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Optical Materials Express

Optical Materials Express

  • Editor: David J. Hagan
  • Vol. 2, Iss. 5 — May. 1, 2012
  • pp: 526–533
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Energy transfer in ZnO-anthracene hybrid structure

Ryoko Shimada, Ben Urban, Mamta Sharma, Akhilesh Singh, Vitaliy Avrutin, Hadis Morkoç, and Arup Neogi  »View Author Affiliations


Optical Materials Express, Vol. 2, Issue 5, pp. 526-533 (2012)
http://dx.doi.org/10.1364/OME.2.000526


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Abstract

Anthracene dispersed in Polyphenylsiloxane (PPS) glass was synthesized on epitaxially grown zinc oxide (ZnO) to realize organic/inorganic hybrid semiconductors for efficient energy transfer. The photoluminescence (PL) from ZnO was modified by the presence of anthracene molecules due to resonant energy transfer. The UV-visible emission from anthracene molecule was also influenced due to resonant coupling with the excitonic and defect bound excitonic states in ZnO. Temperature dependence of PL of the hybrid system showed quenching of the defect bound emission of the ZnO to be due to energy transfer from anthracene. The PL lifetime in ZnO-anthracene/PPS hybrid structure at 4 K is relatively shorter and becomes comparable to the PL lifetimes in ZnO at 77 K. However, at room temperatures the PL lifetime of the hybrid structure is significantly longer than in ZnO and is comparable to the recombination lifetime in anthracene.

© 2012 OSA

1. Introduction

Inorganic/organic hybrid materials have the potential of paving the way for developing new photonic systems [1

1. V. M. Agranovich, R. Atanasov, and F. Bassani, “Hybrid interface excitons in organic-inorganic quantum wells,” Solid State Commun. 92(4), 295–301 (1994). [CrossRef]

]. The dipole-dipole coupling in inorganic/organic structures enables exciton energy transfer from a donor to an acceptor. In recent years, these hybrids have attracted extensive attention due to their potential applications for optical and electronic devices [2

2. G. Heliotis, G. Itskos, R. Murray, M. D. Dawson, I. M. Watson, and D. D. C. Bradley, “Hybrid inorganic/organic semiconductor heterostructures with efficient non-radiative energy transfer,” Adv. Mater. 18(3), 334–338 (2006). [CrossRef]

6

6. O. Seitz, L. Caillard, H. M. Nguyen, C. Chiles, Y. J. Chabal, and A. V. Malko, “Optimizing non-radiative energy transfer in hybrid colloidal-nanocrystal/silicon structures by controlled nanopillar architectures for future photovoltaic cells,” Appl. Phys. Lett. 100(2), 021902 (2012). [CrossRef]

]. The energy transfer is more efficient when the donor molecules in the hybrid system is in the vicinity of the acceptor species and have overlapping exciton emission energy with the absorption energy of the acceptor [7

7. R. J. Lacowicz, Principles of Fluorescence Spectroscopy, 3rd ed. (Springer-Verlag, 2006).

].

III-nitride based semiconductors are much likely to be playing a major role in blue- UV solid-state lighting due to wide bandgap energy and large exciton binding energy at room temperature [8

8. M. H. Crawford, “LEDs for solid-state lighting: performance challenges and recent advances,” IEEE J. Sel. Top. Quantum Electron. 15(4), 1028–1040 (2009). [CrossRef]

10

10. H. Zhao, G. Liu, J. Zhang, J. D. Poplawsky, V. Dierolf, and N. Tansu, “Approaches for high internal quantum efficiency green InGaN light-emitting diodes with large overlap quantum wells,” Opt. Express 19(S4Suppl 4), A991–A1007 (2011). [CrossRef] [PubMed]

]. The lack of suitable lattice matched or high quality substrate makes it relatively expensive compared to GaAs or InP based III-V semiconductor devices. Zinc oxide (ZnO) offers an economically viable alternative wide bandgap semiconductor for solid-state lighting with an energy gap of 3.37 eV (at room temperature). It is classified as one of the wide band-gap semiconductor with a large exciton binding energy (60 meV) compared to GaN or the other II–VI group semiconductors. This large exciton binding energy results in stable excitonic emission at room temperature. This property of ZnO provides the impetus for developing novel optical/electronic devices, inclusive of light emitting diodes (LED), solar cells, and photodetectors [11

11. H. Morkoç and Ü. Özgür, Zinc Oxide (Wiley-VCH, 2009).

]. In addition, ZnO has defect energy levels corresponding to the visible wavelength, which can influence the bandedge optical properties. A reduction of the nonradiative recombination process from the bandedge to the defect level state can be engineered by saturating the population of the defect level states.

The nitrides also have the potentiality of broadband white light emitters. The incorporation of Al and In within GaN which can extends the bandgap emission from the UV to the near-infrared regime [12

12. T. K. Sharma and E. Towe, “On ternary nitride substrates for visible semiconductor light-emitters,” Appl. Phys. Lett. 96(19), 191105 (2010). [CrossRef]

]. However, UV emitting ZnO-based system does not have a viable inorganic ternary system for covering the whole visible spectrum. An efficient light emitting solid state organic molecule is an alternate option.

Anthracene (C14H10) is a polycyclic aromatic compound exhibiting absorption and luminescence in the blue region of the visible spectrum. Polyphenylsiloxane (PPS) glass has a low softening temperature which serves as a host material capable of dispersing anthracene molecules homogeneously without thermal degradation [13

13. B. Menaa, M. Takahashi, Y. Tokuda, and T. Yoko, “High dispersion and fluorescence of anthracene doped in polyphenylsiloxane films,” J. Sol-Gel Sci. Technol. 39(2), 185–194 (2006). [CrossRef]

,14

14. B. Menaa, M. Takahashi, Y. Tokuda, and T. Yoko, “High optical quality spin-coated polyphenylsiloxane glass thick films on polyethyleneterephtalate and silica substrates,” Mater. Res. Bull. 41(10), 1925–1934 (2006). [CrossRef]

]. The energy levels of the anthracene molecules dispersed in this host material are in close proximity of the bandedge and the defect levels of ZnO, which can facilitate efficient resonant energy transfer. This particular feature motivated the present study of the optical properties of hybrid structures composed of the PPS glass homogenously dispersed with anthracene molecules and its cast on ZnO thin films to realize a stable and robust hybrid semiconductor complex for integrated photonics. It was observed that the UV emission from ZnO is enhanced by an order of magnitude at 15 K and the visible emission is quenched at 15 K by the anthracene molecules being in physical proximity to the ZnO surface. The UV-VIS emission from anthracene is also modified by ZnO. In this work, the origin of the modification of the emission properties of the hybrid structure has also been investigated using temperature dependent and time-dependent PL spectroscopy.

2. Sample preparation and experimental procedure

ZnO thin films were grown by molecular beam epitaxy (MBE) on a-sapphire substrates. ZnO growth was carried out at 550 °C following the deposition at 200 °C and annealing up to 650 °C of a low temperature ZnO buffer layer under oxygen rich conditions. ZnO-anthracene/PPS hybrids were prepared by casting of anthracene/PPS solution in a 9/1 acetone/cyclohexane mixed solvent on the ZnO films. PPS was prepared separately using a technique reported in [13

13. B. Menaa, M. Takahashi, Y. Tokuda, and T. Yoko, “High dispersion and fluorescence of anthracene doped in polyphenylsiloxane films,” J. Sol-Gel Sci. Technol. 39(2), 185–194 (2006). [CrossRef]

,14

14. B. Menaa, M. Takahashi, Y. Tokuda, and T. Yoko, “High optical quality spin-coated polyphenylsiloxane glass thick films on polyethyleneterephtalate and silica substrates,” Mater. Res. Bull. 41(10), 1925–1934 (2006). [CrossRef]

]. The required masses of PPS and anthracene were dissolved in this mixed solvent for spin-coating. The structure of the hybrid material is depicted in Fig. 1(a)
Fig. 1 (a) Schematic illustration of the hybrids structure used. (b) Transmittance and PL spectra of Anthracene/PPS and ZnO at room temperature.
. The thickness of the ZnO thin film was about 200 nm, and the anthracene/PPS film cast/dried on the ZnO film had a thickness ~2 μm.

The temperature-resolved PL measurements were carried out with a continuous wave He–Cd laser ~325 nm (~3.81 eV) in a closed cycle helium cryostat at various temperatures ranging from 15 K to 300 K. The time-resolved PL measurements were performed with a 80 MHz Ti:sapphire pulse laser system (pulse width of 80 fs) at the excitation wavelength of 350 nm (~3.54 eV).

3. Results and discussion

Figure 2
Fig. 2 Comparison of PL spectra of ZnO, anthracene/PPS, and the ZnO-anthracene/PPS hybrid at 15 K. The emission of ZnO at 2.21 eV is reduced significantly whereas the emission of ZnO at 3.37 eV is enhanced due to resonant energy transfer.
shows the PL spectra of ZnO and the anthracene/PPS hybrid on sapphire and the ZnO-anthracene/PPS hybrid on the same substrate, measured at 15 K. For interpretation of the spectra changes on the hydridization, it is very useful to refer to the energy levels depicted in Fig. 3
Fig. 3 Schematics of energy levels for ZnO and anthracene in eV range.
[3

3. N. Saito, H. Haneda, T. Sekiguchi, N. Ohashi, I. Sakaguchi, and K. Koumoto, “Low-temperature fabrication of light-emitting Zinc Oxide micropatterns using self-assembled monolayers,” Adv. Mater. 14(6), 418–421 (2002). [CrossRef]

,8

8. M. H. Crawford, “LEDs for solid-state lighting: performance challenges and recent advances,” IEEE J. Sel. Top. Quantum Electron. 15(4), 1028–1040 (2009). [CrossRef]

,15

15. R. Katoh and M. Kotani, “Observation of fluorescence from higher excited states in an anthracene crystal,” Chem. Phys. Lett. 201(1-4), 141–144 (1993). [CrossRef]

]. There are states in anthracene molecules that are energetically higher than the bandedge excitonic states in ZnO. These states can act as donor states for the ZnO bandedge states which then accept the photoexcited carriers due to close proximity of the ZnO and anthracene molecules. Optically induced carrier transfer occurs efficiently at 3.20 eV, when the 0-0 transition energy of S1 → S0 in the anthracene molecules is resonant with the conduction bandgap energy of ZnO. These electrons then rapidly decay to the ground state by radiative recombination as luminescence. There is also the possible energy transfer from the vibrational S1 state of the anthracene molecules to the defect level states in ZnO. The resonance in the energy level is evident from the overlap of the broad absorption in anthracene with the defect level emission peak of ZnO at 2.21 eV in Fig. 1(b). The resonant energy transfer from the S1 states in anthracene to defect states in ZnO can result in saturation of the trapping sites for the electrons. The saturation of available defect level states Zni owing to the energy transfer from S1 is likely to prevent the nonradiative recombination of electrons from the conduction band or excitonic state in ZnO. This results in decrease in green emission from the ZnO-anthracene/PPS hybrid as shown in Fig. 2.

Figure 4
Fig. 4 (a) Temperature dependence photoluminescence spectra of ZnO (b) anthracene/PPS and (c) ZnO-anthracene/PPS hybrid structure.
shows temperature-dependent PL spectra of the ZnO, the anthracene/PPS composites, and the ZnO-anthracene/PPS hybrid material at 15 - 300 K. The near bandedge emission of the ZnO layer shown in Fig. 4(a) is dominated by acceptor-bound exciton (AoX) emission in the entire range of temperatures, T. The shoulder on the donor bound exciton DoX (3.365 eV) peak and the FX (3.377 eV) emission peak are also observed [18

18. A. Teke, Ü. Özgür, S. Dogan, X. Gu, H. Morkoç, B. Nemeth, J. Nause, and H. O. Everitt, “Excitonic fine structure and recombination dynamics in single-crystalline ZnO,” Phys. Rev. B 70(19), 195207 (2004). [CrossRef]

]. The integral PL intensity I (T) usually decreases with T due mainly to thermal quenching and can be described [19

19. X. T. Zhang, Y. C. Liu, Z. Z. Zhi, J. Y. Zhang, Y. M. Lu, D. Z. Shen, W. Xu, X. W. Fan, and X. G. Kong, “Temperature dependence of excitonic luminescence from nanocrystalline ZnO films,” J. Lumin. 99(2), 149–154 (2002). [CrossRef]

] by,
I(T)=I01+Aexp(EakBT)
(1)
where Ea is the activation energy of the thermal quenching process, kB is the Boltzmann constant, I0 is the intensity at 0 K, and A is a constant. The natural logarithm of the integrated intensity of FX versus 1/T plot from the data in Fig. 4(a) estimates Ea to be 36.7 meV. The peak position of AoX, 3.359 eV at 15 K, exhibits a redshift to 3.278 eV at 215 K due to narrowing of the bandgap with increasing T. The linewidth of the dominant AoX emission at 15 K and 215 K is 6.8 meV and 72 meV, respectively [19

19. X. T. Zhang, Y. C. Liu, Z. Z. Zhi, J. Y. Zhang, Y. M. Lu, D. Z. Shen, W. Xu, X. W. Fan, and X. G. Kong, “Temperature dependence of excitonic luminescence from nanocrystalline ZnO films,” J. Lumin. 99(2), 149–154 (2002). [CrossRef]

,20

20. Y. Zhang, D. J. Chen, and C. T. Lee, “Free exciton emission and dephasing in individual ZnO nanowires,” Appl. Phys. Lett. 91(16), 161911 (2007). [CrossRef]

]. This broadening of peaks with increasing T is primarily attributable to the exciton–phonon scattering.

To observe the changes in the free exciton recombination process of the ZnO-anthracene/PPS hybrid structure, time-resolved PL (TRPL) measurements, at various temperatures using a laser power of ~500 mW at 355 nm and corrected emission from 365 to 380 nm, were conducted for ZnO, anthracene/PPS, and ZnO-anthracene/PPS hybrid structure. In general, PL decay rate is a sum of the radiative (R) and nonradiative (NR) decay rates: τ−1 (PL) = τ−1 (NR) + τ−1 (R), where τ (PL), τ (NR), τ (R) are PL, NR and R decay time constants, respectively. Figure 5
Fig. 5 Time-resolved PL measurement for ZnO, anthracene/PPS, and ZnO-anthracene/PPS hybrid structure at various temperature (a) 4 K, (b) 77 K, and (c) 300 K.
shows TRPL measurement results at 4, 77 and 300 K. The PL decay time is relatively longer in anthracene (> 2.0 ns), whereas the carrier recombination in ZnO is relatively shorter (~in the ps domain) due to the strong exciton binding energy and due to the effect of LO phonon mediated interband transitions at higher temperatures. At low temperatures, the PL process in the hybrid system appears to be dictated by the recombination channels in ZnO. The PL decay time in the ZnO-anthracene/PPS hybrid at low temperatures is similar to that of the excitonic or carrier recombination lifetime in ZnO. At 4 K, the PL lifetime in the hybrid system is shorter (0.021ns) than in ZnO (Fig. 5(a)) and it becomes comparable (~0.025ns) at 77 K (Fig. 5(b)). As the temperature is increased to 300 K, the PL decay time in the hybrid structure is over two orders of magnitude longer (~2.39 ns) compared to that at 4 K (~0.02 ns). The emission process at 300 K is dominated by the energy transfer from the ZnO to the anthracene molecules as evidenced by a comparable PL lifetime in the hybrid system and the anthracene molecules (2.33 ns). The PL lifetime of ZnO also exhibits a shorter lifetime (0.274 ns) which can be attributed to the nonradiative recombination process in ZnO at room temperature. The nonradiative recombination process is evidently enhanced due to the presence of the defect level states at 2.21 eV. In the ZnO-anthracene/PPS hybrid structure, these defect levels in ZnO are saturated due to the resonant energy transfer from adjacent donor states in the anthracene molecules (Fig. 1(b)). It results in a single component long PL lifetime from the hybrid structure with negligible nonradiative recombination process. It consequently leads to the enhancement of the PL at 3.37 eV, which is significantly more than it is observed in Fig. 2. The increase in PL intensity does not account for the reduction in the incident pump photons and the emitted PL due to the absorption by the 2 μm-thick anthracene/PPS film overlayered on the ZnO film.

4. Conclusion

Optical properties of ZnO-anthracene/PPS hybrid structures were studied by temperature and time-resolved PL measurements. The band edge PL exhibited enhanced UV emission due to resonant energy transfer from anthracene flourophore to ZnO flourophore at low temperature. The suppression of the defect level emission without the passivation of the actual ZnO defect states at low temperature is due to the saturation of the defect level population in the presence of resonant coupling of donor and acceptor energy levels in ZnO and anthracene. TRPL measurement results at various temperatures suggested efficient energy transfer from ZnO to anthracene at 300 K. At room temperature the carrier recombination is dominated by the non-radiative recombination process as evident by the low emission intensity. As the bandedge emission in ZnO red-shifts and is thermally broadened with temperature in addition to being reabsorbed by the anthracene composite, the room temperature PL enhancement cannot be ascertained. The integrated PL intensity over the UV-blue regime is significantly enhanced in the ZnO-anthracene/PPS hybrid structures. This approach of light harvesting using organic/inorganic hybrid systems may pave the way for the development of new photonic systems.

Acknowledgments

This work was supported by Yamada Science Foundation and The Science Research Promotion Fund in Japan.

References and links

1.

V. M. Agranovich, R. Atanasov, and F. Bassani, “Hybrid interface excitons in organic-inorganic quantum wells,” Solid State Commun. 92(4), 295–301 (1994). [CrossRef]

2.

G. Heliotis, G. Itskos, R. Murray, M. D. Dawson, I. M. Watson, and D. D. C. Bradley, “Hybrid inorganic/organic semiconductor heterostructures with efficient non-radiative energy transfer,” Adv. Mater. 18(3), 334–338 (2006). [CrossRef]

3.

N. Saito, H. Haneda, T. Sekiguchi, N. Ohashi, I. Sakaguchi, and K. Koumoto, “Low-temperature fabrication of light-emitting Zinc Oxide micropatterns using self-assembled monolayers,” Adv. Mater. 14(6), 418–421 (2002). [CrossRef]

4.

M. Law, L. E. Greene, J. C. Johnson, R. Saykally, and P. Yang, “Nanowire dye-sensitized solar cells,” Nat. Mater. 4(6), 455–459 (2005). [CrossRef] [PubMed]

5.

R. K. Das, S. Bhat, S. Banerjee, C. Aymonier, A. Loppinet-Serani, P. Terech, U. Maitra, G. Raffy, J.-P. Desvergne, and A. Del Guerzo, “Self-assembled composite nano-materials exploiting a thermo reversible n-acene fibrillar scaffold and organic-capped ZnO nanoparticles,” J. Mater. Chem. 21(8), 2740–2750 (2011). [CrossRef]

6.

O. Seitz, L. Caillard, H. M. Nguyen, C. Chiles, Y. J. Chabal, and A. V. Malko, “Optimizing non-radiative energy transfer in hybrid colloidal-nanocrystal/silicon structures by controlled nanopillar architectures for future photovoltaic cells,” Appl. Phys. Lett. 100(2), 021902 (2012). [CrossRef]

7.

R. J. Lacowicz, Principles of Fluorescence Spectroscopy, 3rd ed. (Springer-Verlag, 2006).

8.

M. H. Crawford, “LEDs for solid-state lighting: performance challenges and recent advances,” IEEE J. Sel. Top. Quantum Electron. 15(4), 1028–1040 (2009). [CrossRef]

9.

J. Y. Tsao, M. E. Coltrin, M. H. Crawford, and J. A. Simmons, “Solid-state lighting: An integrated human factors, technology, and economic perspective,” Proc. IEEE 98(7), 1162–1179 (2010). [CrossRef]

10.

H. Zhao, G. Liu, J. Zhang, J. D. Poplawsky, V. Dierolf, and N. Tansu, “Approaches for high internal quantum efficiency green InGaN light-emitting diodes with large overlap quantum wells,” Opt. Express 19(S4Suppl 4), A991–A1007 (2011). [CrossRef] [PubMed]

11.

H. Morkoç and Ü. Özgür, Zinc Oxide (Wiley-VCH, 2009).

12.

T. K. Sharma and E. Towe, “On ternary nitride substrates for visible semiconductor light-emitters,” Appl. Phys. Lett. 96(19), 191105 (2010). [CrossRef]

13.

B. Menaa, M. Takahashi, Y. Tokuda, and T. Yoko, “High dispersion and fluorescence of anthracene doped in polyphenylsiloxane films,” J. Sol-Gel Sci. Technol. 39(2), 185–194 (2006). [CrossRef]

14.

B. Menaa, M. Takahashi, Y. Tokuda, and T. Yoko, “High optical quality spin-coated polyphenylsiloxane glass thick films on polyethyleneterephtalate and silica substrates,” Mater. Res. Bull. 41(10), 1925–1934 (2006). [CrossRef]

15.

R. Katoh and M. Kotani, “Observation of fluorescence from higher excited states in an anthracene crystal,” Chem. Phys. Lett. 201(1-4), 141–144 (1993). [CrossRef]

16.

Y. Gong, T. Andelman, G. F. Neumark, S. O’Brien, and I. L. Kuskovsky, “Origin of defect-related green emission from ZnO nanoparticles: effect of surface modification,” Nanoscale Res. Lett. 2(6), 297–302 (2007). [CrossRef]

17.

J. Liu, S. Lee, Y. H. Ahn, J.-Y. Park, and K. H. Koh, “Tailoring the visible photoluminescence of mass-produced ZnO nanowires,” J. Phys. D Appl. Phys. 42(9), 095401 (2009). [CrossRef]

18.

A. Teke, Ü. Özgür, S. Dogan, X. Gu, H. Morkoç, B. Nemeth, J. Nause, and H. O. Everitt, “Excitonic fine structure and recombination dynamics in single-crystalline ZnO,” Phys. Rev. B 70(19), 195207 (2004). [CrossRef]

19.

X. T. Zhang, Y. C. Liu, Z. Z. Zhi, J. Y. Zhang, Y. M. Lu, D. Z. Shen, W. Xu, X. W. Fan, and X. G. Kong, “Temperature dependence of excitonic luminescence from nanocrystalline ZnO films,” J. Lumin. 99(2), 149–154 (2002). [CrossRef]

20.

Y. Zhang, D. J. Chen, and C. T. Lee, “Free exciton emission and dephasing in individual ZnO nanowires,” Appl. Phys. Lett. 91(16), 161911 (2007). [CrossRef]

21.

M. Pope, “Charge-transfer exciton state, ionic energy levels, and delayed fluorescence in anthracene,” Mol. Cryst. 4(1-4), 183–190 (1968). [CrossRef]

OCIS Codes
(250.5230) Optoelectronics : Photoluminescence
(260.2160) Physical optics : Energy transfer

ToC Category:
Fluorescent and Luminescent Materials

History
Original Manuscript: March 20, 2012
Revised Manuscript: April 2, 2012
Manuscript Accepted: April 2, 2012
Published: April 4, 2012

Citation
Ryoko Shimada, Ben Urban, Mamta Sharma, Akhilesh Singh, Vitaliy Avrutin, Hadis Morkoç, and Arup Neogi, "Energy transfer in ZnO-anthracene hybrid structure," Opt. Mater. Express 2, 526-533 (2012)
http://www.opticsinfobase.org/ome/abstract.cfm?URI=ome-2-5-526


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References

  1. V. M. Agranovich, R. Atanasov, and F. Bassani, “Hybrid interface excitons in organic-inorganic quantum wells,” Solid State Commun.92(4), 295–301 (1994). [CrossRef]
  2. G. Heliotis, G. Itskos, R. Murray, M. D. Dawson, I. M. Watson, and D. D. C. Bradley, “Hybrid inorganic/organic semiconductor heterostructures with efficient non-radiative energy transfer,” Adv. Mater. 18(3), 334–338 (2006). [CrossRef]
  3. N. Saito, H. Haneda, T. Sekiguchi, N. Ohashi, I. Sakaguchi, and K. Koumoto, “Low-temperature fabrication of light-emitting Zinc Oxide micropatterns using self-assembled monolayers,” Adv. Mater.14(6), 418–421 (2002). [CrossRef]
  4. M. Law, L. E. Greene, J. C. Johnson, R. Saykally, and P. Yang, “Nanowire dye-sensitized solar cells,” Nat. Mater.4(6), 455–459 (2005). [CrossRef] [PubMed]
  5. R. K. Das, S. Bhat, S. Banerjee, C. Aymonier, A. Loppinet-Serani, P. Terech, U. Maitra, G. Raffy, J.-P. Desvergne, and A. Del Guerzo, “Self-assembled composite nano-materials exploiting a thermo reversible n-acene fibrillar scaffold and organic-capped ZnO nanoparticles,” J. Mater. Chem.21(8), 2740–2750 (2011). [CrossRef]
  6. O. Seitz, L. Caillard, H. M. Nguyen, C. Chiles, Y. J. Chabal, and A. V. Malko, “Optimizing non-radiative energy transfer in hybrid colloidal-nanocrystal/silicon structures by controlled nanopillar architectures for future photovoltaic cells,” Appl. Phys. Lett.100(2), 021902 (2012). [CrossRef]
  7. R. J. Lacowicz, Principles of Fluorescence Spectroscopy, 3rd ed. (Springer-Verlag, 2006).
  8. M. H. Crawford, “LEDs for solid-state lighting: performance challenges and recent advances,” IEEE J. Sel. Top. Quantum Electron.15(4), 1028–1040 (2009). [CrossRef]
  9. J. Y. Tsao, M. E. Coltrin, M. H. Crawford, and J. A. Simmons, “Solid-state lighting: An integrated human factors, technology, and economic perspective,” Proc. IEEE98(7), 1162–1179 (2010). [CrossRef]
  10. H. Zhao, G. Liu, J. Zhang, J. D. Poplawsky, V. Dierolf, and N. Tansu, “Approaches for high internal quantum efficiency green InGaN light-emitting diodes with large overlap quantum wells,” Opt. Express19(S4Suppl 4), A991–A1007 (2011). [CrossRef] [PubMed]
  11. H. Morkoç and Ü. Özgür, Zinc Oxide (Wiley-VCH, 2009).
  12. T. K. Sharma and E. Towe, “On ternary nitride substrates for visible semiconductor light-emitters,” Appl. Phys. Lett.96(19), 191105 (2010). [CrossRef]
  13. B. Menaa, M. Takahashi, Y. Tokuda, and T. Yoko, “High dispersion and fluorescence of anthracene doped in polyphenylsiloxane films,” J. Sol-Gel Sci. Technol.39(2), 185–194 (2006). [CrossRef]
  14. B. Menaa, M. Takahashi, Y. Tokuda, and T. Yoko, “High optical quality spin-coated polyphenylsiloxane glass thick films on polyethyleneterephtalate and silica substrates,” Mater. Res. Bull.41(10), 1925–1934 (2006). [CrossRef]
  15. R. Katoh and M. Kotani, “Observation of fluorescence from higher excited states in an anthracene crystal,” Chem. Phys. Lett.201(1-4), 141–144 (1993). [CrossRef]
  16. Y. Gong, T. Andelman, G. F. Neumark, S. O’Brien, and I. L. Kuskovsky, “Origin of defect-related green emission from ZnO nanoparticles: effect of surface modification,” Nanoscale Res. Lett.2(6), 297–302 (2007). [CrossRef]
  17. J. Liu, S. Lee, Y. H. Ahn, J.-Y. Park, and K. H. Koh, “Tailoring the visible photoluminescence of mass-produced ZnO nanowires,” J. Phys. D Appl. Phys.42(9), 095401 (2009). [CrossRef]
  18. A. Teke, Ü. Özgür, S. Dogan, X. Gu, H. Morkoç, B. Nemeth, J. Nause, and H. O. Everitt, “Excitonic fine structure and recombination dynamics in single-crystalline ZnO,” Phys. Rev. B70(19), 195207 (2004). [CrossRef]
  19. X. T. Zhang, Y. C. Liu, Z. Z. Zhi, J. Y. Zhang, Y. M. Lu, D. Z. Shen, W. Xu, X. W. Fan, and X. G. Kong, “Temperature dependence of excitonic luminescence from nanocrystalline ZnO films,” J. Lumin.99(2), 149–154 (2002). [CrossRef]
  20. Y. Zhang, D. J. Chen, and C. T. Lee, “Free exciton emission and dephasing in individual ZnO nanowires,” Appl. Phys. Lett.91(16), 161911 (2007). [CrossRef]
  21. M. Pope, “Charge-transfer exciton state, ionic energy levels, and delayed fluorescence in anthracene,” Mol. Cryst.4(1-4), 183–190 (1968). [CrossRef]

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