Dahl Young Khang

Finite deformation mechanics in buckled thin films on compliant supports

We present detailed experimental and theoretical studies of the mechanics of thin buckled films on compliant substrates. In particular, accurate measurements of the wavelengths and amplitudes in structures that consist of thin, single-crystal ribbons of silicon covalently bonded to elastomeric substrates of poly(dimethylsiloxane) reveal responses that include wavelengths that change in an approximately linear fashion with strain in the substrate, for all values of strain above the critical strain for buckling. Theoretical reexamination of this system yields analytical models that can explain these and other experimental observations at a quantitative level. We show that the resulting mechanics has many features in common with that of a simple accordion bellows. These results have relevance to the many emerging applications of controlled buckling structures in stretchable electronics, microelectromechanical systems, thin-film metrology, optical devices, and others.

Nanotransfer printing by use of noncovalent surface forces: Applications to thin-film transistors that use single-walled carbon nanotube networks and semiconducting polymers

We report a purely additive nanotransfer printing process that uses noncovalent surface forces to guide the transfer of thin metal films from low-energy surfaces of high-resolution stamps to a variety of substrates. Structures with dimensions as small as a few hundred nanometers, with edge roughness as small as 10nm are demonstrated. Metal multilayer stacks patterned in this way have electrical resistances that are the same as those formed by evaporation and conventional lithography. Thin-film transistors that use source/drain electrodes printed directly onto thin films of the semiconducting polymer regioregular polythiophene and networks of single-walled carbon nanotubes exhibit device mobilities and on/off ratios that are comparable to or higher than those of devices fabricated using standard methods.

Synthesis of linked carbon monolayers: Films, balloons, tubes, and pleated sheets

Because of their potential for use in advanced electronic, nanomechanical, and other applications, large two-dimensional, carbon-rich networks have become an important target to the scientific community. Current methods for the synthesis of these materials have many limitations including lack of molecular-level control and poor diversity. Here, we present a method for the synthesis of two-dimensional carbon nanomaterials synthesized by Mo- and Cu-catalyzed cross-linking of alkyne-containing self-assembled monolayers on SiO2 and Si3N4. When deposited and cross-linked on flat surfaces, spheres, cylinders, or textured substrates, monolayers take the form of these templates and retain their structure on template removal. These nanomaterials can also be transferred from surface to surface and suspended over cavities without tearing. This approach to the synthesis of monolayer carbon networks greatly expands the chemistry, morphology, and size of carbon films accessible for analysis and device applications.

Hydrophobically recovered and contact printed siloxane oligomers for general-purpose surface patterning.

Hydrophobic recovery of elastomeric polydimethylsiloxane (PDMS) has been well-known in various fields, such as microcontact printing (microCP), microfluidics, and electric insulation, etc., which has been believed to be due to the transfer of out-diffused siloxane oligomers in PDMS. The recovery phenomenon has been used to control surface energy of a substrate, due partly to its nanoscale thickness. In this work, we extend the use of recovered oligomers to a general-purpose surface patterning process, in combination with both dry and wet pattern transfer processes. The out-diffused and transfer-printed oligomers play exactly the same role of ink in the conventional microCP; thus, the present method can be termed as inkless microcontact printing (ImicroCP). Also, the detailed nature of recovered oligomers has been investigated, and they are found to have a molecular weight approximately 10 times larger than that of pristine, uncured PDMS oligomers. And the molecular weight distribution is very broad with a polydispersity index of approximately 15. Then, we present and discuss various aspects of the ImicroCP process, such as pattern transfer onto substrate via wet or dry etching, effect of process variables on printing results, minimum feature size achieved by the technique, repeated printing with the same stamp, and the generation of more complex patterns from simpler ones by applying multiple ImicroCP.

Crystallization and microstructure-dependent elastic moduli of ferroelectric P(VDF–TrFE) thin films

The crystallization behaviour of ferroelectric P(VDF–TrFE) copolymer thin films on various substrates has been investigated. The substrates used include bare Si for a high energy surface, and fluorinated self-assembled monolayer deposited Si and elastomer PDMS for low energy surfaces. Low energy surfaces have been found to enhance the crystallization rate. Additionally, the elastic moduli of those thin films, having thicknesses ranging from ∼30 nm to ∼1 μm, have been determined by the buckling method. The moduli have been found to depend strongly on the films microstructure, and have shown a maximum at the crystallization temperatures of 120 °C–140 °C. The films annealed at these temperatures show more than a 5× increase in modulus than that of the room-temperature dried ones. The dependence of the modulus on the film microstructure has been found to be closely related to the change in film crystallinity. The measured moduli of P(VDF–TrFE) thin films can help with the design and practical implementation of emerging applications such as flexible nonvolatile memories and piezoelectric energy harvesting devices.

Glass and Plastics Platforms for Foldable Electronics and Displays

Reversibly, repeatedly foldable electronics and displays are enabled by employing engineered glass or plastics substrates, where folding deformation is localized in thinned parts only. This design concept can further be extended to dual folding, leading to size reduction down to 1/4. Notably, the foldable electronics and displays can be implemented with no need to introduce any novel materials.

Nonsinusoidal buckling of thin gold films on elastomeric substrates

Buckling of stiff thin films on compliant substrates represents a variety of applications, ranging from stretchable electronics to micro-nanometrology. Different but complementary to previously reported sinusoidal buckling waves, this letter presents a nonsinusoidal surface profile of buckled thin Au films on compliant substrates, specifically, a secondary dip on top of buckling wave or rather broadened wave top with very sharp trough. This nonsinusoidal profile is likely due to tension/compression asymmetry, i.e., different strengths in tension and compression resulted from the polycrystalline, grained microstructure of metal film. Finite element analysis with asymmetric tension/compression material model has reproduced the experiments well qualitatively.

Direct Visualization of Etching Trajectories in Metal-Assisted Chemical Etching of Si by the Chemical Oxidation of Porous Sidewalls

We demonstrate a simple method for the visualization of trajectories traced by noble metal nanoparticles during metal-assisted chemical etching (MaCE) of Si. The nanoporous Si layer formed around drilled pores is converted into SiO2 by simple chemical oxidation. Etch removal of the remaining Si using alkali hydroxide leaves SiO2 nanostructures that are the exact replica of those drilled pores or etching trajectories. The differences in etching characteristics between Ag and Au have been investigated using the proposed visualization method. The shape and chemical stability of metal nanoparticles used for MaCE have been found to be critical in determining etching paths. The proposed method would be very helpful in studying the fundamental mechanism of MaCE as well as in micro/nanostructuring of the Si surface for various applications. This approach can also be used for the generation of straight or helical SiO2 nanotubes.

Characterization and theoretical analysis of isoporous cycloaliphatic polyurethane membrane for water treatment

AbstractA narrow pore size distribution is critical for most membrane separation processes even though it is difficult to achieve through conventional methods of membrane synthesis such as phase inversion. Although various technologies have been proposed for the preparation of isoporous membranes, few technologies are available for use in water treatment applications. In our previous works, we have prepared isoporous membranes using a novel technique based on soft lithography. A micro pattern of pyramid shape was applied to produce uniform pores. In this work, we aimed at the characterization of these isoporous membranes using both experimental and theoretical methods. The pore size distributions of the membranes were characterized by scanning electron microscopy image analysis. Using this image analysis technique, the pore size ranging from 2.2 to 21.4u2009μm could be identified. A simple theoretical model was developed to determine the pore size, porosity, and membrane resistance of the isoporous membranes....

Facile generation of surface structures having opposite tone in metal-assisted chemical etching of Si: pillars vs. holes

Easy control on etched surface structures, either pillars or holes, in metal-assisted chemical etching (MaCE) of Si is presented. Starting with Ag mesh film with holes, which is generated by Ag evaporation in combination with a colloidal monolayer mask, the usual MaCE process has led to the vertical arrays of Si pillars. On the other hand, O2 reactive ion etching (RIE) on such Ag mesh film has led to sputtering and re-deposition of Ag on empty sites. During the following MaCE, the damaged Ag mesh spontaneously delaminates off the substrate and thus the etching with remaining Ag dot patterns results in holes array. Mechanistic studies of these etching behaviours have been performed and thus optimized, including the spontaneous mesh delamination. The obtained Si surfaces, either pillars or holes, were found to have comparable anti-reflection properties to other texturing approaches, which would be useful for photon management in applications such as photovoltaics, photoelectrochemical processes, etc.