Since the discovery of highly conducting polyacetylene by Shirakawa, MacDiarmid, and Heeger in 1977, π-conjugated systems have attracted much attention as futuristic materials for the development and production of the next generation of electronics, that is, organic electronics. Conceptually, organic electronics are quite different from conventional inorganic solid state electronics because the structural versatility of organic semiconductors allows for the incorporation of functionality by molecular design. This versatility leads to a new era in the design of electronic devices. To date, the great number of π-conjugated semiconducting materials that have either been discovered or synthesized generate an exciting library of π-conjugated systems for use in organic electronics. 11 However, some key challenges for further advancement remain: the low mobility and stability of organic semiconductors, the lack of knowledge regarding structure property relationships for understanding the fundamental chemical aspects behind the structural design, and realization of desired properties. Organic field-effect transistors (OFETs) are a kind of device consisting of an organic semiconducting layer, a gate insulator layer, and three terminals (drain, source, and gate electrodes). OFETs are not only essential building blocks for the next generation of cheap and flexible organic circuits, but they also provide an important insight into the charge transport of πconjugated systems. Therefore, they act as strong tools for the exploration of the structure property relationships of πconjugated systems, such as parameters of field-effect mobility (μ, the drift velocity of carriers under unit electric field), current on/off ratio (the ratio of the maximum on-state current to the minimum off-state current), and threshold voltage (the minimum gate voltage that is required to turn on the transistor). 17 Since the discovery of OFETs in the 1980s, they have attracted much attention. Research onOFETs includes the discovery, design, and synthesis of π-conjugated systems for OFETs, device optimization, development of applications in radio frequency identification (RFID) tags, flexible displays, electronic papers, sensors, and so forth. It is beyond the scope of this review to cover all aspects of π-conjugated systems; hence, our focus will be on the performance analysis of π-conjugated systems in OFETs. This should make it possible to extract information regarding the fundamental merit of semiconducting π-conjugated materials and capture what is needed for newmaterials and what is the synthesis orientation of newπ-conjugated systems. In fact, for a new science with many practical applications, the field of organic electronics is progressing extremely rapidly. For example, using “organic field effect transistor” or “organic field effect transistors” as the query keywords to search the Web of Science citation database, it is possible to show the distribution of papers over recent years as shown in Figure 1A. It is very clear

Semiconducting π-conjugated systems in field-effect transistors: a material odyssey of organic electronics.

We provide a broad review of fundamental electronic properties of two-dimensional graphene with the emphasis on density and temperature dependent carrier transport in doped or gated graphene structures. A salient feature of our review is a critical comparison between carrier transport in graphene and in two-dimensional semiconductor systems (e.g. heterostructures, quantum wells, inversion layers) so that the unique features of graphene electronic properties arising from its gap- less, massless, chiral Dirac spectrum are highlighted. Experiment and theory as well as quantum and semi-classical transport are discussed in a synergistic manner in order to provide a unified and comprehensive perspective. Although the emphasis of the review is on those aspects of graphene transport where reasonable consensus exists in the literature, open questions are discussed as well. Various physical mechanisms controlling transport are described in depth including long- range charged impurity scattering, screening, short-range defect scattering, phonon scattering, many-body effects, Klein tunneling, minimum conductivity at the Dirac point, electron-hole puddle formation, p-n junctions, localization, percolation, quantum-classical crossover, midgap states, quantum Hall effects, and other phenomena.

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Electronic transport in two-dimensional graphene

A review was presented to demonstrate a historical description of the synthesis of light-emitting conjugated polymers for applications in electroluminescent devices. Electroluminescence (EL) was first reported in poly(para-phenylene vinylene) (PPV) in 1990 and researchers continued to make significant efforts to develop conjugated materials as the active units in light-emitting devices (LED) to be used in display applications. Conjugated oligomers were used as luminescent materials and as models for conjugated polymers in the review. Oligomers were used to demonstrate a structure and property relationship to determine a key polymer property or to demonstrate a technique that was to be applied to polymers. The review focused on demonstrating the way polymer structures were made and the way their properties were controlled by intelligent and rational and synthetic design.

Synthesis of Light-Emitting Conjugated Polymers for Applications in Electroluminescent Devices

This article provides a review about electroluminescence from organic materials and deals in detail with organic light-emitting diodes (OLEDs), light-emitting electro-chemical cells (LECs) and electrogenerated chemilumi-nescence (ECL) reflecting different electrooptical appli-cations of conjugated materials. It is written from an organic chemist's point of view and pays particular attention to the development of organic materials involved in corresponding devices. In recent years a substantial amount of both academic and industrial research has been directed to organic electroluminescence in an effort to improve the processability and tunability of organic materials and the longevity of OLEDs and LECs. On the eve of the commercialization of organic electrolumi-nescence this review provides an overview of lifetimes and efficiencies attained and reflects materials and device concepts developed over the last decade. In this context electrogenerated chemiluminescence is discussed with respect to its importance as a versatile tool to simulate the fundamental electrochemical processes in OLEDs.

The electroluminescence of organic materials

Classical phase transitions occur when a physical system reaches a state below a critical temperature characterized by macroscopic order. Quantum phase transitions occur at absolute zero; they are induced by the change of an external parameter or coupling constant, and are driven by quantum fluctuations. Examples include transitions in quantum Hall systems, localization in Si-MOSFETs (metal oxide silicon field-effect transistors; ref. 4) and the superconductor-insulator transition in two-dimensional systems. Both classical and quantum critical points are governed by a diverging correlation length, although quantum systems possess additional correlations that do not have a classical counterpart. This phenomenon, known as entanglement, is the resource that enables quantum computation and communication. The role of entanglement at a phase transition is not captured by statistical mechanics-a complete classification of the critical many-body state requires the introduction of concepts from quantum information theory. Here we connect the theory of critical phenomena with quantum information by exploring the entangling resources of a system close to its quantum critical point. We demonstrate, for a class of one-dimensional magnetic systems, that entanglement shows scaling behaviour in the vicinity of the transition point.

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Scaling of entanglement close to a quantum phase transition

We have studied the zero magnetic field resistivity \ensuremath{\rho} of unique high-mobility two-dimensional electron systems in silicon. At very low electron density ${\mathit{n}}_{\mathit{s}}$ (but higher than some sample-dependent critical value ${\mathit{n}}_{\mathrm{cr}}$\ensuremath{\sim}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$), conventional weak localization is overpowered by a sharp drop of \ensuremath{\rho} by an order of magnitude with decreasing temperature below \ensuremath{\sim}1--2 K. No further evidence for electron localization is seen down to at least 20 mK. For ${\mathit{n}}_{\mathit{s}}$${\mathit{n}}_{\mathrm{cr}}$, the sample is insulating. The resistance is empirically found to scale with temperature both below and above ${\mathit{n}}_{\mathrm{cr}}$ with a single parameter that approaches zero at ${\mathit{n}}_{\mathit{s}}$=${\mathit{n}}_{\mathrm{cr}}$ suggesting a metal-insulator phase transition.

Possible metal-insulator transition at B=0 in two dimensions.

We have studied the temperature dependence of resistivity, \ensuremath{\rho}, for a two-dimensional electron system in silicon at low electron densities ${\mathit{n}}_{\mathit{s}}$\ensuremath{\sim}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$, near the metal-insulator transition The resistivity was empirically found to scale with a single parameter ${\mathit{T}}_{0}$, which approaches zero at some critical electron density ${\mathit{n}}_{\mathit{c}}$ and increases as a power ${\mathit{T}}_{0}$\ensuremath{\propto}\ensuremath{\Vert}${\mathit{n}}_{\mathit{s}}$-${\mathit{n}}_{\mathit{c}}$${\mathrm{\ensuremath{\Vert}}}^{\mathrm{\ensuremath{\beta}}}$ with \ensuremath{\beta}=16\ifmmode\pm\else\textpm\fi{}01 both in metallic (${\mathit{n}}_{\mathit{s}}$g${\mathit{n}}_{\mathit{c}}$) and insulating (${\mathit{n}}_{\mathit{s}}$${\mathit{n}}_{\mathit{c}}$) regions This dependence was found to be sample independent We have also studied the diagonal resistivity at Landau-level filling factor \ensuremath{\nu}=3/2, where the system is known to be in a true metallic state at high magnetic field and in an insulating state at low magnetic field The temperature dependencies of resistivity at B=0 and \ensuremath{\nu}=3/2 were found to be identical These behaviors suggest a true metal-insulator transition in the two-dimensional electron system in silicon at B=0, in contrast with the well-known scaling theory

Scaling of an anomalous metal-insulator transition in a two-dimensional system in silicon at B =0

Random and alternating fluorene/carbazole (F/Cz) copolymers with various carbazole contents (20−50 mol %) have been designed and synthesized for use as the hole-transporting as well as light-emitting layer in blue light-emitting diodes (LEDs). DSC analysis has indicated the complete suppression of the crystallizability of these polymers by the introduction of 3,6-carbazole linkages into the polymer backbone, which also results in changes in their optical properties. The absorption maximum has been blue-shifted with an increase in the carbazole content due to the interruption in the main chain conjugation. Meanwhile, the photoluminescent properties have been influenced by the sequence distribution of the fluorene segments as well as the carbazole content. The emission maxima and vibronic features of the alternating copolymers have changed with carbazole content, reflecting the differences in the electronic structures of the repeat units. However, in the case of the random copolymers, the emission spectra r...

Synthesis and properties of random and alternating fluorene/carbazole copolymers for use in blue light-emitting devices

We report the synthesis of novel fluorene-based π-conjugated polymers and the investigation of their electroluminescent properties in organic light-emitting devices. We also report on the photo- and electroluminescence of materials of the same class whose synthesis was recently published. The alternated incorporation of phenylene or thiophene moieties in fluorene-based π-conjugated polymers leads to the tunability of the electroluminescent properties. The spectral emission varies from blue to green or yellow, depending on the composition of the copolymers. To enhance the luminous efficiency of the devices, the hole injection and hole transport into the polymer were improved by insertion of an insulating buffer layer and the incorporation of efficient hole transport material in the polymer. The insertion of a charge injection layer such as LiF and a hole transport layer such as N,N‘-diphenyl-N,N‘-bis(3-methylphenyl)-1,1-biphenyl-4,4‘-diamine significantly improves the electroluminescence efficiency of the ...

Light-emitting diodes from fluorene-based π-conjugated polymers

Four novel blue luminescent star-shaped compounds 1,3,5-tris(di-2-pyridylamino)benzene, 1, 1,3,5-tris[p-(di-2-pyridylamino)phenyl]benzene, 2, 2,4,6-tris(di-2-pyridylamino)-1,3,5-triazine, 3, and 2,4,6-tris[p-(di-2-pyridylamino)phenyl]-1,3,5-triazine, 4, have been synthesized and fully characterized. Compounds 1, 2 and 4 were prepared from the reactions of appropriate s-triazine and 1,3,5-trisubstituted benzene compounds with di-2-pyridylamine via copper-mediated Ullmann condensation in good yield (45–85%). Compounds 1, 2
and 4 show glass formation. Compounds 1–4 emit a blue color both in solution and in the solid state. The emission maxima of compounds 1–4 in the solid state are at λ = 412, 409, 393 and 440 nm, respectively. Fluorescence quantum yields of compounds 1–4 are 0.53, 0.16, 0.43 and 0.78, respectively. Electroluminescent devices using compounds 1–4 as the emitters were fabricated.

Marie D'Iorio

Papers

Possible metal-insulator transition at B=0 in two dimensions.

Scaling of an anomalous metal-insulator transition in a two-dimensional system in silicon at B =0

Synthesis and properties of random and alternating fluorene/carbazole copolymers for use in blue light-emitting devices

Light-emitting diodes from fluorene-based π-conjugated polymers

Syntheses, structures, and electroluminescence of new blue luminescent star-shaped compounds based on 1,3,5-triazine and 1,3,5-trisubstituted benzene