LETTER pubs.acs.org/NanoLett
Charge State Control of Molecules Reveals Modification of the Tunneling Barrier with Intramolecular Contrast Ingmar Swart,* Tobias Sonnleitner, and Jascha Repp Institute of Experimental and Applied Physics, University of Regensburg, 93053 Regensburg, Germany ABSTRACT: From scanning tunneling microscopy and spectroscopy experiments it is shown that control over the chargestate of individual molecules adsorbed on surfaces can be obtained by choosing a substrate system with an appropriate workfunction. The distribution of the additional charge is studied using difference images. These images show marked intramolecular contrast. KEYWORDS: Molecules, charge-state, control, intramolecular, contrast, STM
M
any properties of atoms and molecules adsorbed on surfaces are drastically affected by their charge state.1,2 In this respect, the recently reported control of charge-states of individual Au adatoms on insulating films has attracted considerable attention.3 6 Soon after the first controlled manipulation and identification of charge states had been reported for individual Au atoms, it could also be achieved for other metal atoms and, more recently, for magnesium porphyrin molecules adsorbed on certain sites of an alumina film grown on NiAl(110).1 3,7 9 The demonstration of the charge state switching in the case of molecules is particularly interesting, as in molecules there are more degrees of freedom to distribute charges. Furthermore, molecules offer the prospect of implementing charge state control based functionality in the framework of molecular electronics. Hence, it is important to establish if it is possible to choose/design a system to show charge bistability and how the additional charge is distributed spatially within a molecule. In this Letter, we show from scanning tunneling microscopy (STM) and spectroscopy experiments that it is possible to controllably switch the charge state of single molecules adsorbed on insulating films by choosing a substrate system with an appropriate workfunction, such that a molecular orbital (MO) is sufficiently close to the Fermi-level of the substrate. We study the spatial distribution of the additional charge by suitable subtraction of low-voltage STM images of the adsorbates in their different charge states. These difference images show marked intramolecular contrast. The experiments were carried out with a modified commercial (SPS-CREATEC) low-temperature STM operated at 5 K. NaCl was evaporated thermally onto clean Cu(111) and Cu(100) crystals such that defect-free (100)-terminated NaCl bilayer islands (unless stated otherwise) were formed.10 Copper(II)tetraazaphthalocyanine (4NCuPc, structure shown in inset of Figure 1b) and copper(II)phthalocyanine (CuPc) molecules were deposited with the sample inside the STM at T < 10 K. All voltages refer to the sample bias with respect to the tip. r 2011 American Chemical Society
An STM image of two 4NCuPc molecules adsorbed on NaCl/ Cu(111) is shown in Figure 1a. The molecules appear as crosses, resembling the geometric structure. In addition, a standing wave pattern due to scattering of electrons in the interface state (IS) of NaCl/Cu(111) can be seen.1,11 An I(V) curve, measured at the center of a molecule, is shown in Figure 1b. A hysteresis in the current voltage characteristic is observed between 0.20 and 0.15 V. From the I(V) curves it can be concluded that the molecule switches between two states with different conductance. Conductance switching of molecules has been observed with STM before.7,12,13 In the negative voltage sweep direction (dark colors and corresponding arrows in Figure 1b), a sudden increase in the absolute value of the current is observed at negative bias, signaling that the molecule switched to another state. In the other sweep direction (light colors and corresponding arrows Figure 1b), the current gradually changes until at positive bias there is a sudden decrease in the current, switching the molecule back to the original state. Spectra recorded away from any molecules do not show hysteresis, demonstrating that the observed switching behavior originates from the molecules. As suggested by the hysteretic I(V) curve, as long as no voltages outside the bistable range are applied, both states are stable. This holds true also in absence of the STM tip. This behavior is qualitatively different from temporary charging of molecules due to the presence of the STM tip14 and allows the molecule to be imaged in both states at the same voltage inside the range of bistability. The molecule in the low-conductance state has an apparent height of 0.75 Å (Figure 1a). The molecule was switched to the high-conductance state by temporarily changing the voltage to 0.3 V and back to the original value with the tip positioned above the center of the molecule. In the subsequent image (Figure 1c), the molecule is still located at the same position and has the same orientation but the apparent height has increased to 1.1 Å. In addition, the standing wave pattern due to scattering of Received: December 21, 2010 Revised: March 4, 2011 Published: March 23, 2011 1580
dx.doi.org/10.1021/nl104452x | Nano Lett. 2011, 11, 1580–1584
Nano Letters
LETTER
Figure 1. STM images and spectrum of 4NCuPc molecules on NaCl/Cu(111). (a) Image of two 4NCuPc molecules acquired at 20 mV and I = 5 pA (low-conductance state) (b) I(V) spectrum acquired with the tip positioned above the center of a 4NCuPc molecule, showing hysteresis. The red and green curves indicate the two states involved in the switching process. Voltage sweep directions of decreasing and increasing bias are indicated by dark and light colors (and arrows in the corresponding colors), respectively. A model of a 4NCuPc molecule is given. Color coding: gray, C; blue, N; white, H; and orange, Cu. The inset shows a histogram of the observed switching times obtained at V = 140 mV and I = 5 pA. The dashed line shows an exponential fit of the data. (c) Image of the same molecules as in (a) but now with one molecule in the high-conductance state (V = 20 mV, I = 5 pA) (d) Difference image constructed by subtracting image (c) from image (a) (after drift correction), demonstrating the effects of the manipulation. (e) Image of a 4NCuPc molecule adsorbed on NaCl/Cu(100) (V = 750 mV, I = 2 pA). (f h) High-contrast STM images (V = 50 mV, I = 1 pA) showing two 4NCuPc molecules in different states on NaCl/Cu(111) (green dot, high-conductance state, red stripe, low-conductance state). All scale bars: 30 Å.
electrons in the IS band has changed.1,11 The molecule can be switched back to the previous state by applying the same procedure but with a voltage pulse of opposite polarity. Imaging of both states under exactly the same conditions allows the effects of the manipulation to be visualized directly by constructing a difference image. First, the images of the molecule in the two states were drift-corrected. Then, the image of the molecule in the high-conductance state (Figure 1c) was subtracted from the image of the molecule in the low-conductance state (Figure 1a), leading to an image in which only the effects of the manipulation appear. This difference image is shown in Figure 1d. The molecule appears as a dark depression and is surrounded by concentric deformed ring-like standing waves, demonstrating that the scattering of the IS electrons from the adsorbate has changed as a result of the manipulation. The corrugation of this scattering pattern (0.1 Å, first depression to first maximum) is similar to the one observed for negatively charged Au atoms,1 indicating that the switching of the molecule is associated with a change in the charge state of the molecule. As will be discussed later, the assignment of the switching phenomenon to a change in the charge state is consistent with the differential conductance spectra.
Next, we address the question which charge states are involved in the switching process. Since neutral molecules do not strongly scatter IS band electrons, the scattering pattern around them should be random,15 whereas concentric features will be observed around charged molecules. As is clearly visible in the highcontrast images of two 4NCuPc molecules shown in Figure 1f, g,h, the scattering pattern around molecules in the high-conductance state (green dot) appears random (Figure 1f), hence these molecules must be neutral. In contrast, concentric features appear around the molecules upon switching to the low-conductance state (red stripe), indicating that they are charged. For molecules in the low-conductance state, no interface state localization is observed and hence they must be negatively charged.16 The assignment that molecules in the high- and low-conductance state are neutral and negatively charged, respectively, is consistent with the polarity of the voltage pulses that are needed to switch the charge state in comparison to previous results.1,2,7 We have statistically analyzed the switching behavior. A typical histogram of switching times, that is, the time it takes to observe a switch after applying a fixed voltage, is shown in the inset of Figure 1b (V = 140 mV, I = 5 pA). From the histogram, it can be 1581
dx.doi.org/10.1021/nl104452x |Nano Lett. 2011, 11, 1580–1584
Nano Letters concluded that switching between the two states is a statistically independent process, consistent with the electron capture mechanism as proposed in ref 1. Since the switching rate is exponentially dependent on bias voltage, the bias voltage window in which switching is observed in I(V) curves for one identical molecule, such as the one shown in Figure 1b, is on the order of 10 meV. (The details of the switching process depend on the Moire pattern.) For charged metal atoms, the charge is highly localized and this gives rise to a circularly symmetric scattering pattern.1,2 In contrast, the scattering pattern of charged 4NCuPc molecules shows deviations from circular symmetry (see Figure 1d), implying a noncircularly symmetric charge distribution. In a single particle picture, two factors may contribute to this observation. First, the additional electron can occupy a delocalized orbital and second, the electron-density distribution in the molecule will change due to the presence of the additional electron. We now turn to the question of which orbital the additional electron occupies. Neutral 4NCuPc molecules have an odd number of electrons, hence there is a singly occupied MO. In general, transition metal-phthalocyanine molecules have both highly localized metal centered MOs, as well as delocalized ligandcentered MOs. Both the relaxation energy, which stabilizes the charge due to ionic screening, as well as the Coulomb charging energy will be higher for strongly localized orbitals. Therefore, it is not a priori obvious which of the MOs will be occupied by the additional electron. To address this issue, the occupied density of states of the anion has to be investigated. For 4NCuPc, this is not possible on NaCl/Cu(111), since at bias voltages < 0.2 V, the molecules are switched to the neutral state. This limitation was overcome by investigating the same molecules on NaCl/ Cu(100). Because of the lower workfunction of Cu(100) (ΦCu(111) = 5 eV, ΦCu(100) = 4.6 eV17,18 4NCuPc molecules adsorbed on this surface are always negatively charged. Indeed, 4NCuPc molecules do not exhibit charge state bistability on this substrate system. The first resonance at negative bias of 4NCuPc on NaCl/Cu(100), and hence the orbital that the additional electron occupies is a ligand-centered orbital (Figure 1e), which is in agreement with the noncircularly symmetric scattering pattern that is observed on NaCl/Cu(111). One requirement for charge-state bistability is that one of the MOs is shifted across the Fermi-level upon charging as a result of electronic screening and relaxations in the molecule and the ionic film. This is only possible if one of the frontier molecular orbitals of the molecule is located sufficiently close to the Fermi-level of the substrate.2 Compared to 4NCuPc, the orbitals of CuPc are shifted to higher energy. Hence, it is expected that the charge state of CuPc molecules adsorbed on NaCl/Cu(111) cannot be switched. Indeed, the corresponding I(V) curves do not show hysteresis. To compensate the energy offset in the molecular orbitals, CuPc molecules adsorbed on a substrate system with a smaller workfunction (NaCl/Cu(100)) were studied. The I(V) curves of these molecules exhibit a hysteresis (Figure 2a), similar to what was observed for 4NCuPc. As will be shown below, the assignment of two charge-states is consistent with the differential conductance spectra. An image of two CuPc molecules acquired at a voltage (V = 0.9 V) corresponding to the onset of the first peak at positive bias in the dI/dV spectrum is shown in Figure 2b. At this voltage, the molecules are charged and they appear 2-fold symmetric, but rotated 90° with respect to each other. The LUMO of neutral CuPc in the gas-phase is 2-fold degenerate with one orbital primarily localized on each of the two perpendicular long axes of
LETTER
Figure 2. Spectrum and STM images of CuPc molecules on NaCl/ Cu(100). (a) I(V) spectrum taken at the center of a CuPc molecule, showing current-hysteresis. The red and green curves indicate the two states involved in the switching process. Voltage sweep directions of decreasing and increasing bias are indicated by dark and light colors (and arrows in the corresponding colors), respectively. (b) At V = 0.9 V, tunneling occurs predominantly through the LUMO (I = 4 pA). (c,d) Images acquired at 150 mV and I = 5 pA when the molecule is in the lowand high-conductance state respectively. (e) Difference image, demonstrating the effects of the charging. The largest contrast in this image corresponds to Δz = 0.29 Å. (f,g) Laplace filtered zoom of the left and right molecule shown in (e), respectively. Scale bars: 15 Å.
the molecule.19 Upon adsorption on NaCl/Cu(100), the symmetry is reduced and the degeneracy is lifted. For negatively charged CuPc, the additional electron occupies the lower of the two former LUMO orbitals, which becomes a singly occupied MO (SOMO).20,21 The same procedure as described for 4NCuPc can be used to switch the charge state of CuPc molecules. Images of CuPc (V = 150 mV) in the anionic and neutral charge state are shown in Figure 2c,d, respectively. As observed for 4NCuPc, the neutral molecule has a larger apparent height than the anion. In contrast to Cu(111), Cu(100) does not have a surface state close to the Fermi level and therefore no scattering pattern is observed in the images. In the difference image (anion minus neutral), the molecules appear as dark rectangles. Figure 2f,g shows zoom ins with a Laplace filter applied. This highlights the marked intramolecular contrast in these images. Note that the five dark spots in Figure 2f,g can faintly also be seen in Figure 2e. As for the resonance image, both molecules appear 2-fold symmetric but rotated 90° with respect to each other, ruling out that the intramolecular contrast is due to tip effects.22 In a double-barrier tunneling junction geometry, the contrast in such images acquired at bias voltages well below the molecular resonances may be interpreted as resulting from local modifications of the tunneling barrier. This reasoning is consistent with the observation of a smaller apparent height, that is, a larger barrier for tunneling electrons due to the presence of an additional electron of the anion compared to that of neutral 4NCuPc, CuPc, and Au.1 This offers the prospect of analyzing such difference images in terms of the spatial distribution of the additional charge with intramolecular resolution, such that the dark rectangle in Figure 2e including the five dark spots would be tentatively assigned to areas of increased electron density. Alternatively, image contrast can be interpreted in terms of virtual occupation of, or cotunneling through, nonresonant states. Also in this picture such difference images provide important information about electron localization, electron 1582
dx.doi.org/10.1021/nl104452x |Nano Lett. 2011, 11, 1580–1584
Nano Letters
LETTER
intramolecular contrast. This contrast is due to the local modification of the barrier for tunneling electrons due to the presence of one additional charge. We believe that in conjunction with detailed theoretical investigations and/or atomically resolved Kelvin-probe microscopy6,27 experiments, the features shown in Figure 2e can be interpreted further to reveal valuable information about charge distribution and electron correlation at a fundamental level.
’ AUTHOR INFORMATION Corresponding Author Figure 3. (a) dI/dV spectra of CuPc molecules on a bilayer (solid green, red) and trilayer (dotted black) of NaCl grown on Cu(100). (b) dI/dV spectra of 4NCuPc molecules adsorbed on a bilayer of NaCl grown on Cu(100) (dotted black) and Cu(111) (solid green, red). At positive bias, the offset between the curves equals the difference in workfunction, ΔΦ.
correlation, and polarization at a fundamental level. By virtue of the image subtraction, such information obtained using difference images is relatively insensitive to disturbing geometrical effects, as only the differences in the molecular structure (due to relaxations) can contribute.23,24 The electronic screening in the metallic substrate underneath the insulating film also contributes to the energy balance for charge bistability. This screening decreases with increasing thickness of the insulating films,25 which lead to the theoretical prediction26 and experimental observation3 that the most stable charge state can also be selected by changing the layer thickness. This effect can also be exploited in the present case. When performing similar experiments on a trilayer of NaCl on Cu(100) instead of a bilayer, no charge bistability is found. From the above arguments it can be expected that the molecule is only stable in its neutral state. The dI/dV spectra of CuPc adsorbed on a bilayer and a trilayer of NaCl on Cu(100) are shown in Figure 3a (solid green/red and dotted black curves, respectively). On the negative bias side, where the molecule on the bilayer is neutral (green part of the curve), the spectra are the same, except for a small offset due to the difference in electronic screening and slight differences in workfunction between bilayer and trilayer. In contrast, on the positive bias side (red part of the curve), where the molecule on the bilayer is negatively charged, the spectra are qualitatively different. This is consistent with the assumption that CuPc adsorbed on a trilayer is always neutral and strongly supports our assignment of the different charge states on the bilayer. dI/dV spectra of individual 4NCuPc molecules adsorbed on NaCl/Cu(111) and on NaCl/Cu(100) are shown in Figure 3b (solid green/red and dotted black curves, respectively). If a molecule remains in the same charge state, there is a rigid shift in the spectra acquired on the two surfaces, due to the different workfunction of Cu(100) and Cu(111).25 The spectra show such a rigid shift at positive bias, but they are qualitatively different at negative bias. In conjunction with the hysteretic I V curve seen only on the NaCl/Cu(111) substrate, this behavior strongly supports that charge state switching occurs. In summary, the experiments described in this Letter demonstrate that control over the charge state of molecules can be obtained by choosing a substrate system with a workfunction such that the Fermi-level is sufficiently close to one of the MOs. Difference images, constructed by subtracting low-voltage STM images of molecules in both charge-states, show marked
*E-mail:
[email protected].
’ ACKNOWLEDGMENT We are grateful for discussions with A. Donarini, F. Mohn, L. Gross, and G. Meyer and for funding from the Volkswagen Foundation (Lichtenberg program), the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (Rubicon-Grant 680.50. 0907), and the Deutsche Forschungs Gemeinschaft (Research Training group 1570 and Sonderforschungsbereich 689). ’ REFERENCES (1) Repp, J.; Meyer, G.; Olsson, F. E.; Persson, M. Science 2004, 305, 493. (2) Olsson, F. E.; Paavilainen, S.; Persson, M.; Repp, J.; Meyer, G. Phys. Rev. Lett. 2007, 98, 176803. (3) Sterrer, M.; Risse, T.; Pozzoni, U. M.; Giordano, L.; Heyde, M.; Rust, H. P.; Pacchioni, G.; Freund, H. J. Phys. Rev. Lett. 2007, 98, 096107. (4) Ryndyk, D. A.; D’Amico, P.; Cuniberti, G.; Richter, K. Phys. Rev. B 2008, 78, 085409. (5) Fu, Y.-S.; Zhang, T.; Ji, S.-H.; Chen, X.; Ma, X.-C.; Jia, J.-F.; Xue, Q.-K. Phys. Rev. Lett. 2009, 103, 257202. (6) Gross, L.; Mohn, F.; Liljeroth, P.; Repp, J.; Giessibl, F. J.; Meyer, G. Science 2009, 324, 1428. (7) Wu, S. W.; Ogawa, N.; Nazin, G. V.; Ho, W. J. Phys. Chem. C 2008, 112, 5241. (8) Wu, S. W.; Ogawa, N.; Ho, W. Science 2006, 312, 1362. (9) Maddox, J. B.; Harbola, U.; Mayoral, K.; Mukamel, S. J. Phys. Chem. C 2007, 111, 9516. (10) Bennewitz, R.; Barwich, V.; Bammerlin, M.; Loppacher, C.; Guggisberg, R.; Baratoff, A.; Meyer, E.; Guntherodt, H. J. Surf. Sci. 1999, 438, 289. (11) Repp, J.; Meyer, G.; Rieder, K.-H. Phys. Rev. Lett. 2004, 92, 036803. (12) Donhauser, Z. J.; Mantooth, B. A.; Kelly, K. F.; Bumm, L. A.; Monnell, J. D.; Stapleton, J. J.; Price, D. W.; Rawlett, A. M.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 2001, 292, 2303. (13) Qian, G.; Saha, S.; Lewis, K. M. Appl. Phys. Lett. 2010, 96, 243107–3. (14) Mikaelian, G.; Ogawa, N.; Tu, X. W.; Ho, W. J. Chem. Phys. 2006, 124, 131101. (15) Because of the long coherence length and the presence of many scatters, a complicated scattering pattern is observed everywhere on the surface. (16) Repp, J.; Meyer, G.; Paavilainen, S.; Olsson, F. E.; Persson, M. Phys. Rev. Lett. 2005, 95, 225503. (17) Gartland, P. O. Phys. Norv. 1972, 6, 201. (18) For other molecules a rigid shift is observed in the spectra acquired on these two surfaces, indicating that NaCl affects the workfunction of the two substrates similarly.25 (19) Liao, M. S.; Scheiner, S. J. Chem. Phys. 2001, 114, 9780. (20) Neutral CuPc has an odd number of electrons, hence negatively charged CuPc/NaCl/Cu(100) has two SOMOs. 1583
dx.doi.org/10.1021/nl104452x |Nano Lett. 2011, 11, 1580–1584
Nano Letters
LETTER
(21) Repp, J.; Meyer, G.; Paavilainen, S.; Olsson, F. E.; Persson, M. Science 2006, 312, 1196. (22) There are slight differences between the two molecules, which are thought to be primarily due to experimental noise associated with the very small changes in apparent height in difference images and the subsequent Laplace filtering. (23) The complicated Moire-pattern of NaCl/Cu(111) seems to influence the difference images of 4NCuPc/NaCl/Cu(111) making their interpretation difficult. (24) Vitali, L.; Levita, G.; Ohmann, R.; Comisso, A.; De Vita, A.; Kern, K. Nat. Mater. 2010, 9, 320. (25) Repp, J.; Meyer, G.; Stojkovic, S. M.; Gourdon, A.; Joachim, C. Phys. Rev. Lett. 2005, 94, 026803. (26) Pacchioni, G.; Giordano, L.; Baistrocchi, M. Phys. Rev. Lett. 2005, 94, 226104. (27) Gross, L.; Mohn, F.; Moll, N.; Liljeroth, P.; Meyer, G. Science 2009, 325, 1110.
1584
dx.doi.org/10.1021/nl104452x |Nano Lett. 2011, 11, 1580–1584
