(Invited) Silicon Nanostructures: A Versatile Material for Photonics
Authors: Bisadi Zahra , Piotto Chiara , Fontana Giorgio , Scarpa Marina , Pavesi Lorenzo , Bettotti Paolo 
Published: February 2020

The increased control over nanoscale materials structuring is reshaping a number of applications and technologies at a fast pace. Semiconducting materials are playing an increasingly important role and Silicon is so far the most investigated one thanks to its broad availability and its fundamental role in microelectronics.
Despite all its favorable properties, a major drawback of silicon is the lack of an efficient light emission mechanism that prevents its use in active photonics. Quantum size effects has been demonstrated a viable strategy to overcome this limit and to achieve light emission from silicon nanostructures. Si nanostructures are well controlled and can be tuned to achieve the desired properties, often exploiting the mature CMOS technological platform.
Simultaneously several synthetic strategies have been developed in order to fabricate silicon quantum dots from chemical synthesis. Silicon chemistry is (rather) well known and permits to synthesize nanoparticles and to functionalize their surface using a broad number of reactions. Such flexibility allows for the grafting of both organic and inorganic species onto the Si QDs surface, to add functionalities and to protect the nanoparticle from their natural oxidations that, in turn, quenches the photoluminescence properties.
Thus, despite the initial research was focused on the possibility to fabricate inorganic light sources that might rival with III-V or II-VI semiconductors. More recently a large research effort has been devoted to the use of Silicon nanostructures in biological-related applications. In such research topics Silicon has several properties take renders it an ideal candidate: it is considered a biocompatible material; it is readily dissolved in biological environment without releasing toxic byproducts; its red and tunable luminescence make it an interesting fluorescent tag able to exploit the tissue transparency window.
During the presentation I will make a brief review of the fabrications methods proposed so far and I will discuss in details some of the recent research activities performed in our group in ICT- as well as in bio-related topics, all sharing the common denominator of Si QDs as active optical element.
About the ICT-related topics I will discuss two applications of Si QDS. The first is about the use of Si QDs in quantum technologies. Data encryption and cryptography are core technologies hidden in our daily life and their importance is constantly growing. We have developed a robust physical random number generator that exploits Si QDs as a random bit source. The preliminary results are promising and the random sequences pass the standard NIST test suite. Furthermore the system is fully compatible with CMOS technology and can be scaled up to achieve large bit-rate generation. The second case in point is about the large optical nonlinearities possessed by Si QDs and their exploitation to develop integrated photonic devices.
As a bio-related application I will discuss how nanostructured silicon microparticles can be used as fluorescent tags and are successfully up-taken by human dendritic cells without stimulating apoptosis up to a very large concentration. Porous silicon micro-particles (micro-pSi) with size in the range of 1-10 um are functionalized obtaining an interface that exposes either positive or negative charges at neutral pH. Their porous structure enables their use as drug carriers. Moreover, the surface modification stabilizes their intense photoluminescence and suggests their use for simultaneous imaging and drug delivery.
The few cases discussed underline the great potentialities of Silicon nanostructures in different technologies and suggest its possible implementation as a functional interface between organic and inorganic systems.