## 4 Experimental Results and Discussion
### TEM Images of Heterogeneous Nanoparticles  
We considered the following mechanism via which AgPd and AuPd may form alloys. When a laser beam irradiates a solid target, particles are generated by ablation and Pd ions reduced by electrons through laser irradiation are precipitated almost simultaneously. Both the particles and the precipitated Pd are affected by subsequent laser pulses, which concentrate the electric field around the nanoparticles, which thus become nano-sized alloys. If the pulse width is over nanoseconds, it the nanoparticles should reach thermal equilibrium. In such a case, the nanoparticles could grow due to diffusion. However, our pulse width was relatively short such as femtoseconds, so such nanoparticle growth was not possible.  
Figure 4 shows TEM images of nanoparticles of various sizes and shapes created by varying the laser pulse, target material, and metal ions in solution. When there were no metal ions, most nanoparticles were spheres or polyhedrons (Figure 4a,b). Most particles were independent, but some were connected in chains, particularly Au particles. The Ag and Au nanoparticles were 5-30 nm in diameter. The shapes and sizes of particles may depend on the laser irradiation time, but we did not vary this parameter.  
The elements of heterogeneous nanoparticles can be distinguished in terms of brightness (light or dark). As the atomic numbers of Au and Pd differ greatly, these two elements can be distinguished. In general, an atom with a low atomic number exhibits high contrast.  
Figure 4: TEM images of synthesized nanoparticles. (**a**) Ag particles of nanosecond process and femtosecond process, (**b**) Au particles of nanosecond process and femtosecond process, (**c**) AgPd process particles of nanosecond process, (**d**) AuPd particles of nanosecond process, (**e**) AgPd particles of femtosecond, and (**f**) AuPd particles of femtosecond process.  
Au is darker than Pd. In the solution with Pd ions, the shapes and sizes of nanoparticles generated by the two lasers differed. In the presence of Pd ions, the shapes of AgPd and AuPd nanoparticles were random and colloidal; the clustered, heterogeneous nanoparticles had a diameter of 30-50 nm. This was particularly clear when nanosecond pulses were delivered (Figure 4c,d). AgPd and AuPd particles synthesized by the femtosecond laser also formed similarly shaped heterogeneous nanoparticle clusters, in which metal and Pd nanoparticles were homogeneously mixed (Figure 4e,f). However, femtosecond-generated heterogeneous nanoparticle clusters were about 1.5 to 2.5 times larger than nanosecond-generated clusters (cf. Figure 4c-f). The effect of Pd ions on an Ag target can be understood by comparing Figure 4a to Figure 4c,e, and Figure 4b to Figure 4d,f. When Pd ions were present in solution, the size of the nanoparticles was 3-5 nm in diameter. This means smaller nanoparticles were produced in existence of Pd ions. These ions enhance LAL fragmentation, as predicted by modeling (Figure 3c). The metal ions induce LSPR coupling effect around the nanoparticles, thereby affecting their size and contributing to the formation of colonies of heterogeneous nanoparticles.  
Such nanoparticle alloys would be expected to behave differently from the core-shell nanoparticles prepared via conventional synthesis. For the latter nanoparticles, the elemental properties of the shell are stronger than those of the core when encountering waves delivered by a laser or microwave. However, the properties of both metals of an alloy will affect the responses to wave energy. In addition, the fact that the core and shell are formed from different metals limits nanoparticle size, but not the sizes of heterogeneous nanoparticles that are alloys. Even heterogeneous nanoparticles made of the same elements would be expected to differ in terms of their chemical and optical properties, depending on the bond type.