Imagine creating novel devices with amazing and exotic optical properties not found in Nature—by simply evaporating a droplet of particles on a surface.
By chemically building clusters of nanospheres from a liquid, a team of Harvard researchers, in collaboration with scientists at Rice University, the University of Texas at Austin, and the University of Houston, has developed just that.
The finding, published in the May 28 issue of Science, demonstrates simple scalable devices that exhibit customizable optical properties suitable for applications ranging from highly sensitive sensors and detectors to invisibility cloaks. Using particles consisting of concentric metallic and insulating shells, Jonathan Fan, a graduate student at the Harvard School of Engineering and Applied Sciences (SEAS), his lead co-author Federico Capasso, Robert L. Wallace Professor of Applied Physics and Vinton Hayes Senior Research Fellow in Electrical Engineering at SEAS, and Vinothan Manoharan, Associate professor of Chemical Engineering and Physics at SEAS and Harvard's Physics Department, devised a bottom-up, self-assembly approach to meet the design challenge.
"A longstanding challenge in optical engineering has been to find ways to make structures of size much smaller than the wavelength that exhibit desired and interesting properties," says Fan. "At visible frequencies, these structures must be nanoscale."
In contrast, most nanoscale devices are fabricated using top-down approaches, akin to how computer chips are manufactured. The smallest sizes that can be realized by such techniques are severely constrained by the intrinsic limits of the fabrication process, such as the wavelength of light used in the process. Moreover, such methods are restricted to planar geometries, are expensive, and require intense infrastructure such as cleanrooms.
"With our bottom-up approach, we mimic the way nature creates innovative structures, which exhibit extremely useful properties," explains Capasso. "Our nanoclusters behave as tiny optical circuits and could be the basis of new technology such as detectors of single molecules, efficient and biologically compatible probes in cancer therapeutics, and optical tweezers to manipulate and sort out nano-sized particles. Moreover, the fabrication process is much simpler and cheaper to carry out."
The researcher's self-assembly method requires nothing more than a bit of mixing and drying. To form the clusters, the particles are first coated with a polymer, and a droplet of them is then evaporated on a water-repellent surface. In the process of evaporation, the particles pack together into small clusters. Using polymer spacers to separate the nanoparticles, the researchers were able to controllably achieve a two nanometer gap between the particles—far better resolution than traditional top-down methods allow.
Two types of resulting optical circuits are of considerable interest. A trimer, comprising three equally-spaced particles, can support a magnetic response, an essential property of invisibility cloaks and materials that exhibit negative refractive index.
"In essence, the trimer acts as a nanoscale resonator that can support a circulating loop of current at visible and near-infrared frequencies," says Fan. "This structure functions as a nanoscale magnet at optical frequencies, something that natural materials cannot do."
Heptamers, or packed seven particle structures, exhibit almost no scattering for a narrow range of well-defined colors or wavelengths when illuminated with white light. These sharp dips, known as Fano resonances, arise from the interference of two modes of electron oscillations, a "bright" mode and a non-optically active "dark" mode, in the nanoparticle.
"Heptamers are very efficient at creating extremely intense electric fields localized in nanometer-size regions where molecules and nanoscale particles can be trapped, manipulated, and detected. Molecular sensing would rely on detecting shifts in the narrow spectra dips," says Capasso.
Ultimately, all of the self-assembled circuit designs can be readily tuned by varying the geometry, how the particles are separated, and the chemical environment. In short, the new method allows a "tool kit" for manipulating "artificial molecules" in such a way to create optical properties at will, a feature the researchers expect is broadly generalizable to a host of other characteristics.
Looking ahead, the researchers plan to work on achieving higher cluster yields and hope to assemble three-dimensional structures at the macroscale, a "holy grail" of materials science.
"We are excited by the potentially scalability of the method," says Manoharan. "Spheres are the easiest shapes to assemble as they can be readily packed together. While we only demonstrated here planar particle clusters, our method can be extended to three-dimensional structures, something that a top-down approach would have difficulty doing."
Fan, Capasso, and Manoharan's co-authors included Chihhui Wu and Gennady Shvets of University of Texas at Austin; Jiming Bao of the University of Houston; and Kui Bao, Rizia Bardhan, Naomi Halas, and Peter Norlander, all of Rice University.
The researchers acknowledge the support of National Science Foundation, the Air Force Office of Scientific Research; the U.S. Department of Defense; the Robert A. Welch Foundation; and the Center for Advanced Solar Photophysics, a U.S. Department of Energy Frontier Research Center. The work was carried out at the Center for Nanoscale Systems at Harvard, a member of the National Nanotechnology Infrastructure Network.
Michael Patrick Rutter | EurekAlert!
Magnetic nano-imaging on a table top
20.04.2018 | Georg-August-Universität Göttingen
New record on squeezing light to one atom: Atomic Lego guides light below one nanometer
20.04.2018 | ICFO-The Institute of Photonic Sciences
University of Connecticut researchers have created a biodegradable composite made of silk fibers that can be used to repair broken load-bearing bones without the complications sometimes presented by other materials.
Repairing major load-bearing bones such as those in the leg can be a long and uncomfortable process.
Study published in the journal ACS Applied Materials & Interfaces is the outcome of an international effort that included teams from Dresden and Berlin in Germany, and the US.
Scientists at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) together with colleagues from the Helmholtz-Zentrum Berlin (HZB) and the University of Virginia...
Novel highly efficient and brilliant gamma-ray source: Based on model calculations, physicists of the Max PIanck Institute for Nuclear Physics in Heidelberg propose a novel method for an efficient high-brilliance gamma-ray source. A giant collimated gamma-ray pulse is generated from the interaction of a dense ultra-relativistic electron beam with a thin solid conductor. Energetic gamma-rays are copiously produced as the electron beam splits into filaments while propagating across the conductor. The resulting gamma-ray energy and flux enable novel experiments in nuclear and fundamental physics.
The typical wavelength of light interacting with an object of the microcosm scales with the size of this object. For atoms, this ranges from visible light to...
Stable joint cartilage can be produced from adult stem cells originating from bone marrow. This is made possible by inducing specific molecular processes occurring during embryonic cartilage formation, as researchers from the University and University Hospital of Basel report in the scientific journal PNAS.
Certain mesenchymal stem/stromal cells from the bone marrow of adults are considered extremely promising for skeletal tissue regeneration. These adult stem...
In the fight against cancer, scientists are developing new drugs to hit tumor cells at so far unused weak points. Such a “sore spot” is the protein complex...
13.04.2018 | Event News
12.04.2018 | Event News
09.04.2018 | Event News
20.04.2018 | Physics and Astronomy
20.04.2018 | Interdisciplinary Research
20.04.2018 | Physics and Astronomy