Optical and thermal manipulation of molecular spin states on graphite surface
The paper in Nature Communications is here: go.nature.com/2vgoElD
The great feature of the microelectronics revolution, namely, its scalability –- expressed best by Moore’s law –- has led to a rapid increase in computing capacity. But with great computing capacity comes great energy consumption. If the current trend in technology development (aka Moore’s law) continues, it is estimated that electronic devices will burn-off more than half of the Earth’s energy budget within the next two decades . Thus, there is a need to weaned from the existing silicon-based technology, simply because it is not sustainable.
Molecules exhibiting properties that can be controlled externally by light or electric field are seen as an alternative to overcoming the approaching energy crunch. Spin-crossover molecules offer an attractive prospect: their spin states –- so-called high-spin and low-spin states –- can be reversibly switched by temperature, light, pressure, and electric field. In addition, spin-crossover molecules exhibit different electrical conductance in the two spin states: high-conductance in the high-spin state and low-conductance in the low-spin state. Due to this, they are envisioned to serve as basic building blocks for future technologies, in a bottom-up approach.
However, the molecules’ sensitivity to external stimuli -- more often than not –- has proven to be a double-edged sword upon contact with solid surfaces. Spin-crossover molecules are rather fragile, and when they come in contact with solid surfaces –- which is a prerequisite for any device integration –- the spins are quenched in either one or both of the states (coexist), or only a small fraction of the molecules retain their bistability. The reasons may vary; for one, even an inert surface like gold could cause fragmentation of the molecules, while molecular distortion from the usual octahedral symmetry –- responsible for the bistability –- due to interaction with surfaces could also result in the loss of bistability. In fact, the coexistence of the spin states on surfaces is so common that it came to be regarded as the true thermodynamic phase.
In this work –- which is a culmination of years of research on trying to gain control over the spin-crossover property on surfaces, mainly between our research group at Freie Universität Berlin and our collaborators at Christian-Albrecht Universität Kiel –- we showed the complete and reversible switching between the two spin states of spin-crossover molecules on a highly oriented graphite surface, with coverages ranging from submonolayers to multilayers. These results, along with the earlier works by Bernien et al. , debunked the more prominently held view of the spin-state coexistence as being an intrinsic property of spin-crossover molecules when in contact with solid surfaces, but rather a property dependent upon molecule-substrate combinations. It is also interesting to note that spin-crossover molecules in reduced dimensions retain much of their behavior from the bulk, like exhibiting cooperative effects in the temperature induced, and non-cooperative effects in the light-induced spin transitions. These findings are expected to give a renewed impetus to research in this field.
 Bernien, M. et al. Highly efficient thermal and light-induced spin-state switching of an Fe(II) complex in direct contact with a solid surface. ACS Nano 9, 8960-8966 (2015).http://dx.doi.org/10.1021/acsnano.5b02840