Nov 27, 2003
Molecular memories tough it out
Researchers at the University of California, Riverside, US, and North Carolina State University, US, claim to have shown, for the first time, that an organic molecule can compete with semiconductors in terms of stability. The finding could have implications for the development of hybrid molecular-semiconductor devices.
“We have demonstrated that porphyrin, a particular type of organic molecule that we have previously shown might be useful for information storage, can undergo trillions of write/read cycles and is extremely stable under harsh conditions,” David Bocian of the University of California, Riverside told nanotechweb.org. “We anticipate that the first generation of devices that use molecules will be hybrid designs where molecules are integrated with semiconductors. This leverages the vast infrastructure of semiconductor technology with certain advantages that might be afforded by molecules.”
According to Bocian, porphyrins offer the possibility of multibit storage at relatively low potentials (below about 1.6 V). The information is stored by the oxidation state of the molecule. Using the molecules as charge-storage reservoirs differs from most approaches in molecular electronics, which tend to use molecules as switching elements. “Porphyrins are relatively good charge-storage media, indeed nature capitalizes on this in a number of biological processes,” said Bocian. “On the other hand, organic molecules are in general very poor electrical conductors. Hence, strategies that rely on switching typically involve the application of large voltages, which in turn can lead to degradation of the molecules.”
The scientists subjected a monolayer of porphyrin molecules covalently attached to a silicon substrate to temperatures of 400 °C for half an hour. The cyclic voltammetric behaviour of the monolayer after the treatment was the same as that of an untreated monolayer. In contrast, a monolayer of electroactive ferrocene molecules decomposed when treated at temperatures above 200 °C for just five minutes - there was visible charring and the characteristic ferrocene voltammetric signature disappeared.
Bocian and colleagues also tested the robustness of the molecules under repeated cycles of oxidation and reduction. Oxidation is equivalent to writing a bit of information, while reduction equates to erasing or destructively reading out that bit. The researchers stopped the cycling after a total of more than 1010 cycles: the system did not show any signs of degradation. In fact, the charge-storage characteristics of the monolayer only varied by a few per cent during the cycling experiment, with the voltammetric response stabilizing after about 107 cycles.
“One possible application is to integrate the porphyrin with semiconductor technology to form a hybrid molecular-semiconductor DRAM type cell,” said Bocian. “As the porphyrins can function as multibit information storage media, more information can be stored in a single location than with a conventional single-bit memory cell. If two or three bits can be stored in a single location, this obviates the need to shrink the footprint of the memory cell by a factor of two or three.” So scientists could achieve the equivalent of 0.09 or 0.07 micron technology, which is not currently in production, using a 0.13 micron platform, which is.
Now, Bocian and colleagues are investigating other types of porphyrinic molecules and semiconductor platforms. They’d like to understand the factors that control parameters such as electron transfer rate, which limits write/read times, and charge retention time, which determines refresh rates.
The researchers reported their work in Science.
About the author
Liz Kalaugher is editor of nanotechweb.org.