Nanoparticles based molecular electronic devices with tunable molecular functionalization shell and gas sensing measurements

The idea to use molecules as a basic building block in electronic circuits was developed about 50 years ago when a molecular rectifier was developed, but it has been a challenge for this field to make its way to real-world application. Now, due to the advancement in technologies, the properties of single molecules are better understandable and controllable. Some of the main motivations to build molecular electronics devices are that the conductive molecules can be as small as about 1 nm, that they are stable objects and can be tailor-made with desired electronic properties. This small size of molecule poses a challenge in their usage, one solution is to develop the hybrid devices whose properties are based on single and few molecules. In this study, a portable hybrid device is used and further developed called a nanoMoED device, a nano-molecular electronic device. These devices consist of gold nanoparticles (AuNPs), gold nanoelectrodes and conjugated organic molecules. The electrical resistance of the device depends on the molecules functionalizing it and, in this work, they contain phenyl rings such as 4,4’-biphenyldithiol (BPDT), p-ter-phenyl-4,4''-dithiol and oligo phenylene-ethynylene. The 20 nm wide nanogaps are fabricated by a focused ion beam (FIB) creating thus true nanodevices. The molecular nanojunctions are formed by dielectrophoretic trapping of molecule functionalized AuNPs into a nanogap. The distance between the NPs, measured from transmission electron microscopy images is similar to the size of the targeted functionalizing organic molecule that shall bridge the NP-NP gap. We have reported that the primary molecular ligand shell of the AuNPs can be tuned in the synthesis process by the secondary molecular functionalization process. The experimental results showed that this process depends on the interparticle spacing and the structure of the primary functionalizing molecules. The nanoMoED devices showed a successful cyclic molecular place exchange process where alternately BPDT and octanethiol (OT) were moved into the devices. This is confirmed by a change in the electrical resistance of devices showing higher conductance for BPDT than OT. The nanoMoED devices when tested in NO2, ethanol, and NH3 gas atmosphere showed a significant change in device electrical resistance. Density functional theory calculations explain this observation. The analyte molecules bind with the aromatic conjugated molecule and induce additional charge transport channels near the Fermi level of the sensing molecule. In graphene-based, i.e., 2D, micron-sized devices, we could show that the non-covalent molecular functionalization of graphene improves its NH3 gas sensing response by 3 times as compared to pristine graphene. Further experiments are required to understand the device properties under different working conditions as well as to evidence different functionalities for example as a switch.