Organic solar cells have caught considerable attention in the past few years due to their potential for providing environmentally safe, flexible, lightweight, inexpensive, and roll-to-roll feasible production solar cells. However, the efficiency achieved in current organic solar cells is quite low, yet quick and successive improvements render it as a promising alternative. A hopeful approach to improve the efficiency is by exploiting the tandem concept which consists of stacking two or more organic solar cells in series. One important constituent in tandem solar cells is the middle electrode layer which is transparent and functions as a cathode for the first cell and an anode for the second cell. Most studies done so far have employed noble metals such as gold or silver as the middle electrode layer; however, they suffered from several shortcomings especially with respect to reproducibility. This thesis focuses on studying a new trend which employs an oxide material based on nano-particles as a transparent cathode (such as Zinc-oxide-nano-particles) along with a transparent anode so as to replace the middle electrode. Thus, this work presents a study on solution processable zinc oxide (ZnO) nanostructures, their proper handling techniques, and their potential as a middle electrode material in Tandem solar cells in many different configurations involving both polymer and small molecule materials. Moreover, the ZnO-np potential as a candidate for acceptor material is also investigated.

The electronic structure of some transition metal compounds, specifically, Ca-doped LaMnO3, fundamental Mn oxides (MnO, Mn2O3, Mn3O4, and MnO2), and Fe-doped ZnO is studied using a combination of soft X-ray spectroscopy and atomic multiplet calculations. X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) are used as experimental tools to probe the unoccupied and occupied partial density of electronic states,respectively.<p> Ca-doped LaMnO3 perovskites have attracted great attention due to their colossal magnetoresistance and a wide range of magnetic and structural transitions. The magnetic and charge transport properties of these perovskites are directly related with Mn 3d-occupancy or Mn-valency and therefore, an investigation of the Mn-valence at Ca-doped LaMnO3 system is important. In this system, the Mn-valency is generally considered as a mixture of Mn3+ and Mn4+. But my research suggests the presence of Mn2+ at the surface of Ca-doped LaMnO3 samples. It is observed that increasing Ca-doping decreases Mn2+ concentration, and conversely, increases Mn3+ concentration. High temperature annealing at 1000 °C in air leads to the full reduction of surface Mn2+. Mechanisms for these observations are proposed in this study.<p> Mn oxides (MnO, Mn2O3, Mn3O4, and MnO2) are often used as reference standards for determining the Mn-valency in Mn-related complex systems and therefore a detailed understanding of their electronic structure is necessary. The Mn L2,3 XAS and O K XAS are measured for the four Mn oxides consisting of three common Mn oxidation states (Mn2+ in MnO, Mn3+ in Mn2O3, mixture of Mn2+ and Mn3+ in Mn3O4, and Mn4+ in MnO2). A significant energy shift with a systematic trend is observed in measured Mn L2,3 and O K absorption edges. These energy shifts are identified as a characteristic shift for different Mn oxidation states. Mn L2,3 Resonant Inelastic X-ray Scattering (RIXS) spectroscopy is demonstrated as a powerful tool in describing low energy excitations, e.g. d-d excitations and charge-transfer excited states in Mn oxides. For the first time, a RIXS study of Mn2O3,Mn3O4, and MnO2 is accomplished. Atomic multiplet calculations are used to successfully reproduce the energy positions and intensity variations of d-d excitation peaks observed in the experiment, and thus to describe the experimental RIXS spectra.<p> Finally, the local electronic structure of Fe implanted ZnO samples, a useful diluted magnetic semiconductor for spintronics, is investigated to shed light on the existing debate about the origin of ferromagnetism in these materials. Fe L2,3 XAS reveals that doped Fe ions are present in both Fe2+ and Fe3+ valence states. A combined theoretical and experimental study shows that doped ions are incorporated into Zn-sites of ZnO in tetrahedral symmetry. Fe L3- RIXS measurements demonstrate that a high Fe-ion dose of 8 × 107 cm-2 causes formation of FeO clusters, while low dose samples exhibit more free carriers.

This thesis reports on the design and fabrication of micro-electro-mechanical (MEM) resonators on silicon that are piezoelectrically-transduced for operation in the very high frequency (VHF) range. These devices have a block-type or beam-type design, and are designed to resonate in their in-plane and out-of-plane bulk extensional modes. Two piezoelectric materials were taken into consideration, zinc-oxide (ZnO) and lead-zirconate-titanate (PZT). The resonators are fabricated on silicon-on-insulator (SOI) wafers and the metal/piezo/metal stack of layers forming the device is built and patterned on the device layer silicon via photolithography techniques, RF sputtering (for the piezo-layer) and electron-beam evaporation (for the metal layers). The designing aspect involved ANSYS simulations of the mode-shapes and estimation of frequencies, and these have correlated well with experimental results. Devices with RF sputtered ZnO were successfully fabricated and tested to give high quality factors at reasonably high frequencies. A gold ground plane was implemented to reduce the feed-through level and increase the signal-to-noise ratio. Extensive characterization of PZT was also done as a replacement for ZnO, as the former material has a much higher piezoelectric coefficient (~20X that of ZnO) and can therefore extend the operation of these MEM resonators into the UHF range. Although the basic design of the device remains the same, incorporation of PZT complicates the process flow considerably with respect to the chemistry now involved with the patterning of different layers. The frequency response for ZnO-based resonators as well as all the characterization data for PZT has been reported.

M-plane (101 ¡Â0) ZnO thin films were grown on m-plane sapphire (101 ¡Â0) substrates by RF magnetron sputtering. We varied the RF power, working pressure, and O2/Ar ratio to obtain the best growth conditions. Structural properties were investigated by X-ray diffraction(XRD). XRD measurements showed that the crystal orientation of ZnO films was non-polar m-plane (101 ¡Â0). In addition, photoluminescence (PL) spectrum showed the bandgap energy of ZnO films was about 3.24 eV. PL spectrum showed zinc vacancy signal for films grown in oxygen rich condition. Carrier concentration was measured by hall measurement as well as FTIR spectrometry. The results showed the carrier concentration calculated by optical measurements was higher than hall measurements. One possibility for this could be the band tail at the bottom of conduction band. This band tail can make the effective mass larger and thus influencing the optical carrier concentration.

This article describes that we grew InGaN ternary films by Plasma- Assisted Molecular Beam Epitaxy (PAMBE) on the ZnO substrate O-polar (0001) surface. Before we grew the films, we grew the InN films on the ZnO substrate to find out the interface reaction conditions. We used Double Crystal X-ray Diffraction (XRD) to analyze the diffraction peak of InGaN films after we grew them. We found it was very hard to grow the single content InGaN films by generally methods. We tried period shutter control method to grow films, and we succeeded to grow the single phase films. We analyzed the morphologies by AFM and SEM, the microstructures by TEM, the electric properties by Hall measurement, and the fluorescent characteristics by PL.

In this study, zinc oxide (ZnO) nanotip and nanorod were grown on glass substrate by aqueous solution deposition (ASD). Both characteristics of the two nanostructures were investigated. For fabrication of ZnO nanostructure UV photodetector, In-Zn inter-digitated metal electrode was evaporated on the top of the grown ZnO nanostructure to form the contact via. Compared with the common value (375 nm), both the peaks from the PL spectra of ZnO nanotip and nanorod are red-shifted (409 nm) due to the massive defects in nanotip and nanorod. In order to improve the photosensiblity, heterojunction of ZnO nanostructure/TiO2 film was prepared and were made into UV photodetector. Photoresponses of both nanotip and nanorod were improved after N2O annealing at 300oC. With the heterojunction of ZnO 1D nanostructure on TiO2 film, the photoresponses of both ZnO nanotip/TiO2 film can reach to 22.85, and the rise time and decay time are 40 and 82 seconds, respectively. On the other side, the photoresponses of both ZnO nanorod/TiO2 film can reach to 27.44, and the rise time and decay time are 22 and 133 seconds, respectively.

Utilizing bulk micromachining technology, this thesis aimed to develop a flexural plate-wave(FPW) device with novel groove reflection microstructure for high-sensitivity and low insertion-loss biomedical microsystem applications. The influences of the amount and depth of the groove and the distance between the groove and the boundary of ZnO piezoelectric thin-film (DGB) on the reduction of insertion-loss and the enhancement of quality factor (Q) and electromechanical coupling coefficient (K2) were investigated. Three critical technology modules established in this thesis are including the development of (1) a sputtering deposition process of high C-axis (002) orientation ZnO piezoelectric thin-film, (2) an electrochemical etch-stop technique of silicon anisotropic etching and (3) an integration process of FPW device. Firstly, under the optimized conditions of the sputtering deposition process (300¢J substrate temperature, 200 W radio-frequency (RF) power and 30/70 Ar/O2 gas flow ratio), a high C-axis (002) orientated ZnO piezoelectric thin-film with a high X-ray diffraction (XRD) intensity (50,799 a.u.) and narrow full width at half maximum (FWHM = 0.383¢X) can be demonstrated. The peak of XRD intensity of the standard ZnO film occurs at diffraction angle 2£c = 34.422¢X, which matches well with our results (2£c = 34.357¢X). Secondary, an electrochemical etch-stop system with three electrode configuration has been established in this research and the etching accuracy can be controlled to less than 1%. Thirdly, this thesis has successfully integrated the main fabrication processes for developing the FPW device which are including six thin-film deposition processes and six photolithography processes. The implemented FPW device with RIE etched groove reflection microstructure presents a low insertion-loss of -12.646 dB, center frequency of 114.7 MHz, Q factor of 12.76 and K2 value of 0.1876%.

The roles of hydrogen induced defects in pure ZnO has been studied extensively. However, in a transition metal, such as Co, doped ZnO thin films the effect of hydrogen in electric conduction and magnetic coupling is still unclear and needs further study. Recently model predicts that hydrogen can be a shallow donor as well as an agent to induce ferromagnetism coupling between two adjacent Co ions which substitute the Zn sites at room temperature in a ZnO sample with a high Co doping ratio. However, the experimental supports is rare. In this study, Co-doped(5%) ZnO films are grown by a RF-magnetron sputtering system on glass substrate at room temperature. The growth condition is fixed for RF power in 200W, working press of 70 mtorr and various mixing ratio of H2/Ar+H2 gas. The crystal structure, electric and optical properties and the influence of vacuum annealing on the samples are studied. In this research, we found that the doping of hydrogen in Co-doped ZnO thin films truly increases the electric conductivity which is proportional to the H2/(Ar+H2) ratio. When the ratio of hydrogen is low, the (002) peak taken by a Glazing Angle X-ray Diffractometer dominates, while increasing hydrogen ratio other diffraction peaks appear, indicating an enhancement of crystal structure in all directions, and grain sizes and unit cell volume decrease. From the optical transmittance measurement, it is found that the color of films turned into metallic like and the optical band gap increases linearly with H2 ratio which can be attributed to the Burstein-Moss effect that corresponds to the increasing of carriers in the conduction band by doping of H2. The transmittance data provides the information of the ratio of crystalline and amorphous, which can also be correlated to the AFM results. When the H2 ratio is higher than 30%, more crystals and larger sizes of grains were formed in the films, such that carriers did not need to pass grain boundaries so frequently and experienced less scattering that was actually improve the electric conductivity. The electric conductivity can be even improved by post annealing in H2 environment. Moreover, the Magnetic circular dichroism (MCD) measurement shows that the Co2+ ions does truly substitute on Zn sited in Td symmetry during thin film deposition. The resistance measurement as a function of temperature found the hydrogenated Co-doped thin films are semiconductor conductive. More works are needed to determine the magnetization, identify second phases and Vo by SQUID and X-ray photoelectron spectroscopy.

The fate and transformation of laboratory-prepared nano-ZnO, nano-Ag and nano-CeO2 in three aqueous solutions under different environmental conditions were investigated in this work. Over the past decades nanomaterials have been widely used in different technical fields and consumer goods. As a result, nanomaterials might enter the environmental media via different routes and then posed potential hazards to the environment and human health. Researches in this regard have received much attention worldwide. In this work it was found that the solubility of each nanomaterial was highly influenced by the solution pH, but not by the solution temperature. The maximal solubility for the tested nanomaterials was obtained at pH 3, namely about 100% for nano-ZnO and lower than 2% for both nano-Ag and nano-CeO2. The solution pH and ionic strength were found to affect the stability of nanoparticles in different aquatic environments. For the solution pH of higher than the isoelectric point of the concerned nanomaterial, the higher the solution pH is, the greater the degree of stabilization of nanoparticles would be. On the contrary, nanoparticles aggregated as the ionic strength of the solution exceeded its critical aggregation concentration (CAC). CAC for each concerned nanomaterial could also be graphically determined as the attachment efficiency (£\) of nanoparticles increased with increasing ionic strength of the solution and then leveled off after reaching CAC. Experimental results also showed that Zn(OH)2(s) would form when nano-ZnO was in the solution of pH 10. The crystalline structure of the said precipitates was confirmed by X-ray diffraction. Likewise, Ce4+ dissolved from nano-CeO2 reacted with SO42- in aqueous solution yielding Ce(SO4)2(s). Clearly, transformation of nanomaterials might take place when they are in contact with various species in different aquatic environments. Humic acid in aqueous solution was found to be beneficial to the stability of nanomaterial of concern. Efforts have also been made to study the reaction behaviors among di(2-ethylhexyl)phthalate, erythromycin, and selected nanomaterials when they co-existed in the same solution. Their interactions, however, seemed to be unobvious. In this work it was found that under sunlight irradiation nano-ZnO did show its antibiotic effect due to photocatalysis. Nano-Ag was proven to have a strong antibacterial ability even in natural aquatic environments. It yielded the total bacteria survival ratio of less than 2% within one hour of reaction. In summary, the findings of this study showed that the behaviors of nano-ZnO, nano-Ag, and nano-CeO2 in aqueous solutions could be greatly influenced by different factors in different reaction systems.

The electro-optical properties of the ZnO/Si heterojunction embedded with self-assembled gold nanoparticles on patterned silicon substrate are investigated in this master thesis. High quality n-type ZnO film is deposited on patterned p-type silicon substrate by radio-frequency sputtering to form a ZnO/Si pn junction. The patterned silicon substrates are prepared by ICP-RIE using self-assembled nickel metal dot and silicon dioxide as etching mask. The optimum ICP process conditions of silicon nanopillars are CF4/Ar ~ 40/40 sccm and bias/RF power 400/400 W. Silicon nanopillars of diameter ~ 50 nm and height 100~400 nm are formed on the substrate surface. ZnO film is then deposited of a growth rate ~ 12 nm/min at the substrate temperature = 200oC. The plasmonic effects on the electro-optical properties, including photoluminescence (PL), reflection, and electrical characteristics, are studied by adding self-assembled gold nanoparticles within the ZnO film. The self-assembled gold nanoparticles are formed by thermal deposition and rapid thermal annealing at 700oC. The gold nanoparticles are observed by scanning electron microscopy (SEM) and particles of diameter about 100 nm. The PL intensity of ZnO is enhanced more than ten times at the peak wavelength = 380 nm by adding the gold nanoparticles and silicon nanopillars. Strong blue emission light could be saw with the naked eyes. For the electric characteristics, self-assembled gold nanoparticles in patterned ZnO/Si heterojunction show photoelectric conversion phenomenon because of high electromagnetic absorption and plasmonic effects.