TY - JOUR
T1 - Low-Dose, Low-Temperature Convergent-Beam Electron Diffraction and Multiwavelength Analysis of Hydrocarbon Films by Electron Diffraction
AU - Wu, Jinsong
AU - Spence, John
PY - 2003/10
Y1 - 2003/10
N2 - Aromatic hydrocarbon (perylene, coronene) and tetracontane films are shown to produce useful convergent-beam electron diffraction (CBED) patterns under low-dose and low-temperature conditions. These were obtained using a Zeiss LEO-921 electron microscope with an omega energy filter at liquid helium and nitrogen temperatures. The usefulness of patterns showing CBED disks of constant intensity ("blank disks," indicating kinematic scattering) for structure analysis is investigated, with the aim of avoiding film-bending artifacts. Using CBED patterns from thicker areas, sample thickness was experimentally determined using either two-beam or three-beam patterns. Koehler mode illumination (a new form of SAD pattern offering smaller areas) was also used, and the possibility of obtaining structure factor moduli using the kinematic and two-beam approximations was investigated by comparing measured diffraction intensities with experimental ones for these known structures. The commonly used approximation |F| ∼ Ig (intended to account for bending) was found to be a worse approximation than the two-beam approximation with well-defined excitation error for these microdiffraction experiments. A new multiwavelength method of retrieving structure factor moduli and thickness from microdiffraction patterns using two-beam theory is demonstrated for tetracontane.
AB - Aromatic hydrocarbon (perylene, coronene) and tetracontane films are shown to produce useful convergent-beam electron diffraction (CBED) patterns under low-dose and low-temperature conditions. These were obtained using a Zeiss LEO-921 electron microscope with an omega energy filter at liquid helium and nitrogen temperatures. The usefulness of patterns showing CBED disks of constant intensity ("blank disks," indicating kinematic scattering) for structure analysis is investigated, with the aim of avoiding film-bending artifacts. Using CBED patterns from thicker areas, sample thickness was experimentally determined using either two-beam or three-beam patterns. Koehler mode illumination (a new form of SAD pattern offering smaller areas) was also used, and the possibility of obtaining structure factor moduli using the kinematic and two-beam approximations was investigated by comparing measured diffraction intensities with experimental ones for these known structures. The commonly used approximation |F| ∼ Ig (intended to account for bending) was found to be a worse approximation than the two-beam approximation with well-defined excitation error for these microdiffraction experiments. A new multiwavelength method of retrieving structure factor moduli and thickness from microdiffraction patterns using two-beam theory is demonstrated for tetracontane.
KW - Aromatic hydrocarbons
KW - CBED
KW - Electron microdiffraction
KW - Organic molecular crystal
KW - Radiation damage
KW - Structure factors
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U2 - 10.1017/S1431927603030368
DO - 10.1017/S1431927603030368
M3 - Article
C2 - 19771699
AN - SCOPUS:0242276027
SN - 1431-9276
VL - 9
SP - 428
EP - 441
JO - Microscopy and Microanalysis
JF - Microscopy and Microanalysis
IS - 5
ER -