Multifunctional Dendrimers Incorporating Diphenylaminopolyenylic Chromophores
- PDF / 286,582 Bytes
- 6 Pages / 431 x 650 pts Page_size
- 104 Downloads / 207 Views
chamber of the H W E system two H W E reactors and one preheating/annealing oven were installed [11]. Each of the reactors consisted of two separately heated ovens for the wall and source zone, which could be kept at different suitable temperatures during growth. The quartz tube with the source material at the bottom was mounted inside the ovens. The substrate was placed close above the tube end and could be heated separately. The sample holder could be moved from one H W E reactor to another one using a computer controlled step-motor, which allows also the growth of multilayer structures . The vacuum during growth was about 6x10-6 mbar. The films were grown at a fixed PHPand C60 -source temperature of 240°C and 400°C, respectively. The wall temperature was in the range of 240-260°C for PHP growth and 400-420°C for C60. The substrate temp eratures were varied in a 50-180°C range. These growth parameters resulted in a low deposition rate of about 2-3 nm/min. The film morphology was imaged using optical microscopy and atomic force microscopy (AFM ). The AFM was conducted using NanoScope IIIa M icroscope operated in contact mode in air. The crystalline quality of the PHP-films was investigated by a conventional X-ray diffractometer in coupled θ /2θ reflection mode using CuKα radiation. Polarized photoluminescence spectra were measured on a Hitachi F-4010 Fluorescence Spectrometer at normal incidence. All optical measurements were performed at room temperature . RESULTS AND DISCUSSION The growth process strongly depends on the substrate material and substrate temperature . Figure 1 shows typical A FM images of monolayer samples of PHP grown on crystalline mica substrates . In contrast to α-sexithienyl (T6) thin films on mica [8] and PHP films on GaAs [6,9] grown by other techniques , a large scale ordered structure (see
Figure 1. AFM images (so called “deflection or error” images of the feed back signal) chracterizing the “needles”morphology of PHP films grown on mica at 90°C. a) 20x20 µm image, deposition time-60 minutes; b) 30x30 µm image, deposition time-10 minutes.
figure 1a) with a strongly expressed preferential direction is observed. This structure consists of large oriented crystallites looking like „needles „, separated by relatively flat areas . AFM line profile measurements show that these „needles„ depicted in figure 1a are typically about 130 nm high and up to 800 nm wide with flat areas of 300-400 nm in between. Needles with a length of up to ≈100 µm were detected. The crystallites are strongly anisotropic and their axes align according to one preferential direction. On perfectly cleaved mica this preferential direction is not changed for the whole film surface of 20 mm2, as found by light-microscope studies . On terraces, separated by big cleavage steps, the preferential direction of the „needles „ can change by 120° without any other changes in its morphology. A typical example is shown in figure 1b. The roughness of the space between the needles is determined to 3-4 nm that is comparable to the roug
Data Loading...
