A06: Electron Microscopy on Functional Materials and Electron Crystallographic Image Processing

Members:Li Jian-qi,Yang Huai-xin, Li Fang-hua, Wang Yu-mei ,Ma Chao

Research Field: Electron microscopy on functional materials and electron crystallographic image processing

Various kinds of functional materials, such as high-Tc superconductors, CMR manganites, magnetic materials and quantum devices, show complex microstructural features and specific physical properties. Theoretical and experimental investigations revealed that the fundamental properties of these materials couldn’t be well interpreted by Landau Fermi theory which is successfully used for understanding the physical properties of conventional solid materials. Experimental results on these kinds of materials suggest that the local structural features are critical factors for understanding the extraordinary properties, e.g. electronic phase separation, charge ordering, and cooperative JT effects. In our recent works, we have focused on the experimental investigations of local microstructure features and their effects on physical properties.

The electron crystallographic image processing and its applications are the significant research fields in our lab and meaningful for atomic structure investigation. Firstly, this analysis technique, especially for high-resolution electron microscopy, will be developed so that the resolution of structure images obtained with relatively cheap medium-voltage field-emission electron microscopes will be comparable with those obtained with the very expensive high-voltage high-resolution electron microscopes. Secondly, the crystal structures of some minute crystals and crystal defects with atomic resolution, which can hardly be determined with other methods, can be obtained. This is essential to understand the relationship among the property, structure and technology of some functional materials, to improve the quality of materials and develop new devices.

Electron energy-loss spectroscopy and electronic structure of strongly correlated systems.

Electron energy-loss spectroscopy (EELS), which contains a wealth of information on local bonding, electronic structure, and optical property, is a powerful tool to improve our knowledge of materials. The recently developed TEM instruments implemented with a monochromator especially broaden the application of EELS, which now can record high-resolution EELS (HREELS) data with an energy resolution of ~0.1eV, allowing to explore the finer electronic structures of various materials with a spatial resolution in the sub-nanometer range. The density functional theory (DFT) calculations have been proven to be effective theoretical methods to simulate and interpret the experimental EELS. Combining the experimental EELS with the theoretical DFT calculations, we can perform the simulation of experimental data and give good explanation for the spectrum, which will be great helpful to understanding the novel properties in materials.

The study of strongly correlated systems is the foremost area of the contemporary condensed-matter physics. In this kind of materials, such as high-temperature superconductors and colossal magnetoresistance manganites, the electron correlation strength (U) is a key parameter for understanding their characteristic electronic structures. We performed the measurements of high-resolution electron energy-loss spectroscopy (HREELS) on the charge-ordered insulator Na0.5CoO2, which reveals notable spectral structures. The splitting of the first peak in the oxygen K-edge yields two clear sub-peaks with ~1eV apart. Calculations by density functional theory plus Hubbard U (DFT+U) demonstrate that these spectral features are essentially in connection with the presence of electron correlation in addition to the crystal-field effect in Na0.5CoO2. We find that U≈3.0eV provides the best explanation of the experimental spectra for Na0.5CoO2. The electronic structures for this correlation strength give good interpretations for the physical properties observed in this charge-ordered insulator.

(a) Experimental HREELS for O K edge in Na0.5CoO2. The spectrum obtained without monochromator is shown for comparison. (b) Simulated spectra for O K edge with 0≤U≤5.0eV, illustrating the fine structures of peak a. The experimental spectrum obtained with monochromator is shown for comparison.

1.C. Ma, R. J. Xiao, H. X. Geng, H. X. Yang, H. F. Tian, G. C. Che and J. Q. Li*, Investigation of Hole States Near Fermi Level in Nb1-xMgxB2 by Electron Energy-loss Spectroscopy and First-Principles Calculations, Ultramicroscopy, (in press)

2.R. J. Xiao, H. X. Yang and J. Q. Li*, Influence of water intercalation on the electronic structure of the hydrated Na0.3CoO2yH2O using a local spin density approximation, Phys. Rev. B 73 (2006) 092517.

3.C. Ma, R. J. Xiao, H. X. Yang, Z. A. Li, H. R. Zhang, C. Y. Liang, and J. Q. Li*, Electronic structure of the quasi-one-dimensional β-Na0.33V2O5, Solid state communication, 138 (2006) 563。

4.R. J. Xiao, K.Q. Li, H. X. Yang, G.C. Che, H.R. Zhang, C. Ma, Z.X. Zhao, and J.Q. Li*, Correlations among superconductivity, structural instability, and band filling in Nb1-xB2 at the critical point x0.2, Phys. Rev. B 73 (2006), 224516.

5.H.X. Yang, J.Q. Li*, R.J. Xiao, Y.G. Shi and H.R Zhang，Electron energy loss spectra of Na0.33CoO2yH2O (y = 0, 0.6 and 1.3), Phys. Rev. B, 72 (2005), 075106.

Electron Holography

Electron holography is an interference technique which allows the phase change of the object wave to be determined. It is increasingly being employed as a technique for the measurement of electrostatic and magnetic fields in a variety of samples.

Off-axis electron holography uses a biprism to interfere the object wave (electron wave that has passed through the sample) with a reference wave which has passed through vacuum; the overlapping waves create an interference pattern of parallel fringes. These fringes (hologram) are changed in position and contrast, depending upon how the specimen affects the electron beam. The pattern is recorded onto a digital CCD camera system and then processed to yield separate amplitude and phase images. The resulting of interfere can be used to measure the phase shift of the electron wave which is sensitive to the electrostatic potential. If the sample is free from magnetic elds the phase difference between the object wave and the reference wave is proportional to the electrostatic in the specimen projected in the electron beam and given by

Where x, y, is the coordinates in the projected plane, z is the coordinate perpendicular to the plane and CE is a constant depending on the microscopy accelerating voltage [CE=7.295×10-3rad/ (V nm) for 200 keV electron].

Schematic diagram showing the formation of an off-axis hologram

Recently, our electron holography study mainly focus on the electric potentials distributions at the manganite-based heterojunctions, composed of p-n junction(La0.9Sr0.1MnO3 and 0.01 wt% Nb-doped SrTiO3) and n-n junction( La0.7Ce0.3MnO3 and 0.5 wt% Nb-doped SrTiO3), In situ cooling experimental measurement had been applied on n-n junctions.

Electron holography had also been used to detect the charge distribution due to the ferroelectric polarization of epitaxial ferroelectric thin films. The Ba0.5Sr0.5TiO3 thin films on the LaAlO3 substrates had been investigated in detail.

Annealing effects on the microstructure and magnetoresistance of magnetic tunnel junctions with MgO-barrier had been investigated by electron holography, an obvious change can be viewed before and after annealing, which is agreement with the data of the I-V characteristics. These results suggest that the microstructure of the MTJs play an important role for the coherent tunneling processes that give rise to the large tunnel magnetoresistance.

Example: Measurement of the electrostatic potential in manganese -based heterojunctions

Since the discovery of colossal magnetoresistance in the perovskite manganese oxides, there has been enormous interest in these kinds of materials and related devices. The interfacial oxygen diffusion during film growth often results in the appearance of a thin SiOx layer in SrTiO3/Si films and related heterojunctions. the interfacial oxygen diffusion also has obvious influence on the charge density (electrons or holes) in the heterojunction.

Reference:

1.H. F. Tian, H.C. Yu, X.H. Zhu, Y.G. Wang, D.N. Zheng, H.X. Yang and J.Q. Li, Off-axis electron holography and microstructure of Ba0.5Sr0.5TiO3 thin films on LaAlO3, Phys. Rev. B 71 (2005)115419

2.H. F. Tian, J. R. Sun, H.B. Lü, K.J. Jin, H.X. Yang, H.C. Yu and J. Q. Li, Electrostatic potential in manganite-based heterojunctions by electron holography, Appl. Phys. Lett. 87 (2005) 164102

3.H. F. Tian, H. X. Yang, H. R. Zhang, Y. Li, H. B. Lu, and J. Q. Li, “The interface of epitaxial La0.9Sr0.1MnO3/SrTiO3 on silicon characterized by transmission electron microscopy, electron energy loss spectroscopy and electron holography”, Phys. Rev. B 73 (2006) 075325

4.H. F. Tian, H. X. Yang, J. Q. Li, Xiaoyong Liu, W. F. Shen, G. Xiao, Annealing effects on the microstructure and magnetoresistance of magnetic tunnel junctions with MgO-barrier, submitted

Multislice Least Squares (MSLS) Procedure for Crystal Structure Refinement

MSLS is a structure refinement method using quantitative electron diffraction, in which multi-slice calculation is combined with least-squares refinement software. With multi-slice algorithm, which is routinely used for image calculations of HREM images, dynamic diffraction is explicitly taken into account, while least-squares software is often used in x-ray crystallography. In this procedure, electron diffraction patterns should be obtained by using an incident beam with small convergence and spot size, the diffraction spots therefore can be sharp and the variation of the crystal thickness and orientation can be small. The integrated intensities of the experimental reflections are then compared with dynamical simulations using the multi-slice algorithm and a possible unit cell as starting structure model. Different parameters in this cell can then be optimized by the least-squares procedure in order to improve the match between the model and the experimental data, much in the same way as is done in X-ray data analysis. Apart from the parameters related to the crystal structure, certain critical parameters, such as crystal thickness and crystal orientation, can also be refined.

Presently, we have used this method to determine the structural changes in association with phase transitions in functional materials, particularly in strongly correlated systems such as charge ordering in colossal magnetoresistance materials. The MSLS method can give us a quantitative data about local? structural modifications in the atomic scale, these results are essentially important for the understanding of the interesting physical phenomena observed in these systems.

Example: Structural modulation and electronic structural features in the charge ordered state of La0.5Sr1.5MnO4

The [001] and [1ī0]zone-axis electron diffraction patterns of La0.5Sr1.5MnO4 obtained respectively at 300K and 100K, and the schematic structural models of La0.5Sr1.5MnO4 projected on the bs-cs plane.

Phase Transition, Charge/Orbital Order, and Physical Properties of Functional Materials

The fundamental properties of the solid state depend strongly upon structural features of materials. For instances, the local structural distortions in the high-Tc La(Sr)2CuO4 superconductors could result in evident change on Tc, and the presence of interface-dislocations could yield notable different properties in semi-conducting heterojunctions. In every case, understanding of their physical properties can only be achieved if the structure and local electronic features are well understood on rang from the atomic to the macroscopic scale. In microstructure physics, transmission electron microscopy (TEM) is a powerful technique for structural characterization of materials.

At present, we mainly focus on the structures, physical properties and their correlations in functional materials, in particular, the structural modulations, phase transitions, ferroelectricity, charge/orbital orders in colossal magnetoresistance materials, this kind of materials in general belong to strongly correlated systems. Our TEM are equipped with a variety kind of holders for in situ TEM observation, including a double-tilt helium low temperature holder (down to 20K), a double-tilt high temperature TEM holder (up to 1100K), and a special double-tilt TEM holder for applying electric field.

In past years, we have been constantly working on structural properties of the perovskite manganites. Our recent work also involves cation, charge and orbital ordering in layered cobalt oxides, e.g. charge/orbital orders in Na0.5CoO2; charge-stripe order in the electronic ferroelectric LuFe2O4 and layered structure in LaOFeP and NaxCoO2.yH2O superconductors.

Example: How electron stripes yield ferroelectricity

A novel type of ferroelectricity, called “electronic ferroelectricity” with an essential connection to charge ordering , is strongly suggested by new micrographic evidence. Conventional theory of solids holds that ferroelectricity in general originates from atomic structural polarizations – a familiar example is the notable off-center shift in the perovskite BaTiO3. Yet our new work appearing in Physical Review Letter reveals that ferroelectric LuFe2O4 has a curious ground state distinguished by electron stripes. We discovered that these electron stripes manifest a frustrated charge density wave with a remarkable ferroelectric polarization. This 3-dimensional charge ordering state, occurring at a low temperature of about 20K, was directly revealed for the first time by our in-situ transmission electron microscopy (TEM). A remarkable series of richly varied structural phenomena were also recorded as we lowered the temperature from 300K to 20K. The clear micrographic results have enabled us to detect new details about spontaneous polarization.

fig.1 (a) Electron diffraction image showing the weak satellite spots from charge stripe order. (b) Model for charge stripes and ferroelectric polarization.

1.Y. Zhang, H. X. Yang , C. Ma, H. F. Tian & J. Q. Li * Charge-stripe order in the electronic ferroelectric LuFe2O4, Phys. Rev. Lett. 98 (2007) 247602.

Selected publications:

1.Y. Zhang, H. X. Yang , C. Ma, H. F. Tian & J. Q. Li * Charge-stripe order in the electronic ferroelectric LuFe2O4, Phys. Rev. Lett. 98 (2007) 247602.

2.H.X. Yang, R.J. Xiao, C. Ma, H.T. Tian, D.Wu and J.Q. Li*, Charge and orbital ordering in Na0.5CoO2, Solid state communication 142 (2007) 718–722.

3.Z.A. Li, H.X. Yang, H.F. Tian, J.Q. Li, J.R.Cheng, J.G. Chen, Transmission electron microscopy study of multiferroic, (Bi1-xLax) FeO3 --PbTiO3 with x=0.1, 0.2 and 0.3, Appl. Phys. Lett. 90 (2007)182904.

4.C .Y. Liang, R. C. Che, H. X. Yang, H. F. Tian, R. J. Xiao, J. B. Lu, R. Li, J. Q. Li, Synthesis and structural characterization of LaOFeP superconductors, Supercond. Sci. Technol. 20 (2007) 687–690.

5.H.X. Yang, Y.G. Shi, X. Liu, R.J. Xiao, H.F. Tian and J.Q. Li, Structural properties and cation ordering in layered hexagonal CaxCoO2, Phys. Rev. B 73 (2006) 014109.

6.H.X. Yang, C.J. Nie, Y.G. Shi, H.C. Yu, S. Ding, Y.L. Liu, D. Wu, N.L. Wang, J.Q. Li, Structural phase transitions and sodium ordering in Na0.5CoO2: combined electron diffraction and Raman spectroscopy study, Solid state communication, 134 (2005) 403.

7.Y.G.. Shi, C. Dong, J.Q. Li, Structure and superconductivity in NaxCoO2.yH2O, Supercond. Sci. Technol, 17 (2004) 42.

8.H.C. Yu, Y.G. Shi, G.C. Che, L.B. Liu, Y.H. Liu, J.Q. Li, Z.X. Zhao, Superconductivity and microstructure of YBaSrCu3-xFexOy system, Physica C-Superconductivity and Its Applications 411 (2004) 94.

9.J. Q. Li , Y. Matsui, S. K. Park and Y. Tokura, Charge ordered states in La1-x SrxFeO3, Phys. Rev. Lett. 79 (1997) 297.

10.J.Q. Li, M. Uehara, C. Tsuruta, Y. Matsui and Z.X. Zhao, Direct observation of small-polaron ordering in manganites, Phys. Rev. Lett. 82 (1999) 2386.