Figure 2

The electron density profiles and atomic displacements determined by COBRA, for various bias voltage conditions.

In the figure legends, O.C.P. = open circuit condition. (a) The electron density profiles and atomic displacements determined by COBRA for the pristine sample along the [0 0 Z] atomic columns of the unit cell. The electron densities are graphed as a function of Z (nm) and Z (unit cells). Z = 0 nm (unit cells) is the position of the geometric interface between the substrate and the 1 UC buffer. For the substrate, the conversion is 1 unit cell = 1.262 nm, while for the film, we used the average film lattice spacing 1 unit cell = 1.315 nm. The substrate, buffer, film and helium regions are indicated for the pristine sample, for other cases helium is replaced by the IL. Note that the last unit cell before the film-helium interface is incomplete or exhibits only partial registry with the substrate. (b) An expanded view within the buffer, film and helium region. Notice the pronounced difference between the data for V_g = 3.2 V and V_g = −2.5 V bias voltage conditions and the similarity between the data for open circuit and V_g = 3.2 V conditions in the region of Z > 6 nm. (c) An expanded view of the electron density profile in the region of 5 nm < Z < 10 nm. The average cumulative displacements (d) of La/Sr atoms above and below Cu, (e) of the Cu atoms and (f ) of the equatorial oxygen (O_P) atoms measured for the atoms in each half unit cell with zero indicating the substrate/buffer interface. All cumulative atomic displacements are relative to a reference grid composed of the La/Sr and Al substrate atoms and extended throughout the entire sample. The slope in the displacement curves indicates that the size of unit cells above the substrate/film interface is larger than that of the substrate by 0.065 nm/unit cell on average.