Journal of Non-Crystalline Solids 354 (2008) 4727–4729
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Structural and magnetic properties of ZnO and Zn1xMnxO nanocrystals N.O. Dantas a, L. Damigo a, Fanyao Qu a, R.S. Silva a,b,*, P.P.C. Sartoratto c, K.L. Miranda c, E.C. Vilela c, F. Pelegrini d, P.C. Morais b a
Universidade Federal de Uberlândia, Instituto de Física, Laboratório de Novos Materiais Isolantes e Semicondutores (LNMIS), Uberlândia MG 38400-902, Brazil Universidade de Brasília, Instituto de Física, Núcleo de Física Aplicada, Brasília DF 70910-900, Brazil c Universidade Federal de Goiás, Instituto de Química, Goiânia GO 74001-970, Brazil d Universidade Federal de Goiás, Instituto de Física, Goiânia GO 74001-970, Brazil b
a r t i c l e
i n f o
Article history: Available online 25 August 2008 PACS: 75.75.+a 76.30.v 77.84.Lf 78.67.Bf 78.70.Ck
a b s t r a c t Chemical precipitation of metal-ions from aqueous solution has been successfully used to produce Zn1xMnxO nanocrystals, in the form of nano-powder. X-ray diffraction (XRD) measurements reveal that the as-prepared samples are single-phase materials and their lattice constant changes with the variation of Mn-concentration, which indicates the incorporation of Mn2+ into the hosting ZnO. These findings are corroborated by the observation of the well defined six hyperfine lines of Mn2+ ion in the electron paramagnetic resonance (EPR) spectra of the samples with a low Mn-concentration, and of a broad EPR line, which manifests the onset of Mn–Mn exchange interaction, in the samples with an elevated value of x. Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Biomaterials Nanocrystals X-ray diffraction Nano-composites
1. Introduction The recent development of diluted magnetic semiconductor (DMS) materials down to the nanoscale dimension, partially in response to demands coming from spintronics for new and improved materials, has extensively spread throughout several fields of science and technology [1–3]. DMS are semiconductors to which a magnetic impurity is intentionally introduced – a small fraction of the native atoms in the hosting non-magnetic semiconductor material is replaced by magnetic atoms. The main characteristic of this new class of compounds is the possibility of the onset of an exchange interaction between the hosting electronic subsystem and electrons originating in the partially-filled d or f levels of the introduced magnetic atom [4–6], which enables a control of both the optical and magnetic properties of the end material using external fields in regimes hardly achieved with other classes of materials. Among semiconductor materials, the wide band-gap (3.37 eV) ZnO has attracted a particular attention due to its special interesting physical properties [7]. In this study, we report on the synthesis of * Corresponding author. Address: Universidade de Brasília, Instituto de Física, Núcleo de Física Aplicada, Brasília DF 70910-900, Brazil. Tel.: +55 34 32394281; fax: +55 34 32394106. E-mail addresses:
[email protected] (N.O. Dantas),
[email protected] (R.S. Silva). 0022-3093/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2008.04.024
Zn1xMnxO (x P 0) nanocrystals (NCs) using the chemical precipitation of metal-ions in aqueous solutions. The structural and magnetic properties of these NCs, observed by X-ray diffraction (XRD) and X-band electron paramagnetic resonance (EPR), are also presented. 2. Experimental details The preparation of the Zn1xMnxO (x P 0) NC samples is based on the chemical transformation of the [Zn(NH3)4]2+ complex in the presence of sodium oleate, hydrazine sulfate, and manganese chlorite at 80 °C. The pH value of the reaction medium is maintained at 8.5 during the entire chemical process, using sodium hydroxide aqueous solution. Shortly, 100 mL of 0.38 mol/L zinc chlorite and 1.6 mol/L ammonium hydroxide aqueous solutions are mixed together and stirred for 30 min to produce the [Zn(NH3)4]2+ complex in solution. Then, 1.0 mL of hydrazine sulfate and 0.08 g of sodium oleate are added into the reaction medium. The obtained final solution was maintained at 80 °C in water-bath to transform the [Zn(NH3)4]2+ complex while the pH value of the reaction medium is maintained at 8.5 using 4 mol/L sodium hydroxide aqueous solution. The reaction is carried out for 2 h, and the resulting white precipitates are percolated and washed with distilled water and absolute ethanol for several times. After that they are dried up at 60 °C by 12 h and successively at 500 °C by 2 h.
3. Results and discussion Fig. 1 shows the XRD patterns of three Zn1xMnxO (x P 0) NC samples with x = 0, 0.0001, and 0.0081. It is noted that all of the Bragg diffraction peaks stem from the hexagonal wurtzite structure of the ZnO, and no evidence of any extra Bragg diffraction peak related with the Mn-doping process can be found. Thus, all samples are single-phase materials with hexagonal wurtzite structure. Although Mn-doping does not induce any change in the crystal structure of hosting material (ZnO), both a and c lattice parameters are strongly impacted. For instance, as the concentration of manganese increases the Zn1xMnxO XRD peaks shift towards lower 2h angle values, which indicates the change of0 both a and c lattice averparameters. Within an uncertainty of 0.001 Å A, the estimated 0 age c-axis lattice constants are 5.207, 5.208, and 5.220 Å A for x = 0, 0.0001, and 0.0081, respectively. This is attributed to that the 0 A in the ZnO crystal lattice are Zn2+ ions with ionic radius of 0.74 Å 0 substituted by Mn-ions with ionic radius of 0.83 Å A. To clearly demonstrate this interesting behavior, the amplified XRD patterns around the (0 0 2) peak is shown in Fig. 2. Moreover, the corresponding upshift of the lattice parameter as the Mn-concentration increases is shown in the inset of Fig. 2. Fig. 3 shows the room-temperature X-band (9.5 GHz) EPR spectra of Zn1xMnxO NC samples with three different concentrations of manganese. The EPR spectrum of the undoped NC sample (x = 0) shows a single sharp line at Landé factor g = 1.9568 [9]. In contrast, the Zn1xMnxO (x = 0.0001) NC samples present a strong dependence of their EPR spectra on Mn-concentration. For instance, the well-defined six lines with a hyperfine interaction splitting of 7.8 mT are found in the EPR spectrum of the NC sample with a low Mn-concentration (x = 0.0001). Whereas, for the sample with a higher value of x, such as, x = 0.0081, a hyperfine structure is completely quenched, and only a broad signal is observed at
Zn1-xMnxO NC (100) (002)
(110)
(103)
Normalized Intensity (a.u.)
(102)
x=0
x = 0.0001
x = 0.0081 40
50
60
2θ(degree) Fig. 1. X-ray diffraction of Zn1xMnxO nanocrystals for x = 0, 0.0001, and 0.0081, respectively.
5.220 5.217 5.214 5.211 5.208 5.205
0.000 0.003 0.006 0.009
Mn concentration (x)
Zn1-xMnxO NC x=0
x = 0.0001
x = 0.0081
33.9
34.2
34.5
35.1
34.8
2θ (degree) Fig. 2. Shift of the (0 0 2) XRD line as a function of the Mn-concentration (x). The inset shows the dependence of lattice parameter on the manganese concentration (x).
Zn1-xMnxO NC
x=0
x = 0.0001
x = 0.0081
275
(101)
30
Normalized Intensity (a.u.)
For each of two synthesized Zn1xMnxO (x > 0) NC samples, the compositions of both Zn and Mn are simultaneously determined by atomic absorption spectroscopy. We found x = 0, 0.0001, and 0.0081, respectively. Furthermore, the average diameters of corresponding NCs’ have also been evaluated by means of Scherer’s equation, based on the X-ray line broadening of the most intense diffraction peak (1 0 1). We found that they are 25, 40, and 56 nm for x = 0, 0.0001, and 0.0081.
Lattice Parameter (Å)
N.O. Dantas et al. / Journal of Non-Crystalline Solids 354 (2008) 4727–4729
EPR Intensity (a.u.)
4728
300
325
350
375
400
425
Magnetic Field (mT) Fig. 3. Room-temperature EPR spectra of Zn1xMnxO nanocrystals for x = 0, 0.0001, and 0.0081, respectively.
g = 2.0033. The underlying physics can be well understood by an analysis of the competition between hyperfine interaction and Mn–Mn exchange interaction. Let’s_ start our analysis from the _ ^Z þ H ^ 0 . HZ ¼ l ^ ~ spin-Hamiltonian [8]: H ¼ H e S g e B is the Zeeman term of electron, where le, ge, S (=5/2) are the Bohr magneton, Lande factor and spin of electron, and ~ B is the external magnetic ^ 0 ¼ D½s2 SðS þ 1Þ=3 þ EðS2 S2 Þþ ^ 0 is described by H field. H x y z S ^I, where I = 5/2 is the manganese Q ½I2z IðI þ 1Þ=3 þPðI2x I2y Þ þ A^ nucleus spin, D, E, Q, and P are constants of proportionality of the ^ 0 describe the zero terms described below. The first two terms in H magnetic field fine-structure splitting due to spin–spin interactions of electrons, which is non-zero only in environments with symmetries lower than cubic. The third and the forth terms denote the nuclear spin–spin interaction. The fifth term is stemmed from the hyperfine interaction between electron and nuclear spins,
N.O. Dantas et al. / Journal of Non-Crystalline Solids 354 (2008) 4727–4729
which leads to a six line pattern [4]. For most material systems under experimental conditions only a small fraction of all allowed nuclear and electron transitions are observed in an EPR spectrum. For a nanocrystal doped with a low concentration of transition-metal ^ 0 are quite small in comparison with ion, the constants P and Q in H the hyperfine interaction terms. Then it can be safely neglected [8]. In this case, only transitions associated to Dms ¼ 1 and Dml ¼ 0, where ms and ml are respectively electron- and nuclear-spin quantum numbers, are observed in the EPR spectrum [8]. Therefore, only hyperfine structure, characterized by well-resolved six lines due to six z-component of Mn spin, is expected [10,11]. As the con^ 0 incentration of manganese increases, constants P and Q in H creases, and a broad background in EPR spectrum originated from an enhanced Mn–Mn interaction emerges. Then the resulting EPR spectrum becomes the sum of hyperfine structure over a broad background. On the other hand, the emergence of this background smears hyperfine structure. When the Mn–Mn interaction becomes strong enough, the hyperfine structure can be completely smeared out. Hence, only a broad EPR line is presented in the spectrum of the NC sample with a high value of x = 0.0081, as shown in Fig. 3. 4. Conclusion ZnO and Zn1xMnxO nanocrystals (x P 0) have been synthesized by precipitation of metal-ions in aqueous medium. X-ray diffraction data reveal that the ZnO hexagonal wurtzite crystal structure is preserved in all Zn1xMnxO nanocrystal samples. Whereas, the lattice parameter increases as the Mn-concentration
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in the hosting ZnO template increases. It implies that the zinc in the wurtzite-like structure is replaced by manganese. Electron paramagnetic resonance spectra of the as-produced Zn1xMnxO nanocrystals present well resolved six hyperfine lines of Mn2+ ion the samples with a low Mn-concentration, and a broad EPR line in the samples with a high value of x. It gives a further support for the presence of Mn2+ ions incorporated in sites of the as-produced Zn1xMnxO nanocrystals. Acknowledgments The authors gratefully acknowledge the partial financial support of the Brazilian Agencies MCT/CNPq, FAPEMIG, and FINATEC. References [1] H. Ohno, Science 281 (1998) 951. [2] S.A. Wolf, D.D. Awschalom, R.A. Buhrman, J.M. Daughton, S. von Molnar, M.L. Roukes, A.Y. Chtchelkanova, D.M. Treger, Science 294 (2001) 1488. [3] D.D. Awschalom, M.E. Flatté, Nature Phys. 3 (2007) 153. [4] Fanyao Qu, P. Hawrylak, Phys. Rev. Lett. 95 (2005) 217206. [5] D.H. Rodrigues, A.M. Alcalde, N.O. Dantas, J. Non-Cryst. Solids 352 (2006) 32. [6] L. Cywinski, L.J. Sham, Phys. Rev. B 76 (2007) 045205. [7] Ü. Özgür, Ya.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Dogan, V. Avrutin, S.J. Cho, H. Morkoç, J. Appl. Phys. 98 (2005) 041301. [8] R.S. Silva, P.C. Morais, Fanyao Qu, A.M. Alcalde, N.O. Dantas, H.S.L. Sullasi, Appl. Phys. Lett. 90 (2007) 253114. [9] H. Zhou, A. Hofstaetter, D.M. Hofmann, B.K. Meyer, Microelectron. Eng. 66 (2003) 59. [10] H.W. Zhang, E.W. Shi, Z.Z. Chen, X.C. Liu, B. Xiao, L.X. Song, J. Magn. Magn. Mater. 305 (2006) 377. [11] N.S. Norberg, K.R. Kittilstved, J.E. Amonette, R.K. Kukkadapu, D.A. Schwartz, D.R. Gamelin, J. Am. Chem. Soc. 126 (2004) 9387.
