Exploring Two-dimensional Nanomaterials for Future Applications

Cation exchange is a versatile method for transforming nanocrystal quantum dots. Despite its broad utility, we lack a clear understanding of how material properties and experimental conditions determine reaction outcomes. This hinders the application of cation exchange to new materials, where fundamental questions like the reaction’s feasibility, its time scale, and the final compositional profile remain unpredictable. Here, we propose a physically transparent mathematical model of cation exchange that directly addresses this gap. Our model reveals that the interaction of three microscopic processes — cation influx, diffusion, and kickout — leads to a rich variety of reaction outcomes. By applying our model to several well-studied nanocrystal materials, we demonstrate agreement with experimental results, thus providing a rational basis for designing exchange reactions in new materials.

Stoichiometric copper chalcogenides such as CuSe in the klockmannite and CuS in the covellite phases are exceptional, quasi-layered, monolithic materials exhibiting both semiconducting and metallic properties in the adjacent visible and near-infrared (NIR) spectral ranges. They readily crystallize as quasi-two-dimensional nanocrystals owing to their anisotropic crystal structure. In this contribution, the spectral features of klockmannite and covellite in the NIR will be analyzed in detail using complex-scale discrete dipole approximation (CSDDA) calculations. These calculations reveal pronounced optical anisotropy and a hyperbolic domain at the boundary of the visible and infrared ranges. The combined effect of propagating and evanescent fields is considered the cause of these modes in the hyperbolic region, exemplarily demonstrated in klockmannite. Finally, the ultrafast photophysical behavior is investigated, including hot hole cooling, trapping, and coherent phonon generation. The results underscore the important role of crystal anisotropy in the physical properties of nanoscale stoichiometric copper chalcogenides.

Interest in the fabrication of thin films of two-dimensional (2D) materials has grown remarkably in recent years due to their unique properties and their potential for large-scale applications.[1][2] In particular, 2D layered crystal structures such as graphene and transition metal dichalcogenides (TMDs) have attracted significant research interest because of their atomic thickness, high specific surface area, mechanical flexibility and strength, transparency, high carrier mobility, and exceptional electronic and optoelectronic properties.[3] For their successful implementation in devices, high-quality networks of these 2D materials are required, which must provide high coverage, low surface roughness, high carrier mobility, and good reproducibility.[4] However, achieving such networks remains challenging, especially for solution-processable deposition techniques like liquid–liquid interface deposition (LLID), as many influencing factors are not yet fully understood. One key factor is the quality and stability of the 2D nanoflake dispersion (“ink”) used for deposition. Therefore, kinetic studies were conducted to identify a long-term stable ink with respect to processing conditions after exfoliation, as well as solvent choice and concentration. [1] F. T. Z. Toma et. al., J. Mod. Nanotechnol., 4(6) (2024) [2] J. Shakya et. al., Advanced Fabrication Methods of 2D Materials. In: Sharma, R., Raghav, S. (eds) 2D Materials. Engineering Materials. Springer, (2025) 33-54 [3] Y. Dai et. al., Chem. Rev., 124, 9 (2024) 5795-5845 [4] J. Neilson et. al., ACS Appl. Mater. Interfaces, 12, 22 (2020) 25125-25134

Layered ultrathin transition metal dichalcogenides (TMDCs) represent a widely adopted material platform for studying optoelectronic processes under confinement at room temperature, owing to their two-dimensional crystal structure and high exciton binding energy.[1,2] Increasing advancement in the colloidal synthesis of these van der Waals materials allows for selective preparation of semiconducting[3] monolayer-dominated[4] wet-chemical solutions with large variety (here: Mo$_{x-1}$W$_x$S$_2$, 0≤x≤1) of TMDC nanoplatelets (NPLs) size-tunable between 3.7 and 21.2 nm.[5] Varying precursor concentrations and addition times allow for control over both, the lateral size[6,7] and the elemental composition of the TMDCs.[8,9] HR-TEM, STEM, steady state absorption, Raman, photoluminescence and femtosecond transient absorption spectroscopy are employed. Global analysis facilitates disentangling the complex decay dynamics consisting of initial trapping (250-300 fs), band gap renormalization (red shift ~30 nm, 10-61 ps) and trapped carrier recombination (2-5 µs) for colloidal TMDCs. We find distinct dependence on both the size and composition of the TMDCs with laterally smaller NPLs and higher molybdenum ratios leading to a faster non-radiative decay (265 fs vs. 741 fs). These findings are underlined by DFT calculations showing mid gap transitions in MoS$_2$ and WS$_2$ to be increasingly localized with stronger confinement (and generally for MoS$_2$).[5] By fundamentally understanding the excited state dynamics of TMDCs, we aim to chemically enhance the photoluminescence efficiency to offer a scalable approach for photoluminescent colloidal TMDCs. [1] Heine. T Acc. Chem. Res. 2015, 48, 1, 65–72.[2] Chhowalla, M. et al. Nature Chem. 2013, 5, 263–275. [3] Mahler, B. et al. J. Am. Chem. Soc. 2014, 136, 40, 14121–14127 [4] Zhao,Y., Fröhlich, M. et al. Nanoscale Horiz., 2025, 10, 3469-3477. [5] Kumar, S., Fröhlich, M. et al. Nano Letters 2026, 26 (9), 3064-3072. [6] Niebur, A.; Söll, A. et al. Nanoscale, 2023,15, 5679-5688. [7] Frauendorf, A.P.; Niebur, A. et al. J. Phys. Chem. C. 2021, 125, (34), 18841-18848. [8] Fröhlich, M. et al. Phys. Chem. Chem. Phys., 2024,26, 13271-13278. [9] Sun Y. et al. J. Am. Chem. Soc. 2017, 139, 32, 11096–11105.

Among the colloidal nanostructures available, nanoplatelets (NPL) are the most impressive material since they offer a compelling combination of the flatness 2D semiconductors to the richness of the versatile world of colloidal nanostructures [1]. The growth along the confinement dimension, ie the thickness, of all the NPLs in the solution can be controlled at the atomic layer scale. This tour de force in the chemical growth is evidenced by a nearly kT-limited emission linewidth in ensemble measurements at room temperature. By contrast with standard 2D semiconductor materials (MoS2,WTe2,..), the properties of NPLs can be further tailored by selectively changing their thickness or the in-plane dimensions. In addition, NPL are also used as seed material to form heteronanostructures that have striking optical properties. To date CdSe NPLs have been the workhorse of the field and have been used as a playground to investigate the changes of charge carrier’s dimensionality in nanostructures [2] or surface-related interactions [3]. While advanced properties of CdSe NPLs are constantly discovered, basics information on their structural properties (shape, trap states) are still missing, which hampers establishing direct correlations between structural and electronic/optical properties. By studying individual CdSe NPL with a scanning tunnelling microscope (STM) we unveil for the first time their real morphology, their edge shape, as well as the spatial and energetic position of electronic trap state at their surface. These results are of tremendous importance to understand the growth mechanism, the nature of the interface for NPL-based HNS or the origin of trap states in NPLs. [1] B. T. Diroll, B. Guzelturk, H. Po, C. Dabard, N. Fu, L. Makke, E. Lhuillier, and S. Ithurria, Chem. Rev. (2022). [2] N. Peric, Y. Lambert, S. Singh, A. H. Khan, N. A. F. Vergel, D. Deresmes, M. Berthe, Z. Hens, I. Moreels, C. Delerue, B. Grandidier, and L. Biadala, Nano Lett. 21, 1702 (2021). [3] E. V. Shornikova, A. A. Golovatenko, D. R. Yakovlev, A. V. Rodina, L. Biadala, G. Qiang, A. Kuntzmann, M. Nasilowski, B. Dubertret, A. Polovitsyn, I. Moreels, and M. Bayer, Nat. Nanotechnol. 15, (2020).

Liquid exfoliation has become an important production technique to give access to large quantities of two-dimensional nanosheets in colloidal dispersion. Importantly, this is a highly versatile technique that can be applied to numerous layered materials beyond graphene. In this talk, I will review the most important steps that allowed us to arrive at a point, where it is possible to produce samples suitable for (device) applications including mechanistic understanding of sonication-based exfoliation and centrifugation-based size selection, reliable determination of nanosheet size and thickness, production of tiled networks with liquid interface assembly. The fundamental understanding of the exfoliation process has created awareness that large and thin nanosheets (i.e. with high aspect ratio) are not accessible through mechanical liquid phase exfoliation (sonication, shear, ball milling) resulting in the need to explore alternatives. A prominent example is electrochemical exfoliation which exploits voltage-driven intercalation to favour delamination over tearing and fragmentation. Here, new challenges arise such as gaining control over doping and surface contamination and to develop means to measure these crucial parameters. In this context, I will summarise our recent contributions in this area which include i) a semi-quantitative assessment of surface adsorbates using diffuse-reflectand IR spectroscopy and ii) identification of additive-free (green) liquid environments that efficiently stabilise electrochemically-exfoliated nanosheets.

Two-dimensional (2D) materials exhibit remarkable electronic, optical, and vibrational properties that emerge at the nanoscale and are often inaccessible using conventional far-field characterization techniques. Scattering-type Scanning Near-field Optical Microscopy (s-SNOM) overcomes the diffraction limit by combining atomic force microscopy with optical spectroscopy, enabling nanoscale imaging of local optical responses with spatial resolution down to tens of nanometers. This presentation highlights the application of s-SNOM for the investigation of 2D materials and van der Waals heterostructures, including the characterization of layer-dependent properties, phonon-polaritons, plasmonic excitations, strain distributions, and local conductivity variations. Through selected case studies, we demonstrate how near-field optical techniques provide unique insights into the fundamental behavior of 2D materials and support the development of next-generation nanoelectronic and nanophotonic devices.

Colloidal transition metal dichalcogenide (TMD) quantum dots (QDs) represent highly tunable platforms for next-generation optoelectronic, catalytic, and spintronic applications. However, their functional performance is often dictated by the interplay between quantum confinement and topology, fundamental material characteristics that are hard to identify and disentangle in experiments. In this talk, I will show how ab initio simulations can shed light on the complex phenomena behind exciton quenching and magnetism in TMD QDs. Focusing on hexagonal nanoplatelets as a relevant model of colloidal TMDs, we identify that edge-located, optically bright hole traps are responsible for sub-picosecond exciton decay. These intrinsic trap states, stemming from metal d-orbitals, outcompete radiative recombination in small nanostructures but are simultaneously responsible for enhanced catalytic activity. Next, we investigate the emergence of intrinsic edge magnetism in triangular MoS$_2$ nanoflakes. Our results reveal a critical size threshold of about 1.5 nm, above which a magnetic ground state emerges due to polar discontinuities at the edges. This results in the formation of magnetic islands localized on specific Mo edge atoms, providing discrete, addressable magnetic units rather than delocalized edge spins. By unraveling the fundamental physics of these systems, we establish a rational basis for the design of multifunctional 2D nanomaterials for future technological applications, including spin filters, memory elements, and spin qubits.

Unpaired spins in solid materials play a role as spectators of local chemical surroundings, mutual dipole interaction, and structural anisotropy, and at the same time, may dictate their fundamental optical, magnetic, and quantum properties. The talk will tour through selective examples (as discussed below) that demonstrate the influence of individual or collective spins on the electronic and optical properties of low-dimension materials. Long-range magnetic order in metal phosphor tri-chalcogenide compounds: Metal phosphor tri-chalcogenides with the general chemical formula MPX3 (M=metal, X=chalcogenide) exhibit ferromagnetic or anti-ferromagnetic ordering. Most recent magneto-optical measurements will be presented, highlighting the dual relation between magnetism and electronic properties . Intrinsic magnetism in perovskite materials: Perovskite materials are composed of organic-inorganic constituents, forming 2D and 3D structures. The discovery of unique magneto-optical properties in those materials reflects the emergence of intrinsic effective magnetic poles, which results from the inversion symmetry breaking (inducing a Rashba effect) or due to the existence of a nuclear field (Overhauser effect). Overall, the observations underscore the fundamental role of spins in governing the optical and magnetic properties of low-dimensional semiconductors, with significant implications for the development of modern spin-electronics and spin-optical devices.

Understanding electronic transport within interconnected networks of van der Waals (vdW) materials is essential for the development of large-scale, flexible 2D electronics. These films, composed of complex topologies of overlapping flakes, present unique challenges in determining how microscopic connectivity dictates macroscopic resistance. In this work, we implement a comprehensive experimental and theoretical framework to characterize the electrical properties of these disordered systems using two complementary methodologies. The first approach utilizes a scanning technique based on a flexible polymer cantilever integrated with graphite electrodes. Fabricated via mechanical exfoliation and patterned through laser engraving, this probe enables high-resolution mapping of electrical resistance across millimetric areas. The use of graphite as a contact material leverages its intrinsic layered structure to provide a robust, low-friction interface, ensuring stable measurements across the inhomogeneous surface of the flake network without compromising sample integrity. The second approach focuses on a stationary device geometry, where the vdW flake film is characterized within a defined channel between fixed electrodes. This configuration allows for precise measurements of the channel resistance and its evolution under varying environmental conditions. Specifically, we conduct temperature-dependent electrical measurements to probe the fundamental transport mechanisms within the network. By analyzing the resistance across a broad temperature range, we are able to extract characteristic activation energies and identify the presence of quantum transport phenomena, such as weak localization, which significantly modify the film's conductive behavior. To provide a deeper physical interpretation of our findings, experimental results from the second method are compared with a theoretical Random Resistors Network (RRN) model. This model serves as a critical bridge, correlating the statistical distribution of flake-to-flake junctions with the emergent macroscopic electrical response. Our integrated methodology combining localized scanning, temperature resolved channel characterization, and RRN simulations offers a powerful toolkit for the optimization and design of next generation electronic devices based on 2D material networks.

Single-photon emitters (SPEs) play a paramount role in quantum photonics and quantum information technologies. SPEs, either naturally occurring as defects or experimentally created, have been investigated in variety of bulk materials as well as low dimensional systems. In 2015, single photon emission tentatively assigned to native atomic or nanoscale defects was observed in atomically thin layers of transition metal dichalcogenides (TMD) [1]. These built-in SPEs, readily integrated in a truly two-dimensional (2D) matrix offer significant advantages for applications, such as an efficient photon extraction and a natural potential for on-chip integration. TMD flakes, either manually exfoliated from bulk crystals or grown for instance using chemical vapour deposition, host defects and/or strain gradients generating localized states that may be described as quantum dots (QDs) with singular physical properties inherited from the “bulk” 2D matrix. Over the last decade, this subfield of 2D materials research has experienced a rapid growth, focusing mainly on the description of these states and on engineering strategies to control their properties [2], [3]. Along these lines, integrating TMD layers within van der Waals heterostructures offers stimulating opportunities to further engineer SPEs in 2D materials. Specifically, graphene and TMD flakes can be stacked to form a building block for optoelectronics, with fast photoresponse times, in the sub-ns regime [4]. Here, we investigate a hexagonal boron-nitride encapsulated WSe$_2$/graphene (Gr) van der Waals heterostructure using optical spectroscopy techniques at cryogenic temperatures ($<$ 10 K). We have observed low-energy localized states in WSe$_2$, whose properties (e.g. FWHM $\sim$100-150 $\mu$eV, fine structure splitting $\sim$600 µeV, polarization properties) are consistent with recent SPE literature. However, while graphene is expected to strongly quench long-lived states such as SPEs [5], intense and stable localized photoluminescence peaks have been observed in WSe$_2$/Gr. Additional measurements in a charge-tunable sample reveals that localized emitters in WSe$_2$ can be electrically controlled by shifting the chemical potential of the neighbouring graphene layer. Unlike previously reported gate tuned SPEs in bare WSe$_2$[6],[7], localized emitters in WSe$_2$/Gr regions light up only within a narrow gate voltage window. This peculiar behaviour may have potential applications for quantum optoelectronics. [1] YM. He et al., Nat. Nanotechnol., 2015, 10, 6 [2] C. Palacios-Berraquero et al., 2017, Nat. Commun., 8, 15093 [3] A. Paralikis et al., npj 2D mater. appl., 2024, 8, 59 [4] M. Massicotte et al., Nat. Nanotechnol., 2016, 11, 1 [5] E. Lorchat et al., Nat. Nanotechnol., 2020, 15, 4 [6] X. Lu et al., Nat. Nanotechnol., 2019, 14, 5 [7] C. Chakraborty et al., Nat. Nanotechol., 2015, 10, 6

Solution-processed 2D colloidal nanocrystals, also called nanoplatelets, offer great prospects to advance quantum dot-based applications such as light-emitting diodes, displays and lighting devices. Compared to spherical quantum dots, their uniform thickness, tuned to the monolayer level, yields narrower emission line widths, and their 2D shape allows for shorter emission lifetimes, as well as polarized emission in oriented films. In addition, 2D confinement of charge carriers enhances the exciton binding energy, which leads to stable excitons, and even biexcitons, at room-temperature. This expands the potential range of applications to for instance lasers and scintillators. Nanoplatelets with precisely controlled length, width and thickness are instrumental to improve our understanding of their photophysical properties. I will discuss the impact of their size on the exciton and biexciton binding energy and fluorescence lifetime. Next, these insights are combined with edge passivation strategies to enhance the fluorescence quantum efficiency and optical gain coefficient. Finally, I will give some examples of targeted photonic applications, such as LEDs, lasers and scintillators, based on colloidal nanoplatelets.

Bismuth (Bi) and its chalcogenide compounds, such as Bi₂Te₃, are promising platforms for quantum, spintronic, and energy technologies owing to their strong spin–orbit coupling, high thermoelectric efficiency, magnetoresistance, and topological properties. However, scalable and phase-selective synthesis of high-quality two-dimensional (2D) Bi-based nanocrystals with controlled size, shape, and structure remains challenging. Here, we report a tunable colloidal synthesis approach for producing high-quality Bi nanosheets and Bi-based chalcogenide nanosheets (including Bi₂Te₃, BiTe, and Bi₄Te₅) with well-defined hexagonal morphology and tunable lateral sizes. The Bi nanosheets exhibit layered single-crystalline structures along the {00l} planes, excellent long-term stability in solution, and surface ligand protection against oxidation. These colloidal nanostructures enable direct characterization by angle-resolved photoemission spectroscopy (ARPES) on individual nanosheets, revealing a band structure in excellent agreement with density functional theory (DFT) calculations and confirming high crystal quality and uniformity. For the bismuth tellurides, a phosphine-free colloidal method allows phase-selective synthesis within the (Bi₂)ₘ(Bi₂Te₃)ₙ homologous series by controlling precursor chemistry and reaction temperature. In situ heating studies demonstrate that BiTe and Bi₄Te₅ nanosheets transform into Bi₂Te₃ at approximately 340 °C, followed by preferential Te sublimation or oxidation at higher temperatures, with mechanisms supported by EDS and STEM-FFT analyses. This unified colloidal platform offers key advantages, including single-crystalline quality, tunability in size, shape, dimensionality, doping, and phase, as well as solution-processability and cost-effective scalability. Our findings advance the fundamental understanding of phase stability and thermal behavior in Bi-based 2D materials while providing a versatile route for integrating high-quality nanocrystals into advanced quantum, spintronic, and thermoelectric devices.

The talk reports studies on the nature and dynamics of photogenerated charge carriers and excitons in 2D transition metal dichalcogenides (TMDCs) MX2, where M=Mo, W, Hf or Re and X= S, Se or Te. We focus on processes relevant for development of highly efficient photovoltaics, or single photon emitters for quantum information technology applications. Excitons and charge carriers are generated by photoexcitation with ultrashort laser pulses and detected by time-resolved optical absorption and terahertz conductivity measurements. We analyze the measured data using electronic band structures and exciton energies that we calculate using density functional theory (DFT), the GW method and by solving the Bethe-Salpeter Equation (BSE) for excitons. The first part of the talk focuses on the generation of two or more electron-hole pairs by the absorption of one photon. This process of carrier multiplication (CM) can boost the efficiency of photovoltaic devices. CM in few-layered TMDCs is experimentally found to be much more efficient than in PbS and PbSe bulk semiconductors. We explain this theoretically on the basis of CM computer simulations, which show that the presence of a large density of bands with weak dispersion in TMDCs promotes CM.[1] According to BSE calculations the binding energy of finite-momentum excitons at the indirect band gap in e.g. MoSe2 is comparable to thermal energy at room temperature.[2] This implies significant thermal exciton dissociation into free electrons and holes. Studies on exciton stability in other TMDCs, charge mobility and effects of structural defects (e.g. S, Se or Te vacancies) will also be presented. Next dynamics of charge carriers and excitons in mechanically exfoliated monolayer TMDCs is discussed. We focus on mobility and decay dynamics into localized sub-gap defect states, which is important for use of TMDCs as single photon emitters. References [1] S. Weerdenburg et al., J. Phys. Chem. C, 128, 3693 (2024). [2] G. Ghosh et al., ACS Nano, 19, 10347 (2025).

Two-dimensional transition metal dichalcogenides (TMDs) represent a highly promising class of atomically thin semiconductors for photonic technologies. In the monolayer limit, these materials exhibit a direct bandgap, strong photoluminescence, pronounced excitonic effects, large optical nonlinearities, and unique valley-selective optical properties that are attractive for both classical and quantum information processing. Their two-dimensional geometry enables efficient integration with nanophotonic platforms, while their exceptional optical response and tunability make them attractive candidates for compact, high-speed, and multifunctional optoelectronic devices. As a result, hybrid nanostructures that combine 2D TMDs with resonant photonic elements have become an increasingly active area of research, with potential applications ranging from light sources and polarization-sensitive detectors to ultrafast wavefront control and optical information processing. Realizing these opportunities requires new fabrication and integration strategies that allow 2D TMDs to be combined with resonant nanophotonic structures. In this talk, we present recent progress in TMD-based hybrid nanophotonic systems, focusing on their coupling to high-index dielectric metasurfaces and plasmonic nanoantennas. We investigate how excitonic properties, carrier dynamics, and coherence effects shape light–matter interactions in these hybrid platforms. Furthermore, we discuss recent developments in integration approaches that enable genuinely scalable incorporation of 2D TMDs while preserving their optical functionality. Overall, we provide an overview of light–matter interactions in hybrid nanostructures, key integration challenges, and recent advances in resonant TMD-based nanophotonic systems.

The quest for atomically precise synthesis of colloidal semiconductor nanostructures has attracted increasing attention in recent years and remains a formidable challenge. Nevertheless, atomically precise clusters of semiconductors with well-defined and specific dimensions, known as “magic-size clusters” (MSCs), are readily accessible. Ultrathin one-dimensional (1D) nanowires and two-dimensional (2D) nanoplatelets and nanosheets can also be categorized as “magic-size” nanocrystals (MSNCs) since their diameter or thickness also vary in discrete steps from one value to the next. However, despite intense research activity, there is still no consensus regarding their formation mechanism, as several possibilities have been proposed. In this lecture, I will first discuss these mechanisms and show that the notion of a single mechanism capable of accounting for the formation of all types of ultrathin 1D and 2D MSNCs is unsubstantiated. I will show that ultrathin MSNCs correspond to metastable states in a multidimensional variable space and therefore may be accessed through different pathways, that may co-exist, interacting dynamically in a synergistic or antagonistic way, depending on the prevailing physical-chemical conditions. Our conclusion is that the “magic” originates from the complexity of a dynamic and multivariate system running under reaction-control through a potential landscape with multiple metastable minima. I will illustrate this by discussing our recent work on the room-temperature formation of CdTe ultrathin magic-size nanowires and their evolution to ultrathin wurtzite nanoribbons or triangular nanosheets depending on the conditions.

The microscopic coupling between lattice vibrations (phonons) and charge carriers are at the origin of a plethora of exotic condensed matter phenomena, such as superconductivity, charge density waves, and soft-mode phase transitions. These phonon-carrier interactions are also essential for explaining electrical and thermal transport, as well as energy conversion processes in materials for photovoltaics or future quantum technologies. Given their central role in nearly all condensed matter phenomena, gaining access to these microscopic interactions is essential. In the first half of this talk, we will show that ultrafast electron diffuse scattering (UEDS) yields insights into the non-equilibrium lattice dynamics in two-dimensional materials by providing access to momentum-resolved electron-phonon and phonon-phonon coupling. We will discuss our recent work which investigates the interplay of carrier screening and electron-phonon interactions in the semiconductor MoS2. In the second part of this talk, we will show how coherent 2D spectroscopy complements UEDS by providing microscopic insights on the electronic/excitonic response of materials. We will present measurements revealing sub-100-femtosecond homogeneous linewidth broadening of the A-exciton resonance in monolayer WSe, indicative of rapid exciton energy fluctuations. These measurements provide insight into exciton dephasing mechanisms, which sets fundamental limits to coherent control and quantum-optical applications.

Light-emitting diodes (LEDs) are a key ingredient in modern display and lighting technologies. Most commercial LEDs based on inorganic III-V semiconductors utilize a blue pump LED combined with color conversion materials to generate white light, or RGB colors by optical filtering. However, this approach is not very energy efficient and lacks in color purity for high-definition displays. In this respect, colloidal quantum dots and, specifically, lead-halide perovskite (LHP) nanocrystals offer the potential of achieving ultra-high color purity with narrow emission linewidth that is tunable over the entire visible spectrum. But despite their excellent optical performance, including high photoluminescence quantum yield and stability under optical excitation, their implementation in electroluminescent devices, like LEDs, is still challenging. A key factor is that these LHP nanocrystals are densely covered by surface ligands to make them stable in solution and passivate them, which poses a major challenge for charge injection in LEDs. In my talk, I will present some of our recent approaches to achieve more balanced charge injection in LEDs by optimizing the organic transport layers and using ligand-exchanged nanocrystals [1-3]. Furthermore, shape-anisotropy and alignment of nanocrystals offer a handle to improve light outcoupling from LEDs [4,5]. All this needs detailed knowledge of their electronic and optical properties, which we obtain by a wide range of experimental techniques combined with numerical simulations. [1] T. Naujoks et al., Quantum efficiency enhancement of lead-halide perovskite nanocrystal LEDs by organic lithium salt treatment, ACS Applied Materials & Interfaces 14, 28985-28996 (2022) [2] C. Kirsch et al., Zwitterionic carbazole ligands enhance the stability and performance of perovskite nanocrystals in light-emitting diodes, ACS Applied Materials & Interfaces 15, 32744-32752 (2023) [3] R. Jayabalan et al., Optimizing carrier balance in CsPbBr3 nanocrystal LEDs: The role of alkyl ligands and polar electron transport layers, Advanced Optical Materials 13, e01361 (2025) [4] T. Morgenstern et al., Elucidating the performance limits of perovskite nanocrystal light emitting diodes, Journal of Luminescence 220, 116939 (2020) [5] M. Jurow et al., Manipulating the transition dipole moment of CsPbBr3 perovskite nanocrystals for superior optical properties, Nano Letters 19, 2489-2496 (2019)

Transition metal dichalcogenides (TMDs) are among the most investigated layered materials due to their highly attractive properties, such as large and direct band gap in the monolayer limit, spin-valley coupling and tightly bound excitons. Stacking TMD monolayers with parallel orientation leads to the buildup of a ferroelectric (FE) polarization at the interfaces of adjacent layers. The orientation of this polarization can be switched by in-plane sliding of the layers, giving rise to a potential novel type of nonvolatile data storage. Remarkably, the ferroelectric order in such structures can be mapped using optical spectroscopy, as it impacts intralayer exciton transition energies and modifies the Raman spectrum. Integrating these ferroelectric layers into van der Waals heterostructures with other TMDs introduces new opportunities for manipulating the properties of interlayer excitons in these systems. In crystallographically aligned heterostructures consisting of MoSe$_2$ monolayers combined with few-layer parallel-stacked MoS$_2$, we find long-lived, optically bright interlayer excitons. The energy of these states correlates with local ferroelectric domain polarization, showcasing distinct domain-dependent interlayer exciton behavior. Gate voltage experiments further demonstrate tunable shifts in interlayer exciton energy driven by polarization changes in the ferroelectric domains. These results highlight the potential of interlayer excitons being controlled by local ferroelectric polarization and establish a foundation for future ferroelectric optoelectronic devices based on vdW heterostructures.

Group II–VI semiconductor nanoplatelets (NPLs) exhibit optical properties that can be tuned through their thickness and composition. Surface chemistry has emerged as an additional lever to control these optical features. In these NPLs, the top and bottom facets are cation-rich and therefore require, on average, one negatively charged ligand per surface atom for charge compensation. As a result, NPLs can be viewed as flexible substrates for the self-assembly of molecules. Here, I will discuss how the shape of NPLs is affected by ligand exchange, replacing native carboxylate ligands with thiolate, tartrate, or halide ligands. I will show that these transformations are initially driven by thermodynamic effects associated with ligand exchange. Under certain conditions, Janus particles can be obtained, enhancing structural asymmetry in these systems.

The direct wet-chemical synthesis of two-dimensional (2D) lead selenide (PbSe) flat quantum dots (fQDs) yields bright materials with strong excitonic contribution at room temperature. [1,2] These highly confined 2D nanocrystals exhibit photoluminescence (PL) in the near-infrared (900 – 1550 nm) with a PL quantum yield of up to 60 $\%$. [1] Their highly efficient PL at fiber-optics-relevant telecommunication wavelengths renders colloidal lead chalcogenide 2D semiconductors intriguing materials for future solution-processable components for fiber optic applications. However, their (time-resolved) absorption characteristics have not been thoroughly investigated up to now, as their excitonic absorbance is rather weakly expressed in conventional steady-state absorbance measurements. Here, we investigate the photophysics and charge carrier dynamics in PbSe fQDs (8.3 nm × 4.9 nm) by femtosecond broadband transient absorption spectroscopy (TAS). Careful spectral tuning of the photoexcitation wavelength allows for probing of the elusive ground state bleach of the first excitonic transition. By probing the full energy range (350 – 2400 nm), we can assign the 1P(e,h) – 1P(h,e) - exciton at 660 nm shifted via biexcitonic interaction, [3] the ground state bleach of the 1Se,h – 1Sh,e – exciton at 1450 nm and excited state absorption from the 1(Se,h) – 1P(e,h) - transition at 2300 nm. [4] By fitting, charge carrier dynamics such as intraband relaxation [4], radiative recombination [1a] and multiexcitonic contribution to the decay were found. [5] Distinct layer thicknesses as well as their respective decay lifetimes are determined with the help of exciton binding energies calculated by tight-binding theory. Our results advance the understanding of near-infrared emissive highly confined 2D PbSe fQDs for the first time. TAS allows us to optimise our synthesis to achieve the mono-dispersity required for telecommunication applications. [1] a) Biesterfeld, L.; Klepzig, L. F.; Niebur, A.; Rosebrock, M.; Lauth, J. J. Phys. Chem. C 2022, 126, 45, 19277-19285. b) Klepzig, L. F.; Biesterfeld, L.; Romain, M.; Niebur, A.; Schlosser, A.; Hübner, J.; Lauth, J. Nanoscale Adv. 2022, 4, 590-599. [2] Biesterfeld, L.; Ngo, H. T.; Addad, A.; Rudolph, D. A.; Leis, W.; Seitz, M.: Ji, G.; Grandidier, B.: Delerue, C.; Lauth, J.; Biadala, L. Nano. Lett. 2025, 25, 31, 12019-12024. [3] Trinh, M.T.; Houtepen, A.J.; Schins, J.M.; Piris, J. & Siebbeles, L. D. A., Nano. Lett. 2008, 8, 2112-2117. [4] De Geyter, B.; Houtepen, A. J.; Carrillo, S.; Geiregat, P.; Gao, Y.; Cate, S.; Schins, J. M.; Van Thourhout, D.; Delerue, C.; Siebbeles, L. D. A. and Hens, Z. ACS Nano 2012, 6, 7, 6067-6074. [5] Schaller, R. D.; Klimov, V. I.; Physical Review Letters 2004, 92, 186601.

NiPS_3, a member of the metal phosphotrichalcogenide family, is a layered semiconductor composed of crystalline sheets with strong in-plane covalent bonding and weak interlayer van der Waals interactions. Its reduced dimensionality promotes exciton formation, while its tunable bandgap of approximately 1.4 eV makes it attractive for optoelectronic and energy-related applications, including transistors, photodetectors, electrocatalysis, and energy conversion devices. In addition, below its Néel temperature of around 150 K, the antiparallel alignment of nickel magnetic moments gives rise to antiferromagnetic order, positioning NiPS₃ as a promising platform for spintronic, magneto-optical, and magnetic storage applications, as well as for fundamental studies of two-dimensional magnetism. Recent observations of many-body phenomena, including collective excitonic states below 150 K, further highlight the rich physics of this material. Here, we use femtosecond transient absorption spectroscopy to probe the ultrafast photoinduced dynamics of NiPS_3 immediately after optical excitation. The transient response exhibits an initial bimolecular relaxation process followed by a slower exponential decay, revealing distinct carrier recombination pathways in this layered antiferromagnetic semiconductor. Our findings provide new insight into the early-time photophysics of NiPS₃ and establish time-resolved optical spectroscopy as a powerful approach to exploring exciton formation and relaxation processes in this material.

Scanning tunneling microscopy and spectroscopy is the only way to relate the atomic structure of a material to its electronic structure, realizing in this way a longstanding dream in nanoscience. With in-situ prepared samples, e.g. atom-by-atom prepared artificial lattices, in vacuum cleaved crystals, and MBE grown materials, the surface of the crystal is devoid of adsorbates and atomic resolution and reliable spectroscopy of the energy levels of electronic band structure has been achieved by many groups. Here, I will demonstrate this with ultra-resolution of superconducting NbSe2/CrBr3 moiré structures. However, ex-situ prepared real materials, especially those obtained by wet chemistry, provide a longstanding challenge to atomically resolved scanning tunneling microscopy and spectroscopy. Colloidal synthesis results in state-of-the-art materials, but solvent molecules and adsorbates made deteriorate the microscopy/spectroscopy. I will demonstrate a special transfer method into ultra-high vacuum to overcome this problem. Using an adapted transfer method, we can map and relate the atomic and electronic structure of platelets of the Bi2X3 family. We show that the electronic structure is position invariant in accordance with the Bloch theorem for a periodic structure. In addition, we reveal and inter-relate the atomic and electronic structure of different types of point defects. We distinguish donor-type of defects that do not affect the position-invariant band structure from defects that result in extra resonances over a broad energy region across the band spectrum. The latter may have a strong effect on the fate of photo-generated carriers or excitons.