Tailoring Plasmonic Fields with Shape-Controlled Single-Crystal Gold Metasurfaces.

Geometry and crystallinity play a critical role in the wavelength-dependent optical responses and plasmonic local near-field distributions of metallic nanostructures. Nevertheless, the ability to tailor the shape and position of crystalline metal surface nanostructures has remained a challenge that limits control of their enhanced local fields and represents a barrier to harnessing their individual and collective responses. Here, we describe a solution deposition method in the presence of anionic additives, which yields shape-controlled, single-crystal plasmonic gold nanostructures on Ag(100) and Au(100) substrates. Use of SO42- ions yields smooth Au(111)-faceted square pyramids with large plasmonic Raman enhancements. Halide additives produce textured hillocks comprising edge- and screw-type dislocations (Cl-), or platelets with large-area Au(100) terraces and (110) step edges (Br-), while SO42- and Br- additive combinations provide Au(110)-faceted square pyramids. With lithographic patterning, this chemistry yields metal deposition with precise geometry and location control to provide single-crystal, plasmonic gold metasurfaces with tailored optical response. The appropriately designed metasurfaces can then generate large Raman scattering enhancements, far greater than high density gold square pyramids with random surface disposition. Shape-controlled single-crystal plasmonic metasurfaces will thus offer opportunities to tune the characteristics of nanostructures, providing enhanced optical, photocatalytic, and sensory response.

High performance, single crystal gold bowtie nanoantennas fabricated via epitaxial electroless deposition.

Material quality plays a critical role in the performance of nanometer-scale plasmonic structures and represents a significant hurdle to large-scale device integration. Progress has been hindered by the challenges of realizing scalable, high quality, ultrasmooth metal deposition strategies, and by the poor pattern transfer and device fabrication yields characteristic of most metal deposition approaches which yield polycrystalline metal structure. Here we highlight a novel and scalable electrochemical method to deposit ultrasmooth, single-crystal (100) gold and to fabricate a series of bowtie nanoantennas through subtractive nanopatterning. We investigate some of the less well-explored design and performance characteristics of these single-crystal nanoantennas in relation to their polycrystalline counterparts, including pattern transfer and device yield, polarization response, gap-field magnitude, and the ability to model accurately the antenna local field response. Our results underscore the performance advantages of single-crystal nanoscale plasmonic materials and provide insight into their use for large-scale manufacturing of plasmon-based devices. We anticipate that this approach will be broadly useful in applications where local near-fields can enhance light-matter interactions, including for the fabrication of optical sensors, photocatalytic structures, hot carrier-based devices, and nanostructured noble metal architectures targeting nano-attophysics.

Scalable, Green Fabrication of Single-Crystal Noble Metal Films and Nanostructures for Low-Loss Nanotechnology Applications.

The confinement of spatially extended electromagnetic waves to nanometer-scale metal structures can be harnessed for application in information processing, energy harvesting, sensing, and catalysis. Metal nanostructures enable negative refractive index, subwavelength resolution imaging, and patterning through engineered metamaterials and promise technologies that will operate in the quantum plasmonics regime. However, the controlled fabrication of high-definition single-crystal subwavelength metal nanostructures has remained a significant hurdle due to the tendency for polycrystalline metal growth using conventional physical vapor deposition methods and the challenges associated with placing solution-grown nanocrystals in desired orientations and locations on a surface to manufacture functional devices. Here, we introduce a scalable and green wet chemical approach to monocrystalline noble metal thin films and nanostructures. The method enables the fabrication of ultrasmooth, epitaxial, single-crystal films of controllable thickness that are ideal for the subtractive manufacture of nanostructures through ion beam milling and additive crystalline nanostructure via lithographic patterning for large-area, single-crystal metasurfaces and high aspect ratio nanowires. Our single-crystal nanostructures demonstrate improved feature quality, pattern transfer yield, reduced optical and resistive losses, and tailored local fields to yield greater optical response and improved stability compared to those of polycrystalline structures-supporting greater local field enhancements and enabling practical advances at the nanoscale.

Polymer dynamics near the surface and in the bulk of poly(tetrafluoroethylene) probed by zero-field muon-spin-relaxation spectroscopy.

The results of many experiments on polymers such as polystyrene indicate that the polymer chains near a free surface exhibit enhanced dynamics when compared with the bulk. We have investigated whether this is the case for poly(tetrafluoroethylene) (PTFE) by using zero-field muon-spin-relaxation spectroscopy to characterize a local probe, the F-Mu(+)-F state, which forms when spin-polarized positive muons are implanted in PTFE. Low-energy muons (implantation energies from 2.0 to 23.0 keV) were used to study the F-Mu(+)-F state between ∼ 23 and 191 nm from the free surface of PTFE. Measurements were also made with surface muons (4.1 MeV) where the mean implantation depth is on the order of ∼ 0.6 mm. The relaxation rate of the F-Mu(+)-F state up to ∼ 150 K was found to be significantly higher for muons implanted at 2.0 keV than for higher implantation energies, which suggests that the polymer chains in a region on the order of a few tens of nanometers from the free surface are more mobile than those in the bulk.

Shape control of electrodeposited copper films and nanostructures through additive effects.

The use of electrolyte additives to affect nanocrystallite shape and film morphology in electrodeposited copper films is presented. Linear sweep and cyclic voltammetry, atomic force microscopy (AFM), scanning electron microscopy (SEM), and X-ray diffraction (XRD) methods are employed to investigate the effects of alcohol additives and the organic additive malachite green (MG), on copper electrodeposited onto polycrystalline gold electrodes. The use of additives affects the deposition process by increasing cathodic peak potentials and decreasing corresponding peak currents. Copper films deposited from electrolyte solutions with additives show additive-specific nanostructure and crystallite morphology. Film analysis reveals a greater than five times reduction in both film roughness and grain size in the presence of even small concentrations of the additive MG. Use of MG results in the preferential electrodeposition of oriented, square pyramidal crystallites, while alcohol additives result in tetrahedral crystallite textures. These shape-controlled additive effects are supported by additive adsorption energy calculations, which indicate preferential interactions, and differential growth kinetics on different facets of the film's growing nanostructures during electrodeposition. This approach offers a new and cost-effective route to achieve shape-controlled surface nanostructure.

Large scale crystallinity in kinetically stable polythiophene-based Langmuir-Blodgett films.

The first direct observation of large scale molecular level ordering in a complex poly(3-alkylthiophene)-based thin film is reported. The fabrication and characterization of highly ordered thin films made from the regioregular, amphiphilic polythiophene derivative poly(3-(11-(2-tetrahydropyranyloxy)undecyl)thiophene) (PTHPUDT) are described. PTHPUDT affords kinetically stable, high optical quality films deposited layer-by-layer using the Langmuir-Blodgett (LB) method. X-ray diffraction studies confirm the deposition of a film with long-range order normal to the surface, characterized by bilayer separations of approximately 30 A. The films display in-plane anisotropy, associated with the preferential alignment of the polymer main chain in the dipping direction. Molecular resolution atomic force microscopy shows the presence of highly ordered crystalline domains within the plane, consistent with an ordered array of parallel, closed-packed, polythiophene chains. Polarized optical microscopy confirms the formation of large scale domains that display uniform optical retardation across macroscopic length scales. FeCl(3)-doped films yield anisotropic conduction behavior, suggesting higher rates of transport perpendicular to the main chain direction. The kinetic stability of these films is distinct from films of other polythiophenes, other deposition methods, and simpler amphiphiles, all of which tend toward their bulk, thermodynamically stable structures. This stability and long-range order are attributed to the amphiphilic nature of the polymer and the dimensional restrictions of the polymer's main chain, which limits the extent of structural defect-based reorganization, and the limited number of local structures of the alkyl chains. The degree and nature of the ordering in these semicrystalline films make them an ideal model system in which to elucidate the connection between morphology and physical property in complex pi-conjugated polymers.

A low temperature phase transition in Langmuir-Blodgett films.

The influences of temperature on the SFG spectra of Langmuir-Blodgett films of cadmium stearate, ferric stearate, stearic acid and octadecanamide are reported. Upon cooling, all films display reversible discontinuous shifts of approximately 8 cm (-1) in the r+, r- and rfermi modes of the terminal methyl groups at approximately 150 K. Reversible changes in the relative intensities of these methyl group peaks, most pronounced in the PPP spectra, are also observed and attributed to a change in the environment of the methyl group that accompanies a discontinuous transition in the ordering of their alkyl chains. The onset of new spectral features at higher frequency is attributed to the observation of ordered water molecules contained within the films. The correlation between the onset of the water features and the onset of the reversible, discontinuous, spectroscopic changes of the amphiphiles argues for a causal connection between the two. In addition to the discontinuous behavior upon cooling, monolayer films of stearic acid and octadecanamide display activity of methylene modes upon exposure to vacuum. Films displaying SFG-active methylene groups at room temperature had them gradually become completely SFG-inactive by 100 K. Heating the films to room temperature revealed that the methylene group activity was reversible. Monolayer films of cadmium stearate and ferric stearate do not display this methylene activity upon exposure to vacuum, suggesting that this behavior may be linked to solvation of the amphiphile's headgroup. These observations suggest that water plays a key role in the stability and structure of LB supported monolayers, and have important implications to those interested in low temperature (cryogenic) effects of biological systems.

Optical second-harmonic generation study of exfoliated MoS2-malachite green inclusion materials.

We have examined the properties of exfoliated and restacked MoS(2)-malachite green (MG) inclusion compounds to provide insight into the MG-MoS(2) interactions that characterize these materials. The results of X-ray diffraction experiments indicate that MG included into the restacked structure adopts a flat orientation approximately parallel to the MoS(2) sheets. Second-harmonic generation experiments conducted on the exfoliated and restacked materials provide information regarding the averaged orientation of the MG. At low MG coverage, our results support the X-ray diffraction findings, and yield large averaged orientation angles, consistent with a flat orientation of MG between the MoS(2) layers. However, as the MG coverage is increased, the SHG results indicate averaged MG orientations that are much more upright, consistent with the expulsion of excess MG from the layers to the outside of the restacked crystallites. Together with X-ray diffraction and adsorption isotherm data, our SHG results provide a model for the exfoliation, adsorption, and subsequent restacking of these MG-based inclusion materials and demonstrate the utility of nonlinear optical techniques as probes of these interesting layered structures.

Azines possessing strong push-pull donors/acceptors.

Azines (R(2)C[double bond, length as m-dash]N-N[double bond, length as m-dash]CR(2)) are 2,3-diaza analogues of 1,3-butadiene. In this report we show that strong polarisation of the azine imparts structural features consistent with delocalization within the azine fragment; NLO properties for the azines are also reported.