Ultra-small photoluminescent silicon-carbide nanocrystals by atmospheric-pressure plasmas.

Sadegh Askari,Fengjiao Yu,Paul Maguire,Davide Mariotti,Kostya (Ken) Ostrikov,Wuzong Zhou,Manuel Macias-Montero,Igor Levchenko,Atta Ul Haq,Vladimir Svrcek

doi:10.1039/c6nr03702j

Abstract

Highly size-controllable synthesis of free-standing perfectly crystalline silicon carbide nanocrystals has been achieved for the first time through a plasma-based bottom-up process. This low-cost, scalable, ligand-free atmospheric pressure technique allows fabrication of ultra-small (down to 1.5 nm) nanocrystals with very low level of surface contamination, leading to fundamental insights into optical properties of the nanocrystals. This is also confirmed by their exceptional photoluminescence emission yield enhanced by more than 5 times by reducing the nanocrystals sizes in the range of 1-5 nm, which is attributed to quantum confinement in ultra-small nanocrystals. This method is potentially scalable and readily extendable to a wide range of other classes of materials. Moreover, this ligand-free process can produce colloidal nanocrystals by direct deposition into liquid, onto biological materials or onto the substrate of choice to form nanocrystal films. Our simple but efficient approach based on non-equilibrium plasma environment is a response to the need of most efficient bottom-up processes in nanosynthesis and nanotechnology.

Highlights

Elemental silicon nanocrystals (NCs) have been proposed as a promising material for optoelectronic, biomedical imagingThe origin of photoluminescence in NCs in general can vary dramatically depending on the surface states and degree of quantum confinement
The diffraction patterns of silicon carbide (SiC) NCs ensembles is in full agreement with the assignment of the 3C-SiC crystal structure for NCs produced at all three different synthesis conditions
The PL emission of SiC NCs has been attributed to molecular transitions in some cases due to defects,[32] bandgap transitions that are affected by quantum confinement (QC) or transitions taking place at the surface that may be affected by QC.[12,33]

Summary

Introduction

The origin of photoluminescence in NCs in general can vary dramatically depending on the surface states and degree of quantum confinement. In SiC NCs, as well as in most Sibased NCs, the photoluminescence origin and its performance is further complicated by potential transitions between direct and indirect behaviour. While this represents a complex system to be studied, it can offer important application opportunities. These traditional methods do not ensure sufficient level of control over the NCs properties and in particular, over the size distribution, which requires additional size-selective steps. These challenges impede a wide use of SiC NCs in many state-of-the-art applications. The accurate control over the NCs size has allowed for the first time to reveal important insights into the crystal structure and nanocrystal optical properties

Results and discussion

Material characterization

Photoluminescence of the SiC nanocrystals

Plasma-induced crystallization chemistry

Important features of the process

Conclusions