Development of Graphene and Graphene-Nanoparticle Composites for Sensor Applications

The goal of this research was the synthesis of graphene and graphene nanocomposite for use as sensor materials. This dissertation describes the optimization of a novel approach to the synthesis of few layer graphene films on SiC, the modification of the graphene surface by wet chemical methods, the nucleation of nanoparticles to form graphene-nanoparticle composites, the fabrication of chemoresistive sensor structures from these materials, and the characterization of these surfaces and films.;In this work, the basic graphene synthesis method which uses halogen based plasma etching and ultra-high vacuum annealing (UHVA), has been optimized to reliably produce one, two, and three layer graphene on SiC films. The process has also been extended by replacing the UHVA step with rapid thermal annealing (RTA) in atmospheric pressure argon. Graphene films produced by both methods have been characterized using x-ray photoelectron spectroscopy (XPS), Raman microscopy, and atomic force microscopy (AFM). The UHVA process produces films with halogen-based and possibly some oxygen-based defects, whereas the RTA processes produces exclusively oxygen-based defects which include epoxide, hydroxyl, and carbonyl groups similar to, but at much lower levels, than that observed for graphene oxide (GO). As in the case for GO, the defect density was further reduced by wet chemical surface modification.;Nanoparticles (Ag, Au, Pt, Ir) were attached to these surfaces using solution based methods. The particle diameter and height distributions along with surface coverage were characterized using AFM methods. Key parameters in these studies included solution composition and incubation time. For electrical characterization and sensor testing, two structures were then fabricated using lithography free methods and electron beam evaporation. The first of these structures, referred to as the transmission line method (TLM) structure, was used in the present work for electrical characterization. Using the TLM structure, the electrical properties were characterized using two and four point probe methods. The films exhibited semiconducting behavior which is believed to be due to the opening of a band gap by the halogen- and oxygen-based defects. Using the two and four pint methods, the Schottky barrier height, the carrier density, electrical resistivity, and the carrier mobility were determined. The electrical resistivity was found to have an inverse relationship with number of graphene layers for one, two, and three layer films. The second device structure was a simple interdigitated sensor structure which was passed on to other researchers for sensor studies. Overall, reliable and reproducible synthesis and fabrication methods for graphene and graphene-nanoparticle composites have been developed for the next stage of testing and sensor development

Energy dispersive-EXAFS of Pd nucleation at a liquid/liquid interface

Energy dispersive extended X-ray absorption fine structure (EDE) has been applied to Pd nanoparticle nucleation at a liquid/liquid interface under control over the interfacial potential and thereby the driving force for nucleation. Preliminary analysis focusing on Pd K edge-step height determination shows that under supersaturated conditions the concentration of Pd near the interface fluctuate over a period of several hours, likely due to the continuous formation and dissolution of sub-critical nuclei. Open circuit potential measurements conducted ex-situ in a liquid/liquid electrochemical cell support this view, showing that the fluctuations in Pd concentration are also visible as variations in potential across the liquid/liquid interface. By decreasing the interfacial potential through inclusion of a common ion (tetraethylammonium, TEA+) the Pd nanoparticle growth rate could be slowed down, resulting in a smooth nucleation process. Eventually, when the TEA+ ions reached an equilibrium potential, Pd nucleation and particle growth were inhibited

Synthesis of Copper Oxide Nanoparticles in Droplet Flow Reactors

Synthesis of metal oxide nanoparticles within droplet flow reactors is advantageous over batch synthesis due to the elimination of concentration and temperature gradients inside the reactor and prevention of reactor fouling. We present results on the synthesis of copper oxide nanoparticles using aqueous droplets of copper acetate and acetic acid inside a bulk stream of sodium hydroxide in 1-octanol. Varying the copper acetate, acetic acid, and sodium hydroxide concentration resulted in needle-like and plate-like nanoparticles of varying sizes. The rate of mass transfer from the bulk to the droplet phase was found to increase with flow rate and addition of surfactants

Nucleation of Iron Oxide Nanoparticles Mediated by Mms6 Protein in Situ

Biomineralization proteins are widely used as templating agents in biomimetic synthesis of a variety of organic-inorganic nanostructures. However, the role of the protein in controlling the nucleation and growth of biomimetic particles is not well understood, because the mechanism of the bioinspired reaction is often deduced from ex situ analysis of the resultant nanoscale mineral phase. Here we report the direct visualization of biomimetic iron oxide nanoparticle nucleation mediated by an acidic bacterial recombinant protein, Mms6, during an in situ reaction induced by the controlled addition of sodium hydroxide to solution-phase Mms6 protein micelles incubated with ferric chloride. Using in situ liquid cell scanning transmission electron microscopy we observe the liquid iron prenucleation phase and nascent amorphous nanoparticles forming preferentially on the surface of protein micelles. Our results provide insight into the early steps of protein-mediated biomimetic nucleation of iron oxide and point to the importance of an extended protein surface during nanoparticle formation

The role of electron irradiation history in liquid cell transmission electron microscopy.

In situ liquid cell transmission electron microscopy (LC-TEM) allows dynamic nanoscale characterization of systems in a hydrated state. Although powerful, this technique remains impaired by issues of repeatability that limit experimental fidelity and hinder the identification and control of some variables underlying observed dynamics. We detail new LC-TEM devices that improve experimental reproducibility by expanding available imaging area and providing a platform for investigating electron flux history on the sample. Irradiation history is an important factor influencing LC-TEM results that has, to this point, been largely qualitatively and not quantitatively described. We use these devices to highlight the role of cumulative electron flux history on samples from both nanoparticle growth and biological imaging experiments and demonstrate capture of time zero, low-dose images on beam-sensitive samples. In particular, the ability to capture pristine images of biological samples, where the acquired image is the first time that the cell experiences significant electron flux, allowed us to determine that nanoparticle movement compared to the cell membrane was a function of cell damage and therefore an artifact rather than visualizing cell dynamics in action. These results highlight just a subset of the new science that is accessible with LC-TEM through the new multiwindow devices with patterned focusing aides

In situ study of nucleation and aggregation phases for nanoparticles grown by laser-driven methods

In the last decades, nanomaterials and nanotechnologies have become fundamental and irreplaceable in many fields of science and technology. When used in applications, their properties depend on many factors such as size, shape, internal structure and composition. For this, exact knowledge of their structural features is essential when developing fabrication technologies and searching for new types of nanostructures or nanoparticles with specific properties. For the latter, the knowledge of the precise temporal evolution of the growth processes is fundamental when it comes to industrial production and applications. Here we present a method to control, with very high precision, the starting of the aggregation phase during the Laser Ablation in solution growth process. This is obtained by monitoring the optical absorption of the colloidal solution. We apply this control method on the most popular metallic nanoparticle materials (Ag, Al, Co, and Ti) and verify the technique using morphological analysis conducted by AFM and SEM microscopy. The experimental results are explained in terms of Mie extinction theory and Thermal Model for Laser Ablatio

Tunable photochemical deposition of silver nanostructures on layered ferroelectric CuInP2_2S6

2D layered ferroelectric materials such as CuInP$_2$S6 (CIPS) are promising candidates for novel and high-performance photocatalysts, owning to their ultrathin layer thickness, strong interlayer coupling, and intrinsic spontaneous polarization, while how to control the photocatalytic activity in layered CIPS remains unexplored. In this work, we report for the first time the photocatalytic activity of ferroelectric CIPS for the chemical deposition of silver nanostructures (AgNSs). The results show that the shape and spatial distribution of AgNSs on CIPS are tunable by controlling layer thickness, environmental temperature, and light wavelength. The ferroelectric polarization in CIPS plays a critical role in tunable AgNS photodeposition, as evidenced by layer thickness and temperature dependence experiments. We further reveal that AgNS photodeposition process starts from the active site creation, selective nanoparticle nucleation/aggregation, to the continuous film formation. Moreover, AgNS/CIPS heterostructures prepared by photodeposition exhibit excellent resistance switching behavior and good surface enhancement Raman Scattering activity. Our findings provide new insight into the photocatalytic activity of layered ferroelectrics and offer a new material platform for advanced functional device applications in smart memristors and enhanced chemical sensors.Comment: 18 pages, 5 figure

Catalyst nanoparticle growth dynamics and their influence on product morphology in a CVD process for continuous carbon nanotube synthesis

Extrapolating the properties of individual CNTs into macro-scale CNT materials using a continuous and cost effective process offers enormous potential for a variety of applications. The floating catalyst chemical vapor deposition (FCCVD) method discussed in this paper bridges the gap between generating nano- and macro-scale CNT material and has already been adopted by industry for exploitation. A deep understanding of the phenomena occurring within the FCCVD reactor is thereby key to producing the desired CNT product and successfully scaling up the process further. This paper correlates information on decomposition of reactants, axial catalyst nanoparticle dynamics and the morphology of the resultant CNTs and shows how these are strongly related to the temperature and chemical availability within the reactor. For the first time, in-situ measurements of catalyst particle size distributions coupled with reactant decomposition profiles and a detailed axial SEM study of formed CNT materials reveal specific domains that have important implications for scale-up. A novel observation is the formation, disappearance and reformation of catalyst nanoparticles along the reactor axis, caused by their evaporation and re-condensation and mapping of different CNT morphologies as a result of this process.The authors thank Qflo Ltd for providing funding towards this research, C. Hoecker additionally thanks Churchill College Cambridge for financial support, M. Bajada gratefully acknowledges financial support through the 'Master it! Scholarship Scheme'.This is the accepted manuscript. The final version is available at http://dx.doi.org/10.1016/j.carbon.2015.09.05