Super growth of vertically-aligned carbon nanofibers and their field emission properties

CARBON

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Super growth of vertically-aligned carbon nanofibers and their field emission properties JungHo Kang a, Dong Hoon Shin b, Ki Nam Yun b, Felisita Annisanti Masud Cheol Jin Lee b, Myung Jong Kim a,c,*

a,c

,

a

Soft Innovative Materials Research Center, Korea Institute of Science and Technology, Eunha-ri San 101, Bongdong-eup, Wanju-gun, Jeollabuk-do 565-905, South Korea b School of Electrical Engineering, Korea University, Anam-dong, Seongbuk-gu, Seoul 136-713, South Korea c Nanomaterials Science and Engineering, Korea University of Science and Technology (UST), Daejeon 305-350, South Korea

A R T I C L E I N F O

A B S T R A C T

Article history:

We demonstrate a very efficient synthesis of vertically-aligned ultra-long carbon nanofi-

Received 24 March 2014

bers (CNFs) with sharp tip ends using thermal chemical vapor deposition. Millimeter-scale

Accepted 19 July 2014

CNFs with a diameter of less than 50 nm are readily grown on palladium thin film deposited

Available online xxxx

Al2O3 substrate, which activate the conical stacking of graphitic platelets. The field emission performance of the as-grown CNFs is better than that of previous CNFs due to their extremely high aspect ratio and sharp tip angle. The CNF array gives the turn-on electric field of 0.9 V/lm, the maximum emission current density of 6.3 mA/cm2 at 2 V/lm, and the field enhancement factor of 2585.  2014 Elsevier Ltd. All rights reserved.

1.

Introduction

Carbon fibers are one of the most widely used carbon-based materials after over 40 years of research. Carbon nanofibers (CNFs) are different from carbon fibers in their structures and thus possess different physical properties than carbon fibers. While carbon nanotubes (CNTs) have a highly ordered cylindrical shape of sp2 carbon network, CNFs possess various forms of elongated turbostratic stacking of graphitic carbon revealing open-edges to the outer surface. The main difference between CNTs and CNFs is the crystalline structure of the graphitic platelets and the filament axis. While the graphitic layers of CNTs are parallel to the axis, CNFs have angled stacking of less uniform lattice structures [1]. Although CNFs do not show the remarkable electrical/ mechanical properties of CNTs, the existence of dangling bonds at the edges could show promises in energy storage

applications. Baker et al. attributed the growth of carbon filaments by chemical vapor deposition (CVD) technique to the precipitation of diffused carbon on the facets of catalyst particles [2]. The generally accepted growth mechanism of CNFs or CNTs can be summarized as follows: (i) decomposition of hydrocarbon feedstock on catalytic transition metal particles, (ii) diffusion of dissolved carbon atoms on/through the catalysts, (iii) formation of tubular structures by carbon nucleation. Baker et al. calculated the activation energies of CNF growth with Fe, Co, and Ni, and found that they are very close to those of diffusion of carbon in these metals, which implies that the rate-limiting step in forming CNFs is the bulk diffusion of carbon in metal catalysts [3]. Catalysts in CVD play a key role in growing tubular nanostructures. Both floating catalysts and supported catalysts have been used to produce CNTs and CNFs [1,4]. Supported catalysts are capable of growing carpets of vertically aligned

* Corresponding author at: Soft Innovative Materials Research Center, Korea Institute of Science and Technology, Eunha-ri San 101, Bongdong-eup, Wanju-gun, Jeollabuk-do 565-905, South Korea. E-mail address: [email protected] (M.J. Kim). http://dx.doi.org/10.1016/j.carbon.2014.07.054 0008-6223/ 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Kang J et al. Super growth of vertically-aligned carbon nanofibers and their field emission properties. Carbon (2014), http://dx.doi.org/10.1016/j.carbon.2014.07.054

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carbon filaments, and these structures find direct application potentials in microelectronic devices [5]. A dense forest of vertically aligned micrometer-scale CNTs and CNFs can grow on a silicon substrate deposited with metal thin film catalyst, and the as-grown carpet gives field emission characteristics [6,7]. The physical shape of CNTs and CNFs heavily depends on catalyst particle size, precipitation rate which is determined by carbon solubility, diffusion rate of carbon, and catalyst lifetime [8–10]. Transition metals that are commonly used for catalysts have different catalytic performances due to their intrinsic properties. Fe, Co, Ni, Pd, and their alloys are well-known transition metals which grow CNTs or CNFs on substrates [11]. Previous studies showed that Fe particles supported on Al2O3 favor well-organized CNTs [12,13] whereas supported Ni and Pd were prone to produce CNFs [10,14,15]. The complete understanding of bulk diffusion and surface diffusion in relation to the growth of carbon filaments is still under debate, but it seems clear that the bulk diffusion of carbon does influence the growth of multi-walled carbon nanotubes (MWCNTs) as observed in the in-situ transmission electron microscopy study by Rodriguez-Manzo, et al. [16]. The height-to-diameter ratio, (i.e., aspect ratio) of CNFs and CNTs is particularly important for their field emission characteristics. Utsumi claimed that whisker-type electron guns gives the best performance and the field-enhancement factor is roughly proportional to the aspect ratio of an emitter source [17]. Previous works on field emission properties of vertically grown CNTs and CNFs with the aspect ratio of 100 reported field enhancement factors lower than 1300 [6,18]. These results indicate potential improvements in the field emission performance from carbon filaments of higher aspect ratio. The super-growth of vertically aligned CNTs was realized on a common silicon wafer substrate by using water vapor to enhance catalyst lifetime [12,19]. The waterassisted thermal CVD could yield up to 2.5 mm-long singlewalled CNTs and 4 mm-long MWCNTs presumably by inhibiting Ostwald ripening of catalyst particles on the Al2O3 substrate [12,20,21], but the similarly efficient growth of CNFs by thermal CVD has yet to be performed. In this paper, we present the first millimeter-scale synthesis of vertically aligned CNFs without using plasma, and the field emission properties of these CNFs with exceptionally high aspect ratio will be discussed. Using a different catalyst, we are able to prove experimentally that the millimeter-scale growth of vertically aligned CNFs can be achieved by this simpler synthesis technique than in the similar experimental set-up to grow 15– 50 nm thick MWCNTs as reported by Yun et al. [21].

2.

Experimental

To grow vertically aligned CNFs, a p-doped Si substrate topped with 300 nm SiO2 layer was used as the substrate of choice. However, we discovered that a quartz substrate also can yield similar results. 10 nm Al2O3 layer was deposited on top of SiO2 and subsequently 1 nm Pd thin film was deposited by ebeam evaporation. The substrate was annealed in air at 600 C for 10 min. To induce particle shape of the Pd layer by Ostwald ripening, we performed the pretreatment process in H2/Ar atmosphere (400/600 sccm) for 5 min at 780 C. The pretreatment step turned out to be crucial in growing tall CNFs.

Finally CNFs were grown in the flow of C2H4:H2:Ar = 100:400:500 sccm for 40 min at 780 C. Every step was performed in the atmospheric pressure. The structure was characterized by scanning electron microscopy (FEI Nova NanoSEM 450), transmission electron microscopy (FEI Tecnai G2 F20 460L), Raman spectroscopy (Horiba High Resolution Raman Spectrometer), and thermogravimetric analysis (TA-Q50). To measure the work function level of the CNFs to analyze the field emission experiment results, the electronic potential difference of the CNFs and Pd was mapped out by scanning Kelvin probe microscopy (SKPM). We placed dispersed CNFs in ethanol by sonication and spin-coated them on a SiO2/Si substrate. Then 10 nm Pd thin film was e-beam deposited atop with masking small regions of the surface to form ohmic contacts with the CNFs. Pd thin film was biased at 1 V, and the CNFs connected to the Pd thin film were imaged by atomic force microscopy and SKPM on the same region. (Park SYSTEMS NX-10) Field emission measurements were performed on the asgrown CNF carpet in 2 · 107 Torr in a diode configuration. The CNF sample was attached to a stainless steel cathode by carbon tape and silver paste to form a stable mechanical/ electrical contact to the cathode, and a tungsten rod with a diameter of 3 mm was used as the anode. The gap between the CNF tips and the anode was 400 lm. The emission current was monitored with a source meter (Keithley 2400), and the high voltage was controlled by a DC power supply (TECHNIX SR15 P-1500). All the measurements were performed at room temperature.

3.

Results and discussion

Diffusion properties of carbon in Fe, Ni, and Co are quite well known, but only a few studies report the diffusion behaviors of carbon in Pd. Yokoyama, et al. reported three to six times faster bulk diffusion of carbon in Pd than in c-Fe, Ni and Co [22]. Although the phase of pure iron changes at 912 C from body-centered cubic (a-Fe for T < 912 C) to face-centered cubic (c-Fe for T > 912 C), both phases should exist at the growth temperature (780 C) due to nonzero carbon percentage from unavoidable carbon contaminations and increasing carbon content from CNFs [23]. Diffusion of carbon in metal can be described in the Arrhenius equation, D ¼ D0 expðQ d =RTÞ where D0, Qd, R, and T are pre-exponential factor, activation energy, ideal gas constant, and temperature, respectively. Fig. S1 (see Supplementary Data) shows the comparison of previous studies on the bulk diffusion (or volumetric diffusion) of carbon in a-Fe, c-Fe, Pd, Ni, and Co [22,24]. The absolute value of the slope of each curve indicates the activation energy for bulk diffusion of the relevant material. Thus the activation energies of bulk diffusion of a carbon atom in aFe, Pd, Ni, and Co at 780 C are calculated as 0.9 eV, 1.2 eV, 1.5 eV, and 1.5 eV, respectively. a-Fe has the highest bulk diffusivity and the lowest activation energy in the displayed temperature range, which provides the most favorable condition to grow CNFs or CNTs based on the model suggested by Baker et al. This fact is in agreement with the work of other research group in which the authors could grow long vertically-aligned

Please cite this article in press as: Kang J et al. Super growth of vertically-aligned carbon nanofibers and their field emission properties. Carbon (2014), http://dx.doi.org/10.1016/j.carbon.2014.07.054

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Fig. 1 – (a) SEM image of vertically grown CNFs, (b) a zoomed-in image of (a).

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Fig. 2 – (a) TEM image of dispersed CNFs, (b) TEM image of a single CNF, (c) TEM image of the sharp CNF tip, (d) EDX spectrum from the selected region of (a). (A color version of this figure can be viewed online.)

MWCNTs with properly conditioned Fe catalyst [21,25], but no such growth has been reported with any other transition metals.

As the bulk diffusion of carbon is the rate-limiting factor in growing CNFs, Pd is the next candidate after a-Fe to enable super growth of CNFs by thermal CVD. As Fe catalyst particles

Please cite this article in press as: Kang J et al. Super growth of vertically-aligned carbon nanofibers and their field emission properties. Carbon (2014), http://dx.doi.org/10.1016/j.carbon.2014.07.054

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Fig. 3 – (a) Raman spectrum from dispersed CNFs (400–600 cm1 region that corresponds to Si signal was filtered out), (b) TGA curves of the CNFs. (A color version of this figure can be viewed online.)

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Fig. 4 – (a) AFM image showing CNFs on SiO2 (5 lm · 5 lm), (b) potential mapping of (a) by SKPM, (c) potential distribution of many CNFs of the blue line in (b). (A color version of this figure can be viewed online.)

in the reaction step have both a-Fe and c-Fe, the actual diffusivity of carbon in Pd catalysts might be comparable to that in iron catalysts. Fig. 1(a) shows a scanning electron microscopy (SEM) image of the millimeter-scale CNFs. The CNF carpet can grow up to 2 mm in height, and considering that the alignment of each filament is not straight the actual length is significantly longer. Fig. 1(b) shows the blow-up of Fig. 1(a), and it

displays nanofibers of mostly 40 nm thick with a few thinner ones. Noticeably, a transmission electron microscopy (TEM) image of bundled CNFs in Fig. 2(a) does not show any catalyst particles at the sharp tip ends unlike previous studies on CNFs grown with Pd catalysts by plasma enhanced CVD method [14,26]. All CNF filaments exhibit pointed tips without

Please cite this article in press as: Kang J et al. Super growth of vertically-aligned carbon nanofibers and their field emission properties. Carbon (2014), http://dx.doi.org/10.1016/j.carbon.2014.07.054

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catalysts as apparent in Fig. 2(b), which confirms base-growth mechanism as reported elsewhere [11,27]. Fig. 2(c) displays a single nanofiber with inclined graphitic planes and separated hollow cores. The observed nanofiber has a herringbone structure similar to the CNFs reported elsewhere [10], but with much smaller cone angle. The same structure could be repeatedly grow ultra-long CNFs at 750–780 C, but temperature levels outside this range resulted in much shorter carpets. Energy dispersive X-ray (EDX) analysis in Fig. 2(d) confirms that the CNF filaments do not contain any higherindexed elements than carbon except the Cu from TEM grid. Raman spectrum of the CNFs confirms the presence of graphitic layers as shown in G band at 1582 cm1 that corresponds to tangential vibration of carbon atoms in the graphitic lattice (Fig. 3(a)). Fig. 3(b) shows the thermogravimetric analysis (TGA) of the CNFs by changing the temperature at 10 C/min. The weight percent of the CNFs starts to drop at 550 C with 2.5% residue that corresponds to Pd catalyst particles transferred with the CNFs. The derivative weight-loss curve shows a narrow peak with a peak value of 3.0%/C at 675 C. The TGA result confirms fairly uniform crystallinity without any amorphous carbon compared to previous studies [28]. The catalyst life time can be another key factor in growing long carbon filaments. The observed catalyst life time of CNF growth was 40 min that is much longer than of verticallyaligned CNTs with Fe catalysts. Usually, the growth of CNTs with Fe catalysts terminates within 10 min unless assisted by water vapor [20]. In addition, excellent chemical stability of Pd might suppress catalyst poisoning effect in our synthesis and could contribute to the long catalyst lifetime. In addition, Pd is known as an excellent catalyst for dehydrogenation of ethylene [29], and we could see that only Pd thin film on Al2O3 shows very active catalytic activity in decomposing ethylene even at 500 C and under 20 mtorr ethylene forming amorphous carbon whereas a-Fe and Ni could not. This observation can possibly be another factor why Pd is capable of such a long growth without using water vapor or plasma, but the concrete description of the high yield requires further study on the thermodynamic interplay of diffusion and dissociation of carbon-hydrogen bonds on Pd as well as catalyst lifetime. There are several studies on growing CNFs with Pd

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[14,15,26], but none reported the length-scale of the present work without even using plasma. The sharp tip structure and the length of the CNFs apparent in Figs. 1 and 2 are promising characteristics for high field emission performance. In addition, zigzag edges of finite graphite was predicted to have sharp density of states due to the almost flat energy bands at the Fermi level [30], and these CNFs exhibit exceptionally high aspect ratio with plenty of revealed edge sites. To obtain more correct interpretation of the field emission properties of the CNFs, we performed the work function measurement using scanning Kelvin probe microscopy as in Fig. 4. The electronic potential difference between two different regions mapped out by SKPM correlates to the work function difference. The deposited Pd film was biased at 1 V (not shown in the image due to very noisy interface), and the imaged CNFs show 0.5 V down shift in Fig. 4(c). This indicates that the mean work function value of the CNFs is 0.5 eV lower than that of Pd; hence 4.6 eV for the CNFs, which is in very good agreement with a previous study [31]. The field emission current density (J) from the CNF carpet as a function of the applied electric field (F) is shown in Fig. 5(a). The turn-on electric field to obtain 10 nA/cm2 was 0.9 V/lm, and the threshold field for 1 mA/cm2 1.6 V/lm. The turn-on electric field was much lower compared to the previous CNFs’ results about 2–8 V/lm [32–34]. The field enhancement factor can be found to be 2585 by the Fowler– Nordheim (F–N) equation,   c2 F2 6:83  109 u3=2 J ¼ ð1:56  106 Þ exp u cF where J, F, c, and u are emission current density, applied electric field, field enhancement factor, and work function, respectively. We attribute this high emission performance of the CNFs to the extremely high aspect ratio and the sharp tip angle of the CNFs due to base-growth mechanism (Figs. 1(a) and 2(b)). High electric field, which is essential for high emission current density, can induce significant Maxwell stress [35], and therefore strong mechanical bonding of field emitters to a substrate while maintaining a good electrical contact is necessary for high emission current. It is well known that Pd can form strong ohmic contacts with carbon

Fig. 5 – Field emission characteristic of the CNF carpet: (a) J–F curve and (b) F–N plot. (A color version of this figure can be viewed online.) Please cite this article in press as: Kang J et al. Super growth of vertically-aligned carbon nanofibers and their field emission properties. Carbon (2014), http://dx.doi.org/10.1016/j.carbon.2014.07.054

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nanotubes [36] which consist mainly of carbon sp2 networks similarly to the edges of CNFs. As a result, we could obtain high emission current density of 6.3 mA/cm2 (0.44 mA from 0.07 cm2) at 2 V/lm.

[8]

4.

[9]

Conclusion

In conclusion, super growth of vertically aligned CNFs can be realized with palladium by thermal CVD method. High bulk diffusion rate of carbon in Pd and long catalyst life time seem to lead to ultra-long CNFs showing angled graphitic layers at the tip due to the catalyst-faceted stacking. The sharp tip and the exceptional aspect ratio of CNFs induced improved field emission and the field enhancement factor of 2585. The turn-on field and the maximum emission current density of the CNFs are 0.9 V/lm and 6.3 mA/cm2 at 2 V/lm, respectively.

[10] [11]

[12]

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Acknowledgements [14]

This work was supported by the Korea Institute of Science and Technology Institutional Program, International Cooperation of Science & Technology project (KICOS, 2009-00299) funded by the Ministry of Education, Science and Technology, the Converging Research Center Program funded by the Ministry of Science, ICT & Future Planning Technology (2013K000414), and the Graphene Materials/Components Development Project (10044366) funded by the Ministry of Trade, Industry, and Energy (MOTIE), Republic of Korea.

[15]

[16]

[17]

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carbon. 2014.07.054.

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