Size vs Surface: Tuning the Photoluminescence of Freestanding Silicon Nanocrystals Across the Visible Spectrum via Surface Groups

Abstract

The syntheses of colloidal silicon nanocrystals (Si-NCs) with dimensions in the 3-4 nm size regime as well as effective methodologies for their functionalization with alkyl, amine, phosphine, and acetal functional groups are reported. Through rational variation in the surface moieties we demonstrate that the photoluminescence of Si-NCs can be effectively tuned across the entire visible spectral region without changing particle size. The surface-state dependent emission exhibited short-lived excited-states and higher relative photoluminescence quantum yields compared to Si-NCs of equivalent size exhibiting emission originating from the band gap transition. The Si-NCs were exhaustively characterized using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier transformed infrared spectroscopy (FTIR), and their optical properties were thoroughly investigated using fluorescence spectroscopy, excited-state lifetime measurements, photobleaching experiments, and solvatochromism studies.

Cationic Silicon Nanocrystals with Colloidal Stability, pH‐Independent Positive Surface Charge and Size Tunable Photoluminescence in the Near‐Infrared to Red Spectral Range

“…For example, PL quenching in ncSi by primary alkyl amines, mainly attributed to dative surface‐bonded adducts, have been observed by Rho et al28, 29 Furthermore, ncSi with Si–N–R linkages (where R = alkyl or aryl groups) normally have blue or green emission with excitation wavelength‐dependent PL and nanosecond excited‐state lifetimes. These effects were tentatively attributed to surface‐mediated charge transfer processes 30, 31. Previous reports of ncSi with Si–R–NH 2 linkages (where R = alkyl or alkenyl groups) also emit in the blue region.…”

The Interplay of Quantum Confinement and Hydrogenation in Amorphous Silicon Quantum Dots

“…While crystalline Si (c‐Si) QDs have been extensively studied within the last decade,14, 15, 16, 17, 18, 19 investigation of amorphous Si (a‐Si) QDs properties has been very limited. One of the reasons for this limited effort is represented by the synthesis difficulties: during synthesis, it is challenging to preserve both the individuality of the QDs and their amorphous nature as QDs tend to aggregate with consequent loss of the quantum‐confined state.…”

Investigation into Effects of Instability and Reactivity of Hydride-Passivated Silicon Nanoparticles on Interband Photoluminescence

“…40,[43][44][45][46][47][48][49][50][51][52][53] For example, surface reconstructions, chemical impurities, lattice vacancies, dislocations, etc., can produce characteristic energy states (defect states)…”

“…For example, oxidation of high surface area Si occurs readily in ambient and laboratory conditions and is frequently cited to coincide with sudden, drastic changes in the observed PL. 23,51,52,55,[65][66][67] However, the observed increase or decrease in emission energy that oxygen passivation causes varies from study to study.…”

“…However, the formation of such a layer is difficult to achieve experimentally, and surface oxidation typically results in the formation of electronically active states within the bandgap. 23,51,52,55,63,[65][66][67][68]70 Alkyl-passivation has long been used in order to surface stabilize bulk and nanocrystalline silicon surfaces, in particular against ambient oxidation. Alkylpassivation is thought to have negligible effects on the photophysical properties of the Si domains, similar to hydride-passivation.…”