Van Der Waals Mask-Assisted Strategy for Deterministic Fabrication of Two-Dimensional Organic−Inorganic Hybrid Perovskites Lateral Heterostructures

Abstract

Two-dimensional (2D) organic−inorganic hybrid perovskites (OIHPs) have emerged as promising candidates for next-generation optoelectronic applications. While vertical heterostructures of 2D OIHPs have been explored through mechanical stacking, the controlled fabrication of lateral heterostructures remains a significant challenge. Here, we present a lithography-free, van der Waals mask-assisted strategy for the deterministic fabrication of 2D OIHP lateral heterostructures. Mechanically exfoliated 2D materials such as graphene serve as removable masks that enable selective conversion of unmasked perovskite regions via ion exchange reaction. This technique enables the fabrication of various lateral heterostructures, such as BA₂MA₂Pb₃I₁₀/MAPbI₃, PEAPbI₄/MAPbI₃, as well as BA₂MAPb₂I₇/MAPbBr₃. Furthermore, complex multiheterostructures and superlattices can be constructed through sequential masking and conversion processes. Moreover, to investigate the electronic properties and demonstrate potential device applications of the lateral heterostructures, we have fabricated an electrical diode based on a BA₂MA₂Pb₃I₁₀/MAPbI₃ lateral heterostructure. Stable electrical rectifying behavior with a rectification ratio of around 10 was observed. This general and flexible approach provides a robust platform for constructing 2D OIHPs lateral heterostructures and opens new pathways for their integration into high-performance optoelectronic devices.

1. Introduction

The wide and rich physics of two-dimensional (2D) materials have led to the fabrication of heterostructures, multiheterostructures, and superlattices with breakthrough discoveries and applications. Recently, a novel class of 2D materials, known as 2D organic−inorganic hybrid perovskites (OIHPs), has garnered significant attention due to their extensive bandgap tunability and exceptional environmental stability, positioning them as promising candidates for high-performance optoelectronic semiconductors [1,2,3,4]. These materials demonstrate outstanding optoelectronic properties, including long carrier lifetimes [5], high photoluminescence (PL) quantum yields [6], low ionic mobilities [7], and extended carrier diffusion lengths [8], which make them highly suitable for a variety of optoelectronic applications such as solar cells [9,10], photodetectors [11,12], light-emitting diodes [13,14], and lasers [15,16]. The general formula for 2D OIHPs is (RNH₃)₂A_n−1Pb_nX_3n+1 (n = 1, 2, 3, …), where RNH₃ represents a long-chain alkylammonium cation, such as phenylethylammonium (PEA) or n-butylammonium (BA), A is a monovalent cation (e.g., MA⁺ or FA⁺), X is a halide anion, and n denotes the number of [PbX₆]⁴⁻ octahedral layers within each quantum well [5,17,18,19]. The electronic band gaps of 2D OIHPs can be modulated via selecting the long-chain alkylammonium cation, halide anion, as well as the number of [PbX₆]⁴⁻ octahedral layers, which enable the development of a wide range of materials with distinct optical and electronic characteristics [20,21,22]. Such a rich diversity of chemical compositions and electronic band gaps renders 2D OIHPs a self-sufficient family for exploring exciton physics and high-performance optoelectronic devices, as demonstrated in van der Waals heterostructures of transition metal dichalcogenides (TMDs) [23,24,25]. Therefore, the next challenge for 2D OIHPs is the deterministic fabrication of heterostructures with rationally designed electronic structures, either by vertically stacking different layers or by the formation of lateral heterostructures of different semiconductors within a single 2D OIHPs sheet.

The fabrication of vertical heterostructures of 2D OIHPs has been developed by mechanical stacking, which is similar to the construction of van der Waals heterostructures of conventional inorganic 2D materials such as graphene (Gr) and hexagonal boron nitride (h-BN). Compared to vertical heterostructures, the synthesis of lateral heterostructures of 2D OIHPs has been far less developed and primarily realized by the edge epitaxy method [23,26]. In this method, the pregrown 2D OIHP sheets are immersed in a precursor solution, where subsequent perovskites nucleate along the edges of the initial 2D sheets, forming 2D lateral heterostructures directly on the substrate. However, the geometry of the lateral heterostructures is strictly determined by the lattice symmetry so that only thermodynamically allowed configurations, such as concentric square or rectangular lateral heterostructures, can be achieved [27,28], which are often not practical for real-world applications. Similar issues also existed in the synthesis of TMDs lateral heterostructures. This has been solved by lithographic patterning of SiO₂ mask on the pregrown TMDs sheet, followed by selective conversion of the unmasked area to form lateral heterostructures. For instance, Samani et al. synthesized MoSe₂/MoS₂ lateral heterostructures by patterning MoSe₂ and then converting the unmasked MoSe₂ to MoS₂ by pulsed laser vaporization of sulfur [29]. Although the lithographic patterning is an effective method for the formation of TMDs lateral heterostructures, it is not suitable for 2D OIHPs as they undergo severe damage in lithographic solvents such as acetone and water [30,31,32]. Moreover, it is difficult to remove the deposited SiO₂ mask after the formation of lateral heterostructures, which makes device fabrication quite troublesome. Therefore, it is urgent to develop a lithography-free approach to prepare a mask and ideally, the mask can be removed easily without using any solvents after the formation of lateral heterostructures.

In this work, we present a van der Waals mask-assisted strategy for the deterministic fabrication of 2D OIHPs lateral heterostructures. Mechanically exfoliated graphene flakes are employed as removable masks, enabling selective conversion of exposed perovskite regions through ion exchange reactions. This method facilitates the construction of a variety of lateral heterostructures, including BA₂MA₂Pb₃I₁₀/MAPbI₃, PEAPbI₄/MAPbI₃, as well as BA₂MAPb₂I₇/MAPbBr₃. In addition, complex multiheterostructures and superlattices can be engineered through sequential masking and stepwise conversion processes. As examples, BA₂MAPb₂I₇/MAPbI₃/BA₂MAPb₂I₇ multiheterostructure and BA₂MAPb₂I₇/MAPbI₃/BA₂MAPb₂I₇/MAPbI₃ superlattice were fabricated. This general and flexible strategy offers a robust and versatile platform for the construction of 2D OIHP lateral heterostructures.

2. Results and Discussion

Figure 1 schematically illustrates the van der Waals mask-assisted strategy for fabricating 2D OIHPs lateral heterostructures. Using the BA₂MAPb₂I₇/MAPbI₃ lateral heterostructure as a representative example, a BA₂MAPb₂I₇ nanosheet is synthesized on a Si/SiO₂ substrate first (Figure 1a and Figure 2a). A mechanically exfoliated graphene flake on PDMS is then precisely aligned to cover a selected region of the BA₂MAPb₂I₇ sheet using a custom-designed alignment stage (Figure 1b), after which the PDMS is removed (Figure 1c). The sample is subsequently placed in a CVD system, where MAI is introduced under an inert atmosphere. This initiates a gas–solid phase intercalation reaction that selectively converts the exposed BA₂MAPb₂I₇ region into MAPbI₃ (Figure 1d and Figure 2b). Following the conversion, the graphene mask is mechanically removed (Figure 1e), leaving behind the lateral BA₂MAPb₂I₇/MAPbI₃ heterostructure on the substrate (Figure 1f and Figure 2c). The PL spectra acquired from the BA₂MAPb₂I₇ and MAPbI₃ regions (Figure 2d) show distinct excitonic emission peaks at 576 nm and 758 nm, respectively, in good agreement with the literature results [33]. A PL contour plot along the line scan indicated in Figure 2c (Figure 2e) shows spatially uniform emission across each domain, indicating homogenous material quality. At the interface, two clearly resolved peaks are observed due to simultaneous excitation of both domains by the focused laser spot (Figure 2f). Corresponding PL intensity maps at 576 nm and 758 nm further confirm the sharp domain contrast and uniform distribution of optical properties within the lateral heterostructure. Moreover, we measured the thickness of BA₂MAPb₂I₇ before and after conversion by using an atomic force microscope. Before conversion, the thickness of BA₂MAPb₂I₇ is 74.68 nm. After conversion to MAPbI₃, the thickness is 42.51 nm.

This van der Waals mask-assisted strategy enables the deterministic fabrication of a broad range of lateral heterostructures based on 2D OIHPs. By leveraging the rich compositional diversity of 2D OIHPs, we successfully constructed lateral heterostructures with tunable quantum well thickness (n value), long-chain alkylammonium cations, and halide anions. For example, beyond the BA₂MAPb₂I₇/MAPbI₃ heterostructure shown in Figure 1, where the n value of BA₂MAPb₂I₇ is 2, we also fabricated lateral heterostructures involving higher n values. A representative case is the BA₂MA₂Pb₃I₁₀/MAPbI₃ heterostructure, where the n value of the BA₂MA₂Pb₃I₁₀ is 3 (Figure 3a). Although the optical image of this heterostructure (Figure 3b) shows minimal contrast, the two phases are clearly distinguishable in the PL intensity map (Figure 3c), indicating well defined domain boundaries. In addition to varying the n value, we further explored compositional tuning by modifying the organic spacer cation and halide anion. As shown schematically in Figure 3d,g, we fabricated heterostructures such as PEAPbI₄/MAPbI₃ and BA₂MAPb₂I₇/MAPbBr₃. Their optical images (Figure 3e,h) exhibit pronounced contrast. The PL spectra further confirm the successful formation of distinct domains: in the PEAPbI₄/MAPbI₃ heterostructure, the MAPbI₃ region displays its characteristic emission peak at 758 nm (red), while the PEAPbI₄ region emits strongly at 520 nm (purple) (Figure 3f). Similarly, in the BA₂MAPb₂I₇/MAPbBr₃ heterostructure, MAPbBr₃ exhibits a 527 nm emission (green), contrasting with the 576 nm peak (orange) from BA₂MAPb₂I₇.

The high degree of flexibility offered by the van der Waals mask-assisted strategy enables the deterministic fabrication of complex lateral architectures, including multiheterostructures and superlattices, through sequential masking and conversion steps. As an example, Figure 4a–c illustrates the fabrication process for a BA₂MAPb₂I₇/MAPbI₃/BA₂MAPb₂I₇ multiheterostructure. In this process, two graphene masks are applied to protect the terminal regions of a pre-synthesized BA₂MAPb₂I₇ nanosheet, leaving the central region exposed (Figure 4a). The sample is then placed in a CVD system for the intercalation reaction in the unmasked region, converting it into MAPbI₃ (Figure 4b). After the conversion, the graphene masks are mechanically removed, yielding the desired BA₂MAPb₂I₇/MAPbI₃/BA₂MAPb₂I₇ multiheterostructure (Figure 4c). The optical image (Figure 4d) shows distinct color contrast between the bright yellow BA₂MAPb₂I₇ terminal regions and the central yellow–green MAPbI₃ segment. PL intensity map (Figure 4e) confirms this structure, with characteristic emission at 576 nm (orange) from the BA₂MAPb₂I₇ regions and a 758 nm peak (red) from the MAPbI₃ domain, indicating well defined interfaces and compositional integrity.

This strategy can be extended to fabricate even more intricate structures, such as superlattices. For instance, the BA₂MAPb₂I₇/MAPbI₃/BA₂MAPb₂I₇/MAPbI₃ lateral superlattice is constructed by starting from the previously fabricated BA₂MAPb₂I₇/MAPbI₃/BA₂MAPb₂I₇ multiheterostructure (Figure 5a). A portion of this structure is selectively masked with graphene, exposing a segment of the BA₂MAPb₂I₇ region (Figure 5b). The sample is then subjected to the CVD process again, during which the exposed BA₂MAPb₂I₇ is converted into MAPbI₃ via the same intercalation reaction and then the graphene mask is removed, forming the BA₂MAPb₂I₇/MAPbI₃/BA₂MAPb₂I₇/MAPbI₃ lateral superlattice (Figure 5c). The corresponding optical image and PL map (Figure 5d,e) confirm the periodic spatial arrangement and distinct optical characteristics of each segment within the superlattice.

The fabricated 2D OIHPs lateral heterostructures exhibit tunable and promising optical and electronic properties. Notably, their band alignments can be systematically modulated by altering the quantum well thickness (n value), halide anion, or the choice of long-chain alkylammonium cation [34,35]. To further explore their electronic characteristics and evaluate their potential for device applications, we fabricated a proof-of-concept lateral diode based on a BA₂MA₂Pb₃I₁₀/MAPbI₃ heterostructure (Figure 6a), which features a type-II band alignment [36,37] (Figure 6b). The device demonstrates stable rectifying behavior with a rectification ratio of approximately 10, as shown in Figure 6c, highlighting the potential of these heterostructures for integration into functional optoelectronic devices.

3. Experimental Section

Preparation of 2D RP perovskite Nanosheets: The synthesis of BA₂MAPb₂I₇ (BA₂MAPb₃I₁₀, PEA₂PbI₄) thin sheets began with dissolving 0.45 M of lead iodide (PbI₂), 0.31 M of methylammonium iodide (MAI), and 0.31 M of n-butylamine iodide (BAI) (BA₂MAPb₃I₁₀: 0.59 M of PbI₂, 0.40 M of MAI, and 0.17 M of BAI; PEA₂PbI₄: 0.06 M of PbI₂, and 0.06 M of PEAI) precursors in a concentrated aqueous solution of HI and H₃PO₂ (10:1 vol/vol) The mixture was heated at 130 °C until a clear yellow solution formed, then cooled to 35 °C and stored. To initiate nanosheet growth, 2 µl of warm supernatant was deposited onto a glass slide at room temperature (22 °C), causing rapid formation of perovskite sheets at the solution-air interface. These sheets were carefully lifted with a PDMS stamp, aligned, and transferred onto a Si/SiO₂ substrate using a custom-designed transfer stage.

Heterostructure Fabrication: Graphene nanosheets were obtained via mechanical exfoliation from bulk crystals and transferred onto a PDMS stamp. Subsequently, a transfer stage was used to position a selected graphene nanosheet onto a specific BA₂MAPb₂I₇ (BA₂MA₂Pb₃I₁₀, PEA₂PbI₄) nanosheet on the substrate, achieving partial coverage. The exposed portion of the BA₂MAPb₂I₇ (BA₂MA₂Pb₃I₁₀, PEA₂PbI₄) nanosheet was then converted into MAPbI₃ (MAPbBr₃) via a chemical vapor deposition (CVD) process. The target sample on the substrate and MAI (MABr) powder were placed adjacent within a quartz tube and heated to 160 °C for 13 min (BA₂MA₂Pb₃I₁₀/MAPbI₃: 180 °C for 15 min; PEA₂PbI₄/MAPbI₃: 110 °C for 15 min; BA₂MAPb₂I₇/MAPbBr₃: 140 °C for 15 min). During the conversion, the argon gas flow rate, serving as the carrier gas, was maintained at 40 sccm, and the pressure within the tube was kept at 50 mTorr. Finally, the Gr nanosheet was removed via a peeling-off process to complete the BA₂MAPb₂I₇/MAPbI₃, BA₂MA₂Pb₃I₁₀/MAPbI₃, PEA₂PbI₄/MAPbI_3, and BA₂MAPb₂I₇/MAPbBr₃ Heterostructure.

Photoluminescence Characterization: Photoluminescence experiments were performed in a micro-optical spectrometer (Horiba Scientific). Excitation wavelengths of 375 nm and 532 nm, focused with a 100 × objective (numerical aperture = 0.8). During the photoluminescence mapping, the optical path is stationary while moving the sample on a computer-controlled motorized XY stage.

4. Conclusions

In summary, we have developed a versatile, solvent-free van der Waals mask-assisted strategy for the deterministic fabrication of 2D OIHPs lateral heterostructures, including multiheterostructures and superlattices. This method employs mechanically exfoliated 2D materials, such as graphene, as removable masks to enable site-selective conversion of pre-synthesized 2D OIHPs without introducing chemical damage to the fragile perovskite framework. The approach offers precise spatial control over both the composition and band structure of the resulting heterostructures. We demonstrated its generality by successfully fabricating a variety of lateral heterostructures with tunable quantum well thicknesses (n values), halide anions, and long-chain organic spacers. To further explore their functional potential, we fabricated a proof-of-concept lateral diode based on a BA₂MA₂Pb₃I₁₀/MAPbI₃ lateral heterostructure, which exhibited a stable rectifying behavior with a rectification ratio of approximately 10. This work opens new avenues for exploring excitonic physics, charge transport, and band engineering in 2D perovskite systems, offering a generalizable platform for the design and integration of functional heterostructures in advanced photonic and electronic devices.