Band gap variation of size-controlled ZnO quantum dots synthesized by sol–gel method

https://doi.org/10.1016/j.cplett.2005.05.027Get rights and content

Abstract

ZnO quantum dots were synthesized successfully via a simple sol–gel method. The average size of quantum dots can be tailored using well-controlled concentration of zinc precursor. Size-dependent blue shifts of photoluminescence and absorption spectra revealed the quantum confinement effect. The band gap enlargement is in agreement with the theoretical calculation based on the effective mass model. Furthermore, as the particle size decreases, we observed an increase in the size-dependent Stokes shift of the photoluminescence peak relative to the absorption onset.

Introduction

Semiconductor nanoparticles have recently attracted significant attention for their role in fundamental studies and technical applications [1], [2], mainly due to their unusual photonic characteristics. Zinc oxide (ZnO) is a versatile material that has achievable applications in photo-catalysts, varistors, sensors, piezoelectric transducers, solar cells, transparent electrodes, electroluminescent devices and ultraviolet laser diodes. As a result, it has stimulated extensive research [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. Compared to other wide band gap materials, ZnO has a large exciton binding energy of 60 meV, which results in efficient excitonic emission at room temperature. ZnO nanocrystals or quantum dots (QDs) have superior optical properties of the bulk crystals owing to quantum confinement effects. In the past decade, various methods have been employed to produce ZnO quantum QDs [14], [15], [16], [17], [18], [19], [20], [21]. For instance, Guo et al. [15] experimentally established that the third-order nonlinear susceptibility of ZnO nanoparticles is almost 500 times larger than that of bulk ZnO. Vanmaekelbergh and co-workers [22] discovered that the optical transitions in artificial atoms consist of one to ten electrons occupying the conduction levels in ZnO nanocrystals. Fonoberov et al. [23] theoretically investigated that, depending on the fabrication technique and ZnO QD surface quality, the origin of UV photoluminescence (PL) in ZnO QDs is either recombination of confined excitons or surface-bound ionized acceptor–exciton complexes. More and more unique behaviors are continuously being explored.

Understanding the electronic and optical characterizations in ZnO QDs and nanoparticles is important from both a fundamental science and a proposed photonic application point of view. Accordingly, absorption spectra were widely used to investigate the band edge emission from ZnO QDs [14], [15], [16], [17], [18], [19], [20]. However, direct observation of the band gap variation upon particle size from PL is relatively rare [21]. In this Letter, we show growth of high-quality ZnO QDs via a simple sol–gel method. The average size of nanoparticles can be tailored for use of the appropriate concentration of zinc precursor. Furthermore, size-dependent PL and absorption spectra are carefully discussed and compared with the theoretical calculation from the effective mass model.

Section snippets

Experimental

The ZnO colloidal solutions were produced from zinc acetate dihydrate (99.5% Zn(OAc)2, Riedel-deHaen) in diethylene glycol (99.5% DEG, EDTA), similar to what we presented exhaustively before [24]. The slight difference is that we placed the final product in a centrifuge operating 3000 rpm for 30 min. After this procedure, the solution was separated into two gradations. The white bottom layer included the secondary ZnO clusters [24] and the upper suspension was more transparent and included the

Results and discussion

Fig. 1 shows a typical TEM micrograph of the ZnO QDs formed using 0.06 M Zn(OAc)2. The nanoparticles are essentially little aggregated but still appear to be sphere and ellipsoid in shape individually. The mean-particle size is estimated to be ∼4.0 ± 0.3 nm. Presumably due to the viscosity of DEG, the solvent may have modified the Ostwald ripening kinetics such that the growth rate decreases with the size of the ZnO QDs. This would narrow the size distribution of ZnO QDs effectively.

Fig. 2

Conclusion

In summary, we have demonstrated successfully the ZnO QDs synthesized by a simple sol–gel method and the average size of ZnO QDs can be tailored under well-controlled concentration of zinc precursor. Size-dependence of efficient UV photoluminescence and absorption spectra of various QD sizes give evidence for the quantum confinement effect. Furthermore, band gap enlargement is also in agreement with the theoretical calculation based on the effective mass model. We also observed an increase in

Acknowledgements

Authors gratefully acknowledge financial support from the National Science Council (NSC) in Taiwan under Contract No. NSC-93-2112-M-009-035. We also thank the Nano Technology Research Center and Energy & Resources Laboratories of ITRI for facilitates support and S.Y. Lai (TEM group of MRL/ITRI) for great help on electron microscopy measurements.

References (35)

  • F.C. Lin et al.

    Sensors Actuators B

    (1995)
  • T. Makino et al.

    Appl. Surf. Sci.

    (2002)
  • H. Zhou et al.

    Appl. Phys. Lett.

    (2002)
  • H.M. Cheng et al.

    J. Cryst. Growth

    (2005)
  • K.E. Andersen et al.

    J. Non Cryst. Solids

    (2002)
  • J.R. Heath

    Ed. Acc. Chem. Res.

    (1999)
  • A.P. Alivisatos

    Science

    (1996)
  • K.S. Weissenrieder et al.

    Thin Solid Films

    (1997)
  • J. Muller et al.

    J. Anal. Chem.

    (1994)
  • S.C. Minne et al.

    Appl. Phys. Lett.

    (1995)
  • Karin. Keis

    Nanostructured ZnO electrodes for solar cell applications

    (2001)
  • J.B. Baxter et al.

    Appl. Phys. Lett.

    (2005)
  • R.L. Hoffman et al.

    Appl. Phys. Lett.

    (2003)
  • W.I. Park et al.

    Adv. Mater.

    (2003)
  • R. Könenkamp et al.

    Appl. Phys. Lett.

    (2004)
  • Z.K. Tang et al.

    Appl. Phys. Lett.

    (1998)
  • D.W. Bahnemann et al.

    J. Phys. Chem.

    (1987)
  • Cited by (477)

    View all citing articles on Scopus
    View full text