The Working of the Atomic Force Microscope for Chemical Mapping

Since the invention of the scanning tunneling microscope (STM) in 1981 and the atomic force microscope (AFM) in 1986, over 5,000 publications have cited the article “Atomic Force Microscope” by G. Binnig, C.F. Quate, and C. Gerber (published in Physical Review Letters, 1986). This article presents a short review on the operating principle and possible applications of AFM with special attention devoted to chemical mapping. The article would be useful for beginners in AFM technique. In 1959, Nobel Laureate Richard Feynman (Fig. 1), from http://nobelprize.org/nobel_prizes/physics/laureates/1965/ind ex.html) gave a lecture at a meeting of the American Physical Society where he talked about future opportunities for manipulating and controlling things on the very small scale – the nanoscale. It was the first time that the field of nanotechnology was posed to the world. In his talk, Feynman mentioned how in the year 2000 we might look back and wonder why it was not until the year 1960 that we began seriously to move into the field of nanotechnology. Ironically, nanotechnology eventually took off in the year 2000 because back in 1960, we simply did not have the tools to manipulate, control, or characterize things on the nanoscale. The invention of the scanning tunneling microscope (STM) and atomic force microscope (AFM) were major breakthroughs that opened the door to nanotechnology [1, 2]. Since the invention of the STM in 1981 and the AFM in 1986, over 5,000 publications have cited the article “Atomic Force Microscope” by G. Binnig, C.F. Quate, and C. Gerber (published in Physical Review Letters, 1986) [1,3]. And because of the continued research and development of these microscopes, together with their versatility, different surface types can be studied today by over 20 modified force microscopes, such as the magnetic force microscope (MFM), force modulation microscope (FMM), Kelvin probe force microscope (KPFM), electrostatic force microscope (EFM), and others. In this article, we aim to describe how chemical force microscopy (CFM) works and its application in providing chemical information of scanned surfaces. INVENTION OF THE TOOLS Gerd Binnig and Heinrich Rohrer developed a prototype of the STM in 1981 while working at the IBM Zurich Research Laboratory in Switzerland. They were later *Address correspondence to this author at the Department of Chemistry, Faculty of Science, Mahidol University, Rama 6 Rd., Rajchataywee, Bangkok 10400, Thailand; Tel: +662-441-9817, Ext. 1126; Fax: +662-8892337; E-mail: [email protected] Fig. (1). Nobel Laureate Richard P. Feymann, (picture from http://nobelprize.org/nobel_prizes/physics/laureates/1965/index.htm l). His prize was shared with 3 other scientists for “their fundamental work in quantum electrodynamics, with deepploughing consequences for the physics of elementary particles”. rewarded with the Nobel Prize in Physics in 1986 for the invention. (The prize was shared that year with Ernst Ruska for his design of the electron microscope. Fig. (2), from http://nobelprize.org/nobel_prizes/physics/laureates/1986/) Within a year of the invention of the STM, Binnig and coworkers used the microscope to resolve the structure of the Si(111)(7x7) surface, one of the most intriguing unsolved problems in surface science at that time [4]. Despite the success of the STM, it became evident that a similar scanning microscope which could be used for nonconducting surfaces would be more useful. The STM requires that the sample material be electrically conductive, because the microscope uses the tunneling current which flows between a biased tip and the sample. It reflects the strong distance-dependent probability of electron transport through a gap between two conducting solids. With careful observation early in the STM experiments, Binnig became aware of significant forces detected with the tunneling current when the tip-sample distance was small enough for a The Working of the Atomic Force Microscope for Chemical Mapping The Open Materials Science Journal, 2009, Volume 3 51 current to flow. In 1986 (while Gerd Binnig spent a sabbatical at Stanford with Calvin Quate and Christoph Gerber was at the IBM Research Laboratory in Almaden, California), they soon introduced a working prototype of the AFM [2, 5]. The forces were based on the short-range van der Waals interaction when the tip-sample distance was small (see Fig. (3), [3]). Because no electron transport is involved, both insulators and biological samples can be investigated down to atomic resolution. Information on topography [6-11], roughness [9, 10], friction [8], adhesion [8, 11], elastic properties [12, 13], magnetic field [14, 15], resistivity [16, 17], etc. can be obtained by the AFM with nanometer resolution. PRINCIPLES OF ATOMIC FORCE MICROSCOPY Binnig described the AFM as being much like the stylus profilometer, in that both are used to scan the surface, sense the variations of the sample, and generate three-dimensional images [3]. The AFM consists of a probe with a very sharp TIP