Influence of band structure on ballistic transport revealed by molecular nanoprobe

In this study we characterize the tautomerization of HPc on Cu(111) as a charge-carrier-induced reversible one-electron process. An analysis of the bias-dependent tautomerization rate finds an energy threshold that corresponds to the energy of the N-H stretching mode. By using the tautomerization of the molecule as a detector for charge carrier transport in the so-called molecular nanoprobe (MONA) technique, we provide evidence for an inhomogeneous coupling between the fourfold-symmetric molecule and sixfold-symmetric surface. We conclude the study by comparing the energy dependence of charge carrier transport on the Cu(111) to the Ag(111) surface. While the MONA technique is limited to the detection of hot-electron transport for Ag(111), our data reveal that the lower onset energy of the Cu surface state also allows for the detection of hot-hole transport. The influence of surface and bulk transport on the MONA technique is discussed.

Figure 1

(a) Tunneling spectrum of the Cu(111) surface (black line). As indicated by the gray parabola, the steplike increase of the $dI/dU$ signal at $E-E_{\mathrm{F}}=eU\approx 450$ meV represents the onset of the Cu(111) surface state. The threshold energy for HPc tautomerizations which amounts to $E_{\mathrm{th}}=408$ meV is exceeded in the red-shaded regime. (b) Schematic structure of HPc and the Cu(111) surface (carbon: gray; nitrogen: green; hydrogen: white). (c) Topographic scans of the four different tautomers of HPc on Cu(111) ($U=50$ mV, $I=50$ pA). White arrows indicate the binding position of the central hydrogen. (d) Telegraph noise measured with the tip positioned at the green cross in panel (c). The four distinct height levels are labeled with the corresponding tautomeric state.

Figure 2

(a) Current-dependent tautomerization rate of HPc on Cu(111) ($U=-500$ mV). The linear fit with a slope of $m=1.062\pm 0.043$ confirms that the tautomerization is a one-electron process. (b) Injection-energy-dependent HPc tautomerization yield on Cu(111) and Ag(111). The data are fitted with multiple energy thresholds. Literature values of the N-H stretching mode taken from Refs. [27, 28] are indicated by vertical lines at $\pm 408$ and $\pm 816$ meV.

Figure 3

(a) STM topography image ($U=50\mathrm{mV}, I=50\mathrm{pA}$) of a HPc molecule on Cu(111). The injection points are marked as green stars in a distance $d$ from the molecular center. (b) MONA electron yield (injection energy $E=900\mathrm{meV}$) of the HPc detector molecule. A 20% difference is measured between $\alpha =0^{\circ }$ and $\alpha ={90}^{\circ }$, caused by the twofold symmetry of the surface-molecule system. To better visualize the symmetries, measured (black) data points are mirrored (gray) along the $\left[01\overline{1}\right]$ and $\left[2\overline{1}\overline{1}\right]$ axes and superimposed with the schematic structure of the molecule and surface.

Figure 4

Injection-energy-dependent tautomerization rates of HPc on Cu(111) (blue and black black data points indicating different STM tips) and Ag(111) (red) measured with MONA at a tip-sample distance $d=4$ nm along the closed-packed $\left[01\overline{1}\right]$ axis [$0^{\circ }$ in Fig. 3]. Two colored parabolas illustrate the energy range where the Cu(111) and the Ag(111) surface states exist. Their onset energies amount to $E_{\mathrm{on}}^{\mathrm{Cu}}=-450$ meV and $E_{\mathrm{on}}^{\mathrm{Ag}}=-63$ meV, respectively.

Figure 5

Scheme of the tunneling process for bias voltages of (a) $U=-450$ mV and (b) $U=-600$ mV. The surface state of Cu(111) is represented by an upward opened blue parabola; the bulk states are given by the blue shaded area. The length of the arrows symbolizes the amount of tunneling electrons at the given energy. The resulting holes are shaded for bulk states and nonshaded for surface states. States below the threshold energy $E_{\text{th,1}}=-408$ meV are highlighted in gray. For comparison the surface state of Ag(111) is shown in red.

Figure 6

(a) Topograpic scans of tautomers 2 and 3 ($U=50$ mV, $I=50$ pA). White arrows indicate the binding position of the central hydrogen. For better distinction line profiles along the green lines (black for 2, red for 3) are shown in (b). The elongation of arms along the $\left[01\overline{1}\right]$ axis can be seen in the overlap of tautomers in (c). The relevant center of the molecule is enlarged.