Molecular Conductance Research Articles

Modulating Quantum Interference Between Destructive and Constructive States in Double N‐Substituted Single Molecule Junctions (Adv. Electron. Mater. 2/2023)

Quantum Interferences In article number 2201024, Xiangfeng Shao, Colin J. Lambert, Hao-Li Zhang, and co-workers present the significant modulation effect of double N-substitution on quantum interference (QI) in meta-phenylene ethylene oligomer molecular devices, reveal how two N-atoms synergistically modulate the molecular conductance between destructive QI and constructive QI states. The findings of this work pave a path toward utilization of heterocyclic aromatic hydrocarbons in molecular electronics.

Utilizing Noncovalent Conformational Locks to Create Through-Space Charge Transport

Creating through-space electron transport (ET) channels in the molecular backbone has been rarely explored in molecular electronics. Here, we demonstrated that through-space channels could be readily created by substituting the H atom of the central benzene in a starting molecule 1,4-dithiophene-benzen (DT-B) with F, which forms an interaction with the thiophene S and gives rise to the F–S type noncovalent conformational locks (NCLs). Single-molecule conductance investigation showed the molecular conductance was increasingly larger with more F substituents. A theoretical study revealed that NCLs play a major role in enhancing the ET rates, which not only led to better π-conjugation but also created additional through-space transport channels.

Synthesis, spectroscopic elucidation, in vitro antimicrobial, cytotoxic and CT-DNA binding evaluation of heterobimetallic complexes of Ni(II) with main group/transition metal dichlorides

Heterobimetallic complexes of Nickel (II) have been synthesized and characterized of the type [Ni(L)2(M)2R4Cl2], where L = 2,6-diaminopyridine (C5H7N3), M = Sn, Si, Ti and Zr and R = Ph, Me, C5H5 etc. The monometallic complexes of nickel have been synthesized by using 2,6-diaminopyridine in the presence of NiCl2·6H2O dissolved in methanol to form complexes of the type [Ni(L)2]Cl2. To synthesize heterobimetallic complexes, these synthesized complexes were further processed with main group metal or transition metal dichlorides in a 1:2 molar ratio in MeOH. Elemental analyses, molecular weight calculations, electronic, infrared, magnetic susceptibility, conductivity measurements, mass spectra, 1HNMR , 13CNMR , and powder X-ray diffraction studies have been used to evaluate these synthesized compounds. Using a monometallic complex, the heterobimetallic complexes were synthesized, and an octahedral geometry was perceived for these heterobimetallic complexes. The antimicrobial potency of the synthesized complexes has been evaluated against four different bacterial and three fungal strain at different concentrations. The cytotoxicity activity of these resulting complexes against MCF-7, HeLa and HaCaT cell lines using MTT assay has been scrutinized. Also, the interaction of the reported complexes with calf thymus DNA (CT-DNA) by different techniques revealed that the complexes could bind to CT-DNA by intercalative mode. The binding of the reported complexes to CT-DNA has been investigated using UV–vis and fluorescence spectra. The results indicated a much lower binding affinity of monometallic complex than that of the heterobimetallic complexes.

A new understanding of electron affinity of –OCH3 substituent groups based on single molecular conductance

To employ substituent groups to modulate the electronic properties of organic molecules as predicted, learning about their electron affinity is an important prerequisite. Herein, we used conventional electron-donating/withdrawing (ED/EW) substituent groups -OMe/-COOMe to modify TB, finding the resulting molecules OTB and CTB both demonstrated lower molecular conductance than TB. Current-voltage modeling and DFT calculation reveal –OCH3 demonstrated EW features instead of ED characteristics, which was supported by energy gap simulation and light emission as pulsed 365 nm light. This phenomenon was rarely observed, highlighting the importance of substituent groups’ electron affinity investigation in conductance based organic electronics.

Increasing the redox switching capacity of Lindqvist-type hexavanadates by organogold post-functionalisation.

The covalent attachment of organogold(I) moieties to the Lindqvist-type polyoxovanadate results in a measurable charge re-distribution across the formed Au-{V6}-Au linkages. Scanning probe microscopy studies of these hybrid compounds on the Au(111) surface demonstrate the increase in the number of switching states with stepwise increase in molecular conductance, compared with unfunctionalised hexavanadates.

Exploring relationships between chemical structure and molecular conductance: from α,ω-functionalised oligoynes to molecular circuits.

The quantum circuit rule (QCR) allows estimation of the conductance of molecular junctions, electrode|X-bridge-Y|electrode, by considering the molecule as a series of independent scattering regions associated with the anchor groups (X, Y) and bridge, provided the numerical parameters that characterise the anchor groups (aX, aY) and molecular backbones (bB) are known. Single-molecule conductance measurements made with a series of α,ω-substituted oligoynes (X-{(CC)N}-X, N = 1, 2, 3, 4), functionalised by terminal groups, X (4-thioanisole (C6H4SMe), 5-(3,3-dimethyl-2,3-dihydrobenzo[b]thiophene) (DMBT), 4-aniline (C6H4NH2), 4-pyridine (Py), capable of serving as 'anchor groups' to contact the oligoyne fragment within a molecular junction, have shown the expected exponential dependence of molecular conductance, G, with the number of alkyne repeating units. In turn, this allows estimation of the anchor (ai) and backbone (bi) parameters. Using these values, together with previously determined parameters for other molecular fragments, the QCR is found to accurately estimate the junction conductance of more complex molecular circuits formed from smaller components assembled in series.

How an electrical current can stabilize a molecular nanojunction.

The stability of molecular junctions under transport is of the utmost importance for the field of molecular electronics. This question is often addressed within the paradigm of current-induced heating of nuclear degrees of freedom or current-induced forces acting upon the nuclei. At the same time, an essential characteristic of the failure of a molecular electronic device is its changing conductance - typically from a finite value for the intact device to zero for a device that lost its functionality. In this publication, we focus on the current-induced changes in the molecular conductance, which are inherent to molecular junctions at the limit of mechanical stability. We employ a numerically exact framework based on the hierarchical equations of motion approach, which treats both electronic and nuclear degrees of freedom on an equal footing and does not impose additional assumptions. Studying generic model systems for molecular junctions with dissociative potentials for a wide range of parameters spanning the adiabatic and the nonadiabatic regime, we find that molecular junctions that exhibit a decrease in conductance upon dissociation are more stable than junctions that are more conducting in their dissociated state. This represents a new mechanism that stabilizes molecular junctions under current. Moreover, we identify characteristic signatures in the current of breaking junctions related to the interplay between changes in the conductance and the nuclear configuration and show how these are related to properties of the leads rather than characteristics of the molecule itself.

MAVEN/NGIMS Dayside Exospheric Temperatures Over Solar Cycle and Seasons: Role of Dayside Thermal Balances in Regulating Temperatures

AbstractSeveral important processes of the Mars upper atmosphere are regulated by exospheric temperature (Texo) variations, including atmospheric escape rates. From the MAVEN mission, significant and largely periodic variability of Mars dayside Texo is now revealed by Neutral Gas and Ion Mass Spectrometer (NGIMS) data sets collected throughout solar cycle 24. Two complementary methods are utilized to extract temperatures from NGIMS data sets. These NGIMS dayside mean exospheric temperatures are shown to vary by ∼80 K (∼180–260 K) over solar cycle 24. This corresponds to a ΔTexo/ΔEUV sensitivity of ∼38 K m2 mW−1, where Lyman‐α is the solar index. Previous Mars Global Surveyor derived Texo values for solar cycle 23 yielded a sensitivity of ∼45 in the same units. This close correspondence suggests that the underlying dayside energy balances are similar yet slightly different over these two solar cycles. Corresponding Mars Global Ionosphere‐Thermosphere Model simulations show that molecular thermal conduction largely balances EUV heating for the Mars dayside thermosphere, while CO2 15‐µm cooling is secondary in importance, along with global winds. It is valuable to compare this Mars sensitivity to that computed for the dayside thermosphere of Venus. Pioneer Venus dayside data sets imply a sensitivity of ∼17.5 K m2 mW−1 units for solar cycle 21, a factor of ∼2.2 smaller than computed for Mars. This is because Venus CO2 cooling strongly balances EUV heating near its peak, thereby serving as a more efficient thermostat regulating dayside temperatures. CO2 cooling is much weaker for Mars.

Toward a molecular understanding of the conductivity of lithium-ion conducting polyanion polymer electrolytes by molecular dynamics simulation.

With the improved lithium-ion transference number near unity, the low conductivity of single lithium-ion conducting solid polymer electrolytes (SLIC-SPEs) still hinders their application in high-rate batteries. Though some empirical conclusions on the conducting mechanism of SLIC-SPEs have been obtained, a more comprehensive study on the quantitative relationship between the molecular structure factors and ionic conduction performance is expected. In this study, a model structure that contains adjustable main chain and anion groups in the polyethylene oxide (PEO) matrix was used to clarify the influence of molecular structural factors on ionic conductivity and electrochemical stability of SLIC-SPEs. The anionic group was further disassembled into the intermediate group and end group while the main chain structure was distinguished into different degrees of polymerization and various lengths of the spacers between anions. Therefore, a well-defined molecular structure was employed to describe its relationship with ionic conductivity. In addition, the dissociation degree of salts and mobility of ions changing with the molecular structure were also discussed to explore the fundamental causes of conductivity. It can be concluded that the anion group affects the conductivity mainly via the dissociation degree, while the main chain structure impacts the conductivity by both dissociation degree and mobility.

Quantifying polaron densities in sequentially doped conjugated polymers: exploring the upper limits of molecular doping and conductivity

Conjugated polymers are sequentially doped to control film conductivity and optical properties. (1) Carrier fractions (measured via optical absorbance), (2) polymer site densities, and (3) dopant volumes are used to calculate polaron densities.

Charge transport of F4TCNQ with different electronic states in single-molecule junctions.

The molecular conductance of 2,3,5,6-tetrafluoro-7,7,8,8,-tetracyano-quinodimethane (F4TCNQ) with different electronic states (neutral, radical anion, and dianion) was investigated by the scanning tunneling microscope break junction (STM-BJ) technique. These electronic states have distinct conductance, and the conductance decreases in the order of neutral > radical anion > dianion. Surprisingly, the molecular conductance of the neutral F4TCNQ junction reaches 10-1.17G0, attributed to its LUMO energy level being close to the Fermi level of the gold electrode. Moreover, we found that neutral F4TCNQ can be gradually reduced to radical anions under a relatively low bias voltage of 100 mV. These results will advance the development of organic optoelectronic devices and molecule electronics.

Mechanochemical Synthesis of Polyanilines and Their Nanocomposites: A Critical Review.

The mechanochemical synthesis of polyanilines (PANIs), made by oxidative polymerization of anilines, is reviewed. First, previous knowledge of the polymerization reaction in solution is discussed to understand the effect of different parameters: oxidant/monomer ratio, added acid, oxidant, temperature and water content on the properties of the conducting polymers (molecular weight, degradation, doping/oxidation level, conductivity, and nanostructure). The work on mechanochemical polymerization (MCP) of anilines is analyzed in view of previous data in solution, and published data are critically reconsidered to clarify the interpretation of experimental results. A key factor is the production of acids during polymerization, which is often overlooked. The production of gaseous HCl during MCP of aniline hydrochloride is experimentally observed. Since some experiments involves the addition of small amounts of water, the kinetics and heat balance of the reaction with concentrated solutions were simulated. A simple experiment shows fast (<2 min) heating of the reaction mixture to the boiling point of water and temperature increments are observed during MCP in a mortar. The form and sizes of PANI nanostructures made by MCP or solution are compared. The extensive work on the production of nanocomposites by MCP of anilines together with different nanomaterials (porous clays, graphene, carbon nanotubes, metal, and oxide nanoparticles) is also described.

A continuum theory of organic mixed ionic-electronic conductors of phase separation

Organic mixed ionic-electronic conductors (OMIECs) are the core functioning component in the emerging flexible, bio-, and optoelectronics owning to their unique capability of mixed conduction. Of all types, two-phase OMIECs exhibit exceptional performance due to their high stretchability and balanced ionic-electronic conduction. However, the electron-conducting phase may segregate from the ion-conducting phase in a two-phase OMIEC, changing the conducting path and eventually leading to degraded performance and dysfunction of the devices. In this work, we formulate a continuum theory following the thermodynamics framework of a two-phase OMIEC undergoing phase separation. The free energy consists of contributions from the deformation of the polymer chains, the mixing of the polymer with salts and solvents, the electrostatic field, and the two-phase interfaces. The equilibrium conditions and kinetics equations are derived with the constraint of mass conservation, thermodynamics laws, and electrostatics. We implement the theory into a finite element model and study the mechanics and electrochemistry of the OMIEC channel in an electrolyte-gated organic electrochemical transistor (OECT) device. The computational model captures the concurrent transport of charge carriers, mechanical swelling, and phase separation in the OMIEC and replicates the transfer curves of an OECT which agree well with the experiments. More specifically, we reveal the origin of the volumetric capacitance as the accumulation of charge carriers at the two-phase interfaces. We examine the parametric space to elucidate experimental observations such as molecular size-dependent conductivity and substrate-dependent phase separation. The swelling behavior and the transfer curves of OECTs under stretched, free, and constrained states are compared, demonstrating the effects of deformation on the phase dynamics and the electron-conducting behavior. We show that, for volumetric swelling and the electrochemical transfer curves, the effect of stress-transport coupling dominates while the effect of the Maxwell stress is negligible. This work provides a theoretical basis for the mechanics and electrochemistry of two-phase OMIECs.

Optical Trapping of a Single Molecule of Length Sub-1 nm in Solution

Optical Trapping of a Single Molecule of Length Sub-1 nm in Solution

Binding structure, breaking forces and conductance of Au-Octanedithiol-Au molecular junction under stretching processes: a DFT-NEGF study

Au-n-octanedithiol-Au molecular junction (Au-SC8S-Au) has been investigated using density functional theory combined with the nonequilibrium Green’s function approach. Theoretically calculated results are used to build the relationship between the interface binding structures and single-molecule quantum conductance of n-octanedithiol (SC8S) embodied in a gold nanogap with or without stretching forces. To understand the electron transport mechanism in the single molecular nanojunction, we designed three types of Au-SC8S-Au nanogaps, including flat electrode through an Au atom connecting (Model I), top-pyramidal or flat electrodes with the molecule adsorbing directly (Model II), and top-pyramidal Au electrodes with Au atomic chains (Model III). We first determined the optimized structures of different Au-SC8S-Au nanogaps, and then predicted the distance-dependent stretching force and conductance in each case. Our calculated results show that in the Model I with an Au atom bridging the flat Au (111) gold electrodes and the SC8S molecule, the conductance decreases exponentially before the fracture of Au–Au bond, in a good agreement with the experimental conductance in the literature. For the top-pyramidal electrode Models II and III, the magnitudes of molecular conductance are larger than that in Model I. Our theoretical calculations also show that the Au–Au bond fracture takes place in Models I and III, while the Au–S bond fracture appears in Model II. This is explained due to the total strength of three synergetic Au–Au bonds stronger than an Au–S bond in Model II. This is supported from the broken force about 2 nN for the Au–Au bond and 3 nN for the Au–S bond.

Tidal Structures in the Mesosphere and Lower Thermosphere and Their Solar Cycle Variations

We studied the correlations between the migrating and non-migrating tides and solar cycle in the mesosphere and lower thermosphere (MLT) regions between 60° S and 60° N, which are in LAT-LON Earth coordinates, by analyzing the simulation datasets from the thermosphere and ionosphere extension of the Whole Atmosphere Community Climate Model (WACCM-X). A least squares fitting method was utilized to obtain the daily mean migrating tides and non-migrating tides. The Pearson linear correlation coefficient was used to analyze the correlations between tides and solar activity. Our analysis shows that the negative correlations between tides and solar activity are mostly impacted by the first symmetrical structure of the tidal modes for both migrating and non-migrating components. The coefficient of molecular thermal conductivity for the first symmetrical structure is small at low solar flux, so the tides dissipate more slowly when the F10.7 cm radio flux level is low. Thus, the amplitudes of tidal variations under a solar minimum condition are larger than those under a solar maximum condition. The correlation between tides and solar activity could also be influenced by some other factors, such as geomagnetic activity and the density of carbon dioxide CO2 on Earth. The tidal variations can be influenced by westward background wind, which grows stronger as geomagnetic activity rises. Further, dissipation of the tides decreases because the heat conduction and molecular viscosity are weakened in the cooling thermosphere caused by increasing CO2, which results in larger tidal amplitudes under the solar maximum condition. It is found that the correlations between tides and solar cycle vary at different altitudes and latitudes. The negative correlations are most possibly influenced by the first symmetrical structure of tidal variations and may also be impacted by geomagnetic activity. The positive correlations are impacted by the density of CO2.

Molecular Dynamics Study on Structure, Vibrational Properties, and Transport Coefficients of Liquid Alumina

The structure, vibrational density of states, and transport coefficients of liquid alumina were studied using molecular dynamics simulations. At the temperature of 2500 K, 3000 K, 3500 K, and 4000 K, systems with three different densities were constructed, respectively, including the configurations with densities of 2.81 g/cm3 and 3.17 g/cm3, and the relaxed ones with nearly zero pressure at each temperature. With the changes in temperature or density, the transformations on the structural, vibrational and transport properties were discussed. The Born-Mayer-Huggins type of atomic interactions was used, with newly optimized parameters. The analysis of the interatomic correlations indicated that the short-range order of liquid alumina was mainly constructed by AlO4 tetrahedra, also a certain number of AlO3 and AlO5 was present. Meanwhile, the structural transitions on the elemental units occurred as either the temperature or density increased. Two primary frequency bands were observed in each vibrational density of states spectrum, with the higher frequency bands produced by the O atom vibrations, and the lower frequency ones generated by the Al atom vibrations. Self-diffusion coefficients were estimated using the linear behavior of the mean-squared displacement at long time, while by using the Green-Kubo relation during equilibrium molecular dynamics simulations, thermal conductivities and viscosities were calculated. Significantly, the viscosity at 2500 K with a density of 2.81 g/cm3 was equal to 25.23 mPa s, which was very close to the experimental finding.

Molecular dynamics and conduction mechanism of poly(vinyl chloride‐co‐vinyl acetate‐co‐2‐hydroxypropyl acrylate) terpolymer containing ionic liquid

AbstractDielectric spectroscopy (DS) measurements were performed to probe the segmental dynamics and ion mobility of poly(vinyl chloride‐co‐vinyl acetate‐co‐2‐hydroxypropyl acrylate) terpolymer dopped with different amounts of tetrabutylammonium tetrafluoroborate ([TBA] [BF4]) ionic liquid (IL). Differential scanning calorimetry (DSC) was also employed to trace the change in the glass transition temperature (Tg) at different loads of IL. The DSC measurements revealed a remarkable reduction in the PVVH Tg from 344 to 310 K just by adding 20 wt% of IL. The DS measurements revealed three relaxation processes named α, β1, and β2. The α‐process is related to the segmental motion of PVVH while the β1 and β2 are due to the restricted local dynamics of side chains. The segmental relaxation times (α‐relaxation) speed up with increasing the concentration of IL due to the plasticization effect of IL on polymer chains. The temperature dependence of α‐relaxation follows the Vogel‐Fulcher‐Tammann (VFT) relation with dynamic glass transition between 323 and 294 K in agreement with the DSC measurements. The β1 and β2‐relaxations have an Arrhenius temperature dependence. The temperature dependence of ionic conductivity obeys the VFT behavior indicating the coupling between the segmental motion of PVVH chains and ion transport. Polaronic tunneling is the predominant conduction mechanism in PVVH and its composites. The specific capacitance increases with increasing both the temperature and IL concentration.

Hough Mode Extensions (HMEs) and Solar Tide Behavior in the Dissipative Thermosphere

AbstractThis paper quantifies and interprets how thermosphere dissipation in the form of molecular viscosity, molecular thermal conductivity and collisions with ions (“ion drag”) determines the height versus latitude structures of a variety of migrating and non‐migrating solar thermal tides, pole‐to‐pole up to 400 km altitude. This is done through computation of thermosphere Hough Mode Extensions (HMEs); that is, solutions to the linearized momentum, thermal energy, continuity, state and hydrostatic balance equations wherein the horizontal structure of troposphere forcing for each HME corresponds to the eigenfunction (Hough function) of Laplace's tidal equation for a particular tidal mode, and the background state and thermosphere dissipation are specified for nominal solar minimum, average and maximum activity conditions. The broad features revealed, different for each HME, include changes in vertical(horizontal) structure with latitude(height), degree of vertical penetration to the middle and upper thermosphere, changes in vertical wavelength (λz) with height due to the transition in background thermal gradient around the mesopause, and insights into role of ion drag in determining tidal amplitudes and their solar cycle variability. The HMEs are particularly useful for fitting measurements of tides, and for estimating tidal fields beyond those dependent variables actually fit and including those outside the fitting domain. The HMEs reported here were created as part of the Ionospheric CONnection (ICON) mission to serve as observation‐based lower boundary conditions for Thermosphere Ionosphere Mesosphere General Circulation Model‐ICON. Access to the full set of HMEs in tabular and graphical form is provided.

Geometric Structure, Electronic, and Spectral Properties of Metal-free Phthalocyanine under the External Electric Fields.

Here, the ground-state structures, electronic structures, polarizability, and spectral properties of metal-free phthalocyanine (H2Pc) under different external electric fields (EEFs) are investigated. The results show that EEF has an ultrastrong regulation effect on various aspects of H2Pc; the geometric structures, electronic properties, polarizability, and spectral properties are strongly sensitive to the EEF. In particular, an EEF of 0.025 a.u. is an important control point: an EEF of 0.025 a.u. will bend the benzene ring subunits to the positive and negative x directions of the planar molecule. Flipping the EEF from positive (0.025 a.u.) to negative (-0.025 a.u.) flips also the bending direction of benzene ring subunits. The H2Pc shows different dipole moments projecting an opposite direction along the x direction (-84 and 84 Debye for EEFs of -0.025 and 0.025 a.u., respectively) under negative and positive EEF, revealing a significant dipole moment transformation. Furthermore, when the EEF is removed, the molecule can be restored to the planar structure. The transformation of the H2Pc structure can be induced by the EEF, which has potential applications in the molecular devices such as molecular switches or molecular forceps. EEF lowers total energy and reduces highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap; especially, an EEF of 0.025 a.u. can reduce the HOMO-LUMO gap from 2.1 eV (in the absence of EEF) to 0.37 eV, and thus, it can enhance the molecular conductivity. The first hyperpolarizability of H2Pc is 0 in the absence of EEF; remarkably, an EEF of 0.025 a.u. can enhance the first hyperpolarizability up to 15,578 a.u. Therefore, H2Pc under the EEF could be introduced as a promising innovative nonlinear optical (NLO) nanomaterial such as NLO switches. The strong EEF (0.025 a.u.) causes a large number of new absorption peaks in IR and Raman spectra and causes the redshift of electronic absorption spectra. The changes of EEF can be used to regulate the structure transformation and properties of H2Pc, which can promote the application of H2Pc in nanometer fields such as molecular devices.