Field evaporation of oxides: a theoretical study

To understand atom probe results on the field evaporation of oxides we use density functional theory on MgO clusters to follow the structural changes during field evaporation and toobtain potential energy curves, partial charges and desorption pathways. It is straightforward to understand that Mg evaporates doubly charged. We also show that MgO(+), MgO₂(+), MgO(2+) and O(+) ions leave the surface. Two questions are however new for oxides. (1) Where do the electrons go? When the oxides are deposited on a metal tip it can be assumed that the electrons are used to complete the electrical circuit. However this leaves the second question unanswered, namely (2) what happens to the oxygen? We will argue that there are two channels for the oxygen, namely (a) To travel down the (metallic) surface of the tip and eventually to desorb either as atoms or molecules. (b) The oxygen can recombine within the oxide layer itself and desorbs as a neutral molecule accelerated in the inhomogeneous field due to its induced dipole.