Synthesis and characterization of La0.825Sr0.175MnO3 nanowires

Perovskite oxide La0.825Sr0.175MnO3 (LSMO) nanowires are synthesized using the anodized alumina oxide template technique and the characterization of the microstructure, magnetic and photoluminescence properties is performed. The as-prepared LSMO nanowires 50 nm in diameter and tens of microns in length exhibit polycrystalline perovskite structure. The magnetic measurement reveals the competition between antiferromagnetic ordering and spin glass state as ground state in the nanowires, which is different from the magnetic behaviours of bulk ceramic and single crystals. The photoluminescence (PL) spectroscopy demonstrates strong and broadband emissions with two luminescent peaks at 400 and 420 nm, respectively, which are believed to originate from the selftrapped excitons, oxygen vacancies and surface states, respectively. Nanoscale one-dimensional structures have attracted much interest because of their novel magnetic, electronic and optical properties as a result of their low dimensionality and quantum confinement effect. In addition to the well known examples of carbon nanotubes, ZnO nanobelts and so on, nanowires of complicated perovskite functional oxides such as ferroelectric BaTiO3 and Pb(Zr,Ti)O3, as well as colossal magnetoresistance (CMR) manganites, have been synthesized [1–5]. Among the various methods used for fabrication of nanowires, the template synthesis method has been playing an important role in the fabrication of many kinds of nanowires and nanotubes for their interesting and useful features. Possible templates include nuclear track-etched polycarbonate membranes, nanochannel array glasses, 5 Author to whom any correspondence should be addressed. 0953-8984/05/440467+09$30.00 © 2005 IOP Publishing Ltd Printed in the UK L467 L468 Letter to the Editor mesoporous channel hosts, and self-ordered anodized aluminium oxide (AAO) films. It has been found that the AAO template with diameters ranging from below 10 to 200 nm is stable at high temperature and in organic solvents, and that the pore channels in AAO films are uniform, parallel, and perpendicular to the membrane surface. This makes AAO films ideal templates for the synthesis of oxide nanowire [6]. The CMR manganites with the general formula RE1−x AEx MnO3 (where RE and AE are rare and alkaline earth ions, respectively) have unusual magnetic and electronic properties. This effect is attracting considerable interest from both fundamental and practical points of view. Recently, the fabrication of La0.67Ca0.33MnO3 ordered arrayed nanowires was done and it was found that the reported ferromagnetic transition point Tc is higher than that for single crystals [5]. The growth of La0.67Sr0.33MnO3 oriented nanowires within the pores of AAO templates was also reported [7], although not much characterization of the physical properties was presented. These works allow us to expect significant change of the properties of CMR manganites induced by the low dimensionality of nanowire. Here, it should be mentioned that La1−x SrxMnO3, a material of the large bandwidth subset of manganese oxides, is considered as the most representative double-exchange (DE) system. Here one assumes that the doped holes reside on Mn sites, although some measurements show that the doped holes reside on oxygen sites rather than on Mn sites [8–10]. In order to achieve a large CMR effect, the insulating phase is as important as the metallic phase, and the region of the most interest should be the boundary at which metallic and insulating phases coexist and/or ferromagnetic (FM) and antiferromagnetic (AFM) phases co-appear as the ground state. Thus, it is possible for the metallic percolative transitions to take place under a very low external magnetic field. For La1−x SrxMnO3, it was shown that the CMR effect is maximized at the compositional point separating the insulating from metallic states at low temperature (T ), namely x = 0.175 [10]. This point represents the phase boundary at low T , and it is nowadays well accepted that the ground state at low T is a phase separated state in which the FM state and AFM charge-order state coexist [8], indicating a comparable competition between the two phases as the ground state. This allows us to predict that significant fluctuations of the magnetic property for the materials (x = 0.175) in nanowire form, induced by the low dimensionality, may be possible, and also of interest. Although various forms like ceramics and thin films of La0.825Sr0.175MnO3 (LSMO) were synthesized [10, 11], preparation of LSMO quasi-onedimensional nanowires has not yet been reported. On the other hand, the photoemission performance of CMR manganites has been the subject of intensive research. The k-dependent spectral weight at the Fermi level was observed extensively using the angle-resolved photoemission spectroscopy (ARPES) [12]. However, few reports about the photoluminescence (PL) behaviours of CMR manganites were found, and in particular no data on the photoemission of low-dimensional manganite nanowires are available. In this letter, we report the synthesis of LSMO (x = 0.175) nanowires by the AAO template technique and characterization of the magnetic and PL properties. The AAO templates were prepared by means of anodization and the LSMO nanowires were synthesized by the sol–gel method utilizing the AAO templates. The porous AAO templates were fabricated by anodizing pure aluminium foil (purity 99.5%) in a sulfuric acid solution using a two-step process. Aluminium foil was annealed at 500 ◦C for 4 h to form texture, then degreased in acetone. After 2 h anodization at 27 V in 0.4 M sulfuric acid solution at 0 ◦C, the anodic oxide layer was removed in a mixture of 0.4 M H3PO4 and 0.2 M H2CrO4. The specimen was anodized again for 4 h under the same conditions as step one. Then the Al layer was removed in a saturated CuCl2 solution. The pore diameter was adjusted in 6 wt% H3PO4 solution at 30 ◦C for 30 min, forming a through-hole membrane with pore diameters of about 50 nm. Letter to the Editor L469 The LSMO precursor was prepared by dissolving a stoichiometric ratio of lanthanum nitrate [La(NO3)3·6H2O], manganese acetate [Mn(CH3COO)2·4H2O] and strontium acetate [Sr(CH3COO)2] in water and acetic acid, respectively. About 3 ml of acetyl acetone (C5H8O2) was added to the solution to stabilize the LSMO solution. The concentration of the final solution was adjusted to 0.3 M and the pH value of 4–5. The AAO templates were dipped in the precursor solution for 12 h and then subsequently the surfaces were cleaned. Heating the templates containing the precursor in air at 700 ◦C for 30 min using a thermal annealing furnace was sufficient to obtain the desired perovskite phase of LSMO. This annealing temperature is only half of the sintering temperature for LSMO powders using the solid state reaction route (1450 ◦C). The morphology and structure of the LSMO nanowires were investigated using x-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), and highresolution transmission electron microscopy (HRTEM). The magnetic properties of the LSMO nanowires were characterized by a superconductive quantum interference device (SQUID). At room temperature, the PL spectra of the nanowire sample were measured using a visible– ultraviolet spectrophotometer. Figure 1(a) shows the SEM images of the AAO templates. The diameters of the pores and the thicknesses of the AAO templates are about 50 nm and tens of microns, respectively. The SEM observation of the as-prepared nanowires was done along the cross section and compared with the empty AAO templates. It was evident that the pores of the membrane were almost completely filled with nanowires. Figure 1(b) is the corresponding SEM image of the free-standing LSMO nanowires deposited on a silicon substrate after etching away the alumina using a 4 M NaOH solution. These nanowires have almost identical diameters. The XRD spectrum of the LSMO nanowires after dissolving away the surface alumina is shown in figure 1(c). The reflection peaks are clearly distinguishable, and can be indexed as reflections from the perovskite LSMO structures. The microstructure of the nanowires is further analysed using HRTEM. The HRTEM specimens were prepared by dissolving the AAO templates filled with nanowires in the 4 M NaOH solution, then dispersing the LSMO nanowires in ethyl ethanol by ultrasonic vibration, and finally dropping the solution onto carbon films on copper grids. Figure 2(a) shows the TEM image of a single LSMO nanowire, about 50 nm in diameter and tens of microns in length, in good agreement with the pore diameters of the AAO templates. The perovskite structure was further confirmed by the atomic level image shown in figure 2(b), where the well recognized lattice of 3.8 Å corresponds to the {110} atomic plane of LSMO, matching with the XRD results. The insets in figures 2(a) and (b) reveal the polycrystalline structure nature of the LSMO nanowires. For the SQUID measurement of the magnetic behaviours, we collect the nanowires in powder form after dissolving the AAO templates. However, the nanowires in the samples for magnetic measurement are randomly aligned and the measured data represent an average over all orientations of the nanowires. It is well known that LSMO ceramics offer a FM transition at about 270 K, below which there exists a competition between the FM and AFM states due to the phase separation (PS), while the FM state is dominant and the AFM state is partially restrained [10]. In figure 3(a) is plotted the T dependent magnetization M measured under a magnetic field of 100 Oe in a heating path after the sample was zero-field cooled to 5 K. It is observed that a weak AFM transition takes place at T ∼ 150 K, and subsequently a superparamagnet (SPM) transition characterized by the gradually increasing magnetization appears upon further decreasing of T down to 25 K. This demonstrates that in the LSMO nanowires the AFM state as the ground state is dominant while the FM state is restrained to some extent. Here, what should be mentioned is that the electronic PS is constrained to be at nanometre scale because the coulomb interaction restricts the domains size of the two L470 Letter to the Editor