Minwoo Park

Surfactant‐Free Scalable Synthesis of Bi2Te3 and Bi2Se3 Nanoflakes and Enhanced Thermoelectric Properties of Their Nanocomposites

Surfactant-free nanoflakes of n-type Bi2 Te3 and Bi2 Se3 are synthesized in high yields. Their suspensions are mixed to create nanocomposites with heterostructured nanograins. A maximum ZT (0.7 at 400 K) is achieved with a broad content of 10-15% Bi2 Se3 in the nanocomposites.

Ordered Zigzag Stripes of Polymer Gel/Metal Nanoparticle Composites for Highly Stretchable Conductive Electrodes

Stretchable electrodes have gained ever-increasing interest for a wide range of applications including smart clothing, [ 1 ] dielectric elastomer actuators (DEAs), [ 2 ] stretchable and rollable displays, [ 3 ] and fl exible electronics. [ 4 ] The required strain varies depending on the application. For example, a stretchable display needs about 10 to 30% strain, but more than 100% strain is desired for DEAs. Realization of stretchable electrodes has been explored with a number of different approaches. One of them is to generate the stretchable structures of metals. [ 5 ] Inplane S-shaped [ 6 ] or z-shaped [ 7 ] metal strips, or out-of-plane wavy geometry [ 8 ] can have a large net elongation. Although a large strain ( ≈ 60%) without electrical failure has been reported, fabrication of a fi ne-structured electrode is yet to be achieved. Another approach is to use a composite material made of an elastomer and conducting metal fi llers or carbon nanotubes. [ 9 ]

Surfactant-free CuInSe 2 nanocrystals transformed from In 2 Se 3 nanoparticles and their application for a flexible UV photodetector

In(2)Se(3) nanoparticles were synthesized in an aqueous solution without using any surfactant and then chemically transformed into CuInSe(2) nanocrystals. The transformation was thermodynamically favorable and fast. The 93% production yield in mild reaction conditions allowed mass production of the CuInSe(2) nanocrystals. By the virtue of the surface charges, the CuInSe(2) nanocrystals were well dispersed in polar solvents. The surfactant-free nanocrystals enabled the formation of semiconducting CuInSe(2) films on a flexible polymer substrate without any thermal treatment. We took advantage of this to fabricate a flexible UV photodetector. The current and sensitivity of the devices could be improved by utilizing CuInSe(2) nanocrystals annealed at 160 °C in the reaction batch. On bending test, the detection sensitivity remained the same until the bending radius was reduced down to 4 mm. The dynamic response of the film device was stable and reproducible during light illumination (350 nm).

Micropatterned Stretchable Circuit and Strain Sensor Fabricated by Lithography on an Electrospun Nanofiber Mat

This paper describes a novel approach for composite nanofiber mats and its application to fabricate a strain sensor. Electrospun poly(4-vinylpyridine) (P4VP) nanofiber mats are micropatterned by a lithographic approach that includes selective oxidation of the nanofibers and removal of unreacted fibers. The P4VP/HAuCl4 complex is converted to P4VP/Au composites by chemical reduction. We investigate the electrical resistivity of the composite mats according to the number of complexation-and-reduction cycles, the thickness of the fiber mats, and the annealing temperatures which control the percolation of the Au nanoparticles in the fiber mats. Nozzle printing of a polymeric solution on the patterned nanofiber mats simply produces an array of strain-sensitive and strain-invariant units. The patterns demonstrate high strain-sensing performance without any mechanical and electrical failure over 200 bending cycles in the strain range of ε<0.17.

Patterning Materials through Viscoelastic Flow and Phase Separation

Many non-photolithographic techniques have been developed for large-area patterning, with soft lithography being the most successful example. In the form of microcontact printing, nanoimprinting, capillary force lithography, or replica molding, soft lithography uses an elastomeric mold bearing a patterned surface to generate or transfer submicrometer-sized structures without the need to access complicated and often expensive apparatus. In spite of its great potential as a versatile approach to the fabrication of structures down to less than 100 nm, the materials that can be easily and directly patterned using soft lithography are still limited to self-assembled monolayers, polymers, and sol-gel materials. 6] Even though several have employed soft lithography to pattern organic/inorganic hybrid materials or have used a polymer pattern as a sacrificial template for patterning inorganic materials, fine structures of inorganic materials are fabricated by conventional photolithography in combination with vacuum deposition. 12] Here we report a simple and versatile method for patterning inorganic materials. Figure 1 shows the fabrication process. The material to be patterned was randomly deposited on the surface of a substrate that had been patterned with a polymer using capillary force lithography (CFL). The criteria for selecting a polymer were cheap and fast processes, low processing temperatures, and applicability to diverse materials. Specific consideration was placed on wateror alcoholdispersible materials to follow the current trend in practical uses. A crystalline polymer is advantageous for this process because an abrupt drop in viscosity at the melting temperature (Tm) can facilitate the formation of a polymer pattern without a residual layer. A hydrophobic polymer is appropriate to induce phase separation between the polymer and the hydrophilic materials. Thereby, hydrophobic crystalline polymers with a low melting temperature (but higher than room temperature) were selected. Here we used poly(ecaprolactone) (PCL, Tm = 60 8C) to prove the concept. When heated above the Tm of the polymers, the viscoelastic polymer liquid spreads to the recessed area and covers the entire surface of a substrate. If a material placed on the polymer pattern is incompatible to the polymer, it will be phaseseparated and pushed by the polymer flow towards the center of the recessed region. The polymer melt will keep the material in the recessed region until it evolves into a continuous feature. Hard materials with a melting point higher than the annealing temperature tend to sink down to the substrate and the polymer liquid evolves into a thin film with a smooth surface. In contrast, soft materials with a melting point lower or comparable to the annealing temperature are transformed into the liquid phase during heating, then evolve into structures with a sharp interface to the polymer liquid. Amorphous polymers are not appropriate for this purpose because of their high viscosity and slow fluidic velocity (see the Experimental Section, Table S1, and Figure S1 in the Supporting Information). Figure 2a–f shows the patterning of Ag as an example of hard materials. In this case, a solution of AgNO3 in ethanol was spread on the surface of a PCL pattern on a Si wafer. The surface of the PCL pattern had been treated with oxygen plasma. The Ag precursor was then reduced into Ag nanoparticles (30–50 nm in size) by exposure to N2H4 vapor. Figure 2a,b shows atomic force microscopy (AFM) images of the sample in the height and phase modes, respectively. Figure 2c shows a scanning probe microscopy (SEM) image of the same sample, which shows the Ag nanoparticles everywhere on the surface. Figure 2d–f displays the organization of the Ag nanoparticles after the sample had been annealed at 150 8C for 30 s. The AFM image in Figure 2d Figure 1. Self-organization of nanoparticles (NPs) scattered on a polymer pattern. When heated above the melting temperature (Tm), the polymer flows down to the recessed regions of a pattern, pushing the nanoparticles to the center of each recessed region. Hard materials with melting points higher than the annealing temperature sink down to the substrate and evolve into continuous, porous lines. Soft materials form a sharp interface with the polymer liquid and evolve into porous, solid lines.

Thermal Expansion and Contraction of an Elastomer Stamp Causes Position-Dependent Polymer Patterns in Capillary Force Lithography

It is often observed that polymer patterns fabricated by capillary force lithography (CFL) are not identical, position-dependent even in one sample. The drawback has not been successfully explained so far. This paper reveals that the position-dependent pattern is mainly caused by the volume expansion and contraction of the elastomer stamp during heating and cooling in the CFL process. The stamp expands on a polymer liquid on heating, accumulating the polymer at one side-wall of each pattern of the stamp. And the stamp shrinks back to the initial position, accumulating the polymer at the opposite wall of the stamp pattern. For crystalline polymers, the morphology was mainly determined by the annealing temperature, that is, the degree of expansion. The position-dependence of the morphology was enhanced as the annealing temperature was increased. For amorphous polymers, the morphology was sensitive to cooling rate. Fast cooling led to a frozen morphology generated at the hot annealing temperature, while slow cooling produced an opposite morphology from the one at the annealing. The experimental results were theoretically explained by analyzing thermal expansion of the stamp and the shear stress exerted in the polymer layer. In the conclusion, we added our suggestions to avoid the nonuniformity in the polymer pattern by CFL process.

Micropatterning by controlled liquid instabilities and its applications

Robust, reproducible patterning over large areas is essential to the fabrication of miniaturized devices. When production and cost-efficiency are concerned, guided-assembly is a promising strategy for patterning that combines the advantages of both the top-down and bottom-up approaches. Most guided-assembly methods are enabled by controlling the instabilities of liquid solutions or polymer melts to be patterned. These instabilities can be observed in different ways according to the patterning strategies. This article reviews the strategies for micropatterning that are based on the manipulation of liquid instabilities, covering both physical principles and experimental demonstrations. Specifically, we discuss four types of liquid instabilities, which can be controlled for the reliable formation of micropatterns: (i) localization of the instability under an electric field, (ii) adjustment of the evaporation front line during solvent evaporation, (iii) template-directed selective dewetting, and (iv) hierarchical capillary instability for generating complex patterns. We also highlight future prospects of the instability-driven micropatterning techniques.

Growth of Long Triisopropylsilylethynyl Pentacene (TIPS-PEN) Nanofibrils in a Polymer Thin Film During Spin-Coating

This study demonstrates the growth of long triisopropylsilyethynyl pentacene (TIPS-PEN) nanofibrils in a thin film of a crystalline polymer, poly(ε-caprolactone) (PCL). During spin-coating, TIPS-PEN molecules are locally extracted around the PCL grain boundaries and they crystallize into [010] direction forming long nanofibrils. Molecular weight of PCL and weight fraction (α) of TIPS-PEN in PCL matrix are key factors to the growth of nanofibrils. Long high-quality TIPS-PEN nanofibrils are obtained with high-molecular-weight PCL and at the α values in the range of 0.03-0.1. The long nanofibrils are used as an active layer in a field-effect organic transistor.

Ag2Se micropatterns via viscoelastic flow-driven phase separation

A novel approach to prepare micropatterns of metal chalcogenides is proposed by employing viscoelastic flow-driven patterning. A consecutive process involving deposition of the Se precursor on a pattern of a crystalline polymer, chemical reduction of the precursor into amorphous Se (a-Se), and short-time thermal annealing above the melting temperature of the patterned polymer generated regular patterns of a-Se. This work demonstrates patterns of periodic lines and circles which is driven by the viscoelastic polymer flow and the phase separation of Se from the polymer. Additional thermal annealing facilitated the lateral growth of trigonal-Se (t-Se) nanowires from the Se patterns. The growing t-Se nanowires eventually meet each other to produce a 2D network structure. Chemical transformation of the Se into Ag2Se generated metal chalcogenide network structures.

Suppressing Instability of Liquid Thin Films by a Fibril Network and its Application to Micropatterning without a Residual Layer

This study proposes a method to coat thin films of non-volatile solvents on substrates. A small amount of crystalline polymer dissolved in solvents forms a network of crystalline fibrils during the coating process. The network suppresses dewetting of the solvent liquid and helps the liquid film sustaining on the substrate. This strategy can be used in soft lithography to generate micropatterns of diverse materials without having a residual layer. This process does not request etching for achieving residual layer-free micropatterns, which has been a long challenge in soft lithography. As examples, we demonstrate micropatterns of polymer hydrogels and metal oxides (ZnO, In2O3).