Understanding Of Charge Transport Research Articles

Organic Nanowires: Organic Nanowire Crystals Combine Excellent Device Performance and Mechanical Flexibility (Small 2/2011)

The cover picture shows scanning electron microscopy images of organic nanowire crystals, which are viewed by setting the electron beam voltage to the minimum value. The flexibility and elasticity of individual organic nanocrystals are investigated, and the Young's modulus of the nanocrystals is measured to be about 1.9 GPa using atomic force microscopy in tapping mode. The field-effect properties of the organic nanocrystals are studied at both the straight and bent states. The average mobility of bent crystals is higher than that of their straight counterparts, indicating that the intrinsic bending of organic crystals could improve device performance. These investigations facilitate our understanding of charge transport in organic semiconductors. They show considerable promise for a general way to study the mechanical properties of organic nanocrystals and for exciting applications in flexible electronics. For more information, please read the Communication “Organic Nanowire Crystals Combine Excellent Device Performance and Mechanical Flexibility” by W. Hu, T. Bjørnholm, and co-workers, beginning on page 189.

Spectroscopy of molecular junctions

In this tutorial review we present in detail recent studies in which molecular junctions were simultaneously probed by conductance measurements and optical spectroscopy methods such as electroluminescence (EL) and Raman scattering. The advantages of combining these experimental approaches to improve our understanding of charge transport through molecular junctions are discussed and routes for future developments are suggested.

Ballistic Electron Transport Through Au/TiOPc/GaAs and Au/HBC/GaAs Diodes

The understanding of charge transport through metal-organic and/or organic-semiconductor interfaces plays a significant role in the further development and improvement of molecular electronics. This paper presents a theoretical investigation of hot electron transport through organic films grown on GaAs. Ballistic electron transport through Au/TiOPc/GaAs and Au/HBC/GaAs is described by means of the Bell-Kaiser model. Good agreement between experimental results and the prediction of the heterostructure extension of the Bell-Kaiser model is reported showing that the Bell-Kaiser model extension can be used to describe the hot electron transport through metal-organic-semiconductor nanostructures.

Transport in organic semiconductors in large electric fields: From thermal activation to field emission

Understanding charge transport in organic semiconductors in large electric fields is relevant to many applications. We present transport measurements in organic field-effect transistors based on poly(3-hexylthiophene) and 6,13-bis(triisopropyl-silylethynyl) (TIPS) pentacene with short channels, from room temperature down to 4.2 K. Near 300 K transport in both systems is well described by thermally assisted hopping with Poole–Frenkel-type enhancement of the mobility. At low temperatures and large gate voltages, transport in both materials becomes nearly temperature independent, crossing over into field-driven tunneling. These data, particularly in TIPS-pentacene, show that great caution must be exercised when considering more exotic (e.g., Tomonaga–Luttinger liquid) interpretations of transport.

Using Bulk Heterojunction Field Effect Measurements to Understand Charge Transport in Solar Cell Materials

AbstractAmbipolar organic thin-film transistors (OTFTs) have been used to study the transport of charge carriers in bulk heterojunction (BHJ) organic photovoltaic devices. Active layers of phase separated blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM), have been chosen due to their use in performance BHJ organic photovoltaic devices as well as ease of device fabrication. A method for determining recombination rate after exciton dissociation and measurement of excess carrier lifetime has been reported by studying drain current behavior which yields carrier mobility, conductivity, and carrier concentration both in dark and AM1.5g illumination. Channel-length dependent measurements of the photocurrent show that significant recombination of separated charge carriers begins to occur at lengths greater than 10 μm. A recombination rate of cm-3 s-1 and a carrier lifetime of ≥ 8.8 ms has been calculated.

Probing local electric field and conformational switching in single-molecule break junctions

By performing inelastic electron tunneling spectroscopy on single-molecule junctions fabricated through electromigration, we were able to detect polarity-dependent frequency shifts in the molecular vibrational spectra. This observation in conjunction with a simple theoretical model is used to infer the magnitude of local electric field (a crucial and previously inaccessible parameter for understanding charge transport in single-molecule junctions) on the molecule. Electric field induced conformational changes are also detected within this set of measurements.

Deformation and Hyperfine Structures of Dendrimers Investigated by Scanning Tunneling Microscopy

Scanning tunneling microscopy (STM) is known to provide the highest spatial resolution in real space imaging of materials, and its applications are most common among conductive and semiconductive systems. The high tunneling barrier of insulators diminishes the tunneling probability and thus compromises STM's resolution. This work introduces a simple method to approach this problem, by using STM for high-resolution imaging of insulating materials such as the fourth and fifth generations of poly(amidoamine) hydroxyl-terminated dendrimers. The tunneling barrier is lowered by precoordination with Cu(II) or Pt(II) ions, enabling intramolecular hyperfine features be resolved with 0.2 nm resolution. The spatial distribution, size, and overall number of hyperfine features are consistent with the location of dendrimer termini. The immobilization process deforms dendrimers from the spherical geometry in solution phase to asymmetrical domes in ambient. The ultrahigh vacuum (UHV) environment leads to a higher degree of deformation with reduction of volume. The high-resolution images enable the determination of fundamental parameters of individual dendrimers, including axis, height, asymmetry, and volume. From STM spectroscopy and prior knowledge of dendritic systems, the STM imaging mechanism under UHV is consistent with metal(0) nanoparticles encapsulated by dendrimers, while ambient imaging is most likely via metal-ion-facilitated charge transport. The results from this investigation bring us one step closer toward structural characterization at atomistic level and should enable direct comparison of dendrimer structures with simulations, and deepen our understanding of charge transport in dendrimer systems.

The density of states in thin film copper phthalocyanine measured by Kelvin probe force microscopy

The density of states (DOS) is an important factor in understanding charge transport in organic semiconductors. We use Kelvin probe force microscopy to find the DOS in thin films of copper phthalocyanine (CuPC). We find an exponential DOS with a characteristic energy of 0.11 eV over a 0.5 eV range of the highest occupied molecular orbital of CuPC. We also find that the technique is limited by charge trapping and hysteresis at low DOSs, and nonuniform potential profiles within the CuPC film at high DOSs.

Molecular junctions based on aromatic coupling

If individual molecules are to be used as building blocks for electronic devices, it will be essential to understand charge transport at the level of single molecules. Most existing experiments rely on the synthesis of functional rod-like molecules with chemical linker groups at both ends to provide strong, covalent anchoring to the source and drain contacts. This approach has proved very successful, providing quantitative measures of single-molecule conductance, and demonstrating rectification and switching at the single-molecule level. However, the influence of intermolecular interactions on the formation and operation of molecular junctions has been overlooked. Here we report the use of oligo-phenylene ethynylene molecules as a model system, and establish that molecular junctions can still form when one of the chemical linker groups is displaced or even fully removed. Our results demonstrate that aromatic pi-pi coupling between adjacent molecules is efficient enough to allow for the controlled formation of molecular bridges between nearby electrodes.

Electron Transfer Mediating Properties of Hydrocarbons as a Function of Chain Length: A Differential Scanning Conductive Tip Atomic Force Microscopy Investigation

The development of novel molecular and biomolecular devices relies on the understanding of charge transport across molecule-substrate interfaces. However, different strategies adopted so far for fabricating and studying transport through metal-molecule-metal junctions yield values for the transport coefficients that differ by up to orders of magnitude even for the same junction. Conductive tip atomic force microscopy (CT-AFM) allows for the simultaneous measurement of transport and morphological properties of molecular assemblies, but absolute transport measurements depend on the nature of the AFM tip-molecule contact. In this work we present a differential approach to the study of metal-molecule-metal junctions based on the combination of AFM-driven nanolithography and CT-AFM. We nanograft patches of alkanethiol molecules in a self-assembled monolayer of alkanethiol molecules of different chain length and measure by CT-AFM the morphology and the transport properties of the nanopatches and of the reference layer. The method allows for the determination of the differential resistance between the two molecular layers and is thus independent of environmental factors. The validity of this approach is demonstrated by measuring the tunneling decay constant of alkanethiols as a function of their length.

Structure of Rhodamine 6G−DNA Complexes from Molecular Dynamics Simulations

Chromophore-DNA complexes are useful for understanding charge transport along pi-stacks once their structural properties have been clarified. We studied two rhodamine 6G semicapping complexes with 15-mer B-DNA duplexes to determine the preferred orientation of the dye with respect to the neighboring base pair. For each of these systems, two distinct chromophore alignments were identified and quantified in terms of base-step parameters. The obtained geometries agree well with those derived from an NMR structure refinement of similar complexes. Cross-correlation analysis of the base-step parameters shows that slide and twist are highly interdependent during the structural transition from one conformation to the other.

A Kink-Soliton Model of Charge Transport through Microtubular Cytoskeleton

Contemporary trends in science and technology are characterized by integration of biological and technical systems, like in nanotechnology, nanobiology, and quantum medicine. In our case, we were motivated by a necessity to understand charge transport through microtubular cytoskeleton as a constitutive part of acupuncture system. The high frequency component of acupuncture currents, widely exploited in microwave resonance stimulation of acupuncture system in the past decade, implies that explanation of the cytoplasmatic conductivity should be sought in the framework of Frohlich theory. Accordingly, in this paper we critically analyze the problem of the microwave coherent longitudinal electrical oscillations as a theoretical basis for understanding soliton phenomena in microtubules, showing that charged kink-soliton nonlinear microtubular excitations might be a good candidate for charge transport in microtubules.

Low-temperature field effect in a crystalline organic material

Molecular organic materials offer the promise of novel electronic devices but also present challenges for understanding charge transport in narrow band systems. Low temperature studies elucidate fundamental transport processes. We report the lowest temperature field effect transport results on a crystalline oligomeric organic material, rubrene. We find field effect switching with on-off ratio up to 10^7 at temperatures down to 10 K. Gated transport shows a factor of ~10 suppression of the thermal activation energy in 10-50 K range and nearly temperature independent resistivity below 10 K.

Supramolecular rows of discotic liquid crystal on a metal surface

Anchoring of discotic liquid crystals on atomically flat surfaces is important in the field of film growth and for a better understanding of charge transport across interfaces. The presented comparative scanning tunneling microscopy study shows that self-organized monolayers of 2,3,6,7,10,11-undecalkoxy-triphenylene (noted T11) reveal a drastically different packing on Au(111) than on highly oriented pyrolitic graphite (HOPG). A hexagonal structure is found on HOPG while on Au(111) T11 molecules form periodic parallel rows of dimers. These rows consist of molecules lying flat on the surface and aligned along specific directions of the underlying substrate. The presented results suggest that the interactions between the alkoxy chains of T11 and the Au(111) substrate drive the formation of the observed supramolecular rows.

Full band Monte Carlo simulation - beyond the semiclassical approach

A quantum mechanical extension of the full band ensemble Monte Carlo (MC) simulation method is presented. The new approach goes beyond the traditional semi-classical method generally used in MC simulations of charge transport in semiconductor materials and devices. The extension is necessary in high-field simulations of semiconductor materials with a complex unit cell, such as the hexagonal SiC polytypes or wurtzite GaN. Instead of complex unit cells the approach can also be used for super-cells, in order to understand charge transport at surfaces, around point defects, or in quantum wells.

Microscopic Modelling of Opto-Electronic Quantum Devices: A Predictive Simulation Tool

A microscopic analysis of non-equilibrium phenomena in unipolar quantum devices is presented. In particular, a global Monte Carlo simulation scheme (semiclassical as well as quantum) is employed, which allows us to directly access details of the three-dimensional carrier dynamics, without resorting to phenomenological parameters. Applications to state-of-the-art mid-infrared quantum-cascade lasers and novel far-infrared emitters are discussed. The extremely good agreement between theoretical results and experimental findings demonstrates that our approach is a valid and predictive tool for the understanding of charge transport in these quantum devices.

Effect of the polar groups in the non-degenerate π-conjugated pendant of vinyl copolymers

The interest of coploymeric electroluminescent diodes (PELDs) has much increased to improve the quantum efficiency. To understand charge transport and EL properties of the copolymer with non-degenerate π-conjugated pendants, new vinyl copolymers for light-emitting diodes with polar groups were prepared by radical copolymerization of vinyl(9-carbazole) and vinyl derivatives such as vinyl-benzyl chloride, styrene, (4-methyl)styrene.

Time of flight transient analysis in a molecular doped polymer using a dispersive-trapping formalism

The spatio-temporal spreading features of charge packets from time of flight (TOF) current transients in a molecular doped polymer system are analysed based on TOF current modelling and experimental results related to the thermal and electric field dependences of the dispersion parameters as well as the charge carrier mobility. The parameters describing the charge spatial spread are the velocity distributions within the moving charge packet, the trapping time and the trapping lengths. These parameters supplement our understanding of charge transport in these systems in addition to our previous experimental mobility investigations.

13C– 14N REAPDOR and 13C– 2D θ-REDOR NMR on a blend of tri- p-tolylamine and bisphenol- A-polycarbonate

Solid-state NMR experiments are performed on amorphous blends of tri- p-tolylamine (TTA) and polycarbonate. Amines that are similar to TTA are blended with polycarbonates and used as photoconducting components in xerography. Because of its simple molecular structure, TTA is a charge-transporting molecule that is used as a model to understand charge transport in photoconducting materials. Charge transfer between amine molecules must depend on the separation and relative orientation between amine molecules. However, little information is known about the structural relationship between the amine molecules and the polymer host, presumably because of the difficulties associated with characterizing amorphous materials. We report the results of 13C– 14N rotational-echo, adiabatic-passage, double-resonance (REAPDOR) and 13C– 2D θ-rotational-echo, double-resonance (θ-REDOR) magic-angle spinning NMR experiments on a TTA/polycarbonate blend. The experiments provide information about the organization of TTA relative to polycarbonate.

Twenty Years of Charge Transport Studies in Intercalated Graphite

Upon the occasion of the 10th ISIC conference, it seems appropriate to give a brief overview of some of the major advances made in our understanding of charge transport in intercalated graphite since the 1st Franco-American Conference on Graphite Intercalation Compounds held in 1977.