stabilized lasers probing very weak absorptions either in a single cold ion confined in an electromagnetic trap or in an ensemble of cold atoms trapped within an optical lattice. In both cases, different species are under consideration as possible redefinition candidates. In this paper, I consider options for redefinition, contrast the performance of various trapped ion and optical lattice systems, and point to potential limiting environmental factors, such as magnetic, electric and light fields, collisions and gravity, together with the challenge of making remote comparisons of optical frequencies between standards laboratories worldwide.

Influence of preparation method on structural and magnetic properties of nickel ferrite nanoparticles

Abstract: Nickel ferrite nanoparticles of very small size were prepared by sol-gel combustion and co-precipitation techniques. At the same annealing temperature sol-gel derived particles had bigger crystallite size. In both methods, crystallite size of the particles increased with annealing temperature. Sol-gel derived nickel ferrite particles were found to be of almost spherical shape and moderate particle size with a narrow size distribution; while co-precipitation derived particles had irregular shape and very small particle size with a wide size distribution. Nickel ferrite particles produced by sol-gel method exhibited more purity. Sol-gel synthesized nanoparticles were found to be of high saturation magnetization and hysteresis. Co-precipitation derived nickel ferrite particles, annealed at 400°C exhibited superparamagnetic nature with small saturation magnetization. Saturation magnetization increased with annealing temperature in both the methods. At the annealing temperature of 600°C, co-precipitation derived particles also became ferrimagnetic.

Improved accuracy of the NPL-CsF2 primary frequency standard: evaluation of distributed cavity phase and microwave lensing frequency shifts

Abstract: We evaluate the distributed cavity phase and microwave lensing frequency shifts, which were the two largest sources of uncertainty for the NPL-CsF2 cesium fountain clock. We report measurements that confirm a detailed theoretical model of the microwave cavity fields and the frequency shifts of the clock that they produce. The model and measurements significantly reduce the distributed cavity phase uncertainty to $1.1 \times 10^{-16}$. We derive the microwave lensing frequency shift for a cylindrical cavity with circular apertures. An analytic result with reasonable approximations is given, in addition to a full calculation that indicates a shift of $6.2 \times 10^{-17}$. The measurements and theoretical models we report, along with improved evaluations of collisional and microwave leakage induced frequency shifts, reduce the frequency uncertainty of the NPL-CsF2 standard to $2.3 \times 10^{-16}$, nearly a factor of two lower than its most recent complete evaluation.

Anomalously strong pinning of the filling factor nu=2 in epitaxial graphene

Abstract: We explore the robust quantization of the Hall resistance in epitaxial graphene grown on Si-terminated SiC. Uniquely to this system, the dominance of quantum over classical capacitance in the charge transfer between the substrate and graphene is such that Landau levels (in particular, the one at exactly zero energy) remain completely filled over an extraordinarily broad range of magnetic fields. One important implication of this pinning of the filling factor is that the system can sustain a very high nondissipative current. This makes epitaxial graphene ideally suited for quantum resistance metrology, and we have achieved a precision of 3 parts in 1010 in the Hall resistance-quantization measurements.

Metaheuristic optimization: algorithm analysis and open problems

Abstract: Metaheuristic algorithms are becoming an important part of modern optimization. A wide range of metaheuristic algorithms have emerged over the last two decades, and many metaheuristics such as particle swarm optimization are becoming increasingly popular. Despite their popularity, mathematical analysis of these algorithms lacks behind. Convergence analysis still remains unsolved for the majority of metaheuristic algorithms, while efficiency analysis is equally challenging. In this paper, we intend to provide an overview of convergence and efficiency studies of metaheuristics, and try to provide a framework for analyzing metaheuristics in terms of convergence and efficiency. This can form a basis for analyzing other algorithms. We also outline some open questions as further research topics.

Benchmark problems in structural optimization.

Abstract: Structural optimization is an important area related to both optimization and structural engineering. Structural optimization problems are often used as benchmarks to validate new optimization algorithms or to test the suitability of a chosen algorithm. In almost all structural engineering applications, it is very important to find the best possible parameters for given design objectives and constraints which are highly non-linear, involving many different design variables. The field of structural optimization is also an area undergoing rapid changes in terms of methodology and design tools. Thus, it is highly necessary to summarize some benchmark problems for structural optimization. This chapter provides an overview of structural optimization problems of both truss and non-truss cases.

Improved accuracy of the NPL-CsF2 primary frequency standard: evaluation of distributed cavity phase and microwave lensing frequency shifts

Abstract: We evaluate the distributed cavity phase (DCP) and microwave lensing frequency shifts, which were the two largest sources of uncertainty for the NPL-CsF2 caesium fountain clock. We report measurements that confirm a detailed theoretical model of the microwave cavity fields and the frequency shifts of the clock that they produce. The model and measurements significantly reduce the DCP uncertainty to 1.1 ? 10?16. We derive the microwave lensing frequency shift for a cylindrical cavity with circular apertures. An analytic result with reasonable approximations is given, in addition to a full calculation that indicates a shift of 6.2 ? 10?17. The measurements and theoretical models we report, along with improved evaluations of collisional and microwave leakage induced frequency shifts, reduce the frequency uncertainty of the NPL-CsF2 standard to 2.3 ? 10?16, nearly a factor of two lower than its most recent complete evaluation.