J. Cent. South Univ. Technol. (2010) 17: 1144-1147
DOI: 10.1007/s11771-010-0610-5
Low temperature Raman study of PrCoO3 thin films on LaAlO3 (100) substrates grown by pulsed laser deposition
PRAKASH R1, KUMAR S2, LEE C G2, SONG J I1
1. Department of Mechanical Engineering, Changwon National University, Changwon 641-773, Korea;
2. School of Nano and Advanced Materials Engineering, Changwon National University,
Changwon 641-773, Korea
? Central South University Press and Springer-Verlag Berlin Heidelberg 2010
Abstract: Thin films of PrCoO3 were deposited on LaAlO3 substrates by pulsed laser deposition technique. X-ray diffraction result indicates that films are single phase and c-axis textured. To investigate the spin state transition, Raman spectroscopy measurements were performed at different temperatures. The position of the Raman modes is found to increase while full width at half maximum (FWHM) of these modes is found to decrease with the decrease of temperature across spin state transition temperature (220 K) of PrCoO3.
Key words: PrCoO3; thin films; Raman spectroscopy; pulsed laser deposition
1 Introduction
The peroveskite type transition metal oxides materials have been gained much interest in the research activities due to emergence of exotic properties such as charge ordering, orbital ordering, phase separation, and colossal magneto resistance [1]. Cobaltates ACoO3, where A is the rare earth element, form an interesting class of compounds in the perovskite family. These compounds show insulator to metal transition with the increase in temperature [2]. It is believed that such a transition occurs due to the thermally driven spin state transition of Co3+ ions [3-11]. A spin state transition was also proposed for PrCoO3, for which the χ-T (χ denotes magnetic susceptibility; and T denotes temperature) curve exhibits a broad minimum at around 200 K [12]. FIERRO et al [13] reported the reduction study of PrCoO3 using X-ray photo electron spectroscopy and detected two type of oxygen: one was lattice oxygen (binding energy is 528.4 eV), the other was adsorbed oxygen (binding energy is 530.9 eV). PANDEY el al [14] studied the electronic states of PrCoO3 using X-ray photoemission spectroscopy and LDA+U density of states calculation. They found that PrCoO3 was a charge transfer insulator. YOSHII and NAKAMUR [15] studied magnetic behavior and showed that PrCoO3 exhibited no magnetic ordering down to 4.5 K. There are various reports on bulk PrCoO3 properties, but no reports are available on thin films of PrCoO3 while various reports are available on thin films of LaCoO3 compound [16-18]. The thin films of any material have unusual properties compared with the corresponding bulk compound because of its growth condition, deposition induced strain, and method of deposition. SUDHEENDRA et al [19] reported low spin state to intermediate spin state transition of LnCoO3 (Ln=La, Pr and Nd) by variable temperature infrared spectroscopy. They reported spin state transition at 220 K for PrCoO3. The spin state is expected to show considerable lattice distortion because of the strong Jahn-Teller nature of t2g5eg1 configuration. Raman spectroscopy also provides information on local lattice distortions and is a good method for examining the spin state transitions. To study the exotic physical properties of PrCoO3 thin films, the thin films of PrCoO3 were grown on LaAlO3 substrates. LaAlO3 substrates are closely-lattice match ones with PrCoO3. There are various thin film deposition techniques but pulsed laser deposition (PLD) is known to the best technique for deposition of oxide compounds. Therefore, in this work, thin films of PrCoO3 were deposited on LaAlO3 substrates using PLD technique. Low temperature Raman spectroscopy techniques were used to study the spin state transition in PrCoO3 thin films.
2 Experimental
Thin films of PrCoO3 were deposited on LaAlO3 substrates using pulsed laser deposition (PLD) technique (KrF excimer laser, wavelength was 248 nm). The substrate temperature during deposition was kept at 680 ℃ and depositions were done at O2 partial pressure of 40 Pa. Bulk PrCoO3 powder used for target preparation was prepared by combustion method. This pallet for PLD target was obtained by sintering the pallet at 1 200 ℃ for 12 h. The crystal structure and single phase of bulk target were confirmed by X-ray diffraction measurement. The substrate to target distance was kept to be 4 cm and substrate heating was carried out using a heater, and the temperature was controlled with DC power supply. The laser energy density at the target was fixed to 2 J/cm2 and pulse repetition rate was 10 Hz. The deposition was done for 20 min. The thicknesses of the deposited films was about 200 nm as measured by stylus profilometer (Ambios Inc., USA). The crystal structure and phase of deposited thin films were characterized by X-ray diffraction (XRD) measurement. The XRD of the films was carried out using Rigaku diffractometer with Cu Kα radiation. The surface morphology was examined by scanning electron microscope (SEM, Joel model JSM-5600) operating at 20 kV. X-ray photoelectron spectroscopy was employed to get the chemical composition of the thin films. For study of the vibrational properties of these films, laser Raman spectroscopy was employed. Raman spectra were recorded from room temperature to 80 K using micro- Raman spectrometer (Model HR-800, Jobin Yvon) employing He-Ne laser (λ=632.8 nm). The measured resolution of spectrometer was 1 cm-1. Spectra were collected in backscattering geometry using charge- coupled device (CCD) with laser power of 9 mW and incident laser power focused in a diameter of 2 μm. Notch filter was used to suppress the Reighley light.
3 Results and discussion
Fig.1 shows XRD pattern of PrCoO3 thin film deposited on (100) LaAlO3 substrates, which reveals that PrCoO3 film is crystalline and single phase. No impurity peaks are observed other than the substrate peaks. PrCoO3 films on LaAlO3 are oriented to substrate orientation, i.e., along (100) plane. From the XRD pattern, lattice parameter and particle size of the films are calculated. The films have (100), (200) and (300) reflections only. All the observed XRD peaks of PrCoO3 films can be indexed with cubic phase JCPDS No.75-0280. The fitting of highest intensity peak of XRD pattern gives peak position and full width at half maximum (FWHM). These peak positions and FWHM values were used to calculate the lattice parameter and particles size of the films. The lattice parameter estimated from XRD pattern for films deposited on LaAlO3 substrates is 0.378 8 nm, which is very close to
that of bulk PrCoO3 (0.378 0 nm). The grain size (D) of the film is calculated by using Debye-Scherrer formula [20] given by
D=0.94λ/(B cos θ) (1)
where λ is the wavelength of the X-ray source and B2=Γ2-b2 (Γ is FWHM of an individual peak at 2θ; θ is the Bragg angle; and b is instrumental broadening).
Fig.1 XRD pattern of PrCoO3 thin films deposited on LaAlO3 (100) substrates by pulsed laser deposition
The calculated particle size is 38 nm. From XRD data it is clear that films deposited on LaAlO3 have good crystalline quality. After structural characterization, the surface morphologies of deposited film were checked by scanning electron microscope (SEM). Fig.2 shows the SEM image taken for PrCoO3 films deposited on LaAlO3 substrates. It is clear from Fig.2 that the surface of film is smooth and has less particulate in the films. Along with SEM image, spectra of energy dispersive analysis of X-ray (EDAX) were also recorded to get the composition of the film. The observed chemical composition of the film is very close to that of the bulk. The spin state transition has been studied by various methods and no reports are available on Raman spectroscopy across transition temperature of the films. Therefore, Raman spectroscopy measurements were performed across the transition temperature of PrCoO3 thin films. Fig.3 shows the Raman spectra of PrCoO3 film deposited on LAO substrates recorded at temperatures from 80 to 300 K. Raman spectra measured at 300 K shows 9 Raman modes centered at 153, 193, 250, 297, 413, 456, 502, 558 and 651 cm-1. When temperature of measurement decreases to low temperature, one additional Raman mode is observed at 374 cm-1 and a shift towards higher frequency as temperature of the measurement decreases. Raman modes observed at 80 K are centered at 153, 193, 253, 320, 374, 416, 462, 509, 575 and 649 cm-1. It is clear that the maximum change in peak position of Raman spectra is observed in Raman modes that are centered at 297 and 558 cm-1 at 300 K. At around 220 K, which is known to spin state transition temperature for PrCoO3, an abnormal increase in position of Raman mode is observed. This increase in peak position of Raman mode may be due to spin state transition caused by the Jahn-Teller distortions. To further analyze Raman spectra, all the modes of Raman spectra were fitted to get peak positions and FWHM of all the modes. The variation in peak positions with temperature is shown in Fig.4 for five prominent observed modes centered at 297, 413, 456, 502 and 558 cm-1 in Raman spectra recorded at 300 K. The variation of these peak positions with temperature is clearly shown in Fig.4. It is clear that at 220 K, an anomaly is present in all Raman modes and peak position increases below this transition temperature. A similar increase in peak position with the decrease in temperature was also observed by SUDHEENDRA et al [19] in infrared spectroscopy bands of single crystal samples. They interpreted this transition to intermediate spin state to low spin state for PrCoO3.
Fig.2 SEM image of PrCoO3 thin films deposited on LaAlO3 (100) substrates by pulsed laser deposition
Fig.3 Raman spectra of PrCoO3 thin films deposited on LaAlO3 (100) substrates recorded at temperatures from 80 to 300 K
Fig.4 Temperature dependence of Raman mode positions of PrCoO3 thin films across spin state transition
Fig.5 shows the variation of FWHM with temperature of Raman modes corresponding to peak positions shown in Fig.4. From Fig.5 it is clear that the trend of FWHM with temperature across the transition is reverse as in case of peak positions. This trend is also similar to that observed in area of infrared spectroscopy bands by SUDHEENDRA et al [19] for single crystal of PrCoO3. The modes observed at 502, and 558 cm-1 are more intense and the variation in peak position and FWHM of these modes are related to spin state transition of PrCoO3. Further analysis is needed to assign these Raman modes with stretching and bending modes.
4 Conclusions
(1) PrCoO3 thin films on LaAlO3 substrates are deposited by pulsed laser deposition technique.
(2) XRD result indicates that the film is single phase and c-axis textured.
Fig.5 Temperature dependence of FWHM of corresponding Raman modes shown in Fig.4 of PrCoO3 thin films across spin state transition
(3) The surface morphologies of these films indicate that these films are free from particulates and impurity. The peak position of Raman modes increases with the decrease of temperature while FWHM decreases with the decrease of temperature across the spin state transition of PrCoO3 at 220 K.
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(Edited by CHEN Wei-ping)
Foundation item: Project supported by the Second Stage of Brain Korea 21 Project
Received date: 2010-06-29; Accepted date: 2010-09-22
Corresponding author: PRAKASH R, PhD, Research Professor; Tel: +82-55-2133886; E-mail: rpgiuc@gmail.com
