Abstract

Scanning tunneling microscopy (STM) is used to study the structure of {001} hematite single-crystal surfaces. STM images taken at negative sample bias consistently show a hexagonal array ofatomic sites with 3-Å periodicity, as expected for the surface O array and in agreement with photoelectron spectroscopy studies of hematite surfaces after O2 and H2O exposure which show that states at the top of the valence band have O character. Molecular orbital calculations suggest that these states are localized on O sites. In contrast, STM images taken at positive sample bias were variable, sometimes showing hexagonal arrays with O periodicity and sometimes showing arrays with equivalent Fe periodicity. Moreover, because the states at the base of the conduction band are not necessarily localized on atomic sites, the STM images taken at positive sample bias may not show atomic posrtrons.

With the above findings in mind, we used negative bias images to study relaxation of O positions on the hematite {001} surface. Surface O positions are apparently relaxed to a regularized hexagonal array with a single-mode distribution of O-O interatomic distances; in the equivalent bulk plane, O-O distances form a bimodal distribution because of distortion by octahedral face sharing across the (001) plane.

STM images also show that step densities on hematite {001} fracture surfaces are high, often one step every 20–30 Å. The atomic structure near step edges appears to be reconstructed relative to terraces, but convolution of atomic and electronic structure information in STM images makes precise interpretation of step structure difficult.

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