Amitava Patra

Effect of crystal nature on upconversion luminescence in Er3+:ZrO2 nanocrystals

The effects of crystal size and crystal phase on the upconverted emission of Er3+ in ZrO2 oxide nanocrystals are reported. Green (550 nm) and red (670 nm) upconversion emission were observed at room temperature from the 4S3/2 and 4F9/2 levels of Er3+:ZrO2 nanocrystals. It is found that at 850 mW of cw excitation power, the total luminescence was 11960 Cd/m2 for 1000 °C heated sample. We observed that the overall upconversion luminescence intensity depends on crystal structure and particle size. We have also confirmed that upconversion process in all these samples results from a two-photon excited-state absorption process.

Er3+-doped BaTiO3 nanocrystals for thermometry: Influence of nanoenvironment on the sensitivity of a fluorescence based temperature sensor

Frequency upconverted emissions centered at 526 and 547 nm from two thermodynamically coupled excited states of Er3+ doped in BaTiO3 nanocrystals were recorded in the temperature range from 322 to 466 K using a diode laser emitting at 980 nm as the excitation source. The ensemble measurements of the fluorescence intensity ratio (FIR) of the signals at 526 and 547 nm as a function of the temperature showed that the sensitivity (the rate in which the FIR changes with the temperature) of such sensor depends on the size of the nanocrystal. This is explained taking into consideration modifications of nonraditive relaxation mechanisms with the size of the nanocrystals.Frequency upconverted emissions centered at 526 and 547 nm from two thermodynamically coupled excited states of Er3+ doped in BaTiO3 nanocrystals were recorded in the temperature range from 322 to 466 K using a diode laser emitting at 980 nm as the excitation source. The ensemble measurements of the fluorescence intensity ratio (FIR) of the signals at 526 and 547 nm as a function of the temperature showed that the sensitivity (the rate in which the FIR changes with the temperature) of such sensor depends on the size of the nanocrystal. This is explained taking into consideration modifications of nonraditive relaxation mechanisms with the size of the nanocrystals.

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.

Surface energy transfer from rhodamine 6G to gold nanoparticles: A spectroscopic ruler

Here, the authors report the energy transfer from rhodamine 6G dyes to gold nanoparticles. There is a pronounced effect on the photoluminescence and a shortening of the lifetime of the dye when interacting with the Au nanoparticles. The calculated distance (d) between the donor and acceptor varies from 86.06to102.47A with changing the concentrations of Au and dye. Analysis suggests that the energy transfer from dye to the Au nanoparticles is a surface energy transfer process and follows a 1∕d4 distance dependence.

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.

Energy transfer study between Ce3+ and Tb3+ ions in doped and core-shell sodium yttrium fluoride nanocrystals

Here, we report the preparation of Ce(3+) and Tb(3+) co-doped sodium yttrium fluoride nanorods and NaYF(4):Ce(3+)/Tb(3+) core-shell nanoparticles by the emulsion method. The core-shell nanoparticles are confirmed by X-ray diffraction study and transmission electron microscopy (TEM) analysis. The hexagonal crystal phase of Ce(3+)-doped sodium yttrium fluoride nanocrystals is converted to the cubic polymorph after surface coating by TbF(3). Cell volume, cell parameters and lattice strain have been modified due to core-shell structure. The decay times are found to be 8.4 ms and 5.4 ms for doped nanorods and core-shell nanoparticles, respectively, which reveals that non-radiative decay is higher in the case of core-shell nanoparticles than doped nanorods. Energy transfer efficiencies from Ce(3+) to Tb(3+)are 65% and 45% for doped Na(Y(1.5)Na (0.5))F(6):Ce:Tb material and NaYF(4):Ce/Tb core-shell materials, respectively. Quantum yields are found to be 75% and 42% for doped and core-shell samples, respectively.

Hybrid Colloidal Au-CdSe Pentapod Heterostructures Synthesis and their Photocatalytic Properties

In this report, we present a self-driven chemical process to design exclusive Au/CdSe pentapod heterostructures with Au core and CdSe arms. We have analyzed these heterostructures using high-resolution transmission electron microscope (HRTEM), high angle annular dark field-scanning transmission electron microscopic (HAADF-STEM), X-ray diffraction, and X-ray photoelectron spectroscopy (XPS) studies. Microscopic studies suggest that pentapod arms of CdSe are nucleated on the (111) facets of Au and linearly grown only along the [001] direction. From the XPS study, the shifting of peak positions in the higher binding energy region for Au/CdSe heterostructures compared to Au nanoparticles has been found which indicates the charge transfer from CdSe to Au in heterostructures. The steady state and time resolved spectroscopic studies unambiguously confirm the electron transfer from photoexcited CdSe to Au, and the rate of electron transfer is found to be 3.58×10⁸ s⁻¹. It is interesting to note that 87.2% of R6G dye is degraded by the Au/CdSe heterostructures after 150 min UV irradiation, and the apparent rate constant for Au/CdSe heterostructures is found to be 0.013 min⁻¹. This new class of metal-semiconductor heterostructures opens up new possibilities in photocatalytic, solar energy conversion, photovoltaic, and other new emerging applications.

Detection of Hg2+ and F- ions by using fluorescence switching of quantum dots in an Au-cluster-CdTe QD nanocomposite.

A single probe of an Au nanocluster-CdTe quantum dots nanocomposite has been developed by using tripeptide-capped CdTe quantum dots (QD) and bovine serum albumin (BSA) protein-conjugated Au25 nanocluster (NC) for detection of both Hg(2+) ion and F(-) ion. The formation of Au-NC-CdTe QD nanocomposite has been confirmed by TEM, steady state and time resolved spectroscopy, CD and FTIR studies. A significant signal off (74 % PL quenching at 553 nm) phenomenon of this nanocomposite is observed in presence of 6.56×10(-7)  M Hg(2+) ion, due to salt-induced aggregation. However, a dramatic PL enhancement (128 %) of the Au-NC-CdTe QD nanocomposite is observed in presence of 8.47×10(-7)  M F(-) anion. The calculated limit of detections (LOD) of Hg(2+) ion concentration and F(-) ion concentration are found to be 9 and 117 nM, respectively, which are within the safety range set by the United States Environment Protection Agency. Thus, the simple Au-NC-CdTe QD optical-based sensor is very useful to detect both toxic cations and anions.

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.

Fabrication and optical properties of core/shell CdS/LaPO4:Eu nanorods

We report the preparation of CdS/LaPO4:Eu nanorods via a three-step chemical route involving solvothermal preparation of CdS nanorods. Then, the surface functionalisation of CdS nanorods by citric acid is done. Finally, the formation of the LaPO4 shell is achieved. The core/shell nanostructure is confirmed by X-ray diffraction studies and transmission electron microscopy (TEM) analysis. The crystal phase of LaPO4 (shell) changes with changing the temperature of heating. It is interesting to note that the lattice strain of CdS (core) increases from 0.27% to 1.09% for pure CdS nanorods and core CdS nanorods, respectively. The lattice strain can be modified from tensile to compressive by changing the crystal phase of the LaPO4 (shell). It is found that the photoluminescence properties are sensitive to the crystal phase of the LaPO4 shell, which can be tuned by temperature of heating. From the decay time measurements, it is evident that the energy transfer occurs from core CdS nanorods to Eu3+ ions in the LaPO4 shell and the calculated energy transfer efficiency from CdS nanorods to Eu3+ ions is 50.9% for the 600 °C heated core/shell nanostructure. The experimental quantum efficiency is 47% for CdS/LaPO4:Eu samples and this efficiency increases due to energy transfer from CdS to Eu3+ ions in the LaPO4 shell.