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This paper reports a Ni-matrix CNT flexible field emission electron source fabricated by a novel implanting micromachining technology. For the first time, we could implant nano-scale materials into milli-scale metal substrates at room temperature. By embedding CNT roots into Ni film using polymer matrix as transfer media, effective contact between Ni and CNTs was achieved. As a result, our novel emitter shows relatively good field emission properties such as low turn on field and good stability. Moreover, the emitter was highly flexible with preservation of the field emission properties. The excellent field emission characteristics were attributed to the direct contact and the strong interactions between CNTs and the substrate. To check the practical application of the novel emitter, a simplified X-ray imaging system was set up by modifying a traditional tube. The grey shadow that appears on the sensitive film after exposing to the radiation confirms the X-ray generation.
© 2016 Optical Society of America
1. Introduction
Carbon nanotubes (CNT) exhibit excellent field emission characteristics due to their inherent small tip radius and high aspect ratio combined with robust chemical and mechanical stabilities [1]. Flexible electronic devices have recently attracted great attention for their diverse applications such as bendable sensors [2], flexible displays [3] and X-ray radiotherapy [4]. There are many advantages of flexible emitters that show reliable field emission performance under various bending conditions, allowing emitters to be made in any geometry or shape in field emission applications. Although, high performance field emitters with stable emission current of 1 A (current density of 4 A/cm2) have already been reported [5, 6]. It is believed that, as the heart of field emission electronic devices, the carbon nanotube field emitters have not reached their full potential, and there are still lots of challenges in this field [7].
In recent years, several groups have been engaged in fabricating CNT emitters on polymeric substrates for flexibility purposes by using electrophoretic method [8], direct growth [9], transfer [10, 11] and wet coating [3]. However, there are several problems that arise from the polymer substrate hindering its practical application. For weak mechanical adhesion between CNTs and the polymer substrate, the structure and morphology can be easily damaged under complex curving conditions, resulting in a catastrophic vacuum breakdown or arcing during device operation [12]. Low thermal conductivity and low thermal degeneration temperature of the polymer substrate can lead to joule heating of the interface [13], thereby damaging the emitter interface and resulting in the increasing of turn-on field over extended periods [14].
Metal substrates, with good mechanical properties and high thermal conductivity, seem to be the most promising substrate for low cost, flexible and arbitrary shaped emitters. However, weak adhesion between CNTs and the metal substrates is the most crucial limitation blocking their commercial application for flexible emitters [15].
We have used an implanting Micromachining method to solve these problems. With this technique, polymer is firstly used as transfer media, for CNTs could be homogeneously disentangled into the Polyimide matrix. By selective wet etching method, sputtering and electroplating technology, polyimide worked as sacrificial layer, CNTs were transferred into Ni film. This is a relatively simple method that can be easily realized at room temperature. The fabricated Ni film was highly flexible with preservation of the field emission properties. To check the application of this novel flexible film, an X-ray imaging experiment was performed. A simplified diode X-ray source assembly was achieved based on a traditional X-ray tube. By comparing the images that obtained from the X-ray system at different emission current, we confirm the generation of X-rays using our cathode. It is believed that this novel method must be helpful for CNT field emission electron source development.
2. Experimental
2.1 Apparatus
A mechanical ball milling machine (QM-QX04 of Nanjing University Instrument Plant, China) was used for mixing multi-walled carbon nanotube (MWCNT) and polyimide (PI). The morphologies of the fabricated emitters were characterized using a field emission scanning electron microscope (FESEM; Zeiss ultra 55, Germany). The Raman spectrum of the flexible emitters was obtained using a Raman microscope (Ram, Bruker Opties Senterra R200, US) with 10 × and 100 × objectives at a laser wavelength of 532 nm. Spectrum acquisitions were done with a power of 1mW with integration times of 10-60 s depending on the sample examined.
The field emission characteristics of the samples were measured in a vacuum chamber with a parallel diode-type configuration at pressure of 1 × 10−6 Tor. A mica sheet with a round hole (Ø = 3 mm) was used as spacer. A DC voltage was applied by a high-voltage power supply (HBGY HB-2502-100AC, China) across the cathode and the anode with a distance of 150μm. The current was measured and saved by a digit multi-meter (Agilent 34401A, US). In order to protect high-voltage power supply from high-voltage arcing breakdown, a current-limiting resistor (2 MΩ) was used.
2.2 Reagents
Multi-walled carbon nanotubes (MWCNT) with the purity more than 95% (diameter 30-50nm, length 5-15μm) were bought from Timesnano Co., Ltd (Chengdu, China). Polyimide (PI, absolute viscosity 1100-1200 mPa·s) was bought from POME Sci-tech Co., Ltd (Beijing, China).
2.3 Electron source fabrication
The CNT composite paste composed of multi-walled CNTs and Polyimide (PI). To maintain the intrinsic properties of CNTs, no surfactant was added to the paste. And the homogeneous CNT-PI paste was achieved using just ball-milling apparatus. In this research, PI is used as sacrificial medium. The preparation process is presented as follows (Fig. 1):
- (1) The homogeneous CNT-PI paste was spun on the glass wafer, CNT/PI film was formed by baking at 90°C for 2 h, then the film was polished;
- (2) Selective chemical etching was carried out, a thin layer of PI of micron level was etched away from the PI/CNT film, and a flat surface with protruding tips of CNTs was achieved;
- (3) Ni conducting layer was sputtered on the above flat surface and covered the protruding tips of CNTs;
- (4) Photoresist of 50 um was spun on the Ni layer, and lithography was performed to develop the pattern area for Ni substrate;
- (5) Then Ni film of 50 um (the reasons for choosing Ni as the basement were its resistance to corrosion, and favorable mechanical properties) was fabricated by electroplating on the Ni conducting layer;
- (6) Selective chemical etching was carried out to remove the remaining PI and photoresist. The wafer was immersed in the etchant (composed of sodium hydroxide, ethyl alcohol, and sodium hydrogen phosphate), and sonicated for 30 mins and flushed by deionized water, then the Ni film emitter (the image was rotated 180 degrees) with free standing CNTs on its surface was achieved.
All the above steps were carried out at room temperature. And the inset images were CNT morphology before and after sputtering process.
3. Results and discussion
Figure. 2(a) shows the optical image of the glass wafer after processing with Ni film emitters on it. By choosing Ni as the substrate, the emitter showed good mechanical properties for planar supporting and large amplitude bending, it could even be twisted and rolled up (Fig. 2(a) inset). As the CNT roots were firmly embedded in the substrate, strong adhesion could be achieved, avoiding the CNT detachment during the fierce field emission process. The film surface morphology was inspected by SEM. From the images (Fig. 2(b), 2(c)), we can see that the Ni substrate is totally covered by the dense CNTs. The CNT roots are firmly embedded in the substrate and no contamination is induced. In addition, the distribution of CNTs was well controlled by the micromachining process. The vertical alignment and uniform distribution of the CNTs in the Ni substrate were actively contributed to stable electron emission when an electric field was applied.
Fig. 2 (a) Optical image of the glass wafer after processing, the inset shows the optical image of the Ni film, twisted and rolled up; SEM images of the film surface, (b) from top, and (c) from oblique directions.
As shown in Fig. 3, the two sharp peaks at approximate 1350 cm−1 (D bond) and 1580 cm−1 (G bond) representing typical characteristics of amorphous and graphite carbons, respectively. The appearance of those two peaks in the emitter surface indicated that the CNTs were successfully transferred onto the Ni substrate by the implanting process. The D bond at approximate 1350 cm−1 is generally attributed to defects in the curved graphite sheet or other impurities, while the G bond at approximate 1580 cm−1 is corresponding to the opposite direction movement of two neighboring carbon atoms in a graphitic sheet, and it indicates the presence of crystalline graphitic carbon in CNTs. It shows that the IG/ID ratio of pristine CNT is 1.43, and it decreases to 1.18 after the preparation process, indicating decreased crystallinity and improved defects in the CNTs. It is speculated that the main reason for this phenomenon is that the ball milling process was used to disperse CNTs into the transfer media (PI). Due to their small tip radius and high aspect ratio, CNTs have highly entangled structure, which needs to be dispersed, preferably up to single nanotube level, for practical applications. During the ball milling process, large CNT aggregate (composed of many CNTs) died down, and CNTs turned into conglomeration which was closed to granules and sheets because of the friction of rolling between the balls. As a result, the ball milling process not only decreased the CNT aggregate size, but also changed some CNTs into amorphous carbon.
Fig. 3 (a) Raman Spectrum for Ni film, CNT, and Ni-CNT emitter surface; (b) D bond and G bond decomposition for CNT, and Ni-CNT emitter surface.
Field emission of CNTs arrays on the Ni film emitter was carried out in a vacuum chamber. An aging process was carried out with an applied voltage of 550 V for 12 hours before the test. During the aging process, arcing occurred occasionally. Since CNTs of greater heights contribute to higher field emission current, thermal runaway is more serious at longer CNTs. As a result, longer CNTs became short and vertically standing CNTs with more uniform heights remained on the Ni substrate after the aging process. The emission current vs. applied electric voltage were repeatedly measured (in Fig. 4), the I-V curves remained almost constant at the repeated field emission tests. The emission current increases monotonically with the applied field. The turn-on field, that defined as an electric field required to get an emission current of 10 uA, was 1.64 V/um. We simply consider the area of the hole on the mica spacer as the field emission area, as the mica sheet was closely attached to the field emitter. The area S = π*(Ø /2)2 = 0.071 cm2. With the applied field of 3.13 V/um, the field emission current of 0.57 mA and the current density of 8.03 mA/cm2 was achieved. In Fig. 4, the emission current curve seems to show a “linear” relation with the applied field from 400 to 450 V. The phenomenon was caused by the current-limiting resistor who shared the voltage of the field emitter in the circuit. The corresponding Fowler-Nordheim (F-N) plot for the flexible emitter is shown in the inset of Fig. 4. All dots on the curve fit a single straight line well, which implies that the field emission process follows the F-N mechanism.
Fig. 4 Emission current vs. applied voltage curves of the Ni film. The inset represents the FN plots derived from the curves of current vs. electric fields.
Before continuous emission measurement, an aging process was carried out for 12 h with driving condition of a higher applied voltage at 550 V. And the short-term stability of CNT field emitter was evaluated by monitoring emission current under constant DC operation for 40 h.
As shown in Fig. 5, with applied field of 2.05, 2.30, 2.66, 1.43 V/um, emission current of 58.9, 104.4, 204.2, 310.5 uA could be achieved, and they remained almost constant during the 40 h continuous measurement, and the fluctuation width of the emission current for 40 hours were all in ± 5%. One thing to note here is that a few arcing events occurred when the emission current reached higher than 300 uA, however, the emitter could withstand the arcing and the emission current remained constant with time. The stable emission performance was primarily due to the firm, direct bonding between CNTs and Ni substrate, which greatly reduces the contact resistance between CNTs and substrate and the damnification of CNTs escaping from the emitter surface.
The flexible field emission properties of the Ni film emitter were measured using a sandwich structure, two pieces of PET-ITO films acting as the cathode and the anode, a piece of PI film with a thickness of 150 um as a spacer, as described in the inset of Fig. 6(a). The PET-ITO film was used for the cathode substrate and the emitter was adhered onto the PET-ITO film by using conductive adhesive tape.
Fig. 6 (a) Emission current-applied voltage characteristics and (b) the corresponding F-N plots for the Ni film emitter as a function of the bending angle. The inset in (a) shows a schematic diagram of the flexible field emission setup. The inset in (b) shows a photo of the test setup.
The film emitter was highly flexible without incurring a reduction in field emission properties under severe bending conditions. Figure 6(a) and 6(b) show the relationship of emission current with the applied voltage and the corresponding F-N characteristics of the Ni film emitter with respect to the bending angle, respectively. In the flat sample configuration, an emission current density of 8.03 mA/cm2 at an electric field of 3.13 V/um was measured. With the same electric field, the emission current of 7.93, 8.03, 8.10, 7.96 mA/cm2 were achieved at the bending angle of 15°, 30°, 45°and 60°, respectively. And also the release of the sample resulted in the return of the emission current density to its original value. The slopes of the linear F-N regions were also quite similar, regardless of the bending angle. This stable flexibility in the field emission of the sample may have originated from the direct contact as well as the strong interactions between CNTs and the Ni substrate. Consequently, the fabricated Ni film emitter exhibits very stable field emission properties, which are useful for the realization of miniature X-ray tubes that require high-voltage operation.
To check the practical application of the novel emitter, a simplified X-ray imaging system was set up. The electrode structure for X-ray generation was modified based on a traditional hot X-ray tube. The Ni film emitter was used as cathode with bending form as shown in Fig. 7(b). The compact X-ray measurement system was set up, with a simplified diode type (i.e. consisting of a cathode and an anode) configuration of an electron source and a tungsten embedded copper anode. The distance between the cathode and the tungsten anode was maintained at 1.5 mm. The proof of X-ray creation was done by using an X-ray sensitive film. This film was commercially available (Kodak Insight, 31 × 41 mm2) and widely used in dental diagnostics as standard X-ray analogue film plates. The detection was done by placing the film in front of the anode inside the vacuum chamber.
Fig. 7 (a) schematic diagram of the X-ray source; (b) photograph of the X-ray source assembly; (c) photographs of developed film plates exposed at different emission current: 0, 47, 102, 213, 307, and 423 uA.
The X-ray source assembly was successfully operated at 2.5, 2.8, 3.7, 4.1, and 4.2 KV, with an extraction current from the cathode of 47, 102, 213, 307, and 423uA, respectively. As the tungsten anode is surrounded by a copper “hat”, the electric field between cathode and anode is complex. And with the help of the “anode hat”, the electric field around the emitter is significantly improved. As a result, the system can be operated under a relatively low voltage. A round lead aperture (copper film with a pin hole (Φ = 3 mm)) was put in front of the photo sensitive plate in order to create a defined pattern when exposed to radiation, the shadow of the aperture thus unequivocally confirms the X-ray emission. The photosensitive plates placed in front of the source, were exposed to the radiation under emission current for 10 mins to compensate for the X-ray energy loss. The above exposed plates were then developed in accordance with the photographic processing. The photograph of the developed copy can be seen in Fig. 7(c). The gray round shadow on this copy confirms the generation of X-rays. Furthermore, the gray round shape becomes clearer with the increase of the emission current.
Then a Ni badge with a thickness of 100 um and diameter of 3 mm was placed between the pin hole and the sensitive plate. The sensitive plate and the badge were exposed to the radiation under applied voltage of 2.7 kV, emission current of 81 uA for 30 mins to compensate for the X-ray energy loss. Figure 8 shows the X-ray transmission image of the Ni badge, fuzzy image was obtained at relatively low applied voltage.
Fig. 8 Photographs of (a) the developed sensitive plate, the inset shows the magnitude of the Ni badge; (b) magnitude of the blackened shade.
These results above demonstrate that X-rays are generated in our set-up. The Ni film CNTs field emitter with high and stable emission current proved to be available for X-ray generation without any focusing and accelerating installation.
4. Conclusion
In this study, we have introduced a novel method to fabricate Ni based CNT flexible cathode using Micromachining at room temperature. Polymer was used as a transfer media, by using controlled selective wet etching method, sputtering and electroplating technology, CNTs were firmly embedded into the Ni substrate. The emitter shows good mechanical properties for planar supporting and large amplitude bending. Effective direct contact between CNTs and Ni substrate was achieved, which would be crucial for low contact resistance between them. Meanwhile, as the CNT roots were firmly buried into the Ni substrate, there would hardly be any detachment of CNTs from the substrate induced by weak adhesion. As a result, our novel emitter showed relatively good field emission properties such as low turn on field (1.64 V/um), high current density (8.03 mA/cm2 at an applied electric field of 3.13 V/um), and good stability (40 h for 5% fluctuation of emission current around 300 uA). The novel emitter also showed great potential used as an X-ray tube electron source. The round grey pattern that appears on the sensitive film after exposing to the radiation confirms the X-ray generation in a simplified diode system without any focusing and accelerating installation. And the shade of the round pattern darks with the increase of the applied field. From those results, it is believed that, this new method based on Micromachining can be helpful for wide industry application of CNT based cathode in X-ray tube, yet further optimization in device configuration and cathode structure is required.
Acknowledgments
The authors express their sincere gratitude to the colleagues of National Key Laboratory of Nano/Micro Fabrication Technology, thanks for their support and encouragement. The authors would like to appreciate the support from the National Natural Science Foundation of China (No. 51305265, No. 51205390) and the Research Fund for the Doctoral Program of Higher Education of China (No.20120073110061).
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