Field-Electron-Microscopy Studies of Cesium Layers on Various Refractory Metals: Work Function Change

Abstract

The variation of work function φ with Cs coverage θ is investigated for W, Mo, Re, and Ni substrates. Single-crystal-face studies of the φ(θ) relationship performed on the (100) and (110) planes of W show a greater affinity of the (110) plane for mobilely adsorbed Cs at low values of θ. Coverage anisotropies are reduced as θ→1. An empirical relationship between the maximum work function change Δφm and substrate work function φs, and adsorbate ionization potential Ia, of the form Δφm = k1(k2Ia − φs), is established. The constants k1 and k2 are independent of the substrate and adsorbate for periodic Group Ia and IIa. Measurement of the temperature variation of the work function for Cs-coated Mo and W substrates show a negative temperature coefficient of ∼ − 1 × 10−4 eV/deg in the temperature interval 77° − 270°K. The average values of the adsorbate polarizability measured for the (110) and (100) planes of W are 23 ± 2 and 13 ± 0.2 Å3, respectively. The presence of oxygen coadsorbed with Cs on W greatly alters the emission distribution and reduces the minimum work function φm from 1.52 to 1.10 eV; also, the Cs coverage at which φm occurs is decreased from 1.9 to 0.9 × 1014 atoms/cm2.