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High-performing grids for electron microscopy

Atomic layer deposition of alumina on a sacrificial support creates durable, thin grids ideal for the immobilization of nanoparticles

Materials

Optical micrograph of a copper grid after alumina ALD over Formvar. The Formvar is subsequently removed using solvents. The 1-2 nm thick film is robust enough to span 63 µm holes without the benefit of a holey or lacey carbon support. The image was taken at the National Center for Electron Microscopy at Lawrence Berkeley National Laboratory.

Electron microscopy grids support the observation of nanoparticles and biological molecules. Supports are built up from layers of decreasing rigidity and increasing inertness.

Once the more rigid layers are sacrificed in a chemical bath, a final ultrathin film is left for the attachment of nanoparticles of interest, which are studied under an electron microscope. Thinness is particularly important to resolution in transmission electron microscopy (TEM) as electrons must pass through the support and scattering of electrons as a result of the support degrade image quality.

High-resolution TEM images of gold nanoparticles on the alumina supports after argon plasma cleaning (left) and then further heating in air (right). The aggressive cleaning procedure removes the carbon shells, retains the nanoparticles in their original positions, and does not degrade the support since it is an oxide. The photos were taken at the National Center for Electron Microscopy at Lawrence Berkeley National Laboratory.

Recently, graphene and graphene oxide supports have become commercially available as examples of thin supports down to one atom of thickness. However, these supports are often contaminated with carbon from either the preparation process or storage in air. Carbon contamination adds noise to the image due to its random nature.

Furthermore, the thinness of graphene compromises its strength during sample deposition and electron beam analysis. As an alternative, ultrathin (UT) carbon with an additional sacrificial film of amorphous carbon is often employed to image nanometer size samples. While more durable, UT carbon has limitations in that it cannot be readily cleaned. And, like graphene, UT carbon is susceptible to carbon contamination.

To address the above deficiencies in TEM supports, Dr. David Kidwell of the Naval Research Laboratory has developed new robust and flexible TEM supports functionalized to immobilize nanoparticles.

Scanning TEM image of synthetic Au11 clusters on an alumina support. The functionalities on the alumina support help adhere and align the gold clusters. The spacing is determined by the hydrocarbon shell around each gold cluster. The image was taken at the National Center for Electron Microscopy at Lawrence Berkeley National Laboratory.

The new supports are made by atomic layer deposition of alumina (or SiN, SiOx, BN, and mixtures thereof) onto a TEM grid of gold or copper supported by a carbon film. (A thermoplastic resin support such as Formvar may also be used in the buildup.) After the ALD deposition, the sacrificial layers are removed by suitable techniques.

These new TEM supports can be cleaned by plasma treatment, which allows removal of residual carbon contamination leaving only the nanoparticles of interest functionally fixed to the alumina. They can be made carbon-less, which avoids deposition of carbon from conventional ultra-thin carbon supports.

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