Among its well know mechanical and electronic properties, carbon nanotubes also showcase a series of unique optical properties. Whilst a lot of the focus for carbon nanotubes has been on structural materials, they can also be used in specialist optical applications. In this article, we look at the optical properties of carbon nanotubes, as well as looking at some of the areas of optics that they can be used in.
Optical Properties of Carbon Nanotubes
Carbon nanotubes are often known for their high aspect ratio and mechanical strength properties; however, carbon nanotubes also possess a series of beneficial optical properties that mean they can be used in specialist optic applications.
Carbon nanotubes have unique optical absorption properties and differ from those found in bulk materials. However, given the high aspect ratio of nanotubes, many of their properties are only applicable to one direction along the nanotube. These unique absorption properties also arise because carbon nanotubes can often be classified as a metal or a semiconductor, depending on its orientation.
Light will elastically scatter through a carbon nanotube, but it will scatter in an anisotropic fashion. This is attributed to carbon nanotubes possessing non-linear refractive properties (third order non-linearity). These non-linear optical properties arise because the delocalized π-electrons only have a 1D motion within a fixed lattice configuration. These third order non-linear optical properties can also give rise to other phenomenon, such as third harmonic generation (THG), optical Kerr effect, self-focusing and phase conjugation.
Carbon nanotubes also have saturable absorption properties. So, when light comes towards the nanotube, it linearly absorbs the light until it reaches a saturation point. Once the amount of light absorbed is greater than the saturation limit, the nanotubes become transparent.
The absorption of light by a carbon nanotube is also known to produce excitons, which is a charge separation of electrons and holes, where one of each makes up an exciton. Additionally, it has been shown that carbon nanotubes can also form biexciton complexes (4 particle systems of two holes and two electrons) and remain stable. Because these excitation states can be created, carbon nanotubes can also exhibit fluorescence, photoluminescence and electroluminescence properties. Carbon nanotubes generally emit light in the near-infrared range and possess a high photostability.
Given that carbon nanotubes have unique optical properties, there are various applications that they can be used in. Some examples include various types of biosensors, optical rectennas and fiber-optic anemometers.
Carbon nanotube can be employed as part of a sensor to detect various biomolecules, such as those that detect DNA, glucose, hydrogen peroxide, adenosine triphosphate (ATP), nitrogen monoxide and various proteins. To make them more biocompatible, carbon nanotubes are often functionalized with organic molecules, or are composited into a biocompatible matrix. There are various types of carbon nanotube optical sensors to date, and it is perhaps the widest application area that exploits the optical properties of carbon nanotubes.
There are three common sensing mechanisms of carbon nanotubes in these devices, and these are changes in the electro-optical properties, electrochemical fluorescence and quenching mechanisms; and these provide a measurable response that enables the monitoring of exocytosis of biomolecules, the real-time monitoring of the above small molecules, for detecting reactive/radical species, detecting polymorphism in nucleotides, detecting the presence of biomarkers and for detecting the levels of hybridization, sequencing and conformational polymorphism in DNA.
Carbon nanotubes can be used as part of an optical rectenna– which is a device that converts freely propagating electromagnetic waves (at defined optical frequencies) into a direct current (DC). This is a process known as rectification.
In this application, carbon nanotubes can be used as the antenna material and is coupled to PHz-scale metal–insulator–metal tunnel diodes. To act as the antenna, the carbon nanotubes are placed vertically and when they are irradiated with visible and infrared light, a DC open-circuit voltage and open-circuit current can be generated. These rectennas have be shown to also work with solar light without there being any change in the performance of the rectenna.
Carbon nanotubes can also be used within anemometers, i.e. a type of sensor that measures the speed of wind or gas. Carbon nanotubes can be used to coat tilted fiber Bragg gratings (TFBGs) in hot-wire anemometers. These Bragg gratings can be implemented in the sensors to act as both the light coupling and temperature sensing elements of the device. Whilst these Bragg gratings are often used without the carbon nanotubes (usually utilizing a metallic coating), the incorporation of these carbon nanotube coatings have been proven to the increase the light absorption efficiency (up to 93%) of these gratings.
This is because, when the light enters the sensor, it travels down the optical fiber, where it propagates. But because it can’t escape the cladding of the fiber, the carbon nanotubes absorb a significantly high amount of light.
- Aalborg University: http://homes.nano.aau.dk/tgp/nanotube%20research.pdf
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