Discovery of a New Mechanism to Efficiently Induce Luminescence in Carbon Nanotubes
-- Toward the Development of Nanosized Quantum Optical Devices that Can Operate at Room Temperature Without the Use of Rare Elements --
As part of the Japan Science and Technology Agency (JST) Strategic Basic Research Programs, a research team, which consisted of the Institute of Advanced Energy (IAE) specially appointed associate professor Yuhei Miyauchi, IAE professor Kazunari Matsuda, and other researchers from IAE and The University of Tokyo, has made a groundbreaking international discovery of a new mechanism to efficiently induce luminescence in carbon nanotubes, which are expected to have applications in future nanometer-sized quantum optoelectronic devices.
Carbon nanotubes1 are a cylindrical nanomaterial constructed by rolling one-atom-thick sheets of carbon atoms (graphene) into extremely thin wires (quantum wires2). They are about 1 nanometer in diameter, or about 1/100,000 the diameter of a stand of human hair. These extremely thin quantum wires emit near-infrared light (luminesce) when they are excited by light or electric current, and have potential applications in nanosized energy-saving light sources for optical fiber communications and in high-sensitivity photoelectric detectors. Because the luminescence quantum yield3 of carbon nanotubes is typically very low (at about 1%), this value must be significantly increased for practical devices (i.e., they must shine brighter).
In this study, we successfully embedded local zero-dimension-like states (quantum dots4) that locally trap electrons into carbon nanotubes with a small number ratio of about one quantum dot per several hundred nanometers of quantum wire. We then studied the resulting luminescence quantum yield at room temperature, and found that the zero-dimension-like states (quantum dots) exhibit an extraordinarily high luminescence quantum yield of about 18%, which is about 20 times larger than that of the one-dimensional carbon nanotube wire (about 1%). This breakthrough indicates that introducing quantum dots into carbon nanotube quantum wires can produce very high luminescence quantum yields, which greatly surpass the low values inherent to pure carbon nanotubes.
Because carbon nanotubes have a low environmental impact and are made entirely of carbon, which is a ubiquitous element, our findings can help eliminate the need for rare metals and rare earth elements5. For example, carbon nanotubes may provide highly efficient light sources for optical fiber communications. Furthermore, the increased luminescence quantum yield occurs at room temperature, suggesting that the unique properties of the bright zero-dimension-like states (quantum dots) embedded in carbon nanotube quantum wires may realize room temperature operations of quantum optical functions and devices that use the wave properties of electrons. Currently, these have only been realized at extremely low temperatures (typically at the liquid helium temperature of -269°C). In addition to reducing the need for rare elements, our research has the potential to eliminate the need for cooling systems that consume a large amount of energy. This study was published online on July 7, 2013 in Nature Photonics, which is a British peer-reviewed scientific journal.
This study was conducted as a part of the PRESTO program (a component of the JST Strategic Basic Research Programs) under the research area of Nanosystems and Emergent Functions with Yoshihito Osada as the research supervisor.
Explanation of the Terminology
1) Carbon nanotubes
Carbon nanotubes are cylindrical nanomaterials with diameters ranging from one to several nanometers. They are made from one to a few graphene sheets, which consist of six-membered carbon rings arranged in a honeycomb lattice. In particular, carbon nanotubes made from a single sheet of graphene are called single-walled nanotubes. Although their diameter is nanosized, they can be grown to lengths of several micrometers or even several millimeters. They are considered to be the closest man-made material to ideal one-dimensional nanostructures (quantum wires2). Depending on how the graphene sheets are wrapped, carbon nanotubes can be metallic or semiconducting. Our study employed semiconducting carbon nanotubes.
2) Quantum wires
In general, a quantum wire refers to a state where electrons are confined in two dimensions, or to a nanostructure in which this state is realized. In an ideal quantum wire, electrons can only move in one dimension, causing the electrons inside the quantum wire to exhibit characteristics unique to one-dimensional electron systems, which differ from those in the bulk.
3) Luminescence quantum yield
Here, the luminescence quantum yield refers to the efficiency with which an exciton in some state is converted into a photon or a quantum of light. A higher value of luminescence quantum yield indicates that the material luminesces more efficiently and brightly.
4) Quantum dots
In general, a quantum dot refers to a state where electrons are confined in all three dimensions, or to a nanostructure in which this state is realized. Quantum dots are typically synthesized by the self-assembly of compound semiconductors into nanosized particles or disks. They are expected to have a wide variety of applications, including single-electron transistors, single-photon generators, quantum dot lasers, and quantum dot solar cells. In this study, quantum-dot-like states are created in carbon nanotube quantum wires by inhibiting the freedom of motion in the axial direction of the nanotube from electrons already confined in two dimensions.
5) Rare metals and rare earth elements
Rare metals and rare earth elements are elements with a limited distribution and/or production, and consequently, are rare and expensive. Gallium and indium are examples of rare metals, while yttrium and erbium are examples of rare earth elements.