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Growing organic

Nature 412, 517–520 (2 August 2001)

Imagine how useful a plastic transistor could be. Flexible displays and cheap, all-plastic smart cards are just a few possibilities. Last year, the Nobel Prize for chemistry went to Heeger, MacDiarmid and Shirakawa for their discovery of electrically conducting polymers, the committee predicting "consequences of great benefit to mankind".

Pentacene crystals grow in fractal formation on a silicon surface. Dark areas appear as the second layer begins to grow on top of the first layer.

But still most microelectronic devices are made of inorganic semiconducting materials. Plastic transistors are already showing promise for applications , but their speed of operation is slow compared to their inorganic counterparts, limiting their utility. The problem stems from the difficulty in making organic crystals of high quality in thin-film form. But that may be about to change: this week in Nature, Frank Meyer zu Heringdorf and colleagues report progress in understanding organic thin-film growth through detailed studies of the semiconducting molecule pentacene (C₂₂H₁₄), a chain-like aromatic molecule made of five benzene rings.

Using photoelectron emission microscopy, with a field of view of up to 65 µm and a resolution of 125 nm, the authors watched the build-up of pentacene on a Si(001) surface over a timescale of minutes. First, stable two-dimensional islands of pentacene nucleate on the surface, each island forming a one-layer-thick pentacene crystal. The islands branch out like fractals, but before they touch, a second layer nucleates (typically when the first layer coverage reaches about 60%). Eventually, the trenches between islands are filled in, leaving grain boundaries that act as barriers to charge flow — and which would hinder the electrical performance of transistors made from such films.

Meyer zu Heringdorf et al. noticed a dead-time between the start of deposition and the nucleation of pentacene islands. Further imaging revealed that the first-deposited pentacene molecules are adsorbed at reactive sites (where there are 'dangling bonds') on the Si(001) surface. Only when all these sites are neutralized by the adsorption of pentacene can the first layer of the thin crystalline film start to grow. Treating the Si surface with cyclohexene or using a different substrate such as SiO₂, which has no dangling bonds, got rid of the dead-time and produced smoother films.

Also reported in the paper is the successful growth of films with grain sizes reaching about 0.1 mm — about 100 times larger than in previous work, and opening up the possibility of fabricating an organic transistor entirely contained within a single grain. The authors conclude that their observations will speed up the development of workable organic thin-film devices.

letters to nature
Growth dynamics of pentacene thin films
The recent demonstration of single-crystal organic optoelectronic devices has received widespread attention. But practical applications of such devices require the use of inexpensive organic films deposited on a wide variety of substrates. Unfortunately, the physical properties of these organic thin films do not compare favourably to those of single-crystal materials. Moreover, the basic physical principles governing organic thin-film growth and crystallization are not well understood. Here we report an in situ study of the evolution of pentacene thin films, utilizing the real-time imaging capabilities of photoelectron emission microscopy. By a combination of careful substrate preparation and surface energy control, we succeed in growing thin films with single-crystal grain sizes approaching 0.1 millimetre (a factor of 20–100 larger than previously achieved), which are large enough to fully contain a complete device. We find that organic thin-film growth closely mimics epitaxial growth of inorganic materials, and we expect that strategies and concepts developed for these inorganic systems will provide guidance for the further development and optimization of molecular thin-film devices.
FRANK-J. MEYER ZU HERINGDORF, M. C. REUTER & R. M. TROMP
Nature 412, 517-520 (2 August 2001).