18,771,246 research outputs found
Kinetics of island density in thin film growth in the framework of statistical mechanics of rigid disks
No full textThe paper centers on the evaluation of the function n(Theta)=N(Theta)/N-0, that is the normalized number of islands as a function of coverage Theta is an element of[0,1], given N-0 initial nucleation centers (dots) having any degree of spatial correlation. A mean field approach has been employed: the islands have the same size at any coverage. In particular, as far as the random distribution of dots is concerned, the problem has been solved by considering the contribution of binary collisions between islands only. With regard to correlated dots, we generalize a method previously applied to the random case only. In passing, we have made use of the exclusion probability reported in [S. Torquato, B. Lu, and J. Rubinstein, Phys. Rev. A 41, 2059 (1990)], for determining the kinetics of surface coverage in the case of correlated dots, improving our previous calculation [M. Tomellini, M. Fanfoni, and M. Volpe, Phys. Rev. B 62, 11300 (2000)]
Effect of the impingement on the kinetics of island aggregation in the post-nucleation stage of film growth at solid surfaces
No full textThe kinetics of island aggregation in the post-nucleation stage, for a model case system where the impingement mechanism among the islands dominates the film growth, has been measured using scanning electron microscopy (SEM). A theoretical model previously developed is found to be in accordance with the experimental kinetics [M. Fanfoni, M. Tomellini, Appl. Surf. Sci,, 136 (1998) 338.]. (C) 1999 Elsevier Science B.V. All rights reserved
The influence of steps on the island distribution function in thin solid film formation
No full textThe gamma distribution has been shown to be the simplest function for describing the universal size distribution function of InAs quantum dots on GaAs(001) substrate (Fanfoni et al 2007 Phys. Rev. B 75 245312), although there is no reason to believe that it could not be applied to any film growth where the principal feeding of the islands occurs through surface diffusion as proposed, after numerical simulations in the two-dimensional case, by Mulheran and Blackman (1995 Phil. Mag. Lett. 72 55). As far as InAs/GaAs(001) quantum dots are concerned, by fitting the data of Ebiko et al (1999 Phys. Rev. B 60 8234) and Krzyzewski et al (2002 Phys. Rev. B 66 201302(R)) to the gamma function, beta = 4.5 and 2.9 are the respective returned values. The latter value appears anomalous because for a Poissonian distribution of points (centers of islands), beta = 3.5-3.6. Moreover, the greater the spatial correlation the greater the beta value. We prove that the presence of steps alters the distribution of nucleation centers throughout the substrate in such a way that its variance increases with respect to the substrate without steps and, as a consequence, beta decreases
Avenida da Ponte: Knowledge of the City as a Tool for the Design of an Unresolved Place
Get PDFThe article analyses the two unrealised projects - by Fernando Távora in 1955 and Álvaro Siza in 2000 - for Avenida da Ponte in Porto and the public space project underlying the construction of the Casa dos 24. This analysis has made it possible to define how the historical and morphological knowledge of the city becomes an operative and compositional instrument of the project and at the same time is not a presupposition of immobilism, revealing the potential for a process of transformation of urban places updated to today's needs. This article is excerpted from the writer's doctoral thesis: A. Fanfoni, La persistenza del fenomeno dell'architettura nella "Città delle Costanti. L’attualità della lezione di Fernando Távora per la definizione di temi e strumenti utili al progetto degli spazi pubblici e dei luoghi urbani verso le linee guida di Smart City in Emilia-Romagna Università di Parma: Parma 2023 (Tutor Dario Costi, co-tutor Giovanni Leoni)
Dirac delta nucleation in the framework of Avrami's model: The case of diamond growth on deformed Si(100)
No full textThe nucleation and growth kinetics of diamond deposited by hot-filament chemical vapour deposition (HFCVD) on Si(100) substrates, previously deformed by uniaxial compression dong the [100] direction, have been investigated Although the nucleation density at saturation (0.04-0.06 mu m(-2)) was similar to those measured on virgin, as-received Si(100) wafers, the kinetics of stable nucleus formation resembled the fast kinetics observed for substrates which were mechanically abraded prior to CVD in order to enhance diamond nucleation. The results definitely indicate that diamond nucleation occurs randomly with a rate that is a Dirac delta function. The time dependence of the substrate fraction which is covered by islands was measured, and a good agreement with Avrami's kinetics for 2D phase transitions was found. The total island perimeter has also been measured as a function of the covered surface, and is well described by the analytical model recently developed [M. Tomellini and M. Fanfoni, Surf Sci. 349 (1996) L191]. The observed fast nucleation has been attributed to stress-induced defects pre-existing at the surface and which provides suitable sites for diamond growth. (C) 1997 Elsevier Science B.V
Avrami's kinetic approach for describing Volmer Weber growth mode at solid surfaces studied via PES and AES
No full textA model is presented for describing the time dependence of the photoelectron and/or Auger signals during the overlayer formation in the case of Volmer Weber growth mode, i.e. 3D island formation. The impingement among clusters has been taken into account in the framework of Avrami's statistical approach. A first system has been considered in which nucleation occurs at a given number of preexisting sites randomly distributed throughout the whole surface. The results obtained by numerical computations indicate that particular conditions can be indeed realized for which the PES signal is chiefly related to the kinetics of the surface fraction that is covered by islands. A more involved system has been also modeled where nucleation does not occur at preexisting sites but through the formation of stable dimers. Under this circumstance, Avrami's treatment of island impingement can be still retained although now a system of integral differential equations has to be solved to get the kinetics. Such a modelling should be suitable for describing the metallic film growth studied by PES
Modeling adatom life time in thin film growth in case of spatially correlated nuclei
No full textWe model the adatom life time for island growth in the general case of spatially correlated nuclei. The model is based on a stochastic approach, which relies on the use of the m-dots spatial correlation functions. The adatom life time is evaluated in the entire range of surface coverage. The impact of the spatial correlation among nuclei on the life time is analysed and discussed also with reference to the case of random nucleation. © 2004 Elsevier B.V. All rights reserved
L'analogia in fisica come strumento di indagine: il moto casuale e l'interferenza tra onde incoerenti
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