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Title: Alternating current electroluminescent properties of zinc sulfide powders
Author: Salimian, Alireza
Awarding Body: University of Greenwich
Current Institution: University of Greenwich
Date of Award: 2012
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In order to investigate the alternating current electroluminescent properties of zinc sulfide powders the following experiments were conducted: synthesis of zinc sulfide phosphors (comprised of zinc, sulfur and copper dopant); thermal shocking of phosphor materials (sudden cooling, using liquid nitrogen, of phosphor particles heated up to 500oC) and analysis of their alternating current electroluminescent properties as well as studies of particle crystal structures by synchrotron and conventional X-ray powder diffraction techniques. Inductively coupled plasma mass spectrometry was utilized to investigate the concentration of co-activator atoms within the zinc sulphide crystal lattice. Electroluminescent panels were prepared and the emission properties were evaluated theoretically in order to obtain a mathematical relationship between various parameters involved in the electroluminescent process. Thermal quenching of zinc sulfide phosphor alters its photoluminescent and electroluminescent properties. The dominant wavelength of the material alters from 504 nm to 517 nm. It appears that the blue centres are vulnerable to the thermal quenching procedure carried out as the blue emission deteriorates and the overall blue emission of the material is reduced due to the role that the interstitial Cu+ species play in this mechanism. The interstitial Cu+ is not as stable in its location within the lattice compared to substitutional Cu+ and hence a thermal shock is prone to effect its location or association with the surrounding atoms. The green emission centre, however, appears to be unaffected. Results obtained from layer by layer analysis of the material demonstrate that the surface of the phosphor particles contain most of the copper content (copper to zinc molar ratio of 0.08% in the surface compared to 0.06% at inner levels distributed within the lattice). The location of the outer copper layer may play a key role in the alternating current electroluminescence (ACEL) process; further experiments need to be conducted in order to prove the foregoing hypothesis. Irrespective of the amount of copper impurity (dopnat) initially added to the zinc sulphide precursor, prior to synthesis of the phosphor, during the high temperature firing of the material (above 700˚C) a considerable amount of the copper will be ejected from the lattice and be washed off in the latter steps of the synthesis process (where the newly synthesized phosphor is washed in concentrated ammonia solution); an initial copper content of 1.2% molar ratio is reduced to 0.154%; however, the duration of the high temperature firing is a key factor in the final amount of copper present within the lattice. XRPD experiments of a working ACEL device (i.e., when the AC field is applied across the electroluminescent phosphor) show that the diffraction lines all shift, but remain within the region where broad diffraction intensity is observed for a powder sample (i.e. random orientation). Indeed the sharp diffraction lines are observed to span across each broad diffraction area associated with the sphalerite phase. The panel exhibits a different diffraction pattern when the device is powered in an AC field compared to when it is not exposed to the field. This clearly indicates that the particles possess piezoelectric properties and the electric field causes strain on the crystal lattice. When considering the major drawbacks of ACEL technology, i.e. it’s short life time and degradation characteristics, defining a mathematical model of its emission degradation is a step towards understanding part of the mechanism of the ACEL process. Due to the various number of parameters involved in the phenomenon of electroluminescence (such as particle size, copper content and random distribution of crystal planes) and the fact that emissions arise from certain centres randomly distributed over each phosphor particle, mathematical models are only accurate when they are formulated in relation to the analysis of a particular batch of phosphor sample and used to prepare a particular panel. Hence, no overall mathematical formulation can be produced to measure the emission properties of various ACEL panels produced by different batches of zinc sulfide phosphors. The findings of this research indicate that sample preparation technique which involves addition of raw zinc sulfide to an already copper doped zinc sulfide causes an increase in the occurrence of nano p-n junctions species within the lattice where the cupper locations form the p-type and the n type is formed from the release of some sulfur atoms from zinc sulfide structure during the high temperature firing relative to the conventional phosphor preparation methods. Larger particles have a higher probability of contacting interstitial copper sites during firing and preparation as copper atoms tend to migrate out of the zinc sulfide lattice toward the surface. Hence larger particles (commercial phosphors) demonstrate better emission properties. Thermal quenching affects the interstitial copper sites more than the other luminescent centres formed of substitutional copper sites. Hence the lowered blue emission occurs. Due to the probability of high dispersion of Cu atoms within the ZnS lattice a useful mathematical model cannot easily be developed for an EL panel. EXAFS analysis cannot be fully relied up on in respect of the interstitial copper environment in these phosphors considering that a small fraction of the copper impurity in the phosphor exists at interstitial sites. However, the results from experiments using XANES confirm a change in the electronic configuration of Zn atoms when samples are quenched.
Supervisor: Chowdhry, Babur Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QC Physics ; QD Chemistry