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Dr Nimai Mishra, Assistant Professor, Department of Chemistry, SRM University-AP, Andhra Pradesh, along with his research group comprising of students pursuing PhD under his supervision, Ms VG Vasavi Dutt, Mr Syed Akhil, Mr Rahul Singh, and Mr Manoj Paalabathuni have published a research article titled “High-Quality CsPbX3 (X = Cl, Br, or I) Perovskite Nanocrystals Using Ascorbic Acid Post-Treatment: Implications for Light-Emitting Applications” in the Journal “ACS Applied Nano Materials” (published by The American Chemical Society) having an impact factor of ~5.1.

Abstract:

Cesium lead halide perovskite nanocrystals (CsPbX3 PNCs) 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. One of the ways to minimize the surface defects in the surface treatment with suitable ligands is to achieve the PNCs with superior PL properties for light-emitting applications.

In this article, Dr Mishra’s research group addressed the issue of stability in PNCs. We demonstrate to achieve high photoluminescence and stability of CsPbX3 PNCs by incorporating ascorbic acid via post-treatment as a new capping ligand that is abundantly available. Upon addition of ascorbic acid as surface passivation ligand into the oleic acid/oleylamine system to get near-unity photoluminescence quantum yield (PLQY) of CsPbBr3, CsPb(Br/I)3, and for CsPbI3 perovskite NCs. Maintaining stability has become the hotspot of research in this field. Hence, as-a-proof of concept, the stability studies of PNCs in ambient conditions, under continuous UV irradiation, and PL with temperature variations are put forth here. The stability enhancement with post-treatment of ascorbic acid is highly reproducible as we tested for four batches of samples.

Despite the significant advancements of PNCs, there is a challenge afflicting the stability of CsPbI3 PNCs. They are thermodynamically unstable and undergo a non-perovskite phase (δ-phase) transition at room temperature. Many efforts have been reported in the stabilization of iodide perovskite NCs by critically passivating PNCs and applying them for optoelectronics and photovoltaics. On the other hand, mixed halide perovskites like CsPbBrI2 which are relatively stable than CsPbI3 PNCs are a better choice for device applications. But, photo-induced halide segregation is unavoidable which in turn again limit their usage in practical applications. In this manuscript, we demonstrated that the ultra-stable iodide-based PNCs can be achieved by simple and facile surface treatment with ascorbic acid.

The PL intensity of untreated and ascorbic acid-treated PNCs is recorded for 42 days since the date of synthesis. The measurements are carried out for 4 different batches of samples to ensure reproducibility. It is found that the PL intensity is deteriorating rapidly for untreated PNCs while the PL intensity is largely maintained for ascorbic acid treated PNCs. Nearly ̴72% of the initial PL intensity is maintained even after 42 days for the ascorbic acid-treated CsPbBr3 PNCs while the PL intensity is dropped to 24% for untreated PNCs. Ascorbic acid treated CsPbBrI2 PNCs exhibited exceptional ambient stability where ̴69% of the initial PL intensity is maintained after 42 days while the PL of untreated CsPbBrI2 PNCs is degraded rapidly within 2 weeks from the date of synthesis. Moreover, the PL stability of CsPbI3 PNCs is high for ascorbic acid-treated samples even after 55 days while the PL has deteriorated within 4 days for untreated CsPbI3 PNCs. The PL of untreated CsPbI3 PNCs is completely lost in the first 4 hours of UV illumination while ̴ 76.7% remnant PL is observed for ascorbic acid-treated CsPbI3 PNCs. We believe the stabilization of CsPbX3 PNCs of different halide compositions via simple surface treatment with ascorbic acid could form a basis for futuristic light-emitting applications.

Read the full paper: https://pubs.acs.org/doi/full/10.1021/acsanm.1c04312

Dr Nimai Mishra, Assistant Professor, Department of Chemistry, SRM University-AP, Andhra Pradesh, along with his research group comprising of students pursuing PhD under him, Mr Rahul Singh, Mr Syed Akhil, and Ms V. G. Vasavi Dutt, have published a research article titled “Study of Shell Thickness Dependent Charge Transfer Dynamics in Green Emitting Core/Shell Giant Quantum Dots” Nature Index journal, “Inorganic Chemistry” published by The American Chemical Society having an impact factor of 5.1.

About the paper:

The superior photostability enables green-emitting graded alloy core/shell giant quantum dots (g-QDs) for optoelectronic application. 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. The four graded alloy core/shell green-emitting g-QDs with different shell thicknesses were synthesised. The PET and PHT rate constants were obtained from photoluminescence and PL-lifetime decay measurement. Our study concludes that g-QDs with a diameter ~7.14 show a substantial improvement in charge transfer than 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 constant (ket/ht) of g-QD with 7.14 nm in diameter are 10.80 × 107 s-1 and 14 × 107 s-1, which shows 8.5 and 8 times higher compared to g-QDs with a 10.72 nm in diameter.

Industrial implications:

More importantly, 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 photocatalysts, photodetectors, and solar cells.

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SRM University-AP is pleased to announce that Dr Nimai Mishra, Assistant Professor, Department of Chemistry, SRM University-AP, Andhra Pradesh, along with his research group comprising of students pursuing PhD under him, Mr Rahul Singh, Mr Syed Akhil, and Ms V.G.Vasavi Dutt, has published a research article titled “Shell thickness-dependent photostability studies of green-emitting “Giant” quantum dots” in the journal Nanoscale Advances (The Royal Society of Chemistry) with an impact factor of ~4.533.

About the research:

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 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 after a certain thickness of the shell. The QDs diameter of ≥ 8.5 nm shows substantial improvement in photostability compared to QDs with a diameter ≤ 7.12 nm when continuously irradiated under the strong UV light (8 W/cm2, 365 nm) for 48 h. The effect of temperature on the photoluminescence intensities was studied with respect to 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.

More importantly, these results highlight 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.

Read the full paper: https://pubs.rsc.org/en/content/articlelanding/2021/NA/D1NA00663K

SRM University-AP could not be more proud to announce that Dr S Mannathan, Head of Department of Chemistry has made it to the top 5% in the list of the Most Cited Authors by the Royal Society of Chemistry. It is inspiring to have a faculty member in our midst whose work has helped and facilitated the research of so many others.

Dr Mannathan obtained his doctorate from National Tsing Hua University, Taiwan. His research interests primarily lie in Metal-catalyzed organic transformation reactions, Multicomponent reactions, and Asymmetric synthesis. His research followed by scientists all over the world leading him to become one of the top 5% authors in terms of citations

In the field of Transition Metal Complexes as Catalysts in Organic Reactions, he particularly leans towards ‘Nickel-and cobalt-catalyzed three-component coupling and reductive coupling reactions’, and ‘Palladium-catalyzed reductive arylation’. Similarly, in Asymmetric Synthesis, he favours research into ‘Asymmetric reductive Heck reaction for the synthesis of chiral indanones’, and ‘Synthesis of bicyclic tertiary alcohols and its related asymmetric version via reductive [3+2] cycloaddition reaction by using chiral cobalt complexes.’

About the top 5% most cited paper:

In this work, he reported the synthesis and application of a Zn-Bp-BTC MOF (Bp – 4,4′-bipyridine; BTC – 1,3,5-benzene tricarboxylic acid; MOF – metal organic framework) as a heterogeneous catalyst for mediating organic reactions. Initial reaction conditions were optimized for the Knoevenagel condensation reaction using Zn-Bp-BTC as a heterogeneous catalyst. Various factors such as the effect of solvent, temperature and catalyst loading were evaluated. Although the reaction proceeded at room temperature using methanol as the solvent, 60 °C offered the best yield in a shorter duration. Under optimized reaction conditions, a wide range of α,β-unsaturated dicyano compounds were prepared from the corresponding carbonyl precursor and malononitrile, the active methylene counterpart. A systematic investigation was also carried out to assess the role of the ligand and metal salt in the Knoevenagel condensation reaction. It was found that the Zn-Bp-BTC MOF catalyzed the reaction efficiently in comparison to its analogue Zn-BTC MOF and precursor Zn(NO 3 ) 2 ·6H 2 O. Finally, catalytic recycling and stability studies showed that the catalyst is able to mediate the reaction for up to five consecutive cycles without undergoing any significant chemical or morphological changes. Further, the catalyst was tested for its efficacy in a multicomponent reaction (MCR). An MCR with the Zn-Bp-BTC MOF as the catalyst afforded good yields and there was no reaction in the absence of the catalyst. Similarly, the catalyst was tested for its efficiency in benzimidazole synthesis.

Dr Mannathan did this research in collaboration with Dr. Kathiresan Murugavel, Scientist, Electro Organic Division, CSIR-Central Electrochemical Research Institute (Govt of India), Karaikudi.