Last week, researchers at IBM Research-Zurich in Rüschlikon, Switzerland, and the Universities of Basel and Zurich announced in a letter published in Nature a new method for creating electrical contacts to individual molecules on a silicon chip. The advance could open up a promising new way to develop sensors and possibly other electronic or photonic applications of manipulating single molecules.
When, in the mid-1970s, researchers discovered single molecules with interesting electronic properties such as that of a diode, hopes were high that this would spur the development of a new semiconductor technology that might compete with silicon-based electronics. However, establishing electrical contacts to such molecules remained essentially an activity confined to the laboratory. While it is possible to make contact with these molecules from the tips of scanning tunneling microscopes (STMs), these experiments required vacuum and low-temperature conditions. Moreover, single electrical junctions remained difficult to reproduce because they varied widely in the current they admitted to the molecule. These problems were the main reason why, up to now, no molecular-electronic devices were available.
“We needed to fabricate devices that are more or less identical, ambient stable, and that can be placed on a robust platform, such as a silicon chip, in numbers of several billions, to compete with CMOS technology,” says Emanuel Lörtscher, of IBM Research, who is a coauthor of the Nature paper.
To accomplish that, the researchers first turned to a sandwich-on-silicon approach attempted in the past. But that did not work. On a silicon wafer, they created platinum electrodes, which they covered with a dielectric, a thin layer of nonconducting material. Then they created nanopores in this layer, using conventional etching techniques. They filled these pores with a solution of alkane-dithiol molecules and allowed the molecules in the solution to form a self-assembled monolayer in the pores and form a single monolayer of densely packed parallel oriented molecules.
Like the bottoms of wine bottles in a crate, one end of the molecules made contact with the exposed platinum layer at the bottom of the pores. Up to now, researchers had attempted to cover these nanopores with another thin platinum layer to form the upper contact. But the electrical contacts obtained in this way showed a wide variation in contact resistance caused by variations in the distances between the molecules and the contact layer. The resulting device was unusable. They also tried graphene, with the same disappointing results, remembers Lörtscher.
The researchers finally hit upon a solution that was ingenious by its simplicity. Their golden idea: After the pores were filled with the self-assembled monolayer (SAM) material, they covered the SAMs in the pores with gold nanoparticles. These nanoparticles, big enough not to fall in between the self-assembled molecules, made contact with the molecules without destroying them or altering their properties.
“The nanoparticles auto-adjust to the size of the molecules,” says coauthor Marcel Mayor of the University of Basel. “This now looks so simple, and we did so much work to get there.”
The arrow and the dotted circle show where the 10-micrometer-diameter pore sits beneath the bulk metal contact that seals the molecular monolayer film to provide ambient stability. Image: IBM Research-Zurich
The researchers created around 3,000 nanopores on the wafer, each housing self-assembled molecules. When they tested the molecules’ response to applied voltages, they found that for same-size pores, the spread in responses was very small. Although the contact resistance for individual molecules in the pores may differ because of defects, there is an effective all-sample averaging caused by their SAM approach, explains Lörtscher.
Mayor is not sure whether the SAM devices will be able to compete with silicon devices for data storage or switching. Because the electrical properties of SAM molecules are affected by the presence of other molecules, they could be useful in sensing applications, he says. “There are many structures in which this behavior is known,” says Mayor. For example, the SAM molecules are pH sensitive, and they rearrange their structure, or swell, when exposed to certain vapors or solvents. “This is where all the interest from industry in these devices is coming from; they are interested in much more precise analytical devices,” says Mayor. Zhenan Bao, a materials scientist at Stanford University, agrees. “Contacting to single molecules reliably has always been a major challenge. It is impressive that they got very reproducible results and how beautifully the electrical conduction scaled with the length of the molecule. Their approach could be very promising for making molecular memory and circuits in the future,” she says.
However, Youngkyoo Kim, a researcher at Kyungpook National University, in South Korea, expresses some reservations about the SAM devices as sensors: “I feel that the present nanoparticle and self-assembly approach sounds good in terms of large-scale fabrication of electrical contacts in molecular devices but the performance reproducibility and stability may remain a big hurdle to overcome. In the case of the present device structure, both metal electrodes (including metal nanoparticles) and SAM layers need to be well encapsulated for stable operation.”