David Corcoran

Thin and ultrathin polymer films combined with nanoparticles (NPs) are of significant interest as they are used in a host of industrial applications. In this paper we describe the stability of such films (h_poly ≤ 30 nm) to dewetting, specifically, how the development of a spinodal instability in a composite NP–polymer layer is controlled by the embedding of Au NPs. At working temperatures (T = 170 °C) above the polymer glass transition temperature (T_g ≈ 100 °C) the absence of Au NPs leads to film rupture by nucleation dewetting, while their presence over a large surface area enhances the development of a spinodal instability without destroying the film continuity. When the NPs embed, the surface undulations are suppressed. The dynamics change from an unstable to a stable state, and the thin composite NP–polymer layer returns to a flat configuration, while the wavelength of the pattern remains constant. Moreover, we demonstrate from a thermodynamic perspective that NPs will remain on the surface or embed in the polymer film depending on their free energy, which is determined by the NP interactions with the underlying polymer, the native SiO_x layer, and the Si substrate.

We demonstrate control, via electro-hydrodynamic (EHD) induced polymer instabilities and nanoparticle mobility, of hierarchical fractal arrangements of gold nanoparticles on patterned thin polymer films. The induced changes in the film curvature enhance fractal formation for high and not for low mobility nanoparticles. The high mobility nanoparticles cluster in circular fractal networks on the rims of a hexagonally ordered array of EHD-induced polymer peaks. These arrangements exhibit plasmonic properties for surface-enhanced Raman scattering (SERS) spectroscopy.

The stability of thin poly(methyl-methacrylate) (PMMA) films of low molecular weight on a solid substrate is controlled by the areal coverage of gold nanoparticles (NPs) present at the air–polymer interface. As the polymer becomes liquid the Au NPs are free to diffuse, coalesce, and aggregate while the polymer film can change its morphology through viscous flow. These processes lead at the same time to the formation of a fractal network of Au NPs and to the development of spinodal instabilities of the free surface of the polymer films. For thinner films a single wavelength is observed, while for thicker films two wavelengths compete. With continued heating the aggregation process results in a decrease in coverage, the networks evolve into disordered particle assemblies, while the polymer films flatten again. The disordering occurs first on the smallest scales and coincides (in thicker films) with the disappearance of the smaller wavelength. The subsequent disordering on larger scales causes the films to flatten.

The stability of polystyrene thin films of low molecular weight on a solid substrate is shown to be controlled by the presence of uniformly distributed gold sputtered at the air–polymer interface. Continuous gold coverage causes the formation of wrinkles. High coverage and Au nanoparticle (NP) density leads to the development of a spinodal instability while low coverage and NP density retards the nucleation dewetting mechanism that beads up the thin polymer film into drops when no coverage is present. Heating at temperature larger than the polymer glass transition temperature for extended periods allows the gold NPs to coalesce and rearrange. The area of polymer surface covered by NPs decreases as a result and this drives the films from unstable to metastable states. When the gold NPs are interconnected by polymer chains a theoretically predicted spinodal instability that patterns the film surface is experimentally observed. Suppression of the instability and a return to a flat film occurs when the polymer interconnections between particles are broken. While the polymer films maintain their physical continuity changes in their chemical surface composition and thickness are observed. The observed film metastability is nevertheless in agreement with theoretical prediction that includes these surface changes.

Strong electric fields produce forces that can overcome the surface tension in thin liquid polymer films and in this way induce an instability of the free surface of the film, that triggers the formation of structures on a micrometer length scale. Here, we study experimentally a polymer–air–polymer system for several combinations of polymer films. These results are accompanied by theoretical considerations based on coupled long-wave time evolution equations for the two free surface profiles. The linear stability and nonlinear time evolution are investigated and compared to the experimental findings. The prediction that the instability always evolves through a mirror mode that couples the two surfaces in an anti-phase manner agrees well with the experimental results. The model describes well the linear (early stage) evolution of the instability. In the non-linear (later stage) evolution, topographical differences in the instability pattern occur if the mobilities of the two layers significantly differ and an unpredicted acceleration of growth is seen in thinner less mobile films. Possible reasons for the mismatch are discussed.

We have studied here using high resolution resistance measurements the time and statistical behavior of abrupt resistance changes in a thin metal aluminum film undergoing electromigration. We reveal for the first time that early stage electromigration exhibits on-off intermittency. The intermittent resistance fluctuations are also shown to be scale invariant, an effect seen in the fluctuations of several physical systems including earthquakes, superconductor dynamics and stock markets. Finite size scaling of the resistance fluctuations demonstrates that they originate near a critical point.

This work shows the relationship of the state variable rock-friction law proposed by Dieterich to the Carlson and Langer friction law commonly used in the Burridge-Knopoff (BK) model of earthquakes. Further to this, the Dieterich law is modified to allow slip rates of zero magnitude yielding a three parameter friction law that is included in the BK system. Dynamic phases of small scale and large scale events are found with a transition surface in the parameter space. Near this transition surface the event size distribution follows a power law with an exponent that varies as the transition is approached contrasting with the invariant exponent observed using the Carlson and Langer friction. This variability of the power-law exponent is consistent with the range of exponents measured in real earthquake systems and is more selective than the range observed in the Olami-Feder-Christensen model.

We present a Molecular Dynamics study of stresses generated in a metal film (aluminum and copper) on a substrate. It is shown that the surface has limited influence on intrinsic stresses beyond a few atoms distance. Surface relaxation at an open surface was found to be consistent with accepted values for copper but the anomalous outward relaxation of the surface layers reported in some studies of aluminum was not recovered. Instead the surface layer relaxed inwards. The atomic structure surrounding a single vacancy in a crystal at temperatures from 100 to 300 K was also studied. A vacancy was found to occupy a volume less than an atomic volume; the contraction was just 0.95, which is considerably less than the previously‐predicted values.

Slider-block models are often used to simulate earthquake dynamics. However, the models’ origins are more conceptual than analytical. This study uses Navier’s equations of an elastic bulk to derive a one-dimensional slider-block model, the Burridge-Knopoff model. This model exhibits a critical phase transition by varying the friction parameter. Accurate analytical estimates are made of event size limits for the small scale, large scale, and intermediate dynamic phases. The absence of large scale quasiperiodic delocalized events is noted for the parameter set investigated here. The time intervals between large scale events are approximately exponentially distributed for the system in its critical state, in agreement with the theory of nonequilibrium critical systems and earthquake dynamics.