Atom probe microscopy was first performed in 1951 by EW Müller at The Pennsylvania State University when he imaged individual atoms with a field ion microscope. He imaged the atomic structure of a tungsten tip and could illustrate that each bright spot corresponds to a single atom. Since that time, enormous progress has been made in instrumentation, especially in signal detection and processing. The most recent advance is the 3D atom probe technique (3DAP), which is now firmly established. In 1988 Cerezo, Godfrey and Smith at the University of Oxford applied a position sensitive detector to a time-of-flight atom probe, thus determining both the mass-to-charge ratio and the position of the ions simultaneously. This new instrument, which they called the position sensitive atom probe (PoSAP), enabled two-dimensional element mapping with sub-nanometre spatial resolution. Reconstruction of a series of two-dimensional
maps makes it possible to reconstruct a three-dimensional element map, and hence this new generation of atom probes is generally called three-dimensional atom probe (3DAP).
By measuring the time of flight and the coordinates of ions using a PSD, it is possible to map out two-dimensional elemental distributions with near-atomic resolution. The lateral spatial resolution is limited by the evaporation aberration that occurs during the ionization of atoms on the surface. However, the error originating from the evaporation aberration does not exceed 0.2 nm, and this is still the lowest of the errors achieved by existing analytical instruments. The elemental maps can be extended to the depth direction by ionising atoms from the surface of the specimen continuously, by which atom distribution can be reconstructed in a three- dimensional real space. Since field evaporation occurs layer-by-layer in low index planes, the reconstructed 3D elemental map shows the layers corresponding to the atomic planes in the depth direction. Currently, the atom probe is used routinely to examine materials literally on an atom-by-atom basis.
3DAP allows the analysis of cylindrical parts of a specimen (with a diameter of about 30 nm and a length of about 200 nm), probing atom by atom through the specimen. The technique has enormous potential for understanding the fundamental aspects of materials science, including segregation at grain boundaries, processes of nucleation in materials, radiation- induced defects, and decomposition of materials. Impressive results were also obtained on the thermal stability and interdiffusion in thin films. An example of the ultrafine-scale information available with the 3DAP technique is shown in Fig. 5.14, which represents the atomic imaging of a Ω-phase precipitate formed in an Al alloy. Segregation to the surface of the precipitate can easily be recognized. The example illustrates the detailed, ultrahigh-resolution chemical information that is available with an advanced 3DAP. Figure 5.14 also shows the elemental
maps of Zr, Cu, Al and O obtained from 3DAP of a Zr alloy, which shows oxygen segregation into a quasicrystalline particle. This technique could establish that oxygen stabilises the nano- quasicrystalline phase.
5.9 NANOINDENTATION
Nanoindentation is a relatively new technique to obtain mechanical properties of nanometric regions by studying the stress–strain behaviour when a nano-indentor is impressed against the specimen of interest. Usually scanning force microscope (SFM) tips are used both to perform the indentation and to image the sample surface after indentation. The depth of indentation is measured as a function of the force (stress) applied. The mechanical properties including elastic constants can be derived from such loading–unloading curves. A diamond indenter with a small radius of curvature at its tip is generally used. This nanoindentation device can be mounted on the scanner head of the AFM in place of the cantilever. The principle of nanoindentation is similar to other indentation hardness testers, in which an indentor is forced into the material being tested, forming an indent. The hardness is taken as the ratio of load and the area of contact between the sample and the indenter. In nanoindentation, the contact area is measured by a depth-sensing technique. During the test, both the load and the displacement of the indenter are recorded. The load–displacement curve is used to calculate the contact area at maximum load. This presents the problems related to the measurement of extremely small contact areas.
With the help of a nano-indentor, one can measure the elastic modulus also, in addition to hardness. These two properties are a measure of resistance offered by the material to elastic and plastic deformation, respectively. The use of the AFM along with nanoindentation helps in positioning of the tip with accuracy within 20 nm (Fig. 5.15).
In nanoindentation, small loads and tip sizes are used, so that the indentation area may measure only a few square micrometres or even nanometres. Due to this, a typical problem is that the contact area is not easily found. Atomic force microscopy or scanning electron microscopy techniques may be utilised to image the indentation, but can be quite cumbersome. This problem is generally overcome by using high-precision geometry for the indentor, namely, a Berkovich tip, which has a three-sided pyramid geometry. Mechanical properties of the sample are then inferred from the load–displacement curve (such as the one shown in Fig. 5.16). Figure 5.17 shows the nanoindentation results of different phases in the Zr–Pt alloy, which indicates that a nanocomposite of amorphous and nano-quasicrystalline phase has much higher hardness and Young’s modulus in comparison to individual phases.
Fig. 5.16 Load–displacement curve of nanoindentation technique (Source: http://commons.wikimedia.org/wiki/ File:Load-displacement_curve_%2B_onderdelen.JPG).
SUMMARY
• The development of novel tools and instruments for use in nanotechnology is a great challenge.
• The most widely used structural methods for characterizing nanomaterials and nanostructures are: X-ray diffraction (XRD), various electron microscopy (EM), including scanning electron microscopy (SEM), transmission microscopy (TEM), high-resolution scanning electron microscopy (HRSEM), high-resolution transmission microscopy (HRTEM), atomic force microscopy (AFM), scanning tunnelling microscopy (STM) and field ion microscopy (FIM).
• Characterization and manipulation of individual nanostructures require not only extreme sensitivity and accuracy, but also atomic-level resolution.
• Microscopy plays a central role in the characterization and measurement of nanostructured materials.
• Nanoindentation technique is used for the characterization of the mechanical property of nanomaterials by studying the force–displacement curve on application of very small loads.