This lecture will span the discovery of colloidal lead halide perovskite nanocrystals (LHP NCs), as well as our latest work on their synthesis, self-organization, and optical properties, including unpublished work. LHP NCs are of broad interest as classical light sources (LED/LCD displays) and as quantum light sources (quantum sensing and imaging, quantum communication, optical quantum computing). The current development in LHP NC surface chemistry, using designer phospholipid capping ligands, allows for increased stability down to single particle level [1]. The brightness of such a quantum emitter is ultimately described by Fermi’s golden rule, where a radiative rate proportional to its oscillator strength (intrinsic emitter property) and the local density of photonic states (photonic engineering, i.e. cavity). With perovskite NCs, we present a record-low sub-100 ps radiative decay time for CsPb(Br/Cl)₃, almost as short as the reported exciton coherence time, by the NC size increase to 30 nm [2]. The characteristic dependence of radiative rates on QD size, composition, and temperature suggests the formation of giant transition dipoles, as confirmed by effective-mass calculations for the case of the giant oscillator strength. Importantly, the fast radiative rate is achieved along with the single-photon emission despite the NC size being ten times larger than the exciton Bohr radius. When such bright and coherent QDs are assembled into superlattices, collective properties emerge, such as superradiant emission from the inter-NC coupling [3,4].

Colloidal quantum dots (CQDs) that absorb and emit in the short-wave infrared (SWIR, 0.9–1.7 μm) region are critically important in optoelectronics (e.g., SWIR-based LEDs, lasers, photodetectors, telecommunication) and biological imaging. However, SWIR CQD LEDs often underperform due to the low photoluminescence quantum yield (PLQY) of the QDs. Furthermore, many efficient SWIR active QDs demonstrated in application are based on heavy metals such as lead (Pb), cadmium (Cd), and mercury (Hg), which are highly toxic and subject to RoHS (Restriction of Hazardous Substances) regulatory restrictions for consumer electronics applications. This emphasizes the critical need for the development of more SWIR-efficient, environmentally friendly QDs to replace conventional Cd/Pb/Hg-based QDs in various applications.

In my presentation, I will explore the potential of I-III-VI-based nanocrystals, particularly Cu/Ag-In-Se, as eco-friendly alternatives to toxic heavy metal-based QDs. Specifically, I will discuss the development of Cu-In-Zn-Se/ZnS (CIZSe-ZnS) core-shell QDs that emit in the SWIR range. I will detail our synthetic methodologies that enable precise modulation of composition and size tunability, facilitating targeted monitoring of PL emission over a wide range from 915 nm to 1230 nm. To enhance the biocompatibility and chemical stability of the material, we passivated the QDs’ surfaces with amorphous alumina (CIZSe/ZnS/Al₂O₃). This surface passivation not only ensures environmental and photostability but also enhances the PLQY. Notably, we achieved a record PLQY of 53% at 1050 nm and 20% at 1230 nm, the highest reported to date from heavy metal-free QDs. Unlike other indium-based multinary core-shell QDs (e.g., CuInS₂/ZnS), these nanocrystals exhibit a narrow PL full width at half maximum (FWHM) of 102 meV. Finally, I will demonstrate the application of these QDs as efficient SWIR-LEDs, underscoring their practical utility and potential for advancing optoelectronic technologies.

Lead halide perovskites are promising candidates for high-performance light-emitting diodes (LEDs); however, their applicability is limited by their structural instability toward moisture. Although a deliberate addition of water to the precursor solution has recently been shown to improve the crystallinity and optical properties of perovskites, the corresponding thin films still do not exhibit a near-unity quantum yield.[1], [2] Herein, we report that the direct addition of a minute amount of water to post-treated formamidinium lead bromide (FAPbBr₃) nanocrystals (NCs) substantially enhances the stability while achieving a 95% photoluminescence quantum yield in a NC thin film. We unveil the mechanism of how moisture assists in the formation of an additional NH₄Br component. Alongside, we demonstrate the crucial role of moisture in assisting localized etching of the perovskite crystal, facilitating the partial incorporation of NH₄⁺, which is key for improved performance under ambient conditions. Finally, as a proof-of-concept, the application of post-treated and water-treated perovskites is tested in LEDs, with the latter exhibiting a superior performance, offering opportunities toward commercial application in moisture-stable optoelectronics.

Halide perovskite semiconductors can merge the highly efficient operational principles of conventional inorganic semiconductors with the low‑temperature solution processability of emerging organic and hybrid materials, offering a promising route towards cheaply generating electricity as well as light. Following a surge of interest in this class of materials, research on halide perovskite nanocrystals (NCs) has gathered momentum in the last decade. This talk will highlight several findings of our group on their synthesis, for example our recent study on the influence of various exogenous cations and acid-based equilibria on the growth of perovskite NCs, and the preparation of NCs in the strong quantum confinement regime. Superstructures of perovskite nanocrystals are also capturing the attention of the community, for example for what concerns their superfluorescence. I will discuss our findings on the ordering of nanocrystals in superstructures, especially on how this is related to the specific surface ligands and how it influences exciton diffusion processes.

The discovery of 2D materials based on transition metal dichalcogenides (TMDs), has opened up new interesting possibilities in optoelectronic devices, as monolayer TMDs possess direct bandgaps with absorption in the visible to near-infrared (NIR) spectral region. However, monolayer TMDs often exhibit poor photoluminescence quantum yields (PLQEs) and mobilities, which are signs of poor-quality semiconductor material. While there have been advances in materials growth in the past years, our understating of defects and how they degrade performance is still unsatisfactory. Thus, while many defect passivation strategies have been discussed in the literatures, most achieve only moderate PL enhancement. No chemical treatment has yet been able to significantly enhance both the PL and electrical mobility of 2D TMDs. [1]

Here, I will present new chemical functionalization approaches to greatly enhance the PL intensity of mechanically exfoliated monolayer molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂), while simultaneously enhancing the charge and exciton transport properties. [2,3] We propose an atomic-level synergistic defect passivation mechanism of both neutral and charged sulfur vacancies (SVs), supported by ultrafast transient absorption spectroscopy (TA), Hard X-ray photoelectron spectroscopy (HAXPES), and density functional theory (DFT) calculations. In addition, these non-corrosive chemicals are stable and operate in benign solvents under ambient conditions, making them sustainable and suitable for direct use during device fabrication of TMDs. Our findings establish a new performance benchmark for the optical and electronic properties of WS₂ monolayers, paving the way for developing sustainable 2D semiconductor technologies.

Recent developments in the synthesis of colloidal semiconductor nanocrystals (NCs), also known as quantum dots (QDs), have led to an excellent control over their size and shape, often with a (nearly) atomic precision.[1-3] This level of control turned out to be the key to their practical applications in technologies that require precise control over the properties of emitted light, such as its energy, color purity, polarization, directionality, etc.[4,5] In contrast to the great experimental achievements in the colloidal synthesis and characterization techniques, little remains known about the detailed atomistic structure of colloidal QDs, especially the structure of the nanocrystal-ligand interface, which is paramount for controlling their size and shape, optical properties and environmental stability. Focusing on CsPbBr₃ NCs as a representative of a recently discovered family of ionic lead halide perovskite QDs, we will present the first computational investigation of the equilibrium structures of realistically sized (≈4 nm) QDs using large-scale classical force-field molecular dynamics (MD) simulations in explicit solvent. The NCs are predicted to have an inherently CsBr-rich composition and equilibrium nearly cubic shape with the main facets of the {100}_p type (with respect to the primitive unit cell), whereas the analogous PbBr₂-terminated nanocubes are found to be unstable and phase separating with the formation of PbBr₂-rich material. These results agree with previous experimental observations and the fact that nanocube is the most frequently encountered shape for CsPbBr₃ NCs reported in the literature.[6,7] Exploration of the entire phase diagram in terms of NC composition and size further allowed us to shed light on the influence of composition on the NC shape, revealing the presence and structures of the minor {111}_p and {110}_p facets that cause truncation and chamfering of the nanocubes at low and high contents of CsBr. The structures and relative occurrence of different crystallographic facets are rationalized using the concept of nonpolar and polar crystal surfaces. Finally, a preferential binding of organic ligands to the different crystallographic facets and its effect on the NC shape will be discussed. The generated ensembles of representative NC structures will serve as a basis for further investigations of structure-property relationships in these nanomaterials, in particular the influence of surface chemistry and structural defects on the optical properties of the NCs.

Over the past three decades, significant progress has been made in developing colloidal quantum dots (QDs) as efficient and stable light-generating materials. The primary design principle for high quantum yield (QY) of QDs has been a core/shell heterostructure leveraging wide band gap semiconductors to passivate surface states and protect against degradation. Recent advancements have led to high photoluminescence QY for various core/shell heterostructures made of II-VI and III-V compounds. However, in optoelectronic applications, variations and unexpected outcomes in device efficiency, brightness, and operational stability persist, even with high-quality QDs. The absence of standardized protocol for core/shell heterostructures implies that the heteroepitaxial chemistry of QDs remains unclear and is prone to be tarnished by uncontrolled factors.

         Herein, we introduce our recent efforts to elucidate the vailed chemistry in core/shell heterostructures, focusing on the surface chemistry of reactants and the formation of crystalline shells. We investigated InP/ZnSe core/shell QDs synthesized using zinc carboxylate and trialkylphosphine selenide, chosen for their widespread use. Nuclear magnetic resonance spectroscopy revealed that sterically hindered acyloxytrialkylphosphonium and diacyloxytrialkylphosphorane are the main intermediates in the surface reaction of precursors, and their transformation to the coherent crystalline layer is likely to be hindered by surface oxides. Although precise control of the shell growth protocol achieved a near-unity PL QY of 97.3% with a single ZnS epilayer (~0.3 nm), increasing shell thickness deteriorated the luminescent property of QDs.[1] This phenomenon has been understood as a formation of mifit dislocation by the overgrown shell, releasing lattice strain. However, our spectroscopic study on thick-shell QDs uncovered the formation of zinc vacancy during the shell growth.[2] Although the compressive strain on the core deactivates their involvement to the exciton recombination, such defects become apparent at low temperature or high energy excitation, leaving photogenerated hole in the zinc vacancy. Our findings suggest that the PL QY alone cannot determine the success of core/shell QDs, and a deepter understanding is essential for advancing QD-based high performance optoelectronic applications.

Lithium-ion technology is leading the market of energy storage. Direct monitoring of free lithium ions is critical for safety and improved efficiency. We report for the first-time a synthesis of emissive nanocrystals that are sensitive to lithium-ion concentration in their surroundings. These double perovskite nanocrystals with compositions of Cs₂Li_(1-x)Na_(x)InCl₆ can serve as such indicators due to their broad emission spectrum in the visible range and facile cation exchange schemes. Using nanocrystals, we can synthesize a stable cubic phase (with a band gap of 2.78eV).  However, for bulk Cs₂LiInCl₆, this phase was not reported, but a trigonal phase (with a band gap of 3.23eV) is known.

Here, we demonstrate two strategies for stabilizing the emissive cubic phase.

First, we employ the well-documented size-stabilizing effect for colloidal nanocrystals, which asserts that a high surface-to-volume ratio will have a stabilizing surface effect due to the passivating of organic surface ligands. In our case, we can control the size by varying the reaction temperature.

Second, we show another stabilizing mechanism for the cubic phase by alloying between the B site cation (Li and Na). This alloying is possible with both direct synthesis and post-synthesis treatment upon exposure to a free Li or Na ions environment. The evident effect of alloying of the B site, which directly corresponds to a shift in the emission wavelength, suggests that these materials and mechanisms could be used as indicators for monitoring Li-ion content in their surroundings.

We report a colloidal synthesis for nanoparticles of Cs₂LiInCl₆ with a narrow size distribution of ~10nm with access to all Na-Li alloy ratios. The structure and size of the nanocrystals are determined by X-ray diffraction (XRD), a synchrotron high-resolution powder XRD (HRPXRD), and an atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). We show topotactical stability of the cubic phase (100%Na) to the incorporation of Li ions, which does not change the cubic double perovskite structure for the nanoparticles, while for bulk materials at these Li concentrations, only the trigonal phase is stable. We confirm the composition of our resulting NCs via inductively coupled plasma mass spectrometry (ICP-MS), time-of-flight ion mass spectroscopy (TOF-SIMS), and electron energy loss spectroscopy (EELS).

The intrinsic cubic phase is emissive; however, it is still low, and its emission can be further enhanced. The PLQY of many double perovskites was shown to increase with the transfer of free exciton into self-trapped exciton propagated by doping. In previous studies, Sb-doping in Cs₂NaInCl₆ double perovskite was highlighted as a candidate for notable PLQY and controlled emission wavelength through B-site alloying (Shaek et al., 2023). We hypothesize that this doping scheme is also optimal for our Cs₂LiInCl₆ NCs.

The vision is to allow direct detection and monitoring of Li-ion content in energy storage technologies through visible changes of a coupled NCs-based passive indicator.

Two-dimensional (2D) materials have been extensively investigated for the last 20 years. 2D transition metal dichalcogenides (TMDCs) are leading materials due to their extraordinary performance in various applications like optoelectronics, sensing, catalysis and energy etc. The 2D-TMDCs offer advantages such as ultrathin thickness, tuneable band gaps, high surface-to-volume ratio, high current on/off ratio (10⁸), flexibility, high mobility, and polymorphism, which make them ideal for nanoelectronics and optoelectronics. [1]  Semiconducting 2D-TMDCs like WS₂ and MoS₂ have direct band gaps in the visible range, making them highly light-active materials. Despite these advantages, 2D-TMDCs still have limitations in terms of their large-area uniform growth, layer number control, site-specific integration, contamination-free heterostructure formation, etc. The properties of 2D materials are highly influenced by the presence of defects which makes atomic manipulation of defects important.

The 2D materials market is expected to expand from USD 2.71 billion in 2024 to USD 3.62 billion by 2032, with a compound annual growth rate (CAGR) of 3.69% during the forecast period (2024 - 2032). Graphene-based products have already been introduced in the market and are winning the hearts of customers for being strong and lightweight with many intriguing properties. 2D- TMDCs (WS₂ and MoS₂) are next in the line for the industrial trials. For nanoscale material to be introduced into the market, its large-scale fabrication is a bottleneck. While defects such as vacancies and micro-cracks are useful for certain applications, cracks/voids are undesirable and lead to the degradation of performance when used in electronic devices. [2] Defect engineering is extremely important to control the properties and hence the performance of the 2D-TMDCs.

This talk offers a comprehensive overview of the latest advancements in the synthesis, characterization, and application of 2D light-emitting materials, with a particular emphasis on WS₂. We will delve into various synthesis techniques, focusing on chemical vapor deposition (CVD) and gold-assisted mechanical exfoliation. [3] Additionally, the impact of chemical treatments on the photoluminescence properties of 2D-TMDCs will be presented. The discussion will also address the challenges and future directions in the field, including the scalability of production methods and the integration of 2D light-emitting materials with existing semiconductor technologies. This talk aims to provide a foundational understanding of 2D semiconducting light-emitting materials, highlighting their potential and guiding future research and development.

The past decade has seen the discovery and the rapid development of colloidal lead halide perovskite nanocrystals (LHP NCs) of APbX₃ stoichiometry. These materials have captured broad interest due to their straightforward synthesis and outstanding optical properties. The compositional engineering of LHP NCs via the A-site cation  (Cs⁺, formamidinium, and methylammonium) represents a lever to fine-tune their structural and electronic properties. Inspired by recent reports on bulk single crystals with aziridinium (AZ) as the A-site cation, we present a facile colloidal synthesis of AZPbBr₃ NCs with a narrow size distribution and size tunability down to 4 nm, producing quantum dots in the regime of strong quantum confinement with bright photoluminescence and quantum efficiencies of up to 80% [1].

LHP NCs are also attractive blocks for creating controlled NC self-assembly with collective luminescence phenomena, such as superfluorescence. We reported a broad structural diversity in multicomponent, long-range ordered superlattices (SLs) comprising highly luminescent cubic CsPbBr₃ NCs (and FAPbBr₃ NCs) co-assembled with the spherical, truncated cuboid, and disk-shaped NC building blocks such as Fe₃O₄, PbS, NaGdF_4, and LaF₃ NCs [2,3]. These mesostructures also exhibit superfluorescence, characterized at high excitation density, by emission pulses with ultrafast radiative decay. The formation of such SLs was rationalized using entropy-maximization arguments and ligand-deformability. In the multicomponent LHP NC-only SLs comprising CsPbBr₃ NCs of different sizes as building blocks, efficient NC coupling and Förster-like energy transfer from strongly confined 5.3 nm CsPbBr₃ NCs to weakly confined 17.6 nm CsPbBr₃ NCs were observed [4]. The presentation will extend to the most recent work, wherein NCs are co-assembled with molecular entities.

Colloidal quantum dots (CQDs) have attracted considerable attention due to their excellent optoelectronic properties, such as tunable band gap and high optical stability. In the visible regime, CQDs materials have already been successfully employed in optoelectronic devices. On the other hand, the burst of near infrared (NIR) technologies such as detectors, face recognition, food monitoring, and telecommunication are prerequisite materials emitting at the low energy part of the electromagnetic wavelength. Nevertheless, despite the plethora of semiconductors in visible region, only a few examples exist with a tunable band gap in NIR with lead sulphide (PbS) semiconductor CQDs taking the lead due to the high spectral tunability (500-3000nm). Therefore, here we focus on the synthesis of shell engineered  PbS CQDs emitting at telecommunication wavelengths (1500-1620nm) for lasing applications. The high degeneracy of PbS (8-fold) is the main bottleneck for the realization of low-threshold lasing due to the Auger limited gain. Hence, we synthesised a series of core/shell PbS/CdS CQDs with suppressed Auger rates and tunable band-edge absorption across the telecom spectral window. The epitaxial growth of the CdS shell was achieved via cation exchange reaction, producing CQDs of high optical stability, narrow size distribution and low trap state density, reaching Auger lifetimes up to 320 ps.

The pursuit for innovative materials with vibrant, enduring colors resistant to chemical or photobleaching is a critical aim in industries like paints, cosmetics, and displays. This goal has gained importance due to continuous technological progress and to this end nature serves as a significant source of inspiration, showcasing a variety of optical effects developed through evolution, such as the brilliant iridescence of bird feathers and the adaptive colors of camouflaging chameleons. These effects often stem from complex internal structures at the nano- and microscale, resulting in unique light-matter interactions. Particularly intriguing is the structural coloration produced by periodic arrangements of two or more dielectric materials with different refractive indices, where the lattice dimensions are comparable to the wavelength of visible and near-infrared radiation. These photonic materials hold the potential for significant societal benefits due to their light-manipulating capabilities, which are anticipated to drive multiple technological breakthroughs. In this framework we present the fabrication of hybrid organic-inorganic photonic microparticles with unique functionalities achieved through the three-dimensional confined self-assembly of block copolymers (BCPs) within emulsion droplets. By carefully selecting the type of BCP and the processing conditions, we can produce either highly ordered structures (e.g., concentric or stacked lamellae) or quasi-random structures with short-range order. These structures consist of alternating domains with differing refractive indices, resulting in strong light reflection that can be tuned across the visible spectrum. Additionally, we integrate these structurally colored microparticles with various light-emitting inorganic colloidal semiconductor nanomaterials. This integration is achieved quickly through a straightforward, one-step solvent evaporation co-assembly process within emulsion droplets. By fine-tuning the enthalpic and/or entropic interactions between the block copolymers and the ligands on the nanomaterials' surfaces, we control the spatial arrangement of the inorganic nanomaterials within the resulting nanostructured particles. The resulting hybrid photonic structure retains its characteristic structural color while also functioning as a dielectric environment for the embedded quantum dots thus modifying their emission properties.

One of the most distinctive characteristics of semiconductor nanocrystals is their large surface-to-volume ratio. This makes their properties strongly dependent on their size, shape and surface termination (i.e., both stoichiometry and nature of passivating ligands).

The effect of these structural characteristics can be such as to significantly affect the symmetry and nature of the electronic states, modify the radiative lifetimes by orders of magnitude, and even determine the character (direct or indirect) of the optical transitions in these nanostructures.

However, owing to the chemistry involved in their synthesis, in experimental samples these effects are often masked by other factors, such as averages over a large number of dots (in ensemble measurements), shape/size/composition inhomogeneities, incomplete passivation and the presence of trap states, which make it difficult to unambiguously determine the origin of the different features observed, even in single dot experiments.

Theoretical modelling can however come to the rescue by enabling a complete decoupling of all of these effects and the isolation of specific factors.

In this talk I will give an overview of the theoretical work carried out in my group over the last few years on nanocrystals of different materials, shapes and surface termination, highlighting some unexpected properties of these versatile nanostructures and providing guidelines for their effective exploitation in devices.

Low dimensional metal halide hybrids (2D, 1D, 0D) incorporating/doping hetero-metal halide units is of fundamental importance to understanding structure-property relationship that dictates their emergent applications in solid state lighting, scintillation and photodetection. The metal halide local site symmetry has a strong impact on their optical properties showcasing effects of electronic coupling between the constituent metal halide units. However, a clear structure-property correlation in low dimensional hybrids is unavailable. In this talk, I will highlight our current efforts on exploiting various synthetic strategies towards strongly emissive multi-metallic halide hybrids. I will present the photo-physical properties of such multi-metallic halide hybrids unravelling the operative structure-property correlation in such systems. Further, I will demonstrate chemical rationale in successfully supressing melting temperatures of low dimensional metal halide hybrids through exquisite synthetic control and showcase their melt-processability. I will conclude my talk by emphasizing the need of robust theoretical calculations and ultra-fast spectroscopy to provide insight to the observed structure-property correlation in low dimensional metal halide hybrids.

Lead halide perovskite nanocrystals (LHP NCs) exhibit instability due to the dynamic and labile nature of both their inorganic core and the organic-inorganic interface, adversely impacting their optical and electronic properties [1]. The quest for novel capping ligands has not stopped, rather contrary [2], given the ever-expanding expectations for LHP NCs' deployment as classical and quantum light sources [3, 4]. We hypothesized that the facile molecular engineering of sulfonium salts as X-type ligands could enable highly customized surface chemistries for LHP NCs. Molecular dynamics simulations indicated that sulfonium ligands with diverse tail and headgroup structures exhibit equal or greater affinity to CsPbBr₃ surfaces compared to their broadly studied ammonium counterparts. CsPbBr₃ NCs capped with sulfonium bromides exhibit photoluminescence quantum yields exceeding 90% in colloids and enhanced durability in the typical purification processes. The compactness of the headgroup and tail branching significantly govern the long-term colloidal stability and resilience towards dilution and concentration. Further molecular engineering of sulfonium ligands allowed venturing into more demanding MAPbBr₃ and FAPbBr₃ NCs (MA, methylammonium; FA, formamidinium).

Colloidal quantum dots (QDs) have attracted great interest for fundamental studies of exciton and charge dynamics in semiconductor nanostructures, as well as for their applications in various devices. Consequently, extensive studies have provided insights into the exciton and charge carrier dynamics of colloidal QDs. However, exciton and charge carrier transport in QD solids present a different dynamics, as it is dictated by the packing structure of particles and the concomitant coupling between dots. Additionally, the desired exciton transport dynamics of QD solids differs between applications; for example, a photovoltaic cell requires fast exciton transport to the charge-separating interfaces, whereas in a light-emitting diode, this can lead to undesired quenching of luminescence. Therefore, comprehensive understanding of exciton transport physics in QD solids is needed, with the eventual aim of controlling these transport properties in order to optimize device performance.

Despite its importance, true charge carrier transport in QD solids has hardly been reported due to limitations in time- and space-resolved techniques. Here we directly probe the initial exciton dynamics in QD solids at femtosecond (fs) timescales following photo-generation, using a novel integrated time- and space-resolved technique called transient absorption microscopy (TAM). TAM offers dual capabilities: femtosecond time resolution and nanometer-scale spatial resolution. Surprisingly, we find that when the material has a Bohr radius much larger than the QD size, excitons first undergo very fast transport (diffusivity of ~10² cm² s⁻¹) within ~300 fs after photoexcitation and then switch into a much slower transport regime (~10⁻¹–1 cm² s⁻¹). Intriguingly, reducing the interdot distance in the QD solids only enhances transport in the slower regime, while it unexpectedly diminishes the initial fast regime. Both QD packing density and heterogeneity have great impacts on these transport regimes and the transition between them. These findings suggest routes to control the optoelectronic properties of QD solids.

Zinc oxide, ZnO, is an intriguing material with applications spanning from simple to highly advanced and sophisticated technologies. Its wide direct band gap has made it useful as UV-absorbing additive in everything from sunscreens and rubber to advanced plastics.  Various nanoscale morphologies of ZnO have also emerged as promising candidates for a large set of new high-tech applications. Among these are UV-lasers, light-emitting diodes, field emitters, piezoelectric and spintronic devices, gas sensors, transparent conductors, photovoltaics, and photocatalysis.  Several of these nanoscale applications benefit from the control of the energy states where the position, nature, and relation between the states in the material affect the optical behavior. This is especially true for low-dimensional ZnO nanoparticles where the properties of the states will be a function of particle size if dimensions are made small enough.[1] Here we present experimental methods to extract the optical band edges and fluorescing trap states in ZnO quantum dots by combining electrochemistry and UV-spectroscopy.[2,3] We present the shift of band gap with particle size and how the absolute band edges shifts for up to 18 different sizes of low-dimensional  ZnO nanoparticles below the quantum confinement size regime. Time-resolved fluorescence data in the quantum confined regime and the possibility for surface stabilized excitons will be presented.[4]The development of collective vibrations and eventually phonons in the materials are also presented by combining experimental Raman spectroscopy and theoretical simulations where the vibrational quantum confinement regime as larger than the corresponding electronic quantum confinement. [5] We finally present a quantum confined Stark effect that disappears when dimensions of the particles approaches the bulk band gap regime at around 9 nm.[6] We will also briefly touch upon our more recent work on utilizing Raman spectroscopy to extract electron-phonon coupling in ZnO, and optical quantum confinement and exciton emission in 2D lead halide perovskites.

[1] Edvinsson, T. Optical Quantum Confinement and Photocatalytic Properties in Two-, One- and Zero-Dimensional Nanostructures. Royal. Soc. open sci. 2018, 5: 180387.
[2] Jacobsson, T. J.,  Edvinsson, T.  Photoelectrochemical Determination of the Absolute Band Edge Positions as a Function of Particle Size for ZnO Quantum Dots, J. Phys. Chem. C, 2012, 116, 15692.
[3] Jacobsson, T. J.; Edvinsson, T.  A Spectroelectrochemical Method for Locating Fluorescence Trap States in Nanoparticles and Quantum Dots, J. Phys. Chem. C 2013, 117, 5497.
[4] Jacobsson, T. J.; Viarbitskaya, S.; Mukhtar, E.; Edvinsson, T.,  A Size Dependent Discontinuous Decay Rate for the Exciton Emission in ZnO Quantum Dots,  Phys. Chem.
Chem. Phys.  2014, 16, 13849.
[5] Raymand, D.,  Jacobsson, T.J., Hermansson, K., and  Edvinsson, T. Investigation of Vibrational Modes and Phonon Density of States in, ZnO Quantum Dots, J. Phys. Chem. C  2012, 116, 6893.
[6] Jacobsson, T. J.; Edvinsson, T., Quantum Confined Stark Effects in ZnO Quantum  Dots Investigated with Photoelectrochemical Methods, J. Phys. Chem. C 2014, 118, 12061.

Polymorphism in solids refers to the presence of local atomic disorder not discernible in standard diffraction experiments. Although polymorphism is crucial for understanding the electronic structure of halide perovskites, it is frequently overlooked when interpreting their intriguing properties. In this talk, I will present a recently developed ab initio methodology [1,2] for the calculation of transport and optoelectronic properties of perovskites, allowing for the efficient treatment of anharmonic lattice dynamics, electron-phonon coupling, and polymorphism. I will make a connection between theoretical findings and experimental results [3,4,5], beginning with pair distribution functions and expanding to temperature-dependent band gaps, phonons, effective masses, and mobilities. I will also discuss the role of molecular orientation and size in the degree of polymorphism in hybrid compounds, and make contact with the concepts of the lone pair formation and polarons. Our work opens the way for addressing pending questions on perovskites’ technological applications.

“Giant” or thick-shell core/shell quantum dots (gQDs) are an important class of solid-state quantum emitter. Without any encapsulation gQDs are characterized by strongly suppressed blinking, or even non-blinking behavior, and resistance to photobleaching at room temperature. In addition, non-radiative Auger processes are significantly reduced in this class of luminescent nanomaterial. Together, these qualities lead to novel functionality as photon sources for a range of ensemble and single-emitter applications, including down-conversion phosphors, direct excitation light-emitting diodes, single-biomolecule tracking, and single-photon generation for quantum applications. As a single-photon source, with judicious choice of core and shell size and composition, Auger processes can be tuned to either promote or suppress biexciton emission, the latter enabling a photon-pure on-demand single-photon source.

Thus, through chemical synthesis and internal interface control, we have made significant progress toward meeting the demands of an ideal quantum emitter – achieving on-demand, high-purity, room-temperature, spectrally tunable (blue-visible to telecommunications wavelengths) single-photon sources. However, to address other properties, including brightness, on-chip “plug-and-play” integration, chirality, polarization control and photon indistinguishability, we have looked to external environmental control to influence these properties that are not immediately under the influence of the synthetic chemist. Here, I will describe our efforts with collaborators to address these remaining challenges, primarily through integration into nanoantennas or plasmonic cavities. For example, we show the ability to achieve highly directional, radially polarized photons by exploiting the intrinsic stability of the gQD, as well as our developed scanning-probe-enabled strategy for precision placement of single nanocrystals into nanostructured surfaces, e.g., hybrid metal-dielectric bullseye antennas. Alternatively, in a separate collaboration, we have realized for the first time ultrafast (to 65 ps) and ultrabright (to ~12.6 MHz) room-temperature single-photon emission in the O and C telecommunications wavelength bands via coupling to solution-processed plasmonic nanocavities. Lastly, I will also address strategies using surface chemistry or interface modification to achieve circularly polarized emission or chiral quantum light sources, respectively.

Ab initio calculations are becoming more and more efficient and have emerged as an indispensable tool to model, characterise and understand complex systems like halide perovskites and perovskite-like materials. In particular, over the last decade, such computational approaches have been extensively employed and successfully unveiled the underlying atomic-scale physical mechanisms of these exciting materials. In this talk, I will overview our most recent results on the electronic structure of prototypical structures of layered halide perovskites, vacancy ordered double perovskites, and low dimensional halide perovskite-like materials [1-2]. I will present the key details of their electronic structure for each type of system that define their experimentally observed optical properties and achieved performances. Our results show how well (or how bad) these different types of materials can perform for different opto-electronic applications ranging from indoors and outdoors PV, light emitters. Finally, in the last part of my talk, I will focus on our latest state-of-the-art ab initio calculations of the charge carrier transport properties when comparing three-dimensional ABX3 and layered halide perovskites. Our results explore directly the effects of structural dimensionality on the carrier mobilities of a selection of prototypical layered perovskites and identify the importance of the intrinsic carrier density in layered compounds to the exhibited transport properties [3].

Imposing strong quantum confinement in lead halide perovskite nanocrystals enhances the electronic interactions of charge carriers within each nanocrystal and promotes the delocalization of the exciton wavefunction between the nanocrystals in the closely packed quantum dot assemblies. Such enhanced intra- and inter-quantum dot electronic (or exciton) coupling in strong confinement regime can enhance the capability of perovskite quantum dots as the source of hot electrons and coherent photons. Here, we investigated: (i) the generation of hot electrons via Auger hot electron upconversion in strongly quantum-confined cesium lead bromide (CsPbBr₃) nanocrystals doped with Mn²⁺ and (ii) the coherent photon emission as the superradiance from the superlattices of CsPbBr₃ quantum dots. The enhanced exciton-dopant interaction in strongly quantum-confined CsPbBr₃ quantum dots proved beneficial for energetic hot electron generation and allowed for the utilization of the long-lived dark exciton in such processes at low temperatures. The closely-packed QD superlattice CsPbBr₃ quantum dots exhibited coherent photon emission as superradiance significantly better than the larger weakly-confined quantum dots taking advantage of the facilitated exciton delocalization in the superlattices formed from the strongly-confined quantum dots. 

The science and technology of InAs Colloidal Quantum Dots (CQDs) are at the forefront of materials physics, chemistry, and engineering due to their promising applications in near-IR optoelectronics and their non-toxic nature. However, advancing InAs CQD technology is challenging due to complex synthesis processes, poor material quality, surface passivation issues, and surface traps, which result in low quantum yield (QY) and limited ambient stability, thereby restricting device applications.

To address these challenges, we performed advanced theoretical modeling to investigate the impact of surface defects on the electronic and optical properties of InAs CQDs. In particular, by following the Semi-Empirical Pseudopotential Method (SEMP)^1,2 we modelled In-rich spherical-shaped isolated QDs with diameters of, 1.8 nm, 2.36 nm, and 2.96 nm, and In-terminated tetrahedral-shaped isolated QDs with a length of 2.55 nm. We predict the presence of unpassivated surface anions to give rise to states in the gap with an L-like character, resulting in an increase in Stokes' shifts and radiative recombination lifetimes in spherical dots, but having the opposite effect (reduced Stokes' shifts and radiative recombination times) in tetrahedra. We attribute these findings to the specific shape of the QDs³.

These findings offer valuable insights into the surface chemistry of InAs CQDs, particularly regarding traps induced by surface defects. This study provides experimentalists with crucial insights related to the characteristics of InAs CQDs, potentially leading to improved material performance and broader application in optoelectronic devices.

By incorporating optically active materials with high oscillator strength into optical microcavities, it is possible to reach the regime of strong light-matter interaction, whereby exciton-polariton quasiparticles are formed that are composed of both photon and exciton components. These polaritons can undergo non-equilibrium Bose-Einstein condensation at sufficiently high excitation density, exhibiting nonlinear emission, macroscopic coherence, and quantum fluid properties. While room-temperature polariton condensation with bulk crystalline thin films of CsPbBr₃ in microcavities has been achieved, progressing to colloidal quantum dots would not only allow much easier and flexible fabrication, but due to their strong 3D spatial confinement, would have the additional potential to facilitate enhanced polariton interactions.

We present strong light-matter coupling with thin films of colloidal CsPbBr₃ nanocrystals in optical microcavities and exciton-polariton condensation under ambient conditions. This is demonstrated by the observation of nonlinear increases in emission, narrowing of the linewidth, and coherence measurements. A tunable open microcavity based on distributed Bragg reflectors is employed to tune the polariton energy. By means of precise nanofabrication, tiny Gaussian-shaped deformations are created in the mirrors, effectively producing potential landscapes for the polariton condensate. This paves the way for the use of the polariton quantum fluid as an analogue simulator for Hamiltonians.

Quantum confinement is one of the effects dictating semiconductors’ electronic and optical properties. This effect, regulated by the spatial extension of the semiconductor, has the potential to significantly modulate optical and electronic properties of interest. For instance, quantum confinement effects imposed by the small thickness of 2D nanoplatelets (NPLs) result in enhanced oscillator strength, reduced dielectric screening, and increased exciton binding energy. The relative effects are material-dependent and are modulated by the number of electrons and their orbital occupation in the specific material. Here, we investigate the effects of thickness on the vibrational and electronic structure properties of 2D halide perovskite (Csn+1PbnBr3n+1) nanoplatelets (NPLs) with n = 3, 4, and 5. We report the change in electronic structure as a function of platelet thickness as well as changes in vibrations. Our hypothesis is that the Raman intensities ratio between two-dimensionally dependent modes would vary with the layer thickness of the NPLs due to the different vibrational-induced polarizability over n = 3, 4, and 5. To quantify and understand such an effect, Phonon dispersion, electronic structure and Raman intensities of the vibrational modes at the point are computed for each thickness case using density functional perturbation theory (DFPT) and density functional theory (DFT). For this task, we have built slab models from the tetragonal CsPbBr3 phase with a vacuum in the z direction to avoid (as much as possible) the interaction between periodic images. The changes in relative intensities between the modes are in agreement with our experimental data from Raman spectroscopy, revealing the same trend in intensity change between the Raman active modes upon confinement. The effect is attributed to the reported change in electronic structure and subsequent polarizability change upon quantum confinement of the material into fewer layers.

Ten years ago, the hybrid organic-inorganic halide perovskites have emerged in the framework of photovoltaics. But these materials present also relevant physical properties for light emitting devices as suggested more than 20 years ago by pioneer works [1-3]. In particular, just by tuning their composition and/or dimensionality, it is possible to tune easily the band gap and the excitonic properties of this new class of semiconductors. Moreover, these materials can be solution processed and deposited in large surface which is suitable for wide scale wafers and devices, representing a great hope for obtaining large –surface and low-cost emitting devices.

Here, we will focus our attention on perovskites emitting in the green range, addressing the problem of the green gap for lasers and we will study the light-matter interaction in one-dimensional planar microcavities containing them.

A lot of efforts have been done to embed 2D layered perovskites, presenting the electronic structure of a multi-quantum well, in the cavities. Due to very large excitonic effects, the strong coupling regime between the photon mode of the Fabry-Perot cavity and the excitonic mode of the perovskite is obtained at room temperature even with a low quality factor [3]. This leads to the formation of the so-called polaritons, which are a linear and coherent superposition of the exciton and photon states. Nevertheless, it seems difficult to obtain the condensation of these polaritons. Moreover, the transport properties of the 2D perovskites are highly asymmetric due to their layered structure.

We will consider then a microcavity containing a large-surface spin coated thin film of the 3D perovskite CH₃NH₃PbBr₃ as the optical active material. Here again, we show, from both reflectivity and photoluminescence experiments, the strong coupling regime between the photon mode of the Fabry-Perot cavity and the excitonic mode of the perovskite at room temperature [4]. By increasing the incident power, we demonstrate a random lasing emission in the green occurring in the microcavity which is directionally filtered by the lower polariton dispersion curve [5]. In this case, the angle of emission can be controlled by changing the microcavity detuning. Angles of emission as large as 22° are experimentally obtained. This result is interesting from a fundamental point of view because it combines two intriguing physical phenomena: the cavity exciton-polariton and the random lasing, and from a more applied point of view: the control of the random lasing emission direction is a crucial point for optoelectronic applications, such as the LIDAR technology for example.

These works open the route to fundamental studies on the Bose-Einstein condensation of polaritons and to new large-surface opto-electronic devices based on polaritonic effects, working at room temperature, potentially electrically injectable as 3D hybrid perovskites present good transport properties. As perspectives, other configurations, such as nanoimprinted exciton-polariton metasurfaces, are currently being developed to obtain cost-effective and large-scale polaritonic devices operating at room temperature [6].

Indium arsenide (InAs) colloidal quantum dots (QDs) are emerging, heavy-metal free, nanomaterials with tunable bandgap and promising applications across the NIR spectrum. Incorporation of InAs QDs into optoelectronic devices is dependent on the ability to tailor and improve their solid state, transport properties. One of the most efficient ways to achieve this, is by electronically doping the QDs to produce p-n-junctions that traditionally have served as the main building blocks for semiconductor electronic devices.

In this work, we study the solid-state, photophysical properties of colloidal n- and p-doped InAs QDs fabricated via a recently developed synthetic approach¹. Based on such synthetic protocol, the as-grown n-type InAs NCs can be transformed to p-type by appropriate introduction of Zn that serves as a substitutional p-type acceptor within the InAs lattice. Variable temperature, steady-state and transient absorption and photoluminescence spectroscopies are employed to probe the impact of the Zn-doping on the QD optical properties and their variation compared to the respective properties of as-grown n-doped QD films. The work provides insight into the impact of Zn-doping on the exciton recombination, radiative yield and lifetime and exciton-phonon coupling in thin films of doped InAs QDs.

Lead halide perovskites (LHPs) have attracted immense attention via the combination of their exceptional optoelectronic properties (high quantum efficiency, spectral tunability, large carrier diffusion length, high light absorption coefficient, long carrier lifetimes, narrow emission line width, good defect tolerance).^[1] Considering their advantages, LHPs are prospective materials for LEDs, solar cells, and lasers. Particularly, colloidal NCs are widely employed for optoelectronic applications due to size control, compositional mixing, band gap and emission tuning. Meanwhile, they generally have poor stability at high excitation densities.^[2] Typically, this is associated with organic ligands capping the NCs even in dense films. Meanwhile, solution-processed films suffer from defects such as voids, substantially impacting their performance.

This study comprehensively investigates a method of forming compact nanocrystalline thin films through single-source thermal evaporation of CsPbX₃ (X = Cl or Br). Initially, amorphous films are aged at room temperature to form defined nanocrystalline domains with preferential orientation. Moreover, the resulting films show low-threshold amplified spontaneous emission under ambient conditions across and remarkable operating stability surpassing 180 million laser shots (i.e. 5 hours of continuous operation) with a net modal gain of 1240 cm^-1. The results show that evaporated perovskite films are promising optical gain media for lasing applications under ambient conditions.

Lead halide perovskites of the type APbX₃, where Α is an organic/inorganic cation and X a halogen atom, attract wide interest based on their outstanding achievements in the field of photovoltaics. The vast majority of works in the field involve iodine-based perovskites, with an energy gap suitable for solar cell applications. By contrast, relatively few are the works dealing with the wider-gap chlorine-based perovskites emitting in the deep blue-near ultraviolet part of the spectrum.

In this work, we study the lasing process in a vertical-cavity surface-emitting laser structure, containing as active medium 2-10 μm-thick single crystals of MAPbCl₃ in between SiO₂/Ta₂O₅ distributed Bragg reflectors [1],[2]. In such a MAPbCl₃ vertical cavity, we observe for the first-time dual wavelength lasing at 78 K, occurring at 414 and 391 nm at different thresholds. To understand this complex lasing behaviour, the single crystals were extensively studied in terms of micro-photoluminescence and micro-reflectivity experiments as a function of temperature.

In micro-reflectivity spectra at 78 K, aside from a strong exciton feature, marking the excitonic gap at ~385 nm of the orthorhombic phase of MAPbCl₃, we also observe a distinct exciton feature at ~412 nm, next to the second laser line. This second exciton feature, never reported before in any MAPbCl₃ system, strongly suggests the coexistence within the orthorhombic lattice of a “second” crystal phase, which is present in ample quantities to be able to give rise to such pronounced reflectivity signature. We show that this second phase persists up to room temperature, well above the orthorhombic-to-cubic transition at 170K, making it very unlikely that it consists of “cubic” inclusions. Our experiments further suggest that this second phase depends on the growth method and crystal size and that is possibly a more general feature of the MAPbCl₃ system than originally thought. The conclusions drawn from this study are likely to lead to an enhanced understanding of the MAPbCl₃ system and pave the way for new photonic devices in the deep blue-UV region.

The Rise of 2D Halide Perovskites for Efficient & Durable Optoelectronic Devices

Halide perovskites have emerged as a new class of semiconductors with excellent properties such as large tunable band-gaps, large absorption coefficients, long diffusion lengths, low effective mass and long radiative lifetimes. These have resulted in record efficiencies for photovoltaics surpassing that of Si. However, a major challenge for these materials is realizing long-term stability under light, temperature and humidity. In contrast, 2D perovskites are a sub-class of 3D perovskites, have demonstrated excellent stability compared to the 3D perovskites.

In this talk I will describe our work over the past five years on 3D and 2D perovskites ranging from novel fundamental light-induced structural behaviors and its impact on charge transport, solvent chemistry and the synergy between 2D and 3D perovskites in achieving durable and high-efficiency photovoltaic devices. Finally, if time permits, I will also present some new results, which offer an exciting prospects for developing single photon emitters.

The scintillation mechanism in organic scintillator relies on a cascade of energy transfers from the organic matrix to the last wavelength shifter. The last transfer is the least efficient one due to the low concentration in the final emitter. To address this challenge, the distance between the final emitter and the primary dye should be reduced. Nonetheless, this cannot be achieved by increasing the overall final-emitter concentration as this would lead to self-reabsorption. Therefore, the challenge is to increase the local concentration of the final emitter only in the close vicinity of the last energy donor.  We archived this in perovskite nanostructure organic scintillators by binding the organic acceptor directly on the surface of the perovskite nanocrystal. To measure the binding effect on the energy transfer efficiency, a side-by-side comparison of two scintillators was performed. The first one had the chromophore bound to the perovskite and the second contained the same amount of chromophore homogeneously dispersed. The binding resulted in a four-fold enhancement of the energy transfer efficiency, along with a shortening of the scintillator response time. These observations were attributed to the favoring of a Förster mechanism over the trivial energy transfer. Based on the observations we conclude that actively managing the distance between the wavelength shifters is desirable to increase the overall scintillator performance and foresee the binding strategy as transferable to other nanostructure scintillators.

State-of-the-art display screens are only for information display, while a range of extra different sensors are integrated into the screens for additional functions such as touch control, ambient light sensing, and fingerprint sensing. Future ultra-thin and large screen-to-body ratio screens require the development of novel multifunctional light-emitting diodes (LEDs), which both display information and sense signals - a feature hardly possible for conventional LED technologies. [1]

Here, we develop multifunctional displays using highly photo-responsive metal halide perovskite LEDs (PeLEDs) as pixels following our previous publication. [2] With efficient defects passivation within perovskite layers, the red emissive PeLEDs shows an external quantum efficiency (EQE) of 10% when working at LED model and a power conversion efficiency (PCE) of 5.34% at photovoltaic model. Due to the strong photo response of the PeLED pixels, the display can be simultaneously used as touch screen, fingerprint sensor, ambient light sensor, and image sensor without integrating any additional sensors. In addition, decent light-to-electricity conversion efficiency of the pixels also enables the display to act as a photovoltaic device which can charge the equipment. [3] The multiple-functions of our PeLED pixels can not only simplify the display module structure and realize ultra-thin and light-weight display, but also significantly enhance the user experience by these advanced new applications. As such, our results demonstrate great potential of PeLEDs for a new generation of displays for future electronic devices.

Light emitting diodes based on metal halide perovskites have attracted considerable research attention due to the excellent, tunable, optoelectronic properties of perovskite semiconductors. The chemical tunability of perovskite semiconductors results in a wide number of perovskite structures which can be utilised as thin film emitters for LEDs. The quasi-2D perovskites are one such structural group. These are similar to 2D perovskite structures, consisting of sheets of inorganic perovskite octahedra separated by large organic cations but with multiple octahedral layers. LEDs based on quasi-2D perovskites can often achieve high electroluminescence external quantum efficiency yields however they can suffer from issues related to reduced or short operational lifetimes due to poor device stability. These quasi-2D perovskite systems are complex. At present approaches to tackling operational stability are predominantly iterative, focussing on optimising the material system and devices. 

In this talk I will present our recent efforts to understand the relationship between the structure of the quasi-2D perovskite materials and their subsequent device physics. I will discuss our understanding of the interfaces within quasi-2D perovskite thin films and how they influence electronic structure and charge carrier dynamics. Finally, I will discuss routes through which we can advance our understanding to see a significant step change in quasi-2D perovskite LED stability.

Optical lithography has been the key enabler for scaling feature sizes of integrated circuits, allowing the exponential growth of the semiconductor industry. Traditional photolithography involves the use of a photomask and a mask aligner to transfer the desired pattern onto a wafer. An alternative method is maskless lithography, in which the pattern is directly exposed onto the substrate surface using a light sensitizer such as a laser. For many applications this is preferred as it allows to circumvent the lengthy process of designing and manufacturing a photomask to transfer the desired pattern onto the wafer.   

Herein we develop a custom-made laser writer lithography system based on a motorized micro-photoluminescence setup with which various micro – patterns, such us arrays of pixels, dots, waveguides etc. are transferred onto CsPbBr₃ nanocrystal (NC) films deposited on silicon substrates. The structural and optical properties of the patterned NCs are investigated with a combination of atomic force microscopy (AFM), near-field scanning optical microscopy (NSOM) and hyperspectral photoluminescence mapping experiments. Such patterned NC solids can evolve into functional building blocks for solution-processed micro-scale photonic circuits and devices.

All-inorganic two-dimensional (2D) halide perovskite nanoplatelets (PNPls) have recently garnered considerable interest in materials science. These materials exhibit outstanding optoelectronic properties, making them highly promising for applications in LEDs and photodetectors. The appeal of PNPls lies in their exceptional electronic and optical properties, which can be finely tuned across a wide range due to their unique structural characteristics. Initially, tuning the properties of PNPls focused on altering the elemental composition of the perovskite structure, specifically the halide component and/or the primary cation, shifting from organic to inorganic or between various organic molecules. In recent years, extensive research has indicated that another strategy for tuning PNPls' properties is based on the number of monolayers (MLs). A monolayer corresponds to a single microscopic layer of inorganic metal-halide octahedrons surrounded by large organic cations or ligands. While significant research has explored the optical properties of these systems, the role of phonons (lattice vibrations) in different dimensionalities remains underexplored. Additionally, a key scientific goal is a fundamental understanding of the collective carrier-phonon coupling in excited states, the thermalization process, initial charge separation, and final transport, including the mobility of electrons and holes and their relationship to charge carrier-lattice interactions. Since these phenomena are dimensionality- and phase-dependent, where the coupling between the excited state and phonons changes significantly with both dimensionality and phase, it is crucial to investigate the vibrational properties across different system dimensionalities and crystallographic phases to better understand the role of phonons in these materials.

In this work, our main goal is to employ Raman, photoluminescence, UV-Vis spectroscopy for the realization of, firstly, a model for the vibrational properties of 2-6 MLs NPls and larger nanocrystals of oleic acid- and oleylamine-capped CsPbBr₃. Our Raman measurements showed that, by systematically varying the number of monolayers of the nanoplatelets, there is distinct changes in the relative intensities of the vibrational modes that are sensitive to the number of monolayers. These observations can be attributed to the quantum confinement effect, which becomes more pronounced as the thickness of the 2D nanoplatelets decreases. In addition, there is strain generated in the materials upon the formation of lower thickness NPls. This can be identified through the shift of Pb-Br vibrational mode. Secondly, we established a model of the vibrational properties over a wide temperature range to identify changes in phase, strain, and anharmonicity for different system dimensionalities. Raman measurements revealed the tetragonal phase formation at room temperature for all prepared nanocrystal systems is dominant with the orthorhombic and cubic phases at low and high temperatures, respectively. This in addition to the phase-related and/or the anharmonicity-related shift of phonon modes with temperature. Furthermore, there were changes in the peak intensities’ ratios, which provide valuable insights into the structural variations induced by the number of monolayers. Lastly, we investigated the photoluminescence enhancement with different nanoplatelets thicknesses. Photoluminescence measurements revealed that, by changing the number of monolayers from 2-6, there is an enhancement of the photoluminescent intensity exponentially. This is attributed to the increase in the photoactive sites, where the number of the bright excitons increase with increasing the monolayers of the nanoplatelets, considering maintaining all the synthetic parameters the same, such as concentration.

These results provide an essential initial overview of the crucial vibrational and optical properties of the system, paving the way for a better understanding of carrier-phonon coupling, among other phenomena, in these materials. The ability to determine these structural parameters via Raman spectroscopy establishes it as an indispensable characterization technique for rapid and accurate analysis of thickness and confinement regimes in perovskite nanoplatelets and suggests its potential for dimensionality analysis in other nanocrystal families.

The crystalline Silicon (c-Si) / Perovskite (PK) 2 terminal (2T) Tandem solar cells have recently reached a certified power conversion efficiency (PCE) of 34.6% [1], exceeding the theoretical limit of silicon single junction. This makes this tandem a good growth driver for the PV technology. Those results are really promising but are still obtained at lab scale and on small size devices (≤1cm²), the perovskite technology being less mature than the well-established Silicon one. In order to reduce time to market of this new technology, several challenges remain in order to upscale the materials and the processes toward the industry.

The objective of the presented work is to develop efficient Perovskite/Silicon tandem devices that can be manufactured with techniques compatible with the next generation PV industry.

Starting from 8.45 cm² devices with PIN architectures, with PCE above 28 %, CEA is developing materials and processes compatible with industrial requirements. Several bottlenecks are identified and will be discussed.

References

[1] https://www.pv-magazine.com/2024/06/14/longi-claims-34-6-efficiency-for-perovskite-silicon-tandem-solar-cell/

The ultimate goal of quantum optics is to achieve complete control of light-matter interaction at the single-quanta level. Photons carry information encoded in properties such as frequency, amplitude, and phase. Another encoding capacity that can enhance robustness of this qubit is optical chirality. Chiral photons are particularly advantageous because they carry background-free binary data, and the spin-controlled light propagation direction promises powerful nonreciprocal quantum devices. Here, we report the generation of strong chiral emission in single perovskite nanocrystals (NCs), which are bright and efficient quantum emitters but are intrinsically achiral. By placing the quantum emitter in proximity to chiral plasmonic particles, we boosted the degree of circular polarization (DOCP) of single-photon emission by an order of magnitude. Polarization-resolved single-dot spectroscopy at cryogenic temperature revealed that the chirality transfer occurs through the interaction of the local chiral plasmonic field with the photonic states of the perovskite NCs. The handedness anisotropy of both excitation and emission showed an order of magnitude increase at the presence of chiral field, which was accompanied by a 3-4-fold acceleration of the radiative decay rate. This induced chirality was precisely controlled by nanometer-scale engineering of the NC-plasmon spacing. These results provide a significant advance in understanding the largely unexplored mechanism of chiral light-matter interaction, presenting an exciting challenge for future theoretical work and applications.  

The integration of plasmonic nanostructures into perovskites is an effective way to tailor and enhance the light-matter interactions, leading to further improvement of the already impressive intrinsic photonic properties of the perovskites. Despite significant recent progress, the complex mechanisms via which excitons, plasmons and light interact in such hybrid structures are not fully understood. Herein, we study the interactions between formamidinium lead triiodide (FAPbI₃) nanocrystals (NCs) and core-shell Au/SiO₂ nanoparticles (NPs) in a bilayer plasmonic-perovskite structure. By tuning the Au core and SiO₂ shell size, a five-fold increase of the light absorption and luminescence can be achieved compared to the pristine FAPbI₃ NC film. Based on the results from optical spectroscopy and numerical simulations, the enhancement can be attributed to a combination of far-field light scattering by the localised surface plasmon (LSP) and near-field energy transfer from the metal NP to the NC. Far-field light scattering results in photon recycling in the perovskite layer, effectively enhancing light absorption and emission. Near-field energy transfer contributes less, but maximizes for large core and small shell NPs, due to the increased spectral overlap and closer proximity of the LSP and exciton, respectively.

Near-infrared (NIR) light-sources are of interest for a variety of applications such as hyperspectral imaging, night vision, telecommunication systems and point-of-care testing. Colloidal quantum dots (QDs) possess interesting properties for NIR optoelectronics thanks to their tunable photoluminescence, solution processability (which also enables mechanically flexible devices), and capability for CMOS integration. Various QD compositions have been investigated, most of them either including Pb or Hg, with the latter holding promise for extending emission beyond the telecommunication C-band. Yet, QDs based on heavy-metals cannot gain approval for optoelectronic applications due to the European Union’s “Restriction of Hazardous Substances” (RoHS) directive. Colloidal indium arsenide QDs are emerging as a promising substitute to heavy-metal containing compositions as they are fully RoHS-compliant and, thanks to recent progress in material synthesis, they can demonstrate stable and highly efficient emission.Here, I will discuss our recent findings on different NIR-emitting QDs, in particular InAs and CdHgSe ones.

Employing InAs QDs coated with a thick ZnSe shell (7 monolayers) we were able to reach a photoluminescence quantum yield approaching 70% at 906 nm. We have used such QDs in  a light-emitting diode (LED) obtaining an EQE of 13.3%, a turn-on voltage of 1.5V, and a maximum radiance of 12 Wsr^-1m^-2. Such results are comparable to state-of-the-art PbS QD LEDs. Furthermore, not only the fabricated LEDs are fully RoHS-compliant but it employs InAs QDs prepared via a synthetic route based on non-pyrophoric, cheap, and commercially available precursors.^1-4

Similarly,  CdHgSe/ZnCdS nanoplatelets (NPLs) exhibit optical absorption and emission that can be  tuned from the visible to the NIR range through both quantum confinement and adjustment of their composition. By finely control their synthetic parameters, we have obtained CdHgSe/ZnCdS NPLs with a photoluminescence quantum yield of 58% at 1300 nm (O-band). NIR-LEDs based on NPLs demonstrate and EQE of 7.5%, a turn-on voltage of 0.95 V, and a maximum radiance of 1 Wsr^-1m^-2.^5-6

2D Ruddlesden-Popper perovskites (RPP) with formamidinium-cesium (FACs) alloyed cations possess powerful thermal-moisture stability. However, their photovoltaic performance is hindered by the elusive spatial heterogeneity at multiscale lengths, highly associated with their coupled charge selective contacts. Herein, we report on how the rational formation of self-assembly molecules (SAMs) govern the orderliness of the spatial heterogeneity and crystal growth model in FACs-RPP films, which is proposed to dictate the charge-carrier dynamics. Unlike the disordered RPP film driven by the amorphous polymeric contacts, the laterally ordered crystallized SAM contacts facilitate a spatially ordered 2D/3D heterogeneity of FACs-RPP film in long range, where distinct minor 2D phases are tightly coupled in short range. We discovered such ordered heterogeneity can improve the energy transfer efficiency, facilitating the charge-carrier dynamics in a well-aligned 2D/3D landscape. Finally, the champion solar cells using selective SAM based FACs-RPP films (n = 5) yield an efficiency of 18.85% (certified 18.19%) and excellent damp-heat/operational stability, ranking in the top league of reported 2D RPP solar cells. More importantly, this SAM-based principle can be adapted to diverse 2D structures, active areas, and substrate types. This work provides design directions for leveraging bottom-selective contacts to tune spatial heterogeneity in state-of-the-art 2D RPP optoelectronics.

Neuromorphic computing holds the potential to revolutionize traditional computing paradigms, shifting away from the von Neumann architecture towards dynamic, energy-efficient problem-solving. In this talk, I will discuss a new element of photonic neuromorphic computing: Memlumor. It essentially represents a luminescent material with memory. I will show that the metal halide perovskite material class can be used as very efficient Memlumors. The widely considered instability of the physicochemical properties of metal halide perovskites is essential for the operation of Memlumors. To reveal the memory of Memlumors' luminescence, I will additionally present the advanced multi-pulse time-correlated single-photon counting technique. By using this method, I will show the presence of memory in perovskite luminescence over a very wide range of times (from nanoseconds to minutes). The native memory of perovskites, which allows for performing computing operations at the femtojoule energy scale, will revolutionize our perception of luminescence in materials. Further study of memory in the luminescence of different perovskites is essential for making all-perovskite photonic neuromorphic computing processors.

Halide perovskite nanocrystal superlattices exhibit collective superfluorescent emission, due to collective interactions between multiple simultaneously excited nanocrystals [1,2]. This coupling, enabled below a critical temperature of 180-200K, changes both the transition energy and emission rate compared with the emission of individual uncoupled nanocrystals. According to the superradiance model, first described by Dicke in the 1950s, when of several identical emitters are located within a small volume, coherent collective coupling through common vacuum modes of the electromagnetic field result in a faster emission rate [3]. The ensemble of emitters behaves as one large transition dipole with an oscillator strength proportional to the number of coupled emitters. Although Dicke superradiance is commonly observed in dense atomic gases, recent observation of superfluorescent emission in solid state semiconductor systems is nontrivial as they deviate from the ideal superradiance model framework due to strong dipole-dipole interactions between the emitters [2]. We demonstrate how quantum confinement governs the type of coupling through synthetical control over nanocrystal size, and by compositional control over the Bohr radius via anion exchange. Superlattices made of weakly confined nanocrystals, showed a red-shifted collective photon bunching emission bursts with a faster emission rate, showcasing key characteristics of superfluorescence. In contrast, superlattices made of strongly confined nanocrystals showed a blue-shifted collective photon bunching emission bursts, despite having a slower radiative rate. We explain these different modes of collective behavior by suggesting a critical role for exciton dipole-dipole interactions between neighboring nanocrystals in the superlattice. We utilize the exciton interaction theory, which was developed by Kasha in the 1960s to explain attractive (J-type) and repulsive (H-type) dipole-dipole interactions in molecular aggregates [4]. The H/J aggregate behavior switching in perovskite nanocrystals, is the result of modified exciton orientation, due to the quantum confinement. The confinement changes the preferred alignment of transition dipoles in the nanocrystals, shown by analysis of angular-dependent emission patterns, thereby changing the relative dipole orientation between neighbouring nanocrystals, and dictating the resulting optical behavior of the ensemble. Merging Kasha exciton interaction theory with Dicke superradiance model, provides new understanding of exciton interactions and collective emission phenomena at the solid state.

X-ray imaging scintillators are essential in various applications, including medical imaging and security checks. There is a pressing need to develop innovative, environmentally friendly scintillation materials with robust screen fabrication to meet industrial demand and minimize X-ray exposure risks. In this work, we focus on the synthesis and screen fabrication of zero-dimensional manganese hybrid halides with near-unity PLQY emission. We also explored the practical application of Mn(II)-based imaging scintillators for everyday X-ray imaging. Notably, the manganese(II) hybrid bromide scintillators demonstrated an ultrahigh resolution of up to 20.8 lp/mm, surpassing both conventional scintillators and recent alternatives. Additionally, these large-area imaging scintillators achieved a high light yield of 20,000 photons/MeV and an impressively low detection limit of just 180 nGy/s, which is 30 times lower than the typical dose required for medical radiography diagnosis (5.5 µGy/s). Finally, we present compelling evidence of the practicality of using high-resolution Mn(II) hybrid bromide scintillators for X-ray imaging applications.