Field Emission Properties
For field emission experiments a high electric potential (up to 250 V) was applied between the CNT and the counter electrode. The TEM-SPM setup allows accurate control and measurement of the position of the CNT relative to the anode and other surrounding objects (fig 2a). The local field was estimated using [21], where is the applied potential, is the interelectrode distance, is the field enhancement factor. The geometrical field enhancement factor γ for nanotubes was determined by their length L and radius r as γ = (0.87L/r + 4.5) [32,33]. In our case, the field enhancement factor exceeds 100 for most of nanotubes investigated except for CVD grown tube like MWNTs for which γ=20 (table 1).
Figure 2b and table 1 show comparison of field emission currents as a function of local field for all investigated types of carbon nanotubes.

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Figure 2. a) TEM image of a CVD grown multiwall carbon nanotube during field emission measurements; b) current dependence on local field for different types of CNT. Solid lines are fitting with Fowler-Nordheim theory calculated using work function 8.1 eV for bamboo like nanotubes and for others - 5.1 eV.
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Typically, the field emission for MWCNT grown on Co/Mo catalyst sets in at local fields of 1.5 - 4.1 V/nm (table 1) and saturates at local fields of 3.0 – 4.6 V/nm, which is comparable to the literature data for multi-walled carbon nanotubes [21]. Our data for CVD grown SWNTs not filled and filled with C60 molecules shows that field emission starts at almost identical voltages to those observed for MWNTs. Commercial CVD grown nanotubes (from Aldrich) and MWCNTs grown by the novel supercritical fluid (SCF) deposition method exhibit almost identical field emission characteristics to those grown by the CVD method on a Co/Mo catalyst (fig. 2b). Field emission and conductive characteristics of CNTs is affected by the degree of graphitization [34].
According to our findings, the situation is more complicated. Raman spectrum for CVD grown SWCNTs and SCF grown nanotubes (Figure 3) shows difference in G and D band height ratios. G and D bands correspond to sp2 (conductive structures) and sp3 (nonconductive structures) bonding in CNTs. Though G/D band ratio and the consequent higher degree of graphitization is observed for CNTs grown on Co/Mo catalyst, we do not observe remarkable diffrences in field emission properties of CVD and SCF grown nanotubes. G/D ratio for bamboo like structures is even higher than for CVD grown SWCNTs, however, field emission for bamboo shaped MWCNTs grown on MgO supported Pd/Mo catalyst by the CVD method sets up at much higher local fields 7-8 V/nm (figure 2b).

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Figure 3. Raman spectra of CVD-grown single wall CNTs prepared by methane decomposition at 800 C, and SCF-grown multiwall CNTs by CO disproportion at 750 C.
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The dependence of current on local field obtained for all investigated nanotubes fitted well to the Fowler-Nordheim model (see, Figure 2b solid lines), which states that the current (I) per emitter varies with the local field at the emitter’s surface area (A) [36,37]:
(1)
where Φ is the work function, V is the applied voltage; and d is the interelectrode distance.
For all nanotubes, except the BCNTs, field emission characteristics fit to the Fowler-Nordheim model when a value for the work function of graphite 5.1 eV [5] was used. The peculiar local field needed for emission from the BCNTs required a work function value of 8.1 eV to be used. The junctions in these BCNTs are likely to influence the effective L/r ratio, which determines the field enhancement factor, making the emission threshold fields larger.
The average field emission currents were determined at saturation (see table 1) and varied from 10 nA to 500 nA for CVD grown tube like MWCNTs, from 100 nA to 1.5 µA for BCNTs, and from 150 nA to 2 µA for SCF grown MWCNTs. Commercially available (Aldrich) MWCNTs exhibit higher field emission currents - up to 10 μA. Previously reported values for field emission saturation currents for CVD grown nanotubes varied in a very large interval from 2 nA to 9 µA [19]. Field emission saturation currents for CVD grown SWCNTs varied from 100 nA to 5 µA for empty and up to 10 µA for C60 filled SWCNTs. During field emission characterization of individual nanotubes field emission currents at saturation were fluctuating in time for approximately 50% of average values, similar to those reported previously [38].
Table 1. Field emission characteristics of CNTs grown at different conditions.
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Emission start up local field, V/nm
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1.5-4.1
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3.0-4.9
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3.5- .4
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4.5-5.5
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2.5-4.5
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8–10
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Field emission current, μA
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0.01-0.5
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0.1 - 5.0
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0.1 - 10.0
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5 - 15
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0.15 - 2.0
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0.1 – 1.5
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Breaking current, μA
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0.1-1.0
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0.15-10.0
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2.5-10.0
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10-0 25
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0.45-3.5
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0.45-2.0
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Breaking local field, V/nm
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3.0 – 6.0
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4.0 - 6.0
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4.0 - 9.0
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6.0 - 8.0
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3.5 - 8.0
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9.0 - 12.0
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Breaking place
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in contact
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in contact
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gradual destruction
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gradual destruction
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in contact
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in contact, gradual destruction
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Field enhancement factor
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20±5
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110±30
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90±10
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105±15
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135±30
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150±30
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At high local fields failure of the nanotubes was observed (Fig 4, table 1). Failure currents were approximately 2 times higher than saturation currents. Failure local fields are presented in table 1 and varied from 3-4.6 V/nm for tube-like MWCNTs and up to 12 V/nm for bamboo-like MWCNTs. Literature values of breaking local fields for CVD grown nanotubes from different groups varied in the interval of 3 to 10 V/nm [19,21].
Breaking local fields for SWCNTs were comparable with MWCNTs (up to 6 V/nm), while C60-filled SWNTs exhibited a higher stability with breaking local fields up to 9 V/nm. A gradual degradation of C60-filled SWNTS and BCNTs emitters at high applied voltages (200-250V) were observed as shown in figure 4 (see also table 1.) while all other nanotubes failed at the weak contact between the CNT and the gold tip; in this case the CNT fully removes from the gold tip when the emission current is not very high.

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Figure 4. A gradual destruction of a single-walled CNT filled with C60 molecules bundle at constant voltage (200 V) applied. The time between images is 10 s.
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Electrical Characterization Using a Two Probe Method
In order to measure the electric conductivity of the individual CNTs, the nanotube was brought into direct contact with a counter electrode as shown in figure 5a. I(V) characteristics of all types of CNTs addressed in this paper are shown in Figure 5c.
Table 2. Conductive and breaking characteristics of CVD-grown CNTs.
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Breaking voltage, V
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4.5 - 6.0
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5.0 - 11.0
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4.0 - 4.5
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≥25
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0.7 - 8.0
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0.7 - 5.0
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Breaking current, μA
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12 - 19
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7 - 80
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10 - 18
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≥0.3
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0.0003- 0.01
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0.0005 - 0.005
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Breaking place
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near the contact
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near the middle
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near the middle
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near the middle
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near the middle
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near the middle, near the contact
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Accordingly resistances for all nanotubes can be divided into two groups. The resistance of the first group (tube-like MWCNTs and SWCNTs) of nanotubes varied in the interval of 250 kΩ – 1MΩ, which is typical for two-probe characterization of CVD grown CNTs [19]. The resistance in this group decreases in the order of CVD grown tube like MWCNTs, empty and filled with C60 SWCNTs (Figure 5c).
C60-filled SWNTS exhibited lower resistance in comparison to unfilled SWNTS. For CVD grown BCNTs, commercial MWCNTs and SCF grown MWNTs, the resistance measured was higher by 3 orders of magnitude in comparison to data for first group of nanotubes reported above. Reason for such differences for SCF and commercial nanotubes may be a lower degree of graphitization in comparison to nanotubes grown on C0/Mo catalyst.
As shown in Raman spectrums (Figure 3), SCF nanotubes exhibit lower degree of graphitization in comparison to nanotubes grown on Co/Mo catalyst by the CVD method. Our opinion is that the commercial and SCF grown nanotubes have fragmentary outer shells with a high density of defects. In case of a two-point characterization, the counter electrode contacts direct to defective outer shells instead of current-carrying less defective inner shells.
To provide electron transport through inner shells, a higher voltage is required. As reported previously, outer layers of carbon nanotubes are dominant in electron transport and therefore determine the conductivity of the nanotubes [22]. Possibly inner layers are participating in field emission therefore significant differences in field emission between CVD and SCF grown nanotubes was not observed (Figure 2b) in comparison to the difference in resistances (Figure 5c). It is possible that the junctions present in bamboo like structures evenly decrease the conductivity in the outer and inner walls of the CNTs. This property may increase the resistance of field emission and conductivity.
Nanotube Quality
To characterize the quality of the nanotubes, failure currents and voltages were determined (see table 2). Nanotube failure currents were up to 20 µA for CVD-grown SWNTs and MWNTs and up to 80 µA for C60-filled SWNTs. Failure voltages were up to 5 V for SWNTs and up to 10 V for C60-filled SWNTs (see table 2). Bamboo type CNTs are stable even at 25 V which means that these nanotubes can be used in high electric field applications. Figure 5b shows that the nanotube is disrupted near its middle, which has been observed for all types of nanotubes. This result suggests that the tubes were resistively heated and that the temperature becomes locally high enough to vaporize the graphite wall; consequently, the location of damage by Joule heating is not determined by the presence of defects in the CNT. Consequently, the location of the defects does not have an effect on the breaking point of the CNT under observation. In other works [19,21] nanotube failures were found near the middle and in the contact regions.

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Figure 5. a) MWCNT grown by CVD method and b) nanotube breaking during two-probe measurements; c) comparison between I(V) curves of different types of CNTs: 1 - singlewall CNTs filled with C60 molecules, 2 - empty singlewall CNTs, 3 - multiwall CNTs grown on Co/Mo catalyst, 4 - Aldrich commercially available multiwall CNTs, 5 - multiwall CNTs grown on Pd/Mo catalyst, 6 - multiwall CNTs grown by supercritical fluid method.
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