Hexagonal nanodisks of ZnO were fabricated by a solution process using ZnO nanoparticles and their cathodoluminescence characteristics were investigated. Monochromatic cathodoluminescence images showed that luminescence was spatially localized near the boundary of the nanodisk and spectral analysis in conjunction with the intensity profile consistently ascribed the spatial localization of luminescence to whispering-gallery-modelike-enhanced emission. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2174122͔With the successful demonstration of various nanostructures and increasing demand for integrable optoelectronic components with existing silicon technology, small-scale dielectric resonators applicable to photonic nanodevices with low-thresholds have gained importance because they can be used as low-power-consumption light sources in integrated circuits. Reducing the size of photonic devices, however, results in decrease of luminescence intensity in such a way that careful design of intensity enhancement mechanism should be introduced. For this reason, the whispering-gallery mode, which has been particularly utilized in applications to optical communication, has attracted much attention because it is a very efficient mechanism of luminescence enhancement even in small-scale resonators. 1-4 Since a top-down approach, such as lithography, has been among the best technological methods to fabricate tailored nanostructures appropriatue for specific research or application purposes, much effort has been made in order to fabricate nanoresonators via lithography to achieve low-threshold photonic nanodevices. There are, however, a few important generic problems in top-down fabrication of nanoresonators. There is unavoidable damage to films associated with lithographic etching process. An unwanted strain effect is also involved due to the limited choice of substrates to accommodate lattice misfit. In contrast, photonic nanodevices with bottom-up-based nanoscale resonant cavity employing whispering-gallery modes ͑WGMs͒ are very promising integrable components with high luminescence efficiency. More than that, a bottom-up approach typically does not depend on the choice of substrates, which is a major advantage in integration with current silicon technology.Due to direct and wide-band-gap characteristics with a large binding exciton energy, ZnO has drawn much attention for potential application to short-wavelength optoelectronic devices. In addition to single-crystalline ZnO thin films, ZnO nanowires and nanorods with perfect crystallinity were recently fabricated, and newly developed ZnO-based nanostructures demonstrated the possibility for nanoscale optoelectronic devices. [5][6][7] In contrast with one-dimensional nanostructures, such as nanorods, hexagonal nanodisk resonators employing WGMs have a smaller effective volume of gain medium, so that it is easier to fabricate compact photonic nanodevices. In addition, nanodisks are more confined to the surface of substrate and mechanical stability of nanodisks for pos...
We present the results of in situ x-ray scattering experiments that investigate the growth of Pb nanocrystalline islands on Si͑111͒. It is conclusively shown that the Pb nanocrystals do not reside on top of a Pb wetting layer. The nucleating Pb nanocrystals transform the highly disordered Pb wetting layer beneath the islands into well-ordered fcc Pb. The surface then consists of fcc Pb islands directly on top of the Si surface with the disordered wetting layer occupying the region between the islands. As the Pb nanocrystals coalesce at higher coverage we observe increasing disorder that is consistent with misfit strain relaxation. The confinement of the electron wave function in small objects leads to new energy levels that affect the object's total energy. Conversely, confinement influences an object's size by selecting those sizes that minimize the total energy. As a consequence, such "quantum size effects" (QSE) 1 cause metals grown on semiconductors to have certain film thicknesses or island heights that are more stable than others. 2-5
Surface x-ray scattering and scanning-tunneling microscopy experiments reveal novel coarsening behavior of Pb nanocrystals grown on Si 111 -7 7 . It is found that quantum size effects lead to the breakdown of the classical Gibbs-Thomson analysis. This is manifested by the lack of scaling of the island densities. In addition, island decay times are orders of magnitude faster than expected from the classical analysis and have an unusual dependence on the growth flux F (i.e., 1=F). As a result, a highly monodispersed 7-layer island height distribution is found after coarsening if the islands are grown at high rather than low flux rates. These results have important implications, especially at low temperatures, for the controlled growth and self-organization of nanostructures. DOI: 10.1103/PhysRevLett.96.106105 PACS numbers: 68.55.Ac, 81.16.Dn, 61.14.Hg, 68.35.Fx Electron confinement in nanostructures gives rise to new quantized energy levels that are strongly dependent on the nanostructure's dimensions. This means that an object's size or shape is coupled to its total energy. This coupling is referred to as the quantum size effect (QSE) [1]. An example is the growth of Pb nanocrystalline islands on Si(111) [2 -6]. In this system, the height distribution of the grown islands is found to peak in increments of two Pb layers. This bilayer stability is understood in terms of oscillations in the electronic energy as the discrete quantum states fall below the Fermi level approximately every two Pb layers [7][8][9]. While this energetic reason is the driving force for the observed height preference, it is unclear how the preferred islands are assembled and what role kinetic barriers play. The formation of the preferred islands is not exclusively controlled by thermodynamics, since these QSE islands are not in equilibrium.The nucleation, growth, and coarsening of islands have been extensively described by a classical analysis [10]. In this scenario, the initial island nucleation is established by a steady state concentration of adatoms on the surface, which yields stable islands if they exceed a critical size of i atoms. The island density n is predicted and found to scale as the ratio F=D , where i= i 2 , D is the surface diffusion constant, and F is the deposition rate [11]. Once the deposition flux is turned off, the island density slowly begins to decrease due to coarsening (Ostwald ripening), whereby a critical island radius r C is established such that islands having radii larger than r C will slowly grow at the expense of islands having smaller radii. This process, and its inherent dependence on island radius r, is controlled by the chemical potential difference between islands r and a 2D gas of adatoms Free , which is given by the Gibbs-Thomson relation: r ÿ Free 2 =!r (where is the surface tension of an island and ! is the atomic density of Pb) [12]. In particular, the GibbsThomson relation was shown to accurately describe the situation for 2D islands on metal surfaces [10,13]. Such systems have been extensivel...
Synchrotron x-ray diffraction was employed to measure the lattice constants a and c of GaN films grown with an AlN buffer layer on sapphire (0001) over a thickness range of 50 Å to 1 μm. We used multiple reflections and a least-squares fit method for high reliability. As the thickness increased, the lattice constant a increased from 3.133 Å to 3.196 Å and c decreased from 5.226 Å to 5.183 Å. The expected trend was fitted to an equilibrium theory, allowing the critical thickness of GaN on AlN to be estimated at 29 Å ± 4 Å in good agreement with a theoretical prediction.
The authors report the incorporation of unexpectedly large vacancy clusters into homoepitaxial Ag͑001͒ films. These results, which are for a simple noble metal system, have important implications for understanding the atomic-scale kinetics of surfaces where current models have mostly ignored the role of vacancies. For films grown at 150 K, an average vacancy cluster exhibits a local dilatation volume of 750 Å 3 , which leads to a 1% compressive strain of the film. Vacancy clusters are observed even for films grown near room temperature. These in situ diffuse x-ray scattering experiments measure the local deformation around the cluster and, therefore, provide conclusive evidence of vacancy clusters.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.