Molecular Rectification Enhancement Based On Conformational and Chemical Modifications

Design principles for molecules with intrinsic directional charge transport will likely prove crucial for breakthroughs in nanotechnology and other emerging fields like biosensors and advanced photovoltaics. Here, we perform a systematic computational study to characterize the electronic rectification induced by conformational and chemical modifications at low bias potentials and elucidate design principles for intrinsic molecular rectifiers. We study donor–bridge–acceptor (D–B–A) systems that consist of phenylene units with geometrical rotation of the rings and representative electron-donating and -withdrawing substituent groups at the donor and acceptor sites. We calculate transport properties using the non-equilibrium Green’s function technique and density functional theory (DFT-NEGF) and obtain <i>I</i>–<i>V</i> characteristics and rectification ratios. Our results indicate that efficient intrinsic rectification at low bias voltages can only be obtained by combining dihedral angles of 60° between phenyl rings and asymmetric chemical substitution. Together, these structural features cause rectification enhancement by localizing the molecular orbital closer to the Fermi level of the electrode in one end of the molecular device. Our designed systems present rectification ratios up to 20.08 at 0.3 V in their minimum-energy geometry and are predicted to be stable under thermal fluctuations

Effect of Mechanical Stretching on DNA Conductance

Studying the structural and charge transport properties in DNA is important for unraveling molecular scale processes and developing device applications of DNA molecules. Here we study the effect of mechanical stretching-induced structural changes on charge transport in single DNA molecules. The charge transport follows the hopping mechanism for DNA molecules with lengths varying from 6 to 26 base pairs, but the conductance is highly sensitive to mechanical stretching, showing an abrupt decrease at surprisingly short stretching distances and weak dependence on DNA length. We attribute this force-induced conductance decrease to the breaking of hydrogen bonds in the base pairs at the end of the sequence and describe the data with a mechanical model

Tuning the Electromechanical Properties of Single DNA Molecular Junctions

Understanding the interplay between the electrical and mechanical properties of DNA molecules is important for the design and characterization of molecular electronic devices, as well as understanding the role of charge transport in biological functions. However, to date, force-induced melting has limited our ability to investigate the response of DNA molecular conductance to stretching. Here we present a new molecule–electrode linker based on a hairpin-like design, which prevents force-induced melting at the end of single DNA molecules during stretching by stretching both strands of the duplex evenly. We find that the new linker group gives larger conductance than previously measured DNA–electrode linkers, which attach to the end of one strand of the duplex. In addition to changing the conductance the new linker also stabilizes the molecule during stretching, increasing the length a single DNA molecule can be stretched before an abrupt decrease in conductance. Fitting these electromechanical properties to a spring model, we show that distortion is more evenly distributed across the single DNA molecule during stretching, and thus the electromechanical effects of the π–π coupling between neighboring bases is measured

Non-exponential Length Dependence of Conductance in Iodide-Terminated Oligothiophene Single-Molecule Tunneling Junctions

An exponential decrease of molecular conductance with length has been observed in most molecular systems reported to date, and has been taken as a signature of non-resonant tunneling as the conduction mechanism. Surprisingly, the conductance of iodide-terminated oligothiophene molecules presented herein does not follow the simple exponential length dependence. The lack of temperature dependence in the conductance indicates that tunneling still dominates the conduction mechanism in the molecules. Transition voltage spectroscopy shows that the tunneling barrier of the oligothiophene decreases with length, but the decrease is insufficient to explain the non-exponential length dependence. X-ray photoelectron spectroscopy, stretching length measurement, and theoretical calculations show that the non-exponential length dependence is due to a transition in the binding geometry of the molecule to the electrodes in the molecular junctions as the length increases