Nanotube Field Emission Research Articles

SU-D-144-04: Physiologically Gated Microbeam Radiation Therapy Using Electronically Controlled Field Emission X-Ray Source Array

Purpose: Microbeam radiation therapy (MRT) is an experimental pre‐clinical technique using arrays of microscopically‐thin, low‐energy X‐ray radiation to treat radio‐resistant, deep‐seated tumors. All previous MRT experiments were performed using synchrotron radiation. We have developed a compact MRT small animal system using carbon nanotube (CNT) field emission x‐ray technology. Our purpose is to incorporate respiratory gating to the MRT system, minimizing motion blurring and increasing the peak‐to‐valley‐dose ratio (PVDR), which is important for normal tissue sparing. Methods: A prototype CNT MRT system was employed. The intrinsically divergent radiation is collimated into ∼300um microbeams using an external collimator. Parallel microbeam planes were delivered by translating the object perpendicular to the microbeam plane in a step‐and‐shoot fashion. The device generated an average entrance dose rate of 1Gy/minute. For respiratory‐gated irradiation, electron field emission from CNT cathodes was synchronized with the respiratory cycle of the mouse so x‐ray radiation is only extracted during the motionless phase of respiration.A phantom simulating mouse respiration used a servo motor pushing on a pressure sensor in a typical breathing pattern. A control program used the signal to trigger x‐ray irradiation during the motionless phase. Two experiments were run: a single line irradiation at 160 kVp delivering 0.9 Gy/min for 5 minutes, and three such lines of irradiation separated by 900um. Dose profiles were measured with EBT2 Gafchromic dosimetry films. Results: A single beam showed a FWTM increase from 638 to 1088um with un‐gated motion. With gating, the FWTM was 669um, similar to the motionless case.PVDR quantifies MRT effectiveness in the case of multiple lines. Motion reduces PVDR 50 percent; with gating PVDR is almost entirely recovered, 5.5 percent higher than without motion. Conclusion: Gated MRT largely eliminates microbeam broadening from physiological motion, maintaining high PVDR. The technique increases precision and will be tested during MRT treatments.

Effective hybrid graphene/carbon nanotubes field emitters by electrophoretic deposition

Hybrid graphene and carbon nanotube (CNT) field emitters were fabricated with electrophoretic deposition (EPD). The combination of both materials was used to improve the turn-on field for pure carbon nanotubes emitters and the reliability of pure graphene emitters deposited by the same method. The CNT was envisioned to hold down the graphene flakes, like a safety belt or Velcro, at high voltages to prevent an early short circuit at relatively low voltages. These hybrid emitters were studied for their field emission performance in relation to the EPD deposition duration. It was observed that the emitters performed better when the EPD duration was increased due to the increase in the amount and density of graphene flakes. Possible reasons for the improvement of field emission performance were suggested. The roles of graphene and CNT in these hybrid emitters were also discussed.

Enhanced Field Emission from Carbon Nanotubes Coated by Nanoparticles of Turbostratic Stacked Graphenes

The field emission (FE) of the carbon nanotubes (CNTs) modified by hydrocarbon ion treatment with an energy of 80 eV has been demonstrated. Compared with untreated CNTs, the turn-on field and the threshold field of the modified CNTs decreased significantly. Scanning electron microscopy and transmission electron microscopy indicate that, after hydrocarbon ion treatment, the CNTs are coated by amorphous carbon layer at 300°C of substrate temperature and nanoparticles of graphene stacks at 700°C. It is considered that both amorphous carbon and stacked graphene coating layer can decrease the effective surface work function of CNTs and thus increase FE.

Field-emission energy distribution of BN-coated carbon nanotubes

The field-emission energy distribution of BN-coated multiwalled carbon nanotubes was investigated by measuring the energy distribution of emitted electrons. The field-emission energy distribution curve suddenly changes from a single peak to two peaks after the buildup process of carbon nanotube field emitters. The movement of the peaks is different from that in previous reports on coated carbon nanotubes. A possible explanation for this peak movement is proposed here — a thermal effect due to a high emission current during the measurements.

Design of a multi-walled carbon nanotube field emitter with micro vacuum gauge

The variation of vacuum level inside a field emission device when electron is emitted from multi-walled carbon nanotubes (MWCNTs) by electric field was measured where MWCNT gauge packaged with a vacuum device was used to measure the degree of a vacuum until the end of the vacuum device life. It was found that the electrical properties of MWCNTs altered with the degree of a vacuum. We fabricated MWCNT gauge which were printed and pasted by the screen printer. In this paper, we report the successful detection of the ionization of gases in vacuum state.

A digital miniature x-ray tube with a high-density triode carbon nanotube field emitter

We have fabricated a digital miniature x-ray tube (6 mm in diameter and 32 mm in length) with a high-density triode carbon nanotube (CNT) field emitter for special x-ray applications. The triode CNT emitter was densely formed within a diameter of below 4 mm with the focusing-functional gate. The brazing process enables us to obtain and maintain a desired vacuum level for the reliable electron emission from the CNT emitters after the vacuum packaging. The miniature x-ray tube exhibited a stable and reliable operation over 250 h in a pulse mode at an anode voltage of above 25 kV.

Characterization of carbon nanotube field emitters in pulsed operation mode

Carbon nanotubes (CNTs) are promising candidates as electron sources for novel x-ray tubes. Short pulses, high emission currents, and long-term stability are prerequisites for practical applications in medical x-ray imaging. Here, the authors present field emission from CNTs in pulsed operation mode exhibiting very high stability for 200 cumulative hours and a maximum current of 126 mA corresponding to 202 mA/cm2 at an applied field of 9.3 V/μm. They investigated the correlation of classical emitter characteristics such as threshold field and field enhancement factor to the long-term stability and maximum emission current. This correlation was found to be rather poor. Instead, they observed a steady voltage increase for a fixed current during lifetime experiments. This observation allowed to derive a degradation parameter which determines the emitter quality for pulsed applications. Detailed investigations of the degradation in dependency of pressure, duty cycle, and pulse-on time were performed to predict the stability and lifetime of CNT-based field emitters.

Carbon Nanotube Electron Emitter for X-ray Imaging

The carbon nanotube field emitter array was grown on silicon substrate through a resist-assisted patterning (RAP) process. The shape of the carbon nanotube array is elliptical with 2.0 × 0.5 mm2 for an isotropic focal spot size at anode target. The field emission properties with triode electrodes show a gate turn-on field of 3 V/µm at an anode emission current of 0.1 mA. The author demonstrated the X-ray source with triode electrode structure utilizing the carbon nanotube emitter, and the transmitted X-ray image was of high resolution.

Enhancement of electron emission and long-term stability of tip-type carbon nanotube field emitters via lithium coating

Carbon nanotubes (CNTs) were deposited on conical tip-type substrates via electrophoresis and coated with lithium (Li) thin films with diverse thicknesses via electroplating. For the as-deposited (i.e., without Li coating) CNT, the turn-on (or triggering) electric field was 0.92V/μm, and the emission current, which was generated at an applied field of 1.2V/μm was 56μA. In the case of the 4.7nm-thick Li-coated CNT, the turn-on field decreased to 0.65V/μm and the emission current at the same applied field increased more than ten times to 618μA. The analysis based on the Kelvin probe measurement and Fowler–Nordheim theory indicated that the coating of Li caused a loss in the structural-aspect-ratio of the CNTs and it reduced their effective work functions from 5.36eV to 4.90eV, which led to a great improvement of their electron emission characteristics. The results obtained in this study also showed that the long-term emission stability could be enhanced by the coating of thin Li films on CNTs.

Modeling Operational Modes of a Bipolar Vacuum Microelectronic Device

Vacuum microelectronic devices (VMDs) designed for bipolar charge operation hold great promise for applications in radiation-intensive and high-temperature environments. This novel class of devices was first realized in a microelectromechanical platform leveraging integrated carbon nanotube field emitters and an addressable pentode structure for controlling electron-impact dynamics in Ar ambients. That proof of concept demands the development of basic numerical models to aid device optimization. We address this need in the form of a two-fluid model of carrier transport dynamics in a bipolar VMD (BVMD). The fluid model captures the behavior of operational modes demonstrated in previously reported devices. Moreover, this approach promises insight into potentially unforeseen pressure and frequency dependences of the BVMD platform.

Numerical analysis in field emission characteristics of carbon nanotube field emitters and arrays

In this study, the field emission characteristics of carbon nanotube field emitters and arrays fabricated by carbon nanotube implantation process are analyzed by the combination of the finite-difference time-domain and particle-in-cell methods. Carbon nanotubes with different shapes are simulated and their electron trajectory properties are compared. A spacing optimized carbon nanotube field emission array is developed and the impact of random distribution in carbon nanotube length and tilt angle is also investigated. These results are useful to develop an optimized carbon nanotube field emission array that fits current fabrication capabilities.

Study of J-E Curve with Hysteresis of Carbon Nanotubes Field Emitters

We observe hysteresis in J-E plot during field emission measurement of CNTs grown by LPCVD technique. CNTs are synthesized on Fe-coated Si substrate at 650°C. SEM and Raman study confirm that CNTs are successfully grown on Si substrate by LPCVD technique. In this study, we find that ramp-down curve has higher value of current density than ramp-up curve which indicates that CNTs show positive hysteresis. Our results show that a high current density at low turn-on voltage is obtained in ramp-down step of J-E plot which may be since not all CNTs contribute in ramp-up step process. But in ramp-down step all CNTs contribute as field emitters due to high electric field treatment. We also performed stability analysis of CNTs with current at constant applied voltage for 5 hrs and find that the sample shows long-term stability due to increase in emitting site density since a large number of CNTs participate in field emission.

Ultra-high current density carbon nanotube field emitter structure on three-dimensional micro-channeled copper

High-current field emitter with extended stability has been focus of many researchers, which can be fulfilled by application of engineered nanomaterials and beneficial design. In the present research, such a field emitter is designed and fabricated, involving growth of interface controlled carbon nanotubes along micro-channels of 3-dimensional copper template. The emitter offers very high emission current density (270 mA/cm2, peak), low turn-on field (1.1 V/μm), moderate threshold field (1.69 V/μm), emission stability for extended operation, and excellent resistance to structural damage. Higher conductivity of Cu, lower interfacial resistance, strong interfacial bonding, and unique 3-dimensional design leads to such 23-27 times higher current density, providing a potential approach to field emission devices.

Fabrication of carbon nanotubes from conducting polymer precursor as field emitter

Carbon nanotubes with an average diameter of 50nm were produced from polypyrrole nanotubes fabricated with a surfactant mediated microemulsion polymerization. A simple production of polypyrrole nanotube precursor was possible through a proper selection of surfactant and initiator in microemulsion system. A subsequent thermal treatment of conducting polymer precursor generated novel carbon nanotubes. Electron microscopy images confirmed the successful formations of polypyrrole and carbon nanotubes. A series of carbon nanotube pastes were formulated with combination of binders and fillers. The field emission characteristic of these carbon nanotubes was monitored by I–V curves. Constant current tests revealed the lifetime of carbon nanotube field emitters derived from polypyrrole nanotubes.

A vacuum-sealed miniature X-ray tube based on carbon nanotube field emitters

A vacuum-sealed miniature X-ray tube based on a carbon nanotube field-emission electron source has been demonstrated. The diameter of the X-ray tube is 10 mm; the total length of the tube is 50 mm, and no external vacuum pump is required for the operation. The maximum tube voltage reaches up to 70 kV, and the X-ray tube generates intense X-rays with the air kerma strength of 108 Gy·cm2 min−1. In addition, X-rays produced from the miniature X-ray tube have a comparatively uniform spatial dose distribution.

The Fabrication Study of Branched Cathode Electrode for Field Emitter in a FED Panel

Using carbon nanotubes as field emitters, the field emission display panel with branched cathode electrode was fabricated. For the cathode faceplate, the branched cathode electrode was composed of the emitter conducting layer and the emitter supporting layer. On the emitter supporting layer surface the carbon nanotubes field emitters were prepared, and the emitter conducting layer would be covered with the emitter insulation layer. The applied cathode potential would be conducted by the emitter conducting layer, which could decrease the additional voltage drop. The good field emission characteristic for the fabricated field emission display panel was confirmed.

Activation-free printed carbon nanotube field emitters

When a carbon nanotube paste is formulated based on highly functional hyperbranched polymerssuch as dipentaerythritol hexaacrylate, the volume shrinkage during thermal curing buildsup internal stress that generates microcrack patterns on the printed surface. The nanotubesexposed in the cracks emit electrons successfully at such an extremely low electric field as0.5 V µm − 1, and reach25.5 mA cm − 2 of currentdensity at 2 Vµm − 1 from an optimized paste concerning mainly the size and spatial uniformity of the crack. Inaddition to the superior field emission properties with low manufacturing cost, thisactivation-free technology can provide a minimized nanohazard in the device fabricationprocess, compared to those conventional activation technologies developing seriousnanoflakes by using destructive methods.

A Bipolar Vacuum Microelectronic Device

This report provides the first demonstration of a vacuum microelectronic device that utilizes both positive and negative charge states (i.e., a bipolar vacuum microelectronic device), thus enabling the possibility of device designs that are not previously possible in traditional vacuum microelectronics. In the same way that complimentary metal-oxide-semiconductor (complimentary n-channel MOS and p-channel MOS) were required in solid-state electronics before digital logic applications could be addressed, vacuum microelectronic devices benefit from a second charge state to realize many applications. This advance could enable integrated circuits for radiation-intensive environments (nuclear power facilities and space-based communications such as satellites) and high-temperature applications (engines and materials processing). A microelectromechanical systems platform was used to construct pentode structures with integrated carbon nanotube field emitters for electron emission and bias electrodes for separate electron- and ion-current modulation. Ions were generated via electron impact in an argon ambient, and devices were tested in both voltage sweep and pulsed modes. Current that is greater than 2 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> was modulated at the anode between ion and electron collection, demonstrating that this novel platform has the potential to foster a new class of bipolar vacuum microelectronics.

Improvement and Fabrication of Anode Pixel for FED Panel with Cold Cathode Emitters

With high effective screen-printing process, the diode field emission display (FED) panel was fabricated. The soda-lime flat glass was used as panel anode plate, and the indium tin oxide film covered on the anode plate surface was etched, which was used to form the anode electrodes. With simple screen printing method, the phosphor paste was prepared on the exposed anode electrode surface, so the phosphor dots were fabricated. Using insulation slurry as material, the insulation layer was also fabricated on the anode plate. The fabricated FED panel with carbon nanotube field emitters possessed good field emission characteristics and image displaying performance.

A Carbon Nanotube-based NEMS Parametric Amplifier for Enhanced Radio Wave Detection and Electronic Signal Amplification

We propose a scheme for a parametric amplifier based on a single suspended carbon nanotube field-emitter. This novel electromechanical nanotube device acts as a phase-sensitive, variable-gain, band-pass-filtering amplifier for electronic signal processing and, at the same time, can operate as a variable-sensitivity, tuneable detector and transducer of radio frequency electromagnetic waves. The amplifier can exhibit infinite gain at pumping voltages much less than 10 Volts. Additionally, the amplifier's low overhead power consumption (10–1000 nW) make it exceptionally attractive for ultra-low-power applications.