Student authors: Trexler Hirn 2018 & Boxi Li 2019
How AFM works
Atomic force microscopy works by the sample being moved in an X,Y grid beneath a sharp tip attached to the free end of a cantilever being passed along the surface of the material in question. This process is demonstrated in figure 1 below. The cantilever reflects a laser to a photodiode. The contact with the tip to the material can be an atomic scale phenomenon signal from, but the motion of the cantilever is a macro scale movement. This movement is measured by the reflected movement of a laser on the photodiode which is converted to an electrical signal giving information on the measurements. (sometimes there are different ways of measuring the cantilever movement including a direct deflection on an oscillator that converts a change from equilibrium into an electrical signal.)
Figure 1. Scheme of AFD mechanism. 2
For imaging purposes AFM can be used in contact mode (where the tip drags along the surface of the material) tapping mode (where the cantilever oscillates at a specific frequency and the surface disrupts the regularity of the oscillation) and non-contact mode (cantilever oscillates just above the surface of the material and van der waals forces disrupt the oscillation
AFM can also be used for force spectroscopy when the cantilever is extended towards the material and the disruption to the cantilever is disrupted giving information on nanoscale forces such as atomic bonding and van der waals forces.
How to interpret the data
For imaging purposes, the data is usually presented as a topographical “heat map” with a colored scale to show height/depth and a scale for the X,Y plane to show measurements between peaks/valleys etc. Interpreting the data specifically depends on the type of imaging and the material being imaged, but the general output shows the topography with information on smoothness as well as “hardness” due to differing forces experienced by the tip during the scan.
For force microscopy, force curve readouts are available to correspond with loci on the sample giving the ability to correlate the forces with topography.
ERROR: A common source of error to consider is the shape and sharpness of the tip with regard to the surface morphology (as shown in Figure 2 below). If the radius of the tip is too large the refinement of the measurement may be smoothed. Similarly, due to the nature of the tip, AFM cannot measure steep walls/overhangs.
Figure 2. Showing an AFM artefact arising from a tip with a high radius of curvature with respect to the feature which is to be visualized 3
Also the slow scanning time can lead to thermal drift making measurements between topographical features less accurate.
Good literature examples
Figure 3 is from a paper1 that was investigating an organic-inorganic composite of phosphomolybdic acid (PMA) and PEDOT:PSS electron transport layer that is printed on top of the photoactive layer of a semi transparent organic solar cell.
Their use of AFM was to compare the previous technique (just PMA) to their hybrid of PMA:PEDOT:PSS. Figure 2a shows the surface morphology of the photoactive layer which they report is very smooth with a root mean squared roughness of 1.3 nm. Figure 2b shows AFM of just PMA deposited on the photoactive layer and indicates a much rougher surface with a RMS roughness of 4.2 nm, in addition to large isolated islands and non-uniform distribution of PMA. Figure 2c shows AFM of PMA:PEDOT:PSS deposited on the photoactive layer and reveals a fairly smooth and uniform surface. The RMS roughness is slightly greater than the photoactive layer at 1.9 nm, but there are no aggregations and rough islands like just the PMA. This AFM supports the notion that the smoother surface achieved with PMA:PEDOT:PSS will have a good interface contact between the organic photoactive layer and anode.
- Ji, G.; Wang, Y.; Luo, Q.; Han, K.; Xie, M.; Zhang, L.; Wu, N.; Lin, J.; Xiao, S.; Li, Y.-Q.; et al. Fully Coated Semitransparent Organic Solar Cells with a Doctor-Blade-Coated Composite Anode Buffer Layer of Phosphomolybdic Acid and PEDOT:PSS and a Spray-Coated Silver Nanowire Top Electrode. ACS Appl. Mater. Interfaces 2018, 10, 943–954.
- File:Atomic force microscope block diagram.svg - Wikipedia https://en.wikipedia.org/wiki/File:Atomic_force_microscope_block_diagram.svg (accessed May 3, 2019).
- File:Afm artifact2.png - Wikipedia https://en.wikipedia.org/wiki/File:Afm_artifact2.png (accessed May 3, 2019).