Bright excitons in 2D materials, with their transition dipole moments (TDMs) parallel to the surface plane, are crucial for high-efficiency quantum optics and electroluminescent devices. Creating decoupled multi-quantum-well (MQW) superlattices with atomically thin quantum barriers (QBs) between 2D layers is challenging. Current methods, including layer-by-layer assembly, van der Waals epitaxy, intercalation, and colloidal chemistry, haven't yielded scalable, high-order MQW superlattices. A key motivation is to achieve strong spontaneous emission without the detrimental effects of multiexciton quenching mechanisms like the Auger process. Inserting atomically thin QBs to screen interlayer coupling is a promising approach. Interlayer coupling leads to charge-transfer (CT) and dark excitons, hindering radiative recombination. Little is known about the QB requirements for complete decoupling, particularly the relationship between QB and 2D material thicknesses (dQB and d2D). Previous studies on CdSe nanoplatelets and WSe2/MoS2 heterostructures show that even thick QBs may not fully suppress interlayer coupling. This research aims to elucidate the principles of decoupling stacked 2D layers and develop spectroscopic techniques to characterize interlayer coupling. The dipole orientation of bright excitons is predominantly in-plane (IP) in decoupled 2D materials, while interlayer coupling breaks symmetry and induces out-of-plane (OP) components, reducing the IP dipole ratio (RIP). High-order superstructures of decoupled 2D materials have not been previously demonstrated. This study demonstrates that colloidal quantum wells (CQWs) of lead halide perovskites can form fully decoupled MQW superlattices with ultrathin organic QBs. The TDM orientation of bright excitons remains predominantly IP and independent of stacking layer, as confirmed by crystallographic and 2D k-space spectroscopic analysis. The strong ionic dielectric response is believed to screen interlayer electrostatic interactions, leading to the observed exciton localization. The preferential TDM orientation is maintained in mixed-halide superlattices, covering the visible spectrum.

The introduction extensively reviews existing literature on the challenges and limitations of creating high-order MQW superlattices from 2D materials. It cites several studies on various approaches like layer-by-layer assembly and van der Waals epitaxy, highlighting their limitations in achieving scalability and complete interlayer decoupling. The review also discusses the importance of understanding the relationship between quantum barrier thickness and the thickness of the 2D material layers in achieving effective decoupling. Specific examples of previous work on CdSe nanoplatelets and WSe2/MoS2 heterostructures are used to illustrate the complexities of this problem. The literature review sets the stage for the current work by emphasizing the lack of scalable high-order superstructures with fully decoupled layers, positioning the research as a significant advancement in the field.

The study employed a multi-faceted methodology. First, lead halide perovskite MQW superlattices were fabricated using CQWs synthesized in solution with the formula (RNH3)2[CH3NH3PbBr3]nPbBr4, where R is an alkyl group and n is the number of perovskite unit cells. The organic ligands acted as QBs. The QB thickness (dQB) was controlled by varying the length of R. The lateral size of the CQWs exceeded the Bohr radius, ensuring electronic properties were controlled by quantum confinement along the c-axis. Superlattice films were deposited on glass substrates with the c-axis perpendicular to the plane. Film thickness and refractive index were determined by ellipsometry. Synchrotron grazing-incidence wide-angle X-ray (GIWAXS) diffraction confirmed the superlattice structure. The TDM orientation was determined using polarization and angle-dependent PL spectroscopy. The experimental radiation patterns were fitted using a dipole emission model for optical microcavities, with RIP as the fitting parameter. This method is adapted from organic light-emitting devices research. Temperature-dependent PL spectroscopy investigated interlayer coupling mechanisms. The absolute PL quantum yield (ηPL) was characterized using an integrating sphere. Density functional theory (DFT) calculations were performed to investigate the electronic and optical properties of bulk and 2D CH3NH3PbBr3. The calculations included van der Waals interactions and the Bethe-Salpeter equation (GoWo-BSE) to simulate optical absorption. Anion exchange (AE) reactions were used to tune the emission wavelength. The characterization techniques included STEM, GIWAXS, SE, momentum-resolved PL, TRPL, steady-state PL, and DFT calculations. Specific details on the synthesis protocols for CQWs, alkylammonium bromides and AE reactions are provided, along with descriptions of GIWAXS, SE, momentum-resolved PL, TRPL and steady-state PL measurements, and DFT calculation methods.

The key findings demonstrate the successful fabrication of fully decoupled MQW superlattices using lead halide perovskites with ultrathin organic QBs (6.5 Å). The transition dipole moment orientation of bright excitons is predominantly in-plane (RIP = 0.81–0.85), independent of the number of stacking layers (N = 1–19) and QB thickness. This indicates effective interlayer decoupling. High absolute PL quantum yields (up to 0.85) were achieved, exceeding values reported for other 2D material superstructures. Low-temperature PL measurements revealed an additional emission peak (I exciton) at low temperatures, attributed to interlayer coupling that is not present at room temperature. The analysis suggests that the high ionic dielectric response at infrared frequencies effectively screens the electrostatic interactions between layers, localizing the excitons. DFT calculations support the experimental findings by showing that the VBM and CBM for 2D perovskites have predominantly in-plane contributions. The emission wavelength of the superlattices could be continuously tuned from blue to orange using anion exchange while maintaining a high RIP. These results demonstrate scalable miniaturized 2D material superlattices with narrowband emission, high quantum yield, enhanced light outcoupling, and wavelength tunability.

The findings directly address the research question by demonstrating the feasibility of creating scalable, high-order MQW superlattices from 2D perovskites with complete interlayer decoupling. The ultrathin QBs and the strong ionic dielectric screening effectively prevent interlayer coupling even for relatively thick 2D layers. The high PL quantum yield and predominantly in-plane TDM orientation make these superlattices promising for applications in quantum optics and electroluminescent devices. The tunability of the emission wavelength adds further versatility. This work contradicts previous assumptions about the required QB thickness for decoupling and provides valuable insights into the interplay between material properties, quantum confinement, and exciton localization. The discovery of this new materials platform opens up new avenues for creating efficient, scalable photonic sources.

This study successfully demonstrates the fabrication of scalable, high-quality MQW superlattices from 2D lead halide perovskites using ultrathin organic QBs. The superlattices exhibit high PL quantum yields, predominantly in-plane TDM orientation, and tunable emission wavelengths. These findings offer a significant advance in the field of 2D materials-based photonic sources and pave the way for various applications, such as light-emitting diodes and nanoantennas. Future research could focus on exploring other 2D material systems, optimizing the QB design, and integrating these superlattices into functional devices.

The study focuses on a specific type of 2D perovskite material. The generalizability of the findings to other 2D materials requires further investigation. While the anion exchange method allows for wavelength tunability, the long-term stability of the mixed-anion superlattices needs further evaluation. The detailed mechanism behind the observed low-temperature interlayer coupling requires additional experimental and theoretical analysis.