I am presently involved in an EPSRC project on non-radiative transitions of surface defects under an AFM tip. The idea is that the AFM tip may change significantly the adiabatic potential energy of a surface defect thereby affecting its radiative versus non-radiative ratio. The tip creates a non-uniform electrostatic field at the surface area underneath, and therefore a considerable amount of work has been done to take care of the electrostatics in ab initio electronic structure calculations as well as in AFM imaging. Specifically, a theory has been developed [B61,B62] which allows us to account for an induced charge on metal electrodes which has been found to be significant in the surface-tip interaction and thereby is an important contribution to the overall force acting on the tip in non-contact AFM. The computer code IMAGE has been written which calculates the force imposed on the tip and on charges between the electrodes as well as the system total electrostatic energy and the distribution of the potential in the system. Images of an oscillating mode non-contact AFM are presently being calculated for steps at NaCl (001) surface and the TiO2 (110) surface using MARVIN code to take account of the short-range effects and IMAGE code to include the polarisation of the electrodes as well as the non-uniform electrostatic field due to bias (to be submitted shortly).
The IMAGE code is also being incorporated into our embedding code based on GAUSSIAN-94 package and a shell-model semi-classical description of the outer region. The next thing to do is to try out a surface F-centre in alkali halides crystals and see what will be the effect of the tip on the defect ground and excited states. This work is now in progress.
The rate of non-radiative transitions is thought to be calculated using a semi-classical approach based on Frozen Gaussians. This work is in progress.
Ionic and electronic processes at a close contact between the AFM tip and the ionic surface have been studied in [B51] using a combination of semiempirical and ab initio HF and DFT methods. We found a significant distortion of the surface at the tip-surface separation of about 4 Å which may lead to the contamination of the tip and thereby affect the imaging. This work is reviewed in [A6].
Possible modifications of a silicon tip due to adsorption of O, H and OH have been studied in [B54].
An important part of this project related to embedding is described in Section 1.3.