Non-green anti-stokes luminescent substance

Details Non-green anti-stokes luminescent substance Non-green anti-stokes luminescent substance

1999-11-08

2004-10-12

Koslow, C. Melissa (Department: 1755)

Compositions

Inorganic luminescent compositions

Compositions containing halogen; e.g., halides and oxyhalides

C106S031320, C106S031640, C106S031140, C106S031150

Reexamination Certificate

active

06802992

ABSTRACT:

The present invention relates to a non-green anti-Stokes luminescent material, to a process for its production and to its use.
Luminescent materials which are capable of emitting in the visible light range when excited with infrared (IR) radiation are known, and are for example used in IR sensor cards for detection and positioning of IR lasers. Depending on the composition of the active lattices and of the dopants used, these materials briefly emit red, green or blue-green light when stimulated with IR radiation. A disadvantage with these materials is the fact that, using IR radiation, only energy stored beforehand—for example by excitation with visible light—is extracted. For IR detection, it is therefore in each case necessary to charge the materials. During continuous IR stimulation, the stored energy furthermore becomes used up, so that the emission of visible light falls off even after an extremely short time and, in the end, ceases. Continuous emission of visible light under IR radiation is therefore not possible with these IR-stimulable materials. Such luminescent materials based on ZnS:Cu,Co; Ca:Sm,Ce or SrS:Sm,Ce are described, for example, in Ullmann's Encyclopedia of Industrial Chemistry, vol. A15, “Luminescent Materials”, 1990.
On the other hand, infrared-to-visible up-conversion materials, or anti-Stokes luminescent materials, are known which convert IR radiation into visible light without prior charging. These materials use multiphoton excitation of active lattices with dopants from the rare earth metal group, in particular erbium in combination with ytterbium, in order to generate more energetic photons, and therefore visible light, from a plurality of low-energy infrared photons. Materials based on fluorides are known, for example YF
3
: Er, Yb which is described by H. Kuroda et al., J. Phys. Soc. Jpn., vol. 33, 1, pp. 125-141 (1972). Disadvantages with these active lattices are that they are often difficult to produce with the exclusion of oxygen and that there is a tendency, depending on the composition of the active lattice, to instability in practical application, for example in application at high temperatures.
Luminescent materials of the stoichiometric composition Y
l
Yb
m
Er
n
O
2
S with (1+m+n)=2 are, for example, more favourable. Such luminescent materials, or more generally oxysulphides of the general composition Ln
2
O
2
S, Ln standing for the elements Y, Gd, Sc and/or La, which are doped with ytterbium and erbium, are highly stable with respect to organic solvents, alkalis, hot water, heat and atmospheric moisture, and are only slowly dissolved by acids. Such luminescent materials based on active lattices of stoichiometric composition Ln
1
Yb
m
Er
n
O
2
S with (1+m+n)=2, that is to say a ratio of rare earth metals to oxygen to sulphur of 2:2:1 are usually produced by reacting the rare earth metal oxides with alkali metal carbonate and sulphur. On heating, reactions then take place that form alkali metal polysulphides which, in situ or else only when the maximum temperature is reached, have a sulphurating effect on the rare earth metal oxides and lead to the formation of the oxysulphides. Furthermore, only water-soluble compounds remain in the reaction mixture, so that by using known process measures such as leaching and heating with water, washing with acid or alkali, disaggregation or grinding, for example in a ball mill, the reaction product can be subsequently converted on heating to high temperatures in order to reduce the proportion of crystal defects, into the desired oxysulphide with the stoichiometric composition defined above.
When producing such oxysulphides, the proportion of polysulphide—as for example described in Austrian patent 286 468—is adjusted in such a way that the amount of polysulphide is more than the amount needed for converting all the oxides into oxysulphides. The ratio of rare earth metals to sodium carbonate to sulphur is adjusted in such a way that, as for example described in European patent application EP-A 0 410 350, pure-phase products with the aforementioned stoichiometric composition of the oxysulphides result. According to patent 286 468 luminescent materials of the formula M′
(2−x)
M″
x
O
2
S are formed, M′ standing for at least one element from the group Y, Gd and La, M″ for at least one element from the lanthanoid group with atomic number between 57 and 64 or between 64 and 71, and x being a number less than 0.2 and more than 0.0002. By way of example, this application describes the rare earth metal combinations Y and Th, Y and Eu, La and Eu, and Y and Er, which in the case of the combination Y or La and Eu represent luminescent materials with red emission, and for Y and Er and Y and Tb green luminescence.
EP-A 530 807 describes luminescent materials of the formula (Ln
1-x-y
La
x
Ln′
y
)
2
O
2
S, in which Ln is at least one element selected from the group consisting of Y, Gd, Sc and Lu, Ln′ is at least one element selected from among Eu, Th, Sm, Er, Tm, Dy, Ho, Nd and Pr, and x and y represent values in the range 0.005≦x≦0.07 and 0.0001≦y≦0.2. As examples, luminescent materials comprising Y, La and Eu (luminescent material with red emission), Gd, La and Th (luminescent material with green emission) and a luminescent material comprising Y, La and Eu (luminescent material with red emission) are described therein.
GB-B 2 258 659 and GB 2 258 660 describe anti-Stokes luminescent materials based on doped yttrium oxysulphide, which are doped with 4 to 50% by weight Er and/or Yb and 1 to 50 ppm of one or more other lanthanoid elements. Regarding these materials, it is mentioned that they have green luminescence.
Such materials are also described in DE-A 21 58 313.
The materials described above have either anti-Stokes behaviour and green luminescence or non-green luminescence but no anti-Stokes behaviour.
The object of the present invention, in view of the prior art referred to above, is to provide a non-green luminescent material with anti-Stokes behaviour, which is straightforward to produce using standard methods and in departure from hitherto known luminescence, has a different luminescence colour.
It was surprisingly found in this regard that it is possible with the system Ln-O-S-Er-Yb with Ln=Y, Gd, Sc or La, to obtain anti-Stokes luminescent materials which have further non-green luminescence colours under IR excitation.
Accordingly, the present invention relates to a non-green anti-Stokes luminescent material, comprising the elements Ln, erbium (Er) and ytterbium (Yb), Ln representing at least one element which is selected from the group consisting of yttrium (Y), gadolinium (Gd), scandium (Sc) and lanthanum (La).
In another embodiment, the present invention provides a luminescent material which comprises a composition Ln
x
Yb
y
Er
z
O
a
S
b
, where Ln is defined as above, the sum of (x+y+z) is 2 and the sum of (a+b)≦3 and b<1.
The luminescent material here is preferably a luminescent material which has non-green emission on excitation by IR radiation in the wavelength range of approximately 900 to 1100 nm.
The luminescent materials according to the invention can, for example, be used for the detection and positioning of IR-emitting systems, such as for example lasers, laser diodes and LEDs. Compared with other, for example electrical IR detection systems with IR-sensitive photodiodes, IR-excitable materials offer the advantage for this application of simple, visual and cost-effective detection of IR radiation.
Through a combination with a suitable excitation source, such a luminescent material may also be used for the security coding of products and for verifying the originality of valuable and security documents. In this context, it is important to use materials and security features which are very difficult to forge and can be produced in combination with other security features. Besides simple checking by any individual, it is also desirable for the corresponding security features, depe

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