Atomic Force Microscopy

AFM is an analytical technique commonly used to probe surface properties. A Bruker Multimode VIII AFM was used in this thesis to obtain high resolution topographical images and perform quantitative nanomechanical mapping on a variety of samples.

4.4.1.1 Apparatus

An AFM is comprised of a laser, probe, split photodiode detector, piezoelectric scanner and feedback loop as shown in Figure 4.8.

Figure 4.8: Schematic illustration and photo of an AFM. Inset photograph shows a MultiMode 8 AFM, which was used for the majority of AFM work in this thesis.

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The probe used in AFM imaging comprises a sharp tip positioned at the end of a cantilever. The cantilever is usually either rectangular or V-shaped and has a length of around 100 µm, while the tip is usually less than 10 nm in diameter at the apex. The tip and cantilever are manufactured from Si or Si3N4 using conventional semiconductor fabrication methods.

Analysing a sample using AFM involves positioning the cantilever tip over the sample surface, and reflecting the laser beam off the back of the cantilever into the split photodiode detector. As the tip approaches the surface, tip-sample interactions occur, causing the cantilever to deflect. As the cantilever bends, the position of the reflected beam changes on the photodiode detector. The photodiode is split into four quadrants, which are used to determine the position of the reflected laser beam in the lateral and normal directions relative to a reference set-point established at zero deflection. As the path length between the cantilever and photodiode detector is significantly greater than that of the length of the cantilever, deflection of the cantilever results in an amplification of the change in laser position. Consequently, the system is able to accurately detect small changes on the surface in the z-direction, down to the molecular level.

The position of the sample surface relative to the cantilever is controlled by the mechanical movement of the piezoelectric scanner. When a voltage is applied across the piezoelectric stage, the material undergoes displacement proportional to the applied voltage. This enables movement of the sample in the x-y plane and allows the tip to raster across sections of the sample surface during imaging. The piezoelectric scanner also allows the distance between the surface and the tip to be controlled precisely in the z-direction using feedback from the photodiode detector. The photodiode converts a change in the laser spot position to a voltage, which is then passed through the feedback

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loop to the piezoelectric sensor in order to adjust the position of the sample along the z- axis in real time. In this manner, the AFM is able to measure a quantity of interest, such as topographical height at each point on the surface.

4.4.1.2 Basic Operational Modes

AFM are typically capable of operating in a variety of modes which differ according to the way in which the tip contacts the sample. Contact mode and tapping mode are two of the most basic AFM modes upon which other advanced, proprietary scanning modes are based. In contact mode, the tip is in continuous contact with the sample surface at a constant cantilever deflection. To achieve constant deflection of the cantilever, the piezoelectric scanner adjusts the position of the sample along the z-axis in real time, in response to changes in the deflection of the cantilever. As the tip is in constant contact with the sample surface, however, this mode is not well suited to fragile surfaces. In these cases, tapping mode provides an alternative, less destructive mode of operation, avoiding drag and lateral forces along the sample surface. In tapping mode, the cantilever is oscillated near its resonance frequency using a piezoelectric cantilever holder. When the tip makes contact with the sample surface, the amplitude of oscillation decreases. During tapping mode measurements, constant reduced amplitude is maintained throughout the scan using the feedback loop. In this thesis, measurements were performed using two proprietary AFM modes based on tapping mode: ScanAsystTM and PeakForce QNM®.

4.4.1.3 ScanAsystImagingMode

ScanAsyst is an AFM imaging mode that was used in this thesis to obtain topographical images of LbL self-assembled thin film surfaces. In ScanAsyst mode, a system of tip-

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sample interaction feedback and software-based algorithms are used to automatically and continually optimise imaging parameters throughout the scan, providing enhanced force control. As such, ScanAsyst typically enables the tip to selectively apply smaller forces to the sample surface than those typically applied with tapping mode when required, and is therefore ideal for imaging heterogeneous surfaces.

4.4.1.4 PeakForce QNM (Quantitative Nanomechanical Mapping) Mode

PeakForce QNM was used in this thesis to probe the mechanical properties across hydrogen-bonded LbL thin film surfaces. PeakForce QNM is able to perform topographical imaging while simultaneously yielding information on a number of different mechanical surface properties. NanoScope® Analysis software is used to quantify these properties through a variety of parameters including the modulus, adhesion, deformation and dissipation. In order to accurately determine these material properties, calibration of the deflection sensitivity, spring constant and tip radius of the cantilever are required prior to measuring the sample surface. To measure mechanical quantities using PeakForce QNM, a force curve is recorded at each point as the tip is rastered over the sample surface. Sample properties are then extracted in-real time based on specific regions of the AFM force curve (Figure 4.9). For example, in NanoScope Analysis, the pull-off force experienced by the tip during retraction of the tip from the sample surface is quantified by the adhesion parameter. The dissipation parameter corresponds to the energy of adhesion, whilst the deformation parameter is associated with the depth of indentation.

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Figure 4.9: Regions of an AFM force curve with corresponding QNM parameters. The compliance (linear) region of the retract curve fitted to the DMT model provides the Young’s modulus, the area between the approach and retract curves is associated with dissipation, the pull-off force is associated with adhesion while deformation corresponds with depth of indentation.

NanoScope Analysis also provides a number of different theoretical models to calculate the Young’s modulus, each of which is suited to a specific application. The Derjaguin- Muller-Toporov (DMT) approach was used, as it is involves modelling the tip as a sphere and is suitable for generic samples. The DMT modulus parameter is equivalent to the Young’s modulus, and is calculated by first fitting the retract portion of the compliance (linear) region to the DMT model (Equation 4.2) 159:

𝐸𝑟 = 3(𝐹_𝑡𝑖𝑝− 𝐹_𝑎𝑑ℎ) 4√𝑅𝑑3 4.2 Where: 𝐸𝑟 = Reduced modulus (J)

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𝐹𝑎𝑑ℎ = Adhesive force between AFM tip and sample (N)

𝑑 = Deformation depth (m) 𝑅 = Tip radius (m)

The Young’s modulus of a material is then related to the reduced Young’s modulus by Equation 4.3 : 1 𝐸𝑟 = (1 − ν𝑠2) 𝐸𝑠 + (1 − ν_𝐼2₎ 𝐸𝐼 4.3 Where:

𝜈_𝑟 = Poisson’s ratio of sample

𝐸_𝑠 = Young’s modulus of sample (N/m2) 𝜈_𝐼 = Poisson’s ratio of tip (J)

𝐸𝐼 = Young’s modulus of tip (N/m2)