Ioan Baldea
Heidelberg University
Ioan Baldea is principal investigator at the Chair of Theoretical Chemistry, Heidelberg University (Germany) and full Research Professor of Theoretical Physics at the Institute of Space Sciences, National Institute for Lasers, Plasmas, and Radiation Physics, Bucharest-Magurele (Romania). His research work comprises general theory of condensed matter physics, material science, and quantum chemistry. In recent years, he has mainly focused on molecular electronics and nanotransport, with emphasis on quantum dots, quantum dots nanoarrays, transition voltage spectroscopy, solvent effects and reorganization effects in molecules with floppy degrees of freedom.
In the present work I will present results [1] demonstrating a surprising way to control the charge transport in molecular electronic devices based on floppy molecules forming self-assembled monolayers (SAMs) adsorbed on electrodes. To be specific, I will consider two benchmark molecules often used to fabricate molecular junctions: 4-methyl-4'-mercaptobiphenil (CH3-(C6H4)2-SH, BPMT) and 4,4'-dithiol-1,1'-biphenyl (HS-(C6H4)2-SH, BPDT) [2] adsorbed on Au(111). Their most salient structural feature is the twisting angle φ between the two phenyl rings that can easily rotate relative to each other.
Changing φ in molecules of this kind is important e.g. for molecular switches because the low bias conductance G scales as cos2φ but the question is how can one substantially modify the twisting angle. Adding fractional electronic charge q represents a possibility; still, significant φ-variations require transferring of average charge fractions q~80% [3], which is much more that presently achieved (q≤10%) in experiments on molecular junctions.
Results obtained via DFT calculations demonstrate that the SAM coverage can have a strong impact on the torsional angle φ [1]. While the φ-values at low coverages do not significantly differ from the values φ≈36° characteristic for the isolated molecular species, they substantially change in cases of high coverages. To illustrate with a situation typical for the second case, I refer to SAMs with herringbone arrangements. The supercell corresponding to herringbone ordering comprises two crystallographically nonequivalent molecules. Calculations yield values φ1,2≈76° (cf. Tables 1 and 2) for the slightly different twisting angles φ1,2 of the two molecules [1]. This translates into a reduction in conductance by a factor ~10.
So, a SAM herringbone arrangement appears to suppress conduction in junctions based on molecules of the type considered. In fact, for 2BDT junctions this ordering could be ruled out in a recent joint experimental-theoretical study [4].
1. I. Bâldea, Faraday Disc. 2017, DOI 10.1039/C7FD00101K.
2. Z. Xie, I. Bâldea, C. Smith, Y. Wu, and C. D. Frisbie, ACS Nano 2015, 9, 8022.
3. I. Bâldea, RSC Adv. 2016, 6, 111903.
5. Z. Xie, I. Bâldea, A. T. Demissie, C. E. Smith, Y. Wu, G. Haugstad, and C. D. Frisbie, JACS 2017, 139, 5696.
