In 1974, Prof. Aviram and Ratner theoretically suggested that a molecular-scale diode could be realized by a single molecule composed of sigma bridged donor-acceptor molecule (denoted as D-σ-A). After that, the field of molecular electronics has grown dramatically with the advance of nanotechnology. Various molecular-scale diodes have been also developed and demonstrated. Among them, the molecular junctions employing with D-σ-A molecules or ferrocenyl-based alkanethiol molecules have been highlighted as potential candidates for realizing the molecular diode. These ‘specially designed’ molecules have asymmetrically positioned frontier molecular orbital (i.e., highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)) that activates different charge transport pathway according to voltage polarities. As most research did, it is generally considered that synthesizing the specific molecules having asymmetric molecular orbital levels is essential for the molecular-scale diode characteristics.

Figure 1 a. Schematics of molecular heterojunction system, b. I-V characteristics, c. Theoretical prediction of rectifying characteristics with 1_L-MoS₂ for molecular heterojunction system.

However, in this study, we simply implemented a molecular-scale diode without the need of any specifically synthesized molecular component. The molecular heterojunction system we developed is only composed of a non-functionalized molecule and two-dimensional (2D) semiconductor (Fig. 1a). The molecular junction with only non-functionalized self-assembled OPT2 molecules exhibits a symmetric-like tunneling transport (dotted line in Fig. 1b). However, when we inserted 2D semiconductors such as MoS₂ between Au and OPT2 molecules, the distinct rectifying feature was observed (solid red circle in Fig. 1b). This is because different charge transport pathway can be activated according to voltage polarities. In positive bias regime, the conduction band of MoS₂ participated in charge transport mechanism, resulting shortens transport width. In negative bias regime, however, transport width can be longer due to additional Schottky barrier of MoS₂. In addition, the rectification ratio (RR) of the molecular heterojunction was largely changed from 1.24 to 1.83 × 10⁴ depending on molecular species, molecular length, the type of 2D semiconductors, and the number of MoS₂ layers.  

Based on the suggested pathway-dependent transport mechanism, we predicted the RR of molecular heterojunction systems depending on molecular length, barrier height, and the number of MoS₂ layers and confirmed they are in good agreement with the experimental results (Fig. 1c). We believe that our work sets a design rule for implementing tailored-diode function in a molecular heterojunction structure with nonfunctionalized molecular systems.  

For more information, please see our recent publication in Nature Communications:    

Shin, J. et al. Tunable rectification in a molecular heterojunction with two-dimensional semiconductors. Nat. Commun. 11, 1412 (2020). DOI: 10.1038/s41467-020-15144-9   

Gunuk Wang