Mechanically and Electrically Robust Self-Assembled Monolayers for Large-Area Tunneling Junctions

Abstract

This paper examines the relationship between mechanical deformation and the electronic properties of self-assembled monolayers (SAMs) of the oligothiophene 4-([2,2′:5′,2″:5″,2‴-quaterthiophen]-5-yl)butane-1-thiol (T4C4) in tunneling junctions using conductive probe atomic force microscopy (CP-AFM) and eutectic Ga–In (EGaIn). We compared shifts in conductivity, transition voltages of T4C4 with increasing AFM tip loading force to alkanethiolates. While these shifts result from an increasing tilt angle from penetration of the SAM by the AFM tip for the latter, we ascribe them to distortions of the π system present in T4C4, which is more mechanically robust than alkanethiolates of comparable length; SAMs comprising T4C4 shows about five times higher Young’s modulus than alkanethiolates. Density functional theory calculations confirm that mechanical deformations shift the barrier height due to changes in the frontier orbitals caused by small rearrangements to the conformation of the quaterthiophene moiety. The mechanical robustness of T4C4 manifests as an increased tolerance to high bias in large-area EGaIn junctions suggesting that electrostatic pressure plays a significant role in the shorting of molecular junctions at high bias.

Controlling destructive quantum interference in tunneling junctions comprising self-assembled monolayers via bond topology and functional groups

“…S17 and S18 † ), which provide information about the energy offset between E F and the dominant frontier orbital. 53 , 54 Fig. 5a shows the levels for the BDT- n series calculated by DFT with respect to E F (–4.3 eV), clearly predicting LUPS-mediated tunneling for BDT-2 and AQ .…”

Element-Selective Molecular Charge Transport Characteristics of Binuclear Copper(II)-Lanthanide(III) Complexes

“…Such electrical measurements can be performed for common molecular tunnel junctions 7,8 (e.g., Au bottom -SAM-Au top ), spin-polarized junctions 9 (e.g., Au bottom -SAMferromagnetic Ni top ) or hybrid systems involving a conformal electrode (e.g., Au bottom -SAM-EGaIn top (Eutectic Gallium-Indium). 10,11 The latter approach offers particularly interesting perspectives for studying large-area tunneling charge transport 12 across magnetic SAMs, comparing their mechanical 13,14 and electrical properties to those of e.g. widely investigated alkanethiol SAMs.…”

Nanofabrication Techniques in Large-Area Molecular Electronic Devices

“…Moreover, the reader can find elsewhere excellent and comprehensive reviews that describe in detail the methods used to characterize, in the laboratory, molecular electronic properties through the formation of temporal metal-molecule contacts, but that are unsuitable for large-scale integration such as: In situ break junction (STM-BJ), mechanically controlled break junction (MCBJ), I(s) (I = current, s = distance) and I(t) (I = current and t = time) methods based on an STM, electromigration breakdown junction, liquid metal droplets, etc. [9,84,169,341]. In the MCBJ, firstly, a fine metal bridge is formed, and subsequently, this is cleaved upon bending the whole assembly allowing forming the molecular bridge.…”

“…In the self-assembly methodology, the main limitation comes from the number of functional groups showing the specific affinity and robust interactions with the electrode. As a consequence, a vast majority of the SAMs for molecular electronics incorporate molecules having a thiolate derivative as the head group interacting with the metal substrate [159][160][161][162][163][164][165][166][167][168][169][170][171][172][173][174][175]. In fact, most of the pioneering work in the study of electrical properties of molecules done in the late 1990s and early years of the 21st century was based on thiol on gold contacts [176][177][178][179].…”