Feb. 3, 2007 University of Rochester physicists have learned why Pentacene, the leading candidate for developing organic semiconductors, conducts electricity in inconsistent ways rather than with predictable electrical properties. Specifically, Professors Yongli Gao and Yonathan Shapir, with Gao's PhD student Serkan Zorba (now an Assistant Professor at Whittier College), discovered that Pentacene is the first known substance with two basic growth mechanisms that combine to form thin films with unique fractal patterns. These fractal growths are why other researchers have found inconsistent electrical properties in layers of Pentacene.
Organic semiconductor materials have vast potential to transform electronic devices and save energy. For example, experts predict that organics will be used in the near future to create inexpensive, lightweight, flexible organic light-emitting diodes; organic thin film transistors; and organic photovoltaic cells that can power a wide variety of devices.
The processing of organic semiconductors can be done at low temperatures, whereas inorganic semiconductors require high temperatures. However, to create organic semiconductors inexpensively on large areas, fabricators must use evaporative deposition, a common method of placing a thin film on a substrate or on previously deposited layers. Another common method, called sputtering, takes a lot longer.
Pentacene is a compound of carbon and hydrogen (C22H14) with a crystal structure. Most organic materials considered as potential semiconductors are not crystals; rather, they are amorphous. Electricity can move more easily through crystalline materials because atoms are arranged in regular patterns.
As was the case with inorganic semiconductor devices in the twentieth century, the most important factor in developing twenty-first century organic semiconductors is being able to control their electrical properties in thin films. The process of controlling the electrical resistance (or its inverse, which is called mobility) of electrons in a substrate of Pentacene depends on how the material is grown in the laboratory.
Because of Pentacene's importance to the future of the organic semiconductor industry, the University of Rochester team investigated the growth patterns of Pentacene substrates and thin films grown via evaporation. They used a device called an Atomic Force Microscope (AFM), which images surface layers at the level of observing single molecules. Then they created models of the process with numerical simulations and interpreted the results.
Much to their surprise, the researchers discovered that Pentacene has two basic growth mechanisms that together form films with unique fractal patterns.
Diffusion-Limited-Aggregation, or DLA, is one of the most famous fractal cluster structures. It occurs when particles diffuse toward and stick to a cluster of molecules on the surface of a substrate. Many substances exhibit DLA behaviors when used to grow a thin film surface layer. Due to random gaps introduced by the nature of the DLA structure, the fractal dimensions of a two-dimensional layer are 1.6; this means that, given a circle of radius r, the number of molecules inside the circle is proportional to the power of 1.6 rather than 2, which is the regular exponent for a circle.
Many substances grow thin film surface layers using a different mechanism, known as mounded growth, where material deposited grows in mounds, or tiny foot hills, on a substrate. This type of surface growth occurs due to the Schwoebel Effect, where a molecule that is deposited on the surface of a mound is prevented from going downward. As more material is deposited on the substrate, the mounds get higher, and as a result, the film is bumpy rather than smooth and uniform.
Pentacene simultaneously exhibits both Diffusion-Limited-Aggregation growth and mounded growth. The DLA occurs horizontally, while the mound growth occurs vertically.
As Professor Shapir says, "Not only has this never been seen before in any experiment, it has also never been predicted theoretically."
Professor Gao speculates that the manufacturing of the first monolayer of molecules is the key to making a uniform thin film. The random fractal structure in the evaporative deposition of the first layer causes surface gaps with large electrical resistance. The subsequent building of mounds on top of these fractal structures makes the resistance even worse.
The manufacturing of a smooth first monolayer of molecules can be done using Molecular Beam Epitaxy, but that technique is very expensive and can only be used to coat very small areas. Another technique to make a uniform first monolayer is called Self Assembly Monolayer, in which the substrate surface is dipped into a carefully prepared chemical mixture that includes Pentacene. A third technique involves shining linearly polarized light on the surface to organize molecules along straight lines during evaporative deposition. These and other ideas are currently being investigated by researchers worldwide.
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