Electron tomography enables one to measure the morphology and composition of nanostructures in three dimensions (3D), even at atomic resolution. However, an emerging challenge is to fully understand the connection between the 3D structure and properties under realistic conditions, including high temperatures as well as in the presence of liquids and gases. Our recent experiments demonstrate the progress that can be obtained by accelerating both the acquisition and reconstruction during electron tomography [1,2]. In this manner, we were able to perform a dynamic characterisation of shape transformations of metal nanoparticles at high temperatures [1,3]. Moreover, we measured the elemental diffusion dynamics of individual anisotropic bimetallic nanoparticles in 3D and determined the effect of parameters such as type of interfacial facets, aspect ratio, shape and presence of defects [4]. Finally, by combining aberration corrected electron microscopy with a quantitative interpretation, we can provide quantitative measurements of the coordination numbers of the surface atoms of catalytic nanoparticles at high temperatures and in gaseous environments [5].

II-VI semiconductor nanoplatelets are colloidal nanoparticles confined in one dimension which gives them exceptionally narrow optical properties. We are showing that through the design of core/crown/crown heterostructures, it is possible to synthesize nanoplatelets with green and red emissions from a single population. In the CdSe/CdTe/CdSe core/crown/crown NPLs, the exciton either recombine at the interface between 2 semiconductors due to the type II band alignment or in the CdSe area. The ratio of the two emissions can be tuned by the incident power. With increased power, the red emission saturates due to non-radiative Auger recombination thus affecting its emission much stronger than the green one. These NPLs can be introduced in LED. So, the design of the NPLs inorganic core enables to tune their optical properties but their shape can be further modified through the surface chemistry. NPLs with their large lateral dimensions can be considered as flexible substrate for the self-assembly of ligands which originally ensure the colloidal stability of NPLs. The stress brought by surface ligands and the specific zinc blende crystal structure of the NPLs induce a folding of NPLs as helices or twists. We show that an exchange from carboxylate ligands to halides ligands and thiolate ligands enable to unfold CdSe NPLs and to invert the chirality of the helices. Finally, this surface chemistry also enables to control the kinetic of a cation exchange.

Colloidal semiconductor heteronanocrystals (HNCs) exhibit unique optoelectronic properties that are inaccessible to single-component NCs, making them promising materials for a wide range of applications. The optoelectronic properties of HNCs are determined not only by the bandgap and band alignment of its constituent materials but also by their morphology and heteroarchitecture.¹ Applications requiring efficient charge separation (e.g., photovoltaics and photocatalysis) and polarized emission greatly benefit from anisotropic morphologies, such as heteronanorods. Most of the work on heteronanorods has focused on Cd-chalcogenide-based HNCs. However, given the toxicity of Cd, the potential of these materials for large scale applications is limited.

Copper-chalcogenide based NCs have attracted increasing attention as promising alternatives for Cd- and Pb-chalcogenide NCs due to their low toxicity, large absorption cross-sections across a broad spectral range, composition-dependent band gaps in the 1 to 2.5 eV range, and photoluminescence tunability, spanning a spectral window that extends from the UV to the NIR depending on the NC size and composition.^2,3

Several synthetic strategies have been used in the quest for high-quality colloidal copper chalcogenide based HNCs. The most promising ones are based on the multistage approach, which allows the combination of different synthesis techniques (e.g., cation exchange^4,5 or seeded growth^6-9) in a sequential manner in order to achieve the targeted preparation of colloidal HNCs. In this talk, I will discuss a selection of recent examples, chosen to illustrate specific synthesis strategies: CuInSe₂/CuInS₂ dot core/rod shell heteronanorods,⁴ Cu_1.8S-based multicomponent axially segmented heteronanorods,⁵ CuInS₂/ZnS dot core/rod shell heteronanorods,⁶ and Janus-type Cu_2‒xS/CuInS₂ and Cu_2−xS/ZnS heteronanorods.^8,9 I will show that by combining the design principles of post-synthetic heteroepitaxial seeded growth and nanoscale cation exchange into multistage synthesis strategies one can potentially gain access to a plethora of Cu-chalcogenide-based multicomponent heteronanorods with diameters in the quantum confinement regime.

Discovering and developing novel nanomaterials to improve the sustainability of optoelectronic devices, while maintaining reasonable efficiency, is of key importance to drive future research and make commercialization possible. With the criteria of earth-abundant, stable, and low-toxicity elements for nanocrystal (NC) synthesis, we focus on ABX₃ chalcogenides, where A is an alkaline earth metal and B a group four transition metal. We seek to exploit the unique properties of NCs and the perovskite crystal structure while shifting from lead (Pb) and cadmium (Cd) containing materials to more sustainable ternary chalcogenides of low toxicity. Specifically, in this class of materials, BaTiS₃ (in the hexagonal phase) and BaZrS₃ (in the perovskite structure) are promising. A flexible method for controlling the physical and chemical properties of these materials is through colloidal synthesis with organometallic precursors. Synthetic routes using different sulfur and barium sources are investigated. Characterization by TEM, XRD, and absorbance measurements reveal how the shape of the nanocrystals can be tuned from isotropic to anisotropic with aspect ratios ranging from 1:1 to 1:100 by changing the barium precursor from a methylamino compound to a halide salt. Alternatively, adjusting the sulfur source between various thioureas alters the crystallinity and reaction temperature needed to achieve NCs. Transient absorption (TA) spectroscopy shows long excitation lifetime (ca. 1 µs) and clear spectral features resembling PbS quantum dots with respect to spectral width of the band-edge bleach and photoinduced absorption signal. For the application in optoelectronic devices, it is necessary to exchange the long insulating ligands used during synthesis for shorter or more conductive ones. We have developed a ligand exchange procedure to cap the NCs with halides and process the NC solutions as thin films. With the gained knowledge from TA and film formation strategies, first work on the usage of BaTiS₃ NCs in photovoltaic devices and field-effect transistors will be presented.

Ternary alkali-metal dichalcogenides {AMeE (A = Li, Na, K, Rb, Cs, Cu, Ag, Tl; Me= Metals E = S, Se, Te} are an intriguing class of semiconductors with tremendous technological potential.[1,2]  For more than a decade, the compounds of these structural types have been explored as potent thermoelectric materials.[3] High-temperature solid-state synthesis is typically employed to synthesize alkali metal-based chemicals.  However, it naturally forms highly aggregated, polydisperse particles with little control over different phases and understanding in terms of mechanistic insights.  In this case, solution phase synthesis can be advantageous to form uniform nanocrystals.  Herein, we develop a facile colloidal hot-injection strategy to synthesize shape and size-tunable caesium copper selenide nanocrystals comprising earth-abundant and non-toxic elements.  Ex-situ mechanistic investigation reveals that the NCs formation is driven by the dissolution of binary Cu2-xSe, followed by the incorporation of Cs+ to form the ternary CsCu5Se3.  A thorough case study on the addition of alkyl acid and amine ligands indicates that the involvement of both acid and amine functionality is required to synthesize pure-phase NCs.  The current study also reveals that variation in the alkyl chain length of amines influences the size, shape, and formation of distinct phases. Furthermore, the nanocrystals were consolidated as pellets to study thermoelectric transport properties showing ultra-low thermal conductivity of 0.65 W/mK at 728 K, promising for improved thermoelectric applications. Altogether, the present study provides a mechanistic understanding of the factors that impact the synthesis of materials based on alkali metal chalcogenides and exhibit their promising thermoelectric properties.

The activity of electrocatalysts during water electrolysis is controlled by their surface chemistry. The control of the surface structure and composition of an electrocatalyst requires understanding of the solid state reactions that govern the formation of the surface and bulk during synthesis. Probing these reactions requires characterization that can cover multiple length scales, from the nanoscale near-surface layer to the mesoscale phase behavior.

In my talk, I will discuss how the phase evolution of oxide electrocatalysts across different length scales can be probed using a combination of scanning transmission X-ray microscopy (STXM) and transmission electron microscopy (TEM). Specifically, my talk will address the formation of (La,Sr)CoO₃ nano- and meso-scale particles that are often used as electrocatalysts for oxygen evolution reaction.  The impact of the size and phase state of precursors as well as the time-temperature heat treatment profile on the resulting surface and bulk chemistry of the nanoparticles will be discussed.

Colloidal semiconductor nanocrystals (NCs) such as chalcogenides and perovskites have outstanding optical and electronic properties that make them interesting for multiple applications that involve, sensing, solar cells, light-emitting device technology, and more recently photocatalysis. ¹

Tailoring the surface chemistry of colloidal metal halide perovskite NCs (AMX₃; A= monovalent metal such as CH₃NH₃⁺, or Cs⁺; M= Pb²⁺, Sn²⁺; and X= Cl^-, Br^-, I^-) is very challenging and crucial to determine their final properties. Therefore, the nature of the organic capping agents and their interaction with the surface atoms will be relevant to control the photophysical properties of the nanocrystals and prepare novel assemblies.  The most common ligands, according to the covalent bond classification, used to obtain highly emissive and stable colloidal perovskite NCs are X-type and L-type binding ligands (Figure 1).

Several examples of the role of those types of ligands in i) the emissive properties of the NCs²; ii) the formation of blue emissive 2D low-dimensional materials³; iii) the photocatalytic ability of the metal halide perovskite NCs,⁴ and iv) the preparation of nano-heterostructures for NIR light-harvesting applications ⁵ will be discussed.

Ligands play a crucial role in the synthesis and/or stabilization of colloidal nanocrystals. Nevertheless, only a handful molecules are currently used, of which oleic acid being the most typical example. Here we show that monoalkyl phosphinic acids are another ligand class.

We put forward monoalkyl phosphinic acids as an alternative ligand class, similar to carboxylic acids and phosphonic acids. After presenting the ligand synthesis for several selected substrates, we proceed to show the intermediate reactivity of the phosphinic acids in CdSe quantum dot syntheses. The nanocrystals synthesized with phosphinic acids are also easier to purify since there is no gel formation. Very small (2–3 nm) CdSe quantum dots with low polydispersity and high photoluminescence quantum yields can be easily accessed with phosphinic acid ligands. CdSe and CdS nanorods were also synthesized using phosphinic acids, whereby the rods showed high purity an d uniformity.

In addition, we investigate the ligand-NC interaction on both metal oxide (i.e., HfO₂) as metal chalcogenide (i.e., CdSe and ZnS) NCs. Given their intermediate acidity, phosphinic acids (pKa ≈ 3.08) bind to the surface with an affinity between carboxylic and phosphonic acids. Using solution NMR, we quantify the X-for-X ligand exchange via the alkene resonance of the oleyl chain (carboxylic and phosphonic acid), and the ether resonance of the 6-(hexyloxy)hexyl phosphinic acid. We conclude that the monoalkyl phosphinic acids quantitatively displace carboxylate ligands and are in equilibrium with phosphonates (although phosphonate binding is favored). This results show that monoalkyl phosphinic acids are suitable reagents to efficiently functionalize nanocrystal surfaces.

In conclusion, by careful design new type of ligands can be created, and can tailored towards specific functionalities such as solubility matching or intermediate reactivity and binding strength.

The detection of high-energy radiation has gained relevant interest thanks to its application in technologic-relevant fields, such as particle physics, astronomy, geology, medical diagnostics, nuclear monitoring and space exploration [1][2]. In all these areas, the most widely used detectors are scintillating materials that convert the energy deposited by incoming ionizing radiation into visible photons which is finally revealed by coupled photodiodes [3]. Beside typical inorganic materials based on high-Z elements, an alternative class of scintillators which further widens their applicability is plastic scintillators, which exhibit the advantages of a large sizes production, low weight and affordable costs making them particularly adapt for radiation monitors in border and industrial control[4]. Despite these advantages, the low density of plastic materials compared to inorganic crystals limits their interaction with ionizing radiation and typically requires doping these systems with high-Z components, such as organometallic complexes, perovskite or heavy metal chalcogenides nanocrystals (NCs). However, the intrinsic small Stokes shift of these materials represents an issue when used as nanoscintillators in highly dense or large volume detectors because of the strong reabsorption of the scintillation light along its path to the waveguide edges. We aim to takle this issue by developing a new strategy of scintillator consisting in a polymeric scintillating matrix incorporating reabsorption free Cd_0.5Zn_0.5S/ZnS core/shell NCs doped with manganese ions. Doping NCs with Mn is an established approach to activate efficient luminescence at intragap energy arising from the ⁴T₁ →⁶A₁ optical transition, yielding the characteristic Mn-emission at ~ 580-600 nm. Crucially, since such a transition is spin-forbidden, the corresponding optical absorption features negligible oscillation strength, resulting in an apparent Stokes shift between the band-edge absorption of the NCs host and the Mn-related luminescence. In our approach, we further adopted a synergic strategy in which both the plastic matrix waveguide and the NCs interact with incoming ionizing radiation, while the propagating emission is generated by the sole NCs, whose optical properties have been properly engineered to efficiently down-convert matrix emission. The emission efficiency and compatibility of the NCs with the polymer host have been optimized resulting in high optical quality nanocomposites completely transparent in the spectral region of their own emission, with scintillation efficiency comparable to commercial plastic scintillators.

Lead halide perovskites are emerging materials that can be synthesised at low cost on a large scale and have potentially disruptive optical performances in photonic and scintillation applications [1]. On the other hand, these materials are inherently toxic, due to the presence of Pb, and generally unstable, especially when exposed to heat, air and moisture. Therefore, there is vivid research aimed to replace Pb-based perovskites with non-toxic metal halide alternatives that may exhibit similar optical properties and, ideally, greater stability. In this context, the broad family of double perovskites (DPs) is particularly promising and offers fertile ground for new discoveries. Among the various structures, Sb³⁺-doped DPs are attracting increasing interest due to their bright and exceptionally large (more than 1.2eV) Stokes-Shifted PL [2], [3]. Here, we present colloidal nanocrystals (NCs) of Rb3InCl6, composed of isolated metal halide octahedra ("0D"), and dual perovskites of Cs2NaInCl6 and Cs2KInCl6, in which all octahedra share angles and are interconnected ("3D"), with the aim of elucidating and comparing their optical characteristics once doped with Sb3+ ions [4]. Our optical and computational analyses demonstrate the photophysical mechanism underlying PL in these systems, and that it is possible to double the quantum yield through localisation of the exciton in 0D structures by preventing its migration to trap states at the surface. Scintillation properties are evaluated by means of radioluminescence experiments and waveguide performance without resorption in large-area plastic scintillators is assessed by means of Monte Carlo ray-tracing simulations.

Electrically pumped lasers or laser diodes based on solution-processable materials have been long-desired devices for their compatibility with virtually any substrate, scalability, and ease of integration with on-chip photonics and electronics. Such devices have been pursued across a wide range of materials including polymers, small molecules, perovskites, and colloidal quantum dots (QDs). The latter materials are especially attractive for implementing laser diodes as in addition to being compatible with inexpensive and easily scalable chemical techniques, they offer multiple advantages derived from a zero-dimensional character of their electronic states. These include a size-tunable emission wavelength, a low optical-gain threshold, and high temperature stability of lasing characteristics stemming from a wide energy separation between their atomic-like discrete energy levels.

Several challenges complicate the realization of colloidal QD laser diodes (QLDs). These include extremely fast nonradiative Auger recombination of optical-gain-active multicarrier states, poor stability of QD solids under high current densities required to achieve lasing, and unfavorable balance between optical gain and optical losses in electroluminescent (EL) devices wherein a gain-active QD medium is a small fraction of the overall device stack comprising multiple optically lossy charge-transport layers.

Here we resolve these challenges and achieve electrically driven laser action due to amplified spontaneous emission (ASE) in a colloidal-QD optical-gain medium. To demonstrate this effect, we employ compact, continuously graded QDs with strongly suppressed Auger recombination incorporated into a low-loss photonic waveguide integrated into a pulsed, high-current density light-emitting diode. These prototype QLDs exhibit strong, broad-band optical gain and demonstrate low-threshold, room-temperature laser action which leads to intense edge-emitted EL with intensity of more than 100 microwatts.

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Nanostructured semiconductors, or quantum dots (QDs), are heavily investigated for their applications in light emission such as light emitting diodes and lasers. The premise of cost-effective solution processing of such devices based on nanocrystals has recently driven research towards electrically pumped population inversion in laser diode structures. Challenges however remain to achieve net light amplification in the cavity due to a balance between limited material gains and lossy electrical contacts. Further reductions in threshold current densities, mainly limited by the non-radiative cap of ca. 1 nanosecond on the gain lifetime, are also required to achieve stable operation. Finally, color tunability is limited to the red by the gain bandwidth of the red-emitting CdSe/CdS QDs or even core/shell nanoplatelets used.
Here, we show that weakly confined charge carriers in giant CdS quantum dots display disruptive optical gain metrics that could alleviate these remaining issues. Being active in the green part of the spectrum, their properties match and even outcompeting state-of-the-art colloidal materials in the red. Material gain coefficients up to 50.000 cm^-1 combined with a broad gain window of 160 nm are found. Also, a very promising gain lifetime close to 3 ns is found. Invoking a model of stimulated emission based on bulk semiconductor physics, we are able to explain all of these remarkable gain metrics, yet only if a large band gap renormalization effect is invoked. Based on these unique materials, we demonstrate amplified spontaneous emission and lasing action under nanosecond optical excitation. Our results show that weakly confined nanomaterials are excellent gain materials, combining straightforward wet chemical synthesis and the promise of solution processability with beyond state-of-the-art gain metrics.

In condensed matter physics, a stepwise reduction of the dimensionality from bulk to the nanoscale is known to lead to striking electronic and optical properties such as the giant oscillator strength transition for two- dimensional (2D) semiconductor quantum wells and the spin-charge separation in one-dimensional (1D) systems. However, in contrast to metals, the lateral confinement in semiconductor materials competes with the exciton Coulomb interaction, which can be magnified by dielectric mismatch. Due to the complexity of these interactions, the degree of freedom in the electron motion remains poorly understood, whereas its clear circumscription could significantly improve the optoelectronic performances of quasi-2D systems such as CdSe colloidal nanoplatelets (NPLs).

Since the distribution of the quantized states, the so-called density of states (DOS), reflects the restrictions of the electron motion with dimensionality, it can serve as a genuine fingerprint to unambiguously disclose the confinement experienced by the charge carriers. A unique way to probe the DOS consists in measuring single-particle excitation spectra with scanning tunnelling spectroscopy (STS). Ji et al. have demonstrated that the hole DOS is consistent with a free in-plane motion, while the electrons have a more complex DOS that deviates from the simple 2D model. When the width (W) or/and the length (L) are smaller than ten to five times the exciton Bohr radius, the electron behavior, which has a smaller effective mass than the hole, becomes affected, with a significant impact on its interaction with the hole and the dielectric image charges. The already present complexity in understanding electron−hole correlation in these box-shaped nanostructures is further enhanced with the existence of electrical surface traps, which are caused by the sporadic absence of ligands. It makes the electron confinement in the NPLs still unknown, leading to intense debates about their true dimensionality. 

Here we used this technique to study electron confinement in colloidal CdSe nanoplatelets (NPLs), which have a quantized thickness d due to a discrete number of monolayers (MLs) and a rectangular shape with finite length L and width W. The observation of Van Hove singularities in the conduction band implies a paradigm shift on the electronic structure of typical CdSe NPLs considered in the literature. As the electron DOS exhibits a striking modulation that is directly related to the length of the NPLs, delineating the in-plane electron motion at low temperature has important conconsequences for a deeper understanding of the exciton dissociation, diffusive transport, and annihilation in NPLs.
Moreover, it is shown that the side facets of NPLs host electronic deep trap states, which cause a Coulomb blockade in the tunnelling current. As they could be fully removed by the formation of core-crown NPL, our results anticipate a genuine boost for NPL-based lateral heterostructures that will notably allow for the mixing of the dimensionalities to favor specific electronic or optical properties.[1]

When studying the opto-electronic properties of (nanostructured) semiconductors, Transient Absorption (TA) or pump-probe spectroscopy is often used to measure ultrafast changes optical properties due to photo-excitation. Besides making direct conclusions on carrier recombination processes, quantification of net optical gain or studying photo-induced absorption, a typical analysis that is done on such datasets is a Boltzmann fit on the high-energy tail from which the temperature of the photo-excited charge carriers can be extracted. Such information is vital in understanding carrier cooling bottlenecks which are detrimental for optical gain applications, but potentially very useful in solar energy conversion. Usually, this extraction of temperature is done for different pump-probe delays, excitation energies and different excitation powers to see the impact on the thermalization of the carriers.¹ In this work, we take a more detailed look at what this high-energy tail actually entices for a direct gap semiconductor. We show that within this simple fitting approach, one gravely overestimates carrier temperatures and propose a more consistent modelling to find the temperature. Next, we compare this approach to Ultrafast Photoluminescence (UFPL) measurements on the same timescales as TA. We show that extracting the temperature is a more straightforward process for this approach.

Colloidal lead halide perovskites LHP (LHP) nanocrystals (NCs) have recently become popular light-emissive materials, of practical interest for LEDs, LCDs, lasers, as well as single photon light sources.^1,2 Most studies on LHP NCs focus on relatively large cuboidal NC exceeding 10 nm in size, pointing out the inherent challenge of producing small (sub 10 nm), stable and monodisperse LHP quantum dots (QDs). This problem directly originates from the highly ionic lattice of LHPs, generally resulting in sub second reaction dynamics, making it very challenging to control their growth on an atomic level. Consequently, the current generation of LHP QDs (especially the hybrid organic-inorganic ones) show significantly less excitonic absorption landscapes compared to conventional QDs such as CdSe, even though LHPs have more simplistic band structure. This thus hinders studies into the size-quantization of excitons in LHPs (and possible practical use) as well as understanding of the mechanism of LHP QD formation, which still significantly lacks behind compared to conventional CdSe and PbS QDs. To solve this, we developed a room-temperature synthesis, in which the overall QD formation occurred on a time scale of up to 30 min, slowing down the reaction kinetics by several orders of magnitudes compared conventional LHP QD syntheses.³ The size of these QDs were tunable between 3 and 13 nm range and exhibited a rhombicuboctahedral (spheroidal) shape.^3,4 These CsPbBr₃ QDs, as well as FAPbBr₃ and MAPbBr₃ exhibited up to four well-resolved excitonic transitions, finally bringing them on par with the highly excitonic absorption landscapes of CdSe and PbS QDs. This slow growth method also allowed for the first time to direct in-situ study the illusive reaction mechanism of LHP QDs, demonstrating the effective separation of the nucleation and growth stages due to the self-limiting formation of an Cs[PbBr₃] intermediate precursors. The slow growth approach was further extended by using an additional in-situ anion exchange step, resulting also in spheroidal CsPb(Cl:Br)₃ QDs with any Cl:Br ration and sizes from 4-10 nm.^3,5 These quaternary QDs still exhibited up to five sharp excitonic absorption transitions, further demonstrating the versatility of the slow growth method.

Research interest in all inorganic lead halide perovskite nanocrystals (LHP-NCs), featuring general chemical formula of CsPbX_3, has recently grown fast thanks to their outstanding chemical and physical properties that make them optimal candidates for a wide range of technological applications such as photovoltaics¹, light emitting devices² and photodetectors³. In this fast-growing and heterogeneous playground, we report a robust, reproducible, and easy scalable synthetic method allowing the production up to the unprecedented scale of 8g of high quality CsPbBr₃ NCs for either fundamental studies or in-solution and device applications. To this aim, we modified the synthetic procedure reported by Akkerman et al. ⁴ by introducing, for the first time, the use of a turbo emulsifier (Ultra Turrax Homogeneizer) usually employed for the preparation of large batches of formulates, to improve the reaction mixture homogenization and overcome concentration gradients and reproducibility issues that usually affect LHP-NCs liter-scale reaction volumes. We also introduced tetrabutylammonium bromide (TBAB) as extra bromide precursor: working in halogen rich environment is known to help reducing defectivity and this specific quaternary ammonium salt is too bulky to be competitive with other cations in the perovskite crystal lattice. We demonstrated that the amount of recovered solid material is proportional to the volume of solution used at every scale, suggesting that the process is well controlled up to the biggest scale. There are also evidences that increasing the scale, magnetic stirring becomes insufficient where turbo-emulsifier remains reliable. Moreover, the procedure is easy extendable to CsPbCl₃ and mixed phase and other acid and amine ligands. We further pushed the limit by demonstrating that, with our approach, the low-boiling solvents and the excess reactants, in particular lead bromide, can be recovered and reused, thus reducing the environmental impact connected to waste production and moving a step towards final industrial application. We finally preliminary tested radioluminescence properties both in solution and in polymer matrix to apply them to scintillation field: the big amount that can be easily produced, in one pot, with low waste favors many applications, especially the production of wide scintillating windows even with at high concentration.

Recently, lead halide perovskite nanocrystals (LHP NCs) have emerged as a highly promising class of luminescent materials. The optical properties of LHP NCs are easily tunable by their dimensions as well as chemical composition, and they exhibit extremely high photoluminescence quantum yield (PLQY).^{1, 2} Despite great progress in the synthesis and application of LHP NCs, they still suffer from poor surface stability and thus leading to a drastic reduction of PLQY upon washing them.^{3, 4} Interestingly, we find that the PLQY and surface stability of inorganic LHP NCs significantly improves upon replacing some of the inorganic A-cations (Cs⁺) with organic cations (FA⁺ or MA⁺) by cation exchange. Recently, we demonstrated the preparation of mixed A-cation NC systems with a range of compositions by simple room-temperature cation exchange.⁵ Unlike halide segregation, the A-cations in the lattice do not undergo phase segregation under light illumination.⁵ We then systematically studied the surface stability as well as the optical properties of mixed A-cation LHP NCs by washing them several times with an antisolvent.  Surprisingly, the mixed A-cation compositions with a small percentage of organic cation exhibit superior stability and PLQY as compared with inorganic LHP NCs upon washing them with antisolvent. In this presentation, I will discuss the synthesis, optical properties, and stability of mixed A-cation LHP NCs in comparison with individual counterparts. 

Lead halide perovskite (LHP) nanocrystals (NCs) have gained research interest due to their exceptional optoelectronic properties including high photoluminescence (PL) quantum yield and narrow PL linewidth suggesting a promising future in light-emitting technologies [1]–[3]. Emission tunability in colloidal LHP NCs can be achieved by either size [4] or compositional modifications [5]. Since the breakthrough of the LHP NCs family, it has been well-established that the NCs΄ emission can be finely tuned covering the whole visible spectrum by controlling the ratio of the halides (Br/Cl) or (Br/I) forming homogeneous alloys. Particularly, the ionic and labile character of the lattice promotes the facile post-synthesis bandgap engineering via partial or full anion exchange from Br^- to either Cl^- or I^- while preserving the NCs’ original morphology and the 3D perovskite structure [6], [7]. In this work, a systematic study of anion exchange reactions on oleate-capped CsPbCl₃ colloidal NCs is presented. We will introduce novel insights into selective anion exchange manifested by partial exchange presenting either positive cooperativity (all-or-nothing exchange behavior) or segmented nanoheterostructures. We will also address the kinetics of the reaction by means of in-situ spectroscopy and energy-dispersive X-Ray high-resolution scanning transmission electron microscopy.

Colloidal metal halide perovskite nanocrystals (NCs) are regarded as candidates for a variety of applications – spanning from optoelectronics^[1,2] to biotechnology^[3,4] – on account of their many tunable optoelectronic properties^[5–7]. The construction of theoretical models and the investigation of the fundamental atomistic processes influencing their features are paramount for tuning the overall performance of these materials, especially under realistic reaction conditions. An effective tool in the investigation of the dynamical behavior and properties of NC models is provided by classical molecular dynamics (MD) simulations^[8–11].

In this presentation, we will gain insight into anion-exchange reactions occurring in CsPbX₃ (X = Cl, Br) perovskite structures. This multi-step process is composed a series of intermediate states, each one associated to a potential energy barrier^[12]. We employed enhanced sampling methods^[13] to overcome the free-energy barriers associated to the individual reaction steps for the Br-to-Cl anion-exchange starting from an oleate-capped CsPbBr₃ perovskite NC model. We believe that this investigation will shed light on the mechanisms and kinetic rates of this process and on the nature of its intermediate steps, allowing for an appropriate tuning of the most suitable reaction conditions of the process.

Versatile surface functionalization of highly ionic surfaces, ubiquitous among inorganic nanomaterials, remains a formidable challenge in view of inherently non-covalent surface bonding. Colloidal lead halide perovskite nanocrystals (NCs), which are of interest for classical and quantum light generation,[1,2] are one of the examples. Despite some recent empirical progress in surface chemistry of lead halide perovskite NCs, the general strategy towards their robust surface functionalization still remains a challenge.[3] One of the reasons is a limited atomistic understanding of the NC-ligand-solvent interface. Here we would like to present how classical molecular dynamics (MD) simulations can be used in combination with experimental techniques to aid in understanding the surface chemistry of ionic nanomaterials and to guide experimental discovery of new better capping ligands. In particular, we would like to present the first structural investigation of perovskite NC surfaces capped with zwitterionic phospholipid molecules. Combined computational and experimental evidence suggests that the phospholipid ligands bind to the surface of the NCs with both head-groups by displacing native ions of the perovskite. The ligand head-group affinity to the surface is primarily governed by a geometric fitness of its cationic and anionic moieties into the crystal lattice. As a result, stable and colloidally robust nanocrystals of inherently soft and chemically labile lead halide perovskites – FAPbX₃ and MAPbX₃ (X – Br, I) – can be obtained for the first time with a lattice-matched phosphoethanolamine head-group. Stable surface passivation enables excellent optical performance of the NCs. As an example, alkylphospholipid-capped FAPbBr₃ NCs display stable emission with a near-unity photoluminescence quantum yield in a broad concentration range, as well as in thick films. Ligand tail engineering, on the other hand, allows diverse surface functionalization of the NCs, broadening the scope of their potential applications.

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 years. While most of the emphasis has been put on CsPbX3 perovskite NCs, more recently the so-called double perovskite NCs, having chemical formula A+2B+B3+X6, have been identified as possible alternative materials, together with various other metal halides structures and compositions, often doped with different elements. This talk will also discuss the research efforts of our group on these materials. I will highlight how for example halide double perovskite NCs are less surface tolerant than the corresponding Pb-based perovskites. Other topics that will be covered are the role of surface ligands on stabilizing the NCs (including those with alloy compositions), doping, and our ongoing research on various other metal halides (for example the Mn-based ones).

The urgency for affordable and reliable detectors for ionizing radiation in medical diagnostics, nuclear control and particle physics is generating growing demand for innovative scintillator devices combining efficient scintillation, fast emission lifetime, high interaction probability with ionizing radiation, as well as mitigated reabsorption to suppress losses in large volume/high-density detectors. Prized for their solution processability, strong light-matter interaction, large electron-hole diffusion length and tunable, intense luminescence at visible wavelengths, lead halide perovskite nanocrystals (LHP-NCs) are attracting growing attention as highly efficient emitters in artificial light sources and as high-Z materials for next generation scintillators and photoconductors for ionizing radiation detection. Nonetheless, several key aspects, such as the trapping and detrapping mechanisms to/from shallow and deep trap states involved in the scintillation process and the radiation hardness of LHP NCs and LHP NC-based plastic nanocomposites under high doses of ionizing radiation are still not fully understood, leaving scientists without clear indications of the suitability of LHP-NCs in real world radiation detectors or design strategies for materials optimization. In this talk, I will present on our recent strategies for high performance radiation detection schemes1,3 and will report recent spectroscopic results of the scintillation process and its competitive phenomena, ultimately offering a possible path to the realization of highly efficient and extremely radiation hard LHP-NCs.

Long-range coherence and correlations between atoms and electrons in solids are the cornerstones for developing future quantum materials and devices. In 1954, Dicke described correlated spontaneous emission from closely packed quantum emitters, forming the theoretical basis of superradiance and superfluorescence. It has remained an open challenge to observe such phenomena with nanometer spatial resolution, precisely the critical scale at which the collective correlations occur. In my talk, I will demonstrate superfluorescence triggered by ultra-fast free electrons pulses for the first time using superlattices of lead-halide perovskite quantum dots. These new materials have a significant interaction cross-section with electrons, efficient emission, and fast radiative rates that allow the buildup of correlation between the exciton before dephasing. The emission is toggled between spontaneous to superfluorescence by controlling the excitation area. The correlated emission is faster, brighter, and narrower than the spontaneous emission. On-demand correlated emission has applications in non-classical light sources, superfluorescence biomarkers for electron microscopy, and enhanced detectors.

Lead-halide perovskite nanocrystals have been the subject of countless publications. Most of these works report XRD patterns featuring Bragg peaks with unusual shapes, which appear composed of two or more overlapping contributions. These however are too narrow for a nanocrystal, and the material’s structure does not account for their formation. The quest to understand these observations led us to the (re)discovery of Multilayer Diffraction, an intriguing collective interference effect known for epitaxial thin films but never investigated for colloidal nanocrystals before.[1]

Multilayer Diffraction occurs when X-rays are diffracted by nanocrystals packed with a periodic arrangement: here, the X-rays scattered by each particle interfere with those diffracted by neighbors, creating fringes of constructive interference.[2] Since the interfering radiation comes from nanocrystals, the fringes are visible only in correspondence with their Bragg peaks, thus explaining the unusual peak shape. Being a collective interference phenomenon, Multilayer Diffraction is highly sensitive to disorder, and requires precise stacking of nanocrystals to be observed. This condition is typical of self-assembled nanocrystal superlattices or stacks of nanoplatelets. Nevertheless, the Bragg peak split is routinely observed in a variety of samples, and not just in highly ordered superlattices. Why is that so?

This is only one of the many questions we will answer. Together, we will discuss why the peak split affects only some of the Bragg peaks, we will explore the influence of morphology (i.e., nanocrystals vs nanoplatelets) on Multilayer Diffraction,[3] and we will explain why this effect was not reported for popular materials like Au or CdSe, despite the extensive investigations on their superlattices. We will present models for the quantitative description of Multilayer Diffraction, which can be exploited to extract structural information hardly accessible from other techniques, first and foremost the degree of structural disorder in nanocrystal assemblies. Finally, we will demonstrate that Multilayer Diffraction is often observed yet unrecognized in a variety of nanomaterials beyond metal halide perovskites. Your sample might be next!

Cesium lead halide perovskite nanocrystals, owing to their outstanding optoelectronic properties (high oscillator strength of bright triplet excitons, slow dephasing, minimal inhomogeneous broadening of emission lines), are promising building blocks for creating superlattice structures that exhibit collective phenomena in their optical spectra. Thus far, only single-component superlattices with the simple cubic packing have been devised from these nanocrystals, which have been shown to exhibit superfluorescence – a collective emission resulting in a burst of photons with ultrafast radiative decay (ca. 20 ps) that could be tailored for use in ultrabright (quantum) light sources [1]. However, far broader structural engineerability of superlattices, required for programmable tuning of the collective emission and for building a theoretical framework can be envisioned from the recent advancements in colloidal science. We show that the co-assembly of cubic and spherical steric-stabilized nanocrystals is experimentally possible and that the cubic shape of perovskite nanocrystals leads to a vastly different outcome compared to all-spherical systems. Five superlattice structures have been obtained: novel AB₂, ABO₃, ABO₆, besides expected NaCl or common to all-sphere assemblies AlB₂ superlattices [2, 3]. In binary ABO₃ and ABO₆ superlattices, larger spherical nanocrystals occupy the A sites and smaller cubic CsPbBr₃ nanocrystals reside on the B and O sites. Targeted substitution of B-site nanocubes by truncated cuboid PbS nanocrystals leads to the exclusive formation of ternary perovskite-type ABO₃ superlattice. Truncated cuboid PbS NCs can occupy A-sites in binary ABO₃, NaCl, and AlB₂ SLs with smaller CsPbBr₃ nanocubes. All synthesized superlattices exhibit a high degree of orientational ordering of the CsPbBr₃ nanocubes. We also demonstrate the effect of superlattice structure on the collective optical properties. In addition, we explore the on-liquid assembly method that allows for obtaining free-floating SL films comprising perovskite nanocrystals.

Magic-sized clusters (MSC) are identical inorganic cores that maintain a closed-shell stability, inhibiting conventional growth processes. Because MSCs are smaller than nanoparticles they can mimic molecular-level processes, and because of their high organic-ligand/core ratio they have “soft” inter-particle interactions, with access to a richer phase diagram beyond the classical close packed structures. In this talk I will highlight some remarkable behavior we have recently found in both of these areas. MSCs have the ability to undergo a chemically-induced, reversible isomeric transformation between two discrete states. The diffusionless reconfiguration of the inorganic core follows a first order kinetic rate driven by a distortion of the ligand binding motifs. These MSCs also display a surprising ability to self-organize into hierarchical assemblies which span over six orders of magnitude in length scale. The films are optically active with g-factors among the highest reported for semiconductor particles. Since the physical origin of the chirality for  highly-structured films is challenging, we developed a method for extracting the true chiroptic-CD signal from the raw data, derived using Mueller matrix and Stokes vector conventions, and we find that the origin of the chirality is from exciton coupling between adjacent MSCs. Beyond optical properties, the multiscale self-organization behavior of these MSCs provides a new platform for the design and study of complex materials.

References

JACS 140, 3652 (2018) DOI: 10.1021/jacs.5b10006

Science 363, 731 (2019) DOI: 10.1126/science.aau9464

Nat. Mat. 21, 518 (2022) https://doi.org/10.1038/s41563-022-01223-3

ACS Nano (2022 accepted https://doi.org/10.1021/acsnano.2c06730)

Crystal structure plays a central role in the properties exhibited by a material. As such, intense research efforts are focused on polymorph selectivity during materials synthesis in the nano and bulk scales. The MnS system, having three known polymorphs, has received considerable attention towards developing a better understanding of factors that control crystal structure selectivity. The three polymorphs are metastable zincblende- and wurtzite-MnS, with rock salt-MnS as the thermodynamic product. In addition, MnS is an Earth-abundant semiconductor with potential applications that include photovoltaics, batteries, and opto-electronics. This talk focuses on the halide-driven polymorph selectivity of MnS nanoparticles. Specifically, we show that the product can be tuned between the wurtzite and rock salt structures by varying the Mn halide precursor, under otherwise identical conditions. Temperature and aliquot studies were employed to obtain insights into the reaction pathway. The synthesized particles were characterized with powder X-ray diffraction, Transmission Electron Microscopy (TEM) and high-resolution TEM. This halide-driven selectivity can also be extended to the MnSe system.         

Colloidal semiconductor Quantum Dots (QDs) of the III-V family (InP and InAs) are characterized by a large surface-to-volume ratio that renders them extremely sensible to surface processes. Passivating ligands, employed to stabilize QDs in organic solvents, play a pivotal role in influencing the structure and the optoelectronic properties of these materials. Despite major progresses attained in the last years to model the QD surfaces, there are still several key questions to be answered on the nature of the QD-ligand interactions and how trap states, which are deleterious to optical efficiency, develop on the surface. 

A leap forward in solving the above issues is to analyze the surface using first principle simulations, such as Density Functional Theory (DFT). Until now some of the major drawbacks of this approach have been: (i) the size of the system that can be handled that in the best cases is restrained to a few hundredths atoms (i.e. a small sized QD surrounded by short ligands), and (ii) the description of static properties with the absence of dynamic effects. 

Here, I will present a tool to automatically parametrize the force-field of nanoscale semiconductor crystallites, that we can then use to perform multiscale modeling of real sized InP and InAs QDs passivated with chlorine and primary ammine ligands with a simulation box containing up to a million of atoms including the solvent. Molecular dynamics simulations, carried out up to the nanosecond timescale, provide crucial insights on the surface dynamics, and the role of the ligands in influencing the properties of these materials. 

In the short-wavelength infrared range, there is poor or no availability of cheap and optically tunable bulk semiconductors for optoelectronic applications like telecommunications and sensing in automated transport. Semiconducting colloidal quantum dots like lead chalcogenides instead, display bandgap tunability (800-3000 nm) with size and are cheaply synthesized in solution. Unfortunately, due to their deposition techniques, thin films of such materials are disordered and therefore display poor transport properties. Self-assembled arrays of colloidal quantum dots, the so called superlattices, are expected to lead to coherent transport through minibands therefore approaching the electronic properties of the bulk counterparts. Nevertheless, the poor control on the ordering, on the electronic coupling and on surface trap passivation has led to the observation of disappointing charge transport properties.

Here [1] we report the self-assembly of highly ordered 3D superlattices with tunable structure and thickness with single layer precision. The superior quality of such superlattices is demonstrated by a combination of advanced structural characterization techniques like atomic resolution electron microscopy and grazing incidence X-Ray scattering. We measure in-plane coherence lengths up to 100 nm but most noticeably, the superlattices are fully coherent throughout their thickness. This outstanding ordering results in record electron mobilities up to 278 cm²/Vs as measured in a field effect transistor using ionic gel as gate dielectric. This value is not only approaching the bulk mobility, but it is a record among any self-assembled superlattice with fully quantum-confined nanocrystals as building blocks. Such findings demonstrate that we were able to obtain an optoelectronic metamaterial and this could pave the way for a new generation of optoelectronic devices with highly tunable properties.

Solar cells fabricated from metal sulfide quantum dots require facile and reproducible synthetic methods that determine the particle size, distribution, shape and faceting of the quantum dots. These properties will affect the energetic landscape of the nanocrystal thin films, by modifying the bandgap, trap density and surface composition. So far, an established hot injection synthesis route has been adapted for most metal sulfide quantum dots, resulting in solar cells with record efficiencies such as 15.5% for PbS dots (Advanced Energy Materials 2022, 12 (35), 2201676) and 9.2% for environmentally-friendly AgBiS₂ (Nature Photonics 2022, 16, 235). The synthesis of most metal sulfide quantum dot relies on the use of a strongly reactive sulfur source: bis(trimethylsilyl) sulfide (TMS)₂S. This sulfur compound is known to hydrolyze in the presence of water, creating toxic and foul smelling H₂S. Consequently, its storage and handling are problematic even for a small-scale laboratory facility. Moreover, this hydrolyzation causes the synthesis to be irreproducible, since seemingly the same synthetic conditions end up leading to large variations in the quantum dot average size and distribution. These disadvantages, together with its high cost, make (TMS)₂S unsuitable for large scale applications and support the motivation to find alternative sulfur precursors for metal sulfide quantum dot synthesis.

In this study we demonstrate that bis(stearoyl) sulfide (St₂S) is an excellent alternative to (TMS)₂S and illustrate this via the synthesis of PbS and AgBiS₂ quantum dots and their application in photovoltaics. St₂S is a solid, odor-free, air-stable sulfur compound with a low melting point of 60°C, which can be used instead of (TMS)₂S with minimal changes to the synthesis procedure. We characterize the quantum dots by transmission electron microscopy, and confirm the chemical composition by diverse spectroscopic techniques. Furthermore, we prove that without further optimization, PbS quantum dot solar cells made using St₂S are equally efficient as those made using (TMS)₂S. In the case of AgBiS₂, the performance with St₂S is slightly lower than that of the referenced devices, due to a smaller overall size of the nanoparticles. Importantly, photovoltaic cells fabricated using St₂S precursors are more stable because the dots are less prone to surface oxidation. To summarize, the substitution of the unstable (TMS)₂S with the air-stable St₂S is highly promising for the synthesis of metal sulfide quantum dots for a broad range of applications.