Colloidal semiconductor nanocrystals have been widely synthesized in solution. Well-optimized solution synthesis can produce colloidal nanocrystals with great controls on their shapes, sizes, and size distributions. Solution synthesis not only is good in tenability of nanocrystals but also has great advantages in scalability, and consequent cost reduction in both nanocrystal productions and device applications. However, this great method is not applicable to all semiconductor materials. Group XIII nitrides are such materials difficult to produce in desirable quality in solution synthesis. Also solution synthesis is limited in temperature and pressure ranges, due to decomposition of organic molecules at high temperature and use of glass apparatus which cannot bear high pressure difference. Here, two ways to overcome limits in reaction conditions of solution synthesis are discussed: incorporation of biphasic reaction media and high-pressure reaction environment. It is proven that molten salt droplets immiscible with non-polar solvent can be interesting media for nanoparticle growth. Chapter One discusses about synthesis of six-monolayer CdSe nanoplatelets. To the known fact that zinc blende (ZB) CdSe nanoparticles grow laterally into nanoplatelets (NPLs) in molten cadmium acetate droplets, a twist of adding cadmium chloride is added to allow the CdSe NPL to grow thicker. Chapter Two explores solution synthesis of colloidal GaN and AlN nanocrystals enabled with incorporation of molten salt phase to non-polar solution. Chapter Two also discusses about a pressure control of an ammonia reactant, showing that a crystal size of GaN nanoparticles can be systematically tuned with the ammonia pressure. Chapter Three explains high-pressure reactor systems for nanomaterials science in detail.