Youngkyu Hwang

Arrays of Silicon Micro/Nanostructures Formed in Suspended Configurations for Deterministic Assembly Using Flat and Roller‐Type Stamps

The ability to create and manipulate large arrays of inorganic semiconductor micro/nanostructures for integration on unconventional substrates provides new possibilities in device engineering. Here, simple methods are described for the preparation of structures of single crystalline silicon in suspended and tethered configurations that facilitate their deterministic assembly using transfer-printing techniques. Diverse shapes (e.g., straight or curved edges), thicknesses (between 55 nm and 3 μm), and sizes (areas of 4000 μm(2) to 117 mm(2) ) of structures in varied layouts (regular or irregular arrays, with dense or sparse coverages) can be achieved, using either flat or cylindrical roller-type stamps. To demonstrate the technique, printing with 100% yield onto curved, rigid supports of glass and ceramics and onto thin sheets of plastic is shown. The fabrication of a printed array of silicon p(+) -i-n(+) junction photodiodes on plastic is representative of device-printing capabilities.

Transfer of GaN LEDs From Sapphire to Flexible Substrates by Laser Lift-Off and Contact Printing

We fabricate flexible GaN-based light-emitting diode (LED) systems by laser lift-off (LLO) and transfer printing methods. LLO enables transferring a whole GaN LED layer from sapphire onto a silicon handling wafer to provide a stable platform for any shape of LED. Polymer pedestal structures underneath the LEDs support efficient transfer printing of the patterned LED array from the silicon handling wafer to a flexible substrate. We demonstrate the efficacy of the technique by presenting 9 × 9 LED arrays on polyethylene terephthalate and heart-shaped LED pixels on a piece of paper with transfer yields of 92% and 79%, as well as their successful illumination.

Sticker-Type Alq3-Based OLEDs Based on Printable Ultrathin Substrates in Periodically Anchored and Suspended Configurations

Introducing two-dimensional post arrays and a water-soluble sacrificial layer between an ultrathin substrate and a handling substrate provides controllability of the interfacial adhesion in a stable manner. The periodically anchored and suspended configuration after the chemical etching process facilitates the development of, for example, printable Alq3 -based OLEDs that can be attached to unconventional surfaces.

Secondary Sensitivity Control of Silver-Nanowire-Based Resistive-Type Strain Sensors by Geometric Modulation of the Elastomer Substrate

A secondary method for modulation of the sensitivity in silver nanowire (AgNW) resistive-type strain sensors without the need to change the material or coating process in the sensory layer is demonstrated. Instead of using a planar elastomer (polydimethylsiloxane is used in this study) substrate, diverse relief structures are introduced to induce nonuniform and complex strain within the elastic substrate and thereby different distributions of the crack density of the AgNWs upon stretching, which plays an important role in the modulation of the gauge factor (GF). Analysis of the sensory layer and mechanical studies reveal that a lower height ratio and greater number of trenches enhance the sensor sensitivity, for example, reaching a GF of 926 at 9.6% in this study. The demonstration of wrist-motion sensors using the technology illustrates the feasibility of using relief structures for various types of sensors and sensitivity ranges using an identical sensor layer.

Printable ultrathin substrates formed on a concave–convex underlayer for highly flexible membrane-type electrode stickers

The ability to create printable ultrathin devices and transfer printing allows ‘stick and play’ electronics on unusual surfaces where direct device fabrication is not possible. This research describes a systematic method for using an additional handling substrate to mechanically support an ultrathin substrate and printing the final device on a target surface in a deterministic way. Introducing a sacrificial layer and a concave–convex structure with optimized depth, pitch, and shape at the interface between the two substrates provides both stability in device fabrication and high-yield transfer printing in a deterministic manner. To demonstrate the efficacy of this method, we successfully transferred various sizes and layouts of patterns onto various planar and curvilinear substrates. Finally, we demonstrate highly foldable and stretchable membrane-type electrodes that can be attached onto unusual surfaces, such as paper and elastic adhesive tape.