Leica Microsystems EM ACE600 Application Note

Application Note

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

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 HAADF-

STEM image (detail image B) showing the nanorod lattice. (Image courtesy Eva Bladt and Sara Bals, EMAT, University of Antwerp)

LNT Application Note - HOW MUCH IS TOO MUCH CARBON