Vijay Kumar Sharma

Large-area (over 50 cm × 50 cm) freestanding films of colloidal InP/ZnS quantum dots.

We propose and demonstrate the fabrication of flexible, freestanding films of InP/ZnS quantum dots (QDs) using fatty acid ligands across very large areas (greater than 50 cm × 50 cm), which have been developed for remote phosphor applications in solid-state lighting. Embedded in a poly(methyl methacrylate) matrix, although the formation of stand-alone films using other QDs commonly capped with trioctylphosphine oxide (TOPO) and oleic acid is not efficient, employing myristic acid as ligand in the synthesis of these QDs, which imparts a strongly hydrophobic character to the thin film, enables film formation and ease of removal even on surprisingly large areas, thereby avoiding the need for ligand exchange. When pumped by a blue LED, these Cd-free QD films allow for high color rendering, warm white light generation with a color rendering index of 89.30 and a correlated color temperature of 2298 K. In the composite film, the temperature-dependent emission kinetics and energy transfer dynamics among different-sized InP/ZnS QDs are investigated and a model is proposed. High levels of energy transfer efficiency (up to 80%) and strong donor lifetime modification (from 18 to 4 ns) are achieved. The suppression of the nonradiative channels is observed when the hybrid film is cooled to cryogenic temperatures. The lifetime changes of the donor and acceptor InP/ZnS QDs in the film as a result of the energy transfer are explained well by our theoretical model based on the exciton-exciton interactions among the dots and are in excellent agreement with the experimental results. The understanding of these excitonic interactions is essential to facilitate improvements in the fabrication of photometrically high quality nanophosphors. The ability to make such large-area, flexible, freestanding Cd-free QD films pave the way for environmentally friendly phosphor applications including flexible, surface-emitting light engines.

Tunable White-Light-Emitting Mn-Doped ZnSe Nanocrystals

We report white-light-emitting Mn-doped ZnSe nanocrystals (NCs) that are synthesized using modified nucleation doping strategy. Tailoring three distinct emission mechanisms in these NCs, which are MnSe-related blue emission (410 and 435 nm), Zn-related defect state green emission (520 nm), and Mn-dopant related orange emission (580 nm), allowed us to achieve excitation wavelength tailorable white-light generation as studied by steady state and time-resolved fluorescence spectroscopy. These NCs will be promising as single component white-light engines for solid-state lighting.

High brightness formamidinium lead bromide perovskite nanocrystal light emitting devices

Formamidinium lead halide (FAPbX3) has attracted greater attention and is more prominent recently in photovoltaic devices due to its broad absorption and higher thermal stability in comparison to more popular methylammonium lead halide MAPbX3. Herein, a simple and highly reproducible room temperature synthesis of device grade high quality formamidinium lead bromide CH(NH2)2PbBr3 (FAPbBr3) colloidal nanocrystals (NC) having high photoluminescence quantum efficiency (PLQE) of 55–65% is reported. In addition, we demonstrate high brightness perovskite light emitting device (Pe-LED) with these FAPbBr3 perovskite NC thin film using 2,2′,2″-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) commonly known as TPBi and 4,6-Bis(3,5-di(pyridin-3-yl)phenyl)-2-methylpyrimidine (B3PYMPM) as electron transport layers (ETL). The Pe-LED device with B3PYMPM as ETL has bright electroluminescence of up to 2714u2009cd/m2, while the Pe-LED device with TPBi as ETL has higher peak luminous efficiency of 6.4u2009cd/A and peak luminous power efficiency of 5.7u2009lm/W. To our knowledge this is the first report on high brightness light emitting device based on CH(NH2)2PbBr3 widely known as FAPbBr3 nanocrystals in literature.

Ultralow Threshold One-Photon- and Two-Photon-Pumped Optical Gain Media of Blue-Emitting Colloidal Quantum Dot Films

Colloidal quantum dots (QDs) offer advantageous properties as an optical gain media for lasers. Optical gain in the QDs has been shown in the whole visible spectrum, yet it has been intrinsically challenging to realize efficient amplified spontaneous emission (ASE) and lasing in the blue region of the visible spectrum. Here, we synthesize large-sized core/gradient shell CdZnS/ZnS QDs as an efficient optical gain media in the blue spectral range. In this Letter, we demonstrate for the first time that two-photon-absorption-pumped ASE from the blue-emitting QD is achievable with a threshold as low as 6 mJ/cm(2). Utilizing these QDs, we also report one-photon-absorption-pumped ASE at an ultralow threshold of ∼60 μJ/cm(2), which is comparable to the state-of-the-art red-emitting QD-based gain media. This one-photon-pumped ASE threshold is an order of magnitude better than that of the previously reported best blue-emitting QD-based gain media.

Manganese doped fluorescent paramagnetic nanocrystals for dual-modal imaging

In this work, dual-modal (fluorescence and magnetic resonance) imaging capabilities of water-soluble, low-toxicity, monodisperse Mn-doped ZnSe nanocrystals (NCs) with a size (6.5 nm) below the optimum kidney cutoff limit (10 nm) are reported. Synthesizing Mn-doped ZnSe NCs with varying Mn(2+) concentrations, a systematic investigation of the optical properties of these NCs by using photoluminescence (PL) and time resolved fluorescence are demonstrated. The elemental properties of these NCs using X-ray photoelectron spectroscopy and inductive coupled plasma-mass spectroscopy confirming Mn(2+) doping is confined to the core of these NCs are also presented. It is observed that with increasing Mn(2+) concentration the PL intensity first increases, reaching a maximum at Mn(2+) concentration of 3.2 at% (achieving a PL quantum yield (QY) of 37%), after which it starts to decrease. Here, this high-efficiency sample is demonstrated for applications in dual-modal imaging. These NCs are further made water-soluble by ligand exchange using 3-mercaptopropionic acid, preserving their PL QY as high as 18%. At the same time, these NCs exhibit high relaxivity (≈2.95 mM(-1) s(-1)) to obtain MR contrast at 25 °C, 3 T. Therefore, the Mn(2+) doping in these water-soluble Cd-free NCs are sufficient to produce contrast for both fluorescence and magnetic resonance imaging techniques.

High-efficiency CdTe/CdS core/shell nanocrystals in water enabled by photo-induced colloidal hetero-epitaxy of CdS shelling at room temperature

We report high-efficiency CdTe/CdS core/shell nanocrystals synthesized in water by epitaxially growing CdS shells on aqueous CdTe cores at room temperature, enabled by the controlled release of S species under low-intensity ultraviolet (UV) light illumination. The resulting photo-induced dissociation of S2O32− ions conveniently triggers the formation of critical two-dimensional CdS epitaxy on the CdTe surface at room temperature, as opposed to initiating the growth of individual CdS core-only nanocrystals. This controlled colloidal hetero-epitaxy leads to a substantial increase in the photoluminescence (PL) quantum yield (QY) of the shelled nanocrystals in water (reaching 64%). With a systematic set of studies, the maximum PL QY is found to be almost independent of the illuminating UV intensity, while the shell formation kinetics required for reaching the maximum QY linearly depends on the illuminating UV intensity. A stability study of the QD films in air at various temperatures shows highly improved thermal stability of the shelled QDs (up to 120 °C in ambient air). These results indicate that the proposed aqueous CdTe/CdS core/shell nanocrystals hold great promise for applications requiring efficiency and stability.

Sweet Plasmonics: Sucrose Macrocrystals of Metal Nanoparticles

The realization of plasmonic structures generally necessitates expensive fabrication techniques, such as electron beam and focused ion beam lithography, allowing for the top-down fabrication of low-dimensional structures. Another approach to make plasmonic structures in a bottom-up fashion is colloidal synthesis, which is convenient for liquid-state applications or very thin solid films where aggregation problems are an important challenge. The architectures prepared using these methods are typically not robust enough for easy handling and convenient integration. Therefore, developing a new plasmonic robust platform having large-scale dimensions without adversely affecting the plasmonic features is in high demand. As a solution, here we present a new plasmonic composite structure consisting of gold nanoparticles (Au NPs) incorporated into sucrose macrocrystals on a large scale, while preserving the plasmonic nature of the Au NPs and providing robustness in handling at the same time. As a proof of concept demonstration, we present the fluorescence enhancement of green CdTe quantum dots (QDs) via plasmonic coupling with these Au NPs in the sucrose crystals. The obtained composite material exhibits centimeter scale dimensions and the resulting quantum efficiency (QE) is enhanced via the interplay between the Au NPs and CdTe QDs by 58% (from 24% to 38%). Moreover, a shortening in the photoluminescence lifetime from 11.0 to 7.40 ns, which corresponds to a field enhancement factor of 2.4, is observed upon the introduction of Au NPs into the QD incorporated macrocrystals. These results suggest that such “sweet” plasmonic crystals are promising for large-scale robust platforms to embed plasmonic nanoparticles.

Temperature-dependent Optoelectronic Properties of Quasi-2D Colloidal Cadmium Selenide Nanoplatelets

Colloidal cadmium selenide (CdSe) nanoplatelets (NPLs) are a recently developed class of efficient luminescent nanomaterials suitable for optoelectronic device applications. A change in temperature greatly affects their electronic bandstructure and luminescence properties. It is important to understand how and why the characteristics of NPLs are influenced, particularly at elevated temperatures, where both reversible and irreversible quenching processes come into the picture. Here we present a study of the effect of elevated temperatures on the characteristics of colloidal CdSe NPLs. We used an effective-mass envelope function theory based 8-band k·p model and density-matrix theory considering exciton-phonon interaction. We observed the photoluminescence (PL) spectra at various temperatures for their photon emission energy, PL linewidth and intensity by considering the exciton-phonon interaction with both acoustic and optical phonons using Bose-Einstein statistical factors. With a rise in temperature we observed a fall in the transition energy (emission redshift), matrix element, Fermi factor and quasi Fermi separation, with a reduction in intraband state gaps and increased interband coupling. Also, there was a fall in the PL intensity, along with spectral broadening due to an intraband scattering effect. The predicted transition energy values and simulated PL spectra at varying temperatures exhibit appreciable consistency with the experimental results. Our findings have important implications for the application of NPLs in optoelectronic devices, such as NPL lasers and LEDs, operating much above room temperature.

Bio-nanohybrids of quantum dots and photoproteins facilitating strong nonradiative energy transfer

Utilization of light is crucial for the life cycle of many organisms. Also, many organisms can create light by utilizing chemical energy emerged from biochemical reactions. Being the most important structural units of the organisms, proteins play a vital role in the formation of light in the form of bioluminescence. Such photoproteins have been isolated and identified for a long time; the exact mechanism of their bioluminescence is well established. Here we show a biomimetic approach to build a photoprotein based excitonic nanoassembly model system using colloidal quantum dots (QDs) for a new bioluminescent couple to be utilized in biotechnological and photonic applications. We concentrated on the formation mechanism of nanohybrids using a kinetic and thermodynamic approach. Finally we propose a biosensing scheme with an ON/OFF switch using the QD-GFP hybrid. The QD-GFP hybrid system promises strong exciton-exciton coupling between the protein and the quantum dot at a high efficiency level, possessing enhanced capabilities of light harvesting, which may bring new technological opportunities to mimic biophotonic events.

Engineered Peptides for Nanohybrid Assemblies

Inspired by biological material synthesis, synthetic biomineralization peptides have been screened through a laboratory evolution using biocombinatorial techniques. In this study, using the fine examples in nature, silica binding peptides and gold binding peptides were fused together to form a hybrid peptide. We designed fusion peptides with different gold binding and silica binding parts. First, we have tested the binding capability of the fusion peptides using quartz crystal microbalance on gold surface and silica surface. Second, S1G1 hybrid peptide enabled assembly of gold nanoparticles on a silica surface was achieved. Finally, nanomaterial synthesis ability of the S1G1 peptide was presented by the formation of a silica film on a gold surface. In this study, we are presenting a hybrid peptide tool for nanohybrid assembly as a promising route for nanotechnology applications.