Growth process of vapor deposited films of a bisazomethine dye

Graduate School of Environment and Information Sciences, Yokohama National University* Faculty of Education and Human Sciences, Yokohama National University, Japan** Department of Physics and Electronics, Osaka Prefecture University, Japan*** Supramolecular Science Laboratory, RIKEN(The Institute of Physical and Chemical Research), Japan****
â—‹Daisuke Tsuchida* Miei Satoh* Shinya Matsumoto** Takashi Kobayashi*** Tetsuya Aoyama**** Tatsuo Wada****

A bisazomethine dye, N,N'-bis-[4-(N,N'-diethylamino)benzylidene]diaminomaleonitrile (DE2), is known to form J-aggregates in vapor deposited films depending on the film thickness. The J-aggregate films also include non-aggregate forms, and this has become a problem in its applications to optoelectronic devices. The growth of DE2 thin films was then studied to obtain its complete J-aggregate films. The analysis was carried out from the morphological point of view by using AFM. Vapor deposited films were prepared by vacuum deposition onto a slide glass at various film thicknesses. The rate was controlled to be 0.01, 0.1 and 1.0 nm/s.
The J-aggregates appear when the films are deposited at a rate of 0.1 nm/s and their thickness is more than 100 nm. AFM observation showed that the J-aggregate films showed heterogeneous texture composed of prismatic grains and unclear grain boundaries. The J-aggregates were found to be a metastable phase and are considered to form in the grain boundaries and on the grain surfaces. In the film deposited at a rate of 0.01 nm/s, clear crystal grains formed as is the case of 0.1 nm/s. However the J-aggregate formation was not observed in the thick films. At this rate, supersaturation is considered to be too small to form metastable J-aggregates. On the other hand, large supersaturation resulted in many nucleation in the film deposited at a rate of 1.0 nm/s. The substrates were covered by many small crystal grains and they doesn't grow up to large ones as increasing film thickness. The J-aggregate formation was not observed in the thick film at this rate. This result indicates that the J-aggregates cannot form on such small crystal grains. The present study shows that both nucleation and growth processes are important for the J-aggregate formation.