Supramolecular assembly and nanoscale morphologies for organic photovoltaic devices

Organic photovoltaics is a field of rapidly growing activity in both research and industry. Flexibility, light-weight, and low-cost render this technique an appealing alternative to silicon based devices for solar energy power conversion. Recently, several donor-acceptor systems have been investigated as active materials for organic photovoltaic devices and it has been shown that the morphology of these heterojunction systems has a severe impact on the device performance. This work focuses on the correlation of nanoscale morphologies of organic donor-acceptor systems with resulting spectroscopic and electronic properties of organic photovoltaic devices. Discotic molecules are used in the active layer of the devices exhibiting a number of properties highly desired for the application in organic photovoltaic devices: The planar core shape of this class of materials allows an assembly to 1-D molecular wires showing anisotropic and exceptionally high charge carrier mobility.It is the aim to establish supramolecular assemblies of the discotic molecules and to realize nanoscale interface morphologies between donor and acceptor compounds in order to optimize the photovoltaic performance of the resulting devices. The impact of residue modifications attached to the disc shaped molecules on morphology, current generation and recombination is analyzed and design rules for these solution processable small molecule blend mixtures are derived. Vacuum sublimation is discussed as an alternative processing route facilitating the fabrication of devices with mixed but also bi-layered active material stacking. Using a dye sensitization method the exciton harvesting and photovoltaic performance can be significantly increased in these thin film devices. A highly ordered nanoscale morphology at the donor-acceptor interface, demonstrated using a template assisted imprinting approach, offers high potential towards photovoltaic devices with interdigitated interfaces and superior power conversion efficiency

Functional Nanostructure Synthesis and Properties

The dissertation addressed challenges in the nanostructure synthesis, applied the materials to engineering fields, such as lithium battery material, fluorescent and magnetic drug deliveries; and developed new characterization methods to better understand particle properties and formation mechanisms

Functionalization, growth and applications of single wall carbon nanotubes

Because of their remarkable structural, mechanical and electrical properties, carbon nanotubes, and especially single wall carbon nanotubes (SWNTs), represent one of the most widely investigated materials today in the emerging field of nanotechnology. The development of oriented growth of SWNTs critical for applications, novel approaches for the creation of functional SWNTs, and applications of both as-prepared and chemically functionalized SWNTs for electrochemically-induced hydrogen storage, in-situ formation of new polymer and ceramic nanocomposites with SWNTs, and the fabrication and study for the first time, to the best of our knowledge, of a SWNT-based biofuel cell and self-powered biosensor, are the thrusts of the research discussed in this thesis. Introduction to the science and the potential applications of carbon nanotubes are presented in Chapters 1 to 2 of the thesis, and an overview of the methods used in this work is discussed in Chapter 3. Chapters 4 to 6 discuss the results of the work performed. Spin-coating deposition of a polymer-chelated catalyst precursor on conductive silicon wafers developed for the oriented growth of SWNTs, is discussed in Chapter 4. Oriented growth of SWNTs was obtained using chemical vapor deposition with alcohol as the carbon source. For application in biofuel cells and biosensors (discussed in the final segment), the oriented SWNTs on silicon were functionalized with selected redox enzymes to form the fuel cell and sensor electrodes. In initial tests, a substantial open circuit cell voltage of 200 mV, and analyte-sensitive direct electron transfer, were observed from the fabricated biofuel cell and biosensor devices, respectively. Environmentally friendly, rapid and efficient microwave-induced chemical functionalization of SWNTs was achieved for the first time in the course of this work and is described in Chapter 5 of the thesis. The microwave radiation assisted technique has brought down the functionalization time from days using typical chemical methods, such as refluxing, to the order of minutes. Chemical functionalizations by the microwave method achieved include amidation, 1 ,3-dipolar cycloaddition and nitration, with the latter providing SWNTs that are very soluble in water and alcohol. Both microwave-induced and supercritical carbon dioxide approaches were also used to prepare and study the formation of ceramic (silicon carbide, SiC) and polymer (polymethyl methacrylate, PMMA) nanocomposites with SWNTs, respectively. Electrochemically-induced functionalization of SWNTs by nitro groups and enzymes has been studied in some detail, whereas electrochemical hydrogen storage for fuel cell operation using pristine and functionalized SWNTs as the storage medium has also been studied in this work and discussed in Chapter 6 of this thesis. Strong evidence for electrochemically-induced hydrogen uptake approaching 3 wt % based on thermogravimetric measurements has been obtained on SWNT nanopaper membranes on which the nitrogen-containing conducting polymer, polyaniline, was deposited. A summary of the work performed and suggestions for future work are provided in Chapter 7. The schematic molecular structures of the more complex molecules, polymers and enzymes used in this work (except those shown in the tables) are shown schematically in Appendix A

The Importance of Chain Connectivity in the Formation of Non-covalent Interactions between Polymers and Single-walled Carbon Nanotubes and its Impact on Dispersion

Polymer nanocomposites have garnered incredible promise in the field of material science due to the excellent mechanical strength, thermal and electrical conductivities of the nanoparticles and the extension of these properties to the processing flexibility inherent to plastics. However, practical realization of these nanoparticle-based materials has been hindered by the tendency of these nanoparticles to aggregate as a result of strong inter-particle forces. In this dissertation, we investigate the formation of non-covalent charge transfer interactions between polymers and single-walled carbon nanotubes (SWNTs) with the goal of optimizing interfacial adhesion and homogeneity of nanocomposites without modifying the SWNT native surface. Nanocomposites of SWNTs and three sets of polymer matrices with varying composition of electron donating or electron accepting functional groups were prepared. In the first part of this dissertation, quantitative characterization by optical microscopy and Raman spectroscopy and qualitative results through thick film composite visualization show that the existence of a moderate amount of interacting moieties along the polymer chain results in an enhanced intermolecular interaction with SWNT, which translates to an optimum nanoparticle homogeneity. Calculations from density functional theory and Flory-Huggins theory correlate with the experimental results, which illustrate that chain connectivity is critical in controlling the accessibility of the functional groups to form intermolecular interactions. Thus, controlling the amount of interacting functional groups throughout the polymer chain such that an adequate distance between them is realized will direct the extent of charge transfer interaction, which enables tuning the SWNT dispersion. The second part of this dissertation focuses on the elucidation of the morphology of these nanoparticle entities in a polymer matrix. The observance of microphase-separated peaks in the scattering patterns of polyacrylonitrile (PAN) nanocomposites indicate an ordering of the PAN polymer induced by the carbon nanotube cage, which could either be due to a thermodynamically bound layer around the SWNT or the occurrence of SWNT-induced PAN crystallization. Finally, UV-Vis measurements were performed on SWNT-polymer suspension in order to comprehend the interactions that occur during nanocomposite fabrication. These results demonstrate that SWNT dispersions in pure N,N-dimethyl formamide (DMF) are stabilized by the adsorption of polymers onto the SWNTs

Production and processing of graphene and related materials

© 2020 The Author(s). We present an overview of the main techniques for production and processing of graphene and related materials (GRMs), as well as the key characterization procedures. We adopt a 'hands-on' approach, providing practical details and procedures as derived from literature as well as from the authors' experience, in order to enable the reader to reproduce the results. Section I is devoted to 'bottom up' approaches, whereby individual constituents are pieced together into more complex structures. We consider graphene nanoribbons (GNRs) produced either by solution processing or by on-surface synthesis in ultra high vacuum (UHV), as well carbon nanomembranes (CNM). Production of a variety of GNRs with tailored band gaps and edge shapes is now possible. CNMs can be tuned in terms of porosity, crystallinity and electronic behaviour. Section II covers 'top down' techniques. These rely on breaking down of a layered precursor, in the graphene case usually natural crystals like graphite or artificially synthesized materials, such as highly oriented pyrolythic graphite, monolayers or few layers (FL) flakes. The main focus of this section is on various exfoliation techniques in a liquid media, either intercalation or liquid phase exfoliation (LPE). The choice of precursor, exfoliation method, medium as well as the control of parameters such as time or temperature are crucial. A definite choice of parameters and conditions yields a particular material with specific properties that makes it more suitable for a targeted application. We cover protocols for the graphitic precursors to graphene oxide (GO). This is an important material for a range of applications in biomedicine, energy storage, nanocomposites, etc. Hummers' and modified Hummers' methods are used to make GO that subsequently can be reduced to obtain reduced graphene oxide (RGO) with a variety of strategies. GO flakes are also employed to prepare three-dimensional (3d) low density structures, such as sponges, foams, hydro- or aerogels. The assembly of flakes into 3d structures can provide improved mechanical properties. Aerogels with a highly open structure, with interconnected hierarchical pores, can enhance the accessibility to the whole surface area, as relevant for a number of applications, such as energy storage. The main recipes to yield graphite intercalation compounds (GICs) are also discussed. GICs are suitable precursors for covalent functionalization of graphene, but can also be used for the synthesis of uncharged graphene in solution. Degradation of the molecules intercalated in GICs can be triggered by high temperature treatment or microwave irradiation, creating a gas pressure surge in graphite and exfoliation. Electrochemical exfoliation by applying a voltage in an electrolyte to a graphite electrode can be tuned by varying precursors, electrolytes and potential. Graphite electrodes can be either negatively or positively intercalated to obtain GICs that are subsequently exfoliated. We also discuss the materials that can be amenable to exfoliation, by employing a theoretical data-mining approach. The exfoliation of LMs usually results in a heterogeneous dispersion of flakes with different lateral size and thickness. This is a critical bottleneck for applications, and hinders the full exploitation of GRMs produced by solution processing. The establishment of procedures to control the morphological properties of exfoliated GRMs, which also need to be industrially scalable, is one of the key needs. Section III deals with the processing of flakes. (Ultra)centrifugation techniques have thus far been the most investigated to sort GRMs following ultrasonication, shear mixing, ball milling, microfluidization, and wet-jet milling. It allows sorting by size and thickness. Inks formulated from GRM dispersions can be printed using a number of processes, from inkjet to screen printing. Each technique has specific rheological requirements, as well as geometrical constraints. The solvent choice is critical, not only for the GRM stability, but also in terms of optimizing printing on different substrates, such as glass, Si, plastic, paper, etc, all with different surface energies. Chemical modifications of such substrates is also a key step. Sections IV-VII are devoted to the growth of GRMs on various substrates and their processing after growth to place them on the surface of choice for specific applications. The substrate for graphene growth is a key determinant of the nature and quality of the resultant film. The lattice mismatch between graphene and substrate influences the resulting crystallinity. Growth on insulators, such as SiO2, typically results in films with small crystallites, whereas growth on the close-packed surfaces of metals yields highly crystalline films. Section IV outlines the growth of graphene on SiC substrates. This satisfies the requirements for electronic applications, with well-defined graphene-substrate interface, low trapped impurities and no need for transfer. It also allows graphene structures and devices to be measured directly on the growth substrate. The flatness of the substrate results in graphene with minimal strain and ripples on large areas, allowing spectroscopies and surface science to be performed. We also discuss the surface engineering by intercalation of the resulting graphene, its integration with Si-wafers and the production of nanostructures with the desired shape, with no need for patterning. Section V deals with chemical vapour deposition (CVD) onto various transition metals and on insulators. Growth on Ni results in graphitized polycrystalline films. While the thickness of these films can be optimized by controlling the deposition parameters, such as the type of hydrocarbon precursor and temperature, it is difficult to attain single layer graphene (SLG) across large areas, owing to the simultaneous nucleation/growth and solution/precipitation mechanisms. The differing characteristics of polycrystalline Ni films facilitate the growth of graphitic layers at different rates, resulting in regions with differing numbers of graphitic layers. High-quality films can be grown on Cu. Cu is available in a variety of shapes and forms, such as foils, bulks, foams, thin films on other materials and powders, making it attractive for industrial production of large area graphene films. The push to use CVD graphene in applications has also triggered a research line for the direct growth on insulators. The quality of the resulting films is lower than possible to date on metals, but enough, in terms of transmittance and resistivity, for many applications as described in section V. Transfer technologies are the focus of section VI. CVD synthesis of graphene on metals and bottom up molecular approaches require SLG to be transferred to the final target substrates. To have technological impact, the advances in production of high-quality large-area CVD graphene must be commensurate with those on transfer and placement on the final substrates. This is a prerequisite for most applications, such as touch panels, anticorrosion coatings, transparent electrodes and gas sensors etc. New strategies have improved the transferred graphene quality, making CVD graphene a feasible option for CMOS foundries. Methods based on complete etching of the metal substrate in suitable etchants, typically iron chloride, ammonium persulfate, or hydrogen chloride although reliable, are time- and resourceconsuming, with damage to graphene and production of metal and etchant residues. Electrochemical delamination in a low-concentration aqueous solution is an alternative. In this case metallic substrates can be reused. Dry transfer is less detrimental for the SLG quality, enabling a deterministic transfer. There is a large range of layered materials (LMs) beyond graphite. Only few of them have been already exfoliated and fully characterized. Section VII deals with the growth of some of these materials. Amongst them, h-BN, transition metal tri- and di-chalcogenides are of paramount importance. The growth of h-BN is at present considered essential for the development of graphene in (opto) electronic applications, as h-BN is ideal as capping layer or substrate. The interesting optical and electronic properties of TMDs also require the development of scalable methods for their production. Large scale growth using chemical/physical vapour deposition or thermal assisted conversion has been thus far limited to a small set, such as h-BN or some TMDs. Heterostructures could also be directly grown

Carbon Nanotubes

Since their discovery in 1991, carbon nanotubes have been considered as one of the most promising materials for a wide range of applications, in virtue of their outstanding properties. During the last two decades, both single-walled and multi-walled CNTs probably represented the hottest research topic concerning materials science, equally from a fundamental and from an applicative point of view. There is a prevailing opinion among the research community that CNTs are now ready for application in everyday world. This book provides an (obviously not exhaustive) overview on some of the amazing possible applications of CNT-based materials in the near future

Production and processing of graphene and related materials

© 2020 The Author(s). We present an overview of the main techniques for production and processing of graphene and related materials (GRMs), as well as the key characterization procedures. We adopt a 'hands-on' approach, providing practical details and procedures as derived from literature as well as from the authors' experience, in order to enable the reader to reproduce the results. Section I is devoted to 'bottom up' approaches, whereby individual constituents are pieced together into more complex structures. We consider graphene nanoribbons (GNRs) produced either by solution processing or by on-surface synthesis in ultra high vacuum (UHV), as well carbon nanomembranes (CNM). Production of a variety of GNRs with tailored band gaps and edge shapes is now possible. CNMs can be tuned in terms of porosity, crystallinity and electronic behaviour. Section II covers 'top down' techniques. These rely on breaking down of a layered precursor, in the graphene case usually natural crystals like graphite or artificially synthesized materials, such as highly oriented pyrolythic graphite, monolayers or few layers (FL) flakes. The main focus of this section is on various exfoliation techniques in a liquid media, either intercalation or liquid phase exfoliation (LPE). The choice of precursor, exfoliation method, medium as well as the control of parameters such as time or temperature are crucial. A definite choice of parameters and conditions yields a particular material with specific properties that makes it more suitable for a targeted application. We cover protocols for the graphitic precursors to graphene oxide (GO). This is an important material for a range of applications in biomedicine, energy storage, nanocomposites, etc. Hummers' and modified Hummers' methods are used to make GO that subsequently can be reduced to obtain reduced graphene oxide (RGO) with a variety of strategies. GO flakes are also employed to prepare three-dimensional (3d) low density structures, such as sponges, foams, hydro- or aerogels. The assembly of flakes into 3d structures can provide improved mechanical properties. Aerogels with a highly open structure, with interconnected hierarchical pores, can enhance the accessibility to the whole surface area, as relevant for a number of applications, such as energy storage. The main recipes to yield graphite intercalation compounds (GICs) are also discussed. GICs are suitable precursors for covalent functionalization of graphene, but can also be used for the synthesis of uncharged graphene in solution. Degradation of the molecules intercalated in GICs can be triggered by high temperature treatment or microwave irradiation, creating a gas pressure surge in graphite and exfoliation. Electrochemical exfoliation by applying a voltage in an electrolyte to a graphite electrode can be tuned by varying precursors, electrolytes and potential. Graphite electrodes can be either negatively or positively intercalated to obtain GICs that are subsequently exfoliated. We also discuss the materials that can be amenable to exfoliation, by employing a theoretical data-mining approach. The exfoliation of LMs usually results in a heterogeneous dispersion of flakes with different lateral size and thickness. This is a critical bottleneck for applications, and hinders the full exploitation of GRMs produced by solution processing. The establishment of procedures to control the morphological properties of exfoliated GRMs, which also need to be industrially scalable, is one of the key needs. Section III deals with the processing of flakes. (Ultra)centrifugation techniques have thus far been the most investigated to sort GRMs following ultrasonication, shear mixing, ball milling, microfluidization, and wet-jet milling. It allows sorting by size and thickness. Inks formulated from GRM dispersions can be printed using a number of processes, from inkjet to screen printing. Each technique has specific rheological requirements, as well as geometrical constraints. The solvent choice is critical, not only for the GRM stability, but also in terms of optimizing printing on different substrates, such as glass, Si, plastic, paper, etc, all with different surface energies. Chemical modifications of such substrates is also a key step. Sections IV-VII are devoted to the growth of GRMs on various substrates and their processing after growth to place them on the surface of choice for specific applications. The substrate for graphene growth is a key determinant of the nature and quality of the resultant film. The lattice mismatch between graphene and substrate influences the resulting crystallinity. Growth on insulators, such as SiO2, typically results in films with small crystallites, whereas growth on the close-packed surfaces of metals yields highly crystalline films. Section IV outlines the growth of graphene on SiC substrates. This satisfies the requirements for electronic applications, with well-defined graphene-substrate interface, low trapped impurities and no need for transfer. It also allows graphene structures and devices to be measured directly on the growth substrate. The flatness of the substrate results in graphene with minimal strain and ripples on large areas, allowing spectroscopies and surface science to be performed. We also discuss the surface engineering by intercalation of the resulting graphene, its integration with Si-wafers and the production of nanostructures with the desired shape, with no need for patterning. Section V deals with chemical vapour deposition (CVD) onto various transition metals and on insulators. Growth on Ni results in graphitized polycrystalline films. While the thickness of these films can be optimized by controlling the deposition parameters, such as the type of hydrocarbon precursor and temperature, it is difficult to attain single layer graphene (SLG) across large areas, owing to the simultaneous nucleation/growth and solution/precipitation mechanisms. The differing characteristics of polycrystalline Ni films facilitate the growth of graphitic layers at different rates, resulting in regions with differing numbers of graphitic layers. High-quality films can be grown on Cu. Cu is available in a variety of shapes and forms, such as foils, bulks, foams, thin films on other materials and powders, making it attractive for industrial production of large area graphene films. The push to use CVD graphene in applications has also triggered a research line for the direct growth on insulators. The quality of the resulting films is lower than possible to date on metals, but enough, in terms of transmittance and resistivity, for many applications as described in section V. Transfer technologies are the focus of section VI. CVD synthesis of graphene on metals and bottom up molecular approaches require SLG to be transferred to the final target substrates. To have technological impact, the advances in production of high-quality large-area CVD graphene must be commensurate with those on transfer and placement on the final substrates. This is a prerequisite for most applications, such as touch panels, anticorrosion coatings, transparent electrodes and gas sensors etc. New strategies have improved the transferred graphene quality, making CVD graphene a feasible option for CMOS foundries. Methods based on complete etching of the metal substrate in suitable etchants, typically iron chloride, ammonium persulfate, or hydrogen chloride although reliable, are time- and resourceconsuming, with damage to graphene and production of metal and etchant residues. Electrochemical delamination in a low-concentration aqueous solution is an alternative. In this case metallic substrates can be reused. Dry transfer is less detrimental for the SLG quality, enabling a deterministic transfer. There is a large range of layered materials (LMs) beyond graphite. Only few of them have been already exfoliated and fully characterized. Section VII deals with the growth of some of these materials. Amongst them, h-BN, transition metal tri- and di-chalcogenides are of paramount importance. The growth of h-BN is at present considered essential for the development of graphene in (opto) electronic applications, as h-BN is ideal as capping layer or substrate. The interesting optical and electronic properties of TMDs also require the development of scalable methods for their production. Large scale growth using chemical/physical vapour deposition or thermal assisted conversion has been thus far limited to a small set, such as h-BN or some TMDs. Heterostructures could also be directly grown

Dynamics and Limiting Mechanisms of Self-Aligned Carbon Nanotube Growth.

Carbon nanotubes (CNTs) are long, cylindrical molecules, which boast exceptional tensile strength and large thermal and electrical conductivities. Vertically aligned CNT “forests” have promising potential uses, including dry adhesives, electrical interconnects, light emitters, thermal interface materials, gas and liquid filters, composite reinforcements, and photonic crystals. Manufacturing indefinitely long CNTs may realize dreams of CNT-based cables and wires having stiffness, strength, and transport properties exceeding today’s best metal alloys and advanced fibers. However, the functional properties of CNT forests have so far fallen short of those of individual CNTs due to low packing fraction, polydisperse diameters, and relatively short lengths. Toward the eventual goal of bridging this structure-property relationship, my dissertation presents a novel set of in situ and ex situ characterization tools for CNT forest growth by chemical vapor deposition (CVD), as well as the use of these tools to investigate the limiting mechanisms thereof. In situ X-ray scattering reveals the dynamics of catalyst thin film dewetting into nanoparticle growth sites, the initial self-organization of the CNT forest, and the abrupt self-termination of growth. Quantification of catalyst and CNT sizes show that they are inevitably polydisperse, regardless of synthesis conditions. To overcome this, a novel method is introduced for templated dewetting of the catalyst film toward the formation of ordered, monodisperse particles using nanoporous anodic alumina. Further, a map of thermal conditions is explored by independently tuning the temperatures of the catalyst and gaseous precursors, thereby establishing a set of rules for engineering crucial characteristics of forest growth, including CNT diameter, structural quality, vertical alignment, as well as rate and lifetime of the reaction. Finally, aligned CNT ensembles are used as templates to direct the self-assembly of fullerene C60, creating hybrid films with high photoconductive gain, thereby demonstrating an immediate application of this exciting material. These studies represent many new insights into the so-called “birth, life, and death” of CNT growth, and they have important implications for future work in synthesis of advanced carbon materials, including CNTs, fullerenes, and graphene. Meanwhile, these results have immediate applicability to efficient CNT manufacturing, improved characterization, and new hybrid materials for energy conversion.Ph.D.Mechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/91427/1/emeshot_1.pd