Probing Charges on the Atomic Scale by Means of Atomic Force Microscopy

Kelvin probe force spectroscopy was used to characterize the charge distribution of individual molecules with polar bonds. Whereas this technique represents the charge distribution with moderate resolution for large tip-molecule separations, it fails for short distances. Here, we introduce a novel local force spectroscopy technique which allows one to better disentangle electrostatic from other contributions in the force signal. It enables one to obtain charge-related maps at even closer tip-sample distances, where the lateral resolution is further enhanced. This enhanced resolution allows one to resolve contrast variations along individual polar bonds.

Figure 1

KPFS on grids on individual ${\mathrm{F}}_{12}{\mathrm{C}}_{18}{\mathrm{Hg}}_3$ and ${\mathrm{H}}_{12}{\mathrm{C}}_{18}{\mathrm{Hg}}_3$ molecules. (a) Ball models of ${\mathrm{F}}_{12}{\mathrm{C}}_{18}{\mathrm{Hg}}_3$ (left-hand side) and ${\mathrm{H}}_{12}{\mathrm{C}}_{18}{\mathrm{Hg}}_3$ (right-hand side) shown in the same orientation as the molecules under investigation. Carbon, mercury, hydrogen, and fluorine atoms are represented in black, gray, white, and blue, respectively. At each point of a dense grid over the molecules (schematically indicated by red dots), individual spectra are acquired. (b) Current image of the two molecules ($z=9.6\AA$ [22], $V=0.2\mathrm{V}$). (c) $\mathrm{\Delta }f$ image ($z=9.0\AA$). (d) Exemplary KPFS spectrum (blue) of one grid point with parabolic fit (pink) from which $V^{\star }$ is extracted (green). (e)–(h) $V^{\star }$ maps acquired at $z=12.0$, 10.1, 9.8, and 9.6 Å. The experimental data in (h) is partially overlaid with models to indicate the position of individual atoms. All scale bars are 5 Å.

Figure 2

Deducing intramolecular charge distribution from $z$ ramps. (a) Schematic illustration of the data acquisition procedure: For each lateral grid position, two KPFS parabolas at different distances ($\mathrm{I}\to \mathrm{II}$ and $\mathrm{III}\to \mathrm{IV}$) and two $\mathrm{\Delta }f(z)$ curves at different voltages ($\mathrm{II}\to \mathrm{III}$ and $\mathrm{IV}\to \mathrm{I}$) are recorded. (b) Residuals of the parabolic fit to KPFS data show just random noise for a large tip-sample spacing (orange) but systematic deviations for a short one (green). (Data points smoothed over 75 mV.) (c) Background subtracted $\mathrm{\Delta }f(z,V_i)$ curves along with their Lennard-Jones fits. (d) The difference spectrum $\mathrm{\Delta }\mathrm{\Delta }f(z,\mathrm{\Delta }V)$ (blue) is fitted by contributions from electrostatics (green) and vertical relaxation (pink), the sum of which is shown in black (the dashed gray line indicates zero). The green marker in (f) indicates the lateral position of the spectra shown in (a)–(d). (e)–(p) Comparison of dipole-distribution maps extracted from $\mathrm{\Delta }f(z,V_i)$ spectra [(h), (m), (p)] and conventional KPFS maps [(g), (l), (o)] at relatively large distances for PTCDA [(e)–(h)], ClAnCN [(i)–(m)], and ${\mathrm{F}}_{12}{\mathrm{C}}_{18}{\mathrm{Hg}}_3$ and ${\mathrm{H}}_{12}{\mathrm{C}}_{18}{\mathrm{Hg}}_3$ [(n)–(p)] [31].

Figure 3

Highly resolved dipole-distribution map. (a) $\mathrm{\Delta }f$ image recorded at $z=9.6\AA$. (b) Calculated charge distribution deduced from Bader analysis. (For details, see Table S1 in Ref. [22].) (c) Dipole-distribution map extracted from $\mathrm{\Delta }f(z,V_i)$ spectra for ${\mathrm{F}}_{12}{\mathrm{C}}_{18}{\mathrm{Hg}}_3$ and ${\mathrm{H}}_{12}{\mathrm{C}}_{18}{\mathrm{Hg}}_3$ ($9.6\AA \le z\le 10.1\AA$; $V_i=-0.2$ and 0.5 V).