35 research outputs found
Wrinkled few-layer graphene as highly efficient load bearer
Multilayered graphitic materials are not suitable as load-bearers due to
their inherent weak interlayer bonding (for example, graphite is a solid
lubricant in certain applications). This situation is largely improved when
two-dimensional (2-D) materials such as a monolayer (SLG) graphene are
employed. The downside in these cases is the presence of thermally or
mechanically induced wrinkles which are ubiquitous in 2-D materials. Here we
set out to examine the effect of extensive large wavelength/ amplitude
wrinkling on the stress transfer capabilities of exfoliated simply-supported
graphene flakes. Contrary to common belief we present clear evidence that this
type of "corrugation" enhances the load bearing capacity of few-layer graphene
as compared to 'flat' specimens. This effect is the result of the significant
increase of the graphene/polymer interfacial shear stress per increment of
applied strain due to wrinkling and paves the way for designing affordable
graphene composites with highly improved stress-transfer efficiency.Comment: 20 pages, 6 figure
Failure Processes in Embedded Monolayer Graphene under Axial Compression
Exfoliated monolayer graphene flakes were embedded in a polymer matrix and
loaded under axial compression. By monitoring the shifts of the 2D Raman
phonons of rectangular flakes of various sizes under load, the critical strain
to failure was determined. Prior to loading care was taken for the examined
area of the flake to be free of residual stresses. The critical strain values
for first failure were found to be independent of flake size at a mean value of
-0.60 % corresponding to a yield stress of -6 GPa. By combining Euler mechanics
with a Winkler approach, we show that unlike buckling in air, the presence of
the polymer constraint results in graphene buckling at a fixed value of strain
with an estimated wrinkle wavelength of the order of 1-2 nm. These results were
compared with DFT computations performed on analogue coronene/ PMMA oligomers
and a reasonable agreement was obtained.Comment: 28 pages. Manuscript 20 pages, 8 figures. Supporting information 10
pages, 6 figure
Suspended monolayer graphene under true uniaxial deformation
2D crystals, such as graphene, exhibit the higher strength and stiffness of
any other known man-made or natural material. So far, this assertion has been
primarily based on modelling predictions and on bending experiments in
combination with pertinent modelling. True uniaxial loading of suspended
graphene is not easy to accomplish; however such an experiment is of paramount
importance in order to assess the intrinsic properties of graphene without the
influence of an underlying substrate. In this work we report on uniaxial
tension of graphene up to moderate strains of 0.8% ca.. This has been made
possible by sandwiching the graphene flake between two polymethylmethacrylate
(PMMA) layers and by suspending its central part by the removal of a section of
PMMA with e-beam lithography. True uniaxial deformation is confirmed by the
measured large phonon shifts with strain by Raman spectroscopy and the
indication of lateral buckling (similar to what is observed for thin
macroscopic membranes under tension). Finally, we also report on how the stress
is transferred to the suspended specimen through the adhesive grips and
determine the value of interfacial shear stress that is required for efficient
axial loading in such a system
Theoretical study of silicon based nanoparticles and nanosystems
In this work we study a series of representative nanoscale systems based on silicon, in the context of common properties and principles which will assist in future applications in designing molecular materials based on these systems. The categories of the systems which we work on are: (a) hydrogenated and non-hydrogenated silicon nanoclusters and nanocrystallic systems with or without embedded transition metals, which constitute models of metal−semiconductor interfaces, (b) ultrathin hydrogenated silicon nanowires and (c) organometallic silicon−carbon multidecker-sandwiches. In addition to the study of structural, electronic, optical, vibrational and magnetic properties of these systems, we focus on a search for stabilizing mechanisms and in finding and defining rules that can function as “molecular designing tools” with the broadest possible validity. The silicon nanoclusters are stabilized to cage-like structures by the insertion of transition metals and are characterized by high symmetry and large energy gaps, desirable properties for applications in optoelectronics and nanoelectronics. In the study of silicon nanowires we compare the stability between nanowires with different surface structures while we formulate “magicity” rules for nanowires with which we interpret their stability and associate it with their elasticity and the distribution of the surface hydrogen. Finally, based on the isolobal principle the boron connection, we design and study a new class of organometallic multidecker-sandwich type nanostructures. During the elaboration of this dissertation we published overall 19 papers in international scientific journals and conferences’ proceedings.Στην εργασία αυτή μελετάμε μια σειρά από αντιπροσωπευτικά νανοφασικά συστήματα πυριτίου, στο πλαίσιο κοινών ιδιοτήτων και αρχών που θα βοηθήσουν σε μελλοντικές εφαρμογές σχεδιασμού μοριακών υλικών βασισμένων σε αυτά τα συστήματα. Οι κατηγορίες των συστημάτων με τα οποία ασχολούμαστε είναι (α) υδρογονωμένα και μη-υδρογονωμένα νανοσυσσωματώματα και νανοκρυσταλλικά συστήματα πυριτίου με ή χωρίς ενσωματωμένα μέταλλα μετάπτωσης, που αποτελούν χαρακτηριστικά μοντέλα ενδοεπιφάνειας μετάλλου−ημιαγωγού, (β) υπέρλεπτα υδρογονωμένα νανοσύρματα πυριτίου και (γ) οργανομεταλλικά πολλαπλών στρώσεων (multidecker-sandwiches) πυριτίου-άνθρακα. Εκτός από τη μελέτη των δομικών, ηλεκτρονικών, οπτικών, δονητικών και μαγνητικών ιδιοτήτων των συστημάτων, εστιάζουμε στην αναζήτηση μηχανισμών σταθεροποίησης και την εύρεση και καθορισμό κανόνων που μπορούν να λειτουργήσουν ως «εργαλεία μοριακού σχεδιασμού» με τη γενικότερη δυνατή ισχύ. Τα συσσωματώματα πυριτίου σταθεροποιούνται μέσου των μετάλλων μετάπτωσης σε δομές κλωβού και χαρακτηρίζονται συχνά από υψηλή συμμετρία και μεγάλα ενεργειακά χάσματα, ιδιότητες επιθυμητές για εφαρμογές στην οπτοηλεκτρονική και νανοηλεκτρονική. Στη μελέτη των νανοσυρμάτων πυριτίου συγκρίνουμε την σταθερότητα μεταξύ νανοσυρμάτων με διαφορετικές επιφανειακές δομές ενώ διατυπώνουμε κανόνα «μαγικότητας» νανοσυρμάτων με τον οποίο ερμηνεύουμε την σταθερότητά τους και την συνδέουμε με την ελαστικότητα και την κατανομή υδρογόνου στην επιφάνεια τους. Τέλος, βασιζόμενοι στην ισολοβική αρχή the boron connection, σχεδιάζουμε και μελετάμε μια νέα κατηγορία νανοδομών τύπου multidecker sandwiches οργανοπυριτίου. Στη διάρκεια εκπόνησης αυτής της διατριβής δημοσιεύτηκαν συνολικά 19 εργασίες σε διεθνή περιοδικά και σε πρακτικά συνεδρίων