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NEWS & VIEWS LIGHT-EMITTING DEVICES
From nano-optics to street lights Nanocrystal quantum dots are excellent emitters of light, but energizing that emission electrically has proved difficult. A newly discovered non-radiative energy-transfer process that allows electron–hole pairs to be generated in quantum wells, and then transferred into quantum dots, could lead to a new generation of ultra-high-efficiency light sources for use at scales ranging from nano-optics to a city block.
MARK STOCKMAN is at the Department of
Figure 1 A semiconductor quantum well (purple) is electrically excited through the correspondingly doped layers (n-doped indicated by the minus signs and p-doped indicated by plus signs).The injected electron and hole carriers in the quantum well form pairs indicated by ± signs and recombine.The released energy is non-radiatively (by the Förster mechanism) transferred to the NQDs (red spheres) at the top of the system.The NQDs are covered by a stabilizing layer of organic molecules shown as white cones.The Förster energy transfer is indicated by arrows marked ET.The electrical energy source is shown as a battery marked V.The transferred energy is released mostly as optical radiation,shown as the red glow, whose frequency is tunable by the size of the quantum dots.
Physics and Astronomy, Georgia State University, Atlanta, Georgia 30303, USA e-mail:
[email protected]
N
ot since the oil crisis of the 1970s has the need to address the world’s ever-growing demand for energy been greater,yet decisive solutions are still to be found.Feeding this demand with fossil fuels pollutes the environment,exacerbates global warming and is ultimately unsustainable.Increasing our reliance on nuclear power,although greenhouse-neutral,has its own set of environmental concerns,and increases the danger of the proliferation of nuclear weapons,and of the chances of fissile material ending up in the hands of terrorists and rogue states.Until cleaner,safer and sustainable means for energy production are developed to meet this demand,the only option available to help minimize the impact of this crisis is to find ways to use the energy resources we have more efficiently.Writing in Nature,Victor Klimov and colleagues1 describe an effect and device that could provide a promising new route towards achieving unprecedented energy efficiency in a sector that forms a significant part of the world energy demands — lighting.Their approach combines the good electrical behaviour of semiconducting quantumwell structures with the exceptional luminescent properties of nanocrystal quantum dots (NQDs). This bears promise to result in white as well as coloured light sources that could be scaled from nanometre to metre dimensions for applications ranging from nano-optics to optoelectronics,and to street lighting. NQDs are nanometre-scale particles usually made of direct bandgap semiconductors (such as cadmium selenide and zinc sulphide) that are known to be very efficient light emitters, and whose emission wavelength and colour can be controlled by tailoring their size. Moreover, they possess good long-term chemical and luminescent stability, and are relatively easy to produce. Their one drawback for use in lighting sources, however, has been the difficulty in exciting them electrically, owing to the fact that each is generally coated in an insulating layer of organic molecules. This problem is inherent in the previous attempts to make light-emitting
(Picture courtesy of Mollie Boorman and Marc Achermann.)
devices using them — usually based on composite NQD-conjugated polymer structures2–4. The proposal1 by Klimov’s group for overcoming this is simple, elegant, and even somewhat counterintuitive. The structure of the authors’proposed lightemitting device (Fig. 1) consists of two parts, a semiconductor InGaN quantum well, which is injected with electron–hole pairs through doped semiconductor electrodes, and a layer of CdSe/ZnS core–shell NQDs that actually produce light. Semiconductor quantum wells are device structures that are finding increasing use for making solid-state light sources, and most notably for semiconductor lasers. However, when it comes to the emission of light at the blue end of the visible spectrum — crucial for many applications,
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NEWS & VIEWS including the construction of white light sources — the efficiency of quantum-well devices is poor. The introduction of NQDs, whose luminescent efficiency is substantially higher, attempts to address this, but the real trick comes in how the excitation is transferred from one part to the other. Instead of attempting to inject the holes and electrons needed for light emission across the insulating organic layer that coats each NQD — as occurs in more conventional NQD–polymer devices — they take advantage of a long-known non-radiative transfer process known as Förster energy transfer5. The electron–hole pairs generated in the quantum wells recombine there, but do not emit light, as in the known quantum-well light-emitting devices.At the same time, their energy released in the process of this recombination is not wasted but transferred to NQDs by a process of the Förster energy transfer: the oscillating electric fields created by the annihilating electron–hole pairs in the quantum well reach out to the neighbouring NQDs and generate there electron and holes that subsequently relax and form excitons. Moreover, because the luminescent efficiency of NQDs is so high, almost every electron–hole pair excited in this way results in the emission of a photon. This is a counterintuitive effect of the Förster energy transfer: that is, this extra step actually increases efficiency. Another important point is that the emission wavelength (or colour) can be easily tuned by changing the size of the NQDs used — with larger dots emitting towards the red and smaller dots emitting towards the blue. In this way, the authors’device structure has the potential to emit radiation spanning the entire visible and near-infrared region. The quantum-well/NQD system offers a practical solution to the problem of a general lighting source suitable for the illumination of objects from big cities to optical microscopes, because of its efficiency, stability, expected longevity, and potential ease of fabrication in industrial amounts. Moreover, beyond its potential for
general lighting, this approach could also bring new possibilities to the world of nano-optics.At this end of the size spectrum, manipulating light becomes difficult owning to the known impossibility of focusing light to nanoscale regions due to diffraction effects. In answer to this, an excited NQD would create intense, nanometresize optical fields due to its oscillating dipole moment. The possibility for efficient generation of local optical fields — which could be used for carrying out optical probing, manipulation and spectroscopic analysis — at the nanometre scale is moved one step closer to reality by the ability to excite NQDs by the Förster transfer of energy from a quantum well. Finally, by using a cascade of energy transfers along a chain of other NQDs — the feasibility of which has already been established6,7 — the potential functionality of this approach could be extended still further. Although the purpose of the proposed device structure is to achieve high-efficiency, electrically pumped light emission, so far the authors have only demonstrated the optically pumped operation experimentally. This allowed the authors to obtain much clearer insight into the mechanisms of the energy transfer and emission processes taking place. However, there is no reason to doubt that achieving electrically driven operation will be relatively straightforward, and this will be the obvious next step. When this is done, both large-scale and nanoscale quantum-well/NQD-based light-emitting devices should soon follow. References 1. Achermann, M. et al. Nature 429, 642–646 (2004). 2. Colvin, V., Schlamp, M. & Alivisatos, A. Nature 370, 354–357 (1994). 3. Schlamp, M. C., Peng, X. G. & Alivisatos, A. P. J. Appl. Phys. 82, 5837–5842 (1997). 4. Coe, S., Woo, W.-K., Bawendi, M. & Bulovic, V. Nature 420, 800–803 (2002). 5. Basko, D., Rocca, G. C. L., Bassani, F. & Agranovich, V. M. Eur. Phys. J. B 8, 353–362 (1999). 6. Crooker, S. A., Hollingsworth, J. A., Tretiak, S. & Klimov, V. I. Phys. Rev. Lett. 89, 1868021 (2002). 7. Kawazoe, T., Kobayashi, K., Sangu, S. & Ohtsu, M. Appl. Phys. Lett. 82, 2957–2959 (2003).
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