Quantum Confined Excitons in 2-Dimensional Materials PDF
Preview Quantum Confined Excitons in 2-Dimensional Materials
Springer Theses Recognizing Outstanding Ph.D. Research Carmen Palacios-Berraquero Quantum Confined Excitons in 2-Dimensional Materials 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. (cid:129) The theses should have a clearly defined structure including an introduction accessible to scientists not expert in that particular field. More information about this series at http://www.springer.com/series/8790 Carmen Palacios-Berraquero fi Quantum Con ned Excitons in 2-Dimensional Materials Doctoral Thesis accepted by the University of Cambridge, Cambridge, UK 123 Author Supervisors Dr. CarmenPalacios-Berraquero Prof. Mete Atatüre Cavendish Laboratory Cavendish Laboratory University of Cambridge University of Cambridge Cambridge, UK Cambridge, UK Prof. Andrea Ferrari CambridgeGraphene Centre, EngineeringDepartment University of Cambridge Cambridge, UK Dr. Jason Robinson Department ofMaterials Science andMetallurgy University of Cambridge Cambridge, UK ISSN 2190-5053 ISSN 2190-5061 (electronic) SpringerTheses ISBN978-3-030-01481-0 ISBN978-3-030-01482-7 (eBook) https://doi.org/10.1007/978-3-030-01482-7 LibraryofCongressControlNumber:2018955925 ©SpringerNatureSwitzerlandAG2018 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. 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ThisSpringerimprintispublishedbytheregisteredcompanySpringerNatureSwitzerlandAG Theregisteredcompanyaddressis:Gewerbestrasse11,6330Cham,Switzerland ’ Supervisor s Foreword For decades, the concept of a truly two-dimensional sheet of material with only a single atomic thickness remained a curiosity within mathematical physics, until a single sheet of graphene was indeed isolated from graphite and identified unam- biguously.Despitebeingthethemeofthe2010NobelPrize,itsoonbecameevident that graphene was only the very tip of an iceberg of layered materials with prop- erties ranging from metals to superconductors, and insulators to semiconductors. One category of such materials is called the transition metal dichalcogenides (2d-TMDs) which offer both semiconducting properties and direct optical transi- tionsinthelimitofsingleorfewatomiclayers.Thiscombinationis,ofcourse,very attractive for optoelectronics applications and the last few years saw a research boominthequesttounderstandtheutilizethevariousopticallyrelevantproperties of such materials. This boom, while being exciting, remained within the realm of conventional devices until a few years ago. In 2015, five independent research groupsworldwidereportedtheexistenceofrandomlyoccurringlocalizedemission in the form of single photons from a single layer of tungsten diselenide (WSe ) 2 flakeenteringthisclassofmaterialsofficiallyintotheradarofquantumscienceand technologies. Single-photon generation is fundamentally not possible within clas- sicalphysicsandindicatestheexistenceofahighlyconfinedatom-likeenergy-level structurewithopticalselectionrules.Thismuchwasclearinthosepapers,butsome keyquestionshadnotfoundanswers:Whatformsthesequantumemitters(science) and how can they be created deterministically (technology)? Carmen’s thesis addresses these two questions: First, she found a way to create thesequantumemitterswheredesiredwithnotmorethanafewtensofnanometers uncertainty and in large numbers. Carmen and team members worked in close collaboration to fabricate tiny pillars with diameters around a few tens of nanometers on a glass substrate. When a layered material of choice is placed on these nanopillars, it droops down like a tipi tent and a quantum emitter appears at theapexofeverysinglepillar.Buildinguponthenotionthatstraintunestheoptical emission spectrum of layered materials, the application of strain in a spatially confined area on a material offers the potential to create these quantum emitters. Carmen demonstratedthat large-scale arrays of quantumemitters can begenerated v vi Supervisor’sForeword this way, essentially limited by the size of the layered material. She also showed thatthenanopillartechniqueisversatileandisnotrestrictedtotungstendiselenide, theonlylayeredmaterialwherequantumemittershadbeenreported.Whatismore, the deterministically created quantum emitters provide better stability over the naturally occurring counterparts leading to narrower optical emission spectrum for the single photons. Since our first publication of the technique, many research groups worldwide started to employ Carmen’s technique on other materials including hexagonal boron nitride. Second, Carmen identified that these are highly confined excitonic systems similar in nature to the self-assembled quantum dots in conventional III-V mate- rials, rather than the atomistic defects. She applied our nanopillar technique to heterostructures comprising layered materials to impose electric field and charge control.Devicesthatwereoptimizedforelectricfieldcontrolprovedveryusefulin observing both neutral and charged excitonic optical transitions. This capability gaveimportantinsightintothesequantumemitters,andwe,aswellasothergroups, are working with such devices to access their internal degrees offreedom, such as spin and valley orientation. In parallel, devices that were optimized for charge injection yielded the first demonstration of quantum light-emitting diodes where single photons were produced all electrically without the need for an excitation laser.Thisofcoursehasdirectadvantagesinon-chipintegratedquantumphotonics from an applications point of view. While a dissertation is usually a comprehensive body of work that effectively putsanendtoascientificstory,thisisoneofthoselesscommonthesesthatrather starts from a question and leads to many new opportunities of research. In this context,IdohopeyouwillenjoyreadingCarmen’sthesisandthemanyaspectsof quantum optics with layered materials as much as I enjoyed very much working together with her over the last few years. Cambridge, UK Prof. Mete Atatüre September 2018 Preface The two-dimensional semiconductor family of materials called transition metal dichalcogenides (2d-TMDs) offers many technological advantages: low power consumption, atomically precise interfaces, lack of nuclear spins, and ease of functional integration with other 2d materials are just a few. In this work, we harnessthepotentialofthesematerialsasaplatformforquantumdevices:Develop a method by which we can deterministically create single-photon emitting sites in 2d-TMDs, in large-scale arrays. These we call quantum dots (QDs): quantum confinement potentials within semiconductor materials which can trap single excitons. The single excitons recombine radiatively to emit single photons. Single-photon sources are a crucial requirement for many quantum information technology (QIT) applications such as quantum cryptography and quantum communication. The QDs are formed by placing the flakes over substrates nanopatterned with protrusions which induce local strain and provoke the quantum confinement of excitons at low temperatures. This method has been successfully tested in several TMD materials, hence achieving quantum light at different wavelengths. We pre- sent one of the very few systems where quantum confinement sites have been shown to be deterministically engineered in a scalable way. Moreover, we have demonstrated how the 2d-based QDs can be embedded within2d-heterostructurestoformfunctionalquantumdevices:WehaveusedTMD monolayers along with other 2d-materials—graphene and hexagonal boron nitride —to create quantum light-emitting diodes that produce electrically driven single photons. Again, very few single-photon sources can be triggered electrically, and this provides a great advantage when considering on-chip quantum technologies. Finally,wepresentexperimentalstepstowardusingourarchitectureasquantum bits:capturingsinglespinsinsidetheQDs,usingfield-effecttype2d-heterostructures. WeareabletocontrollablychargetheQDswithsingleelectronsandsingleholes—a keybreakthroughtowardtheuseofspinandvalleypseudospinofconfinedcarriers in2d-materialsasanewkindofopticallyaddressablematterqubit. vii viii Preface This work presents the successful marriage of 2d-semiconductor technology with QIT, paving the way for two-dimensional materials as platforms for scalable, on-chip quantum photonics. Cambridge, UK Carmen Palacios-Berraquero Acknowledgements I would like to thank my supervisor Mete, firstly, for his invaluable support when starting this Ph.D.; secondly, for his constant inspiration and enthusiasm; thirdly, for his trust with leading a risky but promising project. It worked! Thank you as well to my second supervisors Andrea Ferrari and Jason Robinson. It is very hard to express in words my gratitude for Dhiren Kara, who has been mymentorduringtheseyears,fromwhomIhavelearnedalmosteverythingIknow aboutopticsandbeyond.Dhiren’swayofdoingscienceiswhatIthinkwouldlead to both genuine groundbreaking scientific knowledge and equality in the scientific community. He has been an incredible colleague and friend. I would like to thank everyone in our research team for making the laboratory such a vibrant place. Special thanks to Helena Knowles, Rob Stockill, Claire Le Gall, and Megan Stanley, for helping me always and being fantastic scientists and rolemodels;tothe2dteam—MatteoBarboneandAlejandroR.P.Montblanchfor many very late nights in the laboratoty; And to all of you, Ben Pingault, Camille Stavrakas, Dorian Gangloff, Lukas Huthmacher, Jeff Holzgraffe, Lucio Stefan, Gabriel Éthier-Majcher, Qiushi Gu, Mustafa Gündogan. Thank you, Sébastien Francoeur and Gang Wang for being incredible teachers, scientists, and people. Thanks,PamSmith,formakingeverythingwork.IsendmylovetoalloftheCiW women, to Sarah Morgan, Hannah Stern, and Hope Bretscher, and especially to Ankita Anirban, who has been the greatest companion during these years. I would like to thank Andy Parker and Rachael Padman for their support of CiW and their time and care. Thank you as well to Aga Iwasiewicz-Wabnig and Karishma Jain, forthehelptheyhaveofferedmethroughouttheseyears.Iwouldalsoliketothank Marko Lonçar for his support and trust and to Pawel Latawieç for being an exceptional collaborator. During my time in Cambridge, I have made and maintained friends that have become my family here, an incredible, infinite source of support and inspiration. My love goes to Rox Middleton, Maire Ní Leathlobhair, and Paul Morris, for making 111 King Street my home; to my adopted family: Dafni Glinos and Brian Balchin; and Jérémie Le Pen and Ana Pinharanda, for their unconditional love; to ix
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