Preparation and characterization of metastable wurtzite cobalt oxide films

Collage of images demonstrating the important findings of the paper: preparation, crystalline structure, semiconduting properties

Materials that appear in different structural phases are of great interest since they test our fundamental understanding of the properties of a substance. The wurtzite phase of cobalt(II) oxide is a rather elusive polymorph of CoO that normally has the simple rocksalt crystal structure. Up to now it could only be produced and characterized in nanocrystalline form where its investigated properties promise application as sensor material, photocatalyst or photoabsorber.
We were now able to produce extended films of the material which is a step forward to be able to understand its physical and chemical properties. We determined its crystal structure with great accuracy by low-energy electron diffraction (LEED). By that the positions of the atoms particularly at the surface of wurtzite CoO were determined with picometer precision, resolving some discrepancies found in literature. The surface structure of a material is particularly important for the above mentioned applications since at the surface the interaction with the surroundings take place.
We further characterized the electronic properties using scanning tunneling microscopy and spectroscopy (STM/STS) and ultraviolet photoelectron spectroscopy (UPS) proving that wurtzite CoO is a semiconductor and determining its energy gap (1.4 eV). The study is completed by a comprehensive theoretical analysis by density functional theory (DFT) that confirmed not only the determined crystal structure and semiconducting nature but also allowed to interpret the experimental findings.
The full paper may be accessed at:
M. Ammon, et al., Epitaxial Cobalt Oxide Films with Wurtzite Structure on Au(111), Phys. Status Solidi RRL 2021, 2100383: (open access)