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A thin film is a layer of material ranging from fractions of a nanometer monolayer to several micrometers in thickness. The controlled synthesis of materials as thin films a process referred to as deposition is a fundamental step in many applications. A familiar example is the household mirror , which typically has a thin metal coating on the back of a sheet of glass to form a reflective interface.

The process of silvering was once commonly used to produce mirrors, while more recently the metal layer is deposited using techniques such as sputtering. Advances in thin film deposition techniques during the 20th century have enabled a wide range of technological breakthroughs in areas such as magnetic recording media , electronic semiconductor devices , LEDs , optical coatings such as antireflective coatings , hard coatings on cutting tools, and for both energy generation e.

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It is also being applied to pharmaceuticals, via thin-film drug delivery. A stack of thin films is called a multilayer. In addition to their applied interest, thin films play an important role in the development and study of materials with new and unique properties. Examples include multiferroic materials , and superlattices that allow the study of quantum phenomena.

The act of applying a thin film to a surface is thin-film deposition — any technique for depositing a thin film of material onto a substrate or onto previously deposited layers. Molecular beam epitaxy , Langmuir-Blodgett method , atomic layer deposition and molecular layer deposition allow a single layer of atoms or molecules to be deposited at a time. It is useful in the manufacture of optics for reflective , anti-reflective coatings or self-cleaning glass , for instance , electronics layers of insulators , semiconductors , and conductors form integrated circuits , packaging i.

Similar processes are sometimes used where thickness is not important: for instance, the purification of copper by electroplating , and the deposition of silicon and enriched uranium by a CVD -like process after gas-phase processing. Deposition techniques fall into two broad categories, depending on whether the process is primarily chemical or physical. Here, a fluid precursor undergoes a chemical change at a solid surface, leaving a solid layer. An everyday example is the formation of soot on a cool object when it is placed inside a flame.

Since the fluid surrounds the solid object, deposition happens on every surface, with little regard to direction; thin films from chemical deposition techniques tend to be conformal , rather than directional. Plating relies on liquid precursors, often a solution of water with a salt of the metal to be deposited. Some plating processes are driven entirely by reagents in the solution usually for noble metals , but by far the most commercially important process is electroplating.

It was not commonly used in semiconductor processing for many years, but has seen a resurgence with more widespread use of chemical-mechanical polishing techniques. Chemical solution deposition CSD or chemical bath deposition CBD uses a liquid precursor, usually a solution of organometallic powders dissolved in an organic solvent.


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This is a relatively inexpensive, simple thin-film process that produces stoichiometrically accurate crystalline phases. This technique is also known as the sol-gel method because the 'sol' or solution gradually evolves towards the formation of a gel-like diphasic system. Langmuir-Blodgett method uses molecules floating on top of an aqueous subphase. The packing density of molecules is controlled, and the packed monolayer is transferred on a solid substrate by controlled withdrawal of the solid substrate from the subphase.

Introduction

This allows creating thin films of various molecules such as nanoparticles, polymers and lipids with controlled particle packing density and layer thickness. Spin coating or spin casting, uses a liquid precursor, or sol-gel precursor deposited onto a smooth, flat substrate which is subsequently spun at a high velocity to centrifugally spread the solution over the substrate. The speed at which the solution is spun and the viscosity of the sol determine the ultimate thickness of the deposited film.

Repeated depositions can be carried out to increase the thickness of films as desired. Thermal treatment is often carried out in order to crystallize the amorphous spin coated film. Such crystalline films can exhibit certain preferred orientations after crystallization on single crystal substrates. Dip coating is similar to spin coating in that a liquid precursor or sol-gel precursor is deposited on a substrate, but in this case the substrate is completely submerged in the solution and then withdrawn under controlled conditions. There are two evaporation regimes: the capillary zone at very low withdrawal speeds, and the draining zone at faster evaporation speeds.

Chemical vapor deposition CVD generally uses a gas-phase precursor, often a halide or hydride of the element to be deposited.

Functional Thin Films and Nanostructures for Sensors

Commercial techniques often use very low pressures of precursor gas. Unlike the soot example above, commercial PECVD relies on electromagnetic means electric current, microwave excitation , rather than a chemical-reaction, to produce a plasma. Atomic layer deposition ALD , and its sister technique molecular layer deposition MLD , uses gaseous precursor to deposit conformal thin films one layer at a time. The process is split up into two half reactions, run in sequence and repeated for each layer, in order to ensure total layer saturation before beginning the next layer.

Therefore, one reactant is deposited first, and then the second reactant is deposited, during which a chemical reaction occurs on the substrate, forming the desired composition. Physical deposition uses mechanical, electromechanical or thermodynamic means to produce a thin film of solid. An everyday example is the formation of frost. Since most engineering materials are held together by relatively high energies, and chemical reactions are not used to store these energies, commercial physical deposition systems tend to require a low-pressure vapor environment to function properly; most can be classified as physical vapor deposition PVD.

The material to be deposited is placed in an energetic , entropic environment, so that particles of material escape its surface.


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Facing this source is a cooler surface which draws energy from these particles as they arrive, allowing them to form a solid layer. The whole system is kept in a vacuum deposition chamber, to allow the particles to travel as freely as possible. Since particles tend to follow a straight path, films deposited by physical means are commonly directional , rather than conformal.

Examples of physical deposition include: A thermal evaporator that uses an electric resistance heater to melt the material and raise its vapor pressure to a useful range. This is done in a high vacuum, both to allow the vapor to reach the substrate without reacting with or scattering against other gas-phase atoms in the chamber, and reduce the incorporation of impurities from the residual gas in the vacuum chamber.

Obviously, only materials with a much higher vapor pressure than the heating element can be deposited without contamination of the film. Molecular beam epitaxy is a particularly sophisticated form of thermal evaporation. An electron beam evaporator fires a high-energy beam from an electron gun to boil a small spot of material; since the heating is not uniform, lower vapor pressure materials can be deposited. Typical deposition rates for electron beam evaporation range from 1 to 10 nanometres per second. In molecular beam epitaxy MBE , slow streams of an element can be directed at the substrate, so that material deposits one atomic layer at a time.

Compounds such as gallium arsenide are usually deposited by repeatedly applying a layer of one element i. If the precursors in use are organic, then the technique is called molecular layer deposition.

Synthesis, characterization and deposition of NPs and thin films of NPs

The beam of material can be generated by either physical means that is, by a furnace or by a chemical reaction chemical beam epitaxy. Sputtering relies on a plasma usually a noble gas , such as argon to knock material from a "target" a few atoms at a time. The target can be kept at a relatively low temperature, since the process is not one of evaporation, making this one of the most flexible deposition techniques. It is especially useful for compounds or mixtures, where different components would otherwise tend to evaporate at different rates.

Note, sputtering's step coverage is more or less conformal. It is also widely used in the optical media. It is a fast technique and also it provides a good thickness control. Presently, nitrogen and oxygen gases are also being used in sputtering. Pulsed laser deposition systems work by an ablation process.

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Pulses of focused laser light vaporize the surface of the target material and convert it to plasma; this plasma usually reverts to a gas before it reaches the substrate. Cathodic arc deposition arc-PVD which is a kind of ion beam deposition where an electrical arc is created that literally blasts ions from the cathode. If a reactive gas is introduced during the evaporation process, dissociation , ionization and excitation can occur during interaction with the ion flux and a compound film will be deposited.

Electrohydrodynamic deposition electrospray deposition is a relatively new process of thin-film deposition. The liquid to be deposited, either in the form of nanoparticle solution or simply a solution, is fed to a small capillary nozzle usually metallic which is connected to a high voltage. The substrate on which the film has to be deposited is connected to ground.


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Through the influence of electric field, the liquid coming out of the nozzle takes a conical shape Taylor cone and at the apex of the cone a thin jet emanates which disintegrates into very fine and small positively charged droplets under the influence of Rayleigh charge limit. The droplets keep getting smaller and smaller and ultimately get deposited on the substrate as a uniform thin layer. Frank-van der Merwe growth [6] [7] [8] "layer-by-layer". In this growth mode the adsorbate-surface and adsorbate-adsorbate interactions are balanced.

This type of growth requires lattice matching, and hence considered an "ideal" growth mechanism. There is an increasing interest in developing and characterizing multiferroic materials, in which both ferromagnetic and ferroelectric orders coexist, as they exhibit rich physical properties and offer exciting opportunities for data storage, spintronics, sensors, electromagnets and photovoltaic PV applications. In addition, multiferroic BiFeO3, with band gap energy of 2.

Moreover, the coupling between ferroic orders in BiFeO3 materials offers new modes for investigating and controlling the PV effect, which may endow next generation solar and photoelectrochemical PEC cells with multiple functionalities. On the other hand, considerable interest has been attributed to multiferroic BiFeO3 nanostructures in the quest of miniaturizing devices and discovering interesting fundamental physics at nanoscale. Therefore, the synthesis of multiferroic BiFeO3 nanostructures and investigation of their functional properties are considered important for both fundamental research as well as designing new multifunctional materials combining magnetic, ferroelectric and optoelectronic properties.

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Meanwhile, recent emergence of a novel double perovskites multiferroic material Bi2FeCrO6 BFCO , with functional properties well above room temperature, opens new opportunities for practical applications of multiferroics. Recent works demonstrated that an ordered Bi2FeCrO6 phase can be obtained in both thin film and nanostructured form using pulsed laser deposition PLD technique. In addition, theoretical studies showed that Fe and Cr mixed d orbital transition allow a small band gap around 2.

The work performed in this thesis was therefore driven by two main objectives: 1 synthesis and understanding the fundamental physical properties i. The results obtained in this work are resumed in two sections as follows: In the first section, we have synthesized and investigated the ferroelectric, magnetic and photocatalytic properties of BiFeO3 nanomaterials 1D nanowires and 2D nanoplates. PFM measurements demonstrated that the assynthesized BiFeO3 nanowires, down to 40 nm in diameter, have components of spontaneous polarization along both in plane and out of plane directions, thereby confirming the ferroelectric nature of the wires.

We explained our results by estimating the shape of the piezoelectric tensor for the rhombohedral symmetry. We have also studied the photocatalytic solar water splitting properties of the BiFeO3 nanowires and discovered that the nanowires exhibit better visible-lightdriven photocatalytic activity for generation of O2 from water than other BiFeO3 materials e.