Field emission behavior of boron nitride nanotubes

Carbon nanotube (CNT) has been considered a promising field emission material, due to its good thermal and electrical conductivities, and high aspect ratio [1–3]. However, critical obstacles still need to be overcome for the real application of CNT field emitters to various vacuum electronic devices. In particular, CNT field emitters have exhibited poor long-term emission stability at high vacuum pressure [4], even though they have indicated remarkable field emission performance at low vacuum pressure, such as a low turn-on electric field, a high emission current, and good long-term emission stability [5–10]. Recently, boron nitride nanotubes (BNNTs) have attracted attention as a new field emission material under harsh ambient condition as an alternative to CNTs, due to their strong oxidation endurance [11]. However, the BNNTs showed much higher turn-on electric field and lower emission current compared to those of CNTs. Some research groups have studied the field emission properties of BNNTs, but field emission performances of the BNNTs under harsh ambient condition have not been reported until now [12–15]. By contrast, the field emission properties of boron carbon nitride (BCN) film have been demonstrated according to vacuum pressure [16]. The obtained results showed that the field emission characteristics of the BCN film were not significantly degraded at high vacuum pressure (∼10⁻⁴ Torr). However, one problem is that they measured emission stability for a very short time of 60 min even though emission stability is normally measured over 10 h to evaluate reasonable emission behavior. In this work, we attempted to understand field emission characteristics of BNNT field emitter according to vacuum pressure during long-term operation for 20 h. Thus, we fabricated the BNNT field emitter using the BNNT film, as demonstrated in our previous work [17], and we also investigated the field emission properties of the BNNT field emitter dependent on vacuum pressure.

The field emission properties of the BNNT field emitters were measured using a diode configuration in a vacuum chamber. A cylindrical stainless steel bar with a diameter of 3 mm was used for the anode electrode, while a stainless steel plate with a rectangular hole was used for the cathode electrode. The BNNT field emitter was fixed into the hole of the stainless steel cathode. The anode electrode was placed above the center of the BNNT field emitter. The distance between the anode electrode and the BNNT field emitter was 750 μm. The vacuum pressure in the chamber was controlled from ∼10⁻⁷ to ∼10⁻⁵ Torr by adjusting the main valve of the vacuum chamber. DC power voltage between the cathode electrode and the anode electrode was applied by a high voltage supplier (TECHNIX SR 15-P-1500), and the emission current was monitored using a current meter (KEITHLEY 2400).

Figures 1(a) and (b) show the surface morphology of the BNNTs that was observed using a scanning electron microscopy (SEM) (Hitachi, S-4800). The SEM images indicate that the BNNTs are of high purity, with a length of a few tens of micrometers, and of uniform diameter. The BNNT film made by a filtration-transfer method was used for the fabrication of the BNNT field emitters [17]. The BNNT suspension made by dispersing the BNNTs in isopropanol (IPA) was poured on a membrane of vacuum filtration equipment, and the BNNT film was formed on the membrane by vacuum suction. The BNNT film was separated from the membrane by dissolving the surface of the membrane using a NaOH solution, and then washed with distilled water and IPA. The separated BNNT film was dried for 2 h in a dry oven at 80 °C. The BNNT film with a thickness of 50 μm was cut into a rectangular shape using a razor. The BNNT field emitter was fabricated by clamping the rectangular shaped BNNT film between two stainless steel plates as shown in figure 1(c). This fabrication method provides good mechanical adhesion and low electrical contact resistance between the BNNT film and the cathode electrode. Figure 1(d) shows that some BNNTs were protruded on the top edge of the BNNT film, which can contribute to electron emission while the electric field is applied.

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Figure 2(a) shows the field emission characteristics of the BNNT field emitter according to vacuum pressure. The emission current was measured by adjusting vacuum pressure from 1.5 × 10⁻⁵ to 1.5 × 10⁻⁷ Torr. The emission area of the BNNT field emitter was about 0.0015 cm⁻². It is noted that the emission current could not be measured in this configuration without the BNNT film. Consequently, the measured emission current in this work originated from BNNT film. The emission current behavior of the BNNT field emitters indicates no significant change according to the vacuum pressure, as shown in figure 2(a). For the current density of about 75 mA cm⁻², the BNNT field emitters need to have almost the same electric field regardless of the vacuum pressure. This result significantly differs from those for the CNT field emitters, for which the emission current was largely dependent on vacuum pressure [4]. Field enhancement factors of the BNNT field emitter according to vacuum pressure were calculated using Fowler–Nordheim plots of the BNNT field emitters as shown in figure 2(b) and the following equation (1):

$$\begin{eqnarray}&&I=aJ=aA{\phi }^{-1}{(\beta {E}_{{\rm{a}}})}^{2}\exp (-B{\phi }^{3/2}/(\beta {E}_{{\rm{a}}})),\end{eqnarray} \tag{ 1 }$$

where A = 1.54 × 10⁻⁶ A V⁻² eV and B = 6.83 ×10⁹ eV^−3/2 V m⁻¹. J is the emission current density, E_a is the applied electric field, a is the total emission area, ϕ is the work function, and β is the field enhancement factor. The value of the field enhancement factors of the BNNT field emitters according to vacuum pressure were almost the same: 1090, 1040, and 1088 at the vacuum pressures of 1.5 × 10⁻⁷, 1.5 × 10⁻⁶, and 1.5 × 10⁻⁵ Torr, respectively. Figure 2(c) shows that the turn-on electric fields at a current density of 0.1 μA cm⁻² and the threshold electric fields at a current density of 1 mA cm⁻² were not significantly changed, regardless of the vacuum pressure. More exactly, the turn-on electric field was slightly increased by 6.78% (0.2 V⁻¹ μm⁻¹) as the vacuum pressure increased, and the threshold electric field was decreased by 6.45% (0.3 V⁻¹ μm⁻¹). These results show that the emission performance of the BNNT field emitter is almost constant, regardless of the vacuum pressure. This type of field emission behavior of BNNT field emitters according to vacuum pressure is mainly attributed to the robustness of the BNNT emitter, which demonstrates the strong oxidation endurance of the BNNT emitter [11, 15].

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The long-term emission stability of the BNNT field emitter was measured at the different vacuum conditions of 1.5 × 10⁻⁷, 1.5 × 10⁻⁶, and 1.5 × 10⁻⁵ Torr. The emission stability measurement commenced at an emission current density of 10 mA cm⁻² under a constant DC voltage bias for 20 h as shown in figure 3. The degradation rates of the emission current were 49.7%, 73.1%, and 96.3% at corresponding vacuum pressures of 1.5 × 10⁻⁷, 1.5 × 10⁻⁶, and 1.5 × 10⁻⁵ Torr, respectively. This result reveals that the degradation of the emission current is acceptable for real applications at a vacuum pressure of 1.5 × 10⁻⁷ Torr, while it is a critical problem at a high vacuum pressure (∼10⁻⁵ Torr). A similar trend was also presented from the CNT field emitters at high vacuum pressure [4]. Even though CNTs indicated good emission stability at low vacuum pressure, the severe degradation of the emission current was occurred at a high vacuum pressure (∼10⁻⁵ Torr), which is caused by structural destruction of a CNT tip, which is mainly induced by the oxidation effect and the ion bombardment effect [18–22]. On the other hand, the BNNTs have a strong oxidation endurance property compared to CNTs, which is also supported by the field emission characteristics of the BNNT field emitters as shown in figure 2(a). Therefore, it is suggested that the current degradation of BNNT field emitters at high vacuum pressure is mainly affected by ion bombardment, rather than by the oxidation of the BNNTs. According to the obtained results, it is proposed that the ion bombardment did not strongly affect the degradation of the field emission behavior of the BNNT field emitter during the short-term emission measurement, while it assumed an important role for the current degradation of the BNNT field emitter during the long-term emission measurement condition.

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The emission current fluctuation of the BNNT field emitters was investigated to evaluate the degradation phenomenon. Figure 4 shows that the current fluctuation was dramatically increased at a high vacuum pressure condition. The average fluctuation rates of the emission current were 3.1%, 8.3%, and 20.1% at vacuum pressure of 1.5 × 10⁻⁷, 1.5 × 10⁻⁶, and 1.5 × 10⁻⁵ Torr, respectively. This result reveals that the degradation of the BNNT emitters is accelerated at the high vacuum condition. According to the previous works, the ion bombardment is a key factor that can induce a high fluctuation rate of the emission current [23]. Therefore, based on the fluctuation of the emission current, it is suggested that the degradation of BNNT field emitters is mainly attributed to the ion bombardment effect rather than to the oxidation effect.

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In order to investigate detailed information for a dominant factor of the emission current degradation, long-term emission stability and emission current fluctuation of the BNNT field emitter were evaluated at high vacuum pressure under Ar ambient condition. In particular, to avoid the oxidation effect, Ar ambient was used for long-term emission stability measurement. Figure 5(a) shows the long-term emission stability of the BNNT field emitter under Ar ambient condition. The degradation rate of the emission current was 97.5% after 20 h, which is an almost similar trend compared to that under Air ambient condition. This result means that the degradation of the emission current of the BNNT field emitters is caused by the ion bombardment effect rather than by the oxidation effect. Figure 5(b) indicates the current fluctuation of the BNNT field emitter during the long-term emission operation under Ar ambient condition. The average fluctuation rate of the emission current was 24.6% and the maximum current fluctuation rate was 196.9%. The average fluctuation rate of the emission current of the BNNT field emitter under Ar ambient was slightly increased compared to that under air ambient condition at vacuum pressure of 1.5 × 10⁻⁵ Torr, while the maximum fluctuation rate of the emission current under Ar ambient was much higher than that under air ambient. This result can be understood as due to the heavy Ar ions, which are a predominant component in the chamber ambient condition, that can induce more damages at the tip of the BNNTs than under air ambient condition. From the above results, it is confirmed that the ion bombardment effect is the dominant factor for the emission current degradation of the BNNT field emitter at high vacuum pressure for long-term emission operation.

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We investigated the field emission behavior of the BNNT field emitters according to vacuum pressure. During the short-term emission operation, the emission performance of the BNNT field emitter showed a only small difference according to the vacuum pressure. The turn-on electric field was slightly increased by 6.78%, and the threshold electric field was decreased by 6.45%, as the vacuum pressure was increased. The small change of the emission performance for the short-term emission measurement implies that the BNNT field emitter has a strong oxidation endurance. On the other hand, for long-term emission stability, the BNNT field emitter showed both the severe degradation and the large fluctuation of emission current at high vacuum pressure. The obtained results for long-term emission stability reveal that the ion bombardment effect is much stronger on the damage of the BNNT emitter tip, rather than the oxidation effect as the vacuum pressure increased. This result was also supported by the field emission behavior under Ar ambient at high vacuum pressure. We believed that our results will be very helpful for the understanding of long-term emission stability of not only BNNT field emitters, but also other nanomaterial field emitters according to the vacuum pressure.

This work was supported by BK21 Plus Humanware Information Technology, ETRI R&D program (Grant 15ZE1140), the Korea Basic Science Institute (KBSI) and Korea University Future Research Grant Program.