Thin Solid Films 520 (2012) 5815–5819

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Thin Solid Films

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Luminescence enhancement due to energy transfer in ZnO nanoparticles and Eu3+

ions co-doped silica

Tao Lin a,b, Xiao-wei Zhang a,b, Yun-ji Wang a,b, Jun Xu a,b,⁎, Neng Wan a,b, Jian-feng Liu a,b,c,Ling Xu a,b, Kun-ji Chen a,b

a School of Electronic Science and Engineering, Joint Research Center Online of Solid State Microstructure and Key Laboratory of Advanced Photonic and Electronic Materials,Nanjing University, Nanjing 210093, Chinab School of Physics, Joint Research Center Online of Solid State Microstructure and Key Laboratory of Advanced Photonic and Electronic Materials, Nanjing University, Nanjing 210093, Chinac Network and Information Center, Nanjing University, Nanjing 210093, China

⁎ Corresponding author at: School of Electronic ScienceCenter Online of Solid StateMicrostructure and Key LaborElectronic Materials, Nanjing University, Nanjing 210093,fax: +86 83595535.

E-mail address: junxu@nju.edu.cn (J. Xu).

0040-6090/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.tsf.2012.04.058

a b s t r a c t

a r t i c l e i n f o

Article history:Received 9 July 2011Received in revised form 17 April 2012Accepted 19 April 2012Available online 30 April 2012

Keywords:Sol–gel processesLuminescenceRare earth ionsNanostructured materials

SiO2 thin films co-doped with ZnO nanoparticles and Eu3+ ions were prepared by sol–gel method. The for-mation of nano-sized ZnO particles was confirmed by X-ray diffraction patterns and transmission electronmicroscopy. The characteristic emission bands from Eu3+ ions can be observed at room temperature andthe luminescence intensity is increased obviously by introducing ZnO nanoparticles into Eu3+-doped silicafilms. The integrated luminescence intensity is influenced by the concentration and size of ZnO particles,suggesting effective energy transfer from nano-sized ZnO to Eu3+ ions. It is argued that the efficient lumines-cence enhancement occurs under the suitable Zn2+ amount and annealing temperature.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Rare-earth (RE) doped silica film materials have attracted much at-tention because of their unique emission in visible to near-infrared lightregion with long luminescence lifetime and narrow emission lines,which can be used in solar cells, display devices and Si-based lightsources [1–4]. However, due to the sharp absorption peaks of RE ions,they cannot be effectively excited. For example, the effective excitationcross section of Er3+ in SiO2 is as low as 10−21 cm2 [5]. In order to en-hance RE emission efficiency, semiconductor materials with large ab-sorption cross sections were introduced, that can act as sensitizers topromote RE emissions by harvesting the excitation photon energy andthen transferring it to the RE ions [6–8]. A good example is the Er3+

doped silicon rich SiO2 film, inwhich the Er3+ ions are sensitized by sil-icon nanoparticles formed in the SiO2 matrix [9]. However, there is astrong thermal quenching of luminescence in this material due to theback energy transfer from Er3+ ions to silicon nanoparticles withsmall band-gaps. Therefore, metal oxide nanoparticles with widerband-gaps can be used instead of Si nanoparticles in order to avoidthe back energy transfer effect. Eu3+ is a kind of RE ion with similar4f5d structures of Er3+ ions. Especially, its electric dipole transition of

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5D0–7F2 at 613 nm and magnetic dipole transition of 5D0–

7F1 at590 nm reveal the different local environment around the ions [6]. So,it can be used to determine the location of RE ions doped in the system.Moreover, Eu3+ doped oxide can be potentially used in many kinds ofoptical and optoelectronic devices, such as waveguide, phosphors andenergy up-conversion solar cells [10,11].

ZnO is an environmentally friendly material and a wide band-gap(Eg=3.37 eV) semiconductorwith extremely large exciton binding ener-gy of 60 meV [12]. Theoretically, ZnO can be used as a good sensitizer forRE ions emission because of its direct band-gap transition and well spec-tral overlapping with RE excitation in emission spectra. Previous studiesdemonstrated that energy transfer from ZnO to RE ions might takeplacewhen the synthesis process iswell designed and controlled [13–16].

In this work, we used sol–gel and spin-coating methods to prepareSiO2 thinfilms co-dopedwith ZnOnanoparticles and Eu3+ ions. The char-acteristic emission of Eu3+ is obviously enhanced after adding ZnOnanoparticles. The influences of the Zn2+ amounts and the post-annealing temperature on the enhanced luminescencewere investigated.The efficient energy transfer from ZnO nanoparticles to Eu3+ ions wasdemonstrated which is responsive for the luminescence enhancement.

2. Experimental details

The sol–gel method was used to prepare the ZnO and Eu3+ co-doped SiO2 thin films. Tetraethyl orthosilicate and Zn(NO3)2 wereused to form the SiO2 host and ZnO nanoparticles, while europium(Eu3+) ions were doped as the luminescence centers using Eu(NO3)3.

Fig. 1. XRD patterns of the 6 mol% Zn2+ and 1 mol% Eu3+ co-doped SiO2 thin filmsannealed at different temperatures for 1 h under ambient air.

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Firstly, the rawmaterials were dissolved in amixture of ethanol (EtOH)and de-ionized water with a molar ratio of 4:4:1 (SiO2:EtOH:H2O)under constant stirring. After complete dissolution, diluted HCl(0.01 M) was dropped into the mixture to adjust the pH value to 2.0.Then the precursor solution was refluxed at 60 °C for about 4 h underrigorous stirring. The obtained solution was clear and transparent. Inthe above process, the amount of Eu3+ added into the precursor solu-tion was fixed at 1 mol% of SiO2 amount, while the amount of Zn2+

was changed from 1.5 to 12 mol%. We use these values to label thefinal samples with different ZnO or Eu3+ doping concentrations. Aftera 2-day storage, the precursor was used for film deposition. The spincoating method was used to prepare thin films on single polishedn-type silicon (100) substrates. Spin coating was performed in aKW-4A spin coater in a rotating speed of 4000 rpm. After spin coat-ing, the prepared films were pyrolyzed at 450 °C in air for 30 min.Several layers were deposited to get film thickness of 500 nm forclear luminescence. The resulting films were annealed at differenttemperatures from 600 to 1100 °C for 1 h at a ramp rate of about6 °C min−1 under ambient air.

The microstructures were investigated by X-ray diffraction (XRD)with θ–2θ geometry under CuKα radiation and a Technai-F20 G2 fieldemission transmission electron microscope (TEM) operated at200 kV. The cross-section TEM samples were prepared by a standardmanual process. A film sample on Si substrate was cut into slices of1 mm width and 4 mm length using a diamond scriber. Two sliceswere glued together to form a pillar inside which the film layers werefaced with each other. Then a small piece of about 300–400 μm thick-ness was cut from top of the pillar using a diamond wire saw, polishedand glued onto a copper holder. The polished cross-section sample wasfinally dimple-grinded by a GATAN 656 dimple-grinder and ion milledby a GATAN 691 precision ion polishing system.

Photoluminescence (PL) spectra and photoluminescence excita-tion (PLE) spectra were obtained using a Jobin Yvon Fluorolog-3 spec-trometer equipped with a 450 W Xe lamp as the light source.

3. Results

3.1. XRD patterns

As previously reported [17], ZnNO3 will decompose into ZnO crys-tals, O2 and NO2 beyond 400 °C. But the hydroxyl group (−OH) maynot be completely removed from the materials at this temperaturewhich may deteriorate the photoluminescence from the Eu3+ ions.On the other hand, high annealing temperature may cause excessivecrystallization and the formation of Zn2SiO4 compound which will re-sult in poor luminescence properties. So the post annealing tempera-ture chosen is important to the experiment. Fig. 1 shows the XRDpatterns of the samples after annealing at different temperatures.The patterns for 400 °C–600 °C annealed samples exhibit the exis-tence of hexagonal phase ZnO crystals (JCPDS No. 36-1451). As canbe seen, the diffraction peaks are broad, indicating the existence ofcrystals with small sizes in the films. The broad diffraction peaks be-come narrower with the increase of annealing temperature, indicat-ing the increase of crystallization and the growth of nano-sizedcrystals, which means than both the density and average size cangrowth with increasing annealing temperature from 400 °C–600 °C.It is noted that some diffraction peaks assigned to another phaseappear at 800 °C. The signals of hexagonal phase ZnO were totallyreplaced by the ones of Zn2SiO4 crystals (JCPDS No. 85-0453) at1000 °C. It means that the Zn2+ ions can react with the amorphousSiO2 matrix and form Zn–Si–O compounds at the annealing tempera-ture above 800 °C. The forming of Zn2SiO4 alloys indicates the breakof SiO2 matrix which will lead to fast growth of crystals without iso-lation. And the band gap energy of Zn2SiO4 is too high to absorb theUV photons. So to avoid the forming of Zn2SiO4 alloys, all of the sam-ples used in the subsequent experiments were prepared at 600 °C.

3.2. Cross-sectional TEM pictures

In order to characterize the formation of ZnO nanoparticles, highresolution TEM measurements were performed. Fig. 2a shows theTEM pictures of 1.5 mol% Zn2+ and 1 mol% Eu3+ co-doped samplesannealed at 600 °C for 1 h. It is demonstrated that ZnO nanoparticleswith sizes around 3–4 nm are formed and distributed homogeneouslyin amorphous SiO2 host matrix. As shown in Fig. 2b, both the formedparticle density and mean particle size increase when the Zn2+

amount goes up to 6 mol%. After further increasing the Zn2+ amountto 12 mol%, ZnO particles with larger sizes can be found with a degreeof agglomeration, as shown in Fig. 2c.

3.3. PL spectroscopy

Room temperature PL spectra of 1 mol% Eu3+ doped SiO2 thinfilms annealed at 600 °C for 1 h with different Zn2+ amounts arepresented in Fig. 3. The excitation wavelength is kept at 370 nm. Nopeak can be found for the samples without ZnO, while a luminescenceband centered at 612 nm can be observed by the introduction of ZnO,which is associated with the 5D0–

7F2 transition of Eu3+ [6]. Well re-solved characteristic emission bands of 5D0–

7FJ (J=0–4) from Eu3+

ions can be identified in the spectra when the ZnO amounts increase.The dependence of the integrated PL intensity of 5D0–

7F2 characteris-tic emission (612 nm) on the amount of Zn2+ is presented in theinset of Fig. 3. It is found that with the increase of the Zn2+ amountfrom 1.5 mol% to 6 mol%, the PL intensity of the 5D0–

7Fj increases lin-early. The integrated PL intensity of 6 mol% Zn2+ doped sample is en-hanced by about 4 times compared to that of 1.5 mol% Zn2+ dopedone. Compared to PL intensity from the Eu3+ doped silica samplewithout Zn2+ co-doping under the optimum excitation condition(say 392 nm excitation as shown by PLE spectra below), the PL inten-sity of 6 mol% Zn2+ co-doped sample is increased by over one orderof magnitude. Moreover, the study of the spectral feature of the5D0–

7Fj(0–4) transitions of the Eu3+ ions reveals the local environ-ment of the Eu3+ ions [16]. In our case, the PL ratio between thetwo peaks 5D0–

7F1 and 5D0–7F2 appears unchanged by increasing

the Zn content. Because the ZnO presents very low solubility for thelanthanide ions owing to the large difference between the radius ofZn2+ and that of the Eu3+, it is difficult for Eu3+ ion entering intothe ZnO nanoparticles and the most possible location is on the sur-face. Further increasing the Zn2+ amount to 12 mol%, the PL intensityis decreased as shown in the inset of Fig. 3. This phenomenon canhardly be explained as back energy transfer from RE ion to ZnO be-cause of the wide band gap of ZnO nanocrystal. Considering theTEM results, the decreasing PL intensity can be explained as the

Fig. 2. TEM images of the film samples that co-doped with (a) 1.5 mol% Zn2+ and 1 mol% Eu3+ annealed at 600 °C, (b) 6 mol% Zn2+ and 1 mol% Eu3+ annealed at 600 °C,(c) 12 mol% Zn2+ and 1 mol% Eu3+ annealed at 600 °C. The above insets give the rough size distributions of ZnO nanoparticles by recording 100 particles in every image.

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over growth and agglomeration of particles and decrease in surface-to-volume radio due to the increased particle size caused by excessiveZn2+ concentration.

In order to further understand the luminescence enhancementmechanism of co-doped silica films, PLE measurements were per-formed. Fig. 4(a) gives the PLE spectra of samples annealed at600 °C with different Zn2+ amounts. The detected emission wave-length kept at 612 nm can be assigned to the 5D0–

7F2 transition ofEu3+. It is found that all the spectra exhibit several sharp peaks owingto the Eu3+ ions, while an additional broad excitation band ranging be-tween 250 nm and 450 nm appears in the spectra from the samplescontaining ZnO nanoparticles. It is an evidence of the energy transferfrom ZnO nanoparticles to Eu3+ ion because this broad band matchesthe band-gap energy level of ZnO nanoparticles as reported previously[11,18]. We can see from the spectra that the intensity of the broad ex-citation band rises rapidly at first, and then reduces with increasing theZn2+ amount, which accords with the PL spectra. Meanwhile, the max-imum of the broad excitation band is red-shifted from 280 nm to370 nm with Zn2+ amount increase. This can be explained in terms ofthe gradual increase of the average sizes of formed ZnO nanoparticles

Fig. 3. Room temperature PL spectra of Zn2+ and 1 mol% Eu3+ co-doped SiO2 thin filmswith different Zn2+ amounts from 0 mol% to 12 mol%. All of the samples wereannealed at 600 °C for 1 h under ambient air and the excitation wavelength was keptat 370 nm. Inset shows the integrated PL intensity for the 5D0–

7F2 characteristic emis-sion of Eu3+ centered at 612 nm as a function of Eu3+ amount.

with increase of the Zn2+ amount. The enlargement of ZnO particlesizes causes the red-shift of the band gap due to the quantum size effectand in turn the optimum excitationwavelength is correspondingly red-shifted. Our experimental results also indicate that there exists a suit-able size of ZnO nanoparticles which can reach the optimum energytransfer efficiency.

Fig. 4(b) shows the PLE spectra of 6 mol% Zn2+ and 1 mol% Eu3+

co-doped samples annealed at different temperatures. It is foundthat 600 °C annealing results in the strongest band located at370 nm and its intensity decreases quickly with the increase of theannealing temperature. The weakening of excitation bands can be as-cribed to the change of film structures. High temperature annealingmay improve the agglomeration of ZnO and combination with thesurrounding amorphous SiO2, form Zn\O\Si bonds as indicated inXRD patterns. It means the decrease of total area of interface betweenZnO nanoparticles and SiO2 matrix, which make important roles inthe energy transfer process. It can be seen from the spectrum thatthe excitation band blue-shift when the annealing temperature in-creases to 700 °C, which is against quantum size effect because theband gap energy should narrow down with the increasing annealing

Fig. 4. (a) Room temperature PLE spectra of Zn2+ and 1 mol% Eu3+ co-doped SiO2 thinfilms with different Zn2+ amount. All of the samples were annealed at 600 °C for 1 hunder ambient air and the emission wavelength was kept at 612 nm. (b) Room tem-perature PLE spectra of 6 mol% Zn2+ and 1 mol% Eu3+ co-doped SiO2 thin filmsannealed at different temperatures for 1 h under ambient air.

Fig. 5. Schematic diagrams illustrating the mechanism of energy transfer process be-tween ZnO nanoparticles and Eu3+ ions.

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temperature and the excitation band should demonstrate weak red-shift. Considering the XRD results, we ascribe the blue-shift of excita-tion band to the forming of Zn2SiO4 phase, which is also the cause ofEu3+ PL quenching. So it can be seen that the excitation band totallydisappear following the further increase of annealing temperature.

4. Discussions

Amechanism to account for the energy transfer behavior in the SiO2

thin films co-dopedwith ZnO nanoparticles and Eu3+ is summarized inFig. 5. ZnO nanoparticles are pumped by incident photons and generateelectron and hole pairs in the ZnO nanoparticles. The photo-excitedelectrons are then trapped by the defect states through the non-radiative decay process. Because of thematching of ZnO defect state en-ergy levels and Eu3+ 5Lj energy levels, the Förster energy transfer canbe occurred between the ZnO nanoparticles and the nearby Eu3+ ions(located at the interface between ZnO nanoparticles and SiO2 matrix,for example.) which cause the electrons jump from the ground stateto the excited ones in Eu3+ ions, and generates the Eu3+ characteristicemissions through the subsequent radiative relaxations. These process-es are dominated by the spectral overlapping, the density of ZnOnanoparticles and the surrounding environment of Eu3+ ions. Asshown in Fig. 6, there exists a wide band emission and the center ofthe emission is located at 390 nm. Based on the previous reports[19,20], the emission band is originated from the defect states on thesurface of the ZnO nanocrystals, such as the oxygen vacancies or dan-gling on the ZnO nanocrystals surface. Moreover, it is meaningful thatthe wide emission band is overlapping with the excitation spectra ofEu3+ ions located around 392 nm, which can be attributed to the possi-ble energy transfer.

Fig. 6. The spectral overlapping between emission peak of ZnO located at 390 nm andthe 4f–4f transition peaks of Eu3+ ions located around 392 nm.

As shown in PL spectra, increasing Zn2+ amount is an effectiveway to enhance luminescence. But superfluous ZnO will lead to lumi-nescent saturation and quench since of the agglomeration of ZnO re-sults in the formation of density particles with larger sizes. Accordingto the Fermi's “golden rule”, the energy transfer probability isgoverned by the space distance between the donor and acceptor. Inour case, Eu3+ ions occupy the SiO2 matrix and the most effective en-ergy transfer occurs between ZnO nanoparticles and the nearest Eu3+

ions. With increasing Zn2+ amount, the ZnO nanoparticles can beformed and increase to a suitable size and density. Thus, the energytransfer is efficient and luminescence can be obviously enhanced. Fur-ther increasing the Zn2+ amount above 6 mol% induces the large-sized and aggregated ZnO nanoparticles, which result in the increaseof the average distance between Eu3+ ions and ZnO particles. As aconsequence, the luminescence becomes weaker due to the reducedenergy transfer probability.

Post annealing is another way to control the ZnO nano-particlesizes and concentrations. We found that the post annealing tempera-ture is very important to get the effective energy transfer and thestrong luminescence of our co-doped thin films. Low annealing tem-perature will induce small particle sizes, but with low density ofZnO nanoparticles as well as lots of organic residuals. On the otherhand, high annealing temperature can effectively improve the crys-tallization of ZnO particles and enlarge the density of ZnO particlesin the films, but exorbitant annealing temperature will make theZn2+ ions agglomerate and react with Si\O bonds. The luminescenceis obviously quenched due to the formation of Zn2SiO4 compounds,decrease of interface and reduced energy transfer probability. There-fore, there exists the most suitable Zn2+ amount and annealing tem-perature to obtain SiO2 film co-doped with high density ZnOnanoparticles and Eu3+ ions which have the effective luminescenceenhancement.

5. Conclusion

In conclusion, SiO2 thin films co-doped with Eu3+ and ZnOnanoparticles were prepared by sol–gel technique. Compact films con-taining ZnO nanoparticles were obtained as revealed by the TEM mea-surements. The Eu3+ characteristic emission is enhanced remarkablywith increasing ZnO concentrations indicating an effective energytransfer from ZnO nanoparticles to Eu3+ ions. The energy transfer effi-ciency is influenced by the density of ZnO nanoparticles and the sur-rounding environment of Eu3+ ions, which can be controlled by theZn2+ amount added into the precursor solution and annealing temper-ature. It is shown that the strongest luminescence enhancement occursunder a Zn2+ amount of 6 mol% and annealing temperature of 600 °C,which is one order ofmagnitude stronger than that of Eu3+ doped silicafilms without ZnO nanoparticles co-doping.

Acknowledgments

Thisworkwas supported byNSF of China (61036001), “333 Project”,NSF of Jiangsu Province (BK2010010), the Fundamental Research Fundsfor the Central Universities (1112021001) and NFFTBS (J0630316).

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