Characterizing the Metal–SAM Interface in Tunneling Junctions

his paper investigates the influence of the interface between a gold or silver metal electrode and an n-alkyl SAM (supported on that electrode) on the rate of charge transport across junctions with structure Met(Au or Ag)TS/A(CH2)nH//Ga2O3/EGaIn by comparing measurements of current density, J(V), for Met/AR = Au/thiolate (Au/SR), Ag/thiolate (Ag/SR), Ag/carboxylate (Ag/O2CR), and Au/acetylene (Au/C≡CR), where R is an n-alkyl group. Values of J0 and β (from the Simmons equation) were indistinguishable for these four interfaces. Since the anchoring groups, A, have large differences in their physical and electronic properties, the observation that they are indistinguishable in their influence on the injection current, J0 (V = 0.5) indicates that these four Met/A interfaces do not contribute to the shape of the tunneling barrier in a way that influences J(V).Chemistry and Chemical Biolog

Charge transport properties of water dispersible multiwall carbon nanotube-polyaniline composites

The transmission electron microscopy images of in situ prepared multiwall carbon nanotubes (MWNTs) and polyaniline (PANI) composites show that nanotubes are well dispersed in aqueous medium, and the nanofibers of PANI facilitate intertube transport. Although low temperature transport indicates variable range hopping (VRH) mechanism, the dc and ac conductivity become temperature independent as the MWNT content increases. The onset frequency for the increase in conductivity is observed to be strongly dependent on the MWNT weight percent, and the ac conductivity can be scaled onto a master curve. The negative magnetoresistance is attributed to the forward interference scattering mechanism in VRH transport.Peer reviewe

The Origin of the Odd–Even Effect in the Tunneling Rates across EGaIn Junctions with Self-Assembled Monolayers (SAMs) of <i>n</i>‑Alkanethiolates

Odd–even effects in molecular junctions with self-assembled monolayers (SAMs) of <i>n</i>-alkanethiolates have been rarely observed. It is challenging to pinpoint the origin of odd–even effects and address the following question: are the odd–even effects an interface effect, caused by the intrinsic properties of the SAMs, or a combination of both? This paper describes the odd–even effects in SAM-based tunnel junctions of the form Ag^A‑TS-SC_<i>n</i>//GaO_<i>x</i>/EGaIn junctions with a large range of molecular lengths (<i>n</i> = 2 to 18) that are characterized by both AC and DC methods along with a detailed statistical analysis of the data. This combination of techniques allowed us to separate interface effects from the contributions of the SAMs and to show that the odd–even effect observed in the value of <i>J</i> obtained by DC-methods are caused by the intrinsic properties of the SAMs. Impedance spectroscopy (an AC technique) allowed us to analyze the SAM resistance (<i>R</i>_SAM), SAM capacitance (<i>C</i>_SAM), and contact resistance, within the junctions separately. We found clear odd–even effects in the values of both <i>R</i>_SAM and <i>C</i>_SAM, but the odd–even effect in contact resistance is very weak (and not responsible for the observed odd–even effect in the current densities obtained by <i>J</i>(V) measurements). Therefore, the odd–even effects in Ag^A‑TS-SC_<i>n</i>//GaO_<i>x</i>/EGaIn junctions are attributed to the properties of the SAMs and SAM–electrode interactions which both determine the shape of the tunneling barrier

Electric field activated nonlinear anisotropic charge transport in doped polypyrrole

Electric field activated nonlinear transport is investigated in polypyrrole thin film in both in-plane and out-of-plane geometries down to 5 K and strong anisotropy is observed. A morphological model is suggested to explain the anisotropy through inter-chain and intra-chain transport. The deviation from the variable range hopping at low temperature is accounted by fluctuation assisted transport. From Zabrodaskii plots, it is found that electric field can tune the transport from insulating to metallic regime. Glazman-Matveev model is used to describe the nonlinear conduction. Field scaling analysis shows that conductance data at different temperature falls on to a single curve. Nonlinearity exponent, m(T) and characteristic length, L-E are estimated to characterize the transport in both the geometries. (C) 2013 AIP Publishing LLC

Tuning charge transport across junctions of ferrocene-containing polymer brushes on ITO by controlling the brush thickness and the tether lengths

This paper describes the electrical characteristics of junctions of ferrocene (Fc)-containing polymer brushes (PBs) grafted on indium tin oxide (ITO) bottom electrodes contacted with eutectic gallium indium (EGaIn) top electrodes. We studied the charge transport phenomena across these junctions as a function of the PB thickness as well as the length of the tether (i.e., an alkyl chain side linker) between the Fc units and the polymethyl methacrylate backbone. Junctions with PBs with a thickness of 20 nm do rectify. Temperature dependent charge transport measurements revealed that the charge transport mechanism is dominated by space charge limited conduction (SCLC).ASTAR (Agency for Sci., Tech. and Research, S’pore)MOE (Min. of Education, S’pore)Published versio

Equivalent Circuits of a Self-Assembled Monolayer-Based Tunnel Junction Determined by Impedance Spectroscopy

The electrical characteristics of molecular tunnel junctions are normally determined by DC methods. Using these methods it is difficult to discriminate the contribution of each component of the junctions, e.g., the molecule–electrode contacts, protective layer (if present), or the SAM, to the electrical characteristics of the junctions. Here we show that frequency-dependent AC measurements, impedance spectroscopy, make it possible to separate the contribution of each component from each other. We studied junctions that consist of self-assembled monolayers (SAMs) of <i>n</i>-alkanethiolates (S­(CH₂)_<i>n</i>−1CH₃ ≡ SC_<i>n</i> with <i>n</i> = 8, 10, 12, or 14) of the form Ag^TS-SC_<i>n</i>//GaO_<i>x</i>/EGaIn (a protective thin (∼0.7 nm) layer of GaO_<i>x</i> forms spontaneously on the surface of EGaIn). The impedance data were fitted to an equivalent circuit consisting of a series resistor (<i>R</i>_S, which includes the SAM-electrode contact resistance), the capacitance of the SAM (<i>C</i>_SAM), and the resistance of the SAM (<i>R</i>_SAM). A plot of <i>R</i>_SAM vs <i>n</i>_C yielded a tunneling decay constant β of 1.03 ± 0.04 <i>n</i>_C^–1, which is similar to values determined by DC methods. The value of <i>C</i>_SAM is similar to previously reported values, and <i>R</i>_S (2.9–3.6 × 10^–2 Ω·cm²) is dominated by the SAM–top contact resistance (and not by the conductive layer of GaO_<i>x</i>) and independent of <i>n</i>_C. Using the values of <i>R</i>_SAM, we estimated the resistance per molecule <i>r</i> as a function of <i>n</i>_C, which are similar to values obtained by single molecule experiments. Thus, impedance measurements give detailed information regarding the electrical characteristics of the individual components of SAM-based junctions