Daktaro disertacijoje pristatomi cinko oksido pagrindu užaugintų puslaidininkinių sluoksnių ir jų įvairialyčių darinių, skirtų taikymams optoelektronikoje, tyrimų rezultatai spektroskopijos metodais. Optiškai charakterizuoti aukštos kokybės cinko oksido sluoksniai ir jų įvairialyčiai dariniai: ZnO, ZnO:Ga ir MgZnO epitaksiniai sluoksniai, auginti molekulinės epitaksijos būdu, ZnO:N sluoksniai, auginti magnetroninio dulkinimo būdu, šviesos diodams paruošti CdZnO/ZnO kvantinių duobių dariniai ant GaN sluoksnių, auginti apjungiant molekulinės epitaksijos ir cheminio metaloorganinio junginio nusodinimo iš dujinės fazės auginimo metodus. Šiame darbe surinkta naujų duomenų apie fotosužadintų krūvininkų dinamiką ir sąveiką cinko oksido dariniuose. Nustatyta, kad netamprioje laisvų eksitonų sąveikoje dalyvaujantys prie priemaišų pririšti eksitonai įtakoja sąveikos liuminescencijos juostos padėtį spektre. Aprašyta krūvininkų lokalizacija MgZnO epitaksiniuose sluoksniuose dviejų skirtungų lokalizacijos centrų lauke. Dėl to žymiai padidėja spindulinės rekombinacijos efektyvumas, kas turi didelės svarbos taikymams optoelektronikos prietaisams. CdZnO/ZnO kvantinių duobių darinių, skirtų žalios spektro srities šviesos diodams, tyrimai parodė, kad mažas spindulinės rekombinacijos efektyvumas kambario temperatūroje yra dėl ženkliai silpnesnio krūvninkų lokalizacijos efekto, kuris užtikrina didelį efektyvumą įprastuose InGaN/GaN kvantinių duobių dariniuose. / In doctoral dissertation, results of investigation on zinc oxide based semiconductor layers and heterostructures for application in optoelectronics by spectroscopy methods are presented. High quality zinc oxide layers and heterostructures were characterized optically. That is ZnO, ZnO:Ga and MgZnO epitaxial layers grown by molecular beam epitaxy technique, ZnO:N layers grown by magnetron sputtering method, CdZnO/ZnO quantum wells structures for light-emitting diodes grown on GaN layers by combined molecular beam epitaxy and metalorganic chemical vapor deposition techniques. In this work, new data on dynamics and interaction of photoexcited carriers in zinc oxide based structures were acquired. It was shown, that the contribution of impurities bound excitons in the inelastic interaction of free excitons influences the position of luminescence band of the interaction in spectrum. In MgZnO epitaxial layers, the localization of carriers in the field of two different localization centers is described. Due to the localization the radiative recombination efficiency increases significantly and it has importance for application in optoelectronic devices. Investigation of the CdZnO/ZnO quantum wells structures for green spectral range light-emitting diodes showed that low radiative recombination efficiency at room temperature is mainly due to weak carrier localization effect, which is responsible for high efficiency of conventional InGaN/GaN quantum wells structures.

Nanotechnology is emerging to be one of the most important scientific disciplines that physics, chemistry and biology truly overlap with each other. Over the last two decades science and technology have witnessed tremendous improvement in the hope of unveiling the true secrets of the nature in molecular or atomic level. Today, the regime of nanometer is truly reached. ZnO is a promising material due to the wide direct band gap (3.37 eV) and the room temperature large exciton binding energy (60 meV). Recent studies have shown considerable attraction towards ZnO nanostructures, particularly on one-dimensional ZnO nanorods, nanowires, and nanotubes due to the fact that, for a large number of applications, shape and size of the ZnO nanostructures play a vital role for the performance of the devices. The noncentrosymmetric property of ZnO makes it an ideal piezoelectric material for nanomechanical devices. Thus, mechanical characterization of one dimensional ZnO nanostructures including strength, toughness, stiffness, hardness, and adhesion to the substrate is very important for the reliability and efficient operation of piezoelectric ZnO nanodevices. Moreover, owing to the large effective surface area with high surface-to-volume ratio, the surface of one dimensional ZnO nanowires, nanorods, and nanotubes is very sensitive to the changes in surface chemistry and hence can be utilized to fabricate highly sensitive ZnO electrochemical sensors. This thesis studies mechanical properties and electrochemical sensor applications of ZnO nanostructures. The first part of the thesis deals with mechanical characterization of vertically grown ZnO nanorods and nanotubes including buckling, mechanical instability, and bending flexibility. In paper I, we have investigated mechanical instability and buckling characterization of vertically aligned single-crystal ZnO nanorods grown on Si, SiC, and sapphire substrates by vapor-liquid-solid (VLS) method. The critical loads for the ZnO nanorods grown on Si, SiC, and sapphire were measured and the corresponding buckling and adhesion energies were calculated. It was found that the nanorods grown on SiC substrate have less residual stresses and are more stable than the nanorods grown on Si and sapphire substrates. Paper II investigates nanomechanical tests of bending flexibility, kinking, and buckling failure characterization of vertically aligned single crystal ZnO nanorods/nanowires grown by VLS and aqueous chemical growth (ACG) methods. We observed that the loading and unloading behaviors during the bending test of the as-grown samples were highly symmetrical and the highest point on the bending curves and the first inflection and critical point were very close. The results also show that the elasticity of the ZnO single crystal is approximately linear up to the first inflection point and is independent of the growth method. In Paper III, we quantitatively investigated the buckling and the elastic stability of vertically well aligned ZnO nanorods and ZnO nanotubes grown on Si substrate by nanoindentation technique. We found that the critical load for the nanorods was five times larger than the critical load for nanotubes. On the contrary, the flexibility for nanotubes was five times larger than nanorods. The discovery of high flexibility for nanotubes and high elasticity for nanorods can be utilized for designing efficient piezoelectric nanodevices. The second part of this thesis investigates electrochemical sensor applications of ZnO nanorods, nanotubes , and nanoporous material. In paper IV, we utilized functionalized ZnO nanorods on the tip of a borosilicate glass capillary coated with ionophore-membrane to construct intracellular Ca2+ selective sensor. The sensor exhibited a Ca2+-dependent electrochemical potential difference and the response was linear over a large dynamic concentration range, which enabled this sensor to measure Ca2+ concentrations in human adipocytes or in frog oocytes. The results were consistent with the values of Ca2+ concentrations reported in the literature. In paper V, ZnO nanotubes and nanorods were used to create pH sensor devices. The developed ZnO pH sensors display good reproducibility, repeatability, and long-term stability. The ZnO pH sensors exhibited a pH-dependent electrochemical potential difference over a large dynamic pH range. We found that the ZnO nanotubes provide sensitivity as high as twice that of the ZnO nanorods. The possible reasons of enhanced sensitivity were explained. Paper VI investigates an improved potentiometric intracellular glucose biosensor based on the immobilization of glucose oxidase on the ZnO nanoporous material. We demonstrated that using ZnO nanoporous material as a matrix material for enzyme immobilization improves the sensitivity of the biosensor as compared to using ZnO nanorods. In addition, the fabrication method of the intracellular biosensor was simple and excellent performance in sensitivity, stability, selectivity, reproducibility, and anti-interference was achieved.

One-dimensional ZnO nanostructures have great potential applications in the fields of optoelectronic and sensor devices.  Therefore, it is very important to realize the controllable growth of one-dimensional ZnO nanostructures and investigate their properties. The main points for this thesis are not only to successfully realize the controllable growth of ZnO nanorods (ZNRs), ZnO nanotubes (ZNTs) and ZnMgO/ZnO heterostructures, but also investigate the structure and optical properties in detail by means of scanning electron microscope (SEM), transmission electron microscope (TEM), resonant Raman spectroscopy (RRS), photoluminescence (PL), time resolved PL (TRPL), X-ray photoelectron spectroscopy (XPS) and Secondary ion mass spectrometry (SIMS). For ZNRs, on one hand, ZNRs have been successfully synthesized by a two-step chemical bath deposition method on Si substrates. The diameter of ZNRs can be well controlled from 150 nm to 40 nm through adjusting the diameter and density of the ZnO nanoparticles pretreated on the Si substrates. The experimental results indicated that both diameter and density of ZnO nanoparticles on the substrates determined the diameter of ZNRs. But when the density is higher than the critical value of 2.3×108cm-2, the density will become the dominant factor to determine the diameter of ZNRs. One the other hand, the surface recombination of ZNRs has been investigated in detail. Raman, RRS and PL results help us reveal that the surface defects play a significant role in the as-grown sample. It is the first time to the best of our knowledge that the Raman measurements can be used to monitor the change of surface defects and deep level defects in the CBD grown ZNRs. Then we utilized TRPL technique, for the first time, to investigate the CBD grown ZNRs with different diameters. The results show that the decay time of the excitons in ZNRs strongly depends on the diameter. The altered decay time is mainly due to the surface recombination process. A thermal treatment under 500°C can strongly suppress the surface recombination channel. A simple carrier and exciton diffusion equation is also used to determine the surface recombination velocity, which results in a value between 1.5 and 4.5 nm/ps. Subsequently, we utilized XPS technique to investigate the surface composition of as-grown and annealed ZNRs so that we can identify the surface recombination centers. The experimental results indicated that the OH and H bonds play the dominant role in facilitating surface recombination but specific chemisorbed oxygen also likely affect the surface recombination. Finally, on the basis of results above, we explored an effective way, i.e. sealing the beaker during the growth process, to effectively suppress the surface recombination of ZNRs and the suppression effect is even better than a 500oC post-thermal treatment. For ZNTs, the structural and optical properties have been studied in detail. ZNTs have been successfully evolved from ZNRs by a simple chemical etching process. Both temperature-dependent PL and TRPL results not only further testify the coexistence of spatially indirect and direct transitions due to the surface band bending, but also reveal that less nonradiative contribution to the emission process in ZNTs finally causes their strong enhancement of luminescence intensity. For ZnMgO/ZnO heterostructures, the Zn0.94Mg0.06O/ZnO heterostructures have been deposited on 2 inch sapphire wafer by metal organic chemical vapor deposition (MOCVD) equipment. PL mapping demonstrates that Mg distribution in the entire wafer is quite uniform with average concentration of ~6%. The annealing effects on the Mg diffusion behaviors in Zn0.94Mg0.06O/ZnO heterostructures have been investigated by SIMS in detail. All the SIMS depth profiles of Mg element have been fitted by three Gaussian distribution functions. The Mg diffusion coefficient in the as-grown Zn0.94Mg0.06O layer deposited at 700 oC is two orders of magnitude lower than that of annealing samples, which clearly testifies that the deposited temperature of 700 oC is much more beneficial to grow ZnMgO/ZnO heterostructures or quantum wells. This thesis not only provides the effective way to fabricate ZNRs, ZNTs and ZnMgO/ZnO heterostructures, but also obtains some beneficial results in aspects of their optical properties, which builds theoretical and experimental foundation for much better understanding fundamental physics and broader applications of low-dimensional ZnO and related structures. / Endimensionella nanostrukturer av ZnO har stora potentiella tillämpningar för optoelektroniska komponenter och sensorer. Huvudresultaten för denna avhandling är inte bara att vi framgångsrikt har realiserat med en kontrollerbar metod ZnO nanotrådar (ZNRs), ZnO nanotuber (ZNTs) och ZnMgO/ZnO heterostrukturer, utan vi har också undersökt deras struktur och optiska egenskaper i detalj. För ZNRs har diametern blivit välkontrollerad från 150 nm  ner till 40 nm. Den storlekskontrollerande mekanismen är i huvudsak relaterad till tätheten av ZnO partiklarna som är fördeponerade på substratet. De optiska mätningarna ger upplysning om att ytrekombinationsprocessen spelar en betydande roll för tillväxten av ZNR. En värmebehandling i efterhand  vid 500 grader Celsius eller användande av en förseglad glasbägare under tillväxtprocessen kan starkt hålla nere kanalerna för ytrekombinationen.För ZNT, dokumenterar vi inte bara samexistensen av rumsliga indirekta och direkta  övergångar på grund av bandböjning, men vi konstaterar också att vi har mindre icke-strålande bidrag till den optiska emissionsprocessen i ZNT. För ZnMgO/ZnO heterostrukturer konstaterar vi med hjälp av analys av Mg diffusionen i den växta och den i efterhand uppvärmda Zn(0.94)Mg(0.06)O filmen, att en tillväxt vid 700 grader Celsius är den mest lämpliga för att växa ZnMgO/ZnO heterostrukturer eller kvantbrunnar.   Denna avhandling ger en teoretisk och experimentell grund för bättre förståelse av grundläggande fysik och för tillämpningar av lågdimensionella strukturer. / SSF, VR

In this thesis, luminescence properties of ZnO nanostructures (nanorods, nanotubes, nanowalls and nanoflowers) are investigated by different approaches for possible future application of these nanostructures as white light emitting diodes. ZnO nanostructures were grown by different growth techniques on different p-type substrates. Still it is a challenge for the researchers to produce a stable and reproducible high quality p-type ZnO and this seriously hinders the progress of ZnO homojunction LEDs. Therefore the excellent properties of ZnO can be utilized by constructing heterojunction with other p-type materials. The first part of the thesis includes paper I-IV. In this part, the luminescence properties of ZnO nanorods grown on different p-type substrates (GaN, 4H-SiC) and different ZnO nanostructures (nanorods, nanotubes, nanoflowers, and nanowalls) grown on the same substrate were investigated. The effect of the post-growth annealing of ZnO nanorods and nanotubes on the deep level emissions and color rendering properties were also investigated. In paper I, ZnO nanorods were grown on p-type GaN and 4H-SiC substrates by low temperature aqueous chemical growth (ACG) method. The luminescence properties of the fabricated LEDs were investigated at room temperature by electroluminescence (EL) and photoluminescence (PL) measurements and consistency was found between both the measurements. The LEDs showed very bright emission that was a combination of three emission peaks in the violet-blue, green and orange-red regions in the visible spectrum. In paper II, different ZnO nanostructures (nanorods, nanotubes, nanoflowers, and nanowalls) were grown on p-GaN and the luminescence properties of these nanostructures based LEDs were comparatively investigated by EL and PL measurements. The nanowalls structures were found to be emitting the highest emission in the visible region, while the nanorods have the highest emissions in the UV region due to its good crystal quality. It was also estimated that the ZnO nanowalls structures have strong white light with the highest color rendering index (CRI) of 95 with correlated color temperature (CCT) of 6518 K. In paper III, we have investigated the origin of the red emissions in ZnO by using post-growth annealing. The ZnO nanotubes were achieved on p-GaN and then annealed in different ambients (argon, air, oxygen and nitrogen) at 600 oC for 30 min. By comparative investigations of EL spectra of the LEDs it was found that more than one deep level defects are involved in the red emission from ZnO nanotubes/p-GaN LEDs. It was concluded that the red emission in ZnO can be attributed to oxygen interstitials (Oi) and oxygen vacancies (Vo) in the range of 620 nm (1.99 eV) to 690 nm (1.79 eV) and 690 nm (1.79 eV) to 750 nm (1.65 eV), respectively. In paper IV, we have investigated the effect of post-growth annealing on the color rendering properties of ZnO nanorods based LEDs. ZnO nanorods were grown on p-GaN by using ACG method. The as grown nanorods were annealed in nitrogen, oxygen, argon, and air ambients at 600 oC for 30 min. The color rendering indices (CRIs) and correlated color temperatures (CCTs) were estimated from the spectra emitted by the LEDs. It was found that the annealing ambients especially air, oxygen, and nitrogen were found to be very effective. The LEDs based on nanorods annealed in nitrogen ambient, have excellent color rendering properties with CRIs and CCTs of 97 and 2363 K in the forward bias and 98 and 3157 K in the reverse bias. In the 2nd part of the thesis, the junction temperature of n-ZnO nanorods based LEDs at the built-in potential was modeled and experiments were performed to validate the model. The LEDs were fabricated by ZnO nanorods grown on different p-type substrates (4H-SiC, GaN, and Si) by the ACG method. The model and experimental values of the temperature coefficient of the forward voltage near the built-in potential (~Vo) were compared. It was found that the series resistance has the main contribution in the junction temperature of the fabricated devices. In the 3rd part of the thesis, the influence of helium (He+) ion irradiation bombardment on luminescence properties of ZnO nanorods based LEDs were investigated. ZnO nanorods were grown by the vapor-liquid-solid (VLS) growth method. The fabricated LEDs were irradiated by using 2 MeV He+ ions with fluencies of ~ 2×1013 ions/cm2 and ~ 4×1013 ions/cm2. It was observed that the He+ ions irradiation affects the near band edge emissions as well as the deep level emissions in ZnO. A blue shift about 0.0347 eV and 0.082 eV was observed in the PL spectra in the near band emission and green emission, respectively. EL measurements also showed a blue shift of 0.125 eV in the broad green emission after irradiation. He+ ion irradiation affects the color rendering properties and decreases the color rendering indices from 92 to 89.

ZnO material based hetero-junctions are a potential candidate for the design andrealization of intrinsic white light emitting devices (WLEDs) due to several advantages overthe nitride based material system. During the last few years the lack of a reliable andreproducible p-type doping in ZnO material with sufficiently high conductivity and carrierconcentration has initiated an alternative approach to grow n-ZnO nanorods (NRs) on other ptypeinorganic and organic substrates. This thesis deals with ZnO NRs-hetero-junctions basedintrinsic WLEDs grown on p-SiC, n-SiC and p-type polymers. The NRs were grown by thelow temperature aqueous chemical growth (ACG) and the high temperature vapor liquid solid(VLS) method. The structural, electrical and optical properties of these WLEDs wereinvestigated and analyzed by means of scanning electron microscope (SEM), current voltage(I-V), photoluminescence (PL), cathodoluminescence (CL), electroluminescence (EL) anddeep level transient spectroscopy (DLTS). Room temperature (RT) PL spectra of ZnOtypically exhibit one sharp UV peak and possibly one or two broad deep level emissions(DLE) due to deep level defects in the bandgap. For obtaining detailed information about thephysical origin, growth dependence of optically active defects and their spatial distribution,especially to study the re-absorption of the UV in hetero-junction WLEDs structure depthresolved CL spectroscopy, is performed. At room temperature the CL intensity of the DLEband is increased with the increase of the electron beam penetration depth due to the increaseof the defect concentration at the ZnO NRs/substrate interface. The intensity ratio of the DLEto the UV emission, which is very useful in exploring the origin of the deep level emissionand the distribution of the recombination centers, is monitored. It was found that the deepcenters are distributed exponentially along the ZnO NRs and that there are more deep defectsat the root of ZnO NRs compared to the upper part. The RT-EL spectra of WLEDs illustrateemission band covering the whole visible range from 420 nm and up to 800 nm. The whitelightcomponents are distinguished using a Gaussian function and the components were foundto be violet, blue, green, orange and red emission lines. The origin of these emission lines wasfurther identified. Color coordinates measurement of the WLEDs reveals that the emitted lighthas a white impression. The color rendering index (CRI) and the correlated color temperature(CCT) of the fabricated WLEDs were calculated to be 80-92 and 3300-4200 K, respectively.

Nanotechnology is an emerging inter-disciplinary paradigm which encompasses diverse fields of science and engineering converge at the nanoscale. This nanoscale science and nanostructure engineering have well demonstrated in the fabrication of sensors/transducers devices with faster response time and better sensitivity then the planer version of the sensor’s configurations. Nanotechnology is not just to grow/fabricate nanostructures by just mixing nanoscale materials together but it requires the ability to understand and to precisely manipulate and control of the developed nanomaterials in a useful way. Nanotechnology is aiding to substantially improve, even revolutionize, many technology and industry sectors like information technology, energy, environmental science, medicine/medical instrumentation, homeland security, food safety, and transportation, among many others. Such applications of nanotechnology are delivering in both expected and unexpected ways on nanotechnology’s promise to benefit the society. The semiconductor ZnO with wide band gap (~ 3.37 eV) is a distinguish and unique material and its nanostructures have attracted great attention among the researchers due to its peculiar properties such as large exciton binding energy (60 meV) at room temperature, the high electron mobility, high thermal conductivity, good transparency and easiness of fabricating it in the different type of nanostructures. Based on all these fascinating properties, ZnO have been chosen as a suitable material for the fabrication of photonic, transducers/sensors, piezoelectric, transparent and spin electronics devices etc. The objective of the current study is to highlight the recent developments in materials and techniques for electrochemical sensing and hetrostructure light emitting diodes (LEDs) luminescence properties based on the different ZnO nanostructures. The sensor devices fabricated and characterized in the work were applied to determine and monitor the real changes of the chemical or biochemical species. We have successfully demonstrated the application of our fabricated devices as primary transducers/sensors for the determination of extracellular glucose and the glucose inside the human fat cells and frog cells using the potentiometric technique. Moreover, the fabricated ZnO based nanosensors have also been applied for the selective determination of uric acid, urea and metal ions successfully. This thesis relates specifically to zinc oxide nanostructure based electrochemical sensors and photonic device (LED) applications.

One dimensional (1-D), zinc oxide (ZnO) and copper (II) oxide (CuO), nanostructures have great potential for applications in the fields of optoelectronic and sensor devices. Research on nanostructures is a fascinating field that has evolved during the last few years especially after the utilization of the hydrothermal growth method. Using this method variety of nanostructures can be grown from solutions, it is a cheap, easy, and environment friendly approach. These nanostructures can be synthesized on various conventional and nonconventional substrates such as silicon, plastic, fabrics and paper etc. The primary purpose of the work presented in this thesis is to realize controllable growth of ZnO, CuO and nanohybrid ZnO/CuO nanostructures and to process and develop white light emitting diodes and sensor devices from the corresponding nanostructures. The first part of the thesis deals with ZnO nanostructures grown under different hydrothermal conditions in order to gain a better understanding of the growth. Possible parameters affecting the growth such as the pH, the growth temperature, the growth time, and the precursors  concentration which can alter the morphology of the nanostructures were investigated (paper 1). Utilizing the advantage of the low temperature for growth we synthesized ZnO nanostructures on different substrates, specifically on flexible substrates, which are likely to be integrated with flexible organic substrates for future foldable and disposable electronics (paper 2, 3). In the second part of the thesis, using the results and findings from the growth of ZnO nanostructures, it was possible to successfully implement ZnO nanostructures for white light emitting diodes (LEDs) on different flexible substrates (paper 4, 5). In paper 4 we realized a ZnO/polymer LED grown on a paper substrate. In paper 5 we extended the idea to print the ZnO nanorods/polymer hybrid LEDs with potential application to large area flexible displays. In the last part of the thesis, CuO and nanohybrid ZnO/CuO nanostructures were utilized to fabricate Ag+ detection and humidity sensors. In paper 6 we reported Ag+ selective electrochemical sensor based on the use of functionalized CuO nanopetals. To combine the advantages of both oxides nanostructures and to improve the performance we fabricated a pn-heterojuction using intrinsic n-ZnO nanorods and p-CuO nanostructures which were then utilized as an efficient humidity sensor (paper 7).

ZnO thin films and nanostructures were grown by eclipse pulsed laser deposition (EPLD) for the first time. On bare sapphire held at 600 °C, a complex nanostructured surface was formed when ablating a metallic Zn target in an oxygen ambient. Nanorods grown by a vapor-solid mechanism clumped together in well separated, micron-sized regions. Nanoscale pyramids with 6 fold symmetry formed between the nanorod clumps by vapor-liquid-solid growth. Strong photoluminescence (PL) was observed from the EPLD grown samples, an order of magnitude stronger than PLD grown nanorods formed under similar growth conditions. Low temperature PL was dominated by the I₇ exciton, which still has an unknown origin. Excitation intensity dependence of I₇ was drastically different than the rest of the nearby excitonic features, behavior which has not been previously reported for bound excitons in ZnO. I₇ also showed large, seemingly random variations in intensity across the surface of each sample compared to the other nearby recombinations, suggesting a structural connection. Introduction of a buffer layer had a profound effect on the morphology and PL from EPLD grown ZnO from a metallic Zn target. Pt has a high melting temperature, which helped suppress the vapor-liquid-solid nanostructure growth resulting in thin-film formation. For standard PLD, the ZnO film showed large grains separated by cracks on the surface. Due to the reduced growth rate in the EPLD geometry, the ZnO layer had a high density of nanoscale pores, reminiscent of the porous Pt buffer layer. Strong PL emission, which was dominated by I₇, was observed from the ZnO/Pt/Al₂O₃ which showed unusual blue/violet emission when the EPLD geometry was used for growth. Thin ZnO buffer layers deposited at reduced temperature also had a profound effect on EPLD grown ZnO, resulting in a random array of nanorods with alignment which was dependent on the growth temperature of the buffer layer. Buffer layers offer another dimension in the control over epitaxial structures and show large potential for EPLD growth of ZnO. Pb was the dominant impurity in the Zn targets used for EPLD growth, hinting at a Pb-related origin for the I7 peak. To explore this idea, hydrothermally grown bulk ZnO was ion-implanted with Pb and then annealed in oxygen at 600 °C to repair damage to the crystal. PL emission intensity was substantially reduced in the Pb-implanted ZnO but the line widths were preserved. No evidence of an I₇ feature was seen for Pb concentrations of up to 0.10%, three orders of magnitude higher than the expected level in the EPLD grown ZnO. However, this does not rule out a Pb-related complex as the origin of I₇ since Pb has complicated interactions with the impurities and native defects in ZnO. Instead of I₇, other sharp excitonic features were observed near the band edge. A bound exciton with a localization energy of 12.4 ± 0.2 meV was observed in the Pb-implanted samples and was attributed to neutral interstitial Pb donors. Pb-implantation produced a clear PL signature which is unique enough to unambiguously detect its presence in ZnO. EPLD also proved successful at depositing oxides of the noble metals. Ir, Pt, Pd, and Ru targets were ablated in oxygen and argon ambients and films were collected on room temperature substrates. Growth in argon resulted in pure metal while oxidized layers were obtained in oxygen. This was clearly evident by the semiconductor-like transmission spectra observed for the oxidized samples. The high fluence used for these growths promoted the oxidation of these resilient metals while the shadow mask blocked most of the molten particulates generated by the high fluence. EPLD is an excellent way to produce oxides from metallic targets, a technique which should be explored in more detail for many material systems.

This thesis is about the growth of ZnO thin films for optoelectronic applications. ZnO thin films were grown using plasma assisted molecular beam epitaxy and were studied using various conventional and novel characterisation techniques. The significance of different growth variables on growth efficiency was investigated. The growth rate of ZnO films was found to be linearly dependent on the Zn flux under O-rich growth conditions. Under Zn-rich conditions, the growth rate was dependent on both atomic and molecular oxygen flux. By characterising the oxygen plasma generated using different RF power and aperture plate designs and correlating the results with the growth rates observed, it was found that atomic oxygen was the dominant growth species under all conditions. Molecular oxygen also participated in the growth process, with its importance dependent on the aperture plate design. In addition, an increase in growth temperature was found to monotonically decrease the growth rate. A growth rate of 1.4 Å/s was achieved at a growth temperature of 650 ℃ by using an oxygen flow rate of 1.6 standard cubic centimetres utilising a plasma source with a 276 hole plate operating at 400 W, and a Zn flux 1.4✕10¹⁵ atoms/cm²⋅s. Characterisation of the MBE grown thin films revealed that the qualities of ZnO thin films were dependent on the growth conditions. Experimental evidence suggested that a maximum adatom diffusion rate can be achieved under Zn-rich conditions, giving samples with the best structural quality. O-rich conditions in general led to statistical roughening which resulted in rough and irregular film surfaces. Experimental results also suggested that by increasing the atomic oxygen content and decreasing the ion content of the plasma, the excitonic emission of the ZnO thin films can possibly be improved. It was also found that the conductivity of the films can possibly be reduced by increasing the plasma ion content. By investigating the evolution of the buffer layer surface during the early stages of growth, dislocation nucleation and surface roughening were found to be important strain relief mechanisms in MBE grown ZnO thin films that affected the crystal quality. The usage of LT-buffer layers was found to improve substrate wetting, and was shown to significantly reduce dislocation propagation. Further strain reduction was achieved via the application of a 1 nm MgO buffer layer, and a significant reduction of carrier concentration and improvement in optical quality was subsequently observed. A carrier concentration of <1✕10¹⁶ cm⁻³ and a near band emission full width half maximum of 2 meV was observed for the best sample. The study of electrical characteristics using the variable magnetic field Hall effect confirmed the existence of a degenerate carrier and a bulk carrier in most MBE grown ZnO thin films. The bulk carrier mobility was measured to be ~120 - 150 cm²/Vs for most as-grown samples, comparable to the best reported value. A typical bulk carrier concentration of ~1✕10¹⁶ - 1✕10¹⁸ cm⁻³ was observed for as-grown samples. Annealing was found to increase the mobility of the bulk carrier to ~120 - 225 cm²/Vs and decrease the bulk carrier concentration by two orders of magnitude. Using time resolved photoluminescence, it was found that the radiative recombination in MBE grown ZnO thin films was dominated by excitonic processes, and followed a T³⁄² trend with temperature. A maximum radiative lifetime of 10 ns was observed for as-grown samples. The non-radiative lifetime in ZnO thin films was dominated by the Shockley-Read-Hall recombination processes. The modelling of the temperature dependence of the non-radiative lifetime suggested that an electron trap at ~0.065 eV and a hole trap at ~0.1 eV may be present in these samples. The application of time resolved photoluminescence also allowed the direct observation of carrier freeze-out in these ZnO films at low temperature.

The shaping of functional materials and the control of their texture at all length scales are sine qua non conditions for the improvement of current systems. This PhD project consists in creating complex solid architectures using interdisciplinary methods such as sol-gel chemistry or complex fluids physics. Therefore, it is possible to synthesize Titanium Dioxide macroscopic fibers or films which possess a hierarchical porosity. This organization allows the optimization of the matter transport (liquid/gaz) for air depollution application (photocatalysis) or dye-sensitized solar cells. In another project, we were able to control the alignment of zinc oxide nanorods within a macroscopic fiber. This alignment provides to the fiber an anisotropic photoluminescence behavior which can be useful for switching devices application. Finally, we synthesized anisotropic particles and nano-sheets of polypyrrole (conducting polymer) in order to obtain smooth thin films presenting interesting electrical properties. The objective was to use them as electrolyte and/or electrode in dye-sensitized solar cells.