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Near-edge x-ray absorption studies of Na-doped tetracyanoethylene films: A model system for the V(TCNE)x room-temperature molecular magnet
Linköping Univ, Dept Sci & Technol ITN.
KTH, School of Biotechnology (BIO), Theoretical Chemistry.
Linköping Univ, Dept Sci & Technol ITN.
Linköping Univ, Dept Phys Chem & Biol IFM.
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2008 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 77, no 5, 054420-1-054420-8 p.Article in journal (Refereed) Published
Abstract [en]

V(TCNE)(x), with TCNE=tetracyanoethylene and x similar to 2, is an organic-based molecular magnet with potential to be used in spintronic devices. With the aim of shedding light on the unoccupied frontier electronic structure of V(TCNE)(x) we have studied pristine TCNE and sodium-intercalated TCNE by near edge x-ray absorption fine structure (NEXAFS) spectroscopy as well as with theoretical calculations. Sodium-intercalated TCNE was used as a model system of the more complex V(TCNE)(x) and both experimental and theoretical results of the model compound have been used to interpret the NEXAFS spectra of V(TCNE)(x). By comparing the experimental and theoretical C K-edge of pristine TCNE, the contributions from the various carbon species (cyano and vinyl) could be disentangled. Upon fully sodium intercalation, TCNE is n doped with one electron per molecule and the features in the C and N K-edge spectra of pristine TCNE undergo strong modification caused by partially filling the TCNE lowest unoccupied molecular orbital (LUMO). When comparing the C and N K-edge NEXAFS spectra of fully sodium-doped TCNE with V(TCNE)(x), the spectra are similar except for broadening of the features which originates from structural disorder of the V(TCNE)(x) films. The combined results from the model system and V(TCNE)(x) suggest that the lowest unoccupied molecular orbital with density on the nitrogen atoms in V(TCNE)(x) has no significant hybridization with vanadium and is similar to the so-called singly occupied molecular orbital of the TCNE anion. This suggests that the LUMO of V(TCNE)(x) is TCNE- or vanadiumlike, in contrast to the frontier occupied electronic structure where the highest occupied molecular orbital is a hybridization between V(3d) and cyano carbons. The completely different nature of the unoccupied and occupied frontier electronic structure of the material will most likely affect both charge injection and transport properties of a spintronic device featuring V(TCNE)(x).

Place, publisher, year, edition, pages
2008. Vol. 77, no 5, 054420-1-054420-8 p.
Keyword [en]
ELECTRONIC-STRUCTURE; SPINTRONICS; MILLENNIUM
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:kth:diva-8341DOI: 10.1103/PhysRevB.77.054420ISI: 000253764000062Scopus ID: 2-s2.0-40749158753OAI: oai:DiVA.org:kth-8341DiVA: diva2:13635
Note
QC 20100726Available from: 2008-05-06 Created: 2008-05-06 Last updated: 2017-12-14Bibliographically approved
In thesis
1. First Principles Studies of Carbon Based Molecular Materials
Open this publication in new window or tab >>First Principles Studies of Carbon Based Molecular Materials
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The aim of this thesis was to investigate carbon based molecular materials at first principles levels. Special attention has been paid to simulations of X-ray spectroscopies, including near edge X-ray absorption fine structure (NEXAFS), X-ray photoelectron, and X-ray emission spectroscopy, which can provide detailed information about core, occupied and unoccupied molecular orbitals of the systems under investigation. Theoretical calculations have helped to assign fine spectral structures in high resolution NEXAFS spectra of five azabenzenes (pyridine, pyrazine, pyrimidine, pyridazine and s-triazine), and to identify different local chemical environments among them. With the help of NEXAFS, the characters of important chemical bonds that might be responsible for the unique magnetic properties of the tetracyanoethylene compound has been revealed. Calculations have demonstrated that X-ray spectroscopies are powerful tools for isomer identification and structure determination of fullerenes and endohedral metallofullerenes. A joint experimental and theoretical study on metallofullerene Gd@C82 has firmly determined its equilibrium structure, in which the gadolinium atom lies above the hexagon on the C2 axis. It is found that the gadolinium atom could oscillate around its equilibrium position and that its oscillation amplitude increases with increasing temperature.

In this thesis, several new computational schemes for large-scale systems have been proposed. Parallel implementation of a central insertion scheme (CIS) has been realized, which allows to effectively calculate electronic structures of very large systems, up to 150,000 electrons, at hybrid density functional theory levels. In comparison with traditional computational methods, CIS provides results with the same high accuracy but requires only a fraction of computational time. One of its applications is to calculate electronic structures of nanodiamond clusters varying from 0.76 nm (29 carbons) to 7.3 nm (20,959 carbons) in diameter, which enabled to resolve the long-standing debate about the validity of the quantum confinement model for nanodiamonds. Electronic structures and X-ray spectroscopies of a series of single-walled carbon nanotubes (SWCNTs) with different diameters and lengths have been calculated, which have made it possible to interpret the existing experimental results.

Place, publisher, year, edition, pages
Stockholm: KTH, 2008. x, 63 p.
Series
Trita-BIO-Report, ISSN 1654-2312 ; 2008:10
Keyword
first principles simulations, carbon based molecular materials, X-ray spectroscopies
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:kth:diva-4724 (URN)978-91-7178-963-1 (ISBN)
Public defence
2008-05-23, FB42, AlbaNova, Roslagstullsbaken 21, Stockholm, 10:00
Opponent
Supervisors
Note
QC 20100727Available from: 2008-05-06 Created: 2008-05-06 Last updated: 2010-07-27Bibliographically approved

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