13349
2008, 112, 13349–13352 Published on Web 08/13/2008
Conductance of Single 1,4-Benzenediamine Molecule Bridging between Au and Pt Electrodes Manabu Kiguchi,*,†,‡ Shinichi Miura,† Takuya Takahashi,† Kenji Hara,†,§ Masaya Sawamura,† and Kei Murakoshi† DiVision of Chemistry, Graduate School of Science, Hokkaido UniVersity, N10W8, Kita, Sapporo, Hokkaido 060-0810, Japan, PRESTO, Japan Science and Technology Agency, N10W8, Kita, Sapporo 060-0810, Japan ReceiVed: July 11, 2008; ReVised Manuscript ReceiVed: August 3, 2008
We investigated the single 1,4-benzenediamine molecule bridging between Au or Pt electrodes. The conductances of the molecular junctions with the Au-NH2 and Pt-NH2 bonds (Au-NH2 and Pt-NH2 molecular junctions) were 1 × 10-2 G0 (2e2/h) and 5 × 10-3 G0, respectively. The stretching lengths of the Au-NH2 and Pt-NH2 molecular junctions were 0.03 and 0.07 nm, respectively. The conductance value of the Au-NH2 molecular junction was unexpectedly larger than the value evaluated with the density of states of the metal electrodes and the molecule-metal bond strength, which have been discussed before. The large conductance value could be explained by the small energy difference between metal and molecular orbitals (∆E) and the high degree of π-conjugation (P) of the Au-NH2 molecular junction, which would be unique characteristics of the Au-NH2 bond. The present study showed the importance of these two factors (∆E, P) in studying the conductance of the single molecular junction. There is a growing interest in electron transport properties through single molecules for the purpose of developing molecular electronic devices.1 In particular, understanding of the effect of the molecule-metal contact on electron transport is one of the important issues in this field because the atomic and electronic structures of the contact plays a decisive role on electron transport through a single molecule. The dependence of the molecule-metal contact on the conductances of single molecules has been studied for molecules with various anchoring groups, including thiol (-SH), isocyanide (-NC), amine (-NH2), carboxyl (-COOH), dimethyl phosphine (P(CH3)2: PMe2), and methyl sulfide (SMe).2-5 Chen et al. investigated the conductances of single disubstituted alkane molecules terminated with -SH, -NH2, and -COOH.2 The conductance decreased in the order of the molecule terminated with -SH > -NH2 > -COOH. Park et al. showed that the conductances of the single disubstituted alkane molecules terminated with -PMe2, -SMe, or -NH2 decreased in that order.3 The dependence of the molecule-metal contact on the conductance was explained by the strength of the molecule-metal bond.2,3 The relative bond strengths were Au-S > Au-NH2 > AuCOOH bond for the former case and Au-PMe2 > Au-SMe > Au-NH2 bond for the latter case. This order of the bond strengths agree with the order of the conductances of the molecules. Recently, we showed that the conductances of the 1,4-disubstituted (-NC and -SH) benzene molecules bridging * To whom correspondence should be addressed. E-mail: kiguti@ sci.hokudai.ac.jp. † Hokkaido University. ‡ PRESTO. § Present address: Catalysis Research Center, Hokkaido University, N21W10, Kita, Sapporo, Hokkaido 001-0021, Japan.
10.1021/jp806129u CCC: $40.75
Figure 1. Schematic view and energy diagram of the single molecular junction. t, F (EF), ∆E, P are the hopping integral between the molecule and metal orbitals, the local density of states value of the contact metal atom at the EF, the energy difference between the EF and conduction MO, and the degree of π-conjugation, respectively.
between Pt electrodes were one order larger than those bridging between Au electrodes.4 This result could be explained by the high local density of states (F: LDOS) of Pt metals at the Fermi level (EF). Up to now, the conductance values of the single molecular junction determined experimentally have been discussed based on these two parameters, i.e., the molecule-metal bond strength and LDOS of the metal electrode.2-4 In the tunneling model, the conductance (G) of a single fully π-conjugated molecule can be represented as G ) A(t2F/∆E)2 (eq 1), where A, t, and ∆E are the constant, hopping integral between the metal and molecular orbitals (MO) and the energy difference between the EF and conduction MO, respectively (see Figure 1).6 The conduction MO is the highest occupied molecular orbital (HOMO) or the lowest unoccupied molecular orbital (LUMO). Because the conduction MO is not always delocalized over the whole molecule in the real molecular 2008 American Chemical Society
13350 J. Phys. Chem. C, Vol. 112, No. 35, 2008
Letters
Figure 2. Typical conductance traces (a-c) and histograms (d-f) of metal point contacts during breaking the contact in 1 mM 1,4-benzenediamine solution. The metal electrodes were Au (a,b,d,e) or Pt (c,f), and the solvent was tetraglyme (b,c,e,f) or 0.1 M NaClO4 (a,d). The dotted line shows the result in the absence of molecules. The conductance histograms were obtained from 1000 traces.
junctions of π-conjugated molecules, an additional parameter (P) is needed to represent the degree of π-conjugation. The equation can thus be replaced by G ) A(t2F/∆E)2P (eq 2). Large F, t, and P values and a small ∆E value are essential for the molecular junctions to achieve large conductivity. A large t value indicates the large bond strength.6 While the conductance of the single molecular junction has been discussed with the F and t values,2-4 two other important parameters, ∆E and P, have not received much attention in discussing the effect of molecule-metal contact on the conductance of the single molecular junction, although they have been discussed for the molecular junctions with a fixed molecule-metal contact.7-9 Here, we focus on the 1,4-disubstituted benzene bridging between metal electrodes to clarify the relationship between the two parameters (∆E and P) and the conductances of single molecular junctions. 1,4-Disubstituted benzene was chosen for the following reasons. First, its HOMO-LUMO gap is small (∼5 eV) compared to that of disubstituted alkane (∼9 eV),5 which provides the small ∆E. The ∆E depends on the choice of the molecule, its anchoring group, and metal electrodes. The relative ∆E difference among the molecule-metal contacts would be larger for molecule junctions with a smaller HOMO-LUMO gap if the ∆E difference does not sensitively depend on the choice of central part of the molecule. Second, 1,4-disubstituted benzene is a π-conjugated molecule. When the molecule interacts with the metal electrode, the spatial extension of the MO (P) is modulated from that of the isolated molecule. Therefore, ∆E and P may play a decisive role in determining the conductance of the single 1,4-disubstituted benzene. In the present study, the conductance of the single 1,4-benzenediamine molecule bridging between Au or Pt electrodes was investigated in comparison with previously reported results for 1,4-benzenedithiol. In addition, the effect of the solution on the conductance of the single molecular junction was investigated. The experiments were performed with the modified scanning tunneling microscope (STM: Pico-SPM, Molecular Imaging Co.) with a Nano ScopeIIIa controller (Digital Instruments Co.) in an electrochemical cell. Details on the experimental design used in this study have been previously reported by our group.4 The STM tip was made of an Au or Pt wire (diameter ∼0.25 mm,
>99%). The substrate was Au or Pt (111), prepared by a flame annealing and quenching method. The solution was either tetraethyleneglycol dimethyl ether (tetraglyme) or 0.1 M NaClO4 containing 1 mM 1,4-benzenediamine. The STM tip was repeatedly moved into and out of contact with a substrate at a rate of 50 nm/s in the solution. Conductance was measured during the breaking process under an applied bias of 20 mV between the tip and substrate. All statistical data was obtained from a large number (over 1000) of individual conductance traces. Figure 2 show the typical conductance traces and histograms of the Au or Pt contacts in solution with and without 1 mM 1,4-benzenediamine. In the absence of molecules, neither plateaus nor peaks were observed both in the conductance traces and histograms. The traces showed a plateau in which the conductance was nearly constant (arrow in Figure 2a-c). The conductance value of the plateau was an integer multiple of 1 × 10-2 G0 for the Au electrode and 5 × 10-3 G0 for the Pt electrode. The corresponding histograms showed the feature at 1 × 10-2 G0 for the Au electrodes and 5 × 10-3 G0 for the Pt electrodes. Neither plateau nor features were observed below 1 × 10-3 G0 in the conductance traces and histograms. In the case of the Au electrode, the conductance value of the feature in 0.1 M NaClO4 was the same as that in tetraglyme. These experimental results indicate that the plateau in the traces and features in the histograms showing the values of 1× and 2 × 1 × 10-2 (5 × 10-3) G0 could be ascribed to one and two 1,4benzenediamine molecules bridging between Au (Pt) electrodes, respectively. The conductances of the single Au/1,4-benzenediamine/Au and Pt/1,4-benzenediamine/Pt junctions were precisely determined to be 1 × 10-2 ((3 × 10-3) G0 and 5 × 10-3 ((1 × 10-3) G0, respectively. These molecular junctions are subsequently referred to as the Au-NH2 molecular junction and the Pt-NH2 molecular junction, respectively. The obtained conductance value was compared with previously reported results. The conductance of the Au-NH2 molecular junction was reported to be 7 × 10-3 G0 in 1,2,4trichrobenzene solution,10 which agreed with our result of 1 × 10-2 ((3 × 10-3) G0 within the experimental errors. For the Pt-NH2 molecular junction, there was no previous study. Under
Letters
Figure 3. Distribution of lengths for the last conductance plateau for the Au-S (O), Pt-S (g), Au-NH2 (b), and Pt-NH2 (f) molecular junctions. The length of the last conductance plateau was defined as the distance between the points (set points) at which the conductance dropped below 4.8 × 10-3 G0 and 3.0 × 10-3 G0 for the Au-S molecular junction. The set points were 1.1 × 10-2 G0 and 0.7 × 10-2 G0 for the Au-NH2 molecular junction, 4 × 10-2 G0 and 2 × 10-2 G0 for the Pt-S molecular junction, and 6.2 × 10-3 G0 and 3.8 × 10-3 G0 for the Pt-NH2 molecular junction.
the same experimental conditions, the conductances of the Au-S molecular junction and Pt-S molecular junction were determined to be 4 × 10-3 G0 and 3 × 10-2 G0, respectively, by our group.5 The series of conductance measurements showed that the ratio of the conductances of the Au-S, Au-NH2, Pt-S, and Pt-NH2 molecular junctions was 1:3:8:1. The conductance of the single 1,4-benzenediamine molecule bridging between the Au electrodes was larger than that when bridging between the Pt electrodes, while the conductance of the single 1,4benzenedithiol molecule bridging between the Au electrodes was smaller than that when bridging the Pt electrodes. Before discussing the conductance value, the strength of the molecule-metal bond was investigated by statistical analysis of the conductance traces. Because the last plateau corresponds to a single molecular junction, the length of the last plateau is the distance over which the single molecular junction can be stretched before breakdown (breakdown distance). The breakdown distance reflects the strength of the molecule-metal bond. Figure 3 shows the distribution of the breakdown distances for the Au-NH2, Pt-NH2, Au-S, and Pt-S molecular junctions. The average lengths were 0.03 ((0.01), 0.07 ((0.02), 0.09 ((0.03), and 0.16 ((0.05) nm for the Au-NH2, Pt-NH2, Au-S, and Pt-S molecular junction, respectively. As for the anchoring group, thiol was shown to provide a longer breakdown distance than amine. As for the metal electrode, Pt provided a longer breakdown distance than Au. The binding energies of the Au-NH2, Pt-NH2, Au-S, and Pt-S bonds were calculated to be 36, 137, 134, and 174 kJ/mol, respectively, using the model clusters.11-13 The order of the binding energies (Au-NH2 < Pt-NH2 ∼ Au-S < Pt-S) agreed with that of the breakdown distance among the molecule-metal contacts. The close agreement between the breakdown distance and the binding energy indicated that the binding energy of the molecule-metal contact could be roughly estimated by the conductance measurement. Now, the conductance value of the single molecular junction is discussed, considering four parameters (t, F, ∆E, P) that determine the conductance of the single π-conjugated molecular junction (see eq 2). Here, the hopping integral (t) was estimated with the binding energy of the metal-anchoring group bond.6 Using the theoretical calculation results for model clusters, the ratio of the t value of the Au-S, Au-NH2, Pt-S, and Pt-NH2 molecular junctions could be determined to be 134:36:174:137, respectively.11-13 The F value of the Pt electrode at the EF was calculated to be two times larger than that of Au.14 If the conductance of the single molecular junction is estimated with
J. Phys. Chem. C, Vol. 112, No. 35, 2008 13351 only two parameters (t and F), the ratio of the conductances of the Au-S, Au-NH2, Pt-S, and Pt-NH2 molecular junctions is 1:0.005:11:4. On the other hand, the series of conductance measurements showed that the ratio was actually 1:3:8:1. The expected ratio can vary within 5 due to the rough estimation of the F and t values. It can be said that the relative conductance values could be explained by the t and F for the Au-S, Pt-S, and Pt-NH2 molecular junctions. On the other hand, the conductance of the Au-NH2 molecular junction was 600 times larger than the expected value. To investigate the origin of the large conductance value of the Au-NH2 molecular junction, the other two factors (∆E, P) are discussed in the following. First, the energy difference between the EF and conduction MO (∆E) is discussed. The large conductance value of the Au-NH2 molecular junction suggests a small ∆E because the conductance is proportional to 1/(∆E)2 (see eq 2). The small ∆E for the Au-NH2 molecular junction can be explained as the following. When a molecule adsorbs on metal electrodes, the MO near the EF (HOMO or LUMO) interacts with the metal orbital near the EF. Bonding and antibonding orbitals are formed due to the interaction, and thus, the MO moves away from the EF, leading to an increase in the ∆E. The magnitude of this energy shift increases with the strength of the interaction between the metal and the molecule.15 The binding energy of the Au-NH2 bond was weak compared to the other bonds (Pt-NH2, Au-S, Pt-S). This weak interaction would lead to the small ∆E. This hypothesis is evaluated with the previously reported theoretical calculation and thermoelectricity measurement results.5,16,17 In the case of the Au-S molecular junction, the HOMO (conduction MO) was 1.2 eV below the EF.5,16 Assuming eq 2 and constant P, the MO is expected to be 0.05 eV above or below the EF, using ∆E ) 1.2 eV for the Au-S molecular junction. Recent theoretical calculation results showed that the HOMO was 1.0 eV below the EF for the Au-NH2 molecular junction.17 The ∆E of the Au-NH2 molecular junction was smaller than that of the Au-S molecular junction, but it was not as small as expected. Thus, it appears difficult to explain the large conductivity of the Au-NH2 molecular junction with only the ∆E. Second, the degree of π-conjugation (P) is discussed. The relatively large conductance value of the Au-NH2 molecular junction suggests that it has a large P because the conductance is proportional to the P value (see eq 2). The large P for the Au-NH2 molecular junction can be explained as the following. In the isolated 1,4-benzenediamine and 1,4-benzenedithiol molecules, a N or S lone pair conjugates with the π-orbital of the benzene ring. This effective conjugation provides the large P. When the molecule adsorbs on the metal electrodes, a N or S lone pair conjugates with the metal orbital. This conjugation between lone pair and metal orbital decreases the degree of conjugation between the lone pair and the π-orbital of the benzene ring. The decrease in the conjugation between the lone pair and the π-orbital of the benzene ring was shown by the theoretical calculation results for the Au-S molecular junction.18 The decrease in the conjugation between a S or N 2p orbital and the π-orbital of the benzene ring (degree of π-conjugation) depends on the strength of the interaction between the molecule and the metal. The binding energy of the Au-NH2 bond was weak compared to the Pt-NH2, Au-S, Pt-S bonds. Because of the small interaction, a N 2p orbital could effectively conjugate with the π-orbital of the benzene ring even after the formation of the single molecular junction for the Au-NH2 molecular junction. Therefore, the Au-NH2 molecular junction showed large conductivity. The weak interaction between the
13352 J. Phys. Chem. C, Vol. 112, No. 35, 2008 metal and the molecule would have an advantage in both the ∆E and P for the Au-NH2 molecular junction. On the other hand, the two parameters, ∆E and P, would not affect the conductances of the other Au-S, Pt-S, and Pt-NH2 molecular junctions. The ∆E would be large for these molecular junctions due to the large interaction between the molecule and the metal. The relative ∆E difference among the molecule-metal contacts would be small for the molecule junction with the large ∆E. The large interaction would also lead to a small P, which would be insensitive to the choice of the molecule-metal contact. Therefore, the ∆E and P would not strongly affect the conductances of the Au-S, Pt-S, or Pt-NH2 molecular junction. The present experimental study showed that not only the F and t values but also the ∆E and P values could play the decisive role in determining the conductance of the single molecular junction. The importance of the ∆E and P on the conductance of the single molecular junction was supported by the previously reported results of diamine molecules bridging between Au electrodes.7,8 Venkataraman et al. investigated the conductances of single substituted 1,4-benzenediamine molecules to clarify the conductance dependence on ∆E, which was tuned by changing the substituents. The conductance of the single molecular junction increased with a decrease in ∆E.7 They also revealed that the conductance of a single substituted 4,4′diaminobiphenyl molecule decreased with an increase in the twist angle between the two benzene rings, that is, the degree of π-conjugation.8 These experimental results showed that the conductance of the single molecular junction depends on not only the F and t but also ∆E and P, as we discussed. The four factors (F, t, ∆E, P) vary with the choice of the molecules and the metal electrode. By considering these factors in choosing the best anchoring group for the individual molecules, the conductance of the single molecule junction can be drastically improved, which leads to a higher performance for molecular devices. In conclusion, we have fabricated well-defined single molecular junctions with Au-NH2 and Pt-NH2 bonds. The conductances of the Au-NH2 and Pt-NH2 molecular junctions were 1 × 10-2 and 5 × 10-3 G0, respectively. In comparison with previous study of benzene dithiol (Au-S molecular junction: 3 × 10-3 G0; Pt-S molecular junction: 3 × 10-2 G0), the conductance value of the Au-NH2 molecular junction was
Letters found to be much larger than the expected value. The large conductance value could be explained by the small energy difference between the EF and the conduction MO (∆E), and the effective conjugation of a N lone pair with the π-orbital of the benzene ring (P). The present experimental study showed that the four factors (F, t, ∆E, P) should be discussed in studying the conductance of the single molecular junction. Acknowledgment. This work was partially supported by a Grant-in-Aid for Scientific Research A (no. 16205026) and Grant-in-Aid for Scientific Research on Priority Areas (no. 17069001) from MEXT. References and Notes (1) Tao, N. J. Nat. Nanotechnol. 2006, 1, 173. (2) Chen, F.; Li, X.; Hihath, J.; Huang, Z.; Tao, N. J. J. Am. Chem. Soc. 2006, 128, 15874. (3) Park, Y. S.; Whalley, A. C.; Kamenetska, M.; Steigerwald, M. L.; Hybertsen, M. S.; Nuckolls, C.; Venkataraman, L. J. Am. Chem. Soc. 2007, 129, 15768. (4) Kiguchi, M.; Miura, S.; Hara, K.; Sawamura, M.; Murakoshi, K. Appl. Phys. Let. 2007, 91, 53110. Kiguchi, M.; Miura, S.; Hara, K.; Sawamura, M.; Murakoshi, K. Appl. Phys. Lett. 2006, 89, 213104. (5) Xue, Y.; Ratner, M. A. Phys. ReV. B 2004, 69, 85403. (6) Tada, T.; Yoshizawa, K. ChemPhysChem 2002, 3, 1035. (7) Venkataraman, L.; Park, Y. S.; Whalley, A. C.; Nuckolls, C.; Hybertsen, M. S.; Steigerwald, M. L. Nano Lett. 2007, 7, 502. (8) Venkataraman, L.; Klare, J. E.; Nuckolls, C.; Hybertsen, M. S.; Steigerwald, M. L. Nature 2006, 442, 904. (9) Li, X.; Hihath, J.; Chen, F.; Masuda, T.; Zang, L.; Tao, N. J. Am. Chem. Soc. 2007, 129, 11535. (10) Venkataraman, L.; Park, Y. S.; Whalley, A. C.; Nuckolls, C.; Hybertsen, M. S.; Steigerwald, M. L. Nano Lett. 2007, 7, 502. (11) Tarazona-Vasquez, F.; Balbuena, P. B. J. Phys. Chem. B 2004, 108, 15992. (12) Nara, J.; Higai, S.; Morikawa, Y.; Ohno, T. J. Chem. Phys. 2004, 120, 6705. (13) Jacob, T.; Blanco, M.; Goddard, W. A. Chem. Phys. Lett. 2004, 390, 352. (14) Cuevas, J. C.; Heurich, J.; Pauly, F.; Wenzel, W.; Schon, G. Nanotechnology 2003, 14, R29. (15) Zangwill, A. Physics at Surfaces; Cambridge University Press: New York, 1998. (16) Reddy, P.; Jang, S.; Segalman, R. A.; Majumdar, A. Science 2007, 315, 1568. (17) Quek, S. Y.; Venkataraman, L.; Choi, H. J.; Louie, S. G.; Hybertsen, M. S.; Neaton, J. B. Nano Lett. 2007, 7, 3477. (18) Delaney, P.; Nolan, M.; Greer, H. C. J. Chem. Phys. 2005, 122, 44710.
JP806129U
