International Research Journal of Pure &
Applied Chemistry
4(1): 97-107, 2014
SCIENCEDOMAIN international
www.sciencedomain.org
Effect of Growth Temperature on Chemical
Synthesis of PbS quantum dots
P. K. Kalita1*, B. Das1 and R. Devi1
1
Department of Physics, Nanoscience Research Laboratory, Guwahati College,
Guwahati-781 021, Assam, India.
Authors’ contributions
This work was carried out in collaboration between all authors. Author PKK designed the
study, managed the analyses of the study and wrote the protocol as well as the manuscript.
Author BD had done the experimental and characterization work and author RD managed
the literature searches and made relevant discussion. All authors read and approved the
final manuscript.
th
Original Research Article
Received 24 June 2013
th
Accepted 13 August 2013
th
Published 11 October 2013
ABSTRACT
PbS quantum dots synthesized at room temperature through CBD technique show a
large blue shift of absorption edge due to formation of very small particle size of the order
of 2nm confirmed by HRTEM measurement. It exhibits a further blue shift when
synthesized in acidic medium (pH 5.5) whereas a red shift for those synthesized in
alkaline medium (pH 9.5) on rise of growth temperature up to 355K. PVA is found to play
a key role in determination of quantum confinement. At high bath temperature the
additional oxide phases that come into surface are also expected to influence the
optoelectronic properties. Photoluminescence clearly exhibits UV emission at 360nm
along with blue-green emissions at 470nm, 480nm, 490nm and 590nm owing to the
doubly and singly ionized oxygen defects. The electrical conductivity in PbS
nanocomposites may be attributed to the carrier tunneling process across the potential
barriers created primarily between the PbS and oxide nanoparticles. The IR spectroscopy
reveals the presence of oxides in PbS nanocomposites.
Keywords: PbS; lead oxides; high temperature bath; hyperbolic band structure.
___________________________________________________________________________________________
*Corresponding author: Email: pkkgc@yahoo.co.in;
International Research Journal of Pure & Applied Chemistry, 4(1): 97-107, 2014
1. INTRODUCTION
Lead chalcogenides PbS, PbSe and PbTe and their ternary compounds are of immense
interest in recent years because of their diverse applications in different type of IR detectors,
sensors and other IR electroluminescence devices [1-8]. Lead sulphide has a cubic rock salt
structure and a small band gap 0.41eV. Because of narrow band gap, it has large Bohr radii
18nm and small effective mass of electron and hole. It also possesses high carrier mobility
2 -1 -1
(0.44Cm V S ) and dielectric constant (17.3) [2,8]. These properties provide a third order
nonlinear optical response that makes PbS a suitable nanomaterial for optical and photonic
device applications [3]. Lead sulphide also have the capability of showing the multiple
exciton generation which in turn multiply can increase the photoconversion efficiency up to
66% in solar cell [4,8]. As PbS shows strong quantum confinement, therefore there is
enough scope for band gap engineering to develop noval nanomaterials suitable for
operating in visible to near infrared regions. Considerable efforts have been devoted to
develop the synthesis of PbS nanoparticles in a controllable manner. Most of the research
workers use organic as well as inorganic polymers for making strong matrix to stabilize the
particles. Polymers offer an active and flexible medium for organizing nanoclusters. The
covalent interaction arising from the surface functionality on the clusters and on the suitable
group on the polymer surface can provide a stable environment for regular dispersion of
shape oriented nanoparticles. In most of the cases the metal is introduced for polymer
support by the reaction of polymer bound functionalities with suitable metal precursors. Metal
precursors can subsequently and conveniently be reduced by varying growth parameters to
form either polymer supported nanoparticles or nanoparticles within the polymer. The
particles embedded in those polymers forms various nanocomposite structures where the
flexibility and easy processing can be profitably used for device fabrication. A variety of
chemical methods such as chemical vapour deposition (CVD), chemical bath deposition
(CBD), electrodeposition, spray pyrolysis, SILAR etc have been employed for preparation of
PbS nanocrystals [2-6,9-14]. Among those the CBD technique is widely used because of its
simplicity, inexpensive and convenient for large area deposition [4,10-12,15]. However there
are also possibilities to form a variety of oxides because of presence of dissolved as well as
atmospheric oxygen atoms if the synthesis is carried out in open environment. The different
oxide traces may co-exist with the prime product in general because of dissolutionprecipitation reaction of metal hydroxide in the precursor solution. Hence the study of growth
parameters mainly the deposition temperature and pH values along with the capping and
surfacent materials are of considerable interest to control the reaction mechanism to yield
stable and reproducible PbS nanostructures. In the present work PbS nanostructures are
prepared through CBD route and a brief report on the effect of bath temperature and pH on
the structural and optical properties has been highlighted.
2. EXPERIMENTAL DETAILS
PbS nanostructures are synthesized through chemical bath deposition method. Equivolume
and equimolar (1M) solution of lead acetate and thiourea were taken for the synthesis. A 3%
solution of poly vinyl alcohol (PVA) using as a capping agent was mixed with the lead salt
solution. The mixture solution was stirred under mild heating. Ammonia solution was then
added to form clear metal complex. The pH values were varied from 5.5 (acidic) to 9.5
(alkaline). Equal volume thiourea was the added drop by drop into that solution under
constant stirring to form the final precursor solution. The solubility product of PbS is very
small. Hence the control precipitation of PbS in the reaction bath is a critical aspect. The
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International Research Journal of Pure & Applied Chemistry, 4(1): 97-107, 2014
++
control of free Pb ions in the solution was done by using ammonia (NH4OH) as complexing
agent. The reaction mechanism can be stated as follows [11]
-
NH2CSNH2 + OH = CH2N2 + H2O + HS
-
HS- + OH = H2O + S
Pb
2+
-
-2
-2
+ S = PbS
2+
-2
When the ionic product of Pb and S exceeds the solubility product of PbS, precipitation of
PbS occurs in the precursor solution. On the other hand if the concentration of Pb and OH
ions in the solution exceeds some critical value, precipitation of PbO starts transforming
Pb(OH)2 into PbO nanocrystals.
Pb(OH)2 = PbO + H2O
In the present synthesis, lead oxide traces were also found with the PbS. Hence the
solubility of Pb(OH)2 which releases the Pb and OH ions are expected to be dependent
upon the bath temperature and pH of the solution.
For deposition of thin films of respective samples, the glass substrates were first kept
overnight in dilute hydrochloric acid then those were washed by rubbing with tissue papers
properly under running distilled water. The substrates were washed several times with
deionized water and finally rinsed in very dilute sodium hydroxide solution. The substrates
were again rinsed with distilled water and dried. Thin films of PbS were allowed to cast on
glass substrates by dip coating in the final matrix solution which was maintained at bath
temperature in the range 303K-355K. The structures of prepared PbS quantum dots were
characterized by XRD (X-Ray Diffraction) and HRTEM (High Resolution Transmission
Microscopy) whereas the optical properties were studied using UV-VIS absorption, PL
(Photoluminescence) and IR (Infra Red) spectroscopy. The I (current)-V (voltage)
measurements were done using a Aplab Picometer in gape type film geometry. A printed
circuit board having parallel copper films separated by 1mm was cleaned with sand papers,
rubbed with acetone and rinsed with deionized water. The PbS/PVA colloidal film was casted
upon two copper electrodes separated by 1mm to form a gape type configuration.
3. RESULTS AND DISCUSSION
3.1 XRD Studies
The X- ray diffraction traces of PbS thin films prepared in acidic and an alkaline medium are
depicted in Figs. 1 and 2. The diffractograms were recorded within the 2 range between 10º
to 80º through X-Ray Powder Diffractometer (Seifert XRD 3003) using Cu K radiation. The
traces show that the all films are polycrystalline having cubic rock salt type structure which is
confirmed from standard data (ICDD-PDF No:01-072-487). The prominent peaks are found
at (2) 26.05º, 30.15º and 43.25º corresponding to (111), (200) and (220) planes. Rock salt
type structure is quite common in chemically synthesized lead sulphide [5,7,9-12,14].
The XRD of room temperature grown PbS films clearly show a broad hump upon which the
diffraction peaks appeared. The broad hump may be attributed to the existence of some of
amorphous phase together with that of crystalline structures. On increasing the temperature
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International Research Journal of Pure & Applied Chemistry, 4(1): 97-107, 2014
of precursor solutions up to 355K, the hump completely disappeared and transforming the
material into a fair crystalline structure irrespective of growth medium either acidic or
alkaline. It is observed that the preferred crystal plane orientation are along (111) and (200)
for room temperature grown films. The other preferred plane along (220) comes into surface
after heat treatment. The diffraction intensity (counts/sec) also increases with the rise of
temperature. However the respective peaks are broaden for films grown in acidic medium.
But those become narrower in an alkaline medium along with the more numbers of chemical
byproducts as a result of fast heterogeneous reaction. It is observed that the heterogeneous
chemical reaction started on heat treatment of final matrix solution which is quite
unavoidable for synthesis in open air environment [15]. Because of the heterogeneous
reaction of residual lead complex ions with oxygen atoms from atmosphere as well as
dissolved oxygen, the lead oxides were formed. The XRD traces after heat treatment
indicate the formation of different prime phases of lead oxides viz: PbO2, Pb2O3, and PbO
[16-22]. Lead di oxides have two structures orthorhombic α-PbO2 and tetragonal β-PbO2
[16,18].
Fig. 1-3. XRD traces of PbS quantum dots synthesized in acidic (Fig. 1) and alkaline
(Fig. 2) medium rising bath temperature from room temperature (300K) to 355K;
HRTEM image of a typical quantum dot (Fig. 3)
In the present case -PbO2 is expected as one of product that can be understood following
its prominent peak positions at 22º and 34º corresponding to orthorhombic crystal planes
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International Research Journal of Pure & Applied Chemistry, 4(1): 97-107, 2014
(011) and (021) respectively. PbO2 is not a stable phase of lead oxide [18]. Therefore the
presences of co-existence of other oxide phases were also seen in the X- ray diffractograms
on rise of growth temperature. The intensity and broadening of diffraction peak
corresponding to (2) at 14º for PbO become prominent than those other oxides on
increasing the ambient temperature up to 355K. This clearly signifies the transformation of
stable PbO nanostructures from other forms of oxides. PVA is amorphous in general.
However it is expected to improve its crystallinity with the rise of temperature. The (2) at 20º
is therefore attributed to that of PVA [13].
The lattice constants for the synthesized PbS films were calculated and the average value
was found to be 5.919A which is in good agreement with that of bulk 5.92A. Assuming
overall line broadening due to size, the average crystallite sizes (D) are estimated according
to Debye-Scherrer formula [ 5,7,9-12]
D= k/  cos
where  is the full with at half maximum (FWHM) of the most preferred prominent peak along
(111) and K is a constant. The crystallite size are found to decrease from 13nm to 7nm for
films deposited at pH 5.5 whereas it is in reverse order from 16nm to 24nm for those
deposited at pH 9.5. This may be attributed to two competent physical processes viz;
nanograin growth governed by capping agent PVA and the rate of reaction on rising the
temperature of precursor solution. In the acidic medium as the reaction rate is slow it can
promote the nanograin growth rather than Ostowald reipening [10]. The seed PbS particles
are better confined because of higher mobility of functional groups or ligands of polymer at
high temperature. The lead oxides as a result of heterogeneous oxidation reaction are also
confined to have nanosize particles because of that similar effect. Alkaline media cause fast
chemical reaction which governs the grain growth mechanism and produces more by
products as a result of oxidation. As the XRD technique is not sensitive enough to measure
particle size, the sizes are not consistent with that of measured from HRTEM (JEM 2100)
image (Fig. 3) and optical absorption. The experiment clearly reveals that the synthesis in
open environment may be carried out at room temperature preferably in acidic medium to
yield good reproducible PbS nanocomposites. However it is observed that if the synthesis is
carried out in nitrogen or other inert gas environment, better quality nanostructured PbS may
also results even at high bath temperature in acidic medium.
3.2 Optical Studies
The absorption spectra of the prepared PbS were taken using a UV-VIS spectrophotometer
(Hitachi U3210) as shown in Fig. 4a & 4b. It shows that all samples irrespective of their
growth condition exhibit a large blue shift of absorption edge. The absorption edges are
found to be within the range 284nm-291nm which implies an enhancement of band gap from
0.41eV (bulk) to 4.44eV-4.55eV. This clearly indicates the occurrence of strong quantum
confinement in these synthesized PbS dots. Similar results are also found by other workers
[1-2]. Considering the hyperbolic band structure of PbS, the particle size can be estimated
from the equation [1,6]
E=[ Eg2 + 2h2Eg (/R)2/m*]1/2
where E is the increment in band gap energy, m* = 0.085me effective mass and Eg is bulk
band gap of PbS. The average particle size (2R) found from above is nearly 1.3nm which is
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well agreement with the HRTEM measurement. However there is a small change of particle
size with the change of pH and growth temperature as shown in Table 1.
Table 1. Variation of absorption edge, band gap and particle size in fresh and
annealed PbS quantum dots
pH
Temp. (K)
a (nm)
Eg (eV)
D (nm)
5.5 (acidic)
Fresh (303K)
289.6
4.47
1.34
Annealed (355K)
285.6
4.53
1.32
9.5 (alkaline)
Fresh (303K)
284.4
4.55
1.31
Annealed (355K)
291.2
4.44
1.35
HRTEM image (Fig. 3) shows spherical particles having average size about 2nm. In
chemical synthesis initial product molecules are called seeds which subsequently grow in
size in a thermodynamically controlled manner to form nanocrystallites. These
nanocrystallites exhibit the most of physical and chemical properties of the material.
However the crystallites have a tendency to grow further in size and if the growth
mechanism is not controlled, then due to Ostwald ripening and Van Der Waals interactions
between particles, they agglomerate to form bigger structures. Hence the particle size
distribution is never uniform. Scherrer formula also measures the average crystal size in
general that is least of agglomerated particle size. The agglomeration can be arrested by
either stabilizing electrostatically or by inducing steric hindrances at appropriate stages in the
precipitation reaction to achieve size selective synthesis. In the present case the steric
hindrance is expected to achieve by the adsorption of PVA molecules on the surface of the
particles. Most of the workers therefore suggest the hyperbolic band theory for a precise
measurement of size [1-2,6]. The present results agree with the reported values of particle
size less than 2nm when band gap rises to around 4.5eV [1,4]. This suggests that there is
strong coulomb interaction between PbS quantum dots.
Fig. 4. Absorption spectra of fresh and annealed PbS quantum dots synthesized in
acidic (a) and alkaline (b) medium
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It is also seen from absorption spectra that the absorption edge of 289.6nm for fresh room
temperature grown films in acidic medium is slightly further blue shifted to 285.6nm on rise
of growth temperature up to 355K whereas those films grown in alkaline medium red shifted
from 284.4nm to 291.2nm. The observation is quite consistent with the respective
broadening of XRD peaks. This type of reverse effect owing to simultaneous change of pH
and growth temperature is not seen earlier. However, the symmetric extended absorption
peak towards UV region may be attributed to the increase absorption at band-tail states of
PbS-PbO nanocomposites. The band-tail states are caused by the disorder/defects at the
lead sulphide and oxide surfaces. A detail study is indeed necessary to understand the
homogeneous growth mechanism of PbS quantum dots governed by polymer capped
precursor solution.
Fluorescence spectroscopy is a kind of electromagnetic spectroscopy which analyses
fluorescence from a sample. The photoluminescence spectrum of PbS nanocomposites
prepared in alkaline medium is shown in Fig. 5. A Hitachi f-2500 spectrophotometer with an
excitation wavelength 325nm was used for taking the spectrum. Photoluminescence clearly
exhibits UV emission at 360nm along with blue-green emission at 470nm, 480nm, 490nm
and 590nm. The UV emission at 360nm is originated from excitonic recombination
corresponding to blue shifted near band-gap emission of the PbS-PbO nanocomposites,
while the other emission peaks are usually referred to as deep-level or trap- state emissions.
It is noted that the UV emission PL peak is quite broad having higher FWHM and is red
shifted with respect to that of absorption edge. This difference in transition energy of
emission and absorption spectrum is the stokes shift which reveals the existence of defect
states in the material.
Fig. 5. Photoluminescence spectrum of PbS dots prepared alkaline medium; Inset
shows energy levels diagram
Native defects arise from positively charged oxygen vacancy (Vo) and lead interstitial (Pbi)
which are electron compensated as well as oxygen interstitial (Oi) and lead vacancy (Vpb)
which are hole compensated. Amongst these Vo is the most stable defect and acts as a
dominant donor species [19]. In the present work the strong blue emission at 470nm
corresponds to the doubly ionized oxygen vacancies in PbS-PbO nanocomposites. Other
emissions at 480nm and 490nm also result from the recombination of photogenerated holes
with the doubly ionized charge states of the oxygen defect [20]. The radiative recombination
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of photo-generated holes and electrons occupying the singly ionized oxygen vacancies (Vo)
may be the cause of that of green emissions at 590nm. An energy level diagram is given in
Inset of Fig. 5 where 3.49eV is the blue shifted band gap, 2.67eV and 2.13eV are the energy
levels corresponding to doubly and singly ionized oxygen defects. Similar PL emissions at
370nm, 473nm and 500nm was reported by A V Borhade et al. [20]
Fig. 6. Infrared spectra of PbS dots prepared in acidic (a) and alkaline (b) medium
The IR spectra of prepared PbS in acidic and alkaline medium are shown in Fig. 6. Spectra
were taken using a Perkin Elmer IR Spectrometer. It shows weak and medium Pb-S bonding
-1
-1
-1
-1
for 414cm , 424cm and 473.5cm . The spectra exhibit vibrational frequency 3445 cm for
-1
OH stretching, 2920 for CH2 stretching, 1595 cm corresponding to vinyl group of dispersed
-1
-1
PVA matrix in the precursor solution [3,10]. It also shows 1160cm and 1096 cm means for
improved crystallinity with CO stretching. The spectra also clearly shows the appearance of
-1
-1
-1
-1
additional vibrational harmonic frequencies 522.6cm , 668.6cm , 728.7cm , 767.9cm
which can be assigned to lead oxides[21]. These functional groups exist in all samples
irrespective of present growth conditions. Similar harmonics were also found by Pasha et al
[22] in mixed phase PbO nanocrystals.
3.3 Electrical Measurements
The I (current)- V (voltage) characteristics of PbS films were studied in the operating voltage
range -100V to 100V. The curves were linear in the range -20V to 20V and became nonlinear afterwards for higher applied bias as shown in the Fig. 7. Nonlinearity leads to a
rectifying nature at sufficient higher order bias. Linear region is attributed to the ohmic
conduction whereas the nonlinearity may arise from different mechanism. Nonlinear
behaviour mainly arises due to thermally activated process, space charge limited conduction
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International Research Journal of Pure & Applied Chemistry, 4(1): 97-107, 2014
(SCLC) process or tunneling effect [14,23-25]. Saraidarov et al studied [23] the I-V
characteristics of PbS hybrid films in sandwich type configuration and suggested that the
rectifying behaviour as a result of resonant tunneling process through the quantized
electronic states in the conduction band of PbS nanocrystals. As the surface of quantum dot
is very sensitive, the surface states dominantly determine the electronic properties. In the
present work additional potential barriers are expected to form across the interfaces of PbS
and those PbO nanoparticles.
Fig. 7. I-V characteristics at 300K (1) and 355K (2) of a typical PbS quantum dots film
prepared in acidic medium. Inset: Measurement technique (E-electrodes; F-film)
The surface states govern the potential barrier and introduce some series resistance in the
conduction path. There also exist inter grain boundary barriers that contain grain boundary
states. All these imperfection states act as traps as well as recombination centres and play
the key role in determination of conduction processes. In the present case more number of
recombination centres is predicted at the barrier of synthesized PbS-PbO nanocomposites
and as a result those lower the conductivity. In addition there exist pores, discontinuity and
other surface disorders especially in gap type film, which make conductivity becomes less
and as a result a high order of bias is usually required for conduction. At sufficient high bias
the enhancement of current and thereby the nonlinearity may be due to the carrier tunneling
process across the potential barriers created primarily between the PbS and oxide
nanoparticles.
4. CONCLUSION
PbS nanocrystals are synthesized through chemical bath deposition. The synthesized PbS
quantum dots are found to be polycrystalline having prominent planes oriented along (111),
(200) and (220) in cubic rock salt type structure. The crystallinity is improved with the rise of
growth temperature. The UV-VIS spectra exhibit clear large blue shifts that enhance the
band gap energy in the range 4.44eV-4.55eV. This indicates a strong quantum confinement.
The average particle size measured from hyperbolic band theory is around 1.34nm which is
consistent with the HRTEM measurement. A rise of growth temperature cause a blue shift of
absorption edge for PbS synthesized in acidic medium whereas it is red shifted for those in
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International Research Journal of Pure & Applied Chemistry, 4(1): 97-107, 2014
alkaline medium. The optical properties are in well agreement with the respective
broadening of XRD peaks. Photoluminescence shows band gap emission at 360nm. A
strong blue emission results at 470nm corresponds to the doubly ionized oxygen vacancies
and other a green emission at 590nm is attributed to singly ionized oxygen vacancies in
PbS-PbO nanocomposites The IR spectroscopy show the relevant functional groups along
with PbS stretching as well as the presence of lead oxides. The I-V characteristics show
ohmic conduction up to low applied bias +20V and thereafter nonlinear in the higher order of
applied bias. Nonlinearity may be attributed to the tunneling of quantized electronic states in
the conduction bands of mainly PbS quantum dots and across the barriers formed by the
interfaces between PbS dots and lead oxide nanoparticles.
ACKNOWLEDGEMENTS
The authors sincerely thank Indian Institute of Technology, Guwahati and Department of
Chemistry, Gauhati University for providing the XRD and optical characterization facilities
respectively.
COMPETING INTERESTS
Authors have declared that no competing interests exist.
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