For large polarons, Fr & ouml;hlich theoretically derived an increased effective mass and a decreased energy as compared to free charge carriers. These changes have hitherto eluded direct observation. Here, we report on the direct observation of large polaron formation after photoexcitation in BiOI nanoplatelets, a system known to host spatially separated e-h pairs. We employ time- and momentum-resolved photoemission electron microscopy with 50-fs pulses to monitor the dynamics of the conduction-band (CB) dispersion E(k). Indeed, we observe a doubling of the effective mass accompanied by a decrease of the CB energy leading to an overall drop of 160 meV.

I-III-VI quantum dots are potential candidates for light-emitting and light-harvesting applications due to their heavy-metal–free composition and chemical tunability. Despite their potential, the investigation of intrinsic exciton and excitonic many-body dynamics has often been complicated because of their extensive defect energy landscape. Here, we investigate bright band-gap-emitting AgInS 2 quantum dots of sizes ranging from ∼ 4 to 7 nm using femtosecond pump-probe spectroscopy to link quantum confinement to exciton formation and many-body interactions. The exciton formation time is governed by inter-valence-band hole relaxation. This increases by more than a factor of 4 with decreasing quantum dot size, reflecting size-dependent exciton-phonon scattering efficiency. By temporally correlating hole relaxation with induced absorption dynamics, we identify different exciton-to-biexciton transitions originating from different exciton states. Excitation-fluence-dependent measurements reveal a biexciton decay time that scales with the quantum dot volume. Smaller quantum dots show fast decay of <100 ps, primarily governed by exciton-exciton annihilation. In larger quantum dots, the biexciton lifetime exceeds 400 ps, indicating reduced annihilation efficiency. Complementary streak-camera photoluminescence experiments provide direct spectroscopic evidence that these long-lived biexcitons are partially emissive. Collectively, we report an in-depth ultrafast-optical study yielding fundamental insights into the complex landscape of exciton formation and biexciton physics, which advances the understanding of intrinsic optical processes in AgInS 2 quantum dots among I-III-VI materials.

Lead-halide hybrid perovskites (RNH3PbX3, X = halide, e.g., Cl, Br, I; R = organic moiety) show promise for next-generation optoelectronic devices due to their simple synthesis routes, strong light absorption, and high photoluminescence quantum yield. However, postsynthetic halide exchange in lead-halide perovskites poses a challenge for the functionality of many perovskite devices. For example, in all-perovskite heterostructures, halide diffusion results in the formation of undesired mixed alloys rather than sharp interfaces required for many optoelectronic applications. To address this issue, we incorporated lysine molecules, one of the 20 common amino acids, into a hybrid perovskite MAPbBr3 (MA = CH3NH3) host and investigated their impact on the host's ability to undergo postsynthetic halide exchange. We immersed lysine-incorporated MAPbBr3 crystals in solutions containing Cl- or I- for varying durations and analyzed subsequent halide exchange-related changes using ion chromatography, high-resolution powder X-ray diffraction, and photoluminescence spectroscopy. Our findings unanimously indicate that incorporated lysine significantly impedes postsynthetic Cl- and I- diffusion into bulk MAPbBr3. Our new bioinspired approach opens a route toward mitigating postsynthetic halide exchange in lead-halide hybrid perovskites and improving the suitability of perovskite devices for optoelectronic applications.

The facile preparation, structural tunability, and unique optical properties of halide perovskites have led to great success in solar cells and light-emitting devices. However, due to their instability in low-cost polar solvents (e.g., methanol and water), their application in photocatalysis is currently lagging and usually requires special encapsulation. Herein, we demonstrate that halide perovskite (MAPbBr3, CsPbBr3) nanocrystals are stable for photocatalytic hydrogen production from methanol/water vapor as proton sources without special protection in the presence of Pt cocatalysts. In methanol vapor, an optimal MAPbBr3 perovskite NC film exhibits steady hydrogen evolution for similar to 20 h under solar irradiation, reaching a marked hydrogen evolution rate of 424 mu molg-1h-1. Notably, time-resolved spectroscopies reveal that the diffusion-limited feeding of electron-hole pairs toward Pt clusters dominates the dynamics of charge transfer. This study shows the potential of directly using halide perovskites for light-driven reactions in polar environments.

I‐III‐VI quantum dots are eco‐friendly alternatives for next‐generation light‐emitting and harvesting devices. The study investigates the optical properties of Ag‐In‐S‐based quantum dots, focusing on excitonic transitions and sub‐bandgap defects. Uniquely synthesized AgInS 2 quantum dots emit via free excitons but dominate with broad, red‐shifted emission from energy‐distributed donor–acceptor pair defects. Photoluminescence excitation spectroscopy unveils charge carrier relaxation pathways with varying efficiencies within the defect states. Gallium sulfide coating forms a core/shell structure, passivating defects and transforming the inhomogeneous sub‐bandgap energy landscape into a more homogeneous one. A narrow free exciton emission and sharp absorption onset with two exciton signatures, X1 and X2, become prominent. Theoretical calculations reveal the band structure, showing that these transitions result from the excitation between two distinct valence band maxima and the conduction band minimum. Femtosecond transient transmission spectroscopy shows a defect trapping time of 590 fs in core quantum dots, while exciton filling prevails in core/shell, with formation times of 560 fs (X1) and 320 fs (X2). This study provides new insights into the optical properties of Ag‐In‐S‐based quantum dots by demonstrating how the computed band structure explains the experimentally observed excitonic features, with ultrafast spectroscopy capturing the exciton formation and trap‐state filling dynamics.

In this work, we analyze the motion of gold nanospheres in orbital angular momentum (OAM)-carrying optical vortex traps in real time using darkfield microscopy and high-speed video analysis. Notably, we observe that optical binding between gold nanoparticles within the ring-shaped laser trap leads to increased orbiting speeds at a lower focal plane for gold nanoparticle dimers compared to monomers. This behavior is attributed to stronger optical scattering forces acting on the dimers driven by the emergence of a coupled plasmon mode. As the particles move closer together, this mode red-shifts, becoming more resonant with the laser wavelength, eventually causing the system to transition from optical trapping to optical printing. This finding suggests a general mechanism for one-step dimer printing based on plasmonic coupling in vortex beams by adjusting the laser wavelength or modifying the dielectric environment of the nanoparticles via a molecular coating. The feasibility of this approach is demonstrated for optical printing and subsequent surface-enhanced Raman scattering (SERS) spectroscopy on gold nanoparticle dimers coated with 4-nitrothiophenol.

Perovskite quantum dots (pQDs) are promising materials for optoelectronic and photocatalytic applications due to their unique optical properties. To enhance charge carrier extraction or injection, donor/acceptor molecules can be tethered to the pQD. These molecules must strongly bind to the ionic surfaces of pQDs without compromising colloidal stability. This is achieved by using multifunctional ligands containing a quaternary ammonium binding group for strong pQD surface attachment, a long tail group for colloidal stability, and a functional group near the pQD surface. Such pQDs with ferrocene-functionalized ligands show fast photoexcited hole transfer with near-unity efficiency. Density functional theory calculations reveal how ferrocene's molecular structure reorganizes following hole transfer, affecting its charge separation efficiency. This approach can also be extended to photoexcited electron and energy transfer processes with pQDs. Therefore, this strategy offers a blueprint for creating efficient pQD-molecular hybrids for applications like photocatalysis.

Mixed halide perovskite nanocrystals in the form of cesium/formamidinium lead triiodide ((Cs:FA)PbI3) offer great potential for efficient and stable solar cells. To date, large-scale production with roll-to-roll compatible deposition methods remains difficult and requires detailed research on each involved processing step. Here, a proof-of-concept study about slot-die coating (printing) the active layer of (Cs:FA)PbI3-based nanocrystal solar cells is presented. Structural and morphological changes during ligand exchange of long-chain oleic acid and oleylamine by Pb(NO3)(2), and top-layer FAI passivation are investigated. Ligand exchange improves the processability of the nanocrystal layer and enhances charge transport. It also changes texture from face-on toward edge-on orientation as grazing-incidence X-ray scattering studies indicate. Ligand exchange and FAI passivation redshift photoluminescence and prolong charge carrier lifetime in the printed nanocrystal films. The proof-of-concept feasibility of printing metal halide perovskite nanocrystal films for solar cells is shown by building 20 devices with a median power conversion efficiency of 6.39%.

Quantum dots (QDs) are semiconductor nanocrystals whose optical properties can be tuned by altering their size. By combining QDs with dyes we can make hybrid QD-dye systems exhibiting energy transfer (ET) between QDs and dyes, which is important in sensing and lighting applications. In conventional QDs that need a shell to passivate surface defects, ET usually proceeds through Förster resonance energy transfer (FRET) that requires significant spectral overlap between QD emission and dye absorbance, as well as large oscillator strengths of those transitions. This considerably limits the choice of dyes. In contrast, perovskite QDs do not require passivating shells for bright emission, which makes ET mechanisms beyond FRET accessible. This work explores the design of a CsPbBr3 QD-dye system to achieve efficient ET from CsPbBr3 QDs to dyes with dimethyl iminium binding groups where the close binding of dyes to CsPbBr3 surface facilitates spatial wavefunction overlap. Using steady-state and time-resolved photoluminescence experiments, we demonstrate that efficient ET from CsPbBr3 to dyes with minimal spectral overlap proceeds via the Dexter exchange-type mechanism, which overcomes the conventional restriction of spectral overlap that severely limits the tunability of these systems. This approach opens new avenues for QD-molecule hybrids for a wide range of applications, such as lighting.

Halide double perovskites (DPs), such as Cs2AgBiX6 (X = Br/Cl), are emerging semiconductors for optoelectronic applications, offering less toxic alternatives compared to lead halide perovskites. Although exhibiting an indirect bandgap, Cs2AgBiCl6 DP nanocrystals (NCs) show bright photoluminescence spectra characterized by a spectrally broad red emission centered approximate to 650 nm and a narrower blue emission band centered at 425 nm. However, the origin of both emission bands is still under debate. In this paper, it is shown that silver (Ag) plays a crucial role in the explanation of both emission bands observed in these DP NCs. The trapping of holes in Ag vacancies leads to a spatial localization of the hole wave function on the scale of the lattice constant. This provides k-values for the hole wave function at all boundaries of the Brillouin zone and thus favors the recombination with electrons at the L-point. Accordingly, a thermally activated behavior of the red photoluminescence with an activation energy of 56 meV is observed. It is further shown that the high-energetic blue emission band originates from lecithin ligands attached to the NC surface. Surprisingly, their emission spectrum coincides exactly with the plasmon resonance of Ag nanoclusters, located on the surface of the DP NCs.

Lead halide perovskite quantum dots (QDs) are a promising material for light amplification devices. In order to improve their optical gain threshold and lifetime, it is essential to understand the underlying gain mechanism. However, there is still debate on the nature of gain in perovskite QDs, which has been attributed to different origins such as biexcitons, trions, and single excitons. Here we study amplified spontaneous emission and optical gain of monodisperse spherical CsPbBr3 QDs and conclusively assign the gain to biexcitons. This is based on the gain threshold and its spectral position which we study via femtosecond transient absorption spectroscopy. Furthermore, the optical gain vanishes within 30 ps, matching the biexciton lifetime, demonstrating the strong correlation to the biexciton population. By identifying the intrinsic mechanism of optical gain in CsPbBr3 QDs and its limiting factors, our findings show the direction for future work on optimizing their gain threshold and lifetime.

In photocatalysis, photogenerated charge separation is pivotal and can be achieved through various mechanisms. Building heterojunctions is a promising method to enhance charge separation, where effective contact and charge exchange between heterojunction components remains challenging. Mostly used synthesis processes for making heterostructures require high temperatures, difficult processes, or expensive materials. Herein, a heterojunction of potassium intercalated graphitic carbon nitride (K-CN) and nanoflakes of iron phosphor trisulfide (FPS) is designed via a simple mechanical grinding process to boost the hydrogen evolution by a factor of more than 25 compared to pure K-CN. This significant improvement is rarely reached by other combinations of two semiconductors without cocatalysts, such as platinum. It can be attributed to the band alignment and band bending of an S-scheme that is validated via optical and X-ray photoelectron spectroscopy. As a consequence, strong quenching of the photoluminescence and significant H2 evolution occur for this unique heterojunction. Furthermore, the excellent durability of the designed photocatalytic heterostructure is confirmed by monitoring the catalysts' H2-evolution rate and crystal structure after 72 h under light illumination. This study opens up promising and simple pathways for constructing efficient S-scheme heterojunctions for photocatalytic water-splitting.

Optically excited electronic excitations are coupled to the soft and polar halide perovskite lattice, generating coherent phonons after subpicosecond interband laser-excitation. In Ag-based halide double perovskites, Ag-vacancies can bind free excitons, resulting in a pronounced bound exciton resonance. Here, we report the detection of three modulation frequencies corresponding to coherent phonons in Ag-based double perovskite nanocrystals at distinct spectral positions at the bound exciton resonance. Two of them are found in oscillatory spectral shifts of the bound exciton resonance and are identified as Cs- and Br-related bulk phonons. Surprisingly, a third frequency is observed as an intensity modulation. We argue that this amplitude oscillation is a consequence of an optically generated vibronic wave packet localized at a Ag-vacancy. Consequently, the localized coherent phonon modulates the giant oscillator strength of the bound exciton. This optically induced and spatially localized lattice shaking could potentially be useful for initiating photochemical reactions with atomic precision.

Manganese doping has been demonstrated as a versatile tool to tune the emission of CsPbCl3 nanocrystals (NCs). Although this has been demonstrated in nanocubes and nanoplatelets, strategies for doping Mn2+ in size-tunable, excitonic CsPbCl3 quantum dots (QDs) remain absent. In this work, we demonstrate the synthesis of size-tunable spheroidal CsPbCl3:Mn2+ QDs, which can be obtained by a water-hexane interfacial combined anion and cation exchange strategy starting from CsPbBr3 QDs. Interestingly, the QDs exhibit a fast 0.2 ms Mn2+ photoluminescence (PL) lifetime and an energy transfer (ET) time of approximately 100 ps from the excitonic state of the QD to the atomic state of the Mn2+ ion. The size dependence observation of the manganese PL efficiency and the slow ET rate suggest that Mn2+ mainly gets incorporated at the QD's surface, highlighting the importance of strategies chosen for the incorporation of Mn2+ into perovskite QDs.

Postsynthetic metalsalt treatments are frequently employed inthe luminescence enhancement of quantum dots (QDs); however, its microscopicpicture remains unclear. CsPbBr3-QDs, featuring strongexcitonic absorption and high photoluminescence (PL) quantum yield,are ideal QDs to unravel the intricate interaction between QDs andsuch surface-bound metal salts. Herein, we study this interactionbased on the controlled PL quenching of CsPbBr3-QDs withBiBr(3). Upon the addition of BiBr3, an instantand complete PL quenching is observed, which can be fully recoveredafter the addition of an excess of PbBr2. This, togetherwith the complete preservation of the excitonic absorption suggestsa surface-driven adsorption equilibrium. Additionally, time-resolvedstudies reveal a non-homogeneous surface trap formation. Based onthe so-called sphere of action model for the adsorption process, weshow that already a single BiBr3 adsorption suffices tocompletely quench a QD's luminescence. This approach is expandedto analyze size-, ligand-, and metal-dependent quenching dynamics.Facet junctions are identified as regions of enhanced surface reactivity.A Langmuir-type ligand coverage is exposed with a strong impact onadsorption. Our results provide a detailed mechanistic insight intopostsynthetic interaction of QDs with metal salts, opening pathwaysfor future surface manipulations.

Correction for 'Atomically flat semiconductor nanoplatelets for light-emitting applications' by Bing Bai et al., Chem. Soc. Rev., 2023, 52, 318-360, https://doi.org/10.1039/D2CS00130F.

Photocatalytic water splitting is a promising approach to generating sustainable hydrogen. However, the transport of photoelectrons to the catalyst sites, usually within ps-to-ns timescales, is much faster than proton delivery (∼μs), which limits the activity. Therefore, the acceleration of abstraction of protons from water molecules towards the catalytic sites to keep up with the electron transfer rate can significantly promote hydrogen production. The photobasic effect that is the increase in proton affinity upon excitation offers means to achieve this objective. Herein, we design photobasic carbon dots and identify that internal pyridinic N sites are intrinsically photobasic. This is supported by steady-state and ultrafast spectroscopic measurements that demonstrate proton abstraction within a few picoseconds of excitation. Furthermore, we show that in water, they form a unique four-level lasing scheme with optical gain and stimulated emission. The latter competes with photocatalysis, revealing a rather unique mechanism for efficiency loss, such that the stimulated emission can act as a toggle for photocatalytic activity. This provides additional means of controlling the photocatalytic process and helps the rational design of photocatalytic materials.