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We report the development of new nanophosphor structures based on the Mn-doped ZnSeS material system to enhance the color properties, luminosity and efficiency of white LEDs. These structures have been demonstrated for phosphor-based white LED applications utilizing both blue and UV LED systems. Bandgap tuning for near UV (405 nm) and blue (460 nm) excitations are reported. Using various optimization procedures, we have produced ZnSe:Mn nanoparticles with an external quantum yield greater than 80%.
©2011 Optical Society of America
1. Introduction
The development of blue and UV GaInN emitting diodes in the 1990’s led to a resurgence in phosphor development, just as occurred in the 60’s and 70’s for fluorescent lighting and color television. Although very complex systems can be developed, there is a strong demand for high performance but simple diodes in which particle phosphors are suspended in epoxy or silicone on top of a GaInN UV or blue emitting chip. The issue is to simultaneously achieve high color performance with high efficacy. The former depends on the “active” luminescent properties and crystallinity of the host and activator, while the latter also depends on the “passive” absorption and refractive index properties. The issues that must be addressed for optimization therefore range from developing highly luminescent material systems to addressing the impact of light coupling (refractive index matching) between the GaInN chip, phosphor materials and binder. Finally the device is usually coupled to the atmosphere by using a curved ‘dome-like” envelope as shown in Fig. 1 . These devices are driven either by using blue light to excite the phosphor(s) to emit yellow or green plus red light, or by using UV light to excite a combination of blue, green and red phosphors.
Fig. 1 Schematic of a typical diode lamp showing the placement of the GaInN drive chip, nanophosphor and host matrix envelope that is typically fashioned to have a dome-like top.
The issues are therefore very complex and involve light coupling from the GaInN chip to the phosphor, light absorption by the phosphor materials, which will depend strongly on whether they are blue or UV pumped, and the refractive index match among the binder, phosphor, and LED die. The distribution of particles within the matrix is also important. The conventional approach is to place most of the phosphor in close contact with the chip. Because of this complexity, one of the advantages of the use of the blue-GaInN pumped yellow-YAG:Ce phosphors is that it offers a very simple, direct, and low-cost solution but not an ideally optimized one. (The Ce activator has an absorption band in the blue and a small Stokes shift that is phonon broadened so that a good color spectrum can be achieved).
Areas available for improvement are a better emission color, higher luminous efficacy, and lower cost per lumen. The broad and red-deficient emission spectrum of the YAG:Ce phosphor, limits the color rending index (CRI) to ~80 while the luminous efficiency of the device is currently limited by a complex set of factors including the GaInN conversion efficiency, phosphor efficiency, package properties and thermal issues. To go beyond these limitations and achieve the goal of high CRI (> 90) and ideal efficacies near 220 lm/W, all of these issues must be addressed.
The potentially high quantum yield (QY) and high degree of color (size) tuning reported for quantum dots (QDs) make them an obvious target of investigation for white light emitting diodes. The use of QDs for color shifting of the UV or blue light emitted by a GaInN diode into the visible with high QY is of interest not only for their high efficiency but because they minimize light scattering and can be uniformly dispersed in either epoxy or silicone [1,2]. For example, some of the most efficient quantum dot systems, such as CdSe and InP, can be tuned from red to blue [3], for particle sizes less than 6 nm. However, the luminescent efficiency of direct band quantum dot systems are limited by self-absorption due to the partial overlap of absorption and emission bands [4] and they exhibit thermal quenching to a greater degree than traditional phosphors [5–7]. Additionally, the inherent toxicity of Cd-based systems prevents their widespread adoption to lighting.
In contrast, doped quantum dots display a Stoke’s shift that minimizes self-absorption [8] and can be designed to maintain their emission efficiency above room temperature [9]. The term “quantum dots” typically refers to semiconductors where the size of the crystals determines their optical properties as a result of the quantum confinement effect. Typically, particles larger than 10 nm behave more like “bulk” crystals and can be referred to as nanocrystals or nanophosphors, in case of luminescent materials. We have therefore investigated both classes of materials, specifically, Mn-doped ZnSe quantum dots (QDs) and nanophosphors (NPs) for UV-blue diode excitation. ZnSe is an efficient near bandgap emitter with a size tunable bandgap, is non-toxic, and emits brightly at 585nm when doped with Mn. Additionally, its refractive index (2.4) closely matches the nitride-based diode material which should enhance the out-coupling efficiency. Furthermore ZnSe can be doped with Cu and Ag to extend its spectral range. Internal and homogeneous doping was recently achieved in this system by separating the host nucleation and growth phases from the doping event [10]. In addition, the intrinsic band edge emission of bulk ZnSe is at 460 nm, comparable to that of most commercial blue LEDs. Thus, NP structures of this material system can be efficiently pumped by a UV (405 nm) source to emit simultaneously at the band edge (460 nm) and at the Mn emission band at 585 nm, whereas larger NP particles can be pumped by a 460 nm LED to again excite the Mn band emission, giving similar spectral components as for the UV excitation. As a consequence, with Mn-doping the spectral emission characteristics of ZnSe:Mn can closely match that of the blue + YAG:Ce system. The challenge then is to develop ZnSe:Mn in a size regime where optical scattering is a minimum for the device environment, but where the full luminescent properties are realized in the structure. From simple extrapolations the energy efficiency of UV pumped ZnSe:Mn is expected to be ~10% less than that possible with a blue pump. However, UV optimized systems will have higher absorption and experience less scattering and so this effect can compensate for a lower estimated energy efficiency. In addition, UV LEDs have higher power conversion efficiencies than equivalent blue LEDs [11]. The challenge is to synthesis nanophosphors that are large enough to optimize luminescent properties – but small enough to minimize optical scattering.
In this work, we report the search for new material structures that offer higher performance. As in many aspects of device development, the optimum structure is a compromise between material properties and the growth and synthesis protocols. For quantum dot synthesis many approaches have been developed and there is currently an emphasis to synthesize nanoparticles by a “one pot” approach. However, in doped quantum dots important differences arise. As first expounded by Erwin et al., particle doping (in the conventional sense) can only be effective once well defined crystal planes have developed, in particular the (100) planes in zincblende lattices that exhibit high binding energies [12]. To overcome this limitation, Peng et al. proposed the nucleus doping technique in which high concentrations of the dopant basically form an alloy with the host crystal at the initiation of growth [13,14]. For isoelectronic dopants, such as Mn in ZnSe, core or nucleus doping can be initiated from MnSe, or a Mn rich MnZnSe alloy. Then these sub-nanometer sized particulate grow larger and, when greater than ~2 nm in diameter, form crystallographic surfaces on which conventional dopant incorporation strategies can be used. Consequently, nanoparticles can be grown with quite complex energy band structures that depend on both the properties of the ZnMnSe alloy and those of the substitutionally incorporated Mn ion.
2. Conventional core-shell doped quantum dots (QDs)
The conventional ZnSe:Mn doped QD system is composed of a 1-2 nm MnSe core covered by a ~2 nm ZnSe shell, therefore a total crystal size less than 10nm. Due to the wider bandgap of MnSe, the energy band diagram is expected to have the form shown in Fig. 2 , where both the energy gaps of MnSe and ZnSe can be larger than in bulk material due to quantization effects. Additionally, Mn will provide in-bandgap radiative recombination states as shown. Following photon excitation electron-hole pairs are expected to collect in the lower band gap ZnSe shell and recombine on Mn ions within the ZnSe/MnSe interface layer between the core and shell and by band-to-band recombination. Thus, the absorption and emission bands of ZnSe shift with particle size below and near the Bohr radius, but the emission band position of Mn is invariant due to its isoelectronic character [10]. Because most e-h pairs are formed near the surface there will also be competing surface recombination effects and so ideally a wider bandgap surface layer should be grown.
Fig. 2 Proposed nanophosphor architecture including Mn ion incorporation within the ZnSe shell for “small” and “large” ZnSe:Mn Nanophosphors.
3. Optical simulation
To evaluate the effect of the use of nanoparticles on LED output optical simulations were carried out using Zemax Optical Design Software. For the purposes of this study, a simple 300 µm square trapezoidal LED die was placed on an absorbing substrate, covered with a 30 μm thick phosphor layer and encapsulated in an epoxy dome. The refractive index of the LED die, epoxy and phosphor were set at 2.4, 1.5 and 2.4 respectively. Scattering efficiencies of the phosphor particles were calculated as a function of nanophosphor refractive index and diameter using Mie theory. Core-shell scattering calculations were also performed but results were similar to those obtained using single particle models due to the relatively small proportion of MnSe core to ZnSe shell thickness in this work. Mean free paths (MFP) were then calculated from scattering results using a fixed 50% by volume loading of the phosphor in the epoxy matrix. An aggregate phosphor/phosphor matrix refractive index was assumed using the rule of mixtures.
Relative LED output efficiencies (of extracted light) were calculated compared to the LED/dome structure without a phosphor layer (where efficiency is set to 1.0) as it is expected that scattering induced in the structure will reduce output efficiency. Calculations were carried out for wavelengths of 405, 460 and 585 nm with the 405 and 460 nm light being generated in the LED die and calculated separately from the 585 nm emission being generated in the phosphor volume. The aggregate output efficiency was then calculated based upon the product of the excitation wavelength efficiency and the phosphor emission efficiency.
Figure 3 shows relative efficiencies simulated as a function of nanoparticle diameter for the two excitation wavelengths, the emission wavelength, and the combined excitation and emission efficiency for the two excitation conditions. The results indicate that either excitation wavelength produces similar results at particle dimensions below ~20 nm. The output efficiency of the phosphor emission on the other hand is relatively insensitive to particle size. The combined excitation and emission efficiency results (normalized to non scattering simulations) indicate that maximum performance is obtained for particle diameters below 15 nm and that only a small decrease occurs for particle sizes 20-25 nm. These results indicate that the use of relatively large nanophosphor particles with bulk-like absorption coefficients, large Stokes shifts and the commensurate reduction in phosphor loading can provide improved performance.
Fig. 3 LED output efficiency as a function of nanoparticle diameter for 405 and 460nm excitation wavelengths, 585 nm emission wavelength, and combined excitation and emission.
In order to study the relative insensitivity of the emission efficiency, simulations were performed to examine a larger range of 585 nm photon MFPs. Figure 4 shows the efficiency of the phosphor emission normalized to the non-scattering value as a function of MFP plotted as a fraction of the phosphor layer thickness. From these results it appears that some scattering can be beneficial with efficiency increasing for a certain range of decreasing MFPs. This may be due to enhanced out-coupling from the higher index phosphor layer into the lower index epoxy dome.
Fig. 4 Relative LED phosphor emission efficiency as a function of MFP normalized to the phosphor layer thickness.
4. Synthesis strategy and protocols
In this study, we used environmentally friendly “green” synthesis techniques that produce higher crystallinity and higher QY with a narrower particle size distribution and at lower cost than aqueous or metalorganic syntheses. The basic synthesis protocol was a modification of the method described by Narayan and Peng [13]. The precursors used were manganese stearate, zinc stearate, zinc acetate and Se powder, and the reaction was moderated by fatty acids of octadecylamine (ODA) or oleylamine (OA) stabilizers in a solvent of Octadecene (ODE) [13]. In this procedure a MnSe nuclei is formed first and then a shell of ZnSe is grown around this core (note that Mn is less reactive than Zn because its outer electrons are d-states). The first process step involves the preparation of Mn and Se precursors. For the Mn precursor, MnSt2 is added to ODE, and heated to 280°C under argon gas to dissociate MnSt2 into cation and anion components according to the following reaction:
For the Se precursor preparation Se is added to TBP and heated to 140°C to form TBPSe:Then the C12H27PSe with excess C12H27P solution is added to the CH3(CH2)17NH2 stabilizer.The Se precursor solution was then injected into the Mn precursor solution to form a mix of Mn2+ cations, 2((C18H35O2)2)2- anions in C18H36 solution, C12H27PSe with excess C12H27P and stabilizer CH3(CH2)17NH2. The free Mn2+ cations and free Se2- anions then combine to form nuclei of MnSe. A precursor solution of ZnSt2 was then added to grow a graded MnZnSe layer. Later to grow a pure ZnSe “shell” more ZnSt2 was injected and a thicker ZnSe shell obtained by adding Zn acetate and TBPSe. During this process, one can also add Mn and finally the Mn doped ZnSe shell was passivated by depositing an overlayer of ZnSSe. The surface nucleation kinetics that control the growth process derive from the fact that the attachment of steric acid to exposed Mn is very facile, whereas Se readily attaches to the Mn core surface as do the stabilizing ligands in solution. This leads to a predominately Se-rich growth surface such that Se attachment controls the growth rate and can be moderated by adjusting the relative concentrations of P/N stabilizers. Finally, the ZnSe:Mn nanoparticle solution was annealed for 30 minutes at 260°C to minimize intra-shell defects and product collected by standard polar/non-polar solvent precipitation techniques and re-dispersed in hexane. To aid the optimization studies, in situ monitoring was used to excite and measure the luminescence during particle growth. To improve product collection, oleylamine (OA) was used rather than octadecylamine as it is a liquid at room temperature. This reduces the collection process to one centrifugation step with a minimum of chloroform stabilization making it easier to synthesize and collect with high yield larger quantities of doped-ZnSe. Particle crystallinity is very dependent on the formation of a single phase MnSe core, which in turn relies on using a pure MnSt2 starting precursor containing little unreacted stearic acid. Accordingly, a vacuum/heating technique was developed to produce high purity MnSt2 containing no excess stearic acid or residual methanol [15].
5. Results
5.1. ZnSe:Mn nanocrystalline phosphors for UV to white LEDs
Various broadband-emitting ZnSe-based nanophosphors were demonstrated, which are particularly applicable for UV LED applications. By controlled Mn co-doping during particle shell growth, some of the ZnSe:Mn nanocrystals exhibited multiple peak emissions in both the blue and yellow-orange parts of the spectrum. The particle size of the nanocrystals having these optical characteristics varied from around 5 nm (quantum confinement mode) to over 20 nm (bulk mode), as was measured from photon correlation spectroscopy (PCS) analysis and evident from size-dependent (< 10 nm) blue ZnSe emission. When pumped using a UV LED or laser (~405nm), the single-component ZnSe:Mn nanophosphor system produced a broad emission spectrum similar to that of conventional white LEDs that are based on a blue LED and a yellow-orange micron-sized phosphor. Apparently, this unique property was the result of strong radiative transitions from both the Mn2+ activator sites as well as the ZnSe host, which typically exhibits weak or non-radiative bandgap emission. By controlling the amounts, ratios, and timing of the precursor injections, it was possible to control both the blue peak emission wavelength (from ~430 to ~468 nm) as well as the blue-to-yellow emission ratio, thus producing a wide range of spectral and color gamuts, as illustrated in Fig. 5 . For example, color chromaticity (CIE 1931) values ranged from (x = 0.289, y = 0.191) to (x = 0.491, y = 0.411) while the correlated color temperature (CCT) varied from around 2318K to 6450K. The color rendering index (CRI) varied from a low of 24 (sample S06) to a high of 81 (sample S02). The red-shifted (~580 nm) spectrum of the Mn2+ emission is particularly suited for warm white LED-based lighting applications, as is shown by the LED photograph in the inset of Fig. 6 . Also shown in Fig. 6 is the emission spectrum and color properties from a conventional YAG:Ce phosphor (~10 microns particle size) pumped by a blue LED around 455 nm. The blue-pumped YAG:Ce spectrum and overall color characteristics are matched very closely by the single component UV-pumped ZnSe:Mn nanophosphor sample S02.
Fig. 6 Broadband emission and spectral control achieved using various single component ZnSe:Mn nanocrystals under 405 nm excitation.
Figure 5 shows particle size distributions and data for samples S01 and S05 obtained using a Coulter DelsaNano dynamic light scattering instrument. In general, most samples have particle sizes over 10 nm, confirming their bulk-like optical properties. However, bimodal distributions were observed for some samples, as illustrated by the data shown for sample S01. A bimodal or a broad size distribution helps create a broader blue emission peak, as shown by some of the spectra in Fig. 6. Particle size control can also be used to customize the blue peak emission properties in order to achieve some color tunability. The small peak present around 405 nm corresponds to light leaking from the UV LED, which can also be controlled and completely eliminated by adjusting the concentration of nanoparticles in the matrix.
Using various optimization procedures, we have produced ZnSe:Mn NPs with external quantum yields greater than 80%, which was measured inside an integrating sphere and compared to a commercial Rhodamine 6G Fluorescence Reference Standard from Anaspec. The optimizations included real-time variation of precursor injection quantities and process time intervals, which was made possible by an in situ fiber optic probe monitoring system connected to a UV LED and a fiber optic spectrometer. The optical feedback provided by this system enabled the operator to adjust and optimize the process conditions in response to the observed emission intensity from the solution [11]. Further optimization to obtain higher efficiencies is continuing using an automated precursor delivery system, also equipped with an in situ optical monitoring.
5.2. ZnSe:Mn nanocrystalline phosphors for blue to white LEDs
Investigations of the doped ZnSe system have also focused on development of a nanophosphor architecture to enhance impurity recombination and allow for larger particle size and longer wavelength blue light absorption without loss in efficiency. The absorption character of the entire structure is a function of the thickness of the ZnSe layer which must be bulk-like to absorb 450-460 nm radiation from a blue LED. Accordingly, as was shown in Fig. 5, some of the ZnSe:Mn had particle sizes far exceeding 10 nm (the bulk-like regime). This made it possible to minimize the ZnSe bandgap and, correspondingly, increase the wavelength of the absorption edge. This was accomplished by the application of several layers of ZnSe in addition to intermittent Mn-doping throughout the host material, similar to what is typically achieved in bulk phosphors. The result was an effective shift of the overall absorption edge into the 450-460 nm spectral region where standard blue diodes emit, as illustrated by Fig. 7 (Top) with the corresponding particle size distributions shown in Fig. 7 (Bottom). The absorption edge is shifted toward the blue region by the growth of the ZnSe shell due (an inverse quantum confinement effect) and band gap recombination from this layer may be suppressed due to the consistent presence of substitutionally doped Mn ions.
Fig. 7 (Top) Relative absorption (dashed lines) and emission (solid lines) of ZnSe:Mn NPs with different particle sizes. (Bottom) Coulter particle size distribution of the same samples showing average sizes of 6.9nm and 17nm.
6. Summary
We have investigated the properties of ZnSe:Mn nanoparticles less than 20 nm in diameter and demonstrated their utilization in white LEDs when excited in the blue (~460 nm) or UV (~400 nm). These materials do not exhibit self-absorption, have low toxicity, and are resistant to thermal quenching at temperatures between room temperature and ~150° C. In addition, some of these nanophosphors exhibited quantum yields over 80% and can be further optimized. We have developed modifications of Mn-doped ZnSeS nanophosphors for application to the (blue diode + yellow emitter) white LED system so as to enable band gap tuning for 460 nm excitation. Modifications have also been developed that enables single component “white” emitters when pumped by UV LEDs around 400 nm. New protocols and in situ monitoring techniques for the synthesis of ZnSe:Mn nanoparticles have been developed and resulted in new nanoparticle structures with higher performance and with the promise for further improvement. This work has helped bring nanophosphor materials and technologies one step closer to commercialization in solid state lighting as well as other potential applications in lighting and imaging products.
Acknowledgments
This work was supported by the U.S. Department of Energy under contract number: DE-FC26-08NT01584.
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