Structural Evaluation and Molecular Control of Vacuum-Evaporated Organic Thin Films
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atoms and molecules in real space under various atmospheres. In addition, STMs can be used as nanoscopic tools for manipulation of individual atoms and molecules,3 thus realizing MEDs and nanotechnology. In this article, we present our recent achievements concerning the STM as well as in situ x-ray diffraction studies on the molecular structure of ultrathin films prepared by vacuum evaporation. STM observations with atomic resolution reveal the mechanism of nuclei formation and the crystal-growth process in organic molecules. Computer simulations based on STM images of polar organic molecules with electronic dipoles have elucidated the role of electronic interaction for their aggregation structures.
Also, nanometer-sized molecular memory can be created by applying an electronic pulse to the evaporated organic films through the STM tip. Furthermore, we discuss the principle of a newly developed in situ total reflection x-ray diffraction (TRXD) apparatus and its application to the evaluation of crystal structure and molecular orientation in organic thin films during the evaporation process, particularly in regard to the role of the substrate, that is, epitaxial growth on organic molecular crystals. STM Observations STMs and their family of scanning probe microscopes (SPMs) are now recognized as some of the most important and powerful research instruments in the fields of atomic and molecular science, as well as in nanotechnology. Among them, the STM with the highest resolution power has been utilized in observing the aggregation structure of various organic molecules. Some research groups 4 " 6 have succeeded in getting STM images of wellordered n-paraffin molecules—among the most simple of organic molecules— and have directly revealed their molecular aggregate structures. Also, liquid crystals of n-alkyl cyanobiphenyl (mCBs where m is the number of carbons in the alkyl group) have been observed extensively with the STM, revealing that the kinds of substrates and the length of alkyl chains affect the aggregation structures of these organic molecules.7"11 However, most of these STM studies have focused on imaging well-established molecular crystals while not elucidating transient features that reveal how individual molecules form nuclei and then grow to two-dimensional crystals, or on the role of intermolecular forces such as an electronic-dipole interaction in the
Figure 1. STM image of n-paraffin, showing successive stages of two-dimensional crystal growth.
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MRS BULLETIN/JUNE 1995
Structural Evaluation and Molecular Control of Vacuum-Evaporated Organic Thin Films
(a)
, N C 12 H M OY()V-Q CN 2.6 nm — ->j I*- 1.1 nm'i
T
Figure 2. (a) Chemical structure of DOPPC, (b) its STM image, and (c) its compute/ simulation.
polar molecular assemblies. In this article, these questions are addressed based on high-resolution STM images and the aid of computer simulation.
Growth Process of TwoDimensional n-Paraffin Crystals Figure 1 shows a series of STM images observed at room temperature in a dried Nj-gas atmosphere
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