Synthesis, structural, thermal and optical studies of inorganic-organichybrid semiconductors, R-PbI4K. Pradeesh, K. Nageswara Rao, and G. Vijaya Prakash Citation: J. Appl. Phys. 113, 083523 (2013); doi: 10.1063/1.4792667 View online: http://dx.doi.org/10.1063/1.4792667 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v113/i8 Published by the American Institute of Physics. Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

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Synthesis, structural, thermal and optical studies of inorganic-organichybrid semiconductors, R-PbI4

K. Pradeesh,a),b) K. Nageswara Rao, and G. Vijaya Prakasha)

Nanophotonics lab, Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016, India

(Received 11 November 2012; accepted 5 February 2013; published online 27 February 2013)

Wide varieties of naturally self-assembled two-dimensional inorganic-organic (IO) hybrid

semiconductors, (4-ClC6H4NH3)2PbI4, (C6H9C2H4NH3)2PbI4, (CnH2nþ1NH3)2PbI4 (where n ¼ 12,

16, 18), (CnH2n-1NH3)2PbI4 (where n ¼ 3, 4, 5), (C6H5C2H4NH3)2PbI4, NH3(CH2)12NH3PbI4, and

(C4H3SC2H4NH3)2PbI4, were fabricated by intercalating structurally diverse organic guest moieties

into lead iodide perovskite structure. The crystal packing of all these fabricated IO-hybrids comprises

of well-ordered organic and inorganic layers, stacked-up alternately along c-axis. Almost all these

hybrids are thermally stable upto 200 �C and show strong room-temperature exciton absorption and

photoluminescence features. These strongly confined optical excitons are highly influenced by

structural deformation of PbI matrix due to the conformation of organic moiety. A systematic

correlation of optical exciton behavior of IO-hybrids with the organic/inorganic layer thicknesses,

intercalating organic moieties, and various structural disorders were discussed. This systematic study

clearly suggests that the PbI layer crumpling is directly responsible for the tunability of optical

exciton energy. VC 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4792667]

I. INTRODUCTION

Current material research is on engineering low dimen-

sional semiconductor structures, for designer optical absorp-

tion/emission properties. In low-dimensional inorganic

semiconductor structures, which are also called as quantum

confined structures, the motion of electrons (and/or holes) is

restricted in one or more dimensions. Based on the confine-

ment along one, two or three dimensions, they are classified

as quantum wells, quantum wires and quantum dots and sev-

eral potential applications of these structures had already

been witnessed in the recent past.1–8 On the other hand, the

low-dimensional organic molecular self-assembled systems

are considered to be an effective alternative to their inorganic

counterparts (e.g., J-aggregates). While organic semiconduc-

tors have obvious advantages, such as high performance and

simple fabrication, the significant issues are lack of thermal/

mechanical durability and the limited life span.9–12

Hybrid inorganic-organic (IO) semiconductors are open-

ing up a new insight to the low dimensional nanostructures.

They deliver as a unique replacement to their inorganic and

organic counterparts and provide significant opportunity as

multifunctional materials for many electronic and optoelec-

tronic applications. Among these hybrids, self-organized low-

dimensional (0D, 1D, and 2D) IO structures, derived from

component 3D networks of RMX3 (R-organic amine and

MX-metal halide) type perovskite, have attracted much atten-

tion because of their unique crystal structures and the modi-

fied optical properties.13–23 Especially 2D IO-hybrids have

shown promising optoelectronic applications recently, which

includes organic-inorganic light emitting diodes (OILEDs),

organic-inorganic field-effect transistors (OIFETs), and non-

linear optical switches based on strong exciton-photon cou-

pling in microcavity photonic architectures.19–22 The basic

structure of these metal halide based 2D hybrids takes the

general form R-MX4 (where R is the mono-/di-amine) con-

sisting of inorganic layers (a network of corner-shared metal

halide octahedra) with mono-/bi-layers of organic cations

stacked between them. They form “natural” multiple quantum

well structures, where inorganic semiconducting layers

behave as “wells” and wider bandgap organic layers as

“barriers.”24,25 The carriers are confined within the inorganic

layers by low dimensionality of inorganic layer (quantum con-

finement effect) combined with the large dielectric mismatch

(dielectric confinement effect) between the inorganic and or-

ganic layers, enabling the formation of stable excitons with

large binding energy even at room temperatures,21,26 exhibit-

ing narrow and strong excitonic absorption and emission fea-

tures within the visible region favourable for optoelectronics

applications.18–21,27–30

The exciton emission and absorption of these IO-Hybrids

is critically dependent on the structural network of metal

halide layers and the size, shape and the driving forces of the

organic spacers.16,31,32 All these factors and vast family of

IO-hybrid materials makes it very complex to explore the

relation between IO-hybrids and its exciton features to sort

out practical device realization problems. Some key factors

like thermal stability and structural correlation of optical

excitonic properties are yet to be studied extensively for a

better understanding to rank their capabilities, according to

the device application requirements. This is challenging since

various potential applications demand freedom of selecting

appropriate organic guest and inorganic host moieties. Our

present study explores lead iodide based different 2D IO-

hybrid semiconductors of the form R-PbI4 containing

a)Authors to whom correspondence should be addressed. Electronic

addresses: pradphysics@gmail.com and prakash@physics.iitd.ac.in.

Telephone: þ91(11) 2659 1326. FAX: þ91(11) 2658 1114.b)Present address: Optoelectronics Research Centre, University of

Southampton, Southampton, United Kingdom.

0021-8979/2013/113(8)/083523/9/$30.00 VC 2013 American Institute of Physics113, 083523-1

JOURNAL OF APPLIED PHYSICS 113, 083523 (2013)

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structurally diverse organic moieties within the PbI layers.

Table I gives brief overview of the synthesized IO-hybrids

with different organic moieties, corresponding chemical for-

mula and the acronyms which will be used hereafter. Though

the synthesis and structural studies were reported for some of

these IO-hybrids,24,33–35 optical studies of the synthesized

hybrids and their structural correlation are less explored.

Here optical and thermal properties of these IO-hybrids were

systematically studied and the observed variation in the exci-

ton features with changes in organic/inorganic layer thick-

nesses, intercalating organic moieties, and various structural

disorders were discussed in detail. A systematic correlation

of exciton energy with one of the structural parameters, Pb-I-

Pb bond angle, of the inorganic network was established. In

addition, effects of IO-hybrid film thickness and local disor-

ders on the exciton features have been discussed.

II. EXPERIMENTAL DETAILS

Synthesis of various low-dimensional IO-hybrids was

carried out by a generalized high product yield, and commer-

cially viable process, which involves mixing of stoichiomet-

ric quantities of organic moiety and inorganic (PbI2) with HI

(57 wt. % in H2O) at 60 �C. The resultant solution was

allowed to rest at 60 �C for an hour and then cooled slowly

to room temperature without stirring. The precipitate thus

obtained was filtered off and dried in the presence of hygro-

scopic material. Single crystals of the respective compounds

were harvested from slow evaporation process by dissolving

them in a sparingly soluble solvent. All reagents and solvents

are used as received, without further purification. However,

the synthesis procedure slightly varies from its generic route

depending on the nature of organic moiety. Synthesized IO-

hybrid crystal/compounds are dissolved in appropriate sol-

vents like C12PI, C16PI, and C18PI in tetra hydrofuran

(THF), DDPI in dimethyl sulphoxide (DMSO), and rest in

acetonitrile. These solutions are spin coated onto glass sub-

strate to get thin films of the IO-hybrids. Among these, thin

films of DDPI were obtained by heating the solution of it on

glass substrate to about 65 �C. Once the solvent starts evapo-

rating, the substrate containing the solution has been quickly

transferred to spin coater, which was spun at 1500 rpm for

30s, whereas the films of rest of the compounds were

obtained by spinning at 3000 rpm for 30s on Photoresist

Spinner (Ducom, Model No. 318). For typical film thickness

of �60-100 nm, 50 ll of the saturated solution is dropped on

the substrate (1� 1 cm2 area) and a spinning rate of

3000 rpm was maintained for 30s. Uniform thin films thus

obtained were used for glancing angle X-ray diffraction

(XRD). The thermal stability of the IO-hybrids is identified

by thermo gravimetric (TG) and differential thermo gravi-

metric (DTG) analysis using Perkin Elmer, Pyris-7 Thermal

Analyzer. Weight loss measurements in the temperature

range 30 �C–800 �C were carried out at a scan rate of 5 �C/

min under N2 atmosphere. Thin film absorption and photolu-

minescence (PL) measurements were carried out using an

Ocean Optics spectrometer (HR2000) and appropriate optics.

PL of thin films/single crystals were carried out in reflection

modes using nitrogen laser (from M/s JSC Plasma Inc.,

Model NPL-3, and average power 3 MW) as an excitation

source. Micro and macro PL spectral and imaging studies of

IO-hybrid single crystals/thin films were carried out by a

modified confocal microscope in which, a conventional laser

scanning confocal microscope (M/s Olympus, Fluoview-

FV1000) is used, which collects PL from high-resolution

images of sizes scaling down to microns. A CW diode laser

of wavelength 447 nm (M/s Coherent cube laser systems,

Model Cube405), is used as an excitation source, this is an

informative imaging tool to observe the effect of local disor-

der in IO-hybrid crystal structures.

III. RESULTS AND DISCUSSION

A. Crystal structure of two-dimensional IO-hybrids

The crystal packing and the PbI network arrangement

vary widely with different organic spacers and the PbI struc-

tural complexity depends on various aspects of organic moi-

ety. In this section structural orientations of the fabricated

2D IO-hybrids are discussed, while the structural correlation

of IO-hybrids with the observed optical features will be dis-

cussed in Sec. III D.

All the IO-hybrids fabricated here are classified as 2D

hybrids where the structure comprises of well-ordered organic

and inorganic layers stacked alternately. Fig. 1(a) exemplifies

the schematic stacked layered arrangement of one of the IO-

hybrids, CAPI. The organic and inorganic species are bonded

TABLE I. The empirical names and chemical formulae of the synthesized IO-hybrids.

S. No Empirical name Chemical formula

1 4-Chloroanilinium tetraiodoplumbate (CAPI) (4-ClC6H4NH3)2PbI4

2 2(1-Cyclohexenyl) ethyl ammonium tetraiodoplumbate (CHPI) (C6H9C2H4NH3)2PbI4

3 Dodecyl ammonium tetraiodoplumbate (C12PI) (C12H25NH3)2PbI4

4 Hexadecyl ammonium tetraiodoplumbate (C16PI) (C16H33NH3)2PbI4

5 Octadecyl ammonium tetraiodoplumbate (C18PI) (C18H37NH3)2PbI4

6 Cyclopropyl ammonium tetraiodoplumbate (CPPI) (C3H5NH3)2PbI4

7 Cyclobutyl ammonium tetraiodoplumbate (CBPI) (C4H7NH3)2PbI4

8 Cyclopentyl ammonium tetraiodoplumbate (CPEPI) (C5H9NH3)2PbI4

9 Phenyl ethylammonium tetraiodoplumbate (PAPI) (C6H5C2H4NH3)2PbI4

10 Dodecyl ammonium lead iodide (DDPI) (NH3(CH2)12NH3)PbI4

11 (2-Thiophene) ethyl ammonium tetraiodoplumbate (TEPI) (C4H3SC2H4NH3)2PbI4

083523-2 Pradeesh, Nageswara Rao, and Vijaya Prakash J. Appl. Phys. 113, 083523 (2013)

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together by hydrogen bonding between the amine group of or-

ganic and the iodide atoms of PbI network.

In the PbI network there are two types of iodine (I) atoms

defined by their different connectivity, terminal and bridging

iodine atoms. Six iodine atoms are bonded to lead (Pb) atoms

to form an octahedron with Pb at the centre and I at the cor-

ners. The trans Pb-I bond angle is 180� and the cis Pb-I bond

angle of all the synthesised 2D IO-hybrids varies from the

ideal angle 90� and hence the Pb-I-Pb bond angle is much less

than 180�. The angle is 155.43� for PAPI, 143.01� for CAPI,

and for other IO-hybrids the angle lies between the values of

PAPI and CAPI (see Table II). The Pb-I-Pb bond angle indi-

cates that the corner sharing of PbI6 octahedra are distorted.

This distortion is both an in-plane (away from the ideal square

grid) distortion as well as a correlated out-of-plane distortion.

The ammonium group of the organic chain can form hydrogen

bond to any of the three iodides out of eight iodides in the

near vicinity by either to bond to two terminal iodides and

one bridging iodide (terminal halogen configuration) (Figs.

1(b) and 1(c)) or to two bridging iodides and one terminal

iodide (bridging halogen configuration) (Fig. 1(d)). Both ha-

lide configurations can adopt either equilateral or right trian-

gle configurations. In DDPI, the hydrogen bridges between

the organic and inorganic entities adopt the terminal halogen

configuration and the right triangle configuration (Fig. 1(b)),

whereas in C12PI it is terminal halogen configuration and

equilateral triangle configuration (Fig. 1(c)). Distinguishably

from DDPI and C12PI, in CPEPI the hydrogen bridging fol-

lows the bridging halogen configuration and the right angled

configuration (Fig. 1(d)), while almost all amino groups of

long chained organics (C12PI, C16PI, and C18PI) prefer ter-

minal halogen configuration with the three iodides.

Fig. 2(a) shows XRD patterns of all the IO-hybrids under

study. As seen, in all these IO-hybrids, cyclo alkyl organic

based (CPPI, CPEPI, CBPI, etc.,), cyclic organic based

hybrids (CHPI and PAPI) and long alkyl chain organic based

(C12PI, C16PI, and C18PI) hybrids show strong diffraction

peaks corresponding to (00 l), (l00), and (002 l) [where l¼ 1,

2, 3…], respectively. Thin film XRD of the prepared IO-

hybrids are compared to that of simulated powder pattern

XRD (Fig. 2(b)) extracted from single crystal XRD

data.17,18,33–36 The corresponding d-spacings matches well

with the simulated powder pattern data and the estimated d-

spacings are tabulated in Table II. Strong presence of (00l)plane diffraction suggests the well-stacked layered structure

of the IO-hybrid thin films when they are spun onto substrate.

Hence on spin coating, these hybrids naturally self assembles

to form alternative layers of organic moiety and inorganic net-

work with the “c” axis perpendicular to the plane of the

FIG. 1. (a) Crystal structure16 of one of the 2D IO-hybrid, CAPI (hydrogen’s

are omitted for clarity) (b) and (c) NH-I terminal halide configuration of

DDPI (right triangle configuration) and C12PI (equilateral triangle configura-

tion), respectively,18 and (d) NH-I bridging halide configuration of CPEPI35

(right triangle configuration).

TABLE II. Empirical names, d-spacing values, both from thin film XRD and single crystal XRD, Pb-I-Pb bond angles, halogen and triangle bonding configura-

tions and decomposition temperatures of all fabricated IO-hybrids.

S. No IO-hybrid

d-spacing

(Thin film XRD) ( �A)

d-spacing

(single crystal XRD) ( �A)

Pb-I-Pb angle

(deg)

Halogen and triangle

bonding configurations

Decom-position

temperature (�C)

1 CAPI 6.508 (100) 7.548 (Ref. 16) (100) 143.01 (Ref. 16) Terminal and Equilateral (Ref. 16) 220

2 CHPI 8.712 (001) 8.618 (Ref. 33) (001) 148.72 (Ref. 33) Terminal and Right (Ref. 33) 220

3 C12PI 6.127 (004) 6.132 (004) (Ref. 34) 150.19 (Ref. 34) Terminal and Equilateral (Ref. 34) 240

4 C16PI 14.78 (002) 14.874 (002) (Ref. 34) 149.52 (Ref. 34) Terminal and Equilateral (Ref. 34) 240

5 C18PI 16.06 (002) 16.121 (002) (Ref. 34) 149.39 (Ref. 34) Terminal and Equilateral (Ref. 34) 240

6 CPPI 5.930 (200) 5.870 (200) (Ref. 35) 147.16 (Ref. 35) Terminal and Equilateral (Ref. 35) 120

7 CBPI 6.351 (200) 6.374 (200) (Ref. 35) 147.27 (Ref. 35) Terminal and Equilateral (Ref. 35) 220

8 CPEPI 6.646 (200) 6.654 (200) (Ref. 35) 154.57 (Ref. 35) Bridging and Right (Ref. 35) 200

9 PAPI 7.170 (001) 7.228 (001) (Ref. 36) 155.43 (Ref. 36) Terminal and Right (Ref. 36) 200

10 DDPI 8.004 (001) 8.021 (001) (Ref. 18) 149.68 (Ref. 18) Terminal and Right (Ref. 18) …

11 TEPI 7.932 … … … 200

083523-3 Pradeesh, Nageswara Rao, and Vijaya Prakash J. Appl. Phys. 113, 083523 (2013)

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substrate in general. However there are diffraction peaks other

than (00l) indicating a partial improper stacking of layers.

B. Thermal analysis

TG and DTG of one of the 2D hybrid, C12PI is shown

in Fig. 3. Figure shows that there was no considerable weight

loss upto 240 �C and as the temperature increases further, a

weight loss of about 64.39% was observed. This suggests

that the IO-hybrid is stable upto 240 �C and only decomposes

above that temperature. The decomposition of C12PI is as

follows

(C12H25NH3)2PbI4 (100%)! PbI2 (35.61%)þ 2C12

H25NH2 (32.19%) þ 2HI (32.2%)In general, the thermal de-

pendence of all the prepared 2D IO-hybrids with mono-amino

organic spacers follows the decomposition process given by

(RNH3)2PbI4 ! PbI2 þ 2RNH2 þ 2HI and 2D hybrids with

di-amino organic spacers is given by NH3RNH3PbI4 ! PbI2

þ NH2RNH2 þ 2HI. The TG and DTG curves of all the syn-

thesised IO-hybrids are shown in Fig. 4. The decomposition

temperatures, up to which the layered structure of IO-hybrid

is unaltered, for all IO-hybrids under study, are reported in

Table II. The data confirms that except for CPPI all the IO-

hybrids are stable up to 200 �C and could very well be utilized

for device applications with operating temperatures less than

decomposition temperatures. However any structural phase

transformations could not be visualized from this TG/DTG

analysis, in such cases differential scanning calorimetry

(DSC) would be of help.

C. Exciton optical features of IO-hybrids: Absorptionand photoluminescence

Two-dimensional IO-hybrids are naturally self-

assembled systems, with alternative stack of organic and inor-

ganic layers. The dimensions of inorganic layers estimated

from single crystal XRD are typically of �6 A thick and the

organic spacer widths vary between 3 and 22 A depending on

the structure of organic moiety. This layered structure resem-

bles multiple quantum wells, where inorganic layer with band

gap of �3 eV forms “well” and organic layer with bandgap

�6 eV forms “barrier.”

Room temperature absorption and photoluminescence of

one of the IO-hybrids, C12PI, is presented in Fig. 5.

Absorption spectrum show two principal absorption lines: a

broad absorption at �400 nm (3.1 eV) and a strong narrow

peak at �490 nm (2.53 eV) in which the former is attributed

to both charge transfer transition between the organic and

inorganic layers and higher order exciton transition energy

levels and the latter narrow absorption peak is attributed to

the lowest exciton within the inorganic layers.16,24,25 Similar

to C12PI all IO-hybrids show strong room-temperature exci-

ton absorption and PL with the peak values ranging between

FIG. 2. (a) Thin film XRD patterns and (b) simulated powder XRD patterns

from single crystal XRD data of synthesized IO-hybrids (where *, �, and �

represents (100), (001), and (002) diffraction peaks, respectively.

FIG. 3. Typical TG (blue) and DTG (red) curves of one of the 2D IO-hybrid

C12PI. FIG. 4. (a) TG and (b) DTG curves of synthesized 2-D IO-hybrids.

083523-4 Pradeesh, Nageswara Rao, and Vijaya Prakash J. Appl. Phys. 113, 083523 (2013)

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480 nm and 525 nm. Both single crystal and thin films of IO-

hybrids show strong room-temperature excitonic features with

symmetric and narrow line shape (FWHM � 20 nm). The PL

and absorption maxima for all the IO-hybrids under study are

given in Table III. The observed oscillator strength of exciton

absorption of C12PI thin film (of thickness �100 nm) is

f¼ 6.5� 1015 cm�2. Since for the film thickness of 100 nm

there are �52 quantum wells (PbI network), the oscillator

strength per quantum-well (fqw) is �12.5� 1013 cm�2. This

value is one order of magnitude higher than conventional

inorganic quantum wells such as InGaAs structure.37

D. Structural correlation with optical exciton features

Despite some understanding on the dependence of exci-

ton energies of 2-D layered IO-hybrids on various parame-

ters, like inorganic well width, organic barrier separation,

dielectric contrast, geometrical arrangements of inorganic

and organic molecules, etc., qualitative details of exciton

binding energies remain out of reach.

The optical excitonic features of these IO-hybrids were ex-

tensively studied both in thin films and single crystals.16–19,38

For thin film fabrication a conventional spin coating method

was employed. Typically, 20 mg of IO-hybrid was dissolved

in 1 ml of appropriate solvent and spun on to a cleaned glass

slide at a spinning rate of 3000 rpm. Absorption and PL

spectra of all the IO-hybrid thin films are recorded and are

presented in Fig. 6. As seen, the peak exciton absorption of

these IO-hybrids vary between 475 nm (2.62 eV) and 520 nm

(2.38 eV) with the replacement of organic spacers. While

CAPI absorbs at 475 nm and emits at �480 nm, other hybrids

PAPI, TEPI, and CHPI show emission wavelengths near to

520 nm with peak absorption around 510 nm. The emission

wavelength shift between CAPI and PAPI is �40 nm.

From Figure 6 and Table III, it is further evident that the

optical features, specially the peak positions and widths of

absorption and PL are different for the IO-hybrids interca-

lated with different organic moieties. From previous reports

on 2D inorganic quantum wells, one of the common features

observed was the inhomogeneous broadening of exciton

transitions due to random fluctuations in the quantum well

widths due to local disorder and the consequence of

FIG. 5. Absorption (black) and PL (red) spectra of the one of the 2-D lay-

ered IO-hybrid (C12PI) thin film.

TABLE III. Empirical names, absorption and PL peak energies, optical band gaps, inorganic “well” widths, organic “barrier” widths, and Pb-I-Pb bond angles

of all the fabricated IO-hybrids.

S. No IO-hybrid Eabs (eV) EPL (eV) Stokes shift (meV) Bandap (eV) Inorganic width (A) Organic width (A) Pb-I-Pb angle (deg)

1 CAPI 2.62 2.56 53.83 3.25 6.34 7.15 143.01

2 CHPI 2.42 2.37 50.93 2.87 6.39 5.29 148.72

3 C12PI 2.53 2.49 35.64 2.75 6.36 14.89 150.19

4 C16PI 2.53 2.51 25.56 2.98 6.36 22.52 149.52

5 C18PI 2.53 2.49 35.64 2.98 6.36 25.12 149.39

6 CPPI 2.56 2.47 91.86 3.04 6.36 3.75 147.16

7 CBPI 2.54 2.49 46.01 2.88 6.40 4.72 147.27

8 CPEPI 2.44 2.43 09.57 2.86 6.39 5.29 154.57

9 PAPI 2.51 2.37 134.64 2.49 6.38 6.44 155.43

10 DDPI 2.46 2.43 33.84 2.99 6.40 9.60 149.68

11 TEPI 2.40 2.36 36.55 … … … …

FIG. 6. (a) Absorption and PL spectra of various 2-D layered IO-hybrid thin

films. The spectra are shifted along y-axis for clarity.

083523-5 Pradeesh, Nageswara Rao, and Vijaya Prakash J. Appl. Phys. 113, 083523 (2013)

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fabrication issues during layer-by-layer growth. Such fea-

tures were directly reflected in the absorption, emission peak

position/width and the energy difference between them

(referred to as stokes’ shift).39,40 Hence, the PL peak broad-

ening and stokes shifts observed in these IO-hybrids could

be directly attributed to the changes in the layered arrange-

ment which is influenced by various factors such as tempera-

ture and compositional fluctuations and defects created

during the growth process.

As explained in Sec. III A, the 2D layered arrangement

in the present IO-hybrids is strongly influenced by the char-

acteristic features of the organic moiety, which results into

various well/barrier widths and structural rearrangements of

PbI network. In our recent works, the influence of local dis-

order in the inorganic network,17 thickness of the film21 and

temperature38 on the absorption and PL features in certain

types of IO-hybrids were discussed. In general, the excitons

absorption is the average effects of all the excitons, whereas

the PL is due to those few excitons migrated to the lowest

occupied energy states and are also influenced by the

involvement of phonons from the extended absorption

(Urbach) tails (due to the local disorder and compositional

random fluctuations). Therefore, the structural re-

arrangement, phase purity, film uniformity, and crystal pack-

ing defects strongly influences the optical features of these

IO-hybrids and understanding of such factors are discussed

in the forthcoming sections.

Though the origin of exciton is essentially dominated by

Inorganic network (the Pb and I electronic states), consider-

able peak position change in the exciton PL with the replace-

ment of different organic spacers suggests that the organic

moiety do play a vital role in modifying the exciton features.

Optical excitons in conventional semiconductor quantum

wells have been fairly understood from the electron and

dielectric confinement point of view. Hong et al.41 reported a

direct relevance of tuning of exciton absorption in some of

the PbI4 based IO-hybrids by using quantum well and barrier

widths. Here an attempt has been made to understand the

tuning of exciton energy with the variation in barrier/well

widths, using the available crystallographic information. The

variation of exciton PL with the well width, barrier width,

well and barrier ratio are shown in Figs. 7(a)–7(d). As seen,

there is less correlation between the PL and the “well” or the

“barrier” or the ratio between well and barrier thickness and

hence attempts were made to understand the exciton tuning

from the structural perception.

Though the exciton and other optical features of PbI2 is

reasonably understood,42–44 it is worth to have a direct com-

parison/correlation between the PbI2 and these IO-hybrids

((R-NH3)2PbI4), since the structural arrangement of PbI

network in PbI2 is different from that of IO-hybrid

((R-NH3)2PbI4). The PbI6 octahedra are corner shared in IO-

hybrids while they are edge shared in PbI2. Schematic of PbI

network of PbI2 and one of the IO-hybrids, PAPI, are pre-

sented in (Figs. 7(e) and 7(f)) which shows the variation in

PbI network. Hence different structural features are to be

considered such as, (1) disorder or conformation of the or-

ganic moiety, (2) crystal packing, (3) arrangement of inor-

ganic layers, (4) position of ammonium group of organic

moiety tagged to the PbI network, etc. After observing

closely, the crystal structure and corresponding exciton fea-

tures, structural rearrangement of PbI octahedra is predicated

as a key factor for the tuneability.38 Since the lowest elec-

tronic bandgap in these hybrids are directly related to the

inorganic (PbI) network, any structural crumpling in Pb-I-Pb

network effectively induces change in the electronic band

structure and related density of states. In order to understand

such structural variation effect on electronic band structure,

Extended H€uckel tight-binding model calculations (using

CAESARTM V2.0 software)45 were performed for all the IO-

hybrids. While the excitons energies could not be easily cal-

culated from such methods, the study of electronic bandgap

and impact of such structural variations in the PbI bong

angles and bond lengths could be indirectly related to exciton

energies.38

To start with, two example IO-hybrids CAPI and PAPI,

whose emission wavelengths are at the extreme, 480 nm and

520 nm, respectively, were selected and electronic band

structures and density of states (DOS) are compared. The

schematic representation of crystal structures and the Pb-I-

FIG. 7. Variation of exciton PL peak

energy vs (a) well (inorganic) width (b)

barrier (organic) widths (c) barrier/well

ratio and (d) well/barrier ratio of various

2-D layered IO-hybrids, (e) and (f) sche-

matic representations of PbI layer

arrangement in parent PbI2 and one of

the IO-hybrids, (PAPI), respectively.

083523-6 Pradeesh, Nageswara Rao, and Vijaya Prakash J. Appl. Phys. 113, 083523 (2013)

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Pb angles of CAPI and PAPI are shown in Fig. 8(a). In both

the cases, side-linked PbI6 octahedra are extended as 2-D

planar sheets and between inter layer, organic moieties are

coupled via N-H� � �I weak hydrogen bonds through the NH3

ligands of the organic cation.

However, the PbI network layers are not completely pla-

nar and possess a bending in the Pb-I-Pb bond angle. The

Pb-I-Pb angles for PAPI and CAPI are 155.42� and 143.01�,respectively, (inset of Fig. 8(a)). On comparing other struc-

tural parameters like well (PbI) and barrier (organic) widths

of PAPI and CAPI, there are hardly any variation in the well

(6.34 A and 6.38 A for CAPI and PAPI, respectively) and

barrier (7.15 A and 6.44 A for CAPI and PAPI, respectively)

widths. But the Pb-I-Pb angles of PAPI and CAPI show

marked variation, with CAPI more crumpled (with 143.01�)than PAPI (157.42�). The estimated band gaps for CAPI and

PAPI were 3.25 eV and 2.49 eV, respectively, (Figs. 8(b) and

8(c)). Thus, one can speculate that there is a direct relevance

of band gap and hence the exciton energy of CAPI and PAPI

to their Pb-I-Pb network crumpling.

Similar to CAPI and PAPI the band gaps of all the syn-

thesized IO-hybrids were estimated using the corresponding

crystal structural data17,18,33–36 and are plotted against the

Pb-I-Pb layer crumbling (i.e., Pb-I-Pb bending angle) (Fig.

9). We have also modeled a pseudo planar PbI network with

the Pb-I-Pb bending 180� and the band gap is estimated as

2.21 eV. As seen, the estimated bandgap values from such

calculations (Fig. 9(b)) also show similar variation with Pb-

I-Pb angle as that of excitons energies (Fig. 9(a)). Band gaps

estimated for all the IO-hybrids are tabulated in Table III.

Some of the close observations are:

(i) An interesting series of IO-hybrids is CnPI (n¼ 12, 16,

and 18 (C12PI, C16PI, and C18PI)), where the organic

spacers are alkyl chained organics CnH2nþ1NH2

(n¼ 12, 16, and 18). The interlayer spacing is substan-

tially increased with the increase of alkyl chain length.

There is hardly any difference in in-plane Pb-I-Pb

bond angles (150.19�, 149.52�, and 149.39� for C12PI,

C16PI, and C18PI, respectively). Therefore, the esti-

mated band gaps are also in close proximity and so

does the experimental exciton absorption and emission

energies.

(ii) Another series is (CnH2n-1NH3)2PbI4 (n¼ 3, 4, and 5)

(named as CPPI, CBPI, and CPEPI), where the or-

ganic spacers are primary cyclic grouped amines,

(CnH2n-1NH3)2PbI4 (n¼ 3, 4, and 5). The exciton

absorption energies are 2.57, 2.55, and 2.43 eV. In

contrast to above CnPI series, in this group, the or-

ganic moiety size is very similar therefore the barrier

layer width is not varied, but the inorganic layer has a

marked crumpling: the Pb-I-Pb bond angles are

147.16�, 147.27�, and 154.57� for n¼ 3, 4, 5, respec-

tively. This change clearly accounts the shift in the

exciton peak energy as well.

(iii) On comparing C12PI and DDPI, C12PI is with mono

terminal amino end group and DDPI is with bi-

terminal amino end groups with same number of car-

bon atoms in the alkyl chain. Both C12PI and DDPI

have similar long-chained organics (n> 12), but the

PbI layers of DDPI are less crumpled, with Pb-I-Pb

bonding angle of DDPI 149.68�. Therefore, the exciton

energy (2.46 eV) is less than that of more crumbled

C12PI (2.56 eV), where the angle is 150.59�.

This systematic study clearly brings out the fact that the

layer crumpling is directly related to the tunability of exciton

energy, apart from other influencing factors such as dielectric

variation and barrier/well widths. A systematic correlation

between the exciton energies and the specific structural

FIG. 8. (a) Schematic crystal packing structures and the Pb-I-Pb bond angles

of CAPI and PAPI (b) and (c) simulated band structure and DOS of CAPI

and PAPI, respectively.FIG. 9. Plot of (a) exciton PL peak energy and (b) estimated bandgap energy

vs Pb-I-Pb in-plane bond angle of various 2-D IO-hybrids. Dashed lines are

guide to eye.

083523-7 Pradeesh, Nageswara Rao, and Vijaya Prakash J. Appl. Phys. 113, 083523 (2013)

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feature, the Pb-I-Pb in-plane bonding angle, was thus estab-

lished. Though this method underestimates the band gap, it

accounts well for the observed correlation between sheet

crumpling and electronic energy levels.

E. Effect of film thickness and local disorderson optical exciton features

1. Effect of film thickness

Even with highly ordered films by spin-coating tech-

nique, exciton features of these IO-hybrids are strongly thick-

ness dependent. To study thickness dependent exciton

features a much precise intercalation technique21 was

employed to precisely control the thickness of IO-hybrid thin

films. Highly ordered and strongly emitting IO-hybrid films of

CHPI and PAPI were fabricated by intercalation technique.

Figure 10 shows the absorption and PL spectra of CHPI

and PAPI thin films of various thicknesses. As seen, the

absorption and PL spectra are significantly thickness depend-

ent. In CHPI, the spectral separation between maximum

absorption and PL (Stokes shift) increases from 5 to 18 nm

as the thickness increases from 40 nm to 350 nm (Figs.

10(a)–10(c)). Similar is the case for PAPI. The Stokes shift

increases from 13.5 nm to 19 nm on with an increase of

the film thickness of PAPI from 40 nm to 120 nm (Figs.

10(d)–10(f)). Since further structural developments are not

observed, possibilities such as formation of bi-layers of inor-

ganic/organic entities46–48 and/or distortion of the 2-D net-

work can be ruled out. Therefore, thickness dependence and

the extra Stokes shift could thus be due to either distorted

bond angles within (PbI6)4� octahedra or due to any possible

low-level presence of unreacted PbI impurities. In general,

present and previous studies49 reveal that few layers are

enough to observe strong exciton related absorption and PL.

Above a certain film thickness (�100 nm) the film results

into imperfect layer stacking and thereby distorted inorganic

network due to heaviness and as a consequence, a significant

shift in the exciton features are observed.

2. Effect of local disorders in single crystals

As discussed previously, the structural distortions and

layer ordering in these IO-hybrids have great impact on exci-

ton properties. While single crystals of all hybrids show

potentially strong-room-temperature exciton absorption and

PL features, stress-related broadband emission was observed

at the edges and grain boundaries of particular IO-hybrid

crystals like CAPI and CHPI. Figures 11(a) and 11(b) show

the PL intensity mapping of CAPI (at �485 nm) and CHPI

(at �520 nm) single crystals, respectively. Both CAPI and

CHPI show fairly characteristic excitonic PL at �485 nm

and ~520 nm, respectively, throughout the crystal, similar to

their respective thin films. Apart from the exciton emission a

broadband peaked at �520 nm and �540 nm for CAPI and

CHPI crystals, respectively (shown in Figs. 11(c) and 11(d)),

was also observed at the edges (marked by squares in Figs.

11(a) and 11(b)) of the crystals.

Such broad band emission along with exciton PL

(recorded at much lower temperatures, at about T¼ 2 K) in

parent PbI2 single crystals, which is also a layered semicon-

ductor, were even reported earlier,50,51 These broad band

emissions have been attributed to the imperfections in the

layer stacking, produce additional red-end excitonic bands

due to radiative recombination of the trapped carriers, apart

from conventional recombination of excitons and self-trapped

excitons. Therefore, the broad red-end PL observed at the

crystal edges is possibly due to imperfect stacking of layers

resulted by the heaviness of the crystal. Such imperfect

FIG. 10. (a) Absorption spectra, (b) normalized PL spectra, and (c) plot of

PL and absorption spectral peak maximum vs film thickness of CHPI thin

films. (d) Absorption spectra, (e) normalized PL spectra, and (f) plot of PL

and absorption spectral peak maximum vs film thickness of PAPI thin films.

FIG. 11. (a) and (b) PL intensity mapping of CAPI (at 485 nm) and CHPI (at

520 nm) single crystals. (c) PL spectra of CAPI thin film and one edge of

CAPI crystal (indicated by square in (a)), and (d) PL spectra of CHPI thin

film and different edges of CHPI crystal (indicated by squares in (b)).

083523-8 Pradeesh, Nageswara Rao, and Vijaya Prakash J. Appl. Phys. 113, 083523 (2013)

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stacking would further misalign the organic moieties and

result in the distortion of Pb-I bonds within the inorganic net-

work followed by changes in the exciton PL peak energies

(Fig. 9).

IV. CONCLUSIONS

Highly emitting, naturally self-assembled, varieties of two

dimensional inorganic-organic (IO) hybrids, (4-ClC6H4

NH3)2PbI4, (C6H9C2H4NH3)2PbI4, (CnH2nþ1NH3)2PbI4 (where

n¼ 12, 14, 16, 18), (CnH2n�1NH3)2PbI4 (where n¼ 3, 4, 5),

(C6H5C2H4NH3)2PbI4, NH2 (CH2)12NH3PbI4, and (C4H3SC2

H4NH3)2PbI4, were fabricated by cost effective method. X-ray

diffraction studies have revealed that there is uniform alternate

layered arrangement in these IO-hybrids. These IO-hybrids

show strong room temperature exciton absorption and photolu-

minescence (PL) features which can be tuned over a span of

�45 nm (480 nm to 525 nm). Almost all the fabricated IO-

hybrids show thermal stability upto 200 �C. While the organic

optoelectronic devices (such as organic light emitting devices

(OLEDS)) had shown remarkable efficiency, the improvement

in thermal stability under continuous operation is still one of

the significant issue. The strong optical exciton features and

thermal stability over wide range of device operating tempera-

tures of the present IO-hybrids are most promising for optoe-

lectronic device applications such as light emitting devices,

low-threshold lasers, etc.32,52,53 A systematic structural correla-

tion with optical exciton features was established highlighting

the Pb-I-Pb bond angle crumpling effects, well/barrier width

influence, thickness, and structural disorder outcome of exciton

features. Thus, the optical exciton versus structure correlation

studies should be important from the device fabrication

perspective.

ACKNOWLEDGMENTS

Authors profoundly thank Professor J. J. Baumberg,

Cavendish Laboratory, University of Cambridge, U.K. and

Professor Ramanan, Department of chemistry, IIT Delhi, India

for their valuable help and discussions. This work was a part

of the UK-India Education and Research Initiative (UK-IERI)

program and High Impact Research Scheme of IIT Delhi.

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