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Atomic resolution EM: Less is More:
How much is too much Carbon?
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Atomic resolution EM: Less is more:
How much is too much Carbon?
Carbon support films are routinely used for high resolution TEM. Thickness is one of the main criteria to assess its usefulness for
a particular experiment. Within that respect graphene (oxide) layers are frequently used. However, charge dissipation and
mechanical stability towards high probe currents and high voltages, including long term acquisition protocols are equally important.
Moreover, contamination issues should be addressed even after deposition of the sample on a substrate. Preparation and use of
ultrathin carbon film is therefore a good complement and in most cases a better alternative.
Multiple evaporation using adaptive carbon thread evaporation was already shown to be a beneficial process for obtaining
ultrathin carbon films with a uniform thickness and good mechanical stability. These films need to be supported by a holey carbon
or Quantifoil grid. The protocol for the synthesis of such carbon films can be found in a previous application note (Leica EM ACE600
Application Note 'Ultra-thin Carbon Films') .
The multiple evaporation process ensures that the carbon film is smooth and has a uniform density, properties that are crucial for
atomic scale experiments. The highly uniform density is due to the lack of large carbon clusters that are usually present when
preparing films with a carbon rod or conventional carbon thread evaporation process.
Figure 1 shows atomic scale imaging data (HAADF-STEM) of CdSe/CdS core/shell nanorods. When first deposited on a freshly
made carbon film, contamination during STEM imaging is a major issue due to contaminants in the nanorod suspension (figure 1A).
Plasma cleaning can be used to break down and desorb contaminants if low power settings are used. However, both the specimen
and the thin carbon film can be adversely affected especially if oxygen is used to create a plasma. An alternative method is to
perform a high vacuum bake out to desorb contaminants. Upon heating the vapour pressure of the contaminants will rise,
facilitating desorption. Figure 1B and 1C show higher magnification images of the treated specimen. Contamination was nearly
eliminated and even a series of 15 projection images could be acquired with an angular range from -70 to 70 degrees. A 3D
reconstruction showing the location of the CdS core of the nanorod can be seen in image 1d.
More information can be found in following article:
Near-Infrared Emitting CuInSe2/CuInS2 Dot Core/Rod Shell Heteronanorods by Sequential Cation Exchange
W van der Stam, E Bladt, FT Rabouw, S Bals.
http://pubs.acs.org/doi/pdf/10.1021/acsnano.5b05496
LNT Application Note - HOW MUCH IS TOO MUCH CARBON
Figure 1. Atomic resolution imaging of CdSe/CdS core/shell nanorods. A. HAADF-STEM imaging before high vacuum bake out showing
contamination build-up. B. HAADF-STEM imaging after high vacuum bake out of several hours at 60°C. C. Atomic resolution HAADFSTEM image (detail image B) showing the nanorod lattice. (Image courtesy Eva Bladt and Sara Bals, EMAT, University of Antwerp)
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Leica EM ACE600 Application Note HOW MUCH IS TOO MUCH CARBON ∙11/16 ∙ Copyright © by Leica Mikrosysteme GmbH, Vienna, Austria, 2015. Subject to modifications. LEICA and the Leica Logo are registered trademarks of Leica Microsystems IR GmbH.
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