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Nanostructured mesoporous nickel oxide thin films

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IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 18 (2007) 115613 (9pp) doi:10.1088/0957-4484/18/11/115613

Nanostructured mesoporous nickel oxide

thin filmsB Sasi and K G Gopchandran

Department of Optoelectronics, University of Kerala, Kariavattom-695 581,

Thiruvananthapuram, India

E-mail: [email protected]

Received 5 October 2006, in final form 19 December 2006Published 7 February 2007Online at stacks.iop.org/Nano/18/115613

AbstractNanostructured nickel oxide thin films were prepared by the pulsed laserablation technique. The effects of annealing on the structural, morphological,electrical and optical properties are discussed. Phase imaging was used toexamine the surface contaminants, adhesion and hardness and height imagingto evaluate the height profile of the films. Morphological investigations usingatomic force microscopy and scanning electron microscopy indicate a stronginfluence of the annealing process on the surface roughness and particle size.A self-assembly of nanocrystals agglomerating together to form anisland-like structure is observed in films annealed at 773 K. X-ray diffractionand x-ray photoelectron spectroscopy investigations indicate the presence ofNi2O3 in the as-deposited films. A transformation to cubic NiO with growthalong (111) and (200) planes with increase of annealing temperature is also

observed.

1. Introduction

Nanostructured oxides prepared in the form of rods, fibres,

ribbons, channels and other shapes display unique properties

that make them suitable for many new applications such

as transparent conductors, sensors, lasers, smart windows,

luminescent materials and solid electrolytes. Because of

quantum and other size effects, the properties of nanosized

oxide materials differ from those of the bulk. Currently,

the vast majority of experimental studies are based onnanocrystalline bulk or thin-film oxide materials characterized

by the presence of a large number of grain boundaries

between regions with dissimilar crystallographic orientations.

Depending on the properties and fabrication route, the crystal

boundaries are associated with various degrees of structural

and compositional disorder. A very simple but successful way

is a slightly controlled oxidation of the surface. The grain

size of materials affects their characteristics. This is related to

the large area of grain boundary per unit weight of material.

More research into these effects will underpin much of the

future applications of materials science. The novel use of

nanocrystalscurrently seen in photovoltaic cells demands more

research into the type of material used, which needs to havea surface for light absorption and charge separation, and may

have major implications for other applications.

Nickel oxide is a transition metal oxide semiconductor,

and is a transparent conducting, electro-chromic and antifer-

romagnetic material having a wide range of technological ap-

plications at the nanoscale [1]. It is a promising material for ap-

plications such as electro-chromic display devices, smart win-

dows, active optical fibres, gas sensors, and solar thermal ab-

sorbers, catalyst for CO oxidation, fuel cell electrodes, and

photo electrolysis [29]. Most of the well-known transparent

conducting oxides, such as indium oxide, indium tin oxide, and

zinc oxide, are n-type semiconductors with free electrons re-sulting from extrinsic donors as well as intrinsic donors [10].

Thin films of p-type semiconductors are required in many op-

toelectronic device applications, which make use of hole in-

jection [11]. Nickel oxide is an interesting candidate for this

class with a wide band gap of 3.64.0 eV [12]. Although

stoichiometric NiO is an insulator, its resistivity can be low-

ered by an increase of Ni3+ ions resulting from the addition

of monovalent atoms or by the introduction of nickel vacan-

cies and/or interstitial oxygen in NiO crystallites [13]. Having

an excellent durability and electrochemical stability, with low

material cost, being a promising ion storage material in terms

of cyclic stability, having a large span optical density, and the

possibility of manufacture by a variety of techniques are themost attractive features of nickel oxide [14]. Many techniques

that involve sputtering, vacuum evaporation, electron beam

0957-4484/07/115613+09$30.00 1 2007 IOP Publishing Ltd Printed in the UK
http://dx.doi.org/10.1088/0957-4484/18/11/115613mailto:[email protected]://stacks.iop.org/Nano/18/1http://stacks.iop.org/Nano/18/1mailto:[email protected]://dx.doi.org/10.1088/0957-4484/18/11/115613


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Nanotechnology 18 (2007) 115613 B Sasi and K G Gopchandran

evaporation, spray pyrolysis, chemical deposition, and solgel

processes [1519] have been used for the preparation of nickel

oxide thin films.

For technological applications, a detailed understanding

of the size, structure, composition, surface morphology and

electrical and optical properties of nanoscale NiO is important.In this study we describe the preparation of nanocrystalline

nickel oxide films by the pulsed laser ablation technique and

the effect of annealing.

2. Experimental details

Nickel oxide thin films were prepared on amorphous fusedquartz substrates by the pulsed laser deposition (PLD)

technique, using a Q-switched Nd:YAG laser, (Quanta-ray

INDI-Series, Spectra-Physics) with 55 mJ of laser energy at

532 nm, pulse width 8 ns, and repetition frequency 10 Hz.

The targetsubstrate distance was 7 cm and the depositiontime was 30 min. The target was rotated with constant speed

to avoid pitting of the target at any given spot and to obtainuniform ablation. The NiO target was prepared from nickel

oxide powder of 99.99% purity (Sigma-Aldrich). The powder

target was isostatically pressed and sintered at 1173 K for

5 h, to form pellets of diameter 1.4 cm. The films were

deposited at room temperature under a vacuum of 106 mbar

and subsequently annealed at different temperatures up to

1173 K, for a period of 2 h. The heat treatment consisted

of raising the temperature at a range of 5 K min1, then

maintaining the temperature for 2 h and gradually lowering

it to room temperature. The structure and crystallinity

of the films were investigated by grazing incidence x-ray

diffraction (GIXRD), using a Siemens D-5000 diffractometer,

operated with a monochromatic Cu K radiation source( = 0.154 18 nm, 40 kV, 30 mA), in step scanning

mode with steps of 0.05 with a scan speed of 4 s/step.

The x-ray reflectivity (XRR) technique was used to determine

the thickness of the films. A chemical binding energy analysis

was performed using x-ray photoelectron spectroscopy (XPS)

using a VSW (UK) system. The x-ray source was Al K,

at 1486.6 eV and base vacuum 6 1010 mbar. Charge

corrections were made by assigning 284.6 eV binding energy

to the C 1s peak. The surface morphology and roughness of

the films at the nanoscale were investigated by atomic force

microscopy (AFM), using Nanoman II, Veeco instruments,and scanning electron microscopy (SEM), using a Quanta

200 system, fitted with an energy-dispersive spectrometer.

Three-dimensional, two-dimensional, phase and height imagesof the samples were studied. Phase imaging is a powerful

extension of tapping mode AFM that provides nanometre-

scale information about surface structure and propertiesoften not revealed by other scanning probe microscopy

(SPM) techniques. By mapping the phase of the cantilever

oscillation during the tapping mode scan, phase imaging goes

beyond simple topographical mapping to detect variations in

composition, adhesion, friction, viscoelasticity, and numerousother properties. The composition of the film was evaluated by

energy-dispersivex-ray analysis(EDX). Optical measurements

were performed in the wavelength range from 300 to 900 nm,

using a double beam UVvis spectrophotometer, Jasco-D 550.

The resistivity of the films was measured by the four-probemethod using a nanovoltmeter model 2182 A and current

source meter model 6430 (Keithley).

0.05 0.10 0.15 0.20 0.25 0.301E5

1E4

1E3

0.01

0.1

1

Inte

nsity(a.u

)

q (A1)

Figure 1. X-ray reflectivity of as-deposited NiO film with curvefitting.

3. Results and discussion

3.1. XRR studies

X-rays are reflected from interfaces between materials of

differing electron density. X-ray reflectivity represents the

interference pattern of the reflected x-rays from the surface and

interfaces of the film and provides an accurate measurement of

the thickness of the film. For a single-layer film, the thickness,

t, can be estimated roughly by

t=

2=

2

qz, (1)

where is the fringe spacing at high angles and qz is the

x-ray momentum transfer along the surface normal direction,

which is given by

qz = 2ksin =4

sin . (2)

However, for higher accuracy, the fitting analysis was done

by fitting the experimental reflectivity pattern with a simulated

one using the program Parratt32. The reflectivity of a system

having n thin layers can be obtained by a recursive method

attributed to Parratt [20]. Figure 1 shows the XRR curve along

with the simulated one of the as-deposited NiO film, from

which the thickness is obtained as 32 nm.

3.2. XRD studies

Figure 2 shows the GIXRD patterns of as-deposited and

annealed nickel oxide thin films. The peaks are indexed

according to ASTM data cards of Ni2O3 (14-481) and NiO

(4-0835). The XRD spectrum of as-deposited film exhibits

the nanocrystalline nature of the film; it shows only a weak

diffraction peak which is from the (200) face of Ni2O3. The

presence of Ni2O3 in the as-deposited film is also confirmed

by XPS measurements. The colour of the as-deposited film

is dark brown, and is due to the presence of Ni 3+ ions in the

Ni2O3 assignment, acting as colour centres [21]. On annealing

to a temperature of 573 K, this peak is found to vanish and

the appearance of two peaks at 2 = 37.28 and 43.33 can

be observed. These peaks are attributed to the diffractionsfrom (111) and (200) planes of the cubic NiO lattice, and the

preferential growth is along the (200) plane. The calculated

2


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20 30 40 50 60

Ni2O3(200)

a

Intensity(arb.u

nits)

2 (deg.)

c

b

NiO(

111)

NiO(

200)

d

Figure 2. GIXRD patterns of NiO thin films. (a) As deposited;annealed at (b) 573 (c) 773 and (d) 1173 K.

value of lattice parameter is found to be 0.417 57 nm, which

is in good agreement with ASTM value. The transformation

of Ni2O3 to NiO in thin films by an annealing process has

already been reported [22, 15]. The intensity of the peaks

in the diffraction patterns is found to increase with annealing

temperature. This may be due to the enhanced oxidation

kinetics and improvement in crystallinity. Grain nucleation and

growth are important phenomena in nanocrystalline materials.

They govern the kinetics of many phase transformations and

recrystallizations that take place during processing.

Despite the various transformation models that have

been proposed in the past, the kinetics of these phase

transformations is still poorly understood. Most of thesemodels arebasedon the classical nucleation theory [23] and the

law of parabolic grain growth as derived by Zener [24], which

describes the behaviour of individual grains in the bulk of the

material. The final average grain size after the transformation

is directly related to the strength of the material. In general,

a smaller average grain size results in a stronger material.

Annealing provides a restructuring of the film, enabling it to

be of a better crystalline nature. In this work the width of

the diffraction peaks are found to decrease with increase of

annealing temperature, which is an indication of increase in

grain size. The grain size of the crystallites was calculated for

the (200) peak using Scherrers formula [25], and was found to

be 16 nm for the film annealed at 773 K and 23 nm for the filmannealed at 1173 K.

3.3. EDX analysis

Energy-dispersive spectroscopy was carried out for elemental

analysis of the film surface. In the EDX spectrum of the

as-deposited NiO thin film shown in figure 3, no other

lines besides those corresponding to nickel and oxygen were

detected. No impurity in the film was observed. The film

exhibits oxygen-rich stoichiometry. The atomic ratio O/Ni was

approximately 93.58/6.42.

3.4. XPS studies

The binding energy measured using XPS allows detection of

different chemical states of bonded elements. Scanning was

0 2

Ni Ni

Ni

O

4 6 8 10keV

Spectrum 1

Full Scale 3737 cts Cursor: 0.000 keV

Figure 3. EDX spectrum of as-deposited NiO thin film.

900 890 880 870 860 850 840

(a)

Intensity(a.u

)

Binding energy (eV)

(b)

Ni 2p880.4

873.2861.9

855.1

880873.7

862

856.5

Figure 4. XPS spectra of NiO films in the Ni 2p range:

(a) as-deposited film and (b) annealed film at 773 K.

done only on the surface layer and not on the entire profile,

because of the possibility of changing the properties of the

material during the ion bombardment etching procedure. XPS

spectra of the as-deposited and film annealed at 773 K in the

Ni 2p and O 1s range are shown in figures 4 and 5 respectively.

The binding energy values of peaks obtained from thespectrum

are summarized in table 1, along with reported XPS peaks

for nickel oxides found in the literature. The peaks obtained

at binding energies of 856.5 eV (Ni 2p3/2) and 873.7 eV (Ni

2p1/2) and the shoulder at 853.4 eV for the as-deposited film

suggest the presence of NiO [26, 27]. For the film annealed

at 773 K, the Ni 2p3/2 peak at 855.1 eV and Ni 2p1/2 peak at873.2 eV indicate the presence of NiO. Satellite peaks due to

shake-up processes [26] appear at the 861862 eV (Ni 2p3/2)

and 879881 eV (Ni 2p1/2) regions in both the films.

Figure 5(a) shows the O 1s spectrum of the as-deposited

NiO film and (b) that of the film annealed at 773 K,

deconvoluted by Gaussian curves corresponding to peaks of

nickel oxides. The spectrum indicates the presence of NiO and

Ni2O3, the major oxygen-containing compounds in the films.

The intense peak having binding energy of 531.54 eV for the

as-deposited film indicates the presence of Ni2O3, consistent

with the results of the XRD analysis. The peak having less

intensity at a binding energy of 529.47 eV corresponds to the

O 1s peak of NiO. This implies that the film contains bothNi2+ and Ni3+. The reason for this is that non-stoichiometric

nickel oxide contains many Ni2+ vacancies, and to keep the

3


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524 526 528 530 532 534 536 538 540

1000

1500

2000

2500

3000

3500

4000 O1s

529.47

531.54

Intensity(cps)

Binding energy (eV)524522 526 528 530 532 534 536 538 540

Binding energy (eV)

1000

1500

2000

2500

3000

3500

4000

4500O1s

531.56529.82

(a)

Intensity(cps)

(b)

Figure 5. XPS spectra of NiO film in the O 1s range: (a) as-deposited film and (b) annealed film at 773 K.

Table 1. XPS binding energies of nickel oxides.

Multiplet MultipletO 1s Ni 2p3/2 satellites Ni 2p1/2 satellites

Material (eV) (eV) (eV) (eV) (eV) Reference

Ni metal 852.3 869.7 [30]NiO 530 855 856.8 861.1 873 879.8 [30, 31]

529.5 854 856.2 861.5 873.5 880 [32]Ni2O3 531.8 857.1 863 [32, 33]

531.5 857.3 [34, 35]As deposited 531.54 856.5 862 873.7 880 This work film 529.47

Annealed film 5 31.56 855.1 861.9 873.2 880.4 This work 529.82

charge near the Ni2+ vacancies neutral, some of the Ni2+ were

oxidized to Ni3+. Every Ni2+ ion lost from NiO will result

in the production of 2Ni3+ ions. Thus mixed states of Ni

appear [28]. The presence of Ni3+ in the Ni2O3 assignment

conforms to the non-stoichiometric nickel oxide. Annealing

the samples gives rise to O 1s peaks corresponding to Ni2O3at 531.56 eV and NiO at 529.82 eV. It can be observed that,

compared to the O 1s peak of the as-deposited film, the

concentration of the Ni2O3 component has been decreased

and that of NiO increased correspondingly, in the annealedfilm. This suggests that Ni2O3 turns into NiO when the

film is annealed. A similar mechanism of transformation has

been reported by Chang et al for sputtered films [29]. The

colour of the as-deposited film in this work is dark brown,

and it becomes more and more transparent with increase in

annealing temperature. It has been reported that the presence of

Ni2O3 in nickel oxide films would give rise to the dark brown

colour [21]. This also supports our observations.

3.5. AFM and SEM studies

AFM observations in tapping mode have been used to

investigate the surface morphology and roughness of the films.Figure 6 shows two-dimensional and three-dimensional AFM

images of the as-deposited and annealed NiO films. The

surface topography reveals the nanocrystalline nature of the

films. The particle size of the films is found to be significantly

influenced by annealing. The roughness of the as-deposited

film is 6.03 nm and it changes to 7.36 and 6.24 nm for

films annealed at 573 and 773 K respectively. The section

analysis shown in figure 7 shows sharp changes in the height

profile for the film annealed at 773 K. This is due to the

formation of a self-assembly of spherical nanocrystals as seen

in its three-dimensional image. On annealing at 773 K the

nanocrystals agglomerate together in groups to form an island-like structure with voids in the nanoscale between them. In

general, a deforming grain is forced to rotate in response to

the external stresses exerted upon it by its neighbours. Grain

rotation during deformation often accompanies the formation

of texture and is accomplished by a microscopic dislocation

glide on multiple active slip systems in the grains. Grain

rotation can also be caused by extensive grain boundary

sliding and diffusion, which usually occur only at elevated

temperatures [36]. Figure 8 shows the SEM photographs

of films annealed at 773 and 1173 K. These photographs

clearly show the increase of porosity between agglomerated

nanograins with increase of annealing temperature, indicating

the formation of a mesoporous film. Figure 9 shows the phaseand height images of the AFM images shown in figure 6.

The particulate formation is a disadvantage of the pulsed laser

4


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19.3 nm

500.0 nm1: Height0.0



19.3 nm

0.0 nm

0.0 nm

0.0 nm

5.0 nm5.0 nm

30.0 nm

30.0 nm

500 nm

500 nm

500 nm

500 nm

500 nm

500 nm

500 nm

500 nm

500 nm

400

400

400

300

300

300

200

200

200

100

100

100

375

375

375

375

375

375

250

250

250

250

250

250

125

125

125

125

125

125

(a)

(b)

(c)

Figure 6. AFM images (2D and 3D) of NiO thin films: (a) as deposited; annealed at (b) 573 and (c) 773 K.

deposition technique. The formation of particulates in the as-

deposited film can easily be seen in the phase image of the as-

deposited film. Phase imaging was used to examine the surface

contaminants, adhesion and hardness, and height imaging to

evaluate the height profile of the films. The phase and height

images shown here are a very good tool for understanding the

presence of particulates and the inhomogeneous height profile.It can be seen that the particulates disappear on annealing at

573 K. The phase image of the film annealed at 773 K exhibits

the high quality of the film in respect of uniform adherence

and hardness. In thermal annealing, the mobility to annihilate

defects is increased, leading to better order, but the height

images of the films show that the constraints to maintain an

equilibrium surface layer remain.

3.6. Optical and electrical properties

The spectral transmittance of the NiO films in the as-depositedstate and annealed at 573, 773, and 1173 K in the wavelength

5


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(a) (b) (c)

50 100 150 200 nm 50 100 150 200 nmnm nm

50 100 150 200 250 nmnm

10

5

0

5

10

15

2

1

0

1

2

3

5

0

10

5

Figure 7. Section analysis of NiO thin films: (a) as deposited; annealed at (b) 573 and (c) 773 K.

(This figure is in colour only in the electronic version)

(a) (b)

Figure 8. SEM micrographs of NiO thin films annealed at (a) 773and (b) 1173 K.

range 300900 nm are shown in the inset of figure 11. A

strong temperature-dependent transmittance can be observed.

An increase in the transmittance is observed with increasing

annealing temperature. The films that are heat treatedto 573 K and above showed an average transmittance of

above 80%. It can be observed that the crystallinity of the

films after heat treatment improved with increasing annealing

temperature. Since a crystallographic structural change of

NiO films was not observed after the heat treatment above

573 K, the improvement in crystallinity may be attributed to

the decrease in native defects such as interstitial oxygen and

nickel vacancies [37].

The absorption coefficient has been calculated from

Lamberts formula,

=1

tln

1

T

, (3)

where T and t are the transmittance and thickness of the films

respectively.

Figure 10 shows the variation of optical density (t) with

photon energy for the NiO films. It is found that the absorption

coefficient decreases with decrease of photon energy, with a

sharp decrease near the band edge in the visible region. The

absorption has its minimum at low energy and increases with

optical energy in a manner similar to the absorption edge

of semiconductors. NiO is a wide band gap semiconductor

with the absorption edge in the UV region and no absorption

in the visible region. But the NiO films deposited in this

study show some absorption in the visible region. Similar

observations of absorption in NiO films prepared by otherdeposition techniques are found in the literature [14, 37]. The

possible reasons for this may be (1) the main stoichiometry

Table 2. Band gap (Eg), transmittance at 550 nm (T), grain size (d),refractive index (n), and porosity (P ) of films annealed at differenttemperatures.

Temperatures Eg

T d P

(K) (eV) (%) (nm) n (%)

573 3.72 75.42 2.12 21.11773 3.68 84.32 16 2.01 31.36

1173 3.65 92.83 23 1.82 47.79

of the film is NiO, and Ni2O3 is present as a minority phase;

(2) two adjacent divalent nickel atoms become Ni3+ due to

the charge transfer process caused by the presence of a nickel

vacancy; and (3) excess oxygen together with hydrogen may

be present in the film as OH groups. The presence of Ni3+

ions in the oxide lattice shows a charge transfer transition, with

consequent absorption in the visible region [14]. The refractive

indices of the films were determined from transmission spectra,using the pointwise unconstrained minimization approach [38].

The porosity of the films was calculated using the following

equation [39]:

p =

1

(n2 1)

(n2T 1)

100(%), (4)

where n is the refractive index of the film and nT is the

refractive index of NiO (nT = 2.33) [40].

The calculated values of refractive index and porosity of

the films are given in table 2. The optical band gap, Eg of the

films was estimated from the optical measurementson the basis

of the relation h = A(h Eg)1/2, (5)

where A is a constant and h is the photon energy.

Figure 11 shows the plot of(h)2 versus photon energy.

The nature of the plotsindicatesthe existence of a direct optical

transition. The band gap is determined by extrapolating the

linear portion of the plot to the energy axis. It is noticed

that the value of the band gap shifts towards lower energy

and the slope of the plot decreases with increasing annealing

temperature. The value of Eg decreases from 3.72 to 3.65 eV

as the annealing temperature increases from 573 to 1173 K.

Reported band gap energies for NiO films are in the range

3.43.8 eV [12], which is in good agreement with our report.

The change in the optical band gap may be due to the changein homogeneity and crystallinity in the film. The optical

properties obtained are listed in table 2. The as-deposited film

6


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(a)

(b)

(c)

69.2

173.0

2: Phase 5.0 m0.0

2: Phase 5.0 m0.0

1: Height 5.0 m

150.0 nm

25.5 nm

100.0 nm 100.0

0.0

1: Height 5.0 m0.0

1: Height 5.0 m0.0 2: Phase 5.0 m0.0

Figure 9. AFM images (height and phase) of NiO thin films: (a) as deposited; annealed at (b) 573 and (c) 773 K.

is dark brown in colour and shows a metallic-like conductivity.

The resistance of the films increases and the colour of thefilms changes from opaque dark brown to transparent as the

annealing temperature increases. The film annealed at 773 K is

found to have a sheet resistance of 2.3 M and a resistivity of

1.88101 m. The increase in resistivity with heat treatment

may be due to the decrease in native defects acting as shallow

acceptors [41].

The electrical resistivity of NiO films has been studied by

many researchers and the reported resistivity is in the range

from 1 101 to 104 m [14]. For transparent conducting

NiO films prepared by the sputtering technique, resistivities

in the range from 1.18 101 to 3 m with transmittance

varying from 55 to 60% in the visible region have been

reported [4245]. Using the sputtering technique, Sato et al[37] reported the preparation of NiO films with a resistivity of

1.4 103 m with a low transmittance of about 40% in the

visible region. Varkey and Fort [18] have deposited insulating

NiO films with a transmittance of 80% using a dip coatingtechnique. Nanocrystalline NiO films prepared by the solution

growth route [46] give a resistance of several M cm2

with a band gap 3.6 eV. Comparing with reported values, it

can be seen that the values obtained in the present study,

for films annealed at 773 K, show a comparable electrical

resistivity, smaller grain size and better transmittance. The

high degree of reproducibility resulting from the absence of

a reactive atmosphere during the film deposition and the use

of a minimum number of growth parameters makes the present

study attractive and suitable for device applications.

4. Conclusions

Nanostructured mesoporous nickel oxide thin films have been

prepared using the pulsed laser ablation technique. Annealing

7


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300 400 500 600 700 800 900

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

d

c

b

a

t

nm

Figure 10. Variation of optical density (t) with wavelength () forthe NiO films (a) as deposited; annealed at (b) 573, (c) 773 and(d) 1173 K.

3.0 3.5 4.00.00

0.01

0.02

0.03

0.04

300 400 500 600 700 800 9000

10

20

30

4050

60

70

80

90

100 d

c

b

a

%

T

nm

c

b

a

(h)2(eV2nm2)

h (eV)

Figure 11. Plot of(h)2 against h for the NiO films annealed at(a) 573, (b) 773 and (c) 1173 K. The inset shows transmittancespectra of NiO films (a) as deposited; annealed at (b) 573, (c) 773and (d) 1173 K.

of as-deposited films at 573 K resulted in the formation of

cubic NiO films with preferential growth along the (200)

crystal plane. A self-assembly of nanocrystals agglomerating

together to form an island-like structure is observed in films

annealed at 773 K. The annealing process was found toinfluence the structural, electrical and optical properties of the

films.

Acknowledgments

The work was supported by the Kerala State Council

for Science Technology and Environment. We gratefully

acknowledge Veeco-India Nanotechnology Laboratory for

AFM measurements. The authors are grateful to ProfessorAjay Gupta, Dr Sreepathy, Dr Ganesan, Dr Raghavendra

Reddy and Dr Phase of the UGC-DAE Consortium, Indore

Centre, for valuable suggestions and help in the measurements.

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nano structured me so porous nickel oxide thin films

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