Carbon nanotube-based electrode, field emission neutralizer assembly and methods of forming the same

Abstract

Various embodiments may relate to a method of forming a carbon nanotube-based electrode. The method may include growing a carbon nanotube array on or over a semiconductor substrate using a chemical vapor deposition process such that the carbon nanotube array is subsequently separated from the semiconductor substrate via self-delamination. The method may also include transferring the carbon nanotube array, the carbon nanotube array being a freestanding carbon nanotube array, to a water bath. The method may further include immersing a target substrate into the water bath. The method may additionally include removing the target substrate with the carbon nanotube array from the water bath to form the carbon nanotube-based electrode, which includes the target substrate and the carbon nanotube array.

Description

CARBON NANOTUBE-BASED ELECTRODE, FIELD EMISSION NEUTRALIZERASSEMBLY AND METHODS OF FORMING THE SAMECROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority of Singapore application No 10202402559P filed August 21, 2024, the contents of it being hereby incorporated by reference in its entirety for all purposes.TECHNICAL FIELD

[0002] Various embodiments of this disclosure may relate to a method of forming a carbon nanotube (CNT)-based electrode. Various embodiments of this disclosure may relate to a carbon nanotube (CNT)-based electrode. Various embodiments of this disclosure may relate to a field emission neutralizer (FEN) assembly. Various embodiments of this disclosure may relate to a method of forming a field emission neutralizer (FEN) assembly.BACKGROUND

[0003] Small satellites (smallsat) electric propulsion methods such as Hall-Effect thrusters (HET) have gained significant importance as compared to conventional chemical propulsion owing to the reduction in propellant mass for accommodating higher payload mass However, HET ejects a highly charged ion plume that requires neutralization, upon which thrust is produced to propel the smallsat. Existing neutralizers such as hot wire or thermionic hollow cathodes, either require a chemical propellant or are energy inefficient, consuming -30% of the overall power required for the operation of the thruster. Further, it is challenging to fabricate thermionic cathodes that can function with a power of less than 30 W. This implies that for thrusters operating in the power range of 40-60 W, the cathode will require at least half of the total power required for the operation of thruster, ultimately making the thruster energy inefficient. The bottleneck herein is that the thermionic cathodes require very high temperatures forthe emission to occur, demanding high power consumption for maintaining the temperature. On the other hand, field emission neutralizers (FEN) require low power and are propellant-less, making them suitable for smallsat propulsion.

[0004] Carbon nanotubes (CNTs) have been investigated as a potential material for the fabrication of FEN upon application of ~109V / m electric field on the emitter, owing to the nanoscale diameter of its tip that concentrates the electric field. A CNT-FEN with a dimension of 88 x 88 mm2was fabricated with a maximum emission current of 20 mA at a potential difference of 500 V between gate and emitter grids. In these approaches, the CNT are deposited via solution process or physically adsorbed on the FEN substrate, where the interfacial adhesion is prone to weaken over time and resulting in the failure of FEN. Furthermore, the adopted processes yield CNT either with an uneven, “fluffy”, or intertwined morphologies, resulting in protrusion of several CNT from the array. These protrusions will lead to concentration of the electric field at the protruded CNT tip surfaces, which induce “burning- off’ the protruded CNT due to the localized heating and reduce the overall lifespan of the FEN.SUMMARY

[0005] Various embodiments may relate to a method of forming a carbon nanotube-based electrode. The method may include growing a carbon nanotube array on or over a semiconductor substrate using a chemical vapor deposition process such that the carbon nanotube array is subsequently separated from the semiconductor substrate via selfdelamination. The method may also include transferring the carbon nanotube array, the carbon nanotube array being a freestanding carbon nanotube array, to a water bath. The method may further include immersing a target substrate into the water bath. The method may additionally include removing the target substrate with the carbon nanotube array from the water bath to form the carbon nanotube-based electrode, which includes the target substrate and the carbon nanotube array.

[0006] Various embodiments may relate to a carbon nanotube-based electrode formed by any method as described herein.

[0007] Various embodiments may relate to a field emission neutralizer (FEN) assembly. The field emission neutralizer (FEN) assembly may include a base plate including one or more cavities. The field emission neutralizer (FEN) assembly may also include one or more carbon nanotube-based electrodes as described herein, or formed by any method as described herein Each of the one or more carbon nanotube-based electrodes may be held within one or more respective cavities The field emission neutralizer (FEN) assembly may further include a maskgrid over the base plate. The field emission neutralizer (FEN) assembly may additionally include an insulator layer on or over the mask grid. The field emission neutralizer (FEN) assembly may further include a gate grid on or over the insulator layer.

[0008] Various embodiments may relate to a method of forming a field emission neutralizer (FEN) assembly. The method may include providing each of one or more carbon nanotubebased electrodes as described herein or formed by any method as described herein into a respective cavity of one or more cavities of a base plate. The method may also include providing a mask grid over the base plate. The method may further include providing an insulator layer over the mask grid. The method may additionally include providing a gate grid over the insulating layer.BRIEF DESCRIPTION OF THE DRAWINGS

[0009] In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily drawn to scale, emphases instead are generally being placed upon illustrating the principles of various embodiments. In the following description, various embodiments of the invention are described with reference to the following drawings.FIG. 1 shows a general illustration of a method of forming a carbon nanotube-based electrode according to various embodiments.FIG. 2 shows a general illustration of a carbon nanotube-based electrode according to various embodiments.FIG. 3 shows a general illustration of a field emission neutralizer (FEN) assembly according to various embodiments.FIG. 4 shows a general illustration of a method of forming a field emission neutralizer (FEN) assembly according to various embodiments.FIG. 5 A shows a schematic of a conventional 0.5 mA-class carbon nanotube (CNT) - field emission neutralizer (FEN).FIG. 5B shows a table comparing the field emission neutralizer assembly according to various embodiments with existing neutralizers.FIG. 6A shows the transfer of the vertically aligned carbon nanotube (VACNT) array according to various embodimentsFIG. 6B shows an image of a 1 cm x 1 cm vertically aligned carbon nanotube (VACNT) array on copper foil according to various embodiments.FIG. 7A shows a scanning electron microscopy (SEM) image of the delaminated vertically aligned carbon nanotube (VACNT) array / membrane according to various embodiments, revealing the vertical alignment of the carbon nanotubes (CNTs).FIG. 7B shows a schematic illustrating contact angle measurements of (a) silicon (Si) and (b) silicon / silicon oxide (Si / SiO2) wafers according to various embodiments.FIG. 7C shows a plot of intensity (in arbitrary units or a.u.) as a function of Raman shift (per centimeter or cm'1) illustrating the Raman spectra of the delaminated vertically aligned carbon nanotube (VACNT) array / membrane according to various embodiments with an etching time of 10 minutes, as measured from the front and back surfaces.FIG. 7D shows a plot of intensity (in arbitrary units or a.u.) as a function of Raman shift (per centimeter or cm'1) illustrating the Raman spectra of the delaminated vertically aligned carbon nanotube (VACNT) array / membrane according to various embodiments with an etching time of 30 minutes, as measured from the front and back surfaces.FIG. 7E shows a plot of intensity (in arbitrary units or a.u.) as a function of Raman shift (per centimeter or cm'1) illustrating the Raman spectra of the delaminated vertically aligned carbon nanotube (VACNT) array / membrane according to various embodiments with an etching time of 60 minutes, as measured from the front and back surfaces.FIG. 7F shows a table illustrating the intensity ratio 1D / IG for samples undergoing etching times of 10 minutes, 30 minutes and 60 minutes according to various embodiments.FIG. 8 shows a schematic illustrating water-assisted transfer of the free-standing vertically aligned carbon nanotube (VACNT) array / membrane 804 onto a glass fabric 802 according to various embodiments.FIG. 9A shows a plot of current (in Amperes or A) as a function of voltage (in volts or V) illustrating the current-voltage (I-V) characteristics of a vertically aligned carbon nanotube (VACNT) array / membrane on a polyethylene terephthalate (PET) substrate according to various embodiments before and after 100 cycles of bending to an angle of 90°.FIG. 9B shows a plot of current (in Amperes or A) as a function of voltage (in volts or V) illustrating the current-voltage (I-V) characteristics of an as-synthesized vertically aligned carbon nanotube (VACNT) array / membrane on a silicon / silicon oxide (Si / SiC>2) wafer and atransferred vertically aligned carbon nanotube (VACNT) array / membrane on a polyethylene terephthalate (PET) substrate according to various embodimentsFIG. 10A shows an exploded view schematic of a carbon nanotube-field emission neutralizer (CNT-FEN) assembly according to various embodiments.FIG. 10B shows the parameters for the mask grid and the gate grid of the carbon nanotubefield emission neutralizer (CNT-FEN) assembly according to various embodiments.FIG. IOC shows a table of the parameters of the gate grid / mask grid according to various embodiments.FIG. 10D shows a schematic illustrating the test bench for the carbon nanotube-field emission neutralizer (CNT-FEN) assembly according to various embodiments.FIG. 1 OE shows a plot of emission current (in milli-Amperes or mA) as a function of open area ratio (OAR) of the carbon nanotube-field emission neutralizer (CNT-FEN) assembly according to various embodiments.FIG. 10F shows a plot of emission current (in milli-Amperes or mA) as a function of silt area of the carbon nanotube-field emission neutralizer (CNT-FEN) assembly according to various embodiments.FIG. 11 shows an overview schematic of another carbon nanotube-field emission neutralizer (CNT-FEN) assembly according to various embodiments.DESCRIPTION

[0010] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.

[0011] Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments. Features that are described in the context of an embodiment may correspondingly be applicable to the other embodiments, even if not explicitly described in these other embodiments. Furthermore, additions and / orcombinations and / or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0012] In the context of various embodiments, the articles “a”, “an” and “the” as used with regard to a feature or element include a reference to one or more of the features or elements.

[0013] In the context of various embodiments, the term “about” or “approximately” as applied to a numeric value encompasses the exact value and a reasonable variance, e.g., within 10% of the specified value.

[0014] As used herein, the term “and / or” includes any and all combinations of one or more of the associated listed items.

[0015] By “comprising” it is meant including, but not limited to, whatever follows the word “comprising”. Thus, use of the term “comprising” indicates that the listed elements are required or mandatory, but that other elements are optional and may or may not be present.

[0016] By “consisting of’ it is meant including, and limited to, whatever follows the phrase “consisting of’. Thus, the phrase “consisting of’ indicates that the listed elements are required or mandatory, and that no other elements may be present.

[0017] Embodiments described in the context of one of the carbon nanotube-based electrodes / field emission neutralizer assemblies are analogously valid for the other carbon nanotube-based electrodes / field emission neutralizer assemblies. Similarly, embodiments described in the context of a method are analogously valid for a carbon nanotube-based electrode / field emission neutralizer assembly, and vice versa.

[0018] FIG. 1 shows a general illustration of a method of forming a carbon nanotube -based electrode according to various embodiments. The method may include, in 102, growing a carbon nanotube array on or over a semiconductor substrate using a chemical vapor deposition (CVD) process such that the carbon nanotube array is subsequently separated from the semiconductor substrate via self-delamination. The method may also include, in 104, transferring the carbon nanotube array, the carbon nanotube array being a freestanding (or freestanding) carbon nanotube array, to a water bath. The method may further include, in 106, immersing a target substrate into the water bath. The method may additionally include, in 108, removing the target substrate with the carbon nanotube array from the water bath to form the carbon nanotube-based electrode, which includes the target substrate and the carbon nanotube array.

[0019] In other words, the method may include forming a carbon nanotube array on or over a semiconductor substrate under conditions such that the formed carbon nanotube array delaminates to form a freestanding carbon nanotube array. The freestanding carbon nanotube array may then be suspended in a water bath. A target substrate is then dipped into the water bath to “scoop” the freestanding carbon nanotube array, such that the carbon nanotube array is on the target substrate, thereby forming the carbon nanotube-based electrode. The carbon nanotube array may include a plurality of vertically aligned carbon nanotubes.

[0020] Self-delamination may be advantageous to manual delamination, since selfdelamination may not cause significant changes to the morphology and / or electrical characteristics of the carbon nanotube array. The carbon nanotube array may not be bent during separation from the semiconductor substrate. Various embodiments may form a carbon-based electrode including an intact carbon nanotube array with well-aligned carbon nanotubes, the carbon nanotube array having improved electrical characteristics compared to those formed by conventional methods. The carbon-based electrode as described herein may be included in a field emission neutralizer (FEN) assembly (alternatively be simply referred to as “field emission neutralizer (FEN)”), which may have electrical characteristics that are superior or on par to that of conventional neutralizers. Various embodiments may consume less power and / or may have improved neutralization capability compared to conventional neutralizers.

[0021] In various embodiments, growing the carbon nanotube array may include providing the semiconductor substrate in a chemical vapor deposition chamber having an inert gas environment, introducing a flow of a carbon precursor (e.g., ethylene, acetylene or methane) to the inert gas environment at an elevated temperature or range of temperatures for a period of time to form the carbon nanotube array on or over the semiconductor substrate, and providing an inert gas (e.g., argon gas (Ar), nitrogen gas (N2), etc) carrying water vapor to the inert gas environment at the elevated temperature or range of temperatures while introducing the flow of the carbon precursor to the inert gas environment. The inert gas environment may refer to any gas (or mixture of gases) that would not react with the semiconductor substrate. The inert gas environment may include a mixture of inert gases such as argon gas (Ar), nitrogen gas (N2), etc. In various embodiments, providing the inert gas carrying water vapor to the inert gas environment may include bubbling the inert gas through water, e g., through a heated bubbler.

[0022] Growing the carbon nanotube array may also include cutting off the flow of the carbon precursor to the inert gas environment after the period of time while continuing to provide the inert gas carrying water vapor to the inert gas environment at the elevated temperature or range of temperatures. Providing the inert gas carrying water vapor to the inert gas environment, after cutting off (i.e., stopping) the flow of the carbon precursor to the inert gas environment may help to contribute to the separation of the carbon nanotube array from the semiconductor substrate via self-delamination. The water vapor and / or the inert gas environment may help to weaken attachment of the carbon nanotube array to the underlying semiconductor substrate (or buffer layer).

[0023] In various embodiments, growing the carbon nanotube array may include depositing a catalyst on (either) the semiconductor substrate or a buffer layer on the semiconductor substrate, before introducing the flow of the carbon precursor to the inert gas environment at the elevated temperature or range of temperatures. The catalyst may be any suitable catalyst, e.g., iron, cobalt or nickel.

[0024] In various embodiments, the buffer layer may include aluminium oxide (AI2O3).

[0025] In various embodiments, the semiconductor substrate may include an oxide layer. The oxide layer may be a silicon oxide layer.

[0026] In various embodiments, the target substrate may be any suitable substrate, e.g., a copper foil, a glass fiber, a polyethylene terephthalate (PET) substrate, or a carbon cloth. The target substrate may be non-reactive with water.

[0027] In various embodiments, the carbon nanotube array may be attached to the target substrate without using an adhesive layer.

[0028] Various embodiments may relate to a carbon nanotube-based electrode formed by any method as described herein. FIG. 2 shows a general illustration of a carbon nanotube-based electrode 200 according to various embodiments. The carbon nanotube-based electrode 200 may include a target substrate 202. The carbon nanotube-based electrode 200 may include the carbon nanotube array 204 on or over the target substrate 202.

[0029] For avoidance of doubt, FIG. 2 is intended to illustrate features of a carbon nanotubebased electrode according to various embodiments, and is not intended to limit, for instance, the dimension(s), orientation etc. of the various features.

[0030] FIG. 3 shows a general illustration of a field emission neutralizer (FEN) assembly according to various embodiments The field emission neutralizer (FEN) assembly may includea base plate 302 including one or more cavities. The field emission neutralizer (FEN) assembly may also include one or more carbon nanotube-based electrodes as described herein, or fonned by any method as described herein. Each of the one or more carbon nanotube-based electrodes 300 may be held by a respective cavity of the one or more cavities. The field emission neutralizer (FEN) assembly may further include a mask grid 304 over the base plate 302. The field emission neutralizer (FEN) assembly may additionally include an insulator layer 306 on or over the mask grid 304. The field emission neutralizer (FEN) assembly may further include a gate grid 308 on or over the insulator layer 306.

[0031] In other words, various embodiments may relate to a FEN assembly including a base plate 302 including a single cavity (e g., groove) to accommodate a single carbon nanotubebased electrode 300 as described herein or formed by a method as described herein, while in various other embodiments, the base plate 302 may include multiple cavities (e.g., grooves) to hold multiple carbon nanotube-based electrodes 300 as described herein or formed by a method as described herein. The FEN assembly may further include a mask grid 304, an insulator layer 306 and a gate grid 308 over the based plate 302 with the carbon nanotube-based electrode(s) 300.

[0032] For avoidance of doubt, FIG. 3 is intended to illustrate features of a FEN assembly according to various embodiments, and is not intended to limit, for instance, the dimension(s), number, orientation etc. of the various features. For instance, while FIG. 3 shows a base plate 302 with two cavities for accommodating two carbon nanotube-based electrodes 300, in various embodiments, the base plate 302 may include any suitable number of cavity or cavities for accommodating any suitable number of carbon nanotube-based electrode(s) 300.

[0033] In various embodiments, the base plate 302 may include any suitable insulating polymeric material(s), for instance, polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE) (i.e., Teflon), and / or polypropylene (PP).

[0034] In various embodiments, the FEN assembly may include one or more spacers such that a respective spacer of the one or more spacers is around each of the one or more carbon nanotube-based electrodes 300. The one or more spacers may include any suitable insulating polymeric material(s), for instance, polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE) (i.e , Teflon), and / or polypropylene (PP) In various embodiments, the base plate 302, the mask grid 304 and / or the one or more spacers may secure the one or more carbon nanotubebased electrodes 300.

[0035] The mask grid 304 may include any suitable electrically conductive material, e.g., copper, aluminum or gold, may be patterned with holes, slits and / or openings. Likewise, the gate grid 308 may include any suitable electrically conductive material, e g., copper, aluminum or gold, may be patterned with holes, slits and / or openings. In various embodiments, the insulator layer 306 may include any suitable insulating polymeric material(s), such as polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE) (i.e., Teflon), and / or polypropylene (PP). The insulator layer 306 may be configured to separate or electrically shield the base plate 302 from the gate grid 308. The insulator layer 306 may be patterned with holes, slits and / or openings. Each of the mask grid 304, the insulator layer 306 and the gate grid may have one or more openings extending through a thickness of the respective grid / layer, such that when the FEN assembly is put together, the one or more carbon nanotube-based electrodes 300 are exposed through the one or more openings.

[0036] In various embodiments, the FEN assembly may further include a receiving plate over the gate grid 08.

[0037] During operation, a power supply which provides a varying negative voltage to the mask grid 304 may be connected between the mask grid 304 and the gate grid 308, while another power supply which provides a fixed positive voltage to the receiving plate may be connected between the receiving plate and the gate grid 308. The gate grid may act as ground for both power supplies.

[0038] FIG. 4 shows a general illustration of a method of forming a field emission neutralizer (FEN) assembly according to various embodiments. The method may include, in 402, providing each of one or more carbon nanotube-based electrodes as described herein or formed by any method as described herein into a respective cavity of one or more cavities of a base plate. The method may also include, in 404, providing a mask grid over the base plate. The method may further include, in 406, providing an insulator layer over the mask grid. The method may additionally include, in 408, providing a gate grid over the insulating layer.

[0039] In other words, various embodiments may relate to forming a FEN assembly as described herein.

[0040] For avoidance of doubt, FIG. 4 is intended to illustrate various steps to form a FEN assembly according to various embodiments, and is not intended to limit the sequence of the various steps. For instance, steps 404, 406 and 408 may occur at the same time, before or after one another.

[0041] Various embodiments may relate to a carbon nanotube (CNT) based field emission neutralizer (FEN) assembly that requires very low power in the order of few watts (W) for neutralization. For the FEN assembly fabrication according to various embodiments, vertically aligned CNT array grown on silicon substrates may be transferred on to copper foil, which is subsequently assembled on the FEN module. Compared to the existing neutralizers, the developed CNT-FEN assembly may possess the potential to consume less power for their operation, enabling energy efficient neutralization of the ion plume for sustainable smallsat propulsion. Various embodiments may relate to a method to grow vertically aligned CNT (VACNT) array, with a more homogenous distribution of CNT height, and to transfer them on to copper foil for the fabrication of FEN. The VACNT may be grown by a chemical vapor deposition (CVD) method on silicon wafers, and may be self-delaminated for transferring them onto copper foil for the fabrication of FEN. The self-delamination may yield an intact and freestanding VACNT array on copper foil with a more homogeneous distribution in CNT height that is desirable for FEN fabrication, with electrical properties similar to that of as-grown VACNT array on silicon wafers. The VACNT array may be secured firmly on copper foil without mechanical disintegration as compared to existing methods. Furthermore, the developed method according to various embodiments may enable tuning of the electrical performances of the CNT-FEN assembly, via modulating the open area ratio of the gate and emitter grids of the FEN module. The neutralization efficiency of the developed CNT-FEN assembly revealed that the proposed method yielded electrical performances that are either superior or as comparable to the exiting configurations. Further, the modular design of the developed CNT-FEN assembly may possess a good scope for improving the electrical properties by increasing the number of individual 1 x 1 cm arrays in the CNT-FEN assembly.

[0042] FIG. 5A shows a schematic of a conventional 0.5 mA-class carbon nanotube (CNT) - field emission neutralizer (FEN) Parameters such as emitter-to-gate separation distance and gate / mask slit aperture width were optimized to yield an emission current of 0.5 mA with an applied voltage of approximately 600 V. The CNT-FEN was then upscaled with an overall dimension of 88 x 88 mm2to yield a maximum emission current of 20 mA at a potential difference of 500 V between the gate and emitter grids. In these reports and similar attempts, the CNT are either obtained via solution process or adsorbed on the FEN substrate, resulting in an intertwined morphologies that concentrate the electric field at the protruding CNT tip surfaces. The reported VACNT growth / transfer processes utilized may also lead to unevensurface topology. These phenomena may lead to the degradation of FEN performance over time owing to the “burning-off’ the VACNT due to the localized heat generation at the protruding tip surfaces. According to various embodiments, an intact VACNT array with uniform morphology and topology may first be obtained on the copper foil. The compact FEN module or assembly may then be fabricated for accommodating the copper foil with enhanced stability and robustness for secure mounting of VACNT array within the FEN module or assembly. The electrical performances of the CNT-FEN module or assembly may be optimized via modulating the open area ratios of mask and grid components of the CNT-FEN module or assembly.

[0043] FIG. 5B shows a table comparing the field emission neutralizer assembly according to various embodiments with existing neutralizers. FIG. 5B reveals that a higher emission current may be obtained by the field emission neutralizer assembly according to various embodiments, as compared to existing neutralizers. The higher emission current of CNT-FEN assembly according to various embodiments may imply that the neutralization capability may be increased while having low power consumption, as compared to thermionic hollow cathode neutralizers, for its operation A drop in efficiency is observed, especially with respect to the neutralizer of Ref. 1, which may be acceptable considering the significant improvement in the emission current that is desirable for improving the overall performance of the CNT-FEN assembly.

[0044] Experiment 1

[0045] 1 cm x 1 cm vertically aligned CNTs (VACNTs) with varying heights between 0.25 mm and 1 mm were grown on silicon wafers with an oxide layer, using iron-aluminum (Fe-Al) as catalysts to form a VACNT array. The VACNT array may be grown using an in-house “FirstNano” chemical vapor deposition (CVD) system. The as-grown VACNT array may be delaminated from the silicon wafer (right) upon their growth, and subsequently transferred onto copper foils for the fabrication of the field emission neutralizer (FEN) assembly using a water- mediated approach. FIG. 6A shows the transfer of the vertically aligned carbon nanotube (VACNT) array 604 according to various embodiments. The free-standing VACNT array 604 may be suspended in water 606 to let the VACNT array 604 float freely on the surface of the water 606. The copper foil 602 may then utilized to draw the VACNT array 604 to form a carbon nanotube-based electrode.

[0046] FIG. 6B shows an image of a 1 cm x 1 cm vertically aligned carbon nanotube (VACNT) array on copper foil according to various embodiments.

[0047] Experiment 2

[0048] 350 pm thick, N type, prime grade silicon / silicon oxide (Si / SiOz) wafers with a resistivity of 0.01-0.05 Q.cm were purchased from Latech, Singapore. The wafers were cleaned with acetone, isopropyl alcohol, followed rinsing in deionized (DI) water and blow drying using nitrogen (N2). An oxide layer of 200 nm thickness was grown on the wafers via a thermal oxidation process. Aluminum oxide (AI2O3) and iron (Fe), with a thickness of ~10 nm and ~1 nm, respectively, were then deposited on the wafers by an electron beam (e-beam) method (Cello Ohmiker-60BL). The wafers were then placed in a commercial CVD system (Firstnano Easytube® 3000) for the VACNT array growth using an optimized growth protocol that enables self-delamination of the VACNT array from the wafer upon growth. In brief, the wafers were annealed in argon (Ar) and hydrogen (H2) environment for 30 min followed by the introduction of ethylene (C2H4) as the carbon precursor to initiate the VACNT growth at 750 °C. Simultaneously, Ar at a flow rate of 330 seem was bubbled through water for controlling the VACNT array growth and to enable self-delamination. Upon VACNT array growth, ethylene flow is cut-off while maintaining the water vapour level and Ar / H? flowrates, which may be hypothesized to enable self-delamination of the VACNT array. VACNT arrays were also grown on wafers without the oxide layer in order to investigate the influence of oxide layer on the efficiency of self-delamination. For characterization, VACNT was grown on smaller sized substrates diced from the Si / SiC>2 wafer. The obtained delaminated, free-standing VACNT membranes were then characterized using scanning electron microscope SEM (SEM, JEOL JSM-m00) and Raman spectroscopy (WITec CRM200 Raman with an neodymium- doped yttrium aluminium garnet (Nd: YAG) 532 nm laser as excitation source) to evaluate their morphology and quality, respectively.

[0049] As for the water-assisted transfer method, a water bath was utilized to float the VACNT arrays / membranes for subsequent transfer on to various substrates. Glass fibers, polyethylene terephthalate (PET) substrates, carbon cloths or copper foil substrates were immersed in the water bath and positioned below the floating VACNT arrays / membranes with an inclination. Upon establishing contact between the substrates and the VACNT arrays / membranes along an edge, the substrates were gradually drawn out of the water bath, while maintaining an angle of inclination of ~30°. The VACNT arrays / membranes were thenlifted off the water bath, and the substrates deposited with the VACNT arrays / membranes were placed on a hotplate at 50 °C for 30 min. Scanning electron microscopy (SEM) and Raman analysis were performed to evaluate the feasibility of the proposed approach for transferring free-standing VACNT array / membrane onto various substrates without compromising the morphology and quality. Mechanical bending tests were carried out with an improvised set-up to estimate the attachment and stability of VACNT array / membrane on various substrates. The substrates with VACNT were mechanically folded at various angles repeatedly using the developed mechanical bending test set-up. Electrical characterization of the substrates with VACNT arrays / membranes prior and post several cycles of folding was performed to estimate the stability and attachment of VACNT arrays / membranes on the substrates.

[0050] Growth of VACNT Arrays / Membranes on Silicon Wafers

[0051] Herein, homogeneous growth of VACNT over Si / SiC>2 wafer may be achieved using an optimized CVD protocol. Uniform VACNTs growth at wafer scale levels with the aspect ratios controllable via the growth time may be possible. Growth of VACNT is achieved using Fe as catalyst (~1 nm) that is uniformly distributed on the wafer via an aluminum oxide AI2O3 (~10 nm) buffer layer. It is hypothesized that the buffer layer controls the migration of Fe particles over the Si / SiCh surface using a process called Ostwald ripening. VACNT of up to 4 mm in height can be grown upon optimization of growth parameters such as temperature, gas flow rates, etc., yielding a broad range for tuning their aspect ratio.

[0052] The VACNT array / membrane may adhere to the Si / SiO2 wafer, which could be attributed to the bonding established between Fe catalyst nanoparticles on the wafer and VACNT array / membrane during the growth process. Although attempts such as scraping, lift off, chemical etching, gas etching, etc. have been made to delaminate the VACNT array / membrane, these processes may cause the VACNT array / membrane to undergo varying levels of deformation depending on the mechanical / chemical forces exerted for delamination. Furthermore, these manual handling processes may damage the VACNT array / membrane, or result in incomplete delamination of VACNT array / membrane from the wafer. This concern may be further exacerbated for longer VACNTs as longer VACNTs are very fragile and are easily damaged if uneven or excessive force is applied. Therefore, developing methodologies to delaminate VACNTs from wafers may be of significant importance for their deployment in high performance electronics.

[0053] Large Area Water-assisted Self-delamination

[0054] Approaches such as oxygen and hydrogen gas assisted delamination of VACNT from Si / SiCh wafers have been explored as alternative delamination methods that do not cause significant deformation to the VACNT, which require manual handling for delamination. The mechanism of delamination is hypothesized to be the etching of carbon atoms at the root ends of the VACNT arrays, which induces cleaving off VACNT from Si / SiOz. Herein, a method for self-delamination of VACNT, which results in a free-standing VACNT membrane upon CVD synthesis, is explored. During the CVD growth process, Ar flow is maintained at 330 seem through a heated water bubbler, along with the C2H4 precursor that controls the VACNT growth. Post VACNT growth at 750 °C, C2H4 flow is cut off while maintaining the temperature and water flow. It may be hypothesized that the water vapor carried into the chamber via Ar forms an oxidative environment that weaken the attachment of VACNT on the Si / SiOz wafer and induces self-delamination of VACNT. SEM observations reveal a homogeneous morphology of VACNT, while maintaining the vertical alignment with no deformities, as observed from the section of the membrane intentionally pulled apart for characterization. FIG. 7A shows a scanning electron microscopy (SEM) image of the delaminated vertically aligned carbon nanotube (VACNT) array / membrane according to various embodiments, revealing the vertical alignment of the carbon nanotubes (CNTs).

[0055] Thus, the water-assisted delamination of VACNT is a “one-step” growth and delamination approach to obtain large area free-standing VACNT. Furthermore, since no manual handling is involved, the freestanding VACNT array / membrane does not undergo any form of deformation, yielding almost similar electrical properties as that of VACNT array / membrane synthesized over the Si / SiC>2 wafer. It is to be emphasized that no delamination was observed without the Ar flow through the water bubbler, even with etching times of up to 1 hour. While there have been reports of using H2 gas to etch VACNT, the VACNT array / membrane does not readily self-delaminate and requires additional process(es) such as lift off, manual scraping, etc. Herein, the water vapor environment, along with Ar and Hz gases, may significantly enhance the efficiency of delamination, resulting in intact, selfdelaminated VACNT arrays / membranes.

[0056] Influence of Substrate on the Efficiency of Delamination

[0057] In order to understand the mechanism of delamination, VACNT was grown on Si wafer without the thermally grown oxide layer (i.e., SiOz layer) with same growth and delamination parameters. VACNT array / membrane was also grown on smaller sized substratesof ~ 1 cm x 1 cm that were diced from the wafer. For small sized substrates, the delamination of VACNT from the Si wafer was relatively successful as compared to substrates of larger area, i.e., over 5 cm x 5 cm. In comparison, large areas of VACNT array / membrane were shown to self-delaminate from Si wafer with a layer of Si O2 This observation illustrates that the Si O2 layer may play a critical role in the delamination of large areas of VACNT array / membrane. A possible explanation is that the hydrophilic nature of the oxide layer, as verified using contact angle measurements, may facilitate the seeping of etchants beneath the edges of the VACNT array / membrane, causing it to self-delaminate from the Si / SiCh wafer, in contrary to the Si wafer. FIG. 7B shows a schematic illustrating contact angle measurements of (a) silicon (Si) and (b) silicon / silicon oxide (Si / SiOi) wafers according to various embodiments. It has also been found that the edges of the VACNT array / membrane on the Si wafer appear to be delaminated, while the VACNT array / membrane is still attached to the center of the Si wafer, therefore requiring manual scraping for delamination and justifying the above hypothesis. Additional experiments with SiCh layer of varying thicknesses (20 nm, 200 nm, and 650 nm) also confirm successful delamination of VACNT array / membrane. Since the same CVD recipe is utilized for VACNT array / membrane growth on Si and Si / SiC>2 wafers, and there are no significant differences in the surface roughness of Si and Si / SiCh, it may be concluded that that the oxide layer (at least with a thickness of 20 nm) may be preferred or required for the proposed water-assisted self-delamination approach. Three different etching times were tested (10 min, 30 min and 60 min) to delaminate 1 cm x 1 cm VACNT array / membrane grown on Si / Si O2 wafer in order to estimate the influence of the proposed water- assisted etching process on the quality of VACNT array / membrane.

[0058] FIG. 7C shows a plot of intensity (in arbitrary units or a.u.) as a function of Raman shift (per centimeter or cm'1) illustrating the Raman spectra of the delaminated vertically aligned carbon nanotube (VACNT) array / membrane according to various embodiments with an etching time of 10 minutes, as measured from the front and back surfaces. FIG. 7D shows a plot of intensity (in arbitrary units or a.u.) as a function of Raman shift (per centimeter or cm' ’) illustrating the Raman spectra of the delaminated vertically aligned carbon nanotube (VACNT) array / membrane according to various embodiments with an etching time of 30 minutes, as measured from the front and back surfaces. FIG. 7E shows a plot of intensity (in arbitrary units or a.u.) as a function of Raman shift (per centimeter or cm'1) illustrating the Raman spectra of the delaminated vertically aligned carbon nanotube (VACNT)array / membrane according to various embodiments with an etching time of 60 minutes, as measured from the front and back surfaces.

[0059] As observed from FIGS. 7C - E, no significant differences in Raman spectra were observed for all the three etching times tested, both on the front and back surfaces of the delaminated VACNT array / membrane. Furthermore, no differences in the ID and IG peak positions and intensities were observed indicating that the etching time does not influence the VACNT array / membrane quality. FIG. 7F shows a table illustrating the intensity ratio ID / IG for samples undergoing etching times of 10 minutes, 30 minutes and 60 minutes according to various embodiments. FIG. 7F shows similar ID / IG ratios for all three etching times tested, suggesting that water induced etching process (up to 1 hour) may not cause degradation of the VACNT array / membrane.

[0060] Water-mediated Transfer of VACNT Arrav / Membrane onto Flexible Substrates

[0061] A method has been developed to transfer the free-standing VACNT array / membrane onto various flexible substrates such as glass fabric, PET substrate, carbon cloth and copper foil. In this method, the delaminated VACNT array / membrane may be placed in a water bath and allowed to freely float on the surface of the water bath. The substrates may be immersed into the water bath to retrieve the VACNT membrane. The delamination process is illustrated in FIG 8. FIG 8 shows a schematic illustrating water-assisted transfer of the free-standing vertically aligned carbon nanotube (VACNT) array / membrane 804 onto a glass fabric 802 according to various embodiments. The 2 cm x 2 cm free-standing VACNT array / membrane 804 may be suspended in water 806 to let the VACNT array / membrane 804 float freely on the surface of the water 806. The glass fabric 802 may then be immersed in the water 806. The glass fabric 802 together with the VACNT array / membrane 804 may be removed from the water 806, followed by drying to obtain the carbon nanotube-based electrode.

[0062] Stability of VACNT Arrav / Membrane on Flexible Substrates

[0063] The robustness of the VACNT membrane transferred on to flexible substrates was evaluated by subjecting them to repeated cycles of bending up to an angle of 90°. The sheet resistance of the VACNT array / membrane on PET flexible substrate upon bending up to 100 cycles was compared with that of as transferred VACNT array / membrane on PET flexible substrate prior bending. FIG 9A shows a plot of current (in Amperes or A) as a function of voltage (in volts or V) illustrating the current-voltage (I-V) characteristics of a vertically aligned carbon nanotube (VACNT) array / membrane on a polyethylene terephthalate (PET)substrate according to various embodiments before and after 100 cycles of bending to an angle of 90°. A decrease in sheet resistance of the VACNT array / membrane was observed, which may be due to the establishment of additional in-plane conducting pathways induced by repeated bending. However, it should be emphasized that a significant change in sheet resistance was not observed upon transfer of VACNT array / membrane on to PET substrate, as compared to as synthesized VACNT array / membrane on Si / SiCh wafer, which may indicate that the VACNT arrays / membranes are intact and are not deformed during the growth and transfer process. FIG. 9B shows a plot of current (in Amperes or A) as a function of voltage (in volts or V) illustrating the current-voltage (I-V) characteristics of an as-synthesized vertically aligned carbon nanotube (VACNT) array / membrane on a silicon / silicon oxide (Si / SiOz) wafer and a transferred vertically aligned carbon nanotube (VACNT) array / membrane on a polyethylene terephthalate (PET) substrate according to various embodiments. Another interesting observation is that the VACNT array / membrane may retain its ohmic conductor characteristics when transferred on to flexible conducting substrates, as illustrated by the obtained linear I-V characteristics. The observations suggest that the proposed water-assisted growth and delamination methodologies yield self-delaminated VACNT arrays / membranes, which may be easily transferred on to flexible substrates without mechanical deformation for deployment in high performance electronics.

[0064] Various embodiments may relate to a CVD based water-assisted method for growth and self-delamination of Si / SiO2 wafers. Wafer-scale level intact and freestanding VACNT arrays / membranes may be obtained using the method as described herein. Characterization experiments ascertained that free-standing VACNT arrays / membranes may retain their electrical and morphological properties, comparable to that of as synthesized VACNT arrays / membranes on Si / SiC>2 wafers. The free-standing arrays / membranes may be transferred onto flexible substrates using a water-assisted method. The stability of the attachment was characterized using mechanical bending tests and determining the current - voltage characteristics of VACNT arrays / membrane prior and after the transfer process. These observations illustrate the potential of the developed water-assisted approaches for facile delamination and transfer of large areas of VACNT arrays / membranes on to various flexible substrates. To further improve the adhesion between larger VACNT arrays / membranes and various substrates, use of water-soluble adhesives and other methods such as surface functionalization may be evaluated.

[0065] Experiment 3

[0066] Fabrication of Small Satellite CNT-FEN Assembly

[0067] FIG. 10A shows an exploded view schematic of a carbon nanotube-field emission neutralizer (CNT-FEN) assembly according to various embodiments. The carbon nanotubefield emission neutralizer (CNT-FEN) assembly may include a base plate 1002 (e.g., Teflon base) including a cavity or groove to accommodate a carbon nanotube-based electrode 1000 (e.g., copper coil and VACNT array / membrane on the copper foil). The CNT-FEN assembly may also include a mask grid 1004 over the base plate 1002, and an insulator layer 1006 (e.g., a Teflon membrane) over the mask grid 1004. The insulator layer 1006 may be used for electrically shielding the base plate 1002 from the gate grid 1008. The gate grid 1008 may be over the insulator layer 1006. The CNT-FEN assembly may further include a spacer 1010 around the carbon nanotube-based electrode 1000 in the cavity or groove of the base plate 1002. The base plate 1002, the spacer 1010 and the mask grid 1004 may secure the copper foil within the cavity or groove, and may improve the robustness of the CNT-FEN neutralizer for facilitating its operation in space environment.

[0068] The CNT-FEN emission current (mA), current density (mA / cm2) and efficiency (mA / W) are evaluated under varying voltage biases between the base plate and the gate grid using a set-up.

[0069] Gate Grid / Mask Grid Design Optimization

[0070] FIG. 10B shows the parameters for the mask grid and the gate grid of the carbon nanotube-field emission neutralizer (CNT-FEN) assembly according to various embodiments. A rectangular mask / gate grid architecture with an open area ratio (OAR) of ~ 80% may be designed for the CNT-FEN fabrication. OAR was calculated by the “total area of holes or openings / total area of the grid”. This investigation revealed that the grid design may involve optimization of three parameters: slit shape, slit area, and OAR, for optimizing the electric field and improving the field emission capabilities of the CNT-FEN assembly. Smaller grid opening areas are expected to yield higher losses as the field emission is masked by the electrode surfaces of the mask grid and gate grid. Conversely, larger openings may increase the field emission and thereby the emission current, but may however cause non-uniform field emission. The larger openings may lead to regions of the carbon nanotube-based electrode near the edge of the silts being subjected to a high electric field gradient, whilst regions at the center of the aperture may experience low electric field gradients This non-uniformity may cause excessivewear and tear on specific regions, and may hence cause a drop in CNT-FEN assembly performance Hence, there may be a trade-off between increasing OAR and providing a uniform electric field for the extraction of electrons.

[0071] Various design components may be taken into consideration during the fabrication of the gate and mask grids as shown in FIG. 10C. FIG. 10C shows a table of the parameters of the gate grid / mask grid according to various embodiments. The width and height of the slits are varied to investigate the performance of the CNT-FEN assembly. The slit area is calculated as the product of W and H, and the OAR is the ratio of the total slit area to the total grid area. As shown in FIG. 10B, a gate grid (gate electrode) of size 1 cm x 1 cm and a mask grid (mask electrode) of size 1 cm * 1 cm was fabricated and integrated with the CNT-FEN assembly. FIG. 10D shows a schematic illustrating the test bench for the carbon nanotube-field emission neutralizer (CNT-FEN) assembly according to various embodiments. The gate grid (gate electrode) may act as act as ground for both the negative power supply and the positive power supply. The VACNT array may be negatively biased via the mask grid as the VACNT array and the mask grid are in electrical contact, while the receiving plate may be positively biased at 400 V.

[0072] FIG. 10E shows a plot of emission current (in milli-Amperes or mA) as a function of open area ratio (OAR) of the carbon nanotube-field emission neutralizer (CNT-FEN) assembly according to various embodiments. FIG. 10F shows a plot of emission current (in milli-Amperes or mA) as a function of silt area of the carbon nanotube-field emission neutralizer (CNT-FEN) assembly according to various embodiments. As observed from FIGS. 10E - F, an OAR ranging from 0.9 to 0.92 and silt area ranging from 2.5 mm2to 4.0 mm2may be optimal, and may yield an average emission current density of -0.35 mA / cm2and an average efficiency of -1.1 mA / W. A carbon nanotube-field emission neutralizer (CNT-FEN) assembly with optimized grids having design parameters shown in FIG. 5B may yield a peak emission current density of -0.8 mA / cm2with an efficiency of 1.3 mA / W.

[0073] Carbon Nanotube-Field Emission Neutralizer Assembly Including Multiple Carbon Nanotube-based Electrodes

[0074] FIG. 11 shows an overview schematic of another carbon nanotube-field emission neutralizer (CNT-FEN) assembly according to various embodiments. The CNT-FEN assembly may include 4 carbon nanotube-based electrodes 1 lOOa-d on a base plate 1102. The mask grid, the insulator layer and the gate grid are not shown in FIG. 1 1 . Each of the 4 carbon nanotube-based electrodes 1 lOOa-d may include up to 3 cm * 3 cm VACNT array / membrane (i.e., each electrode with an area of 9 cm2). The number of carbon nanotube-based electrodes can be varied or extended to achieve the required emission current and efficiency. Multiple carbon nanotube-based electrodes may allow for the individual control of each carbon nanotube-based electrode / VACNT array. Also, if any of the carbon nanotube-based electrodes / VACNT arrays fails, the remaining carbon nanotube-based electrodes / VACNT arrays may still be able to be used for neutralization, thereby ensuring reliability of the carbon nanotube-field emission neutralizer (CNT-FEN) assembly.

[0075] Preliminary optimization experiments have revealed that a catalyst ratio of 10: 1 and a VACNT height of 500 pm may be optimal for fabrication of an emission neutralizer (CNT- FEN) assembly with improved electrical performances. These parameters have been utilized for the fabrication of VACNT arrays and for evaluation of their electrical performance. The area of the individual VACNT arrays and the design of carbon nanotube-based electrodes may yield improved performance of the resultant emission neutralizer (CNT-FEN) assembly, with electrical properties either surpassing or at least on par with existing neutralizers. Further, the VACNT array growth and transfer and the modular design of FEN module may yield a robust neutralizer assembly for applications in harsh environments, such as space, as compared to previous methods.

[0076] Various embodiments may possess the potential to be deployed for neutralization in lower power Hall Effect Thrusters (HETs). Preliminary electrical characterizations have been performed to expose the potential of the developed carbon nanotube-field emission neutralizer assembly according to various embodiments to replace conventional neutralizers in HETs. Emission current and efficiency obtained may be comparable to that of existing neutralizers. There may be good potential to improve the electrical performance via including multiple carbon nanotube-based electrodes / VACNT arrays in a carbon nanotube-field emission neutralizer. The improvements in electrical performance may be mainly attributed to the fabrication of intact and free-standing VACNTs.

Claims

Claims1. A method of forming a carbon nanotube-based electrode, the method comprising: growing a carbon nanotube array on or over a semiconductor substrate using a chemical vapor deposition process such that the carbon nanotube array is subsequently separated from the semiconductor substrate via selfdelamination; transferring the carbon nanotube array, the carbon nanotube array being a freestanding carbon nanotube array, to a water bath; immersing a target substrate into the water bath; and removing the target substrate with the carbon nanotube array from the water bath to form the carbon nanotube-based electrode, which comprises the target substrate and the carbon nanotube array.

2. The method according to claim 1, wherein growing the carbon nanotube array comprises : providing the semiconductor substrate in a chemical vapor deposition chamber having an inert gas environment; introducing a flow of a carbon precursor to the inert gas environment at an elevated temperature or range of temperatures for a period of time to form the carbon nanotube array on or over the semiconductor substrate; providing an inert gas carrying water vapor to the inert gas environment at the elevated temperature or range of temperatures while introducing the flow of the carbon precursor to the inert gas environment; and cutting off the flow of the carbon precursor to the inert gas environment after the period of time while continuing to provide the inert gas carrying water vapor to the inert gas environment at the elevated temperature or range of temperatures.

3. The method according to claim 2, wherein providing the inert gas carrying water vapor to the inert gas environment comprises bubbling the inert gas through water.

4. The method according to claim 2 or claim 3, wherein growing the carbon nanotube array further comprises : depositing a catalyst on the semiconductor substrate or a buffer layer on the semiconductor substrate before introducing the flow of the carbon precursor to the inert gas environment at the elevated temperature or range of temperatures.

5. The method according to claim 4, wherein the buffer layer comprises aluminium oxide (AI2O3).

6. The method according to claim 4 or claim 5, wherein the catalyst is iron, cobalt or nickel.

7. The method according to any one of claims 1 to 6, wherein the semiconductor substrate comprises an oxide layer.

8. The method according to claim 7, wherein the oxide layer is a silicon oxide layer.

9. The method according to any one of claims 1 to 8, wherein the target substrate is a copper foil, a glass fiber, a polyethylene terephthalate (PET) substrate, or a carbon cloth.

10. The method according to any one of claims 1 to 9, wherein the target substrate is non-reactive with water.

11. The method according to any one of claims 1 to 10, wherein the carbon nanotube array is attached to the target substrate without using an adhesive layer.

12. The method according to any one of claims 1 to 11, wherein the carbon nanotube array comprises a plurality of vertically aligned carbon nanotubes.

13. The method according to any one of claims 1 to 12,wherein the carbon nanotube array is not bent during separation from the semiconductor substrate.

14. A carbon nanotube-based electrode formed by any one of claims 1 to 13.

15. A field emission neutralizer assembly comprising: a base plate comprising one or more cavities; one or more carbon nanotube-based electrodes formed by a method according to any one of claims 1 to 13, each of the one or more carbon nanotube-based electrodes held by a respective cavity of the one or more cavities; a mask grid over the base plate; an insulator layer over the mask grid; and a gate grid over the insulator layer.

16. The field emission neutralizer assembly according to claim 15, wherein the base plate comprises polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE), or polypropylene (PP).

17. The field emission neutralizer assembly according to claim 15 or claim 16, further comprising: one or more spacers such that a respective spacer of the one or more spacers is around each of the one or more carbon nanotube-based electrodes.

18. The field emission neutralizer assembly according to any one of claims 15 to 17, wherein the insulator layer comprises polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE), or polypropylene (PP).

19. A method of forming a field emission neutralizer assembly, the method comprising: providing each of one or more carbon nanotube-based electrodes formed by a method according to any one of claims 1 to 13 into a respective cavity of one or more cavities of a base plate; providing a mask grid over the base plate; providing an insulator layer over the mask grid; and providing a gate grid over the insulating layer.

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