Dewetting Of Thin Films Research Articles

De-wetted gold nanostructures for SERS-based sensing of static and dynamic targets

De-wetting of metal thin film is one of the simplest fabrication techniques to produce metallic nanostructures for Surface-Enhanced Raman Spectroscopy (SERS)-based sensing. In this manuscript, we explore the sensitivity of a de-wetted metal SERS substrate by testing it with the organic dye Rhodamine 6G (R6G) as well as with the single amino acids (Glycine, Serine, and Histidine) and a binary mixture of Glycine and Serine mixture. We observe that repeated de-wetting of the metal thin film provides a more than ten times higher SERS enhancement factor compared to a single de-wetting. This is attributed to the presence of more closely spaced metal nanoclusters, enabling a higher electromagnetic field enhancement at the hotspots. We characterized the sensing capability of our SERS substrates by detecting R6G dye drop-casted onto the substrates at sub-femtomolar concentrations. We were also able to detect R6G in a dynamic state while it was transported across a 500 nm diameter pore with a nanomolar detection limit. Finally, we were also able to detect amino acids with a micromolar detection limit using our SERS substrate. These results indicate the promise in the integration of our SERS substrate with solid-state nanopores for SERS-based signal readout where concentrations higher than nanomolar exist in the pore due to high confinement.

Catalytic particles formation from thin nickel films for the synthesis of multi-walled carbon nanotubes

We have investigated the formation of a catalyst for the synthesis of multi-walled carbon nanotubes (MWCNT) using nickel-on-titanium thin films. Nickel decomposes into nanoparticles in the synthesis setup prior to MWCNT growing through the plasma enhanced chemical vapor deposition method. Oxidation and reduction of the catalyst are conducted to control the morphology of the formed nanoparticles array. In this case, the redox treatments required preventing the formation of silicides and Ti–Ni alloys are combined with the well-known effect of low-temperature disintegration (dewetting) of thin films. Detailed studies using a combination of Auger, atomic force, scanning (SEM), and transmission electron microscopy (TEM) show the interaction of metals on a silicon substrate, the influence of treatments parameters on the catalyst's state. The features and morphology of MWCNT synthesized on the obtained catalysts were studied via Raman spectroscopy, SEM and TEM. Overall, showing a link between the catalyst formation process and MWCNT growth, we demonstrate a technological and versatile method for controlling of nanotube growth for various applications.

Uniform large-area surface patterning achieved by metal dewetting for the top-down fabrication of GaN nanowire ensembles

The dewetting of thin Pt films on different surfaces is investigated as a means to provide the patterning for the top-down fabrication of GaN nanowire ensembles. The transformation from a thin film to an ensemble of nanoislands upon annealing proceeds in good agreement with the void growth model. With increasing annealing duration, the size and shape uniformity of the nanoislands improves. This improvement speeds up for higher annealing temperature. After an optimum annealing duration, the size uniformity deteriorates due to the coalescence of neighboring islands. By changing the Pt film thickness, the nanoisland diameter and density can be quantitatively controlled in a way predicted by a simple thermodynamic model. We demonstrate the uniformity of the nanoisland ensembles for an area larger than 1 cm2. GaN nanowires are fabricated by a sequence of dry and wet etching steps, and these nanowires inherit the diameters and density of the Pt nanoisland ensemble used as a mask. Our study achieves advancements in size uniformity and range of obtainable diameters compared to previous works. This simple, economical, and scalable approach to the top-down fabrication of nanowires is useful for applications requiring large and uniform nanowire ensembles with controllable dimensions.

Surface modification of a commercial bone plate (Ti6Al4V) implant for improved antibacterial and cytocompatibility via thermal dewetting of a silver thin film

The high demand for bone grafts has motivated the development of implants with excellent osteogenic activity, whereas the risk of implant-associated infection, particularly given the rise of antimicrobial resistance, has compelled the development of implants with innovative antimicrobial strategies in which a small amount of bactericidal agent can effectively kill a wide range of bacteria. To induce antibacterial property, the surface of Grade-5 bone plate titanium implants used in clinical applications was modified using direct current (DC) sputter coating followed by thermal annealing. The 15 nm silver film-coated implants were thermally annealed in the furnace for 15 min at 750 °C. The modified implant surface’s antibacterial efficacy against Escherichia coli (E. coli), Staphylococcus aureus (S. aureus), Salmonella typhi, and Methicillin-resistant staphylococcus aureus bacteria has been assessed using a colony-forming assay. On the modified implant surface, the growth of E. coli and S. aureus bacteria is reduced by 99.72%, while highly drug-resistant bacteria are inhibited by 96.59%. The MTT assay was used to assess the cytotoxicity of the modified bone-implant surface against NIH3T3 mouse fibroblast cells. The modified bone-implant surface promoted fibroblast growth and demonstrated good cytocompatibility. Furthermore, the mechanical properties of the implant were not harmed by this novel surface modification method. This method is simple and provides new insight into surface modification of commercial metallic implants to have effective antibacterial properties against various classes of bacteria.

Creation of Multi-Principal Element Alloy NiCoCr Nanostructures via Nanosecond Laser-Induced Dewetting.

The multi-principal element alloynanoparticles (MPEA NPs), a new class of nanomaterials, present a highly rewarding opportunity to explore new or vastly different functional properties than the traditional mono/bi/multimetallic nanostructures due to their unique characteristics of atomic-level homogeneous mixing of constituent elements in the nanoconfinements. Here, the successful creation of NiCoCr nanoparticles, a well-known MPEA system is reported, using ultrafast nanosecond laser-induced dewetting of alloy thin films. Nanoparticle formation occurs by spontaneously breaking the energetically unstable thin films in a melt state under laser-induced hydrodynamic instability and subsequently accumulating in a droplet shape via surface energy minimization. While NiCoCr alloy shows a stark contrast in physical properties compared to individual metallic constituents, i.e., Ni, Co, and Cr, yet the transient nature of the laser-driven process facilitates a homogeneous distribution of the constituents (Ni, Co, and Cr) in the nanoparticles. Using high-resolution chemical analysis and scanning nanodiffraction, the environmental stability and grain arrangement in the nanoparticles are further investigated. Thermal transport simulations reveal that the ultrashort (≈100ns) melt-state lifetime of NiCoCr during the dewetting event helps retain the constituent elements in a single-phase solid solution with homogenous distribution and opens the pathway to create the unique MPEA nanoparticles with laser-induced dewetting process.

Decomposition and dewetting of super-saturated Cu-15 at. % Co solid solution film

Nano-structured copper thin films present numerous applications for miniaturization of electronic devices. Their long term stability is key for reliable functionality. This work explores the thermal stability and solid state dewetting of a Cu-Co solid solution thin film and compares it to pure Cu. As according to the phase diagram Co is practically immiscible in Cu at room temperature, a metastable solid solution of Cu-15 at. % Co was deposited as thin film on a sapphire wafer. The microstructural evolution of the Co super-saturated film at temperatures ranging from 673 to 1073 K was evaluated using X-ray diffraction and high resolution microscopy techniques such as scanning electron and transmission electron microscopy. Interestingly, there is a competition between grain growth and phase separation. Co precipitates at grain boundaries acting as pinning sites preventing rapid grain growth during heat treatment at intermediate annealing temperature. However, grain growth occurs very quickly at higher temperatures, once the elemental phases of Cu and Co are formed. The competition between grain growth and phase separation and their consequence for dewetting are discussed.

Extreme Dewetting Resistance and Improved Visible Transmission of Ag Layers Using Sub-Nanometer Ti Capping Layers.

As technology development drives the thickness of thin film depositions down into the nano regime, understanding and controlling the dewetting of thin films has become essential for many applications. The dewetting of ultra-thin Ag (9 nm) films with Ti (0.5 nm) adhesion and capping layers on glass substrates was investigated in this work. Various thin film stacks were created using magnetron sputtering and were analyzed using scanning electron microscopy/energy dispersive X-rays, Vis/IR spectrometry, and four four-point probe resistivity measurements. Upon annealing for 5 h in air at 250 °C, the addition of a 0.5 nm thick Ti capping layer reduced the dewet area by an order of magnitude. This is reflected in film resistivity, which remained 2 orders of magnitude lower than uncapped variants. This Ti/Ag/Ti structure was then deployed in a typical low-emissivity window coating structure with additional antireflective layers of AZO, resulting in a superior performance upon annealing. These results demonstrate an easy, manufacturable process that improves the longevity of devices and products containing thin Ag films.

Dewetting upside-down: two-sided solid state dewetting of thin gold film on soft KBr substrate

Abstract We deposited a 30 nm-thick Au film on single crystalline KBr substrate and studied the solid state dewetting behavior of the film at a temperature of 350 °C. At this temperature, the ions of the KBr compound exhibit significant mobility along the Au–KBr interface, which affects the morphology and kinetics of the solid state dewetting. We performed statistical morphology analysis of the Au–KBr interface by selectively dissolving the KBr substrate after the dewetting heat treatments and subsequent atomic force microscopy imaging of the “upside-down” oriented Au film. We demonstrated that atomic mobility at the interface leads to embedding of the partially dewetted Au film into the KBr substrate. We proposed a quantitative model of the shape evolution of a disc-shaped Au particle on the KBr substrate under the condition of finite interface mobility of the substrate species. The model predictions were consistent with the experimentally observed sinking rates of Au nanostructures.

Control of Self-Assembly and Elemental Mixing of AuNi Bimetallic Nanoparticles via Solid-State and Liquid-State Dewetting of Metal Thin Films

Immiscible Au-Ni alloy thin films undergo phase separation and dewetting because of thermodynamic and morphological instability at elevated temperatures below the miscibility gap. We report the formation and assembly of bimetallic nanoparticles (BNPs) on topographic Si templates. An ordered array of inverted pyramidal pits were produced via solid-state and liquid-state dewetting of a 12-nm-thick Au-Ni thin film by respectively using thermal annealing and laser irradiation. Upon direct thermal annealing at 600 and 800 <sup>o</sup>C, the thin film on the templates self-assembled into an ordered array of BNPs composed of Au-rich and Ni-rich sub-clusters in pits. But the relative proportions of the two sub-clusters varied with annealing temperature due to the additional formation of smaller Ni-rich NPs that were scattered around the BNPs. Laser irradiation of the film, in contrast, formed an ordered array of fully mixed alloy NPs on the template and left no other residues on the surface. Subsequent thermal annealing induced the elements within the NPs to segregate, resulting in Au-rich and Ni-rich sub-clusters. In brief, the combination of solid-state and liquidstate dewetting processes on a topographic template not only enabled the 2-dimesional self-assembly of BNPs but also allowed control of the mixing of alloying elements within the BNPs. These results offer insights into the tailored fabrication of BNPs, which have potential applications in bio-functional catalysts, and plasmonic and chemical sensors.

Hybrid gold-silver nanoparticles synthesis on a glass substrate using a nanosecond laser-induced dewetting of thin bimetallic films and their application in SERS

Hybrid gold-silver nanoparticles synthesis on a glass substrate using a nanosecond laser-induced dewetting of thin bimetallic films and their application in SERS

Regulating thermal diffusion of gold thin films at solid-state interfaces for site-selective decoration of gold nanoparticles on titania nanotubes as an efficient SERS sensing platform

Regulating thermal diffusion and dewetting of Au thin films at the confined ZnO/TiO2 interface favors an optimal distribution of the produced Au nanoparticles for forming localized electromagnetic field hot spots.

Solid state dewetting of semiconductor thin films: From fundamental studies to photonic applications

Here we propose to exploit the natural instability of thin solid films, i.e. solid state dewetting, to form regular patterns of monocrystalline atomically smooth Si, Si1-xGex and Ge nanostructures that cannot be realized with conventional methods. Additionally, the solid-state dewetting dynamics is guided by pre-patterning the sample by a combination of electron-beam lithography and reactive-ion etching, obtaining precise control over number, size, shape, and relative position of the final structures. Methods and structures will be optimized towards their exploitation mainly in photonic devices application (e.g. anti-reflection coatings, colour-filters, random lasers, quantum emitters and photonic sensors).

Solid-State Dewetting of Thin Au Films for Surface Functionalization of Biomedical Implants.

Biomaterial-centered infections of orthopedic implants remain a significant burden in the healthcare system due to sedentary lifestyles and an aging population. One approach to combat infections and improve implant osteointegration is functionalizing the implant surface with anti-infective and osteoinductive agents. In this framework, Au nanoparticles are produced on the surface of Ti-6Al-4V medical alloy by solid-state dewetting of 5 nm Au film and used as the substrate for the conjugation of a model antibiotic vancomycin via a mono-thiolated poly(ethylene glycol) linker. Produced Au nanoparticles on Ti-6Al-4V surface are equiaxed with a mean diameter 19.8 ± 7.2 nm, which is shown by high-resolution scanning electron microscopy and atomic force microscopy. The conjugation of the antibiotic vancomycin, 18.8 ± 1.3 nm-thick film, is confirmed by high resolution-scanning transmission electron microscopy and X-ray photoelectron spectroscopy. Overall, showing a link between the solid-state dewetting process and surface functionalization, we demonstrate a novel, simple, and versatile method for functionalization of implant surfaces.

Solid-State Dewetting of Thin Silver Films into Spherical Nanoparticles under High-Current Pulsed Ion Beam Irradiation.

Noble metal nanoparticles (NPs) are important in many applications, including light trapping of photovoltaic cells, photoelectrochemical applications, etc. The present study reports the formation of silver NPs from the as-deposited silver coatings on fused silica substrates by solid-state dewetting induced by high-current intense pulsed ion beam (IPIB) irradiation. We described the effects of IPIB irradiation with different ion beam current densities and numbers of pulses on NP morphology and compared the results with conventional rapid thermal annealing (RTA). IPIB irradiation enables superfast heating (higher than 109 K/s) and cooling, providing a superfast annealing solid-state dewetting mechanism. Our results demonstrate that the sphericity of silver NPs is enhanced after IPIB irradiation relative to RTA-annealed silver NPs. Our results suggest further possibilities of shape and sphericity control of silver NPs with very fast heating/cooling annealing rates.

Chiral Opto‐Fluidics and Plasmonic Nanostructures as a Functional Nanosystem for Manipulating Surface Deformations

AbstractPhotothermal conversion of energy in plasmonic nanostructures brought a new fascinating field of plasmon‐induced optofluidics to life. Notably, the heat generation by plasmonic nanostructures and consequent fluid convection recently attracted wide attention; however, the possibility of controlling and manipulating liquid surfaces has been sparsely investigated. The plasmon heating and convective fluid flow lead to the emergence of the surface tension gradient on a free liquid surface interface with air, resulting in the Marangoni effect‐driven fluid flow observed as water deformation. Here, nanoscale asymmetric fluid deformation anchored on the chiral plasmon‐induced optofluidic effect is reported on. To understand the fluid dynamics of surface deformation, a theoretical model is developed. The results demonstrate that deformation at different depths can be obtained by adjusting structural parameters or incident light intensity. The fundamental understanding of light‐induced asymmetric deformation will contribute to expanding chirality‐related discipline and open new avenues for controlling fluid flow at the micro‐ or nano‐scale. The proposed method can encourage the research and applications of optofluidics, including integrated fluidic devices, biochemistry, and clinical biology. Moreover, the use of the asymmetric fluid deformation on thermal dewetting of thin films of polymer solutions can be employed for the fabrication of functional and nanopatterned metal/polymer metasurfaces.

A symmetrized parametric finite element method for simulating solid-state dewetting problems

We propose an accurate, area/mass-conservative and energy-dissipative parametric finite element method for solving the sharp-interface continuum model of solid-state dewetting with arbitrary anisotropic surface energy. The model is governed by the anisotropic surface diffusion equation with suitable boundary conditions at the contact points, and could be used to describe the kinetic evolution of the film/vapor interface with contact line migration. The sharp-interface continuum model is firstly reformulated into a conservative form by introducing a symmetric positive definite matrix Zk(n) that depends on the Cahn–Hoffman ξ-vector and a stabilizing function k(n). A new symmetrized variational formulation with arbitrary anisotropic surface energy is built and then its area/mass conservation and energy dissipation properties are proved. By using piecewise linear elements in space, we propose the spatial semi-discretized scheme, and using backward Euler in time with an appropriate approximation of the unit normal vector, we further derive the fully discretized parametric finite element approximation. We show, theoretically, that the semi-discretized and fully discretized schemes are both area/mass-conservative and energy-dissipative. Finally, numerical experiments are reported to show the accuracy and efficiency of the numerical method as well as the anisotropic effects on the morphological evolution processes for solid-state dewetting of thin films.

The effect of cavities produced by focused ion beam milling on solid state dewetting of thin Au films

The goal of this study was to explore the possibility to control the nucleation and expansion of holes during initial stages of solid state dewetting of thin polycrystalline Au film deposited on sapphire substrate. To this end, we fabricated isolated triangular cavities in the film employing Ga+ ion milling in the focused ion beam instrument. The following dewetting heat treatments indicated that the cavities do promote early nucleation and expansion of dewetting holes, yet these holes develop tortuous shapes dominated by a random distribution of nearby hillocks in the film. Deep holes exhibited a triangular patch of amorphous Al-Ga-O phase at their bottom which retained its size and morphology during subsequent thermal treatments. This amorphous phase caused some delay in holes expansion at high temperatures. We proposed a geometrical model of the retracting metal film edge pinning by the amorphous phase. The observed pinning phenomenon can be utilized for the design of patterned thin films with increased thermal stability.

Electric Field Driven Reversible Spinodal Dewetting of Thin Liquid Films on Slippery Surfaces

AbstractThe stability of thin liquid films on a surface depends on the excess free energy of the system involving various short‐range and long‐range interactions. When forced to wet an unfavorable surface, thin liquid films dewet into multiple small‐sized droplets via spinodal, homogeneous, or heterogeneous nucleation process. However, if the total excess free energy of the system can be manipulated using external stimuli, for example, electric field, temperature, or light, one can control the stability of thin liquid films on demand. Here the electric‐field induced reversible dewetting and rewetting of thin liquid films underneath aqueous drops on slippery surfaces is studied. Upon applying voltage, hundreds of nanometer thick stable liquid films dewet in a manner identical to the spinodal dewetting of few nanometer‐thick liquid films following the linear stability analysis. Upon removing the applied voltage, the dewetted droplets spread, coalesce with neighboring ones, and form a uniform film again, albeit taking significantly longer times. The characteristic features defined via spatial and temporal evolution of the dewetting and rewetting processes are present over multiple cycles suggesting the complete reversibility of the process.

Solid-State Dewetting as a Driving Force for Structural Transformation and Magnetization Reversal Mechanism in FePd Thin Films.

In this work, the process of solid-state dewetting in FePd thin films and its influence on structural transformation and magnetic properties is presented. The morphology, structure and magnetic properties of the FePd system subjected to annealing at 600 °C for different times were studied. The analysis showed a strong correlation between the dewetting process and various physical phenomena. In particular, the transition between the A1 phase and L10 phase is strongly influenced by and inextricably connected with solid-state dewetting. Major changes were observed when the film lost its continuity, including a fast growth of the L10 phase, changes in the magnetization reversal behavior or the induction of magnetic spring-like behavior.

Experimental study of dewetting instability in polymer multilayer shear flow

<h2>Abstract</h2> The interfacial stability of multilayer Couette flow is investigated experimentally. A better understanding of this flow is critical for the processing of multilayer polymeric structures, such as multilayer coextrusion. We first look at the liquid film rupture predictions from numerical simulations of the thin film equation that considers capillary and van der Waals forces. With the addition of shear, three regimes have then been evidenced as a function of the shear rate. In the regime of low shear rates the rupture is delayed when compared to the no-shear problem, while at higher shear rates it is suppressed. Then, experiments investigating the effect of shear on the dewetting of thin polymer films at different temperatures by using an optical microscope coupled with a shearing hotstage will be presented. The dewetting dynamics, i.e. the growth of dewetting holes is monitored over time at various shear rates. It is observed that their circularity is modified by shear and that for all temperatures and thicknesses studied, the growth speed of the formed holes rapidly increases with increasing shear rate. A model balancing capillary forces and viscous dissipation while taking into account shear-thinning is then proposed and captures the main features of the experimental data, such as the ellipsoid shape of the holes and the faster dynamics in the direction parallel to the shear.