Aug 3, 2004
Heat treatment tunes conduction for nanocrystal assemblies
Researchers from the NMRC, Ireland, and the University of Hamburg, Germany, have used heat treatment to alter the electrical properties of an array of metal nanocrystals. The assembly moved from acting as an insulator to near-metallic behaviour as the team increased the anneal temperature.
"The unique self-assembly properties of size-selected, organically passivated nanocrystals allow the formation of ordered two-dimensional and three-dimensional arrays that promise nanocrystal-based devices and circuits with novel optical and electronic characteristics," Gareth Redmond of the NMRC told nanotechweb.org. "However, while many future device properties will be governed by inter-nanocrystal coupling mechanisms, as well as individual nanocrystal properties, only a handful of groups have investigated the influence of inter-nanocrystal coupling on the electronic properties of nanocrystal arrays."
Redmond and colleagues created arrays of 3.8 nm diameter CoPt3 nanocrystals coated with organic stabilizers by allowing the crystals to deposit from solution in toluene. The team laid down the nanocrystal superlattices between gold electrodes with an inter-electrode separation of about 70 nm. The lattices had a mean separation between nanocrystals of about 2.5 nm and a room-temperature low-bias resistance of more than 1 TeraOhm.
Thermal annealing under reducing conditions significantly decreased the resistance of the arrays. A heat treatment at 80°C, for example, resulted in a low-bias resistance of 100 MegaOhm. That's more than four orders of magnitude lower than for an untreated device. Analysis of the current-voltage characteristics of the treated device at different temperatures indicated that it was acting as a Mott insulator. The scientists believe the phenomenon was due to annealing having reduced the inter-nanocrystal separation.
An anneal at 100°C reduced the array's resistance by a further three orders of magnitude - down to 190 kOhm. Measuring the resistance as a function of temperature showed that the device had weakly metallic behaviour. An anneal at 150°C, meanwhile, produced a room-temperature resistance of around 1 kOhm and metallic behaviour.
"We have found that in situ mild thermal processing can be exploited to progressively tune device operation in these laterally contacted nanocrystal arrays from a single electron tunnelling regime through to near-metallic behaviour as a function of the anneal temperature," said Redmond. "These changes are consistent with controlled, anneal-induced reduction of inter-nanocrystal distance without nanocrystal melting or sintering. This result represents a key step towards the fabrication of functional nanocrystal-based devices and circuits by providing a simple route to manipulate inter-nanocrystal coupling on chip."
According to Redmond, nanocrystals could be the building blocks for future nanoelectronic and nanophotonic technologies, with applications ranging from logic and memory to data storage and routing. "Our demonstration of the incorporation of nanocrystals as discrete entities into devices while controlling inter-nanocrystal coupling in situ is critical to enabling these future applications," he said. "In the near term our studies will benefit electronics applications where nanocrystals have been proposed as charge storage elements in scaleable alternatives to conventional 'flash' memories."
Now, the researchers say they will focus their efforts on developing new fabrication strategies that will be compatible with microelectronics technologies, in order to develop "hybrid" nanocrystal-based devices, circuits and architectures.
The researchers reported their work in Nano Letters.
About the author
Liz Kalaugher is editor of nanotechweb.org.