The IAC hosts a wide range of advanced materials science analysis techniques. Please contact us for more information.
In addition to conducting materials science research we also perform analysis and consultancy services to industry.
The optical microscope uses a lens, or converging mirror which creates a focal point for the light coming off the sample. More often a lens is used, and in modern microscopes with higher imaging power multiple lenses are used (a compound microscope).
A compound microscope magnifies an object by placing the sample on a stage, which is lit from below. The lens closest to the sample is the objective and the eyepiece (or ocular lens) is located at the opposite end of the microscope.
They are separated by the body tube. The objective lens is actually comprised of a number of different lenses, which work together to form the intermediate image, an enlarged version of the sample. The eyepiece then further magnifies the intermediate image, and is what a person will see.
The scanning electron microscope uses a focused beam of high energy electrons to create a signal at the surface of solid specimens. SEM machines can magnify in the order of 50 to 250,000 times, and generally cover an area from 1 cm to 500 nanometers.
The principal used are secondary electrons, these give the image morphology and topography. This is detected because the angle of secondary electron emission is affected by the angle of incidence.
An important factor in SEM is the accelerating voltage of the ion beams; a greater accelerating voltage gives increased depth penetration. Greater depth penetration can be positive or negative, depending on the aim of the investigation. However, the accelerating voltage also causes a greater interaction volume decreasing the control the SEM has of where signals are generated from.
SEMs often have secondary detectors which use some of the other signals generated to provide further information about the sample. Backscattered Electrons (BEI) and Energy Dispersive X-ray analysis (EDX) are commonly used, they provide information on the atomic number of the sample and compositional information. Electron Backscattered Diffraction (EBSD) detectors generate information on the crystallographic orientation and phases present in the sample.
However, some of these problems can be overcome using variable pressure mode within the Zeiss Sigma SEM.
To use EBSD a beam of electrons is fired on the point of interest of a crystalline sample, which is tilted at approximately 70° to the normal incidence of the electron beam in the SEM, this is to increase the contrast in the diffraction pattern and the fraction of the electrons scattered from the sample. When the electron beam interacts with the sample, at an angle which satisfies Bragg’s equation, the electrons scatter inelastically to form a pair of cones of diffracted electrons.
FIB systems are similar to SEMs but instead of electron guns use a focused beam of gallium ions; by using a low beam current they can be used for imaging and with a higher beam current for site specific sputtering or milling. Sputtering is a process where an atom is ejected from the surface of a sample as it’s bombarded by the high energy particles.
As the ion beam passes over the surface it sputters ions, and produces secondary electrons. The signals from both of these are used to create an image. At low beams the images are useful for looking at grain orientation contrast, morphology and boundary contrasts.
FIBs are typically used for semiconductor fabrication; used for defect analysis, circuit modification, mask repair and TEM slide preparation. They can also be used to deposit materials, for example, to deposit a conductive material. A further use of FIBs is to analyse the secondary ions produced with mass spectroscopy to identify chemical compositions.
FIBs can also use samples which have been cryogenically frozen, so don’t necessarily have to be solid.
Custom built Scanning probe microscope, operating in high speed contact mode.
Making use of a newly discovered physics, our contact mode high-speed atomic force microscope (HS-AFM) is the fastest in the world by several orders of magnitude. The HS-AFM moves the sample in a raster pattern and engages a sharp tip with the surface in order to map the samples topography with nanometre lateral and sub atomic height resolution over millimetre sized areas. The tip can be thought of as a finger passing across a surface, not only is it possible to map the height of the surface, it is also possible to map the local stiffness, thermal and electrical properties at the same time. The microscope does not require the sample to be conductive and does not require a vacuum to operate, indeed the microscope is able to image samples in gaseous and liquid environments.
Ti and collagen: 3D render of HS-AFM data from a 16 x 10µm region of patterned titanium and collagen on silicon. The height of the sample was measured with nanometre lateral and sub-nanometre vertical resolution and used to create the 3D map. The colour scale corresponds to simultaneously collected measurements of the local stiffness of the sample, with stiffer regions having lighter colours. By collecting both height and stiffness information with the same spatial and fast time resolution our HS-AFM stands out as a world leading tool for the nanoscale characterisation of surfaces and materials.
Carburised Steel: Type 316 Steel surface showing carburisation. Using a mechanical polish relatively hard carbides are left proud of the surface allowing them to be mapped using the measured sample topography. The image has a 4nm pixel size and has a total resolution of 625 megapixels and took under 10 minutes to collect.
The HS-AFM typically collects data at 2 million pixels per second, however this can be increased to 10 million pixels per second if required. These pixel rates allow the HS-AFM to image millimetre sized areas in under a day, a task that would take a conventional AFM in excess of a year of continuous imaging. Examples include: vast sampling of nanostructures for unbeatable sample statistics and measurement certainty, and mapping of nano-structures across large areas. In addition to imaging large areas we can use the high pixel rate of the instrument can be used to observe nano and microscale dynamic processes with millisecond temporal resolution. Examples include: corrosion of metallic surfaces, formation of crystals and salts, and bio-molecular processes.
As the HS-AFM physically moves the sample to build up an image the sample mass should be kept below 300g and should be under 2 x 2cm in size. If large areas are to be imaged then it is important that the bottom and top surface of the sample are parallel in order to minimise any slope. As with any scanning probe microscope it is important that the structures to be imaged are not mobile. We have found that if the sample can be imaged with a standard AFM then we are able to image it using the HS-AFM.
Raman spectroscopy is a type of vibration spectroscopy which can look at molecular motion and fingerprint species. A beam of photons is aimed at the sample, which excites a molecule. As the molecule loses the energy a photon is emitted.
Most photons are elastically scattered (Rayleigh scattering) so the scattered photons have the same energy as the incident photons, but some are inelastically scattered (Raman scattering) so that they have a different frequency, usually lower, than the incident photons.
This energy shift (Raman shift) gives information about the vibrational modes in the system.The output (below) shows the intensity versus the Raman shift (in wave numbers), it is also the inverse of IR spectroscopy. The peaks can be identified as different bonds in molecules.
A difficulty in LRS is that Raman scattering is typically weak, so to separate the weak Raman scattered light from the strong Rayleigh light requires special detectors.
A SIMS uses an ion beam to sputter the surface of a sample, then analyses the secondary ions which are ejected. SIMS has a low detection limit, with some instruments being able to detect samples in the parts per billion range.
A schematic of a SIMS is shown below, a large part of the machine acts as a conventional mass spectrometer. It uses an electromagnet which will deflect more charged ions a greater amount than less charged ions, and heavier ions less than lighter ions.
SIMS also use quadrupole mass analysers which separate ions based on their resonant electron field, this allows only selected masses to pass through. Another means of sample identification is “time of flight”, the velocity of the ions are measured and as the ions all have the same kinetic energy, the only factor affecting the velocity is the mass.
The sample is irradiated with a beam of X-rays, which excites an inner shell electron.There are two principal types of X-rays used, monochromatic aluminium Ka X-rays (monochromatic X-rays only have a single wavelength or a narrow band) and non monochromatic X-rays. These X-rays have a known wavelength which corresponds to a known photon energy (Ephoton).
The principal behind BET uses the assumption that if a gas is absorbed onto the surface of a powder, it will be absorbed on a layer a single atom thick. The principle behind the technique relies on the fact that a more stable energy state is reached when every atom/molecule is surrounded by neighbours.
The gases usually used are nitrogen and krypton, so by measuring the volume of gas absorbed the surface area can be determined by some simple calculations. By some additional calculations data on mean pore size and pore size distribution can be determined.
This technique produces challenges in that different materials have different absorption characteristics. For example types III and V are not compatible with the BET method because there are only weak interactions between the solid and gas.
The sample also requires some preparation to remove impurities which are physically bonded to the surface, this is usually done by raising the temperature while in a vacuum or surrounded by an inert gas.
A DSC consists of the sample and a reference, the temperature of the surroundings of both is raised equally and the difference in intrinsic temperature of the sample and reference are measured.
DSCs are commonly used to find the temperature at which phase transitions occur, e.g. solid to liquid, and whether these reactions are exothermic or endothermic (heat releasing or heat using) and the specific heat capacity of a sample.
The output of these techniques gives the temperature against the heat flow or against heat capacity.
The principal of TGA is to use a precision balance and a programmable furnace to measure mass change, temperature and temperature change. The instrument continually weighs a sample while it is being heated up to temperatures of 2000°C.
The output with this method shows temperature on the x-axis and mass loss on the y-axis. To avoid contamination of the sample, as it is heated the chamber should be purged with an inert gas, which will not react with the sample.
TGA is often combined with a gas analysis instrument; they use infrared spectroscopy, mass spectrometry and gas chromatography, which aid in sample identification and quantification.
DC Magnetron Sputtering is a physical vapour deposition technique which is predominately applied to the creation of thin film samples of conductive materials.
The IAC has recently been granted funding by the EPSRC to invest in a new X-Ray Tomography setup with a special focus on examining radioactive samples.
The process of tendering for the system is currently underway.
University of Bristol,
Bristol, BS8 1TH, UK
Tel: +44 (0)117 928 9000