Development of an Ultrafast Low-Energy Electron Diffraction Setup PDF
Preview Development of an Ultrafast Low-Energy Electron Diffraction Setup
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 Aims and Scope The series “Springer Theses” brings together a selection of the very best Ph.D. theses from around the world and across the physical sciences. Nominated and endorsed by two recognized specialists, each published volume has been selected foritsscientificexcellenceandthehighimpactofitscontentsforthepertinentfield of research. For greater accessibility to non-specialists, the published versions includeanextendedintroduction,aswellasaforewordbythestudent’ssupervisor explainingthespecialrelevanceoftheworkforthefield.Asawhole,theserieswill provide a valuable resource both for newcomers to the research fields described, and for other scientists seeking detailed background information on special questions. Finally, it provides an accredited documentation of the valuable contributions made by today’s younger generation of scientists. Theses are accepted into the series by invited nomination only and must fulfill all of the following criteria (cid:129) They must be written in good English. (cid:129) ThetopicshouldfallwithintheconfinesofChemistry,Physics,EarthSciences, Engineeringandrelatedinterdisciplinary fields such asMaterials,Nanoscience, Chemical Engineering, Complex Systems and Biophysics. (cid:129) The work reported in the thesis must represent a significant scientific advance. (cid:129) Ifthethesisincludespreviouslypublishedmaterial,permissiontoreproducethis must be gained from the respective copyright holder. (cid:129) They must have been examined and passed during the 12 months prior to nomination. (cid:129) Each thesis should include a foreword by the supervisor outlining the signifi- cance of its content. 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More information about this series at http://www.springer.com/series/8790 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 ©SpringerInternationalPublishingSwitzerland2015 Thisworkissubjecttocopyright.AllrightsarereservedbythePublisher,whetherthewholeorpart of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission orinformationstorageandretrieval,electronicadaptation,computersoftware,orbysimilarordissimilar methodologynowknownorhereafterdeveloped. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publicationdoesnotimply,evenintheabsenceofaspecificstatement,thatsuchnamesareexemptfrom therelevantprotectivelawsandregulationsandthereforefreeforgeneraluse. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authorsortheeditorsgiveawarranty,expressorimplied,withrespecttothematerialcontainedhereinor foranyerrorsoromissionsthatmayhavebeenmade. Printedonacid-freepaper SpringerInternationalPublishingAGSwitzerlandispartofSpringerScience+BusinessMedia (www.springer.com) ’ 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.
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