Zncds:Cu, Al, Cl: A Near Infra-Red Emissive Phosphor for Marking, Coding, and Identification

The availability of near-ultraviolet light and visible light emitting diodes in addition to infrared emitting systems has made the remote excitation of infrared emitting phosphors a practical possibility.  Infrared emitting powder phosphors have the potential to meet marking and coding requirements, especially in low light and poor visibility. 

The advantages of IR emitting powder phosphors are:-  

1)      That they can easily be deposited by conventional techniques to form large areas, if required, either by simple settling techniques, or by inclusion in a binder suitable for screen or ink-jet printing (the binder must be transparent over the required spectral range).

2)      Such deposited powder layers, depending on thickness and particle size etc., can be used in a transmission or reflective mode with suitable UV or visible excitation sources.

The ease of fabrication and the wide excitation spectra efficiency of the material facilitate a wide range of potential applications, including the following as preliminary examples:

a) For use in coding/marking systems as simple IR sensitive markers.

b) For improved marking/detection in conjunction with night vision systems.¹

It has been reported that increasing the cadmium concentration in Zn_1-xCd_xS solid solutions shifted the photoluminescence emission band to longer wavelength because the band gap energy (E_g) decreases with increasing cadmium concentration, from 3.91eV for ZnS to 2.58eV in CdS.²  Werring et al used Zn_0.4Cd_0.6S:Cu with an emission peak at ~900nm in order to match with the absorption peak of a silicon thyristor for use in an infrared switching device.³

Altering the zinc to cadmium ratio allows fine tuning of the wavelength of the photoluminescent emission band.  The aim of this work was to shift the emission peak of the phosphor sufficiently far into the infra-red region so that visible light emission was minimised and the infra-red emission from the phosphor was clearly detectable by night vision systems.  Zn_1-xCd_xS:Cu phosphors with higher cadmium concentrations higher than Zn_0.4Cd_0.6S:Cu  were selected for study with the aim that the increased cadmium concentration would result in the emission band being moved sufficiently far into the near infrared region.

Zn_1-xCd_xS:Cu_0.03,Al_0.03,Cl and Zn_1-xCd_xS:Cu_0.03,Cl and (where x =0.5-0.9) phosphors were synthesized by an aqueous decomposition method.  The photoluminescent (PL) excitation spectra, at 1000nm emission, of Zn_1-xCd_xS:Cu_0.03,Al_0.03,Cl (where x =0.5-0.9) phosphors are presented in Figure 1a, and the corresponding PL emission spectra in Figure 1b.

For the excitation spectrum of each phosphor (Figure 1a) there was a broad peak at ~365nm, a more intense sharp peak between ~460-505nm which shifted to longer wavelengths with increasing cadmium concentration, and an intense very broad excitation band between ~520nm and ~645nm.  The relatively strong excitation band at ~365nm facilitates the use of long wavelength UV lamps and UV emitting LEDs as backlights for the infrared emission panels and broad excitation in the blue region means that these IR emitting phosphors could also be excited by blue light emitting LEDs.

Addition of aluminium ions as a co-dopant in these phosphors resulted in a blue shift in emission intensity (Figure 1b) compared to the corresponding Zn_1-xCd_xS:Cu_0.03,Cl phosphors (Figure 1c).   The shift to shorter wavelengths was ~70nm for Zn_0.5Cd_0.5S:Cu_0.03,Al_0.03,Cl but decreased in intensity with increasing cadmium concentration.  The blue shift for Zn_0.9Cd_0.1S:Cu,Al,Cl was ~30nm.

A comparison of the emission brightness of Zn_1-xCd_xS:Cu_0.03,Al_0.03,Cl and Zn_1-xCd_xS:Cu_0.03,Cl (where x =0.5-0.9), is presented in Figure 1d.  For the phosphors which did not include aluminium as a co-dopant, there was a small increase in emission intensity with increasing cadmium concentrations; this reached a plateau at the highest cadmium concentrations.  In contrast, the presence of aluminium resulted in a significant increase in the PL emission intensity at the highest cadmium concentrations.  The PL emission peak of Zn_0.1Cd_0.9S:Cu_0.03,Al_0.03,Cl was 219% more intense compared to that of Zn_0.1Cd_0.9S:Cu_0.03,Cl.  At the higher cadmium concentrations, PL emission which involved Al-Cu donor acceptor pairs resulted in significantly brighter PL emission compared to PL emission from Cl-Cu donor-acceptor pairs.

Reference

1. A. Vecht, J. Silver, R. Withnall, and G.R. Fern, DARPA Report, R+D 9350 EE01, (2004)
2. S. Shionoya in Phosphor Handbook, 2^nd ed., W.M. Yen, S. Shionoya, and H. Yamamoto, Editors, p256, CRC Press, Boca Raton (2007).
3. N.J. Werring, A. Vecht, and J.W. Fletcher, Brit. J. Appl. Phys. (J. Phys. D), 2, 509-513, (1969).

Figure 1. (a) PL excitation spectra, at 1000nm emission of Zn_1-xCd_xS:Cu_0.03,Al_0.03,Cl (where x =0.5-0.9) phosphors, (b) normalised PL emission spectra, under 580nm excitation, of Zn_1-xCd_xS:Cu_0.03,Al_0.03,Cl (where x =0.5-0.9) phosphors, (c) normalised PL emission spectra, under 580nm excitation, of Zn_1-xCd_xS:Cu_0.03,Cl (where x =0.5-0.9) phosphors and (d) PL emission intensities of Zn_1-xCd_xS:Cu_0.03,Cl phosphors (à) and Zn_1-xCd_xS:Cu_0.03,Al_0.03,Cl phosphors (ð) with increasing cadmium concentrations (x).