Kuan-Cheng Chiu

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.

Morphological evolution of the self-assembled IrO2 one-dimensional nanocrystals

In this paper, we report on a direct observation of the morphological evolution of one dimensional (1D) nanocrystals via a systematic metal?organic chemical vapour deposition (MOCVD) study. Several IrO2 1D nanostructures have emerged as a result of a decrease in the degree of interface instability. The morphological evolution occurs from triangular/wedged nanorods via incomplete/scrolled nanotubes to square nanotubes and square nanorods according to the morphological stability. The results show that the polycrystalline films composed of continuous 3D grains belong to the most stable form as compared to the 1D nanocrystals. The shape evolution has been found to highly correlate to a morphological phase diagram based on the growth kinetics.

Surface morphology of C60 polycrystalline films from physical vapor deposition

Abstract C 60 polycrystalline films grown from physical vapor deposition are obtained. A surface morphology diagram in terms of the degree of supersaturation and the relative strength of bonding energy vs. thermal energy is determined from scanning electron microscopy, and discussed. The size of the grains in the high quality polycrystalline films is observed to depend sensitively on supersaturation and substrate temperature. X-Ray diffraction is applied, and the preferential orientations of these grains are in the (111), (220) and (311) directions.

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.

Thermally stimulated currents in α-HgI2 polycrystalline films

A study of thermally stimulated currents (TSC) is applied to α-HgI2 polycrystalline films grown by physical vapour deposition with various thermal boundary conditions. Five TSC peaks are clearly observed and numerically fitted. The activation energy and the density of the trapping centre that corresponds to each TSC peak are then calculated. Finally, the effects of the deposition conditions on the TSC results are discussed.

Role of molecular conformations in rubrene polycrystalline films growth from vacuum deposition at various substrate temperatures

We report on the optical and structural characterization of rubrene polycrystalline films fabricated from vacuum deposition with various substrate temperatures (Tsub). Depending on Tsub, the role of twisted and planar rubrene conformational isomers on the properties of rubrene films is focused. The temperature (T)-dependent inverse optical transmission (IOT) and photoluminescence (PL) spectra were performed on these rubrene films. The origins of these IOT and PL peaks are explained in terms of the features from twisted and planar rubrene molecules and of the band characteristics from rubrene molecular solid films. Here, two rarely reported weak-peaks at 2.431 and 2.605 eV were observed from IOT spectra, which are associated with planar rubrene. Besides, the T-dependence of optical bandgap deduced from IOT spectra is discussed with respect to Tsub. Together with IOT and PL spectra, for Tsub > 170 °C, the changes in surface morphology and unit cell volume were observed for the first time, and are attributed to the isomeric transformation from twisted to planar rubrenes during the deposition processes. Furthermore, a unified schematic diagram in terms of Frenkel exciton recombination is suggested to explain the origins of the dominant PL peaks performed on these rubrene films at 15 K.

Effects of Substrate Temperature on the Properties of Alq3 Amorphous Layers Prepared by Vacuum Deposition

In this study, various Alq3 amorphous layers are prepared by vacuum deposition at different substrate temperatures Tsub. The surface morphology, structural information, and electrical and optical properties of these as-deposited layers are studied by atomic force microscopy, X-ray diffraction, J–E curves, and photoluminescence studies, respectively. The optimum deposition conditions for Alq3 amorphous layers with respect to Tsub are also discussed.