Ternary I-III-VI 2 semiconductor nanocrystals (NCs), such as CuInS 2 , are receiving growing attention as they offer the possibility to overcome the toxicity concerns related to heavy metals for numerous technologies spanning from solar cells, luminescent solar concentrators (LSCs) and artificial lighting to bioimaging. Despite the intense research activity, the fundamental mechanisms underpinning the optical properties of CuInS 2 NCs are still not fully understood. Studies suggest that the characteristic Stokes-shifted and long-lived luminescence arises from radiative decay of conduction band electrons to copperrelated defects that are particularly abundant in non-stoichiometric NCs or into a strongly localized HOMO based on Cu(3d) states. However, a recent theoretical model points to a further phenomenon; namely the detailed structure and odd-even parity states of the valence band. Crucially, this model, which has not been experimentally validated, predicts a distinctive optical behaviour in defect-free NCs: the quadratic dependence of both the radiative decay rate and the Stokes shift on the NC radius. If this origin was confirmed, this would have crucial implications for LSC devices as the large solar spectral coverage ensured by low bandgap (large size) NCs would come with a cost in terms of increased reabsorption of the guided near-IR luminescence. Here, we test this hypothesis by studying stoichiometric CuInS 2 NCs of varying sizes. Data reveal, for the first time, the spectroscopic signatures theoretically predicted for the free band edge exciton of I-III-VI 2 NCs, thus providing experimental support to the valence-band structure model. At very low temperatures the same NCs also show dynamic signatures of dark-state emission likely originating from enhanced electron-hole spin interaction. We then evaluated the trade-off between the enhanced solar harvesting of large NCs and their progressively smaller Δ SS on the efficiency of LSCs by performing Monte Carlo ray tracing simulations based on the experimental data that provided useful guidelines for the design of efficient LSCs based on stoichiometric CuInS 2 NCs. Finally, based on such theoretical insights, we fabricated largearea plastic LSC devices showing optical grade quality and an optical power efficiency as high as 6.8%, corresponding to the highest value reported to date for large-area LSC devices.

Electronic doping of colloidal semiconductor nanostructures holds promise for future device concepts in optoelectronic and spin-based technologies. Ag is an emerging electronic dopant in III-V and II-VI nanostructures, introducing intragap electronic states optically coupled to the host conduction band. With its full 4d shell Ag is nonmagnetic, and the dopant-related luminescence is ascribed to decay of the conduction-band electron following transfer of the photoexcited hole to Ag. This optical activation process and the associated modification of the electronic configuration of Ag remain unclear. Here, we trace a comprehensive picture of the excitonic process in Ag-doped CdSe nanocrystals and demonstrate that, in contrast to expectations, capture of the photohole leads to conversion of Ag to paramagnetic Ag. The process of exciton recombination is thus inextricably tied to photoinduced magnetism. Accordingly, we observe strong optically activated magnetism and diluted magnetic semiconductor behaviour, demonstrating that optically switchable magnetic nanomaterials can be obtained by exploiting excitonic processes involving nonmagnetic impurities.

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Semiconducting nanocrystals optically active in the infrared region of the electromagnetic spectrum enable exciting avenues in fundamental research and novel applications compatible with the infrared transparency windows of biosystems such as chemical and biological optical sensing, including nanoscale thermometry. In this context, quantum dots (QDs) with double color emission may represent ultra-accurate and self-calibrating nanosystems. We present the synthesis of giant core/shell/shell asymmetric QDs having a PbS/CdS zinc blende (Zb)/CdS wurtzite (Wz) structure with double color emission close to the near-infrared (NIR) region. We show that the double emission depends on the excitation condition and analyze the electron-hole distribution responsible for the independent and simultaneous radiative exciton recombination in the PbS core and in the CdS Wz shell, respectively. These results highlight the importance of the driving force leading to preferential crystal growth in asymmetric QDs, and provide a pathway for the rational control of the synthesis of double color emitting giant QDs, leading to the effective exploitation of visible/NIR transparency windows.

Two-color emitting colloidal semiconductor nanocrystals (NCs) are of interest for applications in multimodal imaging, sensing, lighting, and integrated photonics. Dual color emission from core- and shell-related optical transitions has been recently obtained using so-called dot-in-bulk (DiB) CdSe/CdS NCs comprising a quantum-confined CdSe core embedded into an ultrathick (∼7-9 nm) CdS shell. The physical mechanism underlying this behavior is still under debate. While a large shell volume appears to be a necessary condition for dual emission, comparison between various types of thick-shell CdSe/CdS NCs indicates a critical role of the interface "sharpness" and the presence of potential barriers. To elucidate the effect of the interface morphology on the dual emission, we perform side-by-side studies of CdSe/CdS DiB-NCs with nominally identical core and shell dimensions but different structural properties of the core/shell interface arising from the crystal structure of the starting CdSe cores (zincblende vs wurtzite). While both structures exhibit dual emission under comparable pump intensities, NCs with a zincblende core show a faster growth of shell luminescence with excitation fluence and a more readily realized regime of amplified spontaneous emission (ASE) even under "slow" nanosecond excitation. These distinctions can be linked to the structure of the core/shell interface: NCs grown from the zincblende cores contain a ∼3.5 nm thick zincblende CdS interlayer, which separates the core from the wurtzite CdS shell and creates a potential barrier for photoexcited shell holes inhibiting their relaxation into the core. This helps maintain a higher population of shell states and simplifies the realization of dual emission and ASE involving shell-based optical transitions.

The nature of the transient species leading to emission from the spin/orbital-forbidden Mn d−d transition in doped semiconductor quantum dots has intrigued scientists for a long time. This understanding is important in the quest for energy efficiency as the energy from the conduction band is transferred efficiently to Mn in the femtosecond time scale overcoming other nonradiative recombination pathways. In this work, we have shown the presence of the transient species using materials with band gaps in resonance with the energy of the Mn emission to understand the nature of the absorbing, transient, and emitting species. Detailed studies lead to the emergence of a transient Mn 3+ state that is further corroborated with spin-dependent density functional theory calculations. This opens up a unique opportunity to realize a reversible photochemical reaction and high radiative efficiency in a semiconductor nanostructure by controlling the spin state of the magnetic ion by external illumination.

Light-driven multi-charge accumulation (i.e., photodoping) of doped metal oxide nanocrystals opens the way to innovative solutions for the direct conversion and storage of the solar energy.

2D semiconducting nanoplatelets (NPLs) are an emerging class of photoactive materials. They can be used as building blocks in optoelectronic devices thanks to their large absorption coefficient, high carrier mobility, and unique thickness‐dependent optical transitions. The main drawback of NPLs is their large lateral size, which results in unfavorable band energy levels and low quantum yield (QY). Here, ultrasmall lead chalcogenide PbSe1–xSx NPLs are prepared, which exhibit an unprecedented QY of ≈60%, the highest ever reported for this structure. The NPLs are applied as light absorber in a photoelectrochemical system, leading to a saturated photocurrent density of ≈5.0 mA cm−2 (44 mL cm−2 d−1), which is a record for NPL‐based photoelectrodes in solar‐driven hydrogen generation. Ultrasmall NPLs hold the potential for breakthrough developments in the field of optically active nanomaterials.