PHOSPHORS WITH NANOSCALE GRAIN
SIZES AND METHODS FOR PREPARING
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to nanocrystalline phosphors. The present invention also relates to a method for 10 preparing such phosphors and devices which contain such phosphors.
2. Description of the Background
High performance cathodoluminescence (CL) phosphors ^ capable of working at low beam voltages have been an objective for field emitter displays (FED). Since FEDs operate at low voltages (<5 kV) under high vacuum (<10~7 Torr), it requires phosphors which emit enough light perceivable by human eyes (>300 cd/m2) and are stable under 20 electron bombardment under high vacuum (low outgassing). A review of cathodoluminescence is provided in L. Ozawa, Cathodoluminescence, Theory and Applications, VCH Publishers, New York, 1990, which is incorporated herein by 25 reference in its entirety.
Existing commercial phosphors, however, are powders with a grain size of a few micrometers which do not meet the requirements for effective use in FEDs. The power efficiency of a phosphor is largely dependent on its effectiveness of 30 materials. In other words, a phosphor will be inefficient if its grains contain a "dead portion." Although the causes of dead portions are yet to be clarified, lattice defects, de-activating impurities, and locally charged sites are thought to be 35 responsible. The dead portion effect becomes especially significant in low voltage electron excitation (beam voltage <1,000 V).
Most phosphors possess activators (or dopants) embedded in a wide band gap semiconductor matrix (or host). The 40 physical mechanisms behind cathodoluminescence are various and remain, at least partly, controversial. Basically, the luminescence results from electron relaxation from an excited level to a ground level within an activator through a 45 highly localized transition path, or from charge carrier recombination following an excitation-created charge separation (the formation of exciton). The latter mechanism is believed to be less localized and hence to be easily perturbed by impurities or other alien quenchers. The matrix crystals, 50 serving as operating media, are required to provide "good" environment for the transitions to happen without dissipation. It has been thought that the presence of perfect crystals of the host would decrease the dead portion effect. Efforts 55 have been, in turn, made toward making phosphor grains in the form of perfect crystals.
The processes adopted so far in phosphor fabrication are "mixing-and-firing" routes based on solid state reactions. The extent to which the reactions are completed is deter- 60 mined by two vital steps: the homogeneous distribution of the solid precursors and the extent of fusion. Since almost all the solid state reactions involved, such as Mn—Zn—Si combination and Ce—Y—Si combination, are slow, the 65 quality of the end productsis governed by efficient and complete incorporation of the dopant at certain concentra
tion levels used in the phosphors. Additionally, the firing-led fusion occurs only between the adjacent partners, and the precursor particles brought together during the "mixing." Any inhomogeneity of the "mixing" results in inefficient matrix formation as well as nonuniform incorporation of the fluorescence centers (dopants) in the matrix. Inefficient fusion leads to the formation of a dead portion even after firing. The dead portion, in this context, includes "dummy parts" of the materials which contain no dopants and in turn yield no light output, and jammed parts of the materials which are overpopulated by the dopants and therefore detrimental to the output light by internal quenching.
Nanocrystals (usually with size of a few nanometers), grown primarily in a quick solution reaction, are defect-free and well-defined chemical entities. Efforts have been invested by Rameshwar N. Bhargava et al (U.S. Pat. No. 5,637,258) in preparing nanosized phosphors of doped-zinc sulfide and yttrium oxides. The resulting phosphors have, in fact, an efficiency which is much lower than for the micron sized samples.
Thus, there remains a need for phosphors which do not contain any dead portions and which exhibit a high efficiency. There also remains a need for a method for preparing such phosphors and for devices which contain such phosphors.
SUMMARY OF THE INVENTION
Accordingly, it is one object of the present invention to provide novel phosphors.
It is another object of the present invention to provide novel phosphors which contain few or no dead portions.
It is another object of the present invention to provide novel phosphors which exhibit high efficiency.
It is another object of the present invention to provide novel nanosized phosphors which contain few or no dead portions.
It is another object of the present invention to provide novel nanosized phosphors which exhibit a high efficiency.
It is another object of the present invention to provide novel methods for preparing such phosphors.
It is another object of the present invention to provide novel devices which contain such phosphors.
These and other objects, which will become apparent during the following detailed description, have been achieved by the inventors' discovery that a method which involves:
(1) forming a solution or slurry which contains nano-sized particles of the phosphor precursors
(2) eliminating the liquid medium in the solution or slurry by heating to an appropriate temperature, preferably slightly above the boiling point of the medium, to obtain a solid residue; and
(3) firing the solid residue, provides phosphors which exhibit an improved efficiency.
In a particularly preferred embodiment of the present method, a first precursor which contains metal-ion-doped metal oxide particles having a greatest dimension of 500 nm or less, preferably 1 to 300 nm , more preferably 2 to 10 nm, in alcosol form is mixed with a second precursor, e.g., containing silica (Si02) having a greatest dimension of 500