Two-dimensional halide perovskite nanomaterials and heterostructures

Enzheng Shi , Yao Gao , Blake P. Finkenauer , Akriti , Aidan H. Coffey and Letian Dou *
Davidson School of Chemical Engineering, Purdue University, West Lafayette, IN 47907, USA. E-mail: dou10@purdue.edu

Received 19th December 2017

First published on 22nd March 2018


Over the last several years, there has been tremendous progress in the development of nanoscale halide perovskite materials and devices that possess a wide range of band gaps and tunable optical and electronic properties. Particularly, the emerging two-dimensional (2D) forms of halide perovskites are attracting more interest due to the long charge carrier lifetime, high photoluminescence quantum efficiency, and great defect tolerance. Interfacing 2D halide perovskites with other 2D materials including graphene and transition metal dichalcogenides (TMDs) significantly broadens the application range of the 2D materials and enhances the performance of the functional devices. The synthesis and characterization of 2D halide perovskite nanostructures, the interface of the 2D halide perovskites with other 2D materials, and the integration of them into high-performance optoelectronic devices including solar cells, photodetectors, transistors, and memory devices are currently under investigation. In this article, we review the progress of the above-mentioned topics in a timely manner and discuss the current challenges and future promising directions in this field.


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Enzheng Shi

Enzheng Shi is currently a postdoc research associate in the Davidson School of Chemical Engineering at Purdue University. He received his PhD degree from the Department of Materials Science and Engineering at Peking University in China in 2015. From 2015 to 2017, he worked as a postdoc research associate in the Department of Chemical Engineering at Iowa State University. His research interests include the synthesis of two-dimensional halide perovskite materials and the corresponding optoelectronic applications, flexible electronics and solar cells based on carbon nanomaterials, and bismuth telluride based thermoelectric materials.

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Yao Gao

Currently, Dr Yao Gao is a postdoc research associate at the Davidson School of Chemical Engineering, Purdue University. He received his BS in Chemistry from Nanjing University in 2011. Yao Gao joined Prof. Yanhou Geng's group at Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, and obtained his PhD degree in Polymer Chemistry and Physics in 2017. Dr Yao Gao is now initiating organic and hybrid materials research in Prof. Letian Dou's group. His current research interest includes the synthesis and characterization of organic conjugated and two-dimensional halide perovskite materials and their applications in optoelectronics.

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Letian Dou

Prof. Letian Dou is currently an assistant professor at the Davidson School of Chemical Engineering, Purdue University. He obtained his BS in Chemistry from Peking University in 2009. He obtained his PhD in Materials Science from UCLA under Prof. Yang Yang's group in 2014 (co-advised by Prof. Fred Wudl @UCSB in 2013). From 2014 to 2017, he was a Postdoc Fellow working with Prof. Peidong Yang in the Department of Chemistry, University of California-Berkeley and Materials Science Division, Lawrence Berkeley National Laboratory and California Research Alliance by BASF. His research interest includes the synthesis and characterization of organic, inorganic, and hybrid nanomaterials and their applications in the next generation optoelectronics.


1. Introduction

The term perovskite refers to the mineral compound CaTiO3 and any structure adopting the same ABX3 three-dimensional (3D) structural framework. Similar to oxide perovskites, halide perovskites have been discovered for more than 3 decades. Recently, halide perovskites have attracted significant attention after their initial application in photovoltaic devices by Kojima et al. in 2009.1 In the case of halide perovskites, typically, 3D halide perovskites with the general formula ABX3 (A = organic ammonium cation, Cs+; B = Pb2+, Sn2+; X = Cl, Br, I) have been widely studied as a new family of semiconductor materials for a variety of applications due to their remarkable features, including long charge carrier lifetime and diffusion length, intense photoluminescence (PL), high light absorption coefficients, easily tunable properties, rich phase diagram, and low-temperature solution processability.2 Tremendous progress has been made after an immense amount of effort has been devoted in pursuing high performance solar cells, light emitting diodes, and photodetectors in the past several years.2–9 Considering the solid-state chemistry of the halide perovskites, the formation of 3D halide perovskites is governed by Goldschmidt's tolerance factor, image file: c7cs00886d-t1.tif, and the octahedral factor, μ = RB/RX, where RA, RB, and RX are the radii for the corresponding ions. The tolerance factor should satisfy 0.80 ≤ t ≤ 1.0 and the octahedral factor should satisfy 0.44 ≤ μ ≤ 0.90 to form 3D halide perovskites. Space filling ionic size constraints determine whether a certain set of “A”, “B”, or “X” ions may form the perovskite framework, which involves a corner-sharing network of [BX6]4− octahedra, with the “A” cations occupying 12-fold coordinated voids within the structure and counterbalancing the charge of the [BX6]4− extended anion.10

Intriguingly, halide perovskites also possess remarkable structural and composition tunability. The basic building blocks of halide perovskites (the metal halide octahedra, [BX6]4−) can be arranged in different ways. They can be connected in 3D, two-dimensional (2D), and one-dimensional (1D) fashions, or isolated to form zero-dimensional (0D) crystal structures (Fig. 1a).11–16 In addition, based on the ABX3 3D halide perovskites, it is possible to control the crystal growth kinetics to form low dimensional nanostructures, such as quantum dots, nanowires, and 2D platelets (Fig. 1b).9,17–27 Furthermore, the composition can be easily tuned by choosing different A site cations (e.g. CH3NH3+: MA, CH(NH2)2+: FA, and Cs+), B site cations (e.g. Pb2+ and Sn2+), as well as X site anions (Cl, Br, and I). The ability of making widely tunable halide perovskite nanostructures leads to a great deal of scientific interest in their fundamental structure–property relationships at nanoscale level. Particularly, atomically thin 2D materials with strong confinement in one dimension and free carrier transport in the other two dimensions have drawn significant attention owing to their unique electrical, optical, magnetic, and mechanical properties. A wide range of layered compounds (e.g. graphene, black phosphorene (BP), transition metal dichalcogenides (TMDs), boron nitride, Mxenes) have been chemically synthesized or physically separated and investigated for photovoltaics, field effect transistors (FETs), photodetectors, light-emitting diodes, chemical sensors, energy storage, and gas separation.28–32 However, most of the current 2D materials are metallic or insulating except for BP and TMDs. There is a strong demand for a new type of semiconducting 2D materials that exhibit better optoelectronic properties and wider tunability. To this end, 2D halide perovskites have emerged as a promising new type of 2D semiconductors for a variety of electronics and optoelectronics applications.


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Fig. 1 (a) Representative crystal structures of halide perovskites in different dimensions; (b) nanoscale morphologies of halide perovskites; (c) schematic representation of the 2D organic–inorganic perovskites from different cuts of the 3D halide perovskite structure. L is a large organic cation, A is a regular cation such as Cs+ or MA+, B is a divalent metal cation such as Pb2+ or Sn2+, and X is a halide.

Compared with conventional 2D materials in which atoms are held together through covalent bonds, 2D halide perovskites are more ionic in nature. The relatively weak ionic bonding features make them easily grown by both vapor transport and solution methods at relatively low temperature.33 In addition to the bonding, there are lots of different fundamental chemistry and physics between halide perovskites and other 2D materials, including chemical reactivity and crystal growth mechanism. More importantly, confining the halide perovskites in the 2D geometry provides opportunities to further tune the band gap, transport properties, charge carrier dynamics, chemical stability, etc. The high PL quantum efficiency and long charge carrier lifetime are associated with the defect tolerant nature of the materials. These excellent optical and electronic properties make 2D halide perovskites particularly attractive candidates for nanoscale optoelectronic devices.34

In this review, we first highlight the recent advancements in the synthesis, characterization, and spectroscopy perspectives of optoelectronic properties of atomically thin 2D halide perovskites. Then, the interface of the 2D halide perovskites with other 2D materials such as graphene and TMDs, and the integration of them into high-performance optoelectronic devices including solar cells, photodetectors, transistors, and memory devices, will be discussed in detail. Finally, we will discuss future directions and challenges towards new 2D structures, complex perovskite heterojunctions, and new applications to shed light on this emerging field. Due to the vast number of existing studies on bulk level layered 2D halide perovskites (bulk crystals and polycrystalline films),35–46 we will primarily focus on the nanoscale structure and property control in this article.

2. Synthesis, structure, and properties of 2D halide perovskites

The structure of 2D halide perovskites is highly tunable. As shown in Fig. 1c, 2D halide perovskites can be thought of as slabs cut from the 3D parent structures (so called Ruddlesden–Popper [RP] phase), and certainly could be excised along different crystal directions. The RP phase layered perovskite has the general chemical formula of L2An−1BnX3n+1, where L is a large cation (mostly large-size or long-chain organic cations, such as butyl ammonium C4H9NH3+ [BA+]), A is a regular cation such as Cs+ or MA+, B is a divalent metal cation such as Pb2+ or Sn2+, and X is a halide. The variable n is an integer, which indicates the number of metal halide octahedral layers between the two L cation layers. When n is infinite, the structure becomes a 3D halide perovskite ABX3. When n = 1, 2, 3, etc., the structure constitutes an ideal quantum well with only a few atomic layers of [BX6]4− separated by L cations. Then many of the repeating units can stack together through van der Waals forces to form bulk crystals. In addition to the RP phase 2D perovskites with long chain “L” cations, 2D perovskites can also be prepared by reducing the thickness of the 3D ABX3 perovskites.47 When the 3D thin sheet is very thin, the chemical formula is no longer ABX3 but with a finite “n” value. Meanwhile, the top and bottom surface of the sheet are terminated by extra “A” instead of “L” cations. Such structural flexibility and tunability of 2D halide perovskites provide a rich and fertile platform for the preparation of interesting nanostructures with excellent electronic properties.
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Fig. 2 (a) Schematic illustration of the solution synthesis of 2D halide perovskite materials. Dark-field optical image (b) and TEM image (c) of solution-synthesized (BA)2PbBr4 2D crystals. (d) AFM image and height profile of several single layers. The thickness is around 1.6 nm. Reproduced from ref. 70 with permission from American Association for the Advancement of Science, copyright 2015. (e) Schematic illustration of the colloidal synthesis of 2D halide perovskite materials. (f) TEM image of CsPbBr3 NS prepared with short-to-long ligands molar ratios equal to 0.67. Scale bar: 1 μm. (g) HRTEM image from a thin region of a CsPbBr3 sheet partly suspended on a hole in the carbon film. Scale bar: 2 nm. The inset shows the low magnification TEM image of the whole NS. The scale bar in the inset is 100 nm. Reproduced from ref. 74 with permission from American Chemical Society, copyright 2016. (h) Study of the influence of reaction temperature on CsPbBr3 colloidal synthesis. (The scale bar is 50 nm.) Reproduced from ref. 75 with permission from American Chemical Society, copyright 2015.

In this section, we review the recent development of 2D halide perovskite nanomaterials. Three major topics will be discussed: new synthetic approaches, structural characterization, and optical and electronic properties. Because of the weak bonding and soft nature of these materials, they exhibit many different properties as compared with some of the most well-studied 2D materials (e.g. graphene and TMDs). To emphasize the differences, we will briefly discuss conventional 2D materials before 2D halide perovskites in those topics. We then summarize the characteristics of different types of 2D materials at the end of this section.

2.1. Synthesis and nanoscale morphology control

As one of the first well-studied 2D materials, graphene has demonstrated extraordinary electronic, thermal, and mechanical properties.48–53 The predominant preparation methods include mechanical exfoliation, chemical vapor deposition (CVD), oxidation-assisted liquid exfoliation and sonication-assisted liquid exfoliation.54 Among these methods, mechanically exfoliated graphene generally demonstrates the highest crystal quality. The liquid exfoliation method generates the highest product yield which can support the practical applications such as battery electrodes55 and transparent electrodes.56 In addition, compared with the exfoliation methods, the CVD method is the most widely used and explored growth approach for high-quality and large-area graphene from single to multi layers. Various kinds of substrates including representative metals (Ni, Cu, Pt, Co, etc.), semiconductors (like Ge) and insulating substrates (like glass) can serve as CVD growth substrates of graphene.57–62 The shape and size of graphene domains can be controlled by varying the pressure and concentration of the methane precursor.

Motivated by the rapid development and success of graphene, there has been increasing interest in semiconducting two-dimensional materials. 2D TMDs (e.g. MoS2, WS2, MoSe2, WSe2, and MoTe2) have been intensively investigated because of their tunable band gap and optical properties.63,64 When the TMDs are scaled down from bulk to mono or a few layers, there is a transition in the band structure of TMDs from indirect to direct band gap due to quantum confinement.65–67 The synthesis methods of 2D TMDs consist of mechanical exfoliation, liquid exfoliation, CVD, etc.54 Similar to the synthesis of graphene, mechanically exfoliated TMDs generally demonstrate the highest crystal quality and cleanest surface, while larger lateral size, tunable layer number and morphology control of TMDs can be realized by the CVD method.

To prepare atomically thin 2D halide perovskite nanomaterials, various approaches have been investigated. Inspired by the methodology widely applied in 2D research, mechanical exfoliation was initially investigated. Niu et al. and Yaffe et al. reported the mechanical exfoliation of ultrathin (BA)2PbI4 2D crystals and studied their optical properties via ultrafast spectroscopy.68,69 Large lateral size can be obtained using this method, but the sheets are typically randomly shaped with a large size distribution. Ultrathin sheets with thickness as small as 2.5 nm were obtained. However, the majority of the sheets are much thicker with a large distribution in thickness. Although the exfoliation method is a convenient way to obtain a large single sheet, it's not suitable for high throughput production and high-density device integration. Recently, Dou et al. employed the solution synthesis of halide perovskites to directly grow large-area atomically thin 2D halide perovskite sheets. In this method, a ternary solvent mixture was used, and the crystals grow during the solvent evaporation process.70Fig. 2a shows an illustration of the crystal growth process and Fig. 2b shows the dark-field optical image of the as-synthesized 2D crystals with well-defined square shapes. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) were also used to investigate the morphology of the individual 2D sheets (Fig. 2c and d). The thinnest sheet was confirmed to be a monolayer structure by AFM showing a thickness of ∼1.6 nm. The ternary solvent was found to be critical, because this method can precisely control the solute solubility and solvent evaporation speed. Controllable crystallization was achieved during the evaporation process. It is realized that the precursors’ solubility should be constantly low so that the solutes will not be concentrated during the solvent evaporation. Compared with the exfoliation methods, better control over the size, shape, and composition of the 2D halide perovskite crystals can be achieved. Later, a more systematic study on solvent engineering during the crystal growth was carried out by varying the solvent type and blending ratio.71 It was found that the growth of the 2D halide perovskite crystals is governed by the synergetic effects of diffusion dominated branched growth and c-axis suppression. Using this method, the sample lateral size was improved up to ∼40 μm. Very recently, 2D halide perovskites with different cations (such as C6H5CH2CH2NH3+, PEA+) were prepared using similar methods.72,73 These studies suggest that solution-processing, rather than exfoliation or CVD growth, might be more suitable to achieve high-quality perovskite 2D crystals.

The above-mentioned approaches provide structurally well-defined 2D halide perovskites with exact thickness of the quantum well (the number “n”) and large lateral dimensions. But such approaches lack processibility after deposition, and the transfer of corresponding samples to other substrates is difficult. Alternatively, colloidal chemistry was applied to directly synthesize halide perovskite thin sheets. The colloidal dispersion can be very uniform and easy to process.76 Recently, several groups reported the colloidal synthesis of MAPbBr3 and CsPbBr3 nanosheets with a few nanometers thickness.35,74,75,77–79 For example, the Manna group improved the synthesis of CsPbBr3 nanosheets by introducing shorter alkyl chain ammonium ligands to the reaction.74 They found that the lateral size of the nanosheets is tunable by varying the ratio of shorter ligands over longer ones, while the thickness is mainly unaffected by this parameter and stays constant at ∼3 nm when the synthesis was conducted with short-to-long ligands ratios below 2/3. Fig. 2e illustrates the colloidal synthesis method and Fig. 2f is a TEM image showing the 2D sheets prepared with short-to-long ligands molar ratios equal to 0.67 and the HRTEM image from a very thin region is recorded in Fig. 2g.

Making even larger sheets using the colloidal method can be challenging and fundamentally difficult, because large sheets tend to either break or aggregate in the colloidal suspension. Compared with non-colloidal solution synthesis, in which it is easy to control the quantum well thickness (the “n” value) by simply blending stoichiometric amounts of LX, AX, and BX2 precursors to the reaction mixture, in the colloidal synthesis of the RP phase 2D halide perovskites it is relatively hard to control the thickness of the ABX3 2D nanosheets, and usually a mixture of thin sheets with different thicknesses (different “n” value) are obtained. Bekenstein et al. reported the colloidal synthesis of CsPbBr3 nanosheets by optimizing the recipe that produced CsPbX3 quantum dots,75 and the morphology of the corresponding sample is shown in Fig. 2h. Reactions conducted at 150 °C produce mostly green-color PL emitting symmetrical nanocubes, while products obtained at lower temperatures output blue-shifted PL spectra, for example, at 130 °C lower symmetry nanoplatelets with cyan emission are formed. Very thin nanoplatelets were detected along with lamellar structures ranging from 200 to 300 nm in length at 90 and 100 °C. Nanoplatelets grow along and inside these lamellar structures, suggesting that organic mesostructures act as growth directing soft templates that break the crystal's intrinsic cubic symmetry and induce 2D growth. They also found that it is possible to change the population by changing the reaction temperature and then separate the colloidal nanosheets with different thicknesses through centrifugation at different speeds. Their method turned out to be a good solution to the above-mentioned issue.

In addition to the wet chemistry approaches, CVD is another important way to prepare large area smooth perovskite sheets. Typically, CVD has been used to prepare perovskite films with thicknesses on the order of a hundred nanometers; however, recent research using 2D materials shows that it is possible to create ultrathin films using CVD. Fig. 3a illustrates a general CVD growth setup for 2D perovskites. Wang et al. used a dual precursor CVD method with a mica substrate temperature of 220 °C. Note that the high temperature used is uncommon in other CVD research studies. An optical image (with the AFM image in the inset) of a CVD-grown MAPbCl3 2D sheet with a thickness of 8.7 nm and lateral dimension of over 20 μm is shown in Fig. 3b.80 A high substrate temperature gives the adatoms higher diffusivity. At first, surface energy dominates and a square sheet, shown in Fig. 3c, is the preferred thermodynamically stable state. As more adatoms are incorporated into the square sheet, the shape becomes kinetically dependent. At the end of a random walk, an adatom has the highest probability to be captured by the edge site closest to it. For a square, the edges have the highest probability to incorporate the adatom. Therefore, the crystal grows into a fraction morphology shown in Fig. 3d. They verified this probability using a static Monte Carlo simulation. It is believed that the formation of larger 2D crystals and a delay in the change of morphology from square to the fractional shape occurred with a slower post-deposition cooling rate, which is similar to the situation in 3D perovskite films.84


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Fig. 3 (a) Illustration of the experimental set-up for the CVD growth of 2D perovskites. (b) Optical image of a CVD-grown MAPbCl3 2D crystal. The inset is the corresponding AFM image. Morphology evolution: (c) a square sheet with {100} facets; (d) protrusion along the 〈110〉 direction. (a–c) Reproduced from ref. 80 with permission from American Chemical Society, copyright 2015. (e and f) AFM topography images of 2D MAPbI3 nanosheets with different thicknesses. Scale bars: 2 μm. Reproduced from ref. 81 with permission from American Chemical Society, copyright 2016. (g) Thickness of PbI2 platelets before (images above the data line) and after being converted to MAPbI3 (images below the data line). Reproduced from ref. 82 with permission from Wiley-VCH, copyright 2014. (h) A high-resolution SEM image and a TEM image of as-synthesized MAPbI3 nanosheets. Reproduced from ref. 83 with permission from Wiley-VCH, copyright 2017.

Nanosheets with thicknesses as low as 1.3 nm have been synthesized using CVD at a low pressure of 40 Torr and 120 °C.81 Decreasing the pressure in the reactor decreases the concentration of the vapor, increases the mean free path of the gas molecules, and thus decreases the intercalating reaction rate. Liu et al. first solution cast PbI2 on SiO2 and then used CVD to create 2D MAPbI3 perovskites. Fig. 3e and f show their produced 2D nanosheets of different thicknesses. The authors noted that the perovskite nanosheets have larger surface roughness than PbI2 films due to the adsorption of MAI. It was also noted that the perovskite nanosheets retained the shape of the PbI2 nanosheet. Along with the shape of the PbI2 sheet, the thickness is also important. Ha et al. created a series of perovskite films of different thicknesses using CVD to coat PbI2 onto mica substrates and then a similar CVD method to convert the films to perovskites.82Fig. 3g shows that the thickness of the PbI2 platelet is linearly correlated to the thickness of the MAPbI3 perovskite platelet by a factor of about 1.8. In this study, the shape of the perovskite also retained the shape of the lead halide platelet. We can see that the color of the platelets is dependent on the thickness. Note that the lowest thickness of the platelet approaches 60 nm. Previous studies show the importance of substrate characteristics and therefore smooth substrates are better. However, Lan et al. show that process pressure can be used to grow PbI2 with high crystallinity on roughened surfaces.83 CVD was then used to convert the as-synthesized nanosheets to MAPbI3. Fig. 3h shows scanning electron microscope (SEM) and dark contrast TEM images of the resulting perovskite nanosheet. The morphology of the freestanding nanosheet is retained, though a bit roughened. The thickness of the nanosheets is about 250 nm.

Another area of research involves the conversion of perovskites into different structured perovskites using CVD. Liu et al. also investigated exchanging the anion in the 2D halide perovskite structure.81 They found that the iodide ions near the edge of the MAPbI3 nanosheet exchange anions first before the center region. Chen et al. synthesized ultrathin (BA)2(MA)n−1PbnBr3n+1 perovskites with thickness down to 4.2 nm and lateral dimension up to 57 μm.85 First they synthesized ultrathin (BA)2PbBr4 perovskites using a ternary solvent solution method and then transformed them using CVD. The vapor phase one-step and two-step synthetic approaches are both powerful, and they together provide extra flexibility in making new low dimensional halide perovskite materials.

2.2. Advanced structural characterization

After obtaining these 2D halide perovskites, it is imperative to conduct corresponding characterization such as X-ray diffraction and TEM to confirm the structure and morphology. Unlike other 2D materials such as graphene or TMDs, whose atomic level images can be easily acquired, the radiation-sensitive nature of halide perovskites has hindered structural studies at the atomic scale because of the destructive electron beam–sample interactions. Making things even more challenging, compared to the bulk material, the electron–beam induced sample degradation occurs much more quickly and severely in low-dimensional halide perovskites. Consequently, structural characterization using electron beams generally needs to be carried out at low magnifications, where the dose-rate required to obtain sufficient contrast is much lower. However, structural damage still occurs and is evident as tiny precipitates or voids in TEM and scanning transmission electron microscope (STEM) images.

Yang's group overcame this obstacle by applying low dose-rate in-line holography, which combines aberration-corrected high-resolution transmission electron microscopy (AC-HRTEM) with exit-wave reconstruction.87 This technique can successfully yield the genuine atomic structure of ultrathin 2D CsPbBr3 halide perovskites. By using the phase information of the reconstructed exit-wave function, quantitative structure characterization was achieved atom column by atom column without introducing structural damage in CsPbBr3. In this work, an extraordinarily high image quality enables an unambiguous structural analysis of coexisting high-temperature and low-temperature phases of CsPbBr3 in single particles. The crystallographic structure of plate shaped nanocrystals can be revealed at atomic resolution with single atom sensitivity. More importantly, the method is applicable to any beam-sensitive material and does not require uncommon TEM attachments so that it can be easily utilized in most of the laboratories.

Recently, using this new technique, Yang's group also reported the direct observation of unusual structure in 2D CsPbBr3 nanosheets, which could be interpreted as the RP phase based on model simulations. Structural details of the plausible RP domains and domain boundaries between the RP and conventional perovskite phases have been revealed on the atomic level.86Fig. 4a shows the typical low magnification STEM morphology of the squared 2D sheets. As halide perovskites are beam-sensitive materials, the electron beam–sample interaction should be carefully controlled. According to their preliminary observations, many of the 2D thin nanosheets are crystallized in the cubic perovskite phase, and orthorhombic phase domains also exist. In their follow-up research, in some of the thick sheets, unusual domains that exhibit different patterns in high-resolution STEM images compared to conventional perovskite phases have been discovered. Fig. 4b shows the high-resolution AC-STEM image of the conventional CsPbBr3 perovskite phase within a 2D sheet. In this Z-contrast image, heavier Pb–Br atom columns display the highest brightness, and relatively lighter Cs atom columns, located at the center of four Pb–Br columns cube, show weaker brightness. Fig. 4c depicts the AC-STEM image of the unusual phase region, in which both the cation atom columns show high contrast. This agrees with the characteristic of RP phases and it can be explained by the structural projections shown in Fig. 4. For RP phases, there is an in-plane (1/2 1/2) shift between two neighbor CsPbBr3 units, so the cation atom columns all become Cs–Pb–Br columns in the [001] projection, regardless of the number n. Therefore, the difference between RP and perovskite phases can easily be distinguished from the intensities of cation atom columns. This promising structural characterization has offered unprecedented opportunities to better understand 2D halide perovskites at the atomic level.


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Fig. 4 (a) Schematic illustration of the crystal structures and the corresponding [001] projection of conventional CsPbBr3 perovskite phases (cubic and orthorhombic) and RP phases (n = 1, 2). STEM images of the conventional perovskite phase and plausible RP phase. (b) Low magnification morphology of the 2D nanosheets. (c and d) Atomic-scale AC-STEM image of the conventional perovskite phase (c) and RP phase (d). [001] structure projections are overlaid in the center. Reproduced from ref. 86 with permission from American Chemical Society, copyright 2017.

Currently this method only applies for all-inorganic 2D halide perovskites, such as CsPbBr3, but not the hybrid ones, which are even more beam sensitive. Another promising approach to achieve atomic resolution imaging is using cryo-electron microscopy at cryo temperature, which has been widely used in biological molecular samples.88,89 Additional care needs to be taken for such measurements; for example, phase transition could occur at low temperature and the crystal structure of the 2D halide perovskites might change at different temperatures.

2.3. Spectroscopy perspectives of optical and electronic properties

Realizing band gap control is critically important for 2D materials for their application in electronic and optoelectronic devices. For graphene, the unique band structure featuring linear dispersion relationship at around K point leads to the formation of a Dirac point with zero band gap. Although the charge carrier mobility and electrical conductivity of graphene are extremely high, opening the band gap for graphene has been proven to be very challenging.90,91 TMDs were found to be promising 2D semiconductors with suitable band gaps. The inversion symmetry breaking in monolayer TMDs and spin–orbit coupling together lead to new spin and valley physics. These properties make TMDs an exciting family of materials for valleytronics and spintronics.92 However, the basic optical and electronic properties of TMDs are not perfect, particularly the exciton lifetime is extremely short and the radiative recombination quantum efficiency is usually very low even for the monolayer sheets. Black phosphorus is another promising semiconducting 2D material. However, it lacks tunability and suffers from poor stability.32,93 Therefore, there is a strong need to seek new semiconducting 2D materials for high performance nanoelectronics and nanophotonics.

The tunable band gaps, high photoluminescence quantum efficiency, and long charge carrier lifetime are three critical features of halide perovskites. These characteristics can still be sustained in the 2D halide perovskite crystals, making them promising candidates for optoelectronic devices. It has been widely reported that mixed halide perovskite alloy thin films or nanocrystals can be easily obtained by mixing different halide precursors during the synthesis. The band gap can be systematically tuned from UV to visible and even to the near-infrared region.94–99 For example, the Haque group have reported a series of lead-free 2D layered perovskite materials (PEA)2SnIxBr4−x, through halide band gap engineering; the class of 2D halide perovskites showed tunable visible light absorption and emission properties in thin films (Fig. 5a).97 More importantly, the superior light emission properties of (PEA)2SnI4 over its 3D counterpart are also demonstrated via steady-state/time-resolved PL spectroscopy. The new substrate synthesis method was also extended to other hybrid perovskites and alloys. Ultrathin 2D sheets of (BA)2PbCl4, (BA)2PbI4, (BA)2PbCl2Br2, (BA)2PbBr2I2, and (BA)2(MA)Pb2Br7 were prepared and their PL spectra with optical images are shown in Fig. 5b.70


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Fig. 5 (a) Schematic of the general crystal structure of a (PEA)2SnIxBr4−x perovskite and electroluminescence spectrum of a (PEA)2SnI4 device, and normalized absorbance (solid lines) and PL (dashed lines) spectra of (PEA)2SnIxBr4−x perovskite thin films processed on glass (photographs of samples shown for each x value). Reproduced from ref. 97 with permission from American Chemical Society, copyright 2017. (b) Photoluminescence and corresponding optical PL images of different 2D hybrid perovskites: (BA)2PbCl4 (i), (BA)2PbBr4 (ii), (BA)2PbI4 (iii), (BA)2PbCl2Br2 (iv), (BA)2PbBr2I2 (v), and (BA)2MAPb2Br7 (vi). Reproduced from ref. 70 with permission from American Association for the Advancement of Science, copyright 2015. (c) The absorption spectra and photoluminescence spectra of 2D MAPbBr3 perovskites with n = 1, 3, 5, 7–10, respectively. Reproduced from ref. 79 with permission from Elsevier Ltd., copyright 2017. (d) Confocal PL mapping of patterns on a CsPbBr3 platelet. (Scale bars, 3 μm.) Reproduced from ref. 100 with permission from National Academy of Sciences, USA, copyright 2017.

Another important method to tune the band gap is controlling the nanoscale quantum confinement.75,79,101,102 Compared to 0D quantum dots or 1D nanowires for halide perovskites, for which precise size control is challenging, 2D geometry provides a natural way to realize precisely controlled quantum well thickness and thus the confinement effects. When the quantum well thickness of 2D halide perovskites gradually approaches the size of the exciton Bohr radius (typically below 10 nm), the quantum confinement effect will begin to affect both absorption and photoluminescence. Huang et al. found that the layer of 2D MAPbBr3 perovskites can be adjusted by the ratio of oleic acid and the equilibrium between the surfactants and precursors across two phases.79Fig. 5c shows the representative absorption and photoluminescence spectra of 2D MAPbBr3 nanosheets with different layer numbers. The layer numbers of these samples from solutions can be assigned via comparison of these peaks with other studies, which resulted in from top to bottom, n = 1, 3, 5, 7–10, respectively. Bulk MAPbBr3 microplate spectra are also plotted for a direct comparison. All these results indicate that the 2D halide perovskites have great composition and color tunability.

The PL quantum efficiency of lead or tin based halide perovskites is surprisingly high. For quantum dots with 5–10 nm in diameter, without any special surface treatment, the colloidal suspension reaches PL quantum efficiency of over 90% easily. Similarly, for 2D halide perovskites, quantum efficiency over 60% in colloidal systems and over 20% in solid states have been observed.70,75 These values are significantly better than those of most of the TMDs and other 2D semiconducting materials. The high PL quantum efficiency of the 2D halide perovskites can be attributed to their direct band gap nature, the increased exciton binding energy and the radiative recombination probability owing to strong dielectric and quantum confinements, and also the shallow charge carrier defect levels associated with the ionic bonding nature. This excellent light-emitting property makes them promising candidates for many solid-state lighting applications.99–101,103–107 The Yang group demonstrated highly spatially resolved heterojunctions in halide perovskites, which are realized by the combination of facile anion-exchange chemistry with nanofabrication techniques.100Fig. 5d shows the confocal PL mapping of the heterojunction pattern obtained on 2D platelets of CsPbBr3. The strong ionic bonding nature of halide perovskites results in highly dynamic crystal lattices, inherently allowing rapid ion exchange at the solid–vapor and solid–liquid interface. To further enhance the PL quantum efficiency of 2D halide perovskites, it is important to understand the exciton relaxation pathway. Recently time-resolved and temperature-dependent PL studies were carried out to elucidate the intrinsic exciton relaxation pathways in (BA)2(MA)n−1PbnI3n+1 (n = 1, 2, 3) perovskite 2D nanosheets.108 It was found that scattering via the deformation potential by acoustic and homopolar optical phonons is the main scattering mechanism for excitons in ultrathin single exfoliated flakes. Surprisingly, polar optical phonon scattering and defect scattering were not observed, and such phenomena were attributed to efficient screening of the Coulomb potential.

Besides the quantum confinement, charge carrier dynamics is another key issue to understand. Despite static and dynamic disorder in hybrid lead halide perovskites, they still exhibit carrier properties similar to those of pristine nonpolar semiconductors. Recently, Zhu et al. revealed the carrier protection mechanism by comparing three single-crystalline lead bromide perovskites.109 As shown in Fig. 6a, the hot fluorescence emission from energetic carriers with ∼102 picosecond lifetimes was observed in MAPbBr3 or FAPbBr3, but not in CsPbBr3. This phenomenon is correlated with liquid-like molecular re-orientational motions, suggesting that dynamic screening protects energetic carriers via solvation or large polaron formation on time scales competitive with that of ultrafast cooling. The exceptionally long lifetimes (∼102 ps) of energetic carriers in hybrid lead halide perovskites may make hot-carrier harvesting possible. And very shortly after this, the Huang group reported the direct visualization of hot carrier migration in MAPbI3 thin films by ultrafast transient absorption microscopy, further suggesting potential applications of hot-carrier devices based on hybrid perovskites.112


image file: c7cs00886d-f6.tif
Fig. 6 (a) Time-dependent PL spectra from FAPbBr3 and CsPbBr3 showing hot PL emission only from the former. Pseudocolor plot of TR–PL spectra for a single-crystal FAPbBr3 microplatelet (left) and a single-crystal CsPbBr3 microplatelet (middle). The excitation photon energy is 3.08 eV, and the excitation density is 1.7 mJ cm−2. (right) PL spectra at indicated delay times for the FAPbBr3 microplatelet. Reproduced from ref. 109 with permission from American Association for the Advancement of Science, copyright 2016. (b) CsPbBr3 nanoplatelet power-dependent emission spectra showing lasing at ca. 545 nm. (inset) Optical image of the same nanoplatelet above the lasing threshold. (Scale bar, 5 μm.) Reproduced from ref. 110 with permission from National Academy of Sciences, USA, copyright 2016. (c) Time-resolved PL measurements showing the bi-exponential decay of ultrathin 2D homologous perovskite (BA)2(MA)n−1PbnBr3n+1. Reproduced from ref. 85 with permission from Wiley-VCH, copyright 2017. (d) PL intensity map of a single exfoliated crystal, probed at 2.010 and 1.680 eV. (right) Microscopy image showing the layer edges of the exfoliated crystal. Scale bar: 10 mm. Reproduced from ref. 111 with permission from American Association for the Advancement of Science, copyright 2017.

In the rapidly growing nanoscale laser field, Eaton et al. reported the low-temperature, solution-phase growth of cesium lead halide nanowires exhibiting low-threshold lasing and high stability.110 Meanwhile, well-faceted CsPbBr3 nanoplatelets were also found to lase at ∼38 μJ cm−2. The power-dependent emission spectra and optical image of the 2D halide perovskite nanoplatelets above the lasing threshold are shown in Fig. 6b. Considering the emission wavelength tunability demonstrated in the 2D halide perovskites, this work suggested that this kind of 2D material can also act as new tunable light sources for nanoscale lasers.

For 2D halide perovskites confinement regime, the binding energy of the photo-generated charge carriers (or excitons) is large. This leads to a relatively faster radiative recombination of the electrons and holes. For instance, the charge carrier lifetime in the 2D sheets is usually a few to tens of nanoseconds, while the lifetime is usually several hundreds of nanoseconds in the bulk MAPbI3 and some high-quality crystals can even reach the microseconds regime. As shown in Fig. 6c, the decay curve of individual ultrathin 2D homologous perovskite (BA)2(MA)n−1PbnBr3n+1 synthesized through self-doping engineering displays a biexponential feature with lifetimes of 0.55 and 5.64 ns.85 It is close to the bulk perovskites, and a well-defined cross star shape shown by the PL mapping (inset in Fig. 6c) indicated its uniform emission. The carrier lifetime is not only dependent on the dimensionality, but is also affected by the local chemical environments. Very recently, Blancon et al. demonstrated that the edge of the RP phase 2D halide perovskite crystals provides states that significantly elongate the carrier lifetime and provide driving force to separate the strongly bonded excitons to dissociate into free carriers.111Fig. 6d shows the PL intensity map of a single exfoliated (BA)2(MA)2Pb3I10 (n = 3) crystal. This phenomenon can only be observed for 2D systems with n > 2. The authors believe that this is counterintuitive to other classical quantum-confined systems where photo-generated electrons and holes are strongly bound by Coulomb interactions or excitons. These unique optical and electronic properties make the 2D halide perovskites very promising candidates for photodetection and energy harvesting applications.113–119

2.4. Comparison of different types of 2D materials

The properties of different 2D materials, including graphene, hexagonal boron nitride (hBN), TMDs, black phosphorus, Mxene and 2D halide perovskites, are summarized in Table 1. In comparison with other materials, the electronic properties can be more easily modulated by tuning the elemental composition and the number of [MX6]4− octahedral layers n for 2D halide perovskites. This leads to a large diversity of 2D halide perovskites with controllable properties. In addition, 2D halide perovskites exhibit much higher PL quantum efficiency, which facilitates their application in lasers and light-emitting diodes. Another advantage is the longer exciton lifetime, which is beneficial for charge separation and collection in solar cells. However, although 2D halide perovskites provide a great platform to study the carrier transport behavior, the electrical conductivity of 2D halide perovskites is relatively low, thus hindering the widespread and in-depth studies on transistors. The low electrical conductivity can be compensated by incorporating other highly conductive 2D materials to form 2D heterostructures, such as graphene and TMDs.
Table 1 The summary of the properties of different kinds of 2D materials
Atomic structure Graphene hBN 2D TMD BP Mxene 2D perovskite

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General chemical formula C B, N MX2: P Mn+1XnTx L2An−1BnX3n+1
M: Mo, W, etc. M: Ti, Zr, Hf, etc. L: long chain organic cations, such as BA+
X: S, Se, Te X: C, N; Tx: surface terminations A: Cs+, MA+; B: Pb2+ or Sn2+; X: halide
Examples Graphene hBN MoS2, WS2, MoSe2, WSe2, MoTe2 BP TiC2, Ti3C2, Ti4C3 (BA)2PbBr4, (BA)2PbI4, (BA)2MAPb2Br7, (BA)2MAPb2I7
Crystal structure Hexagonal Hexagonal Hexagonal (2H phase) Orthorhombic Hexagonal Orthorhombic/tetragonal
Monolayer thickness (nm) ∼0.4 ∼0.4 0.6–0.9 0.7 NA 1.4 (n = 1)
1.9 (n = 2)
Band gap Eg (eV) 0 5.9 1–2.1 (2H phase) 2.0 0–1.8; can be tuned by the surface terminations (1.5–3.5) varying with elemental composition and n
PLQE <0.3% NA <1% (MoS2) NA NA (BA)2PbBr4: ∼26%
Exciton lifetime NA NA 10–100 ps ∼0.1 ps NA ∼1 ns
Room-temperature mobility (cm2 V−1 s−1) ∼2 × 105 NA 10–200 (MoS2) ∼1000 NA 15 for (BA)2SnI4 thin film
Electrical conductivity Excellent Insulating Moderate Moderate Good Low/moderate
Ref. 29, 120 and 121 32 and 122 54, 66, 120 and 123–130 32, 54, 131 and 132 54, 133 and 134 70, 117, 135 and 136


3. 2D halide perovskite–graphene/TMD interfaces and heterojunctions

As mentioned above, 2D materials, including graphene, TMDs, etc., have unique electronic and photonic properties due to the quantum confinement effect. Therefore, the combination of different 2D materials in lateral or vertical geometries offers a wide variety of options in designing new types of heterojunctions with novel functionalities and applications. For example, both graphene and hBN are hexagonal structures with nearly identical lattice constants, which facilitates the epitaxial growth of hBN from graphene edges.137–139 And the incorporation of hBN into graphene can generate a tunable band gap of 0–0.6 eV depending on the hBN ratio.140 In addition, by stacking hBN on graphene, hBN sheets can serve as a protective layer to encapsulate graphene and as a dielectric layer to apply the top-gate voltage.141–145 The introduction of 2D halide perovskites provides many new opportunities to fabricate complex heterostructures with other 2D materials.

3.1. Fundamentals of interfaces and heterojunctions

Different junctions are classified based on the band structures of the two involved materials. These include metal–semiconductor, semiconductor–semiconductor, and insulator–semiconductor interfaces. As graphene is a semi-metallic material (Eg = 0 eV), and TMDs and 2D halide perovskite materials are semiconductors, we will focus our attention on metal–semiconductor and semiconductor–semiconductor interfaces in this review.

Metal–semiconductor interfaces can be categorized into two types depending on the difference of work functions (Φ): Schottky junction and ohmic junction, as illustrated in Table 2 and Fig. 7a, b. When a metal and a semiconductor with different work functions are brought in contact, energy bands in the semiconductor side of the junction will bend to equalize the chemical potentials. Consider the p-type semiconductor as an example. Electrons flow from the semiconductor to the metal to attain charge equilibrium when the work function of the p-type semiconductor is lower than that of the metal (case III). This leaves a negative charge on the metal side and a positive charge on the semiconductor side leading to an ohmic contact. Otherwise, if the work function of the p-type semiconductor is higher than that of the metal, there will be a barrier for holes at the metal–semiconductor interface, which is defined as the Schottky junction (case IV). Band bending for n-type semiconductors is similar to that for p-type semiconductors. The band offset and depletion layer width can be calculated by Poisson's equation following Anderson's rule.

Table 2 Categorization of metal–semiconductor junctions (M = metal, S = semi-conductor)
Cases Relation between work functions Type of semi-conductor Type of junction
I Φ S > ΦM n-Type Ohmic
II Φ S < ΦM n-Type Schottky
III Φ S < ΦM p-Type Ohmic
IV Φ S > ΦM p-Type Schottky



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Fig. 7 Energy band diagrams for metal–semiconductor interfaces and semiconductor–semiconductor heterojunctions. (a) The ohmic junction and (b) the Schottky junction between metal (M) and p-type semiconductor (S): EFM and EFS – the Fermi level of metal and semiconductor, ΦS and ΦM – the work function of metal and semiconductor, Eg – band gap of semiconductor. (c–e) Three types of heterojunctions between two different semiconductors (S-1 and S-2).

Semiconductor–semiconductor interfaces can be categorized into three types: straddling gap (type I), staggered gap (type II) and broken gap (type III).146 This categorization is based on the relative band alignment of conduction and valence bands in the two semiconductors. The three types of semiconductor–semiconductor heterojunctions are shown in Fig. 7c–e. Like metal–semiconductor heterojunctions, the energy bands in semiconductor–semiconductor heterojunctions bend when brought in contact to achieve equilibrium. However, band bending will be observed on both sides of the junction.

In bulk semiconductors, the depletion layer extends within a certain depth in the semi-conductor due to the offset of energy bands. For lateral stitched 2D metal/semiconductor or semiconductor/semiconductor junctions, the depletion layer will be similar to that of bulk junctions. However, the 2D semiconductors will probably be confined within the depletion layer for vertical 2D junctions as the thickness of the involved 2D material is generally smaller than the depletion layer width.

3.2. Heterojunctions in 2D materials

Heterojunctions between different 2D TMD materials are one common type of 2D heterojunctions. TMD materials can be assembled into lateral147,148 and vertical147,149,150 heterostructures according to the interfacing geometries, which opens up numerous opportunities to engineer the properties of TMDs. For example, the lateral epitaxy of WS2 on MoS2 edges is favored by low growth temperature (650 °C), creating atomically sharp in-plane WS2–MoS2 heterostructures that generate strong localized photoluminescence enhancement around the interface and intrinsic p–n junctions. Fig. 8a shows the optical image of a lateral WS2–MoS2 heterostructure. The in-plane heterostructure is further confirmed by the Raman mapping at 351 cm−1 (yellow) and 381 cm−1 (purple) respectively (Fig. 8b). In comparison, vertically 2H stacked bilayers with WS2 epitaxially grown on top of the MoS2 monolayer can be prepared at a higher growth temperature (850 °C). As shown in Fig. 8c, there is a clear contrast between monolayer MoS2 and vertical WS2–MoS2 heterostructure, which is also verified by the Raman spectra in distinct locations (Fig. 8d). From the transfer characteristics of back-gating vertically stacked WS2–MoS2 field-effect transistors (FETs) shown in Fig. 8e, the on/off ratio is larger than 106, and the mobility can reach from 15 to 34 cm2 V−1 s−1, which is much higher than the average mobility of the monolayer MoS2,151 bilayer MoS2, monolayer WS2 and WS2–MoS2 bilayer realized by the transfer method. Furthermore, the lateral TMD heterojunctions can also be formed within one TMD material with the same composition. For example, Sung et al. demonstrated the successful realization of the 2D coplanar heterojunction between distinct metallic (1T′ phase) and semiconducting (2H phase) MoTe2 crystals within the same atomic planes by a heteroepitaxy method.152 The coplanar contact is atomically coherent with an ultralow contact barrier, which leads to higher current density and gate tunability.
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Fig. 8 Heterojunctions between different materials such as MoS2, WS2 and graphene. (a) Optical microscopy image of a triangular in-plane WS2–MoS2 heterojunction. (b) Combined Raman intensity mapping at 351 cm−1 (yellow) and 381 cm−1 (purple), showing the core–shell structure of the lateral WS2–MoS2 heterostructure. (c) Optical microscopy image of a triangular vertical-plane WS2–MoS2 heterojunction. (d) Raman spectra taken from the four positions marked in (c), showing that the outer monolayer region is pure MoS2, whereas the central double layer area is the overlap of MoS2 and WS2 monolayers. (e) Comparison of transfer characteristics of a CVD-grown WS2–MoS2 bilayer, a mechanically transferred WS2–MoS2 bilayer, a MoS2 bilayer and a MoS2 monolayer. Reproduced from ref. 147 with permission from Springer Nature, copyright 2014. (f) Output characteristics of the single-gated graphene–MoS2–Ti device under laser illumination with back gate voltage (VBG) varying from −1 V to 1 V in steps of 0.5 V. The IV curve obtained in the dark at VBG = 1 V passes through the origin. The inset is the schematic illustration of the side view of the device. Red and blue colors denote electrons and holes, respectively. Reproduced from ref. 153 with permission from Springer Nature, copyright 2013. (g) Cross-sectional STEM image of the hBN-encapsulated MoS2 multi-terminal device with multi-layer graphene as electrodes. (h) Hall mobility of hBN-encapsulated MoS2 devices (with different numbers of layers of MoS2) as a function of temperature. Reproduced from ref. 148 with permission from Springer Nature, copyright 2015.

In addition to the combination of different TMD materials, integrating semi-metallic graphene with semiconducting TMD 2D materials vertically offers the possibility for fabricating various kinds of functional devices, including photodetectors,153 solar cells,153,154 and logic and memory devices.155–157 As shown in Fig. 8f, the assembly of Ti–MoS2–graphene vertical heterostructures is achieved by a transfer method and the following evaporation of Ti. As a result, a photocurrent of 3–6 μA and an open voltage of 0.13–0.2 V are detected under laser illumination. In addition, the photocurrent and open-circuit voltage (VOC) can be modulated by back gate voltage. By adjusting the external gate voltage, the graphene–MoS2 based heterojunction exhibits a maximum external quantum efficiency up to 55% and an internal quantum efficiency of 85%.153

In addition, the graphene–MoS2 heterostructure can also be sandwiched by the hBN sheets (Fig. 8g), where the graphene serves as the contact electrode of MoS2 and the hBN helps to encapsulate the device. The hBN encapsulation reduces scattering from substrate phonons and charged impurities, and the highly conductive graphene ensures a high-quality electrical contact with MoS2. A record high Hall mobility of 34[thin space (1/6-em)]000 cm2 V−1 s−1 was achieved for six-layer MoS2 at low temperature (Fig. 8h).148 The van der Waals hBN–Gra–MoS2 heterostructure provides a standard device platform that facilitates the measurement of the intrinsic electrical transport of 2D materials and achieves high-mobility 2D devices to study novel quantum physics and their unique transport properties.

3.3. 2D halide perovskite–graphene junctions

Following the introduction of the junctions constructed by graphene and 2D TMD materials, we will focus this part on the junctions between the 2D halide perovskite and graphene, which should be defined as a Schottky junction due to the zero-band gap of graphene. Recently, there has been significant research interest in halide perovskite–graphene hybrid heterostructures and their applications in photodetectors,158–163 solar cells,164 and transistors.159,162,163,165,166 The most well-studied halide perovskite materials utilized to construct heterostructures with graphene are 3D MAPbX3 and CsPbX3 based thin films158,161,164,166 and nanowires.159 The halide perovskite–graphene Schottky junction will help to separate holes and electrons to different sides under illumination, rendering halide perovskite–graphene heterostructures promising candidates for high-performance photodetectors and solar cells.

Compared with the conventional halide perovskite photodetectors, especially lateral halide perovskite photodetectors with a large channel length, halide perovskite–graphene photodetectors demonstrate much higher photoresponsivity, which is mainly attributed to suppressed charge recombination. As reported by Lee et al., a polycrystalline MAPbI3 film was spin-coated on the CVD-grown mono-layer graphene (Fig. 9a).158 The resulting photodetector exhibited a photoresponsivity of 180 A W−1, an EQE of ∼5 × 104%, and a broad spectral bandwidth across the UV-visible range and a photodetectivity of ∼109 Jones at a relatively high illumination power of 1 μW, as shown in the photo-switching characteristics (Fig. 9b). The high photoresponsivity and EQE derived mainly from the efficient hole transfer from MAPbI3 to graphene, thus effectively restricting the recombination of the photoexcited electron–hole pairs in the perovskite layer. To further improve the photodetection performance, Chang et al. adopted the vapor deposition method to grow ultra-flat MAPbI3 films on graphene sheets.161 Consequently, the stacked halide perovskite–graphene heterostructure has high exciton separation ability under illumination, generating an ultrahigh photoresponsivity of 1.73 × 107 A W−1, a detectivity of 2 × 1015 Jones, and an extremely high effective quantum efficiency of about 108% in the visible range (450–700 nm).


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Fig. 9 2D halide perovskite–graphene heterojunctions. (a) Schematic image of the halide perovskite film–graphene hybrid photodetector. (b) Photo-switching characteristics of the halide perovskite–graphene hybrid photodetector under alternating dark and light illumination (500 μW, 520 nm). The gate and drain voltages were 0 and 0.1 V, respectively. The right panel shows an enlarged view of the temporal photocurrent response during on–off illumination switching. Reproduced from ref. 158 with permission from Wiley-VCH, copyright 2015. (c) PL intensity mapping of the halide perovskite on graphene, the substrate is SiO2/Si. Reproduced from ref. 167 with permission from Wiley-VCH, copyright 2015. (d) Schematic illustration and optical microscopy image of a graphene–perovskite–graphene heterostructure device. The white dashed line outlines the overlapping area; the bottom-graphene and top-graphene are outlined by blue and red solid lines, respectively. (e) IDSVDS curves of the graphene–perovskite–graphene heterostructure device measured at room temperature (298 K) with/without laser illumination. Reproduced from ref. 162 with permission from American Chemical Society, copyright 2016.

In addition to the heterostructures of graphene and polycrystalline halide perovskites, Niu et al. also demonstrated the heterostructures between graphene and single-crystalline MAPbI3 nanoplatelets by the vapor deposition method.167 First, a mono-layer graphene grown by the CVD method is transferred to the target substrate. After that, the growth of highly crystalline PbI2 nanoplatelets is performed via a physical vapor deposition (PVD) process on the graphene coated substrate. Finally, the PbI2 crystals are converted into MAPbI3 nanoplatelets by reacting with MAI vapor under vacuum. The MAPbI3–graphene heterostructure provides a platform to study the carrier transport in the MAPbI3–graphene interface. As shown in the PL intensity mapping of MAPbI3 on graphene (Fig. 9c), there is little contrast between graphene and MAPbI3, indicating that the PL emission from MAPbI3 has been quenched. This is consistent with the PL results in graphene–polycrystalline halide perovskite heterostructures.158,166 And the reduced carrier lifetime (0.42 ns) for perovskite on graphene also suggests the significant carrier transfer at the halide perovskite–graphene interface.

The carrier transport behavior across the perovskite–graphene interface can be directly measured by fabricating a perovskite FET with graphene as the electrodes. As shown in Fig. 9d and e, Cheng et al. performed electrical measurements of MAPbI3 nanoplatelet–graphene heterojunctions.162 The MAPbI3 nanoplatelet was prepared by vapor-phase intercalation of PbI2 with MAI. The vertical graphene–MAPbI3–graphene device comprises graphene as the top and bottom electrodes and the sandwiched MAPbI3 nanoplatelet as the semiconductor and light absorber layer. The graphene electrodes can also protect MAPbI3 from external moisture and oxygen. Upon laser illumination, the device yields a photoresponsivity of ∼950 A W−1 and a photoconductive gain of ∼2200. And most carriers are transported in the vertical direction, which is a shorter charge transfer channel, thus enabling the efficient separation and collection of photo-generated carriers prior to exciton recombination.

2D halide perovskites were reported to have improved moisture resistance compared with 3D halide perovskites in solar cells, making it possible to fabricate highly stable perovskite photodetectors.168 Recently, Shao et al. reported a stable, high gain hybrid photodetector consisting of a monolayer graphene covered by a 2D multiphase perovskite heterostructure.169 Due to the aggregation of hydrophobic BA cations at the surface of the 2D halide perovskite film, the penetration of moisture is dramatically restricted, and the stability of the hybrid photodetector is greatly enhanced. In addition, this 2D multiphase halide perovskite heterostructure photodetector demonstrated superior performance including a high responsivity of ∼105 A W−1, faster photoresponse and smaller 1/f noise compared with its 3D counterparts. This highlights the promising prospects for the future perovskite photodetectors with high performance and high stability.

3.4. 2D halide perovskite–TMD heterojunctions

Different from graphene, 2D TMD materials are semiconductors with a tunable band gap ranging from 1.0 to 2.1 eV. Based on first-principles calculations, the interface between the halide perovskite and TMDs is generally a type-II heterojunction.167 In addition, the dark current will be effectively suppressed in photodetectors because of the high carrier mobility and tunable band gaps in TMD materials. Thereby, there is a strong motivation to explore the applications of halide perovskite–TMD heterostructures in solar cells and photodetectors.

Ma et al. reported the fabrication of MAPbI3–WS2 heterostructures by the vapor deposition method.170 First, one layer of PbI2 film was thermally deposited onto CVD-grown WS2 film, then the PbI2–WS2 film was exposed to MAI vapor, which converted PbI2 into a MAPbI3 film with a thickness of 300 nm, as schematically shown in Fig. 10a. Compared with pure MAPbI3, significant PL quenching was observed in the MAPbI3–WS2 bilayer, indicating efficient charge transfer and exciton dissociation at the interface. Because of the interfacial charge transfer and high-quality interface, the MAPbI3–WS2 bilayer demonstrated suppressed dark current and enhanced photocurrent by more than one order of magnitude. This resulted in an elevated on/off ratio (>105) for the MAPbI3–WS2 bilayer heterostructure, as shown in Fig. 10b. In addition, the detectivity of the heterostructured device can reach as high as 2 × 1012 Jones, and the response speed of the WS2–MAPbI3 bilayer photodetector was increased by four orders of magnitude compared to the pure MAPbI3 layer. The superior photodetector performance was claimed to arise from the efficient carrier separation at the interface as well as the fact that the atomically smooth surface of WS2 could promote the crystallinity of the MAPbI3 nanoplatelets. Additionally, by a two-step vapor deposition method, single-crystalline MAPbI3 nanoplatelets can be directly grown on individually distributed MoS2 nanoplatelets.167 From the corresponding photoluminescence mapping image shown in Fig. 10c, there is a sharp contrast between the SiO2 substrate (purple), monolayer MoS2 (dark blue), and MAPbI3–MoS2 (blue). However, the PL intensity of MAPbI3 on MoS2 is apparently weaker than that on the insulating hBN substrate. In comparison, the lifetimes for the excitons in MAPbI3–MoS2 and MAPbI3–hBN van der Waals stacked bilayers are measured to be 1.3 and 5.8 ns respectively, indicating the existence of charge transfer across the interface between MAPbI3 and MoS2. In combination of the calculation results based on the band structure of MoS2 and MAPbI3, the MAPbI3–MoS2 heterojunction is type II band alignment, which will separate electrons and holes to the opposite sides under illumination. This is different from the halide perovskite–graphene heterojunction, in which both the holes and electrons will recombine in the graphene side, consequently resulting in almost no PL contrast between the halide perovskite and the substrate (Fig. 9c).


image file: c7cs00886d-f10.tif
Fig. 10 2D halide perovskite–TMD heterojunctions. (a) Schematic of the device structure of the hybrid MAPbI3–WS2 photoconductor fabricated on a sapphire substrate. (b) Bias dependence of the on/off ratios measured on the perovskite photoconductor with and without WS2. Reproduced from ref. 170 with permission from Wiley-VCH, copyright 2016. (c) PL intensity mapping of MAPbI3 on a MoS2/SiO2/Si substrate. Reproduced from ref. 167 with permission from Wiley-VCH, copyright 2015. (d) Schematic image of the hybrid bilayer MAPbI3–WSe2 photodetector. (e) Photoresponsivity of pristine MAPbI3–WSe2 (orange color) and laser-healed MAPbI3–WSe2 (cyan color) as a function of incident wavelength. Reproduced from ref. 171 with permission from Wiley-VCH, copyright 2016.

The thickness of the halide perovskite thin film or single nanoplatelet on TMDs discussed above in Fig. 10a–c is over 100 nm, which can be reduced to a few nanometers by controlling thermal evaporation conditions (Fig. 10d).171 Due to the type II band alignment in the MAPbI3–WSe2 heterojunction and the Fermi levels for MAPbI3 and WSe2, the electrons and holes could be separated efficiently. In addition, the carrier diffusion length is about one micrometer in the polycrystalline MAPbI3 film.172 With the decrease of the thickness of MAPbI3 to several nanometers, the carriers would diffuse to the interface and be separated by the built-in electrical field in the depletion layer of the MAPbI3–WSe2 heterojunction more efficiently. With the incorporation of the ultrathin halide perovskite layer, the photoresponsivity increased from 70 to 1780 mA W−1, the EQE increased from 7.0% to 415% and the photodetectivity increased from 1.68 × 109 to 1.5 × 1010 Jones. Additionally, the performance of the halide perovskite–WSe2 photodetector was further improved by laser healing, which reduced unwanted carrier traps via passivating the chalcogen vacancies in WSe2 by oxygen substitution. With the help of laser healing, the optimized photodetector showed an overall improvement with a photoresponsivity of 1.1 × 105 mA W−1, an EQE of 2.5 × 104%, and a photodetectivity of 2.2 × 1011 Jones.

In addition, due to the broad tunability of halide perovskites by controlling dimensionality and composition, it is possible to build heterojunctions between different halide perovskites, either in lateral or vertical layouts. An example of periodic lateral CsPbCl3–CsPbBr3 heterojunctions in a single nanoplatelet by the cation exchange method and nanofabrication technique is shown in Fig. 5d.100 The minimum feature size can be down to several hundreds of nanometers. By tuning the halide elements with different ratios of Cl, Br and I, the emission light can vary over the entire visible range, which makes the halide perovskite heterojunctions promising candidates in light emitting devices for future ultra-high definition displays.

4. Device applications

4.1. Solar cells and photodetectors

As discussed above, efficient charge injection and separation have been discovered at the interface between the halide perovskite and other 2D materials such as graphene and TMDs, triggering the development of novel halide perovskite-based photodetectors and transistors. In addition, halide perovskite materials have long charge carrier lifetimes and diffusion length, high photoluminescence quantum efficiency, high light absorption coefficients, and high defect tolerance. These features make devices based on halide perovskite–graphene or TMD heterojunctions promising candidates in the next-generation solar cells and photodetectors.

Owing to the excellent electronic properties, graphene and TMDs have been incorporated into halide perovskite solar cells to enhance the stability and carrier collection efficiency. For example, the insertion of an ultrathin graphene quantum dot layer between the halide perovskite and TiO2 can effectively increase the power factor of perovskite solar cells.173 Furthermore, CVD-grown graphene can be employed as the top transparent conductive electrode in perovskite solar cells.174,175 By restricting the degradation of the CuSCN–perovskite interface, the introduction of reduced graphene oxide into perovskite solar cells can dramatically increase the stability, allowing the power conversion efficiency to retain >95% of its initial value after aging at a maximum power point for 1000 hours at 60 °C.176 In addition, the graphene/TiO2 composite can be utilized as the superior electron collection layer in perovskite solar cells,177 and graphene oxide can also serve as the hole conductor instead of the widely used spiro-OmeTAD (Fig. 11a).164 The band offset between the thin GO layer and the halide perovskite facilitates the transfer of holes while it blocks electron movement from the halide perovskite to GO. Similarly, it was reported that MoS2 nanoflakes178,179 and MoSe2 layers180 could be adopted as the active buffer layer or hole extraction layer in perovskite solar cells.


image file: c7cs00886d-f11.tif
Fig. 11 2D halide perovskite heterojunctions for solar cells and photodetectors. (a) Cross-sectional SEM image of the optimized planar heterojunction perovskite solar cells employing graphene oxide as the hole conductor. Reproduced from ref. 164 with permission from Royal Society of Chemistry, copyright 2014. (b) Schematic illustration and optical image of a hBN-covered graphene–WSe2–perovskite–graphene device; the white dashed lines outline the overlapping area of the graphene–WSe2–perovskite–graphene structure. (c) The IDSVDS plots obtained under laser (the wavelength is 532 nm) illumination with varied gate voltage (10 V increment). Reproduced from ref. 162 with permission from American Chemical Society, copyright 2016. (d) Schematic illustration and SEM image of a 2D (BA)2PbBr4 photodetector with interdigital graphene electrodes. The scale bar is 1 μm. (e) Time-dependent photocurrent response of a 2D (BA)2PbBr4 photodetector with interdigital graphene electrodes. Reproduced from ref. 163 with permission from American Chemical Society, copyright 2016.

Compared with poly-crystalline films, single-crystalline halide perovskites have much lower trap-state density, which reduces the carrier recombination rate, thus leading to longer diffusion length and lifetime.181 These features are desirable for high-performance solar cells. Cheng et al. fabricated a vertical halide perovskite–WSe2 heterojunction with graphene as back and top electrodes and hBN sheets as an encapsulation layer (Fig. 11b).162 The enrolled halide perovskite is a single-crystalline nanoplatelet. The interface between WSe2 and the halide perovskite can separate electrons and holes effectively, and hence, a VOC of about 0.2 V and a short-circuit current density (JSC) of over 0.5 mA cm−2 were detected upon illumination as shown in Fig. 11c. In addition, as the back-gate voltage varies from −60 to 60 V, a gradual increase in VOC and JSC was observed, indicating that the photovoltaic performance of this microscale halide perovskite–WSe2 solar cell can be tuned by an external gate voltage. This is attributed to the gate modulation of the Schottky barrier height at the bottom-graphene–WSe2 interface and the halide perovskite–WSe2 heterojunction. Even though the realization of solar cells based on the heterojunction sheds light on the possible application of halide perovskite–2D TMD heterojunctions in solar cells, the state-of-the-art performance can’t compete with solar cells based on the polycrystalline halide perovskite film yet. Further optimization of the fabrication technique, crystal quality as well as interface modification is still needed.

In addition to solar cells, the heterojunctions between the 2D halide perovskite and other 2D materials are excellent candidates for high-performance photodetectors. As discussed in Sections 3.3 and 3.4, the performance of the photodetectors can be dramatically enhanced by combining graphene and TMDs with halide perovskite nanoplatelets. The carrier transfer in the interface endows the hybrid photodetector with better detectivity and enhanced photocurrent compared with individual components. As mentioned above, mono-layer or few-layer 2D halide perovskite materials, BA2PbBr4, can be directly grown on substrates.70 Following this work, Tan et al. demonstrated a high-performance photodetector based on the (BA)2PbBr4–graphene heterostructure.163 The (BA)2PbBr4–graphene heterostructure was created by dry-transfer of graphene onto (BA)2PbBr4 crystals and subsequent nanofabrication. The as-fabricated photodetector is comprised of graphene as the electrodes and (BA)2PbBr4 as the light-absorbing element (Fig. 11d). Consequently, an ultrahigh photoresponsivity of up to 2100 A W−1 was observed together with a reasonably high on/off ratio and an ultralow dark current (Fig. 11e). However, for the single-crystal RP phase halide perovskites, the interlayer distance is generally over 1 nm separated by long-chain ligands, making the carrier transfer across the interface between the RP phase halide perovskites and other 2D materials more difficult. It will be important to further elucidate the mechanism of charge transfer in single-crystal RP phase halide perovskite based 2D heterojunctions.

4.2. 2D halide perovskite–graphene field effect transistors

As discussed above, halide perovskite materials showed long charge carrier lifetime/diffusion length and high photoluminescence quantum efficiency, which is evidenced by spectroscopy characterization.172,181,182 In addition, by fabricating and measuring the perovskite field effect transistors (FETs), the carrier concentration and mobility can be deduced from the transfer characteristics of the FET device. However, most of the up-to-date halide perovskite FETs are based on spin-coated halide perovskite films, where the grain boundaries within the polycrystalline film will scatter the charge carriers, thus reducing the carrier transport and mobility.183,184 In addition, the ion migration in 3D halide perovskites will significantly lead to a huge hysteresis and weakening field effect. To this end, Cheng et al. reported the vertical halide perovskite FET utilizing a single-crystalline MAPbI3 nanoplatelet prepared by vapor deposition.162 Graphene is adopted as the top and bottom electrodes because it has good electrical contact with the halide perovskite material and enables efficient carrier transfer across the halide perovskite–graphene interface. Additionally, hBN nanosheets were used to encapsulate the device from external moisture and oxygen, as shown in Fig. 12a. The channel length was defined as the thickness of the MAPbI3 nanoplatelet, which was about 43 nm. From the transfer characteristics of this vertical perovskite FET (Fig. 12b), the on/off ratio of over 100 indicated the effective modulation of carrier transport by gate voltage. Similarly, Li et al. demonstrated the fabrication of a lateral perovskite FET based on a vapor deposition-prepared MAPbI3 nanoplatelet (Fig. 12c).165 The channel length was about 4 μm, which was defined as the interval distance between two graphene electrodes and was much larger than that of the vertical perovskite FET. The temperature-dependent carrier mobility is shown in Fig. 12d, with a maximum value of about 3 cm2 V−1 s−1. The blue arrow indicates the tetragonal to orthorhombic phase transition temperature. However, ion migration is still inevitable in the aforementioned vertical and lateral perovskite FETs because of the weak ionic bonding. Further efforts are necessary to explore the carrier transport properties based on atomically thin perovskite FETs.
image file: c7cs00886d-f12.tif
Fig. 12 2D halide perovskite heterojunctions for transistors. (a) Schematic and optical image of a hBN-covered vertical graphene–perovskite–graphene transistor. The white dashed line outlines the graphene–perovskite–graphene overlapping area. (b) The transfer characteristics of the hBN-covered vertical graphene–perovskite–graphene transistor when biased at −0.8 V (black) and 0.8 V (red). Reproduced from ref. 162 with permission from American Chemical Society, copyright 2016. (c) Schematic and optical images of the lateral hBN encapsulated perovskite FET fabricated on a 300 nm SiO2/Si substrate with monolayer graphene as contact. The red dashed lines indicate graphene contacts. (d) The temperature dependent electron field-effect mobility measured with a source–drain voltage of 10 V. Reproduced from ref. 165 with permission from Wiley-VCH, copyright 2017.

4.3. 2D perovskite–graphene memory devices

As mentioned above, halide perovskites have low electrical conductivity but good ionic transport properties, enabling the potential application in resistive switchable memories. To date, most of the halide perovskite resistive memory devices are based on polycrystalline 3D MAPbX3 or 2D RP phase perovskite films. As shown in Fig. 13a, the polycrystalline film was sandwiched by two planar electrodes, which were generally Ag, Au, Pt or FTO. The halide perovskite resistive memories showed reversible switching behavior by controlling the applied voltage across the vertical sandwiched structure. For example, the Au/MAPbI3/ITO/PET device demonstrated reproducible and reliable memory characteristics in terms of program/erase operations and data retention with a program current of 0.7 mA and an on/off ratio of about 10 (Fig. 13b).185 By adopting the 2D RP phase BA2PbI4 film instead of the MAPbI3 film, the off current can be reduced by 5 orders of magnitude, which is attributed to the higher Schottky barrier, 2D anisotropic structure and electrical thermal activation energy. However, the program current ranging from 10−4 to 10−3 A (set current) is still too large compared with traditional oxide resistive memory devices, which is probably due to parasitic leakage current from grain boundaries inherent to polycrystalline thin films as well as large intrinsic electronic current for 3D halide perovskites.186
image file: c7cs00886d-f13.tif
Fig. 13 Halide perovskites as resistive memory. (a) Schematic of the resistive memory based on polycrystalline halide perovskite film. (b) Data retention characteristics of LRS and HRS states for the Au/MAPbI3 (polycrystalline film)/ITO/PET device. Reproduced from ref. 185 with permission from American Chemical Society, copyright 2016. (c) Endurance characteristics of Ag/2D BA2PbI4(polycrystalline film)/Pt, the set voltage is 0.5 V. Reproduced from ref. 186 with permission from Royal Society of Chemistry, copyright 2017. (d) Cross-sectional TEM image of the graphene/2D (PEA)2PbBr4 (single crystal)/Au resistive memory device, showing the filament shape (white dotted line). (e) Various program or set currents reported in the literature, including those based on TiO2, organic, WOx, AlOx, TaOx, HfOx, MoS2 and 2D halide perovskites. (f) Endurance for resistive switching at 10 pA for 100 cycles, where the red curve stands for LRS and the blue curve stands for HRS. Reproduced from ref. 187 with permission from American Chemical Society, copyright 2017.

In this case, Tian et al. reported the utilization of single-crystalline 2D (PEA)2PbBr4 and graphene for resistive memory with ultra-low operating current.187 As demonstrated by the cross-sectional TEM image of this device (Fig. 13d), graphene and Au were used as the electrodes on both sides of the 2D halide perovskite, which was prepared by mechanical exfoliation. The intrinsic electrical conductivity across the 2D halide perovskite was extremely low because of the existence of multilayer organic ligands and its large band gap (2.9 eV). And there were no current leakage channels from the grain boundaries in the single-crystalline 2D halide perovskite. In this case, the off current could be limited within 1 pA (Fig. 13f). With the increase of the voltage applied between graphene and Au, an ionic conductive filament with a diameter of about 20 nm formed across the 2D halide perovskite layer because of the migration of Br (Fig. 13d). Accordingly, when the ionically conductive filament was formed, the current increased to 10 pA, which was much lower than the set current of resistive memory devices based on polycrystalline halide perovskites and traditional oxides. The difference is indicated by the comparison map of different materials for resistive memories in Fig. 13e. The good reproducibility of switching behavior is demonstrated by switching the device at a 10 pA program current up to 100 cycles (Fig. 13f). These recent studies suggest that 2D perovskites are also promising for low cost high performance computing and memory applications.

5. Conclusions and perspectives

5.1. Novel 2D halide perovskite structures

Currently, 2D halide perovskite nanostructures based on the ultrathin 3D ABX3 or 2D RP phase L2An−1BnX3n+1 structures are being investigated intensively. Within this framework, lead and tin are the most commonly used B site cations. Developing new 2D halide perovskites is an important direction to fully explore this family of materials. There are several promising candidates. For example, the double perovskite Cs2AgBiBr6 and Cs2AgInCl6 bulk crystals have been recently reported and identified to be promising semiconductors.188,189 Reducing the dimensionality of such 3D double perovskites into 2D will open up new space to explore their optical and electronic properties.

Several types of layered halide perovskites have emerged as potential candidates for the active material of semiconductors with some of the more recent ones being comprised of cesium, lead and bromine. A room temperature solution-based method to synthesize Cs4PbBr6, CsPb2Br5 and CsPbBr3 has been presented that only depends on the Cs[thin space (1/6-em)]:[thin space (1/6-em)]Pb ratio.190 The different Cs/Pb/Br nanocrystals displayed a high phase purity and photoluminescence quantum yield with the highest being CsPb2Br5 at 84%, making it an interesting perovskite base to further study.191 In addition, CsPb2Br5 has been shown as an attractive material for optoelectronic nanomaterials due to its stability and ability to form thin nanoplatelets via a solution precipitation method. CsPb2Br5 can be tuned with anion exchanges in the form of CsPb2Br5−xIx and CsPb2Br5−xClx to change the optical absorption of the perovskite. Upon ion exchange with chloride ions, the perovskite layers display a blue shift in photoluminescence under UV irradiation while the iodide ion exchange led to a red shift, indicating that the perovskite can be modified to extend the photoluminescence to cover the visible spectrum. An important note is that after the ion exchange, the nanoplatelet thickness remains at 3 nm as a testament to the stability of the layered halide perovskite structures.190

When the bulk CsPb2Br5 is thinned down to monolayers, calculations have shown that there are two stable phases that feature an indirect band gap of 2.54 eV. While Br vacancies in the monolayer are most likely and lead to trap states, Pb and Cs vacancies can cause the single-layer CsPb2Br5 to behave like a p-type doped semiconductor, adding another dimension to the tunability of CsPb2Br5.192 In addition, the concentration of Cs atoms has been shown to strongly affect the stability of the monolayer CsPb2Br5 in contrast to not significantly affecting the electronic properties in the bulk material.

In another example, the triple perovskite Cs3Bi2Br9 exhibits strong exciton–photon coupling (940 meV), which leads to a strong localized state allowing for well-defined photoluminescence peaks at room temperature.193 The solution-based process and less-toxic components of Cs3Bi2Br9 make it attractive as a new member of light absorbing halide perovskites. As aforementioned, the interest in novel 2D halide perovskites is palpable. The ability to create nanostructured halide perovskites by solution-based methods, combined with further investigation into ion exchange and composition, makes it feasible to find optimized materials for optoelectronics that feature high purity, efficiency, tunability, and scalability.

5.2. Complex heterojunctions and epitaxial growth

As we discussed in Section 3 of this article, stacking 2D halide perovskites with other 2D materials, including graphene, MoS2, and hBN, is now under active investigation. Efficient charge injection, conduction, and extraction have been discovered at halide perovskite–graphene/MoS2 interfaces and high-performance photodetectors, transistors, and memory devices have been fabricated. So far, the heterojunctions are mostly fabricated via transfer methods, which are time-consuming and low-throughput. Fabrication of high quality multi-layer complex heterostructures using well-controlled patterning and sequential growth will be of great interest. Moreover, the combination of halide perovskites with other novel 2D materials has not been well studied. For example, interfacing 2D halide perovskites with BP (Fig. 14a), Mxenes (Fig. 14b), and PtSe2 might lead to new types of heterostructures with unique optical and electronic properties. In addition to interfacing with inorganic materials, introducing functional organic groups (e.g. conducting, semiconducting, chiral, polymerizable moieties) in the “L” or “A” sites is another way to establish complex functional heterojunctions (Fig. 14c). To achieve that, expertise from the organic electronics community is probably needed.
image file: c7cs00886d-f14.tif
Fig. 14 New types of 2D halide perovskite heterostructures. (a) Perovskite–BP heterojunction. Blue dots: cesium or methylammonium; red dots: phosphorus; the black octahedra in between the blue dots represent the metal halide BX6. (b) Perovskite–Mxene heterojunction. Red dots: carbon; purple dots: titanium. (c) Perovskite–organic semiconductor heterojunction. Green ellipses are conjugated organic molecules. (d) Vertical perovskite–perovskite heterojunction. Black and red octahedra indicate different halides. (e) Lateral perovskite–perovskite heterojunction. Black and red octahedra indicate different halides. (f) Epitaxial growth of halide perovskites on NaCl. Grey dots: chloride; yellow dots: sodium.

Another important type of heterojunction is the perovskite–perovskite heterojunction (Fig. 14d and e). So far, vertical perovskite–perovskite heterojunctions have not been achieved due to the difficulty of synthesis and assembly. It might also be limited by the ion inter-diffusion between the adjacent layers. However, this is unclear so far. For lateral heterojunctions, a localized anion exchange approach was used on a CsPbBr3 platelet to create CsPbBr3–CsPbCl3 junctions. Direct growth of a lateral heterojunction along the edge of the halide perovskite remains challenging. Heterojunctions with distinct “A” or “B” cations have not been reported. So far, the reported heterojunctions are all based on relatively thick plates, rather than atomically thin sheets. Stability remains a critical issue in such ultrathin ionic crystals with weak bonding.

Lastly, how to achieve epitaxial growth for halide perovskites on other 2D materials is largely unknown. A recent work by the Shi group demonstrated single-crystalline high-temperature vapor phase epitaxy of halide perovskite thin films on NaCl (Fig. 14f).194 It is shown that the metal alkali halides could be very useful for vapor phase epitaxial growth of halide perovskites due to their similar material chemistry and lattice constants. About 4% lattice mismatch was observed in such systems. Once the thickness of the halide perovskite layer is thinned down to a few nanometers, a larger tolerance to lattice mismatch might be possible. It will be interesting to explore the epitaxial growth of halide perovskites over other semiconductor materials as well.

5.3. New physics and devices

2D halide perovskites and related heterostructures could be excellent candidates for electronic and optoelectronic devices. In addition, the unique bonding and structure of them suggest new physics and applications as well. For example, the soft lattice of halide perovskites is responsive to pressure.195 Pressure induced phase transitions have been observed, indicating the possibility of using 2D halide perovskites in phase change memories and topological insulators.196 Furthermore, thermal (phonon) transport is another area of interest. The heavy metal and halide ions and weak bonding in perovskites make them poor thermal conductors (phonon glass electron crystal).197–200 It will be interesting to see the phonon modes in the 2D extreme. The suppressed thermal transport could be essential in high performance thermoelectric devices. Finally, ferroelectricity and ferromagneticity have also been observed in the bulk layered RP phase halide perovskites.201,202 Thinning down the thickness to one or a few atomic layers may enhance the ferroelectric and ferromagnetic properties because of the strong quantum and dielectric confinements.

5.4. Limitations and challenges

We have summarized the most exciting aspects of the 2D halide perovskites and their heterostructures and interfaces with other 2D materials. Many new opportunities in this promising field are ahead of us. However, 2D halide perovskites, as well as other types of 2D materials, have their own drawbacks and limitations. First, stability of 2D halide perovskites is relatively poor. In general, the metal–halide bond is normally weak and easy to break. Even for 3D halide perovskites, the stability issue hasn’t been fully resolved so far. The stability of the materials usually becomes worse in nanomaterials and this is particularly true for 2D perovskites with only a few atomic layers thick. For example, as shown in Fig. 15, the (BA)2PbI4 2D sheets degraded significantly after storing the sample in air for a few days. The surface became rougher and the PL intensity dropped. These materials also degrade under thermal heating and laser and electron beam irradiation even under an inert environment, making further detailed structure and property characterization difficult. The challenge and opportunity will be synthesizing new 2D halide perovskites with enhanced stability or developing new encapsulation techniques to protect them without influencing or degrading their optoelectronic properties. A second drawback is the poor charge transport in the vertical direction, which limits the performance of solar cells using 2D halide perovskites and related 2D heterojunctions. The vertical transport can be enhanced by decreasing the interlayer distance or inserting a conductive organic layer between the perovskite layers. A third problem is scalability. How to coat a large area film/array of 2D heterostructures is challenging. For a bottom up approach, one can pattern the substrate with certain catalysts to guide the growth of the 2D material. This has been achieved for the thick perovskite flakes. For a top down approach, coating a large area 2D film and then etching it to form a patterned array may also be possible.
image file: c7cs00886d-f15.tif
Fig. 15 Stability of (BA)2PbI4 2D sheets. (a) Optical image of a fresh sample. (b) PL image of the corresponding fresh sample under UV light excitation. (c) Optical image of an aged sample (left in air for 3 days). (d) PL image of the corresponding aged sample. The samples in (b) and (d) were measured under the same excitation intensity and acquisition time. Unpublished data. All scale bars are 20 μm.

In summary, we believe that 2D halide perovskites will be an important new member in the 2D materials family with excellent tunability, highly dynamic structural features, and fascinating optoelectronic properties. What is even more exciting is the possibility of interfacing halide perovskites with other 2D materials to achieve new functional materials and devices. The research topics are interdisciplinary, requiring knowledge and expertise from many different fields. Many challenges remain. More progress towards the development of new materials and interfaces, deeper fundamental understanding of the materials and interfaces, and better-performed devices is anticipated in the near future.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

L. Dou thanks the start-up funding from the College of Engineering and Davidson School of Chemical Engineering of Purdue University. We thank Prof. Libai Huang for the fruitful discussions and proofreading the manuscript. In memory of Professor Gaoquan Shi at Tsinghua University.

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