Simanta Kundu

Nanoscale Strategies for Light Harvesting

Recent advances and the current status of challenging light-harvesting nanomaterials, such as semiconducting quantum dots (QDs), metal nanoparticles, semiconductor-metal heterostructures, π-conjugated semiconductor nanoparticles, organic-inorganic heterostructures, and porphyrin-based nanostructures, have been highlighted in this review. The significance of size-, shape-, and composition-dependent exciton decay dynamics and photoinduced energy transfer of QDs is addressed. A fundamental knowledge of these photophysical processes is crucial for the development of efficient light-harvesting systems, like photocatalytic and photovoltaic ones. Again, we have pointed out the impact of the metal-nanoparticle-based surface energy transfer process for developing light-harvesting systems. On the other hand, metal-semiconductor hybrid nanostructures are found to be very promising for photonic applications due to their exciton-plasmon interactions. Potential light-harvesting systems based on dye-doped π-conjugated semiconductor polymer nanoparticles and self-assembled structures of π-conjugated polymer are highlighted. We also discuss the significance of porphyrin-based nanostructures for potential light-harvesting systems. Finally, the future perspective of this research field is given.

2D Hybrid Nanostructure of Reduced Graphene Oxide–CdS Nanosheet for Enhanced Photocatalysis

Graphene-based hybrid nanostructures have recently emerged as a new class of functional materials for light-energy conversion and storage. Here, we have synthesized reduced graphene oxide (RGO)-semiconductor composites to improve the efficiency of photocatalysis. Zero-dimensional CdS nanoparticles (0D), one-dimensional CdS nanorods (1D), and two-dimensional CdS nanosheets (2D) are grafted on the RGO sheet (2D) by a surface modification method using 4-aminothiophenol (4-ATP). Structural analysis confirms the attachment of CdS nanocrystals with RGO, and the strong electronic interaction is found in the case of a CdS nanosheet and RGO, which has an influence on photocatalytic properties. The degradation of dye under visible light varies with changing the dimension of nanocrystals, and the catalytic activity of the CdS NS/RGO composite is ∼4 times higher than that of CdS nanoparticle/RGO and 3.4 times higher than that of CdS nanorod/RGO composite samples. The catalytic activity of the CdS nanosheet/RGO composite is also found to be ∼2.5 times than that of pure CdS nanosheet samples. The unique 2D-2D nanoarchitecture would be effective to harvest photons from solar light and transport electrons to reaction sites with respect to other 0D-2D and 1D-2D hybrid systems. This observation can be extended to other graphene-based inorganic semiconductor composites, which can provide a valuable opportunity to explore novel hybrid materials with superior visible-light-induced catalytic activity.

Core-size-dependent catalytic properties of bimetallic Au/Ag core-shell nanoparticles.

Bimetallic core-shell nanoparticles have recently emerged as a new class of functional materials because of their potential applications in catalysis, surface enhanced Raman scattering (SERS) substrate and photonics etc. Here, we have synthesized Au/Ag bimetallic core-shell nanoparticles with varying the core diameter. The red-shifting of the both plasmonic peaks of Ag and Au confirms the core-shell structure of the nanoparticles. Transmission electron microscopy (TEM) analysis, line scan EDS measurement and UV-vis study confirm the formation of core-shell nanoparticles. We have examined the catalytic activity of these core-shell nanostructures in the reaction between 4-nitrophenol (4-NP) and NaBH4 to form 4-aminophenol (4-AP) and the efficiency of the catalytic reaction is found to be increased with increasing the core size of Au/Ag core-shell nanocrystals. The catalytic efficiency varies from 41.8 to 96.5% with varying core size from 10 to 100 nm of Au/Ag core-shell nanoparticles, and the Au100/Ag bimetallic core-shell nanoparticle is found to be 12-fold more active than that of the pure Au nanoparticles with 100 nm diameter. Thus, the catalytic properties of the metal nanoparticles are significantly enhanced because of the Au/Ag core-shell structure, and the rate is dependent on the size of the core of the nanoparticles.

Photocatalytic properties of semiconductor SnO2/CdS heterostructure nanocrystals

Here we report the photocatalytic properties of semiconductor SnO2/CdS heterostructure nanocrystals. A type-II SnO2/CdS heterostructure has been synthesized from a single-source precursor and its structure was determined using XRD, HRTEM, elemental mapping and line scan EDAX analysis. A time resolved spectroscopic study revealed the effective charge separation in this heterostructure and the mechanism of charge separation was proposed using a schematic model. It is important to mention that the conductivity was found to be lower in the SnO2/CdS heterostructure compared to uncoated SnO2 nanorods due to efficient charge separation. The efficient charge separation in the SnO2/CdS heterostructure revealed its higher photocatalytic activity and the photocatalytic degradation of Congo red dye was found to be 97% and 20% for the SnO2/CdS heterostructure and SnO2 respectively under UV light irradiation.

Lanthanide-Doped Nanocrystals: Strategies for Improving the Efficiency of Upconversion Emission and Their Physical Understanding

The fundamental understanding of lanthanide-doped upconverted nanocrystals remains a frontier area of research because of potential applications in photonics and biophotonics. Recent studies have revealed that upconversion luminescence dynamics depend on host crystal structure, size of the nanocrystals, dopant concentration, and core-shell structures, which influence site symmetry and the distribution and energy migration of the dopant ions. In this review, we bring to light the influences of doping/co-doping concentration, crystal phase, crystal size of the host, and core-shell structure on the efficiency of upconversion emission. Furthermore, the lattice strain, due to a change in the crystal phase and by the core-shell structure, strongly influences the upconversion emission intensity. Analysis suggests that the local environment of the ion plays the most significant role in modification of radiative and nonradiative relaxation mechanisms of overall upconversion emission properties. Finally, an outlook on the prospects of this research field is given.

Influence of Size and Shape on the Photocatalytic Properties of SnO2 Nanocrystals

Tuning the functional properties of nanocrystals is an important issue in nanoscience. Here, we are able to tune the photocatalytic properties of SnO2 nanocrystals by controlling their size and shape. A structural analysis was carried out by using X-ray diffraction (XRD)/Rietveld and transmission electron microscopy (TEM). The results reveal that the number of oxygen-related defects varies upon changing the size and shape of the nanocrystals, which eventually influences their photocatalytic properties. Time-resolved spectroscopic studies of the carrier relaxation dynamics of the SnO2 nanocrystals further confirm that the electron-hole recombination process is controlled by oxygen/defect states, which can be tuned by changing the shape and size of the materials. The degradation of dyes (90%) in the presence of SnO2 nanoparticles under UV light is comparable to that (88%) in the presence of standard TiO2 Degussa P-25 (P25) powders. The photocatalytic activity of the nanoparticles is significantly higher than those of nanorods and nanospheres because the effective charge separation in the SnO2 nanoparticles is controlled by defect states leading to enhanced photocatalytic properties. The size- and shape-dependent photocatalytic properties of SnO2 nanocrystals make these materials interesting candidates for photocatalytic applications.

Synthesis and characterization of different shaped Sm2O3 nanocrystals

Here, we demonstrate the synthesis of Sm2O3 nanoparticles, nanorods and nanoplates by solution-based techniques. The mechanism related to morphology control is proposed and discussed. The crystal phase of the Sm2O3 nanocrystal is tuned by varying the surfactant in the reaction. It is found that compressive strain is observed for lower temperature (600 °C) heated sample but reversal of strain appears at elevated temperature (900 °C). A chromatic change in photoluminescence (PL) emission with tuning the excitation wavelength has been observed. The PL emission is due to f–f electronic transition from excited states of 4G5/2 of the Sm3+ ion. The PL emission bands at 610 and 656 nm are observed under excitation 370 nm, but bands centred at 545, 610 and 656 nm are observed when the excitation wavelength is tuned to 400 nm. It is seen that the ratio between electric dipole and partially magnetic dipole contributions is maximum for nanorods followed by nanoplates and nanoparticles. Magnetic susceptibility study of the nanocrystals shows that the Sm3+ ion follows the well-known Van Vleck behaviour.

Energy/Hole Transfer Phenomena in Hybrid α-Sexithiophene (α-STH) Nanoparticle–CdTe Quantum-Dot Nanocomposites

Considerable attention has been paid to hybrid organic-inorganic nanocomposites for designing new optical materials. Herein, we demonstrate the energy and hole transfer of hybrid hole-transporting α-sexithiophene (α-STH) nanoparticle-CdTe quantum dot (QD) nanocomposites using steady-state and time-resolved spectroscopy. Absorption and photoluminescence studies confirm the loss of planarity of the α-sexithiophene molecule due to the formation of polymer nanoparticles. Upon photoexcitation at 370 nm, a nonradiative energy transfer (73 %) occurs from the hole-transporting α-STH nanoparticles to the CdTe nanoparticles with a rate of energy transfer of 6.13×10(9) s(-1). However, photoluminescence quenching of the CdTe QDs in the presence of the hole-transporting α-STH nanoparticles is observed at 490 nm excitation, which is due to both static-quenching and hole-transfer-based dynamic-quenching phenomena. The calculated hole-transporting rate is 7.13×10(7) s(-1) in the presence of 42×10(-8)  M α-STH nanoparticles. Our findings suggest that the interest in α-sexithiophene (α-STH) nanoparticle-CdTe QD hybrid nanocomposites might grow in the coming years because of various potential applications, such as solar cells, optoelectronic devices, and so on.

A simple approach to generate efficient white light emission from a ZnO–ionic liquid complex

A simple method has been described for the generation of white light from cationic ionic liquid (IL) compounds 1-n-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4) stabilized ZnO nanocrystals (NCs). Upon mixing of the IL with the ZnO NCs, the absorption band is red shifted from 245 nm to 285 nm which is attributed to the charge-transfer band due to donor–acceptor systems. The blue emission band at 420 nm for pure IL is originated from the π–π* transition in the imidazolium moiety and the strong yellow band at 557 nm is due to the defect emission of the ZnO NCs. In the case of ZnO NCs in the IL, the combination of the blue emission from the ionic liquid and the yellow emission from ZnO gives intense white light. Control experiments have been done with varying concentration of IL and ZnO NCs to understand the best conditions for efficient white light generation. The maximum quantum efficiency of the white light emission was found to be 4.7%.

Photophysical properties of ionic liquid-assisted porphyrin nanoaggregate–nickel phthalocyanine conjugates and singlet oxygen generation

In this report, we demonstrate the formation of ionic liquid (IL)-assisted zinc octaethylporphyrin (ZnOEP) nanoaggregates which is confirmed by field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM) studies. A large red shifted emission of ZnOEP nanoaggregates in comparison to ZnOEP in DCM confirmed the H aggregation which is due to intermolecular porphyrin–porphyrin (such as π–π/hydrophobic) interactions. The steady state and time resolved spectroscopic studies unambiguously confirm the H-aggregation formation of porphyrin molecules during nanoaggregate formation. The significant quenching of the fluorescence spectrum and the shortening of decay time of porphyrin nanoaggregates imply an efficient (89%) energy transfer from porphyrin nanoaggregates to phthalocyanine. Furthermore, the emission band observed at 1270 nm unambiguously confirms the singlet oxygen (1O2) generation from ZnOEP nanoaggregates which opens up further prospects in designing new IL-assisted porphyrin nanoaggregates for their application in photodynamic therapy.