Growth, X-ray peak broadening studies, and optical properties of Mg-doped ZnO nanoparticles Ramin Yousefi a,n , A. Khorsand Zak b , Farid Jamali-Sheini c a Department of Physics, Masjed-Soleiman Branch, Islamic Azad University (I.A.U.), Masjed-Soleiman, Iran b Materials and Electroceramics Lab, Department of Physics, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, Iran c Department of Physics, Ahwaz Branch, Islamic Azad University, Ahwaz, Iran article info Keywords: Mg-doped ZnO nanoparticles X-ray diffraction Size–strain plot Optical properties abstract Undoped and Mg-doped ZnO nanoparticles (NPs) (Zn 1 x Mg x O, x ¼0.01, 0.03, and 0.05) were grown by the sol–gel method. X-ray results showed that the products were crystalline with a hexagonal wurtzite phase. Microscopy studies revealed that the undoped ZnO NPs and Zn 1x Mg x O NPs had nearly spherical and hexagonal shapes. The size–strain plot (SSP) method was used to study the individual contributions of crystal- lite sizes and lattice strain on the peak broadening of the undoped and Mg-doped ZnO NPs. Some physical parameters such as strain, stress, and energy-density values were calculated for all reflection peaks of the XRD corresponding to the wurtzite hexagonal phase of ZnO in the 20–1001 range from the SSP results. The effect of doping on the band- gap was also investigated by a photoluminescence (PL) spectrometer. The PL results showed that Mg 2 þ is a good dopant to control band gap of the ZnO properties. & 2013 Elsevier Ltd. All rights reserved. 1. Introduction ZnO is an important wide-band-gap semiconductor with a direct band-gap of 3.37 eV and a high exciton binding energy of 60 meV, which is greater than the thermal energy at room temperature. ZnO is a promising material for ultraviolet nano-optoelectronic devices and lasers operating at room temperature [1]. By alloying ZnO with another material of a different band-gap, the band- gap of ZnO can be finely tuned; thus affecting the wavelength of exciton emission. In addition, alloying ZnO with MgO or Mg creates Zn 1 x Mg x O, which is a potential candidate for future optoelectronic devices. The addition of MgO, which has a larger band-gap (7.7 eV) than ZnO, results in a widened band-gap. Additionally, MgO has less lattice mismatch with ZnO, as the ionic magnesium and zinc are relatively similar [2–4]. Recently, many researchers have reported the formation of Zn 1x Mg x O nanostructures with different methods such as the metal– organic vapor phase deposition [5–7], pulsed laser deposition [8], and thermal evaporation [9–13]. These researchers studied the effect of Mg element on optical band gap of ZnO nanostructures. We also reported Zn 1x Mg x O nanowires that have been grown by a usual and a modified thermal evaporation method [14–16]. We observed the optical prop- erties of the Zn 1x Mg x O nanostructures that have been affected by different methods. However, we could not obtain the same morphology for the undoped and Mg-doped ZnO nanowires, which is important for a comparative study. Therefore, we decided to grow undoped and Mg-doped ZnO nanostructures by using a sol–gel method in a gelatin media to obtain the same morphology for the undoped and Mg- doped ZnO nanostructures. According to this decision, a comparative study between structural and optical properties of undoped and Mg-doped ZnO nanostructures has been carried out. In fact, such studies could increase our knowledge about optical properties of the Zn 1x Mg x O Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/mssp Materials Science in Semiconductor Processing 1369-8001/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2012.12.025 n Corresponding author. Tel.: þ98 9166224993; fax: þ98 6813330093. E-mail address: Yousefi[email protected] (R. Yousefi). Materials Science in Semiconductor Processing ] (]]]]) ]]]–]]] Please cite this article as: R. Yousefi, et al., Growth, X-ray peak broadening studies, and optical properties of Mg-doped ZnO nanoparticles, Materials Science in Semiconductor Processing (2013), http://dx.doi.org/10.1016/ j.mssp.2012.12.025i
Transcript
Contents lists available at SciVerse ScienceDirect
Materials Science in Semiconductor Processing
Materials Science in Semiconductor Processing ] (]]]]) ]]]–]]]
Ramin Yousefi a,n, A. Khorsand Zak b, Farid Jamali-Sheini c
a Department of Physics, Masjed-Soleiman Branch, Islamic Azad University (I.A.U.), Masjed-Soleiman, Iranb Materials and Electroceramics Lab, Department of Physics, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, Iranc Department of Physics, Ahwaz Branch, Islamic Azad University, Ahwaz, Iran
e cite this article as: R. Yousefi, et ananoparticles, Materials Scie
p.2012.12.025i
a b s t r a c t
Undoped and Mg-doped ZnO nanoparticles (NPs) (Zn1�xMgxO, x¼0.01, 0.03, and 0.05)
were grown by the sol–gel method. X-ray results showed that the products were
crystalline with a hexagonal wurtzite phase. Microscopy studies revealed that the
undoped ZnO NPs and Zn1�xMgxO NPs had nearly spherical and hexagonal shapes. The
size–strain plot (SSP) method was used to study the individual contributions of crystal-
lite sizes and lattice strain on the peak broadening of the undoped and Mg-doped ZnO
NPs. Some physical parameters such as strain, stress, and energy-density values were
calculated for all reflection peaks of the XRD corresponding to the wurtzite hexagonal
phase of ZnO in the 20–1001 range from the SSP results. The effect of doping on the band-
gap was also investigated by a photoluminescence (PL) spectrometer. The PL results
showed that Mg2þ is a good dopant to control band gap of the ZnO properties.
& 2013 Elsevier Ltd. All rights reserved.
1. Introduction
ZnO is an important wide-band-gap semiconductorwith a direct band-gap of 3.37 eV and a high excitonbinding energy of 60 meV, which is greater than thethermal energy at room temperature. ZnO is a promisingmaterial for ultraviolet nano-optoelectronic devices andlasers operating at room temperature [1]. By alloying ZnOwith another material of a different band-gap, the band-gap of ZnO can be finely tuned; thus affecting thewavelength of exciton emission. In addition, alloyingZnO with MgO or Mg creates Zn1�xMgxO, which is apotential candidate for future optoelectronic devices. Theaddition of MgO, which has a larger band-gap (7.7 eV)than ZnO, results in a widened band-gap. Additionally,MgO has less lattice mismatch with ZnO, as the ionic
All rights reserved.
usefi).
l., Growth, X-ray peaknce in Semicondu
magnesium and zinc are relatively similar [2–4]. Recently,many researchers have reported the formation of Zn1�xMgxOnanostructures with different methods such as the metal–organic vapor phase deposition [5–7], pulsed laser deposition[8], and thermal evaporation [9–13]. These researchersstudied the effect of Mg element on optical band gap ofZnO nanostructures. We also reported Zn1�xMgxO nanowiresthat have been grown by a usual and a modified thermalevaporation method [14–16]. We observed the optical prop-erties of the Zn1�xMgxO nanostructures that have beenaffected by different methods. However, we could not obtainthe same morphology for the undoped and Mg-doped ZnOnanowires, which is important for a comparative study.Therefore, we decided to grow undoped and Mg-doped ZnOnanostructures by using a sol–gel method in a gelatin mediato obtain the same morphology for the undoped and Mg-doped ZnO nanostructures. According to this decision, acomparative study between structural and optical propertiesof undoped and Mg-doped ZnO nanostructures hasbeen carried out. In fact, such studies could increase ourknowledge about optical properties of the Zn1�xMgxO
broadening studies, and optical properties of Mg-dopedctor Processing (2013), http://dx.doi.org/10.1016/
R. Yousefi et al. / Materials Science in Semiconductor Processing ] (]]]]) ]]]–]]]2
nanostructures as one of the best optical materials foroptoelectronic devices.
According to above reason, a sol–gel method was usedto obtain undoped and Mg-doped ZnO NPs (Zn1�xMgxO,x¼0.01, 0.03, and 0.05) with high quality in a gelatinmedia. Then an XRD and photoluminescence studies withmore details were presented for the products.
2. Experimental
To begin the synthesis of Zn1�xMgxO NPs, analyticalgrade zinc nitrate hexahydrate (Zn(NO3)2 �6H2O), magne-sium nitrate hexahydrate (Mg(NO3)2 �6H2O) and distilledwater were used as starting materials. All of the materialsused were purchased from Sigma-Aldrich. The precursorswere measured as Zn1�xMgxO, where x¼0, 0.01, 0.03, and0.05 to obtain 5 g of final product. First, a gelatin solutionwas prepared by adding gelatin to distilled water at 60 1C.The metal nitrates were separately dissolved in a minimalamount of distilled water at room temperature thenadded to the gelatin solution. After that, the compoundsolutions were stirred and heated at 80 1C until honey-like gels were obtained. The gels were calcined at 700 1Cfor 2 h at a heating rate of 2 1C/min. The resultingpowders were characterized by several tools to checktheir qualities.
Fig. 1. FESEM images and EDX spectra of (a) ZnO, (b) Zn0.9
Please cite this article as: R. Yousefi, et al., Growth, X-ray peakZnO nanoparticles, Materials Science in Semiconduj.mssp.2012.12.025i
The crystal structure and morphology of the productswere investigated by field emission scanning electronmicroscopy (FESEM, Quanta 200F) and X-ray diffraction(XRD, Siemens D5000). The elemental contents of theproducts were investigated using energy dispersive X-rayanalysis (EDX, Quanta 200F). Room temperature photo-luminescence (Jobin Yvon Horiba HR 800UV) spectro-scopy was employed to study the optical properties ofthe Zn1�xMgxO NPs using a He–Cd laser with a wave-length of 325 nm.
3. Results and discussions
Fig. 1a–d shows FESEM images of the Zn1�xMgxO NPs(x¼0.0, 0.01, 0.03, and 0.05). The scale of all FESEM images is100 nm with the same magnification (�50,000). The FESEMresults indicate that most particles were less than 100 nm indiameter. In addition, EDX spectra that are attached withthese FESEM images indicate Mg contents according to ourmeasurements in the experimental part.
For showing more details of the obtained NPs, TEMstudies have also been carried out. Fig. 2a–d shows theTEM images of the Zn1�xMgxO NPs (x¼0.0, 0.01, 0.03, and0.05). The TEM images reveal that all nanostructures havethe same spherical morphology, approximately. However,the sizes of the undoped and Mg-doped ZnO NPs are not
9Mg0.01O, (c) Zn097Mg0.03O, and (d) Zn0.95Mg0.05 NPs.
broadening studies, and optical properties of Mg-dopedctor Processing (2013), http://dx.doi.org/10.1016/
Fig. 2. TEM images of (a) ZnO, (b) Zn0.99Mg0.01O, (c) Zn097Mg0.03O, and (d) Zn0.95Mg0.05 NPs. The size distribution of (e) ZnO, (f) Zn0.99Mg0.01O,
(g) Zn097Mg0.03O, and (h) Zn0.95Mg0.05 NPs.
Please cite this article as: R. Yousefi, et al., Growth, X-ray peak broadening studies, and optical properties of Mg-dopedZnO nanoparticles, Materials Science in Semiconductor Processing (2013), http://dx.doi.org/10.1016/j.mssp.2012.12.025i
R. Yousefi et al. / Materials Science in Semiconductor Processing ] (]]]]) ]]]–]]] 3
Fig. 3. XRD patterns of the undoped ZnO and Mg-doped NPs.
R. Yousefi et al. / Materials Science in Semiconductor Processing ] (]]]]) ]]]–]]]4
the same. The TEM results indicate that the particle sizesof the NPs decrease with Mg2þ doping. However, thechanges in particle sizes are negligible and according toour measurements, they are around 15% in comparison toundoped ZnO NPs. This could possibly be related to thedecreasing of the self-assembly of the crystals due to theimpurities defects. In addition, Fig. 2e–h shows histo-grams of particle size distribution of the NPs. Thesehistograms indicate that the mean particle sizes of theZn1�xMgxO NPs (x¼0.0, 0.01, 0.03, and 0.05) are approxi-mately 56743, 46710, 4079, and 41710 nm,respectively.
The XRD patterns of the prepared samples in the rangeof 2y¼20–1001 are shown in Fig. 3. All detectable peakscould be indexed as the ZnO wurtzite structure (PDF cardno: 00-005-0664). No further peaks such as Mg, MgO, andZnMgO are detected and the other similar compounds.The inset of Fig. 3 shows a negligible shift in (002) peaksfor the samples that are doped with Mg elements in theZnO matrix, compared to the undoped ZnO-NPs. The ionicradius of the substitute Mg2þ (RMg
2þ¼0.057 nm) is smaller
than that of Zn2þ (RZn2þ¼0.06 nm). Thus, doping with Mg
causes a slight shift of the (002) XRD peaks toward higherdiffraction angles. The extent of these shifts is dependenton the Mg content in the NPs.
Please cite this article as: R. Yousefi, et al., Growth, X-ray peakZnO nanoparticles, Materials Science in Semiconduj.mssp.2012.12.025i
Wurtzite lattice parameters such as the values of d, thedistance between adjacent planes in the Miller indices(hkl) (calculated from Bragg’s equation, l¼2d sin y), lat-tice constants a, b, and c, inter-planar angle (the angle jbetween the planes (h1k1l1), of spacing d1 and theplane (h2k2l2) of spacing d2), and unit cell volumes werecalculated from the lattice geometry equation [17]. Thelattice parameters of the samples are summarized inTable 1.
The Scherrer equation is a common method to calcu-late crystallite size, D¼(kl/bhkl cos y), where D is thecrystallite size in nanometers, l is the wavelength of theradiation (1.54056 A for CuKa radiation), k is a constantequal to 0.94, bhkl is the peak width at half-maximumintensity, and y is the peak position. It is known that someparameters affect the peak width, the most important ofthese parameters is lattice strain. There are a number ofmethods used to calculate the effect of lattice strain onbroadening of the lattice diffraction peaks such as theWilliamson–Hall and the size strain plot (SSP) methods[17]. The SSP method is more accurate, especially athigher diffraction angles. Therefore, the crystallite sizeand lattice strain of the samples are calculated using theSSP method. In this method, the peak broadening due tothe lattice strain is estimated from e¼bs/tan y [17].
broadening studies, and optical properties of Mg-dopedctor Processing (2013), http://dx.doi.org/10.1016/
Fig. 4. SSP plots of the undoped ZnO and Mg-doped NPs.
Table 2Geometric parameters of undoped and Mg-doped ZnO-NPs.
Sample D
(nm)
e�10�4
(no unit)
Y
(GPa)
s(MPa)
u
(KJ m�3)
ZnO 58.23 2.83 146 41.32 5.85
Zn0.99Mg0.01O 42.13 3.46 146 50.50 8.74
Zn0.97Mg0.03O 43.35 4.90 146 71.54 17.53
Zn0.95Mg0.05 41.21 5.66 146 82.64 23.39
R. Yousefi et al. / Materials Science in Semiconductor Processing ] (]]]]) ]]]–]]] 5
Therefore, the total broadening is obtained from
bhkl ¼ bsþbD ð1Þ
where bD is the peak broadening that corresponds tocrystallite size. According to the SSP method, the relationbetween lattice strain and crystallite size is given by
dhklbhklcos y� �2
¼A
Dd2
hklbhklcos y� �
þe2
� �2
ð2Þ
where A is a constant, equal to 3/4 for the spherical shapesamples used in this experiment. In Fig. 4 the term(db cos y)2 is plotted with respect to (d2b cos y) for allthe diffraction peaks of undoped and Mg-doped ZnO NPsfrom 2y¼201–1001. The crystallite size and strain can beobtained from the slope and root of the y-intercept of thelinearly fitted data, respectively. According to Hook’s law,for small dislocations in a lattice a linear relation betweenthe stress and strain is given as s¼Ye, where Y is Young’smodulus and s is the lattice stress. For a hexagonalstructure, Young’s modulus can be obtained from thefollowing relation [18]:
Yhkl ¼h2þ
hþ2kð Þ2
3 þ alc
� �2h i2
s11 h2þ
hþ2kð Þ2
3
� �2þs33
alc
� �4þ 2s13þs44ð Þ h2
þhþ2kð Þ
2
3
� �alc
� �2
ð3Þ
Please cite this article as: R. Yousefi, et al., Growth, X-ray peakZnO nanoparticles, Materials Science in Semiconduj.mssp.2012.12.025i
In the earlier works, the values of s11, s13, s33, and s44,(the lattice compliances of ZnO), were found to be7.858�10�12, �2.206�10�12, 6.940�10�12, and23.57�10�12 m2N�1, respectively [19]. For an elasticsystem, which approximately follows Hooke’s law, theenergy per unit of a lattice can be calculated fromu¼(e2Yhkl)/2. The D, e, s, Y, and energy density (u) of theundoped and Mg-doped ZnO NPs are summarized inTable 2. It can be seen that, Mg2þ affects the structuralparameters of the ZnO NPs.
Photoluminescence (PL) study is a powerful methodfor investigating the effects of impurity doping on opticalproperties of semiconductor nanostructures with a directband gap, because doped nanostructures are expected tohave different optical properties compare to undopednanostructures. Fig. 5 shows the PL spectra of the
broadening studies, and optical properties of Mg-dopedctor Processing (2013), http://dx.doi.org/10.1016/
Fig. 5. PL spectra of the undoped ZnO and Mg-doped NPs.
R. Yousefi et al. / Materials Science in Semiconductor Processing ] (]]]]) ]]]–]]]6
undoped and Mg-doped ZnO NPs. All of the PL spectra ofZnO NPs show a peak in the ultraviolet (UV) regionbetween 368 nm and 382 nm, and a green emission(deep-level emission (DLE)) peak in the visible region atE502–530 nm. It can be seen, the PL spectrum of theundoped ZnO NPs shows a stronger UV peak than the DLEpeak. The ratio of the UV to the DLE peak is one of themain factors used for comparing optical propertiesbetween samples. The UV/DLE ratio of undoped ZnO NPsis bigger than the UV/DLE ratio of the Mg-doped ZnO NPsand it decreases with increasing Mg contents in Mg-doped ZnO NPs. According to Table 2, the undoped ZnONPs have the smallest stress and strain. Therefore, thesetwo factors could be very important in optical propertiesof ZnO nanostructures. In addition, compared with theundoped ZnO NPs, all of the Mg-doped ZnO NPs show anobvious blue-shift in the UV emission. It is known,alloying ZnO with MgO (Eg¼7.7 eV) enables widening ofthe band-gap of ZnO. This widening can be obtained fromfollowing formula [20]:
Eg Zn1�xMgxOð Þ ¼ 1�xð ÞEgðZnOÞ þxEgðMgOÞ ð4Þ
Therefore, the blue-shift in the UV emission is believedto be a result from above relationship between band-gapof ZnO and MgO. It can be seen from the PL results, as theconcentration of Mg increases, the blue-shift of the UVpeak also increases. In fact, the obtained results are ingood agreement with the above relation. Particularly, forthe Mg-doped ZnO NPs (5%), which has a higher Mgcontent than the other Mg-doped ZnO NPs, the UVemission is blue-shifted to 368 nm from 382 nm, due tothe modulation of the band-gap caused by Mg substitu-tion. This result is in good agreement with our old resultfor Zn1�xMgxO nanowires [16]. In fact, the Mg element isone of the best elements to control optical band gap ofZnO nanostructures.
4. Conclusion
Undoped and Mg-doped ZnO-NPs were synthesized bya sol–gel method in a gelatin media. The XRD results
Please cite this article as: R. Yousefi, et al., Growth, X-ray peakZnO nanoparticles, Materials Science in Semiconduj.mssp.2012.12.025i
showed that all of the undoped and doped ZnO NPsexhibited a hexagonal wurtzite structure. The SSP methodwas used to calculate the crystallite size and strain of theundoped and doped lattices. The SSP results revealed thatundoped ZnO NPs had smaller stress than the Mg-dopedZnO NPs. The TEM and XRD results indicated that Mghelped to decrease the crystallite and particles sizes. ThePL results indicated that Mg element as dopant materialcould change oxygen vacancies in ZnO lattice. Further-more, the UV peaks of the Mg-doped ZnO NPs showed ablue-shift in comparison to the undoped ZnO NPs.
Acknowledgment
R. Yousefi and F. Jamali-Sheini gratefully acknowledgethe Islamic Azad University (I.A.U), Masjed-Soleiman andAhwaz Branches, respectively, for their financial supportin this research work.
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