Film-substrate Interaction Research Articles

Coating behavior and surface hardening of Pd77Cu6Si17 thin film metallic glass on AZ31 magnesium alloy

Pd77Cu6Si17 (PCS) thin film metallic glasses (TFMGs) with high glass forming ability and hardness were selected as a hard coating for improving the surface hardness of AZ31 magnesium alloy. Both microindentation and nanoindentation tests were conducted on specimens with various PCS film thicknesses from 30 to 2000 nm. The apparent hardness and the relative indentation depth (β) were integrated using a quantitative model. The interaction parameters involved and relative hardness values were extracted from iterative calculations. According to the results, surface hardness can be enhanced greatly by PCS TFMGs in the shallow region, followed by gradual decrease with increasing β ratio. In addition, specimens with thinner coatings (e.g., 200 nm) showed greater substrate-film interaction and those with thick coatings (e.g., 2000 nm) became prone to film cracking. The optimum TFMG coating thickness in this study was estimated to be around 200 nm.

Influence of thin film nickel pretreatment on catalytic thermal chemical vapor deposition of carbon nanofibers

Nickel and other metal nanoparticles are known to be active as catalysts in the synthesis of carbon nanofibers. In this paper we investigate how dewetting and break-up of nickel thin films depends on film thickness, film–substrate interaction and pretreatment conditions. This is evaluated for films evaporated on oxidized silicon and fused silica substrates with or without tantalum coating, which were subsequently exposed to different pretreatment atmospheres (vacuum, nitrogen, air and hydrogen; 1h, 650°C). Atomic force microscopy, scanning electron microscopy and energy dispersive X-ray analysis were used to characterize the films. Pretreated Ni films were subjected to a thermal catalytic chemical vapor deposition procedure with brief ethylene exposures (0.5–3min, 635°C). It was found that only on the spherical nanoparticles originating from a hydrogen pretreatment of a Ni film with Ta adhesion layer, homogeneously distributed, randomly-oriented, well-attached, and semi-crystalline carbon nanofibers be synthesized.

Magnetic exchange coupling of a synthetic Co(ii)-complex to a ferromagnetic Ni substrate

On-surface assembly of a spin-bearing and non-aromatic porphyrin-related synthetic Co(II)-complex on a ferromagnetic Ni thin film substrate and subsequent magnetic exchange interaction across the interface were studied by scanning tunnelling microscopy (STM), X-ray absorption spectroscopy (XAS), X-ray magnetic circular dichroism (XMCD) and density functional theory +U (DFT + U) calculations.

Possible presence of hydrophilic SO3H nanoclusters on the surface of dry ultrathin Nafion® films: a positron annihilation study

Solutions of Nafion® with an ion exchange capacity (IEC) of 0.91 meq g(-1), which are on the verge of the formation of SO(3)H nanoclusters, were spin coated on silicon (Si), glassy carbon (GC) and platinum/silicon (Pt/Si) substrates to form films of up to 256 nm thickness. Nanostructure of the films was studied using Doppler broadening of annihilation radiation (DBAR), positron annihilation lifetime (PAL), X-ray photoelectron spectroscopy (XPS), an atomic force microscope (AFM) and contact angle measurements. Contact angles as low as 10 degrees indicate that the surface of dry ultrathin Nafion® films on Si is highly hydrophilic. XPS data of 10 nm thick, ultrathin film on Si show that oxygen concentration is enhanced and the SO(3)H group concentration, in other words, IEC on the surface is much higher than other films. The S parameter measured by DBAR of an ultrathin Nafion® film on Si is much higher than that of the films on the other substrates. We consider that a large number of hydrophilic, reversed micelle like SO(3)H groups are on the surface of the ultrathin Nafion® film on Si but not on the surface of other films. Positrons implanted into the film are trapped by the SO(3)H clusters, annihilating with the electrons of oxygen and exhibit the high S parameter. The SO(3)H concentration on the surface of thin Nafion® films on GC and Pt/Si substrates may not be so high as the threshold for the formation of a large number of SO(3)H clusters. Positrons implanted into the films annihilate mostly with fluorine atoms, resulting in a low S parameter. The film-substrate interaction plays an essential role in nanostructuring of Nafion® thin films, which may also be the case for Nafion® on the catalysts of polymer electrolyte fuel cells.

High-Performance Transistors Based on Zinc Tin Oxides by Single Spin-Coating Process

Films of zinc tin oxide (ZTO), grown from solutions with zinc acetate dehydrate and tin(II) 2-ethylhexanoate dissolved in 2-methoxyethanol, have been used to fabricate thin-film transistors in combination with solution-processed aluminum oxide as the gate insulator. And the nonhomogeneity of the single-layer ZTO films, caused by both ZTO film-substrate interaction and surface crystallization, has been studied, which is essential to achieve high performance transistors. In the bottom-contact thin-film transistor based on a Sn-rich layer of ZTO, a high mobility of 78.9 cm(2) V(-1) s(-1) in the saturation region has been obtained, with an on-to-off current ratio of 10(5) and a threshold gate voltage of 1.6 V.

Terahertz transmission characteristics across the phase transition in VO2 films deposited on Si, sapphire, and SiO2 substrates

Vanadium dioxide (VO2) films were deposited on high-purity Si, sapphire, and SiO2 substrates by an organic sol-gel method. The effect of the substrate on the structure, morphology, and phase transition properties of the VO2 films was demonstrated. We proposed that the film-substrate interaction induced the differences in the fraction of the +4 valence state vanadium oxide phase, surface morphology, and grain size for the VO2 films. The VO2 film on the Si substrate exhibited a switching property of about 2 orders of change in electrical resistivity. By contrast, the VO2 films on the sapphire and SiO2 substrates exhibited a switching property of about 3 orders of change in resistivity. The THz transmission across the phase transition in the VO2 films was quite different in the transmission modulation ratio, the width, and the slope of the hysteresis loop. In particular, the VO2 films on the sapphire and SiO2 substrates have the same reduction in THz transmission by about 46% comparing with about 35% in the VO2 film on the Si substrate. Furthermore, the VO2 film on the SiO2 substrate exhibits the widest hysteresis loop with the steepest slope.

Size dependence of static friction between solid clusters and substrates

The existence of the static friction force Fs implies that two surfaces in contact are locked into a free energy minimum. The microscopic origin of this minimum can be easily understood if considering commensurate interfaces where all the slider particles occupy the minima of the substrate potential. On the contrary, in incommensurate interfaces the particle arrangement on the substrate gives rise to an interaction potentialbetweenthesurfaceswhichisflatexceptforboundary effects. 1,2 One proposed explanation for the origin of static friction in macroscopic interfaces is that small molecules, the so-called “third bodies,” adsorbed on surfaces exposed to air can arrange to lock together the contacting surfaces. 3 Yet some observed frictional phenomena remain puzzling, even at the nanoscale. An example is provided by the stick-slip motion of tips sliding on crystalline substrates: The recorded friction force maps usually present the substrate periodicity independently from the tip structure (often amorphous) and environmental conditions, including vacuum. In this Rapid Communication we describe the atomistic mechanisms that determine the effective commensurability of two surfaces in contact, which we show can differ highly from the geometrical commensurability of the separated surfaces. We consider monolayer (ML) islands of krypton on copper. This idealized system has been studied experimentally by means of a quartz crystal microbalance (QCM). 4 We use molecular dynamics simulations based on a first-principles derived potential for the film-substrate interaction. A direct visualization of the particle positions relative to the minima of the substrate potential allowed us to resolve domain structures. Domains are formed in incommensurate interfaces becauseofthecompetitioninminimizingtheinterfacialenergy and elastic strain energy. When the size of the contact is reduced below a critical radius Rc, domains coalesce and the interface becomes commensurate. This structural transition is accompanied by a sharp increase of static friction. It is interesting to notice that the complementary process, i.e., the depinning of a commensurate interface, occurs by the nucleation and growth of domains. 5 We verify that the sizeinducedfrictionalappearanceoccursalsointhree-dimensional systemswithcovalentbondsbyconsideringclustersofsilicon. We describe these results within the nucleation theory and we derive an analytical expression for the critical dimension. It

Topological limit of ultrathin quasi-free-standing Bi2Te3films grown on Si(111)

A fundamental issue for ultrathin topological films is the thickness limit below which the topological surface states become impacted by interfacial interaction. We show that for Bi${}_{2}$Te${}_{3}$ grown on Si(111) this limit is four quintuple layers based on angle-resolved photoemission measurements, using optimized photon energies and polarizations, of the Dirac cone warping and interaction-induced gap as a function of film thickness. The results are close to theoretical predictions for free-standing films. Evidence is presented to show why the substrate-film interaction is actually weak.

Combining X-Ray Diffraction and Substrate Deflection Analysis to Understand Internal Stress in Electroless Copper Films

ABSTRACTElectroless copper films are usually the first conducting layer on the insulating substrates of printed circuit boards. For this and other emerging applications, the internal stress of the copper layer is an important consideration both for film adhesion and film-substrate interaction. We have combined stress/strain analysis based on X-ray diffraction, which is sensitive to the strain of the copper crystallites, with a conventionally used technique that analyses the bending of the substrate (Deposit Stress Analyzer). Both techniques were implemented in such a way that the stress could be monitored continuously during the deposition of the films from the electroless plating bath as well as afterwards. These tests were carried out for three chemical formulations and the results from both techniques agree qualitatively. For one bath, the substrate bending method detects a 60 nm region of local stress at the film-surface interface.

Evolution of magnetic bubble domains in manganite films

We report a thickness-dependent evolution of magnetic domains from long stripe-like to bubble-like entities in La1−xSrxMnO3 (x ∼ 0.3) (LSMO) films grown on LaAlO3 substrates. By using 2-D fast Fourier transformation of magnetic force microscopy images and power spectral density function, we accurately determine the domain width in LSMO films with a wide range of thickness (50–325 nm). We find that the domain size scales with the Kittel’s square root law [C. Kittel, Phys. Rev. 70, 965 (1946).] only when reduced film thicknesses are used, which suggests the critical role of substrate-film interaction in domain formation.

Effect of the substrate on the insulator–metal transition of vanadium dioxide films

Single-phase vanadium dioxide films grown on (0001) sapphire and (001) silicon substrates show a very different insulator–metal electronic transition. A detailed description of the growth mechanisms and the substrate–film interaction is given, and the characteristics of the electronic transition are described by the morphology and grain boundary structure. (Tri-)epitaxy-stabilized columnar growth of VO2 takes place on the sapphire substrate, whereas on silicon the expected Zone II growth is identified. We have found that in the case of the Si substrate the reasons for the broader hysteresis and the lower switching amplitude are the formation of an amorphous insulating VOx (x > 2.6) phase coexisting with VO2 and the high vanadium vacancy concentration of the VO2. These phenomena are the result of the excess oxygen during the growth and the interaction between the silicon substrate and the growing film.

Nonlinear dynamics of phase separation in thin films

We present a long-wavelength approximation to the Navier–Stokes Cahn–Hilliard equations to describe phase separation in thin films. The equations we derive underscore the coupled behaviour of free-surface variations and phase separation. We introduce a repulsive substrate–film interaction potential and analyse the resulting fourth-order equations by constructing a Lyapunov functional, which, combined with the regularizing repulsive potential, gives rise to a positive lower bound for the free-surface height. The value of this lower bound depends on the parameters of the problem, a result which we compare with numerical simulations. While the theoretical lower bound is an obstacle to the rupture of a film that initially is everywhere of finite height, it is not sufficiently sharp to represent accurately the parametric dependence of the observed dips or ‘valleys’ in free-surface height. We observe these valleys across zones where the concentration of the binary mixture changes sharply, indicating the formation of bubbles. Finally, we carry out numerical simulations without the repulsive interaction, and find that the film ruptures in finite time, while the gradient of the Cahn–Hilliard concentration develops a singularity.

Catalytic chemical vapor deposition of methane on graphite to produce graphene structures

Graphene structures, obtained by catalytic chemical vapor deposition of methane on highly oriented pyrolitic graphite (HOPG), were examined using scanning tunneling microscopy. Depending on the Fe catalyst coverage and localization on the substrate steps and terraces, different graphene structures were obtained: curved graphene sheets at the edges of topmost stacked graphene bilayers, laterally grown terraces at the edges of individual graphene layers parallel to the HOPG basal plane and planar graphene islands on the terraces. A growth mechanism is proposed that takes into account the specific features of the spatial distribution of Fe catalytic nanoparticles on the substrate surface, driven by metal film-substrate interaction. The present synthesis approach is promising for the controlled growth and modification of graphene layers, as well as for engineering the edge characteristics of graphene systems at the atomic scales.

Engineering the crystal growth behavior: “On substrate” MOD formation of ZnO hollow spheres

In this paper is described an easy, one-pot synthesis of ZnO hollow spheres with sizes ranging from 300 nm to 500 nm, by spin-coating deposition on aluminum substrate. Simplified models explaining the shape formation based on film–substrate interaction are discussed. The characteristic size and shape of the nanostructures obtained by the described method and the properties of ZnO as a low-cost biocompatible material make this methodology of synthesis interesting for a wide range of applications including optoelectronics, catalysis and (bio)sensors.

GROWTH, OPTICAL IMAGING AND DIELECTRIC PROPERTIES OF FERROELECTRIC BETAINE PHOSPHITE AND TRIGLYCINE SULPHATE FILMS

ABSTRACT Polycrystalline films of betaine phosphite (BPI) and triglycine sulphate (TGS) were grown by the evaporation method on different substrates (α-SiO2, α-Al2O3 and LiNbO3 with interdigital transducers, and Al/SiO2). Large (∼1 mm in BPI and ∼0.2 mm in TGS) single-crystalline blocks were visualized in a polarizing microscope in the reflection mode. Blocks are characterized by preferable in-plane orientation of the ferroelectric axis. Temperature dependences of the BPI and TGS film capacity at frequencies 120 Hz–1 MHz exhibit a strong maximum at the temperature of the ferroelectric phase transition of bulk crystals. The effect of a bias electric field on the dielectric constant and losses was studied. Description of dielectric properties is carried out using Landau-Ginsburg-Devonshire model with account of film-substrate interaction. Low frequency conductivity in BPI films is related to DC conductivity due to thermo-activation process with activation energy Eact ∼ 0.1 eV. AC conductivity associated with the hoping mechanism and the combination DC and AC conductivity was found in TGS films on Al/SiO2 substrates and α-Al2O3 substrates correspondingly.

Disordered nanoparticle interfaces for directed self-assembly

Self-assembly is a promising route for controlling the nanoscale structure and material properties of coatings, yet it remains difficult to control the microstructure of these systems. In particular, self-assembling materials typically have complex and delicate energy landscapes, which are sensitive to defects, making it difficult to control morphology or orientation. We present a simple and robust strategy for modulating the film-substrate interaction, which can bias the self-assembly energy landscape and thus enforce a desired microstructure. The technique uses nanoparticles with tunable surface energy to generate a rough interface with controlled properties. The intentionally disordered interface is tolerant to variation in substrate preparation. We apply this technique to block-copolymer lamellae, and demonstrate a remarkable thickness-dependence of the induced orientation, consistent with theoretical predictions. The simultaneous control of substrate energy and topography enables expression of the vertical lamellae state without rigorous control of the preparation conditions. We measure an 8-fold increase in surface energy tolerance compared to flat substrates.

Ferroelectric betaine phosphite films: Growth, optical imaging, and dielectric properties

Polycrystalline films of betaine phosphite (BPI) were grown by evaporation method on SiO2 and LiNbO3 substrates with interdigital transducers. Large (∼1 mm) single-crystalline blocks were visualized in a polarizing microscope in the reflection mode. Temperature dependences of the film capacity at frequencies 120 Hz–1 MHz exhibit a strong maximum at the temperature of the ferroelectric phase transition of BPI bulk crystals. The effect of a bias electric field was studied. Temperature variations in dielectric permittivity and electric field effect are analyzed using a thermodynamic model accounting for the substrate-film interaction.

Dewetting of thin polymer film on rough substrate: II. Experiment

The theory of the dewetting process developed for a model of substrate–film interaction forces was examined by an experimental investigation of the dewetting process of thin polystyrene (PS) films on chemically etched silicon substrates. In the dependence on PS films thickness and silicon roughness, various situations of dewetting were observed as follows: (i) if the wavelength of the substrate roughness is much larger than the critical spinodal wavelength of a film, then spinodal dewetting of the film is observed; (ii) if the wavelength of the substrate roughness is smaller than the critical wavelength of the film and the substrate roughness is larger in comparison with film thickness, then the dewetting due to substrate roughness is observed and the dewetted film patterns repeat the rough substrate structure; (iii) if the wavelength of the substrate roughness is smaller than the critical wavelength of the film and the substrate roughness is small in comparison with the film thickness, then spinodal dewetting proceeds.

Interplay between structural, magnetic, and electronic properties in aFeO∕Pt(111)ultrathin film

We have investigated the magnetic and electronic properties of a FeO film grown on Pt(111). Coupling first-principles density-functional theory calculations with scanning tunneling microscopy (STM) measurements we have identified the principal mechanisms for the structural and electronic characteristics observed in the Moire unit cell formed between oxide film and metal support. We show that both free and supported FeO(111) monolayers present an antiparallel alignment of the magnetic moments. Due to the lattice mismatch between Pt(111) and the FeO film, the interface properties are different in different regions of the supercell. The resulting modulation of the structural and electronic properties of the film is determined by the strength of the film-substrate interaction in various regions of the supercell. In particular, for the Fe-top site, where this interaction is weak, there is a small rumpling of the FeO layer which increases the interface separation. In correspondence of this structural modification, we observe a change in the work function, coherent with the most recent experimental findings. Our results show that a good agreement with the experimental interpretation of the work function modulation and of the STM images can be obtained only within the more robust, nonpseudomorphic computational models.

Formation of mesoporous silica thin films on oxide substrates by casting

The preparation of mesoporous silica thin films with bi-dimensional hexagonal pores whose channel axis oriented parallel to the plane of the film on oxide substrates has been studied using casting, X-ray diffraction (XRD), scanning electron spectroscopy (SEM), N 2 adsorption–desorption and thermogravimetry (TG). XRD and SEM observations show that the surfactant–silica mesostructured thin films are synthesized on the substrates using a cetyltrimethylammonium bromide (CTMABr)–tetraethyl orthosilicate (TEOS) gel under acidic conditions at ambient temperature. XRD and N 2 adsorption–desorption studies reveal that the powdered mesoporous silica composite remains structurally stable and ordered after template removal and that the supported thin films are plausibly stable after template removal but less stable and ordered than the powdered composite. TG analysis indicates that the template supercritical fluid extraction (SFE) efficiency reaches 87%. The influences of the surfactant-substrate and film-substrate interactions on the formation and structural stability of the thin films are discussed.