NANO LETTERS
Atomistic Mechanics and Formation Mechanism of Metal-Molecule-Metal Junctions
2009 Vol. 9, No. 6 2433-2439
Makusu Tsutsui,† Masateru Taniguchi,*,†,‡ and Tomoji Kawai*,† The Institute of Scientific and Industrial Research, Osaka UniVersity, Ibaraki, Osaka 567-0047, Japan, and PRESTO, Japan Science and Technology Agency, Honcho, Kawaguchi, Saitama 332-0012, Japan Received April 9, 2009
ABSTRACT The present Letter reports a quantitative analysis of contact mechanics in metal-molecule-metal junctions at a single atom/molecule level through investigating their lifetime at cryogenic temperature. We elucidated that the force breaking mechanism of atomic/molecular junctions is stretching speed dependent, attributed to suppression of contact structure relaxation processes at high strain rate conditions. We also provide solid evidence that strain exerted in the preformation stage of molecular junctions poses extra strain energy that accelerates their eventual fracture. Nonetheless, we find that single-molecule junctions subjected to mechanical stretching at 0.6 pm/s can be held for ∼100 s on average at 77 K and for a much prolonged period when freezing the elongation after forming the molecular junctions by virtue of moderate thermal destabilizations, the fluctuation-free condition of which provides optimal experimental platform for performing reliable measurements of single molecule electron transport properties.
The dramatic improvements in fabrication techniques of metal-molecule-metal structures over the past decade have provided new opportunities for studying electron transport through individual molecules.1-6 Particularly, the break junction technique has proven to be a powerful tool for controlling the exact number of molecules bridging two electrodes, thereby allowing the electrical transport measurements at the single-molecule level.6 The method involves sequential breaking of electrode-molecule contacts via the mechanical manipulations of the electrode gap distance. This simple principle of operation led to its wide usage in recent single-molecule experiments;6-16 electrical characteristics of diverse types of molecules have been investigated to date exploiting the scanning probe microscopy setup6,10-12 and mechanically controllable break junctions (MCBJs).13-16 Although easy to perform, however, interpretation and evaluation of the experimental data obtained by the break junction method are often difficult. One of the fundamental issues in the break junction experiments is a lack of understanding of the details of underlying atomistic mechanics during forming and breaking of metal-molecule-metal systems. It is only quite recently that molecular junctions subjected to mechanical stretching have been revealed to suffer considerable deterioration of * Corresponding authors: tel +81-6-6879-4289, fax +81-6-6875-2440, e-mail
[email protected]. † Osaka University. ‡ Japan Science and Technology Agency. 10.1021/nl901142s CCC: $40.75 Published on Web 05/22/2009
2009 American Chemical Society
the metal-molecule contact stability by the accumulated tensile forces even under extremely low strain rate conditions of 10-12 m/s.17-19 As such, it remains an ongoing problem to elucidate the whole picture of molecular junction formation mechanisms. This is in fact an important issue as fabrication of stable and well-defined metal-molecule-metal structures is a prerequisite not only for reliable electron transport measurements but also toward achieving the ultimate goal of practical realization of single molecule devices. The present Letter reports quantitative analyses of contact mechanics in metal-molecule-metal junctions. We examined strain rate dependence of single molecule junction lifetime in a cryogenic vacuum at 77 K. Effects of thermal fluctuations were insignificant under this low temperature condition. Therefore, we were able to evaluate the exact breakdown mechanism of metal-molecule-metal junctions in response to mechanical tension. Reliable investigation of molecular junction formation/ breakdown mechanism requires atomic-scale control of the mechanical deformation processes. Lithographically defined MCBJ was employed for this purpose, which possesses subpicometer mechanical stability.17,20-22 The nano-MCBJ sample was prepared as follows. First, Au junctions of 100 nm × 80 nm cross sections were fabricated on a polyimidecoated phosphor bronze substrate using electron beam lithography and subsequent lift-off. The substrate was then exposed to CF4/O2 reactive ion etching. As a result, we
Figure 1. (a) Schematic illustration of the experimental setup based on nanofabricated mechanically controllable break junctions utilized to measure single molecule junction lifetime at 77 K. (b) Typical G-t curves acquired with (right curve) and without (left curve) BDT molecules being introduced.
obtained free-standing Au nanojunctions. The geometry of the MCBJ samples was configured to provide the displacement ratio of r ) ∆d/∆z ∼ 2 × 10-4, where ∆d and ∆z denote the displacements of the Au junctions and the moving distance of the piezo-actuator-driven pushing rod utilized for bending the substrate, respectively.17,21 The break junction experiment starts by breaking the junctions in a dilute toluene solution of test molecules. We chose benzenedithiolate (BDT), which was well-known to form stable contact with Au electrodes via strong Au-S chemical bonding.23,24 BDT molecules were allowed to spontaneously adsorb onto the fracture surface after the initial junction separation. Subsequently, the sample chamber was evacuated so as to remove the solvent by evaporation. When vacuum pressure reached ∼10-5 Torr, we performed formation and breaking of metal-molecule-metal junctions repeatedly under a constant dc bias voltage Vb of 0.2 V by manipulating the substrate bending through the automated resistance feedback control.17-19 The junction stretching rate Vd is set to 6 nm/s at this stage. After 100 cycles of open/ close processes, we gradually decreased the temperature T0 to 77 K. When stabilized at 77 K, the break junction measurements were exhibited under various Vd conditions covering a wide range from 12 to 0.0006 nm/s (Figure 1a). Temporal conductance change during breaking of the junctions was recorded using a picoammeter and a digital oscilloscope. The breaking procedure begins with elongation of Au nanojunctions. Upon stretching, the cross section shrinks at the narrowest constriction and the conductance decreases accordingly in a stepwise fashion, reflecting the discrete 2434
nature of the atom-sized contact deformation processes.21 A long and flat plateau was often observed at 1G0, which signifies formation of Au single atom contact.21 Here, G0 ) 2e2/h is the quantum conductance where e and h denote the electron charge and Planck’s constant, respectively. Further tension leads to complete separation of Au contacts. We have examined 2000 trials of junction breaking at each Vd condition. In many cases, the conductance rapidly drops to zero after the breakdown indicating electron tunneling across a vacuum gap in the broken junction. Occasionally, however, we observed another series of conductance steps at around 0.01G0 (Figure 2a,b). The corresponding conductance histograms constructed with the trace data demonstrating clear staircase-like structures at the low-conductance regime reveal regularly spaced peaks at 0.011nG0 (n ) 1, 2, 3,...).18,23 It is noticeable that there is little difference in the peak profiles of histograms obtained at different Vd conditions (Figure 2c). These peaks are attributed to conductance states of n molecules bridging over the junction gap in parallel. Therefore, the BDT single molecule conductance GBDT is 0.011G0 irrespective of Vd.23 It should be pointed out that this GBDT at 77 K is equivalent to that at room temperature.18,23 The fact that GBDT varies little with T0 down to 77 K is not unexpected. In general, electron transport in metalmolecule-metal systems is governed by the carrier injection barrier at the metal-molecule link. For Au-BDT-Au junctions, the energy mismatch at the contact is ∼2.5 eV due to the relatively large HOMO-LUMO gap energy of ∼5 eV. The electron transport is thus nonresonant tunneling at the present bias voltage of 0.2 V. This explains why GBDT remains the same at 293-77 K since the electron tunneling transmission is essentially temperature independent. Length of the conductance plateaus at GBDT represents lifetime of BDT single molecule junctions τBDT (Figure 1b).18 It is evident from the traces displayed in Figure 2a,b that τBDT increases with decreasing Vd. This trend can be addressed more definitively by examining the τBDT distributions (Figure 3a). For quantitative analyses of τBDT-Vd dependence, we extracted the peak lifetime τB from each τBDT histograms and plotted as a function of Vd (Figure 3b). τB corresponds to the lifetime of the single molecule junctions of a specific configuration formed with statistical significance. The monotonic decrease of τB with Vd manifests the substantial role of mechanical stretching induced tensile forces on the junction stability. It should be noticed that τB at 77 K is lower than that at 293 K under low Vd conditions (Figure 3b). This result contradicts the previous experimental observations of thermoactivated spontaneous breakdown of molecular junctions,11,18,19 which predicts exponentially prolonged lifetime at low temperatures. We analyzed the junction breakdown length LB ) VdτB aiming to reveal the underlying physics of the unexpected τB-T0 relation. LB is plotted as a function of Vd in Figure 3c. Whereas LB scales linearly with log(Vd) at 293 K, the data at 77 K demonstrate completely different LB-Vd characteristics: LB is constant at ∼0.25 nm when Vd g 0.6 nm/s while it suddenly decreases to ∼0.09 nm at lower Nano Lett., Vol. 9, No. 6, 2009
Figure 2. (a) Examples of conductance curves obtained for Au-BDT-Au junctions at 77 K under various Vd conditions ranging from 6 to 0.0006 nm/s. Color code: Vd ) 6 nm/s (red); Vd ) 0.6 nm/s (green); Vd ) 0.06 nm/s (blue); Vd ) 0.006 nm/s (yellow); Vd ) 0.0006 nm/s (purple). (b) Expanded view of the conductance curves obtained at the high Vd conditions (the same color coding as that in (a)). The clear conductance steps at ∼0.01G0 signify formation of stable Au-BDT-A junctions. (c) Corresponding histograms constructed with traces demonstrating clear conductance steps at the low conductance regime. The solid lines are the Gaussian fit to the peak profiles. The color code follows that applied in (a).
Figure 3. (a) Distributions of Au-BDT-Au single molecule junction lifetime at 77 K under various Vd conditions. Arrows indicate the single peak positions of the histograms. The color code is the same as that in Figure 2. (b) τB-Vd and (c) LB-Vd plots of Au-BDT-Au single molecule junctions obtained at 77 K. Results acquired at 293 K is also shown for comparison.
Vd. We have previously shown that the breakdown mechanisms of molecular junction subjected to mechanical tension can be classified into three distinct groups in terms of LB-Vd characteristics:17 (1) thermoactivated self-breaking at low Vd where junctions fracture thermally under negligible forces with infinitesimal LB; (2) force-accelerated spontaneous breaking at intermediate Vd where mutual effects of Nano Lett., Vol. 9, No. 6, 2009
metal-molecule bond weakening via the mechanical forces and the thermal fluctuations leads to LB ∼ log(Vd); (3) force breaking at high Vd where molecular junctions break almost exclusively by the tensile force accumulated on the junction at a constant LB with little contribution of thermal fluctuations. Likewise, the conspicuous difference in LB-log(Vd) dependence at T0 ) 293 and 77 K is interpreted as reflecting 2435
Figure 4. (a) Distributions of Au single atom contact lifetime at 77 K under various Vd conditions. Arrows indicate the single peak positions. The color code is the same as that in Figure 2. (b) τB-Vd and (c) LB-Vd plots of Au single atom contacts obtained at 77 K. The data at 293 K are also displayed.
influence of thermal fluctuations on the junction stability; the finite slope of LB-log(Vd) at 293 K indicates thermally activated junction breaking in conjunction with nonnegligible mechanical forces while the flat slopes at 77 K evidence force breaking of BDT junctions and hence little effects of thermal fluctuations under 4-fold lower thermal energy. As expected, BDT junctions suffer less significant thermal destabilizations at low temperatures. Then, what makes τBDT at 77 K shorter than that at 293 K? We have performed the lifetime measurements on Au single atom contacts to acquire deeper insight into this anomalous finding. The breakdown mechanism of BDT junctions is closely related to the Au nanocontacts counterpart due to the fact that molecular junctions consisting of Au-S anchoring group tend to fracture at Au-Au bonds in the vicinity of the metal-molecule link when stretched mechanically.23-25 Thus, we can expect to obtain valuable information concerning the BDT junction breakdown mechanism through investigating LB-Vd characteristics of Au contacts. Conductance traces are measured at 77 K without introducing BDT molecules. We deduced lifetimes of Au single atom contacts τAu (Figure 1b) from the length of conductance plateau that emerged at 0.8G0-1.2G0 during junction stretching at Vd ) 12-0.0006 nm/s for each of 2000 conductance curves obtained. Histograms constructed with thus acquired τAu data show a single peak (Figure 4a). We obtained τB of Au single atom contacts from the single peaks in τAu histograms in Figure 4a and plotted as a function of Vd along with that at 293 K17 in Figure 4b,c. The monotonic decrease of τB with Vd manifests considerable role of mechanical tension on fractures of Au single 2436
atom contacts in the Vd range measured (Figure 4b). In contrast, τB-Vd characteristics at 293 K demonstrate a saturation at τB ∼ 10 s, Vd e 0.06 nm/s indicative of the thermoactivated self-breaking as shown in Figure 4b.17 The thermal limit of Au single atom contact lifetime extends exponentially from ∼10 s at 293 K to ∼1040 s at 77 K. Force breaking is thus almost inevitable for Au single atom contacts at 77 K. This explains the absence of τB saturation at 77 K. Correspondingly, LB-Vd plots show a plateau-like feature, which consistently suggests force breaking of Au single atom contacts (Figure 4c). It is interesting to note that the LB plateau emerges at two different levels, ∼0.50 nm at Vd g 0.6 nm/s and ∼0.25 nm at Vd e 0.06 nm/s. We point out that these LB values are close to the critical elongation distance that Au single atom contacts can sustain.21,26 When stretched mechanically, the contacts undergo elastic deformation. Elongating by the amount of the Au-Au distance of the constituent atoms of the single atom contacts dAu ∼ 0.25 nm, the junctions either rupture or release the strain by evolving into a monatomic chain through incorporating an extra atom from the bulk region. Further displacing the single atom chain by dAu leads to the next point of breaking. At this point, the contact has a further chance to survive the breaking by lengthening the chain via the atom-pull-out process. As a consequence of this breakdown mechanism, force-breaking tends to occur at a specific elongation length of LB ∼ ndAu (n ) 1, 2, 3,...).21,26 Thus, the plateaus at LB ∼ 0.25 nm and ∼0.50 nm in the LB-Vd plots imply the existence of two distinct forcebreaking mechanisms involving formation and breaking of one (Vd e 0.06 nm/s) and two (Vd g 0.6 nm/s) atom chains, respectively. Nano Lett., Vol. 9, No. 6, 2009
Figure 5. (a) Distributions of Au single atom contact formed in parallel to BDT junctions at 77 K under various Vd conditions. Arrows indicate the single peak positions. The color code is the same as that in Figure 2. (b) τB-Vd and (c) LB-Vd plots of the parallel Au single atom contacts obtained at 77 K. The data for BDT single molecule junctions and Au single atom contacts are also displayed. Combined LB of parallel Au single atom contacts and BDT single molecule junctions shown by the solid squares well reproduces LB-Vd characteristics of Au single atom contacts.
The stretching rate dependence of single atom chain formations has been studied theoretically.27,28 It has been demonstrated by molecular dynamic simulations that whether single atom contacts evolve into a long monatomic chain is determined by competition between the rate of contact structure relaxations through surface reconstructions and lowenergy defect initiations and the contact elongation speed: low-Vd elongations prefer formations of low-energy short chains by virtue of effective thermodynamic structure relaxations that compensate the strain energy posed by the mechanical stretching, while high Vd conditions exceeding the spontaneous strain energy release rate favor formations of high-energy long single atom chains.28 The two LB regimes in Figure 4c thus indicate suppression of a certain relaxation mode at Vd g 0.6 nm/s that leads to preferential formation of a two-atom chain. Moreover, the double plateau structure has also been observed at room temperatures by Huang et al.29 The transition Vd in their case is around 5-20 nm/s, which is much higher than we find here, 0.6-0.06 nm/s.29 This discrepancy clearly represents that Au atoms are more mobile at room temperature than at 77 K; the resulting shorter relaxation time by virtue of more feasible contact atom rearrangements at room temperatures necessitates higher Vd conditions for attaining formation of long monatomic chains. We point out that LB-Vd characteristics of BDT and Au junctions at 77 K share several aspects in common: there are two well-defined LB plateaus at Vd g 0.6 nm/s and Vd e 0.06 nm/s (Figure 4c). The double-plateau structure of Au single atom contacts is attributed to preferred formation/ breaking of two (one) atom chains at high (low) Vd conditions, as we have explained above. Thus, it is anticipated by Nano Lett., Vol. 9, No. 6, 2009
analogy with the qualitative correspondence in the LB-Vd plots that under the force-breaking conditions, BDT single molecule junctions undergo fracture by the same mechanism as that of Au single atom contacts; mechanical stretching causes elastic elongation of the Au-Au link behind the rigid Au-S anchor that leads to Vd-dependent evolution of a single atom chain through the atom-pull-out process and eventual breakdown. We now discuss the physical origin of the short τBDT at 77 K. BDT junctions are found to undergo force breaking under Vd conditions of 0.0006 nm/s e Vd e 12 nm/s at a Au-Au bond behind a Au-S link by the same mechanism as that of Au single atom contacts at 77 K. It is thus expected that breakdown of BDT junctions occur preferentially at LB ) ndAu (n ) 1, 2, 3,...) as well, which is obviously not as shown by Figure 3c. We speculate that the low-temperature molecular junction stability is already affected in the preformation stage, i.e., point of time before BDT junctions become distinguishable by their conductance. Huisman et al.30 have revealed parallel bridging of molecules to Au contacts during stretching. Here, we propose that the molecular junction instability detected at low temperatures arises as a consequence of strain introduced in BDT junctions during stretching of Au/BDT parallel contacts. Presuming that BDT molecules are already connected to Au electrodes, it is logical to consider that mechanical stretching simultaneously elongates Au contacts and BDT junctions. To test this model, we explored Vd dependence of the Au/BDT parallel junction lifetime τAu/BDT (Figure 1b). τAu/BDT is obtained from the length of 1G0 plateaus that appeared in each of 2000 conductance traces measured for BDT junctions 2437
Figure 6. (a) Assignments of BDT single molecule junction formation/breakdown processes to the experimental G-L curve. (b) Scatter plots of L*Au/BDT versus L*BDT. Color code: 6 nm/s e Vd e 0.6 nm/s (black); 0.06 nm/s e Vd e 0.0006 nm/s (red).
(Figure 1b). A conductance window to extract τAu/BDT is set to 0.8-1.2G0 for GBDT , 1 G0. τB is deduced from the single peak positions in τAu/BDT histograms (Figure 5a). As shown in Figure 5c, taking the extra displacements associated with Au/BDT parallel junction stretching process into account by adding LB of Au/BDT and BDT junctions well-reproduce the double plateau LB-Vd characteristics of Au single atom junctions. That BDT junctions and Au contacts share the same LB-Vd characteristics is consistent with the previous observations that dithiol molecular junctions possess similar mechanical properties as that of Au single atom contacts. To further verify the validity of this molecular junction formation/breakdown mechanism, we investigate correlation between the breakdown length of Au single atom contacts formed during the course of BDT conductance measurements L*Au/BDT ) τAu/BDTVd and that of BDT single molecule junctions L*BDT ) τBDTVd (Figure 6a). It is expected that L*BDT varies inversely with L*Au/BDT since the amount of prestrain exerted on BDT junctions during stretching of the Au/BDT parallel contacts should scale with L*Au/BDT. Indeed, a downward sloping trend is found in the scatter plots of L*Au/BDT versus L*BDT (Figure 6b). This analytical evidence serves to validate that the molecules are indeed bridging in parallel to Au contacts and that the molecular junctions are strained during mechanical stretching of Au/molecule parallel junctions before they become “observable” by their conductance. 2438
On the basis of the above discussion, the peculiar temperature dependence of BDT junction lifetime measured by the break junction method can be explained in terms of Au single atom contact stability. At room temperatures, Au single atom contacts formed by the break junction method fracture thermally under negligible external forces within 10 s on average.17 Therefore, as long as the junction is stretched at slow enough speed, we can have Au contacts rupture at small elongation so as to prevent molecular junctions from suffering destabilization at the stage before they become detectable. Otherwise, large prestrain under stretching at excessively high displacement rate would lead to complete suppression of molecular junction formations. On the other hand, the natural lifetime of Au single atom contacts prolongs exponentially at 77 K. Hence, we cannot exploit the thermoactivated spontaneous breakdown of Au contacts by applying slow junction stretching rate to alleviate the prestraining effects. This tells us that the molecular junction lifetime at 77 K starts to become shorter than at 293 K under low Vd conditions where difference in the effects of thermal fluctuations on Au single atom contact stability becomes notable, the scenario of which plausibly explains the temperature dependence of τB-Vd relation documented in Figure 3b. Although prestraining effect can be circumvented at room temperatures by exploiting thermoactivated self-breaking of Au contacts, it is found to be practically unavoidable at low temperatures. However, from the fact that τBDT monotonically increases with decreasing Vd as is shown in Figure 3b, we can expect that the molecular junctions can hold for at least >100 s by freezing the junction stretching after their formation. Furthermore, it would also be important to actively stabilize molecular junctions through “training”31,32 so as to mechanically relax the prestrain energy. In summary, atomistic contact mechanics of metalmolecule-metal junctions is explored by systematically investigating the stretching rate dependence of single molecule junction lifetime at cryogenic temperatures. We find anomalous temperature dependence of the molecular junction lifetime, which becomes shorter at 77 K compared to that at 293 K under low Vd conditions. To elucidate the underlying physics of this unexpected result, we investigated the lowtemperature molecular junction force-breaking mechanism through examining its resemblance to the stretching rate dependent atom-pull-out chain formation/breakdown characteristics of Au single atom contacts. As a result, we found that the mechanical strain exerted during the preformation stage considerably deteriorates the low-temperature molecular junction stability compared to that at a room temperature. We point out that prestraining effects are practically inevitable in low-temperature break junction experiments. These results suggest the importance of actively stabilizing molecular junctions by using “training” effects in order to obtain stable metal-molecule-metal junctions. References (1) Reed, M. A.; Zhou, C.; Muller, C. J.; Burgin, T. P.; Tour, J. M. Science 1997, 278, 252. (2) Park, H.; Park, J.; Lim, A. K. L.; Anderson, E. H.; Alivisatos, A. P.; McEuen, P. L. Nature 2000, 407, 57. Nano Lett., Vol. 9, No. 6, 2009
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