Adsorption studies of Cr on W (1 1 0) plane by probe hole field emission microscopy

A probe hole field emission microscope has been used to study adsorption of chromium (Cr) on microscopic (1 1 0) plane of tungsten (W) tip. Adsorption is studied by measuring variation in workfunction (Φ) with Cr deposition (θ) at two temperatures, 722 and 300 K. For the 722 K tip, Φ decreases linearly by about 0.5 eV till completion of one monolayer and a very small increase is observed thereafter, indicating that the formation of Cr monolayer is by two-dimensional islanding. The average dipole moment associated with Cr–W pair is calculated to be 6.0×10−31 C m. On a tip held at 300 K, Cr atom population on the (1 1 0) plane is low. Therefore, only a small (0.3 eV) decrease in Φ is observed. Also the ΔΦ–θ behaviour is found to be erratic, indicating crystallite formation on the (1 1 0) plane. These observations are compared with those of Berlowitz and Shinn [Surf. Sci. 209 (1989) 345], who used macroscopic W surfaces to study Cr/W system. The results are discussed in view of the existing literature.