Contrast mechanism in non-contact SFM imaging of ionic surfaces

doi:10.1016/S0169-4332(98)00549-2

Applied Surface Science

Volume 140, Issues 3–4, February 1999, Pages 327-332

https://doi.org/10.1016/S0169-4332(98)00549-2 Get rights and content

Abstract

We analyse the mechanisms of contrast formation in NC-SFM imaging of ionic surfaces and calculate constant frequency shift scanlines of the perfect surfaces of NaCl and MgO. Non-contact SFM operation is modelled in a perturbed oscillator model using an atomistic simulation technique for force–field calculations. We demonstrate that the contrast in NC-SFM imaging of ionic surfaces is based on the interplay between the van der Waals interaction and the electrostatic interaction of the tip with the surface potential and the local surface polarisation induced by the tip. The results emphasise the importance of the tip-induced relaxation of the surface ions in the tip–surface interaction and image contrast.

Introduction

The recent development of non-contact (NC) mode SFM 1, 2, 3, 4, 5, 6 has brought new hope that SFM will finally be able to achieve on insulating surfaces what STM has on conducting surfaces, i.e., to observe and identify individual atoms and point defects. Images obtained with this technique are deemed to be closest to `true' atomic resolution, due to the weak tip–surface interaction, and the fact that several observations of stable point defects have been made 4, 5, 7. Therefore a strong effort to improve the technique and to apply it to different insulators continues (see, for example, Ref. [8]). Part of this effort is the development of a theoretical model which would allow the analysis of the NC-SFM operation and to interpret experimental images [9].

Theoretical modelling of NC-SFM includes two main components: (i) calculation of the tip–surface forces; (ii) modelling of cantilever oscillations using known tip–surface forces. The latter problem has been considered in detail in conjunction with the tapping mode of SFM operation 10, 11, and recently also with respect to NC-SFM 9, 11. However, the chemical component of the tip–surface interaction which is responsible for the contrast formation is difficult to quantify without atomistic modelling of representative systems. As will be shown below, this modelling should include both surface and tip relaxation.

In this paper we analyse the mechanisms of contrast formation in NC-SFM imaging of ionic surfaces and calculate constant frequency shift scanlines of the perfect surfaces of NaCl and MgO, which are treated as model insulators in SFM studies 5, 6.

Section snippets

Model of NC-SFM

For interpretation of NC-SFM images, one needs to know the relation between the tip–surface forces and the parameters of cantilever oscillations.

Results

It is known that an attractive tip–surface interaction decreases the frequency of cantilever oscillations whereas a repulsive interaction makes it bigger than ω₀. Therefore, in the presence of repulsion there is a maximum frequency shift (Δω)_max corresponding to the maximum attraction before the frequency starts to increase thus decreasing Δω. To estimate the maximum scanline corrugation at given cantilever and interaction parameters, we calculated the scanlines for the NaCl surface at the

Summary

The mechanism of the contrast in NC-SFM imaging of ionic surfaces proposed in this paper is based on the interplay between the van der Waals interaction and the electrostatic interaction of the tip with the surface potential and the local surface polarisation induced by the tip. The strength of the electrostatic interaction is determined by the strength of the electric field produced by the tip. In our tip model, this field is produced by the low-coordinated site at the apex of an ionic

Acknowledgements

A.I.L. is supported by EPSRC. A.L.R. would like to acknowledge the financial support of the Australian Federal Government through its Cooperative Research Centres Program. The authors wish to thank F.J. Giessibl, R. Bennewitz, E. Meyer, A. Baratoff, G. Thornton and H. Raza for useful discussions.

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Contrast mechanism in non-contact SFM imaging of ionic surfaces

Abstract

Introduction

Section snippets

Model of NC-SFM

Results

Summary

Acknowledgements

Solid State Ionics

Jpn. J. Appl. Phys.

Science

Z. Phys. B

J. Vac. Sci. Technol. B

Probe Microsc.

Appl. Phys. A

Phys. Rev. Lett.

Rev. Sci. Instrum.

Phys. Rev. B

Nanotechnology

Ann. Physik