High-efficiency upconversion luminescence in UCNPs/CsPbBr3 nanocomposites enhanced by silver nanoparticles.

Upconversion nanocomposites with multiple light-emitting centers have attracted great attention as functional materials, but their low efficiency limits their further applications. Herein, a novel, to the best of our knowledge, system for nanocomposites consisting of upconversion nanoparticles (UCNPs) and perovskite quantum dots (PeQDs) assembled with Ag nanoparticles (NPs) is proposed. Upconversion luminescence (UCL) operation from PeQDs is triggered by near-infrared (NIR) sensitization through Förster resonance energy transfer (FRET) and photon reabsorption (PR). Especially, the photoluminescence (PL) emission efficiency is found to be significantly enhanced due to the increased energy transfer efficiency and radiative decay rate in the UCNPs/CsPbBr3 nanocomposites. The results offer new opportunities to improve the UCL properties of perovskites and open new development in the fields of LED lighting, solar cells, biomedicine, and so on.

Synergistic luminescence effect and high-pressure optical properties of CsPbBr2Cl@EuMOFs nanocomposites.

Metal-organic frameworks (MOFs) are a class of highly porous materials that have garnered significant attention in the field of optoelectronics due to their exceptional properties. In this study, CsPbBr2Cl@EuMOFs nanocomposites were synthesized using a two-step method. The fluorescence evolution of the CsPbBr2Cl@EuMOFs was investigated under high pressure, revealing a synergistic luminescence effect between CsPbBr2Cl and Eu3+. The study found that the synergistic luminescence of CsPbBr2Cl@EuMOFs remains stable even under high pressure, and there is no energy transfer among different luminous centers. These findings provide a meaningful case for future research on nanocomposites with multiple luminescent centers. Additionally, CsPbBr2Cl@EuMOFs exhibit a sensitive color-changing mechanism under high pressure, making them a promising candidate for pressure calibration via the color change of the MOF materials.

Pressure-induced distinct excitonic properties of 2D perovskites with isomeric organic molecules for spacer cations.

Spacer organic cations in two-dimensional (2D) perovskites play vital roles in inducing structural distortion of the inorganic components and dominating unique excitonic properties. However, there is still little understanding of spacer organic cations with identical chemical formulas, and different configurations have an impact on the excitonic dynamics. Herein, we investigate and compare the evolution of the structural and photoluminescence (PL) properties of [CH3(CH2)4NH3]2PbI4 ((PA)2PbI4) and [(CH3)2CH(CH2)2NH3]2PbI4 ((PNA)2PbI4) with isomeric organic molecules for spacer cations by combining steady-state absorption, PL, Raman and time-resolved PL spectra under high pressures. Intriguingly, the band gap is continuously tuned under pressure and decreased to 1.6 eV at 12.5 GPa for (PA)2PbI4 2D perovskites. Simultaneously, multiple phase transitions occur and the carrier lifetimes are prolonged. In contrast, the PL intensity of (PNA)2PbI4 2D perovskites exhibits an almost 15-fold enhancement at 1.3 GPa and an ultrabroad spectral range of up to 300 nm in the visible region at 7.48 GPa. These results indicate that the isomeric organic cations (PA+ and PNA+) with different configurations significantly mediate distinct excitonic behaviors due to different resilience to high pressures and reveal a novel interaction mechanism between organic spacer cations and inorganic layers under compression. Our findings not only shed light on the vital roles of isomeric organic molecules as organic spacer cations in 2D perovskites under pressure, but also open a route to rationally design highly efficient 2D perovskites incorporating such spacer organic molecules in optoelectronic devices.

Multi-color UCNPs/CsPb(Br1-xIx)3 for upconversion luminescence and dual-modal anticounterfeiting.

Advanced hybrid materials have attracted extensive attention in optoelectronics and photonics application due to their unique and excellent properties. Here, the multicolor upconversion luminescence properties of the hybrid materials composed of CsPbX3(X = Br/I) perovskite quantum dots and upconversion nanoparticles (UCNPs, core-shell NaYF4:25%Yb3+,0.5%Tm3+@NaYF4) is reported, achieving the upconversion luminescence with stable and bright of CsPbX3 perovskite quantum dots under 980 nm excitation. Compared with the nonlinear upconversion of multi-photon absorption in perovskite, UCNPs/CsPbX3 achieves lower power density excitation by using the UCNPs as the physical energy transfer level, meeting the demand for multi-color upconversion luminescence in optical applications. Also, the UCNPs/CsPbX3 combined with ultraviolet curable resin (UVCR) shows excellent water and air stability, which can be employed as multicolor fluorescent ink for screen printing security labels. Through the conversion strategy, the message of the security labels can be encrypted and decrypted by using UV light and a 980 nm continuous wave excitation laser as a switch, which greatly improves the difficulty of forgery. These findings provide a general method to stimulate photon upconversion and improve the stability of perovskite nanocrystals, which will be better applied in the field of anti-counterfeiting.

Real-Time Ratiometric Optical Nanoscale Thermometry.

All-optical nanothermometry has become a powerful, remote tool for measuring nanoscale temperatures in applications ranging from medicine to nano-optics and solid-state nanodevices. The key features of any candidate nanothermometer are brightness, sensitivity, and (signal, spatial, and temporal) resolution. Here, we demonstrate a real-time, diamond-based nanothermometry technique with excellent sensitivity (1.8% K-1) and record-high resolution (5.8 × 104 K Hz-1/2 W cm-2) based on codoped nanodiamonds. The distinct performance of our approach stems from two factors: (i) temperature sensors─nanodiamonds cohosting two group IV color centers─engineered to emit spectrally separated Stokes and anti-Stokes fluorescence signals under excitation by a single laser source and (ii) a parallel detection scheme based on filtering optics and high-sensitivity photon counters for fast readout. We demonstrate the performance of our method by monitoring temporal changes in the local temperature of a microcircuit and a MoTe2 field-effect transistor. Our work advances a powerful, alternative strategy for time-resolved temperature monitoring and mapping of micro-/nanoscale devices such as microfluidic channels, nanophotonic circuits, and nanoelectronic devices, as well as complex biological environments such as tissues and cells.

Electrical control of quantum emitters in a Van der Waals heterostructure.

Controlling and manipulating individual quantum systems in solids underpins the growing interest in the development of scalable quantum technologies. Recently, hexagonal boron nitride (hBN) has garnered significant attention in quantum photonic applications due to its ability to host optically stable quantum emitters. However, the large bandgap of hBN and the lack of efficient doping inhibits electrical triggering and limits opportunities to study the electrical control of emitters. Here, we show an approach to electrically modulate quantum emitters in an hBN-graphene van der Waals heterostructure. We show that quantum emitters in hBN can be reversibly activated and modulated by applying a bias across the device. Notably, a significant number of quantum emitters are intrinsically dark and become optically active at non-zero voltages. To explain the results, we provide a heuristic electrostatic model of this unique behavior. Finally, employing these devices we demonstrate a nearly-coherent source with linewidths of ~160 MHz. Our results enhance the potential of hBN for tunable solid-state quantum emitters for the growing field of quantum information science.

Spin defects in hexagonal boron nitride for strain sensing on nanopillar arrays.

Two-dimensional hexagonal boron nitride (hBN) has attracted much attention as a platform for studies of light-matter interactions at the nanoscale, especially in quantum nanophotonics. Recent efforts have focused on spin defects, specifically negatively charged boron vacancy (VB-) centers. Here, we demonstrate a scalable method to enhance the VB- emission using an array of SiO2 nanopillars. We achieve a 4-fold increase in photoluminescence (PL) intensity, and a corresponding 4-fold enhancement in optically detected magnetic resonance (ODMR) contrast. Furthermore, the VB- ensembles provide useful information about the strain fields associated with the strained hBN at the nanopillar sites. Our results provide an accessible way to increase the emission intensity as well as the ODMR contrast of the VB- defects, while simultaneously form a basis for miniaturized quantum sensors in layered heterostructures.

STED microscopy reveals in-situ photoluminescence properties of single nanostructures in densely perovskite thin films.

All-inorganic perovskite nanomaterials have attracted much attention recently due to their prominent optical performance and potential application for optoelectronic devices. The carriers dynamics of all-inorganic perovskites has been the research focus because the understanding of carriers dynamics process is of critical importance for improving the fluorescence conversion efficiency. While photophysical properties of excited carrier are usually measured at the macroscopic scale, it is necessary to probe the in-situ dynamics process at the nanometer scale and gain deep insights into the photophysical mechanisms and their localized dependence on the thin-film nanostructures. Stimulated emission depletion (STED) nanoscopy with super-resolution beyond the diffraction limit can directly provide explicit information at a single particle level or nanometer scale. Through this unique technique, we firstly study the in-situ dynamics process of single CsPbBr3 nanocrystals(NCs) and nanostructures embedded inside high-dense samples. Our findings reveal the different physical mechanisms of PL blinking and antibunching for single CsPbBr3 NCs and nanostructures that correlate with thin-film nanostructural features (e.g. defects, grain boundaries and carrier mobility). The insights gained into such nanostructure-localized physical mechanisms are critically important for further improving the material quality and its corresponding device performance.

Enhanced Emission from Interlayer Excitons Coupled to Plasmonic Gap Cavities.

The emergence of interlayer excitons (IEs) from atomic layered transition metal dichalcogenides (TMDCs) heterostructures has drawn tremendous attention due to their unique and exotic optoelectronic properties. Coupling the IEs into optical cavities provides distinctive electromagnetic environments which plays an important role in controlling multiple optical processes such as optical nonlinear generation or photoluminescence enhancement. Here, the integration of IEs in TMDCs into plasmonic nanocavities based on a nanocube on a metallic mirror is reported. Spectroscopic studies reveal an order of magnitude enhancement of the IE at room temperature and a 5-time enhancement in fluorescence at cryogenic temperatures. Cavity modeling reveals that the enhancement of the emission is attributed to both increased excitation efficiency and Purcell effect from the cavity. The results show a novel method to control the excitonic processes in TMDC heterostructures to build high performance photonics and optoelectronics devices.

Flat Lenses Based on 2D Perovskite Nanosheets.

Ultrathin flat lenses based on metasurfaces or metamaterials have shown great promise in recent years as essential components in nano-optical system, with capability of abrupt changes of light wavefronts. However, such structural designs require complex nanopatterns and a time-consuming nanofabrication process. In this regard, flat lenses are developed based on 2D perovskite nanosheets, using a cost-effective mask-free femtosecond direct laser writing system. The optical properties of the 2D perovskite are rationally adjusted through facile composition engineering as well as thickness-dependent quantum-size confinement. A diffraction theory model is derived to understand the focusing mechanism of the 2D perovskite nanosheets flat lenses. The as-fabricated lenses exploit the tunable material property variations to effectively manipulate not only the amplitude but also the phase of the incident light to focus into a 3D focal spot with a sub-wavelength resolution in the range of 0.5-0.9λ. The results pave the way toward low-cost and large-scale high-resolution imaging applications in the future.

Structured graphene metamaterial selective absorbers for high efficiency and omnidirectional solar thermal energy conversion.

An ideal solar-thermal absorber requires efficient selective absorption with a tunable bandwidth, excellent thermal conductivity and stability, and a simple structure for effective solar thermal energy conversion. Despite various solar absorbers having been demonstrated, these conditions are challenging to achieve simultaneously using conventional materials and structures. Here, we propose and demonstrate three-dimensional structured graphene metamaterial (SGM) that takes advantages of wavelength selectivity from metallic trench-like structures and broadband dispersionless nature and excellent thermal conductivity from the ultrathin graphene metamaterial film. The SGM absorbers exhibit superior solar selective and omnidirectional absorption, flexible tunability of wavelength selective absorption, excellent photothermal performance, and high thermal stability. Impressive solar-to-thermal conversion efficiency of 90.1% and solar-to-vapor efficiency of 96.2% have been achieved. These superior properties of the SGM absorber suggest it has a great potential for practical applications of solar thermal energy harvesting and manipulation.

Template-Free Synthesis of High-Yield Fe-Doped Cesium Lead Halide Perovskite Ultralong Microwires with Enhanced Two-Photon Absorption.

Doping in perovskite is challenging and competitive due to the inherently fast growth mechanism of perovskite structure. Here, we demonstrate successful synthesis of high-yield Fe-doped cesium lead halide perovskite ultralong microwires (MWs) that have diameters up to ∼5 µm and lengths up to millimeters via an antisolvent vapor-assisted template-free method. Microstructure characterization confirms the uniformly doped Fe in the high-quality crystal perovskite MWs. Significantly, doping the Fe(III) concentration can affect both the MW morphology and photoluminescence (PL). The band edge emission of the MW at variable excitation has been accounted for by the superposition and combination of optical transitions of nearby singlet, triplet, and magnetic polaronic excitons. High-quality two-photon PL emission and the enhanced nonlinear absorption coefficient of Fe-doped MWs have been experimentally demonstrated. This superhigh nonlinear absorption coefficient and high-quality optical properties endow it with promising applications in spin-related optical switching and optical limiting devices.

Role of Surface Recombination in Halide Perovskite Nanoplatelets.

Halide perovskites are an extremely promising material platform for solar cells and photonic devices. The role of surface carrier recombination-well known to detrimentally affect the performance of devices-is still not well understood for thin samples where the thickness is comparable to or less than the carrier diffusion length. Here, using time-resolved microspectroscopy along with modeling, we investigate charge-carrier recombination dynamics in halide perovskite CH3NH3PbI3 nanoplatelets with thicknesses from ∼20 to 200 nm, ranging from much lesser than to comparable to the carrier diffusion length. We show that surface recombination plays a stronger role in thin perovskite nanoplatelets, significantly decreasing photoluminescence (PL) efficiency, PL decay lifetime, and photostability. Interestingly, we find that both thick and thin nanoplatelets exhibit a similar increase in PL efficiency with increasing excitation fluence, well described by our excitation saturation model. We also find that the excited carrier distribution along the depth impacts the surface recombination. Using the diffusion-surface recombination model, we determine the surface recombination velocity. This work provides a comprehensive understanding of the role of surface recombination and charge-carrier dynamics in thin perovskite platelets and reveals valuable insights useful for applications in photovoltaics and photonics.

Two-Dimensional CH3NH3PbI3 Perovskite Nanosheets for Ultrafast Pulsed Fiber Lasers.

Even though the nonlinear optical effects of solution processed organic-inorganic perovskite films have been studied, the nonlinear optical properties in two-dimensional (2D) perovskites, especially their applications for ultrafast photonics, are largely unexplored. In comparison to bulk perovskite films, 2D perovskite nanosheets with small thicknesses of a few unit cells are more suitable for investigating the intrinsic nonlinear optical properties because bulk recombination of photocarriers and the nonlinear scattering are relatively small. In this research, we systematically investigated the nonlinear optical properties of 2D perovskite nanosheets derived from a combined solution process and vapor phase conversion method. It was found that 2D perovskite nanosheets have stronger saturable absorption properties with large modulation depth and very low saturation intensity compared with those of bulk perovskite films. Using an all dry transfer method, we constructed a new type of saturable absorber device based on single piece 2D perovskite nanosheet. Stable soliton state mode-locking was achieved, and ultrafast picosecond pulses were generated at 1064 nm. This work is likely to pave the way for ultrafast photonic and optoelectronic applications based on 2D perovskites.

Chemical Stabilization of 1T' Phase Transition Metal Dichalcogenides with Giant Optical Kerr Nonlinearity.

The 2H-to-1T' phase transition in transition metal dichalcogenides (TMDs) has been exploited to phase-engineer TMDs for applications in which the metallicity of the 1T' phase is beneficial. However, phase-engineered 1T'-TMDs are metastable; thus, stabilization of the 1T' phase remains an important challenge to overcome before its properties can be exploited. Herein, we performed a systematic study of the 2H-to-1T' phase evolution by lithiation in ultrahigh vacuum. We discovered that by hydrogenating the intercalated Li to form lithium hydride (LiH), unprecedented long-term (>3 months) air stability of the 1T' phase can be achieved. Most importantly, this passivation method has wide applicability for other alkali metals and TMDs. Density functional theory calculations reveal that LiH is a good electron donor and stabilizes the 1T' phase against 2H conversion, aided by the formation of a greatly enhanced interlayer dipole-dipole interaction. Nonlinear optical studies reveal that air-stable 1T'-TMDs exhibit much stronger optical Kerr nonlinearity and higher optical transparency than the 2H phase, which is promising for nonlinear photonic applications.

Silver nanoparticles inhibit the function of hypoxia-inducible factor-1 and target genes: insight into the cytotoxicity and antiangiogenesis.

Hypoxia-inducible factor-1 (HIF-1) is a transcription factor that is activated upon exposure to hypoxic stress. It modulates a number of cellular responses including proliferation, apoptosis, angiogenesis, and metabolism by activating a panel of target genes in response to hypoxia. The HIF-1 level is often upregulated in the hypoxic microenvironment of solid tumors, which contributes to cancer treatment failure. Here we report that silver nanoparticles (AgNPs), which are widely used as an antimicrobial agent, are an effective inhibitor of HIF-1. AgNPs inhibited the activation of a HIF-dependent reporter construct after the cells were exposed to hypoxic conditions or treated with cobalt chloride, a hypoxia mimetic agent. The AgNPs also interfered with the accumulation of HIF-1α protein and the induction of the endogenous HIF target genes, VEGF-A and GLUT1. Since both HIF-1 and vascular endothelial growth factor-A play an important role in angiogenesis, AgNPs also inhibited angiogenesis in vitro. Our data reveal a new mechanism of how AgNPs act on cellular function, that is, they disrupt HIF signaling pathway. This finding provides a novel insight into how AgNPs can inhibit cancer cell growth and angiogenesis.