A Comment on the Letter by Sungjun Lee et al., [Phys. Rev. Lett. 95, 166402 (2005)]. The authors of the Letter offer a Reply.Peer reviewe

Foster, Adam S.

Krasheninnikov, A. V.

Nieminen, Risto M.

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This is an electronic reprint of the original article.This reprint may differ from the original in pagination and typographic detail.Author(s): Krasheninnikov, A. V. & Foster, Adam S. & Nieminen, R. M.Title: Comment on 'Paired Gap States in a Semiconducting CarbonNanotube: Deep and Shallow Levels'Year: 2007Version: Final published versionPlease cite the original version:Krasheninnikov, A. V. & Foster, Adam S. & Nieminen, R. M.. 2007. Comment on 'PairedGap States in a Semiconducting Carbon Nanotube: Deep and Shallow Levels'. PhysicalReview Letters. Volume 99, Issue 17. 179703/1. ISSN 0031-9007 (printed). DOI:10.1103/physrevlett.99.179703.Rights: © 2007 American Physical Society (APS). This is the accepted version of the following article:Krasheninnikov, A. V. & Foster, Adam S. & Nieminen, R. M.. 2007. Comment on 'Paired Gap States in aSemiconducting Carbon Nanotube: Deep and Shallow Levels'. Physical Review Letters. Volume 99, Issue17. 179703/1. ISSN 0031-9007 (printed). DOI: 10.1103/physrevlett.99.179703, which has been published infinal form at http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.99.179703.All material supplied via Aaltodoc is protected by copyright and other intellectual property rights, andduplication or sale of all or part of any of the repository collections is not permitted, except that material maybe duplicated by you for your research use or educational purposes in electronic or print form. You mustobtain permission for any other use. Electronic or print copies may not be offered, whether for sale orotherwise to anyone who is not an authorised user.Powered by TCPDF (www.tcpdf.org)Comment on ‘‘Paired Gap States in a SemiconductingCarbon Nanotube: Deep and Shallow Levels’’In a recent Letter [1], Lee et al. reported on paired,localized gap states in semiconducting carbon nanotubes.Pairs of gap states were found both near the band edges andin the middle of the gap. The authors explained theirscanning tunneling microscopy (STM) experimental ob-servations in terms of individual point defects, such asStone-Wales (SW) and vacancy-adatom (VA) complexes.Here we point out that such an interpretation of the experi-ments is hardly consistent with the abundance and stabilityof such defects. We also suggest an alternative interpreta-tion of the peaks, which we think is much more realisticand can explain all features reported in Ref. [1] in terms ofa single type of defects.The major problem with the interpretation of the experi-mental results given in Ref. [1] is the high formationenergy of the SW and VA defects (5 eVor larger, the actualvalue depending on the tube diameter and chirality), whichsuggests that the equilibrium concentration of these defectsis extremely low. In particular, the concentration of SWdefects was estimated [2] to be one defect in a few tenths ofa millimeter in the equilibrium state at typical growthtemperature (1500 K).Similar concentrations can be expected for VA defects.Besides, it is not obvious that the barrier Eb for the recom-bination of VA defects is high enough for the defect tosurvive the heat treatment preceding the STM experiments.Indeed, even a value of 1 eV would give the lifetime of thedefect at heat treatment temperature of 150 C of about 1 s.Thus there is no reason to believe such defects would existfollowing the heat treatment. Moreover, the highly reactivedangling bond of the C atom will likely be passivated byH atom, OH, or other chemical groups. Finally, the posi-tions of the theoretical peaks (deep levels) clearly do notmatch the experimental STM spectra, which are almostsymmetric with respect to the band edges.We believe the double-peak structures likely originatefrom two close point defects such as hydrogen adatoms orvacancies or, in principle, any other point defects which,when isolated, result in a single peak near the middle of thegap of a sp2-bonded carbon system [3,4]. This effect canbe expected for both single vacancies and adatoms, as asingle hydrogen adatom chemisorbed on top of asp2-hybridized carbon atom should change the hybridiza-tion of the atom from the sp2- to a sp3- type and thuseffectively remove the atom from the -electron network.Based on experimental data [5,6], one can conclude thatsuch defects must be stable at the temperatures used forheat treatment.Hydrogen adatoms tend to cluster together and formclose metastable pairs on graphite surfaces [7] due tolong-range electronic interaction between the defects,similar to adatoms on metal surfaces [8]. Thus, one canexpect that for two close defects the original state shouldsplit into two states. This can be understood within asimple theoretical picture illustrating the formation of thebonding and antibonding orbitals in a hydrogen molecule,albeit there is no direct overlap of electrons localized atH atoms, and the interaction is mediated by the nanotubeelectrons. Depending on the adatom-adatom separation,the positions of the peaks can be in the middle of the gap(deep levels), or close to the band edges (shallow levels).The splitting should be maximal for two adatoms on ad-jacent atoms and vanish at large separations between theadatoms. This picture is consistent with recent observa-tions of paired gap states in semiconducting nanotubessubjected to hydrogen-plasma treatment, and confirmedby density functional theory calculations [9].In summary, we think that Lee et al. carried out aremarkable experiment demonstrating the possibility ofengineering the local electronic structure of carbon nano-materials through point defects, but the authors misinter-preted their results.A. V. Krasheninnikov,1,2 A. S. Foster,1 andR. M. Nieminen11Laboratory of PhysicsP.O. Box 1100, FI-02015Helsinki University of Technology, Finland2Accelerator LaboratoryP.O. Box 43, FI-00014University of Helsinki, FinlandReceived 21 May 2007; published 26 October 2007DOI: 10.1103/PhysRevLett.99.179703PACS numbers: 71.20.Tx, 61.48.+c, 68.37.Ef, 81.07.De[1] S. Lee et al., Phys. Rev. Lett. 95, 166402 (2005).[2] K. Suenaga et al., Nature Nanotechnology 2, 358 (2007).[3] E. J. Duplock et al., Phys. Rev. Lett. 92, 225502 (2004).[4] A. V. Krasheninnikov, Solid State Commun. 118, 361(2001).[5] A. Nikitin et al., Phys. Rev. Lett. 95, 225507 (2005).[6] F. Banhart, Rep. Prog. Phys. 62, 1181 (1999).[7] L. Hornekær et al., Phys. Rev. Lett. 97, 186102 (2006).[8] J. Repp et al., Phys. Rev. Lett. 85, 2981 (2000).[9] G. Buchs et al. New J. Phys. 9, 275 (2007).PRL 99, 179703 (2007) P H Y S I C A L R E V I E W L E T T E R S week ending26 OCTOBER 20070031-9007=07=99(17)=179703(1) 179703-1 © 2007 The American Physical Society

Comment on 'Paired Gap States in a Semiconducting Carbon Nanotube: Deep and Shallow Levels'

Aaltodoc

This is an electronic reprint of the original article.
This reprint may differ from the original in pagination and typographic detail.
Author(s): Krasheninnikov, A. V. & Foster, Adam S. & Nieminen, R. M.
Title: Comment on 'Paired Gap States in a Semiconducting Carbon
Nanotube: Deep and Shallow Levels'
Year: 2007
Version: Final published version
Please cite the original version:
Krasheninnikov, A. V. & Foster, Adam S. & Nieminen, R. M.. 2007. Comment on 'Paired
Gap States in a Semiconducting Carbon Nanotube: Deep and Shallow Levels'. Physical
Review Letters. Volume 99, Issue 17. 179703/1. ISSN 0031-9007 (printed). DOI:
10.1103/physrevlett.99.179703.
Rights: © 2007 American Physical Society (APS). This is the accepted version of the following article:
Krasheninnikov, A. V. & Foster, Adam S. & Nieminen, R. M.. 2007. Comment on 'Paired Gap States in a
Semiconducting Carbon Nanotube: Deep and Shallow Levels'. Physical Review Letters. Volume 99, Issue
17. 179703/1. ISSN 0031-9007 (printed). DOI: 10.1103/physrevlett.99.179703, which has been published in
final form at http://journals.aps.org/prl/pdf/10.1103/PhysRevLett.99.179703.
All material supplied via Aaltodoc is protected by copyright and other intellectual property rights, and
duplication or sale of all or part of any of the repository collections is not permitted, except that material may
be duplicated by you for your research use or educational purposes in electronic or print form. You must
obtain permission for any other use. Electronic or print copies may not be offered, whether for sale or
otherwise to anyone who is not an authorised user.
Powered by TCPDF (www.tcpdf.org)
Comment on ‘‘Paired Gap States in a Semiconducting
Carbon Nanotube: Deep and Shallow Levels’’
In a recent Letter [1], Lee et al. reported on paired,
localized gap states in semiconducting carbon nanotubes.
Pairs of gap states were found both near the band edges and
in the middle of the gap. The authors explained their
scanning tunneling microscopy (STM) experimental ob-
servations in terms of individual point defects, such as
Stone-Wales (SW) and vacancy-adatom (VA) complexes.
Here we point out that such an interpretation of the experi-
ments is hardly consistent with the abundance and stability
of such defects. We also suggest an alternative interpreta-
tion of the peaks, which we think is much more realistic
and can explain all features reported in Ref. [1] in terms of
a single type of defects.
The major problem with the interpretation of the experi-
mental results given in Ref. [1] is the high formation
energy of the SW and VA defects (5 eVor larger, the actual
value depending on the tube diameter and chirality), which
suggests that the equilibrium concentration of these defects
is extremely low. In particular, the concentration of SW
defects was estimated [2] to be one defect in a few tenths of
a millimeter in the equilibrium state at typical growth
temperature (1500 K).
Similar concentrations can be expected for VA defects.
Besides, it is not obvious that the barrier Eb for the recom-
bination of VA defects is high enough for the defect to
survive the heat treatment preceding the STM experiments.
Indeed, even a value of 1 eV would give the lifetime of the
defect at heat treatment temperature of 150 C of about 1 s.
Thus there is no reason to believe such defects would exist
following the heat treatment. Moreover, the highly reactive
dangling bond of the C atom will likely be passivated by
H atom, OH, or other chemical groups. Finally, the posi-
tions of the theoretical peaks (deep levels) clearly do not
match the experimental STM spectra, which are almost
symmetric with respect to the band edges.
We believe the double-peak structures likely originate
from two close point defects such as hydrogen adatoms or
vacancies or, in principle, any other point defects which,
when isolated, result in a single peak near the middle of the
gap of a sp2-bonded carbon system [3,4]. This effect can
be expected for both single vacancies and adatoms, as a
single hydrogen adatom chemisorbed on top of a
sp2-hybridized carbon atom should change the hybridiza-
tion of the atom from the sp2- to a sp3- type and thus
effectively remove the atom from the -electron network.
Based on experimental data [5,6], one can conclude that
such defects must be stable at the temperatures used for
heat treatment.
Hydrogen adatoms tend to cluster together and form
close metastable pairs on graphite surfaces [7] due to
long-range electronic interaction between the defects,
similar to adatoms on metal surfaces [8]. Thus, one can
expect that for two close defects the original state should
split into two states. This can be understood within a
simple theoretical picture illustrating the formation of the
bonding and antibonding orbitals in a hydrogen molecule,
albeit there is no direct overlap of electrons localized at
H atoms, and the interaction is mediated by the nanotube
electrons. Depending on the adatom-adatom separation,
the positions of the peaks can be in the middle of the gap
(deep levels), or close to the band edges (shallow levels).
The splitting should be maximal for two adatoms on ad-
jacent atoms and vanish at large separations between the
adatoms. This picture is consistent with recent observa-
tions of paired gap states in semiconducting nanotubes
subjected to hydrogen-plasma treatment, and confirmed
by density functional theory calculations [9].
In summary, we think that Lee et al. carried out a
remarkable experiment demonstrating the possibility of
engineering the local electronic structure of carbon nano-
materials through point defects, but the authors misinter-
preted their results.
A. V. Krasheninnikov,1,2 A. S. Foster,1 and
R. M. Nieminen1
1Laboratory of Physics
P.O. Box 1100, FI-02015
Helsinki University of Technology, Finland
2Accelerator Laboratory
P.O. Box 43, FI-00014
University of Helsinki, Finland
Received 21 May 2007; published 26 October 2007
DOI: 10.1103/PhysRevLett.99.179703
PACS numbers: 71.20.Tx, 61.48.+c, 68.37.Ef, 81.07.De
[1] S. Lee et al., Phys. Rev. Lett. 95, 166402 (2005).
[2] K. Suenaga et al., Nature Nanotechnology 2, 358 (2007).
[3] E. J. Duplock et al., Phys. Rev. Lett. 92, 225502 (2004).
[4] A. V. Krasheninnikov, Solid State Commun. 118, 361
(2001).
[5] A. Nikitin et al., Phys. Rev. Lett. 95, 225507 (2005).
[6] F. Banhart, Rep. Prog. Phys. 62, 1181 (1999).
[7] L. Hornekær et al., Phys. Rev. Lett. 97, 186102 (2006).
[8] J. Repp et al., Phys. Rev. Lett. 85, 2981 (2000).
[9] G. Buchs et al. New J. Phys. 9, 275 (2007).
PRL 99, 179703 (2007) P H Y S I C A L R E V I E W L E T T E R S week ending26 OCTOBER 2007
0031-9007=07=99(17)=179703(1) 179703-1 © 2007 The American Physical Society


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