Colloidal lead halide perovskite (LHP) nanocrystals (NCs), with bright and spectrally narrow photoluminescence (PL) tunable over the entire visible spectral range, are of immense interest as classical and quantum light sources. Severe challenges LHP NCs form by sub-second fast and hence hard-to-control ionic metathesis reactions, which severely limits the access to size-uniform and shape-regular NCs in the sub-10 nm range. We show that a synthesis path comprising an intricate equilibrium between the precursor (TOPO-PbBr₂ complex) and the [PbBr₃^-] solute for the NC nucleation may circumvent this challenge [1]. This results in a scalable, room-temperature synthesis of monodisperse and isolable CsPbBr₃ NCs, size-tunable in the 3-13 nm range. The kinetics of both nucleation and therefrom temporally separated growth are drastically slowed, resulting in total reaction times of up to 30 minutes. The methodology is then extended to FAPbBr₃ (FA = formamidinium) and MAPbBr₃ (MA = methylammonium), allowing for thorough experimental comparison and modeling of their physical properties under intermediate quantum confinement. In particular, NCs of all these compositions exhibit up to four excitonic transitions in their linear absorption spectra, and we demonstrate that the size-dependent confinement energy for all transitions is independent of the A-site cation.

   We then show that this synthesis – relying on the labile ligand capping with TOPO-phosphinic acid mixture – makes for a convenient platform for the subsequent surface functionalization with diverse capping ligands [2]. Robust surface functionalization of highly ionic surfaces, as is the case of LHP NCs, has remained a formidable challenge due to the inherently non-covalent weak surface bonding. Leveraging the vast and facile molecular engineering of phospholipids, we present their efficacy as surface capping ligands for LHP NCs. Molecular dynamics simulations and solid-state NMR confirm that the surface affinity of these zwitterionic molecules is primarily governed by the geometric fitness of their anionic and cationic moieties. Judicious selection of the ligands yielded colloidally robust FAPbBr₃ and MAPbBr₃ NCs and enabled colloids in a variety of solvents, from n-hexane to acetone. Robustness of the surface capping is also reflected in optical properties: NCs exhibit PL quantum yield (QY) above 96% after numerous purifications. NCs are essentially blinking-free at a single particle level.

1. Q. Akkerman et al. submitted

2. V. Morad et al. submitted

The flexibility of lead-halide perovskites plays a crucial role in their functional response, but the exact structural distortions and their effects on physical properties are still unclear [1]. Here, we provide for the first time the description of the photoinduced and thermally-activated structural distortions in CsPbBr₃ perovskite nanocrystals with atomic-level precision. We combine time-resolved X-ray absorption spectroscopy (TR-XAS) and ab initio simulations to show that the photoexcitation of CsPbBr₃ nanocrystals (NCs) leads to well-defined polaronic lattice changes, rather than photoinduced structural phase transitions. Additionally, our analysis rules out thermal effects in the photoactivation of the system [2]. We also investigated the purely thermal response of CsPbBr₃ with temperature-dependent XAS and first principles molecular dynamics (MD) simulations across its phase diagram. We show that the thermally-activated lattice cannot be reduced to a cubic average structure, because of the presence of dynamically distorted local configurations and lattice anharmonicity [3].

Our comprehensive investigation demonstrates that the structural changes of CsPbBr₃ NCs induced by light and thermal functional triggers have fundamentally different physical origins. Even though both effects are related to the flexibility of the perovskite lattice, the photoexcitation is selectively driven by electron-phonon coupling, while the thermal activation drives lattice phonon anharmonicity. The latter leads to significant distortions from the CsPbBr₃ space- and time- average lattice symmetry, and it is well rationalized by the soft-mode model in the framework of displacive thermal phase transitions.

These finding clarify the underlying mechanisms of the lattice response under functional activation and offers strategies to control the perovskite nuclear degrees of freedom with different external stimuli. Understanding the thermal processes acting at the atomic-level represents the first step toward a rational design of perovskite-based devices with improved stability.

Traditionally defects have been considered detrimental for the performance of photoactive devices.  For example, in photocatalysts point defects introduce trap states that act as recombination centres lowering quantum yields. However, recent advances in spectroscopy, microscopy and modelling, have opened the door to ground-breaking studies which are challenging our views of disorder. New evidence suggests that defects could play a much more active role than anticipated by controlling parameters as important as the reaction selectivity. The role of disorder is particularly critical in photocatalysis where the act of photoexcitation can, in itself, generate dynamical defects in the active phase.  In this talk I will discuss the many different shapes and forms in which disorder can emerge in photo-electrocatalysis. In particular, I will focus on the formation of electronic defects and structural reconstructions during catalytic operation.  I will present examples of how disorder-associated phenomena can be monitored in situ and operando and how it can be correlated with catalytic function. Moreover, by drawing parallelisms with the biological world I will discuss if disorder is unavoidable to achieve high performance.  Finally, I will contrast the role of disorder in photocatalysis with observations made in other photoactive materials in order to highlight potential routes to use disorder in our favor.

Photocatalytic solar fuel production provides a potential alternative to sustainable energy production. Whilst attention to date has focused on inorganic photocatalysts, carbon-based materials and organic semiconductors have emerged as potential low cost and efficient photocatalyst for hydrogen evolution, mainly due to the tunability of their properties through synthetic control.^[1,2] This allow the design of families of materials with tuned opto-electronic properties by incorporating different building blocks.^[1]  The best performing systems are bulk heterojunctions nanoparticles prepared from a blend of conjugated polymer donor and non-fullerene small molecules acceptor, particularly due to their improved light absorption in the visible range.^[2,3] Despite the efficient performance of the donor/acceptor bulk heterojunction photocatalysts for hydrogen evolution, the fundamental understanding of the photophysical processes that determine their performance remain limited. In this presentation, I will discuss the charge carrier dynamics of donor/acceptor heterojunction nanoparticles photocatalysts with different hydrogen evolution activity.[2,3] Transient and operando photoinduced absorption spectroscopies, on timescales of femtoseconds to seconds after light absorption, were employed to monitor the kinetics of photogenerated charges and their correlation with photocatalytic performance.[3] Differences between the function of Donor/Acceptor bulk heterojunction photocatalysts and single conjugated polymers photocatalyst will be discussed. These results can provide design guidelines towards efficient organic semiconductors photocatalyst.

Semiconductor nanoparticles (NPs) have attracted a great deal of interest in the last decades due to the increasingly range of applications. However, NPs were produced usually in batch processes and the production on a larger scale is still limited. Among n-type semiconductors, Bismuth vanadate (BiVO₄) has emerged as one of the most promising metal oxides as photoanode for water oxidation due to its moderate band gap (2.4 eV), adequate band positions, low-cost, good chemical stability in aqueous medium and environmental inertness.^[1] In this regard, our group designed and studied different methodologies for the preparation of this type of photoelectrodes.^[2] However, the large-scale synthesis of this material is still limited to the use of conventional batch processes, and it is necessary to achieve high-production yields due to the increasing demand in the field of energy materials. Since the automatization of this process has significant advantages with respect to cost, safety and more precise control of the physical and chemical properties, the emergence of continuous flow technology offers an alternative for preparing inorganic nanoparticles (NPs).^[3]

In this communication we show the preparation of BiVO₄ NPs by using a simple continuous-flow method based on two liquid peristaltic pumps. The proposed system allows to up-scale the synthesis through a microreactor and contributes to the possible production of large-scale photoelectrodes.

Two-dimensional (2D) VA group materials, including black phosphorene, bismuthene, and antimonene, attract considerable attention in energy devices because of their unique properties, such as tunable bandgap and superhigh carrier mobility. However, their application in photoelectrochemical devices is rarely reported. In this presentation, we present the successful design of semiconductor /2D VA material/ co-catalyst composite photoanodes, such as BiVO₄/bismuthene/NiFeOOH, BiVO₄/bismuthene/CoPi, and BiVO₄/Sb/NiFeOOH. Specifically, the BiVO₄/bismuthene/NiFeOOH photoanode that achieves 3.7 mA cm^-2 at 0.8 V_RHE are deeply investigated. Comprehensive (photo)electrochemical techniques and surface photovoltage were comprehensively employed to identify the roles of bismuthene and NiFeOOH on BiVO₄. We found that bismuthene increases the density of V_O of BiVO₄ that are beneficial for the oxygen evolution reaction via the formation of oxy/hydroxyl-based water oxidation intermediates. Moreover, bismuthene increases interfacial band bending and fills the V_O-related electron traps with charges, leading to more efficient charge extraction. The NiFeOOH significantly increase the surface hole concentration by passivating the r-ss. The use of 2D VA group materials and co-catalysts as functional layers opens new avenues to tune the surface properties of photoanodes for water oxidation.

  How does the publishing process work behind the scenes? Learn more about what editors look for when they assess manuscripts and hear how the Royal Society of Chemistry is supporting open access publishing.

  How does the publishing process work behind the scenes? Learn more about what editors look for when they assess manuscripts and hear how the Royal Society of Chemistry is supporting open access publishing.

  How does the publishing process work behind the scenes? Learn more about what editors look for when they assess manuscripts and hear how the Royal Society of Chemistry is supporting open access publishing.

  How does the publishing process work behind the scenes? Learn more about what editors look for when they assess manuscripts and hear how the Royal Society of Chemistry is supporting open access publishing.

  How does the publishing process work behind the scenes? Learn more about what editors look for when they assess manuscripts and hear how the Royal Society of Chemistry is supporting open access publishing.

Photoelectrochemical energy conversion from sunlight directly into chemical bonds is a promising path to fabricate alternative solar fuels. But a single semiconductor is not capable to provide enough voltage for the chosen electrochemical reaction and/or to do so with significant productivities. Tandem PEC/PV monolithic devices can generate the necessary voltage meanwhile increasing overall productivity.

BiVO₄, one of the most promising large bandgap (~2.5 eV) photoanode candidates, has been fabricated in tandem with non-fullerene semi-transparent organic (OPV) based thin-film third generation photovoltaics. Although OPVs allow for fine-selection of absorber composition and thus, absorption spectra, the balance is not optimal. Nanophotonic structures are necessary to balance the photon influx to maximize the overall series current in the device.

In this talk we will explain how the many-layers device has been computationally modelled and the nanophotonic structures incorporated, and how the full device has been fabricated and tested at ICFO. The light path for the blue part of the solar spectra has been maximized in the BiVO₄, and the PV part has been adequately selected so that the device provides the necessary voltage for either water splitting or CO₂ reduction. Several parameters have been taking into account in this model, such as the photovoltage loss of the solar cells under reduced photon influx due to the light management.

This many-layers tandem PEC/PV structure can provide >2.5 V of photovoltage and the 1D light management structures have demonstrated 2x increase of the bias-free photocurrent.

In recent years, bismuth-based photocatalysts have been receiving increasing attention in photocatalysis, due to their appropriate bandgap and tunable surface structure, which make them suitable also for the photocatalytic reduction of CO₂¹. Their performances, however, are still limited by the fast charge carriers recombination. Recently, the exploitation of piezo/ferro-electric potentials in photo-active semiconductors has been adopted as an effective strategy to modulate the charge transfer properties both in the bulk phase and at the surface of semiconductors (i.e. piezo-phototronic effect)². In this perspective Bismuth-based Aurivillius compounds, owing to their usually strong spontaneous ferroelectric polarization represent a promising option as cell photo-electrodes, allowing the increase of cell efficiency by electrically polarizing the materials. In addition, thanks to their unique layered structure, this peculiar class of perovskites allows the migration of photo-generated holes and electrons within different areas of the materials, thus intrinsically facilitating charge separation.

In this work, the utilization of different Aurivillius compounds (i.e. Bi₄Ti₃O₁₂ – BiTO, and Bi₂MoO₆ - BiMO) as photo-electrode materials for solar conversion has been studied in detail. The use of BiTO and BiMO for the photocatalytic reduction of CO₂ has recently been reported, however, the main strategies to develop optimized ferroelectric-enhanced photo-electrodes of these materials and their utilization for the ferroelectric potential-assisted CO₂ reduction has never been fully investigated. BiTO and BiMO photo-electrodes were fabricated and accurately optimized to obtain both hierarchically oriented nanostructures with different morphologies (i.e. nanosheet/nanorod arrays), and thin-film layers via in situ hydrothermal deposition and through a sol-gel/spin coating coupled process respectively. The effect of the ferroelectric potential on the photo-electrochemical performances of the optimized photo-electrodes with different architectures was therefore accurately studied. Density current increments and enhanced charge transfer ability were registered under the optimal ferroelectric polarization which was directly reflected in the CO₂ photo-electrochemical reduction performances. This work, therefore, demonstrates the possibility to adopt ferroelectric polarization coupled with an external bias to effectively control the migration of photo-generated charges in bismuth-based Aurivillius semiconductors for the CO₂ photo-electrochemical reduction.

Photoelectrochemical cells (PECs) have been developed as environmentally friendly systems that can directly utilize photogenerated electron-hole pairs for water splitting, fuel production, conversion of carbon dioxide, and pollutant degradation. Most reports on the photocatalytic or PEC hydrogen (H₂) evolution via water splitting have focused on the H2 reduction half-reaction by generating on the photoanode oxygen or using sacrificial agents to consume the generated h+. Lately, much effort has been invested into synthesizing valuable chemicals on the photoanode while retaining the production of H₂ on the cathode.

Over the past few years, polymeric carbon nitrides (CN) have attracted widespread attention due to their outstanding electronic properties, which have been exploited in various applications, including photo- and electro-catalysis, heterogeneous catalysis, CO₂ reduction, water splitting, light-emitting diodes, and PV cells. CN comprises only carbon and nitrogen, and it can be synthesized by several routes. Its unique and tunable optical, chemical, and catalytic properties, alongside its low price and remarkably high stability to oxidation (up to 500 °C), make it a very attractive material for photoelectrochemical applications. However, few reports regarded CN utilization in PECs due to the difficulty in acquiring a homogenous CN layer on a conductive substrate and our lack of a basic understanding of the intrinsic layer properties of CN.

This talk will introduce new approaches to growing CN layers with altered properties on conductive substrates for photoelectrochemical applications. The growth mechanism and their chemical, photophysical, electronic, and charge transfer properties will be discussed. I will show the utilization of PEC with a CN-based photoanode as a stable and efficient platform for the oxidation of water or organic molecules to oxygen or added-value chemicals, respectively, with hydrogen co-production.

Many metal oxide semiconductors, such as TiO₂, require UV excitation in order to drive photocatalytic reactions. However, UV photons account for only 3-5% of the solar spectrum, and thus strategies to extend the range of harvested photons into the visible wavelength range are required. One potential way is to use triplet-triplet annihilation photon up-conversion (TTA-UC). TTA-UC systems generate higher energy photons from lower energy, low intensity, and non-coherent excitation. In this work, we synthesized several 9,10-diphenylanthracene derivatives and attached them to different wide-bandgap metal oxide nanoparticles, TiO₂, ZrO₂, and CeO₂, which were then dispersed in solutions containing triplet sensitizers. Triplet energy transfer leading to triplet-triplet annihilation photon up-conversion and charge generation were probed through steady-state and transient spectroscopy techniques such as photo-induced absorption (PIA), time-resolved photoluminescence (trPL), and transient absorption (TA) spectroscopy. We characterized these TTA-UC systems as films and suspensions and applied them in different photocatalytic processes as proof of concept.

Transition metal-oxides are promising photoelectrode materials for photoelectrochemical water splitting and solar fuel conversion. Several materials with empty or filled d-shell configurations (e.g. TiO₂, BiVO₄) have shown high internal quantum efficiencies, but their band gaps are too large for practical purposes. Metal-oxides with suitable bandgaps generally have open d-shell configurations and suffer from poor photoconversion efficiencies. We have recently introduced spatial collection efficiency analysis, based on optical and external quantum efficiency measurements, as a non-destructive method to probe losses in thin film semiconductor photoelectrodes under operando conditions. [1,2,3] Using hematite as a case study for strongly correlated electron materials, we show that in addition to the well-known recombination losses arising from poor charge transport properties, some of the photon absorption leads to localized excited states rather than to generation of mobile electrons and holes that contribute to the photocurrent. We extract the wavelength-dependent mobile charge carrier photogeneration yield spectrum of hematite and show that it is less than unity across the entire absorption range, fundamentally limiting the attainable photocurrent to roughly half that predicted based on above bandgap absorption.  I will discuss our recent advancements on this front, and the implications of our findings on photoconversion efficiency in open d-shell transition metal-oxides.   

All-Inorganic Halide Perovskite Quantum Dots (QDs) have emerged as a new class of fascinating nanomaterials with outstanding optoelectronic properties, with promise to revolutionize different disciplines like photovoltaics, lasing and emission. In the present talk, we will describe our efforts towards the application of these materials for solar-driven processes spanning from photocatalysis, environmental remediation,[1] H₂ production and organic transformations.[2] We will discuss on the rational design of these fascinating materials towards photocatalytic and photoelectrochemical processes, and the importance of extracting basic electronic and optical information to understand the carrier dynamics,[3] the influence of defect states and to define adequate defect passivation strategies to maximize the performance and stability of these materials.[4] Moreover, we emphasise the need for proper interrogation tools to validate their photoelectrocatalytic activity and selectivity. Our results also highlight the urgent need for stabilization strategies to move beyond the proof-of-concept stage to relevant technological developments.

Since their discovery cadmium chalcogenide nanoplatelets (NPLs) attracted immense attention due to the fact that they exhibit optoelectronic properties, which are superior to the properties of counterpart nanocrystals of other dimensionalities with the same composition. In addition, due to their shape, they can be arranged into oriented assemblies further extending control over the optical and electronic features. One of the particular features of such NPLs is atomic smoothness of their surface and strong one-dimensional quantum confinement in the thickness direction, which can be controlled down to one monolayer. On the other hand, the ability to change thickness only in integer steps becomes a severe disadvantage preventing precise tuning of spectral position of absorption and photoluminescence bands.

To some extent, spectral tunability was achieved by varying the lateral size of NPLs, and their surface modification via ligand exchange or shell growth. However, these approaches are considerably limited and still not nearly as flexible as the simple size-tuning of quantum dots. In addition, they generally result in a red-shift of optical bands of NPLs, thus highlighting another longstanding challenge for NPLs – the lack of bright emitters in the UV-blue region.

Another more powerful approach consists in the modification of nanocrystal composition through the synthesis of alloyed nanoparticles. Although some advances in this direction have been already achieved by modifying existing protocols for the synthesis of CdSe and CdS NPLs, the synthesized nanoparticles either suffered from poor photoluminescence quantum yields of < 5 % or a limited range of available compositions.

In this work, we demonstrate that both of these issues can be overcome by employing highly reactive stearoychalcogenides, which rapidly react with cadmium carboxylate yielding small polydisperse nanocrystals. At later reaction stages, these nanocrystals serve as a monomer source for the expansion of NPLs in the lateral direction in a slow Ostwald-like ripening process guided by acetate ions. The separation of these stages essentially decouples the precursor conversion step and lateral growth of NPLs thus affording much better control over the lateral dimensions of NPLs, which in turn is a significant factor for achieving brightly emitting nanocrystals. Conveniently, unlike in previously reported procedures, the Se/(Se+S) ratio in the synthesized NPLs is essentially the same as in the starting reaction mixture which enables straightforward control over the nanocrystal composition. Moreover, we demonstrate that by using essentially the same procedure and changing the reaction temperature it is possible to control the thickness of NPLs thus further expanding the spectral range of the synthesized nanocrystals.

Optical spectroscopy measurements show that the increase of selenium content in the alloy results in a steady red shift of absorption and photoluminescence bands consistent with the shrinkage of the material bandgap. Due to their relatively small lateral size enabled by controlling the lateral growth, obtained alloyed CdSe_xS_1-x NPLs exhibit bright band-edge emission covering the blue-green region from 380 to 520 nm with quantum yields of around 30–50 %, which are ca. 10 and 2 times higher than previously reported for respectively 3.5 and 4.5-monolayer thick NPLs with similar composition.

Halide perovskite materials have attracted great scientific interest due to their solution-processability and excellent optoelectronic properties. Applying these perovskites photoactive layers for direct solar water splitting has the potential to provide efficient and inexpensive photoelectrodes for solar hydrogen generation. In this presentation all-inorganic CsPbBr₃ perovskite (2.3 eV bandgap) photoelectrodes will be presented, where the photoactive layer is protected from direct contact with the aqueous electrolyte by a graphite sheet and a low-temperature printed carbon layer. The critical role of controlling 2-dimensional (CsPb₂Br₅) and 0-dimensional (Cs₄PbBr₆) perovskite phase formation and their effect on stability and efficiency will be discussed. The low annealing temperature carbon layer enables the deposition of an organic hole transport layer, while the graphite sheet allows for electrodeposition of nanoscale NiFeOOH co-catalyst on its surface. It will be shown that all these developments lead to all-inorganic halide perovskite photoanodes with remarkable days-long stability and photocurrent generation (~10 mA cm^-2) close to the theoretical efficiency limit of CsPbBr₃. Finally, the potential for scaling up these devices will be demonstrated by presenting photoanodes with an area above 1 cm².

The efficient transport of energy carriers plays an essential role in all of optoelectronic technologies. Energy transport characteristics have traditionally been derived from time-resolved spectroscopy or device-level techniques. While such techniques work well for the characterization of the spatially homogeneous lattices of crystalline semiconductors, they fail to capture the spatially varying complexity of nanostructured semiconductors. To tackle this issue, a range of time-resolved microscopy techniques have emerged, capable of directly visualising energy transport with sub-nanosecond and few-nanometer resolution.[1]

In this talk, I will give an overview of the most recent work of my group on improving our understanding of the various forms of anomalous transport phenomena that can be encountered in semiconductors nanomaterials.[2-5] In the first part of the talk, I will discuss the extensive models that we have developed using Brownian dynamics in which we make use of the detailed spatiotemporal information obtained from time-resolved microscopy. In a second part of the talk, I will present recent work in which we use Mn-doped layered perovskites as highly versatile test-bed to understand the role of carrier trapping in energy transport, with Mn sites acting as deep traps for diffusing excitons. By controlling the doping level, we can controllably modify the trap landscape and relate these changes to the observed spatiotemporal dynamics.

Our measurements and the resulting models provide valuable information for the development of optimized nanostructured semiconductors for optoelectronics, including light harvesting and light emitting devices.

The solution−liquid−solid (SLS) mechanism is a well-established method for forming one-dimensional (1D) nanostructures in a solution. Herein, an SLS mechanism is explored for the formation of metal oxides for the first time. Two key synthetic achievements allow this synthesis: (i) the design of a tailored catalyst with a low melting point and high stability and (ii) control over the reactivity and the oxidation of the precursors. Once these conditions are achieved, the SLS growth of indium and tin oxides ensues. Structural characterization of the products at various stages of the growth confirms the formation of 1D In₂O₃ and SnO₂ nanoscale heterostructures using AuIn₂ and Au₇Sn₃ as catalysts. Both metal oxides showed photocatalytic abilities by the photodegradation of 2-mercaptobenzothiazole (MBT). Furthermore, SLS growth was easily adopted to insert SnO₂ rods selectively between two domains of an Au/ZnO rod heterodimer, demonstrating the potential of achieving highly complex multi-component metal oxide nanostructures.

Perovskite solar cells currently demonstrate more than 25% efficiency, however many fundamental processes still remain unclear. In particular, charge carrier mobility and diffusivity are still poorly characterized and understood. The reported mobility values differ by many orders of magnitude depending on the materials preparation, device architecture and measurement techniques. Therefore, carrier motion in real perovskite solar cells still remains far from clear and the lack of suitable carrier mobility investigation techniques is one of the major problems.  Moreover, evaluation of the actual electric field strength in perovskite layer of solar cells is also not a trivial task. Therefore, the charge carrier transport in real operating perovskite solar cells still remains a controversial, heavily disputed   question. Carrier motion is expected to be particularly complex in case of archetypical perovskite solar cell architecture, where majority of the perovskite is embedded into mesoporous TiO₂ (m-TiO₂) layer.

We investigated charge carrier motion and extraction from archetypical methylammonium lead iodide (MAPI) perovskite solar cell. We used an ultrafast electric field-modulated transient absorption technique enabling to directly visualize the carrier motion with subpicosecond time resolution by evaluating electric field dynamics from the time-resolved electroabsorption spectra. We demonstrate that photogenerated holes drift across the mesoporous TiO₂/perovskite layer during hundreds of picoseconds, however their extraction to the Spiro-OMeTAD hole transporting layer takes place during tens of nanoseconds suggesting that the hole extraction is limited by the perovskite/Spiro-OMeTAD interface rather than by the hole transport through the perovskite layer.  Additionally, we use the ultrafast time-resolved fluorescence technique which reveals fluorescence decay during tens of ps, which we attribute to the spatial electron and hole separation.  

Hybrid halide perovskites show heterogeneity at multiple length scales.[1] Cathodoluminescence (CL) in a scanning electron microscope (SEM) can investigate semiconductor materials by scanning an electron beam over a semiconductor. By collecting the optical properties of the material during scanning, CL-SEM can elucidate structure-property relations of halide perovskites at sub-micrometer spatial resolution.

While most CL studies on perovskites have focused on the inorganic compositions, the characterization of hybrid (organic-inorganic) halide perovskites - including the highest-performing compositions - is limited due to their lower stability under the electron beam. To reduce beam damage, we used a pulsed electron-beam (PM) to measure hyperspectral maps of hybrid perovskite compositions.[2] When PM was used, the CL spectra strongly resembled that of pristine perovskite emission, suggesting the use of PM for more robust high spatial resolution spectral mapping of beam-sensitive hybrid halide perovskite materials.

In this presentation, a series of examples will be presented whereby PM and low-current CL aided in the study of luminescence of several different perovskite compositions. For example, the use of CL was crucial to understand the effect that focused ion beam milling has on the perovskite emission, generally considered to be a rough processing step for TEM specimen preparation. Using CL-SEM in PM, the perovskite layer in the lamellae was found to remain optically active, yet with a small blue-shift due to surface amorphization.[3]

Going beyond conventional high-current CL setups, used for traditional semiconductors, enables CL to start to play a role in resolving the complex heterogeneity of hybrid halide perovskites, and it could also enable the elucidation of many other novel beam sensitive “soft” semiconductors.

Simply prepared chalcogenide materials have been investigated as light absorbers for photoelectrochemical water splitting. I will begin the talk with our work on antimony selenide, a promising quasi-1D material whose efficiency has rapidly increased in the past ten years. Thin films of antimony selenide are easily prepared by selenization of antimony metal deposited by electrodeposition or sputtering. The effect of a low temperature sulfurization treatment was investigated with time resolved microwave conductivity, photoluminescence, and low frequency Raman spectroscopy, and the effect on both bulk and surface recombination will be discussed.  The water splitting photocathodes were characterized under operando conditions using electrochemical impedance spectroscopy. We will then explore the related compound antimony sulfide and its band alignment with atomic layer deposited titanium dioxide, which serves both as an electron extracting contact and corrosion protection layer. Finally, a simple solution processing method for phase pure cuprous sulfide will be presented, where the nanostructure can be controlled with the appropriate solvent mixture of the precursor solution.

The interconversion of spin, charge and light are being exploited in perovskite materials leading to efficient, new, and unique device concepts. In this talk, we present the design rules which control and explain the mechanism behind chiral transfer from chiral molecules to the HOIS framework in nanostructured embodiments. Chirality can be imparted by chiral ligands attached during synthesis or in post-synthetic ligand exchanges where the chirality can be studied as a function of the nanocrystal size and surface to volume ratio. We show chirality induced properties from halide perovskite nanocrystals of different sizes and in films and then construct multiple types of prototype devices using heterostructure concepts that harness and display these fascinating properties. Some examples include heterostructured LEDs which emit spin-controlled circularly polarized light at room temperature without external magnets, detectors which couple perovskite nanocrystals and carbon nanotubes to detect and discern circularly polarized photons, neuromorphic devices which exhibit persistent photoconduction lasting thousands of seconds beyond a weak light pulse and can also display read/write capabilities.

Halide perovskite nanocrystals (NCs) have emerged as an intriguing material for optoelectronic applications, most notably for light-emitting diodes (LEDs), lasers, and solar cells. Their emission wavelength depends not only on material composition but also on size and dimensionality, as in the case of two-dimensional (2D) nanoplatelets (NPLs). These colloidal quantum wells have additional appeal for light emission, as the one-dimensional quantum confinement enhances their radiative rates and enables directional outcoupling. On top of this, due to a monolayer-precise control over their thickness, they constitute an intriguing system for spectroscopic studies on their fundamental optical, phononic, and energetic properties.

In this talk, I will explore our recent results on halide perovskite NPLs, including their synthesis and stabilization. I will focus on their interesting excitonic properties, such as the energetic fine structure^[1] with a strong thickness-dependent bright-dark exciton splitting and exciton diffusion in nanocrystal films.^[2]  Further, I will highlight their advantages and disadvantages for integration into optoelectronic applications.

As halide perovskites and their derivatives are being developed for numerous optoelectronic applications, controlling their electronic doping remains a fundamental challenge. We have recently discovered a novel strategy of using redox-active organic molecules as stoichiometric electron acceptors in new expanded perovskite analogs. Compressing the metal-halide framework drives up the valence band relative to the acceptor orbitals of the organic molecules. Thus, the material’s electronic conductivity increases by a factor of 105 with pressure, reaching 50(17) S cm–1 at 60 GPa, exceeding the high-pressure conductivities of most halide perovskites. This conductivity enhancement is attributed to an increased hole density created by reduction of the redox-active molecules. This work elevates the role of organic cations in 3D metal-halides, from templating the structure to serving as charge reservoirs for tuning the carrier concentration.

Redox-active organic cations can act as charge reservoirs in the expanded perovskite analog (dmpz)[Sn2I6]. Material compression increases electronic conductivity by five orders of magnitude. This conductivity rise is attributed to an increased hole density in the valence band, caused by electron transfer to the redox-active molecules