There is plenty of room at the top: generation of hot charge carriers and their applications in perovskite and other semiconductor-based optoelectronic devices

Anisotropic ultrafast hot carrier dynamics of two-dimensional SnS single crystals

In this paper, we investigated the temperature-dependent and anisotropic ultrafast hot carrier decay dynamics of two-dimensional SnS single crystals using femtosecond transient optical spectroscopy. The photo-excited hot carriers in SnS are relaxed via a fast decay (τ1) and a slow decay (τ2), which are contributed by the electron-phonon interactions coupling with high frequency and low frequency optical phonons of SnS, respectively. Both the τ1 and τ2 decay times show anisotropy. In the ab-plane, both τ1 and τ2 decays have a faster relaxion time in the b-axis direction than in the a-axis direction, which is due to the crystal anisotropy of SnS. The crystal anisotropy of SnS gives rise to more phonon vibrations in the b-axis direction than in the a-axis direction, which leads to stronger electron-phonon coupling along the b-axis direction and manifests as a shorter decay time along the b-axis direction. For the τ2 decay process, the femtosecond laser pump induces different dielectric responses in the ab-plane. In the b-axis direction, the pump laser induces a reduction in the dielectric coefficient (Δε < 0), while it induces an increase in the dielectric coefficient (Δε > 0) in the a-axis.

Progress in silicon-based materials for emerging solar-powered green hydrogen (H2) production

The imperative demand for sustainable and renewable energy solutions has precipitated profound scientific investigations into photocatalysts designed for the processes of water splitting and hydrogen fuel generation. The abundance, low toxicity, high conductivity, and cost-effectiveness of silicon-based compounds make them attractive candidates for hydrogen production, driving ongoing research and technological advancements. Developing an effective synthesis method that is simple, economically feasible, and environmentally friendly is crucial for the widespread implementation of silicon-based heterojunctions for sustainable hydrogen production. Balancing the performance benefits with the economic and environmental considerations is a key challenge in the development of these systems. The specific performance of each catalyst type can vary depending on the synthesis method, surface modifications, catalyst loading, and reaction conditions. The confluence of high crystallinity, reduced oxygen concentration, and calcination temperature within the silicon nanoparticle has significantly contributed to its noteworthy hydrogen evolution rate. This review provides an up-to-date evaluation of Si-based photocatalysts, summarizing recent developments, guiding future research directions, and identifying areas that require further investigation. By combining theoretical insights and experimental findings, this review offers a comprehensive understanding of Si-based photocatalysts for water splitting. Through a comprehensive analysis, it aims to elucidate existing knowledge gaps and inspire future research directions towards optimized photocatalytic performance and scalability, ultimately contributing to the realization of sustainable hydrogen generation.

β-Cyclodextrin and reduced graphene oxide loaded Ag-TiO2 composites for enhanced photocatalytic oxidation of urea under sunlight

Urea oxidation is important to increase agricultural growth, which can meet food requirements across the world. It is pivotal for converting nitrogen to nitrate that is usable by crops, thus preventing nitrogen loss to the atmosphere. This study focuses on improving the photodegradation efficiency of TiO2 by incorporating β-CD (beta-cyclodextrin), RGO (reduced graphene oxide), and Ag to enhance nitrate conversion. FT-IR, DRS, PL, EDX, XRD, XPS, HR-TEM DLS, and FESEM were conducted to characterize these materials. Among all the catalysts, the quaternary composite, β-CD/Ag-TiO2/RGO, exhibited superior performance, achieving an 86.2% degradation efficiency with a 27.8% nitrate yield under sunlight irradiation within 150 min of reaction time. Several factors contribute to the enhanced photoactivity of β-CD/Ag-TiO2/RGO, including the high surface area and absorptive power of β-CD, the large electronic mobility of RGO, and the localized surface plasmonic resonance effect of Ag, extending the catalyst's response to visible light. An intriguing aspect of this study is the encapsulation of gaseous nitrogen into the hydrophobic interior cavity of β-CD, contributing to the enhancement of urea oxidation. These findings can be very substantial for both agriculturists and chemists, providing valuable insights into designing novel photocatalysts for improved urea oxidation, thereby enhancing agricultural productivity.

Gate-Tunable Hot Electron Extraction in a Two-Dimensional Semiconductor Heterojunction

Hot carrier solar cells (HCSCs), harvesting the excess energy of hot carriers generated by above-band gap photoexcitation, are crucial for pushing the solar cell efficiency beyond the Shockley-Queisser limit, which is challenging to realize mainly due to fast hot-carrier cooling. By performing transient reflectance spectroscopy in a MoSe2/hBN/WS2 junction, we demonstrate the gate-tunable harvest of hot electrons from MoSe2 to WS2. By spectrally distinguishing hot-electron extraction from lattice temperature increase, we find that electrostatically doped electrons in MoSe2 can boost hot-electron extraction density (nET) by a factor up to several tens. Such enhancement arises from the interaction between hot excitons and doped electrons, which converts the excess energy of hot excitons to excitations of the Fermi sea and hence generates hot electrons. Moreover, nET can be further enhanced by reducing the conduction band offset with an external electric field. Our results provide in-depth insights into the design of HCSCs with electrostatic strategies.

Effects of Surface Plasmon and Catalytic Reactions on Capacitance in Metal-Semiconductor Schottky Diodes

Capacitors, renowned for their high power and energy density attributed to their rapid discharge capabilities, hold significant promise as energy storage devices. While capacitors with various junctions have been the subject of extensive research, there remains a significant knowledge gap concerning the fundamental photoelectron dynamics within metal-semiconductor (MS) junction capacitance, which is crucial for the development of photodevices. Here, we demonstrate the effects of localized surface plasmon resonance (LSPR) on the capacitance behavior of Au/TiO2 Schottky diodes, one of the MS junction diode systems. We have confirmed that the plasmonic Au/TiO2 shows substantial photoresponsiveness, with an increase in net total capacitance under stronger plasmonic effects, as confirmed by wavelength- and power-dependent studies. To enhance our understanding of the electron transfer process occurring at the MS junction, we expand our investigation of capacitance within a catalytic reaction environment, specifically focusing on Pt/TiO2 Schottky diodes. We show the catalytic environment changes capacitance values of the Pt/TiO2, and more active catalytic reactions decrease net total capacitance due to catalytic electrons interfering with diffusion electrons. The LSPR effect and catalytic reaction significantly impact the net total capacitance with a predominant contribution associated with hot electrons, catalytic electrons, and diffusion effects. These findings provide valuable insights for designing and optimizing efficient optical and catalytic devices across diverse applications.

Avalanche Multiplication in Two-Dimensional Layered Materials: Principles and Applications

The avalanche multiplication effect, capable of significantly amplifying weak optical or electrical signals, plays a pivotal role in enhancing the performance of electronic and optoelectronic devices. This effect has been widely employed in devices such as avalanche photodiodes, impact ionization avalanche transit time diode, and impact ionization field-effect transistors, enabling diverse applications in biomedical imaging, 3D LIDAR, high-frequency microwave circuits, and optical fiber communications. However, the evolving demands in these fields require avalanche devices with superior performance, including lower power consumption, reduced avalanche threshold energy, higher efficiency, and improved sensitivity. Over the years, significant efforts have been directed towards exploring novel device architectures and multiplication mechanisms. The emergence of two-dimensional (2D) materials, characterized by their exceptional light-matter interaction, tunable bandgaps, and ease of forming junctions, has opened up new avenues for developing high-performance avalanche devices. This review provides an overview of carrier multiplication mechanisms and key performance metrics for avalanche devices. We discuss several device structures leveraging the avalanche multiplication effect, along with their electrical and optoelectronic properties. Furthermore, we highlight representative applications of avalanche devices in logic circuits, optoelectronic components, and neuromorphic computing systems. By synthesizing the principles and applications of the avalanche multiplication effect, this review aims to offer insightful perspectives on future research directions for 2D material-based avalanche devices.

All-optical diode based on asymmetric nonlinear optical absorption in ternary transition metal chalcogenides

As the typical representative of ternary transition metal chalcogenides, CrPS4 and MnPS3 exhibit unique light-matter interactions, demonstrating great potential in photonic devices. In this work, we systematically studied the nonlinear optical (NLO) responses of CrPS4 and MnPS3 flakes by the I-scan technique. CrPS4 and MnPS3 flakes show saturable absorption and reverse saturation absorption excited by fs laser at wavelengths of 600 nm, respectively. Furthermore, utilizing the non-degenerate transient absorption technology, we observed that the fast and slow carrier relaxation times of CrPS4 at different probe wavelengths were components with constants of about 1.4-3 ps and 490-1420 ps, respectively. Their excellent ultrafast NLO properties imply that they can be applied in photoelectronic fields for advanced and functional devices. Here, we designed and demonstrated an all-optical diode based on a CrPS4/MnPS3 tandem structure by breaking the time-reversal symmetry, and it achieved nonreciprocal transmission of light similar to that of a p-n electron diode.

Tracking and controlling ultrafast charge and energy flow in graphene-semiconductor heterostructures

Low-dimensional materials have left a mark on modern materials science, creating new opportunities for next-generation optoelectronic applications. Integrating disparate nanoscale building blocks into heterostructures offers the possibility of combining the advantageous features of individual components and exploring the properties arising from their interactions and atomic-scale proximity. The sensitization of graphene using semiconductors provides a highly promising platform for advancing optoelectronic applications through various hybrid systems. A critical aspect of achieving superior performance lies in understanding and controlling the fate of photogenerated charge carriers, including generation, transfer, separation, and recombination. Here, we review recent advances in understanding charge carrier dynamics in graphene-semiconductor heterostructures by ultrafast laser spectroscopies. First, we present a comprehensive overview of graphene-based heterostructures and their state-of-the-art optoelectronic applications. This is succeeded by an introduction to the theoretical frameworks that elucidate the fundamental principles and determinants influencing charge transfer and energy transfer-two critical interfacial processes that are vital for both fundamental research and device performance. We then outline recent efforts aimed at investigating ultrafast charge/energy flow in graphene-semiconductor heterostructures, focusing on illustrating the trajectories, directions, and mechanisms of transfer and recombination processes. Subsequently, we discuss effective control knobs that allow fine-tuning of these processes. Finally, we address the challenges and prospects for further investigation in this field.

Photonic Nanomaterials for Wearable Health Solutions

This review underscores the transformative potential of photonic nanomaterials in wearable health technologies, driven by increasing demands for personalized health monitoring. Their unique optical and physical properties enable rapid, precise, and sensitive real-time monitoring, outperforming conventional electrical-based sensors. Integrated into ultra-thin, flexible, and stretchable formats, these materials enhance compatibility with the human body, enabling prolonged wear, improved efficiency, and reduced power consumption. A comprehensive exploration is provided of the integration of photonic nanomaterials into wearable devices, addressing material selection, light-matter interaction principles, and device assembly strategies. The review highlights critical elements such as device form factors, sensing modalities, and power and data communication, with representative examples in skin patches and contact lenses. These devices enable precise monitoring and management of biomarkers of diseases or biological responses. Furthermore, advancements in materials and integration approaches have paved the way for continuum of care systems combining multifunctional sensors with therapeutic drug delivery mechanisms. To overcome existing barriers, this review outlines strategies of material design, device engineering, system integration, and machine learning to inspire innovation and accelerate the adoption of photonic nanomaterials for next-generation of wearable health, showcasing their versatility and transformative potential for digital health applications.

Improving the Efficiency of Semitransparent Perovskite Solar Cell Using Down-Conversion Coating

Perovskite solar cells (PSCs) have demonstrated exceptional efficiency, yet surpassing theoretical performance limits requires innovative methodologies. Among these, down-conversion techniques are pivotal in reducing optical losses and enhancing energy conversion efficiency. In this study, optical modeling, including a generalized transfer-matrix optical model, was employed to meticulously assess optical losses in semitransparent PSCs illuminated from the front and rear sides of the device. To reduce these losses, two down-conversion layers, made of N,N-diphenyl-4-(1,2,2-triphenylethenyl)-benzenamine and 4-(N,N-diphenylamino)benzaldehyde mixed with polymeric binder, were developed, showcasing initial photoluminescence quantum yields of 60% and 50% as films, respectively. The materials luminescence relies on the effect of aggregation-induced emission, which enhances the fluorescence of the dyes within the binder, providing their films with a unique behavior beneficial for photovoltaic applications. An optimization of these layers was performed, which aimed to reduce UV optical losses by adjusting the film thickness atop the PSCs. The refined down-conversion layers yielded a notable increase in the power conversion efficiency by approximately 0.4% for both the front and rear sides of the PSCs, demonstrating their significant potential in pushing the boundaries of solar cell performance.

Plasmonic Bound States in the Continuum Metasurface-Semiconductor-Metal Architecture Enables Efficient Hot-Electron-Based Photodetector

Plasmonic hot-electron-based photodetectors (HEB-PDs) have received widespread attention for their ability to realize effective carrier collection under sub-bandgap illumination. However, due to the low hot electron emission probability, most of the existing HEB-PDs exhibit poor responsivity, which significantly restricts their practical applications. Here, by employing the binary-pore anodic alumina oxide template technique, we proposed a compact plasmonic bound state in continuum metasurface-semiconductor-metal-based (BIC M-S-M) HEB-PD. The symmetry-protected BIC can manipulate a strong gap surface plasmon in the stacked M-S-M structure, which effectively enhances light-matter interactions and improves the photoresponse of the integrated device. Notably, the optimal M-S-M HEB-PD with near-unit absorption (∼90%) around 800 nm delivers a responsivity of 5.18 A/W and an IPCE of 824.23% under 780 nm normal incidence (1 V external bias). Moreover, the ultrathin feature of BIC M-S-M (∼150 nm) on the flexible substrate demonstrates excellent stability under a wide range of illumination angles from -40° to 40° and at the curvature surface from 0.05 to 0.13 mm-1. The proposed plasmonic BIC strategy is very promising for many other hot-electron-related fields, such as photocatalysis, biosensing, imaging, and so on.

Tuning d-Orbital Electronic Structure via Au-Intercalated Two-Dimensional Fe3GeTe2 to Increase Surface Plasmon Activity

While extensive research has been dedicated to plasmon tuning within non-noble metals, prior investigations primarily concentrated on markedly augmenting the inherently low concentration of free carriers in materials with minimal consideration given to the influence of electron orbitals on surface plasmons. Here, we achieve successful intercalation of Au atoms into the layered structure of Fe3GeTe2 (FGT), thereby exerting control over the orbital electronic states or structure of FGT. This intervention not only amplifies the charge density and electron mobility but also mitigates the loss associated with interband transitions, resulting in increased two-dimensional FGT surface plasmon activity. As a consequence, Au-intercalated FGT detects crystal violet molecules as a surface-enhanced Raman scattering substrate, and the detection lines are 3 orders of magnitude higher than before Au intercalation. Our work provides insight for further studies on plasmon effects and the relation between surface plasmon resonance behavior and electronic structures.

Making and Breaking of Exciton Cooling Bottlenecks in Halide Perovskite Nanocrystals

Harnessing quantum confinement (QC) effects in semiconductors to retard hot carrier cooling (HCC) is an attractive approach for enabling efficient hot carrier extraction to overcome the Shockley-Queisser limit. However, there is a debate about whether halide perovskite nanocrystals (PNCs) can effectively exploit these effects. To address this, we utilized pump-probe and multipulse pump-push-probe spectroscopy to investigate HCC behavior in PNCs of varying sizes and cation compositions. Our results validate the presence of an intrinsic phonon bottleneck with clear manifestations of QC effects in small CsPbBr3 PNCs exhibiting slower HCC rates compared to those of larger PNCs. However, the replacement of inorganic Cs+ with organic cations suppresses this intrinsic bottleneck. Furthermore, PNCs exhibit distinct size-dependent HCC behavior in response to changes in the cold carrier densities. We attribute this to the enhanced exciton-exciton interactions in strongly confined PNCs that facilitate Auger heating. Importantly, our findings dispel the existing controversy and provide valuable insights into design principles for engineering QC effects in PNC hot carrier applications.

Extensible Integrated System for Real-Time Monitoring of Cardiovascular Physiological Signals and Limb Health

In recent decades, the rapid growth in flexible materials, new manufacturing technologies, and wearable electronics design techniques has helped establish the foundations for noninvasive photoelectric sensing systems with shape-adaptability and "skin-like" properties. Physiological sensing includes humidity, mechanical, thermal, photoelectric, and other aspects. Photoplethysmography (PPG), an important noninvasive method for measuring pulse rate, blood pressure, and blood oxygen, uses the attenuated signal obtained by the light absorbed and reflected from living tissue to a light source to realize real-time monitoring of human health status. This work illustrates a patch-type optoelectronic system that integrates a flexible perovskite photodetector and all-inorganic light-emitting diodes (LEDs) to realize the real-time monitoring of human PPG signals. The pulse rate of the human body and the swelling degree of finger joints can be extracted and analyzed using photodetectors, thus monitoring human health for the prevention and early diagnosis of certain diseases. Specifically, this work develops a 3D wrinkled-serpentine interconnection wire that increases the shape adaptability of the device in practical applications. The PPG signal sensor reported in this study has considerable potential for future wearable intelligent medical applications.

Decoding the Self-Assembly Plasmonic Interface Structure in a PbS Colloidal Quantum Dot Solid for a Photodetector

Hybrid plasmonic nanostructures have gained enormous attention in a variety of optoelectronic devices due to their surface plasmon resonance properties. Self-assembled hybrid metal/quantum dot (QD) architectures offer a means of coupling the properties of plasmonics and QDs to photodetectors, thereby modifying their functionality. The arrangement and localization of hybrid nanostructures have an impact on exciton trapping and light harvesting. Here, we present a hybrid structure consisting of self-assembled gold nanospheres (Au NSs) embedded in a solid matrix of PbS QDs for mapping the interface structures and the motion of charge carriers. Grazing-incidence small-angle X-ray scattering is utilized to analyze the localization and spacing of the Au NSs within the hybrid structure. Furthermore, by correlating the morphology of the Au NSs in the hybrid structure with the corresponding differences observed in the performance of photodetectors, we are able to determine the impact of interface charge carrier dynamics in the coupling structure. From the perspective of architecture, our study provides insights into the performance improvement of optoelectronic devices.

Hot carrier generation in a strongly coupled molecule-plasmonic nanoparticle system

In strongly coupled light matter systems electronic energy levels become inextricably linked to local electromagnetic field modes. Hybridization of these states opens new relaxation pathways in the system, particularly important for plasmon decay into single electron states, known as hot carriers. We investigate the influence of the coupling strength between a plasmonic resonator and a molecule on hot carrier generation using first principles calculations. An atomistic approach allows the capture of changes in the electronic structure of the system. We show that hot carriers are not only preferably generated at excitation frequencies matching the new polaritonic resonances, but their energy distribution strongly deviates from the one corresponding to the non-interacting system. This indicates existence of new plasmon decay paths due to appearance of hybridized nanoparticle-molecule states. We observe also direct electron transfer between the plasmonic nanoparticle and the molecule. Therefore, we may conclude, that bringing plasmonic nanostructures in strong interaction with molecules gives the ability to manipulate the energy distribution of the generated hot carriers and opens possibility for charge transfer in the system.

Periodic Distributions and Ultrafast Dynamics of Hot Electrons in Plasmonic Resonators

Understanding and managing hot electrons in metals are of fundamental and practical interest in plasmonic studies and applications. A major challenge for the development of hot electron devices requires the efficient and controllable generation of long-lived hot electrons so that they can be harnessed effectively before relaxation. Here, we report the ultrafast spatiotemporal evolution of hot electrons in plasmonic resonators. Using femtosecond-resolution interferometric imaging, we show the unique periodic distributions of hot electrons due to standing plasmonic waves. In particular, this distribution can be flexibly tuned by the size, shape, and dimension of the resonator. We also demonstrate that the hot electron lifetimes are substantially prolonged at hot spots. This appealing effect is interpreted as a result of the locally concentrated energy density at the antinodes in standing hot electron waves. These results could be useful to control the distributions and lifetimes of hot electrons in plasmonic devices for targeted optoelectronic applications.

VSe2-x O x @Pd Sensor for Operando Self-Monitoring of Palladium-Catalyzed Reactions

Operando monitoring of catalytic reaction kinetics plays a key role in investigating the reaction pathways and revealing the reaction mechanisms. Surface-enhanced Raman scattering (SERS) has been demonstrated as an innovative tool in tracking molecular dynamics in heterogeneous reactions. However, the SERS performance of most catalytic metals is inadequate. In this work, we propose hybridized VSe2-x O x @Pd sensors to track the molecular dynamics in Pd-catalyzed reactions. Benefiting from metal-support interactions (MSI), the VSe2-x O x @Pd realizes strong charge transfer and enriched density of states near the Fermi level, thereby strongly intensifying the photoinduced charge transfer (PICT) to the adsorbed molecules and consequently enhancing the SERS signals. The excellent SERS performance of the VSe2-x O x @Pd offers the possibility for self-monitoring the Pd-catalyzed reaction. Taking the Suzuki-Miyaura coupling reaction as an example, operando investigations of Pd-catalyzed reactions were demonstrated on the VSe2-x O x @Pd, and the contributions from PICT resonance were illustrated by wavelength-dependent studies. Our work demonstrates the feasibility of improved SERS performance of catalytic metals by modulating the MSI and offers a valid means to investigate the mechanisms of Pd-catalyzed reactions based on VSe2-x O x @Pd sensors.

Mesoporous CuFe2 O4 Photoanodes for Solar Water Oxidation: Impact of Surface Morphology on the Photoelectrochemical Properties

Metal oxide-based photoelectrodes for solar water splitting often utilize nanostructures to increase the solid-liquid interface area. This reduces charge transport distances and increases the photocurrent for materials with short minority charge carrier diffusion lengths. While the merits of nanostructuring are well established, the effect of surface order on the photocurrent and carrier recombination has not yet received much attention in the literature. To evaluate the impact of pore ordering on the photoelectrochemical properties, mesoporous CuFe₂ O₄ (CFO) thin film photoanodes were prepared by dip-coating and soft-templating. Here, the pore order and geometry can be controlled by addition of copolymer surfactants poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (Pluronic® F-127), polyisobutylene-block-poly(ethylene oxide) (PIB-PEO) and poly(ethylene-co-butylene)-block-poly(ethylene oxide) (Kraton liquid™-PEO, KLE). The non-ordered CFO showed the highest photocurrent density of 0.2 mA/cm² at 1.3 V vs. RHE for sulfite oxidation, but the least photocurrent density for water oxidation. Conversely, the ordered CFO presented the best photoelectrochemical water oxidation performance. These differences can be understood on the basis of the high surface area, which promotes hole transfer to sulfite (a fast hole acceptor), but retards oxidation of water (a slow hole acceptor) due to electron-hole recombination at the defective surface. This interpretation is confirmed by intensity-modulated photocurrent (IMPS) and vibrating Kelvin probe surface photovoltage spectroscopy (VKP-SPS). The lowest surface recombination rate was observed for the ordered KLE-based mesoporous CFO, which retains spherical pore shapes at the surface resulting in fewer surface defects. Overall, this work shows that the photoelectrochemical energy conversion efficiency of copper ferrite thin films is not just controlled by the surface area, but also by surface order.

Schottky barrier effect on plasmon-induced charge transfer

Plasmon-induced charge transfer causes electron-hole spatial separation at the metal-semiconductor interface, which plays a key role in photocatalytic and photovoltaic applications. The Schottky barrier formed at the metal-semiconductor interface can modify the hot carrier dynamics. Taking the Ag-TiO₂ system as an example, we have investigated plasmon-induced charge transfer at the Schottky junction using quantum mechanical simulations. We find that the Schottky barrier induced by n-type doping enhances the electron transfer and that induced by p-type doping enhances the hole transfer, which is attributed to the shift of the Fermi energy and the band bending of the Schottky junction at the interface. The Schottky barrier also modifies the layer distribution of hot carriers. In particular, for the system with a large band bending, there exists electron-hole spatial separation inside the TiO₂ substrate. Our results reveal the mechanism and dynamics of charge transfer at the Schottky junction, and pave the way for manipulating plasmon-assisted photocatalytic and photovoltaic applications.

A Study on Doping and Compound of Zinc Oxide Photocatalysts

As an excellent semiconductor photocatalyst, zinc oxide is widely used in the field of photocatalysis and is regarded as one of the most reliable materials to solve environmental problems. However, because its band gap energy limits the absorption of visible light and reduces the efficiency of catalytic degradation, it needs to be doped with other substances or compounded with other substances and precious metal. This paper summarizes the research on this aspect at home and abroad in recent years, introduces the doping of transition metal ions by zinc oxide, the compounding of zinc oxide with precious metals or other semiconductors, and the prospect of further improving the catalytic efficiency of zno photocatalyst is also put forward.

Photoinduced large polaron transport and dynamics in organic-inorganic hybrid lead halide perovskite with terahertz probes

Organic-inorganic hybrid metal halide perovskites (MHPs) have attracted tremendous attention for optoelectronic applications. The long photocarrier lifetime and moderate carrier mobility have been proposed as results of the large polaron formation in MHPs. However, it is challenging to measure the effective mass and carrier scattering parameters of the photogenerated large polarons in the ultrafast carrier recombination dynamics. Here, we show, in a one-step spectroscopic method, that the optical-pump and terahertz-electromagnetic probe (OPTP) technique allows us to access the nature of interplay of photoexcited unbound charge carriers and optical phonons in polycrystalline CH₃NH₃PbI₃ (MAPbI₃) of about 10 μm grain size. Firstly, we demonstrate a direct spectral evidence of the large polarons in polycrystalline MAPbI₃. Using the Drude-Smith-Lorentz model along with the Frӧhlich-type electron-phonon (e-ph) coupling, we determine the effective mass and scattering parameters of photogenerated polaronic carriers. We discover that the resulting moderate polaronic carrier mobility is mainly influenced by the enhanced carrier scattering, rather than the polaron mass enhancement. While, the formation of large polarons in MAPbI₃ polycrystalline grains results in a long charge carrier lifetime at room temperature. Our results provide crucial information about the photo-physics of MAPbI₃ and are indispensable for optoelectronic device development with better performance.

Formation of carnation-like ZIF-9 nanostructure to achieve superior electrocatalytic oxygen evolution

Rational design and controllable synthesis of metal-organic frameworks nanosheets is critical for electrochemical catalysis. Herein, a carnation-like ZIF-9 nanostructure made of nanosheets is grown on nickel foam (ZIF-9/NF) by a simple one-step solvothermal method, the morphology evolution and the electrocatalytic oxygen evolution properties have been investigated by controlling the solvothermal time. The binder-free ZIF-9-d/NF (60 h, solvothermal time is 60 h) electrode delivers efficient electrocatalytic oxygen evolution reaction activity with low overpotentials of 312 and 337 mV at 50 and 100 mA cm^-2, respectively. Furthermore, ZIF-9-d/NF (60 h) exhibits excellent stability without obvious attenuation for at least 30 h at 200 mA cm^-2. The excellent performances can be attributed to the combination of the highly exposed active sites in the ZIF-9-d nanosheets, as well as the effective electron conduction and mass transfer. This work provides much possibilities for ZIF-9 nanosheets as binder-free electrode for electrocatalyst.