The most studied class of semiconductor nanocrystal—quasi-spherical particles known as colloidal quantum dots—is now commercially used as a fluorescent material. However, despite decades of research, state-of-the-art samples still exhibit a distribution in size and shape, reducing their performance for applications. This leads to a fundamental question: can we achieve true monodispersity in semiconductor nanocrystals via chemical synthesis?  In this talk we will discuss this issue by examining two classes of nanomaterials. First, we will consider thin rectangular particles known as semiconductor nanoplatelets (NPLs). Amazingly, NPL 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 NPLs, 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 monodisperse in 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.

Colloidal CdSe nanoplatelets (NPLs) can be made with a thickness of atomic precision in the range of about one to a few nanometers only. The lateral sizes are of the order of several to tens of nanometers. The thickness is less than the bulk exciton bohr radius and consequently leads to strong effects of spatial confinement on the internal energy of an exciton. Variation of the thickness of a NPL thus allows one to tune the photoluminescence (PL) and optical absorption spectra.

Interestingly, however, the experimental shape of PL and absorption spectra also depends on the lateral sizes of a NPL. To date, the latter has not received much attention, with the exception of a few (mainly theoretical) studies and the origin of this effect has been inconclusive.

We measured the PL and absorption spectra for a series of NPLs with different lateral sizes and find that the dependence of the optical spectra on the lateral size is fully explained by taking into account the quantum-confinement effects on the translational motion of excitons in the plane of the NPLs. The spectra of all samples considered can be reproduced very accurately by a theoretical description of exciton energies and oscillator strengths based on the quantum mechanical particle-in-a-box model and the known size-distribution of the NPLs.

Quantum confinement is certainly the most striking properties of nanocrystal at the nanoscale. In CdSe this is used to tune the energy of the first exciton from green to red enabling their use as downconverter for display. In HgTe the lack of bulk band gap and the weak conduction effective mass makes that the absorption edge can be widely tuned from UV to THz [1]. HgTe 2D nanoplatelets (NPL) are certainly the most confined form of Hgte with up to 1.5 eV of confinement energy. The sysnthesis of these NPLs has recently been reported via a two steps fabrication method including a cation exchange step from CdTe NPL [2]. Such large confinement corresponds to a wavevector range of the HgTe relation dispersion far away from the Γ point, where the latter starts being highly non-parabollic, allowing to explore a strong confinement regime. As a result, the optical properties of the NPL also gets affected. In particular, we report that the temperature dependence of the HgTe NPL optical band gap (dEG/dT<0) is opposite to the one observed in bulk and large HgTe NCs (dEG/dT>0).

To understand this trend, we systematically explore the pressure (0.-4GPa full range of existence of the zinc blende phase) temperature (10-300K) and confinement (0.25-1.5 eV) phase diagram of HgTe and correlate spectroscopic data with a 14 band kp model. The model unveils the critical role plays by the upper conduction band in the curvature of the first conduction band in the strong confinement regime.

These HgTe NPL are latter integrated into field effect transistors and photodetectors [3-4] to reveal the nature of the majority carrier and their photoconductive properties in the near and short wave infrared. 

Ref

[1] Terahertz HgTe nanocrystals: beyond confinement, N Goubet et al, .JACS 140, 5033-5036 (2018)

[5] Strongly confined HgTe 2D nanoplatelets as narrow near-infrared emitters, E Izquierdo et al, JACS 138,  10496-10501 (2016)

[3] Charge dynamics and optolectronic properties in HgTe colloidal quantum wells, C Livache et al, Nano Letters 17 (7), 4067-4074 (2017)

[4] Impact of dimensionality and confinement on the electronic properties of mercury chalcogenide nanocrystals, C Gréboval et al, Nanoscale 11 (9), 3905-3915 (2019)

Layered semiconductors attract significant attention due to their diverse physical properties controlled by their composition and the number of stacked layers, but still obtaining material in large quantity may be a challenge. Liquid exfoliated van der Waals semiconducting crystals have been recently described as the main active material in all-printed devices such as transistors or photodetectors. This technique may lower device preparation cost by accelerating production and omitting expensive methods like lithography.

Herein, large crystals of the ternary layered semiconductor - chromium thiophosphate (CrPS₄) are prepared in big amounts by a vapor transport synthesis. Optical properties are determined using photoconduction, absorption, photoreflectance, and photoacoustic spectroscopy exposing the semiconducting properties of the material. [1] A simple, versatile, one-step protocol for mechanical exfoliation onto transmission electron microscope grid is developed [1], [2] and multiple layers are characterized by advanced electron microscopy methods, including atomic resolution elemental mapping confirming the structure by directly showing the positions of the columns of different elements’ atoms. [1]

CrPS₄ is also liquid exfoliated and then obtained suspension is converted into an ink.  Finally, the CrPS₄ ink in combination with colloidal graphene is used for creating ink-jet printed photodetector. This all-printed graphene/CrPS₄/graphene heterostructure detector demonstrates specific detectivity of 8.3×10⁸ (D*). The study shows a potential application of both bulk crystal as well as individual flakes of CrPS₄ as active components in light detection, when introduced as ink printable moieties with a large benefit for manufacturing. [1]

Directing the self-assembly of colloidal nanocrystals into ordered superstructures is of fundamental and technological interest for creating designer materials that bridge multiple length scales. The assembly of polyhedral nanocrystals at the interface of two immiscible fluids presents a promising approach to create high-fidelity superlattices with exceptional translational order and enables control over the orientational order of constituent building blocks. However, the full potential of this assembly approach remains elusive since despite the ostensible simplicity of the interfacial assembly, many knowledge gaps persist concerning the nuanced physicochemical phenomena that occur during assembly.  

Using synchrotron-source grazing incidence small angle X-ray scattering (GISAXS), the fluid and particle dynamics which lead to the final highly ordered superlattices can be elucidated. In this work, we used high time resolution GISAXS to characterize the spreading and drying dynamics of PbSe nanocrystals assembling from droplet contact with the liquid substrate to the final superlattice structure. Additionally, we explain how tuning the solvent parameters, such as volatility, surface tension and polarity, determines the mesoscale morphology of 2D superlattices on ethylene glycol. Specifically, the solvent interaction with the liquid substrate and ligand shell dynamics during evaporation have significant effects on the final superlattice morphology. Improved understanding of the kinetic phenomena giving rise to superlattice topology will enable growth of high-quality superlattices with long-range order at both nano- and micro- scales.

There is an increasing interest in two-dimensional (2D) Ruddlesden-Popper perovskites for solar harvesting and light emitting applications due to their superior chemical stability as compared to bulk perovskites.[1,2] Both, purely 2D and blends of 2D/3D phases have been successfully employed in solar cells with an efficiencies of >18% and >21%, respectively.[3,4] As with earlier advances in the field of perovskites, these technological improvements are advancing at a pace that far exceeds our understanding of the physical mechanisms underlying their performance. Particularly, the reduced dimensionality in 2D perovskites results in excitonic excited states which dramatically modify the dynamics of charge collection. While the carrier dynamics in bulk systems is increasingly well understood, a detailed understanding about the spatial dynamics of the excitons in 2D perovskites is lacking.[5]

Here, we present the direct measurement of the intrinsic diffusivities and diffusion lengths of excitons in single crystalline 2D perovskites using time-resolved microscopy. Our technique allows us to follow the temporal evolution of a diffraction limited exciton population with sub-nanosecond resolution revealing the spatial and temporal exciton dynamics. We reveal two distinct temporal regimes: For early times excitons undergo unobstructed normal diffusion, while at later times exciton transport becomes subdiffusive as excitons get trapped. Using the versatility of perovskite materials, we study the influence of the organic spacer, cation and dimensionality (n = 1 and 2) on the diffusion dynamics of excitons in 2D perovskites. We find that changes in these parameters can yield diffusivities which differ in up to one order of magnitude. We show that these changes arise due to strong exciton-phonon interactions and potentially with the formation of large exciton-polarons. Our results provide insight into how excitons diffuse through 2D perovskites and yield clear design parameters for more efficient 2D perovskite solar cells and light emitting devices.[6]

Silver phenylselenolate (AgSePh) is an emerging excitonic two-dimensional semiconducting member of a hybrid metal-organic chalcogenolate family. In addition to its two-dimensional structure with high exciton binding energy, strong in-plane anisotropy, and a narrow emission spectrum at 467 nm, AgSePh does not contain any toxic element and is tolerant to both polar and non-polar solvents. AgSePh can be synthesized by a solution-phase reaction as well as a scalable vapor-phase method. Here, we show by testing 24 solvents – with different polarities, boiling points and functional groups – that complexation between silver cations and solvent molecules is the key to an increasing size of AgSePh crystals. With the introduction of amine solvents, we are able to increase the size of AgSePh crystals grown by the solution-phase biphasic reaction from ~3 µm to >200 µm and that of AgSePh thin films prepared by the vapor-phase tarnish reaction from ~200 nm to >1 µm. We also observed that the photoluminescence lifetime of AgSePh is stable after storing under ambient condition and the addition of amines boosts this lifetime from <40 ps to 200 ps. The improved syntheses reported in this work will allow easy integrations of AgSePh in both thin-film electronic and nanoelectronic applications as well as the exploration of strong excitonic anisotropy.

Abstract: Biexcitons in 2D transition metal dichalcogenide from first principle: binding energies and fine structure.

The emerging field of 2-dimensional (2D) materials keeps gaining increasing attention due to the wide range of potential applications in many domains including: optoelectronics, photovoltaic, sensing, quantum computing ...etc. Reducing the dimensionality of a system results in an enhancement of the Coulomb interaction between elementary quasiparticles (i.e. electrons and holes) as it reduces the dielectric screening. This allows for the formation of strongly bounded excitations which can be observed even at room temperature. Among these excitations, biexcitons are of special interest from both the experimental and theoretical perspectives due to its rich physics and potential applications in quantum information and lasing [1]. Understanding biexcitons would be the first step toward a clear understanding of the equilibrium dynamics of photo excited hot carriers and is also relevant in the context of exciton condensation. Moreover, the biexciton, being a complex bound state of 2 electrons and 2 holes, has a rich fine structure and many more degrees of freedom than the simple excitonic case.

First principle treatment of biexcitons, on the same theoretical footing as excitons and trions, is possible thanks to the newly developed methodology of Ref. [2]which uses a hybrid approach combining configuration interaction and green function methods for the description of many-electron many-hole excitations.This methodology has been shown to give reliable results on excitons and trions [2] and it is applied here to study the binding, fine structure and non-equilibrium effects of biexcitons in 2D transition metal dichalcogenide.

In recent year, in response to the request for flexible and sustainable energy storage devices with high electrochemical performance, there has been growing interest in using paper or paper-like substrates for batteries and other energy storage devices such as environmentally friendly supercapacitors [1]. In this context, cellulose-based substrates for energy storage devices could be well-engineered, are light-weight, safe, thin and flexible[2]. We demonstrated a scalable, low cost and easy-to-process approach for the preparation of energy storage devices using large area techniques like spray and blade coating, suitable for smart electronic applications for health monitoring. Following a green strategy, all components were formulated in water-based dispersions. Symmetric paper-based supercapacitors using common copy paper and electronic paper as substrate, and Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) as electrodes, are realized and investigated. The novelty of this work consists in the use of composite based on detonation nanodiamonds (DNDs) and hydroxypropyl cellulose (HPC) as solid state electrolyte and separator. We also prepared devices with solution electrolyte using the same HPC+DND composite but with the addition of sodium sulfate (Na₂SO₄). The performance obtained using solid electrolyte (HPC+DNDs) and liquid electrolyte (HPC+DNDs+Na₂SO₄) on both substrates are comparable in terms of specific capacitance: ~ 0,13 ÷ 0,52 F/g for (HPC+DNDs) and ~ 0,35 ÷ 0,82 F/g for (HPC+ND+Na₂SO₄), with power density in the range of ~19 ÷ 24 µW cm^-2[3].

Nanostructures on the base of lead, tin and copper chalcogenides with defined shape, dimensionality, faceting and surface chemistry are promising building blocks for opto-electronic devices in the near-infrared spectral range. A high degree of control has been already reached within main approaches for the dimensionality control: anisotropic growth, mesophase confined growth due to templating effect and oriented attachment. Here, we demonstrate several examples of fine-tuning of the shape and faceting of CuS, SnS and PbS quasi-two-dimensional structures with impact on electrical and optical properties. We also show synthetic details of the shape transformations combined with simulations which shed light onto the mechanism of the reached control. In case of PbS nanostripes and nanowires we show how the faceting of a nanocrystal dramatically changes its properties from semiconducting to metallic ones and analyze the reasons of the observed behavior.

References

Ramin Moayed, M. M., Kull, S., Rieckmann, A., Beck, P., Wagstaffe, M., Noei, H., ... & Klinke, C. (2020). Function Follows Form: From Semiconducting to Metallic toward Superconducting PbS Nanowires by Faceting the Crystal. Advanced Functional Materials, 30(19), 1910503.

Li, F., Moayed, M. M. R., Gerdes, F., Kull, S., Klein, E., Lesyuk, R., & Klinke, C. (2018). Colloidal tin sulfide nanosheets: formation mechanism, ligand-mediated shape tuning and photo-detection. Journal of Materials Chemistry C, 6(35), 9410-9419.

Li, F., Ramin Moayed, M. M., Klein, E., Lesyuk, R., & Klinke, C. (2019). In-plane anisotropic faceting of ultralarge and thin single-crystalline colloidal SnS nanosheets. The journal of physical chemistry letters, 10(5), 993-999.

Lesyuk, R., Klein, E., Yaremchuk, I., & Klinke, C. (2018). Copper sulfide nanosheets with shape-tunable plasmonic properties in the NIR region. Nanoscale, 10(44), 20640-20651.

The high affinity for the halides ligands with the <100> facets of the zinc blende nanoplatelets (NPLs) lead to a ligand exchange from carboxylate to halides which then partially dissolved the cadmium chalcogenides NPLs through the edges. The released monomers then recrystallized on the large top and bottom facets leading to a growth of NPLs in the thickness. CdSe NPLs with thicknesses from 3 to 9 MLs are synthesized. A direct growth is also achieved when a chalcogenide precursor is jointly introduced with a metal halide. Finally when an incomplete layer is grown, homostructures with a type I band alignement are obtained thus offering a new degree of liberty for the synthesis of structured NPLs.

Two-dimensional (2D) colloidal nanoplatelets (NPLs) are an emerging class of quantum well materials that exhibit many unique properties, including uniform quantum confinement, narrow thickness distribution, large exciton binding energy, giant oscillator strength effect, long Auger lifetime, and high photoluminescence quantum yield. These properties have led to great potentials in optoelectrical applications, such as lasing materials with a low threshold and large gain coefficient. Many of these properties are determined by the structure and dynamics of band-edge excitons in these 2D materials. Motivated by both fundamental understanding and potential applications, the properties of 2D excitons have received intense recent interests. We have carried out a series of recent studies on fundamental exciton properties in 2D NPLs, including lateral size the 2D exciton (i.e. exciton center-of-mass coherent area); exciton in-plane transport mechanism; size and thickness dependence of bi-exciton Auger recombination rate, and optical gain mechanism and threshold. In this talk I will focus on the size, thickness and material dependence of bi-exciton Auger recombination rates. We show that In CdSe NPLs, the biexciton Auger recombination lifetime does not depend linearly on its volume, deviating from the “Universal Volume” scaling law that has been reported for 0D quantum dots. Instead, the Auger lifetime scales linearly with the lateral size, and the Auger lifetime depends sensitively (nonlinearly) on the NPL thickness. In CdPbBr3 1D nanorods and 2D NPLs, the biexciton Auger lifetimes increase linearly with the rod length and NPL lateral areas, respectively, and the lifetimes are much shorter than CdSe nanocrystals with similar volume. These observations can be explained by a model in which the Auger recombination rate for 1D nanorods (NRs ) and 2D NPLs is a product of binary collision frequency in the non-quantum confined dimension, and Auger probability per collision. The former gives rise to the linear dependence on the lateral areas in 2D NPLs and rod length in 1D NRs. The Auger recombination proability per collision depends on material property and the degree of quantum confinement, which gives rise to nonlinear dependence on the thickness of NPLs and diameter of NRs, as well as material dependence of Auger lifetimes. Thus, the Auger lifetimes of 2D NPLs and 1D nanorods deviate from the volume scaling law because of the different dependences on the quantum confined and non-confined dimensions. We believe that his model is generally applicable to all 1D and 2D materials.

Two-dimensional (2D) semiconductors are of a wide interest in recent years due to their unprecedented electrical and optical characteristics. The 2D endeavor, beyond the discovery of graphene, includes the study of inorganic van der Waals (vdW) transition metal dichalcogenides and solution based-2D semiconductors. Despite the striking electronic and optical properties of the mentioned 2D materials, those are lacking long-range magnetic properties or unique magnetic textures. The current study describes the exploration of a new family of semiconductor vdW compounds that possess magnetism along with their electrical and optical properties – these compounds are transition metal phosphorous trichalcogenides (TMPTs) with a honeycomb arrangement of magnetic metal elements.  Furthermore, diamagnetic TMPTs doped with magnetic impurities will be addressed - in comparison with diluted magnetic colloidal nanoplatelets from the II-VI family.

The TMPTs have a chemical formula MPX₃ (X=S, Se) with the metals (M) from the first row of transition metals, arranged in a honeycomb array with P-P at the center of each metal hexagon. These vdW materials permit the isolation of single layers down to a molecular limit via chemical or mechanical exfoliation. The 2D limit ease the Mermin-Wagner thermal agitation restriction and therefore, support intrinsic protected long-range ferromagnetic (FM) or antiferromagnetic (AFM) order, as well as spin textures (e.g., skyrmions) that are impossible in regular 3D materials. The honeycomb arrangement renders some of the TMPTs with a valley degree of freedom, similar to that found in MoS₂ single layers. Above all, TMPT layers permit coupling of long-range magnetic ordering or/and magnetic doping with the semiconducting properties of the materials.

The lecture will include description of experimental observation exposing the magneto-optical properties of FePS₃, MnPS₃ and Mn:ZnPS₃ TMPTs, and Mn-doped II-VI nanoplatelets.  The properties were investigated using circularly polarized magneto-photoluminescence at variable temperatures and optically detected magnetic resonance spectroscopy.  The preliminary observations indicated a removal of valleys' energy degeneracy by the coupling to the AFM magnetic arrangement.  Furthermore, Mn-doped diamagnetic TMPT showed a coupling between dopant and photo-generated carriers with a behavior similar to exciton-polaron found in doped colloidal II-VI nanoplatelets.  

Overall, the TMPT and magnetically doped 2D materials open a new paradigm in science and technology, from the basic understanding of magnetism, to the discovery of a plethora of new physical phenomena, thus being a base for the development of modern memory devices, spintronics, quantum computation and information. 

Colloidal nanoplatelets (NPLs) have become a promising class of semiconductor nanocrystals (NCs) for optoelectronic applications with their distinctly different optical characteristics [1]. They exhibit narrow emission linewidth, large absorption cross-section, giant oscillator strength, and suppressed Auger recombination. However, the first examples of core/shell NPLs synthesized by using a colloidal atomic layer deposition (c-ALD) approach suffered from the low photoluminescence quantum yields (PLQY), decreased crystallinity, and limited stability.  Here, to overcome this issue, we demonstrate a high-temperature shell growth approach that enables the synthesis of NPLs with controlled shell composition [2]. Our proposed CdSe quantum wells with a graded shell, which is composed of CdS buffer interlayer and Cd_xZn_1-xS gradient shell, exhibit highly bright emission (PLQY up to 89%) in the red spectral region (634-648 nm) with a narrow emission linewidth (down to 21 nm). With the smooth confinement potential of graded shell NPLs, hence further suppressing Auger recombination, we obtained a low threshold amplified spontaneous emission (~40 µJ/cm²) under nanosecond laser excitation. We also investigated the electroluminescent performance of graded shell NPLs in solution-processed light-emitting diodes (LEDs). Our NPL-LEDs showed a very high external quantum efficiency (EQE) value of 9.92% with high brightness up to ~46000 cd/m² at 650 nm. These findings show that by carefully designing heterostructures of anisotropically shaped colloidal NPLs, we could obtain highly efficient NPLs with enhanced optical properties to realize their superior performance in optoelectronic applications, overcoming the limitations of the spherically shaped NCs.

Colloidal nanocrystal superlattices are highly ordered aggregates of particles. Crystals are highly ordered aggregates of atoms. However, nanocrystal superlattices are not conventionally considered crystals. But where does the border lie? Previously, we reported that CsPbBr₃ nanocrystal superlattices have a structural perfection comparable with that of epitaxially grown multilayers, which can be considered as full-fledged single-crystals.[1]

In this talk, we will discuss a novel approach to the characterization of periodically stacked colloidal nanocrystals, which was inspired by diffraction experiments on multilayers grown by molecular beam epitaxy.[2] Our method takes advantage of optical interference phenomena arising from the superlattice periodicity, which enrich the profile of Bragg peaks in structural information. By fitting these profiles, collected with a common lab-grade diffractometer, we can extract structural information usually requiring high-end setups such as synchrotrons. Our approach is especially suitable for bidimensional colloidal crystals like nanoplatelets and nanosheets, because they spontaneously assemble into stacked periodic structures thanks to their highly anisotropic shape. However, we expect that our approach can be also extended 2D-layered organic-inorganic materials, which are not considered superlattices but share with them the periodic alternation of different layers.

To demonstrate our approach, we analyzed nanoplatelets of CsPbBr₃ and PbS measuring with high precision thickness, interparticle distance and even distortions in their atomic lattice. In addition, we demonstrated that such nanocrystal superlattices reach stacking displacements as small as 0.3-0.5 Å. This is comparable with atomic displacement parameters found in metal-organic bulk crystals, leading to intriguing questions. For example, how different is a stacking of perovskite nanoplatelets from a bulk crystal of a hybrid Ruddlesden-Popper perovskite? Can we study nanocrystal superlattices as they were bulk crystals? In the end, are nanocrystal superlattices a new class of hybrid organic-inorganic bulk crystals?

Colloidal semiconductor nanoplatelets (NPLs) exhibit strong quantum confinement only along the vertical direction, which can be controlled with atomic precision, and have received significant attention because of their narrow emission spectra and fast fluorescence lifetimes. Here we present a synthetic approach to obtain a ternary two dimensional (2D) architecture consisting of a CdSe core, laterally encapsulated by a type-I barrier of CdS, and finally a type-II outer layer of CdTe. The introduction of CdS leads to the formation of a tunneling barrier between CdSe and CdTe, which modulates the electron-hole overlap as well as the carrier relaxation dynamics. The modulation results in a type-II emission with an extended fluorescence lifetime in addition to the emission from CdSe and CdTe. The synthesis strategies allowed us to tune the indirect and direct transition energies and intensities as a function of the barrier and crown thickness. The different emission peaks of the core/barrier/crown (CBC) heterostructure are corroborated by the photoluminescence (PL) excitation spectroscopy and single particle PL measurements. Furthermore, experimental data are also supported by k.p calculations. To summarize, we have successfully synthesized and characterized CdSe/CdS/CdTe CBC heterostructures, demonstrating that colloidal 2D nanoplatelets offer great flexibility in designing opto-electronic properties toward targeted photonic applications.

Shakeup processes are partly radiative Auger processes whereby an electron-hole pair recombines but transfers a fraction of its energy to excite a third carrier, thus reducing the energy of emitted photons. Two very recent experiments in CdSe and CdSe/CdS core/shell nanoplatelets (NPLs) have hypothesized that such processes are responsible for the multi-peaked fluorescence spectrum observed in these structures at low temperatures.[1,2] Clarifying this point is important, because it would permit defining strategies to narrow down the emission line width of colloidal NPLs, thus improving their efficiency in optical applications where color purity is desirable.

Our work provides the first theoretical description on the origin and behavior of shakeup processes in colloidal nanostructures, defines strategies to control them and assesses on the interpretation of Ref.[1,2] experiments.  The conclusions are:

(1) Shakeup processes are indeed expected in colloidal NPLs charged with trions, unlike in previous colloidal nanostructures. The magnitude of the shakeup lines is strikingly large -over one order of magnitude larger than in epitaxial quantum wells-.

(2) We show that off-centered impurities are a requirement for the processes to take place, as they are needed to break symmetry conservation rules. In doing so, we reconcile two seemingly contradictory interpretations of the asymmetric lineshape of trion emission in core/shell NPLs.[1,3].

(3) We show that the multi-peaked emission in CdSe/CdS NPLs[1] cannot be explained in terms of shakeup processes only, and propose an altermative interpretation involving emission from metastable spin triplet trion states.

The optical absorptance A of a semiconductor layer is the ratio between the absorbed and incident energy. It was shown experimentally that, after corrections due to local-field effects, the absorptance of thin InAs layers is characterized by very clear steps corresponding to nA₀, where n is an integer and A₀ is the product of pi and the fine structure constant [1]. Remarkably, the quantum of absorptance was originally found for graphene monolayers, in a wide energy region [2]. In both cases, the explanation of this observation was provided on the basis of simplified calculations applied to a two-band model. In order to go beyond these approximations, we present atomistic multi-band tight-binding calculations of the absorptance of different types of semiconductor layers. We confirm that, in absence of strong excitonic effects, A is characterized by clear steps which can be related to A₀. The cases of layers of InAs and PbSe are studied in detail, taking into account the complex band structure of these materials. In the case of InAs, remarkable agreement with experiments is found. The origin of the quantization is discussed.

In this talk I will present two routes to computationally develop new photocatalysts. In the first one, layered noble metal chalconides and pnictonides [1], which show potential to be photocatalytically active, are exfoliated, and the resulting layers are investigated with respect to their properties, most importantly their stability and performance to (photo)catalyze hydrogen and oxygen evolution reactions in dependence on the pH and other factors. We have successfully applied this strategy recently to a series of noble-metal chalcogenides [2], phosphochalcogenides [3,4] and pnictonides [5].

In the second route, photoactive molecules, for example phorphyrin derivatives [6], are incorporated into synthetic framework materials such as metal-organic frameworks (MOFs) [7], where stacking provides additional band dispersion and supports charge carrier separation [8]. A similar approach is possible for covalent-organic frameworks (COFs) [9].

We present combined experimental and theoretical studies [1-3], demonstrating that CdSe nanoplatelets are a model system to investigate the tunability of trions and excitons in laterally finite 2D semiconductors. Our results show that the trion binding energy can be tuned from 36 meV to 18 meV with lateral size and decreasing aspect ratio, while the oscillator strength ratio of trion to exciton decreases.[3] In contrast to conventional quantum dots the trion oscillator strength in a nanoplatelet at low temperature is smaller than that of the exciton. The trion and exciton Bohr radii become lateral size tunable, e.g. from ~3.5 to 4.8 nm for the trion. This lateral tunability is practically independent of the transition energy, which is determined by the strong z-confinement in the colloidal wells. We show that dielectric screening has strong impact on these properties. By theoretical modeling of transition energies, binding energies and oscillator strength of trion and exciton and comparison to experimental findings we demonstrate that these properties are lateral size and aspect ratio tunable and can be engineered by the dielectric confinement. The trion binding energy can be tuned below or above the room temperature thermal energy. This allows e.g. together with the size tunable trion to exciton oscillator strength ratio (tunable by more than a factor of three) to suppress detrimental trion emission in devices by the choice of platelet size. [3]

We further show that e.g. the size tunable Bohr radii or wavefunctions together with the appropriate matrix elements for acoustic and polar phonon coupling result in a strong tunability of e.g. the exciton mobility and linewidth.[4] At low temperature e.g. the exciton mobility and diffusion coefficient show an platelet area dependence,  resulting from the inverse area scaling of the exciton-phonon scattering rate. The exciton mobility and diffusion coefficient become size and additionally lateral aspect ratio tunable.

Our results strongly impact further studies, as the demonstrated lateral size and aspect ratio tunable trion and exciton manifold is expected to influence properties like gain mechanisms, lasing, exciton-phonon interaction and transport at low temperature, but also even at room temperature due to the high and tunable exciton and trion binding energies.

Two-dimensional fluorescent colloidal nanocrystals combine the flexibility of solution-processed nanomaterials with the advantages of a (quasi-)2D band structure that offers enhanced optical properties compared to 0D quantum dots. In this presentation, I will discuss the synthesis of a novel ternary heterostructure, composed of a CdSe core, laterally extended by a CdS tunneling barrier, and finally a CdTe crown.[1] The type-II band offset between CdSe and CdTe, in combination with the CdS barrier layer, allows to separate core and crown electron and hole wave functions, yielding an emission spectrum consisting of a long-lived indirect transition at 625 nm, as well as direct CdSe and CdTe transitions around 510 nm and 575 nm, resp. Up to 2% of the total emission can be attributed to CdSe, and we were able to demonstrate two- and even three-photon fluorescence upconversion by exciting the sample with red and near-infrared photons, to yield green emission from the CdSe core.

Colloidal 2D nanosheets and nanoplatelets with thickness- and dimensionality-dependent properties are highly interesting for innovative optoelectronics in the visible and near-infrared.

The first part of my talk is focused on our work in tailoring the synthesis and optoelectronic properties of ultrathin lead chalcogenide nanoplatelets (NPLs). E.g., we have recently shown increased exciton binding energies in thin PbS nanosheets by time-resolved THz spectroscopy.[1] Here, I will disentangle the charge-carrier dynamics of coupled states in ultrathin PbS NPLs with enhanced near-infrared emission by transient absorption spectroscopy.

In the second part of my talk, I will focus on our progress in controlling the formation of ultrathin metallic and semiconducting transition metal dichalcogenide layers by wet-chemical methods.

Our work emphasizes the excellent usability of colloidal chemistry and time-resolved spectroscopy methods for producing tailor-made 2D materials.

[1] J. Lauth*, M. Failla, E. Klein, C. Klinke, S. Kinge, L. D. A. Siebbeles, Photoexcitation of PbS Nanosheets Leads to Highly Mobile Charge Carriers and Stable Excitons, Nanoscale 2019, 11, 21569-21576.