Session G30: Focus Session: Electronic Transport in Organic Films

Scanning Tunneling Microscopy and Spectroscopy of Conjugated Oligomers at the Liquid-Solid Interface

Self-assembly - the spontaneous organization of molecules into stable, structurally well-defined aggregates - has been put forward as a possible paradigm for generating nanoscale templates under ambient conditions. A very convenient method for the formation of extended two-dimensional (2D) networks is physisorption at the liquid-solid interface. The preparation is relatively simple and scanning tunneling microscopy (STM) allows a detailed investigation of the structural aspects of the 2D patterns. A deep understanding and control of the spatial orientation and packing of pi-conjugated oligomers in self-assembled systems is indispensable for the development of future nanodevices. By means of STM, we have investigated the self-assembly of achiral and chiral pi-conjugated small organic molecules at the organic liquid-solid interface with submolecular resolution. In addition, by means of scanning tunneling spectroscopy (STS) we have investigated with molecular and submolecular resolution the electronic properties of isolated and stacked conjugated molecules at the liquid-solid interface. In the first part, we focus on the control of 2D molecular self-assembly of pi-conjugated systems driven by the molecular shape and/or by directional non-convalent interactions such as hydrogen bonding. Examples include alkylated molecules with a rhombus or triangle shaped pi-conjugated core, hydrogen bond forming p-phenylene vinylenes, and oligothiophenes. In the second part, we report on bias dependent imaging and STS experiments revealing information on the electronic properties of electron donor-acceptor-donor triads, and isolated and stacked oligothiophenes.   

Directing molecular traffic by means of a nano-engineered surface

The parallel manipulation of individual molecules is the ultimate goal of many current researches in nanosciences. A first route is the confinement of molecular motion until the continuum approximation breaks down and discrete-molecule effects appear, as observed in zeolite analogs or inside nanotubes. A bottom-up alternative consists in surface self-assemblies tailored at the molecular scale. Although most studies on nanostructured surfaces are still focused on their structure, the demonstrations of selective adsorptions inside single-molecule pores, the observation of rotating molecules inside an accidental bearing in a sub-monolayer film, or within self-assembled monolayers, represent pioneering first steps towards functional systems. In this context, we have demonstrated the sorting and routing of individual molecules via surface diffusion inside bottom-up designed molecular sieves. We monitor the operation in-situ and in real time at the single-molecule scale through fast variable-temperature scanning-tunneling microscopy (STM). These studies unravel the mechanisms of molecular filtering. This permits to derive molecular-design guidelines for tuning cavity or channel selectivity. We discuss the opportunity to control optically and locally the molecular motion, using tip-induced field enhancements.   

Determining the conductance of single molecular wire

We have designed a method for determining the conductance of an isolated molecular wire from the I-V characteristic of molecular junctions. The molecular diodes were 1 mm$^{2}$ in area and consist of self-assembled monolayer (SAM) from a mixture of the molecular wires and non-conducting molecules that are used as spacers; coupled to two opposite gold electrodes. We studied the I-V characteristic dependence of the fabricated diodes on the ratio, $r$ of wires/spacers. To obtain the number of molecular wires in the device we used multiple self-assemblies and titration techniques, as well as AFM of a small portion of the SAM surface. Our method was applied to a mixture of Me-BDT (methyl-bezenedithiol) molecules as wires and PT (pentathiol) molecules as spacers. For 10$^{-8 }<$ r $<$ 10$^{-3}$ we found that the device conductance is dominated by the molecular wires. From the current and obtained number of Me-BDT molecules in the device we determined the molecular conductance of Me-BDT to be 600 M$\Omega $, in good agreement with a theoretical tunneling model.   

Atomic-Scale Spectroscopy of Polydiacetylene Nanowires

In recent years, the appeal of organic electronic devices has spurred interest in conducting polymers, such as polydiacetylene. Polydiacetylene nanowires offer numerous possibilities for application in molecular electronics, and they are a model system for understanding conduction mechanisms in polymers. We have used scanning tunneling microscopy to investigate monolayer films of pentacosadiynoic acid formed on highly ordered pyrolytic graphite using the Langmuir-Blodgett and Langmuir-Schaeffer deposition techniques and subsequently polymerized to form polydiacetylene nanowires. By applying a novel method of microwave frequency mixing at the STM tip junction, we have obtained capacitance-voltage spectroscopic data to characterize the wires across a range of bias voltages, tunneling gap resistances, and microwave mixing frequencies.   

Self-Organized Single-Crystal Polythiophene Microwires

Here we show a well-faceted, high-quality 1D single-crystal poly (3-hexylthiophene), P3HT microwire with unprecedented electrical characteristics such as a low resistance (0.5 M$\Omega )$, a channel current as high as 25 $\mu $A, and a well-resolved gate modulation via solution growth. We find that 1D single-crystal P3HT microwires are formed spontaneously through facile self-assembly of individual polymer chains, adopting preferential well-ordered inter-chain stacking along the wire axis. Our findings indicate that $\pi $-conjugated polymer single-crystals are capable of very efficient charge transport. This approach could lead to the development of chemical and biological sensors which are efficiently capable of electrical and /or optical monitoring. This work was supported by the National Research Laboratory Program, a grant (F0004022) from Information Display R{\&}D Center under the 21st Century Frontier R{\&}D Program, the BK21 Program, and the Pohang Acceleratory Laboratory for providing the synchrotron radiation source at the 4C2, 3C2, and 8C1 beam lines.   

Bond resistances in molecular junctions

The description of molecular contacts is one of the hardest problems in modeling molecular junctions. In common approaches macroscopic leads ensure a finite potential drop and hence a driving force for the current. Recently, a different strategy is emerging where a steady-state DC current is forced in the molecule, by making resort to Lagrange multipliers, or by drawing a magnetic flux through the molecule. The strategy is promising, but two main problems remain to be solved: (1) the calculation of the potential drop needed to sustain the current, and (2) the definition of the potential profile along the molecule. Here the Joule law is used to evaluate the potential drop from the electrical power spent on the molecule, and continuity constraints for steady-state DC current are implemented to get information on the potential profile. Borrowing powerful concepts from the field of molecular spectroscopy, emphasis is put on the molecule, while clamping information about contacts in the molecular relaxation matrix. The molecule is described in a real-space approach, leading to a suggestive analogy between the molecule and an electrical circuit where resistances are associated with chemical bonds.   

Transport Properties of SAM Molecular Diodes with Structural Tunability

A new molecular engineering approach is used to fabricate molecular junctions from self-assembled-monolayers (SAM) sandwiched between gold electrodes, with structural tunability based on two-component solid-state mixtures of molecular wires (1,4 methane benzene-dithiol; Me-BDT, and molecular insulator spacers (1-pentanethiol; PT). The electrical transport of the fabricated SAM diodes was investigated at various temperatures versus the ratio r between the molecular wires and insulators. At r $<$ 10$^{-3}$ the diodes are dominated by the isolated molecular wires dispersed in the dielectric spacer matrix; from the conductivity vs. r we determined the value for the Me-BDT molecular resistance to be 4x10$^{8}$ Ohm. We also found that the activation energy in these devices is $\sim $50 meV at low bias and high temperatures; and injection barrier of $\sim $1.5 eV at intermediate bias and low temperatures. At r $>$ 10$^{-3}$ Me-BDT aggregates are formed in the PT matrix resulting in additional in-plane order and substantive changes in the transport properties.   

Electron Hopping in Conducting Polymers in the Presence Of Mobile Ions

We present the theoretical analysis for electrochemical transistors with the conductivity governed by the gate potential through bulk charging/discharging of the active channel. The predicted I(V) characteristics do not agree with the experimental dependencies for conducting polymer based transistors [1]. We suggest that the field effect in conducting polymers is related to their structural peculiarities. The large free volume within the polymer network enables ions to easily move into and out of the polymers. The main effect of ion insertion is breaking of the percolation network by removing critical hoping sites and, as a result, producing the conductor-nonconductor transition. The application of the present mechanism to the field effect in conducting polymers is discussed. [1] J. Liu, et al., \textbf{J. Appl. Phys. 92}, 6033-6038 (2002); A.J. Epstein, et al., \textbf{Current Applied Physics}, 2, 339-343 (2002); H. Okuzaki, et al., \textbf{Synth. Met. 137}, 947-948 (2003); F.C. Hsu, et al., to be published.