Quantum Confined Excitons in 2-Dimensional Materials

125 Pages·2018·5.539 MB·English

by  Carmen Palacios-Berraquero

Springer Theses
Recognizing Outstanding Ph.D. Research
Carmen Palacios-Berraquero

Quantum Confined
Excitons in
2-Dimensional
Materials
Springer Theses
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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
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’
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
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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
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