Preparation of polytetrafluoroethylene by pulsed electron ablation: Deposition and wettability aspects

Polytetrafluoroethylene (PTFE) has been prepared by pulsed electron deposition technique on glass and silicon substrates. Deposition of the thin films has been carried out in the temperature range from room temperature to 300 °C, pressure range from 133.32 × 10⁻³ Pa to 799.93 × 10⁻³ Pa, and discharge voltages between 10 kV and 16 kV. Argon or nitrogen has been used as a background gas during the deposition of the films. Attenuated Total Reflection Fourier Transform Infrared spectroscopy shows absorption peaks in the films at 644 cm⁻¹, 1154 cm⁻¹ and 1210 cm⁻¹ consistent with those of PTFE target material. Atomic force microscopy and spectroscopic reflectometry reveal the clustered nature of the films and other morphological characteristics. Surface wettability of the films, expressed via the contact angle, has been measured via static angle goniometry. PTFE films increase the contact angle from about 32° (bare glass) and 43° (bare silicon) to up to 90° and 110° for PTFE-coated glass and silicon substrates, respectively. The contact angle decreases with an increase in both pressure and temperature, while it increases then decreases as the discharge voltage increases.     

Highly hydrophilic and superhydrophobic ZnO nanorod array films

The wettability control of solid surfaces is important from the aspects of both science and technology. Herein, we report a surface-modification-induced hydrophilicity to superhydrophobicity transition on well-aligned single-crystalline ZnO nanorod array films (ZnO-NAFs). The ZnO-NAFs were prepared from solution by a hydrothermal method and were characterized by X-ray diffraction and scanning electron microscopy. The surface of transparent ZnO-NAFs was highly hydrophilic with a water contact angle of 9.6 ± 0.8°. However, after being exposed to octadecanethiol solution, the surfaces of the ZnO-NAFs became superhydrophobic with a water contact angle of 156.2 ± 1.8°. The present work offers a technique that has great potentials for preparing two-dimensional micro-patterns with a high wettability contrast for water.     

Conformal polymeric thin films by low-temperature rapid initiated chemical vapor deposition (iCVD) using tert-butyl peroxybenzoate as an initiator

Conformal poly(cyclohexyl methacrylate) (pCHMA) thin films were synthesized via initiated chemical vapor deposition (iCVD), with tert-butyl peroxybenzoate (TBPOB) as the initiator, representing the first time that TBPOB has been used as an initiator for iCVD synthesis. Using TBPOB instead of tert-butyl peroxide (TBPO), the rate of iCVD film growth increased by a factor of up to seven at comparable conformality and lower the filament temperature from 257 to 170 °C at a comparable deposition rate of 3 nm/min. The conformal deposition of functional thin films is desired for applications including microfluidics, medical devices and membranes. Lower filament temperatures reduce the heat load to the deposition surface and thus are advantageous for polymeric substrates that are temperature sensitive or monomers that decompose at high temperatures. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) results demonstrate the similarity of the TBPOB- to the TBPO-initiated pCHMA main chains. However, the aromatic group in TBPOB provided a unique spectral signature of the polymer chain end group in the FTIR and the peak intensity increased with increase of filament temperature. Scanning electron micrographs (SEMs) revealed that the pCHMA coatings are conformal over non-planar structures; however, at identical process conditions, TBPO-initiated films showed a slightly better conformality due to the lower sticking coefficient of TBPO. At a monomer partial pressure of 0.45, TBPOB has a sticking coefficient value of 0.1188 ± 0.0092, which is ～3 times as high as that of TBPO (0.0413 ± 0.0058). The step coverage is insensitive to filament temperature if the surface concentration of the monomer is fixed.     

Artificial petal surface based on hierarchical micro- and nanostructures

In this paper, we present an artificial rose petal composed of hierarchical micro- and nanostructures on a polymethyl methacrylate surface. The petal effect implies that the surface has a high adhesion force in spite of being in a super-hydrophobic state, while the lotus effect implies that the surface has a low adhesion force when it is in a super-hydrophobic state. We have fabricated four different types of surfaces, namely, smooth, nanostructured, microstructured, and hierarchically micro-nanostructured surfaces. Microstructures and nanostructures have a quadrangular pyramid shape (one-side length: 15 ± 2 μm, height: 10.6 ± 1 μm) and a circular bump shape (diameter: 130 ± 10 nm, height: 100 ± 10 nm), respectively. The four types of surfaces are also chemically treated with trichlorosilane in order to reduce the surface energy. The contact angles of the smooth, nanostructured, microstructured, and hierarchically micro-nanostructured surfaces are measured to be 104° ± 2°, 112° ± 2°, 138° ± 4°, and 159° ± 2° after the chemical treatment. In the case of the super-hydrophobic micro-nanostructured surfaces, water droplets remain attached to the surface, even when the surface is turned upside down.     

Low-temperature deposition of iridium thin films by pulsed laser deposition

Extremely smooth iridium (Ir) thin films were deposited on Si(1 0 0) substrate at lower temperature than 300 °C by pulsed laser deposition (PLD) technique using Ir target in a vacuum atmosphere. The crystal orientation, surface morphology, and resistivity of the Ir thin films were systematically determined as a function of substrate temperature. Well-crystallized and single-phase Ir thin films with (1 1 1) preferred orientation were obtained at substrate temperature of 200-300 °C. The surface roughness increased with the increasing of substrate temperature. Likewise, the room-temperature resistivity of Ir thin films decreased with increasing substrate temperature, showing a low value of (10.7±0.1) μΩ cm at 300 °C.     

Water repellent ZnO nanowire arrays synthesized by simple solvothermal technique

The authors report on the water repellent properties of quasialigned zinc oxide (ZnO) nanowire arrays grown by low-temperature solvothermal technique. The uniform and dense ZnO nanowires of average diameter ∼ 75 nm have been found to have hexagonal wurtzite type structure. The as synthesized ZnO nanowire surfaces have hydrophilic nature with a water contact angle of 73° ± 3°. The superhydrophobic behavior with a water contact angle of 145° ± 3° of the nanowire arrays has been realized due to the reduction of surface free energy after being coated with an octadecyltrichlorosilane (OTS) monolayer. This work may be of huge importance from the viewpoint of both the understanding of the mechanisms involved and industrial applications.     

Electroless deposition of CoPtWP magnetic thin films

CoPtWP magnetic thin films were prepared by electroless deposition. The influence of bath pH, deposition temperature and bath composition on the deposition speed, alloy content, microstructure, and magnetic properties of CoPtWP thin films were investigated. It was found that deposition speed increased gradually with the bath pH and deposition temperature. The cobalt content in the CoPtWP thin films varied from 77 at.% to 85 at.% by controlling the bath pH. The microstructure of CoPtWP thin films was dependent on bath pH and deposition temperature: two mixed structures, face centered cubic (fcc) and hexagonal close packed (hcp), were observed at low pH values and low deposition temperature. With the increase of pH values and deposition temperature, the intensity of fcc (111) peak suppressed gradually. The surface morphology was markedly influenced by bath pH, deposition temperature and bath composition. VSM and MFM measurements revealed that perpendicular coercivity had been increased with the bath pH. Unique hard magnetic properties of CoPtWP thin films with large perpendicular magnetic anisotropy were obtained at 90 °C and bath pH 13.0.     

Fabrication of ZnO submicrorod films with water repellency by surface etching and hydrophobic modification

Superhydrophobic ZnO submicrorod films have been fabricated on zinc sheets through an H₂O₂-assisted surface etching process and subsequent surface modification with a monolayer of 1H,1H,2H,2H-perfluorodecyltriethoxysilane (FDS). The crystal structure, chemical compositions, morphologies, and wettability of the resultant ZnO films were analyzed by means of X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and water contact angle measurements. It is found that the surface of the as-prepared ZnO films on zinc substrate was hydrophobic with a water contact angle of 95 ± 2°, whereas after modification with FDS, the film exhibited superhydrophobicity and the water CA increased to 154 ± 2°. It is shown that both the higher surface roughness and the lower surface free energy play an important role in creating the superhydrophobic films.     

Double switching hysteresis loop in a single layer Fe3Pt alloy thin films

The Fe₃Pt alloy thin films were epitaxially grown on MgO(100) substrate by e-beam evaporation. The films were partially ordered at the substrate deposition temperature above 350 °C. These partially ordered films exhibit very large biaxial magnetic anisotropy constant in the order of 10⁵ J/m³ and produce double switching in the hysteresis loops. The difference of the switching field of these films can be up to about 3 × 10⁵ A/m by tuning the angle of the applied field with respect to the easy axes. This double switching behavior stems from the large biaxial magnetic anisotropy of the films.     

Comparative study of resistivity characteristics between transparent conducting AZO and GZO thin films for use at high temperatures

This paper compares in detail the resistivity behavior of transparent conducting Al-doped and Ga-doped ZnO (AZO and GZO) thin films for use in an air environment at high temperatures. AZO and GZO thin films with thicknesses in the range from approximately 30 to 100 nm were prepared on glass substrates at a temperature of 200 °C by rf superimposed dc or conventional dc magnetron sputtering deposition, pulsed laser deposition or vacuum arc plasma evaporation techniques. In heat-resistance tests, the resistivity was measured both before and after heat tests for 30 min in air at a temperature up to 400 °C. The resistivity stability of AZO thin films was found to be always lower than that of GZO thin films prepared with the same thickness under the same deposition conditions, regardless of the deposition technique. However, the resistivity of all AZO and GZO thin films prepared with a thickness above approximately 100 nm was stable when heat tested at a temperature up to approximately 250 °C. It was found that the resistivity stability in both GZO and AZO thin films is dominated by different mechanisms determined by whether the thickness is below or above approximately 50 nm. With thicknesses above approximately 100 nm, the increase in resistivity found in GZO and AZO films after heat testing at a temperature up to 400 °C exhibited different characteristics that resulted from a variation in the behavior of Hall mobility.     

Formation of PTFE-like films in CF4 microwave plasmas

The study focuses on the deposition of films from pure CF₄ using low pressure microwave plasmas (MW) on polystyrene substrate with a subsequent examination of the resulting surfaces. It is generally accepted, that the F/C-ratio of the precursor molecule plays an important role for the outcome of a low pressure plasma process, or the balance between fluorocarbon deposition and material etching. Surprisingly, thin film formation could be observed using MW gas discharges in CF₄, a typical etching gas. Coatings with a thickness of at least 10 nm are formed. The films are polytetrafluoroethylene (PTFE)-like, characterized by an F/C-ratio of 1.8, and a hydrophobic surface with a water contact angle of 110°.     

Electrochemical evaluation of the reliability of plasma-polymerized methylcyclohexane films

Plasma-polymerized thin films are developed for electronic devices to satisfy the important requirement of a low dielectric constant in the interlayer dielectrics. Three types of methylcyclohexane coatings are deposited on copper as interlayer dielectrics by plasma-enhanced chemical vapor deposition at three different deposition temperatures. The coating performance is evaluated by electrochemical impedance spectroscopy and potentiodynamic polarization testing in a 3.5 wt.% NaCl solution. The coatings are also analyzed by surface analyses, including atomic force microscopy, Fourier transform infrared spectroscopy, and contact angle measurements. The electrochemical behavior of the coatings is improved by increasing the deposition temperature. The methylcyclohexane films on the copper substrate show high protective efficiency, charge transfer resistance and low porosity, which indicate that the coating performance increased with increasing deposition temperature. Atomic force microscopy, Fourier transform infrared spectroscopy and contact angle measurements confirm the enhanced formation of C-H, C-C, and CC stretching configurations, improved surface roughness and wettability with increasing deposition temperature.     

Low temperature growth and properties of Cu-In-Te based thin films for narrow bandgap solar cells

Cu-In-Te based thin films were grown onto soda-lime glass (SLG) substrates at 200 °C by co-evaporation using a molecular beam epitaxy system. The microstructural properties were examined by means of scanning electron microscopy, X-ray diffraction and Raman scattering. The crystalline quality of Cu-In-Te based thin films with high Cu/In ratios is superior to that of films with low Cu/In ratios. The films with Cu/In ratios of 0.69 ± 0.04 exhibited a single chalcopyrite phase with random orientation, whereas a defect chalcopyrite phase with a preferred (112) orientation was obtained for thin films with Cu/In ratios of 0.26 ± 0.02. However, the films with high Cu/In ratios of 0.69 ± 0.04 showed nearly constant low resistivity (∼ 10^− 2 Ω cm) at temperatures from 80 to 400 K due to high hole concentration (> 10¹⁹ cm^− 3), resulting in semi-metallic behavior. The hole conduction mechanism of the film (Cu/In atomic ratios = 0.26 ± 0.02) with semi-conductive properties was found to be variable-range-hopping of the Mott type in the wide range of 80-300 K. The optical bandgaps of Cu-In-Te based thin films are determined to be 0.93-1.02 eV at 300 K from transmission and reflection measurements. A solar cell with a ZnO/CdS/CuIn₃Te₅/Mo/SLG structure showed a total area (0.50 cm²) efficiency of 5.1% under AM1.5 illumination (100 mW/cm²) after light soaking. The conduction band offset at the CdS/CuIn₃Te₅ interface was estimated to be − 0.14 eV from X-ray photoelectron spectroscopy analysis.     

Preparation and characterization of chemically-crosslinked polyethyleneimine films on hydroxylated surfaces for stable bactericidal coatings

Polyethyleneimine (PEI) thin films had been built up on aminosilanized glass surfaces via a layer-by-layer process of glutaraldehyde crosslinking, which was stepwise characterized by means of multi-surface analysis including Fourier transform infrared spectroscopy (FT-IR), atomic force microscope, ellipsometer and contact angle measurements. In phosphate buffered saline (PBS) solution, the degradation process of PEI thin films was monitored by FI-IR at 37 °C. As compared with aminosilanized surfaces alone (~ 80% loss after 2 days), chemically-crosslinked PEI films had a good long-term stability (only ~ 10% loss after 30 days). The antibacterial tests futher revealed that these PEI films could fast kill Gram-positive or Gram-negative bacteria (~ 100%) within ~ 10 min. The killing efficiency of the film with five PEI layers treatment was 1.74 × 10⁵ units/cm², and after 30 days of incubation in PBS solution, it still retained the antibacterial activity of 1.72 × 10⁵ units/cm².     

Superhydrophobic polymer films via aerosol assisted deposition — Taking a leaf out of nature's book

Aerosol assisted deposition of three sets of polymer films based on commercially available resins was achieved on various substrates. The films were characterised using a range of methods, including water contact and slip angle to determine water repellent properties. The aerosol assisted deposition inside the chemical vapour deposition reactor was unique in generating a highly rough superhydrophobic surface with water contact angles up to 170°. During the deposition process, two of the polymers were cured resulting in the development of high surface morphology. It was observed that the polymer that did not cure did not develop such a rough surface resulting in a lower water contact angle (∼ 99°). The superhydrophobic films had a Cassie-Baxter type wetting with water failing to penetrate the surface porosity, water spraying on the surface would bounce off. These films had exceptionally low slide angles of ca 1-2° from the horizontal.     

A one-step process to engineer superhydrophobic copper surfaces

Superhydrophobic surfaces are conventionally prepared employing two steps: roughening a surface and lowering their surface energy. In the present work, a direct voltage (DC) is applied between two copper plates immersed in a dilute ethanolic stearic acid solution. The surface of the anodic copper electrode transforms to superhydrophobic due to a reaction between copper and stearic acid solution. The fabrication process of superhydrophobic copper surfaces is simplified in just one-step. The surface of the anodic copper is found to be covered with flower-like low surface energy copper stearate films providing the water contact angle of 153 ± 2° with the roll-off properties.     

Large-scale initiated chemical vapor deposition of poly(glycidyl methacrylate) thin films

The initiated chemical vapor deposition (iCVD) of poly(glycidyl methacrylate) (PGMA) was scaled up using dimensionless analysis. In the first stage, PGMA was deposited onto a large stationary substrate and a deposition rate as high as 85 nm/min was achieved. It was found that the deposition rate increases with increasing filament temperature, whereas the deposition rate and the number-average molecular weight decrease with increasing substrate temperature. In the second stage, PGMA was deposited onto a moving substrate. At speeds between 20 mm/min and 60 mm/min, the deposition rate on the moving substrate was found to be equal to the deposition rate on the stationary substrate. Fourier transform infrared spectroscopy showed that the epoxide functionality of the PGMA films was retained during the iCVD process. Since the iCVD polymerization of different vinyl monomers all use similar parameters, this scale up can be applied to the scale up of other vinyl monomers such as 2-hydroxyethyl methacrylate and perfluoroalkyl ethyl methacrylate.     

Fabrication of hydrophobic inorganic coatings on natural lotus leaves for nanoimprint stamps

Hydrophobic inorganic films were obtained by direct deposition of copper or silicon onto natural lotus leaves by ion beam sputtering deposition technique. Scanning electron microscopy observations showed a lotus-leaf-like surface structure of the deposited inorganic films. Hydrophobic nature of the inorganic films on lotus leaves had been improved compared to the inorganic films deposited on flat silicon substrates. Water contact angles measured on the lotus-leaf-like copper and silicon films were 136.3 ± 8° and 117.8 ± 4.4°, respectively. The hydrophobic lotus-leaf-like inorganic films had been repeated used as nanoimprint stamps. Negative structures of lotus-leaf-like inorganic films were obtained on the polystyrene resist layers.     

Synthesis and characterization of 4H-SiC on C-plane sapphire by C60 and Si molecular beam epitaxy

4H-SiC (silicon carbide) films were grown on (0001) sapphire substrate at rather low temperatures(1000-1100 °C) with relative high deposition rate by using fullerene (C₆₀) and silicon solid sources molecular beam epitaxy with substrate nitridation and aluminum nitride (AlN) buffer layer deposition prior to the SiC deposition. The effects of substrate nitridation and AlN buffer layer to the adhesion of the SiC thin films on sapphire have been studied. X-Ray diffraction, pole figure, atomic force microscope, Fourier transform infrared spectroscopy and photoluminescence were employed for the analysis of composition, orientation of the film and surface morphology. Relative high deposition rate at ∼ 165 nm/h was achieved.     

Initial stages of WO3 growth on silicon substrates

Ultrathin films (5 nm, 10 nm and 20 nm effective thickness) of WO₃ have been deposited in high vacuum (10^− 6 Torr) onto single crystal Si(100) substrates and studied with X-ray diffraction, atomic force microscopy, scanning tunneling microscopy and spectroscopy. The experiments have been carried out on “as-deposited” thin films or after 1 h post-deposition annealing at various temperatures (ranging from 300 °C to 500 °C). A size induced increase of the amorphous to crystalline (monoclinic) phase transition has been observed for the 5 nm and 10 nm films, with a critical crystallite size of 25 ± 5 nm and a critical temperature of 345 ± 5 °C. All the experimental evidences show that, upon annealing, there is a diffusion limited aggregation growth of WO₃ that forms large flat two-dimensional islands composed by aggregates of individual crystallites approximately uniform in size and shape. These islands are isolated in the 5 nm thin films, are connected in the 10 nm case and form a uniform patchwork in the 20 nm thin films. Scanning tunneling spectroscopy shows the opening of a large surface band gap (2.7 eV) in the 500 °C annealed films and the significant presence of in gap states for thin films prepared with a lower (below 400 °C) annealing temperature. These findings are discussed in view of the optimization of the best morphological, structural and electronic parameters to fabricate WO₃ gas sensing devices at the sub-micrometer length scale.