Hector Vazquez

In situ formation of highly conducting covalent Au-C contacts for single-molecule junctions

Charge transport across metal-molecule interfaces has an important role in organic electronics. Typically, chemical link groups such as thiols or amines are used to bind organic molecules to metal electrodes in single-molecule circuits, with these groups controlling both the physical structure and the electronic coupling at the interface. Direct metal-carbon coupling has been shown through C60, benzene and π-stacked benzene, but ideally the carbon backbone of the molecule should be covalently bonded to the electrode without intervening link groups. Here, we demonstrate a method to create junctions with such contacts. Trimethyl tin (SnMe(3))-terminated polymethylene chains are used to form single-molecule junctions with a break-junction technique. Gold atoms at the electrode displace the SnMe(3) linkers, leading to the formation of direct Au-C bonded single-molecule junctions with a conductance that is ∼100 times larger than analogous alkanes with most other terminations. The conductance of these Au-C bonded alkanes decreases exponentially with molecular length, with a decay constant of 0.97 per methylene, consistent with a non-resonant transport mechanism. Control experiments and ab initio calculations show that high conductances are achieved because a covalent Au-C sigma (σ) bond is formed. This offers a new method for making reproducible and highly conducting metal-organic contacts.

Probing the conductance superposition law in single-molecule circuits with parallel paths

According to Kirchhoffs circuit laws, the net conductance of two parallel components in an electronic circuit is the sum of the individual conductances. However, when the circuit dimensions are comparable to the electronic phase coherence length, quantum interference effects play a critical role1, as exemplified by the Aharonov–Bohm effect in metal rings2,3. At the molecular scale, interference effects dramatically reduce the electron transfer rate through a meta-connected benzene ring when compared with a para-connected benzene ring4,5. For longer conjugated and cross-conjugated molecules, destructive interference effects have been observed in the tunnelling conductance through molecular junctions6,7,8,9,10. Here, we investigate the conductance superposition law for parallel components in single-molecule circuits, particularly the role of interference. We synthesize a series of molecular systems that contain either one backbone or two backbones in parallel, bonded together cofacially by a common linker on each end. Single-molecule conductance measurements and transport calculations based on density functional theory show that the conductance of a double-backbone molecular junction can be more than twice that of a single-backbone junction, providing clear evidence for constructive interference. Kirchhoffs conductance superposition law is investigated in single-molecule circuits. A single-molecule junction with two backbones in a parallel configuration can exhibit more than twice the conductance of a single-molecule junction with one backbone, a demonstration of constructive quantum interference.

Highly Conducting π-Conjugated Molecular Junctions Covalently Bonded to Gold Electrodes

We measure electronic conductance through single conjugated molecules bonded to Au metal electrodes with direct Au-C covalent bonds using the scanning tunneling microscope based break-junction technique. We start with molecules terminated with trimethyltin end groups that cleave off in situ, resulting in formation of a direct covalent σ bond between the carbon backbone and the gold metal electrodes. The molecular carbon backbone used in this study consist of a conjugated π system that has one terminal methylene group on each end, which bonds to the electrodes, achieving large electronic coupling of the electrodes to the π system. The junctions formed with the prototypical example of 1,4-dimethylenebenzene show a conductance approaching one conductance quantum (G(0) = 2e(2)/h). Junctions formed with methylene-terminated oligophenyls with two to four phenyl units show a 100-fold increase in conductance compared with junctions formed with amine-linked oligophenyls. The conduction mechanism for these longer oligophenyls is tunneling, as they exhibit an exponential dependence of conductance on oligomer length. In addition, density functional theory based calculations for the Au-xylylene-Au junction show near-resonant transmission, with a crossover to tunneling for the longer oligomers.

Length-Dependent Thermopower of Highly Conducting Au–C Bonded Single Molecule Junctions

We report the simultaneous measurement of conductance and thermopower of highly conducting single-molecule junctions using a scanning tunneling microscope-based break-junction setup. We start with molecular backbones (alkanes and oligophenyls) terminated with trimethyltin end groups that cleave off in situ to create junctions where terminal carbons are covalently bonded to the Au electrodes. We apply a thermal gradient across these junctions and measure their conductance and thermopower. Because of the electronic properties of the highly conducting Au-C links, the thermoelectric properties and power factor are very high. Our results show that the molecular thermopower increases nonlinearly with the molecular length while conductance decreases exponentially with increasing molecular length. Density functional theory calculations show that a gateway state representing the Au-C covalent bond plays a key role in the conductance. With this as input, we analyze a series of simplified models and show that a tight-binding model that explicitly includes the gateway states and the molecular backbone states accurately captures the experimentally measured conductance and thermopower trends.

Conductance of Molecular Junctions Formed with Silver Electrodes

We compare the conductance of a series of amine-terminated oligophenyl and alkane molecular junctions formed with Ag and Au electrodes using the scanning tunneling microscope based break-junction technique. For these molecules that conduct through the highest occupied molecular orbital, junctions formed with Au electrodes are more conductive than those formed with Ag electrodes, consistent with the lower work function for Ag. The measured conductance decays exponentially with molecular backbone length with a decay constant that is essentially the same for Ag and Au electrodes. However, the formation and evolution of molecular junctions upon elongation are very different for these two metals. Specifically, junctions formed with Ag electrodes sustain significantly longer elongation when compared with Au due to a difference in the initial gap opened up when the metal point-contact is broken. Using this observation and density functional theory calculations of junction structure and conductance we explain the trends observed in the single molecule junction conductance. Our work thus opens a new path to the conductance measurements of a single molecule junction in Ag electrodes.

Quantifying through-space charge transfer dynamics in π-coupled molecular systems

Understanding the role of intermolecular interaction on through-space charge transfer characteristics in π-stacked molecular systems is central to the rational design of electronic materials. However, a quantitative study of charge transfer in such systems is often difficult because of poor control over molecular morphology. Here we use the core-hole clock implementation of resonant photoemission spectroscopy to study the femtosecond charge-transfer dynamics in cyclophanes, which consist of two precisely stacked π-systems held together by aliphatic chains. We study two systems, [2,2]paracyclophane (22PCP) and [4,4]paracyclophane (44PCP), with inter-ring separations of 3.0 and 4.0 Å, respectively. We find that charge transfer across the π-coupled system of 44PCP is 20 times slower than in 22PCP. We attribute this difference to the decreased inter-ring electronic coupling in 44PCP. These measurements illustrate the use of core-hole clock spectroscopy as a general tool for quantifying through-space coupling in π-stacked systems.

The electrical properties of biphenylenes.

The effect of the partial antiaromaticity of biphenylene on its substitution chemistry, its oxidation potential, and its single-molecule conductance is explored. Biphenylene and fluorene molecules with linkers of two amino groups or two cyclic thioether groups were synthesized and their conduction properties were investigated using scanning tunneling microscopy (STM) break-junction techniques and DFT calculations. Despite the partial antiaromaticity of biphenylene, which causes the biphenylenes to be much more easily oxidizable, no significant increase in molecular conductance was found.

Highly-conducting molecular circuits based on antiaromaticity

Aromaticity is a fundamental concept in chemistry. It is described by Hückel’s rule that states that a cyclic planar π-system is aromatic when it shares 4n+2 π-electrons and antiaromatic when it possesses 4n π-electrons. Antiaromatic compounds are predicted to exhibit remarkable charge transport properties and high redox activities. However, it has so far only been possible to measure compounds with reduced aromaticity but not antiaromatic species due to their energetic instability. Here, we address these issues by investigating the single-molecule charge transport properties of a genuinely antiaromatic compound, showing that antiaromaticity results in an order of magnitude increase in conductance compared with the aromatic counterpart. Single-molecule current–voltage measurements and ab initio transport calculations reveal that this results from a reduced energy gap and a frontier molecular resonance closer to the Fermi level in the antiaromatic species. The conductance of the antiaromatic complex is further modulated electrochemically, demonstrating its potential as a high-conductance transistor.

A pediatrics-based instrument for assessing resident education in evidence-based practice.

OBJECTIVEnThe principles of evidence-based practice (EBP) are a mandated component of the pediatric residency curriculum; however, a pediatrics-based assessment tool validated with pediatric residents does not exist.nnnMETHODSnWe designed an assessment instrument composed of items in 4 categories: 1) demographics; 2) comfort level; 3) self-reported practice of EBP; and 4) EBP knowledge. This last section required participants to identify best evidence and most appropriate study design by using pediatric-based scenarios, develop searchable questions, and use existing published research to address diagnostic and treatment issues. Four groups completed the instrument: preclinical medical students (MS-2), incoming pediatric interns (PGY-1), incoming second- and third-year pediatric residents (PGY2-3), and expert tutors (expert). We determined internal consistency, interrater reliability, content validity, item difficulty, and construct validity.nnnRESULTSnFifty-six subjects completed tests (MS-2, n = 13; PGY-1, n = 13; PGY2-3, n = 22; expert, n = 8). Internal reliability was good, with Cronbachs alpha = .80. Interrater reliability was high (kappa = 0.94). Items were free of floor or ceiling effects. Comfort level and self-reported practice of EBP increased with expertise level and prior EBP experience (P < .01). Scores on the knowledge section (out of 50 +/- SD) rose with training level (MS-2: 14.8 +/- 5.7; PGY-1: 22.2 +/- 3.4; PGY2-3: 31.7 +/- 6.1; experts: 43 +/- 4.0; P < .01). Scores also correlated with prior EBP education.nnnCONCLUSIONSnWe have developed a reliable and valid instrument to assess knowledge and skill in EBP taught to pediatric residents. This instrument can aid pediatric educators in monitoring the impact of the EBP curriculum.