2015 Science Highlights
Rare-Earth Element Nanoscaffolds to Enhance Energy-Generating Materials
Scientific Achievement: Observed an order of magnitude increase in ionic conductivity in a tunable, strained nanocomposite by varying the type of rare-earth (RE) metal-oxides in the nanoscaffold films.
Significance: Materials with tunable and fast ion-transport (high ionic-conductivity) have applications in energy materials and devices, such as solid oxide fuel cells, sensors, and data storage since they provide high efficiency.
Nanoscaffold films SrZrO3–RE2O3 (where RE = rare-earth elements Sm, Eu, Gd, Dy, Er) were explored to tune the oxygen ionic conductivity of the film.
- The ionic conductivity increased in proportion to tensile strain in the SrZrO3 film, which was induced by increasing the lattice mismatch by using different rare-earth elements in RE2O3.
- The ionic conductivity is highest in the tensile-strained SrZrO3–Sm2O3 nanoscaffold film and lowest in the non-strained SrZrO3–Er2O3 nanoscaffold film by almost an order of magnitude.
Publications: S. Lee, W. Zhang, F. Khatkhatay, Q. X. Jia, H. Wang, and J. L. MacManus-Driscoll, Adv. Funct. Mater., 25, 4328 (2015).
Toward a Low-Cost, Efficient, and Ubiquitous Photovoltaic Material
Scientific Achievement: Achieved previously elusive alloying of germanium with a high concentration of tin (as high as 42%) at the nanoscale level using a new synthetic approach and tested its light absorption properties.
Significance: The inefficient light absorption of germanium and silicon complicates their use in photovoltaic applications (solar energy capture), but it is significantly improved by adding tin. Nanoalloys of germanium and tin are also a potential replacement for pure germanium as an electrode material in lithium battery cells because germanium is brittle.
Developed a new synthesis to incorporate tin into germanium at the nanoscale level
Observed a ten-fold increase in absorption compared with pure germanium
Nanocrystals are solution-processable, making them ideal for solar energy capture applications
Publications: K. Ramasamy, P. G. Kotula, A. F. Fidler, M. T. Brumbach, J. M. Pietryga, and S. A. Ivanov, Chem. Mater. 27, 4640 (2015).
Investigating Effective Light-Harvesting at the Atomic Level
Scientific Achievement: A combined experimental and computational study of energy transfer reveals the relationship between electronic and nuclear forces during light-harvesting processes in dendrimers, a synthetic light-harvesting molecule that can be used in optical-electronic applications.
Significance: Dendrimers contain chromophores and have enormous potential for achieving efficient light capture and transport. This study helps develop improved organic photovoltaics and other solar energy conversion technologies with a new type of dendrimer.
Electronic energy-transfer involves the ultrafast collapse of the photoexcited wave function due to nonadiabatic electronic transitions (no heat energy required)
Computer simulations provide detailed information on quantum-mechanical processes at the nanoscale
Publications: J. F. Galindo, E. Atas, A. Altan, D. G. Kuroda, S. Fernandez-Alberti, S. Tretiak, V. Kleiman, and A. E. Roitberg, J. Am. Chem. Soc. 137, 11637 (2015).
MoS2 as a Powerful Catalyst for Hydrogen Fuels Production and Clean Energy
Scientific Achievement: Theoretical and experimental methods were used to create different MoS2 structures, a catalyst for artificial photosynthesis and hydrogen (H2) fuel production. Both methods revealed that the octahedral structure (1T′ phase) of MoS2 enhances H2 yields by four-fold compared with other MoS2 structures.
Significance: The 1T′ phase of MoS2 is energetically favorable for a catalytic reaction that produces H2. Using this phase as a catalyst produced the highest amount of H2 yet reported for MoS2. High-yielding H2 production is vital toward developing hydrogen fuels that use only solar energy and water (clean energy).
Using advanced microscopy and chemical methods, achieved direct visualization of different atomic arrangements in the MoS2 monolayer
- Used density functional theory (DFT) calculations to predict and rank the most energetically favorable phases/structures of MoS2
Reacted MoS2 with Li to create primarily 1T′ phases of MoS2 and experimentally tested H2 production by investigating the charge transfer between the fluorescent molecule Eosin Y (EY) and the Li-reacted MoS2
Publications: S. S. Chou, N. Sai, P. Lu, E. N. Coker, S. Liu, K. Artyushkova, T. S. Luk, B. Kaehr, and C. J. Brinker, Nat. Comm., 6, 8311 (2015).
Recipe for Elusive Single Photons
Scientific Achievement: A study explains how the competition between diffusion and collision processes of electron-hole pairs (excitons) affects single-photon generation in carbon nanotubes.
Significance: Reveals guiding principles toward using carbon nanotubes as single-photon sources in eavesdropping-proof quantum communication.
- Experimentally investigated diffusion and collision processes of excitons in nanotubes at room temperature
- Developed a new theoretical model to explain the correlation between these two processes
- Theoretical analysis further revealed that
strong localization of excitons could make single-photon generation possible by increasing the exciton annihilation process;
the creation of spatially non-overlapping excitons and an increase in emission efficiency of bi-exciton states make single-photon generation impossible.
Publications: X. Ma, O. Roslyak, J. G. Duque, X. Pang, S. K. Doorn, A. Piryatinski, D. H. Dunlap, and H. Htoon, “Influences of Exciton Diffusion and Exciton-Exciton Annihilationon Photon Emission Statistics of Carbon Nanotubes,” Phys. Rev. Lett. 115, 01741 (2015).
Plasmonic Antenna Nanolaser
Scientific Achievement: The radiative emission from a semiconductor quantum dot can be enhanced by plasmonic field interactions with a nearby metal antenna while keeping the non-radiative loss constant.
Significance: A plasmonic antenna can be used to enhance the emission of two-exciton states for photon-pair generation and plasmon-assisted lasing..
- Semiconductor quantum dots are deterministically placed at the center of the gold gap-bar antennas using a two-step e-beam lithography process
- Up to 9.6 and >3-fold enhancements in radiative decay rate and two-exciton emission efficiency were achieved, respectively
Publications: F. Wang, N.S. Karan, H.M. Nguyen, Y. Ghosh, C.J. Sheehan, J.A. Hollingsworth, and H. Htoon Nanoscale 7 , 9387 (2015).
Terahertz Conductivity in Carbon Microfibers
Scientific Achievement: Terahertz (THz) spectroscopy and imaging measured the resonances of individual, conductive carbon microfibers. Each microfiber “rings” (resonates) at a specific THz frequency.
Significance: Electrical conductivity is analyzed in individual carbon microfibers using THz spectroscopy. This technique is useful for measuring the conductivity of materials from carbon microfibers and can be applied to other semi-conductors.
- Fabricated photoconductive THz near-field probes
- Developed a novel technique for non-invasive probing of conductivity in carbon microfibers using resonant field enhancement and the resonant frequency
- Polymer mobility in the network is different from lipid analogs.
Publications: I. Khromova, M. Navarro-Cía, I. Brener, J. L. Reno, A. Ponomarev, and O. Mitrofanov, Appl. Phys. Lett. 107, 021102 (2015).
Biomolecular Machines Assemble Complex Polymer Networks
Scientific Achievement: Kinesin, one of nature’s biomolecular machines, is shown to both deform and dynamically reorganize highly-ordered polymer nanostructure networks.
Significance: Creates rapid and continuous assembly of nanostructures, which may enable a new class of self-healing electronic/photonic composite materials. Polymer networks display enhanced stability over lipid networks (24 hours vs. 4-5 hours).
- Kinesin/microtubule biomolecular machines exert sufficient mechanical force on polymer micelles to remove polymer nanotubes from micelle. Polymer networks were formed within 30 minutes.
- Polymer network morphology and size increased with the addition of lipid, serving as a fluidizing agent.
- Polymer mobility in the network is different from lipid analogs.
Contact: George Bachand and Wally Paxton
Publications: Paxton, WF, Bouxsein, NF, Henderson, IM, & Bachand, GD., Nanoscale 7, 10998 (2015).
Quantum Dot Molecules Formed in a Plasmonic Gap: A New Building Block for Quantum Information Networks
Scientific Achievement: An enhanced electromagnetic field (plasmonic field) couples quantum dots (QDs) together, forming quantum dot molecules.
Significance: Quantum dot molecules form key building blocks for quantum plasmonic devices (such as single-photon transistors) and quantum communication networks.
Small clusters of 2-3 silica-coated quantum dots “couple” under the influence of a plasmonic field and exhibit photon antibunching, normally a characteristic of single quantum emitters.
- Photon antibunching is observed by separating photons emitted by single-excitons from those produced by bi-excitons.
- Plasmon-enhanced charge interactions cause quantum dot clusters to behave as a single emitter.
Contact: Han Htoon and Jen Hollingsworth
Publications: Wang, F., Karan, N. S., Nguyen, H. M., Ghosh, Y., Sheehan, C. J., Hollingsworth, J. A., Htoon, H., Small, DOI:10.1002/smll. 201500823, (2015).
Doped carbon nanotubes: New building blocks for quantum information technologies
Scientific Achievement: Single photons were generated from oxygen-doped carbon nanotubes in SiO2 at room temperature.
Significance: New, doped nanotubes meet requirements for on-demand single-photon sources that function at room temperature and telecommunications wavelengths, requirements that are essential for quantum photonic devices and quantum information processing.
- Incorporation of carbon nanotubes into a SiO2 matrix leads to solitary oxygen dopant states capable of emission in the 1100-1300 nm wavelength range. The emission could be extended to 1500 nm.
- Nanotubes shows complete photon antibunching (i.e. single photon emission) at room temperature.
- Nanotubes emit strong photoluminescence that is free from fluctuations for long periods of time under continuous, intense laser irradiation.
Publications: Ma, X., Hartmann, N. F., Baldwin, J. K. S., Doorn, S. K. & Htoon, H. Nature Nanotechnology, DOI: 10.1038/NNANO.2015.1136 (2015)