Investigations On The Preparations And The Photoluminescent Properties Of Scheelite-Structured Polycrystalline Films By Electrochemical Methods

Scheelite-structured tungstates and molybdates are important electro-optics materials. Under the excitations of Ultraviolet, X-ray, cathodic ray, etc. tungstates can produce blue emission, and molybdates can generate green light. Hence, scheelite-structured tungstates and molybdates are used in the fields of detections for high energy particles, scintillant arithmometers, X-ray gain screen, luminescence and displays. Moreover, due to the attenuation constants lying in the range from micro seconds to nano seconds, these materials are especially essential in the field of pulsed lasers.To meet the demands of microminiaturization of electron devices, functional devices prepared by thin films are widely concerned due to their better resolution and easier integration with traditional semiconductor technologies. In order to improve the crystallization and performances, thin films prepared by traditional methods, such as physical vapor deposition technologies (for example, evaporation, sputtering, pulsed laser deposition methods), citrate precursor, spray pyrogenation, should be annealed in the temperature rang of 500 - 800°C. However, annealing treatments are apt to produce defects and poor adhesion, even if can make films peel from substrates. Therefore, it seems urgent to develop new ways to prepare thin films without or with lower temperature annealing treatment.Electrochemical techniques have the outstanding advantages in less consumption of energy, easily controlling the reaction, and less pollution to environment etc. They are environmental friendly routines. Hence, they should be used to prepare scheelite-structured films. Compared with other film deposition methods, electrochemical technologies have the character that processes of nucleation, growth, crystallization can be finished in one step and without any annealing treatments; In addition, polycrystalline films prepared by electrochemical methods have single structure and idea stoichiometry.In this thesis, the impacts of electrochemical parameters on the depositions of tungstate and molybdate polycrystalline films have been researched. The parameters are including pH values, current densities, electrode distances, the treatments of substrates and reaction time. At the same time, the growth habits of AMO₄ (A = Ca, Sr, Ba; M = W, Mo) crystals under electrochemical conditions have been studied. What's more, cell electrochemical technologies and cell devices have been invented and used to prepare functional films. Also, how to control the rate of cell reactions and what influences of electrochemical parameters on cell methods have been investigated. Characterizations, such as XRD, SEM, AFM, XPS and EDXA, have been used to study the structures, surface morphologies, compositions and element values of films prepared by cell electrochemical methods. Finally, the effects of electrochemical processes on film luminescence have been researched. The main results are listed in the following:1. The optimum conditions for electro deposition of scheelite-structured alkaline earth tungstate-molybdate polycrystalline films are: to control the pH value (usually in the range from 12 to 13.5); to choose a proper current density; to polish the substrate; and to make electrode distance farther.2. Growth habits of alkaline earth tungstate and molybdate crystals in electrochemical surroundings: (a) Low current densities, smooth substrates and large distances are good for crystals to grow separately in the shapes of tetragonal tapers. In such conditions, the formation of flower-like, fishbone-like and 4-symmetry clusters or dentrites of crystals can be outstandingly restrained; (b) Crystals have the shape of double tetragonal tapers in the early stages; (c) BaWO₄ and BaMoO₄ crystals are inclined to growing in long tapers which have a 4-symmetry; and (d) SrWO₄ and SrMoO₄ crystals usually looks like flower bunches.3. The rate of cell reaction can be well tailored by controlling the reaction temperature and the pH values of electrolytes.4. Highly crystallized and tetragonal-phased polycrystalline films can be prepared by cell routes. The compositions of films are agreement with their stoichiometry. Also, the value of molybdenum or tungsten elements in the films is +6. That is to say, molybdenum and tungsten exist in the form of molybdate and tungstate, respectively.5. The pH value and reaction temperature are the most important factors for cell electrochemical methods; the concentrations of electrolytes and the treatments of substrates will influence such technologies, too. While electrode distances show no effect on cell methods.6. The pH values, current densities, and treatments of substrates have no effect on photoluminescence of scheelite-structured polycrystalline films. That is to say, the band and intensity of luminescence do not change with the changes of the above factors. However, reaction temperature, solution concentrations and reaction time will impact the luminescent intensities.7. Comparisons between the preparations of scheelite polycrystalline films by constant current and cell electrochemical technologies have been investigated in detail. Studies reveal that cell electrochemical technology is a more environmental friendly and green method; the utilization ratio of tungsten or molybdenum is higher than 90% during the syntheses of tungstate or molybdate films by cell route, while it is lower than 70% for constant current method. Moreover, to compare with constant current method, cell route is good to improve the quality of films.The main innovations acquired from the present work are listed in the following:1. The influences of electrochemical factors on the deposition of tungstate and molybdate polycrystalline films have been investigated in detail. Also, the growth habits of AMO₄ (A = Ca, Sr, Ba; M = W, Mo) crystals under electrochemical surroundings have been deeply researched. By controlling electrochemical parameters, morphology-controlled and size-controlled AMO₄ (A=Ca, Sr, Ba; M= W, Mo) polycrystalline films have been prepared by electrochemical methods. Such works were not reported.2. Cell electrochemical technologies and an improved cell device for film deposition have been proposed. The rate of cell reaction can be well tailored by controlling the reaction temperature and the pH values of electrolytes. Such works were not reported.3. Tetragonal-structured alkaline earth tungstate or molybdate polycrystalline films have been prepared on tungsten or molybdenum by cell routes. Their compositions are well agreement with their stoichiometry. Such works were not reported.4. The electrochemical routes, reaction temperature, solution concentrations, and reaction time will impact the intensities of photoluminescence of scheelite-structured alkaline earth tungstate-molybdate polycrystalline films. While the pH values, current densities, and treatments of substrates have no effects on the photoluminescence of such films. Such works were not reported.5. Comparisons between the preparations of scheelite polycrystalline films by constant current and cell electrochemical technologies have been investigated in detail. The utilization ratio of tungsten or molybdenum is higher than 90% during the syntheses of tungstate or molybdate films by cell route, while the ratio is lower than 70% for constant current method. Moreover, to compare with constant current method, cell route is good to improve the quality of films. Such works were not reported.