Molecular electronics: In the right place

06/24/2013

Precise alignment of the electronic states in metal contacts and organic molecules is needed to optimize performance in molecular electronic devices

An illustration of the molecular electronic device’s geometry, with a fullerene (C60) molecule (red, center) embedded within magnetic nickel electrodes. N and S represent the north and south poles, respectively, of the device’s magnetic contacts.
An illustration of the molecular electronic device’s geometry, with a fullerene (C60) molecule (red, center) embedded within magnetic nickel electrodes. N and S represent the north and south poles, respectively, of the device’s magnetic contacts.

Reproduced, with permission, from Ref. 1 © 2013 American Chemical Society

A fundamental discrepancy exists between the basic building blocks that underpin modern engineering and those found in natural organisms: while computers and electronic circuits are based on inorganic materials, such as silicon, biological processes revolve around organic molecules. Increasingly, researchers are striving to combine the two to fabricate complex artificial devices from organic molecules. Ikutaro Hamada, Masaru Tsukada and co-workers from the AIMR at Tohoku University and the University of Tokyo have now shown how the right choice of metal electrodes and operational conditions can enhance electronic performance1.

Molecular electronic devices are particularly interesting for a number of spintronic applications, which exploit the electron’s magnetic property, known as ‘spin’. “The realization of spintronic devices consisting of single molecules promises highly functional sensing, logic (computing) and information-storage devices,” says Hamada, a member of the research team. Until now, however, spin-based effects have not been strong enough to use in practical applications.

While the performance of a molecular electronic device would be chiefly dictated by the organic molecule, contact between the molecule and the adjacent metallic electrodes also plays a key role. The researchers fabricated devices featuring two magnetic nickel electrodes separated by only 1 nanometer — far enough apart to accommodate the fullerene (C60) molecule (see image). To create this precisely sized gap, they used an electromigration-based process to move nickel atoms out of the electrodes, slowly closing the space between them.

The team then studied the effect of the electrodes’ electronic levels on the properties of the spintronic device by measuring the change in electrical resistance when they applied different external magnetic fields, a property known as magnetoresistance. By varying the voltage applied, the researchers could tune the device to observe a change in resistance of up to -80%. Theoretical calculations revealed that hybridization at the nickel–C60 interface may be responsible for the observation of this unusual negative value and, in addition, magnetoresistance was maximal when the applied voltages led to the perfect alignment of the electron spins in the C60 molecule and the nickel electrodes.

These findings demonstrate that molecular electronic devices should be envisaged as a whole since it is crucial to realize an appropriate combination of metallic contacts and organic molecules. Hamada is confident that the team’s approach is applicable to a range of related devices. “Using the theory developed in the present work, I believe that it is possible to predict the properties of spintronic devices made from other molecules and electrodes.”

References

  1. Yoshida, K., Hamada, I., Sakata, I., Umeno, A., Tsukada, M. & Hirakawa, K. Gate-tunable large negative tunnel magnetoresistance in Ni−C60−Ni single molecule transistors. Nano Letters 13, 481−485 (2013). | article

This research highlight has been approved by the author of the original article and all empirical data contained within has been provided by said author.