Semiconductor nanowires (NWs) have been studied extensively for over two decades for their novel electronic, photonic, thermal, electrochemical and mechanical properties. This comprehensive review article summarizes major advances in the synthesis, characterization, and application of these materials in the past decade. Developments in the understanding of the fundamental principles of "bottom-up" growth mechanisms are presented, with an emphasis on rational control of the morphology, stoichiometry, and crystal structure of the materials. This is followed by a discussion of the application of nanowires in i) electronic, ii) sensor, iii) photonic, iv) thermoelectric, v) photovoltaic, vi) photoelectrochemical, vii) battery, viii) mechanical, and ix) biological applications. Throughout the discussion, a detailed explanation of the unique properties associated with the one-dimensional nanowire geometry will be presented, and the benefits of these properties for the various applications will be highlighted. The review concludes with a brief perspective on future research directions, and remaining barriers which must be overcome for the successful commercial application of these technologies.

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25th Anniversary Article: Semiconductor Nanowires – Synthesis, Characterization, and Applications

A most appealing feature of the development of (opto)electronic devices based on conjugated organic materials is the highly visible link between fundamental research and technological advances. Improved understanding of organic material properties can often instantly be implemented in novel device architectures, which results in rapid progress in the performance and functionality of devices. An essential ingredient for this success is the strong interdisciplinary nature of the field of organic electronics, which brings together experts in chemistry, physics, and engineering, thus softening or even removing traditional boundaries between the disciplines. Naturally, a thorough comprehension of all properties of organic insulators, semiconductors, and conductors is the goal of current efforts. Furthermore, interfaces between dissimilar materials-organic/organic and organic/inorganic-are inherent in organic electronic devices. It has been recognized that these interfaces are a key for device function and efficiency, and detailed investigations of interface physics and chemistry are at the focus of research. Ultimately, a comprehensive understanding of phenomena at interfaces with organic materials will improve the rational design of highly functional organic electronic devices.

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Organic Electronic Devices and Their Functional Interfaces

Luminescent materials with efficient solid-state emissions are important for the advancement of optoelectronics. Recently, a new class of propeller-like luminogenic molecules with aggregation-induced emission (AIE) characteristics has drawn increasing research interest. Among them, tetraphenylethene (TPE) is an archetypal luminogen with a simple molecule structure but shows a splendid AIE effect. Utilizing TPE as a building block, an effective strategy to create efficient solid-state emitters is developed. In this feature article, we review mainly our recent work on the construction of luminogenic materials from TPE and present their applications in organic light-emitting diodes. The applicability of the synthetic strategy and the utility of the resulting materials are demonstrated.

Tetraphenylethene: a versatile AIE building block for the construction of efficient luminescent materials for organic light-emitting diodes

Principal goals in organic thin-film transistor (OTFT) gate dielectric research include achieving: (i) low gate leakage currents and good chemical/thermal stability, (ii) minimized interface trap state densities to maximize charge transport efficiency, (iii) compatibility with both p- and n- channel organic semiconductors, (iv) enhanced capacitance to lower OTFT operating voltages, and (v) efficient fabrication via solution-phase processing methods. In this Review, we focus on a prominent class of alternative gate dielectric materials: self-assembled monolayers (SAMs) and multilayers (SAMTs) of organic molecules having good insulating properties and large capacitance values, requisite properties for addressing these challenges. We first describe the formation and properties of SAMs on various surfaces (metals and oxides), followed by a discussion of fundamental factors governing charge transport through SAMs. The last section focuses on the roles that SAMs and SAMTs play in OTFTs, such as surface treatments, gate dielectrics, and finally as the semiconductor layer in ultra-thin OTFTs.

Molecular self-assembled monolayers and multilayers for organic and unconventional inorganic thin-film transistor applications

Considerable research and development means have been focused in the past decade on organic semiconductor thin films and devices with applications to full color displays, flexible electronics and photovoltaics. Critical areas of these thin films are their interfaces with electrodes, with other organic films and with dielectrics, as these interfaces control charge injection and transport through the device. Full understanding of the mechanisms that determine the electronic properties of these interfaces, i.e. the relative position of molecular levels and charge carrier transport states, is an important goal to reach for developing reliable device processing conditions. This report provides an extensive, although probably somewhat biased, review of polymer– and small molecule–metal interface work of the past few years, with emphasis placed specifically on (i) the electronic structure and molecular level alignment at these interfaces, (ii) the perceived differences between small molecule and polymer interfaces, (iii) the difference between organic-on-metal and metal-on-organic interfaces, and (iv) the role played by electrode surface contamination in establishing interface energetics. Environmental conditions, e.g. vacuum vs. ambient, are found to be critical parameters in the processing of polymer and small molecule interfaces with metals. With similar processing conditions, these two types of interfaces are found to obey very similar molecular level alignment rules.

Energetics of metal–organic interfaces: New experiments and assessment of the field

Tunable n‐Type Conductivity and Transport Properties of Ga‐doped ZnO Nanowire Arrays

We report the synthesis of a novel starburst molecule, 4,4‘,4‘ ‘-tris(N-3-methylphenyl-N-(9-ethylcarbazyl-3)amino) triphenylamine (PCATA), and its application in organic light-emitting devices (OLEDs). The introduction of PCATA into the standard NPB/Alq3 OLED as the hole injecting and transporting layer dramatically enhanced the device efficiency to 5.7 cd/A and 2.2 lm/W, which are a factor of 2 higher than those of the standard OLED without the PCATA layer. The performance enhancement is attributed to a better balance of hole and electron injection in the PCATA-added OLED.

Novel Starburst Molecule as a Hole Injecting and Transporting Material for Organic Light-Emitting Devices

This paper reports the synthesis of a new starburst molecule, 4,4′,4′′-tris[(2,3,4,5-tetraphenyl)phenyl]phenylamine (TTPPPA) and its application as a hole-transporting material in organic light-emitting devices (OLEDs). Although TTPPPA has almost the same ionization potential as 1,4-bis(1-naphthylphenylamino)biphenyl (NPB), the TTPPPA-based device of ITO/TTPPPA/Alq3/LiF/Al yields much better efficiency of 5.3 cd/A and 4.3 lm/W than the standard ITO/NPB/Alq3/LiF/Al device (3.0 cd/A and 2.9 lm/W). The remarkable performance enhancement is attributed to a better balance of hole and electron injection in the TTPPPA-based device. Further, TTPPPA has a much higher glass-transition temperature (Tg, 202 °C) than NPB (Tg, 98 °C), suggesting that TTPPPA can be an alternative material to NPB especially for high-temperature applications of OLEDs and other organic electronic devices.

High Tg Triphenylamine-Based Starburst Hole-Transporting Material for Organic Light-Emitting Devices

An efficient organic light-emitting diode with silver electrodes

Several new 1H-pyrazolo[3,4-b]quinoxaline derivatives with N,N-dialkylamino electron-donating groups were prepared; their photoluminescence in solution and electroluminescence in thin-film devices have been investigated. They show emission at 520–540 nm with a fluorescence quantum yield close to unity in moderately polar solvents. The absorption and emission maxima shift to red with increasing solvent polarity. The fluorescence quantum yield also increases with increasing solvent polarity from non-polar to moderately polar solvents, then decreases slightly with further increase of solvent polarity. It indicates that both “positive” and “negative” solvatokinetic effects co-exist. The electroluminescence of OLED devices fabricated by using these compounds as dopants was investigated. All devices with the configuration of indium–tin oxide coated glass/NPB (60 nm)/Alq3 : Dopant (30 nm)/Alq3
(20 nm)/Mg : Ag (200 nm) show bright green emission with efficiencies of 7.5–9.7 cd A−1 and narrow full bandwidth at half intensity of 68–75 nm peaking at 536–552 nm.

J.X. Tang

Papers

Tunable n‐Type Conductivity and Transport Properties of Ga‐doped ZnO Nanowire Arrays

Novel Starburst Molecule as a Hole Injecting and Transporting Material for Organic Light-Emitting Devices

High Tg Triphenylamine-Based Starburst Hole-Transporting Material for Organic Light-Emitting Devices

An efficient organic light-emitting diode with silver electrodes

Synthesis, photoluminescence and electroluminescence of new 1H-pyrazolo[3,4-b]quinoxaline derivatives