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

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

The fundamental chemical, redox, electrochemical, photoelectrochemical, optical and optoelectronic features of carbon nanotubes are surveyed with particular emphasis on the most relevant applications as electron donor/electron acceptor or as electron conductor/hole conductor materials, in solutions and in the solid state. Methods that aim at p- and n-doping as a means to favor hole or electron injection/transport are covered as well (critical review, 208 references).

Carbon nanotubes—electronic/electrochemical properties and application for nanoelectronics and photonics

Carbon nanotube (CNT)- and graphene (G)-based transparent conductive films (TCFs) are two promising alternatives for commonly-used indium tin oxide-based TCFs for future flexible optoelectronic devices. This review comprehensively summarizes recent progress in the fabrication, properties, modification, patterning, and integration of CNT- and G-TCFs into optoelectronic devices. Their potential applications and challenges in optoelectronic devices, such as organic photovoltaic cells, organic light emitting diodes and touch panels, are discussed in detail. More importantly, their key characteristics and advantages for use in these devices are compared. Despite many challenges, CNT- and G-TCFs have demonstrated great potential in various optoelectronic devices and have already been used for some products like touch panels of smartphones. This illustrates the significant opportunities for the industrial use of CNTs and graphene, and hence pushes nanoscience and nanotechnology one step towards practical applications.

25th anniversary article: carbon nanotube- and graphene-based transparent conductive films for optoelectronic devices.

Plasmon resonance is expected to occur in metallic and doped semiconducting carbon nanotubes in the terahertz frequency range, but its convincing identification has so far been elusive. The origin of the terahertz conductivity peak commonly observed for carbon nanotube ensembles remains controversial. Here we present results of optical, terahertz, and direct current (DC) transport measurements on highly enriched metallic and semiconducting nanotube films. A broad and strong terahertz conductivity peak appears in both types of films, whose behaviors are consistent with the plasmon resonance explanation, firmly ruling out other alternative explanations such as absorption due to curvature-induced gaps.

Plasmonic nature of the terahertz conductivity peak in single-wall carbon nanotubes.

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

Bi 1 - x - y Ti x Si y O z (BTSO) thin films were grown by atomic layer deposition using Bi(mmp) 3 , Ti(mmp) 4 , and Si(OEt) 4 (mmp = 1 -methoxy-2-methyl-2-propoxide; Et =ethyl) as metal alkoxide precursors, and water vapor asthe oxidant gas in the temperature range of 200-450°C on 8-in.-diamter Ru/SiO 2 /Si or bare Si wafers. BTSO films were formed at 250°C as a result of self-limiting adsorption and hydrolysis of the alkoxide precursors, wherein the saturated growth rate was about 0.2 A/cycle. At the temperature range of 250-300°C, X-ray diffraction shows that the resultant films are amorphous, and inductively coupled plasma-atomic emission spectroscopy reveals that the ratio of Bi/(Bi + Ti) in BTSO is nearly irrespective of the growth temperature. Rutherford backscattering spectrometry analysis showed that the cation compositional ratio of Bi:Ti:Si in the films grown at 250°C on Ru/SiO 2 /Si substrates is 0.29:0.54:0.17. The leakage current density and dielectric constant of a 30-nm-thick BTSO film are on the order of 10 - 8 A/cm 2 at 1 V and approximately 55, respectively. Uniform thickness and composition were also obtained on a porous anodic aluminum oxide having hole structures with a high aspect ratio of 6.

Ju Hye Ko

Papers

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

Atomic Layer Deposition of Bi1 − x − y Ti x Si y O z Thin Films from Alkoxide Precursors and Water