Sangwoo Shin

Control of Superhydrophilicity/Superhydrophobicity using Silicon Nanowires via Electroless Etching Method and Fluorine Carbon Coatings

Surface roughness is promotive of increasing their hydrophilicity or hydrophobicity to the extreme according to the intrinsic wettability determined by the surface free energy characteristics of a base substrate. Top-down etched silicon nanowires are used to create superhydrophilic surfaces based on the hemiwicking phenomenon. Using fluorine carbon coatings, surfaces are converted from superhydrophilic to superhydrophobic to maintain the Cassie-Baxter state stability by reducing the surface free energy to a quarter compared with intrinsic silicon. We present the robust criteria by controlling the height of the nanoscale structures as a design parameter and design guidelines for superhydrophilic and superhydrophobic conditions. The morphology of the silicon nanowires is used to demonstrate their critical height exceeds several hundred nanometers for superhydrophilicity, and surpasses a micrometer for superhydrophobicity. Especially, SiNWs fabricated with a height of more than a micrometer provide an effective means of maintaining superhydrophilic (<10°) long-term stability.

Size-dependent control of colloid transport via solute gradients in dead-end channels

Significance Dead-end geometries are commonly found in many porous systems. Particle transport into such dead-end pores is often important, but is difficult to achieve owing to the confinement. It is natural to expect that Brownian motion is the sole mechanism to deliver the particles into the pores, but that mechanism is, unfortunately, slow and inefficient. Here, we introduce solute gradient to control the transport of particles in dead-end channels. We demonstrate a size effect such that larger particles tend to focus more and reside deeper in the channels. Our findings suggest a potential pathway to many useful applications that are difficult to achieve in dead-end geometries such as particle sorting and sample preconcentration, which are important in pharmaceuticals and healthcare industries. Transport of colloids in dead-end channels is involved in widespread applications including drug delivery and underground oil and gas recovery. In such geometries, Brownian motion may be considered as the sole mechanism that enables transport of colloidal particles into or out of the channels, but it is, unfortunately, an extremely inefficient transport mechanism for microscale particles. Here, we explore the possibility of diffusiophoresis as a means to control the colloid transport in dead-end channels by introducing a solute gradient. We demonstrate that the transport of colloidal particles into the dead-end channels can be either enhanced or completely prevented via diffusiophoresis. In addition, we show that size-dependent diffusiophoretic transport of particles can be achieved by considering a finite Debye layer thickness effect, which is commonly ignored. A combination of diffusiophoresis and Brownian motion leads to a strong size-dependent focusing effect such that the larger particles tend to concentrate more and reside deeper in the channel. Our findings have implications for all manners of controlled release processes, especially for site-specific delivery systems where localized targeting of particles with minimal dispersion to the nontarget area is essential.

Micro-nano hybrid structures with manipulated wettability using a two-step silicon etching on a large area

Nanoscale surface manipulation technique to control the surface roughness and the wettability is a challenging field for performance enhancement in boiling heat transfer. In this study, micro-nano hybrid structures (MNHS) with hierarchical geometries that lead to maximizing of surface area, roughness, and wettability are developed for the boiling applications. MNHS structures consist of micropillars or microcavities along with nanowires having the length to diameter ratio of about 100:1. MNHS is fabricated by a two-step silicon etching process, which are dry etching for micropattern and electroless silicon wet etching for nanowire synthesis. The fabrication process is readily capable of producing MNHS covering a wafer-scale area. By controlling the removal of polymeric passivation layers deposited during silicon dry etching (Bosch process), we can control the geometries for the hierarchical structure with or without the thin hydrophobic barriers that affect surface wettability. MNHS without sidewalls exhibit superhydrophilic behavior with a contact angle under 10°, whereas those with sidewalls preserved by the passivation layer display more hydrophobic characteristics with a contact angle near 60°.

The Race of Nanowires: Morphological Instabilities and a Control Strategy

The incomplete growth of nanowires that are synthesized by template-assisted electrodeposition presents a major challenge for nanowire-based devices targeting energy and electronic applications. In template-assisted electrodeposition, the growth of nanowires in the pores of the template is complex and unstable. Here we show theoretically and experimentally that the dynamics of this process is diffusion-limited, which results in a morphological instability driven by a race among nanowires. Moreover, we use our findings to devise a method to control the growth instability. By introducing a temperature gradient across the porous template, we manipulate ion diffusion in the pores, so that we can reduce the growth instability. This strategy significantly increases the length of nanowires. In addition to shedding light on a key nanotechnology, our results may provide fundamental insights into a variety of interfacial growth processes in materials science such as crystal growth and tissue growth in scaffolds.

Variations in mechanical and thermal properties of mesoporous alumina thin films due to porosity and ordered pore structure.

Ordered mesoporous aluminum oxide films with porosity ranging between 5% and 37% were synthesized by evaporation-induced self-assembly (EISA) using surfactant templating. To investigate the effects of mesoporous structure on thermal properties, changes in pore structure including pore size, pore distribution, and porosity were monitored as a function of surfactant concentration (Pluronic P-123, poly(ethylene oxide)(20)-poly(propylene oxide)(70)-poly(ethylene oxide)(20)). The ordered mesoporous alumina films were then examined to determine how their morphology influences their thermal properties. These alumina films had a body-centered cubic pore structure or a random-oriented pore structure, depending on the surfactant concentration used, and superior thermal properties were obtained by controlling porosity and pore structure. Therefore, the ordered mesoporous alumina films synthesized in this study can be used as high-temperature thermo-isolating materials.

Tuning the morphology of copper nanowires by controlling the growth processes in electrodeposition

We present an alternative route to fabricate Cu nanowires having various classes of morphologies by controlling the deposition temperature. A rough nanowire with an irregular wire diameter along the wire axis is obtained at a high deposition temperature, whereas a smooth, compact nanowire is obtained as the temperature is lowered. However, as the temperature is dropped further down to subzero degrees, an unusual behavior is observed where the nanowires exhibit rough, dendritic morphologies with relatively uniform wire diameters. We explain this peculiar growth behavior on the basis of kinetic and thermodynamic growth processes.

Phase-dependent thermal conductivity of Ge1Sb4Te7 and N: Ge1Sb4Te7 for phase change memory applications

We report the thermal conductivities of Ge1Sb4Te7 and nitrogen-doped Ge1Sb4Te7 thin films at temperatures ranging from 300 to 520 K using the 3ω method. Thermal conductivity of Ge1Sb4Te7 increases abruptly during the transition from the amorphous to crystalline phase. Nitrogen doping effectively suppresses the crystallization process, resulting reduction of lattice as well as electronic thermal conductivity. These behaviors are confirmed by x-ray diffraction, sheet conductance, and thermal conductivity measurements. Numerical modeling of phase change memory device shows that with nitrogen doping, performance increase in terms of lower reset current and faster reset time can be achieved.

Over 95% of large-scale length uniformity in template-assisted electrodeposited nanowires by subzero-temperature electrodeposition

In this work, we report highly uniform growth of template-assisted electrodeposited copper nanowires on a large area by lowering the deposition temperature down to subzero centigrade. Even with highly disordered commercial porous anodic aluminum oxide template and conventional potentiostatic electrodeposition, length uniformity over 95% can be achieved when the deposition temperature is lowered down to -2.4°C. Decreased diffusion coefficient and ion concentration gradient due to the lowered deposition temperature effectively reduces ion diffusion rate, thereby favors uniform nanowire growth. Moreover, by varying the deposition temperature, we show that also the pore nucleation and the crystallinity can be controlled.

Double-templated electrodeposition: Simple fabrication of micro-nano hybrid structure by electrodeposition for efficient boiling heat transfer

Micro-nano hybrid structure (MNHS) that comprises of microcavities and nanowires is a specific class of MNHS that is considered to be ideal for two-phase boiling heat transfer applications. Realizing MNHS with electrodeposition is favorable in boiling heat transfer, but the realization has been very difficult and time-consuming to achieve. Here, we demonstrate a simple, robust, rapid, and photolithography-free route to fabricate MNHS that consists of individual microcavities and copper nanowires on a large area. We show that this MNHS can be extremely beneficial in boiling heat transfer compared to the state-of-the-art nanowire surface.

A facile route for the fabrication of large-scale gate-all-around nanofluidic field-effect transistors with low leakage current{

Active modulation of ions and molecules via field-effect gating in nanofluidic channels is a crucial technology for various promising applications such as DNA sequencing, drug delivery, desalination, and energy conversion. Developing a rapid and facile fabrication method for ionic field-effect transistors (FET) over a large area may offer exciting opportunities for both fundamental research and innovative applications. Here, we report a rapid, cost-effective route for the fabrication of large-scale nanofluidic field-effect transistors using a simple, lithography-free two-step fabrication process that consists of sputtering and barrier-type anodization. A robust alumina gate dielectric layer, which is formed by anodizing sputtered aluminium, can be rapidly fabricated in the order of minutes. When anodizing aluminium, we employ a hemispherical counter electrode in order to give a uniform electric field that encompasses the whole sputtered aluminium layer which has high surface roughness. In consequence, a well-defined thin layer of alumina with perfect step coverage is formed on a highly rough aluminium surface. A gate-all-around nanofluidic FET with a leak-free gate dielectric exhibits outstanding gating performance despite a large channel size. The thin and robust anodized alumina gate dielectric plays a crucial role in achieving such excellent capacitive coupling. The combination of a gate-all-around structure with a leak-free gate dielectric over a large area could yield breakthroughs in areas ranging from biotechnology to energy and environmental applications.