## 2 Structure, Synthesis, and Properties of Bimetallic CuFe Nanoparticles
### Modeling  
As can be found in the literature, molecular dynamics simulations allow for the creation of a model and determine the properties before starting experiments or full-scale industrial production of metallic nanoparticles [27; 28; 29; 30; 31; 32]. Molecular dynamics calculations were also used to investigate the CuFe system, including the influence of particle size, cooling rate, and Cu concentration on the morphology of the CuFe nanoparticles. [PERSON] et al. studied the dependence of the morphologies of FeCu on different Fe and Cu concentrations and the appearance of possible defects in the stabilized NPs, as well  
Figure 1: CuFe bimetallic nanoparticles, their synthesis, architecture, properties, and applications. as the energy contributions to the bimetallic systems [25]. The researchers studied the Fe\({}_{1-x}\)Cu\({}_{x}\) systems with Cu concentration at 10%, 40%, and 70% (Figure 3). They examined three Fe cores (D\({}_{\rm Fe}\)) with diameters 5, 7, and 9 nm with their respective Cu concentrations. At the low concentration of Cu (10%), the Fe core@shell is created. With increasing the Cu concentration to 40%, core@shell structures are observed for small (5 nm) Fe particles, while Janus structures are preferred for medium (7 nm) and big (9 nm) Fe particles. Seventy percent of Cu leads to segregated Janus (sub-clasters) particles. These simulations show that a segregated (Janus) structure becomes preferred over core@shell architecture when the Cu content increases. In addition, for low Cu concentration, the particles show a bcc phase, at 40% Cu, two phases exist, a bcc phase for Fe, and Cu is mainly ordered as an fcc lattice, high Cu content shows a bcc lattice for Fe and an fcc lattice for Cu. Hexagonal closed-packed (hcp) structures are nucleated only at the Fe\(-\)Cu interface because of twins and stacking faults. Another example of molecular dynamics calculations used to investigate the CuFe system was reported by [PERSON] [26]. He showed that the FeCu bimetallic nanoparticles can be oriented in random alloy, core-shell, and Janus morphologies, depending on their size, and thermal processing (temperature and cooling rate). At a very slow cooling rate (0.1 K/ps), clear interface Fe-Cu atom is created at a temperature of T = 1200 K and lower. As the temperature decreases further below T = 1200 K, the Fe and Cu atoms crystallize in bcc and fcc crystalline structures, respectively (Figure 4a). With an increasing cooling rate (0.5 and 1 K/ps), the immiscibility between Fe and Cu atoms is not complete (Figure 4b,c), and patchy nanoparticles are created. At a cooling rate of 0.5 K/ps, Fe and Cu atoms crystallized in bcc and fcc structures, respectively (Figure 4b), while at cooling rate of 1 K/ps, both Fe and Cu atoms have bcc structures (Figure 4c). The semi-miscible phase of Cu and Fe atoms is the result of very high cooling rates, 5 and 10 K/ps (Figure 4d,e), which do not allow the atoms to crystallize neither in the fcc structure nor in the bcc structure. Figure 2: Types of bimetallic nanoparticles: (**a**) random alloyed, (**b**) ordered alloy, (**c**) sub-clusters/Janus (**d**) core-shell, (**e**) multi-shell core-shell, and (**f**) multiple core materials coated by a single shell material. Yellow and purple spheres represent two different kinds of metal atoms. Reprinted from [5]. Figure 4: Snapshots of FeCu bimetallic nanoparticles during cooling from T = 3000 to 10 K at various cooling rates: (**a**) 0.1 K/ps, (**b**) 0.5 K/ps, (**c**) 1 K/ps, (**d**) 5 K/ps, and (**e**) 10 K/ps (Colors: \(\bullet\) Cu, \(\bullet\) Fe). The rightmost column depicts the evolution of the various crystal structures of FeCu nanoparticles at 300 K (Colors: \(\bullet\) fcc, \(\bullet\) hcp, and \(\bullet\) bcc atoms). Red arrows show the progress of the cooling process. Reproduced with permission from ref. [26]. Copyright 2019 American Chemical Society. Figure 3: (**a**) Cluster structure as a function of size and Cu concentration. (Fe cores (D\({}_{\rm Fe}\)) have diameters 5, 7, and 9 nm).