Atomic force microscopy
Atomic force microscopy (AFM) is a technique that allows the imaging of topography and materials’ properties at the micrometer and nanometer scales. Its operational principle–a sharp tip on a flexible beam that is rastered over a sample surface.
|Atomic force microscopy|
|Other Names||AFM, SPM|
|Equipment||Bruker NanoMan AFM|
|Materials||AFM can be done in air or in liquid, samples must be flat and clean.|
|Warning:||This page has not been released yet.|
Method of operation
An AFM uses a cantilever with a very sharp tip to scan over a sample surface. As the tip approaches the surface, the close-range, attractive forces between the surface and the tip causes the cantilever to deflect towards the surface. However, as the cantilever is brought even closer to the surface, until the tip makes contact with it, increasingly repulsive forces takes over and causes the cantilever to deflect away from the surface. A laser beam is used to detect cantilever deflections towards or away from the surface. By reflecting an incident beam off the flat top of the cantilever, any cantilever deflection will cause slight changes in the direction of the reflected beam. A position-sensitive photo diode (PSPD) can be used to track these changes. An AFM images the topography of a sample surface by scanning the cantilever over a region of interest. The raised and lowered features on the sample surface influence the deflection of the cantilever, which is monitored by the PSPD. By using a feedback loop to control the height of the tip above the surface—thus maintaining constant laser position—the AFM can generate an accurate topographic map of the surface features. Essentially the AFM is probing the surface potential on the sample, if a conductive tip is used, the current flow between the surface and the tip can be measured, thus several parameters such as conductivity and capacitance of the surface can be mapped. When the tip is coated with a well known metal, its vibration close to the surface will act as a Kelvin Probe.
The (AFM) is not only a tool to image the topography of solid surfaces at high resolution. It can also be used, for examnple, to measure force-versus-distance curves . Such curves, briefly called force curves, provide valuable information on local material properties such as elasticity, hardness, Hamaker constant, adhesion and surface charge densities. For this reason the measurement of force curves has become essential in different fields of research such as surface science, materials engineering, and biology.
In all modes of operation, the image obtained is a convolution of the sample signal with the tip signal. Therefore software and especial samples have been developed to deconvolute the signal obtained.
The best way to understand the usefulness of AFM is in the diagram below:
AFM is used to image surfaces when the features present on the sample are in the order of 50 μm in x-y and 7 -9 μm in height. These limitations are set by the x,y,z piezo components of the scanner.
- imaging surfaces
- x-y-z measurements
- film conductivity mapping
- magnetic domains
- piezo electric domains, d33
- surface potential or relative work function
- defect mapping
- i-V curves, electrical testing
- nano manipulation
For each mode (described below) there is a tip. The choice of the tip is crucial for the measurement that needs to be done. As it will be explained below, this techniques requires direct intervention of the operator to optimize the scans. The parameters that matter are listed below.
- laser beam alignment on the cantilever
- resonance frequency of the cantilever
- in tapping mode, the PID control system parameters has to be adjusted manually
- In Force AFM the k of the cantilever must be chosen correctly
- In electrical modes the lock-in adjustment are critical
This is the most common mode for acquiring surface topography. Lateral interaction between the cantilever and the surface can produce problems and reduces resolution of the technique, this is avoided by letting the tip touch the surface for a very short time. This mode, referred to as TappingMode AFM, or AC mode, the cantilever is oscillated at its resonance frequency while is dragged across the surface. Typical TappingMode operation is carried out using amplitude modulation detection with a lock-in amplifier. A typical response curve of a cantilever is shown in the figure.
|Caption = Fig. 1 Resonance curve of a TappingMode cantilever above and close to the surface. Note that the resonance shifts to lower frequencies and exhibits a drop in amplitude.
|Caption = Fig. 2 Force curve highlighting the motion of an oscillating cantilever in TappingMode.
The curve shown in figure 2 (red) is constructed by adding the short-range repulsive and long-range attractive forces. The force curve or direct forces between the tip and the sample is not actually measured by the TappingMode AFM while experiencing the interactions. The TappingMode AFM oscillates back-and-forth on this curve, interacting without being in direct control of the force and only an average response of many interactions though the lock-in amplifier is reported.
The reduction of cantilever amplitude can be measured when the tip and sample approach each other. Though this is not detrimental, it restricts the information beyond sample topography that can be gained and unambiguously assigned to a certain sample property.
The inherently unstable feedback situation in TappingMode operation makes it difficult to automate some of the scan adjustments, interaction of the operator is necessary to optimize the conditions. Forces can vary when going away from a steady-state situation. The higher the tip amplitude, the higher the energy stored in the lever and in the imaging forces. Drift due to temperature changes and/or fluid levels change affects the operation in fluids.
It is essential to adjust the feedback system to achieve reliable information from the AFM. A contact mode (see below) scan can be more easily controlled than a TappingMode scan as TappingMode has complex oscillating system.
TappingMode does, however, offer the undeniable benefit of lateral force free imaging, which has made it the dominant imaging mode in AFM to date.
Contact mode AFM is mainly used to image hard surfaces when the presence of lateral forces is not expected to modify the morphological features. The tip is in permanent contact with the surface, and the small (angular) movement of the lever is commonly measured by a laser beam that is reflected off the cantilever and directed onto a split photodetector, as shown above. The tip-sample forces and their control are the most important issues of AFM. Soft materials can be easy damaged because of the large normal force and/or shearing deformation. And when a sample is rigid, imaging in contact mode may destroy a sharp tip apex. Therefore the control parameters when of the tip is approaching the surface are crucial
Typically, the forces applied to samples in the contact mode are in the range from tens to hundreds of nanoNewtons. Additionally, most surfaces in air are covered by a layer of adsorbed water and other contaminants, whose surface tension pulls the tip and probe downwards. Electrostatic charges on the tip and sample can also give rise to additional long-range forces and complicate the imaging.
This technique is used when a modification to the surface is the goal, for example patterning of a surface layer. All electrical techniques require permanent contact with the surface.
Atomic force microscopy (AFM) force-distance curves have become a fundamental tool in several fields of research, such as surface science, materials engineering, biochemistry and biology. Furthermore, they have great importance for the study of surface interactions.
As the cantilever approaches the surface, when is far from it 1, the forces are too small to give a measurable deflection of the cantilever. At some point the attactive forces (Van der Waals, capillary) overcome the cantilever spring constant and the tip makes contact with the surface 2. Once the tip is in contact with the sample, it remains on the surface as the separation between the cantilever and the samples further decrease, causing a deflection of the cantilever and an increase of the repulsive forces. As the cantilever is retracted, the tip remains on the surface until the adhesion force that retains it is overcome. The Force can be calculated from:
F=kx k is the spring contact of the cantilever and x its displacement which is measured by the AFM
Peak Force Measurements
Kelvin Probe/Surface Potential
This techniques measures the surface potential in a nanometer scale. It is used to measure the relative work function difference of the surface components. The KPFM measures CPD between a conducting AFM tip and a sample. The CPD (VCPD) between the tip and sample is defined as: VCPD = φtip − φsample−e , where φsample and φtip are the work functions of the sample and tip, and e is the electronic charge. When an AFM tip is brought close to the sample surface, an electrical force is generated between the tip and sample surface, due to the differences in their Fermi energy levels. Equilibrium requires Fermi levels to line-up at steady state, if the tip and sample surface are close enough for electron tunneling. Upon electrical contact, the Fermi levels will align through electron current flow, and the system will reach an equilibrium state, this current flow can be nullified. If an external bias is applied. The voltage applied is equal to the work function difference between the tip and sample.
In PFM operation, a conductive AFM tip is brought into contact with the surface of the studied ferroelectric or piezoelectric materials, and a pre-set voltage is applied between the sample surface and the AFM tip, establishing an external electric field within the sample. Due to the electrostriction, or “inversed piezoelectric” effects of such ferroelectric or piezoelectric materials, the sample would locally expand or contract according to the electric field. For example, if the initial polarization of the electrical domain of the measured sample is perpendicular to the sample surface, and parallel to the applied electric field, the domains would experience a vertical expansion. Since the AFM tip is in contact with the sample surface, such domain expansion would bend the AFM cantilever upwards, and result in an increased deflection compared to the status before applying the electric field. Conversely, if the initial domain polarization is anti-parallel to the applied electric field, the domain would contract and in turn result in a decreased cantilever deflection (Figure 1). The amount of cantilever deflection change, in such situation, is directly related to the amount of expansion or contraction of the sample electric domains, and hence proportional to the applied electric field.
- Park Piezoelectric AFM document 
Scanning Tunneling Microscopy
STM (Scanning Tunneling Microscopy) works in a similar manner to AFM but uses a different sensing method. In STM there is a bias voltage set between the probe and the surface and when in atomic distance to the surface a tunneling current can be measured by the probe.
Because both of the techniques involve scanning very close to the surface it is possible to obtain images with atomic resolution.
The big advantage of SPM techniques compared to optical techniques is the ability to obtain height information and the unique capability of obtaining images at atomic resolution. SPM allows a lot of geometrical information to be extracted at a very detailed level.
To obtain geometrically correct images it is crucial that the movement of the probe relative to the surface can be controlled better than the desired resolution, which is a big challenge. It is almost impossible to create images where the pixels are acquired equidistantly and where there is no coupling between the axes. Even in the most perfect instrument problems with environmental noise, vibration and temperature changes will lead to imperfect images.
LNF's AFM is a Bruker NanoMan, Dimension V. This instrument is located in the Clean Room, it seat on a solid floor and is housed under an enclosure that allows its operation under a clean room environment.
In general, all AFM manufacturers companies, have great descriptions of the technique. Below are some that are particularly good.
- How AFM started Bennings, Quate and Gerber, Atomic Force Microscope, Phys. Review Letters, 56(9), 932, 1986.
- Chemical Identification with AFM Y. Sugimoto, P. Pou, M. Abe, P. Jelinek, R. Perez, S. Morita & O. Custance, Chemical Identification of Individual Surface Atoms by Atomic Force Microscopy, Nature 446,64-67,2007
- LNF Tech Talk for Metrology
- Other stuff, e.g. technology workshop slides
- External links (can be in another section below, if appropriate)