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The new darkest manmade material,
with its 0.045 percent reflectance (center), is noticeably darker
than the 1.4 percent NIST reflectance standard (left) and a piece
of glassy carbon (right).
The vertically aligned carbon nanotube samples were
mounted in the center of a integrating sphere, which measured the
material's reflectivity.
A side-view scanning electron micrograph of the darkest material
at a high magnification. The nanotubes are vertically aligned,
forming a highly porous nanostructure.
Images � by
Rensselaer
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�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.
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.
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