Atomic-scale investigation of carbon-based materials by gentle transmission electron microscopy
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摘要: 近年来,石墨烯、金属有机框架(MOFs)、聚合物、生物大分子等碳基材料在物理、化学、材料、生物学等领域受到了持续增长的关注,但由于其微观结构在电子束辐照下的不稳定性导致原子分辨率的实验观察仍面临巨大的挑战。原子结构的认识不足严重限制了对此类材料的深入理解及潜在应用研究。近年来,低加速电压、低电子剂量与低温电子显微学(TEM)亚埃分辨率的革命性突破,极大地促进了对电子束敏感材料结构与化学组成的原子尺度解析。特别是对轻元素原子成像的能力有助于深入研究新型碳基材料的结构和性能。本文总结了先进电子显微学中各种成像与谱学技术的新进展,及其在石墨烯基材料、MOFs、聚合物、生物大分子等碳基材料研究中的应用,并探讨了当前材料研究面临的挑战与电子显微学的发展趋势,以期助力对新型碳基材料构效关系的深入理解和设计开发。Abstract: Although carbon-based materials, such as graphene, metal-organic frameworks (MOFs), polymers and biomolecules, have aroused increasing scientific interest in the fields of physics, chemistry, materials science and molecular biology, their atomic-scale observation is still a challenge due to their structural instability under the electron beam. Ambiguous atomic arrangements have critically limited the fundamental understanding on these materials and their potential applications in electronics, mechanics, thermodynamics, catalysis, bioscience and medicine. Very recently, revolutionary sub-Ångström resolution achievements of transmission electron microscopy (TEM) using a low voltage, a low electron dose, or a cryogenic environment have greatly facilitated the atomic-scale structural and chemical examination of electron beam sensitive materials. In particular, the ability to image light elements atom by atom gives unprecedented insight into the structures and properties of novel carbon-based materials. In this review, the recent developments in advanced TEM combined with various imaging and spectroscopy techniques, and their use in examining graphene-based materials, MOFs, polymers, and biomacromolecules are summarized and discussed. The current challenges in materials research and trends for the future design of TEM equipment are outlined, which is expected to provide a deeper understanding of structure–performance relationships and the discovery of new carbon materials.
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Figure 5. The structure of graphene: (a) Honeycomb structure of graphene, with a hexagonal unit cell and a bond length of 0.142 nm. Graphene has 2 types of common edges: the zigzag edge and armchair edge, (b) Band structure of graphene[46].
Figure 6. Atomic structures of defects in graphene and alike 2D materials. (a) HRTEM image of SW defect in graphene (Scale bar is 0.2 nm)[3]. (b-c) STEM-ADF image of 3 oxygen atoms in graphene multivacancies (Scale bar is 0.2 nm)[10]. (d-f) STEM-ADF image of a nitrogen anti-site NB in a suspended h-BN monolayer, and its edge reconstruction[11] (Red and green dots correspond to B and N atom). (g-i) STEM-ADF image and ELNES for threefold and fourfold coordinated Si impurities in monolayer graphene (Scale bar is 0.2 nm)[51].
Figure 7. STEM characterization of carbon-supported SACs. (a) ADF image of Pt/graphene catalyst for methanol oxidation[55]. (b) HAADF image of single Ni doped graphene for HER[56]. (c) ADF image of Nb atoms anchored in onion-like carbon shells used for ORR catalysis[9]. (d) ADF image of dispersed single W atoms in high-defective graphene layers of carbon onions for ORR catalysis[59]. (f) ADF image Pt atoms in carbon onions, which is catalytically active for chemoselective hydrogenation of nitroarenes[60].
Figure 8. Dynamic structure evolutions of graphene-based materials. (a-c) STEM-ADF images show dynamics of a Ni atom at t = 1 s, 2 s, and 3 s in a graphene nanomesh, respectively[65].
Figure 9. HRTEM image series demonstrate structure evolutions of (a-c) graphitic shells and (d-f) aggregation of Ru nanoparticles in Ru@GNs catalyst during in situ heating[68].
Figure 10. (a, b) ADF STEM images of a wood-based nanoporous carbon with large areas of hexagonal lattice (marked in blue) and a few five- and seven-atom ring defects (marked in red). (c) Segment of a simulated defective graphene sheet, with five to seven dislocation structures arranged similarly to microscopy observations.
Figure 11. Characterization of MOFs with low dose electron microscopy. (a, b) iDPC-STEM image of MIL-101 (111) surface with different termination cages (Scale bars: 5 nm). (c, d) The structures of single unit cells at the two types of surface terminations (Scale bars: 3 nm). (e, f) The structural models show the (111) surfaces terminated by different cages[20]. (g) HRTEM image of sublayer (
$ \bar{1}11 $ ) surface of MIL-101 (Scale bar: 5 nm). (h) Magnified image showing the evolution from sublayer to stable (111) surface (Scale bar: 2 nm).[19](i) Cryo-TEM image of CO2-filled ZIF-8 particle along the <111> projection, with Zn clusters and adsorbed CO2 molecules being identified.[74] In the referred work above, total electron dose for iDPC-STEM is 40 e-Å–2, and electron dose for low dose TEM is usually less than 10.4 e-Å–2 accumulated from tens of dose fractionation frames.Figure 13. Atomic architecture of the authentic SARS-CoV-2 virus, which is responsible for COVID-19 pandemic, resolved by cryo-TEM[80].
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