Ladungstransport in Nanoteilchen-Ketten

• Applicants: Professor Dr. Rolf J. Haug; Professor Dr. Jürgen Parisi
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2007 to 2011
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 42902037

• Final Report Year: 2011

Final Report Abstract

The aim of this project was to get a better understanding of the physics of charge transport in one-dimensional (1D) chains of nanoparticles (diameter 20 nm) stabilized with a citrate coating. We have fabricated three types of devices: (i) the gap between the gold electrodes was bridged by a single gold particle; (ii) the gap between the gold electrodes was bridged by a chain of particles; (iii) a chain of particles located on a SiO2 /Si substrate was contacted via a probing tip setup. The gap between the gold electrodes amounts to several ten nanometers. It was formed by focused ion beam patterning. For the positioning of the NP between the gold electrodes (or on the substrate), we have developed a procedure where, via pinning the edge of the droplet of a particle solution at the appropriate position between the Au electrodes, a concentric ensemble of particles forms along the droplet edge. To understand the physics of charge transport in the chain of particles, we have initially considered a single particle configuration. The current-voltage (I − V ) characteristics are described by I ∼ V ζ with scaling exponent ζ ≈ 1, which we have interpreted as charge transport in a 1D path. We observed temporal current fluctuations at a constant bias voltage, where the amplitude of these is proportional to the voltage applied. A Fourier analysis showed that the higher harmonics sn of the fluctuations follow the power law sn ∼ f µ (with µ=-0.62 in the range 3-30 mHz and µ=-1.08 in the range 30-100 mHz). We found that the current fluctuations derive from conformational changes in the citrate molecules induced by a charge transfer across the molecules. Similar (compared to single particle device) charge transport and current fluctuations at constant bias voltage were found for the 1D chains of NPs (8-10 particles) that have bridged the Au electrodes. Moreover, we found similar fluctuations of the differential conductance attributed to the interaction between mechanical motion of citrate molecules (molecular vibrations) and electron transport at the atomic scale. The electron-vibration interaction leads to an increase in the junction conductance via the opening of an additional tunneling channel for electrons that lost energy to a vibration mode, and to a decrease of junction conductance due to the limit of the electron transmission probability by backscattering of electrons that lose energy to a vibration mode. The observed increase of the current fluctuation in the molecule junctions upon growing electric field originates from inelastic excitations of the vibration modes of single molecules. We found that, for all devices where the particles have bridged the Au electrodes, the conductance of these is about the same at the measurement of the second I − V curve and always decreased at the third and the following measurements. On the contrary, for the devices where the chain of particles was contacted via probing tip setup, the conductance of these always increased at the following measurement of the I − V curve. We assume that both, length and shape of the chain do not return to the initial state after a measurement, and each I − V curve measured afterwards is determined by the new peculiar interplay between the electrostatic, elastic restoring, and damping forces acting between the particles. We have clearly demonstrated that, in particle chains with disorder, one junction can always be predominant in the conductance of the total chain, in particular, that one with the largest value of resistance. The latter exponentially depends on the distance between the particles. The location of the dominant junction in the chain can change along the chain, depending on the bias voltage and on the number of the measurement. We conclude that the negative surface charge of the citrate layer located on the particles is predominant in the electrostatic force, and that causes the strong displacement of particles in chain. The latter leads to additional conformational changes in citrate molecules, and, consequently, to additional fluctuations between high and low conducting states.

Publications

• A simple method for filling nanogap electrodes with polymer, Rev. Sci. Instrum. 80, 033902 (2009)
  L. V. Govor, G. H. Bauer, and J. Parisi
  
• Formation of close-packed nanoparticle chains, ACS Appl. Mater. Interfaces 1 (2), 488 (2009)
  L. V. Govor
  
• Self-assembled patterns from evaporating layered fluids, J. Phys.: Condens. Matter 21, 264015 (2009)
  L. V. Govor, J. Parisi, G. H. Bauer and G. Reiter
  
• Charge transport through a single particle located in between nanogap electrodes, Phys. Lett. A 374, 3328 (2010)
  L. V. Govor, G. H. Bauer, T. Lüdtke, R. J. Haug, and J. Parisi
  
• Charge transport through chains of nanoparticles, Physica E 42, 2830 (2010)
  T. Lüdtke, P. Mirovsky, R. Hüther, L.V. Govor, G. H. Bauer, J. Parisi, and R. J. Haug
  
• Conductance fluctuations in metal-nanoparticle-metal junctions, Phys. Rev. B 82, 155437 (2010)
  L. V. Govor, G. H. Bauer, G. Reiter, and J. Parisi
  
• Controllable positioning of a single particle in between nanogap electrodes, Rev. Sci. Instrum. 81, 106108 (2010)
  L. V. Govor, G. H. Bauer, and J. Parisi
  
• Current fluctuations in chain of nanoparticles, Phys. Lett. A 375, 4041 (2011)
  L. V. Govor, G. H. Bauer, T. Lüdtke , R. J. Haug, and J. Parisi
  
• Transport in nanoparticle chains influenced by reordering, Phys. Lett. A 375, 2079 (2011)
  T. Lüdtke, P. Mirovsky, R. Hüther, L. V. Govor, G. H. Bauer, J. Parisi, and R. J. Haug