Myunghwan Byun

Designing Responsive Buckled Surfaces by Halftone Gel Lithography

Smooth Operator When thin sheets are compressed they can buckle and wrinkle, such as when the edges of a sheet of paper, or two areas of skin, are pushed together. Variations in local thickness and stiffness will alter the buckling patterns, but controlling this in a simple and predictable way is difficult. Kim et al. (p. 1201; see the Perspective by Sharon) used halftone lithography with two photomasks to create highly cross-linked dots embedded in a lightly cross-linked matrix of a swellable polymer. This material could generate “smooth” swelling profiles on thin sheets with arbitrary two-dimensional geometries so that complex three-dimensional structures could be produced. Halftone lithography can pattern two-dimensional swellable gels to produce complex three-dimensional shapes. Self-actuating materials capable of transforming between three-dimensional shapes have applications in areas as diverse as biomedicine, robotics, and tunable micro-optics. We introduce a method of photopatterning polymer films that yields temperature-responsive gel sheets that can transform between a flat state and a prescribed three-dimensional shape. Our approach is based on poly(N-isopropylacrylamide) copolymers containing pendent benzophenone units that allow cross-linking to be tuned by irradiation dose. We describe a simple method of halftone gel lithography using only two photomasks, wherein highly cross-linked dots embedded in a lightly cross-linked matrix provide access to nearly continuous, and fully two-dimensional, patterns of swelling. This method is used to fabricate surfaces with constant Gaussian curvature (spherical caps, saddles, and cones) or zero mean curvature (Enneper’s surfaces), as well as more complex and nearly closed shapes.

Highly‐Efficient, Flexible Piezoelectric PZT Thin Film Nanogenerator on Plastic Substrates

A highly-efficient, flexible piezoelectric PZT thin film nanogenerator is demonstrated using a laser lift-off (LLO) process. The PZT thin film nanogenerator harvests the highest output performance of ∼200 V and ∼150 μA·cm(-2) from regular bending motions. Furthermore, power sources generated from a PZT thin film nanogenerator, driven by slight human finger bending motions, successfully operate over 100 LEDs.

Self-Powered Cardiac Pacemaker Enabled by Flexible Single Crystalline PMN-PT Piezoelectric Energy Harvester

A flexible single-crystalline PMN-PT piezoelectric energy harvester is demonstrated to achieve a self-powered artificial cardiac pacemaker. The energy-harvesting device generates a short-circuit current of 0.223 mA and an open-circuit voltage of 8.2 V, which are enough not only to meet the standard for charging commercial batteries but also for stimulating the heart without an external power source.

A Hyper‐Stretchable Elastic‐Composite Energy Harvester

C. K. Jeong, G.-T. Hwang, D. Y. Park, J. H. Park, Dr. M. Byun, Prof. K. J. Lee Department of Materials Science and Engineering Korea Advanced Institute of Science and Technology (KAIST) 291 Daehak-ro , Yuseong-gu , Daejeon 305-701 , South Korea E-mail: keonlee@kaist.ac.kr Dr. J. Lee, Dr. S. Han, Prof. S. H. Ko Department of Mechanical Engineering Seoul National University 1 Gwanak-ro , Gwanak-gu , Seoul 151-742 , South Korea E-mail: maxko@snu.ac.kr Dr. J. Lee, Prof. S. S. Lee Department of Mechanical Engineering Korea Advanced Institute of Science and Technology (KAIST) 291 Daehak-ro , Yuseong-gu , Daejeon 305-701 , South Korea Dr. J. Ryu Functional Ceramic Group Korea Institute of Materials Science (KIMS) 797 Changwon-daero Seongsan-gu Changwon , Gyeongsangnam-do 642–831 , South Korea

An Unconventional Route to High-Efficiency Dye-Sensitized Solar Cells via Embedding Graphitic Thin Films into TiO2 Nanoparticle Photoanode

Graphitic thin films embedded with highly dispersed titanium dioxide (TiO(2)) nanoparticles were incorporated for the first time into the conventional dye-sensitized solar cells (DSSCs), resulting in a remarkably improved cell efficiency due to its superior electron conductivity. Massively ordered arrays of TiO(2) dots embedded in carbon matrix were fabricated via UV-stabilization of polystyrene-block-poly(4-vinylpyridine) films containing TiO(2) precursors followed by direct carbonization. For dye-sensitized TiO(2) based solar cells containing carbon/TiO(2) thin layers at both sides of pristine TiO(2) layer, an increase of 62.3% [corrected] in overall power conversion efficiency was achieved compared with neat TiO(2)-based DSSCs. Such a remarkably improved cell efficiency was ascribed to the superior electron conductivity and extended electron lifetime elucidated by cyclic voltammetry and impedance spectroscopy.

Topographically-designed triboelectric nanogenerator via block copolymer self-assembly.

Herein, we report a facile and robust route to nanoscale tunable triboelectric energy harvesters realized by the formation of highly functional and controllable nanostructures via block copolymer (BCP) self-assembly. Our strategy is based on the incorporation of various silica nanostructures derived from the self-assembly of BCPs to enhance the characteristics of triboelectric nanogenerators (TENGs) by modulating the contact-surface area and the frictional force. Our simulation data also confirm that the nanoarchitectured morphologies are effective for triboelectric generation.

Robust Self‐Assembly of Highly Ordered Complex Structures by Controlled Evaporation of Confined Microfluids

The evaporative self-assembly of nonvolatile solutes such as polymers, nanocrystals, and carbon nanotubes has been widely recognized as a nonlithographic means of producing a diverse range of intriguing complex structures. The spatial variation of evaporative flux and possible convection mean, however, that these non-equilibrium dissipative structures (e.g., coffee rings, fingering patterns, and polygonal network structures) are often irregular and stochastically organized. Yet for many applications in microelectronics, data storage devices, and biotechnology, it is highly desirable to achieve surface patterns that have a well-controlled spatial arrangement. To date, only a few elegant studies have centered on the precise control of the evaporation process to produce ordered structures. When compared with conventional lithographic techniques, surface patterning by controlled solvent evaporation is simple, cost-effective, and offers a lithography and external-field-free means of organizing nonvolatile materials into ordered microscopic structures over large surface areas. For example, it has been recently demonstrated that constraining a drop of solution in a restricted geometry formed by placing a sphere against a flat substrate results in controlled evaporation. Consequently, the repetitive pinning and depinning of the solution s contact line produces a lateral surface pattern that consists of hundreds of concentric, highly ordered “coffee rings”, the gradients of which vary in width and height. 13, 17] The ability to engineer an evaporative self-assembly process that yields a wide range of complex, self-organizing structures over large areas other than strictly concentric rings 13, 17] offers tremendous potential for applications in electronics, optoelectronics, and sensors. The formation of periodic assemblies of polymeric squares, triangular contour lines, and ellipses would be effectively mediated by controlled solvent evaporation that is precisely guided by the shape of the curved upper surface of a confined curve-on-flat geometry. Herein, we demonstrate a facile, robust, and one-step method of evaporating polymer solutions in curve-on-flat geometries to create versatile, highly regular microstructures in a precisely controlled environment, as well as offering a comprehensive study of the influence of different upper surfaces on complex structure formation by controlled evaporation. Our method further enhances current fabrication approaches to creating highly ordered structures in a simple and cost-effective manner, with the potential to be tailored for use in photonics, electronics, 20] optoelectronics, microfluidic devices, nanotechnology, and biotechnology. A linear conjugated polymer, poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV, molecular weight = 50–300 kg mol ) was used as the nonvolatile solute. The choice of system was motivated by its numerous potential applications in the areas of light-emitting diodes, solar cells, and biosensors. A solution of MEH-PPV in toluene was prepared at a concentration of 0.05 mg mL . The key to our approach is the use of a simple confined geometry consisting of a curved upper surface on a flat lower substrate (curve-on-flat geometry) that forms a microscopic gap in which the MEH-PPV toluene solution is loaded; this results in a capillary-held microfluid (a liquid capillary bridge, see Figure 1a, Figure 3a, and the Experimental Section). The three curved surfaces used in the study were a square pyramid (the area of the sides = 1.0 1.0 cm and Hpyramid (pyramid height) = 100 mm (Figure 1a)), a triangular-slice sphere (R, (the radius of curvature) = 1.65 cm; D, the diameter = 1.5 cm; the arm width at A, C, and D = 800 mm (Figure 3a)), and a chisel lens (D = 1.0 cm, Hchisel (the chisel height) = 100 mm (Figure S2 a in the Supporting Information). The surfaces were made of aluminum, stainless steel, and fused silica, respectively. Unlike previous work in which evaporation occurred over the entire droplet surface, 8] evaporation of the solvent in this case was restricted to the edge of the capillary within the curve-on-flat geometry (Figure 1a and Figure 3a). Figure 1a, b, illustrates the route to concentric square stripes formed by evaporation in the square-pyramid-on-flat geometry. The loss of toluene at the capillary edge by evaporation triggered pinning of the contact line (the “stick”), thereby forming the outermost MEH-PPV square. During deposition of the MEH-PPV, the evaporation of toluene caused the initial contact angle of the capillary edge to gradually decrease to a critical angle, at which the capillary force (the depinning force) became greater than the pinning force. This led the contact line to jump, or “slip,” to a new position, thus developing a new square (Figure 1b). 25, 26] It is worth noting that the pyramid (the upper surface) provided a unique environment to guide the “stick–slip” motions of the contact line of the evaporating MEH-PPV microfluid, thereby forcing MEH-PPV to deposit in a manner that conformed to the square-shaped sides of the pyramid [*] S. W. Hong, M. Byun, Prof. Z. Lin Department of Materials Science and Engineering Iowa State University Ames, IA 50011 (USA) Fax: (+ 1)515-294-7202 E-mail: zqlin@iastate.edu

Flexible Piezoelectric Thin-Film Energy Harvesters and Nanosensors for Biomedical Applications

The use of inorganic-based flexible piezoelectric thin films for biomedical applications has been actively reported due to their advantages of highly piezoelectric, pliable, slim, lightweight, and biocompatible properties. The piezoelectric thin films on plastic substrates can convert ambient mechanical energy into electric signals, even responding to tiny movements on corrugated surfaces of internal organs and nanoscale biomechanical vibrations caused by acoustic waves. These inherent properties of flexible piezoelectric thin films enable to develop not only self-powered energy harvesters for eliminating batteries of bio-implantable medical devices but also sensitive nanosensors for in vivo diagnosis/therapy systems. This paper provides recent progresses of flexible piezoelectric thin-film harvesters and nanosensors for use in biomedical fields. First, developments of flexible piezoelectric energy-harvesting devices by using high-quality perovskite thin film and innovative flexible fabrication processes are addressed. Second, their biomedical applications are investigated, including self-powered cardiac pacemaker, acoustic nanosensor for biomimetic artificial hair cells, in vivo energy harvester driven by organ movements, and mechanical sensor for detecting nanoscale cellular deflections. At the end, future perspective of a self-powered flexible biomedical system is also briefly discussed with relation to the latest advancements of flexible electronics.

Hierarchically Organized Structures Engineered from Controlled Evaporative Self-Assembly

By constraining an asymmetric comb block copolymer (CBCP) toluene solution to evaporate in a wedge-on-Si geometry composed of a wedge lens situated on a Si substrate, gradient concentric stripelike surface patterns of CBCP at the microscopic scale were yielded as a direct consequence of controlled evaporative self-assembly of CBCP. The formation of either straight stripes or jagged stripes was dictated by the height of the wedge. Upon subsequent solvent vapor annealing, hierarchically organized structures of CBCP were produced, resulting from the interplay of solvent vapor assisted, unfavorable interfacial interaction driven destabilization of CBCP from the Si substrate at the microscopic scale and the solvent vapor promoted reconstruction of CBCP nanodomains within the stripes at the nanometer scale. This facile approach of combining controlled evaporative self-assembly with subsequent solvent vapor annealing offers a new platform to rationally design and engineer self-assembling building blocks into functional materials and devices in a simple, cost-effective manner.

Self-powered fully-flexible light-emitting system enabled by flexible energy harvester

Energy-harvesting technology utilising mechanical energy sources is a promising approach for the sustainable, independent, and permanent operation of a variety of flexible electronics. A new concept of a fully-flexible light-emitting system, self-powered by a high-performance piezoelectric thin-film energy harvester has been first established by manipulating highly-robust, flexible, vertically structured light emitting diodes (f-VLEDs). The f-VLEDs fabricated by anisotropic conductive film bonding and entire wafer etching show stable and durable performances during periodic mechanical deformations. A high-output energy harvester capable of generating up to 140 V and 10 μA can be fabricated via laser lift-off (LLO) process widely used in industries, in a safe and robust manner. In particular, this LLO process is of great benefit for the fabrication of mechanically stable, flexible piezoelectric devices, without causing any degradation of piezoelectric properties. In this process, self-powered all-flexible electronic system with light emittance can be spontaneously achieved by the electricity produced from flexible thin-film generator by applying slight biomechanical energy without any externally applied energy storage. This conceptual technology of self-powering based on the conversion of mechanical energy to electrical energy can open a facile and robust avenue for diverse, self-powered, bio-implantable applications, as well as commercial display applications.