Jongho Lee

Stretchable GaAs Photovoltaics with Designs That Enable High Areal Coverage

Recent research in advanced materials and mechanics demonstrates the possibility for integrating inorganic semiconductors with soft, elastomeric substrates to yield systems with linear elastic mechanical responses to strains that signifi cantly exceed those associated with fracture limits of the constituent materials (e.g. ∼ 1% for many inorganics). This outcome can provide stretching to strain levels of tens of percent (in extreme cases, more than 100%), for diverse, reversible modes of deformation, including bending, twisting, stretching or compressing. [ 1–7 ]

Directional adhesion of gecko-inspired angled microfiber arrays

Arrays of angled microfibers with a gecko-inspired structure were fabricated from a stiff thermoplastic polymer (polypropylene) with elastic properties similar to those of β-keratin of natural setae. Friction experiments demonstrated that this fibrillar polymer surface exhibits directional adhesion. Sliding of clean glass surfaces against and along the microfiber direction without applying an external normal force produced an apparent shear stress of 0.1 and 4.5u2002N/cm2, respectively. This directional adhesion is interpreted in the context of a nonlinear elastic bending model of an angled beam. Shearing and normal contact experiments yielded further evidence of the anisotropic adhesion of the fibrillar polymer and revealed the occurrence of a pull-off (adhesive) force at the instant of surface detachment, unlike vertically aligned microfiber arrays of the same material that exhibited a zero pull-off force. The results of this study provide impetus for the design of gecko-inspired adhesives with angled struc...

Sliding-induced adhesion of stiff polymer microfibre arrays. I. Macroscale behaviour

Gecko-inspired microfibre arrays with 42 million polypropylene fibresu200acm−2 (each fibre with elastic modulus 1u200aGPa, length 20u200aμm and diameter 0.6u200aμm) were fabricated and tested under pure shear loading conditions, after removing a preload of less than 0.1u200aNu200acm−2. After sliding to engage fibres, 2u200acm2 patches developed up to 4u200aN of shear force with an estimated contact region of 0.44u200acm2. The control unfibrillated surface had no measurable shear force. For comparison, a natural setal patch tested under the same conditions on smooth glass showed approximately seven times greater shear per unit estimated contact region. Similar to gecko fibre arrays, the synthetic patch maintains contact and increases shear force with sliding. The high shear force observed (approx. 210u200anN per fibre) suggests that fibres are in side contact, providing a larger true contact area than would be obtained by tip contact. Shear force increased over the course of repeated tests for synthetic patches, suggesting deformation of fibres into more favourable conformations.

Contact self-cleaning of synthetic gecko adhesive from polymer microfibers.

Natural gecko toes covered by nanomicro structures can repeatedly adhere to surfaces without collecting dirt. Inspired by geckos, we fabricated a high-aspect-ratio fibrillar adhesive from a stiff polymer and demonstrated self-cleaning of the adhesive during contact with a surface. In contrast to a conventional pressure-sensitive adhesive (PSA), the contaminated synthetic fibrillar adhesive recovered about 33% of the shear adhesion of clean samples after multiple contacts with a clean, dry surface.

Gecko-inspired combined lamellar and nanofibrillar array for adhesion on nonplanar surface.

We report the fabrication from a hard polymer of lamellar structures that act as base support planes for high-aspect ratio nanofiber arrays. We experimentally show that nanofiber arrays on lamellae can adhere to both planar and nonplanar surfaces, exhibiting 5 times greater shear strength on a 100 mum peak-to-peak grating than the arrays without the lamellar support structure. The observed behavior on nonplanar surfaces is attributed to the high compliance of the lamellar flaps. The compliance of the combined lamellae and nanofiber arrays is measured to be about 160 times higher than nanofiber arrays on a flat nonlamellar backing layer.

Stretchable Semiconductor Technologies with High Areal Coverages and Strain‐Limiting Behavior: Demonstration in High‐Efficiency Dual‐Junction GaInP/GaAs Photovoltaics

Notched islands on a thin elastomeric substrate serve as a platform for dual-junction GaInP/GaAs solar cells with microscale dimensions and ultrathin forms for stretchable photovoltaic modules. These designs allow for a high degree of stretchability and areal coverage, and they provide a natural form of strain-limiting behavior, helping to avoid destructive effects of extreme deformations.

Hybrid Core−Shell Nanowire Forests as Self-Selective Chemical Connectors

Conventional connectors utilize mechanical, magnetic, or electrostatic interactions to enable highly specific and reversible binding of the components (i.e., mates) for a wide range of applications. As the connectors are miniaturized to small scales, a number of shortcomings, including low binding strength, high engagement/disengagement energies, difficulties with the engagement, fabrication challenges, and the lack of reliability are presented that limit their successful operation. Here, we report unisex, chemical connectors based on hybrid, inorganic/organic nanowire (NW) forests that utilize weak van der Waals bonding that is amplified by the high aspect ratio geometric configuration of the NWs to enable highly specific and versatile binding of the components. Uniquely, NW chemical connectors exhibit high macroscopic shear adhesion strength (approximately 163 N/cm(2)) with minimal binding to non-self-similar surfaces, anisotropic adhesion behavior (shear to normal strength ratio approximately 25), reusability (approximately 27 attach/detach cycles), and efficient binding for both micro- and macroscale dimensions.

Wet self-cleaning of superhydrophobic microfiber adhesives formed from high density polyethylene.

Biologically inspired adhesives developed for switchable and controllable adhesion often require repetitive uses in general, dirty, environments. Superhydrophobic microstructures on the lotus leaf lead to exceptional self-cleaning of dirt particles on nonadhesive surfaces with water droplets. This paper describes the self-cleaning properties of a hard-polymer-based adhesive formed with high-aspect-ratio microfibers from high-density polyethylene (HDPE). The microfiber adhesive shows almost complete wet self-cleaning of dirt particles with water droplets, recovering 98% of the adhesion of the pristine microfiber adhesives. The low contact angle hysteresis indicates that the surface of microfiber adhesives is superhydrophobic. Theoretical and experimental studies reveal a design parameter, length, which can control the adhesion without affecting the superhydrophobicity. The results suggest some properties of biologically inspired adhesives can be controlled independently by adjusting design parameters.

Flexible Vertical Light Emitting Diodes

Recently developed concepts in materials, manufacturing approaches and mechanics strategies open up opportunities for building optoelectronic systems with high performance, inorganic semiconductors in systems that afford exceptional levels of mechanical deformability (e.g., ability to bend and stretch), diversity in geometric layouts (e.g., large-area coverage, in dense or sparse coverages), and versatility in substrate choices (e.g., plastic, rubber). Each of these capa-bilities offers options in engineering design that are not easily addressed with conventional, wafer-based devices.

Subdermal Flexible Solar Cell Arrays for Powering Medical Electronic Implants

A subdermally implantable flexible photovoltatic (IPV) device is proposed for supplying sustainable electric power to in vivo medical implants. Electric properties of the implanted IPV device are characterized in live animal models. Feasibility of this strategy is demonstrated by operating a flexible pacemaker with the subdermal IPV device which generates DC electric power of ≈647 μW under the skin.