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Plasmonic nanostructures for enhanced LED efficiency

“Shih-Hao Chuang and Dong-Sing Wuu”

“A novel transparent electrode embedded with silver-based nanoparticles enhances the light extraction efficiency of LEDs, improving device performance by 88%.”

Due to the important role that they play in solid-state illumination, LEDs represent a promising candidate for next-generation lighting technology. However, confinement—which occurs due to the large difference in refractive index between the semiconductor and the ambient media—leads to severe total internal reflection at the interface, lowering the light extraction efficiency (LEE) of III-nitride LEDs. The light trapped inside of the LED device is eventually reabsorbed, thereby decreasing its efficiency. To achieve high light excitation and output performance, LEE enhancement is crucial.1

By modifying the chip shape or its surface morphology, the LEE can be improved. Several approaches have been proposed for the fabrication of different structures, either inside of the substrate or on its surface (e.g., photonic crystals, nanopyramids, a patterned substrate, surface roughness, and reflectors). The typical scale of these structures ranges from a few hundred nanometers to a few micrometers, depending on the resolution limit of the optical lithographic technology used. The texturing process still has several disadvantages, however, including non-uniformity, high cost, material degradation, and limited efficiency enhancement.

Due to its high optical transparency and low resistivity, a transparent conductive oxide layer (TCL) can be employed on the LED surface as an effective current-spreading layer and graded refractive index material.2, 3 We have developed a novel TCL embedded with a plasmonic nanostructure to enhance the effective light extraction in LEDs.4

Surface plasmons (collective charge oscillations, SPs), which are excited by the interaction between light and metal surfaces, can enhance light absorption in molecules and increase Raman scattering intensities. Fluctuations of the electromagnetic fields (surface plasmon polaritons, SPPs) accompany the charge fluctuations that arise due to SP oscillation. If the metal/semiconductor surface is flat, extracting light from the SPP mode is difficult, and because the SP is non-radiative, the SPP energy thermally dissipates. However, if roughness or nanostructures are introduced on the surface of the metal layer, the SPP energy can be extracted as light due to scattering of high-momentum SPPs. This scattering causes them to lose momentum and couple, forming radiative light.5

To fabricate the TCL, we embedded silver-based nanoparticles into indium tin oxide (ITO). We grew these nanoparticles on the positively-doped gallium arsenide (p-GaN) layer of blue LED wafers and analyzed them via transmission electron microscopy. Several nanoparticles, with an average size of 25±10nm, were distributed randomly in the ITO film (see Figure 1). These metallic nanoparticles are able to scatter the propagating light and couple the localized surface plasmons (SPs confined to areas on the material surface, LSPs) to the light that is internally trapped in the ITO/p-GaN interface and the ITO layer. This light subsequently radiates outside, enhancing the LEE of the LEDs.

005949_10_fig1

Figure 1. Cross-sectional high-resolution transmission electron microscope images of an indium tin oxide (ITO) film embedded with silver-based nanoparticles. Ag2O: Silver oxide. d: The distance between adjacent planes.4

To embed the nanoparticles, we employed spin coating using a nanosilver solution (particle diameter ∼20nm). The silver layer was subsequently coated uniformly on the p-GaN top layer, leading to the formation of a nanoparticle morphology during the baking process. Because the optical response of the nanoparticles strongly depends on their size, shape, and density, we controlled the nanostructures by modulating the concentration of the nanosilver solution with the appropriate spin coating rate. Deposition of the ITO layer onto the nanoparticles via electron-beam evaporation resulted in the formation of a plasmonic nanostructure embedded within the TCL.

We measured device performance according to the optical output power as a function of the injection current (see Figure 2). An 88.10% enhancement in output power was achieved at an injection current of 350mA due to the excellent electrical characteristics and LSP resonance (LSPR) effects. As Figure 3 illustrates, the normal direction and large-angle far-field pattern of the LEDs were also subject to greater enhancement than those of conventional LEDs. These results indicate that the silver-based nanoparticles efficiently increase light extraction, even at large incident angles, and enhance light diffusion.

2

Figure 2. Light output power as a function of injection current for LEDs with a novel transparent conductive oxide layer (TCL), embedded with (blue line) and without (black line) a plasmonic nanostructure (silver nanoparticles, Ag). The inset shows the current-voltage characteristics of the devices.4

3

Figure 3. Far-field emission patterns of LEDs with (red) and without (black) silver nanoparticles at an injection current of 350mA.4

The transmission spectra of the silver nanoparticles on the glass substrate show an absorption peak at 437nm. This occurs due to the LSPR generated at the plane interface between the nanoparticles and glass under blue light (see Figure 4).6 The effect of LSP-light coupling on the high-energy side of the electroluminescence (EL) spectrum therefore leads to a blueshift of 2nm from its emission peak wavelength with respect to that of a conventional LED (see Figure 4). This spectral blueshift supports the existence of LSP-light coupling, resulting in an effective emission with a 70% increase in EL peak intensity.

005949_10_fig4

Figure 4. Electroluminescence (EL) spectra of the LED samples measured at 20mA. The inset shows the transmission spectra of silver nanoparticles on the glass substrate.4 a.u.: Arbitrary units.

In summary, the excellent performance of our device indicates that the incorporation of a metallic solution via spin coating could enable fabrication of blue LEDs suitable for high-power solid-state lighting. If the resonance wavelength of the LSPs is closer to the LED emission wavelength, it should result in more efficient coupling to LSPs. We are now planning to precisely control the resonance wavelength by incorporating metallic nanoparticles with various sizes, shapes, and periodic distances. Our next challenge is to demonstrate fabrication of ultraviolet LEDs with an LEE lower than that achieved in visible-wavelength LEDs.

 

References:

  1. A. I. Zhmakin, Enhancement of light extraction from light emitting diodes, Phys. Rep.498(4–5), p. 189-241, 2011.doi:10.1016/j.physrep.2010.11.001
  2. C.-Y. Fang, Y.-L. Liu, Y.-C. Lee, H.-L. Chen, D.-H. Wan, C.-C. Yu, Nanoparticle stacks with graded refractive indices enhance the omnidirectional light harvesting of solar cells and the light extraction of light-emitting diodes, Adv. Funct. Mater.23(11), p. 1412-1421, 2013.
  3. J.-Y. Cho, K.-J. Byeon, H. Lee, Forming the graded-refractive-index antireflection layers on light-emitting diodes to enhance the light extraction, Opt. Lett.36(16), p. 3203-3205, 2011.
  4. S.-H. Chuang, C.-S. Tsung, C.-H. Chen, S.-L. Ou, R.-H. Horng, C.-Y. Lin, D.-S. Wuu, Transparent conductive oxide films embedded with plasmonic nanostructure for light-emitting diode applications, ACS Appl. Mater. Interfaces7(4), p. 2546-2553, 2015. doi:10.1021/am507481n
  5. W. L. Barnes, Light-emitting devices: turning the tables on surface plasmons, Nat. Mater.3, p. 588-589, 2004.
  6. K. Okamoto, I. Niki, A. Shvartser, Y. Narukawa, T. Mukai, A. Scherer, Surface-plasmon-enhanced light emitters based on InGaN quantum wells, Nat. Mater.3(9), p. 601-605, 2004. doi:10.1038/nmat1198

 

News source: 19 May 2015, SPIE Newsroom. DOI: 10.1117/2.1201505.005949; http://spie.org/x113613.xml?pf=true&highlight=x2400&ArticleID=x113613

Putting a new spin on plasmonics

Researchers at Aalto University have discovered a novel way of combining plasmonic and magneto-optical effects.

Magnetic nanoparticles arranged in arrays put a twist on light: depending on the distance between the nanoparticles, one frequency of light (visible to the human eye by its colour) resonates in one direction; in the other direction, light (induced by quantum effects in the magnetic material) is enhanced at a different wavelength.

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DNA does design: 3D plasmonic photonic crystals are the first devices prepared by DNA-guided colloidal crystallization

Jan 14, 2015 by Stuart Mason Dambrot

(Phys.org)—As biotechnology and nanotechnology continue to merge, DNA-programmable methods have emerged as a way to provide unprecedented control over the assembly of nanoparticles into complex structures, including customizable periodic structures known as superlattices that allow fine tuning the interaction between light and highly organized collections of particles. Lattice structures have historically been two-dimensional because fabricating three-dimensional DNA lattices has been too difficult, while three-dimensional dielectric photonic crystals have well-established enhanced light–matter interactions. However, the dearth of synthetic means of creating plasmonic crystals (those that exploit surface plasmons produced from the interaction of light with metal-dielectric materials) based on arrays of nanoparticles has prevented them from being experimentally studied. At the same time, it has been suggested that polaritonic photonic crystals (PPCs) – plasmonic counterparts of photonic crystals – can prohibit light propagation and open a photonic band gap (also known as a polariton gap) by strong coupling between surface plasmons and photonic modes if the crystal is in a deep subwavelength size regime. (Polaritons are quasiparticles resulting from strong coupling of electromagnetic waves with an electric or magnetic dipole-carrying excitation.)

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A Highly Tunable and Fully Biocompatible Silk Nanoplasmonic Optical Sensor

Myungjae Lee et al., report a Highly Tunable and Fully Biocompatible Silk  Nanoplasmonic Optical Sensor. Novel concepts for manipulating plasmonic resonances and the biocompatibility of plasmonic devices offer great potential in versatile applications involving real-time and in vivo monitoring of analytes with high sensitivity in biomedical and biological research.

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An adaptive plasmonic lens

” Nature highlights a recent ACS Photonics study on a novel reconfigurable lens” 

In constant search for miniaturizing commercial optical devices, integrated micro-optical elements play a central role in the development of applications that aim to improve high-density data storage and imaging. Until now, such devices presented the drawback of fine aligning and adjusting the focus through mechanically operations, therefore, limiting their accuracy, size and speed.

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HAPPY NOWRUZ

Nowruz (Persian: نوروز‎, IPA: [nouˈɾuːz], meaning “[The] New Day”) is the name of the Iranian New Year. Nowruz marks the first day of spring or Equinox  and the beginning of the year in the Persian calendar.

Norooz

Source: http://en.wikipedia.org/wiki/Nowruz

Plasmonic lasers: On the fast track

The dependence of the output power on the delay indicated that the generated pulse length was less than a picosecond, which suggested an extremely high direct-modulation rate. Moreover, finer interferometric measurements of the spectral composition of the radiation allowed the authors to establish that the generated pulse was even shorter; on a subpicosecond scale. Thus, this spaser has more than a terahertz in the direct modulation bandwidth — a record-setting achievement.

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Nanosphere lithography for device fabrication

Nanosphere lithography (NSL), originally termed ‘ natural lithography’ by its inventors,1 is becoming a widespread bottom-up technique to pattern solid surfaces at the sub-micrometer and nanoscales. Groups such as Van Duyne’s2 at Northwestern University and others3 undertook pioneering work on NSL in the 1990s and early this decade, and a growing number of research laboratories around the globe now use the technique in many scientific disciplines. The approach has applications in various materials systems, is fast and scalable to large surface areas, and is inexpensive in terms of equipment and operation. Some variants of the technique have reached a high level of maturity and control. Therefore, it is likely that it will soon be used in device fabrication.

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