Feasibility Study

Organic Superlattice

Kiyoshi Yase

■Object

In the Feasibility Study titled ﾒOrganic Superlatticeﾓ, basic parameters required for the preparation of molecular thin film and superlattice under the structural control using the organic molecular beam deposition (OMBD) method have been measured and analyzed in three years from Fiscal 1993 through 1995. The thin film of organic compound is usually prepared by letting molecules ﾒflyﾓ through a vacuum to a target. Among millions of kinds of organic molecules, the vapor pressure has been known for volatile compounds, but little works are available for the actual measurement of evaporating rate. Compounds for the vacuum evaporation are generally solid at the normal temperature and pressure, and can evaporate only when heated under a vacuum of 10-5 to 10-8 Torr.

In the vacuum evaporation process, the evaporation rate as a function of the specimen temperature is one of the essential parameters, and the quantitative analysis and control of film growth can be achieved only through he control of molecular beam intensity, that is, the number of molecules arriving at the substrate surface in a unit time. The study has been initiated with a search for the technique of measuring the basic parameter. Then, the quantitative determination has been tried for some parameters of crystal growth, such as the nucleation frequency after the deposition of a certain amount of molecules, and the diffusion range of molecules on the substrate surface. The process of the epitaxial growth of organic molecules on the surface of inorganic crystal has been observed by the atomic force microscopy (AFM), and the growth process of some functional organic molecules has been quantitatively determined by the in-situ measurement with the total reflection X-ray diffraction method. Finally, the molecular configuration of high quality molecular crystalline film prepared under the structural control has been imaged by the scanning tunneling microscopy (STM) and the transmission electron microscopy (TEM).

■Results in Fiscal Year

（(1) Determination of Molecular Beam Intensity

With an ultra-high vacuum system combined with a high quality quadrupole mass spectrometer (Q-MASS), the molecular beam intensity has been measured in association with sublimation and evaporation of functional organic molecules, such as fullerene (C60), phthalocyanine and TIF-TCNQ, and it has been confirmed that these compounds, including charge-transfer type complex such as TTF-TCNQ, evaporate without undergoing pyrolysis, and the saturation vapor pressure and the enthalpy related to sublimation have been determined from the dependence of evaporation rate on the crucible temperatures.

These results have been used as basic quantitative parameter of vacuum evaporation of organic compound for the analysis of film growing mechanism and the construction of high quality film crystals.

Fig. 1 The construction of ultra-high vacuum system combined with a high quality quadrupole mass spectrometer (Q-MASS) and a total reflection X-ray diffraction unit (TRXD).

(2) Nucleation and Surface Diffusion of Organic Molecules

TTF-TCNQ and C60 have been deposited on cleaved face of alkali halide crystal (NaCl, KCl or KBr) using molecular beam, and the size and spacing of microcrystals in the initial stage of growth have been observed by using AFM and TEM. From the temperature dependence of these values, the nucleation frequency, growing rate, activation energy of surface diffusion and dependence of crystalline growth on the axial orientation of molecular crystals have been calculated. In case of TTF- TCNQ, the axial length of crystal differs depending upon the orientation, and some useful findings on the anisotropic crystalline growth have been obtained. In distinction from the previous studies, where the mechanism of epitaxial growth has been related to the mismatch between the substrate surface structure and the molecular crystal lattice, the relation of the substrate surface energy to the growth of organic molecule film has been clarified.

(3) In-Situ Measurement of Epitaxial Growth

The total reflection X-ray diffraction unit (TRXD) has been combined with the ultra-high vacuum organic molecular beam deposition system (UHV-OMBD). The white X-rays obtained with molybdenum as a target have been irradiated onto a specimen at an angle as small as 0.05｡, to cause the total reflection at the surface of inorganic crystal substrate, and the structure of the organic film has been characterized through the energy analysis of diffracted X-ray intensity using a solid-state detector (Fig. 1). While the reflection high energy electron diffraction (RHEED) has been frequently used for the molecular beam epitaxy of inorganic compounds, it is unavoidable to damage the organic film with high energy electron. In the present study, the in-situ structural characterization has been achieved by using X-rays which cause less damage than electron beams.

In the molecular beam deposition of C60 on to a substrate consisting of epitaxially grown film of silver on silicon, the epitaxial growth preserving the orientational relation of the substrate have been successfully measured in-situ.

(4) Observation of Molecular Image of High Quality Organic Film Crystal

Through the examination of growing conditions for wide area epitaxially grown film of TTF-TCNQ and C60, the perfect crystalline film as large as 10μm has been successfully prepared without lattice fault, and the molecular image of this film has been observed by STM and TEM. A TEM image of high quality C60 crystalline film is shown in Fig. 2.

No crystalline film free from fault and dislocation have ever been obtained by other workers. Following the present Feasibility Study, the experiment will be continued toward the goal of making ﾒsuperlatticeﾓ by stacking layers of other organic compounds on the substrate of high quality molecular film.

Fig. 2