Ultrathin sheets of a new 2-D hybrid perovskite are square-shaped and relatively large in area, properties that should facilitate their integration into future electronic devices. Credit: Courtesy of Peidong Yang, Berkeley Lab
These ionic materials exhibit optical properties not found in 2D covalent semiconductors eg graphene, making them promising alternatives to silicon for future electronic devices. To the growing list of 2D semiconductors, eg graphene, boron nitride, and molybdenum disulfide, whose unique electronic properties make them potential successors to silicon in future devices, you can now add hybrid organic-inorganic perovskites. However, unlike the other contenders, which are covalent semiconductors, these 2D hybrid perovskites are ionic materials, which gives them special properties of their own.
Researchers at Berkeley Lab have successfully grown atomically thin 2D sheets of organic-inorganic hybrid perovskites from solution. The ultrathin sheets are of high quality, large in area, and square-shaped. They also exhibited efficient photoluminescence, color-tunability, and a unique structural relaxation not found in covalent semiconductor sheets.
“The results of our study open up opportunities for fundamental research on the synthesis and characterization of atomically thin 2D hybrid perovskites and introduces a new family of 2D solution-processed semiconductors for nanoscale optoelectronic devices, such as field effect transistors and photodetectors.”
Traditional perovskites are typically metal-oxide materials with fascinating electromagnetic properties, including ferroelectricity and piezoelectricity, superconductivity and colossal magnetoresistance. In the past couple of years, organic-inorganic hybrid perovskites have been solution-processed into thin films or bulk crystals for photovoltaic devices that have reached a 20% power conversion efficiency. Separating these hybrid materials into individual, free-standing 2D sheets through such techniques as spin-coating, chemical vapor deposition, and mechanical exfoliation has met with limited success.
Yang’s proposal was free-standing 2D sheets of CH3NH3PbI3, a hybrid perovskite made from a blend of lead, bromine, nitrogen, carbon and hydrogen atoms. “We characterized the structure and composition of individual 2D crystals…and found they have a slightly shifted band-edge emission that could be attributed to structural relaxation. A preliminary photoluminescence study indicates a band-edge emission at 453 nanometers, which is red-shifted slightly as compared to bulk crystals. This suggests that color-tuning could be achieved in these 2D hybrid perovskites by changing sheet thickness as well as composition via the synthesis of related materials.”
The well-defined geometry of these square-shaped 2D crystals is the mark of high quality crystallinity, and their large size should facilitate their integration into future devices. “With our technique, vertical and lateral heterostructures can also be achieved,” Yang says. “This opens up new possibilities for the design of materials/devices on an atomic/molecular scale with distinctive new properties.” http://newscenter.lbl.gov/2015/09/25/a-different-type-of-2d-semiconductor/
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