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, and our attempts to control their growth kinetics. The talk will also discuss post-synthesis strategies to improve the stability of the NCs and their subsequent use in light emitting diodes. The last part of the talk will discuss our findings on the ordering of NCs in superstructures, how for example cryogenic temperatures can influence the degree of ordering, and how the degree of order/disorder can be estimated.

Crystals are defined as solids that display three-dimensional atomic-scale order and produce sharp X-ray diffraction at wide angles. By this definition, colloidal nanocrystal superlattices are not considered true crystals, because cumulative disorder disrupts the atomic periodicity across particles and prevents wide-angle supercrystal diffraction. Thus, superlattices are considered semi-ordered aggregates of nanocrystals, each treated as an independent entity and assumed to remain unaffected by self-assembly. Challenging this assumption, here we show that CsPbBr₃ superlattices meet the formal definition of crystals, and can be isolated and measured in a single-supercrystal diffraction experiment. This analysis revealed striking similarities with protein crystals, whose building blocks are comparable in size and complexity to nanoparticles. Like proteins, CsPbBr₃ nanocrystals undergo conformational changes upon self-assembly, leading to a measurable lattice contraction and a formal transition to cubic supersymmetry. Like proteins, nanocrystals occupy just 50% of the supercrystal volume, with ligands in the remaining space self-organizing in patterns comparable to structural and interstitial water molecules. Like proteins, the nanocrystal electron density can be reconstructed from single-supercrystal X-ray diffraction, using strategies inspired by structural biology. This evidence prompts a rethinking of superlattices as true crystals of hybrid organic-inorganic phases, and demonstrates that they can serve as tools to study the structure of their building blocks. Following the lead of structural biology, we expect that single-supercrystal X-ray diffraction will provide atomic-resolution insight into elusive features of nanocrystals, such as facet-specific surface structure, ligand density and binding motifs, and the presence of strain fields within the inorganic core.

 

Semiconducting colloidal quantum dots (QDs) have garnered great attention for photovoltaics due to their unique properties, including decoupled crystallization from film deposition, size-tunable bandgap, multiple exciton generation, etc. Nanometer-sized colloidal metal halide perovskite QDs have emerged and brought unique opportunities for photovoltaic application due to the high defect tolerance of perovskite and many features that emerge at the nanoscale. Perovskite QDs or more broadly, nanocrystals, show high photoluminescence (PL) quantum yields, spectrally tunable bandgap, flexible compositional control, and crystalline strain benefits. Metal halide perovskite QDs are readily synthesized with exceptional optoelectronic quality opening a route for next generation photovoltaic, as well as exploring LHP physics at the nanoscale.

Since the first report in 2016, perovskite QDs also became a point of interest in photovoltaic research. Currently FAPbI3 QD holds the record efficiency for QD solar cells (over 19%) proving better than any previous QD material composition. This talk will highlight the importance of high-efficiency perovskite QD solar cells, from synthesis to device fabrication. We will discuss current state of the art and lay out many open opportunities in perovskite QD solar cells to achieve approaching 20%, as well as the design and synthesis of conductive perovskite QD ink toward high-efficiency and fast printable solar cells.

Halide perovskite nanocrystals have been extensively studied for their bright and color-tunable emission; however, only limited reports have been presented on their heterostructures with plasmonic metals or other semiconductor nanocrystals. The chemistry governing interface formation—through ionic–covalent or metallic bonding—is considered a key parameter in the design of such hybrid nanostructures. In this presentation, various epitaxial heterostructures of CsPbBr₃ with Pt, Au, Ag, and Bi metal particles, as well as with several chalcogenide and chalcohalide nanocrystals, will be discussed. Surface facets and epitaxial interface bonding will be shown through high-resolution electron microscopic imaging, and the underlying chemistry involved for the formation of these nanostructures will be described. Attention will be extended beyond isotropic nanocrystals to anisotropic morphologies such as rods and platelets. Along with their colloidal synthesis, changes in the optical properties of these heterostructures will also be presented. Overall, this presentation will reflect the synthesis, characterization, and modulation of material properties in the emerging heterostructures of lead halide perovskite nanocrystals.

Recent developments in the synthesis of perovskite nanoplatelets (NPLs) have made them promising candidates for narrow-emissive nanomaterials. The major obstacle to their commercialization is the poor stability. Dynamic ligand binding, combined with the ionic nature of the lattice, causes most perovskite NPLs to degrade within a week. Amines play an indispensable role in NPL synthesis: they provide steric effects that enable anisotropic growth and serve as surface passivating agents, but they are also the main culprit behind the instability. Compared with other ligands, amine ligands dissociate more readily from the NPL surface, promoting aggregation, which compromises quantum confinement and emission purity. In this talk, we will present two complementary approaches to replacing and eliminating ammonium ligands during NPL synthesis. One approach uses a post-synthetic treatment to partially substitute the ammonium ligands with metal cations. The other approach renovates the ligand pair chemistry by employing organosulfonates as the sole ligands, providing both anisotropic growth and effective surface passivation. These amine-reduced or amine-free NPLs exhibit significantly improved stability compared with conventional NPLs, revealing a key strategy for advancing NPLs toward commercialization.

Cesium lead halide has been extensively studied as efficient materials for optoelectronic
device applications. Beyond Cs(I), the exploration of other inorganic A-site monovalent cations
remains limited, though Rb(I) stands out as a potential alternative whose role in colloidal
nanocrystals is largely unexplored. Here, Rb (I) is employed as an effective A-site cation in
forming monoclinic-phase RbPb₂Cl₅, where Pb (II) heptahedrally coordinated. A template
mediated cation exchange strategy is employed where 0D Rb₄CdCl₆ used as host nanocrystals.
Upon introduction of Pb (II), fast Cd to Pb ion exchange triggers and 2D RbPb₂Cl₅ in rhombic
prism, hexagonal prism or hexagonal platelet shaped nanocrystals are formed depending on
the reaction conditions. Further to explore the optoelectronic properties, photo response
measurements are carried out which exhibit significant pyro-photocurrent response from
these nanocrystals even under ultra-low-intensity light illumination (7 nW/cm²) across
ultraviolet (UV) to near-infrared (NIR) spectral range. Despite its centrosymmetric structure,
RbPb₂Cl₅ generates pyro-photocurrent due to surface halide deficiencies, supported by DFT
showing surface polarization of |𝝙P| = 0.173 C/m². These findings highlight the pivotal role of
Rb (I) in stabilizing these nanostructures and open a new avenue for their application in
advanced optoelectronic devices.

The broad-band emision of carbon dots (CDs) has been a key bottleneck restricting their application in high-color-purity optoelectronic devices,such as wide color gamut displays.  To address this fundamental challenge, we propose a rational synthesis strategy to achieve full-color narrow-band emision from carbon dots.  Through meticulous core-shell stucture engineering,by selecting highly symmetric precursors and fine-tuning reaction kinetics, we successfully achieved the tunable fabrication of high-color-purity near-infrared CDs,achieving a maximum emision wavelength of 704 nm and a minimum half-width of 29 nm. The superior performance of these narrow-band CDs has been validated in practical display prototypes. By integrating blue, gren,and red CDs onto a UV chip,a white light-emitting diode (W-LED) was fabricated.  This device exhibits an extremely wide color gamut, covering 114% of the NTSC standard, fully demonstrating the high color purity of our materials and their application potential in next-generation display technologies. In summary,this work provides a fundamental design principle for overcoming the inherent broadband emision problem of carbon dots. It establishes narowband emision carbon dots as a viable,high-performance, heavy metal-free alternative to traditional quantum dots for use in demanding display applications.

Carbon is one of the most abundant elements on Earth and is known for its ability to form a wide range of compounds. At the nanoscale, carbon can also form low-dimensional materials like graphene (2D), carbon nanotubes (1D), fullerenes (0D), and carbon dots (CDots) (0D), depending on how sp² and sp³ hybridized carbon atoms are arranged, which are featured for their unique properties and have become key in materials research.[1] While many carbon-based nanomaterials exhibit weak photoluminescence (PL), CDots are characterized by pronounced and tunable PL properties and are typically classified into four main types: carbon quantum dots (CQDs), graphene quantum dots (GQDs), carbon nanodots (CNDs), and carbonized polymer dots (CPDs).³ Despite extensive efforts towards defining these categories, accurately identifying the specific type of material remains a significant challenge due to the considerable overlap in their morphological and optical properties.[2] Among the challenges in characterizing CDots, a particularly notable feature is their PL dependence on the excitation wavelength. This behavior has been attributed to electronic transitions involving surface states, structural defects, or oxygen-containing functional groups, while others suggest it may result from a distribution of emissive sites or size-related effects.[3]

In this work, we present an ultrarapid microwave-assisted method (60 seconds) that enables the simultaneous synthesis of two distinct carbon-based nanomaterials. A specific purification procedure was developed, allowing their complete separation and revealing that each material exhibits unique physicochemical properties. Through this approach, we successfully obtained green-emissive CQDs with an average size of 7.0 ± 0.9 nm, along with non-emissive black carbon nanospheres of ca. 50 nm in diameter. To gain insight into their structural and functional behavior, an exhaustive characterization was carried out using complementary techniques, including Small-Angle X-ray Scattering (SAXS), Raman spectroscopy, Attenuated Total Reflectance – Fourier Transform Infrared Spectroscopy (ATR-FTIR), High resolution Transmission Electron Microscopy (HRTEM), X-ray Photoelectron Spectroscopy (XPS), and X-ray Diffraction (XRD). This comprehensive analysis provided a detailed understanding of the morphology, composition, and structural features of both nanomaterials. The photocatalytic activity of these materials was evaluated using as proof-of-concept the reduction of methyl viologen (MV²⁺) in aqueous media.

The structural and compositional modifications of CsPbX₃ (X = Cl, Br, I) nanocrystals have been extensively studied for applications such as tailoring optical properties and constructing superlattices. However, the facile synthesis of hollow structured CsPbX₃ nanocrystals remains challenging. Here, we report a simple method to synthesize core@shell AgBr@CsPbBr₃ nanocubes, which serve as templates for producing single-phase hollow CsPbX₃ (X = Cl, Br, I) nanocubes through sequential halide anion exchange reactions. Exchange with Cl⁻ ions leads to Ag@CsPbCl₃, whereas exchange with I⁻ ions results in hollow CsPbI₃ cubes due to the I⁻ etching of the AgBr core. These hollow CsPbI₃ nanocubes can subsequently be transformed into hollow CsPbBr₃ and CsPbCl₃ nanocubes via adjacent halide exchanges. Controlled adjacent halide exchanges (I → Br and Br → I) consistently yield alloyed CsPb(I:Br)₃ and CsPb(Br:Cl)₃ nanocubes. Notably, more complex heterostructures, including CsPbI₃-CsPbCl₃ and CsPbCl₃@CsPbI₃, are formed during jump halide exchanges (I → Cl and Cl → I). In the CsPbI₃-CsPbCl₃ heterostructure, CsPbCl₃ domains, primarily located at the corners, share epitaxial interfaces with CsPbI₃ domains within a single hollow nanocube. In the CsPbCl₃@CsPbI₃ heterostructure, CsPbI₃ layers propagate on the surface, forming a core@shell geometry in a single perovskite nanocube. This discovery provides a platform for exploring the electronic and optical properties of heterostructures featuring epitaxial domains with photoluminescence emissions spanning different regions of the visible spectrum.

Metal borides constitute a broad and technologically significant class of materials defined by their exceptional hardness, thermal stability, electronic tunability, and chemical robustness. These properties make boron-rich compounds—from transition-metal borides such as Ni₃B to rare-earth hexaborides (MB₆)—highly attractive for applications in catalysis, energy storage, heat management, and extreme-environment coatings. Yet, synthesizing metal borides with precise control across multiple length scales remains a key challenge, limiting their wider implementation in functional devices and engineered architectures.

In this work, we establish general strategies to design and process metal borides from the nanoscale to the macroscale. At the nanoscale, we use low-temperature borothermal synthesis to obtain phase-pure Ni₃B and MB₆ (M = Sr, Ca, Ba, La, Ce) nanocrystals with controlled size, crystallinity, and surface chemistry. These nanocrystals can be rendered colloidally stable through tailored inorganic and organic ligand treatments, enabling their dispersion in polar and non-polar solvents and their integration into thin films, composites, and catalytic systems. Advanced electron microscopy and solid-state ¹¹B NMR confirm the formation of crystalline boride cores with surface BₓOᵧ shells, providing insight into boron coordination, stability, and reactivity.

At the macroscale, we demonstrate boron incorporation into 3D metallic architectures by boriding commercial Ni foam. This process forms a conformal nickel-boride layer throughout the porous network, dramatically improving thermal robustness, oxidation resistance, and mechanical performance. The resulting borided foams highlight how boron diffusion can reinforce lightweight metallic scaffolds, complementing the nanoscale functionalities offered by discrete boride nanocrystals.

Together, these approaches present a unified framework for the scalable synthesis, surface engineering, and structural design of metal borides. By bridging nanocrystal chemistry with bulk boriding processes, this work expands the accessible design space for boron-rich materials and opens pathways toward their deployment in high-temperature catalysis, protective coatings, plasmonic and optical devices, energy-storage electrodes, and mechanically resilient porous structures.

We will discuss our work to understand how phase is controlled in nanocrystal synthesis. Substituted thioureas were reacted with iron, cobalt and nickel and copper salts to isolate how reaction kinetics influences phase without the added complication of changing molecular decomposition mechanisms. By mapping out the effects of precursor reactivity and temperature on the phase of the metal sulfides, hypotheses can be developed about the predictable paths that occur between phases based on their anion stacking and hole filling patterns. We discovered that the synthetic behavior of the iron sulfides and cobalt sulfides can mostly be predicted by the anion stacking of hcp or ccp. The nickel sulfides instead show patterns based on the coordination number of the cations, be it 4, 5 or 6 in the product phases. Based on these maps, we developed hypothesis driven syntheses to pyrrhotite (Fe_1-xS), mackinawite (Fe_1+xS), smythite (Fe_3+xS₄), greigite (Fe₃S₄), marcasite (FeS₂), pyrite (FeS₂), jaipurite (CoS), cobalt pentlandite (Co₈S₉), linnaeite (Co₃S₄), cattierite (CoS₂), a- NiS, millerite (NiS), godlevskite (Ni₉S₈), heazlewoodite (Ni₃S₂), polydymite (Ni₃S₄), vaesite (NiS₂). 

Elaborate chemical synthesis methods enable the production of various types of inorganic nanocrystals (NCs) with uniform shapes and size distributions. However, how can we synthesize NCs with thermodynamically metastable phases or highly complex structures? Transforming pre-synthesized NCs through elemental substitutions—such as ion-exchange reactions for ionic NCs [1–4] and galvanic replacement reactions for metallic NCs—can overcome the limitations of conventional one-step syntheses. In particular, ion-exchange reactions have been extensively studied using numerous combinations of foreign ions and ionic NCs of various shapes. The optical properties of the resulting ionic NCs and their assemblies can be readily tuned over a wide range [5]. Here, we focus on full and partial ion-exchange reactions of ionic NCs and their assemblies, emphasizing key aspects such as the preservation of morphology and dimensions [2,3,6,7]. Finally, we discuss the formation of unprecedented Z3-type FePd₃ NCs by substituting a small amount of Pd with In, based on the interelement miscibility among Fe, Pd, and In [8–10].

Colloidal syntheses are used worldwide to prepare various nanomaterials for multiple applications. In most cases, the colloidal syntheses proceed in the liquid phase by reduction of a precursor in presence of reducing agents. Typically, various ligands / capping agents / stabilizers are claimed to be needed for successful syntheses leading to stable colloids with size control [1] Unfortunately, those chemicals that interact with the NP surface can be detrimental for further applications where a clean surface would be preferred, for instance in catalysis or medicine.

Rather than removing those chemicals by tedious chemical- and energy-consuming steps, the Nanomaterials Engineering for Sustainable Technologies (NEST) group aims at developing surfactant-free syntheses, i.e. syntheses with a minimal amount of chemicals that lead to readily active materials, for instance for catalysis. A focus is on processes carried out at room or low temperature and using relatively safe chemicals. This approach not only brings new opportunities for new fundamental insights into nanomaterial synthesis, but also facilitates the general production and use of the nanomaterials, while it facilitates the scaling of the related nanotechnologies.

This talk will provide an overview of our achievements related to metal nanomaterials and in particular our recent findings on gold nanoparticles. Highlights will be on:

Sustainability, with the possibility to develop surfactant-free colloidal Pt [2], Ir[2], Os [3], nanoparticles, but also Au [4]. It will be illustrated how the use of additives can actually be detrimental compared to the surfactant-free approach [5].

Fundamental insights, with the example of Ir and Pt nanoparticle formation [6, 7] but also Au [8].

Size control without surfactant, with the example of Au nanoparticles obtained by ethanol-mediated or NaBH₄-mediated synthesis, leveraging on reducing agent / Au molar ratio, using mixtures of reducing agents, water conductivity and/or inducing the synthesis in different ways [8-12].

Stability, the surfactant-free colloids are typically stable for months even for storage at room temperature and the cations have been overlooked knobs to boost stability and to perform syntheses at higher precursor concentrations. The stability increases in the order K⁺<Na⁺<Li⁺ for various colloidal sytnheses [13].

Catalytic activity, with the example of size effects studies performed for the 4-nitrophenol reduction as a model reaction for water treatment and electrochemical energy conversion with the case studies of the ethanol oxidation reaction (EOR) [4, 9, 14, 15] the oxygen evolution reaction (OER) [2] and the oxygen reduction reaction (ORR) [16]. A focus will be on showing the benefits of surfactant-free syntheses to best design and study the corresponding electrocatalysts [1, 15, 16].

The accent will be on showing that despite the absence of surfactants, the syntheses lead to stable nanoparticles with size control even at relatively high concentration of precursors. The talk will open an emerging case studies where the surfactant-free NPs are being investigated as useful model system for instance in self-driving laboratories [17].

Copper selenide (CuSe) is a quasi-layered monolithic material that uniquely exhibits both semiconducting and metallic properties across the visible and near-infrared (NIR) spectral ranges. This dual behaviour enables excitonic absorption in the visible alongside strong, highly tunable plasmonic resonances in the NIR, providing a versatile platform to explore plasmonic–excitonic interactions, nonlinear optical effects, and anisotropic light–matter coupling.

Despite its potential, synthesising single-phase klockmannite CuSe remains challenging due to its complex layered crystal structure and the variable valence states of copper and selenium. In this work, a simple phosphine- and thiol-free colloidal synthesis is developed using a hot-injection method to produce quasi-2D klockmannite CuSe nanocrystals. Precise morphological control is achieved by adjusting the injection temperature and precursor ratios, without introducing additional ligands. This approach yields large nanosheets with lateral dimensions of ~200 nm to several micrometres, as well as monocrystalline triangular nanoprisms with diagonal sizes of 15–40 nm. Both morphologies feature smooth surfaces and strong NIR plasmonic absorption, which can be tuned by size and shape.

The anisotropic optical response of klockmannite CuSe is further examined through comparison of experimental spectra with theoretical calculations using the complex-scaled discrete dipole approximation (CSDDA). The modelling reveals hyperbolic optical behaviour within the NIR regime, arising from the interplay of propagating and evanescent electromagnetic fields, and leading to characteristic surface modes indicative of strong optical anisotropy.

Finally, ultrafast spectroscopic studies provide further insight into the photophysical behaviour of the material, revealing fast hot-hole cooling, carrier trapping pathways, and the generation of coherent phonons. Together, these results establish a scalable and ligand-minimal route to anisotropic and hyperbolic CuSe nanostructures, offering new opportunities for tunable plasmonic–excitonic coupling and advanced optical functionality in solution-processed nanomaterials.

Colloidal quantum dots (QDs) of III-V semiconductors have attracted wide interest of the last 10 years. These materials comply with regulations on toxic elements and feature band-gaps covering a broad range of the electromagnetic spectrum. Furthermore, recent breakthroughs in colloidal synthesis have extended the range of accessible materials from InP and InAs to InSb and Ga-based semiconductors. However, opposite from QDs made from II-VI, IV-VI or lead halide perovskite materials, InP or InAs core QDs show little or no photoluminescence. While this issue can be adressed through shell growth, applications such as infrared sensing work best with core QDs, for which a lack of photoluminescence implies rapid trapping of photogenerated charge carriers. In this presentation, we discuss the relation between the intrinsic properties of III-V bulk semiconductors, the geometry of III-V QDs and the formation of surface states that result in charge-carrier trapping and non-radiative recombination. Using density functional theory, we show that in the case of III-V semiconductors, the very presence of crystal facets can lead to the formation of surface orbitals. Given the surface chemistry of actual InP and InAs QDs, we show that chemical passivation can suppress such surface orbitals on In-rich facets, but not on P-rich facets. We provide experimental evidence that such surface orbitals exist in a variety of III-V QDs, and we present new directions to suppress the formation of such orbitals in III-V QDs.  

Quantum dots have opened the way to new technologies due to their unique tunable photoelectronic properties. To fabricate devices, they are processed as concentrated inks with short ligands after replacing the insulating ligands used in their colloidal synthesis. Cadmium selenide (CdSe) is a ubiquitously studied quantum dot system with extensive synthetic and surface chemistry understanding. Yet no direct ink synthesis has been developed other than aqueous routes that show poor photoluminescence.

In this work, we develop a direct synthesis of inorganic-passivated CdSe QDs in polar solvents. We identify a thermally activated growth through magic-size clusters, which follows two-step nucleation and growth kinetics. Through liquid state ¹¹³Cd NMR, we show that highly Lewis basic solvents and weakly-coordinating ions favour the formation of high-quality CdSe nanocrystals. Surface characterisations, including ζ-potential, solid-state NMR, Raman and IR spectroscopy, reveal a disordered surface rich in cadmium-oxygen bonds, compromising the photoluminescence and chemical stability of the quantum dots. These results reinvigorate polar synthesis routes for quantum dots and improve our understanding towards a one-step synthesis of inorganic-passivated quantum dots.

Colloidal semiconductor nanocrystals (NCs), particularly CdSe/CdS core/shell systems, represent promising emissive materials for display technologies, bioimaging, and optoelectronic applications owing to their near-unity photoluminescence quantum yield (PL QY) and size-dependent emission tuning. However, their technological implementation is impeded by unpredictable photoluminescence variations under ambient atmosphere, where oxygen and water provoke phenomena such as PL blinking, photobrightening, and photobleaching. Elucidating the fundamental mechanisms is essential for devising stable NC-based devices. By employing single-nanocrystal photoluminescence spectroscopy and ensemble-level optical characterization under controlled atmospheres, we systematically examined the photophysical and photochemical responses of high-quality NCs. Oxygen stabilizes the PL of NCs by deionizing negatively charged NCs (forming negative trions) back to the neutral excitonic state, thereby suppressing blinking via Langmuir-type adsorption kinetics, with superoxide radicals identified as byproducts. Water serves as a reductant, facilitating photoinduced electron transfer to generate charged NCs exhibiting red-shifted, dim emission; under co-existence with oxygen, it initiates a cyclic ionization-deionization process that culminates in irreversible photochemical corrosion. This work provides a unified picture linking diverse photophysical observations to environmental control of charge balance. The derived insights pave the way for formulating effective stabilization strategies, promoting reliable integration of NCs into commercial systems.

Chiral effects are prevalent in natural systems across various length scales and are of fundamental significance in physics, chemistry, and materials science. While molecular chirality has been extensively studied over the past few decades due to its crucial role in biomolecules and pharmaceuticals, achieving large Kuhn dissymmetry factors (g-factors) in individual molecules or their assemblies remains a considerable challenge. In contrast, introducing chirality to nanomaterials with high polarizability offers the potential to create chiral materials with large g-factors, enabling a wide range of applications. In this presentation, we will discuss recent advancements in the assembly of nanocrystals into chiral superstructures through supramolecular interactions. We demonstrate that the co-assembly of nanocrystals with π-conjugated chiral molecules facilitates chirality transfer from molecular assemblies to nanocrystals, resulting in the formation of double-helical and tubular nanocrystal structures. Additionally, we show how anisotropic nanocrystals can be used to enhance the g-factor of nanocrystal assemblies, achieving values as high as ~0.3.

Achieving precise control over chiroptical response in nanocrystal assemblies remains a key challenge for advancing next-generation optoelectronic and photonic materials. While self-assembled colloidal systems, including magic-size clusters (MSCs) and semiconductor nanocrystals, can exhibit chiroptical activity, their optical rotation is typically obscured by circular dichroism and resonant absorption. Here, we present a precision nanochemistry strategy that leverages non-degenerate exciton coupling as a generalizable design principle for engineering pure optical rotation in bottom-up assemblies. Using modeling, we show that breaking energetic degeneracy between interacting chromophores enables strong circular birefringence in spectral regions where ellipticity and absorption are intrinsically minimized. We experimentally validate this concept using precision-synthesized colloidal CdS magic-sized clusters, whose controlled energy detuning (~150 meV) produces non-additive, off-resonant chiroptical behavior consistent with theory. Building on this foundation, we design a layered superstructure that maximizes chromophore interactions in an architecture accessible through colloidal assembly. Simulations predict dispersion-less optical rotation with high transmission and low ellipticity, performance previously limited only to lithographic metamaterials. This work introduces new synthetic and compositional parameters for designing nanocrystal-based chiral materials, enabling scalable fabrication of functional optical components from colloidal building blocks.

HgTe colloidal nanocrystals are promising materials for infrared optoelectronic applications due to their broadband IR absorption.[1] However, conventional HgTe nanocrystals exhibit an irregular multipod shape, which hinders the formation of high-density packed films and consequently reduces their electrical performance.[2] In addition, although their relatively low temperature synthesis reduces energy consumption, these nanocrystals lack thermal stability, leading to aggregation and sintering in devices such as infrared imagers.

Here, we report a two-step synthesis approach that combines seeded-growth for the HgTe spherical cores and low temperature CdS shell growth, leading to thermally stable spherical HgTe@CdS core-shell nanocrystals. In the first step, the decoupling of nucleation and growth processes yields monodisperse spherical HgTe nanocrystals, revealing enhanced carrier mobility compared to multipod nanocrystals.[3] Tuning the growth condition enables to cover both the short and mid wave infrared.

More importantly, these spherical nanocrystals are thermally stable enough to support subsequent low temperature CdS thin shell growth.[4] The presence of this CdS shell induces a 400 cm-1 excitonic redshift. In contrast with the HgTe core, no further shift is observed upon annealing, indicating the integrity of the HgTe core. Furthermore, the presence of the shell drastically reduces the imager dark current, enabling higher contrast, while offering long term operation.

Key words: core shell, HgTe, IR, imaging, thermal stability

Continuous injection reactions are designed to allow for continuous nanocrystal growth as long as monomers are supplied. This strategy is commonly leveraged to synthesize large nanocrystals, of which nanocrystals with sizes larger than their Bohr radius, termed bulk-like nanocrystals (BNCs), have emerged a promising optical gain media for optically pumped lasers [1, 2]. Rationalizing the growth process in continuous injections is thus critical to achieving synthetic protocols to arbitrarily large nanocrystal sizes with narrow size distribution. Herein, we use in-situ small and wide angle X-ray scattering (SAXS/WAXS) to monitor the nucleation and growth of CdSe and CdS BNCs. Contrary to the common assumption of continuous growth we observe a size threshold and continuous nucleation of CdSe nanocrystals. We rationalize this behaviour by invoking a strongly size dependent growth rate—superfocusing—which caps the range of attainable sizes [3]. The reaction profile for CdS is more complex and involves a size-bifurcation convoluted with the presence of both zinc-blende and wurtzite polymorphs. Kinetic reactions simulations qualitatively reproduce this behaviour when weakening the size-dependency of growth to a fraction of the nucleated particles. In sum, our work highlights that continuous injection reaction are complex and motivates further structural and computational studies to evaluate how receptive nanocrystal surfaces are to growth.

Semiconductor nanocrystals have attracted significant interest not only due to the bright emission of individual particles but also to the unique collective optical phenomena they exhibit when assembled into superlattices, such as miniband formation^[1] and superfluorescence^[2]. The collective properties of these superlattices are highly dependent on their overall configuration and scale^[3]. However, the rigidity and polydispersity of conventional inorganic nanocrystals typically restrict superlattices to densely packed, micrometer-scale structures^[4].

Magic-size nanomaterials—such as specific semiconductor nanoclusters^[5] and nanoplatelets^[6], possess atomically precise dimensions in at least one direction. We have employed these magic-size nanomaterials as building blocks for novel superstructures, taking advantages on their atomic-level uniformity and more compliant ligand shells.Series of superstructures with flexible configurations and macroscopic scales have been achieved: (1) One-Dimensional Superlattices with widths of 0.7–1 µm and lengths spanning 10–1500 µm. These structures exhibit near-unity polarized emission (anisotropy factor, P = 0.92), circularly polarized photoluminescence (dissymmetry factor, *g*PL = 0.11), and strong circularly polarized light emission (*g*CPL = 0.3); (2) Centimeter-scale macroscopic helicies with a diameter of ~0.9 cm and a height of ~2.0 cm. These helices display a clear hierarchical organization, progressing from nanoclusters (nm) to filaments (µm), then to ripples (mm), and finally to the complete helices (cm).