Transport and transport-related phenomena in gated junctions based on molecules with floppy vibrational degrees of freedom
Final Report Abstract
This project aimed at theoretically investigating the charge transport and related phenomena in molecular junctions fabricated with floppy molecules, also subject to nanogating and mechanical stretching. An important part of the work within this project was done in close collaboration with the experimental group of Professor C. D. Frisbie (University of Minnesota). Results of these investigations made the object of 33 peer-reviewed publications (many thereof appeared in leading journals with broad audience and high impact factor) and were disseminated in numerous talks at scientific conferences/workshops. Much work was devoted to floppy molecules consisting of aromatic (phenyl) rings that can easily rotate relative to each other. Emphasis was laid on the possibility of tuning the inter-ring torsion angles θ, because this enables to efficiently control the charge transport. For the first time in the literature, three specific manners were indicated on how θ can be modified: (i) by tuning the fractional average redox charge (a goal that can be efficiently achieved via electrochemical gating using ionic liquids/gels as electrolyte, a method pioneered by Frisbie’s group), (ii) by varying the coverage of a self-assembled monolayer fabricated with floppy molecules, and (iii) upon shining floppy molecules with UV/vis radiation. Charge transport through junctions fabricated under CP-AFM testbed using oligophenylene mono (OPTn) and dithiols (OPDn) and electrodes of silver, gold, and platinum were studied in great detail. Nuclear techniques available at the Characterization Facility (University of Minnesota) allowed the precise determination of the number of molecules per CP-AFM junctions, which represents the most important prerequisite for an overall theoretical analysis of the statistical variances in transport and transportrelated properties. An important finding emerging from this work which contradicts previous opposite claims in the literature, was the unambiguous demonstration that all statistical variances are exceptionally small. Even more importantly from a fundamental standpoint, based on new ultraviolet photoelectron spectroscopy (UPS) data reported, these studies validated the approach to charge transport based on the single model level (SLM) in the form put forward earlier by the principal investigator and transition voltage spectroscopy (TVS): first, in the benchmark sets of aromatic OPTn and OPDn molecules, then, in the benchmark sets of aliphatic alkane monothiols (CnT) and dithiols (CnDT) molecules. This adds adds more evidence that combining transport, electron spectroscopy, and analytical theory is a productive strategy for quantitative analysis in molecular electronics, also for settling the long-standing band lineup problem. Results for CP-AFM junctions subject to a mechanical force represent another important piece of new information. Comparison between transport in CnT and OPTn CP-AFM junctions under stretching showed a clear difference between tunneling assisted by a localized “gateway” orbital with σ character (CnT) and a fully delocalized π-conjugated orbital (OPTn). On this basis, another long-standing issue in molecular electronics has been settled: charge transport through alkanethiols is dominated by the localized HOMO; it cannot be mediated by the HOMO-1 extended over the entire backbone. Do molecules embedded in molecular junctions feel the entire perturbation (e.g., bias voltage, temperature difference) applied to electrodes? This is another fundamental issue that plagued molecular electronics since its inception. The detailed characterization of the OPTn junctions achieved thanks to the work in this project allowed to demonstrate that, while subject to the entire bias applied, the molecules are only subject to a rather small fraction of the external temperature difference. An important practical consequence of this finding is that the molecular orbital energy offset deduced from thermopower data is substantially overestimated. Last but not least, insight gathered from studying a sui generis spatial confinement occurring in SAMs of floppy molecules allowed to devise an efficient strategy of fighting against SARS-CoV-2 virus spread, an issue that needs not much explanation amid the ongoing COVID-19 crisis.
Publications
- A surprising way to control the charge transport in molecular electronics: the subtle impact of the coverage of SAM of floppy molecules adsorbed on electrodes, Faraday Discuss. 204 (2017) 35
I. Bâldea
(See online at https://doi.org/10.1039/c7fd00101k) - Effect of heteroatom substitution on transport in alkanedithiol-based molecular tunnel junctions: Evidence for universal behavior, ACS Nano 11 (2017) 569
Z. Xie, I. Bâldea, S. Oram, C. Smith, and C. D. Frisbie
(See online at https://doi.org/10.1021/acsnano.6b06623) - Exceptionally small statistical variations in the transport properties of metal-molecule-metal junctions composed of 80 oligophenylene dithiol molecules, J. Am. Chem. Soc. (Communication) 139 (2017) 5696
Z. Xie, I. Bâldea, A. Demissie, C. Smith, Y. Wu, G. Haugstad, and C. D. Frisbie
(See online at https://doi.org/10.1021/jacs.7b01918) - Why one can expect large rectification in molecular junctions based on alkane monothiols and why rectification is so modest, Chem. Sci. 9 (2018) 4456
Z. Xie, I. Bâldea, and C. D. Frisbie
(See online at https://doi.org/10.1039/c8sc00938d) - Work function and temperature dependence of electron tunneling through an n-type perylene diimide molecular junction with isocyanide surface linkers, Nanoscale 10 (2018) 964
C. E. Smith, Z. Xie, I. Bâldea and C. D. Frisbie
(See online at https://doi.org/10.1039/c7nr06461f) - Determination of energy level alignment in molecular tunnel junctions by transport and spectroscopy: Self-consistency for the case of oligophenylene thiols and dithiols on Ag, Au, and Pt electrodes, J. Am. Chem. Soc. 141 (2019) 497
Z. Xie, I. Bâldea, and C. D. Frisbie
(See online at https://doi.org/10.1021/jacs.8b13370) - Mechanical deformation distinguishes tunneling pathways in molecular junctions, J. Am. Chem. Soc. 141 (2019) 3670
Z. Xie, I. Bâldea, G. Haugstad, and C. D. Frisbie
(See online at https://doi.org/10.1021/jacs.8b11248) - Evidence that molecules in molecular junctions may not be subject to the entire external perturbation applied to electrodes, Langmuir 36 (2020) 1329
I. Bâldea
(See online at https://doi.org/10.1021/acs.langmuir.9b03430)
