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C: Physical Processes in Nanomaterials and Nanostructures
Atomic-layer Dependence of Shear Modulus in Twodimensional Single-crystal Organic-inorganic Hybrid Perovskite Qikun Li, Sheng Bi, Jingyuan Bu, Chaolong Tang, Zhongliang Ouyang, Chengming Jiang, and Jinhui Song J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02080 • Publication Date (Web): 29 May 2019 Downloaded from http://pubs.acs.org on May 30, 2019
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The Journal of Physical Chemistry
Atomic-layer Dependence of Shear Modulus in Two-dimensional Single-crystal Organic-inorganic Hybrid Perovskite
Qikun Li,1,2 Sheng Bi,1,2 Jingyuan Bu,1 Chaolong Tang,3 Zhongliang Ouyang,4 Chengming Jiang1,2,* and Jinhui Song1,2,* 1
Institute of Photoelectric Nanoscience and Nanotechnology, School
of Mechanical Engineering, Dalian University of Technology, Dalian 116024, P. R. China 2
Key Laboratory for Precision and Non-traditional Machining
Technology of the Ministry of Education, Dalian University of Technology, Dalian 116024, P. R. China 3
School of Physical &Mathematical Science, Nanyang Technological
University, 637371, Singapore 4
Department of Electrical and Computer Engineering, Center for
Materials for Information Technology, The University of Alabama, Tuscaloosa, AL, 35487, U.S.A. *E-mail:
[email protected] and
[email protected] 1
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ABSTRACT: Characterizing basic material properties especially, mechanical properties is the prerequisite for building reliable and durable devices. As a promising semiconductor material, two dimensional (2D) organic-inorganic hybrid perovskites possess outstanding optical and electrical properties and have attracted significant interest for their wide energy applications. Therefore, basic mechanical property study of 2D perovskites material will both improve the understandings of atomic-layer dependent property change and guide next generation novel device designs. Here, we report a direct shear modulus characterization of 2D organic-inorganic hybrid (C4H9NH3)2PbBr4 perovskites in direction by atomic force microscopy (AFM). The measured shear modulus of the 2D perovskite increases significantly with the decrease of the atomic-layers, especially as the layer number is less than 60. A composite sandwich model to the free surface contractions of the molecular interaction length is built to reveal this abnormal atomic-layer dependent mechanical phenomenon. The characterization method and the sandwich model with rigid-elastic atomic interaction can be extended to analyze other 2D materials.
2
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The Journal of Physical Chemistry
INTRODUCTION Two-dimensional (2D) organic-inorganic hybrid perovskites, as one promising semiconductor material with large light absorption coefficient, high charge carrier transport, intense photoluminescence, cheap and easy fabrication, and slow non-radiative charge recombination, become ideal building blocks for widespread applications in high-performance optoelectronic and energy generation devices.1–5 Recently, 2D perovskites achieve long-term stability by “slicing” the three-dimensional (3D) into well-defined 2D slabs,6 and thus attract a tremendous research interests.7–10
In
contrast
to
the
bulk
counterpart,
the
novel
organic-inorganic hybrid 2D perovskites , which consists of vertically stacked 2D atomic layers by van der Waals force, exhibit excellent electrical and mechanical properties due to quantum confinement effect and extensively high surface energy.11–14 Additionally, 2D perovskites exhibit more tunable and flexible properties originating from the synergy of
alternately
organic-inorganic
laminated
crystal
structure.
Characterizing the mechanical properties of 2D perovskites plays a key role for building reliable and durable flexible optoelectronics, foldable sensors, wearable and energy devices with desired functions, and it also improves the understanding of the interaction mechanism of the atomic layers.15,16 The mechanical properties of these 3D perovskites have been 3
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well studied in experimentally and theoretically17–24. So far, only Young’s modulus of qusi-2D (CH3(CH2)3NH3)2(CH3–NH3)2Pb3I10 perovskites has been studied by atomic force microscopy (AFM)
25,26.
As a basic but
important property study, shear modulus properties characterization on 2D perovskites has not been carried out due to the materials’ 2D state and the difficulty to induce shear deformation in in-situ measurements. Here, we report a systematic study of shear modulus characterization on 2D perovskites by an in-suit AFM method. For the first time, the shear modulus of 2D hybrid (C4H9NH3)2PbBr4 perovskites in direction is characterized at 82.4 ± 13.2 MPa (12 layers), close to 2.8 times larger than that of 3D counterpart. Furthermore, an atomic-layer dependent shear modulus in 2D perovskites is experimentally revealed, and a sandwich model with quantum correction is proposed to explain the physical origin of the atomic-layer effect.
METHODS Single-crystalline 2D (C4H9NH3)2PbBr4 hybrid perovskites, fabricated by controllable solvent exfoliation method (see in supporting material),27 have well-defined square shape and sharp edges, which have evenly dispersal on the SiO2 substrate as shown in Figure 1a and Figure 1b. The size of the square-shape perovskites ranges from 1 microns to 10 microns, 4
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The Journal of Physical Chemistry
and the number of layers various around 1-200 layers (the thickness of a single (C4H9NH3)2PbBr4 layer is about 1.6nm27). The square shape and the sharp edges of the single crystal 2D (C4H9NH3)2PbBr4 perovskites provide a perfect sample platform for both mechanical characterization and the model calculation. Figure 1c shows the composition and element distribution of carbon, nitrogen, bromine and lead for the 2D (C4H9NH3)2PbBr4 crystals using energy dispersive spectroscopy (EDS). Figure 1d is the transmission electron microscopy (TEM) image of the 2D (C4H9NH3)2PbBr4 perovskite and the inset is the diffraction pattern, which indicates the sample is a single crystal. According to the diffraction pattern, the atomic structure of the 2D (C4H9NH3)2PbBr4 perovskites is shown in Figure 1e, which illustrates that one Pb2+ is surrounded by six Br- and the four in-plane Br- are shared by two octahedrons, forming a 2D sheet of PbBr42– with strong ion bond (at about 260 kJ/mol).28 The interaction between layers is weak van der Waals force (
