Plastic That Performs
Organic transistors get up to speed
Context: Organic transistors, which are made from semiconducting plastics, are cheap to manufacture. Although they enable thin, bendable electronics, so far they can’t implement the fastest, most efficient circuit designs, because the plastics can’t transport electrons. Instead, they rely on a flow of positive charges, or electron “holes,” to pass current, which limits their use. Now, in a surprising discovery, Lay-Lay Chua at the University of Cambridge and Peter Ho at the National University of Singapore have shown that the transistors’ inability to move electrons is due not to the plastic itself but to an interaction with other materials in the transistor.
Methods and Results: In a transistor, current passes through a semiconductor under the control of a gate electrode. The gate electrode is separated from the semiconductor by an insulator, typically silicon dioxide. In conventional silicon transistors, electrons pass through the semiconductor without interacting with the insulator. But in most plastic semiconductors, atoms at the interface between the insulator and the plastic trap electrons, so they can’t flow through the transistor. By carefully designing an alternative insulator to replace silicon dioxide, Chua, Ho, and colleagues demonstrated that organic semiconductors can indeed conduct electrons. The discovery could make for simpler, higher-quality organic transistors that can implement the most commonly used designs.
Why It Matters: Organic transistors can be built using relatively cheap fabrication technologies such as ink-jet printing. The new insulator should let such cheap transistors perform many more tasks. The first application that beckons: electronically active tracking labels known as radio frequency identification (RFID) tags. With the new understanding of organic transistors provided by Chua and his colleagues, fast and cheap plastic electronics could soon be as ubiquitous as ink.
Source: Chua, L.-L., et al. 2005. General observation of n-type field-effect behaviour in organic semiconductors. Nature 434:194-9.
Contents under Pressure
Modeling motor oil could lead to lighter, more efficient engines
Context: Since the late 1930s, zinc dialkyldithiophosphates – or ZDDPs – have been added to motor oils to prevent wear in steel engines, but no one knows why they works so well. The puzzle is more than academic: the auto industry would like to build engines from aluminum, but aluminum wears quickly, and there is currently no antiwear additive that works for aluminum as well as ZDDPs work for steel. Now a group from the University of Western Ontario has developed a computer model that could lead to new additives that make nonsteel engines feasible.
Methods and Results: Nicholas Mosey and colleagues performed a quantum mechanical simulation of how zinc phosphates – the molecules created when ZDDPs decompose in oil – react to the intense pressures generated when engine components slide against each other. They discovered that zinc phosphate molecules, when squeezed together, form a dense network that withstands friction created by rapidly moving engine parts. Strong materials like steel can stand up to the high pressures needed to create this network, but such pressures are too much for aluminum. Before the zinc phosphates organize into the protective network, they become very hard – harder than aluminum – and would cause abrasive wear.
Why It Matters: Using aluminum in engines could boost fuel economy. With an understanding of how ZDDPs work, additives could be designed for aluminum engines, making them more practical.
Source: Mosey, N. J., et al. 2005. Molecular mechanisms for the functionality of lubricant additives. Science 307:1612-5