Nanotechnology for catalysis and solar energy conversion.

This roadmap on Nanotechnology for Catalysis and Solar Energy Conversion focuses on the application of nanotechnology in addressing the current challenges of energy conversion: 'high efficiency, stability, safety, and the potential for low-cost/scalable manufacturing' to quote from the contributed article by Nathan Lewis. This roadmap focuses on solar-to-fuel conversion, solar water splitting, solar photovoltaics and bio-catalysis. It includes dye-sensitized solar cells (DSSCs), perovskite solar cells, and organic photovoltaics. Smart engineering of colloidal quantum materials and nanostructured electrodes will improve solar-to-fuel conversion efficiency, as described in the articles by Waiskopf and Banin and Meyer. Semiconductor nanoparticles will also improve solar energy conversion efficiency, as discussed by Boschloo et al in their article on DSSCs. Perovskite solar cells have advanced rapidly in recent years, including new ideas on 2D and 3D hybrid halide perovskites, as described by Spanopoulos et al 'Next generation' solar cells using multiple exciton generation (MEG) from hot carriers, described in the article by Nozik and Beard, could lead to remarkable improvement in photovoltaic efficiency by using quantization effects in semiconductor nanostructures (quantum dots, wires or wells). These challenges will not be met without simultaneous improvement in nanoscale characterization methods. Terahertz spectroscopy, discussed in the article by Milot et al is one example of a method that is overcoming the difficulties associated with nanoscale materials characterization by avoiding electrical contacts to nanoparticles, allowing characterization during device operation, and enabling characterization of a single nanoparticle. Besides experimental advances, computational science is also meeting the challenges of nanomaterials synthesis. The article by Kohlstedt and Schatz discusses the computational frameworks being used to predict structure-property relationships in materials and devices, including machine learning methods, with an emphasis on organic photovoltaics. The contribution by Megarity and Armstrong presents the 'electrochemical leaf' for improvements in electrochemistry and beyond. In addition, biohybrid approaches can take advantage of efficient and specific enzyme catalysts. These articles present the nanoscience and technology at the forefront of renewable energy development that will have significant benefits to society.

A third of the tropical African flora is potentially threatened with extinction.

Preserving tropical biodiversity is an urgent challenge when faced with the growing needs of countries. Despite their crucial importance for terrestrial ecosystems, most tropical plant species lack extinction risk assessments, limiting our ability to identify conservation priorities. Using a novel approach aligned with IUCN Red List criteria, we conducted a continental-scale preliminary conservation assessment of 22,036 vascular plant species in tropical Africa. Our results underline the high level of extinction risk of the tropical African flora. Thirty-three percent of the species are potentially threatened with extinction, and another third of species are likely rare, potentially becoming threatened in the near future. Four regions are highlighted with a high proportion (>40%) of potentially threatened species: Ethiopia, West Africa, central Tanzania, and southern Democratic Republic of the Congo. Our approach represents a first step toward data-driven conservation assessments applicable at continental scales providing crucial information for sustainable economic development prioritization.

Ciliary beating plane and wave propagation in the bovine oviduct.

The uterine tube is an essential conduit for the gametes and zygote during reproduction. The necessary bidirectional conveyance occurs through peristalsis and ciliary activity, but unlike in respiratory tract, little is known about mucociliary transport in the uterine tube, and the direction of transport and the alignment of oviductal cilia have not been conclusively characterized. This study aimed to determine the uniformity in the axonemal orientation of motile cilia in the bovine uterine tube, to identify the direction of mucociliary transport and to relate the presumptive beating plane and the mucociliary transport direction to the long axis of the uterine tube. The angular spread of oviductal motile cilia was determined by electron microscopy, and by maintaining the accurate alignment of the samples throughout the processing steps, axonemal orientation was determined relative to the long axis of the oviduct. The direction of the effective mucociliary transport was determined by the analysis of video microscopic data recorded on explants. Vector-based analysis of electron micrographs yielded the mean angle of deviation between the 'effective ciliary stroke', as derived from axonemal orientation, and the tubal longitudinal axis pointing towards the uterus to be 0.8°, with a standard deviation of 35.2°. The corresponding angular deviation of the short-wave propagation was -6.8° (SD 34.6°). These results show that oviductal motile cilia are rigorously aligned, that the beating plane of the cilia is parallel to the long axis of the uterine tube and that the 'effective stroke' and mucociliary transport are directed towards the uterus.

Eutectic liquid alloys for plasmonics: theory and experiment.

We report a method based on density functional theory molecular dynamics that allows us to calculate the plasmonic properties of liquid metals and metal alloys from first principles with no a priori knowledge of the system. We show exceptional agreement between the simulated and measured optical constants of liquid Ga and the room temperature liquid In-Ga eutectic alloy (T(m) = 289 K). We then use this method to analyze the plasmonic properties of various alloy concentrations in the In-Ga system. The plasmonic performance of the In-Ga system decreases with increasing In concentration. However, the benefits of a room-temperature plasmonic liquid are likely to outweigh the minor reduction in plasmonic performance when moving from pure Ga to the eutectic composition. Our results show that density functional theory molecular dynamics can be used as a predictive tool for studying the optical properties of liquid metal systems amenable to plasmonics.

Observation of multiple vibrational modes in ultrahigh vacuum tip-enhanced Raman spectroscopy combined with molecular-resolution scanning tunneling microscopy.

Multiple vibrational modes have been observed for copper phthalocyanine (CuPc) adlayers on Ag(111) using ultrahigh vacuum (UHV) tip-enhanced Raman spectroscopy (TERS). Several important new experimental features are introduced in this work that significantly advance the state-of-the-art in UHV-TERS. These include (1) concurrent sub-nm molecular resolution STM imaging using Ag tips with laser illumination of the tip-sample junction, (2) laser focusing and Raman collection optics that are external to the UHV-STM that has two cryoshrouds for future low temperature experiments, and (3) all sample preparation steps are carried out in UHV to minimize contamination and maximize spatial resolution. Using this apparatus we have been able to demonstrate a TERS enhancement factor of 7.1 × 10(5). Further, density-functional theory calculations have been carried out that allow quantitative identification of eight different vibrational modes in the TER spectra. The combination of molecular-resolution UHV-STM imaging with the detailed chemical information content of UHV-TERS allows the interactions between large polyatomic molecular adsorbates and specific binding sites on solid surfaces to be probed with unprecedented spatial and spectroscopic resolution.

Integral and differential cross sections for the S(1D)+HD reaction employing the ground adiabatic electronic state.

We present converged quantum mechanical calculations for the title reaction employing a time-dependent wavepacket method. We obtained integral and differential cross sections over an energy range from 0.23 to 0.35 eV total energy as well as product state distributions for both product channels. The excitation functions decrease with energy and point to statistical dynamics as do the cold vibrational distributions and highly inverted rotational distributions. The differential cross sections oscillate strongly with energy for both product channels. Our differential cross sections for both product channels at 2.5 kcal/mol, one of the experimental energies, compare well to the experimental results. The quantum results obtained in this study are similar to what has been found employing QCT methods, implying that the differences between the experimental and theoretical results are due to the potential energy surface or non-adiabatic effects rather than due to quantum effects or the methods employed.

A charge-dipole interaction model for the frequency-dependent polarizability of silver clusters.

We present a charge-dipole interaction model for the calculation of the frequency-dependent polarizability of silver clusters. The model relies on the representation of silver atoms by both a net electric charge and a dipole. Time variations of the atomic charges are related to the currents that flow through the bonds of the structures considered and the atomic charges and dipoles are eventually determined from the application of a least-action principle. After a generalization that enables the bonds of the bulk and surface atoms to have specific resistances, the model is parameterized on data obtained by the time-dependent density functional theory for tetrahedral Ag(20), Ag(84) and Ag(120) clusters. We then study the polarization properties of dimers of silver clusters. We compare in particular the polarizability of the dimers with that of the isolated clusters, for a range of gap distances and frequencies. We also consider the field enhancements one can achieve with these systems. The results are in good agreement with reference data and enable an extension of these data to a wider range of situations. They show that significant field enhancements are achieved at frequencies associated with resonant polarization along the axis of the dimer.

A method to correlate optical properties and structures of metallic nanoparticles.

The optical response of individual nanoparticles is strongly influenced by their structures. In this report, we present a quick and simple pattern-matching based approach in which optical images of nanoparticles from localized surface plasmon resonance and single-molecule surface-enhanced Raman spectroscopy were used in conjunction with transmission electron microscopy for correlation of optical responses and the nanostructures of exactly the same nanoparticles or clusters of nanoparticles.

Rayleigh anomaly-surface plasmon polariton resonances in palladium and gold subwavelength hole arrays.

Surface plasmon polaritons (SPPs) and Rayleigh anomalies (RAs) are two characteristic phenomena exhibited by periodic grating structures made of plasmonic materials. For Au subwavelength hole arrays, SPPs and RAs from opposite sides of the film can interact under certain conditions to produce highly intense, narrow spectral features called RA-SPP resonances. This paper reports how RA-SPP effects can be achieved in subwavelength hole arrays of Pd, a weak plasmonic material. Well-defined resonances are observed in measured and simulated optical transmission spectra with RASPP peaks as narrow as 45 nm (FWHM). Dispersion diagrams compiled from angle-resolved spectra show that RA-SPP resonances in Pd hole arrays shift in wavelength but do not decrease significantly in amplitude as the excitation angle is increased, in contrast with RA-SPP peaks in Au hole arrays. The apparent generality of the RA-SPP effect enables a novel route to optimize resonances in non-traditional plasmonic media.

Enhanced polarizability of aromatic molecules placed in the vicinity of silver clusters.

We use a charge-dipole interaction model to study the polarizability of aromatic molecules that are placed between two silver clusters. In particular we examine the enhancement in polarizability induced by the clusters at plasmon-like resonant frequencies of the cluster-molecule-cluster system. The model used for these simulations relies on representation of the atoms by both a net electric charge and a dipole. By relating the time variation of the atomic charges to the currents that flow through the bonds of the structures considered, a least-action principle can be formulated that enables the atomic charges and dipoles to be determined. We consider benzene, naphthalene and anthracene for this study, comparing the polarizability of these aromatic molecules when placed in the middle between two Ag(120) clusters, with their polarizability as isolated molecules. We find that the polarizability of these molecules is enhanced by the clusters, and this increases the electromagnetic coupling between the two clusters. This results in significant red-shifting (by up to 0.8 eV) of the lowest energy optical transition in the cluster-molecule-cluster system compared to plasmon-like excitation in the cluster-cluster system. The resulting resonant polarizability enhancement leads to an electromagnetic enhancement in surface-enhanced Raman scattering of over 10(6).

Optical absorption spectra and monomer interaction in polymers: Investigation of exciton coupling in DNA hairpins.

We investigate the effect of exciton coupling on the optical absorption spectra of polymer molecules under conditions of strong inhomogeneous broadening. It is shown that the dependence of the maximum in the rescaled absorption spectrum on the number of monomers is determined by their resonant exciton coupling and is insensitive to inhomogeneous broadening. Thus the absorption spectrum can be used to determine optical interactions between monomers. Using our theory and semiempirical calculations we determine exciton coupling between adjacent AT pairs in DNA to be 0.04 eV and discuss exciton localization in DNA hairpins composed of AT pairs.

Aligning conservation priorities across taxa in Madagascar with high-resolution planning tools.

Globally, priority areas for biodiversity are relatively well known, yet few detailed plans exist to direct conservation action within them, despite urgent need. Madagascar, like other globally recognized biodiversity hot spots, has complex spatial patterns of endemism that differ among taxonomic groups, creating challenges for the selection of within-country priorities. We show, in an analysis of wide taxonomic and geographic breadth and high spatial resolution, that multitaxonomic rather than single-taxon approaches are critical for identifying areas likely to promote the persistence of most species. Our conservation prioritization, facilitated by newly available techniques, identifies optimal expansion sites for the Madagascar government's current goal of tripling the land area under protection. Our findings further suggest that high-resolution multitaxonomic approaches to prioritization may be necessary to ensure protection for biodiversity in other global hot spots.

Resonance vibrational Raman optical activity: a time-dependent density functional theory approach.

We present a method to calculate both on- and off-resonance vibrational Raman optical activities (VROAs) of molecules using time-dependent density functional theory. This is an extension of a method to calculate the normal VROA by including a finite lifetime of the electronic excited states in all calculated properties. The method is based on a short-time approximation to Raman scattering and is, in the off-resonance case, identical to the standard theory of Placzek. The normal and resonance VROA spectra are calculated from geometric derivatives of the different generalized polarizabilites obtained using linear response theory which includes a damping term to account for the finite lifetime. Gauge-origin independent results for normal VROA have been ensured using either the modified-velocity gauge or gauge-included atomic orbitals. For the resonance VROA only the modified-velocity gauge has been implemented. We present some initial results for H(2)O(2) and (S)-methyloxirane and compare with predictions from a simple two-state approximation.

Near-field spectroscopy of surface plasmons in flat gold nanoparticles.

We use near-field interference spectroscopy with a broadband femtosecond, white-light probe to study local surface plasmon resonances in flat gold nanoparticles (FGNPs). Depending on nanoparticle dimensions, local near-field extinction spectra exhibit none, one, or two resonances in the range of visible wavelengths (1.6-2.6 eV). The measured spectra can be accurately described in terms of interference between the field emitted by the probe aperture and the field reradiated by driven FGNP surface plasmon oscillations. The measured resonances are in good agreement with those predicted by calculations using discrete dipole approximation. We observe that the amplitudes of these resonances are dependent upon the spatial position of the near-field probe, which indicates the possibility of spatially selective excitation of specific plasmon modes.

Theory and method for calculating resonance Raman scattering from resonance polarizability derivatives.

We present a method to calculate both normal Raman-scattering (NRS) and resonance Raman-scattering (RRS) spectra from the geometrical derivatives of the frequency-dependent polarizability. In the RRS case, the polarizability derivatives are calculated from resonance polarizabilities by including a finite lifetime of the electronic excited states using time-dependent density-functional theory. The method is a short-time approximation to the Kramers, Heisenberg, and Dirac formalism. It is similar to the simple excited-state gradient approximation method if only one electronic excited state is important, however, it is not restricted to only one electronic excited state. Since the method can be applied to both NRS and RRS, it can be used to obtain complete Raman excitation profiles. To test the method we present the results for the S2 state of uracil and the S4, S3, and S2 states of pyrene. As expected, the results are almost identical to the results obtained from the excited-state gradient approximation method. Comparing with the experimental results, we find in general quite good agreement which enables an assignment of the experimental bands to bands in the calculated spectrum. For uracil the inclusion of explicit waters in the calculations was found to be necessary to match the solution spectra. The calculated resonance enhancements are on the order of 10(4)-10(6), which is in agreement with experimental findings. For pyrene the method is also able to distinguish between the three different electronic states for which experimental data are available. The neglect of anharmonicity and solvent effects in the calculations leads to some discrepancy between theory and experiment.

Finite lifetime effects on the polarizability within time-dependent density-functional theory.

We present an implementation for considering finite lifetime of the electronic excited states into linear-response theory within time-dependent density-functional theory. The lifetime of the excited states is introduced by a common phenomenological damping factor. The real and imaginary frequency-dependent polarizabilities can thus be calculated over a broad range of frequencies. This allows for the study of linear-response properties both in the resonance and nonresonance cases. The method is complementary to the standard approach of calculating the excitation energies from the poles of the polarizability. The real and imaginary polarizabilities can then be calculated in any specific energy range of interest, in contrast to the excitation energies which are usually solved only for the lowest electronic states. We have verified the method by investigating the photoabsorption properties of small alkali clusters. For these systems, we have calculated the real and imaginary polarizabilities in the energy range of 1-4 eV and compared these with excitation energy calculations. The results showed good agreement with both previous theoretical and experimental results.

Measuring the fate of plant diversity: towards a foundation for future monitoring and opportunities for urgent action.

Vascular plants are often considered to be among the better known large groups of organisms, but gaps in the available baseline data are extensive, and recent estimates of total known (described) seed plant species range from 200000 to 422000. Of these, global assessments of conservation status using International Union for the Conservation of Nature (IUCN) categories and criteria are available for only approximately 10000 species. In response to recommendations from the Conference of the Parties to the Convention on Biological Diversity to develop biodiversity indicators based on changes in the status of threatened species, and trends in the abundance and distribution of selected species, we examine how existing data, in combination with limited new data collection, can be used to maximum effect. We argue that future work should produce Red List Indices based on a representative subset of plant species so that the limited resources currently available are directed towards redressing taxonomic and geographical biases apparent in existing datasets. Sampling the data held in the world's major herbaria, in combination with Geographical Information Systems techniques, can produce preliminary conservation assessments and help to direct selective survey work using existing field networks to verify distributions and gather population data. Such data can also be used to backcast threats and potential distributions through time. We outline an approach that could result in: (i) preliminary assessments of the conservation status of tens of thousands of species not previously assessed, (ii) significant enhancements in the coverage and representation of plant species on the IUCN Red List, and (iii) repeat and/or retrospective assessments for a significant proportion of these. This would result in more robust Sampled Red List Indices that can be defended as more representative of plant diversity as a whole; and eventually, comprehensive assessments at species level for one or more major families of angiosperms. The combined results would allow scientifically defensible generalizations about the current status of plant diversity by 2010 as well as tentative comments on trends. Together with other efforts already underway, this approach would establish a firmer basis for ongoing monitoring of the status of plant diversity beyond 2010 and a basis for comparison with the trend data available for vertebrates.

Quantum and classical studies of the O(3P) + H2(v = 0-3,j = 0) --> OH + H reaction using benchmark potential surfaces.

We present results of time-dependent quantum mechanics (TDQM) and quasiclassical trajectory (QCT) studies of the excitation function for O(3P) + H2(v = 0-3,j = 0) --> OH + H from threshold to 30 kcal/mol collision energy using benchmark potential energy surfaces [Rogers et al., J. Phys. Chem. A 104, 2308 (2000)]. For H2(v = 0) there is excellent agreement between quantum and classical results. The TDQM results show that the reactive threshold drops from 10 kcal/mol for v = 0 to 6 for v = 1, 5 for v = 2 and 4 for v = 3, suggesting a much slower increase in rate constant with vibrational excitation above v = 1 than below. For H2(v > 0), the classical results are larger than the quantum results by a factor approximately 2 near threshold, but the agreement monotonically improves until they are within approximately 10% near 30 kcal/mol collision energy. We believe these differences arise from stronger vibrational adiabaticity in the quantum dynamics, an effect examined before for this system at lower energies. We have also computed QCT OH(v',j') state-resolved cross sections and angular distributions. The QCT state-resolved OH(v') cross sections peak at the same vibrational quantum number as the H2 reagent. The OH rotational distributions are also quite hot and tend to cluster around high rotational quantum numbers. However, the dynamics seem to dictate a cutoff in the energy going into OH rotation indicating an angular momentum constraint. The state-resolved OH distributions were fit to probability functions based on conventional information theory extended to include an energy gap law for product vibrations.

Anomalous surface diffusion in nanoscale direct deposition processes.

We report the first observation of anomalous diffusion in nanometer scale direct deposition processes utilizing dip-pen nanolithography (DPN). DPN permits quite general nanostructure patterns to be drawn on flat surfaces. Here we demonstrate experimentally, and discuss theoretically, the situation in which the molecular ink in DPN binds weakly to the surface. We observe, for the weak-binding case of 1-dodecylamine on mica, that anomalous diffusion occurs, leading to nearly fractal deposition patterns.

Photoinduced conversion of silver nanospheres to nanoprisms.

A photoinduced method for converting large quantities of silver nanospheres into triangular nanoprisms is reported. The photo-process has been characterized by time-dependent ultraviolet-visible spectroscopy and transmission electron microscopy, allowing for the observation of several key intermediates in and characteristics of the conversion process. This light-driven process results in a colloid with distinctive optical properties that directly relate to the nanoprism shape of the particles. Theoretical calculations coupled with experimental observations allow for the assignment of the nanoprism plasmon bands and for the first identification of two distinct quadrupole plasmon resonances for a nanoparticle. Unlike the spherical particles they are derived from that Rayleigh light-scatter in the blue, these nanoprisms exhibit scattering in the red, which could be useful in developing multicolor diagnostic labels on the basis not only of nanoparticle composition and size but also of shape.