Ultraviolet Laser Action in Ferromagnetic Zn1−x Fe x O Nanoneedles
© to the authors 2009
Received: 13 August 2009
Accepted: 16 October 2009
Published: 1 November 2009
Fe-doped ZnO nanoneedles (NDs) were fabricated by an Ar+ ion sputtering technique operated at room temperature. The as-grown samples show both ferromagnetic and lasing properties. The saturated magnetization moment was measured from 0.307 to 0.659 emu cm−3 at the field of 10 kOe with various Fe concentrations. Intense ultraviolet random lasing emission was observed from Zn1 − x Fe x O NDs at room temperature. The X-ray photoelectron spectroscopy result reveals that the doped Fe atoms occupy the Zn sites and lead to a decrease in oxygen deficiency.
ZnO-based diluted magnetic semiconductors (DMSs) have attracted great research attention due to a high Curie temperature above 300 K as predicted by theoretical calculation [1, 2]. Recently, ZnO-based DMSs bulk materials and thin films doped with Mn , Cu , Fe , and Co  have been realized by various fabrication methods. On the other hand, developing one-dimensional (1D) DMS materials are of great interest, for the reason that the 1D nanomaterials are ideal research systems for fabricating nanoscale field effect transistors, sensors, optoelectronic devices, logic circuits, and lasers . Hence, 1D ZnO DMS material has been prepared by vapor–solid process and incorporation doping in the precursors [8–10]. However, these fabrication methods may lead to variance in environment of doping element in the matrix or forming second phase. Ideally, the combination of ferromagnetism and optical properties in 1D DMS material can open many new possibilities of freedom and functionality for the fabrication of unique nano-devices. Hence, few research groups have focused their investigation on the study of the photon luminance and stimulated emission from the DMS materials [11, 12]. Nevertheless, there is no report on the observation of the random lasing emissions from DMS materials.
In this letter, we established an effective fabrication method to realize 1D Fe-doped ZnO nanoneedles (NDs) which support random lasing action at ultraviolet (UV) wavelength. The surface morphology of an as-grown sample was characterized by scanning electron microscopy (SEM). X-ray diffraction (XRD) and transmission electron microscope (TEM) were also employed to investigate the crystallinity, which shows the Fe atoms have been doped into the ZnO lattice. Furthermore, the ferromagnetic and lasing properties of Zn1 − x Fe x O NDs have been investigated. The X-ray photoelectron spectroscopy (XPS) was used to find out the oxygen deficiency changing in the slightly doped Zn1 − x Fe x O NDs sample.
In order to analyze the Fe doping of prepared sample, we measured the magnetization versus applied magnetic field curves for the Zn1 − x Fe x O NDs at with different compositions. The measurement was done 300 K by an alternating gradient magnetometer with a maximum field of 10 kOe. The Zn1 − x Fe x O NDs exhibit saturated room temperature magnetic moment (M s) of 0.29, 0.37, and 0.66 emu g−1 with the Fe concentration as 1.1, 1.7, and 3.5 at.%, respectively. The well-defined hysteresis loops with coercive fields (H c) of ~75, 97, and 275 Oe in these three samples implying the ferromagnetic properties at room temperature. These trends with doping concentration have been plot in the insert.
Figure 3d shows the plots of emission intensity versus pumping intensity (i.e., light–light curve) of undoped ZnO, Zn0.989Fe0.011O, and Zn0.983Fe0.017O NDs, respectively. Remarkably, it shows that the lasing threshold is decreased with slightly doped sample (i.e.,x = 0.011), whereas increase to ~0.47 MW cm−2 when x increase to 0.017. It is known that the incorporation of 3d transition ions, such as Fe, generally deteriorates the crystallinity of ZnO due to their low solubility and various valence states , which is the mean reason for the increase of lasing threshold in the Zn0.983Fe0.017O sample. This also suggested the increase in optical properties of Zn0.989Fe0.011O sample. Hence, the slightly doped Fe can improve the crystallinity of undoped ZnO NDs. To compare the defects in the lattice of slightly Fe-doped ZnO and undoped ZnO NDs, we carryout XPS analysis for these two samples.
In summary, we have demonstrated successfully the in situ dope of Fe into ZnO NDs by ion sputtering method. Ferromagnetic characteristics of the Fe-doped ZnO NDs have been observed at room temperature. The UV lasing emissions from these magnetic NDs were also investigated. XPS measurements showed that oxygen deficiencies can be significantly reduced by slightly Fe doping in ZnO NDs. By combining magnetic and lasing functionality, these Fe-doped ZnO NDs have high potential to be used in variety of short wavelength optical devices, such as spin-polarized light emitters, spin-laser diodes, and optical switches and modulators.
This work was supported by LKY PDF 2/08 startup grant.
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