Recent Advances in Thin Films

Thin film is a two dimensional material layer deposited on a substrate in order to achieve properties that cannot be easily achieved or not realized at all by the same material in its bulk form. The distinct property of a thin film is resulting from the unique way of making it, in the form of progressive addition of atoms or molecules. Thickness is the fundamental property of thin film and is closely linked to other properties which scale differently with thickness. Thus, thin films are not defined by their thickness alone. Thin films have a range of properties, based on their thickness used in a number of applications such as in optical coatings, tribological coatings, quantum well structures based on supper lattices, magnetic multilayers, nanoscale coatings, etc. Here effort is made to revisit the definition of thin film by highlighting significance of its thickness and briefly discussed the role of thicknesses in some of the applications such as monolayer, nanoscale coatings and multilayer and superlattice structures.

Complex metallic alloys (CMAs) may be defined as those intermetallic compounds having large (>~nm) unit cell dimensions. This includes quasi crystals as a special case, the unit cell being infinite. The discovery of quasi crystals motivated the study of CMAs, and the surface science community became active in the field once stable samples of sufficient size were produced. While the initial surface science activity centred on clean surface preparation, increasingly the formation of thin films, both metallic and molecular grew in importance. In this chapter, we give a brief introduction to this topic and then focus on several current areas of interest. These include the growth and characterization of ultra-thin metallic films of diverse architectures, the formation through deposition of novel molecular overlayers and thin films, complex intermetallics such as surface alloys and the potential use of intermetallic surfaces for catalytic reactions.

Growth defects are imperfections in coating microstructure at the size level in the order of 0.1–1 µm. Though they are encountered in most techniques of thin film deposition, this paper is generally limited to hard protective coatings deposited by physical vapor deposition. Most results have been obtained by magnetron sputtering. The starting point of a growth defect is a seed, which may have a geometrical origin; it may be an inclusion or a foreign particle. Methods to analyze individual growth defects are presented, with an emphasis on focused ion beam. Using this technique, types of defects are discussed based on seed type, their evolution, and consequences, particularly in terms of corrosion resistance. Experiments involving a single growth defect are presented too. A different approach is a statistical analysis on growth defect density, predominately limited to nodular defects. Stylus profilometry is proposed as the principle technique; however, poor reproducibility should be taken into account and thus interpretation taken accordingly.

In the present scenario, semiconducting material based devices with new capabilities are redefining the existing technologies. The developments in III-Nitride thin-film technology have produced significant advances in high-performance optoelectronic and photovoltaic devices. However, the quality of the material is an important factor for the fabrication of nitride-based efficient devices. For instance, a large difference in the covalent bond radius of Gallium and Nitrogen atoms results in high dislocation densities in the III-Nitride compound, Gallium Nitride (GaN) which is sturdily ruled by residual strain in the GaN films. Another main source of residual strain is the lack of an appropriate lattice-matched substrate that will critically impact the optoelectrical performance of the fabricated devices. So, this chapter illuminates the solution to key challenges and elaborate on various parameters to grow GaN by using plasma-assisted molecular beam epitaxy (PAMBE) technique. Numerous efforts have been made for improving the quality of GaN semiconductor for high performance of device operation. This chapter elucidates the role of the growth variables towards high-quality epitaxial GaN films and discusses the stress-relaxation controlled defect minimization in detail. Also, it provides an in-depth understanding towards structural and interface quality of multilayered GaN/AlN heterostructure grown on c-plane sapphire substrate. Therefore, this chapter contributes distinctly to understanding the growth dynamics in GaN films and subsequently in GaN/AlN based multilayered heterostructures for next-generation promising nitride-based devices.

Volume contraction or higher density of any bulk material is usually obtained by placing the material under high-pressure conditions. For materials in the form of thin films, a compressed (higher density) state may be obtained during growth without the necessity of any external pressure. Here, we present cobalt thin films, grown on silicon, which show the formation of high-density layers within the film. Normal cobalt is ferromagnetic. Theoretical calculations have shown that cobalt can be non-magnetic when its density increases beyond a specific value. Formation of this high-density (HD) non-magnetic (NM) state of cobalt in cobalt thin films has been revealed and confirmed via various experiments. The non-magnetic state of cobalt is of great interest. Ferromagnetism and superconductivity are known to be antagonistic. When ferromagnetic normal cobalt becomes non-magnetic, it raises the possibility of being a superconductor. Indeed both experiments and theory have shown the high-density non-magnetic cobalt to be a superconductor. The cobalt films have grown in a trilayer structure—HDNM Co/normal Co/HDNM Co. Thus, it is a self-organized superconductor (S)/ferromagnet (F)/superconductor (S) hybrid structure. S/F/S hybrid structures have potential applications in areas like spintronics and quantum information technology.

Nitrides of 3d ferromagnetic metals (Fe, Co and Ni) or transition metal nitrides (TMNs) appearing late (Group 8–10) in the 3d series are emerging compounds in a wide range of areas such as spintronics, magnetic devices, hard coatings, catalysts for hydrogen and oxygen evolution reaction for electrochemical water splitting, ion batteries, high energy density materials, etc. Some of these TMNs can be synthesized using non-equilibrium processes such as physical vapour deposition or under extremely high pressure and high temperatures. Thermodynamical constraints arisen due to high enthalpy of formation not only make the synthesis of late TMNs difficult, but also thus formed TMNs are also metastable. The thermal stability of late TMN is closely related to metal and nitrogen self-diffusion processes. We performed both Fe and N self-diffusion measurements in various Fe-N compounds and also explored ways to control it through effective dopants. It was found that in magnetic Fe-N compounds (N\({at.}\)% < 25\(\%\)), N self-diffusion is orders of magnitude faster than Fe, but in non-magnetic Fe-N compounds (N\({at.}\)% \(\approx \)30 and 50\(\%\)), it was surprisingly found to be the other way round. The mechanism related to self-diffusion processes in different Fe-N phases is presented and discussed. This chapter is divided into five sections. In Sect. 1, after a brief introduction of TMNs, different phases and the structure of iron nitrides have been discussed with a brief timeline and applications. Synthesis of different iron nitride thin films using reactive sputtering process, their thermal stability and the effect of dopant on the thermal stability is presented in Sect. 2. Section 3 presents methods and techniques for self-diffusion measurements. In Sect. 4, detailed self-diffusion measurements in magnetic as well as non-magnetic Fe-N compounds are given and also the effect of dopants on self-diffusion process is discussed. This chapter ends with conclusions presented in Sect. 5.

X-ray photoelectron spectroscopy (XPS) used in quantitative chemical analysis of solid surfaces requires subtraction of a broad background, arising from various energy loss mechanisms, to obtain reliable core-level peak intensities. Besides single-electron excitation, collective electron oscillations (plasmons) can be excited in the bulk and at the surface. Photoelectron energy loss spectroscopy (XPS-PEELS) is a non-destructive tool useful for both process control and thin-film metrology. This review emphasizes its versatility to elucidate material research issues. The energy loss function (ELF) is useful for thin-film growth optimization since it gives insight in valence electron density, hardness, optical band gap and interface properties such as adhesion and wetting. XPS-PEELS also provides depth and width of implanted atom profiles in solid targets, e.g. Ar nanobubbles in Al. Special emphasis is given to the retrieval of electronic properties from XPS-PEELS data. Since the ELF, 〈Im[−1/ε(q, ω)]〉_q is related to the q-averaged dielectric function, 〈ε(q, ω)〉_q, the latter can be obtained by taking into account multiple bulk and surface plasmon excitations. This task is rather simple in wide band gap materials, where the ELF and the no-loss peak are clearly separated, as illustrated by amorphous silicon, amorphous carbon or Al oxide data. In contrast, in metals or small band gap materials, the broad asymmetric photoemission peak overlaps the ELF and low-energy features in the ELF may be lost. A Fourier transform (FT) method is proposed to analyse PEELS data, with the objective of retrieving such low-energy excitations, e.g. interband transitions. This FT method is compared with an empirical method based on a smooth cutoff of the zero-loss peak, using PEELS data obtained from Al₂O₃. Current developments of a quantum mechanical theory are crucial to obtain the respective contributions of intrinsic and extrinsic plasmon excitation (along with their interference) and to assess some approximations performed in classical treatments.

The success in exfoliation and later chemical vapour deposition (CVD) of graphene, i.e. single-layer nanosheets of carbon atoms crystallized in hexagonal structure, triggered extensive investigations in non-graphene two-dimensional (2D) materials, driven by their dimension-reduction-induced physically unique and technologically useful properties. This chapter discusses the synthesis of semiconducting MoS₂- and MoO₃-based 2D materials, addressing their fabrication issues in recent bottom-up deposition techniques and/or post-deposition exfoliation processes. Typical applications of the 2D MoS₂–MoO₃ nanosheets composite thin films in optoelectronic devices are also presented and discussed. Through these discussions, we attempt to provide the readers a perspective on recent developments of MoS₂- and MoO₃-based 2D materials as well as their future opportunities towards practical applications in optoelectronic devices.

Superlattice structures consist of alternate layers of two different materials, each having a fixed thickness. These structures thus have an additional periodicity along the growth direction and thus behave like a quasi-crystal with a periodicity much larger than that in single-crystal materials having a periodicity of the order of lattice constant of crystal and exhibit several interesting phenomena. The optical, electrical and other physical properties of these structures are significantly different from those of individual layers. In the present article, we present some interesting experimental results observed for nc-Si/a-Si:H-based superlattice structures. Though the lattice constant and electron affinity of nc-Si/a-Si:H are nearly matched, their structural, electrical and optical properties are significantly different. Our studies show that the electrical transport properties of these structures can be tuned by controlling the thickness of the individual layer. The superlattice structures with thick individual layers show excess conductivity in dark after exposure to light. On the other hand, strong photoluminescence (PL) signal in the visible range is observed for the structures with thin individual layers and the PL peak energy depends upon the thickness of the nc-Si layer. The nc-Si/a-Si:H superlattice structures can be used for silicon-based photonic devices in the integrated circuits.

Nano and subnanometric thin-film multilayer interference optical coatings and devices have been playing key as well as essential roles in the manipulation and transport of electromagnetic radiations in various areas of applications such as lasers, telecommunications, smart windows, astronomy, aerospace, environmental monitoring, display, lighting, etc. There are surmounting challenges in the field of optical coatings due to increasing demands of optics in various fields, therefore innovative millstones are continuously explored, especially in investigations on complex multilayer design of challenging filters, development of advance deposition and characterization techniques, searching of news materials, and tuning of microstructure of thin-film coatings. This book chapter presents the overview of recent trends in design, deposition and characterization methods relevant to multilayer optical coatings.

Organic thin films are playing an important role in our daily life. These films are used as a coating layer for transmission of light, different optical control constituents in microelectronics, detectors for sensing organic, inorganic and gas molecules in biosensor devices, etc. Organic thin films are also used as display materials and electronic circuit elements in transistors, optoelectronic devices, biochips, etc. In addition to the technological applications and having immense importance in materials science, organic thin films are also useful to study different physical phenomena in reduced dimensions.

Currently, many publications report on the electrocaloric effect and its application for cooling devices. This work focusses now on the prospects of using ceramic thin films in electrocaloric cooling. We introduce the electrocaloric effect and examine electrical properties, mechanical boundary conditions as well as the film microstructure affecting electrocaloric activity. Further, multilayer ceramic capacitors are introduced as a promising device concept for electrocaloric refrigeration, and limitations of the related heat transfer in electrocaloric devices will be discussed. Finally, ceramic thin films and bulk ceramics are compared with regard to electrocaloric cooling.

The scarcity of energy is a major constraint in economic growth of any country. Day-by-day increasing demand of energy is deteriorating our environment quality. In order to combat environmental pollution threat, clean and green energy sources are critically enforced. Naturally occurring relative humidity has been probed in the form of humidity sensor. Highly resistive ceria doped magnesium ferrite thin film exhibited a colossal decrease in resistance of the order of 10⁷ with only environmental humidity change from 10 to 95%RH. Such huge colossal humidoresistance of magnesium ferrite paved way to research on energy harvesting from water molecule dissociation at room temperature. Nanoporous and oxygen-deficient lithium substituted magnesium ferrite has been processed to make it highly sensitive towards water molecule dissociation into OH⁻ and H⁺ ions. Electrochemistry has been adapted to collect these dissociated ions by two dissimilar electrodes zinc anode and silver inert cathode. The redox reactions on these electrodes generated power and the device is named ‘Hydroelectric Cell’. The working principle of hydroelectric cell has been also validated in other ferrite and different metal-oxides such as Fe₃O₄, Fe₂O₃, SnO₂, TiO₂, ZnO, Al₂O₃, MgO and SiO₂. The redox reactions at electrodes of hydroelectric cell provide eco-friendly and commercially viable byproducts hydrogen and zinc hydroxide.

Thermoelectric materials can provide a solution to the alarming situation of the energy crisis and global warming by harnessing natural as well as waste heat. Recently many studies are being focused on efficient thermoelectric materials such as chalcogenides, clathrates, half-alloys, skutterudites, etc. However, the chapter presented here discusses the scope of conducting polymers as an emerging class of thermoelectric materials. Conducting polymers owing to their nature friendliness, flexibility, reduced manufacturing and processing cost and low thermal conductivity have recently carved out a special place in the arena of thermoelectricity. Though these organic materials cannot substitute conventional inorganic materials at higher temperatures (in terms of efficiency and stability) but their non-toxicity, plentiful availability and solution processability enable them to overshadow their inorganic counterparts for low-temperature heat recovery programmes. Moreover, their amenability to blend with inorganic materials results in hybrid composites which derive the properties of both the organic and inorganic realms and can be used to develop efficient thermoelectric power generators. Also, diverse morphologies and structures of the conducting polymers can be easily manipulated through many ways such as doping, chain alignment, nanostructuring, etc. to tune their charge transport characteristics. The best thermoelectric figure-of-merit (ZT) ~0.4 (at 300 K) obtained in case of many polymers (PEDOT:PSS and P3HT) suggests that these conducting polymers with their advantages can be a good alternative of Bi₂Te₃ -based alloys (with the highest ZT ~1) that are established thermoelectric materials till date in the lower temperature range (<150 °C). Besides this, conducting polymers can be deposited over large surface areas to be used on curved surfaces (for tapping body/appliance heat), thus providing an additional advantage, which the rigid and brittle Bi₂Te₃ -based alloys cannot provide. With the knowledge of state-of-the-art techniques existing in the field of organic electronics and materials manipulation at nanoscale conducting polymers can really furnish new dimensions to advanced thermoelectric materials. Evolution of high-performance polymer-fabric composites and free-standing films indicates a bright future for conducting polymers-based smart and wearable but inexpensive devices. The chapter attempts to reveal recent advancements that have been attained through conducting polymers in the domain of thermoelectric power generation. In addition, critical analysis of all the problems that may occur while designing conducting polymer based thermoelectric devices is also presented.

Hot-wire chemical vapour deposition technique is the latest tool in the series of low-temperature deposition of thin films by chemical vapour deposition. Since its successful implementation for the preparation of diamond-like films, it has been widely explored for semiconductor thin-film deposition in particular silicon-based thin films. This article gives a detailed report of our efforts to understand this process and its application for depositing thin films of amorphous and microcrystalline silicon as well as depositing silicon nanowires. We also demonstrate that these films and nanowires can be successfully used in devices like solar cells and micro-supercapacitors.

Dye-sensitized solar cells (DSCs) and perovskite solar cells (PSCs), are emerging and promising technologies amongst third-generation solar cells. Recent breakthroughs in achieving a record efficiency of 11.7% in solid DSCs at 1 sun and 32% at 1,000 lx surpass even the performance of conventional crystalline solar cells. In the context of PSCs, recent developments of new methodologies and materials are promising for achieving high efficiency as well as stability. Charge extraction layers, viz. electron (ETLs) and hole transport (HTLs) layers are important constituents of high efficient DSCs and PSCs. These layers aid in the transport of charges selectively whilst block their counterparts for an efficient solar cell. In general, 100-nm-thick TiO₂ is employed to fabricate DSCs and PSCs using conventional methods, such as spin cast, spray pyrolysis and chemical bath deposition. A thinner and compact charge extraction layer (<50 nm) is desirable to reduce the series resistance and improve the transmittance and thereby the device efficiency. This chapter describes basic concepts underlying the role of ETLs in the device DSCs/PSCs. The basic principle and merits of deposition processes to prepare ultrathin films for PSCs/DSCs are discussed in detail in this chapter. Further, ultrathin ETLs prepared by different methods have been reviewed. Future directions to explore unconventional scalable and simpler technologies in the emerging field of DSCs/PSCs are discussed from the point of view of commercialization and fundamental research.

Photovoltaics are now slowly replacing fossil fuels, aiming at higher efficiencies and lower costs to bring PV to cost parity with grid electricity. Solar energy is a clean and renewable energy, which is generated from the natural source sun. Solar cells are devices that convert solar energy into electricity, either directly via the photovoltaic effect, or indirectly by first converting the solar energy to heat or chemical energy. Both inorganic and organic types of solar cells are available. Unfortunately, the solar cells dominating the market are all made of inorganic materials requiring expensive and complicated manufacturing processes and have limited applications basically to the rooftops. One of the promising alternatives to inorganic solar cells is the polymer ones. Polymer solar cells (PSCs) are attracting interest as potential sources of renewable and clean energy because of their attractive advantages of low-cost large-area fabrication on lightweight flexible substrates. Though the efficiency of polymer devices have not yet reached those of their inorganic counterparts (≈10–24%); the perspective of low cost, low temperature and energy processing, low material requirement, can be used on a flexible substrate, can be shaped to suit architectural applications are some advantages of polymer solar cell that drives the development of polymer photovoltaic devices further in a dynamic way. This chapter is devoted to the optimization of layer thickness in a polymer photovoltaic cell. It presents the applied calculation method which is based on the optical transfer matrix 2 × 2 formalism. Optical modelling results show that the distribution of light energy determined by optical interference and optimization of thickness of each layer in the OPV would help in the improvement of its performance. The influence of thickness of active layer, electron transport layer and hole transport layer on the normalized modulus squared of optical electric fields distribution inside devices and on the distributions of exciton generation rate and hence current density has been investigated. A mapping of total and useful absorbed energy and parasitic absorption have been done which helps in accurate measurement of IQE and EQE. The distribution of exciton generation rate has been predicted by modelling.

The world’s energy system is at crossroads as the natural fossil fuels are becoming increasingly unavailable and more expensive. Thus, the usage of various renewable energy (RE) sources to provide environmentally benign, economically feasible and sustainable energy supply is drawing more attention to meet ever-increasing energy demands. Among the various RE sources, solar energy has the potential to provide energy independence and security of supply to every economy. Among the possible solutions, the deployment of photovoltaic (PV) modules to directly convert solar radiation into electricity is one of the best choices. The PV is one of the promising technologies to provide a feasible carbon-free route to replacing nonrenewable power sources worldwide. However, the limited performance to cost ratio for the present market-dominating silicon (Si) wafer PV modules restricts large-scale civil utilization of solar electricity. One of the basic costs for Si PV cells is the starting Si wafer itself, which requires several extensive purification to maintain the reasonable performance of the device. Thus, developing PV devices of high performance to cost ratio has always been in demand. Researchers are trying to explore solar cell designs via an unconventional method, aiming at cost reduction and performance improvement. One of the growing potential approaches is PV designs based on nano-architectured materials with low cost and advanced optical and electrical management properties. Micro-and nanostructured Si surfaces are well known for their applications in Si micro-and optoelectronic devices, particularly in solar PV. A particular class of nanostructured silicon is called black silicon. The black Si concept is a promising approach to eliminate front surface reflection (<2% in broad spectral range) omnidirectionally in PV devices without the need for a conventional anti-reflection coatings (ARC). Besides, strong light-trapping can also be achieved for weakly absorbed photons with energies close to the absorption edge of Si and might lead to both an increase in efficiency as well as a reduction in the fabrication costs of solar cells. The nanostructured black Si surfaces, suitable for solar cell applications, have been fabricated by various methods such as metal-assisted wet-chemical etching (MACE), dry reactive ion etching (RIE), etc. In the chapter, a brief introduction of PV technology is presented along with the current status of Si wafer based solar cells. Various challenges/losses associated with conventional solar cell technology with an emphasis on addressing the optical/reflection losses are also discussed. Efforts made to minimize these losses with an emphasis on minimization of reflection losses through nanostructuring schemes are discussed. Moreover, an attempt is made to present a review on the recent progress of black Si research for solar cell technologies. First, MACE technique for the preparation of black Si is discussed in detail along with a brief critical analysis with respect to advantages and disadvantages for solar PV applications. The applications of black Si in solar cells and the progress over the years are then summarized. In addition, one of the major challenges of black Si cell is enhanced surface recombination which imposes a critical limit to solar cell efficiency especially in thinner solar cells. Hence, an efficient black Si solar cell can only be obtained if an optimal trade-off between light absorption gain and electrical losses (due to recombination) is achieved for the black Si-based PV technology. The dielectric thin films play a critical role in effective surface passivation of the black Si surfaces. Therefore, the current status of passivation schemes and challenges for black Si solar cell technology is also highlighted. Moreover, the current status of black Si solar cells concepts employing both monocrystalline, multicrystalline Si as well as the concept of thin/flexible silicon solar cells towards high-efficiency solar cells is reviewed critically. Finally, conclusions and future prospects of the black Si concept is outlined wherein it is envisaged that nanostructured black Si will play a key role in cost-effective and efficient solar photovoltaic devices in days to come.

Hydrogen sulphide (H₂S) is a colourless, corrosive, flammable and toxic gas with a typical rotten egg smell. Although it has demonstrated wide commercial utility, especially in food processing industry, coal gasification plants and oil refineries, its adverse effect on both environment and human health demands its monitoring and careful usage. It has the ability to block cellular respiration owing to its interaction with iron that is present in the cytochrome enzymes of the mitochondria. Exposure to higher concentrations can adversely affect the nervous and other systems in the body. Consequently, its short-term (15 min) and long-term (8 h) exposure limits have been set to 15 and 10 ppm, respectively. Chemiresistive sensor is one of the simplest sensors that has demonstrated a tremendous potential for H₂S detection in various concentration ranges. It works on the simple principle of resistance change of the sensor due to chemical reaction of H₂S with the sensor surface. The present chapter focuses on the recent advances in the field of chemiresistive sensors for H₂S detection. Care has been taken to give a complete insight about H₂S: its source, applications, dangerous effects and different means of detection. With the upsurge in nanoscience, novel sensor morphologies have been realized with enhanced sensor responses. This chapter will try to cover most of the important works carried out for H₂S detection highlighting the sensor with a potential for commercial application. The chapter will conclude highlighting the challenges that need to be addressed to realize practical application and discussing the future of chemiresistive sensors for H₂S detection.

In recent years, environment monitoring is one of the prerequisites for the welfare of human beings. Environment monitoring leads to the development of sensors that can detect the presence of harmful/toxic gases and vapours. For the detection of poisonous gases, various studies have been reported over the last few years using nanostructured thin films based on various semiconducting oxides, conducting polymers and organic molecules. Among different organic materials, metallo-phthalocyanines (MPcs) based materials are regarded as excellent sensing material, as their electrical conductivity significantly changes on interaction with oxidizing/reducing gases. Sometimes small response characteristics of these sensors at room temperature become a limitation and can be overcome by exploring long-range molecular nanostructure with high surface/volume ratio. In this chapter, we have systematically discussed the development of cost-effective, highly sensitive and reproducible phthalocyanine-based room-temperature chemiresistive sensors capable of detecting harmful/toxic gases up to ppb levels. The primary emphasis will be laid on the formation of different phthalocyanine-based nanostructures, including nanowires, nanoflowers and nanobelts for sensing applications.

A novel flexible, ultra-sensitive, selective and room temperature operable polyaniline-based hybrid (PAni/α-Fe₂O₃ PAni/WO₃) ammonia (NH₃) gas sensors were developed onto a flexible polyethylene terephthalate (PET) substrate by in-situ polymerization process. The observations were recorded to 100 ppm fixed level for various gases including NO₂, CH₃OH, C₂H₅OH, NH₃ and H₂S through monitoring the change in resistance of the developed sensor. The flexible (PAni/α-Fe₂O₃ PAni/WO₃) hybrid sensor demonstrated better selectivity towards NH₃. The synergistic response of the flexible hybrid sensors was remarkable than that of the PAni and α-Fe₂O₃ and WO₃ alone; indicating the effective improvement in the performance of PAni flexible sensor on nanocomposite process. Moreover, the flexible sensor detected NH₃ at low concentration (5 ppm) with a fast response (27 s) and very short recovery time (46 s). Further, PAni/α-Fe₂O₃ and PAni/WO₃ hybrid flexible sensor films were characterized by X-ray diffraction, field-emission scanning electron microscopy, UV–visible and Raman spectroscopy, Fourier transform infrared and X-ray photoelectron for structural analysis, morphological evolution, optical and surface related studies.

The transition metal-oxides are a versatile class of materials with numerous applications. In this chapter, the optical and gas sensing properties of the vanadium oxide thin films have been reviewed keeping in view their applications in energy efficient optical windows and solid state gas sensors. Vanadium exists in variable valence states and exhibit Magneli pahses such as VO₂, V₂O₃, V₆O₁₃, V₂O₅ etc. VO₂ and V₂O₅ are the two most widely studied oxides of vanadium among all of its Magneli phases. VO₂ thin films exhibit metal–insulator transition close to room temperature (68 °C) and find application as thermo-chromic coatings. The changes in the electrical conductivity of V₂O₅ are reported to take place at higher temperatures and are attributed to the creation of oxygen defects and temporary distortion in the structure without any structural phase transition. The changes in optical transmittance of V₂O₅ with temperature are very small as compared to VO₂. However, better thermodynamic stability of V₂O₅ over VO₂ has inspired the research on enhancing the temperature dependent optical properties of the V₂O₅ films, and a correlation between their short-range structural properties and the temperature dependent optical transmission has been found. The results of the previous research carried out on vanadium oxide thin films are discussed in this chapter. V₂O₅ is a good gas sensing material since it shows large changes in the electrical resistance in the presence of some gases. The gas sensing mechanisms and various methods and parameters that have been used to tailor the sensor signal and response time towards different gases are discussed. The atomic structure, surface morphology and composition of the films play important role in determining the gas sensing characteristics. The information provided in this article shows that the physical properties of the V₂O₅ thin films can be modified significantly for their applications as thermo-chromic coatings and gas sensors.