Single-photon emitters in hexagonal boron nitride: a review of progress.

This report summarizes progress made in understanding properties such as zero-phonon-line energies, emission and absorption polarizations, electron-phonon couplings, strain tuning and hyperfine coupling of single photon emitters in hexagonal boron nitride. The primary aims of this research are to discover the chemical nature of the emitting centres and to facilitate deployment in device applications. Critical analyses of the experimental literature and data interpretation, as well as theoretical approaches used to predict properties, are made. In particular, computational and theoretical limitations and challenges are discussed, with a range of suggestions made to overcome these limitations, striving to achieve realistic predictions concerning the nature of emitting centers. A symbiotic relationship is required in which calculations focus on properties that can easily be measured, whilst experiments deliver results in a form facilitating mass-produced calculations

Investigation of Wear Mechanism of Gallium Nitride

The optoelectronic properties of gallium nitride (GaN) has been extensively studied for decades, which has facilitated its application in many different areas, cementing it as one of the most important semiconductor materials in the world. However, in comparison to the study of its optoelectronic properties, there are few studies of its mechanical properties - especially the tribological performance. Knowing the tribological properties of GaN, such as friction and wear, is crucial for understanding its machinability, the implementation of GaN in MEMS, solar cells, and other devices, as well as the wear performance of these GaN-based devices when working under harsh environments. In our study, we reveal that GaN has an ultralow wear nature, and that its wear rate can approach that of diamond. We also discover that the wear rate of GaN is affected by its crystallographic orientation, humidity, and composition.For the crystallographic orientation dependence, we look into the physics by both experimental and computational methods. We demonstrate that both the friction coefficient and wear rate of GaN exhibits a 60° periodicity. We conclude that these periodic variations of wear rate and friction coefficient are the results of a periodic variation of the energy barrier.The moisture dependent wear mechanism of GaN has been investigated under dry, low humidity, and high humidity environments. The results show that the wear rate of GaN perfectly follows an increasing of the humidity which spans over two orders of magnitude when the testing environment switches from dry nitrogen to humid lab air. On the contrary, the friction coefficient gave a contrary response, i.e., the lowest friction coefficient was found under low humidity environment, dry nitrogen had the highest friction coefficient, and the humid environment had its friction in the middle. Various characterization techniques, including SEM/EDS, AFM and TEM were employed to interrogate the worn surfaces under each condition. Based on the results, we hypothesize that the wear under dry nitrogen environment is adhesive in nature whereas grooving abrasive wear dominates the wear behavior of GaN under a humid environment.The compositional study of GaN wear revealed that by alloying different elements into the GaN system, one can not only tune the bandgap, but also modify the wear rate. This finding can be useful for applications and design that require suitable electronic properties while keep the wear rate within an acceptable range.Furthermore, during the investigation of the GaN wear mechanism, we discovered that the tribological sliding can also alter the surface band bending of this material. Our results demonstrate that the environment, number of sliding cycles, and normal loads can effectively tune the surface band bending of GaN. This finding shows the capability of mechanical dynamic contact for surface electronic property modification, which can be used in various applications, such as gas sensing, photocatalysis, and photochemistry.Understanding of the wear mechanism of GaN as well as the shear-induced band bending on GaN can remarkably promote the applications of GaN in various fields other than the optoelectronic area. This also reinforces the important message that tribology is not only a discipline that focuses on investigation of protective coating and lubrication but also can be used in device design and fabrication

FIRST-PRINCIPLES DENSITY FUNCTIONAL THEORY STUDIES OF REACTIVITIES OF HETEROGENEOUS CATALYSTS DETERMINED BY STRUCTURE AND SUBSTRATE

In this dissertation, density functional theory (DFT) calculations were used to investigate (1)NO2 adsorption on BaO in NOx Storage Reduction (NSR) catalyst affected by the morphology of BaO and the γ-Al2O3 support, (2) energy barrier of H2 dissociative adsorption over Mg clusters affected by its electronic structure, and (3) comparison of the activities of CeO2 clusters affected by two different supports--monoclinic ZrO2 and non-spinel γ-Al2O3. Our results showed that the electronic effect caused by the non-stoichiometry of the bare BaO clusters and surfaces improves their reactivities toward NO2 adsorption greatly, whereas the geometric structure of the catalyst has only minor effect on the activity; we also found that the γ-Al2O3 substrate improves the reactivities of the supported BaO clusters and at the same time the interface between BaO and γ-Al2O3 provided a unique and highly reactive environment for NO2 adsorption. Hydrogen dissociation barrier over pure Mg clusters is greatly affected by the electronic structure of the clusters--closed shell clusters such as Mg10 and Mg92- have higher energy barrier toward H2 dissociation; however, H2 dissociation over clusters that are two electrons shy from the closed electronic shell are relatively easier. As substrates, neither ZrO2(111) nor γ-Al2O3(100) affects the reactivity of the supported Ce2O4 toward CO2 adsorption and CO physisorption significantly; whereas the reactivity of Ce2O4 toward CO reactive adsorption were found to be affected by the two substrates very differently

New Trends in Photo(Electro)catalysis

This reprint focuses on new trends in photo-electrocatalysis, specifically addressed to the remediation of wastewater and energy production. The remediation of wastewater, up to a level that is acceptable for discharge into receiving waterbodies, involves an ever-growing demand of energy, so effective and low-energy treatment processes are highly desirable. Among the other treatments, photo- and photo-electrochemical treatment processes may be considered as advanced oxidation processes (AOP), which are based on the generation of OH radicals, strong oxidizing agents able to indiscriminately degrade even the most persistent organic compounds. Photocatalysis and photo-electrocatalysis can be considered as effective methods for organic degradation, especially when the semiconductor is active in the range of visible light. Several results are presented on new morphologies and structures, which allow more photoactive, visibly responsive, and stable materials, as well as studies on combined processes in which photo- or photo-electrochemistry contribute to an increase in the sustainability of the whole process, lowering costs and achieving the most valuable final products. In view of the circular economy concept, microbial fuel cell systems are also considered as possible way to recover energy from organic pollutants contained in wastewater

Nanocrystal

We focused on cutting-edge science and technology of Nanocrystals in this book. "Nanocrystal" is expected to lead to the creation of new materials with revolutionary properties and functions. It will open up fresh possibilities for the solution to the environmental problems and energy problems. We wish that this book contributes to bequeath a beautiful environment and valuable resources to subsequent generations

Kopplung von Stickstoff-Fehlstellenzentren an photonische Kristallresonatoren in Diamant

In this thesis nitrogen-vacancy (NV) centres were coupled to photonic crystal cavities in single-crystal diamond. To this end, the used cavities were first thoroughly characterized in simulations. Subsequently, diamond membranes with a thickness of only a few hundred nanometers were fabricated in two different sample systems and characterized by a methodology that allows the cavities to be fabricated reproducibly therein by FIB-milling. The deployed FIB-milling process was optimized such that fabrication tolerances were minimized and cavity modes with Q-factors up to Q=8250 at modal volumes around one cubic wavelength could be achieved. Furthermore, the directional characteristic of light emission out of the nanostructures was examined in further simulations and a sample system optimized with regard to the collection efficiency was produced. NV-centres were incorporated into the photonic crystal cavities by means of a high resolution implantation technique and subsequently a cavity mode was tuned deterministically into resonance to the zero-phonon line of the incorporated NV-centres. Thus, the spontaneous emission lifetime was reduced from 9.0 ns to 8.0 ns and a spontaneous emission coupling factor of 18.7 % was reached. As an application it was shown that the signal-to-noise ratio of the optical spin readout is almost tripled in resonance to the NV zero-phonon line.In dieser Arbeit wurden Stickstoff-Fehlstellenzentren (NV-Zentren) an photonische Kristallresonatoren in einkristallinem Diamant gekoppelt. Hierzu wurden die eingesetzten Resonatoren in Simulationen zunächst umfassend charakterisiert. Im Anschluss daran wurden wenige hundert Nanometer dicke Diamantmembranen in zwei Probensystemen hergestellt und durch eine Analysemethodik derart charakterisiert, dass die Resonatoren darin reproduzierbar per FIB-Strukturierung hergestellt werden konnten. Der eingesetzte FIB-Prozess wurde optimiert, sodass Fabrikationsabweichungen minimiert und Gütefaktoren der Resonatormoden von bis zu Q=8250 bei Modenvolumina um eine kubische Wellenlänge erzielt werden konnten. Ferner wurde die Abstrahlcharakteristik von Licht aus den Nanostrukturen in Simulationen umfassend untersucht und auch ein hinsichtlich der Sammeleffizienz optimiertes Probensystem hergestellt. Durch zielgenaue Implantation konnten NV-Zentren in die Resonatorflächen eingebracht und im Anschluss daran die Mode eines Resonators gezielt auf die Nullphononenlinie der inkorporierten NV-Zentren abgestimmt werden. Die Lebensdauer des angeregten Zustands konnte dadurch von 9,0 ns auf 8,0 ns verkürzt und ein Kopplungsfaktor der spontanen Emission von 18,7 % erzielt werden. Als Anwendung konnte gezeigt werden, dass das Signal-Rausch-Verhältnis bei der optischen Auslese des NV-Elektronenspins durch das Abstimmen der Mode nahezu verdreifacht werden kann

TiO2 Nanomaterials: Photocatalysis and Multifunctional Water Treatment Applications

Globalization has increased the demand for clean water sources and has increased water pollution due to increased standards of consumption, urbanization and industrial activities. This has necessitated improvements in small-scale and large-scale water treatment processes in terms of improved energy efficiency for treatment and protocols and standards for unmonitored emerging contaminants. One such treatment platform that can be improved is in the area of advanced oxidation processes (AOPs). AOPs are used to decompose organic pollutants into smaller constituents. However, they often use hazardous chemicals that need to be added on a continual basis. TiO2 photocatalysis is an AOP that removes the need for consumable chemicals like hydrogen peroxide and ozone. This work focused on methods that improved the kinetics of TiO2 photocatalysis and combined the application of TiO2 photocatalytic AOP with other water applications, specifically filtration and corrosion protection, in order to increase its attractiveness for use in commercials applications by decreasing the energy required to operate and/or combining more than one application into one process. Two methods were studied to improve the reaction kinetics and limit recombination in TiO2 photocatalysis: (i) modifying physicochemical properties of TiO2 and (ii) altering operational parameters. Physicochemical properties of TiO2 were modified through the synthesis of various nanomaterials that limit recombination and/or increase the number reaction sites. One-dimensional TiO2 (TiO2 nanobelts) were synthesized and increased the electron lifetime compared to nanoparticles. Metal-semiconductor junctions (Ag-TiO2) were also made and they were able to efficiently separate electron and hole via the Schottky barrier effect and limit recombination. This work also investigated the effect of varying operational parameters in order to study their effects on the reaction kinetics. Operational parameters such as pH, light intensity, temperature and catalyst configuration (slurry or membrane) were explored. Of particular interest in this work was altering the light intensity intermittently, which is referred to as controlled periodic illumination (CPI). CPI was used as a means to increase reaction efficiency of TiO2 photocatalysis. The amount of energy required to remove organic contaminants was decreased by lowering the duty cycle and increasing the pulse frequency. In addition, CPI was used to compare the performance of slurry reactors and immobilized TiO2 membrane reactors, in which the latter suffered from mass transport limitations. CPI under mass transfer limitations revealed that the duty cycle may be reduced to 10% and this would not alter its reaction kinetics compared to continuous illumination. Utilization of TiO2¬ photocatalysis¬ was studied in three other water treatment application areas – emerging contaminants, filtration and corrosion protection. Using immobilized TiO2 under UV irradiation, emerging contaminants, specifically pharmaceuticals and personal care products were shown to be degraded based on their pharmaceutical properties (charge, molecular weight, and solubility). The compound charge had the greatest effect on the degradation performance. Another application that was explored was combining TiO2 AOP with filtration using a photocatalytic membrane reactor (PMR). TiO2 coated filters under PMR filtration were shown to increase flux under UV illumination and have higher removals than uncoated filters. Finally, the concurrent degradation of organic compounds and corrosion protection was demonstrated using TiO2 photoanodes coupled with steel. This method reduced the mass loss due to corrosion, while simultaneously degrading organic contaminants. It was shown that TiO2 photocatalysis and the TiO2 AOP process can be utilized in other pertinent areas in water treatment. Overall, the research demonstrated that the efficiency of TiO2 AOPs can be improved through the synthesis of nanomaterials that limit recombination, increasing material surface area, and effectively utilizing light using controlled periodic illumination. Furthermore, TiO2 photocatalysis can be combined with filtration and corrosion protection processes to increase its attractiveness in other water treatment areas

Investigating plasma modifications and gas-surface reactions of TiO2-based materials for photoconversion

2012 Fall.Includes bibliographical references.Plasmas offer added flexibility for chemists in creating materials with ideal properties. Normally unreactive precursors can be used to etch, deposit and modify surfaces. Plasma treatments of porous and compact TiO2 substrates were explored as a function of plasma precursor, substrate location in the plasma, applied rf power, and plasma pulsing parameters. Continuous wave O2 plasma treatments were found to reduce carbon content and increase oxygen content in the films. Experiments also reveal that Si was deposited throughout the mesoporous network and by pulsing the plasma, Si content and film damage could be eliminated. Nitrogen doping of TiO2 films (N:TiO2) was accomplished by pulsed plasmas containing a range of nitrogen precursors. N:TiO2 films were anatase-phased with up to 34% nitrogen content. Four different nitrogen binding environments were controlled and characterized. The produced N:TiO2 films displayed various colors and three possible mechanisms to explain the color changes are presented. Both O2 treated and N:TiO2 materials were tested in photocatalytic devices. Preliminary results from photocatalytic activities of plasma treated P25 TiO2 powders showed that nitrogen doping treatments hinder photocatalytic activity under UV light irradiation, but silicon deposition can improve it. N:TiO2 materials were tested in photovoltaic devices to reveal improved short-circuit current densities for some plasma-modified films. To understand the gas-phase and surface chemistry involved in producing the N:TiO2 films, NH and NH2 species in pulsed NH3 plasmas were explored by systematically varying peak plasma power and pulsing duty cycle. Results from these studies using gas phase spectroscopy techniques reveal interconnected trends of gas-phase densities and surface reactions. Gas-phase data from pulsed plasmas with two different types of plasma pulsing reveal diminished or increased densities at short pulses that are explained by plasma pulse initiation and afterglow effects. Overall this work reveals characteristics of the plasma systems explored, knowledge of the resulting materials, and control over plasma etching, deposition, and modification of TiO2 surfaces

Gas sensors based on carbon nanofibers: a low power consumption approach

[eng] Gas sensors can be found in many activities ranging from environment protection, risk prevention, agriculture and even in food, chemical, and petrochemical industries. There exist different technologies for gas sensors depending on the transduction mechanism: mass-sensitive, optical, calorimetric, magnetic, electrochemical and conductometric. In this work, conductometric (or resistive) gas sensors are studied. Conductometric devices base its operating principle on the variation of the electrical conductivity (resistivity) or conductance (resistance) of a reactive (active) material interacting with gas. A chemical reaction between the active material (surface or bulk) and the gas occurs. This reaction induces a variation on some electrical property of the material resulting in a change on the electrical signal (conductivity or resistivity of the active material) of the sensor. Therefore, the sensor material should be compatible with the mentioned properties above. A carbon based material was chosen to be the reactive compound for the conductometric sensors. This material, a specific type of carbon nanofibers (CNFs), shares some suitable properties with other trendy carbon based materials such as carbon nanotubes or graphene. Conductometric gas sensors usually are composed of two main parts: the already mentioned reactive material and the heater device. The heater is required in order to stabilize the temperature of operation and to activate a desired chemical reaction. Unfortunately, despite the efforts to improve the heater technology, this component is still the most power demanding part of the overall device. The here studied sensors have been characterized with a heater device, but also alternative energy sources and other sensing strategies have been tested in order to reduce the energy cost. Among these, the use of ultraviolet and visible light sources were tested in order to modulate the sensor properties. In addition, another non-common strategy was used to operate the sensor: the so called self-heating effect (or Joule effect). To obtain the electrical signal of a sensor, the reactive material have to be scanned, usually a current (or voltage) is applied to the sensor, then, the voltage (or current) is read. If the probing magnitude is increased, the power dissipation through the sensing material, and its temperature, also increases. Therefore, the sensor could be operated without a heater device with a considerable reduction of its power consumption. Moreover, the self-heating also allows reducing the fabrication complexity, as there is no need of the heater element. In summary, the main objective of this work was to characterize the CNFs as a reactive material for conductometric sensors for low cost applications. First, the CNFs properties (electrical, mechanical, response to light and gases) were screened with the aim to assess the applicability of the sensing material (O. Monereo et al., 2013, Flexible sensor based on carbon nanofibers with multifunctional sensing features). Then, the sensor was tested with the use of temperature modulation (S. Claramunt et al., 2013, Flexible gas sensor array with an embedded heater based on metal decorated carbon nanofibres). At this point, a more detailed characterization of the gas sensing properties with O2, H2O, NO2 and NH3 was conducted. Then, the use of continuous self-heating operation (O. Monereo et al., 2015, Self-heating effects in large arrangements of randomly oriented carbon nanofibers: Application to gas sensors) and pulsed self-heating application (O. Monereo et al., 2016, Self-heating in pulsed mode for signal quality improvement: application to carbon nanostructures-based sensors) were found to be efficient methodologies to modulate the sensing characteristics of sensor devices, based on large arrays of nanostructures. Among the benefits achieved, the sensor presented improvements on stability, specificity, the detection time modulation, all along the simplification of device fabrication and the reduction of the power consumption. Finally, the phenomenon of self-heating in carbon nanofibers and its origin was studied (O. Monereo et al., 2016, Localized self-heating in large arrays of 1D nanostructures). In addition, the use of ultraviolet and visible light as alternative energy sources was also assessed and compared with the self-heating operation. Finally, the applicability of self-heating was also tested in graphene based (reduced graphene oxide) and metal oxide based (ZnO) devices to test the applicability of self-heating in other relevant sensing materials.[spa] El objetivo principal de esta tesis es la caracterización de las nanofibras de carbono (CNFs) como material reactivo para sensores resistivos de gas para aplicaciones de bajo consumo. Primero, las propiedades eléctricas, mecánicas y respuesta a luz y gases de las CNFs fueron evaluadas para comprobar la aplicabilidad del material sensor (O. Monereo et al., 2013, Flexible sensor based on carbon nanofibers with multifunctional sensing features). Posteriormente, la respuesta del sensor a gases fue estudiada con modulación de temperatura (S. Claramunt et al., 2013, Flexible gas sensor array with an embedded heater based on metal decorated carbon nanofibres). En este punto, una caracteritzación más detallada de la respuesta del sensor a gases modulados con temperatura se realizó con O2, H2O, NO2 y NH3. A continuación, el uso de la metodología de auto-calentamiento continuo (O. Monereo et al., 2015, Self-heating effects in large arrangements of randomly oriented carbon nanofibers: Application to gas sensors) y pulsado (O. Monereo et al., 2016, Self-heating in pulsed mode for signal quality improvement: application to carbon nanostructures-based sensors) han sido probados como formas energéticamente eficientes para modular la respuesta de sensores basados en grandes matrices de CNFs. Entre los beneficios encontrados, consta una mejora de la estabilidad, especificidad, la modulación del tiempo de detección; todo añadiendo la simplificación de la fabricación. Finalmente, el origen del fenómeno de auto-calentamiento en CNFs fue estudiado en detalle (O. Monereo et al., 2016, Localized self-heating in large arrays of 1D nanostructures). Además, la aplicabilidad de la metodología fue también probada en nanotubos de carbono, óxido de grafeno reducido y nanohilos de óxido de zinc. Finalmente, el uso de luz ultraviolada y visible ha sido estudiado como a energías alternativas para la modulación de los sensores de gases de CNFs