Tunneling across SAMs Containing Oligophenyl Groups

Abstract

This paper describes rates of charge tunneling across self-assembled monolayers (SAMs) of compounds containing oligophenyl groups, supported on gold and silver, using Ga 2 O 3 /EGaIn as the top electrode. It compares the injection current, J 0 , and the attenuation constant, β, of the simplified Simmons equation, across oligophenyl groups (R = Ph n ; n = 1, 2, 3), with three different anchoring groups (thiol, HSR; methanethiol, HSCH 2 R; and acetylene, HC≡CR) that attach R to the template-stripped gold and silver substrates. The results demonstrate that the structure of the molecules between the anchoring group (-S-or -C≡C-) and the oligophenyl moiety significantly influences charge transport. SAMs of SPh n , and C≡CPh n on gold show similar values of β and log|J 0 | (β = 0.28 ± 0.03 Å -1 and log|J 0 | = 2.7 ± 0.1 for Au/SPh n ; β = 0.30 ± 0.02 Å -1 and log|J 0 | = 3.0 ± 0.1 for Au/C≡CPh n ). The introduction of a single intervening methylene (CH 2 ) group, between the anchoring sulfur atom and the aromatic units to generate SAMs of SCH 2 Ph n , increases β to ~0.6 Å -1 on both gold and silver substrates. (For nalkanethiolates on gold the corresponding values are β = 0.76 Å -1 and log|J 0 | = 4.2). As a generalization, based on this and other work, it seems that increasing the height of the tunneling barrier in the region of the interfaces increases β, and may decrease J 0 ; by contrast, it appears that lowering the height of the barrier at these interfaces has little influence on β or J 0 .

Power Factor of One Molecule Thick Films and Length Dependence

“…(i) Fabrication procedures of S(Ph) n and SC m SAMs, their on-surface (supra)molecular and electronic structures, and the electrical characteristics have been established. 4,24−27 (ii) The thermopower of S(Ph) n SAMs has been widely examined in separate junction platforms, and thus, they could be used as standard molecules in thermopower measurements. 3,4,8 (iii) The tunneling conductance and Seebeck coefficient of these SAMs can be measured on the identical junction platform, an EGaIn-based large-area junction.…”

Anomalously Rapid Tunneling: Charge Transport across Self-Assembled Monolayers of Oligo(ethylene glycol)

“…50 We also approximated d as the length of the transextended molecule from the anchoring atom to the distal hydrogen atom to calculate β in units of Å -1 ; this analysis yields β (EG)n = 0.24 Å -1 (Figure 2b), which is comparable to that describing tunneling across SAMs of oligophenyls on gold (β (CH2)n = 0.28 Å -1 ). 17 S8). Changing the substrate from gold to silver and the anchoring group from thiol to carboxylate influences the structure of the SAM.…”

“…Studies of a range of SAMs based on n-alkanethiolates with different terminal groups (both aliphatic and aromatic), and having the same overall length, have shown remarkably little variation in tunneling current; [15][16] inclusion of groups capable of extended delocalization do lower the barrier to tunneling. [17][18][19][20] This study focuses on the influence of the energy levels of backbone substituents with highlying occupied orbitals on the height of the tunneling barrier and rates of charge transport. We replaced backbone methylene groups (-CH 2 -) with oxygen atoms, and analyzed the influence of this substitution on the rate of charge transport.…”

Understanding, Optimizing, and Utilizing Nonideal Transistors Based on Organic or Organic Hybrid Semiconductors

“…Both generations of SAMs could tune the work function of Au electrodes over a range of 0.9 eV by switching the orientation of a pyrimidine ring . For SAMs with different backbone structures, the OFET treated with the sceond‐generation molecules achieved higher conductance of the BP0‐based SAMs due to the shorter length of the conjugated backbone and the absence of the methylene linker . For SAMs with different dipole orientations, p‐type and n‐type OFETs modified with up–down dipole orientations achieved higher device performance due to the effective modification of injection barriers…”