Imaging nanoclusters in theconstant height modeof the dynamic SFM

Abstract

For the first time, high quality images of metal nanoclusters which were recorded in the constant height mode of a dynamic scanning force microscope (dynamic SFM) are shown. Surfaces of highly ordered pyrolytic graphite (HOPG) were used as a test substrate since metal nanoclusters with well defined and symmetric shapes can be created by epitaxial growth. We performed imaging of gold clusters with sizes between 5 and 15 nm in both scanning modes, constant f mode and constant height mode, and compared the image contrast. We notice that clusters in constant height images appear much sharper, and exhibit more reasonable lateral shapes and sizes in comparison to images recorded in the constant f mode. With the help of numerical simulations we show that only a microscopically small part of the tip apex (nanotip) is probably the main contributor for the image contrast formation. In principle, the constant height mode can be used for imaging surfaces of any material, e.g. ionic crystals, as shown for the system Au/NaCl(001).

Imaging the real shape of nanoclusters in scanning force microscopy

“…The work we present here strongly supports our previous assumptions, 28 and it basically shows that high lateral contrast of nanoclusters can be principally achieved as long as the last nanometer of the tip is physically or chemically sharp. Alongside the topography mode, which provides the height information of objects on the surface, the constant height mode can be used to precisely determine the lateral size and shape of nanoclusters.…”

“…In our recent work we anticipated that this is realized by a so-called nanotip that is located in the short-range interaction between the tip and cluster. 28 In this paper we present a quantitative comparison between experiment and theory, which supports our assumptions. For the comparison the parameters of the cantilever oscillation ͑amplitude, spring constant, detuning͒ but also the parameters of the nanoclusters ͑size, shape, material͒ taken from experimental measurements were used as an input for theory.…”

“…1, we give a brief overview of the imaging modes in dynamic SFM in the following. More details can be found in our recent work 28 and in Ref. 25.…”

“…1͑b͔͒ with an included memory effect of the tip pathway, leading to asymmetric cluster profiles along the fast scanning direction. 28 For very small loop gains and high scanning speeds, the tip follows only the inclination of the surface with respect to the fast scanning direction ͓pathway ͑4͒ in Fig. 1͑b͔͒.…”

“…Recently, we have qualitatively shown that the lateral size and shape of nanoclusters can be more accurately imaged with high scanning speeds if the so-called constant height mode in dynamic SFM is used. 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.…”