Electron−Phonon Coupling and Localization of Excitons in Single Silicon Nanocrystals

Abstract

We report a detailed photoluminescence (PL) study on single silicon nanocrystals produced by laser pyrolysis. The PL spectra reveal nearly homogeneously broadened zero-phonon lines coupled to Si-O-Si phonon transitions in the SiO2 shell. A systematic investigation of electron-phonon coupling is reported on the basis of single nanocrystals. The stepwise localization of electron and hole at the Si-SiO2 interface for nanocrystals smaller than d approximately 2.7 nm is driven by electron-phonon coupling. From the localization energies the effective Bohr radii of the (localized) electron and hole are estimated to be in the range of 1-2 bond lengths of Si-O and Si-Si.

Tunneling of electrons between Si nanocrystals embedded in a SiO2matrix

“…The characteristic parameter of the theory, the polaron binding energy 53 , is approximated by the Fröhlich coupling mechanism in a SiO 2 matrix with a polaron radius of the order of the NC one. Meanwhile the strong coupling of SiO 2 phonons to carriers in Si NCs is confirmed by photoluminescence studies 56 . However, the resulting polaron energies are large compared to the thermal energy k B T , so that the contribution of SiO 2 -phonon-assisted processes remains small.…”

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.…”

“…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 .…”

Designing small silicon quantum dots with low reorganization energy

“…This resulted in a remarkably low excitonic reorganization energy of 36 meV, as shown at right in Figure 8. It is worth noting, though, that the Si − SiO 2 interfaces considered here are free of defects which are experimentally known to have a strong influence on exciton footprint, recombination rate and transport [67][68][69] . The reorganization trend as Si35 becomes more rigid.…”