Kanatzidis International Symposium (4th Intl. Symp. on Materials/Solid State Chemistry and Nanoscience for Sustainable Development)

 2D multilayered perovskites introduced by Calabrese (JACS 1991) share similarities with 3D perovskites including direct electronic band gap, sizeable optical absorption, small effective masses, Rashba-like effects. Calabrese’s Ruddlesden-Popper phases were completed more recently by "Alternative cations in the interlayer" (Soe, JACS 2017) and Dion-Jacobson (Mao, JACS 2018) phases, leading to a consistent classification of multilayered perovskites in relation with the chemistry of the compounds or the crystallographic order along the stacking axis (Blancon, Nature Nano 2020). 2D multilayered thus afford extensive chemical engineering possibilities, and exhibit other features related to tuneable quantum and dielectric confinements, strong lattice anisotropy, strong exciton interactions, more complex combinations of atomic orbitals and lattice dynamics.
Exploring the potential of 2D perovskites for PV and the association of 2D and 3D perovskites in solar cell architectures is a long-term joint project with colleagues in US (Prof. A. Mohite, LANL then Rice Univ., Prof. M. Kanatzidis Northwestern Univ.) that we started years ago including the first breakthrough on 2D perovskite for PV (Tsai Nature 2016). This approach is in line with Snaith’s recent viewpoint (Science 2024) about perovskite solar cell architecture trends: “a growing consensus is forming about the requirements for an ideal perovskite interface: the elimination or repair of surface interface defects, the design of a rational energy landscape to satisfy selective carrier collection, the minimization of strain and stress, and the improvement of physical robustness and adhesion”. 
This will be illustrated by recent combined experimental and theoretical studies on excitons, formation of edge states, hot carrier effects and carrier localization (Blancon Science 2017, Blancon Nature Comm. 2018, Li. Nature Nano 2022, Zhang Nature Phys. 2023).  2D multilayered perovskites have exhibited very early improved device stability under operation. More, combined in 2D/3D bilayer structures using new versatile growth methods, excellent solar cell device stability can be achieved (Sidhik, Science 2022). Band alignment calculations nicely explain the difference of performances for ni-p or p-i-n devices. Our lattice mismatch concept (Kepenekian, Nanoletters 2018) shall provide further guidance for the choice of the proper 2D/3D combinations, leading to enhanced stability for 3D-based solar cells (Sidhik, Science 2024).
 

Fundamental understanding of the nature of chemical bonding and its influence on the electronic structure is paramount to chemistry, solid state physics and materials science. CuBiI₄ has a fascinating structure where Cu and Bi are surrounded by a tetrahedral and octahedral halogen framework respectively. From fundamental inorganic chemistry concepts, it is expected to have symmetry-allowed d-p overlap in the tetrahedral co-ordination and we see here strong Cu (d)- I (p) strong interaction. This rare interaction generates an antibonding state in the valence band just below the Fermi energy in the electronic structure. Electrons filling up the antibonding band  weaken the bond and subsequently the crystal lattice becomes soft and anharmonic giving rise to ultra-low thermal conductivity.¹ In the latter part of my talk, I will be talking about achieving an ultralow value and unusual glass-like temperature dependence of lattice thermal conductivity  in a large single crystal of layered halide perovskite Cs₃Bi₂I₆Cl₃.² Here, Bi-Cl interaction also forms a s-p antibonding state below the Fermi level which renders a soft lattice. While strong anharmonicity originates from the low energy and localized rattling-like vibration of Cs atoms, synchrotron X-ray pair-distribution function analysis further evidences the presence of local structural distortions in the Bi-halide octahedra. We propose that hierarchical chemical bonding, presence of antibonding states near Fermi level and low energy vibrations from selective sublattice in crystalline inorganic halide perovskites open an intriguing avenue for thermal transport research with their unfathomed lattice dynamics and potential applications.^{3, 4} 

We had found it being somewhat surprising that, in the following sequence of “simple” perovskites: PbTiO₃ is tetragonal (c/a = 1.064), PbVO₃ is also tetragonal with quite a higher tetragonal ratio (c/a =1,229) while PbCrO₃ is cubic (¡) [1]. A detailed structural and compositional analysis by X ray and electron diffraction and high-resolution electron microscopy coupled with EDS, of this High-Pressure phase, has shown that the reliability factors of the Rietveld X-ray powder refinement of PbCrO3 could be improved by considering the lead ion in a multi- minimum potential displaced from its special position. Also, the microstructure of this material is a rather complex perovskite superstructure that presents a compositional modulation, within a microdomain distribution in a slightly lead deficient material [2]. The proposed supercell is ~ap x 3ap x (~14-18)a_p; with ap being the average cubic perovskite parameter [3].

An examination of materials discovery processes suggest that there can be a long lag between the creation of compounds and the discovery of their utility that would permit them to be described as materials. The goal of materials-by-design therefore is therefore dictated primarily by the ability to screen materials for function. This is the first step en route to a paradigm of dialing up the optimal material structure and composition to serve a particular function. Several issues that make even this task of screening somewhat complex. The first is that many properties of interest are not tractably calculated in a reliable way, because the underlying science is as-yet not established. The second is that materials optimization is frequently based on much more than a single performance criterion. In this talk, I will describe computational proxies that have allowed us to establish guidelines to find better phosphor materials for solid-state white lighting, better magnetocaloric materials, and some recent work on low-k dielectrics. Separately, I will describe the computational screening of all inorganic photovoltaic materials.

Development of thermoelectric (TE) materials & devices is important, for energy saving via waste heat power generation and IoT power sources [1]. There are a variety of device forms which can be envisioned to be useful. I will present several high-performance materials systems we have been developing such as Mg₃Sb₂-type materials, skutterudites, Heusler alloys, magnetic chalcogenides, etc., and mainly on the development of various TE modules. An initial realistic 8 pair bulk module of our doped Mg-Sb materials exhibited an efficiency of 7.3%@320^oC, with estimated efficiency from the actual materials being ~11%, and a variant exhibited high performance room temperature power generation and cooling [2]. Recently, a modified single element device of Mg₃Sb₂ was able to achieve a TE efficiency ~12% [3]. Design and construction of two different design thin film TEG devices [4] and hybrid flexible TEGs will also be presented. It is also critical to have accurate evaluation of TEGs and we have recently laid out some best practices thereof [5].

The search for new high-performance hybrid metal halide phases with exotic structure types and unique functionalities has emerged recently. With the wide selection pool of metal, halide and organic component choices, targeted syntheses and rational design strategies can expedite the advancement and understanding of these materials. We have been working on a series of new metal halide families gearing towards different properties such as photoluminescence, circularly polarized luminescence and second harmonic generation. Highly luminescent systems combine emissive metal centers Mn, Cu or Sb and bulky rigid organic cations/ligands. Another strategy of controlling the symmetry is through the directional coordination via organic cationic ligands, where asymmetry arises with the complexity of bimetallic halides and cationic ligands. Neutral solvent ligands are used to trap hydroscopic rare earth metals and incorporate them into the bimetallic low-dimensional systems with group V metals. We have successfully demonstrated several new metal halide material systems with tunable and superior optoelectronic properties. 
 

The development of highly active earth-abundant catalysts for solar water splitting is critical for the innovation of noncarbon-based renewable fuels [1]. It is therefore important to determine the mechanisms of these water oxidation catalysts, such as nickel-iron layered double hydroxides ([NiFe]-LDHs), which exhibit low overpotentials, excellent long-term stability, and high current densities and Faradaic efficiencies [2]. In principle, mechanistic insight can pave the way for the development of new materials with enhanced activity.

We have developed a new, magnetic resonance-based technique to monitor the reaction kinetics of [NiFe]-LDH relative to other well-studied catalysts. This technique allows for nanomolar detection of oxygen isotopes and yields important information about the mechanism of these catalysts. Membrane inlet mass spectrometry and differential electrochemical mass spectrometry were instrumental in determining electrochemical properties in situ; however, they are indeed limited in their collection efficiency and quantification of oxygen on the minute timescale [4,5]. Results were paired with computational and kinetic modeling in order to differentiate key O–O bond-forming steps. Nickel-iron-based catalysts were shown to operate by a novel oxo-oxo coupling mechanism, distinct from hydroxide attack proposed for other systems—consistent with previous findings [3]. We present our initial findings and share our efforts at incorporating pulsed EPR experiments for these systems.

LHP NCs are of broad interest as classical light sources (LED/LCD displays) and quantum light sources (quantum sensing and imaging, quantum communication, optical quantum computing). The surface-functionalization of such labile ionic materials poses a formidable challenge, which we address with the library of designer phospholipid capping ligands [1]. Lattice-matched primary-ammonium phospholipids enhance the structural and colloidal integrity of hybrid organic–inorganic NCs [FAPbBr₃ and MAPbBr₃ (FA, formamidinium; MA, methylammonium)] and lead-free metal halide NCs. The molecular structure of the organic ligand tail governs the long-term colloidal stability and compatibility with solvents of diverse polarity, from hydrocarbons to alcohols. These NCs exhibit photoluminescence (PL) quantum yield of more than 96% in solution and solids, as well as hours-long stability at a single-particle level, with minimal PL intermittency, as well as bright and high-purity (about 95%) single-photon emission.  The brightness of such a quantum emitter is ultimately described by Fermi’s golden rule, where a radiative rate proportional to its oscillator strength (intrinsic emitter property) and the local density of photonic states (photonic engineering, i.e. cavity). With perovskite NCs, we present a record-low sub-100 ps radiative decay time for CsPb(Br/Cl)₃ NCs by the NC size increase to 30 nm, owing to the giant oscillator strength [2]. Notably, the fast radiative rate is achieved along with the single-photon emission. When such bright and coherent QDs are assembled into superlattices, collective properties emerge, such as superradiant emission from the inter-NC coupling [3]. In the most recent work [4], we present the formation of multicomponent SLs made from the CsPbBr₃ NCs of two different sizes. The diversity of obtained SLs encompassed the binary ABO₆-, ABO₃-, and NaCl-type structures, all of which contained orientationally and positionally confined NCs. We observed efficient NC coupling and Förster-like energy transfer from strongly confined 5.3 nm CsPbBr₃ NCs to weakly confined 17.6 nm CsPbBr₃ NCs. Exciton spatiotemporal dynamics measurements reveal that binary SLs exhibit enhanced exciton diffusivity compared to one-component SLs. 

Colloidal lead halide perovskite (LHP) nanocrystals (NCs), with bright and spectrally narrow photoluminescence (PL) tunable over the entire visible spectral range, are of immense interest as classical and quantum light sources. Fast (bright) and statistically pure single-photon emission is key for many quantum technologies, from optical quantum computing to quantum key distribution and quantum imaging. The brightness of an emitter is ultimately described by Fermi’s golden rule, with a radiative rate proportional to its oscillator strength (intrinsic emitter property) times the local density of photonic states (photonic engineering, i.e. cavity). With perovskite NCs, we present a record-low sub-100 ps radiative decay time for CsPb(Br/Cl)₃, almost as short as the reported exciton coherence time, by the NC size increase to 30 nm. The characteristic dependence of radiative rates on QD size, composition, and temperature suggests the formation of giant transition dipoles, as confirmed by effective-mass calculations for the case of the giant oscillator strength. Importantly, the fast radiative rate is achieved along with the single-photon emission despite the NC size being ten times larger than the exciton Bohr radius.

  NC self-assembly is a versatile platform for materials engineering, particularly for attaining collective phenomena with perovskite NCs, such as superfluorescence in perovskite NC superlattices.  Thus far, LHP NCs have been co-assembled with building blocks that acted solely as spacers to promote the tuning of the mutual arrangement of LHP nanocubes [2]. However, the functionality of the second SL component can give rise to the enhancement of the LHP NCs properties or the emergence of new collective effects. We present the formation of multicomponent SLs made from the CsPbBr₃ NCs of two different sizes. The diversity of obtained SLs encompassed the binary ABO₆-, ABO₃-, and NaCl-type structures, all of which contained orientationally and positionally confined NCs. For the selected model system, the ABO₆-type SL, we observed efficient NC coupling and Förster-like energy transfer from strongly confined 5.3 nm CsPbBr₃ NCs to weakly confined 17.6 nm CsPbBr₃ NCs. Exciton spatiotemporal dynamics measurements reveal that binary SLs exhibit enhanced exciton diffusivity compared to one-component SLs across the entire temperature range (from 5 K to 298 K). Observed incoherent NC coupling and controllable excitonic transport within the solid NC SLs hold promise for potential applications in optoelectronic devices. 

We also will present a novel library of phospholipid-based capping ligands for LHP NCs [4].

Halide perovskite semiconductors for direct X- and gamma-ray detection have currently attracted enormous attention for medical imaging and nuclear nonproliferation in homeland security, featuring excellent charge transport properties and low cost. As previously evidenced the hole carriers in perovskite semiconductors have better transport properties than electrons carriers. The unipolar sensing strategy could eliminate such challenge induced by the electron trapping issue. However, the development of unipolar detectors for perovskite semiconductors is still at an early stage where substantial efforts are requested for the device optimization. Here, our progress on the unipolar perovskite detectors were reported with the configuration of pixelated and virtual Frisch grid type aiming at their deployment for the high energy resolution gamma-ray spectroscopy. The influence of guard ring electrode on the dark reduction was also investigated. The thickness of single-crystal detectors varied from ~several mm to centimeters which were grown by melt method. The relationship of the carrier drift time and the signal amplitude in various detector configurations were analyzed to estimate the charge transport properties of the whole carrier. The energy resolution was determined based on the signal amplitude analysis. The issues in achieving high energy resolution by unipolar perovskite semiconductors were also analyzed. These results shall be of interest in the applications of high-performance room temperature gamma-ray detectors.

Solid oxide cells (SOCs) are efficient electrochemical systems for electrical power generation in fuel cell mode (SOFC) and hydrogen production in electrolysis mode (SOEC). One solution to increase the lifetime consists of decreasing the operating temperature to 650-750 °C but the electrode reaction kinetics become relatively insufficient [1]. One of the main challenges is to improve the oxygen electrode efficiency by enhancing the oxygen reduction/evolution reaction (ORR/OER). To tackle this issue, it is important to choose suitable materials with adequate physicochemical properties and to optimize the microstructure and architecture to further increase the electrochemical performances [2].

This work aims to design novel optimized oxygen electrodes with improved mixed ionic-electronic properties to be used as more efficient oxygen electrodes in SOCs. Indeed, it is of high importance to control the electrode microstructure and composition to obtain large surface areas. These properties are essential to increase the number of active sites for the ORR/OER and to enhance the ionic transfer at the electrode/electrolyte interface.

Here, we report recent advances in the design of the state-of-the-art La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) [3, 4], La_2-xPr_xNiO_4+δ (LPNO), [5, 6] with 0 ≤ x ≤ 2, and Pr₆O₁₁ [7] oxygen electrodes with grain size and porosity at the nanometre length scales. These active functional layers are fabricated using electrostatic spray deposition (ESD), a unique bottom-up method capable of depositing films with original morphologies by a nano-texturing approach. 

This talk will show our latest electrochemical performance results of these innovative oxygen electrodes investigating the role of the nanostructure and the electrode/electrolyte interface. The correlation between microstructure, composition, grain size, interfaces, and electrochemical properties is discussed in detail for the different investigated oxygen electrodes.

Our investigations suggest that the ESD process is a suitable low-cost method to manufacture unique optimized porous and nanostructured oxygen electrodes with reproducibility. Three MIEC oxygen electrodes have shown one of the lowest values of polarization resistances in the literature and excellent performances in single-cell tests. To conclude, the suitability of these mixed ionic and electronic conductors (MIEC) with innovative and controlled microstructure as durable air electrodes for SOECs has been proven to be promising.

Most large-scale industrial processes rely on heterogeneous catalysts. Among them, supported nanoparticles represent one of the largest classes, for which the desired catalytic performances (activity, selectivity and stability) often relate to specific combination of metals, additives (promoters/poisons) and supports. The complexity of these multicomponent materials often relates to the use of conventional preparation methods in water, associated dissolution/precipitation processes. They thus raise numerous questions on the role of each components, and in particular on the role of specific compositions, interfaces and alloying in driving catalytic properties. 

In this context, our group has developed synthetic methodologies to control the generation of active sites thanks to the concept of surface organometallic chemistry (SOMC). SOMC is anchored on molecular principles with the goal to understand the surface chemistry at a molecular level. It typically relies on controlling the density of functional groups like surface OH groups in oxide materials, grafting molecular precursor to generate isolated metal sites, following in many instances by a thermal treatment that removes the remaining ligands. This approach has been very successful in generating so-called single-site catalysts; it has also recently been shown to grow, in a controlled manner, nanoparticles with tailored compositions, small and nanoparticle size distribution, interfaces, and even alloying. Furthermore, these SOMC catalysts are specifically aimable to detailed characterization and operando spectroscopy, hence the possibility to derive structure-activity relationships.[1] 

This lecture focuses on showing how SOMC combined with state-of-the-art Operando spectroscopies, in particular based on X-Ray Absorption (XAS) and IR, augmented with computational modelling enable to understand the structure and the dynamics of active sites. The lecture will illustrate in particular how interfaces and/or alloys in nanoparticles are created and how these specific sites/interfaces evolve under reaction conditions and contributes to the catalytic events. This lecture will focus on two specific catalytic processes, namely propane dehydrogenation and CO₂ hydrogenation,[2] which are two key industrial processes relevant to current and emerging strategies. Overall, this lecture highlights how interfaces, alloying and dynamics are driving the catalytic performances and how one need to revisit (open) our views on active sites in heterogeneous catalysis.  

Chalcogenide aerogels (chalcogels) are typically synthesized with thiolysis, aggregration of nanoparticles, and metathesis of chalcometallate. Especially, the metathesis of chalcometallate enabled generalized synthesis of chalcogel with flexible choice of central metal cations and chalcometallate. Although recent developments of chalcogel achieved high surface area and unconventional surface functionality with chalcogenide, chalcogels are amorphous structures with lack of localized structural control, which hinder further tuning of pore structure, crystallinity and surface functionality. We have investigated local structure of thiostannate and thiomolybdate chalcogels. In addition to metathesis reaction, the kinetic control of chalcogel formation enables additional reaction pathways, such as condensation with coordination transformation and crystallization. The precise local structure control of thiomolybdate chalcogel enabled high performance electrocatalyst for hydrogen evolution reaction.[1] Furthermore, the coordination transformation of thiostannate enabled new synthetic route and local structure control of chalcogel.[2] For high performance aqueous radionuclide-adsorption, the well-defined crystalline Na-Mn-Sn-S chalcogel enabled efficient Cs⁺ and Sr²⁺ ion exchange reactions.[3]

Fundamental investigations of metal nanolayers and their applications for the oxygen evolution reaction (OER) and ocean wave energy harvesting are presented. First, nonlinear optical laser spectroscopy reveals the number of net-aligned interfacial water molecules and the energetics associated with flipping them as a function of experimental conditions (applied bias, ionic strength, pH). A spectroscopic nonlinear optical autocorrelator approach yields strong signals at wavelengths consistent with high-oxidation states oxo species that are invisible in cyclic voltammetry.  Implications for strategies to lower the OER's overpotential are discussed. Second, metal nanolayers are subjected to  wave action within a wave tank containing ocean water simulant. Electrical measurements using external resistors yield power curves that exceed 50 microwatts per wave event when a nanolayer deposited on a glass microscope slide is paired with a sacrificial anode. Voltages and currents are large enough to light up a blue light emitting diode with each wave event. The linear dependence of output power and wave height velocity is demonstrated. Implications for sustainable energy harvesting are discussed. 

Metal-Organic Frameworks (MOFs) have attracted a tremendous research interest because of their significant potential for practical applications in areas such as gas storage and separation, drug delivery, sensing, catalysis, etc.¹ The crystalline nature of these materials allows them to be characterized via single – crystal X-ray diffraction, which provides valuable insight of their structural features. MOFs with fine-tuned properties can be prepared through a process called post synthesis modification. PSM allows the introduction/exchange of functional groups of a MOF and is preferable to proceed in a single-crystal-to-single-crystal (SCSC) fashion because with this way direct structural information can be provided for the achieved structural modifications via single crystal x-ray crystallography. Several types of SCSC transformations have been reported which include insertion/exchange of organic ligands, exchange of lattice solvent molecules or terminally ligated molecules, transmetallations, metalation of the framework, etc.²

We shall report two families of trivalent rare earth (RE³⁺) MOFs based on a hexanuclear (RE³⁺)₆ SBU and their exchanged analogues. The first one involves 8-connected 2-D MOFs based on an angular dicarboxylic ligand 4,4'-(hydroxymethylene)dibenzoic acid (H₂BCPM), UCY-17(RE). A series of exchanged analogues UCY-17(Tb)/L produced from linker installation SCSC reactions of UCY-17(Tb) with selected dicarboxylic ligands shall also be discussed. The SCSC installation of the dicarboxylic ligands resulted not only to the turn-on of the thermometric properties of these materials but also to a variety of different thermometric performances.³ The second family of compounds with the general formula ((CH₃)₂NH₂)₂[Y₆(μ₃-ΟΗ)₈(bpydc)₆] is based on the linear dicarboxylic ligand H₂bpydc= [2,2'-bipyridine]-5,5'-dicarboxylic acid. Its subsequent metalation with transition metal ions was achieved giving rise to a series of exchanged analogues with various metal ions. Gas sorption measurements of the metalated analogues reveal lower Brunauer - Emmett Teller (BET) surface areas consistent with the complexation of metal ions to the accessible nitrogen atoms of the bpydc^2- ligand whereas the CO₂ uptake of the metalated analogues is increased. Furthermore, gas sensing studies of the pristine and metalated compounds revealed a variety of different gas sensing capabilities. Thus, SCSC transformation reactions allowed not only the targeted modification of the structures of the two MOFs but also the modulation of their temperature and gas sensing properties. 

The introduction of foreign atoms or vacancies into crystal matrices forms atomic-level defects, giving rise to unique defect structures influenced by their coordination preferences and sizes relative to constituent atoms in the matrix. These defects can evolve into more complex structures like one-dimensional dislocations or nanostructures, each interacting uniquely with charge carriers and phonons, thereby significantly impacting the transport properties of bulk solids. Consequently, understanding the formation mechanisms of these defects is essential for developing highly predictable design principles and stabilizing desired defect structures within bulk crystals.

In this presentation, I will discuss our recent research on the deliberate design of multiscale defect structures across various types of crystal lattices. These structures enable independent control over crucial physical parameters that determine the thermoelectric figure of merit (ZT), such as carrier mobility, concentration, electrical conductivity, and Seebeck coefficient. Through this approach, we explore unconventional pathways to enhance ZT, promising significant advancements in thermoelectric materials.

Inorganic fluorine-based compounds are found today as nano-components in many applications, including energy storage and conversion, photonics, electronics, medicinal chemistry, and more [1]. The strategic importance of nano-fluorinated materials can be illustrated by several examples drawn from various scientific fields. In the field of energy storage, fluorinated carbon nanoparticles (F-CNPs) are tested as active materials in primary lithium batteries, while 3d-transition metal fluorides and oxyfluorides, mainly iron-, cobalt- and titanium- based have been proposed as electrodes in secondary batterie(reversible) s. In all-solid-state batteries, materials derived from fluorite- (CaF₂) or tysonite- (LaF₃) structural types can be used as solid electrolytes, provided the F- anions are highly mobile. Nanocrystalline rare-earth fluorides are currently used for their photoluminescent properties at the micro- or nanoscale.

Functionalized nanoparticles and nanostructured compounds based on solid-state inorganic fluorides are used in many other advanced fields, including fluorinated graphene quantum dots (FGQDs), solar cells (DSSC, QDSSC), transparent conducting films (TCF), solid state lasers, nonlinear optics (NLO), UV absorbers, etc.
Their role is also decisive in medicine and biotechnologies [2], where doped rare-earth fluoride nanocrystals serve as luminescent biomarkers thanks to their up- and down-conversion properties, allow fluorine labeling of nanoparticles and in-vivo 19F NMR. Relevant nanotherapeutics include photodynamic therapy (PDT), luminescent thermometry, radiotracers for positron emission tomography (PET), theranostic nano-agents that incorporate both imaging probes and therapeutic media, and are therefore capable of carrying out both diagnosis and therapy within the same nano-object.
 

On the exciting frontier of nanomaterials, we stand on the brink of a revolution. Our journey into the microscopic realm has led us to the discovery and creation of nanoscale structures, with nanowires taking center stage. These nanoscopic wonders, with their superior surface-to-volume ratio and novel properties due to their small size, are poised to reshape the energy-harvesting landscape to power IoT devices by increasing diffusive phonon scattering which decreases the heat conductivity above the amorphous limit.

Our work with Bi2Te3, crafted meticulously within anodic aluminum oxide (AAO) templates, has given birth to structures whose optical response is governed by plasmon resonances. These resonances, a product of nanowire interactions and material properties, can be harnessed to amplify thermal gradients and their associated thermoelectric power, thanks to the thermoelectric properties of Bi₂Te₃ nanowires.

Simultaneously, we are shining a spotlight on passive radiative cooling technology, a game-changer with the potential to revolutionize cooling methods for buildings and devices. This technology, a powerful tool in reducing carbon footprint and energy consumption, capitalizes on the morphological properties and chemical structure of AAO–Al samples to significantly alter their optical properties and cooling performance. The prowess of AAO nanostructures in thermal management applications has been demonstrated through a significant temperature reduction achieved with an AAO–Al sample.

Our exploration into the resistance of 3D-Bi₂Te₃ nanowire nanonetworks at low temperatures has yielded results compatible with the Anderson model for localization. The observed localization effects could potentially enhance the Seebeck coefficient in the 3D-Bi₂Te₃ nanowire nanonetwork compared to individual nanowires. This is particularly relevant as everyday life heavily relies on electricity, necessitating continual study to enhance power generation. Thermoelectric generators (TEGs), which use the Seebeck effect to convert waste energy into electrical energy, are a well-known method of generating electricity. The current state of TEGs, including different geometries and associated issues, as well as new TEG technologies and their challenges, have been analyzed.

Finally, in the case of triboelectric nanogenerators (TENGs), the new kids on the block, offer efficient mechanical energy harvesting through the triboelectric effect and electrostatic induction. Our research into the influence of 3D nanocavities inside polylactic acid (PLA) films on triboelectric power generation has revealed a correlation between the nanocavities and the relative permittivity of the polymer. The combination of 3D-PLA and 3D-AAO yields a fully dielectric composite film that drives power density due to an increase in the relative permittivity of the thin surface layer of the composite. The energy-storing efficiency of the developed PLA films was also studied. This work offers insight into how to use 3D nanocavities to enhance TENG performance and the useful blending of appropriate dielectric proprieties promoting self-power and intelligence of flexible electronic materials.

In conclusion, the future of energy harvesting for IoT devices is being empowered by advancements in nanomaterials. The exploration of nanoscale structures, passive radiative cooling technology, and the development of TENGs and TEGs are paving the way for more efficient and sustainable energy solutions. 
 

Research on solution-processable semiconductors has achieved significant fundamental and technological advancements over the last decade, in large part due to improvements in characterization techniques to understand these materials at different length scales. Notable example include hybrid perovskites and organic semiconductors, which have garnered interest for a wider energy paradigm and sustainability. Recent upserge in the solar-to-enelectrical energy conversion further expands the application space for these materials. Hoever, some fundamental questions regarding to the solar cell efficiencies of are related to morphology, defects, local disorder and interfaces between the semiconductor thin films and charge transport layers. To this end, understanding structure-stability-property relationships in emerging photovoltaics brings new opportunities and challenges to characterization techniques. Synergy between length and timescales of characterization techniques is particularly important.^[1,2] We will present how local structures/morphology and interactions can be resolved by state-of-the-art magnetic resonance spectroscopy and imaging techniques at high fields.^[3-4] Specifically, recent in situ and ex situ capabilities for examining thin films at micron-to-submicron thicknesses will be discussed. Gaining access to the local interfacial structures enables a number of questions to be addressed including a better picture of stacked semiconductor layers in electronic devices, diffusion of electrodes into photo-active layers, and film formation kinetics and molecules aggregation, surfaces/bulk passivation, and instability and degradation reactions and kinetics.^[5-7]

The traditional understanding in materials science considers single crystals nearly perfect in their ordered structures, represented by a unit cell that informs their mechanical and electronic properties. Our studies challenge this paradigm by demonstrating large deviations from the predictions made by the unit cell model to materials properties in systems such as halide perovskites, ion conductors, and organic semiconductors. 

Utilizing Raman spectroscopy, our efforts focus on the detailed examination of thermal motions and their implications on single crystals. The discrepancies between experimental observations and theoretical predictions are explored, particularly emphasizing the interaction between vibrational modes and their impact on material properties.

 A significant aspect of this research is detailed in our recent publication, where we propose a new model for second-order Raman scattering to account for the nonmonotonic temperature dependence observed in perovskite single crystals. This model, supported by numerical simulations, identifies low-frequency anharmonic features as key players in light scattering processes, highlighting a transition between two minima of a double-well potential surface. Our findings provide a more accurate understanding of the structural dynamics within disordered crystals and suggest broader applications for designing materials with enhanced electronic and optical functionalities.

When producing a multi-layer photoelectrode for solar fuel production, selecting appropriate bulk materials to use as a semiconductor, a catalyst, and a protection layer is important. However, optimizing the surface of each component and the interfaces between the components is just as critical to maximize the overall performance of the photoelectrode. Our research team has been at the forefront of demonstrating and elucidating the impact of the photoelectrode surfaces and interfaces on the overall performance of the photoelectrodes. For example, our team has shown that when a ternary oxide containing two different metal ions, such as BiVO₄, is used as a photoanode, the surface metal composition (i.e., the surface Bi:V ratio) may not necessarily be the same as the bulk metal composition (Bi:V = 1:1) and it can also be intentionally modified. We showed that changes in the surface composition while using the same underlying bulk photoelectrode can have an immense impact on the band edge positions and work function, which have a direct impact on electron-hole separation and photocurrent generation, even for the same facet exposed on the surface.[1] This observation made us wonder how varying the surface composition of the same photoelectrode can impact the photoelectrode/catalyst junction when the same catalyst layer is deposited on the photoelectrode. In order to explicitly demonstrate and investigate how the detailed features of the photoanode/OEC interface affect interfacial charge transfer and photocurrent generation for water oxidation, we prepared two BiVO₄(010)/FeOOH photoanodes with different Bi:V ratios at the outermost layer of the BiVO₄ interface (close to stoichiometric vs Bi-rich) while keeping all other factors in the bulk BiVO₄ and FeOOH layers identical. The resulting two photoanodes show striking differences in the photocurrent onset potential and the photocurrent density for water oxidation.[2] In this presentation, we explain the atomic origin of the experimentally observed difference by revealing the impact of the surface Bi:V ratio on the hydration of the BiVO₄surface and bonding with the FeOOH layer, which in turn affect the band alignments between BiVO₄ and FeOOH. 

Characterization and analysis by scanning transmission and transmission electron microscopy (S/TEM) is pervasive in modern materials research. The ongoing work in our group is inspired by innovations in high throughout assays and related automation from biotech. It combines novel design and nanofabrication of in-situ stages with smart imaging to utilize electron exposure in a commensurate manner. It is tailored to “ration” both electrons and time, spatially and temporally, utilizing AI/ML methods. 

  The presentation will cover emerging opportunities in advanced microscopy. In addition to typical static observations of structures and defect phenomena in functional materials (thermoelectrics, energy storage, photovoltaics etc.), it will cover innovative nanofabricated ultra-thin (UT) window fluidic cells for nanoscale discrimination of reactants and products in catalysis with spectroscopy. The presentation will also explore the feasibility of AI/ML-enabled data acquisition approach for rapid and high throughput materials discovery, as well as monitoring of in-situ phenomena in the temporal domain. 

  The presentation will show role of microscopy for energy, environment and sustainability research and innovations for broader societal good. 

In the context of solar cell technology, 2D Ruddlesden-Popper perovskite phases have been utilized alongside polycrystalline (PC) 3D hybrid perovskites (HPs) as ultrathin passivation layers to enhance stability and charge extraction. The majority of the reported 3D/2D heterostructures consist of PC thin films deposited on top of PC 3D HPs. This method offers limited control over the orientation and crystalline phase, leading to a high concentration of defects at grain boundaries and interfaces. These defects promote the presence of traps for charge carriers, ion migration, and water permeation.

On the other hand, pure 2D HPs have been considered less suitable for photovoltaic applications due to their large exciton binding energies, which theoretically hinder charge separation and result in significant energy losses. Surprisingly, the presence of large polarons - charge carriers coupled to lattice deformations - prevents the formation of excitons [1]. One of the first explanations was based on exciton dissociation caused by polycrystalline grains boundaries, suggesting that the formation of free carriers actually requires a defective material [2]. However, fundamental studies performed on singles crystals showed that exciton dissociation into unbound carriers is an intrinsic phenomenon, taking place also in single crystals with low defect densities [3]. This can enable more possibilities for 2D single crystals for optoelectronic applications, including photovoltaic ones.

Despite the potential benefits, the use of single crystal (SC) HPs for both 2D/3D heterostructures and pure 2D film devices remains challenging and, at the moment, the best performances are attributed to polycrystalline films possibly with a 2D passivating layer on top. Indeed, the best performing single crystal solar cells show an efficiency gap with respect to the polycrystalline counterpart which is attributed to the high surface charge trap density that results from the contamination of residual crystal growth solution, strongly affecting the surface quality and charge recombination. 

In this study, we investigate single crystal 2D perovskites and 2D/3D heterostructures. We demonstrate the growth of 2D HP single crystal thin films using various additives and analyze their optical and structural properties together with the electrical characterization. We also present single crystal 2D/3D thin film heterostructures and propose several strategies for interface engineering. Additionally, we provide a critical comparison of the photophysics and transport properties between single crystal and polycrystalline samples.

As chemists and materials scientists, it is our duty to synthesize and utilize materials for a multitude of applications that promote the development of society and the well-being of its citizens. Since the inception of metal-organic frameworks (MOFs), researchers have proposed a variety of design strategies to rationally synthesize new MOF materials, studied their porosity and gas sorption performances, and integrated MOFs onto supports and into devices. MOFs are a class of porous, crystalline materials composed of metal-based nodes and organic ligands that self-assemble into multi-dimensional lattices. In contrast to conventional porous materials, an abundantly diverse set of molecular building blocks allows for the realization of MOFs with a broad range of properties. Efforts have explored the relevance of MOFs for applications including, but not limited to, heterogeneous catalysis, guest delivery, water capture, destruction of nerve agents, gas storage, and separation. For example, we have developed an extensive understanding of how the physical architecture and chemical properties of MOFs affect material performance in applications such as catalytic activity for chemical warfare agent detoxification. Recently, start-up companies have undertaken MOF commercialization within industrial sectors. ION-X™ is used in this talk as an example to show case the way NuMat Technologies is innovating at the intersection of molecular design and precision engineering, to build the products driving the industries of tomorrow.

The performance of thermoelectric materials is mainly governed by the materials’ electrical and thermal conductivity properties and a number of new materials and structures have been exploited in order to optimize the energy conversion efficiency. Especially, nanostructure engineering via dopants, precipitates or phase/twin/grain boundaries is found to be effective in increasing the conversion efficiency by reducing the thermal conductivity. However, a direct correlation of these nanostructures to the material’s property is yet to be elucidated. Nowadays, with the rapid development of aberration-corrected transmission electron microscopy (TEM), the resolution of electron microscopes takes a leap forward to sub-angstrom and sub-eV, which allows a direct access to a material’s structure and chemical composition at an atomic scale.

 The presentation will start with a brief and realistic coverage of the emerging and maturing themes in the context of energy sources, efficiency, charge storage and distribution. It will illustrate GeTe as one example of emerging excitements in nanostructured materials and systems for thermoelectric materials. It will highlight the role of advanced and classical electron microscopy in unravelling the hierarchical architecture of the constituents and their intimate interplay in governing key phenomena in thermoelectric materials.

Two most imminent scientific and technological problems that mankind is facing now are energy and climate. The energy production and utilization in modern society is mostly based on the combustion of carbonaceous fuels like coal, petroleum and natural gas the combustion of which produces CO₂, which alters earth’s carbon cycle. 30 billion of tons of CO₂ per year get emitted globally as waste from the carbonaceous fuel burning and industrial sector, which if converted to valuable chemicals have the potential to change the economy of the world. We, in our lab, are trying to address both issues and are keen upon translating our innovative technologies from the lab to the industrial and commercial scale. In this talk, I will discuss about our recent discoveries of materials based on intermetallics, chalcogenides, oxides, organic-inorganic hybrids, etc as efficient catalysts for the conversion of CO₂ to chemicals/fuels.^[1-15] We are capturing CO₂ from industrial flue stream and converting it to value added chemicals/fuels such as methanol, CO, methane, dimethyl ether, C2-C5 & C5-C11 gasoline hydrocarbons. I will also cover our activities to produce green hydrogen via electrochemical pathway.^[16] The utilization of hydrogen and other fuels like methanol/ethanol through fuel cells also will be discussed.^[17] Catalyst design is at the heart of all these technologies, and we have developed customized catalyst systems for targeted product conversions as per the need of different industries. Development of these catalyst via various methods, the driving force behind the enhancement in activity and the mechanistic pathways will be explained with the support of various in-situ (DRIFTS, IR, XAFS), ex-situ (XPS, XRD, IR, XAFS) and theoretical (DFT calculation) studies. The talk also will cover the industrial viability of these catalysts. 

Thermoelectric energy converters are mostly investigated to recover waste heat from sources such as power plants, factories, vehicles or even transform heat from human bodies into electric power. Besides the energy efficiency of these devices, the selection of materials and processes are also important towards the high-priority pathway of environmentally friendly, earth-abundant, and low-cost materials. Therefore, systems such as silicides attract much attention since they exhibit very promising thermoelectric properties meeting at the same time relative environmental priorities. Furthermore, the proper use of materials as well as the possible reuse/recycling of expensive lost materials from several industries are also main priorities. PV industry is a typical example where high purity Si is required and, at the same time, large amount is wasted as kerf during wafer cutting. 

In this work, our efforts to synthesize thermoelectric silicides based on recycled Si-kerf are presented. n-type and p-type Mg₂Si- based materials as well as p-type Higher Manganese Silicides were prepared using commercial and recyclable Si. The materials were fully characterized in terms of structure and thermoelectric properties and selected compositions were used for prototype module fabrication. 

Thermoelectricity is the process of directly converting heat into electricity and vice versa, offering an environmentally sustainable means to generate electricity from wasted heat. To enhance the efficiency of this conversion, it's essential to precisely control various structural aspects beyond just the crystal structure. These aspects include defects, grain size, orientation, and interfaces. 

In recent years, solution-based techniques have garnered significant interest as a cost-effective and easily scalable approach for manufacturing high-performance thermoelectric materials. In this method, a powdered material is first prepared in a solution and then subjected to purification and thermal processing to produce the desired dense polycrystalline material. Unlike traditional methods, solution-based syntheses offer an exceptional level of control over various particle properties, including size, shape, crystal structure, composition, and surface chemistry. This precise control over the properties of the powder creates distinct opportunities for crafting thermoelectric materials with precisely tailored microstructural characteristics. In this presentation, we will highlight the opportunities and challenges that this synthetic strategy can bring, in particular we will focus on  metal chalcogenides.

Halide perovskites have emerged as a new class of semiconductors with excellent properties such as large tunable band-gaps, large absorption coefficients, long diffusion lengths, low effective mass and long radiative lifetimes. These have resulted in record efficiencies for photovoltaics surpassing that of Si. However, a major challenge for these materials is realizing long-term stability under light, temperature and humidity. In contrast, 2D perovskites are a sub-class of 3D perovskites, have demonstrated excellent stability compared to the 3D perovskites. 

In this talk I will describe our work over the past five years on 3D and 2D perovskites ranging from novel fundamental light-induced structural behaviors and its impact on charge transport, solvent chemistry and the synergy between 2D and 3D perovskites in achieving durable and high-efficiency photovoltaic devices. Finally, if time permits, I will also present some new results, which offer an exciting prospects for developing single photon emitters using a new solid state platform, which allows for ultra stable quantum emitters with high purity photons with unity quantum yield. 

Among several classes of porous materials, metal-organic frameworks (MOFs) are particularly attractive due to their unprecedented internal surface areas (up to 7800 m²/g),^[1] easy chemical tunability, and strong, selective binding of a host of guest species. Through judicious selection of MOF building blocks, which include metal ions and organic ligands, one can readily modify their properties for a variety of potential applications. Despite these attractive features, there are still challenges in the field that limit our ability to use MOFs as a solution for a wide range of industrial problems. For instance, some MOFs have limited mechanical and chemical stability, particularly in highly humid, acidic or basic environments. Overcoming this problem could lead to extended lifetimes and hence increased feasibility for their use in areas where such conditions are required. 

In response to these needs, we have recently begun to combine MOFs and polymers in an effort to boost MOF performance and extend their stability.^[2] Our recent work demonstrates that selected polymers can significantly enhance MOF performance in a number of important liquid and gas separations^[3-6] as well as extend catalyst lifetimes in selected reactions.^[7] In addition to this, controlled polymerization processes were employed to enhance the mechanical stability^[8] of large pore frameworks and extend the chemical stability of a number structurally diverse MOFs not only in humid environments, but also in acidic and basic media.^[9] We hope such work can help bring these frameworks a few steps closer to their deployment into a range of ecologically and economically important applications. In this presentation our recent work devoted to modification of MOFs and their application in several globally relevant separations will be outlined. 

The well-known narrow and intense optical absorption J-band arises as a result of J-aggregation of polymethine dyes in their aqueous solutions. The J-band was discovered experimentally by Jelley and independently Scheibe in 1936. In 1938, Franck and Teller gave a theoretical explanation of the J-band based on the Frenkel exciton model. Subsequently, this explanation was developed in details by many authors. A drawback of this explanation is its inability to explain the shape of optical bands of polymethine dye monomers from which J-aggregates are formed. We give an explanation of the J-band in the framework of a new theory, quantum–classical mechanics [1–3], which includes an explanation of the shape of the bands of polymethine monomers. In quantum–classical mechanics the initial and final states of the “electron+nuclear environment” system for its “quantum” transitions are quantum in the adiabatic approximation, and the transient chaotic electron-nuclear(-vibrational) state due to chaos is classical. This chaos is called dozy chaos. The new explanation of the J-band is based on the so-called Egorov nano-resonance discovered in quantum–classical mechanics [4]. Egorov nano-resonance is a resonance between movements of the electron and the reorganization of the environmental nuclei during quantum–classical transitions in the optical chromophore under weak dozy chaos. In addition to explaining the nature of the J-band of J-aggregates, an explanation is given for the shift of the Egorov nano-resonance observed in polymethine dyes to the long-wavelength region with decreasing polarity of the solvent [5], the shape of the optical absorption bands of their dimers, H- and H*-aggregates [5], the shape of the optical absorption and luminescence bands of J-aggregates in Langmuir films [6], as well as an anomalously small Stokes shift of the J-bands of luminescence and absorption [6]. An explanation is given for the experimentally observed strong parasitic violation of the Egorov nano-resonance during the transition from one-photon to two-photon absorption, and the conditions for its restoration are predicted [7]. The idea of creating “living materials” is put forward, and a method for its practical implementation is indicated by purposefully complicating the design of molecular systems, the heuristic source of which can be the high dynamic organization of quantum-classical transitions in J-aggregates [8].

Chiral metal-halide semiconductors (MHS) have recently developed as promising candidates for spin- and polarization-resolved optoelectronic devices. Although several chiral MHS with rich chemical and structural diversity have been reported lately, the fundamental mechanisms governing their chiroptical activity, namely, circularly polarized absorption and emission, remain elusive. In this talk, I will discuss our recent progress in understanding and tuning the chiroptical activity in chiral MHS. I will first discuss how the chirality is transferred from organic to inorganic component through asymmetric covalent bonding interactions. Their endowed molecular chirality was then studied by circular dichroism (CD). However, we found that the previously reported “apparent” CD in chiral MHS thin films is not an intrinsic chiroptical property, but rather, arising from an interference between the film’s linear birefringence and linear dichroism. We verify the presence of LB and LD effects in both one-dimensional and zero-dimensional chiral MHS thin films. We then establish spectroscopic methods to decouple the genuine CD from other spurious contributions, which allows a quantitative comparison of the intrinsic chiroptical activity across different chiral MHS. The relationship between the structure and the genuine chiroptical activity is then uncovered, which is well described by the chirality-induced spin–orbit coupling in the chiral structures. Meanwhile, we found that high CD signals do not necessarily lead to high circularly polarized luminescence as most of the current chiral MHS display very low photoluminescence quantum yields (PLQY). We will then discuss the reasons of low PLQY in these materials. Finally, we will show our strategies to turn on and amplify the circularly polarized luminescence in chiral MHS.

The chemistry of metal organic frameworks (MOFs) continues to expand rapidly providing materials with diverse structures and properties. The reticular chemistry approach, where well defined structural building blocks are combined together forming crystalline open framework solids, has greatly accelerated the discovery of new and important materials. However, its full potential toward the rational design of MOFs relies on the availability of highly connected building blocks because these are greatly reducing the number of possible structures. Towards this, building blocks with connectivity greater than twelve are highly desirable but extremely rare. We report here the discovery of novel 18-connected, trigonal prismatic, ternary building blocks (tbb) and their assembly into unique MOFs, denoted as Fe-tbb-MOF-x (x: 1, 2, 3) with hierarchical micro- and mesoporosity.¹ The remarkable tbb is an 18-c super-trigonal prism, with three points of extension at each corner, consisting of triangular (3-c) and rectangular (4-c) carboxylate-based organic linkers and trigonal prismatic [Fe₃(μ₃-Ο)(-COO)₆]⁺ clusters. The tbb’s are linked together by an 18-c cluster made of 4-c ligands and a crystallographically distinct Fe₃(μ₃-Ο) trimer, forming overall a 3-D (3,4,4,6,6)-c five nodal net. The hierarchical, highly porous nature of Fe-tbb-MOF-x (x: 1, 2, 3) was confirmed by recording detailed sorption isotherms of Ar, CH₄ and CO₂ at 87, 112 and 195 K respectively, revealing an ultrahigh BET area (4263 - 4847 m² g^-1) and pore volume (1.95 - 2.29 cm³ g^-1). Because of the observed ultrahigh porosities, the H₂ and CH₄ storage properties of Fe-tbb-MOF-x were investigated, revealing well-balanced high gravimetric and volumetric deliverable capacities for cryo-adsorptive H₂ storage (11.6 wt%/41.4 g L-1, 77 K/100 bar – 160 K/5 bar), as well as CH₄ storage at near ambient temperatures (367 mg g-1/160 cm³(STP)cm^-3, 5-100 bar at 298 K), placing these materials among the top performing MOFs. The present work opens new directions to apply reticular chemistry for the construction of novel MOFs with tunable porosities, based on contracted or expanded tbb analogues.

Hybrid halide perovskites are a novel class of semiconductor materials with promising and versatile optoelectronic properties, enabled by their chemically adjustable structures and dimensionality. The diversity in the metal ions, halide anions, and organic spacers enables a wide range of materials with highly tunable properties and variable dimensionalities. These materials are studied for various applications such as solar cells, detectors, and light-emitting diodes. The ability to control and adjust the optical properties for a required application is significant. Thus, an improved understanding of the structure and optical mechanisms is crucial.

Specific low-dimensionality hybrid halide perovskites exhibit white-light emission at room temperature, associated with self-trapped excitons (STE), making them ideal candidates for illumination applications. We study the correlation between structural and chemical motifs of low dimensionality (2D, 1D) halide perovskites and their STE emission. 

Specifically, we have studied how exchanging the halide anions while maintaining the structure affects the STE properties in a unique 1D perovskite structure based on edge-sharing dimers. These structures exhibit strong, broad emission with PLQY of approximately 40%. By changing the halide from I to Br and Cl, we can see the widening of the bandgap, as expected. However, the broad emission shows an anti-correlated behavior, resulting in red-shifted emission for the Cl sample, with a significantly larger stokes shift. We further study how mixing Br and Cl in a single structure affects the broad emission properties and how different synthetic approaches can be utilized for the fabrication of these compounds.