MgO on Mo(001): Local Work Function Measurements above Pristine Terrace and Line Defect Sites

Abstract

We present a local spectroscopy study of thin MgO films on Mo(001), investigated with a combined noncontact atomic force microscope (nc-AFM) and scanning tunneling microscope (STM). The work function of Mo(001) and MgO/Mo(001) was measured with field emission resonance (FER) spectroscopy. The work function of the clean Mo(001) surface ΦMo was determined to be 4.5 eV. After the deposition of an 8 atomic layer thick MgO film, it decreased to ΦMgO/Mo = 3.2 eV (ΔΦFER = −1.3 eV). This work function shift ΔΦFER, introduced by the oxide film on the metal work function, was also investigated with contact potential difference (CPD) measurements. Here, a lowering of the work function by ΔΦCPD = −1.1 eV was measured. Furthermore, the influence of line defects, present in the MgO film, on the local work function was investigated and compared to pristine terrace sites of the oxide film. Here, we measured a slightly higher local work function of +200 meV above line defects as compared to pristine oxide terrace sites.