95 research outputs found
Calculation of the work function with a local basis set
Electronic structure codes usually allow to calculate the work function as a
part of the theoretical description of surfaces and processes such as
adsorption thereon. This requires a proper calculation of the electrostatic
potential in all regions of space, which is apparently straightforward to
achieve with plane wave basis sets, but more difficult with local basis sets.
To overcome this, a relatively simple scheme is proposed to accurately compute
the work function when a local basis set is used, by having some additional
basis functions in the vacuum. Tests on various surfaces demonstrate that a
very good agreement with experimental and other theoretical data can be
achieved.Comment: to appear in Surf. Sci. Let
Biomacromolecular stereostructure mediates mode hybridization in chiral plasmonic nanostructures
The refractive index sensitivity of plasmonic fields has been exploited for over 20 years in analytical technologies. While this sensitivity can be used to achieve attomole detection levels, they are in essence binary measurements that sense the presence/absence of a predetermined analyte. Using plasmonic fields, not to sense effective refractive indices but to provide more âgranularâ information about the structural characteristics of a medium, provides a more information rich output, which affords opportunities to create new powerful and flexible sensing technologies not limited by the need to synthesize chemical recognition elements. Here we report a new plasmonic phenomenon that is sensitive to the biomacromolecular structure without relying on measuring effective refractive indices. Chiral biomaterials mediate the hybridization of electric and magnetic modes of a chiral solid-inverse plasmonic structure, resulting in a measurable change in both reflectivity and chiroptical properties. The phenomenon originates from the electric-dipoleâmagnetic-dipole response of the biomaterial and is hence sensitive to biomacromolecular secondary structure providing unique fingerprints of α-helical, ÎČ-sheet, and disordered motifs. The phenomenon can be observed for subchiral plasmonic fields (i.e., fields with a lower chiral asymmetry than circularly polarized light) hence lifting constraints to engineer structures that produce fields with enhanced chirality, thus providing greater flexibility in nanostructure design. To demonstrate the efficacy of the phenomenon, we have detected and characterized picogram quantities of simple model helical biopolymers and more complex real proteins
Symmetry reduction and shape effects in concave chiral plasmonic structures
Chiral metamaterials have shown a number of interesting properties which result from the interaction of the chiral near-field they produce with light and matter. We investigate the influence of structural imperfections on the plasmonic properties of a chiral gold âgammadionâ, using electron energy loss spectroscopy to directly inform simulations of realistic, imperfect structures. Unlike structures of simple convex geometry, the lowest energy modes of the ideal concave gammadion have a quadrupole and dipole character, with the mode energies determined by the nature of electrostatic coupling between the gammadion arms. These modes are strongly affected by structural imperfections that are inherent to the material properties and lithographic patterning. Even subwavelength-scale imperfections reduce the symmetry, lift mode degeneracies convert dark modes into bright ones and significantly alter the mode energy, its near-field strength, and chirality. Such effects will be common to a number of multitipped concave structures currently being investigated for the chiral fields they support
Active chiral plasmonics: flexoelectric control of nanoscale chirality
The ability to electrically control the optical properties of metamaterials is an essential capability required for technological innovation. The creation of dynamic electrically tuneable metamaterials in the visible and near IR region are important for a range of imaging and fibre optic technologies. However current approaches require complex nanofabrication processes which are incompatible for low cost device production. Here, we report a novel simple approach for electrical control of optical properties which utilises a flexoelectric dielectric element to electromechanically manipulate the form factor of a chiral nanostructure. By altering the dimensions of the chiral nanostructure, we allow the polarisation properties of light to be electrically controlled. The flexoelectric element is part of a composite metafilm that is templated on to a nanostructured polymer substrate. Since the flexoelectric element does not require in situ high temperature annealing it can be readily combined with polymerâbased substrates produced by high throughput methods. This is not the case for piezoelectric elements, routinely used in microelectromechanical (MEM) devices which require high temperature processing. Consequently, combining amorphous flexoelectric dielectric and lowâcost polymerâbased materials provides a route to the high throughput production of electrically responsive disposable metadevices
Roles of superchirality and interference in chiral plasmonic biodetection
Chiral plasmonic nanostructures enable â€pg detection and characterization of biomaterials. The sensing capabilities are associated with the chiral asymmetry of the near fields, which locally can be greater than equivalent circularly polarized light, a property referred to as superchirality. However, sensing abilities do not simply scale with the magnitude of superchirality. We show that chiral molecular sensing is correlated to the thickness of a nanostructure. This observation is reconciled with a previously unconsidered interference mechanism for the sensing phenomenon. It involves the âdissipationâ of optical chirality into chiral material currents through the interference of fields generated by two spatially separated chiral modes. The presence of a chiral dielectric causes an asymmetric change in the phase difference, resulting in asymmetric changes to chiroptical properties. Thus, designing a chiral plasmonic sensor requires engineering a substrate that can sustain both superchiral fields and an interference effect
Biomacromolecular charge chirality detected using chiral plasmonic nanostructures
The charge distributions of solvent exposed surfaces of complex biomolecules such has proteins are unique fingerprints. The chirality of these charge distributions result in stereo-specific electrostatic interactions which help define how proteins interact with each other, contributing to specificity in protein â protein interactions. Thus it is a key concept in understanding chemical processes in biology. There is currently no known spectroscopic phenomenon that allows rapid characterisation of chiral surface charge distributions. We show that this essential property that is currently âinvisibleâ to optical spectroscopy, can be detected by monitoring asymmetries in the chiroptical response of protein-plasmonic nanostructure complexes. The unique capabilities of the phenomenon are utilised to discriminate between a structurally homologous series of proteins, type II dehydroquinase (DHQase) derived from different organisms. The proteins are indistinguishable with conventional structurally sensitive spectroscopy (i.e. circular dichroism). We show that discrimination between proteins can be achieved by detecting differences in chiral surface charge distributions. The phenomenon is explained with a simple model whereby the chiroptical properties of the plasmonic structures are perturbed by the induction of an enantiomeric mirror image charge distribution of the protein in the metal. This new phenomenon has broad impact, it is a powerful analytical tool for discriminating between structurally homologous biomaterials, but will also provide information relevant to macromolecular interactions
Attomole enantiomeric discrimination of small molecules using an achiral SERS reporter and chiral plasmonics
Biologically important molecules span a size range from very large
biomacromolecules, such as proteins to small metabolite molecules.
Consequently, spectroscopic techniques which can detect and characterize the
structure of inherently chiral biomolecules over this range of scale at the
femtomole level are necessary to develop novel biosensing and diagnostic
technologies. Nanophotonic platforms uniquely enable chirally sensitive
structural characterisation of biomacromolecules at this ultrasensitive level.
However, they are less successful at achieving the same level of sensitivity
for small chiral molecules, with less than nanomole typical. This poorer
performance can be attributed to the optical response of the platform being
sensitive to a much larger volume of the near field than is occupied by the
small molecule. Here we show that by combining chiral plasmonic metasurfaces
with Raman reporters, which can detect changes in electromagnetic environment
at molecular dimensions, chiral discrimination can be achieved for attomole
quantities of a small molecule, the amino acid cysteine. The signal-to-noise,
and hence ultimate sensitivity, of the measurement can be further improved by
combining the metasurfaces with gold achiral nanoparticles. This indirect
enantiomeric detection is 9 orders of magnitude more sensitive than strategies
relying on monitoring the Raman response of target chiral molecules directly.
Given the generic nature of the phenomenon,this study provides a framework for
developing novel technologies for detecting a broad spectrum of small
biomolecules, which would be useful tools in the field of metabolomics
The structural analysis of Cu(111)-Te (â3 Ă â3) R30° and (2â3 Ă 2â3)R30° surface phases by quantitative LEED and DFT,
The chemisorption of tellurium on atomically clean Cu(111) surface has been studied under ultra-high vacuum conditions. At room temperature, the initial stage of growth was an ordered 23Ă23R30° phase (0.08 ML). An ordered 3Ă3R30° phase is formed at 0.33 ML coverage of Te. The adsorption sites of the Te atoms on the Cu(111) surface at 0.08 ML and 0.33 ML coverages are explored by quantitative low energy electron diffraction (LEED) and density functional theory (DFT). Our results indicate that substitutional surface alloy formation starts at very low coverages
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