Bo-Yeong Kim

Stretchable, Transparent, Ultrasensitive, and Patchable Strain Sensor for Human–Machine Interfaces Comprising a Nanohybrid of Carbon Nanotubes and Conductive Elastomers

UNLABELLED Interactivity between humans and smart systems, including wearable, body-attachable, or implantable platforms, can be enhanced by realization of multifunctional human-machine interfaces, where a variety of sensors collect information about the surrounding environment, intentions, or physiological conditions of the human to which they are attached. Here, we describe a stretchable, transparent, ultrasensitive, and patchable strain sensor that is made of a novel sandwich-like stacked piezoresisitive nanohybrid film of single-wall carbon nanotubes (SWCNTs) and a conductive elastomeric composite of polyurethane (PU)-poly(3,4-ethylenedioxythiophene) polystyrenesulfonate ( PEDOT PSS). This sensor, which can detect small strains on human skin, was created using environmentally benign water-based solution processing. We attributed the tunability of strain sensitivity (i.e., gauge factor), stability, and optical transparency to enhanced formation of percolating networks between conductive SWCNTs and PEDOT phases at interfaces in the stacked PU-PEDOT:PSS/SWCNT/PU-PEDOT:PSS structure. The mechanical stability, high stretchability of up to 100%, optical transparency of 62%, and gauge factor of 62 suggested that when attached to the skin of the face, this sensor would be able to detect small strains induced by emotional expressions such as laughing and crying, as well as eye movement, and we confirmed this experimentally.

Ultrahigh Responsivity in Graphene–ZnO Nanorod Hybrid UV Photodetector

Ultraviolet (UV) photodetectors based on ZnO nanostructure/graphene (Gr) hybrid-channel field-effect transistors (FETs) are investigated under illumination at various incident photon intensities and wavelengths. The time-dependent behaviors of hybrid-channel FETs reveal a high sensitivity and selectivity toward the near-UV region at the wavelength of 365 nm. The devices can operate at low voltage and show excellent selectivity, high responsivity (RI ), and high photoconductive gain (G). The change in the transfer characteristics of hybrid-channel FETs under UV light illumination allows to detect both photovoltage and photocurrent. The shift of the Dirac point (V Dirac ) observed during UV exposure leads to a clearer explanation of the response mechanism and carrier transport properties of Gr, and this phenomenon permits the calculation of electron concentration per UV power density transferred from ZnO nanorods and ZnO nanoparticles to Gr, which is 9 × 10(10) and 4 × 10(10) per mW, respectively. The maximum values of RI and G infer from the fitted curves of RI and G versus UV intensity are 3 × 10(5) A W(-1) and 10(6) , respectively. Therefore, the hybrid-channel FETs studied herein can be used as UV sensing devices with high performance and low power consumption, opening up new opportunities for future optoelectronic devices.

High-Performance Flexible Ultraviolet (UV) Phototransistor Using Hybrid Channel of Vertical ZnO Nanorods and Graphene

A flexible ultraviolet (UV) photodetector based on ZnO nanorods (NRs) as nanostructure sensing materials integrated into a graphene (Gr) field-effect transistor (FET) platform is investigated with high performance. Based on the negative shift of the Dirac point (VDirac) in the transfer characteristics of a phototransistor, high-photovoltage responsivity (RV) is calculated with a maximum value of 3 × 10(8) V W(-1). The peak response at a wavelength of ∼365 nm indicated excellent selectivity to UV light. The phototransistor also allowed investigation of the photocurrent responsivity (RI) and photoconductive gain (G) at various gate voltages, with maximum values of 2.5 × 10(6) A W(-1) and 8.3 × 10(6), respectively, at a gate bias of 5 V. The UV response under bending conditions was virtually unaffected and was unchanged after 10,000 bending cycles at a bending radius of 12 mm, subject to a strain of 0.5%. The attributes of high stability, selectivity, and sensitivity of this flexible UV photodetector based on a ZnO NRs/Gr hybrid FET indicate promising potential for future flexible optoelectronic devices.

Field-effect Transistor with a Chemically Synthesized MoS2 Sensing Channel for Label-Free and Highly Sensitive Electrical Detection of DNA Hybridization

A field-effect transistor (FET) with two-dimensional (2D) few-layer MoS2 as a sensing-channel material was investigated for label-free electrical detection of the hybridization of deoxyribonucleic acid (DNA) molecules. The high-quality MoS2-channel pattern was selectively formedthrough the chemical reaction of the Mo layer with H2S gas. The MoS2 FET was very stable in an electrolyte and inert to pH changes due to the lack of oxygen-containing functionalities on the MoS2 surface. Hybridization of single-stranded target DNA molecules with single-stranded probe DNA molecules physically adsorbed on the MoS2 channel resulted in a shift of the threshold voltage (Vth) in the negative direction and an increase in the drain current. The negative shift in Vth is attributed to electrostatic gating effects induced by the detachment of negatively charged probe DNA molecules from the channel surface after hybridization. A detection limit of 10 fM, high sensitivity of 17 mV/dec, and high dynamic range of 106 were achieved. The results showed that a bio-FET with an ultrathin 2D MoS2 channel can be used to detect very small concentrations of target DNA molecules specifically hybridized with the probe DNA molecules.

Stretchable, Transparent, and Stretch-Unresponsive Capacitive Touch Sensor Array with Selectively Patterned Silver Nanowires/Reduced Graphene Oxide Electrodes

Stretchable and transparent touch sensors are essential input devices for future stretchable transparent electronics. Capacitive touch sensors with a simple structure of only two electrodes and one dielectric are an established technology in current rigid electronics. However, the development of stretchable and transparent capacitive touch sensors has been limited due to changes in capacitance resulting from dimensional changes in elastomeric dielectrics and difficulty in obtaining stretchable transparent electrodes that are stable under large strains. Herein, a stretch-unresponsive stretchable and transparent capacitive touch sensor array was demonstrated by employing stretchable and transparent electrodes with a simple selective-patterning process and by carefully selecting dielectric and substrate materials with low strain responsivity. A selective-patterning process was used to embed a stretchable and transparent silver nanowires/reduced graphene oxide (AgNWs/rGO) electrode line into a polyurethane (PU) dielectric layer on a polydimethylsiloxane (PDMS) substrate using oxygen plasma treatment. This method provides the ability to directly fabricate thin film electrode lines on elastomeric substrates and can be used in conventional processes employed in stretchable electronics. We used a dielectric (PU) with a Poissons ratio smaller than that of the substrate (PDMS), which prevented changes in the capacitance resulting from stretching of the sensor. The stretch-unresponsive touch sensing capability of our transparent and stretchable capacitive touch sensor has great potential in wearable electronics and human-machine interfaces.