Coalescence and percolation in thin metal films

Control of Ge island coalescence for the formation of nanowires on silicon

Germanium nanowires could be the building blocks of hole-spin qubit quantum computers. Selective area epitaxy enables the direct integration of Ge nanowires on a silicon chip while controlling the device design, density, and scalability. For this to become a reality, it is essential to understand and control the initial stages of the epitaxy process. In this work, we highlight the importance of surface treatment in the reactor prior to growth to achieve high crystal quality and connected Ge nanowire structures. In particular, we demonstrate that exposure to AsH3 during the high-temperature treatment enhances lateral growth of initial Ge islands and promotes faster formation of continuous Ge nanowires in trenches. The Kolmogorov-Johnson-Mehl-Avrami crystallization model supports our explanation of Ge coalescence. These results provide critical insight into the selective epitaxy of horizontal Ge nanowires on lattice-mismatched Si substrates, which can be translated to other material systems.

Time-Resolved Morphology and Kinetic Studies of Pulsed Laser Deposition-Grown Pt Layers on Sapphire at Different Growth Temperatures by in Situ Grazing Incidence Small-Angle X-ray Scattering

Optimizing and monitoring the growth conditions of Pt films, often used as bottom electrodes in multiferroic material systems, represents a highly relevant issue that is of importance for controlling the crystalline quality and performance of ferroelectric oxides such as, e.g. LuFeO₃. We performed a time-resolved monitoring of the growth and morphology of Pt films during pulsed laser deposition (PLD) in dependence on the grown film effective thickness and on the growth temperature Tg using in situ grazing incidence small-angle X-ray scattering (GISAXS). Through real-time analysis and modeling of GISAXS patterns, we could fully characterize the influence of Tg on the morphology and on the growth kinetics of the Pt layers. Consequently, critical and characteristic effective thicknesses for the transitions nucleation phase (I)/coalescence phase (II) and coalescence phase (II)/coarsening phase (III) could be determined. In combination with complementary microscopic imaging and chemical mapping via combined SEM/EDXS, we demonstrate the occurrence of a morphological progression in the Pt PLD-grown Pt films, changing from grains at room temperature to a 3D-island morphology at 300 °C, further to a hole-free structure at 500 °C, and finally to a channel structure for 700 and 900 °C. The film topography, as characterized by atomic force microscopy (AFM), favors the PLD growth of Pt layers at temperatures beyond 700 °C where the film is homogeneous, continuous, and hole-free with a flat and smooth surface. The double dependency of the percolation transition on the film effective thickness and on the growth temperature has been established by measuring the electrical conductivity.

Morphology Evolution of Nanoscale-Thick Au/Pd Bimetallic Films on Silicon Carbide Substrate

Bimetallic Au/Pd nanoscale-thick films were sputter-deposited at room temperature on a silicon carbide (SiC) surface, and the surface-morphology evolution of the films versus thickness was studied with scanning electron microscopy. This study allowed to elucidate the Au/Pd growth mechanism by identifying characteristic growth regimes, and to quantify the characteristic parameters of the growth process. In particular, we observed that the Au/Pd film initially grew as three-dimensional clusters; then, increasing Au/Pd film thickness, film morphology evolved from isolated clusters to partially coalesced wormlike structures, followed by percolation morphology, and, finally, into a continuous rough film. The application of the interrupted coalescence model allowed us to evaluate a critical mean cluster diameter for partial coalescence, and the application of Vincent's model allowed us to quantify the critical Au/Pd coverage for percolation transition.

Development of a superhydrophobic electrospun poly(vinylidene fluoride) web via plasma etching and water immersion for energy harvesting applications

Flexible and breathable electrospun PVDF web with both piezoelectricity and superhydrophobicity for smart textiles was fabricated by a simple process, plasma etching and water immersion, without any additional coatings.

Disposable Plasmonics: Rapid and Inexpensive Large Area Patterning of Plasmonic Structures with CO₂ Laser Annealing

We present a method of direct patterning of plasmonic nanofeatures on glass that is fast, scalable, tunable, and accessible to a wide range of users-a unique combination in the context of current nanofabrication options for plasmonic devices. These benefits are made possible by the localized heating and subsequent annealing of thin metal films using infrared light from a commercial CO2 laser system. This approach results in patterning times of 30 mm(2)/min with an average cost of $0.10/mm(2). Colloidal Au nanoparticles with diameters between 15 and 40 nm can be formed on glass surfaces with x-y patterning resolutions of ∼180 μm. While the higher resolution provided by lithography is essential in many applications, in cases where the spatial patterning resolution threshold is lower, commercial CO2 laser processing can be 30-fold faster and 400-fold less expensive.

Effect of polar surface on the growth of Au

The Au particles are more thermally stable on faceted MgO(111) than on MgO(100).

Critical island size, scaling, and ordering in colloidal nanoparticle self-assembly

In order to obtain a better understanding of short-range (SR) and long-range (LR) nanoparticle (NP) interactions during the self-assembly of dodecanethiol-coated Au NPs in toluene via drop drying, we have investigated the dependence of the island density, scaled island-size distribution (ISD), and scaled capture-zone distribution (CZD) on coverage, deposition flux, and NP size. Our results indicate that, while the critical island size is larger than 1 for all NP sizes studied, due to the increase in the strength of the SR attraction between NPs with increasing NP size, both the exponent describing the dependence of the island density on deposition flux and the critical island-size decrease with increasing NP size. We also find that, despite the existence of significant cluster diffusion and coalescence, the ISD is sharply peaked as in epitaxial growth. In particular, for large NP size, we find good agreement between the scaled ISD and epitaxial growth models as well as good agreement between the scaled CZD and scaled ISD. However, for smaller NPs the scaled ISD is less sharply peaked despite the fact that the critical island size is larger. This latter result suggests that in this case additional effects such as enhanced island coalescence or NP detachment from large islands may play an important role. Results for the ordering of NP islands are also presented which indicate the existence of LR repulsive interactions. One possible mechanism for such an interaction is the existence of a small dipole moment on each NP which arises as a result of an asymmetry, driven by surface tension, in the thiol distribution for NPs adsorbed at the toluene-air interface. Consistent with this mechanism, we find good agreement between experimental results for the nearest-neighbor island-distance distribution and simulations which include dipole repulsion.

Smart composite nanosheets with adaptive optical properties

We report efficient design and facile synthesis of size-tunable organic/inorganic nanosheets, via a straightforward liquid exfoliation-adsorption process, of a near percolating gold (Au) thin film deposited onto a branched polyethylenimine (bPEI) matrix. The nanosheets are stiff enough to sustain their two-dimensional (2D) nature in acidic conditions, yet flexible enough to undergo a perfect reversible shape transformation to 1D nanoscrolls in alkaline conditions. The shape transformations, and associated optical property changes, at different protonation states are monitored by transmission electron microscopy (TEM), atomic force microscopy (AFM), UV-visible spectroscopy and zeta potential measurements. Because of their large surface area, both nanosheets and nanoscrolls could be used as capturing substrates for surface-enhanced Raman scattering (SERS) applications.

Comparison of nanosecond and femtosecond pulsed laser deposition of silver nanoparticle films

Nanoparticle (NP) films of silver were prepared using nanosecond (ns) and femtosecond (fs) pulsed laser deposition (PLD) in vacuum. The flux and energy distribution of the ions in the plasma part of the ablation plume were measured using a Langmuir ion probe. The deposition energy efficiencies of ns and fs silver PLD were also compared. For equivalent thickness up to ∼3 nm the NPs made by ns-PLD are well separated and roughly circular, but for higher thicknesses the NPs begin to coalesce. For equivalent thickness up to 7 nm the fs films are comprised of well separated NPs, though the mean NP size and the surface coverage increase with equivalent thickness. The mean Feret diameter for both ns- and fs-PLD films increases with increasing equivalent solid-density thickness. The surface plasmon resonance peak was observed to red shift for both ns- and fs-PLD films as the equivalent solid-density thickness was increased from 1 nm to 7 nm.

Bimodal island size distribution in heteroepitaxial growth

A bimodal size distribution of two-dimensional islands is inferred during interface formation in heteroepitaxial growth of bismuth ferrite on (001) oriented SrTiO3 by sputter deposition. Features observed by in situ x-ray scattering are explained by a model where coalescence of islands determines the growth kinetics with negligible surface diffusion on SrTiO3. Small clusters maintain a compact shape as they coalesce, while clusters beyond a critical size impinge to form large irregular connected islands and a population of smaller clusters forms in the spaces between the larger ones.

Quantized conductance and switching in percolating nanoparticle films

We demonstrate switching behavior and quantized conductance at room temperature in percolating films of nanoparticles. Our experiments and complementary simulations show that switching and quantization result from the formation of atomic-scale wires in the gaps between particles. These effects occur only when tunnel gaps are present in the film, close to the percolation threshold.

Gold nanoisland arrays by repeated deposition and post-deposition annealing for surface-enhanced Raman spectroscopy

Gold nanoisland arrays with well controlled growth were achieved by repeated sputtering deposition and post-deposition annealing processes. When each deposition was set at 5 nm (nominal thickness based on gold mass), the single deposition and annealing process (single process) yielded gold nanoisland arrays with an average diameter of ~16 nm based on top view scanning electron microscopy (SEM). When the deposition and annealing process was repeated two more times (triple process), top view SEM showed the nanoisland arrays grew to ~38 nm in average diameter. The surface-enhanced Raman spectroscopy (SERS) measurement indicated that triple processed nanoisland arrays led to the highest SERS enhancement, suggesting the necessity of pursuing nanoislands with larger sizes. The gold nanoisland arrays after the triple process were further sputtered with a final layer of gold thin film at different nominal thicknesses. An optimal nominal thickness for SERS was determined experimentally at ~40 nm, as a result of the competition between the positive and negative effects of the final gold deposition. Last, the uniformity of the optimized SERS substrate was investigated on a 5 cm x 5 cm platform. SERS measurements demonstrated a relative standard deviation of ~7% in terms of spectral variation over the entire substrate, rendering the process in the present study a promising fabrication approach for large-scale production of SERS substrates.

Growth kinetics and size-dependent wetting of Ag/α-Al₂O₃(0001) nanoparticles studied via the plasmonic response

The growth of vapour-deposited silver nanoparticles on α-Al₂O₃ was studied in situ from 190 to 675 K by surface differential reflectivity spectroscopy in the UV-visible range. Changes in size, shape and density were derived from the plasmonic response modelled in the framework of interface susceptibilities by assuming that supported clusters were in the form of truncated spheres. The sticking coefficient of silver on alumina is close to one up to T ≃ 575 K before entering a regime of incomplete condensation. The Arrhenius dependence of the saturation density indicates a nucleation on defects at low temperature (T ≤ 300 K) and detrapping above. The particle size D evolution follows temporal power laws, independent of temperature and flux, which characterize the growth (D ∼ t(0.31)) and coalescence (D ∼ t(0.55)) of the film. These are indicative of the growth of isolated particles at constant density and dynamic coalescence, respectively. The wetting angle of the silver clusters is shown to increase during the growth regime, which is assigned to a combination of surface stress and mismatch-induced strain, and to decrease upon coalescence, which is attributed to plastic relaxation. For particles larger than 10 nm in size, the values of contact angle and adhesion energy level off with asymptotic limits (θ(c) = 127.5° ± 1° and 0.48 ± 0.02 J m⁻²) that nicely agree with tabulated data. This work highlights the ability of nanoplasmics to monitor in situ the growth kinetics of thin supported films.

Percolation and jamming in structures built through sequential deposition of particles

The strength of attractive interaction among particles on a surface, which was studied in our previous work, leads to different degrees of clustering and ordering. A growing structure percolates when all clusters connect and become one and finally the structure is jammed when there is no space large enough to accommodate one more particle. The lowest jamming limit reported is for structures from the random sequential adsorption. We studied here, by means of Monte Carlo simulation, structures built through sequential deposition of particles, into which surface diffusion and various degrees of attractive forces are incorporated and reported jamming limits along with the percolation thresholds. The higher the strength of attractive interactions, the larger the percolation densities and jamming limits are. These results were shown in a diagram as a function of temperature (or equivalently the strength of attractive interaction), ranging from very low temperature to very high temperature (RSA limit).

Particle connectedness and cluster formation in sequential depositions of particles: Integral-equation theory

We applied the integral-equation theory to the connectedness problem. The method originally applied to the study of continuum percolation in various equilibrium systems was modified for our sequential quenching model, a particular limit of an irreversible adsorption. The development of the theory based on the (quenched-annealed) binary-mixture approximation includes the Ornstein-Zernike equation, the Percus-Yevick closure, and an additional term involving the three-body connectedness function. This function is simplified by introducing a Kirkwood-like superposition approximation. We studied the three-dimensional (3D) system of randomly placed spheres and 2D systems of square-well particles, both with a narrow and with a wide well. The results from our integral-equation theory are in good accordance with simulation results within a certain range of densities.