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Materials Science in Semiconductor Processing

Materials Science in Semiconductor Processing 15 (2012) 308–313

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Optical studies of nano-structured La-doped ZnO prepared bycombustion method

L. Arun Jose a, J. Mary Linet a, V. Sivasubramanian b, Akhilesh K. Arora c, C. Justin Raj d,T. Maiyalagan e, S. Jerome Das a,n

a Department of Physics, Loyola College, Chennai 600034, Indiab Light Scattering Studies Section, IGCAR, Kalpakkam 603102, Indiac Condensed Matter Physics Division, IGCAR, Kalpakkam 603102, Indiad Pusan National University, Jangjeon, Geumjeong, Busan 609 735, South Koreae School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 639 798, Singapore

a r t i c l e i n f o

Article history:

Received 4 August 2011

Received in revised form

13 March 2012

Accepted 14 March 2012Available online 21 April 2012

Keywords:

Doping

Semiconducting II–VI materials

Nano-structures

combustion

X-ray diffraction spectra

Zinc compounds

Rare earth compounds

01/$ - see front matter & 2012 Elsevier Ltd. A

x.doi.org/10.1016/j.mssp.2012.03.011

esponding author. Tel.: þ91 44 2817 5662;

1 44 2817 5566.

ail addresses: sjeromedas2004@yahoo.com,

loyolacollege.edu (S. Jerome Das).

a b s t r a c t

Coral-shaped nano-structured zinc oxide (ZnO) was successfully synthesized and La-

doped via a facile combustion process using glycine as a fuel. The auto-ignition

(at �185 1C) of viscous reactants zinc nitrate and glycine resulted in ZnO powders.

Hexagonal wurtzite structure of pure and doped ZnO powder was confirmed by X-ray

powder diffraction analysis. The transmission electron micrograph shows that the

nano-structured ZnO is coral-shaped and possess maximal pore (�10–50 nm pore size)

density in it and the grain size is approximately about 15 nm. Addition of dopants

subsequently alters the structural and optical properties which were confirmed by

UV–VIS studies.

& 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Nano-structured metal oxide semiconductors are gain-ing attention due to their wide band-gap and relatedproperties [1]. Recent decades are witnessed withresearchers paying much interest in synthesis and char-acterization of II–VI group semiconducting materials atnano- [2] and bulk [3] levels. Zinc oxide (ZnO) is a widelyexploited, due to its excellent physical and chemicalproperties. Numerous researchers proposed the solutioncombustion method to synthesize simple and mixedmetal oxides [4–9]. Normally ZnO is doped with different

ll rights reserved.

types of metallic ions in order to enhance the optical andconducting properties [10–14]. The exceptional intereston ZnO may be seen in the recent literatures. Themodified ZnO may be used as a base material for dilutedmagnetic semiconductors [15–18], gas sensors [19],photocatalysts [20], field-effect transistors [21,22], light-emitting materials [23–25], solar cells [26,27] and biolo-gical systems (drug delivery, bio-imaging, etc.) [28,29]. Inthe recent times, rare earth metal-doped ZnO (e.g., Tb, Er,Eu, Dy and Sm) has been broadly researched and concen-trated on luminescence properties [24,30–33]. Lantha-num (La)-doped ZnO nano-structures exhibit excellentphotocatalytic activity and gas sensitivity [20,34–36].Nano-sized ZnO has been synthesized by the solutioncombustion method and there are no literature referencesfor La-doped ZnO using this method. Current work is focusedon investigating the result of La doping concentration on the

Heating / Development of precursor

Ignition / Combustion / Burning

Synthesized Material (powder)

Directly mixed with desired (1:09) molar ti

Glycine (NH2CH2COOH) Zinc Nitrate (Zn(NO3)2•6H2O) Lanthanum Nitrate (La(NO3)2•6H2O)

Mixed with (1-x): x molar ratio where x = 0.01, 0.02. 0.03 and 0.05 of (La(NO3)2•6H2O)

Fig. 1. Procedural flow chart for preparation of ZnO with/without La-dopant.

Fig. 2. TEM Images: (a) bright field, (b) dark field, (c) detailed view, (d) diffraction pattern and (e) EDS pattern of pure ZnO.

L. Arun Jose et al. / Materials Science in Semiconductor Processing 15 (2012) 308–313 309

L. Arun Jose et al. / Materials Science in Semiconductor Processing 15 (2012) 308–313310

microstructure and optical properties of ZnO nano-structureprepared by the combustion method.

2. Experimental details

Distinct from usual thermal evaporation, ZnO nano-structures were prepared by the combustion method, whichallows efficient synthesis of nano-size materials. This pro-cess involves a self-sustained reaction in homogeneoussolution of different oxidizers (e.g., metal nitrates) and fuels(e.g., urea, glycine, citric acid, hydrazides). Depending on thetype of precursors, and the suitable conditions for chemicalreaction to take place, zinc nitrate (Zn(NO3)2 �6H2O) waschosen as an oxidizer and glycine (NH2CH2COOH) as a fuel,since its combustion heat (�3.24 kcal/g) is more negative

Fig. 3. TEM images: (a) bright field, (b) dark field, (c) detailed view, (d)

when compared with urea (�2.98 kcal/g) or citric acid(�2.76 kcal/g) [36]. Lanthanum nitrate (La(NO3)2 �6H2O) isadded to zinc nitrate with required molar ratio and glycineis also added along with it, in a molar ratio of 0.9:1 (zincnitrateþ lanthanum nitrate:glycine) and stirred well for 1 hin 100 ml double distilled water. The obtained solution isheated (�185 1C) till combustion reaction occurs. Proce-dural flow chart diagram for the preparation of precursorsand the formation of nano-structures is shown in Fig. 1.Crystallinity of pure ZnO and La-doped ZnO catalysts wereanalyzed by Philips CM 20 Transmission Electron Micro-scope which was operated between 20 and 200 kV. Com-position of the samples were analyzed by energy dispersiveX-ray spectroscopy (EDS) attached to the TEM instrument.X-ray diffraction patterns of the synthesized samples were

diffraction pattern and (e) EDS pattern of 5 mol % La-doped ZnO.

L. Arun Jose et al. / Materials Science in Semiconductor Processing 15 (2012) 308–313 311

recorded using PAN analytical X-ray diffractometer with CuKa (1.5405 A) radiation in the scan range 2y between 301and 701 with a scan speed of 21/min. UV–VIS spectra of pureZnO and La-doped ZnO catalysts were recorded using VarianCARY 5E UV–VIS–NIR Spectrophotometer. The absorbancespectra were then recorded in the range 200–700 nm.Photoluminescence of pure ZnO and La-doped ZnO weremeasured by Jobin Yvon Fluorolog spectrofluorometer andthe results are discussed in detail.

Fig. 4. Powder XRD spectra of samples pure–doped prepared at different

mol percent of La.

3. Results and discussion

TEM analysis shows that the nano-structures whichhad been synthesized using combustion processing arecoral-shaped and porous as shown in Fig. 2. This shapemay be attributed to the thermal fluctuations whilesynthesizing the samples. Grain size is found to be�10–20 nm both in the case of pure and doped ZnO.Porous nature of the nano-structures significantly increasesas the La-dopant concentration increases as shown in Fig. 3.Each individual nano-structure is about 450–1000 nmformed by tiny spherical ZnO nanoparticles. We can alsonotice that the pores are �10–50 nm in diameter whichconsiderably increase the surface to volume ratio. Selectedarea diffraction patterns match very well with wurtziteZnO in both pure and doped ZnO. EDS analysis shows thatsome La3þ ions have been incorporated into the ZnO latticeby substituting zinc ions as shown in Fig. 3(e) and inTable 1. When La is present the composition of oxygenseems to be nearly constant. This may be due to theaddition of oxygen atoms in the La-doped ZnO which wasaccommodated by the additional vacancy in the La3þ ion.Copper peak in the EDS measurement originates from theTEM supporting carbon coated copper grid.

XRD profiles of synthesized pure and doped materialsin appropriate ratio are shown in Fig. 4. The diffractionpeaks and their relative intensities match with the JCPDScard no. 36-1451. Hence the observed patterns can beclearly endorsed to the presence of hexagonal wurzitestructure. XRD peak of lanthanum oxide was not observed

Table 1Composition of elements in La-doped ZnO samples.

La concentration (mol%) Element weight (%) Atomic (%)

0 O 13.30 38.50

Zn 86.70 61.50

1 O 20.30 51.73

La 04.34 01.27

Zn 75.36 47.00

2 O 19.60 51.32

La 08.38 02.53

Zn 72.02 46.15

3 O 18.94 50.92

La 12.15 03.76

Zn 68.91 45.32

5 O 18.30 50.80

La 17.20 05.50

Zn 64.50 43.70

even for the La-doped sample with a high La concentra-tion, suggesting that lanthanum oxide is uniformly dis-persed in the ZnO and no second phase such as La2O3 andLa(OH)3 appears. It is evident that the introduction of Laions does not alter the structure of ZnO and dopantdisperses homogeneously in the ZnO matrix as previouslyreported [37]. Using the Scherrer equations the crystallitesizes were estimated to be around 450 nm from the full-width at half-maximum (FWHM) of diffraction peaks. Thediffraction pattern of ZnO is observed between the 2yvalues of 301 and 701. The peak intensities of doped ZnOincreases with dopant concentration. Therefore, the crys-talline nature of ZnO nanostructure increases with La-dopant in the same manner as previously reported in thecase of Fe doped ZnO [38]. Doping of La ions restrains thegrowth of ZnO grains and dopant with smaller ionicradius has a constructive effect on diffusivity whichpromotes orientation growth and good crystal [39]. Thelattice parameters and the unit cell volume were deter-mined using software program UnitCell method of TJBHolland & SAT Redfern [40]. The determined unit cellparameters, volume and c/a were plotted as a function ofLa concentrations and are shown in Figs. 5 and 6 respec-tively. The lattice constant gradually increases withincrease in concentration of La3þ ions. Consequently, cellvolume and c/a ratio changed, agreeing with the fact thationic radii of La3þ is higher than the Zn2þ ion (0.106 nmfor La and 0.074 nm for Zn) [41,42] but there is a smallvariation in c-axis compared with the results of Chen et al.[37]. This distortion in the lattice parameters confirms theincorporation of La3þ ions up to 5 mol% in ZnO wurzitestructure.

UV–VIS spectrum shows that the absorbance is highbelow 380 nm for pure ZnO and as the La-dopant con-centration increases the absorbance of ZnO decreasesconsiderably below this region as shown in Fig. 7. Thecorresponding band gap values of pure and doped ZnO are

Fig. 7. UV–VIS spectra ZnO with/without dopant.

Fig. 8. Calculated band gap of pure and La-doped ZnO.

Fig. 5. Unit cell parameters a and c were plotted as a function of La

concentration.

Fig. 6. Unit cell volume and c/a were plotted as a function of La

concentration.

Fig. 9. Room temperature PL emission spectra of ZnO with/without

La-dopant.

L. Arun Jose et al. / Materials Science in Semiconductor Processing 15 (2012) 308–313312

presented in Fig. 8. It can be clearly seen that the band gapof La-doped ZnO also increases gradually with increase inLa concentration. After 380 nm, absorbance of pure ZnO is

less compared with La-doped ZnO and absorbanceincreases with increase in dopant concentration.

Photoluminescence (PL) spectra of La-doped ZnOnano-structures were measured with an excitation wave-length of 285 nm and is shown in Fig. 9. The intensity ofPL emission is found to increase with increase in La-dopant, but the intensity of doped ZnO decreases incomparison with pure ZnO between 3.2 and 3.3 eV. ThePL spectrum shows the La characteristic emission band at�2.9 eV and near UV emission between 3.27 and 3.30 eV.There is a shift in the emission spectra for pure and dopedZnO. This may be attributed due to the strain created inthe crystal lattice to accommodate larger La atoms.Spectra in the range of 340–460 nm (2.7–3.6 eV) showsthat a violet peak at about 420 nm (2.95 eV) and theintensity of emission are found to be strongly reliant onthe La concentration. Traps on the grain surface per unitvolume increases with the increase of specific surfacearea. Cordaro et al. [43] assumed that interface traps lie in

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the depletion regions and locate at the ZnO–ZnO grainboundaries when a polycrystalline varistor forms, and thelevel of interface trap was found to be about 0.33 eVbelow the conduction band edge. So violet emission ispossibly attributed to the recombination centers linkedwith interface traps existing at the grain boundaries, andradiative transition occurs between the level of interfacetraps and the valence band.

4. Conclusions

La-doped ZnO was prepared by combustion proces-sing; doping levels included undoped, 1, 2, 3 and 5 molarpercentage. Significant transformation was observed upondifferent doping concentrations. Transmission electronmicrograph shows an enhancement of pore density fordoped ZnO. Lattice parameters and unit cell volume weredetermined from the XRD data and it confirms the entryof La-dopant inside ZnO crystal lattice by the increase inlattice constants. It is evident that the absorbance near UVregion decreases with increase in dopant concentration.The bandgap is found to increase with addition of La. TheLa-doped ZnO nano-structures prepared at low tempera-tures are more suitable for applications such as chemicaland biological sensors, optoelectronic devices, andsolar cells.

Acknowledgments

The authors gratefully acknowledge BRNS (Board ofResearch in Nuclear Sciences—Government of India, Pro-ject no. 2008/37/12/BRNS/1513) for providing financialassistance. They are also thankful to authorities of IndianInstitute of Technology, Chennai 36, for providing TEM,UV–VIS, PL and powder XRD facility.

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