Kyle Mahady

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.

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.

A volume of fluid method for simulating fluid/fluid interfaces in contact with solid boundaries

In this paper, we present a novel approach to model the fluid/solid interaction forces in a direct solver of the Navier-Stokes equations based on the volume of fluid interface tracking method. The key ingredient of the model is the explicit inclusion of the fluid/solid interaction forces into the governing equations. We show that the interaction forces lead to a partial wetting condition and in particular to a natural definition of the equilibrium contact angle. We present two numerical methods to discretize the interaction forces that enter the model; these two approaches differ in complexity and convergence. To validate the computational framework, we consider the application of these models to simulate two-dimensional drops at equilibrium, as well as drop spreading. We demonstrate that the model, by including the underlying physics, captures contact line dynamics for arbitrary contact angles. More generally, the approach permits novel means to study contact lines, as well as a diverse range of phenomena that previously could not be addressed in direct simulations.

Comparison of Navier-Stokes simulations with long-wave theory: Study of wetting and dewetting

The classical long-wave theory (also known as lubrication approximation) applied to fluid spreading or retracting on a solid substrate is derived under a set of assumptions, typically including small slopes and negligible inertial effects. In this work, we compare the results obtained by using the long-wave model and by simulating directly the full two-phase Navier-Stokes equations employing a volume-of-fluid method. In order to isolate the influence of the small slope assumption inherent in the long-wave theory, we present a quantitative comparison between the two methods in the regime where inertial effects and the influence of gas phase are negligible. The flow geometries that we consider include wetting and dewetting drops within a broad range of equilibrium contact angles in planar and axisymmetric geometries, as well as liquid rings. For perfectly wetting spreading drops we find good quantitative agreement between the models, with both of them following rather closely Tanners law. For partially wet...

Instability of nano- and microscale liquid metal filaments: Transition from single droplet collapse to multidroplet breakup

We carry out experimental and numerical studies to investigate the collapse and breakup of finite size, nano- and microscale, liquid metal filaments supported on a substrate. We find the critical dimensions below which filaments do not break up but rather collapse to a single droplet. The transition from collapse to breakup can be described as a competition between two fluid dynamic phenomena: the capillary driven end retraction and the Rayleigh-Plateau type instability mechanism that drives the breakup. We focus on the unique spatial and temporal transition region between these two phenomena using patterned metallic thin film strips and pulsed-laser-induced dewetting. The experimental results are compared to an analytical model proposed by Driessen et al. and modified to include substrate interactions. In addition, we report the results of numerical simulations based on a volume-of-fluid method to provide additional insight and highlight the importance of liquid metal resolidification, which reduces inertial effects.

A numerical approach for the direct computation of flows including fluid-solid interaction: Modeling contact angle, film rupture, and dewetting

In this paper, we present a computationally efficient method for including fluid-solid interactions into direct numerical simulations of the Navier–Stokes equations. This method is found to be as powerful as our earlier formulation [K. Mahady et al., “A volume of fluid method for simulating fluid/fluid interfaces in contact with solid boundaries,” J. Comput. Phys. 294, 243 (2015)], while outperforming the earlier method in terms of computational efficiency. The performance and efficacy of the presented method are demonstrated by computing contact angles of droplets at equilibrium. Furthermore, we study the instability of films due to destabilizing fluid-solid interactions, and discuss the influence of contact angle and inertial effects on film breakup. In particular, direct simulation results show an increase in the final characteristic length scales when compared to the predictions of a linear stability analysis, suggesting significant influence of nonlinear effects. Our results also show that emerging l...

Laser-assisted focused He+ ion beam induced etching with and without XeF2 gas assist

Focused helium ion (He+) milling has been demonstrated as a high-resolution nanopatterning technique; however, it can be limited by its low sputter yield as well as the introduction of undesired subsurface damage. Here, we introduce pulsed laser- and gas-assisted processes to enhance the material removal rate and patterning fidelity. A pulsed laser-assisted He+ milling process is shown to enable high-resolution milling of titanium while reducing subsurface damage in situ. Gas-assisted focused ion beam induced etching (FIBIE) of Ti is also demonstrated in which the XeF2 precursor provides a chemical assist for enhanced material removal rate. Finally, a pulsed laser-assisted and gas-assisted FIBIE process is shown to increase the etch yield by ∼9× relative to the pure He+ sputtering process. These He+ induced nanopatterning techniques improve material removal rate, in comparison to standard He+ sputtering, while simultaneously decreasing subsurface damage, thus extending the applicability of the He+ probe as a nanopattering tool.

On the influence of initial geometry on the evolution of fluid filaments

In this work, the influence of the initial geometry on the evolution of a fluid filament deposited on a substrate is studied, with a particular focus on the thin fluid strips of nano-scale thickness. Based on the analogy to the classical Rayleigh–Plateau (R–P) instability of a free-standing fluid jet, an estimate of the minimal distance between the final states (sessile droplets) can be obtained. However, this numerical study shows that while the prediction based on the R–P instability mechanism is highly accurate for an initial perturbation of a sinusoidal shape, it does not hold for a rectangular waveform perturbation. The numerical results are obtained by directly solving fully three-dimensional Navier–Stokes equations, based on a Volume of Fluid interface tracking method. The results show that (i) rectangular-wave perturbations can lead to the formation of patterns characterized by spatial scales that are much smaller than what is expected based on the R–P instability mechanism; (ii) the nonlinear sta...

Review Article: Advanced nanoscale patterning and material synthesis with gas field helium and neon ion beams

Focused ion beam nanoscale synthesis has emerged as a critical tool for selected area nanofabrication. Helium and neon ion beams from the gas field ion source have recently demonstrated unparalleled resolution among other scanning ion beams. In this review, the authors focus on the nanoscale synthesis applications for these ion species which have been demonstrated to date. The applications and recent work can broadly be grouped into the following categories: (1) Monte Carlo simulations, (2) direct-write milling or sputtering, (3) ion beam lithography, (4) selective ion implantation or defect introduction, and (5) gas-assisted processing. A special emphasis is given toward using He+ and Ne+ for the processing of two dimensional materials, as several groups have demonstrated promising results. Finally, the authors will discuss the future outlook of He+ and Ne+ nanoprocessing techniques and applications.

Room-Temperature Activation of InGaZnO Thin-Film Transistors via He+ Irradiation

Amorphous indium gallium zinc oxide (a-IGZO) is a transparent semiconductor which has demonstrated excellent electrical performance as thin-film transistors (TFTs). However, a high-temperature activation process is generally required which is incompatible for next-generation flexible electronic applications. In this work, He+ irradiation is demonstrated as an athermal activation process for a-IGZO TFTs. Controlling the He+ dose enables the tuning of charge density, and a dose of 1 × 1014 He+/cm2 induces a change in charge density of 2.3 × 1012 cm-2. Time-dependent transport measurements and time-of-flight secondary ion mass spectroscopy (ToF-SIMS) indicate that the He+-induced trapped charge is introduced because of preferential oxygen-vacancy generation. Scanning microwave impedance microscopy confirms that He+ irradiation improves the conductivity of the a-IGZO. For realization of a permanent activation, IGZO was exposed with a He+ dose of 5 × 1014 He+/cm2 and then aged 24 h to allow decay of the trapped oxide charge originating for electron-hole pair generation. The resultant shift in the charge density is primarily attributed to oxygen vacancies generated by He+ sputtering in the near-surface region.