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My research on metal oxide electron-harvesting interlayers for organic solar cells was focused as three interrelated projects in this dissertation: i) development of a chemical vapor deposition (CVD) system for titanium dioxide (TiO₂) film; ii) an electrochemical methodology to evaluate ZnO thin film charge (hole) blocking ability; iii) the effects of plasma modifications on sol-gel ZnO and sol-gel ZnO/organic (active layer) interfaces. In i), we showed that nanoscale (12-36 nm) CVD TiO₂ film deposited at 210 °C from our system obtains properties of conformal growth with ITO substrate, superior hole blocking ability, stoichiometric metal to oxide ratio, and close energetic alignment with electron acceptors, e.g., fullerenes. The introduction of CVD TiO₂ film as an electron transport layer (ETL) into organic solar cell significantly improves its J-V characteristics compared to bare ITO electrode. The optimum TiO₂ thickness in the OPV device applications was found to be 24 nm with a high fill factor (0.58) and power conversion efficiency (3.7%) obtained. In ii), simple electrochemical methods, i.e., cyclic voltammetry, impedance spectroscopy have been used to evaluate sol-gel derived ZnO (sg-ZnO) and sputtered ZnO (sp-ZnO) porosity and pinhole density. We showed that sg-ZnO with high surface area porous structure allows the probe molecules and poly-thiophene (P3HT) thin layer to direct contact ITO substrate, whereas sp-ZnO with dense structural property efficiently eliminates the probe molecule diffusion and the penetration of P3HT layer to ITO substrate. This electrochemical property difference also directly reflects on the device shunt resistance (Rp), where we observed larger leakage current for the devices using sg-ZnO than that of devices using sp-ZnO. We envision these simple electrochemical characterizations can be applied into other similar metal oxide interlayers as well as on flexible TCO substrates, in which pinholes and physical imperfections, e.g., cracking may occur after multiple bending processes. In iii), we demonstrated low power (10.5 watts) radio frequency (RF) O₂ and Ar plasma treatments have significant impacts on sg-ZnO near-surface chemical compositions, which in turn influence the onset potential of sg-ZnO electron injection from the underlying ITO substrate and its energetic alignment with electron acceptors, e.g., C₆₀. Using UPS, we found the presence of localized mid-gap states near the Fermi-level (Ef) of sg-ZnO, which induces the most favorable band bending and the largest vacuum level shift due to significant electron transfer from sg-ZnO to C₆₀. As a result, the resultant solar cells show the best device performance. Upon the plasma treatments, the passivation effects eliminate the mid-gap state. Therefore, we observed less degree of band bending at ZnO/C₆₀ interface and poorer device performance for the plasma treated sg-ZnO. The study demonstrates the importance of oxide/organics interface in operations of organic solar cells and provides a modification method to tune surface properties of oxide materials which can apparently be applied in other organic electronic devices, e.g., field effect transistors (FETs), organic light emitting diodes (OLEDs), etc.

Two sets of nanostructured Zinc Oxide (ZnO) thin films doped with varying nominal concentrations of rare earth (RE) ions were prepared by pulsed laser deposition (PLD). One set was doped with europium ions (ZnO:Eu³⁺) while the other was doped with erbium ions (ZnO:Er³⁺). The nominal concentration of RE ions ranged from 0.025 to 5 atomic %. The produced films were structurally, morphologically and optically characterised using different techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL), combined excitation and emission spectroscopy (CEES) and X-ray photoelectron spectroscopy (XPS). All films were found to possess a single-crystal hexagonal structure and were strongly oriented along the c-axis. However, the crystallinity of the investigated films seemed to deteriorate as the concentration of the rare earth ions increased. This deterioration is assumed to be due to the local distortion of the ZnO structure (host material) caused by the insertion of the relatively large RE ions, hence inducing structural stresses. Importantly, XRD measurements showed that no other crystalline phases related to europium or erbium, such as Eu₂O₃ or Er₂O₃, were observed. Surprisingly, the ZnO lattice constant (c) tended to become smaller as more RE³⁺ ions were added to the films. An explanation is offered whereby this observation can be taken as further evidence that Zn²⁺ ions were successfully substituted by RE³⁺ ions. Interestingly, doping ZnO films with RE³⁺ ions of a nominal concentration of ≥ 0.5 at.% or higher exhibited a drastic effect on the optical properties of the host matrix (ZnO) in which the near band edge luminescence characteristic of pure ZnO completely disappeared. According to SEM images, morphological changes also occur as dopant concentrations increase. Well-defined grains (crystallites) were clearly seen in films doped with ˂ 0.5 at.% of RE ions. However, these grains became hardly distinguishable at higher RE ion concentrations. Typical intra-4f shell transitions of RE³⁺ ions were observed when these ions were non-resonantly excited with UV radiation, indicating that energy had been efficiently transferred from ZnO to the rare earth ions. A plausible physical mechanism for this energy transfer is proposed. The radiative optical centres of rare earth ions were studied by CEES. In these experiments, both sets of films exhibited multiple optical sites. ZnO:Eu³⁺ thin films were found to have two distinct optical sites with differing site symmetries, whereas up to four optical sites were detected in the ZnO:Er³⁺ films.

The aim of this work was the study of sputtered zinc oxide (ZnO) film deposition, the optimisation and characterisation of film properties and applications as a sensing material. In recent years there has been increased interest in ZnO in terms of its potential applications as piezoelectric films (or coatings) for surface acoustic wave devices (SAW), for IR and visible light emitting devices and UV sensing. The electrical, optoelectronic and photochemical properties of undoped ZnO have resulted in its use for solar cells, transparent electrodes and blue/UV light emitting devices. ZnO is a unique material that exhibits both semiconducting and piezoelectric properties. In the past decade, numerous studies have been made on both production and application of one-dimensional ZnO. Compared with other semiconductor materials, ZnO has a higher exciton binding energy of 60 meV, which gives it a high potential for room temperature light emission, is more resistant to radiation, and is multifunctional as it has piezoelectric, ferroelectric, and ferromagnetic properties. ZnO-based semiconductor and nanowire devices are also promising for the integration on a single chip. So far, the various applications of ZnO nanomaterials such as biosensors, UV detectors and field emission displays are being developed. In this work, ZnO was sputtered using both DC and RF magnetron sputtering. Reactive DC sputtering was performed with a Zn target and oxygen plasma, while RF sputtering was performed with a ZnO target. Comparisons between films deposited under different conditions on different substrates were employed to assess film properties. Several experiments were performed on as-grown films as a control for subsequence treatments, other samples were post-annealed in N2 at temperatures up to 1200 ºC, the highest reported annealing temperature and the quality of the deposited films was determined using PL, RBS, XRD, SEM and AFM. The piezoelectric properties (d33) of selected films were determined using single beam interferometry, double beam interferometry, and for the first time, using piezoelectric force microscopy (PFM). It was found that DC sputtered films yielded better quality films as evident by PL and XRD analysis and higher piezoelectric response than RF sputtered films. Films deposited using DC sputtering on Si substrates and followed by post-annealing in N2 atmosphere at 1100 ºC showed the highest recorded PL response, while films deposited on sapphire showed good PL response without any need for post-annealing. The d33 of selected films were determined first using single beam interferometry and inflated results were reported, caused by sample bending/buckling. Double beam interferometry results confirmed d33 values in the range of 3.3 to 4.3 pm/V. Piezoelectric force microscopy (PFM) which is based on AFM, was employed to investigate the local electromechanical (piezoelectric) properties of the ZnO films. UV sensing was demonstrated using Schottky contacts and SAW devices on ZnO deposited on Si and post-annealed. In the first instance, Schottky contacts were fabricated on the films and the I V characteristics determined under exposure of various light sources. The current increased up to one order of magnitude during exposure with a halogen light bulb, which is known to emit energy in the UV band. Another experiment was performed using surface acoustic wave (SAW) devices which were fabricated on the films and interrogated using a network analyser. These SAW devices contain an interdigitated transducer and two reflectors each. The signals sent back from the two reflectors were analysed under various light conditions and gave lower readings during exposure to UV light. In order to enable device fabrication of UV sensors a novel “super coating”, achieving both optimised PL and d33 properties, was designed, fabricated and tested. The structure is based on optically transparent Quartz substrate. During this experiment the first DC sputtered coat was optimised to have high PL response by post-annealing at 900 ºC. Afterwards, the second coat was left as-sputtered in order to have highly piezoelectric properties. Preliminary analysis using XRD showed two peaks corresponding to the annealed and not annealed coat, which suggest the super coating combines the properties of the two individual films. This configuration has the potential to be used as UV sensing material and as piezoelectric substrate for SAW devices.

Semiconductors represent the enabling technology that underpins the many advances that define modern society. One semiconductor that shows considerable promise in the fabrication of new devices is zinc oxide (ZnO). A fundamental understanding of the properties of a material is required in order to exploit its properties. The behaviour of dopants and defects relevant to optoelectronic device fabrication is of particular interest. However, acceptor doping of ZnO is currently controversial, as successful and reproducible acceptor doping has not yet been achieved. Acceptor doping of ZnO using silver (Ag) is explored in this thesis to contribute towards the understanding of defect introduction in ZnO. In addition, there is also increasing interest in exploring materials with unconventional properties, commonly referred to as metamaterials, particularly for optical applications. The previously unexplored unique combination of Ag and ZnO may enable the fabrication of those devices. Several key factors that affect heteroepitaxy film quality, and ultimately its properties, are buffer layers and substrate temperature. A lattice match between sapphire and ZnO was provided by using buffer layers of 1 nm magnesium oxide (MgO) and 7.9 nm low temperature ZnO. The highest quality film was grown at the highest temperature (800°C), with rms roughness of 2.9 nm, carrier concentration of 3.6x10¹⁶ cm⁻³, and mobility of 105 cm²/Vs. In contrast, dopant (Ag) incorporation occurs more readily below 600°C, with dopant incorporation of up to 1020 cm⁻³ measured. Ag manifests as a deep acceptor (up to 94% substitutionally on Zn lattice sites), as evident from decreasing carrier concentration with increasing Ag flux, and DLTS measurements indicating an acceptor trap at 319 meV. This suggests that Ag is suitable for introducing compensation in ZnO, but Ag acceptors are not sufficiently shallow to result in p-type material. However, the unique combination of ZnO and Ag also enables the fabrication of a novel device, namely a superlens. Initial experimental results show the possibility of imaging a 100 nm line as 132 nm, compared with the diffraction-limited resolution of 332 nm for the same line feature.

Engineering particles at the nanoscale demands a high degree of control over process parameters during synthesis. For nanocrystal synthesis, solution-based techniques typically include application of external convective heat. This process often leads to slow heating and allows decomposition of reagents or products over time. Microwave-assisted heating provides faster, localized heating at the molecular level with near instantaneous control over reaction parameters. In this work, microwave-assisted heating has been applied for the synthesis of II-VI semiconductor nanocrystals namely, ZnO nanopods and CdX (X = Se, Te) quantum dots (QDs). Based on factors such as size, surface functionality and charge, optical properties of such nanomaterials can be tuned for application as sensors. ZnO is a direct bandgap semiconductor (3.37 eV) with a large exciton binding energy (60 meV) leading to photoluminescence (PL) at room temperature. A microwave-assisted hydrothermal approach allows the use of sub-5 nm ZnO zero-dimensional nanoparticles as seeds for generation of multi-legged quasi one-dimensional nanopods via heterogeneous nucleation. ZnO nanopods, having individual leg diameters of 13-15 nm and growing along the [0001] direction, can be synthesized in as little as 20 minutes. ZnO nanopods exhibit a broad defect-related PL spanning the visible range with a peak at ~615 nm. Optical sensing based on changes in intensity of the defect PL in response to external environment (e.g., humidity) is demonstrated in this work. Microwave-assisted synthesis was also used for organometallic synthesis of CdX(ZnS) (X = Se, Te) core(shell) QDs. Optical emission of these QDs can be altered ased on their size and can be tailored to specific wavelengths. Further, QDs were incorporated in Enhanced Green-Fluorescent Protein – Ultrabithorax (EGFP-Ubx) fusion protein for the generation of macroscale composite protein fibers via hierarchal self-assembly. Variations in EGFP- Ubx·QD composite fiber surface morphology and internal QD distribution were studied with respect to (i) time of QD addition (i.e., pre or post protein self-assembly) and (ii) QD surface charge — negatively charged QDs with dihydrolipoic acid functionalization and positively charged QDs with polyethyleneimine coating. Elucidating design motifs and understanding factors that impact the protein-nanoparticle interaction enables manipulation of the structure and mechanical properties of composite materials.

ZnO nanowires (NWs) are good candidates for chemical sensing because of their high surface-to-volume ratio. In this work, ZnO nanobridge sensors were fabricated utilizing a novel method which uses carbonized photoresist (C-PR) as a nucleation layer. The use of C-PR allows simultaneous growth and integration of NWs to lithographically-defined features. The nanobridge sensors are shown to be sensitive to the presence of O₂, H₂O, CO, and H₂/N₂ gas. However, since ZnO dissolves in water, a protective layer is necessary for these sensors to be used in the liquid or vapor phase. A chemical vapor deposition (CVD) process for amino-[2,2]paracyclophane (parylene-A) was developed and used to successfully protect the NWs. Gas sensing measurements were performed on bare and parylene-A coated devices with and without UV illumination. The parylene-A layer was found to attenuate sensitivity to O₂ and H₂O, and UV illumination was found to decrease the response time. / Graduation date: 2012

Dissertation (Ph.D.)--Auburn University, 2006. / Abstract. Includes bibliographic references.

Thesis (Ph. D.)--Missouri University of Science and Technology and University of Missouri--St. Louis, 2008. / Vita. The entire thesis text is included in file. Title from title screen of thesis/dissertation PDF file (viewed August 28, 2008) Thesis completed as part of a cooperative degree program with Missouri University of Science & Technology and the University of Missouri--St. Louis. Includes bibliographical references (p. 129-142).

Thesis (Ph. D., Material Sciences and Engineering)--University of Idaho, January 2008. / Major professor: Keith A. Prisbrey. Includes bibliographical references (leaves 85-90). Also available online (PDF file) by subscription or by purchasing the individual file.