Imen Hnid1,Ali Yassin2,David Guérin1,Lionel Sanguinet2,Philippe Blanchard2,Imane Arbouch3,Colin Van Dyck4,Jerome Cornil3,Stephane Lenfant1,Dominique Vuillaume1
IEMN-CNRS1,MOLTECH-Anjou2,Laboratory for Chemistry of Novel Materials, University of Mons3,Laboratory for Theoretical Chemical Physics, University of Mons4
Imen Hnid1,Ali Yassin2,David Guérin1,Lionel Sanguinet2,Philippe Blanchard2,Imane Arbouch3,Colin Van Dyck4,Jerome Cornil3,Stephane Lenfant1,Dominique Vuillaume1
IEMN-CNRS1,MOLTECH-Anjou2,Laboratory for Chemistry of Novel Materials, University of Mons3,Laboratory for Theoretical Chemical Physics, University of Mons4
We study the electron transport properties under a terahertz (THz) irradiation of three molecules that consist of two conjugated systems, which are coupled through a non-conjugated linker. Due to this non-conjugation between them, each molecule can be seen as two weakly coupled sites in series and a resonant transport occurs only when the energy of the two sites are aligned leading to negative differential resistance (NDR) behaviors<sup>[1]</sup>. Moreover, it has been theoretically suggested that switching in such systems, between different current states in transport through molecules, is possible and may be triggered by a passing electromagnetic pulse in the THz range<sup>[2]</sup>. This mechanism relies on a THz-induced resonant electron transfer between the pair of neighboring subunits. As a consequence, the energy of the two sites are misaligned and therefore, the passage of current is turned off by the irradiation pulse and the NDR behaviors are eliminated.<br/><br/>Three molecules that differ by the spacer (phenyl vs. thiophene) and the anchor group (thiol vs. cyanide) were synthesized and corresponding SAMs were formed on very flat template-stripped gold electrodes (RMS<0.5nm). Solid-state molecular junctions (MJs), in which a C-AFM (Conducting-Atomic Force Microscopy) Pt/Ir tip is used as the top electrode, were formed. The electronic properties of the resulting MJs were investigated under an in-situ THz irradiation (30THz), and will be presented for the three molecules.<br/><br/>When the THZ irradiation is OFF, NDR behaviors at the nanoscale (for few tens of molecules) were observed with an unprecedented large peak-to-valley ratios (up to ca. 500 compared to ca. 15 in the MCBJ experiments<sup>[1]</sup>). To gain more insight into the origin of the NDR effect, we also investigated these molecules using density functional theory (DFT) and the non-equilibrium Green’s function formalism (NEGF). The calculations reproduced well the observed experimental NDR behavior, which is explained by breaking the degeneracy of the HOMO and HOMO-1 levels under the effect of the applied voltage.<br/><br/>When the Thz irradiation is ON, the junction is immediately switched and the NDR behaviors are completely suppressed. These phenomena were reproducibly and reversibly observed for all three molecular junctions, whatever the nature of the spacer and anchoring group, and will be presented and discussed.<br/><br/><b>References</b><br/>[1] M. L. Perrin, R. Frisenda, M. Koole, J. S. Seldenthuis, J. A. C. Gil, H. Valkenier, J. C. Hummelen, N. Renaud, F. C. Grozema, J. M. Thijssen, D. Dulić and H. S. J. Van Der Zant, Nat. Nanotechnol., 2014, 9, 830.<br/>[2] P. Orellana and F. Claro. <i>Phys. Rev. Lett.</i> 2003, 90, 4.<br/><br/><i>We acknowledge J.F. Lampin for the loan of the 30 THz laser and help with its set-up. Financial support by ANR (project EVOLMONET, #ANR-20-CE30-0002).</i>