简介概要

Structure and Raman scattering of Mg-doped ZnO nanoparticles prepared by sol-gel method

来源期刊：Rare Metals2018年第10期

论文作者：Zhong-Yu Jiang Ke-Rong Zhu Zhong-Qing Lin Shao-Wei Jin Guang Li

文章页码：881 - 885

摘要：Mg-doped ZnO (Mg_xZn_1-xO, x=0-0.10)nanoparticles were prepared by sol-gel method. Structural characterization by X-ray diffraction （XRD） indicates that the lattice parameter a increases and c decreases linearly with the increase in Mg content （x） due to the substitution of Mg²⁺ for Zn²⁺ in ZnO lattice. The blueshift of Raman modes is observed, impling the increase in force constant of atom vibration in the Mg_xZn_1-xO （MgZnO） nanoparticles. Resonant Raman spectra show longitudinal optical phonon overtones up to fifth order, revealing that the short part of the electron-phonon interaction is enhanced and long-range part is weakened by Mg doping.

稀有金属(英文版) 2018,37(10),881-885

Structure and Raman scattering of Mg-doped ZnO nanoparticles prepared by sol-gel method

Zhong-Yu Jiang Ke-Rong Zhu Zhong-Qing Lin Shao-Wei Jin Guang Li

School of Physics and Materials Science, Anhui University

Modern Experiment Technical Center, Anhui University

作者简介：*Ke-Rong Zhu e-mail: krzhu@ahu.edu.cn;

收稿日期：25 February 2014

基金：financially supported by the National Natural Science Foundation of China (Nos. 11174001 and 11174002);the Science Foundation of Anhui Education (Nos. KJ2013A030);the Scientific Research Startup Outlay for Doctors in Anhui University;

Structure and Raman scattering of Mg-doped ZnO nanoparticles prepared by sol-gel method

Zhong-Yu Jiang Ke-Rong Zhu Zhong-Qing Lin Shao-Wei Jin Guang Li

School of Physics and Materials Science, Anhui University

Modern Experiment Technical Center, Anhui University

Abstract：

Mg-doped ZnO (Mg_xZn_1-xO, x=0-0.10)nanoparticles were prepared by sol-gel method. Structural characterization by X-ray diffraction (XRD) indicates that the lattice parameter a increases and c decreases linearly with the increase in Mg content (x) due to the substitution of Mg²⁺ for Zn²⁺ in ZnO lattice. The blueshift of Raman modes is observed, impling the increase in force constant of atom vibration in the Mg_xZn_1-xO (MgZnO) nanoparticles. Resonant Raman spectra show longitudinal optical phonon overtones up to fifth order, revealing that the short part of the electron-phonon interaction is enhanced and long-range part is weakened by Mg doping.

Keyword：

ZnO nanoparticles; Mg doping; X-ray diffraction; Raman scattering;

Received： 25 February 2014

1 Introduction

ZnO is an n-typeⅡ-Ⅵsemiconducting compound with a wide band gap (3.3 eV) and a large exciton binding energy(60 meV) at room temperature [ 1] .It is recognized as a promising candidate for the development of short-wavelength photonic devices such as ultraviolet detectors,lightemitting diodes and laser diodes owing to their remarkable electrical and optical properties [ 2, 3, 4, 5, 6] .Mg is an ideal dopant to realize a wider band gap.Incorporating Mg into ZnO thin film has already been proved to be a feasible technique to realize band gap tuning since the band gap of wurtzite ZnO is～3.37 eV and that of MgO is～7.4 eV [ 7] .However,the optical and electrical properties of semiconductors are greatly affected by the vibrational properties via the electron-lattice or exciton-phonon interaction.Investigation of the fundamental vibrational properties of Mg-doped ZnO materials is therefore essential to develop a better understanding of its optical properties.Raman spectroscopy is a versatile technique for fast and nondestructive study of phonon scattering processes in materials.The Raman spectra of Mg-doped ZnO nanostructures were reported [ 8, 9, 10] .However,the influence of Mg doping on the lattice dynamics of ZnO was rarely discussed in detail.Besides,the resonant Raman spectra(RRS) of Mg-doped ZnO nanoparticles were not reported yet,although the RRS of ZnO,ZnO quantum dots,ZnO/Ag nanocomposite thin films,Me-doped ZnO nanorods(Me=Mn,Co,Cu and Ni),(Mn,Co)-codoped ZnO films and N-doped ZnO were discussed [ 11, 12, 13, 14, 15, 16, 17, 18] .In this paper,Mg_xZn_1-xO nanoparticles with x ranging from 0 to 0.10were prepared by sol-gel method and the influence of Mg on structure and phonons of ZnO nanoparticles were discussed in detail.It is found that the substitution of Mg²⁺for Zn²⁺results in lattice distortion and variation of the electron-phonon interaction in Mg_xZn_1-xO nanoparticles.

2 Experimental

The Mg_xZn_1-xO nanoparticles with x ranging from 0 to0.10 were prepared by sol-gel method.The precursor materials were zinc nitrate hexahydrate Zn(NO₃)2.6H₂O and magnesium nitrate hexahydrate Mg(NO₃)2.6H₂O.Their molar ratio was (1-x):x with x=0,0.02,0.04,0.06,0.08 and 0.10.Firstly,Zn(NO₃)2.6H₂O and Mg(NO₃)2·6H₂O were dissolved in acetic acid solvent and stirred vigorously at 70℃for 2 h until the homogenous gel formed.Then the gels were dried for 24 h at 100℃.After grinding and annealing as-prepared powder at 650℃for2 h in air atmosphere,the Mg_xZn_1-xO nanoparticles were obtained.

The crystal phases and structures of the samples were determined by a powder X-ray diffractometer (XRD;Pgeneral XD-3) with Cu Kαradiation.The morphologies of the samples were observed by a scanning electron microscope (SEM;Hitachi S4800) with energy-dispersive X-ray spectrometer (EDX).Nonresonant and RRS were obtained by a Raman spectrometer system (Renishaw in Via Reflex) with excitation wavelength of 532 and325 nm,respectively.

3 Results and discussion

Figure 1 shows XRD patterns of the Mg_xZn_1-xO nanoparticles with x=0,0.02,0.04,0.06,0.08 and 0.10.The diffraction lines at the angle (2θ) of 31.69°,34.46°,36.29°,47.58°,56.56°,62.91°,66.45°,69.12°,72.68°and 77.16°are indexed as (100),(002),(101),(102),(103),(200),(112),(201),(004) and (202) lines of the hexagonal wurtzite structure of ZnO (JCPDS:36-1451),respectively.This indicates that Mg doping does not change the hexagonal wurtzite structure.As x increases to 0.10,a new diffraction line (denoted by*) appears and is assigned to (200) line of MgO cubic phase (JCPDS:04-0829),implying that mixed phase emerges in x=0.10 sample.So,only 8%of Zn²⁺can be substituted by Mg²⁺in wurtzite structure of Mg_xZn_1-xO nanoparticles prepared by sol-gel method.There is a significant high angle shift of (002) line with the increase in x in the insert of Fig.1,implicating a change in lattice parameters.The lattice parameters a and c of the Mg_xZn_1-xO nanoparticles were inpidually calculated from(100) and (002) lines in the XRD by and c=λ/sinθ,whereλis the X-ray wavelength andθis the diffraction angle [ 19] ,or obtained by Rietveld analysis of XRD patterns [ 20] .The value of a increases and that of c decreases linearly with the increase in Mg content (x) from0 to 0.08 as shown in Fig.2.This lattice distortion results from the substitution of Zn²⁺by Mg²⁺whose ionic radius(0.066 nm) is smaller than that of Zn²⁺(0.074 nm).Otherwise,MgZnO has wurtzite structure with a larger Mg content of 36 at%and has a cubic phase above that content,because MgO has cubic structure [ 21] .So the lattice distortion is a natural result in the Mg_xZn_1-xO nanoparticles.

Fig.1 XRD patterns of Mg_xZn_1-xO samples (asterisk marked as MaO phase).Inset showing variation of (002) peak with Mg content (x)

SEM images of the Mg_xZn_1-xO nanoparticles with x=0,0.02,0.04,0.06,0.08 and 0.10 are shown in Fig.3a-f,respectively.The morphologies do not almost change with Mg content (x).The average size of the particles is about 70 nm.EDX result reveals magnesium,zinc and oxygen as the observable elements in the sample with Mg content of 0.06 as seen in Fig.3g,confirming that Mg is doped in the samples.

The nonresonant Raman spectra (70-800 cm^-1) of the Mg_xZn_1-xO nanoparticles at room temperature excited by532 nm laser line are shown in Fig.4,from which the influence of Mg doping on lattice dynamics of ZnO was discussed.For undoped samples,Raman modes at 99.5,204,332,379,410,438,529 and 579 cm^-1 are assigned to E₂,2E₂,E_2(high)-E₂,A_1T,E₁,E_2(high),A_1L and E_1L modes of hexagonal wurtzite crystal structure ZnO,respectively [ 22] .The strong and dominant 438 cm^-1 (E₂(high)) mode,which is a characteristic for hexagonal wurtzite crystal structure,is observed in all the samples.Therefore,the wurtzite structure of Mg_xZn_1-xO samples is also confirmed from Raman spectra.As Mg content (x) increases to 0.10,-1an additional mode at 277 cm^-1 appears.For Ga-,Fe-,Sb-,Mn-,Cu-and Al-doped ZnO,a mode around 275 cm^-1 is observed and related to intrinsic host lattice defects [ 23] .For MgO microcrystals,a mode at 280 cm^-1 is observed,enhanced with the decrease in the particle size and discussed as due to the first-order Raman scattering forbidden in the bulk crystals [ 24] .Here,277 cm^-1 mode appears as the MgO emerges in the Mg_xZn_1-xO samples,so this mode arises from MgO.As Mg content (x) increases,the438 cm^-1 mode does not shift,because this E_2(high) mode is associated with the vibration of oxygen atoms which was also confirmed by the isotopic mass dependence of the frequency shift measurements [ 25] .However,the 99.5,204,379,410,529 and 579 cm^-1 modes blue-shift to 102,213,383,415,544 and 590 cm^-1,respectively,as Mg content (x) increases to 0.10.The insert in Fig.4 shows the blueshift of E₂ mode from 99.5 to 102.0 cm^-1 clearly.These blueshifts suggest that the Mg²⁺is substituted for Zn²⁺in ZnO lattice and affects the lattice dynamics of the Mg_xZn_1-xO nanoparticles as a whole.The blueshift of Raman frequency may be induced by reducing atom mass or increasing force constant of atom vibration.The atom mass of Mg (24.3) is smaller than that of Zn (65.4),so the substitution of Mg for Zn can cause blueshift of frequency.The frequency (ω(MgZnO)) of respective mode of Mg_xZn_1-xO can be roughly estimated from the relative frequency (ω(ZnO)) of ZnO using the simple isotopic shift model,as [ 26]

Fig.2 Lattice parameters a and c of Mg_xZn_1-xO nanoparticles versus Mg content (x).Lines being linearly fitted to experimental data

Fig.3 SEM images of Mg_xZn_1-xO samples with different Mg contents:a x=0,bx=0.02,cx=0.04,dx=0.06,e x=0.08 and f x=0.10;g EDX result of Mg_xZn_1-xO with x=0.06

Fig.4 Nonresonant Raman spectra of Mg_xZn_1-xO samples excited by 532-nm laser beam

whereμ_ZnO and,u_MgZnO are the reduced atomic masses of ZnO and Mg_xZn_1-xO,respectively.It can be obtained from Eq.(1) thatω(MgZnO) for Mg_0.10Zn_1-0.10O is 100,205,381,412,532 and 584 cm^-1 by usingω(ZnO)=99.5,204,379,410,529 and 579 cm^-1 measured from undoped ZnO sample,respectively.The calculated frequencies of Mg_0.10Zn_1-0.10O are smaller than the experimental values of 102,213,383,415,544 and 590 cm^-1,respectively.Therefore,it implies that the blueshift is also induced by increasing force constant of atom vibration.This result is different from that of Mn-doped ZnO nanoparticles,in which the force constant of atom vibration is reduced [ 27] .

The electron-phonon interaction could be probed by resonant Raman scattering spectra when the exciting photon energy is resonant with the electronic interband transition energy of wurtzite ZnO.Figure 5 shows the RRS of Mg_xZn_1-xO nanoparticles with Raman shift ranging from200 to 3300 cm^-1 excited by 325 nm.Only E_1Llongitudinal optical mode is observed due to the electronlongitudinal optical phonon interaction.With the increase of x from 0 to 0.10,the first-order (1LO),the second-order(2LO),the third-order (3LO),the fourth-order (4LO) and the fifth-order (5LO) longitudinal optical phonons show significant blueshift from 577,1155,1730,2310 and 2886to 593,1186,1779,2372 and 2965 cm^-1,respectively.This also indicates the substitution of Mg²⁺for Zn²⁺in ZnO lattice and the increase in force constant of atom vibration in the Mg_xZn_1-xO nanoparticles.On the other hand,the maximum frequency shift (n(LO)×ωLO) can be compared with the deformation energy in the material [ 11] ,where n(LO) indicates the nth-order LO phonons,andωLO is the frequency of the nth-order LO phonons.The maximum 5LO can be observed in all the Mg_xZn_1-x O nanoparticles as shown in Fig.5.So,the increase in frequency of 5LO implies that the deformation energy of the Mg_xZn_1-xO nanoparticles increases with Mg doping.The short-range part of electron-phonon interaction is the interaction between the lattice displacement and the electrons,which can be represented by the deformation potential [ 18] .Therefore,the short-range part of electronphonon interaction is enhanced by Mg doping in the Mg_xZn_1-xO nanoparticles.In fact,the deformation potential is caused by the displacement of atoms from equilibrium positions,which affects the electronic band structure of a material.In our case,this can be related to the lattice distortion induced by Mg doping in the Mg_xZn_1-xO nanoparticles.The long-range part of the electron-phonon interaction,the Frohlich interaction,arises from the macroscopic electric filed generated by the LO phonons in polar semiconductors.The intensity ratio of second-to first-order LO phonons (I_2LO/I_1LO) as a function of Mg content is plotted in Fig.6,which reflects the strength of the Frohlich interaction [ 12, 18] .The intensity ratio (I_2LO/I_1LO) decreases with the increase in Mg content (x) as shown in Fig.6.So the long-range part of the electronphonon interaction is weakened by Mg doping in the Mg_xZn_1-xO nanoparticles.As known,the second-order structures are very sensitive to atomic scale disorder,and thus,theI_2LO/I_1LO ratio will be reduced by the compositional disorder resulted from Mg doping.Consequently,the long-range part of the electron-phonon interaction will be weakened in the Mg_xZn_1-xO nanoparticles.

Fig.5 Resonant Raman spectra of Mg_xZn_1-xO samples excited by He-Cd laser with 325-nm laser beam

Fig.6 Ratio ofI_2LO/I_1LO as a function of Mg content (x) for Mg_xZn_1-xO nanoparticles

4 Conclusion

The Mg_xZn_1-xO nanoparticles with x ranging from 0 to0.10 were successfully prepared by sol-gel method.The average diameter of the Mg_xZn_1-xO nanoparticles is about70 nm and almost independent on Mg content (x).The result of XRD shows that the Mg_xZn_1-xO nanoparticles have the hexagonal wurtzite structure and the lattice distortion,the linear increase in parameters a and decrease in c with the increase in Mg content (x),are due to the substitution of Mg²⁺for Zn²⁺in ZnO lattice.Above x=0.08,a mixed phase MgO appears.For the nonresonant Raman,as Mg content (x) increases to 0.10,the 99.5,204,379,529 and579 cm^-1 modes blue-shift to 102,213,383,544 and590 cm^-1,respectively,attributed to the increasing force constant of atom vibration.From RRS analysis,it is discovered that the short part of the electron-phonon interaction is enhanced and long-range part is weakened with the increase in Mg content (x).These results are beneficial to deep understanding of the optical and electrical properties of Mg-doped ZnO materials.

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Nos.11174001 and11174002),the Science Foundation of Anhui Education (Nos.KJ2013A030),and the Scientific Research Startup Outlay for Doctors in Anhui University.