Ultrafast Electron Diffraction of Isolated Molecules | Ultrafast Electron Crystallography of Surfaces and Crystals

Ultrafast 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.

In 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 were studied.

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.

  • Studies 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.
  • Studies 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.
  • The structural dynamics of self-assembled monolayer of 2-mercaptoacitic acid on a gold metal substrate was also studied.

Recently, 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 dynamical structures.

*The text above has been adapted from the following publications.

Selected 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, 8854.

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.