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Structural and luminescence studies on TiO2-MoO3 thin films

  • P. V. Kala
  • K. SrinivasaraoEmail author
Original Research
  • 99 Downloads

Abstract

Thin films of MoO3 and TiO2-MoO3 were deposited on quartz glass and Silicon (100) substrates by dc magnetron sputtering at two substrate temperatures of 300 K and 600 K and at two sputtering pressure of 5 Pa and 10 Pa and sputtering power of 50 W respectively. The deposited films were characterized by X-ray Photo Electron Spectroscopy (XPS), Raman and photoluminescence spectra. The pure MoO3 films deposited at 300 K exhibited the Raman bands at 242 cm−1 and 974 cm−1 indicating the absorption of water molecules and formation of alpha MoO3-H2O. With increasing Ti content the bands are shifted to 224 cm−1 and 960 cm−1 that correspond to h-MoO3 and absorption of water molecules. For Ti at. % of 2.4 the bands were observed only at low frequencies but at high temperatures, a single peak noticed at 954 cm−1 corresponds to α – MoO3 H2O. The Photo-luminescence spectrum reveals that the MoO3 films deposited at 5 Pa and 300 K exhibits Near Band Edge (NBE) transition at 324 nm. The intensity of NBE increases further and with corresponding shift towards higher wavelength with TiO2 doping. The TiO2 doping also introduces Deep Level (DL) emission in TiO2-MoO3 composite film. With increase of deposition temperature the intensity of emission due to NBE in MoO3 films decrease introducing DL emission. The TiO2 doping enhances the intensity of both NBE and DL emission with a shift in emission wavelength towards higher value. The intensity of these NBE and DL emissions are further increasing with increasing sputtering pressure (10 Pa) and deposition temperature (600 K) of MoO3 and TiO2-MoO3.

Graphical abstract

The MoO3 and TiO2-MoO3 thin films were deposited by d.c. magnetron sputtering at different temperatures and sputtering pressures. From XPS, it is observed that there is a shift in binding energy position of Mo characteristic peak towards higher energy due to Titanium doping, when compared to Mo binding energy peak in case of MoO3 films. The Raman spectra results indicates that the TiO2 affecting the crystallization of MoO3 films and crystallizing them from orthorhombic to monoclinic phase with increasing Ti atomic percent The Photoluminescence spectra reveals that a high intense light emission from TiO2 - MoO3 thin films deposited at 10 Pa and 600 K.

Keywords

TiO2 -MoO3 thin films D.c. magnetron sputtering XPS Raman Photo luminescence 

1 Introduction

For the past few decades there is a lot of demand for materials which find applications in various light emitting devices. Among various category of materials oxides are found to be promising and are already proved as potential candidate for various applications, in making gas sensors [1, 2], electrochromic [3, 4], photochromic [5], and thermochromic [6] devices. Apart from the above applications, oxides are also found to be promising candidates as luminescence materials. The two important physical properties which govern the luminosity of a material are optical transmittance, and energy gap. These properties can be tuned in the materials by varying the preparation conditions and can be enhanced further to make a required device by forming a composite with other suitable materials. In this aspect the TiO2 – ZnO [7], TiO2-SiO2 [8], Ag-TiO2 [9] etc. are a few composites which showed enormous improvement in achieving the desired physical property than they are as a single compound. Among these TiO2-MoO3 composite is found [10, 11] to be a promising candidate in terms of the formation of a stable compound, and of improving the luminescence intensity and efficiency of MoO3. Moreover tunability to obtain required structure and energy gap by changing small percentage of TiO2 in the composite is the chief advantage of these materials [12,13]. The composite which is used as a luminescence material should exhibit strong luminescence intensity and emission at required wavelength. This can be achieved by forming composite with TiO2 and MoO3. So, In the present investigation thin films of TiO2-MoO3 thin films were deposited by d.c. magnetron sputtering at two sputtering pressures of 5 Pa, 10 Pa and two substrate temperatures of 300 K and 600 K. The deposited samples were studied for their chemical composition by X-ray Photo Electron Spectroscopy (XPS), structure and modes of vibrations by Raman spectrum and electron transition by study state photo luminescence.

2 Experimental technique

DC magnetron sputtering was used to deposit thin films of TiO2-MoO3 in a vacuum coating unit supplied by VR Technologies, Bengaluru, India. The vacuum chamber was evacuated to a pressure of 10−6 mbar (measured with penning gauge) using a diffusion pump backed by a rotary pump. A high purity Mo target of 3 mm thickness and 2.54 cm diameter and Ti sheet of same dimensions (which is cut in to required size, shape and kept on the top of Mo target) was used to prepare thin films at various sputtering pressures and substrate temperatures. The sputtering process was initiated by flowing Argon gas first and then oxygen was flown in to the vacuum chamber. The target was pre-sputtered for few minutes and the flow rate of both gases was controlled by a needle valve. The substrates were pre-cleaned with deionized water, methanol. Subsequently the substrates were cleaned in ultrasonic agitator. In addition to that the Si substrates were cleaned with HF acid. Later the substrates were fixed to substrate holder and their ion bombardment was also performed before deposition. The sputtering power during the deposition was maintained at 50 W by adjusting the voltage and current in the 1 kV at 1 A dc power supply. The X-ray photo electron spectrum was recorded by using a PHI 5000 Versa Probe II, FEI Inc. The standard procedure was followed to insert the sample in to the high vacuum chamber to record XPS spectrum. The XPS spectrum was recorded in the scanning energy range of 0–1200 eV. The spectral resolution is 1 eV. The photo luminescence measurements were performed by picoseconds time resolved fluorimeter, (Eddinburg Instruments, Model: FSP920) with excitation wavelength of 250 nm, in the scanning range 300–600 nm. The thickness of the films is 3000 Å which was measured by means of a stylus profilometer (Veeco DEKTAK 150). The thickness resolution of the instrument is ±10 Å.

3 Results & discussion

3.1 XPS studies

The X-ray photo electron spectra (XPS) of TiO2 - MoO3 thin films were recorded to know their composition and chemical state. The XPS spectrum was calibrated by the C 1 s peak (284.6 eV).

The XPS spectrum of TiO2 - MoO3 films deposited at 5 Pa and 300 K is shown in Fig. 1. The spectrum indicates the existence of Mo, O and Ti. The calculated at. % percentages of Ti is nearly 1 and Mo and oxygen, calculated by de-convolution method is 23.41 and 75.59 respectively. It is observed that the characteristic shift in binding energy position of Mo peak position towards higher energy due to Titanium doping, when compared to Mo binding energy peak in case of MoO3 films [14]. The binding energies observed at 460 eV and 464 eV are due to Ti 2p3/2 and 2p1/2 states. This indicates the Ti4+ state in TiO2.
Fig. 1

X-ray Photo Electron spectrum of TiO2 - MoO3 thin films deposited at a sputtering pressure of 5 Pa and substrate temperature of 300 K. The atomic percentage of Titanium is 1, Mo is 23.41 and Oxygen is 75.59

The XPS spectrum of TiO2-MoO3 films deposited at 5 Pa and 600 K is shown in Fig. 2. The at. % of Ti is 2.4, Mo is 22.77% and oxygen is 74.84%. The at.% ratio of Oxygen to Mo is 3.12. The binding energy peaks observed at 453 eV and 460 eV are due to Ti. This indicates the characteristic shift in the 2p3/2 and 2p1/2 peaks towards lower energy. The shift in the peak position in case of Mo and Oxygen is towards lower energy. This may be due to the decrease of the energy required to extract an electron from Ti with decreasing Mo at. % in the composite. [15, 16]. But in case of oxygen binding energies the shift in position of binding energy peak is towards higher energy when Ti at.% is low and it is towards lower energy with increasing Ti at.%. The reason may be due to the improvement in the polycrystallanity of the TiO2-MoO3 films deposited at higher temperatures of 600 K [17].
Fig. 2

X-ray Photo electron spectrum of TiO2 - MoO3 thin films deposited at 5 Pa and substrate temperature of 600 K. The atomic percent of Titanium is 2.4, Mo is 22.77 and Oxygen 74.84

3.2 The laser Raman studies

The Raman spectra of MoO3 and TiO2-MoO3 films deposited at 5 Pa and 300 K are shown in Fig.3. The observed numbers of peaks were more than that observed in case of MoO3 films. These characteristic peaks indicates different modes of vibrations of TiO2-MoO3.
Fig. 3

Raman spectra of TiO2 - MoO3 thin films deposited at a substrate temperature of 300 K and at a sputtering pressure of 5 Pa

In case of pure MoO3 films the Raman bands were observed at 242 cm−1 and 974 cm−1. The 242 cm−1 is due to the absorption of water molecules and 974 cm−1 indicates the formation of α-MoO3-H2O [18]. This indicates the presence of water molecules in the films. For 1 at.% of Ti, the peaks were observed at 224 cm−1 which correspond to h-MoO3 and 960 cm−1 is due to the absorption of water molecules [19]. The remaining Raman bands observed at 407 cm−1 and 843 cm−1, which are corresponding to orthogonal and hexagonal phases of MoO3 with a shift of 20 cm−1 in all peak positions due to TiO2.

With further increase in Ti at. % to 2.4 completely low frequency vibrations were observed. These were at 64, 76, 93, 125, 152, 172, 272, 333, 374 cm−1 and were due to monoclinic MoO3 [20,21]. Among these the peaks observed at 76 (Ag 82 cm−1) and 93 cm−1 (B1g, 98 cm−1) is a doublet due to the translational rigid chain modes in the ‘a’ direction with a shift due to TiO2 [22]. This indicates the TiO2-MoO3 layers are weakly interacting along ‘a’ direction. The singlet observed at 152 cm−1 indicates the interaction is strong along TiO2 -MoO3 layer along c and b- directions and the band at 272 cm−1 represents the wagging mode of Mo = O. This is a shift of 11 cm−1 when compared to the bulk value and is due to the bonding between MoO3 with TiO2.

The Raman spectra of the MoO3 and TiO2-MoO3 films deposited at 5 Pa and at 600 K is shown in Fig. 4. The Raman peaks of MoO3 films were observed at 165, 203, 250, 500, 593, 660, 728, 816, 894, 955 cm−1, which are due to orthorhombic α – MoO3. In case of Ti of 2.4 at. % the observed bands were due to monoclinic phase of MoO3.
Fig. 4

Raman spectra of TiO2 - MoO3 thin films deposited at a substrate temperature of 600 K and at a sputtering pressure of 5 Pa

The Raman spectra of the MoO3 and TiO2-MoO3 films deposited at 10 Pa and at 300 K is shown in Fig. 5. The characteristic band of MoO3 was observed at 960 cm−1, indicates the crystallization of films in orthorhombic phase [23]. With increasing Ti at. % to 1, there were no characteristic peaks, may be due to decrement in polycrystallanity due to the TiO2 [24]. With further increasing Ti at. to 2.4 the Raman peaks were observed at 98 and 274 cm−1 which are the characteristic peaks of β – MoO3 [24]. Along with this two peaks were observed at 710 and 819 cm−1 which are the characteristics of α – MoO3. This indicates that the films crystallizes in mixed phases and also the crystallization starts at lower temperatures [23]. Table 1.1 and 1.2 shows the Raman shifts obtained in the current study and reference values for comparison.
Fig. 5

Raman spectra of TiO2 - MoO3 thin films deposited at a substrate temperature of 300 K and at a sputtering pressure of 10 Pa

Figure 6 shows the Raman spectra of the MoO3 and TiO2-MoO3 films deposited at 10 Pa and at 600 K. The Raman spectrum of MoO3 films does not give any characteristic peaks. With increasing Ti at.%, the characteristic peaks were observed at 906 and 963 cm−1 which indicates the films crystallizes in α – MoO3 phase [21]. This indicates the temperature effects significantly the crystallization of the films. With increasing Ti at. % to 2.4 the characteristic peaks were observed at 201, 418 and 783 cm−1, which indicates the films completely, crystallizes in monoclinic phase. Table 2.1 and 2.2 shows the Raman shifts obtained in the current study and reference values for comparison.
Fig. 6

Raman spectra of TiO2 - MoO3 thin films deposited at a substrate temperature of 600 K and at a sputtering pressure of 10 Pa

3.3 Photo luminescence studies

Fig.7 shows the photo luminescence spectra of the MoO3 and TiO2-MoO3 deposited at 5 Pa and at a substrate temperature of 300 K. The excitation wavelength used is 250 nm which is higher than the band gap wavelength of both MoO3 and TiO2-MoO3. More over the energy is also suitable to transfer the electrons to localized levels within the forbidden gap. The PL spectra exhibit a single emission band at 324 nm which corresponds to UV emission. This is attributed to the strong near band edge (NBE) emission due to the free exciton recombination [24].
Fig. 7

Photo luminescence spectrum of MoO3 and TiO2-MoO3 thin films deposited at a sputtering pressure of 5 Pa and substrate temperature of 300 K

By the addition of TiO2 to MoO3 the UV, NBE peak is shifting to 337 nm and the intensity of this peak is strong. Along with this two more peaks were observed near visible region of 396 and 430 nm and are attributed due to the transition of excited optical centres in the deep levels (DL). The reason for increase in the intensity of emission in visible region is due to the decrement of separation between TiO2-MoO3 energy levels [25].

With further increasing Ti at. % to 2.4, the NBE peak is shifting towards lower wavelength with low emission intensity. This may be due to the increment in the gap between TiO2 and MoO3 energy levels, which inhibits the population of more number of electrons to the excited levels [25].

Figure 8 shows the photo-luminescence spectrum of MoO3 and TiO2-MoO3 film deposited at 5 Pa and at 600 K. With increasing substrate temperature to 600 K two deep level transition of low intensity which corresponding to visible region were observed at 414 and 502 nm in case of MoO3 thin films. This may be due to the decrease in the energy gap due to the improvement in the reactivity of Mo with oxygen which in turn improves the crystallinity of the films [26]. The photoluminescence emission wavelengths were given in Table 3.1.
Fig. 8

Photo luminescence spectrum of MoO3 and TiO2-MoO3 thin films deposited at a sputtering pressure of 5 Pa and substrate temperature of 600 K

Table 3.1

: Photo luminescence data of TiO2:MoO3 Thin films

Sputtering pressure, 5 Pa, Substrate temperature 300 K

Atomic percent of Ti

Luminescence emission wavelength (nm)

Reference value

Reference

MoO3

324

Ti(1%): MoO3

337

397

430

343, 345

396

411

[20, 24]

[20,24]

[20, 24]

Ti(2.4%): MoO3

332

450

343

450

[24]

[10]

Sputtering pressure, 10 Pa, Substrate temperature 300 K

 

Atomic percent of Ti

Luminescence emission wavelength (nm)

Reference value

Reference

MoO3

323

400

345

396

[20]

[20]

Ti(1%): MoO3

327

367

345

366

[20]

[24]

Ti(2.4%): MoO3

332

448

471

343

450, 452

470

[24]

[10,20]

[20]

With TiO2 addition the UV peaks reappears at 339 nm with red shift and is high intense. The deep level (DL) transition which is observed at 430 nm at low temperature splits into two emission peaks and is observed at 420 nm and 443 nm. This indicates the TiO2 introduces more number of radiative transitions in the TiO2-MoO3 films [24].

Fig. 9 shows the luminescence spectra of MoO3, TiO2-MoO3 films deposited at 10 Pa and 300 K. The MoO3 films deposited at higher sputtering pressures and low temperatures exhibits NBE peak at 323 nm, and DL at 399 nm, which are the small shifts (blue shift) towards lower wavelengths when compared to films deposited at 5 Pa. At the same time the intensity is high due to improvement in the stoichiometry of the Mo and Oxygen ratio. With increasing Ti at. % to 1 the intensity of NBE is decreasing and shifts towards higher wavelengths of 327 and 365 nm. The shift is further increasing with Ti at. % of 2.4. This may be due to the decrement between TiO2 and MoO3 energy levels which causes population of more number of electrons into MoO3 [27].
Fig. 9

Photo luminescence spectrum of MoO3 and TiO2-MoO3 thin films deposited at a sputtering pressure of 10 Pa and substrate temperature of 300 K

Figure 10 shows the luminescence spectrum of films deposited at 10 Pa and 600 K. The shift of NBE peak position and decrement in intensity is also observed with increase of deposition temperature. This may be due to the improvement in the non-radiative transitions in the UV region and population of less number of excitons in the conduction band [14]. This shift continuous with Ti additions and the intensity of DL emission increases with Ti addition. This may be due to the introduction of sub levels in TiO2-MoO3 due to Ti doping and gap between TiO2 and MoO3 [14]. With further increase of Ti at. % to 2.4 the intensity of NBE emission is decreasing and the peak position is shifting towards higher wavelengths. This may be due to the higher Ti content strongly influencing the crystallographic structure of MoO3 which inturn effecting the energy band structure of TiO2-MoO3. The photoluminescence emission wavelengths were given in Table 3.2.
Fig. 10

Photo luminescence spectrum of MoO3 and TiO2-MoO3 thin films deposited at a Sputtering pressure of 10 Pa and substrate temperature of 600 K

Table 3.2

: Photo luminescence data of TiO2:MoO3 thin films

Sputtering pressure, 5 Pa, Substrate temperature 600 K

Atomic percent of Ti

Luminescence emission wavelength (nm)

Reference value

Reference

MoO3

414

500

411

---

[24]

---

Ti(1%): MoO3

339

420

444

345

411

450,450,452

[24]

[24]

[10,24,20]

Ti(2.4%): MoO3

332

450

345

452

[20]

[20]

Sputtering pressure, 10 Pa, Substrate temperature 600 K

 

Atomic percent of Ti

Luminescence emission wavelength (nm)

Reference value

Reference

MoO3

332

371

345

366

[20]

[24]

Ti(1%): MoO3

335

432

345

452

[20]

[20]

Ti(2.4%): MoO3

337

450

468

345

452

470

[20]

[20]

[20]

Table 1.1

: Modes of vibrations of TiO2:MoO3 thin films

Sputtering pressure, 5 Pa, Substrate temperature 300 K

Atomic percent of Ti

Raman shift (cm−1)

Mode of vibration

Reference value

Reference

MoO3

974

242

α - MoO3 H2O

α - MoO3 H2O

969

250

[3]

[3]

Ti(1%): MoO3

960

843

224

α - MoO3 H2O

β - MoO3 H2O

h-MoO3

969

850

219

[18]

[3]

[22]

Ti(2.4%): MoO3

374

333

272

172

152

125

76

α - MoO3

α - MoO3

β - MoO3

h –MoO3

α - MoO3

h –MoO3

h –MoO3

376

338

284

172

159

121

85

[21]

[21]

[22]

[22]

[22]

[22]

[22]

Table 1.2

: Modes of vibrations of TiO2:MoO3 thin films

Sputtering pressure, 5 Pa, Substrate temperature 600 K

Mole percent of Ti

Raman shift (cm−1) references

Mode of vibration

Reference value

of Raman shift

(cm−1)

Reference

Ti(0%): MoO3

955

α - MoO3 H2O

969

[3]

Ti(1%): MoO3

796

β - MoO3

776

[3]

Ti(2.4%): MoO3

776

705

416

371

250

201

β - MoO3

α - MoO3 H2O

β - MoO3

α - MoO3

α - MoO3

β - MoO3

776

697

418

376

250

202

[3]

[21]

[3]

[22]

[22]

[22]

Table 2.1

: Modes of vibrations of TiO2:MoO3 films

Sputtering pressure, 10 Pa, Substrate temperature 300 K

Atomic percent of Ti

Raman shift (cm−1) references

Mode of vibration

Reference value

Reference

MoO3

960

α - MoO3

951

[24]

Ti(2.4%): MoO3

98

274

710

819

β - MoO3

β - MoO3

α - MoO3

α - MoO3

98

290

700

818

[22]

[24]

[18]

[23]

Table 2.2

Modes of vibrations of TiO2:MoO3 films

Sputtering pressure, 10 Pa, Substrate temperature 600 K

Mole percent of Ti

Raman shift (cm−1) references

Mode of vibration

Reference value

Reference

Ti(0%): MoO3

  

Ti(1%): MoO3

906

963

α - MoO3

α - MoO3

894

951

[28]

[19]

Ti(2.4%): MoO3

201

418

783

β - MoO3

β - MoO3

β - MoO3

200

423

775

[22]

[29]

[21]

4 Conclusions

The MoO3 and TiO2-MoO3 thin films were deposited by d.c. magnetron sputtering at different temperatures and sputtering pressures. The deposited films were characterized to know their oxidation state by XPS, structure by Raman spectrum and electron transitions by photo luminescence spectrum.

The results obtained support the hypothesis that the addition of TiO2 to MoO3 modifies structure, binding energy and luminescence intensity of the composite. The XPS results supports the hypothesis by showing a shift in binding energy position of Mo characteristic peak, when compared to Mo binding energy peak in case of MoO3 films. The observed binding energies of Mo, Oxygen, Ti indicates that they are in their respective oxidation states of 6+, 6- and 4+ with a shift (when compared to the individual binding energies) towards lower energy due to the addition of Ti to MoO3. The another hypothesis has proved by showing increment in binding energies, towards lower energies with the at.% of Ti due to decrement in the polycrystallanity of the films.

The Raman spectra results proves the another hypothesis that the TiO2 effects the crystallization of MoO3 films and crystallizing them from orthorhombic to monoclinic phase with increasing Ti atomic percent. Moreover TiO2 introducing number of vibrational modes in MoO3.

The PL results proves the another hypothesis of band gap tuning by showing the UV and Visible region emissions. The lower Ti at. % (< 1) intensifies the UV emission and shows the blue shift due to strong near band edge emission. Whereas higher Ti at. % (> 1) intensifies the visible region emissions due to transition of excited optical centres in the deep levels (DL). The study reveals that the Ti incorporation can be used to tune chemical state, crystallographic structure and photo emission of MoO3 thin films.

Notes

Acknowledgments

The authors are thank full to the central instrumentation facility, Advanced Centre for Materials Science, IIT Kanpur, Kanpur, India for the XPS measurements. The authors are also thankful to the central instrumentation facility, IIT Guwahati, Guwahati, India for recording phtoluminiscence and Raman spectra.

Compliance with ethical standards

Conflict of interest

The authors have no conflict of interest.

Supplementary material

42114_2019_124_MOESM1_ESM.docx (8 mb)
ESM 1 (DOCX 8158 kb)

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© Springer Nature Switzerland AG 2019

Authors and Affiliations

  1. 1.Department of Basic Sciences & Humanities, Vignan’s Lara Institute of Technology & ScienceVadlamudiIndia
  2. 2.Department of Applied Sciences & Humanities, Sasi Institute of Technology & EngineeringTadepalligudemIndia

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