Observation of fractal patterns in C 60 -polymer thin films
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S. Pang Beijing Laboratory of Vacuum Physics, Academia Sinica, Beijing 100080, People's Republic of China (Received 4 April 1994; accepted 26 April 1994)
We report the observation of fractal patterns in C6o-tetracyanoquinodimethane thin films. The fractal patterns and their microscopic features are described and characterized. The fractal dimension was determined to be 1.69 ± 0.07. According to the characterization results, the observed fractals are compared to the cluster-diffusionlimited-aggregation model. The growth of the fractal patterns in the thin films is also in terms of the existing long-range correlation.
Much effort has been devoted to the understanding of fractal phenomena arising in a wide variety of theoretical and experimental situations in the last decade.1"5 Most of the activity in the field has been mainly on theoretical consideration and computer simulation,6'7 while fewer papers have been published on the observation of the pattern formation.8'9 Up to now, most of the observed patterns can be compared to Witten and Sander's diffusion-limited aggregation (DLA) model6 and the cluster-diffusion-limited-aggregation (CDLA) model.10 Except for a few cases concerning a dense branching morphology (DBM), 1112 the DLA and CDLA models are the most widely adopted by the researchers in analyzing fractal patterns and understanding the growth of the fractals.
the order of 10" 8 Torr. Pure C60 and TCNQ are deposited alternatively onto an amorphous carbon substrate. The vacuum level during deposition was about 5 X 10~ 7 Torr. The films consisted of sandwich-like layers; i.e., the C6o layer lies between two TCNQ layers. The thickness of each individual layer was determined by the deposition rate and time. The relative thickness of the constitution was adjusted to obtain the desired electric properties. The total thickness of the multilayered thin films was about 70 nm. The detailed experimentation can be found in our previous publications.18'19 Scanning tunneling microscopy (STM), scanning electron microscopy (SEM), x-ray diffraction, transmission electron microscopy (TEM), and nuclear magnetic resonance (NMR) were employed to characterize the samples.
Buckminsterfullerene,13 C 60 , has intensively attracted scientists' interest in studying its physical properties as well as its geometrical structures.14'15 In addition to its extraordinary molecular properties such as structural phase transitions, geometrical structures, ultrafast nonlinear optical response, and superconducting behavior caused by alkali metal doping,16 some remarkable phenomena have also been observed in C6o-tetracyanoquinodimethane (C6o-TCNQ) multilayered thin films. In this communication, we report, for the first time to our knowledge, the observation of fractal patterns in the C6o-TCNQ multilayered thin films, which is of vital importance and interest not only for the fractal pattern itself but also the unique form of C6o in nature. This new fractal pattern emerging in the multilayered thin films has a fractal dimension of 1.69 ± 0.07 and
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