Dewetting Of Thin Films Research Articles

Mechano-stimulated equilibration of gold nanoparticles on sapphire

We present a new method for equilibration of faceted nanoparticles obtained by solid state agglomeration (dewetting) of metal thin films. The method relies on crystalline defects introduced in the particles by the atomic force microscopy-based indentation and tapping. The developed method was employed for statistically based determination of the Au {111}/sapphire {0001} interfacial energy, with the result (2.11±0.08J/m2) being in good agreement with the values reported in the literature.

A method for the formation of Pt metal nanoparticle arrays using nanosecond pulsed laser dewetting

Nanosecond pulsed laser dewetting of Pt thin films, deposited on a dimpled Ta (DT) surface, has been studied here in order to form ordered Pt nanoparticle (NP) arrays. The DT substrate was fabricated via a simple electrochemical anodization process in a highly concentrated H2SO4 and HF solution. Pt thin films (3–5 nm) were sputter coated on DT and then dewetted under vacuum to generate NPs using a 355 nm laser radiation (6–9 ns, 10 Hz). The threshold laser fluence to fully dewet a 3.5 nm thick Pt film was determined to be 300 mJ/cm2. Our experiments have shown that shorter irradiation times (≤60 s) produce smaller nanoparticles with more uniform sizes, while longer times (>60 s) give large nanoparticles with wider size distributions. The optimum laser irradiation time of 1 s (10 pulses) has led to the formation of highly ordered Pt nanoparticle arrays with an average nanoparticle size of 26 ± 3 nm with no substrate deformation. At the optimum condition of 1 s and 500 mJ/cm2, as many as 85% of the dewetted NPs were found neatly in the well-defined dimples. This work has demonstrated that pulsed laser dewetting of Pt thin films on a pre-patterned dimpled substrate is an efficient and powerful technique to produce highly ordered Pt nanoparticle arrays. This method can thus be used to produce arrays of other high-melting-point metal nanoparticles for a range of applications, including electrocatalysis, functionalized nanomaterials, and analytical purposes.

Formation, stability, and atomic structure of theSi(111)−(6×6)Ausurface reconstruction: A quantitative study using synchrotron radiation

The conditions of formation of the Au-induced $\mathrm{Si}\text{\ensuremath{-}}(6\ifmmode\times\else\texttimes\fi{}6)$ reconstruction, its stability, as well as its atomic structure are studied experimentally using grazing incidence x-ray diffraction techniques. This reconstruction is found to form at 680 K when cooling down eutectic droplets previously obtained by the dewetting of thin gold films deposited at room temperature on a $\mathrm{Si}(111)$ substrate. The quality of the reconstruction is found to depend on the annealing temperature. The formation of the $(6\ifmmode\times\else\texttimes\fi{}6)$ reconstruction at low temperature after it has been destroyed by ion bombardment, together with the recovery of the Si surface, give evidence of its high stability and highlight the surfactant properties of gold atoms. The determination of the $\mathrm{Si}(111)\text{\ensuremath{-}}(6\ifmmode\times\else\texttimes\fi{}6)\mathrm{Au}$ atomic structure is performed using quantitative surface x-ray diffraction with an existing complex model proposed in the literature as a starting structure. The resulting gold structure is found to be 1 ML thick and consists in two domains related by a mirror. They are composed of trimer and pentagonal units with special sites presenting a partial occupancy of $0.5$. Our experimental data set does not provide enough accuracy to determine the positions of the Si atoms of the substrate, but ab initio calculations tend to confirm that they are only slightly displaced from the bulk position.

Guiding the Dewetting of Thin Polymer Films by Colloidal Imprinting

Micropatterned surfaces are important in many biomedical and bioengineering applications, such as the development of biosensors. An approach for the creation of ordered surface patterns, fabricated combining colloidal crystals, consisting of ordered layers of micrometric particles, with dewetting of bilayers of thin polymer films is introduced. The produced patterns are both topographical and chemical in nature, consisting of ordered arrays of microscale holes imprinted in a polymer film, with tunable size. The spontaneous dewetting of the polymer film enables this tunability, with a maximum sevenfold increase in lateral size of the holes and sixfold increase in depth from imprinting to coalescence with neighboring holes. Polymer dewetting and layer inversion are seen to compete during the annealing of the polymer bilayers, and the optimal conditions for hole growth are identified. An in‐depth investigation highlights the effects of UV‐ozone treatment on the long‐range ordering of the colloidal crystals and on preventing the dewetting of the imprinted bilayers. Ordered patterns of different size and depth are produced over large areas by tuning of the colloidal crystal assembly, UV surface treatment and dewetting conditions. Potential applications of the micropatterns produced in the present work include microarrays for single cell studies and biosensors.

Directed dewetting of amorphous silicon film by a donut-shaped laser pulse

Irradiation of a thin film with a beam-shaped laser is proposed to achieve site-selectively controlled dewetting of the film into nanoscale structures. As a proof of concept, the laser-directed dewetting of an amorphous silicon thin film on a glass substrate is demonstrated using a donut-shaped laser beam. Upon irradiation of a single laser pulse, the silicon film melts and dewets on the substrate surface. The irradiation with the donut beam induces an unconventional lateral temperature profile in the film, leading to thermocapillary-induced transport of the molten silicon to the center of the beam spot. Upon solidification, the ultrathin amorphous silicon film is transformed to a crystalline silicon nanodome of increased height. This morphological change enables further dimensional reduction of the nanodome as well as removal of the surrounding film material by isotropic silicon etching. These results suggest that laser-based dewetting of thin films can be an effective way for scalable manufacturing of patterned nanostructures.

Influence of initiator fragments as chain ends on the Tg-confinement effect and dewetting of thin films of ultralow molecular weight polymer

Prior research on anionically polymerized polystyrene (PS) indicates that there is little or no influence of molecular weight (MW) from 2 to 3000 kg/mol on the Tg-confinement effect of supported PS films. Synthesis by free radical polymerization (FRP) with 2,2′-azobis(2-methylpropionitrile) (AIBN) or benzoyl peroxide (BPO) as initiator incorporates initiator fragments at chain ends that are distinct from PS repeat units and anionic initiator fragments as chain ends. While the influence of different chain ends on the Tg-confinement effect is negligible for high MW PS, we show that it is substantial for ultralow MW PS. As determined by spectroscopic ellipsometry and fluorescence measurements of films of ultralow MW PS (Mn = ∼4 kg/mol) supported on silicon wafers or glass with native oxide surfaces, the magnitude of Tg (21-nm-thick) – Tg (bulk) for PS made by FRP is reduced by 50% from that for anionically polymerized PS. Ellipsometry results indicate that the muted effect for ultralow MW PS made by FRP originates mainly from a weaker perturbation to Tg near the free surface. However, dewetting studies show that PS made by FRP results in enhanced film stability relative to anionically polymerized PS; this indicates that the nitrogen- or oxygen-containing FRP initiator fragments present as chain ends may undergo hydrogen bonding with hydroxyl groups on the substrate surface. In contrast to PS, supported films of ultralow MW poly(methyl methacrylate) show no significant influence of different chain ends on the Tg-confinement effect down to a thickness of 20 nm.

The process of solid-state dewetting of Au thin films studied by in situ scanning transmission electron microscopy

Solid-state dewetting describes the transformation of thin films into a set of particles or droplets at temperatures well below the melting temperature of the bulk. In this work in situ scanning transmission electron microscopy has been used to study the dewetting behavior of discontinuous Au thin films (15nm and 22nm thick) on amorphous silicon nitride membranes at temperatures ranging from 300°C to 600°C. The combination of microscopic and statistical information enabled not only the qualitative discussion of the observed processes but also the quantification of the kinetics as well as the development of a model of the underlying morphological mechanism. A model-free master curve approach to the temporal evolution of the covered area at different temperatures is used to determine the activation energy of dewetting (1.04±0.14eV for the 15nm thick film). A closer inspection reveals a multiple power law behavior, which is discussed in the frame of depercolation. Retraction of finger-like structures is found to be the dominant morphological mechanism based on the observed linear relationship between covered area and boundary length.

Solvent-vapor-assisted dewetting of prepatterned thin polymer films: control of morphology, order, and pattern miniaturization.

Ultrathin (<100 nm) unstable polymer films exposed to a solvent vapor dewet by the growth of surface instability, the wavelength (λ) of which depends on the film thickness (h(f)). While the dewetting of a flat polymer thin film results in random structures, we show that the dewetting of a prepatterned film results in myriad ordered mesoscale morphologies under specific conditions. Such a film undergoes rupture over the thinnest parts when the initial local thickness of these zones (h(rm)) is lower than a limiting thickness h(lim) ≈ 10 nm. Additionally, the width of the pattern grooves (l(s)) must be wider than λ(s) corresponding to a flat film having a thickness of h(rm) for pattern-directed dewetting to take place over surface-tension-induced flattening. We first present an experimentally obtained morphology phase diagram that captures the conditions where a transition from surface-tension-induced flattening to pattern-directed-rupture takes place. Subsequently, we show the versatility of this technique in achieving a variety of aligned mesopatterns starting from a prepatterned film with simple grating geometry. The morphology of the evolving patterns depends on several parameters such as the initial film thickness (h(f)), prepattern amplitude (h(st)), duration of solvent vapor exposure (SVE), and wettability of the stamp used for patterning. Periodic rupture of the film at regular intervals imposes directionality on the evolving patterns, resulting in isolated long threads/cylindrical ridges of polymers, which subsequently disintegrate into an aligned array of droplets due to Rayleigh-Plateau instability under specific conditions. Other patterns such as a double periodic array of droplets and an array of holes are also possible to obtain. The evolution can be interrupted at any intermediate stage by terminating the solvent vapor annealing, allowing the creation of pattern morphology on demand. The created patterns are significantly miniaturized in size as compared to features obtained from dewetting a flat film with the same hf.

Interplay of Substrate Surface Energy and Nanoparticle Concentration in Suppressing Polymer Thin Film Dewetting

It is known that dewetting of a polystyrene (PS) thin film on a silicon substrate gets completely suppressed upon addition of small amount of C60 nanoparticles (NP).1 The NPs migrate to the film–substrate interface and forms an enriched surface layer of the particles that eventually stabilizes the film by apparent pinning. In this article we quantitatively highlight the unexplored effect of substrate surface energy (γS) on the migration of the NPs to the film–substrate interface and their contribution on dewetting suppression. Depending on the relative magnitudes of NP concentration (CNP) and γS, we identify three distinct stability regimes. In regime 1 (CNP < 0.2%) there is no suppression of dewetting and the final polygonal arrangement of droplets closely resemble dewetted structures in particle free films. However, the size of the polygons becomes smaller in NP containing films when γS < γC60 (NP surface energy) and larger as γS exceeds γC60. In regime 2 (0.3% < CNP < 0.75%) the films dewet partially, and the extent of dewetting is seen to strongly dependent on the relative magnitudes of γC60 and γS. While dewetting proceeds up to the stage of partial hole growth and coalescence when γS < γC60, some random isolated holes are seen to form when γS > γC60. On the basis of direct AFM imaging, we show that in both regimes 1 and 2 the NPs migrate to the substrate–film interface only when γS > γC60. We show complete suppression of dewetting in regime 3 (CNP > 1.0%), where the particles are seen to migrate to the substrate for all values of γS. The work highlights that entropy driven migration of particles takes place on substrates with any γS only above a critical NP concentration (CNPC) and only on substrates with γS > γC60 when CNP < CNPC. The findings, apart from dewetting suppressing, can guide potential design criteria for applications such as electron extracting layer in organic photovoltaic.

Gold nanoparticle array formation on dimpled Ta templates using pulsed laser-induced thin film dewetting.

Here we show that pulsed laser-induced dewetting (PLiD) of a thin Au metallic film on a nano-scale ordered dimpled tantalum (DT) surface results in the formation of a high quality Au nanoparticle (NP) array. In contrast to thermal dewetting, PLiD does not result in deformation of the substrate, even when the Au film is heated to above its melting point. PLiD causes local heating of only the metal film and thus thermal oxidation of the Ta substrate can be avoided, also because of the high vacuum (low pO2) environment employed. Therefore, this technique can potentially be used to fabricate NP arrays composed of high melting point metals, such as Pt, not previously possible using conventional thermal annealing methods. We also show that the Au NPs formed by PLiD are more spherical in shape than those formed by thermal dewetting, likely demonstrating a different dewetting mechanism in the two cases. As the metallic NPs formed on DT templates are electrochemically addressable, a longer-term objective of this work is to determine the effect of NP size and shape (formed by laser vs. thermal dewetting) on their electrocatalytic properties.

Dewetting based fabrication of fibrous micro-scaffolds as potential injectable cell carriers.

Although regenerative medicine utilizing tissue scaffolds has made enormous strides in recent years, many constraints still hamper their effectiveness. A limitation of many scaffolds is that they form surface patches, which are not particularly effective for some types of "wounds" that are deep within tissues, e.g., stroke and myocardial infarction. In this study, we reported the generation of fibrous micro-scaffolds feasible for delivering cells by injection into the tissue parenchyma. The micro-scaffolds (widths<100μm) were made by dewetting of poly(lactic-co-glycolic acid) thin films containing parallel strips, and cells were seeded to form cell/polymer micro-constructs during or post the micro-scaffold fabrication process. Five types of cells including rat induced vascular progenitor cells were assessed for the formation of the micro-constructs. Critical factors in forming fibrous micro-scaffolds via dewetting of polymer thin films were found to be properties of polymers and supporting substrates, temperature, and proteins in the culture medium. Also, the ability of cells to attach to the micro-scaffolds was essential in forming cell/polymer micro-constructs. Both in vitro and in vivo assessments of injecting these micro-scaffolding constructs showed, as compared to free cells, enhanced cell retention at the injected site, which could lead to improved tissue engineering and regeneration.

Unusual dewetting of thin polymer films in liquid media containing a poor solvent and a nonsolvent.

We investigate the control of pattern size and kinetics in spontaneous dewetting of thin polymer films (polystyrene) that are stable to thermal annealing by annealing in a poor solvent (acetone)/nonsolvent (ethanol or n-hexane) liquid mixture. Dewetting occurs by the formation and growth of circular holes that coalesce to form droplets. The influence of the nature and the volume fraction of the nonsolvents on the contact angle of polymer droplets, number density of holes, and the kinetics of holes formation and growth is studied. Addition of ethanol greatly increases the hole density and slows down the kinetics substantially, while affecting only a small change in wettability. n-Hexane addition shows an interesting nonmonotonic response in decreasing the hole density and contact angle in the volume fraction range of 0-0.3 but an opposite effect beyond that. Although the two nonsolvents chosen cannot by themselves induce dewetting, their relative affinity for the solid substrate vis-à-vis acetone can strongly influence the observed dewetting scenarios that are not understood by the existing theoretical considerations. n-Hexane, for example, has great affinity for silicon substrate. In addition to the changes in wettability, viscosity, and film interfacial tension engendered by the nonsolvents, the possibility of the formation of adsorbed liquid layers at the substrate-polymer interface, which can modify the interfacial friction and slippage, needs to be considered.

Laser-Induced Self-Assembled Nanostructures on Electron-Transparent Substrates

Currently, one of the challenges in high-resolution transmission electron microscopy (TEM) studies of nanomaterials is to make contamination-free materials in a simple and time-efficient way. Here, a method is demonstrated that combines nanosecond-pulsed laser dewetting of thin films with a film float-off technique to realize nanostructures (NSs) on electron-transparent substrates in a robust and rapid manner. NSs of metal (Ag) and bimetals (AgCo, AuCo) ranging from 20 to 150 nm are synthesized on thin carbon film deposited on mica substrates. The NS/carbon system is subsequently transferred onto TEM grids by a float-off process resulting from debonding of the carbon from mica due to their contrasting hydrophobic nature. This process enables the fabrication of different NSs on flexible and electron-transparent substrates.

In situ control of dewetting of Cu thin films in graphene chemical vapor deposition

Chemical vapor deposition (CVD) on Cu thin films is a promising approach for the large area formation of graphene on dielectric substrates, but a fine control of the deposition parameters is required to avoid dewetting of the Cu catalyst. In this paper we report on the study of the Cu dewetting phenomena by monitoring the intensity of the infra-red emission from the film surface during rapid thermal CVD of graphene. The reduction of Cu film coverage consequent to dewetting is detected as a variation of sample's emissivity. Results indicate three time constants of dewetting, describing three typical stages, hole formation, propagation and ligament breakup. Slowing the first incubation stage by tuning pressure in the chamber allows for an effective surface activation resulting in the deposition of graphene at temperatures lower than those in the case of Cu foils.

Mechanisms of solid-state dewetting of thin Au films in different annealing atmospheres

We studied the initial stages of solid-state dewetting of thin nanocrystalline Au films deposited on sapphire substrates during annealing in air and in the flow of forming gas. The interrupted annealing technique was employed to study the evolution of the size and morphology of individual holes in the film. The holes exhibited comparable expansion rates and similar topographies comprising an elevated rim in both annealing atmospheres. In particular, we observed a terraced topography of the hole rim with sharp steps associated with individual grain boundaries. The holes in the film annealed in air exhibited more branched, tortuous lateral morphology than their counterparts in the film annealed in forming gas. We interpreted these experimental finding in terms of the effect of annealing atmosphere on the surface anisotropy of Au. We developed dewetting models which consider Au self-diffusion along the Au–sapphire interface as a factor controlling the kinetics of the dewetting process. The predictions of our models are in a good agreement with experimental findings.

Metal-catalyzed etching of graphene governed by metal–carbon interactions: A comparison of Fe and Cu

We present a comparative investigation on the etching of graphene catalyzed by Fe and Cu. When Fe or Cu thin film deposited on graphene is rapidly annealed in either N2 or forming gas (10% H2/90% N2), particles are produced due to the dewetting of thin films. Low-voltage scanning electron microscopy reveals different morphology for Fe and Cu particles and their strikingly different catalytic etching behaviors. For the Fe thin film on graphene annealed at 950°C in either gas environment, graphene is severely damaged, suggesting that the etching could occur through catalytic carbon hydrogenation or carbon dissolution into Fe due to the strong Fe–C interactions. In contrast, while no etching takes place for Cu particles on graphene at 1050°C in N2, Cu particles catalytically etch channels in graphene in forming gas through carbon hydrogenation, and the width of the channel is much narrower than the diameter of Cu particle due to the non-wetting behavior of Cu on graphene. The weak interactions between Cu and graphene, along with the low solubility of carbon in Cu, make Cu particles ideal for tracking their etching paths on graphene. This work provides new insights into the metal-catalyzed etching of graphene.

Fabrication of Gallium Phosphide Tapered Nanostructures on Selective Surfaces

We present tapered nanostructures fabricated on a selective area of gallium phosphide substrates for advanced optoelectronic device applications. A lithography-free fabrication process was accomplished by dry etching of metal nanoparticles. Thermal dewetting of micro-patterned metal thin films provides etch masks for tapered nanostructures. This simple process also allows the formation of plasmonic surfaces with corrugated shapes. Rigorous coupled-wave analysis calculations provide design guidelines for tapered nanostructures on gallium phosphide substrates.

Kinetically engendered subspinodal length scales in spontaneous dewetting of thin liquid films.

Numerical simulations reveal emergence of subspinodal length scales in spontaneous dewetting of nonslipping unstable thin liquid films on homogeneous substrates if the liquid viscosity decreases with decrease in film thickness.

Nanoparticle-decorated ceramic as substrate in surface enhanced Raman spectroscopy

Laser-assisted method for fabrication of nanoparticles on the surface of ceramic substrate is presented. The method is based on laser dewetting of thin metal film deposited on the substrate. Using this technique, gold nanoparticles on alumina ceramic surface are fabricated. It is shown that the processing parameters can influence the properties of the fabricated structure, as at certain conditions a homogeneous cover of nanoparticles with narrow size distribution can be achieved. The processed surfaces are tested as substrates in surface enhanced Raman spectroscopy of a standard dye. The efficiency of the substrates is estimated by the ability of detection of the dye at nM concentration and it is compared to that of thin gold film on ceramic substrate. The strongest enhancement is observed for the sample where Au film is not completely decomposed and the structure consists of thin film and nanoparticles aggregates. The proposed method has potential application in fabrication of cheap and reliable substrates for Raman spectroscopy analysis with high sensitivity and also in catalysis.

Asymmetrically Coupled Plasmonic Core and Nanotriplet Satellites

Here, we report asymmetrical multiple electromagnetic coupling of plasmonic core and nanotriplet satellites. Within the plasmonic core and nanotriplet satellites, an enhanced local field is generated which expands across the core due to multiple electromagnetic coupling between a core and nanotriplets. Based on 3D simulations of our plasmonic nanosystem, the overall local field enhancement reaches to over 104 times, compared with that of a single nanoparticle array. A strong local field distribution across the core to nanotriplets as well as the critical role of the plasmonic core is demonstrated through the 3D simulations. It is proposed that a self-assembled nanotriplet array is completed through two stages of dewetting of a gold thin film on an anodic aluminum oxide (AAO) template. Formation of the core–nanotriplet satellites is significantly influenced by geometrical parameters (i.e., the pore diameter and depth) of the AAO template. Our experimental results show that the local field of our plasmonic ...