A unified model for transition-metal mediated electron transport

The 2011 update to the International Technology Roadmap for Semiconductors (ITRS) sets out the need for fundamentally new technologies to replace existing silicon-based device components. Only if this can be achieved will the cost/function ratio of computer components continue to decrease at the rate to which we have become accustomed over the past 30 years (~25-29% per annum, according to the 2011 ITRS). One attractive solution is to develop molecular-scale analogues of key components such as wires, diodes and transistors. The incorporation of such components into realistic devices will face many challenges, not least those related to the manufacture of stable nanoscale arrays with precise and reproducible orientations. Nevertheless, the replacement of the field effect transistor is viewed as 'inevitable' in the most recent ITRS report, albeit probably not within the 15-year horizon that is its remit.
The immediate future is devoted to an exploratory phase where alternative technologies, including molecular electronics, can be assessed. The key challenge for the chemistry and physics communities is to understand the phenomena that control current flow through molecules. Only then can we turn to the engineering issues associated with manufacture. This proposal aims to address a simple yet fundamental question: how do electrons flow through aggregates of transition metal ions? The 'rules' for quantum transport in organic structures are relatively well established and provide direction to the ongoing synthetic effort. In contrast, a similar set of underpinning principles for transition metal-based transport is conspicuously absent. This is surprising given the obvious, if perhaps superficial, resemblance between macroscopic wires and chains of metal atoms that has driven a concerted synthetic effort over the past decade. A glance at the periodic table shows that metal-metal bonds are intrinsically far more diverse and flexible than their carbon-carbon counterparts. Moreover, the wealth of synthetic opportunity offered by coordination and supramolecular chemistry makes it highly unlikely that inorganic structures will not play at least some part in the long-term future of molecular electronics. The major objective of this work is to provide the road-map needed to link the coordination chemistry and physics communities that are central to progress in the field.

Beyond the academic sphere, the potential for impact needs to be viewed in the context of the International Road Map for Semiconductors (ITRS) which identifies the major challenges to the electronics industry over a 15-year horizon. The 2011 edition of the ITRS anticipates that the ultimate replacement of the field effect transistor is inevitable, but also that this is unlikely to be fully realised within the 15-year horizon of the roadmap. Indeed the report argues that current copper-based technology for interconnects is likely to remain dominant for the foreseeable future, and the immediate future (2011-2019) is devoted to exploratory work where alternative technologies can be assessed. It is abundantly clear that there are many hurdles to overcome before molecular electronics can begin to make a real impact on society, many of which relate to reproducibility in the manufacturing stage. The realistic targets for the lifetime of this grant are rather more pragmatic - we need to understand the fundamental chemistry and physics that underpin current flow through molecules. Only then can we aspire to one of the Grand Challenges to Physics - the ability 'to dial up a desired property using new principles rather than proceeding by trial and error'.