1,721,074 research outputs found
Order and randomness in Kolmogorov-Johnson-Mehl-Avrami-type phase transitions
No full textThe distribution of points on a 2D domain influences the kinetics of its coverage when a growth law is attached at each point. This implies that the kinetics of space filling can be adopted as a descriptor of the degree of order of the initial point distribution. In this paper, the degree of order of an initial array of points has been changed following two paths: (i)from a regular square lattice, through increasing displacement assigned to each point, towards Poissonian disorder; (ii)from a Poissonian distribution, by introducing a hard core potential with increasing correlation lengths, towards a more ordered lattice. A linear growth law has been attached to the points of the initial array and the kinetics X(X e), where X e is the extended coverage as defined in the Kolmogorov-Johnson-Mehl-Avrami model, has been monitored. The quantitative analysis has been performed by fitting the kinetics to an equation which we propose for the first time and which has proved to be, in fact, highly suitable for the purpose. The results demonstrate that the gross of variation from order to disorder is obtained for point displacements, u, of the order of a, the latter being the constant of a square lattice. Vice versa, the introduction of a correlation distance in a random distribution provokes at most an order limited to the first neighbors and no real order can ever be reached. Others descriptors have been investigated, all confirming our results. We also developed an analytical description based on the use of the f-functions, as have been defined by Van Kampen, up to the second order terms. Such a description has been shown to work well for u/a<1 within an interval ΔX e which depends on the value. © 2012 IOP Publishing Ltd
Pair interaction between Ge islands on vicinal Si(001) surfaces
No full textThe pair interaction between Ge islands on vicinal Si(001) substrates is investigated by scanning tunneling microscopy measurements as a function of the miscut angle. By the analysis of the nearest-neighbor island distributions, we assess the dependence of the local strain field on the substrate misorientation. We support our results by modeling elastic relaxation for different shapes and arrangements of islands with finite element calculations. © 2010 The American Physical Society
The entangled role of strain and diffusion in driving the spontaneous formation of atolls and holes in Ge/Si(111) heteroepitaxy
No full textWe investigate the interdependent processes of strain and diffusion in the formation of holes and atolls obtained by rapid annealing of Ge/Si(111) islands at T ≈ 970 °C. We show that the shape evolution from islands to atolls and holes is closely captured by an analytical model including strain-driven diffusion. In the model, strain profiles obtained by finite element solutions of continuum elasticity equations are introduced in the diffusion equation as the source of a diffusion flux driven by the strain gradient. When the shape of the elastic field in Ge/Si(111) islands is coupled to diffusion, the morphology of the SiGe nanostructures observed after annealing is reproduced. © 2013 IOP Publishing Ltd
Heteroepitaxy of Ge on singular and vicinal Si surfaces: Elastic field symmetry and nanostructure growth
No full textStarting with the basic definition, a short description of a few relevant physical quantities playing a role in the growth process of heteroepitaxial strained systems, is provided. As such, the paper is not meant to be a comprehensive survey but to present a connection between the Stranski-Krastanov mechanism of nanostructure formation and the basic principles of nucleation and growth. The elastic field is described in the context of continuum elasticity theory, using either analytical models or numerical simulations. The results are compared with selected experimental results obtained on GeSi nanostructures. In particular, by tuning the value of quantities such as vicinality, substrate orientation and symmetry of the diffusion field, we elucidate how anisotropic elastic interactions determine shape, size, lateral distribution and composition of quantum dots
Step-step interaction on vicinal Si(001) surfaces studied by scanning tunneling microscopy
No full textWe report on measurements of step-step interaction on a flat Si(111)-(7x7) surface and on vicinal Si (001) surfaces with miscut angles ranging between 0.2 degrees and 8 degrees. Starting from scanning tunneling microscopy images of these surfaces and describing steps profile and interactions by the continuum step model, we measured the self-correlation function of single steps and the distribution of terrace widths. Empirical parameters, such as step stiffness and step-step interaction strength, were evaluated from the images. The present experiment allows to assess the dependence of the step-step repulsion on miscut angle, showing how parameters drawn from tunneling images can be used to interpolate between continuum mesoscopic models and atomistic calculations of vicinal surfaces
Step-step interaction on vicinal Si(001) surfaces studied by scanning tunneling microscopy
Full text linkWe report on measurements of step-step interaction on a flat Si(111)−(7×7) surface and on vicinal Si(001) surfaces with miscut angles ranging between 0.2° and 8°. Starting from scanning tunneling microscopy images of these surfaces and describing steps profile and interactions by the continuum step model, we measured the self-correlation function of single steps and the distribution of terrace widths. Empirical parameters, such as step stiffness and step-step interaction strength, were evaluated from the images. The present experiment allows to assess the dependence of the step-step repulsion on miscut angle, showing how parameters drawn from tunneling images can be used to interpolate between continuum mesoscopic models and atomistic calculations of vicinal surfaces
Morphological and electronic characterization of functionalized graphene nanoribbons obtained by the unzipping of single-wall carbon nanotubes: a scanning tunneling microscopy study
No full textA few layers of functionalized graphene (FLG) have been obtained by the oxidative
unzipping reaction of single-walled carbon nanotubes. A detailed Scanning Tunneling
Microscopy and Transmission Electron Microscopy study shows that several different
morphologies are observed, depending on the substrate upon which they are
deposited, thus providing useful hints for a “real world” application of these carbon
nanostructures. In addition, a combined Scanning Tunneling Spectroscopy investigation
has shown that the electronic properties of these FLG are affected by their
functionalization degree, opening interesting perspectives for their use in sensor and
biosensor devices
Shaping Ge Islands on Si(001) Surfaces with Misorientation Angle
No full textA complete description of Ge growth on vicinal Si(001) surfaces in the angular miscut range 0°-8° is presented. The key role of substrate vicinality is clarified from the very early stages of Ge deposition up to the nucleation of 3D islands. By a systematic scanning tunneling microscopy investigation we are able to explain the competition between step-flow growth and 2D nucleation and the progressive elongation of the 3D islands along the miscut direction [110]. Using finite element calculations, we find a strict correlation between the morphological evolution and the energetic factors which govern the 105 faceting at atomic scale. © 2010 The American Physical Society
Effects of substrate vicinality on 3D islanding in Ge/Si epitaxy
No full textCommon features of Ge growth on vicinal Si(001) and Si(111) surfaces are discussed. We show that surface vicinality has a twofold effect on three-dimensional islanding in this system. As long as the atomic structure of the singular surface is preserved, epitaxial islands undergo a morphological change which is dictated by the misorientation from the singular surface. When also the surface reconstruction is so affected by miscut that surface and interfacial energies are drastically modified, the islanding pathway diverges from that occurring on flat surfaces. © 2013 Elsevier B.V
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