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LOCATION:ICFO Auditorium
SUMMARY:ICFO | RAJASHREE HALDANKAR
CLASS:PUBLIC
DESCRIPTION:This thesis investigates the mechanical and electrostatic behav
 iour of van der Waals heterostructures based on graphene and hexagonal bor
 on nitride (hBN). The work is centred on two related questions: how fabric
 ation-induced strain affects suspended graphene&ndash\;hBN devices\, and h
 ow scanning-probe measurements can be used to study moir&eacute\; domains 
 in twisted hBN. The first part of the thesis focuses on suspended graphene
 &ndash\;hBN heterostructures. These devices were fabricated by dry transfe
 r onto pre-patterned trenches\, followed by suspension release using super
 critical CO2 drying. After release\, the suspended stacks did not remain f
 lat\, but instead developed smooth out of-plane buckled profiles. Atomic f
 orce microscopy measurements show that this buckling is consistent with bu
 ilt-in compressive strain introduced during fabrication. Thermal-expansion
  mismatch\, transfer-induced stress\, and clamping at the contacts are all
  likely to contribute to the final mechanical state. Electrical measuremen
 ts under gate bias further show that electrostatic loading softens the upw
 ard-buckled configuration and can drive a snap-through transition into a d
 ownward-buckled state. In the present devices\, this transition is observe
 d from the up state to the down state\, while controlled switching back to
  the up state is not demonstrated. The second part of the thesis studies s
 liding ferroelectricity in marginally twisted hBN. In these devices\, the 
 small relative twist between the hBN layers produces a reconstructed moir&
 eacute\; pattern formed by alternating stacking domains. Kelvin probe forc
 e microscopy was used to measure the local electrostatic response of these
  domains. Bias-dependent measurements show that the two domain types have 
 distinct Kelvin- null positions\, with an untwisted hBN reference region l
 ying approximately between them. The comparison between first- and second-
 harmonic responses supports the interpretation that the observed domain co
 ntrast is mainly governed by local contact-potential differences rather th
 an by purely capacitive variations. The KPFM domain image is also analysed
  as a real-space map of the reconstructed moir&eacute\; network. Represent
 ative domain centres are extracted from the image and used to quantify the
  local moir&eacute\; geometry. This analysis provides a moir&eacute\;-scal
 e description of local wavelength\, effective twist variation\, and networ
 k disorder. These quantities are interpreted as geometrical descriptors of
  the reconstructed domain pattern\, not as direct measurements of atomic-s
 cale strain. Overall\, this thesis shows that strain\, electrostatics\, an
 d interfacial polarisation are central to the behaviour of graphene&ndash\
 ;hBN and twisted-hBN heterostructures. The suspended graphene&ndash\;hBN d
 evices demonstrate how residual strain controls mechanical stability and e
 lectrostatic actuation\, while the twisted-hBN measurements show how KPFM 
 can be used to study moir&eacute\; domains. Together\, these results provi
 de a basis for future studies of strain-controlled nanomechanics and elect
 rostatic domain mapping in two-dimensional materials.\nThesis Director: Pr
 of. Dr. Adrian Bachtold
DTSTAMP:20260922T154347Z
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