This PDF file contains the front matter associated with SPIE Proceedings Volume 8987 including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

Ga-doped ZnO films of thicknesses 3 – 500 nm were grown on either Si or ZnO at 200 °C by pulsedlaser deposition in 10 mTorr of Ar. Sheet carrier concentration n_s and mobility μ were measured at room temperature by the Hall effect and were fitted, respectively, to the equations n_s(d) = n(∞)(d - δd) and μ(d) = μ(∞)/[1 + d*/(d - δd)], where n is the volume carrier concentration at d = ∞ (the bulk value), δd is the thickness of the dead layer, μ(∞) is the mobility at d = ∞, and d* is a figure of merit for the electrical properties of the interface. Roughly, d* may be thought of as the minimum layer thickness that will produce good conductance. For GZO/Si, the fitted d* = 23 nm, and for GZO/ZnO, 3 nm. As evidence of the usefulness of d*, a 3-nm layer of GZO/Si showed no measurable conductance (since d << d*), whereas a 5-nm layer of GZO had excellent conductance (since d ≈ d*). In fact, the latter had a resistivity of about 4 × 10-4 Ω-cm at room temperature, possibly the lowest value ever reported in ZnO at this thickness.

Electrochemical performances of different TiO₂ nanostructures, TiO₂/CNT composite and TiO₂ with titanium isopropoxide (TTIP) treatment anode were investigated. For different TiO₂ nanostructures, we investigated vertically aligned TiO₂ nanotubes on Ti foil and TiO₂ nanotube-powders fabricated by rapid breakdown anodization technique. The morphology of the prepared samples was characterized by scanning probe microscopy (SEM). The electrochemical lithium storage abilities were studied by galvanostatic method. In addition, carbon nanotubes (CNT) additives and solution treatment process of TiO₂ anode were investigated, and the results show that the additives and treatment could enhance the cycling performance of the TiO₂ anode on lithium ion batteries.

Extraordinary optical transmission (EOT), through highly conductive ZnO films with sub-wavelength hole arrays is investigated in the long-wavelength infrared regime. EOT is facilitated by the excitation of surface plasmon polaritons (SPPs) on Ga-Doped ZnO films and can be tuned utilizing the physical parameters such as film thickness, period, hole size, and hole shape, as well as doping of the film. Analytical and finite-difference time-domain calculations are completed for 1 micron thick films with square, circular, and triangular hole arrays demonstrating SPP coupling and EOT. The fundamental plasmonic modes are observed in each of these hole shapes at wavelengths that correspond to strong EOT peaks. Doping tunability for these structures is also observed. Ga-doped ZnO films are grown via pulsed laser deposition (PLD) on silicon with plasma frequencies in the near-infrared. The sub-wavelength 2D hole arrays are fabricated in the Ga-doped ZnO films via standard lithography and etching processes. This highly conductive ZnO EOT structure may prove useful in novel integrated components such as tunable biosensors or surface plasmon coupling mechanisms.

We propose a SnO_x | Ag | SnO_x multilayer, deposited in a continuous vacuum atmosphere by E-beam evaporation, as transparent anode for a (poly-3-hexylthiophene):[6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction based Organic Solar Cell (OSC). Optical characterization of the deposited SnO_x is performed to determine the dispersion of the complex refractive index. A Transfer Matrix Method (TMM) numerical optimization of the thicknesses of each layer of the electrode is realized to limit the number of manufactured samples. A numerical study using the morphology of the silver inserted between the oxide layers as input data is performed with a Finite Difference Time Domain (FDTD) method to improve the accordance between measurement and optical model. Multilayers are manufactured with the objective to give to the electrode its best conductivity and transparency in the visible spectral range by using the results of the optical optimization. These bare tri-layer electrodes show low sheet resistance (<10 Ω/□) and mean transparency on [400-700] nm spectral band as high as 67 % for the whole Glass | SnO_x | Ag | SnO_x structure. The trilayer is then numerically studied inside a P3HT:PCBM bulk heterojunction based OSC structure. Intrinsic absorption inside the sole active layer is calculated giving the possibility to perform optical optimization on the intrinsic absorption efficiency inside the active area by considering the media embedding the electrodes.

The interest in the recent years for nanostructure studies has led to the development of a wide palette of characterization techniques such as the electrical modes in scanning probe microscopy (STM, EFM, KPFM...). Optical characterization at nanoscale remains nevertheless a challenge especially for wide gap semiconductors where high energy is required. In this presentation, we will present our work focusing in the development and the improvement of near-field microscopy techniques to investigate nanoscale properties of ZnO nanostructures and related semiconducting objects. For the optical characterization, cathodoluminescence (CL) studies present many advantages over the classical photoluminescence experiments for ZnO analysis. This contribution presents the development of a scanning near-field cathodoluminescence microscope where a bimorph piezoelectric cantilever is simultaneously used for both actuation and oscillation amplitude detection. Operated inside a scanning electron microscope (SEM) it offers the possibility of performing simultaneous topography and cathodoluminescence charting of the sample surface additionally to the SEM imaging with a resolution in the order of several tenths of nanometers. Different measurements of ZnO nanostructures and related objects will be presented to show the potentiality of our optical characterization setup. Complementary STEM-CL measurements at higher beam energy were performed on the ZnO nanowires confirming the good quality of the investigated nanostructures. As for the electrical characterization, we will focus on the local surface potential mapping of ZnO nanowires used for photoconduction using Kelvin Probe Force Microscopy. While ZnO nanowire photoconduction gains as high as 10¹⁰ in the UV region were reported, several issues come into play when it comes to making a precise measurement of a single nanowire. An important issue is the good quality of the injecting contacts on the nanowire and the reproducibility of its characteristics which can be made using KPFM.

Zinc oxide (ZnO) nano/microstructures have been attractive as the building blocks for the efficient opto-electronic devices in the ultraviolet (UV) region. We have succeeded in growing the ZnO micro/nanosphere by a simple laser ablation in the air, and therefore we have obtained UV lasing from the sphere under optical pumping. Recently, large size of several 10 micrometer ZnO microspheres were grown using Nd:YAG laser without Q-switching, and ZnO microsphere/p-GaN heterojunction were fabricated to obtain the electroluminescence (EL) from the microsphere by electrical pumping. Room-temperature EL in near-UV region with peak wavelength of 400 nm is observed under forward bias.

This study explores comprehensively the carrier dynamics in ZnSeO and ZnTeO using photoluminescence (PL) and time-resolved PL spectroscopy. As the O concentration increases, the PL emissions shift toward lower energies. Additionally, the PL lifetime increases with increasing O contents and the decay curves exhibit complex behavior. In the case of ZnSeO, the mechanism of carrier recombination undergoes a complicated change from trapped to free excitons with the increase in temperature. The incorporation of O in ZnTe generates a wide distribution of electron localization below the energy of the E_- conduction subband, and these cause broad PL emission and serve as another intermediate band. Electrons in both the E₊ and the E_- conduction subbands favor rapid relaxation to low energy states. Moreover, temperature-independent long carrier lifetimes (> 130.0 ns) that are induced by localized electrons increase with O concentration.

Optical techniques have been intensively developed for many decades in terms of both experimental and modeling capabilities. In spectroscopy and scatterometry material structures can be measured and modeled from the atomic (binding configurations, electronic band structure) through nanometer (nanocrystals, long range order) to micron scales (photonic structures, gratings, critical dimension measurements). Using optical techniques, atomic scale structures, morphology, crystallinity, doping and a range of other properties that can be related to the changes of the electronic band structure can most sensitively be measured for materials having interband transition energies in the optical photon energy range. This will be demonstrated by different models for the dielectric function of ZnO, a key material in optoelectronics and in numerous other fields. Using polarimetry such as spectroscopic ellipsometry, sub-nanometer precision has long been revealed for the thickness of optical quality layers. The lateral resolution of spectroscopic ellipsometry is limited (> 50 μm) by the use of incoherent light sources, but using single-wavelength imaging ellipsometry, a sub-micron lateral resolution can be reached. In case of sub-wavelength structures, the morphology (of e.g. porous or nanocrystalline materials) can be characterized using the effective medium theory. For structure sizes comparable to the wavelength, scatterometry is applied in a broad versatility of configurations from specular to angle resolved, from coherent to incoherent, from monochromatic to spectroscopic, from reectometric to polarimetric. In this work, we also present an application of coherent Fourier scatterometry for the characterization of periodic lateral structures.

Zinc Oxide (ZnO) is an inexpensive n-type semiconductor having a direct band gap of 3.3eV with a large exciton binding energy of 60meV. Noble metal nanoparticles show a surface plasmon resonance in the visible region due to collective oscillations of electrons at the surface of metal nanoparticles. The unique features in the composite system of dielectrics-metal nanoparticles have potential applications in optoelectronic devices such as transparent conductive films, solar cells, photocatalysts and so on. In this study, ZnO thin films dispersed with Ag or Au nanoparticles were synthesized using a sol-gel technique. X-ray diffraction peaks of ZnO films exhibited a pattern corresponding to the hexagonal wultzite structure. In the TEM analysis of ZnO-Au composite films, spherical Au nanoparticles were observed within the ZnO crystalline matrix. The distribution of the diameter of Au nanoparticles was centered at around 20nm and broadened with the half width of about 20nm. In the ZnO-Ag composite films, Ag nanoparticles grow larger as the annealing temperature becomes higher and various shape of Ag precipitations like triangular and square plates were observed in ZnO-Ag (50:50) composite films. The optical absorption peaks were observed at 580nm and 410nm due to the surface plasmon resonance of gold and silver nanoparticles, respectively. The absorption spectra were analyzed using a typical effective medium approximation of Maxwell-Garnett model and good fitting was obtained for a ZnO-Au composite film assuming spherical Au nanoparticle. The spectra were discussed relating with the size and shape of the nanoparticles, and the refractive index of the matrix.

Effects of laser annealing on electrical and optical properties of Zinc oxide (ZnO) nanocrystals, which are expected as building blocks for optoelectronic devices, have been investigated in this study. In the case of fabricating p-n junction in single one-dimensional ZnO nanocrystal, phosphorus-ions implanted p-type ZnO nanocrystals were recrystallized and recovered in the optical properties by nanosecond-laser annealing using a KrF excimer laser. Antimony-doped p-type ZnO nanocrystals were synthesized by irradiating laminated structure which antimony thin film were deposited on ZnO nanocrystals with the laser beam. Additionally, it is possible to control the growth rate of ZnO nanowires by using laser annealing. Irradiating with pulsed laser a part of ZnO buffer layer deposited on the a-cut sapphire substrate, then ZnO nanowires were grown on the ZnO buffer layer by the nanoparticle assisted pulsed laser deposition method. As a result, the clear boundary of the laser annealed and non-laser annealed area was appeared. It was observed that ZnO nanowires were grown densely at non-laser annealed area, on the other hand, sparse ones were grown at the laser-annealed region. In this report, the possibility of laser annealing techniques to establish the stable and reliable fabrication process of ZnO nanowires-based LD and LED are discussed on the basis of experimental results.

We have demonstrated that fabrication of the ZnO nanowire/GaN hetero-junction light emitting diode (LED) by contacting the tip of the ZnO nanowires with the GaN film, and UV electroluminescence from the p-n junction. In this study, we fabricated the heterojunction by directly-growth of the ZnO nanowires on the GaN film using nanoparticleassisted pulsed laser deposition. Photoluminescence spectrum of the ZnO nanowires showed a weak near-band-edge ultraviolet (UV) emission and a visible broad emission, which was related to transition by ZnO defect state. We applied a selective laser irradiation to the p-n junction of the ZnO-based LED. The UV emission was strongly enhanced from the laser-irradiated p-n junction.

The large bandgap (3.37 eV) and exciton binding energy (60 meV) makes ZnO most promising material in the area of optoelectronic devices. The efficiency of these devices can be enhanced by increasing the bandgap of those materials which is possible by band-gap engineering. It has been found that incorporation of Mg can increase the bandgap of the alloy up to 4 eV and even more. We investigated the optical properties of Zn_1-x Mg_xO film implanted by Li at low energy (40 KeV) with dosage of 5x10¹³ ions/cm² and 10¹⁴ ions/cm² respectively. Prior to implantation 150 nm Zn_1-x Mg_xO (x=0.15) film was deposited on Si substrate followed by annealing at 650°C and 750°C. For dosage of 5x10¹³ ions/cm² and 10¹⁴ ions/cm² the low temperature (15K) and room temperature photoluminescence spectra is dominated by the emission of 3.66 eV which is the band gap energy of Zn_1-xMg_xO, shifts to 3.63 eV at higher dosage of ions. With increasing energy (50 KeV) this peak was revealed only at 5x10¹³ ions/cm^2. At 10¹⁴ ions/cm² no sign of this peak was visible. The splitting of conduction band and valence band into multiple sub-bands causes a transition between the subband of conduction band and sub-band of heavy-hole and an emission occurs at 3.58 eV referred as 11H. The existence of acceptor-bound exciton peak (A°X) around 3.33 eV and the presence of donor-to-acceptor-pair peak at 3.24 eV provide strong evidence of increased acceptor concentration.

Red emitting long-lasting phosphorescence (LLP) material, are useful biomarker for small animal in vivo imaging. We report here our investigations on the optical features of chromium doped AB₂O₄ spinels (A=Zn, Mg and B=Ga, Al…) suitable for such applications. It is possible to tune the emission wavelengths of Cr³⁺ by a crystal field variation to be well centered in the biological window and it is also possible to adjust the traps depth in order to better control the release of the traps. These traps are therefore stable at room temperature and could be emptied by thermal or near infrared source making this material a potential new photostimulated/optically compound. Photoluminescence (PL) and thermally stimulated luminescence (TSL) studies are reported.

The group II chalcogenides are an important class of functional semiconductor materials exhibiting a remarkable diversity in terms of structure and properties. In order to aid the materials design, a consistent set of electronic structure calculations is presented, including data on the polymorphic energy ordering, the band-structures, the band-lineups relative to the vacuum level, surface energies, as well as on the alloy energetics. To this end, current state-of-the-art electronic structure tools are employed, which, besides standard density functional theory (DFT), include totalenergy calculation in the random phase approximation and GW quasiparticle energy calculations. The ionization potentials and electron affinities are obtained by combining the results of bulk GW and surface DFT calculations. Considering both octahedral and tetrahedral coordination symmetries, exemplified by the rock-salt and zinc-blende lattices, respectively, this data reveals both the chemical and structural trends within this materials family.

We investigated the blue excitable persistent luminescence properties in the Ce³⁺-doped garnet ceramics with the composition of Y₃Al_5-xGa_xO₁₂:Ce³⁺ (x=0, 1, 2, 3, 3.5, 4). The persistent luminescence was observed in the sample with x=3 and 3.5 by the blue excitation. In these materials, the energy gap between the lowest 5d₁ excited level of Ce³⁺ and the conduction band is much closer compared with x=0, 1, 2 samples. As a result, the efficient electron transfer to the electron trap occurs through the conduction band by the blue excitation in the x=3 and 3.5 samples. The thermoluminescence (TL) was observed in all the samples by UV excitation and the TL peaks were shifted to lower energy with increasing Ga content. The decreases of the threshold energy of photoionization and the electron trap depth with increasing Ga content can be caused by lowering conduction band. Therefore, we demonstrated that the persistent luminescence properties, such as storagetable wavelength and persistent decay profile, are controlled by changing Ga content. We also discovered that the persistent luminescence intensity and duration time were improved by co-doping with metal ions.

Ga₂O₃ bulk single crystals have been implanted with 300 keV Europium ions to fluences ranging from 1×10¹³ to 4×10¹⁵ at/cm². The damage build-up and Eu-incorporation was assessed by Rutherford Backscattering Spectrometry in the channeling mode (RBS/C). RBS/C results suggest that implantation causes a mixture of defect clusters and extended defects such as dislocations. Amorphisation starts at the surface for fluences around 1×10¹⁵ at/cm² and then proceeds to deeper regions of the sample with increasing fluence. Amorphous regions and defect clusters are efficiently removed during rapid thermal annealing at ~1100 °C; however, Eu diffuses towards the surface. Nevertheless, Eu ions are optically activated and show cathodoluminescence at room temperature. Results in bulk samples are compared to those in Eu-implanted Ga₂O₃ nanowires and despite strong similarities in the structural properties differences were found in the optical activation. Furthermore, damage and dopant incorporation studies were performed using the Perturbed Angular Correlation technique, which allows probing the immediate lattice surroundings of an implanted radioactive probe at the atomic level.

We have determined electronic properties of methyl-phosphonic acid adsorbed on ZnO nanowire structures using semi-local and hybrid Hartree-Fock density functionals. We find a bidentate binding of the molecular groups to the ZnO surface and a strong enhancement of the density of states near the top of the valence band.

The pulsed laser deposition (PLD) growth processes of spinel lithium titanates, Li₄Ti₅O₁₂ and LiTi₂O₄, on MgAl₂O₄ (111) substrates are investigated. Although a Li₄Ti₅O₁₂ target was used for the depositions, the Li/Ti atomic ratio of the species arriving at the substrate during deposition was only ~0.5, enabling high quality LiTi₂O₄ films to be prepared with a rocking curve full-width at half-maximum of ~0.05°. The LiTi₂O₄ epitaxial thin films exhibited high conductivity at room temperature (~3.0 × 10³ Ω⁻¹cm⁻¹) and a superconducting transition temperature of ~13.3 K. These values are the highest recorded for epitaxial thin films. Moreover, the effect of collisions between the atoms in a plume were studied quantitatively. These results demonstrate the importance of the target composition, providing further insight into Licontaining metal-oxide deposition processes using PLD.

Multi-cation oxides with crystalline perfection known from single crystals of Si or Ge are a challenge not only for basic research but also towards a revolution of oxide electronic materials. Here, we present our approach for the synthesis of high-quality thin films of multi-cation oxides. We show that our synthesis method, using state-of the-art molecular beam epitaxy (MBE), facilitates for the design of new materials. We geared our MBE system with a precise rate control system of each constituent cation flux as well as activated oxygen (O*) and ozone (O³). The resulting performances of our MBE setup are unmatched with respect to high-quality film growth as well as multi-cation flexibility by demonstrating growth of various cuprate-, scandate-, argentate-, titanate-, and ruthenate thin films. Such augmented methods are key to novel materials and go well beyond the artificial stacking of known materials and lattices.

We exploit epitaxial relationships of rutile-type VO₂ with (0001) Al₂O₃, (111) LaAlO₃, (10‾10) Al₂O₃, and (10‾12) Al₂O₃ to achieve high-quality VO₂ thin-film synthesis. We show that the deposition temperature can be lowered when these substrates are employed compared to one with no preferred crystallographic relationship with VO₂, such as Si. We also report the first thin-film synthesis of the metastable VO₂(B) polymorph on (001) LaAlO₃ with a strongly preferred (001) out-of-plane orientation. These results are of interest for integrating _VO2 films with other oxides in optoelectronic and reconfigurable device structures.

We summarized our recent studies on the optical properties of SrTiO₃ single crystals and LaAlO₃/SrTiO₃ (LAO/STO) heterostructures using time-resolved photoluminescence (PL) and transient absorption (TA) measurements at low temperatures. We observed sharp band-edge PL peaks both in electron-doped SrTi_1-xNb_xO₃ and LAO/STO samples, corresponding to the radiative recombination of doped electrons in the conduction band and photoexcited holes in the valence band. These results evidence the existence of free electrons in the SrTi _1-xNb_xO₃ single crystal and at the LAO/STO heterointerface. In SrTiO₃ single crystals, TA signal gradually appears within 40 ps, which corresponds to the energy relaxation of photoexcited free electrons into self-trapped polaron states. The polaron formation time is enhanced considerably at the LaAlO₃/SrTiO₃ heterointerface compared to bulk crystals. We discuss the interface effects on the electron relaxation dynamics in terms of the strong interface potential.

For a two-dimensional electron gas, the breaking of inversion symmetry induces a Rashba spin-orbit coupling which modifies its band electronic structure. A clear manifestation of this effect can be observed in the behavior of the electrical resistance in magnetic field. In this paper we study a two-dimensional electron gas at an oxide interface and review the physical properties observed at low temperature upon carrier density modulation achieved by field effect. We analyze magnetoresistance curves measured at different temperatures within the theory of weak localization in the presence of spin-orbit to uncover the strength of the Rashba coupling.

Crystalline lanthanum aluminate (LAO) films were grown epitaxially on SrTiO₃(001) and on Si(001) with a buffer layer of four unit cells of SrTiO₃ by atomic layer deposition. The SrTiO₃ buffer layer was grown by molecular beam epitaxy. Tris(N,N’-diisopropylformamidinate)-lanthanum, trimethylaluminum, and water as co-reactants were employed at 250 °C for atomic layer deposition. Films were characterized using ex-situ reflection high-energy electron diffraction, X-ray diffraction and in-situ X-ray photoelectron spectroscopy. The as-deposited LAO films were amorphous. Different annealing conditions were necessary to realize crystalline films because of different degrees of tensile strain between crystalline LAO and the SrTiO₃ or the Si(001) substrate. When grown on SrTiO₃(001), with a lattice mismatch of 2.9%, annealing temperatures of 750 °C for 2 h were necessary. Crystalline films were realized at 600 °C under vacuum at 2 h for SrTiO₃-buffered Si(001), with a lattice mismatch of 1.3%. By keeping the annealing temperature relatively low (2 h at 600 °C under vacuum), the interfacial amorphous layer at the STO/Si interface was minimized to about one monolayer and an abrupt interface between SrTiO₃ and LAO was maintained.

Heterostructures composed of transition metal oxides with strong electron correlation offer a unique opportunity to design new artificial materials whose electrical, magnetic and optical properties can be manipulated by tailoring the occupation of the 3d-orbitals of the transition metal in the compound. This possibility is an implication of symmetry constraints at interfaces resulting in a delicate interplay of spin-, charge-, orbital and lattice interactions of electrons. In turn, the material properties are sensitive to external perturbations such as strain, electrical and magnetic fields and photon flux as well. In this contribution we use photon flux exposure to explore the consequences of superlattice formation of YBa₂Cu₃O_7–δ/La 2_/3Ca_1/3MnO₃ on the entropy transport, especially on the Seebeck coefficient. In addition to the investigation of the fundamental aspects of entropy transport in oxide superlattices, the driving force for this work is the development of optical sensing devices. The method applied is based on the off-diagonal thermoelectric effect (ODTE) appearing in films deposited on substrates with a vicinal cut. This well-known principle serves as a technique to investigate the anisotropic transport properties and the components of the Seebeck tensor in these superlattices. It could be shown that the normalized ODTE signals scale linearly with the number of interfaces in the structures. We observed an enhancement of the ODTE signals by a factor of four due to superlattice formation. The results are discussed with respect to cross-plane coherent backscattering of phonon waves at the superlattice interfaces and the thermal boundary resistance at the YBa₂Cu₃O_7–δ/La_2/3Ca_1/3MnO₃ interfaces.

Results of Raman studies of magnetic phase transitions of hexagonal LuMnO₃ single crystal and HoMnO₃ thin films are compared directly with the results of magnetic measurements. Our results show that the temperature dependent Raman study of magnon scattering provides a simple and accurate method for investigating magnetic phase transitions, especially in HoMnO₃ thin films. In single crystal, our optical method provides results as good as magnetization measurements.

In this paper we present our recent studies on the crystal, magnetic and dielectric properties of the β-NaMnO₂. Experimental results of neutron powder and electron diffraction combined with measurements of the dielectric permittivity suggest that the β-ΝaMnO2 is an excellent candidate for studying the coupling between the magnetic and electric degrees of freedom. Neutron powder diffraction data reveal the existence of a commensurate and an incommensurate magnetic structure at 200 K and below 100 K, respectively. Dielectric anomalies which appear at the temperature regions where the two magnetic structures emerge, indicate the appearance of magnetodielectric coupling.

Lithium niobate, LiNbO₃, exists in a wide range of compositions, from congruent to stoichiometric. Undoped congruent LiNbO₃ suffers from a relatively low optical damage threshold which constitutes its major disadvantage for optoelectronic devices. The optical damage threshold is dependent on the amount of intrinsic defects, and is considerably increased in stoichiometric material and in congruent material doped with specific impurities, such as Mg, In, Sc and Zn. It has been recently shown that doping with Hf leads to a significant increase of the photorefractive resistance at a threshold concentration of about 3 mol%. The study of the lattice location of Hf in LiNbO₃ and its interaction with other impurities and intrinsic defects had started more than a decade before the discovery of the role of this impurity, as Hf was a convenient probe for combined studies using the nuclear techniques Perturbed Angular Correlations and Rutherford Backscattering Spectrometry/Channeling. An integrated review of the main results obtained with these techniques is presented.

The wide-bandgap semiconductor ZnO has gained major interest in research community for its unique properties and wide range of applications. In this review article, we present synthesis techniques and a few emerging applications for ZnO. Common techniques for growing ZnO films are discussed briefly, and a detailed discussion of MOCVD growth of ZnO is provided citing previous experimental reports on this technique by our group and others. A few important and distinctive uses of ZnO are discussed for various applications focusing on the current limitations of ZnO to realize its feasibility in these applications.

Free-standing (0002)-oriented GaN substrates (φ = 2”) were coated with 200 nm of ZnO and used as templates for the growth of GaN thin films. SEM and AFM revealed that such GaN layers had a relatively homogenous surface morphology with an RMS roughness (5 μm x 5 μm) of less than 4nm. XRD studies revealed strained ZnO growth on the GaN substrate and the reproduction of the substrate rocking curve for the GaN overlayers after only a hundred nm of growth, thus indicating that the GaN films had superior crystallographic quality compared to those grown on sapphire or ZnO/sapphire substrates. Quarter-wafer areas of GaN were removed from the GaN substrate (by selective chemical etching away of the ZnO interlayer). The expensive GaN substrates were then reclaimed/reused (without the need for polishing) for a second cycle of ZnO and GaN growth, which gave similar XRD, SEM, CL and AFM results to the first cycle.

High-quality epitaxial ZnO films on c-plane sapphire substrates have been obtained by utilizing off-axis sputtering configuration together with buffer layers prepared via nitrogen mediated crystallization (NMC). The role of NMC buffer layers is to provide high density of nucleation site and thus to reduce the strain energy caused by the large lattice mismatch (18%) between ZnO and sapphire. The NMC buffer layers allow two dimensional (2D) growth of subsequently grown ZnO films, being particularly enhanced by employing off-axis sputtering configuration, in which the substrate is positioned out of the high-energy particles such as negative oxygen ions originating from the targets. As a result, ZnO films with smooth surfaces (root-mean-square roughness: 0.76 nm) and high electron mobility of 88 cm²/V⋅sec are fabricated. Photoluminescence spectra of the ZnO films show strong near-band-edge emission, and the intensity of the orange-red defect emission significantly decreases with increasing the horizontal distance between the target and the substrate. From these results, we conclude that off-axis sputtering together with NMC buffer layers is a promising method for obtaining high quality epitaxial ZnO films.

ZnO is used in a wide variety of applications owing to the electrical properties. Polycrystalline ZnO ceramics have long been used such as varistor, and ZnO films are currently intensively studied for transparent conductor applications. Grain boundary (GB) in ZnO varistor is believed to be the origin of nonlinear current-voltage characteristics, and GB in ZnO films possibly affects the electrical conductivity. It is therefore important to understand the role of ZnO GB on the electrical properties, which should be closely related with the structure in atomic scale. With these viewpoints, we have studied the atomistic structure of ZnO GBs, where the orientation relations of adjacent crystals are well defined. Single GBs studied were obtained by fabricating ZnO bicrystals and the GBs were characterized by scanning transmission electron microscopy (STEM) and theoretical calculations. It is found that coordination number of ions change in ZnO GBs; there are underfold or overfold coordinated ions that are unusual in bulk inside. It is calculated that these atomistic structures alters the electronic structure but would not create deep states in the band gap. On the other hand, when praseodymium (Pr), which is known to be a key dopant element to obtain nonlinear (I-V) characteristics, is added to the GBs, Pr strongly localizes to the GBs and occupies specific atomic sites. Pr facilitates the formation of the acceptorlike defects such as zinc vacancies, which we think that is an important role of Pr on generation of nonlinear (I-V) characteristics. Furthermore, atomic arrangement and localization behavior of Pr are studied for several GBs to obtain fundamental understanding about GB structure formation.

Lithium is usually added into the solution to improve ZnO hydrothermal growth; however, lithium doping affects the properties of the resulting crystals. Optical and electrical properties of hydrothermal ZnO bulk crystals without lithium, have been studied by photoluminescence and Hall-effect measurements. High quality ZnO crystals without lithium were grown in H₂O/D₂O and in NH₃-H₂O solutions. The crystals grown from H₂O/D₂O are conductive with resistivities of 0.6-0.7 Ωcm and mobilities of ~ 100 cm²/Vs, while lithium doped ZnO crystals typically have resistivities of ~ 10³Ω-cm and mobilities of ~ 200 cm²/Vs, but can be varied from dozens to 10¹⁰ Ω-cm depending on lithium concentration. Lithium-free but nitrogen doped crystals grown in NH₃-H₂O solution have resistivities of 1×10⁰ Ω-cm and sometimes show p-type conduction; the resistivity increases to ~ 1×108 Ω-cm after annealing at 600° C in air. Lithium and nitrogen co-doped ZnO crystals have resistivities of 10⁸-1012 Ω-cm and are semi-insulating after annealling. Electronic irradiation also increases the ZnO resistivity. For lithium-doped samples, a 3.357 eV peak can be seen in the photoluminescence spectra. This is close to the donor-exciton peaks in indium-doped ZnO where 3.3586 eV and 3.357 eV were found on the C⁺ and C^- faces, respectively. More studies are needed to identify lithium-related complexes (defects).

Laser assisted flow deposition (LAFD) is a very high yield method based on a vapor-solid mechanism, allowing the production of ZnO crystals in a very short time. The LAFD was used in the growth of different morphologies (nanoparticles, tetrapods and microrods) of ZnO micro/nanocrystals and their microstructural characterization confirms the excellent crystallinity of the wurtzite structure. The optical properties of the as-grown ZnO crystals investigated by low temperature photoluminescence (PL) evidence a well-structured near band edge emission (NBE) due to the recombination of free (FX), surface (SX) and donor bound (D⁰X) excitons. Among the most representative emission lines, the 3.31 eV transition was found to occur in the stacking faults-free microrods. The luminescence behavior observed in H passivated samples suggests a closer relationship between this optical center and the presence of surface states. Besides the unintentionally doped micro/nanocrystals, ZnO/Ag and ZnO/carbon nanotubes (CNT) hybrid structures were processed by LAFD. The former aims at the incorporation of silver as a p-type dopant and the latter envisaging photovoltaic applications. Silver-related spherical particles were found to be inhomogeneously distributed at the microrods surface, accumulating at the rods tips and promoting the ZnO nanorods re-nucleation. Despite the fact that energy dispersive X-ray measurements suggest that a fraction of the silver could be incorporated in the ZnO rods, no new related luminescence lines or bands were observed when compared with the as-grown samples. For the case of the ZnO/CNT composites two main approaches were adopted: i) a direct deposition of ZnO particles on the surface of vertically aligned multi-walled carbon nanotubes (VACNTs) forests without employing any additional catalyst and ii) new ZnO/CNT hybrids were developed as buckypaper nanocomposites. The use of the LAFD technique in the first approach preserves the CNTs structure and alignment and avoids the collapse of the VACNTs array, which is a major advantage of this method. On the other hand, LAFD grown ZnO nanoparticles and tetrapods were used to produce ZnO/CNT buckypaper nanocomposites. When compared with the as-grown samples the PL spectra of the composites structures behave differently. For the case of the ZnO/VACNTs no changes on the peak position and spectral shape were observed. Only an enhancement of the overall luminescence was found to occur. On contrary, for the buckypaper nanocomposites notable changes on the spectral shape and peak position were observed, likely due to distinct surface band bending effects for the ZnO nanoparticles and tetrapods embedded in the CNTs.

The growth and structural/optical properties of metal-oxide semiconductor nanostructures by a simple, low-cost, and large-scalable fabrication method were studied. These nanostructures were applied to energy and optoelectronic devices, such as piezoelectric nanogenerators and photodetective sensors, to improve the device performance. The morphologies and crystallinity of the fabricated nanostructures were observed from scanning electron microscope/transmission electron microscope images, respectively. The piezoelectric output current and photoresponse property were characterized by manufacturing the nanogenerators and photodetectors with prepared metal oxide nanostructures. These results can provide a fundamental understanding of the mechanisms for improving the device performance in applications of metal-oxide semiconductor nanostructures for energy and sensing devices.

The retention of nanocrystallinity in dense ceramics is still a challenge, even with the application of stress-assisted methods like Spark Plasma Sintering. Starting powder and sintering process strongly affect the evolution of the microstructure and thus, the final properties of ZnO. Control of the microstructure was carried out through the combined effect of high heating rates and the presence of bound water, which seems to significantly promote densification of zinc oxide nanoparticles. Hence, dense nano-grained ZnO could be synthesized at a temperature of only 400 °C. In addition, sintering behavior can be also modified by the use of external electric fields, which can generate a drastic mass diffusion process that is called flash sintering. The current flow through the specimen entails an increment of the temperature produced by Joule heating, enhancing the sintering process. Control of both parameters, heating rates/water content and electric field, leads to dense ZnO compacts with grain size between 150 nm to almost 5 μm.

Er3⁺-doped oxyfluoride glass and glass-ceramics containing SrF₂ nanocrystals have been prepared and investigated their spectroscopic and luminescence properties. The formation of SrF₂ nanocrystals in glass-ceramics were confirmed by Xray diffraction (XRD) and transmission electron microscopy (TEM). Judd-Ofelt parameters have been evaluated from absorption spectra of the Er3⁺-doped glass, which in turn used to predict radiative properties for the fluorescent levels of Er3⁺ ions. The intensities of both Stokes and upconversion (anti-Stokes) emissions significantly increase with increase of the size of the fluoride crystals in the glass matrix. The mechanism of green and red upconversion emissions have been ascribed to two photon processes. The lifetime of the ⁴S_3/2 level of the Er3⁺ ions in glass-ceramics is found to be slightly higher than that of the counter glass, which may be due to the incorporation of Er3⁺ ions into the low phonon sites of SrF₂ nanocrystals.

The Flexible supercapacitor electrode material was prepared by simple spray coating technique. This will provide a greener alternative for the fabrication of binder free composite electrode for supercapacitor applications. A symmetric double layer super capacitor stack was fabricated by using flexible electrodes. The investigation of the capacitance property of the fabricated super capacitor stack was investigated using cyclic voltammetry, chronopotentiometry and electrical impedance spectroscopy studies. The flexible electrode material shows a specific capacitance of 50 Fg^-1 with good cyclibility.

We have studied processing and characteristics of flexible Aluminum-doped Zinc Oxide thin-film transistors (AZO TFTs) fabricated on plastic substrates using radio frequency (rf) magnetron sputtering. To improve the performance of flexible AZO TFT, we studied effects of device structures on characteristics of the aluminum-doped zinc oxide thin film transistors. The electrical properties of top-gate type and bottom-gate type AZO TFTs were investigated, respectively. The top-gate type AZO TFTs shows a threshold voltage of 1.4 V, a I_on/I_off current ratio of 1.0×10⁷, a field effect mobility of 28.2 cm²/ V•s, a subthreshold swing of 0.19 V/decade. And the bottom-gate type AZO TFTs shows a threshold voltage of 1.7 V, a I_on/I_off ratio of 1.0×10⁷, a field effect mobility of 209 cm²/ V•s, a subthreshold swing of 0.16 V/decade, and the off current of less than 10^-11A at room temperature. Both TFTs show low threshold voltage, high I_on/I_off ratio and high field effect mobility. By comparison, the bottom-gate type AZO TFTs shows better characteristics. The flexible AZO-TFT is a very promising low-cost optoelectronic device for the next generation of invisible and flexible electronics due to flexible, transparency, high mobility, and low-temperature processing.

High quality w-Mg_xZn_1-xO thin films were grown epitaxially on c-plane sapphire substrates by plasma-assisted Molecular Beam Epitaxy. ZnO thin films with high crystalline quality, low defect and dislocation densities, and subnanometer surface roughness were achieved by applying a low temperature nucleation layer. By tuning Mg/Zn flux ratio, wurtzite Mg_xZn_1-xO thin films with Mg composition as high as x=0.46 were obtained without phase segregation. Metal- Semiconductor-Metal (MSM) photoconductive and Schottky barrier devices with interdigitated electrode geometry and active surface area of 1 mm2 were fabricated and characterized. Resultant devices showed ~100 A/W peak responsivity at wavelength of ~260nm. We also report on cubic rock salt c-Mg_xZn_1-xO thin films, following a non-traditional approach on MgO substrates, to demonstrate solar-blind photoresponse in MSM photodetectors, realizing a peak responsivity of 460 A/W (@ 250 nm) and 12.6 mA/W (@ 240nm) for mixed phase and single crystal films, respectively. A specific focus of the work is on identifying the impact of various growth parameters on the performance of the c- MgZnO detectors.

This work describes the development of ZnO nanowire (NW) devices for ultraviolet detection and cost-effective gas sensing. A dielectrophoresis (DEP) flow cell fabricated for the integration of NWs on different substrates is presented. The system includes the possibility to set characteristic parameters such as alternating current (AC) frequency, amplitude or flow speed in order to control NW trapping on specific sites defined by micro-gapped electrodes. The electrical characteristics of the rectifying metal/NW contact fabricated by DEP are investigated in darkness and under direct illumination of the metal-NW interface through the ZnO NW. A significant downshift of the turn-on voltage is observed in the current-voltage characteristics during the illumination with photon energies higher than the ZnO bandgap. The reduction is attributed to a barrier height lowering induced by interface charge emission. The effects of AC bias on the thermal drift of the DC average current in NW devices are also discussed. Finally, the reaction kinetics of ethanol and water vapors on the NW surface are compared through the analysis of the DC current under direct exposure to gas flows. Device responses to more complex compound mixtures such as coffee or mint are also monitored over time, showing different performance in both cases.

Electrochemical deposition (ECD) is a versatile technique for the preparation of ZnO nanowires (NWs) and nanorods (NRs) with high structural and optical quality. The bandgap of the ZnO NWs can be engineered by doping. Depending on the doping cation and concentration, the bandgap is increased or decreased in a controlled manner. The NW arrays have been grown on various substrates. The epitaxial growth on single-crystal conducting substrates has been demonstrated. By using p-type GaN layers, heterostructures have been fabricated with a high rectifying electrical behavior. They have been integrated in low-voltage LEDs emitting in the UV or in the visible region depending on the NW composition. For visible-blind UV-photodetector application, ZnO NW ensembles, electrochemically grown on F:SnO₂, have been contacted on their top with a transparent graphene sheet. The photodetector had a responsivity larger than 10⁴ A/W at 1V in the near-UV range. ECD ZnO NWs have also been isolated and electrically connected on their both ends by Al contacts. The obtained nanodevice, made of an individual NW, was shown to be a H₂ gas sensor with a high selectivity and sensitivity. Moreover, it was shown that Cd-doping of ZnO NWs significantly improved the performance of the sensor.

Low applied voltage in electrowetting-on-dielectric (EWOD) can be achieved using thin dielectric. However it is followed by high possibility of dielectric failure. On the other hand, multi-layer dielectric has been known as a way to enhance the dielectric reliability by delaying the dielectric breakdown. In this paper, we report a modified structure of multi-layer insulator called sandwich-like multi-layer structure. This structure is built by dividing one layer into two sections and inserting the other layer between them, resulting a stack with an additional layer but identical in thickness with the conventional multi-layer structure. Using Parylene C and Aluminum Oxide (Al₂O₃), sandwich-like multi-layer structure shows an improvement in dielectric reliability by delaying the occurrence of dielectric breakdown without sacrificing the operational voltage. Dielectric breakdown is investigated by observing the bubbles forming during electrowetting test caused by electrolysis.

Nanowire (NW) based light emitting diodes (LEDs) have drawn great research interest due to many advantages compared to thin film based devices. Marked improved performances are expected from nanostructured active layers for light emission. Semiconducting oxide nanowires can act as direct waveguides and favor emitted light extraction without use of lens and reflectors in LEDs. Moreover, the use of ZnO wires avoids the presence of grain boundaries and then the emission efficiency is boosted by the absence of non-radiative recombinations at the joint defects. In this context, europium (Eu):Chelate/ZnO:Mg-nanowires/p-GaN light-emitting-diode (LED) structures have been fabricated showing near-UV/violet electroluminescence and red emission from trivalent europium. Fabricated LED structures exhibit UV-blue light at about 380 nm coming from the n-(ZnO:Mg)/p-GaN and a sharp red emission at ∼611 nm related to the intra-4f transition of Eu ions. It is found that in the case of the ZnO:Mg, the emission wavelength is slightly shifted to smaller wavelength to be well adapted to the trivalent europium excitation band. Radiative energy transfer is achieved through strong overlap between the emission wavelength from n-(ZnO:Mg)/p- GaN heterojunction and chelate ligand intensive absorption band. Indeed the Eu:chelate/(ZnO:Mg)-nanowires/p-GaN structure appears well adapted to UV/blue and red dual emission. Our results shows that the design of LEDs based on the chelate ligands are important issue to enhance the performance of electroluminescence devices based on ZnO nanowire arrays/p-GaN heterojunction and rare-earth metal complexes.

β-Ga₂O₃ is the most transparent conductive oxide, well known since several decades for its large bandgap of 4.8 eV. Its potential as semiconductor material, however, is just emerging in recent years. Present work shows the development of βGa₂O₃ for semiconductor applications and its current state-of-the-art. The discussion is focused on three different aspects: (1) Advantageous growth from melt of large-size β-Ga₂O₃single-crystals. High-crystalline quality and carrier control make possible the production of conductive and semi-insulating wafers. (2) β-Ga₂O₃as substrate for homoepitaxy as well as for heteroepitaxial deposition of GaN-based devices. High-brightness blue-LEDs with vertical current injection are demonstrated. (3) Potential of β-Ga₂O₃for high-power devices with higher breakdown voltage than GaN and SiC counterparts. The first Schottky barrier diode is shown, as well as first transistors (MESFET and MOSFET) are indicated. Single-crystal phosphors are introduced as novel alternative to currently used powder phosphors. In connection with high-brightness white light-sources, based on LEDs or LDs plus phosphor converters, single-crystal phosphors possess advantageous features. These avoid the use of resins and exhibit a very high internal quantum efficiency, which remains stable with the temperature increase.

Recently, ZnO-based semiconductors have been deposited on various substrates using various methods. Furthermore, they were used in ultraviolet light-emitting diodes (UVLEDs) due to inherent properties including wide direct bandgap and high binding energy. In this work, two different deposition systems were utilized to deposit the ZnO-based films. The resulted films were applied to fabricate the ZnO-based UVLEDs. Firstly, the high quality i-ZnO films were deposited as the active layer by using the vapor cooling condensation system to enhance the internal quantum efficiency. Secondly, the double-heterostructured MgZnO/ZnO/MgZnO layers were deposited as the active layer at low temperature using the vapor cooling condensation system to enhance light intensity. Furthermore, various component ratios of i- MgZnO and i-MgBeZnO films were deposited using a radio frequency (RF) magnetron co-sputter system. Consequently, the deposited films with various energy bandgaps were stacked alternately to form the active layer of multiple-quantum well (MQW) UVLEDs. The light emitting intensity of MQW UVLEDs was better than that of the traditional p-i-n UVLEDs. This phenomenon was attributed to the carrier confinement in well layers and improvement probability of radiative recombination.

By combining inclined pulsed-laser deposition (PLD) with nanoimprint lithography, an original three-dimensional (3D) nanofabrication technique, namely “3D nanotemplate PLD technique E has been established. 3D nanotemplate PLD enables to fabricate the large arrays of programmable ZnO nanostructures: nanoboxes and nanowires with a width of 20nm. Cathodoluminescence (CL) measurements at 300K showed an intense luminescence peak around 380 nm corresponding to near-band-edge (NBE) emission from even a single ZnO nanobox. The CL intensity mapping also showed the brilliant NBE luminescence from the entire single ZnO nanobox. The architecturally designed ZnO nanostructures with an excellent wide-gap luminescent semiconductor character should be good candidates for optoelectronic materials for nanoscale device applications.

Different composite nanostructures based pn-junctions have been synthesized using the low temperature hydrothermal chemical growth. The composite nanostructures based pn junctions demonstrated here include p-NiO/n-ZnO, p-CuO/n- ZnO, and p-NiO/n-TiO₂. Structural characterization of these composite nanostructures based pn-junctions was performed by different complementary tools and the results indicated that reasonable device quality crystals have been achieved. His act was also confirmed by the rectifying electrical behavior observed from these junctions. Further, the different composite nanostructures based junctions were used to demonstrate UV detectors and visible light emitting diodes (LEDs) operating with acceptable performance.

Zinc oxide (ZnO) is a material of great interest for short-wavelength optoelectronic applications due to its wide band gap (3.37 eV) and high exciton binding energy (60 meV). Due to the difficulty in stable p-type doping of ZnO, other p-type materials such as gallium nitride (GaN) have been used to form heterojunctions with ZnO. p-GaN/n-ZnO heterojunction devices, in particular light-emitting diodes (LED) have been extensively studied. There was a huge variety of electronic properties and emission colors on the reported devices. It is due to the different energy alignment at the interface caused by different properties of the GaN layer and ZnO counterpart in the junction. Attempts have been made on modifying the heterojunction by various methods, such as introducing a dielectric interlayer and post-growth surface treatment, and changing the growth methods of ZnO. In this study, heterojunction LED devices with p-GaN and ZnO nanorods array are demonstrated. The ZnO nanorods were grown by a solution method. The ZnO nanorods were exposed to different kinds of plasma treatments (such as nitrogen and oxygen) after the growth. It was found that the treatment could cause significant change on the optical properties of the ZnO nanorods, as well as the electronic properties and light emissions of the resultant LED devices.

Stand-alone heterojunction (HJ) solar cells demonstrated on crystalline germanium (c-Ge) substrates are proposed for usage as the bottom cells of tandem-junction solar cells in various thin-film solar cell technologies. The emitter of the HJ solar cells is formed by growing thin layers of highly doped hydrogenated microcrystalline silicon (μc-Si:H) and further passivated by growing thin layers of hydrogenated amorphous silicon (a-Si:H). The μc-Si:H and a-Si:H layers are grown in the same reactor using plasma-enhanced chemical vapor deposition (PECVD) at temperatures close to 200°C. The quality of the c-Ge surface passivation by μc-Si:H and a-Si:H has a direct impact on the electrical performance of the HJ solar cells. Conversion efficiencies of 5.9% and 7.2% have been achieved for stand-alone c-Ge solar cells on n-type and p-type c-Ge substrates, respectively. These conversion efficiencies are well-comparable with the conversion efficiencies reported for conventional homojunction solar cells fabricated at temperatures as high as 600°C.

The benefit of achieving high electron mobilities in transparent conducting oxides (TCOs) is twofold: they first exhibit superior optical properties, especially in the NIR spectral range, and secondly their low resistivity enables the usage of thinner films. Remarkably high mobilities can be obtained in Al-doped zinc oxide by post-deposition annealing under a protective layer. The procedure has not only shown to increase mobility, but also strongly reduces sub-bandgap absorption. Extensive optical, electrical and structural characterization is carried out in the films in order to clarify the microscopic origins of the changes in material properties. While the annealing of defect states, most likely deep acceptors, seems clear, earlier results also suggest some influence of grain boundaries. Tailing, on the contrary, seems to be linked to extended defects. In application to a-Si:H/μc-Si:H thin film solar cells the films have already shown to increase spectral response. When reducing the film thickness, the main challenge is to provide a suitable light trapping scheme. Normally this is achieved by a wet chemical etching step in diluted HCl, which provides a surface structure with suitable light scattering properties. Therefore a TCO-independent light scattering approach using textures glass was applied in conjunction with the high mobility zinc oxide. The substrate enables the use of very thin TCO layers with a strongly reduced parasitic absorption.

We study light absorption in ZnO nanorod arrays sensitized with CdSe quantum dots as one of the factors affecting solar cell performance in need of improvement given their current performance well below expectations. Light trapping in nanorod arrays (NRAs) as it relates to array density and length as well as quantum dot (QD) loading is studied using the Finite Difference Time Domain model. It is shown that light absorption in such solar cell architecture is a sensitive function of the morphological dimensions and that a higher NRA density does not necessarily correspond to large absorption in the solar cell. Instead, light trapping efficiency depends significantly on the array density, QD axial distribution and refractive index contrast between NR and QDs thus suggesting strategies for improved quantum dot solar cell (QDSC) fabrication. In addition, we present experimental data showing dramatic improvement in photo conversion efficiency performance for relatively short ZnO NRAs (~1 μm) of low NRA density, but whose efficiency improvement can not be solely explained based on our current light trapping estimates from the numerical simulations.

Energy conversion technologies are aiming to extremely high power capacities per year. Nontoxicity and abundance of the materials are the key requirements to a sustainable photovoltaic technology. Oxides are among the key materials to reach these goals. We investigate the influence of thin buffer layers on the performance of an ZnO:Al/buffer/Cu₂O solar cells. Introduction of a thin ZnO or Al₂O₃ buffer layer, grown by thermal ALD, between ZnO:Al and Cu₂O resulted in 45% increase of the solar cell efficiency. VPE growth of Cu₂O employing elemental copper and pure oxygen as precursor materials is presented. The growth is performed on MgO substrates with the (001) orientation. On- and off- oriented substrates have been employed and the growth results are compared. XRD investigations show the growth of the (110) oriented Cu₂O for all temperatures, whereas at a high substrate temperature additional (001) Cu₂O growth occurs. An increase of the oxygen partial pressure leads to a more pronounced 2D growth mode, whereby pores between the islands still remain. The implementation of off-axis substrates with 3.5° and 5° does not lead to an improvement of the layer quality. The (110) orientation remains predominant, the grain size decreases and the FWHM of the (220) peak increases. From the AFM images it is concluded, that the (110) surface grows with a tilt angle to the substrate surface.

ZnO@SnO₂ multilayered network was deposited on fluorine doped tin oxide (FTO) glass and applied as photoanode in dye sensitized solar cells whose functional performances are compared with single oxide-based photoanodes made of SnO₂ nanoparticles and ZnO microparticles. Multi-oxide photoanodes provide for enhanced photoconversion efficiency (3.31%) as compared with bare SnO₂ nanoparticles (1.06%) and ZnO microparticles (1.04%). Improved functional performances of the ZnO@SnO₂ layered network are ascribable to partial inhibition of back electron transfer from SnO₂ to the redox electrolyte, guaranteed by the ZnO, which acts as a capping layer for the underlying SnO₂.

Donor-acceptor (D−A) conjugated polymers have attracted a good deal of attention in recent years. In D−A systems, the introduction of electron withdrawing groups reduces E_g by lowering the LUMO levels whereas, the introduction of electron donating groups reduces E_g by raising the HOMO levels. Also, conjugated polymers with desired HOMO and LUMO energy levels could be obtained by the proper selection of donor and acceptor units. Because of this reason, D−A conjugated polymers are emerging as promising materials particularly for polymer light emitting diodes (PLEDs) and polymer solar cells (PSCs). We report the design and synthesis of four new narrow band gap donor-acceptor (D-A) conjugated polymers, PTCNN, PTCNF, PTCNV and PTCNO, containing electron donating 3,4-didodecyloxythiophene and electron accepting cyanovinylene units. The effects of further addition of electron donating and electron withdrawing groups to the repeating unit of a D-A conjugated polymer (PTCNN) on its optical and electrochemical properties are discussed. The studies revealed that the nature of D and A units as well as the extent of alternate D-A structure influences the optical and the electrochemical properties of the polymers. All the polymers are thermally stable up to a temperature of 300 °C under nitrogen atmosphere. The electrochemical studies revealed that the polymers possess low-lying HOMO energy levels and low-lying LUMO energy levels. In the UV-Vis absorption study, the polymer films displayed broad absorption in the wavelength region of 400−700 nm. The polymers exhibited low optical band gaps in the range 1.70 − 1.77 eV.

The chemistry of graphene oxide (GO) and its response to external stimuli such as temperature and light are not well understood and only approximately controlled. This understanding is however crucial to enable future applications of the material that typically are subject to environmental conditions. The nature of the initial GO is also highly dependent on the preparation and the form of the initial carbon material. Here, we consider both standard GO made from oxidizing graphite and layered GO made from oxidizing epitaxial graphene on SiC, and examine their evolution under different stimuli. The effect of the solvent on the thermal evolution of standard GO in vacuum is first investigated. In situ infrared absorption measurements clearly show that the nature of the last solvent in contact with GO prior to deposition on a substrate for vacuum annealing studies substantially affect the chemical evolution of the material as GO is reduced. Second, the stability of GO derived from epitaxial graphene (on SiC) is examined as a function of time. We show that hydrogen, in the form of CH, is present after the Hummers process, and that hydrogen favors the reduction of epoxide groups and the formation of water molecules. Importantly, this transformation can take place at room temperature, albeit slowly (~ one month). Finally, the chemical interaction (e.g. bonding) between GO layers in multilayer samples is examined with diffraction (XRD) methods, spectroscopic (IR, XPS, Raman) techniques, imaging (APF) and first principles modeling.

Metal oxide-based photoanodes are critical components of dye sensitized solar cells (DSSCs), which are photoelectrochemical cells for the conversion of solar energy, promising to have several benefits as compared with their traditional counterparts. A careful engineering of the wide band gap metal oxide composing the photoanode, as well as their process design, is strategic for improving device performances and for planning a near future production scale up, especially devoted to reducing the environmental impact of the device fabrication. Herein, we present the application of ZnO hierarchical structures as efficient materials to be applied as photoanodes in DSSC, in the perspective of looking for alternative to TiO₂ nanoparticles, currently the most exploited metal oxide in these devices.

NiO was grown on Si (111), c-Al₂O₃ and FTO/glass substrates by pulsed laser deposition (PLD). X-Ray Diffraction (XRD) and scanning electron microscope (SEM) studies revealed that layers grown on c-Al₂O₃ were fcc NiO with a dense morphology of cubic grains that were strongly (111) oriented along the growth direction. The relatively low ω rocking curve linewidth, of 0.12°suggests that there may have been epitaxial growth on the c-Al₂O₃ substrate. XRD and SEM indicated that films grown on Si (111) were also fcc NiO, with cubic grains, but that the grain orientation was random. This is consistent with the presence of an amorphous SiO₂ layer at the surface of the Si substrate, which precluded epitaxial growth. NiO grown at lower temperature (200°C) on temperature-sensitive FTO/glass substrates showed no evidence of crystallinity in XRD and SEM studies. After flash annealing in air, however, peaks characteristic of randomly oriented fcc NiO appeared in the XRD scans and the surface morphology became more granular in appearance. Such layers appear promising for the development of future dye-sensitised solar cell devices based on NiO grown by PLD.