@article{17673, abstract = {Accurate forward modeling of weak lensing (WL) observables from cosmological parameters is necessary for upcoming galaxy surveys. Because WL probes structures in the non-linear regime, analytical forward modeling is very challenging, if not impossible. Numerical simulations of WL features rely on ray-tracing through the outputs of N-body simulations, which requires knowledge of the gravitational potential and accurate solvers for light ray trajectories. A less accurate procedure, based on the Born approximation, only requires knowledge of the density field, and can be implemented more efficiently and at a lower computational cost. In this work, we use simulations to show that deviations of the Born-approximated convergence power spectrum, skewness and kurtosis from their fully ray--traced counterparts are consistent with the smallest non-trivial O(Φ3) post-Born corrections (so-called geodesic and lens-lens terms). Our results imply a cancellation among the larger O(Φ4) (and higher order) terms, consistent with previous analytic work. We also find that cosmological parameter bias induced by the Born approximated power spectrum is negligible even for an LSST-like survey, once galaxy shape noise is considered. When considering higher order statistics such as the κ skewness and kurtosis, however, we find significant bias of up to 2.5σ. Using the LensTools software suite, we show that the Born approximation saves a factor of 4 in computing time with respect to the full ray-tracing in reconstructing the convergence.}, author = {Petri, Andrea and Haiman, Zoltán and May, Morgan}, issn = {2470-0010}, journal = {Physical Review D}, number = {12}, publisher = {American Physical Society}, title = {{Validity of the Born approximation for beyond Gaussian weak lensing observables}}, doi = {10.1103/physrevd.95.123503}, volume = {95}, year = {2017}, } @article{17696, abstract = {We utilize cosmological hydrodynamic simulations to study the formation of Population III (Pop III) stars in dark matter halos exposed to strong ionizing radiation. We simulate the formation of three halos subjected to a wide range of ionizing fluxes, and find that for high flux, ionization and photoheating can delay gas collapse and star formation up to halo masses significantly larger than the atomic cooling threshold. The threshold halo mass at which gas first collapses and cools increases with ionizing flux for intermediate values, and saturates at a value approximately an order of magnitude above the atomic cooling threshold for extremely high flux (e.g. ≈5×108 M⊙ at z≈6). This behavior can be understood in terms of photoheating, ionization/recombination, and Lyα cooling in the pressure-supported, self-shielded gas core at the center of the growing dark matter halo. We examine the spherically-averaged radial velocity profiles of collapsing gas and find that a gas mass of up to ≈106 M⊙ can reach the central regions within 3 Myr, providing an upper limit on the amount of massive Pop III stars that can form. The ionizing radiation increases this limit by a factor of a few compared to strong Lyman-Werner (LW) radiation alone. We conclude that the bright HeII 1640 Å emission recently observed from the high-redshift galaxy CR7 cannot be explained by Pop III stars alone. However, in some halos, a sufficient number of Pop III stars may form to be detectable with future telescopes such as the James Webb Space Telescope (JWST).}, author = {Visbal, Eli and Bryan, Greg L. and Haiman, Zoltán}, issn = {0035-8711}, journal = {Monthly Notices of the Royal Astronomical Society}, number = {2}, pages = {1456--1465}, publisher = {Oxford University Press}, title = {{What is the maximum mass of a Population III galaxy?}}, doi = {10.1093/mnras/stx909}, volume = {469}, year = {2017}, } @article{17698, abstract = {Gaseous circumbinary accretion discs provide a promising mechanism to facilitate the mergers of supermassive black holes (SMBHs) in galactic nuclei. We measure the torques exerted on accreting SMBH binaries, using 2D, isothermal, moving-mesh, viscous hydrodynamical simulations of circumbinary accretion discs. Our computational domain includes the entire inner region of the circumbinary disk with the individual black holes (BHs) included as point masses on the grid and a sink prescription to model accretion onto each BH. The BHs each acquire their own well-resolved accretion discs ("minidiscs"). We explore a range of mass removal rates for the sink prescription removing gas from the central regions of the minidiscs. We find that the torque exerted on the binary is primarily gravitational, and dominated by the gas orbiting close behind and ahead of the individual BHs. The torques from the distorted circumbinary disc farther out and from the direct accretion of angular momentum are subdominant. The torques are sensitive to the sink prescription: slower sinks result in more gas accumulating near the BHs and more negative torques, driving the binary to merger more rapidly. For faster sinks, the torques are less negative and eventually turn positive (for unphysically fast sinks). When the minidiscs are modeled as standard alpha discs, our results are insensitive to the choice of sink radius. Scaling the simulations to a binary orbital period tbin = 1yr and background disc accretion rate Mdot = 0.3MEdd in Eddington units, the binary inspirals on a timescale of 3X10^6 years, irrespective of the SMBH masses. For binaries with total mass <10^7Msun, this is shorter than the inspiral time due to gravitational wave (GW) emission alone, implying that gas discs will have a significant impact on the SMBH binary population and can affect the GW signal for Pulsar Timing Arrays.}, author = {Tang, Yike and MacFadyen, Andrew and Haiman, Zoltán}, issn = {0035-8711}, journal = {Monthly Notices of the Royal Astronomical Society}, number = {4}, pages = {4258--4267}, publisher = {Oxford University Press}, title = {{On the orbital evolution of supermassive black hole binaries with circumbinary accretion discs}}, doi = {10.1093/mnras/stx1130}, volume = {469}, year = {2017}, } @article{17706, abstract = {The Laser Interferometer Gravitational-Wave Observatory, LIGO, found direct evidence for double black hole binaries emitting gravitational waves. Galactic nuclei are expected to harbor the densest population of stellar-mass black holes. A significant fraction (∼30%) of these black holes can reside in binaries. We examine the fate of the black hole binaries in active galactic nuclei, which get trapped in the inner region of the accretion disk around the central supermassive black hole. We show that binary black holes can migrate into and then rapidly merge within the disk well within a Salpeter time. The binaries may also accrete a significant amount of gas from the disk, well above the Eddington rate. This could lead to detectable X-ray or gamma-ray emission, but would require hyper-Eddington accretion with a few percent radiative efficiency, comparable to thin disks. We discuss implications for gravitational wave observations and black hole population studies. We estimate that Advanced LIGO may detect ∼20 such, gas-induced binary mergers per year.}, author = {Bartos, Imre and Kocsis, Bence and Haiman, Zoltán and Márka, Szabolcs}, issn = {0004-637X}, journal = {The Astrophysical Journal}, number = {2}, publisher = {American Astronomical Society}, title = {{Rapid and bright stellar-mass binary black hole mergers in active galactic nuclei}}, doi = {10.3847/1538-4357/835/2/165}, volume = {835}, year = {2017}, } @article{17707, abstract = {The gravitational waves (GWs) from a binary black hole (BBH) with masses between 10^4 and 10^7 Msun can be detected with the Laser Interferometer Space Antenna (LISA) once their orbital frequency exceeds 10^-4 - 10^-5 Hz. The binary separation at this stage is approximately a=100 R_g (gravitational radius), and the orbital speed is of order v/c=0.1. We argue that at this stage, the binary will be producing bright electromagnetic (EM) radiation via gas bound to the individual BHs. Both BHs will have their own photospheres in X-ray and possibly also in optical bands. Relativistic Doppler modulations and lensing effects will inevitably imprint periodic variability in the EM light-curve, tracking the phase of the orbital motion, and serving as a template for the GW inspiral waveform. Advanced localization of the source by LISA weeks to months prior to merger will enable a measurement of this EM chirp by wide-field X-ray or optical instruments. A comparison of the phases of the GW and EM chirp signals will help break degeneracies between system parameters, and probe a fractional difference difference Delta v in the propagation speed of photons and gravitons as low as Delta v/c = O(10^-17).}, author = {Haiman, Zoltán}, issn = {2470-0010}, journal = {Physical Review D}, number = {2}, publisher = {American Physical Society}, title = {{Electromagnetic chirp of a compact binary black hole: A phase template for the gravitational wave inspiral}}, doi = {10.1103/physrevd.96.023004}, volume = {96}, year = {2017}, } @article{17708, abstract = {We explore the sensitivity of weak lensing observables to the expansion history of the universe and to the growth of cosmic structures, as well as the relative contribution of both effects to constraining cosmological parameters. We utilize ray-tracing dark-matter-only N-body simulations and validate our technique by comparing our results for the convergence power spectrum with analytic results from past studies. We then extend our analysis to non-Gaussian observables which cannot be easily treated analytically. We study the convergence (equilateral) bispectrum and two topological observables, lensing peaks and Minkowski functionals, focusing on their sensitivity to the matter density Ωm and the dark energy equation of state w. We find that a cancelation between the geometry and growth effects is a common feature for all observables, and exists at the map level. It weakens the overall sensitivity by up to a factor of 3 and 1.5 for w and Ωm, respectively, with the bispectrum worst affected. However, combining geometry and growth information alleviates the degeneracy between Ωm and w from either effect alone. As a result, the magnitude of marginalized errors remain similar to those obtained from growth-only effects, but with the correlation between the two parameters switching sign. These results shed light on the origin of cosmology-sensitivity of non-Gaussian statistics, and should be useful in optimizing combinations of observables.}, author = {Matilla, José Manuel Zorrilla and Haiman, Zoltán and Petri, Andrea and Namikawa, Toshiya}, issn = {2470-0010}, journal = {Physical Review D}, number = {2}, publisher = {American Physical Society}, title = {{Geometry and growth contributions to cosmic shear observables}}, doi = {10.1103/physrevd.96.023513}, volume = {96}, year = {2017}, } @article{17711, abstract = {The recent discovery of gravitational waves from stellar-mass binary black hole mergers by the Laser Interferometer Gravitational-wave Observatory opened the door to alternative probes of stellar and galactic evolution, cosmology and fundamental physics. Probing the origin of binary black hole mergers will be difficult due to the expected lack of electromagnetic emission and limited localization accuracy. Associations with rare host galaxy types—such as active galactic nuclei—can nevertheless be identified statistically through spatial correlation. Here we establish the feasibility of statistically proving the connection between binary black hole mergers and active galactic nuclei as hosts, even if only a sub-population of mergers originate from active galactic nuclei. Our results are the demonstration that the limited localization of gravitational waves, previously written off as not useful to distinguish progenitor channels, can in fact contribute key information, broadening the range of astrophysical questions probed by binary black hole observations.}, author = {Bartos, I. and Haiman, Zoltán and Marka, Z. and Metzger, B. D. and Stone, N. C. and Marka, S.}, issn = {2041-1723}, journal = {Nature Communications}, number = {1}, publisher = {Springer Science and Business Media LLC}, title = {{Gravitational-wave localization alone can probe origin of stellar-mass black hole mergers}}, doi = {10.1038/s41467-017-00851-7}, volume = {8}, year = {2017}, } @article{17712, abstract = {Multi-frequency gravitational wave (GW) observations are useful probes of the formation processes of coalescing stellar-mass binary black holes (BBHs). We discuss the phase drift in the GW inspiral waveform of the merging BBH caused by its center-of-mass acceleration. The acceleration strongly depends on the location where a BBH forms within a galaxy, allowing observations of the early inspiral phase of LIGO-like BBH mergers by the Laser Interferometer Space Antenna (LISA) to test the formation mechanism. In particular, BBHs formed in dense nuclear star clusters or via compact accretion disks around a nuclear supermassive black hole in active galactic nuclei would suffer strong acceleration, and produce large phase drifts measurable by LISA. The host galaxies of the coalescing BBHs in these scenarios can also be uniquely identified in the LISA error volume, without electromagnetic counterparts. A non-detection of phase drifts would rule out or constrain the contribution of the nuclear formation channels to the stellar-mass BBH population.}, author = {Inayoshi, Kohei and Tamanini, Nicola and Caprini, Chiara and Haiman, Zoltán}, issn = {2470-0010}, journal = {Physical Review D}, number = {6}, publisher = {American Physical Society }, title = {{Probing stellar binary black hole formation in galactic nuclei via the imprint of their center of mass acceleration on their gravitational wave signal}}, doi = {10.1103/physrevd.96.063014}, volume = {96}, year = {2017}, } @article{17936, abstract = {We report that the single‐molecule junction conductance of thiol‐terminated silanes with Ag electrodes are higher than the conductance of those formed with Au electrodes. These results are in contrast to the trends in the metal work function Φ(Ag)<Φ(Au). As such, a better alignment of the Au Fermi level to the molecular orbital of silane that mediates charge transport would be expected. This conductance trend is reversed when we replace the thiols with amines, highlighting the impact of metal–S covalent and metal–NH2 dative bonds in controlling the molecular conductance. Density functional theory calculations elucidate the crucial role of the chemical linkers in determining the level alignment when molecules are attached to different metal contacts. We also demonstrate that conductance of thiol‐terminated silanes with Pt electrodes is lower than the ones formed with Au and Ag electrodes, again in contrast to the trends in the metal work‐functions.}, author = {Li, Haixing and Su, Timothy A. and Camarasa‐Gómez, María and Hernangómez‐Pérez, Daniel and Henn, Simon E. and Pokorný, Vladislav and Caniglia, Caravaggio D. and Inkpen, Michael S. and Korytár, Richard and Steigerwald, Michael L. and Nuckolls, Colin and Evers, Ferdinand and Venkataraman, Latha}, issn = {1521-3773}, journal = {Angewandte Chemie International Edition}, number = {45}, pages = {14145--14148}, publisher = {Wiley}, title = {{Silver makes better eElectrical contacts to thiol‐terminated silanes than Gold}}, doi = {10.1002/anie.201708524}, volume = {56}, year = {2017}, } @article{17937, abstract = {Fabricating nanoscopic devices capable of manipulating and processing single units of charge is an essential step towards creating functional devices where quantum effects dominate transport characteristics. The archetypal single-electron transistor comprises a small conducting or semiconducting island separated from two metallic reservoirs by insulating barriers1,2,3,4,5. By enabling the transfer of a well-defined number of charge carriers between the island and the reservoirs, such a device may enable discrete single-electron operations6,7,8,9. Here, we describe a single-molecule junction comprising a redox-active, atomically precise cobalt chalcogenide cluster wired between two nanoscopic electrodes10,11. We observe current blockade at room temperature in thousands of single-cluster junctions. Below a threshold voltage, charge transfer across the junction is suppressed. The device is turned on when the temporary occupation of the core states by a transiting carrier is energetically enabled, resulting in a sequential tunnelling process and an increase in current by a factor of ∼600. We perform in situ and ex situ cyclic voltammetry as well as density functional theory calculations to unveil a two-step process mediated by an orbital localized on the core of the cluster in which charge carriers reside before tunnelling to the collector reservoir. As the bias window of the junction is opened wide enough to include one of the cluster frontier orbitals, the current blockade is lifted and charge carriers can tunnel sequentially across the junction.}, author = {Lovat, Giacomo and Choi, Bonnie and Paley, Daniel W. and Steigerwald, Michael L. and Venkataraman, Latha and Roy, Xavier}, issn = {1748-3395}, journal = {Nature Nanotechnology}, pages = {1050--1054}, publisher = {Springer Nature}, title = {{Room-temperature current blockade in atomically defined single-cluster junctions}}, doi = {10.1038/nnano.2017.156}, volume = {12}, year = {2017}, } @article{17939, abstract = {We report a series of single-molecule transport measurements carried out in an ionic environment with oligophenylenediamine wires. These molecules exhibit three discrete conducting states accessed by electrochemically modifying the contacts. Transport in these junctions is defined by the oligophenylene backbone, but the conductance is increased by factors of ∼20 and ∼400 when compared to traditional dative junctions. We propose that the higher-conducting states arise from in situ electrochemical conversion of the dative Au←N bond into a new type of Au–N contact. Density functional theory-based transport calculations establish that the new contacts dramatically increase the electronic coupling of the oligophenylene backbone to the Au electrodes, consistent with experimental transport data. The resulting contact resistance is the lowest reported to date; more generally, our work demonstrates a facile method for creating electronically transparent metal–organic interfaces.}, author = {Zang, Yaping and Pinkard, Andrew and Liu, Zhen-Fei and Neaton, Jeffrey B. and Steigerwald, Michael L. and Roy, Xavier and Venkataraman, Latha}, issn = {1520-5126}, journal = {Journal of the American Chemical Society}, number = {42}, pages = {14845--14848}, publisher = {American Chemical Society}, title = {{Electronically transparent Au–N bonds for molecular junctions}}, doi = {10.1021/jacs.7b08370}, volume = {139}, year = {2017}, } @article{17940, abstract = {Single-molecule conductance studies have traditionally focused on creating highly conducting molecular wires. However, progress in nanoscale electronics demands insulators just as it needs conductors. Here we describe the single-molecule length-dependent conductance properties of the classic silicon dioxide insulator. We synthesize molecular wires consisting of Si–O repeat units and measure their conductance through the scanning tunneling microscope-based break-junction method. These molecules yield conductance lower than alkanes of the same length and the largest length-dependent conductance decay of any molecular systems measured to date. We calculate single-molecule junction transmission and the complex band structure of the infinite 1D material for siloxane, in comparison with silane and alkane, and show that the large conductance decay is intrinsic to the nature of the Si–O bond. This work highlights the potential for siloxanes to function as molecular insulators in electronics.}, author = {Li, Haixing and Garner, Marc H. and Su, Timothy A. and Jensen, Anders and Inkpen, Michael S. and Steigerwald, Michael L. and Venkataraman, Latha and Solomon, Gemma C. and Nuckolls, Colin}, issn = {1520-5126}, journal = {Journal of the American Chemical Society}, number = {30}, pages = {10212--10215}, publisher = {American Chemical Society}, title = {{Extreme conductance suppression in molecular siloxanes}}, doi = {10.1021/jacs.7b05599}, volume = {139}, year = {2017}, } @article{17941, abstract = {How heteroatomic substitutions affect electron transport through π-conjugated hydrocarbons has been the subject of some debate. In this paper we investigate the effect of heteroatomic linkers in a molecular junction on the electron-transmission spectrum, focusing on the occurrence of quantum interference (QI) close to the Fermi level, where conductivity can be significantly suppressed. We find that the substitution or addition of heteroatoms to a carbon skeleton at the contact positions does not change the main feature of QI due to the underlying carbon skeleton. QI in the overall system thus remains a robust feature. This empirical observation leads us to derive, in two mathematical ways, that these findings can be generalized. We note that addition or substitution of a carbon atom by a heteroatom at the contact positions will increase or decrease the number of electrons in the π-system, which will lead to a change in the alignment of the molecular orbitals of the isolated system relative to the electrode Fermi level. Both Hückel and density functional theory calculations on model systems probe the effect of this Fermi level change and confirm qualitatively the implications of the underlying mathematical proofs.}, author = {Tsuji, Yuta and Stuyver, Thijs and Gunasekaran, Suman and Venkataraman, Latha}, issn = {1932-7455}, journal = {The Journal of Physical Chemistry C}, number = {27}, pages = {14451--14462}, publisher = {American Chemical Society}, title = {{The influence of linkers on quantum interference: A linker theorem}}, doi = {10.1021/acs.jpcc.7b03493}, volume = {121}, year = {2017}, } @article{17942, abstract = {Quantum interference effects, whether constructive or destructive, are key to predicting and understanding the electrical conductance of single molecules. Here, through theory and experiment, we investigate a family of benzene-like molecules that exhibit both constructive and destructive interference effects arising due to more than one contact between the molecule and each electrode. In particular, we demonstrate that the π-system of meta-coupled benzene can exhibit constructive interference and its para-coupled analog can exhibit destructive interference, and vice versa, depending on the specific through-space interactions. As a peculiarity, this allows a meta-coupled benzene molecule to exhibit higher conductance than a para-coupled benzene. Our results provide design principles for molecular electronic components with high sensitivity to through-space interactions and demonstrate that increasing the number of contacts between the molecule and electrodes can both increase and decrease the conductance.}, author = {Borges, Anders and Xia, Jianlong and Liu, Sheng Hua and Venkataraman, Latha and Solomon, Gemma C.}, issn = {1530-6992}, journal = {Nano Letters}, number = {7}, pages = {4436--4442}, publisher = {American Chemical Society}, title = {{The role of through-space interactions in modulating constructive and destructive interference effects in benzene}}, doi = {10.1021/acs.nanolett.7b01592}, volume = {17}, year = {2017}, } @article{17943, abstract = {This Account provides an overview of our recent efforts to uncover the fundamental charge transport properties of Si–Si and Ge–Ge single bonds and introduce useful functions into group 14 molecular wires. We utilize the tools of chemical synthesis and a scanning tunneling microscopy-based break-junction technique to study the mechanism of charge transport in these molecular systems. We evaluated the fundamental ability of silicon, germanium, and carbon molecular wires to transport charge by comparing conductances within families of well-defined structures, the members of which differ only in the number of Si (or Ge or C) atoms in the wire. For each family, this procedure yielded a length-dependent conductance decay parameter, β. Comparison of the different β values demonstrates that Si–Si and Ge–Ge σ bonds are more conductive than the analogous C–C σ bonds. These molecular trends mirror what is seen in the bulk. The conductance decay of Si and Ge-based wires is similar in magnitude to those from π-based molecular wires such as paraphenylenes However, the chemistry of the linkers that attach the molecular wires to the electrodes has a large influence on the resulting β value. For example, Si- and Ge-based wires of many different lengths connected with a methyl–thiomethyl linker give β values of 0.36–0.39 Å–1, whereas Si- and Ge-based wires connected with aryl–thiomethyl groups give drastically different β values for short and long wires. This observation inspired us to study molecular wires that are composed of both π- and σ-orbitals. The sequence and composition of group 14 atoms in the σ chain modulates the electronic coupling between the π end-groups and dictates the molecular conductance. The conductance behavior originates from the coupling between the subunits, which can be understood by considering periodic trends such as bond length, polarizability, and bond polarity. We found that the same periodic trends determine the electric field-induced breakdown properties of individual Si–Si, Ge–Ge, Si–O, Si–C, and C–C bonds. Building from these studies, we have prepared a system that has two different, alternative conductance pathways. In this wire, we can intentionally break a labile, strained silicon–silicon bond and thereby shunt the current through the secondary conduction pathway. This type of in situ bond-rupture provides a new tool to study single molecule reactions that are induced by electric fields. Moreover, these studies provide guidance for designing dielectric materials as well as molecular devices that require stability under high voltage bias. The fundamental studies on the structure/function relationships of the molecular wires have guided the design of new functional systems based on the Si- and Ge-based wires. For example, we exploited the principle of strain-induced Lewis acidity from reaction chemistry to design a single molecule switch that can be controllably switched between two conductive states by varying the distance between the tip and substrate electrodes. We found that the strain intrinsic to the disilaacenaphthene scaffold also creates two state conductance switching. Finally, we demonstrate the first example of a stereoelectronic conductance switch, and we demonstrate that the switching relies crucially on the electronic delocalization in Si–Si and Ge–Ge wire backbones. These studies illustrate the untapped potential in using Si- and Ge-based wires to design and control charge transport at the nanoscale and to allow quantum mechanics to be used as a tool to design ultraminiaturized switches.}, author = {Su, Timothy A. and Li, Haixing and Klausen, Rebekka S. and Kim, Nathaniel T. and Neupane, Madhav and Leighton, James L. and Steigerwald, Michael L. and Venkataraman, Latha and Nuckolls, Colin}, issn = {1520-4898}, journal = {Accounts of Chemical Research}, number = {4}, pages = {1088--1095}, publisher = {American Chemical Society}, title = {{Silane and Germane molecular electronics}}, doi = {10.1021/acs.accounts.7b00059}, volume = {50}, year = {2017}, } @article{17944, abstract = {The electronic, mechanical, and thermoelectric properties of molecular scale devices have fascinated scientists across several disciplines in natural sciences and engineering. The interest is partially technological, driven by the fast miniaturization of integrated circuits that now have reached characteristic features at the nanometer scale. Equally important, a very strong incentive also exists to elucidate the fundamental aspects of structure-function relations for nanoscale devices, which utilize molecular building blocks as functional units. Thus motivated, a rich research field has established itself, broadly termed “Molecular Electronics,” that hosts a plethora of activities devoted to this goal in chemistry, physics, and electrical engineering. This Special Topic on Frontiers of Molecular Scale Electronics captures recent theoretical and experimental advances in the field.}, author = {Evers, Ferdinand and Venkataraman, Latha}, issn = {1089-7690}, journal = {The Journal of Chemical Physics}, number = {9}, publisher = {AIP Publishing}, title = {{Preface: Special topic on Frontiers in Molecular Scale Electronics}}, doi = {10.1063/1.4977469}, volume = {146}, year = {2017}, } @article{17945, abstract = {We perform temperature dependent conductance measurements on sub-nanometer sized single molecules bound to gold electrodes using a scanning tunneling microscope-based break junction technique in Ultra-High Vacuum (UHV). We find a threefold increase in the conductance of amine-terminated conjugated molecules when the temperature increases from 4 K to 300 K in UHV. Furthermore, the conductance measured at 300 K in UHV is consistent with solution-based measurements under ambient conditions where the transport mechanism corresponds to off-resonant electron tunneling across the molecule. Our measurements indicate that at 300 K, conductance is largely independent of pressure or solvent around the junction. In addition, our data unambiguously show that temperature can affect the tunneling conductance of single molecule-metal junctions. We show that the structure of the metal electrodes that form in these junctions varies systematically with temperature, and hypothesize that this changing structure of the interface alters electron tunneling probability and propose a mechanism to explain our findings.}, author = {Kamenetska, M. and Widawsky, J. R. and Dell’Angela, M. and Frei, M. and Venkataraman, Latha}, issn = {1089-7690}, journal = {The Journal of Chemical Physics}, number = {9}, publisher = {AIP Publishing}, title = {{Temperature dependent tunneling conductance of single molecule junctions}}, doi = {10.1063/1.4973318}, volume = {146}, year = {2017}, } @article{17947, abstract = {We investigate light-induced conductance enhancement in single-molecule junctions via photon-assisted transport and hot-electron transport. Using 4,4′-bipyridine bound to Au electrodes as a prototypical single-molecule junction, we report a 20–40% enhancement in conductance under illumination with 980 nm wavelength radiation. We probe the effects of subtle changes in the transmission function on light-enhanced current and show that discrete variations in the binding geometry result in a 10% change in enhancement. Importantly, we prove theoretically that the steady-state behavior of photon-assisted transport and hot-electron transport is identical but that hot-electron transport is the dominant mechanism for optically induced conductance enhancement in single-molecule junctions when the wavelength used is absorbed by the electrodes and the hot-electron relaxation time is long. We confirm this experimentally by performing polarization-dependent conductance measurements of illuminated 4,4′-bipyridine junctions. Finally, we perform lock-in type measurements of optical current and conclude that currents due to laser-induced thermal expansion mask optical currents. This work provides a robust experimental framework for studying mechanisms of light-enhanced transport in single-molecule junctions and offers tools for tuning the performance of organic optoelectronic devices by analyzing detailed transport properties of the molecules involved.}, author = {Fung, E-Dean and Adak, Olgun and Lovat, Giacomo and Scarabelli, Diego and Venkataraman, Latha}, issn = {1530-6992}, journal = {Nano Letters}, number = {2}, pages = {1255--1261}, publisher = {American Chemical Society}, title = {{Too hot for photon-assisted transport: Hot-electrons dominate conductance enhancement in illuminated single-molecule junctions}}, doi = {10.1021/acs.nanolett.6b05091}, volume = {17}, year = {2017}, } @article{17949, abstract = {Single-molecule electronic devices provide researchers with an unprecedented ability to relate novel physical phenomena to molecular chemical structures. Typically, conjugated aromatic molecular backbones are relied upon to create electronic devices, where the aromaticity of the building blocks is used to enhance conductivity. We capitalize on the classical physical organic chemistry concept of Hückel antiaromaticity by demonstrating a single-molecule switch that exhibits low conductance in the neutral state and, upon electrochemical oxidation, reversibly switches to an antiaromatic high-conducting structure. We form single-molecule devices using the scanning tunneling microscope–based break-junction technique and observe an on/off ratio of ~70 for a thiophenylidene derivative that switches to an antiaromatic state with 6-4-6-π electrons. Through supporting nuclear magnetic resonance measurements, we show that the doubly oxidized core has antiaromatic character and we use density functional theory calculations to rationalize the origin of the high-conductance state for the oxidized single-molecule junction. Together, our work demonstrates how the concept of antiaromaticity can be exploited to create single-molecule devices that are highly conducting.}, author = {Yin, Xiaodong and Zang, Yaping and Zhu, Liangliang and Low, Jonathan Z. and Liu, Zhen-Fei and Cui, Jing and Neaton, Jeffrey B. and Venkataraman, Latha and Campos, Luis M.}, issn = {2375-2548}, journal = {Science Advances}, number = {10}, publisher = {American Association for the Advancement of Science}, title = {{A reversible single-molecule switch based on activated antiaromaticity}}, doi = {10.1126/sciadv.aao2615}, volume = {3}, year = {2017}, } @article{17950, abstract = {Whilst most studies in single-molecule electronics involve components first synthesized ex situ, there is also great potential in exploiting chemical transformations to prepare devices in situ. Here, as a first step towards this goal, we conduct reversible reactions on monolayers to make and break covalent bonds between alkanes of different lengths, then measure the conductance of these molecules connected between electrodes using the scanning tunneling microscopy-based break junction (STM-BJ) method. In doing so, we develop the critical methodology required for assembling and disassembling surface-bound single-molecule circuits. We identify effective reaction conditions for surface-bound reagents, and importantly demonstrate that the electronic characteristics of wires created in situ agree with those created ex situ. Finally, we show that the STM-BJ technique is unique in its ability to definitively probe surface reaction yields both on a local (∼50 nm2) and pseudo-global (≥10 mm2) level. This investigation thus highlights a route to the construction and integration of more complex, and ultimately functional, surface-based single-molecule circuitry, as well as advancing a methodology that facilitates studies beyond the reach of traditional ex situ synthetic approaches.}, author = {Inkpen, Michael S. and Leroux, Yann R. and Hapiot, Philippe and Campos, Luis M. and Venkataraman, Latha}, issn = {2041-6539}, journal = {Chemical Science}, number = {6}, pages = {4340--4346}, publisher = {Royal Society of Chemistry}, title = {{Reversible on-surface wiring of resistive circuits}}, doi = {10.1039/c7sc00599g}, volume = {8}, year = {2017}, }