Synthesis, Magnetism and Photoluminescence of Mn Doped AlN Nanowires

Manganese doped aluminum nitride (AlN:Mn) nanowires were fabricated by direct nitridation of Al and Mn mixed powders. The as-synthesized AlN:Mn nanowires were characterized by X-ray diffraction, Raman, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, scanning and transmission electron microscopy. The measurements reveal the successful incorporation of Mn 2+ ions into AlN nanowires. The magnetic and optical properties of the AlN:Mn nanowires were studied using vibrating sample magnetometer and photoluminescence spectroscopy. The AlN:Mn nanowires exhibit room temperature ferromagnetic behavior and a red emission band centered at 597 nm corresponding to the 4 T 1 ( 4 G)- 6 A 1 ( 6 S) transition of Mn 2+ . The abnormal thermal quenching behavior and long afterglow feature are investigated through the temperature dependent emission and the afterglow spectrum. The thermoluminescence curve shows that AlN: Mn nanowires have a wide trap distribution, which leads to the abnormal thermal quenching behavior and persistent luminescence. Multifunctional AlN:Mn nanowires with magnetic and luminescence properties are expected to be used in spintronic and optoelectronic nanodevices.


Introduction
During the past decades, one-dimensional (1D) nanostructures have attracted much attention because of their unique physical properties and potential applications in the nanodevices and nanocomposites [1,2].
Aluminum nitride (AlN) is a kind of III-V semiconductor with many excellent properties such as widest direct band-gap (6.2 eV), high thermal conductivity, good mechanical strength, high piezoelectric response, and negative electron a nity [3]. Thus, the controllable 1D AlN nanostructures were successfully prepared by different methods, which have desirable applications in the eld of short wavelength light emitting and eld emission devices [4][5][6].
Because doping is an ideal way to improve electrical, optical and magnetic properties of AlN semiconductor, more and more studies on transition metal and rare earth doped AlN are carried out [7][8][9][10][11][12]. Especially, Mn doped AlN (AlN:Mn) has aroused a lot of interest because of its potential applications in diluted magnetic semiconductors (DMS) and red light-emitter. For example, Frazier et al. have prepared AlN:Mn thin lms with different Mn contents through gas-source molecular beam epitaxy [13]. Yang et al. have fabricated AlN:Mn nanowires via a simple vapor phase deposition method [14]. Li et al. have synthesized AlN:Mn polycrystalline powders by solid-state reaction [15]. The room temperature ferromagnetism was observed in AlN:Mn thin lms, nanowires and polycrystalline powders as described above, indicating that ferromagnetism is an intrinsic property of AlN:Mn. On the other hand, the AlN:Mn thin lms with a single emission band around 600 nm has been prepared by chemical vapor deposition [16]. Wang et al. have prepared Mn 2+ doped AlN phosphors with a strong red emission through solid-state reaction method, which has great potential in full-color eld emission display [17]. Later, Xu et al. have studied the afterglow feature of AlN:Mn, indicating its brightness is 0.65 mcd/m 2 after stopping ultraviolet (UV) radiation for 60 minutes, and duration upon 0.32 mcd/m 2 can reach more than 110 minutes [18]. Lei. et al. have also prepared AlN:Mn red phosphors through a solid-state reaction, which can be used as white light-emitting diodes [19]. Although AlN:Mn lms and bulk materials and their properties have been extensively explored, there are very few reports on the synthesis of 1D AlN:Mn nanostructures, due to the limited equilibrium solubility of Mn in AlN semiconductor and the intrinsic di culties in doping nanocrystal. Thus, the preparation and characterization of AlN:Mn nanowires is a challenging work, which is helpful to explore their properties and applications. Also, compared with thin lms and bulk materials, nanowires have many advantages, such as nanoscale size, low dimension, and single crystal, which are potential nanoscale building blocks for electronic and optoelectronic devices. If the DMS nanowires exhibit robust magnetism, they will be important for application in spintronic nanodevices using electronic spin as an additional degree of freedom. Therefore, we decided to explore the possibility of synthesizing AlN:Mn nanowires, which may be a potential magnetic and optical multifunctional nanomaterial.
In previous works, we have used improved arc discharge method for the preparation of multifunctional AlN nanostructures doped with a series of rare earth elements [12,[20][21][22][23][24][25]. In this paper, AlN:Mn nanowires have been synthesized by direct nitridation of Al and Mn mixed powders using similar method.
Various experimental methods are used to characterize the as-prepared nanowires. A promising magnetic and optical properties of the AlN:Mn nanowires is demonstrated. The abnormal thermal quenching behavior and long afterglow feature AlN:Mn nanowires of are also explained. This work provides a new opportunity to design AlN based nanomaterials for optoelectronic and magnetic functionalities on a single chip.

Experimental Section
The sample preparation was carried out in a direct current (DC) discharge plasma setup described previously [26,27]. The anode was Al and Mn mixed powder, while the cathode was a tungsten needle.
The Mn content in AlN:Mn nanowires was controlled by changing the molar ratio of Mn powder to Al powder. The chamber pressure was evacuated to less than 1 Pa, then the N 2 gas was introduced into chamber to 20 kPa. During the experimental process, the current and potential were maintained at 100 A and 25 V for 5 min, respectively. After being passivated in 40 kPa Ar for 24 h, the grey white powder was seen to get settled on the water-cooling wall.

Results And Discussions
The chemical compositions of AlN nanowires with different Mn concentrations were measured by EDS. The at.% of Mn (x) is found to be 0.0, 0.4, 0.9, 1.3 and 2.3 in the samples, respectively. The XRD pattens of AlN nanowires doped with diverse concentrations of Mn ions (AlN: xMn) are presented in Fig. 1 (a). For all samples, the diffraction peaks in patterns can be indexed as wurtzite-structured AlN (PDF card No. 08-0262). Under the resolution of XRD, no peak of impurity phase is detected. As show in Fig. 1  A typical TEM image of an AlN:0.9%Mn nanowire is given in Fig. 5(a), which clearly shows that the diameter of the nanowire is about 50 nm, which is the same as that observed by SEM. Figure 5(b) shows the HRTEM image recorded from the edge of the nanowire. The adjacent lattice spacing is about 0.25 nm, which is consistent with the (001) facet distance of hexagonal AlN, indicating that the nanowire grows along [001] direction. In addition to this, many dark spots can be seen in the lattice, which are attributed to local lattice distortions and defects caused by the larger Mn 2+ ions replacing Al 3+ in the nanowires.
In recent years, the potential applications of AlN-based DMS in spintronics have attracted great interest.
To explore the magnetism of AlN:Mn nanowires, the magnetic measurements of AlN: Mn nanowires with different Mn concentrations are carried out at room temperature using VSM. Figure 6 Table 1.
According to the results of XRD, Raman, XPS and HRTEM, it can be con rmed that the Mn 2+ ions are successfully doped into AlN nanowires without changing wurtzite structure, and excluded the possibility of other impurities. Thus, the observed room temperature ferromagnetism is the intrinsic properties of AlN:Mn nanowires.   Figure 7(b) shows the high resolution PLE spectrum in the wavelength range between 350 and 550 nm. In this range, the PLE spectrum consists of several weak peaks at 400, 455, 489, 520, and 548 nm corresponding to the spin and parity forbidden d-d transitions of Mn 2+ from the ground state 6 A 1 ( 6 S) to the excited state 4 E( 4 D), 4 T 2 ( 4 D), ( 4 A 1 ( 4 G), 4 E( 4 G)), 4 T 2 ( 4 G) and 4 T 1 ( 4 G) [17]. The valency of Mn ions in AlN remains controversial, because Mn 4+ also exhibits red emission in the 560-670 nm range. Also, the ionic radius of Mn 4+ (0.053 nm) is closer to Al 3+ (0.054 nm) than Mn 2+ (0.067 nm), suggesting Mn 4+ is more easily doped into AlN lattice. However, the Mn 4+ ions usually produce line emissions. Therefore, the successful doping of Mn 2+ in AlN nanowires is also evident from the luminescence spectrum.
In order to study the effect of the Mn content on the optical properties, PL measurements were performed for AlN nanowires with Mn doping concentration ranging from 0.0 to 2.3%, as shown in Fig. 8. With increase of Mn 2+ content, the emission intensity increases rapidly, reaches the maximum at 0.9%, and then decreases again at higher doping levels. The deterioration of the emission signal is usually attributed to concentration quenching caused by nonradiative energy transfer between adjacent doped Mn 2+ ions.
The PL decay curves of AlN:xMn nanowires monitored at 597 nm and excited at 266 nm are displayed in Fig. 9. The decay curves are well tted by using the following double-exponential relationship: where I means the luminescence intensity, I 1 and I 2 represent constants, t stands for the time, τ 1 and τ 2 are decay times for exponential components. The detailed tting parameters and lifetime values of AlN:xMn nanowires calculated by Eq. (1) are listed in Table 2. As with other Mn 2+ doped materials, the lifetime is in the range of milliseconds [36]. It is well known that the PL intensity is related to the lifetime of the luminescent center and the radiation velocity [37]. For the same luminescent center, the radiation velocity can be supposed to constant (the amount of radiation photons in a speci c period). Therefore, a stronger luminescence intensity corresponds to a longer uorescence lifetime. The thermal stability of light-emitting materials is a key index to evaluate the potential application of the nal devices, because it has an impact on the different characteristics of the device. Thus, it is necessary to elucidate the thermal stability of light-emitting materials according to the relationship between intensity and temperature. Figure 10 (a) and (b) presents the temperature dependent PL emission spectra and corresponding relative integrated intensity trend for AlN:0.9%Mn nanowires, respectively. It is important to note that the emission intensity of Mn 2+ transition, initially increase slightly (until the temperature reached 353 K), and then gradually drops. When the temperature is below the 413 K, the emission intensity is still higher than initial value at 293 K. In general, an increasing of temperature can increase the population of higher vibration levels, the density of phonons and the probability of nonradiative transfer (energy migration to defects), leading to the emission intensity gradually drops. Here, the AlN:Mn nanowires exhibit an abnormal thermal quenching behavior, which should be related to the traps in AlN band gap. The detail of reason will be discussed later. Figure 11 shows the afterglow decay curves of the AlN:0.9%Mn nanowires after irradiation by a 254 nm UV lamp for 3 min. The decay curve can be successfully tted by Eq. (1), and the tting results are summarized in Table 3. Apparently, the decay curve consists of a rapid attenuation process at rst (τ 1 ) and then a slow decay process (τ 2 ). Due to the presence of the signi cant slow decay component (τ 2 ), the long afterglow features can last more than 30 min with brightness ≥ 0.32 mcd/m 2 in the darkroom.
The observed afterglow feature of AlN:Mn nanowires can signi cantly save power compared with traditional LEDs, which paves the way for further dealing with severe environmental and energy problems. It is considered that the existence of traps is the main reason for the abnormal thermal quenching behavior and afterglow performance. To further verify the presence of traps, the TL curve of AlN:0.9%Mn nanowires is shown in Fig. 12 (a). The TL curve gives an asymmetric shape and a wide temperature region from 300 to 620 K, suggesting a wide trap distribution. However, because the incorporation of is at around 1.9 eV below the CB, and O N -V Al complex level is at around 1.2 eV above the VB [40,41]. Figure 12 (b) exhibits a mechanism diagram for the abnormal thermal behavior and afterglow process. The strong excitation band centered at 266 nm ( Fig. 7 (a)), is attributed to the excitation from the O N -V Al level to its excited state level (process ). After UV light irradiation (process ), most of the excitation energy related to the excited carriers (electrons or holes) will be directly transferred to the luminescence center Mn 2+ (process ), followed by the 4 T 1 ( 4 G)-6 A 1 ( 6 S) emission as the immediate luminescence (process ). However, when some electrons transition into the CB and then are captured by V N or O N levels and the holes are captured by V Al or O N -V Al levels, instead of returning to ground state (progress ). After removing the UV light source, with the thermal disturbances at proper temperature, electrons and holes trapped by the defect levels will be gradually released, transferred to the luminescence center Mn 2+ (process ), and then emit as long afterglow (process ). For thermal quenching process, as the sample is heated to proper temperature, the "stored" electrons in the defect levels can be excited to the excited level (process ) and nally return to ground state. The higher temperature, the more electrons are excited through process and to process . Meanwhile, the higher temperature results in greater energy loss of the electrons in the relaxation process, due to the enhanced nonradiative transition, leading to the decrease of electrons in process from process . Thus, the thermal quenching behavior should be the coupling contribution by the above two effects. When the electrons excited from process to process is higher than that lost in the relaxation process, leading to the increase of emission intensity and the abnormal thermal quenching behavior.

Conclusions
In conclusion, we report on the ferromagnetic and luminescence properties of AlN:Mn nanowires synthesized through direct nitri cation of Al          (a) Thermal quenching behavior of emission spectra for AlN: 0.9 % Mn nanowires. (b) The normalized integrated intensity as a function of temperature.

Figure 11
Afterglow decay curve of the AlN: Mn nanowires after excited by 254 nm light for 3 min. The inset shows optical images taken at different decay times after removal of the excitation source.