Julio L. Palma

Intermediate tunnelling–hopping regime in DNA charge transport

Charge transport in molecular systems, including DNA, is involved in many basic chemical and biological processes, and its understanding is critical if they are to be used in electronic devices. This important phenomenon is often described as either coherent tunnelling over a short distance or incoherent hopping over a long distance. Here, we show evidence of an intermediate regime where coherent and incoherent processes coexist in double-stranded DNA. We measure charge transport in single DNA molecules bridged to two electrodes as a function of DNA sequence and length. In general, the resistance of DNA increases linearly with length, as expected for incoherent hopping. However, for DNA sequences with stacked guanine-cytosine (GC) base pairs, a periodic oscillation is superimposed on the linear length dependence, indicating partial coherent transport. This result is supported by the finding of strong delocalization of the highest occupied molecular orbitals of GC by theoretical simulation and by modelling based on the Büttiker theory of partial coherent charge transport.

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.

Thermoelectric effect and its dependence on molecular length and sequence in single DNA molecules.

Studying the thermoelectric effect in DNA is important for unravelling charge transport mechanisms and for developing relevant applications of DNA molecules. Here we report a study of the thermoelectric effect in single DNA molecules. By varying the molecular length and sequence, we tune the charge transport in DNA to either a hopping- or tunnelling-dominated regimes. The thermoelectric effect is small and insensitive to the molecular length in the hopping regime. In contrast, the thermoelectric effect is large and sensitive to the length in the tunnelling regime. These findings indicate that one may control the thermoelectric effect in DNA by varying its sequence and length. We describe the experimental results in terms of hopping and tunnelling charge transport models.

Gate-controlled conductance switching in DNA

Extensive evidence has shown that long-range charge transport can occur along double helical DNA, but active control (switching) of single-DNA conductance with an external field has not yet been demonstrated. Here we demonstrate conductance switching in DNA by replacing a DNA base with a redox group. By applying an electrochemical (EC) gate voltage to the molecule, we switch the redox group between the oxidized and reduced states, leading to reversible switching of the DNA conductance between two discrete levels. We further show that monitoring the individual conductance switching allows the study of redox reaction kinetics and thermodynamics at single molecular level using DNA as a probe. Our theoretical calculations suggest that the switch is due to the change in the energy level alignment of the redox states relative to the Fermi level of the electrodes.

Piezoresistivity in single DNA molecules.

Piezoresistivity is a fundamental property of materials that has found many device applications. Here we report piezoresistivity in double helical DNA molecules. By studying the dependence of molecular conductance and piezoresistivity of single DNA molecules with different sequences and lengths, and performing molecular orbital calculations, we show that the piezoresistivity of DNA is caused by force-induced changes in the π–π electronic coupling between neighbouring bases, and in the activation energy of hole hopping. We describe the results in terms of thermal activated hopping model together with the ladder-based mechanical model for DNA proposed by de Gennes.

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.

Linker Rectifiers for Covalent Attachment of Transition-Metal Catalysts to Metal-Oxide Surfaces

Linkers that favor rectification of interfacial electron transfer are likely to be required for efficient photo-driven catalysis of multi-electron reactions at electrode surfaces. Design principles are discussed, together with the synthesis and characterization of a specific pair of molecular linkers, related by inversion of the direction of an amide bond in the heart of the molecule. The linkers have a terpyridyl group that can covalently bind Mn as in a well-known water oxidation catalyst and an acetylacetonate group that allows attachment to TiO2 surfaces. The appropriate choice of the sense of the amide linkage yields directionality of interfacial electron transfer, essential to enhance electron injection and slow back-electron transfer. Support comes from electron paramagnetic resonance and terahertz spectroscopic measurements, as well as computational modeling characterizing the asymmetry of electron transfer properties.

Measuring the Spin‐Polarization Power of a Single Chiral Molecule

The electronic spin filtering capability of a single chiral helical peptide is measured. A ferromagnetic electrode source is employed to inject spin-polarized electrons in an asymmetric single-molecule junction bridging an α-helical peptide sequence of known chirality. The conductance comparison between both isomers allows the direct determination of the polarization power of an individual chiral molecule.

Probing the Nature of Charge Transfer at Nano-Bio Interfaces: Peptides on Metal Oxide Nanoparticles.

Characterizing the nano-bio interface has been a long-standing endeavor in the quest for novel biosensors, biophotovoltaics, and biocompatible electronic devices. In this context, the present computational work on the interaction of two peptides, A6K (Ac-AAAAAAK-NH2) and A7 (Ac-AAAAAAA-NH2) with semiconducting TiO2 nanoparticles is an effort to understand the peptide-metal oxide nanointerface. These investigations were spurred by recent experimental observations that nanostructured semiconducting metal oxides templated with A6K peptides not only stabilize large proteins like photosystem-I (PS-I) but also exhibit enhanced charge-transfer characteristics. Our results indicate that α-helical structures of A6K are not only energetically more stabilized on TiO2 nanoparticles, but the resulting hybrids also exhibit enhanced electron transfer characteristics. This enhancement can be attributed to substantial changes in the electronic characteristics at the peptide-TiO2 interface. Apart from understanding the mechanism of electron transfer (ET) in peptide-stabilized PS-I on metal oxide nanoparticles, the current work also has implications in the development of novel solar cells and photocatalysts.

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.