Nonthermal plasma synthesis of semiconductor nanocrystals: Principle, progress and perspectives

Nanotechnology has been thriving for many years, but scientists' pursuit of innovation and breakthrough in the synthesis of various forms of nanomaterials in a controlled manner has never ceased. The 2023 Nobel Prize in Chemistry was awarded to Moungi G. Bawendi, Louis E. Brus, and Alexei I. Ekimov for their pioneer contributions to the discovery and synthesis of quantum dots, also known as semiconductor nanocrystals (NCs), signifying the importance of advanced synthesis methods for nanomaterials in a variety of technical applications. Over the past two decades, nonthermal plasma has been established as an important technique for controlled synthesis of versatile nanomaterials with high quality on a par with conventional colloidal methods. Inherently free of chemical solvents and organic ligands, nonthermal plasma provides a unique non-equilibrium environment for growing high-quality and high-purity semiconductor NCs. First, high-energy electrons in the plasma collide with nanoparticles to negatively charge the nanoparticles, which can effectively reduce agglomeration of the nanoparticles, yielding ultrasmall nanoparticles with a few nanometers accompanied with a narrow size distribution. Second, energetic surface chemical and physical reactions can selectively heat the nanoparticles to temperatures far exceeding the ambient gas temperature, allowing significant deviation of the particle growing from a thermal equilibrium. Third, large difference between chemical potentials of growth species in the gas environment and species bound to the solid nanoparticle surface facilitates hyperdoping of semiconductor NCs for emerging new physical phenomena and applications. Inspired by recent impressive success of nonthermal plasma in the synthesis of semiconductor NCs, it is necessary to summarize latest research progress for on-demand plasma science and technology. Herein the current paperreviews state of the art in nonthermal plasma synthesis of a variety of semiconductor NCs. Fundamental mechanisms of the particle nucleation, growth, and crystallization in nonthermal plasma are discussed in detail in the review. Emphasis is placed on flexible control over the evolution of nanoparticle size, morphology, crystallinity, surface chemistry, and component, as well as recent impressive progress in the synthesis of single-element, multi-elements and complex core-shell structured semiconductor NCs by means of nonthermal plasma. Importantly, nonthermal-plasma-enabled hyperdoping of semiconductor NCs far exceeding the bulk solubility of a dopant as well as their novel optoelectronic properties such as sub-bandgap optical absorption and emission, localized surface plasmon resonance (LSPR), and metal-insulator transition (MIT) is highlighted, together with prospecting the development of nonthermal plasma systems in the future. We are confident that nonthermal plasma technology is expected to evolve into a universal strategy in addition to conventional colloidal methods for controlled synthesis of versatile nanomaterials, and should greatly contribute to the practical implementation of nanomaterials in a wide range of technical fields including electronics, optoelectronics, energy, catalysis, medicine, bioimaging, and beyond.