Topography and work function measurements of thin MgO(001) films on Ag(001) by nc-AFM and KPFM

Abstract

The surface topography and local surface work function of ultrathin MgO(001) films on Ag(001) have been studied by noncontact atomic force microscopy (nc-AFM) and Kelvin probe force microscopy (KPFM). First principles calculations have been used to explain the contrast formation of nc-AFM images. In agreement with literature, thin MgO films grow in islands with a quasi rectangular shape. Contrary to alkali halide films supported on metal surfaces, where the island heights can be correctly measured, small MgO islands are either imaged as depressions or elevations depending on the electrostatic potential of the tip apex. Correct island heights therefore cannot be given without knowing the precise contrast formation discussed in this paper. KPFM shows a silver work function which is reduced by the MgO islands. The values for the work function differences for one and two layer thin films are -1.1 and -1.4 eV, respectively, in good agreement with recent calculations and experiments.

Measuring Electric Field Induced Subpicometer Displacement of Step Edge Ions

“…week ending 5 OCTOBER 2012 146101-3 tips [45,55,56]. The origin of the simulated Áf TR is found entirely in the atomic displacements of the step ions induced by the bias: the ions move mainly along the vertical direction by at most 2 pm [ Fig.…”

“…Interatomic interactions in the surface are included via fitted Buckingham potentials [51][52][53][54]. As the tip is contacted to the surface, we expect the apex to be covered by LiF and model it as a simple dipole [45,55,56]-since we are interested in the interactions at LR, an atomistic model of the tip is unnecessary. The resulting force field is then used to run our home-built virtual DFM [57] capable of simulating Áf TR in the bimodal operation mode, enabling us to correlate the step atom displacements to the frequency shift as well as compare the latter to the experiment.…”

Control of Surface Work Function by Depositing Insulating Oxide Capping Layers

“…3) Contrasted to the barrier layers buried in solid devices, the modulation of work function by depositing an insulating layer at the top of metal surface has recently been attracting much attention. [4][5][6][7][8][9][10] At the surface of metal, electrons are slightly leaked away from the immobile framework of cations, which gives an electrical dipole moment. Due to the presence of such dipole moment, the work function of metal surface is larger than the bulk value.…”

“…Very recently, indeed a steep reduction of the work function as a function of deposition has been experimentally revealed in elemental metals capped with MgO (100) thin films. [8][9][10] While MgO (100) films are nonpolar, there are a variety of polar films among oxide thin films. [15][16][17] It is of considerable interest to study how the work function would be further modulated by depositing polar films rather than nonpolar films.…”

“…How the insulating capping layer affects the work function of elemental metal has been studied by both theoretically [4][5][6][7] and experimentally. [8][9][10] Here we have studied the work function modulation in all-oxide heterostructures, which have been fabricated on conducting single-crystalline oxide substrates and have been terminated with an insulating capping layer. It is quite challenging to realize a very low work function using oxides, which are chemically stable and do not contain rare elements.…”