****NON-TECHNICAL ABSTRACT**** Despite being uninformed by the modern theories of quantum mechanics and relativity, Max Planck's 1900 blackbody law describing thermal radiation appears repeatedly at the forefront of current research. This Faculty Early Career Award funds a project that will critically examine blackbody radiation from nanoscopic objects, and improve science education at several levels. Experimentally, carbon nanotubes will be configured as light bulb filaments, and their temperature will be determined by the color of the radiation they emit. Unlike those in all normal light bulbs, these filaments are narrow compared to the wavelength of the radiation they emit, and thus they are outside the usual regime where classical thermodynamics is expected to be valid. The challenge is to elucidate the modifications to Planck's law that are not material-dependent, but rather functions only of size and dimensionality. A deeper understanding of these effects will impact not only such fundamental areas as the intersection of thermodynamics with quantum mechanics, but also very specific unsolved problems. The graduate students involved with this research will learn skills that will equip them for future scientific careers. The educational improvements funded by this award exploit the ubiquity of blackbody radiation. Courses and activities will be aimed at education at many levels, high school through graduate school, as well as the general public. As blackbody radiation is currently observable coming from sources as varied as the Big Bang and toaster ovens, it forms a natural bridge between the very forefront of physics research and everyday life.
Despite being uninformed by the modern theories of quantum mechanics and relativity, Planck's venerable blackbody radiation law appears repeatedly at the forefront of current research. This Faculty Early Career Award funds a project that will critically examine blackbody radiation in the nanothermodynamic limit, and improve science education at several levels. This project will experimentally investigate thermal radiators that are small compared to their thermal radiation's characteristic wavelength, and hot, with thermal energy scale larger than the quantum level spacings. Carbon nanotubes will be configured as light bulb filaments, and their temperature will be determined by super-resolution optical pyrometry. The challenge is to elucidate the modifications to Planck's law that are not material-dependent, but rather functions only of size and dimensionality. A deeper understanding of these effects will impact not only such fundamental areas as the intersection of thermodynamics with quantum mechanics, but also very specific open problems, such as quantifying the temperatures achieved in sonoluminescing bubbles. The graduate students involved with this research will learn skills that will equip them for future scientific careers. The educational improvements funded by this award exploit the ubiquity of blackbody radiation. Courses and activities will be aimed at education at many levels, high school through graduate school, as well as the general public. As blackbody radiation is currently observable coming from sources as varied as the Big Bang and toaster ovens, it forms a natural bridge between the very forefront of physics research and everyday life.
We have been doing experiments designed to probe systems that are on the boundary between ‘small’ and ‘large’. Small systems contain only a few atoms and are well described by quantum mechanics, while large ones contain many atoms and are well described by thermodynamics. To explore this regime, we have built tiny incandescent lamps with filaments consisting of pure, crystalline carbon. These lamps contain millions of atoms but are still small in comparison to the characteristic wavelengths of the light that they emit. Thus they are simultaneously thermodynamic and quantum mechanical. We have built devices with single carbon nanotubes as their filaments. These devices can be described as the world’s smallest light bulbs. The filaments are small enough that, like individual atoms, they are completely invisible when they are not energized, even when viewed in the best optical microscopes. However, when we apply a voltage to turn them on, they light up bright enough to be seen with the naked eye. While a normal incandescent lamp has an opaque filament that radiates in proportion to its surface area, these nanolamps are partly transparent and radiate in proportion to their volume. Like a normal incandescent lamp, the nanolamps fail when the hot filament starts to evaporate, which in this case occurs at a temperature around 2300 K. We have also made incandescent devices using filaments consisting of atomically-thin, two-dimensional (2D) sheets of carbon instead of carbon nanotubes. This 2D material, a single graphite layer, is considered the parent material of carbon nanotubes and is known as graphene. The discovery in 2004 of a simple technique for producing graphene surprised some scientists, since theoretical results indicated that 2D (and 1D) crystals would be unstable at room temperature. However, the expected melting transition is driven by crystal vibrations, or sound, with very long wavelengths. Because our nanolamp filaments, and indeed any laboratory-scale samples, are too small to support these low-frequency, long-wavelength sound waves, they melt at nearly the same temperature as bulk, 3D graphite. To summarize: 1D and 2D crystals have two and one very small dimension respectively, which makes them less stable than a 3D crystal. But having a small size in the remaining spatial dimensions tends to counteract this effect. For most low-dimensional materials the counterbalancing is incomplete and their melting temperatures are substantially lowered, but the carbon-carbon bond in graphene or carbon nanotubes is so strong that the cancelation of these effects is nearly complete. Thus, surprisingly, even a graphene sheet as large as a football field is not in the (unstable) thermodynamic limit, but rather is a stable (quantum mechanical) object. This award has supported an outreach program that brings science demonstrations to low-performing high schools in the Los Angeles Unified School District, and has helped provide hands-on workshops at UCLA for Latino middle and elementary school students. In addition, this award supported the training and education of graduate and undergraduate students. Our recent graduate at the PhD level is now developing high-efficiency solar cells at Spectrolab, Inc., and our recent graduate at the bachelor’s degree level is in physics graduate school at UC Berkeley.
