One-pot simple synthesis and characterisation of Mn²⁺-doped GaN nanorods by RAPET method

Abstract

In this work, we display one-step reactions under autogenic pressure at elevated temperature (RAPET) method-based synthesis of Mn-doped GaN nanorods by varying the atomic ratio of Mn:Ga as 0, 0.02, 0.04, 0.06 and 0.08, respectively. The synthesised nanorods are characterised by X-ray diffraction, high-resolution transmission electron microscopy (HRTEM) and Raman spectral methods. It is observed that there is a decrease in the lattice constant with an increase in the concentration of Mn (from 0.02 to 0.08 wt%). Moreover, the smaller covalent radius of Mn is the key for the doping process. The Mn-doped GaN nanocrystals show rod-like morphology with a length of 40–50 nm and width of 8–12 nm. This size factor mainly depends on the doping of Mn [from transmission electron microscopy (TEM) analysis] into GaN components. Well-defined lattice fringes are elucidated for the growth of crystalline hexagonal GaN (wurtzite type) nanocrystals.

Keywords:

• Mn²⁺-doped GaN
• nanorods
• RAPET
• HRTEM
• Raman Spectra

1. Introduction

Smart nanomaterials have received enormous attention in recent times due to their application in novel nanoelectronic and optoelectronic devices.[1–8] It has been used in many functional devices owing to their unique one-dimensional nanostructures, excellent electronic and optical properties.[1–8] More specially, wide-band gap semiconductors (III Group-N) such as AlN, GaN (3.4 eV) and InN have attracted extensive attention as they exhibit very high optical efficiencies. Of course, the above-mentioned nanomaterials have been successfully used in many electronic devices such as high-power and high-frequency transistors, blue and ultraviolet light-emitting diodes and laser diodes. Very recently, it is used as the most promising nanomaterial for spintronic device application. This is because of the fact that the dilute magnetic semiconductors (DMS) based on these materials showed the stability of the ferromagnetism at temperatures higher than the room temperature.[9–14] GaN is an excellent host material to be successfully made into DMS according to the computed value at high Curie temperature (T_C).[9] This interesting property created immense interest in the research community. The interest was because of their possible applications for the design of spintronic nanodevices,[15,16] such as electronic-optic switches, ultrasensitive magnetic field sensors, magnetic hard disk media, spin field effect transistors, resonant tunnelling diodes and non-volatile computer memory chips.[17–19] After the discovery of ferromagnetism with a higher temperature in Mn-doped GaN,[9,20] there has been a remarkable interest towards the fabrication of GaN-based DMS. Hence, the research community is extremely interested to prepare DMS. The primary goal of such fabrication of Mn-doped GaN is realising room temperature ferromagnetism and therefore posing it as a potential candidate for future spintronics applications.[21]

The GaN materials have been extensively studied and predicted to be ferromagnetic with a T_C above room temperature. However, many reported experimental values were quite different and even contradictory. Near or above room temperature, ferromagnetism was found by a number of groups,[20,22–29] whereas no ferromagnetism or a very low T_C was reported.[30,31] A number of approaches have been explored to synthesise single-phase Mn-doped GaN, which includes epitaxial growth, chemical vapour deposition, pulse laser deposition, sputtering,[32–35] solvothermal,[36,37] azidothermal metathesis,[38] sublimation sandwich method [39] and carbothermal nitridation.[40] In this work, we demonstrate a simple, efficient and economical synthesis method for the fabrication of nanocrystalline Mn-doped GaN nanorods. The Mn-doped GaN nanorods are obtained in a crystallised form through a single-stage, solvent-free and catalyst-free single precursor reaction. In addition, the reaction is conducted at a relatively lower temperature, i.e. 750 °C, as compared to other methods for the formation of GaN nanocrystals and we named such reaction as reactions under autogenic pressure at elevated temperature (RAPET). Indeed, a survey of the experimental literature reveals that the carbon encapsulated NiO/MgO core/shell structured [41] and Mn-doped CdSe [42] nanostructured materials have been synthesised and reported by the RAPET method.

2. Experimental

Gallium(III) chloride, anhydrous beads – 10 mesh (GaCl₃, Aldrich, 99.99%), manganese(II) chloride, anhydrous beads – 10 mesh (MnCl₂, Aldrich, 99.99%) and sodium azide (NaN₃, Aldrich, 99.99%) were received from Sigma Ltd and were used as the starting precursor materials. In a typical synthesis of Mn-doped GaN nanocrystals, 0.501 g of GaCl₃, 0.061 g of NaN₃ and stoichiometric amount of anhydrous MnCl₂ (according to Mn/Ga atomic ratios of 0, 0.02, 0.04, 0.06 and 0.08) were mixed and introduced into a 2 ml stainless steel Swagelok reactor at room temperature. It is placed inside the glove box under N₂ atmosphere. The filled Swagelok reactor is tightly closed with the other plug and placed at the middle of furnace tube. The temperature of the furnace is raised to 750 °C at a rate of 10 °C min⁻¹ and maintained in this temperature for 7 hr. The Swagelok reactor is gradually cooled (∼5 hr) to room temperature and opened. The yield of the yellowish-green coloured Ga_1−xMn_xN product is ∼52%. The Ga_1−xMn_xN products with Mn/Ga atomic ratios of 0, 0.00, 0.04, 0.06 and 0.08 are determined by inductively coupled plasma (ICP) analysis.

A morphological characterisation of the obtained nanocrystals was done by using high-resolution transmission electron microscopy (HRTEM, JEOL JEM, 2010) with an acceleration voltage of 200 kV measurements. Sample for high-resolution transmission electron microscopy (HRTEM) analysis were prepared by ultrasonically dispersing the product into absolute ethanol followed by placing a 3 μL of nanocrystals suspension onto a copper grid coated with an amorphous carbon film supported and then dried in vacuum. X-ray diffraction (XRD) patterns were obtained on a Bruker AXS D8 diffractometer using CuKα 1.5418 Å radiation at step scan 0.02θ, from 20 to 80θ and energy dispersive X-ray spectroscopy (EDS, JEOL, 2010). Raman spectral measurements were performed by using Olympus BX41 (Jobin-Yvon-Horiba) spectrometer with the wavelength source of 514.5 nm line of an Ar ion laser.

3. Results and discussion

3.1. XRD and elemental analysis (ICP)

The XRD patterns of the nanocrystalline Ga_1−xMn_xN (x = 0.00, 0.02, 0.04, 0.06, 0.08) products were shown in Figure 1(a)–(e). Sharp and well-defined peaks were observed for the earlier mentioned samples at 2θ values of 32.4°, 34.7°, 36.9°, 48.2°, 57.9°, 63.6°, 68.1°, 70.2° and 71.3° corresponding to (100), (002), (101), (102), (110), (103), (200), (112) and (201) nanocrystalline planes. All these peaks could be indexed to a hexagonal GaN phase with lattice constant values of a = 0.3186 nm and c = 0.5178 nm. The obtained lattice constant values matched well with the values of GaN. The calculated lattice constant values of the Mn-doped GaN nanocrystaline powders, with different Mn concentrations are listed in Table 1. It can be seen that the lattice constants, ‘a’ and ‘c’ shrink upon Mn doping and this suggests that the substitution of Mn into the GaN lattice. The shrinkage of both ‘a’ and ‘c’ is due to the covalent radius of Mn (1.17 Å) is smaller than that of Ga (1.26 Å). Furthermore, no diffraction peaks of Ga₂O₃ or other impurities were found in our samples. This result indicates that the as-prepared samples were of highly nanocrystallinity. The average crystallite size was calculated for pure GaN nanocrystals using the Debye–Scherrer formula (i) for the prominent plane of (101).(1)

Table 1. Lattice constant parameters of Mn-doped GaN nanorods versus various atomic compositions.

				Lattice constants
S. no.	Products	Mn/Ga atomic ratio	Mn/GaN weight ratio	a (Å)	c (Å)
1.	GaN	0	0	3.1930	5.1960
2.	Ga0.98Mn0.02N	0.02	0.007	3.1890	5.1890
3.	Ga0.96Mn0.04N	0.04	0.015	3.1860	3.1855
4.	Ga0.94Mn0.06N	0.06	0.022	3.1840	5.1760
5.	Ga0.92Mn0.08N	0.08	0.030	3.1825	5.1722

Figure 1. XRD patterns of Ga_1−xMn_xN nanorods. (a) x = 0.0, (b) x = 0.02, (c) x = 0.04, (d) x = 0.06 and (e) x = 0.08, respectively.

where d is the average particle size (nm), λ is the wavelength of the X-ray used (1.54056 Å), θ is the angle at the maximum of the peak (rad) and β_1/2 is the width of the peak at half-height in radians. The calculated average size of nanocrystals is 50 ± 4 nm, which is in close match with that obtained from HRTEM images (vide infra). The elemental composition of Mn-doped GaN nanocrystals were studied by ICP spectroscopy and summarised in Table 1 also. The results reveal there is a successful doping of Mn in GaN nanocrystals according to the Mn:Ga atomic ratio of 0.00, 0.02, 0.04, 0.06 and 0.08, respectively. Our synthetic method provides nanocrystalline Mn-doped GaN nanorods with no phase separation or grain boundaries, which can induce the ferromagnetism of the DMS to arise from magnetic secondary clusters. XRD patterns of Mn-doped GaN nanorods were perfectly indexed to wurtzite-type GaN nanostructures, revealing that the doping of Mn does not change the nanocrystal (nanorods) structure.

3.2. HRTEM analysis

The HRTEM was used to investigate the shape and size of the nanocrystals by varying the Mn/Ga atomic ratio. Selected area energy dispersive elemental analysis was also used for the identification of the product GaN and crystalline behaviour. The HRTEM images of undoped nanocrystalline GaN nanocrystals were shown in Figure 2, which clearly indicate that the nanocrystals were polydispersed and not uniform. As shown in Figure 2(b), the morphology is capsule-like shape and the dimensions were of 10–20 nm length and 3–5 nm width, respectively. Interestingly, a gradual increase in the nanocrystal size was observed when the concentration of Mn was increasing. The clear nanorods morphology of GaN nanocrystals were observed at the higher concentration of Mn. The TEM images of nanocrystalline GaN shown in Figure 3(a) and 3(b). The Mn-doped GaN nanocrystals showed the rod-shaped morphology and the size range of 40–50 nm of length and 8–12 nm of width. As shown in Figure 3(c), the HRTEM illustrates the well-defined lattice fringes, indicating that the higher nanocrystalline nature of the Mn-doped GaN nanorods. The measured inter-planar d-spacing value of (101) lattice plane is 2.45 Å. The above values were in good agreement with the reported value of d₁₀₁ = 2.43 Å [Joint Committee on Powder Diffraction Standards (JCPDS), No. 074–0243]. The selected area electron diffraction (SAED) pattern showed the formation of pre-dominant GaN (100), (002) and (101) ring patterns indicating the nanocrystalline behaviour of the GaN nanorods (Figure 3(d)).

Figure 2. HRTEM images of undoped GaN nanocrystals.

Figure 3. HRTEM images of Mn-doped (x = 0.08) GaN nanorods (a–c) and the corresponding SAED pattern of Mn-doped GaN nanorods (d).

3.3. Raman spectroscopy

The Raman spectral features of Mn-doped GaN nanorods were shown in Figure 4. The Raman peaks were observed at 420, 560 and 740 cm⁻¹. The Raman spectrum of GaN nanorods showed strong E₂ (high) and A₁ (LO) modes at 560 and 740 cm⁻¹.[43] These modes were in good agreement with Raman selection rules for wurtzite type of GaN nanorods. The high intensity of A₁ (LO) mode in undoped GaN nanorods indicates the low concentration of free carriers in the material. Moreover, a comparison of A₁ (LO) modes in the spectrum of Mn-doped GaN nanorods reveals an increase in the concentration of Mn from 0.02 to 0.08 as shown in Figure 4(b)–(e). The A₁ (LO) mode was decreased due to the doping of Mn into the GaN nanocrystals and a slight shift from 733 to about 720 cm⁻¹ was observed. In addition, the E₂^H mode was also decreased, broadened but not shifted (at 567 cm⁻¹). The shift of A₁(LO) mode was due to the difference in bonding strengths between GaN and MnN compositions in Mn-doped GaN nanorods.[44] The additional peak at ∼420 cm⁻¹ was very close to the overtone processes of the acoustic phonons that are observed in a single crystal of GaN nanorods.[45] The observed results clearly confirmed the successful doping of Mn into GaN nanorods. The synthesised Mn-doped GaN nanorods will be employed in the fabrication of nanoscale magnetic and optical devices. Currently these studies are under way in our laboratory.

Figure 4. Raman spectra of Ga_1−xMn_xN nanorods. (a) x = 0.00, (b) x = 0.02, (c) x = 0.04, (d) x = 0.06 and (e) x = 0.08, respectively.

4. Conclusions

We have demonstrated the RAPET method-based synthesis of Mn-doped GaN nanorods by varying the ratio of Mn:Ga (0, 0.02, 0.04, 0.06 and 0.08) components. Nanocrystalline phase of the GaN nanorods were confirmed by XRD measurements and the relative intensity of the XRD peaks varied by increasing the concentration of Mn. Well-defined Mn-doped GaN nanorods and lattice fringes were illustrated in HRTEM images. This indicates the highly crystalline nature of Mn-doped GaN nanorods. The structural evidence for the formation of pure GaN nanorods was also confirmed from Raman spectral measurements. These well-defined and composition-tuneable Mn-doped GaN nanostructures would be able to provide new building blocks for various optical and magnetic devices.