Article pubs.acs.org/JPCC
Electric Conductance of Single Ethylene and Acetylene Molecules Bridging between Pt Electrodes Tomoka Nakazumi, Satoshi Kaneko, Ryuuji Matsushita, and Manabu Kiguchi* Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 W4-10 Ookayama, Meguro-ku, Tokyo 152-8551, Japan S Supporting Information *
ABSTRACT: We have investigated the conductance and atomic structure of single ethylene and acetylene molecule junctions on the basis of the conductance measurement and vibration spectroscopy of the single molecule junction. Single molecule junctions have a conductance comparable to that of metal atomic junctions (around 0.9G0: G0 = 2e2/h) due to effective hybridization between metal and the π molecular orbital. The ethylene molecules are bound to Pt electrodes via a di-σ bond, while the acetylene molecules are bound to Pt electrodes via di-σ and π bonds. By using the highly conductive single molecule junctions, we investigated the characteristics of vibration spectroscopy of the single molecule junction in an intermediate regime between tunneling and contact. The vibration modes that could modify the conduction orbital were excited for the ethylene and acetylene molecule junctions. The crossover between conductance enhancement and suppression was observed for the single ethylene molecule junction, whereas clear crossover was not observed for the acetylene molecule junction, reflecting the number of conduction orbitals in the single molecule junction.
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INTRODUCTION Molecular electronics have attracted great attention in recent years.1 One of the goals of molecular electronics is the design of molecular wires that can transport charge efficiently over long distances. There are some experimental challenges with the development of highly conductive molecular wires including πconjugated molecules.2−4 While these single molecular wires show small tunneling decay constants, the conductances of these wires are still much lower {200 meV). These highly conductive H2O/Pt and H2/Pt junctions become unstable under high bias due to bias-induced local heating. Ethylene and acetylene have several internal vibration modes in rather low-energy regimes.8,11 It is thus possible to discuss the activity of the vibration modes for the single molecule junction close to the contact regime by investigating ethylene and acetylene molecule junctions. The conductance change caused by the excitation of molecular vibrations is also an interesting topic. For junctions in the tunneling regime, electron−vibration interaction leads to an increase in the junction conductance, whereas it decreases the conductance for junctions in the contact regime.1,24 Recently, theoretical calculations have predicted the crossover between conductance enhancement and suppression at a transition probability of 0.5. A highly conductive single molecule junction (∼0.5G0) is required to investigate this topic.26 In the present study, we fabricated highly conductive single ethylene and acetylene molecule junctions by direct binding of π-conjugated organic molecules to Pt electrodes. The single molecule junctions were characterized with the vibration spectroscopy of the single molecule junction. The ethylene molecule was bound to Pt electrodes via a di-σ bond, whereas the acetylene molecule was bound to Pt electrodes via di-σ and π bonds. The activity of the molecular vibrations and conductance change induced by excitation of the molecular vibration have been discussed by considering the characteristics of the conduction orbital of the single molecule junction. The vibration modes that could modify the conduction orbital were excited for the ethylene and acetylene molecule junctions. The crossover between conductance enhancement and suppression was observed for the single ethylene molecule junction, whereas clear crossover was not observed for the acetylene molecule junction, reflecting the number of conduction orbitals in the single molecule junction.
Figure 1. (a−c) Typical conductance traces and (d−f) conductance histograms of Pt contacts (a,d) before and after introduction of (b,e) ethylene and (c,f) acetylene at the bias voltage of 100 mV. The intensity of the conductance histograms was normalized with the number of the conductance traces. The conductance histograms were constructed without data selection from 1000 conductance traces of breaking Pt contacts. The bin size was 0.004G0. The scale bar (10 nm) represents the distance between the stem parts, which can be converted to the junction stretching distance.
ethylene or the acetylene. Stretch length was the displacement of the distance between the stem parts of the Pt electrodes that were fixed with epoxy adhesive. For clean Pt, the conductance traces plateaued near 1.5G0, and the corresponding conductance histogram showed a peak at 1.5G0.27 The single peak around 1.5G0 is the typical signature of a single atom contact of clean Pt. After introduction of ethylene or acetylene, the contact broke after plateauing near 1G0. In some conductance traces, the conductance decreased in a stepwise fashion, with each step occurring at an integer multiple of 0.9G0, as shown in b and c of Figure 1. While stretching the contact in the presence of molecules, some molecules bridged between metal electrodes just after breaking the metal contact. The number of molecules bridging between metal electrodes decreased while stretching the molecule junction. Finally, the single molecule junction was formed just before breaking the molecule junction. The steps showing the values of 1× and 2 × 0.9G0 could thus be ascribed to one and two ethylene or acetylene molecules bridging Pt electrodes, respectively. The formation of the ethylene and the acetylene molecule junctions was supported by the vibration spectroscopy, as discussed in the following section. The corresponding conductance histograms showed features around 0.9G0. Since the 2× 0.9G0 steps rarely appeared in the conductance traces, only the first feature (0.9G0) appeared in the conductance histogram. In our previous study, the conductance histogram showed features around 0.2G0 and 1.0G0 for the acetylene molecule junctions.16 The feature around 1.0G0 observed in our previous study corresponds to the feature around 0.9G0 observed in the present study. Features below 0.5G0 sometimes appeared in the conductance histogram using the break junction technique,6 however, the features below 0.5G0 were less reproducible in the present study. We, thus, showed the most reliable conductance
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EXPERIMENTAL SECTION The measurements have been performed using the mechanically controllable break junction (MCBJ) technique.27 A notched Pt wire (0.1 mm in diameter, 10 mm in length) was fixed with epoxy adhesive (Stycast 2850FT) on top of a bending beam and mounted in a three-point bending configuration inside a vacuum chamber. In ultrahigh vacuum (UHV) at 10 K, the Pt wire was mechanically broken by bending the substrate. The bending could be relaxed to form atomic-sized contacts between the wire ends using a piezo element for fine adjustment. Ethylene or acetylene was admitted to the contacts via a capillary with a heater. DC two-point voltage-biased conductance was measured during the breaking process under an applied bias voltage in the range from 100 to 300 mV. AC voltage bias conductance measurements were performed using a standard lock-in technique. The conductance was recorded for the fixed contact configuration using an AC modulation 1 mV in amplitude and at a frequency of 7.777 kHz, while slowly ramping the DC bias between −150 and +150 mV. The experiments were performed for six distinct samples. 18251
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of the dI/dV and d2I/dV2 curves supported that the peak or dip in d2I/dV2 originated from the excitation of a vibration mode (see Figure S2 in Supporting Information) In order to accurately determine the associated vibration energy, around 80 differential conductance spectra were collected for the single ethylene and acetylene molecule junctions with conductance of 0.1G0−1.2G0. Figure 3 shows
histogram was obtained by repeated conductance measurements for 6 distinct samples. The conductances of the single ethylene and acetylene molecule junctions were precisely determined to be 0.94G0 and 0.87G0, by statistical analysis of the repeated measurements (more than 2000 times and 6 distinct samples). Here it should be noted that clear features (0.9G0) were observed in the conductance histograms. Most of the single molecule junctions (e.g., alkanedithiol) show various conductance values depending on the atomic configuration of the molecule junctions, and thus, the conductance histograms show several weak features below 0.01G0.1,5 Fabrication of a single molecule junction showing fixed and high conductance values is of great importance in its application to a single molecule electronic device. In the present study, we could fabricate the single molecule junctions showing fixed and high conductance values by direct coupling of the π electron system (ethylene and acetylene) to the metal electrodes. The present results suggest that the direct binding technique is the efficient technique to fix the atomic configuration of the metal− molecule contact. The obtained single ethylene and acetylene molecule junctions were characterized using vibration spectroscopy of the single molecule junction. The vibration spectrum of the single molecule junction (d2I/dV2 curve) was obtained by numerical derivation of the differential conductance (dI/dV) curve of the single molecule junction (see Figure S1 in Supporting Information). The dI/dV curve was measured as a function of voltage across the single molecule junction where the electrodes’ separation was fixed. Figure 2 shows an example
Figure 3. The distribution of vibration energy for Pt contact after the introduction of (a) ethylene or (b) acetylene. Schematic model of the single (c) ethylene and (d) acetylene molecule junction.
the distribution of vibration energy for the single ethylene and acetylene molecule junctions, respectively. The vibration modes were observed around 30, 60, 90−120 meV for the single ethylene molecule junction and around 40, 65, and 95 meV for the single acetylene molecule junction. Our previous study showed only 60 meV modes for the single acetylene molecule junction due to the limited number of experiments.16 The phonon modes are observed below 22 meV for the Pt contacts,28 and thus, the appearance of the vibration modes above 30 meV supported the hypothesis that electrons were transported through ethylene or acetylene molecule bridging between Pt electrodes. The obtained result is discussed by considering the experimental and theoretical calculation results of the ethylene and acetylene molecules on the flat Pt surface,8−15 although the results on the flat surface cannot be simply compared with that of the single molecule junction. The atomic configuration of the ethylene molecule on Pt(111) has been investigated using high resolution electron energy loss spectroscopy (HREELS), ultraviolet photoemission spectroscopy (UPS), and other techniques.8−15 The following adsorption states for the ethylene molecule on Pt(111) are possible: π-bonded ethylene, di-σ-bonded ethylene and ethylidyne. In the π-bonded ethylene case, ethylene is π-bonded to a single Pt atom with its C−C bond parallel to the surface. In the di-σ-bonded ethylene case, ethylene is di-σ-bonded to two Pt atoms with its C−C bond parallel to the surface. In the ethylidyne case, the ethylidyne adsorbs on the hollow site of the Pt surface with its C−C bond perpendicular to the surface. The energy of the vibration modes are 1, 10, 40, 50, 85, 100, and 110 meV for the π bond case; 15, 25, 55, 65, 80, 95, and 115 meV for the di-σ bond case; and 15, 20, 30, 50, and 60 meV for the ethylidyne case.8,11 All of the vibration modes observed for the single ethylene molecule junctions were observed for the di-
Figure 2. Example of d2I/dV2 spectra for Pt contact after the introduction of (a) ethylene and (b) acetylene. The zero bias conductances of the contact were 0.3G0 and 0.6G0 for ethylene and 0.4G0, 0.7G0 and 0.8G0 for acetylene.
of d2I/dV2 curves for the single ethylene and acetylene molecule junctions taken at conductance ranging from 0.3− 0.8G0. The shape of the spectra varied with the single molecule junction. As demonstrated in Figure 2, peaks were observed in d2I/dV2 curves for the single molecule junctions with conductances of 0.3G0 for ethylene and 0.4G0 and 0.7G0 for the acetylene. Dips were observed for the junctions with conductances of 0.6G0 for ethylene and 0.8G0 for acetylene. The peak in d2I/dV2 (increase in dI/dV) is explained by the opening of an additional tunneling channel for electrons that lose energy to a vibration mode. The dip in d2I/dV2 (decrease in dI/dV) is explained by backscattering of electrons that lose energy to a vibration mode.1,16,26 The temperature dependence 18252
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σ bond case. Some vibration modes observed for the single ethylene molecule junctions were not observed for the π bond and ethylidyne cases. Therefore, the ethylene molecule would adsorb onto Pt electrodes via di-σ bond as shown in Figure 3c. The vibration modes of 30, 60, and the 90−120 meV observed for the single ethylene molecule junction would correspond to the vibration modes of 25 (hindered translational), 55 (symmetric Pt−C stretching), 65 (asymmetric Pt−C stretching), 95(CH2 twist), 115 meV (CH2 rocking) observed on the flat Pt surface, respectively.8,11 (see Table S1 in Supporting Information) It should be noticed that the hindered translational mode (15 meV) and hindered rotation mode (80 meV) observed on the flat surface were not observed for the single ethylene molecule junctions. The former vibration mode could be obscured by the large zero bias anomaly feature.29 The hindered rotation mode (80 meV) is discussed in the later section. On the flat Pt(111) surface, ethylene is π-bonded to the Pt atom below 52 K, while it strongly interacts with the Pt substrate and the di-σ bond is formed between ethylene and Pt surface above 52 K. The present study suggests that the ethylene molecule is adsorbed on Pt electrodes via a di-σ bond in the single ethylene molecule junction fabricated at 10 K. The difference in the bonding between the flat surface and the single molecule junction can be explained by the local heating and/or atomic configuration. The single ethylene molecule junctions were investigated under the condition in which electrons were transported through the junction. The single molecule junction was locally heated by the current, and the temperature of the single molecule junction could rise above 52 K. The single molecule junction is a low dimensional structure that has two metal/molecule interfaces, while the molecule adsorbed on flat surface has only one molecule/metal interface. The molecule could more strongly interact with the metal surface for the single molecule junction, and thus, the di-σ bond could be formed between the ethylene and Pt surface below 10 K. The interaction between the C−C double bond and the Pt surface has been investigated for various molecules, including ethylene, butadiene, and benzene.13−15 The π orbital interacts with the vacant bands of metal, while the π* orbital interacts with the occupied bands of metal. These interaction leads to the formation of the di-σ bond, together with the decrease in the C−C bond order and increase in the length of the C−C bond. In the single acetylene molecule junctions, the vibration modes were observed around 40, 65, and 90 meV (see Figure 3b). By considering the vibration spectroscopy result on Pt(111),11 the structure model of the single acetylene molecule junction could be proposed (Figure 3d). The acetylene molecule would adsorb on the Pt electrodes via a di-σ bond and a π bond. On Pt(111), acetylene uses one π orbital to form a di-σ bond with two Pt atoms and the other π orbital to form a π-bond with the third Pt atom. The vibration modes of 40, 65, and 95 meV observed for the single acetylene molecule junction can be assigned to the vibration modes of 40 meV (Pt−C2 stretching), superposition of 65 meV (asymmetric Pt− C stretching) and 70 meV (symmetric Pt−C stretching), 95 meV (CH out-of-plane angle-bending) being observed on the flat Pt surface.11,30 (see Table S2 in Supporting Information) All vibration modes observed on the flat surface were detected for the single molecule junction. The activity of the vibration modes in the vibration spectroscopy of the single molecule junction is discussed by considering the electronic structure of the single molecule
junction. The vibration spectroscopy of the single molecule junction detects the conductance change induced by the excitation of the vibration mode. The vibration mode is excited by conduction electrons, and thus, the character of the conduction orbital would play an important role in the activity of the vibration mode in the vibration spectroscopy of the single molecule junction. The di-σ bond is formed between an unsaturated hydrocarbon and the Pt surface via the interaction between the HOMO and the LUMO, both molecular π orbitals, and the metal d orbital, as discussed in the previous section.13−15 The theoretical calculation results showed that the C 2p and Pt 5d orbitals formed the hybridized orbitals around the Fermi level for the ethylene and acetylene adsorbed on Pt(111).13,14 The theoretical calculation results using a cluster model showed that the hybridized orbital had a significant part at Pt−C−C−Pt, but not at the H atoms.9 Since electrons transport through the molecular orbital around the Fermi level, electrons would transport through the hybridized orbital originating from the C−C π bond for the single ethylene and acetylene molecule junctions. In the present study, all observed vibration modes were the vibration modes which modified the length of the Pt−C bond or C−C bond, and would modify the electronic structure of the hybridized orbitals. On the other hand, the hindered rotation mode did not modify the length of the Pt−C bond nor the C−C bond and, thus, the hindered rotation mode (80 meV) observed on the flat surface which was not observed in the spectra of the single ethylene molecule junction. Next, we discuss conductance enhancement and suppression by the excitation of vibration modes. Figure 4 shows the
Figure 4. Histogram of peak (red) and dip (black) features in the d2I/ dV2 spectra for the (a) ethylene and (b) acetylene molecule junctions as a function of conductance of the single molecule junction. The dotted lines are Gaussian fits to the distribution of peak and dip features.
distribution of the d2I/dV2 curves with peaks and dips according to their conductance of the single ethylene and acetylene molecule junctions (see also Figure S3 in Supporting Information). In the single ethylene molecule junction, curves with peaks were mainly observed below 0.5G0, and curves with dips were mainly observed above 0.5G0. In the single acetylene molecule junctions, curves with peaks were observed below 0.4G0, and both curves with peaks and dips were observed above 0.4G0. Theoretical calculation results predicted the conductance enhancement (peak in d2I/dV2 curves) below a transmission probability of 0.5 and suppression (dip in d2I/dV2 curves) above 0.5 in the case of the single molecule junction in 18253
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Notes
which electrons transport through a single channel and the molecule−metal coupling is symmetric.24,26 This theoretical prediction can explain the experimental result of the ethylene molecule junction, in which electrons would transfer through a single orbital. In contrast, the appearance of the peaks in the d2I/dV2 curves for the acetylene molecule junction with conductance larger than 0.5G0 indicated that the electron transported through more than one channel. The simple singlechannel model cannot be applied to the molecule junction, where electrons transport through more than one channel. Finally, we briefly comment on the low conductance value of the single ethylene and acetylene molecule junctions in which the d2I/dV2 curves were measured. The conductance histogram showed the formation of the molecule junction showed a conductance of 0.9G0. However, this structure was rather unstable. The conductance of the single ethylene and acetylene molecule junctions decreased for most of the junctions while waiting for the stabilization of the junction to measure d2I/dV2 curves. The d2I/dV2 curves were measured for the molecule junction showing a conductance below 0.9G0 in most cases. The conductance traces (histograms) were measured during stretching of the contact, and the metal electrodes deformed via inelastic structural transformation during this time.27 The transformation preferentially occurred in the direction of the close-packed (111) plane for Pt fcc metal, leading to the formation of the molecule junction having a certain atomic configuration of 0.9G0. This structure is the metastable structure and eventually transforms into the most energetically stable structure. This transformation takes some time, and thus, the most energetically stable structure is not formed during the stretching of the contact. On the other hand, the molecule junction is gradually transformed into the most energetically stable structure and showed a conductance below 0.9G0 when we stopped stretching the contact. Most of d2I/dV2 curves were then measured for the molecule junctions showing a conductance below 0.9G0.
The authors declare no competing financial interest.
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CONCLUSION We have investigated Pt contacts in the presence of ethylene and acetylene atmospheres at 10 K. The conductance measurement and vibration spectroscopy of the single molecule junctions showed the formation of the simplest single molecule wires with ethylene and acetylene. These single molecule wires showed a fixed conductance value around 1G0. The vibration spectra of these highly conductive molecule wires showed the modes that modified the conduction orbital. The crossover between conductance enhancement and suppression was observed for the single ethylene molecule junction, while clear crossover was not observed for the acetylene molecule junction, reflecting the difference in the bonding nature of the metal−molecule bond in the single molecule junction. ASSOCIATED CONTENT
S Supporting Information *
The determination of the conductance values of the single molecule junction. The material is available free of charge via the Internet at http://pubs.acs.org.
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ACKNOWLEDGMENTS
This work was supported by Grant-in-Aid for Scientific Research in Innovative Areas (No. 23111706), and Grant-inAid for Scientific Research(B) (No. 21340074) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Telephone: 81-3-5734-2071. FAX: 81-3-5734-2071. 18254
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