Figure 1

Dynamic STM imaging of on-tip ${\mathrm{C}}_{60}$. (A) Schematic illustrations of the $\mathrm{Si}(111)-(7\times 7)$ reconstruction and the experimental tip-sample geometry. (B) STM images acquired before (upper), and after, the transfer of a ${\mathrm{C}}_{60}$ molecule (circled) to the tip. Note the change in the shape of the adatoms following the transfer of the ${\mathrm{C}}_{60}$ molecule. (C) Double-bond-down orientation. The experimental dSTM image [$V=+2.3\mathrm{V}$; $⟨I_t⟩=1\mathrm{nA}$; oscillation amplitude $(A_0)=1.5\mathrm{nm}$] of a silicon adatom (top of frame) comprises two lobes, as predicted by the Hückel theory simulation shown below the experimental data. At the bottom of this frame (and the other frames) we show a direct comparison between a single adatom feature imaged experimentally (left) and the Hückel theory prediction (right). (D) Single-bond-down orientation. dSTM scan parameters: $+1\mathrm{V}$, 100 pA, $A_0=2.25\AA$. (E) Pentagon-down orientation. dSTM scan parameters: $+2.3\mathrm{V}$, 1 nA, $A_0=1.5\mathrm{nm}$.Reuse & Permissions

Figure 2

Atomic resolution imaging of a single molecule probe. The ($7\times 7$) unit cell dimensions are highlighted by dashed lines in (A)–(C). (A) Pentagon-down orientation. (Scan parameters: $A_0=2\AA$, $\Delta f$ set $\mathrm{\text{point}}=-22.3\mathrm{Hz}$). (B) In this case only two atoms of the fullerene cage are observed—the molecule is oriented such that a single or a double bond is closest to the surface. ($A_0=2\AA$, $\Delta f=-46\mathrm{Hz}$). (C) Here there is a larger molecular tilt so that one atom of the ${\mathrm{C}}_{60}$ cage is preferentially imaged. Note also the neighbouring minima (dark “holes” in the image) which arise from the low tip-sample interaction at the centre of the pentagonal and hexagonal faces. ($A_0=2\AA$; $\Delta f=-137.4\mathrm{Hz}$). (D) Profile along the dashed line shown in (C). The inset is an FM-AFM image of the same area as (C) but acquired at a higher frequency set point ($-150\mathrm{Hz}$).Reuse & Permissions

Figure 3

Measuring and calculating the chemical force responsible for submolecular atomic contrast. (A) A comparison of an experimentally determined $F(z)$ curve (open circles) with force-displacement relationships calculated using DFT calculations for a silicon adatom directly below a carbon atom (dotted line) and a $\mathrm{C}\mathrm{C}$ single bond (dashed line). The arrows highlight a “jump-to-contact” event in the experimental and theoretical data, respectively. Insets: Left: Small section of Fig. 2a showing position of experimental spectrum in main figure. Right: Experimental $F(z)$ curves determined with a different tip and sensor for measurements above the primary maximum (squares) and a secondary lobe (circles) of a pentagonal adatom feature. We also include in the inset the $F(z)$ spectrum for a site at the center of the pentagonal face showing only a very weak attractive interaction before the repulsive regime of the ${\mathrm{C}}_{60}\mathrm{Si}$ potential is entered. (B) A charge density difference plot showing the formation of a $\mathrm{Si}\mathrm{C}$ bond at a tip-sample separation of 3.8 Å (see also Ref. 8). [This separation is highlighted with an arrow in (A)]. The red contours represent a charge density difference of $-0.005{\AA }^3$ (density depletion) and the blue contours a difference of $+0.005{\AA }^3$ (density excess). (C) A cartoon representation of the distortion of the ${\mathrm{C}}_{60}$ cage induced via the formation of a bond with a silicon adatom. The solid line represents the carbon atom and bond positions for the molecule at a position before the sharp jump in $F(z)$ [marked by the blue arrow in (A)]; the dotted lines represent the corresponding positions immediately after the jump in $F(z)$, corresponding to a ${\mathrm{C}}_{60}$ position 0.1 Å closer to the adatom.Reuse & Permissions

Figure 4

Determining the ${\mathrm{C}}_{60}{\mathrm{C}}_{60}$ pair potential. After confirming the ${\mathrm{C}}_{60}$ termination of the tip (lower inset, FM-AFM image with $A_0=2\AA$ and $\Delta f=-43\mathrm{Hz}$), a buckminsterfullerene molecule adsorbed on the $\mathrm{Si}(111)-(7\times 7)$ surface was located (upper inset, $\Delta f=-18.5\mathrm{Hz}$) and $\Delta f(z)$ spectra measured at the positions shown in the upper inset. From these measurements we extracted the ${\mathrm{C}}_{60}$ pair potential at various positions on the surface-adsorbed molecule. For comparison we have also plotted the analytical Girifalco potential for the ${\mathrm{C}}_{60}{\mathrm{C}}_{60}$ intermolecular interaction (solid line) with no adjustment or fitting other than an alignment of the minima of the experimental and theoretical curves.Reuse & Permissions