Preparation of a Phosphor, ZnS:Cu 2+ Steven L. Suib a n d John Tanaka Department of Chemistly and Institute of Materials Science, University of Connecticut. Storrs. CT 06268
Phosphorescence is a fascinating phenomenon that we encounter every day. This paper is an outgrowth of an experiment that we have used in an undergraduate advanced inorganic chemistry course tor swrral ).;an. The purpose of this experiment is to famlllnrize the student with methods and equipment not encountered in other chemistry laboratory courses. This particular experiment exposes a student to the preparation of a doped semiconductor, phosphorescence, gettering procedures, reducing atmospheres, and the use of a high temperature furnace with associated thermocouples, temperature controllers, and temperature-sensing devices. There are several variables that can he changed during the course of several semesters or from student to student such as the temoerature. the nature and concentration of the dopant, and Eharacterization methods. The principles of excited states. hand theorv. .. and the structure of solids like ZnS are presented in the corresponding lecture part of this course. Other articles ( 1 . 2 ) have recentlv been ouhlished concerning phosphore&nt ZnS materials: It is the purpose of this paper to discuss the optical properties of inorganic materials, doped ZnS as a specific system and to point out the chemical considerations that need to he made in the preparation of a phosphor. The latter section of this paper dealswith our experimental procedure concerning the preparatiou of Cu2+doped ZnS as a phosphor. The excitement that this experiment generates when a student observes phosphoresence under a black light after preparing this compound is truly amazing. Optical Properties of Inorganic Materials T h e production of light in certain minerals and insects a t normal temperature has fascinated scientists for over four centuries. The first recorded svnthetic luminescent material was prepand in 1fiuR hy Casciarolo, n shoemaker from Holom a . lurl\.. who heiited a mixture o i the minrral l~arirptBaSOJ andcha;coal. T h e resulting product glowed in the dark aftdr exnosure to sunlieht and was eiven the name . ohosohor . which is derived from thr Greek meaning "light bearer." I.att:r in Ififi!!. a German alchemist liennir- Brand discovered a strange . new element while heating "a secret, magical concoction" in a retort. This element which possessed an "eerie glow" was ~~~~~
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named phosphorus (this was the white polymorph of phosphorus, P4). In the middle of the 19th century,Stokes showed that the light emission from the mineral fluorite (CaF2) was a phenomenon involving the conversion of shorter wavelength, incident light into longer wavelength radiation (this is Stokes' law). He called the emitted light "fluorescence" and the incident source "exciting light." In addition, Stokes observed that the incident radiation always corresponded to an absorption hand in the material. Fluorescence is exhibited by many naturally occurring minerals such as scheelite (CaWOd, willemite (ZnnSi03, calcite (CaCOd, halo-apatites (Ca5(P04)3X),fluorite (CaF2), and wurtzite (ZnS). Synthetic samples of all these minerals have been shown to fluoresce when properly doped with selected impurities in the proper concentration. Luminescence is normally not exhibited in a pure, stoichiometric compound. The emission of light by solids, liquids, or gases can be classified as either incandescence or luminescence. While incandescence results from high temperature, which produces a broad continuum of wavelengths, luminescence occurs only from an excitation by absorption of energy followed by emission of light a t wavelengths specific to the compound involved. Different forms of luminescence are differentiated by the type of excitation energy that is employed: photoluminescence uses ultraviolet-visible light, cathodoluminescence uses electrons or a cathode ray, electroluminescence uses an applied voltage, chemiluminescence involves a chemical reaction, triboluminescence requires mechanical energy, X-rays can generate X-ray fluorescence, and thermoluminescence involves thermal enerev -.input. . Fluorescence and .ohosohores. cence are the most common forms of photoluminecence. They are produced hv mechanisticallv different orocesses. which can be distin&hed experimentdly by measiring the lifetime of the excited states (-10'2 to 10-6s for fluorescence; as long as several seconds for phosphorescence). The radiation emitted can he in the ultraviolet, visible, or infrared portion of the spectrum. Luminescent materials are used in fluorescent lights, os-
Volume 61
Number 12 December 1984
1099
CONDUCTION
BAND
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/// VALENCE CONFIGURATION COORDINATE
a
BAND
ii
b
Figue 2. Band diagram fa luminescent centers in ZnS.(a) Model and (b) Madel 6.
Figwe 1. Total energy versus mnfiguration cwrdinatefor (a)groundstate and (b) excited slate.
cilloscope tubes, T.V. screens, night lights, luminescent panels, medical X-rav screens. as laser devices.. on .oostaae - starnos to facilitate processing of mail, and as the basis for several analvtical methods (ex.. . - . fluorescence spectroscoov. ... X-rav fluorescence spectroscopy, etc.). There are three main types of inorganic compounds that luminesce (3).These three main classes involve atoms like Ne in a low pressure gas discharpe; doped insulators like Cr3+ doped al&nina (ruGy);and large hand gap semiconductors like CaAs. The phosphorescence investigated here involves the latter class of inorganic compounds. Processes Involved The energy level diagram in Figure 1qualitatively outlines the nrocesses that occur when a ohoton of enerw is ahsorhed hy a luminescent material. The abscissa is a 'wnfiguration nnrdinate" which defines the confiruration of the ions around the center within the sphere of interaction. The svstem is raised from the eround state to a hieher energy, exiited state by the incide; photon. The excited state can return to its stable mound state hv atw of the several paths shown in the diagram. The favored route is the on; that minimizes the lifetime of the excited state. The center, however, greatly influences the system and often controls the manner in which the system returns to the ground state. Transition 1 to I1 is-called excitation and occurs in ahout lo-'$ s. Transition I I to I is referred to as fluorescrnce and occurs for between 10-% and lo-" s. The mound and exciced states are singlet states. A transition from I 1 to Ill is nonradiative and is often called intersystem crossing. This lattice relaxation is very rapid and occurs in roughly 10-'?s. Transition I11 to IV is an rmission and involves a soin flio from a triplet excited state to a singlet ground state. ?his transition is called nhosohorescence and is a slow orocess which occurs ~ Qabout 60 s. A transitibn from IV to I is also between ~ o - and a lattice relaxation phenomenon and nonradiative in nature. Further details concerning these processes can he found in several texts (3-6). Luminescence in inorganic semiconducting or insulating solids usuallv orininatesat defect sites (centers) in the crystill structure. ~hese'defects may he impurities that are substituted for the components in a structural framework or they may be lattice vacancies or interstitial atoms. The impurity atom is referred to as the activator. Activators introduce (additional) absorption hands in the UV or visible portion of the spectrum, and irradiation of the material leads to excitation and emission from these hands.
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Journal of Chemical Education
The Specific Case of ZnS and Doped ZnS When owe (nonluminescent) ZnS is heated to 8 0 0 - l ~ D C . it phospgoresc& if exposed to uv radiation or to cathode rays: This phenomenon is a .~.~ a r e n tassociated lv with the formation ot'a nonst~ichiometriccompt~iition,therr bring interstitial Zn atoms and/or vacant sulfur ~ositions.If the ZnS is heated with a small amount of C U ~ +this , impurity is ahsorhed into the structure, and the product now phosphoresces as a brilliant yellow-green instead of a blue as in the case of the copper-free, nonstoichiometric material. Several models for the luminrscent processes in ZnS hased on semiconductor theory have heen proposed ( 1 , 2, 7. R J . Whereas in axide-type phosphors the luminescent transitions usually involve excited states of the activator and no charge transfer over lattice sites, the phosphorescence in %nSusually arises from the radiatke recombination ot'electrons and holes a t defect levels located in the forbidden energy gap and may involve charge transfer. Two of the models are shown in Figure 2. Model A describes the ereen luminescence in ZnS:Cu2+ as resulting from the recomh~nationof a photogenerated electron in the conduction hand with a hole trapped at a localized acceptor level (Cu2f activator) above the valence hand. Model B has been used to explain the phosphorescence in ZnS:Cu2+, which is thought to arise by electron transitions between an associated donor-acceptor pair: Unfortunatelv, insufficient definitive data are available to arrive a t an un&higuous assignment of the luminescent transitions. In some cases, electron paramagnetic resonance (EPR) has been useful in identification of the nature and structure of luminescent centers (8).
Chemical Consideration in the Preparation of a ZnS Phosphor Doping of a nonluminescent host with the proper amounts of activators can yield phosphorescent materials. The condition of "orooer" cannot he overemohasized. Manv imnurities. especialiy ;ickel and iron (even at concentrations o i 10-6 g: atom/mole ZnS), can act as "poisons" or "killers" (i.e., they significantly quench phosphorescence). For this reason the zinc sulfide must be carefully purified for use as a host matrix. In our laboratory electronic grade ZnS is used. Phosphorescence is also a function of the concentration of the doping . - ion and the kind of activator used. The preparation of high-purity zinc sulfide (or selenide) by sublimation is an excellent example of a technique that can be used to carefully purify certain inorganic starting materials. However, the level of purity required is so high that it is not feasible to purify small amounts or to work on an individual basis. Apparently, impurities could he introduced into the
NaC1. Drv the d o ~ e dNaCl in an oven ahove 100°C tnkine ~~~~~~~precautiom to exilude dirt or rust particles. Mix the dope; NaCl with 1.0 g ZnS in the clean mortar and triturate well with a small amount of absolute alcohol. Pack the paste into a clean vitreous silica hoat, taking care to keep the outside of the hoat clean. Dry the mixture thoroughly as before. Several tuhe furnaces should he available with vitreous silica tuhes and should he preheated to 900°C. Slide the hoat containing the ZnS-NaCl-CuC12 mixture into one of the silica tubes, up to the edge of the furnace. This can conveniently be done with a hooked quartz rod if the boat also has a hook on one end. Flush the tuhe with nitrogen that has been purged of oxygen by a copper gettering furnace. A simple gettering furnace can he made by adding shredded copper strips to a glass tuhe and heating the tube to 400°C. Nitrogen gas should then be connected to the inlet of the glass tuhe and the outlet can go to several T-tubes for several furnaces. The addition of a rotameter between the nitroeen tank and the eetterine " " furnace is very helpful if one is available. Carefully push the tuhe into the furnace so the hoat is in the hot zone. and set the nitrogen flow so the gas bubbles gently through the water blow-off after passing through the tuhe. Heat the compound a t 900°C-950°C under oxygen-free nitrogen for 30 min, then slide the tuhe so the hoat is out of the hot zone. Allow the product to cool in the nitrogen stream before removing the hoat from the tuhe. Check the product for luminescence with an ultraviolet lamp. Remove the sodium chloride with distilled water and d r y ~ t h ecompound, then recheck for luminescence. The phosphor can be placed in a sample vial and submitted to the lab instructor. Emission and excitation spectra can be ohtained if time permits. ~
Figure 3. Spectral energy distribution lor doped ZnS phosphors under UV excitation. la) Agf doping (b) CuZ+ doping (c)Mn2+ doping. sulfide by the handling procedure and the type of equipment used. F& these reas& commercial, electronic grade zinc sulfide should he supplied as a starting reagent. (Note: This reactant does not phosphoresce prior 6 its use in the experiment, as can he demonstrated by the instructor or student.) Small quantities of electronic grade %nScan be activated bv mixine with NaCl (which will act a s a local tluxl and a few drops o f a dilute cop~er(11)chloride solution. ?;he copper to eive the reauired C U ~ + solution is esoeciallv " oreoared .. concentration' (10-4 g-atomlmole f; host). ~ h : l soptimum concentration of Cu2+ d o ~ i n should e vield a ereen . ohos~hor. . Some reported experiment2 resulh"are &en in Figure 3 below. Manganese-doped ZnS produced an orange phosphor, whereas silver doping generates a blue phosphor upon irradiation with UV liaht. The three emission spectra show the wide range of waverengths accessible by doping with different cations. The introduction of silver ions into the doubly charged framework (Zn2+, S2-) requires valence compensation (electrical neutrality throughout the hulk of thestructure). One purpme of the sodium chloride flux is to provide this valence compensation.
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Conclusion We have outlined the processes that are important in the preparation of a semiconducting phosphor. When the experiment is finished there are several questions and exercises that could be asked concerning the experiment. The following are examples: 1)
Calculate the dopant ion concentration (in ppm Cu2+:Zn2+l in the
3) Write the equation far the chemical reaction between copper and oxygen that takes place in a gettering furnace. Generally, a higher temperature will allow a faster flow rate through the getter, maintaining efficient0 2 removal. But, there is a theoretical temperature beyond which'the getter does not work. Why is this so, and what is that temperature? 41 After the surfaceof the copper is completely oxidized, the getter no longer functions. The copper must be "regenerated." How mieht this best be accomolished?
The silver ions substituting for divalent copper must he charge-compensated by an equivalent amount of chloride ions which substitute for the sulfide ions. A similar process is necessary when ZnS is doped with trivalent ions such as A13+ or Ga3+. Divalent copper substitution in ZnS, however, does not require valence compensation. It is important to dry the mixed sample carefully prior to heating. ZnS reacts with moisture a t high temperature to form ZnO and hydrogen sulfide. Poor phosphorescence may well indicate that the sample was improperly dried prior to Several variations of this experiment can he attempted. heating. id ~111.I ~ ~ r n nand r r usmg othw :nvluding v a r y i ~ tmperature ~r ZnS ordinarily crystallizes with the cubic, zinc hlende dtq),~ntsauch as . 4 y r , hlnlr, and igthvr rations. structure (9).The hexagonal, wurtzite form occurs a t around 1020°C, and is unstable with respect to the low-temperature Literature Cited form. If ZnS is heated ahove 1000". the transition from zinc hlende to wurtzite can occur. On cooling, the reverse transition I11 Schwmkner,R.,Eiawirth,M.,sndVenghaus, H.. J.CHEM. E~uc.,58.808 (1981). 12) Nwrnsrk. G.F.,J. Appi. Phys., 51,3383 119801. is expected but the wurzite form can he t r a. .~ ~ hv e.danenchine . M. D., "Luminescenee Spectroscopy," Academic Press, Now York, 1978, pp. (rapid cwlinxj or by the duping process. Rec;~ust:this s t r ~ ~ e - 13) Lumb. 1-92. tural transformation is a ~ ~ u s ~ i~rohlem. hle u,e t v ~ ~ u ; ~limit ll\. 14) Turru, N. J.. "Modem Molmuiar Photochemiltry." B e n j a m i n I C ~ m m i nPub1 ~ Co., MenloPark. CA, 1978. the upper temperature o i the furnace to 9'500~:' (SI Adamson. A. W.,and Fl'leinchauer,P. D.. "Concepts uflnoqaganic Phafochemirfry: John Wiley & Sons, New Yolk, 1974. Experimental Procedure 161 Pankove. J. I.. ''Optical Pn~cess~s in Semimnducturs: Dover Publiestions. In
