In this work, we develop a novel, in situ characterization method to measure the orientation order parameter and investigate the reorientation and reshaping dynamics of polymer grafted gold nanorods (AuNRs) in polymer nanocomposite (PNC) thin films. The long aspect-ratio of AuNRs results in two well-defined plasmon resonance modes, allowing the optical properties of the PNC to be tuned over a wide spectral range. The alignment of the AuNRs in a particular direction can also be used to further tune these optical properties. We utilize variable angle spectroscopic ellipsometry as a unique technique to measure the optical properties of PNC films containing AuNRs at various angles of incidence, and use effective index of refraction analysis of the PNC to relate the birefringence in the film due to changes of the plasmon coupling to the orientation order parameter of AuNRs. Polymer thin films (ca. 70 nm) of either polystyrene (PS) or poly(methyl methacrylate) (PMMA) containing PS grafted AuNRs are probed with ellipsometry, and the resulting extinction coefficient spectra compare favorably with more traditional analytical techniques, electron microscopy (EM) and optical absorbance (vis-NIR) spectroscopy. Furthermore, variable angle spectroscopic ellipsometry measures optical birefringence, which allows us to determine the in- and out-of plane order of the AuNRs, a property that is not easily accessible using other measurement techniques. Additionally, this technique is applied in situ to demonstrate that AuNRs undergo a rapid (ca. 1–5 hours) reorientation before undergoing a slower (ca. 24 hours) rod to sphere shape transition. The reorientation behavior is different depending on the polymer matrix used. In the athermal case (i.e. PS matrix), the AuNRs reorient isotropically, while in PMMA the AuNRs do not become isotropic, which we hypothesize is due to PMMA preferentially wetting the silica substrate, leaving less vertical space for the AuNRs to reorient.

Here, we report surprisingly weak distance dependence in Raman enhancement from a raspberry-like gold nanoparticle termed raspberry-MM. A raspberry-MM is composed of closely packed gold nanobeads assembled on a polymer core. Due to the abundant “built-in” hot spots between adjacent gold nanobeads, bright and uniform Raman signals were observed from isolated single raspberry-MMs. Interestingly, dimers of raspberry-MMs also showed highly reproducible Raman signals, indicating that the dimer SERS signal is not strongly dependent on the nanoparticle separation. Finite-difference time-domain (FDTD) modeling shows that a strong hot spot is created at the dimer-gap, as expected. However, since there are many more built-in hot spots in each raspberry-MM, the contribution of the dimer-gap hot spot to the total Raman enhancement remains low even for 2 nm separation, which explains the observed weak distance dependence. This result is in stark contrast with many previous SERS studies on nanoparticle dimers and clusters, and provides an important guideline on how to design bright and highly reproducible Raman substrates.

We report the synthesis and characterization of an analogous series of small organic molecules derived from a well-known glass former, 1,3-bis(1-naphthyl)-5-(2-naphthyl)benzene (α,α,β-TNB). Synthesized molecules include α,α,β-TNB, 3,5-di(naphthalen-1-yl)-1-phenylbenzene (α,α-P), 9-(3,5-di(naphthalen-1-yl)phenyl)anthracene (α,α-A), 9,9′-(5-(naphthalen-2-yl)-1,3-phenylene)dianthracene (β-AA) and 3,3′,5,5′-tetra(naphthalen-1-yl)-1,1′-biphenyl (α,α,α,α-TNBP). The design of molecules was based on increasing molecular weight with varied π–π interactions in one or more substituents. The synthesis is based on Suzuki cross-coupling of 1-bromo-3-chloro-5-iodobenzene with arylboronic acids, which allows attachment of various substituents to tailor the chemical structure. The bulk compounds were characterized using NMR spectroscopy and differential scanning calorimetry (DSC). Thin films of these compounds were produced using physical vapor deposition and were subsequently annealed above the glass transition temperatures (Tg). For each molecular glass, cooling rate-dependent glass transition temperature measurements (CR-Tg) were performed using ellipsometry as a high-throughput method to characterize thin film properties. CR-Tg allows rapid characterization of glassy properties, such as Tg, apparent thermal expansion coefficients, apparent activation energy at Tg and fragility. DSC measurements confirmed the general trend that increasing molecular weight leads to increasing melting point (Tm) and Tg. Furthermore, CR-Tg provided evidence that the introduction of stronger π-interacting substituents in the chosen set of structural analogues increases fragility and decreases the ability to form glasses, such that β-AA has the largest fragility and highest tendency to crystallize among all the compounds. These strong interactions also significantly elevate Tg and promote more harmonic intermolecular potentials, as observed by decreasing value of the apparent thermal expansion coefficient.

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.Keywords: gold nanoparticle cluster; magnetic dipole; magnetic quadrupole; magnetic resonance; metamaterial; metamolecule; surface plasmon resonance;

We use cooling-rate-dependent Tg measurements (CR-Tg) to indirectly probe the relaxation dynamics and fragility of thin films of polystyrene (PS)/poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) blends. Thin films of this miscible blend have a single glass transition temperature (Tg) that can be systematically shifted over 100 K simply by varying the composition of the blend. This study shows that the Tg of these blends decreases below the bulk Tg as the film thickness is decreased. Additionally, the degree of change in Tg strongly depends on the cooling rate of the experiment. We show that the Tg of 16 nm films of a 50% PS blend is 15 K lower than that of bulk at a cooling rate of 1 K/min but decreases only 4 K at a cooling rate of 120 K/min. By analyzing the cooling rate dependence of the Tg for various thicknesses of the 50% PS blend, we demonstrate that the fragility of these blends decreases with film thickness. This behavior is similar to what is observed in ultrathin films of polystyrene, which suggests that the deviations from bulk dynamics in PS/PPO blends are due to enhanced mobility near the free surface. Similar to pure PS, if extrapolated to higher temperatures, the dynamics of thin films intersect the bulk dynamics at a temperature a few degrees above bulk Tg. The presence of this temperature (T*) can help explain why some experiments fail to see Tg depression in thin films of these blends. Lastly, we show that while the fragility of the bulk blend changes due to differences in the fragility of the homopolymers, ultrathin films (h = 16 nm) have identical average dynamics and fragilities regardless of the blend composition at the same temperature relative to Tg. This result implies that enhanced mobility near the free surface affects the dynamics of these blend thin films similarly.

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.Keywords: plasmon resonance; QERS; quadrupole resonance; Raman spectroscopy; SERS; spiky nanoshells;

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.Keywords: amyloids; Aβ; fibrils; protofibrils; surface-mediated self-assembly;

Gold nanoshells with varying surface topographies and tunable SPR bands were synthesized in high yields by the templated surfactant-assisted seed growth method. By changing the types and amounts of surfactants and ionic additives in the growth solution, the nanoshell topography was controlled from smooth shells to highly structured nanoshells composed of spherical nanoparticles or sharp spikes of varying aspect ratios. The SPR band of the nanoshells could be tuned over a wide range of wavelengths by varying the nanoshell topography, without significantly changing the amount of gold. Finite-difference time-domain (FDTD) modeling was used to predict and understand the optical properties of nanoshells composed of various subparticles, providing insight into the origins of the tunable SPR band.

Gold nanoshells covered with sharp rods called “spiky gold nanoshells” are synthesized by employing a silver-assisted seed-growth method for heterogeneous nanoparticle syntheses at polymer/water interfaces. It is found that silver ions in the growth solution play an important role in forming uniform gold shells as well as regulating the surface morphology. The optical properties of spiky gold nanoshells are investigated by single-particle scattering measurements, single-particle surface-enhanced Raman scattering measurements, and finite-difference time-domain modeling. The scattering intensities from isolated spiky nanoshells are significantly enhanced compared to those of conventional smooth shells. Moreover, due to the abundant hot spots on spiky nanoshells, the SERS signal is readily observed from single spiky shells with a very small intensity variation (35%), whereas there is no detectable signal from isolated smooth shells. These results demonstrate that our synthetic method provides a straightforward way to organize metal nanoparticles into well-defined assemblies with enhanced scattering properties.