Charge and energy transfer between CdSe quantum dots and polyaniline

Article abstract of DOI:10.1166/jnn.2016.11851

CdSe quantum dots (QDs) and polyaniline (PAni) were mixed to prepare CdSe QDs/PAni complex. PAni can quench the fluorescence of CdSe QDs. Fluorescence intensity of CdSe QDs/PAni complex is related to the size of CdSe QDs and the concentration of PAni. UV-

Full text of DOI:10.1166/jnn.2016.11851

Article
Journal of
Nanoscience and Nanotechnology
Vol. 16, 3909–3913, 2016
Copyright © 2016 American Scientific Publishers
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Charge and Energy Transfer Between CdSe Quantum
Dots and Polyaniline
∗
Liang Xu, Xingbin Huang, Wenjiang Dai, Punan Sun, Xuanlin Chen, and Limin An
College of Physics Science and Technology, Heilongjiang University, Harbin, 150080, China
CdSe quantum dots (QDs) and polyaniline (PAni) were mixed to prepare CdSe QDs/PAni complex.
PAni can quench the fluorescence of CdSe QDs. Fluorescence intensity of CdSe QDs/PAni complex
is related to the size of CdSe QDs and the concentration of PAni. UV-Vis absorption spectra,
fluorescence spectra, time-resolved fluorescence spectroscopy were used to analys the quenching
phenomenon. The mechanism of fluorescence quenching is dependent on two factors: on one hand,
the Förster resonance energy transfer from CdSe to PAni; on the other hand, PAni can intercept
the charge relaxation process of CdSe and lead to the interruption of radiative recombination.
Keywords: Quantum Dots (QDs), CdSe, Polyaniline (PAni), Fluorescence Life, Charge Transfer,
Energy Transfer.
Delivered by Ingenta to: University of New South Wales
1
. INTRODUCTION
IP: 193.105.171.111 On: Sat to, 2y 3e l Al o pw r 2t h0 r1o 6u g1 h9 : c0 h2 a: n0 g9 ing the size of CdSe QDs. In
Copyright: American S hc ii ge hn t vi f oi cl t aP g ue b tl hi seh ed er sv ice emitted green light basically pro-
Conductive polymers have not only the combined
electronic and optical properties of metal and inorganic
semiconductor, but also the mechanical property and man-
ufacturability of organic polymer. In addition they are
an excellent redox active material. Polyaniline (PAni)
is a polymer composite material, commonly known as
the conductive plastic. PAni has become one of the
most promising conductive polymer because of its sim-
ple preparation (the batch productions can be conducted
through the chemical oxidation polymerization), low cost,
and good stability. Semiconductor quantum dots (QDs)
whose radii are smaller than the bulk exciton Bohr radius
constitute a class of materials intermediate between molec-
ular and bulk forms of matter. Consequently semiconduc-
tor QDs exhibit optical and electronical property whose
2
duced by PPV and the brightness can reach 100 cd/m . In
recent years there was a great progress in the field of QDs-
organic electroluminescent devices, especially after white
light emission has been realized in QDs-polymer hetero-
junction electroluminescent devices by changing the size
9
and concentration of QDs.
There are several problems in the research on the elec-
troluminescent devices which are produced through the
mixture of the conductive polymers and QDs, including
the charge carrier transport mechanism between polymer
layer and QDs layers that has not been explained or the
fluorescence efficiency and the life time that still needs
to be improved. Due to the limitation of the resolution of
the transmission electron microscopy, the structure of the
organic molecules can not be observed, so the research on
the interface between QDs and organic molecular can be
conducted only through indirect methods. In this paper,
CdSe QDs/PAni complex is synthesized and the physical
mechanism of energy and charge transfer from CdSe QDs
to PAni is discussed.
1
ꢀ2
energy or color can be easily controlled by adjusting their
sizes.3ꢀ4 We can combine PAni with semiconductor QDs
and get a variety of composite materials which can be
used in solar cells, aerospace, microelectronics, automo-
tive, communication, biomedicine, textile and many other
fields.5
–7
In 1994, Colvin combined CdSe QDs with Poly-
Phenylene Vinylene (PPV) and manufactured double-deck
electroluminescent device for the first time. The fluores-
2. EXPERIMENTAL PROCEDURE
8
2
.1. Reagents and Chemicals
Selenium (100mesh, 99.5%), cadmium oxide (CdO,
9.9%), trioctylphosphine oxide (TOPO, 90%), hexadecy-
lamine (HDA, 90%), technical grade 1-octadecene (ODE)
cence color of the device can be adjusted easily from red
9
∗
Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2016, Vol. 16, No. 4
1533-4880/2016/16/3909/005
doi:10.1166/jnn.2016.11851
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Charge and Energy Transfer Between CdSe Quantum Dots and Polyaniline
Xu et al.
and trioctylphosphine (TOP, 90%) were purchased from
Sigma-Aldrich. Technical grade stearic acid, PAni, toluene,
acetone, hexane, methanol, and chloroform were obtained
from Beijing Reagent Company.
of TOPO as solvent for the synthesis of CdSe QDs with
a small amount of TOPO used as surface ligand to sta-
bilize QDs, so the cost of synthesis was greatly reduced.
The reaction speed with pure TOPO (99.9%) is faster than
with technical grade TOPO (90%), thus it is difficult to
control the growth of QDs. So the technical grade TOPO
2
.2. Synthesis Procedure of CdSe QDs and CdSe
QDs/PAni Complex
(
90%) was added. We can control the growth of CdSe QDs
through the adjustment of different proportion between
HDA and TOPO. The mole ratio of HDA verse TOPO is
CdSe QDs were prepared according to the previous
A stock cadmium stearate solution was pre-
pared by heating a mixture of 0.228 g of stearic acid,
method.4
ꢀ10
4
ꢀ11
4
:1 in this experiment.
The characterization of QDs involves various micro-
0
.0256 g of cadmium oxide and ODE in a 50 ml three-
ꢀ
scopic, spectroscopic, and X-ray techniques. Microscopic
techniques such as TEM, scanning tunneling microscopy
neck flask to 200 C under stirring and continuously flow
of nitrogen until a clear solution was obtained. After this
solution was cooled to room temperature, HDA and TOPO
were added into the flask with a molar ratio of 4:1. Under
(
STM), and atomic force microscopy (AFM) are the most
direct methods for determining particle size and shape. The
particle shape, crystal lattice structures of CdSe QDs can
be clearly determined through TEM and selected area elec-
tron diffraction (SAED), as shown for in Figure 1. CdSe
QDs have a narrow size distribution, averaging (3ꢁ3 ±
0ꢁ4) nm in diameter and a hexagonal wurtzite structure.
The particles appear nearly spherical and no aggregation
was observed.
ꢀ
nitrogen flow, the mixture was reheated to 280 C. At
this temperature, a selenium solution (prepared by dissolv-
ing 0.158 g Se powder into the TOP and ODE mixture
under ultra-sonication) was quickly injected. The temper-
ꢀ
ature was reduced to 240 C within several minutes for
core growth. After the reaction was completed, the reaction
flask was removed from the heating mantle and allowed
ꢀ
Different amount of PAni were respectively mixed with
different sizes of CdSe QDs, and stirred well for 30 min,
thus the CdSe QDs/PAni complex was prepared. The
key problem of directly mixed methods is how to dis-
perse QDs homogeneously under the premise of keeping
to cool to 40 C. The reaction mixture was extracted by
hexane/methanol system to purify the QDs (remained in
the hexane/ODE layer) from side products and un-reacted
precursors (remained in the methanol layer). Then, the
hexane system was extractedDa en ldi v ea cr ee tdo nb ey wI n ag se na dt ad et do :t Uo niversity of New South Wales
the fluorescence. Usually, the mixes of different materi-
IP: 193.105.171.111 On: Sat, 23 Apr 2016 19:02:09
precipitate the QDs. The QDs precipitate was collected by
Copyright: American S ca il es nc tai fui cs ePs ue br i loi su hs ep r hs ase separation and lead to aggregation
centrifugation and re-dissolved in hexane or chloroform.
and deterioration of the original performance. But, in this
experiment we find that the dispersion of QDs in PAni
is homogeneous. The process is similar to the separation
behavior of a blending polymer into an embedded poly-
mer. The flexible polymer chains of PAni interact with
HDA and TOPO on the surface of CdSe QDs, so they
form stable structures.
By systematically changing the nucleation temperature and
growth time, it was possible to finely tune the size of the
CdSe QDs. The CdSe QDs and PAni were mixed and mag-
netically stirred thirty minutes in hexane. Then, the CdSe
QDs/PAni complex is synthesized.
2
.3. Apparatus
The size and morphology of CdSe QDs were characterized
by JEOL-3010 transmission electron microscopy (TEM)
3
.2. Fluorescence Properties
The absorption spectrum of PAni and fluorescence spectra
of different size CdSe QDs are shown together in Figure 2.
The main chain of PAni contains alternate benzene ring
and nitrogen atoms, so PAni is a special type of conduc-
tive polymer similar to p type semiconductor. There is an
(
JEOL Company, Japan). UV-Vis absorption spectra were
recorded with an UV-Vis 3101 spectrometer having a res-
olution of 1.0 nm. Photoluminescence experiment was
conducted on a Hitachi F-4500 fluorescence spectrometer
with a resolution of 2.5 nm. Fluorescence lifetimes were
measured using the time correlated single photon count-
ing technique with FL920-fluorescence lifetime spectrom-
eter. The excitation source was an nF900 nanosecond flash
lamp. Lifetimes were obtained by deconvolution of the
decay curves.
3
. RESULTS AND DISCUSSION
3
.1. CdSe QDs/PAni Complex’s Synthesis
At present, the oil phase synthesis for QDs is a relatively
well established technical method. But the precursors like
TOPO are expensive. As a result, we used ODE instead
Figure 1. TEM and SAEX of CdSe QDs.
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J. Nanosci. Nanotechnol. 16, 3909–3913, 2016

Xu et al.
Charge and Energy Transfer Between CdSe Quantum Dots and Polyaniline
Figure 2. Absorption spectrum of PAni (dash) and fluorescence spectra
of different size CdSe QDs (solid) in CHC .
1
3
Figure 3. Absorption and fluorescence spectra of CdSe QDs/PAni com-
plex with different concentration of PAni.
exciton absorption peak at 450 nm and decreased absorp-
tion up to 600 nm. In the experiment the fluorescence exci-
tation wavelength used was 480 nm (more than the first
exciton absorption peak of PAni), so PAni is not excited.
CdSe QDs can be excited. The fluorescence emission
peaks of CdSe QDs from 506 to 662 nm cover the whole
visible area. It can be seen that the CdSe QDs’ emission
spectra expect the peak in 662 nm have a small overlap
with the PAni absorption spectrum. This is in accord with
the basic rules of fluorescence resonance energy transfer
QDs fluorescence emission peak position does not change,
and new fluorescence peaks do not appear after adding
PAni.
(
2) Excitation competition leads to the decrease of exci-
tation density. This mechanism has little contribution
because at an excitation wavelength of 480 nm (more than
the first exciton absorption peak of PAni), little PAni is
excited.
(
3) Energy transfer. The donor (CdSe QDs) in the excita-
tion state transfers energy to the receptor (PAni) and goes
Delivered by Ingenta to: Univebr as ci tky too f tNh ee wg r So uo nu dt hs tWa t ea .l eT she energy transfer mechanism
words, energy transfer can hap IpPe :n 1 b9 e3 t .w1 e0 e5 n. 1 C7 1d S. 1e 1 1Q OD sn : Sat, 23 Apr 2016 19:02:09
(
Förster resonance energy, FRET) that donor’s emission
spectrum overlap receptor’s absorption spectra, in other
of fluorescence quenching is possible.
and PAni (CdSe QDs as the donor, PA Cn oi pa ys r ri ge ch et p: tAo mr ) .e rican S (c 4i e) n Ct i hf i ac r gP eu bt r lai sn hs fee rr s. That is excited state charge transfers
1
2–14
When the distance between the donor and the receptor is
less than Förster radius, fluorescence energy transfers from
the donor to the receptor, which leads to the decrease of
the donor’s fluorescence emission intensity and the excited
state lifetime, and the increase of the receptor’s fluores-
cence enhancement.
In order to further define the interaction between the
PAni and CdSe QDs, we discuss the influence of the con-
centration of PAni on the CdSe QDs emission. The absorp-
tion and fluorescence spectra of CdSe QDs/PAni complex
with different concentration of PAni are shown in Figure 3.
The shape of CdSe QDs fluorescence spectra have not
significant change with the variation of PAni concentra-
tion, but the fluorescence intensity of CdSe QDs decreases
nonlinearly with the increasing of the PAni concentra-
tion. CdSe QDs in Figure 3 have a fluorescence emission
peak at 553 nm. The same quenching phenomenon is also
observed in other diffedrent size CdSe QDs/PAni complex
although the extent of the quenching is different.
between CdSe QDs and PAni leading to the interruption of
radiative recombination. The charge transfer mechanism
of fluorescence quenching is possible too.
According to the different energy transfer environment
of the donor and receptor, the kinds of energy transfer
mechanism can be: radiative mechanism, resonance mech-
anism and exchange electronic mechanism. The energy
transfer through radiative mechanism cannot change the
lifetime of the donor because the energy transfer is only
the consecutive process to radiative transition. When there
is only energy transfer through radiative mechanism, the
lifetime of the donor is just determined by the rate of
molecular deactivation process. In our experiment the life-
time of the composite system of CdSe QDs and PAni
changes. So we can eliminate the radiation mechanism.
The rate index of energy transfer for electronic exchange
mechanism decreases with the increase of the distance
between donor and receptor ꢂRꢃ. When R is larger than
0
.1 nm, the rate of energy transfer caused by electronic
exchange mechanism can be omitted compared to other
inactivation process. But the distance between CdSe QDs
and PAni is far over 0.1 nm. So the main energy transfer
mechanism is the resonance mechanism on the composite
system of CdSe QDs and PAni.
3
.3. Physical Mechanism of Fluorescence Quenching
The fluorescence intensity of CdSe QDs reduces after
adding PAni as show in Figure 3. The possible physical
mechanisms of fluorescence quenching are listed:
(
1) formation of new radiation center or non-radiation
The transfer efficiency of energy transfer ꢄET is related
mainly to the overlapping degree of the spectra of donor
center. This mechanism can be excluded because CdSe
J. Nanosci. Nanotechnol. 16, 3909–3913, 2016
3911

Charge and Energy Transfer Between CdSe Quantum Dots and Polyaniline
Xu et al.
emission and receptor absorption, the distance between the
donor and the receptor, the relative orientation of transition
dipole moment.
kET
ꢄET
=
(1)
k +k
D
ET
k is the transition rate for the donor itself, k is the elec-
D
ET
tronic energy transfer rate. The interactions between donor
D (CdSe QDs) and receptor A (PAni) is very weak, and
no radiation energy transfer process and the internal filtra-
tion effects are considered. So formula (1) can be rewritten
using the absorbency in excitation wavelength (480 nm)
and the fluorescence intensity of the donor:
Aꢂꢅ ꢃ I ꢂꢅ ꢃ
D
D
0
D
Figure 5. Fluorescence delay curves of CdSe QDs and CdSe QDs/PAni
complex.
ꢄET = 1 −
(2)
A ꢂꢅ ꢃ I ꢂꢅ ꢃ
D
D
D
D
A ꢂꢅ ꢃ is absorbance without receptor, Aꢂꢅ ꢃ is
D
D
D
on the processes of the charge transfer, vibration relaxation
in excited molecules or between the molecules. The curve
in Figure 5 is not a single exponential, but can be fitted
well by double exponents with a fitting formula:
0
absorbance with receptor, I ꢂꢅ ꢃ is the fluorescent integral
D
D
strength of quantum dot without receptor, I ꢂꢅ ꢃ is the
D
D
fluorescent integral strength of CdSe QDs with receptor.
ꢅD is excitation wavelength.
−t/ꢆ1
−t/ꢆ2
e
2
Figure 4 shows the relationship of energy transfer effi-
ciency between different size CdSe QDs and PAni with
the same PAni concentrations. In Figure 4 the fluores-
cence emission peak position of CdSe QDs is horizontal
ordinate, the energy transfer efficiency of CdSe QDs/PAni
polymer is vertical coordinate. The energy transfer effi-
I = A
1
e
+A
(3)
Table I show specific fitting value for fluorescent lifetime.
ꢆ and ꢆ is the lifetime of different component. A and
1
2
1
A is the weighting coefficient of different component. I
2
is fluorescence intensity.
Delivered by Ingenta to: Univers Di t ye co afyN de awt aS soh uo t wh Wt h aa tl e ts he kinetics consists of a fast
ciency is calculated according to formula (2). At a fixed
IP: 193.105.171.111 On: Sa ct ,o 2m 3p oA np ern 2t 0a 1n 6d 1a 9s: l0o 2w :0 c9 omponent attributed to the exci-
PAni concentration, energy transfer efficiency decreases
Copyright: American Scientific Publishers
15
tonic state and the charge trapping state, respectively.
with the increase of the QDs size. The reason is that fluo-
rescence emission wavelength increases with the increase
of the QDs size, and the overlap between PAni absorption
spectrum and the CdSe QDs emission spectrum decreases.
Figure 5 shows the fluorescent lifetime curve of CdSe
QDs and CdSe QDs/PAni complex. The fluorescence emis-
sion peak of CdSe QDs is 553 nm. It can provide important
information of the interaction between different molecules
and the excitation mechanism, and it can be used to study
From Table I and Figure 5, we can see that the lifetime of
CdSe QDs/PAni complex is shorter than the CdSe QDs.
This means that an energy transfer phenomenon appears
after adding PAni. PAni effectively reduces the fluores-
cence intensity of CdSe QDs.
It must be pointed out that the fluorescence spectrum for
the red QDs with the peak at 662 nm and absorption spec-
trum of PAni doesn’t overlap at all (Fig. 2) so it doesn’t
meet the resonance energy transfer conditions. But the flu-
orescence intensity of QDs still declines about 7% (Fig. 3)
so there must also be other mechanism. After CdSe QDs
are excited, the electrons in valence band absorb photon
energy to enter conduction band. At the same time the
holes emerge in the valence band. After a quick vibra-
tion relaxation, the electrons in conduction band release
phonons to go to the bottom of conduction band, then
ꢁ
radiative recombination of electrons and holes emits hꢇ ,
as shown in formula (4). After the addition of PAni, the
holes in the valence band may be transferred because PAni
is a kind of a good p-type conducting polymer, as shown
Table I. Fluorescence lifetime of CdSe QDs and CdSe QDs/PAni.
Sample
A1
ꢆ1/ns
A2
ꢆ2/ns
Figure 4. Energy transfer efficiency of CdSe QDs/PAni with different
QDs size (the fluorescence emission peak position of CdSe QDs is hori-
zontal ordinate, the energy transfer efficiency of CdSe QDs/PAni polymer
is vertical coordinate).
CdSe QDs
CdSe QDs/PAni
155.38
217.07
8.89
6.57
67.82
50.64
48.07
35.91
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J. Nanosci. Nanotechnol. 16, 3909–3913, 2016

Xu et al.
Charge and Energy Transfer Between CdSe Quantum Dots and Polyaniline
in formula (5). Then the radiative recombination process
is interrupted. So the fluorescence quenching effect of
PAni on CdSe QDs contains the resonance energy transfer
mechanism and the charge transfer mechanism.
University), Ministry of Education (2013), the High Level
Innovation Teams of Heilongjiang University under Grant
(HD-028).
+
−
ꢁ
CdSe+hꢇ → CdSeꢂh +e ꢃ → CdSe+hꢇ
(4)
(5)
References and Notes
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+
CdSeꢂh ꢃ+PAni → CdSe+PAni
2
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. CONCLUSIONS
The complex of CdSe QDs and PAni are synthesized and
analyzed the energy and charge transfer between CdSe
QDs and PAni. PAni can quench the fluorescence of CdSe
QDs. The fluorescent intensity of CdSe QDs/PAni complex
is related to the size of CdSe QDs and concentration of
PAni. There are two reasons for the fluorescence quench-
ing mechanism. On the one hand, FRET exists from CdSe
QDs to PAni. On the other hand, PAni can intercept the
charge relaxation process of CdSe QDs and interrupt the
radiative recombination process.
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Acknowledgments: This work was supported by Post-
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Natural Science Foundation of China (21403061,
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2
1173063, U1330106), Natural Science Foundation of
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Delivered by Ingenta to: University of New South Wales
IP: 193.105.171.111 On: Sat, 23 Apr 2016 19:02:09
Received: 15 July 2014. Accepted: 5 December 2014.
Copyright: American Scientific Publishers
J. Nanosci. Nanotechnol. 16, 3909–3913, 2016
3913