Woo Soo Kim

Reversibly stretchable transparent conductive coatings of spray-deposited silver nanowires.

Here, we report the creation of highly adhesive transparent and stretchable coatings via spray-deposition of solution-based silver nanowires (AgNWs). The AgNW dispersion was spray-deposited on a polydopamine-modified stretchable elastomeric substrate to prepare thin, stretchable, transparent, highly conductive films. The polydopamine layer on the elastomeric substrate created a highly hydrophilic surface, which facilitated the subsequent spraying of the AgNW solution. Additionally, the spray-deposited AgNWs demonstrated excellent adhesion to the substrate, which allowed the fabrication of stretchable electrodes with high conductivity. The AgNW-coated elastomeric substrate exhibited ~80% transmittance with an average sheet resistance of ~35 Ω/□, making it suitable for transparent electrode applications. The conductivity of the transparent electrode was maintained up to ~20% mechanical elongation, which demonstrated the stretchable characteristics of the AgNW-coated elastomeric substrate.

Highly sensitive tactile sensors integrated with organic transistors

This paper presents a highly sensitive capacitive pressure sensor composed of a polymer dielectric film with a nano-needle structure. The nano-needle polymer films were prepared by facile fabrication methods including breath figures formation followed by stamping. The pressure sensitivity of the sensor reached 1.76 kPa−1 in the low pressure range (<1 kPa), which is comparable to the sensitivity of human skin. Analysis of the geometries and densities effect was shown, and the nano-needle film showed better sensitivity in comparison to films with hemispherical or conical structures. The pressure sensors were integrated with printed organic thin film transistors to enable flexible, large-area tactile sensing applications.

A Stretchable RF Antenna With Silver Nanowires

A stretchable, flexible antenna fabricated with silver nanowires (AgNW) and polydimethylsiloxane (PDMS) is presented. Highly conductive AgNWs coupled with mechanically flexibility and durable PDMS are shown to produce promising results for RF sensing. The RF antenna is designed as a 1.5-GHz microstrip patch antenna using the transmission-line model. The resonance frequency of the antenna shifts in response to the applied force/strain, making the configuration suitable for wireless sensing applications.

Nanopatterning of photonic crystals with a photocurable silica–titania organic–inorganic hybrid material by a UV-based nanoimprint technique

UV patternable high refractive index inorganic–organic hybrid materials prepared by sol–gel process can be nanoimprinted in order to get photonic crystal nanopatterns. Non-hydrolytic sol–gel process can make a UV curable and relatively high refractive index hybrid material, which can be cured without a significant shrinkage for nanoimprinting. Sol–gel process of heterogeneously functionalized silicon alkoxides can make the organically modified inorganic Si–O–Ti network very well. The UV-based nanoimprint technique utilizes transparent templates and UV curable high refractive index materials to allow pattern replication at room temperature and very low pressure. UV-based nanoimprint technique is an efficient way to fabricate this sort of polymeric photonic crystal nanostructures.

Fabrication of ridge waveguides by UV embossing and stamping of sol-gel hybrid materials

Multimode large core (45/spl times/35 /spl mu/m/sup 2/) optical ridge waveguides are fabricated in the room temperature by ultraviolet (UV) imprinting of soft rubber template with the organic-inorganic sol-gel hybrid materials. The fabrication processes may be either well-known UV embossing or stamping. A new sol-gel hybrid material that is thermally stable has been developed for the fabrication of low-loss optical waveguides. The UV curable methacrylic sol-gel hybrid material is used for the core layer and fluorinated methacrylic sol-gel hybrid material is used for the cladding layer. The optical propagation loss of fabricated waveguide measured by the cutback method is 0.36 dB/cm at 850 nm and the surface roughness of waveguide is about 5 /spl Aring/ root mean square.

Direct stamping of silver nanoparticles toward residue-free thick electrode

Abstract Direct stamping of functional materials has been developed for cost-effective and process-effective manufacturing of nano/micro patterns. However, there remain several challenging issues like the perfect removal of the residual layer and realization of high aspect ratio. We have demonstrated facile fabrication of flexible strain sensors that have microscale thick interdigitated capacitors with no residual layer by a simple direct stamping with silver nanoparticles (AgNPs). Polyurethane (PU) prepolymer was utilized as an adhesive layer to transfer AgNPs more efficiently during the separation step of the flexible stamp from directly stamped AgNPs. Scanning electron microscopy images and energy dispersive x-ray spectroscopy analysis revealed residue-free transfer of microscale thick interdigitated electrodes onto two different flexible substrates (elastomeric and brittle) for the application to highly sensitive strain sensors.

Synthesis of fluorinated hybrid material for UV embossing of a large core optical waveguide structure

A new organic–inorganic hybrid material (HYBRIMER) has been synthesized homogeneously through a non-hydrolytic sol–gel process. The new HYBRIMER shows efficient index control and minor volume contraction behavior during micro-scale molding. In hybrid materials, the methacrylic end-group is considered suitable for cross-linking during the UV curing step, while the fluorinated functionality is appropriate for refractive index control. As the fluorinated silane contents are varied, the structural evolution and refractive index of the synthesized resin are then controlled (1.469 to 1.513 at 850 nm). A large core optical waveguide structure is formed by these hybrid materials, which exhibit low near IR wavelength absorption. A single material system is applied to both the core and the cladding materials of the optical waveguide. The measured optical propagation loss is lower than 0.25 dB cm−1.

Stretchable RFID for Wireless Strain Sensing With Silver Nano Ink

Flexible and stretchable inductor-capacitor (LC) resonator-based chipless radio frequency identification (RFID) tags have been fabricated by the direct stamping with silver nano ink. The tags are optimized based on the sympathetic oscillation of LC circuit at specifically designed resonant frequencies ranged from 1.12 to 1.7 GHz by adjusting dimensions of the inductor and capacitor in a tag. Pressure applied to the layer of silver nano ink during the stamping procedure helps densification of silver nanoparticles inside trenches of the stamp before heat-annealing of them. Transfer stamping process is simulated to demonstrate stress distribution across the layer of silver nanoparticles. Compaction of silver nanoparticles, in turn, positively affects mechanical strength of the final silver electrode and enables RFID strain sensors on polydimethylsiloxane to be stretchable up to 7%. By the stretchable RFID strain sensors, wireless strain sensing is demonstrated with a gauge factor of 0.51. Multiple encoded identifications with combination of double tags and stretching behavior of fabricated tags are also demonstrated. This stretchable RFID sensor is promising for biomedical applications such as real-time monitoring of motion detection.

Photoinduced low refractive index in a photosensitive organic–inorganic hybrid material

A new organic–inorganic hybrid material, that shows photoinduced reduction of refractive index as well as volume contraction, has been prepared using a sol–gel method. Whereas previous photosensitive materials show a photoinduced increase in refractive index, this hybrid material shows a significant decrease in refractive index via the decomposition of methacrylate on light irradiation. Moreover, this hybrid material is patternable. The reduction of refractive index measured with a prism coupler was about 0.045, with a low refractive index value of 1.385 being observed. The structural change of the hybrid material on UV-light irradiation as monitored by FT-IR and energy dispersive X-ray spectroscopy (EDS) is investigated. Direct imprinting of the organic–inorganic hybrid material without any further steps (wet etching, thermal curing, or UV fixing) is also performed utilizing the volume contraction on light irradiation.

Design optimized membrane-based flexible paper accelerometer with silver nano ink

Here we report a highly sensitive single-axis membrane-type paper accelerometer. The accelerometer is consisted of a suspended parallel-plate sensing capacitor prepared by cost-effective nano ink printing technologies on a flexible paper substrate. The proof mass and suspension bridge structures of the accelerometer are designed based on the simulation results for the optimization of sensitivity. Vertical acceleration sensitivity of the optimized design with long ellipse-shaped bridges and round-shaped proof mass can reach 20 fF/g at z-axis acceleration of 1–10 g.