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Image formation mechanisms in scanning electron microscopy of carbon nanotubes,and retrieval of their intrinsic dimensions.
Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
Karlstad University, Faculty of Technology and Science, Department of Mechanical and Materials Engineering.
Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.ORCID iD: 0000-0003-1711-5595
2013 (English)In: Ultramicroscopy, ISSN 0304-3991, Vol. 124, 35-39 p.Article in journal (Refereed) Published
Abstract [en]

We present a detailed analysis of the image formation mechanisms that are involved in the imaging of carbon nanotubes with scanning electron microscopy (SEM). We show how SEM images can be modelled by accounting for surface enhancement effects together with the absorption coefficient for secondary electrons, and the electron-probe shape. Images can then be deconvoluted, enabling retrieval of the intrinsic nanotube dimensions. Accurate estimates of their dimensions can thereby be obtained even for structures that are comparable to the electron-probe size (on the order of 2 nm). We also present a simple and robust model for obtaining the outer diameter of nanotubes without any detailed knowledge about the electron-probe shape.

Place, publisher, year, edition, pages
Elsevier, 2013. Vol. 124, 35-39 p.
Keyword [en]
Scanning electron microscopy; Carbon nanotubes; Imaging mechanisms; Deconvolution; Electron-probe shape
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
URN: urn:nbn:se:kau:diva-16425DOI: 10.1016/j.ultramic.2012.08.012ISI: 000311823700006PubMedID: 23142742OAI: oai:DiVA.org:kau-16425DiVA: diva2:589621
Funder
Swedish Research Council, 2010-4324
Available from: 2014-02-21 Created: 2013-01-18 Last updated: 2015-09-24Bibliographically approved
In thesis
1. Mechanical behaviour of carbon nanostructures
Open this publication in new window or tab >>Mechanical behaviour of carbon nanostructures
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Abstract

Carbon nanotubes (CNTs) have extraordinary mechanical and electrical properties. Together with their small dimensions and low density, they are attractive candidates for building blocks in future nanoelectromechanical systems and for many other applications. The extraordinary properties are however only attained by perfectly crystalline CNTs and quickly deteriorate when defects are introduced to the structure. The growth technique affects the crystallinity where in general CNTs grown by arc-discharge are close to perfectly crystalline, while CVD-grown CNTs have large defect densities. Mechanical deformation also affects these properties, even without introducing defects. When CNTs are bent they behave similarly to drinking straws, i.e. they buckle or ripple and their bending stiffness drops abruptly.

In this thesis, the mechanical behaviour of individual CNTs and vertically aligned carbon nanofibers (VACNFs) has been studied by performing force measurements inside electron microscopes. Cantilevered CNTs, and VACNFs, were bent using a force sensor, yielding force-deflection curves while their structure was imaged simultaneously.

We have found that CNTs grown by arc-discharge have a high enough crystallinity to possess a Young’s modulus close to the ideal value of 1 TPa. CVD-grown CNTs possess a Young’s modulus that is about one order of magnitude smaller, due to their large defect density. The VACNFs are yet another order of magnitude softer as a result of their cup-stacked internal structure.  We also found that a high defect density will increase the critical strain for the rippling onset and the relative post-rippling stiffness. Multi-walled CNTs with a small inner diameter are less prone to ripple and have a larger relative post-rippling stiffness. Our findings show large variations in the onset of rippling and the bending stiffness before and after rippling. These variations open up possibilities of tailoring the mechanical properties for specific applications.

Abstract [en]

Baksidetext

Carbon nanotubes (CNTs) have extraordinary mechanical and electrical properties. Together with their small dimensions and low density, they are attractive candidates for building blocks in nanoelectromechanical systems (NEMS), and many other applications.  In this thesis the mechanical behaviour of individual CNTs and vertically aligned carbon nanofibers has been studied by performing force measurements inside electron microscopes. We have found that the mechanical behaviour is very sensitive to the defect density and the internal structure of the CNTs. The extraordinary properties are only attained by defect free CNTs and quickly deteriorate if defects are introduced to the structure. Mechanical deformations also alter these properties. Single-walled CNTs behave similarly to drinking straws when bent, i.e. they buckle, while the inner tubes of multi-walled CNTs prevent buckling. Instead a more distributed rippling pattern is created for multi-walled CNTs. Both these deformation behaviours will cause an abrupt drop in the bending stiffness, which is detrimental for many applications. The findings in this work will have implications for the design of future NEMS.

Place, publisher, year, edition, pages
Karlstad: Karlstads universitet, 2014
Series
Karlstad University Studies, ISSN 1403-8099 ; 2014:33
Keyword
carbon nanotubes, CNT, multiwalled carbon nanotubes, MWCNT, rippling, buckling, mechanical properties, transmission electron microscopy, TEM, scanning electron microscopy, SEM, atomic force microscopy, AFM, Young’s modulus, in situ TEM, in situ SEM
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:kau:diva-32041 (URN)978-91-7063-566-3 (ISBN)
Public defence
2014-06-13, 21A342, Karlstads universitet, Karlstad, 10:15 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 2010-4324
Note

Artikel 2 Image formation mechanisms tidigare som manuskript, nu publicerad: urn:nbn:se:kau:diva-16425 (MÅ 150924)

Available from: 2014-05-23 Created: 2014-05-06 Last updated: 2017-08-11Bibliographically approved
2. Mechanical properties of carbon nanotubes and nanofibers
Open this publication in new window or tab >>Mechanical properties of carbon nanotubes and nanofibers
2012 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Carbon nanotubes (CNTs) have extraordinary electrical and mechanical properties, and many potential applications have been proposed, ranging from nanoscale devices to reinforcement of macroscopic structures. However, due to their small sizes, characterization of their mechanical properties and deformation behaviours are major challenges. Theoretical modelling of deformation behaviours has shown that multi-walled carbon nanotubes (MWCNTs) can develop ripples in the walls on the contracted side when bent above a critical curvature. The rippling is reversible and accompanied by a reduction in the bending stiffness of the tubes. This behaviour will have implications for future nanoelectromechanical systems (NEMS). Although rippling has been thoroughly modelled there has been a lack of experimental data thus far. In this study, force measurements have been performed on individual MWCNTs and vertically aligned carbon nanofibers (VACNFs). This was accomplished by using a custom-made atomic force microscope (AFM) inside a scanning electron microscope (SEM). The measurements were done by bending free-standing MWCNTs/VACNFs with the AFM sensor in a cantilever-to-cantilever fashion, providing force-displacement curves. From such curves and the MWCNT/VACNF dimensions, measured from SEM-images, the critical strain for the very onset of rippling and the Young’s modulus, E, could be obtained. To enable accurate estimations of the nanotube diameter, we have developed a model of the SEM-image formation, such that intrinsic diameters can be retrieved. We have found an increase in the critical strain for smaller diameter tubes, a behaviour that compares well with previous theoretical modelling. VACNFs behaved very differently, as they did not display any rippling and had low bending stiffnesses due to inter-wall shear. We believe that our findings will have implications for the design of future NEMS devices that employ MWCNTs and VACNFs.

Place, publisher, year, edition, pages
Karlstad: Karlstad University Press, 2012. 60 p.
Series
Karlstad University Studies, ISSN 1403-8099 ; 2012:18
Keyword
atomic force microscopy, bending, carbon nanotubes, deformation, scanning electron microscopy, Young's modulus, carbon nanofibers, mechanical properties
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:kau:diva-12925 (URN)978-91-7063-422-2 (ISBN)
Presentation
2012-06-08, 21A342, Karlstads universitet, Karlstad, 10:15 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 2010-4324
Note

Artikel 2 Image formation mechanisms tidigare som manuskript, nu publicerad: urn:nbn:se:kau:diva-16425 (MÅ 150924)

Available from: 2012-05-24 Created: 2012-04-16 Last updated: 2015-09-24Bibliographically approved

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