Photoluminescent Nanofibers for Solid-State Lighting Applications L. Davis, L. Han, P. Hoertz, K. Guzan, K. Mills, H.Walls, T. Walker, and D. Magnus-Aryitey RTI International, Research Triangle Park, NC, USA Figure 13. Typical spectral radiant flux obtained using the PLN technology. This light source was a neutral white color. Figure 12. Through a judicious choice of nanofiber properties and luminescent particle coatings, virtually any point on the chromaticity axis can be produced. In this example, a blue (450 nm) LED is used to pump green and red PLNs to produce various colors. Figure 1. Size comparison of a human hair with polymeric nanofibers. Figure 2. SEM images of (A) smooth PMMA nanofibers and (B) porous PMMA nanofibers produced through electrospinning. A B As the solution fows to the electrode, the high electric feld deforms each drop of the polymer solution into a conical shape known as a Taylor cone. Above a threshold limit, the electrical forces overcome the surface tension of the solution, and a fne, charged jet is ejected from the electrode and ultimately deposits nanofbers on a grounded substrate. In SSL applications, we have found that nanofber mats serve the following functions: Provide optical fltering of the pump radiation • Difuse light emitted by the structure • Provide a convenient, mass-producible substrate for handling nanoparticles • Conform to geometries imposed by the light fxture • Protect from inadvertent releases of nanoparticles. • Electrospinning Polymer nanofbers are macro-sized objects with nanoscale features. Te length (>> microns) of the nanofbers imparts macro-scale properties, while their diameter (50 nm–500 nm) imparts nanomaterial behavior. In addition, other nanoscale features such as surface pores or nanoparticles (e.g., luminescent quantum dots [QDs]) can be incorporated into the nanofber to provide special physical and optical properties. Nanofbers are typically formed using the process of electrospinning, which involves applying a high voltage to an electrode in contact with a reservoir of polymer solution. Figure 4. Transmittance profiles of smooth and porous nanofibers show that RTI has the capability to tailor the optical properties of the SSL devices via fiber surface morphology. Figure 3. Light diffusion characteristics of a nanofiber structure when illuminated by a commercial LED. The symmetry and gradual decline in light intensity, progressing from the center outward, provides quantitative evidence that this nanofiber structure is a good diffuser of visible light.                 7UDQVPLVVLRQ :DYHOHQJWK QP 6PRRWK 3/1 0DW 3RURXV 3/1 0DW Quantum Dots Figure 5. The emission color of a QD is size- dependent, with green being the color of the smallest QDs. RTI has formulated custom QD solutions for SSL nanofiber coating applications. Te quantum dots used in the spray coating solution have the following properties: Te QD consists of a semiconducting CdSe core that absorbs short wavelengths and emits longer • wavelengths. Te emission color depends on the size of this core. A ZnS shell surrounds the core and provides environmental stability. • A long-chain amine coordination sphere is attached to the ZnS shell to provide compatibility • with various solvents and polymers. Abstract Photoluminescent nanofbers (PLNs) can be formed by combining electrospun polymeric nanofbers and luminescent particles such as quantum dots (QDs). Te physical properties of PLNs are dependent upon many diferent nanoscale parameters associated with the nanofber, the luminescent particles, and their interactions. By understanding and manipulating these properties, the performance of the resulting optical structure can be tailored for desired end-use applications. For example, the quantum efciency of QDs in the PLN structure depends upon multiple parameters including QD chemistry, the method of forming the PLN nanocomposites, and preventing agglomeration of the QD particles. Tis is especially important in solution-based electrospinning environments where some common solvents may have a detrimental efect on the performance of the PLN. With the proper control of these parameters, high quantum efciencies can be readily obtained for PLNs. Achieving high quantum efciencies is critical in applications such as solid-state lighting (SSL), where PLNs are an efective secondary conversion material for producing white light. Methods of optimizing the performance of PLNs through nanoscale manipulation of the nanofber are discussed, along with guidelines for tailoring the performance of nanofbers and QDs for application- specifc requirements. Lighting Applications of PLNs PLNs ofer multiple benefts to lighting applications including: Providing cost-efective, difuse, high refectance light management across the visible spectrum. • Producing appropriate color balance for lighting with high color rendering indices. • Efciently converting the pump wavelength to broad-spectrum visible radiation. • Conforming to geometries imposed by the light fxture, enabling new lighting designs. • Enabling the fabrication of mass-producible substrates for handling luminescent particles. • Substrate Blue LED PLN Layer White Light Figure 10. Schematic diagram of the basic architecture of a lighting system employing PLN technology. Blue LEDs excite the luminescent particles in the PLNs, resulting in broadband color emissions. Through proper choice of excitation and emission color power levels, white light is produced. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 x Chromaticity Coordinate y Chromaticity Coordinate 490 495 500 505 510 515 520 525 530 535 540 545 550 nm 555 560 565 570 575 580 590 600 610 620 nm 640 700 480 470 nm 2000 K 3000 K 5000 K 10,000 K Figure 11. Combining red and green PLNs and exciting them with a blue (450 nm) LED produces an intense white light source that can be used for general illumination. This lighting device has a correlated color temperature of 3,850 K and a color rendering index of 92. 0 500 1000 1500 2000 350 400 450 500 550 600 650 700 750 800 Wavelength (nm) Spectral Radiant Flux (µ฀ W/nm) -80 -60 -40 -20 0 20 40 60 80 -80 -60 -40 -20 0 20 40 60 80 R ef. Test a* b* CIELAB Figure 14. The similarity between the PLN test light and the reference standard demonstrates that highly accurate color rendering is possible using PLN technology. Conclusions PLNs are a new class of materials for SSL applications that can be created by combining polymeric nanofbers and luminescent nanoparticles such as QDs. When dispersed across a fexible panel, PLNs are able to difuse light and provide panel lighting from inorganic LEDs. Te polymeric nature of the PLNs imparts the ability to conform to the luminaire design and also provides a mass-producible substrate for housing QDs. Application of QDs to PLNs can be accomplished using common liquid-phase coating methods such as spray coating. Te composition of the coating formulations will vary depending upon each QD, and each formulation must be optimized to maintain high quantum efciencies and acceptable adhesion. PLNs have been demonstrated to produce high luminous efcacy (>50 lumen/watt) lighting prototypes with excellent color rendering properties. Photoluminescent Nanofibers (PLNs) PLNs are a nanocomposite formed by combining nanofbers with luminescent particles such as QDs. Some solvents used in electrospinning lower the quantum efciency of QDs when incorporated in the interior of the nanofber in a bulk PLN structure. Separating the electrospinning and QD application processes enables each to be optimized separately. Te resulting surface PLN produced by coating nanofbers was found to exhibit the highest quantum efciency as measured by the DILM Method [1]. Figure 7. Comparison of the intensity of light emission from bulk PLNs (in which the QDs reside in the nanofiber interior) to surface PLNs (in which the QDs reside on the nanofiber surface). Figure 8. Transmission electron microscope image of a surface PLN showing low nanoparticle agglomeration. A modifed spray coating technique was developed to produce even coatings of QDs or other luminescent nanoparticles on nanofber substrates. Each QD ink formulation must be customized to the chemistry of each QD. Te advantages of spray coating technologies for this application include: Te method does not contact the nanofbers and does not damage them. • Diferent patterns can be quickly applied and tested. • Multiple layers and geometries can be used to control the quantity of QDs deposited. • Figure 9. A variety of patterns ranging in size from very small to coverage of the entire substrate are being produced with the current RTI technology of spray coating QDs onto nanofiber mats. Bulk PLN Quantum efficiency ~ 0.20 Surface PLN Quantum efficiency up to 0.75 Acknowledgments Partial support for this work was provided by the U.S. Department of Energy Solid-State Lighting Core Technologies Program through award DE-FC26-06NT42860. References 1. International Electrotechnical Commission (IEC) Standard 62607-3-1, Nanomanufacturing: Key Control Characteristics of Luminescent Nanomaterials: Quantum Efciency, in preparation. Contact Information Lynn Davis, PhD Phone: 919.316.3325 E-mail: ldavis@rti.org RTI International 3040 Cornwallis Road, PO Box 12194 Research Triangle Park, NC 27709-2194 www.rti.org RTI International is a trade name of Research Triangle Institute. QDs can be coated on the surface of the nanofbers using various methods such as spray coating and dip coating. For SSL applications, we have determined that a modifed spray coating method produces the best properties in the PLN. Figure 6. Normalized absorbance and emission spectra of the CdSe/ZnS core-shell QDs used in this project. :DYHOHQJWK QP 550 500 600 650 700 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 1RUPDOL]HG $EVRUEDQFH DX *UHHQ 4'V <HOORZ 4'V 2UDQJH 4'V 450 400 :DYHOHQJWK QP 550 500 600 650 700 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 1RUPDOL]HG (PLVVLRQ 450 400 *UHHQ 4'V <HOORZ 4'V 2UDQJH 4'V Table 1. Comparison of lighting device incorporating RTI’s nanofiber technology with conventional lighting devices Incandescent Fluorescent RTI’s Nanofibers Efficiency 10 Lumen/Watt 55 Lumen/Watt >55 Lumen/Watt Heat Production — Color Quality Glare/Eye Strain Dimmability Excellent performance Moderate performance Lower than desired performance