Disordered spontaneously buckled optical gratings for improved lighting applications.

Technologies for directing daylight deeper within a building space are highly sought after for energy efficiency applications in order to offset artificial lighting costs and to improve workplace productivity via the use of natural light. Vertical window coatings that can perform this task by redistributing sunlight deeper into a space are especially attractive as they are significantly more straightforward to incorporate into a wide variety of architectures as well as to retrofit into existing facades as compared to roof-based skylights or bulky horizontal daylight shelf-type options. The potential energy savings are even greater when one takes into account the fact that such technologies would mitigate harsh glare, allowing window shades to be open for longer portions of the day. However, low-cost and readily scalable techniques are essential for widespread adoption of these window coating technologies. Here, we describe a potentially low-cost method to create a window coating that could enhance daylight penetration, requiring only a thin film of polymeric material deposited on an elastomeric substrate. The resulting structure is a disordered, spontaneously buckled optical grating that spreads incident light without noticeable chromatic dispersion due to its stochastic patterning. The described method has the potential to improve energy efficiency while maintaining acceptable optical clarity.

Optical and Chemical Characterization of Uranium Dioxide (UO2) and Uraninite Mineral: Calculation of the Fundamental Optical Constants.

Uranium dioxide (UO2) is a material with historical and emerging applications in numerous areas such as photonics, nuclear energy, and aerospace electronics. While often grown synthetically as single-crystal UO2, the mineralogical form of UO2 called uraninite is of interest as a precursor to various chemical processes involving uranium-bearing chemicals. Here, we investigate the optical and chemical properties of a series of three UO2 specimens: synthetic single-crystal UO2, uraninite ore of relatively high purity, and massive uraninite mineral containing numerous impurities. An optical technique called single-angle reflectance spectroscopy was used to derive the optical constants n and k of these uranium specimens by measuring the specular reflectance spectra of a polished surface across the mid- and far-infrared spectral domains (ca. 7000-50 cm-1). X-ray diffractometry, scanning electron microscopy, and energy-dispersive X-ray spectroscopy were further used to analyze the surface composition of the mineralogical forms of UO2. Most notably, the massive uraninite mineral was observed to contain significant deposits of calcite and quartz in addition to UO2 (as well as other metal oxides and radioactive decay products). Knowledge of the infrared optical constants for this series of uranium chemicals facilitates nondestructive, noncontact detection of UO2 under a variety of conditions.

Single-angle reflectance spectroscopy to determine the optical constants n and k: considerations in the far-infrared domain.

Single-angle infrared (IR) reflectance spectroscopy is a proven and effective method for determining the complex optical constants n and k of condensed matter. The modern method uses a Fourier transform IR spectrometer to record the quantitative reflectance R(ν) spectra followed by application of the Kramers-Kronig transform (KKT) to obtain the complex optical constants. In order to carry out the KKT, it is essential to measure the reflectance spectra to as high and low a frequency (wavenumber) as possible. Traditionally, the reflectance spectra of solid specimens consist of large (typically>10 mm diameter) polished single-crystal faces free of defects or voids. The requirement of a large polished face, however, is not a realistic expectation for many synthetic, geologic, or rare specimens where the size is usually small and the morphology can vary. In this paper we discuss several improvements and considerations to both the hardware and far-IR measurement protocols that lead to more accurate R(ν) values and thus to more accurate n/k values, especially for small (millimeter-sized) specimens where the R(ν) spectrum is concatenated from multiple independent R(ν) spectra from overlapping hardware/spectral domains. Specifically, the improved hardware and analyses introduced here include the following: (1) providing a set of far-IR calibration standards; (2) custom-designing and manufacturing low reflectivity, stray-light reducing sample masks for small specimens; (3) minimizing stray light interaction between the sample mask, the interferometer Jacquinot stop, and the detector; (4) optimizing the methods to "splice" together the spectra from independent domains; (5) discussing what methods one can use to obtain or calculate the important R(0 cm-1) value; (6) using a quartic relationship to extrapolate from the measured R data to R(0); and (7) accounting for the limiting effects of diffraction for the spot size at the sample mask and detector for millimeter-sized specimens, especially at the very long wavelengths. These seven considerations are all highly interconnected and are discussed in turn, as well as their strong interdependencies. This paper presents a holistic approach for determining reliable n/k values of millimeter-sized samples using single-angle reflectance in the mid- and far-IR.

Fabrication of Periodic Gold Nanocup Arrays Using Colloidal Lithography.

Within recent years, the field of plasmonics has exploded as researchers have demonstrated exciting applications related to chemical and optical sensing in combination with new nanofabrication techniques. A plasmon is a quantum of charge density oscillation that lends nanoscale metals such as gold and silver unique optical properties. In particular, gold and silver nanoparticles exhibit localized surface plasmon resonances-collective charge density oscillations on the surface of the nanoparticle-in the visible spectrum. Here, we focus on the fabrication of periodic arrays of anisotropic plasmonic nanostructures. These half-shell (or nanocup) structures can exhibit additional unique light-bending and polarization-dependent optical properties that simple isotropic nanostructures cannot. Researchers are interested in the fabrication of periodic arrays of nanocups for a wide variety of applications such as low-cost optical devices, surface-enhanced Raman scattering, and tamper indication. We present a scalable technique based on colloidal lithography in which it is possible to easily fabricate large periodic arrays of nanocups using spin-coating and self-assembled commercially available polymeric nanospheres. Electron microscopy and optical spectroscopy from the visible to near-infrared (near-IR) was performed to confirm successful nanocup fabrication. We conclude with a demonstration of the transfer of nanocups to a flexible, conformal adhesive film.

Fabrication of large area flexible nanoplasmonic templates with flow coating.

We describe the development of a custom-built two-axis flow coater for the deposition of polymeric nanosphere monolayers that could be used in the fabrication of large area nanoplasmonic films. The technique described here has the capability of depositing large areas (up to 7 in. × 10 in.) of self-assembled monolayers of polymeric nanospheres onto polyethylene terephthalate (PET) films. Here, three sets of films consisting of different diameters (ranging from 100 to 300 nm) of polymeric nanospheres were used to demonstrate the capabilities of this instrument. To improve the surface wettability of the PET substrates during wet-deposition, we enhanced the wettability by using a forced air blown-arc plasma treatment system. Both the local microstructure, as confirmed by scanning electron microscopy, describing monolayer and multilayer coverage, and the overall macroscopic uniformity of the resultant nanostructured film were optimized by controlling the relative stage to blade speed and nanosphere concentration. We also show using a smaller nanoparticle template that such monolayers can be used to form nanoplasmonic films. As this flow-coating approach is a scalable technique, large area films such as the ones described here have a variety of crucial emerging applications in areas such as energy, catalysis, and chemical sensing.

Measuring binding kinetics of aromatic thiolated molecules with nanoparticles via surface-enhanced Raman spectroscopy.

Colloidal plasmonic nanomaterials, consisting of metals such as gold and silver, are excellent candidates for advanced optical probes and devices, but precise control over surface chemistry is essential for realizing their full potential. Coupling thiolated (R-SH) molecules to nanoprobe surfaces is a convenient and established route to tailor surface properties. The ability to dynamically probe and monitor the surface chemistry of nanoparticles in solution is essential for rapidly manufacturing spectroscopically tunable nanoparticles. In this study, we report the development of surface-enhanced Raman spectroscopy (SERS) as a method to monitor the kinetics of gold-thiolate bond formation on colloidal gold nanoparticles. A theoretical model combining SERS enhancement with the Beer-Lambert law is proposed to explain ensemble scattering and absorption effects in colloids during chemisorption. In order to maximize biological relevance and signal reproducibility, experiments used to validate the model focused on maintaining nanoparticle stability after the addition of water-soluble aromatic thiolated molecules. Our results indicate that ligand exchange on gold nanoparticles follow a first-order Langmuir adsorption model with rate constants on the order of 0.01 min(-1). This study demonstrates an experimental spectroscopic method and theoretical model for monitoring binding kinetics that may prove useful for designing novel probes.

Observation of molecular diffusion in polyelectrolyte-wrapped SERS nanoprobes.

The popularity of nanotechnology-based sensing technologies has rapidly expanded within the past decade. Surface-enhanced Raman spectroscopy (SERS) is one such technique capable of chemically specific and highly sensitive measurements. The careful preparation of SERS-active nanoprobes is immensely vital for biological applications where nanoprobes are exposed to harsh ionic and protein rich microenvironments. Encapsulation of optical reporter molecules via layer-by-layer (LbL) polyelectrolyte wrapping is an emerging technique that also permits facile modification of surface chemistry and charge. LbL wrapping can be performed within a few hours and does not require the use of organic solvents or hazardous silanes. Nonetheless, the stability of its products requires further characterization and analysis. In this study, Raman-active methylene blue molecules were electrostatically encapsulated within alternating layers of cationic and anionic polyelectrolytes surrounding gold nanospheres. We observed molecular diffusion of methylene blue through polyelectrolyte layers by monitoring the change in SERS intensity over a period of more than 5 weeks. To minimize diffusion and improve the long-term storage stability of our nanoprobes, two additional nanoprobe preparation techniques were performed: thiol coating and cross-linking of the outer polyelectrolyte layer. In both cases, molecular diffusion is significantly diminished.

Computational study of the surface-enhanced Raman scattering from silica-coated silver nanowires.

Surface-enhanced Raman scattering (SERS) is a popular vibrational spectroscopic technique that can have several applications in chemical and biological sensing. Within the last decade or so, our ability to chemically synthesize nanostructures has improved to the point that the rational design of a variety of SERS substrates is now viable. In this report, we describe a computational study using the finite element method (FEM) to investigate the effects of patchy silica coatings on silver nanowires. We found that varying the degree of silica coating on silver nanowires impacts the enhancement and may be explained through two processes. The first process is a consequence of changes in the dielectric environment surrounding the nanowire due to the silica. As additional layers of silica coat the nanowire, the localized surface plasmon resonance of the nanowire redshifts. The second process is a result of silica distorting the local electric field around the nanowire surface. Anisotropic silica coating can influence anticipated enhancement depending on its spatial localization with respect to excited plasmon modes in the nanowire. We propose that the design of nanostructures with patchy silica coatings can be a viable tool for increasing surface enhancement.

Surface-Enhanced Raman Spectroscopy of Polyelectrolyte-Wrapped Gold Nanoparticles in Colloidal Suspension.

The rapidly expanding field of surface-enhanced Raman spectroscopy (SERS) has helped fuel an intense interest in noble metal nanoparticle synthesis. An in-suspension approach for quantifying SERS enhancement and relating that enhancement to a spontaneous Raman equivalent signal is described. Gold nanoparticles of various shapes were wrapped with polyelectrolyte multilayers that trapped Raman reporter molecules at defined distances from the metal core. Electrospray ionization liquid chromatography mass spectrometry (ESI-LC-MS) on digested samples was employed to measure the average number of bound Raman reporter molecules per gold nanoparticle, and inductively coupled plasma mass spectrometry (ICP-MS) was used to measure the average number of gold atoms per nanoparticle. Using these data, SERS signal intensity was compared to a spontaneous Raman calibration curve to compute a spontaneous Raman equivalent factor. Three different geometries of gold nanoparticles (cubes, spheres, and trisoctahedra) were synthesized to investigate edge and corner effects using these quantitative techniques. Finite element method (FEM) electromagnetic simulations examined the relationship between the different geometries and the observed SERS signal intensities. The experimental observations and theoretical results indicate that cubic gold nanoparticles have the highest effective signal.

Competition Between Extinction and Enhancement in Surface Enhanced Raman Spectroscopy.

Conjugated metallic nanoparticles are a promising means to achieve ultrasensitive and multiplexed sensing in intact three-dimensional samples, especially for biological applications, via surface enhanced Raman scattering (SERS). We show that enhancement and extinction are linked and compete in a collection of metallic nanoparticles. Counterintuitively, the Raman signal vanishes when nanoparticles are excited at their plasmon resonance, while increasing nanoparticle concentrations at off-resonance excitation sometimes leads to decreased signal. We develop an effective medium theory that explains both phenomena. Optimal choices of excitation wavelength, individual particle enhancement factor and concentrations are indicated. The same processes which give rise to enhancement also lead to increased extinction of both the illumination and the Raman scattered light. Nanoparticles attenuate the incident field (blue) and at the same time provide local enhancement for SERS. Likewise the radiation of the Raman-scattered field (green) is enhanced by the near-by sphere but extinguished by the rest of the spheres in the suspension on propagation.

High-index facets in gold nanocrystals elucidated by coherent electron diffraction.

Characterization of high-index facets in noble metal nanocrystals for plasmonics and catalysis has been a challenge due to their small sizes and complex shapes. Here, we present an approach to determine the high-index facets of nanocrystals using streaked Bragg reflections in coherent electron diffraction patterns, and provide a comparison of high-index facets on unusual nanostructures such as trisoctahedra. We report new high-index facets in trisoctahedra and previous unappreciated diversity in facet sharpness.

Off-resonance surface-enhanced Raman spectroscopy from gold nanorod suspensions as a function of aspect ratio: not what we thought.

Design of nanoparticles for surface-enhanced Raman scattering (SERS) within suspensions is more involved than simply maximizing the local field enhancement. The enhancement at the nanoparticle surface and the extinction of both the incident and scattered light during propagation act in concert to determine the observed signal intensity. Here we explore these critical aspects of signal generation and propagation through experiment and theory. We synthesized gold nanorods of six different aspect ratios in order to obtain longitudinal surface plasmon resonances that incrementally spanned 600-800 nm. The Raman reporter molecule methylene blue was trap-coated near the surface of each nanorod sample, generating SERS spectra, which were used to compare Raman signals. The average number of reporter molecules per nanorod was quantified against known standards using electrospray ionization liquid chromatography mass spectrometry. The magnitude of the observed Raman signal is reported for each aspect ratio along with the attenuation due to extinction in suspension. The highest Raman signal was obtained from the nanorod suspension with a plasmon resonance blue-shifted from the laser excitation wavelength. This finding is in contrast to SERS measurements obtained from molecules dried onto the surface of roughened or patterned metal substrates where the maximum observed signal is near or red-shifted from the laser excitation wavelength. We explain these results as a competition between SERS enhancement and extinction, at the excitation and scattered wavelengths, on propagation through the sample.