Jinwoo Sung

Spin cast ferroelectric beta poly(vinylidene fluoride) thin films via rapid thermal annealing

We describe a method of fabricating ferroelectric beta-type poly(vinylidene fluoride) (PVDF) thin films on Au substrate by the humidity controlled spin casting combined with rapid thermal treatment. Our method produces thin uniform ferroelectric PVDF film with ordered beta crystals consisting of characteristic needlelike microdomains. A capacitor with a 160nm thick ferroelectric PVDF film exhibits the remanent polarization and coercive voltage of ∼7.0μC∕cm2 and 8V, respectively, with the temperature stability of up to 160°C. A ferroelectric field effect transistor also shows a drain current bistablility of 100 at zero gate voltage with ±20V gate voltage sweep.

Exploiting the colloidal nanocrystal library to construct electronic devices

Assembling nanocrystal devices A wide range of materials can be grown as high-quality colloidal nanocrystals, with properties spanning from conductors to semiconductors and insulators. Although these materials have been included in electronic devices, they usually only form a single component within the device. Choi et al. took a variety of solution-processable colloidal nanocrystals to form all of the device components. Through the development of the right materials, interfaces, and processing steps, they constructed an all-colloid field effect transistor. Science, this issue p. 205 Semiconductor devices are constructed solely from a range of colloidal nanocrystals. Synthetic methods produce libraries of colloidal nanocrystals with tunable physical properties by tailoring the nanocrystal size, shape, and composition. Here, we exploit colloidal nanocrystal diversity and design the materials, interfaces, and processes to construct all-nanocrystal electronic devices using solution-based processes. Metallic silver and semiconducting cadmium selenide nanocrystals are deposited to form high-conductivity and high-mobility thin-film electrodes and channel layers of field-effect transistors. Insulating aluminum oxide nanocrystals are assembled layer by layer with polyelectrolytes to form high–dielectric constant gate insulator layers for low-voltage device operation. Metallic indium nanocrystals are codispersed with silver nanocrystals to integrate an indium supply in the deposited electrodes that serves to passivate and dope the cadmium selenide nanocrystal channel layer. We fabricate all-nanocrystal field-effect transistors on flexible plastics with electron mobilities of 21.7 square centimeters per volt-second.

AC Field-Induced Polymer Electroluminescence with Single Wall Carbon Nanotubes

We developed a high-performance field-induced polymer electroluminescence (FPEL) device consisting of four stacked layers: a top metal electrode/thin solution-processed nanocomposite film of single wall carbon nanotubes (SWNTs) and a fluorescent polymer/insulator/transparent bottom electrode working under an alternating current (AC) electric field. A small amount of SWNTs that were highly dispersed in the fluorescent polymer matrix by a conjugate block copolymer dispersant significantly enhanced EL, and we were able to realize an SWNT-FPEL device with a light emission of approximately 350 cd/m(2) at an applied voltage of ±25 V and an AC frequency of 300 kHz. The brightness of the SWNT-FPEL device is much greater than those of other AC-based organic or even inorganic ELs that generally require at least a few hundred volts. Light is emitted from our SWNT-FPEL device because of the sequential injection of field-induced holes and then electron carriers through ambipolar carbon nanotubes under an AC field, followed by exciton formation in the conjugated organic layer. Field-induced bipolar charge injection provides great material design freedom for our devices; the energy level does not have to be aligned between the electrode and the emission layer, and the balance of the carrier injected and transported can be altered in contrast to that in conventional organic light-emitting diodes, leading to an extremely cost-effective and unified device architecture that is applicable to all red-green-blue fluorescent polymers.

High Performance AC Electroluminescence from Colloidal Quantum Dot Hybrids

High performance field-induced AC electroluminescence (EL) in a simple ITO/insulator/hybrid emitter/Au structure was demonstrated with efficient control of the brightness and colors based on solution-processed nanohybrids of CdSe-ZnS core-shell colloidal quantum dots and fluorescent polymers.

Micropatterning of thin P3HT films via plasma enhanced polymer transfer printing

We have developed a simple but robust process, plasma enhanced polymer Transfer printing (PEPTP), for fabricating micropatterns of semi-conducting poly(3-hexyl thiophene) (P3HT) thin films. The method is based on transferring thin P3HT film spin cast directly on a pre-patterned elastomeric poly(dimethyl siloxane) (PDMS) mold. Printing is accomplished by the application of oxygen plasma on both P3HT film and substrate for surface energy modulation at ambient conditions without additional pressure. The control of the relative interfacial surface energy by the plasma enables us to micro/nanopattern a wide range of polymers from P3HT to conventional insulating ones on various substrates including Si, glass and polymers over large areas. A bottom contact organic thin film transistor with the printed P3HT patterns using PEPTP exhibits a carrier mobility of approximately 0.02 cm2V−1 s−1 with a relatively high on/off current ratio of 6 × 103.

Extremely Bright Full Color Alternating Current Electroluminescence of Solution-Blended Fluorescent Polymers with Self-Assembled Block Copolymer Micelles

Electroluminescent (EL) devices operating at alternating current (AC) electricity have been of great interest due to not only their unique light emitting mechanism of carrier generation and recombination but also their great potential for applications in displays, sensors, and lighting. Despite great success of AC-EL devices, most device properties are far from real implementation. In particular, the current state-of-the art brightness of the solution-processed AC-EL devices is a few hundred candela per square meter (cd m(-2)) and most of the works have been devoted to red and white emission. In this manuscript, we report extremely bright full color polymer AC-EL devices with brightness of approximately 2300, 6000, and 5000 cd m(-2) for blue (B), green (G), and red (R) emission, respectively. The high brightness of blue emission was attributed to individually networked multiwalled carbon nanotubes (MWNTs) for the facile carrier injection as well as self-assembled block copolymer micelles for suppression of interchain nonradiative energy quenching. In addition, effective FRET from a solution-blended thin film of B-G and B-G-R fluorescent polymers led to very bright green and red EL under AC voltage, respectively. The solution-processed AC-EL device also worked properly with vacuum-free Ag paste on a mechanically flexible polymer substrate. Finally, we successfully demonstrated the long-term operation reliability of our AC-EL device for over 15 h.

Parallel laser printing of nanoparticulate silver thin film patterns for electronics

This report discusses a parallel laser printing process for fabricating metallization patterns, which utilizes a pulsed laser-induced thermoelastic force exerting on nanoparticles. Silver thin films solution deposited on a glass substrate were transferred onto diverse receiver substrates such as Si, glass, and plastics by a spatially modulated Nd:YAG pulsed laser beam (1064nm, 6ns pulse width). High-fidelity patterns at the sub-10μm scales could be printed over several cm2 by a single pulse with 850mJ of energy. The fabrication of organic thin film transistors is demonstrated using printed source and drain Ag electrodes.

Intrinsic memory behavior of rough silicon nanowires and enhancement via facile Ag NPs decoration

In inorganic nanowire memory, both physical features rendered during synthesis and integration processes and intrinsic properties of materials are important because they present appropriate schemes for facile fabrication of an excellent memory device. We demonstrate that silicon nanowires (Si NWs) synthesized by an electroless etching (EE) method intrinsically present memory behavior via charge trapping of water molecules due to the rough surface created during synthesis. Additionally, using an electrochemical reaction of silicon with AgNO3 solution, which produces Ag nanoparticles (NPs) with a blocking SiO2 layer, we easily achieve a Ag NP hybrid Si NW memory device.

Coassembly of Metal and Titanium Dioxide Nanocrystals Directed by Monolayered Block Copolymer Inverse Micelles for Enhanced Photocatalytic Performance

Functional nanostructures of self-assembled block copolymers (BCPs) incorporated with various inorganic nanomaterials have received considerable attention on account of their many potential applications. Here we demonstrate the two-dimensional self-assembly of anisotropic titanium dioxide (TiO(2)) nanocrystals (NCs) and metal nanoparticles (NPs) directed by monolayered poly(styrene)-block-poly(4-vinylpyridine) (PS-b-P4VP) copolymer inverse micelles. The independent position-selective assembly of TiO(2) NCs and silver nanoparticles (AgNPs) preferentially in the intermicelle corona regions and the core of micelles, respectively, for instance, was accomplished by spin-coating a mixture solution of PS-b-P4VP and ex situ synthesized TiO(2) NCs, followed by the reduction of Ag salts coordinated in the cores of micelles into AgNPs. Hydrophobic TiO(2) NCs with a diameter and length of approximately 3 nm and 20-30 nm, respectively, were preferentially sequestered in the intermicelle nonpolar PS corona regions energetically favorable with the minimum entropic packing penalty. Subsequent high-temperature annealing at 550 °C not only effectively removed the block copolymer but also transformed the TiO(2) NCs into connected nanoparticles, thus leading to a two-dimensionally ordered TiO(2) network in which AgNPs were also self-organized. The enhanced photocatalytic activity of the AgNP-decorated TiO(2) networks by approximately 27 and 44 % over that of Ag-free TiO(2) networks and randomly deposited TiO(2) nanoparticles, respectively, was confirmed by the UV degradation property of methylene blue.

Characteristics of localized surface plasmons excited on mixed monolayers composed of self-assembled Ag and Au nanoparticles

The fundamental characteristics of localized surface plasmon resonance (LSPR) excited on mixed monolayers composed of self-assembled Ag and Au nanoparticles (AgNPs and AuNPs, respectively) were investigated. Mixed monolayered films were fabricated at the air-water interface at different mixing ratios. The films retained their phase-segregated morphologies in which AuNPs formed several 10 to 100 nm island domains in a homogeneous AgNP matrix phase. The LSPR bands originating from the self-assembled domains shifted to longer wavelengths as the domain size increased, as predicted by a finite-difference time-domain (FDTD) simulation. The FDTD simulation also revealed that even an alternating-lattice-structured two-dimensional (2D) AgNP/AuNP film retained two isolated LSPR bands, revealing that the plasmon resonances excited on each particle did not couple even in a continuous 2D sheet, unlike in the homologous NP system. The fluorescence quenching test of Cy3 and Cy5 dyes confirmed that the independent functions of AuNPs and AgNPs remained in the mixed films, whereas the AuNPs exhibited significantly higher quenching efficiency for the Cy3 dye compared with AgNPs due to the overlap of the excitation/emission bands of the dyes with the AuNP LSPR band. Various applications can be considered using this nanoheterostructured plasmonic assembly to excite spatially designed, high-density LSPR on macroscopic surfaces.