Plasmonics for nanoimaging and nanospectroscopy

Functional Printing of Conductive Silver-Nanowire Photopolymer Composites

We investigated the fabrication and functional behaviour of conductive silver-nanowire-polymer composites for prospective use in printing applications. Silver-nanowires with an aspect ratio of up to 1000 were synthesized using the polyol route and embedded in a UV-curable and printable polymer matrix. Sheet resistances in the composites down to 13 Ω/sq at an optical transmission of about 90% were accomplished. The silver-nanowire composite morphology and network structure was investigated by electron microscopy, atomic force microscopy, profilometry, ellipsometry as well as surface sensitive X-ray scattering. By implementing different printing applications, we demonstrate that our silver nanowires can be used in different polymer composites. On the one hand, we used a tough composite for a 2D-printed film as top contact on a solar cell. On the other hand, a flexible composite was applied for a 3D-printed flexible capacitor.

Conformation and dynamics of single polymer chain studied by optical microscopy techniques beyond the diffraction limit

The origin of the unique properties of a polymer material is the large entropic term of a single molecule, which has a chain-like structure with a large molecular weight. From the viewpoint of understanding the fundamental polymer physics, conformation of the single polymer chain is one of the most important matters; however, it has been difficult to examine the behavior of a single chain because of the limitation of conventional experimental methods. Recent developments in optical microscopy allow the fluorescence imaging beyond the diffraction limit of light, and the author's group showed that the conformation and the dynamics of a single polymer chain can be examined by the high-resolution fluorescence imaging. This review presents the application of optical microscopy with nanometric spatial resolution to study the polymer materials at the single-chain level.

Surface wave illumination Fourier ptychographic microscopy

We propose a novel microscopy method, combining surface wave illumination and the Fourier ptychographic microscopy (FPM) algorithm to achieve super-resolution (SR) imaging. In our system, an oil-immersion objective lens is used to excite both the total internal reflection (TIR) evanescent waves and the surface plasmon waves (SPWs), which illuminate the sample with large wave vectors. Through the FPM algorithm, a resolution approximately twice that of conventional wide-field microscopy is obtained. Meanwhile, we could retrieve the sample's quantitative phase map in order to obtain its surface profile. Importantly, the field enhancement from a SPW has improved the contrast of the reconstructed images, which is critical for revealing the finer structural details of the specimen. In our experiments, we have imaged metallic gratings with a 120 or 150 nm wide line and trench features. We accurately retrieved their axial dimensions with a lateral resolution better than 240 nm that is close to the theoretical resolution of 215 nm, thus demonstrating the quantitative phase imaging capability of our technique. As this approach provides a label-free solution for intensity and phase imaging of samples with lateral resolution exceeding the limit introduced by the optical system, it can be potentially used in a wide range of noninvasive biological imaging applications.

Spatially resolved spectroscopic differentiation of hydrophilic and hydrophobic domains on individual insulin amyloid fibrils

The formation of insoluble β-sheet-rich protein structures known as amyloid fibrils is associated with numerous neurodegenerative diseases, such as Alzheimer's and Parkinson's disease. A detailed understanding of the molecular structure of the fibril surface is of interest as the first contact with the physiological environment in vivo and plays a decisive role in biological activity and associated toxicity. Recent studies reveal that the inherent sensitivity and specificity of tip-enhanced Raman scattering (TERS) renders this technique a compelling method for fibril surface analysis at the single-particle level. Here, the reproducibility of TERS is demonstrated, indicating its relevance for detecting molecular variations. Consequently, individual fibrils are systematically investigated at nanometer spatial resolution. Spectral parameters were obtained by band-fitting, particularly focusing on the identification of the secondary structure via the amide III band and the differentiation of hydrophobic and hydrophilic domains on the surface. In addition multivariate data analysis, specifically the N-FINDR procedure, was employed to generate structure-specific maps. The ability of TERS to localize specific structural domains on fibril surfaces shows promise to the development of new fibril dissection strategies and can be generally applied to any (bio)chemical surface when structural variations at the nanometer level are of interest.

Enhanced Vibrational Spectroscopies as Tools for Small Molecule Biosensing

In this short summary we summarize some of the latest developments in vibrational spectroscopic tools applied for the sensing of (small) molecules and biomolecules in a label-free mode of operation. We first introduce various concepts for the enhancement of InfraRed spectroscopic techniques, including the principles of Attenuated Total Reflection InfraRed (ATR-IR), (phase-modulated) InfraRed Reflection Absorption Spectroscopy (IRRAS/PM-IRRAS), and Surface Enhanced Infrared Reflection Absorption Spectroscopy (SEIRAS). Particular attention is put on the use of novel nanostructured substrates that allow for the excitation of propagating and localized surface plasmon modes aimed at operating additional enhancement mechanisms. This is then be complemented by the description of the latest development in Surface- and Tip-Enhanced Raman Spectroscopies, again with an emphasis on the detection of small molecules or bioanalytes.

High-resolution Raman imaging of bundles of single-walled carbon nanotubes by tip-enhanced Raman spectroscopy

Bundles of single-walled carbon nanotubes (SWCNTs) prepared by plasma torch method and further purified, are deposited over a glass coverslip to estimate the spatial resolution of tip-enhanced Raman spectroscopy measurements. For this purpose, near-field Raman maps and spectra of isolated bundles of carbon nanotubes are collected using optimized experimental conditions such as a tightly focused beam using a 1.4 numerical aperture oil immersion microscope objective and a gold coated atomic force microscope probe illuminated by a radially polarized 632.8 nm wavelength to selectively excite the localized surface plasmon confined at the extremity of the tip. The near-field nature of the collected Raman signals is evaluated through measuring the decay of the Raman signal with respect to the tip-sample separation.

Separation of time-resolved phenomena in surface-enhanced Raman scattering of the photocatalytic reduction of p-nitrothiophenol

Straightforward analysis of chemical processes on the nanoscale is difficult, as the measurement volume is linked to a discrete number of molecules, ruling out any ensemble averaging over rotation and diffusion processes. Raman spectroscopy is sufficiently selective for monitoring chemical changes, but is not sufficiently sensitive to be applied directly. Surface-enhanced Raman spectroscopy (SERS) can be applied for studying reaction kinetics, but adds additional variability in the signal as the enhancement factor is not the same for every location. A novel chemometric method described here separates reaction kinetics from short-term variability, based on the lack of fit in a principal-component analysis. We show that it is possible to study effects that occur on different time scales independently without data reduction using the photocatalytic reduction of p-nitrothiophenol as a showcase system. Using this approach a better description of the nanoscale reaction kinetics becomes available, while the short-term variations can be examined separately to examine reorientation and/or diffusion effects. It may even be possible to identify reaction intermediates through this approach. With only a limited number of reactive molecules in the studied volume, an intermediate on a SERS hot spot may temporarily dominate the spectrum. Now such events can be easily separated from the bulk conversion process by making use of this chemometric method.

Quantitative understanding of the optical properties of a single, complex-shaped gold nanoparticle from experiment and theory

We report on a combined study of Rayleigh and Raman scattering spectroscopy, 3D electron tomography, and discrete dipole approximation (DDA) calculations of a single, complex-shaped gold nanoparticle (NP). Using the exact reconstructed 3D morphology of the NP as input for the DDA calculations, the experimental results can be reproduced with unprecedented precision and detail. We find that not only the exact NP morphology but also the surroundings including the points of contact with the substrate are of crucial importance for a correct prediction of the NP optical properties. The achieved accuracy of the calculations allows determining how many of the adsorbed molecules have a major contribution to the Raman signal, a fact that has important implications for analyzing experiments and designing sensing applications.

Spectroscopy of scattered light for the characterization of micro and nanoscale objects in biology and medicine

The biomedical uses for the spectroscopy of scattered light by micro and nanoscale objects can broadly be classified into two areas. The first, often called light scattering spectroscopy (LSS), deals with light scattered by dielectric particles, such as cellular and sub-cellular organelles, and is employed to measure their size or other physical characteristics. Examples include the use of LSS to measure the size distributions of nuclei or mitochondria. The native contrast that is achieved with LSS can serve as a non-invasive diagnostic and scientific tool. The other area for the use of the spectroscopy of scattered light in biology and medicine involves using conducting metal nanoparticles to obtain either contrast or electric field enhancement through the effect of the surface plasmon resonance (SPR). Gold and silver metal nanoparticles are non-toxic, they do not photobleach, are relatively inexpensive, are wavelength-tunable, and can be labeled with antibodies. This makes them very promising candidates for spectrally encoded molecular imaging. Metal nanoparticles can also serve as electric field enhancers of Raman signals. Surface enhanced Raman spectroscopy (SERS) is a powerful method for detecting and identifying molecules down to single molecule concentrations. In this review, we will concentrate on the common physical principles, which allow one to understand these apparently different areas using similar physical and mathematical approaches. We will also describe the major advancements in each of these areas, as well as some of the exciting recent developments.

Imaging the optical near field in plasmonic nanostructures

Over the past five years, new developments in the field of plasmonics have emerged with the goal of finely tuning a variety of metallic nanostructures to enable a desired function. The use of plasmonics in spectroscopy is of course of great interest, due to large local enhancements in the optical near field confined in the vicinity of a metal nanostructure. For a given metal, such enhancements are dependent on the shape of the structure as well as the optical properties (wavelength, phase, polarization) of the impinging light, offering a large degree of control over the optical and spatial localization of the plasmon resonance. In this focal point, we highlight recent work that aims at revealing the spatial position of the localized plasmon resonances using a variety of optical and non-optical methods.

A modified transmission tip-enhanced Raman scattering (TERS) setup provides access to opaque samples

The combination of scanning probe microscopy and Raman spectroscopy enables chemical characterization of surfaces at highest spatial resolution. This so-called tip-enhanced Raman scattering (TERS) can be employed for a variety of samples where a label-free characterization or identification of constituents on the nanometer scale is pursued. Present TERS setup geometries are always a compromise for specific dedicated applications and show different advantages and disadvantages: Transmission back-reflection setups, when using immersion objectives with a high numerical aperture, intrinsically provide the highest collection efficiency but cannot be applied for opaque samples. Those samples demand upright setups, at the cost of lower collection efficiency, even though very efficient systems using a parabolic mirror for illumination and collection have been demonstrated. In this contribution it is demonstrated that the incorporation of a dichroic mirror to a transmission TERS setup provides easy access to opaque samples without further modification of the setup.

Localized enhancement of electric field in tip-enhanced Raman spectroscopy using radially and linearly polarized light

Finite-Difference Time-Domain (FDTD) calculations are used to characterize the electric field in the vicinity of a sharp silver or gold cone with an apex diameter of 10 nm. The simulations are utilized to predict the intensity and the distribution of the locally enhanced electric field in tip-enhanced Raman spectroscopy (TERS). A side-by-side comparison of the enhanced electric field induced by a radially and a linearly polarized light in both gap-mode and conventional TERS setup is performed. For this purpose, a radially polarized source is introduced and integrated into the FDTD modeling. Additionally, the optical effect of a thin protective layer of alumina on the enhancement of the electric field is investigated.

Tip-enhanced Raman imaging and nano spectroscopy of etched silicon nanowires

Tip-enhanced Raman spectroscopy (TERS) is used to investigate the influence of strains in isolated and overlapping silicon nanowires prepared by chemical etching of a (100) silicon wafer. An atomic force microscopy tip made of nanocrystalline diamond coated with a thin layer of silver is used in conjunction with an excitation wavelength of 532 nm in order to probe the first order optical phonon mode of the [100] silicon nanowires. The frequency shift and the broadening of the silicon first order phonon are analyzed and compared to the topographical measurements for distinct configuration of nanowires that are disposed in straight, bent or overlapping configuration over a microscope coverslip. The TERS spatial resolution is close to the topography provided by the nanocrystalline diamond tip and subtle spectral changes are observed for different nanowire configurations.