Techniques
Ion milling
A technique for thinning solid samples in which the sample is slowly sputtered by a medium-energy beam of argon ions. Ion milling instruments in the facility includes the Gatan 600 dual ion mill, fitted with a cold stage, a Gatan PIPS precision ion polishing system for rapid milling of samples with a low incidence angle of the primary beam, and a Fischione 1010 ion mill, which also has a cold stage and has low-voltage milling capability.
Microtomy
A technique used to obtain ultra-thin sections for electron microscopy, semi-thin and thick sections for light microscopy. Ultramicrotomy can be used for both room temperature sectioning and cryo-ultramicrotomy at low temperature sectioning of polymers, elastomers, or any materials requiring processing down to -185°C.
Scanning Electron Microscopy
In the scanning electron microscope (SEM), a fine probe of electrons is scanned in a raster pattern, line by line, across the surface of a sample. As a result of the interaction of the beam with the sample, a multitude of secondary emissions and other effects may occur. These include secondary and backscattered electrons, x-rays, light, sound, changes in electrical conductivity, etc., etc., etc. All these effects can be sensed and used - often in more than one way - to provide information about the sample, though it is rare to find more than a few detectors on a single instrument.
In the most basic SEM, secondary electrons are detected, their intensity being used to modulate the brightness of a corresponding spot in an image, thus building up a picture of the surface of the sample. Depending on the sophistication of the instrument, the ultimate resolution in such an image can approach 1nm. Backscattered electrons are more sensitive to the sample composition than the surface topography, though to identify elements it is necessary to analyze the x-ray spectrum with an energy-dispersive x-ray detector. Another relatively common detector system provides electron back-scatter diffraction, which are a powerful way of investigating phases and orientations in polycrystalline materials.
Some SEMs are equipped to allow the examination of a sample in a controlled low-vacuum environment (of the order of 1 Torr, or in the case of the Environmental SEM, more, whereas in the conventional microscope a high vacuum is required). This has a number of potential advantages.
Some instruments, basically SEMs with specialized detector systems, are commonly known by different names, the most obvious examples being the scanning Auger microprobe and the electron microprobe.
Transmission Electron Microscopy
The transmission electron microscope projects energetic electrons (typically 80-200KeV) that penetrate through thin samples (<100nm usually) to give very high- resolution images, diffraction patterns, and/or chemical analyses of samples. Depending on the application, image resolution of better than 0.2 nm can be attained on some of the instruments at MIT, while in ideal cases it is possible to perform chemical analysis to a precision of 0.2 wt. % with a spatial resolution of around 1 nm. Transmission microscopy is widely used in Biology, Materials Science, Biomaterials, Physics, Engineering and Medicine.
Scanning Probe Microscopy (AFM, STM, MFM, etc.)
Scanning Probe Microscopes are a class of instrument derived from the Nobel-Prizewinning Scanning Tunneling Microscope, developed by Binning and Rohrer in the early 1980's. In these instruments, a sharp mechanical sensing tip is scanned - usually in a raster pattern - over the surface of the sample. Some interaction between the probe and the sample is measured, allowing conclusions to be drawn about the sample. The spatial resolution is limited, in many cases (and in suitable installations), by the tip radius of the sensing probe; this can be as small as a single atom.
Versions include:
- Atomic Force Microscopy, a versatile mode in which the force between the tip and the sample is sensed as a function of sample height. In its simplest form, a detailed profile of the sample surface, with height sensitivity of a single atom, can be obtained; other modes include measurement of eleastic properties of samples, interactions between experimental molecules, etc., are possible.
- Scanning Tunneling Microscopy, the original form, in which an electrical bias is set up between the tip and the sample (both of which must be electrical conductors) and the resulting tunneling current measured as a function of sample height. Frequently used by physicists to investigate electron densities of state in a relatively direct and high-resolution way.
- Magnetic Force Microscopy, investigating magnetic interactions at a surface.
Thermal Characterization
In thermal analysis we measure how a sample responds to heat input. We may be interested in how other properties (for example, the mechanical strength) change with temperature, or we may be looking at intrinsic thermal properties, such as the heat capacity. Thermal measurements are frequently made on polymeric samples, in many of which the data can reveal valuable insights about the microstructure. Most thermal analysis is performed at some temperature between 77oK and 1250oK, though individual instruments may not cover this entire range.
The most basic thermal instrument is the Differential Scanning Calorimeter (DSC), which measures heat capacity and/or latent heat, either by supplying heat at a constant rate to a sample of known mass, and measuring the rate of temperature change, or by changing the temperature at a constant rate, and measuring the heat input. (In either case, the heat input may be negative - i.e. the sample may be cooled).
The Thermo-Gravimetric Analyzer (TGA), often coupled with a Differential Thermal Analyzer (DTA) measures the rate of change of mass of a sample as the temperature is changed. Typically, this is a result of sample decomposition, and is usually irreversible. The DTA measures the flow of heat into the sample (rather like the DSC). The TGA/DTA may be coupled with some sort of analytical instrumentation to investigate the gaseous products driven off the sample.
The Dynamic Mechanical Analyzer (DMA) measures, as its name suggests, the mechanical properties of samples in dynamic conditions, as a function of temperature.
Of course, it is possible in many instruments to observe the effects of temperature changes on samples, for example by cooling a sample on the stage of an optical microscope. We do not, though, usually refer to these types of observation as "Thermal Characterization", even though, in a literal sense, the term may be quite apt.
X-Ray Powder Diffraction (XRPD)
XRPD is a non-destructive technique that uses the diffraction of X-rays from any polycrystalline sample, including powders, sintered samples, metal foils, coatings and films, finished parts, etc.
The most common analysis is the determination of the phase composition of a sample; each crystalline phase produces a unique diffraction pattern that can be identified by automated comparison to our databases. More advanced analyses use the shape, intensity and position of diffraction peaks over a large angular range to determine: the average crystallite size of nanocrystalline samples, crystallite microstrain, unit cell lattice parameters, crystal structure, crystallographic texture, and residual stress.
With our in-situ capabilities, any of these analyses can be performed on samples in a temperature range from 11 K to 1473 K.
Single Crystal X-Ray Diffraction (SCD)
SCD uses area detectors and tilting/rotation of the sample to collect as many diffraction spots as possible from a single crystal exposed to X-radiation. CMSE equipment can be used to determine the orientation of large single crystals and, to some extent, the presence of twinning, mosaicity, or other imperfections in the crystal. The Dept. of Chemistry has the equipment necessary for solving the crystal structure of a single crystal.
Small Angle X-Ray Scattering (SAXS)
SAXS instruments are optimized to study X-rays that are only slightly scattered and/or diffracted away from their incident trajectory by the sample. This provides structural information on the nanometer to submicrometer length scale, including the degree of crystallinity of polymers, orientation of polymer chains, average pore or particle size, and ordering in meso- and nanostructured assemblies.
High-Resolution X-Ray Diffraction (HRXRD)
Rocking curves and reciprocal space maps are used to analyze epitaxial thin films, determining the orientation of the thin film with respect to substrate, lattice strain or relaxation of the thin film, lattice mismatch between film and substrate, concentration of stacking faults and mosaicity.
Grazing Incidence Diffraction (GIXD)
This technique duplicates many of the features of XRPD, except that the incident X-ray beam is fixed at a low angle. The depth of penetration of the X-rays into the sample is a function of the incident angle; so this allows the penetration depth to be controlled and the X-rays to be focused in the surface of the sample. Many of the analyses possible with XRPD can be realized with GIXD, but with the added ability to resolve information as a function of penetration depth (depth-profiling).
X-Ray Reflectivity (XRR)
The reflection of X-rays from very smooth surfaces and thin films can be used to determine the roughness of the surface and any interfaces, the thickness of the thin film layers, , and density and composition of thin film layers.
