A. Ishitani et T. Nonaka, PREPARATION AND CHARACTERIZATION OF ORGANIC SUPERLATTICE THIN-FILMS GROWN ON HYDROGENATED SILICON SINGLE-CRYSTAL SUBSTRATES, Surface and interface analysis, 21(6-7), 1994, pp. 356-364
We have succeeded in preparing very thin (nanometre order) alternating
layers that can be called 'organic superlattices' by using a special
combination of materials, substrates and deposition conditions with go
od verification by various characterization techniques. Specially surf
ace-treated single-crystal silicon substrates with hydrogen-terminated
dangling bonds on the surface were used to induce van der Waals heter
oepitaxial growth. Choice of materials with the appropriate molecular
shapes and aggregation force is also important for preparing flat mole
cular films. Tetraphenylporphyrins (H2TPP and ZnTPP) and phthalocyanin
es were used for this purpose. A molecular beam epitaxy chamber with s
pecially designed cells was used with the cooled substrates. Direct pr
oof of the alternating composition was given by transmission electron
microscopy observation, avoiding decomposition by the electron beam, a
nd also by SIMS depth profiling under special conditions to ensure goo
d depth resolution. X-ray diffraction in the small-angle region gives
the periodicity of the alternating layers, as well as their crystal qu
ality. Atomic force microscopy confirms the atomic flatness and also g
ives the molecular orientation in the films. Fourier transform infrare
d spectroscopy examines the chemical stability of the compounds during
deposition and also gives the axial orientation of the molecules in t
he films.