Chemical Coding at the Atomic Scale: Designing Hybrid and Quantum Nanostructures for Applications in Optical Biosensing, Light Harvesting, Chiral Catalysis, and More

Catalytic Functions and Applications of Nanoparticles (Nanozymes) and DNA-Protocell Condensates Itamar Willner Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel (Itamar.willner@mail.huji.ac.il) Polyadenine-aptamer (pA)–conjugated Au-nanoparticles (Au-NPs) act as catalytic nanozymes emulating the active site catalytic/binding functions of native enzymes (“nanoaptazymes”). Conjugation of the dopamine aptamer to the pA-NPs acts as peroxidase aptananozyme catalyzing the H2O2 oxidation of dopamine to aminochrome and the chiro-selective oxidation of D/L-DOPA to dopachrome. The aptananozyme, also, acts as bioreactor system operating the glucose/dopamine cascaded oxidation of dopamine to aminochrome. Moreover, Cu2+- or Co2+-ZIF NMOFs act as aptananozymes catalyzing the oxidation of dopamine, uric acid or polymerization of aniline to polyaniline. The aptananozyme-catalyzed oxidation in the presence of uric acid or D/L-DOPA yields molecularly imprinted polynanozymes catalyzing the selective oxidation of uric acid or the chiro-selective oxidation of D/L-DOPA to dopachrome. DNA condensates present micro-containments emulating native cells, protocells. By integration of circuits and catalysts into the protocell assemblies, biomimetic cell functions are realized. Two kinds of functional protocells will be introduced. One kind of protocells will include all-DNA microdroplets, revealing transient, dissipative, enzyme, DNAzyme, and pH-guided formation and depletion. A second class of protocells will include cyanuric acid (CA) polyadenine-triplex stabilized DNA condensates. Barcode tethers modifying the CA-polyadenine condensates yield functional DNA condensates. Treatment of the condensates with invading nucleic acid strands exhibiting variable lengths of base complementarities leads to compartmentalized domains in the protocell condensates. Encoding the invading DNA strands with stimuli-responsive functionalities allows the triggered dynamic interactions between the protocell compartmentalized domains. Light-triggered programmed dynamic exchange between the microcompartments, control-over biocatalytic functions of the domains and signal-triggered dimerization/separation of protocell containments are demonstrated. The dynamic processes in the protocells are probed by temporal confocal microscopy imaging.

DNA origami, in which a long scaffold strand is assembled with a large number of short staple strands into parallel arrays of double helices, has proven a powerful method for custom nanofabrication of shapes up to 100 nm in size. The scaffold represents about half the mass of an origami, therefore the origami size is restricted by the length of the scaffold. However, it is impractical and prohibitively expensive to scale the length of the scaffold. Here I will discuss a strategy, that we call crisscross polymerization, that combines all-or-nothing scaffold-dependent initiation of folding with scaffold-independent growth, therefore allowing for sizes unbounded by the length of the scaffold. Assembly using single-stranded DNA slats enables digital counting of molecular analytes, where each molecular detection event triggers growth of a single filament resolvable by low-cost microscopy. Assembly using DNA-origami slats, conversely, enables fabrication of fully addressable structures that are twice the mass of the E. coli genome, and that span an area of two microns by two microns.

Nanoparticles can be conjugated with diverse biomolecules and employed in biosensing to detect target analytes in biological samples. This proven concept was primarily used during the COVID-19 pandemic with gold NPs-based lateral flow assays (LFAs). Considering the gold price and its worldwide depletion, here we show that novel plasmonic NPs based on inexpensive metals, titanium nitride (TiN) and copper-covered with a gold-shell (Cu@Au), perform comparable or even better than gold nanoparticles. After conjugation, these novel nanoparticles maintain their plasmonic features and provide high figures of merit for LFA testing, such as high signal, specificity, and robust naked-eye signal recognition. The implementation of bio-conjugated TiN and Cu@Au nanoparticles promises a drastic cost reduction of the LFA technology and may be a promising tool for other bioapplications such as photothermal therapies, delivery, imaging, and sensing technologies.

Wrinkling is widely encountered in nature and can even result in sub- micrometer periodic features e.g. on flower petals. Understanding the physics of wrinkling allows harnessing the process for structure formation in a non-biological context and for metrology of thin film mechanics. The latter is especially interesting for 2D materials and I discuss recent examples including the extension of this work to strain engineering of 2D materials. Controlled wrinkling for structure for- mation on the other hand offers interesting opportunities for scalable surface patterning. I will discuss the use of wrinkling templates for the formation of particle based metamaterials.

Today, innovative tools for diagnostics and bioanalytics are needed. They should be able to detect molecules of interest with sufficient sensitivity, preferably label-free and multiplexed, to be usable outside of dedicated laboratories and with less qualified personnel, at minimal costs. Plasmonic nanostructures promise to provide sensing capabilities with the potential for ultrasensitive and robust assays in a high parallelization and miniaturization, and without the need for markers. When functionalized with certain biomolecules (such as single-stranded DNA or antibodies) which bind the target molecule of interest (e.g., a biomarker like DNA or a protein), they should capture this target molecule with high specificity. Upon binding of target molecules, the localized surface plasmon resonance (LSPR) of these structures is changed, and can be used as sensoric readout. This is possible even on a single nanostructure level, using optical darkfield detection introduced more than 100 years ago, as demonstrated for DNA detection. In contrast to SPR, LSPR senses only in a very thin layer (on the scale of the particle diameter), resulting in an efficient background suppression. In order to multiplex this approach, imaging spectrometer setups, e.g. based on a Michelson interferometer or multiple LEDs have been developed, able to readout a whole array of sensors in one step. On the sensor side, microarrays of gold nanoparticle spots were fabricated using spotting of pre-synthesized gold nanoparticles. Such chemically synthesized particles allow for a cost-efficient generation of highly crystalline particles as nanosensors; by using microfluidic approaches, a higher quality and reproducibility can be achieved. Using this microarray approach, a multiplex DNA-targeting detection of fungal pathogens involved in sepsis could be demonstrated. DNA-based signal amplification, e.g. by hybridization chain reaction, improves the sensitivity. Beyond DNA detection, LSPR sensing is also applicable for the detection of protein targets, such as CRP. In conclusion, the results demonstrate the potential of this LSPR-based array platform for molecular detection of biomolecules of interest, with possible applications in bioanalytics and diagnostics. Acknowledgements: Funding by BMBF (02WIL1521, 01DR20010A and 13N15717) and TAB (2018 VF 0015; 2018 FE 9039).

Ultrathin, atomically flat, two-dimensional semiconductor nanocrystals receive rapidly increasing attention due to their unique physicochemical properties. I will show that ZnS and ZnSe nanoplatelets exhibit sharp excitonic absorption and narrow excitonic emission. I will demonstrate that direct manganese doping leads to tunable emission and photoluminescence lifetime. The initially low dopant PL quantum yield can be dramatically enhanced by passivating the surface trap states of the samples. Using time-resolved PL spectroscopy and density functional theory calculations, a connection between coupling and PL kinetics of Mn ions can be established. We believe that the presented doping strategy and simulation methodology of the Mn-doped ZnS system is a universal platform to study dopant location- and concentration-dependent properties also in other semiconductors and might be an interesting system for catalysis.

The organization of colloidal nanocrystals into assemblies can produce materials imbued with the intrinsic properties of the building blocks and with useful emergent properties that depend on their mesoscale structure. Densely packed nanocrystal superlattices are readily formed by evaporation-driven assembly of particles with uniform morphologies, but they offer limited structural tunability. Assembling nanocrystals into percolated networks results in porous gels with high internal surface areas, widely tunable densities, and easy incorporation of multiple nanocrystal components regardless of their size, composition, or shape. However, the organization of such disordered assemblies can be difficult to characterize or control. We use dynamic covalent chemistry to link functionalized nanocrystals into gel networks. By selecting orthogonal chemistries, nanocrystal assembly can be chemically programmed based on post-synthetic nanocrystal functionalization and the addition of complementary chemical linkers. For example, we demonstrate that chloride ions act as competing “caps” for cobalt ion linkers, thereby tuning the temperature at which gels reversibly form from terpyridine-functionalized nanocrystals. Meanwhile, pairwise nanocrystal interactions between aldehyde-functionalized nanocrystals are tunable by adding a non-coordinating salt. The salt thereby modulations the temporal evolution of gels formed by addition of complementary bishydrazide linkers. Choosing tunable plasmonic metal oxide nanocrystals as the building blocks, these gels exhibit structure-dependent IR optical properties. By varying the building block attributes (size, doping, ligand length) we establish quantitative gel structure-optical absorption correlations that enable the design of reversible gels with distinctive dynamic optical response. Conversely, the optical absorption spectra of the gels can be interpreted with the help of large-scale simulations to augment information gleaned from X-ray scattering and provide conclusive evidence of their structural evolution in varying chemical environments.

The modular and defined arrangement of nanoparticles over several length scales represents an attractive approach for creating an almost infinite variety of architectures from a limited number of building blocks. Among the many possible geometries, three-dimensional and porous structures, such as those found in aerogels, are unique because, on the one hand, the size-specific properties of the nanobuilding blocks are fully preserved in the macroscopic material and, on the other hand, large surface areas are generated, which are crucial for efficient photocatalysis. In this talk, a versatile strategy for synthesizing fully crystalline aerogels by gelation of the corresponding colloidal nanoparticle dispersions will be presented. The composition, transparency and macroscopic geometry of the aerogels can be precisely tuned by carefully selecting the type of nanoparticles and their concentration in the colloidal dispersion as well as by controlling the gelation process. These optimization strategies are demonstrated using the example of aerogels for gas-phase photocatalysis. In this case, it is important that the photoreactors are adapted to the shape of the aerogels. In particular, gas flow through and illumination of the aerogels have a major influence on the efficiency of the photocatalytic gas phase reactions. Based on the example of photocatalytic hydrogen production from methanol, the influence of these parameters is shown and concepts are presented on how the systems can systematically be optimized.

For the objective of driving a complex reaction, e.g., a catalytic reaction involving multiple electron transfer, a nanostructure of comparable complexity is required. In organic chemistry, the precise synthesis of molecules with a defined structure has been perfected for over a century; in colloidal chemistry, the analogous formation of defined nanostructures through multi-step reactions is still at its infancy. In this contribution, a heterostructure is presented that, through a 4-step reaction, produces self-organised nanomatrial consisting of a seeded CdSe/CdS nanorod with a CdTe tip selectively grown on one end, which is then used to nucleate a precious metal. The metal tips then join to form star-like or branched structures, in which several semiconductor particles can donate photo-excited electrons into a shared metal centre. This structure has a high potential for processes like photo-catalytic water splitting. The potentials, possible pitfalls, and important research questions will be discussed.

When discussing plasmonic colloidal nanoparticles, the majority of publications deals with gold or silver nanoparticles. However, many more materials than just these noble metals are capable of showing plasmonic behavior. In this talk, our efforts on the colloid chemical synthesis and structural and optical characterization of such “alternative” plasmonic materials is summarized. Different material classes are invoked such as degenerately doped semiconductors ($Cu_{2-x}Se$) or metallic compounds ($Cu_{1.1}S$ and NiS compounds). Emergent properties are for example a post synthetically tunable plasmon resonance or also a temperature switchable plasmon resonance. Furthermore, ultrafast plasmon/plasmon interaction in dual plasmonic nanoparticles ($Cu_{1.1}S$/Au) are discussed.

In recent decades, metal phosphide, and to a lesser extent arsenide, nanocrystals have emerged as versatile materials for a broad range of energy-related applications, such as light emission, thermoelectrics, catalysis, ion batteries, etc. Along with their exceptional robustness due to high bond covalency, they exhibit tunable electronic, physical, and chemical properties. However, their broader implementation is hindered by many challenges in their syntheses, among which is a limited choice of pnictogen sources. Although some other pnictogen sources were proposed, most of their syntheses employ either slow reactive alkylphosphines or highly reactive tris(trimethylsilyl)pnictines or tris(dimethylamino)pnictines, which on top of having limited availability and high cost, pose significant hazards due to their high flammability and toxicity. In this contribution, I will report recent results on the new class of pnictogen precursors, namely triacylphosphines and triacylarsines, which can be relatively simply prepared on a multigram scale via a one-pot procedure employing inexpensive starting materials. In addition, I will demonstrate two examples of their use in colloidal nanoparticle synthesis. First, triacylphoshines react with nickel and cobalt chlorides to form respective phosphide nanoparticles with controlled size and crystallinity degree. Such phosphide nanocrystals were shown to be highly active in electrocatalysis of hydrogen evolution reaction, and Ni2P/Ni12P5 NPs in particular, exhibit overpotentials of 50 mV at 10 mA/cm2, which is among the best values demonstrated by non-noble metal based systems reported to date. Secondly, benzoylarsine was shown to react with indium oleate (or indium/zinc oleate) to produce very small indium arsenide quantum dots exhibiting photoluminescence in the range of 630 – 780 nm with full width at half maximum of 170 – 210 meV.

Lighting and displays are integral parts of human activities and economic development. Semiconductor nanocrystals, now offering a market volume exceeding 5 Billion Euros annually, have attracted great interest in quality lighting and displays in the last decade. Such colloidal semiconductors enable enriched color conversion essential to superior lighting and displays. These colloids span different types and heterostructures of semiconductors, starting in the form of colloidal quantum dots over three decades ago and more recently extending to the latest sub-family of nanocrystals, the colloidal quantum wells (CQWs). In this talk, we will focus on atomically-flat, tightly-confined, quasi-2-dimensional CQWs, also popularly nick-named ‘nanoplatelets’, particularly for use in lighting and displays [1-6]. Also, we will present a powerful, large-area, orientation-controlled self-assembly technique for orienting these colloidal quantum wells either all face down or all edge up [7]. We will demonstrate three-dimensional constructs of their oriented self-assemblies with monolayer precision [8]. Among their extraordinary features important to applications in lighting and displays, we will show record high efficiency from their colloidal LEDs [9] and record high gain coefficients and record low lasing thresholds from their colloidal laser media [1,10-11] using heterostructures [2-6] and/or oriented assemblies [7,8] of these colloidal quantum wells. Given their current accelerating progress, these solution-processed nanocrystals hold great promise to challenge their epitaxial thin-film counterparts in semiconductor optoelectronics. References [1] B. Guzelturk et al., HVD, Nano Letters 19, 277 (2019) [2] Y. Altıntas et al., HVD, ACS Nano 13, 10662 (2019) [3] N. Taghipour et al., HVD, Nature Communications 11, 3305 (2020) [4] F. Shabani et al., HVD, Small 18, 2106115 (2022) [5] E. G. Durmusoglu et al., HVD, ACS Nano 17, 7636 (2023) [6] S. Delikanli et al., HVD, JACS 145, 12033 (2023) [7] O. Erdem et al., HVD, Nano Letters 19, 4297 (2019) [8] O. Erdem et al., HVD, Nano Letters 20, 6459 (2020) [9] B. Liu et al., HVD, Advanced Materials 32, 1905824 (2020) [10] J. Maskoun et al., HVD, Advanced Materials 33, 2007131 (2021) [11] F. Isik et al., HVD, Small, 2309494 (2024)

In the past decade, the dramatic advents of material science and nanofabrication techniques have been pushing the optical confinement from micro/nano scale down to deep nanoscale or even to the atomic scale. These developments led to the situations to have simultaneous extreme confinement of the optical field and the electrons, where the complex optical responses cannot be described by classical local electromagnetic theory. In this talk, we will discuss the fundamentals and new opportunities associated with this emerging direction. In particular, we report an extended electromagnetic theory for correct and efficient description of the nonlocal and quantum effects associated with the ultrafine features of nanostructured materials. Based on the theory, we will show how one could grasp the optical response from the microscopic features of a macroscopic system. Moreover, we outline some unique opportunities of having photons and electrons co-localized down to the atomic scale.

I will review our work on using colloidal nanostructures for photocatalytic water splitting. We have employed redox shuttles, supramolecular attachments, photobases, built-in electric fields as well as z-schemes in order to enhance efficiencies for light-driven hydrogen evolution. Time-resolved optical spectroscopy studies have led to important insights into limiting microscopic processes for photochemical reactions. More ionic photocatalytic semiconductor materials show strong coupling effects between charge carriers and the lattice leading to ultrafast polaron formation and to the observation of coherent phonons. Key publications: Simon et al., Nature Materials 13, 1013 (2014) Wolff et al., Nature Energy 3, 862 (2018) Fang et al., Nature Comm. 11, 1 (2020) Rieger et al., Nano Letters, 21, 7887 (2021) Fang et al., Angew. Chem. Int. Ed. 62, e202305817 (2023) Bootz et al and Kestler et al, submitted for publication

The nature and timescale of electronic excitation energy transfer in assemblies of semiconductor quantum dots (QD) are crucial parameters for future applications. Because of their simple composition, small oligomers turn out to be ideal model systems to provide novel insights into interparticle couplings. Here, we use a precipitation/redispersion procedure to prepare QD oligomers from a mixture of two sets of core/shell CdSe-QDs of the same size but with absorption and photolumi-nescence (PL) spectra tuned to establish a donor(D)-acceptor(A)-couple for energy transfer. By employing density gradient ultracentrifugation, the fraction of QD dimers could be enriched by up to levels of 50 %.After deposition on a substrate, individual QD monomers, dimers and higher oligomers are discriminated by correlative atomic force and confocal fluorescence microscopy not only in terms of structure but also in terms of D/A-composition. For the case of single D/A-heterodimers, we find that the D-PL decays are accelerated by a factor of 2 compared to D-monomers indicating excitation energy transfer. By applying a force with the AFM cantilever to single dimers, the energy transfer efficiency could be modified. Besides allowing a detailed insight into energy transfer in well-defined QD assemblies, our approach holds promise to externally control the electronic coupling in such entities.

This lecture will span the discovery of colloidal lead halide perovskite nanocrystals (LHP NCs), as well as our latest work on their synthesis, self-organization, and optical properties, including unpublished work. LHP NCs are of broad interest as classical light sources (LED/LCD displays) and as quantum light sources (quantum sensing and imaging, quantum communication, optical quantum computing). The current development in LHP NC surface chemistry, using designer phospholipid capping ligands, allows for increased stability down to single particle level [1]. 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, almost as short as the reported exciton coherence time, by the NC size increase to 30 nm [2]. 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. When such bright and coherent QDs are assembled into superlattices, collective properties emerge, such as as superradiant emission from the inter-NC coupling [3]. In the most recent work [4], 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. [1] V. Morad, A. Stelmakh, M. Svyrydenko, L.G. Feld, S.C. Boehme, M. Aebli, J. Affolter, C.J. Kaul, N.J. Schrenker, S. Bals, Y. Sahin, D.N. Dirin, I. Cherniukh, G. Raino, A. Baumketner, M.V. Kovalenko Nature, 2024, 626, 542–548 [2] C. Zhu, S.C. Boehme, L.G. Feld, A. Moskalenko, D.N. Dirin, R.F. Mahrt, T. Stöferle, M.I. Bodnarchuk, A.L. Efros, P.C. Sercel, M.V. Kovalenko, G. Rainò. Nature, 2024, 626, 535–541 [3] I. Cherniukh, G. Rainò, T. Stöferle, M. Burian, A. Travesset, D. Naumenko, H. Amenitsch, R. Erni, R.F. Mahrt, M.I. Bodnarchuk & M.V. Kovalenko. Nature 2021, 593, 535–542 [4] T.V. Sekh, I. Cherniukh, E. Kobiyama, T.J. Sheehan, A. Manoli, C. Zhu, M. Athanasiou, M. Sergides, O. Ortikova, M.D. Rossell, F. Bertolotti, A. Guagliardi, N. Masciocchi, R. Erni, A. Othonos, G. Itskos, W.A. Tisdale, T. Stöferle, G. Rainò, M.I. Bodnarchuk, and M.V. Kovalenko. ACS Nano 2024, 8423–8436

We employ different types of symmetry breaking in inorganic nanocrystals: 1. Using chiral molecular ligands we break the symmetry in the formation of intrinsically chiral nanocrystals, which are nanocrystals made of materials that form crystals of chiral space groups. I will show a system where complete enantio-selectivity is possible this way. 2. We use circularly polarized light to break the shape symmetry of plasmonic nanostructures and turn them optically active. We show that while obtaining a large shape asymmetry is possible when illuminating the nanocrystals immobilized on a substrate from a single direction, it is also possible to slightly break the symmetry in freely rotating particles in solution.

Owing to their attractive optical and chiroptical properties, chiral metal halide perovskites have received increasing attention, with potential applications ranging from photonics and optoelectronics to spintronics. Metal halide perovskite nanocrystals with either intrinsic or extrinsic (e.g., chiral ligand-induced) chirality have been reported recently, and the interplay between these two types of chirality has yet to be addressed. We report the inversion and tuning of excitonic optical activity in intrinsically chiral perovskite CsPbBr3 nanoplatelets, originating from interactions between their structural chirality (due to the spontaneously formed screw dislocations in the crystalline lattice) and the surface enantiomeric (R/S) chiral ligands R/S-phenylethylammonium bromide [1]. Through post-preparative exposure of the perovskite nanoplatelets to these R/S ligands of varied contents, either chiral ligand-induced intrinsic chirality inversion or negative and positive Cotton effects induced by the ligands via electronic coupling between the ligand and the nanoplatelets have been identified. These findings deepen our understanding of the modulation of excitonic optical activity in chiral perovskites, and can guide the rational design and synthesis of novel chiral materials. In another related study, by constructing hybrid structures of the chiral perovskite CsPbBr3 nanoplatelets and organic molecules, excited state chirality transfer is achieved, either via direct binding or triplet energy transfer, leading to efficient ultraviolet circularly polarized light emission [2]. The underlying photophysical mechanisms of these two scenarios are clarified by comprehensive optical studies. Furthermore, stereoselective photopolymerization of diacetylene monomer is demonstrated by using such efficient ultraviolet emission. This study provides both novel insights and a practical approach for realizing ultraviolet circularly polarized light emission, which can also be extended to other material systems and spectral regions, such as visible and near-infrared. 1. B. Tang, S. Wang, H. Liu, N. Mou, A. S. Portniagin, P. Chen, Y. Wu, X. Gao, D. Lei, A. L. Rogach. Chiral Ligand-Induced Inversion and Tuning of Excitonic Optical Activity in Intrinsically Chiral CsPbBr3 Perovskite Nanoplatelets. Adv. Opt. Mater. 2024, 12, 2301524. 2. B. Tang, Q. Wei, S. Wang, H. Liu, N. Mou, Q. Liu, Y. Wu, A. S. Portniagin, S. V. Kershaw, X. Gao, M. Li, A. L. Rogach. Ultraviolet Circularly Polarized Luminescence in Chiral Perovskite Nanoplatelet-Molecular Hybrids: Direct Binding versus Efficient Space Triplet Energy Transfer. Small, 2024, 2311639.

At the heart of unlocking the potential of global clean, renewable energy is the concerted effort of Advanced Charge Management (ACM) and Advanced Photon Management (APM). Recent advances regarding molecular ACM have documented the maturity of energy conversion schemes. Adding now APM to ACM by means of down- and/or up-conversion and creating synergies is essential to further boost the efficiency of these sun-driven energy conversion schemes. A full-fledged comprehension of APM is essential as an enabler for creating versatile platforms that are broadly applicable not only in the area of solar electricity, but also solar fuels. APM is, in the molecular context, based on either down-converting photons by means of Singlet Fission (SF), on one-hand, or on Triplet Fusion (TF) for up-converting them, on the other hand. To harvest photons in the high-energy regime, SF, the molecular analog to multiple exciton generation, stands out. It allows high-energy, singlet-excited states to be down-converted into twice as many low-energy, triplet-excited states, thereby improving solar-cell performance. This is, however, limited to the part of the solar spectrum, where, for example, the SF-materials feature a significant absorption cross-section. To harvest photons in the remaining regime of the solar spectrum necessitates the use of complementary absorbing chromophores. Our transdisciplinary research has enabled in recent years to gather a comprehensive understanding of molecular down- and up-conversion.

A role of chirality is discussed, being inspired by recent studies on chirality-induced dynamic phenomena. Indeed, charge or thermal flow induces a polarized state of spin or phonon angular momenta in chiral materials. It is important to understand the meaning of such dynamic aspects of chirality, which leads to successful synthesis, quantification, and use of chirality. In the talk, I would introduce some experimental progress on these key ingredients in chiral material science. References: [1] A. Inui et al., PRL 124, 166602 (2020). [2] K. Shiota et al., PRL 127, 126602 (2021), featured in Physics 14, s113. [3] K. Ishito et al., Nat. Phys. 19, 35 (2023); Chirality 35, 338 (2023). [4] K. Ohe et al., PRL 132, 056302 (2024). [5] Y. Kousaka et al., JJAP 62, 015506 (2023). [6] Y. Togawa et al., JPSJ 92, 081006 (2023), Special Topics DMI.

Rare-earth (RE) elements are vital for high technological applications due to their typically large magnetic moments and signature optical transitions associated with the 4f electron shell. In addition to single atoms or ions, one can place them inside coordination complexes to study their behavior. These systems allow for the design and synthesis of new structures with desirable functions. We study different complexes from ions deposited on surfaces to explore their magnetic interactions to RE ionic clusters self-assembled on a gold crystal surface. Experiments on the ionic complexes find them highly mobile at ~100 K, indicating a liquid-like state. Their mobility is greatly reduced below 5K, revealing self-limiting clusters with pairs of RE complexes acting as units joined by electrostatic interactions. The observed RE clusters are found to be stable and conformationally chiral, with strong bonding arising from dispersive forces between aromatic groups in the ligands. We also study possible arrangements of RE ion chains/clusters on gold. The magnetic ground state configurations are found theoretically to be dominated by magnetic anisotropy effects, which we propose can be explored using scanning tunneling techniques to investigate spin-flip excitations. Few RE ion clusters can also be employed to analyze entanglement entropy and their signatures in differential conductance experiments.

Macroscopic Bi2Se3 is a layered material consisting of a high number (>10) Se/Bi/Se/Bi/Se quintuple layers, separated by van der Waals gaps. The material has been characterized as a topological quantum spin Hall insulator, with an inverted bandgap of 300 meV and with protected helical surface states. On the other hand, the absorption spectrum shows intriguing optical transitions in the 0.5 – 3.5 eV energy region: a strong optical transition at 2.8 eV located at the surface, followed by luminescence in the region of 2.3 eV, preserving chirality (1,2). We used colloidal two-dimensional Bi2Se3 nanoplatelets as a model system to study the topological properties and higher-energy excitations of Bi2Se3 in two dimensions (3,4). Using cryogenic tunneling spectroscopy, we characterized the quantum states around the Fermi-level located at the edge of the platelets. Using absorption and transient absorption quenching spectroscopy, we observe more than 10 strong optical transitions in the IR-Visible region. Comparison with DFT-GW theory allowed us to identify these transitions, group them as surface and interior transitions, and locate them in the two-dimensional Brillouin zone. In the time region below 10 ps, the electron and hole cooling and recombination dynamics are very specific for the bands involved, with in some cases signatures of electron-hole separation in momentum space by cooling. 1. Observation of chiral surface excitons in a topological insulator Bi2Se3. Proceedings of the National Academy of Sciences of the United States of America 116, 4006-4011 (2021) 2. Complex optical conductivity of Bi2Se3 thin film: Approaching two-dimensional limit Applied Physics Letters 118, DOI: 10.1063/5.0049170 (2021) 3. Characterization of the edge states in colloidal Bi2Se3 platelets. J. R. Moes et al., Nanoletters 24, 5110 (2024) 4. Identification of high-energy excitations in two-dimensional topological Bi2Se3 platelets. Manuscript in preparation

In this talk, I will discuss some ideas regarding the combination of nanoparticles synthesis, post-synthesis transformations, as well as some examples of nanoparticle organizations and their effects on optical properties. I will show how to use these studies to make useful materials and to gain a deeper understanding of the optics of materials at the nanoscale. For instance, by grouping colloidal nanospheres into clusters of specific geometries, the response of nanoparticles to light can changes dramatically. Highly symmetric configurations have remarkable optical properties that can result in the generation of optical magnetism. This effect can be exploited to produce new light sources. Controlling the degree of disorder over the spatial arrangement of disordered nanoparticles deposited on a layered surface can generate novel visual appearances. We have studied these effects to get abrupt color transitions as a function of observation angle.

The adjustment of the energy levels in semiconductor nanostructures and the distribution of photo excited charge carriers is of major importance for any kind of potential application. For example, if the nanostructures are used in applications such as lighting, the electrons and holes should localize in the same region to achieve a bright luminescence. For catalytic applications such as water splitting a charge carrier separation in different regions of the nanostructure is desired. Here we show that complex elongated nanostructures consisting of a core which is covered with an elongated shell (DotRods, DRs) can be used to tune the degree of charge carrier delocalization. For example, a precise adjustment of the energetic band alignment in DRs can be achieved by tuning the composition of the core material. While in type I CdSe/CdS DRs the photogenerated electrons and holes are mainly localized in the CdSe-core, a strong charge separation is achieved for type-II ZnSe/CdS DRs, where the electron is more strongly localized on the elongated CdS-shell. We show that the first step in a typical synthesis of ZnSe/CdS DRs is a partial cation exchange of Zn- vs. Cd-cations, which effectively reduces the type-II band offset of the conduction bands. Low temperature PL measurements of individual ZnCdS/CdS nanorods show a gradual change of the respective LO-Phonon intensities when the amount of ion exchange in the core is adjusted through a modified synthesis method. Also, the phonon coupling strength is modeled based on finite element calculations of the respective electron and hole wave functions. Finally, we demonstrate that different metals can be grown on the tip of the DRs, which effectively act as a sink for the photogenerated electrons. We show first experiments on the charging of different region of individual semiconductor-metal NRs by KPFM measurements, and also on their long-term stability when used as catalyst for hydrogen production in aqueous solution.

The introduction of chiral organic spacers in low-dimensional metal-halide perovskites triggers chiroptical activity, making these materials of great interest for spintronic applications. To enable such applications, it is necessary to develop a deep understanding of the structure formation of chital two dimensional (2D) perovskites and its impact on their optical properties. In the first part of the talk, I will discuss how changing the processing conditions impacts on the phase purity, microstructure and chiroptical properties of chiral 2D perovskites. In the second part, I will focus on the optical properties of these materials, in particular on the origin of the common observation of asymmetric shape of their photoluminescence.

Quantum dots are nanometer-sized crystallites of semiconductor that have a roughly spherical shape. Due to extensive research, quantum dots are now commercially used as a robust fluorescent material in displays and lighting. However, even with our best procedures, state-of-the-art samples still contain particles with a distribution in size and shape. Because this causes variations in their optical properties, their performance for applications is reduced. This leads to a fundamental question: can we achieve a sample of semiconductor nanocrystals in which all the particles are exactly the same? In this talk we will discuss this possibility by examining two classes of nanomaterials. First, we will consider thin rectangular particles known as semiconductor nanoplatelets. Amazingly, nanoplatelet samples can be synthesized in which all crystallites have the same atomic-scale thickness (e.g., 4 monolayers). This uniformity in one dimension suggests that routes to monodisperse samples might exist. After describing the underlying growth mechanism for nanoplatelets, we will then move to a much older nanomaterial: magic-sized clusters (MSCs). Such species are believed to be molecular-scale arrangements (i.e., clusters) of semiconductor atoms with a specific (“magic”) structure with enhanced stability compared to particles slightly smaller or larger. Their existence implies that MSC samples can in principle be the same size and shape. Unfortunately, despite three decades of research, the formation mechanism of MSCs remains unclear, especially considering recent experiments that track the evolution of MSCs to sizes well beyond the “cluster” regime. Again, we will discuss the underlying growth mechanism and its implications for nanocrystal synthesis. Finally, we will present an outlook if perfect nanomaterials can be obtained.