Liquid Computer: Capture of Ions in Graphene

When we read science fiction or watch a movie of a similar genre, we often come across the computers of the future. The authors of these works endowed their fictional computers with all sorts of properties, from unimaginable computing power to human qualities. What is it worth a completely human disorder like paranoia, which "suffered" HAL 9000 from the series of works of "Space Odyssey" by Arthur Clark. However, today we will focus not on the mental, or rather, computational abilities of the machines of the future, but on their physical structure. What if future computers are no longer tied to silicon, but can function as liquids? This is precisely the main issue of the study, which we will get to know today. Go.

Material base

A “liquid” computer, no matter how wild this phrase sounds, is not a new idea in the world of science. For several decades, research has been underway, trying in one way or another to implement such a futuristic technology. Scientists from NIST (National Institute of Standards and Technology) were no exception. Their study demonstrated that computational logic operations can be performed in a liquid medium by controlled capture of ions in graphene * floating in saline.

Graphene * is a thin film (1 atom thick) of carbon atoms connected to a hexagonal (honeycomb) two-dimensional crystal lattice.

During the experiments, it was noticed that the graphene film acquires the properties of a silicon-based semiconductor, that is, it can serve as a transistor. To control the film, you must change the voltage. And this process is very similar to what happens when the concentration of salts in biological systems changes.


Graphene film: 29 x 29 cm, thickness - 35 microns. By the way, it costs about $ 65 apiece

The focus of course was the graphene film, the dimensions of which were no more than 5.5 at 6.4 nm. In its structure, the film was like an unfinished puzzle, because in the middle of it there was one or more “holes” (pores), more precisely, vacancies surrounded by oxygen atoms. This is a trap for ions. From the point of view of chemistry, such an atomic compound is similar to crown ethers, which are known, among other things, for the fact that they form stable complexes with metal cations. That is, positively charged metal ions “catch” it.


Molecular Structure of Potassium Chloride (KCl) The

second important element of the experiment was a liquid medium, whose role was played by water with potassium chloride ( KCl ), which decays into potassium and chlorine ions.

Crown esters caught potassium ions, since the latter have a positive charge.

Graphene - liquid - voltage

The experiments showed that the main factor affecting the performance of the simplest logical operations is the stress arising on the graphene film. At a low level of concentration of potassium chloride, a direct relationship is shown between the conductivity and the occupancy of the film with ions. With low occupancy, the conductivity level is high, and vice versa. Direct electrical measurement of the voltage level of a graphene film in this experiment is a certain logical operation - reading.


Graphical model of the capture of potassium ions (violet) in pores surrounded by oxygen (red) on a graphene film (gray)

Now let's deal with zeros and ones. If at a certain concentration of potassium chloride on the film the voltage is low (we designate it as “0”), then the film itself is practically non-conductive. In other words, it is off. In this case, the pores are completely filled with potassium ions.

High voltage (more than 300 mV), denoted as "1", increases the conductivity of the film, putting it in the on mode. In this case, not all pores are occupied by potassium ions.

As a result, the input / output ratio can be considered as a logical NOT gate, when the input and output values ​​are reversed. Simply put, 0 enters and 1 exits, and vice versa.

If two graphene films are used, then the logical operation OR (XOR) is possible. In such a situation, the difference in state of two films, called the input value, will be equal to 1 only if one of the films has high conductivity. In other words, we get 1 if the input from the two films is different, and 0 if the data matches.

The experiments also showed the possibility of implementing sensitive switching, since even with a slight change in voltage, the potential charge of the film varies greatly. This led researchers to the idea that custom ion capture can also be used to store information, since sensitive transistors can perform extremely complex computational operations in nanofluidic devices.


Demo video of the process of capturing potassium ions

The process of capturing ions is not as independent as it might seem. It can be adjusted by applying different voltages across the surface of the film.

It was also possible to find out that the ions “stuck” in the pore of the film not only block the penetration of other ions through the film, but also create an electric field around the film. In order for the ion to pass through the film, the voltage must be at the limit level. The electric field of the captured ions increases the voltage by 30 mV, which completely blocks the penetration of other ions.


Logical operations OR (XOR) and NOT

If a voltage of less than 150 mV is applied to the film, the ions will no longer penetrate through it. And the electric field of the captured ions prevents other ions from pushing the first out of the crown ethers. At a voltage of 300 mV, the film begins to pass ions. The higher the voltage, the greater the probability of loss of trapped ions. Wandering ions also begin to actively push the trapped, because the electric field is weaker. These properties make the film an excellent semiconductor for the transfer of potassium ions.

Epilogue

The most important physical moment of a possible device based on this technique is its physical size, which should not exceed several atoms, and the presence of electrical conductivity. Not only graphene can be the basis, but also other materials. As an alternative, researchers offer various types of metal dichalcogenides, as they have water-repellent properties and it is easy to form porous structures from them.

Of course, this is futurism, but not without arguments in its support. Such studies not only provide us with tools for understanding various phenomena, processes or substances, but they also pose tasks that seem insane and impossible to fulfill at first glance, the implementation of which allows us to improve the world around us.

For a long time we will have to wait for “liquid” computers, servers in a glass and flash drives in flasks. However, we are already getting the most important thing for the future of us and the world as a whole - knowledge.

To write the article, materials from the NIST website were used.

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