Shiow-Jing Tang

Effects of substrate temperature on the growth of C60 polycrystalline films by physical vapor deposition

This work examines the growth of C60 polycrystalline films by physical vapor deposition in a vertical chamber. By varying the substrate temperature Tsub, the as-deposited films are grouped into four types, according to surface morphology, elucidated by scanning electron microscopy. X-ray diffraction is then used to determine the degree of crystallinity of the grains in these polycrystalline films. Moreover, the Tsub-dependence of the surface morphology and the growth rate of these films are described by using a model that involves adsorption, surface kinetics, and re-evaporation of the incoming molecules. Finally, the optimal position for vapor deposition is proposed by applying the concept of constitutional supersaturation.

Effects of isomeric transformation on characteristics of Alq3 amorphous layers prepared by vacuum deposition at various substrate temperatures

Organic tris(8-hydroxyquinoline)aluminum (Alq3) amorphous layers are prepared by vacuum deposition at various substrate temperatures Tsub from 30 to 180 °C. The surface morphology and electrical characteristics of these as-deposited layers are studied by atomic force microscopy and current-density versus electric-field (J-E) curves. The temperature dependence of the dark electrical conductivity σ(T) determined from J-E curves is also examined. These experimental results reveal that the surface and electrical properties of Alq3 amorphous layers deposited at Tsub between 90 and 120 °C exhibit an anomalous Tsub dependence. However, this anomalous Tsub dependence is not observed from infrared absorption measurements, and therefore is not the result of chemical degradation. The observed behavior is explained in terms of the property that the vacuum deposition of Alq3 with Tsub between 90 and 120 °C involves a thermal interconversion between meridional and facial Alq3 isomers.

Abrupt Change in Dark Conductivity on C60 Polycrystalline Films at the Rotational Order/Disorder Transition

The temperature dependence of dark conductivity of C60 fullerites in the form of polycrystalline films sandwiched between metal electrodes is systematically studied. At the rotational order/disorder phase transition temperature Tc of about 256 K, an abrupt change in dark conductivity of 4–5 orders of magnitude within a narrow temperature range of less than 2 K is observed. The conductivity for this sample decreases as the temperature crosses Tc from fcc phase to sc phase. From the Arrhenius plot of dark conductivity, the activation energies are estimated to be 53±4 meV for the fcc phase and 111±6 meV for the sc phase. To explain these experimental findings, the electric transport through parallel, extremely pure, and homogeneous C60 monocrystals between two metal contacts is postulated and described, and a model based on the local lattice potential induced by rotational order of C60 molecules is proposed.

Effects of substrate temperature on properties of Alq3 amorphous layers prepared by vacuum deposition

In this report, various organic Alq3 amorphous layers are prepared by vacuum deposition at different substrate temperatures Tsub (from 30 to 180°C). The surface morphology, structural information, electrical and optical properties of these as-deposited layers are investigated by atomic force microscopy, X-ray diffraction, J-E curves, and photoluminescence studies, respectively. Furthermore, a temperature dependence of dark electrical conductivity σ(T) deduced from J-E curves of these organic amorphous layers is presented. Finally, effects from Tsub on the physical properties of these organic Alq3 amorphous layers are discussed and a model based on a thermal interconversion between Alq3 isomers is proposed to explain these experimental results.