12-03 :: March/April 2012

nanotimes News in Brief

the field but at some point you hit a wall unless you understand what is going on at the molecular level, for example, how electrons or holes flow through or across molecules, how the charge transport depends on the structure of the organic layers and the orienta- tion of the molecules, and how the charge transport responds to mechanical forces and chemical inputs,” Salmeron says. “With our experimental results, we have shown that we can now provide answers for these questions.”

Obtaining structural crystallographic maps of mo- nolayer organic films using electron beams posed a major challenge, as researcher Shaul Aloni explains.

“These organic molecules are extremely sensitive to high energy electrons,” he says. “When you shoot a beam of high energy electrons through the film it im- mediately affects the molecules. Within few seconds we no longer see the signature intermolecular align- ment of the diffraction pattern. Despite this, when applied correctly, electron microscopy becomes essential tool that can provide unique information on organic samples.”

Salmeron, Aloni and their colleagues overcame the challenge through the combination of a unique stra- tegy they developed and a transmission electron mi- croscope (TEM) at the Molecular Foundry’s Imaging and Manipulation of Nanostructures Facility. Electron diffraction patterns were collected as a parallel elec- tron beam was scanned over the film, then analyzed by computer to generate structural crystallographic maps.

“These maps contain uncompromised information of the size, symmetry and orientation of the unit cell,

Electron diffraction patterns provide a wealth of infor- mation about the morphology, structure, and quality of monolayer organic thin films. © Berkeley Lab’s Molecular Foundry

Virginia Altoe, Florent Martin, Allard Katan, Miquel Sal- meron, Shaul Aloni: Electron Microscopy Reveals Structure and Morphology of One Molecule Thin Organic Films, In: NANO Letters, Vol. 12(2012), No. 3, Pages 1295-1299, DOI:10.1021/nl203776n: http://dx.doi.org/10.1021/nl203776n

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the orientation and structure of the domains, the de- gree of crystallinity, and any variations on the micro- meter scale,” says first author Altoe. “Such data are crucial to understanding the structure and electrical transport properties of the organic films, and allow us to track small changes driven by chemical modifica- tions of the support films.” In their paper, the authors acknowledge that to gain structural information they had to sacrifice some resolution.