Zahra Fakhraai

Measuring the Surface Dynamics of Glassy Polymers

The motion of polymer chain segments cooled below the glass transition temperature slows markedly; with sufficient cooling, segmental motion becomes completely arrested. There is debate as to whether the chain segments near the free surface, or in thin films, are affected in the same way as the bulk material. By partially embedding and then removing gold nanospheres, we produced a high surface coverage of well-defined nanodeformations on a polystyrene surface; to probe the surface dynamics, we measured the time-dependent relaxation of these surface deformations as a function of temperature from 277 to 369 kelvin. Surface relaxation was observed at all temperatures, providing strong direct evidence for enhanced surface mobility relative to the bulk. The deviation from bulk α relaxation became more pronounced as the temperature was decreased below the bulk glass transition temperature. The temperature dependence of the relaxation time was much weaker than that of the bulk α relaxation of polystyrene, and the process exhibited no discernible temperature dependence between 277 and 307 kelvin.

High-Throughput Ellipsometric Characterization of Vapor-Deposited Indomethacin Glasses

A method for the high-throughput preparation and characterization of vapor-deposited organic glasses is presented. Depositing directly onto a substrate with a large temperature gradient allows many different glasses to be prepared simultaneously. Ellipsometry is used to characterize these glasses, allowing the determination of density, birefringence, and kinetic stability as a function of substrate temperature. For indomethacin, a model glass former, materials up to 1.4% more dense than the liquid-cooled glass can be formed with a continuously tunable range of molecular orientations as determined by the birefringence. By comparing measurements of many properties, we observe three phenomenological temperature regimes. For substrate temperatures from Tg + 11 K to Tg - 8 K, equilibrium states are produced. Between Tg - 8 K and Tg - 31 K, the vapor-deposited materials have the macroscopic properties expected for the equilibrium supercooled liquid while showing local structural anisotropy. At lower substrate temperatures, the properties of the vapor-deposited glasses are strongly influenced by kinetic factors. Different macroscopic properties are no longer correlated with each other in this regime, allowing unusual combinations of properties.

Density and birefringence of a highly stable α,α,β-trisnaphthylbenzene glass

Spectroscopic ellipsometry has been used to understand the properties of α,α,β-trisnaphthylbenzene (ααβ-TNB) glasses vapor-deposited at a substrate temperature of 295 K (0.85 T(g)). In a single temperature ramping experiment, a range of properties of the as-deposited glass can be measured, including density, fictive temperature, onset temperature, thermal expansion coefficient, and birefringence. The vapor-deposited ααβ-TNB glass is 1.3% more dense than the ordinary glass prepared by cooling at 1 K/min, is found to be birefringent, has a fictive temperature 35 K below that of the ordinary glass, and an onset temperature 20 K above that of the ordinary glass. The thermal expansion coefficient of the vapor-deposited ααβ-TNB glass is 14% lower than that of the ordinary glass, indicating that lower portions of the potential energy landscape have more harmonic potential minima than the parts accessible to the ordinary glass.

The effect of periodicity on the extraordinary optical transmission of annular aperture arrays

This work systematically evaluates the effect of array periodicity on the near infrared transmission characteristics of annular aperture arrays (AAAs) in gold films. Both the experimental and theoretical transmission spectra of AAAs are shown to be sensitive to the period and the arrangement of the apertures within the array. The spectra of square arrays with periods ranging from 1400 to 600 nm show a strong correlation with surface plasmon polariton (SPP)-Bloch modes of the metal/dielectric interfaces. For rectangular AAAs the transmission spectra are significantly attenuated and reveal a polarization sensitivity that arises from the breaking of the symmetry and degeneracy of the SPP-Bloch modes.

Raspberry-like Metamolecules Exhibiting Strong Magnetic Resonances

We report a synthetic approach to produce raspberry-like plasmonic nanostructures with unusually strong magnetic resonances, termed raspberry-like metamolecules (raspberry-MMs). The synthesis based on the surfactant-assisted templated seed-growth method allows for the simultaneous one-step synthesis and assembly of well-insulated gold nanoparticles. The aromatic surfactant used for the syntheses forms a thin protective layer around the nanoparticles, preventing them from touching each other and making it possible to pack discrete nanoparticles at close distances in a single cluster. The resulting isotropic gold nanoparticle clusters (i.e., raspberry-MMs) exhibit unusually broad extinction spectra in the visible and near-IR region. Finite-difference time-domain (FDTD) modeling showed that the raspberry-MMs support strong magnetic resonances that contribute significantly to the broadband spectra. The strong magnetic scattering was also verified by far-field scattering measurements, which show that in the near-IR region the magnetic dipole resonance can be even stronger than the electric dipole resonance in these raspberry-MMs. Structural parameters such as the size and the number of gold nanoparticles composing raspberry-MMs can be readily tuned in our synthetic method. A series of syntheses with varying structure parameters, along with FDTD modeling and mode analyses of corresponding model structures, showed that the close packing of a large number of metal nanoparticles in raspberry-MMs is responsible for the unusually strong magnetic resonances observed here.

Comparing surface and bulk flow of a molecular glass former

In this work we measure the response of the molecular glass former 1,3-bis-(1-naphthyl)-5-(2-naphthyl)benzene (TNB Tg = 347 K) to the presence of 20 nm gold nanoparticles placed on the material surface. At times ranging from a few minutes to many hundreds of minutes at temperatures below Tg − 2 K the surface evolves with no change in the apparent height of the nanoparticle. At temperatures Tg − 9 K < T < Tg, and after sufficiently long times, the nanospheres are observed to embed into the material. We employ a simple model for embedding in order to estimate a bulk material viscosity (the material properties ∼10–20 nm into the film) and obtain good agreement with previously reported values over the temperature range 338–345 K. The surface evolution that is observed prior to nanoparticle embedding has a much weaker temperature dependence than the embedding process. The surface evolution is modelled as a thin film with uniformly enhanced mobility, and alternately as surface diffusion. In the context of a decreased viscosity in the entire film, the measured time scales correspond to a viscosity value of 107–1010 Pa·s. Restricting the surface flow to a smaller layer results in correspondingly decreased viscosity values. In the context of a surface diffusion model, the timescale for surface evolution corresponds to a range of surface diffusion coefficients of Ds from 10−14 (at 318 K) to 10−11 m2/s (at 345 K). By measuring both surface and bulk dynamics we provide a quantitative measure for the enhancement of surface dynamics relative to the bulk.

Imaging Secondary Structure of Individual Amyloid Fibrils of a β2-Microglobulin Fragment Using Near-Field Infrared Spectroscopy

Amyloid fibril diseases are characterized by the abnormal production of aggregated proteins and are associated with many types of neuro- and physically degenerative diseases. X-ray diffraction techniques, solid-state magic-angle spinning NMR spectroscopy, circular dichroism (CD) spectroscopy, and transmission electron microscopy studies have been utilized to detect and examine the chemical, electronic, material, and structural properties of amyloid fibrils at up to angstrom spatial resolution. However, X-ray diffraction studies require crystals of the fibril to be analyzed, while other techniques can only probe the bulk solution or solid samples. In the work reported here, apertureless near-field scanning infrared microscopy (ANSIM) was used to probe the secondary structure of individual amyloid fibrils made from an in vitro solution. Simultaneous topographic and infrared images of individual amyloid fibrils synthesized from the #21-31 peptide fragment of β(2)-microglobulin were acquired. Using this technique, IR spectra of the amyloid fibrils were obtained with a spatial resolution of less than 30 nm. It is observed that the experimental scattered field spectrum correlates strongly with that calculated using the far-field absorption spectrum. The near-field images of the amyloid fibrils exhibit much lower scattering of the IR radiation at approximately 1630 cm(-1). In addition, the near-field images also indicate that composition and/or structural variations among individual amyloid fibrils were present.

Chemical Imaging of the Surface of Self-Assembled Polystyrene-b-Poly(methyl methacrylate) Diblock Copolymer Films Using Apertureless Near-Field IR Microscopy

The nanoscale chemical composition variations of the surfaces of thin films of polystyrene- b-poly(methyl methacrylate) (PS- b-PMMA) diblock copolymers are investigated using apertureless near-field IR microscopy. The scattering of the incident infrared beam from a modulated atomic force microscopy (AFM) tip is probed using homodyne detection and demodulation at the tip oscillation frequency. An increase in the IR attenuation is observed in the PMMA-rich domains with a wavenumber dependence that is consistent with the bulk absorption spectrum. The results indicate that even though a small topography-induced artifact can be observed in the near-field images, the chemical signature of the sample is detected clearly.

Quadrupole-Enhanced Raman Scattering

Dark, nonradiating plasmonic modes are important in the Raman enhancement efficiency of nanostructures. However, it is challenging to engineer such hotspots with predictable enhancement efficiency through synthesis routes. Here, we demonstrate that spiky nanoshells have designable quadrupole resonances that efficiently enhance Raman scattering with unprecedented reproducibility on the single particle level. The efficiency and reproducibility of Quadrupole Enhanced Raman Scattering (QERS) is due to their heterogeneous structure, which broadens the quadrupole resonance both spatially and spectrally. This spectral breadth allows for simultaneous enhancement of both the excitation and Stokes frequencies. The quadrupole resonance can be tuned by simple modifications of the nanoshell geometry. The combination of tunability, high efficiency, and reproducibility makes these nanoshells an excellent candidate for applications such as biosensing, nanoantennaes, and photovoltaics.

Surface Effects Mediate Self-Assembly of Amyloid-β Peptides

Here we present a label-free method for studying the mechanism of surface effects on amyloid aggregation. In this method, spin-coating is used to rapidly dry samples, in a homogeneous manner, after various incubation times. This technique allows the control of important parameters for self-assembly, such as the surface concentration. Atomic force microscopy is then used to obtain high-resolution images of the morphology. While imaging under dry conditions, we show that the morphologies of self-assembled aggregates of a model amyloid-β peptide, Aβ12–28, are strongly influenced by the local surface concentration. On mica surfaces, where the peptides can freely diffuse, homogeneous, self-assembled protofibrils formed spontaneously and grew longer with longer subsequent incubation. The surface fibrillization rate was much faster than the rates of fibril formation observed in solution, with initiation occurring at much lower concentrations. These data suggest an alternative pathway for amyloid formation on surfaces where the nucleation stage is either bypassed entirely or too fast to measure. This simple preparation procedure for high-resolution atomic force microscopy imaging of amyloid oligomers and protofibrils should be applicable to any amyloidogenic protein species.