Introduction

Atomic force microscopy (AFM) is one type of scanning probe microscopy (SPM) and a characterization method used for nanoscale and atomic scale 3D imaging. Atomic force microscopy can also be called scanning force microscopy.^{[1, p.8-12]}In SPM, the principle is to use a small probe to detect the properties at a surface or interface of the sample.^{[1, p.3]}

Imaging individual atoms became possible in 1982 when the first kind of scanning probe microscope was invented. This microscope invented by Binnig and Rohrer is called scanning tunneling microscope (STM) and the inventors were rewarded with a Nobel prize in 1986. STM reached rapid success because of the unparalleled spatial resolution and simplicity of the technique.^{[1, p.4][2]}

However, STM has one significant limitation. It can only be used for conducting samples since STM requires electrical conduction. STM technique is based on measuring the tunneling current between the sample and the probe. AFM was invented and a working prototype introduced shortly after STM in 1986 by Binnig, Quate and Gerber. ATM can also be used for insulating samples, because the measured quantity in this technique is the force between the tip (probe) and the sample instead of tunneling current. Today atomic force microscopes are very commonly used tools for example in materials science, chemistry, biology and physics all over the world.^{[1, p.8-11][2][3]}

Principle of AFM

Atomic force microscope resembles very closely scanning tunneling microscope since only the probe tip is different. In general setup of scanning tunneling microscope a sharp tip is mounted on a scanning device. This scanner allows 3D positioning with subatomic precision. In STM the tip is usually a sharpened wire, while in AFM, this tunneling tip is replaced with a force-sensing cantilever, which is a cuboid shaped flat spring typically made of silicon.^[2] The image is achieved when this cantilever is scanned over the surface of a sample. There is a sharp tip (probe) at the end of the cantilever that contacts the surface of the sample. The force between the tip and the sample is measured and it is dependent on the tip-sample distance. The tip-sample force can be determined by measuring the deflection of the cantilever by using a laser beam reflected from the back of it into a photodiode (figure 1).^{[1, p.8-10]}

Figure 1. Schematic image of AFM function (static mode). Figure: Lotta Laihotie

Atomic force microscope can also be used in so called dynamic mode, which means that the cantilever is oscillating. It is exited to vibrate close to its free resonance frequency and when the AFM tip is close to the sample surface, the tip-sample interaction will lead to a change of the resonance frequency and amplitude of the cantilever. The change of amplitude can then be used as a detection signal. ^{[1, p.8]}

Important tip-sample forces are van der Waals forces (long-range attractive), chemical bonding forces and the Pauli repulsion (short range forces) and also electrostatic and capillary forces. If the force gradient of the tip-sample interaction becomes larger than the spring constant of the cantilever, instabilities can occur. Stiff cantilevers can be used to prevent that. Also in dynamic mode large oscillation amplitudes of the cantilever keep the cantilever force larger than the tip-sample force.^{[1, p.175-176][2]}

Example

It took almost ten years from the invention of AFM to resolve the first reactive surface by AFM with atomic resolution. Franz J. Giessibl was able to image the reconstructed silicon (111)-(7 x 7) surface in a noncontact mode of AFM with atomic resolution. Imaging Si(111)-(7 x 7) was considered a touchstone of the AFM’s feasibility as a tool for surface science. Hence this noncontact mode, dynamic atomic force microscopy, was proved to work as a standard method.^[2][4]

References