This PDF file contains the front matter associated with SPIE Proceedings Volume 10105, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

Here we review the results of a comprehensive study of high-temperature superconductivity in cuprates that took over ten years to complete. It required development of the technique, for synthesis as well as for measurements of the key physical properties of the superconducting and the normal states, in order to establish their precise dependence on doping, temperature, and external fields. We use atomic-layer-by-layer molecular beam epitaxy to synthesize atomically perfect thin films and multilayers of high-T_c cuprates. We use the mutual inductance technique refined to measure the absolute value of penetration depth 𝜆 to accuracy better than 1%. We have synthesized and studied over 2,000 cuprate films. The large statistics reveals clear trends and intrinsic properties; this is essential when dealing with complex materials such as cuprates. The findings bring in some great surprises, challenge the commonly held beliefs, rule out many models, and point to an unexpected answer to the question why is T_c so high in cuprates.

We report the resistance noise spectroscopy technique and its applications in detecting the nature of high- temperature superconducting transition in cuprates. In underdoped La_2-xSr_xCuO₄ thick films, in which inter- layer coupling is relatively weak and thus the system is effectively two-dimensional, resistance noise spectroscopy shows the first experimental evidence of intrinsic correlated dynamics near the superconducting transition of the Berezinskii-Kosterlitz-Thouless type. In particular, resistance fluctuations in the low-temperature ordered phase are correlated, slow and nonergodic, i.e., the system exhibits properties that cannot be fully detected in any reasonable experimental time frame.

Large-bipolaron superconductivity is plausible with carrier densities well below those of conventional metals. Bipolarons form when carriers self-trap in pairs. Coherently moving large-bipolarons require extremely large ratios of static to optical dielectric-constants. The mutual Coulomb repulsion of a planar large-bipolaron’s paired carriers drives it to a four-lobed shape. A phonon-mediated attraction among large-bipolarons propels their condensation into a liquid. This liquid’s excitations move slowly with a huge effective mass. Excitations’ concomitant weak scattering by phonons produces a moderate low-temperature dc resistivity that increases linearly with rising temperature. With falling temperature an energy gap opens between large-bipolarons’ excitations and those of their self-trapped electronic carriers.

Electrons in metal clusters organize into quantum shells, akin to atomic shells in the periodic table. Such nanoparticles are referred to as “superatoms”. The electronic shell levels are highly degenerate giving rise to sharp peaks in the density of states, which can enable exceptionally strong electron pairing in certain clusters containing tens to hundreds of atoms. A spectroscopic investigation of size – resolved aluminum nanoclusters has revealed a sharp rise in the density of states near the Fermi level as the temperature decreases towards 100 K. The effect is especially prominent in the closed-shell “magic” cluster Al66 [1, 2]. The characteristics of this behavior are fully consistent with a pairing transition, implying a high temperature superconducting state with Tc < 100K. This value exceeds that of bulk aluminum by two orders of magnitude. As a new class of high-temperature superconductors, such metal nanocluster particles are promising building blocks for high-Tc materials, devices, and networks. -------------------- 1. Halder, A., Liang, A., Kresin, V. V. A novel feature in aluminum cluster photoionization spectra and possibility of electron pairing at T 100K. Nano Lett 15, 1410 – 1413 (2015) 2. Halder, A., Kresin, V. V. A transition in the density of states of metal “superatom” nanoclusters and evidence for superconducting pairing at T 100K. Phys. Rev. B 92, 214506 (2015).

High-temperature superconducting (HTS) cuprates are highly anisotropic materials which exhibit metallic-like behavior in the CuO₂ planes while retaining dielectric properties in the perpendicular, c-axis, direction. Experimental data show however that in HTS systems the in-plane electronic excitations are strongly coupled to c-axis polarized vibrations. This interaction is manifest in various settings, for example in the resonant Raman profile of phononic excitations, inelastic quasi-particle tunneling, as observation of notch-like features and forbidden scattering for in-plane optical conductivity, colossal c-axis photo-expansion upon in-plane illumination as well as in high-resolution electron energy-loss spectra. We propose that this anisotropic coupling is driven by strong unscreened Coulomb interactions and the preponderance of the Madelung component to the cohesion energy, in particular by the large atomic displacements in the spacer layers induced by charge redistribution within the CuO₂ planes.

Zinc and tin oxides are both earth-abundant materials with demonstrated applicability as electrodes in several optoelectronic devices. The presence of grain boundaries in these polycrystalline films generally limits the electron mobility. By a combinatorial study of ZnO and SnO₂, a transparent conducting amorphous zinc tin oxide (ZTO) electrode, free of grain boundaries, with a dense (void-free) microstructure has been developed. We show how tuning the stoichiometry (Zn_4.5Sn_30.2O_65.3) and film’s microstructure during sputtering deposition, allows achieving electron mobilities up to 25 cm²/Vs and free carrier concentrations of ~ 7 x 10¹⁹ cm^-3. The effects of post-deposition thermal treatments are furthermore studied. The ZTO films keep their dense amorphous microstructure upon annealing up to 500°C, as confirmed by cross-section TEM and XRD, while presenting a clear improvement in electron mobility up to 35 cm²/Vs when annealed in oxygen-rich atmospheres.

In recent years, infrared plasmonics has turned towards materials that are wavelength and application tailorable, and which are geared towards CMOS processing. The transparent conductive oxides are very favorable towards infrared plasmonic applications for a number of reasons, one of which being the natural visible transparency due to their relatively large bandgap. Fluorine-doped tin oxide (FTO) is one such transparent and doping-tunable material that in addition is low cost due to spray deposition techniques that result in perfectly conformal coatings. In this work, a deposition recipe that gives high free carrier concentration was used to fabricate structures for demonstration of surface plasmon excitation. 1D gratings with a range of structural parameters were etched in silicon. Then the gratings were conformally coated with FTO by aqueous spray deposition. Excitation of surface plasmon polaritons (SPP) at mid- and long- wave infrared wavelengths on these gratings was demonstrated. The observed (SPP) excitation resonances agree will with analytical excitation calculations and numerical simulations. We show that grating heights of ~10-15% of the wavelength are optimum for achieving the strongest sharpest coupling to plasmonic resonances in the mid- and longwave infrared. The presented results are compared with similar etched silicon gratings coated with Ga-doped ZnO (GZO). The dominant difference between our FTO and GZO measurements is the free carrier concentration. The useful wavelength range is predicted for FTO based plasmonics and compared with other plasmonic host materials. The work presented here could play a key role in novel decreased-cost detectors, filters, and on-chip optoelectronics.

Undoped and Ga-doped ZnO (002) films were grown c-sapphire using the pulsed laser deposition (PLD) method. Znvacancy related defects in the films were studied by different positron annihilation spectroscopy (PAS). These included Doppler broadening spectroscopy (DBS) employing a continuous monenergetic positron beam, and positron lifetime spectroscopy using a pulsed monoenergetic positron beam attached to an electron linear accelerator. Two kinds of Znvacancy related defects namely a monovacancy and a divacancy were identified in the films. In as-grown undoped samples grown with relatively low oxygen pressure P(O₂)≤1.3 Pa, monovacancy is the dominant Zn-vacancy related defect. Annealing these samples at 900 ^oC induced Zn out-diffusion into the substrate and converted the monovacancy to divacancy. For the undoped samples grown with high P(O₂)=5 Pa irrespective of the annealing temperature and the as-grown degenerate Ga-doped sample (n=10²⁰ cm^-3), divacancy is the dominant Zn-vacancy related defect. The clustering of vacancy will be discussed.

Using simulations from first principles we investigate the microscopic role of doping on the optoelectronic properties of X-doped ZnO (XZO, X=Al, F), as transparent conductive oxide for energy applications. We show how the interplay between (co)dopants and defects affects TCO characteristics of the samples. Finally, we study the plasmonic activity of XZO in the near-IR/visible range and in particular at wavelength relevant for telecommunications (1.5 μm), confirming recent experimental results.

For single slabs of uniform material, such as bulk semiconductors, we derive closed-form expressions for absorption and reflection coefficients, ∝ and R, respectively, in terms of measured reflectance and transmittance, R_m and T_m. The formula for α can replace the several commonly used approximations for ∝ as a function of T_m, and in particular does not require ∝d >> 1, where d is the thickness. Thus, it can be applied to weak impurity absorptions, such as Fe absorption in Fe-doped GaN. Finally, the real (η) and imaginary (κ) parts of the index of refraction (n = η + iκ) can be obtained from ∝ and R and agree well with η and κ results obtained from other experiments. For multi-layer structures, “effective” values of ∝, R, η, and κ are obtained, but they can often be assigned to a particular layer. This new technique has been successfully applied to many bulk and layered structures.

The variation of oxygen concentration in the Indium Tin Oxide (ITO) structure highly impacts its electrical and optical characteristics. In this work, we investigated the effect of oxygen partial flow (O₂/O₂+Ar) and deposition pressure (p) on the refractive index (n) of reactive sputtered ITO thin films. A statistical study with a Genetic Algorithm (GA) optimization was implemented to find optimal deposition conditions for obtaining particular refractive indices. Several samples of ITO thin films with refractive indices ranging from 1.69 - 2.1 were deposited by DC sputtering technique at various oxygen concentrations and deposition pressures, in order to develop the statistical database. A linear polynomial surface was locally fitted to the data of O₂/O₂+Ar, p, and n of deposited films. This surface was then used as the fitness function of the GA. By defining the desired n as the objective value of the GA, the optimized deposition conditions can be found. Two cases were experimentally demonstrated, with the GA determining the needed process parameters to deposit ITO with n=2.2 and n=1.6. Measured results were very close to desired values, with n=2.25 and n=1.62, demonstrating the effectiveness of this method for predicting needed reactive sputtering conditions to enable arbitrary refractive indices.

We investigated the crystal structure, growth kinetics and electrical properties of BeMgZnO/ZnO heterostructures grown by Molecular Beam Epitaxy (MBE). Transmission Electron Microscopy (TEM) studies revealed that incorporation of Mg into the BeZnO solid solution eliminates the high angle grain boundaries that are the major structural defects in ternary BeZnO. The significant improvement of x-ray diffraction intensity from quaternary BeMgZnO alloy compared to ternary BeZnO was attributed to the reduction of lattice strain, which is present in the latter due to the large difference of covalent radii between Be and Zn (1.22 Å for Zn, 0.96 Å for Be). Incorporation of Mg, which has a larger covalent radius of 1.41Å, reduced the strain in BeMgZnO thin films and also enhanced Be incorporation on lattice sites in the wurtzite lattice. The Zn/(Be + Mg) ratio necessary to obtain single-crystal O-polar BeMgZnO on (0001) GaN/sapphire templates was found to increase with increasing substrate temperature:3.9, 6.2, and 8.3 at substrate temperatures of 450°C, 475°C, and 500°C, respectively. Based on analysis of photoluminescence spectra from Be_0.03Mg_yZn_0.97-yO and evolution of reflection high-energy electron diffraction patterns observed in situ during the MBE growth, it has been deduced that more negative formation enthalpy of MgO compared to ZnO and the increased surface mobility of Mg adatoms at elevated substrate temperatures give rise to the nucleation of a MgO-rich wurtzite phase at relatively low Zn/(Be + Mg) ratios. We have demonstrated both theoretically and experimentally that the incorporation of Be into the barrier in Zn-polar BeMgZnO/ZnO and O-polar ZnO/BeMgZnO polarization doped heterostructures allows the alignment of piezoelectric polarization vector with that of spontaneous polarization due to the change of strain sign, thus increasing the amount of net polarization. This made it possible to achieve Zn-polar BeMgZnO/ZnO heterostructures grown on GaN/sapphire templates with two-dimensional electron gas densities substantially exceeding those in Zn-polar MgZnO/ZnO and O-polar ZnO/MgZnO heterostructures with similar Mg content.

We demonstrate that exceptional points exist in fully transparent, optically ”effectively” biaxial, anisotropic micro-cavities, fabricated using an uniaxial cavity material with its axis inclined to the Bragg mirror growth direction. This is similar to the existence of singular (optic) axes in absorbing biaxial crystals, but the lack of time reversal symmetry is mediated by the mode broadening, i.e. the photon escape from the – in principle – open cavity system. As a consequence the eigenmodes are generally elliptically polarized, and completely circularly polarized eigenmodes are expected in certain directions. Via geometric and chemical composition design degrees of freedom, the spectral and angular position of these chiral modes can be rationally designed. Possible applications arise from the use of such directions for circularly polarized emission without the use of spin injection or internal or external magnetic fields. Also the coupling of such modes to excitons, adding oscillator strength to the system, seems a promising avenue of research.

Thin films of wide band-gap oxides grown by Atomic Layer Deposition (ALD) are suitable for a range of applications. Some of these applications will be presented. First of all, ALD-grown high-k HfO₂ is used as a gate oxide in the electronic devices. Moreover, ALD-grown oxides can be used in memory devices, in transparent transistors, or as elements of solar cells. Regarding photovoltaics (PV), ALD-grown thin films of Al₂O₃ are already used as anti-reflection layers. In addition, thin films of ZnO are tested as replacement of ITO in PV devices. New applications in organic photovoltaics, electronics and optoelectronics are also demonstrated Considering new applications, the same layers, as used in electronics, can also find applications in biology, medicine and in a food industry. This is because layers of high-k oxides show antibacterial activity, as discussed in this work.

The development of Zinc Oxide (ZnO)-based applications have been strongly limited due to the lack of reproducible p-type doping. Here we present novel opportunities in the field of unipolar oxide wide band gap semiconductors. First we have developed the growth of nonpolar ZnO/ZnMgO multiple quantum wells (MQWs) and then we demonstrate that the structural and optical properties of the MQWs are reaching the required level for intersubband devices in terms of defects, surface and interface roughness and doping. We will show and discuss the most recent results as, for instance, intersubband transitions which have been observed in such structures. This "Zoterac" project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 665107

ZnO has great potential for devices in the mid IR and the THz range through the use of intersubband (ISB) transitions in multiple quantum wells (MQWs), although exploiting these transitions requires great control of the epitaxial layers as well as of the physics involved. In this work we present an analysis of non-polar ZnO grown homoepitaxially by molecular beam epitaxy on m-plane ZnO substrates as an ISB optical absorber. The MQWs were characterized under a 45°-bevelled multi-pass waveguide configuration allowing the observation at room temperature of an ISB transition in the 4-6 μm region for p-polarized incident light.

We address the electronic, phononic, and thermal properties of oxide based superlattices and multi quantum well heterostructures. In the first part, we review the present understanding of phonon coupling and phonon propagation in superlattices and elucidate current research aspects of phonon coherence in these structure. Subsequently, we focus on the experimental study of MBE grown ZnO/ZnMgO multi quantum well heterostructures with varying Mg content, barrier thickness, quantum well thickness, and number of periods. In particular, we discuss how the controlled variation of these parameters affect the phonon dispersion relation and phonon propagation and their impact on the thermal properties.

The plasmonic resonance wavelength λres in ZnO doped with 3wt%Ga₂O₃ can be controlled over the range 1 – 4 μm by simple furnace annealing in flowing Ar. For each annealing temperature T_A, the reflectance Rm and transmittance Tm are measured over a wavelength range, λ = 185 – 3200 nm, (energy range, E = 6.7 – 0.387 eV), and the reflectance coefficient R is calculated from R_m and T_m. The value of λres is then determined from a Drude-theory analysis of R vs E that yields fitting parameters n_opt (optical carrier concentration), μ_opt (optical mobility), high-frequency dielectric constant ε_∞, and thickness d, at each annealing temperature T_A. The validity of this process is confirmed by comparison of ε_∞ with literature values, and comparison of n_opt and μ_opt with analogous quantities n and μH measured by the Hall-effect.

Given the increasing demand for optical in vivo bioimaging materials, persistent luminescence in the red/near infrared range is receiving particular attention. Within this work, several elaboration ways of ZnGa₂O₄:Cr³⁺, respectively by solid state, by microwave assisted hydrothermal synthesis and by glass crystallization have been carried out. Structural analysis shows that all methods lead to pure spinel structure. Powder micrometric material is obtained by solid state reaction while nanoparticles are obtained by the two other methods. In the case of the glass-ceramics process, the nanocrystals obtained are trapped in a glassy matrix. This kind of elaboration leads to nanoparticles with lower amount of defects, no surface contact with air and an increase signal of the persistent luminescence compared to disperse nanoparticles. These good persistent luminescence properties are well correlated to thermoluminescence glow curves analysis. Moreover, such glass ceramics can be used as a good tool to study more deeply the persistent luminescence process of nanoparticles in a transparent environment and samples are easier to characterize.

One of a wide-bandgap semiconductor, Zinc oxide (ZnO) has a near ultraviolet bandgap (3.37 eV) and an exciton binding energy of 60 meV at room temperature (RT), and has several favorable properties, such as high electron mobility, high oscillator strength, and good transparency. In the photoluminescence (PL) spectra of ZnO nanoparticles, the near band edge ultraviolet (UV) emission at 378 nm relevant to direct bandgap of ZnO, and blue light emissions centered at 410, 435, and 465 nm corresponding to Zn interstitial (Zni) to valence band maximum (VBM), and to Zn vacancies (VZn) and green light emission at 540 nm corresponding to conduction band maximum (CBM) to oxygen vacancy (Vo). Ultra-small size quasi consolidated ZnO-graphene nanoparticles was synthesized in which graphene outer layer was chemically attached with ZnO inner core. After attaching graphene to ZnO, green emission completely disappeared whereas the intensity of blue emission was greatly increased. Enhanced blue emission could be well described by both fast electron transfer from CBM of ZnO to graphene having similar molecular energy level with Zni and transition to VBM and Vzn. Glass/ITO/PEDOT:PSS/poly-TPD/ZnO-graphene/Cs2CO3/Al were fabricated and showed the blue emission centered at 435 nm with FWHM of about 90 nm.

Persistent luminescence and phosphorescence, both yields afterglow luminescence, but are completely different mechanisms. Persistent luminescence involves a slow thermal release of trapped electrons stored in defect states, whereas the phosphorescence is caused due to triplet to singlet transition [1,2]. Many persistent luminescence phosphors are based on oxide inorganic hosts, and exhibit long afterglow luminescence after ceasing the excitation. We observed intense and long afterglow luminescence in sol-gel/pechini grown inorganic oxides, and as a first interpretation thought to be due to persistence mechanism. However, some of these materials do not exhibit defect trap centers, and a detailed investigation suggested it is due to phosphorescence, but not the persistence. Phosphorescence is not common in inorganic solids, and that too at room temperature, and therefore usually misinterpreted as persistence luminescence [3]. Here we present a detailed methodology to distinguish phosphorescence from persistence luminescence in inorganic solids, and the process to harvest highly efficient long phosphorescence afterglow at room temperature. 1. Jian Xu, Setsuhisa Tanabe, Atul D. Sontakke, Jumpei Ueda, Appl. Phys. Lett. 107, 081903 (2015) 2. Sebastian Reineke, Marc A. Baldo, Scientific Reports, 4, 3797 (2014) 3. Pengchong Xue, Panpan Wang, Peng Chen, Boqi Yao, Peng Gong, Jiabao Sun, Zhenqi Zhang, Ran Lu, Chem. Sci. (2016) DOI: 10.1039/C5SC03739E

ZnO has attracted growing research attention as a strong candidate material for various optoelectronic device applications. It is important to understand and control the interactions between surface plasmons (SPs) and charge carriers in metal-ZnO hybrid nanostructures to improve the optical characteristics. In this work, we fabricated ZnO/Ag nanogratings using patterned polymer and Si templates. Excitation of the surface plasmon polaritons (SPPs) well explained the optical reflectance and photoluminescence spectra of the ZnO/Ag nanogratings [1,2]. Nanoscopic mapping of surface photovoltage (SPV), i.e., changes in the surface potential under illumination, obtained by Kelvin probe force microscopy (KPFM) enabled us to investigate the local behaviors of the photo-generated carriers. The magnitude and relaxation time of the measured SPV depended on the wavelength and polarization of the incident light [3]. This showed that the SP excitation in the nanogratings directly affected the creation and recombination processes of the charge carriers. All of these results suggested that SPV measurements using KPFM should be very useful for studying the SP effects in metal/semiconductor hybrid nanostructures. References [1] Gwon et al., Opt. Express 19, 5895 (2011). [2] Gwon et al., ACS Appl. Mater. Interfaces. 6, 8602 (2014). [3] Gwon et al., Sci. Rep. 5, 16727; doi: 10.1038/srep16727 (2015).

The controlling mechanism for determining the growth direction of a Ga-doped ZnO (GaZnO) nanoneedle (NN) by using an Ag nanoparticle (NP) as vapor-liquid-solid (VLS) growth catalyst is disclosed. It is found that the local Ag (111) orientation of the catalytic Ag portion in an Ag NP determines the ZnO (002) orientation of the grown GaZnO and hence the NN growth direction. The ZnO (002) plane of the grown GaZnO is always parallel with the Ag (111) planes of the Ag portions involved in VLS growth in either the top or bottom Ag NP of an NN. When GaN is used as NN growth template, at a sufficiently high temperature (350-450 degrees C), a small Ag NP can become a quasi-single crystal with its Ag (111) plane consistent with the GaN (002) plane and hence results in the growth of a vertical GaZnO NN. However, tilted NNs can be grown from a larger Ag NP or a cluster of Ag NP on GaN due to its non-uniform Ag (111) orientation distribution. At the early stage of GaZnO growth, GaZnO precipitation can be observed between Ag layers within an Ag NP, indicating the growth of a semiconductor on Ag. On other templates, like Si, sapphire, or silicon diode, single-crystal Ag NP cannot be formed such that GaZnO NNs of random orientations are grown.

Zinc oxide (ZnO) in its nanostructure form is a promising material for visible light emission/absorption and utilization in different energy efficient photocatalytic processes. We will first present our recent results on the effect of varying the molar ratio of the synthesis nutrients on visible light emission. Further we will use the optimized conditions from the molar ration experiments to vary the synthesis processing parameters like stirring time etc. and the effect of all these parameters in order to optimize the efficiency and control the emission spectrum are investigated using different complementary techniques. Cathodoluminescence (CL) is combined with photoluminescence (PL) and electroluminescence (EL) as the techniques to investigate and optimizes visible light emission from ZnO/GaN light emitting diodes. We will then show and discuss our recent finding of the use of high quality ZnO nanoparticles (NPs) for efficient photo-degradation of toxic dyes using the visible spectra, namely with a wavelength up to 800 nm. In the end, we show how ZnO nanorods (NRs) are used as the first template to be transferred to bismuth zinc vanadate (BiZn₂VO₆). The BiZn₂VO₆ is then used to demonstrate efficient and cost effective hydrogen production through photoelectrochemical water splitting using solar radiation.

The breadth of circuit fabrication opportunities enabled by metal-oxide thin-film transistors (MO-TFTs) is unprecedented. Large-area deposition techniques and high electron mobility are behind their adoption in the display industry, and substrate agnosticism and low process temperatures enabled the present wave of flexible electronics research. Reports of circuits involving complementaryMO-TFTs, oxide-organic hybrid combinations, and even MO-TFTs integrated onto Si LSI back end of line interconnects demonstrate this technology’s utility in 2D and 3D monolithic heterogeneous integration (HI). In addition to a brief literature review focused on functional HI between MO-TFTs and a variety of dissimilar active devices, we share progress toward integrating MO-TFTs with compound semiconductor devices, namely GaN HEMTs. A monolithically integrated cascode topology was used to couple a HEMT’s >200 V breakdown characteristic with the gate driving characteristic of an IGZO TFT, effectively shifting the HEMT threshold voltage from -3 V to +1 V.

Despite recent advances in metal oxide thin-film transistor technology, there are no foundry processes available yet for large-scale deployment of metal oxide electronics and photonics, in a similar way as found for silicon based electronics and photonics. One of the biggest challenges of the metal oxide platform is the stability of the fabricated devices. Also, there is wide dispersion on the measured specifications of fabricated TFT, from lot-to-lot and from different research groups. This can be partially explained by the importance of the deposition method and its parameters, which determine thin film microstructure and thus its electrical properties. Furthermore, substrate pretreatment is an important factor, as it may act as a template for material growth. Not so often mentioned, plasma processes can also affect the morphology of deposited films on further deposition steps, such as inducing nanoparticle formation, which strongly impact the conduction mechanism in the channel layer of the TFT. In this study, molybdenum doped indium oxide is sputtered onto ALD deposited HfO₂ with or without pattering, and etched by RIE chlorine based processing. Nanoparticle formation is observed when photoresist is removed by oxygen plasma ashing. HfO₂ etching in CF₄/Ar plasma prior to resist stripping in oxygen plasma promotes the aggregation of nanoparticles into nanosized branched structures. Such nanostructuring is absent when oxygen plasma steps are replaced by chemical wet processing with acetone. Finally, in order to understand the electronic transport effect of the nanoparticles on metal oxide thin film transistors, TFT have been fabricated and electrically characterized.

Brillouin light scattering (BLS) was conducted on melt-grown ZnO bulk crystals and ZnO thin films grown by pulsed laser deposition. The bulk ZnO crystals presented both longitudinal and transverse bulk acoustic waves. Theoretical calculations agreed well with there being one piezoelectric longitudinal branch and two transverse branches. BLS measurements conducted on ZnO thin films also revealed Rayleigh surface acoustic waves (R-SAW) guided by only the surface of the layer and Sezawa modes, guided by the film thickness. Measurements were conducted for three incidence angles in order to investigate different SAW wave numbers. Higher frequency features were identified as being related to a new class of guided longitudinal (LG) SAW modes which are not usually detected for ZnO thin films. The LG-SAW modes were observed for two incidence angles (θ=45° and 55°) corresponding to frequencies of 17.88 and 20.75 GHz, respectively. BLS measurements enable us to estimate the LG-SAW velocity as 6500 m/s. This value is three times higher than that of the currently used R-SAW. Theoretical simulations were coherent with the presence of LG modes in the ZnO layers. Such LG-SAW modes are promising for the development of novel, higher-speed SAW devices operating in the GHz-band and which could be readily incorporated in Si-based integrated circuitry.

Interfaces with subtle difference in atomic and electronic structures in perovskite ABO₃ heterostructures often yield intriguingly different properties, yet their exact roles remain elusive. In this article, we report an integrated study of unusual transport, magnetic, and structural properties of Pr_0.67Sr_0.33MnO₃ (PSMO) films and La_0.67Sr_0.33MnO₃ (LSMO) films of various thicknesses on SrTiO₃ (STO) substrate. In particular, using atomically resolved imaging and electron energy-loss spectroscopy (EELS), we measured interface related local lattice distortion, BO₆ octahedral rotation and cation-anion displacement induced polarization. In the very thin PSMO film, an unexpected interface-induced ferromagnetic polaronic insulator phase was observed during the cubic-to-tetragonal phase transition of the substrate STO, due to the enhanced electron-phonon interaction and atomic disorder in the film. On the other hand, for the very thin LSMO films we observed a remarkably deep polarization in non-ferroelectric STO substrate near the interface. Combining the experimental results with first principles calculations, we propose that the observed deep polarization is induced by an electric field originating from oxygen vacancies that extend beyond a dozen unit-cells from the interface, thus providing important evidence of the role of defects in the emergent interface properties of transition metal oxides.

Integration of perovskite oxides with silicon and germanium can enable the realization of novel electronics device designs and the improvement of device performance. In particular, the wide variety of perovskite oxides and their ability to grow epitaxially on silicon and germanium allows the design of monolithically integrated semiconductor devices. The fabrication of monolithically integrated metal-ferroelectric-semiconductor structures is reported. Out-of-plane orientation of BaTiO₃ ferroelectric film is demonstrated, and process considerations to ensure oxide electrode conductivity are discussed. The structures reported here demonstrate the feasibility of fabricating ferroelectric field effect devices that are monolithically integrated into silicon and/or germanium platforms.

We present several designs and experimental implementations of optical power diodes – devices that are designed to be transparent from one direction, but opaque from the other, when illuminated by a beam with sufficient intensity. Optical power diodes can be used to protect optical devices that both detect and transmit light. Our designs are based on phase-change material vanadium dioxide (VO2), which undergoes an insulator-to-metal transition (IMT) that can be triggered thermally or optically. Here, VO2 films serve as nonlinear elements that can be transformed from transparent to opaque by intense illumination. We build thin-film metallic structures on top of the VO2 films such that the optical absorption becomes asymmetric – light impinging from one direction is absorbed at a higher rate than from the other direction, triggering the transition, and turning the device opaque. This results in asymmetric transmission. The designs are optimized with finite-difference time-domain (FDTD) simulations, using optical constants of VO2 extracted using ellipsometry, and are shown to be scalable across the near- and mid-infrared. Our initial experimental results using a simple design comprised of metal and VO2 films on sapphire, designed for an operating wavelength of 1.35µm, show a transmission asymmetry ratio of ~2, and experiments with superior designs are ongoing. Future work will include the use of defect-engineered VO2 to engineer the intensity threshold of optical power diodes.

Zinc nitride films can be deposited by radio frequency magnetron sputtering using a Zn target at substrate temperatures lower than 250°C. This low deposition temperature makes the material compatible with flexible substrates. The asgrown layers present a black color, polycrystalline structures, large conductivities, and large visible light absorption. Different studies have reported about the severe oxidation of the layers in ambient conditions. Different compositional, structural and optical characterization techniques have shown that the films turn into ZnO polycrystalline layers, showing visible transparency and semi-insulating properties after total transformation. The oxidation rate is fairly constant as a function of time and depends on environmental parameters such as relative humidity or temperature. Taking advantage of those properties, potential applications of zinc nitride films in environmental sensing have been studied in the recent years. This work reviews the state-of-the-art of the zinc nitride technology and the development of several devices such as humidity indicators, thin film (photo)transistors and sweat monitoring sensors.

Quantum cascade (QC) lasers opens new prospects for powerful sources operating at THz frequencies. Up to now the best THz QC lasers are based on intersubband emission in GaAs/AlGaAs quantum well (QW) heterostructures. The maximum operating temperature is 200 K, which is too low for wide-spread applications. This is due to the rather low LO-phonon energy (36 meV) of GaAs-based materials. Indeed, thermal activation allows non-radiative path through electron-phonon interaction which destroys the population inversion. Wide band gap materials such as ZnO have been predicted to provide much higher operating temperatures because of the high value of their LO-phonon energy. However, despite some observations of intersubband absorption in c-plane ZnO/ZnMgO quantum wells, little is known on the fundamental parameters such as the conduction band offset in such heterostructures. In addition the internal field inherent to c-plane grown heterostuctures is an handicap for the design of QC lasers and detectors. In this talk, we will review a systematic investigation of ZnO/ZnMgO QW heterostructures with various Mg content and QW thicknesses grown by plasma molecular beam epitaxy on low-defect m-plane ZnO substrates. We will show that most samples exhibit TM-polarized intersubband absorption at room temperature linked either to bound-to-quasi bound inter-miniband absorption or to bound-to bound intersubband absorption depending on the Mg content of the barrier material. This systematic study allows for the first time to estimate the conduction band offset of ZnO/ZnMgO heterostructures, opening prospects for the design of QC devices operating at THz frequencies. This was supported by the European Union’s Horizon 2020 research and innovation programme under grant agreement #665107.

ZnO as an alternative electron transport layer (ETL) material for perovskite solar cell applications has drawn increasing research interest due to its comparable energy levels to TiO₂, relatively high electron mobility, as well as its feasibility to be processed at low temperatures for potential applications in flexible devices. Nevertheless, ZnO based perovskite devices usually exhibit inferior performance and severe stability drawbacks which are related to the surface defects of ZnO ETL. In this study, to investigate the correlation between ZnO defect composition and resulting device performance, different approaches of preparing ZnO ETL are compared in terms of the perovskite morphology and device performance. In addition, direct manipulations of ZnO surface defects are performed by various surface treatments, and the photovoltaic performance of devices with ZnO ETL subjected to different surface treatments is compared. Surface modification of ZnO ETL by ethanolamine (EA) is demonstrated to efficiently enhance the photovoltaic performance of resulting ZnO based devices.

ZnO-based materials show promise in energy harvesting applications, such as piezoelectric, photovoltaic and thermoelectric. In this work, ZnO-based vertical Schottky barrier solar cells were fabricated by MOCVD de- position of ZnO thin films on ITO back ohmic contact, while Ag served as the top Schottky contact. Various rapid thermal annealing conditions were studied to modify the carrier density and crystal quality. Greater than 200 nm thick ZnO films formed polycrystalline crystal structure, and were used to demonstrate Schottky solar cells. I-V characterizations of the devices showed photovoltaic performance, but but need further development. This is the first demonstration of vertical Schottky barrier solar cell based on wide bandgap ZnO film. Thin film and bulk ZnO grown by MOCVD or melt growth were also investigated in regards to their room- temperature thermoelectric properties. The Seebeck coefficient of bulk ZnO was found to be much larger than that of thin film ZnO at room temperature due to the higher crystal quality in bulk materials. The Seebeck coefficients decrease while the carrier concentration increases due to the crystal defects caused by the charge carriers. The co-doped bulk Zn_0:96Ga_0:02Al_0:02O showed enhanced power factors, lower thermal conductivities and promising ZT values in the whole temperature range (300-1300 K).

High efficiency solar conversion requires collection of a broad spectrum of wavelengths from the ultra-violet into the infrared. Solar collector mirrors must provide high reflection across this spectral band without degrading over time. This work presents the results of a high-performance 200 mm parabolic mirror coated with an ultra-wide broadband dielectric reflector. The mirror was developed to demonstrate high efficiency broadband solar collection and power conversion. Mirror reflection was measured within the limits of NIST capabilities, and averaged over 99.65% from 400 to 1800 nm with an acceptance angle of 30°. Plasma-assisted reactive magnetron sputtering was used to produce these high density and environmentally stable films. These hard oxide films can be repeatedly cleaned in the field. Salt spray, humidity and angle performance results are presented.

High quality germanium doped β-Ga₂O₃ epitaxial film was grown by PMBE technique and fabricated into a vertical type Schottky photodiode with a Pt/nGa₂O₃/n⁺Ga₂O₃(010) structure. The photodiode exhibited excellent rectifying characteristics with a turn on voltage ~ 1V and near zero bias leakage current ~ 100 fA. The photoresponse measurement showed a true solar blind sensitivity with cutoff wavelength ~260 nm and an out of band rejection ratio of ~10⁴. A maximum responsivity of 0.09 A/W at 230 nm was measured at zero bias, corresponding to an external quantum efficiency of ~52 %. The time response of the photovoltaic diode is in the millisecond range and has no long-time decay component which is very common in the MSM photoconductive wide bandgap devices. The photodiode performance remains stable up to 300°C, suggesting its potential use for high temperature applications.

MgZnO is an attractive semiconductor alloy for UV optoelectronic and electronic devices. Due to recent progress and availability of high quality Ga₂O₃ substrates and its high solar-blind bandgap of ~4.9 eV, it is desirable to investigate its application for solar-blind applications as a potential substrate alternative to sapphire for MgZnO. MgZnO alloys have been grown using plasma-assisted molecular beam epitaxy on Sn doped n-type (010) β-Ga₂O₃ substrates. It was found MgZnO growth with a MgO buffer layer has a rocksalt lattice structure. In-situ RHEED observations show that the sample grown with a MgO buffer shows two-dimensional growth and a surface roughness with root-mean-square (RMS) below 2 nm. On the other hand, MgZnO grown without a MgO buffer has a mixed phase of rocksalt and wurtzite lattice structures. Additionally, as the initial step for the fabrication of tunable wavelength solar-blind photodetectors, Schottky barrier photodetectors have been fabricated, demonstrating zero (0 V) bias responsivity of 0.1 μA/W (rocksalt MgZnO), 0.7 μA/W (mixed phase MgZnO) and 1.3 μA/W (mixed phase MgZnO) at 230 nm, 310 nm and 335 nm, respectively.

Deep-ultraviolet (DUV) photodetectors based on wide bandgap (WB) semiconductor materials have attracted strong interest because of their broad applications in military surveillance, fire detection and ozone hole monitoring. Monoclinic β-Ga2O3 with ultra-wide bandgap of ~4.9 eV is a promising candidate for such application because of its high optical transparency in UV and visible wavelength region, and excellent thermal and chemical stability at elevated temperatures. Synthesis of high qualityβ-Ga2O3 thin films is still at its early stage and knowledge on the origins of defects in this material is lacking. The conventional epitaxy methods used to grow β-Ga2O3 thin films such as molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD) still face great challenges such as limited growth rate and relatively high defects levels. In this work, we present the growth of β-Ga2O3 thin films on c-plane (0001) sapphire substrate by our recently developed low pressure chemical vapor deposition (LPCVD) method. The β-Ga2O3 thin films synthesized using high purity metallic gallium and oxygen as the source precursors and argon as carrier gas show controllable N-type doping and high carrier mobility. Metal-semiconductor-metal (MSM) photodetectors (PDs) were fabricated on the as-grown β-Ga2O3 thin films. Au/Ti thin films deposited by e-beam evaporation served as the contact metals. Optimization of the thin film growth conditions and the effects of thermal annealing on the performance of the PDs were investigated. The responsivity of devices under 250 nm UV light irradiation as well as dark light will be characterized and compared.

Nominally-undoped Ga₂O₃ layers were deposited on a-, c- and r-plane sapphire substrates using pulsed laser deposition. Conventional x-ray diffraction analysis for films grown on a- and c-plane sapphire showed the layers to be in the β-Ga₂O₃ phase with preferential orientation of the (-201) axis along the growth direction. Pole figures revealed the film grown on r-plane sapphire to also be in theβ-Ga₂O₃ phase but with epitaxial offsets of 29.5°, 38.5° and 64° from the growth direction for the (-201) axis. Optical transmission spectroscopy indicated that the bandgap was ~5.2eV, for all the layers and that the transparency was > 80% in the visible wavelength range. Four point collinear resistivity and Van der Pauw based Hall measurements revealed the β-Ga₂O₃ layer on r-plane sapphire to be 4 orders of magnitude more conducting than layers grown on a- and c-plane sapphire under similar conditions. The absolute values of conductivity, carrier mobility and carrier concentration for the β-Ga₂O₃ layer on r-sapphire (at 20Ω^-1.cm^-1, 6 cm²/Vs and 1.7 x 10¹⁹ cm^-3, respectively) all exceeded values found in the literature for nominally-undoped β-Ga₂O₃ thin films by at least an order of magnitude. Gas discharge optical emission spectroscopy compositional depth profiling for common shallow donor impurities (Cl, F, Si and Sn) did not indicate any discernable increase in their concentrations compared to background levels in the sapphire substrate. It is proposed that the fundamentally anisotropic conductivity in β-Ga₂O₃ combined with the epitaxial offset of the (-201) axis observed for the layer grown on r-plane sapphire may explain the much larger carrier concentration, electrical conductivity and mobility compared with layers having the (-201) axis aligned along the growth direction.

The effects of microwave radiation on transport properties of atomically thin superconducting La_2xSr_xCuO₄ films were studied in the 0.1 -13 GHz frequency range in the BKT state above T_c. The microwave nonlinear response is found to decrease by several orders of magnitude within a few GHz of the cutoff frequency f≈2 GHz. The results indicate that two-dimensional superconductivity is resilient against the high-frequency radiation, which is related to strong suppression of the dissociation of the vortex-antivortex pairs in 2D superconducting condensates oscillating at high frequencies.

We have grown few unit cells of epitaxial LaAlO₃ (LAO) on TiO₂ terminated SrTiO₃ (STO) substrates using oxide MBE technique, which shows an interface superconductivity below about 0.3 K. By fabricating a back gate electrode via the STO substrate, the superconductor-to-insulator transition was observed by applying gate voltages on a macroscopic size of the two-dimensional electron liquid (2DEL) at the interface of LAO/STO. From the superconducting critical field anisotropy measurements, a sizable spin-orbit coupling is found to present in the superconducting phase, where the upper limit of the spin-orbit coupling strength can be largely tuned by gate voltages. In addition, magnetotransport anomaly was observed when depleting the electron density and thus driving the 2DEL into insulating phase, suggesting an inhomogeneous density distribution and also a possible multiband conduction in the 2DEL.

In this work dependences of the electron band structure and spectral function in the HTSC cuprates on magnitude of electron-phonon interaction (EPI) and temperature are investigated. We use three-band p-d model with diagonal and offdiagonal EPI with breathing and buckling phonon mode in the frameworks of polaronic version of the generalized tight binding (GTB) method. The polaronic quasiparticle excitation in the system with EPI within this approach is formed by a hybridization of the local multiphonon Franck-Condon excitations with lower and upper Hubbard bands. Increasing EPI leads to transfer of spectral weight to high-energy multiphonon excitations and broadening of the spectral function. Temperature effects are taken into account by occupation numbers of local excited polaronic states and variations in the magnitude of spin-spin correlation functions. Increasing the temperature results in band structure reconstruction, spectral weight redistribution, broadening of the spectral function peak at the top of the valence band and the decreasing of the peak intensity. The effect of EPI with two phonon modes on the polaron spectral function is discussed.

The conventional theory of superconductivity says that charge carriers in a metal that becomes superconducting can be either electrons or holes. I argue that this is incorrect. In order to satisfy conservation of mechanical momentum and of entropy of the universe in the superconductor to normal transition in the presence of a magnetic field it is necessary that the normal state charge carriers are holes. I will also review the empirical evidence in favor of the hypothesis that all superconductors are hole superconductors, and discuss the implications of this for the search for higher T_c superconductors.

For the first time, Neel, the winner of the Nobel Prize, has applied sublattice theory to explain the magnetism of multicomponent systems. Within the bioscillation electron model a superconducting phase transition in the crystal AB is accomplished by break valence ties, the formation of paired electrons or molecule sublattices of A2 and B2: 2AB=A2+B2. Energy Φ balance equations are 2Φ²[AB]≤Φ²[A2]+Φ²[B2], Φ²[AB]≤Φ²[A2], Φ²[AB]≤Φ²[B2]. The mechanism of the superconducting phase transition in the yttrium-barium YBaCuO or other cuprates under poly oscillation electron model is examined. In the first stage there are formed yttrium, barium (or other elements) and copper oxides, in the second stage the oxides are dissociated. The molecules are formed, provided that the atom association energy is more gap energy of valence bonds in oxides. Calculations of quadratic energies for the oxides and cuprates to room temperature and 90K are performed. To superconducting phase transition has been occurred, the quadratic energy must be greater than the criterion. The cuprate with a stoichiometric composition is not a superconductor according to experimental data. The balance equations at 90K are consistent with the experimental data 406.4256*2 ─ (328.482+400.6432) = 83.726 eV². The total quadratic energy required for education Y2 and Ba2 molecules is equal to 812.8512 eV². Cuprates with the introduction of additional oxygen typeYBa2Cu3O6.5 + 0.5 are superconductors. The energies of the valence bonds are reduced the introduction of oxygen above stoichiometric values by expanding crystal lattice.

There is an increasing demand for sensors to monitor environmental levels of ultraviolet (UV) radiation and pollutant gases. In this work, an individual nanowire of Pd modified ZnO nanowire (ZnO:Pd NW) was integrated in a nanosensor device for efficient and fast detection of UV light and CH₄ gas at room temperature. Crystalline ZnO:Pd nanowire/nanorod arrays were synthesized onto fluorine doped tin oxide (FTO) substrates by electrochemical deposition (ECD) at relative low-temperatures (90 °C) with different concentrations of PdCl₂ in electrolyte solution and investigated by SEM and EDX. Nanodevices were fabricated using dual beam focused electron/ion beam (FIB/SEM) system and showed improved UV radiation response compared to pristine ZnO NW, reported previously by our group. The UV response was increased by one order in magnitude (≈ 11) for ZnO:Pd NW. Gas sensing measurements demonstrated a higher gas response and rapidity to methane (CH₄ gas, 100 ppm) at room temperature, showing promising results for multifunctional applications. Also, due to miniature size and ultra-low power consumption of these sensors, it is possible to integrate them into portable devices easily, such as smartphones, digital clock, flame detection, missile lunching and other smart devices.

Solution deposition has potential for highly cost-effective fabrication of thin film transistors (TFTs) on flexible substrates. Shape memory polymer (SMP), with improved thermal mechanical response, may enable large-area flexible devices, as well as add control to the product shape and modulus. Until date, TFTs made on SMP substrates have been limited to vacuum-deposition methods. While TFTs processed through more economical solution-based techniques achieve device performance close to their vacuum-processed counterparts, they have not yet been demonstrated on SMP substrates due to the required high calcination temperatures (> 500 °C). To take full advantages of SMP, low temperature (< 200 °C) solution-based processing is highly desirable. Compatibility of the deposition process with the substrate and previously deposited films is essential. Here, we develop a process that incorporates direct UV patterning that would allow for fabrication of oxide TFTs on SMP using a reduced number of processing steps. Rigid In₂O₃ TFTs, deposited from solution-combustion synthesis, are fabricated on Si substrates with different solution-deposited dielectrics to evaluate their potential for transferring to SMP.

In this ultra fast computing era power optimization is a major technological challenge that requires new computing paradigms. Conservative and reversible logic opens up the possibility of ultralow power computing. In this paper, basic reversible logic gate (double Feynman gate) using the lithium-niobate based Mach-Zehnder interferometer is proposed. The results are verified using beam propagation method and MATLAB simulations.

ZnO is gaining substantial interest day by day because of its wide bandgap (3.4 eV) and large exciton binding energy (60 meV) due to which lasing emission is possible from ZnO based materials even above room temperature. Here we are reporting the influence of growth temperature and annealing ambient on photoluminescence properties, crystalline size and surface morphology of ZnO thin films deposited on Si substrates at 200°C by RF sputtering. Achieved thickness is 198 nm as confirmed by Profilometer. Grown samples were further rapid thermal annealed at 800°C in Ar, N₂, O₂, and in vacuum ambient. The as-grown sample did not exhibit any near band edge emission peak due to presence of deep level defects. Low temperature (18 K) photoluminescence spectra exhibited strong emission peak around 3.32 eV when the as-grown sample was annealed at 800° C in oxygen ambient which indicates defects state passivation. A lowest full width half maximum (FWHM) of 73.85meV was achieved for sample annealed in O₂ ambient .Sample annealed in vacuum showed peak with highest intensity at 3.25eV, which corresponds to donor-bound-acceptor (DAP). High resolution Xray diffraction measurement exhibited a dominant <002> peak. Atomic Force Microscopy also revealed surface roughness of 7.72 nm for sample annealed in O₂ ambient.

Mn₂O₃ is a promising anode material for lithium ion battery. Two different kinds of structures of Mn₂O₃ were synthesized via solution processes, the Mn₂O₃ porous cubes and hollow spheres. Scanning electron microscope images and transmission electron microscopy images clearly show the structures. Electrochemical impedance spectroscopy and cyclic voltammetry measurements were used to characterize their electrochemical properties. As anode materials for lithium ion batteries, Mn₂O₃ porous cubes performed similarly as Mn₂O₃ hollow spheres. Both samples started with high initial capacities (1583.2 mAh/g and 1550.7 mAh/g) which were reduced to 173.3 mAh/g and 162.0 mAh/g at 100th cycle at a current density of 100 mA/g. The decrease is likely due to morphology destruction the materials in charging and discharging process.

Zn _0.85Mg _0.15O a promising material for the future in the area of the optoelectronic devices due to the flexibility of changing bandgap. The impact of thermal annealing on Zn _0.85Mg _0.15O thin films grown by RF sputtering on intrinsic Si substrate by RF sputtering at constant temperature 400°C. During deposition gas flow 80% Argon and 20% oxygen was used. The samples were rapid thermal annealed at 900°C (20 sec) and 950°C for 20 and 30 sec to yield samples A, B and C, respectively. Low temperature photoluminescence (PL) measurements show presence of violet emission around 3.1 eV in as-grown sample due to the presence of zinc interstitial defects. Near-band-edge emission was found at around 3.65 eV for sample A. However, for sample B this peak was redshifted and found around 3.63 eV but with much higher intensity. Further increase on annealing time (30 sec) sample was further red-shifted (sample C). On comparing with sample a, sample B showed 3 times enhancement in PL intensity and 30 times enhancement compared to as grown sample. X-ray diffraction measurements confirmed the growth of highly c-axis oriented <002> Zn _0.85Mg _0.15O thin films for all samples. Uniform lattice constant (a= 0.29 and c= 0.51 nm) was achieved for all annealed samples. The <002> peak for all annealed samples shows higher intensity in comparison with the as-grown. A slight shift in the peak was observed which is due to presence of strain. For sample B surface roughness were measured 6.34nm.

We have investigated the preparation of NiO layers by cathodic electrodeposition in various organic-based solvents, namely ethanol, dimethyl sulfoxide (DMSO), DMSO/2 vol.% H₂O and DMSO/25 vol.% H₂O mixtures. The layers were formed from the electrochemical reduction of nickel nitrate precursor. We show that, depending on the solvent used, various nickel compounds were deposited. In the case of ethanol, a transparent precursor layer was obtained that was transformed into NiO after an annealing treatment at 300°C. For DMSO and DMSO with 2 volume % of H2O, adherent, well-covering, mesoporous and rather thick NiO layers were obtained after an annealing treatment at 450°C. These layers, after growth, contained nickel oxide or hydroxide, metallic nickel and DMSO. The solvent acted as a blowing agent, being included in the deposit and giving rise to a mesoporous film after its elimination by thermal annealing. These porous layers of p-type oxide have been successfully sensitized by a push-pull dye (P1 dye) and showed photocurrent generation and an open circuit voltage (V_oc) up to 167 mV in p-type dye-sensitized solar cells (p-DSSCs). For DMSO with 25 volume % of H₂O, the deposited layers contained more metallic nickel and were dense even after annealing. They were unsuitable in p-DSSCs.

In this work, we study the crystal morphology, electron transport and photo-electroluminescence of erbium-doped nanolaminates of ZnO, TiO₂ and SiO₂, deposited by RF-sputtering, with and without an annealing step. The effect of the nanolaminate on the interface roughness, erbium distribution on the laminate and its correlation to the photo- and electroluminescence (visible and infrared domains) is presented. A discussion on potential nanoscale electrical excitation pathways of active erbium species is also presented.

Graphene/oxide composite structures are attracting increasing attention for many advanced applications. In the present work, mesoporous layers composed of TiO₂ nanoparticles and graphene at various concentrations have been coated on conductive glass substrates. They have been tested for the photocatalytic degradation of 4-chlorophenol used as a model compound of an eco-persistent pollutant dilute in water. The formation of intermediate degradation products, namely, hydroquinone and benzoquinone, has been followed. The results show the high photocatalytic activity of the layers and a beneficial effect of graphene for an optimum concentration of 1.2 wt. %. The decrease in the activity observed at higher graphene content is assigned to the light absorption by this component. The key parameters for the enhancement of the photocatalytic performance are discussed.

The most important parameters of gas sensors are sensitivity and especially high selectivity to specific chemical species. To improve these parameters we developed sensor structures based on layered semiconducting oxides, namely CuO/Cu₂O, CuO:Zn/Cu₂O:Zn, NiO/ZnO. In this work, the ZnO/Cu_xO (where x = 1, 2) bi-layer heterostructure were grown via a simple synthesis from chemical solution (SCS) at relatively low temperatures (< 95 °C), representing a combination of layered n-type and p-type semiconducting oxides which are widely used as sensing material for gas sensors. The main advantages of the developed device structures are given by simplicity of the synthesis and technological cost-efficiency. Structural investigations showed high crystallinity of synthesized layers confirming the presence of zinc oxide nanostructures on the surface of the copper oxide film deposited on glass substrate. Structural changes in morphology of grown nanostructures induced by post-grown thermal annealing were observed by scanning electron microscopy (SEM) investigations, and were studied in detail. The influence of thermal annealing type on the optical properties was also investigated. As an example of practical applications, the ZnO/Cu_xO bi-layer heterojunctions and ZnO/CuO/Cu₂O three-layered structures were integrated into sensor structures and were tested to different types of reducing gases at different operating temperatures (OPT), showing promising results for fabrication of selective gas sensors.

Methods for simultaneously increasing the conductivity and the porosity of NiO layers grown by pulsed laser deposition (PLD) were investigated in order to develop improved photocathodes for p-DSSC applications. NiO:Li (20at%) layers grown on c-Al₂O₃ by PLD showed a sharp drop in conductivity with increasing substrate temperature. Layers grown at room temperature were more than two orders of magnitude more conductive than undoped NiO layers but did not show evidence of any porosity in Scanning Electron Microscope (SEM) images. A new method for imposing a nanoporosity in NiO was developed based on a sacrificial template of nanostructured ZnO. SEM images and EDX spectroscopy showed that a nanoporous morphology had been imprinted in the NiO overlayer after preferential chemical etching away of the nanostructured ZnO underlayer. Beyond p-DSSC applications, this new process could represent a new paradigm for imprinting porosity in a whole range of materials.