Intermolecular and Electrode-Molecule Bonding in a Single Dimer Junction of Naphthalenethiol as Revealed by Surface-Enhanced Raman Scattering Combined with Transport Measurements

Abstract

Electron transport through noncovalent interaction is of fundamental and practical importance in nanomaterials and nanodevices. Recent single-molecule studies employing single-molecule junctions have revealed unique electron transport properties through noncovalent interactions, especially those through a π-π interaction. However, the relationship between the junction structure and electron transport remains elusive due to the insufficient knowledge of geometric structures. In this article, we employ surface-enhanced Raman scattering (SERS) synchronized with current-voltage (I-V) measurements to characterize the junction structure, together with the transport properties, of a single dimer and monomer junction of naphthalenethiol, the former of which was formed by the intermolecular π-π interaction. The correlation analysis of the vibrational energy and electrical conductance enables identifying the intermolecular and molecule-electrode interactions in these molecular junctions and, consequently, addressing the transport properties exclusively associated with the π-π interaction. In addition, the analysis achieved discrimination of the interaction between the NT molecule and the Au electrode of the junction, i.e., Au-π interactions through-π coupling and though-space coupling. The power density spectra support the noncovalent character at the interfaces in the molecular junctions. These results demonstrate that the simultaneous SERS and I-V technique provides a unique means for the structural and electrical investigation of noncovalent interactions.

Figure 1

Schematic illustration of the mechanically controllable break-junction (MCBJ) combined with surface-enhanced Raman scattering (SERS) measurements. The inset represents the possible interactions involved in the formation of the NT molecular junction.

Figure 2

SERS and electric conductance measurement for the NT molecular junctions. (a) Time course of the conductance (G) and SERS intensity, accompanied by the typical SERS spectrum and simultaneously acquired current–voltage (I–V) curves. (b) Histogram of the Raman shift of the observed peak obtained by the fitting with the Lorenz function. (c) Histogram of Raman shift and conductance was constructed from 8985 SERS spectra and I–V curves.

Figure 3

Structural analysis of NT molecular junctions by SERS spectra. (a) Two-dimensional (2D) histogram for Raman shift and conductance, along with the one-dimensional histogram of Raman shift. (b) Calculated spectra for the ring breathing mode. (c) Optimized structural models of the NT molecular junctions. Their assignments are also shown.

Figure 4

Simulation for the electron transport properties. (a) Transmission spectra of NT monomer junction with M1 and M2 structures. (b) Transmission spectra of NT dimer junction with D1 and D2 models. (c) The separation distance dependence of the transmission coefficient at the Fermi level (τ₀). The green and blue curves indicate the monomer and dimer junctions, respectively. The points corresponding to the M1, M2, D1, and D2 structures were marked by circles. The inset illustrates electron transport trajectory of the NT dimer junction with the D2 structure at the zero-bias voltage. Colored lines represent the direction of the electron transport. Blue lines represent transport in forward direction, while red one represents that in the opposite direction.

Figure 5

2D histogram of the flicker noise power (S_FN) and conductance. White and orange circles indicate the L and H states, respectively.