Synthesis and characterization of highly efficient CdSe/CdS core/shell nanocrystals with silar technique

Ankara : The Materials Science and Nanotechnology Program of the Graduate School of Engineering and Science of Bilkent University, 2012.Thesis (Master's) -- Bilkent University, 2012.Includes bibliographical references leaves 67-75.Owing to their size tunable electronic structure and optical properties, semiconductor nanocrystal quantum dots (NQDs) have become attractive for a wide range of device applications ranging from life sciences to electronics in the last two decades. However, highly efficient and stable NQDs are essential to reaching high performance with these devices utilizing NQDs. In this thesis, to meet these requirements, a new class of CdSe/CdS core/shell NQDs are studied including their colloidal synthesis and nanocharacterization. In this work, CdSe/CdS core/shell NQDs were synthesized with successive ion layer adsorption and reaction (SILAR) technique, which enabled highly precise shell thickness control and uniform coating of the shell material. When compared to the most commonly used CdSe/ZnS core/shell NQDs, CdSe/CdS core/shell NQDs were found to provide important advantages. First, the lattice mismatch within CdSe and CdS (3.9%) is lower than that within CdSe and ZnS (12%), which was very critical for obtaining highly efficient NQDs. Second, as a result of having lower bandgap in CdS, great enhancement in absorption cross section was achieved with more red-shifted emission, which is not possible with CdSe/ZnS core/shell NQDs. Moreover, suppression of Auger recombination was successfully observed with the partial separation of electron and hole wavefunctions in the synthesized CdSe/CdS core/shell NQDs. With all these attractive properties that were experimentally measured, CdSe/CdS core/shell NQDs were found to make better alternatives to CdSe/ZnS core/shell for numerous applications.Keleştemur, YusufM.S

Exciton Dynamics of Colloidal Semiconductor Quantum Well Stacks

Colloidal semiconductor nanoplatelets (NPLs) have recently emerged as a new class of colloidal nanocrystals. NPLs are quasi two-dimensional nanocrystals having atomically flat surfaces and have unique properties such as narrow photoluminescence (PL) emission (similar to 10 nm) and giant oscillator strength. NPLs can be self-assembled into stacks. These are one-dimensional superstructures that can contain tens or hundreds of NPLs in one chain

SILICA NANOPARTICLE FORMATION BY USING DROPLET-BASED MICROREACTOR

This paper describes a method for the synthesis of silica nanoparticles that can be later used for coating of quantum dots inside a microfluidic reactor. Here, a droplet-based system is used where two reagents were mixed inside the droplets to obtain silica. Particles in the size range of 25 +/- 2.7 nm were obtained with comparable size distribution to controlled batch wise synthesis methods. This method is suitable to be used later to coat CdSe nanoparticles inside the microreactor

Energy-saving quality road lighting with colloidal quantum dot nanophosphors

Here the first photometric study of road-lighting white light-emitting diodes (WLEDs) integrated with semiconductor colloidal quantum dots (QDs) is reported enabling higher luminance than conventional light sources, specifically in mesopic vision regimes essential to street lighting. Investigating over 100 million designs uncovers that quality road-lighting QD-WLEDs, with a color quality scale and color rendering index >= 85, enables 13-35% higher mesopic luminance than the sources commonly used in street lighting. Furthermore, these QD-WLEDs were shown to be electrically more efficient than conventional sources with power conversion efficiencies >= 16-29%. Considering this fact, an experimental proof-of-concept QD-WLED was demonstrated, which is the first account of QD based color conversion custom designed for street lighting applications. The obtained white LED achieved the targeted mesopic luminance levels in accordance with the road lighting standards of the USA and the UK. These results indicate that road-lighting QD-WLEDs are strongly promising for energy-saving quality road lighting

Excitonic improvement of colloidal nanocrystals in salt powder matrix for quality lighting and color enrichment

Here we report excitonic improvement in color-converting colloidal nanocrystal powders enabled by co-integrating nonpolar green-and red-emitting nanocrystal energy transfer pairs into a single LiCl salt matrix. This leads to nonradiative energy transfer (NRET) between the cointegrated nanocrystals in the host matrix. Here we systematically studied the resulting NRET process by varying donor and acceptor concentrations in the powders. We observed that NRET is a strong function of both of the nanocrystal concentrations and that NRET efficiency increases with increasing acceptor concentration. Nevertheless, with increasing donor concentration in the powders, NRET efficiency was found to first increase ( up to a maximum level of 53.9%) but then to decrease. As a device demonstrator, we employed these NRET-improved nanocrystal powders as color-converters on blue light-emitting diodes ( LEDs), with the resulting hybrid LED exhibiting a luminous efficiency > 70 lm/Welect. The proposed excitonic nanocrystal powders potentially hold great promise for quality lighting and color enrichment applications. (C) 2015 Optical Society of Americ

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 similar to 60 mu J/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

Quantum Dot/Light-Emitting Electrochemical Cell Hybrid Device and Mechanism of Its Operation

A new type of light-emitting hybrid device based on colloidal quantum dots (QDs) and an ionic transition metal complex (iTMC) light-emitting electrochemical cell (LEC) is introduced. The developed hybrid devices show light emission from both active layers, which are combined in a stacked geometry. Time-resolved photoluminescence experiments indicate that the emission is controlled by direct charge injection into both the iTMC and the QD layer. The turn-on time (time to reach 1 cd/m(2)) at constant voltage operation is significantly reduced from 8 min in the case of the reference LEC down to subsecond in the case of the hybrid device. Furthermore, luminance and efficiency of the hybrid device are enhanced compared to reference LEC directly after device turn-on by a factor of 400 and 650, respectively. We attribute these improvements to an increased electron injection efficiency into the iTMC directly after device turn-on

Nonradiative energy transfer in colloidal CdSe nanoplatelet films

Nonradiative energy transfer (NRET) has been extensively studied in colloidal nanocrystal (quantum dots) and nanorod (quantum wires) assemblies. In this work, we present the first account of spectroscopic evidence of NRET in solid thin films of CdSe based colloidal nanoplatelets (NPLs), also known as colloidal quantum wells. The NRET was investigated as a function of the concentration of two NPL populations with different vertical thicknesses via steady state and time resolved spectroscopy. NRET takes place from the NPLs with smaller vertical thickness (i.e., larger band gap) to the ones with a larger vertical thickness (i.e., smaller band gap) with efficiency up to similar to 60%. Here, we reveal that the NRET efficiency is limited in these NPL solid film assemblies due to the self-stacking of NPLs within their own population causing an increased distance between the donor-acceptor pairs, which is significantly different to previously studied colloidal quantum dot based architectures for nonradiative energy transfer

Alloyed Heterostructures of CdSexS1-x Nanoplatelets with Highly Tunable Optical Gain Performance

Here, we designed and synthesized alloyed heterostructures of CdSexS1-x nanoplatelets (NPLs) using CdS coating in the lateral and vertical directions for the achievement of highly tunable optical gain performance. By using homogeneously alloyed CdSexS1-x core NPLs as a seed, we prepared CdSexS1-x/CdS core/crown NPLs, where CdS crown region is extended only in the lateral direction. With the sidewall passivation around inner CdSexS1-x cores) we achieved enhanced photoluminescence quantum yield (PL-QY) (reaching 60%), together-with increased absorption cross-section and improved stability without changing the emission Spectrum of CdSexS1-x, alloyed core NPLs. In addition, we further extended the spectral tunability of these solution-processed NPLs with the synthesis of CdSexS1-x/CdS core/shell NPLs. Depending on the sulfur composition of the CdSexS1-x, core and thickness of the CdS shell, CdSexS1-x/CdS core/shell NPLs possessed highly tunable emission characteristics within the spectral range of 560-650 nm. Finally, we studied the optical gain performances of different heterostructures of CdSexS1-x, alloyed NPLs offering great advantages, including reduced reabsorption and spectrally tunable optical gain range. Despite their decreased PL-QY and reduced absorption cross-section upon increasing the sulfur composition, CdSexS1-x based NPLs exhibit highly tunable amplified spontaneous emission performance together with low gain thresholds down to similar to 53 mu J/cm(2)

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 Mn2+ 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 Mn2+ doping is confined to the core of these NCs are also presented. It is observed that with increasing Mn2+ concentration the PL intensity first increases, reaching a maximum at Mn2+ 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 (approximate to 2.95 mM(-1) s(-1)) to obtain MR contrast at 25 degrees C, 3 T. Therefore, the Mn2+ doping in these water-soluble Cd-free NCs are sufficient to produce contrast for both fluorescence and magnetic resonance imaging techniques