Perovskite-based light effect could be the basis for optical data storage
With perovskites, science already succeeded in increasing electrical efficiency to more then 20 percent. An interdisciplinary research team of the University of Bayreuth revealed that these materials also meet the prerequsites to be used for the design of innovative data storage types. A Perovskite compound that combines methyl ammonium, lead, and iodine could be used for an optical data storage device that works according to the principle “write – read –erase”.
Currently this effect can only be realized at very low temperatures. Nevertheless the researchers are confident that on the basis of their findings it will be possible to develop a cost-effective hybride perovskite capable of working at significantly higher temperatures. Such an entirely optical storage type would open up new perspectives for data storage and processing.
It has been known for quite some time that perovskites of methyl ammonium, lead and iodine changes its appearance depending o temperature. At temperatures above 164 Kelvin (about minus 110 degrees Celsius) it has a tetragonal crystal structure. With right angles and edges that either are parallel or rectangular to each other it has the shape of an elongated cube, resembling a short, upright pillar with square base. If however the temperature is lowered further, the perovskite assumes an orthorhombic crystal structure. In this case, the pillar does no longer has a square base but one of its edges slightly changes its length. This phenomenon, also observed in the transition between two crystal phases, has consequences for the perovskite material as soon as it is irradiated with laser light.
If a crystal is exposed to a laser beam it absorbs energy of the light. In this moment, some electrons switch from their initial state to a higher energy level. If they fall back to their initial level, they emit light. The wavelengths of this light provide reliable information as to the inner structure of the crystal. A special case of this phenomenon is Amplified Spontaneous Emission (ASE). In this situation the laser beam is triggering a chain reaction during which very many electrons are brought to a higher energy level almost at the same instance. Since they also fall back collectively into their initial energy level, they light they are emitting at this point in time is very intensive.
With the abovementioned perovskite material consisting of methyl ammonium, lead and iodine the researchers observed this phenomenon in each of the two crystal states. However, the wavelength of the light is different: If the crystal has been cooled to temperatures below 163K, they generated an ASE that emitted light in the orange part of the spectrum. At higher temperatures, he crystals emitted red light. According to the Bayreuth team, they were the first ones who were able to combine both types of ASE effects in the smallest space. Through very well defined laser radiation, the scientists were able to create tiny tetragonal “island” within the environment that remains in the orthogonally structured environment. After the laser radiation ends, these islands are retained. Thus, the crystal grains can be brought to an energetic state in which both ASE effects are occurring at the same time. The crystal emit red and orange light at the same time, however in different spatial locations. To make the abovementioned islands disappear again, the temperature of the crystal has to be changed or it has to be irradiated with laser light again. Then, they take the uniform orthorhombic structure again.
These properties represent all the prerequisites to develop a data memory unit that works entirely on optical principles. Regarded from the perspective of information technology, the creation of tetragonally structured islands is a write process. The result can be read by measuring the red light. The memory cell will be erased in the moment the crystals are falling back into the uniform orthorhombic structure. Thus, it is possible to create multiple binary sequences with orange light representing the 0 whereas the simultaneous emission of orange and red light represents the 1. This effect occurs within a space of a few nanometers.
According to the Bayreuth researchers, this opens the perspective to data storage and processing based completely on optical principles without the use of any electronics involved. For the time being, the most significant roadblock for industrial application is the low temperature for the transition between the crystal phases. Ideally, this transition would happen at room temperature. Anna Köhler from the chair of experimental physics of the University of Bayreuth and coordinator of the research works, is nevertheless convinced that there is a realistic chance to develop a material that meets the requirements for such an industrial application.
The study is the result of a broad interdisciplinary collaboration of six research teams of the Bayreuth university. It has been published under the title “Reversible Laser Induced Amplified Spontaneous Emission from Coexisting Tetragonal and Orthorhombic Phases in Hybrid Lead Halide Perovskite” (Adv. Optical Materials 2016, DOI: 10.1002/adom.201500765)
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