LEED AND PHOTOEMISSION-STUDY OF THE STABILITY OF VO2 SURFACES

We present low-energy electron-diffraction (LEED) and photoemission me asurements of single-crystal surfaces of VO2, made possible using samp les oriented and characterized in situ. LEED shows that the (100) surf ace has a (1 x 2) reconstruction, but the (110) and (101) surfaces hav e 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 cleav ed single crystals results in irreversible changes in photoemission sp ectra, which we interpret as surface relaxation of (001) facets. At th e metal-insulator transition, photoemission spectra of the nearly sixf old coordinated stable (110) and (101) surfaces change greatly, but sp ectra of the (001) surface change only slightly. We suggest a coupling between the electronic and structural instabilities of VO2 may be res ponsible.