ADSORPTION OF NICKEL ON TUNGSTEN - A PROBE-HOLE FIELD-EMISSION MICROSCOPY STUDY

In a probe-hole field emission microscopy experiment, submonolayer amo unts of high-purity nickel were deposited on to a tungsten tip in clea n, ultra high vacuum conditions and observations were made on W(1 1 0) , (1 2 1)and (1 1 1) planes. The changes in the work function, Delta p hi, and corresponding pre-exponential Factors,ln B, have been obtained by ten point Fowler-Nordheim plots. The results show that the change in average work function of nickel covered tungsten surface increases and attains a saturation value of + 0.52 eV. This is accompanied by a decrease in the pre-exponential factor. Adsorption of nickel on the W( 1 1 0) plane causes a decrease in Delta phi in the lower coverage regi on and then exhibits an abrupt change to reach a saturation value of 0.5 eV as the coverage increases. This has been attributed to surface reconstruction. In the case of the W(1 2 1) plane Delta phi decreases with the number of doses to a value of -0.45 eV. On the W(1 1 1) plan e, the Delta phi increases and saturates to a value of + 0.50 eV after a brief initial decrease in the low-coverage region. The changes in t he work function are explained in terms of atomic and electronic struc ture of nickel overlayers on various single-crystal planes of tungsten . A comparison of nickel with iron and cobalt adsorption on tungsten s hows that the behaviour of nickel is similar to iron. (C) 1998 Elsevie r Science B.V. All rights reserved.