Perovskite quantum dots (QDs) are now emerging as low cost functional materials for many photonic applications due to their superior optical properties and easy fabrication. Recently, we developed the in-situ fabrication of hybrid perovskite QDs in polymeric films (PQDF) with high transparency, superior photoluminescence emission and additional processing benefits for down-shifting applications. The potential use of PQDF as color converters in LCD backlights was successfully demonstrated, showing bright potential in display technology. Very recently, a dual green and red emissive composite film was developed for Mini-LED backlighting applications. Moreover, we further explored the electroluminescence devices based in-situ fabricated perovskite QDs. We illustrated the enhancing role of ligand-assisted reprecipitation (LARP) process and obtained uniform FAPbX3 (X=Br, Cl, I) perovskite QDs films with photoluminescence quantum yield up to 78%. The electroluminescence devices with a maximum external quantum efficiency (EQE) of 16.3%, 15.8% and 8.8% were achieved for green, red and blue devices, showing the promising to achieve high efficiency. In all, the in-situ fabrication strategy provides very convenient route for display technology. In this talk, I will introduce the progress on the development of in-situ fabricated perovskite quantum dots toward other photonic and optoelectronic applications.

Nanomaterials have attracted enormous attention over the last few decades thanks to their improved properties and utility in a plethora of scientific areas including catalysis and electrochemistry. Different parameters such as structure, morphology, chemical composition and the presence of defects directly affect the properties of nanomaterials. A detailed characterization of these parameters is of uttermost importance if one wants to obtain a better insight concerning the structure-to-property connection. Transmission Electron Microscopy (TEM) is an ideal technique to investigate materials at both the nanometer and atomic scale and has therefore been widely used in the study of nanomaterials. [1] By combining the technique with tomography, a technique which derives three-dimensional (3D) information from two-dimensional (2D) projections, one is able to determine the structure and shape of nanostructures in 3D, even with atomic resolution. [2-3] Furthermore, when combined with spectroscopic techniques such as energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) can be used for the determination of the composition and even oxidation state of nanomaterials both in 2D and 3D. [4-5] Nanomaterials are widely used in many electrochemical and catalytic reactions, which typically occur at elevated temperatures, pressures and liquid environments. In such conditions unwanted structural, morphological and compositional changes take place which in turn affect dramatically the catalytic performance. Indeed catalyst degradation is a major problem and has been a matter of intense study for many years. However, only little is known about the changes occurring at the nanometer and atomic scale when nanomaterials are exposed to aggressive chemical environments. During the last decade, specialized in-situ gas and liquid cell, as well as heating TEM holders are available. By using such holders, one can reach higher pressures and temperatures and also introduce liquids in the microscopes, and therefore create an environment which is identical to that during actual catalytic reactions. [6-8] In this talk, an introduction to the fundamentals of TEM and its capabilities will be given, followed by an overview of the most recent advances in the field of in-situ characterization both at the nanometer and the atomic scale in 3D.

References [1] Adv. Mater., 2012, 24, 5655 [2] Nature, 2011, 470, 374 [3] Nat. Mater. 2012, 11, 930 [4] Nano Lett., 2014, 14, 2747 [5] ACS Nano, 2014, 8, 10878 [6] Acc. Chem. Res., 2016, 49, 2015 [7] Nano Lett., 2019, 19, 477-481 [8] Acc. Chem. Res., 2021, 54, 1189-1199

Ligands (aka surfactants, adsorbents, etc) are an essential part of Quantum Dots. In this tutorial talk, I will focus on organic ligands and first explain how they influence the Quantum Dot solubility (aka dispersibility or colloidal stability) in nonpolar and polar solvents. Second, I will introduce Nucleation Magnetic Resonance (NMR) spectroscopy and show how this powerful tool allows (i) discerning bound from free ligands, (ii) quantifying the solvation of the ligand shell (iii) providing access to the (heterogeneous) thermodynamics of ligand binding, etc. We will also discuss the classification of different types of ligands and how one can use this to predict ligand exchange. Finally, note that none of the above is specific for quantum dots but actually applies to all types of nanocrystals.

In this seminar we will explore quantum dot nucleation and growth, with a particular focus on mechanisms beyond the classical regime. Over the past several decades of colloidal semiconductor research, a wide variety of chemical systems have been explored to access II/VI, IV/VI, and III/V semiconductor nanomaterials. A classical understanding of nucleation and growth provides a reasonable explanation for some of these systems. It has become increasingly apparent, however, that the behavior of particle growth in many systems appreciably deviates from that predicted by classical nucleation theory. This deviation can simply affect particle size or can lead to entirely new morphologies and materials. While a complete, systematic understanding of these nonclassical pathways is an ongoing research effort, we will focus on two main concepts: the formation of kinetically persistent nuclei, that is, nanoclusters, and the non-atomistic growth of larger particles and assemblies.

Two-dimensional colloidal nanoplatelets (NPLs) are solution-processed materials with a particular shape, that can be designed toward unique absorption and emission properties. Importantly, optimization of the chemical synthesis goes hand in hand with insights into the optoelectronic properties, obtained from linear and nonlinear absorption spectroscopy, fluorescence spectroscopy and calculations of the nanocrystal band structure.

In this talk, I will discuss recent progress made for CdSe-based 2D nanocrystals, featuring CdSe/ZnS type-I core/shell NPLs with fast, color-tunable emission, suitable as phospors for lasing or LEDs, as well as doped CdSe:Ag NPLs and CdSe/CdTe NPLs with a type-II band offset, which in both cases exhibit fluorescence that is strongly Stokes shifted and has a lifetime beyond 100 ns, which can find application in energy harvesting. For the latter, we were able to insert an intermediate CdS tunneling barrier at the CdSe/CdTe interface, and obtain red and near-infrared to green fluorescence upconversion. This flexibility to design NPL heterostructures has led to a wide range of applications, from light emitting devices, to ultrafast scintillators and luminescent solar concentrators.

While experimental protocols aimed at the synthesis and characterization of colloidal semiconductor nanocrystals are now well established, theoretical calculations of the same materials still present many challenges. Some of these are: (1) the compromise in the size of the modelled systems, which have to be large enough to avoid too strong quantum confinement effects but small enough to be described computationally; (2) the construction of a nanocrystal model system that represent closely that found in the experiment, and (3) the possibility of adding surface ligands that increase dramatically the computational requirements.

A successful methodology to tackle some of the above issues is Density Functional Theory (DFT), which scales cubically with the number of atoms, but that in recent years was capable to provide important insights in the electronic structure and geometry of several colloidal nanocrystals of up to 3nm in size. However, a strong limitation of DFT is the high computational cost, which prevent on the one hand the inclusion of solvent and ligand molecules as in the experiments and on the other, performing long timescale simulations to study rare events like ligand binding at the surface, trap formation rates and phonon induced non-radiative quenching. To address these chemico-physical processes, the field moved to computationally cheaper alternatives such as those based on classical force fields. These methods however have also important drawbacks, such as the development of accurate force-field parameters and more importantly the lack of information on the electronic structure of the material studied. In this framework, novel machine-learning techniques are poised to bridge the gap between DFT and classical force fields to enable DFT quality simulations on colloidal nanocrystals also for long timescale events.  

In this talk I will therefore discuss the most important achievements obtained in the semiconductor nanocrystal field by theoretical methodologies and an outlook of future developments.

The tremendous advancement in material growth by colloidal synthetic procedures has allowed the properties of several compounds to be tailored with nanoscale precision, accelerating the development of optoelectronic devices, such as laser, LEDs and TV display technologies. At the single particle level, individual nanocrystal acts as an artificial atom, and they are capable to generate quantum light (e.g. single photons). In this lecture, we will review the main excitations in nanoscale materials, the radiative and non-radiative exciton recombination dynamics and their impact on the generation of quantum light. Examples of efficient quantum light sources will be presented, their peculiar photon statistics, with a detailed description of the experimental methods used to characterize them.

Semiconductor nanomaterials such as zero-dimensional quantum dots and two-dimensional quantum well-like nanoplatelets can be produced colloidally on large scale with precise control over ensemble optical properties. Quantum confinement in such systems offers size-tunable energy gap, strong photoluminescence, and, in some cases desirable properties such as optical gain and lasing. The role of thermal energy deposition and dissipation in these nanoscopic structures can impact including radiative rate and material stability, but has not been substantively probed. We have pursued ultrafast optical pump, X-ray diffraction probe experiments as well as optical studies on nanoparticle dispersions as functions of particle size, polytype, and pump intensity to examine lattice response. Shifts of diffraction peaks relate lattice heating and peak amplitude reduction conveys transient lattice disordering (or melting). Intraband and Auger-derived heating is clearly observed in lattice dynamics, and disordering is observed upon absorption of larger numbers of photons excitations per NC on average. Diffraction intensity recovery kinetics, attributable to recrystallization, occur over hundreds of picoseconds with slower recoveries for larger particles. Solid-solid phase transitions can also become apparent with disappearance/appearance of particular diffraction peaks.[1] Transient optical studies using mid-infrared pump photons can monitor thermalization timescales via vibrational excitation of ligands followed by heating of the inorganic core, thus revealing information regarding the inorganic-organic interface. These methods and findings suggest means to probe nanomaterial physical response, stability and transient electronic structure applications such as lasing and solid-state lighting.

Colloidal quantum dots (CQDs) are versatile solution-processable nanocrystals which have great promise for optoelectronic applications. Among the properties that make them unique are the large spectral tunability, the outstanding monodispersity, the solution processability and the good transport properties in thin films. Due to the quantum confinement effect, CQDs give rise to large variations in the bandgap. When using Pb chalcogenides the NIR and part of the visible spectra can be covered, which make them very interesting for many optoelectronic applications among which solar cells.  After a brief introduction to the physical properties of colloidal QDs, the state of the art and challenges in colloidal QD solar cells will be discussed.

Nanometer-scale crystals of group IV, III-V, II-VI, IV-VI, I-III-VI₂, and metal-halide perovskite semiconductors, capped by organic or inorganic ligands and dispersed in solvents, are known as colloidal nanocrystals (NCs) [Figure 1A,B] [1], [2]. Colloidal NCs form an excellent, solution-processable materials class for thin film and flexible electronics [Figure 1C-E]. In this tutorial, I will begin by describing the device physics of the thin-film, field-effect transistor. The field-effect transistor is a three-terminal, semiconductor device having metallic source, drain, and gate electrodes, a semiconductor channel (e.g., in this case a NC thin film), and gate dielectric layer which separates the gate from the semiconductor channel [Figure 1D]. Applying a voltage to the gate electrode creates an electric field across the gate dielectric material and modulates the carrier concentration and thus current between source and drain electrodes. I will then review the size, composition, and surface chemistry-dependent physical properties of semiconductor NCs and the arrangement and interparticle distance of NCs in thin films [Figure 1C], and connect these physio-chemical characteristics to the electronic properties of NC thin films and ultimately to advances reported in the performance of NC field-effect transistors. Next, I will share an accessible introduction to NC field-effect transistors as building blocks of electronic circuitry, giving examples of analog and digital circuit topologies and their functions. Device design and fabrication methods will be described that have enabled the scaling up in complexity and area and scaling down in device size of flexible, colloidal NC integrated circuits. Finally, taking stock of the advances made in the science and engineering of NC systems, I will describe challenges and opportunities to develop next-generation, colloidal NC electronic materials and devices, important to their potential in future computational and in Internet of Things applications.

Figure 1. (A) Schematic of a colloidal semiconductor NC composed of a nanometer-scale crystalline semiconductor core capped by ligands. (B) Photograph of a colloidal NC dispersion and (C) SEM images of glassy and ordered NC thin films. (D) Schematic of NC field-effect transistor device structure and (E) photograph of analog and digital NC integrated circuits fabricated on a 4” diameter flexible substrate.

Colloidal semiconductor nanocrystals or ‘quantum dots’ (QDs) comprise an inorganic semiconductor core encased into a shell of organic ligand molecules. As a result, they combine superior light-emission characteristics of quantum-confined semiconductors with chemical flexibility and processability of molecular systems. Highly efficient, size-tunable emission from colloidal QDs has been already exploited in commercial televisions and displays. These materials can also enable highly flexible, solution processable laser diodes with an ultrawide range of accessible colours. However, the realization of such devices has been hampered by several problems including very fast optical gain decay due to nonradiative Auger recombination, and low conductivity and poor thermal stability of QD solids, that complicate achieving high current densities required for the lasing regime. Recently, these challenges have been successfully tackled via novel approaches to Auger decay engineering and special strategies for boosting current densities to ultrahigh values of ~1,000 A cm-2. These advances have brought the QD lasing community very close to its ultimate objective, which is the realization of a QD laser diode (QLD). Here, we overview the principles of light amplification with colloidal QDs, assess the status of the QD lasing field, examine the remaining challenges on a path to a QLD, and, finally, discuss practical strategies for attaining electrically pumped QD lasing.