DOI: 10.1021/cg100755t
Epitaxial Growth by Chemical Solution Deposition of (110) NdNiO3-δ Films with a Sharp Metal-Insulator Transition Annealed under Ambient Oxygen
2010, Vol. 10 4682–4685
Xuebin Zhu,*,† Xianwu Tang,† Bosen Wang,† Yankun Fu,‡ Jianming Dai,† Wenhai Song,† Zhaorong Yang,† Xiaoguang Zhu,† Li Chen,† and Yuping Sun† †
Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China, and ‡College of Science, Shandong University of Science and Technology, Qingdao 266510, People’s Republic of China Received June 7, 2010; Revised Manuscript Received September 13, 2010
ABSTRACT: Epitaxial-stabilized (110) NdNiO3-δ films were first realized successfully by chemical solution deposition under ambient oxygen annealing. Cross-section transmission electronic microscopy observation demonstrates the interface is sharp and coherent due to epitaxial stabilization. The resistance change across the metal-insulator transition, the transition sharpness, as well as the metallic state at 300 K are about 500, 0.14 - ln Ω/K ln K, and 0.03 1/K2, respectively, which are comparable to the values for the films formed by vacuum-based methods, indicating the high-quality of the derived films. The successful achievement of (110) NdNiO3-δ films with a sharp metal-insulator transition will provides an alternative route for preparation of high-quality perovskite nickelate films using ambient oxygen annealing. Chemical solution deposition (CSD) has been widely used to deposit films in various fields.1 However, one of the problems for CSD processing is to obtain epitaxial-stabilized thin films, which have been realized by vacuum-based routes.2 It should be pointed out that the CSD processing is different from the vacuum-based methods.3 In CSD processing, the film is crystallized from amorphous/organic phases, and the crystallization can be thought as a simultaneous occurrence within the whole film thickness. Since the epitaxial-stabilized growth needs the preferred nucleation as well as grain growth near the interface, the simultaneous nucleation and grain growth within the whole film lead to a very rough challenge to prepare epitaxial-stabilized films. It is desirable to investigate the possibility for epitaxial stabilization of films using CSD, which will provide an alternative route for thin films preparation as well as investigation about the relative physiochemical properties.4 Rare-earth nickelates RNiO3 (R, rare-earth) are among the few perovskite oxides showing a very sharp metal-insulator transition (MIT), except for LaNiO3, which is a pure metal.5,6 The crystallographic unit cell of RNiO3 can be visualized as a network of corner sharing NiO6 octahedra forming a threedimensional perovskite structure. At the common apex of two adjacent octahedra sits an oxygen ion making up a Ni-O-Ni bond. The MIT is known to arise because of the closing of the charge transfer gap between the oxygen 2p and the Ni 3d upper Hubbard band. At low temperatures, there is a charge transfer gap between the occupied oxygen 2p band and the unoccupied Ni 3d band. The Fermi level lies in this gap, and hence, the material is insulating. When temperature increases, because of thermal broadening of the bandwidths, it leads to a decrease in the charge transfer gap and eventually, at the MIT temperature, this gap becomes zero, giving rise to a metallic state.7 The sharp MIT in RNiO3 can be used as switches, sensors, or thermochromic coatings to construct smart intelligent devices such as fire alarms and smart radiation devices.8-12 NdNiO3 as a typical RNiO3 compound has been widely investigated, which is orthorhombic, with the room temperature lattice constant of a = 5.3903(6) A˚, b = 5.3787(5) A˚, c = 7.6097(8), and the MIT temperature is about 200 K.13 If a pseudocubic structure is used for NNO, the *To whom correspondence should be addressed. E-mail: xbzhu@ issp.ac.cn. pubs.acs.org/crystal
Published on Web 09/23/2010
lattice constant is about 3.8 A˚. The necessity of high oxygen pressure to crystallize the NdNiO3 phase with a sharp MIT is pointed out.13-15 Although it has been presented that NdNiO3 ceramics can be prepared under ambient pressure annealing, the MIT is marginal due to high oxygen deficiency, which prevents applications based on the sharp MIT.16 As for NdNiO3 films, the preparation is focused on vacuum-based methods and there was no report about CSD processing. It is realized that NdNiO3 films derived by vacuum-based methods show a sharp MIT by optimization of processing parameters, which can be attributed to the epitaxial growth of NdNiO3 films and stabilization of the high oxidation Ni3þ state during the deposition processing.10,11,14,17,18 In order to widen the applications as well as to investigate the relative physicochemical properties, it is desirable to prepare NdNiO3 films with sharp MIT characteristics annealed under ambient oxygen pressure using nonvacuum-based methods such as CSD. In this Communication, epitaxial growth of (110) NdNiO3-δ (NNO) films is first achieved by CSD under ambient oxygen pressure annealing. The results show that the derived films show a very sharp MIT, which will provide an alternative route to prepare NNO films with a sharp MIT using a CSD method under ambient oxygen annealing. Nd nitrate and Ni acetate were dissolved in a mixed solution of 2-methoxyethanol and acetic acid (ratio of 3:2 in volume). The final solution concentration was kept as 0.1 M. The spin-coating processing was used to deposit NNO films on LaAlO3 (LAO, 110) substrates using the spin speed of 4000 rpm and the time of 60 s. The deposited NNO films were baked at 300 °C in air for 30 min in order to expel the organics. For thicker films, the depositing and baking procedures are repeated until the desired number of layers are formed. Finally, the baked films were crystallized at 850 °C under ambient oxygen atmosphere (flowing oxygen atmosphere with 1 atm of pressure). The microstructures were analyzed by high-resolution X-ray diffraction (XRD) using Cu KR radiation, field-emission scanning electron microscopy (FE-SEM), and cross-section transmission electronic microscopy (TEM). The temperature dependence of the resistance, R-T, was measured on a physical properties measurement system (PPMS) using the four-probe method. Figure 1 shows the XRD result for the NNO/LAO (110) film. It is seen that the derived film is highly (110)-oriented without any undesirable phases such as Nd2O3 and NiO. Inset a is the enlarged r 2010 American Chemical Society
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Figure 1. XRD result of the derived NNO/LAO (110) film in log scale. Inset a is the enlarged XRD result in order to show the NNO peak; inset b is the FE-SEM of the derived NNO film.
XRD result, and it is seen that the NNO peak can be observed clearly. The lattice constant calculated from the NNO (220) peak by the Bragg diffraction equation is ∼3.90 A˚ using the pseudocubic structure of NNO, which is similar to previous reports about RNiO3 films on LAO substrates.14,19 The surface morphology determined by FE-SEM as shown in inset b of Figure 1 shows a very dense and uniform microstructure, indicating the relative high-quality. The enhanced lattice constant as compared to that of bulk material should be attributed to two combined reasons: the oxygen deficiency resulting in the lattice expansion16 and the in-plane compressive stress resulting in the lattice compression due to the lattice mismatch between the NNO film and the LAO substrate.14 From Figure 2a, it is shown that the thickness is about 10 nm for the one layer NNO/LAO (110) film, and the film is crystallized in the whole thickness. The high-resolution TEM as shown in Figure 2b gives the coherent interface between the LAO and NNO, and the lattice constant is about 3.9 A˚, which is same as the value from the XRD result. The Nd/Ni ratio from the energy dispersion spectrum (EDS) as shown in Figure 2c is about 1: 1, indicating the stoichiometry of the cations. Moreover, the selected area diffraction pattern (SADP) as shown in Figure 2d shows an overlapped pattern for the NNO and LAO, further suggesting the epitaxial growth of the NNO film. Figure 3 is the temperature dependence resistivity, F-T, of the derived (110)-NNO film. A clear MIT is observed, and the MIT temperature is ∼146 K (in the heating curve), which is similar to that of the previous report,14 further suggesting the successful achievement of stabilized NNO films due to epitaxial growth. The lower TIM as compared to that of the bulk materials should be attributed to two combined reasons. First, it is suggested that there exists oxygen deficiency to some extent, which will lead to the enhanced TIM;8 on the other hand, the larger lattice constant of NNO film as compared to that of the LAO substrate will lead to in-plane compressive stress in the NNO film, which will lead to the decrease of TIM.14 Considering that the TIM of the PLDderived 8-nm-thick NNO/LAO film is about 120 K, it is thought that the 146 K of MIT in this experiment is the combined effects of the in-plane compressive stress and oxygen deficiency. The resistance at 300 K and 10 K is 2.6 10-4 Ω cm and 1.23 Ω cm, respectively, which is also comparable to that of the films by vacuum-based methods.14,20 The resistance change across the MIT, which is defined as the ratio of the resistances at the beginning and end of the hysteresis in the F-T plot, is ∼500, which is remarkable as compared to those of the sol-gel derived NNO ceramics with a marginal MIT. In order to give a further
Figure 2. TEM results of the NNO/LAO (110) film: (a) TEM, giving the thickness as about 10 nm; (b) high-resolution TEM near the interface, with the arrows giving the interface between the film and substrate; (c) EDS result, including both substrate and film; and (d) SADP near the interface.
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Figure 3. F-T result of the NNO/LAO (110) film with heating and cooling processing, with the arrows giving the temperature processes. Inset a is the result of -d(ln R)/dT vs T on the heating curve, giving the transition sharpness, and inset b is the result of 1/R (dR/dT ) at 300 K, giving the quality of the metallic state at 300 K.
estimation of the derived NNO, the transition sharpness (peak value of -d(ln R)/dT vs T on the heating curve) and the quality of the metallic state at 300 K (the normalized resistance slope at 300 K, 1/R (dR/dT) at 300 K) are plotted in insets a and b. Compared with the previous report about the NNO/LAO (100) film prepared by other methods,14 the sharpness and the quality of the metallic state for the CSD-derived NNO/LAO (110) film are also comparable, suggesting the relative high-quality. In order to study the thickness effects on the MIT, NNO/LAO (110) films with two and four layers have been prepared, which is about 20 and 40 nm in thickness, respectively. Figure 4a shows the F-T results for the NNO/LAO (110) films with two and four layers. It is seen that, with increasing thickness, the MIT becomes marginal as compared to that of the one layer NNO film, which is different from the case of NNO films formed by vacuum-based processing in that the MIT becomes more obvious with thickness.14 The different tendency for the CSD and vacuumbased derived NNO films is attributed to different growth mechanisms. For CSD processing, the nucleation and grain growth occurred simultaneously within the whole film thickness and the epitaxial strain due to film/substrate lattice match will be weakened gradually with thickness. On the other hand, in vacuum-based processing such as PLD, the subsequent adsorbed atoms will be crystallized onto the already-crystallized epitaxialgrown layer. The different growth mechanisms lead to different tendencies with thickness for CSD- and PLD-derived NNO films. As shown in Figure 4b and c, it is seen that the films become granular-like, and more small grains are observed with thickness increasing. The appearance of small grains is attributed to the nucleation within the bulk films due to strain relaxation with thickness. The critical thickness for epitaxial growth films is hc = Kbdf/4πMεsds ln(βhc/b), where K is the isotropic energy factor, b is Burgers vector, df is the film lattice constant, ds is the substrate lattice constant, M is the biaxial film modulus, and β is a dimensionless constant (∼4).21 In general, the epitaxial growth will become difficult with thickness.21 Additionally, it is reported that some metastable systems can be stabilized for very small grains.2 Moreover, it has been reported that the NNO ceramics can be obtained using the sol-gel route.16 Combined with the above results, the appearance of small grains for thick NNO films is a compromise result. The appearance of small grains will lead to the flattening of the MIT.15 Before we give the conclusion, we want to point out that the resistance change across the MIT is ∼6 and ∼23 for the films of
Figure 4. F-T results of NNO/LAO (110) films with two and four layers for both heating and cooling processing (a). FE-SEM of the films with two layers (b) and four layers (c).
NNO/LAO (100) and NNO/LAO (111) prepared by the same processing, respectively (not shown here). The reasons are not clear currently, but the critical thickness of epitaxial growth and the anisotropy of oxygen diffusion for different orientations in NNO should play the determinable roles.22 As for the annealing temperature effects, it is observed that 850 °C is the optimized annealing temperature; lower or higher annealing temperature
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will lead to the decrease of MIT sharpness due to lower crystallization and higher oxygen deficiency, respectively. Also, with the annealing oxygen partial pressure decreasing, the MIT will be flattened. In summary, a 10-nm-thick (110)-epitaxial NNO film with a sharp MIT has been successfully obtained under ambient oxygen annealing using the CSD processing method. The resistance change across the MIT, the transition sharpness, and the metallic state at 300 K are comparable to the results of NNO films derived by vacuum-based methods. The results will provide an effective route for preparation of RNiO3 films with a sharp MIT by CSD. Acknowledgment. This work was supported by the National Natural Science Foundation of China under Contract No. 50802096 and the Director’s Fund of the Chinese Academy of Sciences.
References (1) Schwartz, R. W. Chem. Mater. 1997, 9, 2325–2340. (2) Gorbenko, O. Yu.; Samoilenkov, S. V.; Graboy, I. E.; Kaul, A. R. Chem. Mater. 2002, 4, 4026–4043. (3) Zhu, X. B.; Shi, D. Q.; Dou, S. X.; Sun, Y. P.; Li, Q.; Wang, L.; Li, W. X.; Yeoh, W. K.; Zheng, R. K.; Chen, Z. X.; Kong, C. X. Acta Mater. 2010, 58, 4281–4291. (4) Naganuma, H.; Kovacs, A.; Harima, T.; Shima, H.; Okamura, S.; Hirotsu, Y. J. Appl. Phys. 2009, 105, 07D915. (5) Zhou, J. S.; Goodenough, J. B.; Dabrowski, B. Phys. Rev. Lett. 2005, 95, 127204. (6) Zhou, J. S.; Goodenough, J. B.; Dabrowski, B. Phys. Rev. B 2003, 67, 020404(R).
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(7) Garcı´ a-Mu~ noz, J. L.; Rodrı´ guez-Carvajal, J.; Lacorre, P.; Torrance, J. B. Phys. Rev. B 1992, 46, 4414–4425. (8) Conchon, F.; Boulle, A.; Guinebretiere, R.; Dooryhee, E.; Hodeau, J. L.; Girardot, C.; Pignard, S.; Kreisel, J.; Weiss, F.; Lee, T. L. J. Appl. Phys. 2008, 103, 123501. (9) Conchon, F.; Boulle, A.; Girardot, C.; Pignard, S.; Kreisel, J.; Weiss, F.; Dooryhee, E.; Hodeau, J. L. Appl. Phys. Lett. 2007, 91, 192110. (10) Tiwari, A.; Jin, C.; Narayan, J. Appl. Phys. Lett. 2002, 80, 4039– 4041. (11) Ambrosini, A.; Hamet, J. F. Appl. Phys. Lett. 2003, 82, 727–729. (12) Capon, F.; Howeat, D.; Pierson, J. F.; Zaghrioui, M.; Laffez, P. J. Phys. D: Appl. Phys. 2009, 42, 182006. (13) Alonso, J. A.; Mu~ noz, A.; Largeteau, A.; Demazeau, G. J. Phys.: Condens. Matter 2004, 16, S1277–S1281. (14) Catalan, G.; Bowman, R. M.; Gress, J. M. Phys. Rev. B 2000, 62, 7892–7900. (15) Nikulin, I. V.; Novojilov, M. A.; Kaul, A. R.; Mudretsova, S. N.; Kondrashow, S. V. Mater. Res. Bull. 2004, 39, 775–791. (16) Tiwari, A.; Rajeev, K. P. Solid State Commun. 1999, 109, 119–124. (17) Kaur, D.; Jesudasan, J.; Raychaudhuri, P. Solid State Commun. 2005, 136, 369–374. (18) Staub, U.; Meijer, G. I.; Fauth, F.; Allenspach, R.; Bednorz, J. G.; Karpinski, J.; Kazakov, S. M.; Paolasini, L.; d’Acapito, F. Phys. Rev. Lett. 2002, 88, 126402. (19) Novojilov, M. A.; Gorbenko, O. Yu.; Graboy, I. E.; Kaul, A. R.; Zandbergen, H. W.; Babushkina, N. A.; Belova, L. M. Appl. Phys. Lett. 2000, 76, 2041–2043. (20) Goudeau, P.; Laffez, P.; Zaghrioui, M.; Elkaim, E.; Reullo, P. Cryst. Eng. 2002, 51, 317–325. (21) Langhahr, P. A.; Lange, F. F.; Wagner, T.; R€ uhle, M. Acta Mater. 1998, 46, 773–785. (22) Inoue, S.; Kawai, M.; Ichikawa, N.; Kageyam, H.; Paulus, W.; Shimakawa, Y. Nat. Chem. 2010, 2, 213–217.
