(587p) Size-Dependent Second-Order-like Phase Transitions in Fe Nanocluster Melting from Low-Temperature Structural Isomerization

Metal nanoparticles (NPs) under 100 nm have interesting properties desirable for catalytic applications like the growth of carbon nanotubes (CNTs) with floating catalyst chemical vapor deposition (FCCVD) [1]. FCCVD can be enhanced with plasma by using spark discharge to generate nanoparticles in the vapor phase with narrow size distributions [2]. While the mechanism of CNT production depends on many factors, it is thought that carbon precursors interact with an NP with a disordered surface and ordered core: decomposing on, diffusing on, and dissolving inside the NP—eventually forming graphitic carbon that grows the CNT or encapsulates the NP, thereby terminating CNT growth [3]. These factors depend on the NPs melting characteristics, which are strongly size-dependent for clusters under 2 nm relevant to single-wall CNT growth [4]. Sulfur is often employed in FCCVD as a CNT growth promoter and is proposed to influence NP melting properties [5].

Here, classical molecular dynamics (MD) simulations are used to elucidate size-dependent Fe nanocluster melting alone and in the presence of FCCVD species like carbon and sulfur. Classical MD with Embedded Atom Method Finnis-Sinclair interatomic potentials are used to simulate NP melting thermodynamics and mechanisms for NPs up to 100 atoms in size. Parallel tempering simulations are employed to find global minimum energy configurations and energetic melting points (T_Cv) from cluster heat capacity curves. Core (T_core) and surface (T_surface) melting points are calculated from NVT simulations of cluster melting from minimum energy configurations.

Single-shell cluster sizes with closed-shell structures have high T_surface, while cluster sizes with near-closed structures have lower T_surface by hundreds of Kelvin. Small double-shell cluster sizes have high densities of low-energy configurations, showing mostly second-order-like melting behavior. Large double-shell cluster sizes exhibit premelting (T_surface << T_core) due to the presence of an ordered core structure with a less-ordered surface.

Size-dependent melting properties in Fe nanoparticles appear to have predictable trends in optimal structural configurations, second-order-like melting, and behavior of surface and core melting, within certain size regimes. These size-dependent properties may complicate the synthesis of single-walled CNTs. The influence of carbon and sulfur on Fe cluster melting properties is under investigation.

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[2] Malekzadeh, Swihart., Chem. Soc. Rev., 50.12 (2021): 7132-7249.

[3] Puretzky et al., Appl. Phys. A, 81 (2005): 223-240.

[4] Gaston, Adv. Phys. X, 3.1 (2018): 1401487.

[5] Bogdanova et al., Carbon, (2023): 118051.