Transparent electrodes are a necessary component in many modern devices such as touch screens, LCDs, OLEDs, and solar cells, all of which are growing in demand. Traditionally, this role has been well served by doped metal oxides, the most common of which is indium tin oxide, or ITO. Recently, advances in nano-materials research have opened the door for other transparent conductive materials, each with unique properties. These include CNTs, graphene, metal nanowires, and printable metal grids. This review will explore the materials properties of transparent conductors, covering traditional metal oxides and conductive polymers initially, but with a focus on current developments in nano-material coatings. Electronic, optical, and mechanical properties of each material will be discussed, as well as suitability for various applications.

Emerging Transparent Electrodes Based on Thin Films of Carbon Nanotubes, Graphene, and Metallic Nanostructures

Functional carbon-based nanomaterials (CBNs) have become important due to their unique combinations of chemical and physical properties (i.e., thermal and electrical conductivity, high mechanical strength, and optical properties), and extensive research efforts are being made to utilize these materials for various industrial applications, such as high-strength materials and electronics. These advantageous properties of CBNs are also actively investigated in several areas of biomedical engineering. This Perspective highlights different types of carbon-based nanomaterials currently used in biomedical applications.

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Carbon-based nanomaterials: multifunctional materials for biomedical engineering.

The underlying physics of the application of low-temperature, low-pressure reactive plasmas in various nanoassembly processes is described. From the viewpoint of the ``cause and effect'' approach, this Colloquium focuses on the benefits and challenges of using plasma-based systems in nanofabrication of nanostructured silicon films, low-dimensional semiconducting quantum structures, ordered carbon nanotip arrays, highly crystalline ${\mathrm{TiO}}_{2}$ coatings, and nanostructured hydroxyapatite bioceramics. Other examples and future prospects of plasma-aided nanofabrication are also discussed.

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Colloquium: Reactive plasmas as a versatile nanofabrication tool

Carbon nanotubes (CNTs) promise to advance a number of real-world technologies. Of these applications, they are particularly attractive for uses in chemical sensors for environmental and health monitoring. However, chemical sensors based on CNTs are often lacking in selectivity, and the elucidation of their sensing mechanisms remains challenging. This review is a comprehensive description of the parameters that give rise to the sensing capabilities of CNT-based sensors and the application of CNT-based devices in chemical sensing. This review begins with the discussion of the sensing mechanisms in CNT-based devices, the chemical methods of CNT functionalization, architectures of sensors, performance parameters, and theoretical models used to describe CNT sensors. It then discusses the expansive applications of CNT-based sensors to multiple areas including environmental monitoring, food and agriculture applications, biological sensors, and national security. The discussion of each analyte focuses on the strategies used to impart selectivity and the molecular interactions between the selector and the analyte. Finally, the review concludes with a brief outlook over future developments in the field of chemical sensors and their prospects for commercialization.

Carbon Nanotube Chemical Sensors

Vibrations of carbon nanotubes and their composites: a review

We discuss the central issues to be addressed for realizing carbon nanotube (CNT) nanoelectronics We focus on selective growth, electron energy bandgap engineering and device integration We have introduced a nanotemplate to control the selective growth, length and diameter of CNTs Vertically aligned CNTs are synthesized for developing a vertical CNT-field effect transistor (FET) The ohmic contact of the CNT/metal interface is formed by rapid thermal annealing Diameter control, synthesis of Y-shaped CNTs and surface modification of CNTs open up the possibility for energy bandgap modulation The concepts of an ultra-high density transistor based on the vertical-CNT array and a nonvolatile memory based on the top gate structure with an oxide–nitride–oxide charge trap are also presented We suggest that the deposited memory film can be used for the quantum dot storage due to the localized electric field created by a nano scale CNT-electron channel

https://iopscience.iop.org/article/10.1088/0957-4484/15/10/003/pdf

Aligned carbon nanotubes for nanoelectronics

The effect of oxygen adsorption on a nanotube-based field effect transistor have been controversial as to whether it induces p-type doping of the nanotube body or the work function increase in the metal electrode. Here we report a transport measurement showing that a long individual single-walled nanotube can be doped as p-type upon oxygen adsorption. We discuss that, despite the fact that the charge transfer between the nanotube and O2 adsorbator has not been agreed to date, the effect of oxygen adsorption should still be interpreted as inducing p-type doping in the nanotube body. The n-type doping by NH3 adsorption is also measured for the purpose of comparison. Based on these observations, we suggest that, while the Schottky barrier management could be more effective for the transistor with a short nanotube, the doping effect could be more influential in devices with longer nanotubes.

https://iopscience.iop.org/article/10.1088/0957-4484/16/8/008/pdf

Oxygen-induced p-type doping of a long individual single-walled carbon nanotube

We have fabricated a single-wall carbon-nanotube (CNT)-based nonvolatile memory device using SiO2–Si3N4–SiO2 (ONO) layers as a storage node. The memory device is composed of a top gate structure with a channel width of a few nanometers and the ONO layer embedded between CNT and gate electrode. When the bias voltage between the CNT and gate electrode increases to 4 V, charges are tunneled out from CNT surfaces and captured to the traps in the ONO layers. Stored charges on the trap sites make the threshold voltage shift of 60 mV and is independent of charging time, suggesting that the ONO has traps with a quasiquantized energy state. The quantized state is related to the localized high electric field associated with CNT channel. The CNT-field-effect transistor with an ONO storage node could be used for an ultrahigh-density nonvolatile memory.

Carbon-nanotube-based nonvolatile memory with oxide–nitride–oxide film and nanoscale channel

The selective growth of vertically aligned carbon nanotubes (CNTs) and their application as field-effect transistors (FETs) are demonstrated. Vertically aligned carbon nanotubes were selectively grown in nanoholes formed in an anodized aluminum oxide (AAO) template. Each device element is formed on a vertical carbon nanotube attached to bottom (source) and upper (drain) electrodes and a gate electrode, which can be integrated in large arrays with the potential for tera-level density (1012 cm–2). Simulation of the potential distribution shows that the direction of potential formation would depend on the polarity of the gate bias, which is consistent with an experimental result of CNT-FET operation.

Selective Growth of Carbon Nanotubes for Nanoscale Transistors

We investigate ambipolar to unipolar transition by the effect of ambient air on the carbon nanotube field-effect transistor. A unipolar transport property of the double-walled nanotube field-effect transistor and its conversion from ambipolar behavior are observed. We suggest that adsorptions of oxygen molecules, whose lowest-unoccupied-molecular-orbital state is around the midgap of the carbon nanotube, could suppress the electron channel formation and, consequently, result in the unipolar transport behavior.

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Eunju Bae

Papers

Aligned carbon nanotubes for nanoelectronics

Oxygen-induced p-type doping of a long individual single-walled carbon nanotube

Carbon-nanotube-based nonvolatile memory with oxide–nitride–oxide film and nanoscale channel

Selective Growth of Carbon Nanotubes for Nanoscale Transistors

Adsorption-induced conversion of the carbon nanotube field effect transistor from ambipolar to unipolar behavior