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Contrasting Transport and Electrostatic Properties of Selectively Fluorinated Alkanethiol Monolayers with Embedded Dipoles

Published

Author(s)

Robert C. Bruce, Lin You, Anja Forster, Sujitra J. Pookpanratana, Olivia Pomerenk, Han Ju Lee, Maria D. Marquez, Rashid Ghanbaripour, Oussama Zensani, T R. Lee, Christina A. Hacker

Abstract

Surface dipoles are a powerful tool in interfacial modification for improving device output via energy level matching. Fluorinated alkanethiols show strong promise for these applications as they can generate large and tunable dipoles based on fluorine location and chain length. Furthermore, these chains can be designed to possess fluorocarbons solely along the backbone, enabling an ‘embedded’ configuration that generates a significant dipole effect from the fluorines while maintaining surface chemistry to prevent deleterious side effects from altered surface interactions. However, fluorine insertion can modify other molecular electronic properties, and it is important to consider the transport properties of these interfacial modifiers so that any impact on device performance is properly attributed. In this paper, we report the transport properties of self-assembled monolayers derived from a series of fluorinated alkanethiols, both with and without the embedded dipole structure. Photoelectron spectroscopy and Kelvin probe force microscopy (KPFM) show significant work function modification from all fluorine-containing molecules compared to purely hydrocarbon thiols. However, while embedded fluorocarbons generate a smaller electrostatic effect than terminal fluorocarbons, they yield higher tunneling currents across Au/monolayer/eutectic-gallium indium (EGaIn) junctions compared to both terminal fluorocarbon and purely hydrocarbon alkanethiols. Computational studies show that the location of the fluorine constituents is capable of modifying not only dipoles and energy levels but also molecular orbitals, enabling the presence of delocalized LUMO levels within the alkanethiol backbone and, thereby, the appearance of larger tunneling currents compared to other alkanethiols. Ultimately, we show that fluorinated alkanethiols and the embedded dipole architecture are both powerful tools, but they must be thoroughly analyzed for proper utilization in device setting.
Citation
Journal of Physical Chemistry C

Citation

Bruce, R. , You, L. , Forster, A. , Pookpanratana, S. , Pomerenk, O. , , H. , Marquez, M. , Ghanbaripour, R. , Zensani, O. , Lee, T. and Hacker, C. (2018), Contrasting Transport and Electrostatic Properties of Selectively Fluorinated Alkanethiol Monolayers with Embedded Dipoles, Journal of Physical Chemistry C, [online], https://doi.org/10.1021/acs.jpcc.7b11892 (Accessed July 29, 2021)
Created February 7, 2018, Updated November 10, 2018