Tuning the Tunnelling Decay Coefficient Using Single Polarizable Atoms via Energy Level Engineering of the Frontier Orbitals

Xiaoping Chen^a, Harshini Annadata^a, David Egger^b, Christian Nijhuis^a^,^c^,^d

^aDepartment of Chemistry, National University of Singapore, 21 Lower Kent Ridge Road, Singapore, 119077, Singapore

^bInstitute of Theoretical Physics, University of Regensburg, Universitätsstraße 31, 93040 Regensburg, Germany

^cCentre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 6 Science Drive 2, Singapore 117546, Singapore

^dNUSNNI-Nanocore, National University of Singapore, Singapore 117411, Singapore *Correspondence to: chmnca@nus.edu.sg

Materials for Sustainable Development Conference (MATSUS)
Proceedings of nanoGe Fall Meeting 2018 (NFM18)

S5 Charge Carrier Dynamics at the Nanoscale

Torremolinos, Spain, 2018 October 22nd - 26th

Organizers: David Egger, Arjan Houtepen and Freddy Rabouw

Oral, Harshini Annadata, presentation 198
DOI: https://doi.org/10.29363/nanoge.nfm.2018.198
Publication date: 6th July 2018

Understanding the charge transport rates across molecules and molecule-electrode interfaces is important in many areas of research including chemistry, biology, and nanoscience.¹^-² A crucial parameter is the tunnelling decay coefficient (β, in Å^-1 or n_C^-1) which determines how quickly the current across the junction decreases as a function of the length of the molecule. Usually, the value of β can be changed by changing the chemical structure of the molecular backbone,³^-⁴ but β also depends on the type of the binding with the electrodes for conjugated systems.³ We have reported that SAMs of S(CH₂)_nX where X = H, F, Cl, Br, or I, have increasingly high currents with increasing polarizability of X.⁵ Here we report a new approach to tune β by simply changing X. In this system, eutectic gallium-indium alloy (EGaIn) was used as the top electrode, a monolayer of S(CH₂)_nX was self-assembled on a Ag surface which also served as the bottom electrode. We found that as the polarizability of X increases from X = F to I, β decreased from 0.97 ± 0.04 n_C^-1 to 0.34 ± 0.01 n_C^-1 and the dielectric constant ε_r increased from 2.5 ± 0.6 to 8.9 ± 1.6, respectively. DFT calculations show that the electrostatic potential profile of the SAM depends on X. More specifically, we found that the HOMO-1 is the dominant conduction orbital that is highly effected by X resulting in the tunnelling barrier height and thus the decay coefficient. In other words, the value of β can be controlled by using one polarizable atom without the need to change the molecular backbone.

References:

(1) Stubbe, J. et al. Chem. Rev. 2003, 103, 2167.

(2) Heitzer, H. M. et al. Acs Nano 2014, 8, 12587.

(3) Kim, B. et al. Am. Chem. Soc. 2011, 133, 19864.

(4) Xie, Z. et al. Acs Nano 2015, 9, 8022.

(5) Wang, D. et al. Adv. Mater. 2015, 27, 6689.

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