Contrast formation in atomic resolution scanning force microscopy on CaF₂(111): experiment and theory

Abstract

We investigate mechanisms of contrast formation in atomic resolution imaging of flat terraces on the CaF 2 (111) surface with scanning force microscopy operated in the dynamic mode. Experimental results are interpreted with a theory based on atomistic modelling. Experiments reveal characteristic contrast features in the form of triangles that can be explained by theory as being due to the interaction of a positively terminated tip with fluorine ions from two different sublattices. Results for a tip with negative termination are found not to be compatible with experiments. We demonstrate that theory correctly predicts the trend in contrast changes when varying the tip-surface distance but is also limited in quantitative agreement due to the non-ideal atomic structure of real tips. In a distance range where such peculiarities do not play a major role, however, we find good quantitative agreement between theoretical predictions and experimental results. The validity of the comparison between theory and experimental scan lines is discussed in detail using an extensive statistical image analysis.

Unambiguous Interpretation of Atomically Resolved Force Microscopy Images of an Insulator

“…Details of the procedure are discussed in Ref. [14], while we here just note that the tip turned out to be rather blunt. This is in accordance with our experience from many measurements, that best atomic resolution is obtained with tips that are blunt on the macroscopic scale but have a stable cluster at their end.…”

“…These images are reproducible, and a detailed analysis of images taken in different conditions is given in Ref. [14]. For quantitative characterization, we made a statistical analysis of 73 pairs of primary and secondary maxima in 15 separate scan lines and determined the position of the secondary features and their height relative to the primary peaks.…”

“…It should be noted that problems related to thermal drift make it difficult to establish a reference height when calculating the ratio of peak heights, and this reduces the accuracy of this measurement. This problem has been addressed in a more extensive statistical analysis [14].…”

Imaging the real shape of nanoclusters in scanning force microscopy

“…28 The constant height mode is similar to the one frequently used in scanning tunneling microscopy ͑STM͒, 29,30 and was already used for atomic resolution imaging in dynamic SFM. 31 It is a complementary mode to the topography mode that permits imaging of the lateral dimensions of objects at nanometer and Angstrom scale. 23,31 In the constant height mode, the tip is scanned at a constant height above the surface with a high scanning speed of up to 50 Hz or more, where all information of the tip-surface interaction is included in images of the detuning ⌬f.…”

Repulsive interaction and contrast inversion in noncontact atomic force microscopy imaging of adsorbates

“…Noncontact atomic force microscopy ͑NC-AFM͒ facilitates atomic resolution imaging first demonstrated for the Si͑111͒-͑7 ϫ 7͒ surface 2 and is now obtained on a routine basis. The mechanisms governing atomic contrast formation in NC-AFM imaging especially for semiconducting and insulating surfaces have been investigated by both experiment 3,4 and theory. 5,6 Also, manipulation of individual molecules 7 and atoms 8 has been achieved by using NC-AFM.…”