Degradation of Two-Dimensional CH3NH3PbI3 Perovskite and

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Degradation of two-dimensional CH3NH3PbI3 perovskite and CH3NH3PbI3/graphene heterostructure Ziyu Wang, Qingdong Ou, Yupeng Zhang, Qianhui Zhang, Huiying Hoh, and Qiaoliang Bao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04310 • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018

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ACS Applied Materials & Interfaces

Degradation of two-dimensional CH3NH3PbI3 perovskite and CH3NH3PbI3/graphene heterostructure

Ziyu Wanga,c, Qingdong Oua,c, Yupeng Zhangb,*, Qianhui Zhangd, Hui Ying Hohb, and Qiaoliang Baoa,c*

a

Department of Materials Science and Engineering, Monash University, Wellington Road, Clayton,

Victoria 3800, Australia. b

College of Electronic Science and Technology, Shenzhen University, Shenzhen 518000, P. R. China

c

ARC Centre of Excellence in Future Low-Energy Electronics Technologies (FLEET), Monash University,

Clayton, Victoria, Australia d

Department of Civil Engineering, Monash University, Clayton, VIC, Australia

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Abstract Hybrid organic–inorganic metal halide perovskites have been considered as promising materials for boosting the performance of photovoltaics and optoelectronics. Reduceddimensional condiments and tuneable properties render two-dimensional perovskite as novel building blocks for constructing micro/nano scale devices in high performance optoelectronic applications. However, the stability is still one major obstacle for long-term practical use. Herein, we provide micro-scale insights into the degradation kinetics of two-dimensional CH3NH3PbI3 (MAPbI3) perovskite and CH3NH3PbI3/graphene heterostructures. It is found that the degradation is mainly caused by cation evaporation, which consequently affect the micro-structure, light-matter interaction and the PL quantum yield effiency of the 2D perovskite. Interestingly, the encapsulation of perovskite by monolayer graphene can largely preserve the structure of the perovskite nanosheet and maintain reasonable optical properties upon exposure to high temperature and humidity. The heterostructure consisting of perovksite and another two-dimensional impermeable material affords new possibilities to construct high-performance and stable perovskite-based optoelectronic devices.

KEYWORDS: Hybrid organic-inorganic perovskite, Graphene, Stability, Optical properties,

Heterostructure, Photoluminescence


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1. INTRODUCTION Recent progress of hybrid organic-inorganic perovskite based photovoltaic devices with high power efficiencies (>22%) has resulted an emerging class of semiconductor materials with next generation high-performance and low-cost photovoltaic devices gained extensive research effort since the reported low-cost efficient solar cell based on the hybrid perovskite at 2011.1-2 Their unique properties, including sizable bandgap, strong absorption, high photoluminescence quantum efficiency, reduced exciton binding energy and remarkable diffusion length promise numerous applications in optoelectronic and photonic.3 In particular, metal-halide perovskite has been successfully implemented in high efficient light-emitting devices4-5, photodetectors6-9, optical waveguide6 and nano/micro laser10-11. Along with rapid development on preparation methods and synthetic conditions, perovskite have been subject to various dimensionalities,

from zero-dimensional (0D) quantum dots13 (QD)

and

nanoparticle, one-dimensional (1D) nanowires10, 12-13 and nanorods14-15, to two-dimensional (2D) nanosheets16 and nanoplates17. Unique features from low-dimensionality (LD) such as quantum confinement effects18 and very high photoluminescence (PL) quantum yield19-20, tunable band structures suggest a great promise in integration in modern high performance photonic and optoelectronic devices in the near future.21 However, it is increasingly recognised that practical use of this kind of hybrid perovskites is significantly hindered by poor long-term operational stability and durability.22 Due to the induced organic component within inorganic ordered frame, the organic part in perovskite is volatile under high temperature ( > 130 °C ), high humility and strong light illumination. Moreover, the LD perovskite materials with ionic crystal structure would lead to inevitable dangling bond and vacancy defects on the nanocrystal surface, further reducing the stability of LD perovskite materials.3 For instant, moisture was believed to be the main factor leading to degradation of LD perovskites by formation of the intermediate hydrated phase and end products PbIOH with ingress of water.23-24 Thermal induced degradation from CH3NH3PbI3 to PbI2 by release of organic components have been discovered as another important cause for deterioration in optoelectronic properties and relevant device performance.25 Moreover, field-assisted ion migration in an on-working LD perovskite based optoelectronics will also decrease the stability significantly. However, it still remains elusive about the understanding of the micro-scale evolvement of the morphology and properties while LD perovskite is degrading upon external stimulus.



In these regards, it is crucial to experimentally demonstrate the degradation kinetics of the LD perovskites and related devices upon external humidity, heat and applied voltage26-27, as well as the influence of degradation on the micro-structure, light-matter interaction, and the PL quantum yield efficiency of perovskite.

Herein, we reveal the photo-physical and

structural changes of LD perovskite nanostructures under high temperature, light illumination and humidity from micro-scale perspectives with optical characterization methods. Taking as the 2D CH3NH3PbI3 as example because not only CH3NH3PbX3 represent the recent research trend of efficient photovoltaic deceives but also 2D CH3NH3PbI3 has been demonstrated strong light integration and opto-electronic device with good performance. Moreover, the two-step method used in this work is essentially different from that reported for layered 2D perovskite. Our method is highly adjustable to fabricate a broad range of 2D hybrid perovskites with similar crystalline by intercalating various organic or halogen components into the lattice of templating precursors. By tunning the parameters of the solution process, the morphology, thickness and lateral dimension of PbI2 as well as corresponding 2D perovskite nano-sheets could be well adjusted. The experimental results the show a rapid decrease in PL emission and thickness reduction of 2D perovskite nanocrystals under abovementioned conditions. In order to improve the stability and reduce cost, Matsuo’s group proposed stable lead halide perovskite solar cells with sandwiched structure comprising of C60 layer and single-walled CNTs as charge collecting and moisture protecting layers, respectively, which results in 100-min steady operation. Moreover, combined with hydrophilicity of fullerene layer embedded Li+ and oxidation resistance of Li@C60 layer, the stability of the modified perovskite solar cells would be boosted up to 1000 h.28-29 Here, we show that the encapsulation of perovskite by monolayer graphene can largely preserve the structure of the perovskite nanosheet and maintain reasonable optical properties upon exposure to high temperature and humidity, owing to the impermeability of graphene and enhanced charge transfer in the heterostructures. This novel strategy manifest a critical step towards commercialization of perovskite-based optoelectronics by resolving stability issues.

2. RESULTS AND DISCUSSION Here, we adopt previously developed two-step method for the preparation of the high quality 2D hybrid perovskite nanosheets.6 Generally, the process starts with the formation of 2D PbI2 nano-sheet template on a substrate after evaporation of the solvent from saturated PbI2 aqueous solution upon heating. Following, the CH3NH3I molecules diffuse into the spacing ACS Paragon Plus Environment

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between PbI6 octahedrons layers in a chemical vapour deposition (CVD) furnace, forming the 2D CH3NH3PbI3 perovskite nanocrystals afterwards. The heating zone of CVD system was elevated to 120°C under low pressure (40-50 torr) and maintained for 40 minutes. The asgrown CH3NH3PbI3 perovskite still retains the well-defined 2D structure with sizes ranging from a few to a hundred micrometres. However, surface dangling bonds tend to induce unavoidable lattice strain in the 2D perovskite nanosheets, as evidenced by PL peak position change30, which may further affect the stability of perovskite materials. When exposed to ambient environment in the dark (RH~ 30%), 2D perovskite nanosheets degrade rapidly in two days, as illustrated in Figure 1 (a). Time-dependent PL spectra were recorded to investigate the degradation of nanosheet. A laser with wavelength of 532 nm was used to excite our 2D perovskite samples and the laser power and integration time was kept at 1 µW and 0.01s to minimise the impact of laser heating. As shown in Figure 1 (b), PL intensity declines rapidly under ambient environment within two days. The corresponding PL mapping images (Inset in Figure 1(b)) show no detectable PL signal after two-day exposure, suggesting total degradation of 2D perovskite nanosheets. The difference in optical contrast between pristine and degraded perovskites also indicates the change in chemical composition and sample thickness after degradation. To investigate the degraded products, we performed Raman measurements using a 532 nm laser in air. The black trace in Figure 1(c) represents the Raman spectrum of the aged perovskite nanosheets. Four predominating sharps and broad peak are observed at 75, 94, 110 and 215 cm-1 in the 50-350 cm−1 region, which could be assigned to E12 (shear deformation mode), A11 (breathing deformation mode) and A21 mode.31 All characteristic Raman bands of the aged perovskite nanocrystals are in good agreement with previously reported spectral bands for PbI2 crystals, confirming the degraded products of perovskite nanosheet after aging experiments.31 For comparison, we also carried out Raman measurements for pristine 2D perovskite nanosheets, in which there are no observed Raman bands as indicated in the red trace in Figure 2(b), suggesting that 2D PbI2 are the degradation products of 2D perovskite nanosheets. Additionally, Figure 1 (d) shows the Raman mapping with respect to the characteristic A11 Raman mode. Recently, it was reported that a crystal structure transformation from MAPbI3 (tetragonal) to layered PbI2 (trigonal) happens during degradation process, which was confirmed by in-situ selected-area diffraction (SAED) patterns. Detailed SAED analysis shows uniform and continuous transition MAPbI3 to PbI2 with time, suggesting a layer-by-layer breakage of Pb-I-Pb bonds and consequent release of organic molecules along the [001] direction.32 The relatively uniform Raman intensity over the entire degraded 2D perovskite product and uniform surface reduction of 2D perovskite ACS Paragon Plus Environment


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nanosheet under ambient conditions (as shown in Figure S2) suggests that the organic component escapes from the samples continually and uniformly during aging process. Figure 2 presents control experiments of the stability of 2D perovskite nanocrystals at 150 and 200 °C temperature in dark environments. The optical images of samples show a continuous colour change, in which a typical 80 nm-thickness sample changes from brown to green in 20 minutes as shown by Figure 2 (a-d). These results suggest a continuous colour change accompanying conversion from PbI2 to CH3NH3PbI3, which could suggest a thickness change during heat treatment.17, 33 The in-situ thickness changes in perovskite samples with different thicknesses were measured by atomic force microscopy (AFM) at room temperature after the 2D perovskite samples were heated at 150 and 200 °C, respectively. As presented in Figure 2 (e), the relative thicknesses, defined as the ratio of measured thickness and original thickness, decrease with times at elevated temperatures in ten minutes. The thicknesses reduce more significantly and reach its final value (52% of original thickness) in shorter time at higher temperature (200 °C). These observations are also consistent with recent reports, which suggest that the thickness of synthetic CH3NH3PbI3 increased by a factor of about 1.8 after the introduction of organic component in the octagonal lead halide during a fast reverse process of conversion from PbI2 to CH3NH3PbI3.17, 33 Moreover, XRD results in Figure S3 suggests the complete decomposition from 2D MAPbI3 to layered PbI2 because of tetragonaltrigonal structure changes upon heat treatment. The resulting reduction in thickness can well explain the changing contrast shown in Figure 2(a-d). The thickness changes of a typical sample are further determined by contrast spectroscopy, as shown in Figure 2f. The contrast is defined as: Contrast =

I − Ib Ib

(1)

where I and Ib refer to the reflected signal from 2D perovskite nanosheet and the background, respectively.34 The contrast value at around 700 nm decrease, indicating sample thickness reduction upon heating at 150 °C. Moreover, it is found that contrast spectrum of 2D perovskite nanosheet is blue-shifted with the decrease in sample thickness. Corresponding photoluminescence (PL) spectroscopy were performed under the illumination of 532 nm laser. Figure 2 (g) depicts PL spectra of the 2D perovskite crystals over various heating times at 150 °C, in which a strong emission peak at around 755 nm can be observed; this is consistent with recent report.6 With increasing heating times, the PL intensity decrease significantly


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with broadening spectral width and blue-shifting, resulting into wider bandgap, as shown in Figure 2 (h). This observation indicate surface reconstruction induced by water molecules.35 The blue shift in PL peaks is consistent with previous theoretical investigation on moistureinduced instability of CH3NH3PbI3.35

In this regard, decomposition based on cation

evaporation leads to the change in electronic structure, which results into significant impacts on the light-matter interactions of perovskite materials. To investigate the impact of missing organic cation on the light-matter interaction in the 2D perovskite nanocrystals, scanning photocurrent mapping (SPCM) was performed on lateralstructure CH3NH3PbI3 devices (Figure 3 (b)) under a confocal optical microscope, as schematically depicted in Figure 3 (a).36-37 In order to promote photocurrent generation, 1 V was applied between two electrodes. All SPCM data was attained by collecting the drain current (ID-S) at the drain-source bias (VD-S) of 1V under laser illumination (wavelength: 532 nm; spot size: ~ 500 nm). Similar to previous reports, photo-generated current is observed predominantly in the proximity of Au electrodes, where exciton separation and carrier collection happen at the same area especially under the drain bias, as shown in purple region in Figure 3 (d).38-39 Interestingly, after poling at a relatively high field of 2 Vµm-1 for 1 minute, few micro-sized areas at the top electrode interface showed apparent colour change from yellow to green, revealing possible field-induced strain and ion migration.36, 40 Figure 3 (e) presents corresponding SPCM of the 2D perovskite samples under high field poling process. The photo-generated current in the proximity of negative electrode is decreased substantially after this electric poling process. However, the contrast change did not revert after the poling bias was turned off, unlike previous reports.41-42 This irreversible process can be attributed to larger field-induced strain due to thinner sample thickness compared to that in previous report. The photocurrent profiles through the centre of SPCMs (as indicated by the white dash lines in Figure 3 (d, e) are plotted in Figure 3 (f) Apart from the obvious decease in photocurrent generation, the peak shifts towards the negatively charged electrode, suggesting reduced carried diffusion length after strong electrical poling process. AFM was performed on a 2D perovskite device to evaluate the physical change caused by field-induced strain. As depicted in Figure3 (g-h), there is a 15 nm thickness reduction in proximity of the electrode, which represents approximately 40% loss of thickness upon electrical poling process. However, optical image of the post-annealed sample suggest a recovery in 2D perovskite crystal in excess methyl-ammonium iodide (MAI) vapour for 1 hour (Figure. S5). In other words, 2D perovskite crystal lattice is less likely to suppress the release of organic



component due to field-induced strain. Therefore, the poling-induced strain would be a major consideration for stable and proper device operation. It is crucial to inhibit the evaporation of organic cations to ensure long-term operation of perovskite-based optoelectronic devices. Herein present a dry transfer approach to construct 2D perovskite/graphene on varying substrates, which is presented schematically in Figure S4. Figure 4 (a) represents schematic diagram of 2D perovskite/graphene structure, where the encapsulated graphene layer functions to hinder leakage of the organic molecules and water ingress at ambient environment. The optical images of 2D perovskite / 1L or 2L graphene are shown in Figure S6 (a). To determine the coverage of graphene, spatially-resolved PL images were performed to delivery visual message in regard to PL quenching effect on 2D perovskite/graphene structure samples after dry transfer of graphene, as shown in Figure 4 (b). Two distinct zones could be found in the PL images, where brighter areas (yellow colour) represents exposed 2D perovskite and the rest of areas (orange) stands for the 2D perovskite covered by graphene where PL quenching is resulted from charge transfer between graphene and perovskite. After two days in a dark but otherwise ambient environment, PL mapping shows zero PL emission intensity in the exposed areas, as expected in abovementioned aging results in the pristine 2D perovskite as present in Figure S6. By contrast, strong PL emission could be observed in the remaining graphene-protected area, indicating a good protection from graphene layer. The stability enhancement could be attributed to nanobubbles across the graphene sheet, which are included by lattice mismatch between perovskite and graphene. The organic components can be entrapped in the nanobubbles due to the impermeability of graphene, which resulting into enhance stability of 2D perovskite coved by graphene. The topological change of the 2D perovskite sample was investigated using AFM during the two-day aging process, as shown in Figure 4 (c-d). The white dash line marked areas in both Figure 4 (c-d) corresponding exactly to the graphene-protected zone on 2D perovskite as indicated in PL images (Figure 4 (b), marked by black dash line.) The height profiles cross interface show a graphene/perovskite step of 3 nm and 20 nm respectively (Figure 4 (e)), with the 3 nm step matching the thickness of transferred graphene onto the 2D perovskite nanosheet. Moreover, the graphene-coated perovskite maintained its original thickness even after two-day exposure to air whilst unprotected perovskite area shrunk to around 30-40 % of its original thickness, indicating good structural preservation due to graphene protection. In order to evaluate longterm effects, PL spectra on graphene/2D perovskite were collated constantly during an additional 28-day aging process. The remarkable stability difference in protected and


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unprotected area is depicted in Figure 4 (f). Even the PL intensity decrease can be observed in 2D perovskite partially covered by a graphene layer, organic components may diffuse to surface vacancies in 2D perovskite and release laterally along the graphene and perovskite interface. In this regard, to stack multiple graphene layers as protection film seems to be a solution for further stability enhancement.45 However, approximate 35 % of original PL intensity was retained in the graphene/2D perovskite after 30 days while rapid PL intensity decrease could be observed in pristine 2D perovskite in two days, revealing significant enhancement in stability with graphene protection. Apart from stability in ambient conditions, the protection of perovskite by graphene was further evaluated via heat treatment at 150°C for 20 minutes. The optical images of the graphene/2D perovskite before and after heat treatment were shown in Figure 5(a) and Figure 5(d) respectively, in which area 1 shows graphene on perovskite and area 2 represents the pristine perovskite without graphene protection. Corresponding scanning PL mapping was performed, as shown in Figure 5(b) and Figure 5(e) respectively. Owing to PL quenching effect on graphene (Figure 5(c)), similar to Figure 4 (b), area 1 and 2 could be easily distinguished based on the difference in PL intensity in Figure 5 (b). After the 20-minute heat treatment, there is almost zero PL emission in area 2 while PL intensity in area 1 is about 50 times higher than area 2 (Figure 5 (f)), suggesting a surprising PL preservation on graphene/2D perovskite. Moreover, Raman spectra of graphene on SiO2/Si substrate and graphene on perovskite indicate good quality of graphene on perovskite after the dry transfer process. (Figure 5 (g)) A comparison between the pristine perovskite and graphene/perovskite is shown in Figure 5 (h). After heat treatment for an hour, rapid PL decline due to degradation is observed in unprotected perovskite while a much slower PL delay is seen in graphene/perovskite heterostructure. Finally, as shown in Figure 5(i), the PL intensity gradually decreases and the peak blue-shifts with increase in heating times. Since this observation is similar to the PL peak shift seen in the pristine 2D perovskite under ambient condition (Figure 2(h)), we propose that there may be water molecules trapped between graphene and 2D perovskite. Moreover, it is believed that the stability of perovskite devices can be further enhanced by covering graphene with less defects and gain boundaries if the graphene transfer process is further optimised. Previous reports suggest that the defects and grain boundaries in graphene could offer favourable paths for the gases leakage, which in return promotes direct absorption of moisture on the perovskite nanosheet surface.43



Obviously, how the quality of graphene affects the stability of 2D perovskite/graphene is worthy of further study in the future.

3. CONCLUSIONS In summary, we evaluated the evolution of photo-physical properties and structure instability in 2D hybrid perovskite nanocrystals and graphene/perovskite heterostructure. Cation evaporation induces decomposition, resulting in major degradation of the pristine perovskite crystals. Humility, heat and electrical field are fundamental factors which lead to the significant degradation to the photo-properties, crystal structural and device performance such as rapid PL intensity decrease and crystal thickness reduction upon heat. By trasnfering graphene on 2D perovskite crystals, it is found that 2D perovskite/graphene can preserves the structure of the 2D perovskite upon exposure to high temperature and humidity and maintains desirable optical properties of 2D perovskite. The heterostructure consisting of perovskite and another two-dimensional impermeable material such as graphene or h-BN may bring combined benefits to perovskite-based optoelectronic devices and afford long-term operation.


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METHODS Synthesis of the 2D perovskite nanosheet. The 2D CH3NH3PbI3 perovskite nanosheets were fabricated by previous reported two-step method (i.e., a combined solution and vapour phase approach).6 The saturated PbI2 solution was produced via dissolution of 1 mg PbI2 powder (Sigma-Aldrich) into 1 mL heated DI water at 100 °C for 1 hour. The PbI2 solution was then cooled down naturally to around 40 °C. The cooled PbI2 solution was drop-casted onto target substrates for 1 minutes and excess solvent was removed by air blowing. The isolated 2D PbI2 nanosheets will precipitate on the substrate. During vapour process, the CH3NH3I powder produced by previously reported method44, was located at the centre of a tube furnace. Meanwhile the solution-processed 2D PbI2 on substrates were mounted downstream of the tube furnace. The central CVD furnace was heated up to 120 °C slowly under low pressure and maintained for 40 minutes. Ar was used as the carrier gas with a rate of 30 sccm. Graphene Dry transfer process. 1L/2L graphene film on copper foil (Graphene Supermarket) was attached to a piece of Gel-film and copper foil was etched in 0.1 M/L ammonium persulfate (APS, Sigma-Aldrich) solution. The graphene layer was transferred on 2D perovskite by pressing Gel-film on SiO2/Si substrate gently and followed by slowly peeling off the Gel-film. Characterization of the 2D Perovskite nanosheet. The topography and thicknesses of the 2D CH3NH3PbI3 perovskite nanosheets were investigated by optical microscope and atomic force microscope (AFM, Bruker, Dimension Icon SPM). Raman /PL spectroscopy was carried out with a confocal microscope system (WITec, alpha 300R) with a ×100 objective lens (NA = 0.9), equipped with a 532 nm laser, in ambient conditions. The spectra were obtained by using 600 line/mm gratings with spectral resolution < 0.09 nm. A low laser power (1 µW) was applied while characterizing 2D Perovskite nanosheets to avoid sample damage. The exposure time was kept at 0.01 s and the accumulation number was 10. Random PL measurements were carried out on whole sample and the average PL intensity of each excited point was taken. The scanning photocurrent mapping measurements were performed by combining confocal Raman system with a source meters. Specifically, a 532 nm laser was fiber-coupled (Thorlabs P5-405BPM-FC-2) through a fiber bench (Thorlabs FBP-A-FC) and an optical chopper (C995, Terahertz Technologies, Inc.) was used to improve signal-to-noise ratio. The X−Y piezo-stage was adopted to acquire two-dimensional spectral maps. The photocurrent was amplified by a preamplifier (FEMTO DLPCA-200, Messtechnik GmbH)



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and collected using a lock-in amplifier (FEMTO LIA-MV-150) in the dark with electrical bias of 1V. Post-annealing Treatment for the 2D Perovskite nanosheet. The 2D perovskite samples, to which high electrical poling was applied, were placed at the center of a CVD furnace. The central heating zone was heated to 120 °C and maintained at this temperature for 1 h. Ar was introduced into the quartz tube as carrier gas, with a flow rate of 30 sccm, during the postannealing process. The furnace was then cooled down to room temperature naturally

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxx. Schematic sample preparation and transfer process; AFM images; XRD result; Optical images during electrical poling; PL images of 2D Perovskite/ Graphene (PDF)

AUTHOR INFORMATION Corresponding Author *Correspondence:

(Yupeng

Zhang)

[email protected];

(Qiaoliang

Bao)

[email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGEMENTS


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We acknowledge financial support from the Australian Government through the Australia Research Council (IH150100006, FT150100450, and CE170100039), and the National Natural Science Foundation of China (No. 51702219 and 91433107). This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF). Q. Bao acknowledges the support from the MCN Tech Fellowship. Z. W and Q. O acknowledge support from the MCATM. .

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Figure 1. Photo-physical and structural stability of 2D perovskite nanosheet under ambient environments. (a) Evolution of perovskite PL intensity as a function of time under ambient conditions. (b) The changes in PL spectra in two days and corresponding PL images and optical images of 2D perovskite nanosheet as a function of time. Scale bars are 10 µm. (c) Raman spectra of 2D perovskite and fully degraded perovskite (PbI2) nanocrystal in ambient air, measured with a 532 nm laser Raman laser. (d) Raman mapping of a fully degraded 2D perovskite nanosheet at Raman band 94 cm-1. Scale bar is 5 µm.


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Figure 2. (a-d) Optical images depicting change in perovskite crystals at 150 °C after heating for 0, 5, 15 and 20 min. Scale bars are 10 µm (e) Thickness changes for 2D perovskite nanosheet as a function of heating time at 150 and 200 °C, respectively. (f) Contrast spectra of 2D perovskite sheet with different heating times at 150 °C. (g) PL spectra of 2D perovskite sheet with increasing heating time at 150 °C. The intensity dip at 780 nm is due to the CCD detector limit. (h) The PL intensity declines and PL peak position shifts with heating time.



Figure 3. (a) Schematic of experimental set up for scanning photocurrent mapping. (b-c) Optical images of 2D perovskite nanosheet (b) before and (c) after electrical poling (2 V/µm) for 1 mins. Scale bars are 8 µm. (d-e) Photocurrent mappings of 2D perovskite nanosheet (d) before and (e) after poling. (f) Measured photocurrent along the profile in (d) and (e). (g) AFM image of 2D perovskite nanosheet after electrical bias and (h) height profile along the red line. Scale bar is 5 µm

Figure 4. (a) Schematic diagram of 2D perovskite (P)/graphene (G) heterostructure. (b) PL image mapping of as-fabricated P+G samples. (c) AFM images of 2D P+G nanosheet and (d) after 2 days under ambient conditions. Scale bars are 8 µm. (e) Height profile along the red dashed line in (c) and (d). The height profiles of exposed and graphene-protected areas are indicated by black and red line respectively. (f) A significant enhancement in perovskite stability with graphene coating under ambient conditions.


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Figure 5. Enhanced stability of 2D perovskite/graphene in heat treatment (150°C for 20 mins). Optical images of 2D perovskite/graphene (a) before and (d) after heat treatment. PL images of 2D perovskite/graphene (b) before and (e) after heat treatment. Scale bars are 10 µm PL spectra of 2D perovskite/graphene (c) before and (f) after heat treatment. (g) Raman Spectrum of graphene and graphene/2D perovskite. (h) A comparison between pristine perovskite and perovskite/graphene stability under 150°C within 1 hour. (i) Evolution of 2D perovskite/graphene PL spectra as a function of time under ambient conditions.



TOC:

In this work, we demonstrate micro-scale insights into the degradation kinetics of twodimensional CH3NH3PbI3 perovskite. It is found that the degradation is mainly caused by cation evaporation, which consequently photo-properties, crystal structural and device performance such as rapid PL intensity decrease and crystal thickness reduction. Interestingly, the encapsulation of perovskite by monolayer graphene can largely preserve the structure of the perovskite nanosheet and maintain reasonable optical properties upon exposure to high temperature and humidity. The heterostructure consisting of perovskite and another twodimensional impermeable material such as graphene or h-BN may bring combined benefits to perovskite-based optoelectronic devices and afford long-term operation.


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Degradation of Two-Dimensional CH3NH3PbI3 Perovskite and

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