Investigation of film surface roughness and porosity dependence on lattice size in a porous thin film deposition process

Transition from compact to porous films in deposition with temperature-activated diffusion

We study a thin-film growth model with temperature activated diffusion of adsorbed particles, allowing for the formation of overhangs and pores, but without detachment of adatoms or clusters from the deposit. Simulations in one-dimensional substrates are performed for several values of the diffusion-to-deposition ratio R of adatoms with a single bond and of the detachment probability ε per additional nearest neighbor, respectively, with activation energies are E(s) and E(b). If R and ε independently vary, regimes of low and high porosity are separated at 0.075≤ε(c)≤0.09, with vanishingly small porosity below that point and finite porosity for larger ε. Alternatively, for fixed values of E(s) and E(b) and varying temperature, the porosity has a minimum at T(c), and a nontrivial regime in which it increases with temperature is observed above that point. This is related to the large mobility of adatoms, resembling features of equilibrium surface roughening. In this high-temperature region, the deposit has the structure of a critical percolation cluster due to the nondesorption. The pores are regions enclosed by blobs of the corresponding percolating backbone, thus the distribution of pore size s is expected to scale as s(-τ̃) with τ̃≈1.45, in reasonable agreement with numerical estimates. Roughening of the outer interface of the deposits suggests Villain-Lai-Das Sarma scaling below the transition. Above the transition, the roughness exponent α≈0.35 is consistent with the percolation backbone structure via the relation α=2-d(B), where d(B) is the backbone fractal dimension.

Nanoporous thin films with controllable nanopores processed from vertically aligned nanocomposites

Porous thin films with ordered nanopores have been processed by thermal treatment on vertically aligned nanocomposites (VAN), e.g., (BiFeO(3))(0.5):(Sm(2)O(3))(0.5) VAN thin films. Uniformly distributed nanopores with an average diameter of 60 nm and 150 nm were formed at the bottom and top of the nanoporous films, respectively. Controllable porosity can be achieved by adjusting the microstructure of VAN (BiFeO(3)):(Sm(2)O(3)) thin films and the annealing parameters. In situ heating experiments within a transmission electron microscope (TEM) column at temperatures from 25 to 850 degrees C, provides significant insights into the phase transformation, evaporation and structure reconstruction during the annealing. The in situ experiments also demonstrate the possibility of processing vertically aligned nanopores (VANP) with one phase stable in a columnar structure. These nanoporous thin films with controllable pore size and density could be promising candidates for thin film membranes and catalysis for fuel cell and gas sensor applications.

Thin-film growth by random deposition of linear polymers on a square lattice

We present some results of Monte Carlo simulations for the deposition of particles of different sizes on a two-dimensional substrate. The particles are linear, height one, and can be deposited randomly only in the two x and y directions of the substrate and occupy an integer number of cells of the lattice. We show there are three different regimes for the temporal evolution of the interface width. At the initial times we observe an uncorrelated growth, with an exponent β1 characteristic of the random deposition model. At intermediate times, the interface width presents an unusual behavior, described by a growing exponent β2, which depends on the size of the particles added to the substrate. If the linear size of the particle is two we have β2 < β1, otherwise we have β2 > β1, for all other particle sizes. After the growth reaches the saturation regime where the interface width becomes constant and is described by the roughness exponent α, which is nearly independent of the size of the particle. Similar results are found in the surface growth due to the electrophoretic deposition of polymer chains. Contrary to one-dimensional results the growth exponents are nonuniversal.