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Reaction Spectroscopy of Frontier Orbitals J€org Henzl and Karina Morgenstern* Leibniz Universit€at Hannover, Institut f€ur Festk€orperphysik, Abteilung f€ur atomare und molekulare Strukturen (ATMOS), Appelstr. 2, D-30167 Hannover, Germany ABSTRACT: Single anilinonitroazobenzene molecules are investigated on the reconstructed Au(111) surface by low-temperature scanning tunneling microscopy, scanning tunneling spectroscopy, and inelastic electron tunneling manipulation. The molecules attach with their anilino end to the point dislocation of the reconstructed surface and with their nitro end to a gold adatom. These complexes are induced to rotation, diffusion, and isomerization by injecting electrons into the lowest unoccupied molecular orbitals or by injecting holes into the highest occupied molecular orbitals. The reaction yield thereby reflects the spatial distribution of these orbitals. This suggests that frontier orbitals and their spatial extent can be mapped via molecular reactions induced by transient population of these orbitals by tunneling electrons.
’ INTRODUCTION In scanning tunneling spectroscopy (STS), the first derivative of the tunneling current (dI/dV) is measured. The spectra yield information about the electronic structure of the substrate.13 For adsorbed molecules, the spectra reveal the energetic position of their frontier orbitals with respect to the Fermi energy and variations of the position, which is dependent on binding site and environment.The molecular orbitals can be mapped by spatially mapping the STS signal, as demonstrated for C60 on Ag(100)4 or, more recently, by measuring it with the tip at different positions above the molecule.5 This study investigated a molecule (pyridine-2,5-dicarboxylic acid) on Cu(110) that chemisorbs with its molecular plane perpendicular to the surface. In combination with first-principles theory, individual orbitals were identified and their spatial distribution determined. The method, however, is not applicable for π-bonded and thus weakly adsorbed single molecules on metal surfaces. These molecules are not bound strongly enough to survive the electron dose necessary to acquire the spectra. Such spectra are thus dominated by strong changes in the IV characteristics due to changes in the adsorption site of the molecule or due to conformational changes within the molecule itself. Consequently, the electronic structure of the molecules cannot be extracted in STS. On the other hand, the scanning tunneling microscope allows one to inject electrons precisely into frontier orbitals of adsorbed molecules. Though the lifetime of the electrons on a molecule adsorbed on a conducting surface is rather short, the molecule might be left in an excited state that triggers surface reactions. This has been demonstrated already in 1992 for the dissociation of decaborane on the Si(111)-(7 7) surface with electron energies above 4 eV.6 More recently, benzene was desorbed from a Si(100) surface.7 A theoretical description confirmed that the process is initiated by transient ionization of the molecule.8 The same molecule was dissociated upon formation of a negative ion on a metal surface, Cu(110).9 r 2011 American Chemical Society
On the example of an azobenzene derivative, we show here that the reactions induced by the injected electrons reflect the electronic structure of the adsorbed molecule and might be used to extract the position and spatial extent of the frontier orbitals. A similar approach was taken for the vibrational spectrum of molecules that are changed upon their vibrational excitation. Originally, the reasoning was in the opposite way; that is, a threshold found for electron induced manipulation was interpreted with the aid of vibrational spectra measured by other means in order to prove the excitation pathway.1013 Today, reaction spectroscopy has developed into a tool to detect some vibrational modes of adsorbed molecules that are not accessible in the vibrational spectra gathered by STM.14,15 Azobenzene (C6H5NdNC6H5) is considered to be the model switch based on isomerization since the first description of its isomerization in 1937.16 The reversible photoisomerization was studied extensively for the molecule in a solvent1722 and on top of self-assembled monolayers.23,24 These dye molecules make a light-induced configurational transition between an extended (trans) and a compact (cis) isomer at two different wavelengths.2528 The light thereby transfers in both cases an electron to the LUMO (lowest unoccupied molecular orbital) of native azobenzene or the orbital derived from this LUMO for its derivatives. More recently, azobenzene derivatives in direct contact with a surface were explored. This contact with a surface is expected to alter the isomerization properties substantially, for example, by steric hindrance or by bond formation. Furthermore, faster alternative deexcitation might quench the photoexcited adsorbed molecule as isomerization is a rather slow process. To clarify these issues, azobenzene molecules adsorbed on metal surfaces have been Received: December 23, 2010 Revised: March 2, 2011 Published: May 04, 2011 10056
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Figure 1. Anilinonitroazobenzene in elbow sites of the reconstructed Au(111) surface. (ad) Adsorbed molecules. (a, b) 0.69 nA, 100 mV. Inset: ball-and-stick model of surface adapted trans isomer from ref 35. (c) Molecules at Y elbows, 35 pA, 300 mV. (d) Molecules at X elbows, 12 pA, 133 mV. (e, f) Elbow sites in atomic resolution. (e) Y elbow, 13 nA, 10 mV. (f) X elbow, 48 pA, 100 mV.
explored extensively by different methods, that is, scanning tunneling microscopy (STM),2935 theory,3638 high-resolution electron energy loss spectrometry,39 two-photon photoemission,40,41 and near-edge X-ray adsorption fine structure measurements;42 see also ref 43. These have demonstrated that the adsorbed molecules isomerize after excitation by electrons for a number of derivatives. In this article, we demonstrate that the transient ionization of anilinonitroazobenzene adsorbed on Au(111) leads to diffusion, rotation, and isomerization of this molecule. Diffusion and rotation are induced by injection of electrons into the LUMO or holes into the HOMO (highest occupied molecular orbital). The yield is higher at positions at which the orbitals have more intensity. The conformational change is possible only if electrons are injected into the orbital that is also populated in photoisomerization. The article is organized as follows: To interpret the manipulation experiments described in this article, a good knowledge of the adsorption of the molecules on the reconstructed Au(111) surface and their appearance in STM images is indispensable. This is the topic of the Identification of Molecules section. The results of the manipulation experiments are described and compared to spectra of molecules in supramolecular structures in the Manipulation of Molecules section.
’ EXPERIMENTAL METHODS Scanning tunneling microscopy (STM) and spectroscopy (STS) measurements are performed with a low-temperature STM under ultra-high-vacuum (UHV) conditions (base pressure below 8 1010 mbar; during sample preparation, the pressure is reduced below 4 1010 mbar by a cold trap). A clean herringbone reconstructed Au(111) substrate is obtained by repeated cycles of Neþ sputtering and annealing. The sputtering is performed for 30 min at a partial neon pressure of 3 105 mbar. The acceleration of the Neþ ions with 1.3 keV results in a
sputtering current of 2 μA. The sample is then annealed to 900 K for 30 min. 4-Anilino-40 -nitroazobenzene molecules (trade name: Disperse Orange 1, from Acros Organics) are evaporated from a thoroughly outgassed Knudsen cell at a temperature of 410 K. The Au(111) surface is held during evaporation at 245 K, just below the desorption temperature of the molecules, as determined by mass spectrometry. An evaporation time of 30 s results in single molecules with a coverage of ≈0.01 molecules/ nm2, which corresponds to the point dislocation density of the reconstructed Au(111) surface. For a control experiment, we furthermore repeat the preparation of larger coverages of ≈0.1 molecules/nm2, at which the molecules form supramolecular structures.35 After exposure, the sample is transferred to the STM that is operated at a temperature of 5 K. STM images are taken in constant current mode. Scanning tunneling spectroscopy (STS) recorded in the lock-in technique is used to measure the first derivative of the tunneling current (dI/dV). For a spectrum, the tip is positioned above a specific part of the molecule and the feed-back loop is switched off. The bias voltage is then swept over the energy range of interest (typically 12 V) with a sinusoidal modulation of 1030 meV and a typical frequency in the range of 900 Hz. This leads to a modulation of the tunneling current with the same frequency, which is then detected by the lock-in amplifier. This signal is proportional to the first derivative of the tunneling current. To manipulate the molecules, the inelastic electron tunneling manipulation technique (IET manipulation) is applied.44 For this manipulation, the tip is positioned above a specific part of the molecule and the feed-back loop is switched off. A voltage of a few volts is then applied for up to a few seconds. During the manipulation, the tunneling current is recorded. A steplike change in the tunneling current indicates a successful manipulation. A subsequent STM image verifies the result of the manipulation. The time span until the steplike change in the current is 10057
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Figure 2. Identification of individual structures; protrusions are named A, B, C, and D for comparison: (a) STM image, 53 pA, 90 mV; (b) line scan along the line in (a) with Gaussian fits; (c) STM image, 69 nA, 100 V; (d) line scans along lines in (c); and (e) STM image, 75 pA, 100 mV.
used to determine the number of electrons necessary to induce the change. Because of the statistical nature of the process, this number should follow an exponential. The slope of the exponential determines the time constant, τ, which is related directly to the yield via Y = e/Iτ.44
’ RESULTS AND DISCUSSION Identification of Molecules. 4-Anilino-40 -nitroazobenzene is
an azobenzene derivative with two functional groups in the para position, namely, a nitro group (NO2) and an anilino group (NHphenyl) (Figure 1a, inset). At low coverage, the molecules are adsorbed predominantly at the elbow sites of the reconstructed Au(111) surface (Figure 1a,b). Elbow sites on Au(111) are known for their reactivity leading to, for example, nucleation in metal growth.4649 Two elbow sites, X and Y, are conventionally discriminated, as marked in Figure 1a and shown in atomic resolution in Figure 1e,f. At the so-called X elbow, the hcp domain narrows and the fcc domain widens. The opposite is observed at the so-called Y elbow. At both elbows, one of the reconstruction lines bends without any point defect. The other line exhibits a point defect that differs for the two types of elbows sites. At the Y elbow, an atom exists with seven (instead of six) neighbors in the plane. This atom and its direct neighbors appear brighter in the STM image (Figure 1e). At the X elbow, an atom exists with only five neighbors in the plane. This atom and its nearest neighbors give rise to a depression in the STM image (Figure 1f). Here, we observe the molecules to cross over to the hcp and fcc domains at the X and Y elbows, respectively (Figure 1c,d). In fact, the positions of the point defect are not exactly in the middle of the domain wall. Furthermore, higher coverage experiments revealed that the molecules avoid the domain boundaries,35 as observed for other organic molecules.50 The observed crossing of the molecules at the two different elbows is in agreement with the asymmetry of the point defects within the domain boundaries (Figure 1c,d). At low coverage, the molecules thus preferentially adsorb at the point defects of the elbow sites.
The same molecule forms at higher coverage supramolecular structures35 (at the same temperature, c.f. Figure 7a below). The structures are predominantly adsorbed on the fcc domains of the Au(111) surface and are clearly not attached to the point defects. Within these structures, each molecule is bound to two other molecules by two hydrogen bonds each. Thus, the bonding energy to the elbow site observed for single molecules is smaller than the one of four hydrogen bonds. A close-up view shows that each individual structure consists of four protrusions, named AD for reference purposes (Figure 2). Three of the protrusions have an apparent height of ≈130 pm and the fourth one of around 85 pm (Figure 2b). Thereby, the apparent heights differ only slightly for molecules in different bonding orientations with respect to the domain boundaries (Figure 2c,d). The four protrusions are surprising; within supramolecular structures, the molecule showed only three protrusions with a total distance of 1.4 nm,35 in contrast to the distance of the outermost protrusions here of 2 nm (as determined by multiple Gaussian fits). In fact, the three protrusions, AC, here very closely resemble the three protrusions within the supramolecular structures both in apparent height and in distance.35 The additional protrusion D can be dissected from the molecule via IET manipulation with electrons at energies at which the electrons attach to the LUMO þ 1 orbital (Figure 3a, b). Though usually on a line with protrusions B and C, sometimes different positions of D are observed (Figure 3c, left molecule). Furthermore, at low voltages, three protrusion are observed (Figure 3d,e). In all cases, the additional protrusions connect to the nitro end of the molecule. We suggest that these additional protrusions are gold atoms attached to the electronegative end group of the molecule. Scanning at low voltages moves this atom between three possible attachment sites. Such formation of a metalloorganic complex is plausible based on previous studies: It has been shown before that an adatom gas on a surface may lead to the formation of metalorganic complexes in two dimensions.5355 In break junction experiments, nitro groups have been shown to strongly couple 10058
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Figure 3. Identification of protrusion D. (a, b) IET induced dissection with 2.8 V and 4.6 nA for 90 μs; the cross in (a) indicates the position of the tip during manipulation; 22 pA, 134 mV. (c) Two molecules at close distance, 31 pA, 90 mV. (d, e) Two different conformations of the molecules imaged at low bias. (d) 31 pA, 90 mV. (e) 13 pA, 25 mV.
Figure 4. Voltage-dependent imaging of the structure at 22 pA: (a) 0.4, (b) 1.2, and (c) 2.8 V. (d) Line scans across major protrusions of the molecule at indicated voltages; the inset shows the LUMO of the surface adapted molecule as calculated semiempirically using the Parametric Method 3 parametrization of MNDO for the Hamiltonian.51 (e) Difference of line scans at indicated voltages.
to Au electrodes, stronger than, for example, sulfur or cyano groups.56 Furthermore, it is well established that, at the preparation temperature, a low-density atomic gas of gold adatoms is released from the preexisting step edges, as shown by the development of nonequilibrium structures.5759 Diffusion of Au/Au(111) has been calculated a number of times. Though the energies spread, they are in the range of 100300 meV.60 The Au adatoms are thus highly mobile at the deposition temperature and avialable for attachement to the nitro group. The formation of a metallicorganic complex here is corroborated by low-temperature adsorption, at which similar complexes are not observed. After having established that protrusions AC correspond to one molecule, we now determine the orientation of the molecule
within these protrusions. The different distances between these protrusions (ΔCB > ΔBA) allow us to identify the phenyl, to which the anilino and the nitro end groups are attached, as protrusions A and C, respectively. Note that, in images of lesser resolution, A and B are imaged as one ellipsoidal protrusion, indicating the anilino end of the molecule. The molecules are thus adsorbed with their anilino end to the point defects of the reconstruction (c.f. Figure 1c,d) The assignment of protrusion C to the nitro group is corroborated by the voltage-dependent imaging shown in Figure 4. Region C of the molecule more than doubles in apparent height from below 100 pm at around þ0.8 V to above 2050 pm at 2 V (see line scans in Figure 4d and difference of line scans in Figure 4e). For this molecule, the LUMO is situated at the electron-pulling nitro group (Figure 4d, inset). This LUMO was determined by STS to be situated at ≈1.40 V within the supramolecular assemblies.35 The position of the LUMO is expected to shift by a few 100 meV at most because of the hydrogen bonding within these assemblies.52 The large increase in apparent height above 1.2 V at C is thus consistent with it being the nitro group. A further increase in apparent height is observed at 2.2 V (Figure 4d). Thereby, the region between protrusions B and C, that is, the region of the azo group, and the region close to protrusion D increase in apparent height (Figure 4e). Within supramolecular structures, the LUMO þ 1 orbital rises at 2.5 V with a maximum at 2.9 V.35 The second change in apparent height is thus attributed to tunneling into the LUMO þ 1. The usually observed four protrusions are thus indicative of a Auanilinonitroazobenzene complex. Note that the thresholds discussed below have exemplarily been compared between the metalmolecule complex and a molecule, from which the Au atom was dissected. The thresholds were identical within the error bars. Thus, though likely to be a Au atom, the exact identity of protrusion D is not essential for the major topic of this article. Manipulation of Molecules. After having established the identity of the four protrusions, we now manipulate the complex 10059
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Figure 5. IET-induced manipulation of the metalorganic complex. (a, b) Two manipulations with 2 V and 0.2 nA for 1.7 s with the tip at the position of the cross; 133 mV, 12 pA. (c) False color superposition of (a) and (b); green and pink show the molecule before and after manipulation. (d, e) Manipulation with 1.8 V and 0.3 nA for 0.45 s with the tip at the position of the cross; 134 mV, 22 pA. (f) False color superposition of (d) and (e). (g, h) Manipulation with 2.8 V and 1.2 nA for 0.09 s with the tip at the position of the cross; 134 mV, 22 pA. (i) False color superposition of (g) and (h).
by injecting electrons into the different protrusions corresponding to the Au atom (D), the phenyl ring with the nitro group (C), and the two phenyl rings attached to the nitrogen atom of the anilino group (A and B). Furthermore, we inject electrons in the middle between protrusions B and C, that is, into the azo group; between protrusions A and B, that is, into the nitrogen atom of the anilino group; and between C and D, the nitro group bonding to the Au atom (see sketch in Figure 6c). Different processes are observed that are shown in Figure 5. We may induce rotation of the molecule (Figure 5ac, left molecule), its diffusion (Figure 5ac, right molecule), simultaneous diffusion and rotation (Figure 5df), or a conformational change of the molecule (Figure 5gi). Rotation and diffusion can thereby be initiated at similar energies. The conformational change demands higher energies. Note that molecules that are induced to diffusion are stable on all surface regions at the measurement temperature of 5 K. We now analyze the It curves recorded during the manipulation (Figure 6a) in order to determine the threshold barriers for the reactions. The area below the curve up to the first sudden change gives the number of electrons needed to initiate the rotation or diffusion or both. We calculate the yield and display it against the manipulation voltage in Figure 6b. The increase in yield is asymmetric with respect to 0 V. It increases strongly at 1.2 and at 2 V. A more shallow increase is observed at 1.3 V. The asymmetry with respect to the Fermi energy is indicative of an electronic excitation. Indeed, the first value is consistent with the LUMO of this molecule, as observed in the increase in apparent height in Figure 4d. STS is not possible on single molecules because of the induced processes. However, the increase in yield in the positive voltage range is consistent with the first increase in the STS spectra within supramolecular structures (Figure 7c).35 In the negative voltage range, the first increase in dI/dV spectra of these molecules was observed at (1.67 ( 0.10) V and was attributed
to the HOMO orbital. The position of the strong increase in yield at ≈2 V (Figure 6b) can be correlated to this orbital. However, there is a slight increase in dI/dV at 1.2 V in the spectra measured at the anilino end of the molecule (Figure 7b). A possible explanation of this kink in the spectra is that the degeneracy of the HOMO is split by adsorption. Such a splitting should lead to a kink within the peak indicating the orbital. Regardless of its origin, this slight increase in dI/dV spectroscopy is reflected in the small increase in yield in the negative voltage range (Figure 7b). A slight shift in energy of both this kink and the main peak of the HOMO is attributed to hydrogen bonding within the supramolecular structures that does not exist for the single molecules.52 Concerning the processes induced, diffusion and rotation are equally likely above the threshold in the positive voltage range. However, there is mainly diffusion observed above the lower threshold in the negative voltage range. Above the second threshold, diffusion and rotation are approximately equally likely. Furthermore, the yield depends strongly on the injection point (Figure 6c). For injection into the LUMO, no process can be induced, if injecting electrons in any part of the anilino group or in the phenyl of the azobenzene core adjacent to it (protrusions A and B). Injection into the other parts of the molecule induces rotation and diffusion. This is consistent with the localization of the LUMO at the nitro end of the molecule (Figure 6d). The conformational change of anilinonitroazobenzene was not possible within the supramolecular structures by injection into the HOMO or the LUMO, but only into the LUMO þ 1 orbital.35 This latter orbital is derived from the π* orbitals of the native azobenzene, the orbital that is populated during photoisomerization. As best seen in the apparent height difference in Figure 4e, this orbital has weight at the azo group at which the isomerization takes place and between protrusions C and D. Indeed, we are able to induce the conformational change of the 10060
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Figure 6. Analysis of IET manipulation. (a) It curves during manipulation with 2.8 V for different processes and different current set points. (b) Yield for all rotation and diffusion events for all injection points dependent on manipulation voltage, V, based on 562 analyzed events. (c) Table for yield, Y, in 109/e at 1.2 V for rotation and diffusion events for different injection points, as indicated by the crosses. (d) LUMO as calculated semiempirically using the Parametric Method 3 parametrization of MNDO for the Hamiltonian51 for the surface adapted molecule from ref 35.
Figure 7. STS spectra of anilinonitroazobenzene in supramolecular structures on Au(111) at a coverage of ≈0.1 molecules/nm2. (a) STM image of supramolecular structures, 539 mV, 91pA. (b) Average of several dI/dV spectra in the negative voltage range measured on a molecule at the end of a supramolecular structure as in (a). (c) dI/dV spectrum in the positive voltage range reproduced from ref 35.
complex here by manipulating at 2.8 V. The yield is ≈1.7 109 per electron at this voltage, in the same range as the other processes at lower voltage. Note that this yield is approximate, because of the other possible reactions, diffusion and rotation. Concerning the position of the tip, the conformational change could exclusively be initiated by injecting electrons into the region of the nitro group and the connected Au atom, that is, between protrusions C and D. Again, this is a region in which this orbital has considerable weight (Figure 4e). Finally, we report that all processes can be induced more indirectly by injecting electrons into the Au atom with a quite substantial yield of the same order of magnitude as on the molecule. This could be considered as a lead to the molecule in molecular electronics. We suggest investigating this in more detail for such complexes adsorbed on insulating layers.
’ CONCLUSION In conclusion, we have demonstrated electron-induced reactions initiated by injecting electrons into frontier orbitals of a moleculemetal complex. The yield closely resembles the electronic structure as measured in the STS spectra, not only in position but also in the intensity of the maxima. The yields closely follow the dI/dV spectra of these molecules in supramolecular structures. We suggest that mapping of this IET manipulation could be used in
order to determine the position of the orbitals in cases in which dI/ dV spectroscopy is obstructed by the induced processes. Thus, the orientation of the molecule within the protrusions imaged by STM could be determined via “reaction mapping”, similar to the determination of vibrational modes through the reactions induced by their excitation.1015,45,61,62
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors thank the Deutsche Forschungsgemeinschaft for financial support. ’ REFERENCES (1) Wagner, C.; Franke, R.; Fritz, T. Phys. Rev. B 2007, 75, 235432. (2) Schulze, G.; Franke, K. J.; Gagliardi, A.; Romano, G.; Lin, C. S.; Rosa, A. L.; Niehaus, T. A.; Frauenheim, Th.; Di Carlo, A.; Pecchia, A.; Pascual, J. I. Phys. Rev. Lett. 2008, 100, 136801. (3) Tao, C.; Sun, J.; Zhang, X.; Yamachika, R.; Wegner, D.; Bahri, Y.; Samsonidze, G.; Cohen, M. L.; Louie, S. G.; Tilley, T. D.; Segalman, R. A.; Crommie, M. F. Nano Lett. 2009, 9, 3963–3967. 10061
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