LITERAL b)

Atomic force microscopy is not only limited to image morphology of a surface but widely used for determining the magnetic structures, friction measurement and surface charge distributions [269]–[271]. Artefacts are the features appear in the image that are not real [272], [273]. The artefacts introduced in the AFM images may perturb the measurements. It is important to recognise and minimise the source of artefacts [274]–[281]. The artefacts which appear in AFM imaging may originate from the probing tip, scanning system, external vibrations, thermal gradients, feedback circuitry and image processing software [282], [283].

3.20.1 Probing Tip Artefacts

The geometry and dimension of the probe used for measuring the features may introduce artefacts into an AFM image. The accurate imaging of lateral dimension depends on the sharpness and inclination angle of the tip used. If the radius of curvature of the tip is greater than the features of the image (e.g. nanotube and nanosphere), the scanning probe overestimates the lateral dimensions (see Figure 3.30) [282], [284]. Measurements taken with conical tips produce images with fewer artefacts than pyramidal tips [283]. If the tip cannot reach the sidewalls of valley, the AFM tip then underestimates the details of the sample such as size and depth (see Figure 3.30) [283]. However, the height of the object to be measured is independent of the geometry of the tip [283]. The probe artefacts can be minimised by using a sharp tip with a radius of curvature much less than the details of the image being measured. The effect has been illustrated schematically in Figure 3.31. A damaged or contaminated AFM tip may produce blurred images [283]. Shadows or double image can also appear in images and described as double tip effects [5], [283]. While

imaging flat samples featuring fine details with large tips, the image can contains artefacts known as tip self-imaging due to the sidewall angle of the tip as illustrated in Figure 3.31.

Figure 3.30: Schematic of how an artefact can be introduced by a large

diameter tip: (a) overestimating and (b) underestimating the lateral dimensions. Arrow shows the scanning direction and trace. Redrawn from [8].

Figure 3.31: Schematic showing the traces of two tips with different sidewall

angles. The side walls cause broadening of the lateral dimensions in the image. Redrawn from [283].

3.20.2 Scanner Artefacts

Atomic force microscopy uses a piezoelectric scanner that moves the probe in three dimensions (x, y, and z). The geometry of the tip and the scanner may introduce artefacts into AFM images. The most frequent artefacts are edge overshoot, edge elevation, thermal drift and creep as discussed below.

Piezoelectric ceramics are capable of moving the tip by very small distances by applying a linear voltage [284]. Edge overshoot artefact arises in AFM imaging due to hysteresis of the piezoelectric scanner that moves the cantilever in the z-direction (see Figure 3.32). The artefact can be seen in the narrow hills and valleys at the edges of the image and can be minimised by slowing down the scanning speed [283].

Edge elevation is another artefact in AFM imaging and can be caused by the tip- sample attractive forces [282]. The probe in an AFM moves in a curved path over the surface and cause bows in the imaging. If the tip-sample angle is not perpendicular, a tilt is introduced. If the bows and tilts are larger than the features of interest, then the artefacts can be rectified in post processing of the images by subtracting the background bows and tilt and this is called levelling or flattening [284].

Figure 3.32: Schematic of edge overshoot caused by a fast scan speed. Arrow

shows the scanning direction and trace. Redrawn from [282].

Thermal drift and creep of the AFM cantilever caused by the laser or by contact with the sample may introduce artefacts [282]. These artefacts distort the straight lines into curved lines in the AFM imaging as shown in Figure 3.33 and 3.34. This artefact and can be minimised by using a fast scan speed, sample position sensors and improved feedback control loops [282].

Figure 3.33: A topographic image of LiF crystal surface showing distortions

due to thermal drift (left) with bowing apparent in the bottom of the image. The image without thermal drift (right). Reproduced from [282].

Figure 3.34: Distortion of features in an AFM image caused by creep.

Reproduced from [283].

3.20.3 Features along the Scan Direction

Sometimes, triangular and oval features appear along the scan direction which can be due to the high set point, low amplification and high scanning speed [279]. Furthermore, irregular edges and dips can appear along scan direction and are due to a large amplitude of the cantilever and small set point, respectively [279]. Also, plastic deformation of the sample can cause vertical ridges to appear in the scan direction when imaging in contact mode [285].

3.20.4 External and Internal Noise

Mechanical noise arises from factors such as acoustic vibrations, floor vibrations, electromagnetic interference and temperature fluctuations which can introduce artefacts into the image [14]. Furthermore, electronic noise caused by the piezoelectric scanner, amplifiers and optical systems may lead the probe to vibrate and cause artefacts. External noises may influence the texture, sensitivity and resolution of the AFM [282], [286]. Such noise can be removed from AFM images with image enhancement techniques by using proper filtering – for example, 2D Fourier filtering [287].