Electronic Pulses from Pulsed Field Emission of CNT Cathodes

1Department of Microelectronics, School of Electronics and Information Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China 2Science School, Huaihai Institute of Technology, Lianyungang, Jiangsu 222005, China 3Research Institute of Xi’an Jiaotong University, No. 328 Wenming Road, Xiaoshan District, Hangzhou City, Zhejiang Province 311215, China 4Guangdong Shunde Xi’an Jiaotong University Academy, No. 3 Deshengdong Road, Daliang, Shunde District, Foshan City, Guangdong Province 528300, China


Introduction
In the applications of ultrafast electron microscopy, free electron laser, satellite communication, picosecond cathodoluminescence, particle accelerators and THz vacuum microelectronics, and electron sources are limited to ultrafast electron pulses with the stringent requirements of high-power and high-frequency [1][2][3][4][5][6][7].However, with the advent of electron beam from Nanometer-scale metallic tip of cathode emitters induced by femtosecond laser, conventional continuous electron sources are gradually replaced by ultrafast pulsed electron sources [8][9][10][11][12][13].Sharp metallic tip of cathode emitters irradiated by laser pulses has represented a feasibility and implementation of the method for realizing ultrafast pulsed electron sources in recent years [14].With the generation of pulsed electron source, much attention has been paid to explore cathode emitters of pulsed field emission in the related subjects, such as electron rescattering at sharp metallic tips, emission spectrum of laser-matter interaction, coherent field emission images, laser acceleration of relativistic electrons, and emission mechanism [15][16][17][18][19].Although conventional metallic nanotip of cathode emitters takes advantage of an electric field enhancement both in the static and optical electric field, it is certainly worth considering that the complicated process of manufacture and thermal ablation of metallic nanotips served as cathode emitters of pulsed field emission.To extend the studies on conventional metallic nanotip of cathode emitters, we studied the pulsed electron cathode emitters in a popular material of carbon nanotubes (CNTs).
The sharp apex of individual CNTs is an intrinsic property as a nanomaterial, while conventional metallic tips, for example, tungsten, gold, molybdenum, or copper, were produced by electrochemical etching [16][17][18][19].Contrasting to the tip apex of individual CNTs, the diameter of metallic tips is much larger and has to employ a complicated manufacturing process.The nanometer-sized tip apex of CNTs and their high aspect ratio result in a strong field enhancement at the sharp tip of cathode emitter.Besides, another key advantage of CNTs is that of providing a new method to control surface morphology of the cathode emitters by doping with other elements on the growth process [20].And CNTs are a desired flexibility material with the convenient and simply preparation process, which has started to receive significant interest in application of flexible display, flexible sensor, radio frequency identification, bullet-proof vest, spacesuit, wearable devices, and other smart electronic devices in comparison with metallic material [21][22][23][24].Capitalizing on the atomic/macroscopic duality of structure of the nanotip of cathode emitters, CNTs can be modeled from an atomic point of view, unlike conventional metallic tip of cathode emitters which can only be modeled in bulk [19], giving great help to theoretical research.In this paper, we have designed a pulsed field emission system in which CNT cathodes are irradiated by pulsed laser and investigated the property of emission electronic pulses so as to gain insight into the pulsed field emission mechanism for preparing ultrafast pulsed electron source.

Experimental
CNTs were synthesized on SiO 2 substrate by chemical vapor deposition (CVD) using iron phthalocyanine.The detailed information about the growth of CNTs was reported in our previous research [25].Field emission tests were performed in a vacuum chamber with pressure about 5 × 10 −5 Pa.A DC bias voltage was applied between the anode and cathode to establish a static electric field by Stanford Research Systems MODEL PS325.The emission current was monitored by a Keithley 2000 multimeter or an Oscilloscope TDS 3014B.The cathodes of CNT arrays were irradiated by Ytterbium Fiber Laser to realize the pulsed field emission when the bias voltage was applied.In the experiment, we presented the pulsed field emission system based on photofield emission mechanism.Irradiating CNT cathodes with infrared laser pulses led to pulsed field emission [10,18,26].Figure 1 shows the experimental set-up of pulsed field emission system based on CNT cathodes.The output from an Ytterbium Fiber Laser was focused on the anode plate with 2 mm spot size and faced the cathode plate in an ultrahigh vacuum chamber of 5 × 10 −5 Pa.The laser operated at 1 watt with a center wavelength of 1064 nm and produced a train of pulse from 50 ms to 10000 ms which could be tuned by a control panel.The CNT cathodes were mounted on the cathode plate in the ultrahigh vacuum chamber and located 350 m away from the anode plate.The intensity and width of laser pulse which propagated through anode plate were carefully controlled by the collimation system and control panel.And then the collimated laser was used to irradiate CNT cathodes of area 2 mm × 2 mm to excite electrons by absorbing photons for emitting electronic pulses, when the applied voltage was on.

Results and Discussions
In field emission, electron emission happens when the tip bias voltage of cathode emitters is sufficiently large for electrons in the vicinity of Femi-level to tunnel through the barrier.
By applying a bias voltage to the tip of cathode emitters, a tunneling barrier is formed and tunneling electron emission occurs.In the photofield emission, electrons are excited to an intermediate state by one or several photons absorption and subsequent tunneling through the surface barrier into vacuum by field emission, when a laser irradiates the cathodes [14,19,26,27].Figure 2 shows electronic pulses curve from the pulsed field emission of CNT cathodes, when the width of laser pulses (defined as the pulse-on 1) and off-time interval (defined as the pulse-off 2) are both 10000 ms.Interestingly, the width of electronic pulses is also 10000 ms or so, almost synchronous with laser pulses.The average amplitude of electronic pulses is 0.027 mA/cm 2 and the fluctuation is less than 20% on applied voltage of 50 V.More important, the emission current density is nearly zero on laser pulse-off with applied voltage of 50 V, proving that electron emissions are excited only on the condition of laser pulse-on (1).The preliminary results show the realization of electron emission pulses from CNT cathodes and turn-on electric field less than 0.14 V/m (turn-on electric field is defined as the electric field required to produce a current density of 0.01 mA/cm 2 ), which is incomparable with reference to previous studies, such as CNTs on nickel foam (0.53 V/m) [28], graphene-CNT (0.73 V/m) [29], CNTs on silicon pillars (2.16 V/m) [30], and multilayer graphene-CNTs (0.93 V/m) [31].Actually, the turn-on voltage of the CNT cathodes was about 80 V (0.23 V/m) by field emission.On applied voltage of 50 V (0.14 V/m), electron emission would not happen if without laser.This result definitely proved the photofield emission mechanism of which electrons were excited to an intermediate state by laser and subsequently tunneled through barrier into vacuum by field emission.Figure 3 shows electronic pulses curve from pulsed field emission of CNT cathodes irradiated by laser pulses, on the same working condition of laser pulses except different applied voltage of 100 V, 400 V, 700 V, and 1000 V, respectively, contrasting to the insert of emission current density of cathodes without laser illumination by field emission without laser illumination.With applied voltage increase, the amplitude ( on −  off ) of electronic pulses increases on lower applied voltage of 100 V and 400 V, as shown in Figures 3(a) and 3(b).However, the amplitude decreases with the applied voltage continuously increasing to 700 V and 1000 V, as shown in Figures 3(c) and 3(d).The corresponding values of amplitude are listed in Table 1. on is the average amplitude of emission electronic pulses on electron pulse-on, and  off is the average valley value on electron pulse-off.The electronic pulses signals appear obviously on low applied voltages, while the pulses cannot be distinguished on the high applied voltage of 1000 V ( on −  off ≈ 0).Interestingly, the amplitude of  on −  off increases firstly and then decreases until electronic pulses signals are gradually buried in the DC signal of emission current with applied voltage continuous increase, as showed in Table 1.Practically, the emission current of fabricated CNT cathodes was the result of the combined effect of infrared laser and applied voltage but not the simple sum of emission current caused by photofield emission and field emission.On low applied voltage, photofield emission was dominated in emission process, and applied voltage just narrowed the potential barrier in the vicinity of threshold.On high voltages, emission mechanism was dominated by conventional field emission rather than pulsed field emission.Nevertheless, the laser field instantaneously wiggled the barrier.Emission electrons caused by laser pulses just led to a fluctuation for emission current of the field emission.With the applied voltage continuing to increase, emission electrons caused by laser pulses could be ignored due to being too weak comparing with emission current caused by the field emission.The obtained results helped to understand and realize the emission mechanism of pulsed field emission from CNT cathode.
To increase the frequency of electronic pulses, shortening the period of electronic pulses is one of popular methods for pulsed field emission as an ultrafast pulsed electron source, especially in decreasing width of electronic pulses.Figure 4 shows electronic pulses curve from CNT cathodes irradiated by the laser pulse of 1 = 50 ms, while 2 are 1000 ms and 100 ms, respectively.The results showed the width of electronic pulses was about 50 ms, and electronic pulse-off was 1000 ms as shown in Figure 4(a) and 100 ms in Figure 4(b), respectively.However, pulse broadening was obviously observed as shown in Figure 4(b).For the further study, the response time was investigated, and the results showed the pulse broadening arose from the thermal effect.Thermal effect which came from laser heating up tips of CNT cathodes existed in the pulsed field emission and could enhance the emission current.By virtue of the very fast sampling rate of 2.5 GS/s, oscilloscope was used to detect response time of electronic pulses.Figure 5 shows a single electronic pulse with the laser pulse-on of 1 = 5000 ms and applied voltage of 400 V.It is found that emission current increases sharply when the laser pulse of 1 = 5000 ms is on and remarkably drops down when the laser pulse is off.Moreover, the evolution process of emission current is not instantaneous on laser pulse-on and pulse-off.The rise time of emission current jumping to plateau takes 2 s or so when laser pulse is on, and the fall time spends more than 2 s when laser pulse is off.Besides, the amplitude is much higher than that of value in Figure 4 on the same applied voltage 400 V. Working conditions of the electronic pulse in Figure 4 (1 = 50 ms; 2 = 1000 ms, 100 ms) and Figure 5 (1 = 5000 ms; 2 = 10000 ms) are mainly associated with 1 and 2 of laser pulses on the same applied voltage, but the amplitudes are quite different.

Journal of Nanomaterials
Irradiating of the CNT cathodes by laser pulses heated the tips and caused the apex electron temperature to evolve periodically in time [9], so thermally enhanced field emission increased drastically obeying laser excitation and caused the pulse broadening.A major reason for pulse broadening was thermal effect which played an important role in the field emission mechanism, especially in short electronic pulses occurring.In Figure 5, the electronic pulse had enough time to reach to the saturation value on laser pulse-on of 1 = 5000 ms.However, electronic pulses were compelled to drop down before reaching to the saturation value on the short laser pulse-on of 1 = 50 ms in Figure 4(b) and compelled to rise up before dropping to the valley value on condition of 2 = 100 ms.After repeatedly verifying, the optimum working condition of laser pulse irradiated CNT cathodes was that the laser pulse-off 2 was greater than or equal to 2 times the laser pulse-on 1 for realizing ultrafast electronic pulses.These results showed that thermal effect enhanced the pulsed field emission of CNT cathodes irradiating by pulsed infrared laser and resulted in a pulse broadening especially on short electronic pulses.

Conclusion
Irradiating CNT cathodes with laser pulses led to pulsed field emission.We investigated the emission electronic pulses from pulsed field emission of carbon nanotube (CNT) cathodes by tuning the pulsed laser field and attained the short electronic pulses of 50 ms and turn-on field below 0.14 V/m.With the applied voltage increase, electronic pulses amplitude increased firstly and then decreased till it was gradually buried in the emission current.With the width of electronic pulses decrease, pulse broadening existed in the pulsed field emission caused by thermal effect which played an important role in the field emission mechanism, especially in short electronic pulses occurring.To reduce the electronic pulse distortion, the optimum working condition of laser pulse irradiated CNT cathodes was 2 ≥ 1 for realizing short electronic pulses.These results extended conventional electron sources and provided a possibility of realizing ultrashort electronic pulses as a high-frequency electron source.

Figure 1 :
Figure 1: Schematic diagram of pulsed field emission system experimental set-up based on carbon nanotube (CNT) cathode emitters.

2 ) 2 ) 2 ) 2 )Figure 3 :Figure 4 : 2 )Figure 5 :
Figure3: (Black line) Plots of field emission electronic pulses profile on CNT cathodes irradiated by pulsed laser on different applied voltage of 100 V, 400 V, 700 V, and 1000 V, respectively.The width of emission electronic pulse is 10000 ms.The laser working condition is that the pulse width of 1 and laser pulse-off of 2 are both 10000 ms.The insert (blue squares) is corresponding to the emission current density without laser illumination by field emission.
)Figure2: Plot of field emission electronic pulses on CNT cathodes irradiated by pulsed laser, on applied voltage of 50 V.Black line represents the emission electronic pulse profile with width of 10000 ms (1 = 2).Red line represents a train of laser pulses with the width of 10000 ms.