Building a Single-molecule Transistor from Scratch

Circuits with single atoms and molecules: the ultimate of Moore's Law?

3 min read
Building a Single-molecule Transistor from Scratch
Image: PDI

An international team of researchers has demonstrated for the first time that a single molecule can operate as a field-effect transistor when surrounded by charged atoms that operate as the gate. The team published its results in the August 2015 issue of the journal Nature Physics

The experiments were performed in Berlin at the Paul-Drude-Institut für Festkörperelektronik (PDI), in collaboration with researchers at the Free University of Berlin (FUB), the NTT Basic Research Laboratories (NTT-BRL) in Japan, and the U.S. Naval Research Laboratory (NRL) in Washington, D.C.

The researchers used a technique first demonstrated by researchers at IBM in 1990 when they created the letters I, B, and M by moving single atoms around on a metal surface with a scanning tunneling microscope (STM). In order for the molecule to function as a transistor, the researchers had to deposit it—as well as the charged indium atoms that surround it, forming the gate—on a semiconductor surface (in this case, indium arsenide) instead of a metal. 

Doing so was expected to be more difficult, because molecules on semiconductor surfaces usually attach themselves by covalent bonds, which are very strong and make it difficult to move the atoms around with the STM tip. But the molecule they used, a dye called copper phthalocyanine, is attached to the semiconductor surface by van der Waals forces, explains Stefan Fölsch at PDI who led the research. These forces are much weaker, and allow the molecule to be moved around easily. The weak bond also allows a current to flow from the tip of the STM through the molecule. “You have sequential tunneling between the tip, the molecule and the surface,” says Fölsch. In this way, the molecule functions as the channel, with the tip and the semiconductor substrate behaving as the ‘source’ and 'drain' electrodes. 

In a normal transistor, the current through the channel is controlled by modulating the gate voltage. Clearly, this is not possible with this setup, where the gating atoms have fixed charges. Instead, it’s possible to mimic the modulation of the electric field by varying the distance between the channel and the gate. 

“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,” says Fölsch. “In each new position, the molecule feels a different electrostatic potential created by these atomic-scale gates.”

The gating of the channel is different from what we are used to with conventional transistors; the mechanism of how the intensity of electrostatic field controls the current through the channels is just as unfamiliar, in that it changes the quantum state of the molecule. The gate controls the charge state in the molecule, which in turn controls the ability of electrons that tunnel between the gate and the STM tip to hop via close molecular orbitals in the molecule.

The results of the experiments are still far from finding applications in real-world devices, not only because of the complexity of the experiments, but also because much of the physics involved is still not fully understood. “These are very basic experiments where we have very ‘ideal’ systems, and it is important  to gain a detailed understanding of what is going on,” says Fölsch, who adds that without theoretical work performed at the NRL and FUB, the experiments wouldn't have gone far. Still, the path from their work, performed with a high-precision STM at a temperature of 4 degrees Kelvin under ultra high-vacuum conditions, to a practical device will involve a lot of engineering, admits Fölsch. "It is a long way to go,” he says.

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This CAD Program Can Design New Organisms

Genetic engineers have a powerful new tool to write and edit DNA code

11 min read
A photo showing machinery in a lab

Foundries such as the Edinburgh Genome Foundry assemble fragments of synthetic DNA and send them to labs for testing in cells.

Edinburgh Genome Foundry, University of Edinburgh

In the next decade, medical science may finally advance cures for some of the most complex diseases that plague humanity. Many diseases are caused by mutations in the human genome, which can either be inherited from our parents (such as in cystic fibrosis), or acquired during life, such as most types of cancer. For some of these conditions, medical researchers have identified the exact mutations that lead to disease; but in many more, they're still seeking answers. And without understanding the cause of a problem, it's pretty tough to find a cure.

We believe that a key enabling technology in this quest is a computer-aided design (CAD) program for genome editing, which our organization is launching this week at the Genome Project-write (GP-write) conference.

With this CAD program, medical researchers will be able to quickly design hundreds of different genomes with any combination of mutations and send the genetic code to a company that manufactures strings of DNA. Those fragments of synthesized DNA can then be sent to a foundry for assembly, and finally to a lab where the designed genomes can be tested in cells. Based on how the cells grow, researchers can use the CAD program to iterate with a new batch of redesigned genomes, sharing data for collaborative efforts. Enabling fast redesign of thousands of variants can only be achieved through automation; at that scale, researchers just might identify the combinations of mutations that are causing genetic diseases. This is the first critical R&D step toward finding cures.

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