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Metal oxide nanocrystals are emerging as an extremely versatile material for addressing many of the current challenging demands of energy-conversion technology. Being able to exploit their full potential is not only an advantage but also a scientific and economic ambition for a more sustainable energy development. In this direction, the photodoping of metal oxide nanocrystals is a very notable process that allows accumulating multiple charge carriers per nanocrystal after light absorption. The reactivity of the photodoped electrons is currently the subject of an intense study. In this context, the possibility to extract efficiently the stored electrons could be beneficial for numerous processes, from photoconversion and sunlight energy storage to photocatalysis and photoelectrochemistry. In this work we provide, via oxidative titration and optical spectroscopy, evidence for multi-electron transfer processes from photodoped Sn : In2O3 nanocrystals to a widely employed organic electron acceptor (F4TCNQ). The results of this study disclose the potential of photodoped electrons to drive chemical reactions involving more than one electron., Photodoped ITO nanocrystals (NCs) enables multi-electron transfer processes unveiled by F4TCNQ titration analysis. Metal oxide NCs are emerging as extremely versatile materials for energy storage and conversion technologies.

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 (VS) that introduce a donor state below the conduction band (CB), resulting in long-lived intragap luminescence. VS-localized electrons are almost unaffected by trapping, and suppression of thermal quenching boosts the emission efficiency to 85%. Magneto-optical measurements indicate that the VS 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.

In the research field on materials for energy solutions, the exploitation of organic compounds and polymers is of great interest, especially in view of a more eco-sustainable development. In this context, the 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) molecule actively participates in the optimization of manyoptoelectronic devicesbased on such elements, thanks to its electrical properties. In this paper, we report a study on its optical properties, in particular demonstrating fluorescence emission from solutions containing F4TCNQ. This phenomenon is equally relevant, together with the effects on electricalconductivity, in the design of systems for energy technology. The results we present here offer new insights in the phenomenological picture of this molecule and can sharpen its applicability.

"Charge engineering" of semiconductor nanocrystals (NCs) through so-called electronic impurity doping is a long-standing challenge in colloidal chemistry and holds promise for ground-breaking advancements in many optoelectronic, photonic, and spin-based nanotechnologies. To date, our knowledge is limited to a few paradigmatic studies on a small number of model compounds and doping conditions, with important electronic dopants still unexplored in nanoscale systems. Equally importantly, fine-tuning of charge engineered NCs is hampered by the statistical limitations of traditional approaches. The resulting intrinsic doping inhomogeneity restricts fundamental studies to statistically averaged behaviors and complicates the realization of advanced device concepts based on their advantageous functionalities. Here we aim to address these issues by realizing the first example of II-VI NCs electronically doped with an exact number of heterovalent gold atoms, a known p-type acceptor impurity in bulk chalcogenides. Single-dopant accuracy across entire NC ensembles is obtained through a novel non-injection synthesis employing ligand-exchanged gold clusters as "quantized" dopant sources to seed the nucleation of CdSe NCs in organic media. Structural, spectroscopic, and magneto-optical investigations trace a comprehensive picture of the physical processes resulting from the exact doping level of the NCs. Gold atoms, doped here for the first time into II-VI NCs, are found to incorporate as nonmagnetic Au + species activating intense size-tunable intragap photoluminescence and artificially offsetting the hole occupancy of valence band states. Fundamentally, the transient conversion of Au + to paramagnetic Au 2+ (5d 9 configuration) under optical excitation results in strong photoinduced magnetism and diluted magnetic semiconductor behavior revealing the contribution of individual paramagnetic impurities to the macroscopic magnetism of the NCs. Altogether, our results demonstrate a new chemical approach toward NCs with physical functionalities tailored to the single impurity level and offer a versatile platform for future investigations and device exploitation of individual and collective impurity processes in quantum confined structures.

A wide variety of materials with nanometre dimensions are increasingly explored for photonic applications. Among them, semiconductor nanocrystals (NCs) are very promising for a variety of uses, including light emission devices (LEDs), lasers, detectors, photovoltaic cells, biological labelling and sensing [1]. Key advantage of NCs is the possibility to tailor their optical response by controlling the electronic structure (“wave function engineering”) through the choice of composition, size and shape. Significant and interesting results have been obtained with heterostructured and doped NCs. Beyond single wavelength tuneable band-edge emission, other regimes have been demonstrated such as intragap emission, simultaneous emission on two different wavelengths, amplified spontaneous emission and laser emission. The luminescent properties are governed by exciton decay, which can proceed through radiative or nonradiative pathways, following different routes. The study of exciton dynamics can allow elucidating the processes connected to single or dual emission and to optical gain. This, in turn, can lead to the identification of the functional and structural characteristics that are responsible for these behaviors. Exciton relaxation occurs on picosecond timescales, so ultrafast optical techniques are required to perform these studies. In this talk, we present studies carried out by ultrafast pump-probe spectroscopy technique, with 100-fs time resolution, on CdSe/CdS and PbS/CdS heterostructured NCs, with different geometries (core/shell, dot-in-rod, dot-in-bulk, with sharp or graded interface) [2-6] and CdSeS and CdZnSe doped NCs [7,8]. These NCs are optically active in the visible and near-infrared spectral region, show single and dual colour photoluminescence emission, optical gain, laser emission and intragap emission [2-9]. The analysis of the experimental data allowed us to unravel the decay processes: the initials take place in a few ps, leading to the ultimate emitting state whose lifetime can extend to hundreds of ps to few ns, allowing for efficient luminescence and optical gain. Our data on heterostructures allowed us to clarify the role of the volume and of the shape of the outer component and the effect of the interface [2-4]. We found that dual emission is possible for both thick and thin quantum-confined shells, and for different interfaces. We studied the decoupling of excitons lying in the two different component of the NC (core exciton and shell exciton) and we revealed the evolution of the exciton barrier known as dynamic hole-blockade effect. We showed that these phenomena are strictly connected to dual emission and optical gain and we identified the condition for their maximum efficiency, in term of band alignment and band transitions. Our results provide a comprehensive understanding of the physical phenomena governing dual-emission mechanisms, suppression of Auger recombination, optical gain and laser emission in heterostructured NCs. Experiments on CdZnSe NCs doped with Mn and on CdSeS NCs engineered with sulfur vacancies, enabled us to disclose donor and acceptor localized states in the band gap. We observed the carrier dynamics responsible for intragap emission which is associated to the emergence of a transient Mn3+ state [7], in the first case, and to a donor state below the conduction band introduced by sulfur vacancies [8], in the latter case. In conclusion, the study of the exciton dynamics in different NCs allowed us to elucidate the relation between structural-morphological characteristics (shape, volume, and interface) and unconventional emission capabilities (dual emission and optical gain) in heterostructures and the photophysics of electronic states introduced by doping. This knowledge is very important to control NC functionalities toward new multilevel electronic or photonic schemes and in applications such as lasers [9], photoelectrochemical (PEC) cell [10], white light emission [11], ratiometric sensing [12]. [1] P. V. Kamat and G. D. Scholes, J. Phys. Chem. Lett. 7, 584 (2016) [2] G. Sirigu et al., Phys. Rev. B 96, 155303 (2017) [3] V. Pinchetti et al., ACS Nano 10, 6877-6887 (2016) [4] H. Zhao et al., Nanoscale 8, 4217-4226 (2016) [5] M. Zavelani-Rossi et al., Nano Lett. 10, 3142-3150 (2010) [6] R. Krahne et al., Appl. Phys. Lett. 98, 063105 (2011) [7] K. Gahlot et al., ACS Energy Lett. 4, 729−735 (2019) [8] F. Carulli et al., Nano Lett. 21, 6211−6219 (2021) [9] M. Zavelani-Rossi et al., Laser & Photonics Reviews 6, 678-683 (2012) [10] L. Jin et al., Nano Energy 30, 531-541 (2016) [11] S. Sapra et al., Adv. Mater. 19, 569 (2007) [12] J. Liu et al., ACS Photonics, 2479 (2019)

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.

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 Validerad;2021;Nivå 2;2021-11-29 (johcin);Artikeln har tidigare förekommit som manuskript i avhandling

Ternary I-III-VI 2 nanocrystals (NCs), such as AgInS 2 and CuInS 2 , are garnering interest as heavy-metal-free materials for photovoltaics, luminescent solar concentrators, LEDs, and bioimaging. The origin of the emission and absorption properties in this class of NCs is still a subject of debate. Recent theoretical and experimental studies revealed that the characteristic Stokes-shifted and long-lived luminescence of stoichiometric CuInS 2 NCs arises from the detailed structure of the valence band featuring two sublevels with different parity. The same valence band substructure is predicted to occur in AgInS 2 NCs, yet no experimental confirmation is available to date. Here, we use complementary spectroscopic, spectro-electrochemical, and magneto-optical investigations as a function of temperature to investigate the band structure and the excitonic recombination mechanisms in stoichiometric AgInS 2 NCs. Transient transmission measurements reveal the signatures of two subbands with opposite parity, and photoluminescence studies at cryogenic temperatures evidence a dark state emission due to enhanced exchange interaction, consistent with the behavior of stoichiometric CuInS 2 NCs. Lowering the temperature as well as applying reducing electrochemical potentials further suppress electron trapping, which represents the main nonradiative channel for exciton decay, leading to nearly 100% emission efficiency.

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 (30x30 cm(2)) 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.

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 cm(2)) surface and provides a substantial advancement toward scalable ultrathin photodetection devices based on hot-electrons technology.

Optoelectronic devices based on all-inorganic perovskite systems are an energy-efficient source of lighting due to their high photoluminescence quantum yield (QY). However, dominant surface trapping continues to plague the field, despite their high defect tolerance, as evidenced by the several-fold improvements in the external quantum efficiency of perovskite nanocrystals (NCs) upon appropriate surface passivation or physical confinement between high-band-gap materials. Here, we introduce the concept of drip-feeding of photoexcited electrons from an impurity-induced spin-forbidden state to address this major shortcoming. An increased and delayed (about several milliseconds) excitonic QY, Raman spectroscopy demonstrating specific vibrational modes of the PbX6 octahedra, and density functional theory establish the electron back-transfer, signifying an efficient recombination. We term this electron back-transfer from Mn2+ to the host conduction band in this prototypical example of Mn-doped CsPbX3 (X = Cl, Br) NCs through vibrational coupling as vibrationally assisted delayed fluorescence (VADF).

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 Mn3+ 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.

The interfacial structure in "giant" PbS/CdS quantum dots (QDs) was engineered by modulating the Cd:S molar ratio during in situ growth. The control of the gradient interfacial layer could facilitate hole transfer, regulate the transition from double- to single-color emission, as a consequence. These QDs are optically active close-to-the near-infrared (NIR) spectral region and are candidates as absorber materials in solar energy conversion. Photoinduced charge transfer from "giant" QDs to electron scavenger can still take place despite the ultra-thick (~5 nm) shell. The hybrid architecture based on a TiO2 mesoporous framework sensitized by the "giant" QDs with alloyed interface can produce a saturated photocurrent density as high as ~5.3 mA/cm2 in a photoelectrochemical (PEC) cell under 1 Sun illumination, which is around 2 times higher than that of bare PbS and core/thin-shell PbS/CdS QDs sensitizer. The as-prepared PEC device presented very good stability thanks to the "giant" core/shell QDs architecture with tailored interfacial layer and a further coating of the ZnS shell. 78% of the initial current density is kept after 2-h irradiation at 1 Sun. Engineering of electronic band structure plays a key role in boosting the functional properties of these composite systems, which hold great potential for H2 production in PEC devices.

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.

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 Ag2+. 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.

Two-dimensional transition metal dichalcogenide (MX 2 ) are receiving lot of attention in recent years due to their excellent electronic and optical properties which make them feasible for advanced photonics applications [1]. In particular Molybdenum-Disulfide (MoS 2 ) has shown large exciton binding energy, strong spin-orbit interaction, and high carrier mobilities.

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.

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 Zincblende (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 of 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 a rational control of the synthesis of double color emitting giant QDs, leading to the effective exploitation of visible/NIR transparency windows. Validerad; 2016; Nivå 2; 20160119 (jacnas)

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 (similar to 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 similar to 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.

2D semiconducting nanoplatelets (NPLs) are an emerging class of photo-active 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 approximate to 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 approximate to 5.0 mA cm(-2) (44 mL cm(-2) d(-1)), which is a record for NPL-based photo-electrodes in solar-driven hydrogen generation. Ultrasmall NPLs hold the potential for breakthrough developments in the field of optically active nanomaterials.