Fabrication and characterization of plasma polymer thin films from monoterpene alcohols for applications in organic electronics and biotechnology

After more than twenty years of basic and applied research, the use of nanotechnology in the design and manufacture of nanoscale materials is rapidly increasing, particularly in commercial applications that span from electronics across renewable energy areas, and biomedical devices. Novel polymers are attracting significant attention for they promise to provide a low−cost high−performance alternative to existing materials. Furthermore, these polymers have the potential to overcome limitations imposed by currently available materials thus enabling the development of new technologies and applications that are currently beyond our reach. This work focuses on the development of a range of new low−cost environmentally−friendly polymer materials for applications in areas of organic (flexible) electronics, optics, and biomaterials. The choice of the monomer reflects the environmentally−conscious focus of this project. Terpinen−4−ol is a major constituent of Australian grown Melaleuca alternifolia (tea tree) oil, attributed with the oil's antimicrobial and anti−inflammatory properties. Plasma polymerisation was chosen as a deposition technique for it requires minimal use of harmful chemicals and produces no hazardous by−products. Polymer thin films were fabricated under varied process conditions to attain materials with distinct physico−chemical, optoelectrical, biological and degradation characteristics. The resultant materials, named polyterpenol, were extensively characterised using a number of well−accepted and novel techniques, and their fundamental properties were defined. Polyterpenol films were demonstrated to be hydrocarbon rich, with variable content of oxygen moieties, primarily in the form of hydroxyl and carboxyl functionalities. The level of preservation of original monomer functionality was shown to be strongly dependent on the deposition energy, with higher applied power increasing the molecular fragmentation and substrate temperature. Polyterpenol water contact angle contact angle increased from 62.7° for the 10 W samples to 76.3° for the films deposited at 100 W. Polymers were determined to resist solubilisation by water, due to the extensive intermolecular and intramolecular hydrogen bonds present, and other solvents commonly employed in electronics and biomedical processing. Independent of deposition power, the surface topography of the polymers was shown to be smooth (Rq <0.5 nm), uniform and defect free. Hardness of polyterpenol coatings increased from 0.33 GPa for 10 W to 0.51 GPa for 100 W (at 500 μN load). Coatings deposited at higher input RF powers showed less mechanical deformation during nanoscratch testing, with no considerable damage, cracking or delamination observed. Independent of the substrate, the quality of film adhesion improved with RF power, suggesting these coatings are likely to be more stable and less susceptible to wear. Independent of fabrication conditions, polyterpenol thin films were optically transparent, with refractive index approximating that of glass. Refractive index increased slightly with deposition power, from 1.54 (10 W) to 1.56 (100 W) at 500 nm. The optical band gap values declined with increasing power, from 2.95 eV to 2.64 eV, placing the material within the range for semiconductors. Introduction of iodine impurity reduced the band gap of polyterpenol, from 2.8 eV to 1.64 eV, by extending the density of states more into the visible region of the electromagnetic spectrum. Doping decreased the transparency and increased the refractive index from 1.54 to 1.70 (at 500 nm). At optical frequencies, the real part of permittivity (k) was determined to be between 2.34 and 2.65, indicating a potential low-k material. These permittivity values were confirmed at microwave frequencies, where permittivity increased with input RF energy – from 2.32 to 2.53 (at 10 GHz ) and from 2.65 to 2.83 (at 20 GHz). At low frequencies, the dielectric constant was determined from current−voltage characteristics of Al−polyterpenol−Al devices. At frequencies below 100 kHz, the dielectric constant varied with RF power, from 3.86 to 4.42 at 1 kHz. For all samples, the resistivity was in order of 10⁸−10⁹ _m (at 6 V), confirming the insulating nature of polyterpenol material. In situ iodine doping was demonstrated to increase the conductivity of polyterpenol, from 5.05 × 10⁻⁸ S/cm to 1.20 × 10⁻⁶ S/cm (at 20 V). Exposed to ambient conditions over extended period of time, polyterpenol thin films were demonstrated to be optically, physically and chemically stable. The bulk of ageing occurred within first 150 h after deposition and was attributed to oxidation and volumetric relaxation. Thermal ageing studies indicated thermal stability increased for the films manufactured at higher RF powers, with degradation onset temperature associated with weight loss shifting from 150 ºC to 205 ºC for 10 W and 100 W polyterpenol, respectively. Annealing the films to 405 °C resulted in full dissociation of the polymer, with minimal residue. Given the outcomes of the fundamental characterisation, a number of potential applications for polyterpenol have been identified. Flexibility, tunable permittivity and loss tangent properties of polyterpenol suggest the material can be used as an insulating layer in plastic electronics. Implementation of polyterpenol as a surface modification of the gate insulator in pentacene-based Field Effect Transistor resulted in significant improvements, shifting the threshold voltage from + 20 V to –3 V, enhancing the effective mobility from 0.012 to 0.021 cm²/Vs, and improving the switching property of the device from 10⁷ to 10⁴. Polyterpenol was demonstrated to have a hole transport electron blocking property, with potential applications in many organic devices, such as organic light emitting diodes. Encapsulation of biomedical devices is also proposed, given that under favourable conditions, the original chemical and biological functionality of terpinen−4−ol molecule can be preserved. Films deposited at low RF power were shown to successfully prevent adhesion and retention of several important human pathogens, including P. aeruginosa, S. aureus, and S. epidermidis, whereas films deposited at higher RF power promoted bacterial cell adhesion and biofilm formation. Preliminary investigations into in vitro biocompatibility of polyterpenol demonstrated the coating to be non−toxic for several types of eukaryotic cells, including Balb/c mice macrophage and human monocyte type (HTP−1 non-adherent) cells. Applied to magnesium substrates, polyterpenol encapsulating layer significantly slowed down in vitro biodegradation of the metal, thus increasing the viability and growth of HTP−1 cells. Recently, applied to varied nanostructured titanium surfaces, polyterpenol thin films successfully reduced attachment, growth, and viability of P. aeruginosa and S. aureus.

Combined effect of double antireflection coating and reversible molecular doping on performance of few-layer graphene/n-silicon Schottky barrier solar cells

Few-layer graphene films were grown by chemical vapor deposition and transferred onto n-type crystalline silicon wafers to fabricate graphene/n-silicon Schottky barrier solar cells. In order to increase the power conversion efficiency of such cells the graphene films were doped with nitric acid vapor and an antireflection treatment was implemented to reduce the sunlight reflection on the top of the device. The doping process increased the work function of the graphene film and had a beneficial effect on its conductivity. The deposition of a double antireflection coating led to an external quantum efficiency up to 90% across the visible and near infrared region, the highest ever reported for this type of devices. The combined effect of graphene doping and antireflection treatment allowed to reach a power conversion efficiency of 8.5% exceeding the pristine (undoped and uncoated) device performance by a factor of 4. The optical properties of the antireflection coating were found to be not affected by the exposure to nitric acid vapor and to remain stable over time.

Tissue thickness calculation in ocular optical coherence tomography

Thickness measurements derived from optical coherence tomography (OCT) images of the eye are a fundamental clinical and research metric, since they provide valuable information regarding the eye’s anatomical and physiological characteristics, and can assist in the diagnosis and monitoring of numerous ocular conditions. Despite the importance of these measurements, limited attention has been given to the methods used to estimate thickness in OCT images of the eye. Most current studies employing OCT use an axial thickness metric, but there is evidence that axial thickness measures may be biased by tilt and curvature of the image. In this paper, standard axial thickness calculations are compared with a variety of alternative metrics for estimating tissue thickness. These methods were tested on a data set of wide-field chorio-retinal OCT scans (field of view (FOV) 60° x 25°) to examine their performance across a wide region of interest and to demonstrate the potential effect of curvature of the posterior segment of the eye on the thickness estimates. Similarly, the effect of image tilt was systematically examined with the same range of proposed metrics. The results demonstrate that image tilt and curvature of the posterior segment can affect axial tissue thickness calculations, while alternative metrics, which are not biased by these effects, should be considered. This study demonstrates the need to consider alternative methods to calculate tissue thickness in order to avoid measurement error due to image tilt and curvature.

Thickness metrics in posterior segment optical coherence tomography

To develop and compare a set of metrics for calculating tissue thickness in wide-field OCT data.

Automatic wildfire detection and simulation using optical information from unmanned aerial systems

In many parts of the world, uncontrolled fires in sparsely populated areas are a major concern as they can quickly grow into large and destructive conflagrations in short time spans. Detecting these fires has traditionally been a job for trained humans on the ground, or in the air. In many cases, these manned solutions are simply not able to survey the amount of area necessary to maintain sufficient vigilance and coverage. This paper investigates the use of unmanned aerial systems (UAS) for automated wildfire detection. The proposed system uses low-cost, consumer-grade electronics and sensors combined with various airframes to create a system suitable for automatic detection of wildfires. The system employs automatic image processing techniques to analyze captured images and autonomously detect fire-related features such as fire lines, burnt regions, and flammable material. This image recognition algorithm is designed to cope with environmental occlusions such as shadows, smoke and obstructions. Once the fire is identified and classified, it is used to initialize a spatial/temporal fire simulation. This simulation is based on occupancy maps whose fidelity can be varied to include stochastic elements, various types of vegetation, weather conditions, and unique terrain. The simulations can be used to predict the effects of optimized firefighting methods to prevent the future propagation of the fires and greatly reduce time to detection of wildfires, thereby greatly minimizing the ensuing damage. This paper also documents experimental flight tests using a SenseFly Swinglet UAS conducted in Brisbane, Australia as well as modifications for custom UAS.

Lowering grain boundary resistance of BaZr0.8Y0.2O3−δ with LiNO3 sintering-aid improves proton conductivity for fuel cell operation

A novel sintering additive based on LiNO3 was used to overcome the drawbacks of poor sinterability and low grain boundary conductivity in BaZr0.8Y0.2O3-δ (BZY20) protonic conductors. The Li-additive totally evaporated during the sintering process at 1600°C for 6 h, which led to highly dense BZY20 pellets (96.5% of the theoretical value). The proton conductivity values of BZY20 with Li sintering-aid were significantly larger than the values reported for BZY sintered with other metal oxides, due to the fast proton transport in the "clean" grain boundaries and grain interior. The total conductivity of BZY20-Li in wet Ar was 4.45 × 10-3 S cm-1 at 600°C. Based on the improved sinterability, anode-supported fuel cells with 25 μm-thick BZY20-Li electrolyte membranes were fabricated by a co-firing technique. The peak power density obtained at 700°C for a BZY-Ni/BZY20-Li/La0.6Sr0.4Co0.2Fe 0.8O3-δ (LSCF)-BZY cell was 53 mW cm-2, which is significantly larger than the values reported for fuel cells using electrolytes made of BZY sintered with the addition of ZnO and CuO, confirming the advantage of using Li as a sintering aid.

Sinteractivity, proton conductivity and chemical stability of BaZr 0.7In0.3O3-δ for solid oxide fuel cells (SOFCs)

In3+ was used as dopant for BaZrO3 proton conductor and 30 at%-doped BaZrO3 samples (BaZr0.7In 0.3O3-δ, BZI) were prepared as electrolyte materials for proton-conducting solid oxide fuel cells (SOFCs). The BZI material showed a much improved sinteractivity compared with the conventional Y-doped BaZrO 3. The BZI pellets reached almost full density after sintering at 1600 °C for 10 h, whereas the Y-doped BaZrO3 samples still remained porous under the same sintering conditions. The conductivity measurements indicated that BZI pellets showed smaller bulk but improved grain boundary proton conductivity, when compared with Y-doped BaZrO3 samples. A total proton conductivity of 1.7 × 10-3 S cm -1 was obtained for the BZI sample at 700 °C in wet 10% H 2 atmosphere. The BZI electrolyte material also showed adequate chemical stability against CO2 and H2O, which is promising for application in fuel cells.