Growth and physical property study of single nanowire (diameter ~ 45nm) of half doped Manganite

We report here the growth and characterization of functional oxide nanowire of hole doped manganite of La0.5Sr0.5MnO3 (LSMO). We also report four probe electrical resistance measurement of single nanowire of LSMO (diameter ~ 45nm) using FIB fabricated electrodes. The wires were fabricated by hydrothermal method using autoclave at a temperature of 270 oC. The elemental analysis and physical property like electrical resistivity were studied at individual nanowire level. The quantitative determination of Mn valency and elemental mapping of constituent elements was done by using Electron Energy Loss Spectroscopy (EELS) in the Scanning Transmission Electron Microscopy (STEM) mode. We addressed the important issue of whether as a result of size reduction the nanowires can retain the desired composition, structure and physical properties. The nanowires used were found to have a ferromagnetic transition (TC) at around 325 K which is very close to the bulk value of around 330 K found in single crystal of the same composition confirming that the functional behavior is likely to be retained even after size reduction of the nanowires to a diameter of 45 nm. The electrical resistivity shows insulating behavior within the temperature range measured, which is very much similar to the bulk system.


Introduction
Nanowires of functional oxide materials because of their unique one-dimensional like structural characteristics and size effects exhibit many novel physical properties that are different from their bulk counterparts. It had been shown before that down to size range of 40 nm; the ferromagnetic T C is enhanced in nanowires of La 0.67 Ca 0.33 MnO 3 [1]. In past studies nanowires of La 0.67 Ca 0.33 MnO 3 , were grown within templates [1,2]. The enhancement in T C of nanowires corroborates the enhancement in T C of nanoparticles of same materials [3]. Our motivation is to investigate in what extent nanowires of complex oxides such as manganites, retain their functionality and physical properties on size reduction when the wires are grown by hydrothermal method. The 1D nano structures with well-controlled size, phase purity, crystallinity and chemical composition are synthesized by hydrothermal method in this paper. As a result, the retention of the composition, structural and physical properties on size reduction are important issues that need to be established.
Often such structural characterizations are done at average level on an ensemble of nanowires. However, characterizations at the level of a single nanowire using spatially resolved tools are needed. In the present work we have used spatially resolved technique like TEM based EELS to investigate the chemical composition along with other structural and microscopic tools. Importantly we are able to carry out four-probe electrical measurement using FIB grown contacts on a single nanowire of diameter ~ 45nm.
The nanowires of functional oxides can be fabricated by different methods like template-assisted growth along with chemical solution processing [4], laser-assisted vapor-liquid-solid phase growth [5], lithography, solvothermal method [6] and hydrothermal process [7] etc. All the above mentioned methods have their own advantages and disadvantages. We have already reported template assisted growth of functional oxide nanowires by sol-gel synthesis method and magnetic properties of such nanowires [3,8]. We report here template free fabrication of single-crystalline nanowires (~ 50 nm, length ~ 1-10 µm) of hole doped manganite of La 0.5 Sr 0.5 MnO 3 (LSMO) using hydrothermal method. Though this method is suffering from size (diameter) dispersion, it has the advantage of high through put.

Experimental Details:
Functional oxide nanowires of hole doped manganite La 0.5 Sr 0.5 MnO 3 (LSMO) was fabricated by hydrothermal synthesis using autoclave. MnCl 2 , 4H 2 O, La(NO 3 ) 3 ,6H 2 O and Sr(NO 3 ) 2 were used as precursor materials, KOH was served as mineralizer, while KMnO 4 was used as oxidizer. The precursor materials were dissolved in de-ionized water, KOH was added while stirring to adjust the alkalinity of the solution [7]. The initial mole ratios of the input species were 0.6 KMnO 4 :1. which has to be placed in a stainless steel autoclave for higher temperature hydrothermal treatment.
The Teflon vessel was filled till 80% of its volume, and then this vessel was placed in the stainless steel container. The crystallization reaction was performed at 270 o C for 30 hours. After the reaction, the autoclave was cooled and depressurized; the products were washed with de-ionized water and dried in an oven at 120 o C for overnight. A black powder that contains the nanowires was finally To make the four-wire electrical contacts on a single nanowire, we have dispersed the nanowires on SiO 2 substrate. A single nanowire of LSMO of diameter ~ 45 nm was connected to Cr/Au contact pads by interconnectors made of Pt deposited by FIB (FEI -HELIOS 600) using Ga ions at a voltage of 30 keV and beam current of 80 pA. The temperature variation of resistivity of a single nanowire down to liquid helium temperature (5 K) was done by using cryogenic cryocooler model SRDK 305.

Structural and elemental characterization:
A typical collection of nanowires obtained from autoclave is given in Fig. 1. Phase formation and phase purity were checked with X-Ray diffraction measurement shown in Fig. 2. It is the tetragonal structure of the space group I4/mcm and it compares well with bulk values [9] and it matches well

Transmission Electron Microscopy:
The wires were grown of different diameters and lengths ranges from 20-50 nm and 1-10 micron respectively. The TEM image of a single nanowire is shown in Fig. 3(a). The single crystalline nature of the nanowires was confirmed from the diffraction pattern and HRTEM images shown in

Elemental analysis using EELS:
The elemental analysis of these nanowires was done by EELS on different single nanowires repeatedly and estimated the valency of Mn from the calibration curve shown in Fig. 4(a). We have determined Mn valency using the white lines (L 2 , L 3 ionization edges of Mn) and the intensity ratio of L 3 and L 2 lines [11]. The intensities of L 3 and L 2 lines are related to the unoccupied states in the 3d bands. Transition from Mn 2p shell is actually split into two components separated by spin orbit splitting of the ionized 2p core level. Transition from 2p 3/2 to 3d 3/2 3d 5/2 and from 2p 1/2 to 3d 1/2 are L 3 and L 2 lines respectively. Comparing the intensity L 3 /L 2 ratio of Mn of LSMO nanowire and that of Mn with known valency of some compounds, quantitative determination of Mn valency of our sample was evaluated [10]. The intensity ratios of L 3 and L 2 lines of different Mn oxide compounds as a function of their known valency are plotted in Fig. 4(a). This curve serves as the calibration curve from which the valence state of unknown materials can be obtained by using the observed intensity ratios. From the calibration curve, we have estimated the valency of Mn of LSMO nanowire. The Mn valency is of the order of ~ 3.5 and it is very close to its bulk value as shown in the Table 1. Energy Filtered TEM image was taken to check the homogeneity of the elemental distribution in each nanowire. Fig. 4(b)

Magnetic measurements:
The results of magnetic measurements are shown in Fig. 5. It has been observed from the temperature variation of magnetization data that the LSMO nanowires undergo a transition from ferromagnetic (FM) to paramagnetic (PM) phase around 325 K shown in Fig. 5(a). The observed T C was compared with the phase diagram of bulk LSMO [12], shown in Table 1. Field variations of magnetization (M vs. H) were done at 300 K and 5 K in Fig. 5(b). The nanowires show ferromagnetic behavior with large coercivity ~ 645 Oe at 5 K shown in the inset of Fig. 5(b).  [14][15][16]. Around 45 K there is one maximum in the magnetization curve, this behavior is not seen in bulk. We have explained this transition as FM transition as observed in the magnetic entropy change plot [13]. From the magnetic measurements and from our previous magnetocaloric study [13]; we can infer that the hydrothermally grown nanowires show the basic ferromagnetic property as seen in bulk.

Fabrication of contact pads using FIB:
For resistivity measurement Cr/Au contact pads are deposited on SiO 2 substrate by thermal evaporation using hard mask as shown in Fig. 6(a). Using Helios Dual beam system consisting of a FEG source and an ion beam source, we have located one single nanowire of diameter ~ 45nm as shown in Fig. 6(b). The electrical contact pads were deposited by using a Pt source by Focused Electron Beam (FEB) and then these four pads were finally connected to the pre-fabricated gold contact pads by FIB, shown in Fig. 6(c). The separation between two consecutive probes is around ~ 300 nm. The each contact pad has width of ~ 300 nm. is connected to four contact pads by Focused electron beam (FEB) and after that these are connected to gold contact pads by FIB.

Transport measurement on single nanowire:
The resistivity data are plotted in Fig. 7(a) as a function of temperature which were taken at a current of 1 µA.  [14].
We are able to make proper electrical contact and have measured the resistivity of single nanowire of LSMO (x = 0.5). Variation of resistivity as a function of temperature plotted in Fig.   7(a) shows insulating behaviour within the measured temperature range. The behaviour is similar to that seen in the bulk LSMO (x = 0.5) system [14]. The comparison of the resistivity of nanowire compared to the bulk ceramic sample is depicted quantitatively in Fig. 7(b). The bulk sample was prepared by ceramic method which shows quite higher resistivity than the nanowire. The  at 5 K is around 2.7 × 10 -3 -m measured at 1 µA current and the resistivity at highest temperature (310 K) is 4× 10 -5 -m. We have fitted our conductivity data with variable range hopping model ( =  0 exp (-(T 0 /T)) 1/4 ) appropriate for whole temperature ranges ( 10 K -310 K) as shown in inset of Fig.   7(a) [17]. We have found the characteristic temperature (T 0 ) and density of states (DOS) at

Conclusions:
We have demonstrated the fabrication of functional oxide nano wires of Sr doped lanthanum manganite system using autoclave and have used EELS as a tool to study the elemental composition of LSMO nanowires of diameter ~ 45 nm. From the EELS and magnetic measurements it has been observed that the nanowires retain the desired composition as well as basic ferromagnetic property, though H C increases even after the size reduction down to ~ 45 nm. Finally we are able to make four probe contacts to a single nanowire and perform the electrical transport measurements on a single nanowire down to 5 K. The resistivity from 310 K down to 10 K shows variable range hopping.

Acknowledgements:
B. Ghosh wants to thank DST, Govt. of India under UNANST Phase II for financial support. S.
Datta would like to thank DST for financial support. The work has been done in the DST supported Unit for Nano science. S. Chandra and H. Srikanth would like to thank US Department of Energy (Grant no. DE-FG02-07ER46438), for the work at USF.