Advanced atomic force microscopy techniques

Surface science and nanotechnology are inherently coupled because of the increased surface-to-volume ratio at the nanometer scale. Most of the exciting and astonishing properties of nanoscale materials are related to certain surface reconstructions and nanoscale geometries. New functionality is achieved by combinations of nanoscale materials or by structuring their surfaces. The unrivaled tools for measurements of all kind of nanoscale properties are scanning probe microscopy (SPM) techniques, which were triggered by the invention of the scanning tunneling microscope (STM) in 1982 [1–3] and of the atomic force microscope (AFM) in 1986 [4]. These tools opened a huge field of nanoscale studies, from metal surfaces and clusters, molecular structures, insulators to liquid and electrochemical environments and even allowed the integration of various SPM techniques into biological and chemical experiments. 
 
The second volume of the Thematic Series “Advanced atomic force microscopy techniques”, which is presented here, compiles again exciting developments in nanoscale research based on SPM techniques addressed by the scientific community within the last years. Similar to the first volume [5], the development of advanced techniques and their application is the focus of this Thematic Series. Contributions related to energy conversion and storage systems have been addressed, e.g., the analysis of cathodes of lithium–sulfur batteries for a comparison of their nanoscale electrical, electrochemical, and morphological properties [6] or the analysis of CdS quantum dots on TiO2 by a combination of AFM and X-ray photoelectron spectroscopy [7]. The folding and rupture of graphene on SiC analyzed by non-contact AFM and Kelvin probe force microscopy [8] are presented as well as molecular structures such as the self-assembly of multidentate organothiols onto Au(111), which were studied in situ by using scanning probe nanolithography and time-lapse AFM [9]. Patterns of thiol-based self-assembled monolayers for the site-selective growth of metal-organic frameworks have been created and analyzed by a nanografting technique by using an AFM as a structuring tool [10]. The effect of Cu intercalation at the interface of self-assembled monolayers and a Au(111)/mica substrate was analyzed by STM [11] as well as the growth behavior of PTCDA islands [12]. 
 
Furthermore, the analysis of mechanical properties of either nanoparticles [13] or biological systems [14–16] is covered by several articles and reviewed by Cohen and co-workers [17]. Especially the application of advanced SPM techniques in biology provides exciting new results and clearly shows a route for development for the next years. All of the new applications and experiments are strongly dependent on theoretical and technical developments. Virtual AFMs used to simulate AFM measurements [18–20] and to deconvolute complex correlations between various surface properties [21] are based on the implementation of proportional-integral controllers to give realistic feedback behaviours. Stirling proposed a theoretical model for studying the SPM feedback in the context of control theory providing the possibility to understand and model the performance from SPM systems with real parameters [22]. Furthermore, technical contributions discuss the impact of thermal frequency drift of quartz-based force sensors at low temperatures to the accuracy of the force measurements [23] and the trade-offs in sensitivity and sampling depth in bimodal and trimodal AFM [24]. The examples mentioned give a first impression of this collection of high quality research provided to the Beilstein Journal of Nanotechnology, the open-access journal for publication and dissemination of nanoscience research results. We are convinced that the articles presented here will stimulate new ideas in the research field. 
 
We would like to thank all of the authors for their excellent contributions and the referees for their comprehensive and valuable reports, sustaining a journal that is attractive for contributors. Finally, we would also like to thank the team at the Beilstein-Institut for their excellent support and acknowledge the open-access policy of the Beilstein Journal of Nanotechnology, which provides the professional framework and support allowing the collection, review, publishing, and distribution of research results in an easy and excellent way. 
 
Thilo Glatzel, Ricardo Garcia & Thomas Schimmel 
 
November 2014

Although its conceptual approach is as simple as the technique used in record players already introduced in the 19th century, the invention of the atomic force microscope (AFM) in 1986 by Binnig, Quate, and Gerber was a milestone for nanotechnology. The scanning tunneling microscope (STM), introduced some years earlier, had already achieved atomic resolution, but is limited to conductive surfaces. Since its operational principle is based on the detection of the forces acting between tip and sample, this restriction does not exist for the AFM. Consequently, atomic force microscopy quickly became the standard tool for nanometer-scale imaging of all types of surfaces in all environments. True atomic resolution was first achieved in the 1990s. The most convincing results, however, were restricted to the so-called noncontact mode in vacuum for a long time, but recent technical developments overcame this limitation, and atomic-resolution imaging is now also a standard in liquids.
Beyond pushing the resolution limit to the picometer range, the invention of the AFM triggered the development of a growing number of new scanning probe methods and approaches, ranging from an expansion of the properties that can be mapped to the active manipulation of surfaces and small particles. Practically every month, reports on the growing capabilities of AFMs appear. Nearly every physical effect that influences the tip-sample interaction has been used to improve existing modes and to develop new ones. For example, many recently presented techniques include the excitation of higher cantilever oscillation modes; it is amazing in how many ways the shaking of a simple cantilever can improve our knowledge about the tip-sample interaction. Another direction is high-speed atomic force microscopy, which is one of the eminent challenges that need to be solved in order to allow the in situ observation of biological processes. Data acquisition times have already reached the millisecond range, enabling the visualization of the dynamic behavior of biological molecules and cells. Other recent accomplishments include imaging of organic molecules with unprecedented resolution, full three-dimensional mapping of surface force fields, and the imaging and discrimination of individual chemical bonds.
The development of advanced techniques is the focus of this Thematic Series, following the Thematic Series "Scanning probe microscopy and related techniques" edited by Ernst Meyer and the Thematic Series "Noncontact atomic force microscopy" edited by Udo Schwarz. The articles that are part of the series demonstrate that, despite its 25 years of history, the AFM is still far from reaching its limits, and today's developments are far-reaching. As the number of research groups utilizing advanced atomic force microscopy techniques increases with each passing year, the technical improvements, data-acquisition approaches, analysis procedures, user friendliness, and application areas of the technique further diversify. With this Thematic Series, it is our intention to stimulate these improvements.
We thank all authors for contributing their excellent work to this series. Furthermore, we acknowledge all referees for their promptly provided reports keeping the publication times short and attractive for contributors. Finally, we are grateful to the open access policy of the Beilstein Journal of Nanotechnology providing the ground for unrestricted discussions on advanced atomic force microscopy techniques.

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This is an Open Access article under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
The license is subject to the Beilstein Journal of Nanotechnology terms and conditions: (http://www.beilstein-journals.org/bjnano) The definitive version of this article is the electronic one which can be found at: doi:10.3762/bjnano.3.99