Silicon Nanocrystals with enhanced photophysics

Abstract

Silicon is the cornerstone of microelectronics, photovoltaics and optoelectronics, however, due to its indirect band-gap, there is no efficient silicon based LED and laser. Importantly, Si laser is the last missing key device for building optical computer CPUs – an alternative and high performance computer architecture that is within our reach. Also, non-toxicity and abundance of silicon make it a prime candidate for various bio-medical and large-scale applications. Through the past 30 years, attention turned to Si nanocrystals (SiNCs), where the quantum confinement and broken symmetry enhances the radiative rates. However, these effects on their own are not enough to achieve rates as high as we can find in the direct band-gap materials.

In our work, we pursue additional path towards the direct-band-gap-like Si materials – we make use of the surface moeities that are covalently bonded and contribute new states to the band-gap edge [1-3]. In all semiconductor NCs, surface to volume ratio increases with decreasing core diameter and larger fraction of the constituent atoms resides directly on the surface. In case of 2 nm SiNCs, 50% of the Si atoms come from the surface. In silicon, the chemistry is covalent and electrons from attached ligands strongly contribute in the electronic structure (band-structure). This offers an additional degree of freedom to manipulate the optoelectronic properties. We have performed Tight Binding [1,2] and Density Functional Theory [3] simulations showing that the covalently bonded electronegative ligands enhance the radiative rates to the same level as in the direct bandgap materials. Even though the synthesis of such materials remains unsolved, some of the available SiNCs materials come close enough. In the experimental part of our work, we study radiative rate, emission efficiency and spectral properties of such SiNCs on ensemble and single-dot levels. We have confirmed fast radiative rate in organically capped SiNCs [4] and a new way of making multi-chromatic SiNCs by electron beam irradiation [5]. Furthermore, we have uncovered a strong artifact in the standardized quantum yield (QY) methodology that has been misinterpreted in the past as various novel effects in SiNCs (and other materials) and can lead to considerable underestimation of the QY [6].

References:
[1] A. N. Poddubny and K. Dohnalova, Phys. Rev B 90 (2014) 245439
[2] K. Dohnalova et al., Light: Science & Applications 2 (2013) e47.
[3] K. Dohnalova, P. Hapala and I. Infante, manuscript in preparation 2018.
[4] B. van Dam et al., ACS Photonics 5 (2018), 2129.
[5] B. Bruhn et al., Light: Science & Applications 6 (2017) e17007.
[6] B. van Dam et al., AIP Advances 8, 085313 (2018).