Cryst. Res. Technol. 46, No. 5, 523 – 528 (2011) / DOI 10.1002/crat.201100029

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Optical and magnetic properties of (Zn,Mn)O nanostructures

synthesized by CVD method

V. K. Sharma*1, B. K. Gupta

2, and G. D. Varma

3

1 University of Petroleum and Energy Studies, Dehradun-248007, India 2 National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi-110012, India 3 Department of Physics, Indian Institute of Technology Roorkee, Roorkee-247667, India

Received 25 January 2011, revised 22 March 2011, accepted 2 April 2011

Published online 29 April 2011

Key words nanorods, nanowires, chemical vapor deposition, room temperature ferromagnetism.

We report on microstructural, optical and magnetic properties of (Zn,Mn)O nanostructures synthesized by a

chemical vapor deposition (CVD) technique. Average diameters of the as grown (Zn,Mn)O nanorods and

nanowires were ~400 nm and ~50 nm, respectively. X-ray diffraction (XRD) and photoluminescence (PL)

spectra provided the evidence that Mn was incorporated into ZnO lattice. PL spectra of the (Zn,Mn)O

nanostructures showed shift in near band edge (NBE) emission at 396 nm together with a green band (GB)

emission at 510 nm and a blue band (BB) emission at 460 nm. Magnetic measurements revealed mixed

magnetic phases (ferromagnetic and antiferromagnetic) in the (Zn,Mn)O nanostructures. Vapor-solid-solid

(VSS) mechanism was thought to be responsible for the growth of the nanostructures at low temperatures.

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction

Diluted magnetic semiconductors (DMS), obtained by combining charges and spins in a single substance, give rise to a new era of spintronic applications, including spin-valve transistors, spin-emitting diodes, nonvolatile storage and logic devices [1]. Recently, Mn-doped ZnO stimulated considerable research effort since theoretical calculation predicted that it can show ferromagnetic behavior with a Curie temperature (TC) well above room temperature [2]. The prediction has been proven by many experimental studies [3-5]. However, the results concerning the existence of ferromagnetism have been rather controversial. Some groups reported ferromagnetism in ZnO-based DMSs with TC higher than 300 K [1,6,7], while others reported observations of antiferromagnetic and spin-glass behavior [8,9]. In addition, a number of studies indicated that room temperature ferromagnetism (RTFM) may originate from magnetic clusters or secondary magnetic phases [10,11]. Mn doped ZnO also has been regarded as an ideal material for short wave magneto-optical devices [12,13] because of the wide band gap of ZnO [14] and the high thermal solubility of Mn in ZnO [15].

Nowadays, intensive research activities are being directed towards one-dimensional (1D) nanostructures of Mn-doped ZnO, which are considered to be well-defined building blocks for the fabrication of various nanoscale devices. Chang et al. [16] demonstrated that the Zn1−xMnxO nanowires formed by an in situ doping method exhibited ferromagnetic behavior with TC of about 37 K. Philipose et al. [17] synthesized Mn-doped ZnO nanowires by Au-catalyzed vapor–liquid–solid (VLS) growth and ferromagnetism was observed at temperatures up to 400 K. Liu et al. [18] fabricated well-aligned Zn1−xMnxO nanowires via chemical vapor deposition having a TC of ~44 K. This group also synthesized Mn doped ZnO nanorods [19] and found that they exhibit low temperature ferromagnetism at 5 K with TC of about 37 K. Ip et al. [20] observed high-TC (225-300 K) ferromagnetism in (Zn,Mn)O nanorods implanted with Mn at high doses. Baik et al. [5] synthesized well aligned (Zn,Mn)O nanorods possessing RTFM with a TC higher than 350 K. Wu et al. [21] also observed RTFM in Mn doped ZnO nanorods synthesized by radio frequency plasma deposition and suggested that the observed ferromagnetism was due to the bound magnetic polaron (BMP). Deka et al. [22] found paramagnetic behavior at 300 K in Zn1-xMnxO (0 ≤ x ≤ 0.25) nanowires synthesized using autocombustion method. Yan et al. [23] reported RTFM in Mn doped ZnO nanorods with saturation ____________________

* Corresponding author: e-mail: vks80dph@gmail.com

524 V. K. Sharma et al.: (Zn,Mn)O nanostructures synthesized by CVD method

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.crt-journal.org

magnetization (MS) of 0.39 µB/Mn and a coercive field (HC) of 50 Oe. Zhang et al. [24] fabricated Mn doped ZnO nanostructures (nanobelts, nanorods and nanowires) by a chemical vapor transport. It was found that despite their different morphologies, these nanostructures possessed similar magnetic properties, i.e. no ferromagnetism was observed even at ~5 K. Recently, Zhang et al. [25] fabricated single phase Mn doped ZnO nanowires by a one-step vapor-solid process at 500 °C and demonstrated using electrical measurements that Au electrodes made good ohmic contacts with the Zn1-xMnxO nanowires. Thus, there is no consensus on the magnetic results of the Mn doped ZnO nanostructures. In the present work, (Zn,Mn)O nanostructures have been grown by chemical vapor deposition (CVD) technique and their structural, optical and magnetic properties are studied.

2 Experimental

A powder mixture of zinc metal and manganese chloride (MnCl2·4H2O) with atomic ratio of Zn:Mn=10:1 was placed in an alumina boat as source material. The boat and three gold (Au) coated Si substrates were placed into a quartz tube of diameter of ~1 inch in a tubular furnace. The complete experimental set up is shown in figure 1. The temperature of the central zone of the furnace, i.e. the temperature of boat, was maintained at ~ 800 °C for 2 h with continuous flow of argon gas from one end of the quartz tube. The temperatures of the three substrates were maintained at 690, 570 and 450 °C during the experiment by adjusting their positions from centre of the quartz tube. Crystal structures of the material deposited on the Au-coated Si substrates were studied by using an X-ray diffractometer (XRD) employing Cu-Kα (1.54 Å) radiation. Microstructural and chemical analyses of the samples were carried out by using a field emission scanning electron microscope (FESEM) equipped with an Oxford Inca energy dispersive X-ray (EDAX) detector. Optical properties of the samples were studied by using a photoluminescence (PL) spectroscopy. Magnetic measurements of the samples were carried out by using a superconducting quantum interference device (SQUID) magnetometer.

Fig. 1 Experimental setup used for growth of

the nanostructures.

2 Results and discussion

FESEM micrographs of the (Zn,Mn)O samples grown on the Au coated Si substrates are shown in figure 2. There is no formation of any nanostructures at 690 °C. The samples grown at 570 °C shows the growth of well aligned nanorods with average diameter of ~400 nm and length of ~3 µm (Fig. 2b). All the nanorods have a hexagonal shape (Fig. 2c), which is similar to that reported by Garces et al. [26]. The samples grown at 450 °C are nanowires with an average diameter of ~50 nm and length of ~2 µm (Fig. 2d). Magnified views of the nanowires are shown in figure 2e and 2f. We didn’t find any growth of nanostructures on bare Si substrates. Thus, it is concluded that both the catalyst (Au) and substrate temperature are important for the growth of nanostructures.

A typical EDAX spectrum of the sample is shown in figure 3. Besides Si and Au peaks, Zn, O and Mn peaks are also present in the spectrum. We have observed uniform chemical compositions along the length of the nanowires. The atomic ratio of Zn to Mn in the nanowires is roughly ~10:1 i.e., same as the starting composition.

The XRD pattern of the (Zn,Mn)O the nanowires grown at 450 °C is shown in figure 4. Majority of diffraction peaks corresponds to wurtzite structure of ZnO. Lattice parameters of the (Zn,Mn)O nanowires i.e., a=3.2497 Å, c=5.2086 Å and V=47.634 Å3 are greater than those of pure ZnO nanowires i.e., a=3.2454 Å, c=5.2079 Å and V=47.502 Å3 [27]. This confirms that Mn2+ (ionic radius ~0.66 Å) ions substituted for Zn2+ (ionic radius ~0.60 Å) of ZnO [18]. There is a weak peak at 2θ=41.5o, which could not be assigned to ZnO, Au

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or Si (Fig. 4). This peak is attributed to the (330) plane of the γ-brass Au-Zn alloy [28]. Optical properties of the (Zn,Mn)O nanostructures at 300 K are shown in figure 5.

Fig. 2 FESEM micrographs of the (Zn,Mn)O

nanostructures: (a) film grown at 690 °C; (b) normal and

(c) top view of the nanorods grown at 570 °C; (d)

nanowires, (e) magnified view of the nanowires and (f) a

single nanowire grown at 450 °C.

Fig. 3 EDAX spectrum of the (Zn,Mn)O nanowires.

(Online color at www.crt-journal.org)

Fig. 4 XRD pattern of the (Zn,Mn)O nanowires.

Fig. 5 PL spectra of the (Zn,Mn)O nanostructures. (Online

color at www.crt-journal.org)

The (Zn,Mn)O nanostructures have a strong green band (GB) emission at 510 nm and a blue band (BB) emission at 460 nm, together with a usual near band edge (NBE) emission at 396 nm. The UV emission peak at about 396 nm is explained by a NBE transition, which exhibits a redshift of 16 nm compared with the PL emission at about 380 nm usually observed in undoped ZnO nanowires [29-31] and ZnO films [32]. The shift of the NBE towards higher wavelength in the (Zn,Mn)O nanostructures is possibly due to lattice expansion of

526 V. K. Sharma et al.: (Zn,Mn)O nanostructures synthesized by CVD method

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.crt-journal.org

ZnO caused by Mn doping. The observation of BB emission in ZnO film has also been reported using cathodoluminescence [33]. However, the mechanism of this emission is not yet clear. There are different hypotheses on the origin of the green-light emission [16,34-36]. Since the GB is associated with the presence of defects induced by Mn, the green peak may be attributed to singly ionized oxygen vacancies [16] and zinc interstitials [36] in ZnO. Therefore, the stronger the intensity of the green luminescence, the more singly ionized oxygen vacancies are there. It is noted that the luminescence of manganese is not observed in the (Zn,Mn)O nanostructures. Thus, the PL results also confirm that Mn2+ ions substituted for Zn2+ in ZnO.

Magnetization versus magnetic field (M-H) curves of the (Zn,Mn)O nanostructures at 300 K are shown in figure 6. The diamagnetic background of the gold coated Si substrate was subtracted. A weak RTFM is observed both in the (Zn,Mn)O nanorods and nanowires. However, the nanowires exhibit stronger ferromagnetic interactions as compared to the nanorods. This is because of higher surface to volume ratio for the nanowires as compared to the nanorods. Since at surface of the nanostructures the concentration of defects is higher, stronger ferromagnetic interactions are expected in nanowires [37]. These results are also supported by the PL data. Remanent magnetization (Mr) ~ 1 x 10-5 emu/cm3 is same for both (Zn,Mn)O nanostructures, whereas coercive fields (HC) are 960 and 1170 Oe for the nanorods and nanowires, respectively.

Fig. 6 M-H curves of (Zn,Mn)O nanostructures. Inset

shows magnified view of M-H curves around H=0 Gauss.

(Online color at www.crt-journal.org)

Fig. 7 ZFC-FC M-T curves for the (Zn,Mn)O nanowires.

Inset shows expansion of the ZFC-FC curves in the region

of 250-300 K. The blue line in the figure is the fit of the FC

M-T data using eq. 1. (Online color at www.crt-journal.org)

Zero-field-cooled (ZFC) and field-cooled (FC) M-T curves, measured by applying a magnetic field of 500 Oe, are shown in figure 7. We see a clear deviation in the ZFC-FC curves at spin-freezing temperature Tf of ~40 K. The irreversibility of the ZFC-FC curves and a cusp at low temperature are features of spin glass behavior [38,39]. As demonstrated by PL, the nanowires possess defects, so this spin glass phase at low temperature is thought to arise from the random distribution of Mn atoms into the ZnO lattice. Spin glass is a magnetic state in which the magnetic moments of ions are frozen in a random direction [40]. The difference between the ZFC and FC persists up to RT (see inset of figure 7) for the ZnMnO nanowires, which is also supported by the M-H curve recorded at 300 K. The difference between ZFC and FC gives a net magnetization value (ΔM = FC - ZFC) corresponding to ferromagnetic contribution in the sample by eliminating the para-and diamagnetic contributions [41]. The net magnetization ΔM is, however, very small at 300 K. This suggests nanowires at RT are weak FM.

We also found that FC M-T data for the ZnMnO nanowires can be fitted by a standard Bloch spin-wave model (1st term of eq. 1) [42] added with a Curie-Weiss model (2nd term of eq. 1), revealing a mixed magnetic phase in the samples. The fitting equation is as follows

3/2 *(1 )

o

H CM M aT

T θ= −

+

+ , (1)

where, θ is Curie-Weiss temperature, C is curie constant, H is the applied field (500 Oe), a is a constant which depends on the exchange interaction between the number of nearest neighbors and M

o is zero temperature

magnetization. Value of the parameters are, Mo = 5.14 x 10-8 emu/cm3, a = 9.89 x 10-3 K2/3, C ~ 2.21 x10-6 emu

K g-1 Oe-1 and θ ~ -111 K. Thus, the samples contain mixed magnetic phases, i.e. a ferromagnetic phase plus a

Cryst. Res. Technol. 46, No. 5 (2011) 527

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antiferromagnetic one. Similar results have been reported by other groups [43, 44]. It is concluded that only a fraction of Mn2+ ions are involved in ferromagnetic ordering and the rest is still in paramagnetic or antiferromagnetic ordering. This may be the reason for the weak RTFM in the (Zn,Mn)O nanostructures.

3 Conclusions

We have synthesized nanowires and nanorods of (Zn,Mn)O by CVD method. XRD and PL spectra confirmed that Mn2+ ions were incorporated into ZnO lattice. Magnetic measurements revealed the presence of mixed magnetic phases (FM + AFM) in the (Zn,Mn)O nanostructures, thus resulting in an overall weak RTFM. Acknowledgements One of the authors (V. K. Sharma) is grateful to C.S.I.R., Government of India, for a research

fellowship. Financial support from BRNS (DAE), Government of India is highly acknowledged.

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