Light-emitting diodes enhanced by localized surface plasmon resonance
© Gu et al; licensee Springer. 2011
Received: 21 September 2010
Accepted: 8 March 2011
Published: 8 March 2011
Light-emitting diodes [LEDs] are of particular interest recently as their performance is approaching fluorescent/incandescent tubes. Moreover, their energy-saving property is attracting many researchers because of the huge energy crisis we are facing. Among all methods intending to enhance the efficiency and intensity of a conventional LED, localized surface plasmon resonance is a promising way. The mechanism is based on the energy coupling effect between the emitted photons from the semiconductor and metallic nanoparticles fabricated by nanotechnology. In this review, we describe the mechanism of this coupling effect and summarize the common fabrication techniques. The prospect, including the potential to replace fluorescent/incandescent lighting devices as well as applications to flat panel displays and optoelectronics, and future challenges with regard to the design of metallic nanostructures and fabrication techniques are discussed.
Light-emitting diodes [LEDs] have attracted much scientific and commercial interest since the realization of a practical LED device with emission frequencies in the visible region of the electromagnetic spectrum . Since then, research activities have been focusing on how to produce economical LEDs with the desired colors as well as white light sources . The strong demand has also driven materials technology, and new emitting materials and configurations have been proposed to enhance the performance. For example, the use of a polymer instead of small molecules opens the door to flexible, large-area, and stable organic LEDs [OLEDs] [ 3]. In the past 15 years, low-dimensional emitting devices incorporating quantum dots [QDs] and quantum wells [QWs] have been extensively investigated in order to achieve the desirable emission color and enhance device efficiency [4–10]. However, LEDs suffer from inherently low efficiency due to the sometimes low internal quantum efficiency [IQE] and difficulty extracting the generated photons out of the device. Although the use of electro-phosphorescent materials with proper management of both singlet and triplet excitons has brought IQE in OLEDs to almost unity [11–13], that of LEDs with inorganic emitting materials such as GaN, CdSe, and Si QDs or QWs remains unsatisfactory because non-radiative electron/hole pair recombination dominates. Another channel of energy loss is total internal reflection at the emitter/air interface because of the typically high refractive index of the emitting materials. Several methods have been proposed to enhance the overall efficiency of LEDs, and they include substrate modification and incorporation of scattering medium, micro-lenses, nanogratings, corrugated microstructures, photonic crystals, and so on [14–17]. In spite of some efficiency enhancement, spectral changes and angle-dependent colors associated with the substrate modification techniques, the high precision needed to produce nanogratings and the high cost of photonic crystals are still challenging issues plaguing commercial applications.
Summary of representative experimental results showing the important relationship between the LSPR energy and emission energy in order to attain the best enhancement
Emitting materials and configurations
Peak emission energy (nm)
Optical properties of metal layer
InGaN/GaN multiple QW
Transmittance exhibits absorption from 396 to 455 nm
32.2% with an 100-mA current
Kwon et al. 
Alq3 thin film
A peak in absorption at 510 nm
Fujiki et al. 
A dip in transmission at about 550 nm
150% in peak intensity with a 20-mA current
Yeh et al. 
A dip in transmission in 511 nm
180% with a 20-mA current
Sung et al. 
Large absorption from 330 to 500 nm
Qiu et al. 
A peak in absorption at 535 nm
Reaches maximum at 530 nm
Kim et al. 
Extinction band near 350 nm
You et al. 
Extinction band near 370 nm
Cheng et al. 
A dip in transmission at about 520 nm
2.5-fold in peak intensity
Pillai et al. 
A peak in absorption at about 600 nm
Pompa et al. 
A dip in transmission at 710 nm
Twofold, with the peak blue shifts
Biteen et al. 
A dip in transmission at about 440 nm
Mak et al. 
Enhancement of IQE
The local electric field and magnitude of the extinction spectrum are significantly enhanced at the LSPR frequency . This effect has been broadly studied and utilized in many fields such as surface-enhanced Raman spectroscopy, solar cells, and biosensors [27–30]. With regard to the efficiency improvement rendered by LSPR, it is supposed that the enhanced electric field interacts with the emitting materials, increasing the spontaneous emission rate and consequently enhancing the IQE of the device. This assumption can be partly verified by experiments showing that the radiative decay rate and spontaneous emission rate of the light emitters can be improved in the presence of silver SPPs since the enhanced local electric field at the LSPR frequency plays a similar role as the evanescent wave induced by SPPs [31, 32]. An example of the IQE enhancement in LSPR-enhanced LEDs is the GaN-based LED developed by Kown et al. . The optical output power increases by 32.2% at an input current of 100 mA, and the time-resolved PL measurement shows that the PL decay time in the presence of Ag NPs is significantly reduced. As a result, the spontaneous emission rate and the IQE are better.
The effect of varying the distance between the active emitting region and metallic NPs on the overall device efficiency enhancement also reveals that the larger local field plays an important role. Coupling and enhancing vanish as the distance is increased above a certain threshold, and the distance cannot be too small or too large. If it is too small, the non-radiative quenching process dominates and most of the energy is dissipated accordingly. On the contrary, if the distance is too large, the coupling effect vanishes since only the electron/hole pairs near the metallic NPs can effectively couple to increase the IQE. Fujiki et al.  have introduced a copper phthalocyanine hole transport layer in their LED structure as a spacer to avoid non-radiative quenching. In order to retain the effect, a 20-nm-thick film is used and smaller enhancement is observed if the thickness is larger. The experiment provides direct evidence of the role played by the higher local electric field in the IQE enhancement effect.
Enhancement of LEE
From the viewpoint of energy transfer, nanoscaled antennae can be incorporated to enhance the absorption of generated photons and subsequent reemission. As the boundary separating the fields of electronics and photonics is becoming more blurred, the concept of antennae developed for radio frequency and microwave communication has been extended to optical frequencies, and one potential application of these nanoscaled antennae is to enhance light extraction from light emitters [38, 39]. In the paper by Bakker et al. , a nanoantenna system consisting of two gold elliptical NPs is shown to enhance the LEE of a fluorescent dye by a factor of 20 to 100. Near-field measurements show that the enhanced emission is localized and polarized. Another group has reported an organic emitter coupled with silver nanoantenna arrays . In addition to increased light absorption in the ultraviolet [UV] range and enhanced PL efficiency, an energy transfer process responsible for light extraction is suggested. In fact, the effect of increased UV absorption is less dominant compared to the enhancement of LEE, as reported in similar experiments, and the results demonstrate the efficacy of nanoantennae in efficiently extracting generated photons.
Vacuum deposition such as sputtering or electron beam evaporation sometimes accompanied by post thermal processing [PTP], electron beam lithography [EBL], nanosphere lithography [NSL], nanoimprinting, and chemical synthesis are common methods to produce metallic NP arrays. However, not all of these techniques have been used to make LEDs due to various technological considerations. For instance, many of these methods involve chemical processing, which can harm the LED structure. At present, vacuum deposition is the most successful in fabricating EL LEDs, whereas only PL results have been obtained from samples produced by other techniques. Nonetheless, the PL results still have considerable value as they enable better understanding of the nature of the LSPR-induced enhancement and spur further development in the fabrication methods. In this section, we not only describe these various methods but also discuss the compatibility of each method from the perspective of LED fabrication, ease of implementation, and production cost.
Under ultrahigh vacuum conditions, a thin metal film can be formed on the LED structure via the condensation of atoms produced by evaporation. The film thickness is governed by factors such as the substrate type, ambient pressure, and sputtering time. One advantage of the technique is that it does not require chemical processing, and so chemical damage to the emitting region can be avoided. This method has been employed to produce Ag NP arrays on a silicon QD LED . To alter the optical properties such as the extinction spectrum of the materials for better enhancement, PTP is often applied. By conducting annealing at different temperatures and for different time durations on metal films with different thicknesses, NPs with the desirable sizes and heights can be produced in order to achieve the best coupling effect with the light emitters. Yeh et al.  have observed that the size and heights of Ag NPs are significantly increased after annealing three samples with initial film thicknesses of 5, 10, and 15 nm at 200°C for 30 min. Annealing enhances the PL intensity of the LSPR light emitters due to enhanced LSP/QW coupling. Other PL experiments conducted on Pt/ZnMgO films , Ag/ZnO films , and conjugated polymers  also demonstrate the constructive role of annealing after vacuum deposition in enhancing the coupling between the emitters and metallic NPs.
Electron beam lithography
Although EBL has some benefits over other fabrication methods, for instance, the high resolution and the ability to produce NPs with different shapes, it has disadvantages. For instance, large-area fabrication is difficult. The NPs produced are only two-dimensional and the equipment is expensive. Another disadvantage is that the acetone lift-off process harms the electron or hole transport layer. Future research needs to focus on introducing some protection during patterning of the metallic NPs.
Using patterned templates, various nanostructures, both two- and three-dimensional, for example cylinders, squares grooves, and pyramids, can be easily fabricated. Frequently used templates are often produced from porous anodic alumina [PAA] or by lithography techniques [41, 63, 64]. The advantage of this method is avoidance of chemical peel-off, thus making incorporation of metallic NPs into EL LEDs possible. At present, only PAA-based templates have been applied to the fabrication of PL light emitters , and the bright future of using templates to fabricate EL LEDs is to be explored.
The fabrication techniques aforementioned are mainstream techniques that have been applied to NP fabrication to improve light emission efficiency. Nanoimprint lithography and chemical synthesis are alternatives and have been extensively studied [65–67]. However, these two techniques are too costly or difficult to control, and so neither of them has been used successfully to produce NP arrays with improved light emission. More work is required in order to produce large-area, cost-effective, and easily tunable metallic NPs with enhanced light emission efficiency. One promising proposal for achieving this goal is to use self-assembled nanorods, nanowires, or nanotubes because of the ease of production and high controllability [68–71]. Ag nanorods formed by heating of AgNO3 in pores of PAA template or oblique angle deposition, nanotubes by shadow evaporation, and Ag NPs on stacked carbon nanotube layers have already found intriguing applications in surface-enhanced Raman scattering, and we anticipate that these proposals may shortly be employed to LED efficiency enhancement with proper modifications.
LEDs have found major commercial applications in three areas: general lighting, flat panel displays, and optoelectronic chips.
The use of LSPR in fabricating white LEDs has been described by Yeh et al. . Here, mixing of the red light converted from blue/green photons and residual blue light leads to white light emission. Instead of enhancing the IQE or LEE of the device, this configuration enhances light absorption by the CdSe/ZnS QWs via coupling between the QWs and LSP generated by the Au NPs. Spectral tunability, high quantum efficiency, and photo-stability are the advantages of this method. Another possible but seldom reported method to increase the efficiency of white light emitters is to incorporate NPs of different geometries in a device. As demonstrated in microwave frequencies, combination of NPs with various geometries results in several transmission dips corresponding to the type of NPs . Hence, if different NPs with properly designed geometries are introduced into a white light emitter, the various light components can be enhanced selectively, thereby producing light emitters giving the desirable spectrum.
Flat panel displays
Among the various techniques to improve the efficiency of LEDs, LSPR-based methods have great promise due to the high degree of enhancement and reasonable cost. The enhancement is generally attributed to the increase in the IQE or LEE of the device. By using traditional nanotechniques such as sputtering/deposition, EBL, and NSL, LEDs with a myriad of metallic NP geometries have been fabricated. By carefully examining their PL or EL properties, the enhancement mechanism has been elucidated, further confirming the immense potential of LSPR LEDs.
One of the current tasks is to identify the suitable fabrication parameters and optimize the size and shape of the structure in order to achieve the best enhancing effect experimentally. Analytical expressions have been obtained for the interaction between simple nanostructures like nanoholes and light . More powerful are computer softwares, such as finite difference time domain [FDTD] tools, for it is flexible to simulate any structures with them, from regular to highly disordered. As a common practice in the emerging field of metamaterials such as sub-wavelength metallic structures, it is routine to first simulate and examine the efficacy of the structures before actual fabrication and performance assessment [84–88]. Similarly here at optical frequencies, simulated and measured electromagnetic waves near metallic NPs irradiated by light often agree well; for example, simulation results from nanoscaled antennae which can effectively enhance light emission have been reported [38, 39]. As the gap between antennas in microwave engineering and those in optical frequencies is being bridged, nanoantenna structures are promising in further enhancing the LEE in a LED, especially OLED, in which the IQE of the device is already high and cannot be significantly increased. For an LED with a mediocre IQE, nanoantennae can possibly be used to accomplish both IQE and LEE enhancement. Another challenge lies in the fabrication techniques. That is, even though the structures can be designed, they may not be produced using current technology. With regard to the fabrication challenges, efforts are expected to extend existing methods proven useful for arraying ordered metallic arrays such as nanoimprint and chemical synthesis to LED fabrication. The key is to protect the LED structures during chemical processing.
This work was jointly supported by the National Natural Science Foundation of China under grant no. 50801013 and no. 51071045; Natural Science Foundation of Jiangsu Province, China, under grant no. BK2009291; Specialized Research Fund for the Doctoral Program of Higher Education under grant no. 200802861065; Excellent Young Teachers Program of Southeast University; Hong Kong Research Grants Council (RGC) General Research Fund (GRF) no. CityU 112307; and City University of Hong Kong Strategic Research Grant (SRG) no. 7008009.
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