Electron field emission was investigated from multi-walled carbon nanotubes (MWCNT)/polyethylene oxide (PEO) composites with (5, 15, 25, 33, 40 and 50wt.% of MWCNTs). The resulted samples were characterized using Raman spectroscopy to estimate The MWCNTs-PEO (Nanotubes-Polymer) interaction through the shifting or width alteration of the peaks. The SEM micrographs were issued to investigate the surface morphology of the samples. In addition, and in order to gain additional understanding of the filler content and the samples thermal stability; the TGA technique was applied. The current density versus applied electric field J(E) and current stability of the prepared composites were measured using a range of voltages. Here, the 33wt.% composite displayed the lowest turn on field of 6.51V/μm, whereas the highest current density achieved was 1.17mA/cm2 by the 40wt.% composite

Benkara Mohammed, N.

Benouattas, N.

Md Nor, R.

Electronic Sumy State University Institutional Repository

JOURNAL OF NANO- AND ELECTRONIC PHYSICS ЖУРНАЛ НАНО- ТА ЕЛЕКТРОННОЇ ФІЗИКИ Vol. 10 No 4, 04006(5pp) (20186) Том 10 № 4, 04006(5cc) (2018)   2077-6772/2018/10(4)04006(5) 04006-1  2018 Sumy State University Electrons Extraction from MWCNT/PEO Composites  N. Benkara Mohammed1,2,, R. Md Nor1, N. Benouattas3  1 Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia  2 Department of Sciences and Techniques, Faculty of Sciences and Technology, University of Mohamed Bachir El-Ibrahimi, Bordj-Bou-Arreridj 34000, Algeria  3 Laboratory of Surfaces and Interfaces of Solid Materials, University of Mohamed El-Bachir El-Ibrahimi,  Setif I 19000, Algeria  (Received 14 March 2018; revised manuscript received 15 August 2018; published online 25 August 2018)  Electron field emission was investigated from multi-walled carbon nanotubes (MWCNT)/polyethylene oxide (PEO) composites with (5, 15, 25, 33, 40 and 50wt.% of MWCNTs). The resulted samples were char-acterized using Raman spectroscopy to estimate The MWCNTs-PEO (Nanotubes-Polymer) interaction through the shifting or width alteration of the peaks. The SEM micrographs were issued to investigate the surface morphology of the samples. In addition, and in order to gain additional understanding of the filler content and the samples thermal stability; the TGA technique was applied. The current density versus ap-plied electric field J(E) and current stability of the prepared composites were measured using a range of voltages. Here, the 33wt.% composite displayed the lowest turn on field of 6.51V/µm, whereas the highest current density achieved was 1.17mA/cm2 by the 40wt.% composite.  Keywords: MWCNTs, PEO, Nanomposites, Field electron emission.  DOI: 10.21272/jnep.10(4).04006 PACS number: 78.67.Sc                                                                 nadjima2004@yahoo.com 1. INTRODUCTION  In order to develop a novel, strong, light, conduc-tive, smart and multifunctional composite materials, polymer matrices have been reinforced with carbon nanotubes. These tubes have exhibited excellent field emission features, a low threshold field, and a high current density [1]. On the other hand, and in order to exploit the flexibility of polymers, earlier studies have focused on integrating CNT emitters into flexible de-vices [2-4]. Hence, the objective of this study is to in-vestigate field electron emission of MWCNTs embedded in polyethylene oxide thermoplastic matrix, the loading of MWCNTs is between 5 and 50 wt.%. PEO is chosen, as it is miscible with water in all ratios due to hydra-tion of its ether oxygen [5]. To ensure MWCNTs disper-sion; the tubes were non-covalently functionalized with sodium dodecyl sulfate (SDS) anionic surfactant. In this work, ultrasonic dispersion technique was applied, because wrapping the nanotubes with SDS, guarantees that the tubes previously separated by ultrasonication will not rejoin [6].  2. EXPERIMENTAL SET-UP    MWCNTs used as received, they were of 95 % puri-ty,  50 nm diameter and lengths between 10 and 15 m, purchased from Chengdu Organic Chemicals Limited, China. MWCNT/PEO composites were fabri-cated by solution casting method. To obtain stable suspension; MWCNTs were ultrasonically dispersed using sodium dodecyl sulfate as surfactant; typically 1g MWCNTs and 1g SDS were dissolved in 100ml of de-ionized (DI) water, the solution was ultrasonicated for 2 hours at 25W. To fabricate composites; 1g PEO was thoroughly dissolved in 50ml DI water and mixed with solutions having different MWCNTs weights, making six mixtures containing between 5 and 50 wt.% of MWCNTs. Each mixture was thoroughly mixed and subjected to ultrasonic agitation for 2 hours.  The produced liquid composites were poured in Petri dishes and left for several days to dry in air and room temperature. Dried thick films of about 60m thickness were cut in rectangular shapes with surface areas be-tween 0.13 and 0.32 cm2, then pasted on polished N-type Si (100) substrates. The composite films were analyzed using scanning electron microscopy (SEM), Raman spec-troscopy at 532 nm and thermogravimetry. Field electron emission measurements were conducted at vacuum level of about 110 – 7 mbar, utilizing a source meter unit con-nected to a computer. The I-V characteristics were ob-tained at bias voltages up to 1.1 kV and stability curves were measured for about 34 minutes at 600 V.  3. RESULTS AND DISCUSSION  Figure 1 shows raman spectra of MWCNT/PEO thick film composites before and after electrons extraction (Figure 1(a),(b)). In Figure 1(a), the uppermost plot is obtained from the pure MWCNTs. The second trace shows the spectrum obtained from composite sample with 50wt.% MWCNTs loading. The D, G and G’ peaks are present in addition to C-H stretching bands of PEO, the latest bands intensities are much lower than the G’ band intensity. The third trace shows that 40 wt.% sample exhibits the same features as 50 wt.% with higher intensities in C-H stretching bands. The same thing can be said for 33, 15 and 5 wt.%, where the C-H bands increase in intensi-ties compared to G’ whenever the carbon portion de-creases, which implies that a well dispersion of the nanotubes within the polymer matrix was achieved. PEO skeletal vibrations between 400 and 1500 cm – 1 and far infrared region below 400 cm – 1 appear clearly  N. BENKARA MOHAMMED, R. MD NOR, N. BENOUATTAS J. NANO- ELECTRON. PHYS. 10, 04006 (2018)   04006-2 in the low loading samples, with reduced intensities when the MWCNTs fractions augment.  1000 2000 300024681012G'G(a) SDSPEO5%33%25%15%40%50%MWCNTIntensityWavenumber(cm-1)D  1000 2000 300024681012 G'G (b) SDSPEO5%15%25%33%40%50%MWCNTIntensityWavenumber(cm-1)D  Fig. 1 – Raman spectroscopy of MWCNT/PEO thick films (a) before FEE measurements and (b) after FEE measurements  The D and G peaks have up-shifted in all prepared films; the G peak shifting was as high as 16.66 cm – 1 in 40 and 50 wt.% composites. The D and G shifting to upper frequencies was probably on one hand due to the nanotubes dispersion and disentanglement and on the other hand to the compression force exerted by the polymer molecules on the carbon tubes walls, as it was reported by McNally et al. [6] in a previous study but using MWCNT/PE composite.   After FEE measurement (Figure 1(b)), the same band features with slight differences were obtained. However, the spectra appear smoother than before FEE measurement, particularly in 5 and 40 wt.% composites. Here, contrary to the before FEE measurement case, the G-band down shifted in 40 and 50 wt.% and reached 8 cm – 1 in the two samples. Whereas D-band showed a down shift in four out of the six samples (5, 15, 25 and 33 wt.%) and no change in two (40, 50 wt.%). This D and G bands down shifting infers that less compressive force is exerted on the carbon tubes after exposure to FEE measurement. We relate it to the evaporation of the cathode material (prepared composites) to the anode during exposure to high voltage and low pressure. In fact, Purcell et al.[7] have reported that the temperature at the tip of a single MWCNT could go up to 2000K. In addition, PEO polymer holds a low melting point of 57-73 °C [5]. The mechanical stress demonstrates its influence on the FEE results.  The intensity ratio ID/IG suggests that almost all the emitters exhibitted higher disorder after FEE measurement. For instance; 5 and 40 wt.% samples show that their ID/IG increased from 0.56 to 0.87 and from 0.52 to 0.64 respectively. This augmentation sig-nifies the increment of sp3 hybridized carbon and spe-cifically represents damage and defect sites in the tubes resulted from high voltage and localized heat. Figure 2 (a-f) shows the SEM micrographs of 5 to 50 wt.% MWCNT/PEO composites respectively. 5 wt.% sample exhibits a smooth surface containing pores of about 8µm length and 2 µm width. The size of these pores diminishes gradually until they disappear for 50 wt.%, leading to rougher and rougher surfaces having more compacted nanotubes with nano and micro protrusions that formed after MWCNTs addition. It is noted that these protrusions are necessary for local electric field intensification in the electron field emission [8, 9]. Extremely, in Figure 2(f) corresponding to 50 wt.% sample, some MWCNTs agglomerates are seen in the matrix, usually resulting from the Van der Waals in-teraction. Apparently, in 50 wt.% the CNTs could not be wrapped perfectly, totally due to the high quantity of MWCNTs. Generally, the SEM micrographs indicate that a well and homogeneous dispersion was achieved.  Figure 3 shows TGA curves of the pure PEO and MWCNT/PEO composites. PEO started an abrupt deg-radation at about 335 °C and lost 95.5 wt.% of its weight at 401 °C, the DTG curve displayed a sharp intensive and symmetric peak at 346 °C. Thus, the decomposition of the polymer PEO appeared in only one stage, whereas all the prepared composite films showed two stages decomposition. We remarked that the first stage was in a long inter-val of time with a slow process of weight loss. The DTG of this first transition yielded more than one peak for all samples, except for 5 wt.% which resembles the DTG of PEO as the MWCNTs portion is very small. The presence of more than one peak in the DTG curve indicates the presence of multiple components which confirms the oc-currence of different carbonaceous structures in the sam-ples. This is due to the interaction between the MWCNTs and the polymer chains which caused changes in MWCNTs C-C bonds. This result is emphasized by the Raman spectroscopy, which showed relatively high ID/IG ratio. It is reported that changes in C-C bonding indicate the adhesion of PEO chains to MWCNTs sidewalls [10]. The second stage of TGA decomposition displayed at 309-411 °C, was abrupt and steep and is due to breaking of the main chain of PEO in the composite.  The current density-electrical field J(E) curves  from MWCNT/PEO cold cathodes (Figure 4), were modeled according to Fowler–Nordheim theory [9, 11].   2 2 3/2( ) expA E BJ EE      , (1)  where A  1.5610 – 6 AeVV – 2, B  6.83107 eV – 3/2Vcm – 1,  is the work function, taken to be equal to that of graph-ite (5 eV) and  is the field enhancement factor. The known linear Fowler-Nordheim plot is as follows:   2 3/22lnJ A BEE    , (2)    JOURNAL OF NANO- AND ELECTRONIC PHYSICS ЖУРНАЛ НАНО- ТА ЕЛЕКТРОННОЇ ФІЗИКИ Vol. 10 No 4, 04006(5pp) (20186) Том 10 № 4, 04006(5cc) (2018)   2077-6772/2018/10(4)04006(5) 04006-3  2018 Sumy State University   Fig. 2 – SEM micrographs of the six MWCNT/PEO thick films composites (a) 5 wt.%, (b) 15 wt.%, (c) 25 wt.%, (d) 33 wt.%, (e) 40 wt.% and (f) 50 wt.%  200 300 400020406080100 CNT5 CNT15 CNT25 CNT33 CNT40 CNT50 PEOWeight Loss %Temperature oC   Fig. 3 – TGA of pure PEO and 5 wt.%, 15 wt.%, 25 wt.%, 33 wt.%, 40 wt.% and 50 wt.% MWCNT/PEO composites  where, linear fitting of equation (2) would obtain the field enhancement factor . The approximately linear attitude of the Fowler-Nordheim (FN) (Figure 4(b)) suggests that the yielded current is produced by the field electron emission mechanisms. The turn-on field (Eto), threshold field (Eth) and the field enhancement factor (); are useful parameters, which are practical for comparison with other emitters. Hence, the values of Eto (the applied field to produce a current density equal to 10 µAcm – 2), the Eth (the ap-plied field at an emission current density of 0.1 mAcm – 2) and the  for each film, deduced from current density-electrical field curves, are drawn in Figure 5. It can be seen that the 33 wt.% composite displayed the lowest turn on field of 6.51 V/µm, whereas the highest    9x1041x1051x1050.00.30.60.91.2     5%  15%  25%  33%  40% 50%J(A/cm2) x10-3E(V/cm)(a)  1.0x10-51.2x10-51.4x10-5-36-34-32-30   5% 15% 25% 33% 40% 50%ln(J/E2)1/E(b)  Fig. 4 – (a) Current density versus the applied electric field and (b) Fowler-Nordheim plot of the films with different MWCNTs weight concentrations  N. BENKARA MOHAMMED, R. MD NOR, N. BENOUATTAS J. NANO- ELECTRON. PHYS. 10, 04006 (2018)   04006-4 80012001600200010 20 30 40 50789 Enhancement Factor  Enhancement Factor Turn on FieldThreshold FieldTurn on/Threshold Fields(V/um)MWCNTs (wt.%) Fig. 5 – Turn on field, threshold field and enhancement fac-tors for the different MWCNTs wt.%.  current density was 1.17 mA/cm2, achieved by the 40 wt.% composite, which showed 8.36 and 8.76 V/µm turn on and threshold fields respectively. To our knowledge, field electron emission from MWCNTs/PEO thick films (60 m) has not been investigated so far. However, regarding just MWCNTs, Lyth et al. [3] have investigated the FEE of acid oxidized MWCNTs depos-ited by means of dip coating on paper substrates, the Eth ranged between 0.8 and 11.6 V/µm. Where, the Eth is defined as the applied field at an emission current of 1 nA. Obviously, always regarding Figure 5, there is no line-ar relationship between the MWCNTs loading and field emission characteristics, as was observed in earlier stud-ies regarding other polymer composites [12,13]. Since the CNTs are homogeneously dispersed, the nano-tubes/polymer interface is strong enough to effectively enhance the electrical, thermal and mechanical properties of the composite [14]. However, when nanotubes density increases, the electric field on a singular nanotube become screened by the CNTs in the vicinity [15]. Nilsson et al. found out that highly packed CNT films did not provide the best FEE results [16].  The stability of our samples was tested by current in-tensity measurements while maintaining constant voltage for 2000 s (Figure 6). Generally, all the samples exhibited a satisfaction in stability.  Samples with 25 wt.% and 33 wt.% showed a great stability for the whole period. While, after 420 s; 5 and 15 wt.% maintained a significant stability. The current of 40 and especially 50 wt.% had dramatic up and down fluctuations. The normal trend in current decreasing  0 500 1000 1500 20001E-91E-81E-71E-61E-51E-41E-3  Current Intensity (A)Time (S)40% 5%33%15%25%50%y/10y/100y/1000  Fig 6 – Current stability in about 2000s period time for thick films with different WCNTs weight concentrations  and some fluctuations; is probably the result of dynam-ic rearrangement of emission sites. The local Joule heating and the large electrostatic force have their influence on the films. In deed, the substantial electro-static force can even peel the CNTs from the substrate and deteriorate the film’s performance [17].  4. CONCLUSIONS  MWCNT/PEO composite thick films were prepared by the ultrasonication technique. The SEM showed roughness and protrusions resulted from MWCNTs addition and played the key role in electrons emission. TGA and Raman spectroscopy emphasized the good adhesion of the polymer chains to the nanotubes walls. The thermal stability of PEO was improved by the incorporation of MWCNTs in the polymer matrix. Re-garding field electron emission, Regarding field elec-tron emission; the best enhancement factor (β) and the lowest turn on field were 1853 and 6.51V/µm respec-tively, achieved by 33wt.% emitter. The highest current density (1.17mA/cm2) was produced by 40wt.% compo-site. The MWCNT/PEO thick films emitters presented a high degree of stability during 34min emission, ex-cept for 50wt.% which had a dramatic up and down fluctuations, most obviously at about 1400s. In this view, we are confident that the devices based on MWCNT/PEO will show enhancements in the FEE field.  ACKNOWLEDGEMENT  This work is funded by The Ministry of Science and Innovation of Malaysia under the FRGS grant No. FP008/2010B.     REFERENCES  1. S.D. Liang, L. Chen, Phys. Rev. Lett. 101, 027602 (2008). 2. Y.J. Jung, S. Kar, S. Talapatra, C. Soldano, G. Viswanathan, X. Li, Z. Yao, F.S. Ou, A. Avadhanula, R. Vajtai, S. Curran, O. Nalamasu, P.M. Ajayan, Nano Lett. 6, 413 (2006). 3. S.M. Lyth, S.R.P. Silva, Appl. Phys. Lett. 90, 173124 (2007). 4. H.S. Sim, S.P. Lau, H.Y. Yang, L.K. Ang, M. Tanemura, K. Yamaguchi, Appl. Phys. Lett. 90, 143103 (2007). 5. M.M. Crowley, F. Zhang, J.J. Koleng, J.W. McGinity, Biomaterials 23, 4241 (2002). 6. T. McNally, P. Postschke, P. Halley, M. Murphy, D. Martin, S.E.J. Bell, G.P. Brennan, D. Bein, P. Lemoine, J.P. Quinn, Polymer 46, 8222 (2005).  ELECTRONS EXTRACTION FROM MWCNT/PEO COMPOSITES J. NANO- ELECTRON. PHYS. 10, 04006 (2018)   04006-5 7. S. Purcell, P. Vincent, Phys. Rev. Lett. 88, 105502 (2002). 8. O. Gröning, O.M. Küttel, P. Gröning, L. Schlapbach,  J. Vac. Sci. Technol. B 17, 1970 (1999). 9. J.M. Bonard, J.P. Salvetat, T. Stockli, W.A. de Heer, L. Forro, A. Chatelain, Appl. Phys. Lett. 73, 918 (1998). 10. Y.L. Liu, Y.H. Chang, M. Liang, Polymer 49, 5405 (2008). 11. R.B. Rakhi, K. Sethupathi, S. Ramaprabhu, Appl. Surf. Sci. 254, 6770 (2008). 12. H. Jung, S.Y. An, D.M. Jang, J.M. Kim, J.Y. Park, D. Kim, Carbon 50, 987 (2012).  13. R.C. Smith, J.D. Carey, R.J. Murphy, W.J. Blau, J.N. Coleman, S.R.P. Silva, Appl. Phys. Lett. 87, 263105 (2005). 14. Z. Xu, Y. Niu, L. Yang, W. Xie, H. Li, Z. Gan, Polymer 51, 730 (2010). 15. Y. Li, Y. Sun, J.T.W. Yeow, Nanotechnology 26, 242001 (2015). 16. L. Nilsson, O. Groening, C. Emmenegger, O. Kuettel, E. Schaller, L. Schlapbach, H. Kind, J.M. Bonard, K. Kern, Appl. Phys. Lett. 76, 2071 (2000).  17. Y. Cheng, O. Zhou, Comptes Rendus Physique 4, 1021 (2003).    

Electrons Extraction from MWCNT/PEO Composites

Mohammed, Nadjima Benkara

Nor, Roslan Md

Benouattas, Noureddine

UM Digital Repository

SocioEconomic Challenges Journal (Sumy State University)

JOURNAL OF NANO- AND ELECTRONIC PHYSICS ЖУРНАЛ НАНО- ТА ЕЛЕКТРОННОЇ ФІЗИКИ 
Vol. 10 No 4, 04006(5pp) (20186) Том 10 № 4, 04006(5cc) (2018) 
 
 
2077-6772/2018/10(4)04006(5) 04006-1  2018 Sumy State University 
Electrons Extraction from MWCNT/PEO Composites 
 
N. Benkara Mohammed1,2,, R. Md Nor1, N. Benouattas3 
 
1 Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia  
2 Department of Sciences and Techniques, Faculty of Sciences and Technology, University of Mohamed Bachir El-
Ibrahimi, Bordj-Bou-Arreridj 34000, Algeria  
3 Laboratory of Surfaces and Interfaces of Solid Materials, University of Mohamed El-Bachir El-Ibrahimi,  
Setif I 19000, Algeria 
 
(Received 14 March 2018; revised manuscript received 15 August 2018; published online 25 August 2018) 
 
Electron field emission was investigated from multi-walled carbon nanotubes (MWCNT)/polyethylene 
oxide (PEO) composites with (5, 15, 25, 33, 40 and 50wt.% of MWCNTs). The resulted samples were char-
acterized using Raman spectroscopy to estimate The MWCNTs-PEO (Nanotubes-Polymer) interaction 
through the shifting or width alteration of the peaks. The SEM micrographs were issued to investigate the 
surface morphology of the samples. In addition, and in order to gain additional understanding of the filler 
content and the samples thermal stability; the TGA technique was applied. The current density versus ap-
plied electric field J(E) and current stability of the prepared composites were measured using a range of 
voltages. Here, the 33wt.% composite displayed the lowest turn on field of 6.51V/µm, whereas the highest 
current density achieved was 1.17mA/cm2 by the 40wt.% composite. 
 
Keywords: MWCNTs, PEO, Nanomposites, Field electron emission. 
 
DOI: 10.21272/jnep.10(4).04006 PACS number: 78.67.Sc 
 
 
                                                             

 nadjima2004@yahoo.com 
1. INTRODUCTION 
 
In order to develop a novel, strong, light, conduc-
tive, smart and multifunctional composite materials, 
polymer matrices have been reinforced with carbon 
nanotubes. These tubes have exhibited excellent field 
emission features, a low threshold field, and a high 
current density [1]. On the other hand, and in order to 
exploit the flexibility of polymers, earlier studies have 
focused on integrating CNT emitters into flexible de-
vices [2-4]. Hence, the objective of this study is to in-
vestigate field electron emission of MWCNTs embedded 
in polyethylene oxide thermoplastic matrix, the loading 
of MWCNTs is between 5 and 50 wt.%. PEO is chosen, 
as it is miscible with water in all ratios due to hydra-
tion of its ether oxygen [5]. To ensure MWCNTs disper-
sion; the tubes were non-covalently functionalized with 
sodium dodecyl sulfate (SDS) anionic surfactant. In 
this work, ultrasonic dispersion technique was applied, 
because wrapping the nanotubes with SDS, guarantees 
that the tubes previously separated by ultrasonication 
will not rejoin [6]. 
 
2. EXPERIMENTAL SET-UP  
  
MWCNTs used as received, they were of 95 % puri-
ty,  50 nm diameter and lengths between 10 and 
15 m, purchased from Chengdu Organic Chemicals 
Limited, China. MWCNT/PEO composites were fabri-
cated by solution casting method. To obtain stable 
suspension; MWCNTs were ultrasonically dispersed 
using sodium dodecyl sulfate as surfactant; typically 1g 
MWCNTs and 1g SDS were dissolved in 100ml of de-
ionized (DI) water, the solution was ultrasonicated for 
2 hours at 25W. To fabricate composites; 1g PEO was 
thoroughly dissolved in 50ml DI water and mixed with 
solutions having different MWCNTs weights, making 
six mixtures containing between 5 and 50 wt.% of 
MWCNTs. Each mixture was thoroughly mixed and 
subjected to ultrasonic agitation for 2 hours.  
The produced liquid composites were poured in Petri 
dishes and left for several days to dry in air and room 
temperature. Dried thick films of about 60m thickness 
were cut in rectangular shapes with surface areas be-
tween 0.13 and 0.32 cm2, then pasted on polished N-type 
Si (100) substrates. The composite films were analyzed 
using scanning electron microscopy (SEM), Raman spec-
troscopy at 532 nm and thermogravimetry. Field electron 
emission measurements were conducted at vacuum level 
of about 110 – 7 mbar, utilizing a source meter unit con-
nected to a computer. The I-V characteristics were ob-
tained at bias voltages up to 1.1 kV and stability curves 
were measured for about 34 minutes at 600 V. 
 
3. RESULTS AND DISCUSSION 
 
Figure 1 shows raman spectra of MWCNT/PEO 
thick film composites before and after electrons 
extraction (Figure 1(a),(b)). In Figure 1(a), the 
uppermost plot is obtained from the pure MWCNTs. 
The second trace shows the spectrum obtained from 
composite sample with 50wt.% MWCNTs loading. The 
D, G and G’ peaks are present in addition to C-H 
stretching bands of PEO, the latest bands intensities 
are much lower than the G’ band intensity. The third 
trace shows that 40 wt.% sample exhibits the same 
features as 50 wt.% with higher intensities in C-H 
stretching bands. The same thing can be said for 33, 15 
and 5 wt.%, where the C-H bands increase in intensi-
ties compared to G’ whenever the carbon portion de-
creases, which implies that a well dispersion of the 
nanotubes within the polymer matrix was achieved. 
PEO skeletal vibrations between 400 and 1500 cm – 1 
and far infrared region below 400 cm – 1 appear clearly 
 N. BENKARA MOHAMMED, R. MD NOR, N. BENOUATTAS J. NANO- ELECTRON. PHYS. 10, 04006 (2018) 
 
 
04006-2 
in the low loading samples, with reduced intensities 
when the MWCNTs fractions augment. 
 
1000 2000 3000
2
4
6
8
10
12
G'G(a)
 
SDS
PEO
5%
33%
25%
15%
40%
50%
MWCNT
In
te
n
s
it
y
Wavenumber(cm
-1
)
D
 
 
1000 2000 3000
2
4
6
8
10
12 G'G (b)
 
SDS
PEO
5%
15%
25%
33%
40%
50%
MWCNT
In
te
n
s
it
y
Wavenumber(cm
-1
)
D
 
 
Fig. 1 – Raman spectroscopy of MWCNT/PEO thick films (a) 
before FEE measurements and (b) after FEE measurements 
 
The D and G peaks have up-shifted in all prepared 
films; the G peak shifting was as high as 16.66 cm – 1 in 
40 and 50 wt.% composites. The D and G shifting to 
upper frequencies was probably on one hand due to the 
nanotubes dispersion and disentanglement and on the 
other hand to the compression force exerted by the 
polymer molecules on the carbon tubes walls, as it was 
reported by McNally et al. [6] in a previous study but 
using MWCNT/PE composite.   
After FEE measurement (Figure 1(b)), the same 
band features with slight differences were obtained. 
However, the spectra appear smoother than before FEE 
measurement, particularly in 5 and 40 wt.% composites. 
Here, contrary to the before FEE measurement case, the 
G-band down shifted in 40 and 50 wt.% and reached 
8 cm – 1 in the two samples. Whereas D-band showed a 
down shift in four out of the six samples (5, 15, 25 and 
33 wt.%) and no change in two (40, 50 wt.%). This D and 
G bands down shifting infers that less compressive force 
is exerted on the carbon tubes after exposure to FEE 
measurement. We relate it to the evaporation of the 
cathode material (prepared composites) to the anode 
during exposure to high voltage and low pressure. In 
fact, Purcell et al.[7] have reported that the temperature 
at the tip of a single MWCNT could go up to 2000K. In 
addition, PEO polymer holds a low melting point of 57-
73 °C [5]. The mechanical stress demonstrates its 
influence on the FEE results.  
The intensity ratio ID/IG suggests that almost all the 
emitters exhibitted higher disorder after FEE 
measurement. For instance; 5 and 40 wt.% samples 
show that their ID/IG increased from 0.56 to 0.87 and 
from 0.52 to 0.64 respectively. This augmentation sig-
nifies the increment of sp3 hybridized carbon and spe-
cifically represents damage and defect sites in the 
tubes resulted from high voltage and localized heat. 
Figure 2 (a-f) shows the SEM micrographs of 5 to 
50 wt.% MWCNT/PEO composites respectively. 5 wt.% 
sample exhibits a smooth surface containing pores of 
about 8µm length and 2 µm width. The size of these pores 
diminishes gradually until they disappear for 50 wt.%, 
leading to rougher and rougher surfaces having more 
compacted nanotubes with nano and micro protrusions 
that formed after MWCNTs addition. It is noted that 
these protrusions are necessary for local electric field 
intensification in the electron field emission [8, 9]. 
Extremely, in Figure 2(f) corresponding to 50 wt.% 
sample, some MWCNTs agglomerates are seen in the 
matrix, usually resulting from the Van der Waals in-
teraction. Apparently, in 50 wt.% the CNTs could not 
be wrapped perfectly, totally due to the high quantity 
of MWCNTs. Generally, the SEM micrographs indicate 
that a well and homogeneous dispersion was achieved.  
Figure 3 shows TGA curves of the pure PEO and 
MWCNT/PEO composites. PEO started an abrupt deg-
radation at about 335 °C and lost 95.5 wt.% of its 
weight at 401 °C, the DTG curve displayed a sharp 
intensive and symmetric peak at 346 °C. Thus, the 
decomposition of the polymer PEO appeared in only 
one stage, whereas all the prepared composite films 
showed two stages decomposition. 
We remarked that the first stage was in a long inter-
val of time with a slow process of weight loss. The DTG of 
this first transition yielded more than one peak for all 
samples, except for 5 wt.% which resembles the DTG of 
PEO as the MWCNTs portion is very small. The presence 
of more than one peak in the DTG curve indicates the 
presence of multiple components which confirms the oc-
currence of different carbonaceous structures in the sam-
ples. This is due to the interaction between the MWCNTs 
and the polymer chains which caused changes in 
MWCNTs C-C bonds. This result is emphasized by the 
Raman spectroscopy, which showed relatively high ID/IG 
ratio. It is reported that changes in C-C bonding indicate 
the adhesion of PEO chains to MWCNTs sidewalls [10]. 
The second stage of TGA decomposition displayed at 309-
411 °C, was abrupt and steep and is due to breaking of the 
main chain of PEO in the composite.  
The current density-electrical field J(E) curves  
from MWCNT/PEO cold cathodes (Figure 4), were 
modeled according to Fowler–Nordheim theory [9, 11]. 
 
 
2 2 3/2
( ) exp
A E B
J E
E


 
     
, (1) 
 
where A  1.5610 – 6 AeVV – 2, B  6.83107 eV – 3/2Vcm – 1,  
is the work function, taken to be equal to that of graph-
ite (5 eV) and  is the field enhancement factor. The 
known linear Fowler-Nordheim plot is as follows: 
 
 
2 3/2
2
ln
J A B
EE


 
  
 
, (2) 
 
 
 
JOURNAL OF NANO- AND ELECTRONIC PHYSICS ЖУРНАЛ НАНО- ТА ЕЛЕКТРОННОЇ ФІЗИКИ 
Vol. 10 No 4, 04006(5pp) (20186) Том 10 № 4, 04006(5cc) (2018) 
 
 
2077-6772/2018/10(4)04006(5) 04006-3  2018 Sumy State University 
 
 
Fig. 2 – SEM micrographs of the six MWCNT/PEO thick films composites (a) 5 wt.%, (b) 15 wt.%, (c) 25 wt.%, (d) 33 wt.%, (e) 
40 wt.% and (f) 50 wt.% 
 
200 300 400
0
20
40
60
80
100
 CNT5
 CNT15
 CNT25
 CNT33
 CNT40
 CNT50
 PEO
W
e
ig
h
t 
L
o
s
s
 %
Temperature 
o
C  
 
Fig. 3 – TGA of pure PEO and 5 wt.%, 15 wt.%, 25 wt.%, 
33 wt.%, 40 wt.% and 50 wt.% MWCNT/PEO composites 
 
where, linear fitting of equation (2) would obtain the 
field enhancement factor . The approximately linear 
attitude of the Fowler-Nordheim (FN) (Figure 4(b)) 
suggests that the yielded current is produced by the 
field electron emission mechanisms. 
The turn-on field (Eto), threshold field (Eth) and the 
field enhancement factor (); are useful parameters, 
which are practical for comparison with other emitters. 
Hence, the values of Eto (the applied field to produce a 
current density equal to 10 µAcm – 2), the Eth (the ap-
plied field at an emission current density of 0.1 mAcm – 2) 
and the  for each film, deduced from current density-
electrical field curves, are drawn in Figure 5. It can be 
seen that the 33 wt.% composite displayed the lowest 
turn on field of 6.51 V/µm, whereas the highest  
 
 
9x10
4
1x10
5
1x10
5
0.0
0.3
0.6
0.9
1.2
 
 
   5% 
 15% 
 25% 
 33% 
 40%
 50%
J
(A
/c
m
2
) 
x
1
0
-3
E(V/cm)
(a)
 
 
1.0x10
-5
1.2x10
-5
1.4x10
-5
-36
-34
-32
-30
 
 
 5%
 15%
 25%
 33%
 40%
 50%
ln
(J
/E
2
)
1/E
(b)
 
 
Fig. 4 – (a) Current density versus the applied electric field 
and (b) Fowler-Nordheim plot of the films with different 
MWCNTs weight concentrations 
 N. BENKARA MOHAMMED, R. MD NOR, N. BENOUATTAS J. NANO- ELECTRON. PHYS. 10, 04006 (2018) 
 
 
04006-4 
800
1200
1600
2000
10 20 30 40 50
7
8
9
 E
n
h
a
n
c
e
m
e
n
t 
F
a
c
to
r 
 Enhancement Factor
 
Turn on Field
Threshold Field
T
u
rn
 o
n
/T
h
re
s
h
o
ld
 F
ie
ld
s
(V
/u
m
)
MWCNTs (wt.%)
 
Fig. 5 – Turn on field, threshold field and enhancement fac-
tors for the different MWCNTs wt.%. 
 
current density was 1.17 mA/cm2, achieved by the 
40 wt.% composite, which showed 8.36 and 8.76 V/µm 
turn on and threshold fields respectively. To our 
knowledge, field electron emission from MWCNTs/PEO 
thick films (60 m) has not been investigated so far. 
However, regarding just MWCNTs, Lyth et al. [3] have 
investigated the FEE of acid oxidized MWCNTs depos-
ited by means of dip coating on paper substrates, the 
Eth ranged between 0.8 and 11.6 V/µm. Where, the Eth 
is defined as the applied field at an emission current of 
1 nA. 
Obviously, always regarding Figure 5, there is no line-
ar relationship between the MWCNTs loading and field 
emission characteristics, as was observed in earlier stud-
ies regarding other polymer composites [12,13]. Since the 
CNTs are homogeneously dispersed, the nano-
tubes/polymer interface is strong enough to effectively 
enhance the electrical, thermal and mechanical properties 
of the composite [14]. However, when nanotubes density 
increases, the electric field on a singular nanotube become 
screened by the CNTs in the vicinity [15]. Nilsson et al. 
found out that highly packed CNT films did not provide 
the best FEE results [16].  
The stability of our samples was tested by current in-
tensity measurements while maintaining constant 
voltage for 2000 s (Figure 6). Generally, all the samples 
exhibited a satisfaction in stability.  
Samples with 25 wt.% and 33 wt.% showed a great 
stability for the whole period. While, after 420 s; 5 and 
15 wt.% maintained a significant stability. The current 
of 40 and especially 50 wt.% had dramatic up and down 
fluctuations. The normal trend in current decreasing  
0 500 1000 1500 2000
1E-9
1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
 
 
C
u
rr
e
n
t 
In
te
n
s
it
y
 (
A
)
Time (S)
40%
 5%
33%
15%
25%
50%
y/10
y/100
y/1000
 
 
Fig 6 – Current stability in about 2000s period time for thick 
films with different WCNTs weight concentrations 
 
and some fluctuations; is probably the result of dynam-
ic rearrangement of emission sites. The local Joule 
heating and the large electrostatic force have their 
influence on the films. In deed, the substantial electro-
static force can even peel the CNTs from the substrate 
and deteriorate the film’s performance [17]. 
 
4. CONCLUSIONS 
 
MWCNT/PEO composite thick films were prepared 
by the ultrasonication technique. The SEM showed 
roughness and protrusions resulted from MWCNTs 
addition and played the key role in electrons emission. 
TGA and Raman spectroscopy emphasized the good 
adhesion of the polymer chains to the nanotubes walls. 
The thermal stability of PEO was improved by the 
incorporation of MWCNTs in the polymer matrix. Re-
garding field electron emission, Regarding field elec-
tron emission; the best enhancement factor (β) and the 
lowest turn on field were 1853 and 6.51V/µm respec-
tively, achieved by 33wt.% emitter. The highest current 
density (1.17mA/cm2) was produced by 40wt.% compo-
site. The MWCNT/PEO thick films emitters presented 
a high degree of stability during 34min emission, ex-
cept for 50wt.% which had a dramatic up and down 
fluctuations, most obviously at about 1400s. In this 
view, we are confident that the devices based on 
MWCNT/PEO will show enhancements in the FEE 
field. 
 
ACKNOWLEDGEMENT 
 
This work is funded by The Ministry of Science and 
Innovation of Malaysia under the FRGS grant No. 
FP008/2010B. 
 
 
 
 
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