High-Resolution Carbon Coating: How much Carbon is too much?

Application Note for Leica EM ACE600 - Material Research

November 23, 2016

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. Howewer, 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 benefical 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 of the synthesis of such carbon films can be found in this Leica EM ACE600 application note: 'Ways to Reveal More from your Samples: 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 rodor 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 this publication: Near-Infrared Emitting CulnSe2/CulnS2 Dot Core/Rod Shell Heteronanorods by Sequential Cation Exchange. W van der Stam, E Bladt, FT Rabouw, S Bals, Celso de Mello Donega. ACS NANO (2015).

Fig.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)