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 colloidal 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 of acid-based equilibria on the growth of perovskite NCs, which can lead to the formation of NCs with peculiar shapes (for example hollow structures) and to NC heterostructures (for example CsPbBr3/PbS heterostructures) by promoting/suppressing the heterogenous nucleation of selected materials. I will also discuss our findings on the ordering of NCs in superstructures, and how cryogenic temperatures can influence the degree of ordering.

While fluoride nanocrystals (e.g., NaYF₄) are popular host materials for lanthanide-based up-and downconversion, ceramic oxide hosts (e.g., ZrO₂ and HfO₂) have found less widespread use, due to the synthetic challenge of producing colloidally stable oxide nanocrystals with a complex (e.g., core/shell) architecture. The oxides are however, more chemically and thermally stable.

We first present the results of our mechanistic investigation in the synthesis of zirconium and hafnium oxide from metal chloride and metal alkoxide in trioctylphosphine oxide (TOPO). We study the metal speciation in solution, we determined the decomposition kinetics and its mechanism and we study the nucleation and growth. We find evidence for an E1 elimination mechanism and the occurrence of amorphous particles as intermediate on route the final highly crystalline particles. This is a consequence of the rate imbalance between a fast precursor decomposition and a slow crystallization process.

Second, we demonstrate the epitaxial growth of hafnia shells onto zir-conia cores, and pure zirconia shells onto europium doped zirconia cores. The core/shell structures are fully crystalline. Upon shelling, the optical properties of the europium dopant are dramatically improved (featuring a more uniform coordination and a longer photoluminescence lifetime), indicating the suppression of non-radiative pathways.

Third, we dive deeper in lanthanide doped zirconia nanocrystals. We determine the incorporation efficiency of multiple lanthanide in zirconia. We show the influence of the surface chemistry on the emission spectrum and lifetimes of Eu³⁺ of Tb³⁺. Time resolved emission spectra allow us to differentiate different europium sites (surface and bulk).

These results launch the stable zirconium and hafnium oxide hosts as alternatives for the established NaYF₄ systems.

Colloidal lead halide perovskites (LHP) nanocrystals (NCs) are popular light-emissive materials for optoelectronic devices, of interest for LEDs, LCDs, lasers and quantum light sources. Most studies on LHP NCs focus on relatively large NCs exceeding 10 nm in size, exhibiting weak to no quantum confinement effects. Recently, we showed that perovskite quantum dots (QDs) can be synthesized using a newly developed synthesis route, resulting in perovskite QDs that are tunable between 3 and 13 nm range.[1] Further exploring the tunability of these QDs, we extend the emission of these materials through doping, as well as by coupling these QDs to anchored luminescent dyes.[2,3] First, we demonstrate the synthesis of size-tunable spheroidal CsPbCl₃:Mn²⁺ QDs, which can be obtained by a water–hexane interfacial combined anion and cation exchange strategy starting from CsPbBr₃ QDs.[2] The size dependence observation of the manganese PL efficiency and the slow ET rate suggest that Mn²⁺ mainly gets incorporated at the QD’s surface, highlighting the importance of strategies chosen for the incorporation of Mn²⁺ into perovskite QDs.

In a second approach to tune and extend the perovskite QD´s emission, we combined the perovskite QDs with dye molecules, make hybrid QD-dye systems that exhibit efficient ET from the QDs to the dye molecules.[3] With most QDs, ET usually proceeds through Förster resonance energy transfer (FRET), which requires significant spectral overlap between the QD emission and dye absorbance, and large oscillator strengths of those transitions which severely limits the choice of suitable dyes. As the perovskite QDs do not require passivating inorganic shells for bright emission, we can attach dye molecules directly to their surface, making ET mechanisms beyond FRET accessible. With the CsPbBr₃-ATTO610 QD-dye system we achieved efficient ET from CsPbBr₃ QDs to dyes with dimethyl iminium binding groups. The close binding of dyes to the CsPbBr₃ surface facilitates spatial wavefunction overlap, resulting in efficient ET from CsPbBr₃ to dyes with bright emission from the dye molecules, even with minimal spectral overlap. With steady-state and time-resolved photoluminescence experiments, we show that the ET proceeds via the Dexter exchange-type mechanism which significantly improves the tuneability of such QD-dye systems, and opening avenues for QD-molecule hybrids in a wide range of such as lighting applications

Study of Nanoscale Atomic Diffusion in Metal-Semiconductor Core-Shell Nanoparticles

Sharona Horta¹, Seungho Lee¹, Rhys Bunting¹, Mario Palacios¹, Jani Kotakoski² & Maria Ibáñez¹*

¹Institute of Science and Technology Austria

²University of Vienna

Abstract

Core-shell nanocrystals have garnered significant attention due to their potential for multifunctionality and synergistic effects across various fields including electronics, biomedical applications, pharmaceuticals, optics, and catalysis.  To take full advantage of the functionality of these multi-component nanoparticles, it is imperative to possess a comprehensive understanding of their dynamic structural behavior and stability when exposed to external stimuli.

In this study, we focus on metal-semiconductor nanoparticles, in particular Cu_1-x Au_x@PbS, and investigate the differences in their dynamic structural evolution depending on the core composition using in-situ high-resolution transmission electron microscopy during heating. Our findings reveal that these nanoparticles transform from core-shell to Janus structure via the diffusion of core metal atoms towards the nanoparticle surface. The diffusion of the core atoms can occur either collectively or individually depending on the copper content within. To understand these differences, density functional theory (DFT) calculations were employed. The DFT calculations indicate that the diffusion mechanism is determined by the competition between the surface energy of the core and the interface energy between the core and shell. When the interface energy value is larger, the collective movement of the core atoms is favored, whereas when surface energy predominates, atomic diffusion becomes more favorable. Furthermore, strain mapping of this particle unveils significant changes in interface strain as temperature changes and core diffuses in a manner that ultimately minimizes overall strain.  The insights obtained from these results contribute to the advancement of understanding diffusion at the nanoscale and the dynamic behavior of strain with temperature at the nanoscale.

Colloidal nanocrystals (NCs), with their tunable optical properties, have emerged as essential components in modern optoelectronic devices [1]. While traditional optimization approaches focus on pristine material properties, they often neglect the influence of the complex operational environment, where NCs interact with transport layers, electrodes, and applied electric fields. This gap highlights the need for advanced techniques capable of probing the electronic structure of devices in situ and operando.

In this talk, I will show how scanning photoemission microscopy [2] addresses this need, providing unique insights into the energy landscape of NC-based devices during operation with sub-micron spatial resolution [3]. By unveiling the whole device energy landscape, this technique reveals key parameters such as diode built-in potential and transistor lever arm. Beyond scalar information, the high spatial resolution of this technique enables access to the vectorial distribution of the electric field, uncovering the current pathways within the NC film [4].

This approach significantly advances our ability to study nanocrystal-based devices under realistic conditions, fostering a deeper understanding that can drive rational design and performance optimization.

In the context of nanocrystal-based optoelectronic devices, the design of photodiodes faces a major challenge related to the design of building blocks as simple as the p-n junction that require some control on the carrier density profile. Gating appears as a possible alternative method [1] to shape the doping profile. The main limitations of the latter are related to the continuous application of DC bias, which increases energy consumption and can be a noise source.

In this work, we develop a general approach to achieve carrier density control by coupling the nanocrystal layer to a ferroelectric material. By utilizing the remnant polarization of the latter, the need for the application of gate bias is eliminated. The idea is to use the up and down change in the ferroelectric polarization to directly form a lateral p-n junction in the nanocrystal film, as has already been achieved for 2D materials.

We start by designing a ferroelectric-NC heterostructure made of Lead Zirconium Titanate (PZT) and HgTe NCs that act as an infrared active spin-coatable ink. The effect of the ferroelectric polarization on the HgTe band offsets is revealed using nano-beam X-ray photoemission microscopy. A photodiode is then built and enhanced photoresponse and reduced noise are obtained thanks to the built-in potential of the diode. [2] We then expand the concept to a large scale substrate using the commercially available periodically poled lithium niobate (PPLN) substrate. [3]

How can boron-rich nanocrystalline films be optimized to meet the stringent mechanical demands of extreme environment applications?

Modern advances in clean energy, hypersonic travel, and nuclear technologies place extraordinary demands on materials' thermal and mechanical durability. High-stakes fields, such as aerospace and space exploration, require materials that withstand extreme conditions, often exceeding 4,000 °C, with substantial mechanical strength and oxidation resistance. Refractory materials like ultra-high temperature ceramics (UHTCs), while promising, are limited by high production costs and challenging synthesis processes. This study seeks to address this challenge by exploring nanoscale metal boride materials—specifically, strontium hexaboride (SrB6) nanocrystals (NCs)—as a cost-effective, mechanically robust alternative.

Nanocrystals (NCs) offer unique advantages due to their high surface area, tunable crystallization, and the ability to form films with nanoscale precision, which is critical for enhancing mechanical properties in thin coatings. Here, we investigate the potential of surface-modified SrB6 NCs, blade-coated onto silicon and sapphire substrates, as a pioneering solution for boron-rich, super-hard thin films. Through ligand modification with BF4 and BI3, these NCs achieve distinct structural formations on different substrates, significantly impacting their mechanical performance.

Our findings demonstrate that SrB6-BI3 films on silicon reach up to 10 GPa hardness and a Young's modulus between 180 and 200 GPa. In comparison, SrB6-BF4 films attain 5 GPa hardness and 170 GPa modulus on silicon, with a notably higher modulus of 300 GPa on sapphire, suggesting enhanced stiffness through substrate optimization. Atomic force microscopy (AFM) revealed crystallization patterns where SrB6-BI3 formed micron-sized crystals on silicon, while SrB6-BF4 created spherical clusters, further affecting mechanical properties.

This study highlights that by optimizing ligand choice, substrate selection, and minimizing defects, boron-rich metal boride nanomaterials can be tailored for demanding applications. These findings position SrB6 NC-based films as a promising, cost-efficient alternative to conventional super-hard materials like diamond, with potential breakthroughs in extreme environment applications.

The recent emergence of quantum confined nanomaterials in the field of radiation detection [1], in particular lead halide perovskite nanocrystals, offers scalability [2] and performance advantages [3] over conventional materials. This development raises fundamental questions about the mechanism of scintillation itself at the nanoscale and the role of particle size, arguably the most defining parameter of quantum dots. Understanding this is crucial for the design and optimisation of future nanotechnology scintillators.

In this work, we address these open questions by theoretically and experimentally studying the size-dependent scintillation of CsPbBr₃ nanocrystals using a combination of Monte Carlo simulations, spectroscopic, and radiometric techniques. The results show that the simultaneous effects of size-dependent energy deposition, (multi-)exciton population, and light emission under ionizing excitation, typical of confined particles, combine to maximize the scintillation efficiency and time performance of larger nanocrystals due to greater stopping power and reduced Auger decay.

The agreement between theory and experiment produces a fully validated descriptive model that predicts the scintillation yield and kinetics of nanocrystals without free parameters, providing fundamental guiding for the rational design of nanoscale scintillators.

Perovskite nanocrystals (NCs) are emerging as promising materials for next-generation light-emitting diodes (LEDs) and scintillators, offering significant potential for optoelectronics and radiation detection [1,2]. This presentation explores recent advancements and challenges in developing perovskite-based materials for these applications. We focus on Ruddlesden–Popper (RP) interfaces in mixed halide perovskites (MHPs), which play a critical role in optimizing device performance [1,2]. Using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy, we examine atomic-level defects and strain at these interfaces, revealing how halide migration affects the structural integrity and electroluminescence of MHP-based devices. In addition to lead halide perovskite NCs, we explore the growing interest in low-dimensional, lead-free copper halide perovskite (CHP) NCs for scintillation applications [3]. By employing mechanochemical methods, we synthesize 1D and 0D CHP NCs and investigate their scintillation properties, including decay times and Stokes shifts. Our findings show that tuning the I:Br composition ratio enhances scintillation performance, providing insights into the design of fast-decaying, ultrasensitive X-ray detectors. The presentation will also highlight ongoing research into nanophotonic structures, such as photonic crystals and plasmonic structures, that enhance light emission via Purcell effects, offering the potential for faster, brighter LEDs and scintillators in future technologies [4].

In recent decades, lead halide perovskite nanocrystals (PNCs) have emerged as promising high-atomic-number scintillating materials for next-generation scintillators and photodetectors in ionizing radiation detection. Their strong light-matter interaction, tunable radioluminescence, and solution processability position them as key candidates for advanced applications. However, preserving their luminescence while ensuring environmental stability remains a challenge.  Innovative strategies for embedding PNCs in impermeable host matrices have been developed [1], These approaches preserve the luminescence properties of the NCs in harsh environments and also prevent Pb dispersion, thus enabling their safe use in biological applications. On the one hand, study reveals the crucial parameters in the design of scintillation detector on isolated NC level [2] Here, we explore innovative embedding strategies using mesoporous silica to enhance PNC stability and performance. By spatially engineering PNCs, we can manipulate their collective scintillating behaavior, leading to cascading scintillation effects while maintaining a low overall lead content. Also, we propose a design of bismuth-loaded PNC plastic scintillator for gamma and neutron detection. Given that bismuth is known to quench the photoluminescence (PL) of perovskites instantaneously, to the best of our knowledge, our work is the first to successfully integrate perovskites with bismuth in a scintillation system, demonstrating a novel approach that overcomes the challenge and advancing perovskites’ potential for.broader radiation detection technologies.

Colloidal coupled quantum dots molecules (CQDMs) were recently synthesized by “nano-chemistry”.[1-3] These CQDMs, made by fusing two CdSe/CdS core/shell quantum dots, uniquely consist of two coupled emitting centers. Such CQDMs raise significant fundamental interest as they manifest optical properties differing from those of the constituent quantum dots, which has been taken as a signature of electronic coupling, akin to that in molecules constructed from real atoms.

Recent theoretical studies, however, indicate that the distinct behavior of CQDMs arises mainly from relaxed exciton confinement, rather than molecular-like electronic coupling.[4] The reason is that CQDMs are designed to favor electron tunneling between the dots, but holes remain strongly localized inside the core of the individual dots. Consequently, upon photoexcitation, holes capture electrons and prevent them from tunnel-coupling. The same holds when CQDMs are populated with biexcitons (forming an artificial H₂ molecule).[5]

Herein, we present a strategy to restore electronic coupling in CQDMs. By means of k·p simulations, we show that using trions (charged excitons) instead of neutral excitonic complexes, the ground state of the CQDM acquires strong molecular (bonding) character. This is because uncompensated Coulomb repulsions prevent carriers from localizing in individual dots. Positive trions (an artificial H₂⁺ molecule)  are found to be particularly apt for the practical realization of CQDMs with substantial electronic coupling.

Quantum dots (QDs) as scintillators for radiation detection have garnered significant attention due to their unique photophysical properties. Previous studies have shown nanocrystal scintillators for X-rays, gamma, neutrons, and electrons[1][2][3][4]. The European Pathfinder project Unicorn proposes to develop novel nano-scintillators for detection of neutrinoless double beta decay(0νββ) for the first time. 0νββ, a rare nuclear process, is at the frontier of neutrino physics, offering insight into the fundamental nature of neutrinos. Cadmium isotopes, such as ¹¹⁶Cd, are promising candidates for such studies due to their favorable properties, including high natural abundance, a large Q_ββ value, and potential for inverse beta decay detection[5]. In this work, we developed a novel scintillator Cu-doped CdZnS/ZnS QDs for this application. This study presents a comprehensive investigation of the fundamental optical properties and scintillation dynamics of Cu-doped CdZnS/ZnS QDs. The results demonstrate that these QDs exhibit high quantum efficiency, ensuring a strong light yield. Additionally, their large Stokes shift effectively suppresses reabsorption, a critical feature for high-loading and large-volume detectors. Furthermore, we evaluated the performance of this material in gamma spectroscopy highlighting its potential as a scintillator for gamma detection, compared to conventional plastic scintillators. This work represents a step forward in the development of advanced scintillators for neutrino physics and rare event detection technologies.

Nanostructured semiconductors dominate opto-electronics to date, mainly in the framework of quantum wells and dots grown by vacuum epitaxial methods. A next (r)evolution in this field is happening as we speak through the use of solution processable nanostructured materials. Indeed, by combining low temperature, substrate independent processing with size-tunable optical properties, such as luminescence and optical amplification (gain), these ‘nanocrystals’ are excellent building blocks to realize small footprint LEDs and lasers. In this talk, I will overview our efforts towards integrated laser sources based on colloidal nanocrystals. To do so, I will start out with discussing how optical gain, a premise to build lasers, is measured and quantified in these materials. Building on this framework, I will explain how net stimulated emission develops in and how we reached after nearly a decade of research a set of materials with excellent gain metrics on all fronts using so-called ‘bulk nanocrystals’ as a new design route. Next, I will show a few results of combining these flexible materials with integrated photonic cavities, thereby realizing ultra-small footprint lasers operating under quasi-CW pumping. Finally, I will present a route forward to transition these RGB lasers into the infrared spectrum where a select pick of ultrafast mid-infrared spectroscopy experiments gives a first flavor of the challenges , but also massive opportunities, lying ahead in the ongoing ERC project ‘NOMISS’.

The interconversion between singlet and triplet spin states of photogenerated radical pairs is a genuine quantum process, which can be harnessed to coherently manipulate the recombination products through a magnetic field. This control is central to such diverse fields as molecular optoelectronics, quantum sensing, quantum biology and spin chemistry, but its effect is typically fairly weak in pure molecular systems. Here we introduce hybrid radical pairs constructed from semiconductor quantum dots (QDs) and organic molecules. The large g-factor difference allows us to directly observe the radical-pair spin quantum beats usually hidden in previous studies, which are further accelerated by the strong exchange coupling of the radical pairs enabled by quantum confinement of QDs. The rapid quantum beats enable efficient and coherent control of charge recombination dynamics at room temperature, with the modulation level of the yield of spin-triplet products reaching 400%. As spin-triplets are ubiquitously involved in molecular and hybrid inorganic/organic photocatalytic and photonic devices, an efficient, quantum-coherent control over the triplet recombination yields of radical pairs could offer disruptive solutions to engineering the performance of such devices. Additionally, hybrid radical pairs constitute a unique material platform to merge the field of emerging molecular quantum sciences with solid-state quantum platforms to enable novel physical phenomena and quantum technologies.

The current spectral coverage of colloidal quantum dot (CQD) lasers spans from the visible, using zinc and cadmium chalcogenide-based quantum dots, all the way through to the short-wave infrared (SWIR) region, up until telecoms wavelengths using lead sulphide (PbS). However, the full spectral tuneable range of the PbS quantum confinement (up to 2.5 μm) has yet to be explored. The so-called extended SWIR region (1.7 μm – 2.5 μm), has many applications such as in LIDAR, biological imaging and environmental monitoring and is currently served only by exotic and costly materials with limited-scalability. We extend the gain coverage of PbS CQDs up until 2500 nm and report lasing with emission tuned between 2150 nm and 2500 nm using distributed feedback cavities. A detailed size-dependent study of the gain behaviour of these PbS CQDs is performed, investigating the absorption cross-section and excited state dynamics of the CQDs. The optical gain threshold of the largest dots emitting in the extended SWIR is seen to reduce by a factor of 36 compared to smaller-sized CQDs emitting at telecoms, reaching an amplified spontaneous emission (ASE) threshold down to 42 µJ/cm². From this study we were then able to predict and realise for the first time, optical gain and lasing from PbS CQDs under nanosecond pumping. This major milestone paves the way for the realization of compact and practical CQD infrared lasers in the extended SWIR region and potentially towards electrically driven laser diodes.

Colloidal quantum dots (QDs) show great promise as active gain materials for integrated photonics due to their solution processability. Unlike conventional semiconductors, colloidal QD layers can be directly deposited on various substrates using simple coating techniques such as spin coating, inkjet printing, or drop-casting.  Recent advances in QD research have demonstrated their utility as low-threshold laser materials with remarkable robustness and stability.^1,2 In this study, we utilized so-called type-(I+II) QDs² to demonstrate well-behaved microdisk lasers.

Type-(I+II) QDs were recently introduced in the context of research into liquid QD lasing.^2,3 They feature large optical-gain cross-sections and long optical gain lifetimes of ~3 ns. The latter property stems from a novel optical gain mechanism in which optical gain species are hybrid direct/indirect biexcitons with slow, trion-like Auger recombination. Due to their excellent optical-gain properties, type-(I+II) QDs are well suited for implementing both liquid- and solid-state lasers, including laser diodes.     

To prepare microdisk lasers, we use standard photolithography and dry etching techniques. When excited using short optical pulses, QD-based microlasers exhibit low-threshold whispering gallery mode lasing. We applied similar fabrication techniques to incorporate microring resonators into high-current density electroluminescent structures. The developed devices exhibit strong dual-band electroluminescence due to the band-edge (1S) and first excited (1P) electronic states. This indicates the realization of population inversion in the QD medium. The next step is to achieve a net positive overall gain by optimizing the design of our fully stacked devices to enhance the modal gain and, in parallel, reduce the parasitic optical loss. The expected result of this work will be an electrically excited microdisk laser.

Artificial quantum systems with synthetic dimensions enable the exploration of novel quantum phenomena that are challenging to create in conventional materials. The synthetic degrees-of-freedom effectively increase the system's dimensionality without altering its physical structure, allowing for the study of higher-dimensional physics in lower-dimensional setups. However, synthetic quantum systems can suffer from intrinsic disorder, leading to rapid decoherence and limiting their scalability and collective coherence. This disorder-induced decoherence is a major obstacle in quantum information science and technology. Here we show that long-range interactions can mitigate decoherence and create persistent collective coherence preserved in highly symmetric collective excited states. These states exhibit exceptional robustness, capable of storing long-lived quantum memory despite disorder. We introduce an order parameter to quantify collective coherence, identifying a sharp transition from a zero to a non-zero collective coherence at a critical interaction strength or critical disorder. We denote this phenomenon as “supercoherence” and show its universality in a wide range of systems. This phenomenon not only challenges traditional views on the inevitability of decoherence in disordered quantum systems but also opens new avenues for harnessing collective coherence for quantum information storage and processing.

In lead halide perovskites (APbX₃), the effect of the A-site cation on optical and electronic properties has initially been thought to be marginal. Yet, evidence of beneficial effects on solar cell performance and light emission is accumulating. Here, we report that the A-cation in soft APbBr₃ colloidal quantum dots (QDs) controls the phonon-induced localization of the exciton wavefunction. Insights from ab initio molecular dynamics and single-particle fluorescence spectroscopy demonstrate that anharmonic lattice vibrations and the resulting polymorphism act as an additional confinement potential. Avoiding the trade-off between single-photon purity and optical stability faced by downsizing conventional QDs into the strong confinement regime, dynamical phonon-induced confinement in large organic-inorganic perovskite QDs enables bright (10⁶ photons/s), stable (>1h), and pure (>95%) single-photon emission in a widely tuneable spectral range (495-745 nm). Strong electron-phonon interaction in soft perovskite QDs provides an unconventional route toward the development of scalable room-temperature quantum light sources.

Supercrystals represent three-dimensional orderings of colloidal nanocrystals (NCs), showcasing collective properties in photonics, phononics, and electronics applications.^1,2 Recent studies have shown that such assemblies are directly produced during nanocrystal reactions.^3–6 However, a fundamental understanding of in situ formed supercrystals that withstand typical NC purification processes remains underexplored, which is important for further use. Herein, we report the reaction precursor-mediated formation of stable PbTe supercrystals. Rationalizing the formation of these assemblies through small-angle x-ray scattering (SAXS) measurements, we unveil their formation mechanism. Our findings reveal that the supercrystal formation occurs in the presence of an excess of lead oleates in the crude solution. It should be noted that the formed supercrystals can be stabilized under specific conditions determined by the lead oleate cluster concentration, content of trioctylphosphine telluride (TOP-Te), NC size and the need of an annealing step at mild conditions. Furthermore, this approach allows for the continuous growth of a secondary phase within the supercrystal; for example in the case of PbTe supercrystals, a PbS shell can be grown on each PbTe NC constituent, resulting in core-shell PbTe-PbS supercrystals. Our work elucidates that reaction precursors play an important role in in situ SC formation and stabilization, implying the possibility of applying this knowledge to other NC reactions.

Optical studies of colloidal nanocrystals (NCs) at cryogenic temperatures in high magnetic fields provide valuable information about their emission properties. Low temperatures (typically 1.7 – 4 K) are required to reduce the population of acoustic phonons and narrow the linewidth, and to reduce the population of the exited states. These conditions enable ensemble measurements. However, ensemble techniques generally give an “average” estimate of the underlying photophysics. Single NC spectroscopy, on the other hand, offers valuable additional insight into the properties of these materials, but has several disadvantages. First, the experimental complexity increases, and second, typically only NCs with bright and stable emission are selected for single-NC measurements, therefore, their properties not always represent the properties of all NCs in ensemble. Therefore, a combination of ensemble and single-NC measurements can provide a comprehensive understanding of the properties of these materials.

InP-based colloidal NCs are being developed as an alternative to cadmium-based materials. Since it is a relatively new material, understanding of their magneto-optical properties is still at an early stage. We have performed both ensemble and micro-photoluminescence measurements of InP/ZnSe/ZnS NCs [1]. The ensemble measurements revealed excitonic emission and provided information about hole ground state and exciton fine structure splitting. On a single-NC level, the trion emission has been discovered. The Zeeman splitting between the trion emission lines provided electron and hole Lande g-factors.

Colloidal CdSe-based nanoplatelets (NPLs) are quasi-two-dimensional nanocrystals with a strong quantum confinement in only one direction. They benefit from a lack of inhomogeneous broadening and have narrow and bright photoluminescence. To increase photostability of NPLs and enhance their quantum yield, it is desired to coat the CdSe core with a larger bandgap semiconductor shell. However, the shell growth leads to a large increase of the linewidth (>40 meV in core/shell NPLs versus ~10 meV in bare NPLs at cryogenic temperatures). Understanding of the broadening mechanisms is important, since narrow ensemble emission spectra are required in a lot of applications. We have demonstrated that at cryogenic temperatures the emission spectra of individual CdSe/CdZnS NPLs exhibit multiple isolated emission lines separated by several meVs spanning a range of 30 – 60 meV [2]. We believe that these features stem from trions localized in shallow (a few meV deep) traps. We have demonstrated that the population of occupied traps can be controlled optically.

Group II-VI semiconductor nanoplatelets present narrow optical properties thanks to their thickness, which is the only confined direction, defined at the atomic scale without roughness. It makes them appealing for optoelectronic devices. Besides, in NPLs, the wide top and bottom facets are extended over 1000s of nm². Thus, these facets can be assimilated to substrates for the self-assembly of molecules. At the end of the syntheses, the carboxylate ligands induce a tensile in-plane stress. These pristine ligands can be exchanged with X-type ligands, which can be halide, thiolate, phosphonate or chiral ligands. And, depending on their anchoring groups, the in-plane stress can vary from a tensile to a compressive stress, inducing thus a distortion of the crystal structure.

Here, we will first show that in CdSe NPLs capped with in-plane compressive stress thiolate ligands, Cd²⁺ are effectively exchanged by Cu⁺ cations, thus enabling the synthesis of Cu_2-xSe NPLs with thicknesses comprise between 2 monolayers (MLs) and 5 MLs. Indeed, cation exchange is an interesting method to obtain nanoparticles which shape cannot be directly synthesized. In this case, the exchange most probably happens through the edges and its kinetic can be modified depending on the aliphatic chain length of the thiolate ligands. Such Cu_2-xSe NPLs can be used as template for further cation exchange with Hg²⁺, leading to the synthesis of HgSe NPLs. However, the process is tedious and requires 2 consecutive steps including one in the glovebox. So we have demonstrated that a cation exchange from Cd²⁺ to Hg²⁺ could be co-catalyzed with Ag⁺ cations. Through this method, HgCdSe NPLs are obtained and enable to overcome the limitation of the direct cation exchange from Cd²⁺ to Hg²⁺ to only two cationic planes. The obtained NPLs are now showing a variation of the optical properties with quantum confinement.

Finally, I will show that pristine ligands can be exchanged with chiral ligands, in the aim of circular dichroism optical properties. We are showing a direct ligand exchange in ethanol from carboxylate to tartrate ligands, inducing the self-assembly of the tartrate ligands in two different conformations exhibiting opposite circular dichroism signal. The emergence of the circular dichroism optical properties comes from a good coupling of the ligands on the surface and an orthorhombic crystal distorsion. The dissymmetric factor can reach values as high as 1.2% for extended NPLs.

Traditional infrared (IR) technology has relied on the epitaxial growth of CdHgTe (MCT) and InGaAs since 1960s. 
While these epitaxial materials are well-established, they have not become cost-effective products for the consumer 
market, limiting their applications. Solution-processed nanocrystals (NCs) exhibit tunable optical properties 
spanning from the UV to THz spectra, making them suitable for applications such as single photon emission, 
biolabeling, and down conversion for displays. Although the visible spectrum is commonly utilized, the potential of 
NCs in the IR range has been largely overlooked. For the infrared spectral range, organic semiconductors are 
intrinsically inefficient, leaving inorganic NCs as the most cost-effective and efficient option. Here, we present our 
latest achievements in infrared light-emitting diodes (LEDs) and photodiodes (PDs), operating across various 
wavelengths using tunable emissive colloidal materials. Our focus has been on CdHgSe nanoplatelets (NPLs) and
RoHS-compliant InAs quantum dots (QDs).
Mercury chalcogenides (eg. HgSe and HgTe) exhibit the most efficient emission in the near-IR to mid-IR range. 
However, these materials are inherently fragile, making it extremely challenging to grow a shell over them. To 
address this, we developed an innovative synthesis method starting with CdSe NPLs. We performed cation 
exchange, replacing Cd with Hg, and subsequently grew a thick layer of CdZnS over the resulting Cd_xHg_1-xSe NPLs. 
This procedure allows fine tuning by adjusting the concentration of Hg cations and the stoichiometric ratio of Cd/Hg, 
mirroring the epitaxial growth of MCT. Utilizing this tunable short-wave infrared (SWIR) emitting material, which 
achieves a photoluminescence quantum yield (PLQY) of 55%, we designed and fabricated LEDs that emit at 
wavelengths ranging from 1200 nm to 1700 nm. This builds on our previous work, where we achieved an external 
quantum efficiency (EQE) of 7.5% at 1300 nm. [1] Here, we also showed the potential of using the same material as 
the active layer of PD, with an EQE of 25% at 1200 nm wavelength.
In parallel, with an innovative synthesis of InAs/ZnSe core/shell QDs we increased the thickness of ZnSe thick shell. 
Synthesizing InAs QDs has traditionally been a complex process, typically requiring the use of highly reactive, 
flammable, toxic, and expensive chemicals such as tris-trimethylsilyl arsine (TMS-As). In recent years, researchers 
have sought to replace TMS-As with cheaper, safer, and less reactive arsenic precursors. Among the alternatives 
explored, tris(dimethylamino)-arsine (amino-As) has shown the most promise. Using amino-As along with Alane 
N,N-dimethylethylamine as a reducing agent and ZnCl₂ as an additive, we developed a method to synthesize InAs 
QDs and InAs/ZnSe core/shell QDs with a shell thickness 1.5 monolayers. These QDs exhibit photoluminescence (PL) at 860 nm and a PLQY of 42% ± 4%.[2] Utilizing these QDs, we produced an LED with a turn-on voltage of 2.7V, an EQE of 5.5%, and a maximum radiance of 0.2 Wsr⁻¹cm⁻². [3] Building on these findings, we refined our synthesis 
process to create InAs/ZnSe QDs with a tunable ZnSe shell thickness of up to 7 monolayers, achieving a remarkable 
PLQY of approximately up to 70% ± 7% and a PL peak at 900 nm in solution. [4] The electronic structure of the thickshell QDs resembles that of type-I heterostructures, enhancing exciton confinement in the core region due to the ZnSe layer. We utilized these efficient QDs for making LEDs. The champion LED reaches an EQE of 13.3% and radiance of 12 Wsr⁻¹cm⁻², figures-of-merit that are comparable to devices based on complex core/multi-shell InAs QDs obtained via a tris-trimethylsilyl (TMS) arsine route. [5]

II-VI semiconductor nanoplatelets (NPLs) present optical features lacking of inhomogeneous broadening thanks to their 2D shape. Their thicknesses only present few atomic planes, such that any modifications of the surface chemistry induces a modification of their optical features. Recent studies have been dedicated to induce chiral light-matter interactions on these particles, to reach strong circular dichroism (CD) and circularly polarized luminescence (CPL) features, for example by grafting cysteine ligands on their surface.

Here, we propose to use chiral tartrate ligands [1]. Surprisingly, the exchange undergoes several stages, with an increase of the CD feature at the position of the heavy hole which red shifts over time followed by an inversion of the CD signal when the absorption saturates. The dissymmetry factor can reach values as high as 1.2 x 10^-2. The peak inversion global aspect is influenced by the lateral aspect of the initial particle, the former surface chemistry, and the synthesis conditions.

This inversion is attributed to a mechanical relaxation of the ligand assembly that induces a different coupling between the inorganic core and the chiral ligands. This hypothesis is supported by surface chemistry characterization and XRD analysis.