Deposition, diffusion, and aggregation of atoms on surfaces: A model for nanostructure growth

A theory of coalescence of signaling receptor clusters in immune cells

A theory of coalescence of signal receptor clusters in mast cells is developed in close connection with experiments. It is based on general considerations involving a feedback procedure and a time-dependent capture as part of a reaction-diffusion process. Characteristic features of observations that need to be explained are indicated and it is shown why calculations available in the literature are not satisfactory. While the latter involves static centers at which the reaction part of the phenomenon occurs, by its very nature, coalescence involves dynamically evolving centers. This is so because the process continuously modifies the size of the cluster aggregate which then proceeds to capture more material. We develop a procedure that consists of first solving a static reaction-diffusion problem and then imbuing the center with changing size. The consequence is a dependence of the size of the signal receptor cluster aggregate on time. A preliminary comparison with experiment is shown to reveal a sharp difference between theory and data. The observation indicates that the reaction occurs slowly at first and then picks up rapidly as time proceeds. Parameter modification to fit the observations cannot solve the problem. We use this observation to build into the theory an accumulation rate that is itself dependent on time. A memory representation and its physical basis are explained. The consequence is a theory that can be fit to observations successfully.

Catalyst-free growth of single- to few-layered graphene on ionic liquid surfaces at room temperature

Single- to few-layered graphene is successfully fabricated on ionic liquid surfaces by a modified arc-discharge evaporation method without the assistance of catalysts and at room temperature.

Non-ohmic behavior and resistive switching of Au cluster-assembled films beyond the percolation threshold

Networks based on nanoscale resistive switching junctions are considered promising for the fabrication of neuromorphic computing architectures. To date random networks of nanowires, nanoparticles, and metal clusters embedded in a polymeric matrix or passivated by shell of ligands or oxide layers have been used to produce resistive switching systems. The strategies applied to tailor resistive switching behavior are currently based on the careful control of the volume fraction of the nanoscale conducting phase that must be fixed close to the electrical percolation threshold. Here, by blending laboratory and computer experiments, we demonstrate that metallic nanostructured Au films fabricated by bare gold nanoparticles produced in the gas phase and with thickness well beyond the electrical percolation threshold, show a non-ohmic electrical behavior and complex and reproducible resistive switching. We observe that the nanogranular structure of the Au films does not evolve with thickness: this introduces a huge number of defects and junctions affecting the electrical transport and causing a dynamic evolution of the nanoscale electrical contacts under the current flow. To uncover the origin of the resistive switching behavior in Au cluster-assembled films, we developed a simple computational model for determining the evolution of a model granular film under bias conditions. The model exploits the information provided by experimental investigation about the nanoscale granular morphology of real films. Our results show that metallic nanogranular materials have functional properties radically different from their bulk counterparts, in particular nanostructured Au films can be fabricated by assembling bare gold clusters which retain their individuality to produce an all-metal resistive switching system.

Substrate-independent and catalyst-free synthesis of magnesium nanowires

We report a catalyst free and substrate independent synthesis of magnesium nanowires using a simple thermal evaporation method. The produced Mg nanowires have a size of 8-60 nm with a crystalline MgO shell of ∼2-5 nm thickness. The synthesized nanowires grow along the [001] direction and horizontal to the substrate. Moreover, from ex situ TEM investigation the various sequential stages involved in the nanowire formation process were identified. The experimental outcome indicates the sequential stages including (i) formation of Mg nanoparticles, (ii) coarsening of Mg nanoparticles to microparticles via deposition diffusion aggregation (DDA) and the orientation attachment (OA) process, and (iii) nucleation and growth of Mg nanowires. In depth analysis confirms two types of nanowires, straight and serpentine-like, where the latter dominates as the holding duration/temperature of the synthesis increases. The straight nanowires are formed by the direct attachment of nanodroplets from the core to the surface and serpentine-like wires are formed on the surface of a microparticle which is in a quasi-melted state. Moreover, nanowires were produced by confining the Mg vapour to the substrate using a curved quartz bottle, thereby controlling the supersaturation in the absence of any inert/reactive gas during the synthesis. Our synthesis method is cost effective and can be applied to other low melting point elements for producing various nanostructures. Finally based on the experimental results a possible growth mechanism is proposed.

UV-Protective TiO2 Thin Films with High Transparency in Visible Light Region Fabricated via Atmospheric-Pressure Plasma-Enhanced Chemical Vapor Deposition

This article focuses on control of film thickness and roughness to improve the ultraviolet (UV)-protective performance of TiO₂ films prepared by atmospheric-pressure plasma-enhanced chemical vapor deposition using titanium(IV) isopropoxide (TTIP) as the precursor and argon as the plasma working gas. The relationship between the film morphology and UV-protective performance suggested that a decrease in roughness is the key factor to achieve performance improvement. The effects of substrate temperature and precursor concentration were investigated, and the results showed that an increase in both substrate temperature and precursor concentration reduced the roughness and improved the transparency to visible light without reducing the ability to block UV light. Finally, a TiO₂ film with greater than 99% UV light blockage and greater than 95% transmittance of visible light was obtained.

Formation of nanostructured metallic glass thin films upon sputtering

Morphology evolution of the multicomponent metallic glass film obtained by radio frequency (RF) magnetron sputtering was investigated in the present work. Two modes of metallic glass sputtering were distinguished: smooth film mode and clustered film mode. The sputtering parameters, which have the most influence on the sputtering modes, were determined. As a result, amorphous Ni-Nb thin films with a smooth surface and nanoglassy structure were deposited on silica float glass and Si substrates. The phase composition of the target appeared to have a significant influence on the chemical composition of the deposited amorphous thin film. The differences in charge transport and nanomechanical properties between the smooth and nanoglassy Ni-Nb film were also determined.

Site-specific colloidal crystal nucleation by template-enhanced particle transport

The monomer surface mobility is the single most important parameter that decides the nucleation density and morphology of islands during thin-film growth. During template-assisted surface growth in particular, low surface mobilities can prevent monomers from reaching target sites and this results in a partial to complete loss of nucleation control. Whereas in atomic systems a broad range of surface mobilities can be readily accessed, for colloids, owing to their large size, this window is substantially narrow and therefore imposes severe restrictions in extending template-assisted growth techniques to steer their self-assembly. Here, we circumvented this fundamental limitation by designing templates with spatially varying feature sizes, in this case moiré patterns, which in the presence of short-range depletion attraction presented surface energy gradients for the diffusing colloids. The templates serve a dual purpose: first, directing the particles to target sites by enhancing their surface mean-free paths and second, dictating the size and symmetry of the growing crystallites. Using optical microscopy, we directly followed the nucleation and growth kinetics of colloidal islands on these surfaces at the single-particle level. We demonstrate nucleation control, with high fidelity, in a regime that has remained unaccessed in theoretical, numerical, and experimental studies on atoms and molecules as well. Our findings pave the way for fabricating nontrivial surface architectures composed of complex colloids and nanoparticles as well.

Surface effects mediate self-assembly of amyloid-β peptides

Here we present a label-free method for studying the mechanism of surface effects on amyloid aggregation. In this method, spin-coating is used to rapidly dry samples, in a homogeneous manner, after various incubation times. This technique allows the control of important parameters for self-assembly, such as the surface concentration. Atomic force microscopy is then used to obtain high-resolution images of the morphology. While imaging under dry conditions, we show that the morphologies of self-assembled aggregates of a model amyloid-β peptide, Aβ(12-28), are strongly influenced by the local surface concentration. On mica surfaces, where the peptides can freely diffuse, homogeneous, self-assembled protofibrils formed spontaneously and grew longer with longer subsequent incubation. The surface fibrillization rate was much faster than the rates of fibril formation observed in solution, with initiation occurring at much lower concentrations. These data suggest an alternative pathway for amyloid formation on surfaces where the nucleation stage is either bypassed entirely or too fast to measure. This simple preparation procedure for high-resolution atomic force microscopy imaging of amyloid oligomers and protofibrils should be applicable to any amyloidogenic protein species.

Performance of a size-selected nanocluster deposition facility and in situ characterization of grown films by x-ray photoelectron spectroscopy

We report here on a newly installed gas aggregation type nanocluster deposition unit based on magnetron sputtering ion source with mass selection of the clusters by quadrupole mass filter. The system is ultra high vacuum compatible and is equipped with an in situ X-ray Photoelectron Spectroscopy facility, giving compositional information of the films formed by nanoclusters deposition on a substrate. Detailed descriptions and working of the components of the system are presented. For the characterization of the nanocluster source and associated mass filter for size selected clusters, the dependence of output performance as a function of aggregation length, sputter gas flow and magnetron power of the cluster source have been studied. Copper nanoclusters deposited on Silicon (100) surface and on transmission electron microscope grids are, respectively, studied with scanning electron microscopy and transmission electron microscopy for the morphology.

Silanization of quartz, silicon and mica surfaces with light-driven molecular motors: construction of surface-bound photo-active nanolayers

The attachment of molecular rotary motors containing triethoxysilane functional groups to quartz, silicon and mica surfaces is described. Motors containing silane coupling agents in their structure form stable molecular layers on quartz and silicon surfaces. Motors attached to these surfaces were found to undergo photochemical and thermal isomerization steps similar to those observed in solution. Additionally, successful formation of molecular "carpets" on atomically flat mica extending micrometer-sized length scales is presented. These "carpets" were found to undergo morphological changes upon irradiation with UV-light.

Coarsening effect on island-size scaling: the model case InAs/GaAs(001)

The distributions of the size of islands and of the capture zones are discussed comparatively, both experimentally and numerically, for the case of a sudden nucleation process with and without coarsening. The experiments were performed by growing InAs islands on GaAs(001) and the coarsening was altered by varying the temperature. In the two-dimensional kinetic Monte Carlo simulations a single-species diffusing adatom was taken into account, and the coarsening was altered in this case by modifying the binding energy between adatoms and islands. The results show that size and capture zone distributions overlap only when coarsening can be disregarded.

Controlling self-assembly of engineered peptides on graphite by rational mutation

Self-assembly of proteins on surfaces is utilized in many fields to integrate intricate biological structures and diverse functions with engineered materials. Controlling proteins at bio-solid interfaces relies on establishing key correlations between their primary sequences and resulting spatial organizations on substrates. Protein self-assembly, however, remains an engineering challenge. As a novel approach, we demonstrate here that short dodecapeptides selected by phage display are capable of self-assembly on graphite and form long-range-ordered biomolecular nanostructures. Using atomic force microscopy and contact angle studies, we identify three amino acid domains along the primary sequence that steer peptide ordering and lead to nanostructures with uniformly displayed residues. The peptides are further engineered via simple mutations to control fundamental interfacial processes, including initial binding, surface aggregation and growth kinetics, and intermolecular interactions. Tailoring short peptides via their primary sequence offers versatile control over molecular self-assembly, resulting in well-defined surface properties essential in building engineered, chemically rich, bio-solid interfaces.

Formation of peelable rough gold patterns on an ionic liquid template

The ability to control metal patterns at the micro- and nanoscales, along with the development of a simple fabrication method, is important to many applications in the fields of materials science, biological sensing, electronics, and photonics. Herein, a simple approach to fabricating gold micropatterns with controlled roughness is reported. In this approach, gold is evaporated onto a striped liquid micropattern formed on self-organized microwrinkles. Gold nanoribbons with higher roughness form on the liquid part of the substrate because the deposited gold atoms can diffuse, grow, and aggregate at the liquid-air interface, whereas flat gold films form on the solid part. The rough gold nanoribbons formed on the liquid can then be peeled off through contact with water. The extinction spectrum of the rough gold nanoribbons suggests characteristic surface-plasmon absorption. This shows the possibility of using rough gold nanoribbons with controlled shape in plasmonic technology.

Fractal and Dendritic Growth of Surface Aggregates

The similarity of patterns formed in non-equilibrium growth processes in physics, chemistry and biology is conspicuous and many attempts have been made to discover common mechanisms underlying their growth. The central question in this context is what causes some patterns to be dendritic, as e.g. snowflakes, while others grow fractal (randomly ramified). Here we report a crossover from fractal to dendritic patterns for growth in two dimensions: the diffusion limited aggregation of Ag atoms on a Pt(111) surface as observed by means of variable temperature STM. The microscopic mechanism of dendritic growth can be analyzed for the present system. It originates from the anisotropy of the diffusion of adatoms at corner sites which is linked to the trigonal symmetry of the substrate. This corner diffusion is observed to be active as soon as islands form, therefore, the classical DLA clusters with the hit and stick mechanism do not form. The ideas on the mechanism for dendritic growth have been verified by kinetic Monte-Carlo simulations which are in excellent agreement with experiment.

Development of metal nanocluster ion source based on dc magnetron plasma sputtering at room temperature

A simple and cost effective nanocluster ion source for the deposition of size selected metal nanocluster has been developed based on the dc magnetron discharge (including pulsed dc discharge). The most important and interesting feature of this cluster source is that it is working at room temperature, cooled by chilled water during the experiment. There is no extraction unit in this device and the cluster streams flow only due to the pressure gradient from source chamber to substrate via quadrupole mass filter. It has provision of multiple substrate holders in the deposition chamber, which can be controlled manually. The facility consists of quadrupole mass filter (QMF 200), which can select masses in the range of 2-125 000 atoms depending on the target materials, with a constant mass resolution (M/DeltaM approximately 25). The dc magnetron discharge at a power of about 130 W with Ar as feed/buffer gas was used to produce the Cu nanocluster in an aggregation tube and deposited on Si (100) wafer temperature.

New insights on growth mechanisms of protein clusters at surfaces: an AFM and simulation study

Despite its relevance to a wide range of technological and fundamental areas, a quantitative understanding of protein surface clustering dynamics is often lacking. In inorganic crystal growth, surface clustering of adatoms is well described by diffusion-aggregation models. In such models, the statistical properties of the aggregate arrays often reveal the molecular scale aggregation processes. We investigate the potential of these theories to reveal hitherto hidden facets of protein clustering by carrying out concomitant observations of lysozyme adsorption onto mica surfaces, using atomic force microscopy, and Monte Carlo simulations of cluster nucleation and growth. We find that lysozyme clusters diffuse across the substrate at a rate that varies inversely with size. This result suggests which molecular scale mechanisms are responsible for the mobility of the proteins on the substrate. In addition the surface diffusion coefficient of the monomer can also be extracted from the comparison between experiments and simulations. While concentrating on a model system of lysozyme-on-mica, this 'proof of concept' study successfully demonstrates the potential of our approach to understand and influence more biomedically applicable protein-substrate couples.

Inorganic metallodielectric materials fabricated using two single-step methods based on the Tollen's process

Two methods for preparing polycrystalline silver shells on colloidal silica spheres are reported. These do not include the use of organic ligands or metal seeding steps and are based on the Tollen's process for silvering glass. Reaction parameters such as temperature and reactant concentrations are adjusted to slow the reaction kinetics, which we find leads to preferential silver growth on the spheres. The resulting shells are polycrystalline and granular, showing highly uniform sphere coverage. Surface morphologies range from sparsely interconnected grains for shells approximately 20 nm thick, to complete (yet porous) shells of interconnected silver clusters which are up to approximately 140 nm in thickness. The extinction spectra of the core-shell materials are markedly different from those of smooth continuous shells, showing clear evidence that the granular shell geometry influences the plasmon resonance of the composite system. Spheres coated with shells 20-40 nm thick are also suitable for colloidal crystallization. Monolayers of self-assembled spheres with long-range ordering are demonstrated.

Coherent light scattering from semicontinuous silver nanoshells near the percolation threshold

We report on measurements of visible extinction spectra of semicontinuous silver nanoshells grown on colloidal silica spheres. We find that thin, fractal shells below the percolation threshold exhibit geometrically tunable plasmon resonances. A modified scaling theory approach is used to model the dielectric response of such shells, which is then utilized to obtain the extinction cross section in a retarded Mie scattering formalism. We show that such spherical resonators support unique plasmon dynamics: in the visible there is a new regime of coherently driven cluster-localized plasmons, while crossover to homogeneous response in the infrared predicts a delocalized shell plasmon.

Adsorption-induced conformational changes in protein diffusion-aggregation surface assemblies

Two-dimensional rigid colloid aggregation models may be applied to protein layers when no large conformational change is involved. Yet, following adsorption, several proteins undergo a conformational transition that may be involved in aggregative structures. Our focus here is how a conformational change might influence surface clustering in a diffusion-aggregation model. We propose a model including diffusion, aggregation, and unfolding of proteins that are randomly adsorbed onto a surface. Our model allows simulating the case where protein-protein interaction favors unfolding and the case where this interaction prevents it. We study the effect of a simple disk-to-rod unidirectional unfolding and investigate the morphology of the resulting clusters in the diffusion- and reaction-limited regimes. A rich variety of structures is produced, with fractal dimension differing from that in universal diffusive aggregation models. Increasing unfolding probability shifts the system from the neighbor-induced to the neighbor-prevented unfolding regime. The intermediate structures that arise from our model could be helpful in understanding the assembly of different observed protein structures.

Regular Arrays of 2 nm Metal Nanoparticles for Deterministic Synthesis of Nanomaterials

A novel method is developed to enable the formation, positioning, and patterning of individual metal nanoclusters with controllable and monodispersed sizes down to 1-2 nm scale. The method is generic for fabricating designed arrays of virtually any type of metal nanoparticles well below 10 nm. Among the wide range of potential applications in surface and materials science, the nanoparticle arrays are used for deterministic synthesis of monodispersed single-walled carbon nanotubes at individually controlled locations with near 1:1 yield, reaching one of the ultimate goals of nanotube synthesis.

Anomalous roughness with system-size-dependent local roughness exponent

We note that in a system far from equilibrium the interface roughening may depend on the system size which plays the role of control parameter. To detect the size effect on the interface roughness, we study the scaling properties of rough interfaces formed in paper combustion experiments. Using paper sheets of different width lambda L0, we found that the turbulent flame fronts display anomalous multiscaling characterized by nonuniversal global roughness exponent alpha and the system-size-dependent spectrum of local roughness exponents zeta(q) (lambda) = zeta(1) (1) q(-omega) lambda(phi) <alpha, whereas the burning fronts possess conventional multiaffine scaling. The structure factor of turbulent flame fronts also exhibit unconventional scaling dependence on lambda. These results are expected to apply to a broad range of far from equilibrium systems, when the kinetic energy fluctuations exceed a certain critical value.

Influence of adsorption and diffusion rates on the growth of Pb1-x Fex S nanoparticle films

We study the effect of adsorption rate on the particle size distribution in solution-grown ternary Pb1-x Fex S nanoparticle films. Computer simulations of a stochastic lattice model with adsorption and mass dependent diffusion have been performed to mimic the underlying mechanism of particle growth. The experimental as well as numerical data exhibit identical scaling with respect to the incident flux rate. A transmission electron microscope analysis of Pb1-x Fex S nanoparticle films reveals self-similarity in the particle size distributions corresponding to different adsorption rates as a manifestation of the observed scaling.

Percolation in models of thin film depositions

We have studied the percolation behavior of deposits for different (2+1)-dimensional models of surface layer formation. The mixed model of deposition was used, where particles were deposited selectively according to the random (RD) and ballistic (BD) deposition rules. In the mixed one-component models with deposition of only conducting particles, the mean height of the percolation layer (measured in monolayers) grows continuously from 0.898 32 for the pure RD model to 2.605 for the pure BD model, but the percolation transition belongs to the same universality class, as in the two-dimensional (2D) random percolation problem. In two-component models with deposition of conducting and isolating particles, the percolation layer height approaches infinity as concentration of the isolating particles becomes higher than some critical value. The crossover transition from 2D to 3D percolation was observed with increase of the percolation layer height.

Monte Carlo simulation of submonolayer vapor-deposition polymerization

In this paper, we propose a Monte Carlo simulation model for the initial growth of polymer films by considering only monomer surface diffusion in the vapor-deposition polymerization process. In the model, monomers are deposited randomly on a two-dimensional square lattice with periodic boundary conditions and are allowed to diffuse with nearest neighbor hops. Whenever monomers meet, they stop diffusing and form a stable dimer. When a diffusing or deposited monomer encounters one of the ends of a polymer (L>1), it stops moving and attaches to the polymer. Attachment of monomers or other polymers is allowed only at the two ends of the polymer. We have shown that there are three distinct growth regimes for surface coverages theta<0.5: a low-coverage initiation regime (I), a chain propagation regime (P), and a saturation regime (S). In both regimes I and P, the growth is similar to the molecular beam epitaxy model. We examine in detail the scaling relations for the chain length distribution, which agree quite well with results of a rate equation. However, in regime S, our model gives very different kinetics. The breakdown of scaling at higher coverages is due to the fact that long-chain polymers have partitioned the lattice with inactive sites. This inhibits further polymer growth and enhances production of dimers, shifting the chain distribution to favor shorter polymers and driving the average molecular weight down. The chain configuration initially is similar to a path taken in a diffusion-limited self-avoiding walk. However, at high coverages, due to the correlation of long polymer chains, the polymer chains tend to be compact.

An AFM Study of the Effects of Silanization Temperature, Hydration, and Annealing on the Nucleation and Aggregation of Condensed OTS Domains on Mica

Partial monolayers of octadecyltrichlorosilane (OTS) were formed on mica under different reaction conditions in which the silanization temperature, time, and amount of water adsorbed on the mica substrates were varied. OTS surface coverage increased with silanization time for all samples; however, the amount and distribution of adsorbed OTS varied greatly under these different reaction conditions. AFM analysis showed that OTS formed two phases on mica silanized at 25°C: condensed "island-like" domains and expanded "liquid-like" domains. Partially dehydrated mica silanized at 9°C, however, displayed only condensed domains which were of smaller size compared to those on the 25°C samples. The lateral diffusion and aggregation of small condensed OTS domains to form larger aggregates was evident on all surfaces except the 25°C partially dehydrated mica. A uniform distribution of many small condensed domains surrounded by expanded OTS phases was seen instead. Extended annealing resulted in surface diffusion and aggregation of these domains and nucleation of new condensed domains from the surrounding expanded OTS phases. These observations are consistent with a deposition, diffusion, and aggregation model (DDA) which allows for activated diffusion; however, rigorous modeling is not presented here.