Metal-Molecule Contacts and Charge Transport across Monomolecular Layers: Measurement and Theory

Charge transport studies across molecular length scales under symmetric and asymmetric metal-molecule contact conditions using a simple crossed-wire tunnel junction technique are presented. It is demonstrated that oligo(phenylene ethynylene), a conjugated organic molecule, acts like a molecular wire under symmetric contact conditions, but exhibits characteristics of a molecular diode when the connections are asymmetric. To understand this behavior, we have calculated current-voltage ($I\mathrm{\text{−}}V$) characteristics using extended Huckel theory coupled with a Green’s function approach. The experimentally observed $I\mathrm{\text{−}}V$ characteristics are in excellent qualitative agreement with the theory.

Figure 1

(a) Schematic representation of the experimental setup (not to scale). The $I\mathrm{\text{−}}V$ characteristics of the junction are obtained by ramping the bias voltage ($V$) while monitoring the current flow ($I$) across the junction. In all cases the bias voltage is applied to the wire that was initially modified with the self-assembled monolayer. All measurements were made in a nitrogen purged Faraday cage at room temperature. (b) Junction resistance (calculated from the linear fits of the $I\mathrm{\text{−}}V$ characteristics at low bias voltage) as a function of applied force for a Au-1-Au junction. The $I\mathrm{\text{−}}V$ characteristics reported in this Letter (Fig. 2) were obtained in the low force regime which exhibits no force dependence. (c) Structure of the molecules investigated in this study.

Figure 2

Transport characteristics of Au-1-Au (a) and Au-2-Au (b) junctions. The open symbols in this figure and in Fig. 3 correspond to the current for the positive bias (applied to the gold wire onto which the molecules are first assembled), while the solid symbols correspond to the current for negative applied bias. The Au-1-Au junction yields $I\mathrm{\text{−}}V$ and $dI/dV\mathrm{\text{−}}V$ (numerically differentiated) characteristics that are symmetric with respect to bias polarity. The large asymmetry apparent in the transport characteristics for the Au-2-Au junction results from the different metal-molecule contacts on either side of the junction. The positive branch of the $I\mathrm{\text{−}}V$ trace (open symbols) corresponds to electron injection at the Au-phenylene interface.

Figure 3

Theoretically predicted transport characteristics for dithiolate (a) and monothiolate (b) oligo(phenylene ethynylene) molecules. The calculated $I\mathrm{\text{−}}V$ and $dI/dV\mathrm{\text{−}}V$ transport characteristics are in excellent qualitative agreement with the corresponding experimentally observed behavior (Fig. 2). (See text for details of the theory.)