Study of Shell Thickness-Dependent Charge Transfer Dynamics in Green-Emitting Core/Shell Giant Quantum Dots.

Owing to their superior photostability, green-emitting graded alloy core/shell giant quantum dots (g-QDs) can be applied in optoelectronics. However, it is essential to understand how the shell thickness affects interfacial charge separation. This work explores the impact of shell thickness on photoinduced electron transfer (PET) and photoinduced hole transfer (PHT) with an electron acceptor benzoquinone and a hole acceptor phenothiazine, respectively. Four graded alloy core/shell green-emitting g-QDs with different shell thicknesses were synthesized. The PET and PHT rate constants were obtained from photoluminescence and PL lifetime decay measurements. Our study concludes that g-QDs with a diameter of ∼7.14 show a substantial improvement in charge transfer compared with g-QDs ≥8.5 nm in diameter. Similarly, the PET and PHT rates are 3.7 and 4.1 times higher for 7.14 nm g-QDs than for the 10.72 nm sample. The calculated electron and hole transfer rate constants (ket/ht) of g-QDs with 7.14 nm in diameter are 10.80 × 107 and 14 × 107 s-1, which are 8.5 and 8 times higher compared to g-QDs with 10.72 nm in diameter. These results highlight the impact of shell thickness on the excited-state interactions of green-emitting g-QDs and conclude that g-QDs with a relatively thin shell can be a better choice as photoactive materials for photocatalyst, photodetector, and solar cells.

Completely Amine-Free Open-Atmospheric Synthesis of High-Quality Cesium Lead Bromide (CsPbBr3 ) Perovskite Nanocrystals.

Cesium lead halide perovskite nanocrystals (NCs) CsPbX3 (X=Cl, Br, and I) have been prominent materials in the last few years due to their high photoluminescence quantum yield (PLQY) for light-emitting diodes and other significant applications in photovoltaics and optoelectronics. In colloidal CsPbX3 synthesis, the most commonly used ligands are oleic acid and oleylamine. The latter plays an important role in surface passivation but may also be responsible for poor colloidal stability as a result of facile proton exchange leading to the formation of labile oleylammonium halide, which pulls halide ions out of the NC surface. Herein, a facile, efficient, completely amine-free synthesis of cesium lead bromide perovskite nanocrystals using hydrobromic acid as halide source and tri-n-octylphosphane as ligand under open-atmospheric conditions is demonstrated. Hydrobromic acid serves as labile source of bromide ion, and thus this three-precursor approach (separate precursors for Cs, Pb, Br) gives more control than a conventional single-source precursor for Pb and Br (PbBr2 ). The use of HBr paved the way to eliminate oleylamine, and thus the formation of labile oleylammonium halide can be completely excluded. Various Cs:Pb:Br molar ratios were studied and optimum conditions for making very stable CsPbBr3 NCs with high PLQY were found. These completely amine-free CsPbBr3 perovskite NCs synthesized under bromine-rich conditions exhibit good stability and durability for more than three months in the form of colloidal solutions and films, respectively. Furthermore, stable tunable emission across a wide spectral range through anion exchange was demonstrated. More importantly, this work reports open-atmosphere-stable CsPbBr3 NCs films exhibiting strong PL, which can be further used for optoelectronic device applications.

Exploring the Role of Short Chain Acids as Surface Ligands in Photoinduced Charge Transfer Dynamics from CsPbBr3 Perovskite Nanocrystals.

The most commonly used surface capping ligands, like oleic acid and oleylamine, passivate the surface of perovskite nanocrystals (PNCs) to enhance their stability and optical properties. However, due to their inherent insulating nature, charge transport across the surface of the PNCs is hindered, limiting their application in devices. In this study, we have post-treatment CsPbBr3 PNCs with short chain ligands benzoic acid (BA) and ascorbic acid (AA) and observed that both acid-treated PNCs show enhanced stability and optical properties. Still, BA-treated PNCs show the highest charge transport rate due to their conjugating nature. The photoelectrochemical measurements also show the most efficient electron flow across the surface of the PNC with BA-treated PNCs. A longer carrier lifetime and fast charge transfer make BA-treated PNCs ideal candidates for application in real-life devices.

Amine-Free Synthetic Route: An Emerging Approach to Making High-Quality Perovskite Nanocrystals for Futuristic Applications.

In recent years, colloidal cesium lead halide (CsPbX3) perovskite nanocrystals (PNCs) have attracted significant attention from researchers due to their unique optical properties and potential use in optoelectronic applications. In colloidal synthesis, oleic acid and oleylamine are commonly used as surface-capping ligands. Although oleylamine plays a crucial role in maintaining the colloidal stability and surface passivation of PNCs, its dynamic equilibrium with oleic acid leads to the formation of labile oleylammonium, which pulls halides from the surface of PNCs and thus degrades the crystals. In this Perspective, we summarize the various approaches for eliminating the amines to make high-quality, photostable, and amine-free CsPbX3 PNCs. In addition, we look over the prospects of these PNCs regarding stability in different environmental conditions, photoluminescence properties, and optoelectronic device performance. This perspective will give a broad overview of amine-free PNCs starting from their synthesis, challenges, and optoelectronic properties to their future prospects.

In situ synthesis of high-quantum-efficiency and stable bromide-based blue-emitting perovskite nanoplatelets.

We present a facile synthetic approach for the growth of two-dimensional CsPbBr3 nanoplatelets (NPLs) in the temperature range of 50-80 °C via the vacuum-assisted low-temperature (VALT) method. In this method, we utilized the solubility of the PbBr2 precursor at temperatures high than the reaction temperature, thus making Br available during the reaction to form NPLs with fewer defects. The high chemical availability of Br during the reaction changes the growth dynamics and formation of highly crystalline nanoplatelets. Using this method, we have synthesized NPLs with an emission wavelength range of 450 to 485 nm that have high photoluminescence quantum yields (PLQY) from 80 to 100%. The synthesized NPLs retain their initial PLQY of about 80% after one month at ambient conditions. The formation of NPLs with fewer defects and enhanced radiative recombination was further confirmed by X-ray diffraction (XRD), reduced Urbach energy, time-resolved photocurrent measurements, X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared (FTIR) spectroscopy. Additionally, we utilized the synthesized NPLs for the fabrication of down-conversion light emitting diodes (LEDs), and the electroluminescence peak was barely shifted compared to the photoluminescence peak.

Surface modification for improving the photoredox activity of CsPbBr3 nanocrystals.

In recent years inorganic lead halide perovskite nanocrystals (PNCs) have been used in photocatalytic reactions. The surface chemistry of the PNCs can play an important role in the excited state interactions and efficient charge transfer with redox molecules. In this work, we explore the impact of CsPbBr3 nanocrystal surface modification on the excited state interactions with the electron acceptor benzoquinone (BQ) for three different ligand environments: as oleic acid/oleylamine (OA/OAm), oleic acid (OA)/trioctylphosphine (TOP), and oleic acid (OA)/oleylamine (OAm)/trioctylphosphine (TOP) ligands. Our finding concludes that amine-free PNCs (OA/TOP capped) exhibit the best excited state interactions with benzoquinone compared to the conventional oleylamine ligand environment. The photoinduced electron transfer (PET) rate constants were measured from PL-lifetime decay measurement. The amine-free PNCs show the highest PET which is 9 times higher than that of conventional ligand capped PNCs. These results highlight the impact of surface chemistry on the excited-state interactions of CsPbBr3 NCs and in photocatalytic applications. More importantly, this work concludes that amine-free PNCs maintain a redox-active surface with a high photoinduced electron transfer rate which makes them an ideal candidate for photocatalytic applications.

Enhancement of photoluminescence and the stability of CsPbX3 (X = Cl, Br, and I) perovskite nanocrystals with phthalimide passivation.

Cesium lead halide perovskite nanocrystals (CsPbX3 NCs) have been the flourishing area of research in the field of photovoltaic and optoelectronic applications because of their excellent optical and electronic properties. However, they suffer from low stability and deterioration of photoluminescence (PL) properties post-synthesis. In this work, we demonstrate that incorporating an additional ligand can further enhance the optical properties and stability of NCs. Here, we introduced phthalimide as a new surface passivation ligand into the oleic acid/oleylamine system in situ to get near-unity photoluminescence quantum yield (PLQY) of CsPbBr3 and CsPbI3 perovskite NCs. Phthalimide passivation dramatically improves the stability of CsPbCl3, CsPbBr3, and CsPbI3 NCs under ambient light and UV light. The PL intensity was recorded for one year, which showed a dramatic improvement for CsPbBr3 NCs. Nearly 11% of PL can be retained even after one year with phthalimide passivation. CsPbCl3 NCs exhibit 3 times higher PL with phthalimide and retain 12% PL intensity even after two months, while PL of as-synthesized NCs completely diminishes. Under continuous UV light illumination, the PL intensity of phthalimide passivated NCs is well preserved, while the as-synthesized NCs exhibit negligible PL emission in 2 days. About 40% and 25% of initial PL is preserved for CsPbBr3 and CsPbCl3 NCs in the presence of phthalimide. CsPbI3 NCs with phthalimide exhibit PL even after 2 days, while PL for as-synthesized NCs rapidly declined in the first 10 h. The presence of phthalimide in CsPbI3 NCs could maintain stability even after a week, while the as-synthesized NCs underwent a transition to the non-luminescent phase within 4 days. Furthermore, blue, green, yellow, and red-emitting diodes using CsPbCl1.5Br1.5, CsPbBr3, CsPbBr1.5I1.5, CsPbI3 NCs, respectively, are fabricated by drop-casting NCs onto blue LED lights, which show great potential in the field of display and lighting technologies.

Bromopropane as a novel bromine precursor for the completely amine free colloidal synthesis of ultrastable and highly luminescent green-emitting cesium lead bromide (CsPbBr3) perovskite nanocrystals.

Recently, lead halide perovskite nanocrystals (PNCs) have attracted intense interest as promising active materials for optoelectronic devices. However, their extensive applications are still hampered by poor stability under ambient conditions. Oleic acid and oleylamine are the most commonly used ligands in colloidal CsPbX3 (X = Cl, Br, and I) synthesis. Oleylamine plays a dual role as it stabilizes the surface but in the long run or post-synthesis, it may disturb the colloidal stability due to facile proton exchange leading to the formation of labile oleylammonium halide, which detaches the halide ions from the NC surface. To address these issues, herein, we report an open-atmospheric, facile, efficient, and completely amine-free synthesis of cesium lead bromide perovskite nanocrystals using a novel bromine precursor, bromopropane, which is inexpensive and available at hand. The reaction mechanism follows a trioctylphosphine/oleic acid-mediated surface passivation route that provides an amine-free reaction environment to stabilize ligand capping on the NC surface. Uniform, highly monodisperse NCs of size ∼29 nm were obtained. The as-synthesized NCs have a high photoluminescence quantum yield (PLQY) of around 80%, and especially, exhibited strong stability under ambient conditions and continuous UV irradiation. The PLQY can maintain 83% of the initial one even after 120 days. Furthermore, after 96 h of continuous irradiation under UV light at 365 nm (8 W cm-2) under open ambient conditions, the photoluminescence (PL) intensity showed retention of 68% of its original value with no significant changes in the full width at half-maximum, whereas the amine-based sample retains only 5% of its original PL intensity. Furthermore, we have utilized these NCs to fabricate stable down-converted LED devices. The present work demonstrated the synthesis of ultra-stable CsPbBr3 NCs that can be an ideal candidate for display applications.

Shell thickness dependent photostability studies of green-emitting "Giant" quantum dots.

Highly efficient green-emitting core/shell giant quantum dots have been synthesized through a facile "one-pot" gradient alloy approach. Furthermore, an additional ZnS shell was grown using the "Successive Ionic Layer Adsorption and Reaction" (SILAR) method. Due to the faster reactivity of Cd and Se compared to an analogue of Zn and S precursors it is presumed that CdSe nuclei are initially formed as the core and gradient alloy shells simultaneously encapsulate the core in an energy-gradient manner and eventually thick ZnS shells were formed. Using this gradient alloy approach, we have synthesized four different sized green-emitting giant core-shell quantum dots to study their shell thickness-dependent photostability under continuous UV irradiation, and temperature-dependent PL properties of nanocrystals. There was a minimum effect of the UV light exposure on the photostability beyond a certain thickness of the shell. The QDs with a diameter of ≥8.5 nm show substantial improvement in photostability compared to QDs with a diameter ≤ 7.12 nm when continuously irradiated under strong UV light (8 W cm-2, 365 nm) for 48 h. The effect of temperature on the photoluminescence intensities was studied with respect to the shell thickness. There were no apparent changes in PL intensities observed for the QDs ≥ 8.5 nm, on the contrary, for example, QDs with <8.5 nm in diameter (for ∼7.12 nm) show a decrease in PL intensity at higher temperatures ∼ 90 °C. The synthesized green-emitting gradient alloy QDs with superior optical properties can be used for highly efficient green-emitters and are potentially applicable for the fabrication of green LEDs.