High-Temperature Cadmium-Free Nanophosphors for Daylight-Quality White LEDs

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1 High-Temperature Cadmium-Free Nanophosphors for Daylight-Quality White LEDs Anngelique H. Canty*, Brian A. Akins, Marek Osiński Center for High Technology Materials University of New Mexico, Albuquerque, NM *Undergraduate student of Department of Electrical and Computer Engineering University of Texas at San Antonio San Antonio, Tx Abstract A white light emitter was successfully made from combining color-tuned, cadmium-free colloidal quantum-dots excited by a blue light. Several different techniques were used to synthesize Mn-doped ZnSe nanocrystals with ZnSe shells. One technique resulted in an efficient green light emitter which is believed to be a quantum-dot quantum well. It had a peak emission at 499 nm and an initial quantum yield of 7%. Another technique resulted in the yellow emission (583 nm) expected from ZnSe:Mn. The red emitter was InP:Cu grown with ZnSe shells. The mixture of these three nanocrystals resulted in a white light emission and a PL range from 48-8 nm with three distinct peaks. Polysiloxane was investigated as a possible medium to disperse the nanocrystals. Introduction Due to the higher efficiencies and lifetimes of light-emitting diodes (LEDs) there has been an increased interest in switching from fluorescent to LED lighting. Currently, commercial white LEDs are made by combining a yellow phosphor with a blue InGaN chip. The resulting LED produces a white light with an intense dominating blue emission. 1 The human body is highly sensitive to the blue emission within the nm range. 2 Blue light can disrupt circadian rhythms and disruption has been linked to a variety of ailments such as breast and colon cancer, cardiovascular disease, mood disorders, obesity, and type-2 diabetes. This project sought to overcome the problem of intense blue light LEDs by using colloidal quantum-dots (QDs) to make a daylight-quality white light. The QDs are semiconductor nanocrystals (NCs) which, due to the quantum confinement effect, can be tuned to emit different colors of visible light by changing their shapes and sizes. 3 In the past, cadmium- Graduate Mentor Faculty Mentor

2 based QDs were studied for the use in LEDs but the toxicity of cadmium limits its commercial and biological applications. In addition, those NCs were not very thermally stable. Instead, this project investigated non-toxic alternatives, such as ZnSe and InP, doped with either manganese or copper. Not only do these alternatives exhibit the color-tunability of cadmium-based nanocrystals, but they also have been found to be more chemically and thermally stable. 4 Their ability to emit light at high temperatures is particularly important if they are to be used in LEDs. Our goal was to synthesize QDs so that they spanned the visible spectrum, from green to red, and could be used together along with a blue excitation to produce a daylight-quality white light. We made two different versions of ZnSe:Mn to act as a green and a yellow light emitter, and InP:Cu was the red light emitter. In addition, we synthesized polysiloxane as a possible medium to hold the crystals, and co-doped ZnSe:Mn:Cu was synthesized to see if we could achieve a single nanocrystal white-light emitter. Experimental Description Materials We used 1-octadecene (ODE), indium acetate (In(Ac) 3 ), tributylphosphine (TBP), tri-noctylphosphine (TOP), stearic acid (SA), myristic acid, zinc stearate, tris(trimethylsilyl)phosphine (P(TMS) 3 ), 1-octylamine, oleylamine, selenourea, vinyltrimethoxysilan (VTMS), diphenylsilanediol (DPSD), barium hydroxide monohydrate (BH), p-xylene, phenyltris(dimethylsiloxy)silane (PTDMSS), platinum()-1,3,-divinyl-1,1,3,3- tetramethyldisiloxane (Pt catalyst), poly-vinyl-oligosilane (PVO), octadecylamine (ODA), and selenium powder (Se). Manganese stearate (MnSt 2 ) and copper (II) stearate (CuSt 2 ) were prepared in our lab. Synthesis of ZnSe:Mn Nanocrystals 5 Zn precursor was prepared in a three-neck flask. The solution was degassed and then heated to 26 C on a schlenk-line under argon gas. Selenium precursor was prepared in a glovebox. The precursor was taken out of the glove-box and injected into the three-neck flask before reducing the temperature to 24 C. In the glove-box, MnSt 2 was dissolved in ODE and then brought out and injected into the three-neck flask. Once the dopant emission appeared under a UV lamp, the solution was heated to 26 C. Additional ZnSe shells were grown on the doped crystals by loading zinc stearate and SA in a 25 ml three-neck flask. The mixture was dissolved in ODE inside the glove-box. The flask was sealed, brought out of the glove-box and connected to the schlenk-line under argon. The solution was heated to 8 C and then a small amount of it was injected into the doped NCs at 24 C. The NCs were heated to 26 C again for 15 minutes before being cooled to 24 C so a bit more ZnSe shell precursor could be injected again. This process was repeated three more times with progressively increasing amounts of precursor. The solution was then cooled to room temperature. The NCs were washed three times with acetone and then stored in toluene. Synthesis of InP:Cu Nanocrystals 6

3 In the glove-box, phosphine precursor was prepared by mixing P(TMS) 3 and 1- octylamine with ODE. This was brought out in a 3 ml syringe. In a 1 ml three-neck flask, In(Ac) 3 was added to myristic acid and ODE. The flask was heated to 188 C under argon before the phosphine precursor was injected. After ten minutes, the flask was cooled to 13 C. A Cu dopant made from CuSt 2 dissolved in ODE was prepared in the glove-box and then drawn out in a 3mL syringe and injected into the flask. The flask was heated to 21 C and then cooled to 15 C. The ZnSe shells were prepared by mixing zinc stearate with ODE. Separately, Se powder was dissolved in TOP. Once both solutions were well mixed, a small amount of Zn precursor was injected into the three-neck flask. After ten minutes, the same amount of Se precursor was injected. The flask was heated to 22 C for thirty minutes. This was repeated four times with increasing amounts of precursor solutions before cooling the flask to room temperature. The NCs were washed three times with acetone. Synthesis of Polysiloxane 7 Polysiloxane was made by first synthesizing PVO from VTMS and DPSD mixed with BH loaded into a three neck flask. The mixture was heated to 8 C on a schlenk-line for thirty minutes before p-xylene was added. After 3.5 hours, the BH was filtered out by vacuum filtration and the by-product methanol was removed with vacuum heating at 8 C. Next, PTDMSS and Pt catalyst were added to 5. g of the PVO. The mixture was stirred for 15 minutes and then put in a glass mold or put on a glass slide and heated in a Carbolite oven at 18 C for two hours. Synthesis of ZnSe:Mn:Cu 8 In a three-neck flask, Se, ODA, and TBP were mixed and heated to 7 C. In the glove-box, MnSt 2 and ODE were put in a flask, brought out, and then heated under vacuum to 1 C for 2 minutes, and then it was heated under argon to 29 C. The Se precursor was injected into the Mn precursor and then cooled to 28 C. In another flask, ZnSt 2, SA and ODE were mixed and heated to 1 C under vacuum for 2 minutes. Then it was heated to 16 C under argon until the solution was clear. The Zn precursor was then injected into the MnSe flask, which was then cooled to 26 C for 1 minutes. After the ten minutes, the flask was cooled to 18 C. In the glove-box, CuSt 2 was mixed with TBP. The Cu precursor was brought out of the glove-box and injected into MnSe. A UV lamp was used to watch for emission and then the flask was heated to 23 C for 5 minutes. Afterwards it was cooled to 18 C. In the glove-box, Zn(Ac) 2 was mixed with TOP. The solution was syringed out and injected into the MnSe flask at 18 C. The temperature was maintained for 3 minutes. Lastly, sulfur was dissolved in ODE and injected dropwise at 18 C into the flask. The temperature was maintained at 18 C for 3 minutes and then cooled to room temperature. Synthesis of InP:Cu/ZnSe:Mn 9 In the glove-box, phosphine precursor was prepared by mixing P(TMS) 3 and 1- octylamine with ODE. This was brought out in a syringe. In a 1 ml three-neck flask, In(Ac) 3 was added to myristic acid and ODE. The flask was heated to 188 C under argon before the phosphine precursor was injected. After ten minutes, the flask was cooled to 13 C. A Cu dopant

4 made from CuSt 2 dissolved in ODE was prepared in the glove-box and then drawn out in a syringe and injected into the flask. The flask was heated to 21 C and then cooled to 15 C. The ZnSe shells were prepared by mixing zinc stearate with ODE. Separately, Se powder was dissolved in TOP. Once both solutions were well mixed, Zn precursor was injected into the three-neck flask. After ten minutes, the same amount of Se precursor was injected. The flask was heated to 22 C for thirty minutes. In the glove-box, MnSt 2 was mixed with ODE. The solution was brought out and injected into the InP flask. A UV lamp was used to watch for emission and then the flask was heated to 26 C. The flask was then cooled to 15 C. The alternating Zn and Se precursor injections were repeated four times progressively increasing amounts of precursor solutions before cooling the flask to room temperature. The NCs were washed three times with acetone. Results and Discussion Synthesis of ZnSe:Mn_3 Fig. 1: ZnSe:Mn_3 NC s yellow emission excited by UV lamp, Left at 24 C, Right at 22 C. Figure 1 includes two photos demonstrating the thermal stability of non-cadmium based nanocrystals. In the left picture, the emission of ZnSe:Mn can be seen in the flask, especially in the left-most neck. The right picture shows how the NCs become brighter as they cool. Figure 2 shows the absorption and photoluminescence spectra of ZnSe:Mn NCs synthesized using procedure three. These NCs were expected to have a yellow emission but instead they emitted an intense green light with a peak at 499 nm. Procedure three followed the steps summarized in the experimental description above. The green emission is currently suspected to be the result of adding all of the Se precursor at once. Doing so allowed manganese selenide to form instead of doping ZnSe. The result is believed to be a ZnSe/MnSe/ZnS quantum-dot quantum well (QDQW), though more research is needed to confirm this. The QE was initially 7% but after two months, the sample had a QE of 36%. The loss of efficiency may be due to photo-oxidation. Future experiments include attempting to color-tune the emission by modifying the size of the MnSe layer to confirm that this is a QDQW. In addition, the loss of QE might be addressed by adding protective shell layers.

5 Fig. 2: Absorption and Photoluminescence spectra of ZnSe:Mn_3 NCs. Synthesis of ZnSe:Mn_4 Fig. 3: ZnSe:Mn_4 NC s yellow emission excited by UV lamp, Left at 24 C, Right at 22 C. Figure 3 demonstrates the thermal stability and bright yellow emission of ZnSe:Mn nanocrystrals made using procedure four. Figure 4 is the absorption and photoluminescence spectra of ZnSe:Mn_4. Here, unlike with procedure three, the PL peak is well within the yellow range and it ranges from green to red. Procedure four followed most of the experimental description but, instead of adding the Se precursor all at once, it was added in alternating and progressively increasing injections alongside the shell injections. This seems to have allowed Mn to dope ZnSe and prevented MnSe from forming. Future experiments may investigate ways, such as protective shells, to make the NCs brighter and more efficient.

6 Fig. 4: Absorption and Photoluminescence spectra of ZnSe:Mn_4 NCs. Synthesis of InP:Cu/ZnSe Fig. 5: InP:Cu/ZnSe NC s red emission excited by UV lamp, Left at 126 C, Right at 22 C. Figure 5 shows the red emission of InP:Cu/ZnSe NCs during synthesis at 126 C on the left and after being cooled to room temperature on the right. Figure 6 shows the absorption and photoluminescence spectra of InP:Cu/ZnSe NCs. The PL is broad, ranging from 5-8 nm, and it has a peak at around 7 nm. These NCs were synthesized before we began adding sulfide shells to protect the surfaces from defects and degradation. Future work may include adding a sulfide shell and methods to improve brightness and efficiency.

7 ABS [A.U.] PL Intensity [A.U.] InP:Cu/ZnSe Abs PL Wavelength [nm] Fig.6: Absorption and Photoluminescence spectra of InP:Cu/ZnSe NCs. Mixture of ZnSe:Mn_3, ZnSe:Mn_4, and InP:Cu/ZnSe Fig. 7: White light emitted from a mixture of ZnSe:Mn_3, ZnSe:Mn_4, and InP:Cu/ZnSe NCs in Toluene. Figure 7 shows the white light emitted by a mixture of ZnSe:Mn_3, ZnSe:Mn_4, and InP:Cu/ZnSe nanocrystrals in toluene when exposed to a blue light. Figure 8 is the PL of the mixture of ZnSe:Mn_3, ZnSe:Mn_4, and InP:Cu/ZnSe NCs in toluene. Each substances peak can be clearly seen in the PL. On the left is the green peak of

8 ZnSe:Mn_3, and in the middle is the yellow peak of ZnSe:Mn_4. The right peak is the red emission of InP:Cu/ZnSe NCs Fig. 8: Photoluminescence of ZnSe:Mn_3, ZnSe:Mn_4 & InP:Cu/ZnSe NC s mixed in toluene. Synthesis of Polysiloxane Fig. 9: Glass slide with polysiloxane and nanocrystals Figure 9 shows a glass slide with polysiloxane and NCs. On the left of the slide, the NC clusters can be seen. One of the major problems with polysiloxane was that it would push the NCs out as it polymerized. The right side of the slide demonstrates the other problem with polysiloxane: it would only fully polymerize as a very thin film. Here, the polymer was still liquid. Figure 1 shows the PL of polysiloxane, polysiloxane containing ZnSe:Cu_3, and polysiloxane with InP:Cu/ZnSe. Polysiloxane did not fluoresce. The ZnSe:Cu NCs normally had a green emission which can be seen in the figure. InP:Cu/ZnSe NCs normally had a red emission; however, when it was dispersed in polysiloxane, the PL peaked in the green range. This may have been because of the lack of a protective sulfide shell on the InP:Cu NCs.

9 Intensity (CPS) Intensity [CPS] Polysiloxane s tendency to push out NCs and its inability to polymerize in thicker forms make it seem inappropriate to use as medium to hold nanophosphors within a LED. A different substance, perhaps an epoxy already in use in LEDs, might be investigated to act as a medium for the NCs but more work should be done to fully confirm whether or not polysiloxane is insufficient Polysiloxane Polysiloxane_ZnSe:Cu_3 Polysiloxane_InP:Cu/ZnSe_ Wavelength [nm] Fig. 1: Photoluminescence spectra of polysiloxane, polysiloxane containing ZnSe:Cu_3 NCs, an d polysiloxane containing InP:Cu/ZnSe NCs. Synthesis of ZnSe:Mn:Cu_ ZnSe_Mn_Cu_1 BAACGA_72511_1 In Toluene PL BA_72611 Exc. 4nm Em. 4nm Excitation 365 nm 22 C Fig. 11: PL spectra of ZnSe:Mn:Cu_1 using a 365 nm excitation Figure 11 shows the PL spectra of ZnSe:Mn:Cu_1 when using a 365 nm excitation. Three peaks can be seen in the spectra: a violet peak at 41 nm, a bluish-green peak at 487 nm, and a greenish-yellow peak at 573 nm. The intense violet peak was unexpected.

10 Intensity (CPS) Intensity (CPS) ZnSe_Mn_Cu_1 BAACGA_72511_1 In Toluene PL BA_72611 Exc. 3nm Em. 3nm Excitation 415 nm 22 C Fig. 12: PL spectra of ZnSe:Mn:Cu_1 using a 415 nm excitation Figure 12 shows the PL spectra of ZnSe:Mn:Cu_1 using a 415 nm excitation. The two co-doped peaks can be seen here, shifted slightly to the right. These NCs were not very bright and had insignificant quantum efficiency. Synthesis of ZnSe:Mn:Cu_2 We modeled procedure two after our successful ZnSe:Mn_4 procedure and just added more ZnSe shells to dope with Cu ZnSe:Mn:Cu_2 BAACGA_71811_1 Acetone/Methanol wash In Toluene PL BA_71911 Exc. 1nm Em. 2nm Excitation 45 nm Fig. 13: PL spectra of ZnSe:Mn:Cu_2 Figure 13 shows the PL spectra of ZnSe:Mn:Cu_2. The peak was very similar to our QDQW peak from before and the expected co-doped peaks were not present. This was the most efficient of the attempted co-doped NCs and it had a QE of 4%.

11 Intensity (CPS) Synthesis of ZnSe:Mn:Cu_3 For procedure three, we returned to modifying the literature s procedure but this time we used more Mn and less Cu ZnSe:Mn:Cu_3 BAACGA_71311_1 In Hexane PL BA_71511 Exc. 2nm Em. 2nm Excitation 345 nm 22 C Fig. 14: PL spectra of ZnSe:Mn:Cu_3 Figure 14 shows the PL spectra of ZnSe:Mn:Cu_3 and a broad violet peak can be seen at 397 nm. These NCs were not bright or efficient. In addition, none of the co-doped peaks were seen. Synthesis of ZnSe:Mn:Cu_4 For procedure four, we continued modifying the literature procedure. This time we changed the ratio of ZnSt 2 and Cu so that there was more ZnSt 2. When we synthesized these NCs, we found two different types of substances. One was separated out by washing with acetone and methanol. They appeared white in color and their PL can be seen in figure 15. The second substance was harvested by dissolving unwashed NCs in toluene and then filtering the solution. They appeared yellow in color and their PL can be seen in figure 16.

12 Intensity (CPS) Intensity (CPS) ZnSe_Mn_Cu_4 BAACGA_72711_1 Acetone then Methanol washed In Toluene PL BA_72811 Exc. 2 nm Em. 2 nm Excitation 35 nm 22 C Fig. 15: PL spectra of ZnSe:Mn:Cu_4 washed with acetone and methanol Figure 15 shows the PL spectra of ZnSe:Mn:Cu_4 after it was washed with acetone and methanol. These NCs have a violet emission at 378 nm and appear to be ZnSe NCs produced from the shelling process of the procedure ZnSe_Mn_Cu_4 BAACGA_72711_1 Aliquot dissolved in toluene & filtered In Toluene PL BA_72811 Exc. 2 nm Em. 3 nm Excitation 413 nm 22 C Fig. 16: PL spectra of ZnSe:Mn:Cu_4 dissolved in toluene and filtered Figure 16 shows the PL spectra of ZnSe:Mn:Cu_4 after it was dissolved in toluene and filtered. There is a broad green peak at 52 nm. Though it isn t seen here, another PL spectra was taken of ZnSe:Mn:Cu_4 before it was washed. The PL showed three peaks: one intense violet peak and two tiny green and yellow peaks. The yellow peak appears to have been lost after filtering and cannot be seen in figure 16. A violet peak was present both in procedure one (fig. 11) and in procedure four (fig. 15). We believe that both of these peaks are due to ZnSe, which emits blue, being produced as a side

13 Intensity (CPS) product during shell growth. The reason we were able to see, and filter out, the ZnSe in the procedure four was because we added more Zn precursor and so the NCs were larger. Since we were able to remove the violet peak, we concluded that the literature was not producing single co-doped nanocrystals as claimed but rather two different NCs: ZnSe and ZnSe:Mn:Cu. The emissions of the blue ZnSe and the green/yellow co-doped NCs combined with the UV excitation used in the literature produced a white light. 1 Since we were looking for single codoped NCs that could be combined with a blue excitation to produce a white light, ZnSe:Mn:Cu was no longer relevant to this project. Synthesis of InP:Cu/ZnSe:Mn InP:Cu/ZnSe:Mn_1 BAAC_8111_1 Acetone wash In toluene PL BA_8211 Exc. 2nm Em. 3nm Excitation 359 nm 22 C Fig. 17: PL spectra of InP:Cu/ZnSe:Mn_1 using a 359 nm excitation Figure 17 shows the PL spectra of InP:Cu/ZnSe:Mn using a 359 nm excitation. A violet peak can be seen at 377 nm.

14 Intensity (CPS) InP:Cu/ZnSe:Mn_1 BAAC_8111_1 Acetone wash In toluene PL BA_8211 Exc. 2nm Em. 3nm Excitation 45 nm 22 C Fig. 18: PL spectra of InP:Cu/ZnSe:Mn_1 using a 45 nm excitation Figure 18 shows the PL spectra of InP:Cu/ZnSe:Mn using a 45 nm excitation. A broad red peak can be seen at 79 nm. The PL of InP:Cu was also red but peaked around 69 nm (fig. 6). The PL of InP:Cu/ZnSe:Mn appears to simply be a red-shift of InP:Cu. This might be due to InP:Cu being sensitive to size changes. Adding the doped Mn layer increased the size of the NCs so the PL shifted farther out into the red. A future experiment might involve shelling ZnSe:Mn with InP:Cu to see if a green/red PL can be achieved. Summary and Conclusion To summarize, three different doped, cadmium-free nanocrystals were successfully synthesized. The green emitter, ZnSe:Mn_4, is suspected to be a QDQW and was the most efficient nanophosphor with a initial QE of 7% and a final of 36%. A yellow emitter, ZnSe:Mn_3, and a red emitter, InP:Cu, were also successfully produced. The polymer, polysiloxane, was synthesized but more work must be done to confirm if it can act as a medium to disperse the nanocrystals in. Though several co-doped nanocrystals were made, none of them were successful. They were either not efficient or were simply not co-doped. In conclusion, different doped, cadmium-free nanocrystals can be mixed together and combined along with a blue excitation to make a white light. In addition, the mixture of doped nanocrystals is preferable to co-doped nanocrystals. The co-doped NCs did not emit the correct wavelengths to achieve a white light with a blue excitation and they were also not as efficient as the mixture of doped NCs.

15 Acknowledgements I want to thank the UNM REU Program for this wonderful opportunity and the National Science Foundation for funding the program. I am also grateful to Linda Bugge for all the dedicated planning and working she did for all of us involved with REU program. I thank my faculty mentor, Marek Osiński, for his support and my graduate mentor, Brian Akins, for guiding and teaching me throughout the program. I am grateful to Gema Alas for working on the project with us and helping me prepare for my presentations. I am thankful to my fellow REU students for being a supportive group and sharing their different experiences and views with me. Finally, I want to thank my dad for always encouraging me to pursue my interests in science and my sister for always being strong enough to lovingly shame me when encouragement was not enough. References 1 J. K. Sheu, S. J. Chang, C. H. Kuo, Y. K. Su, L. W. Wu, Y. C. Lin, W. C. Lai, J. M. Tsai, G. C. Chi, R. K. Wu, White-light emission from near UV InGaN-GaN LED chip precoated with blue/green/red phosphors, Photon. Technol. Lett., 15 (#1), 18-2 (23). 2 G. C. Brainard, J. P. Hanifin, J. M. Greeson, B. Byrne, G. Glikman, E. Gerner, and M. D. Rollag, Action spectrum for melatonin regulation in humans: evidence for a novel circadian photoreceptor, J. Neurosci., 21 (16), (21). 3 A. M. Smith, and S. Nie, Semiconductor nanocrystals: Structure, properties, and band gap engineering, Accounts of Chemical Research, 43 (#2), 19-2 (Feb. 21). 4 Modified after: R. G. Xie and X. G. Peng, Synthesis of Cu-doped InP nanocrystals (d-dots) with ZnSe diffusion barrier as efficient and color-tunable NIR emitters, J. Am. Chem. Soc. 131 (#3), (29). 5 Modified after: S. Acharya, D.D. Sarma, N.R. Jana and N. Pradhan, An Alternate Route to High-Quality ZnSe and Mn-Doped ZnSe Nanocrystals, J. Phys. Chem. Lett., 1, (21). 6 Modified after: R. G. Xie and X. G. Peng, Synthesis of Cu-doped InP nanocrystals (d-dots) with ZnSe diffusion barrier as efficient and color-tunable NIR emitters, J. Am. Chem. Soc. 131(#3), (29). 7 Modified after: J.-S. Kim, S. C. Yang, and B.-S. Bae, Thermally stable transparent sol-gel based siloxane hybrid material with high refractive index for light emitting diode (LED) encapsulation, Chem. Mater. 22, (21). 8 Modified after: S. K. Panda, S. G. Hickey, H. V. Demir and A. Eychmuller. Bright White-Light Emitting Manganese and Copper Co-Doped ZnSe Quantum Dots Angew. Chem. Int. Ed., 5, pp. 1 6 (211). 9 Modified after: R. G. Xie and X. G. Peng, Synthesis of Cu-doped InP nanocrystals (d-dots) with ZnSe diffusion barrier as efficient and color-tunable NIR emitters, J. Am. Chem. Soc. 131(#3), (29). 1 S. K. Panda, S. G. Hickey, H. V. Demir and A. Eychmuller. Bright White-Light Emitting Manganese and Copper Co-Doped ZnSe Quantum Dots Angew. Chem. Int. Ed., 5, pp. 1 6 (211).