Roughness exponents and grain shapes

Deposit of Red Blood Cells at low concentrations in evaporating droplets is dominated by a central edge growth

Evaporation of blood droplets and diluted blood samples is a topic of intensive research, as it is considered a potential low-cost diagnostic tool. So far, samples with a volume fraction down to a few percent of red blood cells have been studied, and these were reportedly dominated by a "coffee-ring" deposit. In this study, samples with lower volume fractions were used to investigate the growth of the evaporative deposit from sessile droplets in more detail. We observed that blood samples and salt solutions with less than 1% volume fraction of red blood cells are dominated by a central deposit. We characterized the growth process of this central deposit by evaporating elongated drops and determined that it is consistent with the Kardar-Parisi-Zhang process in the presence of quenched disorder. Our results showed a sensitivity of the deposit size to fibrinogen concentration and the shape of red blood cells, suggesting that this parameter could be developed into a new and cost-effective clinical marker for inflammation and red blood cell deformation.

Colloidal model of two-step protocol for epitaxial growth in one dimension

We explore the application of a two-step growth protocol to a one-dimensional colloidal model. The evolution of the system is described in terms of the time-dependence of both monomer and island densities,N₁andN, while its structure is characterized by using distributions of the gap length, the capture zone, the inter-island distance, and the island length. Analytical results obtained from rate equations are compared with these from molecular dynamics simulations. Since the two-step growth protocol deals with nucleation and aggregation processes in two completely separated time regimes, it makes possible to gain better understanding and control on the island formation mechanism than the standard one-step protocol. The predicted features and advantages of the two-step process could be experimentally tested using deposition of colloidal spheres on pattern substrates.

Arylenevinylene Oligomer-Based Heterostructures on Flexible AZO Electrodes

We investigated the optical and electrical properties of flexible single and bi-layer organic heterostructures prepared by vacuum evaporation with a p-type layer of arylenevinylene oligomers, based on carbazole, 3,3' bis(N hexylcarbazole)vinylbenzene = L13, or triphenylamine, 1,4 bis [4 (N,N' diphenylamino)phenylvinyl] benzene = L78, and an n-type layer of 5,10,15,20-tetra(4-pyrydil)21H,23H-porphyne = TPyP. Transparent conductor films of Al-doped ZnO (AZO) with high transparency, >90% for wavelengths > 400 nm, and low resistivity, between 6.9 × 10-4 Ω·cm and 23 × 10-4 Ω·cm, were deposited by pulsed laser deposition on flexible substrates of polyethylene terephthalate (PET). The properties of the heterostructures based on oligomers and zinc phthalocyanine (ZnPc) were compared, emphasizing the effect of the surface morphology. The measurements revealed a good absorption in the visible range of the PET/AZO/arylenevinylene oligomer/TPyP heterostructures and a typical injection contact behavior with linear (ZnPc, L78) or non-linear (L13) J-V characteristics in the dark, at voltages < 0.4 V. The heterostructure PET/AZO/L78/TPyP/Al showed a current density of ~1 mA/cm2 at a voltage of 0.3 V. The correlation between the roughness exponent, evaluated from the height-height correlation function, grain shape, and electrical behavior was analyzed. Consequently, the oligomer based on triphenylamine could be a promising replacement of donor ZnPc in flexible electronic applications.

Initial pseudo-steady state & asymptotic KPZ universality in semiconductor on polymer deposition

The Kardar-Parisi-Zhang (KPZ) class is a paradigmatic example of universality in nonequilibrium phenomena, but clear experimental evidences of asymptotic 2D-KPZ statistics are still very rare, and far less understanding stems from its short-time behavior. We tackle such issues by analyzing surface fluctuations of CdTe films deposited on polymeric substrates, based on a huge spatio-temporal surface sampling acquired through atomic force microscopy. A pseudo-steady state (where average surface roughness and spatial correlations stay constant in time) is observed at initial times, persisting up to deposition of ~10⁴ monolayers. This state results from a fine balance between roughening and smoothening, as supported by a phenomenological growth model. KPZ statistics arises at long times, thoroughly verified by universal exponents, spatial covariance and several distributions. Recent theoretical generalizations of the Family-Vicsek scaling and the emergence of log-normal distributions during interface growth are experimentally confirmed. These results confirm that high vacuum vapor deposition of CdTe constitutes a genuine 2D-KPZ system, and expand our knowledge about possible substrate-induced short-time behaviors.

Universality and dependence on initial conditions in the class of the nonlinear molecular beam epitaxy equation

We report extensive numerical simulations of growth models belonging to the nonlinear molecular beam epitaxy (nMBE) class, on flat (fixed-size) and expanding substrates (ES). In both d=1+1 and 2+1, we find that growth regime height distributions (HDs), and spatial and temporal covariances are universal, but are dependent on the initial conditions, while the critical exponents are the same for flat and ES systems. Thus, the nMBE class does split into subclasses, as does the Kardar-Parisi-Zhang (KPZ) class. Applying the "KPZ ansatz" to nMBE models, we estimate the cumulants of the 1+1 HDs. Spatial covariance for the flat subclass is hallmarked by a minimum, which is not present in the ES one. Temporal correlations are shown to decay following well-known conjectures.

Width and extremal height distributions of fluctuating interfaces with window boundary conditions

We present a detailed study of squared local roughness (SLRDs) and local extremal height distributions (LEHDs), calculated in windows of lateral size l, for interfaces in several universality classes, in substrate dimensions d_{s}=1 and 2. We show that their cumulants follow a Family-Vicsek-type scaling, and, at early times, when ξ≪l (ξ is the correlation length), the rescaled SLRDs are given by log-normal distributions, with their nth cumulant scaling as (ξ/l)^{(n-1)d_{s}}. This gives rise to an interesting temporal scaling for such cumulants as 〈w_{n}〉_{c}∼t^{γ_{n}}, with γ_{n}=2nβ+(n-1)d_{s}/z=[2n+(n-1)d_{s}/α]β. This scaling is analytically proved for the Edwards-Wilkinson (EW) and random deposition interfaces and numerically confirmed for other classes. In general, it is featured by small corrections, and, thus, it yields exponents γ_{n} (and, consequently, α,β and z) in good agreement with their respective universality class. Thus, it is a useful framework for numerical and experimental investigations, where it is usually hard to estimate the dynamic z and mainly the (global) roughness α exponents. The stationary (for ξ≫l) SLRDs and LEHDs of the Kardar-Parisi-Zhang (KPZ) class are also investigated, and, for some models, strong finite-size corrections are found. However, we demonstrate that good evidence of their universality can be obtained through successive extrapolations of their cumulant ratios for long times and large l. We also show that SLRDs and LEHDs are the same for flat and curved KPZ interfaces.

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.

Effect of the local morphology in the field emission properties of conducting polymer surfaces

In this work, we present systematic theoretical evidence of a relationship between the point local roughness exponent (PLRE) (which quantifies the heterogeneity of an irregular surface) and the cold field emission properties (indicated by the local current density and the macroscopic current density) of real polyaniline (PANI) surfaces, considered nowadays as very good candidates in the design of field emission devices. The latter are obtained from atomic force microscopy data. The electric field and potential are calculated in a region bounded by the rough PANI surface and a distant plane, both boundaries held at distinct potential values. We numerically solve Laplace's equation subject to appropriate Dirichlet's condition. Our results show that local roughness reveals the presence of specific sharp emitting spots with a smooth geometry, which are the main ones responsible (but not the only) for the emission efficiency of such surfaces for larger deposition times. Moreover, we have found, with a proper choice of a scale interval encompassing the experimentally measurable average grain length, a highly structured dependence of local current density on PLRE, considering different ticks of PANI surfaces.

Statistical analysis of mounded surfaces: application to the evolution of ultrathin gold film morphology with deposition temperature

Extracting characteristic dimensions from mounded surfaces such as grain size or intergrain lengths is usually made by statistical analysis. Different statistical functions are used in the literature to extract characteristic lengths. The main issue is that depending on the choice of the statistical function the results can be very different. In this paper, we demonstrate using a series of model mounded surfaces for which characteristic dimensions are known, that a method (namely, interfacial differential function, IDF) is the most effective method to determine the different characteristic lengths. The influence on the statistical treatment of the variation of the different characteristic lengths is then studied and confirms the ability of the IDF analysis. The IDF method was used to analyze the evolution of ultrathin gold film morphology as function of deposition temperature. This approach allows us to demonstrate that the roughness increase with deposition temperature is mainly due to a grain height increase and not to a grain coarsening phenomena as it was claimed before.