Atomic force microscopy (AFM) 47

With the invention of the scanning tunnelling microscope (STM) by Binnig and Rohrer in 1981,[167] the first surface imaging technique of atomic resolution set the basis for a whole scanning probe microscope (SPM) family and nanotechnology as a new hot spot in science.[168] The imaging capability of STM relies on an electrically biased, conductive scanning tip, which detects very low tunnelling currents from conducting or semiconducting samples with atomic spatial resolution.

In 1986, Binnig, Quate and Gerber extended the idea with the development of the AFM as an alternative to STM.[169] AFM is also capable of scanning non-conducting surfaces with high spatial resolution on the nanometre scale to image surface structures and morphology. In contrast to STM, AFM is based on probing tip-sample surface interatomic force interactions when brought very close together.

When in close proximity to the surface the probing tip can experience attractive and repulsive forces depending on its distance to the surface.[170, 171] Attractive forces are of longer range and include van der Waals (vdW) interactions, capillary forces, chemical forces and electrostatic attraction. Repulsive forces, such as hard sphere repulsion and electron-electron Coulomb repulsion are of much shorter range due to a high exponential decay law with increasing distance. The probing tip is attached to a cantilever (see Figure 2.7c), which acts as a spring with a well defined spring constant. Attractive forces bend the cantilever and the attached tip towards the surface and repulsive forces push it away. This attraction-repulsion force potential in conjunction with the classical Hooke’s law describes the interaction behaviour and lateral movement of the tip, which leads to force and surface profile height measurements.

Chapter 2: Experimental

Figure 2.7 a) Schematic and b) photograph of an AFM. Photograph of c) an AFM tip. (The photographs in

b) and c) were adapted from the MFP-3D Manual.)[172]

The complete setup of an AFM by Asylum Research is shown schematically and as a photograph in Figure 2.7a and 2.7b respectively.[172] _{Depending on the scanning} mode the tip follows the surface contours either in contact with a constant force applied or in intermittent contact in tapping mode. A light beam is targeted onto the reflective backside of the tip and the resulting reflected beam is focused and positioned by a mirror for detection by the position sensitive photo-detector. The detector monitors the deflection of the tip, which is caused by surface–tip interactions. During a scan a feedback loop adjusts the tip height to maintain a constant amplitude or deflection, and therefore a measure of surface height is obtained. By scanning a local area a height profile can be determined by the computer software. The z-position of the tip is controlled by a piezo moving only along the z-axis with sub-nanometre precision. The tip holder, the deflection mirror and the photo-detector monitoring the tip position are all situated in the scanner head. The sample is mounted on a table with piezos moving it with nanometre precision along the x- and y-axis, placed in the base (see Figure 2.7b).

Scanner head Base b) c) Tip Cantilever Mirror Cantilever Detector x z y Z-value a) Tip Sample Tip holder Z-piezo Y-piezo Scanner head Cantilever motion Set point Controller Computer Feedback loop X-piezo

The two most common AFM scanning modes are contact mode and alternating contact (AC) or tapping mode. Contact mode is used for hard surfaces and conductive AFM. The tip is always kept in the repulsive force regime. It is referred to as being in contact to the surface with a constant force. Repulsive and attractive surface forces cause a bending of the cantilever with the feedback loop maintaining a constant cantilever deflection. This mode can damage soft surface structures such as thin film organic material layers. However, in tapping mode the tip is not constantly in contact to the surface. The cantilever is held very close but with a constant distance to the surface. It oscillates around its own resonance frequency and taps the surface gently during the scanning process. Interactions between the tip and the surface induce a slight change of the oscillation amplitude, which is restored by the feedback loop, and hence a height profile can be determined. Changes in phase and amplitude can give valuable information about the tip-surface interaction and add details to the pure topography based height image.

The surface topography of the samples was studied using an Asylum Research MFP-3D in tapping mode. Standard tapping mode tips (AC240TS) equipped with a silicon probe had a resonance frequency of 70 kHz and a tip radius of 9 nm. MFP-3D software based on Igor Pro was used for image reconstruction and analysis. The presented images have been line and plane filtered. By using the software, the surface roughness parameter, Rq, was determined. It is defined as the root mean square (RMS) of the sum over all N points for the surface height difference of each point Zi and the average height Zavg of the surface, as shown in Equation 2.4.

∑ (Equ. 2.4)