Atomic force microscopy (AFM) produces images of nanoscale and atomic surfaces through the use of a tip at the end of a cantilever raster scanned over the surface.
In its most basic form, forces between the tip and the surface result in the deflection of the cantilever. A laser beam is focused onto the backside of the cantilever (the side which faces away from the surface), and reflected onto a photodetector. This can be seen in the basic diagram of an AFM setup in Fig. 1.
Figure 1. Simplified AFM setup showing position shift of laser incident on the photodetector as the tip moves across the sample surface.
Deflection of Cantilever in Atomic Force Microscopy
The deflection of the cantilever differs in relation to the height of the surface features. As a result of this variation in deflection, the position of the reflected laser beam incident on the photodetector shifts, as seen in Fig. 1. The position shift of the beam as the tip is scanned over the surface creates an image of the surface.
The backside of the cantilever is usually covered with a reflective metal coating with a thickness of a few tens of nanometers, shown in Fig. 1, intended to amplify the signal of the laser beam reflecting off the cantilever. This enhances the signal-to-noise ratio of the photodetector and as a result, its sensitivity.
Where silicon is used to produce the cantilever, a portion of the laser light will be reflected from the backside, as well as from the tip-side as a result of the semi-transparency of silicon to the utilized light wavelength.
Advantages of Reflective Coating
Additionally, if the sample is strongly reflective, there could be a level of reflection of the laser light from the sample surface. The reflective coating is designed to avert interference between the reflected beams from these surfaces. Figures 2 (a) and (b) demonstrate AFM images of a platinum thin film captured using an AFM probe with a coated and uncoated cantilever, respecticely.
Figure 2. AFM image of a platinum thin film taken using a probe with an (a) coated and (b) uncoated cantilever.
The intermittent pattern of lines in Fig. 2 (b) is typical of laser beam interference. Such interference cannot be seen in Fig. 2 (a), where a coated cantilever is utilized for imaging. NuNano offer a reflective backside coating with their Scout 350 and Scout 70 probes, which mitigates this interference.
It is, however, important to note that using a reflective coating does result in a number of disadvantages. It can result in strain on the cantilever, causing it to curve. When the laser beam strikes the metal coating, it increases in heat, which results in temperature gradients through the cantilever.
This in turn leads to thermal drift whereby the laser beam might drift in placement on the photodetector, and thermal stress, which results in an increase in cantilever noise.
Additionally, a reflective coating lessens the Q-factor (quality factor), as there is an upsurge in damping of the cantilever. In addition to the power of the laser signal, possible deterioration of the reflective coating as a result of other experimental conditions should be considered when opting to use either a coated or uncoated cantilever. Moreover, it is crucial to calibrate the properties of a coated cantilever before carrying out imaging.
Using Aluminum and Gold Films as Reflective Coatings
Aluminum and gold films are often utilized as reflective coatings for silicon AFM probes. Aluminum is lower in cost and has higher reflectivity, but is unsuitable for use in some biological buffers and solvents as a result of its lack of stability and its dissolution into the liquid environment.
Gold, however, is both chemically and biologically inert, and thus, a gold-coated cantilever is most frequently used for the study of biological samples.
NuNano offer AFM probes with or without an aluminium or gold reflective backside coating, increasing the laser signal incident on the photodetector by about 2.5 times.
This information has been sourced, reviewed and adapted from materials provided by Nu Nano Ltd.
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