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IMAGING
Rigid tip improves atomic force microscopy ning a buckyball. The bond lengths in the AFM images were within two tenths of an angstrom of the bond lengths obtained by X-ray diffraction and predicted by density functional theory. CO tips overestimate ably image bonds without false positives Atomic force microscopy (AFM) can get these bond lengths by as much as an angand can accurately measure bond lengths. chemists pretty close to actually seeing strom. The stiffer copper oxide tips did The copper oxide tip is about one order the atoms and bonds of molecules. The sacrifice some sharpness in imaging relaof magnitude more rigid than its CO technique produces images a bit like a tive to CO tips. counterpart. record player generates sound. An atomThe researchers also successfully im“This is a very significant paper,” says ic-scale needle at the end of a cantilevered aged a predicted nitrogen-gold-nitrogen Xiaohui Qiu, a physical chemist at the Naarm scans across a sample, moving up and bond between bis(para-pyridyl)acetylene down according to the shape and electron- tional Center for Nanoscience & Technoland a gold surface. Chemists had debated ic properties of the surface. Measuring and ogy, explaining that the work represents the existence of this bond recording that deflection because of discrepancies produces a three-dimenbetween simulations and sional representation of AFM images made with a the molecules within the CO tip. sample. Finally, the group Minimizing interacimaged intermolecular hytions between the needle drogen bonding between and the sample can lead 3,4,9,10-perylene-tetrato sharper images. In carboxylic-dianhydride 2009, IBM researchers (PTCDA) molecules. The proposed using a carbon flexibility of CO tips can monoxide tip to signifiproduce lines of contrast cantly improve AFM resthat look like bonds between olution. The drawback is atoms in AFM images, making that atoms on a sample’s it hard to distinguish between surface can tug on the actual hydrogen bonds and CO dangling from the An AFM image (left) made using a rigid tip shows hydrogen bonds artifacts. Mönig’s group found end of a metal needle, (identified with arrows) between molecules of PTCDA (right). that copper oxide tips did not making bonds look lonproduce those kinds of artifacts in the a significant advance in noncontact AFM ger than they actually are or creating artiPTCDA images. imaging. Qiu says Mönig’s paper resolves facts that look like bonds between atoms. To make these new rigid tips, Mönig’s a debate that has been ongoing since 2013 Now, researchers have swapped this team first presses a blunt tungsten needle about whether hydrogen bonds in AFM flexible CO tip for a more rigid one to into a copper surface to pick up a pyrimages are real or artifacts, and the work improve AFM’s ability to image inter- and amid of copper atoms. They then dip it shows that AFM can reliably characterize intramolecular bonds (Nat. Nanotechnol. just tenths of angstroms deep in a copper both inter- and intramolecular bonds. 2018, DOI: 10.1038/s41565-018-0104-4). oxide surface. The process reliably adds Mönig’s group had previously shown Taking inspiration from a 2012 paper copper oxide to the tip, although Mönig that their copper oxide-tipped needle in Physical Review B that proposed using a says it can be challenging to make sure the could be used in AFM. In the new paper, copper oxide tip for AFM, Harry Mönig of copper oxide pyramid has one oxygen aimthey first demonstrate the tip’s ability to the University of Münster and colleagues ing straight down.—SAM LEMONICK accurately measure bond length by scandemonstrated that such a needle can reli-
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C&EN | CEN.ACS.ORG | APRIL 16, 2018
C R E D I T: NAT. NA NOT EC H NOL ./ H A R RY MO N I G
Inflexible copper oxide tip allows accurate imaging of bonds within and between molecules
