Development of an Ultrafast Low-Energy Electron Diffraction Setup

149 Pages·2015·7.099 MB·English

by  Max Gulde (auth.)

Springer Theses
Recognizing Outstanding Ph.D. Research
Max Gulde

Development
of an Ultrafast
Low-Energy
Electron
Diffraction Setup
Springer Theses
Recognizing Outstanding Ph.D. Research
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Max Gulde
Development of an Ultrafast
Low-Energy Electron
Diffraction Setup
Doctoral Thesis accepted by
ö
the University of G ttingen, Germany
123
Author Supervisor
Dr. MaxGulde Prof. ClausRopers
4th Physical Institute 4th Physical Institute
University of Göttingen University of Göttingen
Göttingen Göttingen
Germany Germany
ISSN 2190-5053 ISSN 2190-5061 (electronic)
SpringerTheses
ISBN978-3-319-18560-6 ISBN978-3-319-18561-3 (eBook)
DOI 10.1007/978-3-319-18561-3
LibraryofCongressControlNumber:2015939820
SpringerChamHeidelbergNewYorkDordrechtLondon
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’
Supervisor s Foreword
Time-resolved spectroscopy offers unique experimental possibilities to investigate
the dynamical processes in physics, chemistry, and biology. In the past, numerous
optical pump-probe techniques were established to obtain access to electron
dynamicsinexcitedmedia.Withtheaidoftheoreticalmodels,theopticalresponse
also offers insight into the associated structural dynamics. However, X-ray and
electron diffraction provide the most direct approach to observe ultrafast structural
changes. Using laboratory scale sources, ultrashort pulses of electrons are very
attractive structural probes, due to their relatively large atomic scattering cross
sections and the availability of powerful electron optics. In the field of time-
resolved electron diffraction, significant progress was made in the past decade,
which nowadays enables the observation of various structural phase transitions in
thetimedomain,forexample,inhighlycorrelatedmaterials.Practicallyallprevious
ultrafast electron diffraction studies were carried out at high electron energies of
tens to hundreds of keV electron energy, aiming at the observation of bulk struc-
tural dynamics. Notable exceptions are recent developments of grazing incidence
electrondiffraction,whichprovidestructuralinformationatandnearsurfacesunder
certain conditions. The ultimate surface sensitivity, however, is obtained in the
diffractionoflow-energyelectronsnearnormalincidence.Requiringthegeneration
ofacollimatedbeamofslowelectronswithultrashortpulsedurationatthesample
location, the realization of ultrafast low-energy electron diffraction (ULEED) pre-
sents tremendous challenges, encountered by several research groups working
toward this direction now and in the past.
This outstanding dissertation by Max Gulde documents the development and
applicationofthefirstexperimentalsetup tosuccessfullytrackstructural dynamics
using ultrafast low-energy electron diffraction. The thesis is remarkable in many
respects, establishing both the conceptual foundations of ULEED using nanoscale
electron emitters, and presenting an exciting new model system composed of a
polymer-graphene bilayer studied in transmission diffraction. The observation
of the structural dynamics of this ordered polymer layer on freestanding graphene
with few picosecond temporal resolution demonstrates the richness of information
v
vi Supervisor’sForeword
provided by low-energy electron diffraction from ultrathin samples, and the suit-
ability of the technique for soft matter systems susceptible to electron beam dam-
age. The outlook presenting first low-energy backscattering diffraction images
using a laser-triggered source further illustrates that this work opens up new
frontiers in ultrafast structural dynamics at surfaces.
For me as his supervisor and for his colleagues in the laboratory, it has been a
tremendouspleasuretoworkwithMaxGulde,toexperiencehisscientificcreativity
and enthusiasm, his positive attitude in confronting obstacles, and his incredible
ability to “get things done.” This thesis is a great evidence of his achievements,
which I hope the readers will enjoy.
Göttingen Prof. Claus Ropers
May 2015
Abstract
Quasi two-dimensional systems such as surfaces and atomically thin films can
exhibit drastically different properties relative to the material’s bulk, including
complex phases and transitions only observable in reduced dimensions. However,
while methods for the structural and electronic investigation of bulk media with
ultrahigh spatio-temporal resolution have been available for some time, there is a
striking lack of methods for resolving structural dynamics at surfaces.
Here, the “Development of an Ultrafast Low-Energy Electron Diffraction
Setup” is presented, offering a temporal resolution of a few picoseconds in com-
bination with monolayer structural sensitivity. In particular, a detailed account is
given on the defining beam properties of the electron source, based on a nonlin-
early driven nanometric photocathode. The emitter parameters within an electro-
static lens assembly are studied by means of a finite element approach. In
particular, the optimal operation regime as well as achievable temporal resolution
are determined. A prototype emitter comparable to the one used in the simulation
is designed, characterized and applied within an ultrafast low-energy diffraction
experiment. Specifically, the superstructure dynamics of an ultrathin bilayer of
polymer crystallites adsorbed on free-standing graphene are investigated upon
strong out-of-equilibrium excitation. Different processes in the superstructure
relaxation are identified together with their respective timescales between 40 and
300ps,includingtheenergytransferfromthegraphenetothepolymer,thelossof
crystalline order and the formation of extended amorphous components. The
findings are subsequently discussed in view of an ultrafast melting of the super-
structure. To conclude, the contribution of the approach to time-resolved surface
science is discussed and an outlook is given in terms offuture systems to inves-
tigate and further developments of the apparatus.
vii
Acknowledgments
Atthispoint,Iwouldliketothankallthosepeoplewhocontributedtothesuccess
of the dissertation project in one way or the other.
My special thanks go toClausRopers, who excellently supervised and directed
meoverthecourseofthepastyearsandwhoencouragedmetopresentthefindings
on many international conferences and workshops. Not least, the many bets (for
pizza) and attempts at bribery (with pizza) resulted both in exciting working con-
ditionsaswellasafamiliaratmosphere,makingthedailysciencebusinessalwaysa
fun one.
EquallywellhaveIbeensupportedbySaschaSchäfer,whosehelpfulcomments
and profound knowledge guided me though many difficult problems. Thank you
and “High five”!
Additionally,IwouldliketothankTimSaldittandKlausSokolowski-Tintenfor
volunteering as co-referents for the dissertation.
TheuncomplicatedconceptualandfinancialsupportofmyworkbytheGerman
National Academic Foundation is thankfully acknowledged. This support not only
allowedmetofocusmorestronglyonmywork,butalsoenabledmetopresentthe
results in front of an international audience on several occasions.
Moreover, I would like to thank Simon Schweda, Manisankar Maiti, and Gero
Storeckfortheproductive,creative,andalwaysfunteamworkinthelabandoffice.
Especially, the alternating measurement runs (with and without measurement cap)
havebeenboostingtheproject’sprogress.BeforeIgottoknowyouguys,hoodies
were just cool, but now I finally understand what they are really made for.
For the excellent sample preparation and fun hours in the laboratory as well as
excellent and often entertaining discussions, I would like to thank Manisankar
Maiti, Hakki Yu, Alec Wodtke and Simon Schweda: “Sample? You can have it!”
Furthermore, I would like to acknowledge Philipp Kloth for introducing me to
the secret art of STM tip preparation, as well as Philipp Willke, Philipp Ansorg,
Thomas Kotzott, and Martin Wenderoth for the STM measurements.
Besides, I would like to express my appreciation to Holger Stark from the MPI
for Biophysical Chemistry for taking so much time to characterize the samples
under the cryo TEM.
ix
x Acknowledgments
Without thehelpoftheworkshopsofthe4thPhysicalInstitute,theInstitutefor
MaterialPhysics,aswellasthecentralworkshop,theprojectwouldnothavebeen
thesuccessitis.MyspecialthanksaregoingtoAlexanderGehrtaswellasChristof
Schmidtfortheirsupportinconstructingthevacuumchamberandsampleholders.
In addition, I would like to express my thanks to Felix Schenk, Thorsten
Hohage,andAxelMunkforprovidinguswiththepowerfulFEMsoftwareusedin
the simulations in this work.
ForclarifyingdiscussionsaboutpolymerdynamicsandtheirsimulationIwould
like to thank Marcus Müller and Kostas Daoulas. In this view I would also like to
mention the productive and fun cooperation with the University of Crete, Greece.
Thank you, Vagelis Harmandaris, Tasia Rissanou, Albert “Party” Power, Veronas
Petrakis,andDespoinaTzeli,forintroducingmetoGROMACSandnotlettingme
starve while trying to adapt to the Greek lunch and dinner times: “Time is an
illusion. Lunchtime doubly so.”
Discussions with Melanie Müller and Alexander Paarmann about surface sci-
ence, publications, as well as about African and Japanese food are thankfully
acknowledged.
Moregenerally,IwouldliketothankthewholeworkgroupofClausRopersfor
a great, creative, and, most of all, fun time. I will never forget the crossing of the
fire protection pond in a self-built sailing vessel, or joint activities such as bad-
minton, barbecuing,skiing,kartracing,rockclimbingtoname justafew.Staythe
way you are!
Also, I would like to thank Daniel Solli not only for proofreading the thesis at
ludicrous speed (“ain’t found no more mistakes”), but also for helping me convert
all units into the imperial system.
Theatmosphereinthe4thPhysicalInstituteisalwaysveryhelpfulandfamiliar,
which in my opinion makes it a very special place to work. Thank you.
Additionally, I would like to thank the Institute for Material Physics to have
integrated us so cordially.
Moreover, I would like to thank my friends for helping me strike (“ouch”) the
balance between physics and free time. My special thanks go to the couch, which
providedbeddingforsomanyyears,butalsotothecouch’sroomies,namelyHajo,
Reiner, Christian, Carla, and somehow also Sergey, with whom I collectively
enjoyed sophisticated movies and pizza.
Finally,mydeepest thanks go to myfamily, which supportedme overall those
years,especiallywhenthingsdidnotgosoultrafast.Andlastbutnotleast,Ithank
Julia,whohasalwaysbeenthereforme,beitinaNorwegianswamp,onapassin
the Alps, or just at home during dinner.