Scientists image the charge distribution within a single molecule for the first time

For their experiments the IBM scientists used their home-built combined scanning tunneling microscope (STM) and atomic force microscope (AFM). In this focused ion beam micrograph, the tip attached to a tuning fork can be seen. The tuning fork measures a few millimeters in length. The tiny tip measures only a single atom or molecule at its apex

IBM scientists were able to measure for the first time how charge is distributed within a single molecule. This achievement will enable fundamental scientific insights into single-molecule switching and bond formation between atoms and molecules. Furthermore, it introduces the possibility of imaging the charge distribution within functional molecular structures, which hold great promise for future applications such as solar photoconversion, energy storage, or molecular scale computing devices.
As reported in in the journal Nature Nanotechnology, scientists Fabian Mohn, Leo Gross, Nikolaj Moll and Gerhard Meyer of IBM Research – Zurich directly imaged the charge distribution within a single naphthalocyanine molecule using a special kind of atomic force microscopy called Kelvin probe force microscopy at low temperatures and in ultrahigh vacuum.
Whereas scanning tunneling microscopy (STM) can be used for imaging electron orbitals of a molecule, and atomic force microscopy (AFM) can be used for resolving its molecular structure, until now it has not been possible to image the charge distribution within a single molecule.
“This work demonstrates an important new capability of being able to directly measure how charge arranges itself within an individual molecule”, states Michael Crommie, Professor for Condensed Matter Physics at the University of Berkeley. “Understanding this kind of charge distribution is critical for understanding how molecules work in different environments. I expect this technique to have an especially important future impact on the many areas where physics, chemistry, and biology intersect.”
In fact, the new technique together with STM and AFM provides complementary information about the molecule, showing different properties of interest. This is reminiscent of medical imaging techniques such as X-ray, MRI, or ultrasonography, which yield complementary information about a person’s anatomy and health condition.

Schematic of the measurement principle. At each tip position, the frequency shift is recorded as a function of the sample bias voltage (inset, red circles). The maximum of the fitted parabola (inset, solid black line) yields the KPFM signal V* for that position. Image courtesy of IBM Research - Zurich

“The technique provides another channel of information that will further our understanding of nanoscale physics. It will now be possible to investigate at the single-molecule level how charge is redistributed when individual chemical bonds are formed between atoms and molecules on surfaces. This is essential as we seek to build atomic and molecular scale devices,” explains Fabian Mohn of the Physics of Nanoscale Systems group at IBM Research – Zurich…..
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Microscopy, quantum-style: Atomic stacks imaged in real space

Illustration of QPCM on a Cu(111) surface. (A) Schematic model demonstrating the working principle of QPCM. The gray circles and arrows indicate the motion of the tip and the Cu atomic contact. (B) Conductance G as a function of tip approaching distance d acquired with the tip on top of a Cu adatom. (C) QPCM image with the same scan size as the inset in (B); the forward scan (from left to right) is shown. (D) Backward scan (from right to left) acquired simultaneously with the image shown in (C). (E) Constant current image of a step edge on Cu(111). Standing wave patterns originating from the surface state are clearly visible in the image. (F) QPCM image of the same area as shown in (E). The conductance decrease from top to bottom of the image is due to the plane in which the tip scans being slightly tilted with respect to the surface. Reprinted with permission from Quantum Point Contact Microscopy, Yong-hui Zhang et al., Nano Letters, July 26, 2011, Copyright © 2011 American Chemical Society

Since the first optical microscopes appeared in the late 1600s – an exact date and original inventor elude precise identification – microscopy has evolved dramatically. Scanning tunneling microscopy (STM), atomic force microscopy (AFM) and (although not generally recognized as an established method) point contact microscopy (PCM) allow scientists to view objects inaccessible to optical microscopes, with images of atoms now commonplace. Nevertheless, even this inexorable march towards ever-smaller scales has encountered limitations. (For example, STM does not provide information on local chemistry, while PCM cannot adequately image individual atoms due to it not having a single-atom contact.)….

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