Chemical and optical properties of mno2 thin films

IJRET: International Journal of Research in Engineering and Technology

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CHEMICAL AND OPTICAL PROPERTIES OF MnO2 THIN FILMS PREPARED BY REACTIVE EVAPORATION OF MANGANESE A. H. Y. Hendi11, M. F. Al-Kuhaili2, S. M. A. Durrani3

Lecturer, Physics Department, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia Professor, Physics Department, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia 3 Professor, Physics Department, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia 1

2

Abstract

Thin films of manganese oxide were deposited by thermal reactive evaporation of pure manganese. The films were deposited at room temperature under various oxygen partial pressures. Structural analyses were performed by X-ray diffraction and atomic force microscopy. The films were polycrystalline and the surface roughness of the films was oxygen partial pressure-dependent.

Chemical analysis, studied by x-ray photoelectron spectroscopy, exhibited the control of stoichiometry in the films by varying the oxygen partial pressure, where fully oxidized MnO2 films were obtained at higher oxygen partial pressures. Optical measurements revealed a slight decrease in the transmittance of the films prepared at higher oxygen partial pressures. Gradual increase in the direct band gap of the films with oxygen partial pressure was noted. Based on the structural and chemical characterizations, the optical properties (refractive index and extinction coefficient), of the stoichiometric MnO2 film were investigated.

Keywords: Manganese Oxide; Evaporation; Oxidation

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1. INTRODUCTION

Manganese oxide is a promising material for electrochemically lithium-ion batteries [1-4]. It has also improved high performance in the electrochromic devices through smooth reversible insertion and extraction of lithium [5]. Furthermore, manganese oxide thin films are used in catalysis [6-9] and as substrates in the growth of magnetic oxide perovskite materials [10-12]. These applications for manganese oxide thin films are based on the capability of manganese to change its valence state. Indeed, manganese oxide can be found in various valence states including, MnO (Mn+2), Mn3O4 (Mn+2.67), Mn2O3 (Mn+3), and MnO2 (Mn+4) [13-14]. The valence state specifies the field of application. For instance, MnO2 is used as an electrode in lithium-ion batteries and as a catalytic material in oxidation–reduction reactions [15-16]. The presence of Mn+3 ions in the MnO2 lattice as MnO-OH leads to the transfer of electrons between Mn+4 and Mn+3 ions. Such a mechanism is responsible for the catalytic activity in nonstoichiometric manganese oxide [17]. Mn2O3 proved its importance to the manufacture of soft magnetic materials, and it is also used as a catalyst for the removal of carbon monoxide from waste gas [13]. Finally, Mn3O4 (Mn+2.67), has attractive electrochromic properties [18] such as the change of its optical transmittance in the visible spectrum with an applied voltage which makes it candidate for window applications.

Several techniques were employed to deposit manganese oxides thin films including atomic layer deposition [14,19], liquid-phase electrochemical method [20-21], pulsed laser deposition [13, 22], chemical bath deposition [23], electron beam deposition [24], thermal evaporation [25], plasma assisted molecular beam epitaxy [26], hydrothermal method

[27], sol–gel [28-29], and chemical spray pyrolysis [30].

Previously, our research group has prepared manganese oxide thin films by thermal evaporation of MnO2 powder, demonstrating the dependence of the optical constants (refractive index and extinction coefficient) on the substrate temperature, oxygen partial pressure and deposition rate [31]. In this work, we report the preparation and optical characteristics of MnO2 thin films prepared by the reactive evaporation of pure manganese. This is a simpler and more economical technique for the preparation of MnO2 thin films, where the stoichiometry of the films is controlled through the oxygen partial pressure during deposition. The optical properties of the films are determined and are interpreted in terms of the chemical and structural properties of the films.

2. EXPERIMENTAL

2.1 Deposition of MnO2 Thin Films

MnO2 films were deposited in a Leybold L560 coater by thermal reactive evaporation of metallic Mn powder (Alfa Aesar, 99.8 % purity) using a molybdenum boat. The vacuum chamber was initially evacuated to a base pressure of 3× 10-5 mbar. The deposition was conducted on fused silica and tantalum substrates in an oxygen ambient. Fused silica substrates were used for structural and optical measurements; however, tantalum substrates were used for chemical analysis. Five samples of MnO films called F1, F2, F3, F4 and F5 were prepared at room temperature with an oxygen partial pressure of 2.5×10-4, 5.0×10-4, 7.5×10-4, 1.0×10-3 and 1.5x10-3 mbar, respectively. During deposition, the substrates were rotating and the distance between the source and substrate was kept at 40 cm. Film thickness and evaporation rate of 0.2 nm/s were monitored by an oscillating quartz crystal.

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2.2 Material Characterization

To investigate the structural properties of the films, X-ray diffraction (XRD) data were collected by Rigaku Ultima IV diffractometer, equipped with Cu Kα (1.54 Å) radiation. The diffraction angles (2θ) were scanned in the range 20°- 80°.

Atomic force microscopy (AFM; Veeco Innova diSPM) operated at contact mode was employed for the examination of the surface morphology of the prepared films. The sample surface was probed with a silicon tip of 10 nm radius oscillating at its resonant frequency of 300 kHz. The scan area was 2×2 µm2 and scan rate was 1 Hz. A Jasco V-570 double beam spectrophotometer was used for the measurements of normal-incidence transmittance in the wavelength range 200–2000 nm. X-ray photoelectron spectroscopy (XPS) (Thermo Scientific ESCALAB 250 Xi spectrometer), equipped with an Al Kα (1486.6 eV) X-ray source was employed for the investigation of the chemical composition of the films. During the XPS analysis, the samples were maintained at ambient temperature at a pressure of 8×10-9 mbar.

3. RESULTS AND DISCUSSION 3.1 Structural Properties

XRD analysis revealed the formation of different phases of polycrystalline manganese oxide depending on the oxygen partial pressures, as shown in Fig. 1. No other significant peaks assigned to metallic Mn were observed. Phase identification was based upon the comparison of the measured XRD peaks with the database distributed by the International Center for Diffraction Data (ICDD) [32]. For F1-F4, the diffraction peaks (110) and (130) assigned to MnO2 and the (220) peak assigned to MnO [32] confirm that these films are composed of MnO2/MnO multiphases. The enhancement of the MnO2 phase with the oxygen partial pressure in these films is evident by the sharpness of (110) peak. At the highest oxygen partial pressure (F5), the absence of (220) peak of MnO is observed, revealing the formation of pure MnO2. This indicates that the increase of the oxygen partial pressure causes the oxidation of Mn to yield MnO2 as can be later confirmed by the XPS analysis.

Moreover, it can be noted that the intensity of the (110) diffraction peak increases gradually with the increase of the oxygen partial pressure in the films (F1-F4), and then decreases for the film deposited at the highest oxygen partial pressure (F5). This could be attributed to the reduction of the adhesion coefficient for the evaporated species that reaches the substrate due to excess of oxygen that interferes with the nucleation and growth of the films [33].

3.2 Morphological Properties

Figure 2 shows three dimensional AFM images of the films.

The surface roughness of the films was characterized by the root-mean-square (Rrms) values. The film prepared at the lowest oxygen partial pressure (F1) had a dense columnar structure and high roughness. As the oxygen partial pressure increased (F2-F5), a porous columnar structure with lower roughness was developed and the lateral growth of the columns was apparent. The variation of Rrms with the oxygen

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partial pressure is presented in Fig.3. The higher the oxygen partial pressure, the higher smoothness of the films.

3.3 Chemical Properties

The surface chemistry of the prepared films was investigated by XPS. The adventitious carbon C1s peak, centered at binding energy of 284.4 eV was used as a reference for charge correction. A typical XPS spectrum for MnO2 thin film is shown in Fig. 4a, indicating that the constituent elements were Mn and O in the films. The O1s spectrum (Fig.4b) was resolved into three components positioned at lower (OI), intermediate (OII) and higher (OIII) binding energies. The binding energy and the weight of each component in each film are listed in Table 1. The OI component, of binding energies in in the range 529.7-530.3 eV, is attributed to lattice oxygen; while the OII and OIII components in the range 530.9-531.7 and 531.8-532.4 eV are ascribed to hydroxide and adsorbed water, respectively [34-35]. The Mn2p core level consists of two main peaks (as shown in Fig.5a) centered at binding energies of 642.0 eV and 653.6 eV with an energy separation of 11.6 eV, assigned to Mn2p3/2 and Mn2p1/2 in MnO2, respectively [3638]. The binding energies of Mn+2 and Mn+4 in the Mn2p3/2 region are in the range 640.2-640.9 and 642.0-646.0 eV [3642], respectively. The Mn2p3/2 peak was resolved into three components of binding energies corresponding to the three valence states mentioned above. The net valence state of each film was determined from the weighted average of these components. Fig. 5b shows a representative resolved Mn2p3/2 XPS spectrum. The binding energy and the weight of each component as well as the net valence state of each film are listed in Table 2. It is apparent from Table 2 that manganese changes its valence state as a function of the oxygen partial pressure. The net valence state in sample F1 was 3 which indicate the formation of Mn2O3 (Mn+3). Then, the net valence state increased gradually with the oxygen partial pressure as can be seen in F2, F3 and F4. At the highest oxygen partial pressure (F5), the net valence state of manganese raised up to 4. This confirms that the growth conditions employed in this sample fully oxidized manganese to form MnO2.

3.4 Optical Properties

The transmittance spectra of the films are shown in Fig. 6.

The films deposited at lower oxygen partial pressures (F1F3) were transparent down to a wavelength of 700 nm. The optical transmittance decreased at higher oxygen partial pressures (F4-F5) with a blue shift of the absorption edge.

This observation may be accounted for the reducing of adatoms mobility at higher oxygen partial pressures [33].

Film F5, that was reactively-evaporated under an oxygen partial pressure 1.5 × 10-3 mbar, was used to derive the optical constants of MnO2 thin films in its transparency range, i.e. λ≥ 500 nm. The transmittance spectrum of the film was fitted using the equation for the transmittance of a thin film on a transparent substrate [43-44]:

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T 

16ns (n 2  k 2 )  A  B 2  2 [C cos(4nd /  )  D sin(4nd /  )]

with

A  [(n  1) 2  k 2 ][( n  ns )  k 2 ]

(1)

B  [(n  1) 2  k 2 ][(n  n s )  k 2 ]

D  2kns ( n 2  1  k 2 )  2k ( n 2  n s2  k 2 )

(2)

where Eo is the effective dispersion oscillator energy, Ed is the dispersion energy related to the inter-band transition strength, and E is the incident photon energy. The extinction coefficient of the film can be expressed as:

k () 

ko



2



k1

3

= ln

(α ) =

where n = refractive index β = exp (–4πkd/λ). In order to fit the experimental transmittance spectra using Eq. (1), models for the dispersion of n and k must be implemented. The refractive index of the films was modeled by a single oscillator model [45]:

Eo E d 1 / 2 ) Eo2  E 2

The relationship between the absorption coefficient (α) and the transmittance of the films in the fundamental absorption region (λ< 400) is given by (4)

where, d is the thickness of the film.

The band gap of the films was determined using the following formula [51]

C  ( n 2  1  k 2 )( n 2  n s2  k 2 )  4k 2 ns

n  (1 

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(3)

where ko and k1 are constants. The second term in the above equation represents absorption due to scattering processes, whereas all other processes (such as defect absorption or multiphoton processes) are represented by the first term.

The dispersion curves of the refractive index and the extinction coefficient are shown in Fig. 7. It can be observed that the refractive index decreases as wavelength increases, displaying the normal dispersion behavior of the material.

The refractive index value was 2.13 at a wavelength of 500 nm, which agrees with the reported values for MnO2 thin film [46-47] and is slightly less than the previous value reported by our group [31]. It was reported that the presence of sub-oxidized material in the deposited film leads to the increase of the refractive index [48-49]. Hence, the lower value of the refractive index in the current study may be attributed to the stoichiometric nature of the film as evident from the XPS results. In addition, it is well-known that the increase of the oxygen partial pressure decreases the density of the deposited films [50]. This is confirmed by AFM result which revealed the porous property of the film. Hence, the decrease of the film density leads to lower refractive index.

The extinction coefficient of the films was small, indicating the decrease of the absorption in the film. The decrease of the absorption is explained by the decrease of the scattering due to the decrease of defects in the film as confirmed by OII component of O1s spectrum as well as the decrease of the surface roughness as revealed by AFM result.

( −

)

(5)

where (E) is the incident photon energy, is the optical band gap of the material and A is a constant which does not depend on ( E ). The value of n is ½ for a direct allowed transition and 2 for an indirect allowed transition. Thus, the direct band gap of the films can be evaluated by plotting (αE)2 versus (E). and extrapolating the linear portion of the plot to intercept the horizontal axis at (αE )2 = 0, as shown in Fig. 8. The values of the extrapolated intercepts for the direct transition are 2.60, 2.62, 2.65, 2.67 and 2.69 eV, corresponding to F1, F2, F3, F4 and F5, respectively. The band gap value of 2.6 eV corresponding to F1 is assigned to Mn2O3 [52]. With increasing the oxygen partial pressure, the band gap increased gradually to reach 2.69 eV at the highest oxygen partial pressure (F5). This band gap value is attributed to MnO2 [53-54], which confirms the oxidation states obtained by XPS analysis.

4. CONCLUIONS

Manganese oxide thin films were deposited by the reactive evaporation of manganese. All films were polycrystalline as revealed by XRD analysis. AFM revealed that the surface roughness decreased with the increase of the oxygen partial pressure. The stoichiometry of the films was controlled by varying the oxygen partial pressure during deposition. XPS analysis showed that stoichiometric Mn2O3 films were formed at an oxygen partial pressure of 2.5×10-4 mbar.

Then, increasing the oxygen partial pressure led to formation of nonstoichiometric MnOx films until at a higher value of 1.5×10-3 mbar, stoichiometric MnO2 films were obtained. Optical measurements revealed that the direct band gap of the films increased from 2.60 to 2.69 eV as the oxygen partial pressure increased from 2.5×104 mbar to 1.5×10-3 mbar. The optical constants (refractive index and extinction coefficient) of the stoichiometric MnO2 films were determined. The refractive index of manganese oxide was found to be 2.13 at a wavelength of 500 nm, which is in a good agreement with other works reported in the literature and less than that value previously reported by our group.

The low value of the refractive index in this study may be attributed to the improvement of stoichiometry and the decrease of the film density. The extinction coefficient was small due to the decrease of defects and smoothness of the film that decreases the scattering and absorption.

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ACKNOWLEDGEMENT

The support to this work by the Physics Department of King Fahd University of Petroleum and Minerals is acknowledged.

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BIOGRAPHIES

Mr. Abdulmajeed H. Hendi, is a Lecturer in the Physics Department at King Fahd University of Petroleum and Minerals (KFUPM). Besides, he is doing his PhD in the area of thin films and nanomaterials.

Professor Dr. Mohammad F. Al-Kuhaili at Physics Department at King Fahd University of Petroleum and Minerals (KFUPM). He is specialized in thin films and materials physics. Director, Energy Research Center, KFUPM, 2004 â&#x20AC;&#x201C; 2007.

Coordinator, Thin Films and Materials Group, Physics Department, KFUPM, 2005 â&#x20AC;&#x201C; now. He is also supervising many Msc and PhD students.

Professor Dr. Sardar Mohammad Ayub at Physics Department in KFUPM. He is specialized in thin films and materials science. He was also involved in developing the IR laser and thin film laboratory at Physics department.

Fig. 1. XRD patterns of the prepared manganese oxide thin films.

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F1

F2

F4

F3

F5

Fig. 2. Three dimensional AFM images of the prepared films.

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Fig. 3.Variation of Rrms of the prepared films with oxygen partial pressure.

Fig. 5. (a) Typical XPS spectrum in the Mn 2p region. (b) Resolved Mn2p3/2XPS spectrum of MnO2thin film (F5).

Fig. 4. (a) Typical XPS survey spectrum. (b) Resolved O1s XPS spectrum of MnO2 thin film (F5).

Fig. 6. Transmittance spectra of the prepared films at different oxygen partial pressures.

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IJRET: International Journal of Research in Engineering and Technology 2.14

2.10

0.08

Extinction Coefficient k

Refractive Index n

0.10

n

2.12

0.06

k

2.08

0.04

2.06

0.02

2.04

0.00

2.02

500

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550

600 650 700 Wavelength (nm)

750

800

Fig. 7. The optical constants of MnO2 thin film (F5).

Fig. 8. Tauc plots for the determination of the direct band gaps of manganese oxide films.

Table 1: XPS characteristics in the O1s region for manganese oxide thin films prepared at various oxygen partial pressures.

Sample

(mbar)

OI

F1

2.5x10-4

F3

7.5x10-4

F2 F4 F5

5.0x10-4

Peak Binding Energy (eV)

1.0x10 1.5x10-3

W% (OII)

W% (OIII)

OII

OIII

529.7

530.9

531.8

78.8

17.2

4.0

529.9

531.2

532.0

81.0

13.9

5.1

529.7

531.0

529.9 530.3

-3

Weight (%)

W% (OI)

531.8

531.1 531.7

79.0

531.8 532.4

16.7

84.0 91.0

4.3

8.7 6.2

7.3 2.8

Table 2: XPS characteristics in the Mn2p3/2 region for manganese oxide thin films prepared at various oxygen partial pressures.

Sample

Peak Binding Energy (eV) (mbar)

Mn+2

(Mn+4)

A

B

(Mn+4)

C

Weight (%) (Mn+4)

F1

2.5x10-4

640.8

642.2

-

644.2

F3

7.5x10-4

640.8

642.0

-

644.2

F2 F4 F5

5.0x10-4 1.0x10 1.5x10-3

-3

640.9 640.9 -

642.4 642.0 642.1

A, B and C: all peaks are ascribed to MnO2

643.4

644.7 644.8 644.9

W% (Mn+2) 49.1 41.2 35.3 24.8 -

W% (Mn+4)

A

39.2 48.4 52.0

66.5 43.3

B

W% (Mn+4) -

40.5

C

W% (Mn+4)

Net oxidation state

11.7

3.0

12.7

3.3

10.4 8.7 16.2

3.2 3.5 4.0

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