Malcolm Kadodwala

Ultrasensitive detection and characterization of biomolecules using superchiral fields.

The spectroscopic analysis of large biomolecules is important in applications such as biomedical diagnostics and pathogen detection, and spectroscopic techniques can detect such molecules at the nanogram level or lower. However, spectroscopic techniques have not been able to probe the structure of large biomolecules with similar levels of sensitivity. Here, we show that superchiral electromagnetic fields, generated by the optical excitation of plasmonic planar chiral metamaterials, are highly sensitive probes of chiral supramolecular structure. The differences in the effective refractive indices of chiral samples exposed to left- and right-handed superchiral fields are found to be up to 10(6) times greater than those observed in optical polarimetry measurements, thus allowing picogram quantities of adsorbed molecules to be characterized. The largest differences are observed for biomolecules that have chiral planar sheets, such as proteins with high β-sheet content, which suggests that this approach could form the basis for assaying technologies capable of detecting amyloid diseases and certain types of viruses.

Induced chirality through electromagnetic coupling between chiral molecular layers and plasmonic nanostructures.

We report a new approach for creating chiral plasmonic nanomaterials. A previously unconsidered, far-field mechanism is utilized which enables chirality to be conveyed from a surrounding chiral molecular material to a plasmonic resonance of an achiral metallic nanostructure. Our observations break a currently held preconception that optical properties of plasmonic particles can most effectively be manipulated by molecular materials through near-field effects. We show that far-field electromagnetic coupling between a localized plasmon of a nonchiral nanostructure and a surrounding chiral molecular layer can induce plasmonic chirality much more effectively (by a factor of 10(3)) than previously reported near-field phenomena. We gain insight into the mechanism by comparing our experimental results to a simple electromagnetic model which incorporates a plasmonic object coupled with a chiral molecular medium. Our work offers a new direction for the creation of hybrid molecular plasmonic nanomaterials that display significant chiroptical properties in the visible spectral region.

Chiral electromagnetic fields generated by arrays of nanoslits

Using a modal matching theory, we demonstrate the generation of short-range, chiral electromagnetic fields via the excitation of arrays of staggered nanoslits that are chiral in two dimensions. The electromagnetic near fields, which exhibit a chiral density greater than that of circularly polarized light, can enhance the chiroptical interactions in the vicinity of the nanoslits. We discuss the features of nanostructure symmetry required to obtain the chiral fields and explicitly show how these structures can give rise to detection and characterization of materials with chiral symmetry.

The adsorption of methanol on Ag(111) studied with TDS and XPS

Abstract The adsorption of methanol on Ag(111) is studied using thermal desorption spectroscopy (TDS) and X-ray photoelectron spectroscopy (XPS). It is found that methanol adsorbs reversibly. The TDS spectra reveal three different states. These states can be assigned to molecules desorbing from the monolayer, the crystallised multilayers and the two amorphous multilayers which are located between the two other phases. This is the first time that these three states are seen by means of TDS. This multilayer behaviour of methanol most likely also applies to other metals. The sticking coefficient is found to be coverage independent from the submonolayer into the multilayer regime. The desorption energy is found to be 0.41 eV. XPS shows for both the oxygen and carbon a small shift to higher binding energies with increasing exposure.

Disposable plasmonics: plastic templated plasmonic metamaterials with tunable chirality

Development of low-cost disposable plasmonic substrates is vital for the applicability of plasmonic sensing. Such devices can be made using injection-molded templates to create plasmonic films. The elements of these plasmonic films are hybrid nanostructures composed of inverse and solid structures. Tuning the modal coupling between the two allows optimization of the optical properties for nanophotonic applications.

A NIXSW and NEXAFS investigation of thiophene on Cu(111)

The local registry and geometry of thiophene at sub-monolayer coverages on Cu(111) has been probed with normal incidence X-ray standing wavefield absorption (NIXSW) and near edge X-ray absorption fine structure (NEXAFS). Thiophene was found to adopt a flat geometry on the surface with the sulphur atom in an atop position. The relatively short data collection times required for the NIXSW technique were found to be beneficial when looking at thiophene overlayers which are sensitive to X-ray induced damage.

Tert-butyl nitrite surface photochemistry: The transition from submonolayer to multilayer behavior

The photochemistry of tert-butyl nitrite, (CH3)3CONO, adsorbed on Ag(111) at 355 nm has been studied using time-of-flight and reflection adsorption infrared spectroscopy (RAIRS) for coverages ranging from 0.1 to 50 ML. For all coverages a thermal and hyperthermal NO desorption distribution is observed due to photodissociation. The thermal component originates from hyperthermal NO molecules from the first and second layer, which were trapped by the silver substrate. The cross section for photodissociation is independent of coverage and matches the gas phase value. This is consistent with a direct excitation into the S1 state. For coverages exceeding 5 ML the outermost layers still dissociate, but RAIRS shows that in the more inward layers caging is preventing dissociation. Due to caging only isomerization of tert-butyl nitrite is allowed, as is evident from the RAIRS.

Bromine adsorption on Cu(111)

Abstract The dissociative chemisorption of molecular bromine on Cu(111) at 300 K has been studied using ultraviolet photoelectron spectroscopy (UPS), Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and work function change measurements. A (√3 × √3)R30° structure is formed initially at a bromine coverage of 0.33 ML. This then converts to a (9√3 × 9√3)R30° compression structure with a coverage of 0.41 ML. The coincidence distance of the compression structure is determined entirely by the van der Waals diameter of adsorbed bromine. The applicability of using the van der Waals diameters of the three halogens, Cl, Br and I, to predict the saturation compression structures on Cu(111), is discussed.

Surface scienceStructural determination of the Cu(111) -(√3 × √3) R30°-ClBr surface using the normal incidence X-ray standing wave method

Abstract A full structural determination has been made of the Cu (111)(√3 × √3)- R 30°- Cl Br surface, which comprises a mixed Cl Br overlayer with a nominal coverage of one third of a monolayer and a 1:1 stoichiometry, using the normal incidence X-ray standing wave (NIXSW) method. Two Bragg reflections were used, the (111) which uses reflecting planes parallel to, and the (111) which are tilted at an angle of 70.5° to, the (111) surface. Using the (111) data, adsorbate-substrate outermost layer spacings were found to be 1.81 ± 0.05 A for chlorine and 2.00 ± 0.05 A for bromine assuming zero substrate relaxation. These results, coupled with the (111) data, are consistent with a model in which both halogens, each of which has a coverage of 1 6 monolayer, occupy three-fold coordinated hollow sites on the surface. The adsorbates occupy primarily fcc hollows (directly above third-layer Cu atoms) but some occupation of hcp hollows (above second-layer Cu atoms) is also observed. For chlorine the fcc:hcp population ratio is 75%:25% (± 10%), while for bromine the rato is 80%:20% (± 10%). Possible origins of the minor species hcp hollow site occupation are discussed.

The photodissociation of tert-butyl nitrite adsorbed on Ag(111): bimodal velocity distributions of the photoproducts

Abstract In this study the photodissociation of physisorbed tert-butyl nitrite, (CH 3 ) 3 CONO, adsorbed on Ag(111) by 355 nm light is investigated at coverages between 0.15 and 50 layers at 85 K. Upon irradiation, time-of-flight spectroscopy showed desorbing NO molecules due to the photodissociation of the tert-butyl nitrite. These molecules show a bimodal velocity distribution with a thermal and a hyperthermal component at all the investigated coverages. The yield and the velocity of the hyperthermal component saturated at a coverage of 5 layers. The conclusion is made that if more than 5 layers are adsorbed the hyperthermal component is produced in the top 5 layers of the film via a direct excitation mechanism. The thermal component consisted of NO photoproducts which had equilibrated to the substrate temperature.