Transmission Electron Microscopy

A Jeol 2100 200 kV fitted with a LaB6 filament giving a point resolution of 0.13 nm.  It is equipped with bright and dark field STEM detectors and an Oxford Instruments EDS detector.  It is housed in LG26, in the basement.

The 2100 is very heavily used, often booked up 5 – 6 weeks in advance.

The pressure inside the TEM column is of the order of 10-6 Pa, so the sample must be dry before being inserted into these TEMs.

Samples must be thinner than about 100 nm, or the electrons will not pass through them. This means films on a substrate cannot be imaged, but most powders can be. If the particle size is greater than 100 nm the powder can be sonicated in e.g. methanol or hexane to form a suspension. One or two drops of this suspension are put onto a grid and left to evaporate. Large particles or clusters will just produce a silhouette image, but it is often possible to get images of details at the edges of such large particles, or simply to find smaller particles on the grid.

To put the samples into the TEM, they must be mounted onto a sample grid. These grids are ca. 3 mm diameter circular mesh of copper or gold, with a layer of carbon to support the sample. It is important to put the sample on the darker, carbon coated, side of the grid. Samples are usually prepared by sonicating the powder in a volatile liquid such as hexane or methanol to produce a suspension. Two or three drops of this suspension are dropped onto the dark side of the grid and left to dry. Thin films cannot be imaged directly, but can be scraped off the substrate and the scraping sonicated as above. Alternatively, the film and substrate can be sonicated together, and anything that comes into suspension can be dropped onto the grid as above. Of course, these methods destroy the integrity of the film. When the sample grids are dry, they are mounted onto a sample rod and inserted into the TEM for imaging. The sample rod for the 2100 will only hold one sample grid, so each sample must be imaged individually.

When the samples have been inserted into the TEM they are left for two or three minutes for the pressure in the column to return to the operating pressure. Then the filament is turned on to generate the electron beam and the beam is aligned down the microscope column. These alignment procedures will be shown and explained during the training session. For the 2100, the training to prepare and insert the sample, align the microscope and image the sample will take three to four hours. The training will be done on your sample, and the rest of the session can be spent imaging another of your samples.