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Bingqian Xu

Publications and source records attributed to Bingqian Xu.

11 recordsLinked to original sources
PubMed2006

Conductance of single alkanedithiols: conduction mechanism and effect of molecule-electrode contacts.

The conductance of single alkanedithiols covalently bound to gold electrodes has been studied by statistical analysis of repeatedly created molecular junctions. For each molecule, the conductance histogram reveals two sets of well-defined peaks, corresponding to two different conductance values. We have found that (1) both conductance values decrease exponentially with the molecular length with an identical decay constant, beta approximately equal to 0.84 A(-1), but with a factor of 5 difference in the prefactor of the exponential function. (2) The current-voltage curves of the two sets can be fit with the Simmons tunneling model. (3) Both conductance values are independent of temperature (between -5 and 60 degrees C) and the solvent. (4) Despite the difference in the conductance, the forces required to break the molecular junctions are the same, 1.5 nN. These observations lead us to believe that the conduction mechanism in alkanedithiols is due to electron tunneling or superexchange via the bonds along the molecules, and the two sets of conductance peaks are due to two different microscopic configurations of the molecule-electrode contacts.

Journal Article↗
PubMed2006

Measurement of current-induced local heating in a single molecule junction.

We have studied the current-induced local heating effects in single molecules covalently bound to two electrodes by measuring the force required to break the molecule-electrode bonds under various conditions. The breakdown process is thermally activated, which is used to extract the effective temperature of the molecular junction as a function of applied bias voltage. We have also performed first-principles calculations of both local heating and current-induced force effects, and the results are in good agreement with the experimental findings.

Electric Impedance↗
PubMed2006

Controlling charge transport in single molecules using electrochemical gate.

We have studied charge transport through single molecules covalently bound to two gold electrodes in electrolytes by applying a voltage between the two electrodes and a reference electrode (gate). This electrochemical gating can effectively control the current through the molecules, depending on the electronic properties of the molecules. For electrochemically inactive molecules, such as 4,4'-bipyridine and 1,4'-benzenedithiol, the gate voltage influences the transport current only slightly (less than 30%). This lack of significant gate effect is attributed to the large LUMO-HOMO gaps of the molecules and the screening of the gate field by the two electrodes. For nitro-oligo(phenylene ethynylene) (OPE-NO2), which undergoes multiple irreversible reductions at negative gate voltages, the current through the molecules can be modulated several folds by the gate. This gate effect is irreversible and associated with the reduction of the NO2 group to different products that have different electron withdrawing capabilities from the conjugate backbone of the molecule. The most interesting molecules are perylene tetracarboxylic diimide compounds (PTCDI), which exhibit fully reversible redox reactions. The current through PTCDI can be reversibly varied and controlled over three orders of magnitude with the gate. Such a large gate effect is related to a redox state-mediated electron transport process.

Journal Article↗
PubMed2005

Study of single-nucleotide polymorphisms by means of electrical conductance measurements.

Understanding the complexities of DNA has been a hallmark of science for over a half century, and one of the important topics in DNA research is recognizing the occurrence of mutations in the base-stack. In this article, we present a study of SNPs by direct-contact electrical measurements to a single DNA duplex. We have used short, 11- and 12-bp dsDNA to investigate the change in conductance that occurs if a single base pair, a single base, or two separate bases in the stack are modified. All measurements are carried out in aqueous solution with the DNA chemically bound to the electrodes. These measurements demonstrate that the presence of a single base pair mismatch can be identified by the conductance of the molecule and can cause a change in the conductance of dsDNA by as much as an order of magnitude, depending on the specific details of the double helix and the single nucleotide polymorphism.

DNA↗
PubMed2005

Large gate modulation in the current of a room temperature single molecule transistor.

We have demonstrated a single molecule field effect transistor (FET) which consists of a redox molecule (perylene tetracarboxylic diimide) covalently bonded to a source and drain electrode and an electrochemical gate. By adjusting the gate voltage, the energy levels of empty molecular states are shifted to the Fermi level of the source and drain electrodes. This results in a nearly 3 orders of magnitude increase in the source-drain current, in the fashion of an n-type FET. The large current increase is attributed to an electron transport mediated by the lowest empty molecular energy level when it lines up with the Fermi level.

Journal Article↗
PubMed2004

Conductance titration of single-peptide molecules.

We have measured the conductance of single peptides covalently bonded to two Au electrodes via S-Au bonds by repeatedly forming a large number of molecular junctions. The conductance decreases exponentially with the peptide length, with a decay constant of beta = 0.9 +/- 0.1 A-1, suggesting that tunneling is the mechanism of electron transport in the peptides. The conductance of the peptides is sensitive to the solution pH, due to the protonation/deprotonation of the amine and carboxyl groups of the peptides, which provides titration measurements based on single-molecule conductance.

Electric Conductivity↗
PubMed2003

Measurements of single-molecule electromechanical properties.

The electromechanical properties of a single molecule covalently attached to two gold electrodes are studied by simultaneously measuring the conductance and the force during the stretching of the molecule. The conductance, the spring constant of the molecular junction, and the dependence of the conductance on the stretching force are determined. Like the conductance, the spring constant of a molecule depends also on the molecule-electrode contacts. The forces required to break the molecule-gold contacts are 1.5 nN for alkanedithiols and 0.8 nN for 4,4' bipyridine, indicating that the breakdowns take place at the Au-Au bond and at the N-Au bond, respectively.

Journal Article↗
PubMed2003

Measurement of single-molecule resistance by repeated formation of molecular junctions.

The conductance of a single molecule connected to two gold electrodes was determined by repeatedly forming thousands of gold-molecule-gold junctions. Conductance histograms revealed well-defined peaks at integer multiples of a fundamental conductance value, which was used to identify the conductance of a single molecule. The resistances near zero bias were 10.5 +/- 0.5, 51 +/- 5, 630 +/- 50, and 1.3 +/- 0.1 megohms for hexanedithiol, octanedithiol, decanedithiol, and 4,4' bipyridine, respectively. The tunneling decay constant (betaN) for N-alkanedithiols was 1.0 +/- 0.1 per carbon atom and was weakly dependent on the applied bias. The resistance and betaN values are consistent with first-principles calculations.

Journal Article↗
PubMed2002

Controlling the conductance of atomically thin metal wires with electrochemical potential.

We report on the study of quantum transport in atomically thin Au wires suspended between two Au electrodes by modulating the electrochemical potential of the wires in various electrolytes. The potential modulation induces a conductance modulation with a phase shift that is always approximately 180 degrees, meaning that an increase in the potential always causes a decrease in the conductance. The amplitude of the induced conductance modulation, however, depends on several parameters. First, it depends on the atomic configurations of the individual wires. Second, the relative amplitude, defined as the ratio of the conductance modulation amplitude to the conductance, decreases as the diameter of the wire increases. Third, it depends on whether anion adsorption is present. In the absence of anion adsorption, it is approximately 0.55G(0) (G(0) = 2e(2)/h) per V of potential modulation, for a wire with conductance quantized near 1G(0). This double layer charging-induced conductance modulation can be attributed to a change in the effective diameter of the wire. In the presence of anion adsorption, the amplitude is much larger (e.g., approximately 1.6G(0)/V when I(-) adsorption takes place) and correlates well with the strength of the adsorption, which is due to the scattering of conduction electrons by the adsorbed anions.

Journal Article↗