Recognition of surface species in atomic force microscopy: Optical properties of a Cr3+ defect at the MgO (001) surface - art. no. 184111

We show using stare-of-the-art theoretical methods how scanning force micro scopy (SFM) can be extended to spectroscopic defect properties (radiative a nd nonradiative) with atomic resolution. This extra information offers one route to the identification of certain defect or impurity species using sca nning probe microscopy. As a case study, we consider the Cr3+ ion in the Mg 2+ lattice site at the MgO (001) surface. Our calculations cover two major topics. First, we calculate the noncontact SFM (NCSFM) image of this defect . Secondly, we show how the SFM tip can affect the impurity's optical prope rties. The NCSFM topographic image is predicted using classical atomistic s imulation methods; the effect of the tip on the defect spectroscopic proper ties is studied using an ab initio quantum-mechanical embedded cluster meth od. The electrostatic force due to the applied bias and to the image intera ction from polarization of the conducting electrodes are included self-cons istently in the calculation of the system geometry. The predicted defect NC SFM image can be used for defect identification in conventional NCSFM exper iments. Our electronic structure calculations show that an oxidized tip can significantly affect the oscillator strength and energy of the well-locali zed Cr ion d-d transitions. These effects can be used to identify a topogra phic defect image with a specific luminescence signal. The defect spectrosc opic properties can depend strongly on the local electric field, significan tly altering the branching ratios between radiative and nonradiative transi tions. We suggest that this effect could also be used to study local electr ic fields at surfaces due to proximity of surface steps or dislocations.