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Wafer-scale monolayer two-dimensional (2D) materials have been realized by epitaxial chemical vapor deposition (CVD) in recent years. To scale up the synthesis of 2D materials, a systematic analysis of how the growth dynamics depend on the growth parameters is essential to unravel its mechanisms. However, the studies of CVD-grown 2D materials mostly adopted the control variate method and considered each parameter as an independent variable, which is not comprehensive for 2D materials growth optimization. Herein, we synthesized a representative 2D material, monolayer hexagonal boron nitride (hBN), on single-crystalline Cu (111) by epitaxial chemical vapor deposition and varied the growth parameters to regulate the hBN domain sizes. Furthermore, we explored the correlation between two growth parameters and provided the growth windows for large flake sizes by the Gaussian process. This new analysis approach based on machine learning provides a more comprehensive understanding of the growth mechanism for 2D materials. © 2023 American Chemical Society.

© 2020 American Physical Society. We report three-pulse photodope-pump-probe measurements on photocarrier dynamics in semiconducting transition metal dichalcogenide monolayers of MoS2, WS2, MoSe2, and WSe2. The samples are fabricated by metal-organic chemical vapor deposition and mechanical exfoliation techniques and characterized by photoluminescence spectroscopy. In the time-resolved measurement, the samples are first photodoped by a prepulse, which injects background photocarriers of various densities. A pump pulse then injects photocarriers, whose dynamics is monitored by measuring a differential reflection of a time-delayed probe pulse. We found that the ultrafast decay component of the differential reflection signal, which has been widely reported before, shows minimal dependence on the background exciton density. This observation shows that a previously suggested carrier-trapping model cannot account for this component. The results thus further support an exciton-formation model that was previously proposed based on spectroscopic evidence.

In this study, monolayer hexagonal boron nitride (h-BN) grown via chemical vapor deposition (CVD) as an effective electron blocking layer (EBL) for the organic photovoltaics (OPVs) is proposed. Unexpectedly, it is found that h-BN can replace the commonly used hole transport layers (HTLs), i.e., molybdenum trioxide (MoO3) and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) in an inverted device architecture. Here, a wet-transfer technique is employed and a single layer of h-BN on top of the PV2000:PC60BM blend is successfully placed. Analysis of the bandgap diagram shows that the monolayer h-BN makes smaller barrier for holes but significantly larger barrier for electrons. This makes the h-BN effective in blocking electrons while creating a possible path for the holes through tunneling to the electrode, due to the low energy barrier at the PV2000/h-BN interface. Using h-BN as an EBL, efficient inverted OPVs are achieved with an average solar-to-power conversion efficiency of 6.13%, which is comparable with that of reference devices based on MoO3 (7.3%) and PEDOT:PSS (7.6%) as HTLs. Interestingly, the devices with h-BN shows great light-soak stability. The study reveals that the monolayer h-BN grown by CVD could be an effective alternative EBL for the fabrication of efficient, lightweight, and stable OPVs.

In the last few years, two-dimensional (2D) metal halide perovskites (MHPs) are gaining tremendous attention and are replacing their three-dimensional (3D) congeners rapidly. These next-generation halide perovskites can circumvent the limitations of the 3D MHPs such as stability and structural diversity. The incorporation of bulky organic cation can not only prevent the moisture penetration into the crystal lattice and thus providing greater stability but also expands the field of hybrid semiconducting materials by offering structural diversity. These unique features render even higher tuneability and improved photophysical properties and as a result intensive investigations have been made for 2D MHP–based polycrystalline thin films. However, single crystals based on two-dimensional halide perovskites are still emerging and have great potential for future device applications. Along with the hybrid halide perovskites, the all inorganic halide perovskites are also blossoming. In this review, we have discussed exclusively the development of hybrid as well as all inorganic two-dimensional halide perovskites. First, we have discussed the crystal structure of 2D perovskites. In the next section, different growth procedures reported for preparation of single crystals are discussed. We then highlight the effect of doping on single crystals and their optoelectronic properties. Finally, we discuss the current challenges and future perspectives to further develop 2D single crystals for their efficient use in various devices.

© 2019 American Physical Society. We report an experimental study on charge transfer properties of mixed-dimensional heterostructures formed by zero-dimensional PbS quantum dots and two-dimensional transition metal dichalcogenides. Monolayers of MoSe2 and MoS2 were fabricated by mechanical exfoliation and chemical vapor deposition techniques, respectively. PbS quantum dots with diameters of 2.3 and 5 nm were synthesized by a hot-injection method and characterized by optical absorption spectroscopy and ultraviolet photoemission spectroscopy. The quantum dots were deposited on the MoS2 and MoSe2 monolayers to form heterostructures. Photoluminescence and transient absorption measurements were performed on the heterostructures as well as individual materials to reveal their photocarrier dynamics. We found that the holes excited in MoSe2 can efficiently transfer to both 2.3-and 5-nm PbS quantum dots, while electrons in these quantum dots cannot transfer to MoSe2. Similar charge transfer properties were observed between MoS2 and the 5-nm PbS quantum dots, while no charge transfer was observed between MoS2 and the 2.3-nm quantum dots. These results provide useful information for understanding the physical mechanism of charge transfer in mixed-dimensional heterostructures and for developing PbS quantum-dot-based mixed-dimensional materials.

Here, highly efficient and stable monolithic (2-terminal (2T)) perovskite/PbS quantum dots (QDs) tandem solar cells are reported, where the perovskite solar cell (PSC) acts as the front cell and the PbS QDs device with a narrow bandgap acts as the back cell. Specifically, ZnO nanowires (NWs) passivated by SnO2 are employed as an electron transporting layer for PSC front cell, leading to a single cell PSC with maximum power conversion efficiency (PCE) of 22.15%, which is the most efficient NWs-based PSCs in the literature. By surface passivation of PbS QDs by CdCl2, QD devices with an improved open-circuit voltage and a PCE of 8.46% (bandgap of QDs: 0.92 eV) are achieved. After proper optimization, 2T and 4T tandem devices with stabilized PCEs of 17.1% and 21.1% are achieved, respectively, where the 2T tandem device shows the highest efficiency reported in the literature for this design. Interestingly, the 2T tandem cell shows excellent operational stability over 500 h under continuous illumination with only 6% PCE loss. More importantly, this device without any packaging depicts impressive ambient stability (almost no change) after 70 days in an environment with controlled 65% relative humidity, thanks to the superior air stability of the PbS QDs.