Long-Term Oxidation Behaviour of Lead Sulfide Surfaces

Abstract

We have applied nanospectroscopy to examine the surface oxidation of the prototypic mineral lead sulfide in two states: ‘oxidised’ (oxidised in air for one year); and ‘clean’ (cleaved in air and inserted into the vacuum system within about 10 minutes). The elements detected on the surfaces were Pb, S, O, Cl (a natural impurity) and C (adventitious or natural, carbonate). Images showed strong topographic contrast and weak chemical contrast for the oxygen 1s maps. Near defects such as steps, there is a higher concentration of oxygen, indicating a higher oxidation rate in this zone. From the O 1s and Pb 4f binding energies, it is concluded that the oxidation products present are Pb₃O₄, PbCO3 and Pb(OH)₂, while PbO, PbSO₄ and PbO2 were excluded. Sulfur is present on the oxidised sample in two chemical states: one with the same binding energy as for the clean sample, due to the native PbS below the oxide film; and another of higher binding energy. The binding energy is assigned to S present as polysulfide and/or sulfur located at the oxide/sulfide interface. Spectra taken at different points on the surface show energy shifts up to about 0.5 eV, due to band bending and pinning of the Fermi level by defects or impurities.