NANO LETTERS
Effect of Local Environment on Molecular Conduction: Isolated Molecule versus Self-Assembled Monolayer
2005 Vol. 5, No. 1 61-65
Yoram Selzer,†,| Lintao Cai,‡ Marco A. Cabassi,‡ Yuxing Yao,§ James M. Tour,*,§ Theresa S. Mayer,*,‡ and David L. Allara*,† Department of Chemistry and Materials Research Institute and Department of Electrical Engineering, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802, and Department of Chemistry and Center for Nanoscale Science and Technology, Rice UniVersity, Houston, Texas 77005 Received October 1, 2004
ABSTRACT Developing a fundamental understanding of molecular conduction in different device environments is essential to the advance of molecular electronics. We show through a quantitative comparison of two types of junctions with the same molecule − one based on an isolated individual molecule and the other on a self-assembled monolayer − that intrinsic differences in the conduction per molecule as large as several orders of magnitude can exist simply as a function of the presence or absence of neighboring molecules. This behavior can be understood on the basis of thermal and electrostatic effects that depend critically on the local molecular environment. These results will help to unify data obtained from disparate device structures and to provide an improved basis for designing future molecular electronic devices.
A central question in the science and technology of molecular electronics is: “what is the conductivity of a junction containing an individual molecule?”1 While individual molecules can serve as electronic devices,2 presently it is not clear how to make quantitative comparisons between electrical measurements of molecular junctions that contain large ensembles of molecules and individual molecule junctions, since this inevitably relies on normalization to a single molecule.3 On one hand, it has recently been shown that the current scales with the number of molecules in the junction when individual and small ensembles of molecules are embedded in an insulating matrix, i.e., N molecules carry N times as much current as an individual molecule.4-6 On the other hand, measurements of junctions fabricated using a variety of other methods have normalized currents that vary by orders of magnitude.3 While small differences might be explained on the basis of an uncertainty in the number of molecules contacted and the quality of the contacts, it is unlikely that such factors alone can account for these large discrepancies. * Corresponding authors. E-mail:
[email protected]. E-mail:
[email protected]. † Department of Chemistry and Materials Research Institute, PSU. ‡ Department of Electrical Engineering, PSU. § Rice University. | Permanent address: School of Chemistry, Faculty of Exact Science, Tel Aviv University, Israel. 10.1021/nl048372j CCC: $30.25 Published on Web 12/02/2004
© 2005 American Chemical Society
Recent theoretical developments suggest that differences in the local molecular environment can have a profound influence on molecular conduction. These differences can be attributed to effects such as the electrostatic potential distribution across the molecular bridge,7,8 local heat conduction,9,10 coupling to the contacts and the surrounding thermal bath,11 and the vibrational density of states (VDOS) of the molecule,12 which is related to the thermal conduction properties and electron-vibrational mode coupling. The widest variation in these effects can be expected between the cases of junctions containing an individual isolated molecule and an individual molecule embedded in a densely packed monolayer. Thus, we explored the quantitative magnitude of these effects by directly comparing the temperature-dependent current-voltage I-V(T) behavior of two types of junctions that are composed of the same molecule, 1-nitro-2,5-di(phenylethynyl-4′-mercapto)benzene (see molecule 1 in Figure 1). One type of junction contains individual isolated molecules that bridge across a gap in electromigrated gold break junctions,13-16 and the other type contains several thousand molecules self-assembled between two gold nanowire segments (in-wire junction).17,18 The results of this study show that the normalized coherent conduction at low applied bias and low temperature is essentially the same in both junction
Figure 1. (a) Schematic of molecule 1 (1-nitro-2,5-di(phenylethynyl-4′-mercapto)benzene). (b) Cartoon of an electromigrated break junction (not to scale) and corresponding SEM image. (c) Cartoon of an in-wire molecular junction (not to scale) and SEM image of a junction aligned between two large-area electrodes.
configurations. In contrast, the conduction through an individual isolated molecule increases rapidly with increasing bias and becomes comparable to the conductance of several thousand otherwise identical molecules packed in a selfassembled monolayer (SAM). These results underscore the practical importance of correlating local environment with electrical properties when designing molecular electronic devices and offer a basis for comparing conduction in different types of molecular junctions and test configurations. Molecule 1 was selected because it is a well-characterized, conjugated, π-bonded molecular wire from which a highquality monolayer can be prepared.19,20 In-wire junctions containing a densely packed monolayer of 1 were prepared by electrochemical template replication following published procedures.17,18 Briefly, 2 µm long single-crystal Au nanowires were electrodeposited into polycarbonate membranes with 40 nm nominal pore diameter. After metal deposition, molecule 1 was self-assembled on all of the nanowire tips (1010-1011 cm-2) using chemical assembly techniques. The remaining 2 µm long metal nanowire segments were grown by either directly electrodepositing Au on top of molecule 1 or by first incorporating a thin layer of coalesced Pd nanoparticles on the free thiol end of molecule 1 prior to Au electrodeposition. Electrical measurements were conducted on individual nanowires immediately after they were released from the template and aligned between pairs of large-area lithographically defined Au electrodes by ac electric-field-assisted assembly.21 Both methods of depositing top contacts resulted in junctions with comparable I-V(T) characteristics. The room-temperature breakdown voltages of the in-wire junctions reported here were within the narrow range 2.5-2.8 V, consistent with the breakdown field strength expected for a single monolayer junction of molecule 1.17 Individual isolated molecule electromigrated junctions were fabricated using a method based on published procedures.13-16 Data in addition to our previous study15,16 were obtained in parallel with the in-wire junction measurements and the results were in full agreement with the earlier ones. All electrical measurements on both types of junctions were conducted in the same evacuated chamber with variable 62
Figure 2. Representative I-V curves measured at 10 K for an in-wire junction (solid black) and for an electromigrated individual molecule junction (red) comprising molecule 1. The dashed curve shows the in-wire current normalized to that of a single molecule, determined by dividing the solid black curve by the estimated SAM number of molecules in the junction, defined as I10K (see text).22 The upper inset, an expanded view of the -0.3 to +0.3 V range, shows that at low bias (e0.1 V) individual molecules in both junctions (dashed black and red curves) conduct similarly. With increasing bias it is evident that individual molecules in each case behave differently; until at bias values close to 1.0 V, an isolated molecule conducts such as a few thousand molecules in a monolayer.
temperature control between 10 and 300 K and voltage bias sweeps of 0 to (1 V.22 First we consider the data obtained at the lowest temperature of our experiments, 10 K. The I-V characteristics shown in Figure 2 are representative of those collected on in-wire junctions (black curve) and individual isolated molecule junctions (red curve). The current per molecule in the SAM, (ISAM 10K ) (dashed black curve), is obtained by dividing the in-wire curve by the estimated number of molecules in the junction, ∼5 × 103, which is determined based on the 40 nm diameter of the nanowires18 and the known packing density of ∼4.6 molecules/nm.20 While the current through an individual isolated molecule, Iisolated 10K , for SAM bias values less than 0.1 V is comparable to (I10K ) (see inset of Figure 2), with increasing bias the individual isolated molecule becomes much more conductive than an identical molecule surrounded on all sides by neighboring molecules in a densely packed SAM. Eventually, at a bias of 1.0 V, SAM 3 Iisolated 10K /I10K >10 , viz., the current through an individual isolated molecule increases up to a point at which it is comparable to that measured through a few thousand Nano Lett., Vol. 5, No. 1, 2005
otherwise identical molecules packed in a SAM. Thus, these data suggest that the conductivity of a bridge molecule can shift by several orders of magnitude under a fixed bias and temperature simply by changing its environment from isolated to being surrounded by neighboring molecules. To understand these observations in terms of transport mechanisms it is necessary to examine the full temperaturedependent I-V(T) characteristics. The two junctions behave very differently in two distinct ways, as can be seen in the Arrhenius plots shown in Figure 3. First, while the current through an in-wire junction shown in Figure 3a reveals a temperature independent behavior characteristic of coherent tunneling through the molecules, Figure 3b shows that for an individual isolated molecule there is a transition from coherent conduction at low temperatures (T < 100 K) to a thermally activated sequential hopping mechanism at temperatures higher than ∼100 K.15,16,23 Second, the observation at 10 K of orders of magnitude enhancement of the isolated molecule conduction over a SAM molecule with increasing bias holds over the entire coherent tunneling conduction regime (compare black and red curves in Figure 3b). This implies that there are inherent differences in the coherent transport through these junctions. Before proceeding to a discussion of the fundamental basis for these differences it is essential, however, to first understand why an isolated molecule 1 experiences a transition between coherent conduction to sequential hopping. We have previously argued that this transition results from the onset of torsional fluctuations of the phenyl rings in this molecule as the temperature increases.15,16 The comparison between the two types of junctions in this study gives further evidence to support this argument according to the following semiquantitative reasoning. If the tunneling time through a molecule (τ0) is comparable to the period of a characteristic vibrational mode (1/ν), inelastic processes during tunneling are probable.24 The activation barrier for hopping, ∆E, calculated from the slope of the red curves in Figure 3b at low bias (0.1 V) is 80 ( 20 meV.25 Approximating the tunneling barrier to be this value, associating τ0 with the uncertainty time p/∆E, and applying the molecule length N (taken here as 5 interacting π-units) according to τ0 ) pN/ ∆E,26 we find that τ0 ∼ 0.1 ps. The lower barrier for rotation of the phenyl rings in a free molecule 1 is estimated to be ∼40 meV, which, based on molecular dynamics simulations, corresponds to a rotational time of ∼0.1 ps at 100 K.12 This can explain the transition to the inelastic conduction process of sequential hopping with increasing temperature in the isolated molecule, because once fluctuations of the phenyl rings around the dihedral angle of θ ) 0 are initiated, electronic coupling between adjacent rings is decreased, dephasing is initiated, and the rise in kT allows sequential hopping to become a more efficient mechanism of transmission than direct tunneling.11,23 The barrier for rotation in this molecule is calculated to be >1.0 eV once it is immersed in a SAM.12 This high barrier could result in the failure to observe a transition to hopping in the in-wire junction due to increased frequencies and restricted amplitudes of the torsional modes in a close packed matrix. Nano Lett., Vol. 5, No. 1, 2005
Figure 3. Comparison of in-wire SAM junctions and isolated molecule junctions as a function of temperature (10-300 K) and bias (0.1-1.0 V in steps of 0.1 V). The lowest curve in both panels is for V ) 0.1 V. (a) I-V(T) of in-wire SAM junction and (b) total current of isolated molecule junction (red curve) compared to normalized current-per-molecule for in-wire SAM junction (black). The in-wire junctions conduct coherently over the entire temperature range studied, while the isolated molecule junctions show a transition at high temperatures to activated hopping conduction. At a bias of 0.1 V, considering the error bars, the currentper-molecule in both cases is comparable. However, at larger biases (>0.4 V), the individual isolated molecule conducts like several thousand molecules assembled in a monolayer in the in-wire junctions. The error bars in panel b show the distribution of currents measured for the in-wire junctions over the entire temperature range, and through the isolated molecule junctions for T e 100 K. Error bars in the activated regime were not inserted for clarity and can be found in ref 16.
We now turn to discuss the marked difference between the two types of junctions in the low temperature (0.4 V, which is essentially where all fundamental vibrational modes of the molecule 64
Figure 4. IET spectrum collected at 10 K from an in-wire junction by directly measuring d2I/dV2 with a lock-in amplifier (2 mV steps; 4 mV excitation amplitude). Peaks were assigned based on DFT calculations (see ref 20) and some of the typically visible features are labeled in the spectrum. Note the expected inverted symmetry around zero bias. Examples of key features related to molecular conductance are the ring modes (defined by the Wilson-Varasanyi terminology for aromatic rings): (a) 138 mV, υ(8a) in ring 2 and, υ(18a) in rings 1 and 3, (b) 180 mV, υ(18a) in all rings.
have been excited by inelastic tunneling (Figure 4) and the inelastic heating is maximized. The results presented in this article show that significant differences in molecular conduction behavior can result from simple changes in the molecular environment. While these effects are somewhat complex in terms of the temperature and bias dependence, it is encouraging that the growing body of theory describing molecular conduction provides a sound, even semiquantitative, basis for the observed behavior, in particular, the relationship between the local environment of the molecules and the electrostatic potential distribution across the molecule, density of excited vibrational modes, heat dissipation to the surroundings, and electron-phonon coupling. Any understanding of molecular conductance or quantitative comparison between different molecular devices will depend on a detailed understanding of such effects. Acknowledgment. The authors acknowledge support from the Defense Advanced Research Project Agency/Office of Naval Research and the Air Force Office of Scientific Research (Y.S., DA). Y.S. thanks Profs. M. Ratner and A. Nitzan for helpful comments on the initial manuscript. References (1) Nitzan, A.; Ratner, M. A. Science 2003, 300, 1384. (2) Joachim, C.; Gimzewski, J. K.; Aviram, A. Nature 2000, 408, 541. (3) Salomon, A.; Cahen, D.; Lindsay, S. M.; Tomfohr, J.; Engelkes, V. B.; Frisbie, C. D. AdV. Mater. 2003, 15, 1881. (4) Cui, X. D.; Primak, A.; Zarate, X.; Tomfohr, J.; Sankey, O. F.; Moore, A. L.; Moore, T. A.; Gust, D.; Harris, G.; Lindsay, S. M. Science 2001, 294, 571. (5) Xu, B. Q.; Tao, N. J. J. Science 2003, 301, 1221. (6) Kushmerick, J. G.; Naciri, J.; Yang, J. C.; Shashidhar, R. Nano Lett. 2003, 3, 897. (7) Nitzan, A.; Galperin, M.; Ingold, G.; Grabert, H. J. Chem. Phys. 2002, 117, 10837. (8) Liang, G. C.; Ghosh, A. W.; Paulsson, M.; Datta, S. Phys. ReV. B 2003, 69, 115302. (9) Segal, D.; Nitzan, A.; Hanggi, P. J. Chem. Phys. 2003, 119, 6840.
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(10) Segal, D.; Nitzan, A. J. Chem. Phys. 2002, 117, 3915. (11) Segal, D.; Nitzan, A. Chem Phys. 2001, 268, 315. (12) Seminario, J. M.; Derosa, P. A.; Bastos, J. L. J. Am. Chem. Soc. 2002, 124, 10266. (13) Park, H.; Park, J.; Kim, A. K. L.; Anderson, E. H.; Alivisatos, P. A.; McEuen, P. L. Nature 2000, 407, 57. (14) Park, J.; Pasupathy, A. N.; Goldsmith, J. I.; Chang, C.; Yaish, Y.; Petta, J. R.; Rinkoski, M.; Sethna, J. P.; Abruna, H. D.; McEuen, P. L.; Ralph, D. C. Nature 2002, 417, 722. (15) Selzer, Y.; Cabassi, M. A.; Mayer, T. S.; Allara, D. L. J. Am. Chem. Soc. 2004, 126, 4052. (16) Selzer, Y.; Cabassi, M. A.; Mayer, T. S.; Allara, D. L. Nanotechnology 2004, 15, S483. (17) Mbindyo, J. K. N.; Mallouk, T. E.; Mattzela, J. B.; Kratochvilova, I.; Razavi, B.; Jackson, T. N.; Mayer, T. S. J. Am. Chem. Soc. 2002, 124, 4020. (18) Cai, L. T.; Skulason, H.; Kushmerick, J. G.; Pollack, S. K.; Naciri, J.; Shashidhar, R.; Allara, D. L.; Mallouk, T. E.; Mayer, T. S. J. Phys. Chem. B 2004, 108, 2827. (19) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour J. M. Science 1999, 286, 1550. (20) Stapleton, J.; Harder, P.; Daniel, T.; Reinard, M. D.; Yao, Y.; Price, D.; Tour, J. M.; Allara, D. L. Langmuir 2003, 19, 8245; a more recent report contains minor corrections and full Raman mode calculations [see, Richter, L. J.; Yang, C. S.-C.;. Wilson, P. T.; Hacker, C. A.; van Zee, R. D.; Stapleton, J. J.; Allara, D. L.; Tour, J. M. J. Phys. Chem. B 2004, 108, 12547-12559]. (21) Smith, P. A.; Nordquist, C. D.; Jackson, T. N.; Mayer, T. S.; Martin, B. R.; Mbindyo, J.; Mallouk, T. E. Appl. Phys. Lett. 2000, 77, 1399. (22) Based on SEM images, the in-wire junctions have a diameter of 40 nm ( 5 nm. With ∼4.6 molecules/nm2 (as measured by AFM, e.g., see ref 20) there are ∼5000 ( 1000 molecules in these junctions. This 20% error is much smaller than the variation in the current at low bias and low temperature (see black error bar on the bottom right of Figure 3). As for the isolated molecule junctions: the variation in currents is ∼5 (see red error bar in Figure 3), and hence we conclude that the number of bridging molecules in these junctions is small and very close to the single molecule limit. Due to the sudden and statistical nature of the method used to fabricate the latter junctions (instantaneous eruption to form a gap in the gold wire), it is reasonable to assume that any bridging molecules cannot be organized and that they behave independently. The larger variance in the in-wire currents is a result of two contributions. (i) The exact number of molecules in each junction is not known. (ii) A random distribution of the dihedral angle, θ, between adjacent phenyl rings
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(23) (24) (25)
(26) (27) (28) (29) (30)
(31) (32)
in each molecule is also expected. It has been theoretically shown that by changing the value of θ for one phenyl ring from 0 to π/2, the electronic coupling through the molecule can be reduced by a factor of ∼8 (see for example Newton, M. D. Int. J. Quantum Chem. 2000, 77, 255). In contrast, an isolated molecule is expected to be coplanar at 10 K, as no local energetic minimum at θ*0 is expected for this molecule (see ref 28). The results and errors in Figure 2 are based on five junctions of each type. Segal, D.; Nitzan, A.; Davis, W. B.; Wasielewski, M. R.; Ratner, M. A. J. Phys. Chem. B 2000, 104, 3817. Burin, A. L.; Berlin, Y. A.; Ratner, M. A. J. Phys. Chem. A 2001, 105, 2652. This value is low considering that the theoretical HOMO-LUMO gap of molecule 1 is ∼3.0 eV (see Seminario, J. M.; Zacarias, A. G.; Tour, J. M. J. Am. Chem. Soc. 2000, 122, 3015). However, vibrational coupling is not included in these estimations, and so the barrier could be very different once the molecule is “virtually” charged, see ref 24 for details. The change in barrier height should be in the order of the reorganization energy. While no specific calculation of this property for molecule 1 is currently available, we note that it has been suggested that the HOMO-LUMO gap in this molecule is decreased to ∼1.0 eV when charged by 1 electron (see Seminario, J. M.; Zacarias, A. G.; Tour, J. M. J. Am. Chem. Soc. 2000, 122, 3015). Image charge also affects the size of the gap. Recently it has been shown to decrease the gap of a related molecule from the theoretically expected value of ∼2.5 eV to experimentally measured value of ∼200 meV (see Kubatkin, S.; Danilov, A.; Hjort, M.; Cornil, J.; Breda, J.; Stuhr-Hansen, N.; Hedegard, P.; Bjornholm, T. Nature 2003, 425, 698). Nitzan, A.; Jortner, J.; Wilkie, J.; Burin, A. L.; Ratner, M. A. J. Phys. Chem. B. 2000, 104, 5661. Troisi, A.; Ratner, M. A.; Nitzan, A. J. Chem. Phys. 2003, 119, 5782. Pecchia, A.; Gheorghe, M.; Di Carlo, A.; Lugli, P.; Niehaus, T. A.; Frauenheim, T.; Scholz, R. Phys. ReV. B 2003, 68, 235321. Emberly, E. G.; Kirczenow, G. Phys. ReV. Lett. 2003, 91, 188301. When the energy of the injected charge into a molecule is within few kT from a resonance level, there is no clear distinction between transmission in-resonance and in off-resonance. We artificially make such a distinction in order to simplify the discussion and to explain various heat dissipation mechanisms. Chen, Y.; Zwolak, M.; Di Ventra, M. Nano. Lett. 2003, 3, 1391. Kushmerick, J. G.; Lazorcik, J.; Patterson, C. H.; Shashdihar, R.; Seferos, D. S.; Bazan, G. C. Nano Lett. 2004, 4, 639.
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