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TEM - Transmission Electron Microscopy
research, projects and collaborations
Chiral TEM
The CME is part of the CHIRALTEM project.Dichroism is the property of certain materials whose photon absorption spectrum depends on the polarisation of the incident radiation. In the case of X-Ray Magnetic Circular Dichroism (XMCD) the absorption cross section of a ferromagnet or a paramagnet in a magnetic field changes when the helicity of a circularly polarised probing photon is reversed relative to the magnetisation. Although the similarities between X-ray absorption (XAS) and electron energy loss spectra (EELS) in the transmission electron microscope (TEM) have long been recognised, it was presumed that extending such equivalence to circular dichroism would require a beam of spin polarised electrons. Recently, it was argued on theoretical grounds that this is probably wrong [1]. Within the CHIRALTEM project it has been demonstrated the first direct experimental proof of magnetic circular dichroism in the TEM by comparing Electron Energy Loss Magnetic Chiral Dichroism (EMCD) with XMCD spectra from the same specimen together with theoretical calculations [2]. The experiment shows that chiral atomic transitions in a specimen are accessible with inelastic electron scattering under particular scattering conditions. This result bears dramatic consequences for the study of magnetism on the nm and sub-nm scale, as EMCD offers the potential of spatial resolutions down to the nanometre scale and provides depth information, overcoming the major limitations of X-ray methods.
[1] C.Hebert
,P.Schattschneider Ultramicroscopy 96 (2003), p.
463
[2] P. Schattschneider, S. Rubino, C. Hébert, J. Rusz, J. Kune, P. Novák, E. Carlino, M. Fabrizioli, G. Panaccione, G. Rossi.
Experimental proof of circular magnetic dichroism in the electron microscope
Nature 441, pp. 486-488 (2006)
Figure 2 EMCD. Measured Fe L2,3 edges for 10 nm Fe on GaAs (001) in the two configurations shown in figure 1. The difference (magnified by a factor of 5 in the figure) is 0.07 for the measured spectra and 0.32 for the simulations. The r.m.s of the noise is ±8% of the difference. The experiment was performed at the Technical University of Vienna by the group of P. Schattschneider.
Application to Italian and European funds for research
The CEM facility is a national resource for INFM
research programs in semiconductor physics (FIRB collaborative program,
72k€), magnetic materials (FIRB collaborative
program90k€), carbon nanotubes, nanowires and nanocontacts
(FIRB collaborative program, 100 k€).
CEM presented at the end of 2002 to MIUR a project in collaboration
with the University of Trieste and the École Centrale du
Paris to study the ultimate performances of the electron energy loss
spectr oscopy (EELS) at the highest spatial resolution (520
k€). This project would hopefully allow to one of the most
important international scientist in the field of EELS to join CEM in
2003.
CEM is a core partner for the INFM-PRA 2003 named NANOBIO
(60k€);
CEM is a core partner in an approved INFM-PAIS project named HINCANA
(45k€)
CEM is a partner in a European project proposal in the framework of
MEDEA action of ST microelectronics (160k€);
CEM is part of the European network of excellence named TEMNET;
CEM is part of a project approved by the Austrian Science Fund to study
dichroic effects during TEM experiments;
CEM is part of a NEST project presented in the VI EU WP to study
dichroic effects during TEM experiments;
CEM has been invited by the Polish Academy of Science to be part of the
“Centre of Excellence in nano and micro scale
characterization and development of advanced materials”
(NAMAM) already approved in the VI EU WP.
TEM methodologies
Quantitative atomic resolution z-contrast in STEM (also known as HAADF imaging).
HAADF imaging is a field of large interest in which the European community has some delay with respect to USA or Japan. In particular, CME is the first laboratory in Italy to achieve a resolution of 0.126 nm with this approach. It has also demonstrated how HAADF can be used to gauge the Si distribution in GaAs on the atomic scale even without any image simulation (Fig. 1).
FIGURE 1 - (a) High-resolution z-contrast HAADF image in [110] zone axis of a Si quantum well. (b) Intensity profile (a. u.) as measured on the image in Fig. 2(a)
On the side of the simulation of the results, software with multislice approach and the frozen phonon approximation is being developed starting from the codes of E. J. Kirkland. The limit of this calculation is in the huge CPU time necessary to get reliable simulation. CEM has also developed a procedure to reduce the calculation time by means of parallel computing. By comparison of the experimental and simulated images it has been possible to add a quantitative value to the chemical information. The quantitative agreement with STM measurement is embarassing...
The coupling of STEM HAADF imaging with high spatial resolution energy dispersive x-ray spectroscopy allows further possibilities in studying materials as demonstrated [1], [2]; in the case of the GaAs/ZnSe heterostructure (Fig. 2) where the interface chemistry and structure of low defect density epilayer was addressed.
FIGURE 2 - Atomic resolution HAADF image, in [001] zone axis, of the GaAs/ZnSe interface along with the image intensity profile as measured along the region marked in the image. The sensitivity of the HAADF to the atomic number and the geometry of the experiment permit to distinguish the cation/anion atomic columns with a spatial resolution of 0.16 nm. The coupling between the HAADF result and the EDS spectra allows one to address the composition of the interface layer.
[1] - A. Colli, E. Carlino, E. Pelucchi, V. Grillo and A. Franciosi
Local interface composition and native stacking fault density in ZnSe/GaAs (001) heterostructure
Journal of Applied Physics, 96 (5) 2004, p. 2592
[2] - E. Carlino, D. Furlanetto, A. Colli and A. Franciosi
High spatial resolution TEM studies of GaAs/ZnSe interfaces grown by different MBE procedures
Inst. Phys. Conf. Ser. 180, pp. 183-186 (2003); in Mic. of Semicond. Materisls 2003; A.G. Cullis and P.A. Midgley, Eds.
It is worthwhile to remark how tailored experiments to explore the ultimate performance of TEM can be addressed thank to the synergy with the growth facility at TASC laboratory and, on the other side, how new materials can be tailored thank to the knowledge gathered by TEM.
In situ microscopy (collaboration within TASC). The aim is to measure the properties of nanotubes or nanowires stressed by STM tip or by micro electromechanical systems (MEMs) while viewing by TEM. This project is part of a collaborative project FIRB approved in 2002 by MIUR. To this aim a dedicated specimen holder is under construction in the technical support group of the TASC laboratory. The TEM specimen holder will give the possibility to drive an STM or MEMs and also to measure conductivity of nanostructured materials under stress. The structural modification of the materials can be studied in the meanwhile by TEM. This project is a good example of how new TEM experiments can be realised thanks to the know-how at TASC laboratory.
Methodological studies: Picometer range resolution by coherent diffraction imaging in transmission electron microscopy
The in-depth study of nanoscale matter represents a
burning issue in modern materials science. Along with development of
tools enabling programmable material fabrication with nanometer-level
compositional and geometric precision, the unfolding of the local
structure of nanomaterials with atomic resolution is increasingly
emerging as a fundamental transition pathway towards control of their
unique size-dependent properties and realization of their technological
potential.
Coherent diffractive imaging (CDI) relies on recording the coherent
diffraction pattern of an isolated object at a sampling frequency
smaller than the reciprocal of its size (Nyquist sampling), on the
basis of which the image of the object is deduced by computational
phase retrieval, instead of using a lens to back-transform the
diffraction pattern. A particular CDI technique that can be performed
in a transmission electron microscope (TEM) experiment is the electron
diffractive imaging (EDI), whereby some of the information missing in
the electron diffraction (ED) pattern, relative to the target object,
can be deduced from its corresponding phase-contrast high-resolution
TEM (HRTEM) image. The latter can be used as input data in the EDI
experiment, from which a final phase-retrieved picture of the specimen
can be eventually extracted with a resolution higher than that
characterizing the original HRTEM image.
Herein, we demonstrate an EDI methodology, performed in a Jeol 2010F
UHR microscope (spherical aberration coefficient Cs = 0.47±0.01
mm), by which the crystal structure of transition-metal oxide
nanocrystals can be determined at 70 pm of resolution while
unambiguously revealing the presence and location of light elements
atomic columns in the relevant lattice. This approach, applied as a
case study to TiO2 in the form of organic-capped nano-rods, also allows
appreciating subtle alterations in the unit cell structure of the
nano-crystals, relative to that inherent to the bulk material
counterpart, which would not be otherwise detectable by conventional
HRTEM. Such structural deviations could be at the origin of peculiar
size-dependent physical-chemical properties of the concerned oxide
material in the nanoscale regime. In addition, it is worthwhile to
remark that this result has been achieved exposing the specimen to an
electron dose as low as 106 e/nm2. The latter condition usually
prevents the specimen against possible structural damages under
exposure to 200 keV electrons, the induction of which remains one of
the key issues in the ultimate accuracy achievable in the structural
determination of materials.
Fig. 1a displays an experimental phase-contrast HRTEM image of an
individual TiO2 nanorod found on an isolated area of the TEM grid. The
small dimensions of the nanocrystal assure kinematical scattering of
electrons, while the optic conditions enable an interpretable image
resolution of 0.19 nm; Fig. 1b is obtained as a combination of the fast
Fourier transform (FFT) of a) with the nanodiffraction pattern as
obtained illuminating a region of about 40 nm centered on the rod and
subtracted from the contribution of the amorphous carbon substrate;
Fig. 1c is a magnified view of the HRTEM contrast shown in fig. 1a. A
new phase retrieval algorithm was then applied which allowed to extend
the crystal information from the 0.19 nm HRTEM image (Fig. 1a) to the
maximum spatial resolution of the diffraction data (here 70 pm), shown
in Fig 1b. Some iterations of Hybrid-Input-Output and of
Error-Reduction (ER) algorithms have been cyclically alternated for
allowing a faster convergence. Any simultaneous modulus and phase
updating for the unobserved data, used in the phasing process, was
avoided. Missing data at high resolution were estimated only at half of
the ER cycles. Among these unobserved data estimates, only the
strongest ones were used in the phasing process [5]. Fig. 1d shows the
phase-retrieved diffractive image of the nanorod crystal structure,
which has been rotated so as to exhibit the c-axis vertically oriented.
Figure 1: a) HRTEM image of a TiO2 rod in [100] zone axis; b) Combination of the FFT of a) with the nanodiffraction pattern of the relevant rod subtracted from the contribution of the amorphous carbon substrate; c) Magnified view of the HRTEM image contrast in a); d) Retrieved image, the rectangular box is the TiO2 in [100] projection (blue: O atoms, red: Ti atoms).
The retrieved result has been averaged over 10 cells to improve the display but producing a symmetric pattern. The elongated spots visible in fig 1d, are formed by two Ti and two O atomic columns that are not completely resolved, as schematically shown by comparison with the partially overlapped tetragonal structure of bulk anatase TiO2 highlighted in the rectangular box. Indeed, the approach allows one to visualize the oxygen atomic column at a resolution of 70 pm [5], not detectable in the relevant HRTEM image. In conclusion, electron coherent diffraction imaging in the TEM represents an approach to improve the capability to study the matter at the highest spatial resolution and accuracy and its development can push further the limits of today microscopy.
For further details see:
Liberato De Caro, Elvio Carlino, Gianvito Caputo, Pantaleo Davide Cozzoli, Cinzia GianniniElectron diffractive imaging of oxygen atoms in nanocrystals at sub-ångström resolution
Nature Nano. 5 (2010) 360 DOI: 10.1038/NNANO.2010.55
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Projects:
| March 2010 - February 2011 | Regional Project funded by Regione Friuli -
Venezia Giulia; Bando 1022/LAVFOR/2009 D. P. Reg. n. 0165/Pres.
dd 23.06.2009. Project title: Sviluppo di nuovo materiale fotovoltaico nanostrutturato - Principal investigator for INFM - CNR |
| July 2004 - July 2007 | 6th EU WP STREP project 508971 - CHIRALTEM - Demonstration of a new physical effect and development of a new methodology to study the chiral properties of magnetic materials in a TEM - Principal investigator for INFM and WP leader |
| January 2001 - January 2005 |
MUR FIRB on Nanostructured carbon - Principal investigator for TEM studies |
| January 2001 - January 2005 |
MUR FIRB project on Nanotecnologie e nanodispositivi optoelettronici e spintronici - Principal investigator for TEM studies |
| January 2001 - January 2005 |
MUR FIRB project on magnetism - Principal investigator for TEM studies |
| January 2002 - January 2004 |
INFM project E5 for the quantitative study of the strain effect in TEM experiments - Principal investigator |
| February 2000 - February 2003 | Project CIPE - INFM P5AW2 - Potenziamento di strutture di ricerca per servizio e trasferimento tecnologico - Principal investigator for WP P5AW2A1 : New facility for atomic resolution TEM/STEM study of the matter |
Collaborations with Italian and foreign groups:
- Università di Trieste
- Università di Ferrara
- Università di Brescia
- Università di Trento
- Università di Lecce
- Università di Roma
- University of Antwerp
- University of Vienna
- École Centrale du Paris
- University of Sheffield
- Politecnico di Torino
- CNR-IMEM Lecce
- CNR-IMEM Bologna
- Università di Camerino
- Università di Ancona
- University of Krakow
- Polish Academy of Science
- University of Zagreb
- CNR-NNL Lecce
- CNR-IC Bari
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