3.1 Thickness Measurement:-
We use following formula in order to calculate thickness of deposited thin film:-
t = m /Aρ
Where t is the thickness of film, m- is the mass of deposited material, A is the area of deposition, ρ is the standard value of density of material. The thickness of prepared undoped sample (Cd-Zn)S is 1.17 µm and the La Doped Sample (Cd-Zn)S:La thickness is 1.12 µm. (Khare et al. 2006 reported thickness as 1–2 µm)
3.2 Absorption Spectra:-
The absorption coefficient ‘α’ depends on the transition probability between ground state and excited state. The exponential equation that absorption coefficient satisfied is :-
I = I 0 exp(-αt)
Where I is attenuated intensity of radiation, I0 is the initial intensity of radiation, and t is thickness of sample.
(Table 3.1- Absorption and Band gap data of prepared thin film)
S.NO.
|
Sample Name
|
Absorption edge (nm)
|
Band gap (eV)
|
1
|
(Cd-Zn)S
|
550 nm
|
2.12
|
2
|
(Cd-Zn)S:La
|
520 nm
|
2.26
|
The optical absorption spectra of undoped and La doped (Cd-Zn)S films deposited at room temperature in water bath are presented in Fig. 3.1.The absorption edge found at 550nm for undoped (Cd-Zn)S thin film where as in doped film it is 520nm which are in the visible region. The presence of La in host material (Cd-Zn)S reflect information that the absorption shifted to the lower wavelength side that means blue shift observation found. This shift is because of the expansion in the band gap. The expansion of the band gap with the adjustment in the bearer fixation can be clarified by the M-B impact (Moss-Burstein impact). The Moss-Burstein effect comes due to the Pauli Exclusion Principle and mostly observed in semiconductors materials. Increasing band-gap reflects information the separation in energy between the top of the valence band and the unoccupied energy states in the conduction band. The shift observed when the Fermi energy (EF) lies in the conduction band for heavy n-type doping (or in the valence band for p-type doping). The filled states therefore block thermal or optical excitation. Consequently the measured band gap determined from the onset of interband absorption moves to higher energy. In the consequence of increased band gap values observed in doped films might also be due to the decreased crystallite size value. Shift toward shorter wavelength side in absorption edge not only shows reduce particle size but also because of the individual containments of the electron and holes.
For determination of the band gap and nature of materials prepared, plots a curve between (αhν)2 and hν. This plot is known as tauc’s plot. The following equation
α = c(hν-E g ) 1/2 /hν……………..(1)
Where α is the absorption coefficient, Eg is the direct band gap. The extrapolation of the linear portion of the graph to the energy axis at α = 0 gives the band gap energy Eg. The energy band gap value presented in table 3.1. Figure 3.2 (A) and (B) presented the tauc’s plot of undoped and La doped (Cd-Zn)S film. Doping of La caused increased in band gap from 2.12eV to 2.26eV. This reflect the size quantization found in the (Cd-Zn)S:La films which also influence the other optical properties. The presence of the rare earth not only reduces the absorption but also increases band gap value.
3.3 Photoluminescence Spectra :-
The following graphs contain the PL spectra of doped and undoped (Cd-Zn)S thin film under an excitation wavelength region from 200 nm to 800 nm are recorded.
(Table 3.2 - Observed peak positions of PL emission spectra of doped and undoped (Cd- Zn)S)
Sample
|
Peak Position ( nm)
|
Intensity (a.u.)
|
(Cd-Zn)S
|
|
691.687
|
|
|
(Cd-Zn)S:La
|
|
436.5
|
|
1015.5
|
The PL spectra of (Cd- Zn)S under an excitation wavelength of 274nm are recorded and are presented in Figs. 3.2(a). The spectra display strong peaks centered at 560nm and 720nm due to high energy photon in the undoped case and 720nm peaks correspond to band to band transition and other peaks at 560nm might be correspond to excitonic level emission. In case of doped material again two prominent peaks are observed which are 680nm and 520nm presented in Figs. 3.2(b). The peaks are shifted to lower wavelength side due to the doping of La in the host material. This indicate that blue shift of emission found in the prepared sample and that should be indicated to the increased band gap of (Cd- Zn)S. This shows that the quantum confinement effect occurs in the prepared sample when doing comes in picture. It is clear that the change in band gap is result of change in particle size i.e. increases in Eg with reducing particle size and discreteness in the continuum of valence band and conduction bands. The value of excitonic level is found to be 3.09eV. This emission in shorter wavelength side reflects information about the discreteness of energy states and the transition involving levels higher than lowest unoccupied CB.
4. XRD Study:-
The X-ray diffractograms of the undoped and La doped nanocrystalline (Cd- Zn)S) films prepared by CBD method on the glass substrate at room temperature is presented in Fig. 1(a,b,c,d). There are large numbers of peaks present in the XRD pattern which indicate the films of Ag2S are crystalline in nature. The assignments of peaks are made by comparing with JCPDS information and count of grid consistent (a) and their examination with the detailed qualities. The estimation of 2θ, intensity, interplannar distance (d), lattice parameter and phase are recorded in table 1–3. There are number of peaks observed in this sample. The synthesized material (Cd-Zn)S exhibit orthogonal, tetragonal, hexagonal, and cubic structure. The value of miler indices are (110)O, (012)T, (112)O, (022)C , (013)H.
It is observed that orthogonal phase is dominant in the prepared sample. The particle size calculated from the Scherer formula D = Kλ/βCosθ, where β is Full width half maxima which is strongly recommended in case of calculation of grain size of crystalline size of material. The crystalline sizes of the relative peaks are 16.30nm, 32.71nm, 32.89nm, 16.46nm, 22.06nm, respectively.
(Cd-Zn)S
|
Angle (2θ)
|
Intensity (a.u)
|
hkl
|
distance
d(Å)
|
Lattice Constance (Å)
|
25.2182
|
149.64
|
110 (O)
|
3.5286
|
a = 4.5100
b = 5.5870
c = 9.7000
|
29.3196
|
5923.99
|
012 (T)
|
3.0437
|
a = 6.7300
c = 6.8300
|
31.7795
|
842.43
|
112 (O)
|
2.8135
|
a = 4.5100
b = 5.5870
c = 9.7000
|
44.2548
|
1003.61
|
022 (C)
|
2.04504
|
a = 5.7790
|
51.8531
|
1230.79
|
013 (H)
|
1.76182
|
a = 3.8220
c = 6.2500
|
Formula λ = 2dsinθ used for the calculation of the average interplannar distance. The silver sulphide is known to exhibits both monoclinic and orthogonal structures. The XRD pattern of nanocrystalline Ag2S films shows hexagonal, tetragonal and cubic phase. The peak (200)H, (100)H (110)H and (104)H associated with hexagonal phase and the intensity of (100)H peaks are very high. The tetragonal phase corresponds to (200)T, (002)T, (112)T, (312)T and (111)C, (312)T and (111)C (200)C plane fall on cubic phase. It is observed that the hexagonal phases are dominant in the XRD spectra of Ag2S film. These spectra reflect information that the Ag2S films have nanocrystalline structure with different phases. By applying Debye Scherer’s formula for the XRD pattern, the size of the particle can be estimated:
D = Kλ/(β cos θ) ……(2)
Where, D is the mean size of crystallites in nm, K is crystallite shape factor a decent estimate is 0.9, λ is the X-ray wavelength, β is the full width at a half maximum (FWHM) in radians of the X-ray diffraction Pattern and θ is the Bragg's angle (deg.)
The lattice parameter of the thin films was calculated for the cubic, tetragonal, orthorhombic and hexagonal structure using the following relation:
Doped (Cd-Zn)S:La (3ml) 0.001M
|
Angle (2θ)
|
Intensity (a.u)
|
hkl
|
Interplaner distance d(Å)
|
Lattice Constance (Å)
|
26.94
|
89.23
|
111 (C)
|
3.336
|
a = 5.7790
|
44.03
|
56.36
|
202 (O)
|
2.0447
|
a = 4.5100, b = 5.5870, c = 9.7000
|
52.36
|
54.46
|
020 (H)
|
1.6549
|
a = 3.8220, c = 6.2500
|
For Cubic
1/d2 hkl = (h2 + K2 + l2)/a2 ………..(3)
For Tetragonal
1/d2 hkl = {(h 2 + K2)/ a2} +l2/c2 ……… (4)
For Orthorhombic:
1/d2 hkl =(h2 /a2 )+(k2 /b2 )+(l2 /c2 )……(5)
For Hexagonal:
1/d2 hkl = 4/3{(h2 + k2 + hk)/ a2}+l2/c2 ……(6)
The average value of lattice parameter was found to be 6.189 Å. The tetragonal, cubic and hexagonal phases depend upon the arrangement of the atomic layers. Using the Scherer’s formula particle size from the intense peaks of Ag2S film is calculated. The particle size is found to be 28.29nm, 28.39nm, 28.71nm, 28.77nm, 29.01nm, 29.17nm, 29.81nm, 29.87nm,30.94nm and 33.08nm for the (111)C, (200)T, (200)H, (002)T, (100)H, (112)T (110)H,(200)C (312)T and (104)H peaks respectively. All the value of particle size is smaller than the value calculated from the SEM image of the Ag2S films. The deviation of the value comes because of non-spherical morphology of the particle in the materials.
Particle Size (nm)
(Cd-Zn)S
|
Particle Size (nm)
(Cd-Zn)S:La
|
Difference of Size (nm)
|
16.30
|
7.93
|
8.37
|
32.71
|
6.30
|
26.41
|
32.86
|
7.02
|
25.84
|
(Table 3.4 - Particle Size Comparison of (Cd-Zn)S & (Cd-Zn)S:La)