Self-assembled semiconducting materials have been rapidly developed for a range of applications. This work aims to control the morphology of nanostructured semiconductors to understand how their functions arise from the structural properties.

The first part of this dissertation focuses on the formation of a bulk-heterojunction (BHJ) in the active layer of organic photovoltaics (OPV). A BHJ is a bicontinuous interpenetrating network of organic components. The phase separation of the electron donor and the acceptor is required to achieve a BHJ structure in the nanostructured morphology, which promotes an efficient charge transportation. The use of solvent additive is one of the strategies to control the spontaneous phase separation during the film formation. Low vapor pressure solvent additives are introduced to a polymer casting solution in a sequentially processed OPV system, to study the swelling effect on the phase separation. In particular, the change in crystallinity and vertical mixing will be intensively studied upon polymer swelling. As another strategy, we introduce a molecular structure change to fullerene derivatives. A small structural variation leads to a large enough contrast of their surface energy, which is attributed to different vertical phase separation in the active layer. It eventually allows us to examine photovoltaic performance and device physics.

In the second part, mesoporous inorganic films are investigated by preparation from a nanocrystal solution or sol-gel precursors for solar energy applications. Mesoporous nanocrystal-based titania is synthesized for inorganic/organic hybrid solar cells. The effect of surface modification is examined by anchoring a fullerene derivative on to titania surface. 3D interconnected mesoporous tantalum nitride films are prepared via sol-gel method as photoanodes in solar water splitting. The simple synthetic method using polymer template enables us to successfully prepare nitride films with excellent pore periodicity. The porous tantalum nitride film is examined with photoelectrochemical measurement to investigate the correlation between nanostructuring and photocatalytic activity.

For the final part of this dissertation, porous cobalt ferrite and cadmium sulfide films are studied using ellipsometric porosimetry. Understanding the nature of their pores allows us to tune the intrinsic properties of the materials or prove the newly designed synthetic method.

Magnetic nanomaterials are an important and widely studied class of materials with a wide variety of applications. The work presented here is aimed at both developing techniques to control the nanoscale structure of these materials and understanding the relationship between that structure and the overall material properties. The techniques used here are primarily solution-phase methods which offer a high degree of control and versatility.

The first part of this work is focused on thin films of magnetic oxide materials which are particularly applicable to radio frequency (RF) devices. Here, both sol-gel and nanocrystal precursors are used to create thin films where the film composition, grain size and porosity are controllably tuned. We then investigated both the static and dynamic magnetic properties of the films to better understand how the nanoscale structure impacts the overall properties. These investigations provide valuable insights that can allow us to design materials with properties tailored to meet the requirements of individual devices. Importantly, these insights are applicable to a wide variety of magnetic materials and are not limited to the specific materials studied here.

The second part of this work is focused on metallic alloy nanocrystals which have potential applications as elements in high density data storage devices. First, in chapter 5, the magnetoelectric properties of FePd nanocrystals is investigated. FePd is a good candidate for use in magnetoelectric memory devices which are highly energy efficient. By using nanocrystals of FePd, we hope to find a route to potentially reducing bit size in those devices which can lead to increased data storage densities. Then, in chapter 6, we move on to explore shape effects by looking at FePt nanorods. FePt has a very high magnetic anisotropy which in memory devices translates to increased bit stability and potentially allows for smaller bit sizes. In nanorods, shape anisotropy can enhance the already high magnetic anisotropy to create even stronger nanomagnets.

Electrochemical energy storage devices with both high energy density and high power density are technologically necessary for the electrification of transportation and many other applications. This dissertation focuses on the development of fast-charging Li-ion batteries through a fundamental understanding of structural parameters that allow for fast and reversible redox reactions in electrode materials. Using solution-based routes, the atomic and nanoscale structure of electrode materials was controlled and connected to their electrochemical characteristics, including specific capacity, rate capability, and cycling longevity. Additionally, advanced in operando characterization techniques were employed to probe how materials' structure and properties evolve during cycling. Overall, these findings provide guiding principles for the design of fast-charging electrode materials going forward. The first two sections focus on size-dependent phase transition behavior in MoO2, a model tunnel structure anode (Chapters 2 and 3). Chapter 2 shows how the large first-order phase transition in bulk MoO2 becomes systematically suppressed in a series of size controlled nanoarchitectures. The phase transition remains first-order, but shrinks dramatically, in intermediate-sized nanoporous MoO2 and becomes entirely continuous solid-solution in smaller MoO2 nanocrystals. Accordingly, the signatures of slowed charge storage from the bulk phase transition disappear in the nanomaterials. In chapter 3, we employ this suite of materials to show that this change phase transition behavior is key to the development of pseudocapacitive properties in nanoscale MoO2 using electrochemical impedance spectroscopy. Chapters 4 and 5 characterize the charge storage properties of V9Mo6O40, which transforms into a disordered rock salt structure during cycling. In chapter 4, the crystal structure of V9Mo6O40 is shown to govern its transformation pathway and resulting morphology compared to a well-studied analog, V2O5, while chapter 5 highlights the role of optimized Li+ diffusion distances to realize fast-charging using nanoporous V9Mo6O40. Chapter 6 details the use of a newly developed technique to measure the insulating to conductive transition in two high rate anode materials, T-Nb2O5 and Nb18W16O93. The rate of the increase in conductivity is shown to explain the difference in performance. Finally, Chapter 7 focuses on synthesis and characterization of (W0.2V0.8)3O7, a unique V-based Wadsley-Roth shear structure that shows high rate capability.

The properties of a material as given by the crystal structure can be modified or changed completely by the microstructure architecture. Properties attributed to an insignificant fraction of the material, such as the surface atoms or defect sites, can become material-dominant properties if that fraction of the material is amplified through an appropriate architecture. High surface area forms, in which the material is predominantly surface sites, can have completely different properties than a non-architectured material of the same composition.

In the first part of this work, we discuss coatings to enable microarchitectures for 3D microbatteries. 3D batteries require high surface area structures in order to expand their electrode mass per limited footprint. Increased electrode magnitudes allow a battery to store more energy, and utilizing a high surface area design allows this without increasing ion diffusion distances as would thick monoliths of equivalent electrode mass. A variety of anodes and cathodes have been designed and tested following this principle, and require new electrolyte coatings to be developed to suit them. We design and instrument to deposit these coatings, and demonstrate plasma polymerized PEO-like polymer coatings as versatile solid state electrolyte films that should be compatible with a variety of 3D microbattery architectures. Our electrolyte coatings are analyzed by impedance spectroscopy, and their ionic conductivity is compared amongst various classes of depositions and various Li-ion concentrations.

In the second part of this work, we explore the versatility of nano-architectured materials. High surface area iron oxide aerogels have already been discovered to be useful scaffolds for the paramagnetic iron oxide nanoparticles that form in situ in aerogel synthesis. We have found that the high concentration of surface defects in this material give the aerogels high capacities >100 mAh/g as Li-ion cathodes. Additionally, nanoporous silicon has already been discovered to add stability to and extend the cycle life of silicon as a Li-ion anode. We explore the versatility of this material for the use as anodes in other battery chemistries.

This dissertation compiles work on sol-gel syntheses of multiferroic materials and applications thereof. Multiferroics, or materials that simultaneously exhibit multiple order parameters such as ferromagnetism, ferroelectricity, or ferroelasticity, may be fabricated by solution processing techniques. Specifically, these techniques may be used to control both the atomic and the nanoscale structures of piezoelectric lead zirconate titanate (PbZrxTi1−xO3 or PZT) and magnetostrictive cobalt ferrite (CoFe2O4 or CFO).

The first part of this work focuses on strain-coupling PZT and CFO into a magnetoelectric composite. A mesoporous CFO framework was synthesized using block copolymer templating, which was subsequently conformally filled by PZT by atomic layer deposition (ALD). The final porosity of the film is controlled by the ALD PZT layer, and we show that this porosity influences the magnetoelectric coupling of the composite. An ex situ external electric field is applied to the composite, and samples with the greatest porosity, and thus greatest mechanical flexibility, were able to accommodate strain transfer to the CFO, resulting in a greater reduction of the sample saturation magnetization.

The second part of this work focuses on using solution processing to control domain-level contributions to the material’s ferroic properties. An iterative spin coating process can be used to create PZT films of arbitrary thickness. Electric domains are generally pinned in nanoscale PZT thin films, but models of PZT films on the mesoscale must consider domain reorientation. As for CFO, solution processing may be used to control the CFO grain size, which in turn limits the size of its magnetic domains, and subsequently its static magnetic properties.

Novel battery technology must be capable of providing both increased energy density and power density to keep up with global energy demand. This dissertation addresses a variety of structural challenges present in energy storage materials for both positive and negative electrode applications. Several materials (LiMn2O4, MoS2, and SbSn) have been synthesized as nanoporous architectures to reduce ion diffusion length, mitigate volume changes, and enhance structural stability. The first part of this work describes a facile synthesis for nanoporous LiMn2O4 and how the self-discharge problem is mitigated by doping with Al3+. Through x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), and electrochemical rate cycling, we show that the Al3+ dopant preferentially substitutes Mn3+ at the surface, stabilizes the spinel structure at lower temperature (200˚C), mitigates manganese dissolution, while still maintaining good rate cycling up to current densities of 20C. The second and third chapters of this dissertation focus on de-convoluting the effects of reducing crystal size and lattice disorder in mesoporous MoS2 pseudocapacitors. A series of mesoporous MoS2 powders with different sizes and degrees of crystallinity are synthesized through a sulfurization reaction. Crystal size is controlled by the size of mesoporous MoO2 precursor used, and disorder is controlled by higher annealing temperatures. Ambient synchrotron x-ray total scattering / pair distribution function (PDF) analysis techniques enable us to quantify the extent of layer shifting compared to layer expansion disorder in the matrix of samples. Operando x-ray diffraction (XRD) demonstrates that both size and disorder effects cause the suppression Li-intercalation induced phase transitions in MoS2. The reduced size and increased disorder are correlated to enhanced fast-charging performance up to rates of 100C. Kinetic analyses show size and disorder effects increase the fraction of capacitive current in mesoporous MoS2, demonstrating that reducing crystal size and increasing lattice disorder are both effective methods for introducing pseudocapacitive charge storage. Using operando PDF to study our matrix of size and disordered MoS2, further understanding into the dynamic evolution of disorder and local structure is achieved. The final chapter elucidates the formation of both crystalline and amorphous phases evolved during the cycling of nanoporous SbSn alloying anode through both operando XRD and PDF. The nanoporous architecture of SbSn, intermetallic phase, and ductile amorphous intermediates all provide means of buffering the volume expansion during cycling, resulting in improved long-term cycling stability.

This thesis focuses on studying and controlling the structure of pristine and doped semiconducting polymers. Semiconducting polymers have many applications in flexible electronics due to their structural tunability, low cost and solution processability. Intrinsically, semiconducting polymers have poor conductivities due to a lack of mobile carriers. Charge transfer between a semiconducting polymer and a dopant molecule is necessary to introduce carriers into a polymer system. If an electron is fully transferred, commonly called “integer charge transfer (ICT)”, this will result in a polaron and a dopant anion. On the other hand, the electron charge could be shared between the polymer and a dopant molecule to form a “charge transfer complex (CTC)”. In the first part of the thesis, we explored factors that affect the charge transfer pathways in doped semiconducting polymers and were able to control the formation of CTCs. Semiconducting polymers are composed of both crystalline and amorphous parts. Compared to crystalline regions, amorphous polymer parts are disordered, thus the dopant anion is usually close to the polarons, resulting in poor carrier mobility due to Columb attraction between polarons and counterions. CTCs also tend to form in amorphous polymer regions compared to crystallites and result in less carriers due to the charge-sharing nature of CTC. In our second project, we explored ways to suppress the formation of both CTCs and localized carriers even in highly amorphous polymer films, using large boron cluster-based dopants. The electron density of these dopants is core-localized and is shielded from the holes on the polymer, resulting in increased crystallinity and higher film conductivities. In our third project, we further explored how polymer crystallite orientation influences the ease of doping and found that polymer regions with structures similar to the final doped structure could be doped more easily. In the last chapter, we designed amphiphilic semiconducting polyelectrolytes that form ordered cylindrical micelles in water. Our results demonstrate that we can achieve relatively precise control between electron donor and acceptor co-assemblies by varying the structural properties of component amphiphilic polymers and acceptors, which can provide guidelines for designing systems with controllable excited-state transfers.

Semiconducting polymers are a versatile class of materials used for a variety of electronic applications. They are a cheap source of material relative to their inorganic counterparts, are flexible, and with solution processing, are easily scalable However, they are intrinsically poor conductors, which can result in low device efficiencies and shorter carrier lifetimes. The conductivity can be improved using a variety of methods, including controlling the morphology to improve carrier transport or introducing charge carriers using chemical dopants. In this dissertation, we describe experiments that use a combination of X-ray and neutron scattering techniques to understand how we can use morphology in semiconducting polymers to improve their charge transport properties.

The first part of this dissertation focuses on the design of an amphiphilic conjugated polyelectrolyte model system to control the aggregation of polymer chains in solution, with the goal of straightening chains to reduce carrier trap sites caused by kinks that disrupt the π-conjugation. After showing good control of the nanoscale morphology in a well-defined, ideal system in solution, we then look at the controllably improving the electron transfer process in an actual organic photovoltaic (OPV) device. We then look to control and improve the electron transfer process in full organic photovoltaic devices. We show that using sequential processing, where the polymer and fullerene are deposited in two separate steps, we can control the device level mixing of polymer and fullerene.

We end with a discussion of studies focused on the morphological effects of molecular doping of semiconducting polymers. Using small molecule dopants, we show that with SqP, the polymer morphology is maintained as compared to conventional blend casting (BC) method. This allows more dopant to incorporate into thin films, further increasing the conductivity. Thereafter, we focus on how SqP provides the opportunity to tune the polymer morphology prior to doping and investigate how controlling the polymer crystallinity affects the optical and electronic properties in its doped state. Using statistical copolymers, we investigate the effect of polymer crystallinity and energy level offset between polymer and dopant. Finally, we conclude with a study on how the interplay between polymer chain ordering and the location of the dopant counterion in the lattice controls polymer conductivity. Overall, these results emphasize the importance of understanding and controlling the morphology of semiconducting polymers on multiple length-scales.

Transparent, insulating coatings can be applied to windows to increase the energy efficiency of buildings. Amorphous material like silica make good thermal insulators due to their local atomic disorder that impedes heat conduction. Pores can additionally be added to the material to further reduce heat conduction by decreasing the material density while adding interfaces that scatter heat carriers. This concept has been extensively used in highly porous silica aerogels, which are valued for their ultra-low thermal conductivities. However, these aerogels significantly scatter light, and cannot be used for applications that require high optical transparency. This thesis examines four different nanoporous silica networks, synthesized using a combination of template-assisted and template-free methods, to understand the relationship between the structure of each network and its thermal conductivity, and to explore effective, scalable synthetic methods for producing nanoporous materials with optimized porosity. In the first part of this dissertation (Chapters 2 to 5), we explore the effect of nanoscale architecture on polymer-templated silica-based networks. Silica-based thin films with various types of precursors, pore sizes, particle sizes and dopants in the network are studied. We found that although silica is amorphous, the change in the nanoscale architecture at fixed porosity and changes in the chemical composition of the walls can both be used to tune the thermal conductivity. In the second part (Chapter 6), we combined the knowledge gained from our thin film studies to synthesize hollow silica shells that can be assembled to produce transparent, thermally insulating monoliths. We optimize the synthesis of hollow silica shells to reduce their sizes from c.a. 30 nm to below 15 nm, and then demonstrate a method to assemble those shells into mechanically robust, monoliths. In the final part of this work (Chapter 7 and 8), we use small angle X-ray scattering to understand the structural changes that occur when silica precursors react to form wet gels and when those gels are dried to produce monoliths. Insights into the changes in the nanoscale architecture allow us to further optimize the optical transparency and minimize the thermal conductivity of the final monoliths.

Developing novel functional materials for applications in energy storage and conservation is essential to solving today’s problems of meeting growing energy demands with minimal environmental impact. The first part of this dissertation describes pseudocapacitive Li-ion electrodes for fast charge storage. Pseudocapacitance, a charge-transfer process that is not diffusion-limited by definition, can be achieved by utilizing short diffusion lengths in nanostructures. In this section, three distinct transitional metal oxides with nanoscale architecture, Nb2O5, MoO2, and LiMn2O4, are examined. We focus on correlating charge storage mechanisms with structural properties, and exploring different aspects of pseudocapacitive charge storage with each system. First, we developed a core/shell Nb2O5/Nb4N5 composite via partial nitridation, achieving specific capacity over 500 C/g within 90 seconds. Our use of the nitride shell provides excellent electronic pathways that connect Nb2O5 nanoparticles and improve the interfacial electrical conductivity. The charge storage mechanism was found to depend on the thickness of the nitride shell. For MoO2, we developed an interconnected porous network that provides facile electrical and ionic pathways for fast charge storage. We also used it as a model system to understand the effects of structural variations on charge storage behavior. Phase transition suppression was found to be associated with highly capacitive systems, and a strong correlation between charge storage properties and crystallite size was established. Last, we developed needle-like nanostructures with selectively engineered surface facets to overcome its capacity loss due to surface Mn dissolution. This unique structure allowed for a higher capacity and less surface Mn dissolution due to a dominant presence of dissolution-resistant facets.

The second part of this dissertation studies thermal transport in mesoporous SiO2. SiO2 is widely used in many thermal insulating applications due to its facile synthesis and easy-to-control of structure. Porous amorphous SiO2 has strong boundary scattering effects that lead to significantly low thermal conductivity. In this section, we examined the effect of porosity, pore size, and framework texture (i.e., continuous or nanoparticulate) on thermal conductivity in mesoporous SiO2 thin films. Sol-gel and nanoparticle-based mesoporous SiO2 thin films were synthesized by evaporation-induced self-assembly using tetraethyl orthosilicate and pre-made SiO2 nanoparticles as the framework precursors, respectively, and a variety of template reagents. The thermal conductivity was measured by time-domain thermoreflectance at room temperature in vacuum. A porosity weighted simple effective medium approximation was employed to explain trends in thermal conductivity. The results give new insight into thermal transport in nanostructured amorphous materials, and suggest design rules of the nanoscale architecture to control the thermal conductivity of mesoporous materials for a wide range of applications.