Abstract
Polyoxometalates (POMs) are promising candidates for molecular electronic applications because (1) they are inorganic molecules, which have better CMOS compatibility compared to organic molecules; (2) they are easily synthesized in a one-pot reaction from metal oxides (MOx) (where the metal M can be, e.g., W, V, or Mo, and x is an integer between 4 and 7); (3) POMs can self-assemble to form various shapes and configurations, and thus the chemical synthesis can be tailored for specific device performance; and (4) they are redox-active with multiple states that have a very low voltage switching between polarized states. However, a deep understanding is required if we are to make commercial molecular devices a reality. Simulation and modeling are the most time efficient and cost-effective methods to evaluate a potential device performance. Here, we use density functional theory in combination with nonequilibrium Green’s function to study the transport properties of [W18O54(SO3)2]4–, a POM cluster, in a variety of molecular junction configurations. Our calculations reveal that the transport profile not only is linked to the electronic structure of the molecule but also is influenced by contact geometry and presence of ions. More specifically, the contact geometry and the number of bonds between the POM and the electrodes determine the current flow. Hence, strong and reproducible contact between the leads and the molecule is mandatory to establish a reliable fabrication process. Moreover, although often ignored, our simulations show that the charge balancing counterions activate the conductance channels intrinsic to the molecule, leading to a dramatic increase in the computed current at low bias. Therefore, the role of these counterions cannot be ignored when molecular based devices are fabricated. In summary, this work shows that the current transport in POM junctions is determined by not only the contact geometry between the molecule and the electrode but also the presence of ions around the molecule. This significantly impacts the transport properties in such nanoscale molecular electronic devices.
Highlights
The semiconductor industry is driven by the ambition to extend Moore’s law
All density functional theory (DFT) studies rely on the choice of the functional and basis set and its ability to accurately describe the system of interest
It is vital that the highest occupied molecular orbital (HOMO)−lowest unoccupied molecular orbital (LUMO) gap is accurately calculated and the molecular geometry is successfully reproduced
Summary
Further miniaturization of CMOS technology is reaching its inherent limitations.[1] There is a significant research effort to overcome these limitations through a new approach. One such approach is from “the bottom up”, creating electronic circuits from single molecules. This is the goal of the field of molecular electronics.[2] While a lot of progress has been made since the discovery of molecular rectifiers in 1974,3 a commercial molecular based electronic device is still out of reach. For this reason various computational modeling techniques such as density functional theory (DFT)[5] are an efficient way to probe molecular electronic systems and gain a more fundamental understanding of electron transport and charge distribution
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