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Control of current-induced heat exchange in molecular junctions by molecular scale design of the electronic properties

Periodic Reporting for period 1 - HEATEXMOL (Control of current-induced heat exchange in molecular junctions by molecular scale design of the electronic properties)

Reporting period: 2016-07-01 to 2018-06-30

The interaction of an electrical current with atomic vibrations is responsible for the Joule heating mechanism. At the nanoscale, due to the high current densities, the power dissipated in the system is very high and it can lead to conformational changes or even the breakdown of the system.
The HEATEXMOL project is aimed at investigating through atomistic, ab-initio calculations the complex interplay between electronic structure and current-induced heating and cooling dynamics in molecular junctions. Depending on this interplay, the current passing through the molecule can heat the junction, but also cool it.
Concerning the first part of WP1, we studied the effect of the atomistic structure of electrode terminations on the HCD of a series of biscarbene molecular junctions.
We found that sharp, elongated tip terminations result in a more efficient heat dissipation under an applied bias compared to the cases of blunt tips.
These qualitatively different behaviors are related to different coupling regimes of the molecule to the electrodes. These findings provide new insight to the current-induced heating and cooling dynamics of molecular junctions under an applied bias and pave the way to the rational design of electrode structure for the efficient heat removal in molecular circuits. As a result of this work we published an article:

G. Foti and H. Vázquez, J. Phys. Chem. C 121, 1082, 2017

In the second part of WP1 we explored the effect of an NH2 molecule adsorbed on only one electrode, near a biscarbene-metal junction.
We found that energy stored in the vibrational modes of the molecule is lower in presence of the adsorbate than in a “clean” junction with no adsorbed species. This means that the NH2 adsorbate effectively cools the conducting molecule. We explain this cooling in terms of the changes in the electronic structure of the junction induced by the adsorbate.
These results were published in a paper accepted in the Beilstein Journal of Nanotechnology:

G. Foti and H. Vázquez, Beilstein J. Nanotechnol. 8, 2060, 2017

In WP2 we focused on the origin and localization of vibrational modes on the inelastic vibrational signal, in collaboration with experiment. In recent experiments it was found that the inelastic signal associated to the frustrated translational (FT) mode of CO depends on the relative position of tip and an underlying iron phtalocyanine (FePc) molecule deposited on a Au(111) surface.
This gives a contrast for the sub-molecular imaging of the structure of the underlying FePc molecule. The intensity of the frustrated rotational (FR) mode, on the other hand, does not change as a function of tip position. Thus it cannot be used for the imaging of molecular structure.
Using a method we developed [1] we found that FT mode has contributions from the tip and CO atoms, and from the coupling between CO molecule and the benzene molecule. This makes FT mode more sensitive to the local “environment”. The FR mode, on the other hand, has its origin in the tip-CO region alone and it is thus insensitive to the substrate potential.

These results were published on Physical Review Letters:

B. de la Torre, M. Švec, G. Foti, O. Krejči, P. Hapala, A. García-Lekue, T. Frederiksen, R.
Zbořil, A. Arnau, H. Vázquez and P. Jelínek, Phys. Rev. Lett. 119, 166001, 2017

[1] G. Foti and H. Vázquez, Phys. Rev. B 94, 045418 (2016)

In the third work package we investigated the presence of current-induced vibrational instabilities in molecules where transport is controlled by separated, empty electronic resonances. We showed that these instabilities are much more general then previously predicted.
Using first-principles and model system calculations we found that, from the point of view of the electronic structure, the only requirement to observe instabilities is to have empty separated molecular states confined each at the one side of the junction. Using the model system we also generated a stability diagram revealing the important role of the electron-vibration matrix elements.
This shows that the electronic structure is only one of the factors contributing to the instability which crucially depends also on the elements of the electron-vibration coupling matrix.
We also analyzed the evolution of bias-induced instability for a wide range of the energy offset between left and right resonance, which is not accessible from DFT-NEGF simulations.
This last calculation clearly demonstrates the presence of instabilities for systems with reduced conjugation, LUMO-dominated transport and without the condition of population inversion.

These results were published in the Journal of Physical Chemistry Letters:

G. Foti and H. Vázquez, J. Phys. Chem. Lett., 9, 2791, 2018
We shed light on the importance of the atomistic structure of electrode terminations for the heating and cooling dynamics of molecular junctions. Our results open the way to the atomic-scale design of contact structure for the efficient heat removal in molecular junctions with important implications for the stability and correct functioning of future molecular circuits.
The role of contaminants adsorbed near the junction had not been explored in the context of the energy exchange processes in molecular junctions.
Our study shows that adsorbates can influence not only the elastic transport properties but also the heating and cooling dynamics of nanoscale systems.
Also, we demonstrated the utility of a local interpretation of the inelastic tunneling mechanism in molecular junctions.
We demonstrated both analytically and numerically that vibrational instabilities can be expected in a much broader class of systems than previously reported and without the rather restrictive condition of population inversion of previous predictions.