PbS spontaneous ?precipitation in order to form a

PbS is an
important binary IV-VI semiconductor material with a
rather small band gap (0.41 eV at 300K) and relatively large excitation Bohr
radius (18-20 nm) 1, which results in good quantum confinement
of both holes and electrons in nanosized structures 2. These inherent properties make PbS one of the
most important functional materials used in as thin films for several
applications such as IR detectors 3, photovoltaic cells 4, thin films transistors 5, LED 6, gas and biosensors 7-12 and photonic crystals 13.

In recent years, various
techniques have been used to deposit PbS thin films including
microwave assisted chemical bath deposition 2, successive ionic layer adsorption and reaction
(SILAR) technique 14-17, atomic layer epitaxial process
18, pulse electro deposition 19, spray pyrolysis 20 and chemical bath deposition.

Chemical bath deposition? ?method, also
called chemical solution deposition ?technique, has become an attractive method due
to many reasons, including ?low cost, no requirement of sophisticated
instruments, freedom?
?to deposit ?materials
on a variety of substances, suitability for large scale deposition? ?areas, and ability of tuning thin film properties? ?by adjusting and controlling ?the deposition
experimental? ?parameters. 21 ?

It was realized that changing CBD parameters
such as temperature, ?deposition time 
and solution composition leads to nanoparticles with ?different
sizes and shapes 22, which change the value of the band gap with ?respect
to the effective mass model.?

CBD process uses a controlled chemical reaction
to achieve thin film ?deposition by precipitation. It is necessary to
eliminate spontaneous ?precipitation in order to form a thin film 23.
Chemical deposition of films ?on solid substrates can take place via two major
mechanisms.?

The first mechanism is ion-by-ion mechanism, in
which the film is formed ?by sequential ionic reactions. If the reaction
progresses in alkaline medium, ?a complex agent is required to prevent the
formation of hydroxide ?precipitates 24.?? ?If the complex concentration is not adequate to completely
prevent ?formation of metal hydroxides, cluster mechanism occurs. In
this case, a ?small amount of colloidal hydroxide will be
formed, which then reacts with ?anions generated in the bath and produces the
final product. On the other ?hand, research has shown that the dominant
mechanism of deposition is ?dependent on the reaction conditions and
changing the dominant ?mechanism during deposition is possible 24.?

Each mechanism ?leads to
different particle size and morphology, which affects films ?properties.
Therefore, for synthesis of a film with specific properties, we ?should
be able to predict the effect of deposition parameters on reaction ?mechanism.
We have previously shown that deposition temperature is ?effective
in determining dominant deposition mechanism 26.

PbS thin films were deposited on clean,
spectroscopic glass substrates at ?different
deposition time (30 to 150 min). All the reagents were purchased ?from Merck Chemical Co. and were
used without further purification. ?

According to previous studies, the aging of
precursor solutions will affect ?deposition
rate 27??.?? Therefore, fresh precursor solutions were
utilized to ?remove probable noise caused by
aging. Prior to deposition, substrates were ?cleaned with the cleaning procedure of Obeid et al. In brief, substrates
were ?washed with hot distilled water,
immersed in 20% HCl for 24 hr, and washed ?with acetone. Then the substrates were cleaned ultrasonically with DI
water for ??20 min 2. ?

To prepare the reactive solution, 40 mL of
0.146 M NaOH and 100 mL of DI ?water
were mixed. After drop wise addition of 8 mL of 0.175 M lead nitrate to ?the stirring mixture, pure N2 was
passed through the reaction solution for 1 hr ?in order to diminish levels of dissolved O2 and CO2. Then 8 mL of 1 M ?thiourea solution was then added
to reaction mixture. Finally, the clean glass ?substrates were placed in the solution at 70ºC with respect to the
horizon using ?the Plaxi holder to prevent large
particles from adhering to the growing film. ?The samples were taken out after deposition time (30, 60, 90,120 and 150
min) ?rinsed with DI water and then air
dried. The grayish obtained films were well ?adherent to the substrate and homogenous. The reactions process for
synthesis ?of lead sulfide films through ion
by ion and cluster mechanisms have been ?previously reported28, 29.?

Structural characterizations of the films
were determined by X-ray diffraction ?method using a Philips PW3710 at room temperature with Cu K? radiation (?
??= 1.5405 ?A, Time/step=0.5S, Step
Size=0.02). In order to determine crystallites ?size from XRD, the Scherrer formula was used. Field emission gun scanning
?electron microscopy (FE-SEM)
studies were carried out using a HITACHI S-??4160 microscope, in order to determine the morphology of the films. Film ?thickness was measured from cross
sections while surface topography was ?observed in plans view. The surface morphology of the thin films was ?characterized with an Auto probe
CP (Park Scientific Instruments) scanning ?electron microscope. AFM imaging was performed under ambient conditions ?using commercial Si3N4 cantilevers
in contact mode at a scan rate of 1 Hz. The ?optical transmittance and reflectance spectrum were recorded on a Perkin
Elmer ?Lambda950 spectrophotometer in the
wavelength range of 200-3100 nm. ?

Results and Discussion

Figure 1 shows the XRD pattern of a PbS film
deposited on a glass substrate ?at room temperature for
30 to 150 minutes. As shown in Fig. 1, with ?increasing
deposition time, the intensity of the peaks and the crystallinity of ?the films increased. This issue can be attributed to
increased thickness and ?increased particle size with
increasing reaction time.?? ?

According to the identification with X’pert
HighScore software, all reflections ?corresponding to
rocksalt phase of PbS (JCPDS powder diffraction file #5-??0592).
The absence of any other diffraction peaks indicates that no other ?crystalline phases, such as oxides or carbonates of Pb,
exist with detectable ?concentration within the layers.?

The XRD spectra indicate an increase in grain
size with increasing deposition ?time, and a gradual
transition to <100> texture, which likewise strengthens ?with
deposition time.

The evolution of the film topography with
deposition time is illustrated by ?AFM surface plot
images shown in Fig.2??.?
Fig. 2a displays the initial nucleation stage, whereas the subsequent images ??(Fig. 2b–d) show films which gradually developed with
increasing ?deposition time. The plot of the surface
roughness vs. deposition time for ?layers deposited at
room temperature shown in Fig. 2f indicates that simultaneously with complete
shift of particle shape, the roughness ?value of the
film increases sharply from 20 nm to 65 nm;?? which
falls back to around 30 nm?, with further increasing
deposition time. Similar behavior ?has been reported
for samples deposited at lower deposition temperatures (10 ??ºC)
on the GaAs (100) substrate, however, due to the lower deposition ?temperature, these changes occurred over longer periods of
time30. ?

In the period from 90 to 120 minutes, island
growth has occurred, resulted in a ?significant
increase surface roughness of the film, but over a period of 120 to ??150 minutes, the growth process has progressed through
layer by layer growth ??(Frank–van der Merwe) resulted
in a significant reduction in RMS.?

Sample deposited for 30 min (Fig. 3a) showed a
discontinuous nano-crystalline ?film consisting of
round particles with typical size of 100 nm. Increase of the ?deposition
time to 60 min (Fig. 3b) resulted in relatively continuous and dense ?film. In addition nuclei with typical 20-30 nm have appeared
on the primary ?film. Within 90 minutes a well
adherent, dense compact layer which covers the ?entire
substrate surface was achieved (Fig. 3c) the first signs of change in ?particle shape have appeared in this stage. Due to the
compactness of the film, ?distinguishing of particle
boundaries and determination of particle size are ?difficult.
Further increase in deposition time to 120 min (Fig. 3d) results in ?complete transition to faceted cubic particles with typical
size of 500 nm. The ?boundaries of particles are quite distinctable,
it can be attributed to ?the ?dominance
of columnar growth (versus layer by layer growth) at this stage. ?Further increase in deposition time to 150 minutes, was not
varied the film ?morphology significantly.?

Figure 3f
shows the thickness the film as a function of the deposition time. ?Change in the growth rate with the reaction time is
illustrated by this curve. ?Different slope of the
graph represent the different stages of the reaction. The ?initial
slope can be attributed to the nucleation stage or incubation time; at this ?stage, as ?the time increases, the
thickness increases slightly because the ?primary ?nuclei are forming. The formation of these nuclei provides
fast growth ?rate of the film during the next stage. In
the third stage, due to the depletion of ?the reaction
solution from the reactants, the deposition rate is less than the ?previous stage.? Each deposition
mechanism has a characteristic growth rate, grain size and shape, which
directly affect the nature and properties of the films 21. Hence, it can be concluded that the changes
in the deposition rate and particle shape is due to the transition in
deposition mechanism. On the other hand, it is well understood previously that
cluster mechanism has higher growth rate; so, the high rate of deposition in
the second stage (60-90 min) can be attributed to the dominance of cluster
mechanism.

The
deposition rate declines, tendency to form larger particles and columnar ?growth in the third stage of deposition are evidences to
transition from the ?cluster growth mechanism in the
initial stages of growth to ion-by-ion growth.?

In fact,
this time-dependent transition from cluster to ion-by-ion mechanism is ?expected due to depletion of lead ions in solution (e.g.
increase in complex-to-?metal ion concentration ratio)
as the reaction proceeds.?

The
gradual change in film morphology, accompanied by enhancement of (200) ?preferred orientation, occurs with increasing film
thickness.? This observation consistent with the AFM
results; with increasing deposition ?time from 90 to
120 minutes, roughness of the samples increases sharply (from ?about
20 to about 65 nm), which can be attributed to the columnar growth, ?whereas the columnar growth is characteristic of the
ion-by-ion mechanism, it ?would be suggested than after
90 min from the beginning  of the
reaction active ?mechanism altered to ion-by-ion.?

Deposition
mechanism depends on the reaction conditions and specifies the ?product characteristics. Previous studies on the PbSe films
deposited via CBD, ?revealed that texture development
which observed with increasing thickness is ?also
related to the change of the dominant mechanism 31. The results of this ?study show that increasing deposition time leads to (200)
texture ?developments.?

Conclusions

The thin film of lead sulfide was deposited on
a glass substrate using the CBD ?method for different deposition times. It was observed
that morphology of the ?samples
depends on deposition time. As the deposition time increases, the ?shape of the ?particles
changes from round to cubic, texture ??(200) ?develops Furthermore the roughness of the film changes ?during the
deposition. These ?changes
are attributed to the change in the dominant deposition mechanism. ?This study
showed that deposition time is an important parameter in ?determining the dominant mechanism of
deposition and consequently the ?characteristics of the film.?

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