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Preparation and characterization of natural lower dimensionallayered perovskite-type compounds

Y. Tabuchia,b,* , K. Asaia,b, M. Rikukawab,c, K. Sanuib,c, K. Ishigurea

aDepartment of Quantum Engineering and Systems Science, University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 1130033, JapanbCREST Japan, Science and Technology Corporation (JST), 7-3-1, Hongo, Bunkyo-ku, Tokyo 1130033, Japan

cDepartment of Chemistry, Sophia University, 7-1, Kioi-cho, Chiyoda-ku, Tokyo 1028554, Japan

Received 9 July 1999; accepted 12 November 1999

Abstract

Thin films of layered perovskite compounds (CnH2n11NH3)2(CH3NH3)m21PbmBr3m11 �m� 1; 2 and 3) were prepared by thespin-coating method from their DMF (N,N-dimethylformamide) solutions. The control of the inorganic layer thickness inlayered perovskites can be achieved by changing the ratio of the two amines (CnH2n11NH3/CH3NH3). The layered perovskitefilms showed a strong, clear exciton absorption peak at room temperature due to their large exciton binding energy. Withincreasing numbers of inorganic layers, a red shift of the exciton absorption was observed due to the decrease in transfer energy.X-ray diffraction patterns clearly demonstrated that the spin-coated film was highly oriented normal to the surface, which isconsistent with ‘mono’, ‘bi’, and ‘tri’ layer structures, respectively. The effect of the alkyl chain length of the organic part wasalso investigated.q 2000 Elsevier Science Ltd. All rights reserved.

Keywords: A. Quantum wells; A. Thin films; C. X-ray diffraction

1. Introduction

Over the past 10 years, much attention has been paid tofinding suitable materials with large nonlinearity and shortresponses for optical signal processing and optical comput-ing [1]. The optical properties of exciton materials such assemiconductor Multiple Quantum Wells (MQW) have beenintensively studied [2]. The multi-quantum-well structure isvery useful for optical application because the exciton bind-ing energy and optical nonlinearity have been found tobecome larger in two-dimensional (i.e. low dimensional)than in three-dimensional materials. Extensive studies onsemiconductor quantum wells have resulted in better under-standing of the low-dimensional electronic systems.

Hanamura et al. [3] have proposed a new two-dimen-sional system in which the quantum well is sandwiched bya barrier material of a large bandgap and smaller dielectricconstant to increase the exciton binding energy and optical

nonlinearity. Such low-dimensional electronic systems andrelated quantum-well structures can be fabricated by Mole-cular Beam Epitaxy (MBE), Metal Organic Chemical VaporDeposition (MOCVD) or Electron Beam Lithography tech-niques. Even with these techniques, nanometer structuresare still difficult to realize because of the inevitable mono-layer fluctuation.

On the other hand, organic–inorganic perovskites haverecently attracted much attention due to their unique elec-trical and optical properties, as well as their film process-ability [4–6]. Hybrid organic–inorganic materials offerscientifically and technologically significant opportunitiesfor combining the attractive features of both systems withina single material. Layered perovskite-type compounds withthe general formula (CnH2n11NH3)2(CH3NH3)m21MmX3m11

(M � group IVB metal, X� halogen) are such materialsand naturally form the quantum-well structure consistingof a metal halide semiconductor sheet sandwiched betweenorganic insulator layers. Due to the low dimensionality ofthese semiconductor sheets, the exciton has a large bindingenergy of several millielectron volts, which enables strongphotoluminescence even at room temperature sufficientto offer potential applications in emitter materials in

Journal of Physics and Chemistry of Solids 61 (2000) 837–845

0022-3697/00/$ - see front matterq 2000 Elsevier Science Ltd. All rights reserved.PII: S0022-3697(99)00402-3

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* Corresponding author. Tel.:181-3-5841-6978; fax:181-3-3818-3455.

E-mail address:[email protected](Y. Tabuchi).

electroluminescent devices [7,8]. Moreover, the organic–inorganic layered perovskites can be easily obtained byconventional spin-coating method. One can prepare thinfilms of perovskite structure with good optical quality anda plain surface at an atomic level without difficulty.

Among these compounds, optical properties of(CnH2n11NH3)2(CH3NH3)m21PbmI3m11 have been mainlydiscussed because they show a strong exciton peak in thevisible region [7–14]. However, it has been observed thatthe crystal of (CnH2n11NH3)2(CH3NH3)m21PbmI3m11 is notstable toward moisture and light and gradually decomposes[15]. In this study, we tried to fabricate a series of layeredperovskites, (CnH2n11NH3)2(CH3NH3)m21PbmX3m11 filmswhich have [PbBr6]

42 as the semiconductor layers to

improve the stability. The films were characterized by X-ray diffraction and optical absorption.

2. Layered perovskite compounds

In general, the layered perovskites have the chemicalformula (RNH3)2MX4, which corresponds to the simplestmaterial �m� 1� of the (CnH2n11NH3)2(CH3NH3)m21

MmX3m11, where M is a divalent metal, X is a halide andR can be a wide range of organic moieties. The schematicstructure of these layered perovskite compounds is shown inFig. 1. The two-dimensional inorganic layer and an organicammonium layer are alternately stacked. The inorganiclayer is comprised of a two-dimensional sheet of [MX6]

42

octahedra which are connected at the four corners withhalide ions on the plane. The six halogen ions X2 surroundM21, forming an octahedral [MX6]

42 cluster as shown inFig. 2. Therefore, the inorganic layer is as thin as a fewatomic layers. The –NH3

1 ends of the cations bind to theanion layers of [MX6]

42 in a specific orientation determinedby hydrogen bonding with both equatorial and axial halideions.

A multi-layer structure is organized by neutralizing[MX 6]

42 with alkylammonium ions. The adjacent alkylchains interact with each other by van der Waals interaction,and they may or may not overlap, depending on the size ofthe ions in the inorganic layer and the cationic molecules.This perovskite family based on [MX6]

42 has the possibilityof considerable structural flexibility in the nature of theorganic cations. In order to form a crystal out of the buildingblocks, cations are required to neutralize the electricalcharge. When they combine with a cation having a longtail such as ann-decylammonium ion (C10H21NH3

1), theysandwich a two-dimensional plane to form a two-dimen-sional structure as shown in Fig. 3(a). On the contrary, inthe case where they combine with small cations such as theNH4

1 or CH3NH31, the cations are embedded into the voids to

form a three-dimensional structure as shown in Fig. 3(b).Recently, a group at Du Pont expanded the field by

showing that there exists a family of layered inorganic–organic perovskites, (CnH2n11NH3)2(CH3NH3)m21MmX3m11,CnMmX3m11 for short, which has CH3NH3MX3 perovskitesheets with the accumulation number ofm sandwichedwith alkylammonium bilayers. Here,m is the number ofthe metal-ion monolayer sheets. The organic–inorganiccompounds (CnH2n11NH3)2(CH3NH3)m21MmX3m11 formnatural multi-quantum-well structures, with variable wellthickness (controlled bym) and even well depth (controlledby metal), providing an ideal model for tailoring the opticalproperties [16–20]. Methylammonium fits in the center ofeight MX6 corner-shared octahedra, while a long-chainalkylammonium fits only the periphery of a set of fourMX6 octahedra. To maintain the charge neutrality, methyl-ammonium ions incorporate into the structure and functionas counterions to cohere the two layers. The extreme

Y. Tabuchi et al. / Journal of Physics and Chemistry of Solids 61 (2000) 837–845838

Fig. 1. Schematic structure of layered perovskite compound(CnH2n11NH3)2MX4.

Fig. 2. Unit structure of perovskites: closed circle, ammonium ion;shaded circle, metal cation M21; open circle, anion X2.

member of this class of compounds is CH3NH3MX3 for m�∞: This structure lacks barrier layers and can be consideredas a three-dimensional variation of the generic compounds,where [MX6]

42 octahedra share no corners and are isolated.In this study, we tried to fabricate (CnH2n11NH3)2

(CH3NH3)m21PbmBr3m11 which have a ‘mono’ �m� 1�;‘bi’ �m� 2�; or ‘tri’ �m� 3� layer of inorganic two-dimen-sional sheets. The structure of the thin films was character-ized by X-ray diffraction and optical absorption. The effectof the length of the alkylamines on the formation of two-dimensional materials was also investigated.

3. Experimental

3.1. Sample preparation

3.1.1. Amine hydrobromidesN-alkylamines (CnH2n11NH2), and hydrobromic acid

(HBr) (aqueous solution, 48%) are commercially available.Stoichiometric amounts of hydrobromic acid and alkyla-

mine are mixed in a flask cooled with water in order toremove the reaction heat. After the reaction, water shouldbe evaporated by elevating the temperature followed bywashing with diethylether.

3.1.2. Tetrabromoplumbate perovskite compoundsThe monolayer (i.e.�m� 1� compounds, (CnH2n11NH3)2

PbBr4 �n� 2; 3, 4, 6 and 10)), were prepared by treatingpurified CnH2n11NH3Br with a stoichiometric amount ofPbBr2 in N,N-dimethylformamide (DMF) at room tempera-

ture in N2 atmosphere. The bilayer�m� 2� perovskites,(CnH2n11NH3)2(CH3NH3)Pb2Br7 �n� 2;3,4 and 6), wereprepared by reacting CnH2n11NH3Br and CH3NH3Br withPbBr2 in DMF. As the reaction proceeds, PbBr2 dissolvesin DMF. The monolayer and bilayer compounds areobtained as a precipitate, when the reaction solutions werepoured into a poor solvent such as acetone. The trilayer�m� 3� compound, (CnH2n11NH3)2(CH3NH3)2Pb3Br10

�n� 6�; was obtained by adding excess amount ofCH3NH3Br to stoichiometry. The DMF solutions withCH3NH3Br/CnH2n11NH3Br in a molar ratio of 1.5 weremixed with PbBr2 at 358C.

Thin films of microcrystalline (CnH2n11NH3)2

(CH3NH3)m21PbmBr3m11 were fabricated on glass slides bythe spin-coating method. The scan speed was electricallycontrolled to approximately 6000 rpm.

3.2. Measurements

Optical absorption spectra of the films on glass or SiO2

substrates were taken using a conventional UV–visiblespectrophotometer (HITACHI, U-3300) at room tempera-ture. Room temperature photoluminescence (PL) spectrawere also measured. A monochromated Xe lamp was usedas an excitation source for the PL measurement (HITACHIspectrophotometer). The film on the substrate was irradiatedby the light beam at an angle of 458. Excited wavelengthsranged from 250 to 550 nm, depending on the samples.

The crystallinity and orientation of the powder samplesand spin-coated films were determined by X-ray diffraction.X-ray diffraction measurements recorded with a 2u ranging

Y. Tabuchi et al. / Journal of Physics and Chemistry of Solids 61 (2000) 837–845 839

Fig. 3. Structures based on [MX6]42 octahedra: (a) CnMBr4; and (b) CH3NH3MBr3.

from 1.5 to 408 were performed on a Rigaku RAD-IIA witha CuKa target�l � 1:5418 �A� at 40 kV and 25 mA. All thesamples were measured at room temperature.

Differential scanning calorimetry (DSC) measurementswere performed in the temperature range between2150and 208C under N2 atmosphere (SEIKO Instruments).

4. Results and discussion

4.1. Control of the thickness of the inorganic quantum-wellin perovskites

Layered perovskite compounds, (CnH2n11NH3)2

(CH3NH3)m21PbmBr3m11 �m� 1–3�; which consist of differ-ent inorganic thicknesses were successfully obtained usingdifferent sizes of alkylammonium ions with a controlledratio of the two amines. Mono-�m� 3� and bilayer�m�2� are prepared by stoichiometric reaction. On the contrary,pure trilayer�m� 3� compounds can be obtained only whenan excess of CH3NH3Br is added. Optimum experimentalconditions for preparing the trilayer films involve usingCnH2n11NH3Br/CH3NH3Br in a molar ratio at 1.5 and main-taining the temperature at 358C.

4.1.1. X-ray diffractionFig. 4 shows X-ray diffraction profiles of a powder

and a spin-coated film of monolayer perovskites,(C6H13NH3)2PbBr4 which has hexylamine as a longer alky-lamine (abbreviated C6PbBr4). In the diffraction profiles ofthe powder sample, a series of (001) diffractions corre-sponding to the interlayer spacing is mainly observed withsmall peaks assigned to other reflection planes in the low 2u

region. This result shows that the perovskite structure withchemical formula of (CnH2n11NH3)2(CH3NH3)m21PbmBr3m11

has already formed in the powder sample. In contrast, onlythe diffraction patterns from the (001) plane are observed inthe spin-coated film. This evidence led us to stronglysupport the fact that these films were highly oriented withthe c-axis perpendicular to the substrate. Moreover, theobservation of higher order peaks indicates that the perov-skite films are well crystallized. From the diffraction peaks,the long cell dimension along thec-axis was calculated to be18.0 A.

X-ray diffraction profiles of (C6H13NH3)2(CH3NH3)m21

PbmBr3m11 �m� 2 and 3) fabricated under optimum con-ditions are depicted in Fig. 5. The (C6H13NH3)2

(CH3NH3)Pb2Br7 �m� 2� films also exhibited an X-raydiffraction pattern which corresponded to a layered perovs-kite structure suggesting primarilyc-axis orientation. Noevidence for lower order (i.e.m� 1� or higher order�m .2�members of the perovskite family could be detected usingX-ray diffraction. Similarly, a clear diffraction pattern isobserved in the case of (C6H13NH3)2(CH3NH3)Pb3Br10 �m�3�: However, the half width at half maximum of the peak ofthe trilayer compound is wider than that of the mono- andbilayer compounds. This is thought to be derived from thesmaller grain size of the perovskite crystal or distortion ofthe inorganic layers.

The interlayerd-spacing values calculated from the X-raydiffraction are 23.2, and 28.5 A˚ for C6Pb2Br7 and C6Pb3Br10

spin-coated films, respectively. An increase in layer spacingwas observed. The increase from then� 1 to the n� 2structure is 5.2 A˚ , and that fromn� 2 to n� 3 is 5.3 A.These values are nearly equal to the lattice constant 5.8 A˚ ofthe cubic perovskite, CH3NH3PbBr3. The increase observed


Fig. 4. X-ray diffraction profiles of (a) powder; and (b) spin-coated film of (C6H13NH3)2PbBr4(C6PbBr4).

in these systems is the result of inserting two or three moreCH3NH3PbBr3 perovskite layers per unit cell.

These X-ray diffraction results clearly demonstrated thatthe spin-coated film was highly oriented normal to thesurface, which is consistent with ‘mono’, ‘bi’, and ‘tri’layer structures, respectively.

4.1.2. Absorption spectraFig. 6 shows the optical absorption spectra of

(C6H13NH3)2(CH3NH3)m21PbmBr3m11 �m� 1–3� at roomtemperature. The monolayer�m� 1� exhibits a strong andsharp absorption at 395 nm even at room temperature asshown in Fig. 6(a), which is assigned to the exciton from


Fig. 6. Absorption spectra of spin-coated films of (C6H13NH3)2(CH3NH3)m21PbmBr3m11 �m� 1–3�:

Fig. 5. X-ray diffraction patterns of spin-coated films of (C6H13NH3)2(CH3NH3)m21PbmBr3m11 �m� 2 and 3).

the [PbBr6]42 inorganic layer. The exciton is considered to

be formed by the transition of the Pb21 (6s) orbital hybri-dized with the Br (4p) orbital to the Pb21 (6p) orbital and isconfined two-dimensionally by the organic layers.

Thin films of the bilayer�m� 2� and trilayer�m� 3�also show strong, sharp exciton absorption due to theirlarge exciton binding energy. A red shift of the excitonabsorption was observed with increasing numbers ofinorganic layers,m. There might be two reasons forthe red shift. The first reason is the decrease in thequantum confinement effect because of the expansion ofthe exciton Bohr radius, which causes a decrease in excitonbinding energy. The second reason is the decrease in thebandgap. As the inorganic sheet thickness increases, theconduction and the valence bands become wider becausethe transfer energy among [PbBr6]

42 octahedra becomeslarger.

These perovskite films showed strong photoluminescenceeven at room temperature sufficient to detect with an aidedeye. The luminescence originates from electronic transitionswithin the inorganic perovskite layer, rather than the organiclayer, because the simple organic molecules beingemployed are transparent in the visible reigon.

4.2. The effect of alkyl sidechain length on the two-dimensional structure

Fig. 7 demonstrates the dependence of the layerd-spacingof ‘mono’ and ‘bi’ layer perovskites upon alkyl chain length.The d-spacing values of each layer with various alkylam-monium molecules are summarized in Table 1.

Layer spacing is increased linearly with increasing alkylchain length in monolayer perovskites. This result showsthat the distances between the inorganic layers are control-lable by changing the alkyl chain length. The relationshipbetween thed-spacing and alkyl chain length is derivedfrom the results for monolayer compounds expressed asd � �A� � 8:051 1:59× n (here,n is the number of carbonatoms of the alkylamines). Considering that the chain lengthof a normal hydrocarbon increases by 2.5 A˚ per carbonatom, the alkyl chain is supposed to be tilted or interdigi-tated. A usual result was obtained with the bilayer system.Although the d-spacing increases linearly similar to themonolayer system in the case that alkylamines are longerthan propylamine�n� 3�; C2Pb2Br7 deviates from linearity.

The absorption spectra of the monolayer, CnPbBr4 �n� 2and 6), are shown in Fig. 8. Fig. 9 displays the excitonabsorption peaks of the monolayer, CnPbBr4, as a functionof alkyl chain length. C6PbBr4 showed a single clear excitonpeak at 395 nm due to the two-dimensionality as describedpreviously. All of the CnPbBr4 films, except forn� 2; showan exciton peak at around 390 nm. Although the distancebetween the adjacent lead bromide layers changes from12.62 A for n� 3 to 18.02 A for n� 6; the qualitativefeatures of the absorption spectra are essentially the same.This demonstrates that the interaction between the layers isweak enough to treat them as two-dimensional. On thecontrary, C2PbBr4 showed two absorption peaks at 390


Fig. 7.d-spacing value of CnPbBr4 and CnPb2Br7 spin-coated films as a function of carbon atoms in the alkyl chains.

Table 1TheD-spacing value of CnPbBr4 and CnPb2Br7 spin-coated films

Monolayer d-spacing (A) Bilayer d-spacing (A)

C2PbBr4 11.62 C2Pb2Br7 22.64C3PbBr4 12.62 C3Pb2Br7 18.39C4PbBr4 13.80 C4Pb2Br7 19.62C6PbBr4 18.02 C6Pb2Br7 23.23C10PbBr4 23.86

and 402 nm. This result revealed that C2PbBr4 does not haveenough distance to neglect the percolation of wave function.

4.3. Thermal analysis of layered perovskite-type compounds

Fig. 10 shows the simultaneous thermogravimetric analy-sis (TGA) and differential thermal analysis (DTA) scan forthe C6PbBr4. All of the layered perovskite compounds trea-ted in this paper are stable to heat up to 2008C but graduallydecompose above that point.

DSC studies on the perovskites were undertaken demon-strating structural transitions below the melting/decomposi-tion point for many of these compounds. These transitionsare driven primarily by order–disorder transitions of the

alkylammonium cations, and for the longer alkyl chaincompounds, by conformational and melting transitionswithin the organic layer. Decomposition of the(CnH2n11NH3)2MX4 compounds often occurs through thesimultaneous loss of organic amines and HX from thecompound below the melting temperature.

Fig. 11 shows the DSC thermographs of CnPbBr4 withn� 4 and 6, respectively. All of the monolayer perovskites,CnPbBr4, except for then� 4 compound, are found to showa structural phase transition under room temperature. On theother hand, CnPbBr4 shows a structural transition at around1108C as shown in Fig. 11(a). This is an important merit forapplication to an optical device because of the exciton peakdoes not skip at the low temperature.


Fig. 8. Absorption spectra of CnPbBr4 �n� 2 and 6) spin-coated films.

Fig. 9. Exciton absorption peaks of CnPbBr4 as a function of the carbon number in the alkyl chains.


Fig. 10. Tg–DTA for C6PbBr4.

Fig. 11. DSC curves of CnPbBr4 for (a) n� 4; and (b)n� 6:

5. Conclusion

The layered perovskite compounds (CnH2n11NH3)2

(CH3NH3)m21PbmBr3m11, of which the inorganic layer thick-ness in the layered perovskites ranges from one to three, canbe achieved by changing the ratio of the two amines(CnH2n11NH3/CH3NH3). The layered perovskite filmsshowed strong, clear exciton absorption peak at roomtemperature due to their large exciton binding energy.With an increasing number of inorganic layers, a red shiftof the exciton absorption was observed due to the decreasein transfer energy. No difference in the absorption peakwas observed in the case of alkylamines as long aspropylamine.

Acknowledgements

This work was supported by the Core Research for Evolu-tion Science and Technology (CREST) of the Japan Scienceand Technology Corporation (JST).

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