Structural, optical and electrical properties of PbS and PbSe [602238]

Structural, optical and electrical properties of PbS and PbSe
quantum dot thin films
E. M. El-Menyawy1•G. M. Mahmoud1•R. S. Ibrahim1•F. S. Terra1•
H. El-Zahed2•I. K. El Zawawi1
Received: 11 April 2016 / Accepted: 26 May 2016
/C211Springer Science+Business Media New York 2016
Abstract PbS and PbSe were prepared by hot injection
method. The powders were used for preparing the corre-
sponding films by using thermal evaporation technique.
The structural, optical and electrical properties of PbS and
PbSe thin films were investigated. The structural properties
of PbS and PbSe were investigated by X-ray diffraction,
transmission electron microscopy and energy dispersive
X-ray techniques (EDX). PbS and PbSe films were found
to have cubic rock salt structure. The particles size ranged
from 1.32 to 2.26 nm for PbS and 1.28–2.48 nm for PbSe.
EDX results showed that PbS films have rich sulphur
content, while PbSe films have rich lead content. The
optical constants (absorption coefficient and the refractive
index) of the films were determined in the wavelength
range 200–2500 nm. The optical energy band gap of PbS
and PbSe films was determined as 3.25 and 2.20 eV,
respectively. The refractive index, the optical dielectric
constant and the ratio of charge carriers concentration to its
effective mass were determined. The electrical resistivity,
charge carriers concentration and carriers mobility of PbS
at room temperature were determined as 0.55 Xcm,
1.791016cm-3and 656 cm2V-1s-1, respectively, and
for PbSe films they were determined as 0.4 Xcm,
991015cm-3and 1735 cm2V-1s-1, respectively.
These electrical parameters were investigated as a function
of temperature.1 Introduction
Lead chalcogenide bulk materials have received great
attention as semiconductors which are used in different
applications such as gas sensors [ 1], photoresistors,
chemical sensors, lasers, thermoelectric devices and other
electronic devices [ 2–7] such as photovoltaic devices
[8,9]. They are thermoelectric materials that can be used to
convert directly thermal energy to electrical energy. This
behaviour can be applied to generate electric power by
recycling waste heat [ 10]. Bulk PbSe and PbS are narrow
energy band gap, materials with high dielectric constant
and high carriers mobility, which leads to extensive com-
mercial applications as infrared detectors [ 11–13].
PbS and PbSe nanocrystals have attracted attention due
to their size-tuneable optical and electrical properties.
Their controlled size can be used in preparing modified
optical switches, light emitting diodes, telecommunications
and optical amplification [ 14–16]. Synthesis of PbS and
PbSe nanomaterials with different properties are important
for modern applications. PbS and PbSe have exciton Bohr
radius of 18 and 46 nm, respectively [ 17,18]. By reducing
the particle size of PbS and PbSe the direct band gap can be
widened to the visible region. PbS and PbSe nanocrystals
show strong electrons-holes quantum confinement, which
causes multiple exciton generation leading to an enhance-
ment in the photo-conversion efficiency of solar cells
[19–21].
PbS and PbSe films were prepared by many deposition
techniques such as chemical vapour deposition [ 22],
chemical bath deposition [ 23], pulsed laser ablation [ 24],
thermal evaporation [ 25], sputtering evaporation [ 26],
electron beam evaporation [ 27], molecular beam epitaxy
[28] and flash evaporation [ 29]. In the present work, it is
aimed to prepare PbS and PbSe nanomaterial by hot
&E. M. El-Menyawy
emad_elmenyawy@yahoo.com
1Solid State Electronics Laboratory, Physical Research
Division, Solid State Physics Department, National Research
Centre, 33 El-Bohouth St., Dokki, Giza 12622, Egypt
2University Girls College for Arts, Science and Education, Ain
Shams University, Helioples, Cairo, Egypt
123J Mater Sci: Mater Electron
DOI 10.1007/s10854-016-5080-6

injection method. The resulting powders were used to
prepare their corresponding films by thermal evaporation
technique. The structural, optical and electrical properties
of these films were investigated.
2 Experimental
2.1 Synthesis of PbS and PbSe nanocrystals
PbSe was synthesized by hot injection method as previ-
ously reported [ 30]. The synthesis was carried out in a
three-neck-flask and under argon controlled rate flow. In
typical synthesis, a mixture of 2.5 g of PbO, 7.5 g of oleic
acid (OA), and 18.5 g of 1-octadecene were heated to150/C176C. The temperature was raised to 180 /C176C and 1.5 g of
Se powder in 20 mL of trioctylphosphine (TOP) was
quickly injected. The flask was quickly removed from theheater and cooled down to 80 /C176C using a cold water bath.
25 mL of n-hexane was added until the temperature
dropped to 30 /C176C. The resulting PbSe were isolated with
acetone then re-dispersed in chloroform and dried. For
preparing PbS nanomaterial the same procedure was fol-
lowed except that selenium powder was replaced by sul-phur powder (0.61 g).
2.2 InstrumentalThermal evaporation technique was used for the prepara-
tion of thin films on glass and quartz substrates by usingcoating unit model (Edwards E 306 A). The pressure inside
the chamber were pumped down to about 10
-5Torr before
starting the evaporation process. The thickness of both PbSand PbSe films was 115 nm. There films were used to study
the structural, optical and electrical properties. They were
deposited under vacuum at the same time.
Compositional analysis of the prepared thin films was
carried out by using scanning electron microscope (Qunta
FEG 250). The structural analysis of the films was carriedout by Bruker D8 diffractometer and CuK aradiation of
wavelength 1.5481 A ˚. The diffraction angle (2 h) ranges
from (10–100 /C176) with a speed of the detector 2 /C176per min.
The tube current and voltage were 30 mA and 40 kV,
respectively. The transmission electron microscope, (Joel
JEM 2100), was used to determine the morphology andparticle size of the films.
The optical properties of thin films were studied by using
spectrophotometer measurements (JASCO model V-570-UV-VIS-NIR), in the wavelength range 200–2500 nm.
Ohmic contacts for electrical measurements were made
by depositing indium (of purity 99.999 %) onto PbS andPbSe films. The electrical resistivity measurements were
achieved with high impedance electrometer (model,Keithly 617). An evacuated metallic cryostat (model
CROYO industry, USA) supplied with a heater and a
platinum sensor was used. An automatic temperature
controller (model, Lake Shore, 321, USA) connected to thecrystal was used for measuring the resistivity and Hall
effect as a function of temperature. For this purpose high
direct current power supply (model, Ealing, 014/029,England), was connected to an electromagnet (model,
Oxford, Newport Instruments, England). The gap width
between the magnetic poles is 4 cm. The reversal magnet
polarity for Hall-effect measurements is carried out by a
switch key.
3 Results and discussions
3.1 Structural properties
Figure 1shows the energy dispersive X-ray (EDX) spectra
of PbS and PbSe thin films. The spectrum of PbS thin films
shows Pb:S ratio as 48.57 : 51.43 respectively. This indi-cates that the PbS films have slight excess sulphur. The ratio
of Pb:Se is 58.11:41.89, for which Pb is higher than Se in
PbSe nanomaterial. It is expected that Se has high volatilitycompared with Pb and there is a difference in the conden-
sation coefficients of Pb and Se [ 31]. This may give n-type
conductivity due to Se deficiency which agrees with [ 32].
The X-ray diffraction (XRD) patterns of as-deposited
PbS and PbSe thin films are shown in Fig. 2. The broad
peaks are related to the small crystallites due to nanos-tructure. The XRD PbS pattern shows some peaks which
were indexed to be of cubic rock salt structure according to
JCPDS card No. 05-0592. These peaks are characteristicpeaks corresponding to (111), (200) and (220) lattice
planes where (111) are the predominant plane. The XRD
spectrum has also two high intensity peaks of sulphur,which correspond to (113) and (112) lattice planes as
confirmed with JCPDS card No. 89-6764 (Table 1). From
the EDX data there is excess sulphur which agrees withXRD data. The PbSe films have also cubic rock salt phase
as matched with JCPDS card no. 65-2941 (Table 2). It is
seen that, there are three characteristics peaks of PbSe thinfilms. These peaks correspond to (200), (220) and (111)
lattice planes where (200) are the predominant plane.
The lattice parameters ( a) of cubic structure are calcu-
lated for PbS and PbSe films using the following formula
[33]:
a¼d
hklh2țk2țl2/C0/C1 0:5ð1Ț
where dis the inter planar lattice spacing and ( hkl) are the
Miller indices. The lattice parameters were calculated for
PbS as 5.9305 A ˚which agrees with standard JCPDS card
No. 05-0592 (5.9362 A ˚). The lattice parameter of PbSeJ Mater Sci: Mater Electron
123

thin films is deduced as 6.162 A ˚which is comparable that
the standard data of JCPDS card No. 65-2941 (6.134 A ˚).
The slight deviation of the lattice constant can be related to
the presence of the lattice strain in the films [ 34] and the
difference in the ratio of Pb and Se as obtained from EDXresults.The transmission electron microscopy images of PbS
and PbSe films are shown in Figs. 3and4, respectively.
The micrographs of PbS show that, the films are homo-
geneous, continuous, well grown with well oriented
nanoparticles in the form of rows with size ranging from1.32 to 2.26 nm for PbS as shown at the inset which con-
firms the nanosized material. The parallel rows of
nanoparticles are seemed to be the planes of the lattice. ForPbSe films, particles with diameter in the range of
1.28–2.48 nm were obtained. The low particle size of PbS
and PbSe indicate that the films are composed of quantumdots. The electron diffraction of both films shows spotty
rings indicating crystalline nature of the nanoparticles
which support the XRD data.
3.2 Optical properties
The optical transmittance, T, and reflectance, R, spectra for
PbS and PbSe thin films deposited onto quartz substrates
are shown in Fig. 5. The two films show strong absorption
at low wavelengths in which the transmittance edge of PbS
lies in the ultraviolet region, whereas that for PbSe lies in
the visible light region. Above the transmittance edges, thefilms show weak absorption and tend to be transparent at
Fig. 1 EDX spectra of aPbS
andbPbSe films
Fig. 2 X-ray diffraction patterns of PbS and PbSe filmsJ Mater Sci: Mater Electron
123

high wavelengths. In this region, the reflectance of PbSe is
higher than that for PbS, and the transmittance of PbSe is
lower than that for PbS.
The absorption coefficient, a, of PbS and PbSe films is
estimated using the expression [ 35]:
a¼1
dlnð1/C0RȚ2
2Tțffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ð1/C0RȚ4
4T2țR2s 2
43
5 ð2Ț
where dis the film thickness. Figure 6shows the absorp-
tion coefficient spectra of PbS and PbSe films. The main
observation is that the two films have high value ofabsorption coefficient ( *10
5cm-1) in strong absorption
region, which is higher for PbSe films than PbS ones.
Information concerning the band structure can be obtainedby the analysis of absorption coefficient spectra. The nature
of inter-band electronic transition through the optical
energy band gap can be determined by using Tauc’s rela-tion [ 36]:ahm¼Bhm/C0E
g/C0/C1Yð3Ț
where Bis a constant, his the Planck’s constant, mis the
frequency of the photon and Yequals to 1/2 and 2 for direct
allowed and indirect allowed band gap, respectively. Theband gap width of PbS and PbSe can be determined by
extrapolating the straight line of ( ahm)
2versus hmcurve at
a=0 as shown in Fig. 7. The direct band gap of PbS and
PbSe films are calculated as 3.25 and 2.20 eV, respectively
which were larger than that for corresponding bulk mate-
rials as 0.41 and 0.29 eV, respectively [ 37]. The properties
of nanomaterials differ from those of bulk structure sinceTable 1 Crystalographic data
of PbS filmsExperimental Standard data
(JCPDS card No. 05-0592) (JCPDS card No. 89-6764)
a=5.936, P-1 PbS S
d-spacing
(A˚)I/Io(%) d-spacing
(A˚)I/Io
(%)h k l d-spacing
(A˚)I/Io
(%)hkl
4.080 160.02 4.027 385 1 1 2
3.664 131.21 3.616 27 1 1 33.424 100 3.4290 84 1 1 12.982 26.42 2.9690 100 2 0 02.103 21.76 2.0990 57 2 2 01.794 9.32 1.7900 35 3 1 11.714 6.21 1.7140 16 2 2 21.487 3.88 1.4840 10 4 0 01.364 3.88 1.3620 10 3 3 11.329 5.44 1.3270 17 4 2 0
Table 2 Crystalographic data of PbSe films
Experimental JCPDS card (No 65-2941)
a=6.134, Fm–3m
d-spacing (A ˚) I/I o(%) d-spacing (A ˚) I/I o(%) h k l
3.541 25.9 3.5414 360 1 1 1
3.081 100 3.0670 999 2 0 02.164 36.6 2.1687 672 2 2 01.849 9.24 1.8494 162 3 1 11.770 11.6 1.7707 227 2 2 21.371 8.97 1.3716 257 4 2 01.250 5.05 1.2521 179 4 2 2
Fig. 3 HRTEM images of PbS thin filmJ Mater Sci: Mater Electron
123

the sizes of the crystallites become comparable to the Bohr
exciton radius. The calculated Egvalue of PbS is compa-
rable with that obtained for PbS nanoparticles prepared by
chemical method [ 38]. However, the obtained value for
PbSe is lower than that prepared recently [ 39] which can be
attributed to the dependence on the particle size and/or the
excess lead content.
The refractive index, n, of PbS and PbSe thin films was
estimated using the expression [ 40]:
n¼1țR
1/C0Rțffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
4R
ð1/C0RȚ2/C0k2s
ð4Țwhere kis the extinction absorption coefficient ( k=ak/
4p). The refractive index spectra of PbS and PbSe films are
shown in Fig. 8. It is observed that the refractive index of
PbSe films is much higher than that of PbS films. Therefractive index decreases with increasing wavelength in
weak absorption region showing normal dispersion.
The dispersion has a great importance in studding the
optical properties of different materials, which it is con-
sidered as the main factor in optical communication and in
designing devices for spectral dispersion. The normal dis-persion is analyzed according to the expression given by
[37]:
n
2¼e1/C0e2
4p2eoc2N
m/C3k2ð5Ț
where e?is the optical dielectric constant, eis the charge
of electron, Nis the free charge-carrier concentration, eois
the vacuum permittivity, m*is the effective mass of the
electron and cis light velocity. From the extrapolation of
the straight line portion of n2versus k2plot at longer
wavelengths as shown in Fig. 9, the ratio of N/m*can be
determined for PbS and PbSe thin films by obtaining the
slopes of these lines. Table 3shows that the optical
dielectric constant and the ratio N/m*for PbSe films are
higher than the corresponding values for PbS films.
3.3 Electrical properties
The electrical measurements of PbS and PbSe films
including the electrical resistivity ( q), the carrier concen-
tration ( N) and carriers mobility ( l) were obtained using
Hall effect experiment. The measurements were performedfor PbS and PbSe films with thickness of 115 nm. The
logarithm of resistivity versus 10
3/T for PbS and PbSe
films are shown in Fig. 10. The electrical resistivity at
Fig. 4 HRTEM images of PbSe thin film
Fig. 5 The transmittance and reflectance spectra of PbS and PbSe
filmsFig. 6 The variation of absorption coefficient ( a) with photon energy
for PbS and PbSe filmsJ Mater Sci: Mater Electron
123

room temperature of PbS and PbSe are 0.55 and 0.4 Xcm,
respectively. The resistivity of both films decrease withincreasing temperature. The relation can be fitted with
straight line according to Arrhenius equation. The activa-
tion energy was calculated as 0.18 and 0.22 eV for PbSand PbSe thin films, respectively. Figure 11shows that the
PbS and PbSe carriers concentration are slightly affected
by heating from temperature 300 to 320 K then itdecreased at 340 K followed by a saturation at about
5910
17cm-3for PbS and 4 91015cm-3for PbSe . The
PbS and PbSe carriers concentration at room temperatureare 1.7 910
16cm-3and 9 91015cm-3respectively. The
carriers mobility ( l) of PbS at room temperature is
656 cm2V-1s-1which was affected with heating between300 to 320 K then increases fo llowed by nearly constant
value as shown in Fig. 12. For PbSe the carriers mobility
at room temperature is 1735 cm2V-1s-1and is nearly
unaffected by heating in the studied temperature range.
The carriers mobility of PbS and PbSe films are higherthan the previous data [ 41,42] which can be related to the
nonstoichiometery continuity and homogeneity of the
films as confirmed by TEM study. The effective mass ofPbS and PbSe is determined from the ratio N/m
*after
substituting the experimen tally obtained values of the
carriers concentration. Th ee f f e c t i v em a s so fP b Sa n d
PbSe is determined as 3.2 910-34and 1.03 910-35kg,
respectively. These values a re low compared to published
values which can be attributed relatively to high mobilityof the films.
Fig. 7 The variation of ( ahm)2
with photon energy for PbS and
PbSe films
Fig. 8 The variation of refractive index with wavelength for PbS and
PbSe filmsFig. 9 The variation of n2withk2for PbS and PbSe filmsJ Mater Sci: Mater Electron
123

4 Conclusion
PbS and PbSe quantum dot thin films with particle size less
than 3 nm were successfully prepared on glass and quartz
substrates with thickness of about 115 nm. PbS films growwith slight sulphur content, whereas PbSe grow with
excess lead content. It was found that the structures of PbS
and PbSe thin films are related to rock salt cubic structure.PbS showed preferred orientation along (111) direction,whereas PbSe films showed preferred orientation along
(200) direction. The direct energy band gap of PbS and
PbSe films are calculated as 3.25 and 2.20 eV, respec-
tively. The electrical resistivity, carriers concentration andcarriers mobility of PbS films were determined 0.55 Xcm,
1.7910
16cm-3and 656 cm2V-1s-1and for PbSe films
were 0.4 Xcm, 9 91015cm-3and 1735 cm2V-1s-1,
respectively. Correlating the structural, optical and elec-
trical properties with each other, low particle size of both
films result in relatively high value of optical band gap
compared to bulk material. The nonstoichiometery of the
films is responsible for obtaining high mobility and loweffective mass values.
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