Li-Li Lin

Inelastic Electron Tunneling Spectroscopy of Gold-Benzenedithiol-Gold Junctions : Accurate Determination of Molecular Conformation

The gold-benzenedithiol-gold junction is the classic prototype of molecular electronics. However, even with the similar experimental setup, it has been difficult to reproduce the measured results because of the lack of basic information about the molecular confirmation inside the junction. We have performed systematic first principles study on the inelastic electron tunneling spectroscopy of this classic junction. By comparing the calculated spectra with four different experimental results, the most possible conformations of the molecule under different experimental conditions have been successfully determined. The relationship between the contact configuration and the resulted spectra is revealed. It demonstrates again that one should always combine the theoretical and experimental inelastic electron tunneling spectra to determine the molecular conformation in a junction. Our simulations have also suggested that in terms of the reproducibility and stability, the electromigrated nanogap technique is much better than the mechanically controllable break junction technique.

Formation and electronic transport properties of bimolecular junctions based on aromatic coupling

A systematic first-principles study on conductance-voltage characteristics of bi-(quasi)oligo(phenylene ethynylene)-monothiol molecular junctions reported by Wu et al (2008 Nat. Nanotechnol. 3 569) is presented. The so-called ortho- and para-conformations of the bimolecular junction are considered. Our calculation indicates that the bimolecular junction prefers to take the ortho-conformation because of its lower energy. The simulation supports the experimental findings that aromatic coupling between two molecules is strong enough to induce the formation of molecular junctions. By comparing with experimental results, structure parameters for a probable bimolecular junction are determined. The underlying mechanism for formation of the bimolecular junction and its electron transport is discussed.

Effects of the basis set and of the exchange–correlation functional on the Inelastic Electron Tunneling signatures of 1,4-benzenedithiol

The effects of the atomic basis set and of the exchange-correlation (XC) functional on the Inelastic Electron Tunneling (IET) spectra have been investigated by considering the prototypical 1,4-benzenedithiol molecule. These studies have been completed by tackling the reliability of the same methods for predicting the IR absorption spectrum of the same molecule. The main conclusions are (i) the B3LYP XC functional is suitable to predict the relative vibrational frequencies, (ii) provided a scaling factor is used, the root mean square error on the vibrational frequencies goes down to 18 cm(-1), (iii) triple-ζ basis sets and in particular the cc-pVTZ basis set is a good compromise between accuracy and computational needs, (iv) basis set effects on the IET intensities are larger than those of the XC functional, and (v) the cc-pVTZ, cc-pVQZ, and aug-cc-pVDZ basis sets provide consistent IET intensities.

Theoretical insight into the inelastic electron tunneling spectra of an anil derivative.

First-principles simulations have been employed to simulate the inelastic electron tunneling (IET) spectra of the enol and keto forms of an anil molecular switch and to analyze them with respect to the character of the vibrational normal modes. When the molecules are sandwiched between Au plates, the dominant IET signatures appear at very similar voltages for both forms, but their intensities are clearly different, which makes IET an efficient technique to probe the molecular state of the switch. The IET-active modes are also similar for both anil forms and consist of in-plane molecular motions, CC and ring stretching, and C-H bending motions. Moreover, the IET activity of the vibrational modes specific to the enol and keto forms, i.e., those involving bending motions of the C-O-H and C-N-H groups, respectively, demonstrates that IET spectroscopy is an efficient technique to distinguish unambiguously between the two states of the keto/enol switch.