"Liquid" computer: the capture of ions in graphene

When we read science fiction or watch a movie of this 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 worth quite a human disorder as paranoia, which "suffered" HAL 9000 from the series of works "Space Odyssey" by Arthur Clarke. However, today we are not going to talk about the mental, or rather, the computing capabilities of the machines of the future, but about their physical structure. What if future computers are no longer bound to silicon, but can function as a liquid? This is the main issue of the study, which we will meet today. Go.

Material base

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

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

During the experiments, it was observed that the graphene film acquires the properties of a silicon-based semiconductor, that is, it can perform the function of a transistor. To control the film, you need to 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. It costs, by the way, about $ 65 per piece

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


Molecular structure of potassium chloride (KCl)

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

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

Graphene - liquid - voltage

Experiments have shown that the main factor influencing the performance of the simplest logical operations is the voltage arising on the graphene film. At low levels of potassium chloride concentration, a direct relationship between the conductivity and the filling of the film with ions appears. At 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.


Graphic model of potassium ion (purple) capture 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 denote 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 into 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 gate NOT, when the input and output values ​​are reversed. Simply put, 0 comes in, and 1 comes out, and vice versa.

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

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 prompted researchers to the idea that tunable 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 potassium ion trapping process

The process of ion capture is not as independent as it may seem. It can be adjusted by applying different voltages across the surface of the film.

We also managed 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 a limit. 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 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 of the crown ethers. At a voltage of 300 mV, the film begins to pass ions. The higher the voltage, the greater the likelihood 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 options for metal dichalcogenides, since 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. This kind of research not only gives us the tools to understand certain phenomena, processes or substances, but also sets us tasks, seemingly insane and impracticable, the implementation of which allows us to improve the world around us.

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

For writing the article materials from the NIST website were used.

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