Green and red electroluminescence from CdS powder electrodes in aqueous solutions

https://doi.org/10.1016/S0927-7757(98)00233-7Get rights and content

Abstract

The room temperature electroluminescence properties of CdS powder (polycrystalline) electrodes have been investigated in aqueous solutions under various experimental conditions. Red emission is observed from all the CdS electrodes annealed at 400–600°C, whereas green emission, which has previously been observed only from single crystal CdS electrodes, is also observed from the powder electrodes annealed at 600°C. Both the red and the green emission intensities increase with an increase in the applied (negative) potential up to a critical value (−1.6 V vs Ag/AgCl); above this value there is a decrease in the intensity. The increase in the green emission is much stronger than the increase in the red emission at higher applied potentials. The effects of several electrolytes (Na2S2O8 and NaOH) on the electroluminescent properties of CdS powder electrodes have also been studied.

Introduction

Electroluminescence (EL) and photoluminescence (PL) characteristics of many transition metal compounds have been extensively investigated over the last few decades 1, 2, 3, 4, 5, 6, 7, 8, 9, 10owing to their application as the active medium in electro-optical devices such as LEDs. EL properties of semiconductor materials provide direct information on the electronic structure of the bulk and surface states [11]and provide information on the mechanisms of electron transfer at the semiconductor–water interface [12].

The luminescent properties of CdS electrodes have already been extensively studied 13, 14, 15, 16, 17. In the case of PL and EL from polycrystalline CdS in aqueous solutions at room temperature [11], two broad emission bands are observed in the red (R) and the infrared (IR) regions in both the EL and PL spectra: these are attributed to radiative recombination transitions involving band-gap states generated during the annealing treatment. Conversely, in the case of CdS single crystals, the IR band appears to be unimportant, but there is, in addition to the red emission, a green band whose intensity is a strong function of the applied potential and surface pretreatment 18, 19, 20. Green EL is observed from Cd-rich CdS diodes [18], and CdS–CuGaS2 heterodiodes prepared by vapour deposition methods also emit green EL at 77 K [19]. The light emission results from electron injection into the CuGaS2, and the active luminescent centre appears to involve Cd atoms which have diffused into the CuGaS2 during the growth of the CdS film. A sharp green emission band observed from a single crystal CdS electrode has been assigned to band-to-band recombination by Streckert et al. [20]. The green emission from these electrodes is found to be affected by Te doping and applied potential. More recently, the band-gap energy dependence of the emission spectra in doped Zn–CdS thin film EL devices has been used to predict the different electronic transitions responsible for green and red light emissions [21].

To date, green emission has only been observed from single crystals, and most of the EL investigations reported in the literature deal primarily with the red light emission from CdS polycrystalline materials and s. In this paper, we investigate the EL properties of CdS powder electrodes (polycrystalline material) and report on the first observations of both green and red light emission in aqueous solutions from powdered CdS electrodes. The effects of annealing temperature, applied potential, electrolyte concentration and NaOH addition on the EL properties of CdS powder electrodes are discussed.

Section snippets

Preparation of CdS powder electrodes

CdS pellets of ∼2 mm thickness and 10 mm diameter were prepared by compressing 0.4 g of CdS powder (Aldrich; 5 μm, 99.995%) with a load of 8 t. These pellets were then annealed in an oven at a desired temperature (400–600°C) for 1–3 h in air and cooled to room temperature. One face of each of the pellets was attached to a copper wire using conducting silver paint for electrical contact. The copper wire was then inserted through a glass tube and one end of the glass tube was glued (with epoxy resin)

Results

Room temperature EL spectra from CdS electrodes have been recorded under a variety of experimental conditions. Typical behaviour, showing the current flowing through the cell and the maximum EL intensity detected by the PMT when a potential of −1.0 V (vs Ag/AgCl) is applied to a CdS electrode, is shown in Fig. 1. As observed in this figure, there is an instantaneous response in both current and PMT signals. Nevertheless, the response behaviour for the relaxation of the current is different to

Discussion

Gerischer and coworkers demonstrated that EL in solution at n-type semiconductor interfaces could be facilitated by the presence of powerful oxidants such as persulphate ion or peroxide. Normally a depletion layer is present at the n-type semiconductor surface in aqueous solution due to trapping of conduction band electrons in surface traps or by adsorbed oxygen. This creates a space charge layer which retards cathodic electron transfer to oxidants such as persulphate. Under a cathodic bias,

Conclusions

The experimental investigations on the room temperature EL spectra of CdS powder electrodes annealed at 400–600°C reveal that the emission properties of CdS depend upon the preparation conditions. Electrodes annealed at higher temperatures for longer duration show green emission, due to radiative recombination processes involving band-to-band transitions or bulk exciton formation. High temperature annealing also causes a drastic increase in the concentration of sulphur vacancies, with the

Acknowledgements

P.M. acknowledges support of this work through an ARC Grant. Part of this project has been carried out through the University of Melbourne and Tokyo Institute of Technology postgraduate student exchange program. We thank Professor Geoff Stevens and Professor T. Kajiuchi for their support of this work.

References (33)

  • N. Saito et al.

    Chem. Phys. Lett.

    (1993)
  • M. Ashokkumar et al.

    Chem. Phys. Lett.

    (1994)
  • G.C. Papavassiliou

    J. Solid State Chem.

    (1981)
  • T. Yamase et al.

    Ber. Bensenges. Phys. Chem.

    (1983)
  • E. A-Shalom et al.

    J. Phys. Chem.

    (1983)
  • K. Uosaki et al.

    J. Am. Chem. Soc.

    (1986)
  • S. Wagner

    J. Appl. Phys.

    (1974)
  • C.A. Koval et al.

    Chem. Rev.

    (1992)
  • V.B. Singh et al.

    J. Phys. D: Appl .Phys.

    (1970)
  • A. Adachi et al.

    Bull. Chem. Soc. Jpn.

    (1995)
  • M. Fukui et al.

    Japanese Journal of Applied Physics

    (1971)
  • I.J. Ferrer et al.

    J. Appl. Phys.

    (1989)
  • H.W. Lewerenz, An Introduction to Luminescence of Solids, Dover, New York,...
  • B.R. Karas et al.

    J. Am. Chem. Soc.

    (1980)
  • A. Henglein

    J. Phys. Chem.

    (1982)
  • A. Henglein

    Ber. Bunsenges. Phys. Chem.

    (1982)
  • Cited by (6)

    • Electronic band alignment at CuGaS<inf>2</inf> chalcopyrite interfaces

      2016, Computational Materials Science
      Citation Excerpt :

      According to the results obtained in this work, in a photovoltaic device using as light absorber CuGaS2, CuAlSe2 can be used to achieve selective hole collection, while ZnSe can be used for selective electron collection. For CuGaS2/CdS interface, for which the Anderson’s rule predicts a staggered gap, meaning that the valence and conduction bands in CdS are lower in energy than the valence and conduction-bands in CuGaS2, our result shows on the contrary, as said above, a straddling gap, which is an incorrect band alignment for photovoltaic applications, but is often used to fabricate light emitting diodes and lasers [35,36]. This example shows how the effects of the lattice mismatch can modify the valence and conduction-bands discontinuities at which the CuGaS2/CdS interface switches from type II to type I. Finally, the structural and electronic results suggest that the nanostructure CuAlSe2/CuGaS2/ZnSe may improve the carrier collection efficiency, which would allow to build a p–i–n heterostructure.

    • Synthesis and characterization of organic-inorganic hybrid materials prepared by sol-gel and containing Zn<inf>x</inf>Cd<inf>1-x</inf>S nanoparticles prepared by a colloidal method

      2013, Journal of Luminescence
      Citation Excerpt :

      The incorporation of the nanoparticles in the matrix did not significantly alter their emission properties, as suggested by the similarity in the shape and in the varying position, with composition x of the nanoparticles, of the PL spectra both in the colloidal solution and in the doped xerogels. This PL behaviour, with broad bands and large Stokes shifts is characteristic of nanoparticles synthesized in reverse micelles [17] and capped with thiols [50] and results mainly from defects on the surface of the nanoparticles [48,51,52]. The most common defects in CdS nanoparticles consist of sulfur or cadmium vacancies, cadmium atoms adsorbed on the surface of the nanoparticle and interstitial sulfur [53], and the same defects could also be present in the ZnxCd1−xS nanoparticles.

    • Quantized electroluminescence from Q-CdS films immersed in aqueous electrolytes

      1999, Colloids and Surfaces A: Physicochemical and Engineering Aspects
    • Cadmium Sulphide (CdS) nanotechnology: Synthesis and applications

      2008, Journal of Nanoscience and Nanotechnology
    View full text