abstract: The molecular beam epitaxy growth of the III-V semiconductor alloy indium arsenide antimonide bismide (InAsSbBi) is investigated over a range of growth temperatures and V/III flux ratios. Bulk and quantum well structures grown on gallium antimonide (GaSb) substrates are examined. The relationships between Bi incorporation, surface morphology, growth temperature, and group-V flux are explored. A growth model is developed based on the kinetics of atomic desorption, incorporation, surface accumulation, and droplet formation. The model is applied to InAsSbBi, where the various process are fit to the Bi, Sb, and As mole fractions. The model predicts a Bi incorporation limit for lattice matched InAsSbBi grown on GaSb.The optical performance and bandgap energy of InAsSbBi is examined using photoluminescence spectroscopy. Emission is observed from low to room temperature with peaks ranging from 3.7 to 4.6 μm. The bandgap as function of temperature is determined from the first derivative maxima of the spectra fit to an Einstein single oscillator model. The photoluminescence spectra is observed to significantly broaden with Bi content as a result of lateral composition variations and the highly mismatched nature of Bi atoms, pairs, and clusters in the group-V sublattice. A bowing model is developed for the bandgap and band offsets of the quinary alloy GaInAsSbBi and its quaternary constituents InAsSbBi and GaAsSbBi. The band anticrossing interaction due to the highly mismatched Bi atoms is incorporated into the relevant bowing terms. An algorithm is developed for the design of mid infrared GaInAsSbBi quantum wells, with three degrees freedom to independently tune transition energy, in plane strain, and band edge offsets for desired electron and hole confinement. The physical characteristics of the fundamental absorption edge of the relevant III-V binaries GaAs, GaSb, InAs, and InSb are examined using spectroscopic ellipsometry. A five parameter model is developed that describes the key physical characteristics of the absorption edge, including the bandgap energy, the Urbach tail, and the absorption coefficient at the bandgap. The quantum efficiency and recombination lifetimes of bulk InAs0.911Sb0.089 grown by molecular beam epitaxy is investigated using excitation and temperature dependent steady state photoluminescence. The Shockley-Read-Hall, radiative, and Auger recombination lifetimes are determined. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2020

京都大学 / 新制・課程博士 / 博士(理学) / 甲第22991号 / 理博第4668号 / 新制||理||1670(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 柳瀬 陽一, 教授 川上 則雄, 教授 石田 憲二 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

With the soaring advancement of organolead halide perovskite solar cells rising from a power conversion efficiency of merely 3% to more than 22% shortly in five years, researchers’ interests on this big material family have been greatly spurred. So far, both in-depth studies on the fundamental properties of organolead halide perovskites and their extended applications such as photodetectors, light emitting diodes, and lasing have been intensively reported. The great successes have been ascribed to various superior properties of organolead halide hybrid perovskites such as long carrier lifetimes, high carrier mobility, and solution-processable high quality thin films, as will be discussed in Chapter 1. Notably, most of these studies have been limited to their polycrystalline thin films. Single crystals, as a counter form of polycrystals, have no grain boundaries and higher crystallinity, and thus less defects. These characteristics gift single crystals with superior optical, electrical, and mechanical properties, which will be discussed in Chapter 2. For example, organolead halide perovskite single crystals have been reported with much longer carrier lifetimes and higher carrier mobilities, which are especially intriguing for optoelectronic applications. Besides their superior optoelectronic properties, organolead halide perovskites have shown large composition versatility, especially their organic components, which can be controlled to effectively adjust their crystal structures and further fundamental properties. Single crystals are an ideal platform for such composition-structure-property study since a uniform structure with homogeneous compositions and without distraction from grain boundaries as well as excess defects can provide unambiguously information of material properties. As a major part of work of this dissertation, explorative work on the composition-structure-property study of organic-cation-alloyed organolead halide perovskites using their single crystals will be discussed in Chapter 3 and 4. Despite their outstanding charge transport characteristics, organolead halide perovskite single crystals grown by hitherto reported crystallization methods are not suitable for most optoelectronic devices due to their small aspect ratios and free standing growth. As the other major part of work of this dissertation, explorative work on growing organolead halide perovskite monocrystalline films and further their application in solar cells will be discussed in Chapter 5.

Kerr frequency combs enable compact and robust platforms for applications such as data communications and spectroscopy. Initial demonstrations used dissipative soliton combs operating in the anomalous group velocity dispersion (GVD) regime to illustrate the promising capabilities of Kerr-comb-based technologies. However, many real-world applications, such as wavelength division multiplexing and LiDAR, benefit from higher comb-line powers that are inherently inaccessible to Kerr frequency combs. Nevertheless, nonsolitonic Kerr combs operating in the normal GVD regime offer promise as a platform for the integration of such applications due to their ability to easily access high pump-to-comb conversion efficiencies and spectral profiles with slower power falloffs. Unlike a dissipative Kerr soliton which is a single cycle periodic pattern on top of a homogenous background, a nonsolitonic Kerr comb arises through the interlocking of two switching waves of opposite polarity, each of which connects the two homogenous state solutions of the bistable cavity response. Here, we present a systematic approach for turn-key generation of Kerr combs in the normal GVD regime. We use a coupled ring geometry to induce and control avoided mode crossings for the generation of low-noise frequency combs with conversion efficiencies of up to 57%. Moreover, we explore the regime of synchronization for these nonsolitonic Kerr combs. Synchronization is a universal mechanism of coupled nonlinear oscillators that manifests itself as the spontaneous appearance of order within nature's tendency for disorder and chaos. It is a process by which the natural rhythms of interacting oscillators adjust to a common frequency and produce a mutually phase-locked state. In the realm of Kerr frequency combs, synchronization allows for the repetition rates, or equivalently the comb-line spacings, of individual Kerr combs to become identical. We reveal the universality of Kerr comb synchronization by synchronizing two nonsolitonic Kerr combs and thereby extending the scope of synchronization beyond the soliton regime. Furthermore, we introduce a method to overcome the inherently low output power of Kerr combs while maintaining the high conversion efficiency of a normal GVD Kerr comb. We demonstrate efficient comb-line power enhancement by coherently combining two nonsolitonic Kerr combs via on-chip synchronization. Our on-chip synchronization design removes the requirement for dispersion compensation of the coupling signal while increasing the overall stability of the system. Our techniques of comb generation, synchronization, and coherent combining enable nonsolitonic Kerr combs as a platform to achieve a fully integrated, high-power Kerr comb source.

In this dissertation, the design and fabrication of deep-ultraviolet photodetectors were investigated based on gallium oxide and its alloys, through the heterogeneous integration with metallic and other inorganic materials. The crystallographic properties of oxide films grown directly and indirectly on silicon, magnesium oxide, and sapphire are examined, and the challenges that hinder the realization of efficient and reliable deep-ultraviolet photodetectors are described. In recent years, single-crystalline heterojunction photodiodes employing beta-polymorph gallium oxide thin films as the main absorption layers have been studied. However, reports in the literature generally lack a thorough examination of epitaxial growth processes of high-quality single-crystalline beta-polymorph gallium oxide thin films on metals, such as transition metal nitrides. My research was initiated by demonstrating an ultraviolet-C photodetector based on an amorphous aluminum gallium oxide photoconductive layer grown directly on (100)-oriented silicon. The solar-blind photodetector exhibited a peak spectral responsivity of 1.17 A/W. This is the first reported gallium oxide-based photodetector to have been grown and fabricated directly on silicon. The growth of high-quality monoclinic crystals on cubic silicon is a challenging process, which is largely due to the large lattice mismatch that compromises the crystal quality of the oxide layer, and leads to the degradation of device performance. This issue was addressed by growing the material on substrates with metal nitride templates, which resulted in improvements to the oxide crystal quality. Consequently, high optical gain ultraviolet-C photodetectors were fabricated based on a beta-polymorph gallium oxide photoconductive layer grown on magnesium oxide and silicon substrates with titanium nitride templates. The enhanced solar-blind photodetectors exhibited peak spectral responsivity levels as high as 276 A/W. Moreover, thin polymorphic gallium oxide films were grown on c-plane sapphire using pulsed laser deposition for the first time. The stacked thin films, namely epsilon- and beta-polymorph gallium oxide, were sequentially grown under the same conditions. X-ray diffraction measurements and transmission electron microscopy micrographs confirmed a heteroepitaxially grown beta-polymorph gallium oxide on a heterogeneously nucleated epsilon-polymorph gallium oxide polymorphic heterostructure on c-plane sapphire, which had rocking-curve widths of 1.4° (β-Ga2O3 (−603)) and 0.6° (ε-Ga2O3 (006)).

Optical wireless communication, taking advantage of the unlicensed ultraviolet-to visible wavelength region of the electromagnetic spectrum, had been coined as the next-generation wireless communication technology and holds promises to deliver a high-speed, reliable, and secured broadband experience. The push towards the optical-based medium is manifested by the demand for additional channel bandwidth to accommodate the rapid growth of the Internet-of-Things (IoT) and Internet-of-Underwater-Things (IoUT). Therefore, high-bitrate optoelectronics devices and components forming the transceiver units used in an optical wireless communication system require substantial progression to accelerate the development of this paradigm-shifting technology. In this dissertation, we demonstrated a plethora of optical detection platforms to circumvent the existing long-standing issues related to modulation bandwidth, wavelength-selectiveness, and solar-blind ultraviolet-C detection found in conventional planar silicon-based optical detectors. Herein, we presented the semipolar group-III-nitride-based micro-photodiodes for enabling up to Gbit/s optical detection in the ultraviolet-to-violet domain. The wavelength-selectiveness nature of the micro-photodiodes enabled a bitrate of up to 1.5 Gbit/s based on a power-saving on-off-keying modulation scheme. While it offers a high bitrate for the optical communication link, it restricts its detection size and angle-of-view due to the conventional resistance-capacitance and étendue limits. Therefore, we also explored using polymer-based scintillating fibers as a high-speed and near-omnidirectional optical detection platform to cater to various dynamic scenarios in optical wireless communication. The detection platform formed by the scintillating fibers enabled near-omnidirectional and large-area optical detection without sacrificing the modulation bandwidth. These investigations paved the way towards relieving the resistance-capacitance limit while addressing the pointing, acquisition, and tracking issue in underwater wireless optical communication. Subsequently, we also presented a novel wavelength-converting mechanism based on halide-perovskite nanocrystals and a conventional silicon-based platform. This demonstration addressed the lack of ultraviolet-C optical detectors in the existing market and enabled future solar-blind optical communication links. Finally, we also presented on halide-perovskite polymer-based scintillating fibers as the high-bitrate and near-omnidirectional optical detection platform. Our studies successfully addressed the existing inadequacy for high-bitrate photodetection. These works could play a significant role in progressing the technology forward, based on bottom-up material and devices innovation, to offer a reliable internet connection to the future highly interconnected society.

La photonique silicium est reconnue comme la technologie à même de répondre aux nouveaux défis des interconnexions optiques. Néanmoins, la photonique silicium doit faire face à d'importants défis. En effet, le Si ne peux pas émettre ou détecter de la lumière dans la plage de longueurs d'onde des télécom (1,3 µm à 1,5 µm). Par conséquent, les sources et les détecteurs sont mis en œuvre avec du Ge et des matériaux III-V. Cette approche multi-matériaux complique la fabrication des dispositifs et augmente le coût final du circuit. Cependant, les nanomatériaux ont été identifiés comme alternative pour la mise en œuvre d’émetteurs-récepteurs moins chers et plus petits.Cette thèse est dédiée à l'étude et au développement de dispositifs optiques et optoélectroniques sur la plateforme photonique silicium basés sur l’utilisation de nanotubes de carbone mono paroi (SWCNT). L’objectif principal est de démontrer les blocs fonctionnels de base qui ouvriront la voie à une nouvelle technologie photonique dans laquelle les propietés actives proviennent des nanotubes de carbone.Les nanotubes de carbone ont été étudiés comme matériaux pour la nanoélectronique avec la démonstration de transistors ultra-compacts à hautes performances. De plus, les SWCNTs semi-conducteurs (s-SWCNTs) sont également des matériaux très intéressants pour la photonique. Les s-SWCNTs présentent une bande interdite directe qui peut être ajustée dans la gamme de longueurs d'onde du proche infrarouge en choisissant le bon diamètre. Les s-SWCNT présentent une photoluminescence et une électroluminescence, pouvant être exploitées pour la mise en œuvre de sources de lumière. Ils présentent également diverses bandes d’absorption pour la réalisation de photodétecteurs. Ces propriétés font que les nanotubes de carbone sont des candidats très prometteurs pour le développement de dispositifs optoélectroniques pour la photonique.Le premier objectif de la thèse était l'optimisation des solutions de nanotubes de carbone. Une technique de tri par ultra-centrifugation assistée par polymère a été optimisée, donnant des solutions de haute pureté en s-SWCNT. Sur cette base, plusieurs solutions de s-SWCNTs ont été élaborées pour obtenir des SWCNTs émettant dans les longueurs d'onde comprise entre 1µm et 1,6µm.Le deuxième objectif était d’étudier l'interaction des s-SWCNT avec des guides d'onde silicium et des résonateurs optiques. Plusieurs géométries ont été étudiées dans le but de maximiser l'interaction des s-SWCNT avec le mode optique en exploitant la composante transverse du champ électrique. D'autre part, une approche alternative a été proposée et démontrée en utilisant la composante longitudinale du champ électrique. En utilisant la composante longitudinale, une amélioration de la photoluminescence, un seuil d’émission avec la puissance de pompe ainsi qu’un rétrécissement de la largeur spectrale des résonances dans les microdisques ont été observés. Ces résultats sont un premier pas très prometteur vers la démonstration d’un laser intégré à base de SWNTs.Le troisième objectif était d'étudier les dispositifs optoélectroniques à base de s-SWCNTs. Plus spécifiquement, une diode électroluminescente (DEL) et un photodétecteur ont été développés, permettant la démonstration du premier lien optoélectronique sur puce basé sur les s-SWCNT.Le dernier objectif de la thèse était d'explorer le potentiel de s-SWCNT pour l’optique non linéaire. Il a été démontré expérimentalement, qu’en choisissant la chiralité des s-SWCNTs, le signe du coefficient Kerr pouvait être soit positif ou négatif. Cette capacité unique ouvre un nouveau degré de liberté pour contrôler les effets non linéaires sur puce, permettant de compenser ou d'améliorer les effets non linéaires pour des applications variées. / Silicon photonics is widely recognized as an enabling technology for next generation optical interconnects. Nevertheless, silicon photonics has to address some important challenges. Si cannot provide efficient light emission or detection in telecommunication wavelength range (1.3μm-1.5μm). Thus sources and detectors are implemented with Ge and III-V compounds. This multi-material approach complicates device fabrication, offsetting the low-cost of Si photonics. Nanomaterials are a promising alternative route for the implementation of faster, cheaper, and smaller transceivers for datacom applications.This thesis is dedicated to the development of active silicon photonics devices based on single wall carbon nanotubes (SWCNTs). The main goal is to implement the basic building blocks that will pave the route towards a new Si photonics technology where all active devices are implemented with the same technological process based on a low-cost carbon-based material, i.e. SWCNT.Indeed, carbon nanotubes are an interesting solution for nanoelectronics, where they provide high-performance transistors. Semiconducting SWCNT exhibit a direct bandgap that can be tuned all along the near infrared wavelength range just by choosing the right tube diameter. s-SWCNTs provide room-temperature photo- and electro- luminescence and have been demonstrated to yield intrinsic gain, making them an appealing material for the implementation of sources. SWCNTs also present various absorption bands, allowing the realization of photodetectors.The first objective of this thesis was the optimization of the purity of s-SWCNT solutions. A polymer-sorting technique has been developed and optimized, yielding high-purity s-SWCNT solutions. Based on this technique, several solutions have been obtained yielding emission between 1µm and 1.6µm wavelengths.The second objective was the demonstration of efficient interaction of s-SWCNT with silicon photonics structures. Different geometries have been theoretically and experimentally studied, aiming at maximizing the interaction of s-SWCNT with optical modes, exploiting the electric field component transversal to light propagation. An alternative approach to maximize the interaction of s-SWCNT and the longitudinal electric field component of waveguide modes was proposed. Both, a power emission threshold and a linewidth narrowing were observed in several micro disk resonators. These results are a very promising first step to go towards the demonstration of an integrated laser based on CNTs.The third objective was to study optoelectronic SWCNT devices. More specifically, on-chip light emitting diode (LED) and photodetector have been developed, allowing the demonstration of the first optoelectronic link based on s-SWCNT. s-SWCNT-based LED and photodetector were integrated onto a silicon nitride waveguide connecting them and forming an optical link. First photodetectors exhibited a responsivity of 0.1 mA/W, while the complete link yielded photocurrents of 1 nA/V.The last objective of the thesis was to explore the nonlinear properties of s-SWCNT integrated on silicon nitride waveguides. Here, it has been experimentally shown, for the first time, that by choosing the proper s-SWCNT chirality, the sign of the nonlinear Kerr coefficient of hybrid waveguide can be positive or negative. This unique tuning capability opens a new degree of freedom to control nonlinear effects on chip, enabling to compensate or enhancing nonlinear effects for different applications.

Converting solar energy to clean fuel has gained remarkable attention as an emerged renewable energy resource but optimum efficiency in photocatalytic applications has not yet been reached. One of the dominant factors is designing efficient photocatalytic semiconductors. The research reveals a theoretical investigation of optoelectronic properties of bismuth-based metal oxide and oxysulfide semiconductors using highly accurate first-principles quantum method based on density functional theory along with the range-separated hybrid HSE06 exchange-correlation functional. First, bismuth titanate compounds including Bi12TiO20, Bi4Ti3O12, and Bi2Ti2O7 were studied in a combined experimental and theoretical approach to prove its photocatalytic activity under UV light. They have unique bismuth layered structure, tunable electronic properties, high dielectric constant and low electron and effective masses in one crystallographic direction allowing for good charge separation and carrier diffusion properties. The accuracy of the investigation was determined by the good agreement between experimental and theoretical values. Next, BiVO4 with the highest efficiency for oxygen evolution was investigated. A discrepancy between the experimental and theoretical bandgap was reported and inspired a systematic study of all intrinsic defects of the material and the corresponding effect on the optical and transport properties. A candidate defective structure was proposed for an efficient photocatalytic performance. To overcome the carrier transport limitation, a mild hydrogen treatment was also introduced. Carrier lifetime was enhanced due to a significant reduction of trap-assisted recombination, either via passivation of deep trap states or reduction of trap state density. Finally, an accurate theoretical approach to design a new family of semiconductors with enhanced optoelectronic properties for water splitting was proposed. We simulated the solid solutions Bi1−xRExCuOS (RE = Y, La, Gd and Lu) from pure BiCuOS to pure RECuOS compositions. Starting from the thermodynamic stability of the solid solution, several properties were computed for each system including bandgaps, dielectric constants, effective masses and exciton binding energies. Several compositions with specific organization and density of Bi and RE atoms, were found to be appropriate for water splitting applications. In General, the presented results give further insights to the experimentalists and recommendations for appropriate future application and defect-design of each material.

Halide perovskites have recently emerged as the state-of-the-art semiconductors with the unique combination of outstanding optoelectronic properties and facile solution synthesis. Within only a decade of research, they have witnessed a remarkable success in photovoltaics and shown great potential for applications in light-emitting devices, photodetectors, and high-energy sensors. Yet, the majority of current perovskite-based devices still rely on polycrystalline thin films which, as will be discussed in Chapter 2, exhibit inferior charge transport characteristics and increased tendency to chemical degradation compared to their single-crystalline analogues. In this regard, unburdened from the effects of grain boundaries, single crystals demonstrate the upper limits of semiconductor performance. Their study is, thus, important from both fundamental and practical aspects, which present the major objectives of this dissertation. In Chapter 3, we study the intrinsic charge transport and recombination characteristics of single crystals of formamidinium lead halide perovskites. While, in Chapter 4, we investigate the mechanistic origins of rapid synthesis of halide perovskite single crystals by inverse temperature crystallization. Understanding the nucleation and growth mechanisms of halide perovskites enables us to design strategies toward integrating their single crystals into device applications. Namely, in Chapters 5 and 6, we demonstrate crystal engineering approaches for tailoring the thicknesses and facets of halide perovskite single crystals to make them suitable for, respectively, vertical and planar architecture optoelectronic devices. The findings of this dissertation are expected to benefit future studies on fundamental characterization of halide perovskites, as well as motivate researchers to develop perovskite-based optoelectronic devices with better crystallinity, performance and stability.

Indiana University-Purdue University Indianapolis (IUPUI) / Semiconductor nanocrystals (SNCs) are a class of material with one dimension <100 nm, which display size, shape, and composition dependent photophysical (absorption and emission) properties. Ultrasmall SNCs are a special class of SNCs whose diameter is <3.0 nm and are strongly quantum confined leading to a high surface to volume ratio. Therefore, their electronic and photophysical properties are fundamentally dictated by their surface chemistry, and as such, even a minute variation of the surface ligation can have a colossal impact on these properties. Since the development of the hot injection-method by Bawendi et al., the synthetic methods of SNCs have evolved from high-temperature, highly toxic precursors to low-temperature, relatively benign precursors over the last 25 years. Unfortunately, optimization of their synthetic methods by appropriate surface ligation is still deficient. The deficiency lies in the incomplete or inappropriate surface passivation during the synthesis and/or post-synthetic modification procedure, which due to the high surface to volume ratio of ultrasmall SNCs, is a significant problem. Currently, direct synthetic methods produce SNCs that are either soluble in an aqueous media or soluble in organic solvents therefore limiting their applicability. In addition, use of insulating ligands hinder SNCs transport properties and thus their potential application in solid state devices. Appropriate choice of surface ligation can provide 1) solubility, 2) stability, and 3) facilitate exciton delocalization. In this dissertation, the effects of appropriate surface ligation on strongly quantum confined ultrasmall SNCs was investigated. Due to their high surface to volume ratio, we are able to highly control their optical and electronic properties through surface ligand modification. Throughout this dissertation, we utilized a variety of ligands (e.g. oleylamine, cadmium benzoate, and PEGn-thiolate) in order to change the solubility of the SNC as well as investigate their optical and electronic properties. First delocalization of the excitonic wave function 1) into the ligand monolayer using metal carboxylates and 2) beyond the ligand monolayer to provide strong inter-SNC electronic coupling using poly(ethylene) glycol (PEG)-thiolate was explored. Passivation of the Se sites of metal chalcogenide SNCs by metal carboxylates provided a two-fold outcome: (1) facilitating the delocalization of exciton wave functions into ligand monolayers (through appropriate symmetry matching and energy alignment) and (2) increasing fluorescence quantum yield (through passivation of midgap trap states). An ~240 meV red-shift in absorbance was observed upon addition of Cd(O2CPh)2, as well as a ~260 meV shift in emission with an increase in PL-QY to 73%. Through a series of control experiments, as well as full reversibility of our system, we were able to conclude that the observed bathochromic shifts were the sole consequence of delocalization, not a change in size or relaxation of the inorganic core, as previously reported. Furthermore, the outstanding increase in PL-QY was found to be a product of both passivation and delocalization effects. Next we used poly(ethylene) glycol (PEG)-thiolate ligands to passivate the SNC and provide unique solubility properties in both aqueous and organic solvents as well as utilized their highly conductive nature to explore inter-SNC electronic coupling. The electronic coupling was studied: 1) as a function of SNC size where the smallest SNC exhibited the largest coupling energy (170 meV) and 2) as a function of annealing temperature, where an exceptionally large (~400 meV) coupling energy was observed. This strong electronic coupling in self-organized films could facilitate the large-scale production of highly efficient electronic materials for advanced optoelectronic device applications. Strong inter-SNC electronic coupling together with high solubility, such as that provided by PEG-thiolate-coated CdSe SNCs, can increase the stability of SNCs during solution-phase electrochemical characterization. Therefore, we utilized these properties to characterize solution-state electrochemical properties and photocatalytic activity of ternary copper indium diselenide (CuInSe2) SNCs as a function of their size and surface ligand chemistry. Electrochemical characterization of our PEG-thiolate-coated SNCs showed that the thermodynamic driving force (-ΔG) for oxygen reduction, which increased with decreasing bandgap, was a major contributor to the overall photocatalytic reaction. Additionally, phenol degradation efficiency was monitored in which the smallest diameter SNC and shortest chain length of PEG provided the highest efficiency. The information provided herein could be used to produce superior SNC photocatalysts for a variety of applications including oxidation of organic contaminants, conversion of water to hydrogen gas, and decomposition of crude oil or pesticides. Therefore, we believe our work will significantly advance quantitative electrochemical characterization of SNCs and allow for the design of highly efficient, sustainable photocatalysts resulting in economic and environmental benefits.