Non-contact atomic force microscopy
[citation needed] Another potential problem with amplitude modulation is that a sudden change to a more repulsive (less attractive) force can shift the resonance past the drive frequency causing it to decrease again.
The reason for the higher stiffness is stop the probe snapping to contact with the surface due to Van der Waals forces.
By doping the silicon, the sensor can be made conductive to allow simultaneous scanning tunneling microscopy (STM) and nc-AFM operation.
The AFM deflection signal is generated by the piezoelectric effect, and can be read from the two electrodes on the tuning fork.
The tip can either be electrically connected to one of tuning fork electrodes, or to a separate thin (~30μm diameter) gold wire.
New versions of the qPlus sensor with one or several integrated service electrodes as proposed in reference [13] and implemented in [14] solve that problem.
The Bergman reaction has recently been imaged by the IBM group in Zurich using such a qPlus sensor with integrated STM electrode.
During the ramp the amplitude or frequency shift (depending on the mode of operation) is recorded to show the strength of the interaction at different distances.
This fitting, however, is very sensitive to where the cut-off between long and short range forces is chosen, causing results of questionable accuracy.
Usually the most appropriate method is to perform two spectroscopy measurements, one over any molecule under study, and a second above a lower section of the clean surface, then to directly subtract the second from the first.
This method is not applicable to features under study on a flat surface as no lower section may exist.
These experiments can take a considerable time, often over 24 hours, thus the microscope is usually cooled with liquid helium or an atom tracking method is employed to correct for drift.
[27] It is possible to perform lateral force measurements using a nc-AFM probe oscillating normal to the surface under study.
[19] A direct measurement of lateral forces is possible by using a torsional mode with a silicon cantilever [29] or by orienting the sensor to oscillate parallel to the surface.
[30] Using the latter technique, Weymouth et al. measured the tiny interaction of two CO molecules as well as the lateral stiffness of a CO terminated tip.
The frequency shift is then independent of the amplitude and is most sensitive to short-range forces,[32] possibly yielding atomic scale contrast within a short tip-sample distance.
CO molecule has shown to be a prominent option for the tip functionalization,[34] but also other possibilities have been studied, such as Xe atoms.
Reactive atoms and molecules, such as halogens Br and Cl or metals have been shown not to perform as well for imaging purposes.
[34][36][37] Van der Waals interaction, on the other hand, merely adds a diffuse background to the total force.
During the pick-up, the CO molecule orients itself such that the carbon atom attaches to the metal probe tip.
[38][39] The CO molecule, due to its linear structure, can bend while experiencing varying forces during the scanning, as shown in the figure.
[40] Additionally, the bending of the CO molecule adds its contribution to the images, which may cause bond-like features in locations where no bonds exist.
[36][41] Thus, one should be careful while interpreting the physical meaning of the image obtained with a bending tip molecule such as CO. nc-AFM was the first form of AFM to achieve true atomic resolution images, rather than averaging over multiple contacts, both on non-reactive and reactive surfaces.
[32] nc-AFM was the first form of microscopy to achieve subatomic resolution images, initially on tip atoms [42] and later on single iron adatoms on copper.
