EYE ON TECHNOLOGY

Richard Feynman’s 1959 lecture “There’s Plenty of Room at the Bottom” suggests how manipulation of materials on the scale of tens to hundreds of atoms can be used to achieve unique properties.1 In the range of approximately one nanometer (nm) to several hundred nm, the number of surface atoms is a significant proportion of the total number of atoms in a given volume. Since surface atoms have higher reactivity than atoms that are fully bound, behaviors can be achieved that are very different from those found in bulk materials. The unique physical, electrical and chemical properties enabled by nanotechnology have become more common and producible. And with advances in the ability to model, manipulate and characterize materials at the nanoscale, implementation of nanotechnology across defense and commercial sectors continues to expand.

Engineers and scientists at Raytheon are leveraging nanomaterials and nanotechnologies to advance our next generation systems and manufacturing capabilities. For example, two-dimensional (2D) materials are enabling epitaxial growth and transfer of semiconductors for new integration opportunities. Controlling composition and grain growth of nanopowder-based ceramics has resulted in highly transparent and mechanically robust materials for Mid-Wave Infrared (MWIR) applications. Nanostructured coating technologies that inhibit moisture and dirt retention, as well as fouling and bacterial growth, are being implemented for improved sustainability of fielded systems. Thermal Interface Materials (TIMs) benefit from nanoscale and 2D dopants that enhance thermal transport. Additionally, emerging capabilities in quantum dots (QDs) and nanophotonics have potential impact in many Raytheon applications. This article discusses several of these nanotechnologies and their role in advancing Raytheon product capabilities.

2D Materials

2D materials enable novel electronics and sensor technologies not previously possible. A recent joint study led by Raytheon BBN and Harvard University indicates that 2D electrons flow like a fluid rather than like a gas.2 This discovery describes a new electrical transport regime in which the interactions among the electrons are much stronger than those between the electrons and the impurities inside the materials. The Raytheon team, in collaboration with MIT and Harvard University, is now using this electron fluid concept to develop electronics with lower power consumption as well as a new ultrawide bandwidth amplifier enabled by steering the electron fluid. The amplifier design is analogous to modulating the direction of a jet stream, achieving wide-bandwidth amplification to more than 100 gigahertz (GHz).

Figure 1: Graphene-based single-photon detector in a twin-slot antenna.

Graphene is the first material in the entire 2D material family to be exfoliated down to mono-atomic layer thickness, opening up a new area of nanotechnology. In addition to its outstanding mechanical properties and thermal conductivity, graphene possesses some remarkable properties based on its electron behaviors. The electrons can move as massless particles and follow relativistic dynamics rather than Newtonian dynamics. This departure from conventional conductors gives rise to new fundamental physics in graphene, such as high electronic mobility, wide bandwidth electromagnetic wave absorption, and fast thermal response. These properties translate to innovations in ultrafast infrared sensing and single-photon detection. Raytheon BBN is a prominent player in the research and development of these new nanotechnologies with recent publications in the Physical Review Applied and Nature Nanotechnology scientific journals.3,4 The graphene-based single-photon detector (Figure 1), for example, shows a sensitivity approximately 10,000 times higher than current state-of-the-art detectors, and promises to enable novel solutions and products in sensing, imaging, and communication applications for Raytheon in years to come.

2D materials, such as graphene and hexagonal boron nitride, offer strong in-plane bonding that can lead to high thermal and electrical transport, as well as mechanical strength. However, these materials have relatively weak bonds between planes, facilitating the ability to form films with a thickness of a single or small controlled number of monolayers. 

Figure 2. Covalent field is unaffected by the weak Van der Waals forces (Bottom). Epitaxial growth of gallium nitride (GaN) material with graphene transfer layer (Top).

The Nanoelectronics Group at MIT, led by Dr. Jeehwan Kim, Associate Professor of Mechanical Engineering, has published work leveraging a 2D Material Layer Transfer (2DLT) process for remote epitaxial growth of single-crystal membranes that can readily be removed from their substrates for use as freestanding films or for integrating onto other materials.5 Raytheon is partnering with Professor Kim’s group to evaluate gallium nitride (GaN) films grown on silicon carbide (SiC) substrates utilizing graphene for the 2DLT process (Figure 2). This technology has demonstrated that the strong atomic interactions between the substrate and epitaxial material dominate the epitaxial growth, which is not impacted by the weak Van der Waals forces presented by the intermediate graphene film. This work has been demonstrated for a number of substrate and epitaxial materials, as well as for varying monolayers of graphene and hexagonal boron nitride.6 Potential applications for Raytheon include flexible electronics, heterogeneous integration of passive or active electronics, and integration of active devices onto high thermal conductivity substrates.

Structuring at the Nanoscale

Figure 3. A scanning electron microscope (SEM) image of two-phase nanocomposite material at 30,000 times magnification.

In 2007, Raytheon began a DARPA-funded program to develop nanocomposite optical ceramic windows (NCOC)7 for MWIR sensors. The objectives included increasing transmittance beyond approximately 5 microns, and reducing emittance at elevated temperatures — two properties that were lacking in commonly used MWIR materials like Sapphire, Spinel and ALON (aluminum oxynitride). A material system based on yttrium oxide (Y2O3) and magnesium oxide (MgO) was chosen. These materials are mutually insoluble. Therefore, they could be densified at high temperature and pressure without exhibiting the grain growth usually promoted by these conditions. Restricting the grain growth results in higher strength materials and reduces optical scattering. Grain size must be limited to approximately one twentieth of the wavelength (  /20) to minimize scattering caused by refractive index differences in the two constituent materials. For MWIR wavelengths of 3 to 5 microns, this means grain size must be
no larger than approximately 150nm. Figure 3 shows the microstructure of Raytheon’s Y2O3-MgO nanocomposite material. In Figure 4 is the resulting transmission of the two-phase material, as well as a comparison to Spinel, ALON and Sapphire in the MWIR band.

Figure 4. Transmission curves for two-millimeter thick nanocomposite optical ceramic (NCOC) material (top), and comparison through the mid-wave infrared (MWIR) spectrum to Spinel, aluminum oxynitride (ALON) and Sapphire (bottom).

Optical Sensing

Nanotechnologies enable Raytheon to deliver lower cost, high performance optical instruments for earth science, defense applications and commercial sensing. Carbon-based materials, such as carbon nanotubes (CNTs), vertically aligned CNT (VACNT) forests and graphene can absorb over 99.5% of incident photons. These carbon-based materials are often used as light absorption coatings on baffles and interior surfaces of the optical assembly. The National Institute of Standards and Technology (NIST) and the National Aeronautics and Space Administration (NASA) are exploiting the broadband absorption properties of VACNT forests as an absorption coating on the next generation of space-based bolometers for radiometry.8 VACNTs are also replacing black paints and polyurethane coatings in state-of-the-art blackbody infrared calibrators. 

Optical calibration supports improved imagery and scene analytics by providing flat field correction to remove artifacts from the image and a reference datum for measuring the spectral radiance of the scene. Multispectral and hyperspectral calibration systems consist of an integrating sphere for visible calibration, and one or more blackbody targets held at precise temperatures for long-wave infrared (LWIR) calibration. For large apertures, integrating spheres quickly become impractical, because of the high volume, mass and power required. Co-locating multiband sources for visible-through-LWIR calibration results in large size, mass, power and cost impacts to the optical instrument. 

Figure 5. A compact ultraviolet (UV) through long-wave infrared (LWIR) Hyperspectral Calibrator shown on a smallsat optical instrument.

Leveraging available nanotechnologies, Raytheon has developed a low-cost compact onboard calibrator for ultraviolet (UV) to LWIR hyperspectral imaging. The calibrator illustrated in Figure 5 is approximately the size of a ping-pong paddle, with patented calibration methods utilizing VACNTs, graphene, colloidal quantum dots (CQDs), and engineered phosphors.9 Compared to traditional calibration systems, the compact calibrator is approximately 80% smaller, is lighter and has a lower orbit average power. These characteristics enable installation on small satellites, calibration within a single orbit, and the ability to scale to large apertures.

Nanotechnologies in the longwave calibrator include a patented VACNT high emissivity top layer and internal graphene layers for heat spreading.10 Graphene is lightweight and very thermally conductive, improving both thermal agility and temperature uniformity across the calibrator.

Shortwave calibration is provided by an array of light emitting diodes (LEDs) that provide a relatively flat illumination pattern across the field of view when placed at the entrance pupil. The LEDs can contain engineered phosphors and/or CQDs that down-convert the ultraviolet (UV) light emitted by the diodes to longer wavelengths. While phosphors are widely used in commercial LEDs to down-convert from the emitted blue wavelength to yellow or green and red, phosphorescent materials provide wideband emission and may continue to fluoresce after the excitation source is removed. Alternatively, CQDs provide narrow and ultra-narrowband down-conversion as well as quicker photon re-emission, making them more suitable for fast imaging detectors.

Summary

The ability to manipulate matter at the atomic and molecular scale has opened new doors to research and development across a broad range of the physical, electronic and chemical sciences. With applications throughout the commercial and military industries, nanomaterials and nanotechnologies are enabling capabilities never before possible. In collaboration with academic and industry partners, Raytheon applies its expertise in advanced materials and manufacturing across product lifecycles, exploiting the benefits of nanoscale technology to advance product capabilities and create value for the customer.  

— K.C. Fong
— C. Haynie
— M. Herndon
— C. Koontz

 

Feynman, Richard P. (1960) “There’s Plenty of Room at the Bottom.” Engineering and Science, 23 (5). pp. 22-36.
Observation of the Dirac fluid and the breakdown of the Wiedemann-Franz law in graphene, Science 351, 1058 (2016).
Graphene-Based Josephson-Junction Single-Photon Detector, Physical Review Applied 8, 024022 (2017).
Fast thermal relaxation in cavity-coupled graphene bolometers with a Johnson noise read-out, Nature Nanotechnology 13, 797 (2018).
Remote epitaxy through graphene for two-dimensional material based layer transfer, Nature 544, 340 (2017).
Polarity governs atomic interaction through two-dimensional materials,  Nature Materials 17, 999 (2018).

Properties of an Infrared‐Transparent MgO:Y2O3 Nanocomposite, Journal of the American Ceramic Society 96, 3828 (2013).
“NIST-on-a-Chip: Quantum Optics and Radiometry – Chip-scale Radiometers and Detectors.”  https://www.nist.gov/pml/productsservices/nist-chip-portal/nist-chip-quantum-optics-and-radiometry/nist-chip-quantum.
U.S. Patents # 9459154, 9086327, 10054485, 10139287.
10 U.S. Patent # 9459154.