LETTER pubs.acs.org/NanoLett
Two-Level Conductance Fluctuations of a Single-Molecule Junction N. Neel,*,†,§ J. Kr€oger,‡ and R. Berndt† † ‡
Institut f€ur Experimentelle und Angewandte Physik, Christian-Albrechts-Universit€at zu Kiel, D-24098 Kiel, Germany Institut f€ur Physik, Technische Universit€at Ilmenau, D-98693 Ilmenau, Germany ABSTRACT: The conductance of a single-molecule junction in a lowtemperature scanning tunneling microscope has been measured at nanosecond time resolution. In a transition region between tunneling and contact the conductance exhibits rapid two-level fluctuations which are attributed to different geometries of the junction. The voltage dependence of the fluctuations indicates that electrons injected into the lowest unoccupied molecular orbital may efficiently couple to molecular vibrations. KEYWORDS: Scanning tunneling microscope, single-molecule junction, two-levels fluctuations
andom fluctuations between discrete conductance levels have long been reported for nanoscale junctions such as submicrometer Si metal oxide field effect transistors1 and are believed to be at the origin of flicker noise in electronic devices.2 Fluctuations observed from point contacts have been discussed in terms of nanomechanical oscillations due to inelastic electron tunneling processes,3 molecular motion due to conformational changes,4,5 charge trapping in defect states of the molecular junction,6 bond fluctuations that result from molecule attachment to or detachment from the contacting electrodes,7 and, most recently, resonant coupling of transmitted electrons to molecular vibrational degrees of freedom.8 Scanning tunneling microscopy (STM) has been used to measure time-dependent processes on surfaces, which include molecular conformational changes,9 11 rotary motions of molecules,12 vertical13,14 and lateral15 17 translations of atoms, and the thermal switching behavior of the magnetization of ferromagnetic nanostructures.18 Indication of the role of a molecular orbital in heating has recently been obtained from the decomposition of C60 molecules on Cu(110).19 At voltages corresponding to the second lowest unoccupied molecular orbital (LUMO+1) of C60 the electrical power necessary for decomposition of the molecule was found to be reduced. This observation has been attributed to enhanced vibrational excitation of C60 by inelastic electron scattering. In this article, excitation of single-C60 junctions is investigated in a nondestructive manner. Two-level conductance fluctuations, which occur at the transition between tunneling and contact, are investigated directly in the time domain. For these measurements, the time resolution of the current measurement of our STM was extended to ≈10 ns. A wide range of junction voltages is probed, and a drastic variation of the rate of fluctuations is found, which is interpreted in terms of electron injection into the LUMO and coupling to molecular vibrations. The experiments were performed with a custom-built scanning tunneling microscope operated at 7 K and in ultrahigh vacuum with a base pressure of 10 9 Pa. Cu(100) surfaces and
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chemically etched tungsten tips were cleaned by argon ion bombardment and annealing. C60 molecules were sublimated from a tantalum crucible and deposited on Cu(100) held at room temperature. The deposition rate was ≈1 ML min 1 as calibrated using a quartz microbalance and STM images. A monolayer (ML) is defined as one C60 molecule per 16 Cu atoms. During the deposition the residual gas pressure stayed below 5 10 8 Pa. An ordered C60 superstructure was obtained by subsequent annealing of the sample at 500 K for several minutes. Prior to conductance measurements, the tip was controllably indented into the substrate surface until STM images of C60 exhibited submolecular resolution and spectra of the differential conductance (dI/dV) acquired from Cu(100) were featureless. To acquire time-resolved conductance fluctuations, a transimpedance amplifier with a 3 dB cutoff frequency of 30 MHz at 100 pF was used. The input capacitance is due to the shielded cable connecting the amplifier to the STM tip. The output of the transimpedance amplifier was fed into an oscilloscope with a sampling rate of 100 MHz as well as into the low-pass filtered (3 dB at 10 kHz) feedback loop input. Figure 1 shows a constant-current STM image of Cu(100) covered with C60. The bright and dim rows correspond to chains of C60 molecules that are oriented along the indicated crystallographic direction. These stripe patterns have been attributed to a C60-induced missing row reconstruction of the underlying substrate surface.22 Molecules adsorbed on single and double missing rows of reconstructed Cu(100) appear high and low, respectively. The intramolecular structure visible in the STM image is mainly due to the spatial distribution of the LUMO+1,20 which appears as a torus centered at the C60 pentagons. Thus, different adsorption orientations of the fullerene cage on the Cu(100) substrate can be inferred from the different motifs in the Received: April 20, 2011 Revised: July 26, 2011 Published: August 19, 2011 3593
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Nano Letters STM image (molecules 1 4, indicated by dashed circles in Figure 1). 21 The conductance G = I/V (I, current; V, sample voltage) of a C60 molecule adsorbed on a single missing row was recorded while approaching the tip from tunneling toward the molecule (Figure 2a, approach from left to right). The initial tip molecule distance, which is determined by the parameters I and V prior to opening the STM feedback loop, corresponds to a tip displacement Δz = 0. Dots show data sampled at an elevated time resolution of 10 ns. A solid line depicts conductance data recorded at a more conventional time resolution using a low-pass filter with
Figure 1. Constant-current STM image of C60-covered Cu(100) at 7 K (1.6 V, 0.1 nA, 90 nm 80 nm). Different structural motifs correspond to different C60 orientations:20,21 molecules 1 and 2 expose a carbon hexagon or pentagon, respectively, toward vacuum, while molecules 3 and 4 expose a C C bond between a carbon pentagon and hexagon or two carbon hexagons, respectively. Single (smr) and double (dmr) missing rows of Cu(100) are indicated.
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a 10 kHz roll-off. There are three characteristic distance ranges, which can be defined by the intersections of exponential fits to the low-pass filtered conductance data in the different ranges.23,24 The tunneling range (Δz < 55 pm) is separated from the contact range (Δz > 78 pm) by the intermediate transition range. Conductance data acquired at high time resolution (dots in Figure 2a) show two-level fluctuations, which notably occur in the transition range and extend in the contact range, too. Individual events can be discerned more easily in close-up views of time-resolved data at different displacements (Figure 2b). These data reconcile the results of previous density functional calculations,23 which predicted conductance fluctuations due to thermally activated abrupt transitions between two configurations of the tip molecule junction, with an apparently smooth transition in experimental data recorded at low time resolution (solid line in Figure 2a).23 Below we discuss geometric rearrangements of the junction, which may be at the origin of the distinct conductance levels. Compared to rigidity of the C60 cage and the Cu substrate, the STM tip apex and the bond of the molecule to the substrate are expected to be more flexible. Relaxations of the outermost atoms of the tip were reported to lead to clear deviations from an exponential current distance relation in metallic contacts.25 In the present case of a metal molecule contact, however, we hint that these relaxations are less significant. Whereas fluctuations are observed from molecules 3 and 4, they are absent at molecules 1 and 2, even when the tip is approached deep into the contact range. This difference is difficult to reconcile with a relaxation of the tip apex. It is consistent, however, with the different bonding of the C60 molecules to the Cu substrate. Molecules 1 and 2 bind with a C hexagon and a pentagon, respectively, to a double missing row and more atoms are involved in these bonds than for molecules 3 and 4, which are located at single missing rows, with a C C bond closest to the
Figure 2. (a) Conductance of a single C60 molecule adsorbed on Cu(100) versus vertical displacement, Δz, of the STM tip. Dots show data recorded at a sampling interval of 10 ns. A solid line depicts low-pass filtered data recorded with a 10 kHz roll-off frequency and sampled at 50 μs intervals. Zero displacement corresponds to the tip molecule distance defined by feedback loop parameters of 0.3 V and 500 nA. Conductance regions denoted tunneling, transition, and contact have been defined by the intersections of exponential fits to low-pass filtered data as reported previously.23,24 Inset: Constant-current STM image (0.31 nm 0.19 nm, 1.6 V, 0.1 nA) showing the position (cross) where the conductance data on molecule 3 were acquired. The contacted molecule exposes a C C bond between a pentagon and hexagon toward the tip. (b) Close-up views of the conductance data in (a) displayed in the time domain. During the time interval shown (120 μs) the tip displacement was constant at 60 (top), 68 (middle) and 80 pm (bottom). τl and τh denote the residence times in the low and high conductance levels, respectively. (c) Averaged conductances, ÆGlæ and ÆGhæ, of low and high levels, respectively, on a logarithmic scale. The conductance of each level has been averaged over a time interval of 125 μs. The solid line depicts low-pass filtered data as in (a). 3594
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Figure 3. Duty cycle τh/(τh + τl) versus sample bias. The data were obtained from a contact at Δz ≈ 68 pm, i.e., in the transition range.
metal. We therefore tentatively suggest that the low and high conductance configurations correspond to molecules which are slightly tilted with respect to each other. While only one of the configurations is stable on the pristine surface, the presence of the STM tip induces a second state.26 Depending on the tip molecule distance, either of these states may be energetically more favorable. A low conductance is observed in the initial junction configuration with the tip centered above a C C bond. At high conductance, the molecule is rotated to approach one of the C atoms of the C C bond closer to the tip apex. To remove high-frequency noise from the conductance data, we separately time-averaged conductances in the low and high levels, ÆGl(Δz)æ and ÆGh(Δz)æ. In a displacement range between ≈55 and ≈70 pm, ÆGlæ and ÆGhæ evolve exponentially at almost identical slopes. This similarity is consistent with the above suggested junction geometries; i.e., in this displacement range the molecule appears to fluctuate between two orientations, which result in different tunneling currents. For larger tip displacement, smaller slopes of the conductances of high and low levels observed between ≈70 pm and ≈80 pm suggest that the molecule fluctuates now between two contact configurations. Previously it was proposed that the transitions between junction configurations are driven by inelastic electron scattering.23 The present data provide further and more direct support to this interpretation. Figure 3 shows the so-called duty cycle, τh/(τh + τl),27 where τh and τl denote the mean residence times in the high and low conductance levels, respectively (Figure 2b). The duty cycle was evaluated near the middle of the transition region at Δz = 68 pm. At this tip molecule separation, for voltages below 0.05 V, the junction is predominantly in its high conductance state, τh/(τh + τl) ≈ 100%. The duty cycle decreases at positive voltages; i.e., the energetically less favorable geometry occurs more frequently. The excitation process may be further characterized from the voltage dependence of the conductance fluctuations (Figure 4). These data have been obtained by moving the tip approximately to the middle of the transition range, disabling the feedback loop, and then varying the sample voltage. As the voltage rises the current increases as well (Figure 4a). Figure 4b shows the evolution of the fluctuation rate ν. Fluctuations are rare at V < 0.05 V but become more frequent for 0.05 V e V e 0.3 V. The rate ν stays almost constant for V > 0.3 V. Sample voltages exceeding 0.6 V typically led to a deterioration of the contact, either by removal of the molecule from the surface or by transfer of tip material to the sample. As the current, which serves to excite the molecule, varies over the voltage range of Figure 4, it is interesting to consider the normalized rate ν/I in Figure 4c (dots).28 It shows a similar evolution as ν but drops at V > 0.3 V. Given that C60 exhibits vibrational modes with energies up to ≈200 meV,19,29 the rise of the normalized rate could reflect the
Figure 4. (a) Current I versus sample voltage V measured at a tip molecule distance in the transition region (Δz ≈ 68 pm). (b) Rate of fluctuations (evaluated from 3 ms time intervals). (c) Normalized rate of fluctuations ν/I (dots) along with a dI/dV spectrum of the C60 LUMO (solid line). The spectrum was acquired in the tunneling range (1 nA at 0.55 V).
fact that a wider range of modes may be excited at increased bias. In contradiction to such a scenario, however, fluctuations are absent at reversed bias. The polarity dependence of the cross sections for inelastic tunneling processes is too small to explain the asymmetry observed here.30 Instead, we suggest that the excitation of the molecule is due to electrons from the tip which are injected into the LUMO and efficiently couple to molecular vibrations thus inducing fluctuations. This interpretation is corroborated by the dI/dV spectrum of C60 (Figure 4c, solid line). The spectrum shows the LUMO of the molecule and essentially matches the normalized rate.31 Heating of molecules by resonant tunneling through molecular states and enhanced emission of vibrational modes has indeed been proposed theoretically.32 In agreement with this model, the current required to decompose C60 molecules was found to depend on the bias voltage.19 It should be noted, however, that model calculations also indicated the possibility of molecular cooling.33,29 Injection of electrons with energies slightly below a molecular resonance may occur along with the absorption of vibrational quanta thus cooling the junction.33,29 As the present data (Figure 4c) do not exhibit a decrease of ν/I close to the LUMO maximum a resonant cooling effect appears to be too small to be detected in our measurements of the fluctuation rate. The electric field between tip and sample appears to play a minor role for conductance fluctuations. Conductance data acquired at voltages between 0.1 and 0.5 V showed that fluctuations always occur close to contact formation. No fluctuations were observed in the tunneling range. If the electric field played a major role, then higher voltages would be expected to induce fluctuations at larger tip molecule distances. In summary, the conductance of a single-molecule junction in a low-temperature STM has been measured at nanosecond time resolution. Two-level conductance fluctuations between tunneling and contact have been observed from molecules adsorbed to single missing rows of the Cu(100) substrate. The fluctuations reflect reversible conversions between different tip molecule junction configurations. The fluctuation rate varies significantly with the voltage applied to the junction. Its similarity to the spectral signature of the C60 LUMO indicates that electrons injected to the LUMO efficiently couple to molecular vibrations. 3595
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Nano Letters No fluctuations occur for molecules at double mission rows, presumably owing to their different interaction with the substrate. It will be interesting to extend the experimental approach presented here to other molecules to gain further insight into the role of molecular geometry and bonding in controlling energy transfer in single-molecules junctions.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Present Addresses §
Institut f€ur Physik, Technische Universit€at Ilmenau, D-98693 Ilmenau, Germany.
’ ACKNOWLEDGMENT Financial support by the Deutsche Forschungsgemeinschaft through SFB 677 and discussions with Th. Frederiksen (Donostia International Physics Center, San Sebastian) and M. Brandbyge (Technical University of Denmark, Lyngby) are acknowledged. ’ REFERENCES (1) Ralls, K. S.; Skocpol, W. J.; Jackel, L. D.; Howard, R. E.; Fetter, L. A.; Epworth, R. W.; Tennant, D. M. Phys. Rev. Lett. 1984, 52, 228–231. (2) Dutta, P.; Horn, P. M. Rev. Mod. Phys. 1981, 53, 497–516. (3) Kaun, C.-C.; Seidemann, T. Phys. Rev. Lett. 2005, 94, 226801. (4) Donhauser, Z. J.; Mantooth, B. A.; Kelly, K. F.; Bumm, L. A.; Monnell, J. D.; Stapleton, J. J.; D. W. Price, J.; Rawlett, A. M.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 2001, 292, 2303–2307. (5) Wassel, R. A.; Fuierer, R. R.; Kim, N.; Gorman, C. B. Nano Lett. 2003, 3, 1617–1620. (6) He, H. X. J. Am. Chem. Soc. 2001, 123, 7730–7731. (7) Ramachandran, G. K.; Hopson, T. J.; Rawlett, A. M.; Nagahara, L. A.; Primak, A.; Lindsay, S. M. Science 2003, 300, 1413–1416. (8) Secker, D.; Wagner, S.; Ballmann, S.; H€artle, R.; Thoss, M.; Weber, H. B. Phys. Rev. Lett. 2011, 106, 136807. (9) Lastapis, M.; Martin, M.; Riedel, D.; Hellner, L.; Comtet, G.; Dujardin, G. Science 2005, 308, 1000–1003. (10) Nacci, C.; Lagoute, J.; Liu, X.; F€olsch, S. Phys. Rev. B 2008, 77, 121405(R). (11) Wang, Y. F.; Kr€oger, J.; Berndt, R.; Hofer, W. A. J. Am. Chem. Soc. 2009, 131, 3639–3643. (12) Gao, L.; Liu, Q.; Zhang, Y. Y.; Jiang, N.; Zhang, H. G.; Chang, Z. H.; Qiu, W. F.; Du, S. X.; Liu, Y. Q.; Hofer, W. A.; Gao, H.-J. Phys. Rev. Lett. 2009, 101, 197209. (13) Eigler, D. M.; Lutz, C. P.; Rudge, W. E. Nature 1991, 352, 600–603. (14) Agraït, N.; Rodrigo, J. G.; Vieira, S. Phys. Rev. B 1993, 47, 12345–12348. (15) Stroscio, J. A.; Celotta, R. J. Science 2004, 306, 242–247. (16) Stroscio, J. A.; Tavazza, F.; Crain, J. N.; Celotta, R. J.; Chaka, A. M. Science 2006, 313, 948–951. (17) Sperl, A.; Kr€oger, J.; Berndt, R. Phys. Rev. B 2010, 81, 035406. (18) Krause, S.; Herzog, G.; Stapelfeldt, T.; Berbil-Bautista, L.; Bode, M.; Vedmedenko, E. Y.; Wiesendanger, R. Phys. Rev. Lett. 2009, 103, 127202. (19) Schulze, G.; Franke, K. J.; Gagliardi, A.; Romano, G.; Liu, C. S.; Rosa, A. L.; Niehaus, T. A.; Frauenheim, Th.; Carlo, A. D.; Pecchia, A.; Pascual, J. I. Phys. Rev. Lett. 2008, 100, 136801. (20) Lu, X.; Grobis, M.; Khoo, K. H.; Louie, S. G.; Crommie, M. F. Phys. Rev. Lett. 2003, 90, 096802. (21) Neel, N.; Kr€oger, J.; Limot, L.; Berndt, R. Nano Lett. 2008, 8, 1291–1295.
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(22) Abel, M.; Dimitriev, A.; Fasel, R.; Liu, N.; Barth, J. V.; Kern, K. Phys. Rev. B 2003, 67, 245407. (23) Neel, N.; Kr€oger, J.; Limot, L.; Frederiksen, T.; Brandbyge, M.; Berndt, R. Phys. Rev. Lett. 2007, 98, 065502. (24) Kr€ oger, J.; Neel, N.; Limot, L. J. Phys.: Condens. Matter 2008, 20, 223001. (25) Olesen, L.; Brandbyge, M.; Sorensen, M. R.; Jacobsen, K. W.; Laegsgaard, E.; Stensgaard, I.; Besenbacher, F. Phys. Rev. Lett. 1996, 76, 1485. (26) Lang, N. D. Phys. Rev. B 1992, 45, 13599. (27) Ralls, K. S.; Ralph, D. C.; Buhrman, R. A. Phys. Rev. B 1989, 40, 11561–11570. (28) In the experiment, current, voltage, and distance are linked. Therefore, it is not clear whether the relation between the fluctuation rate and the current is linear. At the currents used in the transition region, the rate of electrons arriving at the molecule is comparable to vibrational frequencies suggesting that two-electron processes are not unlikely to occur. (29) Romano, G.; Gagliardi, A.; Pecchia, A.; Carlo, A. D. Phys. Rev. B 2010, 81, 115438. (30) Lauhon, L. J.; Ho, W. Rev. Sci. Instrum. 2001, 72, 216–223. (31) The rising edge of ν/I occurs ≈0.1V below the rising edge of the LUMO feature. It should be noted, however, that the spectrum in Figure 4c was recorded in the tunneling range. At contact, the LUMOrelated signature is shifted to lower voltages.21 (32) Pecchia, A.; Pecchia, G. R. A.; Carlo, A. D. Phys. Rev. B 2007, 75, 035401. (33) Galperin, M.; Saito, K.; Balatsky, A. V.; Nitzan, A. Phys. Rev. B 2009, 80, 115427.
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