Inelastic electron tunneling process for alkanethiol self-assembled monolayers

Abstract

Recent investigations of inelastic electron tunneling spectroscopy (IETS) for alkanethiol self-assembled monolayers (SAMs) are reviewed. Alkanethiol SAMs are usually prepared by immersing a gold substrate into a solution of alkanethiol molecules, and they are very stable, even under ambient conditions. Thus, alkanethiol SAMs have been used as typical molecules for research into molecular electronics. Infrared spectroscopy and electron energy loss spectroscopy (EELS) have frequently been employed to characterize SAMs on the macroscopic scale. For characterization of alkanethiol SAMs on the nanometer scale region, or for single alkanethiol molecules through which electrons actually tunnel, IETS has proven to be an effective method. However, IETS experiments for alkanethiol SAMs employing different methods have shown large differences, i.e., there is a lack of standard data for alkanethiol SAMs with which to understand the IET process or to satisfactorily compare with theoretical investigations.An effective means of acquiring standard data is the formation of a tunneling junction with scanning tunneling microscopy (STM). After explanation of the STM experimental techniques, standard IETS data are presented whereby a contact condition between the tip and SAM is tuned. We have found that many vibrational modes are detected by STM-IETS, as is also the case for EELS. These results are compared with IET spectra measured with different tunneling junctions. In order to precisely investigate which vibrational modes are active in IETS, isotope labeling of alkanethiols with specifically synthesized isotopically substituted molecule has been examined. This method provides unambiguous assignments of IET spectra peaks and site selectivity for alkanethiol SAMs such that all parts of the alkanethiol molecules almost equally contribute to the IET process. The IET process is also discussed based on density functional theory and nonequilibrium Green's function calculations. These results quantitatively reproduce many the experimentally observed features, whereas Fermi's golden rule for IETS qualitatively explains the propensity rule and site selectivity observed in the experiments. However, comparison between experiment and theory reveals a large difference in IETS intensity for the C-H stretching mode that originates from the side chains of the alkanethiol molecules. In order to explain this difference, we discuss the importance of an intermolecular tunneling process in the SAM. Application of STM-IETS to identify a hydrogenated alkanethiol molecule inserted into a deuterated alkanethiol SAM matrix is also demonstrated.

Vibrations of a molecule in an external force field

“…To measure the conductance (dI/dV) and IET signal (d 2 I/dV 2 ), a modulation voltage (2,338.7 Hz and 1-mV rms amplitude) is added to the sample bias, and the first and second harmonics in the current are detected by a lock-in amplifier (HF2LI; Zurich Instruments). In the text, the IETS signal is normalized to the conductance (6,21), i.e., (d 2 I/dV 2 )/(dI/dV) has been plotted. The complete set of the IETS are shown in SI Appendix, Figs.…”

“…The energy loss of those particles is related to the excitation of vibrations (1,2). Isotope-specific vibration spectra of individual molecules have been measured with a combination of scanning tunneling microscopy (STM) and inelastic electron tunneling spectroscopy (IETS), pioneered by Stipe et al (3), where the excitation of a vibration appears as a peak in the second derivative of the current-voltage curve (4)(5)(6). STM-IETS also enables us to measure the vibrational state of a single molecule interacting with a nearby atom or molecule, e.g., the catalytic reaction between a CO molecule and O atoms on a silver surface can be controlled by changing their distance and tracked by STM-IETS (7).…”

Influence of atomic tip structure on the intensity of inelastic tunneling spectroscopy data analyzed by combined scanning tunneling spectroscopy, force microscopy, and density functional theory

“…By studying these subtle changes of the vibrational energy using STM-IETS with a molecularfunctionalized tip, it has been demonstrated that STM-IETS can provide information on the inner structure of a molecule [4,5] similarly to atomic force microscopy (AFM) [6]. These advantages of STM-IETS have accelerated research in related fields [7][8][9][10][11][12][13][14][15][16]. Owing to recent progress in the theoretical description of IETS [17][18][19][20][21][22], the qualitative understanding has been improved considerably: the symmetry of the wave functions of a tip and a molecule on a substrate and a vibrational mode of the molecule are predicted to influence the efficiency of the inelastic process (γ inel ) for the tunneling current involving the molecule.…”

Current-induced phonon renormalization in molecular junctions

“…Molecular vibrations are directly related to the molecule geometry and inter/intramolecular bonds. Hence, vibrational spectroscopy methods, such as inelastic electron tunneling, infrared, and Raman, have been widely employed to investigate the conduction mechanism and the stability of molecular devices [11][12][13][14][15][16][17][18][19]. There is also some experimental evidence that the current can modify the vibrational modes of the molecule in a junction [17][18][19].…”