Why do Si quantum dots with stronger fast emission have lower external photoluminescence quantum yield?

Silicon quantum dots (QDs) are a promising non-toxic alternative to the already well-developed platform of light-emitting semiconductor QDs based on III-V and II-VI materials. Oxidized SiQDs or those surface-terminated with long alkyl chains typically feature long-lived orange-red photoluminescence originating in quantum-confined core states. However, sometimes an additional short-lived PL band, whose mechanism is still highly debated, is reported. Here, we perform a detailed study of the room-temperature PL of SiQDs using samples covering three main fabrication techniques. We find evidence for the presence of only one set of radiative processes in addition to the typical long-lived PL. Moreover, we experimentally determine the ratio between the short- and long-lived PL component, obtaining a wide range of values (0.003 - 0.1) depending on the type of sample. In accordance with an already published report, we observe a tendency of SiQDs with stronger short-lived PL to have lower external quantum yield. We explain this trend using a model of the optical performance of an ensemble of QDs with widely varying optical characteristics through a mechanism we call selective lifetime-based quenching.

The red and blue luminescence in silicon nanocrystals with an oxidized, nitrogen-containing shell.

Traditionally, two classes of silicon nanocrystals (SiNCs) are recognized with respect to their light-emission properties. These are usually referred to as the "red" and the "blue" emitting SiNCs, based on the spectral region in which the larger part of their luminescence is concentrated. The origin of the "blue" luminescence is still disputed and is very probably different in different systems. One of the important contributions to the discussion about the origin of the "blue" luminescence was the finding that the exposure of SiNCs to even trace amounts of nitrogen in the presence of oxygen induces the "blue" emission, even in originally "red"-emitting SiNCs. Here, we obtained a different result. We show that the treatment of "red" emitting, already oxidized SiNCs in a water-based environment containing air-related radicals including nitrogen-containing species as well as oxygen, diminishes, rather than induces the "blue" luminescence.

A complex study of the fast blue luminescence of oxidized silicon nanocrystals: the role of the core.

Silicon nanocrystals (SiNCs) smaller than 5 nm are a material with strong visible photoluminescence (PL). However, the physical origin of the PL, which, in the case of oxide-passivated SiNCs, is typically composed of a slow-decaying red-orange band (S-band) and of a fast-decaying blue-green band (F-band), is still not fully understood. Here we present a physical interpretation of the F-band origin based on the results of an experimental study, in which we combine temperature (4-296 K), temporally (picosecond resolution) and spectrally resolved luminescence spectroscopy of free-standing oxide-passivated SiNCs. Our complex study shows that the F-band red-shifts only by 35 meV with increasing temperature, which is almost 6 times less than the red-shift of the S-band in a similar temperature range. In addition, the F-band characteristic decay time obtained from a stretched-exponential fit decreases only slightly with increasing temperature. These data strongly suggest that the F-band arises from the core-related quasi-direct radiative recombination governed by slowly thermalizing photoholes.

Brightly luminescent organically capped silicon nanocrystals fabricated at room temperature and atmospheric pressure.

Silicon nanocrystals are an extensively studied light-emitting material due to their inherent biocompatibility and compatibility with silicon-based technology. Although they might seem to fall behind their rival, namely, direct band gap based semiconductor nanocrystals, when it comes to the emission of light, room for improvement still lies in the exploitation of various surface passivations. In this paper, we report on an original way, taking place at room temperature and ambient pressure, to replace the silicon oxide shell of luminescent Si nanocrystals with capping involving organic residues. The modification of surface passivation is evidenced by both Fourier transform infrared spectroscopy and nuclear magnetic resonance measurements. In addition, single-nanocrystal spectroscopy reveals the occurrence of a systematic fine structure in the emission single spectra, which is connected with an intrinsic property of small nanocrystals since a very similar structure has recently been observed in specially passivated semiconductor CdZnSe nanoparticles. The organic capping also dramatically changes optical properties of Si nanocrystals (resulting ensemble photoluminescence quantum efficiency 20%, does not deteriorate, radiative lifetime 10 ns at 550 nm at room temperature). Optically clear colloidal dispersion of these nanocrystals thus exhibits properties fully comparable with direct band gap semiconductor nanoparticles.