Table Of ContentSpringer Theses
Recognizing Outstanding Ph.D. Research
Matthew Holwill
Nanomechanics
in van der Waals
Heterostructures
Springer Theses
Recognizing Outstanding Ph.D. Research
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Matthew Holwill
Nanomechanics in van der
Waals Heterostructures
Doctoral Thesis accepted by
the University of Manchester, Manchester, UK
123
Author Supervisors
Dr. MatthewHolwill Prof. Sir KostyaNovoselov
Schoolof Physics andAstronomy Schoolof Physics andAstronomy
University of Manchester University of Manchester
Manchester, UK Manchester, UK
Dr. Artem Mishchenko
Schoolof Physics andAstronomy
University of Manchester
Manchester, UK
ISSN 2190-5053 ISSN 2190-5061 (electronic)
SpringerTheses
ISBN978-3-030-18528-2 ISBN978-3-030-18529-9 (eBook)
https://doi.org/10.1007/978-3-030-18529-9
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The comfort zone is a beautiful place, but
nothing ever grows there.
Unknown
To Alan and Joyce
’
Supervisor s Foreword
The thesis explores interaction, surface reconstruction and micromechanical prop-
ertiesof2DmaterialsinvanderWaalsheterostrucutres.Whenseveral2Dmaterials
are brought together and start to interact, their structure can change, which
potentiallychangesother(electronic,optical,chemicalandother)propertiesofsuch
stacks. Control and dynamic change of the stacking arrangement between the
crystalsisachallengingtask,which,however,offershugepotentialbenefitsbothin
terms of the expansion of the range of possible physical experiments and in
applications. ThethesisofMatthewHolwilllays thefoundation for thetypeofthe
experimentswhichcanbeperformed,thetypeofheterostructuressuitableforsuch
experiments, and, themostimportant, developsthemethodology andprotocolsfor
nanotechnological procedures which allow fabrication of such heterostructures.
Thisworkwillpavethewayforthenewdirectioninscienceandtechnologyof2D
materials—van der Waals heterostructures with dynamically controlled interfaces.
Manchester, UK Prof. Sir Kostya Novoselov
January 2019
ix
Abstract
Due to the small lattice mismatch, graphene crystallographically aligned to
hexagonal boron nitride (hBN) forms an hexagonal moiré superlattice whose
periodicity depends on the relative crystal orientation. Near perfect alignment, the
periodicity of the superlattice is *13.9 with central regions characterised by the
graphene lattice stretching to fit the potential of the underlying hBN. Small
out-of-planedeformationsoccurringattheedgesofthesuperlatticeaccumulatethis
built up strain. These two van der Waals materials are the subject of study in this
thesis due to their large intrinsic strengths (70.5–130 GPa) and Young’s moduli
(0.865–1 TPa).
Frictional forces between hBN and graphene flakes are increased as atoms are
‘pinned’ near the potential trough. In the language of the Frenkel–Kontorova
model, graphene is said to be in the commensurate state separated by out-of-plane
kinks/solitons.Potentialbarrierstokinkmotionaremuchlowerthanthoseseenby
individual commensurate atoms. By introducing new kinks or destabilizing those
present, flake motion may be possible using minimal forces. It is these kink exci-
tations that are the topic of study in this thesis.
New fabrication procedures for producing complex suspended van der Waals
heterostructuresareanimportantadditionalaimofthisproject.Theywillhelppush
the boundaries of nano-fabrication in the realm of two-dimensional materials.
Alignedgraphene/hBNdevicesarefabricatedincorporatingasuspendedregion.
Anelectrostaticforceisappliedtothesuspendedregionandusedtoimpartalateral
force tothe graphene/hBN stack.In situ longitudinalresistance measurements of a
*2 lm2 device identify the need for reduced sample sizes or alternative
higher-resolution characterisation methods.
High-resolution atomic force microscopy measurements are used to track the
changesofthesuperlatticeperiodunderincreasingloadsina300-nmscanwindow.
However, thermal drift becomes an acute adversary that prevents conclusive ver-
dicts on the nature of kink motion.
xi
xii Abstract
Scanning tunnelling microscopy measurements afford the most promise and
althoughthereareimportantchallengestoovercome,namelyaccuratelypositioning
the tip, it offers the best opportunity of studying graphene/hBN kinks.
