[ad_1]
Researchers around the world have been studying new materials to develop neuromorphic computers with a design based on the human brain.
A memristive device is a vital component of neuromorphic computers, and its resistance depends on the history of the device, quite similar to human neurons, whose response depends on previous input.
Materials scientists at the University of Groningen studied the behavior of strontium titanium oxide, a platform material for memristor research, and used 2D material graphene to investigate it. The results of the study were reported in ACS applied materials and interfaces official on 11 Novemberth, 2020.
Computers are huge calculators with numerous switches with a value of 0 or 1. They use many of these binary systems to perform calculations very quickly. But, in other respects, computers aren’t that efficient.
Compared to a standard microprocessor, the human brain uses less energy to recognize faces or perform other complex tasks. This is because the brain is made up of neurons that can take on different values besides 0 and 1 and because the output of the neurons depends on the previous input.
Oxygen vacancies
Strontium titanium oxide (STO) is often used to develop memristors, switches with a reminder of past events. This material is a perovskite, whose crystalline structure depends on temperature and can transform into an incipient ferroelectric at low temperatures. Above temperatures of 105 K, the material loses its ferroelectric behavior.
The domains and domain walls associated with these phase transitions are under active study. However, it is still not clear why the material behaves the way it does. “It is in a league of its own“Said Tamalika Banerjee, professor of functional materials spintronics at the Zernike Institute for Advanced Materials, University of Groningen.
Its behavior seems to depend more on the oxygen atoms in the crystal.
Oxygen vacancies can travel through the crystal and these defects are important. Additionally, the domain walls are present in the material and these move when a voltage is applied.
Tamalika Banerjee, Professor of Spintronics of Functional Materials, Zernike Institute for Advanced Materials, University of Groningen
Although several research works have tried to determine how this happens, looking into this material is not that easy. But Banerjee’s research team has successfully used another material that is in a league of its own: graphene, the two-dimensional carbon sheet.
Conductivity
“The properties of graphene are defined by its purity“Banerjee added,”while the properties of STO derive from imperfections in the crystal structure. We found that combining them leads to new insights and possibilities. “
An important part of this study was performed by Si Chen, a PhD student from Banerjee. Chen placed strips of graphene on an STO flake and quantified conductivity at various temperatures by shifting a gate voltage between positive and negative values.
When there is an excess of electrons or positive holes, created by the gate voltage, graphene becomes conductive. But at the point where there are very small amounts of electrons and holes, the Dirac point, conductivity is limited.
Si Chen, PhD Student, Banerjee’s Lab, Zernike Institute for Advanced Materials, University of Groningen
Under normal conditions, there is no change in the minimum conductivity position with the sweep direction of the gate voltage. But in the case of the graphene strips placed above the STO, there is a huge separation between the positions of minimum conductivity for the forward and backward sweeps.
The effect is quite noticeable at 4K, but less clear at 105K or 150 K. Analysis of the results, together with theoretical studies conducted at Uppsala University, reveals that oxygen vacancies near the surface of the STO are behind this effect.
Memory
According to Banerjee, “Phase transitions less than 105 Kelvin lengthen the crystal structure, creating dipoles. We show that oxygen vacancies accumulate on the domain walls and that these walls provide the channel for the movement of oxygen vacancies. These channels are responsible for memorizing behavior in the STO. “
The shift in the position of the minimum conductivity is explained by the accumulation of vacant oxygen channels in the crystal structure of STO.
Chen also performed another experiment: “We kept the STO gate voltage at -80 V and measured the resistance in the graphene for nearly half an hour. In this period, we observed a change in resistance, indicating a shift from the conductivity of the hole to the electronic conductivity. “This effect is mainly due to the accumulation of oxygen vacancies on the STO surface.
Overall, the experiments show that the properties of the combined STO / graphene material vary due to the movement of electrons and ions, each at different time scales.
Collecting one or the other, we can use the different response times to create memristive effects, which can be compared to short or long term memory effects. And the combination with graphene opens a new path to memristive heterostructures that combine ferroelectric and 2D materials.
Tamalika Banerjee, Professor of Spintronics of Functional Materials, Zernike Institute for Advanced Materials, University of Groningen
The study offers a new understanding of the behavior of STO memristors.
Journal reference:
Chen, S., et al. (2020) Presentation of temperature-induced structural domains and the movement of oxygen vacancies in SrTiO3 with graphene. ACS applied materials and interfaces. doi.org/10.1021/acsami.0c15458.
Source: https://www.rug.nl/?lang=en
Source link
