Tendency in tip polarity changes in non-contact atomic force microscopy imaging on a fluorite surface

Abstract

We investigate the impact of tip changes on atomic-scale non-contact atomic force microscopy (NC-AFM) contrast formation when imaging a CaF₂(111) surface. A change of the atomic contrast is explained by a polarity change of the tip-terminating cluster or by a polarity-preserving tip change via the re-arrangement of the foremost atoms. Based on the established understanding of the unique contrast patterns on CaF₂(111), polarity-preserving and polarity-changing tip changes can be identified unambiguously. From analyzing a large set of images, we find that the vast majority of tip changes tend to result in negative tip termination. This analysis delivers hints for tip configurations suitable for stable imaging of CaF₂(111) surfaces.

Keywords: atomic resolution imaging; calcium fluoride surface; interaction force; non-contact atomic force microscopy (NC-AFM); tip change.

Figure 1

Distance-dependent contrast formation on CaF₂(111) for positively and negatively terminated tips (vertical columns) as well as transitions between contrast modes due to tip changes. The contrast modes C₁, C₃, and C₄* (C₂ and C₄) are assigned to a positive (negative) tip termination [10]. Solid black arrows indicate experimentally observed changes of the tip polarity, while grey arrows denote polarity-preserving tip changes. Dashed black arrows indicate tip changes that were observed but are excluded from the discussion herein.

Figure 2

Examples of polarity-preserving tip changes on CaF₂(111) at room temperature. (a, d) Δf data acquired at (a) Δf_set = −109 Hz and (d) −82.1 Hz with a top view CaF₂(111) surface model overlaid. (b, c) and (e, f) Contrast profiles extracted along the direction of the unit-cell averaged data from image regions indicated by the square brackets. represents the distance between equivalent atoms along ⟨11−2⟩ directions, where a₀ is the bulk lattice constant of CaF₂. Atomic assignment follows the model introduced in [10], with the solid line in panels (b, c, e, f) representing a polynomial fit of degree seven as a guide to the eye.

Figure 3

Examples of polarity-changing tip changes on CaF₂(111). (a) Δf data acquired at Δf_set = −30.9 Hz (RT) and (d) Δf data acquired on a thin film sample at Δf_set = −12.0 Hz (77 K). Top-view CaF₂(111) surface models are overlaid. (b, c) and (e, f) show line profiles extracted along the direction of the unit-cell averaged data in the regions indicated by the square brackets. represents the distance between equivalent atoms along ⟨11−2⟩ directions, where a₀ is the bulk lattice constant of CaF₂. Atomic assignment follows the model introduced in [10], with the solid line in panels (b, c, e, f) representing a polynomial fit of degree seven as a guide to the eye.

Figure 4

Tip changes leading to a successively stabilizing negative tip termination. (a–c) and (g–i) Δf data acquired while step-wise decreasing the tip–sample distance (Δf_set = −60.2 Hz to −78.0 Hz). (d–f) and (j–l) Line profiles extracted along the direction of the unit-cell averaged data. represents the distance between equivalent atoms along ⟨11−2⟩ directions, where a₀ is the bulk lattice constant of CaF₂. Atomic assignment follows the model introduced in [10], with the solid line in panels (d–f) and (j–l) representing a polynomial fit of degree seven as a guide to the eye.