Fabrication and characterization of tungsten oxide and molybdenum oxide multilayer thin films

EXPERIMENTAL SETUP

Different Thin Film Deposition Methods

Thin-film deposition active methods are based on the nature of the deposition process and can be divided into two groups i.e., physical and chemical methods. They probably used deposition processes

The physical deposition processes comprise vacuum evaporation laser ablation, molecular beam epitaxy (MBE), and sputtering, The chemical deposition processes include gas-phase deposition methods and solution techniques. Chemical vapors deposition (CVD) and atomic layer epitaxy (ALE) while spray pyrolysis, sol-gel, dip-coating, spin coating, methods employ precursor solutions and drop-casting method are the gas phase methods.

Dip coating Technique 

There are many methods for depositing the film e.g., spin coating, knife edging drop-casting and dip coating, etc. but dip-coating gives the best result among the above-mentioned technique. In which, the substrate holder moves constantly up and down and deposits the film in a very smooth, uniform, and fine way. Substrate tight with the sample holder. The solution is inserted in a bath a little bigger in size than the substrate. Si is attached to the substrate holder and the solution bath is placed exactly below the substrate holder. Substrate holder moving at a constant speed and dipped uniformly substrate in a solution bath. Four samples were prepared by using different speeds (100,110,120 and 130 mm/min) of dip coater. The condition is when the substrate holder moves with uniform speed towards the solution bath and deposits the film on it.

Advantages of dip coating

• Low-temperature processing
• Large area films can be fabricated
• Control of composition is much easier.
• Better homogeneity
• Large and complex-shaped substrates are easily coated using this process.

Tungstic oxide Solution Preparation

 To grow the phase of tungsten oxide, tungstic acid, hydrogen peroxide, and ethanol for stabilizing and deionized water is to be required for this purpose. Tungstic acid is in powder form and hydrogen peroxide is in liquid form. This all process was carried out at 80℃ temperatures by using a hot plate.

Tungstic acid is a yellow color powder that is used as a precursor in this method. An aqueous solution of peroxo-tungstic acid was obtained by dissolving fresh water-prepared tungstic acid into a hydrogen peroxide solution. Firstly, add a small amount of tungstic oxide powder to a beaker. At the same time, hydrogen peroxide few ml as per required for this solvent, and then add ethanol as a stabilizer. Tungstic acid and hydrogen peroxide was taken as precursor solution. The atomic masses of tungstic acid are 245.86 g/mol and hydrogen peroxide is 34.01486g/mol. By using the weighing balance, I took a very precise measurement of both materials and then add the required amount of both chemicals to make a fine solution by dissolving the precipitates of powder. Then, stir this solution with a magnetic stirrer for 2 hours by maintaining the heating temperature at 80 ℃ by covering the beaker with aluminum foil to avoid the evaporation of the solution. 

After stirring, blend these two solutions and again kept back on stirring for 20 minutes at room temperature. More stirring causes increased smoothness and can unify the solution more effectively thus a solution of tungstic oxide is formed. The color of the yellow solution turned into milky yellow and then kept it back on stirring for 20 minutes again at room temperature whenever you are going to use this solution. Add an equal amount of ethanol as a stabilizer. Note that this all process occurs at room temperature. Now, the solution is ready for dip coating.

Solution preparation of molybdenum oxide

The transition metal oxides i.e., Molybdenum trioxide (MoO₃), received substantial attention over the previous few years because of their numerous applications in different fields. MoO₃ thin films were prepared using the drop-casting method. It is a white color powder that is used as a precursor in this method.

First, the different molarity precursor solution was prepared by dissolving 1g and 2g of ammonium pentamolybdate tetrahydrate (NH₄)₆Mo₇O₂₄.4H₂O) in 10 ml of deionized water and then isopropyl was added as stabilizer separately. Second, the aqueous solution was continuously stirred for 1h at 600℃ to yield a clear and homogeneous solution. After that, the solution was aged for 24h before coating

(NH₄)₆Mo₇O₂₄ .4H₂O+H₂O  7MoO₃+6NH₃+8H₂O                                                      

The molecular weight of molybdenum oxide is 1235.86g/mol. By using the weighing balance, I took a very precise measurement of both materials and then add the required amount of both chemicals to make a fine solution by dissolving the precipitates of powder. Then, stir this solution with a magnetic stirrer for 60 minutes at room temperature by covering the beaker with aluminum foil to avoid the evaporation of the solution.

Table 2.1 solution preparation summary

Sr. No	Name of solution	Pre cursor Materials	Molarity	Stirring time (Hours)	Temperature (°C)
1	WO3	Tungstic Acid (solute) hydrogen peroxide (solvent) ethanol (stabilizer)		2	80
2	MoO 1	Ammonium Molybdate (solute)                           distilled water (solvent) Isopropyl (stabilizer)	1 M	1	60
3	MoO 2	Ammonium Molybdate (solute)      distilled water (solvent) Isopropyl (stabilizer)	2 M	1	60

Target Preparation

Sample Preparation

The glass substrate is cut by using a glass cutter in a very small strip size of 1×1 in

Substrate Cleaning

In this Experiment substrate preparation and cleaning is the most important process. It should be performed carefully. Following are some steps involved in substrate cleaning. Distilled isopropyl is used for cleaning the substrate. And later it is cleaned with distilled water.

1)      Wash the substrate with soap.

2)      After that clean it with acetone.

3)      Wipe it out with cotton tissues gently.

4)      Then dry it.

Dip coating Procedure

The first sample was prepared at 120 mm/min speed. Substrate dip in tungstic acid solution bath and then withdraw with a uniform speed and then kept for drying for10 minutes at 160℃ temperatures. After that leave the sample in the furnace for cooling. After drying solution was again immersed in the bath and withdrawn keeping the same speed thus, Film was deposited on the substrate. A total of 5 coatings were done to achieve the required thickness by keeping the constant speed of the dip coater at 120 mm/min. The films were annealed at 600℃ after five coatings for 10mints in the furnace under an air atmosphere.

Now is the time to deposit molybdenum oxide over this sample to get a multilayer oxide thin film. The same pattern was observed for the next 5 coating by changing the drying temperature to about 110℃ and then annealing it at about 60mint at 500℃ when the sample was ready to leave it for cooling for about 24 hours.

Post deposition annealing of all the samples was accomplished at different annealing temperatures for one hour in the electric furnace. 

Table 2.2 film deposition summary

Sr No	Name of Slide	Technique	Drying Temperature (°C)	Drying Time (minutes)	No. of Layers	Annealing Temperature (°C)	Annealing Time (minutes)
1	Glass					600	10
2	WO3	Dip coating	160	10	5	600	10
3	MoO	Drop casting	110	10	5	600	10
4	M11	Dip coating, drop casting	160            110	10	10	500	60
5	M22	Dip coating, drop casting	160            110	10	10	500	60
6	B11	Dip coating, drop casting	160            110	10	10	500	60
7	B22	Dip coating, drop casting	160            110	10	10	500	60

Characterization techniques

The properties of the thin films and aspects of the growth mechanism can be well understood by the characterization of the films. The characterization techniques used for the thin films are thickness measurement, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive analysis of X-ray (EDAX), optical absorption, and electrical resistivity measurement techniques utilized in the present study. A brief discussion of these techniques is given below:

X-ray Diffraction (XRD)

X-ray diffraction is a very vastly used and multipurpose technique for the crystalline structure determination of solid and powdered materials. The high range of material properties can be analyzed. It is a non-destructive technique that works on Bragg’s law used for the identification of phases and to characterize the material. It is normally used to obtain information about crystal structure and different orientations of material. The particle size (D) is generally calculated using the Scherer formula given in eq. (2):

                                                         D=kλ/βCosθ                                       (2)

While K refers to the shape constant for a factor having a value of 0.9; λ is the wavelength of the incident X-rays beam, θ is Bragg’s angle and β symbolizes full width at half maximum (FWHM) of X-rays peaks.  A graph is plotted between resultant diffracted X-ray intensities and angle of scattering (2θ) that give the distinction between samples. Bragg’s law equation for diffraction is given by:

                                                        2dSinθ=nλ                                                                     (3)

Where λ is the wavelength of the incident X-rays beam, θ is the angular positions in degrees, and d is spacing between adjoining crystal planes. Integer n is used that refers to diffraction order.

UV visible spectroscopy

Atomic spectroscopy includes the investigation of the collaboration of Ultraviolet (UV)- Visible radiation with particles. Ultraviolet light and visible light have quite recently had the correct vitality to cause an electronic Transition of electrons starting with one filled orbital and then onto the next of higher Energy unfilled orbital. We get information about the energy band gap by utilizing this method.

FTIR Spectroscopy

Fourier transformed infrared spectroscopy is a technique that is s used to identify the materials and provide information about the absorption of infrared radiations by the sample material versus wavelength. Chemical analysis was done by using FTIR. At the point when a material is illuminated with infrared radiation, absorbed IR radiation usually excites molecules into a higher vibrational state. The wavelength of light consumed by a specific atom is a component of the energy difference between the very still and excited vibrational states. 

RESULTS AND DISCUSSION

After analyzing the microstructural properties of tungsten and molybdenum oxide multilayer thin film by using x-ray diffraction and FTIR. Their optical properties and UV-visible can be analyzed by using a spectrophotometer. The detail of these techniques is given in the following section.

XRD for Single Films

X-ray diffraction spectra of multilayer thin films prepared at different deposition conditions are the various crystal parameters obtained from the XRD data reported in Table

Table 3.1: Molybdenum Oxide

Sample ID	2 Theta	Standard Value	Phase Identified	Crystal Structure	FWHM	Crystalline Size	2 Theta Shift
Peak 1	25.45	25.70	Molybdenum Oxide	Orthorhombic	0.1097	77.57	0.25
Peak 2	27.10	27.33	Molybdenum Oxide	Orthorhombic	0.2044	41.77	0.23
Peak 3	38.75	38.97	Molybdenum Oxide	Orthorhombic	0.0589	149.39	0.22
Peak 4	58.55	58.80	Molybdenum Oxide	Orthorhombic	0.7033	13.53	0.25

Tungsten Oxide

Sample ID	2 Theta	Standard Value	Phase Identified	Crystal Structure	FWHM	Crystalline Size	2 Theta Shift
Peak 1	21.59	20.35	Tungsten oxide	Orthorhombic	10.795	73.93	-1.24
Peak 2	24.00	24.22	Tungsten oxide	Orthorhombic	12	36.16	0.22
Peak 3	32.78	33.04	Tungsten oxide	Orthorhombic	16.39	59.06	0.26
Peak 4	36.36	35.95	Tungsten oxide hydrate	Orthorhombic	18.18	54.17	-0.41
Peak 5	44.20	44.15	Tungsten oxide	Orthorhombic	22.1	33.13	-0.05
		44.24	Tungsten oxide hydrate	Monoclinic			0.04
Peak 6	49.25	49.20	Tungsten oxide hydrate	Orthorhombic	24.625	26.85	-0.05
		49.13	Tungsten oxide	Monoclinic			-0.12
		49.11	Tungsten oxide	Hexagonal			-0.14
Peak 7	56.20	56.15	Tungsten oxide hydrate	Orthorhombic	28.1	42.12	-0.05
		56.16	Tungsten oxide	Monoclinic			-0.04

Sample ID	Peak No.	2θ (degree)	Standard Value	Hkl values	Phase Identified	Crystal Structure	FWHM (degree)	Crystallite Size (n m)	2θ Shift
M11	1	16.33	16.35	(011)	Wo3	orthorhombic	0.218	38.47	0.02
	2	24.32	24.22	(200)	WO3	orthorhombic	0.126	67.39
	3	26.06	26.11 26.13	(-111) (011)	Moo2, wo3	Monoclinic, monoclinic	0.167	51.02
	4	27.71	27.74	(021)	Moo2	monoclinic	0.153	55.88
	5	34.19	34.14	(140)	Moo2	monoclinic	0.214	40.58
	6	39.34	39.32	(150)	Moo2	monoclinic	0.191	46.15
	7	40.79	40.08	(-104)	H2O5W	Monoclinic	0.174	50.90
	8	49.31	49.32	(002)	moo3	orthorhombic	0.079	115.62
	9	49.44	49.63	(400)	Wo3	Orthorhombic	0.093
M22	1	16.37	16.36	(011)	Wo3	orthorhombic	0.104	83.86
	2	23.76	22.92	(002)	WO3	orthorhombic	0.183	46.35
	3	26.09	26.47	(120)	Wo3	orthorhombic	0.149	57.18
	4	27.72	27.75	(021)	Moo2	Monoclinic	0.132	64.77
	5	34.16	34.09	(140)	Moo3	Orthorhombic	0.199	43.64
	6	39.35	39.36	(150)	Moo3	Orthorhombic	0.212	41.58
	7	40.76	40.08	(-104)	H2O5W	monoclinic	0.069	128.34
	8	49.29	49.29	(002)	moo3	orthorhombic	0.074	123.42
	9	49.42	49.63	(400)	Wo3	orthorhombic	0.074

 

Graph 1 shows the XRD spectra of tungsten, molybdenum thin films, and the glass substrate which were annealed at 600°C. The main peaks in the diffractograms are labeled as (1 1 1), (2 0 0), (0 2 2), and (1 2 3) for 2θ values 21.59, 24.0, 32.78, and 44.20 respectively for orthorhombic crystal structures of tungsten Oxide. The peaks labeled as (0 1 2), (0 4 2), and (3 1 1) with their 2θ values of 36.36, 44.20, and 49.25 respectively show the orthorhombic crystal structures of tungsten oxide hydrate. The peaks labeled as (2 1 1), (0 1 2), and (4 2 0) are for the monoclinic crystal structure of tungsten oxide and the last (3 1 0) is the peak labeled for the hexagonal crystal structure of tungsten oxide. The 5 peaks of the graph line b are indexed as (0 4 0), (0 4 0), (0 2 1), (0 6 0) and (0 8 1) for orthorhombic crystal structures of molybdenum oxide.

XRD for multilayer

XRD for multilayer thin films can be described according to the table and the graphs are as follows: The XRD spectra of M11 and M22 are shown in fig 1 and 2 respectively. The peaks showing the planes of tungsten oxide for orthorhombic crystal structure are (011), (200), and (400) for M11. Similarly, for M22 the planes are (011), (022), (120) and (400) for the orthorhombic crystal structure of tungsten oxide. The molybdenum oxide orthorhombic crystal structure was observed at the planes (002) for M11 and (140), (150), and (002) for M22.

The sharp peaks at 2θ value 49° show the high crystalline orthorhombic crystal structure for both tungsten oxide and molybdenum oxide. There are a few peaks between 26° and 40° for M11 which show monoclinic structures for both tungsten and molybdenum. The peaks between 2θ 30° and 45° for both M11 and M22 are weak with low intensities. The XRD results show that both the film is crystalline in nature and shows both orthorhombic and monoclinic crystal structures.

FTIR for single films

FT–IR spectra give us information about the vibration of atoms or molecules with associated energies in the infrared regime. The FTIR measurements can be projected as stretching, bending, and bonding.

Sample ID	Wavelength cm−1	Standard value cm−1	Phase identified
MoO3 (2M) peak 1	755.2	710	Mo-O-Mo
Peak 2	887.5	882	Mo-O-Mo
WO3 peak 1	758.9	700	W-O-W
Peak 2	855.3	861	W-O-W

The FTIR spectrum of MoO₃ and wo₃ peaks appear in the 500–2000 cm⁻¹ range. FTIR provides information about the presence of bonds. Two sharp bands were detected at 755.7 cm⁻¹ and 887.8 cm⁻¹ corresponding to the bending vibrations, associated respectively with the stretching mode of oxygen linked with three metal atoms, the stretching mode of oxygen in Mo–O–Mo units, the specification of a layered orthorhombic MoO₃ phase.

For wo3 the peaks exhibit the stretching mode at 855.3cm^-1 and 758.9cm^-1 in a w-o-w group which shows the bridging mode.

FTIR for multilayer

Chemical analysis was done by using the FTIR spectrum of α-MoO3 and wo3 peaks appearing in the 500–2000 cm⁻¹ range. 

The peak at 991.5 cm⁻¹ of molybdenum with double bond oxygen (Mo=O), at peak 1401.5 cm⁻¹ and 1637.9 cm⁻¹ single bond of a hydroxyl group (Mo-OH) and at 2329.9 cm⁻¹ (O-H) exhibits stretching bond. Bending vibration is observed at 1401.5 cm⁻¹ and 1625.1 cm⁻¹ corresponding to (the OH, H-O-H) plane. The band at 3548.4 cm⁻¹ was attributed to the symmetric stretching bond of (OH) in W-OH…..H₂O. The peak at 656.2 cm⁻¹ exhibits a Mo-O bond which represents deformation modes. The spectrum of molybdenum oxide shows that the highest peak is at 872.2 cm⁻¹ due to the Mo-O-Mo bond. The peaks at 745.5 cm⁻¹, 872.2 cm^−1, and 738 cm⁻¹ are due to the W-O-W stretch which shows the bridging mode in M11 and M22.

Table 3.3 FTIR multilayer

Sample	Peaks cm−1	Standard Beak cm−1	bond	Assignment
M11	738	700	W-O-W	Stretching bond
	991.5	991	Mo=O	Stretching bond
	1401.5	1433	OH, H-O-H	Bending vibration OH in W-OH
	1401.5	1384	Mo-OH	Stretching bond (Mo-O-Mo)
	1625.1	1633	OH, H-O-H	Bending vibration OH in W-OH
	1637.9	1643	Mo-OH	Stretching bond (Mo-O-Mo)
	2329.9	2312	O-H	Stretching bond
	3548.4	3454	W-OH..H2O	Symmetric Stretching bond (OH)
M22	656.2	619	Mo-O	Deformation bond
	745.5	700	W-O-W	Stretching bond
	872.2	861	W-O-W	Stretching bond
	872.2	882	Mo-O-Mo	Bending (Mo-O)

UV Visible for Multilayer

The optical properties of as-deposited tungsten and molybdenum oxide thin films with different numbers of laser pulses are studied by using a UV-Visible spectrophotometer. The optical transmission spectra of tungsten and molybdenum oxide thin films are in the range of 200 to 800 nm.

It can be seen that the percentage transmission decreases constantly for all films when the wavelength decreases from 800 nm to 200 nm. The absorption edge is observed near 270 nm. The percentage transmission becomes zero below 200 nm for all the thin film samples. It can also be seen that the percentage transmission increases as the number of pulses increase from …… to ……..

 The fundamental formulas used to calculate the absorption coefficient (α) are given below

I= I_o e^αt ,  A = log (I_o/I)           ………….       (1)

α = 2.303 (Abs/t)                     ………….       (2)

where (t) shows the thickness and Abs represents the absorbance.

The Tauc’s relation has been used to determine the optical band gaps for all the films.

(αhν) ⁿ = A (hν – E_g)               ………….       (3)

Where A is the constant, hν is the photon energy and n is an index and it defines the optical absorption process and is theoretically equal to n=2 for direct and n = 1/2 for indirect band gap. The energy band gap values have been deduced by plotting hν vs. (αhν)². The linear extrapolation at the energy axis gives the value of the band gap energy. The UV-visible transmission spectra are shown in the figure which shows the high transmission in the UV region. The fundamental formulas used to calculate the absorption coefficient (α)

Optical band gap energy is estimated through Tauc's curves by considering two portions of Tauc's curves. The first portion is taken by considering the linear portion of the curve (green line) and is denoted by E_g

Sample Name	Wavelength (nm)	Band gap Energy (eV)
M11	261.56	4.70
M22	263.1	4.65
B11	266.5	4.68
B22	266.11	4.63
MoO	263.1	4.66
WO3	261.56	4.70