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We study the optical properties of Molybdenum Disulphide nanosheets deposited by chemical vapor deposition onto a nanopatterned substrate endowed with a uniaxial corrugation. The uniaxial nanocorrugation leads to a polarization dependence in steady state and an anisotropic relaxation around the C resonance. Finite element numerical simulations allow to better disclose the scenario. Our findings point towards new possibilities for the manipulation of the optical properties of two-dimensional transition metal dichalcogenides via substrate nanopatterning.

Characterizing carrier redistribution due to optical field modulation in a plasmonic hot-electron/semiconductor junction can be used to raise the framework for harnessing the carrier decay of plasmonic metals in more efficient conversion systems. In this work we comprehensively studied the carrier redistribution mechanisms of a 1-dimensional (1D) metal-semiconductor Schottky architecture, holding the dual feature of a hot-electron plasmonic system and a simple metal/semiconductor junction. We obtained a strongly enhanced external quantum efficiency (EQE) of the plasmonic Ag decorated ZnO semiconductor in both the band-edge region of ZnO and the corresponding plasmonic absorption profile of the Ag NPs (visible region). Simultaneously, the insertion of an insulating Al2O3 intermediate layer between Ag NPs and ZnO resulted in a parallel distinction of the two main non-radiative carrier transfer mechanisms of plasmonic NPs, i.e. direct electron transfer (DET) and plasmonic induced resonance energy transfer (PIRET). The multi-wavelength transient pump–probe spectroscopy indicated the very fast plasmonic radiative transfer dynamics of the system in <500 fs below 389 nm. We demonstrate a 13% increase of photogenerated current in ZnO upon visible irradiation as a result of non-radiative plasmonic hot-electron injection from Ag NPs. Overall, our device encompasses several effective solutions for designing a plasmonic system featuring non-radiative electron–electron plasmonic dephasing and high photoconversion efficiencies. [ABSTRACT FROM AUTHOR]

A cost effective method to tailor the optical response of large‐area nanosheets of 2D materials is described. A reduced effective metalayer model is introduced to capture the key‐role of the out‐of‐plane component of the dielectric tensor. Such a model indicates that the optical extinction of 2D materials can be strongly altered by controlling the geometry at the local (i.e., subwavelength) scale. In particular, a giant linear optical dichroism at normal incidence is demonstrated, with major features around the excitonic peaks, that can be tailored by acting on the average curvature and slope of the nanosheets. The approach is experimentally demonstrated in few‐layer MoS2 grown by chemical vapor deposition on cm‐scale anisotropic nanopatterned substrates prepared by a self‐assembling technique, based on defocused ion beam sputtering. Major variations in the photoluminescence spectrum as a function of the average curvature and slope are also revealed. A full‐vectorial numerical study beyond the effective metalayer model and comprising strain effects induced by the geometry turns out to be consistent with such a complex scenario. The results demonstrate that the extrinsic geometrical engineering definitely opens viable way to tailor the optical properties and modulate bandgap of low dimensional materials. [ABSTRACT FROM AUTHOR]

Hybrid plasmonic‐semiconductor assemblies are receiving considerable attention due to the possibility to achieve hot‐carrier‐based photodetection. In this context, 2D transition metal dichalcogenides (TMDs) coupled to metal nanostructures are very promising. However, the plasmon‐to‐TMD carrier injection process is extremely challenging to achieve and even to reveal in a clear‐cut way. Herein, a report of multiple transient absorption ultrafast measurements, with tunable pump excitation, enabling quantitative comparison between the ultrafast behavior of metal nanostructures, TMDs, and their assembly is shown. This allows to provide the evidence of plasmon‐enhanced charge injection from Au nanostripes to a rippled‐shaped molybdenum disulfide (MoS2) few‐layer nanosheet. Finite element method numerical simulations and modeling of the transient optical response corroborate the charge transfer mechanism, showing that the experimental data cannot be described in terms of the thermomodulational nonlinearity of gold nanostripes or by simple superposition of metal and semiconductor responses. The sample is obtained by a self‐organization process on a large area; this demonstrates that plasmon‐enhanced photon harvesting exploiting hot‐electron injection can be achieved on a large area (approximately cm2) surface and provides a substantial advancement toward scalable ultrathin photodetection devices based on hot‐electrons technology. [ABSTRACT FROM AUTHOR]

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- xS x 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. [ABSTRACT FROM AUTHOR]

Ternary I‐III‐VI2 nanocrystals (NCs), such as CuInS2, are receiving attention as heavy‐metals‐free materials for solar cells, luminescent solar concentrators (LSCs), LEDs, and bio‐imaging. The origin of the optical properties of CuInS2 NCs are however not fully understood. A recent theoretical model suggests that their characteristic Stokes‐shifted and long‐lived luminescence arises from the structure of the valence band (VB) and predicts distinctive optical behaviours in defect‐free NCs: the quadratic dependence of the radiative decay rate and the Stokes shift on the NC radius. If confirmed, this would have crucial implications for LSCs as the solar spectral coverage ensured by low‐bandgap NCs would be accompanied by increased re‐absorption losses. Here, by studying stoichiometric CuInS2 NCs, it is revealed for the first time the spectroscopic signatures predicted for the free band‐edge exciton, thus supporting the VB‐structure model. At very low temperatures, the NCs also show dark‐state emission likely originating from enhanced electron‐hole spin interaction. The impact of the observed optical behaviours on LSCs is evaluated by Monte Carlo ray‐tracing simulations. Based on the emerging device design guidelines, optical‐grade large‐area (30×30 cm2) LSCs with optical power efficiency (OPE) as high as 6.8% are fabricated, corresponding to the highest value reported to date for large‐area devices. [ABSTRACT FROM AUTHOR]

Colloidal semiconductor nanocrystals (NCs) emitting simultaneously at two distinct wavelengths are of great interest for multiple applications ranging from ultrasensitive self-calibrating nanoscale sensing to light-emitting diodes and white-light emission. The physical mechanism governing dual emission in core/shell NCs composed of two materials is still under investigation; in particular, the roles of the volume of the shell and of the core/shell interface are not well understood. Here we compare three different PbS/CdS NCs with identical cores, but shells of different thicknesses and different core/shell interfaces. We study photoluminescence (PL) characteristics and ultrafast exciton dynamics with subpicosecond time resolution. We find that dual emission is possible for both thick and thin quantum-confined shells, and for different core/shell interfaces. We show that very thick shells allow the decoupling of shell excitons from core ones and we fully describe the dynamic exciton barrier, connected to efficient shell PL. We discuss the efficiency of shell PL in relation to the properties of the NCs and show that it can give rise to optical gain. Our results provide a comprehensive understanding of the physical phenomena governing dual-emission mechanisms in NCs. [ABSTRACT FROM AUTHOR]

The combination of semiconductors and redox active molecules for light-driven energy storage systems has emerged as a powerful solution for the exploitation of solar batteries. On account of this, transparent conductive oxide (TCO) nanocrystals (NCs) demonstrated to be interesting materials, thanks to the photo-induced charge accumulation enabling light harvesting and storage. The charge transfer process after light absorption, at the base of the proper use of these semiconductors, is a key step, often resulting in non-reversible transformations of the chemicals involved. However, if considering the photocharging through TCO NCs not only as a charge provider for the system but potentially as part of the storage role, the reversible transformation of the redox compound represents a crucial aspect. In this paper, we explore the possible interaction of indium tin oxide (ITO) NCs and typical redox mediators commonly employed in catalytic applications with a twofold scope of enhancing or supporting the light-induced charge accumulation on the metal oxide NC side and controlling the reversibility of the whole process. The work presented focuses on the effect of the redox properties on the doped metal oxide response, both from the stability point of view and the photodoping performance, by monitoring the changes in the optical behavior of ITO/redox hybrid systems upon ultraviolet illumination., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Rebecchi, Rubino, Camellini and Kriegel.)

Controlled insertion of electronic states within the band gap of semiconductor nanocrystals (NCs) is a powerful tool for tuning their physical properties. One compelling example is II-VI NCs incorporating heterovalent coinage metals in which hole capture produces acceptor-bound excitons. To date, the opposite donor-bound exciton scheme has not been realized because of the unavailability of suitable donor dopants. Here, we produce a model system for donor-bound excitons in CdSeS NCs engineered with sulfur vacancies ( V S ) that introduce a donor state below the conduction band (CB), resulting in long-lived intragap luminescence. V S -localized electrons are almost unaffected by trapping, and suppression of thermal quenching boosts the emission efficiency to 85%. Magneto-optical measurements indicate that the V S are not magnetically coupled to the NC bands and that the polarization properties are determined by the spin of the valence-band photohole, whose spin flip is massively slowed down due to suppressed exchange interaction with the donor-localized electron.

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.