Abstract
The origin of bond-resolved atomic force microscope images remains controversial. Moreover, most work to date has involved planar, conjugated hydrocarbon molecules on a metal substrate thereby limiting knowledge of the generality of findings made about the imaging mechanism. Here we report the study of a very different sample; a hydrogen-terminated silicon surface. A procedure to obtain a passivated hydrogen-functionalized tip is defined and evolution of atomic force microscopy images at different tip elevations are shown. At relatively large tip-sample distances, the topmost atoms appear as distinct protrusions. However, on decreasing the tip-sample distance, features consistent with the silicon covalent bonds of the surface emerge. Using a density functional tight-binding-based method to simulate atomic force microscopy images, we reproduce the experimental results. The role of the tip flexibility and the nature of bonds and false bond-like features are discussed.
Highlights
We show that following ex situ cleaning with ebeam and field ion microscopy (FIM), a qPlus senor with a tungsten tip can be prepared in situ with the hydrogen-terminated silicon surface to obtain either a reactive or a passivated tip, both identified from the typical force curves they generate
We address the effect of tip flexibility in the imaging of this surface and in enhancing the atomic force microscopy (AFM) topographic feature registered between adjacent dimers in different dimer rows, where we know with certainty there is no hydrogen bond or covalent bond
Are we seeing the silicon covalent bonds? To highlight the difference in the high-resolution AFM images between features corresponding to real chemical bonds and those appearing in the silicon inter-dimer region, we present additional calculation results using two different systems
Summary
Since the pioneering work of Gross et al.[1], many studies have reported submolecular resolution atomic force microscopy (AFM) images of different molecules and molecular assemblies revealing a chemical bond contrast[2,3,4,5,6,7,8,9,10] Observing such contrast usually requires the use of a qPlus sensor[11] at liquid helium temperature to achieve: (i) high stability and low signal to noise, (ii) small tip-sample distances and (iii) the controlled functionalization of the tip apex, usually with a CO molecule. This was confirmed in a more recent study by Monig et al.[17] where a bond-like contrast could be seen experimentally and reproduced theoretically using a rigid Cu tip with an oxygen apex
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