Browsing by Author "Liu, Yifeng"

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    Compositional and Heterostructure Engineering in Low Dimensional Metal Halide Perovskites
    (2024-01-22) Liu, Yifeng; Lou, Jun
    Metal halide perovskites (MHPs) have become the preeminent semiconducting materials serving for photovoltaics, light-emitting diodes, photodetectors, lasers and photocatalysis, attributed to their superior optoelectronic properties. Low-dimensional MHPs with unique properties originating from quantum confinement and dimension dependence have garnered considerable attention in photonic and electronic studies. To enrich the function versatility and performance optimization to address the practical challenges, chemical engineering strategies have been applied in MHPs including compositional modification, heterostructure fabrication, and composite synthesis. This thesis will center on the compositional and heterostructure engineering in low-dimensional MHPs, specifically lead-free bismuth halide perovskites, for the unprecedented MHP research on photonics, spin physics and energy conversion. First, chemical vapor deposition and anion exchange protocols to synthesize bismuth halide perovskite nanoflakes with controlled dimensions and variable compositions are established. In particular, the gradient bromide distribution by controlling the anion exchange and diffusion is spatially resolved by the time-of-flight secondary ion mass spectrometry. Moreover, the optical waveguiding properties of bismuth halide perovskites can be modulated by the flake thickness and anion composition. Next, a novel metal ion doping protocol through the vapor phase metal halide insertion reaction to the CVD-grown ultrathin Cs3BiBr6 perovskites is presented. The Fe-doped Cs3BiBr6 (Fe: Cs3BiBr6) II perovskites demonstrate that the iron spins are successfully incorporated into the lattice, as revealed by the spin-phonon coupling below the critical temperature Tc around 50 K observed through the temperature-dependent Raman spectroscopy. Furthermore, the phonons exhibit significant softening under the applied magnetic field, possibly originating from magnetostriction and spin exchange interaction. In addition to the compositional engineering in ultrathin bismuth halide perovskite crystals, the direct CVD growth of the Cs3Bi2I9 based heterostructures is introduced. Cs3Bi2I9-MoSe2 and Cs3Bi2I9-MoS2 heterostructures are synthesized to investigate the strong interlayer couplings within the heterostructures. Cs3Bi2I9-graphene heterostructures are developed for future applications in X-ray sensing transistors. Finally, the synthesis of lead-free bismuth halide perovskite nanocrystals encapsulated by the covalent organic frameworks (PNCs- COFs) via an in-situ growth approach is reported. The PNCs-COFs were further applied as photocatalysts for free-radical polymerizations, photo-activating different co-initiators via both hole and electron transfer mechanisms in aqueous and organic phases. The excellent photocatalytic performance of PNCs-COFs was confirmed by high monomer conversion (up to 97.5%), diverse functional group tolerance and recyclability. These protocols provide facile, universal, well-adaptable and efficient avenues for exploring the versatilities of Bi-based perovskite materials in low dimensions. The endeavors devote to shedding light on the potential of low-dimensional MHPs (bismuth halide perovskites) as promising semiconductors for optoelectronics, spin physics and solar energy harvesting.
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    High-performance 2D electronic devices enabled by strong and tough two-dimensional polymer with ultra-low dielectric constant
    (Springer Nature, 2024) Fang, Qiyi; Yi, Kongyang; Zhai, Tianshu; Luo, Shisong; Lin, Chen-yang; Ai, Qing; Zhu, Yifan; Zhang, Boyu; Alvarez, Gustavo A.; Shao, Yanjie; Zhou, Haolei; Gao, Guanhui; Liu, Yifeng; Xu, Rui; Zhang, Xiang; Wang, Yuzhe; Tian, Xiaoyin; Zhang, Honghu; Han, Yimo; Zhu, Hanyu; Zhao, Yuji; Tian, Zhiting; Zhong, Yu; Liu, Zheng; Lou, Jun; Rice Advanced Materials Institute
    As the feature size of microelectronic circuits is scaling down to nanometer order, the increasing interconnect crosstalk, resistance-capacitance (RC) delay and power consumption can limit the chip performance and reliability. To address these challenges, new low-k dielectric (k < 2) materials need to be developed to replace current silicon dioxide (k = 3.9) or SiCOH, etc. However, existing low-k dielectric materials, such as organosilicate glass or polymeric dielectrics, suffer from poor thermal and mechanical properties. Two-dimensional polymers (2DPs) are considered promising low-k dielectric materials because of their good thermal and mechanical properties, high porosity and designability. Here, we report a chemical-vapor-deposition (CVD) method for growing fluoride rich 2DP-F films on arbitrary substrates. We show that the grown 2DP-F thin films exhibit ultra-low dielectric constant (in plane k = 1.85 and out-of-plane k = 1.82) and remarkable mechanical properties (Young’s modulus > 15 GPa). We also demonstrated the improved performance of monolayer MoS2 field-effect-transistors when utilizing 2DP-F thin films as dielectric substrates.
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    Metal Oxide Catalysts for the Synthesis of Covalent Organic Frameworks and One-Step Preparation of Covalent Organic Framework-Based Composites
    (American Chemical Society, 2021) Zhu, Yifan; Zhu, Dongyang; Yan, Qianqian; Gao, Guanhui; Xu, Jianan; Liu, Yifeng; Alahakoon, Sampath B.; Rahman, Muhammad M.; Ajayan, Pulickel M.; Egap, Eilaf; Verduzco, Rafael; Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment
    The integration of covalent organic frameworks (COFs) with inorganic materials provides opportunities to develop a new class of composite materials with high surface areas and novel functionalities relevant to photocatalysis, chemical adsorption, and magnetic resonance imaging. However, current methods for the preparation of COF-based composites require challenging, multistep synthetic protocols. Herein, we report a one-pot synthesis approach using a wide range of metal oxides to catalyze the synthesis of highly crystalline and porous COFs. We found that a large variety of metal oxides served as effective catalysts for the synthesis of imine COFs, including niobium(V) oxide (Nb2O5), nickel(II) oxide (NiO), manganese(IV) dioxide (MnO2), ruthenium(IV) oxide (RuO2), zinc(II) oxide (ZnO), lead(II) oxide (PbO), tellurium(IV) dioxide (TeO2), tin(IV) oxide (SnO2), manganese(III) oxide (Mn2O3), zirconium(IV) dioxide (ZrO2), and aluminum(III) oxide (Al2O3). Nb2O5 was effective for the synthesis of a wide range of COFs with different functional groups and pore sizes, and these reactions produced a metal oxide/COF composite. By using Fe3O4 nanoparticles (NPs) as the catalyst, we produced COF-based nanocomposites with Fe3O4 NPs distributed throughout the final COF product. The Fe3O4/COF nanocomposite had a high surface area of 2196 m2 g–1. This work demonstrates a class of novel, low-cost catalysts for synthesizing COFs and a new approach to produce metal oxide/COF composite materials.
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    Porphyrin-based donor–acceptor COFs as efficient and reusable photocatalysts for PET-RAFT polymerization under broad spectrum excitation
    (Royal Society of Chemistry, 2021) Zhu, Yifan; Zhu, Dongyang; Chen, Yu; Yan, Qianqian; Liu, Chun-Yen; Ling, Kexin; Liu, Yifeng; Lee, Dongjoo; Wu, Xiaowei; Senftle, Thomas P.; Verduzco, Rafael
    Covalent organic frameworks (COFs) are crystalline and porous organic materials attractive for photocatalysis applications due to their structural versatility and tunable optical and electronic properties. The use of photocatalysts (PCs) for polymerizations enables the preparation of well-defined polymeric materials under mild reaction conditions. Herein, we report two porphyrin-based donor–acceptor COFs that are effective heterogeneous PCs for photoinduced electron transfer-reversible addition–fragmentation chain transfer (PET-RAFT). Using density functional theory (DFT) calculations, we designed porphyrin COFs with strong donor–acceptor characteristics and delocalized conduction bands. The COFs were effective PCs for PET-RAFT, successfully polymerizing a variety of monomers in both organic and aqueous media using visible light (λmax from 460 to 635 nm) to produce polymers with tunable molecular weights (MWs), low molecular weight dispersity, and good chain-end fidelity. The heterogeneous COF PCs could also be reused for PET-RAFT polymerization at least 5 times without losing photocatalytic performance. This work demonstrates porphyrin-based COFs that are effective catalysts for photo-RDRP and establishes design principles for the development of highly active COF PCs for a variety of applications.
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    Superior mechanical properties of multilayer covalent-organic frameworks enabled by rationally tuning molecular interlayer interactions
    (PNAS, 2023) Fang, Qiyi; Pang, Zhengqian; Ai, Qing; Liu, Yifeng; Zhai, Tianshu; Steinbach, Doug; Gao, Guanhui; Zhu, Yifan; Li, Teng; Lou, Jun
    Two-dimensional (2D) covalent-organic frameworks (COFs) with a well-defined and tunable periodic porous skeleton are emerging candidates for lightweight and strong 2D polymeric materials. It remains challenging, however, to retain the superior mechanical properties of monolayer COFs in a multilayer stack. Here, we successfully demonstrated a precise layer control in synthesizing atomically thin COFs, enabling a systematic study of layer-dependent mechanical properties of 2D COFs with two different interlayer interactions. It was shown that the methoxy groups in COFTAPB-DMTP provided enhanced interlayer interactions, leading to layer-independent mechanical properties. In sharp contrast, mechanical properties of COFTAPB-PDA decreased significantly as the layer number increased. We attributed these results to higher energy barriers against interlayer sliding due to the presence of interlayer hydrogen bonds and possible mechanical interlocking in COFTAPB-DMTP, as revealed by density functional theory calculations.