How to... use plasma treatment in electron microscopy

Materials World magazine
,
1 Sep 2018

Henniker Scientific Ltd, UK, explains the advantages of using plasma treatment to maintain microscopy equipment.

The technique known as transmission electron microscopy (TEM) produces images that are of a significantly higher resolution than light microscopes. They enable users to capture even the finest details, closing in on atomic levels. Capturing these images without contamination is critical, as is the ability to control the environment that surrounds them.

Plasma cleaning

Plasma cleaning is a proven, effective, economical and environmentally safe method for critical surface preparation. With oxygen plasma, it eliminates natural and technical oils and grease at the nanoscale, and reduces contamination up to sixfold when compared with traditional wet cleaning methods, including solvent cleaning. Plasma cleaning produces a pristine surface, ready for further processing, without any harmful waste material or organic residue.

To effectively examine a specimen under TEM conditions, operators will prepare sample holders, usually grids, ready for species study. Plasma cleaning is a fast and effective method of preparation – here we discuss how the user is able to deliver precise and controlled removal of carbon contamination on such a grid.

Plasma cleaner model

In this example, the Henniker HPT-100 Plasma Cleaner with Jeol TEM sample holder adapter was used, enabling a continuously variable power output from 0–100W and dual digital mass flow controller gas mixing. Samples were introduced via the front feedthrough adapter and cleaned with Argon:Oxygen 95:5 mixtures at low power – 20W and 50W – in 30-second time steps.

Preparation and examination

A holey carbon coated copper TEM grid was used to hold the samples. These were then examined in the JEM 2800 TEM using the following instrumental conditions – Voltage (kV) 200, C2 aperture (um) 70 and 40, bright-field imaging mode using charge-coupled devices in a high magnification lattice resolution imaging mode.

Effective cleaning

As hydrocarbon contamination results in poorer TEM image quality, it is critical to be able to effectively clean the carbon grids that contain the samples. The images taken from an identical location, using a finder holey carbon grid, showed a gradual loss of carbon from a small-thinned area. First the thinned carbon was removed, or eaten away, and then the thick walls were observed to lose carbon from the edges.

The results show the treatment capability of the system in eradicating these issues at different times and power levels, giving an alternative to other methods of treatment and other systems on the market.