Many functional nanomaterials used for displays, lighting, photodetectors, catalysts, and other applications are synthesized by colloidal methods. The scope of chemical transformations accessible to colloidal chemists is determined by the thermal and chemical stability of the solvents and surfactants employed. For example, very few traditional solvents can tolerate temperatures above 400 °C, whereas the temperatures used in CVD and MBE growth of GaAs and other important semiconductors typically exceed 500 °C.
To expand the range of synthesizable nanomaterials, we are developing a comprehensive understanding of a novel class of colloidal systems: colloids in molten inorganic salts. Nanoparticles of various transition metals, semiconductors, oxides, and magnetic materials can form stable colloids in these highly unusual solvents. Their colloidal stability in molten salts cannot be explained by traditional electrostatic and steric stabilization mechanisms. Our experimental and computational studies suggest that long-range ion correlations in the molten salt near the nanocrystal interface play a crucial role.
In parallel with our fundamental exploration of these new colloidal systems, molten salts broaden the scope for solution-based synthesis of many nanomaterials that have been beyond the reach of traditional colloidal chemistry. We have used molten salts to synthesize colloidal GaAs, GaP, GaN, InₓGa₁₋ₓP, InₓGa₁₋ₓAs, and InₓGa₁₋ₓSb quantum dots, which resisted numerous synthetic attempts for decades. Most recently, we have turned toward the synthesis of colloidal nitride nanomaterials. By advancing colloidal chemistry in molten salts, we aim to enable synthetic routes to functional materials previously regarded as unsynthesizable by colloidal methods.

This year marks the first decade of colloidally synthesized lead halide perovskite quantum dots (LHP QDs), defining QDs as size- and shape-uniform ensembles with tunable quantum confinement and single-photon emission. Gradually, during this period, practically the entire compositional within a general formula APbX3 was thoroughly studied, with A being cesium (Cs), methylammonium (MA), formamidinium (FA), and azeridinium (AZ) was produced as high-quality nanocrystals. This journey is, arguably, at its very beginning. The LHP QDs are vastly different from conventional, more covalent semiconductors – they are ionic compounds with much lower formation energies, entropically stabilized, and structurally dynamic. The design of surface capping ligands turned out to be decisive for their stabilization at the nanoscale and for taming their photophysics. Currently, LHP NCs are prototyped as primary green emitters for television displays owing to facile and scalable production, higher emissivity-per-mass under blue excitation, and narrow emission linewidth. Their excitonic characteristics exceed initial expectations in many regards, opening opportunities as quantum light sources. In particular, at cryogenic temperatures, LHP QDs exhibit long excitonic coherence times, which start to match the fast sub-100 ps radiative rates. Both characteristics are optimized, to our surprise, in larger CsPbX3 QDs beyond the quantum confinement, namely, 20-40 nm, owing to the single-photon superradiance effect (giant oscillator strength at the single-exciton per NC regime). Single-component and multicomponent QD superlattices exhibit collective emission, known as superfluorescence, characterized by the oscillating, ultrafast (10-30 ps) radiative decays. This presentation will walk you through both the most essential progress over this first decade, including our current work, and outline future prospects. 

The synthetic tunability of electronic structure and surface chemistry of semiconductor nanocrystals make them attractive light absorbers for light-driven chemistry. Nanocrystals can drive a variety of photochemical transformations and they have been coupled with redox enzymes to drive reactions like H₂ generation, CO₂ reduction, and N₂ reduction. In this talk, I will focus on our efforts to understand the properties of semiconductor nanocrystals that are essential for these light-driven transformations.

Plasmonic excitations in nanomaterials enable strong light-matter coupling, electromagnetic field enhancement, and novel energy transfer pathways. While localized surface plasmon resonances are well-studied, the controlled manipulation of surface plasmon polaritons (SPPs) in two-dimensional materials presents both fundamental challenges and opportunities for next-generation optical technologies. In this talk, I will demonstrate how polarization-dependent, time-resolved photoemission electron microscopy (PD-TR-PEEM) enables direct visualization of SPP propagation with nanometer spatial and femtosecond temporal resolution in layered 2D materials. Our investigations reveal conventional SPP behavior in transition metal carbides and nitrides (MXenes), while identifying hyperbolic plasmon polaritons (HPPs) with extremely long propagation lengths in the anisotropic layered material MoOCl₂. These HPPs exhibit unique electromagnetic properties including directional propagation and enhanced light confinement not observed in isotropic materials. I will discuss how detailed theoretical and computational modeling of interfacial interactions is critical to describing SPP behavior in these systems and highlight emerging opportunities for exploiting hyperbolic dispersion in applications ranging from advanced sensing to quantum information processing and nanophotonic devices.

Colloidal semiconductor nanocrystals are prized in optoelectronics for their bandgap tunability, high photoluminescence quantum yield, and ease of colloidal processing. However, rapid nonradiative Auger recombination (AR) can negatively impact device efficiencies at high excitation intensities. In bulk semiconductors, AR is temperature-dependent, but in zero-dimensional quantum dots (QDs), it becomes temperature-independent due to the discretized band structure. The two-dimensional morphology of nanoplatelets (NPLs) complicates predictions of their photophysical behaviors.
We examined temperature dependent excited-state lifetime and fluence-dependent emission in CdSe NPLs, comparing them with QDs. As temperature decreases, the biexciton lifetime in NPLs surprisingly decreases, becoming shorter than trion emission, while emission intensity increases nearly linearly with fluence, indicating dominance of radiative recombination over AR. This contrasts fundamentally with core-only QDs.
Building on this, we found that photoexcitation of high-density 4 or 5 monolayer (ML) thick CdSe NPL films at temperatures below 200K results in pump-intensity-dependent, superlinear, and progressively red-shifted light output due to amplified spontaneous emission (ASE) from polyexcitonic species, distinct from biexcitonic ASE. These polyexcitonic species, known as "quantum droplets," form when multiexciton binding energy exceeds thermal energy fluctuations.
The ASE threshold for close-packed films decreases significantly with lower temperatures, attributed to extrinsic (trapping) and intrinsic (phonon-derived line width) factors. For pump intensities exceeding the ASE threshold, intense emission shifts to lower energy, particularly when the film temperature is ≤200 K. In contrast, NPLs diluted in an inert polymer suppress both biexcitonic ASE and low-energy emission, indicating reliance on high chromophore density and rapid, collective processes.
These findings enhance understanding of quantum droplet optical nonlinearities and the utility of photogenerated excitons and multiexcitons in optoelectronic applications.
 

In this talk, I will discuss our recent efforts on utilizing liquid-phase TEM imaging and associated machine-learning or computational simulation methods to map the fundamental forces involved in the equilibrium and out-of-equilibrium assemblies of nanoparticles. In equilibrium assembly, we study the nonclassical nucleation pathways, the growth habits, and the phononic relaxation of the nanoparticle self-assemblies. We show that colloidal interactions at the nanoscale can be mapped from the statistical sampling of single particle trajectories, or as effective springs by fitting phonon dispersion spectra. Going beyond equilibrium dynamics, we also study the external field driven assembly of nanoparticles, where electroosmosis effects drive the nanoparticles into active “swarms” with rapidly-changing patterns. We will show new structural control and new functional relevance in these particulate systems, when we consider both the colloidal interactions and all the other factors such as diffusivity, many-body effects, and ionic flows. 

Degenerately doped semiconductor nanocrystals exhibit tunable localized surface plasmon resonances with strong optical absorption cross sections and band gaps. Upon excitation, these materials produce non-equilibrium carrier distributions that rapidly relax, offering a brief window for utilization. These features mark plasmonic nanocrystals as promising candidates for new applications that require efficient light absorption and highly directed energy and carrier utilization, such as photodetectors, photovoltaics, and photocatalysts, if we can understand and engineer efficient transport mechanisms. To date, studies on hot carrier technologies have focused on coinage metals, which have orders of magnitude higher carrier concentrations than semiconductor plasmonic nanomaterials. It is unclear how lower carrier concentrations influence hot carrier generation or if transfer is feasible in these systems.

Here, we investigate hot carrier generation and transfer from prototypical tin-doped indium oxide (ITO) nanocrystals to adsorbates as a model system for harvesting and utilizing light in plasmonic semiconductor nanocrystals. Using transient absorption spectroscopy, we track carrier and energy transfer from ITO nanocrystals to adsorbates and evaluate the impact of aliovalent dopant concentration, wavelength, and energy level alignment. We find hot thermal carriers, rather than athermal carriers, transfer to adsorbates with external quantum efficiencies ~ 1%. Two-temperature and Marcus Theory modeling show that this is due to the low carrier concentration of ITO. Combining these results, we propose general design rules to optimize carrier energy transfer from plasmonic semiconductors including scenarios in which hot thermal carriers may transfer more efficiently than athermal carriers. We conclude with new results demonstrating unique pathways for semiconductor plasmonic nanocrystals.

Dynamics and spectral features of exciton emission in strongly quantum-confined metal halide quantum dot systems on two different platforms will be presented. The first involves cooperative photon emission in the form of superradiance in an electronically coupled superlattice, which exhibits polarized superradiance. The second concerns the emission of bright and dark excitons from quantum dots fabricated on a micro-ring cavity, which shows an unequal Purcell effect on bright and dark excitons.

Discovering and improving new semiconductor nanomaterials made in solutions is often a slow process that relies on trial and error. Traditional methods using batch reactors can be inconsistent, especially with heating and mixing, making it hard to explore all the possible ways to create and process these materials. Even though colloidal semiconductor nanomaterials have remarkable properties and are widely used in energy, quantum, and chemical technologies as well as photonic devices, we need better approaches to speed up their discovery and development. Recent advances in reaction miniaturization, automated experiments, in-situ multi-modal characterization, and artificial intelligence (AI)-assisted experimentation offer exciting new opportunities to accelerate nanomaterials discovery and development. In my talk, I'll present how combining continuous-flow reactors with autonomous experimentation—what we call a Self-Driving Fluidic Lab—can accelerate research in colloidal nanoscience. By breaking down the steps of making nanomaterials and processing into separate modules, using methods that can run up to 100 experiments per minute, and applying AI to help model the processes in real-time and make informed decisions about future experiment(s), we can efficiently navigate complex and high-dimensional experimental spaces. Specific examples will be shared to show how these self-driving fluidic labs can autonomously and precisely create metal halide perovskites, as well as II–VI and III–V semiconductor nanocrystals, reducing the development timeline from more than a decade to just a few weeks.

Colloidal quantum dots (QDs) exhibit complex electronic, optical, and structural properties, making them essential in optoelectronics, photovoltaics, and nanomedicine. Despite advances in understanding QD surface chemistry and trap state formation, key questions remain, particularly regarding surface effects on electronic properties. Addressing these challenges requires accurate theoretical modeling.
In our group, extensive density functional theory (DFT) studies have explored QDs up to ∼4.5 nm, revealing that increasing system size leads to band gap collapse and facet-specific localization of frontier orbitals. We also found that introducing surface vacancies induces reconstructions that widen the band gap and delocalize charge carriers, emphasizing the critical role of surface geometry in defining QD properties. However, DFT-based approaches are computationally expensive, limiting their application to larger, more realistic systems and longer timescales.
To overcome these limitations, we are preparing to employ machine learning force fields (MLFFs) trained on DFT datasets. A key aspect of this approach is the inclusion of long-range electrostatic interactions during the training process to ensure the structural stability of QDs, which is crucial for accurately capturing their surface and bulk properties. These MLFFs aim to provide DFT-level accuracy at significantly reduced computational costs, enabling the study of larger QD systems and their dynamic behavior over extended timescales.
We anticipate applying ML-based models to various QD compositions, including CdSe, InP, PbSe, and CsPbBr₃, facilitating more efficient investigations into QD growth and optoelectronic integration. This transition highlights the transformative potential of machine learning in advancing the computational toolkit for nanomaterial design.
 

Maturation of QDs in displays has led to several implementation methods using QDs as down-converters, each with their own advantages, drawbacks, and challenges to implementation. QD films, extruded plastic diffusers, and QD inks directly on top of OLED pixels have all become commonplace in displays. As the regulatory landscape and technical requirements evolve, QDs must evolve. Future implementation such as electroluminescent QLED has the potential to disrupt the display industry, but after decades of research this technology still falls short. This talk will provide a commercial and regulatory status update of QDs in display, and needs of the display community that are addressable by fundamental QD-related research.

In recent years, CuInSeS/ZnS semiconductor quantum dots (QDs) have shown improvements that have unlocked new capabilities and end markets. These materials exhibit tunable photoluminescence in the visible and near-IR, with near unity quantum yield. They can have efficient broad emission, but also narrow emission, large Stokes shifts, and high optical extinction coefficients.  Moreover, for applications in sunlight, their scalable synthesis (enabling low cost) and good stability, both in terms of field us and manufacturing, enable this class of dots to be deployed at scale in cost-sensitive applications like solar energy. UbiQD, a materials technology and QD manufacturing company spun out of Los Alamos National Laboratory, has pioneered the industrialization and scale-up of these materials, starting with critical end uses related to spectrum optimization for greenhouses and solar energy. This presentation will give an overview of how QDs are being used to enhance spectral quality of sunlight for improved photovoltaics (energy) and photosynthesis (food), which is driving a new order of magnitude of QD demand and production. We’ll discuss sunlight optimization and how the industry is adapting to delver in excess of 100 MT of QD solids per year for non-display applications.

Precursor Chemistry of Metallic and Semiconducting Nanocrystals

Two of the primary methods used to control the size and shape of nanocrystals include using precursor chemistry to influence precursor conversion rate or the addition of surface-binding ligands. This talk will focus on the chemistry of zinc sulfide nanocrystals, where we have quantitatively measured the precursor conversion rate of a library of substituted thiourea precursors and shown that unlike the cadmium and lead chalcogenides counterparts, zinc sulfide shows little to no influence of precursor conversion rate on nanocrystal size. Instead, adjusting the structure of the zinc precursor can be used to influence the nanocrystal size, likely through a surface effect. Interestingly, while the effect of electronics on the precursor structure is the same across the materials, adjusting the steric bulk of the precursors has an opposite effect for lead sulfide and zinc sulfide. This observation points to differences in the underlying mechanism between the materials. Finally, we will conclude with some work on silver nanocrystals that again emphasizes the role of surface chemistry on influencing nanocrystal shape and size.

There is a huge diversity of transaction metal chalcogenides, varying in composition and symmetry. When we put reagents in the flask for colloidal synthesis, how do we select for one crystalline phase over another? I will discuss our work with transition metal sulfides and selenides to answer these questions. We use libraries of reagents and ligands to examine kinetic and surface chemistry effects, and careful analysis of the molecular transformations that preclude nanocrystal formation and growth. Or main lessons have been:
1) Ligands change the rules of the game before it even starts.
Often ubiquitous ligands react with our reagents before nanocrystal formation, changing their nature, which in turn influences the final crystalline phase.
2) Structural motifs of the first nucleated phase are often retained in the product phase, even if the stoichiometry is different. Understanding the structural relationships between phases such as cation coordination or anion stacking patterns can be used, in concert with chemical potential and the nucleating phase, to rationally develop syntheses to each of the phases in a family.
3) The structural motif of cation coordination number on the crystal surface can be influenced by ligand coordination. The motif can become translated through the nanocrystal, controlling phase.
4) The over-growth and isolation of metastable polytypes require slow reaction conditions. Diffusion of reagents through the ligand shell is one route.

Colloidal quantum dots (QDs) have been a subject of extensive research for several decades for their unique optical properties and versatile applications. Ongoing refinements in their synthesis have enabled the production of high-quality QDs with precise control over their size, shape, and compositions. Specifically, core-shell and other types of heterostructure QDs have garnered significant attentions, resulting in advancements that enhance their optical properties, morphology control, material stability, and their diverse applications in real-life. In this talk, I will discuss about our works on the synthesis of cadmium-chalcogenide and perovskite-type QDs and QD-metal heterostructure nanomaterials. This includes a discussion on how these anisotropic QDs can be used as fundamental building blocks for constructing intricate superlattice materials. I will also talk about the potential applications of the colloidal QD-metal hybrid nanocrystals in photocatalytic reactions, presenting our experimental findings alongside theoretical insights. 

Shape control in colloidal III-V semiconductor nanocrystals, such as InAs and InP quantum dots, provides a powerful approach to tailor their optoelectronic properties. By precisely tuning synthesis parameters—including ligand selection, reaction kinetics, and precursor chemistry—nanocrystal morphology can be carefully controlled, selectively exposing crystal facets and minimizing surface defects. In this talk, I will highlight recent strategies developed in our group, emphasizing facet-specific reactivity, optimized surface passivation, and controlled precursor manipulation. In particular, modulation of pnictogen precursors through controlled redox chemistry allows refined morphological and compositional precision. Moreover, incorporating Zn significantly enhances surface passivation, reducing trap densities and improving photophysical stability. Special attention will be given to elucidating the relationships among nanocrystal morphology, surface chemistry, and key optoelectronic characteristics, including charge-carrier dynamics, trap state management, and optical absorption.

Post-synthetic transformations are a means to design nanoparticles that would be difficult to synthesize directly due to their shape, phase, or the presence of multiple chemical components. We have developed two anionic post-synthetic transformations of copper sulfide nanorods to create Cu2-xS/Cu2-xTe and Cu2-xS/Cu2-xSe nanoheterostructures. Tellurium anion exchange creates Cu2-xS/Cu2-xTe nanoheterostructures with various regioselectivities. Selenium transformations proceed either through a dissolution/reprecipitation mechanism to create Cu2-xS/Cu2-xSe nanoheterostructures with various shapes or through an anion exchange mechanism to create Cu2-x(S,Se) alloyed nanorods. Here we present our latest work to understand how we can extend these anionic post-synthetic transformations to develop a variety of nanoheterostructures. We have investigated the results of consecutive cation and anion transformations to reveal new materials design rules. Combination of tellurium anion exchange with cadmium cation exchange alters ion mobility and reveals a combined regioselectivity that is distinct from each individual transformation. Combination of selenium transformation with cadmium cation exchange reveals the importance of the order of when applying consecutive post-synthetic transformations.

Metal halide perovskite quantum dots (QDs) are bright narrowband emitters that offer outstanding optoelectronic properties based on size manipulation [1,2]. Despite this, size control has relied on empirical selection of binary acid-base pairs (e.g., trioctylphosphine oxide (TOPO) and oleic acid (OA))  to solubilise precursor metal halide salts (PbX₂) alongside unsystematic reaction conditions due to sub-second crystallization rates restricting the evaluation of precursor transformation kinetics thus evolution of QD size (dispersity) [3,4,5]. To address limited access to early-time kinetics and precursor chemistry, a bespoke helical microfluidic platform is developed to spectroscopically (in-situ) monitor the growth evolution of crystallites across three orders of magnitude, from milliseconds to minutes, of synthesis time using dynamic flow ramping [6]. We find that crystallisation proceeds through dissociative ligand interchange in which abstraction of Br anions from PbX₂(TOPO)_n complexes, via deprotonation of OA, allows the sub-second evolution of PbB species; a predecessor to monomer formation. Through kinetic analysis of preceding steps, modest activation energies (5-15 kJ/mol) are extracted elucidating their fast formation under the availability of excess OA, [TOPO]:[OA] < 1, otherwise restricted through the generation of hydrogen bonded adducts, [TOPO]:[OA] > 1. Moreover, we showcase a simultaneous fast equilibrium between PbX2 and TOPO whereby the degree of coordination (n = 1-4) is found to destabilise Pb-X bonds whilst concurrently strengthening Pb-O restricting TOPO removal hindering QD growth as supported by quantum chemical calculations and ²⁰⁷Pb NMR. In turn, early-stage (300-1000 ms) growth of highly confined quasi-spherical QDs (~2 nm) are accelerated by excess addition of OA whilst increased [TOPO]:[PbX₂] depresses growth. Beyond the role in precursor conversion, OA further facilities late-stage (> 5 s) surface reconstruction (ripening) in weakly confined (~8 nm) crystallites. Correlating time-resolved bulk compositional analysis with time-resolved photoluminescence reveals healing of Cs-vacancies yielding a twofold increase in PLQY, 20% to 50%. Overall, our study exacerbates the kinetic role of solubilising ligands in synthesising crystallising bright and monodispersed QDs of the desired size.

Achieving atomic precision in semiconductor nanomaterials is important for reproducible synthesis, consistent properties, and enhanced performance in various applications. However, precision synthesis of nanomaterials is challenging because of their complex structures and entangled reactions. Here, I will introduce our recent progress toward precision synthesis and atomic engineering of semiconductor nanoclusters. By combining coordination, cluster, and colloidal chemistry, we “precisionalize” the colloidal cation exchange reaction. Atomically precise semiconductor clusters are obtained at near-unity yields by using atomically defined clusters as anion templates and metal-ligand complexes as cation carriers. X-ray crystallographic structures provide atomic-level insights into the transformation mechanisms, the core and surface structures, and the origins of chirality and polarity in semiconductor nanomaterials. Our results represent an important step in extending precision chemistry from the molecular scale to the nanoscale, providing a designable approach to accessing a library of atomically precise semiconductor nanoclusters with tailored cores and surfaces for their existing and emerging applications.

Metal-organic frameworks (MOFs), zeolites, and other high-surface-area materials become even more useful when prepared as colloidal nanoparticles. They can be processed into coatings, membranes, and thin films via solution-state methods, such as spray coating or 3-D printing, at industrial scales. Nanosizing porous materials also improves their bioavailability for drug delivery, while enhancing the kinetics of gas sorption in carbon capture, water harvesting, and chemical separations technologies. Fundamentally, the presence of both external and internal surfaces challenges basic notions of interfacial science. I will describe advances from the Brozek lab towards the synthesis of monodisperse MOF nanocrystals and analytical techniques to understand their internal and external surface chemistry. Aided by sum-frequency generation spectroscopy,  electrochemical measurements, and synthetic molecular approaches, we observe unexpected size-dependent optical properties, redox processes driven by cooperative supramolecular interactions, and tunable phase change behavior.

A cloneable NanoParticle (cNP) is an inorganic nanoparticle that is synthesized by a protein. The protein determines the physicochemical properties of the particle, such as elemental composition, size, and morphology. Recent work on cloneable nanoparticles comprised of Se, ZnSe, and Bi will be presented, including the synthetic paradigm and the use of these cNPs in correlated light and electron microscopy.

This talk will describe recent results in which ytterbium doping has been used to generate very unusual photonic properties in otherwise (reasonably) well-understood inorganic materials. One example involves Yb3+ doping of lead-halide perovskite (CsPbX3) semiconductors to form new materials that excel at “quantum cutting”. A second example involves Yb3+ doping of 2D van der Waals magnets to generate unique optical manifestations of magnetic exchange. Both new families of materials present fascinating electronic-structure challenges.

The discovery and development of new materials tailored to a specific function remain as one of the grand challenges for materials’ scientists. To this end, layered materials have been extensively studied due to the sundry of properties that emerge from chemical and physical modifications. Unfortunately, little is still known of the fundamental requirements to generate pronounced and tailored alterations to the electronic structure upon dimensional reduction or functionalization of their active sites. Comprehensive nanoscale characterization is essential to understand how the loss of 3D structural coherence and further modifications of the surface, induced because of ion intercalation, exfoliation to 2D sheets, or functionalization of the basal planes, alter the electronic structure of these materials which translates into physical properties. In this talk, I will discuss the use of layered VS2 structures as seed to nucleate VOOH nanostars and the precise control of ligand modification on Fe-based layered double hydroxides as novel catalysts. With these modifications, we seek to understand changes in the capabilities of these materials to serve as pollution remediation catalysts, especially for emergent pollutants. 

While fluoride nanocrystals (e.g., NaYF₄) are popular host materials for lanthanide-based up-and downconversion, ceramic oxide hosts (e.g., ZrO₂ and HfO₂) have found less widespread use, due to the synthetic challenge of producing colloidally stable oxide nanocrystals with a complex (e.g., core/shell) architecture. The oxides are however, more chemically and thermally stable.

We first present the results of our mechanistic investigation in the synthesis of zirconium and hafnium oxide from metal chloride and metal alkoxide in trioctylphosphine oxide (TOPO). We study the metal speciation in solution, we determined the decomposition kinetics and its mechanism and we study the nucleation and growth. We find evidence for an E1 elimination mechanism and the occurrence of amorphous particles as intermediate on route the final highly crystalline particles. This is a consequence of the rate imbalance between a fast precursor decomposition and a slow crystallization process.

Second, we demonstrate the epitaxial growth of hafnia shells onto zir-conia cores, and pure zirconia shells onto europium doped zirconia cores. The core/shell structures are fully crystalline. Upon shelling, the optical properties of the europium dopant are dramatically improved (featuring a more uniform coordination and a longer photoluminescence lifetime), indicating the suppression of non-radiative pathways.

Third, we dive deeper in lanthanide doped zirconia nanocrystals. We determine the incorporation efficiency of multiple lanthanide in zirconia. We show the influence of the surface chemistry on the emission spectrum and lifetimes of Eu³⁺ of Tb³⁺. Time resolved emission spectra allow us to differentiate different europium sites (surface and bulk).

These results launch the stable zirconium and hafnium oxide hosts as alternatives for the established NaYF₄ systems.

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). At the same time, they remain wide bandgap semiconductors, so they are transparent to visible light, offering unique spectrally selective opportunities to control light. In nanocrystals of the prototypical material tin-doped indium oxide (ITO), for example, the strength and spectrum of light absorption are tunable across the mid- and near-infrared by varying the amount of tin incorporated during synthesis and the nanocrystal size and shape. These nanocrystals are ideal building blocks for optical metamaterials, where the spectra of the components and the nature of the coupling between them determine the effective optical response. We build photonic materials by assembling superlattices of ITO nanocrystals layer-by-layer and integrating them with thin films of metals and dielectric materials. The resulting cavity-coupled plasmonic metamaterials provide unparalleled control over infrared reflectance and absorption. For example, we show that photonically integrated single monolayers of ITO nanocrystals can act as spectrally tunable infrared perfect absorbers. Adding nanocrystal layers of variable composition expands the design space, with opportunities for multi-resonant structures and ultrathin optics.

Semiconductor nanocrystals have proven to be a promising material platform for a range of optoelectronic applications including in photovoltaic devices and light emitting technologies. However, one major drawback of many of the best performing materials is the presence of toxic, heavy metals (e.g. cadmium or lead). We present the hot injection synthesis of iron sulfide nanocrystals, which are formed from earth-abundant, heavy-metal free sources. Through size control, we shift the band gap from the near-infrared bulk value (0.95 eV for pyrite) into the visible range (>1.7 eV). We investigate the synthetic control of these materials, their atomistic structure, and the self-assembly into nano to microscale structures. We further demonstrate the formation of a magic sized cluster which acts as an intermediate in the formation of larger nanocrystals (>10 nm). This material platform shows promise for control of confinement effects in iron sulfide nanocrystals and tuning of optical properties through interparticle interactions in mesoscale assemblies.

Inorganic nanomaterials display unusual size-dependent properties that differ from the same material in bulk. Non-thermodynamic crystal structures inaccessible in bulk can be obtained as nanocrystals, aided by the energetic impact of the large surface area to volume ratio present in nanoscale solids. This presentation will highlight our group’s efforts to isolate unusual metastable crystal structures as colloidal nanocrystals, with work specifically targeting underexplored composition spaces in nanosynthesis. I will highlight the discovery of a series of rare-earth containing ternary metal chalcogenide nanocrystals that adopt a novel hexagonal crystal structure that does not have a bulk analog in the studied phase space.

Combining the remarkable semiconducting properties of metal halide semiconductors with  synthetically tunable chirality produces a new family of chiral semiconductors that offer the ability to control spin at room temperature through the chirality-induced spin selectivity (CISS) effect.  We have studied CISS in 2D chiral metal halide semiconductors (CMHS) where the chiral organic sub-component induces chirality into the two dimensional inorganic metal halide halides.  We developed spin-valves that exhibit a room-temperature CISS-induced magnetoresistance (CISS-MR) of >300%  and consists of a ferromagnet (FM), tunneling barrier, and CMHS. The large CISS-MR results from the formation of an interfacial spin-selective tunneling barrier due to CISS, which can produce spin-polarization and MR that surpass the limit generally assumed in the Jullière model. We also developed two non-contact ultrafast probe of CISS. One is through the inverse-CISS effect where by purely injected spin-current via ultrafast excitation of the FM induces a charge current in the CHMS.  The charge current is read out by THz emission spectroscopy, which we show is sensitive to the charge current polarity and direction. The second involves induces an ultrafast charge current in a chiral metal and measuring picosecond spin-polarization. We will discuss the implications of these measurements on the CISS mechanism. 

Since the end of the 1990s, the inorganic core of CQDs has been their key appeal as infrared solution processed optical materials. Striking milestones were PbS CQD short wave infrared imaging in 2008, background limited detection with HgTe CQDs in 2015, and thermal imaging a year later.  A growing number of companies around the world work on infrared imagers with simply spun-on CQD inks on CMOS chips.  
Work on CQD detectors is well advanced, and the goals are to reach relevant performances and extend the coverage to longer wavelengths.  In an exciting new development, biased CQDs can readily produce intraband cascade electroluminescence in the mid-infrared.  As a result, CQDs now provide both mid-infrared detectors and light sources. 
I will then discuss the central challenge of making brighter CQDs. Experiments point to energy transfer to yet uncontrolled/unspecified IR absorption that bypasses far-field emission.  This has a strong impact on detector sensitivity,  and HgTe CQDs for 5 microns, 8.5 microns, and 12 microns show the correlation of poorer sensitivity with weaker emission.   While we still struggle to find a materials solution,  nanoantenna and the Purcell effect already allow CQD mid-infrared detectors and emitters with impressive performance.