Oxygen and nitrogen doping in single wall carbon nanotubes: An efficient stable field emitter

Journal of Alloys and Compounds 711 (2017) 85e93

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Oxygen and nitrogen doping in single wall carbon nanotubes: An efficient stable field emitter Avshish Kumar a, Shama Parveen a, Samina Husain b, M. Zulfequar a, Harsh a, Mushahid Husain a, * a b

Department of Physics, Jamia Millia Islamia (A Central University), New Delhi, 110025, India Centre for Nanoscience and Nanotechnology, Jamia Millia Islamia (A Central University), New Delhi, 110025, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 January 2017 Received in revised form 16 March 2017 Accepted 28 March 2017 Available online 29 March 2017

Vertically aligned single wall Carbon Nanotubes (v-SWCNTs) were synthesized by plasma enhanced chemical vapour deposition system. Oxygen and nitrogen molecule inclusion in v-SWCNTs was performed in ultra high vacuum by radio frequency (RF) sputtering system. The creation of defects induced by oxygen and nitrogen plasma ions also drives the formation of different oxygen and nitrogen species at the SWCNTs sidewall surface as well as SWCNTs tips where incorporation is more efficient. The G/D ratio, radial breathing mode and other oxygen and nitrogen characteristic Raman modes were analyzed from Raman spectra. The type of attachments was also interpreted from fourier transform infrared spectroscopy and the electronic 1s core levels of oxygen and nitrogen with carbon in X-ray photoelectron spectroscopy. The effect of oxygen and nitrogen inclusion on the surface of SWCNTs was investigated in terms of its correlation in the enhancement of field emission characteristics. The analysis was done on the comparison of change in field enhancement factor of as-grown SWCNTs and plasma induced O-&NSWCNTs. The results show drastic enhancement in current density at low turn on field with long term emission current stability. Our results give very unique improvement in field emission display devices using SWCNTs with simple doping effects. © 2017 Elsevier B.V. All rights reserved.

Keywords: Carbon nanotubes Chemical vapour deposition X-ray photoelectron spectroscopy Field emission Raman spectroscopy

1. Introduction Doping in carbon nanotubes (CNTs) with foreign atoms has attracted a large attention of the researchers in the last decade [1,2]. It is not only an effective method to alter electronic and mechanical properties of CNTs for optimizing a targeted application but also doped carbon nanotubes can be used as an organic catalysts for energy conversion system, such as water oxidation and oxygen reduction reactions [3,4]. The properties of single wall carbon nanotubes (SWCNT) are dependent of its diameter and chirality. If some dopant of high curvature is incorporated in SWCNT then its properties are enhanced. Comparatively, the impact of doped SWCNTs is greater than that of doped multiwall carbon nanotubes (MWCNTs). Therefore, the doping of oxygen (O2) and nitrogen (N2) into SWCNTs intended to enhance the electronic properties of the nanotubes so that they would easily be applicable in electronic

* Corresponding author. E-mail address: [email protected] (M. Husain). http://dx.doi.org/10.1016/j.jallcom.2017.03.324 0925-8388/© 2017 Elsevier B.V. All rights reserved.

devices and other applications [5e8]. The electronic structure of SWCNTs prevents the easy binding of nitrogen than oxygen towards its external surface due to their electronic configurations. The results shown by the previous researchers indicate that the interaction of nitrogen (having triple bond) with SWCNTs is more constructive at open end than its external surface [9,10]. However, the binding of oxygen atoms (having double bond) are more effective at the external surface of SWCNTs. It is still an open question whether oxygen and nitrogen atoms are actually incorporated into the carbon nanotubes lattice or it creates certain amorphous regions within the nanotubes or at the external surface of the nanotubes. If we consider sp2 electronic structure of CNT, then it is easy to understand that adsorption of N2 molecules is more difficult than adsorption of O2 molecule at the external surface of CNTs. It is now well known that, electronic structure of O2 is KK (s2s)2 (s2s*)2 (s2pz)2 (p2px)2 (p2py)2 (p2px*)1 (p2py*)1 and transferred electron will surely occupy the half-filled anti-bonding orbitals of oxygen molecule and therefore will weaken the OeO bond. Whereas, the electronic structure of N2 is KK (s2s)2 (s2s*)2 (s2pz)2 (p2px)2 (p2py)2

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and the binding orbital of nitrogen molecule is filled, therefore, the transferred electron can't occupy this binding orbital [11,12]. Basically, three distinct types of nitrogen molecules incorporations in graphitic structures exist; pyridinic, quaternary and pyrrolic. The main disparity between pyridinic and pyrrolic defects is associated with the number of contributed electrons in the p-system. The pyridinic nitrogen contributes one electron to the p-system whereas pyrrolic nitrogen contributes with two electrons and therefore no pyrrolic nitrogen atom needs to be located in a pyrrole ring [13]. It is therefore to be noted that for a plane structure, pyridinic, pyrrolic and quaternary nitrogen atoms are in sp2 hybridized configurations, but as the curvature is increased such as in CNTs, the admixture of sp3 character is increased with these bonds. In view of the fact that pyrrolic nitrogen atoms are understood to increase the curvature to a higher level and therefore can easily be connected with a stronger sp3 character [14,15]. We have shown a schematic representation of possible common interaction of oxygen and nitrogen species with carbon atoms in SWCNTs (Fig. 1). In this work, the as-grown SWCNTs were doped with oxygen and nitrogen using plasma functionalization process. Since, doping of oxygen and nitrogen affects the electron transport characteristics of SWCNTs, therefore, field emission properties of doped SWCNTs were analyzed and it was observed that doped SWCNTs are excellent field emitters for future display devices. SWCNTs as field emitting material have already possessed many excellent properties such as high field emission, low turn-on field, long lifetime, narrow energy distribution, high brightness and stable emission current due to its small diameter which provides the highest aspect ratio compared with MWCNTs [16e20]. Any system that uses an electron source could potentially host a SWCNT based field emission device. But, for excellent field emitters, an optimal combination of high density array of vertically aligned SWCNTs with large scale control of diameter, length, alignment, orientation and more importantly good adhesion with the substrate is required [21e27]. The most favourable method to achieve the above specific synthesis requirement of SWCNTs is plasma enhanced chemical vapour deposition (PECVD) which can be used to synthesize SWCNTs at very low temperature [28e40]. Therefore, firstly, in this work, we aimed to synthesize vertically aligned SWCNTs with optimum density of array with tight control of diameter and length. For better adhesion with substrate, Fe/Al catalyst coated Si substrates were used. Secondly, the as-grown SWCNTs were doped with oxygen and nitrogen species using

plasma functionalization process in order to enhance the field emission characteristic properties of SWCNTs based display devices. Thirdly, field emission studies were recorded using field emission setup and we observed that after doping of oxygen and nitrogen species, the SWCNTs have shown long-term emission current stability and also improvement in the field emission factor. In order to verify enhanced field emission properties of SWCNTs, a comparative study was performed among as grown SWCNTs, oxygen and nitrogen doped SWCNTs. 2. Experimental 2.1. Synthesis of SWCNTs During synthesis of SWCNTs using PECVD system, Fe/Al coated Si substrate was placed upon a graphite heater fitted inside the quartz belljar chamber of PECVD system (Black Magic 2 inch system, AIXTRON, Cambridge, UK). The chamber was evacuated to a pressure of the order of 15 mbar. Direct current (DC) plasma at a power of 40 W was used, to assist uniform and vertically aligned growth of the SWCNTs. We followed two steps to synthesize the SWCNTs. In the first step, pre-treatment of the Fe/Al substrate was done under hydrogen (H2) gas atmosphere having continuous flow rate of 750 sccm for 10 min at 450
C. The temperature was monitored using thermocouple connected to the graphite heater cum substrate holder. In the second step, after pre-treatment, high purity acetylene (C2H2) gas at a flow rate of 20 sccm was inserted into the chamber in continuation with increased H2 flow rate of 1380 sccm. During growth process, the heater temperature was quickly raised from 450
C to 500
C. The growth time was kept 15 min. After growth process is over, the samples were then cooled down to normal room temperature. 2.2. Doping process In order to improve the field emission properties of as-grown SWCNTs, the samples were subjected to oxygen and nitrogen doping using plasma functionalization process for 10 min. The radio frequency (RF) sputtering system used for plasma functionalization process was operated at RF power of 100 W and at a frequency of 13.56 MHz. During plasma functionalization, excited electrons, ions, and free radicals are generated through inelastic collisions between energetic electrons and molecules [41e43]. These plasma

Fig. 1. In SWCNTs, Schematic representation of possible common configuration of (A1) carbon atoms (in grey) with oxygen atoms (in red) (A2) oxygen molecular type interaction with Carbon atoms and (B1) Carbon atoms (in grey) with nitrogen atoms (in blue); the sp2 hybridized (a) pyridinic nitrogen atoms, (b) graphitic and, (c) sp3 hybridized pyrrolic (B2) Nitrogen molecular type interaction with carbon atoms. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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species (oxygen and nitrogen) creates the defects and also drives the formation of different oxygen and nitrogen species at the sidewall surface of SWCNTs as well as at the tips of SWCNTs.

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check the repeatability of the results and were carried out under identical environment conditions. Finally, current density with turn on field was measured using J-E plot and field enhancement factor was calculated by Fowler-Nordheim (F-N) plots.

2.3. Characterization of as-grown and doped SWCNTs 3. Results and discussion Different characterization techniques were used to analyze the samples. Field emission scanning electron microscope (FESEM) of FEI (model: Nova Nano) was used to study the morphology of the SWCNTs. High resolution transmission electron microscope (HRTEM), Tecnai G2 F30 S-Twin (FEI; Super Twin lens with Cs ¼ 1.2 mm) instrument operating at an accelerating voltage of 300 kV, was used to assess the structure of SWCNTs before and after plasma functionalization. The structural analysis of grown SWCNTs was studied by Raman spectrometer of HORIBA Jobin Yvon (LABRAM HR 800 JY) at a wavelength of 633 nm. FTIR spectroscopy was performed using Biorad FTS 40 spectrometer for the identification of chemical group attached with SWCNTs. X-ray photoelectron spectroscopy (XPS) studies of the samples were made using Omicron NanoTechnology GmbH, Germany with EA 125 energy analyzer. 2.4. Field emission studies of as-grown and doped SWCNTs The field emission measurements of as-grown SWCNTs, oxygen and nitrogen doped SWCNTs were measured in a planar diode configuration (Fig. 2) at room temperature (25
C). The field emission studies were done under high vacuum of the order of 106 Torr so that electron scattering and degradation of the emitters can be minimized. The samples was pasted on copper plate as cathode using silver epoxy and dried at 80
C for 1 h in order to make good adhesion and ohmic contact between substrate and copper plate. The anode plate with area 78.5 mm2 was made up of canonical shaped stainless steel. The distance between cathode and anode was kept at 500 mm and the diode arrangement was fixed inside the vacuum chamber of field emission set up. The anode was subjected to high voltage DC power supply and emission current was measured by keithley multimeter. All the measurements were repeatedly observed to

3.1. SEM study The morphology of the as-grown and doped samples was observed using FESEM (Fig. 3(aed)). From Fig. 3(a), we can clearly observe the vertically aligned SWCNTs with uniform distribution and length approximately more than 50 mm. Fig. 3(b) reveals the high resolution image of the as-grown SWCNTs. It was observed that the film is essentially composed of high density, long and vertically aligned SWCNTs. The high resolution micrograph (Fig. 3(b)) also gives a rough idea about the diameter distribution of the SWCNTs in the range less than 2 nm. The morphological changes of the oxygen and nitrogen doped SWCNTs are shown in Fig. 3(c) and (d) respectively. From these micrographs, some attachments of oxygen and nitrogen species around the surface of SWCNTs can be clearly observed. As discussed above, the binding of oxygen is very much effective at the external surface of SWCNTs because the adsorption of oxygen is chemisorptions. Whereas, in case of nitrogen doped SWCNTs (Fig. 3(d)), very less number of nitrogen seems to be attached on the surface of SWCNTs. It is because of the electronic structure of SWCNTs which prevents the easy binding of nitrogen than oxygen towards its external surface. However, the binding of nitrogen may be more constructive at the tip of SWCNTs. The binding of oxygen and nitrogen have also been discussed in XPS studies of the samples. 3.2. HRTEM study HRTEM was used to characterize the as-grown as well as doped SWCNTs. All the samples were collected from the surface of the substrates and ultrasonically dispersed in ethanol for 20 min. Then grids (carbon coated) were used to prepare the samples of dispersed SWCNTs. The HRTEM micrograph (Fig. 4(a)) reveals that the as-grown SWCNTs sample consisted of long bundles of SWCNTs. However, due to the tendency of self agglomeration of the SWCNTs, it is very difficult to separate a single SWCNT. These as-grown SWCNTs, after doping, were found to be structurally deformed as shown in Fig. 4(b and c). HRTEM image in Fig. 4(b) shows the presence of defective regions with no alignment in the wall structure of SWCNTs. This is because of the binding of oxygen molecules which causes a significant interaction with the tube structure. The inclusion of oxygen molecules is also confirmed by XPS (discussed below). Similarly, the surface of SWCNTs doped with nitrogen also (Fig. 4(c)) seems to be deformed. It is likely that nitrogen molecules inclusions are not homogenous in our tubes and confirmed by XPS. 3.3. Raman spectroscopic analysis

Fig. 2. Schematic diagram of planar diode configuration used for field emission studies.

The Raman spectroscopy has been widely used to assess the individual SWCNT structure and its quality [44,45]. Fig. 5 shows the Raman spectra of SWCNTs at a laser excitation energy of 1.96 eV (l ¼ 633 nm from helium-neon (HeeNe)). The main characteristic features of Raman spectra, such as radial breathing mode (RBM) mode, the D-band, the G-band and the G'-band, can be easily seen with many weak Raman modes in the zone between the RBM mode and the D-band. RBM is a Raman active normal vibration mode of SWCNTs which is consequent to the coherent movement of carbon atoms in the radial direction and act as an indicator about the

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Fig. 3. FESEM micrographs of (a, b) as-grown SWCNTs, (c) oxygen doped SWCNTs and (d) nitrogen doped SWCNTs.

quality of SWCNTs and its geometry. The RBM frequency is inversely proportional to the diameter of the SWCNT and lies in the range between 110 and 300 cm1, which makes it an important feature for determining the SWCNTs diameter distribution in the grown sample. The diameter of SWCNTs can be estimated using the correlation as suggested by Jorio et al. [46], d ¼ 248 n where d the diameter of SWCNT in nm and n is the Raman shift in cm1. Raman spectra of as-grown SWCNTs and plasma functionalized SWCNTs are shown in Fig. 5(aec). From Fig. 5(a), we observe that the spectrum is mainly divided into four regions. The first region is the low frequency region from 100 to 450 cm1 which is basically RBM region, modes appearing at 181, 218, and 283 cm1 corresponding to the diameter 1.37, 1.13, and 0.87 nm, respectively, indicating the existence of SWCNTs. The second region is the intermediate region from 500 to 1050 cm1. In this region, the peak at 521 cm1 is attributed due to the Si substrate. Some other peaks at 853, 938 and 1038 cm1 may be due to the overtones and phonon modes scattered from tubes of different diameters with different chiral index as suggested by various researchers [47]. Third region is the high frequency region from 1050 to 2000 cm1. In this region, the peak at 1301 cm1 is due to the photon defect interaction and called Dband induced by double-resonant Raman scattering which involves phonons from the graphite K-point. There is another tangential mode with splitting of two modes at 1551 and 1588 cm1. The Gband or Tangential Mode (TM) sometimes also called the high energy band corresponds to the stretching modes in the graphite plane. The G/D ratio is generally the consequence of quality of the grown SWCNTs because the G-band, being a first-order Raman scattering process is not affected by defects. However, the D-band is a second-order Raman scattering process, enhanced by the presence of defects in the grown sample. From Fig. 5(a), we conclude that G/D ratio shows the good quality of as-grown SWCNTs. In the

third region (Fig. 5(a)), we also observe two peaks at 1705 and 1917 cm1. The band at 1705 cm1 is termed as M-band and originates due to the combination of 1551 cm1 of G-band and the RBM peak at 181 cm1 [48,49]. Another peak at 1900 cm1 is due to the overtone of the mode at 938 cm1 in the intermediate range. The forth region is the second order scattering region from 2000 to 3000 cm1. In this region, the peak intensity at 2591 cm1 correspond to the G0 -band and arises due to photon-second phonon interaction. After oxygen and nitrogen doping, the Raman spectra of doped SWCNTs with the same laser excitation wavelength of 633 nm, was recorded. Fig. 5(bec) shows the Raman spectra of oxygen and nitrogen doped SWCNTs respectively. From Fig. 5(b), we observe RBM Peak intensity at 180, 200, 242 and 257 cm1 corresponds to 1.37, 1.24, 1.02 and 0.96 nm tube diameter which is almost same as observed in as-grown sample. After oxygen doping, we observed the presence of an extra peak at 981 cm1. The existence of this peak shows the attachment of O2 molecule as reported by Lee et al. [50]. However, if we compare the G/D ratio of oxygen doped SWCNTs with the as-grown SWCNTs; we conclude that the quality of the oxygen doped SWCNTs is low because of the high peak intensity of D band. The Raman spectra of the nitrogen doped SWCNTs (Fig. 5(c)) was also recorded and the RBM peak intensity observed at 179, 201, 243 and 257 corresponds to 1.38, 1.23, 1.02 and 0.96 nm tube diameter which is exactly as same as observed in oxygen doped SWCNTs. In this spectrum (Fig. 5(c)), we observe a new peak at 1461 cm1and it appears that the peak is related to CeN bonds. This new peak at 1461 cm1 has already been mentioned by Matsui et al. [51] and Yang et al. [52], as an attachment of N2 molecule. On the other hand, we also observed that peak intensity of D-band has increased in height which indicates that nitrogen doping may have cause defects/disorder in the SWCNTs.

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Fig. 4. HRTEM micrograph of (a) as-grown SWCNTs, (b) oxygen doped SWCNTs and (c) nitrogen doped SWCNTs.

XPS studies of a-sgrown and doped SWCNTs were made using Omicron Nano Technology GmbH, Germany with EA 125 energy analyzer. It is a multi-technique surface analysis system to analyze the elemental composition of a material in its as-received state or after doping and confirms the bonding of doped species with SWCNTs. XPS spectra of as-grown, and oxygen & nitrogen doped SWCNTs are shown in Fig. 6(aec). From the XPS spectra, we observe the peaks at 289, 399 and 531 eV which were identified as electronic 1s core levels of carbon, nitrogen and oxygen atoms respectively. The sharp peak at 289 eV, correspond to C 1s which is observed in as-grown SWCNTs. The peak at 399 and 531 eV indicates N 1s and O 1s levels respectively, which clearly confirms the bonding of nitrogen- and oxygen- functionalities with carbon nanotubes.

except for indication of CeC stretch. After oxygen and nitrogen doping, we observed few new peaks in the FTIR spectra (Fig. 7(bec)). From Fig. 7 (b), we observe the presence of infrared bands around 1650-1740 cm1 which indicates the presence of C] O oxygen functionalities in the nanotube surface. However, the infrared band around 1390-1650 cm1, can be associated with carbon double bond (C]C) stretching, which is typical of CNTs. From Fig. 7 (b), we also observe a band in the range 3000e3500 cm1 which is related to OeH stretching and is also associated with the formation of amorphous carbon because amorphous carbon easily forms a bond with atmospheric air. Band at 2970 cm1 is the characteristic of CeH. In Fig. 7 (c), we observed a band at 2100-2400 cm1 pointing towards the presence of C]N nitrogen functionalities in the nanotube surface. Therefore, FTIR spectrum clearly indicates the presence of oxygen- and nitrogencontaining functionalities after doping.

3.5. FTIR spectroscopic study

3.6. Field emission studies

FTIR spectrometer was used to identify the chemical group attached with SWCNTs [53]. All the samples (as-grown SWCNTs and doped SWCNTs) were scratched and the quantity of SWCNTs was added to KBr to prepare pellets under hydraulic press force of 10 ton. The spectrometer in the range 400e4000 cm1 was used to identify the oxygen and nitrogen functional groups attached on the surface of the SWCNTs. Fig. 7(a) shows the FTIR spectra of as-grown SWCNTs samples whereas Fig. 7(b) and (c) shows infrared spectrum of SWCNTs doped with oxygen and nitrogen respectively. The FTIR spectra of as-grown SWCNTs (Fig. 7(a)) shows very weak intensity peaks and a small hump around 1450 - 1650 cm1 corresponding to aromatic stretching of SWCNTs and there was almost no signal

The FowlereNordheim (F-N) theory [54] has been used to describe field emission from the as-grown as well as doped SWCNTs based electron emitters. According to this theory, it has been shown that work function is the relevant physical parameter for the field electron emitters. CNTs have much smaller values of work function than conventional field emitters such as silicon, diamond and molybdenum [55]. The theoretical studies render the information that defects and imperfections in the CNT structure may lead to dramatic reduction in its work function. It also depends on the type of bonding in CNTs formed when bending the graphene sheet, sp2 hybridizing characteristics change from sp2 to sp3 and atom lies along the nanotube surface. Consequently, it leads to

3.4. X-ray photoelectron spectroscopic (XPS) studies

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Fig. 6. XPS spectra of (a) as-grown SWCNTs, (b) oxygen doped and (c) nitrogen doped SWCNTs.

SWCNTs at their ends. According to F-N theory, emission current density (J) as a function of the electric field (E) and work function (f) of the emitting material, can be expressed as:

Bf3=2 J ¼ AE exp E 2

! (1)

where A ¼ 1.56  106 AeVV2, B ¼ 6.83  107 eV3/2V cm1 are constants and applied electric field (E) is defined as bdV , where V is the voltage between anode and the CNT emitters as cathode, d is the distance between cathode and anode, and b is field enhancement factor. The value of b has been determined from the slope (m) of FN plot by using the following relation

B¼

Fig. 5. Raman spectra of (a) as-grown SWCNTs (b) oxygen doped SWCNTs (c) nitrogen doped SWCNTs.

decrease in work function and effective surface barrier. Some reports have shown that the work function for SWCNTs is attributed to be an electrostatic effect due to the small radius of curvature of

Bf3=2 d m

(2)

J-E plots of as-grown, oxygen and nitrogen doped SWCNTs emitters were recorded in order to measure the field emission characteristics and a comparison among J-E plots is shown in Fig. 8. From the graph (Fig. 8), a good current density of 22.3 mA/cm2 at 1.6 V/mm field with a turn-on field (Eto) of 1.2 V/mm was recorded for the as-grown SWCNTs emitters. Further, a drastic enhancement in the current density of 116.59 mA/cm2 at 2.0 V/mm with a very low turn-on field of 0.8 V/mm, was recorded after oxygen doping. Moreover, after nitrogen doping, a high current density of 101 mA/ cm2 at 2.2 V/mm with a turn-on field of 1.2 V/mm, was observed. In order to calculate the field enhancement factor, the observed results were plotted on semi-log scale as shown in Fig. 9. The b estimated from slope of F-N plots were calculated to be 9.8  103, 2.2  104 and 1.5  104 for as-grown, oxygen and nitrogen doped SWCNTs respectively. The comparison of typical results obtained is shown in Table 1. Thus, the comparison among these three typical results indicates drastic improvement in the field emission properties after doping with oxygen and nitrogen. The enhancement in the field emission properties of the SWCNTs after doping may be attributed due to an increase in the number of activated SWCNTs tips by ion

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Fig. 8. J-E Plots of as-grown SWCNTs, and doped SWCNTs.

Fig. 9. F-N Plots of as-grown SWCNTs and doped SWCNTs.

Table 1 Field emission parameters of as-grown SWCNTs and doped SWCNTs.

Fig. 7. FTIR spectra of (a) as-grown SWCNTs (b) oxygen doped SWCNTs (c) nitrogen doped SWCNTs.

bombardment, as well as by the removal of amorphous carbon from the surface of the samples. During oxygen and nitrogen doping using plasma functionalization process, the as-grown SWCNTs

SWCNTs

Turn-on field

As-grown Oxygen doped Nitrogen doped

1.3 V/mm 0.8 V/mm 1.2 V/mm

Current density 2

22.3 mA/cm 116.59 mA/cm2 101 mA/cm2

Beta (b) 9.8  103 2.2  104 1.5  104

emitters with a smaller diameter, longer length and with large enhancement factor (b) appears to be deformed by the positive ions and with the initial removal of the amorphous carbon from the surface of the SWCNTs, it further creates large number of emitters having smaller values. Thus, a large number of emitters are activated which in turn enhance the current density with low turn-on field and improve the emission stability which is a favourable requirement for field emission display devices. Zhi et al. [56] in their work have shown an enhancement in the emission characteristics after hydrogen plasma functionalization of CNTs grown by CVD and reported that the enhancement was observed as a result of increase in the number of activated CNT tips and the removal of the catalyst particles remaining on the CNT tips. Gohel et al. [57] also

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(FEDs) devices in terms of low operating voltage, good emission stability, long operating life and high current density. Acknowledgements The authors are thankful to Department of Electronics and Information Technology (DeitY), Govt. of India (Grant no. 20(10) / 2007-NANO) for the financial support in the form of major research project and one of the authors, Dr. Avshish Kumar is also thankful to CSIR for providing fellowship as a Research Associate. References

Fig. 10. Emission current stability of as-grown SWCNTs emitters and doped SWCNTs emitters.

reported that after N2 plasma functionalization, an improvement in the field emission properties of CNTs grown by CVD was observed and concluded that it was due to the reduction in the nanotube density and nitrogen doping in the functionalized CNTs. 3.7. Emission current stability The emission current stability of the as-grown and doped SWCNTs emitters were measured at their constant applied voltage. The current stability as a function of time is shown in Fig. 10. Fig. 10 reveals the significant degradation in the current density observed in the case of as-grown SWCNTs. In the case of nitrogen doped SWCNTs emitters, we observe the degradation in the current density lesser than as-grown SWCNTs. Therefore, if we compare the emission stability of the as-grown, oxygen and nitrogen doped SWCNTs, we simply observed that the emission current stability of oxygen and nitrogen doped SWCNTs emitters were constant in comparison with as-grown SWCNTs emitters. However, oxygen doped SWCNTs emitters showed more stability throughout with almost constant emission current density of 116 mA/cm2. An increase in the electron emission stability and an improvement in the emission uniformity after oxygen and nitrogen doping have not been reported by any research group. 4. Conclusions Highly vertically aligned SWCNTs with diameter 1e2 nm and length of several micrometers were grown on Fe/Al substrate using PECVD system. The as-grown SWCNTs were found to have good field emission characteristic properties with long term emission current stability. The field emission characteristics of SWCNTs were further enhanced dramatically after oxygen and nitrogen doping. The oxygen doped SWCNTs showed greater current density as compared with nitrogen doped SWCNTs. After comparing the results with as-grown SWCNTs, it was observed that improvement in current density was almost 6 times in case of oxygen doped SWCNTs and almost 5 times in case of nitrogen doped SWCNTs. After doping, it was also observed that SWCNTs showed long term emission current stability due to strength of adhesion with substrate and increase in number of emitters participated in field emission process. Therefore oxygen and nitrogen doped SWCNTs will enhance the performance of future field emission display

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