Publications by authors named "Latha Venkataraman"

110 Publications

Destructive quantum interference in heterocyclic alkanes: the search for ultra-short molecular insulators.

Chem Sci 2021 Aug 30;12(30):10299-10305. Epub 2021 Jun 30.

Department of Applied Physics and Applied Mathematics, Columbia University, New York New York 10027 USA

Designing highly insulating sub-nanometer molecules is difficult because tunneling conductance increases exponentially with decreasing molecular length. This challenge is further enhanced by the fact that most molecules cannot achieve full conductance suppression with destructive quantum interference. Here, we present results for a series of small saturated heterocyclic alkanes where we show that conductance is suppressed due to destructive interference. Using the STM-BJ technique and density functional theory calculations, we confirm that their single-molecule junction conductance is lower than analogous alkanes of similar length. We rationalize the suppression of conductance in the junctions through analysis of the computed ballistic current density. We find there are highly symmetric ring currents, which reverse direction at the antiresonance in the Landauer transmission near the Fermi energy. This pattern has not been seen in earlier studies of larger bicyclic systems exhibiting interference effects and constitutes clear-cut evidence of destructive σ-interference. The finding of heterocyclic alkanes with destructive quantum interference charts a pathway for chemical design of short molecular insulators using organic molecules.
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http://dx.doi.org/10.1039/d1sc02287cDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8386164PMC
August 2021

Voltage-Induced Single-Molecule Junction Planarization.

Nano Lett 2021 01 18;21(1):673-679. Epub 2020 Dec 18.

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

Probing structural changes of a molecule induced by charge transfer is important for understanding the physicochemical properties of molecules and developing new electronic devices. Here, we interrogate the structural changes of a single diketopyrrolopyrrole (DPP) molecule induced by charge transport at a high bias using scanning tunneling microscope break junction (STM-BJ) techniques. Specifically, we demonstrate that application of a high bias increases the average nonresonant conductance of single Au-DPP-Au junctions. We infer from the increased conductance that resonant charge transport induces planarization of the molecular backbone. We further show that this conformational planarization is assisted by thermally activated junction reorganization. The planarization only occurs under specific electronic conditions, which we rationalize by ab initio calculations. These results emphasize the need for a comprehensive view of single-molecule junctions which includes both the electronic properties and structure of the molecules and the electrodes when designing electrically driven single-molecule motors.
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http://dx.doi.org/10.1021/acs.nanolett.0c04260DOI Listing
January 2021

Single-molecule conductance in a unique cross-conjugated tetra(aminoaryl)ethene.

Chem Commun (Camb) 2021 Jan;57(5):591-594

Department of Physical Chemistry, University of Málaga, Andalucía-Tech, Campus de Teatinos s/n, Málaga 29071, Spain.

A 1,1,2,2-tetrakis(4-aminophenyl)ethene with three paths of π-conjugation, linear-cis, linear-trans and a cross-conjugation, has been prepared. The molecule is able to bind to gold electrodes forming molecular junctions for single-molecule conductance measurements. Only two regimes of conduction are found experimentally. The modelling of the conductance allows to assign them to through-bond transmission in the linear case, while the cross-conjugated channel is further assisted by through-space transmission, partially alleviating the destructive quantum interference.
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http://dx.doi.org/10.1039/d0cc07124bDOI Listing
January 2021

Highly nonlinear transport across single-molecule junctions via destructive quantum interference.

Nat Nanotechnol 2021 Mar 7;16(3):313-317. Epub 2020 Dec 7.

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

To rival the performance of modern integrated circuits, single-molecule devices must be designed to exhibit extremely nonlinear current-voltage (I-V) characteristics. A common approach is to design molecular backbones where destructive quantum interference (QI) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) produces a nonlinear energy-dependent tunnelling probability near the electrode Fermi energy (E). However, tuning such systems is not straightforward, as aligning the frontier orbitals to E is hard to control. Here, we instead create a molecular system where constructive QI between the HOMO and LUMO is suppressed and destructive QI between the HOMO and strongly coupled occupied orbitals of opposite phase is enhanced. We use a series of fluorene oligomers containing a central benzothiadiazole unit to demonstrate that this strategy can be used to create highly nonlinear single-molecule circuits. Notably, we are able to reproducibly modulate the conductance of a 6-nm molecule by a factor of more than 10.
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http://dx.doi.org/10.1038/s41565-020-00807-xDOI Listing
March 2021

Too Cool for Blackbody Radiation: Overbias Photon Emission in Ambient STM Due to Multielectron Processes.

Nano Lett 2020 12 18;20(12):8912-8918. Epub 2020 Nov 18.

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

Light emission from tunnel junctions are a potential photon source for nanophotonic applications. Surprisingly, the photons emitted can have energies exceeding the energy supplied to the electrons by the bias. Three mechanisms for generating these so-called overbias photons have been proposed, but the relationship between these mechanisms has not been clarified. In this work, we argue that multielectron processes provide the best framework for understanding overbias light emission in tunnel junctions. Experimentally, we demonstrate for the first time that the superlinear dependence of emission on conductance predicted by this theory is robust to the temperature of the tunnel junction, indicating that tunnel junctions are a promising candidate for electrically driven broadband photon sources.
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http://dx.doi.org/10.1021/acs.nanolett.0c03994DOI Listing
December 2020

Cyclopropenylidenes as Strong Carbene Anchoring Groups on Au Surfaces.

J Am Chem Soc 2020 Nov 11;142(47):19902-19906. Epub 2020 Nov 11.

CNR-IOM Laboratorio Nazionale TASC, Basovizza SS-14, km 163.5, 34149 Trieste, Italy.

The creation of stable molecular monolayers on metallic surfaces is a fundamental challenge of surface chemistry. N-Heterocyclic carbenes (NHCs) were recently shown to form self-assembled monolayers that are significantly more stable than the traditional thiols on Au system. Here we theoretically and experimentally demonstrate that the smallest cyclic carbene, cyclopropenylidene, binds even more strongly than NHCs to Au surfaces without altering the surface structure. We deposit bis(diisopropylamino)cyclopropenylidene (BAC) on Au(111) using the molecular adduct BAC-CO as a precursor and determine the structure, geometry, and behavior of the surface-bound molecules through high-resolution X-ray photoelectron spectroscopy, atomic force microscopy, and scanning tunneling microscopy. Our experiments are supported by density functional theory calculations of the molecular binding energy of BAC on Au(111) and its electronic structure. Our work is the first demonstration of surface modification with a stable carbene other than NHC; more broadly, it drives further exploration of various carbenes on metal surfaces.
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http://dx.doi.org/10.1021/jacs.0c10743DOI Listing
November 2020

Cumulene Wires Display Increasing Conductance with Increasing Length.

Nano Lett 2020 11 23;20(11):8415-8419. Epub 2020 Oct 23.

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

One-dimensional sp-hybridized carbon wires, including cumulenes and polyynes, can be regarded as finite versions of carbynes. They are likely to be good candidates for molecular-scale conducting wires as they are predicted to have a high-conductance. In this study, we first characterize the single-molecule conductance of a series of cumulenes and polyynes with a backbone ranging in length from 4 to 8 carbon atoms, including [7]cumulene, the longest cumulenic carbon wire studied to date for molecular electronics. We observe different length dependence of conductance when comparing these two forms of carbon wires. Polyynes exhibit conductance decays with increasing molecular length, while cumulenes show a conductance increase with increasing molecular length. Their distinct conducting behaviors are attributed to their different bond length alternation, which is supported by theoretical calculations. This study confirms the long-standing theoretical predictions on sp-hybridized carbon wires and demonstrates that cumulenes can form highly conducting molecular wires.
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http://dx.doi.org/10.1021/acs.nanolett.0c03794DOI Listing
November 2020

Tight-binding analysis of helical states in carbyne.

J Chem Phys 2020 Sep;153(12):124304

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

Carbyne is a linear allotrope of carbon that is composed of a chain of sp-hybridized carbon atoms. Through appropriate engineering of the chain termination, carbyne can harbor helical states where the π-electron delocalization twists along the axis of the chain. Herein, we present a comprehensive analysis of these helical states at the tight-binding level. We demonstrate that, in general, the molecular orbital coefficients of the helical states trace out an ellipse, in analogy to elliptically polarized light. Helical states can be realized in a model, inspired by the structure of cumulene, which considers a chain terminated by sp-hybridized atoms oriented at a nontrivial dihedral angle. We provide a complete analytic solution for this model. Additionally, we present a variation of the model that yields perfect helical states that trace out a circle as opposed to an ellipse. Our results provide a deeper understanding of helical states and lay a foundation for more advanced levels of theory.
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http://dx.doi.org/10.1063/5.0021146DOI Listing
September 2020

Single-Electron Currents in Designer Single-Cluster Devices.

J Am Chem Soc 2020 Sep 18;142(35):14924-14932. Epub 2020 Aug 18.

Department of Chemistry, Columbia University, New York, New York 10027, United States.

Atomically precise clusters can be used to create single-electron devices wherein a single redox-active cluster is connected to two macroscopic electrodes via anchoring ligands. Unlike single-electron devices comprising nanocrystals, these cluster-based devices can be fabricated with atomic precision. This affords an unprecedented level of control over the device properties. Herein, we design a series of cobalt chalcogenide clusters with varying ligand geometries and core nuclearities to control their current-voltage (I-V) characteristics in a scanning tunneling microscope-based break junction (STM-BJ) device. First, the device geometry is modified by precisely positioning junction-anchoring ligands on the surface of the cluster. We show that the I-V characteristics are independent of ligand placement, confirming a sequential, single-electron tunneling mechanism. Next, we chemically fuse two clusters to realize a larger cluster dimer that behaves as a single electronic unit, possessing a smaller reorganization energy and more accessible redox states than the monomeric analogues. As a result, dimer-based devices exhibit significantly higher currents and can even be pushed to current saturation at high bias. Owing to these controllable properties, single-cluster junctions serve as an excellent platform for exploring incoherent charge transport processes at the nanoscale. With this understanding, as well as properties such as nonlinear I-V characteristics and rectification, these molecular clusters may function as conductive inorganic nodes in new devices and materials.
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http://dx.doi.org/10.1021/jacs.0c04970DOI Listing
September 2020

Mechanically Tunable Quantum Interference in Ferrocene-Based Single-Molecule Junctions.

Nano Lett 2020 Sep 6;20(9):6381-6386. Epub 2020 Aug 6.

Institute of Theoretical Physics, University of Regensburg, 93040 Regensburg, Germany.

Ferrocenes are ubiquitous organometallic building blocks that comprise a Fe atom sandwiched between two cyclopentadienyl (Cp) rings that rotate freely at room temperature. Of widespread interest in fundamental studies and real-world applications, they have also attracted some interest as functional elements of molecular-scale devices. Here we investigate the impact of the configurational degrees of freedom of a ferrocene derivative on its single-molecule junction conductance. Measurements indicate that the conductance of the ferrocene derivative, which is suppressed by 2 orders of magnitude as compared to a fully conjugated analogue, can be modulated by altering the junction configuration. transport calculations show that the low conductance is a consequence of destructive quantum interference effects of the Fano type that arise from the hybridization of localized metal-based d-orbitals and the delocalized ligand-based π-system. By rotation of the Cp rings, the hybridization, and thus the quantum interference, can be mechanically controlled, resulting in a conductance modulation that is seen experimentally.
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http://dx.doi.org/10.1021/acs.nanolett.0c01956DOI Listing
September 2020

Using Deep Learning to Identify Molecular Junction Characteristics.

Nano Lett 2020 05 9;20(5):3320-3325. Epub 2020 Apr 9.

Department of Chemistry, Columbia University, New York, New York 10027, United States.

The scanning tunneling microscope-based break junction (STM-BJ) is used widely to create and characterize single metal-molecule-metal junctions. In this technique, conductance is continuously recorded as a metal point contact is broken in a solution of molecules. Conductance plateaus are seen when stable molecular junctions are formed. Typically, thousands of junctions are created and measured, yielding thousands of distinct conductance versus extension traces. However, such traces are rarely analyzed individually to recognize the types of junctions formed. Here, we present a deep learning-based method to identify molecular junctions and show that it performs better than several commonly used and recently reported techniques. We demonstrate molecular junction identification from mixed solution measurements with accuracies as high as 97%. We also apply this model to an electric field-driven isomerization reaction of a [3]cumulene to follow the reaction over time. Furthermore, we demonstrate that our model can remain accurate even when a key parameter, the average junction conductance, is eliminated from the analysis, showing that our model goes beyond conventional analysis in existing methods.
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http://dx.doi.org/10.1021/acs.nanolett.0c00198DOI Listing
May 2020

Unsupervised feature recognition in single-molecule break junction data.

Nanoscale 2020 Apr 2;12(15):8355-8363. Epub 2020 Apr 2.

Department of Physics, Budapest University of Technology and Economics, 1111 Budapest, Budafoki ut 8, Hungary.

Single-molecule break junction measurements deliver a huge number of conductance vs. electrode separation traces. During such measurements, the target molecules may bind to the electrodes in different geometries, and the evolution and rupture of the single-molecule junction may also follow distinct trajectories. The unraveling of the various typical trace classes is a prerequisite to the proper physical interpretation of the data. Here we exploit the efficient feature recognition properties of neural networks to automatically find the relevant trace classes. To eliminate the need for manually labeled training data we apply a combined method, which automatically selects training traces according to the extreme values of principal component projections or some auxiliary measured quantities. Then the network captures the features of these characteristic traces and generalizes its inference to the entire dataset. The use of a simple neural network structure also enables a direct insight into the decision-making mechanism. We demonstrate that this combined machine learning method is efficient in the unsupervised recognition of unobvious, but highly relevant trace classes within low and room temperature gold-4,4' bipyridine-gold single-molecule break junction data.
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http://dx.doi.org/10.1039/d0nr00467gDOI Listing
April 2020

Gold-Carbon Contacts from Oxidative Addition of Aryl Iodides.

J Am Chem Soc 2020 Apr 6;142(15):7128-7133. Epub 2020 Apr 6.

Aryl halides are ubiquitous functional groups in organic chemistry, yet despite their obvious appeal as surface-binding linkers and as precursors for controlled graphene nanoribbon synthesis, they have seldom been used as such in molecular electronics. The confusion regarding the bonding of aryl iodides to Au electrodes is a case in point, with ambiguous reports of both dative Au-I and covalent Au-C contacts. Here we form single-molecule junctions with a series of oligophenylene molecular wires terminated asymmetrically with iodine and thiomethyl to show that the dative Au-I contact has a lower conductance than the covalent Au-C interaction, which we propose occurs via an oxidative addition reaction at the Au surface. Furthermore, we confirm the formation of the Au-C bond by measuring an analogous series of molecules prepared with the complex Au(PPh) in place of the iodide. Density functional theory-based transport calculations support our experimental observations that Au-C linkages have higher conductance than Au-I linkages. Finally, we demonstrate selective promotion of the Au-C bond formation by controlling the bias applied across the junction. In addition to establishing the different binding modes of aryl iodides, our results chart a path to actively controlling oxidative addition on an Au surface using an applied bias.
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http://dx.doi.org/10.1021/jacs.0c01466DOI Listing
April 2020

Visualizing Quantum Interference in Molecular Junctions.

Nano Lett 2020 04 16;20(4):2843-2848. Epub 2020 Mar 16.

Department of Chemistry, Columbia University, New York, New York 10027, United States.

Electron transport across a molecular junction is characterized by an energy-dependent transmission function. The transmission function accounts for electrons tunneling through multiple molecular orbitals (MOs) with different phases, which gives rise to quantum interference (QI) effects. Because the transmission function comprises both interfering and noninterfering effects, individual interferences between MOs cannot be deduced from the transmission function directly. Herein, we demonstrate how the transmission function can be deconstructed into its constituent interfering and noninterfering contributions for any model molecular junction. These contributions are arranged in a matrix and displayed pictorially as a QI map, which allows one to easily identify individual QI effects. Importantly, we show that exponential conductance decay with increasing oligomer length is primarily due to an increase in destructive QI. With an ability to "see" QI effects using the QI map, we find that QI is vital to all molecular-scale electron transport.
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http://dx.doi.org/10.1021/acs.nanolett.0c00605DOI Listing
April 2020

Solitonics with Polyacetylenes.

Nano Lett 2020 04 9;20(4):2615-2619. Epub 2020 Mar 9.

Institute of Theoretical Physics, University of Regensburg, 93040 Regensburg, Germany.

Polyacetylene molecular wires have attracted a long-standing interest for the past 40 years. From a fundamental perspective, there are two main reasons for the interest. First, polyacetylenes are a prime realization of a one-dimensional topological insulator. Second, long molecules support freely propagating topological domain-wall states, so-called "solitons," which provide an early paradigm for spin-charge separation. Because of recent experimental developments, individual polyacetylene chains can now be synthesized on substrates. Motivated by this breakthrough, we here propose a novel way for chemically supported soliton design in these systems. We demonstrate how to control the soliton position and how to read it out via external means. Also, we show how extra soliton-antisoliton pairs arise when applying a moderate static electric field. We thus make a step toward functionality of electronic devices based on soliton manipulation, that is, "solitonics".
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http://dx.doi.org/10.1021/acs.nanolett.0c00136DOI Listing
April 2020

The importance of intramolecular conductivity in three dimensional molecular solids.

Chem Sci 2019 Oct 28;10(40):9339-9344. Epub 2019 Aug 28.

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

Recent years have seen tremendous progress towards understanding the relation between the molecular structure and function of organic field effect transistors. The metrics for organic field effect transistors, which are characterized by mobility and the on/off ratio, are known to be enhanced when the intermolecular interaction is strong and the intramolecular reorganization energy is low. While these requirements are adequate when describing organic field effect transistors with simple and planar aromatic molecular components, they are insufficient for complex building blocks, which have the potential to localize a carrier on the molecule. Here, we show that intramolecular conductivity can play a role in controlling device characteristics of organic field effect transistors made with macrocycle building blocks. We use two isomeric macrocyclic semiconductors that consist of perylene diimides linked with bithiophenes and find that the -linked macrocycle has a higher mobility than the -based device. Through a combination of single molecule junction conductance measurements of the components of the macrocycles, control experiments with acyclic counterparts to the macrocycles, and analyses of each of the materials using spectroscopy, electrochemistry, and density functional theory, we attribute the difference in electron mobility of the OFETs created with the two isomers to the difference in intramolecular conductivity of the two macrocycles.
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http://dx.doi.org/10.1039/c9sc03144hDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7006630PMC
October 2019

Enhanced coupling through π-stacking in imidazole-based molecular junctions.

Chem Sci 2019 Nov 16;10(43):9998-10002. Epub 2019 Sep 16.

Department of Chemistry , Queensborough Community College of the City University of New York , Bayside , New York 11364 , USA . Email:

We demonstrate that imidazole based π-π stacked dimers form strong and efficient conductance pathways in single-molecule junctions using the scanning-tunneling microscope-break junction (STM-BJ) technique and density functional theory-based calculations. We first characterize an imidazole-gold contact by measuring the conductance of imidazolyl-terminated alkanes (, = 3-6). We show that the conductance of these alkanes decays exponentially with increasing length, indicating that the mechanism for electron transport is through tunneling or super-exchange. We also reveal that π-π stacked dimers can be formed between imidazoles and have better coupling than through-bond tunneling. These experimental results are rationalized by calculations of molecular junction transmission using non-equilibrium Green's function formalism. This study verifies the capability of imidazole as a Au-binding ligand to form stable single- and π-stacked molecule junctions at room temperature.
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http://dx.doi.org/10.1039/c9sc03760hDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6979055PMC
November 2019

Directing isomerization reactions of cumulenes with electric fields.

Nat Commun 2019 10 2;10(1):4482. Epub 2019 Oct 2.

Department of Applied Physics, Columbia University, New York, New York, USA.

Electric fields have been proposed as having a distinct ability to catalyze chemical reactions through the stabilization of polar or ionic intermediate transition states. Although field-assisted catalysis is being researched, the ability to catalyze reactions in solution using electric fields remains elusive and the understanding of mechanisms of such catalysis is sparse. Here we show that an electric field can catalyze the cis-to-trans isomerization of [3]cumulene derivatives in solution, in a scanning tunneling microscope. We further show that the external electric field can alter the thermodynamics inhibiting the trans-to-cis reverse reaction, endowing the selectivity toward trans isomer. Using density functional theory-based calculations, we find that the applied electric field promotes a zwitterionic resonance form, which ensures a lower energy transition state for the isomerization reaction. The field also stabilizes the trans form, relative to the cis, dictating the cis/trans thermodynamics, driving the equilibrium product exclusively toward the trans.
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http://dx.doi.org/10.1038/s41467-019-12487-wDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6775130PMC
October 2019

Permethylation Introduces Destructive Quantum Interference in Saturated Silanes.

J Am Chem Soc 2019 10 17;141(39):15471-15476. Epub 2019 Sep 17.

Nano-Science Center and Department of Chemistry , University of Copenhagen , Universitetsparken 5, 2100 Copenhagen Ø , Denmark.

The single-molecule conductance of silanes is suppressed due to destructive quantum interference in conformations with cisoid dihedral angles along the molecular backbone. Yet, despite the structural similarity, σ-interference effects have not been observed in alkanes. Here we report that the methyl substituents used in silanes are a prerequisite for σ-interference in these systems. Through density functional theory calculations, we find that the destructive interference is not evident to the same extent in nonmethylated silanes. We find the same is true in alkanes as the transmission is significantly suppressed in permethylated cyclic and bicyclic alkanes. Using scanning tunneling microscope break-junction method we determine the single-molecule conductance of functionalized cyclohexane and bicyclo[2.2.2]octane that are found to be higher than that of equivalent permethylated silanes. Rather than the difference between carbon and silicon atoms in the molecular backbones, our calculations reveal that it is primarily the difference between hydrogen and methyl substituents that result in the different electron transport properties of nonmethylated alkanes and permethylated silanes. Chemical substituents play an important role in determining the single-molecule conductance of saturated molecules, and this must be considered when we improve and expand the chemical design of insulating organic molecules.
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http://dx.doi.org/10.1021/jacs.9b06965DOI Listing
October 2019

In Situ Coupling of Single Molecules Driven by Gold-Catalyzed Electrooxidation.

Angew Chem Int Ed Engl 2019 Nov 11;58(45):16008-16012. Epub 2019 Jul 11.

Department of Applied Physics and Applied Mathematics, Columbia University, New York, USA.

A single-molecule method has been developed based on the scanning tunneling microscope (STM) to selectively couple a series of aniline derivatives and create azobenzenes. The Au-catalyzed oxidative coupling is driven by the local electrochemical potential at the nanostructured Au STM tip. The products are detected in situ by measuring the conductance and molecular junction elongation and compared with analogous measurements of the expected azobenzene derivatives prepared ex situ. This single-molecule approach is robust, and it can quickly and reproducibly create reactions for a variety of anilines. We further demonstrate the selective synthesis of geometric isomers and the assembly of complex molecular architectures by sequential coupling of complementary anilines, demonstrating unprecedented control over bond formation at the nanoscale.
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http://dx.doi.org/10.1002/anie.201906215DOI Listing
November 2019

Resolving the Unpaired-Electron Orbital Distribution in a Stable Organic Radical by Kondo Resonance Mapping.

Angew Chem Int Ed Engl 2019 Aug 28;58(32):11063-11067. Epub 2019 Jun 28.

Institute of Experimental and Applied Physics, University of Regensburg, 93053, Regensburg, Germany.

The adsorption geometry and the electronic structure of a Blatter radical derivative on a gold surface were investigated by a combination of high-resolution noncontact atomic force microscopy and scanning tunneling microscopy. While the hybridization with the substrate hinders direct access to the molecular states, we show that the unpaired-electron orbital can be probed with Ångström resolution by mapping the spatial distribution of the Kondo resonance. The Blatter derivative features a peculiar delocalization of the unpaired-electron orbital over some but not all moieties of the molecule, such that the Kondo signature can be related to the spatial fingerprint of the orbital. We observe a direct correspondence between these two quantities, including a pronounced nodal plane structure. Finally, we demonstrate that the spatial signature of the Kondo resonance also persists upon noncovalent dimerization of molecules.
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http://dx.doi.org/10.1002/anie.201904851DOI Listing
August 2019

The Environment-Dependent Behavior of the Blatter Radical at the Metal-Molecule Interface.

Nano Lett 2019 04 25;19(4):2543-2548. Epub 2019 Mar 25.

Department of Chemistry , Columbia University , New York , New York 10027 , United States.

Stable organic radicals have potential applications for building organic spintronic devices. To fulfill this potential, the interface between organic radicals and metal electrodes must be well characterized. Here, through a combined effort that includes synthesis, scanning tunneling microscopy, X-ray spectroscopy, and single-molecule conductance measurements, we comprehensively probe the electronic interaction between gold metal electrodes and a benchtop stable radical-the Blatter radical. We find that despite its open-shell character and having a half-filled orbital close to the Fermi level, the radical is stable on a gold substrate under ultrahigh vacuum. We observe a Kondo resonance arising from the radical and spectroscopic signatures of its half-filled orbitals. By contrast, in solution-based single-molecule conductance measurements, the radical character is lost through oxidation with charge transfer occurring from the molecule to metal. Our experiments show that the stability of radical states can be very sensitive to the environment around the molecule.
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http://dx.doi.org/10.1021/acs.nanolett.9b00275DOI Listing
April 2019

Non-chemisorbed gold-sulfur binding prevails in self-assembled monolayers.

Nat Chem 2019 04 4;11(4):351-358. Epub 2019 Mar 4.

Department of Applied Physics, Columbia University, New York, NY, USA.

Gold-thiol contacts are ubiquitous across the physical and biological sciences in connecting organic molecules to surfaces. When thiols bind to gold in self-assembled monolayers (SAMs) the fate of the hydrogen remains a subject of profound debate-with implications for our understanding of their physical properties, spectroscopic features and formation mechanism(s). Exploiting measurements of the transmission through a molecular junction, which is highly sensitive to the nature of the molecule-electrode contact, we demonstrate here that the nature of the gold-sulfur bond in SAMs can be probed via single-molecule conductance measurements. Critically, we find that SAM measurements of dithiol-terminated molecular junctions yield a significantly lower conductance than solution measurements of the same molecule. Through numerous control experiments, conductance noise analysis and transport calculations based on density functional theory, we show that the gold-sulfur bond in SAMs prepared from the solution deposition of dithiols does not have chemisorbed character, which strongly suggests that under these widely used preparation conditions the hydrogen is retained.
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http://dx.doi.org/10.1038/s41557-019-0216-yDOI Listing
April 2019

Breaking Down Resonance: Nonlinear Transport and the Breakdown of Coherent Tunneling Models in Single Molecule Junctions.

Nano Lett 2019 04 5;19(4):2555-2561. Epub 2019 Mar 5.

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

The promise of the field of single-molecule electronics is to reveal a new class of quantum devices that leverages the strong electronic interactions inherent to subnanometer scale systems. Here, we form Au-molecule-Au junctions using a custom scanning tunneling microscope and explore charge transport through current-voltage measurements. We focus on the resonant tunneling regime of two molecules, one that is primarily an electron conductor and one that conducts primarily holes. We find that in the high bias regime, junctions that do not rupture demonstrate reproducible and pronounced negative differential resistance (NDR)-like features followed by hysteresis with peak-to-valley ratios exceeding 100 in some cases. Furthermore, we show that both junction rupture and NDR are induced by charging of the molecular orbital dominating transport and find that the charging is reversible at lower bias and with time with kinetic time scales on the order of hundreds of milliseconds. We argue that these results cannot be explained by existing models of charge transport and likely require theoretical advances describing the transition from coherent to sequential tunneling. Our work also suggests new rules for operating single-molecule devices at high bias to obtain highly nonlinear behavior.
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http://dx.doi.org/10.1021/acs.nanolett.9b00316DOI Listing
April 2019

Determination of the structure and geometry of N-heterocyclic carbenes on Au(111) using high-resolution spectroscopy.

Chem Sci 2019 Jan 5;10(3):930-935. Epub 2018 Nov 5.

Department of Applied Physics and Applied Mathematics , Columbia University , New York , New York 10027 , USA . Email:

N-heterocyclic carbenes (NHCs) bind very strongly to transition metals due to their unique electronic structure featuring a divalent carbon atom with a lone pair in a highly directional sp-hybridized orbital. As such, they can be assembled into monolayers on metal surfaces that have enhanced stability compared to their thiol-based counterparts. The utility of NHCs to form such robust self-assembled monolayers (SAMs) was only recently recognized and many fundamental questions remain. Here we investigate the structure and geometry of a series of NHCs on Au(111) using high-resolution X-ray photoelectron spectroscopy and density functional theory calculations. We find that the N-substituents on the NHC ring strongly affect the molecule-metal interaction and steer the orientation of molecules in the surface layer. In contrast to previous reports, our experimental and theoretical results provide unequivocal evidence that NHCs with N-methyl substituents bind to undercoordinated adatoms to form flat-lying complexes. In these SAMs, the donor-acceptor interaction between the NHC lone pair and the undercoordinated Au adatom is primarily responsible for the strong bonding of the molecules to the surface. NHCs with bulkier N-substituents prevent the formation of such complexes by forcing the molecules into an upright orientation. Our work provides unique insights into the bonding and geometry of NHC monolayers; more generally, it charts a clear path to manipulating the interaction between NHCs and metal surfaces using traditional coordination chemistry synthetic strategies.
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http://dx.doi.org/10.1039/c8sc03502dDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6346291PMC
January 2019

Large Variations in the Single-Molecule Conductance of Cyclic and Bicyclic Silanes.

J Am Chem Soc 2018 11 29;140(44):15080-15088. Epub 2018 Oct 29.

Nano-Science Center and Department of Chemistry , University of Copenhagen , Universitetsparken 5 , 2100 Copenhagen Ø , Denmark.

Linear silanes are efficient molecular wires due to strong σ-conjugation in the transoid conformation; however, the structure-function relationship for the conformational dependence of the single-molecule conductance of silanes remains untested. Here we report the syntheses, electrical measurements, and theoretical characterization of four series of functionalized cyclic and bicyclic silanes including a cyclotetrasilane, a cyclopentasilane, a bicyclo[2.2.1]heptasilane, and a bicyclo[2.2.2]octasilane, which are all extended by linear silicon linkers of varying length. We find an unusual variation of the single-molecule conductance among the four series at each linker length. We determine the relative conductance of the (bi)cyclic silicon structures by using the common length dependence of the four series rather than comparing the conductance at a single length. In contrast with the cyclic π-conjugated molecules, the conductance of σ-conjugated (bi)cyclic silanes is dominated by a single path through the molecule and is controlled by the dihedral angles along this path. This strong sensitivity to molecular conformation dictates the single-molecule conductance of σ-conjugated silanes and allows for systematic control of the conductance through molecular design.
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http://dx.doi.org/10.1021/jacs.8b10296DOI Listing
November 2018

Resonant Transport in Single Diketopyrrolopyrrole Junctions.

J Am Chem Soc 2018 10 5;140(41):13167-13170. Epub 2018 Oct 5.

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

We study the single-molecule transport properties of small bandgap diketopyrrolopyrrole oligomers (DPP n, n = 1-4) with lengths varying from 1 to 5 nm. At a low bias voltage, the conductance decays exponentially as a function of length indicative of nonresonant transport. However, at a high bias voltage, we observe a remarkably high conductance close to 10 G with currents reaching over 0.1 μA across all four oligomers. These unique transport properties, together with density functional theory-based transport calculations, suggest a mechanism of resonant transport across the highly delocalized DPP backbones in the high bias regime. This study thus demonstrates the unique properties of diketopyrrolopyrrole derivatives in achieving highly efficient long-range charge transport in single-molecule devices.
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http://dx.doi.org/10.1021/jacs.8b06964DOI Listing
October 2018

Near Length-Independent Conductance in Polymethine Molecular Wires.

Nano Lett 2018 10 11;18(10):6387-6391. Epub 2018 Sep 11.

Department of Chemistry , Columbia University , New York , New York 10027 , United States.

Polymethine dyes are linear π-conjugated compounds with an odd number of carbons that display a much greater delocalization in comparison to polyenes that have an even number of carbon atoms in their main chain. Herein, we perform scanning tunneling microscope based break-junction measurements on a series of three cyanine dyes of increasing length. We demonstrate, at the single molecule level, that these short chain polymethine systems exhibit a substantially smaller decay in conductance with length (attenuation factor β = 0.04 Å) compared to traditional polyenes (β ≈ 0.2 Å). Furthermore, we show that by changing solvent we are able to shift the β value, demonstrating a remarkable negative β value, with conductance increasing with molecular length. First principle calculations provide support for the experimentally observed near-uniform length dependent conductance and further suggest that the variations in β with solvent are due to solvent-induced changes in the alignment of the frontier molecular orbitals relative to the Fermi energy of the leads. A simplified Hückel model suggests that the smaller decay in conductance correlates with the smaller degree of bond order alternation present in polymethine compounds compared to polyenes. These findings may enable the design of molecular wires without a length-dependent decay for efficient electron transport at the nanoscale.
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http://dx.doi.org/10.1021/acs.nanolett.8b02743DOI Listing
October 2018

In Situ Formation of N-Heterocyclic Carbene-Bound Single-Molecule Junctions.

J Am Chem Soc 2018 07 3;140(28):8944-8949. Epub 2018 Jul 3.

Institute of Physics , Academy of Sciences of the Czech Republic , Cukrovarnická 10 , Prague 16200 , Czech Republic.

Self-assembled monolayers (SAMs) formed using N-heterocyclic carbenes (NHCs) have recently emerged as thermally and chemically ultrastable alternatives to those formed from thiols. The rich chemistry and strong σ-donating ability of NHCs offer unique prospects for applications in nanoelectronics, sensing, and electrochemistry. Although stable in SAMs, free carbenes are notoriously reactive, making their electronic characterization challenging. Here we report the first investigation of electron transport across single NHC-bound molecules using the scanning tunneling microscope-based break junction (STM-BJ) technique. We develop a series of air-stable metal NHC complexes that can be electrochemically reduced in situ to form NHC-electrode contacts, enabling reliable single-molecule conductance measurements of NHCs under ambient conditions. Using this approach, we show that the conductance of an NHC depends on the identity of the single metal atom to which it is coordinated in the junction. Our observations are supported by density functional theory (DFT) calculations, which also firmly establish the contributions of the NHC linker to the junction transport characteristics. Our work demonstrates a powerful method to probe electron transfer across NHC-electrode interfaces; more generally, it opens the door to the exploitation of surface-bound NHCs in constructing novel, functionalized electrodes and/or nanoelectronic devices.
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http://dx.doi.org/10.1021/jacs.8b05184DOI Listing
July 2018

Comprehensive suppression of single-molecule conductance using destructive σ-interference.

Nature 2018 06 6;558(7710):415-419. Epub 2018 Jun 6.

Nano-Science Center and Department of Chemistry, University of Copenhagen, Copenhagen, Denmark.

The tunnelling of electrons through molecules (and through any nanoscale insulating and dielectric material ) shows exponential attenuation with increasing length , a length dependence that is reflected in the ability of the electrons to carry an electrical current. It was recently demonstrated that coherent tunnelling through a molecular junction can also be suppressed by destructive quantum interference , a mechanism that is not length-dependent. For the carbon-based molecules studied previously, cancelling all transmission channels would involve the suppression of contributions to the current from both the π-orbital and σ-orbital systems. Previous reports of destructive interference have demonstrated a decrease in transmission only through the π-channel. Here we report a saturated silicon-based molecule with a functionalized bicyclo[2.2.2]octasilane moiety that exhibits destructive quantum interference in its σ-system. Although molecular silicon typically forms conducting wires , we use a combination of conductance measurements and ab initio calculations to show that destructive σ-interference, achieved here by locking the silicon-silicon bonds into eclipsed conformations within a bicyclic molecular framework, can yield extremely insulating molecules less than a nanometre in length. Our molecules also exhibit an unusually high thermopower (0.97 millivolts per kelvin), which is a further experimental signature of the suppression of all tunnelling paths by destructive interference: calculations indicate that the central bicyclo[2.2.2]octasilane unit is rendered less conductive than the empty space it occupies. The molecular design presented here provides a proof-of-concept for a quantum-interference-based approach to single-molecule insulators.
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http://dx.doi.org/10.1038/s41586-018-0197-9DOI Listing
June 2018
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