TY - JOUR
T1 - Quantifying Image Charge Effects in Molecular Tunnel Junctions Based on Self-Assembled Monolayers of Substituted Oligophenylene Ethynylene Dithiols
AU - Xie, Zuoti
AU - Diez Cabanes, Valentin
AU - Van Nguyen, Quyen
AU - Rodriguez-Gonzalez, Sandra
AU - Norel, Lucie
AU - Galangau, Olivier
AU - Rigaut, Stéphane
AU - Cornil, Jérôme
AU - Frisbie, C. Daniel
N1 - Publisher Copyright:
© 2021 American Chemical Society.
PY - 2021/12/1
Y1 - 2021/12/1
N2 - A number of factors contribute to orbital energy alignment with respect to the Fermi level in molecular tunnel junctions. Here, we report a combined experimental and theoretical effort to quantify the effect of metal image potentials on the highest occupied molecular orbital to Fermi level offset, ϵh, for molecular junctions based on self-assembled monolayers (SAMs) of oligophenylene ethynylene dithiols (OPX) on Au. Our experimental approach involves the use of both transport and photoelectron spectroscopy to extract the offsets, ϵhtrans and ϵhUPS, respectively. We take the difference in these quantities to be the image potential energy eVimage. In the theoretical approach, we use density functional theory (DFT) to calculate directly eVimage between positive charge on an OPX molecule and the negative image charge in the Au. Both approaches yield eVimage ∼-0.1 eV per metal contact, meaning that the total image potential energy is ∼-0.2 eV for an assembled junction with two Au contacts. Thus, we find that the total image potential energy is 25-30% of the total offset ϵh, which means that image charge effects are significant in OPX junctions. Our methods should be generally applicable to understanding image charge effects as a function of molecular size, for example, in a variety of SAM-based junctions.
AB - A number of factors contribute to orbital energy alignment with respect to the Fermi level in molecular tunnel junctions. Here, we report a combined experimental and theoretical effort to quantify the effect of metal image potentials on the highest occupied molecular orbital to Fermi level offset, ϵh, for molecular junctions based on self-assembled monolayers (SAMs) of oligophenylene ethynylene dithiols (OPX) on Au. Our experimental approach involves the use of both transport and photoelectron spectroscopy to extract the offsets, ϵhtrans and ϵhUPS, respectively. We take the difference in these quantities to be the image potential energy eVimage. In the theoretical approach, we use density functional theory (DFT) to calculate directly eVimage between positive charge on an OPX molecule and the negative image charge in the Au. Both approaches yield eVimage ∼-0.1 eV per metal contact, meaning that the total image potential energy is ∼-0.2 eV for an assembled junction with two Au contacts. Thus, we find that the total image potential energy is 25-30% of the total offset ϵh, which means that image charge effects are significant in OPX junctions. Our methods should be generally applicable to understanding image charge effects as a function of molecular size, for example, in a variety of SAM-based junctions.
KW - Fermi level-HOMO offset
KW - charge transport
KW - image charge
KW - molecular junction
KW - photoelectron spectroscopy
KW - single level model
KW - tunneling
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U2 - 10.1021/acsami.1c16398
DO - 10.1021/acsami.1c16398
M3 - Article
C2 - 34783518
AN - SCOPUS:85119962836
SN - 1944-8244
VL - 13
SP - 56404
EP - 56412
JO - ACS Applied Materials and Interfaces
JF - ACS Applied Materials and Interfaces
IS - 47
ER -