Annealing temperature and environment effects on ZnO nanocrystals embedded in SiO2: a photoluminescence and TEM study
© Pita et al.; licensee Springer. 2013
Received: 28 October 2013
Accepted: 27 November 2013
Published: 6 December 2013
We report on efficient ZnO nanocrystal (ZnO-NC) emission in the near-UV region. We show that luminescence from ZnO nanocrystals embedded in a SiO2 matrix can vary significantly as a function of the annealing temperature from 450°C to 700°C. We manage to correlate the emission of the ZnO nanocrystals embedded in SiO2 thin films with transmission electron microscopy images in order to optimize the fabrication process. Emission can be explained using two main contributions, near-band-edge emission (UV range) and defect-related emissions (visible). Both contributions over 500°C are found to be size dependent in intensity due to a decrease of the absorption cross section. For the smallest-size nanocrystals, UV emission can only be accounted for using a blueshifted UV contribution as compared to the ZnO band gap. In order to further optimize the emission properties, we have studied different annealing atmospheres under oxygen and under argon gas. We conclude that a softer annealing temperature at 450°C but with longer annealing time under oxygen is the most preferable scenario in order to improve near-UV emission of the ZnO nanocrystals embedded in an SiO2 matrix.
Recently, ZnO nanocrystals (ZnO-NCs) have attracted a lot of interests because of their promising applications in optoelectronic devices, such as light-emitting devices or UV photodetectors [1, 2]. The near-UV emission of ZnO-NC can also be utilized for efficient energy transfer to rare earth ions (e.g., Eu3+ and Er3+ ions) to obtain emission in the visible (for lighting) or in the near-infrared (for telecommunications) regions [3, 4]. In order to facilitate the energy transfer, the emission band from the excited ZnO must overlap with the absorption band of the rare earth ions. In our earlier work , for example, the ZnO films were doped with Cd ions to maximize the overlap between the emission of Cd-doped ZnO and the absorption of Eu3+ ions. We propose here the development and study of ZnO-NC embedded in a SiO2 matrix to have a broadband near-UV emission from ZnO to facilitate and optimize the energy transfer to rare earth ions without introducing doping ions such as Cd ions . It is desirable to embed ZnO-NCs in a dielectric matrix, such as SiO2, to provide both chemical and physical protection for the ZnO-NCs  and also to incorporate rare earth ions.
Many existing studies have already intensively reported on the various fabrication techniques and optical properties of ZnO-NCs embedded in SiO2[5–15]. Nonetheless, a complete investigation on the growth of ZnO-NCs as a function of annealing temperature under different annealing environments is essential to understand the influence of various annealing conditions on the optical properties of ZnO-NC:SiO2 systems. Through this understanding, the emission of ZnO-NCs can be engineered to provide optimum energy transfer to rare earth ions as mentioned above. We report in this article the study on optical and structural properties of ZnO nanocrystals embedded in SiO2 matrix using the low-cost sol–gel technique. We show that annealing temperature and annealing atmosphere are crucial parameters that can be optimized in order to maximize the near-UV emission from the ZnO-NCs. Transmission electron microscopy (TEM) images as well as photoluminescence (PL) spectra are studied in order to find the right conditions for obtaining a maximized emission. A blueshifted emission at 360 nm was necessary to account for the emission of the smallest-size NCs. Such a result is in agreement with earlier-reported blueshifted transmission spectra observed for ZnO-NCs but diluted in solution, not in thin films .
Results and discussion
TEM of ZnO nanocrystals embedded in SiO2 matrix
Average sizes and corresponding standard deviations of the ZnO-NCs for various annealing temperatures
Average size (nm)
Standard deviation (nm)
Photoluminescence of ZnO-NCs in SiO2 at various annealing temperatures
On the other hand, the few ZnO-NCs that exist in the sample give rise to some UV emission, which results in the broad PL spectrum. At 500°C annealing temperature, the PL spectrum exhibits an overall blueshift which is due to the increase of the UV-blue emission in the sample. As shown in Figure 3c, the RTP annealing at 500°C is accompanied by an increase of the blue and UV emission between 360 and 450 nm and a decrease of defect emissions at higher wavelengths. The drastic change in the emission spectrum of the sample can be attributed to an increase in the ZnO-NCs and the decrease of ZnO clusters in the sample (Figure 2b), which should in turn increase the ZnO near-band-edge emission in the UV region. The emission peak at 378 nm can be related to ZnO near-band-edge (excitonic) emission [19, 20]. The emission peak at 396 nm could possibly be related to the electron transition from Zn interstitial to Zn vacancy as reported by Panigrahi et al.. While being relatively weak, it is worth noting the appearance of a peak at 360 nm for the smallest NCs for which quantum confinement is expected to occur as already reported in a transmission experiment in solution . Further analysis and especially low-temperature PL measurement are needed to confirm the peak origin. For annealing temperatures higher than 550°C, no drastic change is observed in the shape of the emission spectra, as seen in Figure 3a. Instead, the PL spectra mainly exhibit a decrease in the emission intensity. Indeed the Gaussian fitting analysis shows that the peak amplitudes decreased by the same proportion compared to its value at 500°C. However, the analysis shows that the decrease of the defect emission is slightly stronger than that of the UV emission contribution. The overall decrease of the emission intensity is consistent with the reduction of the ZnO-NC average volume (i.e., size) with increasing annealing temperature, as shown in Figure 3c. The decrease of the ZnO-NC average volume normally results in a decrease of the ZnO-NC absorption cross section, leading to a weaker ZnO-NC luminescence.
Photoluminescence of ZnO-NCs in SiO2 after the second annealing step in O2 or Ar atmosphere
For the sample annealed in RTP at 450°C, the PL spectra (see Figure 4a) show a remarkable change in the emission characterized by a decrease of the defect (i.e., visible) emission and the appearance of the UV emission around 378 and 396 nm. Compared to the post-annealing in Ar, the post-annealing in O2 results in a stronger decrease of the defect emission around 500 and 575 nm. This behavior strongly indicates that oxygen vacancies are at the origin of the defect emissions in the visible region, which supports our analysis above that the defects are due to the oxygen vacancies. For the samples annealed in RTP at 500°C, the PL spectra present a slight change in the shape of the emission. Nonetheless, the post-annealing in Ar results in an overall decrease of the emission intensity, while the post-annealing in O2 leads to an increase in the UV emission and a comparatively slight decrease in the defect emissions. The slight decrease in the defect emissions indicated that the RTP annealing at 500°C for 1 min is sufficient to form the ZnO-NC and significantly reduces the oxygen deficiency. For the sample annealed in RTP at 550°C, the post-annealing in Ar and O2 hardly presents any change in the emission spectra, except for a slight change in the intensity of the UV emission. The post-annealing in Ar and O2 has no effect on the sample after the RTP annealing at 550°C.
To conclude, we studied ZnO nanocrystals embedded in SiO2 matrix fabricated by the sol–gel method. We have analyzed the effects of temperature and atmosphere on the annealing of such thin films. We post-annealed the samples from 450°C to 700°C under O2 or under Ar atmosphere. By looking at the effect of such annealing conditions using TEM images and PL spectra, we identify the best annealing temperature for maximizing the near-UV emission of the ZnO nanocrystals. We show that an annealing temperature of 450°C under longer annealing time and under oxygen is preferable to higher annealing temperatures and shorter times. By maximizing the near-UV emission of the ZnO nanocrystals, which produce a relatively wide emission band centered at ~398 nm, the spectral overlapping with rare earth ions like Eu3+ (which has an absorption band at 395 nm) can be greatly enhanced. These results are important in the process of making efficient luminescent thin films (including energy transfer to other species such as rare earth ions) for future applications in lighting and telecommunication based on ZnO-NCs.
We thank the SINGA programme for the financial support to P. Baudin. K. Pita would like to thank the Singapore MoE for the Tier 1 programme for financing this work. C. Couteau and G. Lérondel would like to acknowledge the France-Singapore programme Merlion for contributing to the collaboration of this work.
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