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Energy Express

  • Editor: Christian Seassal
  • Vol. 22, Iss. S2 — Mar. 10, 2014
  • pp: A276–A281
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Red-emitting silicon quantum dot phosphors in warm white LEDs with excellent color rendering

Chang-Ching Tu, Ji H. Hoo, Karl F. Böhringer, Lih Y. Lin, and Guozhong Cao  »View Author Affiliations


Optics Express, Vol. 22, Issue S2, pp. A276-A281 (2014)
http://dx.doi.org/10.1364/OE.22.00A276


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Abstract

We demonstrate red-emitting silicon quantum dot (SiQD) phosphors as a low-cost and environment-friendly alternative to rare-earth element phosphors or CdSe quantum dots. After surface passivation, the SiQD-phosphors achieve high photoluminescence quantum yield = 51% with 365-nm excitation. The phosphors also have a peak photoluminescence wavelength at 630 nm and a full-width-at-half-maximum of 145 nm. The relatively broadband red emission is ideal for forming the basis of a warm white spectrum. With 365-nm or 405-nm LED pumping and the addition of green- and/or blue-emitting rare-earth element phosphors, warm white LEDs with color rendering index ~95 have been achieved.

© 2014 Optical Society of America

1. Introduction

In this paper, we demonstrate red-emitting phosphors based on silicon quantum dots (SiQDs) as a low-cost and environment-friendly alternative to the red-emitting Eu-doped phosphors or CdSe QDs. After passivation with silicon oxide and alkyl silanes, the red-emitting SiQD-phosphors have high PLQY = 51% with 365-nm excitation, which is comparable to the quantum efficiencies achieved by other direct bandgap semiconductor QDs. The red phosphors also have a peak photoluminescence (PL) wavelength at 630 nm and a full-width-at-half-maximum (FWHM) of 145 nm. The deep red emission, although relatively broadband compared to CdSe QDs which usually have FWHM < 50 nm, is ideal for forming the basis of a warm white spectrum. With UV (365 nm) or near-UV (405 nm) LED pumping and the addition of some green- and/or blue-emitting REE phosphors, warm white LEDs with excellent color rendering capability have been achieved. Noticeably, fluorescent SiC is also an alternative to the current REE phosphors [6

6. M. Syväjärvi, J. Müller, J. W. Sun, V. Grivickas, Y. Ou, V. Jokubavicius, P. Hens, M. Kaisr, K. Ariyawong, K. Gulbinas, P. Hens, R. Liljedahl, M. K. Linnarsson, S. Kamiyama, P. Wellmann, E. Spiecker, and H. Ou, “Fluorescent SiC as a new material for white LEDs,” Phys. Scr. T 148, 014002 (2012). [CrossRef]

]. In addition to the phosphor application, SiQDs as efficient electroluminescent materials in LEDs have been demonstrated [7

7. F. Maier-Flaig, J. Rinck, M. Stephan, T. Bocksrocker, M. Bruns, C. Kübel, A. K. Powell, G. A. Ozin, and U. Lemmer, “Multicolor silicon light-emitting diodes (SiLEDs),” Nano Lett. 13(2), 475–480 (2013). [CrossRef] [PubMed]

].

2. Experiment

We synthesized the red-emitting SiQD-phosphors in three major steps. Firstly, a 6-inch p-type silicon wafer was electrochemically etched in the electrolyte comprising HF and methanol, and the resulting porous silicon layer was mechanically harvested from the wafer surface [8

8. C.-C. Tu, Q. Zhang, L. Y. Lin, and G. Cao, “Brightly photoluminescent phosphor materials based on silicon quantum dots with oxide shell passivation,” Opt. Express 20(S1), A69–A74 (2012). [CrossRef] [PubMed]

10

10. K. Kůsová, O. Cibulka, K. Dohnalová, I. Pelant, J. Valenta, A. Fucíková, K. Zídek, J. Lang, J. Englich, P. Matějka, P. Stepánek, and S. Bakardjieva, “Brightly luminescent organically capped silicon nanocrystals fabricated at room temperature and atmospheric pressure,” ACS Nano 4(8), 4495–4504 (2010). [CrossRef] [PubMed]

]. Secondly, the silicon powders were collected, sonicated and dispersed in a mixture of HNO3 and diluted HF for isotropic etching, followed by HNO3 etching for passivating the silicon powders with a silicon oxide layer. Finally, the silicon powders were refluxed in 185 mM diphenylsilanediol in ethanol, followed by surface treatment with chlorotrimethylsilane in toluene. Generally, about 100 mg of phosphors was obtained per wafer per synthesis, and the concentration of about 2.5 mg mL−1 in toluene was used for all optical characterizations. The PL and excitation spectra were measured using a Jobin Yvon Horiba Fluorolog FL-3 fluorometer system. The PLQYs were measured using a HAMAMATSU External Quantum Efficiency Measurement System (C9920) with quartz cuvettes. The absorbance spectra were measured using an Agilent 8453 Diode Array UV-Vis Spectrophotometer. The FTIR-ATR spectra were recorded using a Perkin Elmer FTIR Spectrum RX I system. The PL lifetimes were measured using a PicoQuant Time-Correlated Single Photon Counting System.

3. Results and discussion

The photographs and PL spectra of the red-emitting SiQD-phosphor suspension in toluene are shown in Fig. 1(d) and Fig. 2(a), respectively. Despite different excitation wavelengths, the PL spectra have almost the same peak wavelengths at about 630 nm and FWHM of about 145 nm. The PLQY, measured by the integrating sphere method, of the same phosphor suspension as a function of excitation photon energy ranging from 2.70 eV (460 nm) to 4.13 eV (300 nm) is shown in Fig. 2(b). Particularly, the PLQY readings at 365 nm, 405 nm and 460 nm, the emission wavelengths of GaN or InGaN LEDs, are equal to 51%, 41% and 37%, respectively. In general, when the excitation photon energy increases, the PLQY of molecules or quantum dots is expected to remain constant (Kasha-Vavilov rule) or decrease as a result of new non-radiative channels being activated [12

12. R. A. Cruz, V. Pilla, and T. Catunda, “Quantum yield excitation spectrum (UV-visible) of CdSe/ZnS core-shell quantum dots by thermal lens spectrometry,” J. Appl. Phys. 107(8), 083504 (2010). [CrossRef]

]. In contrast, the PLQY of the SiQD-phosphors is higher at higher excitation energies. Previously, an analogous trend of PLQY increase was observed for porous silicon grain suspension in ethanol, made by a similar electrochemical etching method [13

13. D. Timmerman, J. Valenta, K. Dohnalová, W. D. A. M. de Boer, and T. Gregorkiewicz, “Step-like enhancement of luminescence quantum yield of silicon nanocrystals,” Nat. Nanotechnol. 6(11), 710–713 (2011). [CrossRef] [PubMed]

]. The enhancement of PLQY was attributed to carrier multiplication, where the excess energy of an excitation photon overcomes the threshold of a multiple of the band gap energy to produce an additional electron-hole pair, leading to more chances of radiative recombination and higher PLQY [13

13. D. Timmerman, J. Valenta, K. Dohnalová, W. D. A. M. de Boer, and T. Gregorkiewicz, “Step-like enhancement of luminescence quantum yield of silicon nanocrystals,” Nat. Nanotechnol. 6(11), 710–713 (2011). [CrossRef] [PubMed]

]. The PL spectrum of the SiQD-phosphors with its energy axis multiplied by two is placed at the bottom of Fig. 2(b), to show that the increase of PLQY starts to become obvious when the excitation photon energy is about two times of the SiQD band gap energy. Moreover, since the non-radiative microns-size cores are also present in the integrating sphere when measuring PLQY, the PL photons emitted by the SiQDs can potentially be re-absorbed by the cores which have a small energy band gap (1.11eV) of bulk silicon. Therefore, the actual PLQY without the re-absorption effect should be higher than the measured values in Fig. 2(b).
Fig. 2 (a) PL spectra of the SiQD-phosphor suspension in toluene, with 320-nm (orange curve), 365-nm (red curve), 400-nm (green curve) and 440-nm (blue curve) excitations, respectively. (b) PLQY versus excitation photon energy. (c) Excitation (red solid line) and absorbance (blue dotted line) spectra. (d) FTIR-ATR spectra of the dried SiQD-phosphor powders at different stages of the synthesis, as-harvested (blue dotted line), after HNO3 etching (green dotted line) and after alkyl silane treatment (red solid line). ν and δ mean stretching mode and deformation mode, respectively. (e) Electronic band structure of the SiQD-phosphors. The green, red, blue rectangles represent the energy band gaps of the silicon core, SiQD and silicon oxide, respectively.

The excitation spectrum of the red-emitting SiQD-phosphor suspension is shown in Fig. 2(c). During the excitation measurement, the PL intensity at 630 nm reached its maximum when the excitation was at around 375 nm. At 405-nm and 450-nm excitation, the PL intensity was 90% and 55% of the maximum, respectively. The absorbance (or extinction) has a much broader spectrum than the excitation, due to a combination of scattering and absorption effects by the micron-size cores [14

14. C.-C. Tu, L. Tang, J. Huang, A. Voutsas, and L. Y. Lin, “Solution-processed photodetectors from colloidal silicon nano/micro particle composite,” Opt. Express 18(21), 21622–21627 (2010). [CrossRef] [PubMed]

]. Especially at wavelengths longer than 550 nm, the absorbance of the phosphor suspension simply does not contribute to PL. Previously, single luminescent porous silicon nanoparticles which do not have the micron-size cores were estimated to have PLQY as high as 88% [15

15. G. M. Credo, M. D. Mason, and S. K. Buratto, “External quantum efficiency of single porous silicon nanoparticles,” Appl. Phys. Lett. 74(14), 1978–1980 (1999). [CrossRef]

]. Therefore, to further improve PLQY, decreasing the proportion of non-radiative bulk silicon in the SiQD-phosphors is critical. Noticeably, PLQY > 60% of SiQDs measured with 380-nm LED excitation in an integrating sphere has been achieved [16

16. D. Jurbergs, E. Rogojina, L. Mangolini, and U. Kortshagen, “Silicon nanocrystals with ensemble quantum yields exceeding 60%,” Appl. Phys. Lett. 88(23), 233116 (2006). [CrossRef]

]. However, the PL wavelengths of these SiQDs are within the near-IR range, and hence are not suitable for lighting applications which require high spectral efficiencies, i.e. high luminous efficacy of optical radiation (LER, lm Wopt−1).

4. Conclusion

Acknowledgments

The authors thank NSF SBIR program, Washington Research Foundation, the W Fund, and Center for Commercialization (C4C) and Buerk Center for Entrepreneurship of the University of Washington for their funding support.

References and links

1.

“The new oil?” Nat. Photonics5(1), 1 (2011). [CrossRef] [PubMed]

2.

T. Erdem and H. V. Demir, “Semiconductor nanocrystals as rare-earth alternatives,” Nat. Photonics 5(3), 126 (2011). [CrossRef]

3.

H.-S. Chen, C.-K. Hsu, and H.-Y. Hong, “InGaN–CdSe–ZnSe quantum dots white LEDs,” IEEE Photon. Technol. Lett. 18(1), 193–195 (2006). [CrossRef]

4.

S. Nizamoglu, G. Zengin, and H. V. Demir, “Color-converting combinations of nanocrystal emitters for warm-white light generation with high color rendering index,” Appl. Phys. Lett. 92(3), 031102 (2008). [CrossRef]

5.

K. T. Yong, W. C. Law, R. Hu, L. Ye, L. W. Liu, M. T. Swihart, and P. N. Prasad, “Nanotoxicity assessment of quantum dots: from cellular to primate studies,” Chem. Soc. Rev. 42(3), 1236–1250 (2013). [CrossRef] [PubMed]

6.

M. Syväjärvi, J. Müller, J. W. Sun, V. Grivickas, Y. Ou, V. Jokubavicius, P. Hens, M. Kaisr, K. Ariyawong, K. Gulbinas, P. Hens, R. Liljedahl, M. K. Linnarsson, S. Kamiyama, P. Wellmann, E. Spiecker, and H. Ou, “Fluorescent SiC as a new material for white LEDs,” Phys. Scr. T 148, 014002 (2012). [CrossRef]

7.

F. Maier-Flaig, J. Rinck, M. Stephan, T. Bocksrocker, M. Bruns, C. Kübel, A. K. Powell, G. A. Ozin, and U. Lemmer, “Multicolor silicon light-emitting diodes (SiLEDs),” Nano Lett. 13(2), 475–480 (2013). [CrossRef] [PubMed]

8.

C.-C. Tu, Q. Zhang, L. Y. Lin, and G. Cao, “Brightly photoluminescent phosphor materials based on silicon quantum dots with oxide shell passivation,” Opt. Express 20(S1), A69–A74 (2012). [CrossRef] [PubMed]

9.

J. L. Heinrich, C. L. Curtis, G. M. Credo, M. J. Sailor, and K. L. Kavanagh, “Luminescent colloidal silicon suspensions from porous silicon,” Science 255(5040), 66–68 (1992). [CrossRef] [PubMed]

10.

K. Kůsová, O. Cibulka, K. Dohnalová, I. Pelant, J. Valenta, A. Fucíková, K. Zídek, J. Lang, J. Englich, P. Matějka, P. Stepánek, and S. Bakardjieva, “Brightly luminescent organically capped silicon nanocrystals fabricated at room temperature and atmospheric pressure,” ACS Nano 4(8), 4495–4504 (2010). [CrossRef] [PubMed]

11.

A. G. Cullis and L. T. Canham, “Visible light emission due to quantum size effects in highly porous crystalline silicon,” Nature 353(6342), 335–338 (1991). [CrossRef]

12.

R. A. Cruz, V. Pilla, and T. Catunda, “Quantum yield excitation spectrum (UV-visible) of CdSe/ZnS core-shell quantum dots by thermal lens spectrometry,” J. Appl. Phys. 107(8), 083504 (2010). [CrossRef]

13.

D. Timmerman, J. Valenta, K. Dohnalová, W. D. A. M. de Boer, and T. Gregorkiewicz, “Step-like enhancement of luminescence quantum yield of silicon nanocrystals,” Nat. Nanotechnol. 6(11), 710–713 (2011). [CrossRef] [PubMed]

14.

C.-C. Tu, L. Tang, J. Huang, A. Voutsas, and L. Y. Lin, “Solution-processed photodetectors from colloidal silicon nano/micro particle composite,” Opt. Express 18(21), 21622–21627 (2010). [CrossRef] [PubMed]

15.

G. M. Credo, M. D. Mason, and S. K. Buratto, “External quantum efficiency of single porous silicon nanoparticles,” Appl. Phys. Lett. 74(14), 1978–1980 (1999). [CrossRef]

16.

D. Jurbergs, E. Rogojina, L. Mangolini, and U. Kortshagen, “Silicon nanocrystals with ensemble quantum yields exceeding 60%,” Appl. Phys. Lett. 88(23), 233116 (2006). [CrossRef]

17.

B. Averboukh, R. Huber, K. W. Cheah, Y. R. Shen, G. G. Qin, Z. C. Ma, and W. H. Zong, “Luminescence studies of a Si/SiO2 superlattice,” J. Appl. Phys. 92(7), 3564–3568 (2002). [CrossRef]

18.

M. Sykora, L. Mangolini, R. D. Schaller, U. Kortshagen, D. Jurbergs, and V. I. Klimov, “Size-dependent intrinsic radiative decay rates of silicon nanocrystals at large confinement energies,” Phys. Rev. Lett. 100(6), 067401 (2008). [CrossRef] [PubMed]

19.

S. Godefroo, M. Hayne, M. Jivanescu, A. Stesmans, M. Zacharias, O. I. Lebedev, G. Van Tendeloo, and V. V. Moshchalkov, “Classification and control of the origin of photoluminescence from Si nanocrystals,” Nat. Nanotechnol. 3(3), 174–178 (2008). [CrossRef] [PubMed]

20.

K. Dohnalová, K. Kůsová, and I. Pelant, “Time-resolved photoluminescence spectroscopy of the initial oxidation stage of small silicon nanocrystals,” Appl. Phys. Lett. 94(21), 211903 (2009). [CrossRef]

21.

J. Valenta, A. Fučíková, F. Vácha, F. Adamec, J. Humpolíčková, M. Hof, I. Pelant, K. Kůsová, K. Dohnalová, and J. Linnros, “Light-emission performance of silicon nanocrystals deduced from single quantum dot spectroscopy,” Adv. Funct. Mater. 18(18), 2666–2672 (2008). [CrossRef]

22.

H. S. Jang, H. Yang, S. W. Kim, J. Y. Han, S.-G. Lee, and D. Y. Jeon, “White light-emitting diodes with excellent color rendering based on organically capped CdSe quantum dots and Sr3SiO5:Ce3+,Li+ phosphors,” Adv. Mater. 20(14), 2696–2702 (2008). [CrossRef]

23.

J. Ziegler, S. Xu, E. Kucur, F. Meister, M. Batentschuk, F. Gindele, and T. Nann, “Silica-coated InP/ZnS nanocrystals as converter material in white LEDs,” Adv. Mater. 20(21), 4068–4073 (2008). [CrossRef]

OCIS Codes
(160.2540) Materials : Fluorescent and luminescent materials
(250.5230) Optoelectronics : Photoluminescence
(160.4236) Materials : Nanomaterials

ToC Category:
Light-Emitting Diodes

History
Original Manuscript: December 9, 2013
Revised Manuscript: January 17, 2014
Manuscript Accepted: January 18, 2014
Published: January 28, 2014

Citation
Chang-Ching Tu, Ji H. Hoo, Karl F. Böhringer, Lih Y. Lin, and Guozhong Cao, "Red-emitting silicon quantum dot phosphors in warm white LEDs with excellent color rendering," Opt. Express 22, A276-A281 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-S2-A276


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References

  1. “The new oil?” Nat. Photonics5(1), 1 (2011). [CrossRef] [PubMed]
  2. T. Erdem and H. V. Demir, “Semiconductor nanocrystals as rare-earth alternatives,” Nat. Photonics5(3), 126 (2011). [CrossRef]
  3. H.-S. Chen, C.-K. Hsu, and H.-Y. Hong, “InGaN–CdSe–ZnSe quantum dots white LEDs,” IEEE Photon. Technol. Lett.18(1), 193–195 (2006). [CrossRef]
  4. S. Nizamoglu, G. Zengin, and H. V. Demir, “Color-converting combinations of nanocrystal emitters for warm-white light generation with high color rendering index,” Appl. Phys. Lett.92(3), 031102 (2008). [CrossRef]
  5. K. T. Yong, W. C. Law, R. Hu, L. Ye, L. W. Liu, M. T. Swihart, and P. N. Prasad, “Nanotoxicity assessment of quantum dots: from cellular to primate studies,” Chem. Soc. Rev.42(3), 1236–1250 (2013). [CrossRef] [PubMed]
  6. M. Syväjärvi, J. Müller, J. W. Sun, V. Grivickas, Y. Ou, V. Jokubavicius, P. Hens, M. Kaisr, K. Ariyawong, K. Gulbinas, P. Hens, R. Liljedahl, M. K. Linnarsson, S. Kamiyama, P. Wellmann, E. Spiecker, and H. Ou, “Fluorescent SiC as a new material for white LEDs,” Phys. Scr. T148, 014002 (2012). [CrossRef]
  7. F. Maier-Flaig, J. Rinck, M. Stephan, T. Bocksrocker, M. Bruns, C. Kübel, A. K. Powell, G. A. Ozin, and U. Lemmer, “Multicolor silicon light-emitting diodes (SiLEDs),” Nano Lett.13(2), 475–480 (2013). [CrossRef] [PubMed]
  8. C.-C. Tu, Q. Zhang, L. Y. Lin, and G. Cao, “Brightly photoluminescent phosphor materials based on silicon quantum dots with oxide shell passivation,” Opt. Express20(S1), A69–A74 (2012). [CrossRef] [PubMed]
  9. J. L. Heinrich, C. L. Curtis, G. M. Credo, M. J. Sailor, and K. L. Kavanagh, “Luminescent colloidal silicon suspensions from porous silicon,” Science255(5040), 66–68 (1992). [CrossRef] [PubMed]
  10. K. Kůsová, O. Cibulka, K. Dohnalová, I. Pelant, J. Valenta, A. Fucíková, K. Zídek, J. Lang, J. Englich, P. Matějka, P. Stepánek, and S. Bakardjieva, “Brightly luminescent organically capped silicon nanocrystals fabricated at room temperature and atmospheric pressure,” ACS Nano4(8), 4495–4504 (2010). [CrossRef] [PubMed]
  11. A. G. Cullis and L. T. Canham, “Visible light emission due to quantum size effects in highly porous crystalline silicon,” Nature353(6342), 335–338 (1991). [CrossRef]
  12. R. A. Cruz, V. Pilla, and T. Catunda, “Quantum yield excitation spectrum (UV-visible) of CdSe/ZnS core-shell quantum dots by thermal lens spectrometry,” J. Appl. Phys.107(8), 083504 (2010). [CrossRef]
  13. D. Timmerman, J. Valenta, K. Dohnalová, W. D. A. M. de Boer, and T. Gregorkiewicz, “Step-like enhancement of luminescence quantum yield of silicon nanocrystals,” Nat. Nanotechnol.6(11), 710–713 (2011). [CrossRef] [PubMed]
  14. C.-C. Tu, L. Tang, J. Huang, A. Voutsas, and L. Y. Lin, “Solution-processed photodetectors from colloidal silicon nano/micro particle composite,” Opt. Express18(21), 21622–21627 (2010). [CrossRef] [PubMed]
  15. G. M. Credo, M. D. Mason, and S. K. Buratto, “External quantum efficiency of single porous silicon nanoparticles,” Appl. Phys. Lett.74(14), 1978–1980 (1999). [CrossRef]
  16. D. Jurbergs, E. Rogojina, L. Mangolini, and U. Kortshagen, “Silicon nanocrystals with ensemble quantum yields exceeding 60%,” Appl. Phys. Lett.88(23), 233116 (2006). [CrossRef]
  17. B. Averboukh, R. Huber, K. W. Cheah, Y. R. Shen, G. G. Qin, Z. C. Ma, and W. H. Zong, “Luminescence studies of a Si/SiO2 superlattice,” J. Appl. Phys.92(7), 3564–3568 (2002). [CrossRef]
  18. M. Sykora, L. Mangolini, R. D. Schaller, U. Kortshagen, D. Jurbergs, and V. I. Klimov, “Size-dependent intrinsic radiative decay rates of silicon nanocrystals at large confinement energies,” Phys. Rev. Lett.100(6), 067401 (2008). [CrossRef] [PubMed]
  19. S. Godefroo, M. Hayne, M. Jivanescu, A. Stesmans, M. Zacharias, O. I. Lebedev, G. Van Tendeloo, and V. V. Moshchalkov, “Classification and control of the origin of photoluminescence from Si nanocrystals,” Nat. Nanotechnol.3(3), 174–178 (2008). [CrossRef] [PubMed]
  20. K. Dohnalová, K. Kůsová, and I. Pelant, “Time-resolved photoluminescence spectroscopy of the initial oxidation stage of small silicon nanocrystals,” Appl. Phys. Lett.94(21), 211903 (2009). [CrossRef]
  21. J. Valenta, A. Fučíková, F. Vácha, F. Adamec, J. Humpolíčková, M. Hof, I. Pelant, K. Kůsová, K. Dohnalová, and J. Linnros, “Light-emission performance of silicon nanocrystals deduced from single quantum dot spectroscopy,” Adv. Funct. Mater.18(18), 2666–2672 (2008). [CrossRef]
  22. H. S. Jang, H. Yang, S. W. Kim, J. Y. Han, S.-G. Lee, and D. Y. Jeon, “White light-emitting diodes with excellent color rendering based on organically capped CdSe quantum dots and Sr3SiO5:Ce3+,Li+ phosphors,” Adv. Mater.20(14), 2696–2702 (2008). [CrossRef]
  23. J. Ziegler, S. Xu, E. Kucur, F. Meister, M. Batentschuk, F. Gindele, and T. Nann, “Silica-coated InP/ZnS nanocrystals as converter material in white LEDs,” Adv. Mater.20(21), 4068–4073 (2008). [CrossRef]

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