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Electroluminescence from ZnO nanoparticles/organic nanocomposites

Chun-Yu Lee

Graduate Institute of Electro-optical Engineering, National Taiwan University, Taipei, Taiwan 10617, Republic of China

Yau-Te Haung

Graduate Institute of Electronics Engineering, National Taiwan University, Taipei, Taiwan 10617, Republic of China

Wei-Fang Su

Graduate Institute of Materials Science Engineering, National Taiwan University, Taipei, Taiwan 10617, Republic of China and Department of Materials Science Engineering, National Taiwan University, Taipei, Taiwan 10617, Republic of China

Ching-Fuh Lina兲

Graduate Institute of Electro-optical Engineering, National Taiwan University, Taipei, Taiwan 10617, Republic of China; Graduate Institute of Electronics Engineering, National Taiwan University, Taipei, Taiwan 10617, Republic of China; and Department of Electrical Engineering, National Taiwan University, Taipei, Taiwan 10617, Republic of China

共Received 14 September 2006; accepted 30 October 2006; published online 7 December 2006兲 The authors report ultraviolet electroluminescence from ZnO nanoparticle-based devices prepared by the phase-segregation technique. The conditions for phase segregation are investigated using confocal microscopy. With proper parameters for phase segregation, the ZnO nanoparticles and

N , N

-diphenyl-N , N

-bis共3-methylphenyl兲-1,1

-biphenyl-4 , 4

-diamine: poly共methyl methacrylate兲 can be separated into two layers upon spin-coating process. The method allows electrons and holes to recombine in the ZnO nanoparticles. The I-V curve shows stable and excellent rectification. For the device with 90 nm ZnO nanoparticles, it exhibits a very narrow spectrum with a peak at 392 nm and no defect-related emission. The emission peak well corresponds to the ZnO band-gap energy. © 2006 American Institute of Physics.关DOI:10.1063/1.2404614兴

Zinc oxide is attracting extraordinary attention due to its promising properties such as wide direct band gap of 3.3 eV and very high exciton binding energy of 60 meV. These properties make ZnO light emitting diodes potentially useful in efficient solid state lighting, which has been the subject of many recent studies.1–3 In the past, the light emission from ZnO is mainly based on optical pumping method.4–7 Opti-cally pumped lasing had also been observed in ZnO nanocrystals.8,9However, electrically pumped light emission from ZnO has remained a challenge. Only a few electrolu-minescent 共EL兲 devices were fabricated successfully.10,12,13 In most of the ongoing works, ZnO EL devices are prepared in epitaxy films and single crystalline nanorods. In 2006, Ye

et al. reported on the electroluminescence at the wavelength

of 600 nm from n-ZnO / p-Si heterojunction by metal-organic chemical-vapor deposition technique.11In 2004, Park and Yi reported on the fabrication of n-ZnO / p-GaN nanorod electroluminescent devices by catalyst-free metal-organic vapor-phase epitaxy.12 The EL spectra show a weak blue emission peak at 450 nm and a relatively strong yellow emission centered at 560 nm. The above works involve the epitaxial growth of ZnO, which is usually inconvenient and expensive. In this work, we report the use of ZnO nanopar-ticles to fabricate the ZnO EL devices with ultraviolet共UV兲 emission by spin-coating method. The method has the promi-nent advantage of making the cost of devices less expensive.

We utilized organic-inorganic composite film, combining spin-coating method, to prepare ZnO EL devices. The composite film consists of the ZnO nanoparticles and

N , N

-diphenyl-N , N

-bis共3-methylphenyl兲-1,1

-biphenyl-4 , -biphenyl-4

-diamine 共TPD兲: poly共methyl methacrylate兲 共PMMA兲. The strong UV emission peak was observed at 392 nm when we applied forward-bias voltage at 7 V. Because the ZnO nanoparticle solubility in chloroform is different from TPD:PMMA, it is possible to use such an organic hole-transporting material that phase segregates from the ZnO nanoparticles during the spin-coating step. A layer of ZnO nanoparticles is formed on top of the TPD:PMMA film. The ZnO nanoparticle concentration and proportion in TPD:PMMA were experimented to successfully achieve phase segregation for EL devices with UV emission.

The procedure of device fabrication is as follows. First, we cleaned the indium tin oxide共ITO兲 glass using de-ionized water, acetone, and isopropyl alcohol sequentially. Then the EL film was fabricated by spin-coating process. The solution was formed by dissolving the ZnO nanoparticles, TPD, and PMMA in chloroform or mixture of chloroform and toluene with proper proportion. The ZnO nanoparticles were pur-chased from Aldrich. The ZnO nanoparticles were made by vapor phase synthesis. Two kinds of the ZnO nanoparticles were used. The diameters of the ZnO nanoparticles are 90± 10 and 20± 5 nm, respectively. The concentrations of ZnO nanoparticles, TPD, and PMMA in the solution were varied in the range of 0.7– 1.2 wt %. The solution was then spin coated onto an ITO coated glass substrate with a sheet resistance of 7⍀/䊐. The thickness of the ZnO composite a兲Author to whom correspondence should be addressed; electronic mail:

cflin@cc.ee.ntu.edu.tw

APPLIED PHYSICS LETTERS 89, 231116共2006兲

0003-6951/2006/89共23兲/231116/3/$23.00 89, 231116-1 © 2006 American Institute of Physics

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film is estimated to be about 1.5– 2␮m. The sample was subsequently annealed at 60 ° C for 2 h to remove the sol-vent. Afterwards, 2000 Å of Al was deposited onto the ZnO composite layer using thermal evaporation. The emitting area is 0.7⫻0.3 cm2.

With phase segregation, the ZnO nanoparticles and TPD:PMMA will be divided into two layers upon spin coat-ing. This method can improve the probability that electrons and holes recombine in the ZnO nanoparticles. The TPD works as the hole-transporting material. Holes are injected from the ITO contact into the highest occupied molecular orbital of the TPD matrix and are transported towards the valence band of the ZnO nanoparticles. In the same way, electrons are injected from the Al cathode into the conduc-tion band of the ZnO nanoparticles. Finally, holes and elec-trons form the excitons in the ZnO nanoparticles and recom-bine immediately. Hence, for the EL device of ITO/ TPD:PMMA/ZnO nanoparticles/Al, the spectral peak of the device corresponds to the ZnO band-gap energy.

In the past, the phase-segregation technique has been applied to fabricate EL devices of CdSe nanoparticles.14 However, the condition of the phase segregation for ZnO nanoparticle devices is very different from that for CdSe nanoparticle devices. It is necessary to investigate the proper proportion among ZnO nanoparticles, TPD, PMMA, chloro-form, and toluene. At the beginning, we dissolved the ZnO nanoparticles and TPD together in chloroform. Then we de-posited the solution on ITO glass by spin coating. Neverthe-less, the film was broken and very rough. This means that the film is discontinuous. Afterwards, we modified the param-eters of phase segregation for the ZnO nanoparticles. We used PMMA as a host matrix in the ZnO nanoparticles/TPD mixture. PMMA can improve the film-forming property.15 Therefore, we could get the unbroken film.

The solvent selection is also an important issue for phase segregation. When we only used chloroform as the solvent, the distribution of ZnO nanoparticles was not uniform on the TPD/PMMA film although the film was not broken. The 100⫻ optical image in Fig. 1共a兲shows that the distribution of ZnO nanoparticles forms many streaks of structures. To further improve the phase segregation, we added toluene into the solvent of chloroform. The purpose is to reduce the solu-bility of the ZnO nanoparticles in the solvent. On the other hand, toluene is still a good solvent for PMMA and TPD. The optical image in Fig.1共b兲shows that the distribution of the ZnO nanoparticles is improved significantly, compared to Fig.1共a兲.

Various parameters for the phase segregation had been tried. The confocal microscopy共WiTec, Alpha SNOM兲 was utilized to further examine all of the composite thin films.

When the concentration of the mixture solution and the ratio of the ZnO nanoparticles to TPD:PMMA are properly se-lected, the ZnO nanoparticles will aggregate on top of the film. Figures2共a兲and2共b兲 show the confocal images in the

x-y plane that are 0 and 1000 nm below the top surface,

respectively. More ZnO nanoparticles are shown in Fig.2共a兲 than in Fig.2共b兲. In other words, the ZnO nanoparticles tend to distribute on top of the film. This causes the nanoparticles/ organic film to exhibit the layer structure shown by the sche-matic diagram in Fig. 3共a兲. To confirm that the ZnO nano-particles aggregate on top of the film, the depth-profile confocal image was also taken, as shown in Fig.3共b兲. On the contrary, when the ratio of the ZnO nanoparticles to TP-D:PMMA is not appropriate, the ZnO nanoparticles will not separate from the TPD:PMMA layer. Figure3共c兲shows the corresponding depth profile of the thin film taken from the confocal microscopy. The ZnO nanoparticles aggregate in block.

In this experiment, TPD is used as the hole-transporting material. Although it is blue-emission material,16 it has no contribution to light emission in our devices by spinning process. We used the same parameters to fabricate devices without using the ZnO nanoparticles. The device shows no light emission and no electrical rectification.

The I-V characteristics of the ZnO nanoparticle devices were measured. Figure 4 shows the I-V curve for the ZnO nanoparticle devices, using Al as the cathode material. For good film-forming property, such as that of the ZnO nano-particles:TPD:PMMA film shown in Fig.3共b兲, the I-V curve 共curve a兲 shows stable and excellent rectification. The turn-on voltage is about 4 V. However, for bad film-forming property, the corresponding I-V curve共curve b兲 exhibits no rectification behavior. Because the film is broken, it leads to no current injecting into the ZnO nanoparticles. Hence the FIG. 1. 100⫻ optical images of the ZnO/organic thin film for 共a兲 chloroform

solvent and共b兲 the solvent of chloroform and toluene.

FIG. 2. Confocal images of the TPD:PMMA/ZnO nanoparticles thin film for the solvent of chloroform and toluene in the x-y plane:共a兲 the surface and共b兲 1000 nm below the surface.

FIG. 3. 共a兲 Schematic diagram of the ZnO nanoparticles/organic thin film and共b兲 confocal image 共depth profile兲 of the ZnO devices with phase seg-regation;共c兲 confocal image 共depth profile兲 of the ZnO devices without phase segregation.

231116-2 Lee et al. Appl. Phys. Lett. 89, 231116共2006兲

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phase segregation plays a very important role for device performance.

The normalized electroluminescence spectra of the ZnO nanoparticle-based devices at forward bias of 7 V are shown in Fig.5. For the device with 90 nm ZnO nanoparticles, its emission spectrum is very narrow. The emission peak is at 392 nm, which corresponds to the band-gap energy of ZnO pretty well. The full width at half maximum共FWHM兲 of the spectrum is 35 nm. The inset is a photograph of light

emis-sion from the EL device with 90 nm ZnO nanoparticles at a forward bias of 7 V. It is worth noting that this device has very different spectral behaviors from those reported previ-ously in ZnO nanorod and ZnO thin film EL devices.10–13 This device shows a UV electroluminescence peak at 392 nm and has no broad defect-related band at longer wave-lengths. For the 20 nm ZnO device at the same forward bias of 7 V, it also has a peak around 392 nm. However, the electroluminescence spectrum showed the broad defect-related band at longer wavelengths, presumably due to the high concentration of defects共oxygen vacancies兲.10–13 There-fore, the qualities of the ZnO nanoparticles influence the electroluminescence spectrum.

In conclusion, we report the use of phase-segregation technique to fabricate the ZnO nanoparticle EL devices. The UV-emission peak has a FWHM of 35 nm at a drive voltage of 7 V. To optimize the phase segregation, we take the con-focal microscopy for the ZnO nanoparticle film. When the phase segregation is achieved, the ZnO nanoparticles and TPD:PMMA separate into two layers. The I-V curve exhibits excellent rectification. The optimized film shows a narrow UV EL peak at 392 nm, which corresponds to the band-gap energy of ZnO. The processing procedure revealed in this work shows a convenient way to fabricate ZnO EL devices with a very low cost.

This work is supported by the National Science Council, Taiwan, Republic of China, with Grant NOs. NSC94-2120-M-002-010 and NSC94-2112-M-002-009.

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FIG. 4. Current-voltage characteristics of the ZnO devices:共a兲 with phase segregation and共b兲 without phase segregation.

FIG. 5.共Color online兲 Normalized electroluminescence spectra for ZnO EL devices:共a兲 ITO/ZnO nanoparticles 共90 nm兲/TPD:PMMA/Al and 共b兲 ITO/ ZnO nanoparticles共20 nm兲/TPD:PMMA/Al. The applied voltage is about 7 V.共Inset: photograph of light emission from the EL device with 90 nm ZnO nanoparticles.兲

231116-3 Lee et al. Appl. Phys. Lett. 89, 231116共2006兲

數據

FIG. 2. Confocal images of the TPD:PMMA/ZnO nanoparticles thin film for the solvent of chloroform and toluene in the x-y plane: 共a兲 the surface and 共b兲 1000 nm below the surface.
FIG. 4. Current-voltage characteristics of the ZnO devices: 共a兲 with phase segregation and 共b兲 without phase segregation.

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