Memristors combine ferroelectrics and graphene for neuromorphic chips
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为了创建备忘录，经常使用氧化钛（Sto）的过去事件记忆的开关。该材料是一种钙钛矿，其晶体结构取决于温度，并且可以在低温下成为初期的铁电。然而，铁电行为丢失了105开尔文，而这些相变的域和域壁是主动研究的主题。
“It is in a league of its own,” says Tamalika Banerjee, Professor of Spintronics of Functional Materials at the Zernike Institute for Advanced Materials, University of Groningen.
The oxygen atoms in the crystal appear to be key to its behaviour. “Oxygen vacancies can move through the crystal and these defects are important,” she says. “Furthermore, domain walls are present in the material and they move when a voltage is applied to it.”
Instead Banerjee’s team used graphene.
Banerjee说：“石墨烯的特性是由其纯度定义的，而STO的性质是由晶体结构中的缺陷引起的。我们发现将它们结合起来会带来新的见解和可能性。”
Placing graphene strips on top of a flake of STO and measuring the conductivity at different temperatures by sweeping a gate voltage between positive and negative values showed the potential. ‘When there is an excess of either electrons or the positive holes, created by the gate voltage, graphene becomes conductive,’ said researcher Si Chen. “But at the point where there are very small amounts of electrons and holes, the Dirac point, conductivity is limited.”
在正常情况下，最小电导率位置不会随栅极电压的扫描方向而变化。但是，在STO顶部的石墨烯条中，向前扫描的最小电导率位置和向后扫描之间存在很大的分离。在4开尔文处的效果非常明显，但在105开尔文或150 kelvin处的效果较不明显。对结果的分析以及在乌普萨拉大学进行的理论研究表明，STO表面附近的氧气空位是负责的。
“The phase transitions below 105 Kelvin stretch the crystal structure, creating dipoles. We show that oxygen vacancies accumulate at the domain walls and that these walls offer the channel for the movement of oxygen vacancies. These channels are responsible for memristive behaviour in STO,” said Banerjee. Accumulation of oxygen vacancy channels in the crystal structure of STO explains the shift in the position of the minimum conductivity.
Chen also carried out another experiment: “We kept the STO gate voltage at -80 V and measured the resistance in the graphene for almost half an hour. In this period, we observed a change in resistance, indicating a shift from hole to electron conductivity.,” she said. This effect is primarily caused by the accumulation of oxygen vacancies at the STO surface.
实验表明，组合的STO/石墨烯材料的性能通过电子和离子的运动在不同的时间尺度上变化。Bannerjee说：“通过收获一个或另一个，我们可以使用不同的响应时间来产生回忆作用，这可以与短期或长期记忆效应进行比较。”
The study creates new insights into the behaviour of STO memristors. “And the combination with graphene opens up a new path to memristive heterostructures combining ferroelectric materials and 2D materials,” she added.
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