Fluorination, and Tunneling across Molecular Junctions

Abstract

This paper describes the influence of the substitution of fluorine for hydrogen on the rate of charge transport by hole tunneling through junctions of the form Ag TS O 2 C(CH 2 ) n (CF 2 ) m T//Ga 2 O 3 /EGaIn, where T is methyl (CH 3 ) or trifluoromethyl (CF 3 ). Alkanoate-based selfassembled monolayers (SAMs) having perfluorinated groups (R F ) show current densities that are lower (by factors of 20− 30) than those of the homologous hydrocarbons (R H ), while the attenuation factors of the simplified Simmons equation for methylene (β = (1.05 ± 0.02)n CH 2 −1 ) and difluoromethylene (β = (1.15 ± 0.02)n CF 2 −1 ) are similar (although the value for (CF 2 ) n is statistically significantly larger). A comparative study focusing on the terminal fluorine substituents in SAMs of ω-tolyl-and -phenyl-alkanoates suggests that the C−F//Ga 2 O 3 interface is responsible for the lower tunneling currents for CF 3 . The decrease in the rate of charge transport in SAMs with R F groups (relative to homologous R H groups) is plausibly due to an increase in the height of the tunneling barrier at the T//Ga 2 O 3 interface, and/or to weak van der Waals interactions at that interface. ■ INTRODUCTIONStudies of charge tunneling through metal−molecule−metal (MMM) junctions have focused predominately on testing hypotheses that correlate the chemical and electronic structure of the molecules with current densities (or in the case of singlemolecule studies, with currents). A convenient, semiquantitative theoretical framework around which to organize trends relating measurable parameters (e.g., the length of a (CH 2 ) n group) to experimental data (e.g., current densities at a fixed applied voltage) has been the simplified Simmons equation (eq 1). 1−11 In this approximation, the tunneling barrier is approximated as rectangular, with width d, and a height related to the attenuation factor β. 12,13 J(V) is current density (A/cm 2 ) at an applied bias V, and J 0 is loosely interpretable as the injection current for a hypothetical junction with d = 0. Changes in the topography of the barrier, the energies of the frontier orbitals, molecular dipoles, and polarizabilities of the insulating molecules in the junctions, are ignored or considered as part of J 0 . 14−18We have studied this type of system using self-assembled monolayer (SAM)-based junctions of the structure Au TS or Ag TS /A−R 1 −M−R 2 −T//Ga 2 O 3 /EGaIn; previous papers describe these studies. 11,19−25 We have used a variety of polar, aromatic, and aliphatic groups for the "anchoring" (A), "middle" (M), and "terminal" (T) groups. One of the unexpected implications of these studies has been that increasing the strength of the interaction across the T//Ga 2 O 3 interface does not seem to increase the tunneling current density of n-alkyl SAMs; 20 decreasing this strength does, however, seem to decrease the tunneling current. The topography of these tunneling barriers seems to be dominated by the electronic structure of the insulating alkyl chains. A theoretical study by Nij...

(Invited) Interface Engineering for Nanoelectronics

“…The largest contact resistance was obtained from the FH 19 SH molecular layer. This could be related to the terminal fluorine group and is consistent with previous reports showing that the fluorine-EGaIn interface is more resistive (33). Further work is underway to understand the full details of these molecular layers and the impact of the dipoles, fluorine, and interfacial electronic properties (29).…”

EGaIn Microelectrode for Electrical Characterization of ITO-Based van der Waals Interface and Airborne Molecular Contamination of ITO Surface

“…On the other hand, it has been well-documented that EGaIn microelectrode is nontoxic and easy to prepare, and EGaIn-based junctions work with high yields (80-100%) of working junctions for various substrates. [8][9][21][22]31,33,[36][37]44 These high yields of working junctions for the EGaIn-based junction allow statistical analyses when examining the structure-property relationships of the junctions. These advantages make the EGaInbased junction distinguishable from the mercury drop-based junction, and particularly attractive for investigating the electrical characteristics of various types of surfaces.…”

Physical‐Organic Chemistry: A Swiss Army Knife

“…-O 2 C-, or acetylene [49,50] ; the "middle" group M can be any one of a number of well-understood organic groups (e.g., amide, phenyl, ether) [51] , and the "terminal" group T can have even wider latitude [52][53][54][55][56] . Studies of this system, when combined with outstanding work by Cahen [57] , Nijhuis [58] , Frisbie [59] , and a number of others, have been most useful in detailing the connection between the structure of the SAM and the tunneling current.…”