Matching Items (5)
Description
Studying charge transport through single molecules tethered between two metal electrodes is of fundamental importance in molecular electronics. Over the years, a variety of methods have been developed in attempts of performing such measurements. However, the limitation of these techniques is still one of the factors that prohibit one from gaining a thorough understanding of single molecule junctions. Firstly, the time resolution of experiments is typically limited to milli to microseconds, while molecular dynamics simulations are carried out on the time scale of pico to nanoseconds. A huge gap therefore persists between the theory and the experiments. This thesis demonstrates a nanosecond scale measurement of the gold atomic contact breakdown process. A combined setup of DC and AC circuits is employed, where the AC circuit reveals interesting observations in nanosecond scale not previously seen using conventional DC circuits. The breakdown time of gold atomic contacts is determined to be faster than 0.1 ns and subtle atomic events are observed within nanoseconds. Furthermore, a new method based on the scanning tunneling microscope break junction (STM-BJ) technique is developed to rapidly record thousands of I-V curves from repeatedly formed single molecule junctions. 2-dimensional I-V and conductance-voltage (G-V) histograms constructed using the acquired data allow for more meaningful statistical analysis to single molecule I-V characteristics. The bias voltage adds an additional dimension to the conventional single molecule conductance measurement. This method also allows one to perform transition voltage spectra (TVS) for individual junctions and to study the correlation between the conductance and the tunneling barrier height. The variation of measured conductance values is found to be primarily determined by the poorly defined contact geometry between the molecule and metal electrodes, rather than the tunnel barrier height. In addition, the rapid I-V technique is also found useful in studying thermoelectric effect in single molecule junctions. When applying a temperature gradient between the STM tip and substrate in air, the offset current at zero bias in the I-V characteristics is a measure of thermoelectric current. The rapid I-V technique allows for statistical analysis of such offset current at different temperature gradients and thus the Seebeck coefficient of single molecule junctions is measured. Combining with single molecule TVS, the Seebeck coefficient is also found to be a measure of tunnel barrier height.
ContributorsGuo, Shaoyin (Author) / Tao, Nongjian (Thesis advisor) / Bennett, Peter (Committee member) / Ning, Cun-Zheng (Committee member) / Yu, Hongbin (Committee member) / Arizona State University (Publisher)
Created2012
Description
Charge transport in molecular systems, including DNA (Deoxyribonucleic acid), is involved in many basic chemical and biological processes. Studying their charge transport properties can help developing DNA based electronic devices with many tunable functionalities. This thesis investigates the electric properties of double-stranded DNA, DNA G-quadruplex and dsDNA with modified base.
First, double-stranded DNA with alternating GC sequence and stacked GC sequence were measured with respect to length. The resistance of DNA sequences increases linearly with length, indicating a hopping transport mechanism. However, for DNA sequences with stacked GC, a periodic oscillation is superimposed on the linear length dependence, indicating a partial coherent transport. The result is supported by the finding of delocalization of the highest occupied molecular orbitals of Guanines from theoretical simulation and by fitting based on the Büttiker’s theory.
Then, a DNA G4-duplex structures with a G-quadruplex as the core and DNA duplexes as the arms were studied. Similar conductance values were observed by varying the linker positions, thus a charge splitter is developed. The conductance of the DNA G-tetrads structures was found to be sensitive to the π-stacking at the interface between the G-quadruplex and DNA duplexes by observing a higher conductance value when one duplex was removed and a polyethylene glycol (PEG) linker was added into the interface. This was further supported by molecular dynamic simulations.
Finally, a double-stranded DNA with one of the bases replaced by an anthraquinone group was studied via electrochemical STM break junction technique. Anthraquinone can be reversibly switched into the oxidized state or reduced state, to give a low conductance or high conductance respectively. Furthermore, the thermodynamics and kinetics properties of the switching were systematically studied. Theoretical simulation shows that the difference between the two states is due to a difference in the energy alignment with neighboring Guanine bases.
First, double-stranded DNA with alternating GC sequence and stacked GC sequence were measured with respect to length. The resistance of DNA sequences increases linearly with length, indicating a hopping transport mechanism. However, for DNA sequences with stacked GC, a periodic oscillation is superimposed on the linear length dependence, indicating a partial coherent transport. The result is supported by the finding of delocalization of the highest occupied molecular orbitals of Guanines from theoretical simulation and by fitting based on the Büttiker’s theory.
Then, a DNA G4-duplex structures with a G-quadruplex as the core and DNA duplexes as the arms were studied. Similar conductance values were observed by varying the linker positions, thus a charge splitter is developed. The conductance of the DNA G-tetrads structures was found to be sensitive to the π-stacking at the interface between the G-quadruplex and DNA duplexes by observing a higher conductance value when one duplex was removed and a polyethylene glycol (PEG) linker was added into the interface. This was further supported by molecular dynamic simulations.
Finally, a double-stranded DNA with one of the bases replaced by an anthraquinone group was studied via electrochemical STM break junction technique. Anthraquinone can be reversibly switched into the oxidized state or reduced state, to give a low conductance or high conductance respectively. Furthermore, the thermodynamics and kinetics properties of the switching were systematically studied. Theoretical simulation shows that the difference between the two states is due to a difference in the energy alignment with neighboring Guanine bases.
ContributorsXiang, Liming (Author) / Tao, Nongjian (Thesis advisor) / Lindsay, Stuart (Committee member) / Gould, Ian (Committee member) / Arizona State University (Publisher)
Created2016
Description
Studying charge transport through single molecules is of great importance for unravelling charge transport mechanisms, investigating fundamentals of chemistry, and developing functional building blocks in molecular electronics.
First, a study of the thermoelectric effect in single DNA molecules is reported. By varying the molecular length and sequence, the charge transport in DNA was tuned to either a hopping- or tunneling-dominated regimes. In the hopping regime, the thermoelectric effect is small and insensitive to the molecular length. Meanwhile, in the tunneling regime, the thermoelectric effect is large and sensitive to the length. These findings indicate that by varying its sequence and length, the thermoelectric effect in DNA can be controlled. The experimental results are then described in terms of hopping and tunneling charge transport models.
Then, I showed that the electron transfer reaction of a single ferrocene molecule can be controlled with a mechanical force. I monitor the redox state of the molecule from its characteristic conductance, detect the switching events of the molecule from reduced to oxidized states with the force, and determine a negative shift of ~34 mV in the redox potential under force. The theoretical modeling is in good agreement with the observations, and reveals the role of the coupling between the electronic states and structure of the molecule.
Finally, conclusions and perspectives were discussed to point out the implications of the above works and future studies that can be performed based on the findings.
First, a study of the thermoelectric effect in single DNA molecules is reported. By varying the molecular length and sequence, the charge transport in DNA was tuned to either a hopping- or tunneling-dominated regimes. In the hopping regime, the thermoelectric effect is small and insensitive to the molecular length. Meanwhile, in the tunneling regime, the thermoelectric effect is large and sensitive to the length. These findings indicate that by varying its sequence and length, the thermoelectric effect in DNA can be controlled. The experimental results are then described in terms of hopping and tunneling charge transport models.
Then, I showed that the electron transfer reaction of a single ferrocene molecule can be controlled with a mechanical force. I monitor the redox state of the molecule from its characteristic conductance, detect the switching events of the molecule from reduced to oxidized states with the force, and determine a negative shift of ~34 mV in the redox potential under force. The theoretical modeling is in good agreement with the observations, and reveals the role of the coupling between the electronic states and structure of the molecule.
Finally, conclusions and perspectives were discussed to point out the implications of the above works and future studies that can be performed based on the findings.
ContributorsLi, Yueqi, Ph.D (Author) / Tao, Nongjian (Thesis advisor) / Buttry, Daniel (Committee member) / Mujica, Vladimiro (Committee member) / Arizona State University (Publisher)
Created2017
Description
This thesis is devoted to the theoretical and computational study of electron transport in molecular junctions where one or more hydrogen bonds are involved in the process. While electron transport through covalent bonds has been extensively studied, in recent work the focus has been shifted towards hydrogen-bonded systems due to their ubiquitous presence in biological systems and their potential in forming nano- junctions between molecular electronic devices and biological systems.
This analysis allows us to significantly expand our comprehension of the experimentally observed result that the inclusion of hydrogen bonding in a molecular junc- tion significantly impacts its transport properties, a fact that has important implications for our understanding of transport through DNA, and nano-biological interfaces in general. In part of this work I have explored the implications of quasiresonant transport in short chains of weakly-bonded molecular junctions involving hydrogen bonds. I used theoretical and computational analysis to interpret recent experiments and explain the role of Fano resonances in the transmission properties of the junction.
In a different direction, I have undertaken the study of the transversal conduction through nucleotide chains that involve a variable number of different hydrogen bonds, e.g. NH···O, OH···O, and NH···N, which are the three most prevalent hydrogen bonds in biological systems and organic electronics. My effort here has fo- cused on the analysis of electronic descriptors that allow a simplified conceptual and computational understanding of transport properties. Specifically, I have expanded our previous work where the molecular polarizability was used as a conductance de- scriptor to include the possibility of atomic and bond partitions of the molecular polarizability. This is important because it affords an alternative molecular descrip- tion of conductance that is not based on the conventional view of molecular orbitals as transport channels. My findings suggest that the hydrogen-bond networks are crucial in understanding the conductance of these junctions.
A broader impact of this work pertains the fact that characterizing transport through hydrogen bonding networks may help in developing faster and cost-effective approaches to personalized medicine, to advance DNA sequencing and implantable electronics, and to progress in the design and application of new drugs.
This analysis allows us to significantly expand our comprehension of the experimentally observed result that the inclusion of hydrogen bonding in a molecular junc- tion significantly impacts its transport properties, a fact that has important implications for our understanding of transport through DNA, and nano-biological interfaces in general. In part of this work I have explored the implications of quasiresonant transport in short chains of weakly-bonded molecular junctions involving hydrogen bonds. I used theoretical and computational analysis to interpret recent experiments and explain the role of Fano resonances in the transmission properties of the junction.
In a different direction, I have undertaken the study of the transversal conduction through nucleotide chains that involve a variable number of different hydrogen bonds, e.g. NH···O, OH···O, and NH···N, which are the three most prevalent hydrogen bonds in biological systems and organic electronics. My effort here has fo- cused on the analysis of electronic descriptors that allow a simplified conceptual and computational understanding of transport properties. Specifically, I have expanded our previous work where the molecular polarizability was used as a conductance de- scriptor to include the possibility of atomic and bond partitions of the molecular polarizability. This is important because it affords an alternative molecular descrip- tion of conductance that is not based on the conventional view of molecular orbitals as transport channels. My findings suggest that the hydrogen-bond networks are crucial in understanding the conductance of these junctions.
A broader impact of this work pertains the fact that characterizing transport through hydrogen bonding networks may help in developing faster and cost-effective approaches to personalized medicine, to advance DNA sequencing and implantable electronics, and to progress in the design and application of new drugs.
ContributorsWimmer, Michael (Author) / Mujica, Vladimiro (Thesis advisor) / Wolf, George (Committee member) / Chizmeshya, Andrew (Committee member) / Arizona State University (Publisher)
Created2017
Description
Understanding charge transport in single molecules covalently bonded to electrodes is a fundamental goal in the field of molecular electronics. In the past decade, it has become possible to measure charge transport on the single-molecule level using the STM break junction method. Measurements on the single-molecule level shed light on charge transport phenomena which would otherwise be obfuscated by ensemble measurements of groups of molecules. This thesis will discuss three projects carried out using STM break junction. In the first project, the transition between two different charge transport mechanisms is reported in a set of molecular wires. The shortest wires show highly length dependent and temperature invariant conductance behavior, whereas the longer wires show weakly length dependent and temperature dependent behavior. This trend is consistent with a model whereby conduction occurs by coherent tunneling in the shortest wires and by incoherent hopping in the longer wires. Measurements are supported with calculations and the evolution of the molecular junction during the pulling process is investigated. The second project reports controlling the formation of single-molecule junctions by means of electrochemically reducing two axial-diazonium terminal groups on a molecule, thereby producing direct Au-C covalent bonds in-situ between the molecule and gold electrodes. Step length analysis shows that the molecular junction is significantly more stable, and can be pulled over a longer distance than a comparable junction created with amine anchoring bonds. The stability of the junction is explained by the calculated lower binding energy associated with the direct Au-C bond compared with the Au-N bond. Finally, the third project investigates the role that molecular conformation plays in the conductance of oligothiophene single-molecule junctions. Ethyl substituted oligothiophenes were measured and found to exhibit temperature dependent conductance and transition voltage for molecules with between two and six repeat units. While the molecule with only one repeat unit shows temperature invariant behavior. Density functional theory calculations show that at higher temperatures the oligomers with multiple repeat units assume a more planar conformation, which increases the conjugation length and decreases the effective energy barrier of the junction.
ContributorsHines, Thomas (Author) / Tao, Nongjian (Thesis advisor) / Li, Jian (Thesis advisor) / Mujica, Vladimiro (Committee member) / Allee, David (Committee member) / Arizona State University (Publisher)
Created2013