TY - JOUR
AU - Ball, Philip
AB - Abstract Quantum dots introduce a new form of quantum engineering of materials properties based purely on size and dimensionality. Whereas we traditionally think of materials as having intrinsic properties such as strength, electrical conductivity and light absorption or emission spectra due to their composition and atomic-scale structure, when their size is very small, such properties can be altered by quantum-mechanical effects, for example due to confinement of the quantum wavefunctions that determine the energies of charge carriers like electrons. As it became possible in the 1990s to control the size of material structures with a precision of less than a nanometre in all three dimensions of space, quantum effects began to be exploited for technological applications. Quantum dots are particles with a nanoscale extent in three dimensions—in effect, tiny fragments of material, often made of one type of semiconductor embedded in another. Quantum dots in which the wavelength of absorption and fluorescent emission of light is tuned by the nanoparticle size are being explored for uses ranging from colour displays to biomedical imaging. Films of semiconducting materials thin enough to exhibit one-dimensional quantum-confinement effects, meanwhile, are routinely used to make efficient optical devices such as lasers. Such applications have depended on the availability of techniques for precise control of particle size, composition and thickness. NSR spoke to two leading researchers in this field of quantum engineering about its development and prospects. Dieter Bimberg is founding director of the Center of Nanophotonics at the Technical University of Berlin in Germany. He pioneered the use of quantum dots in photonic devices such as lasers and optical amplifiers, as well as developing non-volatile ‘dynamic’ random-access memories (DRAMs), which retain their information when the power is switched off. Kang Wang is a professor of electrical engineering at the University of California at Los Angeles, whose work on nanoscale quantum devices has focused on electronic and magnetic properties, and in particular on the development of non-volative RAMs and the manipulation and control of electron spin as a new parameter for information processing (a technology called spintronics). NSR: When did quantum dots really begin to progress from an interesting theoretical idea to structures of real practical importance for device technology? Wang: People have used quantum dots for hundreds of years without realizing their true nature. Nanoscale metal clusters create colours in antique glass—in particular, gold nanoparticles give a ruby red colour. Systematic experimental study of quantum dots thrived in the 1980s. Quantum confinement was first observed in gadolinium sulfide quantum dots: as the particle sizes shrinks, the photo-emission is blue-shifted because of changes in the quantum-confined energy levels. After three decades of development, quantum dots have found applications in various fields, such as drug delivery. Techniques of modern microscopy and elemental analysis have dramatically boosted progress in research on quantum dots. Meanwhile, more powerful computer-simulation methods can reveal the physical and chemical processes occurring in quantum dots, helping us develop a deep understanding of their nature. Bimberg: In 1986 Japanese electronic engineer Yasuharu Suematsu and colleagues published a theoretical paper predicting enormous advantages for photonic devices if conventional ‘quantum wells’ [electrons confined to thin films] could be replaced by quantum dots. Such lasers were first reported in the early 1990s, and in 1994 we showed the first quantum-dot lasers based on Stranski-Krastanov (SK) growth [where deposition of thin layers on a substrate occurs initially as small islands]. These operated at ultra-low current density, and can now achieve continuous-wave operation [a steady beam] at current densities even lower than the theoretical predictions. The discovery and realization of the SK growth mode, which requires no lithography for making the quantum dots, was the breakthrough. Open in new tabDownload slide Kang Wang is a professor of electrical engineering at the University of California at Los Angeles (Courtesy of Kang Wang). Open in new tabDownload slide Kang Wang is a professor of electrical engineering at the University of California at Los Angeles (Courtesy of Kang Wang). NSR: There are many ways to make these structures: via wet chemistry, chemical vapour deposition (CVD), nanolithography and so on. Which methods have proved most important? Are cheap, large-scale methods essential for applications, or can cost-intensive approaches still be valuable in the marketplace? Wang: The best way to fabricate quantum dots depends strongly on the application and research objective. For commercialization, of course price and fabrication scale are important, but the fabrication method is mainly determined by the application. In biological applications, people would like to use quantum dots for drug delivery; in catalysis, people want them to have reactive sites. In both cases, having a large surface area is the most important consideration. Generally, people want good sample yields, but a uniform morphology is not crucial. So wet chemistry and CVD methods are preferred in those cases because of the fast preparation process and high yields. But for applications such as photonics, precisely patterned quantum dots with well-controlled morphology are required. So here, fabrication needs precise lithographic processes. Bimberg: At present, metal-organic chemical vapour deposition (MOCVD) and molecular-beam epitaxy (MBE) are the growth methods of choice if you want to have real-world semiconductor devices superior to classical ones, and want to get accepted by industry. Both are inexpensive, large-volume technologies which are already well developed. NSR: What do you personally find to be the most attractive characteristics of quantum dots for applications? What are the important features that one needs to be able to control in order to get useful behaviour? Bimberg: It depends on the device. For lasers, you gain an ultra-low threshold current, high-speed cut-off [switching], high temperature stability, and the ability to extend the range of gallium-arsenide-based devices to the commercially relevant 1.3-μm wavelength band. For amplifiers, you need outstanding performance for linear (for example, wavelength multiplexing) and nonlinear applications (for example, wavelength switching). And for memories, it would be great to have the potential to merge DRAM and flash memory into one device, which would revolutionize computer design. Finally—and most attractively to many researchers today—the photon emitted from a single quantum dot can represent a quantum bit for quantum communication technologies. Open in new tabDownload slide Dieter Bimberg is founding director of the Center of Nanophotonics at the Technical University of Berlin in Germany (Courtesy of Dieter Bimberg). Open in new tabDownload slide Dieter Bimberg is founding director of the Center of Nanophotonics at the Technical University of Berlin in Germany (Courtesy of Dieter Bimberg). Wang: Quantum dots have many attractive aspects. In my personal research the value comes from quantum-confinement effects in these and other quantum hetero-structures, which permit precise control of energy levels. In some semiconductor quantum dots, it's possible to control the number of charge carriers in individual dots, which is important for quantum information processing. Other important phenomena include the Coulomb blockade [stepwise electron transport owing to electrostatic repulsion], exchange interactions and spin interactions [both strictly quantum-mechanical properties of electrons]. NSR: How important do you think quantum dots are likely to be for your fields of photonic information processing, memories and spintronics? What do you think will be the first devices and applications to find a market in this field, and how close are we to that? Bimberg: The window in time for applications is open now, and the introduction of lasers and amplifiers in emerging fields such as access and local-area networks and ‘fibre to the home’ systems is approaching. Big Chinese companies are starting now not only to become interested but to invest. Directly modulated lasers at 1.3-μm wavelength have been commercially available for several years, as has wafer growth by MBE. For memories, meanwhile, we’re still in the research phase. The window in time for applications is open now. Big Chinese companies are starting not only to become interested but to invest. —Dieter Bimberg Wang: Nanoscale engineering is playing a crucial role in modern spintronics and information technology. Here I’m not just talking about quantum dots; for example, thanks to nanotechnology we can fabricate magnetic tunnelling junctions and make use of so-called giant magnetoresistance (GMR) technology to increase the memory volume of a single hard drive to the order of terabytes. Single quantum dots can be treated as an isolated spin system—a single quantum bit for information storage—and they can talk to each other. Such systems would enable great increases in memory volume. What's more, it is possible to manipulate the magnetism in quantum dots with small electrical fields, in order to realize spintronic transistors for quantum information processing. In the health sciences, meanwhile, magnetic nanoparticles may be used to deliver drugs and for monitoring living organisms and tissues, including brain imaging. NSR: This is a field in which close collaboration between theory and experiment seems to be vital. Do you feel that we can now model quantum dots in sufficient detail to guide experiment? Wang: This is a good question. Quantum dots are very hard to simulate. We can predict the behaviour of an atom very precisely with quantum mechanics, because it is a single-body problem. For large crystals, we can approximate them as a single-body problem too, or use statistical physics to treat them as many-body systems. But the hardest problem is few-body systems—which are nanoscale ones. The objects and the interactions between them are more complicated than single-body systems, but not enough to permit a statistical treatment. So far, theoretical simulations of quantum dots can provide valuable guidelines for researchers, but generally speaking they may be not accurate enough to explain all the details of experimental observations, especially for few-atom clusters. Bimberg: We have developed excellent numerical models first for single-particle effects and then for many-particle effects (including exchange interaction) in quantum dots over the past 15 or more years. NSR: Have you seen significant new developments in this field coming out of China? Where in your view are the most important Chinese laboratories? Wang: China is nowadays one of the leading countries in the field of nanotechnology. The Chinese government devotes large efforts to building top-level laboratories and universities and attracting the best talent. And Chinese researchers are always intelligent and diligent. Ever more high-impact publications are coming out of Chinese institutions and universities. Most of the key labs are in the major cities along the coast and inland. It is really hard to list all my collaborators; they are in nearly every province in China. They have the state-of-art facilities to support the most advanced research. Bimberg: There has been basic research going on for many years now at various universities, and at the Chinese Academy of Sciences in Beijing. Among the most important applied research are device developments at the Changchun Institute of Optics, Fine Mechanics and Physics of the Chinese Academy of Sciences in Changchun, Jilin. NSR: This field is regularly tipped for a Nobel prize. Who should it go to, in your view? Wang: This is a hard question, and only the Nobel committee can answer it! It is true that a few Nobel prizes have been awarded in the nano field. But nanoscience features in every scientific discipline right now, from physics to chemistry, biology and medicine. Nanoscience features in every scientific discipline now, from physics to chemistry, biology and medicine. —Kang Wang However, I think such an award should satisfy these criteria. The research should offer a dramatic and fresh viewpoint, and have a strong impact on humankind. Or it should be instrumental in promoting the development of a research area, not only in terms of basic research but also engineering and product commercialization. The physics Nobel prizes awarded for GMR [2007] and blue light-emitting diodes [2014] are both examples of this. [Both involve precision semiconductor engineering.] Bimberg: Leave that to the Nobel committee! NSR: In the 1990s, this was a hot new field, but now it is a rather mature one. What advice would you give to a young researcher entering the field today? Bimberg: Only the InAs/GaAs growth technology is mature, and to a much lesser extent the InAs/InP technology. Gallium nitride technology [for green-light sources] is not really well explored. SK growth is a general concept that can be extended to any strained heterostructure. Young researchers should look for novel nanoheterostructures—too few are already explored—and for novel devices, and find out for which systems (such as energy-efficient photonics) they are advantageous. Wang: It is not the field itself that became mature, so much as the attitude of researchers. Hype always accompanies a new field—quantum dots weren’t unique in that regard. It takes time for people to understand the potential and limitations. For young researchers, it is very easy to follow temporary trends without clear understanding and deep thinking. I have seen many young scholars follow the newest publications, but it's not unusual for those to contain misunderstanding and even mistakes. If young scholars just want to ride a fashion and get high-impact publications, it's unhealthy for both personal development and the progress of the field. So I would suggest that young scholars study fundamental textbooks and classical references too when choosing their research topic, so that they can have a fresh view of what to pursue. © The Author(s) 2016. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is properly cited. for commercial re-use, please contact journals.permissions@oup.com © The Author(s) 2016. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com
TI - Quantum engineering of matter from the laboratory to the market: an interview with Dieter Bimberg and Kang Wang
JO - National Science Review
DO - 10.1093/nsr/nww067
DA - 2017-03-01
UR - https://www.deepdyve.com/lp/oxford-university-press/quantum-engineering-of-matter-from-the-laboratory-to-the-market-an-HF2deR1eTT
SP - 210
EP - 212
VL - 4
IS - 2
DP - DeepDyve
ER -