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Topic Name: Researchers Develop a Thin Coating Arrays of Loosely Vertically-Aligned Carbon Nanotubes that Absorbs light, could Boost Solar Energy Conversion

Category: Nanobiotechnology

Research persons: Shawn-Yu Lin, Pulickel Ajayan

Location: Rensselaer Polytechnic Institute, United States

Details

Researchers at Rensselaer Polytechnic Institute and Rice University have created the darkest material ever made by man.

The material, a thin coating comprised of low-density arrays of loosely vertically-aligned carbon nanotubes, absorbs more than 99.9 percent of light and one day could be used to boost the effectiveness and efficiency of solar energy conversion, infrared sensors, and other devices. The researchers who developed the material have applied for a Guinness World Record for their efforts.

“It is a fascinating technology, and this discovery will allow us to increase the absorption efficiency of light as well as the overall radiation-to-electricity efficiency of solar energy conservation,” said Shawn-Yu Lin, professor of physics at Rensselaer and a member of the university’s Future Chips Constellation, who led the research project. “The key to this discovery was finding how to create a long, extremely porous vertically-aligned carbon nanotube array with certain surface randomness, therefore minimizing reflection and maximizing absorption simultaneously.” 

The research results were published in the journal Nano Letters. 

All materials, from paper to water, air, or plastic, reflect some amount of light. Scientists have long envisioned an ideal black material that absorbs all the colors of light while reflecting no light. So far they have been unsuccessful in engineering a material with a total reflectance of zero.

The total reflectance of conventional black paint, for example, is between 5 and 10 percent. The darkest manmade material, prior to the discovery by Lin’s group, boasted a total reflectance of 0.16 percent to 0.18 percent.

Lin’s team created a coating of low-density, vertically aligned carbon nanotube arrays that are engineered to have an extremely low index of refraction and the appropriate surface randomness, further reducing its reflectivity. The end result was a material with a total reflective index of 0.045 percent — more than three times darker than the previous record, which used a film deposition of nickel-phosphorous alloy.

“The loosely-packed forest of carbon nanotubes, which is full of nanoscale gaps and holes to collect and trap light, is what gives this material its unique properties,” Lin said. “Such a nanotube array not only reflects light weakly, but also absorbs light strongly. These combined features make it an ideal candidate for one day realizing a super black object.”

“The low-density aligned nanotube sample makes an ideal candidate for creating such a super dark material because it allows one to engineer the optical properties by controlling the dimensions and periodicities of the nanotubes,” said Pulickel Ajayan, the Anderson Professor of Engineering at Rice University in Houston, who worked on the project when he was a member of the Rensselaer faculty. 

The research team tested the array over a broad range of visible wavelengths of light, and showed that the nanotube array’s total reflectance remains constant.  

“It’s also interesting to note that the reflectance of our nanotube array is two orders of magnitude lower than that of the glassy carbon, which is remarkable because both samples are made up of the same element — carbon,” said Lin.

This discovery could lead to applications in areas such as solar energy conversion, thermalphotovoltaic electricity generation, infrared detection, and astronomical observation.

Other researchers contributing to this project and listed authors of the paper include Rensselaer physics graduate student Zu-Po Yang; Rice postdoctoral research associate Lijie Ci; and Rensselaer senior research scientist James Bur.

Note for Carbon Nanotube
Carbon nanotubes (CNTs) are allotropes of carbon. This results in a nanostructure where the length-to-diameter ratio exceeds 1,000,000. Such cylindrical carbon molecules have novel properties that make them potentially useful in many applications in nanotechnology, electronics, optics and other fields of materials science. They exhibit extraordinary strength and unique electrical properties, and are efficient conductors of heat. Inorganic nanotubes have also been synthesized.
Nanotubes are members of the fullerene structural family, which also includes buckyballs. Whereas buckyballs are spherical in shape, a nanotube is cylindrical, with at least one end typically capped with a hemisphere of the buckyball structure. Their name is derived from their size, since the diameter of a nanotube is in the order of a few nanometers (approximately 1/50,000th of the width of a human hair), while they can be up to several millimeters in length. Nanotubes are categorized as single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs).
The nature of the bonding of a nanotube is described by applied quantum chemistry, specifically, orbital hybridization. The chemical bonding of nanotubes are composed entirely of sp2 bonds, similar to those of graphite. This bonding structure, which is stronger than the sp3 bonds found in diamond, provides the molecules with their unique strength. Nanotubes naturally align themselves into "ropes" held together by Van der Waals forces. Under high pressure, nanotubes can merge together, trading some sp² bonds for sp³ bonds, giving great possibility for producing strong, unlimited-length wires through high-pressure nanotube linking.

Note for Solar Energy
Solar energy is energy directly from the Sun. This energy drives the climate and weather and supports virtually all life on Earth. Heat and light from the sun, along with solar-based resources such as wind and wave power, hydroelectricity and biomass, account for most of the available flow of renewable energy.
Solar energy technologies harness the sun's energy for practical ends. These technologies date from the time of the early Greeks, Native Americans and Chinese, who warmed their buildings by orienting them toward the sun. Modern solar technologies provide heating, lighting, electricity and even flight.
Solar power is used synonymously with solar energy or more specifically to refer to the conversion of sunlight into electricity. This can be done either through the photovoltaic effect or by heating a transfer fluid to produce steam to run a generator.

Note for Observational Astronomy
Observational astronomy is a division of the astronomical science that is concerned with getting data, in contrast with theoretical astrophysics which is mainly concerned with finding out the measurable implications of physical models. It is the practice of observing celestial objects by using telescopes and other astronomical apparatus.
As a science, astronomy is somewhat hindered in that direct experiments with the properties of the distant universe are not possible. However, this is partly compensated by the fact that astronomers have a vast number of visible examples of stellar phenomena that can be examined. This allows for observational data to be plotted on graphs, and general trends recorded. Nearby examples of specific phenomena, such as variable stars, can then be used to infer the behavior of more distant representatives. Those distant yardsticks can then be employed to measure other phenomena in that neighborhood, including the distance to a galaxy.

Note for Scanning Electron Microscope
The scanning electron microscope (SEM) is a type of electron microscope that creates various images by focusing a high energy beam of electrons onto the surface of a sample and detecting signals from the interaction of the incident electrons with the sample's surface. The type of signals gathered in a SEM varies and can include secondary electrons, characteristic x-rays, and back scattered electrons. In a SEM, these signals come not only from the primary beam impinging upon the sample, but from other interactions within the sample near the surface. The SEM is capable of producing high-resolution images of a sample surface in its primary use mode, secondary electron imaging. Due to the manner in which this image is created, SEM images have great depth of field yielding a characteristic three-dimensional appearance useful for understanding the surface structure of a sample. This great depth of field and the wide range of magnifications are the most familiar imaging mode for specimens in the SEM. Characteristic x-rays are emitted when the primary beam causes the ejection of inner shell electrons from the sample and are used to tell the elemental composition of the sample. The back-scattered electrons emitted from the sample may be used alone to form an image or in conjunction with the characteristic x-rays as atomic number contrast clues to the elemental composition of the sample.

The project was funded by the U.S. Department of Energy’s Office of Basic Energy Sciences and the Focus Center New York for Interconnects.

Lin’s research was conducted as part of the Future Chips Constellation at Rensselaer, which focuses on innovations in materials and devices, in solid state and smart lighting, and applications such as sensing, communications, and biotechnology. A new concept in academia, Rensselaer constellations are led by outstanding faculty in fields of strategic importance. Each constellation is focused on a specific research area and comprises a multidisciplinary mix of senior and junior faculty, as well as postdoctoral researchers and graduate students.

About Rensselaer
Rensselaer Polytechnic Institute, founded in 1824, is the nation’s oldest technological university. The university offers bachelor’s, master’s, and doctoral degrees in engineering, the sciences, information technology, architecture, management, and the humanities and social sciences. Institute programs serve undergraduates, graduate students, and working professionals around the world. Rensselaer faculty are known for pre-eminence in research conducted in a wide range of fields, with particular emphasis in biotechnology, nanotechnology, information technology, and the media arts and technology. The Institute is well known for its success in the transfer of technology from the laboratory to the marketplace so that new discoveries and inventions benefit human life, protect the environment, and strengthen economic development.

In figure 1, The new darkest manmade material, with its 0.045 % reflectance (center), is noticeably darker than the 1.4% NIST reflectance standard (left) and a piece of glassy carbon (right). This photo was taken under a flash light illumination
In figure 2, Scanning electron micrograph (SEM) of the darkest material.
In figure 3, The vertically aligned carbon nanotube samples were mounted in the center of a integrating sphere, which measured the material's reflectivity.