Abstract

In this work, the structural properties of the monocrystalline vanadium pentoxide have been presented. Vanadium pentoxide (V₂O₅) films were deposited by using a DC reactive magnetron sputtering system at a working pressure of 8.5x10^-2mbar.The sputtered vanadium atoms were sputtered and oxidized in presence O₂:Ar gas mixture by (5/95,10/90, 20/80,30/70,50/50). Employment of magnetron results in the formation of V₂O₅ in the final samples according to the XRD analysis, increase the roughness and hence surface area of the produced V₂O₅ nanostructures. The results of X-rays are shown to us, the deposited films were formed by nanoparticles with an average grain size in the range of (52.11nm to 98.03) nm and roughness Ave (nm) in the range of (1.04nm to 8.88nm). The deposited films are identified to be polycrystalline nature with a cubic structure along ((001), (111)) and ((200)) orientation also MonoV2O5, Cub VO were found as deposited. The texture of the films was observed using SEM and AFM, it was observed that the grain size was increased with increasing the O₂ percentage. These improvements in the structural properties of the produced vanadium pentoxide make these nanostructures good candidates for specific applications, such as photo detectors, solar cells, electro chromic smart window and gas sensor.

1. Introduction

Vanadium creates a wide range of composites with oxygen which have variation structural and different valence states and founds in a many phases of oxide comprise of vanadium oxide, There are at least 15 variation vanadium oxides communally to now, for example; VO, V₂O₃, VO₂, V₂O₅, V₆O₁₃ and so on ¹. V₂O₅ are known to be the most stable compound form for vanadium metal , but other non-stable stoichiometry can be constrained under particular oxidation conditions. Different phases of vanadium oxides can be obtained by controlling the deposition method parameters, or by proportion process manipulate, e.g., additional thermal annealing .properties of these phases of vanadium oxides depend on their structure, which determines other properties which exhibit a complex stoichiometric composition ². Thin-walled oxide nanoparticles belong to this smart inventory list, and these thin-layer oxide structures have been accelerated for practical applications such as electronics and optoelectronics. Most of these vanadium oxides are characters for the transfer of semiconductor phases to metal to reversible ³, which is accompanied by large changes in infrared optical properties and their electrical resistance ^{4, 5}. Vanadium oxide (V/O) structures are a well-known catalyst among various metal oxides, and so many fundamental studies have been developed wide-spreading centering on catalytic oxidation ⁶. They show reversible metal-semiconductor transition through a wide band of temperatures depending on the ratio of O/V ^{7, 8}, Which results in sudden changes that are accompanied by large changes in infrared optical properties and electrical resistance ⁷. The thin layer of vanadium / oxide with defined stoichiometry with the appropriate selection of the network simulation subsystem and the adjustment of many parameters including sedimentation, precipitate rate, pressure and temperature. However, the difficulty of precisely controlling all the parameters and preventing the emergence of defect results in several cases in the stoics is impossible and challenges the synthesis of these oxides as an additional task. Due to these problems, vanadium oxide is mixed with valentine for complex synthesis and research has been reduced. However, vanadium oxide mixed Usually in the process of oxidation / reduction VO₂ – V₂O₅ ⁹. Vanadium pentoxide (V₂O₅) has been broadly studied because of its highest oxidation state in the V / O system, a wide band gap, a better stability and its electro thermal effects it is useful for device applications ¹⁰. Except for the pure values of vanadium 0, +2, +3, +4 and +5, respectively, are related to phases V, VO, V₂O₃, VO₂ and V₂O₅. Two sets of compost properties are identified as stable phases. These series VnO2n-1 and VnO2n + 1 are defined as magnetism ^{11, 12}. Wesley phases, respectively, and they connect the three most common phases, V₂O₃, VO₂ and V₂O₅ ^{11, 13, 14}. The working mechanism of electro chromic materials consists of electrolyte, intercalated mobile ions which performs intercalation/de intercalation while a potential difference is applied. For this purpose, layered structure of V₂O₅ is a promising material. The most commonly used phases, which found in wide range of applications due to their novel properties, are the VO, VO₂, V₂O₃, and V₂O₅ oxide phases ¹⁵.

2. Experimental Details

Vanadium pure was chosen as the primary material to study for this project. Sputtering processes normally uses argon gas because it does not react with the target material allowing thin films to be formed. The addition of a gas that reacts with the target material, such as oxygen, will form compounds of the material and the reactant. To determine how vanadium oxide is formed using reactive magnetron sputtering a reactive gas test was performed. Because the sputtering system uses a DC power supply that allows both voltage and current to float while maintaining constant power, the target voltage can be easily measured and used to infer changes in the impedance of the sputtering plasma. The gun in which the target is mounted is designed to provide a narrow range of impedance to the power supply. With the addition of a reactive gas the impedance of the target changes. Using the fixed conditions of 660V of DC power and a chamber pressure of 0.08 mb while introducing increased amounts of oxygen the voltages were recorded.

3. Results and Discussion:

Figure 2 shows the XRD patterns for the VO_X nanoparticles coating layer deposited at various Ar/O₂ ratio, ranging from 5% to 50 %. For a deposition time of 120 min and a sputtering power of 680V, within the range of Ar pressure, the thickness of the VO_X films ranges from 59.6 to 212 nm. The intensity of the (001) peak for the V₂O₅ nanoparticles coating layer deposited at lower Ar/O₂ ratio was observed to be higher compared with those deposited at higher Ar/O₂ ratio. The intensity of a peak in the X-ray diffraction pattern is a direct measure of the film crystallinity. We attribute this phenomenon to the increase of the surface mobility of the adatoms with higher energy and decreasing Ar/O₂ pressure, which is necessary to form a highly crystalline film. In this respect, high Ar/O₂-pressure in the sputtering process infers the decrease in the surface mobility of the adatoms after series of particle collisional events during the travel from the deposition source to the substrate . In other words, at low pressure the electron temperature is high. at high temperature, more energy should be available for the atoms to acquire so that they may diffuse and occupy the correct site in the crystal lattice. This result is consistent with previous study ¹⁶.

The VOX films exhibit peaks corresponding to the mono V₂O₅ phase at 2θ = 18.3105°, 18.3105°,18.3509°, 36.8051° and 36.6840° direction identify with standard peaks[ card. No (96-901-2222) and also the XRD pattern illustrates that the vox films had a polycrystalline structure with peaks attributed to Cub VO phase at 2θ =44.5100° and 44.4697° direction identify with standard peaks [card. No(96-900-8767). The XRD pattern illustrates that the (V₂O₅) films have polycrystalline (111), (001) and The X-ray diffractions of the Vanadium target is also displayed cub (VO). All the XRD patterns show same behavior where the FWHM decreases with increase of the different Ar:O₂ ratio with increasing the crystallite size ^{17, 18}.

Figure 3 shows the two-dimensional (2D) and three- dimensional (3D) AFM images of (VOx) deposited at different gas (Ar/O₂) mixture ratio on the glass substrate. The 2D images show that the films are uniform and the substrate surface is well covered with grains that are nearly uniformly distributed over the surface. the surface morphology reveals the Nano-crystalline (VOx) grains. The 3D images reveal that there is a large sharp island or particles formation observed in deposited films microstructure. From the images, it was observed that the surfaces of the films exhibited a certain degree of roughness. The surface roughness gives further information about surface morphology of thin films. The variation in surface roughness of thin film plays an important role in optical coatings. It improves the optical absorbance and influences the optical properties of thin films. In addition, Surface roughness is directly proportional to the gas sensitivity of the film since larger roughness results in a larger contact area with gaseous species ¹⁹. The average grain size and root mean square roughness (RMS) of these films are shown in the Table 2.

Two and three-dimensional AFM images of the as-deposited vanadium oxide films with constant discharge current, and different gas Ar/O₂ mixture flow. The average grain size and root mean square roughness (RMS) of these films are shown in table 4.17. The AFM images of all samples displayed are granular structure. The granular films show a higher surface area, which is conducive for film-gas interaction and results in higher gas sensitivity ²⁰. The gas sensitivity has a proportional relationship with film roughness ²¹. All the images appeared in figure (4.17) show homogeneous cluster distribution with columnar structure; where the average grain size values decrease slightly with increasing oxygen flow. Increasing oxygen flow leads to increase in the number of oxygen molecules adsorbs on the substrate surface which acts as traps for atoms causing to reduce grain size for thin films by increasing the number of nucleation centers, Also, increases roughness with increasing oxygen flow which leads to increase the vertical growth on the substrate surface.

Figure 4 - Figure 8 show the surface morphology images of VOX thin film with different gas(Ar/O2) mixture ratio. We can see clearly demonstrated particles having the uniform, homogenous distribution. FESEM micrograph shows that the deposited coating layer consists of VO_X nanoparticles with very small diameters at different gas (Ar/O₂) mixture and constant working pressure. It shows that the sputtered particles diameter are increased, from 36.9 nm to 251.5 nm. This may be due to increasing the sputter power will increase the deposition rate and therefore diminishes the time available for the deposited particles to diffuse via (surface) diffusion processes ²². From the images, it proved that the grain size of nanoparticles was observed and estimated to be increased when the gas rate increased and the surface morphology mostly irregular shape. When the gas rate has increased the nanoparticle of Vanadium oxide becomes rough. The higher the gas rate of Vanadium oxide thin film, the larger the grain size of Vanadium oxide nanoparticle ^{23, 24, 25, 26}. crystallite size and grain size of samples calculated by XRD and attributed to the fact that in the case of doped systems, the columnar (nanorods type) growth is always inhibited during incorporation of the doping element in to the matrix due to the formation of either defects or aggregation after doping. Thus, for the deposition of composite films, heterogeneous or homogeneous nucleation may occur on the substrate, and the coarsening process (through atomic diffusion) of small particles proceeds with difficulty, in contrast to the undead V₂O₅ deposition at the same annealing temperature rather than a columnar structure is formed.

4. Conclusion

FE-SEM investigations showed that the nanoparticles obtained at gas rate mixture (50%) more larger in size than obtained at gas rate mixture (5%) and the particle size increase with increasing gas mixture ratio. An analysis of the structural properties of vanadium powder oxide produced by reactive spectrometry has been observed. An increase in the surface roughness of the samples was observed with an increase in the ratio of the mixture of gases. This allows for the use of samples prepared in the use of gas sensors. The FE-SEM images showed that the particles size increase from 36.9 to 251.5 nm when the gas mixture rate increased from 5% to 50%. The X-ray diffraction studies have confirmed that VO_X sample has the Mono (V₂O₅) phase while remaining samples have a cubic(VO) phase. The intensity of the major peaks was decreased and shifted toward higher 2θ angles when Oxygen ions increase.

Acknowledgements

The search is performed as part of the execution PhD "program of the Ministry of Higher Education Education//2019/01/1. "Developing methodology Nanoparticles for thin films.

References