Inorganic perovskites display an enticing foreground for their wide range of optoelectronic applications. Recently, supercrystals (SCs) of inorganic perovskite nanocrystals (NCs) have been reported to possess highly ordered structure as well as novel collective optical properties, opening new opportunities for efficient films. Here, we report the large-scale assembly control of spherical, cubic, and hexagonal SCs of inorganic perovskite NCs through templating by oil-in-oil emulsions. We show that an interplay between the roundness of the cubic NCs and the tension of the confining droplet surface sets the superstructure morphology, and we exploit this interplay to design dense hyperlattices of SCs. The SC films show strongly enhanced stability for at least two months without obvious structural degradation and minor optical changes. Our results on the controlled large-scale assembly of perovskite NC superstructures provide new prospects for the bottom-up production of optoelectronic devices based on the microfluidic production of mesoscopic building blocks.

Nanoscale dielectric resonators and quantum-confined semiconductors have enabled unprecedented control over light absorption and excited charges, respectively. In this work, we embed luminescent silicon nanocrystals (Si-NCs) into a 2D array of SiO2 nanocylinders and experimentally prove a powerful concept: the resulting metamaterial preserves the radiative properties of the Si-NCs and inherits the spectrally selective absorption properties of the nanocylinders. This hierarchical approach provides increased photoluminescence (PL) intensity obtained without utilizing any lossy plasmonic components. We perform rigorous calculations and predict that a freestanding metamaterial enables tunable absorption peaks up to 50% in the visible spectrum, in correspondence with the nanocylinder Mie resonances and of the grating condition in the array. We experimentally detect extinction spectral peaks in the metamaterial, which drive enhanced absorption in the Si-NCs. Consequently, the metamaterial features increased PL intensity, obtained without affecting the PL lifetime, angular pattern, and extraction efficiency. Remarkably, our best-performing metamaterial shows +30% PL intensity achieved with a lower amount of Si-NCs, compared to an equivalent planar film without nanocylinders, resulting in a 3-fold average PL enhancement per Si-NC. The principle demonstrated here is general, and the Si-NCs can be replaced with other semiconductor quantum dots, rare-earth ions, or organic molecules. Similarly, the dielectric medium can be adjusted on purpose. This spectral selectivity of absorption paves the way for an effective light down-conversion scheme to increase the efficiency of solar cells. We envision the use of this hierarchical design for other efficient photovoltaic, photocatalytic, and artificial photosynthetic devices with spectrally selective absorption and enhanced efficiency.

Energy exchange between closely packed semiconductor quantum dots allows for long-range transfer of electronic energy and enables new functionalities of nanostructured materials with a huge application potential in photonics, optoelectronics, and photovoltaics. This is illustrated by impressive advances of quantum-dot solids based on nanocrystals (NCs) of direct bandgap materials, where this effect has been firmly established. Regretfully, the (resonant) energy transfer in close-packed ensembles of NCs remains elusive for siliconthe main material for electronic and photovoltaic industries. This is the subject of the present study in which we conclusively demonstrate this process taking place in dense dispersions of Si NCs in an SiO2 matrix. Using samples with different NC configurations, we can directly determine the wavelength dependent energy transfer rate and show that it (i) can be modulated by material parameters, and (ii) decreases with the NCs size, and thus being consistent with the energy flow proceeding from smaller to larger NCs. This result opens the way to new applications of Si NCs, requiring energy transport and extraction. In particular, it forms a fundamental step toward development of an excitonic all-Si solar cell, operating in some analogy to polymer devices.

Si nanocrystals (NCs) are often prepared by thermal annealing of multiple stacks of alternating sub-stoichiometric SiOx and SiO2 nanolayers. It is frequently claimed that in these structures, the NC diameter can be predefined by the thickness of the SiOx layer, while the NC concentration is independently controlled by the stoichiometry parameter x. However, several detailed structural investigations report that the NC size confinement to within the thickness of the SiOx layer is not strictly obeyed. In this study we address these contradicting findings: based on cross-correlation between structural and optical characterization of NCs grown in a series of purposefully prepared samples of different stoichiometry and layer thickness, we develop a comprehensive understanding of NC formation by Si precipitation in multinanolayer structures. We argue that the narrow NC size distribution generally observed in these materials appears due to reduction of the Si diffusion range, imposed by the SiO2 spacer layer. Therefore, both the SiOx layer thickness and composition as well as the actual thickness of the SiO2 spacer play an essential role in the NC formation.

Carrier multiplication (CM) is an interesting fundamental phenomenon with application potential in optoelectronics and photovoltaics, and it has been shown to be promoted by quantum confinement effects in nanostructures. However, mostly due to the short lifetimes of additional electron–hole (e-h) pairs generated by CM, major improvements of quantum dot devices that exploit CM are limited. Here we investigate CM in SiO2 solid state dispersions of phosphorus and boron codoped Si nanocrystals (NCs): an exotic variant of Si NCs whose photoluminescence (PL) emission energy, the optical bandgap, is significantly red-shifted in comparison to undoped Si NCs. By combining the results obtained by ultrafast induced absorption (IA) with PL quantum yield (PL QY) measurements, we demonstrate CM with a long (around 100 μs) lifetime of the additional e-h pairs created by the process, similar as previously reported for undoped Si NCs, but with a significantly lower CM threshold energy. This constitutes a significant step toward the practical implementation of Si-based NCs in optoelectronic devices: we demonstrate efficient CM at the energy bandgap optimal for photovoltaic conversion.

We investigated experimentally optical generation of electronhole pairs in layers of silicon nanocrystals co-doped with phosphor and boron, dispersed in a solid-state matrix of silicon dioxide. The study was performed at room temperature. From the red-shift of the photoluminescence spectrum and the enhanced absorption at low energies appearing upon doping, the formation of additional levels inside the bandgap has been confirmed. By comparing the transient induced absorption at two excitation energies, below and well above twice the emission energy, the evidence of carrier multiplication in co-doped silicon nanocrystals has been obtained for the first time.Effect of doping on optical characteristics of silicon nanocrystals

Perovskite nanocrystals (NCs) have seen unprecedented interest in recent years for their numerous applications in optoelectronic devices. Out of all-inorganic perovskites, CsPbI3 NCs possess a bandgap closest to the ideal...

Past research has shown that indirect excitation of Er 3þ ions in SiO 2 solid-state matrix with Si nanocrystals can be achieved by different pathways. Here, we investigate the impact excitation mechanisms in detail by means of time-resolved photoluminescence spectroscopy. We explicitly demonstrate that the free carrier impact excitation mechanism is activated as soon as the carriers obtain sufficient excess energy. The "hot" carriers with the above-threshold energy can be created upon optical pumping in two ways: either upon absorption of (i) a single photon with an energy exceeding a certain threshold h > Eth or (ii) following absorption of multiple photons of lower energy in a single nanocrystal, h < Eth, followed by an Auger recombination of the generated multiple e-h pairs. In addition, we show that the impact excitation dynamics by hot carriers are similar, regardless of the mode in which they have been created.