Dongho Park

Influence of postannealing on polycrystalline pentacene thin film transistor

We studied systematically the influence of in situ postannealing treatment of ultrahigh vacuum grown polycrystalline pentacene thin film transistor. The gradual grain growth with the elimination of defects and misoriented crystallites is confirmed in x-ray diffraction (XRD) data and the atomic force microscopy image as the annealing temperature increases. The XRD data reveal that the pentacene molecules are packed parallel to each other in an upright position with a tilting angle of 15.5°. The postannealing results in the enhanced field effect mobility of pentacene organic thin film transistors increases from 0.19±0.04 to 0.49±0.05 cm2/V s after annealing at 90 °C. We suggest that the abnormally small on/off current ratio (∼103) due to the large leakage current is attributed to the conduction via impurity levels originated from the structural isomers of pentacene.

Enhancing the electroluminescent properties of organic light-emitting devices using a thin NaCl layer

We report on the fabrication of organic light-emitting devices (OLEDs) using a thin NaCl interlayer as an electron-injection medium. The results show that the device containing the NaCl layer has a higher brightness and electroluminescent efficiency than the device without this layer. We also fabricated similar-structured comparable devices, which were prepared with a LiF layer as a different electron-injection medium. The maximum electroluminescent efficiency of the NaCl (1 nm)/Al cathode device was 2.85 cd/A, which is higher than the 2.25 cd/A of the LiF (1 nm)/Al cathode device. The ultrathin NaCl layer modified the carrier injection properties. In conclusion, the NaCl layer between a cathode and an emitting layer of OLEDs can be used as the carrier injection layer to improve the EL properties.

Buffer layer effect on the structural and electrical properties of rubrene-based organic thin-film transistors

The structural and electrical properties of organic thin-film transistors with rubrene/pentacene and pentacene/rubrene bilayered structures were investigated using x-ray diffraction, atomic force microscopy, and x-ray emission spectroscopy. High-quality rubrene thin films with orthorhombic structure were obtained in the rubrene/pentacene bilayer while the pentacene/rubrene bilayer only had an amorphous rubrene phase present. The rubrene/pentacene thin-film transistor shows more desirable current-voltage characteristics compared to the pentacene/rubrene transistor. The overall results suggest that the presence of a chemically active organic buffer layer and its associated crystal structure are crucial in enhancing the structural and electrical properties of rubrene-based transistors.

Micromachined cascade virtual impactor with a flow rate distributor for wide range airborne particle classification

This letter reports a module for airborne particle classification, which consists of a micromachined three-stage virtual impactor for classifying airborne particles according to their size and a flow rate distributor for supplying the required flow rate to the virtual impactor. Dioctyl sebacate particles, 100–600nm in diameter, and carbon particles, 0.6–10μm in diameter, were used for particle classification. The collection efficiency and cutoff diameter were examined. The measured cutoff diameters of the first, second, and third stages were 135nm, 1.9μm, and 4.8μm, respectively.

Formation of Ceramic Nanoparticle Patterns Using Electrohydrodynamic Jet Printing with Pin-to-Pin Electrodes

As one of the direct write technologies, electrohydrodynamic jet printing was used in obtaining fine ceramic lines. We used pin electrodes of various diameters, each of which was located below the substrate, and analyzed the effects of pin diameter on Al2O3 nanoparticle one- and two-dimensional patterns formed with pin (nozzle)-to-pin (ground) electrodes. The onset voltage required to start the formation of a pattern for a 1-µm-diameter electrode was fourfold lower than the voltage required for a 1000-µm-diameter electrode. Additionally, an Al2O3 nanoparticle pattern with a uniform width as fine as 25 µm was obtained despite using the very large diameter of the nozzle (920 µm) used.

Electronic structure of pentacene/ultrathin gate dielectric interfaces for low-voltage organic thin film transistors

This paper describes the fabrication of pentacene-based thin film transistors (TFTs) with ultrathin (4.5nm) SiO2 and SiON gate dielectric layers for low-voltage operations. The device with the SiON gate dielectric layer operated at gate voltages lower than −3.0V, showing a threshold voltage of −0.45V, which was lower than the threshold voltage of the SiO2 device (−2.5V). The electronic structures of the interface between the pentacene and dielectric layers were investigated by in situ ultraviolet photoelectron spectroscopy (UPS) and x-ray photoelectron spectroscopy (XPS) to determine the reason for the lower operating voltage. The UPS and XPS results demonstrated that the interface dipole modified the potential of the dielectric layer, explaining the lower operating voltage. The electronic structure allowed for band bending at the interface, resulting in complete energy level diagrams for pentacene on SiO2 and SiON. The shifts in the threshold and turn-on voltages were explained by the energy level diagrams.

Micromachined cascade virtual impactor for aerodynamic size classification of airborne particles

We report design, fabrication and experimental results of a micromachined cascade virtual impactor with flow rate distributor for classifying airborne particles according to their sizes. The fabricated device consists of two components: a cascade virtual impactor for classifying particles according to their own inertia, and a flow rate distributor to maintain specific flow rates at each channel. Proposed device is fabricated using polymer-based micromachining. Due to the flow rate distributor, only one external pump was used for controlling major flow rates and minor flow rates. DOS particles ranged from 100 to 600 nm and carbon particles from 0.6 to 10 mum were used for the classification experiments. Particle size distribution at the inlet and each minor flow were measured by SMPS for DOS particles and APS for carbon particles. Classification of airborne particles with 3 cut-off diameters is successfully demonstrated. The measured cut-off diameters were 135 nm, 1.9 mum, and 4.8 mum.

Micromachined electrical mobility analyzer for wide range airborne particle classification

This paper reports a micromachined nano electrical mobility analyzer (nEMA) for nano- sized airborne particle classification. The micromachined nEMA is a particle classifier that uses both the inertia and electrical mobility of the particles for the classification. The microchannel of the nEMA is defined by silicon bulk micromachining. The collection efficiency of the solid particle, NaCl with a diameter of less than 50 nm, was examined using the nEMA by applying an electrical potential. For various electric field magnitude ranging from 10 to 160 V, the particle diameter collected in the outlet was measured. The collection efficiency of 40 nm particles was investigated by increasing an electric field, and it was highest at 70 V.

A hybrid chip based on aerodynamics and electrostatics for the size-dependent classification of ultrafine and nano particles.

Conventional virtual impactors experience a large pressure drop when they classify particles according to size, in particular ultrafine particles smaller than 100 nm in diameter. Therefore, most virtual impactors have been used to classify particles larger than 100 nm. Their cut-off diameters are also fixed by the geometry of their flow channels. In the proposed virtual impactor, particles smaller than 100 nm are accelerated by applying DC potentials to an integrated electrode pair. By the electrical acceleration, the large pressure drop could be significantly decreased and new cut-off diameters smaller than 100 nm could be successfully added. The geometric cut-off diameter (GCD) of the proposed virtual impactor was designed to be 1.0 microm. Performances including the GCD and wall loss were examined by classifying dioctyl sebacate of 100 to 600 nm in size and carbon particles of 0.6 to 10 microm in size. The GCD was measured to be 0.95 microm, and the wall loss was highest at 1.1 microm. To add new cut-off diameters, monodisperse NaCl particles ranging from 15 to 70 nm were classified using the proposed virtual impactor with applying a DC potential of 0.25 to 3.0 kV. In this range of the potential, the new cut-off diameters ranging from 15 to 35 nm was added.

The Control of Particle Size Distribution for Fabricated Alumina Nanoparticles using a Thermophoretic Separator

The control of particle size distribution of fabricated alumina nanoparticles from general alumina that has a large geometric standard deviation (GSD) is studied. A thermophoretic separator was used to control the GSD of the fabricated alumina nanoparticles. Unevaporated particles and primary particles were separated to yield a small GSD for fabricated alumina nanoparticles. The fabricated alumina nanoparticles were characterized by using a field emission scanning electron microscope and scanning mobility particle sizer. The GSD of the fabricated alumina nanoparticles was confirmed to be controlled by the thermophoretic separator. The temperature difference applied to the thermophoretic separator for the control of GSD of the fabricated alumina nanoparticles was between 79 K and 151 K. The GSD of the fabricated alumina nanoparticles was improved from 1.74 to 1.44.