Identifying Molecular Orbital Energies by Distance-Dependent

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Identifying Molecular Orbital Energies by Distance-Dependent Transition Voltage Spectroscopy M. Christina Lennartz,† Nicolae Atodiresei,‡ Vasile Caciuc,‡ and Silvia Karth€auser*,† †

Peter Gr€unberg Institut (PGI-7), ‡Peter Gr€unberg Institut (PGI-1) and Institute for Advanced Simulation (IAS-1), Forschungszentrum J€ulich GmbH and JARA, 52428 J€ulich, Germany ABSTRACT: Besides current voltage spectroscopy, also transition voltage spectroscopy (TVS) becomes an interesting tool to investigate the energetic position of the molecular orbitals involved in the tunneling process. We used scanning tunneling spectroscopy to perform both spectroscopy techniques as a function of the tip substrate distance. Employing our model system, benzoic acid on a Cu(110) surface, we could observe a step in the transition voltage using distance-dependent TVS. Combining the spectroscopic results with density functional theory based calculations, it was possible to identify the molecular orbitals responsible for charge transport. Moreover, it was found that different molecular orbitals are responsible for charge transport if varying STM tip substrate distances are examined.

’ INTRODUCTION Molecular components with their inherent scalability are supposed to be promising candidates for future nanoscale electronic devices. The electronic structure of the combined electrode/molecule/electrode system defines the charge transport properties through the molecular device. Thus, the molecular energy levels and especially the position of the frontier molecular orbitals relative to the electrode Fermi level are of high importance. Besides current voltage spectroscopy (I(V) spectroscopy), currently, transition voltage spectroscopy (TVS)1 becomes an interesting tool to investigate these molecular energy levels.2,3 TVS is based on a simple transformation of the normal I(V) transport characteristic into a plot of ln(I/V2) versus 1/V, that is, a Fowler Nordheim plot. It was found by Beebe et al.1 that the replotted data show a characteristic minimum at a specific bias voltage, Vtrans, which scales linearly with the energy of the highest occupied molecular orbital (HOMO) obtained by ultraviolet photoelectron spectroscopy. A theoretical description was first given based on the Simmons model,4 and later on, a coherent molecular transport model was employed.5 Whereas Beebe et al.1 predicted a one-to-one relation between the TVS minimum and the tunneling barrier height, ΦB, that is, the energetic distance between the HOMO position and the electrode Fermi level, this is currently under debate.5 7 In the theoretical work by Chen et al.,6 it was suggested that the TVS minimum could be used as a direct measure of the molecular energy levels since the molecular level broadening due to the coupling to the electrodes does not affect the minimum. However, importantly, it was shown that the ratio between the tunneling barrier height, ΦB, and the transition voltage value, Vtrans, depends strongly on the junction asymmetry.6 r 2011 American Chemical Society

Up to now, further experimental TVS investigations were performed for several molecules, for example, dithiols coupled symmetrically to gold electrodes,8,9 monothiols coupled asymmetrically only by one sulfur endgroup to a gold electrode,10,11 and cyanides12 or phosphonic acids13 coupled asymmetrically to various conducting materials. In all these cases, TVS was applied successfully, revealing a transition from direct tunneling to Fowler Nordheim tunneling. In most of these articles, Vtrans was related to the position of the molecular levels. In this paper, we report on scanning tunneling spectroscopy (STS) investigations performed as a function of the tip substrate distance using benzenecarboxylic acid (BCA) chemisorbed on a Cu(110) substrate. This simple Cu(110)/ BCA/W-tip system allows us to compare directly the molecular energy levels obtained from peaks in the dI/dV curve (and thus ΦB values) with the Vtrans values deduced from Fowler Nordheim plots. This is possible because our investigations show that the energy levels of this molecular system are located within the accessible voltage window of current voltage measurements.14,15 Supporting the experimental data by density functional theory (DFT) calculations, it is further possible to identify the hybrid molecule-surface states relevant for the charge transport and to correlate orbital mediated I(V) spectroscopy with TVS. Moreover, a change in the transport process depending on the tip substrate distance can be identified.

Received: May 6, 2011 Revised: June 22, 2011 Published: June 28, 2011 15025

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’ EXPERIMENTAL SECTION Sample Preparation. The organic monolayers were vapordeposited onto the Cu(110) single-crystal substrates in a separated chamber of a UHV-System. BCA was purchased from Sigma-Aldrich with a purity of g99.5% and was used for evaporation after further purification in vacuum by freezing thawing cycles. The evaporation was performed at a substrate temperature of 400 K, a molecule temperature of 293 K, a background pressure of 1  10 5 mbar, and a deposition time of 30 min. The used Cu(110) single-crystal substrates were previously prepared by repeated cycles of Ar+-sputtering (1 keV) and annealing at 800 K to obtain a high level of surface cleanliness. XPS and LEED were performed as control measurements. Spectroscopic Measurements. STS studies were performed at room temperature with a JEOL JSPM-4500S STM head under UHV conditions of 9  10 11 mbar using homemade electrochemically etched tungsten tips. The I(V) curves were recorded with the tip at a predefined position above the sample. The feedback loop was switched off, and a voltage ramp ( 3.0 to + 3.0 V) was applied with a typical step size of 10 mV. With completion of the ramp, the feedback loop was turned on and the tip height was readjusted. The recorded I(V) curves at one position were averaged over about 10 successive ramp repetitions, and the derivatives were calculated numerically. Distancedependent I(V) measurements were realized by changing ISet at a constant set-point voltage VSet. Measurements were repeated several times, with different tips at various positions and also for positive and negative set-point voltages. In this way, an unambiguous assignment of the I(V) curves to the electronic structure of the BCA/Cu(110) system is possible. The UV/vis adsorption spectra were taken with a PerkinElmer Lambda 900 spectrometer. Computational Methods. Our ab initio total-energy calculations have been performed in the framework of DFT by using the projector augmented wave method16 to describe the electron ion interactions as implemented in the VASP code.17,18 In our study, we used the Perdew Burke Ernzerhof19 (PBE) exchange-correlation energy functional while the plane-wave basis set includes all plane waves up to a kinetic energy of 500 eV. The molecule/Cu(110) system was modeled within the supercell approach and contains five atomic layers of copper with the adsorbed molecule on one side of the slab. For the adsorbate/ substrate system investigated in our study, the corresponding supercell was generated with the theoretical bulk copper lattice parameter of 0.363 nm and a p(4  5) in-plane surface unit cell. To obtain the ground-state adsorption geometry, in our firstprinciples simulations, the uppermost two copper layers and the molecule atoms were allowed to relax until the atomic forces are lower than 0.05 eV/nm.

’ RESULTS/DISCUSSION Benzenecarboxylic acid (BCA) is the smallest carboxylatebased molecule with a delocalized π-system. It self-assembles on Cu(110) surfaces and builds close-packed monolayers of standing-up molecules with a c(8  2) structure.14,20 Because of chemical bonds of both oxygen atoms of the carboxylate group to the structured Cu(110) template, a rigid, stable, and highly ordered molecular assembly is present.21 Because the BCA BCA distance is determined by the Cu(110) template and amounts to 0.36 nm, only weak π π interactions are possible ( ΦB/e). The exact values of the transition voltages were determined for each I(V) curve and then averaged for one special tip sample distance (ISet = const., VSet = const.). The obtained standard deviation in Vtrans is approximately (0.12 V. The transition voltages obtained at different tip substrate separations are listed in Table 1. At the moment, the relation between the experimentally determined Vtrans and the tunneling barrier height, ΦB, associated with the transport through molecular junctions, given by the difference between the electrode Fermi energy, EF, and the energy of the nearest molecular orbital, either HOMO or LUMO, is under discussion. Whereas a direct relation of Vtrans and ΦB is assumed by Beebe et al.,1 according to Chen et al.,6 the asymmetry of the molecular junction has to be taken into account, if the values should be compared quantitatively. Different transition voltage values are expected for positive and negative bias voltages corresponding to different tunneling barrier heights for electron (LUMO EF) or hole injection (HOMO EF). We first focus on this point for large tip sample separations. Whereas for positive bias voltages, the value of the transition voltage is around Vtrans,pos = 1.3 V, we observe for negative bias a value of Vtrans,neg = 1.5 V. According to ref 6, an asymmetry factor of η ∼ 0.33 corresponding to a ΦB/Vtrans,neg ratio of around 1.6 can be assumed for benzene-based molecules in an STM-like configuration, as in our experimental setup. When 15028

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The Journal of Physical Chemistry C this ratio is applied to our obtained transition voltage minima at negative bias, Vtrans,neg, a tunneling barrier height for hole transport of ΦB,hole = 2.4 eV results. This value is larger than the value of V1 = 2.0 V that was obtained from differential conductance measurements (Figure 1). Calculating the tunneling barrier height for electron transport from Vtrans,pos and applying the same ratio of 1.6 leads to ΦB,elec = 2.1 eV, which is in good agreement with V2 = 2.2 V. It is worth to note that Chen et al. deduced the asymmetry factor η for benzene coupled via a sulfur endgroup to a Au(111) surface and that we report here about a BCA/Cu(110) system, which is quite a remarkable difference, and that a full agreement by applying the thus obtained asymmetry factor to our setup is not expected. Moreover, it is an open question, if the same asymmetry factor can be applied for electron and hole injection barriers. However, our experimental results confirm that a kind of correction has to be considered if the transition voltage should be transferred into tunneling barrier heights. In contrast, the HOMO LUMO gap of the molecule under consideration can be determined directly from differential conductance measurements, as elaborated above. Another important issue becomes visible, if the transition voltage is compared for different tip substrate separations (Table 1). While Vtrans,pos is constant for almost the whole investigated tip substrate distance range, also pointing to a negligible influence of the changing asymmetry in this range, the value of the transition point Vtrans,neg changes considerably. At a large tip sample separation, for example, at a set-point current of 0.1 nA, a value of Vtrans,neg = 1.49 V is obtained, whereas for smaller separations (ISet = 0.5 nA), the transition occurs already at around 0.8 V. In the latter case, the transition voltage corresponds to ΦB,hole = 1.28 eV, which is in very good agreement with 1.25 V, the energy of the σ1-MO. This points to the fact that, for large tip sample separations, the π1-orbital mediates the charge transport through the Cu(110)/BCA/W-tip system and determines the tunneling barrier height ΦB, whereas for small separations, the σ1-MO is addressed directly, as indicated by a lower tunneling barrier height. Accordingly, it is possible to unambiguously identify by TVS the molecular orbitals that are responsible for the conductance in the given measurement setup. In our case, it is obvious from the charge density plots (Figure 2b) that the highly delocalized π-MOs are available for the tunneling process at large tip substrate distances, while the σ1-MO localized at the molecule substrate interface is screened. Thus, the relevant MOs for conduction in this case are the geometrically available π-MOs due to their significant spatial extent at the molecule vacuum interface. However, the situation changes for small tip substrate separations when the σ1-MO is also geometrically available; then, of course, only the lower tunneling barrier height is relevant. Coming back again to Figure 3b and looking at the Fowler Nordheim plot for the shortest tip substrate distance, not only one but two minima can be identified. The new flat minima corresponds to a Vtrans,pos value of 0.7 V. In analogy to the discussion above, this second minima can be interpreted as the opening of a second conduction channel through the σ1*-MO. The appearance of a double minimum points to an energy range between the two conducting channels where no molecular orbital is available. These results reveal that the molecular orbitals with a low tunneling barrier are of high importance for molecular conductance. Thus, a conductance change, that is, a switching from, for

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example, a π-orbital-mediated conductance to a σ-orbitalmediated conductance regime, can be achieved also by a shift in the tip position. Considering the theoretical data obtained for our BCA/Cu(110) model system, each hybrid molecule-surface electronic state is available in a certain energy range as a consequence of molecular orbital broadening due to coupling to the substrate. Moreover, these energy ranges corresponding to different orbitals overlap to some extend, for example, the π1- and the πd-MO. A comparison of these theoretical data with our experimental ones reveals that the Vtrans values correspond approximately to the low-energy limit of the energy range describing a specific molecular orbital. This is in accordance with the definition for Vtrans:1 voltage at which a mechanistic transition from direct tunneling to field emission occurs. In particular, an important feature of the TVS method is the possibility to identify the onset of the transport through the HOMO and LUMO. In contrast, the peaks in the experimentally obtained differential conductivity curves (V1 or V2) can be related to the center of the respective HOMO or LUMO energy level. Thus, energies corresponding to peaks in the first derivative of the I(V) curves relate to maximal charge transport through the molecule metal system. In conclusion, we have shown, using our model system, BCA on a Cu(110) surface, that differential conductance spectroscopy is a method to reliably determine the HOMO LUMO gap of molecules chemisorbed on a surface. On the other hand, we assume that the energy necessary for the onset of charge transport through the respective MO results from TVS. Most interestingly, we observed a change in the transition voltage of BCA on Cu(110) caused by the approaching tip. In combination with density functional theory based calculations, this step in transition voltage is assigned to a change of the charge transport determining molecular orbital. Whereas for large tip sample distances, the delocalized π1-orbital determines the transport properties, this situation changes for short tip sample distances. Here, the bonding carboxylate copper orbital with σ-character becomes available and determines the charge transport due to the lower MO energy. Thus, not only the energy but also the spatial distribution of MOs has to be taken into account when the most suitable molecular orbital for charge transport is selected.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was funded by the DFG (Grant SPP1243). The computations were performed at JUROPA and JUGENE supercomputers at the J€ulich Supercomputer Centre, Forschungszentrum J€ulich, Germany. ’ REFERENCES (1) Beebe, J. M.; Kim, B.; Gadzuk, J. W.; Frisbie, C. D.; Kushmerick, J. G. Phys. Rev. Lett. 2006, 97, 026801. (2) Beebe, J. M.; Kim, B.; Frisbie, C. D.; Kushmerick, J. G. ACS Nano 2008, 2, 827–832. (3) Choi, S. H.; Kim, B.; Frisbie, C. D. Science 2008, 320, 1482–1486. (4) Simmons, J. G. J. Appl. Phys. 1963, 34, 1793–1803. (5) Huisman, E. H.; Guedon, C. M.; van Wees, B. J.; van der Molen, S. J. Nano Lett. 2009, 9, 3909–3913. 15029

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