The picosecond kinetic of luminescence in conglomerations of hydrophilic colloidal CdS quantum dots
with an average diameter of 2.5 nm in gelatin was investigated. It was observed in the recombination luminescence
band with a maximum at 580 nm. A complicated character of depending in the time interval
from 300 ps to 1800 ns was found. Obtained dependences were interpreted in terms of radiative recombination
at the donor-acceptor pairs (different sizes), complicated non-radiative transitions involving localized
charge carriers on deeper levels.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3527

Ambrozhevitch, S.A.

Katsaba, A.V.

Ovchinnikov, O.V.

Shatskikh, T.S.

Smirnov, M.S.

Vitukhnovsky, A.G.

SocioEconomic Challenges Journal (Sumy State University)

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE  
NANOMATERIALS: APPLICATIONS AND PROPERTIES 
Vol. 2 No 3, 03NCNN16(3pp) (2013) 
 
 
2304-1862/2013/2(3)03NCNN16(3) 03NCNN16-1  2013 Sumy State University 
The Picosecond Kinetic of Luminescence in Hydrophilic Colloidal CdS Quantum Dots 
 
M.S. Smirnov1,*, O.V. Ovchinnikov1,†, A.G. Vitukhnovsky2, S.A. Ambrozhevitch2, A.V. Katsaba2, 
T.S. Shatskikh1 
 
1 Voronezh state university, 1, Universitetskaya Pl., 394006 Voronezh, Russian Federation 
2 P.N. Lebedev Physical Institute of the Russian Academy of Sciences, 53, Leninskiy Pr., 119991 GSP-1, Moskow, 
Russian Federation 
 
(Received 19 May 2013; published online 31 August 2013) 
 
The picosecond kinetic of luminescence in conglomerations of hydrophilic colloidal CdS quantum dots 
with an average diameter of 2.5 nm in gelatin was investigated. It was observed in the recombination lu-
minescence band with a maximum at 580 nm. A complicated character of depending in the time interval 
from 300 ps to 1800 ns was found. Obtained dependences were interpreted in terms of radiative recombi-
nation at the donor-acceptor pairs (different sizes), complicated non-radiative transitions involving loca-
lized charge carriers on deeper levels. 
 
Keywords: Quantum dots, Auger recombination, Kinetics of luminescence decay, Donor-acceptor recom-
bination. 
 
 PACS numbers: 72.20.Jv, 78.68.Hc, 61.72.uj 
 
 
                                                        
* Smirnov_M_S@mail.ru 
† Ovchinnikov_O_V@rambler.ru 
1. INTRODUCTION 
 
Optical properties of colloidal quantum dots (QDs) 
are the basis for the development of one of the im-
portant areas of modern physics-nanophotonics. Practi-
cal interest in the photophysics of colloidal QDs and 
nanostructures with their participation is caused by a 
large range of practical applications for medicine, biol-
ogy, optoelectronics and photocatalysis [1-6]. 
Processes of radiation and non-radiation degradation 
of electronic excitations in quantum dots play the im-
portant role in the formation of optical properties. The 
absorption and luminescence spectra provide information 
about the energy structure of QDs "bands", structural and 
impurity defects [7,8]. Non-radiative processes are multi-
phonon relaxation of electronic excitations on the levels of 
structural and impurity defects, localized mainly on the 
interface of colloidal QDs; non-radiative Auger recombina-
tion involving free charge carriers [9] and charge carriers, 
localized on the defects [10]. 
Photoluminescence properties are largely deter-
mined by the synthesis method of QDs [1]. It is high-
temperature organometallic synthesis of colloidal QDs 
in a highly toxic solvent Trioctylphosphinoxide (TOPO). 
For this method there is predominantly "exciton" lumi-
nescence with a high quantum yield. The second meth-
od is to use sol-gel techniques in aqueous or aqueous 
ethanol mixtures in the presence of nontoxic polymer. 
These QDs have the luminescence associated with im-
purity-defect levels, and do not have the "exciton" lu-
minescence. 
This paper presents the results of a study of lumi-
nescence decay kinetics in colloidal CdS QDs in the 
gelatin in the time interval from 0.3 to 1800 ns. We tied 
to do model explanation of observed regulates. The cur-
rently known data about nature of luminescence cen-
ters and the picture of levels of structural and impurity 
defects in colloidal CdS QDs were considered. 
2. INVESTIGATION METHODS AND SIMPLES 
 
Investigation simples were colloidal CdS QDs, pre-
pared by sol-gel method in an aqueous solution of gela-
tin. This method was described in detail in [11]. The 
preparing gelatin sol of colloidal CdS QDs was applied 
to a quartz plate of 2  2 cm2 and dried. 
The identification of colloidal CdS QDs was made 
using (TEM) LEO ~ 912AB ~ OMEGA transmission 
electron microscope with an accelerating voltage of 
100 kV. Structural investigations were carried out by 
electron scattering method and the x-ray diffraction, 
using ARL X'TRA (Switzerland) diffractometer for Kα1 
copper. The absorption spectra of CdS QDs were ob-
tained using Shimadzu BioSpec mini spectrometer (Ja-
pan). The luminescence spectra were investigated with 
help of Ocean Optics Maya ~ Pro 2000 spectrofluorome-
ters. The luminescence decay kinetics was studied us-
ing PicoQuant TimeHarp ~ 100 TCSPC-system. QDs 
luminescence was excited by PicoQuant PDL 800-B 
pulsed diode laser (wavelength   405 nm, duration of 
a pulse — 75 ps). The presented results were obtained 
at 300 K, 77 K and 10 K. 
 
3. RESULT AND DISCUSSION  
 
3.1 The Structural Properties of CdS QDs  
Samples 
 
Fig. 1 inset 2 shows the distribution of CdS QDs in 
size, obtained by analysis of TEM images.  
The average value of the diameter of CdS QDs was 
2.5 nm. A half-width of the size distribution was  35 %. 
Analysis of reverse electron scattering and X-ray dif-
fraction (Fig. 1, inset 1) shows that the CdS QDs have a 
cubic lattice. Also the value of QDs size was obtained 
from half-width X-ray peaks using the Debye-Scherrer 
equation. It was shown that QDs have a size of a few 
nanometers. 
 M.S. SMIRNOV, O.V. OVCHINNIKOV, A.G. VITUKHNOVSKY, ET AL. PROC. NAP 2, 03NCNN16 (2013) 
 
 
03NCNN16-2 
 
 
Fig. 1 – Absorption and luminescence spectra of CdS QDs 
(d  2.5 nm). Insert 1 – X-ray diffraction image for CdS QDs, 
insert 2 – microscopic image and size distribution of CdS QDs 
 
3.2 Luminescent Properties of CdS QDs Samples  
 
For CdS QDs of average size (Fig. 1) we showed a 
broad absorption band with a special feature at 
3.20 eV. This feature is the result of contribution to the 
absorption of the most probable "exciton" transition. 
For synthesized colloidal QDs the volume of effective 
band gap is much greater than the band gap of cubic 
single CdS crystal (2.42 eV). 
A single broad band with a large Stokes shift 
(0.95 eV) of the luminescence band maximum relative 
to the exciton transition in absorption was a character-
istic feature of observed CdS QDs luminescence 
(Fig. 1). At the same time, the absence of significant 
contribution of the exciton luminescence band in the 
resulting spectrum shows inhibition of this lumines-
cence mechanism due to the rapid capture of photo car-
riers by defect-impurity traps. 
Preliminary analysis of the size dependence of lu-
minescence bands for CdS QDs in [7] and QDs synthe-
sized by sol-gel technique in gelatin [11] showed that 
the maximum of the observed luminescence band is 
located between two bands of donor-acceptor recombi-
nation. These bands are characteristic for CdS. This 
comparison indicates that the observed photolumines-
cence band not elementary. And its appearance is the 
result of radiative recombination (D-A) pairs. According 
to [7] recombination is implemented with the participa-
tion of shallow electron (~ 0.1 eV) and deep hole traps. 
The bands associated with band-impurity transi-
tions (D-h and e-A) completely absent for available av-
erage size of QDs. In our case, it should be in the dark 
blue and blue-green spectrum regions.  
Below we discuss the results of investigation of CdS 
QDs luminescence kinetics in the range 450-700 nm. 
 
3.3 General Regularities of Luminescence  
Decay Kinetic in Colloidal CdS QDs  
 
CdS QDs luminescence decay kinetic at 300 K is 
shown in Fig. 2. From the observed dependence we can 
do the following conclusions: 
 The luminescence decay time for CdS QDs 
(d  2.5 nm) for all investigated wavelength is in the 
nanosecond range. 
 The luminescence decay kinetic slows down as we 
move to longer wavelengths. 
 These dependencies are not described by one or 
two exponential functions. This indicates a complex 
pattern of recombination transitions in hydrophilic CdS 
QDs. 
 There is a transformation of the luminescence 
spectrum of CdS QDs with the luminescence decay. 
There is long-wave shift of the maximum of the lumi-
nescence band with the luminescence decay. This shift 
reaches a value of 0.36 eV for the decay time of about 
1000 ns. 
 
 
 
Fig. 2 – The luminescence decay kinetic in CdS QDs at 300 K 
 
3.4 Temperature Dependence of the Picosecond 
Kinetic of Luminescence in CdS QDs 
 
The luminescence spectra and luminescence decay 
kinetic of colloidal CdS QDs are noticeably changed 
when the temperature is decreased. 
Changes in the luminescence spectra can be summa-
rized as: 
 The temperature decrease leads to a slight and 
non-monotonous change of position of the luminescence 
band maximum. At 80 K the maximum is shifted by 
15 nm to longer wavelengths, and at 10 K it is shifted 
by 10 nm back. 
 The luminescence quenching is observed when the 
temperature increase. It begins to manifest at 50 K. The 
luminescence intensity decreases by a factor of 3 at the 
temperature of 10-300 K. 
The luminescence decay kinetic with decreasing 
temperature changes along the time scan range of 
0.3 ns to 1800 ns. For normalized per unit of kinetics 
there are several features. 
 Luminescence decay kinetics are complex. They 
can be described by superposition of 5 exponents. 
 A view of the luminescence kinetic can clearly dis-
tinguish two regions. They are "fast" (at times up to 
50 ns), and "slow" (at times more than 150 ns). The fast 
component has the greatest statistical weight for the 
short-wave part of the spectrum. With decreasing tem-
perature, its contribution to the total kinetic picture is 
noticeably increased for all wavelengths, particularly 
for 450 nm and 500 nm (Fig. 2, 3). 
 The statistical weight of the slow component with 
decreasing temperature (for times  50 ns) decreases 
from 0.1 at 300 K to 0.01 at 77 K and to 0.001 at 10 K. 
 For the slow component the time constant is 
aligned with decreasing temperature to 10 K in the case 
of luminescence kinetics, measured for all wavelengths 
(Fig. 3).The kinetics are parallel. 
 THE PICOSECOND KINETIC OF LUMINESCENCE… PROC. NAP 2, 03NCNN16 (2013) 
 
 
03NCNN16-3 
 
 
Fig. 3 – The luminescence decay kinetic in CdS QDs at 10 K 
 
3.5 Discussion of observed kinetic regularities  
 
The main feature was a slowdown of luminescence 
decay kinetic along the time scan range when we move 
to longer wavelengths. It leads to long-wave shift of the 
emission spectrum as damping. This regularity can 
have several reasons: 
– The recombination mechanism. The center of re-
combination luminescence in QDs CdS, as noted in [8], 
is the donor-acceptor. This qualitative dependence is 
characteristic for such luminescence mechanism. It is 
due to the recombination of donor-acceptor pairs located 
at different distances from each other for variety quan-
tum dots. As is known, the donor-acceptor nature of the 
emission allows us to estimate the energy of the Cou-
lomb interaction between donor and acceptor in the 
pair.  
 
2
lum g d a
d a
e
E E E
r r


   

, (3.1) 
 
where gE  is the band gap energy, dE  is the ionization 
energy of donor, aE  is the ionization energy of 
acceptor, lum  is the energy of emitted photon, e  is 
the electron charge, d ar r  is the distance between 
donor and acceptor,   is permittivity. 
For QDs of 2.5 nm, the Coulomb interaction energy 
was in the range from 0.30 eV (for distances donor - 
acceptor of 0.5 nm) to 0.06 eV (for a distance of 2.5 nm). 
Then, the maximum shift of the luminescence spectrum 
can reach the value of 0.24 eV. It is of the order of mag-
nitude as the observed changes. In addition, D-A pairs 
at different distances have different recombination time 
[12]. It leads to complex no-mono exponential depend-
ence. 
– The size distribution of QDs in the conglomeration, 
which determines the position of the luminescence spec-
trum and its half-width. The luminescence decay time 
for QDs of different sizes in the test conglomeration is 
different. It leads to a complication of the decay kinetic. 
– Interaction of QDs in the conglomeration during 
light absorption and transfer of electronic excitation en-
ergy from the luminescence centers of smaller QDs to the 
luminescence centers of larger QDs with the participa-
tion of several phonons [13]. Despite the fact that the 
centers have the same structure. Then the luminescence 
kinetic at long wavelengths should be the slowest. We 
observe it in the experiment. 
– Non-radiative Auger recombination involving lo-
calized charge carriers. If the excitation power of used 
semiconductor laser is 5 mW at 4  107 pulses/s, irradia-
tion area of the sample is 4 mm2, the absorption coeffi-
cient is 104 cm – 1, then an electron-hole pair will be born 
in one of 104 QDs. Then the probability of Auger process 
involving two excitons in a single QDs is negligible. On 
the other hand, as a result of Auger recombination the 
electron can transfer energy to electron, localized on 
acceptor. Luminescent centers will this acceptor. 
– The slow component of luminescence decay can be 
caused by the trapping of electrons into shallow traps. 
They are not part of the luminescence center. Then for 
recombination electron must overcome the potential 
barrier. It will be accompanied by a slowing of the lu-
minescence kinetic with lowing temperatures. The mac-
roscopic local states due to a jump of the dielectric con-
stant on «QD-matrix» boundary can be these shallow 
traps.. Usually they are small with ionization energy 
less than 0.1 eV [14]. 
Apparently, in the case of hydrophilic CdS QDs in 
all of the above processes are implemented with some 
probably. 
 
AKNOWLEDGEMENTS 
 
The work was supported by RFBR project №12-02-
90827-mol-rf-nr and partially supported by RFBR pro-
ject №12-02-31735-mol-a. 
 
 
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The Picosecond Kinetic of Luminescence in Hydrophilic Colloidal CdS Quantum Dots

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The Picosecond Kinetic of Luminescence in Hydrophilic Colloidal CdS Quantum Dots

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