Scanning Quantum Dot Microscopy

We introduce a scanning probe technique that enables three-dimensional imaging of local electrostatic potential fields with subnanometer resolution. Registering single electron charging events of a molecular quantum dot attached to the tip of an atomic force microscope operated at 5 K, equipped with a qPlus tuning fork, we image the quadrupole field of a single molecule. To demonstrate quantitative measurements, we investigate the dipole field of a single metal adatom adsorbed on a metal surface. We show that because of its high sensitivity the technique can probe electrostatic potentials at large distances from their sources, which should allow for the imaging of samples with increased surface roughness.

Figure 1

Working principle of SQDM. (a)–(c) Energy diagrams showing the QD attached to a scanning probe tip. (a) In the absence of a sample bias, a given level of the QD is occupied (QD charge state $N$). (b) When a critical sample bias $V^{-}$ is reached, one electron tunnels from the QD into the tip (QD charge state $N-1$). (c) If a local charge at the surface modifies the potential in the junction, the QD level shifts and becomes reoccupied (QD charge state $N$). (d) STM image of monolayer PTCDA islands on Ag(111). Here, and on all further images, a 5 nm scale bar is shown. (e) Const. height $\mathrm{\Delta }f$ image of the area in (d) recorded at $z_{\mathrm{tip}}=3\mathrm{nm}$ [for definition, see Fig. 2] and $V=-990\mathrm{mV}$. Prominently visible in red are lines where the QD changes its occupation between $N$ and $N-1$. The charge states of the QD in the different regions are labeled. (f) Same as (e), but recorded at $V=-910\mathrm{mV}$.

Figure 2

SQDM with a molecular QD and a NC-AFM. (a) Schematic view of the QD sensor: A single PTCDA molecule is chemically bonded to the AFM tip via a corner oxygen atom. The definitions of $z_{\mathrm{tip}},z$, and $d$ are indicated. The calibration of $z_{\mathrm{tip}}$ was performed as described in Ref. [16]. (b) Schematics of the energy level alignment of the PTCDA and NTCDA QD tips. (c) $\mathrm{\Delta }f(V)$ spectra taken with the PTCDA QD tip at $z_{\mathrm{tip}}=3\mathrm{nm}$ above the clean Ag(111) surface (blue) or above a PTCDA island (orange). The center of each dip determines $V^{-}$. The voltages used for scanning Figs. 1 and 1 are indicated at the top. (d) $\mathrm{\Delta }f(V)$ spectra, recorded with PTCDA (red) and NTCDA (blue) QD tips above the bare Ag(111) surface. The NTCDA spectrum is multiplied by 4. For the PTCDA spectrum, recorded at $z_{\mathrm{tip}}=26\AA$, the QD charge states $N-1$, $N$, and $N+1$ are indicated.

Figure 3

PTCDA on Ag(111): separation of topographic and electrostatic contrasts in SQDM. (a)–(c) Experimental $\mathrm{\Delta }V(x,y)$ maps (related to topography, cf. text) recorded with the PTCDA QD tip above an isolated PTCDA molecule adsorbed on the Ag(111) surface. (a) A white rectangle outlines the size of the PTCDA molecule. In the upper right corner, an enlarged structure formula, on which the quadrupolar charge distribution is indicated, is displayed. The inset in the bottom left corner shows an STM image [scale as in (a)]. (d)–(f) Experimental $V^{-}/\mathrm{\Delta }V(x,y)$ maps (related to electrostatic potential, cf. text) of the same area as in (a)–(c). Maps (a) and (d) were recorded at $z_{\mathrm{tip}}=24\AA$, (b) and (e) at 28 Å, and (c) and (f) at 36 Å. (g)–(i) Simulated electrostatic potential of adsorbed PTCDA at $z=16\AA$ (g), 22 Å (h), and 28 Å (i). The color scales in (d)–(i) were adjusted to optimize the contrast of each figure.

Figure 4

Smoluchowski dipole of an adatom: quantitative electrostatic potential measurements with SQDM. (a)–(c) Electrostatic potential maps $\mathrm{\Phi }(x,y)$ measured above a silver adatom on Ag(111) at $z=21\AA$ (a), 29 Å (b), and 37 Å (c). The scale bars show the absolute values of the electrostatic potential $\mathrm{\Phi }$, obtained as described in the text. Line profiles through the adatom are shown in white. (d) Comparison of the experimental ${\mathrm{\Phi }}^{\star }$ (blue), $\mathrm{\Phi }$ (red), and DFT-calculated (black) potentials vertically above the adatom. The red line shows a $1/(z-z_0{)}^2$ fit of the experimental data, where $z_0$ was obtained by fitting the DFT dataset. For the experimental data, $z$ is the distance between the point inside the QD, at which the electrostatic potential is measured, and the surface. For DFT, $z$ is the distance from the surface at which the potential of the adatom was calculated. (Inset) Constant height raw $\mathrm{\Delta }f$ image recorded at $z=6.3\mathrm{nm}$ with an applied bias of $V=9.6\mathrm{V}$, close to $V^{+}$ for this $z$. (Bottom left corner) STM image of the adatom.