Influence of O2 and N2 on the conductivity of carbon nanotube networks

1.1 MB·English

by  D. J. Mowbray

InfluenceofO andN ontheconductivityofcarbonnanotubenetworks
2 2
D. J. Mowbray1,∗ C. Morgan2, and K. S. Thygesen1
1Department of Physics, Center for Atomic-scale Materials Design (CAMD),
Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark
2Department of Physics, Molecular and Materials Physics Group,
Queen Mary University of London, Mile End Road, London E1 4NS, United Kingdom
(Received11February2009;Published22May2009)
We have performed experiments on single-wall carbon nanotube (SWNT) networks and compared with
density-functionaltheory(DFT)calculationstoidentifythemicroscopicoriginoftheobservedsensitivityof
thenetworkconductivitytophysisorbedO andN .PreviousDFTcalculationsofthetransmissionfunctionfor
2 2
isolatedpristineSWNTshavefoundphysisorbedmoleculeshavelittleinfluenceontheirconductivity.However,
bycalculatingthefour-terminaltransmissionfunctionofcrossedSWNTjunctions,weshowthatphysisorbed
O andN doaffectthejunction’sconductance.Thismaybeunderstoodasanincreaseintunnelingprobability
2 2
due to hopping via molecular orbitals. We find the effect is substantially larger for O than for N , and for
2 2
semiconductingratherthanmetallicSWNTsjunctions,inagreementwithexperiment.
0
1
PACSnumbers:73.63.–b,68.43.–h,73.50.Lw
0
2
n I. INTRODUCTION (7,7)metallicarmchair,(12,0)semimetalliczigzag,and(13,0)
a semiconducting zigzag SWNT junctions, shown schemati-
J cally in Fig. 1. Comparing our theoretical results for SWNT
Usingsingle-wallcarbonnanotubes(SWNTs)asnanosen-
4 junctions with experimental measurements for SWNT net-
sors, both individually and in SWNT networks, has been
1 works suggests that the surprising sensitivity to O and N
one of the most promising potential applications of SWNTs 2 2
] since their discovery.1,2 Several experimental studies have may be partially due to an increased tunneling probability
l demonstratedthattheconductanceofSWNTsystemsisrather throughO2andN2physisorbedatSWNTjunctions.
l
a sensitive to the presence of even single-molecule concentra- In Sec. II we describe experimental measurements of the
h influence of both O and N on the conductivity of SWNT
tions of physisorbed gas molecules such as O and N .3–9 2 2
- 2 2 networksandthecharacterizationofthesenetworksusingRa-
s Further, by measuring conductivity of individually charac-
e terized SWNTs,10 as well as thick (metal-like) and thin manspectroscopy.AdescriptionoftheDFTandNEGFmodel
m used to describe the microscopic origin of this effect is then
(semiconductor-like) SWNT networks,8,9,11 the response of
t. SWNTstocontaminantshasbeenshowntocorrelatewiththe provided in Sec. III. In Sec. IV we compare our theoretical
a intrinsic electronic properties of the material. For example, results for the SWNT junction transmission with the SWNT
m networkexperiments,followedbyaconcludingsection.
ithasbeenfoundthatthepresenceoflow-O concentrations,
2
- independentoftemperature,introducesanincreaseinconduc-
d
tance of approximately 20% on thin SWNT networks, while
n
II. EXPERIMENTALRESULTS
o anincreaseinconductanceofonlyabout1%isfoundforthick
c SWNTnetworks.8
[ Ontheotherhand,previoustheoreticalstudieshavefound Below we give a brief discussion of our experiments on

1 that SWNTs are rather inert, so that gases tend only to ph- SWNT network conductivity. A more detailed description
v ysisorb to the SWNT surface.12–19 For this reason, it was may be found in Refs. 8 and 9. Figure 2 shows experimen-
8 suggested that O2 should not effect conductance through talmeasurementsoftheconductancesensitivitytoO2 andN2
9 SWNTs, but only influence conductance at either SWNT-
4
SWNT junctions, at the SWNT-metal contacts, or at SWNT
2 defect sites.12,20 Although the conductivity of SWNTs with (a) Physisorbed O2 (b) Physisorbed N2
.
1 molecules physisorbed at defect sites has been extensively
0 studied,21–23 theconductivityoffour-terminalSWNT-SWNT
0
junctions has been previously studied only for small pristine
1
metallic SWNTs.24,25 The possible influence of physisorbed
:
v molecules on SWNT-SWNT junctions has not been investi-
i
X gated.
r In this paper we address the microscopic origin of the in-
a crease in conductance of SWNT networks when exposed to
O or N gas. To this end, we have performed density-
2 2
functional theory (DFT) calculations of the intratube trans-
mission within a SWNT and the intertube transmission be-
tween two SWNTs in the nonequilibrium Green’s function FIG.1: (Coloronline)Schematicsofa(13,0)SWNTjunctionwith
(a)physisorbedO and(b)physisorbedN .
(NEGF) formalism for O and N molecules physisorbed in 2 2
2 2
2
Diameter (nm)
0)) 4 3 2 1
(G 2.0 l = 532 nm
( exc
G 1.8 l = 785 nm
∆ exc
e 1.6
c
n
n Conducta e Intensity 111...024 (13,0) SWNT ((172,7,0) )S SWWNNTT
ge i ativ 0.8
n el
a R
h 0.6
C0.001
0.4
Time (h)
0.2
Time (s) 0.0
50 100 150 200 250 300 350 400
FIG. 2: (Color online) Fractional change in conductance ∆G = Raman Shift (cm-1)
G/G(0)−1 versus time t in seconds and hours (inset) following Diameter (nm)
exposuretoO andN forthin((cid:3),(cid:2))andthick( ,⊗)SWNTnet-
2 2 1 2 3 4
worksrespectively,onlog-logandlinear(inset)s(cid:35)cales(Refs.8and
9). 1.0 l =632.5 nm
exc
0.8
exposureforthickandthinSWNTnetworks. Thesesamples (12,0) SWNT
y (13,0) SWNT
wereinitiallyplacedundervacuum(∼1×10−6mbar)andir- sit (7,7) SWNT
n 0.6
radiatedbyaUVlight-emittingdiode(LED)(λ∼400nm)at nte
lowintensity(∼0.03mW/cm2)forapproximately12htodes- e I
orbsurfaceandinterbundleadsorbates(surfacedopants)from ativ 0.4
el
the SWNTs. Once the SWNT network’s conductance stabi- R
lized,thesampleswereexposedtoeitherO (99.5%pure)or 0.2
2
N (99.998% pure) at 1 atm. The conductance of the sam-
2
pleswasthenmonitoredbyperiodicallysampling(∆t≈1s) 0.0
the current while applying a fixed bias of 1 mV to the thick
(metal-like) SWNT network (R ≈ 1 kΩ) and 10 mV to the -500 -400 -300 -200 -100
thin(semiconductor-like)SWNTnetwork(R≈1000kΩ),as Raman Shift (cm-1)
showninFig.2.
After 5 min of exposure to O , the thin network shows an
2 FIG.3:(Coloronline)Ramanspectraandapproximatediameterdis-
increase in conductance of about 13% while the thick net-
tributionofHiPcoSWNTsampleforanexcitationwavelength(top)
work’s conductance changes by about 7%. For the same ex-
λ ≈532nm(lowerblackcurve),λ ≈785nm(upperredcurve),
exc exc
posure to N2, both networks show substantially smaller con- and(bottom)λexc ≈632.5nm(——). TheDFTcalculateddiame-
ductancechangesof2%–3%. However,atexposuretimesof tersofd≈9.76A˚,9.79A˚,and10.66A˚ for(7,7),(12,0),and(13,0)
morethan2h,thethinSWNTnetworkresponsetoN issim- SWNTs,respectively,areprovidedforcomparison(dashedlines).
2
ilartothatofthethickSWNTnetworktoO . Thismightbe
2
caused by a weaker physisorption of N to the SWNT net-
2
works than O2. The inset of Fig. 2 also shows that at very the high-pressure carbon monoxide (HiPco) method. Fig-
long exposure times the fractional change in conductance, ure 3 shows the radial breating mode (RBM) Raman sig-
∆G=G/G(0)−1,becomessaturatedafter24h.Further,the nals of HiPco samples at excitation wavelengths λ ≈ 532
exc
responsetoO2 depictedinFig.2showsthattheconductance nm, λexc ≈ 632.5 nm and λexc ≈ 785 nm. The van Hove
changeforathinSWNTnetworkisabouttwotothreetimes singularity energy separation was calculated using the tight-
that of the thick SWNT network at all times. This suggests bindingapproximationwiththecarbon-carboninteractionen-
thattheconductancechangeunderO2exposureisanintrinsic ergyγ0 ≈2.9eVandcarbon-carbonbondlengthaC−C ≈1.44
propertyoftheSWNTnetworks,presentevenatverylowO2 A˚.TheSWNTdiameterddependenceoftheRBMfrequency
concentrations. Hereinweshallfocusonthemicroscopicori- ν for isolated SWNTs on SiO has been shown26 to be-
RBM 2
ginofthenetworksensitivitytoO2andN2,withthetemporal have as νRBM ≈ 248/dt. The DFT calculated diameters for
behaviorofthenetworksdiscussedelsewhere.8,9 (7,7),(12,0),and(13,0)SWNTsofd≈9.76,9.79,and10.66
We have performed the Raman spectroscopy to character- A˚,respectively,arefoundtocorrelatewellwiththeHiPcoRa-
ize our SWNT network samples, which were produced via manshift,asshowninFig.3. Thisshouldensureagoodde-
3
Leads Central Region TABLEI:Changeinintertubeconductance∆G atthevalence-
inter
band maximum ε relative to the pristine junction for O and
Frozen VB 2
N physisorbed in SWNT junctions of (7,7), (12,0), and (13,0)
Layers 2
SWNTs, with the respective SWNT-O and SWNT-N separations
2 2
dinAngstrom.
beort
up
IntratTrans SWNTJunction O∆2Ginter(%N)2 SWNT-O2d[A˚]SWNT-N2
beort (7,7)Armchair 30 6 2.3 2.8
up
IntertTrans ((1123,,00))ZZiiggzzaagg 1184000 11340 22..65 22..88
O
2
Frozen
Layers
of each of the four SWNT layers adjacent to the boundaries
Leads HCprin,tobewithin0.1eVoftheHamiltonianfortherespective
leads H , so that max|Hprin − H | < 0.1 eV. In this way
α C α
theelectronicstructureattheedgesofthecentralregionwas
FIG.4: (Coloronline)Schematicof(13,0)SWNTjunctionconsist-
ingoffourSWNTleads(lightgray)coupledtothecentralscattering ensuredtobeconvergedtothatintheleads.
regionviafourSWNTprimitiveunitcellsorlayers(gray)“frozen” The Landauer-Bu¨tticker conductance for a multi-terminal
attheirrelativepositionsintheisolatedSWNT.Theatomicpositions systemcanbecalculatedfromtheGreen’sfunctionofthecen-
oftheremainingeightSWNTlayers(darkgray)andthephysisorbed tralregion,G ,accordingtotheformula32–34
C
O moleculehavebeenrelaxed.ASWNTseparationconsistentwith
2
experiments of approximately 3.4 A˚ has been used. The intratube G= G Tr[G Γ G† Γ ](cid:12)(cid:12) , (1)
transportthroughtheSWNTandtheintertubetransportbetweenthe 0 C in C out (cid:12)ε=εF
SWNTshasbeenshownschematically.
where the trace runs over all localized basis functions in the
central region. To describe the conductance at small bias
scription of the SWNT network’s work function, which may for semiconducting systems, the Fermi energy εF should be
besignificantlydifferentforsmallertubes. taken as the energy of the valence-band maximum εVB or
conduction-band minimum ε for p-type and n-type semi-
CB
conductors,respectively. ThecentralregionGreen’sfunction
III. BASICTHEORY iscalculatedfrom
(cid:34) (cid:35)−1
OurDFTcalculationshavebeenperformedwiththeSIESTA G (ε)= zS −H −(cid:88)Σ (ε) , (2)
C C C α
DFT code27,28 using a double-zeta polarized (DZP) basis set
α
for the physisorbed molecules (O and N), and a single-zeta
polarized(SZP)basissetfortheSWNTs(C),andthePerdew- where z = ε+i0+, SC and HC are the overlap matrix and
Burke-Ernzerhof exchange-correlation functional.29 We note Kohn-Sham Hamiltonian matrix of the central region in the
here that the DZP and SZP SIESTA basis sets have recently localizedbasis,Σαistheself-energyofleadα,
been shown to yield transmission functions in quantitative
agreement with plane-wave codes and maximally localized Σα(ε)=[zSCα−HCα][zSα−Hα]−1[zSC†α−HC†α], (3)
Wannierfunctions.30 WhenmodelingO wehaveperformed
2
spin-polarizedcalculations12 buthaveperformedspin-unpo- andthecouplingelementsbetweenthecentralregionandlead
α for the overlap and Kohn-Sham Hamiltonian are S and
larizedcalculationsotherwise. Cα
H respectively.
We have modeled the junction system using 6(11) primi- Cα
The coupling strengths of the input and output leads are
tiveunitcellsorlayersforeachzigzag(armchair)SWNTper
supercell, with a separation of approximately 3.4 A˚, as de- then given by Γin/out = i(Σin/out − Σ†in/out). For a four-
pictedfora(13,0)SWNTjunctioninFigs.1and4.31Thefour terminal SWNT junction, the intratube and intertube trans-
SWNT layers at the boundaries of the central region, shown missionfunctionsarecalculatedbychoosingtheappropriate
ingrayinFig.4,werekeptfixedattheirrelaxedpositionsin outputlead,asdepictedinFig.4.
the isolated SWNT. At the same time the central 4(9) primi-
tive unit cells from each tube, shown in dark gray in Fig. 4,
andthephysisorbedmoleculeswererelaxeduntilamaximum IV. THEORETICALRESULTS&DISCUSSION
force of less than 0.1 eV/A˚ was obtained. Since the super-
cellhasdimensionsof(cid:38) 25A˚ foreachSWNTjunction,aΓ ForeachofthethreetypesofSWNTjunctionsconsidered,
point calculation was sufficient to describe the periodicity of wefindthatbothO andN arephysisorbedwithbindingen-
2 2
thestructure. ergiesof∼0.2eV,asdepictedinFig.1. Further,theSWNT–
Such a large supercell was necessary for the Hamiltonian O andSWNT–N equilibriumseparationdistancedisinthe
2 2
4
N 2 HOMO O2 HOMOs (↑) O2 LUMOs(↓)
16
(a) (7,7) SWNT Junction Metallic
12
8
4
n
ssio 0 (b) (12,0) SWNT Junction Semimetallic
mi 12
s
n
Tra 8
be 4
u
ntrat 0 (c) (13,0) SWNT Junction Semiconducting
I
12
8
4
0
100 (d) (7,7) SWNT Junction Metallic
10-2
n10-4
o
missi100 (e) (12,0) SWNT Junction Semimetallic
s
n
Tra10-2
e
ub10-4
ert
Int100 (f) (13,0) SWNT Junction Semiconducting
10-2
10-4
-4 -3 -2 -1 0 1 2
Energy (eV)
FIG. 5: (Color online) (a)–(c) Intratube transmission and (d)–(f) intertube transmission vs energy in eV relative to the Fermi energy for a
pristine(blacksolidcurves)SWNTjunction,andwithN (bluedashedcurves),andO (reddash-dottedcurves)physisorbedconsistingof[(a)
2 2
and(d)]metallic(7,7)SWNTs,[(b)and(e)]semimetallic(12,0)SWNTs,and[(c)and(f)]semiconducting(13,0)SWNTs. Schematicsand
isosurfacesof±0.02e/A˚3formolecularorbitalsonthephysisorbedmoleculesareshownabovethecorrespondingeigenenergies(dottedlines).
range2.3—2.8A˚,asgiveninTableI.Theseresultsagreequal- curs. This is only slightly smaller than the expected band
itativelywithprevioustheoreticalstudiesforO bindingdis- gap of ∼ 0.7 eV, based on a d−1 fit to experimental data.36
2
tancesandenergiesonisolatedSWNTs.12–19 TheseresultsfortheintratubetransmissionofpristineSWNTs
alsoagreequalitativelywithpreviousDFTstudiesofisolated
Figures5(a)-5(c)showtheintratubetransmissionforthree
(5,5), (10,10), (10,5), (11,0), and (12,0) SWNTs.25,37,38 We
prototypicalSWNTscommonlyfoundinexperimentalHiPco
samples,35 as shown in Fig. 3. In Fig. 5(a) we see that for a alsofindinFigs.5(a)–5(c)thatneitherO2norN2physisorbed
at a SWNT junction noticeably influence the intratube trans-
metallic armchair (7,7) SWNT, transmission occurs through
mission.
two channels at the Fermi level. We see in Fig. 5(b) that
the conductance for the semimetallic zigzag (12,0) SWNT In Fig. 5 we also show isosurfaces and eigenenergies for
resembles that found in Fig. 5(a) for a metallic SWNT, ex- the highest occupied and lowest occupied molecular orbitals
cept for a tiny band gap of (cid:46) 0.05 eV at the Fermi level. In (HOMO and LUMO) on physisorbed O and N . For these
2 2
Fig. 5(c) we find for a semiconducting zigzag (13,0) SWNT weaklycoupledmolecules,therenormalizedmolecularlevels
a band gap of approximately 0.6 eV between the valence mayeasilybeidentifiedwiththemolecularorbitalsofthefree
and conduction bands, through which no transmission oc- O and N molecules.39 Since the position of the molecular
2 2
5
levels is rather insensitive to the type of junction, it should onlyslightly,alsoinqualitativeagreementwithexperiment,5
also be insensitive to the exact binding geometry. This sug- asshowninFig.2.
geststhatadditionalphysisorbedmoleculeswillinfluencethe Althoughitiswell-knownDFTcalculationsunderestimate
intertubetransmissionsimilarly. bandgaps,37,42,43 sinceweareprimarilyinterestedinhowthe
We find the intertube transmission is proportional to the presenceofO orN qualitativelychangestheDOSandcon-
2 2
density of states (DOS) for the system with peaks in the ductance,suchcalculationsarestilluseful.
transmissionatthevanHovesingularities. Thisisconsistent
withtransportbetweentheSWNTsoccurringinthetunneling
regime,asexpectedforaSWNTseparationofapproximately
V. CONCLUSIONS
3.4A˚.
The presence of physisorbed molecules in the SWNT-
Inconclusion,wehaveproposedapossiblemicroscopicex-
SWNT gap should then increase the tunneling probability at
planation for the experimentally observed sensitivity of the
energiesneartheeigenenergiesofthemolecularorbitals.This
electrical conductance of carbon nanotube networks to oxy-
is evidenced by the distinct peaks in the intertube transmis-
gen and nitrogen gases. Our DFT calculations suggests that
sionforeachSWNTjunctionatenergiescorrespondingtothe
O andN physisorbedincrossedSWNTjunctionscanhave
HOMO of N and the spin polarized HOMOs and LUMOs 2 2
2 a large influence on the intertube conductance. In particular,
of O , as seen in Figs. 5(d)–5(f). Under such conditions, a
2 forO thecloseproximityofthehighestoccupiedmolecular
SWNT junction behaves as a simple tunneling electron mi- 2
orbitals with the Fermi level of the SWNT significantly in-
croscopy (TEM) tip. By applying appropriate bias voltages,
creaseselectrontunnelingacrossthegap. Theeffectisfound
one may potentially probe the molecular orbitals of a ph-
tobelargerforO thanforN andforsemiconductingrather
ysisorbedmoleculetodetermineitschemicalcomposition. 2 2
thanmetallicSWNTs,inagreementwiththeexperimentalob-
For this reason, the sensitivity of SWNT network conduc-
servations. Our results suggests that the electrical properties
tivitytoO isatleastpartlyduetothecloseproximityofthe
2 ofSWNTnetworksaretoalargeextentdeterminedbycrossed
O HOMOs to the Fermi energies of typical SWNTs (≈ 0.6
2 SWNTjunctions.
eV), as shown in Fig. 5. Further, it has been shown experi-
mentallythatdefectsinherentinphysicallyrealizableSWNTs
yieldp-typesemiconductors.1,40,41Theconductivitymeasured
experimentally at small bias is thus at the energy of the va- Acknowledgments
lencebandε .
VB
As seen in Fig. 5(f), the O HOMO eigenenergy is only We thank E. I. Kauppinen, and K. W. Jacobson for useful
2
about 0.3 eV below ε for a semiconducting (13,0) SWNT discussions.D.J.M.andK.S.T.acknowledgefinancialsupport
VB
junction. As shown in Table I, this yields a substantial in- from NABIIT and the Danish Center for Scientific Comput-
creaseintheintertubeconductanceatzerobiasforsemicon- ingunderGrantNo. HDW-1103-06. TheCenterforAtomic-
ducting junctions in the presence of O , while much smaller scale Materials Design (CAMD) is sponsored by the Lund-
2
increases are found for the metallic and semimetallic junc- beckFoundation. C.M.acknowledgesfinancialsupportfrom
tions, inagreementwithexperiment.8 Ontheotherhand, we theNationalPhysicalLaboratory(NPL)andtheEngineering
alsofindphysisorbedN increasestheintertubeconductance andPhysicalSciencesResearchCouncil(EPSRC-GB).
2
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