Photobase effect for just-in-time delivery in photocatalytic hydrogen generation

Carbon dots (CDs) are a promising nanomaterial for photocatalytic applications. However, the mechanism of the photocatalytic processes remains the subject of a debate due to the complex internal structure of the CDs, comprising crystalline and molecular units embedded in an amorphous matrix, rendering the analysis of the charge and energy transfer pathways between the constituent parts very challenging. Here we propose that the photobasic effect, that is the abstraction of a proton from water upon excitation by light, facilitates the photoexcited electron transfer to the proton. We show that the controlled inclusion in CDs of a model photobase, acridine, resembling the molecular moieties found in photocatalytically active CDs, strongly increases hydrogen generation. Ultrafast spectroscopy measurements reveal proton transfer within 30ps of the excitation. This way, we use a model system to show that the photobasic effect may be contributing to the photocatalytic H-2 generation of carbon nanomaterials and suggest that it may be tuned to achieve further improvements. The study demonstrates the critical role of the understanding the dynamics of the CDs in the design of next generation photocatalysts

Disorder and Confinement Effects to Tune the Optical Properties of Amino Acid Doped Cu2O Crystals

Biominerals are organic-inorganic nanocomposites exhibiting remarkable properties due to their unique configuration. Using optical spectroscopy and theoretical modeling, it is shown that the optical properties of a model bioinspired system, an inorganic semiconductor host (Cu2O) grown in the presence of amino acids (AAs), are strongly influenced by the latter. The absorption and photoluminescence excitation spectra of Cu2O-AAs blue-shift with growing AA content, indicating band gap widening. This is attributed to the void-induced quantum confinement effects. Surprisingly, no such shift occurs in the emission spectra. The theoretical model, assuming an inhomogeneous AA distribution within Cu2O-AAs due to compositional disorder, explains the deviating behavior of the photoluminescence. The model predicts that the potential causing the confinement effects becomes a function of the local AA density. It results in a Gaussian band gap distribution that shapes the optical properties of Cu2O-AAs. Imitating and harnessing the process of biomineralization can pave the way toward new functional materials

Interfacial Manganese-Doping in CsPbBr3 Nanoplatelets by Employing a Molecular Shuttle

Mn-doping in cesium lead halide perovskite nanoplatelets (NPls) is of particular importance where strong quantum confinement plays a significant role towards the exciton-dopant coupling. In this work, we report an immiscible bi-phasic strategy for post-synthetic Mn-doping of CsPbX3 (X=Br, Cl) NPls. A systematic study shows that electron-donating oleylamine acts as a shuttle ligand to transport MnX2 through the water-hexane interface and deliver it to the NPls. The halide anion also plays an essential role in maintaining an appropriate radius of Mn2+ and thus fulfilling the octahedral factor required for the formation of perovskite crystals. By varying the thickness of parent NPls, we can tune the dopant incorporation and, consequently, the exciton-to-dopant energy transfer process in doped NPls. Time-resolved optical measurements offer a detailed insight into the exciton-to-dopant energy transfer process. This new approach for post-synthetic cation doping paves a way towards exploring the cation exchange process in several other halide perovskites at the polar-nonpolar interface

Manganese‐Doping‐Induced Quantum Confinement within Host Perovskite Nanocrystals through Ruddlesden–Popper Defects

The concept of doping Mn2+ ions into II–VI semiconductor nanocrystals (NCs) was recently extended to perovskite NCs. To date, most studies on Mn2+ doped NCs focus on enhancing the emission related to the Mn2+ dopant via an energy transfer mechanism. Herein, we found that the doping of Mn2+ ions into CsPbCl3 NCs not only results in a Mn2+‐related orange emission, but also strongly influences the excitonic properties of the host NCs. We observe for the first time that Mn2+ doping leads to the formation of Ruddlesden–Popper (R.P.) defects and thus induces quantum confinement within the host NCs. We find that a slight doping with Mn2+ ions improves the size distribution of the NCs, which results in a prominent excitonic peak. However, with increasing the Mn2+ concentration, the number of R.P. planes increases leading to smaller single‐crystal domains. The thus enhanced confinement and crystal inhomogeneity cause a gradual blue shift and broadening of the excitonic transition, respectively

Lead-Free Halide Perovskite Materials and Optoelectronic Devices: Progress and Prospective

Halide perovskites, in the form of thin films and colloidal nanocrystals, have recently taken semiconductor optoelectronics research by storm, and have emerged as promising candidates for high-performance solar cells, light-emitting diodes (LEDs), lasers, photodetectors, and radiation detectors. The impressive optical and optoelectronic properties, along with the rapid increase in efficiencies of solar cells and LEDs, have greatly attracted researchers across many disciplines. However, most advances made so far in terms of preparation (colloidal nanocrystals and thin films), and the devices with highest efficiencies are based on Pb-based halide perovskites, which have raised concerns over their commercialization due to the toxicity of Pb. This has triggered the search for lower-toxicity Pb-free halide perovskites and has led to significant progress in the last few years. In this roadmap review, researchers of different expertise have joined together to summarize the latest progress, outstanding challenges, and future directions of Pb-free halide perovskite thin films and nanocrystals, regarding their synthesis, optical spectroscopy, and optoelectronic devices, to guide the researchers currently working in this area as well as those that will join the field in the future.I.L.-F., D.V., C.-Y.W., S.S., T.O., Y.-T.H., K.S., Y.L., V.S.C., J.Z., L.D.T., and D.G. contributed equally to this work. L.P. acknowledges the support from the Spanish Ministerio de Ciencia e Innovación through the Ramón y Cajal grant (RYC2018-026103-I) and the Spanish State Research Agency (Grant No. PID2020-117371RA-I00; TED2021-131628A-100), as well as the grant from the Xunta de Galicia (ED431F2021/05). C.-Y.W. acknowledges the financial support from Alexander von Humboldt Foundation. K.S. acknowledges the financial support from China Scholarship Council (CSC), and P.M.-B. acknowledges support from Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany's Excellence Strategy – EXC 2089/1–390776260 (e-conversion). V.B. and T.O. acknowledge the MEXT JSPS Grants 20J00974, 21K14580, and 23H01781. H.Z acknowledges the financial supported by NSFC (62222405, 52131304), the Natural Science Foundation of Jiangsu Province (BK20220142), the Fundamental Research Funds for the Central Universities (30922010713), and NSFC-RGC (62261160392). H.-T.S. acknowledges the financial support from JSPS KAKENHI (21H01743). Y.-T.H and R.L.Z.H. would like to thank the Engineering and Physical Sciences Research Council (EPSRC) for funding (no. EP/V014498/2). R.L.Z.H. also thanks the Royal Academy of Engineering through the Research Fellowships scheme (no. RF∖201718∖17101). D.V. and E.D. acknowledge financial support from the Research Foundation – Flanders through an FWO doctoral fellowship to D.V. (FWO Grant Number 1S45223N) and the KU Leuven Internal Funds (Grant Numbers STG/21/010, C14/23/090, and CELSA/23/018). T.D. acknowledges the Department of Science and Technology (DST) and the Science and Engineering Research Board (SERB) for the Ramanujan Fellowship Award (RJF/2021/000125). I.M.-S. acknowledges Ministry of Science and Innovation of Spain under Step-Up (TED2021-131600B-C31) project and by Generalitat Valenciana under Print-P (MFA/2022/020) project. V.S.C., I.M.-S. and J.P.M.-P acknowledges the support of the Horizon 2020 research and innovation program through the DROP-IT project (grant agreement no. 862656)

CuInS₂‑Decorated Perovskite Nanoarchitecture: Halide-Driven Energy and Electron Transfer

Perovskite nanocrystals (NCs) are an emergent and game-changing entrant in semiconductor research, yet the research on the corresponding nanoheterostructures remains in its infancy. In this work, we fabricate a type II nanoarchitecture of CsPbX3 NCs (where X = Cl, Br, or I) and CuInS2 quantum dots to investigate the energy and charge transfer (ET and CT, respectively) processes. Optical measurements of CsPbX3/CuInS2 show efficient photoluminescence (PL) quenching when X = Br or I, while the PL quenching efficiency of the X = Cl compound is 2 orders of magnitude lower. We argue the drastic PL quenching in the X = I compound is solely due to the CT process, while for the X = Br compound, a predominantly ET process is active. In contrast to the driving force (−ΔG) for the CT process, we observe the reverse order of the electron transfer process, for which we propose the electron transfer occurs in the Marcus inverted region. Our halide-dependent controlled regulation of CT and ET processes in these nanoarchitectures may find promising optoelectronic applications

Super sensitization: grand charge (hole/electron) separation in ATC dye sensitized CdSe, CdSe/ZnS type-I, and CdSe/CdTe type-II core-shell quantum dots

Ultrafast charge-transfer dynamics has been demonstrated in CdSe quantum dots (QD), CdSe/ZnS type-I core–shell, and CdSe/CdTe type-II core–shell nanocrystals after sensitizing the QD materials by aurin tricarboxylic acid (ATC), in which CdSe QD and ATC form a charge-transfer complex. Energy level diagrams suggest that the conduction and valence band of CdSe lies below the LUMO and the HOMO level of ATC, respectively, thus signifying that the photoexcited hole in CdSe can be transferred to ATC and that photoexcited ATC can inject electrons into CdSe QD, which has been confirmed by steady state and time-resolved luminescence studies and also by femtosecond time-resolved absorption measurements. The effect of shell materials (for both type-I and type-II) on charge-transfer processes has been demonstrated. Electron injection in all the systems were measured to be <150 fs. However, the hole transfer time varied from 900 fs to 6 ps depending on the type of materials. The hole-transfer process was found to be most efficient in CdSe QD. On the other hand, it has been found to be facilitated in CdSe/CdTe type-II and retarded in CdSe/ZnS type-I core–shell materials. Interestingly, electron injection from photoexcited ATC to both CdSe/CdTe type-II and CdSe/ZnS type-I core–shell has been found to be more efficient as compared to pure CdSe QD. Our observation suggests the potential of quantum dot core–shell super sensitizers for developing more efficient quantum dot solar cells

Ultrafast hole-and electron-transfer dynamics in CdS–dibromofluorescein (DBF) supersensitized quantum dot solar cell materials

Ultrafast charge-transfer (CT) dynamics has been demonstrated in CdS quantum dot (QD)–4′,5′-dibromofluorescein (DBF) composite materials, which form a strong CT complex in the ground state. Charge separation in the CdS–DBF composite was found to take place in three different pathways, by transferring the photoexcited hole of CdS to DBF, electron injection from photoexcited DBF to the CdS QD, and direct electron transfer from the HOMO of DBF to the conduction band of the CdS QD. CT dynamics was monitored by direct detection of the DBF cation radical and electron in the QD in the transient absorption spectra. Electron injection and the electron-transfer process are found to be pulse-width-limited (<100 fs); however, the hole-transfer time was measured to be 800 fs. Charge recombination dynamics has been found to be very slow, confirming spatial charge separation in the CdS–DBF supersensitized quantum dot system. Grand charge separation process suggests that the CdS–DBF supersensitized quantum dot system can be used as superior materials for quantum dot solar cells (QDSCs)