In a proof of concept study, scientists created self-assembled protein circuits that can perform simple logical functions. This work shows that it is feasible to use the characteristics of electrons on the quantum scale to create a stable digital circuit. One of the stumbling blocks in creating molecular circuits is that as the circuit size decreases, the circuit becomes unreliable. This is because the electrons needed to create an electric current behave like waves, not particles, on a quantum scale.
For example, in a circuit with two wires one nanometer (one billionth of a meter) apart, electrons can shuttle through the "tunnel" between the two wires and effectively appear in both places at the same time, making it difficult to control the direction of current. Molecular circuits can alleviate these problems, but due to the challenge of manufacturing electrodes on this scale, the effective existence time of single molecular junctions is short or low yield.
Ryan chiechi, an associate professor in the Department of chemistry at North Carolina State University, said: "our goal is to try to create a molecular circuit that takes advantage of the tunnel rather than fight it."
Chiechi and Xinkai Qiu, co-author of Cambridge University, first constructed these circuits by placing two different types of fullerene cages on patterned gold substrates. They then immersed the structure in a solution of photosystem unification (PSI), a commonly used chlorophyll protein complex.
Different fullerenes induce psi proteins to self assemble on the surface in a specific direction. Once the top contact of gallium indium liquid metal eutectic is printed on it, diodes and resistors will be generated. This process not only solves the shortcomings of single molecule junction, but also retains the function of molecular electron.
"Where we want a resistor, we print a fullerene on the PSI self-assembled electrode, and where we want a diode, we print another type," chiechi said. "Directional psi rectifies, which means it only allows electrons to flow in one direction. By controlling the net direction of PSI aggregates, we can determine how charges flow through them."
The researchers combined the self-assembled protein combination with human made electrodes, and made a simple logic circuit to regulate the current by using the behavior of electron tunneling.
"These proteins scatter electron wave functions that mediate tunneling behavior in a way that is not yet fully understood. The result is that despite the thickness of 10 nm, this circuit works at the quantum level and operates in the tunneling system. And because we use a group of molecules rather than a single molecule, the structure is stable. We can actually print electrodes on the top of these circuits and build devices," chiechi said
Researchers created a simple diode based and / or logic gate from these circuits and incorporated it into a pulse modulator, which can encode information by turning on or off one input signal according to the voltage of another input signal. The logic circuit based on PSI can switch the input signal of 3.3khz -- although it cannot be compared with modern logic circuit in speed, it is still one of the fastest molecular logic circuits reported so far.
"This is a proof of concept primary logic circuit that relies on both diodes and resistors," chiechi said. "This may indicate that proteins can be used to build powerful integrated circuits that can work at high frequencies. In terms of immediate utility, these protein-based circuits may lead to the development of electronic devices to enhance, replace and / or expand the functions of classical semiconductors."
The study was published in nature communications. Co authors chiechi and Qiu worked at the University of Groningen in the Netherlands.