Structural characterization and EXAFS wavelet analysis of Yb doped ZnO by wet chemistry route

doi:10.1016/j.jallcom.2014.09.186

Journal of Alloys and Compounds

Volume 622, 15 February 2015, Pages 115-120

https://doi.org/10.1016/j.jallcom.2014.09.186 Get rights and content

Highlights

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Optical and electrical properties of ZnO are influenced by lanthanide doping.
•
Optical and electrical properties of ZnO are influenced by lanthanide positioning.
•
Yb is incorporated in the O_h sites of the wurtzite structure.
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There is not Yb₂O₃ clustering or segregation for treatments below 800 °C.

Abstract

Lanthanide doped ZnO are interesting materials for optical and electrical applications. The wide band gap of this semiconductor makes it transparent in the visible range (E_gap = 3.2 eV), allowing a sharp emission from intra shell transition from the lanthanides. From the electrical side, ZnO is a widely used material in varistors and its electrical properties can be tailored by the inclusion of lanthanides. Both applications are influenced by the location of the lanthanides, grain boundaries or lattice inclusion. Yb doped ZnO samples obtained by wet chemistry route were annealed at different temperatures and characterized by Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD), Rietveld refinement of XRD data, and X-ray Absorption Fine Structure (XAFS). These techniques allowed to follow the changes occurred in the matrix and the Yb environment. The use of the Cauchy continuous wavelet transform allowed identifying a second coordination shell composed of Zn atoms, supporting the observations from XRD Rietveld refinement and XAFS fittings. The information obtained confirmed the incorporation of Yb in O_h sites of the wurtzite structure without Yb₂O₃ clustering in the lattice.

Introduction

ZnO is an interesting material due to its structural, electrical and optical properties. The structural properties are related to its wurtzite structure (space group P6₃mc), that allows obtaining varied morphologies of the oxide: colloidal nanoparticles [1], nanostructured colloids [2], nanorods [3], [4], [5], [6], nanowires [7], [8], [9], and tetrapods [10], thin films [11], among another complex nanostructures [12], [13], [14]. To obtain these morphologies several methods such as: thermal evaporation [7], [10], [13], laser ablation [4], hydrothermal synthesis [3], [6], [8], [14], forced hydrolysis[2], [12], precipitation [1] and electro-deposition [5], [15], are implemented.

The nonlinear current–voltage (I–V) properties of ZnO, are known since the ’50 [16], however it was not until the ’70s, when Matsuoka [17] rediscovered its electrical properties settling the basis for the implementation of ZnO in varistors. In spite of the origin of the nonlinear I–V behavior that is not completely understood, it was reported that ion segregation at the interfaces has a great influence in this phenomena. Just as an example, Sato et al. showed that in the ZnO:Pr system, the Pr atoms are located at the grain boundaries [18], and Jiang et al. demonstrated that for ZnO:Yb, the Yb presence induces oxygen rich zones by means of a detailed HRTEM–EELS analysis [19].

The good optical properties of ZnO have intensified the interest in this material for applications in hybrid light-emitting diodes [20], [21], [22], and its doping with lanthanides (Ln) as activators [23], [24], [25] has a potential use in flat displays. One drawback for these last applications is that the energy transfer from the host to the Ln ion is not efficient enough, making them not suitable to be used in devices [26]. The origin of this observation may be attributed to the fact that Ln ions occupy an interstitial site in the lattice [27].

To have a deeper knowledge on the electrical and optical properties of Ln doped ZnO, it is important to determine whether the Ln ions are located into the structural framework, or whether they are segregated in the grain boundaries [28]. In this work, we present a soft chemistry route for the synthesis of ZnO:Yb and its characterization based on Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD), Rietveld refinement of XRD data and X-ray Absorption Fine Structure Spectroscopy (XAFS).

The X-ray diffraction technique (XRD) provides information of the host matrix. Differences in the cell parameters of pure and doped samples, treated at different temperatures, supplied experimental evidence of the incorporation of Yb atoms in the ZnO lattice. On the other hand, the X-ray absorption measurements (X-ray Absorption Near Edge Spectroscopy and XANES, and Extended X-ray Absorption Fine Structure, EXAFS) are specific to the Yb atoms environment providing structural information of the changes taking place in samples with different thermal treatment.

Taking into account the EXAFS equation [29], [30]: $X (k) \approx \sum_{j} S_{i} (k) \frac{N_{j}}{{kR}_{j}^{2}} |f_{i} (k, π)| e^{- \frac{2 R_{j}}{λ_{j} (k)}} e^{- 2 σ_{j}^{2} k^{2}} \sin (2 {kR}_{j} + \sum ϕ_{ij} (k))$ where S_i is the amplitude reduction factor for the total central atom loss, N_j and R_j are the number of neighbor atoms and the distance of each one from the central atom, |f_i(k,π)| is the effective curved-wave backscattering amplitude function, $σ_{j}^{2}$ is the Debye–Waller term, λ_j is the photoelectron mean-free path, and ∑ϕ_ij(k) is the sum over all the phase shifts.

Based on this, the identity of the backscattering atoms can be determined by the backscattering function. Wavelet analysis allows improving the data analysis due to the possibility to observe all the EXAFS contributions simultaneously, obtaining a tridimensional representation of k (reciprocal) and R (real) space.

Several reports can be found about the fundamentals of wavelet analysis [31], [32], [33] and its implementation [30], [34], [35]. In our case, we applied the Continuous Cauchy Wavelet Analysis (CCWT) to the EXAFS spectra which allows determining the identity of the atoms in noisy signals [30], [36].

The correlation of the experimental results provided a conclusive structural evidence of the Ln incorporation in the structural framework and a range of temperature for thermal annealing to obtain ZnO:Yb without phase segregation.

Section snippets

Experimental section

Zinc acetate dihydrate (Zn(AcO)₂⋅2H₂O, (puriss. – Sigma–Aldrich) (ZAD), ytterbium acetate octahydrated (Yb(AcO)₃⋅8H₂O) (99.9% – Sigma–Aldrich), potassium hydroxide (KOH) (p.a. – Merck), absolute ethanol (EtOH) (p.a. – Merck), dimethylformamide (DMF) (p.a. – J.T. Baker) and hexane (p.a. – Riedel-de Haen) were used without further purification. Hydration water from lanthanides acetates were quantified by thermogravimetric analysis up to 1000 °C for full conversion into sesquioxides.

The synthetic

TEM analysis

The HRTEM images (Fig. 1) shows the good crystallinity of the agglomerated nanoparticles. This is corroborated by FFT (Fast Fourier Transform) that shows a sharp ring pattern. The size distribution of nanoparticles ranges within a 4–8 nm interval.

Nanoparticles agglomeration did not show any preferential orientation in spite of some specific orientations such as [0 0 1], [1 0 1] and [1 1 0] zone axis that can be observed in the nanoparticles at the edge of the colloidal particles.

XRD analysis

The ZnO:Yb-070 sample

Conclusions

ZnO:Yb with good crystallinity and free of crystalline impurities was synthesized by a soft chemistry route and structurally characterized by X-ray techniques. The combined utilization of Rietveld refinement, XAS characterization and data analysis techniques proves the insertion of lanthanide ions in the O_h sites of the ZnO structure without phase segregation. Differently from our previous paper using XRD [43], where only the interstitial insertion could be postulated. The CCWT proved that the

Acknowledgments

This work was financially supported by the YPF foundation (José A. Estenssoro PhD Grant), CONICET (Type 2 PhD Grant) and CITEDEF. The authors are grateful for being given access to the facilities of the XAFS (D04B – XAFS1 #6673, #7732 and #8239) beamlines at the LNLS. Also to Dr. Gustavo Azevedo from LNLS for his invaluable help in the XAFS measurements, Raul Tarulla from CITEDEF for ICP-EOS analysis and Matias Jobbágy from INQUIMAE-FCEN-UBA for the generous help in the thermal analysis of

References (47)

Y. Dai et al.
Synthesis and optical properties of tetrapod-like zinc oxide nanorods
Chem. Phys. Lett.
(2002)
D.C. Kim et al.
Fabrication of the hybrid ZnO LED structure grown on p-type GaN by metal organic chemical vapor deposition
Physica B
(2007)
S. Bachir et al.
Photoluminescence of polycrystalline zinc oxide co-activated with trivalent rare earth ions and lithium. Insertion of rare-earth ions into zinc oxide
J. Lumin.
(1997)
H.L. Han et al.
Up-conversion luminescence switching in Er³⁺-containing ZnO nanoparticles through Li⁺ co-doping
Opt. Mater.
(2008)
J. Timoshenko et al.
Wavelet data analysis of EXAFS spectra
Comput. Phys. Commun.
(2009)
E.H. Otal et al.
Metastability of heavy lanthanides in the ZnO wurtzite structure
J. Alloys Compd.
(2011)
D.A. Schwartz et al.
Magnetic quantum dots: synthesis, spectroscopy, and magnetism of Co²⁺- and Ni²⁺-doped ZnO nanocrystals
J. Am. Chem. Soc.
(2003)
E.H. Otal et al.
Nanostructured colloidal crystals from forced hydrolysis methods
Langmuir
(2009)
B. Liu et al.
Hydrothermal synthesis of ZnO nanorods in the diameter regime of 50 nm
J. Am. Chem. Soc.
(2003)
A.B. Hartanto et al.
Growth mechanism of ZnO nanorods from nanoparticles formed in a laser ablation plume
Appl. Phys. A
(2004)

Y. Tang et al.

Fast growth of high work function and high-quality ZnO nanorods from an aqueous solution

J. Phys. Chem. C

(2011)

Y. Wang et al.

Highly oriented 1-D ZnO nanorod arrays on zinc foil: direct growth from substrate, optical properties and photocatalytic activities

J. Phys. Chem. C

(2008)

M.H. Huang et al.

Room-temperature ultraviolet nanowire nanolasers

Science

(2001)

L.-Y. Chen et al.

Influence of polyethyleneimine and ammonium on the growth of ZnO nanowires by hydrothermal method

J. Phys. Chem. C

(2011)

D. Wang et al.

Defects-mediated energy transfer in red-light-emitting Eu-doped ZnO nanowire arrays

J. Phys. Chem. C

(2011)

G. Malandrino et al.

Relationship between nanostructure and optical properties of ZnO thin films

J. Phys. Chem. C

(2008)

Z.R. Tian et al.

Complex and oriented ZnO nanostructures

Nat. Mater.

(2003)

Han et al.

Controllable synthesis and optical properties of novel ZnO cone arrays via vapor transport at low temperature

J. Phys. Chem. B

(2005)

N. Wang et al.

Hexagonal ZnO bipyramids: synthesis, morphological evolution, and optical properties

J. Phys. Chem. C

(2009)

O. Lupan et al.

Controlled mixed violet–blue–red electroluminescence from Eu:nano-phosphors/ZnO-nanowires/p-GaN light-emitting diodes

J. Phys. Chem. C

(2013)

K.S. Valeyev et al.

elektrichestvo

(1964)

M. Matsuoka

Nonohmic properties of zinc oxide ceramics

Jpn. J. Appl. Phys.

(1971)

Y. Sato et al.

Role of Pr segregation in acceptor-state formation at ZnO grain boundaries

Phys. Rev. Lett.

(2006)

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Structural characterization and EXAFS wavelet analysis of Yb doped ZnO by wet chemistry route

Highlights

Abstract

Introduction

Section snippets

Experimental section

TEM analysis

XRD analysis

Conclusions

Acknowledgments

Chem. Phys. Lett.

Physica B

J. Lumin.

Opt. Mater.

Comput. Phys. Commun.

J. Alloys Compd.

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J. Am. Chem. Soc.

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Appl. Phys. A

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J. Phys. Chem. C

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