Cadmium sulfide membranes

A method is described for the creation of novel q-effect cadmium sulfide membranes. The membranes are made by first creating a dilute cadmium sulfide colloid in aqueous suspension and then removing the water and excess salts therefrom. The cadmium sulfide membrane thus produced is luminescent at room temperature and may have application in laser fabrication.

FIELD OF THE INVENTION 
The present invention relates to membranes of semi-conductor materials in 
general and relates, in particular, to the creation of a new form of 
cadmium sulfide membrane. 
BACKGROUND OF THE INVENTION 
Cadmium sulfide is a known semi-conductor material available conventionally 
in crystalline form. Cadmium sulfide crystals can be used for various 
semi-conductor applications and are normally grown by crystalline growth 
by withdrawing a seed crystal from a molten pool of elemental cadmium 
sulfide heated to liquid form. Cadmium sulfide can be fabricated into 
wafers or other materials having various uses in semi-conductor and 
integrated circuit technologies. 
Cadmium sulfide crystals have also been investigated for various practical 
applications. Various investigators have created the possible use of 
polished cadmium sulfide single crystals for applications in laser 
development Aasov investigated pulsed two-photon optical pumping in 
polished cadmium sulfide crystals forming a Fabry-Perot cavity which was 
investigated for its laser activity. Fuchs et al. developed an argon 
pumped CW-CdS platelet ring laser with the hope of combining the 
advantages of an increased spectral range over dye lasers and with the 
possibility of intra-cavity tuning elements which were not yet available 
in semi-conductor diode lasers at the time. Other investigators reported 
that Xenon flash-induced lasing could be created in cooled cadmium sulfide 
single crystal and electron beam pumped uncooled multi-element cadmium 
sulfide lasers were reported. Unfortunately, the low lasing efficiency and 
optical damage place the cadmium sulfide material in a category far from 
commercial application and comparatively unattractive in comparison to 
other lasing materials. 
It has been previously demonstrated that dilute colloidal suspensions of 
extremely small cadmium sulfide particles have been created which exhibit 
unique and interesting size quantization effects. See Henglein "Mechanism 
of Reactions on Colloidal Microelectrodes and Size Quantitation Effects," 
Topics in Current Chemistry, Vol. 143, pp. 115-116, 129-132, 165-180 
(1988). These particles are referred to as "Q" particles. These 
size-quantization effects become noticeable when the particle size is 
comparable or smaller than the diameter of the exciton in a semi-conductor 
macrocrystal. Dilute colloidal suspensions of Q-CdS particles can be shown 
to have unique quantum mechanical behavior deriving from the extremely 
small particle size which can be observed by their ultraviolet and visible 
and luminescence spectra which can be observed in aqueous suspension. 
SUMMARY OF THE INVENTION 
The present invention is summarized in that a quantum effect cadmium 
sulfide membrane is created consisting of a porous membrane of cadmium 
sulfide particles wherein the particle size is sufficiently small that 
quantum mechanical behavior can be observed in the ultraviolet and visible 
spectra of the resulting membrane. 
It is an object of the present invention to provide a quantum effect 
cadmium sulfide membrane having unique photophysical properties and very 
intense luminescence at room temperatures. Such a membrane holds potential 
application as a cadmium sulfide based laser. 
The present invention is also directed to a method of creating new size 
quantization effect cadmium sulfide membranes. 
It is another object of the present invention to provide such membranes 
which may have potential uses in photo-electro-optical and lasing 
applications. 
Other objects, advantages and features of the present application will 
become apparent from the following specification when taken in conjunction 
with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
The fabrication of the quantum effect cadmium sulfide membranes in 
accordance with the present invention takes place, in essence, in a series 
of three steps. The first step is the preparation of a highly diluted 
colloid of extremely small cadmium sulfide particles. The second step is 
the concentration of this colloidal suspension under controlled ionic 
strength conditions to make a concentrated colloid. The third step is the 
controlled evaporation of the solvent from the concentrated colloidal 
suspension under conditions such as to leave a solvent-free membrane. 
Under proper conditions, the material proceeds irreversibly from 
step-to-step until a stable and useful quantum effect cadmium sulfide 
membrane is created. 
The preparation of Q effect cadmium sulfide colloids has been previously 
described by others. 
Henglein, Topics in Current Chemistry, Vol. 143, pp. 113-180 (1988). The 
process essentially involves dissolving a cadmium salt in aqueous solution 
together with a quantity of a stabilizer, typically sodium polyphosphate. 
The cadmium salt and polyphosphate solution is purged by percolation of 
nitrogen gas to remove excess oxygen. Then, while the pH of the solution 
is held constant, hydrogen sulfide gas in limited quantities is percolated 
through the solution preferably diluted again in nitrogen. The solution is 
vigorously stirred while this process proceeds. The resulting colloid will 
have a characteristic color which varies somewhat depending on the 
relative concentrations of the cadmium ion and the polyphosphate in the 
beginning salt solution. The color is related to the size of the particles 
in the colloid and is characteristic of the quantum effects associated 
with particles of certain sizes. The colors are characteristic since the 
size of the actual particles themselves is on the same order of magnitude 
as the wave length of visible light which can be absorbed by the particles 
in the colloid. Typically the colloids can range from colorless through 
yellow-greenish to a deeper color and are in all cases optically 
transparent. 
The size quantization effects from such a highly diluted colloid can be 
demonstrated while the particles are still in solution. It is possible to 
observe characteristic luminescence of such colloidal solutions and the 
characteristic luminescence spectra of the colloids will be characteristic 
of the colloids as determined by the relative concentrations of the 
cadmium and polyphosphate in the initial salt solution. 
The polyphosphate appears to stabilize the cadmium-sulfide in solution. In 
the absence of polyphosphate, the cadmium sulfide will precipitate from 
the medium in particulate form and the result will not be a colloid but a 
precipitated powder. 
A highly diluted cadmium sulfide colloid as described above can then be 
concentrated through a process of desalting and concentration. Several 
alternative approaches are possible to achieve the desirable concentrated 
colloidal suspension. A typical methodology might involve beginning with a 
rotary evaporator to reduce the volume of the colloid by approximately a 
factor of five, or, in other words to remove an initial 80% of the water. 
The remaining suspension can then be placed in a dialysis assembly and 
dialyzed against ultrapure water. Such a dialysis would remove additional 
water from the colloidal suspension at the same time. The dialysis would 
also remove the excess salts from the initial suspension not involved in 
the cadmium sulfide and polyphosphate matrix. The colloidal suspension can 
then be further concentrated by roto-evaporation. 
An alternative technique for concentrating the dilute colloidal suspension 
is through the use of a micro filtration cell. A micro filter such as the 
finest ultra filtration membranes currently available can be used to, in 
essence, filter the colloid against the ultrafine micro filter. This can 
be accomplished by forcing the suspension against the ultrafine micro 
filter and then washing it continually with ultra pure water in a nitrogen 
atmosphere under pressure. 
The result of either of these techniques will be a highly concentrated 
colloidal suspension. The purification should continue until the 
concentrated colloidal suspension exceeds a given value of conductivities, 
such as twenty to thirty micro-mhos. The resulting colloidal solutions 
made by this process can vary from optically transparent to 
yellow-greenish or greenish in color and are stable for days at room 
temperature without flocculation or particle growth. 
The next part of the process is the procedure to evaporate the solvent from 
the concentrated colloidal suspension to create the solid membrane. This 
process begins by spreading the concentrated colloidal solution in a 
suitable receptacle and keeping it at room temperature in a confined 
container under approximately sixty percent relative humidity. After two 
days of such storage, a super saturated xerogel is obtained. Such a 
xerogel is a dry gel form or precursor to a completed dry membrane. The 
transition from a concentrated colloid to a xerogel can be reversible or 
non-reversible depending on the relative concentrations of the initial 
cadmium ion and polyphosphate in the salt solution. If the concentration 
of polyphosphate is higher in the original salt solution and thus in the 
original colloid, the formation of the xerogel will be reversible, that 
is, additional water will allow the xerogel to be transferred back to a 
state of colloidal suspension. If the concentration of cadmium salt is 
higher in the initial salt solution compared to the polyphosphate, the 
transition from a colloid to a xerogel becomes completely irreversible and 
the resulting xerogel becomes a rigid solid-appearing structure. At 
intermittent levels of the ratio between the cadmium ion and the 
polyphosphate, an elastic xerogel is created which will not completely 
reverse in state but is also flexible and elastic when placed in water. 
The xerogel is then placed at a room temperature of humidity, i.e. of a 
humidity of approximately twenty to thirty percent until seemingly dry. To 
completely remove the solvent from the membrane, the xerogel can be dried 
in a high vacuum (10.sup.-5 torr) at a slightly elevated temperature of 
40.degree. C. 
The resulting cadmium sulfide membrane will exhibit size quantization 
effects similar to those previously obtained with the Q-effect colloidal 
suspension of cadmium sulfide particles. The membrane can be relatively 
crack free and completely crack free membranes can be obtained of 
reasonable sizes by this process. The resulting membrane is completely 
water free and, most importantly, exhibits extremely intensive 
room-temperature luminescence properties. It is these optical properties 
that make the membrane of interest for a variety of applications including 
potentially as a lasing material. 
Shown in FIG. 1 is a comparison of the optical characteristics of a 
Q-effect cadmium sulfide colloid and a Q-effect cadmium sulfide membrane. 
The graph of FIG. 1 illustrates that the absorption onset occurs in both 
samples at around 460 nm indicating that both samples contain Q-effect 
particles of approximately the same size which would calculate to be 
approximately 30 Angstroms. This is in contrast to the absorption onset of 
cadmium sulfide macrocrystals which occurs around 515 nm. The strong blue 
shift of the colloidal particles is due to the confined "exciton in the 
small particle box" as has already been demonstrated in previous 
literature regarding the Q-effect cadmium sulfide colloids. The existence 
of this blue shift in the cadmium sulfide membrane and its strong 
correlation with the observed Q-effect in the cadmium sulfide colloidal 
suspension is strong evidence that the membranes are in fact Q-effect 
materials exhibiting similar luminescence properties to the Q-effect 
colloidal suspension materials although at a greatly increased intensity. 
Shown in FIGS. 2 and 3 are similar plots of luminescence and corrected 
luminescence-excitation spectra detected for fresh Q-effect cadmium 
sulfide colloids and Q-effect cadmium sulfide membranes respectively. One 
immediate observation is that unlike the instance of Q-effect colloids in 
suspension, the shape and maximum of the luminescence spectra in the 
Q-effect membrane is independent of the excitation wave length. The high 
surface to volume ratio of the membranes permits the chemical or physical 
tailoring of particle size and surface chemistry in such a fashion as to 
control the spectral distribution of luminescence. 
One interesting phenomenon of the Q-effect cadmium sulfide membrane is that 
in the absence of water they exhibit high emissivity and little or no 
conductivity. By contrast, when the Q-effect cadmium sulfide membranes are 
saturated with water, the emissivity is virtually quenched and the 
conductivity increases to practical levels of electrical conductivity. The 
mechanism by which this phenomenon occurs is obscure. 
The size of the particles in the cadmium sulfide Q-effect membrane can be 
adjusted by means of adjusting the concentration of cadmium ions in the 
initial salt solution. It has been discovered that concentrations on the 
order of 3.times.10.sup.-4 molar cadmium ions result in smaller particle 
size (around 20 Angstroms) while higher concentrations of cadmium, for 
example about 2.5.times.10.sup.-3 molar result in slightly larger particle 
sizes (30 to 50 Angstroms). As particle size increases, there is a red 
shift in the emission spectra of the type illustrated in FIG. 3, with the 
emission spectra of a slightly larger particle size illustrated by the 
dashed lines in FIG. 3. 
EXAMPLES 
1. Preparation of 20 Angstrom Q-CdS Colloid 
An optically transparent Q-CdS colloid was prepared in a three-neck-flask 
at room temperature. The flask was equipped with a pH electrode so as to 
be able to closely follow pH of the solution, and also with a septum 
through which sodium hydroxide or hydrochloric acid could be injected to 
adjust pH. A gas inlet dispersion tube was also provided in the flask. 
Stock solutions used were 0.2M Cd (CIO.sub.4).sub.2 (Alfa), 0.1M sodium 
polyphosphate stabilizer (Sigma, fed. grade F.W. 1592 g/mol). The gases 
H.sub.2 S (99.9%) and N.sub.2 (99.9%) were used without further 
purification. 
The colorless Q-CdS sol preparation began with the creation of a 1 liter 
aqueous solution of 3.times.10.sup.-4 M Cd.sub.2+ and 1.times.10.sup.-4 M 
polyphosphate (PP). Through this solution, N.sub.2 was bubbled for 25 
minutes to displace dissolved oxygen. The pH of the solution was adjusted 
to between 8.8 and 9.2 with NaOH. Then a small quantity of the H.sub.2 S 
gas (2.times.10.sup.-4 M) was injected into the continuing nitrogen stream 
at the lowest possible rate while the solution was vigorously stirred with 
a magnetic stirring bar. Simultaneously, the pH was held close to 8.5 by 
the dropwise addition of NaOH. The reaction continued for 20 minutes after 
which the resulting colloid was purged with nitrogen for 20 minutes to 
remove any residual oxygen or carbon dioxide. The result was an optically 
clear CdS colloid having an average particle size of around 20 Angstroms 
and a concentration of 10.sup.-4 M. 
2. Preparation of 30-50 Angstrom Q-CdS Colloid 
The procedure of Example 1 was repeated with the only principle difference 
being differing concentrations of the starting materials. In this example, 
2.5.times.10.sup.-3 M, Cd.sup.2+, 5.times.10.sup.-4 M PP and 44 ml of 
H.sub.2 S were used. The ratio of Cd to PP was thus increased 
substantially. The H.sub.2 S gas was directly injected into the gas phase 
above the nitrogen saturated reaction solution was stirred vigorously. The 
resulting colloid was yellow-greenish indicating a particle size of 
between 30 and 50 Angstroms. The concentration was 2.times.10.sup.-3 M. 
3. Desalting and Concentrating 
One liter of fresh Q-CdS colloid prepared as in examples 1 and 2 were 
concentrated in a rotary evaporator (12 Torr, 28.degree. C.) by a factor 
5, thus expelling 80% of the water. The remaining 200 ml colloid 
suspension (pHF) was placed in a cleaned molecularly porous dialysis tube 
(regenerated natural cellulose from Spectra/Par with a molecular weight 
cut-off of 3,500). The colloidal suspension was dialyzed against 
ultra-pure "Millipore" water for 24 hours. The conductivity of the colloid 
was monitored periodically. The final conductivity of the purified 
concentrated colloid did not exceed 30 micro mho. Subsequently, the 
solution was concentrated by rotary evaporation until the colloidal 
suspension had a volume of 10 ml. 
4. Desalting and Concentrating 
As an alternative to the process of Example 3, a 1 liter quantity of fresh 
dilute Q-CdS colloid, prepared as in Example 1, was concentrated to 200 ml 
and then introduced into a stirred microfiltration cell. The cell (Micro 
Filtration Systems) was equipped with a Teflon coated stirring rod, a 
safety relief valve, and a gas or liquid inlet. The micro filter used was 
an ultrafiltration membrane, polymer type UH, MWCO=1,000. The concentrated 
colloidal suspension was washed continuously with ultrapure "Millipore" 
water under a nitrogen atmosphere (N.sub.2 pressure=55 psi). In the first 
stage of this procedure, the conductivity in the salt solution leaving the 
stirred cell was measured. When the conductivity approached 20 micro-mho, 
the procedure was stopped. Then, in a second stage, the cell was connected 
directly to the nitrogen cylinder and the desalted colloid was 
concentrated to 10 ml. The resulting optically transparent colloid was 
stable for days against flocculation and particle growth. 
5. Q-CdS Membrane Preparation 
Aliquots of 5 ml of the concentrated and purified colloid 
(2.times.10.sup.-2 M to 2.times.10.sup.-7 M Q-CdS) were spread over a flat 
commercial glass plate or plastic Petri dish. The homogeneously covered 
carriers were kept in a plexiglass box under 60% relative humidity. After 
two days a supersaturated optically transparent xerogel was obtained. The 
xerogels recovered were dry gels the stability of which depended on the 
ratio of Cd.sup.2+ to PP in the initial solution for high levels of the 
ratio of Cd.sup.2+ to PP, the transition from a colloid to a xerogel was 
irreversible and the xerogel was insoluble and retained its shape in 
water. For very low rations, the xerogel could be resuspended as a 
colloid. For intermediate values of the ratio, the xerogel was rigid when 
dry but became elastic and flexible when in water although it would not 
resuspend. 
To finalize the transformation to membrane form, the humidity of drying was 
lowered to 20-30% (i.e. ambient room humidity) and continued. In a last 
drying step, the membrane was dried in a high vacuum (10.sup.-5 torr) at a 
slightly elevated temperature (40.degree. C.). 
6. Results 
The resulting CdS membranes were generally obtained in reasonably 
crack-free condition. They exhibit strong luminescence at room temperature 
and vary in color from colorless to yellow-green. 
Shown in FIG. 1 is the optical spectrum of a Q-CdS membrane compared to a 
Q-CdS colloidal suspension. The onset of adsorption is about 460 nm for 
both samples, indicating a particle size of approximately 30 Angstroms. 
This is in strong contrast to the absorption onset for CdS macrocrystals 
at 515 nm thus demonstrating the size quantization effect presumably due 
to the exciton in the particle box phenomenon described in the literature, 
Henglein supra. 
FIGS. 2 and 3 shows luminescence (excitation at 515 nm) and corrected 
luminescence - excitation (emission at 505 nm) spectra in a fresh Q-CdS 
colloid and a Q-CdS membrane respectively. FIG. 3 also illustrates that 
the increasing particle size in the membrane causes a red-shift in the 
spectral characteristics of the membrane. Unlike the Q-CdS colloids, in 
the membranes the shape and maximum of the luminescence spectra is 
independent of the excitation wavelength. 
Interestingly, the Q-CdS membranes produced as described here are highly 
emissive when dry and are non-conductive In the presence of water, the 
emissivity is quenched and conductivity increases.