Elsevier

Materials Letters

Volume 60, Issue 4, February 2006, Pages 551-554
Materials Letters

Low temperature synthesis of hexagonal phase ZnS nanocrystals by thermolysis of an air-stable single-source molecular precursor in air

https://doi.org/10.1016/j.matlet.2005.09.033Get rights and content

Abstract

Pure metastable hexagonal phase ZnS nanocrystals with the sizes in the range of 2.8 to 6.6 nm were synthesized by thermolysis of an air-stable, easily obtained single-source molecular precursor (zinc diethyldithiocarbamate, Zn-(DDTC)2)) in air at 280 °C for 12 h, and characterized by means of powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and selected-area electron diffraction (SAED), and UV–Vis absorption spectrum.

Introduction

ZnS is a direct wide band gap (for the bulk cubic and hexagonal phases of ZnS, Eg = 3.68 and 3.80 eV, respectively [1], [2]) semiconductor that has been widely used as a phosphor in luminescent devices due to its emission of in the visible range. Recently, nanocrystalline ZnS powders have drawn tremendous interest for the optoelectronic applications, because their optical property, due to quantum confinement effect, dramatically changes and, in most cases, improves as compared with their bulk counterparts [1], [2], [3], [4], [5]. ZnS crystal usually exhibits a polymorphism of two phases with different stacking sequences of close-packed planes to each structure [6]: one is the cubic phase with a zinc blende structure (C-ZnS); and the other is the hexagonal phase with a wurtzite structure (H-ZnS). At atmospheric pressure, C-ZnS is more stable at low temperatures and transforms to H-ZnS only at ≥ 1023 °C [6]. Since the inherent crystal structures of ZnS play an important role in its physical and chemical properties [7], [8], the preparation of ZnS nanocrystals with controllable phase is vital to develop them as building blocks in constructing the future nanoscale optoelectronic devices using the so-called “bottom–up” approach. For instance, the thermal stability is crucial for reliable optoelectronic device operation, so the preparation of H-ZnS, the high temperature stable form of ZnS crystal, is highly desirable in such a case [8], [9], [10]. However, the high growth temperature has proved to be an obstacle to synthesizing pure H-ZnS nanocrystals. So far, to our knowledge, there were only a few examples [2], [11], [12], [13], [14], [15], [16], [17], [18], [19], where pure H-ZnS nanocrystals were obtained with either special solvothermal reactions or modified colloid chemistry methods or single-source molecular precursor routes at temperatures below 500 °C.

The single-source molecular precursor (an individual molecule containing all the elements required in the final product) route has several appealing features in the synthesis of semiconducting chalcogenide materials. First of all, it offers the potential advantages of mildness, safety and simplified fabrication procedure and equipment, when compared with the use of multiple sources requiring exact control over stoichiometry, and it is a one-step synthesis compatible with the established metallorganic chemical vapor deposition [20], [21], [22]. Another important motivation for the utilization of single-source molecular precursors may be found in the observation of unusual crystal growth selectivity or metastable phase formation of the resultant products, which are sometimes unattainable with conventional synthetic techniques [11], [12], [13], [14], [23], [24], [25]. In this paper, we report the synthesis of pure H-ZnS nanocrystals by thermal decomposition of an air-stable, easily obtained single-source molecular precursor (Zn-(DDTC)2) in air at a low temperature of 280 °C, as well as the characterization results of the product by XRD, XPS, TEM and SAED, UV–Vis absorption spectrum.

Section snippets

Experimental procedure

All the chemical reagents used in our experiments are of analytical grade. The single-source molecular precursor, Zn-(DDTC)2, was prepared directly through the precipitation reaction of 0.01 mol Zn(CH3COO)2·2H2O and 0.02 mol sodium diethyldithiocarbamate ((C2H5)2NCS2Na·3H2O) in 100 ml of distilled water under ambient condition for 12 h. Then, pure ZnS nanocrystals could be obtained by subjecting 1.0 g of the Zn-(DDTC)2 precursor to heat treatment in air at 280 °C for 12 h. An average

Results and discussion

Fig. 1 shows the XRD pattern of the single-source molecular precursor resulting from the precipitation reaction of 0.01 mol Zn(CH3COO)2·2H2O and 0.02 mol (C2H5)2NCS2Na·3H2O in 100 ml of distilled water under ambient condition for 12 h. All of its XRD peaks can be indexed to monoclinic structure (Space group: P21/c no. 14) of Zn-(DDTC)2 (molecular formula: C10H20N2S4Zn) with the lattice constants of a = 10.015 Å, b = 10.661 Å, and c = 16.357 Å, which agree with those of Zn-(DDTC)2 reported in the

Conclusions

The synthesis of pure H-ZnS nanocrystals was realized by thermal decomposition of an air-stable, easily obtained single-source molecular precursor (Zn-(DDTC)2) in air at 280 °C for 12 h. XRD, TEM and SAED, XPS, and UV–Vis absorption spectrum have been used to characterize the resultant product. The surprising ability of this single source molecular precursor route in attaining a high-temperature (≥ 1023 °C) stable phase at a much lower temperature (280 °C) not only provides economically viable

References (27)

  • Y. Li et al.

    J. Phys. Chem. Solid

    (1999)
  • S.D. Scott

    Geochim. Cosmochim. Acta

    (1972)
  • D. Moore et al.

    Chem. Phys. Lett.

    (2004)
  • C. Lan et al.

    Solid State Commun.

    (2003)
  • S.H. Yu et al.

    Chem. Phys. Lett.

    (2002)
  • Y. Dong et al.

    Inorg. Chem. Commun.

    (2004)
  • Q. Zhao et al.

    Inorg. Chem. Commun.

    (2003)
  • Z. Qiao et al.

    J. Solid State Chem.

    (2002)
  • O. Osasona et al.

    Opt. Mater.

    (1997)
  • Y.C. Zhang et al.

    Growth

    (2005)
  • Z. Qiao et al.

    J. Solid State Chem.

    (2002)
  • Q. Xiong et al.

    Nano Lett.

    (2004)
  • W.T. Yao et al.

    Small

    (2005)
  • Cited by (0)

    View full text