THE POWER OF A SCANNING FORCE MICROSCOPE
The atomic force microscope has still another name – the scanning force microscope. When imaging with it, feedback must be used to control the interaction force between the probe and the sample. Generally, it must be kept at a minimum value so as not to deform the sample and probe during measurements. In contrast, in nanolithography and nanoindentation a large force must be applied to produce a clear pattern on the surface. In this paper we consider the forces that are generated and how much energy is expended when a typical defect in the form of a six-pointed star on graphite occurs.
According to one hypothesis, the star-shaped defect is formed when graphite splits in places where there are defects binding the carbon layers together. The elastic strain energy arising in these areas during graphite cleavage is then spent on cracking and bending the lobes. After clevage, several stars and corrugated stripes may be present in the frames at once.
The experimenter can also create such a defect. For example, it is possible to approach the probe with a sample inaccurately. The probe touches the graphite surface during the approach and then under the influence of feedback abruptly flies away. At the moment of touching the probe can get stuck due to adhesive forces that lead to entrapment of the upper graphite layer by the probe with the subsequent tearing of the graphite layer along its crystallographic axes. This is how a "star" defect appears. An image of such a defect is shown in Fig.1. It is possible to read more about graphite defects in .
In the atomic force microscope forces are measured by the cantilever deflection. As the cantilever deflection dZ and its mechanical stiffness k are known, it is easy to determine the force F by multiplying F=kdZ. It should be remembered that the force value measured in this way does not determine the magnitude of the force occurring at the probe-sample contact point. In order to keep this in mind, one should watch the video clip "Atomic Force Microscope. How does it work? What is it made of?" about atomic force microscopy on Youtube channel "An insight into the nanoworld" .
Let's measure the force curve in an atomic force microscope. A typical view is shown in Fig.2. At first, there is no interaction between the probe and the sample at a large distance (about 1 µm). Then the probe is slightly attracted to the sample by Van der Waals forces. Thereafter the probe and the sample move synchronously. When the probe retracts, it is drawn further in the opposite direction due to adhesion forces. At the moment of detachment the elastic force on the cantilever side becomes greater than the sum of the adhesion and Van der Waals forces. The area of the formed triangle equals the work of the force that pulls the cantilever away from the sample. For the force curve in Fig.2 it equals about 1.2 keV, i.e. this is the energy that could be used to break bonds in graphite and form a "star".
Knowing the perimeter of the star and the number of its constituent layers, it is possible to estimate the energy of star formation for two cases when the breaking of layers occurs so as to form zigzag and armchair type stages (Fig.3). The perimeter of the star shown in Fig.1 is 6,079 nm and it consists of 5 carbon layers. The bond energy between carbon atoms is about 5 eV. Thus, the formation energy of such a defect with zigzag edges is approximately 300 keV, with armchair edges about 350 keV. It is evident that a difference in the formation energies of stars with two types of edges is very small and, consequently, the probability of occurrence of both defects when scanning the surface of graphite is great.
If we compare the energy of star formation and the work of the adhesion force, a single-layer star with a lobe size of about 5 nm could be created in our experiment with the aid of an atomic force microscope probe.
In this paper we have studied how the force interaction between the probe and the sample surface occurs, using the formation of a typical six-pointed star defect on the surface of graphite as an example.
The study was completed with the financial support of the RFBR, the London Royal Society No. 21-58-10005, and from the Foundation for the Promotion of Innovation, Project No. 71108, and Ministry of Science and Higher Education of the Russian Federation.
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Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.