structural and optical properties of spin coated iron ... · hematite is a naturally occurring...

The 2018 World Congress on Advances in Civil, Environmental, & Materials Research (ACEM18) Songdo Convensia, Incheon, Korea, August 27 - 31, 2018

Structural and Optical Properties of Spin Coated Iron Oxide Thin

Films – Effect of Mn Doping

Hassan Yousaf1)*, Saira Riaz2), S. Sajjad Hussain2), Zohra N Kayani3), Shahzad Naseem2)

1) 2)

Centre of Excellence in Solid State Physics, Punjab University, Lahore-54590, Pakistan

3)Department of Physics, LCWU, Lahore, Pakistan 1)

[email protected]

ABSTRACT

Iron oxide comprises an important class of materials that are omnipresent in

nature. From an optical standpoint, α-Fe2O3 exhibits band gap (∼2.2 eV) that lies in

the visible range and has a relatively high refractive index. Therefore, it has been explored as a prospective nominee for several optical applications, such as solar energy conversion, photocatalysis, electrochromism, photo-oxidation of water and interference filters. Properties of iron oxide can be tuned with use of dopant. For optical tunning/enhancement sol-gel synthesized iron oxide films with manganese (Mn) doping (Fe2-xMnxO3) is reported by varying concentration of dopant (x) as 0.04-0.1 (interval 0.04). Hematite phase of iron oxide is observed from XRD analysis. Increase in crystallinity and crystallite size is observed by increasing dopant concentration from 0.04 to 0.1. Ferromagnetic behavior is observed for the synthesized films with highest saturation magnetization of 0.20196emu obtained for films with dopant concentration 0.08. Highest transmission of ~85% (λ = 700nm) has been obtained for films with dopant concentration 0.08. Band gap of films is in the range of 2.213eV to 2.09eV. Films with dopant concentration 0.08 results in high refractive index and low extinction coefficient of 2.12 and 0.013 (λ = 700nm), respectively. 1. INTRODUCTION

Iron oxide comprises an important class of materials that are omnipresent in nature. Naturally, 16 different forms of iron oxide and oxyhydroxides exists that exhibit wide range of physical characteristics. Due to vast range of properties from insulting to semiconducting and conducting and from antiferromagnetic to ferrimagnetic iron oxide have found its applications in photovoltaic devices, electronic devices, sensors and

1)

Graduate Student 2)

Professor


spintronic devices (Unni et al. 2017; Onn et al. 2014; Riaz et al. 2014a,b; Akbar et al. 2014; Akbar et al. 2015).

Hematite is a naturally occurring polymorph of iron oxide and possesses highest thermal stability in comparison with other polymorphs of iron oxide. Hematite crystal structure is most commonly described by rhombohedral system belonging to R3c space group or by hexagonal system belonging to D6

3d space group. Crystallographic structure of hematite is composed of oxygen anions arranged in hexagonal close packing. Fe3+ cations are orderly arranged on 2/3 of the octahedral sites. This results in formation of oxygen anion and iron cation layers that are set normal to 3-fold axis. Chains of face sharing octahedral are arranged along c-axis. Within each chain, Fe3+ cations are arranged in the form of pairs that are alienated by interstitial sites. These interstitial sites are empty. Hematite is paramagnetic above its Neel temperature of 965K. At room temperature it exhibits weak magnetic behavior. At temperature below room temperature (256K) it undergoes magnetic transition from weak ferromagnetic to antiferromagnetic state (Varshney and Yogi 2013; Balout et al. 2017).

Substitution of metal cations at iron site has a strong impact of physical, magnetic, optical and electrical properties. Properties of undoped hematite are well established where as doped hematite has attracted the researchers attention during the last few years. Doping of 4d or 5d metal cations in hematite structure leads to enhancement in magnetic anisotropy due to spin orbit interaction between dopant cation and iron cation of host lattice. This results in changes in magnetic properties as well as changes in Morin temperature (Shimomura et al. 2015).

In this work, Mn doped iron oxide thin films by varying concentration of dopant as 0.04, 0.06, 0.08 and 0.1 is reported. 2. EXPARIMENTAL DETAILS Nitrates of iron and manganese were dissolved in water and ethylene glycol. At first stage, nitrate of iron was mixed in DI H2O and stirred for 30mins. At second stage of sol preparation, ethylene glycol (EG) was mixed during first stage of sol synthesis. To obtain iron oxide sol it was heat treated at 80˚C.For doping purpose sol was synthesized at room temperature. This solution was then mixed to iron oxide sol for doping purpose. Doping concentration (x) in Fe2-xMnxO3 is varied as 0.04, 0.06, 0.08 and 0.1. Prior to film deposition, it is important to obtain clean substrate. For deposition of Mn doped iron oxide sols copper was selected as substrate. These substrates were then cleaned with the help of acetone and isopropyl alcohol for 10mins and 15mins, respectively using ultrasonic vibrator and etched with HCl (Asghar et al. 2006a,b). These annealed films were characterized for their structural properties using X-ray Diffractometer. Magnetic properties were studied using Lakeshore’s 7407 Vibrating Sample Magnetometer. Optical properties were studied with the help of M-2000 Variable Angle Spectroscopic Ellipsometer.

3. RESULTS AND DISCUSSION

The patterns of XRD for Mn doped iron oxide films are shown in Fig 1 and peaks are indexed (JCPDS card # 87-1165). Presence of diffraction peaks indexed as (110), (006), (202), (024), (214) and (217) at dopant concentration 0.04 confirm the formation of an important polymorph of iron oxide i.e. Hematite phase. Fig. 1(a) shows that no peaks for


manganese oxide were observed. This indicates that manganese ions have occupied the substitutional sites. By increasing dopant concentration from 0.04 to 0.1 (Fig. 1(a-d)) two effects were observed: 1) Increase in peak intensities associated with plane (202) was observed by increasing dopant concentration from 0.04 (Fig. 1(a)) to 0.08. Whereas (Fig. 1(c)) indicating increase in crystallinity of the films. At high dopant concentration of 0.1 (Fig. 1(d)) decrease in peak intensities of plane (202) was observed along with elimination of small diffraction peaks corresponding to planes (018), (214) and (217). This indicates decrease in crystallinity of the films at high dopant concentration. Higher the dopant concentration, higher is the probability that dopant ions take place on the grain boundaries. This results in decrease in crystalline order (Riaz et al. 2014a,b; Azam et al. 2015, Riaz et al. 2015; Cullity et al. 1956); 2). This shift is owing to difference in ionic radius of host and dopant ion as Mn2+ ions (0.645Å ) have smaller radius as compared to Fe3+ ions (0.69Å ) that results in decrease in d-spacing and unit cell volume (Table 1).

Fig. 1 XRD patterns for Mn doped iron oxide thin films with dopant concentration (a) 0.04; (b) 0.06; (c) 0.08; (d) 0.1.

Table 1 Structural parameters for Mn doped iron oxide thin films.

Dopant concentration

Lattice parameter (Å )

Unit cell volume (Å 3)

a c

0.04 5.025 13.610 297.6104

0.06 5.023 13.605 297.2643

0.08 5.021 13.603 296.984

0.1 5.018 13.601 296.5856

Crystallite size “t” (Cullity 1956), dislocation density “δ” (Riaz et al. 2014,) and strain

(Cullity 1956) were determined using euations (1-3).


cos

9.0

Bt (1)

t

1 (2)

hkl

hkl

d

dd

d

dStrain

exp (3)

Where, B is FWHM. For 0.04 to 0.08 concentration of dopant increase in crystallite size was observed. This increase in crystallite size is because of the temperature gradient that is built up in the sol due to replacement of larger ionic radius ion (Fe3+) with ion of smaller radius. This temperature gradient results in increase in crystallite size due to ostwald ripening mechanism (Azam et al. 2015; Riaz and Naseem 2015). In addition, it also depends on surrounding grains. There exists difference in energies due to difference in strain energy (Riaz et al. 2007). Fig. 2(b) shows that strain decreases from 1.932×10-3 to 1.658×10-3 by increasing concentration of dopant from 0.04 to 0.08. This results in decrement of strain energy along with decrement of dislocations (Fig. 2(b)). At higher concentration dopant 0.1 there are chances that dopant atoms occupy the position on grain boundary that results in decrease in crystallite size.


Fig. 2 (a) Crystallite size (b) dislocation density for plotted as function of concentration dopant

Films of iron oxide doped with Mn is shown in Fig. 3(a). Films depicts strong ferromagnetic (FM) behavior as compared to weak magnetic behavior of bulk hematite at room temperature.

Hematite, an important phase of iron, antiferromagnetic in nature. The variation in

properties is owing to canting in spins where spins in the contiguous planes are coupled ferromagnetically and antiferromagnetic coupled in adjacent planes that give rise to uncompensated spins of Fe3+ cations (Bhowmik and Saravanan 2010) and is the cause of FM (Varshney and Yogi 2013; Bhowmik and Saravanan 2010).

Spins are aligned ferromagnetically within the plane however, antiparallel in the adjoining planes that results in the production of strong exchange coupling (Ziese and Thornton 2007). The hopping is responsible for the increase in magnetization with increase of dopant concentration to 0.08 (Fig. 3). However at 0.1, number of imperfections are large that results in insufficient alignment.


Fig. 3 (a) M-H curves for Mn doped iron oxide thin films; (b) Saturation magnetization for Mn doped iron oxide thin films plotted vs. dopant concentration.

Transmission curves for manganese (Mn) doped iron oxide thin films can be observed in Figure 4 at low wavelengths (λ<530nm) no transmission was observed for Mn doped iron oxide thin films. Sharp increase in transmission of the films was observed within λ= 530-600nm. This sharp increase in transmission indicates onset of optical absorption in Mn doped iron oxide thin films. At higher wavelengths λ>600nm films show high transmission. Two effects can be observed with changes in dopant concentration from 0.04 to 0.1: 1) Shit of absorption edge to higher wavelength 2) Increase in transmission from 76% to 86% (λ=800nm) by increasing concentration dopant from 0.04 to 0.08. As it was observed in Fig. 2(a), that decrease in dislocation density takes place with increase in concentration of dopant to 0.08. This resulted in decreased scattering from the dislocations (Riaz and Naseem 2015; Riaz et al. 2016).


Thus, optical transmission increases as concentration of dopant was increased to 0.08. Increased dislocations resulted in decreased transmission to 67% (λ=800nm).

From transmission spectra in Fig. 4(a), absorption coefficient (α) of Mn doped iron oxide thin films was calculated using Eq. (5) (Riaz and Naseem 2015, Riaz et al. 2016).

4222

2

)1(4)1(

2ln

'

1

RTRR

TR

t (4)

Where, tʹ is the film thickness determined using Spectroscopic Ellipsometer, T is the

transmission and R is the reflection of Mn doped iron oxide thin films measured for wavelength. Optical absorption edge (Eg) was determined by extrapolation method. Direct band gap of 2.213eV, 2.16eV, 2.14eV and 2.09eV was obtained with dopant concentration 0.04, 0.06, 0.08 and 0.1, respectively. Due to creation of shallow trapping levels decrease in band gap is observed by increasing dopant concentration (Riaz et al. 2016, Riaz et al. 2014c).

Fig. 4 Transmission curves for Mn doped iron oxide thin films

4. CONCLUSIONS

Sol-gel method was used for the preparation of manganese (Mn) doped iron oxide thin films. Concentration of dopant (Mn) was changed as 0.04, 0.06, 0.08 and 0.1. Increase in concentration of dopant from 0.04 to 0.08 resulted in increased crystallinity and increased crystallite size of Mn doped iron oxide thin films. Mn doped iron oxide thin films showed ferromagnetic behavior with highest saturation magnetization observed for 0.08. Synthesized films show high transmission in the visible and infrared region with band gap in the range of 2.213-2.09eV.


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structural and optical properties of spin coated iron ... · hematite is a naturally occurring...

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