By using advanced technology such as a scanning tunneling microscope, physicists succeeded to create a miniaturised transistor consisting just of a single molecule and a few atoms. The transistor functions significantly different from what is its conventionally expected behavior. This technology breakthrough could be important for fundamental studies on molecular nanostructures' electron transport, as well as the development of future device technologies.
A team of researchers in physics from the Freie Universität Berlin and the Paul-Drude-Institut für Festkörperelektronik, in Germany, the U.S. Naval Research Laboratory, United States and the NTT Basic Research Laboratories, Japan, has used a scanning tunneling microscope for the creation of a miniaturised transistor made up of a single molecule and just a small number of atoms. The results of their research are published in the journal Nature Physics, in the August 2015 issue.
Transistors contain an electrical gate electrode and a channel region between two external contacts. The electrical gate electrode has the role to modulate the flow of electrical current through the channel. This current is extremely sensitive in atomic-scale transistors to single electrons hopping via discrete energy levels.
Molecular transistors and their single-electron transport have been studied previously by using some top-down approaches, like break junctions and lithography. However, it is not possible to control the transistor's gate in an atomically precise way with these previous approaches. Precise control of the gate to the atomic level is crucial to transistor function at the smallest size scales.
In the latest study, the research team made up of physicists used a highly stable scanning tunneling microscope in order to create the small scale transistor made of just a single organic molecule and a few positively charged metal atoms. The atoms were positioned with the stable scanning tunneling microscope's tip on the surface of an indium arsenide crystal.
The growth technique of molecular beam epitaxy was employed in order to prepare this surface. The stable scanning tunneling microscope approach allowed the researchers to assemble electrical gates with atomic precision from the +1 charged atoms and to place the molecule at various close to the gates desired positions.
According to the researchers, the molecule is only weakly bound to the indium arsenide crystal template so by bringing the STM tip very close to the molecule and applying to the tip-sample junction a bias voltage, single electrons can tunnel between tip and template by hopping via nearly molecular orbitals.
This technique is similar to the working principle of an external electrode gating a quantum dot. In the case of the study performed by the physicists team, the electrostatic gate potential is provided by the charged atoms nearby. The electrostatic gate potential regulates the charge state of the molecule and the electron flow.
The present case of a surface bound molecule and a conventional semiconductor quantum dot present a substantial difference, as explained by the researcher team. Depending on its charge state, the molecule can adopt different rotational orientations.
The physicists taking part in this study have developed a generic model that accounts for the orientation dynamics and coupled electronic of the molecule and reproduces the experimentally observed characteristics in the single-molecule transistor characteristics. It is expected that a better understanding and controlling capacity over these processes will lead to future applications of the molecule based devices within the existing semiconductor technologies.
