January 8, 2016

Visualising Atoms of Perovskite Crystals

Topography image of atoms of the perovskite crystal and calculated images
with position of atoms and molecules indicated.

(January 8, 2016)  Organic-inorganic perovskite materials are key components of the new generation of solar cells. Understanding properties of these materials is important for improving lifetime and quality of solar cells. Researchers from the Energy Materials and Surface Sciences (EMSS) Unit, led by Prof. Yabing Qi, at the Okinawa Institute of Science and Technology Graduate University (OIST) in collaboration with Prof. Youyong Li’s group from Soochow University (China) and Prof. Nam-Gyu Park’s group from Sungkyunkwan University (Korea) report in the Journal of the American Chemical Society the first atomic resolution study of organic-inorganic perovskite.

Perovskites are a class of materials with the general chemical formula ABX3. A and B are positive ions bound by negative ions X. Organic-inorganic perovskites used in solar cells are usually methylammonium lead halides (CH3NH3PbX3, where X is bromine, iodine, or chlorine). The OIST scientists used a single crystal of methylammonium lead bromide (CH3NH3PbBr3) to create topographic images of its surface with a scanning tunneling microscope.

The researchers discovered that methylammonium molecules (represented by a ball-and-stick model
in the centre) can rotate and that they favour specific orientations that lead to two types of surface
structures with distinctly different properties (left and right images).

This microscope uses a conducting tip that moves across the surface in a manner very similar to a finger moving across a Braille sign. While the bumps in Braille signs are a few millimetres apart, the microscope detects bumps that are more than million times smaller — atoms and molecules. This is achieved by the quantum tunneling effect — the ability of an electron to pass through a barrier. The probability of an electron passing between the material surface and the tip depends on the distance between the two. The resulting atomic-resolution topographic images reveal positions and orientations of atoms and molecules, and also provide a detailed look at structural defects in the surface.
Dr Robin Ohmann, first author of the paper, transfers a sample into the scanning tunneling microscope.

"At room temperature atoms and molecules are quite mobile, so we decided to freeze the crystal to almost absolute zero (-269ºC) to get a good picture of its atomic structure,” says Dr Robin Ohmann, a member of the EMSS Unit and the first author of the paper. The crystal was cut and studied in a vacuum to avoid contamination of the surface. Dr Ohmann's colleagues from Soochow University calculated atomic structures using principles of quantum physics and then compared them with scanning tunneling microscopy data.

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