Solid-state Dewetting Research Articles

Growth evolution and infrared response of thermally dewetted Au nano-structures for bolometric applications

This paper discusses the growth and characterization of thermally dewetted gold nanostructures for enhanced infrared absorption in bolometer applications. The Au nanostructures are fabricated by solid-state dewetting of ultrathin (18 nm) Au film on Si substrate. The duration of dewetting is varied from 1 min to 30 min. The Au film exhibited the highest X-ray diffraction (XRD) peak intensity along the (111) plane; while the Au nanostructure showed the rising XRD peak along the (220) plane with the dewetting duration. The dewetting process also yielded grain growth and recrystallization of Au nanostructure. The effect of the dewetting on surface morphologies of the Au nanostructure is studied. The Au nanostructures exhibited broadband infrared absorption in the 500–6500 cm−1 range. The sample dewetted for 10 min exhibited up to 200% enhanced absorbance (with respect to the silicon substrate) in the IR range due to the well-suited dimensions of the nanostructures. The hypothesis is also substantiated by examining the plasmonic response of the nanostructure by simulation.

Ion-beam-induced structure fragmentation and dewetting in Au/Ti films

We investigate the formation of Au structures by continuing fragmentation of a Au film separated from Si (100) substrate by a Ti adhesion layer. The ion-beam processing uses 2 keV Cs+ projectiles. It induces the formation of a large variety of structures with dimensions from tens of microns down to a few nanometers, and shapes that include flat Au islands and clusters of Au nanoparticles. We identify different fragmentation mechanisms. In one pathway, Au structures are formed as pinch-off droplets detached from large Au islands undergoing solid-state dewetting. In a second pathway, a continuing cracking fragmentation produces Au structures with a wide range of sizes and shows self-similarity, expressed in a power law for volume distribution of fragments Vα with α = −2.03 ± 0.05. The measured α value is consistent with theoretical results in the literature modeling strong impact fragmentation. In the last stage of fragmentation evolution, Au nanostructures are spread over a texturized Si background. They assume shapes close to hemispherical, reflecting the increasing relevance of surface tension as dimensions decrease.

An energy-stable parametric finite element method for anisotropic surface diffusion

We propose an energy-stable parametric finite element method (ES-PFEM) to discretize the motion of a closed curve under surface diffusion with an anisotropic surface energy γ(θ) – anisotropic surface diffusion – in two dimensions, while θ is the angle between the outward unit normal vector and the vertical axis. By introducing a positive definite surface energy (density) matrix G(θ), we present a new and simple variational formulation for the anisotropic surface diffusion and prove that it satisfies area/mass conservation and energy dissipation. The variational problem is discretized in space by the parametric finite element method and area/mass conservation and energy dissipation are established for the semi-discretization. Then the problem is further discretized in time by a (semi-implicit) backward Euler method so that only a linear system is to be solved at each time step for the full-discretization and thus it is efficient. We establish well-posedness of the full-discretization and identify some simple conditions on γ(θ) such that the full-discretization keeps energy dissipation and thus it is unconditionally energy-stable. Finally the ES-PFEM is applied to simulate solid-state dewetting of thin films with anisotropic surface energies, i.e. the motion of an open curve under anisotropic surface diffusion with proper boundary conditions at the two triple points moving along the horizontal substrate. Numerical results are reported to demonstrate the efficiency and accuracy as well as energy dissipation of the proposed ES-PFEM.

Solid-state thermal dewetted silver nanoparticles onto electrochemically grown self-standing vertically aligned ZnO nanorods for three-dimensional plasmonic nanostructures

Three dimensional (3D) plasmonic nanostructures composed of silver nanoparticles decorated ZnO NRs arrays, have been fabricated by a process combining the electrochemical growth of ZnO NRs and further formation of Ag nanoparticles by the solid-state thermal dewetting (SSD) process. The effect of SSD parameters on the morphological, structural and optical properties of the Ag NPs decorated ZnO NRs arrays has been investigated. It is possible to tune the bandgap of the Ag NPs@ZnO nanorods array 3D plasmonic nanostructure by tailoring the Ag nanoparticle sizes, allowing light manipulation at the nanoscale. The silver nanoparticles attached to the ZnO NRs arrays experienced surface plasmonic coupling effect, causing enhancement in the room temperature photoluminescence (PL) UV emission and quenching the corresponding visible light one. An enhancement in the near band edge emission PL intensity of ZnO to the deep level emission PL intensity ratio after Ag NPs decoration of the ZnO nanostructures corresponding to ca. 11 folds has been observed, indicating that the defect emission is obviously suppressed.

Solid-state synthesis, dewetting, and magnetic and structural characterization of interfacial FexSn1−x layers in Sn/Fe(001) thin films

Solid-state synthesis, dewetting, and magnetic and structural characterization of interfacial FexSn1−x layers in Sn/Fe(001) thin films

Combining in situ heating with transmission diffraction and imaging in SEM for investigation of early stages of solid-state dewetting

Combining in situ heating with transmission diffraction and imaging in SEM for investigation of early stages of solid-state dewetting

Scalable Disordered Hyperuniform Architectures via Nanoimprint Lithography of Metal Oxides.

Fabrication and scaling of disordered hyperuniform materials remain hampered by the difficulties in controlling the spontaneous phenomena leading to this novel kind of exotic arrangement of objects. Here, we demonstrate a hybrid top-down/bottom-up approach based on sol-gel dip-coating and nanoimprint lithography for the faithful reproduction of disordered hyperuniform metasurfaces in metal oxides. Nano- to microstructures made of silica and titania can be directly printed over several cm2 on glass and on silicon substrates. First, we describe the polymer mold fabrication starting from a hard master obtained via spontaneous solid-state dewetting of SiGe and Ge thin layers on SiO2. Then, we assess the effective disordered hyperuniform character of master and replica and the role of the thickness of the sol-gel layer on the metal oxide replicas and on the presence of a residual layer underneath. Finally, as a potential application, we show the antireflective character of titania structures on silicon. Our results are relevant for the realistic implementation over large scales of disordered hyperuniform nano- and microarchitectures made of metal oxides, thus opening their exploitation in the framework of wet chemical assembly.

Ion-Beam-Induced Structure Fragmentation and Dewetting in Au/Ti Films

We investigate the formation of Au structures by continuing fragmentation of a Au film separated from Si (100) substrate by a Ti adhesion layer. The ion-beam processing uses 2 keV Cs+ projectiles. It induces the formation of a large variety of structures with dimensions from tens of microns down to a few nanometers, and shapes that include flat Au islands and clusters of Au nanoparticles. We identify different fragmentation mechanisms. In one pathway, Au structures are formed as pinch-off droplets detached from large Au islands undergoing solid-state dewetting. In a second pathway, a continuing cracking fragmentation produces Au structures with a wide range of sizes and shows self-similarity, expressed in a power law for volume distribution of fragments Vα with α= -2.03±0.05. The measured α value is consistent with theoretical results in the literature modelling strong impact fragmentation. In the last stage of fragmentation evolution, Au nanostructures are spread over a texturized Si background. They assume shapes close to hemispherical, reflecting the increasing relevance of surface tension as dimensions decrease.

Effect of Gold Nanoparticles on Optoelectronic Properties of Thermally Reduced Graphene Oxide

Gold nanoparticles (AuNPs)-decorated reduced graphene oxide (rGO) is a compelling material from both aspects of processing and functionality. This paper presents an alternative method to prepare and alter the optoelectronic properties of the AuNPs-rGO hybrid film. Au was sputtered for a duration of 0–60 s on graphene oxide (GO) film before subjected to thermal reduction at 700∘C. The surface of rGO was decorated by spherical AuNPs with a mean particle size of around 10[Formula: see text]nm due to the solid-state dewetting process. The surface plasmon resonance (SPR) of AuNPs around 550[Formula: see text]nm was intense when the Au sputter duration exceeded 20[Formula: see text]s. Remarkably, the AuNPs layer retarded the de-oxygenation of GO during thermal annealing. The AuNPs-rGO hybrid film with long-duration Au sputtering exhibited low optical constants and thicker rGO film. The AuNPs also played the role in lowering the bandgap, increasing the lattice disorder and assisting the high-energy photon absorption in the thermally reduced rGO film. The sample of 30[Formula: see text]s and 40[Formula: see text]s Au sputtering was optimum to obtain the lowest sheet resistance of 59[Formula: see text]k[Formula: see text]/sq and 56[Formula: see text]k[Formula: see text]/sq, respectively. The sheet resistance was bargained between de-oxygenation retardation and doping by the AuNPs. Both samples of 30 s and 40 s Au sputtering were having a unique and satisfyingly strong absorption band around 260[Formula: see text]nm, 350[Formula: see text]nm and 555[Formula: see text]nm corresponding to [Formula: see text]* transition, [Formula: see text]* transition and SPR of AuNPs, respectively. Such properties are desirable in several applications such as a transparent electrode and optoelectronic sensor.

Retarded solid state dewetting of thin bismuth films with oxide capping layer

We present a study of the impeding influence of a capping, native oxide layer on the solid state dewetting of thin bismuth films on silicon(111) in vacuum. We study the temperature dependence of the film thickness and strain of the thin films through the analysis of crystal truncation rods of clean and capped bismuth films. This analysis reveals a dewetting temperature difference of 40°C between capped and uncapped films. The results are supported by scanning electron microscopy and x-ray photoelectron spectroscopy experiments. Furthermore, a model for the retarding effect of the oxide layer and the final shape of the thin film is presented.

Pulsed laser-induced dewetting and thermal dewetting of Ag thin films for the fabrication of Ag nanoparticles

Two different dewetting methods, namely pulsed laser-induced dewetting (PLiD)—a liquid-state dewetting process and thermal dewetting (TD)—a solid-state dewetting process, have been systematically explored for Ag thin films (1.9–19.8 nm) on Si substrates for the fabrication of Ag nanoparticles (NPs) and the understanding of dewetting mechanisms. The effect of laser fluence and irradiation time in PLiD and temperature and duration in TD were investigated. A comparison of the produced Ag NP size distributions using the two methods of PLiD and TD has shown that both produce Ag NPs of similar size with better size uniformity for thinner films (<6 nm), whereas TD produced bigger Ag NPs for thicker films (≥8–10 nm) as compared to PLiD. As the film thickness increases, the Ag NP size distributions from both PLiD and TD show a deviation from the unimodal distributions, leading to a bimodal distribution. The PLiD process is governed by the mechanism of nucleation and growth of holes due to the formation of many nano-islands from the Volmer−Weber growth of thin films during the sputtering process. The investigation of thickness-dependent NP size in TD leads to the understanding of void initiation due to pore nucleation at the film-substrate interface. Furthermore, the linear dependence of NP size on thickness in TD provides direct evidence of fingering instability, which leads to the branched growth of voids.

In Situ Assembly of Ordered Hierarchical CuO Microhemisphere Nanowire Arrays for High-Performance Bifunctional Sensing Applications.

Seeking a facile approach to directly assemble bridged metal oxide nanowires on substrates with predefined electrodes without the need for complex postsynthesis alignment and/or device procedures will bridge the gap between fundamental research and practical applications for diverse biochemical sensing, electronic, optoelectronic, and energy storage devices. Herein, regularly bridged CuO microhemisphere nanowire arrays (RB-MNAs) are rationally designed on indium tin oxide electrodes via thermal oxidation of ordered Cu microhemisphere arrays obtained by solid-state dewetting of patterned Ag/Cu/Ag films. Both the position and spacing of CuO microhemisphere nanowires can be well controlled by as-used shadow mask and the thickness of Cu film, which allows homogeneous manipulation of the bridging of adjacent nanowires grown from neighboring CuO hemispheres, and thus benefits highly sensitive trimethylamine (TMA) sensors and broad band (UV-visible to infrared) photodetectors. The electrical response of 3.62 toward 100ppm TMA is comparable to that of state-of-the-art CuO-based sensors. Together with the feasibility of in situ assembly of RB-MNAs device arrays via common lithographic technologies, this work promises commercial device applications of CuO nanowires.

Ag droplets nano-shape design on SiC: Study on wetting and energetics

This work describes the effect of the annealing temperature on the size and wetting of nanostructures obtained on 4H–SiC substrate by the dewetting process of a deposited nanoscale-thick Ag film. The work focuses, in particular, on the difference between solid-state dewetting (below Ag melting temperature) and liquid-state dewetting (above Ag melting temperature). Following the deposition of a 58 nm-thick Ag film on the SiC substrate, annealings were perfomed at 800 °C, 850 °C and 885 °C so to induce the Ag film solid-state dewetting. In addition, annealing at 1040 °C was performed so to induce the Ag film liquid-state dewetting. The dewetting processes led to the formation of nanostructures on the SiC surface. Plan-view and cross-view scanning electron microscopy analyses were carried out to quantify the evolution of the average planar size and of the average contact angle (to the SiC surface) of the Ag nanostructures versus the annealing temperature. These analyses allowed us to extract information on the wetting behavior of the nanostructures on the SiC surface by calculating the temperature-dependent adhesion work. In fact, the nanostructures obtained by the 800 °C and 850 °C processes show mean contact angle of 118,9° and 117,4°, respectively, and mean work of adhesion of 0,45 and 0,48 J/m2, respectively. On the other hand, the nanostructures obtained by the 885 °C and 1040 °C show mean contact angle of 48,3° and 52,2° respectively, and work of adhesion of 1,45 and 1,40 J/m2, respectively. The variation of the wetting properties is identified in Si diffusion from the SiC substrate to the Ag nanostructures at the higher temperatures, as confirmed by energy dispersive x-ray analyses. Finally, the optical properties of the Ag nanostructures/SiC systems were probed by absorbance and transmittance measurements allowing to recognize the occurrence of a peak around 460 nm which is attributed to localized surface plasmon resonances due to the Ag nanostructures.

Simultaneous Multi-Bragg Peak Coherent X-ray Diffraction Imaging

The simultaneous measurement of two Bragg reflections by Bragg coherent X-ray diffraction is demonstrated on a twinned Au crystal, which was prepared by the solid-state dewetting of a 30 nm thin gold film on a sapphire substrate. The crystal was oriented on a goniometer so that two lattice planes fulfill the Bragg condition at the same time. The Au 111 and Au 200 Bragg peaks were measured simultaneously by scanning the energy of the incident X-ray beam and recording the diffraction patterns with two two-dimensional detectors. While the former Bragg reflection is not sensitive to the twin boundary, which is oriented parallel to the crystal–substrate interface, the latter reflection is only sensitive to one part of the crystal. The volume ratio between the two parts of the twinned crystal is about 1:9, which is also confirmed by Laue microdiffraction of the same crystal. The parallel measurement of multiple Bragg reflections is essential for future in situ and operando studies, which are so far limited to either a single Bragg reflection or several in series, to facilitate the precise monitoring of both the strain field and defects during the application of external stimuli.

Morphological and compositional mapping of supersaturated AuNi alloy nanoparticles fabricated by solid state dewetting

The solid state dewetting (SSD) of metallic bilayers is a straightforward method for the fabrication of alloy nanoparticles. In particular, alloys that present a gap of miscibility offer a rich phenomenology regarding not only the particle formation but also the composition of their phases. In the present work, AuNi precursor bilayers have been annealed at different temperatures and times to produce AuNi alloy nanoparticles. The evolution of the shape, size, and interparticle distance as well as the composition of the different phases formed in the nanoparticles, allow to unravel the role of the annealing temperatures and times for the fabrication of AuNi supersaturated alloys. Furthermore, the results offer a morphological and compositional map for the fabrication of AuNi alloys nanoparticles of different shapes, sizes, and compositions. Therefore, this map is a useful tool for the tailored design of supersaturated or decomposed nanoparticles by SSD.

Nano-shaping of gold particles on silicon carbide substrate from solid-state to liquid-state dewetting

This work reports on the effect of annealing temperature on the size, shape and wetting of particles obtained on 4H-SiC substrate by the dewetting process of a deposited nanoscale-thick Au film, with focus on the difference between solid-state dewetting (below Au melting temperature) and liquid-state dewetting (above Au melting temperature).After depositing nanoscale-thick Au film on the SiC substrate, annealings are perfomed so to induce the solid-state or liquid-state dewetting process of the film with the consequent formation of particles. Plan-view and cross-view scanning electron microscopy analyses are carried out to quantify the evolution of the average planar size and vertical size of the particles and of the average contact angle of the particles to the SiC surface versus the annealing temperature. These analyses allow us to extract quantitative information on the wetting behaviour of the particles on the SiC surface by calculating the adhesion work versus the annealing temperature. Energy dispersive x-ray analyses are, also, performed on the dewetted particles to analyze their composition in the various annealing conditions. Overall, we set a general framework connecting process parameters to the nano-shape of the dewetted particles towards specific shape design for selected applications.

Solid-State Dewetting of Gold on Stochastically Periodic SiO2 Nanocolumns Prepared by Oblique Angle Deposition.

Solid-state dewetting (SSD) on patterned substrates is a straightforward method for fabricating ordered arrays of metallic nanoparticles on surfaces. However, a drawback of this procedure is that the patterning of substrates usually requires time-consuming and expensive two-dimensional (2D) fabrication methods. Nanostructured thin films deposited by oblique angle deposition (OAD) present at the surface a form of stochastically arranged periodic bundles of nanocolumns that might act as a patterned template for fabricating arrays of nanoparticles by SSD. In this work, we explore this concept and investigate the effect of three different types of OAD SiO2 thin films on the SSD of Au deposited on their surface. We demonstrate that the size and spatial distribution of the particles can be tailored through the surface morphology of these OAD film substrates. It has been found that the SSD of the evaporated Au layer gives rise to a bimodal size distribution of particles. A majority of them appeared as mesoparticles with sizes ≳100 nm and the rest as nanoparticles with ∼10 nm, respectively, located either on top of the nanocolumns following their lateral distribution (i.e., resulting from a patterning effect) or incorporated inside the open mesopores existing among them. Moreover, on the SiO2-OAD thin films where interconnected nanocolumnar bundles arrange in the form of discrete motifs, the patterning effect gave rise to the formation of approximately one Au mesoparticle per motif, which is one of the assets of patterned SSD. The morphological, optical (i.e., plasmon resonance), and crystalline structural characteristics of Au mesoparticles suggest that the interplay between a discontinuous nanocolumnar surface acting as a template and the poor adhesion of Au onto SiO2 are key factors for the observed template effect controlling the SSD on the surface of OAD thin films.

Fabrication of hybrid Pd@Ag core-shell and fully alloyed bi-metallic AgPd NPs and SERS enhancement of Rhodamine 6G by a unique mixture approach with graphene quantum dots

In this work, an innovative and simple growth approach of two-step solid state dewetting (SSD) has been introduced for the fabrication of bimetallic and core-shell nanoparticle (NP) configurations. A significant surface-enhanced Raman spectroscopy (SERS) enhancement on the hybrid Pd@Ag core-shell NP substrate is systematically demonstrated by a unique hybrid approach of mixing the graphene quantum dots (GQDs) and Rhodamine 6G (R6G). Due to the substantially increased electromagnetic hotspots in the hybrid Pd@Ag core-shell NP configuration and also the charge transfer enhancement by the addition of GQDs through the chemical enhancement mechanism, over 100 times of SERS signal enhancement is observed for the detection of R6G. The hybrid Pd@Ag core-shell NPs with the background Ag NPs and fully alloyed bi-metallic AgPd NPs without the background Ag NPs are demonstrated on sapphire (0001). The bimetallic and core-shell NPs demonstrate much enhanced localized surface plasmon resonance (LSPR) and thus significantly improved SERS responses. In specific, the hybrid Pd@Ag core-shell NPs demonstrate a gradual blue-shift and narrowing of LSPR peaks while the fully alloyed bi-metallic AgPd NPs show a continuing redshift and broadening of LSPR peaks.

Thermal stability enhancement of ultrathin Ag film electrodes by incorporating atomic oxygen

The structural deformation of a continuous thin film to discrete nanoscopic particles due to solid-state thermal dewetting has been considered an inevitable consequence of using ultrathin Ag films under severe thermal and electrical loading conditions. Thermal dewetting diminishes the electrical and optical performance of Ag film electrodes. Herein, a highly smooth, less defective, completely continuous ultrathin Ag film was used to prove the effectiveness of the atomic O-mediated Ag growth mode in suppressing thermal dewetting. The strong O interference at the outermost surfaces of the Ag nanoscopic geometries provided morphological perfection and chemical modification that contributed to the suppression of the migration of thermally activated Ag atoms. The alleviation of thermal dewetting could be attributed to the reduction in free energies because of the incorporation of the minimal amount of oxygen atoms into interstitial Ag lattice sites without forming oxide phases. The ultrathin (4.5 nm) Ag films sandwiched between thin ZnO films maintained a strong structural integrity and high electrical endurance by successfully circumventing thermal dewetting after exposure to high annealing temperatures up to 673 K and under heavy voltage loading conditions. This facilitated the integration of the heat generating and saving functions, while maintaining optical transparency.

Thermally Structured Gold Films for SERS Substrates—Patterns With and Without Solid-state Dewetting

This paper presents an experimental study on the manufacturing process of surface enhanced Raman scattering (SERS) substrates obtained by the thermal structuring of gold films. Three such samples were fabricated by depositing gold on glass substrates, inside an ultra high vacuum (UHV) installation. The UHV facility also included a sample heater, an argon ion gun and an atomic force microscope (AFM). As the efficiency of a SERS substrate is optimum for a particular value of the horizontal granularity of the metallic film, which is linked to the laser wavelength used to probe the sample, we aimed to gradually increase the horizontal granularity, through several stages of thermally induced structuring, so to get as close as possible to this optimum value. The horizontal granularity of the gold film was expressed in terms of the surface correlation length, calculated for a 20% reference threshold of the surface correlation function, which we denoted by SCL20. This parameter can be measured from the AFM images of the gold films, which are taken at various stages in the fabrication process, and it gives meaningful values for the horizontal granularity, irrespective of the fact that the thermally induced pattern exhibits solid-state dewetting or not. Therefore, this parameter offers a useful unique measuring criterion for the horizontal granularity, as it can be applied to both continuous and discontinuous films. After fabrication, the SERS performance of each sample was measured and compared with that of a commercial sample, which had a paper based active region impregnated with gold nanoparticles.