Synthesis and Characterization of Silver Aluminium Selenide (AgAlSe2), Absorbent Materials prepared by Electrodeposition Technique.

Synthesis and Characterization of Silver Aluminium Selenide (AgAlSe2), Absorbent Materials prepared by Electrodeposition Technique. 1Muomeliri B. C., 2Okereke N. A. and Chibuogwu I.U2 1Department of Physics and Industrial Physics, Nnamdi Azikiwe University, Awka Anambra State, Nigeria. 2Department of Industrial Physics, Chukwemeka Odumegwu Ojukwu University, Anambra State, Nigeria. Email: [email protected] ABSTRACT, Silver Aluminium selenide (AgAlSe2) thin films have been successfully grown through the reaction of AgNO3, AlCl3.6H2O and Selenium metal powder unto glass substrates using electrochemical deposition technique and annealed at 150°C. The resulting thin films were characterized using Rutherford Backscattering Spectroscopy (RBS), UV-VIS Spectrophotometers and X-ray diffractometer to determine the films composition, the optical and structural properties and then to determine the possible areas of application of the films through the analysis of the results of the characterization. Optical analysis of the films revealed high absorbance in the UV region which decreased in the visible and infrared regions as the concentration of selenide ion increases and increased when annealed. The structural analysis indicates that AgAlSe2 films are of tetragonal crystal structure with lattice constant a = 5.956 Å, b = 5.956 Å and c = 10.750 Å respectively. The result of the RBS showed that the grown films are rich in silver. The band gap of the deposited films was found to decrease from the range of 3.7eV to 3.95 eV to the range of 3.3 eV to 3.8 eV respectively as a result of annealing at varying concentration of selenide ion. The optical and structural analyses of the films revealed significant effect of annealing and varying concentration of selenide ion on the properties of AgAlSe2 films. The thin films that exhibit these properties are desirable for photovoltaic devices, optical and opto-electronic applications. Keywords: Thin Films, electrochemical deposition, AgAISe2, absorbance, band gap energy. 1. INTRODUTION Thin film technology plays an important role in almost electronic and optical devices. Many industries throughout the world are involved with thin films. It has been applied as electroplated films for decoration and protection (Heaven, 1970). Thin films have long been used as anti-reflection coatings on window glass, video screens, camera lenses and other optical devices. Thin films can be defined as thin materials that form by process of atom-byatom, molecule-by-molecule, ion-by-ion or cluster of species-by-cluster of species condensation process. Also, a thin film is a very thin layer of a substance on a supporting material; especially: a coating (as of a semiconductor) that is deposited in a layer one atom or one molecule thick. It is layer of material ranging from fractions of a nanometre (monolayer) to several micrometers in thickness. Electronic semi-conductor devices and optical coating are the main applications benefiting from thin film deposition. These films are less than 100nm thick and are made from dielectric transparent materials. They have refractive index less than that of the substrate (Pentia et al, 2004). Thin films have found many applications in today’s energy research and other industrial applications. Renewable energy sources that can be developed include continuous natural energy flows such as sunlight (solar energy), ocean currents, wave, falling water, wind energy whose replenishment is far greater than projected human use. Out of these energy choices, solar energy is certainly one of the most attractive. Since sunlight is mostly abundant in most countries of the world especially the less developed countries mostly Africa; it is hoped that if this particular source of energy is researched into and developed, the bridging of the technological gap between the third world countries and most of the developed countries is then feasible (Nnabuchi, 2004). Aluminium selenide (Al2Se3) has been used as a precursor to produce hydrogen selenide, which is released when the solid is treated with acids. It should be stored away from moisture and air as it is hydrolytically unstable. Heterostructures combining III-VI semiconductors with silicon have attracted attention (Brinker et al., 1991) due to their close lattice matching and promising optoelectronic properties. Al2Se3 is the least studied of the group III-VI chalcogenide (Chopra and Das, 1983) when compared to other members of the III-VI family of semiconductors. The first growth of aluminum selenide on silicon is reported by Cui et al, (2003). Silver selenide, a group I-VI semiconductor compound is a mixed ionic conductor (Kumar and Pradeep 2002). Ag2Se undergoes a polymorphic phase transition, a lowtemperature orthorhombic phase (βAg2Se) at 0 K, and a high temperature cubic phase (α-Ag2Se) with a transition temperature of 135 ˚C (Santhosh and Pradeep, 2002; Abdullaev, et al., 2014). Silver selenide exhibits many interesting and useful properties (Sreenivas et al, 1997). A low temperature phase (β-Ag2Se) is a narrow band gap and n-type semiconductor (Rao et al., 2002) with resistivity of 10‾3 and 10‾4 Ωcm (Shukla et al., 1981). β - Ag2Se is used as a photo-sensitizer in photographic films and in thermochromic materials due to its relatively high Seeback coefficient, low lattice thermal conductivity, and high electrical conductivity (Cui et al, 2003). A high temperature phase (α-Ag2Se) is a superionic conductor. It finds application in solid electrolytes in photochargable secondary batteries (Kobayashi, 1990). Besides, the present research describes synthesis of Silver Aluminium Selenide (AgAlSe2) thin films by electrochemical technique, using selenium dioxide (powder) as source of Se2ion and Silver chloride as source Ag+ and Al3S as source Al3+ ion. The AgAlSe2 (Ternary) compound belong to group I-III-VI semiconductor thin film. The preparative parameters such as molar concentration of precursor of selenide is to be studied and reported. 2. EXPERIMENTAL DETAIL The AgAlSe2, (AAS) thin films were deposited by by Electrodeposition Technique on glass substrates. A good effort was made to synthesize silver Aluminumselenide from selenium (Se) metal powder, aluminum chloride (AlCl3) and silver trioxonitrate (v) (AgNO3) employing electrochemical deposition method. Many trial attempts were made to synthesize AgAlSe2 from different ratios of the above named materials and finally the following ratio were selected to obtain AgAlSe2 by electrodeposition techniques: 1. 0.1M AlCl3. 6H2O (Aluminum chloride hexahydrate) 2. 0.1M AgNO3 (Silver trioxonitrate (v)) and 3. 0.1M selenium metal powder (of various molar concentration for variation of selenide source). Saturated calomel electrode (SCE) and carbon electrode were used as reference electrode and counter electrode respectively. Glass substrates were chosen to be the working electrodes. They were degreased and cleaned with acetone and methanol and allowed to dry in air before electrodeposition. Substrate Pre-Treatment In the electrodeposition technique, usually there is need to subject the substrates to distinct pre-treatment to ensure that the presence of catalytic surface is removed to improve the adhesion of the films to the substrates particularly as film thickness increases. This pretreatment of glass slides substrates involves the following steps; i. Cleaning the glass slides with detergents. ii. Soaking the slides in acetone for about 15 minutes for degreasing. iii. Ultrasonicating the substrates/slides for about 10 minutes in an ultrasonic bath. iv. Drying the substrates in an electronic thermal heated dryer called an oven for about 510 minutes at a temperature of about 500C to 60°C. Preparation of Precursors 0.1 M of AgNO3 solution was prepared by dissolving 1.7 g of AgNO3 salt in 100 ml of distilled water. 0.1 M of AlCl3.6H2O solution was prepared by dissolving 2.41 g of the salt (AlCl3.6H2O) in 100 ml of distilled water. These two solutions prepared (ie AgNO3 and AlCl3.6H2O solutions) served as the cationic precursor for precipitation of Al3+ and Ag+ ions. Also, 0.1 M selenium solution was prepared by dissolving 0.79 g of selenium metal powder in 100 ml of distilled water. Variation of molar concentration of selenide ion AAS (AgAlSe2) films were deposited at room temperature at a constant voltage of 10 Volts for 5 minutes at a five variable/different molar concentration of selenide source as seen in table one below. The three chemicals prepared were kept in three different beakers. 12 ml of each of the chemicals/sample was collected and transferred to the electrochemical deposition apparatus containing pure carbon (graphite) electrode, saturated calomel electrode (SCE) and working electrode, Glass substrates. Also, power supply and multimeter which is set to measure voltage and current were connected to the apparatus as shown above in fig 3:1 The molar concentration of selenide source was varied at five different concentration of selenide metal powder, that is, 0.1 M, 0.2 M, 0.4 M and 0.5 M while the molar concentration of AgNO3 and AlCl3.6H2O remain unchanged throughout the experiment. After stirring the mixture of the three samples/reagents collected together, all the three electrodes were immersed into the big container containing the reagents. Wooden guide/non-conductor was used to fasten the electrode erect in place in the container. The DC voltage supply was then switched on at constant voltage of 10 volts and constant time of 5 minutes. The samples B1, B2, B4 and B5 were annealed at 150oC for 5 minutes, while the samples A1, A2, A4, A5 are un-annealed for the same concentrations 0.1 M, 0.2 M, 0.4 M and 0.5 M of selenium source. Table 3.1: Variation of Molar Concentration of Selenide ion which was annealed at 1500C S/N SAMPLE (SP) CONC. SELENIUM /MOLE (M) WV / V WORKING VOLTAGE WC/A WORKING CURRENT 1 B1 0.1 1.00 0.50 2 B2 0.2 1.00 0.50 4 B4 0.4 1.00 0.50 5 B5 0.5 1.00 0.50 Table 3.2: Variation of Molar Concentration of Selenide ion un-annealed. S/N SAMPLE (SP) CONC. SELENIUM /MOLE (M) WV / V WORKING VOLTAGE WC/A WORKING CURRENT 1 A1 0.1 1.00 0.50 2 A2 0.2 1.00 0.50 4 A4 0.4 1.00 0.50 5 A5 0.5 1.00 0.50 4. RESULTS AND DISCUSSION ABSORBANCE. Figure 4.1: Graph of Absorbance against Wavelength for un-annealed samples (A) Figure 4.2: Graph of Absorbance against Wavelength for annealed samples (B) The spectral absorbance of the thin films of Silver Aluminum Selenide (AgAlSe) as a function of wavelength for the un-annealed and annealed samples were displayed in figures 4.1 and 4.2 respectively. The graphs shown that generally the absorbance of the film is high in the UV regions but decreases gradually towards the visible and near infrared regions of the 0 20 40 60 80 100 120 140 160 180 300 400 500 600 700 800 900 1000 1100 Absorbance A Wavelength λ (nm) 0.1M 0.2M 0.4M 0.5M 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 300 400 500 600 700 800 900 1000 1100 Absorbance A Wavelength λ (nm) 0.1M 0.2M 0.4M 0.5M electromagnetic regions. Besides, the absorbance increases as the molar concentration of the selenide ion increases for the un-annealed sample and decreases for the annealed sample. 4.2: TRANSMITTANCE Figure 4.3: Graph of %Transmittance against Wavelength for un-annealed samples (A). Figure 4.4: Graph of %Transmittance against Wavelength for annealed samples (B). Figures 4.3 and 4.3 are plots of the transmittance against the wavelength of the as deposited thin film samples. The plot indicates that the transmittance of the films is low in the UV region but increases in the VIS and NIR regions. Furthermore, the transmittance of the films increases as the molar concentration of selenide ion increased for the annealed samples but films decreases as the molar concentration of selenide ion increases for the un-annealed sample. BAND GAP ENERGY. 0 10 20 30 40 50 60 70 300 450 600 750 900 1050 1200 % Transmittance Wavelength λ (nm) 0.1M 0.2M 0.4M 0.5M 0 10 20 30 40 50 60 300 400 500 600 700 800 900 1000 1100 % Transmittance Wavelength λ (nm) 0.1M 0.2M 0.4M 0.5M Figure 4.19: Graph of square of absorption coefficient (αh)2 against Photon Energy (h) for un-annealed samples (A). The plot of square of absorption coefficient (αhʋ)2 against photon energy (h) for determination of band gap value of the deposited AgAlSe2 thin films was displayed in figure 4.19 for un-annealed. The band gap of the deposited thin film is in the range of 3.7 to 3.9 eV. The graph also shows that the band gap energy increases with increase in selenide ion concentration for un-annealed samples. Figure 4.20: Graph of square of absorption coefficient (αhʋ)2 against Photon Energy (h) for annealed samples (B). Figure 4.20 shows the plot of square of absorption coefficient (αhʋ)2 against photon energy (h) for determination of band gap value of the AgAlSe2 thin film for the annealed samples. The band gap of the annealed thin film ranges from 3.5 to 3.7 eV as a result of annealing. COMPOSITIONAL ANALYSIS 0 100 200 300 400 500 600 0 0.4 0.8 1.2 1.6 2 2.4 2.8 3.2 3.6 4 4.4 4.8 Absorption coefficient (αhʋ) X1012 Photon energy (hʋ) 0 100 200 300 400 500 600 700 800 900 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4 4.2 4.4 Square of absorption Coeffcient (αhʋ)2 X1012 Photon Energy (hʋ) 0.1 M 0.2 M 0.4 M 0.5 M Figure 4.25: Compositional analysis of the un-annealed AgAlSe2 thin film 0.1M (sample A1). LAYER 1: THICKNESS 1527.10x1015 Atoms/cm2 or 364 nm Compo: Ag 70.65% Se 2.60% Al 26.75% LAYER 4: THICKNESS 14000x1015 Atoms/cm2 or 3333 nm Compo: Si 15.16% O 55.38% Ca 1.20% Na 22.59% Al 3.55% K 0.53% Fe 0.41% Mg 0.14% Ti 0.95% S 0.11% SAM-A1.dat Simulated O Na Mg Al Si S K Ca Ti Fe Se Ag Channel 1,8001,7001,6001,5001,4001,3001,2001,1001,000900800700600500400 Counts 150 145 140 135 130 125 120 115 110 105 100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0 600 800 1000 1200 1400 1600 1800 2000 2200 Energy [keV] Figure 4.26: Compositional analysis of the annealed AgAlSe3 thin film 0.1M (sample B). LAYER 1: THICKNESS 1657.81x1015 Atoms/cm2 or 395 nm Compo: Ag 57.82% Se 12.36% Al 29.82% LAYER 4: THICKNESS 14000x1015 Atoms/cm2 or 3333 nm Compo: Si 15.16% O 55.38% Ca 1.20% Na 22.59% Al 3.55% K 0.53% Fe 0.41% Mg 0.14% Ti 0.95% S 0.11% The Rutherford Backscattering Spectroscopy (RBS) spectra showing the elements in the grown AgAlSe2 film were shown in figures 4.25 and 4.26. The composition ratio was found to be 27.2:10.3:1 of atomic mass percent for Ag, Al and Se respectively for the un-annealed film while the composition ratio of 4.6:2.4:1 of atomic mass percent for Ag, Al and Se was obtain for the annealed film. This indicates that the grown films are rich in silver. The silicon, oxygen, calcium, iron, sodium, magnesium, tin, potassium, etc with various percentage detected in layer 4 are believed to have come from the glass substrates used for the deposition. SAM-B1.dat Simulated O Na Mg Al Si S K Ca Ti Fe Se Ag Channel 1,8001,7001,6001,5001,4001,3001,2001,1001,000900800700600500400 Counts 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 600 800 1000 1200 1400 1600 1800 2000 2200 Energy [keV]