Optical and magnetic properties of Co-doped ZnO synthesized by magnetic assisted hydrothermal method

Materials Science in Semiconductor Processing 74 (2018) 303–308

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Optical and magnetic properties of Co-doped ZnO synthesized by magnetic assisted hydrothermal method ⁎

Libei Huanga, Yongde Haoa, , Mingzhe Hub, a b

MARK

⁎

School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, PR China Department of Phys. & Electronic Sci, Liupanshui Normal University, Minghu Road 1, Liupanshui 550025, Guizhou, PR China

A R T I C L E I N F O

A B S T R A C T

Keywords: Magnetic field pretreatment Diluted magnetic semiconductor Co-doped ZnO Optical properties Magnetic properties

Cobalt(Co) doped ZnO nanoparticles were synthesized by a hydrothermal method with a magnetic field pretreatment(in the hydrolysis reaction). The pretreatment time(t) was set as 0 h(no pretreatment), 1 h, 2 h and 4 h. The crystallite structure, the lattice constant have been estimated by X-ray diffraction (XRD) with Rietveld refinement, which shows slight lattice expansion due to Co doping. Scanning electron microscopy (SEM) reveals flower-like microstructure and transmission electron microscopy (TEM) indicates the single crystalline in nature. The defect, the optical and magnetic properties of the four obtained samples were systematically investigated by Raman spectroscopy, photoluminescence (PL) spectra and vibrating sample magnetometer (VSM) respectively. The results show that the pretreatment time of magnetic field has a significant influence on optical and magnetic properties of Co-doped ZnO, and the effective time of magnetic field in the hydrolysis reaction on the magnetic property is between 1 h and 2 h. When 1 h ≤ t ≤ 2 h, more Co atoms are incorporated into ZnO and the saturation magnetization (MS) increases by nearly three times, comparing with the sample without pretreatment. Samples of different pretreatment time show changes in the emission band of PL spectra and defect related bands of Raman spectra. The characterization of Raman scattering combined with the analysis of PL spectra suggests that the original of room temperature ferromagnetism(RTFM) may arise from the Co2+–VO+–Co2+ bound magnetic polaron(BMP).

1. Introduction Zinc oxide (ZnO) is a transparent semiconductor compound with a direct electronic transition [1–3]. It has a wide band gap of 3.37 eV and large excitonic binding energy of 60 meV at room temperature, which makes it a potential candidate for ultraviolet optoelectronic devices [4–6]. ZnO is a kind of diluted magnetic semiconductor (DMS) that possesses both charge and spin degrees of freedom when small amounts of transition metal atoms (Mn [7–11], Co [12–20], Ni [21–26], Gd [27–29], or Fe [30–34]) are introduced into ZnO, opening up new prospects for applications in spintronic devices. The main challenge in the practical applications of the DMS materials is the attainment of ferromagnetism above room temperature [35], also the mechanism of ferromagnetism is still inconsistent or controversial. Based on its specific applications, numerous techniques have been utilized for the fabrication of ZnO nanostructures including sol-gel method [36,37], vapor phase growth [38], solvothermal method [39], thermal decomposition [40,41], hydrothermal method [42,43]. Among these methods, hydrothermal method is an ideal method for deriving uniform doping

⁎

Corresponding authors. E-mail addresses: [email protected] (Y. Hao), [email protected] (M. Hu).

http://dx.doi.org/10.1016/j.mssp.2017.08.032 Received 3 June 2017; Received in revised form 8 August 2017; Accepted 27 August 2017 1369-8001/ © 2017 Published by Elsevier Ltd.

DMS because it is simple, convenient and environment-friendly. Nowadays, multiple experimental studies have been trying to induce some new ways in the process of growth to change the morphology, structure and the orientation of crystal to investigate the magnetic, optical and electrical properties of ZnO. For example, a pulsed magnetic field was applied in the hydrothermal process in order to control the morphology and regulate the magnetism of DMS [44,45]. In Chu group, a sealed autoclave with reactant mixture was appended in a 12 T superconducting magnet and maintained at 393 K for 5 h to urge the energies of ferromagnetic spin alignment to decrease to a degree that ferromagnetism might be activated [46]. The external magnetic field was involved in the whole hydrothermal process in these studies. In this letter, Co-doped ZnO nanoparticles were prepared by hydrothermal method. Before the heating process, a 0.4 T constant magnetic field was added in the hydrolysis reaction, the time of imposition of the magnetic field was fixed as different hours. The effect of the magnetic field pretreatment time on the morphologies, optical and magnetic properties of Co-doped ZnO were investigated.

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2. Experiments and characterization

3. Results and discussion

2.1. Experiments

3.1. Morphological and structural properties

Zinc acetate dihydrate(0.75 g) and cobalt acetate tetrahydrate (0.0265 g, doping concentration of 3%) powders were thoroughly mixed and dissolved in 70 mL mixed solvent of absolute ethanol and deionized water(mixture ratio is 1:1). All of the reagents were analytical grade and were used without any further purification. And 1.75 mL ammonia buffer solution was slowly dropped into the former solution and mixed by a magnetic stirrer. Then the mixed solution was transferred into an autoclave (100 mL), which was placed in a 0.4 T constant magnetic field subsequently. The pretreatment time was set as 0 h (no pretreatment), 1 h, 2 h and 4 h. The hydrothermal process was carried out at 170 °C for 24 h in an oven. Thereafter the products were cooled to room temperature. Finally, the resulting precipitates were washed several times with deionized water and absolute ethanol and dried at 60 °C for 8 h. The four Zn0.97Co0.03O samples obtained were marked by S(t = 0 h), S(t = 1 h), S(t = 2 h) and S(t = 4 h) respectively.

Fig. 1(a)–(d) shows the SEM image of S(t = 0 h), S(t = 1 h), S(t = 2 h) and S(t = 4 h), and the inset of Fig. 1(a) is the magnification image of a random region. The size and distribution of all samples is uniform, which confirms that the hydrothermal method could prepare good quality particles. As seen from the inset of Fig. 1(a), the particles are flower shaped with seven petals and the length of each petal is around 900 nm. Comparing Fig. 1(a)–(d), no significant change in the microstructure can be observed. Fig. 2 depicts XRD patterns of Zn0.97Co0.03O obtained with different magnetic field time applying in the hydrolysis reaction. To cross-reference, the pure ZnO was prepared by the same reaction conditions. In all cases, only the wurtzite structure of ZnO can be found and indexed as JCPDS card no. 36-1451. Comparing with pure ZnO, there is no structural change and formation of additional phases due to the incorporation of Co in ZnO within the sensitivity of XRD technique, confirming that Co dopants are well substituted in ZnO and doping samples are highly pure and single-phase crystal. The diffraction angle (2θ), full width at half maximum (FWHM), and lattice parameters that given by Rietveld refinement of XRD are illustrated in Table 1. It is observed that peaks shift slightly towards lower diffraction angle due to the small lattice expansion by additional Co atoms. The same results were observed in the reports of the literature [1,47–50]. Lower 2θ value is found in S(t = 2 h) and S(t = 4 h), that is, more Co atoms enter into ZnO with the increase of pretreatment time. Comparing with pure ZnO, the little increase in lattice constant a and c with t could be explained by the lattice distortion [51]. And S(t = 2 h) and S(t = 4 h) have the larger cell volume, the increase of cell volume is due to the lattice expansion. The value of FWHM turns to larger in S(t = 2 h) and S(t = 4 h), which may reveal more defects in S(t = 2 h) and S(t = 4 h). On

2.2. Characterization The crystalline phase structure of the samples was characterized by a Bruker D8 ADVANCE X-ray diffractometer using Cu-Kα radiation （1.5418 Å）in the range of 20° ≤ 2θ ≤ 80°. The morphology and microstructure of the samples were observed by SEM and TEM. Raman spectra was measured by a Horiba LabRAM HR800 Raman spectroscope using a 532 nm Nd-YAG laser as the excitation source in the range of 200–1000 cm−1. PL spectra was recorded by a PerkinElmer fluorescence spectrometer with the excitation wavelength of 320 nm. Magnetization studies were performed through the Vibrating Sample Magnetometer from Quantum Design company in the United States.

Fig. 1. SEM image of Zn0.97Co0.03O (a) S(t = 0 h), (b) S(t = 1 h), (c) S(t = 2 h), (d) S(t = 4 h).

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the basis of the above discussed, it can be concluded that an appropriate increase of magnetic field pretreatment time may contribute to the doping of Co. The transmission electron microscopy images of S(t = 0 h) and S(t = 4 h) are shown in Fig. 3. Both of the two samples in Fig. 3(a) and (b) are in good flower shapes with the length of the petals being around 800 nm, which is consistent quite well with the SEM observations in Fig. 1. High-resolution TEM (HRTEM) image gives deep understanding about the detailed atomic structure of the nanoparticles. Fig. 3(c) and (d) show the HRTEM image of S(t = 0 h) and S(t = 4 h) with measured interplane spacing (d) of 0.21 nm and 0.22 nm, which is in good agreement with the distance between (102) crystal plane respectively. The value of d in doped samples is slightly higher than that of undoped ZnO (0.191 nm), which is consistent with a smaller angle shift of diffraction peaks in XRD analysis. The inset of Fig. 3(c) and (d) shows the selected area electron diffraction (SAED). HRTEM and SAED measurement suggests that samples are single crystalline in nature and there is no evidence for the presence of Co cluster.

Fig. 2. XRD patterns of pure ZnO and Zn0.97Co0.03O samples. Table 1 Parameters of the XRD patterns for pure ZnO and Zn0.97Co0.03O samples.

3.2. Raman studies

Sample

2θ (deg)

FWHM (deg)

a (Å)

c (Å)

v (Å3)

Pure ZnO S(t = 0 h) S(t = 1 h) S(t = 2 h) S(t = 4 h)

36.22 36.2 36.181 36.169 36.176

0.273 0.291 0.281 0.389 0.381

3.252 3.253 3.252 3.255 3.254

5.209 5.213 5.211 5.211 5.213

47.71 47.76 47.74 47.8 47.79

Raman spectra is a versatile method to investigate dopant incorporation and the resulting defects in a host lattice [52,53]. The defect density could be reflected in Raman spectra, even at very low densities. The Raman spectrum of Co-doped ZnO is detected with the frequency range between 200 cm−1 and 1000 cm−1, as shown in Fig. 4. Fig. 3. (a) Low magnification TEM of S(t = 0 h) (b) low magnification TEM of S(t = 4 h) (c) HRTEM and SAED (top right) of S(t = 0 h) (d) HRTEM and SAED (top right) of S(t = 4 h).

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Fig. 7. M–H curves of Zn0.97Co0.03O samples.

Fig. 4. Raman spectra of Zn0.97Co0.03O samples.

nonpolar high-frequency optical phonon branch of E2 mode [E2H] of ZnO, which is sensitive to internal stress and corresponds to a band characteristic of the wurtzite phase [54]. Moreover, the high intensity of this mode indicates the good crystal quality of as-prepared samples [55]. The mode (527 cm−1) is associated with defects in the host lattice induced by the doping, such as shallow-donor defects bound on the tetrahedral Co sites [56]. A broad peak at about 692 cm−1 is also observed. WH Weber proposed that a mechanism called LVM of –Co–O–Zn– in ZnO should be detected at higher wavenumber than the highest phonon frequency of ZnO because of the smaller mass of Co compared to Zn [57], when doping Co was less than 4.7 at%, the author contributed a band around 690 cm−1 to disordered local vibrational modes (LVMs) of –Co–O–Zn– in ZnO [58]. The doping Co concentration of our samples is 3 at%, it is reasonable to ascribe the broad peak at about 692 cm−1 to LVM of –Co–O–Zn–, and this peak turns to broaden and weaken when increases t. To make a more detailed comparison, the value of FWHM of the peak at 692 cm−1 based on Gauss fitting is described in Table 2, which shows an increase of the value of FWHM with t, indicating fewer LVMs in samples with longer pretreatment time. XRD analysis shows that more Co atoms in S(t = 2 h) and S(t = 4 h), so the decrease of LVMs is caused by the absence of O, namely presence of oxygen vacancy(VO, VO+, VO++). Stated thus, S(t = 2 h) and S(t = 4 h) have more oxygen vacancies than S(t = 0 h) and S(t = 1 h).

Table 2 Values of FWHM of peak at 692 cm−1 base on Gaussian fitting. t FHWM(cm

−1

)

0h

1h

2h

4h

90.135

95.257

103.828

100.988

Fig. 5. PL spectra of Zn0.97Co0.03O samples.

3.3. Photoluminescence spectroscopy Photoluminescence spectroscopy is also an effective tool to detect the presence of intrinsic and extrinsic defects in the semiconductors and to investigate their optical properties, which is known as a sensitive nondestructive method [51]. A certain defect could be confirmed by the detailed comparison between the corresponding energy of luminescence band and the energy level of defect. Fig. 5 shows photoluminescence of S(t = 0 h), S(t = 1 h), S(t = 2 h) and S(t = 4 h), in which two peaks are observed. The obvious ultraviolet(UV) emission at about 381 nm is ascribed to the recombination of carriers bound within excitons leading to the near-band-edge(NBE) luminescence [59,60]. The energy of 3.26 eV corresponding to this UV peak is smaller than the band gap energy of ZnO (3.37 eV), informing that the successful doping of Co causes a band-gap narrowing about 0.11 eV [61]. The UV peak becomes broader, and the slight blueshift with t indicates that magnetic field pretreatment time could promote more Co atoms that influence the crystalline quality and arise quantum confinement effect [59,62] to enter into the crystal lattice, which is consistent with XRD results. The broad visible light (VL) emission band from 443 nm (2.76 eV) to 630 nm(1.97 eV), expanding from green band to the red band, is indicative of a large number of quasi-continuous defects and doping energy levels. This band due to the intrinsic defects and oxygen vacancies

Fig. 6. The draft of defect's level in Co-doped ZnO [64].

It should be pointed out that the Raman spectra are normalized by the relation: Inom = (I − Imin)/Imax. For all samples, the Raman spectrum consists of four modes centered at 324, 433, 527 and 692 cm−1. The peak at 324 cm−1 related single crystalline nature of ZnO can be ascribed to difference mode between the E2 high and E2 low frequencies [E2H-E2L] [52]. And the sharpest peak at 433 cm−1 can be assigned to 306

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References

[63] is centered 546 nm(2.27 eV). The growing intensity with t displays more defects in the lattice. Xu et al. calculated the structure of ZnO and its defects [64], and the results are demonstrated in Fig. 6. It is obvious that the energy(2.27 eV) corresponding to the VL emission center is almost equaled to the energy level difference(2.3 eV) between the bottom of conduction band and singly ionized oxygen vacancy(VO+), which is in agreement with previous reports [35,65,66]. As a result, VO+ is the dominant defect, deciding the optical properties of Co doped ZnO. And the intensity around 546 nm becomes stronger in S(t = 2 h) and S (t = 4 h) compared to S(t = 0 h) and S(t = 1 h), that is, much more recombination of electrons with photo-generated holes trapped in VO+ occurs, suggesting higher VO+ concentration in these samples.

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3.4. Magnetization studies The magnetic properties of the Zn0.97Co0.03O are shown in Fig. 7 by measuring magnetization (M) versus magnetic field strength (H) at room temperature. As seen from Fig. 7, all samples exhibit ferromagnetism at room temperature because of well-defined hysteresis loops. The value of saturation magnetization of S(t = 0 h), S(t = 1 h), S (t = 2 h) and S(t = 4 h) is 0.0174, 0.0182, 0.05 and 0.0488 emu/g respectively, MS increases nearly three times. MS of S(t = 0 h) and S(t = 1 h) almost equals while MS of S(t = 2 h) and S(t = 4 h) is close, which reveals that the effective time of magnetic field in the hydrolysis reaction on the magnetic property is between 1 h and 2 h. And the coercivity value is 23, 25, 21 and 41 Oe. The increase in the coercivity value from 23 to 41 Oe may be related to the domain-wall pinning effects. The oxygen vacancies in the samples may serve as pinning sites to impede the domain-wall motion and thus enhance the coercivity [67]. To investigate the origin of room temperature ferromagnetism, it is necessary to combine the former analyses including XRD pattern, PL spectrum and Raman spectroscopy. The magnetic moment of 0.0085 µB/Co (0.0174 emu/g) for S(t = 0 h) and 0.0245 µB/Co (0.05 emu/g) for S(t = 2 h) is much smaller in comparison to the full Co2+ moment (3 μB), which indirectly implies that the ferromagnetism doesn't originate from Co2+ related oxides. Through comprehensive consideration of detected data, defects are taken into account for the origin of RTFM. A steep increase in MS with t may be due to the stronger peaks correlated with defects in Raman and PL spectra. And VO+ predominates among defects, therefore, VO+ may be responsible for the presence of RTFM. The BMP model [68] related to the joint effect of the intrinsic exchange interaction could be used to explain the mechanism of RTFM. The exchange interactions between VO+ and surrounding Co2+ ions form a Co2+–VO+–Co2+ BMPs, neighboring BMPs could overlap and then spur the establishment of long-range ferromagnetic order in Co-doped ZnO [44,58,69–72]. S(t = 2 h) and S(t = 4 h) have more doping Co2+ and VO+ defects than S(t = 0 h), S(t = 1 h), so more Co2+–VO+–Co2+ BMPs are formed in these samples, and then exhibit better magnetic properties. 4. Conclusion We have synthesized Zn0.97Co0.03O nanoparticles by hydrothermal technique while a pretreatment of a constant magnetic field in the hydrolysis reaction was conducted. Four different pretreatment times were set as 0 h, 1 h, 2 h and 4 h. XRD, SEM and TEM characterizations reveal that the samples are single crystalline and flower shaped with a hexagonal wurtzite ZnO structure, and the diffraction peaks shift slightly to a small angle due to lattice expansion. XRD analysis shows more Co atoms while Raman and PL spectra show more VO+ defects in samples with 2 h and 4 h magnetic field pretreatment. VSM measurement exhibits an almost three-fold increase of MS with t. It's believed that applying a constant magnetic field in the hydrolysis reaction could promote the doping of Co and improve ferromagnetism of DMS, and the effective time is considered as 1–2 h. Finally, room temperature ferromagnetism might derive from Co2+–VO+–Co2+ BMP. 307

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