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Sensors andActuators A 203 (2013) 142148

Contents lists available at ScienceDirect

Sensors and Actuators A: Physical

journal homepage: www.elsevier .com/ locate /sna

Design and analysis offunctional multiwalled carbon nanotubes forinfrared sensors

R. Afrina, N.A. Shaha,, M. Abbas a, M. Amin a, A.S. Bhattib

a Thin Film TechnologyLaboratory, Department of Physics, COMSATS Institute of InformationTechnology, Islamabad 44000, Pakistanb Centre for Micro & Nano Devices, Department of Physics, CIIT, Islamabad, Pakistan

a r t i c l e i n f o

Article history:

Received 14May 2013Received in revised form 7 August 2013

Accepted12 August 2013

Available online 6 September 2013

Keywords:

Carbonnanotubes

Scanningelectronmicroscopy

Infrared sensors

a b s t r a c t

In this paper,we report the comparison ofinfrared detection based on buckypapers ofchemicallymodi-

fiedmultiwalled carbonnanotubesby carboxyl and thiol groups.Weobserved that functionalizedcarbonnanotube buckypapers had a significant impact on radiation sensing at room temperature. Nanotubes

buckypapers were formed by successively increasing cross-linker reagent in carbon nanotubes. It wasobserved that infrared sensitivity of resistively read-out of carboxylated and thiolated buckypapers

increased by increasing the cross-linker reagent up to certain limit. Further increase in cross-linkerreagent results in brittleness of buckypaper. The maximum sensitivity of carboxylated and thiolated

buckypapers was 3.4% and 16.07% while minimum response time ofcarboxylated and thiolated bucky-papers was 1.29 s and 6.09s, respectively. X-ray diffraction, scanning electron microscopy, transmission

electron microscopy, Raman spectroscopy and Fourier transform infrared spectroscopy were used foranalysis ofpresences ofnanotubes, structural morphology and defect density to evaluate all themodifi-cations ofthe nanotubes structure and the nature ofthe compounds added to the nanotubes surface by

functionalization treatment. 2013 Elsevier B.V. All rights reserved.

1. Introduction

Carbon nanotubes (CNTs) have been known as the exception-ally promisingmaterial for infrared (IR) sensors. One dimensionalquantum nature and large surface to volume ratio of CNTs makestheir intrinsic properties highly sensitive to small external pertur-

bations [17]. Infraredphotodectors arehighlydesirableforvariousapplications including remote sensing and biological imaging [8].Multiwalled carbon nanotubes (MWCNTs) based infrared detec-tors have receivedmuch attention due to theirmoderate band gap

of 0.46.0eV and high absorption efficiency in IR band [9]. Theextremely small specific heat of a carbon nanotube gives a bolo-metric (change in resistance under heating) thermal detector witha very fast response time and good sensitivity [1012].

CNTs infrared detectors were developed based on thermal andphoto effects. In thermal effect like Bolometers, the output sig-nal (resistance, current or voltage) was produced by temperaturechange due to infrared illumination [1315]. Moreover in photo

detector electronhole pairs aregeneratedanddissociated bypho-ton absorption of nanotubes leading the photo current or photo

Corresponding author. Tel.: +923215105363; fax: +92514442805.

E-mail addresses: raheen 06@yahoo.com (R. Afrin), nabbasqureshi@yahoo.com ,

nazar abbas@comsats.edu.pk (N.A. Shah), manzar@comsats.edu.pk (M. Abbas),

aminislamabad@yahoo.com(M. Amin).

voltage in thedevice [16]. Bolometer deviceswere fabricatedusingsingle or multiple tube arrays of single-walled and multiwalled

carbon nanotubes in the form of pristine or composites as activematerial for infrareddetection [15,1721]. For instance, theMikhailet al. reported the photo response of suspended SWCNTs networkunder vacuum on infrared illumination. The results showed the

Bolometric effect due to ultrafast relaxation of photo excited car-riers under IR illumination and transferring radiation energy tocrystal lattice which leads to change in resistance by rising thetemperature of samples [13]. Gohier et al. reported the Bolometric

effect using MWCNTs deposited on flexible polyimide substrate.The response to IR radiation had a significant resistance drop of0.35% after 10s of illumination at room temperature [9].

Regardless of these remarkable results, scientists are still try-

ing to improve the integrity, detectivity, responsivity, andsignal tonoise ratio of the CNTs-based Bolometers. Alternatively, buckypa-pers of CNTs have provided a potential approach to enhance theintegrity and sensitivity of infrared sensors [22].

CNTs with no intrinsic defects are chemically inert due tosp2 hybridization in C C bonding. Defects were created by acidtreatment which causes the functional group to attach on defectsites along side walls of nanotubes [2325]. The functional groups

induces the localized impurity states near the Fermi level withinthe band gap region along the radial directions on the side wallsof nanotubes and forms the local sp3 rehybridization of C C bond-ing [26]. These impurity states dislocates the conducting and*

0924-4247/$ see front matter 2013 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.sna.2013.08.018

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R.Afrin et al. / Sensors and Actuators A203 (2013) 142148 143

states. The scattering centers were formed due to functionaliza-

tionwhichdisturbs theballisticconductingproperties ofnanotubesand leads the considerable change in electronic states aswell as inthe electrical properties of nanotubes [27]. Furthermore, the elec-trodes on the outermost shells of MWCNTs, which may be either

semiconducting or metallic, plays the dominant role in electricaltransportvia photonabsorptionwhich causesthephonons genera-tion andexciton dissociationin nanotubes[12]. The semiconductorband gap of MWCNTs decreases inversely with the tube diame-

termakingnanotubes the excellent candidate for infrared sensing.Motivated by these advantages, we have investigated the infraredphotoresponse of unsuspended buckypapers of nanotubes.

In thiswork, buckypapers of chemically side-wall surfacemodi-

fied semiconducting CNTsby attachingdifferent functionalgroups:carboxylicacid ( COOH) and thiol ( SH)wereutilized asan impor-tant step for sensing infrared radiations. Buckypapers of function-alized CNTs were obtained by covalent cross-linking of nanotubes.

The cross-linking between multiwall nanotubes was achieved bycontrolled chemical treatments which enhances the mechanicalstrength aswell as to characterize the electrical conduction behav-ior as compared to those which are linked by only weak interac-

tions.Vacuumfiltrationprocesswasused forbuckypaper formationwhich provides the control of homogeneous thickness of buckypa-

per. Thedensity and thickness of buckypaper hasbeenmaintainedby observing the same weight measurements in each chemical

reaction of reactants. The CNTs in random network have beeninterconnected through electrodes for continuous electrical pathsbetween electrodes. It revealed the nonlinear currentvoltagebehavior and showed the semiconducting transport in network.

As functionalized CNTs produces the outer wall defects in nano-tubes, so the incident radiation on buckypaper absorb energy andexcitations and relaxation of valance electrons occurred with thecreation of vibrational modes when the frequency of infrared was

same as the vibrational frequency of attached bonds. This led toincrease in thermal energy with electronic excitations and relax-ation andhence the sensitivity via resistance changewasobservedwhich may be due to the charge carrier density.

Theproductswere characterizedbyusingX-raypowderdiffrac-tion (XRD, XPert PRO Difractometer, PANalytical) with Cu Kradiation( =1.5418A)andFourier transforminfrared(FT-IR) spec-trophotometer Nexus 670 ThermoNicolet in a PotassiumBromide

(KBr) tablet. Field emission scanning electronmicroscopy (FESEM)images were taken on JSM JEOL 7401 with ultra high resolution at3kV. Transmission electron microscopy (TEM) images were takenon a JEOL, JEM-2010 microscope (200kV) with EDX attachment.

Raman spectroscopy of samples was taken on Lab RAM HR800.Electrical measurementswere taken using optical microscope unitFS-70Z with keithley source metermodel 2636A using two-probemethod. Sensormeasurementswere madeusing keithley 6 digi-

talmultimeter 2100.

2. Experimental

2.1. Chemicals

2-Mercaptoethanol (HSCH2CH2OH) 99%pure 4,4-diaminoben-

zophenone (C13H12N2O), nitric acid (HNO3), hydrogen per oxide(H2O2), dimethylformamide (DMF) 99% pure, and all organic sol-vents were purchased from Sigma Aldrich and were used asreceived without further purification.

2.2. Fabrication and sensor setup

MWCNTs were purchased from Beijing DK Nano technology,

China. The diameter of MWCNTs used was about 2560nm and

length was 1020m. The MWCNTs were purified by reflux and

sonication in concentrated hydrochloric acid for 3h at 30C fol-

lowed by washing (pH 7), filtration and drying at 120 C for 24hin oven to remove the metal particles. For carboxylation, 0.5gpurified CNTs were mixed in 5M HNO3 and 35% H2O2 refluxedand sonicated for 5h at 100 C followed by washing (pH 7) with

distilled water, filtration from suspension and drying at 120 Cin oven for 24h. By this method, the obtained CNTs were oxi-dized by grafting COOH groups along the side walls of nanotubesand designated as MWCNTsCOOH. For thiolation of nanotubes,

0.02g of carboxylated CNTs were dispersed in 50mLmethanoland sonicated for 30min to form a stable black suspension. Thesuspension was added to solution of 2-mercaptoethanol (15mL)and methanol (30mL) and so obtained solution was sonicated for

3h at room temperature. The 2-mercaptoethanol modifiedMWC-NTs were achieved and denoted as MWCNTsSH (Fig. 1a). TheMWCNTsSHwere separated from the solution by centrifugationand dispersed in 50mLethanol then dried in oven at 120 C for

24h.For the cross-linking or mat/buckypaper formation of nano-

tubes, 0.02g of functionalizedCNTs ( COOH, SH) weredispersedin 15mLdimethylformamide (DMF) by sonication for 30min to

obtain nanotube suspension. In a separate reaction container, 0.1gof 4,4-diaminobenzophenone was dissolved in 10mLDMF. While

stirring the CNTs, the 4,4-diaminobenzophenone solution wasinjected at a rate of 0.1mL/min into suspension. After the addition

of 4,4-diaminobenzophenone, the reaction mixture was allowedto stir for 12h at 75 C. The solution was then vacuum filtered(Nylon filtration membrane of 0.45m pore size) and dried at120 C for 24h. The obtained buckypapers were designated as

CNTCBPs and CNTTBPs for carboxylated and thiolated nanotubesrespectively. The 4,4-diaminobenzophenone was used as cross-linking agent between nanotubes (Fig. 1b) in order to form thebuckypaper. In buckypaper formation the ratio of CNTs to cross-

linker was 1:5, 1:7, 1:9 and 1:11 by weight. - Fig. 1c showed theexperimental setup for measuring the sensing behavior. A MWC-NTs cross-linked buckypaper and buckypaper on glass substratewith connecting wires is shown in Fig. 1d. Actual image of IR sens-

ing setup and inset showing the sensor with holder are shownin Fig. 1e. The glass substrate was used as a mechanical support.The gold (Au) of 200nm thickness was deposited through sput-tering technique on glass substrate to make good contacts. The

area of one electrode on glass substrate was 5mm5mm whilethe separation between the electrodeswas a single line drawn bypaper cutter on the deposited thin film for the formation of sepa-rate electrodes. The area of CNTs film was about 2mm5mm on

gold electrodes. The unsuspended CNTs film was made by sonica-tion the nanotubes in organic solvent for 15min and suspensiondropwasplacedbetweenelectrodes. Furthermore, the gold coatedelectrodes were connected through copper pressure contacts with

resistancemeasuringdevice (Keithley6digitalmultimeter2100)and graphical read-out display unit. The buckypaper having area

of 2mm5mmwas placed on the glass substrate and the copperpressure contacts were directly made on buckypaper for onward

connection to resistance measuring device. The electrical proper-tiesweremeasured using KItools application software of keithleymultimeter. The infrared lamp (100W) was used to bring out con-

tinuous infrared light for characteristic behavior. The wavelengthof the infrared lamp was in far infrared range (501000m). Allthe measurements were performed in open air at room tempera-ture.

3. Results and discussion

Fig. 2 shows the X-ray diffraction analysis of pristine carbon

nanotubes. The XRD patterns were recorded in angle range (2)

20

80

, where the angle 2is between the incident andscattered

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144 R. Afrin et al. / Sensors and Actuators A203 (2013) 142148

Fig.1. (a)Reaction schemefor functionalizationand cross-linkingofMWCNTs;(b) stackingscheme;(c) schematic diagramof IRsensorsetup;(d) aMWCNTscross-linked

buckypaper andbuckypaper on glass substratewith connecting wires; (e)actual image of IR sensing setup and inset showing sensorwith holder.

beams. The XRD pattern of CNTs samples revealed the presences

of three peaks at 26.1576, 44.3053 and 51.6300 correspondingto d002, d101 and d102 reflections of hexagonal graphite struc-ture of carbon atoms. The structure has p63/mmc space groupwith lattice constant a= b=2.47A and c=6.8 A. The metal catalyst

particles peaks at 2=21.8162, 36.0105 and 43.2058 were also

observed.The chemical bonding status of functionalized nanotubes was

characterized by FT-IR spectroscopyas shown in Fig. 3. After acidictreatment, the CNTCBP showed the peak at 1360cm1 assigned to

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R.Afrin et al. / Sensors and Actuators A203 (2013) 142148 145

20 30 40 50 60 70 80

0

100

200

300

400

500

600

(102)C(1

01)C

(011)Zn

(012)Si

(011)Ni

(002)C

Intensity(a.u

)

2(Degree)

(001)Co

Fig. 2. XRD pattern of MWCNTs showing the catalyst particles and MWCNTs asgraphitic carbon.

O Hbendingvibrationsin plane.ThearomaticC C stretchingpeak

was observed at 1520 cm1.The absorption peak at 1725cm1 corresponds to C O was

attributed to stretching vibrations of carboxylic acid groups( COOH). The peaks at 2910cm1, 2840cm1 and 2850 cm1 cor-

responds to C H stretching in both spectra, may be attributeddue to the cross-linking of nanotubes. In CNTTBP, the bands inthe range 10501200cm1 assigned to C S were attributed to thestretching of thiocarbonyl groups [28]. The peaks at 785cm1 and

2345cm1 assignedtoS Hwere attributedto stretchingvibrationsof thiol groups. Thebroad asymmetrical stretchingbands observedat around 3410 cm1 in CNTCBP and 3390 cm1 in CNTTBP spectrawere assigned to O H stretching bands and might be attributed

to the trace of water in the KBr pellet used for analysis [29]. Fig. 4

showed the SEM micrograph of functionalized multiwall carbonnanotubes.

In Fig. 4a, we observed that MWCNTCOOH were obtained

without shortening of CNTs and inset showed the pristinenanotubes which were purified by concentrated hydrochloricacid to remove the amorphous carbon and metal particles.

Fig. 4b shows the CNTCBP buckypaper of oxidized nanotubes.These nanotubes were cross-linked through side walls via4,4-diaminobenzophenone which increases mechanical strength.Fig. 4c shows the thiol functionalized MWCNTs. The buckypa-

per of MWCNTSH (Fig. 4d) was obtained by crossed linked viachemical reaction and observed stacking morphology which may

500 1000 1500 2000 2500 3000 3500 4000 45008890

92

94

96

9888

90

92

94

96

98100

136029102850

Intensity(

a.u

)

CNTCBP1725

15203410

Wavenumber (cm-1 )

2345

23452345785

CNTTBP

23451080 1160

Fig.3. FT-IRspectra of carboxylic acidattachedbuckypaperand thiolatedbuckypa-

per of multiwall carbon nanotubes.

be due to cross linking and more interactions between thiol

groups.The transmission electron microscopy (Fig. 5) visualizes the

number of walls and linking of nanotubes to each other. Fig. 5ashows the buckypaper of carboxylated functionalized nanotubes,

inset showed the magnified image of single CNT with outer walldefects. Fig. 5b shows thebuckypaper of thiolatedCNTs with outerwall defects (brown arrow) and surface linkage of nanotubes (yel-lowarrow).

The currentvoltage plots for multiwall carbon nanotubescharacterized by IV-system are shown in Fig. 6. The curvesshowed the semimetallic behavior. This semimetallic behaviormight be due to the presence of semiconductive tubes and

metallic tubes at the same time and amorphous carbon compo-nent was probably present [30,31]. IV curve for CNTCOOH wasnearly linear and average resistance was 0.5k while IVcurvesfor CNTCBPs and CNTTBPs showed the nonlinear behavior hav-

ing average resistances of 1.6 k and 3.1k, respectively. Theobservednonlinearbehavior suggests that thenanotubesbuckypa-per behaves as a semiconducting material after functionalization.

IV results were optimized for four samples for each type of

CNTs.The effect of different surface modifications on the chemical

statesof MWCNTssurfacewasinvestigatedby Ramanspectroscopyat 633nm excitation wavelength. The bands of the Raman spectra

for chemically modified MWCNTs are shown in Fig. 7. The mainfeatures in the Raman spectra are the fundamental vibrationalexcitations induced by different types of translational breakingsymmetry perturbations attributed as D and G bands [32]. The

disorder-induced D band around 1330cm1 corresponds to probeall phonon branches of graphite near the and K points ofthe graphite two-dimensional (2D) brillouin zone due to surfacedefects bonding states on nanotubes. The tangential mode G at

1590cm1 which involves theoptical phononmodes betweentwodissimilar carbon atoms in the unit cell corresponds to Ramanenhanced C C bonding oscillation inside the well crystallizedgraphite phase structures of MWCNTs. After chemical modifica-

tion, the noticeable change consisting of increase in intensity ofD-band relative to G band was found in the Raman spectra. Thiswas because of covalently functionalized side walls altered to amore disorderedstructuredue to thiolation andcarboxylation. The

intensity of Raman peaks ID and IG were determined and ratiosof ID/IG can be characterized inversely proportional to the crys-tallinity La of conventional MWCNTs [32,33]. The calculated ID/IGratios of CNTTBP, CNTCBP and CNTCOOHwere 1.28, 1.22,and 1.20,

respectively. The high ID/IG ratio indicates that there were moresurface defects due to surface functional groups onMWCNTs aftermodifications. The intensity ratio of CNTCOOH was less than theintensity ratio of CNTCBP which showed that more defects were

created along side walls due to cross-linking of nanotubes. Theintensity ratio of CNTTBP showedmore defects creation along side

walls due to SH covalent bonding and cross-linking of the nano-tubes.

Fig. 8 compares the photo response in terms of resistancechange of multiwall carbon nanotubes buckypapers. The changein resistance was caused by infrared radiation and given as

R=RR0, Where R0 is the sample resistance before infraredradiation turned on and R is the maximum resistance atinfrared radiations. The sensitivity of sensor was measured bycalculating mean values using R/R0 for each cycle [15]. The

observed response was due to presence of photo excited elec-trons and holes and also due to the rise in temperature [34].Because the metal electrodes on the outermost shells of theMWCNTseithersemiconductingormetallicplay thedominant role

in electrical transport via photons absorption which causes the

phonons generation and exciton dissociation in the nanotubes.

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146 R. Afrin et al. / Sensors and Actuators A203 (2013) 142148

Fig. 4. SEMimages of (a)MWCNTCOOH, (b)buckypaper ofMWCNTCOOH,(c) MWCNTSH, and (d) buckypaper ofMWCNTSH.Scale bars arein micrometers.

As the buckypaper of functionalized nanotubes was formed bychemically treating thenanotubeswith 4,4-diaminobenzophenonwhichincreasedmechanicalstrength anddecreaseselectron trans-port. Since nanotubes were randomly distributed within thebuckypaperand tube-endtobody-endcross-linkingmostfavorable

which slow down the motion of electrons within the nanotubesdue to creation of many defects along the side walls. Also chem-ical treatment can led to the p-doping which down shifts theFermi level in the valance band and may results in weaken-

ing of all interband transitions in semiconducting tubes causingincrease in resistance. This increase in resistance was observed

via infrared sensing of device. During infrared illumination, thedangling bonds created by functional groups also absorb theinfrared radiations along with the excitation of electrons andproduces lattice vibrations resulting, more change in resistancethus increasing the sensitivity of the device. Different prototypes

of each buckypaper were fabricated and optimized for infraredsensing as shown in Fig. 8. In this figure, the ratio of CNTs tocross-linker in buckypaper from bottom to top was 1:5, 1:7, 1:9and 1:11 by weight. The sensitivity (Rs) of carboxylated nano-

tubes (Fig. 8a CNTCOOH) thin film was 0.36% and responsetime (tres) was 1.29s. While the sensitivity of resistively read-out

Fig.5. (a)TEM image ofMWCNTCOOHbuckypaper inset shows themagnifiedimageof singleCNTwith outerwalldefects(b) TEMimageofMWCNTSHbuckypaper. Brown

arrow shows the defects on outerwalls and yellow arrowshows the surface linking of nanotubes. Scale bars are in 50 nanometers. (For interpretation of the references to

color in this figure legend, thereader is referred to theweb version of thearticle.)

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R.Afrin et al. / Sensors and Actuators A203 (2013) 142148 147

Fig. 6. IV characteristic curves showing semimetallic behavior due to presence

of semiconducting and metallic multiwall nanotubes in functionalized multiwall

carbonnanotubes.

of carboxylated buckypapers (CNTCBPs) from bottom to top (CNTto cross-linker ratio 1:5, 1:7, 1:11) was 1.24%, 1.54% and3.4% and response time 1.29s, 3.7s, 7s and 6.05s, respectively

(Fig. 8a). Furthermore the sensitivity of thiolated buckypapers(CNTTBPs) from bottom to top was 3.36%, 3.8%, 8.87% and 16.07%and response time 6.09s, 7.49s, 11.59 s and 6.24s, respectively(Fig. 8b).

The overall decrease in resistance of device was due to envi-ronmental effects and it can be reduced by operating the deviceunder vacuum [35]. We observed that by increasing the CNTsto cross-linker ratio up to 1:9, results in increasing the sen-

sitivity and response time of the device. Further increase incross-linkermakes the buckypaper brittle thus reduced sensitivity

1000 1200 1400 1600 18001000

1200

1400

1600

1800

2000

2200

2400

2600

2800

Intensity(a.u

)

Wave number ( cm-1)

CNTCBP

CNTCOOH

CNTTBP

1329.9

1590.8

Fig. 7. Raman spectra of functionalizednanotubes andbuckypapers.

and the response time. Below this ratio (1:5) no buckypaperwas formed. In case of CNTTBPs samples, we observed larger

resistance change as compare to CNTCBPs samples on infraredillumination. We also observed the change in electrical prop-erties after functionalization and small current flows throughthese devices. When carbon nanotubes were treated for thiola-

tion and buckypaper formation, more defects along side wallswere created as observed in Raman analysis. These defectscauses down shift the Fermi level to the valance band [36]and may minimize the excitation of electrons under illumina-

tion which causes the resistance of the device to be increased asobserved in electrical properties and infrared sensing of CNTTBPssamples.

20 40 60 80 100 120

14.00

14.05

14.10

14.1559

60

61

62

920

940

960

2.5k

2.6k

2.6k

2.7k

2.8k

offoff

offoff

onononononon

Time (sec)

CNTCOOHt

res= 1.29 s, R

s= 0.36%

tres

= 3.7 s, Rs= 1.24%

off

off

offon

on

on

on

CNTCBP2

tres

= 7 s, Rs= 1.54%

offoff

offon

on

on

on

Resistance(ohm)

CNTCBP3

tres

= 6.05 s, Rs= 3.4%

offoffoff

ononon

on

CNTCBP4

on

off

0 20 40 60 80 100 120

370

380

390

760

780

800

820

8407.5k

8.0k

8.5k

9.0k

9.5k

10.0k

100k

120k

140k

tres

= 6.09 s, Rs= 3.36%

Time (sec)

CNTTBP1

tres

= 7.49 s, Rs= 3.8%

CNTTBP2

tres

= 11.59 s, Rs= 8.87%

on

Resi

stance(ohm)

CNTTBP3

on

on

offoff

onon

off

off

tres

= 6.24 s, Rs= 16.07%

off

on

CNTTBP4

offoff

onon

offoff

onon

offoff

on

on onon

offoff

off

on

onon

on

offoff

off

onon

offoff

off

Fig. 8. Infraredsensor responseof differentprototypes of (a) CNTCOOH, CNTCBPs and (b) CNTTBPs.

7/22/2019 Design and analysis of functional multiwalled carbon nanotubes forinfrared sensors

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