Well-Aligned and Densely Packed

Researchers invent a low-tech, solution-based route to high-performance carbon nanotube thin films.

                                           Image courtesy of Weilu Gao, Rice University

A) Metal contacts (gold regions) connect to highly aligned carbon nanotube patterned films (red strips) to allow voltage control of their electrical properties. B) A close-up image of the films shows the high degree of carbon nanotube alignment. C) The cross section of the film shows the nanotubes are densely packed having about 1 million tubes per square micron (~1/10,000th the cross-sectional area of a human hair).

https://science.energy.gov/bes/highlights/2017/bes-2017-05-e/

The Science

Think of a computer chip that bends, rather than breaks. That’s the potential of a new study by scientists at Rice University and Los Alamos National Laboratory. Making such a chip requires a rapid, simple route to self-organizing tiny carbon tubes into thin films. The team’s method produces a film with millions of carbon nanotubes aligned and tightly packed, like water pipes on a flatbed truck. The tubes are orders of magnitude better aligned than previously possible. The team’s method for creating the film balances forces between the tubes and polymer membrane surfaces to harness a spontaneous self-alignment mechanism.

The Impact

Assembly is a key challenge to fabricating flexible computer chips, displays, and other devices from carbon nanotubes. Self-assembly of the carbon nanotubes is particularly vital as one looks to integrate multiple devices into useful systems. The researchers were up to the challenge. They harnessed a spontaneous self-alignment mechanism to create a fast way to produce the needed films. The films have applications in imaging, sensing and security.

Summary

Inside an individual carbon nanotube, electrons, phonons and excitons can travel in only one dimension. This property enables the tube to have electronic, optical and thermal properties that depend on direction. For example, electrical current can flow along the length of the tube but not perpendicular to it. Tightly bundling millions of tubes together would greatly increase the current-carrying capacity. Despite significant efforts to produce large-scale architectures of aligned nanotubes, results have been limited. In this study, scientists showed that films greater than a centimeter squared of aligned single-walled carbon nanotubes are possible by combining surfactant chemistry and slow vacuum filtration.

The nanotubes in the resulting films are aligned and tightly packed, with 1 million nanotubes in a cross-sectional area of one square micron (about 1/10,000 the cross-sectional area of a human hair). The method works for nanotubes created using various methods. In addition, the method works with different lengths of nanotubes, meaning film thickness is controllable from a few nanometers to around 100 nanometers. Combining this method with recently developed carbon nanotube sorting techniques allows for highly aligned and chirality-enriched nanotube thin-film devices. The team demonstrated how the films could be used as efficient terahertz polarizers and thin film transistors for optoelectronic applications.

Contact

Junichiro Kono
Departments of Electrical & Computer Engineering, Physics & Astronomy, and Materials Science & NanoEngineering
Rice University 
kono@rice.edu

Stephen K. Doorn
Center for Integrated Nanotechnologies
skdoorn@lanl.gov 

Funding

This work was supported by Basic Energy Sciences in the U.S. Department of Energy, Office of Science through grant DE-FG02-06ER46308 (for the preparation and characterization of aligned carbon nanotube films) and the Robert A. Welch Foundation through grant C-1509 (for terahertz and infrared characterization). S.K.D. and E.H.H. acknowledge support from the Los Alamos National Laboratory Laboratory Directed Research and Development program. Portions of this work were performed at the Center for Integrated Nanotechnologies, an Office of Science user facility operated for the U.S. Department of Energy Office of Science, project C2011B21. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.  

Publications

X. He, W. Gao, L. Xie, B. Li, Q. Zhang, S. Lei, J.M. Robinson, E.H. Hároz, S.K. Doorn, W. Wang, R. Vajtai, P.M. Ajayan, W.W. Adams, R.H. Hauge, and J. Kono, “Wafer-scale monodomain films of spontaneously aligned single-walled carbon nanotubes.” Nature Nanotechnology 11, 633-638 (2016). [DOI: 10.1038/nnano.2016.44]

Related Links

LA-UR-16-29576. Approved for public release; distribution is unlimited. Los Alamos National Laboratory, an affirmative action/equal opportunity employer, is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy under contract DE-AC52-06NA25396. By acceptance of this article, the publisher recognizes that the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or to allow others to do so, for U.S. Government purposes. Los Alamos National Laboratory requests that the publisher identify this article as work performed under the auspices of the U.S. Department of Energy. The Department of Homeland Security sponsored the production of this material under the Department of Energy contract for the management and operation of Los Alamos National Laboratory.

Nanoscale optical switch breaks miniaturization barrier

by David Salisbury | Posted on Thursday, Mar. 13, 2014 — 2:40 PM

http://news.vanderbilt.edu/2014/03/nanoscale-optical-switch/

Graduate student Kent Hallman checking the sample alignment the vapor deposition machine located in Vanderbilt Institute for Nanoscale Science and Engineering's clean room. (Joe Howell / Vanderbilt)

An ultra-fast and ultra-small optical switch has been invented that could advance the day when photons replace electrons in the innards of consumer products ranging from cell phones to automobiles.

The new optical device can turn on and off trillions of times per second. It consists of individual switches that are only one five-hundredth the width of a human hair (200 nanometers) in diameter. This size is much smaller than the current generation of optical switches and it easily breaks one of the major technical barriers to the spread of electronic devices that detect and control light: miniaturizing the size of ultrafast optical switches.

Physicist Richard Haglund has been studying the properties of vanadium dioxide for more than 20 years. (Joe Howell / Vanderbilt)

The new device was developed by a team of scientists from Vanderbilt University, University of Alabama-Birmingham, and Los Alamos National Laboratory and is described in the March 12 issue of the journal Nano Letters.

The ultrafast switch is made out of an artificial material engineered to have properties that are not found in nature. In this case, the “metamaterial” consists of nanoscale particles of vanadium dioxide (VO2) – a crystalline solid that can rapidly switch back and forth between an opaque, metallic phase and a transparent, semiconducting phase – which are deposited on a glass substrate and coated with a “nanomesh” of tiny gold nanoparticles.

The scientists report that bathing these gilded nanoparticles with brief pulses from an ultrafast laser generates hot electrons in the gold nanomesh that jump into the vanadium dioxide and cause it to undergo its phase change in a few trillionths of a second.

“We had previously triggered this transition in vanadium dioxide nanoparticles directly with lasers and we wanted to see if we could do it with electrons as well,” said Richard Haglund, Stevenson Professor of Physics at Vanderbilt, who led the study. “Not only does it work, but the injection of hot electrons from the gold nanoparticles also triggers the transformation with one fifth to one tenth as much energy input required by shining the laser directly on the bare VO2.”

Left: Illustration of terahertz optical switches shows the vanadium dioxide nanoparticles coated with a "nanomesh" of smaller gold particles. Right: Scanning electron microscope image of the switches at two resolutions. (Haglund Lab / Vanderbilt)

Both industry and government are investing heavily in efforts to integrate optics and electronics, because it is generally considered to be the next step in the evolution of information and communications technology. Intel, Hewlett-Packard and IBM have been building chips with increasing optical functionality for the last five years that operate at gigahertz speeds, one thousandth that of the VO2 switch.

“Vanadium dioxide switches have a number of characteristics that make them ideal for optoelectronics applications,” said Haglund. In addition to their fast speed and small size, they:

• Are completely compatible with current integrated circuit technology, both silicon-based chips and the new “high-K dielectric” materials that the semiconductor industry is developing to continue the miniaturization process that has been a major aspect of microelectronics technology development;
• Operate in the visible and near-infrared region of the spectrum that is optimal for telecommunications applications;
• Generate an amount of heat per operation that is low enough so that the switches can be packed tightly enough to make practical devices: about ten trillionths of a calorie (100 femtojoules) per bit.

“Vanadium dioxide’s amazing properties have been known for more than half a century. At Vanderbilt, we have been studying VO2 nanoparticles for the last ten years, but the material has been remarkably successfully at resisting theoretical explanations,” said Haglund. “It is only in the last few years that intensive computational studies have illuminated the physics that underlies its semiconductor-to-metal transition.”

Graduate student Christina McGahan holding a disk on which centimeter square samples are grown. (Joe Howell / Vanderbilt)

Vanderbilt graduate students Kannatassen Appavoo and Joyeeta Nag fabricated the metamaterial at Vanderbilt; Appavoo joined forces with University of Alabama, Birmingham graduate student Nathaniel Brady and Professor David Hilton to carry out the ultrafast laser experiments with the guidance of Los Alamos National Laboratory staff scientist Rohit Prasankumar and postdoctoral scholar Minah Seo. The theoretical and computational studies that helped to unravel the complex mechanism of the phase transition at the nanoscale were carried out by postdoctoral student Bin Wang and Sokrates Pantelides, University Distinguished Professor of Physics and Engineering at Vanderbilt.

The university researchers were supported by Defense Threat-Reduction Agency grant HDTRA1-0047, U.S. Department of Energy grant DE-FG02-01ER45916, U.S. Department of Education GAANN Fellowship P200A090143 and National Science Foundation grant DMR-1207241. Portions of the research were performed at the Vanderbilt Institute of Nanoscale Science and Engineering in facilities renovated with NSF grant ARI-R2 DMR-0963361, at the Center for Integrated Nanotechnologies at Los Alamos National Laboratory under USDOE contract DE-AC52-06NA25396) and at Sandia National Laboratories under USDOE contract DE-AC04-94AL85000).

Metamaterial flexible sheets from LANL could transform optics

http://www.laserfocusworld.com/articles/2013/06/metamaterial-flexible-sheets-from-lanl-could-transform-optics.html

06/05/2013

Posted by Gail Overton
Senior Editor

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IMAGE: Members of the LANL metamaterials team include, from left, Nathaniel K. Grady, Hou-Tong Chen, and Jane E. Heyes. (Image credit: LANL)

Los Alamos, NM--Research from a team of Los Alamos National Laboratory (LANL) scientists is leading to ultrathin, planar, lightweight, and broadband polarimetric photonic devices and polarimetric optics (http://www.laserfocusworld.com/articles/2011/01/metamaterials-three-layer-metamaterial-lens-focuses-terahertz-radiation.html) that would boost security screening systems, infrared thermal cameras, energy harvesting, and radar systems. The research, titled "Terahertz Metamaterials for Linear Polarization Conversion and Anomalous Refraction," was published online May 16 in Science Express, DOI: 10.1126/science.1235399 and aims to replace bulky conventional optics with flexible metamaterial sheets that are about the thickness of a human hair, weigh a fraction of an ounce, and give scientists new levels of control over light wavelengths.

The team demonstrated broadband, high-performance linear polarization conversion using ultrathin planar metamaterials (http://www.laserfocusworld.com/articles/2009/10/photonic-frontiers-metamaterials-and-transformation-optics-newest-metamaterials-promise-customized-optical-properties.html), enabling possible applications in the terahertz (THz) frequency regime. Their design can be scaled to other frequency ranges from the microwave through infrared. "Conventional methods for advanced polarization control impose very demanding requirements on material properties and fabrication methods, but they attain only limited performance," said Hou-Tong Chen, the senior researcher on the project.

Metamaterial-based polarimetric devices (http://www.laserfocusworld.com/articles/print/volume-47/issue-8/world-news/metamaterials-large-area-printed-3d-negative-index-metamaterial-is-flexible.html) are particularly attractive in the terahertz frequency range due to the lack of suitable natural materials for THz applications. Currently available designs suffer from either very limited bandwidth or high losses. The Los Alamos designs further enable the near-perfect realization of the generalized laws of reflection/refraction. According to the researchers, this can be exploited to make flat lenses, prisms, and other optical elements in a fashion very different from the curved, conventional designs that we use in our daily life.

The Los Alamos National Laboratory Directed Research and Development (LDRD) program funded a portion of the research. Part of the work was performed at the Center for Integrated Nanotechnologies (CINT).

SOURCE: LANL; http://www.lanl.gov/newsroom/news-releases/2013/June/06.05-metamaterial-flexible-sheets-transform-optics.php

Active terahertz metamaterials

http://spie.org/x92945.xml?highlight=x2414&ArticleID=x92945 Active terahertz metamaterials Nathaniel Grady and Hou-Tong Chen Incorporating superconducting and semiconducting elements into electromagnetic metamaterials can achieve active control of terahertz radiation. 18 March 2013, SPIE Newsroom. DOI: 10.1117/2.1201303.004760 Terahertz radiation—electromagnetic waves in the 0.1–10THz frequency range—occupies a middle ground between microwaves and infrared light. The unique properties of terahertz radiation make it very attractive for numerous applications in molecular spectroscopy, biomedical imaging, short-range ultra-high-bandwidth wireless communications, non-destructive inspection, security screening, and even quantum computing in silicon devices. For example, in a similar way to how infrared radiation can probe the vibrations corresponding to the motion of atoms bound together in an individual molecule, terahertz spectroscopy can probe the much weaker forces governing the interaction between molecules. Its use could revolutionize our understanding of how complex biomolecules interact. Terahertz waves are also well suited to non-destructive testing and security screening applications because, like microwaves and millimeter waves, they are non-ionizing, making them much safer than x-rays, and they can travel through many materials including plastics and cloth. However, the shorter wavelength of terahertz waves compared to microwaves and millimeter waves enables significantly higher image resolution.   Figure 1. Microscope image of our superconducting metamaterial. YBCO: Yttrium barium copper oxide.   Figure 2. Transmission through the metamaterial showing control of the resonance by (a) temperature, (b) optical excitation, and (c) high-intensity terahertz fields. Historically, the terahertz range has been largely unexplored due to the difficulty of generating, detecting, and manipulating terahertz radiation. Electronic devices that have driven the widespread use of microwaves are generally limited to much lower frequencies. In addition, a paucity of suitable materials means that photonic technologies that have been wildly successful in the infrared, visible, and ultraviolet regimes run into severe limitations in the terahertz range. One powerful way to circumvent the limitations of existing materials is to create artificial ‘metamaterials.’ These consist of arrays of conducting elements that are small enough to appear as an effectively continuous material to the electromagnetic waves (much as we can often ignore that natural materials are really composed of tiny atoms instead of being continuous). Metamaterials can also be engineered to exhibit exotic electromagnetic phenomena not observed in natural materials, including negative refraction and anomalous reflection. In addition, they can be used to create novel devices such as ultrathin perfect absorbers, planar lenses, and thin layers that rotate the polarization 90°. Metals have been used for the conductive elements in the vast majority of metamaterials. However, just as a metal wire can passively direct the flow of an electrical current but not actively control it, these metallic structures can only passively manipulate electromagnetic waves. The most frequently studied approach to enable active control is to add additional materials, such as semiconductors, to metallic metamaterials.1 Recently, we have focused on entirely replacing the metal with superconducting oxides.2, 3These have a conductivity that can be as high as that of metals, but that can also be easily controlled by temperature, light, a magnetic field, an electric field, or even very intense terahertz radiation. Our initial work showed that the resonance of superconducting split ring resonators (SRRs), the most frequently used metamaterial building blocks, could be tuned or suppressed by varying the temperature.2We used a metamaterial consisting of an array of yttrium barium copper oxide (YBCO) SRRs on a lanthanum aluminate substrate (see Figure 1). Plotting the transmission through the metamaterial at several temperatures ranging from 20K to 100K shows that the SRR resonance causes a dip at around 0.6THz: see Figure 2(a). The change in resonance frequency and strength with temperature arises from the decrease in the density of Cooper pairs, the carriers responsible for the supercurrent. It continues up to the superconducting transition temperature (around 90K for these samples), where the material ceases to be a superconductor. Although temperature tuning nicely demonstrates how sensitive the metamaterial resonance is to the Cooper pair density in the superconductor, thermal control tends to be too slow for many device applications. Recently, we achieved ultrafast switching of the metamaterial resonance by using near-infrared light as the control signal.2 Near-infrared photons are sufficiently energetic to easily break Cooper pairs, which again leads to a change in the YBCO conductivity and therefore the metamaterial resonance: see Figure 2(b). While terahertz photons do not have sufficient energy to break a Cooper pair directly, intense electric fields of 10–102kV/cm can drive the supercurrent into a nonlinear regime, again tuning the SRR resonance on a timescale of a few picoseconds: see Figure 2(c). Thus far, we have shown only superconducting devices that modulate the intensity of terahertz radiation. Although this is in itself a technologically important result, we are now extending our approach of building metamaterials directly out of an inherently controllable material. By applying this to more complex metamaterial devices, we expect in due course to be able to make tunable polarization rotators, beam steering devices, or perhaps even lenses with dynamically controllable focal lengths. Superconductors also exhibit manifestly quantum behaviors in response to a magnetic field or when formed into Josephson junctions, opening exciting possibilities in the emerging field of quantum metamaterials. We acknowledge partial support from the Los Alamos National Laboratory Laboratory Directed Research and Development (LDRD) program. This work was performed, in part, at the Center for Integrated Nanotechnologies, a user facility of the Office of Basic Energy Sciences. Nathaniel Grady, Hou-Tong Chen Center for Integrated Nanotechnologies Los Alamos National Laboratory Los Alamos, NM Nathaniel Grady received his BS from State University of New York Fredonia in 2002 and his PhD from Rice University, TX, in 2010, followed by a year at the Institute of Physics, Chinese Academy of Sciences. He is currently studying terahertz generation, metamaterials, and superconductors. Hou-Tong Chen received his BS and MS from the University of Science and Technology of China in 1997 and 2000, respectively, and a PhD from Rensselaer Polytechnic Institute, NY, in 2004. He is currently an R8D scientist. References: 1. H.-T. Chen, J. F. O'Hara, A. K. Azad, A. J. Taylor, Manipulation of terahertz radiation using metamaterials, Laser Photon. Rev. 5, p. 513-533, 2011.doi:10.1002/lpor.201000043 2. H.-T. Chen, H. Yang, R. Singh, J. F. O'Hara, A. K. Azad, S. A. Trugman, Q. X. Jia, A. J. Taylor, Tuning the resonance in high-temperature superconducting terahertz metamaterials, Phys. Rev. Lett. 105, p. 247402, 2010.doi:10.1103/PhysRevLett.105.247402 3. R. Singh, J. Xiong, A. K. Azad, H. Yang, S. A. Trugman, Q. X. Jia, A. J. Taylor, H.-T. Chen, Optical tuning and ultrafast dynamics of high-temperature superconducting terahertz metamaterials, Nanophotonics 1, p. 117-123, 2012. doi:10.1515/nanoph-2012-0007

Metamaterial is engineered for “Active Slow Light THz devices”

by Tim Palucka

http://www.materials360online.com/newsDetails/38466

Materials Research Society | Published: 07 March 2013

A metamaterial lattice consists of photo-doped silicon islands in the split ring resonator gap. The green pulse is the terahertz wave exciting the metamaterial. The near infrared femtosecond pump laser beam (shown in red) excites the silicon islands in the metamaterial, thus controlling the group velocity of the terahertz pulse transmitting through the metamaterial. Image credit: Jianqiang Gu. Click image to enlarge.

Light moving through a vacuum is the fastest phenomenon we know, but there could be significant technological advantages to applying the brakes occasionally. For applications such as optical computing, sensing, telecommunications and perhaps even quantum computing, controlling the speed of light through various media could be the key to optimum performance. Now, researchers have developed artificially engineered resonant metamaterials that, when illuminated by a femtosecond near-infrared laser light of varying intensity, can actively tune the group velocity of the terahertz light transmitted through the metamaterials. This is achieved by the dynamic tuning of the resonance enhanced dispersion of the effective medium that comprises subwavelength metamaterial unit cells called meta-molecules. The near field coupling between the meta-molecule resonators is exploited to create a resonant transparency window that mimics the quantum phenomena of electromagnetically induced transparency (EIT). 

“The biggest benefit is that if you can slow down light it can interact very strongly with matter, resulting in enhanced optical nonlinearities that would play a major role in the progress of on-chip, all-optical signal processing and quantum computation,” says Ranjan Singh of Los Alamos National Laboratory, one of the lead authors, along with Jianqiang Gu of Tianjin University, of the paper recently published in Nature Communications. Quanta of light— photons—are electromagnetic radiation without any mass or charge, whereas material interactions mostly involve electrons, Singh says; enabling photons to interact more strongly with the electrons in materials “can help us understand the interactions of photons with matter in a much more profound way.”  

Singh and his colleagues at Tianjin University in China; the University of Birmingham and Imperial College London in the UK; and Oklahoma State University and Los Alamos National Laboratory in the U.S., have designed the meta-molecules on a sapphire substrate. The meta-molecule unit cell consists of two square, split ring resonators (SRRs) and one cut wire made of aluminum, with Si filling the “splits” in the SRRs. Each unit cell behaves like an active molecule, according to Singh. They have arranged 10,400 of these unit cells together on one 10 x 10 mm sapphire chip. The sizes of the SRRs and the cut wire are chosen in such a way that their fundamental resonance modes coincide at one single frequency. It is the destructive interference between the non-radiative inductive-capacitive (LC) resonance of SRRs and the radiative dipole resonance of the cut wires that results in the EIT effect. 

When a 2.5-mm diameter terahertz beam is shined perpendicular to the face of the metamaterial chip, the material is strongly transparent at 0.74 THz. No external laser light is used at this point, and the terahertz pulse transmitted through the chip is at its lowest group velocity. By concurrently shining a femtosecond near-infrared laser pulse at a slight angle to this surface, the LC resonance property of the SRRs change due the Si pads, and the electromagnetically induced resonant transparency begins to diminish. As the laser power is increased from 25 mW to 1,000 mW, the transparency peak slowly fades. At 1,350 mW laser power, the EIT peak is gone, and light propagates through the metamaterial as if it were an ordinary medium.  

“The femtosecond infrared laser pulse is photo-exciting the Si, changing it from a semiconductor to a quasi-metal,” Singh says. “The LC resonance of the SRRs gradually quenches with increasing Si conductivity, and that actually tunes the group velocity through the metamaterial. Our metamaterial medium is an active slow light device—you can control how much to slow down the light by using a laser pulse.”  

One technological area in which Singh envisions applications for slow light is telecommunications. If you are transmitting two light pulses containing different information through a high bit rate telecom router, the information can get smeared by overlap of the light pulses, he says.   By letting one light pulse go through the router at its normal speed, and slowing down the other light pulse using this metamaterial chip, you can create a  time delay between the two pulses and  transmit their information through routers without any significant interference. 

Currently, the group is working on improving the performance of a slow light metamaterial device by suppressing the losses and enhancing the operation bandwidth. “If we can do that, I think it’s possible to have a practical slow light metamaterial device that could act as an all-optical tunable delay line for terahertz and microwaves,” Singh says. 

Read the abstract in Nature Communications  here.