Youngjong Kang

Quasi-Amorphous Colloidal Structures for Electrically Tunable Full-Color Photonic Pixels with Angle-Independency

Similar to other PBG materials prepared by self-assembly of block copolymers or by lithographic techniques, colloidal photonic crystals require crystalline or highly ordered periodic structures to exhibit PBG by the collective refraction in longrange. [ 9–11 ] In this case, the PBG can be tuned by modulation of periodicity or/and refractive index contrast. [ 9 , 12 ] While such tunable photonic crystals based on crystalline structures have recently been proposed as active components of display [ 13–16 ]

Full Color Stop Bands in Hybrid Organic/Inorganic Block Copolymer Photonic Gels by Swelling-Freezing

We report a facile way of fabricating hybrid organic/inorganic photonic gels by selective swelling and subsequent infiltration of SiO(2) into one type of lamellar microdomain previously self-assembled from modest-molecular-weight block copolymers. Transparent, in-plane lamellar films were first prepared by assembly of polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP), and subsequently the P2VP domains were swollen with a selective solvent, methanol. The swollen structures were then fixated by synthesizing SiO(2) nanoparticles within P2VP domains. The resulting frozen photonic gels (f-photonic gels) exhibited strong reflective colors with stop bands across the visible region of wavelengths.

Electrically Tunable Hysteretic Photonic Gels for Nonvolatile Display Pixels

Materials that display hysteresis, such as ferromagnetic or ferroelectric materials, have been extensively investigated for their potential engineering applications towards electromagnetic memory devices and switches. While similar hysteretic volume phase transitions are observed in many soft organic materials, including hydrogels and biomaterials, it has not been well exploited but is often considered as problematic because it complicates the calibration procedure and affects signal reproducibility in many conventional applications of hydrogels in sensors and drug delivery. However, such hysteretic volume phase transitions of hydrogels can be utilized in a positive way when they are combined with other functional materials and carefully tuned to exhibit strong bistability. For example, Kim et al. recently reported wet photonic gel memory pixels by combining the hysteresis of hydrogels with photonic crystals. Block copolymer photonic gels consisting of alternating glassy polystyrene (PS) and swellable poly(2-vinyl pyridine) (P2VP) gel layers showed pH-dependent photonic stop bands. The optical responses of the photonic gels varied with the direction of pH changes. This hysteretic optical response of photonic gels was able to be tuned and further optimized by controlling ion-pairing affinity between the protonated pyridine groups and their counteranions. Extending the previous work, herein we report electrochemically controllable nonvolatile photonic pixels and their applications towards e-papers. Our approach is clearly different from the nonvolatile photonic ink demonstrated by Ozin and co-workers in the sense of using hydrogel hysteresis instead of using redox reaction of metallopolymers. Electroactive photonic pixels were achieved by coupling the hysteretic optical properties of PS-b-P2VP block copolymer photonic gels to the electrochemical cells, where the pH gradient can be tuned. Similar to other conventional hydrogels, our photonic gels show volume expansion and contraction in response to the applied electric field, and accordingly exhibit various photonic colors. We assumed that electroactive nonvolatile photonic gels can be achieved when the volume transition of gel layers was tuned to exhibit strong hysteresis with changes of electrochemically driven pH gradient. The electroactive photonic pixel comprises a simple electrochemical cell consisting of two transparent electrodes separated by 1 mm thick spacer and electrolytes (Figure 1a). The block copolymer photonic gel films were coated on the working electrode. The block copolymer photonic gel films were prepared as previously described. Briefly, in-plane oriented lamellar films were first prepared by spin casting

Enhancement of quaternary nitrogen doping of graphene oxide via chemical reduction prior to thermal annealing and an investigation of its electrochemical properties

A simple and efficient method to enhance the quaternary nitrogen doping (N-doping) of graphene has been demonstrated. Recent studies have shown that quaternary N in the graphene network provides more efficient electrocatalytic activity. Therefore, a novel strategy to enhance the quaternary N-doping is currently in high demand. The strategy employed in this work was to modify graphene oxide (GO) prior to thermal annealing so as to provide a more efficient structure for quaternary N doping. GO was first chemically reduced with hydrazine to substantially increase the formation of CC bonds and simultaneously decrease the atomic oxygen concentration. The reduced graphene oxide (RGO) was then annealed in the presence of NH3. Although N-doping via the replacement of oxygen is preferred, the probability of carbon being substituted with N dopants in the graphitic structure of RGO could increase due to the relatively higher content of CC when compared to the atomic oxygen concentration. In addition, due to the decreased atomic oxygen concentration, the electro-conductivity was enhanced. Cyclic voltammograms (CVs) of 5 mM K3Fe(CN)6 and 2 mM H2O2 were used to examine the electrochemical response of the quaternary N-maximized RGO. An improvement in electrocatalytic reduction and a higher electro-conductivity were confirmed based on an analysis of the obtained CVs.

Inkjet‐Assisted Nanotransfer Printing for Large‐Scale Integrated Nanopatterns of Various Single‐Crystal Organic Materials

Inkjet-assisted nanotransfer printing (inkjet-NTP) facilitates spatial control of many arrays of various organic functional materials on a single substrate with a high-throughput integration process, enabling monolithic integration of various organic nanopatterns. Inkjet-NTP enables wafer-scale organic electronic circuits composed of field-effect transistors, complementary inverters, and p-n diodes, demonstrating its capability to produce a high-performance, multifunctional organic chip.

Two-dimensional ordering of benzenethiol self-assembled monolayers guided by displacement of cyclohexanethiols on Au(111)

Although the adsorption of benzenethiols (BT) on Au(111) usually leads to the formation of disordered phases, we demonstrate here that the displacement of preadsorbed cyclohexanethiol self-assembled monolayers (SAMs) on Au(111) by BT molecules can be a successful approach to obtain two-dimensional BT SAMs with long-range ordered domains.

Cross-Stacked Single-Crystal Organic Nanowire p−n Nanojunction Arrays by Nanotransfer Printing

We fabricated cross-stacked organic p-n nanojunction arrays made of single-crystal 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-PEN) and fullerene (C60) nanowires as p-type and n-type semiconductors, respectively, by using a nanotransfer printing technique. Single-crystal C60 nanowires were synthesized inside nanoscale channels of a mold and directly transferred onto a desired position of a flexible substrate by a lubricant liquid layer. In the consecutive printing process, single-crystal TIPS-PEN nanowires were grown in the same way and then perpendicularly aligned and placed onto the C60 nanowire arrays, resulting in a cross-stacked single-crystal organic p-n nanojunction array. The cross-stacked single-crystal TIPS-PEN/C60 nanowire p-n nanojunction devices show rectifying behavior with on/off ratio of ∼ 13 as well as photodiode characteristic with photogain of ∼ 2 under a light intensity of 12.2 mW/cm(2). Our study provides a facile, solution-processed approach to fabricate a large-area array of organic crystal nanojunction devices in a desired arrangement for future nanoscale electronics.

High Performance of Low Band Gap Polymer-Based Ambipolar Transistor Using Single-Layer Graphene Electrodes

Bottom-contact bottom-gate organic field-effect transistors (OFETs) are fabricated using a low band gap pDTTDPP-DT polymer as a channel material and single-layer graphene (SLG) or Au source/drain electrodes. The SLG-based ambipolar OFETs significantly outperform the Au-based ambipolar OFETs, and thermal annealing effectively improves the carrier mobilities of the pDTTDPP-DT films. The difference is attributed to the following facts: (i) the thermally annealed pDTTDPP-DT chains on the SLG assume more crystalline features with an edge-on orientation as compared to the polymer chains on the Au, (ii) the morphological features of the thermally annealed pDTTDPP-DT films on the SLG electrodes are closer to the features of those on the gate dielectric layer, and (iii) the SLG electrode provides a flatter, more hydrophobic surface that is favorable for the polymer crystallization than the Au. In addition, the preferred carrier transport in each electrode-based OFET is associated with the HOMO/LUMO alignment relative to the Fermi level of the employed electrode. All of these experimental results consistently explain why the carrier mobilities of the SLG-based OFET are more than 10 times higher than those of the Au-based OTFT. This work demonstrates the strong dependence of ambipolar carrier transport on the source/drain electrode and annealing temperature.

Creating Patterned Conjugated Polymer Images Using Water-Compatible Reactive Inkjet Printing

The fabrication of patterned conjugated polymer images on solid substrates has gained significant attention recently. Office inkjet printers can be used to generate flexible designs of functional materials on substrates on a large scale and in an inexpensive manner. Although creating patterns of conjugated polymers on paper using common office inkjet printers has been reported, only a few examples exist, such as polyaniline (PANI) and poly(3,4-ethylenedioxythiophene) (PEDOT), because only water-compatible inks can be utilized. Herein, we describe the production of poly(phenylenevinylene) (PPV) patterns on paper by employing a reactive inkjet printing (RIJ) method. In this process, printing of a hydrophilic terephthaldehyde, bis(triphenylphosphonium salt) and potassium t-butoxide using a common office inkjet printer leads to formation PPV patterns as a consequence of an in situ Wittig reaction. In addition, microarrayed PPV patterns are also readily generated on solid substrates, such as glass and PDMS, when a piezoelectric dispenser system is employed. The in situ prepared PPV was found to be insoluble in water and chloroform. As a result, unreacted excess reagents and byproducts can be efficiently removed by washing with these solvents.

Heterogeneous Monolithic Integration of Single-Crystal Organic Materials

Manufacturing high-performance organic electronic circuits requires the effective heterogeneous integration of different nanoscale organic materials with uniform morphology and high crystallinity in a desired arrangement. In particular, the development of high-performance organic electronic and optoelectronic devices relies on high-quality single crystals that show optimal intrinsic charge-transport properties and electrical performance. Moreover, the heterogeneous integration of organic materials on a single substrate in a monolithic way is highly demanded for the production of fundamental organic electronic components as well as complex integrated circuits. Many of the various methods that have been designed to pattern multiple heterogeneous organic materials on a substrate and the heterogeneous integration of organic single crystals with their crystal growth are described here. Critical issues that have been encountered in the development of high-performance organic integrated electronics are also addressed.