Hyunhyub Ko

Nanostructured Surfaces and Assemblies as SERS Media

Metallic nanostructures attract much interest as an efficient media for surface-enhanced Raman scattering (SERS). Significant progress has been made on the synthesis of metal nanoparticles with various shapes, composition, and controlled plasmonic properties, all critical for an efficient SERS response. For practical applications, efficient strategies of assembling metal nanoparticles into organized nanostructures are paramount for the fabrication of reproducible, stable, and highly active SERS substrates. Recent progress in the synthesis of novel plasmonic nanoparticles, fabrication of highly ordered one-, two-, and three-dimensional SERS substrates, and some applications of corresponding SERS effects are discussed.

Optically- and thermally-responsive programmable materials based on carbon nanotube-hydrogel polymer composites

A simple approach is described to fabricate reversible, thermally- and optically responsive actuators utilizing composites of poly(N-isopropylacrylamide) (pNIPAM) loaded with single-walled carbon nanotubes. With nanotube loading at concentrations of 0.75 mg/mL, we demonstrate up to 5 times enhancement to the thermal response time of the nanotube-pNIPAM hydrogel actuators caused by the enhanced mass transport of water molecules. Additionally, we demonstrate the ability to obtain ultrafast near-infrared optical response in nanotube-pNIPAM hydrogels under laser excitation enabled by the strong absorption properties of nanotubes. The work opens the framework to design complex and programmable self-folding materials, such as cubes and flowers, with advanced built-in features, including tunable response time as determined by the nanotube loading.

Metal-catalyzed crystallization of amorphous carbon to graphene

Metal-catalyzed crystallization of amorphous carbon to graphene by thermal annealing is demonstrated. In this “limited source” process scheme, the thickness of the precipitated graphene is directly controlled by the thickness of the initial amorphous carbon layer. This is in contrast to chemical vapor deposition processes, where the carbon source is virtually unlimited and controlling the number of graphene layers depends on the tight control over a number of deposition parameters. Based on the Raman analysis, the quality of graphene is comparable to other synthesis methods found in the literature, such as chemical vapor deposition. The ability to synthesize graphene sheets with tunable thickness over large areas presents an important progress toward their eventual integration for various technological applications.

Giant Tunneling Piezoresistance of Composite Elastomers with Interlocked Microdome Arrays for Ultrasensitive and Multimodal Electronic Skins

The development of flexible electronic skins with high sensitivities and multimodal sensing capabilities is of great interest for applications ranging from human healthcare monitoring to robotic skins to prosthetic limbs. Although piezoresistive composite elastomers have shown great promise in this area of research, typically poor sensitivities and low response times, as well as signal drifts with temperature, have prevented further development of these materials in electronic skin applications. Here, we introduce and demonstrate a design of flexible electronic skins based on composite elastomer films that contain interlocked microdome arrays and display giant tunneling piezoresistance. Our design substantially increases the change in contact area upon loading and enables an extreme resistance-switching behavior (ROFF/RON of ∼10(5)). This translates into high sensitivity to pressure (-15.1 kPa(-1), ∼0.2 Pa minimum detection) and rapid response/relaxation times (∼0.04 s), with a minimal dependence on temperature variation. We show that our sensors can sensitively monitor human breathing flows and voice vibrations, highlighting their potential use in wearable human-health monitoring systems.

Porous Substrates for Label-Free Molecular Level Detection of Nonresonant Organic Molecules

We report on the design of practical surface enhanced Raman scattering (SERS) substrate based upon 3D alumina membranes with cylindrical nanopores chemically modified with polyelectrolyte coating and loaded with gold nanoparticle clusters. These substrates allow for a molecular-level, label-free detection of common plastic explosive materials (TNT, DNT) down to 5-10 zeptograms or 15-30 molecules and a common liquid explosive (HMTD) down to 1 picogram. Such a sensitive detection of organic molecules by utilizing efficient SERS substrates opens the path for affordable and label-free detection of trace amount of practically important chemical compounds.

Tactile-direction-sensitive and stretchable electronic skins based on human-skin-inspired interlocked microstructures.

Stretchable electronic skins with multidirectional force-sensing capabilities are of great importance in robotics, prosthetics, and rehabilitation devices. Inspired by the interlocked microstructures found in epidermal-dermal ridges in human skin, piezoresistive interlocked microdome arrays are employed for stress-direction-sensitive, stretchable electronic skins. Here we show that these arrays possess highly sensitive detection capability of various mechanical stimuli including normal, shear, stretching, bending, and twisting forces. Furthermore, the unique geometry of interlocked microdome arrays enables the differentiation of various mechanical stimuli because the arrays exhibit different levels of deformation depending on the direction of applied forces, thus providing different sensory output patterns. In addition, we show that the electronic skins attached on human skin in the arm and wrist areas are able to distinguish various mechanical stimuli applied in different directions and can selectively monitor different intensities and directions of air flows and vibrations.

Nanoparticle‐Decorated Nanocanals for Surface‐Enhanced Raman Scattering

The surface-enhanced Raman scattering (SERS) effect is considered important for fast detection of characteristic ‘‘fingerprint’’ signatures of analytes. In the SERS effect, a substantial Raman enhancement arises on localized spots (‘‘hot spots’’) in metallic nanostructures owing to strong local electromagnetic fields associated with the surface plasmon resonances of metal nanostructures. SERS on colloidal metal aggregates and nanowires and Raman resonances have been utilized for trace detection of explosives, chemical and biological warfare agents, internal mechanical stress, distribution and stress of carbon nanotubes, and toxic environmental pollutants. However, an outstanding challenge of SERS-based detection is the lack of robust and facile fabrication routines for SERS substrates with considerable enhancements. Traditionally, electrochemically roughened metal surfaces, colloids, island films, nanowires, periodic arrays, and self-assembled nanoparticles are employed as SERS substrates (for a recent review, see Reference [27]). However, the long-term stability of aggregated nanoparticles is the main obstacle for assembled structures, and the fabrication of complex surface structures (e.g. with microfabrication) is laborand cost-demanding and sometimes impossible to extend to large dimensions. Moreover, the sensitivity of simple 2D SERS substrates remains modest owing to a limited number of hot spots (usually below 10). To increase the sensitivity of SERS substrates, 3D porous structures have been suggested as active SERS substrates with the advantage of having a large surface area available for the formation of hot spots and the adsorption of target analytes. Consequently, 3D SERS substrates have been fabricated by depositing Au or Ag films on porous silicon, GaN, and filter paper. Alternatively, deposition of metal nanoparticles on the porous aluminum membranes, colloidal crystal templates, or microwires have been exploited. For instance, nanopores in gold films have been proven to show enhanced Raman scattering owing to an intense electromagnetic field generated by the surface plasmons. However, most of these studies have not fully utilized the advantages of 3D structures for SERS effects, mainly because of limited light propagation through porous materials owing to

Fingertip skin–inspired microstructured ferroelectric skins discriminate static/dynamic pressure and temperature stimuli

Fingertip skin-mimicking ferroelectric skins sensitively detect and discriminate static/dynamic pressure and temperature. In human fingertips, the fingerprint patterns and interlocked epidermal-dermal microridges play a critical role in amplifying and transferring tactile signals to various mechanoreceptors, enabling spatiotemporal perception of various static and dynamic tactile signals. Inspired by the structure and functions of the human fingertip, we fabricated fingerprint-like patterns and interlocked microstructures in ferroelectric films, which can enhance the piezoelectric, pyroelectric, and piezoresistive sensing of static and dynamic mechanothermal signals. Our flexible and microstructured ferroelectric skins can detect and discriminate between multiple spatiotemporal tactile stimuli including static and dynamic pressure, vibration, and temperature with high sensitivities. As proof-of-concept demonstration, the sensors have been used for the simultaneous monitoring of pulse pressure and temperature of artery vessels, precise detection of acoustic sounds, and discrimination of various surface textures. Our microstructured ferroelectric skins may find applications in robotic skins, wearable sensors, and medical diagnostic devices.

Triboelectric generators and sensors for self-powered wearable electronics.

In recent years, the field of wearable electronics has evolved at a rapid pace, requiring continued innovation in technologies in the fields of electronics, energy devices, and sensors. In particular, wearable devices have multiple applications in healthcare monitoring, identification, and wireless communications, and they are required to perform well while being lightweight and having small size, flexibility, low power consumption, and reliable sensing performances. In this Perspective, we introduce two recent reports on the triboelectric generators with high-power generation achieved using flexible and lightweight textiles or miniaturized and hybridized device configurations. In addition, we present a brief overview of recent developments and future prospects of triboelectric energy harvesters and sensors, which may enable fully self-powered wearable devices with significantly improved sensing capabilities.

Capillary Printing of Highly Aligned Silver Nanowire Transparent Electrodes for High-Performance Optoelectronic Devices

Percolation networks of silver nanowires (AgNWs) are commonly used as transparent conductive electrodes (TCEs) for a variety of optoelectronic applications, but there have been no attempts to precisely control the percolation networks of AgNWs that critically affect the performances of TCEs. Here, we introduce a capillary printing technique to precisely control the NW alignment and the percolation behavior of AgNW networks. Notably, partially aligned AgNW networks exhibit a greatly lower percolation threshold, which leads to the substantial improvement of optical transmittance (96.7%) at a similar sheet resistance (19.5 Ω sq(-1)) as compared to random AgNW networks (92.9%, 20 Ω sq(-1)). Polymer light-emitting diodes (PLEDs) using aligned AgNW electrodes show a 30% enhanced maximum luminance (33068 cd m(-2)) compared to that with random AgNWs and a high luminance efficiency (14.25 cd A(-1)), which is the highest value reported so far using indium-free transparent electrodes for fluorescent PLEDs. In addition, polymer solar cells (PSCs) using aligned AgNW electrodes exhibit a power conversion efficiency (PCE) of 8.57%, the highest value ever reported to date for PSCs using AgNW electrodes.