Molecular interaction and assembly is an essential process in various fields such as in chemistry, biology and materials science. As one of sophisticated systems, the process is successfully incorporated in biology to construct functional structures, including the membrane of cells. Through the interactions of each molecule, molecules show assembly to form functional patterns and structures. We are trying to extend the molecular interaction/assembly onto inorganic electronic materials/devices. This presentation shows our recent results of molecular interaction on 2D materials, especially the semiconducting 2D material, transition metal dichalcogenides (TMDCs).^1,2,3

The TMDC is constructed from atomically thin layers (thickness ~0.7 nm) with van der Waals interaction, and is expected to make tiny optoelectronic devices due to their thin nature. However, the essential requirement for controlling carriers is a problem because the conventional methods, such as ion plantation, are not applicable due to its atomically thin nature. We found that mere contact of a dopant molecule with MoS₂, the donor molecule is effective to control the carrier concentration of a TMDC, MoS₂.¹ They formed an organic-inorganic electron transfer interaction (Fig. 1a). As a result, the doping level of MoS₂ can be varied depending on the amount of the donor molecule. We also found that the doping process can make a high-performance MoS₂ transistor with subthreshold swings as low as 77 mV/dec. close to the theoretical limit.

We further examined the assembly and pattern formation of molecules on MoS₂; MoS₂ was used as a new class of substrates for assembling or patterning molecules (Fig. 1b). ² First, we found that MoS₂ is a model substrate for growth of in-plane Au nanowires (NWs). Au NWs show oriented growth despite the nearly 8% lattice mismatch between MoS₂ and Au. Au NWs on the MoS₂ surface are oriented along three symmetrically equivalent directions within the substrate that arise from the symmetry of MoS₂ that templates the oriented growth. In terms of the electronic state point of view, the growth process of Au NWs includes a strong charge transfer process between Au NWs and MoS₂, resulting in degenerate p-doping of MoS₂.

We also found that a dopant molecule showed a pattern formation on MoS₂ as like cellular structure or patterns shown in natural systems. We destabilized an ultrathin molecular film on MoS₂ by external perturbation, induced lateral diffusion of molecules on MoS₂ surface and showed various patterns such as tiny holes, networks, maze structures. Since the lateral undulation of the pattern was on the 100 nm scales, the pattern was revealed to modify MoS₂ characteristics. In the presentation, the details of the above research will be discussed from the structural point of view to the electronic point of view.

References:

¹D. Kiriya et al., J. Am. Chem. Soc. 2014, 136, 7853. ²Adv. Funct. Mater. 2015, 25, 6257. ³M. Amani^*, D.H. Lien^*, D. Kiriya^* et al., Science, 2015, 350, 1065.