Island dissolution during capping layer growth interruption

A possible scenario for the dissolution of partially capped quantum dots was investigated. This model is based on the consideration of the total free energy being a sum of elastic and surface energies as the quantum-dot material redistributes itself as a second wetting layer on top of the capping layer. Quantitative results were obtained for the case of InAs/GaAs quantum dots that are partially capped by GaAs. We compare our results with supporting experimental evidence. Received: 29 January 2001 / Accepted: 30 January 2001 / Published online: 3 April 2001     

A nucleation theory of cell surface capping

We propose a new theory of cell surface capping based on the principles of nucleation. When antibody interacts with cell surface molecules, the molecules initially form small aggregates called patches that later coalesce into a large aggregate called a cap. While a cap can form by patches being pulled together by action of the cell's cytoskeleton, in the case of some molecules, disruption of the cytoskeleton does not prevent cap formation. Diffusion of large aggregates on a cell surface is slow, and thus we propose that a cap can form solely through the diffusion of small aggregates containing just one or a few cell surface molecules. Here we consider the extreme case in which single molecules are mobile, but aggregates of all larger sizes are immobile. We show that a set of patches in equilibrium with a sea of free cell surface molecules can undergo a nucleation-type phase transition in which the largest patch will bind free cell surface molecules, deplete the concentration of such molecules in the sea, and thus cause the other patches to shrink in size. We therefore show that a cap can form without patches having to move, collide with each other, and aggregate.     

Filament capping and nucleation in actin-based motility

Propulsion by actin polymerization is versatilely used in cell motility. Here, we investigate a model of the semi-flexible region of an actin gel close to a propelled object describing the force generation, the dynamics of the propagation velocity, filament attachment to and detachment from the obstacle surface and dynamics of the number of filaments, which result from filament nucleation and capping. The model equations are derived as moment equations of the length distributions. We find a variety of dynamic regimes. The filament number may respond very sensitively to small changes of the attachment rate.     

Magnetism induced by capping of non-magnetic ZnO nanoparticles

Magnetism of 10 nm size capped nanoparticles, NPs, of non-magnetic ZnO is analysed in terms of the surface band since, as magnetic dichroism analysis has pointed out, impurity atoms bonded to the surface act as donor or acceptor of electrons that occupy the surface states. Due to the nanometric scale of the particles the kinetic energy spectrum of the surface states can be considered as discrete. Therefore, the magnetic polarisation cannot be easily induced by pumping electrons to energy levels above the Fermi energy. It is in the Fermi level itself, generally unfilled, that develops a spontaneous magnetic moment similarly to that induced by Hund rules in unfilled atomic orbitals. It is shown, however, that the total magnetic moment of the surface originated at the unfilled Fermi level can reach values as large as 10² or 10³ Bohr magnetons.     

Secondary electron yield enhancement by MgO capping layers

The yield of secondary electrons emitted from an epitaxial three monolayer (3 ML) NiO(1 0 0)/Ag(1 0 0) film excited by soft X-ray linearly polarized synchrotron radiation at the Ni L_2,3 absorption threshold has been measured for different values of the thickness of a MgO(1 0 0) capping layer. Compared with the as grown 3 ML NiO(1 0 0)/Ag(1 0 0) film, we observe a significant enhancement by about a factor 1.2 of the secondary electron emission for the capped 8 ML MgO(1 0 0)/3 ML NiO(1 0 0)/Ag(1 0 0) sample. A further substantial yield enhancement by a factor 1.6 with respect to the uncapped NiO sample is observed after deposition of an additional 8 ML MgO(1 0 0) film, for a total capping layer thickness of 16 ML. The observed secondary electron yield enhancement is discussed in terms of modified electronic structure, surface work function changes, and characteristic electron propagation lengths.     

Optimal capping layer thickness for stacked quantum dots

We study the effect of strain on the vertical and lateral self-organization of nanoscale patterns and stacked quantum dots during epitaxial growth. The computational approach is based on the level set method in combination with an atomistic strain code. Strain changes the energetics of microscopic parameters during growth, and thus determines the nucleation sites and the growth of islands and dots. Our results show that strain can lead to vertical alignment as well as lateral organization. Moreover, our simulations suggest that there is an optimal thickness of the capping layer to get the best alignment and most uniform size distribution of stacked quantum dots, and that its variation can be used to control the formation of interesting structures.     

Synthesis of Mn doped ZnO nanoparticles with biocompatible capping

Free standing nanoparticles of ZnO doped with transition metal ion Mn have been prepared by solid state reaction method at 500 °C. X-ray diffraction (XRD) analysis confirmed high quality monophasic wurtzite hexagonal structure with particle size of 50 nm and no signature of dopant as separate phase. Incorporation of Mn has been confirmed with EDS. Bio-inorganic interface was created by capping the nanoparticles with heteromultifunctional organic stabilizer mercaptosuccinic acid (MSA). The surface morphological studies by scanning electron microscopy (SEM) showed formation of spherical particles and the nanoballs grow in size uniformly with MSA capping. MSA capping has been confirmed with thermo gravimetric analysis (TGA) and FTIR. Photoluminescence (PL) studies show that the ZnO:Mn²⁺ particles are excitable by blue light and emits in orange and red. Occurrence of room temperature ferromagnetism in Mn doped ZnO makes such biocompatible luminescent magnetic nanoparticles very promising material.     

Formation of CdS nanoparticles using starch as capping agent

CdS nanoparticles have been synthesized using starch as capping agent in aqueous solution. The morphology and crystalline structure of such samples were measured by high-resolution transmission electron microscopy and X-ray diffraction, respectively. The average grain size of the nanoparticles determined by these techniques was of the order of 5 nm. Photoluminescence of CdS nanoparticles shows a strong emission peak below to the band gap bulk semiconductor attributed to center trap states, also the broadening peak was interpreted in terms of electron-phonon interaction.     

AlGaAs capping effect on InAs quantum dots self-assembled on GaAs

The effects of AlGaAs capping on InAs quantum dots self-assembled on GaAs are investigated. It is observed that, the photoluminescence intensity becomes stronger up to twice when Al is incorporated into the cap layer. In the mean time, the full width at half maximum of the photoluminescence spectrum becomes narrower, the peak splitting between the ground and first excited exciton levels becomes wider, and the photoluminescence peak wavelength becomes longer. With considerations of the increased barrier height and the changed microstructures of the quantum dots induced by AlGaAs capping, the mechanisms of the observed improvements are discussed.     

Photostabilization of dye on anatase titania nanoparticles by polymer capping

Nanoparticles of titanium dioxide (TiO₂) were prepared by the sol-gel method and characterized by XRD, TEM, FTIR and UV-Visible absorption spectroscopy for their structure, morphology and particle size. Organization, stabilization of the nanoparticles of TiO₂ from further growth, and partially inhibiting the photodegradation was achieved by capping with polymers. Photodegradation of dye on TiO₂ nanoparticles encapsulated in different polymers has been studied by diffused reflectance UV spectroscopy. The role of the polymer on the photodegradation of TiO₂ has been compared and explained.     

Coarse-grained model for gold nanocrystals with an organic capping layer

We have developed a coarse-grained interaction potential between icosahedral nanocrystals and united CH _x or SH atoms that interact via Lennard–Jones interactions. This interaction potential can be used to efficiently compute thermodynamic and structural properties of alkyl-thiol capping layers adsorbed on gold nanocrystals.     

Branching, capping, and severing in dynamic actin structures

Branched actin networks at the leading edge of a crawling cell evolve via protein-regulated processes such as polymerization, depolymerization, capping, branching, and severing. A formulation of these processes is presented and analyzed to study steady-state network morphology. In bulk, we identify several scaling regimes in severing and branching protein concentrations and find that the coupling between severing and branching is optimally exploited for conditions in vivo. Near the leading edge, we find qualitative agreement with the in vivo morphology.     

Extracellular biosynthesis of monodispersed gold nanoparticles by a SAM capping route

Monodispersed gold nanoparticles capped with a self-assembled monolayer of dodecanethiol were biosynthesized extracellularly by an efficient, simple, and environmental friendly procedure, which involved the use of Bacillus megatherium D01 as the reducing agent and the use of dodecanethiol as the capping ligand at 26 °C. The kinetics of gold nanoparticle formation was followed by transmission electron microscope (TEM) and UV-vis spectroscopy. It was shown that reaction time was an important parameter in controlling the morphology of gold nanoparticles. The effect of thiol on the shape, size, and dispersity of gold nanoparticles was also studied. The results showed that the presence of thiol during the biosynthesis could induce the formation of small size gold nanoparticles (<2.5 nm), hold the shape of spherical nanoparticles, and promote the monodispersity of nanoparticles. Through the modulation of reaction time and the use of thiol, monodispersed spherical gold nanoparticles capped with thiol of 1.9 ± 0.8 nm size were formed by using Bacillus megatherium D01.     

Size-shrinkage effect of InAs quantum dots during a GaAs capping growth

The modification of the InAs quantum dots (QDs) by the GaAs capping growth was studied by using cross-sectional STEM and atomic force microscopy. In case of the GaAs capping growth at 450 °C, it was found that the lateral size of the InAs QDs significantly decreases rather than the height and that this size-shrinkage effect is enhanced for the large QDs. The shrinkage behavior is mainly attributed to the indium surface segregation, strongly depending on the surface strain of the QDs. The growth process of the GaAs capping layer plays an important role for achieving the size ordering of the embedded QDs.     

Influences of capping molecules on optical properties of nanocrystalline ZnS:Cu

ZnS:Cu nanocrystals capped with different capping molecules have been successfully synthesized by a simple aqueous method. The prepared nanocrystals were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive analysis by X-rays (EDAX). The surface characterization of the nanocrystals was done by FTIR spectroscopy. The effect of capping agents on absorption and photoluminescence (PL) spectra of the ZnS:Cu nanocrystals was studied. A blue shift of the absorption peaks was observed and attributed to a quantum confinement effect, which increases the band gap energy. The photoluminescence spectra of the capped ZnS:Cu nanocrystals showed a broad peak in the range of 460–480 nm. The intensity of the PL spectra strongly depended on the capping agents.     

The effect of capping layers on low-angle Bragg peaks from copper-cobalt multilayers

Using simulations based on the Parratt theory for grazing incidence X-ray specular reflectivity, we show that small variations in the thickness of the gold cap used to prevent oxidation in Co/Cu multilayers can have a dramatic effect on the height and position of the low-order Bragg peaks from these multilayers. The height of the specular Bragg peak cannot be used as a measured of interface roughness nor its position used to determine multilayer period unless the cap thickness is known or determined precisely by simulation.     

Improved thin film transistor performance of solution-processed-zinc-oxide nanorods with spin-on-glass capping layer

Due to the vulnerability of various organic and oxide materials to the atmosphere, a protective layer is often used to improve device performance and stability. In this study, we use a spin-on-glass (SOG) layer to encapsulate a solution-processed-zinc-oxide (ZnO) film. ZnO loses oxygen very easily to the atmosphere and even loses Zn at relatively low temperatures. An SOG capping layer prevents the loss of oxygen without degrading its crystalline properties. We demonstrate the properties of a bottom-gate transistor with a capping layer; it shows improved electrical properties with a mobility of 0.5 cm²/V.s. and stability. Physical characterization reveals that the defect density with a capping layer is much lower than it is without it. A capping layer can also prevent the loss of oxygen at the annealing temperature of 350 °C.     

Influence of organic capping layers on the performance of transparent organic light-emitting diodes

We investigate a set of transparent organic LEDs (TOLEDs) with different organic capping layer (OC) thicknesses to understand the capping layer effect. We find that thickness variation of the OC strongly influences the emission properties of TOLEDs and exhibits different trends for top or bottom emission. The external quantum efficiency for the top side can be enhanced by a factor of 63%, but that of the bottom side only by 4% compared to a reference device without an OC. Additionally, we demonstrate that the introduction of the OC is an effective method to control the bottom-to-top emission ratio within a measured range from 2.87 to 6.05.     

Combined effects of microcavity and dielectric capping layer on bidirectional organic light-emitting diodes

We report on highly enhanced and controlled light outcoupling of bidirectional organic light-emitting diodes by introduction of an enhanced microcavity structure as well as an organic capping layer (OC). Combining both OC and microcavity, we find that the overall external quantum, as well as current efficiency (CE), can be greatly enhanced. Especially, the CE with an appropriate thickness of OC is almost 1.75 times larger than that of the reference device without OC. Furthermore, we also analyze our devices with a numerical optical model calculating the flux of outcoupled photons, and compare theoretical predictions with our experimental results.