NANO LETTERS
Vibronic Contributions to Charge Transport Across Molecular Junctions
2004 Vol. 4, No. 4 639-642
James G. Kushmerick,* Jason Lazorcik,† Charles H. Patterson, and Ranganathan Shashidhar† Center for Bio/Molecular Science and Engineering, NaVal Research Laboratory, Washington, DC 20375
Dwight S. Seferos and Guillermo C. Bazan Departments of Chemistry and Biochemistry, Materials and Institute for Polymers and Organic Solids, UniVersity of California, Santa Barbara, California 93106 Received January 21, 2004; Revised Manuscript Received February 17, 2004
ABSTRACT We report low-temperature charge transport measurements of metal−molecule−metal junctions. Studies on insulating alkyl and π-conjugated molecular wires provide experimental insight into the coupling between tunnel charge carriers and molecular vibrations in molecular electronic systems. By comparison with other vibrational spectroscopy studies and density functional theory calculations, the observed vibrational peaks have been assigned to longitudinal modes of the molecules.
Numerous experimental studies have focused on understanding the charge transport properties of metal-molecule-metal junctions seen as building blocks for future molecular electronic systems.1-5 However, relatively few measurements have investigated the role of vibronic coupling between the charge carriers and nuclear motions of the molecules.6-8 Although it is known that inelastic electron tunneling spectroscopy (IETS)9,10 can measure the vibrational spectrum of molecular junctions, to date, IETS has not been utilized to measure metal-molecule-metal junctions relevant to molecular electronics. In this letter we present in situ vibrational spectroscopy of metal-molecule-metal junctions containing prototypical molecular wires (Figure 1), and determine that the tunneling charge carriers couple strongly to longitudinal molecular vibrations. Transport measurements were performed with a homebuilt cryogenic crossed-wire tunnel junction, similar to our room temperature apparatus previously reported.5,11,12 Full details of the experimental setup will be published elsewhere.13 Briefly, a crossed-wire tunnel junction consisting of two 10 µm diameter Au wires, one coated with a monolayer of the molecule of interest (Figure 1),14,15 is mounted inside a stainless steel vacuum can. The vacuum can is evacuated and purged repeatedly with He gas before being lowered into a liquid He storage dewar. All data were acquired at a temperature of 4 K. Cryogenic temperatures * Corresponding author. E-mail:
[email protected]. † Current address: Geo-Centers Inc., Maritime Plaza One, 1201 M Street S. E., Suite 050, Washington, DC 20003. 10.1021/nl049871n CCC: $27.50 Published on Web 03/18/2004
© 2004 American Chemical Society
Figure 1. Chemical structures for the three molecules investigated.
are required to minimize the thermal broadening of electrons at the Fermi level of the Au electrodes to enable resolution of molecular vibrations.9,16 Standard ac modulation techniques, along with two lock-in amplifiers, are utilized to enable the first and second harmonic signals (proportional to dI/dV and d2I/dV2, respectively) to be measured simultaneously with the current-voltage characteristics. It should be emphasized that a direct measurement of the junction harmonics is preferential to numerical differentiation of the I-V characteristics since numerical differentiation decreases the signal-to-noise ratio. Figure 2 shows the transport properties for a metalmolecule-metal junction formed from a monolayer of C11. Although the I-V characteristics are linear over the bias range shown, plots of the differential conductance (dI/dV) and d2I/dV2 reveal significant features. The most noticeable attribute, besides the steps in dI/dV and peaks in d2I/dV2 which correspond to molecular vibrations as will be discussed below, is the prominent zero-bias feature (ZBF)17 which
Figure 2. Transport characteristics for a C11 junction. An ac modulation amplitude of 8 mV and lock-in time constants of 1 s was used to obtain the first and second harmonic signals.
represents a significant suppression of the junction conductance within ∼40 mV of zero bias. Although the physical origin of the ZBF is not fully understood,17-20 it does not interfere with our analysis of vibrational peaks that occur at energies larger than 40 meV (∼320 cm-1). Normalizing the d2I/dV2 signal by the differential conductance yields what we will refer to as the IETS spectrum. Molecular vibrations are observed with equal intensity in the positive and negative bias polarity (Figure 2), thus for clarity we only show the positive bias region in the IETS spectrum. In all cases the reported IETS spectra are the average of 4-8 voltage sweeps. The IETS spectrum for the C11 junction is shown in Figure 3. Based on previous IR,21 Raman22 and highresolution electron energy loss spectroscopy23 studies of alkanethiolate monolayers we are able to assign the observed peaks in the C11 junction to specific molecular vibrations. The C-H stretch at 362 mV is the most intense vibrational mode observed, but we also see a number of lower energy vibrations in the region from 70 mV to 200 mV. The fwhm of the C-H stretch is 20 mV consistent with the expected experimental resolution for a measurement at 4 K with a modulation amplitude of 8 mV.16 The low energy portion of the spectrum can be deconvoluted into a number of discrete peaks by constraining the fwhm of the vibrational peaks to that of the resolved C-H stretch.24 From such an analysis we tentatively assign CH2 rocking and wagging modes at 95 mV and 170 mV, respectively, as well as a C-C stretch at 134 mV. A complete analysis of the alkane IETS signature as well as the fitting procedures employed will be reported elsewhere.13 Molecular wires (π-conjugated rigid rod molecules) are attractive candidates for molecular electronics applications since they exhibit more facile charge transport than saturated alkanes.1,11 Molecular systems based on phenylene ethynylene1,5,11 and phenylene vinylene11,25 structures have 640
Figure 3. IETS spectrum of a C11 junction. The dashed line presented in this and other IETS spectra is a simple polynomial background and is presented as a guide to the eye. The upturn at low energies is the result of the ZBF as discussed in the text and visible in Figure 2. Mode assignments are from comparison to previous experimental results (see text).
Figure 4. IETS spectrum of a OPE junction. Mode assignments are from DFT calculations of the free molecule.
received an enormous amount of attention and serve as prototypical molecular wires for this study. The IETS spectra for junctions formed from the conjugated molecular wires OPE and OPV are shown in Figures 4 and 5. To aid in assignment of the vibrational modes for OPE26 and OPV density functional theory (DFT) calculations were performed on the free thiol form of the molecules at the B3LYP/631G* level27 and the calculated vibrational energies were scaled by the recommended factor of 0.961 to compare with the experimental results.28 Mode descriptions for OPE and OPV are in terms of Wilson-Varsanyi terminology for aromatic rings.29 For the OPE junction we observe three prominent peaks in the IETS spectrum at 138 mV, 196 mV, and 274 mV (Figure 4). We assign these vibrations as the V(18a), V(8a) ring modes and the C≡C stretch, respectively. There is also the suggestion of a C-H in plane bending mode (57 mV) Nano Lett., Vol. 4, No. 4, 2004
Figure 5. IETS spectrum of an OPV junction. Mode assignments are from DFT calculations of the free molecule. Table 1. Vibrational Mode Assignments for the Molecular Junctions Investigated peak position molecule
mV
cm-1
mode
activity
C11
95 134 170 362 57 133 138 196 274 96 134 144 174 195
766 1081 1371 2920 463 1074 1114 1582 2211 777 1078 1163 1403 1573
CH2 rocking v(C-C) CH2 wag v(C-H) aryl C-H ip v(18a) v(18a) v(8a) v(CtC) aryl C-H op v(18a) v(18a) v(15) v(8a) & v(CdC)
Raman Raman Raman IR, Raman IR IR Raman IR, Raman Raman IR IR Raman IR, Raman IR, Raman
OPE
OPV
riding on the tail of the ZBF (Figure 4). The OPV spectrum shows some similar structure, namely the V(8a) and CdC combination stretch at 197 mV and the V(18a) peaks at 134 and 144 mV (Figure 5). Weaker V(15) stretches and C-H out-of-plane bending modes can also be seen. Table 1 lists the peak energies and mode assignments for the observed vibrations from the three molecular junctions. One limitation of IETS compared with other vibrational spectroscopies is the lack of known selection rules. While it was determined early on that both IR and Raman active modes are observed with comparable intensity (which is consistent with the results reported here), the isotropic orientation of molecules in metal-insulator-adsorbatemetal junctions made the assignment of selection rules derived from molecular orientation impossible.16 The angstrom level resolution of the scanning tunneling microscope (STM) has enabled IETS on isolated molecules of known orientation.10 While adsorption geometry has been shown to affect the measured IETS signal30-32 and a symmetry selection rule has been proposed,33 the paucity of vibrational modes observed with STM-IETS makes a conclusive deterNano Lett., Vol. 4, No. 4, 2004
mination of selection rules difficult. Our experimental design and ability to resolve numerous vibrational modes per molecular species thus affords us a unique opportunity. The observed molecular vibrations (Table 1) have a strong transition component (either dipole or polarizability) perpendicular to the electrode surface. Given the junction geometry, the observed vibrations can be thought of as longitudinal modes of the metal-molecule-metal junction. From this observation we can state that the tunneling charge carriers couple strongly to dipole or polarizability changes of the molecules along their direction of travel. The only observed vibration that is not consistent with this assignment is the weak C-H out-of-plane bend observed in the OPV junction. The observation of this one weak transverse vibrational mode suggests that the coupling to longitudinal modes is best termed a propensity rather than a strong selection rule. Such a longitudinal propensity rule for metalmolecule-metal junctions is supported by the recent theoretical analysis of Troisi, Ratner, and Nitzan.34 While the theoretical study investigates a number of potential molecular wire structures, their calculations on a biphenyl junction afford the best comparison to our experimental results. For a molecular junction containing a biphenyl molecule, the theory predicts that the V(8a) ring mode should have the strongest vibronic coupling and that there should be significant coupling to the V(18a) ring mode. Neglecting the C≡C stretch of OPE (which is not present in biphenyl) the V(8a) and V(18a) ring modes are the most intense modes observed experimentally for OPE and OPV. Whether or not such a longitudinal propensity rule will apply to STM-IETS35 is unclear since the junction geometry and electric field distribution is very different. In summary, we have demonstrated that inelastic electron tunneling spectroscopy is a powerful tool for investigating metal-molecule-metal junctions relevant to molecular electronics. Longitudinal vibrational modes of the molecular species couple strongly to the tunneling charge carrier consistent with recent theoretical predictions. The ability to measure the vibronic coupling associated with charge transport will provide insight into the issue of local heating in molecular junctions.36,37 We are currently employing this in situ spectroscopic capability to investigate the molecular conductance switching reported by a number of groups.38,39 Acknowledgment. The authors acknowledge support from the Defense Advanced Research Project Agency (J.G.K, J.L., C.H.P., and R.S.), the National Science Foundation (DMR 0097611) and the Office of Naval Research (D.S.S. and G.C.B.). Note: This manuscript is submitted simultaneously with similar IETS results from W. Wang et al. on a monolayer of alkanedithiol molecules.40 The IETS results of these two manuscripts exhibit similarities as well as some differences possibly reflecting differences in device geometries and/or metal-molecule contacts. References (1) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705-1707. 641
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