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Title:Single molecule studies of molecular electronics and biohybrid materials
Author(s):Li, Songsong
Director of Research:Schroeder, Charles M
Doctoral Committee Chair(s):Schroeder, Charles M
Doctoral Committee Member(s):Shim, Moonsub; Evans, Christopher M; Zhang, Yingjie
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Degree:Ph.D.
Genre:Dissertation
Subject(s):molecular electronics
single molecule conductance
biohybrid materials
scanning tunneling microscope break-junction (STM-BJ)
polymers
Abstract:A grand challenge in materials chemistry and physics lies in understanding charge transport, chemical reactions, and polymer chain dynamics at the molecular level. In this thesis, I directly address these challenges using single molecule techniques. Recent advances in single molecule techniques have ushered in a fundamentally new understanding of charge transport at the molecular level, which is a crucial step for designing new materials for molecular electronics and energy storage applications. In Chapter 2, I describe a new anchor-electrode pair for robust and high-conducting covalent contacts for molecular junctions using a scanning tunneling microscope-break junction technique (STM-BJ). Here, metal-carbon covalent bonds are formed to silver electrodes from unprotected terminal acetylene anchors. Single molecule charge transport experiments and molecular simulations were performed on a series of arylacetylenes using gold and silver electrodes. Our results show that molecular junctions on silver electrodes spontaneously form silver-carbynyl carbon (Ag-C) contacts, resulting in a nearly 10-fold increase in conductance compared to the same molecules on gold electrodes. Overall, this work presents a simple, new electrode-anchor pair that reliably forms molecular junctions with stable and robust contacts useful for molecular electronics applications. In Chapter 3, the role of quantum interference (QI) on molecular charge transport is investigated. Here, we show that oxazole serves as an efficient anchor group to form stable gold-molecule-gold junctions. Molecular charge transport is studied using conjugated oligomers with oxazole anchors, focusing on the role of the heteroatom substitution position in terminal oxazole groups. These results reveal QI effects in oxazole-terminated phenylene molecular junctions, including destructive QI in meta-substituted phenyl rings and constructive QI in para-substituted phenyl rings containing terminal oxazole groups with the same chemical constitution on both termini (i.e. O5O5 (5-oxazolyl) or O4O4 (4-oxazolyl) linkages on both termini). Surprisingly, meta-substituted phenyl rings with non-equivalent constitutions (i.e. O4O5 oxazole terminal linkages) show unexpectedly higher conductance compared to para-substituted analogs. These results suggest that charge transport in oxazole-terminated molecules is determined by the heteroatom substitution position of the oxazole anchor in addition to the aryl substitution pattern of the pi-conjugated core. Unexpectedly, these results further show that conjugated molecules with homogeneous oxazole linkages obey a quantum circuit rule. In Chapter 4, the role of electric fields as ‘smart catalysts’ is studied to understand chemical reactions at the molecular level. In particular, we study the charge transport of amine- (NH2) and methylthio (SMe)-terminated oligophenyls in nonpolar solvents at different bias voltages. At a low bias voltage, the contact resistance of SMe is lower than NH2. However, at a high bias voltage, we observe a reverse in the trends of contact resistance. Surprisingly, large conductance jumps are observed in molecular junctions based on terphenyl-NH2 and terphenyl -SMe. We posit that this phenomenon arises due to in situ formation of Au-N and Au-S covalent bonds at one interface induced by electric fields. Density functional theory simulations are used to compare molecular conductance in different solvents using common covalent and dative anchors. In Chapter 5, the role of monomer sequence on charge transport in conjugated oligomers is studied. Our results show that charge transport in molecular junctions based on conjugated oligomers critically depends on the primary sequence of monomers. Importantly, oligomers with specific monomer sequences exhibit unexpected and distinct charge transport pathways that enhance molecular conductance more than 10-fold. A systematic analysis using monomer substitution patterns revealed that sequence-defined pentamers containing imidazole or pyrrole groups in specific locations provide molecular attachment points on the backbone to the gold electrodes, thereby giving rise to multiple conductance pathways. These findings highlight the subtle but important role of molecular structure - including steric hinderance and directionality of heterocycles - in determining charge transport in these molecular junctions. In Chapter 6, charge transport in redox-active pyridinium-based molecular junctions is studied, focused on the role of intermolecular interactions. Our results shows that intramolecular conductance occurs over displacements consistent with the molecular contour length. However, pyridinium-based junctions exhibit charge transport over reduced molecular displacements upon increasing the solution concentration of the charged pyridinium complex, which is attributed to intermolecular electrostatic effects. Interestingly, formation of host-guest complexes via addition of a crown ether resulted in recovery of charge transport over molecular displacements corresponding to single pyridinium junctions at low concentrations, thereby suggesting that host-guest complexes efficiently screen electrostatic repulsions between cationic molecules. In Chapter 7, we study the conformational dynamics of single copolymer molecules using fluorescence microscopy. Aside from studying charge transport, single molecule techniques allow for the direct observation of long chain macromolecules in solution. Our work focused on single molecule studies of chemically heterogeneous copolymers as stimuli-response hybrid materials. Here, I synthesized and directly observed the dynamics of thermo-responsive DNA copolymers using single molecule techniques. Single molecule fluorescence microscopy is used to observe the non-equilibrium stretching and relaxation dynamics of DNA copolymers both below and above the lower critical solution temperature (LCST) of side chains. These results reveal an underlying molecular heterogeneity associated with polymer stretching and relaxation behavior, which arises due to heterogeneous chemical identity on DNA copolymer dynamics.
Issue Date:2020-07-16
Type:Thesis
URI:http://hdl.handle.net/2142/108471
Rights Information:Copyright 2020 Songsong Li
Date Available in IDEALS:2020-10-07
Date Deposited:2020-08


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