Self-assembly versus directed assembly of nanoparticles via pulsed laser induced dewetting of patterned metal films

Self-ordering of small-diameter metal nanoparticles by dewetting on hexagonal mesh templates

Arrays of small-diameter nanoparticles with high spatial order are useful for chemical and biological sensors, data storage, synthesis of nanowires and nanotubes, and many other applications. We show that self-ordered metal nanoparticle arrays can be formed by dewetting of thin films on hexagonal mesh substrates made of anodic aluminum oxide (AAO). Upon heating, the metal (Fe) film dewets onto the interstitial sites (i.e., the node points) between pores on the top surface of the AAO. We investigated the particle morphology and dynamics of dewetting using a combination of atomic force microscopy (AFM), grazing-incidence small-angle X-ray scattering (GISAXS), and numerical simulations. Templated metal particles are more monodisperse and have higher local order than those formed by the same dewetting process on flat, nonporous alumina. The degree of order depends on the initial film thickness, and for the optimal thickness tested (nominally 2 nm), we achieved uniform coverage and high order of the particles, comparable to that of the AAO template itself. Computational modeling of dewetting on templates with various pore order and size shows that the order of AAO pores is primarily influential in determining particle position and spacing, while the variance in pore size is less impactful. Potential uses of these ordered nanoparticle arrays on porous materials include plasmonic sensors and spatially controlled catalysts.

Formation of crownlike and related nanostructures on thin supported gold films irradiated by single diffraction-limited nanosecond laser pulses

A type of laser-induced surface relief nanostructure-the nanocrown-on thin metallic films was studied both experimentally and theoretically. The nanocrowns, representing a thin corrugated rim of resolidified melt and resembling well-known impact-induced water-crown splashes, were produced by single diffraction-limited nanosecond laser pulses on thin gold films of variable thickness on low-melting copper and high-melting tungsten substrates, providing different transient melting and adhesion conditions for these films. The proposed model of the nanocrown formation, based on a hydrodynamical (thermocapillary Marangoni) surface instability and described by a Kuramoto-Sivashinsky equation, envisions key steps of the nanocrown appearance and gives qualitative predictions of the acquired nanocrown parameters.

Directed liquid phase assembly of highly ordered metallic nanoparticle arrays

Directed assembly of nanomaterials is a promising route for the synthesis of nanoscale materials. In this paper, we demonstrate the directed-assembly of highly ordered two-dimensional arrays of hierarchical nanostructures with tunable size, spacing and composition. The directed assembly is achieved on lithographically patterned metal films that are subsequently pulse-laser melted; during the brief liquid lifetime, the pattened nanostructures assemble into highly ordered primary and secondary nanoparticles, with sizes below that which was originally patterned. Complementary fluid-dynamics simulations emulate the resultant patterns and show how the competition of capillary forces and liquid metal-solid substrate interaction potential drives the directed assembly. As an example of the enhanced functionality, a full-wave electromagnetic analysis has been performed to identify the nature of the supported plasmonic resonances.

Rupture mechanism of liquid crystal thin films realized by large-scale molecular simulations

The ability of liquid crystal (LC) molecules to respond to changes in their environment makes them an interesting candidate for thin film applications, particularly in bio-sensing, bio-mimicking devices, and optics. Yet the understanding of the (in)stability of this family of thin films has been limited by the inherent challenges encountered by experiment and continuum models. Using unprecedented large-scale molecular dynamics (MD) simulations, we address the rupture origin of LC thin films wetting a solid substrate at length scales similar to those in experiment. Our simulations show the key signatures of spinodal instability in isotropic and nematic films on top of thermal nucleation, and importantly, for the first time, evidence of a common rupture mechanism independent of initial thickness and LC orientational ordering. We further demonstrate that the primary driving force for rupture is closely related to the tendency of the LC mesogens to recover their local environment in the bulk state. Our study not only provides new insights into the rupture mechanism of liquid crystal films, but also sets the stage for future investigations of thin film systems using peta-scale molecular dynamics simulations.

Coexistence of spinodal instability and thermal nucleation in thin-film rupture: insights from molecular levels

Despite extensive investigation using hydrodynamic models and experiments over the past decades, there remain open questions regarding the origin of the initial rupture of thin liquid films. One of the reasons that makes it difficult to identify the rupture origin is the coexistence of two dewetting mechanisms, namely, thermal nucleation and spinodal instability, as observed in many experimental studies. Using a coarse-grained model and large-scale molecular dynamics simulations, we are able to characterize the very early stage of dewetting in nanometer-thick liquid-metal films wetting a solid substrate. We observe the features characteristic of both spinodal instability and thermal nucleation in the spontaneously dewetting films and show that these two macroscopic mechanisms share a common origin at molecular levels.

Hierarchical nanoparticle ensembles synthesized by liquid phase directed self-assembly

A liquid metal filament supported on a dielectric substrate was directed to fragment into an ordered, mesoscale particle ensemble. Imposing an undulated surface perturbation on the filament forced the development of a single unstable mode from the otherwise disperse, multimodal Rayleigh-Plateau instability. The imposed mode paved the way for a hierarchical spatial fragmentation of the filament into particles, previously seen only at much larger scales. Ultimately, nanoparticle radius control is demonstrated using a micrometer scale switch.

Manipulation of nanoparticles of different shapes inside a scanning electron microscope

In this work polyhedron-like gold and sphere-like silver nanoparticles (NPs) were manipulated on an oxidized Si substrate to study the dependence of the static friction and the contact area on the particle geometry. Measurements were performed inside a scanning electron microscope (SEM) that was equipped with a high-precision XYZ-nanomanipulator. To register the occurring forces a quartz tuning fork (QTF) with a glued sharp probe was used. Contact areas and static friction forces were calculated by using different models and compared with the experimentally measured force. The effect of NP morphology on the nanoscale friction is discussed.

Dewetting properties of metallic liquid film on nanopillared graphene

In this work, we report simulation evidence that the graphene surface decorated by carbon nanotube pillars shows strong dewettability, which can give it great advantages in dewetting and detaching metallic nanodroplets on the surfaces. Molecular dynamics (MD) simulations show that the ultrathin liquid film first contracts then detaches from the graphene on a time scale of several nanoseconds, as a result of the inertial effect. The detaching velocity is in the order of 10 m/s for the droplet with radii smaller than 50 nm. Moreover, the contracting and detaching behaviors of the liquid film can be effectively controlled by tuning the geometric parameters of the liquid film or pillar. In addition, the temperature effects on the dewetting and detaching of the metallic liquid film are also discussed. Our results show that one can exploit and effectively control the dewetting properties of metallic nanodroplets by decorating the surfaces with nanotube pillars.

Instability of liquid Cu films on a SiO2 substrate

We study the instability of nanometric Cu thin films on SiO2 substrates. The metal is melted by means of laser pulses for some tens of nanoseconds, and during the liquid lifetime, the free surface destabilizes, leading to the formation of holes at first and then in later stages of the instability to metal drops on the substrate. By analyzing the Fourier transforms of the SEM (scanning electron microscope) images obtained at different stages of the metal film evolution, we determine the emerging length scales at relevant stages of the instability development. The results are then discussed within the framework of a long-wave model. We find that the results may differ whether early or final stages of the instability are considered. On the basis of the interpretation of the experimental results, we discuss the influence of the parameters describing the interaction of the liquid metal with the solid substrate. By considering both the dependence of dominant length scales on the film thickness and the measured contact angle, we isolate a model which predicts well the trends found in the experimental data.

Numerical simulation of ejected molten metal nanoparticles liquified by laser irradiation: interplay of geometry and dewetting

Metallic nanoparticles, liquified by fast laser irradiation, go through a rapid change of shape attempting to minimize their surface energy. The resulting nanodrops may be ejected from the substrate when the mechanisms leading to dewetting are sufficiently strong, as in the experiments involving gold nanoparticles [Habenicht et al., Science 309, 2043 (2005)]. We use a direct continuum-level approach to accurately model the process of liquid nanodrop formation and the subsequent ejection from the substrate. Our computations show a significant role of inertial effects and an elaborate interplay of initial geometry and wetting properties: e.g., we can control the direction of ejection by prescribing appropriate initial shape and/or wetting properties. The basic insight regarding ejection itself can be reached by considering a simple effective model based on an energy balance. We validate our computations by comparing directly with the experiments specified above involving the length scales measured in hundreds of nanometers and with molecular dynamics simulations on much shorter scales measured in tens of atomic diameters, as by M. Fuentes-Cabrera et al. [Phys. Rev. E 83, 041603 (2011)]. The quantitative agreement, in addition to illustrating how to control particle ejection, shows utility of continuum-based simulation in describing dynamics on nanoscale quantitatively, even in a complex setting as considered here.

Self-organization of multifunctional surfaces--the fingerprints of light on a complex system

Nanocomposite patterns and nanotemplates are generated by a single-step bottom-up concept that introduces laser-induced periodic surface structures (LIPSS) as a tool for site-specific reaction control in multicomponent systems. Periodic intensity fluctuations of this photothermal stimulus inflict spatial-selective reorganizations, dewetting scenarios and phase segregations, thus creating regular patterns of anisotropic physicochemical properties that feature attractive optical, electrical, magnetic, and catalytic properties.

Directed assembly of one- and two-dimensional nanoparticle arrays from pulsed laser induced dewetting of square waveforms

The directed assembly of arrayed nanoparticles is demonstrated by dictating the flow of a liquid phase filament on the nanosecond time scale. Results for the assembly of Ni nanoparticles on SiO2 are presented. Previously, we have implemented a sinusoidal perturbation on the edge of a solid phase Ni, thin film strip to tailor nanoparticle assembly. Here, a nonlinear square waveform is explored. This waveform made it possible to expand the range of nanoparticle spacing-radius combinations attainable, which is otherwise limited by the underlying Rayleigh-Plateau type of instability. Simulations of full Navier-Stokes equations based on volume of fluid method were implemented to gain further insight regarding the nature of instability mechanism leading to particle formation in experiments.

Narrow linewidth picosecond UV pulsed laser with mega-watt peak power

We demonstrate a master oscillator power amplifier (MOPA) burst mode laser system that generates 66 ps/402.5 MHz pulses with mega-watt peak power at 355 nm. The seed laser consists of a single frequency fiber laser (linewidth < 5 KHz), a high bandwidth electro-optic modulator (EOM), a picosecond pulse generator, and a fiber based preamplifier. A very high extinction ratio (45 dB) has been achieved by using an adaptive bias control of the EOM. The multi-stage Nd:YAG amplifier system allows a uniformly temporal shaping of the macropulse with a tunable pulse duration. The light output from the amplifier is converted to 355 nm, and over 1 MW peak power is obtained when the laser is operating in a 5-μs/10-Hz macropulse mode. The laser output has a transform-limited spectrum with a very narrow linewidth of individual longitudinal modes. The immediate application of the laser system is the laser-assisted hydrogen ion beam stripping for the Spallation Neutron Source (SNS).

Laser implantation of plasmonic nanostructures into glass

A laser direct-writing method producing high-resolution patterns of gold, silver and alloy plasmonic nanoparticles implanted into the surface of glass substrates is demonstrated, by scanning a pulsed UV laser beam across selected areas of ultra-thin metal films. The nanoparticles are incorporated beneath the surface of the glass and hence the patterns are scratch-resistant. The physical mechanisms controlling the process are investigated and we demonstrate that this technique can be used to fabricate a wide range of plasmonic optical structures such as wavelength selected diffraction gratings and high-density substrates for lab-on-chip surface-enhanced Raman spectroscopy.

Templated assembly of Co-Pt nanoparticles via thermal and laser-induced dewetting of bilayer metal films

Templated dewetting of a Co/Pt metal bilayer film on a topographic substrate was used to assemble arrays of Co-Pt alloy nanoparticles, with highly uniform particle size, shape and notably composition compared to nanoparticles formed on an untemplated substrate. Solid-state and liquid-state dewetting processes, using furnace annealing and laser irradiation respectively, were compared. Liquid state dewetting produced more uniform, conformal nanoparticles but they had a polycrystalline disordered fcc structure and relatively low magnetic coercivity. In contrast, solid state dewetting enabled formation of magnetically hard, ordered L1(0) Co-Pt single-crystal particles with coercivity >12 kOe. Furnace annealing converted the nanoparticles formed by liquid state dewetting into the L1(0) phase.

Real-time observation of nanosecond liquid-phase assembly of nickel nanoparticles via pulsed-laser heating

Using pump-probe electron microscopy techniques, the dewetting of thin nickel films exposed to a pulsed nanosecond laser was monitored at tens of nanometers spatial and nanosecond time scales to provide insight into the liquid-phase assembly dynamics. Thickness-dependent and correlated time and length scales indicate that a spinodal instability drives the assembly process. Measured lifetimes of the liquid metal are consistent with finite-difference simulations of the laser-irradiated film and are consistent with estimated and observed spinodal time scales. These results can be used to design improved synthesis and assembly routes toward achieving advanced functional nanomaterials and devices.

Parallel assembly of particles and wires on substrates by dictating instability evolution in liquid metal films

Liquid metal wires supported on substrates destabilize into droplets. The destabilization exhibits many characteristics of the Rayleigh-Plateau model of fluid jet breakup in vacuum. In either case, breakup is driven by unstable, varicose surface oscillations with wavelengths greater than the critical one (λ(c)). Here, by controlling the nanosecond liquid lifetime as well as stability of a rivulet as a function of its length by lithography, we demonstrate the ability to dictate the parallel assembly of wires and particles with precise placement.

Competition between collapse and breakup in nanometer-sized thin rings using molecular dynamics and continuum modeling

We consider nanometer-sized fluid annuli (rings) deposited on a solid substrate and ask whether these rings break up into droplets due to the instability of Rayleigh-Plateau-type modified by the presence of the substrate, or collapse to a central drop due to the presence of azimuthal curvature. The analysis is carried out by a combination of atomistic molecular dynamics simulations and a continuum model based on a long-wave limit of Navier-Stokes equations. We find consistent results between the two approaches, and demonstrate characteristic dimension regimes which dictate the assembly dynamics.

Plateau Rayleigh instability simulation

The well-known phenomena of Plateau-Rayleigh instability has been simulated using computational fluid dynamics (CFD). The breakup of a liquid film into an array of droplets on a cylindrical element was simulated using a volume-of-fluid (VOF) solver and compared to experimental observations and existing theory. It is demonstrated that the VOF method can correctly predict the breakup of thins films into an array of either axisymmetric droplets or clam-shell droplets, depending on the surface energy. The existence of unrealistically large films is precluded. Droplet spacing was found to show reasonable agreement with theory. Droplet motion and displacement under fluid flow was also examined and compared to that in previous studies. It was found that the presence of air flow around the droplet does not influence the stable film thickness; however, it reduces the time required for droplet formation. Novel relationships for droplet displacement were derived from the results.

Competing liquid phase instabilities during pulsed laser induced self-assembly of copper rings into ordered nanoparticle arrays on SiO2

Nanoscale copper rings of different radii, thicknesses, and widths were synthesized on silicon dioxide thin films and were subsequently liquefied via a nanosecond pulse laser treatment. During the nanoscale liquid lifetimes, the rings experience competing retraction dynamics and thin film and/or Rayleigh-Plateau types of instabilities, which lead to arrays of ordered nanodroplets. Surprisingly, the results are significantly different from those of similar experiments carried out on a Si surface. We use hydrodynamic simulations to elucidate how the different liquid/solid interactions control the different instability mechanisms in the present problem.

Effect of film thickness on morphological evolution in dewetting and crystallization of polystyrene/poly(ε-caprolactone) blend films

In this Article, the morphological evolution in the blend thin film of polystyrene (PS)/poly(ε-caprolactone) (PCL) was investigated via mainly AFM. It was found that an enriched two-layer structure with PS at the upper layer and PCL at the bottom layer was formed during spinning coating. By changing the solution concentration, different kinds of crystal morphologies, such as finger-like, dendritic, and spherulitic-like, could be obtained at the bottom PCL layer. These different initial states led to the morphological evolution processes to be quite different from each other, so the phase separation, dewetting, and crystalline morphology of PS/PCL blend films as a function of time were studied. It was interesting to find that the morphological evolution of PS at the upper layer was largely dependent on the film thickness. For the ultrathin (15 nm) blend film, a liquid-solid/liquid-liquid dewetting-wetting process was observed, forming ribbons that rupture into discrete circular PS islands on voronoi finger-like PCL crystal. For the thick (30 nm) blend film, the liquid-liquid dewetting of the upper PS layer from the underlying adsorbed PCL layer was found, forming interconnected rim structures that rupture into discrete circular PS islands embedded in the single lamellar PCL dendritic crystal due to Rayleigh instability. For the thicker (60 nm) blend film, a two-step liquid-liquid dewetting process with regular holes decorated with dendritic PCL crystal at early annealing stage and small holes decorated with spherulite-like PCL crystal among the early dewetting holes at later annealing stage was observed. The mechanism of this unusual morphological evolution process was discussed on the basis of the entropy effect and annealing-induced phase separation.

Controlling the velocity of jumping nanodroplets via their initial shape and temperature

Controlling the movement of nanoscale objects is a significant goal of nanotechnology. Dewetting-induced ejection of nanodroplets could provide another means of achieving that goal. Molecular dynamics simulations were used to investigate the dewetting-induced ejection of nanoscale liquid copper nanostructures that were deposited on a graphitic substrate. Nanostructures in the shape of a circle, square, equilateral, and isosceles triangle dewet and form nanodroplets that are ejected from the substrate with a velocity that depends on the initial shape and temperature. The dependence of the ejected velocity on shape is ascribed to the temporal asymmetry of the mass coalescence during the droplet formation; the dependence on temperature is ascribed to changes in the density and viscosity. The results suggest that dewetting induced by nanosecond laser pulses could be used to control the velocity of ejected nanodroplets.