Chapter-IV OPTICAL PROPERTIES

Transcription

1 Chapter-IV OPTICAL PROPERTIES 4.1 Ultraviolet/ Visible Spectroscopy UV- visible spectroscopy of wavelength is shorter than visible light, but longer than X- rays. UV-Violet color is the shortest wavelength in the visible spectrum. The typical range of UV-wavelength is nm.UV is invisible to human eyes. 4.2 UV Absorption UV-absorption can be analyzed when a beam of light is passing through the sample and it is reflected from the surface of the sample. UV-absorption forms at a single or a wide range of spectral wavelengths. During the transition of electrons the excitation takes place to higher energy levels. UV-Visible is used to identify inorganic complexation of molecules and their qualitative and quantitative measurements [77]. The main role for UV-Visible is to identify the properties of optical and electronic materials and to characterize the absorption, transmission, and reflectivity materials Beer- Lambert Law The UV absorption a may fallow the Beer- Lambert Law. Absorbance (A) is directly proportional to the path length (l, cm) and the concentration (c, mol/l) of the absorbance species A= ϵlc, (9) Where ϵ is the molar extension coefficient and it is measure of the amount of light absorbed per unit of concentration. The Beer- Lambert Law expressed by the equation (9.5) is valid at low concentrations. At high concentrations the law is no longer valid because the relationship between A and c are non linear. Where Beer- Lambert Law is invalid, UV-Visible absorption cannot be used for quantitative analysis, before UV-Visible absorption and Beer- Lambert Law to determine the concentration of a substance of intrest in a solution, a calibration curve must be made 71

2 by measuring UV-Visible absorption as a function of concentration for the Beer-Lambert Law. 4.4 UV-Visible Spectra: In a semiconductor material the charge carriers is generated by optical photons. The absorption takes in any material doe to the fallowing aspects i). Charge carriers electrons ii). Valence band electrons iii). Excitation of electrons iv). Inner shell electrons The valance band in a semi conductor is fills with electrons, such that the excitation takes place to higher energy levels during the absorption process. As the photon energy in a semi conducting materials the quantization of energy tends to transfer of electrons from valence band to conduction band. Direct band gap and indirect band gap of optical transitions tales place in a semi conducting materials. This two type band gap involves in the interaction with an electron in the valence band. However, direct band gap involves a vertical transition of electrons from the valence band to the conduction band. Whereas indirect band gap involves simultaneous interaction with lattice vibrations. Direct band gap transition the moment of electrons have stabilized hence energy is conserved, and the moment of electrons takes place by phonons in indirect band gap [78-79]. The working principal of UV-Visible spectra is as shown in Fig 4.1. Absorption coefficient (α) was calculated by using the following equations I=I o exp (-αx) (10) Hence α=2.303/x, log (I/I o ) = (2.303/x) A In the energy level the photon is incident by its energy direct band gap obtained and the absorption coefficient is given by αhυ= C (hυ-e g ) 1/2 (11) Where C is Constant which is dependent on the structure of sample, α is the absorption coefficient and h is a planks constant. 72

3 In an energy level phonon is required for transitions, where the absorption coefficient is fallowing by αhυ= A (hυ-e g -E p ) 2 +B (hυ-e g -E p ) 2 (12) Light Source Diffraction Grating Mirror 1 Slit 1 Slit 2 Filter Mirror 3 Sample Detector 1 Half Mirror Mirror 2 Reference Lens 1 Detector 2 Mirror 4 Lens 2 Fig 4.1. The working principal of UV-Visible spectra The optical absorption takes place in semiconductors and depends on the important points which are given below. i). The electronic transitions of elements are constant 73

4 ii). The absorption and emission process are under consideration. UV-visible Spectroscopy is used for identify the energy band gap values of the materials in the transmitting radiation. In an energy level a photon is absorbed in its orbit. When an electron is jumps from lower energy level to higher energy level. Transitions takes place in a band gap energy as it rise in the absorption process called as absorption edge, where the optical band gap energies be determined [80-82]. Conducting materials are having energy bands and classified in to two types (1) Direct band gap (2) Indirect band gap. The top of the valence band and bottom of the conduction band are same i.e. zero crystal momentum then direct band gap takes place, where as in indirect band gap The top of the valence band and bottom of the conduction band are invariant i.e. zero crystal momentum, which is accumulated with phonon of magnitude of crystal momentum Davis and shalliday [83]. The schematic diagram of Optical direct and indirect band gap is shown in Fig 4.2. Fig Direct inter band optical transitions for a) Direct band b) Indirect band semiconductors. 74

5 Chapter OPTICAL PROPERTIES: PVP complexed with MgCl 2 6H 2 O: Optical analysis is used for identify the vibrational bands and the transitions of electrons in an energy levels. A solid polymer electrolyte thin film has been prepared with the complexation of inorganic salt (MgCl 2 6H 2 O) and the polymer PVP. UV-Visible spectroscopy has been introduced for the optical properties of the given material. Where the optical band gap of absorption coefficient (α) is calculated by the following equitation. I=I o exp (-αx) To calculate band gap energy values graphs were plotted between (α), (αhυ) 2,(αhυ) 1/2 as a function of (hυ). The linear portion of the (α) versus (hυ) curves to zero absorption value. Absorption values for 5%,10%,15wt %,20Wt% of Mgcl 2 6H 2 O doped PVP films lies at 4.84,4.80,4.73 and 4.65 ev, which is observed from Fig (a)pure (b)95:5 (e) 1600 (c)90:10 (d) (d) 85:15 (e) 80: (c) (b) (a) h (ev) Fig 4.3. (α) vs (hυ) (Photon energy) plots of (a) Pure PVP (b) PVP+Mgcl 2 6H 2 O (95:5) (c) PVP+Mgcl 2 6H 2 O (90:10) (d) PVP+Mgcl 2 6H 2 O (85:15) (e) PVP+Mgcl 2 6H 2 O (80:20) 75

6 In the energy level the photon is incident by its energy direct band gap exists and the absorption coefficient is given by αhυ= C (hυ-e g ) 1/2 Where E g is the energy band gap, C is Constant which is dependent on the structure of sample, α is the absorption coefficient and h is a planks constant. From the graph (αhυ) 2 versus hυ direct band gap values are obtained. Energy gap values are 4.99, 4.96, 4.45 and 4.89 ev as shown in Fig E E E+007 (a)pure (b)95:5 (c)90:10 (d)85:15 (e)80:20 (e) (d) ( h 6.00E E E+007 (c) (b) (a) 0.00E h ev) Fig.4.4: (α) vs (αhυ) 2 (Photon energy) plots (a) Pure PVP (b) PVP+Mgcl 2 6H 2 O (95:5) (c) PVP+Mgcl 2 6H 2 O (90:10) (d) PVP+Mgcl 2 6H 2 O (85:15) (e) PVP+Mgcl 2 6H 2 O (80:20) where indirect band gap values are obtained by plotting (αhυ) 1/2 versus hυ as shown in Fig 4.5.Indirect band gap values are obtained from the graph are 4.87,4.86,4.85 and 4.83 respectively. 76

7 110 ( h (a)pure (b)95:5 (c)90:10 (d)85:15 (e)80:20 (e) (d) (c) (b) (a) h ev) Fig.4.5. (α) vs (αhυ) 1/2 (Photon energy) plots (a) Pure PVP (b) PVP+Mgcl 2 6H 2 O (95:5) (c) PVP+Mgcl 2 6H 2 O (90:10) (d) PVP+Mgcl 2 6H 2 O (85:15) (e) PVP+Mgcl 2 6H 2 O (80:20) In an energy level phonon is required for transitions, where the absorption coefficient is fallowing by αhυ= A (hυ-e g -E p ) 2 +B (hυ-e g -E p ) 2 From the obtained values it is clear that the dissolution of salt is completely attains in polymer chains in a host lattice thus the activation energy is decreased such that the ionic conductivity is increased, on doping with Mgcl 2 6H 2 O salt to the polymer the Direct and indirect band gaps values shifted to lower energies. The Direct, Indirect band gaps and Absorption edge values are shown in the Table.1.1. From Table.1.1 the lowest bang gap value is found for the PVP+ Mgcl 2 6H 2 O (85:15) at 4.45 ev. Which reveals that the sample (85:15) has having high ionic conductivity. This is a good agreement with DC conductivity. 77

8 Polymer electrolyte Optical band gap Direct(eV) Indirect (ev) Absorption edge Pure PVP PVP+ Mgcl 2 6H 2 O (95:5) PVP+ Mgcl 2 6H 2 O (90:10) PVP+ Mgcl 2 6H 2 O (85:15) PVP+ Mgcl 2 6H 2 O (80:20) Table.1.1 Optical band gap values of PVP+MgCl 2 6H 2 O solid polymer electrolytes PVP complexed with MgCl 2 6H 2 O+ Nano particles: Optical analysis is used for identify the vibrational bands and the transitions of electrons in an energy levels. A solid polymer electrolyte thin film has been prepared with the complexation of inorganic salt with the addition of nano particles (MgCl 2 6H 2 O+Al 2 O 3 ) and the polymer PVP. UV-Visible spectroscopy has been introduced for the optical properties of the given material. Where the optical band gap of absorption coefficient (α) is calculated by the following equation. I=I o exp (-αx) To calculate band gap energy values graphs were plotted between (α), (αhυ) 2,(αhυ) 1/2 as a function of (hυ). The linear portion of the (α) versus (hυ) curves to zero absorption value. Absorption values for 5%,10%,15wt %,20Wt% of Mgcl 2 6H 2 O +Al 2 O 3 doped PVP films lies at 3.69,3.61,3.42 and 3.50 ev, which is observed from Fig

9 Chapter PurePVP 95:5 90:10 85:15 80: h ev) Fig 4.6. (α) vs (hυ) (Photon energy) plots of (a) Pure PVP (b) PVP+Mgcl 2 6H 2 O+ Al 2 O 3 (95:5) (c) PVP+Mgcl 2 6H 2 O+ Al 2 O 3 (90:10) (d) PVP+Mgcl 2 6H 2 O+ Al 2 O 3 (85:15) (e) PVP+Mgcl 2 6H 2 O+ Al 2 O 3 (80:20) In the energy level the photon is incident by its energy direct band gap exists and the absorption coefficient is given by αhυ= C (hυ-e g ) 1/2 Where E g is the energy band gap, C is Constant which is dependent on the Structure of the sample, α is the absorption coefficient and h is a planks constant. From the graph (αhυ) 2 versus hυ direct band gap values are obtained. Energy gap values are 3.50, 3.48, 3.28 and 3.32 ev as shown in Fig

10 4.5x x x x10 7 PurePVP 95:5 90:10 85:15 80:20 2.5x10 7 h 2.0x x x x h Fig.4.7: (α) vs (αhυ) 2 (Photon energy) plots (a) Pure PVP (b) PVP+Mgcl 2 6H 2 O+ Al 2 O 3 (95:5) (c) PVP+Mgcl 2 6H 2 O+ Al 2 O 3 (90:10) (d) PVP+Mgcl 2 6H 2 O+ Al 2 O 3 (85:15) (e) PVP+Mgcl 2 6H 2 O+ Al 2 O 3 (80:20) Where indirect band gap values are obtained by plotting (αhυ) 1/2 versus hυ as shown in Fig 4.8. Indirect band gap values are obtained from the graph are 3.67, 3.46, 3.31, 3.15 and 3.29 respectively. 80

11 PurePVP 95:5 90:10 85:15 80:20 65 ( h h Fig.4.8. (α) vs (αhυ) 1/2 (Photon energy) plots (a) Pure PVP (b) PVP+Mgcl 2 6H 2 O+ Al 2 O 3 (95:5) (c) PVP+Mgcl 2 6H 2 O+ Al 2 O 3 (90:10) (d) PVP+Mgcl 2 6H 2 O+ Al 2 O 3 (85:15) (e) PVP+Mgcl 2 6H 2 O+ Al 2 O 3 (80:20) In an energy level phonon is required for transitions, where the absorption coefficient is fallowing by αhυ= A (hυ-e g -E p ) 2 +B (hυ-e g -E p ) 2 From the obtained values it is clear that the dissolution of salt is completely attains in polymer chains in a host lattice thus the activation energy is decreased such that the ionic conductivity is increased, on doping with Mgcl 2 6H 2 O+Al 2 O 3 salt to the polymer the Direct and indirect band gaps values shifted to lower energies. The Direct, Indirect band gaps and Absorption edge values are shown in the Table

12 Polymer electrolyte Optical band gap Direct(eV) Indirect (ev) Absorption edge Pure PVP PVP+ Mgcl 2 6H 2 O+Al 2 O 3 (95:5) PVP+ Mgcl 2 6H 2 O+Al 2 O 3 (90:10) PVP+ Mgcl 2 6H 2 O+Al 2 O 3 (85:15) PVP+ Mgcl 2 6H 2 O+Al 2 O 3 (80:20) Table 1.2. Optical band gap values of PVP+MgCl 2 6H 2 O+Al 2 O 3 solid polymer electrolytes PVP complexed with MgSO 4 7H 2 O: Optical analysis is used for identify the vibrational bands and the transitions of electrons in an energy levels. A solid polymer electrolyte thin film has been prepared with the complexation of inorganic salt with the addition of nano particles (MgSO 4 7H 2 O) and the polymer PVP. UV-Visible spectroscopy has been introduced for the optical properties of the given material. Where the optical band gap of absorption coefficient (α) is calculated by the following equitation. I=I o exp (-αx) To calculate band gap energy values graphs were plotted between (α), (αhυ) 2,(αhυ) 1/2 as a function of (hυ). The linear portion of the (α) versus (hυ) curves to zero absorption value. Absorption values for 5%,10%,15wt %,20Wt% of Mgcl 2 6H 2 O doped PVP films lies at 4.93,4.76,4.49,4.54 and 4.36eV, which is observed from Fig

13 Chapter Pure 95:5 90:10 85:15 80: h (ev) Fig 4.9. (α) vs (hυ) (Photon energy) plots of (a) Pure PVP (b) PVP+ MgSO 4 7H 2 O (95:5) (c) PVP+ MgSO 4 7H 2 O (90:10) (d) PVP+ MgSO 4 7H 2 O (85:15) (e) PVP+ MgSO 4 7H 2 O (80:20) In the energy level the photon is incident by its energy direct band gap exists and the absorption coefficient is given by αhυ= C (hυ-e g ) 1/2 Where E g is the energy band gap, C is Constant which is dependent on the Structure of the sample, α is the absorption coefficient and h is a planks constant. From the graph (αhυ) 2 versus hυ direct band gap values are obtained. Energy gap values are 5.02, 4.72, 4.81 and 4.65 ev as shown in Fig

14 h ) 2 Chapter E E E E+008 Pure 95:5 90:10 85:15 80: E E E E h ev) Fig.4.10: (α) vs (αhυ) 2 (Photon energy) plots (a) Pure PVP (b) PVP+ MgSO 4 7H 2 O (95:5) (c) PVP+ MgSO 4 7H 2 O (90:10) (d) PVP+ MgSO 4 7H 2 O (85:15) (e) PVP+ MgSO 4 7H 2 O (80:20) Where indirect band gap values are obtained by plotting (αhυ) 1/2 versus hυ as shown in Fig Indirect band gap values are obtained from the graph are 4.75, 4.43, 4.50 and 4.39 respectively. 84

15 Pure 95:5 90:10 85:15 80:20 h h (ev) Fig (α) vs (αhυ) 1/2 (Photon energy) plots (a) Pure PVP (b) PVP+ MgSO 4 7H 2 O (95:5) (c) PVP+ MgSO 4 7H 2 O (90:10) (d) PVP+ MgSO 4 7H 2 O (85:15) (e) PVP+ MgSO 4 7H 2 O (80:20) In an energy level phonon is required for transitions, where the absorption coefficient is fallowing by αhυ= A (hυ-e g -E p ) 2 +B (hυ-e g -E p ) 2 From the obtained values it is clear that the dissolution of salt is completely attains in polymer chains in a host lattice thus the activation energy is decreased such that the ionic conductivity is increased, on doping with MgSO 4 7H 2 O salt to the polymer the Direct and indirect band gaps values shifted to lower energies. The Direct, Indirect band gaps and Absorption edge values are shown in the Table

16 Polymer electrolyte Optical band gap Direct(eV) Indirect (ev) Absorption edge Pure PVP PVP+ MgSO 4 7H 2 O (95:5) PVP+ MgSO 4 7H 2 O (90:10) PVP+ MgSO 4 7H 2 O (85:15) PVP+ MgSO 4 7H 2 O (80:20) Table.1.3. Optical band gap values of PVP+ MgSO 4 7H 2 O solid polymer electrolytes PVP complexed with MgSO 4 7H 2 O+Nano particles: Optical analysis is used for identify the vibrational bands and the transitions of electrons in an energy levels. A solid polymer electrolyte thin film has been prepared with the complexation of inorganic salt with the addition of nano particles (MgSO 4 7H 2 O+TiO 2 ) and the polymer PVP. UV-Visible spectroscopy has been introduced for the optical properties of the given material. Where the optical band gap of absorption coefficient (α) is calculated by the following equitation. I=I o exp (-αx) To calculate band gap energy values graphs were plotted between (α), (αhυ) 2,(αhυ) 1/2 as a function of (hυ). The linear portion of the (α) versus (hυ) curves to zero absorption value. Absorption values for 5%,10%,15wt %,20Wt% of Mgcl 2 6H 2 O doped PVP films lies at 4.93,4.75,4.51,4.64 and 4.48 ev, which is observed from Fig

17 Chapter PurePVP 95:5 90:10 85:15 80: h Fig (α) vs (hυ) (Photon energy) plots of (a) Pure PVP (b) PVP+ MgSO 4 7H 2 O+TiO 2 (95:5) (c) PVP+ MgSO 4 7H 2 O+TiO 2 (90:10) (d) PVP+ MgSO 4 7H 2 O+TiO 2 (85:15) (e) PVP+ MgSO 4 7H 2 O+TiO 2 (80:20) In the energy level the photon is incident by its energy direct band gap exists and the absorption coefficient is given by αhυ= C (hυ-e g ) 1/2 Where E g is the energy band gap, C is Constant which is dependent on the specimen structure, α is the absorption coefficient is the frequency of light and h is a planks constant. From the graph (αhυ) 2 versus hυ direct band gap values are obtained. Energy gap values are 4.2, 5.23, 5.0, 5.10 and 4.92 ev as shown in Fig

18 ( h Chapter-4 7x10 7 6x10 7 5x10 7 PurePVP 95:5 90:10 85:15 80:20 4x10 7 3x10 7 2x10 7 1x h Fig.4.13: (α) vs (αhυ) 2 (Photon energy) plots (a) Pure PVP (b) PVP+ MgSO 4 7H 2 O+TiO 2 (95:5) (c) PVP+ MgSO 4 7H 2 O+TiO 2 (90:10) (d) PVP+ MgSO 4 7H 2 O+TiO 2 (85:15) (e) PVP+ MgSO 4 7H 2 O+TiO 2 (80:20) Where indirect band gap values are obtained by plotting (αhυ) 1/2 versus hυ as shown in Fig Indirect band gap values are obtained from the graph are 5.1, 4.61, 4.38, 4.49 and 4.27 respectively. 88

19 PurePVP 95:5 90:10 85:15 80:20 60 ( h Fig (α) vs (αhυ) 1/2 (Photon energy) plots (a) Pure PVP (b) PVP+ MgSO 4 7H 2 O+TiO 2 (95:5) (c) PVP+ MgSO 4 7H 2 O+TiO 2 (90:10) (d) PVP+ MgSO 4 7H 2 O+TiO 2 (85:15) h (e) PVP+ MgSO 4 7H 2 O+TiO 2 (80:20) In an energy level phonon is required for transitions, where the absorption coefficient is fallowing by αhυ= A (hυ-e g -E p ) 2 +B (hυ-e g -E p ) 2 From the obtained values it is clear that the dissolution of salt is completely attains in polymer chains in a host lattice thus the activation energy is decreased such that the ionic conductivity is increased. 89

20 Polymer electrolyte Optical band gap Direct(eV) Indirect (ev) Absorption edge Pure PVP PVP+ MgSO 4 7H 2 O+TiO 2 (95:5) PVP+ MgSO 4 7H 2 O+TiO 2 (90:10) PVP+ MgSO 4 7H 2 O+TiO 2 (85:15) PVP+ MgSO 4 7H 2 O+TiO 2 (80:20) Table.1.4. Optical band gap values of PVP+ MgSO 4 7H 2 O+TiO 2 solid polymer electrolytes On doping with MgSO 4 7H 2 O+TiO 2 salt to the polymer the Direct and indirect band gaps values shifted to lower energies. The Direct, Indirect band gaps and Absorption edge values are shown in the above Table