Nanostructured Semiconductors are a playing an increasingly dominant role for next-generation light-harvesting and light-emitting applications. In these materials, quantum confinement effects allow for enhanced control over their optoelectronic properties while reduced processing temperatures provide routes to more flexible integration. However, the reduced dimensionality and increased disorder can significantly impact the spatial dynamics of the energy carriers within the material.

To study these effects, we employ a series of time-resolved microscopy techniques which allow for a direct visualization of the excited state transport with few-nanometer and sub-nanosecond resolution. I will start the talk by giving an overview of some of the surprising effects that can be observed in the presence of energetic disorder using mixed-halide and doped 2D perovskites as an example. Halide mixing is one of the most powerful techniques to tune the optical bandgap of metal-halide perovskites across wide spectral ranges. However, halide mixing has commonly been observed to result in phase segregation, which reduces excited-state transport and limits device performance. Our results show that even in the absence of phase segregation, halide mixing still impacts carrier transport due to the local intrinsic inhomogeneities in the energy landscape. Using Mn-doping, we show how we can engineer local energy landscapes and derive detailed information about the trapping mechanisms of energy carriers.

In the last part of the talk, I will present our most recent efforts using interferometric scattering techniques. The exceedingly high signal-to-noise-ratio that interferometric scattering provides allows not only for the direct imaging of charge carriers, but also for super-resolution imaging of lattice dynamics. 
 

Understanding charge carrier dynamics and transport in halide perovskite semiconductors is essential for optimising their performance in optoelectronic applications, such as photovoltaics, light-emitting diodes, and photodetectors. Here, we employ transient photoluminescence microscopy and optical spectroscopy to directly visualize spatiotemporal carrier dynamics with high spatial and temporal resolution. We first study mixed lead-tin perovskites to elucidate the impact of compositional disorder on carrier transport. [1-5] We observe that increasing the tin-to-lead ratio raises the background hole concentration, while the most alloyed compositions exhibit the lowest diffusion coefficients, likely a consequence of alloy-induced disorder. Separately, we investigate quantum well structures to explore the effects of the quantum and dielectric confinement in thin films. [6-7] We find that confinement leads to a blue shift in optical transitions, enhanced excitonic character, and accelerated charge carrier recombination. Collectively, our results demonstrate how the spatiotemporal dynamics of charge carriers are affected by microscopic material disorder and macroscopic confinement effects, offering insights for the design of high-performance halide perovskite devices.

Solar energy conversion to produce hydrogen using photocatalysis is a promising route for sustainable energy production. However, disordered polymeric photocatalyst systems, such as carbon nitride (CN_x), suffer from low solar-to-hydrogen efficiencies due to rapid recombination of photogenerated charges and interlayer electrostatic barriers, which impede charge transfer to surface reaction sites. Transient Absorption Spectroscopy (TAS) is routinely used to explore these dynamics, revealing insights into properties such as charge carrier lifetimes and trapping in these photocatalysts. However, conventional TAS measurements typically rely on large probe sizes (millimeters to centimeters), which average out the effect of spatial heterogeneity in local charge carrier trapping behaviour. To overcome this limitation, a home-built Transient Absorption Microscopy (TAM) setup was developed, in which we studied the transient absorption behaviour of single CN_x particles with probe beams that allowed micron-level spatial resolution. In our technique, we measure charge carrier dynamics in the μs–s timescale, which is particularly relevant for water splitting reactions. These reactions dispaly slower kinetics, making it essential to probe trapping and detrapping phenomena within this regime. For the first time, μs–s carrier dynamics were explored within individual CN_x particles, revealing: a) significant particle-to-particle heterogeneity in trapped charge densities, and b) spatial heterogeneity in half-lives of the trapped charge populations within the same particle.

With this evidence, we were able to indicate the presence of at least two distinct spatial defects controlling charge trapping in CN_x [1]. Additionally, we proceeded to spatially investigate the charge trapping behaviour of these charges when a cocatalyst, such as Pt is deposited. Pt deposition was found to extend the charge carrier half-lives by around threefold, and Pt displayed a preference for binding in areas with the lowest initial lifetimes [2]. These findings suggest that local chemical environments independently influence charge trapping, which dictates phenomena such as cocatalyst deposition. Our spatiotemporally resolved TA offers a powerful approach for understanding defects, probing operando chemical reactions, and hence laying the foundation for the optimal design of efficient photocatalysts for solar energy conversion using systems like CN_x.

Achieving ballistic charge and energy flow in materials at room temperature is a long-standing goal that could unlock ultrafast, lossless energy and information technologies. The key obstacle to overcome is short-range scattering between electronic particles and lattice phonons. I will describe two promising avenues for realizing ballistic transport in two-dimensional (2D) semiconductors by harnessing hybridization between electronic particles and long-wavelength excitations. First, I will show that non-perturbative interactions between electrons and delocalized phonons in flat-band materials can result in the formation of 2D acoustic polarons. These polarons are protected from scattering, resulting in sustained ballistic transport over macroscopic spatiotemporal scales at room temperature, a remarkable phenomenon we are beginning to harness in electronic devices. I will then focus on hybridization between semiconductor excitons and light to form polaritons, demonstrating that these hybrid quasiparticles display long-range ballistic transport at light-like speeds even in the presence of finite interactions with lattice phonons. I will conclude with new prospects for leveraging polaritons to control material function even in the absence of illumination. In all cases, we develop ultrafast optical imaging capabilities enabling us to track the propagation of these quasiparticles with femtosecond resolution and few-nanometer sensitivity, providing precise measurements of transport dynamics and sensitivity to both static and dynamic disorder.

Perovskite materials have gathered significant attention due to their remarkable optoelectronic properties and potential applications in various fields. Some synthetic methods offer control over the size and shape of perovskite nanostructures [1], facilitating a comprehensive exploration of their optical properties.

This study explores the synthesis and characterization of perovskite materials with a particular emphasis on diverse dimensionalities, especially nanowires. The one-dimensional nature of perovskite nanowires can provide excellent electrical, optical, and physical properties, such as improved light trapping, lower defect density, longer photocarrier lifetime and better mechanical properties [2]. As such, they have been used for solar cells, photodetectors and LEDs, among others [3]. The key question then is understanding how excitation travels through the material and whether this behavior differs from that in the bulk material solely due to the shape.

Using Transient Photoluminescence microscopy,[4] we can directly visualize the spatial movement of energy carriers with sub-nanosecond and few-nanometer resolution in CsPbBr₃ nanowires grown by hot-injection synthesis . Our results reveal efficient transport of energy carriers with high anisotropy imposed by the dimensionality of the structure. Power dependent spectroscopy moreover reveals that energy transport is dominated by free charges.

To further study the influence of morphology on the optoelectronic properties of perovskite nanowires, we use scanning-probe of an atomic force microscope (AFM) to manipulate the nanowires and create complex nanostructures [5]. For efficient manipulation with AFM, we have developed different methods to fine-tune the substrate interaction, including the use of self-assembled monolayers and optimization of the wire surface using oleic acid saturation. We will discuss how mechanically induced strain can influence the transport properties of the nanowires, correlating these effects with high resolution TEM imaging.  

These studies provide crucial insights into the dynamic behavior of energy carriers within these materials and their relationship with the local morphology. The findings from this study not only deepen our understanding of perovskite nanomaterials but also pave the way for their use in advanced optoelectronic devices and applications.

We present an in-situ Lorentz Transmission Electron Microscopy (LTEM) investigation into the cumulative effects of pulsed laser excitation on the metamagnetic transition in freestanding FeRh thin films. Our experiments demonstrate that repeated ultrafast laser pulses progressively introduce defects and associated residual strains, which promote the ferromagnetic phase and change the nucleation process from a homogeneous to a heterogeneous mechanism. This evolution is marked by a significant 20 K reduction in transition temperature, a nearly 50% decrease in the laser fluence threshold required for nucleation, and the emergence of magnetic vortices as dominant nucleation sites that emanate from strain fields of dislocation networks. Complementary thermal modeling and defect analyses identify laser-induced strain and rapid thermal quenching as critical factors promoting defect formation and stabilization. We also conducted ultrafast pump-probe LTEM experiments to further explore the nature of the unique antiferromagnetic to ferromagnetic transition in FeRh. These UTEM experiments aim to solve a long-standing fundamental "chicken-and-egg" dilemma about whether the magnetic order change initiates the structural lattice transformation or structural rearrangements trigger the magnetic phase change. These findings establish a clear connection between defect formation, nucleation energetics, and the microscopic structure of the emerging ferromagnetic phase, offering valuable insights for ultrafast stroboscopic imaging and defect-driven phase transitions in functional materials.

Three-dimensional (3D) morphology and composition govern the properties of nanoparticles (NPs). However, due to their high surface-to-volume ratio, the morphology and composition of nanomaterials are not as static as those for their bulk counterparts. One major influence is the increase in relative contribution of surface diffusion, which underlines rapid reshaping of NPs in response to changes in their environment. If not accounted for, these effects might affect the robustness of prospective NPs in practically relevant conditions, such as elevated temperatures, intense light illumination, or changing chemical environments. In situ techniques are promising tools to study NP transformations under relevant conditions. Among those tools, in situ transmission electron microscopy (TEM) provides an elegant platform to directly visualize NP changes down to the atomic scale. By the use of specialized holders or microscopes, external stimuli, such as heat, or environments, such as gas and liquids, can be controllably introduced inside the TEM. However, standard TEM yields two-dimensional (2D) projection images of 3D objects.With the growing complexity of NP shapes and compositions, the information that is obtained in this manner is often insufficient to understand intricate diffusion dynamics. In this contribution, I will describe recent progress on measuring NP transformations in 3D inside the electron microscope.

By taking advantage of adjustable short-range attractive interactions of electrostatically stabilized colloidal nanocrystals, we demonstrate an unusual degree of control over the phase behaviour of a nanoscale system, studied via in situ small angle X-ray scattering (SAXS). This control is exemplified through the use of a metastable liquid intermediate state that enables varying the colloidal crystallization rate by over three orders of magnitude, along with predictive control of crystal yield, size, and crystallinity. Most strikingly, we reveal that crystallinity can be increased simultaneously with the crystallization rate.

To further elucidate how the short-range interactions dictate the phase behaviour at the nanoscale, we also resolve the microscopic dynamics of colloidal suspensions and liquid droplets of the nanocrystals via MHz X-ray photon correlation spectroscopy (XPCS). The attractive interactions suppress self-diffusion in the liquid state, suggesting design rules for the shape of interaction potentials not only to leverage liquid intermediates in crystallization processes but also to avoid gelation for better control of phase behaviours.

Finally, we show how light absorption and the associated photochemistry systematically alters the system phase behaviour through a combination of optical transient absorption spectroscopy, time-resolved wide-angle X-ray scattering, MHz XPCS, and in situ SAXS. Results suggest electrostatically driven solvation shell redistribution that renormalizes binodal curves on the nanocrystal phase diagram. Ultimately, the multiscale characterization of and manipulation of electrostatically stabilized nanocrystals paves the way to more clearly explain the design rules for nanoscale interaction potentials so that nanomaterial assemblies can achieve more effective functionalities via deterministic and predictive control.

Understanding processes at the nanoscale is essential for the development of next-generation materials. However, conventional techniques often lack the spatial and temporal resolution to probe dynamic changes in individual nanostructures. Interferometric scattering (iSCAT) microscopy overcomes this limitation by enabling label-free optical detection of nanoscale processes with high sensitivity and millisecond time resolution [1, 2]. Its versatility makes it well suited for studying dynamic phenomena across a wide range of material systems at ambient conditions, offering insight into structural, optical, and electrochemical behavior at the single-particle level.

Our work leverages iSCAT to track nanoscale transformations in real time across chemically, optically, and electrochemically active systems. Despite the diversity of materials, the unifying goal is to understand how individual nanostructures evolve and function during key processes relevant to material performance.

We have applied iSCAT across a diverse set of materials challenges to uncover dynamic behavior in real time. In the context of chemical synthesis, iSCAT enables direct visualization of the nucleation and growth of covalent organic frameworks (COFs) [3], revealing kinetic features that can guide more controlled synthesis. For optoelectronic nanomaterials, combining iSCAT with photoluminescence imaging allows in situ characterization of individual perovskite nanocubes, yielding correlative measurements of both size and photoluminescence quantum yield (PLQY) for hundreds of particles. This high-throughput single-particle approach reveals structure–property trends relevant for improving device performance.

To explore electrochemical processes, iSCAT has been coupled with an electrochemical cell to achieve real-time visualization of ion intercalation and structural evolution in individual flakes of MXenes—two-dimensional transition metal carbides, carbonitrides, and nitrides relevant for energy storage applications [4]. These measurements reveal early-stage morphological restructuring that precedes and strongly influences the intercalation behavior. Such nanoscale insights provide a mechanistic understanding of how structural changes govern electrochemical performance and contribute to long-term material degradation and instability.

The study of carrier transport has advanced significantly in recent years with the emergence of a series of transient microscopy techniques that allow for imaging of carriers with few-nanometer and sub-nanosecond resolution¹. The access to time-resolved information of carrier transport provides critical insight into the different transport regimes that carriers may experience during their lifetime. Traditional transient microscopy techniques have relied on photoluminescence^2,3 or absorption⁴ to generate contrast; however, these techniques have major disadvantages due to their reliance on bright samples or high excitation powers, respectively.

Recently, transient scattering microscopy (TScM) has emerged as an alternative to these techniques^6,7, it relies on interference and small changes in the refractive index to generate contrast, as such it is not dependent on photoluminescence and achieves high SNR even at low excitation powers. Importantly though, the sensitivity of TScM to different species of carriers can complicate the interpretation of results, highlighting the need to develop models specifically tailored to TScM.

In this work, we perform TScM on bulk TMDCs. We observe exciton transport characterized by a fast-moving free exciton population and a slow trapped population. We find that the resulting spatial distribution deviates from a gaussian distribution. To better interpret these features, we perform simulations incorporating both shallow traps and Auger recombination and quantify these deviations by analyzing how the kurtosis of the distributions evolves over time.

Our findings show that, except at high excitation fluences, exciton dynamics are well-described by the shallow trap model. This explains the observed non-Gaussian behavior and emphasizes the need for analysis methods beyond traditional Gaussian fitting.

In conclusion, we combine experiment and simulation to demonstrate how multispecies populations affect exciton transport in TScM and introduce modeling approaches that allow us to obtain detailed information about the excited states dynamics in these materials.

Investigating the properties of quasi-particles and their dynamics in a specific environment is an essential part of tailoring functionality in device fabrication. Nonlinear optical spectroscopy, in particular transient absorption, has become a standard tool for probing ultrafast single quasi-particle dynamics by tracking the evolution of the probe pulse spectrum after photoexcitation with a pump pulse. As an extension of this approach, multidimensional electronic spectroscopy enables the simultaneous temporal and spectral resolution of pump and probe interactions so that the spectral overlap of signatures attributed to, e.g., energy transport processes and coupling mechanisms, can be resolved [1].

As most devices operate through solid interfaces, quasi-particle dynamics are usually studied in the solid state. In conventional nonlinear spectroscopy, the signal represents an average over structural inhomogeneities and nanostructured regions of the interface. We have therefore adapted the concept of transient absorption for fluorescence-detected measurements in a light microscope to follow the exciton dynamics of a single molecule [2]. Beyond that, some of us have introduced 2D nanoscopy [3–6], a combination of multidimensional electronic spectroscopy with photoemission electron microscopy (PEEM), to study exciton and surface-plasmon polariton dynamics, coupling and energy transfer processes in extended systems on a spatial scale of a few nanometers.

Because focused beams concentrate light into a small volume, they produce high light intensities that can generate multiple quasi-particle excitations which can interact with each other. Molecular excitons, for instance, can undergo exciton–exciton annihilation, i.e., a process that hinders charge carrier transport in optoelectronic devices. Multiple excitations at high light intensities, which are usually intended to provide a better signal-to-noise ratio, have plagued nonlinear spectroscopy methods for decades. In other cases, many-particle interactions are an essential part of physical systems, e.g., singlet fission, exciton–phonon interactions or Bose–Einstein condensation. Some of us have therefore recently developed the intensity cycling method, which makes it possible to experimentally separate the individual terms of the Taylor expansion in nonlinear light–matter interaction by taking specific linear combinations of measurement data recorded at different excitation intensities [7]. In this way, the signal contribution of N interacting particles can be isolated from the rest of the signal. The method, introduced using transient absorption, is universal and does not depend on the sample system. In addition, the method has just been adapted to multidimensional electronic spectroscopy [8].

Here, I will report on our progress to extend the intensity cycling method to our PEEM setup [9] in order to study quasi-particle interactions with high spatial resolution. In our first steps, we look for exciton–exciton annihilation, a measure for exciton diffusion, in a 5 nm thick film of terrylene bisimide (TBI) molecules on Si(100). As an excitation sequence, we use a 680 nm pump pulse and a 340 nm probe pulse for photoemission.

Time-resolved variants of transmission electron microscopy have started to provide an unparalleled view into the fast and ultrafast dynamics of solid-state nanostructures. A crucial instrumental pre-requisite for constructing the next generation of time-resolved electron microscopes is the development of novel pulsed electron sources, fast detectors and versatile sample excitation schemes. In the first part of the talk, our recent development of a novel laser-driven cold-field emitter source is described [1]. The properties of extracted photoelectron pulses, including the achieved electron pulse duration, spectral width, and electron beam brightness, are characterized in detail, and the advantages of aberration-corretected ultrafast transmission electron microscopy are discussed. 

The second part focusses on the application of event-based TimePix3 electron detectors for the time-resolved probing of nonlinear structural dynamics in nanoscale resonatorss. We demonstrate the phase-resolved mapping of nonlinear Duffing modes in a silicon membrane resonator with quality factors exceeding 10⁵. Higher harmonics of the driving frequency are observed in the structural response, indicatig the emergencs of multi-mode coupling channels with large effective nonlinearities. At the largest driving strengths, periode doubling bifurcations emerge highlighting the onset of temporal symmetry breaking in a simple repetitively driven nanoscale system.

Water’s anomalous behavior becomes especially pronounced in the deeply supercooled regime, where rapid crystallization has long prevented direct structural investigation. In this talk, I present time-resolved electron diffraction experiments inside a TEM that define the kinetic boundaries separating vitrification, crystallization, and metastable liquid persistence, providing new insight into water’s structure in the so-called no man’s land.[1,2]

Using shaped microsecond laser pulses, we precisely measured the critical cooling rate required to vitrify pure water as 6.4 • 10⁶ K/s, thereby resolving long-standing discrepancies in the literature.[3] In contrast, flash-heating amorphous solid water reveals that crystallization can still occur at significantly higher heating rates, unless they exceed a critical threshold of approximately 10⁸ K/s.[4] These results establish both the lower and upper kinetic limits for maintaining water in an amorphous or liquid state.

Structural analyses further show how water transitions between the liquid and glassy states, and how these transitions differ between H₂O and D₂O. Deuterated water consistently exhibits greater structural order and sharper transitions, underscoring the impact of nuclear quantum effects on hydrogen bonding in water.

Our experimental approach offers a new access to water’s supercooled regime and helps unravel the dynamic and structural limits that govern its behavior in this metastable state.

Spatiotemporal microscopy (SPTM) techniques enable the probing of the evolution of photogenerated energy carriers in both space and time [1]. Through the application of ultrafast lasers and time-resolved detection methods, these techniques reach the femtosecond to picosecond time scales of the natural transport phenomena. Nonetheless, in the spatial domain, SPTM is still fundamentally diffraction-limited. Although sensitive to nanoscale signal variations, the diffusion information is averaged over many hundreds of nanometers, preventing the access to the real scale of exciton transfer, where diffusion lengths typically range from 10 to 100nm [2]. To bridge this size gap and access the real nanoscale structure of materials, we present a method to localize the excitation down to a few tens of nanometers 

Our method is based on a nanostructured platform consisting of rectangular nanoslits fabricated through electron-beam lithography on an opaque aluminum thin film on a glass cover slide. Upon illumination, only the light transmitted through a slit reaches the sample on the platform, resulting in a background-free excitation spot confined to the dimension of the slit – as small as 50nm.

Here we show the first successful proof-of-concept experiments using this near-field approach to study the exciton dynamics in the organic semiconductor Y6. By combining this platform with a photoluminescence SPTM configuration, we retrieve the diffusivity constant, the luminescence lifetime, and the diffusion length. Varying excitation fluence and sub-wavelength slit dimensions we easily reached the fluence regime where nonlinear effects, such as singlet-singlet annihilation (SSA), become negligible. As a result, consistent diffusion constants were obtained.

In conclusion, we present a versatile platform where the limits of excitation localization are no longer imposed by diffraction, but only by the fabrication capabilities at hand. Given the cover slide-based design, it is easy to implement with other experimental setups, such as pump-probe SPTM, and thus not limit the sample pool to photoluminescent materials only. Furthermore, the expansion of these sub-diffraction capabilities also to the detection should be possible by combining the slits with nanoantennas strategically fabricated in their vicinity. These will act as localized detection spots, leading to the next generation of ultra-high resolution and sensitivity spatiotemporal studies.

Ultrafast transmission electron microscopy (UTEM) provides access to dynamics in heterogeneous nanomaterials by implementing laser-pump electron-probe spectroscopy, diffraction, and imaging [1]. In particular, tailored optical interactions offers unique insights into nanophotonic systems and promise the coherent control of free electrons and material excitations [2].

Here, I will discuss new opportunities in time-resolved and ultrafast electron microscopy for the study of attosecond phase-resolved optical dynamics and integrated photonics systems. Incorporating electron energy gain spectroscopy (EEGS) and correlated single-particle detection, we implement high-precision photonic mode imaging and establish a novel free-electron-driven quantum light source.

In a first line of experiments, photon-induced near-field electron microscopy (PINEM) [3] using nanometer-focussed electron beams [4] enables mode-resolved analysis of plasmonic nanocavities, and consecutive mixing with a phase-locked reference gives access to attosecond field-driven dynamics [5].

Secondly, high-Q integrated photonic microresonators facilitate efficient electron-light interaction even using a continuous electron beam [6]. High-frequency detuning of the exciting laser and nanosecond detection of electron spectra enable μeV-electron spectroscopy and imaging the buildup of dissipative Kerr solitons (DKSs) [7].

Building on this platform, even the single particle electron-photon interaction becomes accessible, enabling the generation of electron-photon pair [8], and photon Fock states [9] at an empty cavity, which we monitor by nanosecond resolved correlation spectroscopy.

Ultimately, tailored electron-light interactions and the ability to induce highly correlated multi-electron/photon states may provide new avenues in electron microscopy, including new contrast mechanisms and enhanced sensitivity. Establishing single-particle coupling is pivotal for the emerging field of free-electron quantum optics, promising hybrid quantum technology that fuses free electrons and light.