Open access peer-reviewed chapter - ONLINE FIRST
Abstract
Two-dimension (2D) hybrid organic-inorganic perovskites (HOIPs) are formed naturally multiple-quantum-well structures with a much larger carrier binding energy, which possess stable excitons even at room temperature. In addition, 2D HOIPs allow us to exfoliate them into ultrathin flakes and stack them into various heterostructures, extending their photophysical properties. Therefore, 2D HOIPs are promising candidates for optoelectronic device applications, such as light-emitting diodes, lasing, etc. In this chapter, a summary of the crystal structures of 2D HOIP crystals and their heterostructures, excitonic properties, and the current research progress of the 2D HOIPs and their heterostructures are revealed. Next, high-pressure technology will be studied in detail on the effective engineering of crystal structures and exciton properties of 2D HOIPs toward significantly optimizing their functionalities. Finally, a summary is given, and the high-pressure strategy toward manipulation of 2D perovskite-based heterostructures is rationalized for next-generation high-performance excitonic devices.
Keywords
- 2D HOIPs
- perovskite heterostructures
- exciton
- high pressure
- photophysical properties and crystal structures
1. Introduction
Two-dimensional (2D) hybrid organic-inorganic perovskite (HOIPs) have been star materials due to their excellent optoelectronic properties [1, 2], chemical tunability [3], and environmental stability [4, 5]. 2D HOIPs are composed of alternating layers of organic and inorganic parts, which undergo self-assembly in solution to generate regularly arranged stacked with organic and inorganic components. 2D HOIPs present a large compositional and structural phase space as well as exciting physical properties, such as a soft and dynamic structure [6], strong anisotropy in the in-plane and out-of-plane directions [7], strong light harvesting capability [8], high photoluminescence quantum yield (PLQY) [9], controllable charge-carrier mobility [10], stable excitons at room temperature [11, 12, 13, 14], long carrier diffusion length [15, 16], and nonlinear effects [17, 18, 19, 20], etc. Breakthrough performances in 2D HOIP-based devices have been achieved since 2016, including solar cells [21, 22, 23], light-emitting diodes (LEDs) [24, 25, 26, 27], lasing [28, 29, 30, 31] and photodetectors [32, 33, 34, 35, 36, 37]. On the other hand, for integrating 2D HOIPs into electronic devices, precise control over layer thickness is crucial. The modest interaction among organic chains within unit cells facilitates the mechanical exfoliation of thin flakes of 2D HOIPs, employing techniques akin to those developed for graphene and other 2D layered materials [38, 39]. In these exfoliated perovskite flakes, alterations are expected in exciton-binding energy, dielectric constant, and spatial extension of electron and hole wavefunctions, particularly as thickness diminishes toward the atomic scale. Researchers have investigated the nonlinear optical (NLO) effects, such as third-harmonic generation (THG), in these exfoliated thin flakes with thickness in the 20–60 nm range [40, 41]. Furthermore, surface and interface effects become significantly more influential in these molecularly ultrathin flakes, exerting a profound impact on their photophysical and electronic characteristics [42, 43]. For example, surface disorder or lattice reconstructions in these molecularly thin perovskite flakes may disrupt surface-inversion symmetry, potentially resulting in Rashba-band splitting [38]. In addition, 2D HOIPs have become a strong candidate recently as a new building block for fabricating various heterostructures with extended physical properties. Until now, 2D perovskite-based vertical and lateral heterostructures have been fabricated based on multiple methods, such as dry transfer, vapor deposition, and solution-based chemical synthesis [44, 45, 46, 47]. Through modulating materials and the stacking sequence integrated with 2D HOIPs and other 2D materials, various 2D perovskite-based heterostructures have been obtained, which not only provides a unique platform for researchers to explore new fundamental physics but also expands the potential applications of novel optoelectronic devices [47, 48, 49, 50, 51]. It is worth noticing that the novel physical properties observed in 2D HOIP crystals have close relationships with their microscopic crystal structures. For example, a tiny change in their crystal structures, such as bond length and bond angle, will induce a giant change in their optical and electronic properties. Therefore, comprehensively understanding their crystal structures and resolving the structure-property correlations at the atomic level is important for further improving their material properties and device functionalities. Moreover, for the development of heterostructure devices utilizing 2D HOIPs, a comprehensive grasp of exciton interactions, as well as charge and energy transfer mechanisms occurring at the interfaces between 2D layers, is imperative [48].
Pressure engineering as an effective and clean technique can systematically modify the crystal structures and electronic states of solid materials, resulting in new materials, new properties, and new physics that cannot be obtained by using other methods. Particularly, HOIPs with a variety of compositions have soft lattices, where the bond length and bond angle of the inorganic framework will be drastically changed under pressure, significantly modulating their crystal structures and photophysical properties. Therefore,
In this review, we will discuss the current understanding of the crystal structures and physical properties of 2D HOIPs with a particular focus on various crystal structures, heterostructures, excitonic band structures, and light–matter interactions in these materials. Particularly, high-pressure technique will be mainly discussed as an effective tool for the manipulation of crystal structures and excitonic properties of 2D HOIPs. In the final part, we will give a summary, and also provide a perspective on the high-pressure engineering of 2D perovskite-based heterostructures, addressing key scientific questions in the physical properties that are required to be resolved to realize breakthroughs in their functionalities for next-generation high-performance optoelectronic device applications.
2. Crystal structure and physical properties of 2D HOIPs
2.1 Crystal structure of 2D HOIPs
The general chemical formula of 2D HOIPs is (RNH3)2An-1BnX3n + 1, where n represents the number of inorganic layers. RNH3+ is a long-chain alkylammonium organic molecular layer, such as butyl ammonium (BA) and phenylethyl ammonium (PEA), A is a smaller cation filled into the 12-fold coordinated holes wrapped by the BX6 octahedra, such as CH3NH3+ (MA+), formamidinium (FA), B refers to metal cations, usually Pb or Sn, and X stand for halide anion (Figure 1a) [55, 56, 57]. 2D HOIPs can be structurally considered as slicing three-dimensional (3D) perovskite frameworks by inserting organic cation spacers among the BX6 inorganic framework along the (100), (110), and (111) different crystallographic orientations (Figure 1b) [4, 6, 53]. The conversion from pure 2D perovskites to quasi-2D perovskites and then to 3D perovskites will be realized by increasing the number (n) of inorganic layers (Figure 1a) [4, 6, 7]. The extensively studied candidate of 2D HOIPs is the (100) perovskite derivatives, such as Ruddlesden-Popper (RP) phase and Dion-Jacobson (DJ) phase (Figure 1b) [7, 53, 58, 59, 60]. For the RP phase perovskites, the adjacent organic layers are connected via weak Van der Waals (vdW) forces, while there is only one layer of the organic molecules in the DJ phase perovskites (Figure 1c) [54]. Due to the interaction between the organic and inorganic layers occurring via hydrogen bonding, DJ phase 2D perovskites demonstrate superior structural stability compared to RP phase 2D perovskites. Consequently, RP 2D perovskites are more readily mechanically exfoliated and stacked to form diverse 2D heterostructures.
Figure 1.
(a) schematic illustration of the evolution from 2D perovskite to 3D perovskite with key components [6]. Copyright 2018 American Chemical Society, (b) schematic structures of different oriented families of 2D perovskites along (100), (110), and (111) planes, respectively [53]. Copyright 2019 MDPI, and (c) structural details of PR phase and DJ phase 2D perovskite, respectively [54]. Copyright 2019 Elsevier.
2.2 Excitonic properties of 2D HOIPs
2D HOIPs naturally form structures akin to multiple-quantum-wells, with adjacent inorganic layers sandwiched by two layers of long-chain organic spacer cations. Specifically, the inorganic (BX6)4− layers act as the “wells” while the organic spacers act as the “barriers”, giving rise to these quantum-well configurations (Figure 2a) [61, 63]. Such a setup results in large dielectric confinement within 2D HOIPs due to the substantial difference in dielectric constants between the organic layer barriers and the inorganic layers. Consequently, there are huge oscillator strengths and large exciton-binding energy (> 100 meV) [7]. As a result, electrons and holes are strongly confined within the 2D inorganic slabs, leading to stable excitonic emission even at room temperature (Figure 2b) [61]. Given that quantum and dielectric confinement effects strongly rely on the number of inorganic layers (n), the band structure of 2D HOIPs can be efficiently modulated by adjusting this parameter. For example, increasing the value of n results in thicker unit cells, accompanied by a reduction in the band gap and excitonic binding energies (Figure 2c) [1]. Moreover, the electronic band structure of 2D HOIPs is intricately linked with the crystal structural distortion of inorganic sublattices, specifically the bond angle of B-X-B and bond length of B-X. These distortions induce shit in the valence band and conduction bands, thereby causing variations in the bandgap (Figure 2d) [62].
Figure 2.
(a) 2D quantum well structure illustration of 2D HOIPs [6]. Copyright 2018 American Chemical Society, (b) excitonic emission (left) and schematics of the Rydberg of the exciton ground states merging with the continuum (right) [61]. Copyright 2018 springer nature, (c) diagram showing the binding energy and band gap vs. layer number of n [2]. Copyright 2016 advanced materials, and (d) correlation of structure parameter of bond angle and bond length on the excitonic band structure of 2D HOIP [62]. Copyright 2018 American Chemical Society.
Furthermore, the electron-phonon interaction, namely, the interaction of charge carriers with lattice vibrations (phonons), in 2D HOIPs is strong due to the soft lattice nature, leading to the formation of self-trapped excitons (STEs) [64, 65]. The states of STEs are below the intrinsic band edge which affects charge-carrier recombination pathways and dynamics (Figure 3a) [67, 68, 70]. Particularly, the emission of STEs is the physical origin of the intensive broadband emission with a large Stokes shift and even the white-light emission observed in some 2D HOIPs (Figure 3b) [68]. In 2014, there were reports of three-layered perovskites producing broadband white-light emission when excited by near-UV light. These perovskites showcased an impressively high color rendering index, making them particularly suitable for applications in natural white LEDs [67].
Figure 3.
(a) Nuclear coordinate diagram illustrating the dynamic process of exciton self-trapping and de-trapping a 2D perovskite, with labels indicating different states and activation energies. (GS = ground state, FE = free exciton state, STE = self-trapped exciton state, Ea,trap = activation energy for self-trapping, Ea,detrap = activation energy for de-trapping, S=Huang-Phys parameter) [66]. Copyright 2017 the Royal Society chemistry, (b) broadband emission covering the entire visible spectrum from (110) 2D perovskite, accompanied by photographic images of the crystal in the inset [67]. Copyright 2014 American Chemical Society, (c) 2D perovskites with short and long organic ligands (left), showcasing the corresponding narrow and broad PL emission (right). The insets demonstrate the structure-induced color change (from orange to yellow) [68]. Copyright 2016 American Chemical Society, and (d) schematic illustrations of the crystal structures of 2D perovskites with R- and S-MBA chiral organic molecular chains, along with circularly polarized PL spectra of these chiral 2D perovskites at 77 K [69]. Copyright 2019 American Chemical Society.
Although the electronic band structure of 2D HOIPs is primarily governed by the inorganic sublattices, the presence of long organic ligands also holds significance in altering their optical properties. Incorporating diverse long organic chains opens avenues for exploring a wealth of physics within 2D HOIPs. For example, the strength of electron-phonon interaction can be adjusted by varying the organic ligands, leading to enhanced emission of STEs surpassing that of free exciton emission in certain 2D HOIPs (Figure 3c) [70, 71]. Secondly, altering the organic molecules allows for manipulation of the exciton-binding energy in 2D HOIPs due to differing dielectric constants among organic molecules, consequently impacting the PLQY [72, 73]. Thirdly, the substitution of long organic chains with chiral molecules in specific 2D HOIPs has resulted in the observation of circularly polarized light emission. This phenomenon arises from the transfer of chirality from the chiral molecule to the 2D perovskites (Figure 3d) [69, 74, 75, 76]. Such 2D HOIPs exhibit circular dichroism (CD) on account of the different absorption of left and right-handed circularly polarized light, holding significant promise for applications in spintronics and valleytronics [75, 77, 78, 79].
3. Fabrication and excitonic properties of 2D perovskite heterostructures
3.1 Fabrication of 2D perovskite heterostructures
The inherent layered structure and weak interlay interactions of 2D HOIPs, particularly in RP phase perovskites, facilitate their mechanical exfoliation using the conventional tape method, enabling the transformation from bulk crystals down to molecularly thin forms (Figure 4a) [38, 80]. Such thin and soft forms of 2D HOIPs can be easily picked up and transferred onto various substrates, including 2D HOIPs and other 2D layered materials, such as 2D transition metal dichalcogenides (TMDCs) and graphene, to fabricate vertically stacked artificial heterostructures. In addition, researchers have employed solution-based methods (bottom-up approaches) to naturally generate both in-plane and vertical 2D perovskite-based heterostructures.
Figure 4.
(a) Mechanically exfoliated ((C4H9NH3)2PbI4) n = 1 2D RPP nanocrystals, the optical image of the exfoliated perovskite nanoflakes (left), the dark field optical image of the exfoliated perovskite nanoflakes (middle), and the AFM image of the monolayer perovskite sheet (right) [80]. Copyright 2017 the Royal Society of Chemistry, (b) construction of 2D perovskite heterostructures with dry transfer method (top) and the optical and PL images of the staked perovskite heterostructure(bottom) [48]. Copyright 2023 IOP publishing ltd, and (c) schematic illustrations of the (2 T)2PbI4/(2 T)2PbBr4 lateral 2D perovskite heterostructures (top) and the corresponding TEM image and EDS mappings of the 2D perovskite heterostructures (bottom) [48]. Copyright 2023 IOP publishing ltd.
As an illustration, researchers have successfully produced 2D RP phase perovskite multi-heterostructures comprising different layer numbers of n using a simple solution growth technique. This method has enabled the creation of vertically stacked double heterostructures and intricate multilayer heterostructures of 2D lead iodide perovskites through vdW epitaxy [81]. Furthermore, they have achieved controllable direct growth of large-area nanosheets of various phase-pure RP perovskites with thickness down to monolayer. These free-standing perovskite nanosheets, assembled at the air-liquid interface, were lifted using a PDMS stamp and transferred onto Si/SiO2 substrates. Subsequently, a second layer of free-standing perovskite nanosheet was stacked on top to form a heterostructure. By iterative pick-up and transfer steps, they have systematically assembled these nanosheets into diverse 2D perovskite heterostructures (Figure 4b) [48, 82]. In addition, in-plane 2D perovskite heterostructures have also been obtained by a sequential solution phase approach, where two adjoined perovskites will be epitaxially grown in the lateral direction from the center to the edge. They have been demonstrated highly stable a tunable superlattice (Figure 4c) [44, 48, 83].
3.2 Optical properties of 2D perovskite heterostructures
2D perovskite heterostructures incorporating diverse layered configurations offer distinct optical properties, thereby significantly broadening the scope of potential functionalities and applications in devices. Firstly, the varying electronic band structures of 2D perovskite series with different layer numbers of n facilitate energy transfer from the higher energy bandgaps (lower n values) to the lower energy bandgaps (high n values), naturally giving rise to different types of band alignments [84]. For example, in the vertical heterostructure of (PEA)2PbI4/(PEA)2(MA)Pb2I7 (n = 1/n = 2), energy transfer from (PEA)2PbI4 to (PEA)2(MA)Pb2I7 occurs within hundreds of picoseconds after photoexcitation, resulting in the formation of a type I band alignment (Figure 5a) [81]. More importantly, charge-transfer excitons (CTEs) could be effectively formed in 2D perovskite heterostructures, as demonstrated in the PEA2PbI4:PEA2SnI4 heterostructure with the host and guest components, where broad PL occurs in the below-bandgap region (Figure 5b). These CTEs, crucial states formed at interfaces, introduce a novel mechanism for enhancing the multifunctionality of 2D perovskites [81, 85, 87]. Secondly, the interface containing edge-contact organic cations, chirality and symmetry properties allows for the property transfer across the interface by selecting different organic components, thus leading to the emergence of new physics in the interfaces. For example, the chirality can be transferred to the inorganic layers that are interfaced with the chiral molecules, providing a means to break valley or spin degeneracy within the inorganic layer [86]. Additionally, certain 2D perovskite heterojunctions exploit surface-bound ligands as spatial barriers to inhibit ion migration across the junctions, enabling multicolor emission with high spectral purity [81]. Also, the investigations into giant nonlinear optical responses have been conducted in (n-C4H9NH3)2PbI4/(n-C4H9NH3)2(CH3NH3)Pb2I7 heterostructure of centimeter size, revealing giant two-photon absorption and strong multiphoton-induced PL in the heterostructure of varying thickness (Figure 5c) [88]. In the lateral epitaxial 2D perovskite heterostructures, band alignments can be modified either by varying the inorganic composition in the lateral in-plane direction or by altering the molecular structure in the out-of-plane direction, thereby determining PL emission properties. For example, a type-II band alignment is achieved in (4Tm)2SnI4–(4Tm)2PbI4 heterostructure, where exciton emission predominates at the edge and the lifetime is enhanced at the interface of the heterostructure [83].
Figure 5.
(a) Schematic structure illustrating 2D heterostructure composed of RPP with n = 1 and n = 2 (left), along with the corresponding PL spectrum exhibiting two emission peaks at 532 nm and 576 nm assigned to n = 1 and n = 2 RPP (middle). The inset depicts the band alignment within the heterostructure, showcasing energy transfer from n = 1 to n = 2 layer. GS: Ground state, ES: Excited state. Lifetime curves of the heterostructure probed at both components’ emission wavelengths are presented on the right side [81]. Copyright 2018 American Chemical Society, (b) PL spectrum of a 2D perovskite heterostructure, demonstrating a broad emission with the peak position at 697 nm, alongside intrinsic emission peaked at 524 and 627 nm from host PEA2PbI4 and guest PEA2SnI4 components (left). The middle schematic diagram illustrates the formation of CTEs at the interfaces between these 2D perovskite heterostructures. PL lifetimes of the broad emission at 669 nm, intrinsic emission at 524 nm, and 621 nm of this 2D perovskite heterostructures are depicted on the right side [85]. Copyright 2020 springer nature, and (c) schematic illustration of the crystal structures of PEA (non-chiral) and R-/S-MBA perovskites and optical/AFM images of this 2D heterostructure with the PEA layer on top of the R-MBA layer (left). Circularly polarized PL (CPL) spectra of pure PEA exhibit no chirality (middle), whereas CPL spectra of PEA/R-MBA and PEA/S-MBA heterostructures exhibit clear chirality (right) [86]. Copyright 2022 American Chemical Society.
4. High pressure
4.1 High-pressure technique
The diamond-anvil cell (DAC) stands out as a widely employed technique for generating ultra-high hydrostatic pressure on a small number of samples [89, 90]. A typical DAC utilized in laboratory settings comprises several essential components (Figure 6a). At the heart of a DAC lies a pair of diamonds situated on opposing piston and cylinder components, enabling the generation of high pressure at the pointed ends of each diamond by mutual compression facilitated by the metal piston and cylinder (Figure 6b). Sandwiched between the diamond pieces is a stainless-steel gasket featuring a central drilled hole, serving as the sample chamber (Figure 6b) [91]. To ensure effective and uniform transmission of pressure to the sample, the sample space is typically filled with a pressure-transmitting medium, commonly a liquid (Figure 6b). During pressurizing, the pressure within the liquid escalated significantly and was evenly transmitted to the sample in all directions, resulting in hydrostatic pressure. For perovskite samples, silicone oil is frequently employed as the pressure-transmitting medium [93]. The PL peak position of ruby serves as a reference for pressure calibration, exhibiting a nearly linear shift with hydrostatic pressure below 100 GPa (Figure 6c) [92, 94].
Figure 6.
(a) Actual images of a DAC, along with the metal piston, cylinder, and screws, are shown on the right side [91]. Copyright 2021 Elsevier ltd., (b) schematic illustration of the cross-section of a DAC (left), with a zoomed-in view (right) on the diamond/gasket assembly [91]. Copyright 2021 Elsevier ltd., and (c) typical fluorescence spectra of the ruby emission observed within the DAC under compression [92]. Copyright 2022 American Chemical Society.
4.2 High-pressure engineering of excitonic behaviors in 2D HOIPs
Over the past 6 years, there has been considerable research into the impact of pressure on the structure and excitonic properties in 2D HOIPs. Controlled compression-induced modulation of the inorganic sublattices in these materials can finely adjust their photophysical properties. Specifically, lattice compression can effectively alter the excitonic states by narrowing or broadening the band gap and introducing new exciton bands. This presents a promising avenue for engineering 2D HOIPs with tailored properties under ambient pressure conditions. For example, the exciton emission peak is effectively tuned in the mechanically exfoliated thin flakes of (BA)2PbI4 (n = 1) 2D perovskite, which exhibits a blue shift at very low pressure of 0.1 GPa along with a continuous redshift up to 5.3 GPa before the occurrence of amorphization, the total energy change of excitonic emission is about 350 meV within a moderate pressure of 5.3 GPa (Figure 7a) [62]. The emission lifetimes of excitons have also changed accordingly, which is prolongated by 40 ps within 0.4 GPa and is shortened by 100 ps up to 2.3 GPa (Figure 7a) [62]. Besides, the emission linewidth of exciton has been investigated in the exfoliated ultrathin flakes of (BA)2(MA)Pb2I7 (n = 2) 2D perovskite, where the narrow band emission is replaced by a stable broadband emission after pressure-treatment beyond certain pressure threshold (>3 GPa) due to the pressure-induced permanent distortion of inorganic sublattices and the formation of STEs (Figure 7b) [52]. Specifically, free excitons are firmly trapped by strongly distorted inorganic sublattices to form STEs, which cannot revert to free excitons due to the large lattice deformation potential induced by the pressurization process, leading to broadband emission at ambient conditions. Moreover, the emission intensity of excitons in (HA)2(GA)Pb2I7 (HA = n-hexylammonium, GA = guanidinium) 2D perovskite has undergone a remarkably 12-fold enhancement. This enhancement is attributed to the substantial suppression of carrier trapping resulting from lattice compression under mild pressure conditions, consequently leading to significantly heightened free exciton emission [95, 96]. In addition, the manipulation of band-edge states and charge distribution in organic semiconductor-incorporated 2D halide perovskites has been realized recently. This manipulation reveals a switching of band alignment at the organic-inorganic interface, offering control over the emission properties of 2D perovskites (Figure 7c) [78]. More interestingly, the anisotropic structural nature of 2D HOIPs has proved that the compression process is highly anisotropic [97]. For example, the quasi-uniaxial compression primarily squeezes the organic layers in the layer-to-layer direction in (PEA)2PbI4 2D perovskite crystal in a low-pressure region. This compression effectively modulates the quantum confinement effect by 250 meV, owing to the significant reduction in thickness of the organic barrier layers [98]. Pressure-induced emission from initially non-emissive 2D perovskites has also been investigated. For example, the STE emission is observed at 2.5 GPa with a broad emission band and large Stokes shift in initially nonfluorescent (BA)4AgBiBr8 2D perovskite, with further significant enhancement upon compression up to 8.2 GPa. Such pressure-induced emission in these 2D perovskites is attributed to a structural phase transition, namely, [AgBr6]5− and [BiBr6]3− inter octahedra tilt [99].
Figure 7.
(a) Evolution of the PL peak positions (left) and lifetimes (right) of 2D RPP (n = 1) under high pressure [62]. Copyright 2018 American Chemical Society, (b)
5. Conclusion
2D HOIPs emerge through the incorporation of sizable organic cations derived from 3D perovskites, presenting enhanced structural stability as compared to their 3D counterparts. The large binding energy, stemming from the large dielectric constant difference between the inorganic and organic layers, facilitates the formation of stable excitons in 2D HOIPs even at room temperature. This characteristic furnishes a distinctive platform for the development of optoelectronic devices through the manipulation of excitons. Undoubtedly, the intrinsic excitons formed within band structures function as the principle excited states, pivotal for the development of high-performance light-emitting properties in such 2D HOIPs. Moreover, the thickness of 2D HOIPs down to the atomic limit to be stacked with various heterostructures largely expands their material diversity, triggering the emergence of new physics and extending optoelectronic heterostructure devices. In this chapter, we have reviewed recent advances in 2D HOIPs in terms of crystal structures, heterostructures, electronic band structures, and excitonic properties. To achieve high-performance exciton-based light-emitting devices, extensive exploration of the structure-property correlation in this type of perovskites has been conducted to comprehensively understand and precisely establish the relationship between crystal structures and excitonic properties. Among these methods, the utilization of high pressure has been shown to provide a means to regulate the unit cell and interatomic spacing of materials, bypassing the necessity for new growth methods or processing while enabling the examination of materials properties
Fundamentally, the manipulation of intrinsic excitons involving the formation probability, binding energy, and radiative/nonradiative recombination governs the light-emitting properties. It is noteworthy to observe the effective behaviors of CTE/energy transfer in 2D perovskite heterostructures. This donor-acceptor design presents an intriguing aspect, where CTEs/energy transfers can be conveniently adjusted across energy, polarization, and spin parameters by artificially stacking different 2D perovskites together. This phenomenon gives rise to novel physical phenomena and unique capabilities to regulate optical, electrical, and magnetic functionalities through CTEs/energy transfers. Due to their high sensitivity to interlay spacing, CTEs/energy transfers exhibit more tunability, making them well-suited for pressure engineering, thus facilitating the acquisition of robust interlayer excitons. Moreover, high pressure serves as a powerful tool capable of altering the vdW interactions between layers, layer-to-layer sliding, twisted angles, as well as crystal structures in 2D perovskite heterostructures. This transformative capability significantly controls the dynamics of intralayer excitons, interlayer excitons, and free carriers, thus facilitating researchers in systematically and comprehensively unraveling the processes of interlayer exciton formation, relaxation, and transport in 2D perovskite heterostructures. The urgent need for high-pressure engineering of 2D perovskite heterostructures is underscored, both for fundamental research and practical applications. It provides researchers with new opportunities for vdW interaction engineering, effectively forming/controlling interlayer excitons, triggering new exciton physics, and developing novel heterostructure excitonic devices with desired properties.
Acknowledgments
T. Yin gratefully acknowledges strong support from the President Postdoctoral Fellowship of Nanyang Technological University.
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