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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 17, Iss. 19 — Sep. 14, 2009
  • pp: 16436–16443
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Hybrid GaN/organic microstructured light-emitting devices via ink-jet printing

M. Wu, Z. Gong, A.J.C. Kuehne, A.L. Kanibolotsky, Y.J. Chen, I.F. Perepichka, A.R. Mackintosh, E. Gu, P.J. Skabara, R.A. Pethrick, and M.D. Dawson  »View Author Affiliations


Optics Express, Vol. 17, Issue 19, pp. 16436-16443 (2009)
http://dx.doi.org/10.1364/OE.17.016436


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Abstract

We report what we believe to be the first use of organic nanostructures for efficient colour conversion of gallium nitride light emitting diodes (LEDs). The particular nanomaterials, based on star-shaped truxene oligofluorenes, offer an attractive alternative to inorganic colloidal quantum dots in the search for novel and functional ‘nanophosphors’. The truxenes have been formed into a composite with photoresist and ink-jet printed onto microstructured gallium nitride LEDs, resulting in a demonstrator hybrid microdisplay technology with pixel size ~32µm. The output power density of the hybrid device was measured to be ~8.4mW/cm2 per pixel at driving current density of 870.8A/cm2 and the efficiency of colour conversion at drive current of 7mA was estimated to be approximately 50%.

© 2009 Optical Society of America

1. Introduction

Recently, there has been considerable interest in developing ‘hybrid’ light-emitting technologies based upon gallium nitride optoelectronics. Electrically injected gallium nitride hetero-structures offer customised direct band-gaps in the ultraviolet to visible spectral range and can selectively and efficiently transfer excitation either non-radiatively (Förster Resonance Energy Transfer or FRET) [1

1. M. Achermann, M.A. Petruska, S. Kos, D.L. Smith, D.D. Koleske, and V.I. Klimov, “Energy-transfer pumping of semiconductor nanocrystals using an epitaxial quantum well,” Nature 429, 642–646 (2004). [CrossRef] [PubMed]

,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, 334–338 (2006). [CrossRef]

] or radiatively [3

3. G. Heliotis, P.N. Stavrinou, D.D.C. Bradley, E. Gu, C. Griffin, C.W. Jeon, and M.D. Dawson, “Spectral conversion of InGaN ultraviolet microarray light emitting diodes using fluorene-based red-, green-, blue-, and white-light-emitting polymer overlayer films,” Appl. Phys. Lett. 87, 103505 (2005). [CrossRef]

] to an overlayer based on alternative light-emitting materials. This approach has important implications for areas including; colour conversion and white-light generation for solid-state lighting [4

4. S. Nakamura, “Current status of GaN-based solid state lighting,” MRS Bull. 34, 101–107 (2009). [CrossRef]

], microdisplays [5

5. Z. Gong, H.X. Zhang, E. Gu, C. Griffin, M.D. Dawson, V. Poher, G. Kennedy, P.M.W. French, and M.A.A. Neil, “Matrix-addressable micropixellated InGaN light-emitting diodes with uniform emission and increased light output,” IEEE Trans. Electron Devices 54, 2650–2658 (2007). [CrossRef]

], bioscience [6

6. H. Xu, J. Zhang, K.M. Davitt, Y.K. Song, and A.V. Nurmikko, “Application of blue-green and ultraviolet micro-LEDs to biological imaging and detection,” J. Phys. D: Appl. Phys. 41, 094013 (2008). [CrossRef]

], instrumentation [7

7. V. Poher, H.X. Zhang, G.T. Kennedy, C. Griffin, S. Oddos, E. Gu, D.S. Elson, J.M. Girkin, P.M.W. French, M.D. Dawson, and M.A.A. Neil, “Optical sectioning microscopes with no moving parts using a micro-stripe array light emitting diodes,” Opt. Express 15, 11196–11206 (2007). [CrossRef] [PubMed]

] and photo-pumped organic semiconductor lasers [8

8. Y. Yang, G.A. Turnbull, and I.D.W. Samuel, “Hybrid optoelectronics: a polymer laser pumped by a nitride light-emitting diode,” Appl. Phys. Lett. 92, 163306 (2008). [CrossRef]

]. It has to date been embodied primarily using conventional inorganic phosphors [9

9. S.C. Allen and A.J. Steckl, “A nearly ideal phosphor-converted white light-emitting diode,” Appl. Phys. Lett. 92, 143309 (2008). [CrossRef]

], inorganic semiconductor nanocrystals (mainly CdSe/ZnS colloidal quantum dots) [1

1. M. Achermann, M.A. Petruska, S. Kos, D.L. Smith, D.D. Koleske, and V.I. Klimov, “Energy-transfer pumping of semiconductor nanocrystals using an epitaxial quantum well,” Nature 429, 642–646 (2004). [CrossRef] [PubMed]

] and, to a lesser degree, organic polymers [3

3. G. Heliotis, P.N. Stavrinou, D.D.C. Bradley, E. Gu, C. Griffin, C.W. Jeon, and M.D. Dawson, “Spectral conversion of InGaN ultraviolet microarray light emitting diodes using fluorene-based red-, green-, blue-, and white-light-emitting polymer overlayer films,” Appl. Phys. Lett. 87, 103505 (2005). [CrossRef]

]. The case of colloidal quantum dots is particularly interesting, because it offers a means of ‘indirect’ electrical injection into nanostructured light emitters, and is a competitive approach to e.g. microdisplays and nanolasers being developed based on direct electrical injection into quantum dot containing conductive composite thin films [10

10. L.A. Kim, P.O. Anikeeva, S.A. Coe-Sullivan, J.S. Steckel, M.G. Bawendi, and V. Bulovic, “Contact printing of quantum dot light emitting devices,” Nano Lett. 8, 4513–4517 (2008). [CrossRef] [PubMed]

]. Here, we report development of such devices based on organic nanostructures as a technologically and scientifically interesting alternative.

Our demonstrations of this capability here utilise the specific truxene oligofluorene ‘T3’ [11

11. A.L. Kanibolotsky, R. Berridge, P.J. Skabara, I.F. Perepichka, D.D.C. Bradley, and M. Koeberg, “Synthesis and properties of monodisperse oligofluorene-functionalized truxenes: highly fluorescent star-shaped architectures,” J. Am. Chem. Soc. 126, 13695–13702 (2004). [CrossRef] [PubMed]

], wherein three terfluorenyl arms are attached to the central truxene core, giving a 3.1nm molecular radius. These molecules were blended into a photo-resist based on 1,4-cyclohexyldimethanol divinyl ether (CHDV), and the T3/CHDV composite printed onto 370nm-emitting micro-pixellated AlInGaN light emitting devices. The resulting hybrid devices show per pixel output power density of ~8.4mW/cm2 at driving current density of 870.8A/cm2 and reach colour conversion efficiencies of ~50% at drive current of 7mA.

2. Integration experiments

The structure, fabrication and performance of the gallium nitride devices used here has been described in detail elsewhere, together with their use for micro-display and instrumentation purposes [5

5. Z. Gong, H.X. Zhang, E. Gu, C. Griffin, M.D. Dawson, V. Poher, G. Kennedy, P.M.W. French, and M.A.A. Neil, “Matrix-addressable micropixellated InGaN light-emitting diodes with uniform emission and increased light output,” IEEE Trans. Electron Devices 54, 2650–2658 (2007). [CrossRef]

,16

16. Cluster issue on ‘Micro-pixellated LED’s for science and instrumentation’ (Guest Editors,M.D. Dawson and M.A.A. Neil) J. Phys. D: Appl. Phys.41, 090301 (2008). [CrossRef]

]. Briefly, these devices were micro-structured quantum well light emitting diodes (LEDs) made from AlInGaN epi-structures grown on sapphire and designed to emit at ~370nm. They were patterned using inductively-coupled plasma dry etching techniques into matrix-addressable arrays of 64×64 micro-pixels, where each pixel had a 16µm emission aperture and the pixel-to-pixel pitch was 50µm. The structure of these pixels is indicated schematically in Fig. 1, wherein it is seen that a Ti/Au annular contact (thickness 250nm, outer diameter 32 µm, inner diameter 16µm) defined the emission aperture, which consisted of a 200nm thick silicon oxide isolation layer above a Ni (3nm)/Au (9nm) current spreading layer. The individual pixels in this work had turn-on voltages of 3.6V, and emitted continuous wave output powers of 48nW at a current of 7mA (see later).

Fig. 1. Schematic picture of inkjet printing one T3/CHDV blend droplet on one pixel of the matrix addressable micro-pixellated LEDs

Fig. 2. (a) chemical structure of T3 molecule; (b) normalized absorption (black) and emission (red) spectra of T3/CHDV blends and normalized emission (blue) spectrum of micro-LEDs

A T3/CHDV blend with 10 wt% T3 concentration and 0.5 wt% PAG to initiate the photopolymerisation was prepared for the fabrication of hybrid inorganic/organic LED devices. It is estimated that, at this concentration, there are around 1013 T3 molecules in 1 pL of blend. Due to the high miscibility of T3 molecules in the CHDV matrix, it is not necessary to add any solvent into the blend to obtain a phase-uniform solution. This solventless blend facilitated the fabrication of uniform microstructures by preventing the ‘coffee ring’ stain formation due to solvent evaporation [19

19. E. Tekin, P.J. Smith, S. Hoeppener, A.M.J. van den Berg, A.S. Susha, A.L. Rogach, J. Feldmann, and U.S. Schubert, “Inkjet printing of luminescent CdTe nanocrystal-polymer composites,” Adv. Funct. Mater. 17, 23–28 (2007). [CrossRef]

]. After printing the blend droplets onto the top of micro-LED pixels, the whole packaged device was exposed under an ultraviolet (370 nm in wavelength) lamp at an energy density of ~15mW/cm2 for 15min in order to fully polymerize the CHDV monomers.

3. Results

Figure 3(a) is a plan-view optical micrograph under white light illumination, which shows that microstructures of 10wt% T3 in CHDV matrix have been successfully inkjet printed onto the treated LED micropixels. For illustration purposes, we chose here to print alternate pixels to form a simple 3×3 hybrid array. It can be seen that the diameter of the organic structures is ~40µm and that each has not flowed, before photocuring, beyond the boundary of the respective underlying GaN pixel. The close-up image of the microstructure taken by scanning electron microscopy (SEM) (Fig. 3(b)) shows the printed polymer microstructure has a smooth surface and well defined edges, demonstrating the effect of the surface treatment and the accurate printing alignment. The shape of the microstructure is dome-like and the thickness is approximately 2.9µm.

Fig. 3. (a) Plan view optical micrograph of 3×3 array of 10wt% T3 in CHDV matrix integrated on the GaN LED micropixels; (b) oblique SEM image of the inkjet printed T3/CHDV blend microstructure on one single LED micropixel

Figure 4(a) is an optical micrograph comparing a turned on bare GaN pixel (upper) with a turned on T3/CHDV integrated pixel (lower). The bare pixel emits UV light at ~368nm wavelength (see the unconverted light in Fig. 5 later). The integrated pixel emitted blue light from the oligofluorene truxene molecules photo-pumped by the underlying UV LED, showing colour conversion to the visible in an integrated device format. Figure 4(b) shows a demonstration combination of multiple illuminated hybrid pixels.

Fig. 4. Optical micrographs of (a) two pixels: bare micro-LEDs pixel (top) and T3/CHDV blend integrated on the pixel (bottom); (b) three alternating pixels with T3/CHDV blend

Ideally, the electrical properties of the inorganic light emitter should not be influenced by the integration of the organic fluorescent microstructures. We therefore first characterized the current-voltage (I-V) performance of the LED pixels before and after the integration of T3/CHDV blend microstructures. Our measurements show that the turn-on voltage (3.6V) and I-V characteristics of the representative micro-LED pixels did not change after T3/CHDV integration. As the aim of the integration is to down convert the UV light emitted from the inorganic LED to the visible wavelength via optically pumping the organic light emitting molecules, we have carried out spectral and optical power measurements to investigate the colour conversion of the integrated device. Figure 5 is the normalized photoluminescence (PL) spectrum of a representative T3/CHDV blend microstructure pumped by the AlInGaN LED pixel underneath operated at an injection current of 7mA. This spectrum was measured by using a home-built micro-PL system, which allowed the emission from a single target pixel to be imaged and analysed. It is observed that the emission spectrum is composed of unconverted and/or leakage/scattered LED pump light peaked at 368nm together with the characteristic emission of the T3 organic material showing vibronic peaks at 408nm, 428nm and 458nm respectively. As the CHDV matrix is UV-transparent after photocuring [14

14. A.J.C. Kuehne, D. Elfstrom, A.R. Mackintosh, A.L. Kanibolotsky, B. Guilhabert, E. Gu, I.F. Perepichka, P.J. Skabara, M.D. Dawson, and R.A. Pethrick, “Direct laser writing of nanosized oligofluorene truxenes in UV-transparent photoresist microstructures,” Adv. Mater. 21, 781–785 (2009). [CrossRef]

], the unconverted UV light transmits through the matrix, forming the predominant peak in the spectrum. This observation clearly demonstrates the optical pumping of the organic light emitting molecules by the UV micro-LED pixel to achieve UV to blue colour conversion.

Fig. 5. Spectral output of a single hybrid pixel showing integrated photopumping by the underlying electroluminescent gallium nitride ultraviolet LED. The peak at 368nm is unconverted and/or scattered pump light and the broad vibronic-structured emission between 400nm and 600nm is from the T3 organic nanostructures

To further study the emitting performance of the hybrid device, it is necessary to investigate the colour conversion of the T3/CHDV microstructures under various driving currents. Figure 6 shows the experimental spectra of the T3/CHDV blend microstructure pumped by the underlying GaN micropixel under increasing currents. Over this range of currents (1-7mA) the spectra show no obvious peak wavelength shift or broadening at either UV or blue wavelengths. Moreover, the integrated intensity of the 408nm (integrated from 390nm to 418nm) blue peak (ST3) is plotted versus the driving current in the inset of Fig. 6, showing no strong saturation effects over the range though there is some modest saturation observed higher than 2.5mA.

Fig. 6. Photoluminescence spectra of T3/CHDV blend microstructure pumped by the micro-LEDs underneath under increasing current; Inset, the integrated peak intensity of emission at 408nm

Fig. 7. Optical output power plotting of the micro-LEDs pixel before and after T3/CHDV integration via inkjet printing under increasing injection current. The latter have here been corrected for the effects of the filter

4. Conclusions

We have demonstrated that organic nanostructures, here represented by the ‘T3’ type of truxene oligofluorenes, can be successfully used as efficient colour converters for gallium nitride optoelectronics. When incorporated into a novel form of photocurable polymer matrix, these materials are suitable for solventless ink-jet printing to form controlled microstructured nanocomposites on device surfaces. By utilising a micro-pixellated and matrix-addressable format of underlying ultraviolet gallium nitride light-emitting diode, we provide an ideal photopumping device underlayer for the printed organics, allowing a simple hybrid micro-display technology to be demonstrated. This type of approach is anticipated to be of both fundamental interest, in delivering ‘indirect’ electrical excitation to organic nanostructures, and technological interest as a form of micro-display competitive with other approaches [10

10. L.A. Kim, P.O. Anikeeva, S.A. Coe-Sullivan, J.S. Steckel, M.G. Bawendi, and V. Bulovic, “Contact printing of quantum dot light emitting devices,” Nano Lett. 8, 4513–4517 (2008). [CrossRef] [PubMed]

]. As a further interest, the recent demonstration of photo-pumped laser action in the truxene oligofluorenes [15

15. G. Tsiminis, Y. Wang, P.E. Shaw, A.L. Kanibolotsky, I.F. Perepichka, M.D. Dawson, P.J. Skabara, G.A. Turnbull, and I.D.W. Samuel, “Low-threshold organic laser based on an oligofluorene truxene with low optical losses,” Appl. Phys. Lett. 94, 243304 (2009). [CrossRef]

] offers the prospect, using the advances demonstrated here, of integrated photo-pumped organic lasers.

Acknowledgements

This research work is supported by the EPSRC project ‘HYPIX’ and an EPSRC Science & Innovation Award on “Molecular Nanometrology”. The authors thank the supports from M. McGrady for the contact angle measurements, Dr O.J. Rolinski for absorption spectrum measurement and Y.F. Zhang, Dr P.R. Edwards and Prof R.W. Martin for the scanning electron microscopy.

References and links

1.

M. Achermann, M.A. Petruska, S. Kos, D.L. Smith, D.D. Koleske, and V.I. Klimov, “Energy-transfer pumping of semiconductor nanocrystals using an epitaxial quantum well,” Nature 429, 642–646 (2004). [CrossRef] [PubMed]

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, 334–338 (2006). [CrossRef]

3.

G. Heliotis, P.N. Stavrinou, D.D.C. Bradley, E. Gu, C. Griffin, C.W. Jeon, and M.D. Dawson, “Spectral conversion of InGaN ultraviolet microarray light emitting diodes using fluorene-based red-, green-, blue-, and white-light-emitting polymer overlayer films,” Appl. Phys. Lett. 87, 103505 (2005). [CrossRef]

4.

S. Nakamura, “Current status of GaN-based solid state lighting,” MRS Bull. 34, 101–107 (2009). [CrossRef]

5.

Z. Gong, H.X. Zhang, E. Gu, C. Griffin, M.D. Dawson, V. Poher, G. Kennedy, P.M.W. French, and M.A.A. Neil, “Matrix-addressable micropixellated InGaN light-emitting diodes with uniform emission and increased light output,” IEEE Trans. Electron Devices 54, 2650–2658 (2007). [CrossRef]

6.

H. Xu, J. Zhang, K.M. Davitt, Y.K. Song, and A.V. Nurmikko, “Application of blue-green and ultraviolet micro-LEDs to biological imaging and detection,” J. Phys. D: Appl. Phys. 41, 094013 (2008). [CrossRef]

7.

V. Poher, H.X. Zhang, G.T. Kennedy, C. Griffin, S. Oddos, E. Gu, D.S. Elson, J.M. Girkin, P.M.W. French, M.D. Dawson, and M.A.A. Neil, “Optical sectioning microscopes with no moving parts using a micro-stripe array light emitting diodes,” Opt. Express 15, 11196–11206 (2007). [CrossRef] [PubMed]

8.

Y. Yang, G.A. Turnbull, and I.D.W. Samuel, “Hybrid optoelectronics: a polymer laser pumped by a nitride light-emitting diode,” Appl. Phys. Lett. 92, 163306 (2008). [CrossRef]

9.

S.C. Allen and A.J. Steckl, “A nearly ideal phosphor-converted white light-emitting diode,” Appl. Phys. Lett. 92, 143309 (2008). [CrossRef]

10.

L.A. Kim, P.O. Anikeeva, S.A. Coe-Sullivan, J.S. Steckel, M.G. Bawendi, and V. Bulovic, “Contact printing of quantum dot light emitting devices,” Nano Lett. 8, 4513–4517 (2008). [CrossRef] [PubMed]

11.

A.L. Kanibolotsky, R. Berridge, P.J. Skabara, I.F. Perepichka, D.D.C. Bradley, and M. Koeberg, “Synthesis and properties of monodisperse oligofluorene-functionalized truxenes: highly fluorescent star-shaped architectures,” J. Am. Chem. Soc. 126, 13695–13702 (2004). [CrossRef] [PubMed]

12.

W.Y. Lai, R.D. Xia, Q.Y. He, P.A. Levermore, W. Huang, and D.D.C. Bradley, “Enhanced solid-state luminescence and low-threshold lasing from starburst macromolecular materials,” Adv. Mater. 21, 355–360 (2009). [CrossRef]

13.

G. Heliotis, S.A. Choulis, G. Itskos, R. Xia, R. Murray, P.N. Stavrinou, and D.D.C. Bradley, “Low-threshold lasers based on a high-mobility semiconducting polymer,” Appl. Phys. Lett. 88, 081104 (2006). [CrossRef]

14.

A.J.C. Kuehne, D. Elfstrom, A.R. Mackintosh, A.L. Kanibolotsky, B. Guilhabert, E. Gu, I.F. Perepichka, P.J. Skabara, M.D. Dawson, and R.A. Pethrick, “Direct laser writing of nanosized oligofluorene truxenes in UV-transparent photoresist microstructures,” Adv. Mater. 21, 781–785 (2009). [CrossRef]

15.

G. Tsiminis, Y. Wang, P.E. Shaw, A.L. Kanibolotsky, I.F. Perepichka, M.D. Dawson, P.J. Skabara, G.A. Turnbull, and I.D.W. Samuel, “Low-threshold organic laser based on an oligofluorene truxene with low optical losses,” Appl. Phys. Lett. 94, 243304 (2009). [CrossRef]

16.

Cluster issue on ‘Micro-pixellated LED’s for science and instrumentation’ (Guest Editors,M.D. Dawson and M.A.A. Neil) J. Phys. D: Appl. Phys.41, 090301 (2008). [CrossRef]

17.

J.A. DeFranco, B.S. Schmidt, M. Lipson, and G.G. Malliaras, “Photolithographic patterning of organic electronic materials,” Org. Electron. 7, 22–28 (2006). [CrossRef]

18.

G. Raabe and J. Michl, The Chemistry of Organic Silicon Compounds (John Wiley, 1989), Chap. 17.

19.

E. Tekin, P.J. Smith, S. Hoeppener, A.M.J. van den Berg, A.S. Susha, A.L. Rogach, J. Feldmann, and U.S. Schubert, “Inkjet printing of luminescent CdTe nanocrystal-polymer composites,” Adv. Funct. Mater. 17, 23–28 (2007). [CrossRef]

OCIS Codes
(160.5470) Materials : Polymers
(220.4000) Optical design and fabrication : Microstructure fabrication
(230.3670) Optical devices : Light-emitting diodes
(230.3990) Optical devices : Micro-optical devices
(130.7405) Integrated optics : Wavelength conversion devices

ToC Category:
Optical Devices

History
Original Manuscript: June 18, 2009
Revised Manuscript: July 24, 2009
Manuscript Accepted: July 25, 2009
Published: August 31, 2009

Citation
M. Wu, Z. Gong, A. J. Kuehne, A. L. Kanibolotsky, Y. J. Chen, I. F. Perepichka, A. R. Mackintosh, E. Gu, P. J. Skabara, R. A. Pethrick, and M. D. Dawson, "Hybrid GaN/organic microstructured light-emitting devices via ink-jet printing," Opt. Express 17, 16436-16443 (2009)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-19-16436


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References

  1. M. Achermann, M. A. Petruska, S. Kos, D. L. Smith, D. D. Koleske, and V.I . Klimov, "Energy-transfer pumping of semiconductor nanocrystals using an epitaxial quantum well," Nature 429, 642-646 (2004). [CrossRef] [PubMed]
  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, 334-338 (2006). [CrossRef]
  3. G. Heliotis, P. N. Stavrinou, D. D. C. Bradley, E. Gu, C. Griffin, C. W. Jeon, and M. D. Dawson, "Spectral conversion of InGaN ultraviolet microarray light emitting diodes using fluorene-based red-, green-, blue-, and white-light-emitting polymer overlayer films," Appl. Phys. Lett. 87, 103505 (2005). [CrossRef]
  4. S. Nakamura, "Current status of GaN-based solid state lighting," MRS Bull. 34, 101-107 (2009). [CrossRef]
  5. Z. Gong, H. X. Zhang, E. Gu, C. Griffin, M. D. Dawson, V. Poher, G. Kennedy, P. M. W. French, and M. A. A. Neil, "Matrix-addressable micropixellated InGaN light-emitting diodes with uniform emission and increased light output," IEEE Trans. Electron Devices 54, 2650-2658 (2007). [CrossRef]
  6. H. Xu, J. Zhang, K. M. Davitt, Y. K. Song, and A. V. Nurmikko, "Application of blue-green and ultraviolet micro-LEDs to biological imaging and detection," J. Phys. D: Appl. Phys. 41, 094013 (2008). [CrossRef]
  7. V. Poher, H. X. Zhang, G. T. Kennedy, C. Griffin, S. Oddos, E. Gu, D.S. Elson, J. M. Girkin, P. M. W. French, M. D. Dawson, and M. A. A. Neil, "Optical sectioning microscopes with no moving parts using a micro-stripe array light emitting diodes," Opt. Express 15, 11196-11206 (2007). [CrossRef] [PubMed]
  8. Y. Yang, G. A. Turnbull, and I. D. W. Samuel, "Hybrid optoelectronics: a polymer laser pumped by a nitride light-emitting diode," Appl. Phys. Lett. 92, 163306 (2008). [CrossRef]
  9. S. C. Allen and A. J. Steckl, "A nearly ideal phosphor-converted white light-emitting diode," Appl. Phys. Lett. 92, 143309 (2008). [CrossRef]
  10. L. A. Kim, P. O. Anikeeva, S. A. Coe-Sullivan, J. S. Steckel, M. G. Bawendi, and V. Bulovic, "Contact printing of quantum dot light emitting devices," Nano Lett. 8, 4513-4517 (2008). [CrossRef] [PubMed]
  11. A. L. Kanibolotsky, R. Berridge, P. J. Skabara, I. F. Perepichka, D. D. C. Bradley, and M. Koeberg, "Synthesis and properties of monodisperse oligofluorene-functionalized truxenes: highly fluorescent star-shaped architectures," J. Am. Chem. Soc. 126, 13695-13702 (2004). [CrossRef] [PubMed]
  12. W. Y. Lai, R. D. Xia, Q. Y. He, P. A. Levermore, W. Huang, and D. D. C. Bradley, "Enhanced solid-state luminescence and low-threshold lasing from starburst macromolecular materials," Adv. Mater. 21, 355-360 (2009). [CrossRef]
  13. G. Heliotis, S.A. Choulis, G. Itskos, R. Xia, R. Murray, P. N. Stavrinou, and D. D. C. Bradley, "Low-threshold lasers based on a high-mobility semiconducting polymer," Appl. Phys. Lett. 88, 081104 (2006). [CrossRef]
  14. A. J. C. Kuehne, D. Elfstrom, A. R. Mackintosh, A. L. Kanibolotsky, B. Guilhabert, E. Gu, I. F. Perepichka, P. J. Skabara, M. D. Dawson, and R. A. Pethrick, "Direct laser writing of nanosized oligofluorene truxenes in UV-transparent photoresist microstructures," Adv. Mater. 21, 781-785 (2009). [CrossRef]
  15. G. Tsiminis, Y. Wang, P. E. Shaw, A. L. Kanibolotsky, I. F. Perepichka, M. D. Dawson, P. J. Skabara, G. A. Turnbull, and I. D. W. Samuel, "Low-threshold organic laser based on an oligofluorene truxene with low optical losses," Appl. Phys. Lett. 94, 243304 (2009). [CrossRef]
  16. Cluster issue on ‘Micro-pixellated LED’s for science and instrumentation’ (Guest Editors, M.D. Dawson and M.A.A. Neil) J. Phys. D: Appl. Phys. 41, 090301 (2008). [CrossRef]
  17. J. A. DeFranco, B. S. Schmidt, M. Lipson, and G. G. Malliaras, "Photolithographic patterning of organic electronic materials," Org. Electron. 7, 22-28 (2006). [CrossRef]
  18. G. Raabe and J. Michl, The Chemistry of Organic Silicon Compounds (John Wiley, 1989), Chap. 17.
  19. E. Tekin, P. J. Smith, S. Hoeppener, A. M. J. van den Berg, A. S. Susha, A. L. Rogach, J. Feldmann, and U. S. Schubert, "Inkjet printing of luminescent CdTe nanocrystal-polymer composites," Adv. Funct. Mater. 17, 23-28 (2007). [CrossRef]

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