Scientists created simple logic functions using self-assembled protein-based circuits in a proof of concept study. This work shows that stable digital circuits can be made that exploit the quantum-scale properties of electrons.

The problem with creating molecular circuits is the fact that they become less reliable as the circuit’s size decreases. Because electrons used to create current behave at the quantum scale like waves and not particles, this is why it can be so difficult to make molecular circuits. An example is a circuit that has two wires one nanometer apart. The electron can “tunnel,” or travel between the wires, and be in both of these places simultaneously. This makes it difficult to control current direction. These problems can be mitigated by using a molecular circuit, however single-molecule junctions are often short-lived and low-yielding because of the difficulties associated with fabricating electrodes at this scale.

“Our goal was to create a molecular circuit which uses tunneling to benefit us, rather than fighting it,” Ryan Chiechi, associate professor at North Carolina State University, and co-author of the paper that describes the work, says.

Chiechi and Xinkai Qiu, co-corresponding author at the University of Cambridge, built the circuits by first placing two types of fullerene cells on patterned gold substrates. The structure was then immersed in a solution of Photosystem One (PSI), which is a common chlorophyll protein complex.

Different fullerenes induced PSI protein self-assemble on the surface. Once top-contacts of gallium-indium liquid metallic eutectic (EGaIn) are printed on top, diodes and resistors were created. This eliminates the disadvantages of single-molecule junctions while maintaining molecular-electronic functionality.

We patterned one type fullerene on electrodes that PSI self-assembles where we needed resistors. Where we needed diodes, we patterned another type,” Chiechi explains. “Oriented PSI rectifies the current, which means that electrons can only flow in one direction. We can control the net orientation of PSI ensembles to determine how charge flows through them.

Researchers combined self-assembled protein assemblies with human-made electrodes to create simple logic circuits that use electron tunneling behavior as a way to control the current.

Chiechi explains that proteins scatter the electron wave function and mediate tunneling in ways that are not fully understood. The result is that the circuit, despite being only 10 nanometers thick works at the quantum level and operates in a tunneling mode. The structure is stable because it uses a group of molecules rather than single molecules. These circuits can be printed with electrodes to make devices.

These circuits were used to create simple diode-based AND/OR logic gate designs. They were then incorporated into pulse modulators that can encode information by changing one input signal on/off depending on the input voltage. Although not as fast as modern molecular logic circuits, the PSI-based logic circuits were capable of switching a 3.3kHz input signal.