For example, on a circuit with two wires one nanometer (one-billionth of a meter) apart, electrons can “tunnel” between the two wires and effectively be in two places at the same time, allowing current to flow direction is difficult to control. Molecular circuits can alleviate these problems, but due to the challenges of fabricating electrodes at this scale, the effective lifetime of single-molecule junctions is short-lived or low-yield.
“Our goal was to try to create a molecular circuit that takes advantage of tunneling, not against it,” said Ryan Chiechi, an associate professor in NC State’s Department of Chemistry.
Chiechi and co-corresponding author Xinkai Qiu of the University of Cambridge first built these circuits by placing two different types of fullerene cages on a patterned gold substrate. They then dipped the structure into a solution of photosystem one (PSI), a commonly used chlorophyll protein complex.
Different fullerenes induce PSI proteins to self-assemble on the surface in specific orientations, creating diodes and resistors once the top contacts of the gallium-indium liquid metal eutectic are printed on. This process not only solves the shortcomings of single-molecule junctions, but also preserves the function of molecular electrons.
“Where we wanted resistors, we printed one type of fullerene on the PSI self-assembled electrodes, and where we wanted diodes, we printed another type,” Chiechi said. “Oriented PSI rectifies, which means it only allows electrons to flow in one direction. By controlling the net direction of aggregates of PSIs, we can determine how charges flow through them.”
The researchers combined self-assembled protein assemblies with human-made electrodes and fabricated simple logic circuits that used electron tunneling behavior to regulate current flow.
“These proteins scatter electron wavefunctions, mediating tunneling behavior in ways that are still not fully understood,” Chiechi said. “The result is that, despite being 10 nanometers thick, this circuit works at the quantum level, operating in a tunneling system.” Since we’re working with a group of molecules, rather than a single molecule, the structure is stable. We can actually print electrodes on top of these circuits and build the device.”
The researchers created simple diode-based AND/OR logic gates from these circuits and incorporated them into a pulse modulator that encodes information by turning one input signal on or off, depending on the voltage of the other input signal. The PSI-based logic circuit is capable of switching input signals at 3.3kHz — not on par with modern logic circuits in speed, but still one of the fastest molecular logic circuits reported to date.
“This is a proof-of-concept primary logic circuit that relies on both diodes and resistors,” Chiechi said. “This could show that proteins can be used to build robust integrated circuits that can operate at high frequencies. In terms of immediate utility, these protein-based circuits may lead to the development of electronic devices that enhance, replace and/or extend the classical The function of semiconductors.”
The research was published in Nature Communications. Co-authors Chiechi and Qiu worked at the University of Groningen in the Netherlands.