Shichoon Lee

Switchable Transparency and Wetting of Elastomeric Smart Windows

Smart windows with switchable light transmittance properties have recently attracted signifi cant attention because of their many applications, such as architectural or vehicle windows, skylights, and internal partitions. To date, reversible switching has been achieved either by a change in molecular arrange-ments or by the oxidation–reduction reaction of chromogenic materials activated by light, electrical voltage, or tempera-ture.

Paschen breakdown curve by one-dimensional PIC-MCC simulation

The voltage at which a gas confined in a gap ignites, i.e. the breakdown voltage, is a function of the total pressure (p) and the gap size (d). The understanding of gas discharge has been important in applications of weakly ionized plasmas and studying the properties of the plasma medium, such as nanotechnoogy, ion sources and thrusters [1-2]. We obtained the Paschen breakdown curve by considering the energy and angle dependent secondary electron emission coefficient γse for ions impinging on a MgO surface and compared it with the results of fluid simulation. For the dc breakdown, a parallel electrode assumed to be coated with MgO was considered. The gap distance between two parallel electrodes was fixed 100μm and the gas pressure varied between 100 to 1000Torr. For these simulations, Ne-Xe mixture gas was employed and an elastic, excitation and ionization collisions between electrons and neutrals as well as elastic and resonant charge exchange collisions between ions and neutrals were added to the original XPDP1 code [3]. Figure 1 shows the Paschen breakdown curves by one-dimensional PIC-MCC simulation and twodimensional fluid simulation at typical pd values used in PDP cells (1-10Torr cm). Four symbols correspond to the data from one-dimensional PIC simulation results with the energy and angle dependent γse. The solid lines were obtained using our two-dimensional fluid simulation. The constant γse used in fluid simulation are 0.2 and 0.05 for Ne and Xe ions, respectively. While the simulation results had some discrepancies, the trend of these curves is obtained. The breakdown voltage increases as the Xe content is increased and minimum voltages are shifted to higher pd values around 3Torr cm in PIC simulations [4]. The Paschen breakdown curves for Ne-Xe mixture gas have been investigated using one-

pn-Heterojunction Effects of Perylene Tetracarboxylic Diimide Derivatives on Pentacene Field-Effect Transistor

We investigated the heterojunction effects of perylene tetracarboxylic diimide (PTCDI) derivatives on the pentacene-based field-effect transistors (FETs). Three PTCDI derivatives with different substituents were deposited onto pentacene layers and served as charge transfer dopants. The deposited PTCDI layer, which had a nominal thickness of a few layers, formed discontinuous patches on the pentacene layers and dramatically enhanced the hole mobility in the pentacene FET. Among the three PTCDI molecules tested, the octyl-substituted PTCDI, PTCDI-C8, provided the most efficient hole-doping characteristics (p-type) relative to the fluorophenyl-substituted PTCDIs, 4-FPEPTC and 2,4-FPEPTC. The organic heterojunction and doping characteristics were systematically investigated using atomic force microscopy, 2D grazing incidence X-ray diffraction studies, and ultraviolet photoelectron spectroscopy. PTCDI-C8, bearing octyl substituents, grew laterally on the pentacene layer (2D growth), whereas 2,4-FPEPTC, with fluorophenyl substituents, underwent 3D growth. The different growth modes resulted in different contact areas and relative orientations between the pentacene and PTCDI molecules, which significantly affected the doping efficiency of the deposited adlayer. The differences between the growth modes and the thin-film microstructures in the different PTCDI patches were attributed to a mismatch between the surface energies of the patches and the underlying pentacene layer. The film-morphology-dependent doping effects observed here offer practical guidelines for achieving more effective charge transfer doping in thin-film transistors.