(193b) Plasma Synthesis of Metal-Sulfide Nanocrystals

Metal-sulfide nanocrystals (e.g. CdS, PbS, Cu₂ZnSnS₄) are typically produced through liquid solution synthesis in solvents that may pose challenges in achieving high purity materials. Moreover, solution synthesis generates large volumes of hazardous waste and the commonly used hot injection method is difficult to scale up to achieve high throughput.  Gas-phase approaches, such as the continuous nonthermal plasma processes we have been developing, do not require the use of solvents and can offer very high purity and very high throughput.  The benefits of the plasma approach come without sacrificing control over materials properties. In many cases the materials have excellent quality and achieve high performance in applications. For example, intrinsic and doped silicon nanoparticles have been produced and already commercialized for use in solar cells. However, to date no plasma synthesis method has been developed for metal-sulfides.  Starting with a model material, zinc sulfide (ZnS), we have developed a gas phase approach to making binary sulfide nanocrystals.  ZnS is a wide band gap semiconductor (E_g = 3.7 eV) that has been extensively used as a phosphor, buffer layer in thin film solar cells and surface passivation shell for semiconductor quantum dots.  Crystalline ZnS particles with 2-10 nm diameters were synthesized from diethyl zinc and elemental sulfur vapor.   Particle diameter and composition could be controlled through the gas flow rate, feed gas composition and pressure. Even without optimization, production rates were as high as 250 mg/hr with 25% mass yield in our bench scale 8 cm³ reactor.  The nanocrystals could be deposited on substrates directly or formed into stable nanocrystal dispersions (inks) by capping their surfaces with organic ligands such as oleylamine. This, to our knowledge, is the first report of a metal-sulfide nanocrystalline material made using a nonthermal plasma process.  Beyond ZnS, we will also present findings on the Fe, Sn and Cu sulfides.