Electron Diffraction of Isolated Molecules |
Electron Crystallography of Surfaces and Crystals
Electron Crystallography of Surfaces and Crystals*
Electron crystallography, such as the diffraction mode in
transmission electron microscopy (TEM) and reflection high energy
electron diffraction (RHEED), has been developed over the last
30 years as a structure determination method with high sensitivity
and atomic-scale spatial resolution. Its applications include
structural and charge density studies on organic molecules, proteins
and complicated inorganic materials in the amorphous, crystalline,
nano-, meso- and quasi-crystalline state, in many cases beyond
the capacities of X-ray diffraction. Introducing the resolution
in time leads to the realm of ultrafast electron crystallography
(UEC), to visualize real-time structural changes of systems, with
unprecedented spatio-temporal resolutions. It enables us to study
the transient non-equilibrium structures which are crucial to
the understanding of phase transitions and dynamics in solids,
surfaces, and macromolecular systems.
The UEC apparatus, based on three generations of ultrafast electron
diffraction (UED) in this laboratory, is very different from and
much more complex than all previous generations. It includes three
integrated ultrahigh vacuum (UHV) chambers — sample preparation,
load-lock and scattering chamber, and a femtosecond laser system.
The preparation chamber has sputtering, cleaning and characterization
tools for the crystal surface. The diffraction experiments take
place in the scattering chamber, in which the sample is mounted
on a computer-controlled goniometer. The electrons are generated
through the photoelectric effect by back-illumination with a femtosecond
laser pulse on a silver photo-cathode. With the electron gun focusing
system, both the reflection and transmission diffractions can
be obtained, and the patterns are recorded by an intensified CCD
camera assembly capable of single electron detection. The chamber
is augmented with a retractable gas dosing assembly, a residual
gas analyzer and a cooling system which enables temperature-dependent
experiments down to 10 K.
the reflection mode, the conceptual framework is as follows. The
change of the structure is initiated by an ultrafast laser pulse.
An ultrashort packet of high energy electrons then impinges at
a grazing incidence angle on the surface, scattered to form the
diffraction pattern. While the diffraction pattern of a single
crystal surface contains discrete spots or streaks, the diffraction
pattern of a polycrystal or amorphous surface contains a set of
arcs or rings. From the positions, widths and intensities of the
characteristic diffraction spots or rings, the surface structure
can be determined. By changing the time delay between the laser
pulse and the electron pulse, a series of diffraction images are
obtained as a function of time. From these images, the surface
structural change is directly determined in real time.
The first experiments were carried out on crystalline silicon
with different adsorbates: hydrogen, chlorine and molecular trifluoroiodomethane,
aligned by the surface [Proc. Natl. Acad. Sci. USA 2004, 101,
1123]. The structural changes of the Si (111) surfac, bulk and
the phase transition from an amorphous phase to the liquid state
UEC has been applied to study several crystalline or amorphous
systems. Some examples are
Studies of semiconductor Si(111) surfaces with different adsorbates
showed the coherent restructuring of the surface layers with
sub-angstrom displacement of atoms, and the amorphous to liquid
phase transition on the picosecond time scale.
of chlorine terminated GaAs(111) surfaces, by following the
change of Bragg diffraction (shift, width and intensity). The
surface atomic motion and the transient temperature were determined.
of interfacial water on a hydrophilic and hydrophobic surfaces.
The coexistence of ordered surface water and crystallite-like
ice structures and the dynamics after the ultrafast substrate
temperature jump were observed.
structural dynamics of self-assembled monolayer of 2-mercaptoacitic
acid on a gold metal substrate was also studied.
bilayers of fatty acid Langmuir-Blodgett films were studied. The
structure of the 2D assembly was determined. The coherent and
anisotropic dynamical expansion along the aliphatic chains and
the restructuring toward equilibration at longer times were observed.
This is a leap forward for the determination of macromolecular
*The text above has been adapted from the following publications.
Atomic-Scale Dynamical Structures of Fatty-Acid Bilayers Observed
by Ultrafast Electron Crystallography, S. Chen, M. T.
Seidel, A. H. Zewail, Proc. Natl. Acad. Sci. USA 2005, 102,
Ultrafast Electron Crystallography of Interfacial Water,
C.-Y. Ruan, V. A. Lobastov, F. Vigliotti, S. Chen, A. H. Zewail,
Science 2004, 304, 80.
Ultrafast Electron Crystallography of Surface Structural
Dynamics with Atomic-Scale Resolution, F. Vigliotti,
S. Chen, C.-Y. Ruan, V. A. Lobastov, A. H. Zewail, Angew. Chem.,
Int. Ed. 2004, 43, 2705.
Structures and Dynamics of Self-Assembled Surface Monolayers Observed
by Ultrafast Electron Crystallography, C.-Y. Ruan, D.-S.
Yang, A. H. Zewail, J. Am. Chem. Soc. 2004, 126, 12797.
Ultrafast Electron Crystallography: Transient Structures of Molecules,
Surfaces and Phase Transitions, C.-Y. Ruan, F. Vigliotti,
V. A. Lobastov, S. Chen, A. H. Zewail, Proc. Natl. Acad. Sci. USA
2004, 101, 1123.