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CL in STEM

New Video:
Cathodoluminescence (CL) system in Scanning Transmission Electron Microscopy (STEM)

View QuickTime video (3.9 MB)

Created by Sung Keun Lim of the Gradecak group

The video describes the cathodoluminescence (CL) system in STEM.

1) When the converged e-beam scans the sample, the light (cathodoluminescence) is generated as a result of the interaction between the sample and e-beam.
2) The emitted light is guided by paraboloidal shaped mirror and dispersed at the grating in the monochromator.
3) In parallel-detection-mode, the dispersed light is detected by a charge-coupled device (CCD) and three-dimensional data set I(x,y,λ) is acquired as the beam scans across
the sample.
4) In serial-detection-mode, the dispersed light is filtered by a slit and then detected by a photomultiplier tube (PMT) to form monochromatic images or CL spectra.

The detail explanation can be found in the recent publication:
Sung K. Lim et al., Nano Letters 9(11), 3940-3944 (2009).
(http://pubs.acs.org/doi/abs/10.1021/nl9025743)

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.

Polycrystalline X-Ray Diffraction (XRPD)

Commonly called X-ray powder diffraction, though it can also be used for sintered samples, metal foils, coatings and films, finished parts, etc.

Used to determine: phase composition (commonly called phase ID, quantitative phase analysis, unit cell lattice parameters, crystal structure, average crystallite size of nanocrystalline samples, crystallite microstrain, texture, and residual stress (really residual strain)

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 chanelling patterns, 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.

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:

Single Crystal Diffraction (SCD)

Used to determine: crystal structure, orientation, and degree of crystalline perfection/imperfections (twinning, mosaicity, etc.).

Small Angle X-Ray Scattering (SAXS)

Used to determine: crystallinity of polymers, organic molecules (proteins, etc.) in solution, structural information on the nanometer to submicrometer length scale, ordering on the meso- and nano- length scales of self-assembled molecules and/or pores, and dispersion of crystallites in a matrix.

Auger Electron Microscopy

A scanning Auger microscope can be thought of as a scanning electron microscope with a specialized electron energy analyzer attached. Auger electron spectroscopy permits elemental and sometimes chemical analysis with high depth resolution (typically 3 nm), good elemental sensitivity (1.0 to 0.1 atomic percent) and high lateral resolution (minimum 11 nm). The spatial distribution of elements on a surface can be mapped. Changes in elemental composition with depth can be documented by recording surface composition while using an ion gun to gradually remove surface layers. The sample is analyzed in an ultra high vacuum chamber.

X-ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy (XPS, aka ESCA) permits elemental and chemical spectroscopic analysis of both conductive and insulating samples, with high depth resolution (10 nm or less), good elemental sensitivity (0.1 to 0.01 atomic percent), and lateral resolution down to 10 Ám. Changes in elemental composition with depth (to a maximum depth of about 10 nm) can be documented nondestructively by recording surface composition while varying sample tilt relative to the analyzer (this technique is called angle-resolved depth profiling). Compositional changes with depth down to a few hundred nm can be observed by recording surface composition while using an ion gun to gradually remove surface layers. The spatial distribution of elements or chemistries on a surface can be mapped with a lateral resolution of down to ten microns. The sample is analyzed in an ultra high vacuum chamber.

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 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.

Thin Film Diffraction (GIXD)

Grazing Incidence Angle Diffraction (also called Glancing Angle X-Ray Diffaction)

Used to determine: orientation of thin film with respect to substrate, lattice mismatch between film and substrate, epitaxy/texture, macro- and microstrains, and reciprocal space map, as well as many of the analyses possible with XRPD, but with the added ability to resolve information as a function of depth (depth-profiling).

Thin Film Reflectivity (XRR)

Used to determine: thickness of thin film layers, density and composition of thin film layers, and roughness of films and interfaces

Transmission Electron Microscopy

The transmission electron microscope projects energetic electrons (typically 100-250KeV) 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.

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