Recognition of surface species in atomic force microscopy: Optical properties of aCr3+defect at the MgO (001) surface

Abstract

We show using state-of-the-art theoretical methods how scanning force microscopy (SFM) can be extended to spectroscopic defect properties (radiative and nonradiative) with atomic resolution. This extra information offers one route to the identification of certain defect or impurity species using scanning probe microscopy. As a case study, we consider the ${\mathrm{Cr}}^{3+}$ ion in the ${\mathrm{Mg}}^{2+}$ 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 properties. The NCSFM topographic image is predicted using classical atomistic simulation methods; the effect of the tip on the defect spectroscopic properties is studied using an ab initio quantum-mechanical embedded cluster method. The electrostatic force due to the applied bias and to the image interaction from polarization of the conducting electrodes are included self-consistently in the calculation of the system geometry. The predicted defect NCSFM image can be used for defect identification in conventional NCSFM experiments. Our electronic structure calculations show that an oxidized tip can significantly affect the oscillator strength and energy of the well-localized Cr ion $d\ensuremath{-}d$ transitions. These effects can be used to identify a topographic defect image with a specific luminescence signal. The defect spectroscopic properties can depend strongly on the local electric field, significantly altering the branching ratios between radiative and nonradiative transitions. We suggest that this effect could also be used to study local electric fields at surfaces due to proximity of surface steps or dislocations.