Bond-Order Discrimination by Atomic Force Microscopy

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Science  14 Sep 2012:
Vol. 337, Issue 6100, pp. 1326-1329
DOI: 10.1126/science.1225621

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  1. Fig. 1

    Measurements on C60. (A) C60 model. The bonds fusing a pentagon and a hexagon (p) are of smaller bond order compared with the bonds fusing two hexagons (h). (Inset) STM image (sample bias V = 0.2 V, current I = 2 pA, size 24 by 24 Å2). The molecule and tip are identical to those in (B) to (F). (B to E) AFM measurements showing Δf at different tip heights z (27) above C60/Cu(111) using a CO-functionalized tip. Image size 10 by 10 Å2, oscillation amplitude A = 0.36 Å, V = 0 V. (F) Laplace-filtered and flattened image of (E), used to measure the apparent bond length L′ (22). (G) Line profiles Δf(x) across a p and h bond extracted from a three-dimensional (3D) force map (24). The position of the line profiles is indicated in the inset, showing a map of Δf at z = 3.6 Å, extracted from the same 3D force map. The apparent positions of the p and h bonds are indicated by the dotted lines. The x = 0 position corresponds to the molecular center, determined by the minimum of Δf(x) at z = 4.8 Å. Note that p is located at a smaller absolute value of x than h and that Δf(xp) is smaller than Δf(xh) for all plotted values of z, with the maximum difference for z = 3.7 Å.

  2. Fig. 2

    Density functional theory calculations on C60. Calculated interaction energy between CO and C60 at d = 2.9 Å (A) and electron density of C60 at 2.9 Å above the molecule (B); image size 4 by 4 Å2. Using the tip model shown in (C), Δf(x) line profiles along the dashed arrow in (A) were calculated with (solid lines) and without (thin dashed lines) relaxing the tip geometry, respectively (D). The relaxation resulted in a lateral displacement of the oxygen atom Δx(x), as shown in (E). The vertical gray lines in (D) and (E) indicate the positions of the p and h bonds as expected from the atomic model.

  3. Fig. 3

    Hexabenzocoronene model (A) and constant-height AFM measurements (A = 0.35 Å) on HBC on Cu(111) at z = 3.7 Å (B) and 3.5 Å (C). In (C), a pseudo-3D representation is shown to highlight the local maxima. (D) Calculated electron density at a distance of 2.5 Å above the molecular plane. Note that i bonds are imaged brighter (B) and shorter (C) compared with j bonds (22).

  4. Fig. 4

    Model (A) and constant-height AFM measurements of DBNP on bilayer NaCl on Cu(111) (33) at z = 3.6 Å (A = 0.48 Å) (B and C). A pseudo-3D representation of (B) is shown in (C) to highlight the bonds. Measured values of the frequency shift Δf (D) and the apparent bond length L′ (E) for indicated bonds, including HBC in (E), are plotted as a function of the Pauling bond order. (F) Apparent bond length as a function of the realistic bond length obtained by DFT calculations (for DBNP) (22) and from diffraction data (for HBC) (34). Linear regressions are drawn as a guide to the eye.