Robert N. Lamb

POSTDEPOSITION ANNEALING OF RADIO FREQUENCY MAGNETRON SPUTTERED ZNO FILMS

Zinc oxide (ZnO) films have been deposited on 1 μm SiO2/Si (100) substrates by rf magnetron sputtering. Using a sputtering gas of pure oxygen, a pressure regime is found in which the ZnO films grow on room temperature substrates with a single (0001) orientation, small grains (crystallite sizes ∼10–15 nm), and high intrinsic biaxial compressive stress (∼6 GPa). The effects of post‐deposition annealing these films in air was investigated over a range of temperatures (200–1000 °C) and durations (2–2000 min). Annealing resulted in lower biaxial compressive stresses and increased average crystallite sizes in all films. Additional ZnO grain orientations were detected only after annealing above 500 °C for longer than 90 min, and the results are interpreted in terms of film recrystallization. Consequently, a relatively rapid thermal anneal at 1000 °C for 5 min caused grain recovery without recrystallization, resulting in maximum stress reduction (90%–100% of stress was relieved and average crystallized size tripl...

Bactericidal activity of black silicon

Black silicon is a synthetic nanomaterial that contains high aspect ratio nanoprotrusions on its surface, produced through a simple reactive-ion etching technique for use in photovoltaic applications. Surfaces with high aspect-ratio nanofeatures are also common in the natural world, for example, the wings of the dragonfly Diplacodes bipunctata. Here we show that the nanoprotrusions on the surfaces of both black silicon and D. bipunctata wings form hierarchical structures through the formation of clusters of adjacent nanoprotrusions. These structures generate a mechanical bactericidal effect, independent of chemical composition. Both surfaces are highly bactericidal against all tested Gram-negative and Gram-positive bacteria, and endospores, and exhibit estimated average killing rates of up to ~450,000 cells min−1 cm−2. This represents the first reported physical bactericidal activity of black silicon or indeed for any hydrophilic surface. This biomimetic analogue represents an excellent prospect for the development of a new generation of mechano-responsive, antibacterial nanomaterials.

XPS study of Nb-doped oxygen sensing TiO2 thin films prepared by sol-gel method

Abstract Titanium dioxide (TiO2) thin films have been prepared using the sol-gel method and subsequently doped with niobium oxide (Nb2O5) for use in oxygen sensing applications. The chemical composition of the resultant film sensor surface has been investigated using X-ray photoelectron spectroscopy (XPS). Films are essentially stoichiometric with carbon as the dominant impurity at the surface. The film electrical resistance has been examined for detection of oxygen at concentrations of 1 ppm to 1%. Doping resulted in a 40% increase in the oxygen gas sensitivity of the thin films at an operating temperature, as low as 190°C.

Engineering nanoscale roughness on hydrophobic surface—preliminary assessment of fouling behaviour

Abstract A preliminary investigation of the fouling behaviour of smooth and roughened superhydrophobic coatings is reported. The effect of nanoscale interfacial roughness on the adhesion of single (SW8) and mixed cultures of micro-foulant for periods of up to 6 months was assessed using visual and wettability measurements. Detailed analysis indicated virtually no micro-organism attached to the superhydrophobic surfaces in the first weeks of immersion. As a result by comparison with smooth substrates, whichexhibited fouling within a day, very rough (roughness ratio <2.7) surfaces exhibited high resistance to fouling over a 6-month period. However, after periods exceeding 2 months under ocean conditions, both films showed limited anti-fouling properties. There appears to be a correlation between the nature of the nanoscale roughness in the creation of superhydrophobic coatings and their potential anti-fouling properties. The future architecture of such a correlation is investigated.

The role of nano-roughness in antifouling

Nano-engineered superhydrophobic surfaces have been investigated for potential fouling resistance properties. Integrating hydrophobic materials with nanoscale roughness generates surfaces with superhydrophobicity that have water contact angles (θ) >150° and concomitant low hysteresis (<10°). Three superhydrophobic coatings (SHCs) differing in their chemical composition and architecture were tested against major fouling species (Amphora sp., Ulva rigida, Polysiphonia sphaerocarpa, Bugula neritina, Amphibalanus amphitrite) in settlement assays. The SHC which had nanoscale roughness alone (SHC 3) deterred the settlement of all the tested fouling organisms, compared to selective settlement on the SHCs with nano- and micro-scale architectures. The presence of air incursions or nanobubbles at the interface of the SHCs when immersed was characterized using small angle X-ray scattering, a technique sensitive to local changes in electron density contrast resulting from partial or complete wetting of a rough interface. The coating with broad spectrum antifouling properties (SHC 3) had a noticeably larger amount of unwetted interface when immersed, likely due to the comparatively high work of adhesion (60.77 mJ m−2 for SHC 3 compared to 5.78 mJ m−2 for the other two SHCs) required for creating solid/liquid interface from the solid/vapour interface. This is the first example of a non-toxic, fouling resistant surface against a broad spectrum of fouling organisms ranging from plant cells and non-motile spores, to complex invertebrate larvae with highly selective sensory mechanisms. The only physical property differentiating the immersed surfaces is the nano-architectured roughness which supports longer standing air incursions providing a novel non-toxic broad spectrum mechanism for the prevention of biofouling.

The influence of film crystallinity on the coupling efficiency of ZnO optical modulator waveguides

The coupling efficiencies of sputtered, c-axis-orientated zinc oxide (ZnO) films in guided wave resonance optical modulators have been measured. The ZnO planar waveguide is sandwiched between two 3 nm thick chromium layers on top of a SiO2/Si(100) substrate. This novel design facilitates application of a modulation voltage directly across the dielectric film in a prism coupler set-up and thus avoids voltage losses across the silicon dioxide (SiO2) optical isolation layer. We present a comparison of four differently prepared ZnO waveguides/modulators to investigate the influence of film crystallinity on the coupling efficiency and electro-optical parameters. The coupling efficiency of a ZnO waveguide is found to be dependent on both film thickness and average grain diameter measured in the plane of the film. A high efficiency (0.76) can be achieved for film thicknesses below 300 nm when the average grain diameter is 26 nm. This value is comparable to the maximum value of typically around 0.81 expected from model calculations for uniform couplers.

Electronic characteristics of the SrBi2Ta2O9–Pt junction

The voltage and film composition dependence of leakage currents of ferroelectric SrBi2Ta2O9 thin films, sandwiched between Pt has been studied. Schottky emission dominated the leakage current at voltages above the ohmic conduction regime, while space charge limited currents (SCLC), for which the observed temperature dependence is correctly predicted in Rose’s theory, appeared to dominate the leakage current in high conductivity SrBi2Ta2O9 thin films including bismuth-excess samples. A consequence of the latter was the observation of negative differential resistivity in high conductivity SrBi2Ta2O9 thin films. X-ray photoemission spectroscopy depth profiling indicated that Bi has diffused into the ferroelectric–metal interface and also influenced the electronic conduction mechanism of the ferroelectric capacitors. Confirmation of this was found through the current–voltage dependence of Pt/SrBi2Ta2O9/Bi, which is compatible with space charge limited currents. The theory of Rose was successfully applied to t...

An XPS study of zinc oxide thin film growth on copper using zinc acetate as a precursor

Abstract Ultrathin zinc oxide (ZnO) films (10–200 A thickness range) were grown on polycrystalline copper substrates via the sublimation of anhydrous zinc acetate (Zn(CH3COO)2) in high vacuum (5 × 10−7mbar). The thermal decomposition of basic zinc acetate precursor films, deposited on room temperature substrates, to ZnO were examined using X-ray photoelectron spectroscopy (XPS). The transformation to ZnO was monitored as a function of post-deposition sample heating, up to temperatures of 350 °C. ZnO films were grown on copper substrates heated to 400 °C and analysed using angle-dependent XPS and depth profiling. These films were found to contain less than 4% atomic carbon in the bulk and there was no direct evidence for island-type growth.

Investigation on ozone-sensitive In2O3 thin films

Abstract Indium oxide (In2O3) thin films have been prepared by sol–gel and RF sputtering techniques. The sol–gel film appeared to be much more sensitive to ozone compared to the RF sputtered film. The morphology of both films was examined by SEM, while their chemical composition was analyzed using X-ray photoelectron spectroscopy (XPS) in order to understand the properties responsible for the high sensitivity of sol–gel films. The examination results showed that the sol–gel films are very porous and uniform in grain size, but the RF films are relatively dense and of coalescent grains. XPS analysis also highlighted that there may be oxygen vacancies on the surface of sol–gel films. The large surface areas of sol–gel films and oxygen deficiency in the film structure are responsible for its higher sensitivity.

Air-directed attachment of coccoid bacteria to the surface of superhydrophobic lotus-like titanium

Superhydrophobic titanium surfaces fabricated by femtosecond laser ablation to mimic the structure of lotus leaves were assessed for their ability to retain coccoid bacteria. Staphylococcus aureus CIP 65.8T, S. aureus ATCC 25923, S. epidermidis ATCC 14990T and Planococcus maritimus KMM 3738 were retained by the surface, to varying degrees. However, each strain was found to preferentially attach to the crevices located between the microscale surface features. The upper regions of the microscale features remained essentially cell-free. It was hypothesised that air entrapped by the topographical features inhibited contact between the cells and the titanium substratum. Synchrotron SAXS revealed that even after immersion for 50 min, nano-sized air bubbles covered 45% of the titanium surface. After 1 h the number of cells of S. aureus CIP 65.8T attached to the lotus-like titanium increased to 1.27 × 105 mm−2, coinciding with the replacement of trapped air by the incubation medium.