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OÅM

The One-Angstrom Microscope (OÅM) is a mid-voltage transmission electron microscope (TEM) capable of producing images with sub-angstrom resolution. The basic instrument is a modified Philips CM300FEG/UT, a TEM with a field-emission electron source and an ultra-twin objective lens with low spherical aberration (Cs = 0.60 mm) and a point-to-point resolution of 1.7Å. Special, highly stable power supplies for the objective lens and for high-voltage power produce an information limit of 0.8Å. A 2048x2048-pixel charge-coupled device (CCD) camera digitally records images up to a maximum magnification of 38Mx. The microscope can operate at an acceleration voltage of either 300kV or 150 kV and can produce aberration-corrected exit wave images with 0.8Å resolution. The OÅM can operate in any of the following modes:

OAM driver test

Contact: Christian Kisielowski.

 

Specifications

Acceleration Voltage:

300 kV, 150kV

Spherical Aberration Cs:

0.60mm

Chromatic Aberration Cc:

1.27mm


Specimen stage

Philips double-tilt low background holder

±30°/±30°

 

Focal-Series Restoration

Images recorded at different defocus settings contain differently-phased spatial frequencies. Generating one image with sub-Å resolution requires extraction and combination of the correct band of frequencies from members of a focal series. For focal series reconstruction, the OÅM uses the Philips/Brite-Euram software, by Coene and Thust, to process focal series acquired automatically from the microscope under computer control, thereby establishing the exit-surface wave. This process can correct residual lens aberrations due to 2- and 3- fold astigmatism, coma, and spherical aberration.

Holography

Electron holography can produce high-resolution images by recording images (holograms) that have been multiplexed with a suitable high-frequency carrier. The carrier frequency is then removed with image processing, yielding a complex image (amplitudes and phases). The complex image is corrected for the phase changes imposed by the microscope imaging system to produce the corrected image at enhanced resolution. In a lower-resolution mode electron holograms can be used to detect electric and magnetic fields, as well as the sample's mean inner potential.

Energy-filtered Imaging

The 2k Gatan imaging filter (GIF), installed after the microscope column, can be used to map chemical elements at near-atomic resolution (<10Å). Images are acquired with the multiscan CCD camera at sizes up to 2048x2048 pixels and can be read out at speeds up to 2MHz under computer control. Local chemical concentrations can be quantified using the GIF in EELS (electron energy-loss spectroscopy) mode to access the spectroscopic signatures of light elements such as O, N, C, and even He.

Annular Dark Field TEM Imaging

Annular dark field transmission electron microscopy (ADF-TEM) imaging enables Z-contrast imaging without scanning noise and with reduced sample contamination. ADF-TEM uses an objective aperture that acts as a central beam stop in the back focal plane of the objective lens. This causes the central beam and all electrons scattered up to a certain semiangle to be excluded from imaging, and the image formation is entirely nonlinear.

Electron Tomography

Customized sample holders can be used to produce a tilt series of images in steps of 1 to 2 degrees over an annular range that is limited (to ~70 degrees) by shadow effects from the utilized sample grids. Electron tomograms can be reconstructed from such tilt series. Different imaging modes (Bright Field, Annular Dark field, EFTEM) are available to highlight specific aspects of the investigated sample.

High Resolution Results

The OAM reached sub Ångstrom resolution in 2000 by resolving the 0.89Å dumbbell spacing of diamond [110] in a reconstructed phase image of the electron exit wave [C. Kisielowski et al., Microscopy and Microanalysis 6, 2000, 16]. Under optimized conditions it is even possible to separate the 0.78Å dumbbell spacing of Si [112] in a reconstructed phase image [C. Kisielowski, 2001].

The reconstruction of electron exit waves removes effects of delocalization and of phase differences. A reconstructed phase image can directly reveal the projected crystal structure, which isn’t evident from an inspection of lattice images, as demonstrated in the figure [A. Ziegler et al., Acta Materialia 50 (2002) 565].

An ultra-high-resolution reconstruction of electron exit waves from a through-focus series allows separation of amplitude and phase. In each lattice image of the series an information limit of 0.8 Å can be exploited to boost resolution. The depicted phase image of an interface between hexagonal GaN and sapphire proves that the method allows for an unambiguous determination of interface structures [C. Kisielowski, et al.,Ultramicroscopy 89 (2001) 243; M.A. O’Keefe et al.,Ultramicroscopy 89 (2001) 215]

Atomic structure of a Sigma 13 grain boundary in SrTiO3. The reconstructed phase image reveals all atom columns at the boundary, including an Sr column splitting (0.9 Å) and the position of Oxygen columns. The high sensitivity of the microscope allows for an evaluation of the oxygen occupancy in each of the columns [J. Ayache et al., J. Materials Science 40 (2005) 3091].

Operation of the microscope at 150 kV can reduce radiation damage and facilitate the investigation of radiation sensitive material (in this case BN nanotubes). The chemical composition can be revealed by the application of EELS [Courtesy Dr. Weiqiang Han, Berkeley, 2004].

The outstanding signal-to-noise ratio in reconstructed phase images allows identification of column positions with a precision as low as 2.4 pm. As a result, experimental images and first-principles calculations can be compared with pm precision, as in this case of a partial dislocation in GaAs:Be [X. Xu et al., PRL 95 (2005) 145501].

Electron hologram of a polymer sphere and its reconstructed phase image, which can be used to recover the sample’s shape. [Y.C. Wang et al.,Microscopy & Microanalysis 4 (1998) 146].

The shell periodic structure of nanoparticles (FePt) can be studied quantitatively by focal series reconstruction at sub-angstrom resolution. Strain relaxation effects in icosahedral FePt nanoparticles, studied with a precision of ± 0.002 nm, suggested that they occur by element segregation and site-specific atom loss at the nanocrystals surface. As a result, the particles consist of a Pt shell and an iron/platinum core, making them catalytically active nanomagnets. [R. Wang et al., submitted 2005].

TEM bright field image (a) and annular dark field image (b) of a CdSe tetrapod (one “branch” is coming out of the plane) next to a Au nanoparticle and a CdSe nanoparticle. The contrast in the ADF-TEM image (Z-contrast) is chemically sensitive [S. Bals et al., Solid State Communications 130 (2004) 675].

Bright field electron tomograms of tetrapods on a carbon support and a Pt nanocrystal [S. Bals et al., Microsc Microanal 11(Suppl 2), 2005].

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