Photoluminescence evolution of functional silicon quantum dots assembled via a sustainable mechanochemical process

Abstract

In the present work, we report a sustainable, room-temperature mechanochemical approach for synthesizing functional silicon quantum dots (Si QDs) with tunable photoluminescence (PL) in the visible range. Using hydrogen silsesquioxane as a precursor, precise control over the size and surface chemical state of Si QDs is achieved through controlled ball-milling and subsequent chemical etching and hydrosilylation. Discrete element method simulations reveal that the cumulative supra-critical impact energy (E_sup), defined as the total impact energy exceeding a critical threshold (e_crit) required for chemical activation, plays a dominant role in driving the formation of Si radicals and Si–H bond cleavage, which are key steps in crystallite growth. Under high-energy milling conditions using larger balls, a significant portion of collisions exceed e_crit, thereby enhancing the efficiency of solid-state chemical reactions and leading to the formation of larger Si QDs. The PL red shift observed across blue-, green-, and red-emitting Si QDs is attributed to a size–surface coupling mechanism. For smaller Si QDs, PL originates from quantum-confined band-edge transitions and shallow surface states, enabled by high alkyl chain coverage. Larger Si QDs exhibit red-shifted, excitation-independent emission dominated by deep oxide-related surface states, stemming from enhanced oxidation and reduced organic passivation. These findings highlight the interplay between mechanical energy input, structural/size evolution, and surface chemistry in tailoring the optical properties of Si QDs.