A unified model for transition-metal mediated electron transport

Lead Research Organisation: University of Oxford
Department Name: Oxford Chemistry


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.

Planned Impact

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'.


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Description The purpose of this work was to establish a relationship between chemical structure and composition and the ability of molecules to conduct electricity. Molecules that are long and thin are often referred to as 'molecular wires', but the overall morphology is only the starting point for conductivity. Analogies to the macroscopic world are heightened when transition metals ions are involved, because everyone knows that bulk metals are generally good conductors (the copper core in most wiring circuits, for example). A single-atom thick chain of transition metal ions, typically supported by ligands and typically in oxidation states other than neutral, is a world away from this bulk metallic limit, yet this analogy is a powerful motivator for the continuing synthetic effort in this field. Our work has sought to establish the extent to which these simple analogies are in any way reflected at the molecular level, where quantum effects are all-important.
Perhaps the most important general conclusion to emerge from this work is the distinction between first-row transition metals and the heavier metals. In the former, very strong repulsions between the electrons (ultimately caused by the absence of radial nodes in the 3d orbitals) tend to cause localisation which necessarily inhibits conduction (this is embodied in the well-known Mott-Hubbard physics). The same factors make describing the electronic structure within a single-determinant ansatz (i.e. density functional theory) very challenging. We have seen this most strikingly in our work on chrmium chains, where the dominant feature is the weak interactions between the metals. At the opposite extreme, the heavier transitions are much more delocalised as a result of better overal and weaker repulsions. Our work on ruthenium atom chains has highlighted the striking stronger conductivity, but also the much greater tendency to structural distortions which again relates to better overlap. The distortions, most notable the bending the occurs in the Ru3 chains, has a significant impact on conductivity and again invites analogies to the macroscopic world: bends and kinks in wires inhibit conduction, and it seems that the same is true, albeit for rather different reasons, at a molecuar level.
Through the course of the project, we have tried to survey a range of different metals, and as a result we have adapted our methodology to meet different challenges. The extensive use of multi-configurational approaches was not anticipated at the outset, but has proved to be absolutely essential to our understanding of the chrmium chains in particular. Likewise, this greater understanding of the underlying physics has pushed the project in the direction of silicon-based materials, and in particular those where a transition metal ion is encapsulated. The move towards silicon-based materials provides the potential to connect directly with existing transistor architectures (MOSFET), and this will be the focus of the project in the future.
Exploitation Route We hope that the deeper understanding of the relationship between structure and function in molecular conductors will inform the various groups around the world who are synthesising new compounds that fall under the broad umbrella of 'molecular wires'
Sectors Chemicals,Education,Electronics

URL http://research.chem.ox.ac.uk/john-mcgrady.aspx