Quantum-confined CsPbBr3 Research Articles

Intraband Cooling and Auger Recombination in Weakly to Strongly Quantum-Confined CsPbBr3 Perovskite Nanocrystals.

Semiconductor nanocrystals (NCs) with size-tuned energy gaps present unique and desirable properties for optoelectronic applications. Recent synthetic advancements offer routes to spheroidal CsPbBr3 perovskite NCs in the strong quantum confinement regime with narrow size dispersion. Using tunable femtosecond laser pulses, we examine intraband carrier relaxation using transient absorption spectroscopy and show that, across the transition from weak to strong confinement, hot carrier lifetime increases compared to larger bulk-like particles. However, further increases of confinement subsequently lead to a reduction of the hot carrier lifetime and increase of the non-radiative Auger recombination rate. Finally, we show that hot carrier lifetimes increase as a function of excess energy above the band gap less sensitively under high confinement in comparison to the bulk. Understanding such unique trends is important for maximizing hot carrier lifetimes for use in next-generation hot carrier devices as well as evaluating the transition from weak to strong confinement.

Enhancing crystal integrity and structural rigidity of CsPbBr3 nanoplatelets to achieve a narrow color-saturated blue emission

Quantum-confined CsPbBr3 perovskites are promising blue emitters for ultra-high-definition displays, but their soft lattice caused by highly ionic nature has a limited stability. Here, we endow CsPbBr3 nanoplatelets (NPLs) with atomic crystal-like structural rigidity through proper surface engineering, by using strongly bound N-dodecylbenzene sulfonic acid (DBSA). A stable, rigid crystal structure, as well as uniform, orderly-arranged surface of these NPLs is achieved by optimizing intermediate reaction stage, by switching from molecular clusters to mono-octahedra, while interaction with DBSA resulted in formation of a CsxO monolayer shell capping the NPL surface. As a result, both structural and optical stability of the CsPbBr3 NPLs is enhanced by strong covalent bonding of DBSA, which inhibits undesired phase transitions and decomposition of the perovskite phase potentially caused by ligand desorption. Moreover, rather small amount of DBSA ligands at the NPL surface results in a short inter-NPL spacing in their closely-packed films, which facilitates efficient charge injection and transport. Blue photoluminescence of the produced CsPbBr3 NPLs is bright (nearly unity emission quantum yield) and peaks at 457 nm with an extremely narrow bandwidth of 3.7 nm at 80 K, while the bandwidth of the electroluminescence (peaked at 460 nm) also reaches a record-narrow value of 15 nm at room temperature. This value corresponds to the CIE coordinates of (0.141, 0.062), which meets Rec. 2020 standards for ultra-high-definition displays.

Two-Dimensional Quantum-Confined CsPbBr3 in Silicene for LED Applications

Perovskites with reduced dimensions are very attractive for LED applications due to their improved stability and efficient energy transfer. This is exemplified by 2D perovskites reported in the literature, predominantly either Dion-Jacobson or Ruddlesden–Popper systems, which typically use long chain alkyl moieties as spacers in order to form 2D layers. Here, we report an approach which uses an inorganic 2D material─silicene─as a template to form two-dimensional quantum-confined CsPbBr3 (qc-CsPbBr3) layers of less than 5 nm in thickness. This approach can be further generalized to synthesize other types of perovskites with reduced dimensionality. The structural and optical properties of the qc-CsPbBr3 layers inside silicene were characterized. The qc-CsPbBr3-based LEDs demonstrate improved stability under ambient conditions, along with a current efficiency of 13.7 Cd A–1 and external quantum efficiency of 8.5%.

A facile reprecipitation method of Quantum-confined CsPbBr3 perovskite quantum dots for narrow-band deep-blue emission

Quantum-confined cesium lead tribromide perovskite quantum dots (PeQDs) are attractive photonic sources for versatile optoelectronic devices and thus the synthetic routes have been explored via various techniques. However, the lack of a general method to fabricate the quantum-confined PeQD with remarkable stability at colloidal state has been one of the major obstacles to device applications. Herein, we propose a simple and robust ligand-assisted reprecipitation method associated with didodecyl dimethyl ammonium bromide for quantum-confined PeQDs with remarkable stability at colloidal state. This method will shed a light on the field of colloidal PeQDs synthesis for the next generation of photonic sources.

Mechanochemical synthesis of stable, quantum-confined CsPbBr3 perovskite nanocrystals with blue-green emission and high PLQY

Cesium lead halides are a family of bright, visible-light emitting materials with near-unity photoluminescence quantum yield (PLQY) in nanocrystals (NCs). The usual way to achieve visible light-emission tunability is by mixing halides, which often leads to phase separation and poor stability. While the NCs should also show size-dependent PL emission, reports on strong quantum confinement in these materials are scarce. Here, we report the synthesis of quantum-confined cesium lead bromide (CsPbBr3) NCs via a facile, environment-friendly, and scalable high-energy mechanochemical synthesis route. The PLQY measured is ∼85%, even after 90 days of synthesis, and the emission wavelength is shifted from green, 520 nm, to blue, 460 nm by quantum confinement in NCs of size 3–5 nm. Micro-PL optical spectroscopy and atomic force microscopy confirm the size tunability of PL on a single-dot scale. Our work demonstrates the potential of mechanochemical synthesis in the medium-scale production of bright luminescent quantum-confined NCs that could be extended to other materials as well.

Wavelength-Tunable and Water-Stable Cesium-Lead-Based All-Bromide Nanocrystal-Polymer Composite Films Using Ultraviolet-Curable Prepolymer as an Anti-Solvent.

All-inorganic metal halide perovskite nanocrystals (IPeNCs) have become one of the most promising luminescent materials for next-generation display and lighting technology owing to their excellent color expression ability. However, research on IPeNCs with stable blue emission is limited. In this paper, we report stable blue emissive all-bromide IPeNCs obtained through a modified ligand-assisted reprecipitation method using an ultraviolet (UV)-curable prepolymer as the anti-solvent at a low temperature. We found that the blue emission originates from quantum-confined CsPbBr3 nanoparticles formed together with the colorless wide-bandgap Cs4PbBr6 nanocrystals. When the temperature of the prepolymer was increased from 0 to 50 °C, CsPbBr3 nanoparticles became larger and more crystalline, thereby altering their emission color from blue to green. The synthesized all-bromide blue-emitting IPeNC solution remained stable for over 1 h. It also remained stable when it was mixed with the green-emitting IPeNC solution. By simply exposing the as-synthesized IPeNC–prepolymer solutions to UV light, we formed water-stable composite films that emitted red, green, blue, and white colors. We believe that this synthetic method can be used to develop color-emitting composite materials that are highly suitable for application as the color conversion films of full-color liquid crystal display backlight systems and lighting applications.

Entropy-Gated Thermally Activated Delayed Emission Lifetime in Phenanthrene-Functionalized CsPbBr3 Perovskite Nanocrystals.

Charge and electronic energy transfer form the basis of many natural and artificial energy transduction systems. The energy landscapes that drive these transfer processes are often constructed from enthalpy changes. In contrast, the entropic effect, although occasionally invoked to explain some excited-state dynamics, has rarely been used to actively control charge/energy flow. Here we derive a generic formula describing how entropy can quantitatively gate the thermally activated delayed emission lifetime in semiconductor nanocrystal-molecular triplet acceptor complexes and experimentally verify the model using highly emissive, quantum-confined CsPbBr3 nanocrystals surface-functionalized with multiple phenanthrene triplet acceptors. Triplet energy transfer from photoexcited CsPbBr3 nanocrystals to phenanthrene is followed by thermally activated repopulation of nanocrystal excitons, leading to delayed nanocrystal emission. The lifetime of delayed emission increases with the phenanthrene/nanocrystal ratio, due to lowering of the free energy of the acceptor state by entropic gain. This study points toward a direction of using entropy to artificially design donor-acceptor light-emitting materials with predetermined excited-state lifetimes.

Stable Hexylphosphonate-Capped Blue-Emitting Quantum-Confined CsPbBr3 Nanoplatelets.

Quantum-confined CsPbBr3 nanoplatelets (NPLs) are extremely promising for use in low-cost blue light-emitting diodes, but their tendency to coalesce in both solution and film form, particularly under operating device conditions with injected charge-carriers, is hindering their adoption. We show that employing a short hexyl-phosphonate ligand (C6H15O3P) in a heat-up colloidal approach for pure, blue-emitting quantum-confined CsPbBr3 NPLs significantly suppresses these coalescence phenomena compared to particles capped with the typical oleyammonium ligands. The phosphonate-passivated NPL thin films exhibit photoluminescence quantum yields of ∼40% at 450 nm with exceptional ambient and thermal stability. The color purity is preserved even under continuous photoexcitation of carriers equivalent to LED current densities of ∼3.5 A/cm2. 13C, 133Cs, and 31P solid-state MAS NMR reveal the presence of phosphonate on the surface. Density functional theory calculations suggest that the enhanced stability is due to the stronger binding affinity of the phosphonate ligand compared to the ammonium ligand.

Visible-to-Ultraviolet Upconversion Efficiency above 10% Sensitized by Quantum-Confined Perovskite Nanocrystals.

Photon upconversion (UC) based on sensitized triplet-triplet annihilation (TTA), TTA-UC, can potentially alleviate the transmission loss of below-band-gap photons in solar energy conversion. TTA-UC across various spectral windows has been demonstrated, but efficient visible-to-ultraviolet (UV) UC remains a big challenge primarily due to the lack of suitable triplet sensitizers. Here we report a TTA-UC system sensitized by quantum-confined CsPbBr3 perovskite nanocrystals (NCs) that simultaneously achieves a high photon energy gain of up to 0.7 eV (443-355 nm) and a high UC efficiency up to 10.2%. Time-resolved spectroscopy studies reveal that the performance is mainly enabled by ultrafast and efficient triplet energy transfer from the strongly confined NC sensitizers to triplet acceptors.

Triplet Sensitization by Lead Halide Perovskite Thin Films for Efficient Solid-State Photon Upconversion at Subsolar Fluxes

Summary We investigate the rubrene triplet sensitization by perovskite thin films based on methylammonium formamidinium lead triiodide (MAFA) of varying thicknesses. The power-law dependence of both the MAFA photoluminescence (PL) intensity and upconverted emission is tracked as a function of the incident power density. Bimolecular triplet-triplet annihilation (TTA) exhibits a unique power-law dependence with a slope change from quadratic-to-linear at the threshold Ith. The underlying MAFA PL power-law dependence dictates the power law of the upconverted PL: (1) below Ith, the slope of the upconverted PL is twice the value of the MAFA PL; (2) above Ith, it follows the same power law as the underlying recombination of mobile electrons and holes in the MAFA films. We find that the Ith shifts to subsolar incident laser powers when increasing the MAFA thickness above 30 nm. For the thickest MAFA film of 380 nm we find an upconversion threshold of Ith = 7.1 mW/cm2.

Insights of Doping and the Photoluminescence Properties of Mn-Doped Perovskite Nanocrystals.

Doping Mn2+ in semiconductor nanocrystals is widely known forits long-lifetime Mn d-d orange emission. While this had been extensively studied for chalcogenide nanostructures, recently this was also extended to perovskite nanocrystals. Being that CsPbCl3 has a wide bandgap, the exciton energy transfer was found to be more efficient, but the dopant-induced photoluminescence was also obtained for layered perovskites and quantum-confined CsPbBr3 nanocrystals. In recent yearssignificant advances have been achieved in understanding the physical insights of doping following various approaches and optimizing the conditions for obtaining intense dopant emission. In addition, several new properties associated with these doped nanocrystals were also reported, and by modulating the compositions, the host bandgap and the dopant emission positions were also tuned. Keeping all of these developments in mind, this Perspective focuses on the insights of doping and the photoluminescence properties of Mn2+-doped perovskite nanocrystals. In addition, it also proposes possible future prospects of both synthesis and optical properties of these nanomaterials.

Visible-Light-Driven Sensitization of Naphthalene Triplets Using Quantum-Confined CsPbBr3 Nanocrystals.

Recent years have seen the revived interests in the triplet states of polycyclic aromatic hydrocarbons (PAHs) due to their potential applications ranging from photon upconversion to photoredox catalysis. Because of their "dark" nature, these triplets have to be generated via triplet energy transfer (TET) from sensitizers. Traditional sensitization schemes introduce asizable energy loss (ca. ≥ 0.5 eV) resulting from intersystem crossing (ISC) in sensitizers. Consequently, triplets of naphthalene (Nap), the most energetic PAH triplets (∼2.6 eV), have remained relatively underexplored because ultraviolet (UV) photons and wide-energy-gap sensitizers would be required for the sensitization. Here we show that Nap triplets can be efficiently generated using TET from quantum-confined CsPbBr3 perovskite nanocrystals (NCs). Quantum confinement is essential to meet the energetics and electronic coupling requirements in TET. Because of the vanishingly small bright-dark states energy splitting in NCs, this sensitization process can take place at the limit of energy conservation, driven by visible photons.

Triplet Energy Transfer from CsPbBr3 Nanocrystals Enabled by Quantum Confinement.

The spectral properties of lead halide perovskite nanocrystals (NCs) can be engineered by tuning either their sizes via the quantum confinement effect or their compositions using anion and/or cation exchange. To date, the latter is more frequently adopted, primarily because of the ease of ion exchange for lead halide perovskites, making the quantum confinement effect seemingly redundant for perovskite NCs. Here we report that quantum confinement is required for triplet energy transfer (TET) from perovskite NCs to polycyclic aromatic hydrocarbons (PAHs). Static and transient spectroscopy measurements on CsPbBr3 NC-pyrene hybrids showed that efficient TET occurred only for small-sized, quantum-confined CsPbBr3 NCs. The influences of the size-dependent driving force and spectral overlap on the TET rate were found to be negligible. Instead, the TET rate scaled linearly with carrier probability density at the NC surface, consistent with a Dexter-type TET mechanism requiring wave function exchange between the NC donors and pyrene acceptors. Efficient TET funnels the excitation energy generated in strongly light-absorbing perovskite NCs into long-lived triplets in PAHs, which may find broad applications such as photon upconversion and photoredox catalysis.

Rapid synthesis of quantum-confined CsPbBr3 perovskite nanowires using a microfluidic reactor.

Microfluidics has been considered as an effective platform in the mechanism study and large-scale manufacturing of nanomaterials. In this work, we report the facile synthesis of quantum-confined CsPbBr3 nanowires (NWs) by using a continuous-flow microfluidic reactor. The optimized reaction temperature is around 50 °C, and one "synthesis run" by microfluidics requires only ten minutes. This study reveals that the formation of CsPbBr3 NWs takes place by a hybrid growth mechanism of seed-mediated growth and oriented attachment growth. This microfluidic approach benefits the alignment of the short quantum-confined NWs and promotes their oriented attachment to form long NWs, while conventional flask synthesis results in large and irregular nanorods under the same reaction conditions. This work not only provides a new synthetic path for the preparation of CsPbX3 NWs but also sheds some light on the fundamental study of CsPbX3 NWs.

Biexciton Auger recombination in mono-dispersed, quantum-confined CsPbBr3 perovskite nanocrystals obeys universal volume-scaling

Auger recombination has been a long-standing obstacle to many prospective applications of colloidal quantum dots (QDs) ranging from lasing, light-emitting diodes to bio-labeling. As such, understanding the physical underpinnings and scaling laws for Auger recombination is essential to these applications. Previous studies of biexciton Auger recombination in various QDs established a universal scaling of biexciton lifetime (τXX) with QD volume (V ): τXX = γV. However, recent measurements on perovskite nanocrystals (NCs), an emerging class of enablers for light harvesting and emitting applications, showed significant deviations from this universal scaling law, likely because the measured NCs are weakly-confined and also have relatively broad size-distributions. Here we study biexciton Auger recombination in mono-dispersed (size distributions within 1.7%–9.0%), quantum-confined CsPbBr3 NCs (with confinement energy up to 410 meV) synthesized using a latest approach based on thermodynamic equilibrium control. Our measurements clearly reproduce the volume-scaling of τXX in confined CsPbBr3 QDs. However, the scaling factor γ (0.085 ± 0.001 ps/nm3) is one order of magnitude lower than that reported for CdSe and PbSe QDs (1.00 ± 0.05 ps/nm3), suggesting unique mechanisms enhancing Auger recombination rate in perovskite NCs.

Reversible Concentration-Dependent Photoluminescence Quenching and Change of Emission Color in CsPbBr3 Nanowires and Nanoplatelets.

We discuss the photoluminescence (PL) of quantum-confined CsPbBr3 colloidal nanocrystals of two different shapes (nanowires and nanoplatelets) at different concentrations in solution and in solid-state films. Upon increasing the nanocrystal concentration in solution, a constant drop in photoluminescence quantum yield is observed, accompanied by a significant PL red shift. This effect is reversible, and the original PL can be restored by diluting to the original concentration. We show that this effect can be in part attributed to self-absorption and partly to aggregation. In particular, for nanoplatelets, where the aggregation is mostly irreversible, while the self-absorption effect is reversible, the two contributions can be well separated. Finally, when dry solid-state films are prepared, the emission band is shifted into the green spectral region, close to the bulk CsPbBr3 band gap, thus preventing blue emission from such films.