Polar and nonpolar self-assembled Co-doped ZnO thin films: Structural and magnetic study

Accepted Manuscript Polar and nonpolar self-assembled Co-doped ZnO thin films: Structural and magnetic study

E. Abdeltwab, F.A. Taher PII: DOI: Reference:

S0040-6090(17)30438-8 doi: 10.1016/j.tsf.2017.06.004 TSF 36010

To appear in:

Thin Solid Films

Received date: Revised date: Accepted date:

12 January 2017 30 May 2017 1 June 2017

Please cite this article as: E. Abdeltwab, F.A. Taher , Polar and nonpolar self-assembled Co-doped ZnO thin films: Structural and magnetic study, Thin Solid Films (2017), doi: 10.1016/j.tsf.2017.06.004

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ACCEPTED MANUSCRIPT Polar and nonpolar self-assembled Co-doped ZnO thin films: structural and magnetic study E. Abdeltwab1, F. A. Taher2 1

Physics Department, Faculty of Science (Girls Branch), Al -Azhar University, Cairo, Egypt.

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Chemistry Department, Faculty of Science (Girls Branch), Al-Azhar University , Cairo,

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Egypt.

Abstract

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Novel nanostructures of self-assembled Co-doped ZnO films were

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successfully synthesized by hydrothermal synthesis process. Our investigation indicates that the growth reaction rate is the key factor that controls the morphology evolution and crystallographic orientation of self-assembled Co-doped ZnO films via

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changing the reaction solution concentration. The preferential growth direction of Codoped ZnO films alters from polar [0001] to non-polar [10-10] lattice direction by

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increasing the concentration, which facilitates the transformation from 1D nanorods, assembled in straw-like structure, to 2D nanosheets with dendrite-like structure. Interestingly, room-temperature ferromagnetism is observed in both of polar (0001) c-

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plane and non-polar (10-10) m-plane Co-doped ZnO films. As the most promising

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nanostructures, the assembled nanostructured Co-doped ZnO films may find future applications in spintronics devices.

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1. Introduction

Dilute magnetic semiconductors (DMSs) have attracted much interest in

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recent years because their great potential for use as spintronic materials [1, 2]. In particular, transition metal (TM) doped ZnO DMS is of interest because it has ferromagnetism (FM) at or above room temperature. [3-6] Although various theoretical models [7-10] have been proposed to understand FM in DMSs, the nature of FM in are still controversial issue among different groups [11-14], which seems to be strongly dependent on the preparation techniques and growth condition[15, 16]. Among all materials, ZnO probably has the richest family of nanostructures where exhibits the most diverse and abundant configurations of nanostructures shapes ranging from nanowires [17], nanobelts [18] to nanorings [19] and even nanosprings [20]. Amongst various dopants, cobalt is a potential candidate as its ionic radius is 1

ACCEPTED MANUSCRIPT extremely close to that of Zn. The doping of Co atom in ZnO is not only for the successful doping of TM into the ZnO matrix to develop the intrinsic DMS materials, but also to provide a better way for controlling the morphology evolution of the nanostructures. As advanced the synthesis of nanostructured functional DMS materials, with controlled structure and morphology, synthesis one-dimensional (1D) nanorod and

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two- dimensional (2D) nanosheet of Co doped ZnO nanostructures is critical for nanoscale technological application. Utilizing the large surface to volume ratio of nanorods and large surface area of nanosheets, Co-doped ZnO might find future

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applications in nanoelectronics and nanophotonics. Although various hierarchical

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heterostructures have been reported for ZnO, little work has been done to address the effect of doping on morphological evolution of this DMS system.

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In order to investigate the effect of growth process on morphological evolution, structural orientation and magnetic behavior of Co -doped ZnO based DMS nanostructures materials, we chose self-assembled Co-doped ZnO films with fixed Co

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concentration of 0.05 as system. Self-assembled Co-doped ZnO nanostructures were grown on seeded glass substrates by hydrothermal method. The growth mechanism of

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obtained nanostructures will be elaborated. Here, we report varied and unprecedented nanostructures morphology for Co-doped ZnO assembled films with a high value of

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room temperature ferromagnetism.

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2. Experimental details

Co-doped ZnO nanostructures films were grown in two stages, as described in the following:

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The first step was coating ZnO seed layers on glass substrate by a sol gel-spin coating method. The ZnO seed layers were prepared by dissolving (0.01 M) Zinc acetate dihydrate [Zn(CH3COO)2.2H2O ] as a precursor into the 2-methoxyethanol as solvent by stirring at room temperature and then adding the monoethanolamine (MEA) drop by drop as a sol stabilizer. After stirring at 60°C for 2 h and aging for 24 h, a transparent and homogeneous seed solution was formed. Before spin coating, glass substrates were cleaned repeatedly by ultrasonication in alcohol and acetone. Subsequently, the standard cleaned glass substrate was coated with the aged solution by spin coating at 4000 rpm for 30 sec. In order to prevent cracking and peeling, each

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ACCEPTED MANUSCRIPT seed layer was dried at 140 °C–150 °C for 10 min. The coating process was repeated for several times and the resulting films were annealed at 250 °C for 10 h to form the ZnO seed layers. The thickness of the resulting coated ZnO films is estimated to be 380nm. The second step was the formation of self-assembled Co-doped ZnO nanostructures on the as-pretreated glass substrate by hydrothermal growth method.

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The growth process was prepared by immersing the seeded substrate in growth solution that consists of an aqueous solution of zinc nitrate hexahydrate [(Zn(NO3)2.6H2O)] and cobalt nitrate hexahydrate [Co(NO3)2.6H2O)] as precursors

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and hexamethylenetetramine (HMT) (C6H12N4), where the substrate is being placed

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vertically in the growth solution. Cobalt dopant concentration was fixed within the solubility limit of 5%. To test the growth reaction conditions of the Co doped ZnO

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morphology, three equimolar concentrations (0.005, 0.025 and 0.050 M) of the precursors and HMT were mixed. Synthesis was carried out in an autoclave at a temperature of 90°C for 3h. Finally, the growth solution was allowed to cool to room

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temperature and the films were rinsed with deionized water for several times then annealed at 550 °C for 10 min to form the Co-doped ZnO films. For structure and

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magnetic comparison, the pure ZnO and Zn0.95Co0.05O in the powder form derived from similar chemical solution but without substrate are also provided.

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The crystalline structure of the films were examined by the sensitivity of the grazing incidence X-ray diffraction (GIXRD) by an X-ray diffractometer (PANalytical X'pert PRO) with monochromatic CuKα radiation (λ= 1.5406 Å). The

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angle of incidence was fixed at 0.5 deg. The surface morphology was investigated by field emission scanning electron microscopy (FESEM, Quanta FEG 250) with an

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operating voltage of 30 kV. The magnetization measurement was carried out using vibrating sample magnetometer (VSM, Lake Shore 7410) at room temperature.

3. Results and discussion 3.1 SEM observation Fig.1 (a, b, c) shows the SEM images of Co-doped ZnO grown from 0.005 M at 90°C for 3h. Straw-like structure with high density nanorods was obtained on a large scale (Fig. 1a). High-magnification FE-SEM images (Fig.1 b,c) revealed that the

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ACCEPTED MANUSCRIPT individual nanorods have width of 190–200 nm, thickness of 40–60 nm, and extend to several tens micrometers in length. By increasing the concentration to 0.025 M, dendritic morphology (Fig. 2 a, b) was constructed with large numbers of 2D nanosheets arranged in definite symmetry with several micrometers in lengths on a large scale. As can be seen in Fig. 2b, the nanosheets directly collected at the bottom of the structure that might be to lower the

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surface potential. Thereafter, the nanosheets assembled to produce the dendrite-like structure.

With further increasing growth solution concentration (0.05 M), this dendrite-

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like structure became more densely and packed nanosheets with uniform shape and

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size formed over the whole substrate (Fig. 3a). It can be seen from Fig. 3b that with increasing growth solution concentration, the separation between the nanosheets

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decreased and the number of nanosheets increased. High-magnification SEM images (Fig. 3c) revealed that the individual nanosheets have lateral thicknesses of 90–130 nm, widths in the range of 0.5–1 μm, and the lengths of the individual nanosheets are

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usually less than 10 μm.

These results show that the growth solution concentration affected the surface

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morphology of the Co-doped ZnO. Where the increasing of the reaction strength (Co: Zn: HMT) assist the transformation from1D nanorods to 2D nanosheets and caused

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changes in the interplanar separation and the arrangement of the nanosheets to be more uniform than the samples prepared at lower concentration. This could probably due to slower crystal growth kinetics at high reactant concentration (the increase of

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the amount of NH‫‏‬4 ions produced from higher concentration of HMT) [21-23].

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3.2 Crystal Structure The GIXRD patterns of the self-assembled Co-doped ZnO films are shown in Fig. 4. For comparison, the XRD patterns for the ZnO and Co-doped ZnO prepared in the powder form are also provided. The XRD data for ZnO and Co-doped ZnO powdered samples in Fig. 4(a,b) shows that all the diffraction peaks can be indexed as the hexagonal wurtzite structure, which are in good agreement with the standard pattern of ZnO (JCPDS No. 36-1451, a = b =3.249 Å and c = 5.205 Å, space group: P63mc). No other phases were detected, implying that the Co ions might be substituted into Zn sites without changing the wurtzite structure. The lattice 4

ACCEPTED MANUSCRIPT parameters of the ZnO and Co-doped ZnO powdered samples are (a =3.246Å and c = 5.203Å) and (a = 3.232 Å and c = 5.178Å), respectively. The GIXRD patterns (Fig. 4(c-e)) of the Co-doped ZnO films shows that the straw-like structure film (0.005M) grows only in the polar (0002) c-plane orientation with peak position at around 34.43°.While that of the dendrite-like structure (0.025M) and (0.05M) films grow only in the nonpolar (10-10) m-plane orientation with peak

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positions at around 31.72° and 31.64°, respectively. Compared with the (0002) peak position of ZnO, small right shift of the (0002) peak appearing in Co-doped ZnO powdered sample demonstrates the reduction

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of c lattice parameter caused by Co dopants. In contrast, a small left shift of (0002)

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peak position of straw-like structure Co-doped ZnO film was observed, as shown in the inset (a) of Fig. 4, demonstrates the expansion of lattice spacing. The lattice

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constant c of the straw-like structure film is 5.205 Å which is larger than that of ZnO and Co-doped ZnO powdered samples. Regarding the dendrite-like structure (0.025M, 0.05M) films, a shift of the (10-10) peak position towards a lower angle direction as

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shown in the inset (b) of Fig. 4, demonstrates the expansion of a lattice constant caused by Co dopants. While a shift of the (10-10) peak position towards a higher

reduction of a lattice constant.

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angle direction was observed in Co-doped ZnO powdered sample, indicates the

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In fact, the lattice parameters should have been decreased in Co doping ZnO lattice since the ionic radius for tetrahedrally coordinated of Co

2+

(0.58 Å) is

relatively smaller than that of Zn2+ (0.60 Å) [24,25]. It was reported that the

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substitution of Zn2+ ions with smaller Co2+ ions results in the reduction in lattice parameter with large compressive strain [26, 27]. The expansion of the lattice

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parameter is also reported due to the substitution of Zn with larger ions [28]. However, this is in sharp contrast with other work in which it was found that the lattice constant increases linearly with Co concentration [3, 29, and 30]. But that has not fully happened in our experiment. While lattice parameters of Co-doped ZnO powdered sample were reduced, the situation in nanostructured films is different, where expansion in lattice was found. To understand the origin of this difference, we need to look insight the hexagonal wurtzite structure where both Zn and O ions occupy half of tetrahedral sites and octahedral sites remains empty. If Co2+ ions are incorporated into ZnO crystal in such a way that not all Co2+ ions located at tetrahedral sites but small amount of Co2+ ions settled in octahedral position. This can 5

ACCEPTED MANUSCRIPT be explaining the expansion of lattice in nanostructured films. Where, the ionic radius of Co

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in octahedral coordination (0.740 Å) [31] is larger than that of Zn2+. Higher

percentage of Co

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on octahedral position is perhaps the reason for the larger

structural expansion, which may be caused by increasing the reactant concentration during growth process. While the reduction of lattice parameters indicates that the Co ions might be substituted for Zn sites without changing the wurtzite structure. The

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expansion of lattice parameter induced by Co doping suggests that Co ions do not fully substitute Zn in the wurtzite lattice points. Co ions may be exists in interstitial site or octahedral sites of the hexagonal wurtzite structure or both. Therefore, the

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expansion of lattice with large tensile lattice strain is due to lattice distortion induced

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by inhomogeneous distribution of Co in the Co-doped ZnO lattice sites. We will come

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back to this again shortly when we discuss the Magnetization (VSM) results.

3.3 Growth mechanism

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The previous observations indicate the possibility of controlling polarization of Co-doped ZnO DMS films by hydrothermal growth technique. Through changes to reaction conditions became clear that the growth of c-plane polar surface or m-plane

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nonpolar surface could be achieved by controlling reaction strength. Where the

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increasing (Co: Zn: HMT) concentrations facilitates the growth of nanosheets along the [10-10] direction to form Co-doped ZnO with dendritic morphology. In addition, since the growth process is related to the surface energy, that determines the

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preferential growing surfaces, and the growth kinetics that determines the final structure; let us elaborate the growing mechanism from different deposition strength solution. Initially, Zn2+ ions react with OH—groups to form the zinc hydroxide and

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subsequently

ions are introduced into Zn crystal growth unit; then, in

presence of an excess OH—, these growth units form tetrahydroxozincate and cobalt hydroxide due to the increasing of deposition solution pH in constant time. Finally, different Co-doped ZnO like structures are formed. Therefore, OH— group plays a vital role in altering the morphology of Co-doped ZnO thin film. In other word, the more the strength of HMT, the more the evolved ammonia gas and the more the provided continuous OH—groups that is a key factor for controlling the growth rate and thus lead to the formation of various-like structures.

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At low concentration;

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At higher concentrations;

In case of low concentration, Co-doped ZnO has a preference to grow along

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the polar (0001) c-plan in 1D nanorods since (0001) surface is thermodynamically unstable and has higher growth rates to reduce their higher surface energy. This means, at higher concentrations in the hydrothermal process, the interacted or the

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coordinated OH- functional group on the surface of Co-doped ZnO will prevent the

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dissociated Zn(OH)42- ions from growing along the polar (0001) direction; but still grow sideways along the nonpolar (10-10) direction of 2D nanosheets due to its lower surface energy and slower growth velocity, as shown in Fig. 5. Therefore, the Co-

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doped ZnO structure became shorter, thinner and has more width, as discussed in SEM results. Generally, it is worth to mention that the more the dopant concentration,

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the less the overall growth rate along the (0001) direction (i.e. produce shorter crystals). Finally, when the concentration of Zn(NO3)2 is increased to 0.05 M (nonequilibrium process), aligned Co-doped ZnO nanosheets are formed as the crystal growth along the (0001) direction is more effectively suppressed. As a result, the Codoped ZnO morphology evolves from 1D nanorods to 2D nanosheets by increasing the growth solution strength, as depicted in XRD data.

3.4 Magnetization results. The Magnetic characterization of the samples was measured using VSM at room temperature with a maximum applied magnetic field of 2000 kAm-1. Fig. 6(a) 7

ACCEPTED MANUSCRIPT shows the magnetization versus field (M–H) curves for pure ZnO and Co-doped ZnO powdered samples. Fig. 6(b) shows the M–H curves of the nanostructured Co-doped ZnO films, where the magnetic field is applied perpendicular to the film plan [10, 32]. Inset Fig. 6(b) shows the M–H curve of the seeded substrate. The films M–H curves were obtained after correction for the diamagnetic contribution arising from the seeded substrate. The magnetization results are summarized as the following: (i) pure

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ZnO shows a feature of diamagnetism, indicating that the introduction of the Co is the cause of the observed ferromagnetism. The diamagnetic nature of pure ZnO is consistent with the previously reported results [33-35] ; (ii) The magnetization curve

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of the Co-doped ZnO powdered sample demonstrate paramagnetic behavior [13, 36].

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The absence of ferromagnetism in Co-doped ZnO is also consistent with the previously reported results [37-39] ; (iii) straw and dendrite (0.025M and 0.05M)

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nanostructured Co-doped ZnO films showed room-temperature ferromagnetic ordering with saturation magnetization (Ms) of 3.3, 12.5 and 16 (kA/m) , respectively. Our saturation magnetization values show in agreement with the Ms was varied

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between 2 to 20 kAm-1 in 5% Co-doped laser ablated ZnO films [40, 41]. There are several possible origins of the high value of Ms in our films that we

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should consider. The enhancement of RT ferromagnetism in Co-doped ZnO thin films were mostly originated from secondary phases such as metallic Co clusters and CoO.

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Although, no diffraction peaks attributed to the Co-related secondary phases were detected in the samples, their existence cannot be completely excluded due to the sensitivity limits of XRD methods. But According to literature[42, 43], the solubility

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limit of Co atom in ZnO is found to be about 25%, below this limit the Co can be doped homogeneously in ZnO, whereas above the formation of secondary phase is

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expected. It is well known that the Co metal cluster easily forms in vacuum processing environment and at relatively high temperature [5,16,39,44,45]. On the other hand, the observed high Ms value also rules out the possibility that the observed ferromagnetism arising from CoO because the magnitude of magnetization is too large to attribute to the weak ferromagnetism of CoO, where CoO is a well-known antiferromagnetic material with TN of 291 K [46, 47]. Thus, based on our growth conditions along with x-ray diffraction measurements, we can exclude the possibility that the observed FM originates from the secondary phase. These results confirm that the observed high Ms arising from substitutional doping of Co2+ ions into wurtzite ZnO crystal. 8

ACCEPTED MANUSCRIPT The other possible source for RTFM is the in-plane strain and the out-of plane (vertical) strain, where it has found that strain in CoxZn1−xO can result in an enhancement of ferromagnetism [48]. The in-plane strain originated from the lattice mismatch can be excluded as the origin of ferromagnetic in our films where ZnO seed layers was deposited to limit this effect. Whereas that the vertical strain mediated by the matrix can enhance ferromagnetism. Recently, it has been hypothesized that the

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presence of an interphase boundaries and grain boundaries facilitates the transition from diamagnetic state into a ferromagnetic state even in pure nanograined ZnO [49, 50]. Indeed, SEM observations for self-assembled Co-doped ZnO nanostructures (Fig.

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1-3) witnessed that the obtained samples are very fine grained and contains quite

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developed surfaces and interfaces, which is considered as another possible reason for the presence of ferromagnetism that consistent with the reported hypothesis.

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In order to investigate the origin of the observed difference in the magnetic features, we turn to a structural analysis. The fact that only the (0002) reflection of straw -like structure film (Fig. 4) elucidates the grown film is c-plane oriented. As

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compared with the ZnO, we found that the film was tensily strained (0.03%) along the c-axis growth direction. Moreover, the only (10-10)

reflection of dendrite -like

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structure films elucidates the grown film is m-plane oriented with tensile strain (0.24%, 0.49% for 0.025M, 0.05M ) in the growth direction. In addition to the

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expansion of lattice in Co-doped ZnO film, another interesting feature was that the (0002) and (10-10) diffraction peaks of the straw and dendrite-like structure films are asymmetric, where the peaks are broadened at the low diffraction angle side, which

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corresponding to expansion of the lattice along the growth direction. This may be due to tensile strain caused by inhomogeneous distribution of Co in the Co-doped ZnO

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matrix. A similar phenomenon is also reported in Zn96Co0.04O implanted thin film [51]. These strongly indicate that Co doping tends to facilitate lattice defect caused probably by incorporation of Co ions in an octahedral sites of the hexagonal structure or at interstitial site or both, which means that the existence of tensile strain caused by Co doping can enhance the magnetism of Co-doped ZnO film. In sharp contrast to that, Co-doped ZnO powder has symmetric (0002) and (10-10) diffraction peaks. Moreover, we found that the lattice was compressed by 0.48% along the c-axis and 0.43% along the a-axis. Thus, the VSM results, in conjunction with the XRD results, clearly indicate that doping Co atoms into ZnO matrix induced intrinsic structural lattice defect without forming Co metal clusters or Co- oxide secondary phases. These 9

ACCEPTED MANUSCRIPT structural lattice defects have a significant role in the magnetic origin of the Co-doped ZnO nanostructures films [52, 53]. On the other hand, the possibility that the Co ions may exist in octahedral interstitial site of the perfect hexagonal ZnO lattice leading to formation of neighboring Zn vacancy, the Zn vacancies are found to lead to the ferromagnetic state [54]. Therefore we can attribute the observed FM in the Co-doped ZnO films to the

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vacancy induced by the inhomogeneous distribution of cobalt ions inside the perfect hexagonal ZnO lattice. It was reported that the Zn vacancy and higher Zn vacancy concentration can arouse ferromagnetism in ZnO and the main source of the magnetic

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moment mainly comes from unpaired 2p electrons at O sites surrounding the Zn

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vacancy [54]. Although the Zn vacancy under normal conditions is very difficult to form due to its high formation energy, Zn vacancies can be generated during the

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growth process because of thermal fluctuation or growth conditions, as found in experiments [54-59]. Our results indicate that doping of Co atoms into ZnO can facilities the formation of Zn vacancy. This assumption confirms the theoretical

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prediction and experimental investigation which suggestion that the observed ferromagnetism arises not from O vacancies but from Zn vacancies [54, 59].

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While the possibility of presence of Co ions in octahedral interstitial site of the perfect hexagonal ZnO lattice and formation of Zn vacancy can account as origin of

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magnetism in these nanostructured films, we cannot exclude the possibility that

Conclusion

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observed magnetism originating from Co substitution on the Zn sites also contributes.

The structural and magnetic properties of novel nanostructures Co-doped ZnO

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films prepared by hydrothermal growth technique are investigated. Our recent experiment has found that the nanostructures morphology and the growth direction of the grown Co-doped ZnO films could be tuned by the growth reaction rate via controlling growth solution concentration. These results indicate that the polarization of the Co-doped ZnO films can be controlled through preparation condition. The effects of Co doping on the structure and magnetic properties of the assembled nanostructured films were studied. The magnetic measurements show that the straw (0.005)

and dendrite (0.025M and 0.05M) nanostructured Co-doped ZnO films

exhibit room-temperature ferromagnetic ordering with saturation magnetization Ms of 3.3, 12.5 and 16 (kA/m), respectively. 10

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Fig. 3. (a, b) SEM images of the Co-doped ZnO dendrite -like structures grown with 0.05 M at 90°C for 3h. (c) Highmagnification SEM image of the assembled nanosheets.

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Fig. 6. (a) Magnetization curves of ZnO and Co-doped ZnO powder sample measured at room temperature. (b) Magnetization curves of Co-doped ZnO films measured at room temperature. The inset is the magnetic contribution of seeded substrate.

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 Straw and dendrite-like structure Co-doped ZnO films are grown

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 Observation of room-temperature ferromagnetism in c-plane and m-plane films.

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