Low temperature deposition of high-quality epitaxial GaN is crucial for its integration in electronic applications. Chemical vapor deposition at approximately 800 °C using SiC with an AlN buffer layer or nitridized sapphire as substrate is used to facilitate the GaN growth. Here, we present a low temperature atomic layer deposition (ALD) process using tris(dimethylamido)gallium(III) with NH3 plasma. The ALD process shows self-limiting behaviour between 130–250 °C with a growth rate of 1.4 Å per cycle. The GaN films produced were crystalline on Si (100) at all deposition temperatures with a near stochiometric Ga/N ratio with low carbon and oxygen impurities. When GaN was deposited on 4H-SiC, the films grew epitaxially without the need for an AlN buffer layer, which has never been reported before. The bandgap of the GaN films was measured to be ∼3.42 eV and the Fermi level showed that the GaN was unintentionally n-type doped. This study shows the potential of ALD for GaN-based electronic devices.

Nanocomposites of metal nanoparticles (NPs) and bacterial nanocellulose (BC) enable fabrication of soft and biocompatible materials for optical, catalytic, electronic, and biomedical applications. Current BC-NP nanocomposites are typically prepared by in situ synthesis of the NPs or electrostatic adsorption of surface functionalized NPs, which limits possibilities to control and tune NP size, shape, concentration, and surface chemistry and influences the properties and performance of the materials. Here a self-assembly strategy is described for fabrication of complex and well-defined BC-NP composites using colloidal gold and silver NPs of different sizes, shapes, and concentrations. The self-assembly process results in nanocomposites with distinct biophysical and optical properties. In addition to antibacterial materials and materials with excellent senor performance, materials with unique mechanoplasmonic properties are developed. The homogenous incorporation of plasmonic gold NPs in the BC enables extensive modulation of the optical properties by mechanical stimuli. Compression gives rise to near-field coupling between adsorbed NPs, resulting in tunable spectral variations and enhanced broadband absorption that amplify both nonlinear optical and thermoplasmonic effects and enables novel biosensing strategies.

Shape-controlled synthesis of gold nanoparticles generally involves the use of surfactants, typically cetyltrimethylammonium (CTAX, X = Cl-, Br-), to regulate the nucleation growth process and to obtain colloidally stable nanoparticles. The surfactants adsorb on the nanoparticle surface making further functionalization difficult and therefore limit their use in many applications. Herein, the influence of CTAX on nanoparticle sensitivity to local dielectric environment changes is reported. It is shown, both experimentally and theoretically, that the CTAX bilayer significantly reduces the refractive index (RI) sensitivity of anisotropic gold nanoparticles such as nanocubes and concave nanocubes, nanorods, and nanoprisms. The RI sensitivity can be increased by up to 40% by removing the surfactant layer from nanoparticles immobilized on a solid substrate using oxygen plasma treatment. This increase compensates for the otherwise problematic decrease in RI sensitivity caused by the substrate effect. Moreover, the removal of the surfactants both facilitates nanoparticle biofunctionalization and significantly improves their catalytic properties. The strategy presented herein is a simple yet effective universal method for enhancing the RI sensitivity of CTAX-stabilized gold nanoparticles and increasing their potential as transducers in nanoplasmonic sensors, as well as in catalytic and biomedical applications.

In our modern society, we are surrounded by numerous sensors, constantly feeding us information about our physical environment. From small, wearable sensors that monitor our physiological status to large satellites orbiting around the earth, detecting global changes. Although, the performance of these sensors have been significantly improved during the last decades there is still a demand for faster and more reliable sensing systems with improved sensitivity and selectivity. The rapid progress in nanofabrication techniques has made a profound impact for the development of small, novel sensors that enables miniaturization and integration. A specific area where nanostructures are especially attractive is biochemical sensing, where the exceptional properties of nanomaterials can be utilized in order to detect and analyze biomolecular interactions. 

The focus of this thesis is to investigate plasmonic nanoparticles composed of gold or silver and optimize their performance as signal transducers in optical biosensors. Metal nanoparticles exhibit unique optical properties due to excitation of localized surface plasmons, which makes them highly sensitive probes for detecting small, local changes in their surrounding environment, for instance the binding of a biomolecule to the nanoparticle surface. This is the basic principle behind nanoplasmonic sensing based on refractometric detection, a sensing scheme that offers real-time and label-free detection of molecular interactions. 

This thesis shows that the sensitivity for detecting local refractive index changes is highly dependent on the geometry of the metal nanoparticles, their interaction with neighboring particles and their chemical composition and functionalization. An increased knowledge about how these parameters affects the sensitivity is essential when developing nanoplasmonic sensing devices with high performance based on metal nanoparticles. 

The possibility to enhance the local refractive index sensitivity using plasmonic coupling between spherical gold nanoparticles (Au-NPs) has been investigated. A strong and distinct optical coupling between Au-NPs of various sizes was achieved by controlling the interparticle separation using a layer-by-layer assembly of polyelectrolytes. The frequency of the coupled plasmon peak could be tuned by varying either the particle size or the interparticle separation, shown both experimentally and by theoretical simulations. The bulk refractive index (RI) sensitivity for the plasmonic coupling modes was investigated and compared to the RI sensitivity of monolayers of well-separated Au-NPs, and the results clearly demonstrates that the RI sensitivity can be significantly enhanced in plasmonically coupled Au-NPs. The proposed approach is simple and scalable and improves the rather modest RI sensitivity of spherical gold nanoparticles with a factor of 3, providing a new route for fabrication of inexpensive sensors based on plasmonic nanostructures.

The bulk refractive index (RI) sensitivity of dispersed and immobilized silver nanoparticles of three different shapes (spheres, cubes, and plates) is investigated. We demonstrate, both experimentally and theoretically, that the influence of immobilization on the RI sensitivity is highly dependent on the shape of the nanoparticles. A strong correlation is seen between the fraction of the particle surface area in direct contact with the substrate and the decrease in RI sensitivity when the particles are immobilized on a glass substrate. The largest decrease (−36%) is seen for the most sensitive nanoparticles (plates), drastically reducing their advantage over other nanoparticle shapes. The shape-dependent substrate effect is thus an important factor to consider when designing nanoplasmonic sensors based on colloidal noble-metal nanoparticles.

Bulk and surface refractive index sensitivity for localized surface plasmon resonance (LSPR) sensing based on edge gold-coated silver nanoprisms (GSNPs) and gold nanospheres was investigated and compared with conventional surface plasmon resonance (SPR) sensing based on propagating surface plasmons. The hybrid GSNPs benefit from an improved stability since the gold frame protecting the unstable silver facets located at the silver nanoprisms (SNPs) edges and tips prevents truncation or rounding of their sharp tips or edges, maintaining a high refractive index sensitivity even under harsh conditions. By using layer-by-layer deposition of polyelectrolytes and protein adsorption, we found that GSNPs exhibit 4-fold higher local refractive index sensitivity in close proximity (andlt;10 nm) to the surface compared to a flat gold film in the conventional SPR setup. Moreover, the sensitivity was 8-fold higher with GSNPs than with gold nanospheres. This shows that relatively simple plasmonic nanostructures for LSPR-based sensing can be engineered to outperform conventional SPR, which is particularly interesting in the context of detecting low molecular weight compounds where a small sensing volume, reducing bulk signals, is desired.

Coreshell Ag@Au nanoprisms are prepared through a surfactant-free seed-mediated approach by taking advantage of the anisotropic structure of silver nanoprisms as seeds. The gold coating on the silver nanoprism surface is achieved by using hydroxylamine as a mild reducing agent, and the final fully gold-coated prism structures are confirmed by microscopic and spectroscopic characterization. The resulting Ag@Au coreshell structure preserves the optical signatures of nanoprisms and offers versatile functionality and particularly better stability against oxidation than the bare silver nanoprism. The surface plasmon resonances of the coreshell Ag@Au nanoprisms can be tuned throughout the visible and near-IR range as a function of the Au shell thickness. Such tailorable optical features and surfactant-free gold shells have great potential applications in biosensing and bioimaging.

The prognosis for patients suffering from cardiovascular and many other diseases can be substantially improved if diagnosed at an early stage. High performance diagnostic testing using disposable microfluidic chips can provide a platform for realizing this vision. Åmic AB (Uppsala, Sweden) has developed a new microfluidic test chip for sandwich immunoassays fabricated by injection molding of the cycloolefin-copolymer Zeonor™. A highly ordered array of micropillars within the fluidic chip distributes the sample solution by capillary action. Since wetting of the pillar array surface is the only driving force for liquid distribution precise control of the surface chemistry is crucial. In this work we demonstrate a novel protocol for surface hydrophilization and antibody immobilization on cycloolefin-copolymer test chips, based on direct silanisation of the thermoplastic substrate. Dextran is subsequently covalently coupled to amino groups, thus providing a coating with a low contact angle suitable for antibody immobilization. The contact angle of dextran coated chips is stable for at least two months, which enables production of large batches that can be stored for extended periods of time. We demonstrate the utility of the presented platform and surface chemistry in a C-reactive protein assay with a detection limit of 2.6 ng ml-1, a dynamic range of 102 and a coefficient of variance of 15%. © The Royal Society of Chemistry.