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Data reported herein are available in the supplementary materials.
Support from the U.S. Department of Energy Office of Basic Energy
Science to the SUNCAT Center for Interface Science and
Catalysis is gratefully acknowledged. A.J.M. is grateful for
support by the U.S. Department of Defense through the
National Defense Science and Engineering Graduate
Materials and Methods
Figs. S1 and S2
Tables S1 to S3
17 March 2014; accepted 2 June 2014
Ultrafast low-energy electron diffraction
in transmission resolves polymer/
graphene superstructure dynamics
Max Gulde,1 Simon Schweda,1 Gero Storeck,1 Manisankar Maiti,1 Hak Ki Yu,2
Alec M. Wodtke,2,3 Sascha Schäfer,1 Claus Ropers1*
Two-dimensional systems such as surfaces and molecular monolayers exhibit a
multitude of intriguing phases and complex transitions. Ultrafast structural probing of
such systems offers direct time-domain information on internal interactions and couplings
to a substrate or bulk support. We have developed ultrafast low-energy electron diffraction
and investigate in transmission the structural relaxation in a polymer/graphene bilayer
system excited out of equilibrium. The laser-pump/electron-probe scheme resolves the
ultrafast melting of a polymer superstructure consisting of folded-chain crystals registered
to a free-standing graphene substrate. We extract the time scales of energy transfer
across the bilayer interface, the loss of superstructure order, and the appearance of an
amorphous phase with short-range correlations. The high surface sensitivity makes this
experimental approach suitable for numerous problems in ultrafast surface science.
The investigation of atomic-scale dynamics with high spatiotemporal resolution yields insights into ultrafast structural reorgan- izations associated with energy transfer or phasetransitions. Substantial progress
was made in establishing methods for the time-
resolved structural analysis of bulk media, including
ultrafast implementations of x-ray crystallogra-
phy (1–3), high-energy electron diffraction (4–6),
and microscopy (7–9), as well as time-resolved
x-ray and electron spectroscopy (10, 11). In con-
trast, structural dynamics at surfaces, interfaces,
and ultrathin films remain largely elusive, as the
surface signal in both x-ray and high-energy elec-
tron diffraction is typically masked by large bulk
contributions. This limits our ability to study
quasi–two-dimensional (2D) systems exhibiting
characteristic phase transitions and topologically
controlled ordering (12, 13), as well as the dy-
namics of surface reconstructions and complex
adsorbate superstructures (14, 15). Ultrafast elec-
tron scattering in grazing incidence enhances the
surface signal (14, 16) but faces particular chal-
lenges in quantitative diffraction analysis.
Optimal surface sensitivity would be attained
with an ultrafast implementation of low-energy
electron diffraction (LEED). At electron energies of tens to a few hundreds of electron volts,
scattering cross sections are strongly increased,
which allows for probing depths of only a few
monolayers and has made LEED a widely used
tool for surface structure determination. However,
at such low energies, it has proven exceedingly
difficult to implement pulsed electron sources
that fulfill the requirements of an ultrafast diffraction experiment (17–19), that is, short pulse
duration and low beam emittance. Laser-triggered
electron emission from nanoscale photocathodes
(20, 21) is expected to resolve some of these issues
(22–24), providing well-collimated low-energy
electron pulses and a temporal resolution that is
comparable to state-of-the-art ultrafast x-ray or
high-energy electron diffraction. Motivated by these
prospects, we have undertaken the development
of a new diffraction apparatus.
We have developed transmission ultrafast
LEED (T-ULEED) based on a nanometric needle
photocathode and demonstrate its capability to
resolve atomic-scale structural dynamics of surfaces and monolayer films with a temporal resolution of a few picoseconds. Specifically, we
studied the ultrafast laser-driven dynamics of a
polymer superstructure on freestanding monolayer graphene.
In the laser-pump/electron-probe scheme
(Fig. 1A), the sample is excited out of equilibrium
by amplified femtosecond laser pulses (800 nm
wavelength, 80-fs pulse duration, repetition rate
10 kHz, focal diameter about 100 mm). To minimize hot electron emission from graphene (25),
the pump pulse is temporally stretched to 3 ps by
dispersion, which, however, is still sufficiently
short to resolve the processes described below.
The pump-induced structural dynamics are probed
by ultrashort electron pulses emitted from a sharp
tungsten tip (50-nm radius of curvature), triggered by the second harmonic of the laser. These
electron pulses (up to 100 electrons per pulse) are
collimated and focused onto the sample at variable electron energies using an electrostatic lens
assembly in a geometry that we have recently
studied numerically (22). Scattered electrons are
subsequently recorded in a transmission geometry by a phosphor screen microchannel plate
detector (MCP, Hamamatsu F2226-24P). With our
laser-triggered low-energy electron source, diffraction patterns can also be recorded in back-reflection and for a range of electron energies,
as demonstrated in the supplementary materials
(26) (figs. S6 to S8).
The electron pulse duration and the spatial
and temporal overlap (delay time, Dt = 0) of the
laser-pump and electron-probe pulses are determined via a transient-electric-field effect near a
bare transmission electron microscopy (TEM)
copper grid. Upon excitation of a single copper
grid bar with high peak intensity (fluence up to
30 mJ/cm2, unstretched pump pulses), a dense
electron cloud is emitted (25, 27), which may lead
to a spatial deflection of the passing electron pulse
(Fig. 2A). Projection images of the grid before
and after Dt = 0 are shown in Fig. 2B, acquired by
defocusing the pulsed electron beam. The central
distortion in the lower image indicates the extension of the pump-induced electron cloud. Using a
collimated electron beam passing a single mesh
200 11 JULY 2014 • VOL 345 ISSUE 6193 sciencemag.org SCIENCE
14th Physical Institute, University of Göttingen, 37077
Göttingen, Germany. 2Max Planck Institute for Biophysical
Chemistry, 37077 Göttingen, Germany. 3Institute for Physical
Chemistry, University of Göttingen, 37077 Göttingen,
*Corresponding author. E-mail: firstname.lastname@example.org