Full-color emission and temperature dependence of the luminescence in poly-P-phenylene ethynylene-ZnS/Mn2+ composite particles

Article abstract of DOI:10.1021/jp034476r

The synthesis of a nanocomposite material composed of anionic poly(phenylene ethynylene) (aPPE) polymer particles and ZnS/Mn²⁺ nanoparticles is described, and its luminescence properties are investigated. aPPE particles have two emissions, one in the blue (460 nm) and the other in the green (490 nm), that are assigned to the 0-0 transition and an excimer, respectively. ZnS/Mn²⁺ nanoparticles have an emission at 596 nm that is due to the ⁴T₁-⁶A₁ transition of Mn²⁺ and an emission at 706 nm that is ascribed to defect-related luminescence. The blue, green, yellow, and red emissions make the composite a potential material for full-color displays. More interestingly, the relative intensities of the different emissions may be varied by changing the excitation energy. Infrared spectra reveal that interactions exist between the two particles; however, photoluminescence excitation and emission spectra as well as observations of luminescence lifetimes indicate that there is negligible energy transfer from the polymer particles to the ZnS/Mn²⁺ nanoparticles. Temperature studies reveal that the ZnS/Mn²⁺ particles in the nanocomposite have a significantly reduced thermal quenching energy relative to that of bare ZnS/Mn²⁺ nanoparticles. In addition, between room temperature and 90 ?°C, the luminescence of the ZnS/Mn²⁺ nanoparticles at 596 nm increases in intensity with increasing temperature. This surprising phenomenon is attributed to thermoluminescence and thermal curing of the particle surface upon heating.

Full text of DOI:10.1021/jp034476r

6544
J. Phys. Chem. B 2003, 107, 6544-6551
Full-Color Emission and Temperature Dependence of the Luminescence in
2
+
Poly-P-phenylene ethynylene-ZnS/Mn Composite Particles
,†
‡
§
§
†
Wei Chen,* Alan G. Joly, Jan-Olle Malm, Jan-Olov Bovin, and Shaopeng Wang
Nomadics, Inc., 1024 South InnoVation Way, Stillwater, Oklahoma 74074, Pacific Northwest National
Laboratory, P.O. Box 999, Richland, Washington 99352, and Department of Materials Chemistry,
Chemical Center, UniVersity of Lund, P.O. Box 124, S-22100 Lund, Sweden
ReceiVed: February 25, 2003; In Final Form: May 6, 2003
The synthesis of a nanocomposite material composed of anionic poly(phenylene ethynylene) (aPPE) polymer
particles and ZnS/Mn2 nanoparticles is described, and its luminescence properties are investigated. aPPE
particles have two emissions, one in the blue (460 nm) and the other in the green (490 nm), that are assigned
+
2
+
to the 0-0 transition and an excimer, respectively. ZnS/Mn nanoparticles have an emission at 596 nm that
4
6
2+
is due to the T
1
1
- A transition of Mn and an emission at 706 nm that is ascribed to defect-related
luminescence. The blue, green, yellow, and red emissions make the composite a potential material for full-
color displays. More interestingly, the relative intensities of the different emissions may be varied by changing
the excitation energy. Infrared spectra reveal that interactions exist between the two particles; however,
photoluminescence excitation and emission spectra as well as observations of luminescence lifetimes indicate
2+
that there is negligible energy transfer from the polymer particles to the ZnS/Mn nanoparticles. Temperature
studies reveal that the ZnS/Mn2 particles in the nanocomposite have a significantly reduced thermal quenching
+
2
+
energy relative to that of bare ZnS/Mn nanoparticles. In addition, between room temperature and 90 °C,
the luminescence of the ZnS/Mn2 nanoparticles at 596 nm increases in intensity with increasing temperature.
This surprising phenomenon is attributed to thermoluminescence and thermal curing of the particle surface
upon heating.
+
1. Introduction
porating semiconductor nanoparticles such as CdS into a
polymer such as poly(N-vinylcarbazole) (PVK) may enhance
Currently, nanostructured materials form a new branch of
the photoconductivity of the polymer resulting from charge
generation, charge transfer, and charge separation at the
materials science that is under extensive investigation. For
semiconductor nanoparticles, perhaps the most striking property
is the massive change in optical properties as a function of size,
which is most notably observed as a blue shift in the absorption
5
interfaces between the polymer and the nanoparticles. Semi-
conductor-polymer nanocomposites may show larger nonlinear
susceptibilities relative to those of bulk semiconductors and may
1
spectrum with the decrease in the particle size. As the particle
exhibit a higher refractive index than the polymer alone.6
size approaches the exciton Bohr radius, there are drastic
changes in the electronic structure and physical properties such
as a shift of the energy levels to higher energy, the development
of discrete features in the spectra, and the concentration of the
Therefore, these nanocomposites may find applications in optical
amplification, optical storage, antireflection coatings, and optical
6
lenses. Nanocomposites have been used for flat-panel displays
by taking advantages of luminescence from both the semicon-
2
oscillator strength into just a few transitions. These novel
7-10
ductor nanoparticles and the luminescent polymers.
Recently,
characteristics enable semiconductor nanoparticles to have a
bright future in practical applications such as lasers, flat-panel
displays, optical storage, single-electron transistors, and biologi-
highly fluorescent polymer particles composed of anionic poly-
phenylene ethynylene) have been used to capture Cy-5-labeled
(
11
oligonucleotides. By using the subsequent fluorescence quench-
ing of the complex, a detection sensitivity of over 2 orders of
magnitude relative to that of direct Cy-5 excitation has been
3
cal probes.
Often during nanoparticle preparation, organic polymers are
4
used to terminate growth. This allows for the fabrication of a
11
realized. In addition to chemical sensors, aPPE particles may
new type of composite materialsnanocomposites consisting of
an organic polymer and inorganic nanoparticles. In these
nanocomposites, the function of the organic polymer is not only
to stabilize the nanoparticles, but the coupling of the organic
and the inorganic components usually results in some novel
properties with potential applications. Nanocomposite structures
have been used to create optically functional materials. Incor-
find applications in displays and lighting because they have a
11,12
strong blue emission peaking at 460 nm.
In this paper, we
describe the formation and luminescence of a composite
luminescent thin film composed of aPPE blue polymer particles
2+
and ZnS/Mn -doped nanoparticles with emission in the orange
and attempt to develop these composite luminescent materials
for applications in displays and temperature sensing.
2
. Experimental Section
.1. Preparation of Poly(P-phenylene ethynylene). Anionic
*
Corresponding author. E-mail: wchen@nomadics.com. Tel: 405-372-
9
535. Fax: 405-372-9537.
2
†
Nomadics, Inc.
Pacific Northwest National Laboratory.
University of Lund.
‡
§
poly(phenylene ethynylene) (aPPE) particles possessing pendant
sulfonate groups were prepared by the copolymerization of
1
0.1021/jp034476r CCC: $25.00 © 2003 American Chemical Society
Published on Web 06/18/2003

Poly-P-phenylene ethynylene-ZnS/Mn2+
J. Phys. Chem. B, Vol. 107, No. 27, 2003 6545
SCHEME 1
gas evolution ceased, and then the reaction volume was doubled
by adding additional water. Initially, a white precipitate formed
but dissolved on dilution and stirring. The mixture was acidified
with concentrated HCl, yielding a clear, light-brown solution.
The product was recovered by precipitation in 1.5 L of acetone
and subsequent filtration. The crude product was recovered as
a fine, off-white powder (37.4 g, 85% yield) and was used
1
without further purification in the subsequent iodination. H
NMR (D2O): δ 6.88 (s, 4H), 4.01 (t, 4H), 2.96 (appt. t, 4H),
2
.06 (appt. p, 4H).
1
,4-(Bis-3-sulfopropyloxy)-2,5-diiodobenzene, B. A 1-L,
three-necked round-bottom flask with an affixed reflux con-
denser was charged with compound A (37.4 g, 0.106 mole),
KIO3 (9.0 g), and iodine (53.6 g). To this, 290 mL of acetic
acid, 35 mL of deionized water, and 2 mL of concentrated
sulfuric acid were added. The contents of the flask were heated
with stirring to reflux for 6 h. After the mixture was allowed to
cool to room temperature, residual iodine was consumed by the
addition of 20% sodium hydrosulfite. A fine silt-like precipitate
was collected by filtration and recrystallized twice from deion-
ized water to yield large needle-like crystals (16 g, 20% yield).
1
H NMR (D2O): δ 7.53 (s, 2H), 4.24 (t, 4H), 3.24 (t, 4H), 2.29
(appt. t, 4H). The lithium salt was prepared by heating B (1.0
g) and LiOH (0.075 g) in approximately 15 mL of deionized
water until both dissolved. On cooling, fine white needle-like
crystals formed and were collected by filtration and dried under
vacuum.
aPPE Particles. A Schlenk reaction tube equipped with a
magnetic stirbar was charged with monomers B (14.8 mg, 2.40
-
5
-5
×
10 mole), C (15 mg, 2.40 × 10 mole), and D (22.9 mg,
-
5
4
.79 × 10 mole). The reaction tube was transferred into an
inert atmosphere glovebox where Pd(P(C6H5)3)4 (5.5 mg) and
CuI (5.5 mg) were added and the tube was sealed. Argon
sparged 2:3 (v/v) diisopropylamine/dimethylformamide (3 mL
total) was added via cannula. The reaction mixture was heated
to 65 °C with stirring for 20 h. At the end of the polymerization,
the copolymer was recovered by precipitation in ∼150 mL of
acetone. The yellow-green, fluffy solid was collected by
filtration and dried under vacuum. On drying, the solid turned
black. The molecular weight was determined by GPC using 0.1
M LiBr in DMF as the eluent and monodispersed poly(methyl
methacrylate) standards with Mn ) 17 000/Mw ) 123 000.
SCHEME 2
2
+
2
.2. Preparation of ZnS/Mn Nanoparticles. The ZnS/
sulfonated monomer (B) and glycol monomer (C) with penti-
ptycene monomer (D) (Scheme 1).
The synthesis of B, 1,4-(bis-3-sulfopropyloxy)-2,5-diiodo-
benzene, is described below (Scheme 2).
2
+
Mn nanoparticles were prepared as follows: The contents of
a four-necked flask charged with 400 mL of deionized water
and 2 g of poly(vinyl alcohol) (PVA) were stirred under N2 for
2
.5 h. An aqueous solution of 1.6 g of Na2S and an aqueous
The syntheses of monomers C and D have been reported
1
3
solution of 5.8 g of Zn(NO ) ‚6(H O) and 0.26 g of Mn(NO )
elsewhere. Reagents and solvents were purchased from Ald-
3 2
2
3 2
2
+
2+
1
(Mn /Zn molar ratio 5:95) were prepared and added to the
first solution simultaneously via two different necks. After the
addition, the resulting solution was stirred constantly under N2
at 80 °C for 4 h, and a white colloid of ZnS/Mn was formed.
The pH value of the final solution was 4. This relatively low
pH value is required to prevent the precipitation of unwanted
Mn species. The nanoparticles were separated from the
solution by centrifugation and dried in vacuum at room
temperature.
2.3. Formation of aPPE Particles-ZnS/Mn2 Composite
Particles. A polymer particle solution was made by dissolving
0.05 g of aPPE particles in 5 mL of DMF and 10 mL of water.
The particle size of the polymer prepared in this way is about
rich. H NMR (400 MHz) spectra were obtained from Spectra
Data Services (Champaign, IL).
1,4-(Bis-3-sulfopropyloxy)benzene, A. NaH (60% in oil; 9.9
2
+
g, 0.248 mole) was placed in a clean, oven dried, three-necked
round-bottom flask that was then sealed and purged with argon.
Anhydrous THF (50 mL) was added to disperse the NaH, and
the oil was extracted from the resulting dispersion. Hydro-
quinone (13.36 g, 0.121 mole) was placed in a clean oven-dried
flask and dissolved in 50 mL of anhydrous THF. The hydro-
quinone solution was added slowly to the NaH dispersion over
1
3
+
1
h, resulting in vigorous gas evolution and the formation of a
white gelatinous precipitate. An additional 100 mL of anhydrous
THF was added to form a free flowing, white slurry that was
stirred for an additional hour. Propanesultone (22.4 mL, 0.255
mole) was added slowly via syringe, and the mixture was stirred
for 1 h. The mixture was quenched cautiously with water until
1
1
500 nm. A semiconductor nanoparticle solution was made by
dissolving 0.5 g of the PVA-stabilized ZnS/Mn nanoparticle
powder in 10 mL of water and 10 mL of ethanol. The two
2
+

6546 J. Phys. Chem. B, Vol. 107, No. 27, 2003
Chen et al.
Figure 1. HRTEM image of the aPPE-nanoparticle composite. The
Figure 2. HRTEM image of the aPPE-nanoparticle composite
showing a dark, carbon-like material that is likely formed by the
decomposition of PVA.
2
+
estimated size of the ZnS/Mn nanoparticles is about 4.5 nm. The
111) lattice planes of the ZnS nanoparticles can be observed in some
(
of the particles.
solutions (1:1) were mixed, stirred, and heated to 60 °C under
nitrogen protection for 1 h. A thin film was made by dropping
the solution on a glass substrate and drying at room temperature.
2
.4. HRTEM, Infrared Spectra, and Luminescence of
2+
aPPE-ZnS/Mn Nanocomposites. The composite material
was dissolved in a water solution by sonication and was placed
onto holey-carbon-covered copper grids for high-resolution TEM
(HRTEM) observations. The HRTEM images of the particles
were obtained with a JEM-4000EX electron microscope (400
kV) with a structural resolution of 0.16 nm.
2
+
2+
IR spectra of aPPE, ZnS/Mn , and aPPE-ZnS/Mn
Figure 3. Absorption spectrum (- - -) and luminescence spectrum
composite particles were recorded on a Nicolet Magna FTIR
spectrometer using a liquid-nitrogen-cooled detector. Solid
samples were prepared as KBr pellets. The photoluminescence
excitation and emission spectra at different temperatures were
recorded on a SPEX fluorolog 3 fluorescence spectrophotometer.
The fluorometer was equipped with a 450-W xenon arc lamp,
double monochromators (SPEX 1680) for excitation and emis-
sion, and a cooled photomultiplier tube. The nanoparticle sample
was mounted on the coldfinger of a liquid-helium flow-through
cryostat using indium metal for thermal contact. The coldfinger
was equipped with a heating element, and the temperature was
controlled by a Lakeshore model 330 temperature controller that
monitored the temperature with a calibrated silicon diode
attached to the indium metal near the sample position.
(s, excitation at 275 nm) of aPPE.
average size are easily observed. In each image, the (111) lattice
2+
planes of some ZnS/Mn particles can be observed. The (111)
lattice spacing of the particles was estimated to be around 0.31
nm from the HRTEM images. This is consistent with the (111)
spacing of bulk ZnS (0.312 nm). In some areas, as shown in
Figure 2, a carbon-like material can be observed. This material
14
is likely produced by the dissociation of PVA during heating.
Because of the large size (∼500 nm) and likely low contrast of
the polymer, aPPE particles are not observed in the HRTEM
images.
Figure 3 shows the absorption and emission spectra of aPPE
particles in DMF. The strongest peaks in the absorption spectrum
around 430 nm and in the emission spectrum at 460 nm are
A pulsed nanosecond optical parametric oscillator/amplifer
(OPO) (Spectra-Physics MOPO-730) operating at a 10-Hz
11,12
attributed to 0-0 band absorption and emission, respectively.
repetition rate was used to collect the photoluminescence (PL)
lifetime data. The output of the OPO was frequency doubled in
KDP to produce the PL excitation light. The excitation light
was directed onto the particles, and emission was collected at
The green emission shoulder at 490 nm may be from an excimer
11,12
formed within the polymer,
and the emission band peaking
at 370 nm is due to the solvent, DMF.
Figure 4 shows the photoluminescence excitation (PLE) and
1
right angles to the excitation and focused into a /8-m mono-
2
+
emission spectra of ZnS/Mn nanoparticles. The emission at
chromator equipped with a standard photomultiplier tube. The
photomultiplier tube output was directed into a digital oscil-
loscope to record the emission decays. The response time of
the system was measured to be about 15 ns fwhm.
4
6
2+ 13
5
90 nm is attributed to the T1- A1 transition of Mn . The
absorption band in the excitation spectrum is due to the band-
to-band transition of ZnS. Energy transfer from the ZnS exciton
2
+
2+
trapped at the Mn site leads to efficient excitation of the Mn
2
+
2+
center. In ZnS/Mn -doped nanoparticles, the Mn ions may
3
. Results and Discussion
2
+
occupy either Zn sites or surface sites. The presence of the
Figure 1 shows a HRTEM image of the aPPE-ZnS/Mn2
+
Mn emission at 590 nm following excitation into the absorp-
tion band of ZnS indicates that Mn ions occupy Zn sites.
2+
2
+
2+
particles prepared in dispersions. ZnS particles of 4.5-nm

Poly-P-phenylene ethynylene-ZnS/Mn2+
J. Phys. Chem. B, Vol. 107, No. 27, 2003 6547
Figure 4. Excitation (a) and emission (b) spectra of ZnS/Mn2+
nanoparticles. The peak at 330 nm in the excitation spectrum is due to
the band-band transition of ZnS. The emission at 590 nm is attributed
4
6
2+
to the T
1
1
- A transition of Mn .
Figure 6. aPPE-ZnS/Mn2 nanocomposite excitation spectra monitor-
ing the luminescence at 590 nm (590-nm PLE), 490 nm (490-nm PLE),
and 460 nm (460-nm PLE). Also shown is the luminescence spectrum
obtained following 370-nm excitation (370-nm PL).
+
6
2+
A1 transition of Mn . When monitoring the emissions of aPPE
particles at 460 or 490 nm, only the 420-nm peak is observed
in the excitation spectra, indicating that the 420-nm excitation
peak is from aPPE particles. When monitoring the emission of
2
+
Mn at 596 nm, two excitation peaks are observed at 330 and
20 nm. The 330-nm peak is due to the ZnS band-to-band
transition. The 420-nm peak may be due to a variety of sources.
4
2+
First, energy transfer from the aPPE particles to Mn would
2
+
result in increased Mn luminescence. Second, the aPPE
emission is broad, and the tail extends well into the red region
of the spectrum. Excitation at 420 nm causes increased aPPE
luminescence in the red (as well as blue) region of the spectrum,
leading to an observed peak in the excitation spectrum. Third,
the peak in the excitation spectrum may be due to a d-d
Figure 5. Infrared spectra of aPPE, ZnS/Mn2+, and aPPE-ZnS/Mn2+
2+
transition in Mn because it is consistent with a known d-d
nanocomposite particles.
2+
transition in ZnS/Mn particles. Two results indicate that there
2+
is little or no energy transfer from aPPE particles to Mn in
Otherwise, the Mn2+ emission is observed at 350 nm if Mn2+
ions exist at the surface sites.15
2+
the ZnS/Mn nanoparticles or that the energy transfer is a minor
2+
contribution to the Mn luminescence in the nanocomposite.
2
+
2+
Figure 5 shows the infrared spectra of aPPE, ZnS/Mn , and
aPPE-ZnS/Mn2 composite particles, respectively. ZnS is an
IR window material that is transparent in this measurement
range; therefore, the IR peaks from ZnS/Mn2+ arise from the
stabilizer (PVA) incorporated into the nanoparticles. The
The first is that very little Mn emission is observed following
+
excitation at 420 nm whereas the emission of aPPE particles is
the highest at this excitation wavelength. The second is that in
2+
the aPPE-ZnS/Mn nanocomposite the luminescence quantum
2
+
efficiency of Mn is not influenced by the concentration of
aPPE particles. Therefore, it is likely that the 420-nm peak in
-
1
fingerprint region (600-1500 cm ) is quite different for the
three samples. The spectrum of the aPPE-ZnS/Mn2 particles
is not simply a superposition of the individual spectra of aPPE
and the nanoparticles. This indicates that there are likely
interactions between aPPE and ZnS/Mn2 particles. The alkyl
+
2+
this excitation spectrum is due to either a d-d transition in Mn
and/or the red tail of the aPPE particle emission.
A new emission band is observed at 706 nm. This emission
+
2+
band has not been reported in ZnS/Mn nanoparticles; however,
2
+
C-H stretch peaks of aPPE and PVA in the ZnS/Mn
an emission at 709 nm has been reported in undoped ZnS
nanoparticles at 2865 cm-1 are shifted to 2850 cm in the
composite material, which may be due to the coupling between
the two kinds of particles. In addition, some new IR peaks
appear in the nanocomposite particles. These new IR peaks
might arise from the coupling or new chemical bonding between
the polymer particles and the semiconductor nanoparticles. The
-1
16
nanoparticles and has been assigned to impurities. In addition,
2
+
in CdS/Mn nanoparticles, an emission around 700 nm has
been observed and attributed to a defect-related CdS emission.18
This emission is strongest at temperatures below room temper-
ature; above room temperature, it is quenched rapidly. This
behavior is similar to the luminescence of most defects in
-
1
17
strong, broad -OH peak from 3100 to 3600 cm in the ZnS/
semiconductors. Accordingly, the emission at 706 nm in the
2
+
2+
Mn sample is most likely due to residual water in the sample
and the -OH group in PVA. Most likely, the coupling between
the polymer particles and ZnS/Mn2+ nanoparticles is via
chemical bonding through the stabilizer (PVA).
aPPE-ZnS/Mn composite is assigned to an impurity or a
defect in the ZnS nanoparticles.
As displayed in Figure 7, the overall emission color of the
nanocomposite changes for different excitation wavelengths
2
+
Figure 6 shows the excitation and emission spectra of aPPE-
because the relative emission intensities of Mn and aPPE
particles change. As shown in the inset of Figure 7, as the
excitation wavelength varies from 280 to 420 nm, the orange
2
+
ZnS/Mn composite particles. Four emission peaks are ob-
served at 460, 490, 596, and 706 nm. As before, the two
emission peaks at 460 and 490 nm are assigned to aPPE
2
+
(596-nm) emission intensity of Mn decreases, and the blue
(460-nm) and green (490-nm) emissions of aPPE particles
4
particles, and the emission at 596 nm is assigned to the T1-

6548 J. Phys. Chem. B, Vol. 107, No. 27, 2003
Chen et al.
Figure 8. Luminescence spectra of the aPPE-ZnS/Mn2 nanocom-
+
posite at different temperatures ranging from 11 to 276 K.
Figure 7. Photoluminescence spectra of the aPPE-ZnS/Mn2+ nano-
composite following excitation at different wavelengths. The inset
displays the variation in luminescence intensity of the different
emissions as a function of excitation wavelength.
increase. This change in intensity is related to the energy
structure of ZnS/Mn2+ nanoparticles and aPPE particles in the
composite. The conduction band of aPPE particles partially
2+
overlaps with the excited states of ZnS/Mn nanoparticles, and
the energy gap of ZnS/Mn2 nanoparticles (around 330 nm) is
larger than that of aPPE particles (around 420 nm). In this case,
if the excitation energy is greater than the energy gap of ZnS/
+
2
+
Mn nanoparticles, then the induced emission is mainly from
2
+
2+
Mn particles and the emission of Mn is stronger than that
of aPPE particles in intensity. If the excitation energy is less
than the energy gap of the ZnS/Mn2 nanoparticles but greater
than the energy gap of aPPE particles, then the induced emission
is mainly from aPPE particles. In semiconductors, it has been
demonstrated that the excitation rate is greater when the
excitation energy is closer to the absorption edge.17 This may
explain why the emission intensity of aPPE particles increases
with increasing excitation wavelength from 280 to 420 nm. If
+
Figure 9. Emission-energy maxima of the defect ((), Mn2+ (9), 490-
nm aPPE (*), and 460-nm aPPE (2) emissions at different temperatures
below room temperature.
2
+
the excitation energy is less than the energy gap of ZnS/Mn
2+
nanoparticles, then the emission of Mn may be due to the
2+
excitation of d-d transitions of Mn . Because the d-d
absorption cross section of Mn2 is smaller than the band-
band transition of ZnS, the d-d excited luminescence is weaker
in intensity than that from the band-band excitation. This
phenomenon provides a possible mechanism for adjusting the
emission color of the composite by varying the excitation
wavelength, thus making these materials useful in applications
involving emission displays.
+
Luminescence lifetime measurements (not shown) reveal that
the majority of the luminescence from aPPE particles decays
on a time scale faster than the instrument response (<15 ns),
although there is a smaller amplitude component with a lifetime
of ∼200 µs. Previous aPPE luminescence results have also
Figure 10. Emission intensity of the defect ((), Mn2 (9), 490-nm
aPPE (*), and 460-nm aPPE (2) emissions at different temperatures
below room temperature. The error is within 2%.
+
1
2
shown subnanosecond lifetimes for both emissions. The
luminescence lifetime of Mn2 emission in the ZnS/Mn
+
2+
nanoparticles is likewise multiexponential, as has been observed
previously, with the longest lifetime (∼1.5 ms) consistent with
nanoparticles of similar size.1 There is, therefore, no evidence
of energy transfer in the luminescence decay data, consistent
with the results from the excitation and emission spectra.
Figures 8-10 show the emission spectra, emission wave-
length maxima, and the emission intensity at different temper-
atures. All of the emission bands shift to the red with decreasing
temperature although the shift in the peak position is not very
dramatic. All of the emissions decrease in intensity with
increasing temperature, with the decrease of the Mn2 emission
at 596 nm being much larger than the decrease of the emissions
of aPPE particles.
+
9,20
2
+
2+
As in bulk ZnS/Mn , the emission-energy shifts of Mn in
2
+
ZnS/Mn nanoparticles with temperature can be described by
2
1,22
2+
crystal field theory.
The red shift of the Mn emission
wavelength with decreasing temperature is due to the enhance-
ment of the crystal field strength at lower temperatures resulting
from crystal lattice contraction. As a consequence, the emitting

Poly-P-phenylene ethynylene-ZnS/Mn2+
TABLE 1: Simulated Parameters for the Temperature Dependence of Emissions in ZnS/Mn2+-aPPE Particle Nanocomposites
J. Phys. Chem. B, Vol. 107, No. 27, 2003 6549
wavelength (nm)
460
1.9 ( 0.8
27 ( 8
1.04 ( 0.03 × 10
490
596
6 ( 1
27 ( 3
1.43 ( 0.03 × 10
706
29 ( 7
47 ( 4
3.15 ( 0.05 × 10
a
E
27 ( 27
49 ( 16
4.9 ( 0.3 × 10
b
(meV)
5
4
5
4
I
0
4
2+
state, T1(G) of Mn , shifts to lower energies with decreasing
temperature, shifting the emission to longer wavelengths.
2+
The temperature dependence of the Mn emission intensity
in ZnS/Mn2 nanoparticles has been discussed,22-25 and the
proposed mechanisms can reasonably explain the temperature
behavior of Mn2 emission observed here. On the basis of the
theory of thermal quenching, the temperature dependence of
the emission intensity, I(T), can be described by26
+
+
I0
I(T) )
(1)
-Eb/kT
1
+ ae
where Eb is the activation energy (thermal quenching energy),
k is the Boltzmann constant, a is a constant related to the ratio
of the nonradiative rate to the radiative rate, and I0 is the
emission intensity at 0 K. The solid lines in Figure 10 represent
a simulation of the intensities using this theory. The parameters
used to fit the four emissions are shown in Table 1. The excellent
agreement between theory and experiment suggests that the
intensity decrease of the emissions with increasing temperature
is mainly due to thermal quenching.
Figure 11. Intensity-temperature dependence of the aPPE emission
2+
at 460 nm and the Mn emission from room temperature to 60 °C.
The 460-nm emission decreases in intensity upon heating (9); however,
2
+
the intensity does not recover upon subsequent cooling (b). The Mn
emission increases upon heating (2) but only partially recovers upon
subsequent cooling (1). The error is within 2%.
The thermal quenching energy is related to the bound exciton
2
3
2+
binding energy. For bulk ZnS/Mn , the exciton binding
energy is about 60 meV.24 This value is very close to the thermal
quenching energy (57 meV), as estimated from the temperature
dependence of Mn2 luminescence in bulk ZnS/Mn . Re-
cently, however, a markedly lower value of 23 ( 5 meV has
been reported for the thermal quenching energy of bulk ZnS/
Mn . For ZnS/Mn nanoparticles with an average size of
.5 nm, the thermal quenching energy reported for the T1-
A1 transition of Mn is ∼70 meV, which is larger than the
exciton binding energy in the bulk. This increase in exciton
binding energy has been attributed to the increase in the overlap
of the electron and hole wave functions as a result of quantum
size confinement. The thermal quenching energy for the T1-
A1 transition of Mn
+
2+ 24
2
+ 27
2+
4
3
6
2+
25
4
6
2+
2+
emission in the ZnS/Mn -aPPE
nanocomposite film is approximately 27 meV, which is
significantly less that the result from 3.5-nm ZnS/Mn nano-
particles. Most likely, this large difference is related to the
2
+
2
5
Figure 12. Intensity-temperature dependence of the aPPE emission
2
+
2+
interaction of Mn with aPPE particles in the nanocomposite
as well as the surface characteristics and defects in the
nanoparticles. The exciton binding energy in nanoparticles is
related not only to the particle size but also to their surface
characteristics. The differences in surface coating and structure
may cause differences in the lattice strain and therefore
differences in the binding energy of up to 0.1 eV. In addition,
the temperature dependence of the emission in a material such
as PbWO4 has been attributed to traps or defects in the
material. Photoinduced carriers (electrons or holes) may be
trapped at these defects or traps, and at certain temperatures,
the trapped carriers may be released to the conduction band,
resulting in luminescence. Because of the smaller surface/
volume ratio, 4.5-nm particles are expected to have fewer surface
traps relative to 3.5-nm particles.2 The aPPE polymer may
affect the nature and number of defect or surface states in the
nanocomposite; consequently, these factors will affect the
temperature dependence of the luminescence and the thermal
quenching energy (exciton binding energy).
at 460 nm and the Mn emission from room temperature to 140 °C
following one cycle of heating and cooling described in Figure 11.
Above 60 °C, the emission of aPPE upon heating ([) decreases steadily
and is completely quenched at 140 °C. The luminescence intensity does
2
+
not recover following subsequent cooling (×). The Mn emission
intensity also decreases from 60 to 140 °C (+), upon which it is
completely quenched. The intensity also does not recover upon cooling
28
28
(*). The error is within 2%.
Figures 11 and 12 show the luminescence intensity temper-
2
9
ature dependence of the emission at 460 and 596 nm at
temperatures above room temperature for the nanocomposite
material. Figure 11 shows the dependence from room temper-
ature to 60 °C. It is surprising to observe that the emission of
30
2
+
Mn at 596 nm (Figure 11, 2) increases while the emission of
aPPE particles at 460 nm (Figure 11, 9) decreases with
increasing temperature. Upon subsequent cooling, the emission
intensity at 596 nm (Figure 11, 1) decreases, although the
intensity does not recover to its original value. Similarly, the
intensity of the aPPE emission at 460 nm (Figure 11, b) does
1

6550 J. Phys. Chem. B, Vol. 107, No. 27, 2003
Chen et al.
not recover to its initial value upon cooling. Figure 12 shows
the intensity dependence up to 140 °C following one cycle of
heating and cooling described above. The blue emission of aPPE
particles decreases gradually with increasing temperature (Figure
ment in practical applications. Variations of the optical path,
such as the bend of an optical fiber or skin penetration, could
change the detected fluorescence intensity easily, but the ratio
of the two peak intensities is much less dependent on such
2+
34
12, [). However, the luminescence of Mn at 596 nm increases
factors. This potential application is particularly interesting
with increasing temperature up to 90 °C (Figure 12, +). When
the temperature is higher than 90 °C, the luminescence is
quenched rapidly. At 140 °C, both the emissions of aPPE
particles and the Mn2 emission are quenched completely. In
either species, the luminescence does not recover following
subsequent cooling (Figure 12, ×, *).
and suitable for temperature measurement in laser cooling
systems where temperature detection is still an unsolved
problem. In addition, the nanocomposite exhibits properties
that may make it practical for use in full-color displays.
3
5
+
4
. Summary
Theoretically, luminescence will decrease in intensity with
2
6
A composite material composed of aPPE polymer particles
increasing temperature because of thermal quenching and the
2+
and ZnS/Mn nanoparticles is described, and its luminescence
properties are investigated. The composite has blue (460-nm),
green (490-nm), orange (596-nm), and red (706-nm) emissions,
and the relative intensities of the three emissions change with
the excitation energy. Observations of luminescence lifetimes
as well as excitation and emission spectra indicate that there is
little or no energy transfer from the blue polymer particles to
increase in the nonradiative rate as a result of a stronger
electron-phonon interaction.3
1,32
Three possible reasons may
cause the increase in luminescence intensity with increasing
temperature observed here: energy transfer, thermal lumines-
cence, and thermal curing of the nanoparticle surface. We have
demonstrated that there is negligible energy transfer from aPPE
2+
particles to Mn . Most likely, the luminescence enhancement
2+
of Mn2+ with increasing temperature up to 90 °C is due to
ZnS/Mn particles. However, infrared investigation reveals that
there are interactions or coupling between the two particles.
thermoluminescence and surface curing. Upon heating, carriers
at some traps are released to the conduction band and contribute
to the luminescence; as a result, the luminescence increases with
increasing temperature. As the trapped sites are emptied, the
luminescence enhancement decreases in intensity.
2
+
Temperature studies suggest that the ZnS/Mn nanoparticles
are more stable than the aPPE polymer particles with respect
to decomposition and that the thermal quenching energy of ZnS/
2
+
Mn nanoparticles within the composite is smaller than that
for naked nanoparticles. From room temperature to 90 °C, the
In addition, ultraviolet curing has been observed to increase
2
+
luminescence of ZnS/Mn nanoparticles at 596 nm increases
in intensity with increasing temperature. This interesting result
is attributed to thermoluminescence and thermal curing of the
particle surface upon heating. At temperatures higher than 90
2+
33
the luminescence of ZnS/Mn nanoparticles. This is due to
polymerization and surface passivation that occurs along with
the polymerization. Similarly, heating can induce polymerization
and surface passivation, resulting in luminescence enhancement.
Therefore, it is reasonable to assign the luminescence enhance-
ment of Mn2 in the nanocomposite to thermoluminescence and
surface curing with increasing temperature up to 90 °C.
°C, the luminescence is quenched, and the emissions from both
the polymer particles and the nanoparticles are quenched
completely at 140 °C.
+
The luminescence quenching of both emissions at increased
temperature may be due to phonon quenching and/or chemical
dissociation or oxidation of the compounds in the composite.
Phonon quenching has already been discussed, and the same
arguments apply above room temperature. If phonon quenching
were solely responsible, then the luminescence intensity would
be expected to return to its original value upon cooling from
elevated temperatures. However, this is not the case with either
Acknowledgment. We thank Nomadics, Inc., the National
Science Foundation (grant DMI-0060254), the NIH, and the Air
Force Office of Scientific Research (contract no. F49620-00-
C-0058) for financial support. Part of the research described in
this paper was performed at the W.R. Wiley Environmental
Molecular Sciences Laboratory, a national scientific user facility
sponsored by the Department of Energy’s Office of Science and
located at the Pacific Northwest National Laboratory (PNNL).
PNNL is operated by Battelle for the U.S. Department of Energy
under contract DE-AC06-76RLO1830. J.-O.M. and J.-O.B.
thank the Swedish Natural Science Research Council and the
Foundation for Strategic Research of Sweden for financial
support. We thank B. Deans and L. Hancock for their help in
aPPE particle preparation and G. H Li and F. H. Su for their
help with the fitting of Figure 10. We thank M. Gheith for his
help in IR spectra measurement.
2
+
the aPPE or Mn luminescence. Therefore, it is likely that the
luminescence quenching is mainly due to chemical dissociation
or oxidation of the compounds in the composite. It is more likely
that some decomposition occurs within the aPPE particles,
decomposing the polymer into species that serve as efficient
energy quenchers. From our measurements, as the polymer
begins to decompose, there is quenching of the aPPE lumines-
2
+
cence. There may also be some quenching of the Mn
luminescence, but this may be masked by the increase due to
the thermal curing and thermoluminescence. At temperatures
2+
above 90 °C, the Mn luminescence is quenched rapidly. Thus,
References and Notes
2
+
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