Efficient plasmon-hot electron conversion in Ag-CsPbBr3 hybrid nanocrystals

g-C3N4 as ballistic electron transport "Tunnel" in CsPbBr3-based ternary photocatalyst for gas phase CO2 reduction

Perovskite CsPbBr3 quantum dot shows great potential in artificial photosynthesis, attributed to its outstanding optoelectronic properties. Nevertheless, its photocatalytic activity is hindered by insufficient catalytic active sites and severe charge recombination. In this work, a CsPbBr3@Ag-C3N4 ternary heterojunction photocatalyst is designed and synthesized for high-efficiency CO2 reduction. The CsPbBr3 quantum dots and Ag nanoparticles are chemically anchored on the surface of g-C3N4 sheets, forming an electron transfer tunnel from CsPbBr3 quantum dots to Ag nanoparticles via g-C3N4 sheets. The resulting CsPbBr3@Ag-C3N4 ternary photocatalyst, with spatial separation of photogenerated carriers, achieves a remarkable conversion rate of 19.49 μmol·g-1·h-1 with almost 100 % CO selectivity, a 3.13-fold enhancement in photocatalytic activity as compared to CsPbBr3 quantum dots. Density functional theory calculations reveal the rapid CO2 adsorption/activation and the decreased free energy (0.66 eV) of *COOH formation at the interface of Ag nanoparticles and g-C3N4 in contrast to the g-C3N4, leading to the excellent photocatalytic activity, while the thermodynamically favored CO desorption contributes to the high CO selectivity. This work presents an innovative strategy of constructing perovskite-based photocatalyst by modulating catalyst structure and offers profound insights for efficient CO2 conversion.

Defect Engineering of Metal Halide Perovskite Nanocrystals via Spontaneous Diffusion of Ag Nanocrystals

Perovskite nanocrystals (NCs) have emerged as a promising building block for the fabrication of optic-/optoelectronic-/electronic devices owing to their superior characteristics, such as high absorption coefficient, rapid ion mobilities, and tunable energy levels. However, their low structural stability and poor surface passivation have restricted their application to next-generation devices. Herein, a drug delivery system (DDS)-inspired post-treatment strategy is reported for improving their structural stability by doping of Ag into CsPbBr3 (CPB) perovskite NCs; delivery to damaged sites can promote their structural recovery slowly and uniformly, averting the permanent loss of their intrinsic characteristics. Ag NCs are designed through surface-chemistry tuning and structural engineering to enable their circulation in CPB NC dispersions, followed by their delivery to the CPB NC surface, defect-site recovery, and defect prevention. The perovskite-structure healing process through the DDS-type process (with Ag NCs as the drug) is analyzed by a combination of theoretical calculations (with density functional theory) and experimental analyses. The proposed DDS-inspired healing strategy significantly enhances the optical properties and stability of perovskite NCs, enabling the fabrication of white light-emitting diodes.

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.

All-in-one, all-optical logic gates using liquid metal plasmon nonlinearity

Electronic processors are reaching the physical speed ceiling that heralds the era of optical processors. Multifunctional all-optical logic gates (AOLGs) of massively parallel processing are of great importance for large-scale integrated optical processors with speed far in excess of electronics, while are rather challenging due to limited operation bandwidth and multifunctional integration complexity. Here we for the first time experimentally demonstrate a reconfigurable all-in-one broadband AOLG that achieves nine fundamental Boolean logics in a single configuration, enabled by ultrabroadband (400-4000 nm) plasmon-enhanced thermo-optical nonlinearity (TONL) of liquid-metal Galinstan nanodroplet assemblies (GNAs). Due to the unique heterogeneity (broad-range geometry sizes, morphology, assembly profiles), the prepared GNAs exhibit broadband plasmonic opto-thermal effects (hybridization, local heating, energy transfer, etc.), resulting in a huge nonlinear refractive index under the order of 10-4-10-5 within visual-infrared range. Furthermore, a generalized control-signal light route is proposed for the dynamic TONL modulation of reversible spatial-phase shift, based on which nine logic functions are reconfigurable in one single AOLG configuration. Our work will provide a powerful strategy on large-bandwidth all-optical circuits for high-density data processing in the future.

Epitaxial Heterostructures of CsPbBr3 Perovskite Nanocrystals with Post-transition Metal Bismuth

The facet chemistry of halide perovskite nanocrystals plays a key role in designing nanoscale epitaxial heterostructures. However, despite significant successes achieved in designing these nanocrystals, their heterostructures with several leading transition metals could not be established yet. Herein, the possible heterostructures of metals beyond transition metals are explored and the epitaxial combinations of soft CsPbBr3 nanocrystals with the post-transition metal Bi(0) are reported. These heterostructures are built with interfacing facets having hexagonal atomic configurations of both the rhombicuboctahedron CsPbBr3 and octahedral Bi(0). A high reaction temperature and the presence of alkylamine kept Bi(III) in reduced form and helped in sustaining these CsPbBr3-Bi(0) heteronanocrystals. Since understanding of and synthesis optimization of metal-halide perovskite heterostructures are limited, this finding adds a new fundamental insight in designing ionic and nonionic materials heterojunctions. Furthermore, oxidation and sulfidation of Bi(0) are studied, and the possible oxide/sulfide heterostructures with CsPbBr3 are discussed.

Pt-CsPbBr3 Perovskite Nanocrystal Heterostructures: All Epitaxial

Designing heterostructures of soft ionic nanocrystals with metallic or covalent nanostructures having epitaxial junctions in solution poses several fundamental challenges. Hence, in spite of large successes in developing lead halide perovskite nanocrystals, the chemistry of formation of their facet-directive epitaxial growth of noble metals cannot be explored yet. To address this, herein, epitaxial heterostructures of orthorhombic CsPbBr3 and cubic Pt in multiple directional approaches are reported. Appropriate facets of perovskite nanocrystals and high-temperature reaction are the key parameters for obtaining such nanocrystal heterostructures. Interfacial planes at the junctions having ideal lattice matching helped in establishing the epitaxial relations of (110) of orthorhombic (space group Pbnm) CsPbBr3 with {020} of cubic Pt and again (011) of CsPbBr3 with {111} of Pt. These results provided strong fundamental insights that ionic halide perovskite nanostructures and materials having different crystal phases can be placed in a single building block with continuous sublattice structures.

Mitigating halide ion migration by resurfacing lead halide perovskite nanocrystals for stable light-emitting diodes

Lead halide perovskite nanocrystals are promising for next-generation high-definition displays, especially in light of their tunable bandgaps, high color purities, and high carrier mobility. Within the past few years, the external quantum efficiency of perovskite nanocrystal-based light-emitting diodes has progressed rapidly, reaching the standard for commercial applications. However, the low operational stability of these perovskite nanocrystal-based light-emitting diodes remains a crucial issue for their industrial development. Recent experimental evidence indicates that the migration of ionic species is the primary factor giving rise to the performance degradation of perovskite nanocrystal-based light-emitting diodes, and ion migration is closely related to the defects on the surface of perovskite nanocrystals and at the grain boundaries of their thin films. In this review, we focus on the central idea of surface reconstruction of perovskite nanocrystals, discuss the influence of surface defects on halide ion migration, and summarize recent advances in resurfacing perovskite nanocrystal strategies toward mitigating halide ion migration to improve the stability of the as-fabricated light-emitting diode devices. From the perspective of perovskite nanocrystal resurfacing, we set out a promising research direction for improving both the spectral and operational stability of perovskite nanocrystal-based light-emitting diodes.

Vibrationally Hot Reactants in a Plasmon-Assisted Chemical Reaction

Recent studies on plasmon-assisted chemical reactions postulate that the hot electrons of plasmon-excited nanostructures may induce a non-thermal vibrational activation of metal-bound reactants. However, the postulate has not been fully validated at the level of molecular quantum states. We directly and quantitatively prove that such activation occurs on plasmon-excited nanostructures: The anti-Stokes Raman spectra of reactants undergoing a plasmon-assisted reaction reveal that a particular vibrational mode of the reactant is selectively excited, such that the reactants possess >10 times more energy in the mode than is expected from the fully thermalized molecules at the given local temperature. Furthermore, a significant portion (∼20%) of the excited reactant is in vibrational overtone states with energies exceeding 0.5 eV. Such mode-selective multi-quantum excitation could be fully modeled by the resonant electron-molecule scattering theory. Such observations suggest that the vibrationally hot reactants are created by non-thermal hot electrons, not by thermally heated electrons or phonons of metals. The result validates the mechanism of plasmon-assisted chemical reactions and further offers a new method to explore the vibrational reaction control on metal surfaces.

Tandem Efficient Bromine Removal and Silver Recovery by Resorcinol-Formaldehyde Resin Nanoparticles

Halogen wastewater greatly threatens the health of human beings and aquatic organisms due to its severe toxicity, corrosiveness, and volatility. Efficient bromine removal is therefore urgently required, while existing Br₂-capture materials often face challenges from limited water stability and possible halogen leaking. We report a facile and efficient aqueous Br₂ removal method using submicron resorcinol-formaldehyde (RF) resin nanoparticles (NPs). The abundant aromatic groups dominate the Br₂ removal by substitution reactions. An excellent Br₂ conversion capacity of 7441 mg g_RF^-1 was achieved by RF NPs that outperform state-of-the-art materials by ∼2-fold, along with advantages including good water stability, low cost, and easy fabrication. Two recycling-coupled (electrochemical or H₂O₂-involved) Br₂ removal routes further reveal the feasibility of in-depth halogen removal by RF NPs. The brominated resin can be downstream upcycled for silver recovery, realizing the harvesting of precious metal, reducing of heavy-metal pollution, and resource utilization of brominated resin.

Deciphering Adverse Detrapped Hole Transfer in Hot-Electron Photoelectric Conversion at Infrared Wavelengths

Hot-carrier devices are promising alternatives for enabling path breaking photoelectric conversion. However, existing hot-carrier devices suffer from low efficiencies, particularly in the infrared region, and ambiguous physical mechanisms. In this work, the competitive interfacial transfer mechanisms of detrapped holes and hot electrons in hot-carrier devices are discovered. Through photocurrent polarity research and optical-pump-THz-probe (OPTP) spectroscopy, it is verified that detrapped hole transfer (DHT) and hot-electron transfer (HET) dominate the low- and high-density excitation responses, respectively. The photocurrent ratio assigned to DHT and HET increases from 6.6% to over 1133.3% as the illumination intensity decreases. DHT induces severe degeneration of the external quantum efficiency (EQE), especially at low illumination intensities. The EQE of a hot-electron device can theoretically increase by over two orders of magnitude at 10 mW cm^-2 through DHT elimination. The OPTP results show that competitive transfer arises from the carrier oscillation type and carrier-density-related Coulomb screening. The screening intensity determines the excitation weight and hot-electron cooling scenes and thereby the transfer dynamics.

Ultralong Lifetime of Plasmon-Excited Electrons Realized in Nonepitaxial/Epitaxial Au@CdS/CsPbBr3 Triple-Heteronanocrystals

Combination of the strong light-absorbing power of plasmonic metals with the superior charge carrier dynamics of halide perovskites is appealing for bio-inspired solar-energy conversion due to the potential to acquire long-lived plasmon-induced hot electrons. However, the direct coupling of these two materials, with Au/CsPbBr₃ heteronanocrystals (HNCs) as a prototype, results in severe suppression of plasmon resonances. The present work shows that interfacial engineering is a key knob for overcoming this impediment, based on the creation of a CdS mediate layer between Au and CsPbBr₃ forming atomically organized Au-CdS and CdS-CsPbBr₃ interfaces by nonepitaxial/epitaxial combined strategy. Transient spectroscopy studies demonstrate that the resulting Au@CdS/CsPbBr₃ HNCs generate remarkably long-lived plasmon-induced charge carriers with lifetime up to nanosecond timescale, which is several orders of magnitude longer than those reported for colloidal plasmonic metal-semiconductor systems. Such long-lived carriers extracted from plasmonic antennas enable to drive CO₂ photoreduction with efficiency outperforming previously reported CsPbBr₃ -based photocatalysts. The findings disclose a new paradigm for achieving much elongated time windows to harness the substantial energy of transient plasmons through realization of synergistic coupling of plasmonic metals and halide perovskites.

Facets-Directed Epitaxially Grown Lead Halide Perovskite-Sulfobromide Nanocrystal Heterostructures and Their Improved Photocatalytic Activity

Lead halide perovskite nanocrystal heterostructures have been extensively studied in the recent past for improving their photogenerated charge carriers mobility. However, most of such heterostructures are formed with random connections without having strong evidence of epitaxial relation. Perovskite-chalcohalides are the first in this category, where all-inorganic heterostructures are formed with epitaxial growth. Going beyond one facet, herein, different polyhedral nanocrystals of CsPbBr₃ are explored for facet-selective secondary epitaxial sulfobromide growths. Following a decoupled synthesis process, the heterojunctions are selectively established along {110} as well as {200} facets of 26-faceted rhombicuboctahedrons, the {110} facets of armed hexapods, and the {002} facets of 12-faceted dodecahedron nanocrystals of orthorhombic CsPbBr₃. Lattice matching induced these epitaxial growths, and their heterojunctions have been extensively studied with electron microscopic imaging. Unfortunately, these heterostructures did not retain the intense host emission because of their indirect band structures, but such combinations are found to be ideal for promoting photocatalytic CO₂ reduction. The pseudo-Type-II combination helped here in the successful movement of charge carriers and also improved the rate of catalysis. These results suggest that facet-selective all-inorganic perovskite heterostructures can be epitaxially grown and this could help in improving their catalytic activities.

Organic Photodetectors with Extended Spectral Response Range Assisted by Plasmonic Hot-Electron Injection

Organic photodetectors (OPDs) have aroused intensive attention for signal detection in industrial and scientific applications due to their advantages including low cost, mechanical flexibility, and large-area fabrication. As one of the most common organic light-emitting materials, 8-hydroxyquinolinato aluminum (Alq₃) has an absorption wavelength edge of 460 nm. Here, through the introduction of Ag nanoparticles (Ag NPs), the spectral response range of the Alq₃-based OPD was successfully extended to the near-infrared range. It was found that introducing Ag NPs can induce rich plasmonic resonances, generating plenty of hot electrons, which could be injected into Alq₃ and then be collected. Moreover, as a by-product of introducing Ag NPs, the dark current was suppressed by around two orders of magnitude by forming a Schottky junction on the cathode side. These two effects in combination produced photoelectric signals with significant contrasts at wavelengths beyond the Alq₃ absorption band. It was found that the OPD with Ag NPs can stably generate electric signals under illumination by pulsed 850 nm LED, while the output of the reference device included no signal. Our work contributes to the development of low-cost, broadband OPDs for applications in flexible electronics, bio-imaging sensors, etc.

Silver based photocatalysts in emerging applications

The infinite availability of solar energy grants the potential of fulfilling the energy demands and environmental sustainability requirements with more feasible and reliant renewable energy forms through photocatalysis. In the past decade, the intensive plasmonic effect, suitable work function, superior electrical conductivity and physiochemical properties have made Ag-based photocatalysts attractive components for emerging applications. The local surface plasmon resonance effect (LSPR) provides extra hot-carriers to participate in the photocatalytic process, and Schottky/Ohmic contacts would facilitate charge transfer. Here, recent studies focused on Ag-based photocatalysts for emerging applications are reviewed. Notably, the mechanisms of LSPR, the Schottky barrier and ohmic contacts are introduced together with urgent issues in CO₂ reduction, antibacterial application, H₂ generation, and environmental hazard removal. Additionally, some perspectives and directions on more comprehensive designs on material system, band alignment and functionalization are given to further the exploration in this research area.

All Optical Switching through Anistropic Gain of CsPbBr3 Single Crystal Microplatelet

Perovskite micro/nanostructures have recently emerged as a highly attractive gain material for nanolasers. To explore their applications and further improve performance, it is essential to understand the optical gain and the anisotropic properties. Herein, we obtained high quality CsPbBr₃ microplatelets (MP) with anisotropic orthorhombic phase. Optical gain of CsPbBr₃ single crystal MP was investigated via microscale variable stripe-length measurement. A polarization-dependent optical gain was observed, and the gain along [002] was larger than that of [1-10]. The behavior was attributed to the lowest energy transition dipole moment of [002] induced by the smaller deviation of Br-Pb-Br bond from the perfect lattice. Along the [002] direction, we obtained the optical gain value up to 5077 cm^-1, which is the record value ever reported. Moreover, all optical switching of lasing is realized by periodical polarized excitation. Our results provide new perceptions in the design of novel functional anisotropic devices based on perovskite micro/nanostructures.

Wavelength-Dependent Bifunctional Plasmonic Photocatalysis in Au/Chalcopyrite Hybrid Nanostructures

Excited, or "hot" charge carrier generation and transfer driven by the decay of localized surface plasmon resonances (LSPRs) are key steps in plasmonic photocatalysis. Hybrid structures that contain both metal and semiconductor building blocks facilitate the extraction of reactive charge carriers and their utilization for photoelectrocatalysis. Additional functionality arises from hybrid structures that combine noble metal nanostructures with semiconductor components, such as chalcopyrite (CuFeS₂) nanocrystals (NCs), which by themselves support quasistatic resonances. In this work, we use a hybrid membrane to integrate Au nanorods (NRs) with a longitudinal LSPR at 745 nm and CuFeS₂ NCs with a resonance peak at 490 nm into water-stable nanocomposites for robust and bifunctional photocatalysis of oxygen and hydrogen evolution reactions in a wavelength-dependent manner. Excitation of NRs or NCs in the nanocomposite correlates with increased hydrogen or oxygen evolution, respectively, consistent with a light-driven electron transfer between the metal and semiconductor building blocks, the direction of which depends on the wavelength. The bifunctional photoreactivity of the nanocomposite is enhanced by Cu(I)/Cu(II)-assisted catalysis on the surface of the NCs.

Plasmon-induced trap filling at grain boundaries in perovskite solar cells

The deep-level traps induced by charged defects at the grain boundaries (GBs) of polycrystalline organic-inorganic halide perovskite (OIHP) films serve as major recombination centres, which limit the device performance. Herein, we incorporate specially designed poly(3-aminothiophenol)-coated gold (Au@PAT) nanoparticles into the perovskite absorber, in order to examine the influence of plasmonic resonance on carrier dynamics in perovskite solar cells. Local changes in the photophysical properties of the OIHP films reveal that plasmon excitation could fill trap sites at the GB region through photo-brightening, whereas transient absorption spectroscopy and density functional theory calculations correlate this photo-brightening of trap states with plasmon-induced interfacial processes. As a result, the device achieved the best efficiency of 22.0% with robust operational stability. Our work provides unambiguous evidence for plasmon-induced trap occupation in OIHP and reveals that plasmonic nanostructures may be one type of efficient additives to overcome the recombination losses in perovskite solar cells and thin-film solar cells in general.

Enhancing Photoelectric Response of an Au@Ag/AgI Schottky Contact through Regulation of Localized Surface Plasmon Resonance

Carrier generation and migration are both pivotal to photoelectric (PE) response. Formation of a Schottky contact is conducive to promote carrier migration but cannot fundamentally magnify carrier generation, limiting the eventual PE performance. In this work, an Au@Ag/AgI Schottky contact is established by in situ growth of AgI nanotriangles on the surface of Au@Ag nanoparticles (NPs), and PE enhancement of the Schottky contact is realized by regulating localized surface plasmon resonance (LSPR) properties. In comparison with Ag/AgI Schottky contact, assembly of Au NPs in the center of Ag NPs adjusts the dominated LSPR property from hot-electron transfer (HET) to plasmon-induced resonance energy transfer (PIRET). With the concurrent manipulation of HET and PIRET, additional energy can be employed for carrier generation, while photogenerated electrons offset by hot electrons are reduced, which jointly enlarges PE responses of the Au@Ag/AgI Schottky contact up to 4 times. Benefitted from the etching of thiols to Ag-based materials, the Au@Ag/AgI Schottky contact is further applied to the construction of a photoelectrochemical cysteine sensor. This work proposes a general strategy to enhance PE responses of Schottky contacts, which may advance the design of LSPR-related PE systems.

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

Hot charge carriers (HC) are photoexcited electrons and holes that exist in nonequilibrium high-energy states of photoactive materials. Prolonged cooling time and rapid extraction are the current challenges for the development of future innovative HC-based optoelectronic devices, such as HC solar cells (HCSCs), hot energy transistors (HETs), HC photocatalytic reactors, and lasing devices. Based on a thorough analysis of the basic mechanisms of HC generation, thermalization, and cooling dynamics, this review outlines the various possible strategies to delay the HC cooling as well as to speed up their extraction. Various materials with slow cooling behavior, including perovskites and other semiconductors, are thoroughly presented. In addition, the opportunities for the generation of plasmon-induced HC through surface plasmon resonance and their technological applications in hybrid nanostructures are discussed in detail. By judiciously designing the plasmonic nanostructures, the light coupling into the photoactive layer and its optical absorption can be greatly enhanced as well as the successful conversion of incident photons to HC with tunable energies can also be realized. Finally, the future outlook of HC in optoelectronics is highlighted which will provide great insight to the research community.

Construction of Single-Atom Platinum Catalysts Enabled by CsPbBr3 Nanocrystals

Lead halide perovskite nanocrystals (CsPbX₃ NCs) have been regarded as promising materials in photocatalysis. Combining metal single atoms with CsPbX₃ NCs may be a practical way in exploring perovskite-based catalysts. However, such hybrids have not been achieved experimentally yet, mainly due to the weak interaction between the metal atom and the CsPbX₃ surface. Here, we demonstrate that Pt single atoms can be deposited on CsPbBr₃ NCs through a photoassisted approach, in which the surface was partially oxidized first, followed by the anchoring of Pt single atoms through the formation of Pt-O and Pt-Br bonds. The deposition of Pt single atoms can significantly change the photophysical properties of CsPbBr₃ NCs by inducing the generation of deep trap states in the band gap. The as-prepared Pt-SA/CsPbBr₃ can be used as efficient and durable catalysts for photocatalytic semi-hydrogenation of propyne. A CsPbBr₃ nanocrystal might be a suitable substrate for anchoring other metal single atoms, such as Cu, Au, Ag, Pd, and so on.

Plasmon Energy Transfer in Hybrid Nanoantennas

Plasmonic metal nanoparticles exhibit large dipole moments upon photoexcitation and have the potential to induce electronic transitions in nearby materials, but fast internal relaxation has to date limited the spatial range and efficiency of plasmonic mediated processes. In this work, we use photo-electrochemistry to synthesize hybrid nanoantennas comprised of plasmonic nanoparticles with photoconductive polymer coatings. We demonstrate that the formation of the conductive polymer is selective to the nanoparticles and that polymerization is enhanced by photoexcitation. In situ spectroscopy and simulations support a mechanism in which up to 50% efficiency of nonradiative energy transfer is achieved. These hybrid nanoantennas combine the unmatched light-harvesting properties of a plasmonic antenna with the similarly unmatched device processability of a polymer shell.

Quantitative Single-Particle Fluorescence Imaging Elucidates Semiconductor Shell Influence on Ag@TiO2 Photocatalysis

The understanding of the structure-reactivity relationship is helpful for the nanocatalyst (NC) design. However, though precisely parse, this information is challenging due to the heterogeneity of NCs and the complex mechanism of energetic charge carrier (e^-/h⁺ pairs) generation and transfer within the catalysts upon light irradiation. Here, the effect of the semiconductor shell on the photocatalytic redox reaction is probed at the single-Ag@TiO₂ NC level with single-molecule imaging. By engineering the TiO₂ shell thickness, catalytic activities of the NCs are precisely controlled and quantitatively measured to show a parabolic-like distribution with increasing TiO₂ thickness. Besides, the varied activity among different NCs and the dynamic activity fluctuation of single NCs during continuous redox conversion are observed. Mathematical analysis indicates that the TiO₂ layer affects the activity of the core-shell NCs by simultaneously affecting the fate of photo-induced e^-/h⁺ pairs and hot electrons generated at the Ag core. This work sheds light on molecular-scale elucidation of the impact of metal-semiconductor NC structures on their reactivities.

Prospects of Coupled Organic-Inorganic Nanostructures for Charge and Energy Transfer Applications

We review the field of organic-inorganic nanocomposites with a focus on materials that exhibit a significant degree of electronic coupling across the hybrid interface. These nanocomposites undergo a variety of charge and energy transfer processes, enabling optoelectronic applications in devices which exploit singlet fission, triplet energy harvesting, photon upconversion or hot charge carrier transfer. We discuss the physical chemistry of the most common organic and inorganic components. Based on those we derive synthesis and assembly strategies and design criteria on material and device level with a focus on photovoltaics, spin memories or optical upconverters. We conclude that future research in the field should be directed towards an improved understanding of the binding motif and molecular orientation at the hybrid interface.

Photon management to reduce energy loss in perovskite solar cells

Despite the rapid development of perovskite solar cells (PSCs) over the past few years, the conversion of solar energy into electricity is not efficient enough or cost-competitive yet. The principal energy loss in the conversion of solar energy to electricity fundamentally originates from the non-absorption of low-energy photons ascribed to Shockley-Queisser limits and thermalization losses of high-energy photons. Enhancing the light-harvesting efficiency of the perovskite photoactive layer by developing efficient photo management strategies with functional materials and arrays remains a long-standing challenge. Here, we briefly review the historical research trials and future research trends to overcome the fundamental loss mechanisms in PSCs, including upconversion, downconversion, scattering, tandem/graded structures, texturing, anti-reflection, and luminescent solar concentrators. We will deeply emphasize the availability and analyze the importance of a fine device structure, fluorescence efficiency, material proportion, and integration position for performance improvement. The unique energy level structure arising from the 4fn inner shell configuration of the trivalent rare-earth ions gives multifarious options for efficient light-harvesting by upconversion and downconversion. Tandem or graded PSCs by combining a series of subcells with varying bandgaps seek to rectify the spectral mismatch. Plasmonic nanostructures function as a secondary light source to augment the light-trapping within the perovskite layer and carrier transporting layer, enabling enhanced carrier generation. Texturing the interior using controllable micro/nanoarrays can realize light-matter interactions. Anti-reflective coatings on the top glass cover of the PSCs bring about better transmission and glare reduction. Photon concentration through perovskite-based luminescent solar concentrators offers a path to increase efficiency at reduced cost and plays a role in building-integrated photovoltaics. Distinct from other published reviews, we here systematically and hierarchically present all of the photon management strategies in PSCs by presenting the theoretical possibilities and summarizing the experimental results, expecting to inspire future research in the field of photovoltaics, phototransistors, photoelectrochemical sensors, photocatalysis, and especially light-emitting diodes. We further assess the overall possibilities of the strategies based on ultimate efficiency prospects, material requirements, and developmental outlook.

Shape-controlled synthesis of Ag/Cs4PbBr6 Janus nanoparticles

The poor light absorption of visible light for Cs4PbBr6 nanocrystals (NCs) has severely impeded their practical applications. Although the semiconductor/perovskite heterostructure holds great promise for enhancement in absorption, it has remained a serious challenge for synthesizing a semiconductor/perovskite heterostructure. In this work, monodispersed Janus heterostructures composed of Cs4PbBr6 decorated with either multiple Ag or single Ag on its surface (named as mAg/Cs4PbBr6 and sAg/Cs4PbBr6 respectively), are successfully prepared. The size of Ag seeds has an important effect on the shape of the products. Small-sized Ag seeds lead to the formation of mAg/Cs4PbBr6 Janus NCs, while relatively large-sized Ag seeds produce sAg/Cs4PbBr6 Janus NCs. It is noted that this work not only provides a novel method for the modification of individual Cs4PbBr6 NCs, but also enhances the absorption of the Cs4PbBr6 in the visible region, indicating great potential for optoelectronic applications, such as photocatalysis and solar cells.

Extremely Low Program Current Memory Based on Self-Assembled All-Inorganic Perovskite Single Crystals

Memory devices based on lead halide perovskite have attracted great interests because of their unique current-voltage hysteresis. However, current memory devices based on polycrystalline perovskites usually suffer from large intrinsic electronic current and parasitic leakage current due to the existence of grain boundaries, which further leads to high power consumption. Here, a low-power resistance switching random-access memory device is demonstrated by assembling single-crystalline CsPbBr₃ on Ag electrodes. The assembled structure serves as a bipolar nonvolatile resistance switching memory device with a low program current (∼10 nA), good endurance, long data retention (>10³ S), and big on/off ratio of ∼10³. The low program current results in a power of ∼3 × 10^-8 W, which is much lower than that of polycrystalline perovskite-based devices (10^-1-10^-6 W). It is found that the formation and annihilation of Ag and bromide vacancy conductive filaments contribute to the significant resistive switching effect. At a low resistive state, the conductive filaments originate from the accumulation of Br^- ions at the drain. Furthermore, the conductive filaments are proved to be a cone shape, shrinking from the drain to the source.

Surface modification of all-inorganic halide perovskite nanorods by a microscale hydrophobic zeolite for stable and sensitive laser humidity sensing

Perovskite materials are very sensitive to the environment which is beneficial for humidity sensing. However, the existing illuminating humidity sensor has low luminous efficiency and sensitivity. Besides, the stability of perovskite materials remains a key issue to be resolved. Compared to luminescence, lasing is much more sensitive to the surrounding environmental situation. However, humidity sensing based on perovskite lasing has not been reported so far. In this work, all-inorganic halide perovskite CsPbBr3 nanorods with an optical gain coefficient as high as 954 cm-1 were designed and fabricated. Moreover, a microscale hydrophobic zeolite was introduced to modify perovskites for improved stability. It is interesting to note that the hydrophobic zeolite introduces strong scattering which is beneficial for three-dimensional random lasing with a quality (Q) factor of 2263. Through the strategy of using lasing instead of luminescence, optical stability and sensitive laser humidity sensing were demonstrated, and it exhibits high sensitivity and good reliability. This work provides a new idea of improved stability of perovskites, which will promote the practical application of perovskite materials and devices.

Recent Advances in Plasmonic Perovskite Solar Cells

Perovskite solar cells (PSCs) have emerged recently as promising candidates for next generation photovoltaics and have reached power conversion efficiencies of 25.2%. Among the various methods to advance solar cell technologies, the implementation of nanoparticles with plasmonic effects is an alternative way for photon and charge carrier management. Surface plasmons at the interfaces or surfaces of sophisticated metal nanostructures are able to interact with electromagnetic radiation. The properties of surface plasmons can be tuned specifically by controlling the shape, size, and dielectric environment of the metal nanostructures. Thus, incorporating metallic nanostructures in solar cells is reported as a possible strategy to explore the enhancement of energy conversion efficiency mainly in semi-transparent solar cells. One particularly interesting option is PSCs with plasmonic structures enable thinner photovoltaic absorber layers without compromising their thickness while maintaining a high light harvest. In this Review, the effects of plasmonic nanostructures in electron transport material, perovskite absorbers, the hole transport material, as well as enhancement of effective refractive index of the medium and the resulting solar cell performance are presented. Aside from providing general considerations and a review of plasmonic nanostructures, the current efforts to introduce these plasmonic structures into semi-transparent solar cells are outlined.

Dynamics of Photo-Generated Carriers across the Interface between CsPbBr3 Nanocrystals and Au-Ag Nanostructured Film, and Its Control via Ultrathin MgO Interface Layer

The dynamics and control of charge transfer between optoelectronically interesting and size-tunable halide perovskite quantum dots and other juxtaposed functional electronic materials are important issues for the emergent device interest involving such a family of materials in heterostructure configurations. Herein, we have grown bimetallic Au-Ag thin films on glass by pulsed laser deposition at room temperature, which bear nanoparticulate character, and the corresponding optical absorption spectra reveal the expected surface plasmon resonance signature(s). Subsequently, spin-coated CsPbBr₃ nanoparticle films onto the bimetallic Au-Ag films exhibit surface-enhanced Raman scattering as well as strong photoluminescence quenching, the latter reflecting highly efficient transfer of photo-generated carriers across the CsPbBr₃/Au-Ag interface. Surprisingly, when an ultrathin MgO (insulating) layer of optimum thickness is introduced between the CsPbBr₃ and Au-Ag films, the charge transfer is further facilitated with the average lifetime of carriers becoming even shorter. By changing the thickness of the thin MgO layer, the carrier lifetime can in fact be tuned; with the charge transfer getting fully blocked for thick enough MgO layers, as expected. Our study thus throws light on the charge-carrier dynamics in halide perovskites, which is of importance to emergent optoelectronic applications.

Gas-assisted transformation of gold from fcc to the metastable 4H phase

The metastable hexagonal 4H-phase gold has recently attracted extensive interest due to its exceptional performance in catalysis. However, gold usually crystallizes to its lowest free energy structure called face-centered cubic (fcc). The phase transformation from the stable fcc phase to the metastable 4H phase is thus of great significance in crystal phase engineering. Herein, we report this unusual phenomenon on a 4H gold nanorod template with the aid of CO gas and an electron beam. In situ transmission electron microscopy was used to directly visualize the interface propagation kinetics between the 4H-Au-nanorod and fcc-Au nanoparticle. Epitaxial growth was initiated at the contact interface, and then propagated to convert all parts of these fcc nanoparticles to 4H phase. Density functional theory calculations and ab initio molecular dynamics simulations show that the CO molecules can assist the Au diffusion process and promote the flexibility of Au particles during the epitaxial growth. The phase transformation was driven by the reduction of Gibbs free energy by eliminating the interface between fcc and 4H phases.

Crystal phase engineering enables the growth of nanostructures with controlled crystal phases that show superior functional properties. Here, the authors find that CO gas-metal atom interactions combined with the electron beam can trigger phase transformations of precious metals at room temperature.

Exploring Energy Transfer in a Metal/Perovskite Nanocrystal Antenna to Drive Photocatalysis

The use of all-inorganic perovskite nanocrystals (PeNCs) in photocatalytic systems has been limited because of their instability in polar solvents. Encapsulation of PeNCs in inorganic or polymeric matrices has been shown to be effective in overcoming such instability issues, yet studies on charge and energy extraction from these composite systems are still rare. Herein, we explore the capacity of CsPbBr₃ PeNC/AlO_x composite films to drive chemical reactions by coupling them to plasmonic AgNCs. AlO_x is used both as a stabilizing layer and as a spacer to study distance-dependent excitation energy transfer, which reveals a migration of energy from the PeNCs toward the AgNCs. We then utilize this pooled energy for a plasmon-mediated methylene blue desorption where we demonstrate enhancement effects of spectral and spatial absorption on the reaction outcome due to the coupling to PeNCs.

Multicomponent Plasmonic Nanoparticles: From Heterostructured Nanoparticles to Colloidal Composite Nanostructures

Plasmonic nanostructures possessing unique and versatile optoelectronic properties have been vastly investigated over the past decade. However, the full potential of plasmonic nanostructure has not yet been fully exploited, particularly with single-component homogeneous structures with monotonic properties, and the addition of new components for making multicomponent nanoparticles may lead to new-yet-unexpected or improved properties. Here we define the term "multi-component nanoparticles" as hybrid structures composed of two or more condensed nanoscale domains with distinctive material compositions, shapes, or sizes. We reviewed and discussed the designing principles and synthetic strategies to efficiently combine multiple components to form hybrid nanoparticles with a new or improved plasmonic functionality. In particular, it has been quite challenging to precisely synthesize widely diverse multicomponent plasmonic structures, limiting realization of the full potential of plasmonic heterostructures. To address this challenge, several synthetic approaches have been reported to form a variety of different multicomponent plasmonic nanoparticles, mainly based on heterogeneous nucleation, atomic replacements, adsorption on supports, and biomolecule-mediated assemblies. In addition, the unique and synergistic features of multicomponent plasmonic nanoparticles, such as combination of pristine material properties, finely tuned plasmon resonance and coupling, enhanced light-matter interactions, geometry-induced polarization, and plasmon-induced energy and charge transfer across the heterointerface, were reported. In this review, we comprehensively summarize the latest advances on state-of-art synthetic strategies, unique properties, and promising applications of multicomponent plasmonic nanoparticles. These plasmonic nanoparticles including heterostructured nanoparticles and composite nanostructures are prepared by direct synthesis and physical force- or biomolecule-mediated assembly, which hold tremendous potential for plasmon-mediated energy transfer, magnetic plasmonics, metamolecules, and nanobiotechnology.

Deciphering the Ultrafast Nonlinear Optical Properties and Dynamics of Pristine and Ni-Doped CsPbBr3 Colloidal Two-Dimensional Nanocrystals

While the unabated race persists in achieving record efficiencies in solar cells and other photonic/optoelectronic devices using lead halide perovskite absorbers, a comprehensive picture of the correlated third-order nonlinear optical (NLO) properties is yet to be established. The present study is aimed at deciphering the role of dopants in multiphoton absorption properties of intentionally engineered CsPbBr₃ colloidal nanocrystals (NCs). The charge separation of the plasmon-semiconductor conduction band owing to the hot electron transfer at the interface was demystified using the dynamics of the bleached spectral data from femtosecond (fs) transient absorption spectroscopy with broadband capabilities. The NLO properties studied through the fs Z-scan technique revealed that Ni-doped CsPbBr₃ NCs exhibited strong third-order NLO susceptibility of ∼10^-10 esu. The exotic photophysical phenomena in these pristine and Ni-doped CsPbBr₃ colloidal two-dimensional (2D) NCs reported herein are believed to provide the avenues to address the critical variables involved in the structural differences and their correlated optoelectronic properties.

Perovskites with d-block metals for solar energy applications

Pb2+ halide organic-inorganic perovskites are excellent semiconductors for use in solar energy applications, but at the expense of robustness and environmental compatibility. Tin (Sn), which sits just above lead in the periodic table, forms pure (or mixed with lead) perovskites when at the 2+ or 4+ oxidation state. It can act as a promising alternative; however, there are still some serious concerns regarding its suitability. This presents a major challenge; viable metal cations have to be identified. A good number of elements, originating from a large range of d-block metal ions, with adequate oxidation states, moderate toxicity, and relative abundance, seem ideal for this purpose. In this review, we present the most characteristic perovskites (conventional perovskites, layered, or double perovskites) that can be formed with the help of these metals. We focus on d-block metal ions with stable oxidation states, such as Ag+ or Ti4+, which have exhibited satisfactory photovoltaic properties until now. Further, we highlight the results involving compounds other than halide perovskites, such as oxides, chalcogenides, and nitrides (as well as oxyhalides, oxysulfides, and oxynitrides); a few of them are ferroelectric (based on Ti4+, Zr4+, Fe3+, and Cr3+) and can yield a photovoltage that exceeds the bandgap of the material. Finally, we present the critical challenges that currently limit the efficiency of these systems and propose prospects for future directions.

Luminescent perovskites: recent advances in theory and experiments

This review summarizes previous research on luminescent perovskites, including oxides and halides, with different structural dimensionality. The relationship between the crystal structure, electronic structure and properties is discussed in detail.