**4. Advantages and limitations**

Atomic force microscopy works by running a sharp tip attached to a cantilever and sensor over the sample surface and measures the surface forces between the probe and the sample. As the cantilever runs laterally the sample surface, it moves up and down due to the surface features and the cantilever deflects accordingly. This deflection is computed using an optical sensor, with the laser beam being reflected on the back of the cantilever onto the light detector. AFM provides various advantages over other

techniques. AFM can operate in ambient air or under liquid, it does not need to be operated in a vacuum hence it is increasingly being used to image biological samples as well as nanoparticles. AFM has resolution in the order of fractions of a nanometer and provide a 3D imaging technique. The AFM allows the topographic characterization of surfaces at resolutions not attainable by optical microscopy. The lateral resolution of the AFM is limited by the tip size and shape and is typically on the order of a few nanometers. The height (*z*) resolution in AFM is nearly 1 Å, limited only by electronic and thermal noise in the system. FM can only scan a single nanosized image at a time of about 150 × 150 nm and possess chances of damaging the tip and the sample during detection. Further it has a limited magnification and vertical range. Furthermore the speed of scanning of AFM is very slow compared to SEM and sample analysis cannot be done for areas greater than 100 μm. The images also gets affected by non-linearity, hysteresis, and creep of the piezoelectric material. Another drawback is that the images are generated because of the interaction of probe with sample which might not be the true topography of the sample. Also the probe tip can results in shape changes in samples like fine powder. The surface roughness over which a forensic traces are deposited may obstruct a proper examination of an image height. This problem could partly be overcome by use of phase imaging. Nevertheless, the addition of optical microscopy and Raman spectroscopy or surface enhanced Raman spectroscopy along with the AF can prove to be useful for forensic examinations.
