Understanding nanoparticle (NP) growth mechanisms is crucial for comprehending mineral formation and natural environment evolution, as well as for controlling the size, morphology, and properties of synthetic nanomaterials. Classical crystal growth models emphasizing surface reactions and monomer diffusion are being challenged by evidence supporting nonclassical growth, including particle-based aggregation. This aggregation encompasses various mechanisms like oriented attachment (OA), near-OA, and non-OA processes, each requiring detailed investigation due to the complexity of atomic diffusion, particle movement, interfacial interactions, and grain boundary evolution. Five-fold twin (5-FT) structures, exhibiting unique properties due to their pentagonal symmetry and inherent lattice strain, offer a promising approach to tune nanocrystal properties. However, understanding their nonclassical growth, particularly given their small thermodynamically stable size and complex twin configurations, remains challenging. This study utilizes in situ HRTEM and MD simulations to elucidate the atomic-scale mechanisms governing size-dependent and twin configuration-related aggregation phenomena between 5-FT and other nanoparticles.

Extensive research has explored 5-FT structures in various natural and synthetic systems, highlighting their enhanced mechanical, catalytic, and optical properties. The twin boundaries and lattice strain in 5-FT structures offer an avenue for tailoring nanocrystal configurations and properties. However, the atomic-scale formation and growth mechanisms of 5-FT structures remain experimentally challenging due to their small size (3–14 nm), complex twin structures, and NP movement. Previous work using electron-beam-induced decomposition of organic ligands revealed atomic formation mechanisms in Au, Pd, and Pt nanomaterials, but nonclassical growth mechanisms, especially under coupled thermodynamic and kinetic effects, remain elusive. This includes understanding the evolution of the complex twin structure (twinning and de-twinning), atom surface diffusion, and the relative slip and configuration modulation of NPs.

This research employed in situ high-resolution transmission electron microscopy (HRTEM) combined with molecular dynamics (MD) simulations to study the atomic aggregation growth and evolution mechanisms involving 5-FT and diverse NPs (varying size ratios and twinned configurations: single crystal (SC) and 5-FT). The impact of thermodynamic and kinetic landscapes on aggregation evolution was systematically investigated. Gold nanoparticles (Au NPs) were synthesized using a TDAB method, with variations in size and structure achieved by controlling the synthesis parameters and post-synthesis treatments. In situ HRTEM experiments were performed using an aberration-corrected TEM at 300 kV with a frame rate of 0.5 s and an optimized electron-beam dose rate of (2-4) × 10⁶ e nm⁻² s⁻¹. NP aggregation was induced by decomposing organic ligands under electron-beam irradiation. MD simulations were performed using LAMMPS code with an Au embedded atom potential to investigate the 3D configuration and energy evolution of aggregated NPs. Two types of models were used, one with 5-FT NPs attaching to SC NPs and another with 5-FT NPs attaching to other 5-FT NPs. The simulations were conducted at 1100 K with a time step of 1 fs. Van der Waals (vdW) force calculations were performed using Hamaker's approach to understand the interparticle forces.

The in situ HRTEM observations revealed that aggregation between a 5-FT NP and a small single-crystal (SC) NP (size ratio R < 0.76) resulted in surface diffusion dominating the aggregation growth process, leading to the formation of symmetrical or asymmetrical 5-FT NPs. However, when the SC NP was larger (R > 0.76), surface diffusion dominated the initial stage, followed by a partial dislocation-induced de-twinning process leading to the formation of a SC NP. The MD simulations confirmed these findings. For small size ratios (R < 0.76), surface diffusion from the small NP to the large 5-FT NP was the dominant mechanism, with mutual diffusion also observed. For larger size ratios (R > 0.76), simulations showed three distinct stages: initial relative slip, TB migration-dominated de-twinning process driven by partial dislocations, and finally, surface diffusion-dominated de-twinning. The de-twinning process involved the migration of twin boundaries (TBs) closely associated with the nucleation and slip of partial dislocations. The analysis involved the Thompson tetrahedron model to explain the dislocation reactions and TB migration. Aggregation between 5-FT NPs also showed varying outcomes. For smaller size ratios, symmetrical 5-FT NPs formed. For larger size ratios, intricate twin structures were observed, involving a sealed region between the NPs that stabilized the structure initially before eventually leading to a complicated twin structure. The size effect on aggregation was also studied. For 5-FT NPs attaching to SC NPs, a symmetrical 5-FT formed when the size ratio was less than 0.83. For 5-FT NPs attaching to other 5-FT NPs, a symmetrical 5-FT formed when the size ratio was less than 0.72. Larger ratios led to asymmetrical 5-FT or complex twin structures. Van der Waals force calculations revealed stronger interaction between 5-FT and SC NPs compared to two 5-FT NPs, influencing the aggregation process.

This study's findings provide fundamental insights into the atomic-scale mechanisms governing the aggregation growth and evolution of 5-FT NPs. The interplay between surface diffusion and TB migration determines the final configuration. Surface diffusion promotes symmetrical 5-FT formation, especially at smaller size ratios. TB migration, primarily driven by partial dislocations, contributes significantly to de-twinning and the formation of SC or simple twinned structures when larger NPs are involved. The formation of a sealed region during 5-FT NP aggregation affects the evolution pathway, leading to complex twin structures. The stability of the 5-FT structure and the differences in vdW forces between 5-FT/SC and 5-FT/5-FT interactions also play critical roles. These findings contribute to constructing quantitative models of twinned mineral formation and provide guidance for the innovative synthesis of twinned crystals with controlled morphologies and properties.

This research successfully revealed the atomic-scale mechanisms governing the aggregation growth and evolution of 5-FT NPs, highlighting the crucial interplay of surface diffusion and twin boundary migration. These findings provide fundamental insights into the formation of twinned structures and offer a path towards deterministic manipulation of NP growth for advanced material synthesis. Future research could explore the influence of other factors, such as temperature and pressure, on the aggregation processes and expand the study to other materials.

The MD simulations were performed at a relatively high temperature (1100 K) to accelerate the simulation process. This might influence the exact representation of the low-temperature growth processes. The electron beam in the in situ HRTEM experiments may induce additional effects on the NP behavior, although efforts were made to optimize the dose rate. The study focused primarily on Au NPs, and further investigations are needed to determine the generalizability of these findings to other materials.