Hydrodynamic pattern formation (PF) and dewetting resulting from pulsed laser induced melting of nanoscopic metal films have been used to create spatially ordered metal nanoparticle arrays with monomodal size distribution on SiO 2 /Si substrates. PF was investigated for film thickness h ≤ 7 nm < laser absorption depth ∼ 11 nm and different sets of laser parameters, including energy density E and the irradiation time, as measured by the number of pulses n. PF was only observed to occur for E ≥ E m , where E m denotes the h-dependent threshold energy required to melt the film. Even at such small length scales, theoretical predictions for E m obtained from a continuumlevel lumped parameter heat transfer model for the film temperature, coupled with the 1-D transient heat equation for the substrate phase, were consistent with experimental observations provided that the thickness dependence of the reflectivity of the metal-substrate bilayer was incorporated into the analysis. The model also predicted that perturbations in h would result in intrinsic thermal gradients ∂T /∂h whose magnitude and sign depend on h, with ∂T /∂h > 0 for h < h c and ∂T /∂h < 0 for h > h c ≈ 9 nm. For the thickness range investigated here, the resulting thermocapillary effect was minimal since the thermal diffusion time τ H ≤ the pulse time. Consequently, the spacing between the nanoparticles and the particle diameter were found to increase as h 2 and h 5/3 respectively, which is consistent with the predictions of the thin film hydrodynamic (TFH) dewetting theory. PF was characterized by the appearance of discrete holes followed by bicontinuous or cellular patterns which finally consolidated into nanoparticles via capillary flow rather than via Rayleigh-like instabilities reported for low temperature dewetting of viscous liquids. This difference is attributed to the high capillary velocities of the liquid metal arising from its relatively large interfacial tension and low viscosity as well as the smaller length scales of the liquid bridges in the experiments. The predicted liquid phase lifetime τ L was between 2 − 15 ns, which is much smaller than the dewetting time τ D ≥ 25 ns as predicted by the linear TFH theory. Therefore, dewetting required the application of multiple pulses. During the early stages of dewetting, the ripening rate, as measured by the rate of change of characteristic ordering length with respect to n, increased linearly with E due to the linear increase in τ L with increasing E as predicted by the thermal model. The final nanoparticle spacing was robust, independent of E and n, and only dependent on h due to the relatively weak temperature dependence of the thermophysical properties of the metal (Co). These results suggest

We have observed nanopattern formation with robust and controllable spatial ordering by laser-induced dewetting in nanoscopic metal films. Pattern evolution in Co film of thickness 1≤h≤8 nm on SiO(2) was achieved under multiple pulse irradiation using a 9 ns pulse laser. Dewetting leads to the formation of cellular patterns which evolve into polygons that eventually break up into nanoparticles with unimodal size distribution and short range ordering in nearest neighbour spacing R. Spatial ordering was attributed to a hydrodynamic thin film instability and resulted in a predictable variation of R and particle diameter D with h. The length scales R and D were found to be independent of the laser energy. These results suggest that spatially ordered metal nanoparticles can be robustly assembled by laser-induced dewetting.

We show here that the morphological pathway of spontaneous dewetting of ultrathin Ag films on SiO 2 under nanosecond laser melting is found to be film thickness dependent. For films with thickness h between 2 ≤ h ≤ 9.5 nm, the morphology during the intermediate stages of dewetting consisted of bicontinuous structures. For films 11.5 ≤ h ≤ 20 nm, the intermediate stages consisted of regularly-sized holes. Measurement of the characteristic length scales for different stages of dewetting as a function of film thickness showed a systematic increase, which is consistent with the spinodal dewetting instability over the entire thickness range investigated. This change in morphology with thickness is consistent with observations made previously for polymer films [A. Sharma et al, Phys. Rev. Lett., v81, pp3463 (1998); R. Seemann et al, J. Phys. Cond. Matt., v13, pp4925, (2001)]. Based on the behavior of free * Corresponding author, ramki@utk.edu 1 energy curvature that incorporates intermolecular forces, we have estimated the morphological transition thickness for the intermolecular forces for Ag on SiO 2 . The theory predictions agree well with observations for Ag. These results show that it is possible to form a variety of complex Ag nanomorphologies in a consistent manner, which could be useful in optical applications of Ag surfaces, such as in surface enhanced Raman sensing.

When an ultrathin metal film of thickness h (<20 nm) is melted by a nanosecond pulsed laser, the film temperature is a nonmonotonic function of h and achieves its maximum at a certain thickness h*. This is a consequence of the h and time dependence of energy absorption and heat flow. Linear stability analysis and nonlinear dynamical simulations that incorporate such intrinsic interfacial thermal gradients predict a characteristic pattern length scale Lambda that decreases for h>h*, in contrast to the classical spinodal dewetting behavior where Lambda increases monotonically as h2. These predictions agree well with experimental observations for Co and Fe films on SiO2.

Self-assembly of surfactant molecules into micelles of various shapes and forms has been extensively used to synthesize soft nanomaterials. Translucent solutions containing rod-like surfactant micelles can self-organize under flow to form viscoelastic gels. This flow-induced structure (FIS) formation has excited much fundamental research and pragmatic interest as a cost-effective manufacturing route for active nanomaterials. However, its practical impact has been very limited because all reported FIS transitions are reversible because the gel disintegrates soon after flow stoppage. We present a new microfluidics-assisted robust laminar-flow process, which allows for the generation of extension rates many orders of magnitude greater than is realizable in conventional devices, to produce purely flow-induced permanent nanogels. Cryogenic transmission electron microscopy imaging of the gel reveals a partially aligned micelle network. The critical flow rate for gel formation is consistent with the Turner-Cates fusion mechanism, proposed originally to explain reversible FIS formation in rod-like micelle solutions.