A broad range of modern analytical technology exists enabling us to understand matter both structurally and chemically. Here are descriptions of some of the techniques I have had the opportunity to apply in my research.
The FIBSEM (Focused Ion Beam Scanning Electron Microscope) is a dual beam microscope, one beam has an electron source and when using this beam it functions as a scanning electron microscope (SEM) producing high magnification images.
The backscattered electrons produced can be recorded these show contrast based on compositional differences within the sample being analysed or secondary electrons can be recorded which show structural details on a surface.
The other beam is composed of Gallium ions, these can be used to generate images in a similar fashion to SEM or it can be focused onto a defined area to mill away sample atoms from this region allowing us to engineer structures into the surface on a very small scale.
Sometimes we use the FIB to prepare very thin wafers of materials which we then use a micro-manipulator to extract and attach it to a grid for analysis in other devices such as transmission electron microscopes. In order for us to be able to transmit electrons through these wafers they must be thin, typically they are only about 100nm thick, this corresponds to only approximately 400 atoms.
SEM imaging is non-destructive to the sample but the FIB is destructive on a small scale, even recording images using the FIB will cause some alteration of the sample surface.
Energy Dispersive X-ray Spectroscopy (EDS) is a technique used for the elemental characterisation of materials in order to produce quantitative measurements at specific points on a surface or to define the elemental distribution across a surface.
A high energy beam of electrons is focused onto the sample for analysis, atoms in the sample contain electrons at discrete energy levels or shells. The beam of incoming electrons may hit one of the inner shell electrons transferring energy and liberating it from its shell leaving behind a hole for one of the outer electrons to fill. The energy difference in these two shells can be released as an x-ray. The energy difference in electron shells is characteristic of each element, so measuring the energy of an x-ray precisely can tell us which element is present and in what quantities.
X-ray Micro Computed Tomography (x-MCT) is a technique used to visualize materials in 3D making use of different composition materials having different x-ray attenuation. An object for analysis is placed between an x-ray source and x-ray detector, the x-ray beam is switched on and a projected image of the object is recorded on the detector. the object is rotated very slightly and the process repeated to produce a series of projected images from different angles. As the x-rays travel in straight lines each image is a representation of the structure and composition of the sample. With the knowledge of the angular differences in an image series we are able to form virtual cross sections and 3D models with use of graphics processing software.
Atomic Force Microscopy (AFM) is a method of very high resolution imaging of a surface making use of the electrostatic forces existing between atoms as a means to generate signal defining the position of matter. It depends on the exact mode of imaging and scanning tip in use that defines the type of force being measured, it is capable of atomic resolution in the x-y dimensions and subatomic scale resolution in z.
It works by the principle of a small sharp tip being positioned very close to the sample surface then moved across the surface, as the sample surface topography varies the tip reacts and deflects in response, it is this deflection that is recorded. As the two atoms approach each other at large separation distances exist long range attractive forces such as van der Waals forces which dominate most strongly within approximately 10 nm, at shorter distances the electron shells of the tip and sample atoms start to overlap, causing a strong short range strongly repulsive force.
Scanning Tunneling Microscopy (STM) is a very high resolution surface imaging method. It involves generating a signal by use of quantum tunneling, this occurs when two atoms are brought close to each other, a small potential difference is applied across them to encourage the flow of electrons in a preferred direction, it allows the electrons to quantum tunnel through the atomic potential barriers that would otherwise confine them. The tip atoms transfer the current flow to a measuring system which has a feedback loop controlling the positioning of the tip to keep it in position allowing the tip to receive a constant tunneling current. The feedback movement therefore defines changes in the sample surface topography with atomic resolution capability.
Surface reconstruction lines are caused by a surface undulation produced from the re-arrangement of the highly ordered atomic lattice at the crystal surface – air interface. A change in direction of these lines occurs at point dislocations, the atomic arrangement between the two sets of lines is hexagonal close packed structure outside this is face centered cubic. This surface undulation has a magnitude smaller than the diameter of an atom hence only extremely sensitive imaging techniques are capable of imaging these structures.