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.

Functionalized acenes and heteroacenes for organic electronics.

The science and technology of ultracapacitors are reviewed for a number of electrode materials, including carbon, mixed metal oxides, and conducting polymers. More work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and an organic electrolytes. The energy density of these devices is 3¯5 Wh/kg with a power density of 300¯500 W/kg for high efficiency (90¯95%) charge/discharges. Projections of future developments using carbon indicate that energy densities of 10 Wh/kg or higher are likely with power densities of 1¯2 kW/kg. A key problem in the fabrication of these advanced devices is the bonding of the thin electrodes to a current collector such the contact resistance is less than 0.1 cm2. Special attention is given in the paper to comparing the power density characteristics of ultracapacitors and batteries. The comparisons should be made at the same charge/discharge efficiency.

Ultracapacitors: Why, How, and Where is the Technology

Acenes have long been the subject of intense study because of the unique electronic properties associated with their pi-bond topology. Recent reports of impressive semiconductor properties of larger homologues have reinvigorated research in this field, leading to new methods for their synthesis, functionalization, and purification, as well as for fabricating organic electronic components. Studies performed on high-purity acene single crystals revealed their intrinsic electronic properties and provide useful benchmarks for thin film device research. New approaches to add functionality were developed to improve the processability of these materials in solution. These new functionalization strategies have recently allowed the synthesis of acenes larger than pentacene, which have hitherto been largely unavailable and poorly studied, as well as investigation of their associated structure/property relationships.

The larger acenes: versatile organic semiconductors.

Organic semiconducting oligomers for use in thin film transistors.

Synthesis and characterization of electron-deficient pentacenes.

Hemp-derived activated carbons for supercapacitors

Knowledge of the carrier type (p- or n-type) and the carrier density of carbon nanotubes is critical in semiconductor applications such as thermoelectric power generation and Peltier cooling. In this paper, an experimental procedure for electrochemically characterizing multiwalled carbon nanotubes (MWCNTs) doped with boron or nitrogen has been presented. The carrier type and carrier density of the doped MWCNTs were determined by generating Mott-Schottky plots. The boron-doped nanotubes were synthesized using the substitution reaction method and the nitrogen-doped MWCNTs were synthesized using the continuous-feed chemical vapor deposition (CVD) method using pyridine or acetonitrile as the carbon precursor and ferrocene as the metal catalyst particle precursor. The nitrogen-doped nanotubes were synthesized at different CVD reaction temperatures: 650, 700, 800, and 900°C, and the effect of reaction temperature on the carrier densities of the carbon nanotubes was examined. Thermoelectric devices were constructed using pyridine nanotubes synthesized at different synthesis temperatures, and it was found that with any change (increase/decrease) in the carrier densities reflected a corresponding change in the thermoelectric power of the nanotubes.

Determination of Carrier Densities of Boron- and Nitrogen-Doped Multiwalled Carbon Nanotubes Using Mott–Schottky Plots

The performance of carbon-supported cobalt catalysts was compared with that of Co/γ-Al2O3 reference catalysts for the Fischer–Tropsch synthesis (FTS) reaction. The carbon support (CS) was prepared using a hydrothermal method that formed mostly spherical ∼300–800 nm carbons that were first carbonized at 900 °C and then partially graphitized at 1900 °C. The FTS study was conducted using a continuously stirred tank reactor, and the cobalt catalysts were promoted with Pt (0.2% Pt–10% Co) to facilitate the reduction of cobalt oxides. Catalysts were prepared by an evaporative method (Co/CS-IWI) and by a chemical vapor deposition technique (Co/CS-CVD). The CVD technique led to a higher CO conversion (26.5%) relative to the conventional evaporative (IWI) method (7.4%) at the same temperature (220 °C) and space velocity (1.5 NL/gcath). Remarkably, the Co/CS-CVD displayed a high oxygenate selectivity (∼10%) in comparison with cobalt alumina catalysts (i.e., including one having similar Pt and Co loadings, as well a...

Fischer–Tropsch Synthesis: Higher Oxygenate Selectivity of Cobalt Catalysts Supported on Hydrothermal Carbons

Enhanced methods for preparing activated carbons have been discovered. In order to form an activated carbon, a carbon precursor material is coated with a phosphorus based chemical solution and physically activated. An activated carbon may also be formed by coating a green carbon precursor with a chemical solution that chemically reacts with carbon, carbonizing the resulting material, and physically activating the material during at least a portion of the carbonizing step. An activated carbon may also be formed by milling a carbon material to a predetermined particle size, then activating the milled particles. In another enhancement, an activated carbon is formed by coating a carbon or carbon precursor with nanoparticles, carbonizing if the carbon is a carbon precursor, then catalytically activating in air and an inert gas, and physically activating in steam or carbon dioxide. An activated carbon may also be formed by physically activating a previously chemically activated carbon.

Christopher R. Swartz

Papers

Synthesis and characterization of electron-deficient pentacenes.

Hemp-derived activated carbons for supercapacitors

Determination of Carrier Densities of Boron- and Nitrogen-Doped Multiwalled Carbon Nanotubes Using Mott–Schottky Plots

Fischer–Tropsch Synthesis: Higher Oxygenate Selectivity of Cobalt Catalysts Supported on Hydrothermal Carbons

Methods of forming activated carbons