Eur. Phys. J. Appl. Phys. (2015) 69: 10601 DOI: 10.1051/epjap/2014140185
THE EUROPEAN PHYSICAL JOURNAL APPLIED PHYSICS
Regular Article
Effects of hydrogen annealing and codoping (Mn, Fe, Ni, Ga, Y) of nanocrystalline Cu-doped ZnO dilute magnetic semiconductor Mohamed Bououdina1,2,a and Aqeel Aziz Dakhel1 1 2
Nanotechnology Centre, College of Science, University of Bahrain, P.O. Box 32038, Kingdom of Bahrain Department of Physics, College of Science, University of Bahrain, P.O. Box 32038, Kingdom of Bahrain Received: 4 May 2014 / Received in final form: 19 November 2014 / Accepted: 2 December 2014 c EDP Sciences 2015 Published online: 7 January 2015 – Abstract. Zinc oxide (ZnO) codoped with Cu and M ions (M = Mn, Fe, Ni, Ga, Y) powders were synthesised by simultaneous thermal co-decomposition of a mixture of zinc and metal complexes. The synthesised chemical formula for the prepared solid solution is Zn0.97 Cu0.01 M0.02 O. X-ray diffraction (XRD) analysis confirms the formation of single nanocrystalline structure of the as-prepared powders, thus, both Cu and M ions were incorporated into ZnO lattice forming solid solutions. Magnetic measurements reveal that all the as-synthesised doped ZnO powders gained partial (RT-FM) properties but with different strength and BH-behaviour depends on the nature of the doping (M). Furthermore, H2 post-treatment was subsequently carried out and it was found that the observed RT-FM is enhanced. Very interestingly, in case of Ni dopant, the whole powder becomes completely ferromagnetic with coercivity (Hc ), remanence (Mr ) and saturation magnetisation (Ms ) of 133.6 Oe, 1.086 memu/g and 4.959 memu/g, respectively. The value of Ms was increased by ∼ 95% in comparison with as-prepared.
1 Introduction Dilute magnetic semiconductors (DMSs) oxides with partial replacement of cations by magnetic atoms are of great interest as magnetic components in spintronics [1,2]. The practical applications of DMS in spintronics require that the DMS should exhibit ferromagnetism at and above room temperature. On other hand, the discovery of room-temperature ferromagnetism (RT-FM) in transition metal-doped transparent conducting oxides (TCOs) like ZnO or CdO broaden the possibility of application of DMS in modern technology. Wurtzite ZnO has an energy band-gap in the ultraviolet region (∼3.37 eV), resistivity (∼10−2 Ω cm), and large exciton binding energy (60 meV) [3]. Films of ZnO could be made highly conductive with good optical transparency by appropriate doping. Therefore, ZnO has novel applications in optoelectronic devices such as lasers and solar cells, smart windows, as well as sensor devices for UV-visible photodetection and gas/liquid sensing [3]. The n-type electrical conduction properties of ZnO are caused by its natural intrinsic structural defects like oxygen vacancies (VO ) and zinc interstitials (Zni ), that could be controlled and modified by doping with foreign metallic ions. On the other hand, doping can create stable exotic properties within ZnO, such as magnetic, a
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mechanical, etc. that would diverse its field of applications. This present work is focusing on tailoring the RTFM properties of doped ZnO, aiming to use it in the field of dilute magnetic semiconductors (DMS). Of course, the strength of RT-FM depends on the quantity and quality of dopant metallic ions, that should be comparatively investigated together with almost identical conditions of preparation, that is the subject of the present work. In many previous research work, ZnO was studied doped with different metallic ions, like Cu [4–6], Fe [7], Mn [8], Ni [9,10], Ga [11] and Y [12]. It is important to note that among the large number of research papers published so far in the literature, we have mentioned those references particularly, simply because they are directly related to the present work, from which the possible ion valences were used. In summary, DMS ZnO needs dopants from transition metals (TMs) like Fe, Mn, Ni, Co or rare earth ions, which create RT-FM. Moreover, the creation of stable RT-FM in doped ZnO needs also, an internal medium from itinerant charges as carrier for exchange magnetic interactions between localised spins of dopant ions. Indeed, more experimental and theoretical research work are needed to explain the still challenging cause of RT-FM in DMS materials, although there are many previous papers reported in the literature dealing with that issue. It is important to note that RT-FM was also observed in un-doped oxides (d0 FM) [13], which were known for a long time to be
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diamagnetic (DM) or paramagnetic (PM) materials. Some research findings attribute that d0 FM to the defects, such as oxygen vacancies [13–16], and even what is called size effect (quantum effect) due to the formation of nanosized grains (particles) within the material. The present study aims to compare the stable RT-FM property created by codoping ZnO powder with Cu2+ and different metallic ions (Fe, Mn, Ni, Ga and Y). Doping ZnO with Cu ions was described variously in different studies. With light doping of ZnO, Cu ions substitute for Zn ions [17] with Cu2+ state [18,19], Cu1+ state [20], or a mixed valence state of Cu1+ /Cu2+ [21,22]. By using X-ray absorption near-edge structure (XANES) [23], it was found that Cu2+ ions substitute at Zn sites, in agreement with the results reported by reference [24]. In addition to Cu2+ , Cu1+ is also present and substitutes for Zn2+ , that creates oxygen vacancies (VO ) which assist magnetic properties created in ZnO by other dopant. Furthermore, it was observed and studied that the electrical properties of ZnO:Cu has a direct affect on its magnetic properties [19], where a paramagnetic behavior (PM) for Cu content < 0.75% mole related with n-type conduction, was observed. Thus doping of Cu could assist or support the medium for magnetic mutual interactions. Additionally, it must be mentioned here the obvious fact that neither metallic copper nor copper-related oxides are ferromagnetic [19]. Thus in general, Cu ion doping can be considered as magneto-catalyst [25,26] that will be used in the present work. Although many reports on the ZnCuO doped with transition elements exist, to the best of our knowledge, the comparative study on the doping effects of transition elements into ZnCuO has not been reported yet. It would be therefore instructive and interesting to systematically estimate and compare such doping effects on the structural and magnetic properties of ZnCuO powder.
2 Experimental part The following organic complexes: bis(acetylacetonato)zinc, bis(acetylacetonato)copper, tri(acetylacetonato)iron, bis (acetylacetonato)nickel, tri(acetylacetonato)manganese, tri(acetylacetonato)gallium, tri(acetylacetonato)ytterium (purity 99.995%) were used as starting materials to synthesise ZnO powder codoped with common Cu and M of solid solution molar formula Zn0.97 Cu0.01 M0.02 O, where M is Mn, Fe, Ni, Ga and Y. The method of “thermal co-decomposition of a mixture of complexes” was used to synthesis the samples (solid solutions). A mixture of controlled amounts of the above complexes were totally dissolved in methanol in a ceramic crucible with continuous magnetic stirring at room temperature and then slowly increasing its temperature up to ∼90 ◦ C. The stirring procedure was prolonged until a gel was obtained. The gel was flash sintered in air at temperature of 500 ◦ C for 1 hr followed by natural cooling within the oven to room temperature. The sintering was able to decompose the complexes leading to the formation of crystalline solid
(a)
(b)
Fig. 1. (a) XRD patterns of as-synthesised undoped and Cu- and M-codoped SS ZnO powders. (b) XRD patterns of H2 -annealed undoped and Cu- and M-codoped SS ZnO powders.
solutions (SSs). Some amount of each as-synthesised powder was post-annealed in hydrogen (H2 ) atmosphere at 300 ◦ C for 30 min. Finally, the resulting powders were grounded and pelletized using 750 MPa. Un-doped ZnO powder prepared by the same procedure was also added to the set of the investigated samples, for comparison (for shorting, powders identified as ZnC-M, where ZnC stands for ZnO:Cu, the common root for all synthesised SSs). Structural analysis was performed using a Rigaku Ultima VI θ-2θ X-ray diffractometer equipped with Cu Kα radiation (0.15406 nm). Magnetic characterization was measured by using a vibrating sample magnetometer type Micro-Mag Model 3900 with a sensitivity of 0.5 μemu using step field of 25 Oe and an averaging time of 1 s.
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M. Bououdina and A.A. Dakhel: Effects of H2 annealing and codoping (Mn, Fe, Ni, Ga, Y) of nanocrystalline Cu-doped ZnO DMS Table 1. Lattice parameters (a, c), unit-cell volume (Vcell ), mean crystallite size (CS), internal structural strain, and ionic radius rion quoted from reference [16] of the as-synthesised and H2 -treated ZnO powders codoped with Cu and M(Mn, Fe, Ni, Ga, Y). Sample
a (Ang)
c (Ang)
ZnO Zn0.97 Cu0.01 Mn0.02 O Zn0.97 Cu0.01 Fe0.02 O Zn0.97 Cu0.01 Ni0.02 O Zn0.97 Cu0.01 Ga0.02 O Zn0.97 Cu0.01 Y0.02 O
3.2513 3.2549 3.2559 3.2542 3.2563 3.2542
5.21294 5.21660 5.2142 5.2180 5.2138 5.2099
ZnO-H Zn0.97 Cu0.01 Mn0.02 O-H Zn0.97 Cu0.01 Fe0.02 O-H Zn0.97 Cu0.01 Ni0.02 O-H Zn0.97 Cu0.01 Ga0.02 O-H Zn0.97 Cu0.01 Y0.02 O-H
3.2531 3.2552 3.2566 3.2545 3.25711 3.2536
5.2116 5.2181 5.2148 5.2185 5.2144 5.2137
Vcell (Ang3 ) As prepared 47.72231 47.86177 47.86827 47.85149 47.87545 47.78031 Hydrogenated 47.7621 47.8817 47.8943 47.8665 47.9057 47.7970
c/a
CS (nm)
Strain (%)
rion (Ang)
1.6033 1.6027 1.6015 1.6035 1.6012 1.6010
13.1 12.6 13.3 12.7 10.2 11.7
0.38 0.50 0.32 0.30 0.32 0.40
Zn2+ 0.60 Mn2+ 0.67 Fe2+ 0.63 Ni2+ 0.69 Ga3+ 0.62 Y3+ 0.90
1.6020 1.6030 1.6013 1.6034 1.6009 1.6024
13.0 13.0 10.6 10.5 11.8 10.6
0.34 0.60 0.30 0.4 0.30 0.44
Magnetization curves were measured at room temperature (T = 294 K) in the field range +1 to −1 T.
3 Results and discussion 3.1 Structural characterisation of as-synthesised powders XRD patterns for the as-synthesised un-doped as well as Cu- and M-codoped ZnO are shown in Figure 1a. The patterns show that all the investigated powder samples are polycrystalline of wurtzite ZnO structure. No diffraction peaks arising from pure dopant metals or any related phases were detected so that no trace of secondary phase was found within XRD detection limit. Table 1 presents the results of the structural analysis; lattice parameters (calculated by Rietveld refinements), microstrain and crystallite size (calculated by Halder-Wagner method). The calculated lattice parameters for un-doped ZnO powder is close to the JCPDS data having hexagonal wurtzite structure (P63 mc) with lattice constants a = 3.2539 ˚ A, c = 5.2098 ˚ A [27]. A peak shift related to lattice spacing changes is clearly observed by changing dopant ions, which indicates that doping phenomenon has taken place. However, the lattice parameters and the unit-cell volume (Vcell ) of host ZnO, in general, were increased with the incorporation of both Cu and M ions, due to the smaller ionic radius of Zn2+ (0.6 nm [6,19, 28,29]) in comparison with other dopant ions mentioned in Table 1. To find some systematic relationship between the experimental data results, the Shannon ionic radius (rion ) were used. Thus, it was found that by increasing of rion , the parameter c increases almost linearly, the parameter a slightly decreased, and, therefore, the unit cell volume (Vcell = 0.866a2 c) slightly and almost linearly decreases, as shown in Figure 2. These structural variations means that there are several factors controlling
Fig. 2. The rion – dependence of unit-cell volume Vcell . The inset shows the rion – dependence of lattice parameters (a and c).
simultaneously the variation of Vcell : the co-doping effect, creation of structural vacancies, and type of dopant ion incorporation (substitutional doping or interstitial incorporation) as well as the properties of the dopant ion itself like its valency during the preparation process. The crystallite size was found to be in the range 10–13 nm, almost independent of M-type. Let us discuss the possible kinds of incorporation of Cu and M ions in ZnO lattice. As long as their ionic radii is different than that of Zn2+ ions by ∼5–15% (except Y), then the substitutional incorporation MZn is most likely to occur or these ions can form a substitutional solid solutions (SSSs) with Zn2+ without strong distortion of the crystalline structure, according to the well-known Hume-Rothery rules [30]. Of course, doping by interstitial mechanism can also occur. Therefore, the charge disturb of the unit
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(a)
(c)
(b)
(d)
Fig. 3. MH dependence of the as-measured and FM component of; (a) ZnO:Cu:Ga, (b) ZnO:Cu:Mn, (c) ZnO:Cu:Fe, (d) ZnO:Cu:Ni.
cell due to the dopant charge would be balanced and controlled naturally by the creation or/and annihilation of oxygen/Zn vacancies (VO and VZn ). All these factors altogether (incorporation and vacancy creation) control the change in Vcell . 3.2 Structural characterisation of H-annealed powders XRD patterns of hydrogenated Cu- and M -codoped ZnO H2 -treated powders are shown in Figure 1b. It shows the appearance of additional weak peak due to free Zn nanograins that was zoomed by 15 times in Figure 1b around Zn(1 0 1) zone. The higher intensity of Zn(1 0 1) peak was observed with Ga, Fe and Ni dopants. This can be explained by the interaction of H2 with structural oxygen that liberate metal zinc, according to the following chemical reaction: ZnO + H2 → Zn + H2 O. The efficiency and even the presence of such interaction depends on the kind of the doping metal (M) as catalyst for the
reaction, temperature and time of annealing, and powder state (such as particle size, purity, etc.). It should be mentioned that for Y dopant, the above mentioned reaction seems not to occur for Mn dopant. Furthermore, no reflections arising from dopant metals, their oxides, their hydrides, or any related phases have been detected referring to the stability of ZnO:Cu:M powders when subjected to annealing under H2 atmosphere. Table 1 presents the results of structural analysis. It shows that the volume of unit cell Vcell was slightly increased with H2 posttreatment, as shown in Figure 2. The inset of Figure 2 shows that the parameter c increased with rion while the parameter a remains without change, similarly as in the case of as-synthesised powders. Annealing in H2 gas slightly (∼0.06%) increases the unit cell volume for all dopant metals. This may be explained as follows: H2 molecules might dissociate into H atoms on the surface of nanocrystallites due to the presence of dopant ions, serving as active centers and playing the role of catalyst (transition metals are well know to posses high catalytic effect
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for H2 molecules dissociation [31]); then H atoms will be adsorbed at the surface of NPs then diffuse into the bulk by occupying interstitial sites within ZnO crystal lattice resulting in the formation of a solid solution ZnO:Hx accompanied by a lattice volume expansion. it is well know that H2 absorption by some compounds induces important volume expansion depending on the amount of absorbed H2 [32]. Furthermore, in addition to the liberation of Zn atoms by the interaction with structural oxygen (ZnO + H2 → Zn + H2 O), it is very important to note that H2 -treatment has great structural influence on doped host ZnO by removing some of the structural defects like dangling ions/atoms, structural defects and dislocations. These structural variations should have a great influence on the properties of the medium of exchange magnetic interactions inside the crystallites that might enhance the FM behaviour.
(a)
(b) 3.3 Magnetic properties 3.3.1 As-synthesized powders It has been well-known for a long time that pure un-doped ZnO shows diamagnetic behaviour [33]. However, recently, small amount of ferromagnetic (FM) component was observed in pure ZnO at room temperature and explained by its structural defects (vacancies) and grain size, that overlap with its major diamagnetic characteristic [34]. However, with Cu and M (Mn, Fe, Ni, Ga and Y) codoping of ZnO, different magnetic behaviours were obtained; DM + FM was obtained with Ga or Y doping (Fig. 3a), PM + FM with Mn or Fe doping (Figs. 3b and 3c), and only FM with Ni doping (Fig. 3d). It is clear that these different behaviours are due to different magnetic and other electronic properties of the dopant ion. For example, as Y and Ga ions have no intrinsic magnetic moment so that it is not expectable to obtain PM behaviour. The FM components (after the removal of the dia-/para-magnetic component) for the samples are given in Figure 4a, for comparison. Table 2 shows all magnetic parameters determined from M-H curves: coercive force (Hc ), remanence (Mr ), saturation magnetization (Ms ) and paramagnetic susceptibility (χp ) as well as the energy product parameter (Hc Mr ) that is considered as created magnetic figureof-merit [35]. FOM is usually used to quantify the quality of created FM produced in one system and thus to compare the quality of FM properties. Figure 5 shows the dependence of FOM = Hc Mr on the ionic radius of the used ions. It is clear that the higher FOM among the dopant ions is that with Mn2+ and the lower value is that with Y3+ dopant, which is similar to undoped ZnO. However, the saturation magnetization Ms parameter has the highest value with Fe dopant. Total transformation to FM bahaviour was observed only with Ni dopant defeating all DM or PM behaviours, as shown in Figure 3d. The magnetic behavior of ZnO:Cu:Mn and ZnO:Cu:Fe powders PM properties (overlapped with FM) as shown in Figures 3b, 3c and Table 2, defeating the basic DM properties of host ZnO, since both incorporated Mn2+ and
Fig. 4. (a) MH-dependence of FM component of all assynthesised powders. (b) MH-dependence of FM component of all H2 -annealed powders.
Fe2+ ions have intrinsic magnetic moments. It is possible to estimate the dopant ion effective magnetic moment, μ for samples having PM properties by using the measured PM susceptibility χp and applying Curie equation for volume susceptibility (χv ) in cgs units: χv = N μ2 /3kB T,
(1)
where N is the volume concentration of dopant ions, μ is the magnetic moment of dopant ion, kB is the Boltzmann constant, and T is the measuring temperature. Thus, the calculated effective μ was 5.0 μB for Mn ion and 4.67 μB for Fe ion. However, the previously observed magnetic moment for Mn2+ was ∼5.9 μB and for Fe2+ was 5.1– 5.5 μB [36,37]. Thus, the calculated effective values of the magnetic moments are less than the previously observed magnetic moments of Mn2+ by ∼15% and of Fe2+ by ∼8%, indicating that part of PM property spend to
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The European Physical Journal Applied Physics Table 2. Magnetic behaviour and Magnetic parameters; coercivity (Hc ), remanence (Mr ), saturation magnetization (Ms ), energy product (Hc Mr ), PM susceptibility (χp ), and ionic magnetic moment of as-synthesised and H2 -annealed ZnO powders codoped with Cu and ions M(Mn, Fe, Ni, Ga, Y). Sample ZnO ZnO-Cu-Mn ZnO-Cu-Fe ZnO-Cu-Ni ZnO-Cu-Ga ZnO-Cu-Y ZnO-H ZnO-Cu-Mn-H ZnO-Cu-Fe-H ZnO-Cu-Ni-H ZnO-Cu-Ga-H ZnO-Cu-Y-H
Behaviour Hc (Oe) Mr (memu/g) Ms (memu/g) D+F P+F P+F F D+F D+F D+F P+F P+F F D+F D+F
295 208.1 100.3 133.6 131.3 461.7 262.3 198.7 84.51 117.6 161.5 52.08
0.588 2.564 2.631 1.083 0.145 0.373 0.303 2.943 1.985 1.643 1.359 0.230
1.728 5.606 6.880 4.959 2.663 0.678 1.402 8.052 13.52 9.683 4.161 2.930
Energy product Mr/Ms Slope (Hc Mr ) (Oe emu/g) χp × 10−6 0.173 0.34 1.166 0.45 2.61 0.690 0.38 1.65 0.663 0.22 0.349 0.05 0.172 0.55 0.367 0.21 1.60 0.36 4.07 1.142 0.15 1.85 1.138 0.17 0.672 0.32 0.152 0.08
μ (in μB )
5.0 4.67
6.23 4.95
case of Ni doping that might give sight in the explanation of set of Figure 3. 3.3.2 Annealing of of H2 -annealed powders
Fig. 5. The rion – dependence of magnetic energy product for all as-synthesised and H2 -annealed powder.
defeat the basic DM property of ZnO in addition to the usually observed fact that not all doped ions participate in the measured susceptibility. By comparing set of Figure 3, it is clear that the considerable and pronounced FM hysteresis loop is that obtained in the case of Ni doping, comparing to Fe and Mn doping. This might be explained by studying the ratio of dopant ion-ion distance (R) to ionic radius, (R/rion ) for Ni, Fe and Mn doping samples of the present work. Let us first estimate R value by considering a sphere of “FM influence” of radius R around each M (Ni, Fe, Mn) ion corresponding to M-M distance of magnetic exchange interaction (dEx ). The value of R can be estimated by considering a uniform distribution of dopant ions within the host medium by: Nion V = 1, where Nion is the dopant ionic M concentration and V = (4/3)πR3 , thus R = 0.66 nm for all studied SSs. The ratio(R/rion ) is 9.85, 10.4 and 9.56 for Mn, Fe and Ni doping. Thus the lowest ratio is that in
Figure 4b and 3d show the experimental results of M-H curves for the as-synthesized powders after annealing in H2 gas, where one can observe some FM enhancement by comparing with Figure 4a for as-prepared or Table 2. Figure 5 shows the variation of FOM among the dopant ions for the as-synthesised and H2 -treated powders. The greatest enhancement of FOM and Ms under hydrogenation was observed with Fe and Ni doping. The enhancement of FOM and Ms after annealing under H2 gas is substantial, 65% and 96% in case of Fe dopant and 71% and 95% in case of Ni dopant, respectively. Thus, the magnetic enhancement with H2 posttreatment depends on dopant type and can be explained by convey improvement of the medium through which the M-M magnetic exchange interaction takes place. The magnetic moment per ion was calculated for Mn and Fe ions after H2 -annealing according to equation (1), and the results are given in Table 2. It is clear that the effective magnetic moment per ion increased becoming 4.95 μB and 6.23 μB for Fe2+ and Mn2+ which is close to the known observed magnetic moment, indicating that almost all dopant ions participate in magnetic susceptibility.
4 Conclusion Copper and transition (Mn, Fe, Ni, Y) with Ga ions were successfully doped into ZnO crystal lattice by thermal co-decomposition of their complexes. Structural study shows the formation of solid solution (SS) of the form Zn0.97 Cu0.01 M0.02 O. Subsequent H2 post-treatment of the powders produce small amount of free metallic Zn nanoparticles especially with Ga, Fe and Ni. The magnetic
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study prove the creation of stable partial FM in all the studied ZnO:Cu:M powders with different strength depending on M type. PM phase (overlapped with FM) was observed with Mn and Fe dopants, however with Ni dopant the entire powder became ferromagnetic. H2 -treatment was found to enhance FM phase at the expense of dia-/para-magnetic components, so that the value of saturation magnetisation was increased by ∼95.2% in comparison with as-prepared SS with Ni ions. The study shows that the greatest figure-of-merit of FM property was with Mn dopant, which can be considered as a potential candidate for future DMS applications.
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