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The study of the formation of monolayers of quantum dots at different temperatures
Abstract
The process of formation of Langmuir monolayers of quantum dots at the different subphase temperatures was studied by means of compression isotherm, Brewster angle microscopy and transmission electron microscopy. The increasing of the maximum surface pressure from 32 to 44 mN/m takes place with decreasing the temperature from 34 to 11°C. This is due to a decrease in the rate of dissolution of surfactant molecules in water. The increasing of a filling degree of monolayer by the quantum dots and increasing of it uniformity in thickness takes place in this temperature range. The area of bilayer and multilayer film of quantum dots decreasing and the area of quantum dots monolayer is increasing. This change explained by the difference in the phase condition of oleic acid molecules, which stabilized quantum dots.
... В работе [11] исследованы процессы самоорганизации на поверхности водной субфазы КТ, стабилизированных полимерными молекулами, что позволило сформировать нанопровода, геометрия которых зависит от концентрации внесенного на поверхность раствора и соотношения объема КТ с определенным типом стабилизатора в рабочем растворе. Были проведены исследования влияния температуры водной субфазы на процесс формирования монослоев КТ и на их стабильность [12]. Установлено, что в случае изменения температуры водной субфазы происходит изменение типа жидкого состояния монослоя КТ, стабилизированных олеиновой кислотой, с жидкоконденсированного, при температурах, меньших 25 °С на жидкорасширенному, при больших значениях температуры. ...
Colloidal nanocrystals of semiconductor quantum dots (QDs) are gaining
prominence among the optoelectronic materials in the photonics industry.
Among their many applications, their use in artificial lighting and
displays has attracted special attention thanks to their high efficiency
and narrow emission band, enabling spectral purity and fine tunability.
By employing QDs in color-conversion LEDs, it is possible to
simultaneously accomplish successful color rendition of the illuminated
objects together with a good spectral overlap between the emission
spectrum of the device and the sensitivity of the human eye, in addition
to a warm white color, in contrast to other conventional sources such as
incandescent and fluorescent lamps, and phosphor-based LEDs, which
cannot achieve all of these properties at the same time. In this review,
we summarize the color science of QDs for lighting and displays, and
present the recent developments in QD-integrated LEDs and display
research. First, we start with a general introduction to color science,
photometry, and radiometry. After presenting an overview of QDs, we
continue with the spectral designs of QD-integrated white LEDs that have
led to efficient lighting for indoor and outdoor applications.
Subsequently, we discuss QD color-conversion LEDs and displays as
proof-of-concept applications - a new paradigm in artificial lighting
and displays. Finally, we conclude with a summary of research
opportunities and challenges along with a future outlook.
The application of colloidal nanocrystals in various devices requires their assembly into well-defined mono- or multilayers. We explore the possibilities of the Langmuir-Schaefer technique to make such layers, using CdSe quantum dots as a model system. The layer quality is assessed using atomic force microscopy, transmission electron microscopy, and UV-vis absorption spectroscopy. For hydrophobic substrates, we find that the Langmuir-Schaefer technique is an excellent tool for controlled multilayer production. With hydrophilic substrates, dewetting induces a cellular superstructure. Combination with photolithography leads to micropatterned multilayers, and combination of different nanocrystal sizes allows for the formation of 2D binary superstructures.
Quantum dots are ultimate in quantum-confined structures. They provide the basis for a new class of semiconductor devices. This chapter describes quantum-confined structures; it discusses the fabrication and modeling of quantum dots, and their potential applications in a post-VLSI generation of integrated circuits. Microfabrication techniques have permitted the formation of semiconductors structures that are small enough to be comparable with the Bloch wavelength of electrons. Boundary conditions that must be met in one or more dimensions by electrons in these structures create quantized energy levels. Structures small enough in all three dimensions are known as “quantum dots” and have discrete energy levels. Experiments on quantum dots have confirmed these discrete energy levels. Quantum dots can provide the basis for a post-very-large-scale integration (VLSI) technology. Keys to developing such a technology are heterostructure charge barriers, resonant-tunneling device physics, limited interconnect, parallel circuit architectures, and modeling of quantum devices.
Synthesis and application of CuInS2/ZnS core/shell quantum dots with varying [Cu]/[In] ratios were conducted using a stepwise solvothermal route. CuInS2 core QDs with varying [Cu]/[In] ratios exhibited deep-red emissions as a result of donor-acceptor pair recombination. The bandgap and emission energies of the CuInS2 QDs increased with the decrease in Cu content. Upon ZnS shell overcoating, the bandgap energies of the CuInS2/ZnS were tuned from 550 nm to 616 nm by controlling the [Cu]/[In] ratio. The CIS QDs model was developed to elucidate the observed structural and photoluminescence emission evolutions of the QDs with various [Cu]/[In] ratios. Temperature-dependent photoluminescence spectra of the CIS/ZnS QDs were also investigated. The temperature dependency of the photoluminescence energy and intensity for various CIS/ZnS QDs were studied from 25°C to 200°C. Efficient white light-emitting diodes with high color rendering index values (Ra = 90) were fabricated using CIS/ZnS QDs as color converters in combination with green light-emitting Ba2SiO4:Eu2+ phosphors and blue light-emitting diodes.
A multilayer of quantum dots (QDs) is preferred for QD-sensitized solar cells over a monolayer counterpart to fully utilize the sunlight incident into a relatively thin-film-based photovoltaic device. A controlled assembly of QD multilayers such as layer-by-layer (LbL) assemblies can provide a model system to study the interactions between the QD layers and can offer an optimal device configuration for efficient solar power conversion. Recently, we have proposed a LbL QD assembly using electrostatic interactions of the surface charges and have successfully prepared a controlled multilayer of QD on the surface of mesoporous metal oxide films. The as-prepared tailor-made QD multilayers not only guaranteed the sufficient absorption of incident solar light but also provided a toolbox for the study and optimization of electron/energy transfers between QD layers.
We propose to use the self-assembly ability of a block copolymer combined with compression-expansion cycles to obtain CdSe quantum dots (QDs) structures of different morphology. The methodology proposed consists in transferring onto mica mixed Langmuir monolayers of QDs and the polymer poly (styrene-co-maleic anhydride) partial 2 buthoxy ethyl ester cumene terminated, PS-MA-BEE previously sheared by 50 compression-expansion cycles. Results indicate that the shear stress takes out nanoparticles at the air-water interface from metastable states and promoted a new equilibrium state of the Langmuir monolayer, then it was transferred onto mica by the Langmuir-Blodgett (LB) methodology and the morphology of the LB films was analyzed by Atomic Force Microscopy and Transmission Electron Microscopy measurements. Our results show that when the amplitude strain increases the QDs domain size decreases and the LB film becomes more compact. The dynamic of the surface pressure relaxation after cycling involves at least three timescales which are related to the damping of surface fluctuation, raft rearrangement and component movements inside each raft. Brewster Angle Microscopy allowed visualizing in situ the raft rearrangement at the air-water interface.
We report a hybrid, quantum dot (QD) based, organic light-emitting diode architecture using a non-inverted structure with the QDs sandwiched between hole transporting layers (HTLs) outperforming the reference device structure implemented in conventional non-inverted architecture by over five folds and suppressing the blue emission that is otherwise observed in the conventional structure due to the excess electrons leaking towards the HTL. It is predicted in the new device structure that 97.44% of the exciton formation takes place in the QD layer, while 2.56% of the excitons form in the HTL. It is found that the enhancement in the external quantum efficiency is mainly due to the stronger confinement of exciton formation to the QDs.
A method employing Brewster angle microscopy was developed to measure the relative thickness of monolayers and multilayers at the air−water interface. Results proved that multilayers in ultrathin films of 8CB (4‘-n-alkyl[1,1‘-biphenyl]-4-carbonitrile; nCB, n = 8) are built up from one or two stacked bilayers on top of a monolayer at the air−water interface covered by a continuous bilayer. In addition, the fan texture of the mixed splay-bend disclination defects, which are formed in 10CB monolayers on reverse collapse of a continuous trilayer, was used to obtain estimates for the components of the anisotropic dielectric constant of the monomolecular film. This estimate was made by comparing experimental images with computationally generated model images based on an assumed director distribution around the defect. The results support the validity of the folding mechanism of multilayer generation and reverse collapse as opposed to the condensation mechanism. Finally, comparison of the experimental and theoretical values of the ratio of the multilayer thickness and the monolayer thickness indicates that at this level the water−film interface cannot be assumed to be sharp or well defined.
The hazards of oleic acid are described. Keywords (Audience): High School / Introductory Chemistry
Thin UV-blocking films of poly(methyl methacrylate) (PMMA) and ZnO quantum dots (QDs) were built-up by spin-coating. Ellipsometry reveals average thicknesses of 9.5 and 8.6 nm per bilayer before and after heating at 100 °C for one hour, respectively. The surface roughness measured by Atomic force microscopy (AFM) was 3.6 and 8.4 nm for the one and ten bilayer films, respectively. The linear increase in thickness as well as the low surface roughness increment per bilayer indicates a stratified multilayer structure and a smooth interface without aggregation. The absorption of UV radiation increased with increasing number of bilayers. At the same time, transmission was damped at wavelengths shorter than 375 nm. The thin films had a high and constant transparency in the visible region. Green-light emitting QDs could be detected by confocal microscopy at a concentration of 20% in a single layer of PMMA/ZnO. PMMA/ZnO QDs thin films are hydrophobic, as indicated by contact angle measurements.
Quantum dots show great promise for fabrication of hybrid bulk heterojunction solar cells with enhanced power conversion efficiency, yet controlling the morphology and interface structure on the nanometer length scale is challenging. Here, we demonstrate quantum dot-based hybrid solar cells with improved electronic interaction between donor and acceptor components, resulting in significant improvement in short-circuit current and open-circuit voltage. CdS quantum dots were bound onto crystalline P3HT nanowires through solvent-assisted grafting and ligand exchange, leading to controlled organic-inorganic phase separation and an improved maximum power conversion efficiency of 4.1% under AM 1.5 solar illumination. Our approach can be applied to a wide range of quantum dots and polymer hybrids and is compatible with solution processing, thereby offering a general scheme for improving the efficiency of nanocrystal hybrid solar cells.
Monolayers of PbSe and PbS quantum dots and PbSe/CdSe core/shell quantum dots made by Langmuir-Blodgett deposition are compared, with a focus on the formation, the morphology and the photoluminescence properties of the films. It is shown that PbSe quantum dots suffer from oriented attachment and a complete quenching of their photoluminescence after Langmuir-Blodgett processing. While the oriented attachment can be resolved by growing a CdSe shell around the PbSe core QDs, the photoluminescence of PbSe/CdSe Langmuir-Blodgett monolayers remains largely quenched. In the case of PbS quantum dots, the formation of a close-packed monolayer is more difficult, yet the resulting films show no sign of oriented attachment and their photoluminescence is comparable to that of the original, suspended quantum dots. In spite of their slow oxidation in a matter of weeks, these results mark PbS quantum dots as the preferred material for light-emitting applications in the near IR based on Langmuir-Blodgett monolayers of lead chalcogenide quantum dots.
Successive ion layer adsorption and reaction (SILAR) originally developed for the deposition of thin films on solid substrates from solution baths is introduced as a technique for the growth of high-quality core/shell nanocrystals of compound semiconductors. The growth of the shell was designed to grow one monolayer at a time by alternating injections of air-stable and inexpensive cationic and anionic precursors into the reaction mixture with core nanocrystals. The principles of SILAR were demonstrated by the CdSe/CdS core/shell model system using its shell-thickness-dependent optical spectra as the probes with CdO and elemental S as the precursors. For this reaction system, a relatively high temperature, about 220-240 degrees C, was found to be essential for SILAR to fully occur. The synthesis can be readily performed on a multigram scale. The size distribution of the core/shell nanocrystals was maintained even after five monolayers of CdS shell (equivalent to about 10 times volume increase for a 3.5 nm CdSe nanocrystal) were grown onto the core nanocrystals. The epitaxial growth of the core/shell structures was verified by optical spectroscopy, TEM, XRD, and XPS. The photoluminescence quantum yield (PL QY) of the as-prepared CdSe/CdS core/shell nanocrystals ranged from 20% to 40%, and the PL full-width at half-maximum (fwhm) was maintained between 23 and 26 nm, even for those nanocrystals for which the UV-vis and PL peaks red-shifted by about 50 nm from that of the core nanocrystals. Several types of brightening phenomena were observed, some of which can further boost the PL QY of the core/shell nanocrystals. The CdSe/CdS core/shell nanocrystals were found to be superior in comparison to the highly luminescent CdSe plain core nanocrystals. The SILAR technique reported here can also be used for the growth of complex colloidal semiconductor nanostructures, such as quantum shells and colloidal quantum wells.
The precipitation of calcium oxalate monohydrate (COM) at phospholipid monolayers confined to the air/water interface is observed in situ with the aid of Brewster angle microscopy. COM crystals appear as bright objects that are easily identified and quantified to assess the effects of different conditions on crystallization. Crystal precipitation was monitored at monolayers of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) in liquid condensed (LC) and liquid expanded (LE) phases. Within the LC phase, higher pressures reduce the incidence of crystallization at the interface, implying that within this phase precipitation is enhanced by higher compressibility or fluidity of the monolayer. Precipitation at biphasic LC/LE and LE/gas (G) monolayers was also studied. COM appears preferentially at phase boundaries of the DPPC LC/LE and LE/G monolayers. However, when an LC/LE phase boundary is created by two different phospholipids that are phase segregated, such as DPPC and 1,2-dimyristoyl-sn-glycero-3-phosphocholine, crystal formation occurs away from the interface within the DPPC LC phase. It is suggested that COM growth at phase boundaries is preferred only when there is molecular exchange between the phases.
We demonstrate the organization of nearly monodisperse colloidal InP quantum dots at the air/water interface in Langmuir monolayers. The organization of the particles is monitored in situ by surface pressure-surface area measurements and ex situ by AFM measurements on films transferred to mica by Langmuir-Blodgett deposition. The influence of different ligands on the quality of the monolayer formed has been studied. We show that densely packed monolayers with little holes can be formed using short chain ligands like pyridine and pentamethylene sulfide. The advantage of using short chain ligands for electron tunneling to or from the quantum dots is demonstrated using scanning tunneling spectroscopy.
- A W Adamson
- A P Gast
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