ARTICLE pubs.acs.org/JPCC
Length and Temperature Dependent Conduction of Ruthenium-Containing Redox-Active Molecular Wires Liang Luo,† Ahmed Benameur,‡ Pierre Brignou,‡ Seong Ho Choi,† Stephane Rigaut,*,‡ and C. Daniel Frisbie*,† † ‡
Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, United States UMR 6226, CNRS-Universite de Rennes 1, Sciences Chimiques de Rennes, Campus de Beaulieu; F-35042 Rennes Cedex, France ABSTRACT: We report the electrical transport characteristics of two series of linear ruthenium(II) bis(σ-arylacetylide) molecular wires, RunM and RunH (n = 1, 2, 3), consisting of multiple redox-active Ru(II) centers and different saturated side chains (M = CH2, and H = O(CH2)6) with lengths up to 6.0 nm. The self-assembled monolayers of these molecular wires on Au surfaces were comprehensively characterized by ellipsometry, X-ray photoelectron spectroscopy (XPS), reflection absorption infrared spectroscopy (RA-FTIR), and cyclic voltammetry. The resistance and current (I)voltage (V) characteristics of these molecular wires were measured as a function of length and temperature using conducting probe atomic force microscopy (CP-AFM). Both series of molecular wires exhibited very weak length dependence of the wire resistance, the β value of RunM being 1.02 nm1, and that of RunH being 1.64 nm1, indicating a high degree of electronic coupling between the redox centers. Further analysis of IV characteristics revealed that the charge transport in RunM junctions was direct tunneling, but in RunH (n = 2, 3) junctions with the long chains the mechanism was thermally activated hopping, consistent with the temperature-dependent conduction measurement.
’ INTRODUCTION Molecular electronics is an active domain in the science of nanometer-scaled systems14 and is motivated by a variety of intriguing discoveries in molecular junctions including electrical rectification,58 switching,911 and Coulomb blockade effects.1214 A central focus of molecular electronics is to measure, understand, and control charge transport through molecules connected to electrodes.2,3,15 Connecting molecules between electrodes remains a clear challenge because of the nanoscale dimensions. Nevertheless, multiple strategies have been developed to form such molecular junctions reliably.1518 Direct measurements of molecular transport characteristics enable better understanding of the dependence of molecular conduction on bonding architecture and molecular energy levels, which will ultimately influence the design of molecular conductors for the applications of photovoltaics,19,20 lightemitting diodes,21,22 and transistors.2325 Recently, there has been increased interest in investigating length- and temperature-dependent conduction of long conjugated molecular wires connected to metal electrodes.2631 It is well understood that with increased molecular length, there is a transition of conduction mechanism from direct tunneling to hopping, where charge is injected into the molecular wires. Improved charge transport of molecular wires has also been achieved in several studies by incorporation of single or multiple redox centers.3235 In addition, incorporating redox centers into molecular wires can enhance the localization of charge carriers within the molecular wires. As a result, the conduction mechanism changes to sequential tunneling controlled by distinct charged states, r 2011 American Chemical Society
and Coulomb blockade behavior is observed when the charging energy overcomes thermal energy.1214 We have previously reported the transport behavior of a series of redox active conjugated molecular wires as a function of molecular length.32 The molecular wires consist of Ru(II) bis(σarylacetylide) covalently coupled to isocyanide head groups and ranging in length from 2.4 to 4.9 nm. The conduction of these wires has very weak length dependence, consistent with a high degree of electronic coupling between redox centers.36,37 Coulomb blockade-like behavior was observed in low-temperature (5 K) experiments. To investigate further the transport mechanism of molecular wires incorporating redox centers, we have performed a more comprehensive study on length- and temperature-dependent conduction of Ru(II)-incorporated redox-active molecular wires, the structures of which are shown in Figure 1. The new molecular wires, RunM and RunH (M = CH2, and H = O(CH2)6), have the same covalently coupled Ru(II) bis(σ-arylacetylide) complexes as before, but the surface linking group has been changed to thioacetate to reduce contact resistances.38 In addition, saturated aliphatic side chains with different lengths (CH2 and O(CH2)6) are inserted between the redox centers and the surface linkers to enhance the localization of charges on the redox sites.12,13,39,40
Received: August 1, 2011 Revised: September 2, 2011 Published: September 06, 2011 19955
dx.doi.org/10.1021/jp207336v | J. Phys. Chem. C 2011, 115, 19955–19961
The Journal of Physical Chemistry C
ARTICLE
Figure 1. Structures of Ru(II)-incorporated molecular wires used in the study.
Prior to junction formation, the self-assembled monolayers (SAMs) of all molecular wires on Au substrates were comprehensively characterized using reflectionabsorption infrared spectroscopy (RA-FTIR), ellipsometry, X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry. The characterization results indicated that all wires were well-assembled with a surface coverage of (0.8 to 2) 1013 molecules/cm2. Using conducting probe atomic force microscopy (CP-AFM) to measure the current (I)voltage (V) characteristics, we found that both series of molecular wires exhibit very weak length dependence of the wire resistance at room temperature, consistent with a high degree of electronic communication between the redox centers. Further analysis of IV characteristics and temperaturedependent studies demonstrated that charge transport on RunM wires was still dominated by direct tunneling, but on Ru2H and Ru3H wires the charge transport was thermally activated hopping, indicating that charges were injected into the wires. These studies demonstrate that the charge transport mechanism and IV characteristics in these conductive, multimetal center wires can be controlled by proper designation of molecular wire architectures, especially the structure of the electrode linking group.
’ EXPERIMENTAL SECTION Sample Preparation. The synthesis of these molecular wires has been reported elsewhere.41 To prepare the molecular wire SAMs, we dissolved the desired Ru molecule (1 mg) in 5 mL of toluene at room temperature, followed by the addtion of triethyl amine (15 μL). The mixture was incubated for 10 min before immersing a freshly evaporated Au substrate. After different growth periods (18 h for Ru1M, Ru2M, and Ru3M; 9 h for Ru1H, Ru2H, and Ru3H) in the absence of light, the substrates were rinsed thoroughly with toluene and ethanol and then dried under a gentle stream of nitrogen. For molecular wires Ru3M and Ru3H, a mixed solvent of toluene and chloroform (v/v 2:1), was used instead of pure toluene to increase the solubility of the molecules. SAM Characterization. Prior to junction formation and electrical measurement, the SAMs of different molecular wires on Au substrates were extensively characterized by RA-FTIR, ellipsometry, XPS, and cyclic voltammetry. To monitor the
acetylide stretching modes (CtC) in SAMs of the Ru wires, we took RA-FTIR with a Nicolet Series II Magna-IR system 750 FTIR with a Harrick Seagull accessory for grazing angle specular reflectance measurements. The infrared beam was incident at 84° with respect to the surface normal. A total of 2000 scans was collected at 1.0 cm1 resolution. Monolayer thicknesses were determined by ellipsometry and XPS. Ellipsometry measurements of the polarization angles Ψ and Δ were taken as a function of wavelength (λ) between 600 and 990 nm at an incident angle of 65°. The indices of refraction (n) and extinction coefficients (k) were assumed to be 1.45 and 0 based on previous experience. XPS spectra were taken on an SSX-100 system (Surface Science Instruments) equipped with a monochromatic Al Kα X-ray source (200 W), a hemispherical sector analyzer (HAS), and a resistive anode detector under ultrahigh vacuum (
