Atomic force microscopy

Atomic force microscopy (AFM) is a versatile technique that can image, characterise and manipulate matter with high spatial resolution. It was invented by Binning et al. in 1986 and was first commercially available in 1989.207 The main benefit of AFM is that it can probe the features of both conductive and non-conductive samples,

61 monitoring the close range tip-sample interatomic interactions. This generates a topographical map of surface features. The technique provides vital information in the field of OPVs as the surface properties of a layer can have a significant effect on cell performance.

An AFM set up is shown schematically in Figure 2.8. Located at the end of cantilever is the tip which is typically ~ 3-6 microns long. The amount of force between the tip and sample at any given time follows Hooke’s law and is dependent on the spring constant of the cantilever and distance between the tip and sample. The forces measured depend on the influence of attractive and repulsive forces as the tip approach the surface, shown in Figure 2.8 [a]. In the non-contact region (typically hundreds of angstroms from surface) the interactions between the cantilever and the surface are attractive, including electrostatic and van der Waals interactions. As the tip is brought closer (less than a few angstroms from the surface) the interatomic interactions between the cantilever and the surface are dominated by repulsive forces including hard sphere repulsion, repulsive van der Waals and electron-electron coulombic interactions. These repulsive forces cause the tip to bend up.

When imaging the motion across the surface is controlled using a feedback loop and piezoelectronic scanners. The output from the laser is positioned on the parabolic end of the cantilever which is reflected into a photodetector with four quadrants, which generate a voltage proportional to the light hitting each quadrant. Any differences measured between the four segments indicate the position of the laser spot, this is relayed to the controller, ensuring the set point for deflection or amplitude is kept constant. A topographic image is obtained by the distance the scanner moves in the z direction being stored in the computer relative to the spatial variation in the x-y direction.208 Different scanning modes operate in different

62 regions of the curve, contact mode operates in the repulsive region, non-contact mode (where the tip does not contact the surface but oscillates above the absorbed fluid layer) uses the attractive region whereas tapping mode (also known as alternating contact mode) operates between the two. In contact mode, a tip is brought to the surface, ensuring that a tip-sample distance is maintained depending on the set point used. When the spring constant of the cantilever is less than the forces between the probe and sample, the tip starts to bend towards the surface. The sample is moved laterally and a feedback loop is used to keep the cantilever deflection constant by changing the probe height z while scanning in x and y and therefore a nearly constant force is maintained between tip and surface during imaging. The advantages of contact mode are the high lateral resolution and quick imaging of hard samples. Therefore, contact mode was utilised for imaging of some metal oxide thin films and when using conductive AFM. The drawbacks however is that contact mode can lead to large shear forces on the surface, damaging softer samples.

All images of organic materials that would be susceptible to such damage were taken using tapping mode AFM. In tapping mode, the cantilever is oscillated close to its resonant frequency using a piezoelectric element. When the oscillating tip interacts with the surface the amplitude of the oscillations is dampened. An oscillation amplitude set point is chosen by the user, which ensures a constant tip-sample interaction is maintained by monitoring the oscillation amplitude using the four- quadrant photodiode and controlled using the z-piezo and feedback loop. This allows an image of the surface to be obtained.

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Figure 2.8 A schematic showing [a] the force curve obtained with increasing tip-sample distance (z) and [b] the AFM set-up.

All images were taken using an Asylum Research MFP-3D using Olympus AC240- TS Si cantilevers with a resonant frequency of 70 kHz and a tip radius of 9 nm.

2.2.5.1 Conductive AFM

Conductive atomic force microscopy (CAFM) can be used to characterise electrical properties of a sample at high resolution. A voltage is applied between the sample and the conductive AFM tip and the current flow is measured as a function of tip location whilst scanning the sample surface in contact mode. This results in simultaneously obtained topography images and current distribution maps. This is a useful property as it allows any changes in conductivity to be compared to any differences in features on the samples surface.

Photodiode Laser beam Tip Cantilever Surface Tip atoms Surface atoms Force Tip-sample distance ( z ) For ce ( F ) Repulsive regime Attractive regime 0 [a] [b]

64 The MFP-3D was fitted with an ORCA integrated tip-holder and current preamplifier (20 nA) for CAFM measurements. Both the topography and current distribution images were obtained simultaneously using Au/Cr coated cantilevers which act as a nanoelectrode and form the top contact (Olympus TR400PB, tip radius < 40 nm, spring constant 0.06 N m-1). A contact of silver paint was added to the film of interest and a sample bias was applied via a wire from the OCRA tip holder. A 500 MΩ resistor was used to limit the current.