Table of contents

The Special issue presents the papers for the INERA Workshop entitled "Transition Metal Oxides as Functional Layers in Smart windows and Water Splitting Devices", which was held in Varna, St. Konstantin and Elena, Bulgaria, from the 4th-6th September 2014.

The Workshop is organized within the context of the INERA "Research and Innovation Capacity Strengthening of ISSP-BAS in Multifunctional Nanostructures", FP7 Project REGPOT 316309 program, European project of the Institute of Solid State Physics at the Bulgarian Academy of Sciences.

There were 42 participants at the workshop, 16 from Sweden, Germany, Romania and Hungary, 11 invited lecturers, and 28 young participants. There were researchers present from prestigious European laboratories which are leaders in the field of transition metal oxide thin film technologies. The event contributed to training young researchers in innovative thin film technologies, as well as thin films characterization techniques. The topics of the Workshop cover the field of technology and investigation of thin oxide films as functional layers in "Smart windows" and "Water splitting" devices. The topics are related to the application of novel technologies for the preparation of transition metal oxide films and the modification of chromogenic properties towards the improvement of electrochromic and termochromic device parameters for possible industrial deployment.

The Workshop addressed the following topics:

• Metal oxide films-functional layers in energy efficient devices;

• Photocatalysts and chemical sensing;

• Novel thin film technologies and applications;

• Methods of thin films characterizations;

From the 37 abstracts sent, 21 manuscripts were written and later refereed.

We appreciate the comments from all the referees, and we are grateful for their valuable contributions.

Guest Editors:

• Assoc. Prof. Dr.Tatyana Ivanova

• Prof. DSc Kostadinka Gesheva

• Prof. DSc Hassan Chamatti

• Assoc. Prof. Dr. Georgi Popkirov

Workshop Organizing Committee

• Prof.DSc Kostadinka Gesheva, Central Laboratory of Solar Energy and New Energy Sources, Bulgarian Academy of Sciences (CL SENES-BAS) - Chairperson

• Assoc. Prof. Dr Anna Szekeres - Institute of Solid State Physics- BAS

• Assoc. Prof Dr. Tatyana Ivanova - CL SENES -BAS

• Assist. Prof. Radostina Kamburova - ISSP-BAS

All papers published in this volume of Journal of Physics: Conference Series have been peer reviewed through processes administered by the proceedings Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

Vanadium dioxide, VO₂, is a widely studied thermochromic material with potential applications in energy efficient window technology. It undergoes a first-order metal-to- insulator transition, accompanied by a crystal structure transformation from monoclinic to tetragonal rutile, at a critical temperature of 68 °C. Below this temperature, VO₂ is semiconducting and infrared transmitting whereas it is metallic and infrared reflecting above the transition temperature. However, in order to achieve significant thermochromic switching, the luminous transmittance of thin films will typically be less than 50%. Here we report on recent research to improve the luminous transmittance as well as the transmittance change at the transition temperature. We systematically evaluate the effect of antireflection coatings, doping with Mg and the performance of coatings comprising thermochromic nanoparticles in a transparent matrix. The last option is shown to give the best performance and holds great promise for practical applications.

Transition metal oxides (TMO) exhibit electrochromic effect. Under a small voltage they change their optical transmittance from transparent to collored (absorbing) state. The individual material can manifest its electrochromic properties only when it is part of electrochromic (EC) multilayer system. Smart window is controlling the energy of solar flux entering the building or car and makes the interiors comfortable and energy utilization more effective. Recently the efforts of material researchers in this field are directed to price decreasing. APCVD technology is considered as promissing as this process permits flowthrough large-scale production process. The paper presents results on device optimization based on WO₃-MoO₃ working electrode. Extensive research reveals that WO₃-MoO₃ structure combines positive features of single oxides: excellent electrochromic performance of WO₃ and better kinetic properties of MoO₃ deposition. The achieved color efficiency of APCVD WO₃-MoO₃ films is 200cm²/C and optical modulation of 65-70% are practically favorable electrochromic characteristics. To respond to low cost requirement, the expensive hexacarbonyl can be replaced with acetylacetonate. We have started with this precursor to fabricate mixed W_xV_1-xO₃ films. The films possess excellent surface coverage and high growth-rate. CVD deposition of VO₂, a promissing thermochromic thin film material is also presented.

One of the most studied classes of materials in the modern microelectronic devices are the metal oxides. There are different metal oxide films, such as electrodes, charge injecting and electrochromic coatings for displays or "smart" windows applications. This paper aims to describe the recent achievements for oxide coating deposition for flexible electrochromic displays. Although many deposition methods for production of such films have been developed, some of the achievements in the field of RF sputtering of transparent electrodes from indium-tin oxide on low-cost polyethyleneterephthalate substrate are presented. Attention is paid on some critical issues, such as films electro-optical parameters (sheet resistance, transparency in the visible range), adhesion, degradation due to stress and patterning ability.

We report on electrochromic properties of stoichiometric and oxygen-deficient amorphous films, denoted aWO_3-x and aTiO_2-x, under Li⁺-ion-electron inter/deintercalation. Optical characterization of the films in their as-deposited, fully intercalated (dark), and bleached states were performed by in-situ optical transmittance measurements. We explore electrochromism and optical absorption phenomena in the context of oxygen deficiency and nanostructure. Studies by cyclic voltammetry suggest good optical modulation and charge capacity upon Li⁺-ion-electron inter/deintercalation for almost stoichiometric films.

An explorative study was performed on sputter-deposited thermochromic VO₂ films with top coatings of Al oxide and Al nitride. The films were exposed to dry air at a high temperature. Bare 80-nm-thick VO₂ films rapidly converted to non-thermochromic V₂O₅ under the chosen conditions. Al oxide top coatings protected the underlying VO₂ films and, expectedly, increased film thickness yielded improved protection. Specifically, it was found that a 30-nm-thick sputter-deposited Al oxide top coating delayed the oxidation by more than one day upon heating at 300°C. The results demonstrate the importance of protective layers in thermochromic windows for practical application.

Porous Ni oxide thin films were deposited on unheated ITO/glass substrates by sputtering in argon-oxygen. The as-deposited thin films have a cubic NiO structure and still exhibit such a structure after 10,000 electrochemical cycles in 1 M LiClO₄ in propylene carbonate in the range between 2.0 and 4.1 V vs Li/Li⁺. Electrochromic performance showed a rapid drop of charge density over the first hundreds of cycles and subsequently a very slow decrease. The charge density was 87% of the initial one after 1,000 cycles and 82% after 10,000 cycles, indicating an extremely slow decay after 1,000 cycles. Optical modulation was also slightly decreased after 10,000 cycles, which is due to the drop of charge density.

Photonic polystyrene (PS) opals with face-centered cubic structure were fabricated by convective evaporation. The influences of substrate and its physical properties, as well as deposition conditions were investigated. It is shown that the surface roughness must be less than about 30% of the bead diameter to form well- ordered opals, rendering substrates such as glass, ITO and quartz superior to thick Kapton, ordinary tape, thermal release semiconductor tape, plastic and hydrophilic plastic HHNW W (Kemafoil). Periodic stripe-like structures were found to form perpendicular to the growth direction defined by the receding meniscus of the solution front when the PS concentration is lower than 1.0 w/v%. Finally, we present the principles and results of using soft sacrificial layer deposited on quartz substrates to fabricate free standing inverse opal structures.

In this paper we investigate the possibility of incorporating nitrogen into amorphous, photocatalytic TiO₂ thin films, prepared at room temperature, during the growth process. The aim is to reduce the bandgap of the UV active thin films. Physical vapor deposition experiments employing a titanium vacuum arc with gas backfill ranging from pure oxygen to pure nitrogen, are carried out. The resulting films are characterized for chemical composition, phase composition, optical properties and hydrophilicity in order to determine a correlation between gas composition and thin film properties. The experimental results point that a visible change in the band structure of the deposited layers is achieved.

The possibility to increase human comfort in buildings is a powerful driving force for the introduction of new technology. Among other things our sense of comfort depends on air quality, temperature, lighting level, and the possibility of having visual contact between indoors and outdoors. Indeed there is an intimate connection between energy, comfort, and health issues in the built environment, leading to a need for intelligent building materials and green architecture. Photocatalytic materials can be applied as coatings, filters, and be embedded in building materials to provide self-cleaning, antibacterial, air cleaning, deodorizing, and water cleaning functions utilizing either solar light or artificial illumination sources – either already present in buildings, or by purposefully designed luminaries. Huge improvements in indoor comfort can thus be made, and also alleviate negative health effects associated with buildings, such as the sick-house syndrome. At the same time huge cost savings can be made by reducing maintenance costs. Photocatalytic oxides can be chemically modified by changing their acid-base surface properties, which can be used to overcome deactivation problems commonly encountered for TiO₂ in air cleaning applications. In addition, the wetting properties of oxides can be tailored by surface chemical modifications and thus be made e.g. oleophobic and water repellent. Here we show results of surface acid modified TiO₂ coatings on various substrates by means of photo-fixation of surface sulfate species by a method invented in our group. In particular, we show that such surface treatments of photocatalytic concrete made by mixing TiO₂ nanoparticles in reactive concrete powders result in concrete surfaces with beneficial self-cleaning properties. We propose that such approaches are feasible for a number of applications in the built environment, including glass, tiles, sheet metals, plastics, etc.

This paper reviews the various lD nanostructures, which were prepared by electrospinning and atomic layer deposition (ALD). On the one hand, electrospinning served to make sacrificial polymer templates for the ALD growth; and thus various single or multilayer inorganic nanotubes were obtained. On the other hand, polymer, polymer/inorganic or inorganic nanowire templates were produced by electrospinning. By a consecutive ALD reaction various core/shell nanowires were synthesized.

Different crystal facets of anatase TiO₂ are known to have different chemical reactivity; in particular the {001} facets which truncates the bi-tetrahedral anatase morphology are reported to be more reactive than the usually dominant {101} facets. Anatase TiO₂ thin films were deposited by reactive DC magnetron sputtering in Ar/O₂ atmosphere and were characterized using Rietveld refined grazing incidence X-ray diffraction, atomic force microscopy and UV/Vis spectroscopy. By varying the partial O2 pressure in the deposition chamber, the degree of orientation of the grains in the film could be systematically varied with preferred <001> orientation changing from random upto 39% as determined by March-Dollase method. The orientation of the films is shown to correlate with their reactivity, as measured by photo-degradation of methylene blue in water solutions. The results have implications for fabrication of purposefully chemically reactive thin TiO₂ films prepared by sputtering methods.

Formaldehyde is a volatile organic compound and a harmful indoor pollutant contributing to the "sick building syndrome". We used advanced gas deposition to fabricate highly porous nickel oxide (NiO) thin films for formaldehyde sensing. The films were deposited on Al₂O₃ substrates with prefabricated comb-structured electrodes and a resistive heater at the opposite face. The morphology and structure of the films were investigated with scanning electron microscopy and X-ray diffraction. Porosity was determined by nitrogen adsorption isotherms with the Brunauer-Emmett-Teller method. Gas sensing measurements were performed to demonstrate the resistive response of the sensors with respect to different concentrations of formaldehyde at 150 °C.

Very thin titanium dioxide (TiO₂) films of less than 10 nm were deposited by atomic layer deposition (ALD) in order to study their gas sensing properties. Applying the quartz crystal microbalance (QCM) method, prototype structures with the TiO₂ ALD deposited thin films were tested for sensitivity to NO₂. Although being very thin, the films were sensitive at room temperature and could register low concentrations as 50-100 ppm. The sorption is fully reversible and the films seem to be capable to detect for long term. These initial results for very thin ALD deposited TiO₂ films give a promising approach for producing gas sensors working at room temperature on a fast, simple and cost-effective technology.

The research was fixed on sensing behavior of ZnO nanostructured (NS) films to NO₂ concentrations in the environment. The ZnO NS layers are deposited by electrochemical method on quartz resonators with Au electrodes. The sorption properties of ZnO layers were defined by measuring the resonant frequency shift (Δf) of the QCM-ZnO structure for different NO₂ concentrations. The measurements were based on the correlation between the frequency shift of the QCM and additional mass loading (Δm) on the resonator calculated using Sauerbrey equation for the AT-cut quartz plate. Frequency – Time Characteristics (FTCs) of the samples were measured as a function of different NO₂ concentrations in order to define the sorption abilities of ZnO layers. The experiments were carried out on a special set up in a dynamical regime. From FTCs the response and the recovery times of the QCM-ZnO structure were measured with varying NO₂. Frequency shift changed from 23 Hz to 58Hz when NO₂ was varied in the range of 250ppm – 5000ppm. The process of sorption was estimated as reversible and the sorption as physical. The obtained results demonstrated that QCM covered with the electrochemically deposited nanostructured ZnO films can be used as application in NO₂ sensors.

In this paper microsensing elements with thin piezoelectric films are presented. Three different materials from the basic types – ceramic, metal oxide and polymer, in microsystems for environment parameters control are used.

Zinc oxide (ZnO) and lead zirconium titanate (PZT) by sputtering technology from hot targets are deposited. The optimal parameters of the process and thickness of the layers are presented. Layers of piezoelectrical polymer poly (vinylidene fluoride) (PVDF) were prepared by spray deposition technique.

The microelements (MEMS) with these piezoelectric films are proposed for direct and indirect measuring of parameters as pressure, vibration, gas and liquid sensing, fluid flow, etc.

This paper show results from the development of transparent conductive oxides (TCO's) on large areas for the use as front electrode in thin film silicon solar modules. It is focused on two types of zinc oxide, which are cheap to produce and scalable to a substrate size up to 6 m². Low pressure CVD with temperatures below 200°C can be used for the deposition of boron doped ZnO with a native surface texture for good light scattering, while sputtered aluminum doped ZnO needs a post deposition treatment in an acid bath for a rough surface. The paper presents optical and electrical characterization of large area samples, and also results about long term stability of the ZnO samples with respect to the so called TCO corrosion.

For analyzing the long-term behavior of thin film a-Si/μc-Si photovoltaic modules, it is important to observe the light-induced degradation (LID) in dependence of the temperature for the parameters of the one-diode model for solar cells. According to the IEC 61646 standard, the impact of LID on module parameters of these thin film cells is determined at a constant temperature of 50°C with an irradiation of 1000 W/m² at open circuit conditions. Previous papers examined the LID of thin film a-Si cells with different temperatures and some others are about a-Si/μc-Si. In these previous papers not all parameters of the one-diode model are examined. We observed the serial resistance (Rs), parallel resistance (Rp), short circuit current (Isc), open circuit voltage (Uoc), the maximum power point (MPP: Umpp, Impp and Pmpp) and the diode factor (n). Since the main reason for the LID of silicon-based thin films is the Staebler Wronski effect in the a-Si part of the cell, the temperature dependence of the healing of defects for all parameters of the one-diode model is also taken into account. We are also measuring modules with different kind of transparent conductive oxides: In a-Si thin film solar cells fluorine-doped tin oxide (FTO) is used and for thin film solar cells of a-Si/μc-Si boron- doped zinc oxide is used. In our work we describe an approach for transferring the parameters of a one-diode model for tandem thin film solar cells into the one-diode model for each part of the solar cell. The measurement of degradation and regeneration at higher temperatures is the necessary base for optimization of the different silicon-based thin films in warm hot climate.

The results of study of the optical and structural properties of ZnO nanorods (NR) arrays electrochemically deposited on two type substrates – the ITO surface on the front side of Si heterojunction (SHJ) solar cells and on stainless steel plate used for formation of a-Si:H thin film solar cells, are reported. The surface morphology of the NS arrays is examined by Scanning Electron Microscopy and AFM. The spectra of specular diffused and total reflection, and haze ratio in reflectance are compared before and after deposition of the ZnO NR arrays. In the case of deposition on ITO surface of SHJ solar cells the values of the direct and diffused reflection of the ZnO NR array decrease demonstrating good antireflection properties. Deposition of ZnO NS arrays on stainless steel plates leads to increasing the values of the diffused reflection and the total reflectance. Possible application of ZnO NS structures for the processing of advanced Si based solar cells for increasing light harvesting is discussed.

The aim of this study is to obtain and characterize the ferric oxides/(oxy)hydroxides formed after cultivation of bacteria under laboratory conditions. The pure cultures of these bacteria isolated from natural habitats are identified by the methods of classical and molecular taxonomy as strains of the Leptothrix genus. Adler (AM) and Silicon iron glucose peptone (SIGP) media are the most appropriate ones for obtaining the iron oxides. The characterization of the oxides and sheaths is performed by different physical methods. The sheaths are formed in a SIGP medium. Light micrograph images and SEM revealed the average size and diameter of the sheaths. The XRD measurements showed the composition of the oxides obtained, as well as the average size of the iron particles (up to 30 nm). The TEM micrographs showed the shape of the biogenic nanoparticles, while the magnetic measurements demonstrated the superparamagnetic character of the magnetic part of the biomaterials. The new biogenic materials are promising for application in magneto electronic for building biosensors.

ZnO nano structured (NS) arrays are grown by electrochemical method on conductive seeding layers of ZnO doped with V (ZnO:V) deposited by magnetron sputtering on glass substrates. The surface morphology of the ZnO NS is studied by Scanning Electron Microscopy. The spectra of transmittance, diffused and specular reflectance are measured for the ZnO NS arrays deposited for 30 and 60 min. and are compared to the corresponding spectra of the substrate with seeding ZnO:V film. The values of diffused reflection are higher than those of the seeding layers due to the grown of nanostructured ZnO arrays. Increasing of the diffused reflection with the time of electrochemical growth is observed and is explained by increasing size of the grains. The results show the possibility for deposition of nanostructured ZnO arrays on low cost ZnO:V seeding layers with large effective surface area that could be applied in thin film solar cells for increasing light harvesting.

In this work we examine the opportunity to combine the advantages of two well- known experimental techniques – ellipsometry and scanning electron microscopy (SEM). Ellipsometry has very high sensitivity in the direction normal to the sample's surface while the lateral resolution is limited by the width of the probe beam, while the scanning electron microscopy has better lateral, but limited in-depth resolution. The electron beam can induce local changes in the electron density and the temperature of the sample, which alters the reflectance coefficient and can be potentially detected by optical methods and ellipsometry in particular.