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Hugely enhanced electroluminescence from mesoporous ZnO particles
Optical Materials 28 (2006) 385–390 www.elsevier.com/locate/optmat
Hugely enhanced electroluminescence from mesoporous ZnO particles Guang-hui Ning, Xiao-peng Zhao *, Jia Li, Chang-qing Zhang Institute of Electrorheological Technology, Department of Applied Physics, School of Science, Northwestern Polytechnical University, Xi’an 710072, P.R. China Received 1 October 2004; accepted 29 January 2005 Available online 17 March 2005
Abstract Using octadecylamine (ODA) and dodecylamine (DDA) as template, nanostructured porous ZnO particles were synthesized by sol–gel method. The results of experiments show that the density of ZnO processed with ODA, DDA and without template is 5.31, 5.37 and 5.42 cm2/g respectively. The surface analysis proves that the ZnO particles processed with ODA and DDA hold porous structure. Hugely enhanced electroluminescence (EL) was observed from the porous ZnO particles under direct current electric field from 2–4.66 V/lm. Comparing with the low emission intensity of the ZnO without porous structure, the emission intensity of the ZnO sample processed with DDA and DDA were enhanced 12 times and 20 times respectively at the voltage of 4.66 V/lm. The EL spectrum shows mainly broad peak emission feature with a peak at 556 nm. The threshold voltage is just 2 V/lm. The results indicate that the porous structure in ZnO particles can enhance EL intensity. 2005 Elsevier B.V. All rights reserved. PACS: 78.60.Fi; 78.55.Et; 82.70.Uv Keywords: Electroluminescence; Zinc oxide; Porous; Sol–gel
1. Introduction Zinc oxide is an interesting direct-band-gap II–VI semiconductor with wurtzite-type structure, which consists of the hexagonal lattice with a c/a ratio of 1.602. Its band energy is 3.3 eV and excitonic binding energy is 60 meV at room temperature. It is stable for chemical as well as thermal fluctuations, with a reasonably good electrical conductivity. The stability of ZnO material is unsurpassable comparing with nitride in III–V group and selenide in II–VI group. In particular, its large exciton binding energy (60 meV) and superadding with
*
Corresponding author. Tel./fax: +86 29 88495950. E-mail address: [email protected] (X.-p. Zhao).
0925-3467/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.01.024
quanta confinement effect provide a possibility to produce light-emitting diodes and diode lasers [1–4]. On the other hand, ZnO emits a broad luminescence emission in the green–yellow region, and, as a result, it is a potential material for use in white light sources [5]. The excitonic lasing excited by electron-beam was observed at low temperature many years ago [6]. The emission could be quenched with the increased temperature. Due to this deficiency, people have showed less interest in ZnO as an optoelectronic material for a long time. Recently, there has again been great interest in the research of ZnO material because of the upsurge on GaN material [7–11]. Most of the researches have centered around the studies on photoluminescence, namely, power provided by optical pumping. If the luminescence or laser light from ZnO was achieved with electron injection, then ZnO phosphor can be applied in widespread
386
G.-h. Ning et al. / Optical Materials 28 (2006) 385–390
fields such as optical computing, super-high resolution laser printers and laser-powered biochips. ZnO exhibits good piezoelectric, photoelectric and optical properties, and might be a good candidate for electroluminescence devices. Few articles have been reported on the electroluminescence of ZnO [12–14], especially on ZnO powder EL. Powder EL displays made from inorganic EL materials have virtually unlimited potential, i.e., uniform light emission, thin profile and low power consumption. Such materials are important for both fundamental study and their technological application in catalysis, micro- and opto-electronics, and liquid crystal display. The biggest hurdle to ZnO powder EL is its low intensity. The EL properties might be improved if there is porous structure in ZnO particles. Base on the ideas, the intent of this paper is to synthesis porous ZnO particles by sol–gel template method and to improve EL properties of ZnO particles by using the porosity in ZnO nanoparticles. The experimental results show that the enhanced EL is observed with the ZnO particles possessing porous structure. To the best of our knowledge, this is a new result that has not been reported before.
molar ratio). The products were obtained after washing the white precipitation with aether by soxhlet extractor and then drying them in vacuum at 60 C. The samples used as measurement of EL were annealed at 500 C. 2.2. Fabrication of EL device
2. Experimental
The structure of EL device was fabricated as follows: indium-tin-oxide (ITO) was used as the anode and stable Al metal was used as the cathode. The porous ZnO powders were used as luminescence layers. The detailed preparation is stated as follows: A piece of insulating paper with a circular hole (diameter 4 mm) was adhered onto a cleaned surface of aluminum, and then a retuse pothole was formed. Subsequently, a little ZnO powder was planished into the pothole to form a luminescent layer whose thickness was about 0.3 mm. The preparation of the devices was carried out after covering an ITO glass onto the luminescent layer. The fabricated EL device is driven by a high volts d.c. The electric field is applied to provide a current through the ZnO powder from aluminum electrode. The EL experiments were performed at room temperature. Light emitted from the sample passed through ITO glass. The schematic structure of the devices is shown in Fig. 1.
2.1. Preparation of ZnO powder
2.3. Characterizations
The preparation procedure followed in this study is essentially the same as that of Meulenkamp [15]. The whole sampling procedure consists of three major steps: (1) preparation of pure nano-sized ZnO particles; (2) preparation of nano-sized ZnO processed with template (ODA and DDA); (3) annealing all the ZnO samples at high temperature. Zinc acetate, Zn(CH3COO)2 Æ 2H2O, and absolute ethanol are used in the synthesis without further purification. For a typical preparation, 5 mmol of zinc acetate dihydrate is dissolved in 50 ml of boiled absolute ethanol in a covered flask under vigorous stirring at 80 C. After cooling to room temperature, the solution is further cooled to 0 C in an ice-water bath. 7 mmol of lithium hydroxide ethanol solution was then added to the zinc acetate solution under vigorous stirring to yield a clear and homogeneous solution. A white precipitation was obtained after adding 200 ml hexane and washing with ethanol and hexane alternately. The ZnO processed without template was obtained after drying the precipitation in vacuum at 60 C. The ZnO processed with template (dodecylamine, DDA; octadecylamine, ODA) were prepared in a similar way with the procedure described above. ODA and DDA were added into ethanol before adding lithium hydroxide in proportion as 1:10(ODA or DDA:Zn,
The typical XRD pattern of the powder was characterized by D/max-3C automatic X-ray diffractometer with CuKa line (k = 0.15406 nm) operated at 40 kV, 40 mA. The intensity was determined in the range 25 < 2h < 75 with Si powders as a standard for calibration in a 0.02 step size. The powder was planished into a 10 · 10 · 1 mm glass square groove and its surface was kept as smooth as possible. The crystallite size of particles was determined by using the Scherrers equation,
Fig. 1. Configuration of the EL device.
G.-h. Ning et al. / Optical Materials 28 (2006) 385–390
L = kk/(b cos h), where L is the crystal size, k is the X-ray wavelength, b is the broadening of the diffraction peak and h is the diffraction angle [16]. In general, the intensity and the full-width at half maximum (FWHM) of (0 0 2) peak can show the structural properties of the ZnO. The solid density of ZnO powders was measured using the liquid displacement technique by a pycnometer. N2 adsorption/desorption isotherms were obtained at liquid N2 temperature of 77 K using a NOVA 2000e surface area and pore size analyzer (Quantachrome instruments). Prior to the adsorption measurements, the samples were degassed in vacuum at 120 C for 1 h. Surface areas were calculated by the Brunauer– Emmett–Teller (BET) method. The pore size distributions were determined from the adsorption isotherms by the Barrett–Joyner–Halenda (BJH) method. The pore volume was estimated from the adsorbed amount of N2 at P/P0 = 0.9814. Fourier transform infrared (FTIR) spectra were obtained on a Bruker Equinox 55 FT-IR spectrometer using KBr pellets. All the EL spectra were measured in the upright direction of the ITO face of the EL device with a USB 2000 spectrophotometer (Ocean company). The detected distance between the device and the receiving end of optic fiber was about 0.2 mm The EL stability of ZnO sample was assessed on an IFM-D Flow Injection Chemiluminescence Meter.
3. Results and discussion
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67.82 and 68.96 correspond to (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2) and (2 0 1) directions of ZnO hexagonal structure. This indicated that ZnO particles possessed polycrystalline hexagonal wurtzite structure [17]. Comparison of the diffraction for curve (b), (c) with that for (a) indicated that the particle annealed at 500 C (curve b and c) was better crystallized than that without annealing (curve a). The size of crystal is about 2 nm for the ZnO processed without template (curve a), 40 nm for the ZnO processed with DDA (curve b) and 32 nm for the ZnO processed with ODA (curve c) figured out by Scherrers equation. The structure of sample without being annealed can be characterized by infrared ray (IR) spectra. The IR spectra of the ZnO processed without template (curve a), the ZnO processed with DDA/ODA (curve b/c) and the pure ODA (curve d) in the region of 4000– 500 cm 1 are shown in Fig. 3. Four absorption bands were exhibited. For curve a, the absorption peaks centered at 3418 cm 1, 1575 cm 1, 1413 cm 1 and 423 cm 1 can be attributed to the stretching vibration of hydroxyl group of alcohols, asymmetrical and symmetrical stretching vibration of C@O bond in –CH3COO [18] and Zn–O stretching vibration [19]. It indicates that ZnO crystals produced via the sol–gel technique are not pure but have acetate (CH3COO ) groups, which originate from the reagent materials adsorbed on the surface of the crystals [20]. Comparing curve a, b with c, a difference can be found that some IR absorb peaks in curve b and c shifted to high wavenumber. From the above facts, a consequence can be derived that the species of ZnO is negative because of absorbing the acetate groups. In the solution, the front group of amidocyanogen of ODA or DDA is positive. The interaction between the opposite charged species made the ODA or DDA link up with ZnO.
Transmittance (%)
(200)
(103)
(110)
(102)
c
(112) (201)
d
(002)
Intensity (a.u.)
(100)
Fig. 2 shows the diffraction patterns of the ZnO processed without template (curve a), the ZnO processed with DDA/ODA then followed with annealing at 500 C (curve b/c). As can be clearly seen from Fig. 2, ZnO XRD peaks were well indexed. Peaks at 2h = 31.7, 34.36, 36.2, 47.54, 56.6, 62.76, 66.34,
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a b c
b a 30
3500 40
50
60
70
2500
1500
500
-1
Wavenumber (cm )
2θ (degrees) Fig. 2. XRD pattern of ZnO processed without template (a), processed with DDA (b) ODA (c).
Fig. 3. FT-IR spectra of ZnO processed without template (a), processed with DDA (b), ODA (c) and before annealing and pure ODA (d).
G.-h. Ning et al. / Optical Materials 28 (2006) 385–390
120 0.025
100
0.010
0.000
1
10 Dp (nm)
100
40 Adsorption Desorption
20
0 0.0
EL Intensity (a.u.)
4.66V/µm
80
80
40
2.5
3.0
4V/µm
3.5 4.0 4.5 Voltage (V/µm)
5.0
60 3.33V/µm
40 20 0
2.66V/µm baseline
400
500
(a)
600 700 Wavelength (nm)
ZnO (ODA)
4.66V/µm
EL Intensity (a.u.)
200
200
λ=406nm λ=556nm
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2
4V/µm
100
800
3 4 Voltage (V/µm)
5
3.33V/µm 2.66V/µm
50
2V/µm baseline
0
400
(b)
500
600 700 Wavelength (nm)
800
Fig. 5. EL spectra of ZnO processed with DDA (a) and ODA (b). The inset is the ratio of the EL intensity peaked at 556 nm to that peaked at 406 nm.
Table 1 Comparison of the emission characteristics of our measurement and the literature Peak (nm)
Method
Ref.
480 590 500 556
Electron-beam evaporation MOCVD Solid-state chemical reaction Sol–gel
[12] [13] [21] Present study
0.015
0.005
60
100
λ=406nm λ=556nm
120 DDA-ZnO
ZnO (DDA)
0.020 dV(d) (cc/g.nm)
Volume (cc/g)
80
120
EL Intensity (a.u.)
The N2 adsorption–desorption isotherms of the ZnO processed with template are shown in Fig. 4. The ZnO exhibits an isotherm similar to type-IV with H2-type hysteresis loop. The corresponding pore size distribution curve (see the inset to Fig. 4) indicates that the material has narrower pore size distributions with an average pore diameter around 3.83 nm. Such a value confirms that the material has mesoporosity. The BET surface areas of ZnO calculated from these isotherms are 94.72 m2 g 1 and the pore volume was 0.18 cm3 g 1. EL measurements were performed on powder of nanocrystalline ZnO particles of different treatment. The EL spectra of ZnO processed with DDA (Fig. 5(a)), ODA (Fig. 5(b)) are shown in Fig. 5. All samples show two emission bands: a relatively weak violet emission band at 406 nm and a more intense and broad green–yellow emission band at 556 nm. No light emission was observed in the measurement of visible spectrum range for reverse-bias voltage up to electric breakdown. The EL was strong enough to be observed clearly by the naked eye when the applied forward-bias voltage reached a certain values. By increasing the applied bias, the EL intensity of green emission enhanced faster than that of violet emission (inset graph). The EL intensity of ZnO processed with ODA is 1.5 or 2 times larger than that of ZnO processed with DDA under the same applied bias. For the location of the visible emission peak, the comparison to some reported results has been made in Table 1. It indicates that different preparation methods for ZnO result in the different locations of the visible emission peak. It has been reported that the emission properties of ZnO are strongly dependent on the growth conditions including growth temperature and growth ambient [21–24]. In order to study the EL from porous ZnO par-
EL Intensity (a.u.)
388
0.2
0.4
0.6
0.8
1.0
Relative Pressure (p/po)
Fig. 4. N2 adsorption–desorption isotherm of mesoporous ZnO. The inset to the figure shows the pore size distribution curve.
ticles in detail, we make the growth conditions stable and just change the porosity of ZnO sample. Fig. 6 correspond to the blue emission at 4 V/lm dc bias from the ZnO processed without template, the ZnO, ZnO processed with DDA and ODA respectively. One can clearly see that very weak emission was observed in the measurement of visible spectrum range for the ZnO with no porosity. On the contrary, strong EL emissions were seen in the ZnO processed with template and the ZnO processed with ODA emitted higher EL intensity than that processed with DDA. This suggests that the EL emission can be enhanced by the porosity in the ZnO particles.
G.-h. Ning et al. / Optical Materials 28 (2006) 385–390
389
150
150
100
100
100
50 c 0
50
onset
baseline 400
500
600
700
800 0
Wavelength (nm)
0 1
Fig. 6. EL spectra of ZnO processed with ODA (a) DDA (b) processed without template (c) at 4 V/lm volts d.c.
The excitation properties of the devices were also studied. In Fig. 7, the time evolution of the EL signal at switch on is reported for a current density of 0.3 A/ cm2. At time s = 0 the voltage is switched on and the signal is seen to saturate in a characteristic time son of 5 s. The devices are very stable and exhibit a strong EL signal at room temperature. The EL signal dropped down just 5.5% of saturate value in 1 h. After the voltage is switched off, the EL signal goes to zero with a lifetime soff of 5 s. Current–voltage (I–V) and luminance–voltage (L–V) curves of porous ZnO are shown in Fig. 8. When applied voltage is above 2 V/lm, both the light emission intensity and the current increased with the voltage. As the applied forward-bias voltage increased from 2 to 4.66 V/lm, the EL emission showed a drastic increase in its intensity without exhibiting any peak shift. By further increasing the forward-bias voltage to 5 V/lm, ZnO sample was broken down.
2
3
4
5
Voltage (V/µm)
Fig. 8. Static I–V curves from the device of Fig. 1. The onset of EL emission was at 2 V/lm.
4. Conclusion Nanostructure porous ZnO particles were synthesized by sol–gel template method. The results of experiments show that the density of ZnO processed with ODA, DDA and without template is 5.31, 5.37 and 5.42 cm2/g respectively. The surface analysis proved that the ZnO particles processed with ODA and DDA hold porous structure. The ZnO particles processed with ODA has a surface area of 94.72 m2/g, a pore volume of 0.18 cc/g and a pore diameter of 3.831 nm. Comparing with the low emission intensity of the ZnO without porous structure, the emission intensity of the ZnO sample processed with DDA and ODA was enhanced 12 and 20 times respectively at the field of 4.66 V/lm. The EL spectrum shows mainly broad peak emission feature with a peak at 556 nm. The threshold emission field is just 2 V/lm. Based on the analysis of property and structure of porous ZnO particles, a conclusion has been reached that the enhanced EL has relationship with the porous structure.
Acknowledgements
300 EL intensity (a.u.)
50
Current (µA)
b EL intensity (a.u.)
EL intensity (a.u.)
a
The National Natural Science Foundation of China under grant no. 50272054 and the Natural Science Foundation of China for Distinguished Young Scholars under grant no. 50025207 as well as Doctorate Foundation of Northwestern Polytechnical University (200241) are all gratefully acknowledged.
200
100
0 0
1000
2000
3000
References
Time (s)
Fig. 7. Time evolution of the room temperature EL signal at the device switch on for a volts intensity of 3.33 V/lm.
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Hugely enhanced electroluminescence from mesoporous ZnO particles Guang-hui Ning, Xiao-peng Zhao *, Jia Li, Chang-qing Zhang Institute of Electrorheological Technology, Department of Applied Physics, School of Science, Northwestern Polytechnical University, Xi’an 710072, P.R. China Received 1 October 2004; accepted 29 January 2005 Available online 17 March 2005
Abstract Using octadecylamine (ODA) and dodecylamine (DDA) as template, nanostructured porous ZnO particles were synthesized by sol–gel method. The results of experiments show that the density of ZnO processed with ODA, DDA and without template is 5.31, 5.37 and 5.42 cm2/g respectively. The surface analysis proves that the ZnO particles processed with ODA and DDA hold porous structure. Hugely enhanced electroluminescence (EL) was observed from the porous ZnO particles under direct current electric field from 2–4.66 V/lm. Comparing with the low emission intensity of the ZnO without porous structure, the emission intensity of the ZnO sample processed with DDA and DDA were enhanced 12 times and 20 times respectively at the voltage of 4.66 V/lm. The EL spectrum shows mainly broad peak emission feature with a peak at 556 nm. The threshold voltage is just 2 V/lm. The results indicate that the porous structure in ZnO particles can enhance EL intensity. 2005 Elsevier B.V. All rights reserved. PACS: 78.60.Fi; 78.55.Et; 82.70.Uv Keywords: Electroluminescence; Zinc oxide; Porous; Sol–gel
1. Introduction Zinc oxide is an interesting direct-band-gap II–VI semiconductor with wurtzite-type structure, which consists of the hexagonal lattice with a c/a ratio of 1.602. Its band energy is 3.3 eV and excitonic binding energy is 60 meV at room temperature. It is stable for chemical as well as thermal fluctuations, with a reasonably good electrical conductivity. The stability of ZnO material is unsurpassable comparing with nitride in III–V group and selenide in II–VI group. In particular, its large exciton binding energy (60 meV) and superadding with
*
Corresponding author. Tel./fax: +86 29 88495950. E-mail address: [email protected] (X.-p. Zhao).
0925-3467/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.01.024
quanta confinement effect provide a possibility to produce light-emitting diodes and diode lasers [1–4]. On the other hand, ZnO emits a broad luminescence emission in the green–yellow region, and, as a result, it is a potential material for use in white light sources [5]. The excitonic lasing excited by electron-beam was observed at low temperature many years ago [6]. The emission could be quenched with the increased temperature. Due to this deficiency, people have showed less interest in ZnO as an optoelectronic material for a long time. Recently, there has again been great interest in the research of ZnO material because of the upsurge on GaN material [7–11]. Most of the researches have centered around the studies on photoluminescence, namely, power provided by optical pumping. If the luminescence or laser light from ZnO was achieved with electron injection, then ZnO phosphor can be applied in widespread
386
G.-h. Ning et al. / Optical Materials 28 (2006) 385–390
fields such as optical computing, super-high resolution laser printers and laser-powered biochips. ZnO exhibits good piezoelectric, photoelectric and optical properties, and might be a good candidate for electroluminescence devices. Few articles have been reported on the electroluminescence of ZnO [12–14], especially on ZnO powder EL. Powder EL displays made from inorganic EL materials have virtually unlimited potential, i.e., uniform light emission, thin profile and low power consumption. Such materials are important for both fundamental study and their technological application in catalysis, micro- and opto-electronics, and liquid crystal display. The biggest hurdle to ZnO powder EL is its low intensity. The EL properties might be improved if there is porous structure in ZnO particles. Base on the ideas, the intent of this paper is to synthesis porous ZnO particles by sol–gel template method and to improve EL properties of ZnO particles by using the porosity in ZnO nanoparticles. The experimental results show that the enhanced EL is observed with the ZnO particles possessing porous structure. To the best of our knowledge, this is a new result that has not been reported before.
molar ratio). The products were obtained after washing the white precipitation with aether by soxhlet extractor and then drying them in vacuum at 60 C. The samples used as measurement of EL were annealed at 500 C. 2.2. Fabrication of EL device
2. Experimental
The structure of EL device was fabricated as follows: indium-tin-oxide (ITO) was used as the anode and stable Al metal was used as the cathode. The porous ZnO powders were used as luminescence layers. The detailed preparation is stated as follows: A piece of insulating paper with a circular hole (diameter 4 mm) was adhered onto a cleaned surface of aluminum, and then a retuse pothole was formed. Subsequently, a little ZnO powder was planished into the pothole to form a luminescent layer whose thickness was about 0.3 mm. The preparation of the devices was carried out after covering an ITO glass onto the luminescent layer. The fabricated EL device is driven by a high volts d.c. The electric field is applied to provide a current through the ZnO powder from aluminum electrode. The EL experiments were performed at room temperature. Light emitted from the sample passed through ITO glass. The schematic structure of the devices is shown in Fig. 1.
2.1. Preparation of ZnO powder
2.3. Characterizations
The preparation procedure followed in this study is essentially the same as that of Meulenkamp [15]. The whole sampling procedure consists of three major steps: (1) preparation of pure nano-sized ZnO particles; (2) preparation of nano-sized ZnO processed with template (ODA and DDA); (3) annealing all the ZnO samples at high temperature. Zinc acetate, Zn(CH3COO)2 Æ 2H2O, and absolute ethanol are used in the synthesis without further purification. For a typical preparation, 5 mmol of zinc acetate dihydrate is dissolved in 50 ml of boiled absolute ethanol in a covered flask under vigorous stirring at 80 C. After cooling to room temperature, the solution is further cooled to 0 C in an ice-water bath. 7 mmol of lithium hydroxide ethanol solution was then added to the zinc acetate solution under vigorous stirring to yield a clear and homogeneous solution. A white precipitation was obtained after adding 200 ml hexane and washing with ethanol and hexane alternately. The ZnO processed without template was obtained after drying the precipitation in vacuum at 60 C. The ZnO processed with template (dodecylamine, DDA; octadecylamine, ODA) were prepared in a similar way with the procedure described above. ODA and DDA were added into ethanol before adding lithium hydroxide in proportion as 1:10(ODA or DDA:Zn,
The typical XRD pattern of the powder was characterized by D/max-3C automatic X-ray diffractometer with CuKa line (k = 0.15406 nm) operated at 40 kV, 40 mA. The intensity was determined in the range 25 < 2h < 75 with Si powders as a standard for calibration in a 0.02 step size. The powder was planished into a 10 · 10 · 1 mm glass square groove and its surface was kept as smooth as possible. The crystallite size of particles was determined by using the Scherrers equation,
Fig. 1. Configuration of the EL device.
G.-h. Ning et al. / Optical Materials 28 (2006) 385–390
L = kk/(b cos h), where L is the crystal size, k is the X-ray wavelength, b is the broadening of the diffraction peak and h is the diffraction angle [16]. In general, the intensity and the full-width at half maximum (FWHM) of (0 0 2) peak can show the structural properties of the ZnO. The solid density of ZnO powders was measured using the liquid displacement technique by a pycnometer. N2 adsorption/desorption isotherms were obtained at liquid N2 temperature of 77 K using a NOVA 2000e surface area and pore size analyzer (Quantachrome instruments). Prior to the adsorption measurements, the samples were degassed in vacuum at 120 C for 1 h. Surface areas were calculated by the Brunauer– Emmett–Teller (BET) method. The pore size distributions were determined from the adsorption isotherms by the Barrett–Joyner–Halenda (BJH) method. The pore volume was estimated from the adsorbed amount of N2 at P/P0 = 0.9814. Fourier transform infrared (FTIR) spectra were obtained on a Bruker Equinox 55 FT-IR spectrometer using KBr pellets. All the EL spectra were measured in the upright direction of the ITO face of the EL device with a USB 2000 spectrophotometer (Ocean company). The detected distance between the device and the receiving end of optic fiber was about 0.2 mm The EL stability of ZnO sample was assessed on an IFM-D Flow Injection Chemiluminescence Meter.
3. Results and discussion
(101)
67.82 and 68.96 correspond to (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2) and (2 0 1) directions of ZnO hexagonal structure. This indicated that ZnO particles possessed polycrystalline hexagonal wurtzite structure [17]. Comparison of the diffraction for curve (b), (c) with that for (a) indicated that the particle annealed at 500 C (curve b and c) was better crystallized than that without annealing (curve a). The size of crystal is about 2 nm for the ZnO processed without template (curve a), 40 nm for the ZnO processed with DDA (curve b) and 32 nm for the ZnO processed with ODA (curve c) figured out by Scherrers equation. The structure of sample without being annealed can be characterized by infrared ray (IR) spectra. The IR spectra of the ZnO processed without template (curve a), the ZnO processed with DDA/ODA (curve b/c) and the pure ODA (curve d) in the region of 4000– 500 cm 1 are shown in Fig. 3. Four absorption bands were exhibited. For curve a, the absorption peaks centered at 3418 cm 1, 1575 cm 1, 1413 cm 1 and 423 cm 1 can be attributed to the stretching vibration of hydroxyl group of alcohols, asymmetrical and symmetrical stretching vibration of C@O bond in –CH3COO [18] and Zn–O stretching vibration [19]. It indicates that ZnO crystals produced via the sol–gel technique are not pure but have acetate (CH3COO ) groups, which originate from the reagent materials adsorbed on the surface of the crystals [20]. Comparing curve a, b with c, a difference can be found that some IR absorb peaks in curve b and c shifted to high wavenumber. From the above facts, a consequence can be derived that the species of ZnO is negative because of absorbing the acetate groups. In the solution, the front group of amidocyanogen of ODA or DDA is positive. The interaction between the opposite charged species made the ODA or DDA link up with ZnO.
Transmittance (%)
(200)
(103)
(110)
(102)
c
(112) (201)
d
(002)
Intensity (a.u.)
(100)
Fig. 2 shows the diffraction patterns of the ZnO processed without template (curve a), the ZnO processed with DDA/ODA then followed with annealing at 500 C (curve b/c). As can be clearly seen from Fig. 2, ZnO XRD peaks were well indexed. Peaks at 2h = 31.7, 34.36, 36.2, 47.54, 56.6, 62.76, 66.34,
387
a b c
b a 30
3500 40
50
60
70
2500
1500
500
-1
Wavenumber (cm )
2θ (degrees) Fig. 2. XRD pattern of ZnO processed without template (a), processed with DDA (b) ODA (c).
Fig. 3. FT-IR spectra of ZnO processed without template (a), processed with DDA (b), ODA (c) and before annealing and pure ODA (d).
G.-h. Ning et al. / Optical Materials 28 (2006) 385–390
120 0.025
100
0.010
0.000
1
10 Dp (nm)
100
40 Adsorption Desorption
20
0 0.0
EL Intensity (a.u.)
4.66V/µm
80
80
40
2.5
3.0
4V/µm
3.5 4.0 4.5 Voltage (V/µm)
5.0
60 3.33V/µm
40 20 0
2.66V/µm baseline
400
500
(a)
600 700 Wavelength (nm)
ZnO (ODA)
4.66V/µm
EL Intensity (a.u.)
200
200
λ=406nm λ=556nm
150 100 50
150
2
4V/µm
100
800
3 4 Voltage (V/µm)
5
3.33V/µm 2.66V/µm
50
2V/µm baseline
0
400
(b)
500
600 700 Wavelength (nm)
800
Fig. 5. EL spectra of ZnO processed with DDA (a) and ODA (b). The inset is the ratio of the EL intensity peaked at 556 nm to that peaked at 406 nm.
Table 1 Comparison of the emission characteristics of our measurement and the literature Peak (nm)
Method
Ref.
480 590 500 556
Electron-beam evaporation MOCVD Solid-state chemical reaction Sol–gel
[12] [13] [21] Present study
0.015
0.005
60
100
λ=406nm λ=556nm
120 DDA-ZnO
ZnO (DDA)
0.020 dV(d) (cc/g.nm)
Volume (cc/g)
80
120
EL Intensity (a.u.)
The N2 adsorption–desorption isotherms of the ZnO processed with template are shown in Fig. 4. The ZnO exhibits an isotherm similar to type-IV with H2-type hysteresis loop. The corresponding pore size distribution curve (see the inset to Fig. 4) indicates that the material has narrower pore size distributions with an average pore diameter around 3.83 nm. Such a value confirms that the material has mesoporosity. The BET surface areas of ZnO calculated from these isotherms are 94.72 m2 g 1 and the pore volume was 0.18 cm3 g 1. EL measurements were performed on powder of nanocrystalline ZnO particles of different treatment. The EL spectra of ZnO processed with DDA (Fig. 5(a)), ODA (Fig. 5(b)) are shown in Fig. 5. All samples show two emission bands: a relatively weak violet emission band at 406 nm and a more intense and broad green–yellow emission band at 556 nm. No light emission was observed in the measurement of visible spectrum range for reverse-bias voltage up to electric breakdown. The EL was strong enough to be observed clearly by the naked eye when the applied forward-bias voltage reached a certain values. By increasing the applied bias, the EL intensity of green emission enhanced faster than that of violet emission (inset graph). The EL intensity of ZnO processed with ODA is 1.5 or 2 times larger than that of ZnO processed with DDA under the same applied bias. For the location of the visible emission peak, the comparison to some reported results has been made in Table 1. It indicates that different preparation methods for ZnO result in the different locations of the visible emission peak. It has been reported that the emission properties of ZnO are strongly dependent on the growth conditions including growth temperature and growth ambient [21–24]. In order to study the EL from porous ZnO par-
EL Intensity (a.u.)
388
0.2
0.4
0.6
0.8
1.0
Relative Pressure (p/po)
Fig. 4. N2 adsorption–desorption isotherm of mesoporous ZnO. The inset to the figure shows the pore size distribution curve.
ticles in detail, we make the growth conditions stable and just change the porosity of ZnO sample. Fig. 6 correspond to the blue emission at 4 V/lm dc bias from the ZnO processed without template, the ZnO, ZnO processed with DDA and ODA respectively. One can clearly see that very weak emission was observed in the measurement of visible spectrum range for the ZnO with no porosity. On the contrary, strong EL emissions were seen in the ZnO processed with template and the ZnO processed with ODA emitted higher EL intensity than that processed with DDA. This suggests that the EL emission can be enhanced by the porosity in the ZnO particles.
G.-h. Ning et al. / Optical Materials 28 (2006) 385–390
389
150
150
100
100
100
50 c 0
50
onset
baseline 400
500
600
700
800 0
Wavelength (nm)
0 1
Fig. 6. EL spectra of ZnO processed with ODA (a) DDA (b) processed without template (c) at 4 V/lm volts d.c.
The excitation properties of the devices were also studied. In Fig. 7, the time evolution of the EL signal at switch on is reported for a current density of 0.3 A/ cm2. At time s = 0 the voltage is switched on and the signal is seen to saturate in a characteristic time son of 5 s. The devices are very stable and exhibit a strong EL signal at room temperature. The EL signal dropped down just 5.5% of saturate value in 1 h. After the voltage is switched off, the EL signal goes to zero with a lifetime soff of 5 s. Current–voltage (I–V) and luminance–voltage (L–V) curves of porous ZnO are shown in Fig. 8. When applied voltage is above 2 V/lm, both the light emission intensity and the current increased with the voltage. As the applied forward-bias voltage increased from 2 to 4.66 V/lm, the EL emission showed a drastic increase in its intensity without exhibiting any peak shift. By further increasing the forward-bias voltage to 5 V/lm, ZnO sample was broken down.
2
3
4
5
Voltage (V/µm)
Fig. 8. Static I–V curves from the device of Fig. 1. The onset of EL emission was at 2 V/lm.
4. Conclusion Nanostructure porous ZnO particles were synthesized by sol–gel template method. The results of experiments show that the density of ZnO processed with ODA, DDA and without template is 5.31, 5.37 and 5.42 cm2/g respectively. The surface analysis proved that the ZnO particles processed with ODA and DDA hold porous structure. The ZnO particles processed with ODA has a surface area of 94.72 m2/g, a pore volume of 0.18 cc/g and a pore diameter of 3.831 nm. Comparing with the low emission intensity of the ZnO without porous structure, the emission intensity of the ZnO sample processed with DDA and ODA was enhanced 12 and 20 times respectively at the field of 4.66 V/lm. The EL spectrum shows mainly broad peak emission feature with a peak at 556 nm. The threshold emission field is just 2 V/lm. Based on the analysis of property and structure of porous ZnO particles, a conclusion has been reached that the enhanced EL has relationship with the porous structure.
Acknowledgements
300 EL intensity (a.u.)
50
Current (µA)
b EL intensity (a.u.)
EL intensity (a.u.)
a
The National Natural Science Foundation of China under grant no. 50272054 and the Natural Science Foundation of China for Distinguished Young Scholars under grant no. 50025207 as well as Doctorate Foundation of Northwestern Polytechnical University (200241) are all gratefully acknowledged.
200
100
0 0
1000
2000
3000
References
Time (s)
Fig. 7. Time evolution of the room temperature EL signal at the device switch on for a volts intensity of 3.33 V/lm.
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