Kinetic roughening in polymer film growth by vapor deposition

Chemical synthesis and optical, structural, and surface characterization of InP-In2O3 quantum dots

InP-In₂O₃ colloidal quantum dots (QDs) synthesized by a single-step chemical method without injection of hot precursors (one-pot) were investigated. Specifically, the effect of the tris(trimethylsilyl)phosphine, P(TMS)₃, precursor concentration on the QDs properties was studied to effectively control the size and shape of the samples with a minimum size dispersion. The effect of the P(TMS)₃ precursor concentration on the optical, structural, chemical surface, and electronic properties of InP-In₂O₃ QDs is discussed. The absorption spectra of InP-In₂O₃ colloids, obtained by both UV-Vis spectrophotometry and photoacoustic spectroscopy, showed a red-shift in the high-energy regime as the concentration of the P(TMS)₃ increased. In addition, these results were used to determine the band-gap energy of the InP-In₂O₃ nanoparticles, which changed between 2.0 and 2.9 eV. This was confirmed by Photoluminescence spectroscopy, where a broad-band emission displayed from 2.0 to 2.9 eV is associated with the excitonic transition of the InP and In₂O₃ QDs. In₂O₃ and InP QDs with diameters ranging approximately from 8 to 10 nm and 6 to 9 nm were respectively found by HR-TEM. The formation of the InP and In₂O₃ phases was confirmed by X-ray Photoelectron Spectroscopy.

Anomalous kinetic roughening in growth of MoS2 films under pulsed laser deposition

Growth dynamics of thin films expressed by scaling theory is a useful tool to quantify the statistical properties of the surface morphology of the thin films. To date, the growth mechanism for 2D van der Waals materials has been rarely investigated. In this work, an experimental investigation was carried out to identify the scaling behavior as well as the growth mechanism of 2D MoS₂ thin films, grown on glass substrates by pulsed laser deposition for different deposition time durations, using atomic force microscopy images. The growth of MoS₂ films evolved from layer-by-layer to layer plus island with the increase in deposition time from 20 s to 15 min. The film surface exhibited anisotropic growth dynamics in the vertical and lateral directions where RMS roughness varied with deposition time as w ∼ t^β with the growth exponent β = 0.85 ± 0.11, while the lateral correlation length ξ was ξ = t^1/z with 1/z = 0.49 ± 0.09. The films showed a local roughness exponent α_loc = 0.89 ± 0.01, global roughness exponent α = 1.72 ± 0.14 and spectral roughness exponent α_s = 0.85 ± 0.03, suggesting that the growth of MoS₂ thin films followed intrinsic anomalous scaling behavior (α_s < 1, α_loc = α_s ≠ α). Shadowing owing to conical incoming particle flux distribution towards the substrate during deposition has been attributed to the anomalous growth mode. The optical properties of the films, extracted from ellipsometric analysis, were also correlated with RMS roughness and cluster size variation which unveiled the important role played by surface roughness and film density.

MoS₂ films grown on glass by pulsed laser deposition technique evolve from bilayer to bulk-like structure with time following intrinsic anomalous scaling behaviour caused by shadowing effect during deposition.

Multi-facets of kinetic roughening of interfaces

In this review, the authors are going to explore the intriguing aspects of kinetic roughening of interfaces. Interface roughness dynamics connected with various physical processes have been studied through novel microscopic models in connection with experiments. The statistical properties of such rough interfaces appearing in wide range of physical systems are observed to belong to different universality classes characterized by the scaling exponents. With the advancement of characterization techniques, the scaling exponents of thin-film surface (or the morphological evolution of amorphous surfaces eroded by ion bombardment) are easily computed even in situ during the growing (erosion) conditions. The relevant key physical parameters during the dynamics crucially control the overall scaling behaviour as well as the scaling exponents. The non-universal nature of scaling exponents is emphasized on the variations of the physical parameters in experimental studies and also in theoretical models. Overall, this review containing both theoretical and experimental results will unfold some novel features of surface morphology and its evolution and shed important directions to build an appropriate theoretical framework to explain the observations in systematic and consistent experiments.

Fabrication of N-doped multidimensional carbon nanofibers for high-performance cortisol biosensors

Cortisol is an hormone that regulates blood pressure, glucose levels and carbohydrate metabolism in humans. Abnormal secretion of cortisol can cause various symptoms closely linked to psychological and physical health. In this study, high-performance field-effect transistor (FET)-based biosensors for cortisol detection were fabricated from N-doped multidimensional carbon nanofibers. Nanofiber morphology was controlled by tailoring the pressure conditions during vapor deposition polymerization (VDP). Thereafter, conductive channels of FET were completed by thermal annealing, acid treatment, and antibody attachment. Changes associated with chemical processes were characterized by various instruments. The resulting transducers exhibited a rapid response toward cortisol molecules with accurate selectivity, stable reusability, and high sensitivity. Minimum detection level were as low as 100 aM with a wide linear detection range of 100 aM to 10 nM due to the large surface area of the transducer and a correspondingly high number of antibody labels. The response and applicability of these cortisol biosensors were also assessed using saliva as a test matrix.

Nonuniversality of front fluctuations for compact colonies of nonmotile bacteria

The front of a compact bacterial colony growing on a Petri dish is a paradigmatic instance of non-equilibrium fluctuations in the celebrated Eden, or Kardar-Parisi-Zhang (KPZ), universality class. While in many experiments the scaling exponents crucially differ from the expected KPZ values, the source of this disagreement has remained poorly understood. We have performed growth experiments with B. subtilis 168 and E. coli ATCC 25922 under conditions leading to compact colonies in the classically alleged Eden regime, where individual motility is suppressed. Non-KPZ scaling is indeed observed for all accessible times, KPZ asymptotics being ruled out for our experiments due to the monotonic increase of front branching with time. Simulations of an effective model suggest the occurrence of transient nonuniversal scaling due to diffusive morphological instabilities, agreeing with expectations from detailed models of the relevant biological reaction-diffusion processes.

Growth kinetics of plasma-polymerized films

The growth kinetics of polymer thin films prepared by plasma-based deposition method were explored using atomic force microscopy. The growth behavior of the first layer of the polythiophene somewhat differs from that of the other layers because the first layer is directly deposited on the substrate, whereas the other layers are deposited on the polymer itself. After the deposition of the first layer, each layer is formed with a cycle of 15 s. The present work represents the growth kinetics of the plasma-polymerized films and could be helpful for further studies on growth kinetics in other material systems as well as for applications of plasma-polymerized thin films.

A bifractal nature of reticular patterns induced by oxygen plasma on polymer films

Plasma etching was demonstrated to be a promising tool for generating self-organized nano-patterns on various commercial films. Unfortunately, dynamic scaling approach toward fundamental understanding of the formation and growth of the plasma-induced nano-structure has not always been straightforward. The temporal evolution of self-aligned nano-patterns may often evolve with an additional scale-invariance, which leads to breakdown of the well-established dynamic scaling law. The concept of a bifractal interface is successfully applied to reticular patterns induced by oxygen plasma on the surface of polymer films. The reticular pattern, composed of nano-size self-aligned protuberances and underlying structure, develops two types of anomalous dynamic scaling characterized by super-roughening and intrinsic anomalous scaling, respectively. The diffusion and aggregation of short-cleaved chains under the plasma environment are responsible for the regular distribution of the nano-size protuberances. Remarkably, it is uncovered that the dynamic roughening of the underlying structure is governed by a relaxation mechanism described by the Edwards-Wilkinson universality class with a conservative noise. The evidence for the basic phase, characterized by the negative roughness and growth exponents, has been elusive since its first theoretical consideration more than two decades ago.

On growth and form of Bacillus subtilis biofilms

A general feature of mature biofilms is their highly heterogeneous architecture that partitions the microbial city into sectors with specific micro-environments. To understand how this heterogeneity arises, we have investigated the formation of a microbial community of the model organism Bacillus subtilis. We first show that the growth of macroscopic colonies is inhibited by the accumulation of ammoniacal by-products. By constraining biofilms to grow approximately as two-dimensional layers, we then find that the bacteria which differentiate to produce extracellular polymeric substances form tightly packed bacterial chains. In addition to the process of cellular chaining, the biomass stickiness also strongly hinders the reorganization of cells within the biofilm. Based on these observations, we then write a biomechanical model for the growth of the biofilm where the cell density is constant and the physical mechanism responsible for the spreading of the biomass is the pressure generated by the division of the bacteria. Besides reproducing the velocity field of the biomass across the biofilm, the model predicts that, although bacteria divide everywhere in the biofilm, fluctuations in the growth rates of the bacteria lead to a coarsening of the growing bacterial layer. This process of kinetic roughening ultimately leads to the formation of a rough biofilm surface exhibiting self-similar properties. Experimental measurements of the biofilm texture confirm these predictions.

Local diffusion induced roughening in cobalt phthalocyanine thin film growth

We have studied the kinetic roughening in the growth of cobalt phthalocyanine (CoPc) thin films grown on SiO2/Si(001) surfaces as a function of the deposition time and the growth temperature using atomic force microscopy (AFM). We have observed that the growth exhibits the formation of irregular islands, which grow laterally as well as vertically with coverage of CoPc molecules, resulting rough film formation. Our analysis further disclosed that such formation is due to an instability in the growth induced by local diffusion of the molecules following an anomalous scaling behavior. The instability relates the (ln(t))(1/2), with t as deposition time, dependence of the local surface slope as described in nonequilibrium film growth. The roughening has been characterized by calculating different scaling exponents α, β, and 1/z determined from the height fluctuations obtained from AFM images. We obtained an average roughness exponent α = 0.78 ± 0.04. The interface width (W) increases following a power law as W ∼ t(β), with growth exponent β = 0.37 ± 0.05 and lateral correlation length (ξ) grows as ξ ∼ t(1/z) with dynamic exponent 1/z = 0.23 ± 0.06. The exponents revealed that the growth belongs to a different class of universality.

Molecular dynamics simulations of the growth of poly(chloro-para-xylylene) films

Parylene C, poly(chloro-para-xylylene) is the most widely used member of the parylene family due to its excellent chemical and physical properties. In this work we analyzed the formation of the parylene C film using molecular mechanics and molecular dynamics methods. A five unit chain is necessary to create a stable hydrophobic cluster and to adhere to a covered surface. Two scenarios were deemed to take place. The obtained results are consistent with a polymer film scaling growth mechanism and contribute to the description of the dynamic growth of the parylene C polymer.

Mechanism for the formation of isolated poly(p-xylylene) fibrous structures under shadowing growth

Because of the low sticking coefficient, conventional parylene deposition is known to achieve the conformal coating on corrugated or patterned surfaces. However, recently, it has been shown that in contrary to the conformal coating, extremely nonconformal and isolated fibrous parylene structures can be formed on surfaces if it is deposited at an oblique angle using a directional flux. We demonstrate that directional flux can create a high local vapor pressure facing the flux, while the reflection of monomers because of a small sticking coefficient would generate a background vapor pressure. The parylene oblique angle deposition is a combination of the shadowing growth and a much slower conformal coating process, which together give rise to the isolated fibrous structure.

Chemical vapor deposition of conformal, functional, and responsive polymer films

Chemical vapor deposition (CVD) polymerization utilizes the delivery of vapor-phase monomers to form chemically well-defined polymeric films directly on the surface of a substrate. CVD polymers are desirable as conformal surface modification layers exhibiting strong retention of organic functional groups, and, in some cases, are responsive to external stimuli. Traditional wet-chemical chain- and step-growth mechanisms guide the development of new heterogeneous CVD polymerization techniques. Commonality with inorganic CVD methods facilitates the fabrication of hybrid devices. CVD polymers bridge microfabrication technology with chemical, biological, and nanoparticle systems and assembly. Robust interfaces can be achieved through covalent grafting enabling high-resolution (60 nm) patterning, even on flexible substrates. Utilizing only low-energy input to drive selective chemistry, modest vacuum, and room-temperature substrates, CVD polymerization is compatible with thermally sensitive substrates, such as paper, textiles, and plastics. CVD methods are particularly valuable for insoluble and infusible films, including fluoropolymers, electrically conductive polymers, and controllably crosslinked networks and for the potential to reduce environmental, health, and safety impacts associated with solvents. Quantitative models aid the development of large-area and roll-to-roll CVD polymer reactors. Relevant background, fundamental principles, and selected applications are reviewed.

Dynamic scaling study of vapor deposition polymerization: a Monte Carlo approach

The morphological scaling properties of linear polymer films grown by vapor deposition polymerization are studied by 1+1D Monte Carlo simulations. The model implements the basic processes of random angle ballistic deposition (F) , free-monomer diffusion (D) and monomer adsorption along with the dynamical processes of polymer chain initiation, extension, and merger. The ratio G=D/F is found to have a strong influence on the polymer film morphology. Spatial and temporal behavior of kinetic roughening has been extensively studied using finite-length scaling and height-height correlations H(r,t). The scaling analysis has been performed within the no-overhang approximation and the scaling behaviors at local and global length scales were found to be very different. The global and local scaling exponents for morphological evolution have been evaluated for varying free-monomer diffusion by growing the films at G=10 , 10(2), 10(3), and 10(4) and fixing the deposition flux F. With an increase in G from 10 to 10(4), the average growth exponent beta approximately 0.50 was found to be invariant, whereas the global roughness exponent alpha(g) decreased from 0.87 (1) to 0.73 (1) along with a corresponding decrease in the global dynamic exponent z(g) from 1.71(1) to 1.38(2). The global scaling exponents were observed to follow the dynamic scaling hypothesis, z(g)=alpha(g)/beta. With a similar increase in G however, the average local roughness exponent alpha(l) remained close to 0.46 and the anomalous growth exponent beta(*) decreased from 0.23(4) to 0.18(8). The interfaces display anomalous scaling and multiscaling in the relevant height-height correlations. The variation in H(r,t) with deposition time t indicates nonstationary growth. A comparison has been made between the simulational findings and the experiments wherever applicable.

Scaling of surface roughness and polymer structure in a model for film growth and polymerization

We study a model of growth of polymer films using numerical simulations and scaling concepts. During the deposition, each new monomer flows in a direction perpendicular to the substrate, aggregates at the first contact with the deposit and executes up to G steps along the polymers, propagating an existing chain or nucleating of a new polymer. Some qualitative results agree with those of a previous model for vapor deposition polymerization (VDP) with collective diffusion, such as the roughness increase and density decrease with G . This supports the interpretation of G as a ratio between diffusion coefficient and monomer flux. We perform a systematic study of scaling properties of the outer surface roughness and of polymer size and shape. For large G , the polymers are stretched in the direction perpendicular to the substrate and have typical size increasing as G(1/2). This is explained by the solution of the problem of random walk trapping, which illustrates the connection of surface processes and bulk properties. The distributions of polymer sizes are monotonically decreasing for all G and very broad, thus a large number of small chains and of chains much larger than the average is found in typical samples. The outer surface roughness obeys Kardar-Parisi-Zhang scaling, in contrast to the apparent anomalous scaling of previous VDP models with oblique monomer flux. However, the calculation of reliable exponents requires accounting for huge finite-size corrections. Possible applications and extensions of this model are discussed.

Anomalous scaling for thick electrodeposited films

Anomalous surface roughness scaling, where both the local and the large-scale roughness show a power-law dependence on the film thickness, has been widely observed. Here we show that the value of the local roughness exponent in the early stages of Cu electrodeposition depends on the deposition potential. However, initial anomalous scaling can lead to two qualitatively different types of behavior for large film thickness (t>/ or =4 microm). For Cu films electrodeposited with forced convection at high potential and current density, the anomalous scaling is transient: the local roughness saturates for the thickest films studied. When Cu films are electrodeposited at similar potential and current density but with reduced convection, no saturation of the local roughness is observed. Instead the film forms overhangs such that the surface height becomes a multivalued function of the lateral position.

Rapid roughening in thin film growth of an organic semiconductor (diindenoperylene)

The scaling exponents alpha, beta, and 1/z in thin films of the organic molecule diindenoperylene deposited on SiO2 under UHV conditions are determined. Atomic-force microscopy, x-ray reflectivity, and diffuse x-ray scattering were employed. The surface width displays power law scaling over more than 2 orders of magnitude in film thickness. We obtained alpha = 0.684+/-0.06, beta = 0.748+/-0.05, and 1/zeta = 0.92+/-0.20. The derived exponents point to an unusually rapid growth of vertical roughness and lateral correlations. We suggest that they could be related to lateral inhomogeneities arising from the formation of grain boundaries between tilt domains in the early stages of growth.

Scaling of the interface roughness in Fe-Cr superlattices: self-affine versus non-self-affine

We have analyzed kinetic roughening in Fe-Cr superlattices by energy-filtered transmission electron microscopy. The direct access to individual interfaces provides both static and dynamic roughness exponents. We find an anomalous non-self-affine scaling of the interface roughness with a time dependent local roughness at short length scales. While the deposition conditions affect strongly the long-range dynamics, the anomalous short-range exponent remains unchanged. The different short- and long-range dynamics outline the importance of long-range interactions in kinetic roughening.