Seeing dynamic phenomena with live scanning tunneling microscopy
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uction Scanning tunneling microscopy (STM) was introduced in the early 1980s.1,2 The ability to “see” atoms directly at the surfaces of a wide variety of materials provided the surfacescience community with an enormous boost and also became indispensable in other fields of science. Complex structures were easily recognizable, which greatly helped solve the geometrical puzzles of several surface reconstructions.2,3 STM images also provided the first realistic views of the defects that often dominate the behavior of surfaces. Where ideal, flat surfaces had monopolized the descriptions of surfaces thus far, STM made it impossible to ignore the “omnipresence” of steps.4 In addition, point defects, such as vacancies, adatoms, and kinks became familiar elements. In many practical processes in which surfaces play an essential role, such as crystal growth, sintering, or heterogeneous catalysis, this role can be traced back to these defects.5–7 Early STM images immediately fueled the dream that this technique would make it possible to follow some of these processes dynamically and reveal in detail where and how they take place. This would necessitate making STM observations in the conditions under which these processes take place. Technically, this turned out to be far from trivial, as STM is a
delicate measurement technique that is “vulnerable” to many external influences. These can make STM imaging challenging or even practically impossible. In this article, we concentrate on two of these challenges, namely, those introduced by high and even varying temperatures and those involved in high gas pressures (and flows), as well as their combination. We describe the instruments that we have developed to face these challenges and illustrate their application with two examples, namely the growth of graphene by low-pressure chemical vapor deposition (CVD) and the surface structure of catalysts under operation conditions.
Variable-temperature STM Thermal drift Early STM configurations were designed for imaging at room temperature. A common problem was that even minor temperature variations in the laboratory or the last few degrees of cooling down of the specimens after a high-temperature preparation step would lead to a noticeable, continuous translation of the STM tip with respect to the imaged surface, both parallel and perpendicular to the surface.8 This drift results from the differences in thermal expansion between the components
Joost W.M. Frenken, Advanced Research Center for Nanolithography, The Netherlands; [email protected] Irene M.N. Groot, Leiden Institute of Chemistry, Leiden University, The Netherlands; [email protected] doi:10.1557/mrs.2017.239
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• VOLUME 42 • NOVEMBERUniversity 2017 • www.mrs.org/bulletin 2017 Materials Downloaded MRS fromBULLETIN https://www.cambridge.org/core. of New England, on 13 Nov 2017 at 07:00:19, subject to the Cambridge Core terms of© use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/mrs.2017.239
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SEEING DYNAMIC PHENOMENA WI
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