Giuliana Di Martino et al. / Nature electronics, 2020
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The method of optical spectroscopy for studying the properties of memristors made it possible to take a fresh look at the processes taking place in them. Physicists have used this non-destructive method to study the mechanisms of drift and the formation of oxygen bubbles in the active region of memristors. Job published In the magazine Nature electronics.
Properties memristora hidden in its name – it has memory and resistance. Memory is directly related to hysteresis the nature of the dependence of the current through the memristor on the voltage applied to it. This means that the resistance of the element will depend on what happened to it before. If you first pass the current through the memristor in one direction and then measure its resistance, then it will not coincide with what happens when the current is passed in the opposite direction. Thus, it is possible to set a certain state of the memristor, which it will store even without energizing it. And computers with memristors as RAM will not require the system to be loaded, because even after shutdown they will store information about their last state.
It took scientists 37 years to move from theory to practice and create an element that could demonstrate some of the properties of a memristor. Typically, a memristor consists of two electrodes, to which a voltage can be applied, and an active layer between them. Conductive filaments can appear or disappear between the electrodes, which are most often represented by bridges of oxygen ions. Sometimes ions can oxidize and lead to the formation of oxygen bubbles, which affects the operation of the device as a whole. All these processes need to be studied and investigated in order to create more perfect elements. However, methods for measuring the characteristics of memristors often have many disadvantages and can be destructive.
–(a) geometry of resistive memory with a plasmon field inside the working layer, (b) scheme of optical spectroscopy using a cantilever coated with a layer of conductive material and optically transparent, (c) image of the structure through the cantilever
Giuliana Di Martino et al. / Nature electronics, 2020
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-Scientists led by Jeremy Baumberg of the University of Cambridge used optical spectroscopy to study the processes that occur in the memristor. They took a memristor with electrodes in the form of a titanium nitride plate and a gold nanoparticle as a sample, and strontium titanate served as an active zone. Conductive and optically transparent cantilever touched a gold nanoparticle to apply voltage.
–(a) the location of the gold nanoparticle relative to the area of accumulation of oxygen vacancies, (b) a schematic representation of the bridge of oxygen vacancies, and (c) the formation of an oxygen bubble
Giuliana Di Martino et al. / Nature electronics, 2020
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– The presence of gold nanoparticles makes it possible to monitor the plasmon resonance. Depending on the state of the memristor, the intensity of two peaks in the scattering spectrum of gold nanoparticles changes – at wavelengths of 610 and 780 nanometers. In this case, the disappearance and appearance of these peaks occurs in different ways for different thicknesses of the active layer. For an eight-nanometer layer, the peak at 610 nanometers disappears when the memristor is turned on, and the second appears at this time and remains after turning off. And for a layer four nanometers thick, the peak at 780 nanometers disappears after the voltage is turned off. Repeated switching on and off of the memristor led to a gradual decrease in the intensity of the 610 nm peak.
In addition, the authors studied how the plasmon peak is affected by the presence of a bridge of oxygen ions. If the gold particle is immersed inside strontium titanate, then together with the titanium nitride surface they form a plasmon resonator. Moreover, its frequency depends on the optical parameters of what is between the nanoparticle and the surface. A change in this frequency makes it possible to register the formation of the titanium oxide layer and the thickness of the bridge. Interestingly, over time, a new peak appeared in the spectrum of such a structure, which scientists associated with the formation of oxygen bubbles.
–Simulation of the first cycle of memristor operation, (a) scattering intensities of structures at different stages: oxygen vacancies are uniformly distributed over the sample (1); just before the start of the cycle (2); stage of work with h = 2 nanometers and b = 30 nanometers (3); transition to the off state with h = 4 nanometers (4)
Giuliana Di Martino et al. / Nature electronics, 2020
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– Thanks to data from an optical spectrometer, physicists were able to describe and simulate the formation of conductive filaments when voltage is applied to the memristor electrodes, the formation of a titanium oxide sublayer and an oxygen bubble. A memristor with a thinner strontium titanate layer showed greater promise, so in the future the authors plan to work with it or try to reduce it as well.
One of the interesting uses of the memristor is to use it as the basis for artificial neural networks. The first such network created American engineers, and a group of scientists from Great Britain was able to tie artificial neuron with natural.
Oksana Borzenkova
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