Preparation, characterization, Raman, and terahertz spectroscopy study on carbon nanotubes, graphene nano-sheets, and onion like carbon materials

Accepted Manuscript Preparation, characterization, Raman, and terahertz spectroscopy study on carbon nanotubes, graphene nano-sheets, and onion like carbon materials A. Abouelsayed, Badawi Anis, Safwat Hassaballa, Ahmed S.G. Khalil, Usama M. Rashed, Kamal A. Eid, Emad Al-Ashkar, W. El hotaby PII:

S0254-0584(16)30977-4

DOI:

10.1016/j.matchemphys.2016.12.065

Reference:

MAC 19396

To appear in:

Materials Chemistry and Physics

Received Date: 14 June 2016 Revised Date:

8 December 2016

Accepted Date: 28 December 2016

Please cite this article as: A. Abouelsayed, B. Anis, S. Hassaballa, A.S.G. Khalil, U.M. Rashed, K.A. Eid, E. Al-Ashkar, W. El hotaby, Preparation, characterization, Raman, and terahertz spectroscopy study on carbon nanotubes, graphene nano-sheets, and onion like carbon materials, Materials Chemistry and Physics (2017), doi: 10.1016/j.matchemphys.2016.12.065. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Intensity (a.u.)

D/G = 0.19

1600

2550

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1 (-1cm-1) OLC2 (Ni catalyst)

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Raman Shift (cm )

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MWCNTs Graphene nanosheets OLC1 (large cages)

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MWCNTs Graphene nano-sheets OLC1 (large cages)

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Intensity (a.u.)

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Intensity (a.u.)

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ACCEPTED MANUSCRIPT MWCNTs

(b)

2D-mode Fit

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Graphene Nanosheets

(a)

OLC2 (small cages)

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MWCNTs

Graphene

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Preparation, Characterization, Raman, and Terahertz Spectroscopy Study on Carbon Nanotubes, Graphene Nano-sheets, and Onion like Carbon Materials. A. Abouelsayed*a , Badawi Anisa , Safwat Hassaballab , Ahmed S. G. Khalilc,d,e , Usama M.Rashedb , Kamal A. Eida , Emad Al-Ashkara , W. El hotabya Department, Physics Division,National Research Centre, 33 El Bohouth st. (fromer El Tahrir st.)- Dokki - Giza - Egypt P.O. 12622 b Physics Department, Faculty of Science, Al-Azhar University, Cairo 11884, Egypt. c Center for Environmental and Smart Technology, Fayoum University, Fayoum, Egypt. d Egypt Nanotechnology Center, Cairo University, Giza, Egypt. e Arab Academy for Science, Technology and Maritime Transport, Smart Village Campus, Giza, Egypt.

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a Spectroscopy

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Abstract

We present the optical properties of carbon nanotubes, graphene nanosheets, and onion like carbon (OLC) samples

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with different cages size in wide frequency range from 0.06 - 1650 THz. The samples were characterized by high resolution transmission electron microscope (HRTEM), Raman, and UV-Vis-IR-THz spectroscopy. The broad absorption bands centered at around 10, 3, 2.5, 1.5, and 1.8 THz for SWCNTs, MWCNTs, graphene nanosheets, large cages (OLC1 ), and small cages (OLC2 ) samples, respectively, are assigned to plasmon resonance due to the localization of free carriers in a finite length. For SWCNTs, both the plasmon band position and the Drude weight (D) are located at higher values as compared with MWCNTs, graphene nanosheets, and OLC sample, suggesting that the dimensionality of the system plays a major role regarding the carrier mobility of the graphene structure. The differences in the estimated values of

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D, the Fermi energy (Ef ), and density of carriers (N) in case of OLC samples can be due to the variation in sizes of the cages and the variation of the defects in the structure of the outermost layers of cages, where each cages consist of multi-layers of graphene enclosed one into another.

2010 MSC: 00-01, 99-00

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1. Introduction

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Keywords: Terahertz, Plasmon, Nanostructured Graphene, Carbon like Onion, Carbon Nanotubes.

10

otubes (SWCNTs and MWCNTs) have fascinating optical

The plasmonic resonance observed in one dimensional

and electronic properties which are mainly distinct from

(1D) and two dimensional (2D) materials provide great op-

their graphene sheet. Similarly, the 2D graphene exhibits

portunities to construct new optical device such as detec5

interesting optical and electronic properties which are fun-

tors [1, 2, 3, 4], sources [1, 2, 4, 5, 6], polarizers [2, 7, 8, 9], and antennas [2, 10, 11]. Therefore, the field of plasmon-

graphene. The single-walled and multi-walled carbon nan-

15

damentally diverse from their bulk graphite.

ics in 1D and 2D materials has attracted a great deal of

Large number of experimental studies showed that, the

interest, particularly after the recently observation of the

plasmon resonance can be tuned at terahertz frequencies

terahertz plasmon peak in 1D carbon nanotubes and 2D

through changing the width of graphene micro-ribbon and electrical gating [12, 13, 14, 15, 16, 17]. The THz radia-

∗ Ahmed

Abouelsayed Email addresses: [email protected] (A.

Abouelsayed*), a− [email protected] (A. Abouelsayed*) Preprint submitted to Journal of LATEX Templates

20

tion incident onto a finite width of graphene micro-ribbon launches collective electrons oscillations (plasmons) along December 29, 2016

ACCEPTED MANUSCRIPT the the width of the micro-ribbon with a frequency given 60 different cage sizes in a wide frequency range from 0.06

25

by the charge density (n) and micro-ribbon width (w).

to 1650 THz. We address the effect of the sp2 defects,

Long et al. [16] showed that such plasmon frequency (W p)

the hexagonal domain size, and the dimensionality on the

of graphene micro-ribbons scales as w−1/2 , where the W p

structure and optical properties by presenting Raman and

was shifted from 3 to 6 THz when micro-ribbon width

THz spectroscopy results. We estimate the Drude weight

decreased from 4 to 1 µm [16]. Furthermore, the plas- 65 D, the charge density N, the Fermi energy EF ), and the

tral weight with increasing n by using electrostatic gating. 30

Also, it has been found that the shift of plasmon reso1/4

nance scales as n

2. Experiment

. The pwer law dependenc of plasmon 2.1. SWCNTs

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excitation on w and n was expalined a signature of two-

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dc conductivity σdc for the samples under investigation.

mon peak was shifted to higher energy and gained spec-

dimensional massless Dirac electrons [18, 19].

Arc discharge SWCNTs were purchased from Carbon

For SWCNTs, broad plasmon absorption band centered 70 Solutions, Inc. (Type P2 and average diameter 1.4 nm). at ≈ 5.6 THz was observed in both metallic and doped

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35

2.2. Preparations of MWCNTs

semiconducting SWCNTs [20]. The origin of the plas-

al. [20]. Two scenario regarding to the origin of this peak

ide (MgO) have been prepared by combustion route [29,

have been proposed, whether it is due to inter-band ab-

30].

authors [21, 22, 23, 24] or due to the plasmon resonance

(NH4 )6Mo7 O24 .4H2 O was added to an aqueous solu-

along their finite lengths [25, 26, 27, 28]. Zhang et al. [20]

tion containing ferric nitrate Fe(NO3 )3 .9H2 O and magne-

concluded that this peak can not be attributed to cur-

sium nitrate Mg(NO3 )2 .6H2 O, keeping the molar ratio of

vature induced gap because of the following reasons: (i)

Mo:Fe:MgO at 1:1:13. Three times the stoichiometric ra-

The intensity of the THz broad absorption band for semi- 80 tio of urea was then added to the mixture and the mixture conducting SWCNTs was noticeably decreased after the

was heated at 70 ◦ C for 12 h. The solution was inserted in

carriers have been removed by annealing. (ii) THz peak

a preheated furnace at 550 ◦ C for 10 min. Then the com-

showed a very weak temperature dependence. Further-

bustion product was baked at 550 ◦ C for 3 h and ground

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more, the THz absorption band was observed in MWC50

55

The typical preparation method is as follows:

sorption across curvature induce gap as confirmed by many 75 weighted amount of ammonium molybdate hydrate

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45

Fe and Mo nanoparticles supported on Magnesium Ox-

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40

mon peak in SWCNTs have been discussed by Zhang et

to a fine powder.

NTs, where the outer tube diameter is almost larger than 85

Synthesis of MWNTs was carried out an in-house-

3 nm and the curvature induced gap is almost neglected

assembled CVD setup. The catalyst was spread uniformly

at this diameter [21, 24].

onto a quartz boat. The quartz boat was inserted into the

Several THz spectroscopic studies on different sp2 ma-

center of the quartz tube mounted inside an electrical tube

terials such as carbon nanotubes, graphene nano-sheets,

furnace. Subsequently, the furnace was heated to 950 ◦ C

and onions like carbon (OLC) samples have been done. 90 in an argon atmosphere. Then, a mixture of gases, typiThe previous studies have been performed on small THz

cally methane and argon was introduced into the reactor.

frequency range. Here, we present result of a THz spec-

The flow rates of methane and argon were maintained at

2

troscopy study on different sp such as SWCNTs, MWC-

50 and 100 sccm, respectively. After 30 min, the reactor

NTs, graphene nano-sheets, and two OLC samples with

was cooled to room temperature in an Argon atmosphere. 2

ACCEPTED MANUSCRIPT 95

The resulting black dense material containing carbon nan-

tained when a mixture of Ni and Co mixtures were used.

otubes (CNTs) around the oxide grains was ground care-

A mixture of Co and Ni catalysts was mounted in graphite

fully to a fine powder. The as-prepared CNTs were treated

rood inside a hole of 2 mm diameter and 20 mm depth.

with concentrated hydrochloric acid (HCl) at 60 ◦ C for 24135 The arc was generated between the two graphite electrodes in a chamber under argon atmosphere (500 mbar

times to the pH reaches 7. Then the product was dried,

and 130 sccm argon flow). The cathode and the anode

dispersed in ethanol under sonication, and filtered using

were graphite rods with (15 mm diameter, 30 mm long)

Millipore (0.2 µm) filter paper. The filtered product was

and (5 mm diameter, 120 mm long), respectively. Carbon

◦

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100

h. The product was washed with distilled water several

dried in an oven at 100 C for 2 h. In order to effectively140 like onion sample with small cage size (OLC2 ) was produced under the same conditions but only Ni powder was

remove the amorphous carbon present on the nanotubes walls, the dried sample was heated to 850 C in a furnace

used as a catalyst.

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105

◦

in flowing hydrogen and held at that temperature for 6 h.

2.4. Samples Characterization The samples were characterized with a Jeol -JEM-

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2.3. Preparation of Graphene nanosheets and Onion like Carbon

145

(HRTEM). The microscope was operated at 100 KV. The

Arc discharge method was used to synthesis graphene 110

Samples were dispersed in distilled water using an ultra-

nanosheets and OLC samples with different cage sizes. For

sonic probe and subsequently few two drops were put onto

graphene nanosheets the preparation method is as follows:

a copper TEM grid coated with amorphous carbon. Ra-

the arc is generated between two graphite rod i.e; cathode

150

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(15 mm diameter, 30 mm long) and anode (5 mm diameter, 120 mm long). The arc was generated in a reactor chamber 115

with three branches under argon atmosphere at ambient

pressure and flow rate 330 sccm. A current of about 90 A was applied between the two electrodes, where a constant

155

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distance (about 0.2 mm) between the cathode and anode

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man spectrometer(BRUKER, SENTERRA), interfaced to a microscope (100 MPLN Olympus objective). The infrared transmittance measurements were carried out at room temperature in the energy range 200-4000 cm−1 us-

tra 3000 system (Teraview Ltd. England)) by using the

trodes. During the arc generation process, the produced

ATR unit (350) with silicon crystal and under Nitrogen

carbon species by the vaporization were pushed from the 160

gas purging. The measuring range is in between 60 GHz and 3 THz (2 cm−1 - 100 cm−1 ) and the number of scans

and collected on a water-cooled holder. 125

perature and the spectra were recorded by a micro Ra-

hertz measurements were carried out using (TPS spec-

maintain a voltage drop of 40 V between the two elec-

high-temperature arc zone of the chamber by argon gas

man spectra were excited by 633 nm laser at room tem-

ing V-630 Spectrophotometer (Jasco, Japan). All tera-

were kept fixed by continuously translating the anode to 120

1011 high resolution transmission electron microscope

1800 scan/sec with spectral resolution 1.2 cm−1 .

Nickel powder (extrapure 99.5%, 100 mesh A.W. 58.71, Sisco research laboratories pvt. ltd. India) and Cobalt

3. Results and discussion

powder (99+, A.W. 59.93- Merck, Germany) mixture were used as a catalyst for OLC samples preparation. The pow-

Fig. 1 (a) and (b) shows the HRTEM images of the pu-

ders were ball-milled for 10 h to reduce the size of both165 rified MWCNTs and graphene nano-sheets, respectively. 130

metals to ≈ 1µm.

From Fig. 1 (a) one can observe the formation pure MWC-

Onion like carbon with large cage size (OLC1 ) was ob-

NTs without any significant amout of amorphous carbon. 3

ACCEPTED MANUSCRIPT (b)

(a)

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Defects in the most outer shell of the closed carbon cages

Figure 1: HRTEM images of (a) Multi walled carbon nanotubes obtained by CVD method over Fe1 :Mo1 :Mg13 catalyst.(b) Graphene

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nano-sheets prepared by arc discharge method: The upper right inset shows a well-ordered graphite structure. The upper left inset show a magnified HRTEM image of few layer graphene sheets.

Figure 2: HRTEM images of the OLC1 (large cages size) Onion-Like

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carbon synthesized by using Cobalt and Nickel catalyst.

The diameters of MWCNTs are ranged from 4 to 6 nm

170

with 1.9 nm inner tube diameter. The tubes wall thick-

well-aligned graphite layers toward the center of the cages

nesses are ranged from 1.03-2 nm. The HRTEM images of

are well shown in Fig. 2. The results are in consistence

the graphene nano-sheets are shown in Fig. 1 (b). It is The

with with those published in the literature [32, 33]. The

sample exhibits well-ordered graphite structure as shown195 inner diameters of the cavities take the values from 2 to in the upper right inset of the figure with 0.14 nm inter-

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175

10 nm and the wall thicknesses of the cages are ranged

atomic distance. The upper left inset of the Fig. shows few

from 2.5 to 10 nm diameter. The large diameters of the

multi layers of graphene sheets arranged over each other.

cages are almost similar to that demonstrated by Zhao et.

Plachinda et. al. [31] showed that the atomic arrange-

al. [33]. From Figs. 2 and 3, one can observe that the cages

ment in crystal lattice of nanographene sample appeared200 size estimated form the HRTEM images were found to be ber layers. The authors showed that a useful information

process. The cages sizes of the OLC1 sample prepared by

about the symmetry of the atomic arrangement in crystal

using Co-Ni catalyst are larger than the OLC2 sample pre-

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dependent on the catalysis used during the preparation

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180

in HRTEM images are strongly dependent on the num-

lattice of nanographene sample can be provide by adjusting the HRTEM contrast [31].

185

pared by Ni catalyst. Also, it is clear that the onion-like 205

carbon samples contain defects at the outermost layers.

A typical structure of OLC1 and OLC2 are appeared

These defects decreased in the case of OLC2 , where Co-Ni

in Figs. 2 and 3, respectively. As shown in the figures,

catalyst was used. It means that the addition of Co to

the two samples exhibit polyhedral-like shape or almost

the catalyst decreases the amount of defects. This could

spherical structure, where the carbon atoms are arranged

be attributed to the ability of Co atoms to enhance the

in nanoshape cages of ≈ 5-40 nm diameter. Each cage con-210 aromatisation and formation of crystalline graphite struc-

190

sists of multi-layers of graphene enclosed one into another.

ture [34]. The volume fraction of the onion-like carbon

It is clear from the Figs. that the interatomic distance

nanoparticles in the two two samples (Figs. 2 and 3) ap-

is ≈ 0.14 nm i.e; the width of the bright lines. Clearly,

pears to be the same. Only, there is a difference in the

a preferential curved orientation of the lattice fringes and

amount of catalyst impurities cobalt and nickel in the two 4

ACCEPTED MANUSCRIPT Graphene Nanosheets

D/G=0.012

2500

Defects in the most outer shell of the closed carbon cages

2800

1400

1600

2550

MWCNTs

D/G=0.66

(d)

OLC1 (Ni -Co catalyst)

D/G = 0.19

1200

2700

Intensity (a.u.)

Intensity (a.u.)

(c)

2600

Intensity (a.u.)

Cages inner diameter

(b)

2D-mode Fit

OLC2 (Ni catalyst)

D/G = 0.62

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Intensity (a.u.)

(a)

2700

2850

3000

-1

1200

1400

1600

2550

2700

2850

3000

-1

Raman Shift (cm )

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Raman Shift (cm )

Figure 4: Raman spectra recorded with a 633 nm excitation wavelength of (a) graphene nano-sheets, (b) MWCNTs, (c) OLC1 ) synthesized by using Cobalt-Nickel catalyst, and (c) (OLC2 ) synthesized

cages size) Onion-Like carbon synthesized by using Nickel catalyst.

nickel catalyst. Inst: Raman spectrum of the 2D mode together with

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Figure 3: HRTEM images of the samples synthesized OLC2 (small

the fitting (short dotted red lined) using Lorentzian oscillators.

215

samples. From HRTEM images, a rough estimation of the content of the onion-like carbon nanoparticles was found

the other hand, the D-band and the 2D originate from

to be about 85%.

a second-order process, involving two in-plane transverse

The vibrational modes of multi walled carbon nan-240 optic (iTO) phonons near the K point for the 2D band or one iTO phonon and one defect in the case of the D-

otubes and carbon related nanomaterials are well defined by Raman spectroscopy, particularly for determi-

band. However, the 2D band is allowed in the second order

nation of well ordered and disordered graphene struc-

Raman spectra of graphite materials without any kind of

tures [35, 36, 37, 38]. Fig. 4 (a) to (d) shows Raman spec-

disorder or defects. This process is called an inter-valley

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220

tra of graphene nanosheets, MWCNTs, OLC1 , and OLC2 ,245 process because it connects the two inequivalent K and K0 points in the first Brillouin zone of graphene structure.

namely, D mode ≈ 1340 cm−1 , G mode ≈ 1575 cm−1 , and

[41, 42, 43, 39].

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225

respectively. In general the spectra consist of three peaks,

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2D mode ≈ 2650 cm−1 . These values are consistent with

For highly crystalline single layer graphene, the 2D

those of graphite materials measured with 633 nm laser excitation [39].

230

mode has large intensity compared to the G and D modes. 250

The origin of the frequency in the 2D mode originates from

The G mode at ≈ 1575 cm−1 in the Raman spectra

a triple resonance (TR) Raman process. Where the inci-

of carbon related nanomaterials point out the presence of

dent or scattered photon and the two scattered phonons

the sp2 phase of graphite structure. This mode is related

are resonant with electronic levels in the graphene band

2

to the in-plane bond stretching of the sp bonded carbon

structure [39, 44]. As the number of graphene layers in-

atoms in hexagonal rings, and corresponds to the first or-255 creases as in the case of bi-layer graphene, a parabolic band

235

der scattering of the E2g phonon mode at Γ point of the

structure emerges with two valence bands and two conduc-

graphene Brillouin zone [38, 40]. It well known that, the

tion bands. This splitting in the valence and conduction

G-band is the only Raman band coming from a normal

bands results in four different double resonance (DR) Ra-

first order Raman scattering process in graphene [39]. On

man scattering processes giving rise to four Raman peaks 5

ACCEPTED MANUSCRIPT

1x10

structure of the outer nanotube during the purification

3

Reference Signal MWCNTs Graphene Nanosheets OLC1 (Large cages)

3

(a)

process. But in general the ratio of ≈ 0.66 has been used to estimate the quality of CNTs and the absence of any

OLC2 (Small cages)

significant amorphous carbon in the sample [45].

0

275

In the case of OLC samples shown in Fig. 4 (c) and (d), the D/G ratio is higher than the bilayers graphene [Fig. 4

-1x10

3

-2x10

3

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Terahertz signal (a.u.)

2x10

(a)], indicating the presence of considerable amount of de-

95 96 Scanner position (mm) : Sample-Reference 5

3x10

5

2x10

5

(b)

280

MWCNTs Graphene Nanosheets OLC1 (Large cages)

sation and formation of crystalline graphite structure [34].

OLC2 (Small cages)

Fig. 2and Fig. 3].

This result is inconsistent with the HRTEM images [see

The time-dependent electric field E(t) of the THz wave

285

1x10

5

shown in Fig. 5 (a) were recorded with and without the samples by using attenuation total reflection (ATR) unit with silicon crystal. The electric field E0 (t) through the

0

1

2 Terahertz (THz)

silicon substrate crystal were used as a reference. The cor-

3

responding electric field frequency-domain E(ω) for MWC-

Response of the SWCNTs,

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290

Figure 5:

addition of Co atoms to the catalyst enhances the aromati-

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4x10

case of OLC2 compared to OLC1 sample. In general the

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Sample Spectrum: Electric field (a.u.)

fects in both samples. The D/G ratio is higher in the

MWCNTs,

graphene

nanosheets, OLC1 and OLC2 samples at THz frequencies. (a) Mea-

NTs, graphene nanosheets, OLC1 , and OLC2 were then obtained by applying Fourier transformation as shown in Fig. 5 (b). Obviously, one can observe immediately a significant attenuation of THz radiation transmitted through

sured THz electric-field waveforms attenuated through the silicon substrate crystal (dash blue line) and the silicon crystal with sam-

260

carriers in the carbon related nano-materials. The attenuation spectra (-log10 T), where T is the transmittance, in the THz-Uv range for SWCNTs and MWC-

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ing time-domain signals.

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ples. (b) The samples spectra: Fourier transforms of the correspond-295

the samples due to the high response of electronic charge

in the 2D mode spectrum with intensity lower than the G

NTs, OLC1 and OLC2 samples are shown in Fig. 6 (a)

mode [41, 39]. As shown in the inset of Fig. 4 (a), the

and (b), respectively. For SWCNTs, the NIR-VIS ab-

2D mode can be fitted with four Lorentzian contributions300 sorption bands are attributed to the inter-band transi-

265

with relatively low intensity compared to the G-mode. In

tions across the van Hove singularities in the density of

addition, one can observe the very low intensity of the D-

states (DOS) [46]. These peaks correspond to both semi-

mode, where the D/G modes ratio ≈ 0.012. Accordingly,

conducting and metallic nanotubes. The peaks related to

we can conclude that the graphen sample consists of highly

semiconducting tubes are located around 0.82-1.23 eV and

crystalline bilayers graphene.

270

305

1.4-19 eV and coresspond to ES1 and ES2 groups, respec-

For the case of MWCNTs, one can observe that the

tively, while the peaks related to the metallic tubes are ap-

D/G ratio is ≈ 0.66. The increase of this ratio could

peared at about 2.05-2.4 eV (EM 1 group). These absorp-

be attributed to the destruction of the external hexagon

tion bands show spread in energy due to the different diam6

ACCEPTED MANUSCRIPT 1E-3 (a)

3

garding origin of this band in CNTs: due to the curvature 1

SWCNTs Drude

Fitting E00

S11

S22

M11



induced gap or plasmon resonance absorbtion. 320

In nonarmchair metallic SWCNTs, a narrow band gap induced is opened due to the lattice distortion in the rolledup graphene sheet. The lattice distortion is mainly due

2

to strong curvature in small nanotubes, where the bond

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Absorbance

4

Energy (eV) 0.01 0.1

length between the two carbon atom ac−c in the rolled 1

325

graphene sheet are slightly modified. The curvature induced gap is given by: Eg =

1

10

100

= 0.142 nm, γ◦ = 3.2 eV, and θ are the nanotube diam-

1000

Frequency (THz) 1

MWCNTs Graphene Nanosheets OLC1 (with large cages)

330

distortion in small diameter SWCNTs, the ac−c is slightly modified and subsequently the lengths of the two units vec-

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Absorbance

0.9

(b)

eter, the interatomic distance in graphene, the hopping integral, and the chiral angle, respectively. Due the lattice

Energy (eV) 0.01 0.1

1E-3

where d, aC−C

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0 0.1

3γ◦ a2c−c 4d2 cos3θ,

tors a1 and a2 in the graphene unit cell are also modified.

OLC2 (with small cages)

As a result, the hopping integral γ0 takes different values.

0.6

This yields a modification of the band structure around

335

0.3

the Fermi energy, except for the armchair nanotubes with chirality (n,m) where n=m. Therefore, a very small gap is

0.0 0.1

1

10

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opened at EF for all the metallic tubes rather than arm-

100

1000

Frequency (THz)

340

tube diameter 1/d2 as confirmed by scanning tunneling spectroscopy [23, 24]. For a SWCNT with d=1.5 nm the Eg is about 20 meV. This assumption can be correct, if the THz peak appears only in SWCNTs not also in MWC-

graphene nanosheets, OLC1 and OLC2 samples with the fitting curve

NTs. The observation of the THz peak in both doped

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Figure 6: Absorbance spectra of the (a) SWCNTs (b) MWCNTs,

with Drude-Lorentz function. In the spectra, there is a black window

semiconducting SWCNTs, metallic SWCNTs, and MWC-

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between 3 THz (upper limit of the THz range) and 5.5 THz (lower 345

limit of the FTIR range).

at around 4.5 eV corresponds to the π − π

NTs strongly suggest that this band is attributed to the plasmonics resonance. In SWCNTs, the peak significantly

eters and chiralities in the sample [46]. The band appeared 310

chair nanotubes. This gap is inversely proportional to the

∗

located at higher energy around 7.3 THz as compared to

electronic

MWCNTs. However, for small diameter nanotubes i.e.

transitions in the graphene structure. For the MWCNTs,

strong curvature, some re-hybridization between σ and π

graphene nanosheets, and OLC samples (Fig. 6 (b)), the350 states is also expected. Such re-hybridization between the

315

ES1 , ES2 , and EM 1 are not observed but only the π − π ∗

σ and the π states are strongly modified the lower lying

at 4.5eV can be observed.

conduction band energy levels and shifts them towards the

In Fig. 6 (a) and (b), one can observe two terahertz

Fermi level. Thus the energy gaps of some small diam-

bands at around 7.3 THz and 4.3 THz for SWCNTs and

eter nanotubes are decreased according to local density

MWCNTs, respectively. There are two assumptions re-355 functional (LDA) calculations[47]. Accordingly, there are 7

ACCEPTED MANUSCRIPT diameter ≥ 2 nm. In addition, the THz band is also ob-

2000

1 (-1cm-1)

(a) SWCNTs 1500

1

Drude

E00

S11

S22

M11

served in the case of MWCNTs at about 4.3 THz, while the broad plasmon band appeared at 1.4-2.8 THz in case

-*

of onion like carbon samples (see Fig. 6 (b)). This strongly

1000

365

suggest that the observed peaks are attributed to the plasmon resonance, where the THz radiation launches collec-

500

frequency related to the nanotube length and charge den0.01

0.1 1 10 Frequency (THz)

100

1000 370

MWCNTs Graphene nanosheets OLC1 (large cages)

9.0

sity. Therefore, the lower values of the plasmon beaks in

(b)

the case of MWCNTs and OLC samples could be related to the large confinement of the charge in small 1D scale

SC

1 (-1cm-1)

0 1E-3

RI PT

tive electrons oscillations along the nanotube axis with a

in the case of SWCNTs compared to the large diameter

OLC2 (small cages)

6.0

MWCNTs and the 2D graphene.

M AN U

The THz conductivity of the synthesized samples was 3.0

375

obtained from the complex transmission coefficient t(ω) = (T (ω))0.5 eiφ(ω) with the phase spectra φ(ω) and amplitude

0.0 1E-3

0.01

0.1 1 10 Frequency (THz)

100

the measurements. We first merged the data in the region

0.4 (c)

MWCNTs Graphene nano-sheets OLC1 (large cages)

between the THz, IR, and Vis-Uv by interpolation and

380

TE D

1 (-1cm-1)

0.3

|T (ω)| obtained directly from the THz spectrometer during

1000

OLC2 (small cages)

0.2

0.1

1 10 Frequency (THz)

100

385

1000

AC C

0.0 0.01

EP

0.1

extrapolation of the T (ω) spectra. To prevent spikes, we applied four-pass binomial smoothing. By applying the standard thin-film approximation, one can obtain the real and the imaginary part of the optical conductivity σ1 (ω), σ2 (ω) through [48, 49]: t(ω) =

1 + nsub 1 + nsub + Z◦ σ(ω)d

where d ≈ 0.001 mm is the samples thickness.

(1) The

impedance of free space Z◦ is 377 Ω [50], and n is the re-

Figure 7: The optical conductivity together with the fit with Drude-

fractive index of the silicon substrate n=3.5. Fig. 7 (a) and

Lorentz function and the fit components of (a) SWCNTs (b) MWC-

(b) show the real part of the conductivity for SWCNTs and

NTs, graphene nanosheets, OLC1 and OLC2 samples (c) The Drude-

390

contribution from free electron response.

MWCNTs, graphene nanosheets, and OLC samples in the broad spectral range, respectively. Obviously from Fig. 7 (a), the real part of the conductivity of the SWCNTs sam-

probably two competing effects: the energy gap is open-

ple shows a higher value of σdc in comparison with that of

ing around Ef due to the curvature induced effect and

MWCNTs, graphene nanosheets, and OLC samples. The

gap closing due to re-hybridization between the σ and395 σdc in the DC limits take the values of 1400Ω−1 cm−1 ,

360

the π states. Both the curvature-induced gap and the re-

25Ω−1 cm−1 , 30Ω−1 cm−1 , 18Ω−1 cm−1 , and 22 Ω−1 cm−1

hybridization effects are almost neglected for large tubes

for SWCNTs, MWCNTs, graphene nanosheets, OLC1 , 8

ACCEPTED MANUSCRIPT MWCNTs

Graphene nano-sheet

OLC1 (large cages)

OLC2 (small cages)

D (×103 G◦ cm−1 )

3.5

0.6

0.4

0.2

0.3

σDrude (Ω−1 cm−1 ) √ ωp = D(cm−1 )

1400

25

30

18

22

58

24

5

13

14

ωp /2π, 1 = 0(T Hz)

10

3

2.5

1.5

1.8

3.2

0.09

0.045

0.0073

0.011

230

39.5

27.5

12

N (×10 cm

−2

Ef (meV)

)

RI PT

SWCNTs

10.99

13.7

Table 1: Parameters estimated from fitting the real part of the optical conductivity spectra, where (D , σDrude , ωp , N , EF , and G◦ ) are the Drude weight, Drude conductivity in the DC limit, plasmon frequency, charge density, Fermi energy, and the quantum of conductance,

SC

respectively.

with Drude-Lorentz functions [48, 50, 57, 58].

and OLC2 , respectively. These values are listed in table 1. The large conductivity is expected in case of SWCNTs because of the quasi-one dimensional nature of SWCNTs in425 comparison with MWCNTs and 2D graphene, and onion

nitude of the response of free carriers absorption. The

level of THz conductivity (below 10 THz) indicates an in-

Drude weight can be written as 4πN e2 /m = ωp2 in con-

crease of the localization effects in the case of MWCNTs,

ventional semiconductors or metals, where Γ = 1/τ is the

OLC samples, and graphene nanosheets. These results

430

TE D

were found to be in good agreement with experimental

results as well with the theoretical calculations on the localization effects in MWCNTs nanotubes [51, 52, 53, 54].

According to the theoretical prediction, there is an increase 410

of the localization effect of the carriers in MWCNTs, lead-

435

EP

ing to a decrease in the drude conductivity [52, 53, 54, 55].

the density of carriers and carrier effective mass, respectively, and ωp is the plasma frequency [48, 50, 57, 58]. The Drude-Lorentz model fits the experimental results of the σ1 (ω) well within the measured spectral range of 6 GHz to 1500 THz, where the signal-to-noise ratio is con-

for graphene structure samples with sp2 hybridization and

AC C

massless electrons can be written as [12, 49, 59]:

et. al. [56] where the distances between individual tubes in

Z

the bundles are decreased by applying external hydrostatic

D=8

ω

σ1 (ω)d(ω) = (υf e2 /¯h)(πN )1/2

(3)

0

pressure. The results showed that the distance between the individual tubes in the bundles are tuned by apply-440

420

average scattering rate of charge carriers. N and m are

siderably good as shown in Fig. 7 (a) and (b). The D

A similar observation was also reported by Abouelsayed

415

(2)

Here the Drude weight (D) πσ◦ /τ characterizes the mag-

like carbon samples with different cage sizes. The lower

405

D 1 π (Γ − iω)

σ(ω) =

M AN U

400

with υf

=

1.1 × 106 m/s is the Fermi veloc-

ing external hydrostatic pressure and approaches to the

ity [59].

The estimated D values are (3.5 , 0.6, 0.4,

distance between the walls of MWCNTs, leading to red-

0.2, 0.3) ×103 G◦ cm−1 for SWCNTs, MWCNTs, graphene

shift, suppress the interband transitions, and decrease the

nanosheets, OLC1 , and OLC2 samples, respectively. The

level of the Drude conductivity peak (EM 0 ) in SWCNTs

D values and the carrier scattering rates Γ are listed

bundled [56]. Quantitative information on the THz Drude445 in table 1.

We can estimate the charge density N

contribution and plasmon peaks of the graphene structure

by applying the relation of the D, where the esti-

samples were obtained by fitting the conductivity spectra

mated N values were found to be about (3.2, 0.09, 9

ACCEPTED MANUSCRIPT

450

0.045, 0.0073, 0.011) ×1012 cm−2 for SWCNTs, MWC-

terlayer attraction between the outermost shells. It means

NTs, graphene nanosheets, OLC1 , and OLC2 samples,

that the interlayer attraction could even induce a metal-

respectively. By using the expression for the Fermi en-

to-insulator transition or vice-versa depending on the chi-

ergy EF = (N ¯ h2 υf2 π/e2 )1/2 , the Fermi velocity υf , and

rality [52, 60]. Therefore, we may assign this decrease

the carrier density N, one can obtain the Fermi energy490 in the THz conductivity in the case of MWCNTs to the transformation of the metallic and small-gap MWCNTs

(230, 39.5, 27.5, 10.99, 13.7) meV which is partially con-

to large-gap tubes caused by the increases of the number

sistent with the results obtained by Jnawali et. al. [49] for

of walls in the case of the MWCNTs. In FIG. 7, the de-

graphene sample. In fact, the results obtained by Jnawali

crease in the plasmon peak (E00 ) in the case of MWCNTs

RI PT

455

EF . The estimated Fermi energy were found to be about

et. al. [49] showed higher values of charge density N and495 menas that there is ni significant charge transfer between

cm−2 and 275 meV, respectively. The authors calculated

with the theoretical calculations [53]. In this way, the plas-

the values by simultaneous fitting of the conductivity of

mon peak (E00 ) decreased and the interband transitions

single-layer graphene sample synthesized by chemical va-

smeared out for all bands in the case of the MWCNTs.

SC

the outermost and the inner tubes, which is in agreement

M AN U

460

Fermi energy EF in graphene samples, i.e. 4.6 × 1012

por deposition (CVD) grownn over a Cu foil. It is obvious500

465

For OLC1 and OLC2 samples, the estimated D, N, and

from Fig. 7 (a) and (c) that the D for SWCNTs shows

EF show different values as the size of the cages changes.

higher value compared to those of MWCNTs, graphene

The effect of the significant decrease of the size of the

nanosheets, and OLC samples, which suggests that the di-

sp2 domains in the outermost carbon shells of the closed

mensionality of the system plays a major roll regarding the

carbon cages are clearly observed on the estimated values

carrier mobility of the graphene structure sample with sp2 505 of the D, N, and EF . The σdc in the DC limits have been estimated for OLC1 and OLC2 . The conductivity is

SWCNTs typical for one-dimensional systems smears out

expected to be larger in case of OLC1 than OLC2 , however

towards a step-like behavior for the two-dimensional sys-

the σdc estimated in case of OLC2 is larger than the σdc

tems such as MWCNTs, graphene nanosheets, and OLC

of OLC1 . This could be attributed to the large number

TE D

470

hybridization. The singularity in the density of states for

EP

samples. In this case, the D as well as the spectral weight510 of defects in the case of OLC2 than the OLC1 sample. In addition, the lower-dimensional nature of the OLC2 in

ergy range. Thus, the Drude weight D and the spectral

comparison with OLC1 might affect the carrier mobility of

weights for plasmon peaks decrease for two-dimensional

cages.

AC C

475

of the plasmone peaks is distributed over much wider en-

samples. These results are in a good agreement with the

The plasmon frequency ωp is further illustrated by the

absorption spectra obtained by Fourier transform infrared 515

spectroscopy by Abouelsayed et. al. [56] in which the spec-

index (nandK) as a function of THz frequency. These

tral weights for all inter-bands transition decrease, when 480

plotting the dielectric response (1 and2 ) and refractive

are presented in FIG. 8 (a)- (d) for SWCNTs, MWC-

the dimensionality of the system deviates from 1D under

NTs, graphene nanosheets, and OLC samples with dif-

the application of hydrostatic pressure.

ferent cages.

The real and imaginary parts of the di-

Furthermore, a considerable number of theoretical stud-520 electric function (1 , 2 ) were obtained by fitting the con-

485

ies have investigated the localization effect on the elec-

ductivity spectra with Drude-Lorentz function, and by

tronic structure close to the Fermi level. It was suggested

considering the zero-crossing of 1 . One can obtain the

that the a gap can be open at the Fermi level due to in-

plasma frequency as illustrated in the inset of FIG. 6 (a), 10

ACCEPTED MANUSCRIPT 3.0

2 100

1 0

300

OLC2 small cages

0.01

0.1

4

1 10 Frequency (THz)

(c)

Refractive index n

200

MWCNTs Graphene OLC1 large cages

100

OLC2 small cages

1

1.0

1.5

2.0

MWCNTs Graphene Nanosheets OLC1 large cages

2.0

OLC2 small cages

(OLC2 ) in the THz-Uv frequency range. The THz bands

545

1.5

2.5

3.0

appeared in all samples are attributed to the plasmon resonance results from the localization of charge carriers in

1.0

a finite length not to curvature induced gab. The plas-

0.5 0.1

1 10 Frequency (THz)

mon band can be tuned by changing the dimensionality

100

2.0

2

0.5

2.5

0.0

MWCNTs Graphene Nanosheets OLC1 large cages

3

0

dielectric function 

(b)

3

Extinction coefficient K

Dielectric function 

(a)

(d)

of graphene system, where the Drude weight (D) and the

MWCNTs Graphene Nanosheets OLC1 large cages

1.5

OLC2 small cages

550

plasmon band shows higher value in case of SWCNTs as

RI PT

4

1.0

compared with MWCNTs, graphene nanosheets, and OLC 0.5

sample. The differences in the estimated values of the D, 0.0

0.1

1

Frequency (THz)

10

100

the Fermi energy (Ef ) and density of carriers (N) in case

Frequency (THz)

555

Figure 8: (a) Real 1 and (b) imaginary parts 2 of dielectric function

σdc in the DC limits for SWCNTs, MWCNTs, graphene

spectra using the DL model. (c) Real and (d) imaginary parts of

nanosheets, OLC1 , and OLC2 show that the value of the

refractive index.

conductivity in case of SWCNTs is larger than other sam-

M AN U

with different size cages obtained from the fit of the conductivity

560

where the estimated values of the plasma frequency from

nance of the sp2 nano-structured materials opens the door

for the implementation of such materials in many indus-

TE D

tive to the plasma edge. The plasma frequency calculated from the spectral weight of the D components are 10, 3,

2.5, 1.5, and 1.8 THz for SWCNTs, MWCNTs, graphene

565

trial applications, specially the micro-structured antennas.

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ACCEPTED MANUSCRIPT

Preparation and spectroscopic studies on carbonaceous materials were performed.

•

Drude-Lorentz model were used for fitting the optical conductivity spectra.

•

The plasmonic resonances have been observed in THz frequency range.

•

The charge density N has been effected by disordered of the grapheme structure.

•

The σDC values is decreased in case of 2D carbonaceous materials.

AC C

EP

TE D

M AN U

SC

RI PT

•