By Gary Thomas
Using noncontact atomic force microscopy (AFM), IBM researchers have for the first time differentiated the individual molecules’ chemical bonds, thus extending the boundaries of analysis involving atoms and molecules at the smallest scale possible.
This work is also a significant step towards exploring graphene devices, which are the hot topic of both academia and industry for developing applications such as electronic displays and high-bandwidth wireless communication.
Leo Gros, a researcher at IBM, informed that the researchers were able to identify two contrast mechanisms to differentiate the chemical bonds. The first second contrast mechanism is based on slight variations in the force calculated above the bonds. Although the researchers anticipated this type of contrast, it was difficult to be resolved. The second one was a surprise wherein different length bonds appeared in AFM measurements. Using ab initio calculations, the researchers identified that this contrast was the result of a carbon monoxide molecule’s tilting at the tip apex.
The study findings have appeared in the Science magazine under the title ‘Bond-Order Discrimination by Atomic Force Microscopy.’ In the paper, the researchers provided the images of the bond length and order of individual carbon-carbon bonds in bucklyballs (C60) and in two planar polycyclic aromatic hydrocarbons (PAHs) that look like small graphene flakes.
Centro de Investigacion en Quimica Bioloxica e Materiais Moleculares synthesized the PAHs at the Universidade de Santiago de Compostela and Centre National de la Recherche Scientifique in Toulouse. The slight differences in the strength and length of the individual carbon-carbon bonds in such molecules are responsible for their optical, electronic and chemical properties.
In this study, the IBM researchers utilized an AFM with a single carbon monoxide (CO) molecule-terminated tip, which produces an image by oscillating with small amplitude over the sample such as a molecule to make measurements of force between the sample and the tip. The tip’s CO termination serves as a robust magnifying glass that exposes the molecule’s atomic structure, including its bonds. The researchers confirmed their experimental findings by performing first-principles density functional theory calculations.
The IBM Research scientists’ demonstration to detect these differences in individual molecules and bonds provides new insights at the individual molecule level. This knowledge is helpful in conducting research on innovative OLEDs, organic solar cells and electronic devices and especially useful in exploring bond changes in excited states and in chemical reactions as well as bond relaxation around defects in graphene.