An international team of scientists has created a field-effect transistor (FET) with a channel that consists of just a single molecule. The transistor is switched on and off by the arrangement of individual atoms around the channel. This isn’t the first single-molecule transistor, though the individual-atom gating is rather novel.
Take a moment to appreciate the image at the top of the story, captured with a scanning tunnelling microscope (STM). In the middle is a phthalocyanine (H2Pc) molecule, a large molecule that is the basis of many blue/green dyes. Around the outside, arranged in a hexagon, are 12 charged indium (In+1) atoms. The molecule and atoms rest on an indium arsenide substrate.
Don’t get too excited about the prospect of single-molecule transistors being commercialised, though. As you may know, a transistor consists of more than just a channel (the molecule) and the gate (the hexagon of atoms). There also needs to be a source and a drain. The drain is the indium arsenide substrate; no big deal. The source, however, is the tip of a giant STM: a room-sized cryogenic multi-million-pound device that can only really be found at specialised research institutions.
Putting that caveat aside, though, the gating method is pretty cool. In a normal transistor (inside your PC, for example), the flow of electricity through the channel is controlled by the voltage across the gate. In this case, the nearby indium ions alter the energy levels of the H2Pc molecule’s electrons, changing the “potential landscape” slightly. Eventually, when enough charged indium ions are in the molecule’s vicinity, electrons from the STM can tunnel through the molecule to the substrate below: voilà, a transistor.
“We created a certain electrostatic potential ‘landscape’ on the surface by placing [charged] atoms in a certain geometry through which we are moving the molecule on a fixed line,” Stefan Fölsch, one of the authors of the research, told IEEE Spectrum. The indium atoms are moved around the substrate with the STM. (The STM is a wonderfully flexible piece of apparatus). “In each new position, the molecule feels a different electrostatic potential created by these atomic-scale gates.”