Approximately thirty-five years ago, two theorists at International Business Machines (IBM) were among the first to propose electronic functions for molecules. Until about fifteen years ago, progress of this idea had been very slow. However, in 1997, Professor Mark Reed, Yale’s Harold Hodgkinson Chair of Engineering and Applied Science, examined the conductance of individual molecules and created a molecular resistor. After this discovery, Reed and an international collaboration involving a number of scientists at Gwangju University in South Korea sought to pursue further developments in the use of molecules as electrical components; today, they have succeeded in creating the first molecular transistor.
The molecular transistor functions similarly to a macroscopic transistor. A conductor is placed between two leads, which for a molecular transistor are elemental solids. The electrical properties of the conductor are changed when a voltage is applied across the leads. In the case of the molecular transistor, the electron orbital structure is altered and displays transistor-like properties.
The apparatus consists of two gold plates placed within nanometers of each another. Specific distances between the plates are based on which molecule is being tested for the properties of a transistor. The experiments test several molecules, including an alkane and benzene. The alkane is used as a control because its chemical properties do not allow for it to be an effective transistor, while the benzene is actively tested due to the properties of its orbital structures.
Designing the apparatus to test on such a small scale is certainly no small task. For this reason, the idea took many years to develop and prove. Because the team had to assemble and measure molecules, standard measuring devices such as Scanning Electron Microscopes and Transmission Electron Microscopes would not suffice. The team turned to a new technique known as Inelastic Electron Tunneling Microscopy (IETS), which involves passing current through a molecule and looking for resonance patterns in order to identify the molecule in question. The team implemented countless setups until they found several that could sustain the experiment.
The properties of gold allowed the team to attach two sulfur atoms at both ends of the gold plates. The benzene molecule could then connect to the plates by binding to these two sulfur atoms. The distance across the benzene molecule is only eight angstroms, and the apparatus had to be precisely set up to achieve accuracy on such a small scale.
Since observing the transistor-like behavior in benzene, Reed has been excited about the prospects of future research in the field. “This opens up a whole new area that was not accessible before,” Reed noted of the discovery. The team’s next step is to test molecules of varying orbital types and varying end-groups.
Reed explained that the experiment has no clear implications for technology yet, and was quick to put his work in perspective, “This is a science experiment. Maybe someone down the road will learn how to fabricate [commercial molecular transistors]. We wanted to understand the physics.”
Reed observed that the most fascinating aspect of widely used transistors today is the ease and regularity of manufacture. Even though it is unlikely these characteristics will be developed for molecular transistors in the near future, the possibilities for technology in this field seem quite promising. Reed mused that “it is important to be clear about what is science and what is potential technology,” but that “[the discovery] lays foundations for progress.”
Extra Readings:
Song, H., Kim, Y., Jang, Y.H., Jeong, H., Reed, M.A., Lee, T. (2009) Observation of molecular orbital gating. Nature, 462 (7276), pp. 1039-1043.
M. A. Reed, C. Zhou, C. J. Muller, T. P. Burgin, J. M. Tour. (1997) Conductance of a Molecular Junction. Science, 278 (5336), pp. 252-254.