A Novel Approach for Fabricating LaMnO 3 Thin Films Using Combined Microwave Combustion and Pulsed Electron Deposition Techniques

Hanoi University of Mining and Geology, Duc ang, Tu Liem, Hanoi, Vietnam VNU University of Science, Vietnam National University, 334 Nguyen Trai, anh Xuan, Hanoi, Vietnam Quang Tri Teacher Training College, Quang Tri, Vietnam Department of Physics, Faculty of Basic–Fundamental Sciences, Vietnam Maritime University, 484 Lach Tray, Le Chan, Hai Phong, Vietnam Hong Duc University, 565 Quang Trung, Dong Ve, anh Hoa, Vietnam Duy Tan University, 03 Quang Trung, Danang, Vietnam


Introduction
Recently, semiconductor magnetic nanomaterials have received much awareness than that of their same bulk materials, due to their size and surface effects, which exhibit unique properties such as photoluminescence, magnetooptical, electrochemical, and photocatalytic activity [1][2][3][4][5][6][7][8][9][10][11][12][13].ese unique properties of nanomaterials in turn help to solve a lot of urgent problems related to exhaustion of fossil fuels and environment pollution.In this aspect, production and application of clean fuels based on nanomaterials become more and more critical for the development of human society [14][15][16][17].In particular, solid oxide fuel cells (SOFCs) have been extensively studied and attracted much attention as a promising way to generate electricity at high efficiency and low cost [18][19][20].For such applications, the cathode material must satisfy several criteria such as good electrical conductivity, high porosity to allow gas diffusion, thermal expansion coefficient matching well with that of the solid electrolyte, and chemical stability at a high temperature [21][22][23].Most of the requirements listed above are satisfied by LaMnO 3 perovskites, making it one of the most suitable materials for cathode in SOFCs [24].
Among many effective methods developed to produce nanopowders, microwave-assisted synthesis was first developed in 1985 and has quickly become a major area of study.Since microwave radiations were discovered, these have been widely utilized for synthetic chemistry in order to activate chemical reactions more rapidly and homogeneously [25][26][27][28][29].Moreover, a self-combustion reaction assisted with microwave is extremely suitable for preparation of perovskite nanomaterials due to notable advantages such as simple and convenient experimental set-up, extremely time, and energy saving, as well as offering homogeneous product [30][31][32].ese advantages result from a thorough blending among the constituents in an aqueous media under microwave irradiation.Furthermore, an exothermic redox reaction between the fuel and an oxidizer provides a great amount of heat which is necessary for the formation of the perovskite phase [33,34].
In general, thin films can be manufactured by a variety of methods such as sputtering, thermal evaporation, and molecular-beam epitaxy [35][36][37].Pulsed electron deposition (PED) is also a physical thin film deposition technique offering many advantages such as high film homogeneity and low cost.However, this method seems to be not explored for preparation of perovskite thin films even though it is convenient to transfer the target stoichiometry to the deposited films by PED [38].
In this paper, LaMnO 3 (LMO) nanopowder was prepared by microwave combustion where glycine was used as a fuel in a combustion reaction with nitrate salts of lanthanum and manganese.e as-prepared LMO powder was pressed and annealed at 1000 °C for 8 hours to make a target for thin film deposition.e fabrication of LMO thin films by PED was investigated in details.
We first prepared a set of samples with different molar ratios of glycine to the metal ion (F � 2.5, 3, 3.5, 4, and 4.5) to study the effect of fuel on the nanoproduct.After heating on a hot plate at 150 °C, the light pink solution evolved into a colorless transparent one and then viscous brown gel.Gel was stored in a glass container covered with an open lid and then quickly transferred to a microwave oven.After a few seconds, the viscous gel bubbled up and autoignited to produce brown fine powder.
e LMO powder was milled, pressed, and annealed at 1000 °C for 8 hours to make a target for fabrication of the thin film by pulsed electron deposition system PEBS-20 from Neocera, Inc.All the LaMnO 3 films were deposited on silicon substrates with the repetition rate of pulses maintained at 5 Hz, pulse width of 100 ns, and 20 000 pulses.
Two sets of thin film samples were prepared to study the effect of discharge voltage (LMO-1) and O 2 /N 2 flux ratio (LMO-2).e deposition of the LMO-1 films was carried out at room temperature and at five discharge voltages: 11, 12, 13, 14, and 15 kV.e N 2 gas and O 2 gas were introduced at a pressure of 9 × 10 −3 Torr for enhancing the electron beam and stabilizing the beam propagation to the target with a gas flux of 10 sccm and 15 sccm for oxygen and nitrogen, respectively.LMO-2 films were fabricated at 15 kV with various flux ratios of oxygen and nitrogen: 0 : 25; 5 : 20; 10 : 15; 20 : 5; and 25 : 0 sccm.During the deposition process, the pressure was maintained by balancing between the rate of the turbopump and the flow rate of oxygen and nitrogen gas introduced into the chamber.e as-deposited thin films were annealed at 400, 600, and 800 °C for 2 h.e crystallinity of the thin films was characterized by the X-ray diffraction system (Siemens D5005, Bruker, Germany) with Cu K α1 (λ � 0.154056 nm) radiation.e morphology of the product was investigated by a scanning electron microscope (Nova Nano SEM 450).e composition of the samples was verified by energy dispersive X-ray (EDX) spectrometry (Oxford Isis 300) integrated into the JEOL-JSM 5410 scanning electron microscope.

Results and Discussion
Figure 1 shows XRD patterns of LaMnO 3 nanoparticles prepared by the microwave irradiation method when using glycine as fuel in the combustion reaction with the glycinenitrate molar ratio (G/N) (F � 2.5, 3, 3.5, 4, and 4.5).
e sample prepared with F � 2.5 is amorphous because the released energy is not enough for formation of the perovskite phase.With F � 3, 3.5, 4, and 4.5, the LaMnO 3 perovskite phase has formed.Sharp and intense XRD peaks of the samples with F � 3 suggested that crystal quality of those samples is better.e combustion reaction with a higher value of F liberated more heat.However, gas generated during the reaction can bring out heat from the reaction chamber and reduced the heat used for synthesis of the perovskite material [39].Heat generated by the reaction might be reduced too much and resulted in an amorphous state of samples with F greater than 4. XRD patterns indicate that LaMnO 3 nanoparticles, prepared with F � 3; 3.5; and 4, crystallized in a hexagonal structure.Lattice parameters and crystalline size of the samples prepared with different ratios of F are shown in Table 1.
XRD analysis of the LaMnO 3 nanoparticles prepared by combustion assisted with microwave irradiation also shows the presence of both LaMnO 3 and La 2 CO 5 .La 2 CO 5 is an unwanted phase due to the unavoidable reaction of metal ions with organic precursors.
A pure perovskite is necessary for making cathode of good performance fuel cell.erefore, such secondary phases need to be removed.As an unsolvable material, La 2 CO 5 cannot be washed away using water, so the nanopowder was annealed at 1000 °C to convert the remaining La 2 CO 5 into LaMnO 3 .Figure 2 shows the XRD patterns of LaMnO 3 samples with a molar ratio of glycine to metal ion of 3 and 3.5 before and after annealing at 1000 °C in 8 h.After annealing, no peak related to La 2 CO 5 can be seen in the XRD pattern.
e result implies the purity of the nal nanopowder.Sharper and stronger di raction peaks show that crystalline sizes of the LaMnO 3 powder increase clearly after annealing, as expected due to the crystal growth during the annealing process at a high temperature.It should be also noted that, after annealing, a clear peak shift to a higher angle was observed.e reason for lattice expansion in the sample should be taken into account.One possible explanation for lattice expansion in the sample before annealing is small particles size.In other words, in nanomaterials of poor structural order, defect might be introduced at a high concentration, which results in an internal stress in the lattice due to the presence of large La 3+ at interstitial sites in the lattice.e internal stress is responsible for the lattice expansion, as observed for as-prepared nanopowder samples.e corresponding decrease of the lattice parameter is shown in Table 2.
However, the results show that annealing the samples at 1000 °C does not result in transition of the phase structure.Such peak shift to a higher angle, combining with the disappearance of some peaks in the XRD pattern, can be an indication of a phase transition.e change in the lattice parameter suggests that annealing phase transition from hexagonal to other structures can occur at a higher temperature or in longer time.However, this is not being mentioned in this paper but would be reported in another study.
e energy dispersive X-ray spectra (Figure 3) of the LaMnO 3 nanopowder with F 3 before and after annealing at 1000 °C in 8 hours only show the peaks of La, O, and Mn. is result indicates that samples are clean and pure.LaMnO 3 nanopowder prepared with a molar ratio of glycine/nitrate (F 3) was used to make a target because the results showed that better crystallinity was achieved for this sample.e as-prepared LMO powder was remilled, pressed at high pressure, and annealed at 1000 °C for 8 hours to make a target.e structure and phase purity of the LMO target was examined by XRD measurements, as shown in Figure 4.
e XRD patterns reveal that the structure of the LMO target is still hexagonal.
Even though the LMO target can be fabricated by the normal solid-state reaction method from oxides precursors, using nanopowder of LMO to make the target help in reducing the treating temperature to 1000 °C, the target prepared by the solid-state reaction normally required a much higher annealing temperature (1300-1400 °C) and longer time (12-24 h).
Figure 5 shows XRD patterns of the LaMnO 3 thin lms deposited at 15 kV before and after annealing at di erent temperatures.It can be seen that the as-deposited lms and lms annealed at 400 °C and 600 °C were amorphous because no di raction peak is observed.As annealing temperature is   raised up to 800 °C, the thin lm is crystallined in the hexagonal structure.Figure 6 shows SEM images of LMO thin lms deposited at 15 kV before and after annealing at di erent temperatures.
SEM images of thin lms deposited at 15 kV show that the particle size distribution is quite uniform and postannealing does not change the morphology of the produced thin lms.e lm is crack free both before and after being annealed.SEM images also reveal that the lms have uniform particulates which are uniformly distributed on the surface.XRD analysis (Figure 7) shows that all thin lms are amorphous for all discharge voltage.Postannealing at a temperature higher than 800 °C is required to obtain crystalline lms.However, transition from the amorphous to hexagonal structure occurs only in lms deposited at 14 and 15 kV.Other lms prepared at voltage lower than 14 kV remains in the amorphous state despite postannealing.e di raction peaks were higher at 15 kV which is the maximum voltage available for the used PED system.e above results can be understood that discharge voltage is a critical factor determining the growth of thin lms by PED.Voltage higher than 14 kV is required to preserve the    Journal of Chemistry stoichiometry composition of the target to the thin lms.However, the energy transferred by the electron beam is not enough for the atom to arrange on the substrate in a crystalline structure.Hence, postannealing is necessary to activate the crystallization of the thin lms.Lattice parameters of the lms prepared at 14 and 15 kV after annealing are shown in Table 3.
In vacuum deposition techniques, gas ambient may also have a great in uence on the structure and morphology of the thin lms [40].We also studied the e ect of the N 2 /O 2 ux ratio on the LMO thin lm prepared by pulse electron deposition.Figure 8 shows XRD patterns of the LaMnO 3 thin lms deposited at 15 kV and 5 Hz with di erent ratios of N 2 /O 2 ux (25 : 0, 20 : 5, 10 : 15, and 0 : 25 sccm).
It is clear that the lms after postannealing are well crystallined at all di erent ratios of N 2 /O 2 ux.e highest intensity was achieved for lm grown at N 2 /O 2 ux (10 : 15 sccm), indicating that partial pressure of oxygen also has a slight e ect on the structure of the thin lms.
Figure 9 shows SEM images of the LMO-2 lms, which were prepared at a di erent oxygen partial pressure.While the sample prepared in pure nitrogen ambient possesses ne particulates, the samples prepared in oxygen show more particulates.is could be due to the fragmentation of the oxide particulates in the plasma region at higher oxygen partial pressures [41].

Conclusion
LaMnO 3 nanopowder was successfully synthesized using combustion assisted with microwave irradiation approach.Using nanopowder helps to lower the temperature of heat treatment of the target.Detailed investigation of LaMnO 3 thin films preparation shows that discharge voltage is a critical parameter for film deposition by PED. e obtained films were amorphous but could be converted into the crystalline phase after annealing at relatively a low temperature of 800 °C.Suitable ratio of oxygen/nitrogen flux also contributes to better crystal quality of the thin films.e results demonstrate the potential of using PED as a tool for fabrication of perovskite thin films of high quality for various applications in electronic fields.

3 Figure 2 :
Figure 2: XRD patterns of LaMnO 3 samples with F 3 and 3.5 before and after annealing at 1000 °C in 8 h.

Figure 3 :
Figure 3: EDS spectra of the LaMnO 3 nanopowder with F 3 (a) before and (b) after annealing at 1000 °C.

Figure 4 :Figure 5 :
Figure 4: XRD pattern of the LaMnO 3 target made of LaMnO 3 nanoparticles prepared by self-combustion assisted with microwave irradiation.

Table 1 :
e lattice parameters and crystallite size of the LaMnO 3 samples with di erent ratios of glycine/nitrate molar (F 3, 3.5, and 4).

Table 2 :
e lattice parameter of the LaMnO 3 samples with di erent ratios of glycine/nitrate molar (F 3 and 3.5) before and after annealing at 1000 °C for 8 h.

Table 3 :
e lattice parameters of the LaMnO 3 thin lms deposited at 14 and 15 kV after annealing at 800 °C for 2 h.