Publications by authors named "Brian Capozzi"

10 Publications

  • Page 1 of 1

Tuning the polarity of charge carriers using electron deficient thiophenes.

Chem Sci 2017 Apr 28;8(4):3254-3259. Epub 2017 Feb 28.

Department of Chemistry , Columbia University , 3000 Broadway, MC3124 , New York , NY 10027 , USA . Email:

Thiophene-1,1-dioxide (TDO) oligomers have fascinating electronic properties. We previously used thermopower measurements to show that a change in charge carrier from hole to electron occurs with increasing length of TDO oligomers when single-molecule junctions are formed between gold electrodes. In this article, we show for the first time that the dominant conducting orbitals for thiophene/TDO oligomers of fixed length can be tuned by altering the strength of the electron acceptors incorporated into the backbone. We use the scanning tunneling microscope break-junction (STM-BJ) technique and apply a recently developed method to determine the dominant transport channel in single-molecule junctions formed with these systems. Through these measurements, we find that increasing the electron affinity of thiophene derivatives, within a family of pentamers, changes the polarity of the charge carriers systematically from holes to electrons, with some systems even showing mid-gap transport characteristics.
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http://dx.doi.org/10.1039/c6sc05283eDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5465950PMC
April 2017

Mapping the Transmission Functions of Single-Molecule Junctions.

Nano Lett 2016 06 19;16(6):3949-54. Epub 2016 May 19.

Molecular Foundry, Lawrence Berkeley National Laboratory , Berkeley, California 94720, United States.

Charge transport phenomena in single-molecule junctions are often dominated by tunneling, with a transmission function dictating the probability that electrons or holes tunnel through the junction. Here, we present a new and simple technique for measuring the transmission functions of molecular junctions in the coherent tunneling limit, over an energy range of 1.5 eV around the Fermi energy. We create molecular junctions in an ionic environment with electrodes having different exposed areas, which results in the formation of electric double layers of dissimilar density on the two electrodes. This allows us to electrostatically shift the molecular resonance relative to the junction Fermi levels in a manner that depends on the sign of the applied bias, enabling us to map out the junction's transmission function and determine the dominant orbital for charge transport in the molecular junction. We demonstrate this technique using two groups of molecules: one group having molecular resonance energies relatively far from EF and one group having molecular resonance energies within the accessible bias window. Our results compare well with previous electrochemical gating data and with transmission functions computed from first principles. Furthermore, with the second group of molecules, we are able to examine the behavior of a molecular junction as a resonance shifts into the bias window. This work provides a new, experimentally simple route for exploring the fundamentals of charge transport at the nanoscale.
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http://dx.doi.org/10.1021/acs.nanolett.6b01592DOI Listing
June 2016

Solvent-dependent conductance decay constants in single cluster junctions.

Chem Sci 2016 Apr 11;7(4):2701-2705. Epub 2016 Jan 11.

Department of Chemistry , Columbia University , New York , New York 10027 , USA . Email:

Single-molecule conductance measurements have focused primarily on organic molecular systems. Here, we carry out scanning tunneling microscope-based break-junction measurements on a series of metal chalcogenide CoSe clusters capped with conducting ligands of varying lengths. We compare these measurements with those of individual free ligands and find that the conductance of these clusters and the free ligands have different decay constants with increasing ligand length. We also show, through measurements in two different solvents, 1-bromonaphthalene and 1,2,4-trichlorobenzene, that the conductance decay of the clusters depends on the solvent environment. We discuss several mechanisms to explain our observations.
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http://dx.doi.org/10.1039/c5sc02595hDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5477014PMC
April 2016

Single-molecule diodes with high rectification ratios through environmental control.

Nat Nanotechnol 2015 Jun 25;10(6):522-7. Epub 2015 May 25.

1] Department of Applied Physics and Applied Mathematics, New York, New York 10027, USA [2] Department of Chemistry, Columbia University, New York, New York 10027, USA.

Molecular electronics aims to miniaturize electronic devices by using subnanometre-scale active components. A single-molecule diode, a circuit element that directs current flow, was first proposed more than 40 years ago and consisted of an asymmetric molecule comprising a donor-bridge-acceptor architecture to mimic a semiconductor p-n junction. Several single-molecule diodes have since been realized in junctions featuring asymmetric molecular backbones, molecule-electrode linkers or electrode materials. Despite these advances, molecular diodes have had limited potential for applications due to their low conductance, low rectification ratios, extreme sensitivity to the junction structure and high operating voltages. Here, we demonstrate a powerful approach to induce current rectification in symmetric single-molecule junctions using two electrodes of the same metal, but breaking symmetry by exposing considerably different electrode areas to an ionic solution. This allows us to control the junction's electrostatic environment in an asymmetric fashion by simply changing the bias polarity. With this method, we reliably and reproducibly achieve rectification ratios in excess of 200 at voltages as low as 370 mV using a symmetric oligomer of thiophene-1,1-dioxide. By taking advantage of the changes in the junction environment induced by the presence of an ionic solution, this method provides a general route for tuning nonlinear nanoscale device phenomena, which could potentially be applied in systems beyond single-molecule junctions.
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http://dx.doi.org/10.1038/nnano.2015.97DOI Listing
June 2015

Molecular length dictates the nature of charge carriers in single-molecule junctions of oxidized oligothiophenes.

Nat Chem 2015 Mar 2;7(3):209-14. Epub 2015 Feb 2.

Department of Chemistry, Columbia University, New York 10027, USA.

To develop advanced materials for electronic devices, it is of utmost importance to design organic building blocks with tunable functionality and to study their properties at the molecular level. For organic electronic and photovoltaic applications, the ability to vary the nature of charge carriers and so create either electron donors or acceptors is critical. Here we demonstrate that charge carriers in single-molecule junctions can be tuned within a family of molecules that contain electron-deficient thiophene-1,1-dioxide (TDO) building blocks. Oligomers of TDO were designed to increase electron affinity and maintain delocalized frontier orbitals while significantly decreasing the transport gap. Through thermopower measurements we show that the dominant charge carriers change from holes to electrons as the number of TDO units is increased. This results in a unique system in which the charge carrier depends on the backbone length, and provides a new means to tune p- and n-type transport in organic materials.
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http://dx.doi.org/10.1038/nchem.2160DOI Listing
March 2015

Length-dependent conductance of oligothiophenes.

J Am Chem Soc 2014 Jul 15;136(29):10486-92. Epub 2014 Jul 15.

Department of Applied Physics and Mathematics, Columbia University , New York, New York 10027, United States.

We have measured the single-molecule conductance of a family of oligothiophenes comprising 1-6 thiophene moieties terminated with methyl-sulfide linkers using the scanning tunneling microscope-based break-junction technique. We find an anomalous behavior: the peak of the conductance histogram distribution does not follow a clear exponential decay with increasing number of thiophene units in the chain. The electronic properties of the materials were characterized by optical spectroscopy and electrochemistry to gain an understanding of the factors affecting the conductance of these molecules. We postulate that different conformers in the junction are a contributing factor to the anomalous trend in the observed conductance as a function of molecule length.
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http://dx.doi.org/10.1021/ja505277zDOI Listing
July 2014

Breakdown of interference rules in azulene, a nonalternant hydrocarbon.

Nano Lett 2014 May 22;14(5):2941-5. Epub 2014 Apr 22.

Department of Chemistry and ‡Department of Applied Physics and Applied Mathematics, Columbia University , New York, New York 10027, United States.

We have designed and synthesized five azulene derivatives containing gold-binding groups at different points of connectivity within the azulene core to probe the effects of quantum interference through single-molecule conductance measurements. We compare conducting paths through the 5-membered ring, 7-membered ring, and across the long axis of azulene. We find that changing the points of connectivity in the azulene impacts the optical properties (as determined from UV-vis absorption spectra) and the conductivity. Importantly, we show here that simple models cannot be used to predict quantum interference characteristics of nonalternant hydrocarbons. As an exemplary case, we show that azulene derivatives that are predicted to exhibit destructive interference based on widely accepted atom-counting models show a significant conductance at low biases. Although simple models to predict the low-bias conductance do not hold with all azulene derivatives, we demonstrate that the measured conductance trend for all molecules studied actually agrees with predictions based on the more complete GW calculations for model systems.
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http://dx.doi.org/10.1021/nl5010702DOI Listing
May 2014

Tunable charge transport in single-molecule junctions via electrolytic gating.

Nano Lett 2014 Mar 5;14(3):1400-4. Epub 2014 Feb 5.

Department of Applied Physics and Mathematics and ‡Department of Chemistry, Columbia University , New York, New York, United States.

We modulate the conductance of electrochemically inactive molecules in single-molecule junctions using an electrolytic gate to controllably tune the energy level alignment of the system. Molecular junctions that conduct through their highest occupied molecular orbital show a decrease in conductance when applying a positive electrochemical potential, and those that conduct though their lowest unoccupied molecular orbital show the opposite trend. We fit the experimentally measured conductance data as a function of gate voltage with a Lorentzian function and find the fitting parameters to be in quantitative agreement with self-energy corrected density functional theory calculations of transmission probability across single-molecule junctions. This work shows that electrochemical gating can directly modulate the alignment of the conducting orbital relative to the metal Fermi energy, thereby changing the junction transport properties.
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http://dx.doi.org/10.1021/nl404459qDOI Listing
March 2014

Impact of molecular symmetry on single-molecule conductance.

J Am Chem Soc 2013 Aug 6;135(32):11724-7. Epub 2013 Aug 6.

Department of Chemistry, Columbia University, New York, New York 10027, USA.

We have measured the single-molecule conductance of a family of bithiophene derivatives terminated with methyl sulfide gold-binding linkers using a scanning tunneling microscope based break-junction technique. We find a broad distribution in the single-molecule conductance of bithiophene compared with that of a methyl sulfide terminated biphenyl. Using a combination of experiments and calculations, we show that this increased breadth in the conductance distribution is explained by the difference in 5-fold symmetry of thiophene rings as compared to the 6-fold symmetry of benzene rings. The reduced symmetry of thiophene rings results in a restriction on the torsion angle space available to these molecules when bound between two metal electrodes in a junction, causing each molecular junction to sample a different set of conformers in the conductance measurements. In contrast, the rotations of biphenyl are essentially unimpeded by junction binding, allowing each molecular junction to sample similar conformers. This work demonstrates that the conductance of bithiophene displays a strong dependence on the conformational fluctuations accessible within a given junction configuration, and that the symmetry of such small molecules can significantly influence their conductance behaviors.
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http://dx.doi.org/10.1021/ja4055367DOI Listing
August 2013

Electronic transport and mechanical stability of carboxyl linked single-molecule junctions.

Phys Chem Chem Phys 2012 Oct 31;14(40):13841-5. Epub 2012 Jul 31.

Department of Chemistry, Columbia University, New York, NY, USA.

We characterize electron transport across Au-molecule-Au junctions of heterogeneous carboxyl and methyl sulfide terminated saturated and conjugated molecules. Low-bias conductance measurements are performed using the scanning tunneling microscopy based break-junction technique in the presence of solvents and at room temperature. For a series of alkanes with 1-4 carbon atoms in the hydrocarbon chain, our results show an exponential decrease in conductance with increasing molecule length characterized by a decay constant of 0.9 ± 0.1 per methylene group. Control measurements in pH 11 solutions and with COOMe terminations suggest that the carboxylic acid group binds through the formation of a COO(-)-Au bond. Simultaneous measurements of conductance and force across these junctions yield a rupture force of 0.6 ± 0.1 nN, comparable to that required to rupture a Au-SMe bond. By establishing reliable, in situ junction formation, these experiments provide a new approach to probe electronic properties of carboxyl groups at the single molecule level.
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http://dx.doi.org/10.1039/c2cp41578jDOI Listing
October 2012
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