Title: Anisotropic Lead Halide Perovskite Nanocrystals for Light-Emitting Applications

Abstract: In recent years, lead halide perovskite nanocrystals (NCs) have attracted much interest due to their outstanding optoelectronic properties, including facile band gap tunability, high exciton binding energies, and ultrafast radiative recombination. While green and near-infrared perovskite emitters are well established, blue-emitting CsPbCl₃ NCs suffer from insufficient stability and low photoluminescence quantum yields.

Herein, we present anisotropic CsPbBr₃ NCs as a promising alternative for light-emitting applications. Quasi-1D and quasi-2D NCs are prepared at room temperature following a wet synthesis approach. As a result of size-dependent quantum confinement effects, anisotropic CsPbBr₃ NCs with 2-8 monolayer (ML) thickness exhibit a tunable photoluminescence (PL) between 430 to 510 nm. We implement a data-efficient machine-learning algorithm to optimize synthesis parameters and enhance the quality of resulting perovskite NCs. Not only are their PL profiles significantly narrower, but for the first time, we synthesized monodisperse 7ML and 8ML perovskite NCs. Furthermore, we successfully studied the synthesis with 10 ms time resolution by combining in situ small-angle X-ray scattering (SAXS) and PL spectroscopy. We find that, contrary to commonly assumed mechanisms, the formation of anisotropic perovskite NCs proceeds via growth of intermediate perovskite clusters in well-ordered superlattices. Interestingly, both 2D nanoplatelets and 1D nanorods were obtained from the same synthesis approach by varying the antisolvent. Lastly, CsPbBr₃ nanorods exhibit isotropic growth after synthesis when prepared under Cs-rich reaction conditions. This process results in a gradual PL redshift but can be easily terminated by adding an enhancement solution, thus allowing for nm-precise tuning of blue-green emission.

Title: Chiral Perovskite Nanoplatelets Exhibiting Circularly Polarized Luminescence Through Ligand Optimization

Abstract: Chiral halide perovskite nanocrystals have many applications in next-generation optoelectronic devices due to their interaction with circularly polarized light. Through the careful selection of chiral organic surface ligands, control over the circular dichroism (CD) and circularly polarized luminescence (CPL) of these materials can be achieved. However, while recent developments of CD-active perovskites have seen significant advances, effective CPL remains a challenge. Here, we synthesize colloidal perovskite nanoplatelets exhibiting room temperature CPL with dissymmetry factors up to g_lum=4.3×10^-3 and g_abs=8.4×10^-3. Methylammonium lead bromide nanoplatelets are synthesized with a mixture of chiral dimethyl benzyl ammonium ligands and achiral octylammonium ligands, the precise ratio of which is shown to be critical to achieving high g-factors. We investigate the competitive binding of these surface ligands using ¹H NMR, and use an equilibrium model to demonstrate the ligand affinity. The magnitude of CPL and CD is quantitatively shown to exhibit a linear correlation, such that g_lum=0.4×g_abs. Lastly, by screening several amines with close structures, we show that subtle differences in ligand structure have significant impact on the resulting CD signal of the nanoplatelets. Our findings provide new insights for the effective design of perovskites exhibiting CPL and can facilitate the development of high-performance devices based on circularly polarized luminescence.

Title: Characterization of CuInS₂ Quantum Dots by SAXS/WAXS Methods

Abstract: In the quest for quantum dots with green formulation, Cu-based materials turn out to be potential candidates.

Our group has combined reciprocal space X-ray total scattering techniques in the wide- (WAXS) and small-angle regions (SAXS) to achieve a characterization of size, morphology, and defects in CuInS₂ as a promising alternative to more well-known quantum dots (QDs), that possess favorable photophysical properties combined with low toxicity.

We have optimized a colloidal synthesis approach to obtain QDs with selected size and morphology. This can be achieved by tuning the reaction conditions, such as – reaction temperature, ratio of precursor’s concentration. CuInS₂ can be synthesized following high-temperature colloidal pathways using octadecene as solvent, oleic acid (OA) as capping agent, and dodecane thiol (DDT) as both sulfur source and capping agent. By changing the ratio of two precursors, the photophysical properties of the samples can be tuned.

The purification protocol was specially designed to enable the size selection of the QDs by exploiting their dispersibility in different solvents by fractional separation and centrifugation. The process was closely monitored by a combination of laboratory X-ray diffraction and absorption/emission spectroscopies.

Finally, a thorough structural and microstructural characterization of the QDs was carried out by SAXS/WAXS reciprocal space total scattering methods, based on the Debye Scattering Equation (DSE).

The results of this study may serve as an important reference point to further the understanding of the correlation between the structural and functional properties of this important class of materials.

Title: Synthesis of III-V Quantum Dots Using Indium Monohalides and Aminopnictogen Precursors

Abstract: III-V semiconductor QDs such as InP and InAs hold great promises for use in displays, LEDs, biological imaging, photocatalysis, and photodetectors. They are widely considered a safer alternative to toxic cadmium- and lead-based QDs. Significant progress has been made during the past decade in their synthesis using aminopnictogen E(NR²)₃ compounds (with E = P, As and R= Me, Et) as non-pyrophoric group V precursors. However, it is still highly challenging to achieve narrow emission line widths and a broad QD size range with these approaches.

We present a novel synthetic scheme relying on the co-reduction of aminophosphines via two different pathways by replacing indium trihalides, routinely used as the In precursors, with indium monohalides. This method gives access to tetrahedral InP QDs whose sizes can be tuned from around 3 to 10 nm. Narrow emission line widths down to 110 meV at 730 nm and photoluminescence quantum yields up to 80% are obtained. Finally, the extension of this approach to InAs QDs is also discussed.

Title: Towards Understanding the Chemistry of Tin Halide Perovskite Nanocrystals

Abstract: Tin halide perovskites are deemed to be as a more sustainable alternative to the lead-halide perovskites. However, a clear understanding and control of their mercurial chemistry remains slender as compared to the Pb counterparts in bulk as well as in nano owing to their inability to provide a stable Sn(II) oxidation state to maintain pristine crystal structure phase in ambient conditions. In this work, I will discuss the physical and chemical parameters which can help us to achieve a stable, tunable and monodisperse CsSnX₃ perovskite nanocrystals with defined optical features. Pertaining to the formation energies of nanostructures, the interplay of the 2D Ruddlesden-Popper (L₂Cs_n-1Sn_nX_3n+1) phases with 3D CsSnX₃ nanocrystals is apparent with respect to ligand combinations (ammonium, carboxylate, phosphinate), precursor ratios and temperature when SnX₂ salt is used as a precursor. X-ray diffraction and scattering studies conjoined with optical spectroscopy and electron microscopy helps us in acquiring the useful insights into directing the chemistry of Sn-halide perovskites at nanoscale. This research work showcases the insistent necessity for the development of mechanistic understanding to design efficient synthetic routes in order to achieve high-quality tin-halide perovskite nanocrystals.

Title: Colloidal Supercrystal growth studied in-situ by SAXS

Abstract: Chemical synthesised colloidal nanocrystals (NCs) offer the opportunity for realising novel materials with tailored functionalities. Especially an inner core/shell structure of the semiconducting NCs leads to an increased photoluminescence (PL) output, but also, the NCs’ shape determines their optical or magnetic performance. The NC’s shape can also significantly influence the super-crystal structure of colloidal supercrystals SCs, where NCs act as building blocks to form 3D nanocrystal solids with designed properties. To what extent these properties are enhanced depends on the specific ordering of the NCs within the superstructure.

Recently, we used faceted semiconducting PbTe/PbS NCs as building blocks, which are synthesised by the Ibáñez-Group at ISTA and can be used for thermoelectric applications. The supercrystals’ structure was investigated during growth by time resolved SAXS measurements at lab and synchrotron (ELETTRA) sources. We tried to reveal the impact of the NCs&apos shape on the superstructure and thus to understand not only the positional ordering of the NCs, but also their orientation ordering within the supercrystals. We determined the atomic crystal structure (with XRD/WAXS) of the NCs within the supercrystals and demonstrated the connection between crystal structure and superstructure via the NCs’ shape. Finally, we try to relate the micrometer sized shape of the SCs investigated by optical microscopy to their internal supercrystal structure.

Title: Anion-Assisted Yb³⁺ Doping of 2D Organic-Inorganic Perovskite Nanoplatelets Resulting in Near-Infrared Emission

Abstract: Currently, lead halide perovskite nanocrystals are in a focus in terms of design of materials with attractive optical properties. Photoluminescence of perovskite nanocrystals can be tuned by several means: changing their dimension, anion exchange, and different doping strategies where lead ions are replaced with various metal ions (so-called B-site doping). Recently, it has been shown that the B-site doping can be more effective with the assistance of simultaneous anion exchange, since it leads to the effective surface passivation and greater implementation of metal ions. However, the approaches of anion driven cation exchange for nanoplatelets and nanowires are still under-developed because of the usage of polar solutions as precursors of metal halides, what leads to high possibility of rearrangement of the brittle structure of 2D nanocrystals. Moreover, Yb³⁺ doping usually requires high temperature and occurs during nanocrystal formation. Herein, we report a non-destructive doping strategy of organic-inorganic FAPbBr₃ nanoplatelets (FA: formamidinium cation) with Yb³⁺ cations, which allows us to shift their photoluminescence into the near-infrared (NIR) spectral range. Perovskite nanoplatelets with a thickness of 2 monolayers were synthesized by the ligand-assisted reprecipitation (LARP) method and treated with YbCl₃ to achieve the NIR emission, whose emerging band was studied by means of steady-state and time-resolved spectroscopy. The preservation of the 2D shape of perovskite nanoplatelets during anion assisted Yb³⁺ doping was confirmed by TEM. The obtained results open new ways towards fabrication of perovskite nanocrystals with controlled dimensions and attractive optical properties.

Title: Exciton-Phonon Coupling and Charge-Carrier Localization in ZnCdSe/CdS Dot-in-Rod Nanostructures

Abstract: The properties of heteronanostructures are determined by their band-alignment. A type-I band alignment is beneficial high fluorescence quantum yields, while a type-II band alignment is ideal for catalytic applications. The corresponding excitonic properties can be revealed by performingtime- and energy-resolved photoluminescence (PL) spectroscopy. Investigating individual nanoparticles circumvents inhomogenous broadening effects of the characterized properties. Carrying out the measurements at low temperatures around 10 K reduces the linewidths of the emission spectra and allows to resolve spectral diffusion, as well as additional emission lines originating for example from exciton-phonon coupling. Here, we report on a study with a set of heteronanostructures consisting of Zn_1-xCd_xSe cores embedded in rod-shaped CdS shells. With increasing x, these samples change their conduction and valence band alignments across the heterointerface from type-I to type-II, which in turn changes the localization of photoexcited excitions. We analyze the exciton-phonon coupling strength by determining the intensity of the LO-phonon-replica of core and shell in low-temperature single-nanoparticle PL spectra. By comparing our spectroscopical results with quantum mechanical calculations within the effective mass approximation, which are performed by using COMSOL Multiphysics, we find that the exciton-phonon coupling is a good measure for the localization of excitons within the heteronanostructures.

Title: Two Biexciton Types Coexisting in Coupled Quantum Dot Molecules

Abstract: Extensive research of colloidal quantum dots (CQDs) gave rise to the development of nano-chemistry as a method to fuse two CQDs to form coupled QD molecules (CQDMs). While QDs are often described as “artificial atoms” due to their discrete electronic states, CQDMs are analogous to artificial molecules, manifesting hybridization of the electron wave-function, as evidenced by a red shifted spectrum. The joining of two light emitting centers and the increase in the volume stabilize charged- and multi-excitons, relative to such states in the parent QDs. The unique structure of the CQDMs can accommodate new types of excitonic states and different relaxation pathways. An interesting example is biexcitons (BXs), as they have two possible configurations in the CQDMs: localized BX, with two excitons in one QD, and segregated BX, with one exciton in each QD.

In this study, we aim to characterize the two types of BX in CQDMs and compare their properties to BXs in the parent monomer QDs. Recently, Heralded spectroscopy, a direct approach to probe the BX events was introduced, and resolved the ambiguity of previous indirect methods. Heralded spectroscopy is based on an array of avalanche photodiodes at the output of a grating spectrometer, which enables the post-selection of BX events in a time-resolved-spectrally-resolved manner. Herein, we use Heralded spectroscopy to explore the BX events in CQDMs. We find evidence to the coexistence of the two BX configurations in a model system of CdSe/CdS core/shell CQDMs, according to their different BX binding energies and BX lifetimes. We support our findings with theoretical analysis.

Title: Quantum Wells for Recording Cell Membrane Potential

Abstract: Studying the brain and understanding its complexity has been one of the greatest scientific challenges of our time. One of the limitations to achieve a better understanding of neuronal activity is the lack of tools for accurate mapping of neuronal circuits. Inorganic voltage nanosensors, in particular quantum dots (QDs), show great potential for non-invasive recording of electrical activity with high spatial resolution and fast temporal response in neuronal systems. The transmembrane electric fields, which are characteristic of neuronal spiking, can modulate electronic structure of QDs and lead to changes in the intensity and position of emission wavelength (Quantum Confinement Stark Effect), providing a means to report voltage dynamics with millisecond precision. Since electric fields outside the lipid bilayer of the cellular membrane decay exponentially due to Debye screening, a precise transmembrane targeting of QDs is mandatory that remains a long-lasting challenge holding back the successful use of this material in neuroscience.

In this presentation, we share our research on the use of spherical quantum wells (SQWs) composed of CdS/CdSe/CdS for measuring changes in membrane potential of self-spiking HEK. A focus is put on phase transfer of nanocrystals from non-aqueous to aqueous medium through their encapsulation within octyl glucoside micelles to increase their integration into the lipid bilayer without compromising optical properties.

Title: Exploring the Photoluminescence of Gold Nanoclusters and Ag2S Nanoparticles to Boost their SWIR Emission

Abstract: Current challenges and objectives for non-invasive optical bioimaging are deep tissue penetration, high detection sensitivity, high spatial and temporal resolution, and fast data acquisition. A promising spectral window to tackle these challenges is the short-wave infrared (SWIR) ranging from 900 nm to 1700 nm where scattering, absorption, and autofluorescence of biological components are strongly reduced compared to the visible/NIR. At present, the best performing SWIR contrast agents are based on nanomaterials containing toxic heavy-metal ions like cadmium or lead, which raises great concerns for biological applications. Promising heavy-metal free nanoscale candidates are gold nanoclusters (AuNCs) and Ag₂S nanoparticles (NPs). The photoluminescence (PL) of both types of nanomaterials is very sensitive to their size, composition of their surface ligand shell, and element composition, which provides an elegant handle to fine-tune their absorption and emission features and boost thereby the size of the signals recorded in bioimaging studies.

Aiming for the development of SWIR contrast agents with optimum performance, we dived deeper into the photophysical processes occurring in these nanomaterials, thereby exploring in depth how the environment, surface ligand composition, and the incorporation of transition metals influence the optical properties of AuNCs and Ag₂S NPs. We observed a strong enhancement of the SWIR emission of AuNCs upon exposure to different local environments (in solution, polymer, and in the solid state). Addition of metal ions such as Zn²⁺ to Ag₂S based NPs led to a strong PL enhancement, yielding PL quantum yields of about 10% and thus making them highly suitable for non-invasive deep imaging of vascular networks and 3D fluid flow mapping.

Title: Nanoengineering of low-dimensional porous silica with multi-hydrophilic block copolymer

Abstract: Porous silica nanoparticles have become highly relevant materials in the fields of nanomedicine for their potential in targeted drug delivery strategies. Their synthesis through micelle soft-templating enables precise control over pore sizes, accessibility and tailored nanotextures. Recent progress in the field has led to the formation of new low-dimensional structures from single micelle systems. In this context, polyion complex micelles (PICs), which are obtained through the complexation of a multi-hydrophilic block copolymer with a micellization partner, are emerging as a novel structure directing agent in alternative to traditional surfactant micelles. They offer a versatile platform to modulate the structure of porous materials by adjusting each block individually.

Through the combination of silica sol-gel chemistry and PICs, we introduce a new route for the synthesis of hybrid mesoporous silica nanostructures with controlled size and shape. While most previous studies have focused on double-hydrophilic block copolymers, we used for the first time a triple-hydrophilic block copolymer (Fig. 1a), which remarkably led to the formation of 0D mesoporous nanoparticles around individual micelles and hence without 3D periodicity (Fig. 1b). The cage-type structure of these nanoparticles was resolved by single particle 3D reconstruction from TEM images (inset Fig. 1b). Varying the synthesis condition further led to the formation of first-of-its-kind 1D mesoporous nanostructures (Fig. 1c). To shed light on the formation mechanism of these low-dimensional structures, we performed in-situ SAXS measurements, revealing a change in form factor with micelle concentration and during their concerted self-assembly with silica species (Fig. 1d,e).

Title: Scalable Synthesis of Hard Ferromagnetic ε-Fe₂O₃ Nanoparticles For 6G Applications

Abstract: Ferrites showing natural ferromagnetic resonances (NFMR) in millimeter wave frequencies, particularly in 95-220 GHz are interesting because of their potential applications in self-biased non-reciprocal devices for the next generations wireless communication (6G and beyond). ε-Fe₂O₃ is an orthorhombic iron oxide phase with a sizeable magnetization (M_S ~ 100 emu/cm³) and a strong uniaxial magnetic anisotropy reflected in an enormous coercivity (H_C ~ 20 kOe) at room temperature and NFMR at around 180 GHz. The resonance frequency can be controlled by substitution of Fe³⁺ with other trivalent metals which is interesting for the above mentioned application.⁵ To be successfully used in self-biased non-reciprocal devices such as circulators, ε-Fe₂O₃ nanoparticles have to retain their remanent magnetization state. We have studied the relaxation of ε-Fe₂O₃ nanoparticles as a function of their size, finding that relaxation effects are largely supressed for particles above 25 nm. The current state of art allows preparing larger particle sizes but using techniques that are not compatible with a large scale production maintaining a sustainable approach.  In this contribution, we will discuss our newly developed method to obtain in one batch large amounts of ε-Fe₂O₃ particles which are not affected by relaxation processes, overcoming an important limitation for its application in 6G devices.

Title: Dual Cationic Ligand Passivated Stable Double Perovskite Nanocrystals For Optoelectronic And Photocatalysis Application

Abstract: The doping and compositional engineering of Cs₂AInCl₆ (A = Ag, Na) double perovskite nanocrystals (DP NCs) are under spotlight due to their enhanced self-trapped excitons (STEs) emission properties and dopant-induced extended light harvesting abilities for a wide range of applications including optoelectronic devices and photocatalysis. However, desorption of surface capping ligands during isolation and purification results in the loss of colloidal, photophysical and structural stabilities and thus limits their application in optoelectronic devices. Moreover, easy reduction of a silver ions by oleylamine leads to low photoluminescence quantum yield (PLQY) and poor stability of silver based DP NCs.

In this talk, I will present the colloidal synthesis and purification of highly emissive and stable Bi and Sb-doped Cs₂AInCl₆ (A = Ag, Na) DP NCs by using a combination of strongly coordinating silver-trioctylphosphine (Ag-TOP) complex along with additional TOP ligand. The Ag-TOP complex efficiently prevent the reduction of silver ions into metallic silver. While TOP facilitate the nucleophilic reaction with chloride source such as benzoyl chloride during nucleation and growth stage and forms benzoyl trioctylphosphonium chloride intermediate that serve as both halide source and surface capping ligand. The antisolvent washed DP NCs shows high colloidal, structural and photophysical stability due to the tight binding of dual cationic ligand of benzoyl trioctylphosphonium and oleylammonium cations together with oleate anion to the surface of DP NCs. Moreover, I will explore our recent results on Fe³⁺ doped Cs₂Ag_xNa_1-xInCl₆ DP NCs, including synthesis, surface chemistry, optical properties and their application in photocatalytic CO₂ reduction.

Title: Local light-controlled precipitation and dissolution of CaCO₃

Abstract: Light-activated processes enable high spatial and temporal control over the production and modification of materials. For this reason, stereolithography technology frequently relies on photochemical processes, usually involving organic resins, in some cases modified with inorganic fillers. In our work, we demonstrate, for the first time, that light activation can be used to control the precipitation and the dissolution directly of inorganic materials. This is possible by using a photoactive molecule that, when irradiated, triggers an event that induces the material formation/modification. We first applied this principle to the precipitation and dissolution of CaCO₃. CaCO₃ crystallization was obtained by using Ketoprofen, which releases CO₂ under irradiation, and CaCO₃ dissolution by using Diphenyliodonium ion, which induces a decrease of pH when irradiated. These photo-induced processes allow to obtain CaCO₃ precipitation and dissolution just in the illuminated area of the precursor’s solution, with high precision (Figure 1), making this technique suitable for application in photolithography. The photo-precipitation method was also investigated for other inorganic materials, such as calcium phosphate, for dentistry application. Our “photo-precipitation/photo-dissolution” principle provides a simple method to obtain local control on the formation of inorganic structures, laying the foundations to a new class of materials for several applications.

Title: Role of Water in Self-Assembly of Colloidal CdSe Nanoplatelets

Abstract: CdSe nanoplatelets (NPL) have aroused a growing interest over the past few years thanks to their ultrathin thickness that can be controlled to the atomic layer. Excitons quantum confinement imposed by this thickness leads to an intense fluorescence with a narrow spectral width and a large spectral overlap. Thus, inter-NPL ultrafast FRET can occur when NPLs in stacks and can reach distances of 500 nm² when happening in micrometer-long nanoplatelet threads. Such assemblies are promising objects to present novel collective photophysical properties involving hundreds of emitters but are difficult to synthesize because mechanisms to explain their formation remains unclear.

In this work we explored the experimental conditions to obtain long stacks with high yield through the slow evaporation of a dispersion of NPL in presence of oleic acid. We tuned the evaporation rate, the concentration in oleic acid and the relative humidity of the atmosphere during evaporation. We found that water captured from atmosphere by oleic acid during evaporation is essential to obtain long threads. Understanding this effect is a step forward to synthesize new long threads made of NPLs with higher quantum yield or smaller ligands leading to new opto-electronics properties.

Title: Photoluminescence Quantum Yields of Luminescent Nanocrystals and Particles in the UV/vis/NIR/SWIR in Dispersion and the Solid State

Abstract: The rational design of functional luminescent materials such as semiconductor quantum dots and lanthanide-based upconversion nanoparticles, all photophysical and mechanistic studies, and the comparison of different emitters require accurate and quantitative photoluminescence measurements. Particularly the reliable determination of the key performance parameter photoluminescence quantum yield (Φ_f), the number of emitted per absorbed photons, and the brightness are of special importance for luminescence applications in the life and material sciences and nano(bio)photonics. In this context, examples for absolute measurements of the photoluminescence quantum yields of UV/vis/NIR/SWIR emissive semiconductor quantum dots and rods, made from different materials, and spectrally shifting lanthanide upconversion nanocrystals with different surface chemistries in transparent matrices are presented including excitation wavelength and power density dependent studies utilizing integration sphere spectroscopy. In addition, procedures for the absolute determination of the photoluminescence quantum yields of scattering dispersions of larger size quantum rods and differently sized inorganic particles have been developed as well as procedures for the characterization of solid luminescent nanomaterials such as different perovskites and YAG:Cer converter materials. Thereby, challenges and pitfalls of Φ_f measurements in different wavelength regions including the SWIR and material-specific effects related to certain emitter classes are addressed, achievable uncertainties are quantified, and relative and absolute measurements of photoluminescence quantum yield measurements are compared to underline limitations of the former approach. Finally, a set of novel UV/vis/NIR quantum yield standards is presented including their certification with a complete uncertainty budget.

Title: Polarimetric Measurements of The Bright Triplet Emission of Single Cesium Lead Halide Perovskite Quantum Dots at Cryogenic Temperature

Abstract: Cesium lead halide perovskite quantum dots (QDs) have recently emerged as promising platform for quantum light sources. Indeed, they exhibit exceptional photoluminescence properties thanks to the emission from a bright triplet exciton state. The energetic fine structure splitting, in the order of a few millielectronvolts, can be revealed by performing single QD spectroscopy at cryogenic temperature. In particular, depending on the QD orientation and on the observation direction, up to three orthogonal emitting dipoles can be spectrally detected, each of them with a high degree of linear polarization. To measure the polarization of the exciton fine structure, typically a rotating linear polarizer was used. However, this method cannot provide a complete characterization of the polarization state of the emitted light, and the polarization properties of the small, non-linearly polarized fraction of the emission were unclear. That is why in this work we are investigating the polarization properties of individual cesium lead halide perovskite QDs by more advanced polarimetric techniques that allow to measure the complete Stokes polarization vector at cryogenic temperature for each emission line. For these measurements it is crucial to perform a careful calibration of the optical path with respect to residual, unwanted birefringent phase shifts that are often occurring from optical coatings. Moreover, these measurements require colloidal QDs with very low intermittent emission fluctuations. In my presentation I will report the results of the polarimetry of single perovskite QDs, shedding light on the non-linearly polarized emission fraction and the nature of their peculiar emission properties.

Title: First Principles Study of HgTe Nanocrystals: Understanding Its Atomic, Electronic And Optical Properties

Abstract: Mercury-telluride (HgTe) nanocrystals (NC) have, over the past decades, demonstrated great potential as building blocks for low-cost-high-performing infrared (IR) sensors covering spectrum from short-wave-infrared (SWIR) to terahertz frequencies (THz). Despite the great progress in applied technology, fundamental mechanisms of HgTe NCs remain unveiled. These include atomic/electronic/optical structures, charge carrier transport and non-radiative recombination in HgTe NC thin films. We propose a systematic investigation into these properties based on density functional theory (DFT) calculations. Based on experimental observations, HgTe NCs in different sizes, shapes and surface terminations are computationally constructed. By applying DFT calculations on these NC series, we show that our results not only agree well with experiments in absorption and photoluminescence (PL), but also give insights on atomic structures of NC surfaces, electronic structures and optical coupling. We further demonstrate the impact of surface termination and dopants/defects on the electronic coupling between neighboring HgTe NCs and optical coupling in intraband transitions (1s®1p above LUMO), which plays an important role on the enhanced magnitude of charge carrier mobility. Finally, we show electron-phonon (EP) coupling of HgTe NCs and how surface termination can influence. Our study, bridging computational results and experimental observations, will be vital for guiding the optimization of next generation low-cost-high-performing NC-based IR photodetectors.

Title: Confocal Imaging of Unlabelled Nanoparticles in Cells and Biological  Tissues

Abstract: Noble metal nanoparticles (NPs), particularly gold (Au) and silver (Ag) NPs, have recognized relevance in chemistry, physics, and biology because of their outstanding optical, electrical, and photothermal properties. Optical properties, as localized surface plasmon resonance, are easily measurable signatures indicative of their morphology (size and shape), composition, surface chemistry, aggregation state and physical environment that can be used to identify molecular targets and chemical transformation processes. Due to the growing interest in the use of metal NPs in medicine and biology, detailed cellular studies are required before their application in vivo for treatment or diagnostic purposes. Herein, we present the observation of unlabelled Au NPs on Confocal Laser Scanning Microscopy (CLSM), by using the light reflectance instead of the commonly used fluorescence mode. The NPs size resolution limits for CLSM observation is studied experimentally. Theoretical calculations, of the size-dependent optical properties are also presented to support this argument. The Au NPs used were synthesized using the seeded growth citrate reduction method, and the NPs sizes range from 15nm to 150nm. Full characterization of the produced NPs was also performed by Transmission Electron Microscopy (TEM), UV-Vis spectroscopy and Dynamic Light Scattering (DLS). Further, the intracellular observation of different sizes of Au NPs using the reflectance mode is also presented in cultured cells and tissue sections. This work reveals as a method to observe NPs in living systems in real-time and non-invasive way, which can be extended to other inorganic NPs.

Title: Surface Enhanced Raman Scattering-Based Lateral Flow Immunoassay For Multiplex Detection And Quantification of Histamine And Parvalbumin
Abstract: Nowadays, the quality controls in the agri-food industry are increasingly rigorous. Detecting food allergens in meat or fish is very important for population’s health. Point of care (PoC) devices are highly needed to detect contaminants as histamine and parvalbumin in this work. Histamine is a biogenic amine produced by bacteria that causes food poisoning when it is consumed in high concentrations, it can be found in fermented or preserved foods, such as wine, cheese or fish. Parvalbumin is a protein that can cause an allergic reaction in some individuals, considering it an allergen that can be found in certain fish species such as tuna.

Herein, we have combined indirect SERS technology with LFIA (lateral flow immunoassay) to develop an ultrasensitive test for PoC applications. SERS-based LFIA will allow us to overcome some drawbacks of traditional colorimetric LFIA such as low sensitivity and/or limit of detection/quantification. This test has been designed to specifically detect and quantify histamine and parvalbumin in canned tuna fish. Thus core-shell Au@Ag nanoparticles (Au@Ag NPs) codified with Rhodamine B Isothiocyanate (RBITC) and Malachite Green Isothiocyanate (MGI), as Raman reporter molecules, and bioconjugated with antibodies against both contaminants have been obtained. Next, the performance of the SERS-LFIA for histamine and parvalbumin detection has been evaluated obtaining calibration curves for each analyte. The results revealed that our SERS-LFIA meets a successful range of quantification considering the legal concentrations of both allergens and improves the limit of detection up to 1.18 x 10^-4 and 1.91 x10^-3 mg/mL for histamine and parvalbumin respectively.

Title: Nanoparticle-Based Gel Structures For Photoelectrochemical Applications

Abstract: Controlled destabilization of a ligand-stabilized nanocrystal solution with an oxidizing agent can be used to build up a macroscopic highly porous self-supporting nanocrystal network with good accessibility to the surface. These networks provide charge carrier delocalization beyond a single nanocrystal building block which extends the charge carrier lifetimes and increases the probability of photocatalytic reactions. With the advances in colloid chemistry made in the last decades various ligand-stabilized nanocrystals with specific physicochemical properties can be synthesized. Combining properties of tailor-made nanocrystals and nanoparticles-based gel structures leads to novel materials with optimized photocatalytic properties. In this study, nanocrystal-based hydrogels are presented which exhibit a higher hydrogen production rate compared to their ligand-stabilized nanocrystal solutions. The gel synthesis through controlled destabilization by ligand oxidation preserves the high surface-to-volume ratio, ensures the accessible surface area and facilitates photocatalytic hydrogen production without a co-catalyst. Especially with such self-supporting networks of nanocrystals, the problem of colloidal (in)stability in photocatalysis is circumvented. The advantageous properties of the nanocrystal-based gel structures are revealed using photocatalytic measurements, X-ray photoelectron spectroscopy and photoelectrochemical measurements.

Title: Outperforming Catalytic Activity of Au_xPt_1-X Nanostructures by a Chlorine-Assisted Synthesis in Hydrogen Evolution Reaction

Abstract: Despite the highest activity of Pt-based electrocatalysts for hydrogen evolution reaction (HER), its scarcity and elevated cost calls for a rational use and for the development of nanocatalysts outperforming commercial Pt/C. Herein, we present the synthesis of Au_xPt_1-x nanocrystals by a chlorine-mediated microwave (MW)-assisted route. The addition of chlorine ions does control the final structure of the nanocomposite leading to a Au-Pt core-shell structure with enhanced electrocatalytic activity for HER. Particularly, AuPt structures with a Pt loading of 6.86% display overpotentials of 24 and 51 mV at 10 and 100 mA cm^-2 respectively, as well as a Tafel slope of 13 mV dec^-1. Those values are far lower compared to the samples produced without chlorine (32 and 81 mV at 10 and 100 mA cm^-2 respectively, Tafel slope 26 mV dec^-1) and benchmark Pt/C (31, 72 mV at 10, 100 mA cm^-2 respectively, Tafel slope 30 mV dec^-1). Interestingly, mass activity for that NCs were 13.7 A mg^-1_Pt which is 3 and 7 times higher compared to that of samples produced without chlorine (4.4 A mg^-1_Pt) or commercial Pt/C (2.0 A mg^-1_Pt) respectively. The high catalytic activity of the AuPt core-shell structure may result from the combination of a high exposure of Pt atoms and electron transfer between the Au core and the Pt shell. Finally, the MW-assisted synthesis developed to produce this AuPt nanostructures alloyed electrocatalysts allows for a mass production (500 mg per synthesis) in rather short time (ca. 15 min).

Title: Fast And Selective Reduction of Nitroarenes Under Visible Light With an Earth-Abundant Plasmonic Photocatalyst

Abstract: Reduction of nitroaromatics to the corresponding amines is a key process in the fine and bulk chemicals industry to produce polymers, pharmaceuticals, agrochemicals and dyes. However, their effective and selective reduction requires high temperatures, pressurized hydrogen and involves noble metal-based catalysts. Here we report on an earth-abundant, plasmonic nano-photocatalyst, with an excellent reaction rate towards the selective hydrogenation of nitroaromatics. With solar light as the only energy input, the chalcopyrite catalyst operates through the combined action of hot-holes and photothermal effects. Ultrafast laser transient absorption and light-induced electron paramagnetic resonance spectroscopies unveiled the energy matching of the hot holes in the valence band of the catalyst with the frontier orbitals of the hydrogen and electron donor, via a transient coordination intermediate. Consequently, the reusable and sustainable copper-iron-sulfide (CuFeS₂) catalyst delivers previously unattainable turn-over-frequencies, even in large-scale reactions, while the cost-normalized production rate stands an order of magnitude above the state-of-the-art.

Title: Precursor Chemistry-Dependent Efficiency of Anion Exchange Reactions in Perovskite Nanocrystals

Abstract: In perovskite nanocrystals, anion exchange reactions are very broadly utilized to tune the compositions, which finally results in effective modifications of electronic structures and thus emission colors. Anion exchange reactions can be simply implemented by introducing halide source and/or precursors to host perovskite nanocrystals. For example, all inorganic CsPbBr₃ nanocrystals can be rapidly transformed to CsPbI₃ nanocrystals when iodide anionic precursor source is introduced. In such anion exchange reactions, the most widely utilized halide source is the mixture of oleylamine and iodine. Although anion exchange reaction itself has been investigated by many researchers, understanding on the effect of halide precursor source in anion exchange reactions is lacking.

In this study, we show the effect of halide precursor source in anion exchange reaction of all inorganic perovskite nanocrystals. We reveal, for the first time, that the efficiency of anion exchange reaction strongly depends on halide precursor chemistry. We propose the mechanism how halide precursors affect anion exchange reactions in perovskite nanocrystals. Finally we develop the strategy to improve the efficiency of anion exchange reaction by boosting the reaction occurred in halide precursor source solutions.

Title: 980nm Light Activated SWIR-Emitting Gold Nano-Probes For Bio-Imaging

Abstract: Non-invasive photoluminescence imaging (PLI) has emerged as a powerful technique with clinical translation for diagnosis where it enables monitoring the progression of diseases such as cancer and helps surgeons during the tumor resection. Seminal work from Dai and Bawendi’s teams have demonstrated the benefit of shifting to shortwave infrared region SWIR (900-1700 nm) that allows to obtain detailed information with higher resolution up to few microns at higher depth down to few millimeters compared to the first near-infrared window (700-900nm). Since then, several inorganic materials such Quantum dots or lanthanide-doped nanoparticles have been designed to strongly emit in the SWIR providing high resolution of the vascular networks. However, such nanoprobes still require several chemical steps to make them biocompatible as they end up in the liver which can cause some severe toxicity.

Noble metal Nanoclusters, mainly gold, are ultrasmall nanoparticles composed of tens to hundred metal atoms stabilized by capping ligands. They exhibit bright emission tunable in the SWIR, are biocompatible with high renal clearance hence promising candidates for fluorescence bio-imaging and cellular labeling. A library of functionalized atomically precise SWIR-emissive gold nanoclusters with tunable near infrared absorbance has been synthesized by wet chemistry using a monothiol ligand (6-Mercaptohexanoic acid) and a dithiol co-ligand (hexa-ethylene glycol dithiol). Physico-chemical and optical characterizations indicates that the size and rigidity of the ligand shell protecting the gold core (~1.5 nm) can lead to a striking enhancement of the absorption at 980 nm and a shift of PL to higher wavelength due to possible changes of non-radiative processes. These nanoclusters could be detected by SWIR imaging in capillaries down to 6 mm in an intralipid solution mimicking living tissues using a 980 nm laser excitation. We also demonstrated the ability to monitor these emitters by SWIR imaging in small animals with a pharmacokinetic highly depend on their size.

Title: Opto-Electronics Properties of (2,5dmpz)PbX₄ Perovskite: From Single Halide to Mix Halides
Abstract: Low-dimensionality hybrid metal-halide perovskites are a promising family of light emitters, offering highly tunable optoelectronic properties. Low-dimensionality perovskite compounds with broadband emission, associated with self-trapped excitons, can be formed in different structure types such as: face-sharing 1D chains (Fig.1A), edge-sharing double chains and corner-sharing 1D chains (Fig.1B), edge-sharing double 1D chains (Fig.1C) which found to be the most efficient broadband emitters, to date.

(2,5dmpz) PbBr₄ forms 1Dl structure with edge-sharing double chains and exhibit bright, broad emission at room temperature with PLQY>40%. In this work, we study the effect of halide on the optical properties, specifically, broadband emission. We exchange the halides, mixing them, and changing the halide ratio, in order to gain control over their color rendering and bandgap. We synthesize different low-dimensionality (2,5dmpz)PbX₄ compounds (X = I, Br, Cl), or a mixture of halides in different ratios, through solution- and mechano-synthesis. These compounds’ structural properties are studied using powder X-ray diffraction. Their optical properties were studied by diffuse reflectance, photoluminescence, time-resolved photoluminescence, temperature-dependent photoluminescence, and photoemission spectroscopy. The structural characterization has shown that moving from big to small halide maintains the same structure, with a smaller unit cell, as expected. For the same transition, the optical characterization exhibited an interesting trend, according to which, while generally, the band gap increases, a redshift of the emission (lower energy) occurs. This trend was preserved during the mixing of the halide in different ratios and is still under investigation.

Title: 2D Metal Halide Perovskites: Synthesis and Characterization

Abstract: 2D metal halide perovskites (HPs) are recently emerging as promising materials for optoelectronic, and spintronic applications, due to their structural versatility that allows tunning of its photophysical properties. In these HPs, inorganic and organic layers are alternately stacked, creating an electronic band structure, resembling classical multiple quantum wells. The mentioned electronic band structure can be manipulated by the change of the composition either of the inorganic or the organic moieties. Furthermore, our laboratory recently demonstrated that the heterogeneity of a soft structural nature enables a motion degree of freedom, which is responsible for the breakage of inversion symmetry and the consequent formation of the Rashba effect. The last is pronounced as a split emission band at a band-edge energy with opposing circular polarization, hence beneficial for spin-related technologies.

The current work focuses on the manipulation of the electronic properties of 2D HP’s single crystals via the variation of either the organic spacers or the inorganic constituent.  The organic spacers will vary by using different solvents of variable polarities. Interestingly, we discovered that the polarity of the solvent regulates crystal crystallization and symmetry, as will be elaborated on at the meeting.  The metal content will be altered by embedding magnetic impurities.  The laboratory has implemented magnetic impurities in the past in 3D HPs, in which impurity-to-host carrier interaction has been exposed, and consequent changes in the carrier’s g-factor.  Similar work regarding the 2D HPs is presently in process, and some of those results will be reported at the meeting.

Title: Direct Observation of Charge Carrier Distribution in Contacted Nanowires Under Local Illumination

Abstract: Kelvin probe force microscopy (KPFM) is a method of measuring the surface potential. Being based on atomic force microscopy, KPFM enables the investigation of nanostructures. Among these nanostructures are semiconductor nanowires and nanosheets. Here, we report on combining optical, electrical, and surface potential measurements in one setup to investigate the charge separation in these materials and bring insight into the properties of semiconductor-metal interfaces. Nanowires on transparent substrates are electrically contacted using lithography. This allows for optical excitation simultaneous to current and KPFM measurements.

The surface potential of semiconductors, measured by KPFM in air, is influenced by ligands and adsorbates, because a surface charge region is formed. Under Illumination, this region is filled with charge carriers, changing the surface potential. We analyze this change using KPFM, which gives insight into properties of the semiconductor, such as n- or p-type, charge carrier mobility and its surface ligands.

KPFM directly reveals how the potential drops along a contacted CdS nanowire depending on the direction of an applied bias, and the contact material. This is due to the formation of Ohmic and Schottky contacts at the semiconductor-metal interface. A Schottky contact biased in reverse impedes the current flow, and no charging is observed by KPFM. By locally illuminating the space charge region of the Schottky contact, charge carriers are spatially separated, either charging the wire, or forming a current along the nanowire. Characterizing this space charge region is necessary to tailor the properties of nanowire devices such as transistors or solar cells.

Title: An Isotopic-Labeling Study to Investigate the Primary Source of Hydrogen in a Photocatalytic System of Pt-Tipped CdSe/CdS Dot-In-Rod Nanostructures in Alkaline Aqueous Alcohol Solution

Abstract: Hybrid nanostructures that combine semiconductor and metal materials, such as Pt-tipped CdSe/CdS dot-in-rod stabilized by 11-mercaptoundecanoic acid (DR), exhibit promising potential as photocatalysts for generating hydrogen from alkaline aqueous alcohol solutions when exposed to light. In this process, the alcohol acts as sacrificial agent because CdS cannot undergo water splitting without appropriate co-catalysts. The oxidation of alcohols releases protons that can potentially be reduced to hydrogen. However, it remains unclear which of the two reacting substances, water or alcohol, serves as the primary source of hydrogen for this system. Here, we present findings obtained by analyzing the production rates of deuterium, deuterium hydride, and hydrogen from various mixtures of isotopically labeled substances. Similar to Kandiel et al., who investigated pH-neutral aqueous solutions containing TiO₂/Pt hybrid nanostructures as photocatalysts and methanol as a sacrificial agent, our results confirm water as the primary source of hydrogen. Our findings provide important insights into the mechanism of this photocatalytic system and contribute to a better understanding of the roles of water and alcohol in this context.

Title: Organic Reactions Photocatalytically-Driven by Lead Halide Perovskite Quantum Dots

Abstract: Photocatalysis is a cost-effective and valuable method that is used for the synthesis of important organic molecules and critical/major bond formations such as C-C, C-N and C-O bonds. Over the last decades, a remarkable improvement was achieved in the field using purely organic, organometallic or transition metal catalysts. However, photocatalysts often comprise expensive transition metals (Ru, Ir, Au), are elaborate to prepare, require air-free conditions to yield high conversions, and may be non-reusable. Thus, the design and preparation of practically usable photocatalysts remain a formidable challenge. Lead halide perovskite quantum dots (LHP-QDs) are promising candidates featuring very high absorption coefficients (~10⁶ M^-1cm^-1), near-unity photoluminescence (PL) quantum yield (>95%) and tunable emission wavelength and high surface area. These nanocrystals can act as oxidative and reductive catalysts simultaneously, their reduction and oxidation potentials can be tuned, they can be separated from the reaction mixture and regenerated similarly to bulkier heterogeneous catalysts, and their surface offers a platform for the separation of the photoexcited carriers.

Here we report on using surface-engineered LHP-QDs in dimerization reactions of benzyl bromide derivatives with high-yield and fast conversion into the desired products. Coating of the particles with various metal oxides alters the surface properties, widening the scope of reactions to conduct. The regeneration of ligand-capped particles is successfully achieved using a post-treatment method and control experiments show that these reactions are solely photocatalytic.

Title: Ultrafast Spin-Exchange Auger Recombination and Carrier Multiplication in Manganese-Doped Colloidal Quantum Dots

Abstract: Manganese-doped CdSe colloidal quantum dots (CQDs) have been used in photoemission and photocatalysis  thanks to their ability to generate hot carriers. Recent studies ascribed the observed photoemission to very fast spin-exchange Auger ionization which occurred via ejection of either a ‘hot’ electron (single-step process) or a band-edge electron (two-step process).^2,4 In both cases, Auger re-excitation must compete with intra-band cooling which normally impedes electron ejection. However, in Mn-doped QDs, spin-exchange energy transfer is much faster than phonon-assisted cooling, which leads to highly efficient photoemission.

Carrier multiplication (CM) occurs via impact ionization or inverse Auger recombination. During this process, the kinetic energy of a carrier relaxes via generation of additional excitons. The ultrafast character of spin-exchange Auger recombination suggests that the rate of spin-exchange impact ionization can also be high, leading to highly efficient CM. To demonstrate spin-exchange CM (SE-CM), we use specially engineered Mn-doped core/shell PbSe/CdSe CQDs. In these structures,  SE-CM occurs via two spin-exchange steps: (1) exciton transfer from a CdSe shell to a Mn dopant, followed by (2) spin-flip relaxation of the excited Mn ion to create two excitons in a PbSe core. The observed CM benefits from extremely fast rates of spin-exchange interactions leading to a three-fold enhancement of the CM yield versus undoped CQDs, accompanied by a considerable reduction of the electron-hole pair creation energy. These observations indicate a significant potential of spin-exchange interactions as a highly effective driver of CM for applications in advanced photoconversion.