Scanning tunneling microscopy and photoemission electron microscopy studies on single crystal Ni2P surfaces

The surface structures of nickel phosphide (Ni2P) single crystals were studied by scanning tunneling microscopy (STM) and photoemission electron microscopy (PEEM). Atomically resolved 1 x 1 images of the Ni2P(0001) and (1010) surfaces are successfully obtained with STM, whose respective dimensions of (0.59 nm x 0.59 nm) and (0.34 nm x 0.59 nm) match the unit cell lengths. The Ni2P(0001) surface has two possible terminations in which the Ni:P ratios are 3:1 (Ni3P termination) or 3:2 (Ni3P2 termination). In the Ni3P terminated surface the Ni atoms have a square pyramidal structure, and in the Ni3P2 terminated surface the Ni atoms have a tetrahedral structure. Only the P is visible in the STM for Ni2P(0001) and this is explained as being due to the greater extension of the phosphorus p orbitals than the Ni d orbitals at the surface. The surface domain sizes of the Ni3P and Ni3P2 termination structures of Ni2P(0001) are determined to be 500 microm by means of PEEM using a Hg-Xe lamp through a low-pass UV-filter. Evidence is found for dissociative adsorbed hydrogen on the Ni3P termination surface of Ni2P(0001), but not on the Ni3P2 surface, indicating that the square pyramidal sites on the Ni3P surface have high reactivity.