Chirality transitions and transport properties of individual few-walled carbon nanotubes as revealed by in situ TEM probing

Growth Mechanism of Carbon Nanotubes Revealed by in situ Transmission Electron Microscopy

Elucidating the growth mechanism of carbon nanotubes (CNTs) is critical to obtaining CNTs with desired structures and tailored properties for their practical applications. With atomic resolution imaging, in situ transmission electron microscopy (TEM) has been a key technique to reveal the microstructure and dynamics of CNTs in real time. In this review, recent advances in the development of in situ TEM with different types of environmental reactors will be introduced. The catalytic growth mechanisms of CNTs revealed by in situ TEM under realistic conditions are discussed from fundamental thermodynamics and kinetics to the detailed nucleation, growth, and termination mechanisms, including the state and phase of active catalysts, interfacial connections between catalyst and growing CNTs, and catalyst-related growth kinetics of CNTs. Great progresses have been made on how a CNT nucleates, grows and terminates, focusing on the interface dynamics and kinetic fluctuations. Finally, challenges and future directions for understanding the atomic dynamics under the real growth conditions are proposed. It is expected that breakthroughs in the fundamental growth mechanisms will pave the way to the ultimate goal of designing and controlling the atomic structures of CNTs for their applications in various devices.

Breaking the Axis-Symmetry of a Single-Wall Carbon Nanotube During Its Growth

The asymmetrical growth of a single-wall carbon nanotube (SWCNT) by introducing a change of a local atomic structure, is usually inevitable and supposed to have a profound effect on the chirality control and property tailor. However, the breaking of the symmetry during SWCNT growth remains unexplored and its origins at the atomic-scale are elusive. Here, environmental transmission electron microscopy is used to capture the process of breaking the symmetry of a growing SWCNT from a sub-2-nm platinum catalyst nanoparticle in real-time, demonstrating that topological defects formed on the side of a SWCNT can serve as a buffer for stress release and inherently break its axis-symmetrical growth. Atomic-level details reveal the importance of the tube-catalyst interface and how the atom rearrangement of the solid-state platinum catalyst around the interface influences the final tubular structure. The active sites responsible for trapping carbon dimers and providing enough driving force for carbon incorporation and asymmetric growth are shown to be low-coordination step edges, as confirmed by theoretical simulations.

Semiconductor nanochannels in metallic carbon nanotubes by thermomechanical chirality alteration

[Figure: see text].

Developments and advances in in situ transmission electron microscopy for catalysis research

Recent developments and advances in in situ TEM have raised the possibility to study every step during the catalysts' lifecycle. This review discusses the current state, opportunities and challenges of in situ TEM in the realm of catalysis.

Growth and Termination Dynamics of Multiwalled Carbon Nanotubes at Near Ambient Pressure: An in Situ Transmission Electron Microscopy Study

Understanding the growth mechanism of carbon nanotubes (CNTs) has been long pursued since its discovery. With recent integration of in situ techniques into the study of CNT growth, important insights about the growth mechanism of CNT have been generated, which have improved our understanding significantly. However, previous in situ experiments were mainly conducted at low pressures which were far from the practical conditions. Direct information about the growth dynamics under relevant conditions is still absent and thus is highly desirable. In this work, we report atomic-scale observations of multiwalled CNT (MWCNT) growth and termination at near ambient pressure by in situ transmission electron microscopy. On the basis of the real-time imaging, we are able to reveal that the working catalyst is constantly reshaping at its apex during catalyzing CNT growth, whereas at the base the catalyst remains faceted and barely shows any morphological change. The active catalyst is identified as crystalline Fe₃C, based on lattice fringes that can be imaged during growth. However, the oscillatory growth behavior of the CNT and the structural dynamics of the apex area strongly indicate that the carbon concentration in the catalyst particle is fluctuating during the course of CNT growth. Extended observations further reveal that the catalyst splitting can occur: whereas the majority of the catalyst stays at the base and continues catalyzing CNT growth, a small portion of it gets trapped inside of the growing nanotube. The catalyst splitting can take place multiple times, leading to shrinkage of both, catalyst size and diameter of CNT, and finally the growth termination of CNT due to the full coverage of the catalyst by carbon layers. Additionally, in situ observations show two more scenarios for the growth termination, that is, out-migration of the catalyst from the growing nanotube induced by (i) Oswald ripening and (ii) weakened adhesion strength between the catalyst and CNT.