**3. Nanomaterials synthesis and characterization**

In this section, we give an overview of the application of PLA in high-density media for the fabrication of nanomaterials—mainly not only metallic and semiconductor nanoparticles but also organic molecules. We focus on the effect of the pressurized fluid on the particle characteristics, and advantages compared with PLA in atmospheric pressure liquids or gases.

While PLA in high-density media offers many advantages, the nucleation and growth mechanisms are still not understood very well. Methods that enable in situ characterization of nanoparticle formation, namely, fast X-ray Radiography (XRR) and Small-angle X-ray Scattering (SAXS) will be discussed in Section 3.2.
