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Carbon–ZnO nanocomposite thin films for enhanced electron field emission characteristics prepared by continuous wave CO2 laser ablation
Vacuum 106 (2014) 21e26
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
CarboneZnO nanocomposite thin films for enhanced electron field emission characteristics prepared by continuous wave CO2 laser ablation Vishakha Kaushik, A.K. Shukla, V.D. Vankar* Thin Film Laboratory, Indian Institute of Technology, Delhi, New Delhi 110016, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 January 2014 Received in revised form 25 February 2014 Accepted 1 March 2014
A composite structure made by blending of nanostructured carbon (nC) and ZnO, was efficiently grown by a continuous wave CO2 laser assisted laser ablation method. The synthesis includes the growth of this composite material in a single experiment. A detail mechanism of interaction between the target material and impinging laser beam is explained. By adjusting the laser power nCeZnO composites were efficiently fabricated. Scanning and transmission electron microscopy studies showed that the ZnO formed bead-shaped nanoparticles on carbon matrix. The experimental results showed that the composite structure (nCeZnO) exhibit improved turn on field w2.2 V/mm compared to carbon matrix without ZnO coating (w3.7 V/mm). It was observed that the presence of ZnO on the carbon matrix reduces the work function of composite structure due to charge transfer process at interface. The strong van der Walls forces act at the interface result the quite fast charge transfer at interface. Thus, the electron field emission studies present the efficient electronic properties of nCeZnO composites and have the potential for future application in display devices. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Carbon films Nanocomposites Scanning electron microscopy Electron field emission
1. Introduction Recently, researchers have focused their attention on the carbon nanocomposites because of their splendid physical, chemical and mechanical properties [1e4]. Several groups have reported synthesis of carbon-oxide composites including, amorphous carbon, Al2O3, TiO2, SnO2, NiO film coated nanowires [5e8]. Apart from these composites, ZnOegraphene nanocomposites have been considered the potential contenders for electron field emission as well as for optoelectronic applications. It is well established that the carbon based nanostructures act as electron acceptors, whereas, ZnO structures behave as electron donors. Thus, the formation of composite by blending these two components would present many adorable advantages. Yan et al., studied the field emission properties of CNTeZnO heterojunction arrays [9]. In their studies, they observed efficient field emission with low turn-on field (1.8 V/mm), low threshold field (2.7 V/mm), and excellent emission current
* Corresponding author. Tel.: þ91 11 26591329; fax: þ91 11 26581114. E-mail addresses: [email protected], [email protected], [email protected] (V.D. Vankar). http://dx.doi.org/10.1016/j.vacuum.2014.03.002 0042-207X/Ó 2014 Elsevier Ltd. All rights reserved.
stability (700 min). In another study, Yu et al. reported the lower values of turn-on field (1.3 V/mm) and the threshold field (2.5 V/mm) of ZnO nanostructures grown on screen-printed CNT film as compared to ZnO nanomaterials [10]. First principle studies of grapheneeZnO nanocomposites have been carried out by Zhang et al. [11]. In their studies, they have reported electric structure and field emission characteristics using density functional theory. They showed that the work function of grapheneeZnO nanocomposites gets reduced up to 3.6 eV in the absence of electric field which will improve electron field emission [11]. Further, as the morphology of grown structures is directly influenced by the growth technique, therefore, reliable fabrication technique is the crucial parameter which decides their particular practical application. Several techniques are available to synthesize carbon based composites, such as, arc discharge, laser ablation and chemical vapour deposition (CVD), with or without plasma assistance [12e15]. Out of all these techniques, laser ablation is one of the most commonly employed methods for controlled growth of carbon based composites. Various interesting morphologies and microstructures of the films can be obtained by varying power, components of target material and pressure [16]. However, the growth parameters, such as, substrate temperature, laser power,
22
V. Kaushik et al. / Vacuum 106 (2014) 21e26
chamber pressure, carbon precursor and deposition time etc. must be optimized for their use in all the mentioned applications. The tuning of growth parameters, results in attractive morphologies and structures. The various techniques used for the growth of carboneZnO composites suffer from two major drawbacks. Firstly, the ZnO nanostructures are physically adsorbed on the surface of nanostructured carbon materials. It is well known that physical adsorption is weak, and hence, the composite structures are not stable. Therefore, a strong interaction is required between n-C and ZnO nanocomposites for improved electron field emission characteristics. Secondly, all available reports to synthesize a nanocomposite based on blending of both materials involve a complicated or multistep process [17,18]. In which, either gas phase or solution routes are adopted for the preparation of these nanocomposites. Both routes involves a high temperature (w1000 C) reduction of ZnO with graphite followed by a subsequent reoxidation to form ZnO and the colloidal nanoparticles of ZnO as precursors or zinc nitrate Zn(NO3)2 or acetylacetonate complexes of zinc, either by thermal decomposition or by employing solegel process for the deposition of ZnO on nanostructured carbon. In the context to the above drawbacks, laser ablation is a simple and effective technique; here we get the final product in a single step process. Altogether, this technique provides favourable tools to self-organization of ablated species into ZnOecarbon composites. Thus, it offers mass production in a short span of time and a strong interaction between the two species, which is important in terms of electrons interaction at the interface. The exploration on the synthesis of nanostructured carboneZnO (nCeZnO) nanocomposites by laser ablation method is important and needs investigation. In this study, we demonstrate a novel method for the fast, homogeneous and large area growth of thin composite films using continuous wave CO2 laser assisted laser ablation technique. Moreover, we shed light on the nanostructured carbon layer ZnO composites formed directly on Si substrates, without any substratetransfer technique. Further, we also report electron field emission properties of nCeZnO composites synthesized by the laser ablation process. 2. Experimental An indigenously developed experimental setup was used for this study and is shown in Fig. 1. A stainless steel vacuum chamber was evacuated by the turbo molecular pump to the base pressure at w107 torr after loading a pressed graphite powder and carbone ZnO pellet as the target. The nCeZnO composites were deposited on n-Si (100) substrates. A continuous CO2 laser (wavelength 10.6 mm and 400 W power) was used to ablate the target surface. The laser beam was focused to w1 mm and used to irradiate the surface of pellet at an incident angle of 45 . The total irradiation time was w5 min. Irradiation was carried out at a base pressure of w5 106 torr. The ablated material was deposited on the substrates kept at w50 mm away from the target. The deposition rate and the film thickness were controlled by the laser power and the deposition time which was kept constant throughout the experiments. The average growth rate was found to be dependent on the substrate temperature, the pressure inside the chamber, and the distance between the target and the substrate. The laser power was varied from 80 W to 140 W and the deposited samples were named as A, B, C, D and E respectively and are described in Table 1. A scanning electron microscope (Zeiss EVO 50 operating at 20 KV) was used to investigate the surface morphology of these films. A transmission electron microscope (Philips TM 12 operating at 200 KV) was used to carry out the micro-structural investigation
Fig. 1. A Schematic view of continuous wave CO2 laser assisted laser ablation set up.
of the samples. A diode set up was used to study the electron field emission characteristics of nCeZnO composites at a base pressure of 2 106 torr. Composite samples were used as a cathode and a stainless steel plate was used as an anode. The distance between the cathode and the anode was kept at w250 mm. The experiments were carried out using a high voltage dc power supply (H5KO2 N) and a current meter (Keithley 196 system DMM). The emission current density was calculated by dividing the current by the area of the sample. Fig. 1 shows the schematic view of the laser ablation system. Laser target material interaction occurs in two steps. Initially, the continuous laser beam strikes on the target material surface, where one part of this incident laser beam gets reflected and another part pervades into the target material i.e. absorbed. Now this absorbed radiation plays a complex role to decide the whole process. Since, in our experiments a continuous wave CO2 laser with wavelength 10.6 mm was used. The absorbed part of radiation is less piercing to the target and therefore, heats the target material by increasing its temperature up to its vapour pressure. This results in melting and simultaneous ablation of target material in the form of plasma plume. The laser beam, which is continuously impinging on the target material, interacts with the plasma plume. The plume absorbs a part of this radiation and shields the target surface resulting into reduced interaction with the target surface. The density of the plume is very critical in deciding the nature of species generated and reading over to the substrate. Altogether, this method offers sufficient and continuous temperature to selforganization of ablated species into ZnOecarbon composites. Various groups have reported the synthesis of carbon based
Table 1 Parameters used for the growth of nanostructured carboneZnO composite films by continuous wave CO2 laser assisted laser ablation technique. Sample name
Details
Pressure (torr)
A B C D E
Pristine carbon ZnOeC composites ZnOeC composites ZnOeC composites ZnOeC composites
5 5 5 5 5
106 106 106 106 106
Deposition time (min)
Laser power (watt)
3 3 3 3 3
80 80 100 120 140
V. Kaushik et al. / Vacuum 106 (2014) 21e26
structures using pulsed laser deposition and high power continuous wave laser [19e22]. In this way, we have demonstrated an easy method to create favourable conditions for directly synthesizing the ZnO particulates embedded in the network of carbon forming nCeZnO composites using a continuous CO2 laser vaporization reactor at low power. 3. Results and discussion 3.1. Surface morphology of nCeZnO composites Fig. 2 shows the surface morphology of the nCeZnO composites samples A, B C and D under laser irradiation of different power values. SEM micrographs have shown the ZnO particulates onto the carbon surface. In our experiments it is found that the irradiation of pure graphite pellet target at laser power up to 60 W does not lead to any deposition on the substrate. Fig. 2(a)e(d) clearly show the pristine carbon platelets and ZnO particulates deposited on the surface of the carbon platelets of samples A, B, C and D respectively. Here in our case, ZnO has less conductivity as well as absorption as compared to carbon for infrared continuous CO2 laser having 0.117 eV photon energy. Thus, carbon absorbed mostly energy and issued directly from solid to vapour state. After this the released ZnO vapour from the target condenses in liquid nano-aggregates and solubilize in carbon and ZnO composite aggregates. A model is presented here in Fig. 3. 3.2. Micro-structural study of nC-ZnO composites Fig. 4(a)e(f) shows the TEM micrographs of samples. Pristine carbon platelets can be seen in Fig. 4(a). Fig. 4(b) shows particles and spring of ZnO on carbon matrix and a chain of bead shape ZnO
23
particulates deposited on the carbon matrix is shown in Fig. 4(c). Further increasing the power at w140 W a very good film is deposited on the substrate with almost spherical shaped ZnO particles on the very thin carbon film (shown in Fig. 4(d)). The enlarged view of Fig. 4(d) is shown in Fig. 2(e). The sizes of the ZnO particulates on carbon film are in the range of w40e100 nm with a narrow size distribution. At w160 W powers the structure gets damaged corresponding to Fig. 2(f). It is speculated that as the laser power increases to an extent that the grain boundaries formation slows down at this point and crystalline structure deteriorates and formation of amorphous structure takes place. It can be clearly seen from Fig. 4(d) that a large area of carbon film/graphene layers as well as graphene composites can be obtained by using laser ablation technique. 3.3. Structural and X-ray photoelectron spectral study of nCeZnO composites The EDX spectrum shows clearly the presence of ZnO and carbon in the nCeZnO composite samples (Fig. 5(a)). The X-ray powder diffraction pattern of nCeZnO composite is confirmed the identification of crystalline phase of ZnO and carbon. Fig. 5(b) shows the X-ray photoelectron spectra (XPS) of ZnOeC composites. The spectra of Zn 2p doublets Zn2p3/2 and Zn2p1/2 is at w1022.38 eV and w1044.9 eV respectively. These values are consistence with the previously reported results [23]. The peak located at w284.6 and is assigned to the carbon element peak of C1s. Fig. 5(c) shows the asymmetry in the O1s spectra which is deconvoluted into two Gaussian peaks centred at w531.3 eV and w532.7 eV respectively. The peak of O1s at 531.3 eV belongs to oxygen ions of ZnO [23]. The higher binding energy spectrum of O1s is attributed due to the presence of some defects.
Fig. 2. SEM micrographs showing pristine carbon platelets (a) and ZnO particulates on carbon matrix (b, c, d) at 80 W, 100 W and 120 power respectively.
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V. Kaushik et al. / Vacuum 106 (2014) 21e26
Fig. 3. Illustration of the growth process steps for nCeZnO composites via laser ablation technique.
3.4. Electron field emission Electron field emission studies for nCeZnO composite films have been carried out using a diode configured set-up. A schematic of the electron field emission from nCeZnO composites is shown in Fig. 6(a). Fig. 6(b) shows the macroscopic current density versus macroscopic field plots for samples A, B, C and D and the inset image shows the corresponding FeN plots for all the samples. The turn on (Eto) values are taken at 10 mA/cm2. The electron field emission results show that the Eto values of the sample D is 2.2 V/ mm, which increase to 3.2 V/mm in sample B and 3.5 V/mm in sample C. Whereas, for sample A the turn on field 3.7 V/mm is observed to be maximum (Fig. 6) amongst all the samples. The electron field emission behaviour for nCeZnO composite samples was analysed using Fowler Nordheim theory [24].
JM zlM af1 gC 2 E2 exp nF bf3=2 =gC E In
this
equation,
JM
The constant term a ¼ 1.54 106 AeV V2 and b ¼ 6.83 107 aV3/ V cm1. The slope of the FN plots from the plot ln {J/E2}versus 1/E, is given by:
2
¼
macroscopic
current
density,
lM ¼ macroscopic pre-exponential correction factor and f ¼ emitter’s local work function. E ¼ applied macroscopic electric field and gC the field enhancement factor. nF is a correction factor.
S ¼ ðs=gC Þbf3=2 From this value of slope, the field enhancement factor is calculated. During calculation, we used the approximate value of slope correction factor s ¼ 1. It is found that Eon decrease and b-factor increase after grafting of ZnO on the carbon film samples and the values are tabulated in Table 2 and also are compared with the results from the literature. Fig. 7 shows the variation in the turn on field Eto with the increasing laser power. The increase in b-factor leads to electron field emission enhancement from ZnO grafted carbon film samples. It was found that carbon films grafted with ZnO at laser power 30% (sample D) showed better emission as compared with the other samples (grown for different times). The values obtained are found to be quite lower than the best values reported for graphene sheets, FLG
Fig. 4. TEM micrographs showing pristine carbon platelets (a), ZnO particulates on carbon layer (b), a chain of bead shape ZnO particles on carbon layer (c), a large folded graphene layer with almost spherical ZnO nanoparticles (d), the magnified view is shown in (e) and amorphous structure (f) at 80 W, 100 W, 120 W and 140 W respectively.
V. Kaushik et al. / Vacuum 106 (2014) 21e26
25
Fig. 5. EDAX image of nCeZnO composite to confirm the presence of C and ZnO in the samples (a), XPS spectra of Zn (2p) at the w1022 eV and w1046 eV core level (b), O 1s spectra at w531.3 eV and w532.7 eV (c) and C (1s) spectra at w284.7 eV (d) in nCeZnO composite sample.
and single layer graphene. The observed low value of turn on field and high enhancement factor in the present work is thus noteworthy and probably results from the unique characteristics of carboneZnO interaction. Santandrea et al., reported the turn on field (w600 V/mm) at 10 pA current from the inner flat part of single and few layer of graphene [25]. The enhancement factor for ZnO nanowires calculated by Kim et al., is quite lower than our calculated results [26]. In present study, we demonstrate that a composite of carbon and ZnO structure having advantage over flat graphene also. In the case of vertically aligned carbon films, the electron field emission originates from tips and edges. As the tips of carbon films get smaller and edges get sharper, the electron field emission of sample improves and the highest current density is
obtained. Various groups have reported the high current density as well as high enhancement factor and low turn on field values for vertically aligned carbon films [27e29]. But the case of horizontally aligned carbon films is totally different from the vertically aligned carbon films in terms of sharper edges and tips as well as aspect ratio. It is well known that the work function (f) of any material affects the corresponding field emission characteristics significantly. To understand the f dependence of electron field emission behaviour several studies has been carried out. The lowering of work function of material could result in improved electron field emission characteristics. We speculate that the enhanced electron field emission from nCeZnO composites may be due to lowering of
Fig. 6. (a)Schematic diagram of electron field emission from nCeZnO nanocomposites. (b) The macroscopic emission current density (J) as a function of applied macroscopic field (E) from pristine carbon platelets and nCeZnO nanocomposite samples A, B, C and D prepared at different laser power 80 W, 100 W and 120 W respectively. The inset shows the FeN plots for the same samples.
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V. Kaushik et al. / Vacuum 106 (2014) 21e26
Table 2 Field emission parameters showing Eon and enhancement factor for pristine and nanostructured carboneZnO composite films. Eon was taken at 10 mA/cm2 and values are compared with reported results from the literature. Sample details
Turn on field (Eon) V/mm
Enhancement factor (b)
Sample A Pristine C platelets (@80 W) Sample B nCeZnO composites (@80 W) Sample C nCeZnO composites (@100 W) Sample D nCeZnO composites (@120 W) ZnO/graphene hybrid taken at 1 mA/cm2 (From Ref. No. [31]) ZnO tip coated CNT (From Ref. No. [32])
3.7
4125
3.5
5178
3.0
6245
2.1
8100
2.7
3102
3.2
e
the work function. When the electric field is applied across the ZnO grafted carbon film, the charge is transferred from carbon to ZnO film interface and is clearly explained in the band diagram shown in Fig. 8. This will further increase the applied field strength and electron emission will be enhanced. This is consistent with the observation of Ho et al. [30]. The nCeZnO composite is regarded as a metal-semiconductor junction i.e. Schottky barrier. As the value of electron affinity of ZnO (w4.1 eV) is lower than the value of work function of carbon (w5 eV), this reduces the barrier and helps electrons to eject from ZnO to vacuum. These results are consistence with the earlier reported studies [30]. 4. Conclusion In summary, a new and simple process window is enlightened for the combined synthesis of nCeZnO composites by continuous wave CO2 laser assisted laser ablation technique. The electron field emission characteristics of nCeZnO composites were explored. The nano-particulates of ZnO are grown on top of the carbon matrix network. The results present a direct correlation between the synthesis of both the materials i.e. nCeZnO composites in an easy and single step process. A feasible growth mechanism is explained on the basis of electron microscopy results. The nCeZnO composites have improved turn on, threshold field and field enhancement factor as compared to carbon matrix. This is due to the presence of ZnO nano-particulates on the nC network, where lower barrier is formed for electrons to tunnel from ZnO to vacuum and they have a strong contact at interface due to which charge transfer is prominent. Additionally, the nano-particulates of ZnO on the nC network act as new emission sites. In this way, a single step grown special
Fig. 8. Band diagram of electron field emission from nCeZnO composite structure.
nCeZnO composites would present potential in electron emission devices. Acknowledgements One of the authors (V.K.) is highly thankful to director IIT Delhi for providing a research scholarship. Authors are also thankful to Prof. Vikram Kumar for useful discussions and suggestions. The financial support of the Ministry of Information Technology (MIT), Government of India, is gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
Fig. 7. Variation in turn on field parameter with increasing the power of CO2 laser.
[30] [31] [32]
Zhi M, Xiang C, Li J, Li M, Wu N. Nanoscale 2013;5:72. Liu X, Pan L, Zhao Q, Lv T, Zhu G, Chen T, et al. Chem Eng J 2012;183:238. Xu P, Tang Q, Zhou Z. Nanotechnology 2013;24:305401. Son DI, Kwon BW, Park DH, Seo W-S, Yi Y, Angadi B, et al. Nat Nanotechnol 2012;7:465. Yamamoto G, Omori M, Hashida T, Kimura H. Nanotechnology 2008;19: 315708. Tatsuda N, Itahara H, Setoyama N, Fukushima Y. Carbon 2005;43:2358. Han F, Li W-C, Li M-R, Lu A-H. J Mater Chem 2012;22:9645. Kuang D, Xu L, Liu L, Hu W, Wu Y. Appl Surf Sci 2013;273:484. Yan X, Tay B-K, Miele P. Carbon 2008;46:753. Cao Y, Flores RL, Xu Y-Q. Appl Phys Lett 2013;103:183103. Zhang S, Zhang Y, Huang S, Liu H, Wang P, Tian H. J Phys Chem C 2010;114: 19284. Journet C, Maser WK, Bernier P, Loiseau A, De La Chapelle ML, Lefrant S, et al. Nature 1997;388:756. Zhang Y, Gu H, Suenaga K, Iijima S. Chem Phys Lett 1997;279:264. Kaushik V, Shukla AK, Vankar VD. Carbon 2013;62:337. Coleman JN, Khan U, Blau WJ, Gun’ko YK. Carbon 2006;44:1624. Chen A, Bi Z, Tsai C-F, Lee J, Su Q, Zhang X, et al. Adv Funct Mater 2011;21:2423. Li B, Cao H. J Mater Chem 2011;21:3346. Xu T, Zhang L, Cheng H, Zhu Y. Appl Catal B Environ 2011;101:382. Sarath Kumar SR, Alshareef HN. Appl Phys Lett 2013;102:012110. Mortazavi SZ, Parvin P, Reyhani A. Laser Phys Lett 2012;9:547. Zhang H, Ding Y, Wu C, Chen Y, Zhu Y, He Y, et al. Phys B Condens Matter 2003;325:224. Morjan I, Voicu I, Dumitrache F, Sandu I, Soare I, Alexandrescu R, et al. Carbon 2003;41:2913. Zhang R, Fan L, Fang Y, Yang S. J Mater Chem 2008;18:4964. Forbes RG. Nanotechnology 2012;23:095706. Santandrea S, Giubileo F, Grossi V, Santucci S, Passacantando M, Schroeder T, et al. Appl Phys Lett 2011;98:163109. Kim SH, Lee CS, Kim J, Cho JW, Kim YK. AIP Adv 2013;3:092132. Srivastava SK, Shukla AK, Vankar VD, Kumar V. Thin Solid Films 2005;492:124. Cheng CY, Nakashima M, Teii K. Diam Relat Mater 2012;27e28:40. Takeuchi W, Kondo H, Obayashi T, Hiramatsu M, Hori M. Appl Phys Lett 2011;98:123107. Ho YM, Zheng WT, Li YA, Liu JW, Qi JL. J Phys Chem C 2008;112:17702. Ding J, Yana X, Xuea Q. Mater Chem Phys 2012;133:405. Tan Z, Chua DHC. J Electrochem Soc 2011;158:K112.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
CarboneZnO nanocomposite thin films for enhanced electron field emission characteristics prepared by continuous wave CO2 laser ablation Vishakha Kaushik, A.K. Shukla, V.D. Vankar* Thin Film Laboratory, Indian Institute of Technology, Delhi, New Delhi 110016, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 January 2014 Received in revised form 25 February 2014 Accepted 1 March 2014
A composite structure made by blending of nanostructured carbon (nC) and ZnO, was efficiently grown by a continuous wave CO2 laser assisted laser ablation method. The synthesis includes the growth of this composite material in a single experiment. A detail mechanism of interaction between the target material and impinging laser beam is explained. By adjusting the laser power nCeZnO composites were efficiently fabricated. Scanning and transmission electron microscopy studies showed that the ZnO formed bead-shaped nanoparticles on carbon matrix. The experimental results showed that the composite structure (nCeZnO) exhibit improved turn on field w2.2 V/mm compared to carbon matrix without ZnO coating (w3.7 V/mm). It was observed that the presence of ZnO on the carbon matrix reduces the work function of composite structure due to charge transfer process at interface. The strong van der Walls forces act at the interface result the quite fast charge transfer at interface. Thus, the electron field emission studies present the efficient electronic properties of nCeZnO composites and have the potential for future application in display devices. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Carbon films Nanocomposites Scanning electron microscopy Electron field emission
1. Introduction Recently, researchers have focused their attention on the carbon nanocomposites because of their splendid physical, chemical and mechanical properties [1e4]. Several groups have reported synthesis of carbon-oxide composites including, amorphous carbon, Al2O3, TiO2, SnO2, NiO film coated nanowires [5e8]. Apart from these composites, ZnOegraphene nanocomposites have been considered the potential contenders for electron field emission as well as for optoelectronic applications. It is well established that the carbon based nanostructures act as electron acceptors, whereas, ZnO structures behave as electron donors. Thus, the formation of composite by blending these two components would present many adorable advantages. Yan et al., studied the field emission properties of CNTeZnO heterojunction arrays [9]. In their studies, they observed efficient field emission with low turn-on field (1.8 V/mm), low threshold field (2.7 V/mm), and excellent emission current
* Corresponding author. Tel.: þ91 11 26591329; fax: þ91 11 26581114. E-mail addresses: [email protected], [email protected], [email protected] (V.D. Vankar). http://dx.doi.org/10.1016/j.vacuum.2014.03.002 0042-207X/Ó 2014 Elsevier Ltd. All rights reserved.
stability (700 min). In another study, Yu et al. reported the lower values of turn-on field (1.3 V/mm) and the threshold field (2.5 V/mm) of ZnO nanostructures grown on screen-printed CNT film as compared to ZnO nanomaterials [10]. First principle studies of grapheneeZnO nanocomposites have been carried out by Zhang et al. [11]. In their studies, they have reported electric structure and field emission characteristics using density functional theory. They showed that the work function of grapheneeZnO nanocomposites gets reduced up to 3.6 eV in the absence of electric field which will improve electron field emission [11]. Further, as the morphology of grown structures is directly influenced by the growth technique, therefore, reliable fabrication technique is the crucial parameter which decides their particular practical application. Several techniques are available to synthesize carbon based composites, such as, arc discharge, laser ablation and chemical vapour deposition (CVD), with or without plasma assistance [12e15]. Out of all these techniques, laser ablation is one of the most commonly employed methods for controlled growth of carbon based composites. Various interesting morphologies and microstructures of the films can be obtained by varying power, components of target material and pressure [16]. However, the growth parameters, such as, substrate temperature, laser power,
22
V. Kaushik et al. / Vacuum 106 (2014) 21e26
chamber pressure, carbon precursor and deposition time etc. must be optimized for their use in all the mentioned applications. The tuning of growth parameters, results in attractive morphologies and structures. The various techniques used for the growth of carboneZnO composites suffer from two major drawbacks. Firstly, the ZnO nanostructures are physically adsorbed on the surface of nanostructured carbon materials. It is well known that physical adsorption is weak, and hence, the composite structures are not stable. Therefore, a strong interaction is required between n-C and ZnO nanocomposites for improved electron field emission characteristics. Secondly, all available reports to synthesize a nanocomposite based on blending of both materials involve a complicated or multistep process [17,18]. In which, either gas phase or solution routes are adopted for the preparation of these nanocomposites. Both routes involves a high temperature (w1000 C) reduction of ZnO with graphite followed by a subsequent reoxidation to form ZnO and the colloidal nanoparticles of ZnO as precursors or zinc nitrate Zn(NO3)2 or acetylacetonate complexes of zinc, either by thermal decomposition or by employing solegel process for the deposition of ZnO on nanostructured carbon. In the context to the above drawbacks, laser ablation is a simple and effective technique; here we get the final product in a single step process. Altogether, this technique provides favourable tools to self-organization of ablated species into ZnOecarbon composites. Thus, it offers mass production in a short span of time and a strong interaction between the two species, which is important in terms of electrons interaction at the interface. The exploration on the synthesis of nanostructured carboneZnO (nCeZnO) nanocomposites by laser ablation method is important and needs investigation. In this study, we demonstrate a novel method for the fast, homogeneous and large area growth of thin composite films using continuous wave CO2 laser assisted laser ablation technique. Moreover, we shed light on the nanostructured carbon layer ZnO composites formed directly on Si substrates, without any substratetransfer technique. Further, we also report electron field emission properties of nCeZnO composites synthesized by the laser ablation process. 2. Experimental An indigenously developed experimental setup was used for this study and is shown in Fig. 1. A stainless steel vacuum chamber was evacuated by the turbo molecular pump to the base pressure at w107 torr after loading a pressed graphite powder and carbone ZnO pellet as the target. The nCeZnO composites were deposited on n-Si (100) substrates. A continuous CO2 laser (wavelength 10.6 mm and 400 W power) was used to ablate the target surface. The laser beam was focused to w1 mm and used to irradiate the surface of pellet at an incident angle of 45 . The total irradiation time was w5 min. Irradiation was carried out at a base pressure of w5 106 torr. The ablated material was deposited on the substrates kept at w50 mm away from the target. The deposition rate and the film thickness were controlled by the laser power and the deposition time which was kept constant throughout the experiments. The average growth rate was found to be dependent on the substrate temperature, the pressure inside the chamber, and the distance between the target and the substrate. The laser power was varied from 80 W to 140 W and the deposited samples were named as A, B, C, D and E respectively and are described in Table 1. A scanning electron microscope (Zeiss EVO 50 operating at 20 KV) was used to investigate the surface morphology of these films. A transmission electron microscope (Philips TM 12 operating at 200 KV) was used to carry out the micro-structural investigation
Fig. 1. A Schematic view of continuous wave CO2 laser assisted laser ablation set up.
of the samples. A diode set up was used to study the electron field emission characteristics of nCeZnO composites at a base pressure of 2 106 torr. Composite samples were used as a cathode and a stainless steel plate was used as an anode. The distance between the cathode and the anode was kept at w250 mm. The experiments were carried out using a high voltage dc power supply (H5KO2 N) and a current meter (Keithley 196 system DMM). The emission current density was calculated by dividing the current by the area of the sample. Fig. 1 shows the schematic view of the laser ablation system. Laser target material interaction occurs in two steps. Initially, the continuous laser beam strikes on the target material surface, where one part of this incident laser beam gets reflected and another part pervades into the target material i.e. absorbed. Now this absorbed radiation plays a complex role to decide the whole process. Since, in our experiments a continuous wave CO2 laser with wavelength 10.6 mm was used. The absorbed part of radiation is less piercing to the target and therefore, heats the target material by increasing its temperature up to its vapour pressure. This results in melting and simultaneous ablation of target material in the form of plasma plume. The laser beam, which is continuously impinging on the target material, interacts with the plasma plume. The plume absorbs a part of this radiation and shields the target surface resulting into reduced interaction with the target surface. The density of the plume is very critical in deciding the nature of species generated and reading over to the substrate. Altogether, this method offers sufficient and continuous temperature to selforganization of ablated species into ZnOecarbon composites. Various groups have reported the synthesis of carbon based
Table 1 Parameters used for the growth of nanostructured carboneZnO composite films by continuous wave CO2 laser assisted laser ablation technique. Sample name
Details
Pressure (torr)
A B C D E
Pristine carbon ZnOeC composites ZnOeC composites ZnOeC composites ZnOeC composites
5 5 5 5 5
106 106 106 106 106
Deposition time (min)
Laser power (watt)
3 3 3 3 3
80 80 100 120 140
V. Kaushik et al. / Vacuum 106 (2014) 21e26
structures using pulsed laser deposition and high power continuous wave laser [19e22]. In this way, we have demonstrated an easy method to create favourable conditions for directly synthesizing the ZnO particulates embedded in the network of carbon forming nCeZnO composites using a continuous CO2 laser vaporization reactor at low power. 3. Results and discussion 3.1. Surface morphology of nCeZnO composites Fig. 2 shows the surface morphology of the nCeZnO composites samples A, B C and D under laser irradiation of different power values. SEM micrographs have shown the ZnO particulates onto the carbon surface. In our experiments it is found that the irradiation of pure graphite pellet target at laser power up to 60 W does not lead to any deposition on the substrate. Fig. 2(a)e(d) clearly show the pristine carbon platelets and ZnO particulates deposited on the surface of the carbon platelets of samples A, B, C and D respectively. Here in our case, ZnO has less conductivity as well as absorption as compared to carbon for infrared continuous CO2 laser having 0.117 eV photon energy. Thus, carbon absorbed mostly energy and issued directly from solid to vapour state. After this the released ZnO vapour from the target condenses in liquid nano-aggregates and solubilize in carbon and ZnO composite aggregates. A model is presented here in Fig. 3. 3.2. Micro-structural study of nC-ZnO composites Fig. 4(a)e(f) shows the TEM micrographs of samples. Pristine carbon platelets can be seen in Fig. 4(a). Fig. 4(b) shows particles and spring of ZnO on carbon matrix and a chain of bead shape ZnO
23
particulates deposited on the carbon matrix is shown in Fig. 4(c). Further increasing the power at w140 W a very good film is deposited on the substrate with almost spherical shaped ZnO particles on the very thin carbon film (shown in Fig. 4(d)). The enlarged view of Fig. 4(d) is shown in Fig. 2(e). The sizes of the ZnO particulates on carbon film are in the range of w40e100 nm with a narrow size distribution. At w160 W powers the structure gets damaged corresponding to Fig. 2(f). It is speculated that as the laser power increases to an extent that the grain boundaries formation slows down at this point and crystalline structure deteriorates and formation of amorphous structure takes place. It can be clearly seen from Fig. 4(d) that a large area of carbon film/graphene layers as well as graphene composites can be obtained by using laser ablation technique. 3.3. Structural and X-ray photoelectron spectral study of nCeZnO composites The EDX spectrum shows clearly the presence of ZnO and carbon in the nCeZnO composite samples (Fig. 5(a)). The X-ray powder diffraction pattern of nCeZnO composite is confirmed the identification of crystalline phase of ZnO and carbon. Fig. 5(b) shows the X-ray photoelectron spectra (XPS) of ZnOeC composites. The spectra of Zn 2p doublets Zn2p3/2 and Zn2p1/2 is at w1022.38 eV and w1044.9 eV respectively. These values are consistence with the previously reported results [23]. The peak located at w284.6 and is assigned to the carbon element peak of C1s. Fig. 5(c) shows the asymmetry in the O1s spectra which is deconvoluted into two Gaussian peaks centred at w531.3 eV and w532.7 eV respectively. The peak of O1s at 531.3 eV belongs to oxygen ions of ZnO [23]. The higher binding energy spectrum of O1s is attributed due to the presence of some defects.
Fig. 2. SEM micrographs showing pristine carbon platelets (a) and ZnO particulates on carbon matrix (b, c, d) at 80 W, 100 W and 120 power respectively.
24
V. Kaushik et al. / Vacuum 106 (2014) 21e26
Fig. 3. Illustration of the growth process steps for nCeZnO composites via laser ablation technique.
3.4. Electron field emission Electron field emission studies for nCeZnO composite films have been carried out using a diode configured set-up. A schematic of the electron field emission from nCeZnO composites is shown in Fig. 6(a). Fig. 6(b) shows the macroscopic current density versus macroscopic field plots for samples A, B, C and D and the inset image shows the corresponding FeN plots for all the samples. The turn on (Eto) values are taken at 10 mA/cm2. The electron field emission results show that the Eto values of the sample D is 2.2 V/ mm, which increase to 3.2 V/mm in sample B and 3.5 V/mm in sample C. Whereas, for sample A the turn on field 3.7 V/mm is observed to be maximum (Fig. 6) amongst all the samples. The electron field emission behaviour for nCeZnO composite samples was analysed using Fowler Nordheim theory [24].
JM zlM af1 gC 2 E2 exp nF bf3=2 =gC E In
this
equation,
JM
The constant term a ¼ 1.54 106 AeV V2 and b ¼ 6.83 107 aV3/ V cm1. The slope of the FN plots from the plot ln {J/E2}versus 1/E, is given by:
2
¼
macroscopic
current
density,
lM ¼ macroscopic pre-exponential correction factor and f ¼ emitter’s local work function. E ¼ applied macroscopic electric field and gC the field enhancement factor. nF is a correction factor.
S ¼ ðs=gC Þbf3=2 From this value of slope, the field enhancement factor is calculated. During calculation, we used the approximate value of slope correction factor s ¼ 1. It is found that Eon decrease and b-factor increase after grafting of ZnO on the carbon film samples and the values are tabulated in Table 2 and also are compared with the results from the literature. Fig. 7 shows the variation in the turn on field Eto with the increasing laser power. The increase in b-factor leads to electron field emission enhancement from ZnO grafted carbon film samples. It was found that carbon films grafted with ZnO at laser power 30% (sample D) showed better emission as compared with the other samples (grown for different times). The values obtained are found to be quite lower than the best values reported for graphene sheets, FLG
Fig. 4. TEM micrographs showing pristine carbon platelets (a), ZnO particulates on carbon layer (b), a chain of bead shape ZnO particles on carbon layer (c), a large folded graphene layer with almost spherical ZnO nanoparticles (d), the magnified view is shown in (e) and amorphous structure (f) at 80 W, 100 W, 120 W and 140 W respectively.
V. Kaushik et al. / Vacuum 106 (2014) 21e26
25
Fig. 5. EDAX image of nCeZnO composite to confirm the presence of C and ZnO in the samples (a), XPS spectra of Zn (2p) at the w1022 eV and w1046 eV core level (b), O 1s spectra at w531.3 eV and w532.7 eV (c) and C (1s) spectra at w284.7 eV (d) in nCeZnO composite sample.
and single layer graphene. The observed low value of turn on field and high enhancement factor in the present work is thus noteworthy and probably results from the unique characteristics of carboneZnO interaction. Santandrea et al., reported the turn on field (w600 V/mm) at 10 pA current from the inner flat part of single and few layer of graphene [25]. The enhancement factor for ZnO nanowires calculated by Kim et al., is quite lower than our calculated results [26]. In present study, we demonstrate that a composite of carbon and ZnO structure having advantage over flat graphene also. In the case of vertically aligned carbon films, the electron field emission originates from tips and edges. As the tips of carbon films get smaller and edges get sharper, the electron field emission of sample improves and the highest current density is
obtained. Various groups have reported the high current density as well as high enhancement factor and low turn on field values for vertically aligned carbon films [27e29]. But the case of horizontally aligned carbon films is totally different from the vertically aligned carbon films in terms of sharper edges and tips as well as aspect ratio. It is well known that the work function (f) of any material affects the corresponding field emission characteristics significantly. To understand the f dependence of electron field emission behaviour several studies has been carried out. The lowering of work function of material could result in improved electron field emission characteristics. We speculate that the enhanced electron field emission from nCeZnO composites may be due to lowering of
Fig. 6. (a)Schematic diagram of electron field emission from nCeZnO nanocomposites. (b) The macroscopic emission current density (J) as a function of applied macroscopic field (E) from pristine carbon platelets and nCeZnO nanocomposite samples A, B, C and D prepared at different laser power 80 W, 100 W and 120 W respectively. The inset shows the FeN plots for the same samples.
26
V. Kaushik et al. / Vacuum 106 (2014) 21e26
Table 2 Field emission parameters showing Eon and enhancement factor for pristine and nanostructured carboneZnO composite films. Eon was taken at 10 mA/cm2 and values are compared with reported results from the literature. Sample details
Turn on field (Eon) V/mm
Enhancement factor (b)
Sample A Pristine C platelets (@80 W) Sample B nCeZnO composites (@80 W) Sample C nCeZnO composites (@100 W) Sample D nCeZnO composites (@120 W) ZnO/graphene hybrid taken at 1 mA/cm2 (From Ref. No. [31]) ZnO tip coated CNT (From Ref. No. [32])
3.7
4125
3.5
5178
3.0
6245
2.1
8100
2.7
3102
3.2
e
the work function. When the electric field is applied across the ZnO grafted carbon film, the charge is transferred from carbon to ZnO film interface and is clearly explained in the band diagram shown in Fig. 8. This will further increase the applied field strength and electron emission will be enhanced. This is consistent with the observation of Ho et al. [30]. The nCeZnO composite is regarded as a metal-semiconductor junction i.e. Schottky barrier. As the value of electron affinity of ZnO (w4.1 eV) is lower than the value of work function of carbon (w5 eV), this reduces the barrier and helps electrons to eject from ZnO to vacuum. These results are consistence with the earlier reported studies [30]. 4. Conclusion In summary, a new and simple process window is enlightened for the combined synthesis of nCeZnO composites by continuous wave CO2 laser assisted laser ablation technique. The electron field emission characteristics of nCeZnO composites were explored. The nano-particulates of ZnO are grown on top of the carbon matrix network. The results present a direct correlation between the synthesis of both the materials i.e. nCeZnO composites in an easy and single step process. A feasible growth mechanism is explained on the basis of electron microscopy results. The nCeZnO composites have improved turn on, threshold field and field enhancement factor as compared to carbon matrix. This is due to the presence of ZnO nano-particulates on the nC network, where lower barrier is formed for electrons to tunnel from ZnO to vacuum and they have a strong contact at interface due to which charge transfer is prominent. Additionally, the nano-particulates of ZnO on the nC network act as new emission sites. In this way, a single step grown special
Fig. 8. Band diagram of electron field emission from nCeZnO composite structure.
nCeZnO composites would present potential in electron emission devices. Acknowledgements One of the authors (V.K.) is highly thankful to director IIT Delhi for providing a research scholarship. Authors are also thankful to Prof. Vikram Kumar for useful discussions and suggestions. The financial support of the Ministry of Information Technology (MIT), Government of India, is gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
Fig. 7. Variation in turn on field parameter with increasing the power of CO2 laser.
[30] [31] [32]
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