Article pubs.acs.org/JPCC
Experimental Observation of Real Time Molecular Dynamics Using Electromigrated Tunnel Junctions Darin O. Bellisario,‡ Albert Tianxiang Liu,‡ Daichi Kozawa,‡ Rebecca Han,£ Jasmine K. Harris,‡ Robert Brandon Zabala,‡ Qing Hua Wang,§ Kumar V. Agrawal,‡ Youngwoo Son,‡ and Michael S. Strano*,‡ ‡
Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States Department of Chemical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States § School of Engineering of Matter, Transport & Energy, Arizona State University, Tempe, Arizona 85281, United States £
S Supporting Information *
ABSTRACT: Single molecule tunnel junctions (SMTJs) can provide important physical insights into electronic and vibrational phenomena at the molecular scale. However, observations and analysis are typically confined to sufficiently low temperatures as to suppress molecular motion and the resulting stochastic fluctuations in the tunneling current. In this work, we introduce and experimentally validate a methodology whereby a slightly higher temperature (9 K) compared to a typical SMTJ study can be used to induce sparse fluctuations in the inelastic tunneling current and provide the fingerprints of dynamics between the conformational states of the molecule. Two examples of benzene dithiol and cysteine are studied in electromigratively formed W/Au nanowire SMTJs on SiO2 at 9 K. The second-order transform of the tunneling current reveals the expected vibrational spectra. However, we show that temporal fluctuations can be analyzed using a hidden Markov Model to reveal dynamics assigned to millisecond rearrangements of the molecule, with apparent energy barriers ranging from 35 to 66 meV, consistent with theoretical predictions. The observed transitions are consistent with a model of lateral migration of the thiol-anchored molecules in an asymmetric junction. The use of temperature in SMTJs in this way can provide new insights into molecule dynamics in confined volumes and at electrode interfaces.
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INTRODUCTION Single molecules trapped in on-chip tunnel junctions (SMTJs) have led to important physical discoveries1−16 and hold promise for single-molecule electronics (SMEs),13,17,18 optoelectronics,14 and biomolecule assays.16,19 To these ends, they have been studied to show Coulomb blockade,1−4 the Kondo effect,2,5−7 inelastic tunneling spectroscopy (IETS),8,9 surfaceenhanced Raman spectroscopy (SERS),10,11 superconductivity,12 spin-selective transport,13 optical rectification,14 thermoelectricity,15 and biomolecule fingerprinting.16 To date, however, most studies are conducted and analyzed assuming static molecular configurations presenting time-invariant inelastic tunneling states.20−25 It is common to treat current fluctuations as a source of confounding noise to be suppressed26−28 or analyzed with the goal of extracting periods of invariant properties.29−33 Alternatively, in this work, we develop a methodology employing millisecond conductance and differential conductance measurements capable of observing the complex, time-resolved dynamics of molecules within SMTJs at low temperature as they transition between configurations that present different inelastic tunneling states. SMTJ studies are typically conducted at low temperature (4 K) as to suppress molecular motion, since the resulting noise © XXXX American Chemical Society
confounds the analysis of the inelastic tunneling current. Several studies have observed such a noise and attempted to suppress it. Peterfalvi et al., for example, have demonstrated that single molecules with planar anchor groups attached to carbon-based electrodes are more resilient to atomic-scale variation in the contacts and exhibit significantly lower conductance fluctuations.26 Chen et al. have reported nonequilibrium “excess” noise in ensembles of atomic-scale gold junctions as a function of bias conditions at room temperature and observed suppression of the noise near conductances associated with conductance quantization in such junctions.34 Scanning tunneling microscopy (STM) studies provide further evidence that even at liquid He temperatures an electric field can promote molecule dynamics,35,36 but even in these cases, such dynamics typically confound analysis. Some researchers have offered analytical methods to get around the problems associated with this noise.32 Jiang et al., for instance, have measured the electrical conductance of a series of thiol-terminated alkanes in STM break junctions and Received: August 17, 2017 Revised: September 14, 2017 Published: September 25, 2017 A
DOI: 10.1021/acs.jpcc.7b08228 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
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