Optical Studies of Ag 2 O Thin Film Prepared by Electron Beam Evaporation Method

Silver oxide (Ag2O) thin films were deposited on glass substrates using electron beam gun evaporation techniques without oxygen atmosphere. The deposited films were post annealed at 100 ̊C, 150 ̊C, and 200 ̊C, respectively. The surface morphologies, structural and optical properties at different annealing temperatures were studied using scanning electron microscopy (SEM), X-ray diffraction (XRD), and ultra-violet-visible spectroscopy. The XRD results show that the intensity of (200) plane intensified as the annealing temperature increased from 100 ̊C to 200 ̊C. The XRD spectra reveal that the films are polycrystalline, having cubic structure irrespective of post annealing temperatures. The optical band gap of Ag2O thin films decreases from 1.716 eV to 1.559 eV as the annealing temperature increases from 100 ̊C to 200 ̊C.


Introduction
Silver has been valued throughout history for many of its properties that are useful to humans.It is used as a precious commodity in currencies, ornaments, jewellery, electrical contacts and photography.One of the most beneficial uses of silver has been as a potent antibacterial agent that is toxic to fungi, viruses and algae.Silver has long been used as a disinfectant; for example, the metal has been used in treating wounds and burns because of its broad-spectrum toxicity to bacteria as well as because of its reputation of limited toxicity to humans.
Development of new nanomaterials with metal nanoparticles (Ag, Au, Cu, Rh, Pd, etc.) deposited on glass surfaces, have gained much attention in material science because of expanding applications of such composites in optics, medical diagnostics, analytical chemistry, catalysis, photo catalysis etc.In this work we present improved photo-thermo chemical production of AgO and Ag 2 O 3 , metal functional films, as well as binary and ternary oxide composites via glass substrate assisted electron beam gun evaporation method with Ag nanoparticles (NPs) [1].
Argentous oxide is also described as silver (1) oxide or Ag 2 O with the 2 representing the number of silver atoms combining with each oxygen atom.Argentic oxide or silver (2) oxide is also known as plain silver oxide or silver peroxide.The peroxide here indicates that the compound contains more oxygen than usual.Metal oxides in the Ag-O system, including Ag 2 O, AgO, Ag 3 O 4 and Ag 2 O 3 , constitute a fascinating group of inorganic materials [2].Silver oxides crystallize in various types of crystal structures, leading to a variety of interesting physico chemical properties such as catalytic, electrochemical, electronic and optical properties.Therefore, silver oxide nanocrystals and thin films have been intensively pursued for promising applications as a catalyst for ethylene and methanol oxidation [3,4], as a sensor for the detection of carbon monoxide and ammonia [5][6][7], as photovoltaic materials [8][9][10], as important components in optical memories [11] and plasmon photonic devices [12], or as active cathode materials in silver oxide/zinc alkaline batteries [13,14].The properties of silver oxide nanostructures and thin films are found to be vitally sensitive to the growth method and the experimental conditions applied.
Park et al. [15] reported that the occurred band was at-tributed to the localized state of oxygen vacancies in AgO cubic-like structure.A weak shoulder peak seen at 440 nm was the bluish green emission due to the presence of defects in the films.Without oxygen partial pressure at RT, the peak shifted to 618 nm attributed to blue emission due to the reduction of oxygen vacancies.Niederberger et al. [16] and Luo et al. [17] suggested that the blue emission was attributed to the band-to-band transition.The possible reason that sufficient oxygen is available in the sputter chamber leads to formation of stoichiometric Ag 2 O films as explained in EDAX analysis and the films have O/W ratio of 2.98 at oxygen partial pressure of Pascal.

Experimental
Electron gun evaporation technique is that one of the oldest techniques used for depositing thin films [18,19], which is still widely used in the laboratories and in industries for depositing 54 metals and metal alloys.The following sequential basic steps take place: 1) a vapour is generated by boiling or subliming a source material; 2) the vapour is transported from the source to the substrate; and 3) the vapour is condensed to a solid film on the substrate surface.Thin films of Ag 2 O were prepared by EBE technique using a HINDHI-VAC vacuum unit (model: 12A4D) fitted with electron beam power supply (model: EBG-PS-3K).Well degreased microscopic glass plates have been employed as the substrates in the present work.500 mg of spectroscopically pure Ag 2 O (99.99%) was mixed well using a pestle and mortar.The mixture was pressed into pellets by hydraulic method to get pellet with a pressure of 500 kg/cm 2 , which was used as the source material for evaporation.The pellet was taken in a graphite crucible and kept in water cooled copper hearth of the electron gun.The pelletized Ag 2 O targets were heated by means of an electron beam collimated from the dc heated tungsten filament cathode.The surface of the Ag 2 O pellet was bombarded by 180˚ deflected electron beam with an accelerating voltage of 5 kV and a power density of ~1.5 kW/cm 2 .The evaporated species from Ag 2 O pellet were deposited as thin films on the substrates in a pressure of about 1 × 10 −5 mbar.Each substrate was placed normal to the line of sight from the evaporation source at a polar angle to avoid shadow effects and also to obtain uniform deposition.The different preparation parameters such as source to substrate distance (15 cm) and partial pressure (10 −5 mbar) have been varied and optimized for depositing uniform, well adherent and transparent films.The rate of evaporation (0.5 nm/s) was used to deposit all Ag 2 O films.The as deposited Ag 2 O films were annealed at 100˚C, 200˚C and 300˚C, respectively, to study the effect of annealing temperature on the structural, optical, and morphological properties.
The structural properties of Ag 2 O thin films were in-vestigated using PAN analytical X-ray diffractometer with CuKα radiation at λ = 1.5406Å.The X-ray diffraction (XRD) pattern was recorded in the range of 20 to 60 at a scanning rate of 5˚ min −1 .Morphological studies were performed using scanning electron microscopy (SEM, Hitachi S-3400N, Japan).For the optical characterization, the transmittance and absorbance spectra of the Ag 2 O thin films were recorded using a UV-Vis spectrophotometer (Schimadzu 160A).

Structural Characterisation
Figure 1 shows the XRD pattern of Ag 2 O films annealed at 100˚C, 150˚C and 200˚C respectively.
The peak intensity at the (200) plane increased with an increase in annealing temperature.The intensification of the peak appearing at (200) plane indicates that the crystal quality of the Ag 2 O thin films improves with the increase in annealing temperature which is shown in Figure 2. The crystalline character of Ag 2 O is greatly enhanced with smaller width of the peaks [20].The average grain size was calculated by Scherrer's equation, Where D is the grain size, β the full width at half maxima, θ is the angle of diffraction, and λ is the wavelength of X-ray (1 × 5409 Å).
The crystal lattice presents a series of parallel reflecting planes to the incident X-ray beam.The intensity of the reflected beam at certain angles will be maximum when the path difference between two reflected waves from two different crystal planes is an integral multiple of λ.This condition is termed as Bragg's law and is given by, n 2d sin where, n is the order of diffraction, λ is the wavelength of X-rays, d is the spacing between consecutive parallel planes and θ is the glancing angle (or the complement of the angle of incidence) [21,22].The lattice parameter c of the as-deposited room temperature Ag 2 O thin film is calculated using the formula for the hexagonal crystal structure.For the hexagonal crystal system, the lattice parameter c is related to d with the following equation Where h, k, and l are all integers, (hkl) is the lattice plane index, a and c are lattice constants, d hkl is distance between two consecutive planes (m = 1) with lattice plane index (hkl).

Open Access
OJMetal  By using the standard formula [23], the values of strain (є), dislocation density (δ) and number of crystallites (N) were calculated.
The strain value ε can be evaluated by using the following relation:  . ( The dislocation density (δ) has been calculated by using the formula for thin films with cubic structure: Using grain size (D) and film thickness (t), the number of crystallites N can be estimated using the relation 3 From the results of without oxygen atmosphere Ag 2 O thin film and different annealing effects of RT, 100˚C, 150˚C and 200˚C, we conclude that the process does improve the ordering of Ag 2 O structure; the optical properties of Ag 2 O thin films deposited without oxygen pressure can be greatly improved which is shown in Table 1.The annealing speed and annealing temperature are both important in improving the optical quality and crystallinity of the thin film.If the annealing time is constant (1 hr), the optical quality of the thin film cannot be greatly improved.If the annealing time is too high for example above 1 hr, the thin film will be broken because of the stress inside the thin film and gets peeled off from the substrate.Similarly, when the annealing temperature was used beyond 250˚C the films are non-adherent to the substrate.So we maintained the annealing temperature to a maximum of 200˚C only.

SEM Studies of Ag 2 O Thin Film
Figure 1 depicts the SEM images of the Ag 2 O thin films annealed at different temperatures.It was observed that as the annealing temperature increases from 100˚C to 150˚C, the film displays small particles on the surface of the film.Further increase in temperature from 150˚C to 200˚C, the films shows the accumulation of the Ag 2 O thin film.That accumulation may be attriburted due to the melting point of Ag 2 O.

Optical Studies
Figure 3 shows the optical absorbance spectra of Ag 2 O thin films annealed at 100˚C, 150˚C and 200˚C, respectively.The p-type conducting and reasonably high transparency, 62% at 550 nm, before and after annealing, the change of optical constants is very small.As observed in other work [24,25], there is an optical absorption occurring in the 1.5 -1.6-eV photon-energy range for the Ag 2 O sample prepared by the chemical methods are depicted in Table 1.We did not see the strong optical absorption in this range, due to mixing of the microstructures of the sample prepared by the sputtering method.Through the annealing process, some of the Ag 2 O components in the as-deposited structure can be thermally decomposed into Ag 2 O and oxygen.The oxygen will react with a small amount of silver particles in the film.Such a chain reaction process may occur as: AgO + Ag → Ag 2 O.
Therefore, the annealing process will certainly improve the crystallization of the film, to cause a change of the optical properties, especially in the high-energy range.Themicro-phase structure of the film, however, might still be a mixture of the amorphous and crystalline structures, depending on the sample preparation and annealing conditions.In the amorphous structure, Urbach tail [26] will induce a new density of states into the band gap, and thus the absorption spectra of Ag 2 O will be influenced.
To date, there is still a lack of detailed band-structure calculations to compare with the experimental data for Ag 2 O materials.For the direct or indirect interband optical transitions, the spectra of the optical absorption coefficient can be analyzed by using the following equations:  The extrapolations of these linear sections yield band gaps: Eg 100C = 1.716 eV, E 150C = 1.6098 eV and E 200C = 1.559 eV.The 0.066 eV-interband transitions tend to be attributed to the free-carrier response.In the data analysis of this work with the Tauc-Lorentz oscillator model, the value of E g2 is close to E g3 given in Figure 4. E g2 is larger than the Ag 2 O bandgap of E g1 eV.The optical band gap of the transmitting thin films calculated from the optical absorption spectra (using a Tauc plot) increases slightly (from 1.507 to 1.573 eV for AgO monoclinic structure table without the effect of oxygen pressure.

Conclusion
Ag 2 O thin films were successfully synthesized on glass substrate using ebeam evaporation technique.The effect of post annealing temperature on the morphological, structural, and optical properties of Ag 2 O thin films were investigated using X-ray diffraction, SEM and UV-Vis spectroscopy.The optical bandgap energy of the Ag 2 O thin films decreased as the annealing temperatures increased.The XRD intensity of (2 0 0) plane increased as annealing temperature increased.

Figure 1 .
Figure 1.SEM Images of Ag 2 O thin film for different annealing temperatures.

Figure 2 .
Figure 2. XRD pattern of Ag 2 O thin film for different annealing temperatures.

Figure 3 .
Figure 3. Absorbance curve of Ag 2 O thin film for different annealing temperatures.

4 .
photon energy as shown in Figure 4.The intercept of the tangent to the plot provides a good approximation of the bandgap energy of Ag 2 O thin films coated on glass substrate.The band gap of the Ag 2 O thin film annealed at 100˚C was 1.716 eV.This value gradually decreased to 1.6098 and 1.559 eV as the annealing temperature increased to 150˚C and 200˚C, respectively.The two linear sections shown in the curve indicate the allowed indirect

Figure 4 .
Figure 4. Band gap values of Ag 2 O thin films with different annealing temperature.