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Molecular memories tough it out - monolayer of porphyrin molecules covalently attached to a silicon substrate...

Molecular memories tough it out - monolayer of porphyrin molecules covalently attached to a silicon substrate... - Physics Forum

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Old 11-29-2003, 02:23 PM
Do Wah Ditty
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Default Molecular memories tough it out - monolayer of porphyrin molecules covalently attached to a silicon substrate...



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Molecular memories tough it out
27 November 2003

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.

--
Do Wah Ditty

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far the most generous nation in the world when it comes to that, and I'm
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just telling you as an aside. We're generous. We shouldn't be bragging about
it. But we are. We're very generous."

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