Imaging the charge distribution within a single molecule

Journal name:
Nature Nanotechnology
Volume:
7,
Pages:
227–231
Year published:
DOI:
doi:10.1038/nnano.2012.20
Received
Accepted
Published online

Abstract

Scanning tunnelling microscopy and atomic force microscopy can be used to study the electronic and structural properties of surfaces, as well as molecules and nanostructures adsorbed on surfaces, with atomic precision1, 2, 3, 4, 5, 6, 7, but they cannot directly probe the distribution of charge in these systems. However, another form of scanning probe microscopy, Kelvin probe force microscopy, can be used to measure the local contact potential difference between the scanning probe tip and the surface, a quantity that is closely related to the charge distribution on the surface8, 9, 10, 11, 12. Here, we use a combination of scanning tunnelling microscopy, atomic force microscopy and Kelvin probe force microscopy to examine naphthalocyanine molecules (which have been used as molecular switches13) on a thin insulating layer of NaCl on Cu(111). We show that Kelvin probe force microscopy can map the local contact potential difference of this system with submolecular resolution, and we use density functional theory calculations to verify that these maps reflect the intramolecular distribution of charge. This approach could help to provide fundamental insights into single-molecule switching and bond formation, processes that are usually accompanied by the redistribution of charge within or between molecules14, 15, 16.

At a glance

Figures

  1. STM and AFM imaging of naphthalocyanine on NaCl(2 ML)/Cu(111).
    Figure 1: STM and AFM imaging of naphthalocyanine on NaCl(2 ML)/Cu(111).

    a, STM constant-current image (I = 2 pA, V = 0.25 V) recorded at the edge of a 2 ML NaCl island on Cu(111). A single naphthalocyanine molecule (yellow) and a single CO molecule (depression close to the NaCl step edge) can be identified. The crystallographic directions of the topmost NaCl layer are indicated. b, STM constant-current image of a naphthalocyanine molecule (I = 2 pA, V = 0.6 V). The image was recorded with a CO-terminated tip. The positions of the central hydrogens and the tautomerization path are highlighted in red, and the definition of the H-lobes and N-lobes is illustrated. c,d, Constant-height AFM frequency-shift images of the same molecule as in b, measured with a CO-terminated tip. The images were recorded at distances z = 0.145 nm (c) and z = 0.175 nm (d) above the height determined by the STM set point (I = 2 pA, V = 0.2 V) over the substrate. e,f, Cuts through the DFT-calculated electron density of a naphthalocyanine molecule at distances d = 0.2 nm (e) and d = 0.3 nm (f) from the molecular plane. g,h, Asymmetry of the calculated electron density at d = 0.1 nm (g) and d = 0.4 nm (h) from the molecular plane. Scale bars: 2 nm in a and 0.5 nm elsewhere. The DFT-calculated atomic positions are overlaid in b and in the upper halves of ch. Carbon, hydrogen and nitrogen atoms are in grey, white and blue, respectively.

  2. LCPD images of the tautomerization switching of naphthalocyanine.
    Figure 2: LCPD images of the tautomerization switching of naphthalocyanine.

    a, 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 V* and Δf* for that position. b,c, LCPD images of naphthalocyanine on NaCl(2 ML)/Cu(111) before (b) and after (c) switching the tautomerization state of the molecule. The images were recorded with a copper-terminated tip on a 64 × 64 lateral grid at constant height (z = 0.1 nm above the height determined by the STM set point (I = 3 pA, V = 0.2 V) over the substrate). d, Difference image obtained by subtracting c from b. e, DFT-calculated asymmetry of the z-component of the electric field above a free naphthalocyanine molecule at a distance d = 0.5 nm from the molecular plane. All scale bars: 0.5 nm. The DFT-calculated atomic positions are overlaid in the upper halves of be. Carbon, hydrogen and nitrogen atoms are in grey, white and blue, respectively.

  3. Enhanced resolution in LCPD images by tip functionalization with CO.
    Figure 3: Enhanced resolution in LCPD images by tip functionalization with CO.

    ah, LCPD images of naphthalocyanine on NaCl(2 ML)/Cu(111) measured with a CO-terminated tip. The images were recorded on a 40 × 40 lateral grid at constant height, for distances decreasing from a to h (z = 0.29, 0.27, 0.25, 0.23, 0.22, 0.21, 0.20 and 0.19 above the height determined by the STM set point (I = 2 pA, V = 0.2 V) over the substrate). The colour scale ranges from 240 mV (black) to 340 mV (white). i, LCPD image recorded with the same tip and imaging parameters as in h, but on a 92 × 92 lateral grid. j, DFT-calculated z-component of the electric field above a free naphthalocyanine molecule at a distance d = 0.3 nm from the molecular plane. All scale bars: 0.5 nm. The DFT-calculated atomic positions are overlaid in the upper halves of i and j. Carbon, hydrogen and nitrogen atoms are grey, white and blue, respectively.

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Affiliations

  1. IBM Research–Zurich, 8803 Rüschlikon, Switzerland

    • Fabian Mohn,
    • Leo Gross,
    • Nikolaj Moll &
    • Gerhard Meyer

Contributions

F.M., L.G. and G.M. performed the experiments. F.M. and N.M. carried out the DFT calculations. F.M. wrote the manuscript. All authors discussed the results and commented on the manuscript.

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The authors declare no competing financial interests.

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