748 NATURE PHOTONICS | VOL 8 | OCTOBER 2014 | www.nature.com/naturephotonics

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Solid-state lighting based on white LEDs is an attractive solution for next-generation illumination, owing

to its outstanding energy effi ciency1,2. However, a serious issue that must be addressed is the trade-off between optimal luminous efficacy and acceptable colour-rendering index in conventional phosphor-converted white LEDs; one usually comes at the expense of the other3. Now, writing in Nature Materials, Philipp Pust and colleagues4 report how a new narrowband, red-emitting phosphor — Sr(LiAl3N4):Eu2+ (named SLA for convenience) — overcomes this limitation. The result is a phosphor-converted warm-white LED that simultaneously offers excellent colour characteristics and efficient operation.

White LEDs are typically fabricated by combining a blue GaN-based LED with a single yellow–green phosphor or a combination of red and green phosphors. The phosphor materials play a key role in developing high-quality white LEDs because their performances directly determine the luminous efficacy, colour quality, reliability and lifetime of the final devices5,6. Desirable LED phosphors must possess strong broadband absorption in the region of the LED emission, high luminescence efficiency, short decay times to avoid saturation at high excitation flux, excellent chemical and thermal stability, low-cost and environmental friendliness. Such requirements place stringent constrains on activator ions and host materials, thus limiting the number of suitable LED phosphors.

To suit use in demanding commercial and residential applications, high-power illumination-grade phosphor-converted white LEDs must not only be highly efficient but also offer a high-quality warm-white light with a colour rendering index of more than 80 and a low correlated colour temperature of 2,700–4,000 K (ref. 2). However, conventional white LEDs cannot currently satisfy all of these requirements simultaneously. At present, nearly all commercial phosphor-converted white LEDs consist of a blue LED chip and a YAG:Ce3+ yellow phosphor. Although

such an LED is efficient, its cold-white light has a poor colour rendering index (~70–80) and is thus not ideal for general illumination purposes. One proposed solution is LEDs that make use of both yellow (YAG:Ce3+) and red (typically CaAlSiN3:Eu2+) phosphors7, but unfortunately such devices have lower luminous efficiencies because a large portion of the broadband emission (full-width at half-maximum (FWHM) of ~90 nm) from CaAlSiN3:Eu2+ lies beyond the human eye’s long-wavelength sensitivity limit (~700 nm). Generally speaking, a high colour rendering index requires a broad emission profile that covers the full visible spectrum, whereas narrow-linewidth visible sources facilitate the maximum luminous efficacy by minimizing the spillover of light to wavelengths — particularly in the deep blue and deep red — where the human eye’s sensitivity is very poor5,8. It is therefore possible to optimize colour rendering index and luminous efficiency simultaneously by employing suitable red-emitting phosphors with narrowband emission spectra.

As a proof-of-concept, Pust et al.4 fabricated a prototype warm-white LED with an excellent colour rendering index of up to 91 (Fig. 1) by incorporating SLA as a red component. Impressively, this prototype LED device offers a higher luminous efficacy than state-of-the-art commercial devices, producing 14% more light than conventional warm-white LEDs with a

comparable colour rendering index, but with similar power consumption.

This result is important because it clearly demonstrates that high luminous efficacy and high colour rendering index are no longer mutually exclusive properties for phosphor-converted white LEDs, if carefully designed red phosphors are employed. The key feature of the new SLA phosphor is its intriguing narrowband deep-red emission at ~654 nm with a FWHM of only ~50 nm. Strikingly, this red emission is located within the sensitivity range of the human eye, thus minimizing energy loss in the near-infrared region, which occurs with the commercial red phosphor CaSiAlN3:Eu2+. Furthermore, the broadband emission spectrum of the fabricated prototype LED covers the entire visible spectral range, which is why it achieves a high colour rendering index.

The rare-earth ion Eu2+ has proved popular as a broadband emitter (FWHM greater than 70 nm) because its 4f 65d1 → 4f 7 emission transition is sensitive to local crystal field variations and can thus be manipulated by the choice of host material8,9. The success of Pust and colleagues in observing the narrowband emission of Eu2+ ions comes from their careful choice of SLA as a host. This compound consists of a highly condensed, rigid framework of AlN4 and LiN4 tetrahedra. Such a rigid structure results in a weak electron–phonon interaction for Eu2+, which significantly reduces the probability of unwanted non-radiative relaxation processes from excited Eu2+ ions. Moreover, the Eu2+ ion occupies only one type of strontium crystallographic site and is coordinated by eight nitrogen atoms in a highly symmetric cuboid-like environment, which is beneficial for minimizing the inhomogeneous line broadening effects often caused by different local crystal field environments around the activator sites. All of these factors together contribute to the small Stokes-shifted narrowband red emission of SLA. We can expect that other nitride compounds with similar crystal structures may be potential candidates capable of emitting attractive narrowband

SOLID-STATE LIGHTING

Red phosphor converts white LEDsThe development of high-performance red-emitting phosphors provides new opportunities for fabricating white LEDs with both high colour rendering index and high luminous efficacy.

Xiaoyong Huang

Figure 1 | Red-phosphor LED lamp with a correlated colour temperature of 2,700 K and a lumen equivalent of 282 lm W–1.

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NATURE PHOTONICS | VOL 8 | OCTOBER 2014 | www.nature.com/naturephotonics 749

news & views

red light from Eu2+ ions. Interestingly, the emission properties of SLA vary only slightly with Eu2+ concentration, thus ensuring colour stability and consistency for the resulting LEDs.

Another useful aspect of the SLA red phosphor is its thermal and chemical stability. Its luminescence quenching temperature is very high, and the temperature-induced colour fluctuation can be negligible. Furthermore, this phosphor shows a remarkable insensitivity to air and moisture. Such intrinsic durability originates from the high degree of condensation of the host lattice.

Fortunately, it seems that the synthesis of this new red phosphor is straightforward and convenient. Indeed, SLA powders can be fabricated at much lower temperatures than previous nitride-based phosphors6,7 and at normal pressures, thus allowing for lower manufacturing costs and providing exciting opportunities for large-scale manufacturing.

There is still room to improve this fascinating new red phosphor. First, its particle morphology could be better and its quantum efficiency is modest, but both of these may be able to be improved through optimization of the preparation conditions and post-treatment processes. Second, the peak of its red emission at ~654 nm is not optimal — a slightly

shorter wavelength would be preferable. A blue-shift in the emission peak position is needed to enhance the luminous efficacy while maintaining a high colour rendering index. Cross-substituting alkaline earth ions (replacing Sr2+ by Ba2+) is perhaps a promising way to achieve this10. Finally, additional experiments are also needed to investigate the long-term reliability of SLA before it suits use as an industrial product.

In a separate report in Nature Communications, Haomiao Zhu and co-workers11 also describe an interesting development in the area of red phosphor research. The researchers proposed a facile method for preparing highly efficient K2TiF6:Mn4+ narrow-line red-emitting phosphors (emission peak ~630 nm, FWHM less than 10 nm) with a quantum yield close to unity. Using the phosphor blend of YAG:Ce3+ and K2TiF6:Mn4+, they fabricated a high-performance warm-white LED with a low correlated colour temperature of 3,556 K, a high colour rendering index of 81 and a high luminous efficacy of 116 lm W‒1. This device did not exhibit luminescence saturation even at a drive current of 120 mA, despite the slow decay time of Mn4+ (5.70 ms).

Overall, Pust et al. have made important progress in finding high-performance red-emitting phosphors for high-power,

illumination-grade phosphor-converted white LEDs. The researchers have cracked the puzzle of how phosphor-converted LEDs are able to generate a pleasing warm-white light with both a high colour rendering index and high luminous efficacy. This breakthrough will definitely encourage further discovery and development of highly efficient phosphors for white LEDs, and strengthens the case for LED lighting as the first choice source for illumination in the future. ❐

Xiaoyong Huang is at the College of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, P. R. China. e-mail: huangxy04@126.com

References1. Humphreys, C. J. MRS Bull. 33, 459–470 (2008).2. Crawford, M. H. IEEE J. Sel. Top. Quant. Electron.

15, 1028–1040 (2009).3. Erdem, T. & Demir, H. V. Nature Photon. 5, 126 (2011).4. Pust, P. et al. Nature Mater. 13, 891–896 (2014).5. Smet, P. F., Parmentier A. B. & Poelman, D. J. Electrochem. Soc.

158, R37–R54 (2011).6. George, N. C., Denault, K. A. & Seshadri, R.

Annu. Rev. Mater. Res. 43, 481–501 (2013).7. Lin, C. C. et al. J. Electrochem. Soc. 157, H900–H903 (2010).8. Phillips, J. M. et al. Laser Photon. Rev.

1, 307–333 (2007).9. Huang, X. Y., Han, S. Y., Huang, W. & Liu, X. G. Chem. Soc. Rev.

42, 173–201 (2013).10. Xie, R.-J., Hirosaki, N., Takeda, T. & Suehiro T.

ECS J. Solid State Sci. Technol. 2, R3031–R3040 (2013).11. Zhu, H. et al. Nature Commun. 5, 4312 (2014).

INTEGRATED QUANTUM PHOTONICS

On-chip teleportation Quantum teleportation is achieved on a photonic chip, paving the way for scalable quantum information processing based on linear optical networks.

Xiao-Song Ma

The no-cloning theorem forbids the perfect copying of an unknown quantum state1. However, in 1993

Bennett and colleagues2 proposed a quantum teleportation protocol that can circumvent the no-cloning theorem and faithfully reproduce the quantum state of a particle at a distant location. Quantum teleportation is not only useful for transferring an unknown quantum state, but is also of crucial importance in quantum computation, a direction that may provide an exponential speed increase for certain computation tasks.

Writing in Nature Photonics, Benjamin Metcalf and colleagues3 report quantum teleportation on an integrated photonic chip — a milestone for integrated

quantum photonics. Their work may help to enable scalable quantum information processing based on linear optics.

Quantum teleportation uses both quantum and classical channels between two communication parties, typically called Alice and Bob. Alice and Bob use the quantum channel to share the maximally entangled bipartite state, or, the so-called Bell state (involving qubits Q2, belonging to Alice, and Q3, belonging to Bob). Qubit Q1 is the input (the one that is teleported) and its quantum state can be arbitrary and is unknown to Alice and Bob. Alice performs a two-qubit Bell-state measurement (BSM) on the input Q1 and half of the entangled state Q2. This operation has two consequences: first, Q1 and Q2 are

entangled and projected into one of the four Bell states with equal probabilities; second, Q2 and Q3 will be disentangled. By obtaining Alice’s BSM result, Bob can correspondingly transform the quantum state of Q3 so that it is identical to the original input state of Q1. Therefore, to achieve ideal quantum teleportation, it is necessary that Alice sends her BSM result to Bob using a classical communication channel. Bob can apply the required unitary transformation to his qubit, Q3, according to the information he has obtained from Alice and hence reproduce the quantum state of Q1. Here, the quantum resource — the entanglement between Q2 and Q3 — is crucial to the entire protocol. Without entanglement, one can only

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