Two-Photon-Pumped Perovskite Semiconductor Nanocrystal Lasers

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.

In Situ Compositing CsPbBr3 with Exfoliated Layered-Perovskite CsCa2Ta3O10: Interfacial Interaction and Enhanced Stability

Cesium lead halide (CsPbX₃, X = Cl, Br, I) perovskite quantum dots (QDs) have been intriguing optoelectronic materials for applications in various devices owing to their superior electronic and optical properties. However, poor resistance to humidity and light irradiation impedes their promotion. Herein, bulk perovskite-type layered CsCa₂Ta₃O₁₀ is exfoliated into two-dimensional (2D) negatively charged Ca₂Ta₃O₁₀^- (CTO) nanosheets as seeds to in situ synthesize and composite CsPbBr₃. The as-synthesized CsPbBr₃/CTO nanocomposites possess improved green emission with apparently prolonged decay time with reference to bare CsPbBr₃ QDs. The decay time can retrieve to a normal state when the nanocomposites are treated with some water. It is found that the CTO acts as a defect to trap the bound exciton of the loaded CsPbBr₃. Protons from water can preferably replace Cs⁺ at the interface of the nanocomposites, resulting in the separation of the nanosheets and CsPbBr₃ and retrieving the decay time. X-ray photoelectron spectroscopy results also indicate the strong interaction between CsPbBr₃ and CTO with reference to the physical mixing sample of bare CsPbBr₃ QDs and CTO nanosheets. The decoration of ultrathin 2D charge-bearing oxide nanosheets on the QDs benefits significant improvements in humidity resistance and photostability performance in light-emitting diode devices. This research offers a distinct strategy to modify the surface of perovskite QDs.

Amplified Spontaneous Emission Threshold Reduction and Operational Stability Improvement in CsPbBr3 Nanocrystals Films by Hydrophobic Functionalization of the Substrate

The use of lead halide perovskites in optoelectronic and photonic devices is mainly limited by insufficient long-term stability of these materials. This issue is receiving growing attention, mainly owing to the operational stability improvement of lead halide perosvkites solar cells. On the contrary, fewer efforts are devoted to the stability improvement of light amplification and lasing. In this report we demonstrate that a simple hydrophobic functionalization of the substrates with hexamethyldisilazane (HMDS) allows to strongly improve the Amplified Spontaneous Emission (ASE) properties of drop cast CsPbBr₃ nanocrystal (NC) thin films. In particular we observe an ASE threshold decrease down to 45% of the value without treatment, an optical gain increase of up to 1.5 times and an ASE operational stability increase of up to 14 times. These results are ascribed to a closer NC packing in the films on HMDS treated substrate, allowing an improved energy transfer towards the larger NCs within the NC ensemble, and to the reduction of the film interaction with moisture. Our results propose hydrophobic functionalization of the substrates as an easy approach to lower the ASE and lasing thresholds, while simultaneously increasing the active material stability.

Effective Control of the Growth and Photoluminescence Properties of CsPbBr3/Cs4PbBr6 Nanocomposites by Solvent Engineering

Metal halide perovskites exhibit small exciton binding energy, which leads to a low electron-hole capture rate for radiative recombination and accordingly decreases the luminescence efficiency. Reducing the thickness of the perovskite film or the size of the perovskite crystal is found to be an effective method to spatially confine the electrons and holes to promote the bimolecular radiative recombination. Here, we fabricate CsPbBr₃/Cs₄PbBr₆ nanocomposites, applicable for light emission diodes, by a simple self-assembly method. We effectively reduce the critical size of the CsPbBr₃ nanocrystals in the CsPbBr₃/Cs₄PbBr₆ nanocomposites by adding a certain amount of dimethyl sulfoxide into the perovskite precursor solution. Accordingly, the photoluminescence quantum yield of the CsPbBr₃/Cs₄PbBr₆ nanocomposites increased from 56 to 91% due to the quantum size effect. In situ observation of the growth of CsPbBr₃/Cs₄PbBr₆ nanocomposites reveals that the reduction of the CsPbBr₃ crystal size is due to the change of the chemical reaction speed during the two-step growth process of the CsPbBr₃/Cs₄PbBr₆ nanocomposites.

The importance of relativistic effects on two-photon absorption spectra in metal halide perovskites

Despite intense research into the optoelectronic properties of metal halide perovskites (MHPs), sub-bandgap absorption in MHPs remains largely unexplored. Here we recorded two-photon absorption spectra of MHPs using the time-resolved microwave conductivity technique. A two-step upward trend is observed in the two-photon absorption spectrum for methylammonium lead iodide, and some analogues, which implies that the commonly used scaling law is not applicable to MHPs. This aspect is further confirmed by temperature-dependent conductivity measurements. Using an empirical multiband tight binding model, spectra for methylammonium lead iodide were calculated by integration over the entire Brillouin zone, showing compelling similarity with experimental results. We conclude that the second upward trend in the two-photon absorption spectrum originates from additional optical transitions to the heavy and light electron bands formed by the strong spin-orbit coupling. Hence, valuable insight can be obtained in the opto-electronic properties of MHPs by sub-bandgap spectroscopy, complemented by modelling.

Ferroelectric Polarization Suppresses Nonradiative Electron-Hole Recombination in CH3NH3PbI3 Perovskites: A Time-Domain ab Initio Study

The ordered alignment of polar organic cations in hybrid organic-inorganic perovskites causes a ferroelectric phase, which is believed to be benign for photovoltaic devices. Using a combination of time-domain density functional theory and nonadiabatic molecular dynamics, we study the influence of the interactions of polar MA (CH₃NH₃⁺) cations and between their inorganic counterparts on the nonradiative electron-hole recombination in the room-temperature ferroelectric MAPbI₃ perovskite. We show that ferroelectric alignment of the polar C-N bonds favors charge separation and reduces nonadiabatic coupling. Symmetry breaking enhances low-frequency collective motions and introduces additional high-frequency vibrations, thus accelerating quantum decoherence. Both factors contribute to suppressing the nonradiative electron-hole recombination, extending the charge carrier lifetimes to several nanoseconds and showing a factor of 3 increase compared to the pristine system. Consequently, ferroelectric engineering provides an excellent route to improve hybrid perovskite device performance as a result of long-range charge separation and slow charge recombination.

White light-emitting devices based on ZnCdS/ZnS and perovskite nanocrystal heterojunction

Perovskite white light-emitting devices (WLEDs) without intercalation layers have not been achieved due to the ion exchange. Although the intercalation layers prevent ion exchange between perovskite nanocrystals (NCs), it also creates a new problem of charge imbalance and the structure becomes more complex. In this study, blue emitting ZnCdS/ZnS NCs with high quantum yield and stability are introduced to work with the yellow emission from CsPb(Br/I)₃ perovskite NCs for WLEDs. The WLEDs are constituted of ITO/ZnO/PEI/ZnCdS/ZnS NCs/CsPb(Br/I)₃ NCs/TCTA/MoO₃/Au. This design avoids ion exchange between different perovskites NCs, and realizes white light emission by simple fabrication. As a result, we achieved the white light coordinates of (0.34, 0.34) and a correlated color temperature of 5153 K.

Unconventional route to dual-shelled organolead halide perovskite nanocrystals with controlled dimensions, surface chemistry, and stabilities

The past few years have witnessed rapid advances in the synthesis of high-quality perovskite nanocrystals (PNCs). However, despite the impressive developments, the stability of PNCs remains a substantial challenge. The ability to reliably improve stability of PNCs while retaining their individual nanometer size represents a critical step that underpins future advances in optoelectronic applications. Here, we report an unconventional strategy for crafting dual-shelled PNCs (i.e., polymer-ligated perovskite/SiO₂ core/shell NCs) with exquisite control over dimensions, surface chemistry, and stabilities. In stark contrast to conventional methods, our strategy relies on capitalizing on judiciously designed star-like copolymers as nanoreactors to render the growth of core/shell NCs with controlled yet tunable perovskite core diameter, SiO₂ shell thickness, and surface chemistry. Consequently, the resulting polymer-tethered perovskite/SiO₂ core/shell NCs display concurrently a stellar set of substantially improved stabilities (i.e., colloidal stability, chemical composition stability, photostability, water stability), while having appealing solution processability, which are unattainable by conventional methods.

Two Photon-Pumped Whispering-Gallery Mode Lasing and Dynamic Regulation

Realizing the dynamic regulation of nonlinear optical signals has a great scientific significance for the development of new-type nonlinear optoelectronic devices and expands its application in the field of laser technology, spectroscopy, material structure analysis, etc. Here, two photon absorption-induced whispering-gallery mode lasing from a single ZnO microresonator with a relatively low lasing threshold (15 µW) and high quality factor (Q ≈ 3200) under ambient conditions is demonstrated. Furthermore, success is achieved in obtaining the dynamic regulation of two photon-pumped lasing mode in the UV gain region. The corresponding resonant wavelength can be tuned dynamically from 388.99 and 391.12 to 390.01 and 392.12 nm for TE33 and TE32 modes, respectively. This work provides a new strategy for building high-performance mode-adjustable frequency upconversion lasers.

Two-photon excited photoluminescence of single perovskite nanocrystals

Lead-halide perovskite nanocrystals (NCs) have emerged as a novel type of semiconductor nanostructure, attracting great research interests in both fundamental science and practical applications. Here, we compare the optical properties of single CsPbI₃ NCs under both one-photon and two-photon excitations, mainly including the photoluminescence (PL) blinking and PL decay dynamics. By means of the PL saturation effect caused by multi-exciton Auger recombination, we have also estimated a two-photon absorption cross section of ∼6.8 × 10⁶ GM for single CsPbI₃ NCs. The ability to realize efficient two-photon excitation of single perovskite NCs with strongly suppressed background fluorescence will help not only to promote their bio-imaging and biolabeling applications but also to reveal and manipulate their delicate electronic structures for potential usage in quantum information processing.

Stable Perovskite Quantum Dots Coated with Superhydrophobic Organosilica Shells for White Light-Emitting Diodes

Metalammonium lead perovskite (MAPbX₃ , MA=CH₃ NH₃ ⁺ ; X=Cl, Br, I) quantum dots (QDs) have attracted tremendous attention due to their outstanding optical properties. However, they usually suffer from poor stability towards water or moisture, which seriously limits their practical applications. Here, we report a simple and effective approach to improve the stability of MAPbBr₃ QDs by encapsulating them with superhydrophobic fluorinated organosilica (FSiO₂ ) shells. The water-resistant stability of the superhydrophobic MAPbBr₃ QDs/FSiO₂ is significantly enhanced and they display strong fluorescence even after immersion in water for 12 hours. This method is readily extended to prepare superhydrophobic MAPbBr_2.4 Cl_0.6 QDs/FSiO₂ and MAPbI₃ QDs/FSiO₂ powders. These superhydrophobic MAPbX₃ QDs/FSiO₂ can be further used to fabricate white light-emitting diodes (LEDs) with comparable color to pure white emission.

Single-Exciton Amplified Spontaneous Emission in Thin Films of CsPbX3 (X = Br, I) Perovskite Nanocrystals

CsPbX₃ perovskite nanocrystals (PNCs) have emerged as an excellent material for stimulated emission purposes, with even more prospective applications than conventional colloidal quantum dots. However, a better understanding of the physical mechanisms responsible for amplified spontaneous emission (ASE) is required to achieve more ambitious targets (lasing under continuous wave optical or electrical excitation). Here, we establish the intrinsic mechanisms underlying ASE in PNCs of three different band gaps (CsPbBr₃, CsPbBr_1.5I_1.5, and CsPbI₃). Our characterization at cryogenic temperatures does not reveal any evidence of the biexciton mechanism in the formation of ASE. Instead, the measured shift toward long wavelengths of the ASE band is easily explained by the reabsorption in the PNC layer, which becomes stronger for thicker layers. In this way, the threshold of ASE is determined only by optical losses at a given geometry, which is the single-exciton mechanism responsible for ASE. Experimental results are properly reproduced by a physical model.

Achieving long carrier lifetime and high optical gain in all-inorganic CsPbBr3 perovskite films via top and bottom surface modification

Solution-processed all-inorganic CsPbX3(X = Br, I, and Cl) perovskites are proven to be promising materials for various optoelectronic applications. However, CsPbX3 films as optical gain media were confronted with unsatisfactory surface coverage and inferior photoluminescence performance when compared with their colloidal nanocrystal counterparts. Herein, we demonstrate a strategy for improving the optical properties via modification of both top and bottom surfaces of CsPbBr3 films. The treated perovskite films show ultra-smooth morphology and a carrier lifetime of 44 ns, more than one order of magnitude longer than the untreated one. Meanwhile, a mixed polymer layer on the top of the perovskite film could combine surface passivation with symmetric waveguide effects, leading to an outstanding net gain coefficient of 694 cm-1. These merits predict the great potential of all-inorganic perovskite films to support high efficiency charge transport or stimulated emission.

Enhanced stability of the optical responses from all-inorganic perovskite nanocrystals embedded in a synthetic opal matrix

Nanostructured luminescent materials based on perovskite nanocrystals (p-NCs) are attractive since their optical properties can be tuned in a wide spectral range with high luminescence quantum yields and lifetimes, however, they lack stability. In this work, the optical properties of highly luminescent colloidal p-NCs (CsPbX₃, where X = Cl/Br, Br, I) embedded in porous opal matrices are presented. It is shown that the photoluminescence of the p-NCs embedded into opal matrices possess increased longtime stability of its spectral and kinetic parameters under ambient conditions. LEDs based on the developed materials show pure color p-NC emission with stability of its parameters. The results of this work may expand the knowledge of interactions between luminescent nanoparticles within multicomponent nanostructured materials for further photonic applications.

Highly Luminescent and Water-Resistant CsPbBr3-CsPb2Br5 Perovskite Nanocrystals Coordinated with Partially Hydrolyzed Poly(methyl methacrylate) and Polyethylenimine

All inorganic lead halide perovskite nanocrystals (PNCs) typically suffer from poor stability against moisture and UV radiation as well as degradation during thermal treatment. The stability of PNCs can be significantly enhanced through polymer encapsulation, often accompanied by a decrease of photoluminescence quantum yield (PLQY) due to the loss of highly dynamic oleylamine/oleic acid (OLA/OA) ligands. Herein, we propose a solution for this problem by utilizing partially hydrolyzed poly(methyl methacrylate) (h-PMMA) and highly branched poly(ethylenimine) (b-PEI) as double ligands stabilizing the PNCs already during the mechanochemical synthesis (grinding). The hydrophobic polymer of h-PMMA imparts excellent film-forming properties and water stability to the resulting NC-polymer composite. In its own turn, the b-PEI forms an amino-rich, strongly binding ligand layer on the surface of the PNCs being responsible for the significant improvement of the PLQY and the stability of the resulting material. Moreover, the introduction of b-PEI promotes a partial phase conversion from CsPbBr₃ to CsPb₂Br₅ to obtain CsPbBr₃/CsPb₂Br₅ nanocrystals with a core-shell-like structure. As-prepared PNCs solutions are directly processable as inks, while their PLQY drops only slightly from 75% in colloidal solution to 65% in films. Moreover, the final PNC-polymer film exhibits excellent stability against water, heat, and ultraviolet light irradiation. These superior properties allowed us to fabricate a proof of concept thin film OLED with h-PMMA/b-PEI-stabilized PNCs as an easily processable, narrowly emitting color conversion composite material.

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.

Single Halide Perovskite/Semiconductor Core/Shell Quantum Dots with Ultrastability and Nonblinking Properties

The further practical applications of halide perovskite quantum dots (QDs) are blocked by problems of instability and nonradiative Auger recombination manifested as photoluminescence blinking. Here, single core/shell structured perovskite semiconductor QDs are successfully fabricated by capping CsPbBr3 QD core with CdS shell. It is demonstrated that CsPbBr3/CdS core/shell QDs exhibit ultrahigh chemical stability and nonblinking photoluminescence with high quantum yield due to the reduced electronic traps within the core/shell structure. Efficiency of amplified spontaneous emission exhibits obvious enhancement compared to that of pure CsPbBr3 QDs, originating from the mitigated competition between stimulated emission and suppressed nonradiative biexciton Auger recombination. Furthermore, low-threshold whispering-gallery-mode lasing with a high-quality factor is achieved by incorporating CsPbBr3/CdS QDs into microtubule resonators. Density functional theory (DFT)-based first-principles calculations are also performed to reveal the atomic interface structure, which supports the existence of CsPbBr3/CdS structure. An interesting feature of spatially separated charge density at CsPbBr3/CdS interface is found, which may greatly contribute to the suppressed Auger recombination. The results provide a practical approach to improve the stability and suppress the blinking of halide perovskite QDs, which may pave the way for future applications for various optoelectronic devices.

Instantaneous, room-temperature, open-air atmosphere, solution-phase synthesis of perovskite quantum dots through halide exchange employing non-metal based inexpensive HCl/HI: ensemble and single particle spectroscopy

Herein, the instantaneous synthesis of highly crystalline, uniform-sized (ca. 11.3 ± 0.1 nm), blue-to-green to yellow to red-emitting all-inorganic perovskite quantum dots (CsPbBr₃ and mixed halide PQDs) was achieved at room temperature under an open-air atmosphere (no glove box) through halide exchange in the solution phase employing easily available, inexpensive non-metal-based halide sources such as HCl and HI. No complicated pre-treatment of the halide source was required. Moreover, these PQDs were stable for a few weeks under an open-air atmosphere. The PL emission spectra are quite narrow, and the PLQYs are quite high (80% for even Br/I mixed PQDs). At the single particle level, the 〈ON fraction〉 has been noted to vary from 75% to 85% for different PQDs, the m _ON values are close to 1.0, and the m _OFF values are >1.5. The latter indicates that long ON durations are more probable. The increase in the ON event truncation time (from 2.7 to 4.0 s) and the concomitant decrease in the OFF event truncation time (from 6.6 to 4.3 s) could be correlated with the increase in the PLQY (from 0.55 to 0.75). In addition, an interesting memory effect could be observed in both the ON and the OFF event durations.

Stable, Strongly Emitting Cesium Lead Bromide Perovskite Nanorods with High Optical Gain Enabled by an Intermediate Monomer Reservoir Synthetic Strategy

One-dimensional (1D) semiconductor nanorods are important for numerous applications ranging from optics and electronics to biology, yet the direct synthesis of high-quality metal halide perovskite nanorods remains a challenge. Here, we develop an intermediate monomer reservoir synthetic strategy to realize the controllable growth of uniform and low-defect CsPbBr₃ perovskite nanorods. Intermediates composed of CsPb₂Br₅ and Cs₃In₂Br₉ are obtained through the substitution of Pb²⁺ with In³⁺ cations in the template of CsPbBr₃ nanocubes and act as a precursor reservoir to gradually release monomers, ensuring both the slow growth rate and low defects of nanorods. We have used branched tris(diethylamino)phosphine as a ligand, which not only has unequal binding energies with different crystal faces to promote the orientation growth but also provides strong steric hindrance to shield the nanorods in solution. Because of minor amount of defects and an effective ligand passivation, in addition to significantly enhanced stability, the perovskite nanorods show a high photoluminescence quantum yield of up to 90% and exhibit a net mode gain of 980 cm^-1, the latter being a record value among all the perovskite materials. An extremely low amplified spontaneous emission threshold of 7.5 μJ cm^-2 is obtained under excitation by a nanosecond laser, which is comparable to that obtained using femtosecond lasers in other recent studies.

Size dependent charge separation and recombination in CsPbI3 perovskite quantum dots

CsPbI₃ perovskite quantum dots (QDs) have shown great potential in light-harvesting and light-emitting applications, which often involve the transfer of charge carriers in and out of these materials. Here, we studied size-dependent charge separation (CS) and charge recombination (CR) between CsPbI₃ QDs and rhodamine B (RhB) molecules, using transient absorption spectroscopy. When the average size decreases from 11.8 nm to 6.5 nm, the average intrinsic CS time constant decreases from 872 ± 52 ps to 40.6 ± 4.3 ps and the corresponding charge recombination time constant decreases from 3829 ± 51 ns to 1384 ± 54 ns. The observed trend of size-dependent CS and CR rates can be well explained by Marcus theory using the theoretically calculated CS and CR driving forces (ΔG_CS and ΔG_CR), molecular reorganization energy (λ_RhB), and electronic coupling strength between QD and RhB (H_CS and H_CR). Unlike the extensively studied more strongly quantum confined Cd chalcogenide QDs, the CsPbI₃ QDs are in a weak quantum confinement regime in which size-dependent coupling strength plays a dominant role in the size-dependent charge transfer properties.

Size- and Morphology-Dependent Auger Recombination in CsPbBr3 Perovskite Two-Dimensional Nanoplatelets and One-Dimensional Nanorods

CsPbX₃ (X = Cl, Br, I) perovskite nanocrystals (NCs), including zero-dimensional (0D) quantum dots (QDs), one-dimensional (1D) nanorods (NRs), and two-dimensional (2D) nanoplatelets (NPLs), have shown promising performances in light-emitting diode (LED) and lasing applications. However, Auger recombination, one of the key processes that limit their performance, remains poorly understood in CsPbX₃ 2D NPLs and 1D NRs. We show that the biexciton Auger lifetimes of CsPbBr₃ NPLs (NRs) scale linearly with the NPL lateral area (NR length) and deviates from the "universal volume scale law" that has been observed for QDs. These results are consistent with a model in which the Auger recombination rate for 1D NRs and 2D NPLs is a product of binary collision frequency in the nonquantum confined dimension and Auger probability per collision. Comparisons of Auger recombination in CsPbBr₃ NCs of different dimensionalities and similar band gaps suggest that Auger probability increases in NCs with a higher number of confined dimensions. Compared to CdSe and PbSe NCs with the same dimensionalities and similar sizes, Auger recombination rates in 0D-2D CsPbBr₃ NCs are over 10-fold faster. Fast Auger recombination in CsPbBr₃ NCs shows their potentials for Auger-assisted up-conversion and single photon source, while suppressing Auger recombination may further enhance their performances in LED and lasing applications.

Giant two- to five-photon absorption in CsPbBr2.7I0.3 two-dimensional nanoplatelets

CsPbBr_2.7I_0.3 two-dimensional (2D) nanoplatelets (NPs) with emission wavelengths of 469 nm and 527 nm were synthesized and characterized. Femtosecond transient absorption spectra revealed hot carrier cooling times of ∼368  fs and ∼438  fs for 469 nm and 527 nm 2D NPs, respectively. Importantly, the 2D NPs exhibit giant two-, three-, four-, and five-photon absorption cross-sections, reaching ∼4.1×10⁶ GM at 830 nm, ∼2.3×10^-74  cm⁶ s² photon^-2 at 1300 nm, 2.06×10^-104  cm⁸ s³ photon^-3 at 1600 nm, and 1.50×10^-136  cm¹⁰ s⁴ photon^-4 at 2200 nm, respectively, which are 3-8 orders of magnitude larger, compared to specially designed organic molecules.

Anti-Stokes Ultraviolet Luminescence and Exciton Detrapping in the Two-Dimensional Perovskite (C6H5C2H4NH3)2PbCl4

Recently, it has been found that low-dimensional organometallic halide perovskites can be adopted as nonlinear monolayer emitters because of their efficient spontaneous anti-Stokes visual luminescence under visual or near-infrared laser excitation. Herein, we demonstrate a luminescence up-conversion process from the visible self-trapped exciton (STE) to an ultraviolet (UV) free exciton (FE) in the two-dimensional perovskite (C₆H₅C₂H₄NH₃)₂PbCl₄ quantum wells excited by nanosecond pulse laser excitation. An ultraviolet 347 nm near-band-edge FE emission is obtained under the excitation of 579 nm dye laser at 10 K by a two-step, two-photon absorption process from the real intermediate exciton state. In addition, the decay rise time of higher-laying states of STE indicates the excitonic detrapping procedure could occur by the annihilation of phonons. Our results suggest that the low-dimensional halide perovskites with deformable structure are able to be applied in visible light-pumped UV-emitting devices.

Phase Regulation Strategy of Perovskite Nanocrystals from 1D Orthomorphic NH4 PbI3 to 3D Cubic (NH4 )0.5 Cs0.5 Pb(I0.5 Br0.5 )3 Phase Enhances Photoluminescence

This work reports this first synthesis of 1D orthomorphic NH₄ PbI₃ perovskite nanocrystals (NCs) considering the role of inorganic ammonium ions at the nanoscale. The addition of bromide ions at the halogen site did not improve the photoluminescence properties. Furthermore, the 3D cubic phase of (NH₄ )_0.5 Cs_0.5 Pb(I_0.5 Br_0.5 )₃ NCs with bright photoluminescence was synthesized by adding Cs ions into the crystal lattice of (NH₄ )Pb(I_0.5 Br_0.5 )₃ . Moreover, the photophysical properties of different phase structures were studied using femtosecond transient absorption (FTA) spectroscopy. The ultrafast trap state capture process is a key factor in the change of photoluminescence properties and the cubic phase may be the best structure for photoluminescence. These results suggest that the ammonium ion perovskite (AIP) nanocrystals could be potential materials for optoelectronic applications through A-site cation substitution.

Improving the Stability of Metal Halide Perovskite Quantum Dots by Encapsulation

Metal halide perovskite quantum dots (PQDs), with excellent optical properties and spectacular characteristics of direct and tunable bandgaps, strong light-absorption coefficients, high defect tolerance, and low nonradiative recombination rates, are highly attractive for modern optoelectronic devices. However, the stability issue of PQDs remains a critical challenge of this newly emerged material despite the recent rapid progress. Here, the encapsulation strategies to improve the stability of PQDs are comprehensively reviewed. A special emphasis is put on the effects of encapsulation, ranging from the improvement of chemical stability, to the inhibition of light-induced decomposition, to the enhancement of thermal stability. Particular attention is devoted to summarizing the encapsulation approaches, including the sol-gel method, the template method, physical blending, and microencapsulation. The selection principles of encapsulation materials, including the rigid lattice or porous structure of inorganic compounds, the low penetration rate of oxygen or water, as well as the swelling-deswelling process of polymers, are addressed systematically. Special interest is put on the applications of the encapsulated PQDs with improved stability in white light-emitting diodes, lasers, and biological applications. Finally, the main challenges in encapsulating PQDs and further investigation directions are discussed for future research to promote the development of stable metal halide perovskite materials.

The ground exciton state of formamidinium lead bromide perovskite nanocrystals is a singlet dark state

Lead halide perovskites have emerged as promising new semiconductor materials for high-efficiency photovoltaics, light-emitting applications and quantum optical technologies. Their luminescence properties are governed by the formation and radiative recombination of bound electron-hole pairs known as excitons, whose bright or dark character of the ground state remains unknown and debated. While symmetry analysis predicts a singlet non-emissive ground exciton topped with a bright exciton triplet, it has been predicted that the Rashba effect may reverse the bright and dark level ordering. Here, we provide the direct spectroscopic signature of the dark exciton emission in the low-temperature photoluminescence of single formamidinium lead bromide perovskite nanocrystals under magnetic fields. The dark singlet is located several millielectronvolts below the bright triplet, in fair agreement with an estimation of the long-range electron-hole exchange interaction. Nevertheless, these perovskites display an intense luminescence because of an extremely reduced bright-to-dark phonon-assisted relaxation.

Silicon nitride nanobeam enhanced emission from all-inorganic perovskite nanocrystals

Optically active perovskite nanocrystals have shown considerable promise for a myriad of applications, such as single photon source, light-emitting diodes and nanophotonics. Coupling those nanocrystals to photonic micro- and nanostructures will offer additional degrees of freedom to manipulate their optical properties. Herein, we demonstrate the coupling of perovskite nanocrystals to a mechanically robust, poly(methyl-methacrylate) (PMMA)-encapsulated silicon nitride nanobeam photonic crystal cavity at room temperature. As determined from the time-resolved photoluminescence decay measurements, we observed enhanced spontaneous emission from the perovskite nanocrystals by a factor of 1.4, consistent with finite difference time domain simulation. In addition, by varying the concentration of the perovskite nanocrystal in the PMMA layer, the effective index of the layer can be modified, allowing us to tune the cavity mode resonance. Our results show that solution-processable perovskite nanocrystals hold a promising prospect for applications such as on-chip light sources, optoelectronic devices and photonic integrated circuits.

Ultralow-Threshold and Color-Tunable Continuous-Wave Lasing at Room-Temperature from In Situ Fabricated Perovskite Quantum Dots

Room-temperature-operated continuous-wave lasers have been intensively pursed in the field of on-chip photonics. The realization of a continuous-wave laser strongly relies on the development of gain materials. To date, there is still a huge gap between the current gain materials and commercial requirements. In this work, we demonstrate continuous-wave lasers at room temperature using rationally designed in situ fabricated perovskite quantum dots in polyacrylonitrile films on a distributed feedback cavity. The achieved threshold values are 15, 24, and 58 W/cm² for green, red, and blue lasers, respectively, which are one order lower than the reported values for the conventional CdSe quantum dot-based continuous-wave laser. Except for the high photoluminescence quantum yields, smooth surface, and high thermal conductivity of the resulting films, the key success of an ultralow laser threshold can be explained by the interaction of polyacrylonitrile and perovskite induced "charge spatial separation" effects. This progress opens up a door to achieve on-chip continuous-wave lasers for photonic applications.

Giant reduction of the random lasing threshold in CH3NH3PbBr3 perovskite thin films by using a patterned sapphire substrate

Hybrid organic-inorganic metal halide perovskites are currently arousing enthusiasm and stimulating huge activity across several fields of optoelectronics due to their outstanding properties. In this study, we present the incoherent random lasing (RL) emissions from CH3NH3PbBr3 perovskite thin films on both planar fluorine-doped tin oxide (FTO) substrates and patterned sapphire substrates (PSSs). A detailed examination of the spectral evolution indicates that inelastic exciton-exciton scattering called P-emission is the most plausible mechanism accounting for the lasing emissions. The RL threshold of the perovskite films on PSSs is found to be effectively reduced by more than one order of magnitude from 2.55 to 0.15 μJ per pulse compared to that on FTO substrates. The giant threshold reduction is ascribed to the enhanced random scattering of light and the photon recycling induced by the multireflection processes at the perovskite/PSS interface, which increases the likelihood that the inoperative random rays will re-enter the possible optical loops formed among the perovskite particles, resulting in considerable optical resonance enhancement. The simulation results reveal that the light extraction efficiency on the top facet of the perovskites is significantly increased by approximately 155% by utilizing the PSS instead of the FTO substrate. Moreover, the first direct experimental observation of the multireflection phenomenon of light, as well as the dynamic processes of photon propagation in the composite PSS structure, is presented by Kerr-gate-based time-resolved photoluminescence. Our results provide an effective strategy to achieve high-performance perovskite random lasers and novel light-emitting devices for speckle-free full-field imaging and solid-state lighting applications, by introducing ingeniously designed periodic nano-/microscale optical structures.

Ultrastable CsPbBr3 Perovskite Quantum Dot and Their Enhanced Amplified Spontaneous Emission by Surface Ligand Modification

The poor stability and aggregation problem of CsPbBr₃ quantum dots (QDs) in air are great challenges for their future practical application. Herein, a simple and effective ligand-modification strategy is proposed by introducing 2-hexyldecanoic acid (DA) with two short branched chains to replace oleic acid (OA) with long chains during the synthesis process. These two short branched chains not only maintain their colloidal stability but also contribute to efficient radiative recombination. The calculations show that CsPbBr₃ QDs with DA modification (CsPbBr₃ -DA QDs) have larger binding energy than CsPbBr₃ QDs with OA (CsPbBr₃ -OA QDs), resulting in significantly enhanced stability. Due to the strong binding energy between DA ligands and QDs, CsPbBr₃ -DA QDs exhibit no aggregation phenomenon even after stored in air for more than 70 d, and CsPbBr₃ -DA QDs films can maintain 94.3% of initial PL intensity after 28 d, while in CsPbBr₃ -OA QDs films occurs a rapid degradation of PL intensity. Besides, the enhanced amplified spontaneous emission (ASE) performance of CsPbBr₃ -DA QDs films has been demonstrated under both one- and two-photon laser excitation. The ASE threshold of CsPbBr₃ -DA QDs films is reduced by more than 50% and their ASE photostability is also improved, in comparison to CsPbBr₃ -OA QDs films.

Stable Two-Photon Pumped Amplified Spontaneous Emission from Millimeter-Sized CsPbBr3 Single Crystals

Metal-halide perovskites are promising optical gain materials because of their excellent photophysical properties. Recently, large perovskite single crystals with phase purity, less defects, and over millimeter dimensions have been successfully synthesized. However, the optical gain effect from these large-size single crystals has not yet been realized. Herein, we for the first time report efficient two-photon pumped amplified spontaneous emission (ASE) from millimeter-sized CsPbBr₃ single crystals (SCs) with a low threshold of 0.65 mJ cm^-2 and an optical gain of 38 cm^-1. Furthermore, the CsPbBr₃ SCs also exhibit ultrastable ASE under continuous laser irradiation for more than 40 h (corresponds to 1.5 × 10⁸ laser shots) at ambient condition. This work suggests the potential application of large-size perovskite single crystals in practical nonlinear optical devices.

Photoluminescence Characteristics of Sn2+ and Ce3+-Doped Cs2SnCl6 Double-Perovskite Crystals

In recent years, all-inorganic lead-halide perovskites have received extensive attention due to their many advantages, but their poor stability and high toxicity are two major problems. In this paper, a low toxicity and stable Cs₂SnCl₆ double perovskite crystals were prepared by aqueous phase precipitation method using SnCl₂ as precursor. By the XRD, ICP-AES, XPS, photoluminescence and absorption spectra, the fluorescence decay curve, the structure and photoluminescence characteristics of Ce³⁺-doped and undoped samples have been investigated in detail. The results show that the photoluminescence originates from defects. [ S n S n 4 + 2 + +V_Cl] defect complex in the crystal is formed by Sn²⁺ substituting Sn⁴⁺. The number of defects formed by Sn²⁺ in the crystal decreases with Ce³⁺ content increases. Within a certain number of defects, the crystal luminescence is enhanced with the number of [ S n S n 4 + 2 + +V_Cl] decreased. When Ce³⁺ is incorporated into the crystals, the defects of [ C e 3 + S n 4 + +V_Cl] and [ S n S n 4 + 2 + +V_Cl] were formed and the crystal show the strongest emission. This provides a route to enhance the photoluminescence of Cs₂SnCl₆ double perovskite crystals.

Controllable Growth of Aligned Monocrystalline CsPbBr3 Microwire Arrays for Piezoelectric-Induced Dynamic Modulation of Single-Mode Lasing

CsPbBr₃ shows great potential in laser applications due to its superior optoelectronic characteristics. The growth of CsPbBr₃ wire arrays with well-controlled sizes and locations is beneficial for cost-effective and largely scalable integration into on-chip devices. Besides, dynamic modulation of perovskite lasers is vital for practical applications. Here, monocrystalline CsPbBr₃ microwire (MW) arrays with tunable widths, lengths, and locations are successfully synthesized. These MWs could serve as high-quality whispering-gallery-mode lasers with high quality factors (>1500), low thresholds (<3 µJ cm^-2 ), and long stability (>2 h). An increase of the width results in an increase of the laser quality and the resonant mode number. The dynamic modulation of lasing modes is achieved by a piezoelectric polarization-induced refractive index change. Single-mode lasing can be obtained by applying strain to CsPbBr₃ MWs with widths between 2.3 and 3.5 µm, and the mode positions can be modulated dynamically up to ≈9 nm by changing the applied strain. Piezoelectric-induced dynamic modulation of single-mode lasing is convenient and repeatable. This method opens new horizons in understanding and utilizing the piezoelectric properties of lead halide perovskites in lasing applications and shows potential in other applications, such as on-chip strain sensing.

Ligand-Induced Tunable Dual-Color Emission Based on Lead Halide Perovskites for White Light-Emitting Diodes

Cesium lead halide perovskites (CsPbX₃, X = Cl, Br, and I) have emerged as an important class of color-tunable light-emitting materials in the past 4 years. However, single-CsPbX₃ nanostructures with dual-color emission remain scarce. Here, we demonstrate dual-color emission from lead halide perovskite nanowires induced by the surface ligands, that is, oligomeric methoxypolyethylene glycol (MEOPEG). In addition to the characteristic emission from the host lattice, an unprecedented emission from the expanded band gap caused by MEOPEG is observed. The ratio of the two emission intensities can be easily adjusted by changing the concentration of the surface ligands. Moreover, the band gaps of CsPbX₃-MEOPEG can be further fine-tuned by a simple postsynthetic anion exchange process. As a result, white light-emitting diodes (WLEDs) with high-quality CIE coordinates of (0.33, 0.29) and a high color rendering index value (84) are realized. These CsPbX₃-MEOPEG materials, with tunable dual-color emission, may serve as ideal model systems for WLEDs, which will undoubtedly expand the applications of cesium lead halide perovskites.

O2 as a molecular probe for nonradiative surface defects in CsPbBr3 perovskite nanostructures and single crystals

Lead halide perovskites, owing to their flexible, scalable chemistry and promising physical properties are attracting increasing attention for solution-processed optoelectronic and photonic technologies. Despite their well-known 'defect tolerant' electronic structure, studies highlighted the active role of shallow and deep defect states, as well as of oxidative environmental conditions, on the optical and electrical behavior of perovskite nanocubes, films and single bulk crystals. To date, however, no in-depth systematic study of the surface trap-mediated processes in perovskite materials of different dimensionality has been conducted. In this work, we aim to bridge this gap by using O2 as a molecular probe for the effects of surface states on the exciton recombination processes of nanocubes (NCs), nanowires (NWs), nanosheets (NSs) and bulk single crystals (SCs) of CsPbBr3 perovskite. Continuous wave and time-resolved photoluminescence (PL) experiments in a controlled O2 atmosphere reveal the opposite optical response of NCs with respect to higher dimensional perovskites directly deriving from the different nature of the material surfaces. Specifically, O2 passivates surface hole-traps in NWs, NSs and SCs, leading to PL brightening with unaltered recombination dynamics. Conversely, NCs appear to be free from such surface hole-traps and exposure to O2 leads to direct extraction of photogenerated electrons that competes with radiative exciton recombination, leading to dimmed PL efficiency in atmospheric conditions. This opposite oxygen PL response demystifies the critical role of surface passivation in perovskite NCs in stark contrast to higher dimensional nanostructures and single crystals.

Stable and enhanced frequency up-converted lasing from CsPbBr3 quantum dots embedded in silica sphere

Perovskites have emerged as a class of cutting-edge light-emitting materials; however, their poor stability, due to the high sensitivity to moisture in the ambient environment, severely hinders their further application. Here, to obtain stable perovskite-based laser with excellent optical performance, all-inorganic perovskite CsPbBr₃ quantum dots (QDs) evenly distributed into sub-micro silica sphere (CsPbBr₃-SiO₂) have been used as laser gain medium. The single silica sphere embedded by plentiful CsPbBr₃ QDs demonstrates frequency up-converted lasing with compounded mode of random and whispering-gallery-mode (WGM) at room temperature. Furthermore, by incorporating the CsPbBr₃-SiO₂ spheres into a microtubule, the frequency up-converted WGM lasing has been successfully achieved under two-photon excitation. Notably, the CsPbBr₃-SiO₂ microtubule resonator exhibits a low lasing threshold of 430 μJ/cm², mostly due to the enhanced gain for CsPbBr₃ QDs inside the silica sphere. Moreover, stable WGM lasing is observed under continuous optical pump for 140 min, benefited from the protection of silica shells, which isolate the QDs from the environmental conditions. The enhanced lasing performance provides an effective way for further exploration and application of perovskite-based micro/nano photonic devices.

Optical Gain from Biexcitons in CsPbBr3 Nanocrystals Revealed by Two-dimensional Electronic Spectroscopy

Perovskite semiconductor nanocrystals (NCs) exhibit highly efficient optical gain, which is promising for laser applications. However, the intrinsic mechanism of optical gain in perovskite NCs, particularly whether more than one exciton per NCs is required, remains poorly understood. Here, we use two-dimensional electronic spectroscopy to resonantly probe the interplay between near-band-edge transitions during the buildup of optical gain in CsPbBr₃ NCs. We find compelling evidence to conclude that optical gain in CsPbBr₃ NCs is generated through stimulated emission from strongly interacting biexcitons. The threshold is largely determined by the competition between stimulated emission from biexcitons and excited-state absorption from single exciton to biexciton states. The findings in this work may guide future explorations of NC materials with low-threshold optical gain.

The top-down synthesis of single-layered Cs4CuSb2Cl12 halide perovskite nanocrystals for photoelectrochemical application

The development of an all-inorganic lead-free perovskite nanocrystal is of crucial importance to solve the instability and lead toxicity of organic-inorganic lead hybrid perovskites. Herein, single-layered Cs4CuSb2Cl12 nanocrystals (NCs) with a narrow band gap of 1.6 eV were prepared for the first time via an ultrasonic exfoliation technique. This powerful top-down method was further generalized to synthesize a class of lead-free perovskite (Cs3Bi2X9 and Cs3Sb2X9) NCs. The experimental and theoretical studies revealed that not only inter-layer van der Waals forces but also in-plane chemical bonds played a critical role in the exfoliation process. Specifically, smaller uniform-sized NCs were observed for Cs4CuSb2Cl12 (∼3 nm) as compared to those for Cs3Sb2Cl9 (∼20 nm) although both Cs4CuSb2Cl12 and Cs3Sb2Cl9 exhibited a similar exfoliation energy (∼0.310 J m-2). This can be ascribed to the weaker in-plane chemical bonds of Cu-Cl (2.808 Å) and Sb-Cl (2.924 Å) in Cs4CuSb2Cl12 than the uniform Cl-Sb bond (2.69 Å) in Cs3Sb2Cl9 that allow for an easier exfoliation process. In addition, exfoliation of the Cs4CuSb2Cl12 microcrystal into NCs results in an indirect-to-direct bandgap transition and a reduced electron effective mass, which provides a rapid and steady photoelectrochemical response, demonstrating that Cs4CuSb2Cl12 NCs are a promising candidate for optoelectronic applications.

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

Hybrid metal/semiconductor nano-heterostructures with strong exciton-plasmon coupling have been proposed for applications in hot carrier optoelectronic devices. However, the performance of devices based on this concept has been limited by the poor efficiency of plasmon-hot electron conversion at the metal/semiconductor interface. Here, we report that the efficiency of interfacial hot excitation transfer can be substantially improved in hybrid metal semiconductor nano-heterostructures consisting of perovskite semiconductors. In Ag–CsPbBr₃ nanocrystals, both the plasmon-induced hot electron and the resonant energy transfer processes can occur on a time scale of less than 100 fs with quantum efficiencies of 50 ± 18% and 15 ± 5%, respectively. The markedly high efficiency of hot electron transfer observed here can be ascribed to the increased metal/semiconductor coupling compared with those in conventional systems. These findings suggest that hybrid architectures of metal and perovskite semiconductors may be excellent candidates to achieve highly efficient plasmon-induced hot carrier devices.

Proposed devices exploiting the strong exciton-plasmon coupling are limited by the low efficiency of hot carrier generation. Here, Huang et al. study the efficiencies of different plasmon-hot electron conversion processes in metal/perovskite semiconductor nanocrystals to address this problem.

The luminescence properties of CsPbxM1-xBr3 perovskite nanocrystals transformed from Cs4PbBr6 mediated by various divalent bromide MBr2 salts

A novel high concentration doping method based on the transformation from Cs₄PbBr₆ nanocrystals (NCs), which reacted with divalent metal bromide MBr₂, to CsPb_xM_1-xBr₃ NCs was developed. Two types of M²⁺ and Zn²⁺ which cannot emit light and Mn²⁺ and Eu²⁺ which can be used as the luminous centres, were chosen to trigger the transformation of Cs₄PbBr₆ NCs to CsPb_xM_1-xBr₃ NCs. CsPb_xZn_1-xBr₃ NCs maintained high photoluminescence quantum yields (PLQY) (>75%) and had good dispersion in hexane without obvious dissolution or agglomeration after two weeks. By adjusting the reaction temperature, the intrinsic band edge luminescence and the emission of Mn²⁺ ions CsPb_xMn_1-xBr₃ NCs show different colours of light from green, green-yellow, pink, and orange-red to purple under an excitation of 365 nm. CsPb_xEu_1-xBr₃ NCs were synthesized for the first time, and a weak luminescence around 618 nm from Eu³⁺ was detected in addition to the band edge luminescence of NCs. X-ray photoelectron spectroscopy (XPS) data showed that Zn²⁺, Mn²⁺ and Eu³⁺ (Eu²⁺) doping concentrations are up to 80%, 75% and 50%, respectively. We also analysed the doping mechanism and compared the new method with the traditional high temperature injection method. The lead-depleted perovskite NCs transformed from Cs₄PbBr₆ can provide a feasible pathway to reduce the lead toxicity of perovskite NCs and expand their applications.

Layered Hybrid Perovskites for Highly Efficient Three-Photon Absorbers: Theory and Experimental Observation

Multiphoton absorption may find many technological applications, such as enhancing the conversion efficiency of solar cells by the utilization of sub-band-energy photons, below-bandgap photodetection through the simultaneous absorption of several infrared photons for photocurrent generation, or light frequency upconversion for high-resolution, 3D imaging. To enhance multiphoton absorption in semiconducting materials, one of the strategies is to explore low-dimensional excitons. Here, a quantum perturbation theory on a giant enhancement in three-photon absorption (3PA) arising from 2D excitons in multilayered crystals of organic-inorganic hybrid perovskites is presented. The maximal 3PA coefficient is predicted to be in the range of 2-7 cm3 GW-2 at 1100 nm, the largest values reported so far for any 2D and bulk semiconductors at room temperature. Excellent agreement between theory and the experimental findings unambiguously demonstrates a pivotal role in the enhancement of 3PA played by 2D excitons. The theory predicts that the resonant 3PA coefficient should be enhanced further by at least two orders of magnitude with very low temperature. The findings are essential for understanding giant 3PA arising from 2D excitons in layered hybrid perovskites and may open new pathways for highly efficient conversion from infrared light energy to either electrical energy or higher-frequency light emission/lasing.