Silicon-Tin Alloyed Nanocrystals by Femtosecond Laser Plasma

Abstract

Silicon’s (Si) indirect bandgap limits its absorption and possible applications, and therefore remarkable efforts are put into the possibility of fabricating a material compatible with the existing silicon technology that is also a direct bandgap material. Tin (Sn) alloying with silicon represents an exciting possibility to tune Si properties. In addition, at the quantum confinement size (<10 nm) when the material becomes a quantum dot, it becomes possible to control the electronic structure of a material by varying the size. Although the possibility to tailor the bandgap of elemental Si over a wide range has been demonstrated, it is very challenging to obtain stable alloyed SiSn quantum dots. In particular, alfa Sn has a crystal structure similar to Si with zero band gap: a diamond cubic crystal structure, and importantly affected by the crystal configuration and a possible change to a direct energy gap at sufficiently high Sn concentration. This therefore makes Sn a favorable element to create an alloy with a lower bandgap than Si. Exploiting these features would make alloyed SiSn NCs an important candidate for a broad range of applications such as next generation photovoltaic cells based on carrier multiplication, super high efficient photodetectors, fluorescent tags for biomedical applications etc. In this contribution we will discuss the synthesis paths and optoelectrical properties in the context of silicon, tin, and silicon/tin alloyed nanocrystals at high alloying concentrations. Our results show that plasma-based processes allow the synthesis of alfa tin alloyed SiSn QDs with unprecedented dimensions and alloying concentrations. We will present SiSn-QDs as the world’s highest alloys fabricated by confined plasma from femtosecond (fs) laser in liquid media with a high tin concentration of approximately 17% ever reported in literature to our knowledge. We have demonstrated for the first time the experimental and exact theoretical calculation determination of the thermal stability of the SiSn-NCs at those concentrations. We study their electronic properties by means of an approximate band structure suitable for finite-sized systems (quasi-band structure) derived from first-principles calculations. This work therefore advances the current state of the art, provides opportunities to compare and verify theoretical results and offer important directions for applications.