We present a synthesis of highly durable and active electrocatalysts based on ordered fct-PtFe nanoparticles and FeP nanoparticles coated with N-doped carbon shell (J. Am. Chem. Soc. 2015, 137, 15478; J. Am. Chem. Soc., 2020, 142, 14190; J. Am. Chem. Soc. 2017, 139, 6669). We report on the design and synthesis of highly active and stable Co-N4(O) moiety incorporated in nitrogen-doped graphene (Co1-NG(O)) that exhibits a record-high kinetic current density (2.84 mA cm-2 at 0.65 V vs. RHE) and mass activity (277.3 A g-1 at 0.65 V vs. RHE) with unprecedented stability (>110 h) for electrochemical hydrogen peroxide (H2O2) production (Nature Mater. 2020, 19, 436). We report on the design and synthesis of highly active TiO2 photocatalysts incorporated with site-specific single copper atoms (Cu/TiO2) that exhibit reversible & cooperative photoactivation process, and enhancement of photocatalytic hydrogen generation activity (Nature Mater. 2019, 18, 620). We synthesized multigrain nanocrystals consisting of Co3O4 nanocube cores and Mn3O4 shells. At the sharp edges of the Co3O4 nanocubes, we observed that tilt boundaries of the Mn3O4 grains exist in the form of disclinations, and we obtained a correlation between the defects and the resulting electrocatalytic behavior for the oxygen reduction reaction (Nature 2020, 359, 577).
We fabricated ultraflexible and/or stretchable soft-electronic and optoelectronic devices integrated with various functional nanomaterials and their applications to wearable and implantable medical and healthcare devices. We introduced electromechanical cardioplasty using an epicardial mesh made of electrically conductive and elastic Ag/Au nanowire-rubber composite material to resemble the innate cardiac tissue and confer cardiac conduction system function (Science Transl. Med. 2016, 8, 344ra86; Nature Nanotech. 2018, 13, 1048). We fabricated highly conductive and elastic nanomembrane for skin electronics  (Science 2021, abh4357). We reported graphene-hybrid electrochemical devices integrated with thermo-responsive micro-needles for the sweat-based diabetes monitoring and feedback therapy (Nature Nanotech. 2016, 11, 566).

Technology at the nanoscale has become one of the main challenges in science as new physical effects appear and can be modulated at will. Especially 2D nanomaterials can be designed and engineered in order to improve their performance and efficiency for energy and environmental applications. In this way, a proper selection of defects, grain boundaries and surfaces, or the right selection of dopants allow a major increase on the properties of a new generation of (photo)electrocatalysts.

In the present work, by using powerful advanced electron microscopy related techniques, we will move to the atomic scale in order to visualize the beauty of such nanostructures. Modified nanostructures as support for single atom catalysts with a great enhancement on lithium-sulphur batteries stability and performance or CO2 Reduction will be shown [1]. Nanoengineered atom-thin transition metal dichalcogenides (MoS2 and WS2) showing a high density of grain boundaries acting as efficient active sites for the hydrogen evolution reaction (HER) will be also studied at the atomic scale [2]. A glimpse on the latest single atom catalysts developed in the group for CO2 Reduction (CO2RR) will be also shown [3]. Atomic resolution electron microscopy analyses will help us to visualize such fancy nanostructures and allow us to create 3D atomic models in order to understand not only the growth  mechanisms implied, but also to be used as input models for further DFT simulations, which will allow us to gain knowledge on the novel catalytic mechanisms achieved.

We will show our latest results on direct visualization and modelling of nanomaterials at atomic scale, which will help to understand their growth mechanisms (sometimes complex) and also correlate their chemical properties ((photo)electrocatalytic) at sub-nanometer scale with their atomic scale structure.

The ability to move energy and electrons, under the influence of light, in an efficient manner is one of the fundamental challenges in the area of energy research. Our group is interested in designing principles based on the interplay of forces at the nanoscale, to improve the light harvesting properties of hybrid nanomaterials. For instance, the potency of electrostatic forces was elegantly explored to demonstrate efficient light induced energy and electron transfer processes in cationic quantum dots (QDs), in water. Further, a fine control over such interaction driven photophysical processes helped in the creation of high-contrast multicolor luminescent patterns from a single QD nanohybrid film. For this, the energy and electron transfer processes in QD thin films were regulated through the selective and controlled photodegradation of organic acceptor molecules. Similar control over interparticle interactions helped in outplaying the ligand poisoning effect in nanoparticle catalyzed photochemical transformations, and some of these aspects will be covered in the presentation. Such advancements in the existing light harvesting properties of nanomaterials, through the fine control of interactions at the nanoscale, can expand the scope of nanoscience in energy research.

In this talk I would like to present a perspective on the application of micro- Angle-Resolved PhotoEmission Spectroscopy (µ-ARPES) to study two-dimensional (2D) materials heterostructures. [1]

ARPES allows the direct measurement of the electronic band structure of solid-state materials, generating extremely useful insights into their electronic properties. The possibility to apply it to 2D materials is of paramount importance because these ultrathin layers are considered fundamental for future electronic, photonic and spintronic devices.

Moreover, recent technical developments of ARPES allow to visualize the entire band structure (including empty states) [2] and to obtain spin, real-space, many-body effects and electron-dynamics information. Direct measurements of k-space position, binding energy, effective mass, spin and dynamics of electrons at Valence Band Maximum and Conduction Band Minimum, are making ARPES an unmissable tool of investigation for 2D materials.

In this context, µ-ARPES has been developed for micro-area analysis, allowing the measurements of small samples and detecting variation of their properties in the micro-scale. It is routinely applied for exfoliated 2D materials and can complete other µ-investigations such as µ-Raman, µ-photoluminescence and scanning probe microscopies. µ-ARPES is very useful to understand electronic, spintronic and optoelectronic measurements for 2D heterostructures.

Because lateral dimensions and quality of sample are crucial for µ-ARPES, I would also briefly explain the preparation methods of 2D materials and heterostructures, presenting pros and cons of “bottom-up” [3] and “top-down” methods.

Some of the most interesting results obtained by ARPES for 2D heterostructures will be reported.

Transition metal dichalcogenide (TMD) nanosheets have become an intensively investigated topic in the field of 2D nanomaterials, due to their semiconductor nature, the direct band gap transition and the broken inversion symmetry going from bulk to monolayer. These properties makes TMDs suitable for different technological applications such as photovoltaics, valleytronics, or hydrogen evolution reactions (HER), or transistors. Among them, MoX₂ (X = S, Se) are only direct-gap semiconductors when their thickness is reduced to a monolayer, hence an important effort is devoted to obtain single layer TMDs. Colloidal synthesis of TMDs has been developed in recent years as it provides a cost-efficient and scalable way to produce few-layer TMDs having homogenous size and thickness, yet obtaining a monolayer has proved challenging. Here we present a general method for the colloidal synthesis of mono- and few-layer MoX₂ (X = S, Se) using elemental chalcogenide and metal chloride as precursors. Using a synthesis with slow injection of the MoCl₅ precursor under nitrogen atmosphere, and optimizing the synthesis parameters with a Design Of Experiments (DOE) approach, we obtained a monolayer MoX₂ sample with the required semiconducting (2H) phase, a band gap of 1.96 eV for 2H-MoS₂ and 1.67 eV for 2H-MoSe₂, respectively, both displaying fluorescence at cryogenic and elevated temperatures. A correlation between the blue shifted absorption spectrum and the spectral difference between the Raman modes was established and confirmed that a single-layer thickness was obtained.

Liquid phase exfoliation (LPE) offers a scalable method to prepare colloids of two-dimensional (2D) Van der Waals (VdW) solids. Over the years, LPE has been rationalized within the context of solution thermodynamics. We outline this approach by introducing a method for the LPE of rhenium disulfide (ReS₂), a promising novel semiconducting 2D material. By screening LPE in various solvents, we retrieve optimal conditions for high yield ReS₂ exfoliation in solvents characterized by similar Hildebrand and Hansen solubility parameters – a result in agreement with the solution thermodynamics approach. Besides, our thermodynamic analysis points out that the current method to describe LPE fails to account for the considerable tolerance on the solubility parameter mismatch. In other words, the window for solvent exfoliation is much broader than solution thermodynamics allows for based on the size of the colloid.

We combine our experimental dataset on ReS₂ with an extensive survey of literature studies to reconcile the too-broad exfoliation window observed for 2D VdW solids. Starting from standard Flory-Huggins theory, we address inconsistencies in the current line of thought and propose a more coherent way of working based on the critical exchange parameter χ_c – an idea inspired by polymer physics. In such a framework, suitable exfoliation solvents result in exchange parameters below a critical value χ_c. Notably, the predicted solubility parameter window for suitable exfoliation solvents based on such a criterium agrees well with the experimental data available for a wide range of 2D VdW solids. Therefore, we conclude that solution thermodynamics describes LPE, provided that the solubility parameter window is linked to the critical exchange parameter for solvent/solute immiscibility.

We investigate the charge carrier mobility in 1D and 2D II-VI semiconductor nanoparticles. Based on a quantum mechanical modeling of CdSe Nanorods and -platelets, we provide a microscopic understanding of the frequency-dependent charge carrier transport in structures of finite lateral size. In contrast to Drude-type terahertz conductivity models, which imply a quasi-continuous density of states, the domain size of 1D or 2D nanoparticles strongly affects the frequency-dependent mobility and strong confinement can further result in oscillations for sub-resonant THz-probing, as seen in experiments. In 2D systems the mobility is further governed by transitions in the two orthogonal x-
and y-directions and depends nontrivially on the THz polarization, as well as the quantum well lateral aspect ratio, defining the energetic detuning of the lowest THz-photon transitions in both directions.

Indium Phosphide Quantum Dots (InP QDs) are  promising non-toxic alternatives to the more established cadmium-based QDs for applications such as light-emitting diodes and display technologies. However, as-synthesized InP QDs exhibit low photoluminescent efficiencies and broad emission linewidths, hindering their successful implementation in commercial products. Those opto-electronic characteristics are strongly influenced by the surface chemistry of the QDs. Most importantly, surface defects such as under-coordinated atoms lead to localized trap states, strongly decreasing the emission efficiencies. To improve the properties of InP QDs, it is thus of uppermost importance to understand the termination of the inorganic QD cores, the binding modes of organic ligands, and to explore ways of tweaking the surface chemistry on demand.

The InP QDs studied in this work are synthesized via the aminophosphine-route, resulting in tetrahedrally shaped QDs, colloidally stabilized by oleylamine (OLA), and co-passivated by halide anions. In contrast to the traditional carboxylate-passivated InP QDs, the surface chemistry of the halide-amine InP QDs is still unexplored, and strategies to improve their emission qualities by surface treatments are limited.

To address this, we first performed ligand titration experiments with thiols and carboxylic acids to investigate the binding mode of OLA. Monitoring by quantitative nuclear magnetic resonance (NMR) spectroscopy and elemental analysis, we confirmed that OLA binds as a neutral L-type ligand, and can be replaced in an acid-base mediated ligand exchange. We further point out that surface-bound thiolates and carboxylates respectively decrease and increase the band edge emission of the InP QDs. Density functional theory (DFT) calculations confirm our finding, suggesting that thiolates easily form localized surface trap states.

In a second step, we added ZnCl₂ as Z-type ligand to  further increase the PLQY of the halide-amine co-passivated InP QDs. Most importantly, we find that pure metal chlorides strip OLA from the surface in a Z-type mediated L-type ligand replacement reaction, thus destabilizing the QDs. Interestingly, the complexation of ZnCl₂ with OLA prevents spurious aggregation. In this case, the binding of ZnCl₂ to the surface  triggers band edge luminescence. DFT calculations support these findings, and show how the adsorption of ZnCl₂ to under-coordinated P atoms can remove hole trap states.

Colloidal InAs quantum dots are ideal candidates for QD based technologies in the near infrared region of the electromagnetic spectrum. Despite significant effort, scalable synthesis methods that produce large InAs nanocrystals absorbing beyond 1400 nm are scarce, rely on harsh precursors, require long reaction times and typically yield poorly luminescing QDs.  In this work we overcome these challenges and report a 1-step colloidal synthesis of tetrahedrally shaped InAs QDs with tunable edge lengths between 4.5 and 9.5nm, relative size dispersions of 10% and excitonic absorption features with maxima as far as 1550 nm. With tris(diethylamino)phosphine as a mild reducing agent to synthesize InAs from InCl3 and tris(trimethylamino)arsine, we identify that the growth of InAs is triggered only at high temperatures. As a consequence, dilution and a steep temperature ramp can be used to suppress nucleation and promote growth, respectively. With these simple strategies, different sizes of InAs QDs can be synthesized in a reaction time of just 1 hour. Furthermore, after ligand exchange reactions with alkanethiols or zinc oleate the already luminescent InAs core is passivated better to yield PLQYs of 9 and 52% for the respective ligands. This work demonstrates the value of identifying and optimizing fundamental nanocrystal synthesis conditions, which culminates here in a meticulous control over InAs QDs with application ready optical properties.

Quantum dots (QDs) are novel, nano-sized semiconductors with a size-tunable bandgap due to the quantum confinement effect [1]. Over the past few decades, the surface passivation of Cd- and Pb-based QDs has been studied using both experimental and computational methods, leading to significant advancements in the preparation of high-quality Cd- and Pb-based QDs [2,3]. Unfortunately, the toxicity of Cd- and Pb-based QDs often limits the utility of these QDs in consumer devices [4]. In recent years, indium phosphide (InP) has rapidly gained attention as a non-toxic alternative to Cd-based QDs [5]. However, as-synthesized InP QDs have a large density of trap states originating from unsaturated surface atoms, resulting in low PLQYs [5,6].

We report post-synthetic surface passivation utilizing benzoic acid (BZA) as an X-type surface ligand. To understand how BZA impacts their electronic structure, we conducted spectroscopic studies on InP QDs with various surface modifications, including in-situ fluorination and post-synthetic BZA treatment. Comparison of a variety of time-resolved spectroscopic techniques reveals that BZA can selectively remove electron trap states in InP QDs by passivating unsaturated indium atoms at the QD surface. When the BZA treatment is used in combination with a well-established fluoride treatment, the photoluminescence quantum yield of these unshelled InP QDs exceeds 20%. Compared with previous post-synthetic methods to increase the performance of InP QDs, including treatment with Z-type ligands and HF etching, the BZA treatment is green, safe, and easy to use. This research advances our understanding of the function of X-type ligands as passivants for unsaturated indium atoms for the post-synthetic treatment of InP QDs.

Photo-induced charge transfer is at the heart of many solar harvesting technologies including ones that involve semiconductor nanocrystals. To increase the efficacy of these technologies, it is imperative that the energetic barriers to charge separation and transfer are reduced. We have examined the unexplored role of the redox acceptor’s internal reorganization energy in effecting the charge transfer rates from quantum dots. The charge transfer rate to cobalt complexes having an electrochemical reduction potential difference of only 350 mV, but having nearly 2 eV difference in the reorganization energy, was experimentally studied and modelled with Marcus Theory. While driving force is important to the electron transfer rates, the difference in reorganization energy proved to have a more profound effect altering charge transfer rates by several orders of magnitude. The design of redox mediators to minimize reorganization energy rather than focusing solely on driving force would seem to be a previously ignored method to increase the efficiency of charge transfer from quantum dot applications.

Sorting through nanoparticles and nanoplatelets with first principles calculations

Using first principles molecular dynamics and electronic structure calculations, we investigate fundamental properties of complex semiconducting nanoparticles and nanoplatelets and their relevance to the design of electronic and solar energy conversion devices. In particular, we discuss electronic properties [1] and structure-function relationships [2], including ensemble of nanoparticles [3-5].

[1] Stoichiometry of the Core Determines the Electronic Structure of Core-Shell III–V/II–VI Nanoparticles, Mariami Rusishvili, Stefan Wippermann, Dmitri Talapin and Giulia Galli, Chem. Mater., 32, 22, 9798-9804 (2020).

[2] Determining the Structure–Property Relationships of Quasi-Two-Dimensional Semiconductor Nanoplatelets, Arin Greenwood, Sergio Mazzotti, David Norris and Giulia Galli, J. Phys. Chem. C, 125, 8, 4820 (2021).

[3] Modelling Superlattices of Dipolar and Polarizable Semiconducting Nanoparticles, Sergio Mazzotti, Federico Giberti and Giulia Galli, Nano Letters 19(6), 3912-3917 (2019).

[4] Surface Chemistry and Buried Interfaces in All-Inorganic Nanocrystalline Solids, Emilio Scalise, Vishwas Srivastava, Eric M. Janke, Dmitri Talapin, Giulia Galli, and Stefan Wipperman, Nature Nanotechnology, 13, 841-848 (2018).

[5] Optical Absorbance Enhancement in PbS QD/Cinnamate Ligand Complexes, Daniel Kroupa, Márton Vörös, Nicholas Brawand, Noah Bronstein, Brett McNichols, Chloe Castaneda, Arthur Nozik, Alan Sellinger, Giulia Galli and Matthew Beard, J. Phys. Chem. Lett., 9(12), 3425-3433 (2018).

The surface structure of nanocrystals (NC) strongly impacts their chemical and optolectronic properties because of their large surface area to volume ratios. However, techniques that can provide experimental atomic-level surface structures of NC are lacking. Here, we show how magic angle spinning (MAS) dynamic nuclear polarization (DNP) solid-state NMR spectroscopy can be used to determine the surface structures of some of the most widely investigated nanocrystals, zinc blende CdSe NCs with spheroidal and plate morphologies.^[1] 1D ¹¹³Cd and ⁷⁷Se cross-polarization magic angle spinning (CPMAS) NMR spectra reveal distinct signals from Cd and Se atoms on the surface of the nanoparticle, and those residing in bulk-like environments below the surface. ¹¹³Cd magic-angle-turning NMR experiments identifies CdSe₃O and CdSeO₃ coordination environments from {111} facets and CdSe₂O₂ coordination environments from {100} facets, where the oxygen atoms are from coordinated oleate ligands. The sensitivity gains from DNP enables acquisition of natural isotopic abundance 2D homonuclear ¹¹³Cd and ⁷⁷Se and heteronuclear ¹¹³Cd-⁷⁷Se correlation solid-state NMR experiments. 2D homonuclear ¹¹³Cd and ⁷⁷Se correlation spectra reveal the connectivity of the surface and core Cd and Se atoms. Importantly, ⁷⁷Se{¹¹³Cd} scalar heteronuclear multiple quantum coherence (J-HMQC) experiments illustrate the connection between the various Cd and Se environments and can be used to selectively measure one-bond ¹¹³Cd-⁷⁷Se scalar coupling constants (¹J_Se-Cd). With knowledge of ¹J_Se-Cd, ⁷⁷Se{¹¹³Cd} heteronuclear spin echo (J-resolved) NMR experiments are then used to determine the number of Se atoms bonded to Cd atoms, and vice versa. The J-resolved experiments directly confirm that major Cd and Se surface species have CdSe₂O₂ and SeCd₄ stoichiometries, respectively. Considering the crystal structure of zinc blende CdSe and the NMR data obtained from NC with spheroidal and platelet morphologies, we conclude that the surface of the spheroidal CdSe nanocrystals is primarily composed of {100} and {111} facets. We will also present preliminary data showing how these methods can be used to probe the surface structure of ligand exchanged CdSe nanocrystals that are passivated with phosphine, amine and chloride ligands. The methods outlined here will be generally applicable to obtain detailed surface structures of a wide variety of main group semiconductors.

Metal oxide nanocrystals doped with a few percent of aliovalent dopants become electronically conducting and support strong light-matter interactions in the infrared due to localized surface plasmon resonance (LSPR). In the prototypical material tin-doped indium oxide (ITO), we explored the influence of the spatial distribution of electrons on optical and electronic properties. In as-synthesized nanocrystals, electrons are largely confined to a plasmonic core, surrounded by an electron-depleted shell governed by the electronic states at the nanocrystal surface. Deliberately sequestering dopants in either the core or shell of the nanocrystals modulates the electrostatic landscape and shapes the plasmonic volume, including compression or expansion of the depletion layer. Core- or shell-doped nanocrystals exhibit multimodal optical spectra that respond strongly to changes in the dielectric environment. In thin films of nanocrystals, electronic conductivity is greatly enhanced in shell-doped nanocrystals wherein the barrier to nanocrystal-nanocrystal electron transfer is minimized.

We study liquid inks that form hybrid functional electronic films when drying. The interplay between organic and inorganic components affects structure and electronic properties. In this talk, I will focus on the case of metal or semiconductor particles and discuss how non-polar organic shells change their agglomeration and assembly into dielectric films [1].

Experiments that monitor the agglomeration and self-assembly in non-polar dispersions allow us to follow the film formation of spherical and anisometric particles. Systematic variation of the geometry of spherical particles with metal or semiconductor cores and dense alkyl shells indicate a transition between core- and shell-dominated assembly of non-polar dispersions that I will outline [2]. The comparison of spherical particles with ultrathin wires indicates that entropic effects strongly depend on solvent molecule geometry and that they can induce nanowire agglomeration or “bundling” [3]. Comparatively little is known on the assembly of highly concentrated dispersions. I will introduce new types of experiments that probe this regime using small, evaporating droplets of dispersions that we bring into the X-ray beam of a small-angle beamline [4].

Complexity increases when using electronically conductive organic shells on inorganic particles. I will briefly discuss hybrids of gold spheres that are coated with polythiophenes. The molecular arrangement of the monomer units on the metal surface depends on the core geometry and affects bulk conductivity [5].

Conforming thermal energy to electricity and vice versa through solid-state thermoelectric devices is appealing for many applications. Not only because thermal waste energy is generated in many of our most common industrial and domestic processes but also because thermoelectric devices can be used for temperature sensing, refrigeration, etc. However, their extended use has been seriously hampered by the relatively high production cost and low efficiency of thermoelectric materials. The problem is that thermoelectric materials require high electrical conductivity, high Seebeck coefficient, and low thermal conductivity, three strongly interrelated properties.¹

Thermoelectric materials are often dense, polycrystalline inorganic semiconductors. Usually, the processing of such materials has two steps: preparing the semiconductor in powder form and consolidating the powder into a dense sample. The most common route to prepare powders among the thermoelectric community is through high-temperature reactions and ball milling. Alternatively, solution methods to produce powders with much less demanding conditions (e.g. lower reagent purity, lower temperatures, shorter reaction times) have been explored to reduce the production costs. These methods also provide opportunities to produce particles with better-controlled features, such as crystallite size, shape, composition, and crystal phase, which allow modifying the properties of the consolidated material. However, when dealing with powders produced in solution, one should pay special attention to potential undesired elements coming from the reactants. Those elements may not affect the crystal structure and bulk composition of the powder but can be present as surface adsorbates. The composition, chemical stability, and bonding nature of surface species can influence the sintering process, and reaction byproducts can determine the final properties of the consolidated material.

Herein, we will demonstrate the importance of surface species in the use of solution-processed particles as precursors for bulk thermoelectric materials. In particular, we will provide examples in which surface species are used to deliberate control of the type and density of major carriers, engineer the electronic band structure, define composite microstructure, and hence charge carrier mobility and phonon transport.

In the past few decades, there have been many advances in the development of plasmonic nanomaterials and their application in chemical sensing, photothermal therapy, enhancement of photovoltaic device performance, catalysis, solar-driven desalination, among others. To date, nano-sized metals such as gold, silver, and copper have been well-explored for plasmonic applications, however these can suffer from poor thermal and/or chemical stability. Gold is also expensive making it non-ideal material for large scale applications. In a quest to develop plasmonic nanomaterials that are low cost and chemically stable, transition metal nitrides have emerged as strong contenders based on computational studies performed in the last decade. However, the experimental studies on these materials remain scarce. This presentation will highlight some of the recent results from our group on the solid-state synthesis of free-standing plasmonic transition metal nitride nanoparticles [1], their chemical and thermal stability, and photothermal properties [2,3]. 

Colloidal CdSe quantum rings (QRs) are a new class of nanomaterials synthetized via thermo-chemical edge reconfiguration of thinner CdSe nanoplatelets [1],[2]. In the latter, the photo-physics is consistently dominated by strongly bound electron-hole pairs, so-called excitons, that can merge to form excitonic molecules (biexcitons), giving rise to net stimulated emission along the molecule-to-exciton recombination pathway.[3] On the other hand, little is known on the nature of elementary excitations in thicker CdSe QRs - whether they are excitons or free electron-hole pairs- and their behavior at high density regime. Here, we show that charge carriers in QRs condense into a hot uncorrelated electron-plasma at high density opposed to the stable exciton gas found in thinner nanoplatelets. Through strong band gap renormalization, this plasma state is able to produce sizable optical gain with a broadband spectrum. Next, we show that the typical signatures of excitonic transitions are indeed absent in QRs. The gain is limited by a second order radiative recombination process and the buildup is counteracted by a typical charge cooling bottleneck. Overall, our results show that weakly confined QRs are a unique system to study uncorrelated electron-hole dynamics in nanoscale materials.

 Based on synthesis and the sample preparation procedure, the surface of colloidal nanocrystal is being covered by organic ligands on a regular manner. The ligand structure and the way of ligand-surface interaction can be crucial for  the physicochemical and optical properties of nanocrystal as a system.  In this work, we investigate the binding of zinc oleate to ZnSe nanocrystals from the perspective of its physicochemical properties. ZnSe nanocrystals have been used as narrow blue/violet emitters, as hosts for luminescent ions, or as large bandgap shell material, for example to form InP/ZnSe core/shell QDs [1]. In this study, we focus on neat ZnSe nanocrystals synthesized by injecting black selenium powder in a solution of zinc oleate in octadecene. By use of quantitative solution Nuclear Magnetic Resonance spectroscopy, we confirm that the ZnSe nanocrystals, obtained through described synthesis, are capped by zinc oleate. Following resembling work on CdSe quantum dots and nanoplatelets, we have shown that the addition of butylamine, an organic Lewis base or L-type ligands, leads to the displacement of zinc oleate from the ZnSe surface. By gradually increasing the concentration of butylamine, we record a displacement isotherm to represent the dynamic displacement equilibrium between bound zinc oleate and the desorbed zinc oleate butylamine complex. Going through quantitative analysis of these displacement isotherms and compare it with similar II-VI material we can conclude  that zinc oleate is more prone to displacement from ZnSe as, for example,  cadmium oleate ligand on the surface of CdSe quantum dots [2]. The estimation of the binding energy and the distribution of binding energies of zinc oleate over the surface of ZnSe nanocrystals can be done by using DFT-calculations on ZnSe model of the same crystal size.

(1)  Tessier, M.D.; Dupont, D.;  De Nolf, K.;  De Roo, J.; and Hens, Z. Economic and Size-Tunable Synthesis of InP/ZnE (E = S, Se) Colloidal Quantum Dots. Chem. Mater. 2015, 27, 13, 4893–4898.

(2)  Drijvers, E.; De Roo, J.;  Martins, J.C.; Infante, I,;  Hens, Z. Ligand Displacement Exposes Binding Site Heterogeneity on CdSe Nanocrystal Surfaces. Chem. Mater. 2018, 30, 3, 1178–1186.

The optoelectronic and chemical properties of semiconductor nanocrystals depend on their composition, size, shape and surface functionality. Cadmium telluride nanoplatelets (CdTe NPLs) are one of these semiconductor nanocrystals which, because of their relatively small bandgap (E_g = 1.44 eV) in bulk CdTe[1], are considered a promising material for photonic applications across the visible and near-infrared spectral range. Colloidal synthesis is a powerful strategy to alter the NPL optoelectronic and chemical properties, yet most efforts have been dedicated to CdSe NPLs[2]. In this work, we focused on the synthesis of size-controlled CdTe NPLs, and show that the chemical reactivity of the tellurium precursor plays a role in controlling the area of 3.5 monolayer (ML) CdTe NPLs. We built a novel 3.5 ML CdTe NPLs model based on a definitive screening design (DSD) to optimize the 3.5 ML CdTe NPLs synthesis and investigate the effects of additives on Cd precursor reactivity, such as oleic acid [OA], acetic acid [AcAc] and water [H₂O]. Our results show that there is an optimal combination of additives that reduces the NPL area to a minimum. This benefits the fluorescence quantum efficiency (PL QE), as we observed an inverse relationship between the  NPL area and the PL QE. Finally, we obtained 3.5 ML CdTe NPLs with up to 5% PL QE, and a reduced trap band emission.

Colloidal nanocrystals of different metals, semiconductors, magnets, and other functional materials can self-assemble into long-range ordered crystalline and quasicrystalline phases. However, insulating organic ligands present at nanocrytal surfaces prevent the development of extended electronic states (minibands) in ordered supercrystalline materials. Here we report self-assembly of nanocrystals with compact and conductive inorganic ligands,⁶ with optical and electronic measurements confirming strong electronic coupling between neighboring nanocrystals and showing evidence of metallic transport. Structural characterization of the resulting all-inorganic assemblies reveals faceted supercrystalline structures with a high degree of internal crystalline order. The assembly of charge-stabilized nanocrystals can be rationalized and navigated using phase diagrams computed for spherical particles interacting through short range attractive potentials. nanocrystals with large static dielectric constants have a unique propensity to form long-range ordered structures because of image-charge induced ion structuring and short-range repulsive forces that prevent gelation and glass formation. The assembly occurs in proximity to a binodal line separating two metastable colloidal fluids, and the conditions can be tuned to enable either one-step nucleation of supercrystalline solids or non-classical two-step nucleation.  We envision that the ability to grow all-inorganic long range ordered assemblies of strongly coupled nanoscale building blocks demonstrated in this work, combined with already available synthesis toolset for engineering nanocrystal size, shape, and functionality, will offer endless possibilities for engineering hierarchical solids.

A “Holy Grail” of particle science has been to be able to predict and control particle average size and particle size distribution (PSD) for the myriad of particles formed synthetically, or naturally, throughout nature. We have recently been able to make a significant advance on this problem following (i) 25 years of developing the minimum mechanisms for particle formation and agglomeration—now a total of ≥96 distinct mechanism when including A. Karim’s important ligand-based pseudoelementary steps; and most recently (ii) the combination of those experimentally based, Ockham’s razor obeying, hence deliberately minimum mechanisms with classic population balance modeling, what we have termed Mechanism-Enabled Population Balance Modeling (ME-PBM). The talk will strive to present the essential features of ME-PBM and its main implications. Time permitting, the highlights of two recent comprehensive reviews of the ~2000 papers in the literature citing LaMer’s model of particle formation, as well as a review of the use of synchrotron radiation for XAFS and SAXS studies of particle formation, will be briefly summarized.