Growth Of Metal Films Research Articles

Metal Films on Two-Dimensional Materials: van der Waals Contacts and Raman Enhancement.

Electronic devices based on two-dimensional (2D) materials will need ultraclean and defect-free van der Waals (vdW) contacts with three-dimensional (3D) metals. It is therefore important to understand how vdW metal films deposit on 2D surfaces. Here, we study the growth and nucleation of vdW metal films of indium (In) and non-vdW metal films of gold (Au), deposited on 2D MoS2 and graphene. In follows a 2D growth mode in contrast to Au that follows a 3D growth mode. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to image the morphology of metal clusters during growth and quantify the nucleation density. As compared to Au, In atoms exhibit nearly 50 times higher diffusivity (3.65 × 10-6 μm-2 s-1) and half the nucleation density (64.9 ± 2.46 μm-2), leading to larger grain sizes (∼60 nm for 5 nm In on monolayer MoS2). The grain size of In can be further increased by reducing the 2D surface roughness, while the grain size for Au is limited by its high nucleation density due to the creation of interface defects during deposition. The vdW gap between In and MoS2 and graphene leads to strong enhancement (>103) in their Raman signal intensity due to localized surface plasmon resonance. In the absence of a vdW gap, the plasmon-mediated enhancement in Raman does not occur.

Novel Insights into Atomic-Scale Interface Reconstruction during Epitaxial Growth of Metallic Delafossite Thin Films

Novel Insights into Atomic-Scale Interface Reconstruction during Epitaxial Growth of Metallic Delafossite Thin Films

Metal-organic framework template-guided electrochemical lithography on substrates for SERS sensing applications

The templating method holds great promise for fabricating surface nanopatterns. To enhance the manufacturing capabilities of complex surface nanopatterns, it is important to explore new modes of the templates beyond their conventional masking and molding modes. Here, we employed the metal-organic framework (MOF) microparticles assembled monolayer films as templates for metal electrodeposition and revealed a previously unidentified guiding growth mode enabling the precise growth of metallic films exclusively underneath the MOF microparticles. The guiding growth mode was induced by the fast ion transportation within the nanochannels of the MOF templates. The MOF template could be repeatedly used, allowing for the creation of identical metallic surface nanopatterns for multiple times on different substrates. The MOF template-guided electrochemical growth mode provided a robust route towards cost-effective fabrication of complex metallic surface nanopatterns with promising applications in metamaterials, plasmonics, and surface-enhanced Raman spectroscopy (SERS) sensing fields.

Growth of hard metal films by deep oscillation magnetron sputtering

ABSTRACT Pulsed vapour fluxes with proper controlled time duration were proved to be an important controlling dimension for the deposition of a desired thin film. In this work, deep oscillation magnetron sputtering was employed to tailor the ionized pulsed flux for Nb and Cr thin film grown on AISI 304 stainless steel. Through tuning the micropulse oscillating voltage-off time from 10 to 40 μs, Nb and Cr thin films showed an obvious variation on microstructure owing to the deposition flux transited from continuous to chopped ones. The deposited Nb thin films showed Nb(110) preferred orientation experienced a nanocrystallization process with compressive residual stress showing a slightly decrease. While Cr thin films showed Cr(110) preferred orientation without an obvious nancrystallization trend with its compressive stress turning to be tensile. The tailoring of the microstructure and properties relied on interval time between the two interrupted ionized deposition flux.

Influence of Potassium Metal-Support Interactions on Dendrite Growth.

Combined synchrotron X-ray nanotomography imaging, cryogenic electron microscopy (cryo-EM) and modeling elucidate how potassium (K) metal-support energetics influence electrodeposit microstructure. Three model supports are employed: O-functionalized carbon cloth (potassiophilic, fully-wetted), non-functionalized cloth and Cu foil (potassiophobic, nonwetted). Nanotomography and focused ion beam (cryo-FIB) cross-sections yield complementary three-dimensional (3D) maps of cycled electrodeposits. Electrodeposit on potassiophobic support is a triphasic sponge, with fibrous dendrites covered by solid electrolyte interphase (SEI) and interspersed with nanopores (sub-10 nm to 100 nm scale). Lage cracks and voids are also a key feature. On potassiophilic support, the deposit is dense and pore-free, with uniform surface and SEI morphology. Mesoscale modeling captures the critical role of substrate-metal interaction on K metal film nucleation and growth, as well as the associated stress state.

Influence of Potassium Metal‐Support Interactions on Dendrite Growth

AbstractCombined synchrotron X‐ray nanotomography imaging, cryogenic electron microscopy (cryo‐EM) and modeling elucidate how potassium (K) metal‐support energetics influence electrodeposit microstructure. Three model supports are employed: O‐functionalized carbon cloth (potassiophilic, fully‐wetted), non‐functionalized cloth and Cu foil (potassiophobic, nonwetted). Nanotomography and focused ion beam (cryo‐FIB) cross‐sections yield complementary three‐dimensional (3D) maps of cycled electrodeposits. Electrodeposit on potassiophobic support is a triphasic sponge, with fibrous dendrites covered by solid electrolyte interphase (SEI) and interspersed with nanopores (sub‐10 nm to 100 nm scale). Lage cracks and voids are also a key feature. On potassiophilic support, the deposit is dense and pore‐free, with uniform surface and SEI morphology. Mesoscale modeling captures the critical role of substrate‐metal interaction on K metal film nucleation and growth, as well as the associated stress state.

Experimental and numerical study of different metal contacts for perovskite solar cells

Various metal layers grown by e-beam evaporator have been studied to be used as metal contacts for scalable perovskite photovoltaic (PV) devices. The evaporated films consisting of gold (Au), silver (Ag), nickel (Ni), titanium (Ti), tin (Sn), copper (Cu), and molybdenum (Mo) were grown on glass substrates at room temperature with an optimized thickness. Later, the measured optical properties such as transmission and absorptance of such films were used computationally to extract the optimum device performance using SCAPS-1D software. Among all the layers, Ti-based perovskite solar cells outperform other metal contacts with a power conversion efficiency of (>27%). The films were characterized optically, topologically, structurally, and morphologically using ultraviolet—visible (UV—Vis) spectrometry, atomic force microscopy, x-ray photoelectron spectroscopy (XPS), three-dimensional (3D) profilometry, and scanning electron microscopy. The morphological data confirm the growth of compact, uniform, and defect-free metal films as confirmed by the field emission scanning electron microscopy. Contact angle measurement was also performed to determine the wettability of metal surfaces. Both Au and Ni films were found semi-hydrophilic which shows the adaptability of better stability through repelling water from the surface. The computational analysis confirms that screening of suitable metal back contact is necessary to increase device performance and stability significantly.

Structure Formation of C60 on Insulating CaF2 Substrates: Matching Experiments with Simulations

AbstractThe epitaxial growth of metallic thin films has been studied intensively, leading to computational models that can predict diverse morphologies depending on thermodynamic and kinetic growth parameters. Much less is known about thin films of organic molecules. Here kinetic Monte Carlo simulations to study buckyball C60 molecules on dielectric CaF2(111) substrates is employed. This system undergoes a transition from mostly one‐layered growth to two‐layered growth of triangular cluster morphologies as the temperature is raised. Even though simplified, these simulations still require a large set of parametrized rates for the elementary transitions, the determination of which has been challenging. Here recent results for these parameters to measure cluster densities and second layer occupations are employed. Simulation snapshots of cluster morphologies are compared in a wide range of temperatures to test to which extent rate models can match the experimental observations. While adjustments to free diffusion parameters are necessary to reproduce the experimentally observed cluster densities and sizes, the cluster morphologies are excellently reproduced without further adjustments.

Effect of chitosan coating on polypropylene fibers on the deposition of copper ions

AbstractComposites based on chitosan coatings and metal ions are attracting great attention due to their antibacterial properties and biocompatibility. It is known that the presence of chitosan helps to stabilize the growth of metal films or individual nanoparticles during their deposition. In this work chitosan coating obtained from an aqueous carbonic acid solution is applied to fibers of a polypropylene nonwoven membrane. Aqueous carbonic acid solution is a clean and cheap solvent for chitosan that can effectively impregnate a porous polypropylene matrix. Before chitosan deposition, the fibers were pretreated in the presence of peroxycarbonic acid (i.e., in an aqueous hydrogen peroxide solution saturated with pressurized carbon dioxide). Copper ions were then deposited on the chitosan‐coated fibers. It was shown that the presence of a chitosan interlayer can significantly improve adhesion and reduce the aggregation of growing particles, which results in the decoration of the fibers with uniform thin copper‐containing films. The proposed method for creating composite materials can serve as a new approach to antibacterial polymer‐inorganic composites manufacturing.

Fluorinated β-diketonate complexes M(tfac)2(TMEDA) (M = Fe, Ni, Cu, Zn) as precursors for the MOCVD growth of metal and metal oxide thin films.

Partially fluorinated β-diketonate complexes M(tfac)2(TMEDA) (M = Fe 1, Ni 2, Cu 3, Zn 4; tfac = 1,1,1-trifluoro-2,4-pentanedionate; TMEDA = N,N,N′,N′-tetramethylethylenediamine) were synthesized and structurally (sc-XRD) and thermochemically (TGA) characterised. A new polymorph of Fe(tfac)2(TMEDA) was found. The structural and physicochemical properties of 1–4 were compared with related M(acac)2(TMEDA) and M(hfac)2(TMEDA) (acac = 2,4-pentanedionate, hfac = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate) β-diketonate complexes to evaluate the effect of the degree of fluorination. A positive effect on the thermal behaviour of the metal acetylacetonates was observed, but no discernible trends. Application of complexes 1–4 as precursors in a MOCVD process yielded either metal (Ni, Cu) or metal oxide thin films (Fe3O4, ZnO), which were further oxidized to NiO, CuO and α-Fe2O3 films by calcination in air at 500 °C.

Epitaxial growth of copper, gold, and silver on vicinal NaCl surfaces

Epitaxial growth of metal films on (100) cleaved NaCl surfaces has been extensively studied. However, attempts to grow films on other crystallographic surfaces of rock salt are limited and typically require sophisticated control over the polishing process. This work uses a simple approach of polishing NaCl crystals approximately towards a specific crystallographic plane followed by the water treatment or air annealing. Epitaxial Ag, Au, and Cu films were grown quite reproducibly on these surfaces through physical vapor deposition. The technique is quite robust against misalignment of the deposition surface from the intended crystallographic plane. This tolerance towards surface misorientation was observed not only for the high atomic-density planes such as (111) and (110) but also for the previously unexplored and relatively high-index (112) surface.

Thermal laser evaporation for the growth of oxide films

Thermal laser evaporation (TLE) is a particularly promising technique for the growth of metal films. Here, we demonstrate that TLE is also suitable for the growth of amorphous and polycrystalline oxide films. We report on a spectrum of binary oxide films that have been deposited by laser-induced evaporation of elemental metal sources in oxygen–ozone atmospheres. The oxide deposition by TLE is accompanied by an oxidation of the elemental metal source, which systematically affects the source molecular flux. Fifteen elemental metals were successfully used as sources for oxide films grown on unheated substrates, employing one and the same laser optic. The source materials ranged from refractory metals with low vapor pressures, such as Hf, Mo, and Ru, to Zn, which readily sublimates at low temperatures. These results reveal that TLE is also well suited for the growth of ultraclean oxide films.

Integrated Wafer Scale Growth of Single Crystal Metal Films and High Quality Graphene.

We report on an approach to bring together single crystal metal catalyst preparation and graphene growth in a combined process flow using a standard cold-wall chemical vapor deposition (CVD) reactor. We employ a sandwich arrangement between a commercial polycrystalline Cu foil and c-plane sapphire wafer and show that close-spaced vacuum sublimation across the confined gap can result in an epitaxial, single-crystal Cu(111) film at high growth rate. The arrangement is scalable (we demonstrate 2″ wafer scale) and suppresses reactor contamination with Cu. While starting with an impure Cu foil, the freshly prepared Cu film is of high purity as measured by time-of-flight secondary ion mass spectrometry. We seamlessly connect the initial metallization with subsequent graphene growth via the introduction of hydrogen and gaseous carbon precursors, thereby eliminating contamination due to substrate transfer and common lengthy catalyst pretreatments. We show that the sandwich approach also enables for a Cu surface with nanometer scale roughness during graphene growth and thus results in high quality graphene similar to previously demonstrated Cu enclosure approaches. We systematically explore the parameter space and discuss the opportunities, including subsequent dry transfer, generality, and versatility of our approach particularly regarding the cost-efficient preparation of different single crystal film orientations and expansion to other material systems.

Anomalous versus Normal Room-Temperature Diffusion of Metal Adatoms on Graphene.

Fabrication of high-performance heterostructure devices requires fundamental understanding of the diffusion dynamics of metal species on 2D materials. Here, we investigate the room-temperature diffusion of Ag, Au, Cu, Pd, Pt, and Ru adatoms on graphene using ab initio and classical molecular dynamics simulations. We find that Ag, Au, Cu, and Pd follow Lévy walks, in which adatoms move continuously within ∼1–4 nm2 domains during ∼0.04 ns timeframes, and they occasionally perform ∼2–4 nm flights across multiple surface adsorption sites. This anomalous diffusion pattern is associated with a flat (<50 meV) potential energy landscape (PEL), which renders surface vibrations important for adatom migration. The latter is not the case for Pt and Ru, which encounter a significantly rougher PEL (>100 meV) and, hence, migrate via conventional random walks. Thus, adatom anomalous diffusion is a potentially important aspect for modeling growth of metal films and nanostructures on 2D materials.

Nucleation and Growth of Metal Films and Nanocrystals on Two-dimensional Materials

Nucleation and Growth of Metal Films and Nanocrystals on Two-dimensional Materials

Methanol solvent effect on photosensing performance of AZO thin films grown by nebulizer spray pyrolysis

Dopant concentration variation or solvent change is commonly used for the growth and characterization of doped metallic thin films. In contrast, we have studied the effects of varying methanol solvent volume ratio for aluminum doped zinc oxide (AZO) thin film growth. AZO thin films were coated on glass and ITO substrates by nebulizer spray pyrolysis method. The effect of different solvent concentrations and the water to methanol ratio on the resultant film properties were analyzed. The structural and lattice constants of the prepared films were investigated by x-ray diffraction (XRD). Pure methanol solvent enhances the structural quality of the AZO film, as confirmed from the XRD and x-ray photoelectron spectroscopy analyses. The atomic force microscopy images showed thinner and smooth film surface for the pure methanol solvent condition without water. The optical absorption study showed a high transmittance (∼93%) in the visible region with a bandgap of 3.30 eV, which is close to bulk ZnO (3.36 eV). The photoluminescence spectra showed a powerful UV emission at 390 nm, indicating that the films were of good quality. Ag/AZO/Cu2O/ITO structured photodiodes were fabricated using ITO substrate and their photosensing properties were studied. All the prepared films (S1, S2, S3, S4 and S5) showed an increase in current level for both dark and light conditions, with an increase in methanol solvent ratio. The film S5 (pure methanol solvent) under the dark and light illumination conditions at a bias of 1.0 V, showed current values of ∼6.5 × 10–5 A and ∼1.85 × 10–4 A, respectively.

Pulsed-laser epitaxy of metallic delafossite PdCrO2 films

Alternate stacking of a highly conducting metallic layer with a magnetic triangular layer found in delafossite PdCrO2 provides an excellent platform for discovering intriguing correlated quantum phenomena. Thin film growth of delafossites may enable not only the tuning of the basic physical properties beyond what bulk materials can exhibit, but also the development of novel hybrid materials by interfacing with dissimilar materials, yet this has proven to be extremely challenging. Here, we report the epitaxial growth of metallic delafossite PdCrO2 films by pulsed laser epitaxy (PLE). The fundamental role of the PLE growth conditions, epitaxial strain, and chemical and structural characteristics of the substrate is investigated by growing under various growth conditions and on various types of substrates. While strain plays a large role in improving the crystallinity, the direct growth of epitaxial PdCrO2 films without impurity phases was not successful. We attribute this difficulty to both the chemical and structural dissimilarities with the substrate and volatile nature of the PdO sublayer, which make nucleation of the right phase difficult. This difficulty was overcome by growing CuCrO2 buffer layers before PdCrO2 films were grown. Unlike PdCrO2, CuCrO2 films were readily grown with a relatively wide growth window. Only a monolayer thick buffer layer was sufficient to grow the correct PdCrO2 phase. This result indicates that the epitaxy of Pd-based delafossites is extremely sensitive to the chemistry and structure of the interface, necessitating near perfect substrate materials. The resulting films are commensurately strained and show an antiferromagnetic transition at 40 K that persists down to as thin as 3.6 nm in thickness. This work provides key insights into advancing the epitaxial growth of the broader class of metallic delafossites for both studying the basic physical properties and developing new spintronic and computing devices.

Surface and mechanical analysis of metallized poly(dimethylsiloxane) gel for varifocal micromirrors

Varifocal micromirrors are alternative and miniaturized mirror systems that are used for imaging procedures in biomedical diagnostics, mirror‐based tunable lenses, or the correction of spherical aberration. In this study, we demonstrate a new concept for focal length variation by electrostatic mirror deformation of a compact and integrated electroactive polymer actuator based on metallized poly(dimethylsiloxane) (PDMS) gel thin films. The varifocal micromirrors have a lateral size of 70 × 70 μm2 and are capable of deforming in either convex or concave direction, depending on a defined surface treatment by O2‐plasma or UV/ozone of the PDMS prior to metallization by physical vapor deposition (PVD). Surface and interface analysis by wetting experiments, sputter depth‐profiling X‐ray photoelectron spectroscopy (XPS) combined with scanning electron microscopy (SEM) on cross‐sections processed with focused ion beam (FIB) as well as polymer and metal surfaces are used to understand and to improve the metal film growth and quality with respect to high reflectivity and conductivity. In addition to the high quality of the metallic mirror layer, the mirror displacement is important and inevitably depends on the gel stiffness of the actuator. Therefore, we investigate the gel mechanics and the performance of the actuator with rheology, confocal microscopy, and image formation on the electrically deformed mirrors. Our concept of varifocal micromirrors offers a wide range of applications for tunable mirror‐based optics due to their simple and compact design.

The effect of kinetics on intrinsic stress generation and evolution in sputter-deposited films at conditions of high atomic mobility

Vapor-based metal film growth at conditions that promote high atomic mobility is typically accompanied by compressive stress formation after completion of island coalescence, while an apparent stress relaxation is observed upon deposition interruption. Despite numerous experimental studies confirming these trends, the way by which growth kinetics affect postcoalescence stress magnitude and evolution is not well understood, in particular, for sputter-deposited films. In this work, we study in situ and in real-time stress evolution during sputter-deposition of Ag and Cu films on amorphous carbon. In order to probe different conditions with respect to growth kinetics, we vary the deposition rate F from 0.015 to 1.27nm/s, and the substrate temperature TS from 298 to 413K. We find a general trend toward smaller compressive stress magnitudes with increasing TS for both film/substrate systems. The stress-dependence on F is more complex: (i) for Ag, smaller compressive stress is observed when increasing F; (ii) while for Cu, a nonmonotonic evolution with F is seen, with a compressive stress maximum for F=0.102nm/s. Studies of postdeposition stress evolution show the occurrence of a tensile rise that becomes less pronounced with increasing TS and decreasing F, whereas a faster tensile rise is seen by increasing F and TS. We critically discuss these results in view of ex situ obtained film morphology which show that deposition-parameter-induced changes in film grain size and surface roughness are intimately linked with the stress evolution.

Dendrite-Free Potassium Metal Anodes in a Carbonate Electrolyte.

Potassium (K) metal anodes suffer from a challenging problem of dendrite growth. Here, it is demonstrated that a tailored current collector will stabilize the metal plating-stripping behavior even with a conventional KPF6 -carbonate electrolyte. A 3D copper current collector is functionalized with partially reduced graphene oxide to create a potassiophilic surface, the electrode being denoted as rGO@3D-Cu. Potassiophilic versus potassiophobic experiments demonstrate that molten K fully wets rGO@3D-Cu after 6 s, but does not wet unfunctionalized 3D-Cu. Electrochemically, a unique synergy is achieved that is driven by interfacial tension and geometry: the adherent rGO underlayer promotes 2D layer-by-layer (Frank-van der Merwe) metal film growth at early stages of plating, while the tortuous 3D-Cu electrode reduces the current density and geometrically frustrates dendrites. The rGO@3D-Cu symmetric cells and half-cells achieve state-of-the-art plating and stripping performance. The symmetric rGO@3D-Cu cells exhibit stable cycling at 0.1-2 mA cm-2 , while baseline Cu prematurely fails when the current reaches 0.5 mA cm-2 . The half-cells cells of rGO@3D-Cu (no K reservoir) are stable at 0.5 mA cm-2 for 10 000 min (100 cycles), and at 1 mA cm-2 for 5000 min. The baseline 3D-Cu, planar rGO@Cu, and planar Cu foil fails after 5110, 3012, and 1410 min, respectively.