Insight on the optoelectronics and enhanced dielectric properties of strontium decorated PbI2 nanosheets for hot carrier solar cell applications

Article abstract of DOI:10.1016/j.jallcom.2020.157762

Dielectric properties determine by electric field distributions play a decisive role in energy harvesting and storage applications. In this context, strontium decorated lead iodide nanosheets (Sr:PbI₂) are prepared, and its vibrational and dielectric properties are studied. The bandgap changes are explained by Brustein-Moss effect and renormalization process. Raman spectral studies reveal that introducing Sr atoms into PbI₂ lattice enhance the lifetime of LO phonon through bottleneck effect. Subsequently, the dielectric constant (ε′) values of 5 wt% Sr:PbI₂ are increased 20% than pure PbI₂. The results conclude that the vibrational properties of Sr decorated PbI₂ nanosheets are much significant for hot carrier solar cell devices.

Full text of DOI:10.1016/j.jallcom.2020.157762

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Insight on the optoelectronics and enhanced dielectric properties of strontium
decorated PbI nanosheets for hot carrier solar cell applications
2
S. Muthu Mariappan, Mohd. Shkir, T. Alshahrani, V. Elangovan, H. Algarni, S. AlFaify
PII:
S0925-8388(20)34126-8
DOI:
https://doi.org/10.1016/j.jallcom.2020.157762
JALCOM 157762
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To appear in: Journal of Alloys and Compounds
Received Date: 4 July 2020
Revised Date: 9 September 2020
Accepted Date: 28 October 2020
Please cite this article as: S.M. Mariappan, M. Shkir, T. Alshahrani, V Elangovan, H. Algarni, S.
AlFaify, Insight on the optoelectronics and enhanced dielectric properties of strontium decorated
PbI nanosheets for hot carrier solar cell applications, Journal of Alloys and Compounds, https://
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Insight on the optoelectronics and enhanced dielectric properties of strontium decorated
PbI2 nanosheets for hot carrier solar cell applications
S. Muthu Mariappan1, Mohd. Shkir*2, T. Alshahrani3, Vinoth Elangovan4, H. Algarni2, S.
AlFaify**2
1Department of Physics, Vel Tech Multitech Dr. Rangarajan Dr. Sakunthala Engineering
College, Chennai-600 062, Tamilnadu, India.
2Advanced Functional Materials & Optoelectronics Laboratory, Department of Physics,
College of Science, King Khalid University, Abha 61413, Saudi Arabia.
3Department of Physics, College of Science, Princess Nourah Bint Abdulrahman University,
Riyadh, 11671 Saudi Arabia.
4Functional Materials and Energy device Laboratory, SRM Institute of Science and
Technology, Kattankulathur, Chennai, Tamilnadu, India.
*Corresponding author
E-mail: shkirphysics@kku.edu.sa
1

Abstract
Dielectric properties determine by electric field distributions play a decisive role in
energy harvesting and storage applications. In this context, strontium decorated lead iodide
nanosheets (Sr:PbI2) are prepared, and its vibrational and dielectric properties are studied.
The bandgap changes are explained by Brustein-Moss effect and renormalization process.
Raman spectral studies reveal that introducing Sr atoms into PbI2 lattice enhance the lifetime
of LO phonon through bottleneck effect. Subsequently, the dielectric constant (ε′) values of 5
wt.% Sr:PbI2 are increased 20% than pure PbI2. The results conclude that the vibrational
properties of Sr decorated PbI2 nanosheets are much significant for hot carrier solar cell
devices.
Keywords: Lead iodide (PbI2); optoelectronics; Burstein-Moss effect; dielectrics; electrical
properties.
1. Introduction
Fascinating properties of various oxide and nitride based wide bandgap
semiconductors are being studied predominantly over past few decades. Validating this, an
improved trend in device performances of such semiconducting materials can be observed
from plenty of reports [1–7]. On the other hand, interest shown on halide-based
semiconductors is comparatively low and most of their properties are yet to be discovered.
Among different halide-based semiconductors, lead iodide (PBI2) is a noteworthy material
due to its technological viability for various applications. Indeed, PbI2 is a layered compound
with alternative stacked I-Pb-I layers arranged sequentially along vertical axis (i.e. ꢀ-axis)
[8]. Resembling graphite structure, atoms in same layers of PbI2 are strongly glued together
with covalent bonding and nearby layers atoms are weakly hold by Van der Waals force [9].
Precise control on PbI2 number of layers by heat treatment process and the corresponding
changes in absorption bands were examined by Goto et al. [10]. Insight on radiative
2

mechanism of PbI2 have confirmed the existence of both bi-excitonic and exciton-exciton
collisions, which makes their emission properties quiet interesting [11]. Ando et al.
demonstrated that, bottleneck effect in PbI2 provides missing link between its anomalous
emission properties and optical phonons [12]. The observed bottleneck effect in PbI2 are
much essential to improve the performance of lead halide based perovskite solar cells [13].
Confirming this, very recently the decisive role of PbI2 on the actions of perovskite solar cells
was revealed by Gujar et al. [14]. Yet, a profound analysis on the absorption properties of
PbI2 are much needed to effectively use them in optoelectronic devices like photodetectors
and photovoltaics, etc. Besides, their dielectric properties are also much reliable for energy
storage applications [15].
Owing to its quasi two dimensional (2D) anisotropic layered structure, PbI2 can be
easily modified into novel morphologies such as thin nanowires, nanorods, and nanosheets,
etc [16, 17]. Though 1D materials disclose important properties like high surface to volume
ratio, its electronic properties are dubious with predominant noise arising through carrier
trapping at defect sites, also defect free nanowires generally require expensive preparation
technique [18, 19]. But dielectric applications require a smooth, high density, and low
conducting material with large surface area. Hence, 2D-PbI2 nanosheets with larger bandgap
and reasonable dielectric constant (ε′) are ideal candidates for such applications.
In earlier days, considerable amount of research works on PBI2 have focussed its
nuclear radiation detection properties due to high stability of lead against nuclear reactions
and its strong electron density [20, 21]. But in contemporary days, profound knowledge on
engineering materials with cutting edge tools and available state of the art characterization
techniques have turned out to be a driving factor for researchers to unravel its nano-regime
properties. Modification in intrinsic semiconductors like (i) doping foreign atoms at different
lattice sites (or at interstitial position), (ii) making composites with suitable compounds, and
3

(ii) tuning its morphology to provide high surface to volume ratio could results in interesting
properties [22, 23]. Various reports have demonstrated the importance of dopant elements in
both n-type and p-type semiconductors. For example, in oxide-based semiconductors,
presence of oxygen defects play a positive role in photocatalysis via inducing redox reaction.
However, same defects act as a recombination site for excitonic carrier and degrade the
performance of photovoltaic devices. Hence, depending upon application purpose, defects
should be tuned with suitable dopant atoms at optimum composition. Similarly, halide
semiconductors with different dopants can provide improved performance with enriched
properties. Our previous works on noble metal nanoparticles decorated PbI2 sheets exhibits
improved photo-detecting performance by virtue of surface plasmon resonance (SPR) of
metallic nanoparticles in near infrared region [24]. Also, we examined optoelectronic and
improved dielectric properties of neodymium (lanthanide group) doped PbI2 nanorods [25].
However, to the best of our facts there is no exclusive report to monitor the optical, phonon,
and dielectric properties of alkaline metal decorated PbI2 nanosheets. To fill this void, we
prepared strontium decorated PbI2 nanosheets (Sr:PbI2) with microwave assisted technique
and analysed its structural and optoelectronic properties for hot carrier solar cells. In the final
section, the impact of Sr atoms on the dielectric properties of PbI2 are studied.
2. Experimental procedures
Fabrication and investigational details
For the preparation of pure PbI2 and Sr:PbI2 samples, we initially procured high grade
lead acetate (LA), sodium iodide (NaI), strontium nitrate (SN), cetyl trimethylammonium
bromide (CTAB), and acetic acid (AA) from Sigma and Alfa Aesar Pvt. Ltd. Com. In the first
step, solutions of 0.5M LA and 1M NaI were dissolved separately in 50 ml of DDW at 60 ºC
and respective solutions were named A and B. Followed by that, 50 ml of CTAB solution
from a stock of CTAB (30g/1000 ml) was added to A, and stirred for 15 min. Then, 50 ml of
4

AA was added to same solution to catalyst the reaction. Next, solution B was poured into
solution A under same temperature and stirring conditions. Subsequently, a yellow colour
precipitate appears while pouring first drop of NaI itself, clearly indicates the formation of
aimed PbI2. Later, the solution was collected in cylindrical flask and placed in indigenously
modified microwave system of 700 W power and irradiated for 15 min. Finally, the solution
was cool down to room temperature and the precipitate was collected after washing with
DDW several times. Finally, the precipitate is dried at 80 ºC in an oven for 24 h. Similar
process is followed to prepare Sr:PbI2 samples with additional solution of 1 wt.%, 3 wt.% and
5 wt.% of SN into the LA solution. The synthesis procedure is represented in Fig.1.
Fig. 1: Schematic of the synthesis procedure and the formation of pure PbI2 and Sr:PbI2
samples
All these prepared samples were inspected by Lab X6000 XRD system for phase
identifications [CuKα1, λ=1.54056 Å; operating conditions- 40kV/30A; scan rate - 2 º/min;
scan range - 5 to 70º]. Additionally, Rietveld refinement process was carried out through
MAUD version 2.33 of Rietveld [L. Lutterotti: program Maud (version 2.33), city)] and real-
time refinement of structure was done by a least-squares refinement progression. Optical
5

absorption analysis performed with JASCO V570 system was used to analyse the impact of
Sr atoms on the band structure of PbI2. The surface morphology, elemental composition and
homogeneity in final products were recorded by JEOL JSM6360 SEM/EDX system. For
vibrational modes identifications, a high-precision DXR FT-Raman setup with 532 nm laser
operated at 0.2 mW power was used. The dielectric properties were measured from 4200-SC
setup and prior to measurements all samples were converted into a pellet of dimensions 10
mm diameter and 0.70 mm of thickness by applying 5 ton of pressure with a pelletizer.
3. Outcomes and discussion
3.1. Phase identification
Structural properties of all samples are inspected from XRD profiles. As shown in
Fig. 2(a), peaks obtained for pure PbI2 and Sr:PbI2 are exactly matching with its standard
diffraction values [JCPDS 7-0235]. The lack of additional phases and well defined sharp
peaks decline the presence of impurities during growth process and the same results are
further confirmed with EDX/Raman measurements.
Fig. 2: (a) XRD pattern of pure PbI2 and Sr:PbI2. (b) Magnified view of (001) peak. (c-f)
Rietveld refinement data for pure PbI2 and Sr:PbI2.
6

PbI2 is an iso-structural material with over 50 different polytypes have been displayed
so far, and the corresponding space groups are determined by number of symmetry elements
exist in the structure [26]. Majority (i.e. > 95%) of PbI2 compound possess a smaller number
of symmetry element and leads to the formation of 2H-P3m1 space group as reported in this
article. The term 2H represent that Pb layers are stacked between two hexagonal honeycombs
shaped I layers with different stacking orientation as shown in Fig. 4.
Further, changes in crystallite size (ꢁ) of prepared samples are calculated by Scherrer
ꢂ.ꢃꢄ
formula: ꢁ = ꢅ ꢆꢇꢈꢉ , where ꢊ, ꢋ, ꢌ represent the wavelength of the Cu-Kα radiation
(1.5406Å), diffraction angle and the full width half maximum (FWHM), one-to-one. As
shown in table 1, the crystallite size is found to decrease with increasing Sr wt.%, whereas
the lattice constants increases. Such phenomenon is termed as size-induced negative pressure
effect. It’s a commonly occurring process in covalent semiconductors to maintain the
symmetry to a long range. Such observation has been previously reported in other transition
semiconductors like Fe2O3, CuO and CeO systems [26].
Also, the lattice constants ꢍ, ꢎ are calculated by standard equations through
POWDERX software, see Table 1. The ratio between ꢎ/ꢍ are approximately 1.53, which
exactly matches with the previously reported value [27]. The similar dopant dependent
variations in the structural parameters are also confirmed by Rietveld refinement process
[Fig. 2 (c-f)]. The error in the refinement parameters are minimized to about ± 0.001 Å by
carefully performing the experiments and repeated refinements.
7

Table 1: Calculated structural parameters for all samples from POWDERX software and
Rietveld refinement processes.
Sample
POWDERX software
V (Å3)
Rietveld refinement
parameters
D
δ10-14
ε10-13
×10
×10
Pb1-xSrxI2
x=0.0 wt. %
a
c
a
c
(Å)
V (Å3)
(nm)
(nm-2)
(Å)
(Å)
(Å)
4.55636 6.97999 125.49349 4.5556 6.9756 125.3754
56.276
47.658
52.038
52.913
3.158
4.403
3.693
3.572
2.757
3.257
2.982
2.935
x= 1.0 wt. % 4.55628 6.98211 125.52738 4.5548 6.9766 125.3460
x= 3.0 wt. % 4.55711 6.98233 125.57702 4.5555 6.9773 125.3951
x= 5.0 wt. % 4.55892 6.98294 125.68798 4.5550 6.9765 125.3554
The influence of dopant atoms on preferential growth direction of PbI2 is monitored
from intensities of different diffraction peaks. Although there are no significant changes has
been observed in intensities of most of the peaks, there is a slight increase can be noticed in
(102) plane of the 1 wt.% and 3 wt.% Sr:PbI2 samples. In addition, there is a considerable
amount of reduction has been observed in the intensities of (001) plane of all Sr:PbI2, along
with a small shift towards lower diffraction angle. This decrement in the (001) plane intensity
suggest that, growth of nanorods are highly restricted with the presence of Sr dopants and the
same fact has been confirmed from SEM image. Typically, the lower angle shifts in the XRD
peaks are mainly attributed to the biaxial tensile stress induced inside the PbI2 lattice due to
the dopant atoms (see Fig. 2(b)) [28]. These observations imply that the ratio of dopant to the
parent concentration plays a fair role on the preferential growth directions. As the
experimental conditions for all samples are similar, the dopant atoms introduced into PbI2
structure should be the only reason for inducing stress and influence the growth directions.
The increased lattice parameter consequently increases the values of calculated unit cell
volume (see Table 1). Insight on the broadening of (001) plane and increased intensity in
(102) plane is done by calculating the lattice strain (ε) and dislocation density (δ) with
8

ꢏ × ꢐꢑꢒꢓ
ꢔ
ꢗ
respective formulas; ε =
ꢍꢕꢖ δ = ꢙ , where ꢌ ꢍꢕꢖ ꢋ corresponds to the FWHM
ꢘ
and the corresponding diffraction angle respectively. The calculated values of ε and δ are
given in Table 1. It can be observed that dopant atom causes an increased ε and δ values.
Vegard’s law suggests that variations in the unit cell dimensions are linked to ionic radii of
dopant atom and provide a pathway to reveal the exact location of dopant atom. As the ionic
radii of Sr2+ are almost equal to Pb2+ ions, it can be speculated that increase in unit volume is
due to the replacement of Sr atoms with the Pb atoms. But it is not possible due to the strong
electronegativity values of Pb2+ (2.33) compared to Sr2+ (1.00). Therefore, the increments in
the unit cell dimensions are only due to the interstitial occupancy of Sr atoms and caused the
observed stress.
3.2. Optical absorption spectrum analysis
Modifications in band structure of PbI2 with interstitial Sr atoms could be ascertained
by optical absorption spectra. The recorded absorption spectra are shown in Fig. S1 (see
supplementary information). For precise calculation of band gap (ꢚꢛ) we adopted Tauc’s plot
approach instead of energy-wavelength relation (ꢚꢛ = ꢜꢆ). Since PbI2 is a direct bandgap
ꢝ
semiconductor, ꢚꢛ values are measured by extrapolating linear section of (ꢞℎν)ꢟ curve to
intersect the horizontal axis of ℎν vs (ꢞℎν)ꢟ graph, where ꢞ is absorption coefficient of PbI2
and ℎν is the energy of incident photon. The intersecting point of extrapolating line on ℎν
axis (i.e. (ꢞℎν)ꢠ = 0) gives accurate band gap. For calculation purpose, the value of ꢞ is
obtained by Beer- lamberts law; α = 2.303A/d where A is absorbance value and d is path
length (i.e. thickness of the cuvette, 10 mm). As shown in Fig. 3, bandgap of pure PbI2, 1
wt.%, 3 wt.% and 5 wt.% Sr:PbI2 are calculated as 3.26 eV, 3.29 eV, 3.28 eV, and 3.26 eV
respectively. Zhang et al. reported that, bandgap of PbI2 will decrease with the increasing
concentration of iodine concentration when PbI2 is sandwiched between two perovskite
9

layers. The decrease may be attributed to the cationic exchange/ interactions between PbI2
and the perovskites [29]. However in our case, compared to pure PbI2, widening of ꢚꢛ of
about 0.03 eV is observed in 1wt.% Sr:PbI2, and followed by a narrowing down of ꢚꢛ occurs
for 3 wt.% and 5 wt.% Sr:PbI2.
Fig. 3: Tauc’s plots for pure PbI2 and Sr:PbI2, corresponding bandgap values are marked.
Fig. 4: Schematic representation of bandgap widening and narrowing by Burstein-Moss
mechanism and renormalization process. (Iodine - dark blue and cyan blue, Pb- pink, Sr-
green).
10

The band gap widening is commonly attributed to different mechanisms such as (i)
Burstein-Moss (BM) shift, (ii) quantum confinement effect, or (iii) excitation dependent
absorption widening, etc. [30, 31]. But the mechanism (ii) and (iii) will be effective only
when particle size is curtailed to the Bohr radius diameter and the surface to volume ratio of
the samples are very high. Therefore, those mechanisms can be deliberately excluded as no
such 0D or 1D morphology is observed from SEM image. Hence the only pathway for
widening of ꢚꢛ can be the Burstein-Moss (BM) shift. Though widening in ꢚꢛ by BM shift
has been reported in surplus of n-type of materials, to the best our knowledge this is one of
the infant observations in a direct p-type material. However, to understand the increase in ꢚꢛ
of p-type material by BM shift mechanism, it is significant to recall the same mechanism in
n-type material [32]. With the Fermi level near to conduction band edge, carrier
concentration of n-type semiconductors will be increased with electron donor dopant atoms.
Owing to filled density of states near the conduction band edge, Fermi level will shift into
conduction band with an upward transition of
absorption takes places between the top of valence band edge to the Fermi level inside
conduction band. Hence, band gap is apparently increased to ꢣꢚꢛ + ꢚꢡꢢꢤ rather than the
∆ꢚꢡꢢ (Brustein Moss shift). Consequently,
∆
actual band gap (ꢚꢛ). On the other hand, introducing hole dopants like Sr into PbI2 (p-type
semiconductors), where Fermi level located near the valence band edge, the added holes
recombine with the electrons near the upper electronic states of valence band [33]. Thus, the
Fermi level makes a downward transition and leads to an increased band gap as shown in Fig
4. Therefore, with the addition of 1 wt.% Sr dopant, it increases the band gap of PbI2 by
injecting holes into the parent semiconductor. However, with the higher concentration of Sr
dopant atoms, the band gap shrinks due to a renormalization process which generally results
from phenomena like (i) carrier-carrier thermalization, (ii) carrier- ionic impurity interactions,
and (iii) carrier- optical phonon interactions etc. Such renormalization process downshift the
11

bottom edge of conduction band and reduce the bandgap of the sample [34, 35]. Theoretical
modelling of such shift has been reported by Zhu et al. in pure CdO films [35]. But due to the
difficulties in developing such unequivocal theory to connect ꢚꢛ values of PbI2 with Sr wt.%,
we analysed the bandgap variations by numerical methods. The ꢚꢛ values correspond to
different Sr concentrations are obtained by cubic spline plot analysis (see supplementary Fig.
S2), and it is clear that the ꢚꢛ values starts to decrease at 1.58 wt.% of Sr dopant. In addition,
linear curve fit methods show that observed variation in ꢚꢛ can be related with the
supplementary equation 1 (see supporting data).
3.3. Morphological and EDX analysis
Fig. 5: (a) EDX profile (b-e) SEM-mapping images for Pb (blue), I (green), Sr (red) and the
overlay of Pb, I, Sr for 3.0 wt.% Sr:PbI2.
12

Elemental compositional analysis and morphological imaging were performed with
EDX/SEM elemental mapping (e-mapping) and SEM imaging, respectively. The peaks in
EDX spectra [Fig. 5(a)], confirm the presence of Pb, I, and low weight percentage of Sr
atoms respectively. Further, SEM e-mapping endorse the homogenous distribution of
crystallites without any agglomerations, even in 3 wt.% Sr:PbI2 [Fig.5 (b-e)]. Then, surface
morphologies of pure PbI2 and Sr:PbI2 are recorded with different SEM resolutions (see Fig.
6).
Fig. 6: SEM micrographs of pure PbI2 and Sr:PbI2. The corresponding sheet thickness
variations are shown below in the respective samples.
It can be observed that, pure PbI2 consists of mixed spherical nanoparticles with few
protruding nanorods of thickness 0.17 µm and width 0.574 µm, which indicates that particles
tend to grow along ꢎ-axis for given experimental conditions. However, morphology of 1
wt.% Sr:PbI2 spreads into two dimensional (2D) nanosheets of width 1.913 µm and thickness
of 0.354 µm. This observation implies, growth along ꢎ-axis is highly restricted in 1 wt.%
Sr:PbI2 and similar observation is evidenced from XRD analysis. The same trend has been
observed with increase in Sr dopant concentration and to understand such morphological
13

changes, a possible growth process is much needed from the formation of PbI2 seed layers as
provided by following equation,
Pb(CH3COO)2 + 2NaI → PbI2 + 2NaCH3COO.
The cationic exchange model suggests that formed nanocrystals tend to elongate into
1D nanorods with the presence of surfactant like CTAB. Though various parameters can
influence preferential growth direction such as temperature, nature of surfactant, and pH
values of the reaction media, the resultant PbI2 nanosheets are quite surprising because of the
similar experimental conditions for all samples. Hence, the big picture of such observation is
that, “dopant Sr atom must be the cause for the change in morphologies of Sr:PbI2”. In
general, planes normal to preferred growth direction possess minimum surface area and
eventually sample morphologies are determined by the available formation energy along each
direction. Hence a time resolved growth mechanism with the presence of Sr atoms is
necessary to unfold the evolution process. To start with, the formation energy of pure PbI2
along ꢎ-axis must be initially higher to kinetically favoured its growth along (001) plane.
However, with the addition of Sr atoms 1D nanorods experience high reactivity at the ridges
which is the main reason for the spreading of nanosheets [36, 37]. Moreover, as evidenced
from XRD analysis, incorporating Sr atoms into PbI2 will create defects along (001) plane
which further decreases the formation energy along (001) plane and facilitates growth along
different direction other than ꢎ-axis. Thus, in a binary step growth process, growth along ꢎ-
axis is initially hindered and then favours the growth along (102) plane to achieve a thin
nanosheets. Such observation has been noticed previously in the Fe doped ZnO lattice [38].
3.4. Vibrational analysis:
Raman spectroscopy is a significant characterization to analyse the number of layers
in 2D structured materials. The relative intensity changes, frequency shift, and broadening in
Raman bands could provide information about phonon lifetime and carrier-phonon
14

interactions in the semiconductor. The recorded Raman spectra are shown in supplementary
Fig. S3 (see supporting information) and in-order to extensively analyse the system, we de-
convolute the bands and presented in Fig. 7. As the unit cell in 2H-PbI2 consists of 3 atoms,
the possible 3N vibrational modes are represented by 6 irreducible phonon modes: Γ =
ꢥꢗꢛ+ 2A ꢠꢦ+2ꢚꢧ + ꢚꢛ. Hence, the peaks obtained at 73 cm−1, 93 cm−1, 110 cm−1, 167 cm−1,
and 216 cm−1 corresponds to Eꢦ(TO), Aꢠꢦ(TO), Aꢠꢦ(LO), and the overtone modes,
respectively. Yagmurcukardes et al. has reported the position of Aꢠꢦ(TO) mode, for different
layer of PbI2 [39]. According to their report, our nanosheet should possess less than 5 layers.
The vibrational properties of absorbent layer play a crucial role in determining the
performance of perovskite hot carrier solar cells. Especially the phonon bottleneck effect
arising as a result of weak carrier-phonon interaction facilitates the confinement of incident
light energy inside the system.
15

Fig. 7: Fitted Raman data after base line corrections and the de-convoluted peaks are
differentiated with different colours.
Table 2: FWHM and Phonon lifetime of A2u (TO) and A2u (LO) modes
Samples
A2u (TO)
A2u (LO)
FWHM
Peak
Position
(cm-1)
FWHM Phonon
Peak
Phonon
life time
(ps)
life time Position
(ps)
(cm-1)
Pure- PbI2
93.63
8.29
0.6402
107.90
16.74
0.31
1 wt.% Sr- PbI2
3 wt.% Sr- PbI2
5 wt.% Sr- PbI2
93.96
94.2
93.98
7.85
8.32
8.53
0.6761
0.6379
0.6222
108.53
108.63
108.26
15.17
15.26
15.41
0.35
0.34
0.34
(
)
Table 2 shows the phonon lifetime (τ) of longitudinal optical phonon Aꢠꢦ LO and
ꢗ
(
)
transverse optical phonon Aꢠꢦ TO calculated by the equation: = 2ꢩꢎГ, where ꢎ, Г are the
ꢨ
16

velocity of light and the FWHM of corresponding band. The lifetime of Aꢠꢦ(LO) is found to
be slightly increased for 1 wt.% Sr:PbI2. In general, the properties of LO phonon is very
significant, because, when the incident light photons create excitons immediately its energy is
transferred into LO phonon and finally dissipates as heat via acoustic phonon. However, if
the carrier concentration is very high, it forms a bottleneck like structure to escape into LO
phonon, consequently the lifetime of excitonic carrier and LO phonon will be increased.
Thus, the increased lifetime of Aꢠꢦ(LO) is attributed to bottleneck effect induced by large
carrier density inside Sr:PbI2. Such phonon bottleneck effect in PbI2 with enormous carrier
density has previously confirmed with photoluminescence studies by Ando et al [12].
Contrarily, at higher doping concentration, large carrier-carrier interaction results in
renormalization process as discussed in absorption studies. Also, the intensity of the Raman
band at Aꢠꢦ(TO) at 96 cm−1 is highly enhanced in 1 wt.% Sr:PbI2 samples due to the
improved crystalline quality. A similar observation has been revealed by Baibarac et al. due
to the exciton-phonon coupling in PbI2, yet a lower temperature analysis is required to
intuitively study such changes [40]. These observations have revealed that, role of Sr dopant
in PbI2 could play a crucial role on the efficiency of the photovoltaic devices.
3.5. Dielectric studies
Owing to the dependency of charge storage ability on its dielectric properties, its
associated parameters such as dielectric constant (ε′), dielectric loss (tan
δ), and AC
conductivity gains attention as it directly infers about the feasibility of PbI2 to apply in energy
storage devices. The dielectric studies are carried out by metal-insulator-metal (MIM)
technique, where samples are initially made into pellets and then sandwiched between Cu
electrodes on both sides of pellets. Later, the required bias voltage is applied across metallic
electrodes to provide electric field across the sample. In general, ε′ is a complex quantity
17

where its real part signifies degree of polarization (i.e. stored electrical energy) and imaginary
part reveal the dissipated energy.
Fig. 8(a,b) show the values of ε′ and tan
δ of pure PbI2 and Sr:PbI2 samples over a
wide frequency range. All dielectric parameter calculations were performed as from our
previous report [25]. A 10% and 20% increase in ε′ has been observed for 3 wt.% and 5 wt.%
Sr:PbI2 samples, meanwhile ε′ values of 1 wt.% Sr:PbI2 and pure PbI2 samples are almost
overlapping. Except for a small hump at low frequency region, the values of ε′ are almost
constant in entire frequency range. The variation trend in ε′ values insists that the dielectric
mechanism is same for all sample at a given frequency. The derivative of ε′ values are shown
in Fig. 8c. Thus, unravelling reason for increasing values of ε′ at high sr wt.% and small
hump at low frequency region are much needed. To understand the dielectric behaviour of
sample, insight on the conduction mechanism is crucial which generally involves (i) hopping
conduction, (ii) space charge induced conduction, (iii) Poole-Frenkel emission, and (iv)
Ohmic conduction [41]. Due to the large bandgap of PbI2, current could not increase
significantly for small bias voltage. Hence the Ohmic conduction might not be the reason for
downward trend in ε′ value after the small hump in low frequency region. Similarly, the
Poole-Frenkel conduction, often called intrinsic Schottky conduction require high
temperature for inducing sufficient carriers; hence it can also be conveniently omitted as all
experiments are carried out in room temperature. Hence, the observed hump in low frequency
region can be only due to core intrinsic polarization properties and not due to experimental
parameters and external conditions. Maxwell-Wagnel model suggest that, lattice is composed
of different grain separated by walls/boundaries.
18

Fig. 8: Plots for (a) ꢪ′ vs. ln(ω), (b) ꢫꢍꢕꢬ vs. ln(ω), (c) ꢪ′′ vs. ln(ω), (d) ꢭꢮꢆ vs. ln(ω) of0 wt.%
1 wt.%, 3 wt.% and 5 wt.% Sr decorated PbI2 nanosheets.
The carrier can freely move only inside the volume of given grain and cannot
percolate into grain boundaries. Thus, a polarization will induce across the grain boundary for
an applied electric field. On the other hand, it’s a well-established fact that at low frequency
region the ε′ and tan δ values are highly dominated by space charge results from commonly
observed lattice vacancy and possibly some dangling bonds. Further the hopping carriers
trapped inside grain could deteriorate the conduction and results in small hump at low
frequency region. Also, the presence of sr atoms at high concentration could possibly
increases the existing ionic vacancies. Hence large Sr interstitials significantly improve
oriental polarization values due to the difference in electronegativities in Sr2+ and Pb2+ [42].
Consequently, the ε′ values are increased for 3 wt.% and 5 wt.% Sr:PbI2. However, in high
19

frequency region, the ε′ values are almost constant, suggesting that the dipole does not obey
the applied field in that region. In fact, large damping of dipoles results in slight downward
trend in the ε′ values at higher frequencies. Desirably the dielectric losses are very low in all
cases as shown in Fig. 8(b).
Further, the AC conductivity (σꢯꢰ) of pure PbI2 and Sr:PbI2 samples are calculated
and corresponding values are plotted in Fig. 8(d). Irrespective of dopant concentration, all the
samples show improved σꢯꢰ values at high frequencies. Such increase can be attributed to the
enhanced drift mobility of the carrier as a result of hopping conduction [43]. As shown in
inset of Fig. 8(d), there is a slight increase in conductivity of 3 wt.% and 5 wt.% Sr:PbI2
samples. This can be explained by the large number of carriers induced by S interstitial atoms
and the obtained results are consistent with the dielectric values. Further, the frequency
( )
dependent σꢯꢰ conductivity is analyzed by the standard Jonscher’s equation: σꢯꢰ ω =
σ
ꢱꢰ + Aωꢲ, where σꢱꢰ is the DC conductivity (i.e, σ at ω‰ꢂ) and ꢳ, A are the frequency
exponent and a constant respectively. The value of ꢳ is obtained by fitting the slope of ln
σꢴꢵ vs ln ω as reported in our previous work [24, 25]. As shown in supplementary Fig. S4
(see supporting information), ꢳ value initially increases and then slowly decreased at high Sr
wt.%. The average value of ꢳ is around unity and in general it’s a temperature dependent
quantity. Hence a detailed work on the temperature dependent analysis of ε′ is required. The
schematic diagram shown in Fig.9. illustrates the charge separation phenomena in Sr:PbI2.
20

Fig. 9: Schematic representation of charge storage mechanism in PbI2 nanosheets with
applied electric field (shown by lightning).
4. Conclusions:
In summary, Sr:PbI2 were prepared by microwave assisted synthesis process. XRD
analysis suggest that, Sr atoms suppress the tendency of pure PbI2 to grow along the (001)
plane, rather it favours the (102) plane as preferential growth directions. Based on the stress
induced on PbI2 structure by Sr atoms as confirmed by structural analysis, a possible
mechanism has furnished for the spreading of nanosheets. Compared to pure PbI2, the
bandgap values are found to be increased of 0.03 eV at 1wt.% Sr:PbI2, however the trend
starts decreasing at higher wt.% of Sr atoms. These observations are explained by the
Burstein-Moss shift and the renormalization process respectively. The dielectric constant
values are found to be increased for the 3 and 5 wt.% of Sr dopants PbI2 and the observed
increases is attributed to the hopping conduction and the space charge region developed at the
PbI2 surface. The outcomes signify that the fabricated Sr:PbI2 are useful in optoelectronic &
PV applications.
Acknowledgement
21

This research was funded by the Deanship of Scientific Research at Princess Nourah bint
Abdulrahman University through the Fast-track Research Funding Program.
Declaration of conflict of interest
Authors have no conflict to declare in this work
References
1. Cui, J. Zinc oxide nanowires. Mater. Charact., 2012, 64, 43–52.
2. Kandalkar, S. G.; Dhawale, D. S.; Kim, C. K.; Lokhande, C. D. Chemical synthesis of
cobalt oxide thin film electrode for supercapacitor application. Synth. Met., 2010, 160, 1299–
1302.
3. Lee, C. T.; Chen, H. W.; Lee, H. Y. Metal-oxide-semiconductor devices using Ga2O3
dielectrics on n-type GaN. Appl. Phys. Lett., 2003, 82, 4304–4306.
4. Lokhande, C. D.; Dubal, D. P.; Joo, O. S. Metal oxide thin film based supercapacitors.
Curr. Appl. Phys., 2011, 11, 255–270.
5. So, H.; Park, W. Attachable freezing-delayed surfaces for ultraviolet sensing using GaN
photodetector at low temperature in air. Appl. Surf. Sci., 2019, 473, 261–265.
6. Zhang, B.; Liu, Y. A review of GaN-based optoelectronic devices on silicon substrate.
Chinese Sci. Bull., 2014, 59, 1251–1275.
7. Zhang, L.; Wang, S.; Shao, Y.; Wu, Y.; Sun, C.; Huo, Q.; Zhang, B.; Hu, H.; Hao, X. One-
step fabrication of porous GaN crystal membrane and its application in energy storage. Sci.
Rep., 2017, 7, 44063.
8. Beckmann, P. A. A review of polytypism in lead iodide. Cryst. Res. Technol., 2010, 45,
455–460.
9. Goto, T.; Tanaka, H.; Exciton study in PbI2 microcrystallites by pump-probe method. Solid
State Commun., 1994, 89, 17–21.
10. Goto, T.; Saito, S.; Tanaka, H. Size quantization of excitons in PbI2 microcrystallites.
Solid State Commun., 1991, 80, 331–334.
22

11. Tanaka, K.; Hosoya, T.; Fukaya, R.; Takeda, J. A new luminescence due to an exciton-
exciton collision process in lead iodide induced by two-photon absorption. J. Lumin., 2007,
122(123), 421–423.
12. Ando, M.; Yazaki, M.; Katayama, I.; Ichida, H.; Wakaiki, S.; Kanematsu, Y.; Takeda. J.
Photoluminescence dynamics due to biexcitons and exciton-exciton scattering in the layered-
type semiconductor PbI2. Phys. Rev. B., 2012, 86, 155206.
13. Yang, Y.; Ostrowski, D. P.; France, R. M.; Zhu, K.; van de Lagemaat, J.; Luther, J. M.;
Beard, M. C. Observation of a hot-phonon bottleneck in lead-iodide perovskites. Nat.
Photonics. 2016, 10, 53–59.
14. Gujar, T. P.; Unger, T.; Schönleber, A.; Fried, M.; Panzer, F.; van Smaalen, S.; Köhler,
A.; Thelakkat, M. The role of PbI2 in CH3NH3PbI3 perovskite stability, solar cell parameters
and device degradation. Phys. Chem. Chem. Phys., 2018, 20, 605–614.
15. Gon, H. B.; Veeraiah, N. Dielectric properties of LiF single crystals. J. Mater. Sci., 1981,
16, 2571–2574.
16. Liu, J.; Liang, Z.; Xu, B.; Xiang, H.; Xia, Y.; Yin, J.; Liu, Z. Synthesis of PbI2 nanowires
for high sensitivity photodetectors. RSC Adv., 2016, 6, 59445–59449.
17. Kaviyarasu, K.; Sajan, D.; Selvakumar, M. S.; Thomas, S. A.; Anand, D. P. A facile
hydrothermal route to synthesize novel PbI2 nanorods. J. Phys. Chem. Solids, 2012, 73,
1396–1400.
18. Reza, S.; Bosman, G.; Islam, M. S.; Kamins, T. I.; Sharma, S.; Williams, R. S. Noise in
silicon nanowires. IEEE Trans. Nanotechnol., 2006, 5, 523–528.
19. Bid, A.; Bora, A.; Raychaudhuri, A. K. 1/F noise in nanowires. Nanotechnology, 2006,
17, 152–156.
20. Shoji, T.; Hitomi, K.; Tiba, T.; Suehiro, T.; Hiratate. Y. Fabrication of a nuclear radiation
detector using the PbI2 crystal and its response characteristics for gamma-rays. IEEE Trans.
23

Nucl. Sci., 1998, 45, 581–584.
21. Liu, J.; Zhang, Y. Growth of lead iodide single crystals used for nuclear radiation
detection of Gamma-rays. Cryst. Res. Technol., 2017, 52, 1600370.
22. Sivalingam, M. M.; Balasubramanian, K. Influence of the concentration of reducing agent
on gold nanoparticles decorated reduced graphene oxide and its ammonia sensing
performance. Appl. Phys. A, 2017, 123, 281.
23. Sivalingam, M. M.; Balasubramanian, K. Morphological tuned preparation of zinc oxide:
reduced graphene oxide composites for non-enzymatic fluorescence glucose sensing and
enhanced photocatalysis. Appl. Phys. A, 2016, 122, 294.
24. Shkir, M.; Yahia, I. S.; Ganesh, V.; Bitla, Y.; Ashraf, I. M.; Kaushik, A.; Alfaify, S. A
facile synthesis of Au- nanoparticles decorated PbI2 single crystalline nanosheets for
optoelectronic device applications. Sci. Rep., 2018, 8, 13806.
25. Shkir, M.; Alfaify, S. Tailoring the structural, morphological, optical and dielectric
properties of lead iodide through Nd3+ doping. Sci. Rep., 2017, 7, 16091.
26. Prabhu, D,; Narayanasamy, A.; Shinoda, K.; Jeyadeven, B.; Greneche, J-M.; and
Chattopadhyay, K. Grain size effect on the phase transformation temperature of
nanostructured CuFe2O4, J. Appl. Phys., 2011, 109, 013532.
27. Shah, M. A.; Wahab, M. A. Growth rate and symmetry of polytypes in MX2-compounds.
J. Mater. Sci. Lett., 2000, 19, 1813.
28. Li, Y. F.; Yao, B.; Lu, Y. M.; Cong, C. X.; Zhang, Z. Z.; Gai, Y. Q.; Zheng, C. J.; Li, B.
H.; Wei, Z. P.; Shen, D. Z.; Fan, X. W.; Xiao, L.; Xu, S. C.; Liu, Y. Characterization of
biaxial stress and its effect on optical properties of ZnO thin films. Appl. Phys. Lett., 2007,
91, 021915.
24

29. Zhang, X.; Ye, J.; Zhu, L.; Zheng, H.; Liu, G.; Liu, X.; Duan, B.; Pan. X.; and Dai, S.
High-efficiency perovskite solar cell prepared by a sandwich structure MAI-PbI2-MAI
precursor film. Nanoscale, 2017, 9, 4691-4699.
30. Liu, X.; Zhang, Q.; Yip, J. N.; Xiong, Q.; Sum, T. C. Wavelength tunable single nanowire
lasers based on surface plasmon polariton enhanced Burstein-Moss effect. Nano Lett., 2013,
13, 5336–5343.
31. Guo, M; Zhang, X. D. Band gap widening and quantum confinement effects of ZnO
nanowires by first-principles calculation. Mater. Sci. Forum., 2011, 675-677, 243–246.
32. Gibbs, Z. M.; LaLonde, A.; Snyder, G. J. Optical band gap and the Burstein-Moss effect
in iodine doped PbTe using diffuse reflectance infrared Fourier transform spectroscopy. New.
J. Phys., 2013, 15, 075020.
33. Yogamalar, N. R.; Bose, A. C. Burstein-Moss shift and room temperature near-band-edge
luminescence in lithium-doped zinc oxide. Appl. Phys. A., 2011, 103, 33–42.
34. Zhu, C.; Liu, Y.; Xu, J.; Nie, Z.; Li, Y.; Xu, Y.; Zhang, R.; Wang, F. Bandgap
renormalization in single-wall carbon nanotubes. Sci. Rep., 2017, 7, 11221.
35. Zhu, Y.; Mendelsberg, R. J.; Zhu, J.; Han, J.; Anders, A. Dopant-induced band filling and
bandgap renormalization in CdOꢀ: In films. J. Phys. D. Appl. Phys., 2013, 46, 195102.
36. Qin, J.; Qiu, G.; Jian, J.; Zhou, H.; Yang, L.; Charnas, A.; Zemlyanov, D.Y.; Xu, C.Y.;
Xu, X.; Wu, W.; Wang. H.; Ye. P. D; Controlled growth of a large-size 2D selenium
nanosheet and its electronic and optoelectronic applications. ACS Nano., 2017, 11(10),
10222–10229.
37. Qiu, J.; Li, X.; He, W.; Park, S. J.; Kim, H. K.; Hwang, Y. H.; Lee, J. H.; Kim, Y. D. The
growth mechanism and optical properties of ultralong ZnO nanorod arrays with a high aspect
ratio by a preheating hydrothermal method. Nanotechnology, 2009, 20, 155603.
38. Iqbal, J.; Jan, T.; Ronghai, Y.; Naqvi, S.H.; Ahmad, I. Doping induced tailoring in the
25

morphology, band-gap and ferromagnetic properties of biocompatible ZnO nanowires,
nanorods and nanoparticles. Nano-Micro Lett., 2014, 6, 242–251.
39. Yagmurcukardes, M.; Peeters, F. M.; Sahin, H. Electronic and vibrational properties of
PbI2: From bulk to monolayer. Phys. Rev. B., 2018, 98, 085431.
40. Baibarac, M.; Smaranda, I.; Scocioreanu, M.; Mitran, R.A.; Enculescu, M.; Galatanu, M.;
Baltog, I. Exciton-phonon interaction in PbI2 revealed by Raman and photoluminescence
studies using excitation light overlapping the fundamental absorption edge. Mater. Res. Bull.,
2015, 70, 762–772.
41. Chiu, F. C. A review on conduction mechanisms in dielectric films. Adv. Mater. Sci.
Eng., 2014, 2014, 578168.
42. Wang, Y.; Xu, C.; Zhang, J.; Du, X.; Yan, Y. Electronic band structure and magnetic
properties of I vacancy and nonmetallic atoms doped single layer PbI2. J. Magn. Magn.
Mater., 2018, 463, 36–43.
43. Cherifi, Y.; Chaouchi, A.; Lorgoilloux, Y.; Rguiti, M.; Kadri, A.; Courtois, C. Electrical,
dielectric and photocatalytic properties of Fe-doped ZnO nanomaterials synthesized by sol
gel method. Process. Appl. Ceram., 2016, 10, 125–135.
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Research highlights
1) Facile one pot microwave synthesis of Sr@PbI2 nanosheets was achieved.
2) XRD, Vibrational studies revealed monophasic PbI2 synthesis at all Sr contents, EDX
spectra and SEM e-mapping studies reveals Sr content and its regular dispersal in the
final products.
3) SEM study confirms the formation of well-defined nanosheets (NSs) and the optical
band gap was varied between 3.26 to 3.29 eV.
4) Dielectric constant and electrical conductivity values are found to be enriched at 3 and
5 wt.% Sr doping.
5) Sr atoms enhance the lifetime of LO phonon by inducing bottleneck effect with
enormous charge carriers and such phenomena is very significant for hot carrier solar
cell devices.