Energetic and Spatial Mapping of Resonant Electronic Excitations

Jun 2, 2016 - ABSTRACT: In this article, we present a detailed study of the rotational movement of transition metal phthalocyanine molecules ((TM)Pc) ...
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Energetic and Spatial Mapping of Resonant Electronic Excitations Andreas Krönlein,† Jens Kügel,*,† Konstantin A. Kokh,‡,¶,§ Oleg. E. Tereshchenko,¶,§,∥ and Matthias Bode†,⊥ †

Physikalisches Institut, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences 630090 Novosibirsk, Russia ¶ Novosibirsk State University, 630090 Novosibirsk, Russia § Saint-Petersburg State University, 198504 Saint-Petersburg, Russia ∥ A. V. Rzanov Institute of Semiconductor Physics, Siberian Branch, Russian Academy of Sciences 630090 Novosibirsk, Russia ⊥ Wilhelm Conrad Röntgen-Center for Complex Material Systems (RCCM), Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany ‡

ABSTRACT: In this article, we present a detailed study of the rotational movement of transition metal phthalocyanine molecules ((TM)Pc) on topological insulators by means of scanning tunneling microscopy and spectroscopy. Our data taken on two different systems, namely FePc on Bi2Te3 and MnPc on Sb2Te3, reveal that the energetic onset of the rotational motion coincides with the energetic position of the lowest unoccupied molecular orbital (LUMO). The strong correlation between molecular motion and tunneling into molecular orbitals is corroborated by a very good agreement between the spatial distribution of the switching frequency and the differential conductance map of the LUMO. These results lead to the conclusion that the driving mechanism of the rotational motion is resonant electronic excitation, which we were able to resolve energetically and spatially.

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INTRODUCTION Single particle manipulation offers unique possibilities to tune electronic properties of molecular and atomic assemblies and thereby opens new paths for future electronic devices.1,2 Hence, the understanding of the underlying physical mechanisms that lead to and allow for the deliberate motion of adsorbates is of significant importance. It is generally envisioned that the detailed understanding of the underlying processes will allow for the controlled assembly of artificial molecular structures and devices in terms of the bottom-up approach.3 Although there exist numerous examples where single atoms and small molecules have been routinely manipulated by applying an electrostatic potential to overcome the adsorption energy and then carry it with a STM tip to the desired location,4 the same approach turned out to be much more delicate for larger molecules. Depending on the particular molecular species and the chemical nature of the substrate different approaches for lateral manipulation have been employed, such as pushing the molecule in the constant-height mode5 or injecting inelastically tunneling electrons into the molecules.6 In addition to lateral manipulation the controlled rotation of molecules may represent another possibility to realize molecular switches in nanoscale electronics.7−10 The rotation process is usually induced by the excitation of vibrational modes. Two fundamentally different mechanisms have been discussed in literature: The molecular rotation process may (i) © 2016 American Chemical Society

either be achieved by dipolar coupling of the tunneling electrons11 or (ii) by resonant electron excitation.12 While the former is driven by the electric field of the tunneling electron and its image charge which couple to the molecule’s dynamic dipole moment,13 the latter process is induced by electrons that tunnel into an unoccupied molecular orbital, thereby forming a negatively charged molecular species. As the charge is transiently trapped the molecule relaxes into a new equilibrium state. Eventually, the charged intermediate molecular state is released as the electron jumps from the molecular orbital into an unoccupied state of the substrate, a process which is often accompanied by a vibrational excitation.14 The reverse process, i.e., resonant electronic excitation of an occupied molecular orbitals, is also possible, thereby forming a hole-like intermediate state.15 The resonant electronic excitation mechanism is usually identified by comparing the energetic position of molecular orbitals with the energetic onset of molecular excitation, which is either done by action spectroscopy16−18 or by inelastic electron tunneling spectroscopy (IETS).19 While in the case of action spectroscopy, molecular excitations are mapped indirectly by analyzing the energy-dependent time trace of molecular motion or switching processes,7 the energy of Received: May 23, 2016 Revised: May 31, 2016 Published: June 2, 2016 13843

DOI: 10.1021/acs.jpcc.6b05177 J. Phys. Chem. C 2016, 120, 13843−13849

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excitations can directly be detected in IETS as a step-like increases in the differential conductance.20 In addition to the determination of energy thresholds, both methods were also used to visualize the spatial distribution of excitations such as the vibrational excitation of Cu(II) etioporphyrin-I on Cu(001) 21 and O 2 molecules on Ag(110),22 the rotational motion of CuPc on Cu(111),23 or for the hydrogen tautomerization of naphthalocyanine molecules on a NaCl(100) bilayer on Cu(111).24 However, the direct correlation which compares the spatial distribution of the excitation with the spatial distribution of the molecular orbital has not yet been achieved. In fact, in the case of a resonant electronic excitation one would expect that the electron involved in the excitation process tunnels into a molecular orbital, such that the spatial distribution of the excitation should be identical or at least very similar to a differential conductance map of the orbital. Here, we present a detailed study of the rotational motion of transition metal phthalocyanines on topological insulators, namely FePc on Bi2Te3 and MnPc on Sb2Te3. Topological insulators are suitable substrates not because of their peculiar electronic and transport properties but rather because they consist of quintuple layers which are bound by van der Waals forces. This weak interaction between adjacent unit cells allows for the easy cleaving of TIs and results in phthalocyanine molecules which are not chemi- but only physisorped. As will be shown below Pc molecules are admitted to rotate around their central transition metal ion on many TI surfaces, a behavior which is usually not observed for chemisorption, e.g., Pcs on metal surfaces. In these molecule-topological insulator systems the energetic onset of the rotational movement is found to match very well with the energetic position of the respective LUMO. The correlation of the rotational motion of the molecules with molecular orbitals is further supported by the fact that the spatial distribution of the switching probability overlaps very well with the differential conductance map of the LUMO. Therefore, resonant electronic excitation is unambiguously identified as the driving mechanism of the rotational movement of transition metal phthalocyanine ((TM)Pc) molecules on topological insulator surfaces.

Article

RESULTS AND DISCUSSION

It is well-known that the electronic properties of adatoms and molecules crucially depend on the substrate’s crystallographic and chemical quality. To safely exclude any unwanted influence of substrate imperfections in our study, every molecule to be investigated was initially moved to a location far away from any defect. For example, it is not clear a priori whether the FePc molecule imaged in the topographic image of Figure 1a is adsorbed on defect-free Bi2Te3. Two typical defects, which probably represent an antisite and a vacancy defect,27 are marked by white arrows. After completion of the scan shown in

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MATERIALS AND METHODS MnPc (Strem Chemicals; 97% initial purity) and FePc (Alfa Aesar; 96% initial purity) molecules were purified by sublimation in a tube furnace followed by extended degassing in UHV environment. Topological insulator single-crystals (pdoped) were grown using a modified Bridgman technique. A more detailed description of the growth procedures can be found elsewhere.25,26 Surface generation and molecule deposition was conducted in a UHV chamber with a base pressure p ≤ 5 × 10−11 mbar. The topological insulators were cleaved in situ at room temperature, immediately followed by the evaporation of (TM)Pc molecules. Within 2 min after preparation the sample was transferred into a home-build lowtemperature scanning tunneling microscope (LT-STM) operating at a temperature of T ≈ 4.5 K. Topographic images were measured in the constant current mode with the bias voltages U applied to the sample. In order to obtain the scanning tunneling spectroscopy (STS) data, the differential conductance dI/dU was measured with lock-in technique by adding a small voltage modulation at a frequency of f = 811.7 Hz.

Figure 1. STM images of a single FePc molecule on Bi2Te3 before (a) and after (b) manipulation along the path marked blue (scale bar = 5 nm). Although occasional defects can be found on the Bi2Te3 surface (white arrows), this procedure guarantees that the molecule is eventually located at a defect-free destination. (c) FePc molecule imaged at 1.6 V ≤ U ≤ 2.1 V (I = 50 pA). Discontinuities along the fast scan (horizontal) direction indicate the rotational movement of the molecule. Obviously, the rotation frequency increases as the bias voltage is raised (scale bar = 1 nm). (d) Topography scans of the three different orientations (labeled A, B, and C) of a FePc molecule on Bi2Te3 [I = 70 pA, U = 1.2 V and same frame size as part c]. (e) Temporal evolution of the tip height at U = 1.6 V has been recorded with the tip parked at the position of the black dot. The three different orientations can clearly be distinguished based on the different tip heights. 13844

DOI: 10.1021/acs.jpcc.6b05177 J. Phys. Chem. C 2016, 120, 13843−13849

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Figure 2. (a) Bias voltage dependence of the rotation rate f rot for FePc/Bi2Te3. Inset: Zoom into the gray-shaded area at the onset of rotational switching. For comparison the differential conductance dI/dU as measured with the STM tip positioned above the isoindole group is displayed (blue points, stabilization parameters: I = 50 pA, U = 2 V). Two peaks can be recognized at about 1.2 and 1.9 V, corresponding to an interface resonance (IR) and the LUMO, respectively. Obviously, the threshold for rotational switching coincides with the onset of the LUMO. (b) Topography and (c) dI/dU map of an FePc cluster at U = 1.9 eV. The 4-fold symmetric intensity surrounding the molecules is consistent with the LUMO’s 2eg symmetry (scale bar = 4 nm). (d) Topography and (e) rotation rate f rot measured at U = 1.9 V, corresponding to the binding energy of the LUMO (scale bar = 1 nm). The absence of the four-lobed structure characteristic for (TM)Pc molecules indicates numerous rotational switching events. f rot exhibits a marked annulus-shaped structure. (f) Topography and (g) dI/dU map of FePc taken at U = 1.9 V (scale bar = 1 nm). The LUMO of the rotating molecule is localized in an annulus-shaped region of enhanced dI/dU signal. As marked by blue circles in both parts e and g, f rot and the dI/dU signal are enhanced within the same region.

The resulting one-by-one correlation between the tip height and the molecule orientation can be used to distinguish the rotational states A, B, and C without the need of scanning the complete (TM)Pc molecule. Figure 1e displays the temporal evolution of the tip height as measured on a FePc molecule at U = 1.6 V at the position marked by black dots in Figure 1d. Obviously, the tip height exhibits three different levels with abrupt changes. This technique allows for an accurate determination of the rotation rate f rot as obtained by dividing the number of rotational switching events n by the probing time. Typically, measurements were performed for a duration of 12 min. Since we expect the events to be Poisson distributed the error bar is derived from the standard deviation given by n1/2. FePc on Bi2Te3. Figure 2a summarizes our experiments performed to understand the bias voltage dependence of the rotation rate f rot of FePc adsorbed on Bi2Te3. All data were taken on the same molecule with the tip positioned at the same nominal position with respect to the central metal ion. The results show a negligible rotation rate ( f rot ≈ 0) for negative and small positive bias voltages. The threshold bias voltage required for molecule rotation is determined to UTh ≈ 1.5 V. Such a large threshold bias clearly speaks against a rotation process which is caused by directly exciting molecular vibration modes. For example, the threshold energy associated with hindered rotations and stretching modes detected in earlier IETS and Raman scattering experiments is much lower (typically several tens of millielectronvolts).31−33 Instead, a UTh value beyond 1 eV is consistent with a resonant electronic excitation as previously observed for biphenyl molecules adsorbed on a Si(100) surface.17

Figure 1a, the molecule was deliberately moved laterally (tunneling parameters during manipulation: I = 3···10 nA, U = 1 V) by about 3 nm to a location which showed no hint of any defect in Figure 1a. The final result with the molecule safely positioned on clean Bi2Te3 is shown in Figure 1b. Similar to observations made on several other substrates9,28 we observe discrete rotational instabilities of isolated molecules around the central transition metal ion as the sample bias U is enhanced above a threshold value. Typical data for a FePc molecule on Bi2Te3 acquired at bias voltages 1.6 V ≤ U ≤ 2.1 V and a set-point current I = 50 pA are shown on Figure 1c. Rotation events appear as discontinuities along the fast scan (horizontal) direction. Inspection of Figure 1c reveals that above the threshold bias voltage UTh the rate of rotational switching events f rot, referred to as rotation rate hereafter, increases with increasing U within the bias range covered by Figure 1c. As f rot approaches the horizontal scanning frequency (about 1 Hz) rotation events are so frequented that the usual cross-like shape of the molecule is blurred to an almost circular symmetric appearance. Imaging (TM)Pc molecules at U < UTh reveals that they adsorb in three orientations (termed A, B, and C) which are rotated by 120° (see Figure 1d). This behavior follows from the 6-fold symmetry of the substrate in combination with the 4-fold symmetry of the molecule.29 In order to quantify the rotation rate f rot between these states we used a simplified version of the so-called scanning noise microscopy.30 For this purpose we positioned the STM tip at a position in between the highsymmetry axes of any of the three orientations as indicated by black dots in Figure 1d. 13845

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Figure 3. (a) Voltage dependence of the rotation rate of MnPc on Sb2Te3. Inset: Rotation rate (blue) and STS data in the voltage range between the Fermi level (U = 0 V) and U = 2 V. dI/dU peaks at U = 0.7 V and U = 1.0 V correspond to the LUMO and LUMO+1, respectively. (b) Topography and (c) dI/dU map (U = 0.7 V) of a MnPc molecule stabilized by a nearby molecule (scale bar = 1 nm). (d) Topography and (e) rotation rate of an isolated MnPc molecule (scale bar = 1 nm). Comparison with the corresponding (f) topography and (g) dI/dU map confirms that rotational motion is induced by electrons tunneling from the tip into the LUMO (scale bar = 1 nm).

FePc on Bi2Te3 is expected at a binding energy just below the Fermi level EF. Unfortunately, this expectation could not be verified by STS data which is essentially featureless in the voltage range of interest (data not shown here). We also never observed any rotational motion at negative bias voltage. As we will discuss in more detail below this is probably caused by the low binding energy of the HOMO in FePc molecules on Bi2Te3. So far we have shown that the threshold bias voltage for the tunneling current-induced rotational motion of FePc molecules on Bi2Te3 agrees well with the onset of the molecule’s LUMO. In order to strengthen the apparent causality, we have performed additional spatially resolved measurements. They were conducted by measuring the rotation rate f rot and the differential conductance dI/dU of single molecules within a 3 nm × 3 nm scan frame at a 15 × 15 x−y-grid pixel resolution. Even though a higher resolution might be desirable these values represent a reasonable compromise between resolution and acquisition time per pixel which eventually determines the statistical standard deviation. Figure 2 shows (d) the topography and (e) a color-coded map of the rotation rate of an FePc molecule on Bi2Te3 measured at a bias voltage U = 1.9 eV, i.e., close to the binding energy of the LUMO found in the inset of Figure 2a. While the ligands of (TM)Pc molecules typically lead to a characteristic four-lobed appearance in topographic images,29,36,37 Figure 2d manifests an almost perfect circularsymmetric protrusion, indicating the presence of numerous rotational movements without any translational motion and at negligible piezo drift. Interestingly, f rot peaks within an annulusshaped region at a radius of about 1 nm from the central binding site. For comparison we have performed similar experiments to record the appearance of the LUMO for an individual FePc molecule. Parts f and g of Figure 2 display the topography and the dI/dU map as obtained at the LUMO energy of U = 1.9 V,

To assess this hypothesis we compare the observed biasdependent rotation rate with STS data taken with the probe tip positioned above the FePc ligand. As can be recognized in the inset of Figure 2a, the tunneling spectrum (blue data points) exhibits two relatively broad but clearly distinct peaks which are energetically located at about 1.2 and 1.9 V above the Fermi level. On the basis of previous investigations29 these features can be identified with an interface resonance (IS), i.e., a hybrid molecule−surface state and the lowest unoccupied molecular orbital (LUMO) of FePc, respectively. While no rotational motion was observed around bias voltages where the IS is found, the onset of an LUMO corresponds very well with UTh = 1.5 V. The identification of the STS feature at 1.9 V with the LUMO is also corroborated by spatially resolved dI/dU maps taken at the same bias voltage. Since the tunneling currentinduced rotation inhibits the stable imaging of a single molecule, the experiment has been performed on the cluster displayed in Figure 2b. It consists of several molecules which stabilize each other in their respective adsorption sites. The simultaneously recorded dI/dU map is shown in Figure 2c. It exhibits a 4-fold symmetric high intensity signal surrounding each molecule, as expected for the 2eg symmetry of most (TM)Pc LUMOs.29,34 Interestingly, the rotation rate f rot markedly decreases as the bias voltage is raised to values well above the position of the LUMO. This may point to the resonant character of the rotation excitation process. We speculate that at bias voltages exceeding the LUMO energy electrons relax into final states which less likely induce the molecular rotation, thereby strongly reducing the frequency of rotational events. This explanation is also consistent with the absence of rotation events at bias voltages corresponding to the IS which is strongly hybridized with the substrate. It is well-known that the highest occupied molecular orbital (HOMO) is typically located around 2 eV below the LUMO.29,35 On the basis of this argument the HOMO of 13846

DOI: 10.1021/acs.jpcc.6b05177 J. Phys. Chem. C 2016, 120, 13843−13849

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which for MnPc/Sb2Te3 coincidentally occurs at the same binding energy as the LUMO. Again we have performed spatially resolved measurements of the rotation rate f rot and the differential conductance dI/dU at a bias voltage that corresponds to the binding energy of the LUMO, i.e. U = 0.7 V. The results obtained on an isolated MnPc molecule on Sb2Te3 are displayed in Figure 3d−g. Also, for this combination of a (TM)Pc molecule adsorbed onto a topological insulator surface, we can recognize annulus-shaped regions with enhanced signal intensity for both, the rotation rate map (Figure 3e) and the dI/dU (Figure 3g). Within the measurement accuracy of about 2 Å their respective inner and outer diameters are identical. Therefore, these data fully confirm our above-mentioned hypothesis of a molecule rotation triggered by the relaxation of electrons injected into LUMO in (TM)Pc molecules. It remains to be solved if the relaxation of other molecular or hybrid substrate-molecule states at a binding energy of 0.7 eV above the Fermi level may also lead to molecule rotation events. For example, in addition to the off-center dI/dU signals in Figure 3c indicate LUMO, an enhanced dI/dU intensity can also be seen in the central region of the MnPc molecule. It potentially arises from the central transition metal ion’s dz2 -orbital or indicates an enhanced tunneling probability into conduction band onset of the Sb2Te3 substrate. Since our experimental approach relies on the detection of tip height variations at the ligand position it is insensitive, however, to rotational switching events as the tip is parked over the molecule center. Our spatially resolved data of Figure 3e show a continuously decreasing rotation rate as the STM tip is moved from the annulus toward the position of the TM ion, thereby indicating that the two mechanisms mentioned above are insignificant. To obtain better data, however, further experiments are required where the excitation and detection of rotational switching events are performed at different locations. Another difference with respect to FePc on Bi2Te3 is the observation of a nonzero tunneling current-induced rotation rate on MnPc/Sb2Te3 at negative bias voltages U = −1.2 V and around U ≈ −2.0 V. Even though f rot is at least 1 order of magnitude lower than the corresponding rates at positive bias voltages the signal is found to be statistically relevant. We speculate that the signal at U = −1.2 V most likely originates from the HOMO of MnPc. As already mentioned above, the HOMO of (TM)Pc molecules is typically found about 2 eV below the LUMO, in rough agreement with the situation observed for MnPc on Sb2Te3. Possibly, the different rotational switching behavior of FePc on Bi2Te3 as compared to MnPc on Sb2Te3 is caused by their strongly different HOMO binding energies. While it is expected to be very low for FePc on Bi2Te3 (just below EF based on the position of the LUMO) the removal of an electron from the HOMO of MnPc on Sb2Te3 represents a much stronger distortion. Therefore, it appears quite reasonable that the following relaxation process potentially induces a molecular rotation event.

respectively. Again, the circular-symmetric topography indicative for molecule rotation and a annulus-shaped enhanced dI/ dU signal can be recognized. To highlight the close correlation between the radial position of the LUMO and the locations where rotational switching occurs two annular-shaped regions have been marked by two identical sets of blue circles in Figure 2, parts e and g. Because of the molecule’s rotation at the applied bias voltage, no statement can be made about angular variations. The observed fluctuations are within the error bar expected for Poisson statistic. MnPc on Sb2Te3. To exclude an accidental coincidence between the LUMO energy and the threshold for rotational motion in the case of FePc on Bi2Te3 we have conducted analogous measurements for another (TM)Pc molecule, i.e. MnPc. Because of strong coupling between MnPc and the Bi2Te3 substrate,29 the molecules showed no rotational motion. For this reason, we performed the measurements on the surface of a different topological insulator, namely Sb2Te3. Similar to FePc on Bi2Te3 the MnPc molecules on Sb2Te3 also exhibit rotational motion at certain bias voltages. Furthermore, translational movement is absent for sufficiently low set point currents (I ≤ 70 pA). Figure 3a displays a plot of the bias voltage-dependent rotation rate f rot. No rotational switching events, i.e., f rot = 0, were observed in the voltage range −1.0 V ≤ U ≤ + 0.6 V. In the positive bias range the threshold for molecule rotation amounts to UTh ≈ + 0.7 V. As U is enhanced beyond this value, f rot first rises rapidly, but then decreases for U > 1.5 V, a behavior also observed for FePc on Bi2Te3. In contrast to FePc on Bi2Te3, however, for MnPc on Sb2Te3, molecule rotation was also observed at negative bias voltage. Although the rotation rate is much lower than for positive bias voltage, weak maxima can clearly be recognized at about U = −1.2 V and U = −2.0 V. In analogy to Figure 2a, the inset of Figure 3a shows the voltage range around the pronounced onset of molecule rotation at UTh ≈ + 0.7 V. Within the same plot blue data points represent STS data measured with the probe tip positioned above the ligand. Three peaks can be recognized which originate from the IS (U = 0.2 V), the LUMO (U = 0.7 V), and the LUMO + 1 (U = 1.0 V). In agreement with the observations made for FePc on Bi2Te3, it is found that also for MnPc on Sb2Te3 the threshold bias UTh coincides with the LUMO energy. Tunneling into the IS at U = 0.2 V does not trigger any molecular rotation. It appears that the f rot-peak is wider and possibly exhibits a shoulder at about U = 1.0 V. This may be caused by the fact that for MnPc, in contrast to FePc, tunneling, not only into the LUMO but also into the LUMO + 1, leads to rotation events. The assignment of the differential conductance peak at U = 0.7 V to the LUMO is corroborated by the simultaneously acquired topography and dI/dU map displayed in Figure 3, parts b and c, respectively. In this case molecule rotation is probably hindered by the presence of a nearby molecule partially visible in the bottom right corner of Figure 3b. The dI/ dU map shown in Figure 3c exhibits four slightly outward curved segments of enhanced differential conductance at a distance of about 0.8 nm from the molecule center. This appearance is characteristic for the 2eg symmetry of the LUMO in most (TM)Pc molecules.38 In addition, an enhanced dI/dU signal is also observed in the center of the molecule. We speculate that this signal originates from the Mn dz2 -orbital

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CONCLUSION In summary, the rotational movement of FePc on Bi2Te3 and MnPc on Sb2Te3 was investigated by scanning tunneling microscopy and spectroscopy. It has been shown by direct correlation that the rotational motion of these systems is induced by electrons tunneling into molecular orbitals. This finding is evidenced by the detailed comparison of rotational molecular motion with the LUMO orbital regarding their 13847

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(9) Stipe, B. C.; Rezaei, M. A.; Ho, W. Inducing and Viewing the Rotational Motion of a Single Molecule. Science 1998, 279, 1907− 1909. (10) Stipe, B. C.; Rezaei, M. A.; Ho, W. Coupling of Vibrational Excitation to the Rotational Motion of a Single Adsorbed Molecule. Phys. Rev. Lett. 1998, 81, 1263−1266. (11) Persson, B. N. J.; Demuth, J. E. Inelastic Electron Tunnelling from a Metal Tip. Solid State Commun. 1986, 57, 769−772. (12) Gata, M. A.; Antoniewicz, P. R. Resonant Tunneling Through Adsorbates in Scanning Tunneling Microscopy. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 13797−13807. (13) Roy, S.; Mujica, V.; Ratner, M. A. Chemistry at Molecular Junctions: Rotation and Dissociation of O2 on the Ag(110) Surface Induced by a Scanning Tunneling Microscope. J. Chem. Phys. 2013, 139, 074702. (14) Persson, B. N. J.; Baratoff, A. Inelastic Electron Tunneling from a Metal Tip: The Contribution from Resonant Processes. Phys. Rev. Lett. 1987, 59, 339−342. (15) Salam, G. P.; Persson, M.; Palmer, R. E. Possibility of Coherent Multiple Excitation in Atom Transfer with a Scanning Tunneling Microscope. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 49, 10655−10662. (16) Huang, T.; Zhao, J.; Feng, M.; Popov, A. A.; Yang, S.; Dunsch, L.; Petek, H. A Multi-State Single-Molecule Switch Actuated by Rotation of an Encapsulated Cluster within a Fullerene Cage. Chem. Phys. Lett. 2012, 552, 1−12. (17) Lastapis, M.; Martin, M.; Riedel, D.; Hellner, L.; Comtet, G.; Dujardin, G. Picometer-Scale Electronic Control of Molecular Dynamics Inside a Single Molecule. Science 2005, 308, 1000−1003. (18) Henzl, J.; Morgenstern, K. Reaction Spectroscopy of Frontier Orbitals. J. Phys. Chem. C 2011, 115, 10056−10062. (19) Qiu, X. H.; Nazin, G. V.; Ho, W. Vibronic States in Single Molecule Electron Transport. Phys. Rev. Lett. 2004, 92, 206102. (20) Stipe, B. C.; Rezaei, M. A.; Ho, W. Single-Molecule Vibrational Spectroscopy and Microscopy. Science 1998, 280, 1732−1735. (21) Wallis, T. M.; Chen, X.; Ho, W. Single Molecule Vibrational Spectroscopy and Microscopy: Cu(II) Etioporphyrin-I on Cu(001). J. Chem. Phys. 2000, 113, 4837−4839. (22) Hahn, J. R.; Lee, H. J.; Ho, W. Electronic Resonance and Symmetry in Single-Molecule Inelastic Electron Tunneling. Phys. Rev. Lett. 2000, 85, 1914−1917. (23) Schaffert, J.; Cottin, M. C.; Sonntag, A.; Karacuban, H.; Bobisch, C. A.; Lorente, N.; Gauyacq, J.-P.; Möller, R. Imaging the Dynamics of Individually Adsorbed Molecules. Nat. Mater. 2013, 12, 223−227. (24) Liljeroth, P.; Repp, J.; Meyer, G. Current-Induced Hydrogen Tautomerization and Conductance Switching of Naphthalocyanine Molecules. Science 2007, 317, 1203−1206. (25) Kokh, K. A.; Makarenko, S. V.; Golyashov, V. A.; Shegai, O. A.; Tereshchenko, O. E. Melt Growth of Bulk Bi2Te3 Crystals with a Natural p-n Junction. CrystEngComm 2014, 16, 581−584. (26) Sessi, P.; Storz, O.; Bathon, T.; Wilfert, S.; Kokh, K. A.; Tereshchenko, O. E.; Bihlmayer, G.; Bode, M. Scattering Properties of the Three-Dimensional Topological Insulator Sb2Te3: Coexistence of Topologically Trivial and Nontrivial Surface States with Opposite Spin-Momentum Helicity. Phys. Rev. B: Condens. Matter Mater. Phys. 2016, 93, 035110. (27) Bathon, T.; Achilli, S.; Sessi, P.; Golyashov, V. A.; Kokh, K. A.; Tereshchenko, O. E.; Bode, M. Experimental Realization of a Topological p-n Junction by Intrinsic Defect Grading. Adv. Mater. 2016, 28, 2183−2188. (28) Nacci, C.; Erwin, S. C.; Kanisawa, K.; Fölsch, S. Controlled Switching within an Organic Molecule Deliberately Pinned to a Semiconductor Surface. ACS Nano 2012, 6, 4190−4195. (29) Sessi, P.; Bathon, T.; Kokh, K. A.; Tereshchenko, O. E.; Bode, M. Probing the Electronic Properties of Individual MnPc Molecules Coupled to Topological States. Nano Lett. 2014, 14, 5092−5096. (30) Schaffert, J.; Cottin, M. C.; Sonntag, A.; Karacuban, H.; Utzat, D.; Bobisch, C. A.; Möller, R. Scanning Noise Microscopy. Rev. Sci. Instrum. 2013, 84, 043702.

respective spatial distribution and energy. Our results indicate that molecule rotation processes can be initiated most effectively for source/drain electrodes positioned at maxima of the LUMO density of states. For sufficiently high HOMO binding energies it is found that electron removal may also trigger molecular rotation, although this process appears to be about an order of magnitude less efficient. We believe that the tunneling-induced mechanisms leading to molecular rotation identified here are not restricted to topological insulators but potentially to a wider range of materials where van der Waals forces lead to physisorption due to relatively weak hybridization with substrate states. Further research will be necessary to better understand the impact of parameters not systematically investigated here, such as the respective roles of the substrate’s gap width or the energetic positions of valence and conduction band onsets.

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AUTHOR INFORMATION

Corresponding Author

*(J.K.) E-mail: [email protected]. Telephone: +49 931 31 85085. Author Contributions

J.K. conceived the experiment. J.K. and A.K. performed and analyzed the measurements. K.A.K. and O.E.T. synthesized the crystal. A.K wrote the manuscript. M.B. supervised the project. All authors discussed and commented on the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Paolo Sessi for helpful discussions. This work was supported by DFG within SPP 1666 (Grant No. BO1468/21-2) and through SFB 1170 “ToCoTronics” (project A02). Partial support by the Saint Petersburg State University (Grant Number 15.61.202.2015) is also acknowledged.

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DOI: 10.1021/acs.jpcc.6b05177 J. Phys. Chem. C 2016, 120, 13843−13849