ATOMIC FORCE MICROSCOPE
The atomic force microscope (AFM), is a type of scanning probe microscope which has a resolution in the nanometer range. It is a widely used technique for nano structure surface studies. The AFM has a much wider range of applicability than the other scanning probe microscopes. The atomic force microscope monitors the force exerted between the sample surface and the probe tip. Carbon nanotubes are used as tip material. AFM can be used to observe and manipulate nanometer sized objects of conductive and insulating nature. It can work in vacuum, air, gaseous and liquid environments.
Principle
A sketch of an AFM experiment is shown in Fig. It consists of a nanometer sized tip mounted on a sharp silicon or SiO 2 or silicon nitride elastic cantilever. Cantilevers are fabricated by photolithographic methods. They are typically 100 um long and 1 um tick.
A piezoelectric xyz scan driver is used to scan the tip across the sample. This tip is brought close to the surface of the sample under study so that atomic forces occur between tip and the sample. The distance between the tip and the sample can be anywhere between 0 and 100 nm. These atomic forces can be either repulsive or attractive.
The force acting on the tip is function of the topography and contents of the surface of the sample and changes during the scan which causes in a position dependent deflection of the elastic cantilever. T
he deflection produced in the cantilever is monitored by reflecting a laser beam on the cantilever and recording the movement of the reflected beam with a position sensitive photodetector. The atomic force microscope is sensitive to the vertical component of the surface forces. The signal from the photodetector can be used in afeedback loop with the xyz scan driver and the motion of the tip can be controlled.
The spring constant (usually between 0.1 and 1.0 N/m) of the cantilever shouldbe small enough so that minute atomic forces can be detected. The tip should be assharp as possible to provide atomic resolution.
To minimize sensitivity of AFM toexternal mechanical vibrations, the resonance frequency of the cantilevershould be as high as possible. The tip should be as narrow as possible so that it can penetrate into deep troughs on the surface of the sample. Small cantileversfulfill the requirements of small spring constant and high resonance frequency.Silicon is used for fabricating such cantilevers.
Modes of Operation of AFM
1. Contact Mode
In the repulsive or contact mode of operation, the tip position over the surface of the sample is kept constant through a feedback mechanism (9.13b). In this mode the AFM tip is forced down into the surface until repulsive contact with the core electrons of the surface atoms is obtained. During a scan the AFM tip ollows the topographic (surface) features of the sample with atomic resolution. The Lennard-Jones potential
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2. Non-Contact Mode
In the non contact or attractive mode, attractive forces bend the tip bringing it closer to the sample but the AFM-tip does not touch the surface. It isinfluenced by the long range van der Waals forces. The advantage of the non-contact mode is that it does not destroy even the softest samples. So, soft specimens are analysed by AFM by operating it in the non-contact mode.
3. Tapping Mode
This mode combines the good things of the contact and the non-contact mode ofAFM operation. The AFM tip is oscillated with large peak to peak amplitude resulting in brief forrays into the repulsive regime. The tip gently touches the sample once during each cycle (9.13c). The tapping mode has nearly the same resolution as the contact mode. Being in gentle-touch of the sample, can be used for soft samples. It is a simple and robust mode of operating the AFM. Tappingmode provides most sensitive measurements.
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