Microscopic Origin of the Fast Blue‐Green Luminescence of Chemically Synthesized Non‐oxidized Silicon Quantum Dots

Abstract

The microscopic origin of the bright nanosecond blue-green photoluminescence (PL), frequently reported for synthesized organically terminated Si quantum dots (Si-QDs), has not been fully resolved, hampering potential applications of this interesting material. Here a comprehensive study of the PL from alkyl-terminated Si-QDs of 2-3 nm size, prepared by wet chemical synthesis is reported. Results obtained on the ensemble and those from the single nano-object level are compared, and they provide conclusive evidence that efficient and tunable emission arises due to radiative recombination of electron-hole pairs confined in the Si-QDs. This understanding paves the way towards applications of chemical synthesis for the development of Si-QDs with tunable sizes and bandgaps.

Ultrafast optical spectroscopy of surface-modified silicon quantum dots: unraveling the underlying mechanism of the ultrabright and color-tunable photoluminescence

“…23 Furthermore, although the possible modulation mechanisms of the carrier wave function in Si NCs still remain controversial, some researchers have achieved the effect of surface ligands on the low-energy electron states of Si NCs, which would lead to phononless excitonic recombination. 24,25 Thus, the major drawback of Si NCs could be overcome by these effects solely or synergistically. Nevertheless, insight into the PL mechanism of Si NCs remains elusive, 26231 in particular for ultrabright Si NCs.…”

Bright trions in direct-bandgap silicon nanocrystals revealed by low-temperature single-nanocrystal spectroscopy

“…Interestingly, a similar spectral pattern was observed not only in II-VI-based semiconductor NCs 13 , being interpreted as the emission of a trion, but also by independent groups studying SiNCs, this time being assigned to phonon replicas. In particular, basically identical single-NC spectra with nanosecond dynamics were observed in both nominally SiO 2 -surface-capped SiNCs 4,22,23 and SiNCs capped with n-butyl 6 . In these cases, the low-energy satellites were identified with processes that involved the emission of a mixture of SiO 2 TO and LO phonons 22 and the Si-C stretching-vibration phonon 6 , respectively (for an overview of single-NC experiments in SiNCs, see Supplementary Table S1, Fig.…”

“…The use of SiNCs instead of the bulk form of silicon, however, can alleviate the problem of poor light emission only partly, because SiNCs preserve the indirect nature of the bandgap 1,2 , and the light-emission rate thus remains low, on the order of 10 4 s 21 or less 3 . However, when passivated by a "suitable" organic material, SiNCs can have fast macroscopic radiative lifetime [4][5][6][7] and can actually be transformed into a direct-bandgap material 5,[8][9][10] . In particular, we have recently shown both theoretically and experimentally that properly capped SiNCs can be strain-engineered into a material with fundamental direct C 15 -C 25 ' bandgap 5 and the corresponding fast electron-hole radiative recombination rate of 10 8 s 21 (see also Figure 1a and the section on "Results overview").…”

“…With respect to single-NC spectroscopy in SiNCs, several types of samples have already been probed by different groups 3,4,6,[18][19][20][21][22][23][24][25][26][27] . The most commonly observed spectral shape was a mixture of broader unstructured spectra and structured spectra with peaks approximately 150 meV apart.…”

“…Although the spectral structure looks almost identical in all the measurements, no clear agreement on the mechanism behind it exists. Some groups have suggested that this structure arises due to the involvement of surface-related SiO 2 18,22 or SiC phonons 6,22 . In contrast to the phonon-based explanation, our group noted not only the similarities of the spectral structures observed in SiNCs prepared by various methods but also the similarity of the spectral structures observed in SiNCs and II-VI NCs 13 .…”