LEED and photoemission study of the stability of VO₂ssurfaces

We present low-energy electron-diffraction (LEED) and photoemission measurements of single-crystal surfaces of VO₂ , made possible using samples oriented and characterized in situ. LEED shows that the (100) surface has a (1×2) reconstruction, but the (110) and (101) surfaces have diffraction patterns consistent with those expected from termination of the bulk. The (001) surface is evidently unstable: gentle in situ heating of our samples causes faceting, while similar heating of cleaved single crystals results in irreversible changes in photoemission spectra, which we interpret as surface relaxation of (001) facets. At the metal-insulator transition, photoemission spectra of the nearly sixfold coordinated stable (110) and (101) surfaces change greatly, but spectra of the (001) surface change only slightly. We suggest a coupling between the electronic and structural instabilities of VO₂ may be responsible.