CHARACTERISTICS

STATEMENT OF FEDERAL FUNDING

[0001] The invention described herein was made in the performance of work under a NASA contract NNN12AA01 C, and is subject to the provisions of Public Law 96-517 (35 USC 202) in which the Contractor has elected to retain title.

FIELD OF THE INVENTION

[0002] The present invention generally relates to the adhesion of vertically aligned carbon nanotubes to a substrate.

BACKGROUND

[0003] Field electron emission generally regards the emission of electrons, e.g. from a solid surface into a vacuum, based on the influence of an electric field. Field electron emission is relied on in a number of applications including microscopy, spectroscopy, and display technology. Recently, carbon nanotubes (CNTs) have been studied for their potential to provide for viable electron emitters. In particular, carbon nanotubes exhibit a host of properties that would suggest that they could make for excellent field emitters in a variety of applications.

SUMMARY OF THE INVENTION

[0004] Systems and methods in accordance with embodiments of the invention implement enhanced carbon nanotube-based field emitters. In one embodiment, a method of fabricating a structure characterized by a plurality of carbon nanotubes adhered to a substrate includes: growing a plurality of vertically aligned carbon nanotubes to a height of at least approximately 25 μηι on a first flat substrate; where the grown plurality of vertically aligned carbon nanotubes define a pattern characterized by at least two regions, each region including at least one carbon nanotube, where the at least two regions are spaced apart by at least 1 μηι; affixing a second substrate to the free ends of the plurality of the grown carbon nanotubes without disturbing said pattern; and detaching the first flat substrate from the carbon nanotubes affixed to the second substrate.

[0005] In another embodiment, the first flat substrate is optically flat.

[0006] In yet another embodiment, the first flat substrate has a surface roughness of less than approximately 25 nm root mean squared.

[0007] In still another embodiment, the pattern is characterized by at least two bundles of carbon nanotubes spaced apart by at least 1 μηι.

[0008] In still yet another embodiment, the pattern is characterized by an array of bundles of carbon nanotubes spaced apart by at least 1 μηι.

[0009] In a further embodiment, the bundles of carbon nanotubes are characterized by a diameter of between approximately 1 μηι and approximately 2 μηι.

[0010] In a still further embodiment, the bundles of carbon nanotubes are spaced apart by at least approximately 5 μηι.

[0011] In a yet further embodiment, affixing the second substrate to the free ends of the plurality of grown carbon nanotubes includes welding the second substrate to the free ends of the plurality of grown carbon nanotubes.

[0012] In a still yet further embodiment, affixing the second substrate to the free ends of the plurality of grown carbon nanotubes includes curing an epoxy-based substrate when the carbon nanotubes are rooted in the epoxy-based substrate.

[0013] In another embodiment, affixing the second substrate to the free ends of the plurality of grown carbon nanotubes includes attaching an adhesive surface to the free ends of the plurality of grown carbon nanotubes.

[0014] In yet another embodiment, the adhesive surface is a copper tape.

[0015] In still another embodiment, the second substrate is affixed to the plurality of carbon nanotubes to an extent that an applied pressure of less than 60 kPa is insufficient to detach a plurality of the carbon nanotubes from the second substrate. [0016] In still yet another embodiment, the second substrate is affixed to the plurality of carbon nanotubes to an extent that an applied electric field of less than 3 V/μΓη is insufficient to detach a plurality of carbon nanotubes from the affixed second substrate.

[0017] In a further embodiment, the first substrate includes silicon.

[0018] In a yet further embodiment, the second substrate is electrically conductive.

[0019] In a still further embodiment, the second substrate includes titanium and copper.

[0020] In a still yet further embodiment, the method further includes coating the aggregate of the plurality of carbon nanotubes and the affixed second substrate with a layer using atomic layer deposition.

[0021] In another embodiment, the coating layer includes one of: titanium nitride, vanadium nitride, tungsten, and mixtures thereof.

[0022] In yet another embodiment, a method of fabricating a structure characterized by a plurality of carbon nanotubes adhered to a substrate, includes: growing a plurality of vertically aligned carbon nanotubes on a substrate; and coating the aggregate of the grown plurality of vertically aligned carbon nanotubes and substrate with a layer using atomic layer deposition.

[0023] In still another embodiment, the grown plurality of carbon nanotubes are characterized by a height of at least approximately 25 μηι .

[0024] In still yet another embodiment, the grown plurality of vertically aligned carbon nanotubes define a pattern characterized by at least two regions, each region including at least one carbon nanotube, where the at least two regions are spaced apart by at least 1 μΓΠ.

[0025] In a further embodiment, the pattern is characterized by at least two bundles of carbon nanotubes spaced apart by at least 1 μηι .

[0026] In a yet further embodiment, the pattern is characterized by an array of bundles of carbon nanotubes spaced apart by at least 1 μηι.

[0027] In a still further embodiment, the bundles of carbon nanotubes are characterized by a diameter of between approximately 1 μηι and approximately 2 μηι . [0028] In a still yet further embodiment, the bundles of carbon nanotubes are spaced apart by at least approximately 5 μηι .

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] FIG. 1 illustrates a method of adhering a plurality of carbon nanotubes to a substrate so as to achieve a structure characterized by relatively uniform tip protrusion in accordance with certain embodiments of the invention.

[0030] FIG. 2 illustrates a method of growing carbon nanotubes on a substrate, the method incorporating atomic layer deposition, in accordance with certain embodiments of the invention.

[0031] FIGS. 3A-3F schematically illustrate a method of adhering a plurality of carbon nanotubes to a substrate so as to achieve a structure characterized by relatively uniform tip protrusion in accordance with an embodiment of the invention.

[0032] FIGS. 4A-4H are SEM images of carbon nanotube-based field emitters fabricated in accordance with certain embodiments of the invention.

[0033] FIGS. 5A-5E illustrate the testing of carbon nanotube-based field emitters fabricated in accordance with certain embodiments of the invention.

[0034] FIGS. 6A-6E illustrate data pertaining lifetime tests of carbon nanotube-based field emitters fabricated in accordance with certain embodiments of the invention.

DETAILED DESCRIPTION

[0035] Turning now to the drawings, systems and methods for implementing carbon nanotube-based field emitters are illustrated. Carbon nanotubes (CNTs) have many exceptional properties that make them attractive for a variety of applications. For instance, CNTs are amongst the strongest materials, as measured by tensile strength, and amongst the stiffest materials, as measured by elastic modulus. Additionally, CNTs have also been determined to possess outstanding electrical field emission properties, with high emission currents at low electric field strengths (e.g., applied field from 1 - 3 ν/μΓη and an emission current ~0.1 mA from a single nanotube, as measured in conventional configurations). Accordingly, CNTs are thereby attractive as cold-cathode field emission sources, especially for applications requiring high current densities (e.g. hundreds to thousands of amperes per cm ² ) and lightweight packages (high frequency vacuum tube sources). Indeed, in U.S. Patent Application Number 1 1/137,725 (issued as U.S. Patent Number 7,834,530 - the "'530 patent"), Manohara et al. disclose particular configurations for a high density carbon nanotube-based field emitter that provide favorable performance characteristics. For example, Manohara et al. disclose that field emitters that include a plurality of bundles of vertically aligned CNTs disposed on a substrate, where the diameter of the bundles is between approximately 1 μηι and 2 μηι, and where the bundles of CNTs are spaced at a distance of approximately 5 μηι from one another, demonstrate particularly advantageous performance characteristics. The '530 patent is hereby incorporated by reference in its entirety, especially as it pertains to the aforementioned beneficial configurations.

[0036] Notably, carbon nanotube-based field emitters can be used in a variety of applications. For example, in U.S. Pat. App. No. 13/796,943 (issued as U.S. Patent Number 8,796,932 - "the '932 patent"), Manohara et al. disclose microscale digital vacuum electronics that can advantageously incorporate carbon-nanotube based field emitters. Such microscale digital vacuum electronics can be beneficial relative to conventional CMOS-based electronics, as they can readily be developed to be high- temperature tolerant and radiation insensitive. The '932 patent is hereby incorporated by reference in its entirety, especially as it pertains to the implementation of carbon nanotube-based field emitters in microscale digital vacuum electronics.

[0037] Nonetheless, even though CNT-based field emitters have been proposed and developed, in many instances these field emitters have been deficient in a number of respects. For example, in many previous instances, the CNTs were not sufficiently bonded to the underlying substrate. For instance, in many cases, when the field emitters were subject to an electric field during operation, the electric field would cause at least some of the carbon nanotubes to detach from the substrate. In some instances, it was determined that pressures as light as between approximately 20 kPa and approximately 60kPa were sufficient to detach CNTs from the underlying substrate. Thus, gently rubbing the surface of the substrate of these field emitters with a Q-tip could have been sufficient to dislodge CNTs from the substrate. This fragility can be problematic in a number of respects. For example, the field emission performance of the field emitter can degrade as a function of the number of detached CNTs. Moreover, detached CNTs can potentially short circuit associated circuitry. In essence, the weak bond between CNTs and associated substrates of these CNT-based field emitters can undermine their potential to serve as robust field emitters that can withstand rigorous operating conditions. To address this issue, in U.S. Pat. App. No. 14/081 ,932 (issued as U.S. Patent Number 9,064,667 - "the '667 patent"), Manohara et al. disclose particular methods for better adhering CNTs to a respective underlying substrate. For example, the '667 patent discloses heating the substrate so as to soften it and thereby allow a plurality of the CNTs to become at least partially enveloped by the softened substrate. The disclosure of the '667 patent is hereby incorporated by reference in its entirety, especially as it regards methods enabling the substrate to envelop at least a portion of a plurality of CNTs.

[0038] Even with the above-described enhancements, the field emission characteristics of CNT-based field emitters can still be improved. For instance, while better adhering CNTs to an underlying substrate can enhance the viability of the respective structure, such structures can further be enhanced if the CNTs can be made to have a more uniform topography - e.g. with a plurality of the vertically aligned carbon nanotubes protruding from the underlying substrate at a relatively uniform extent. In other words, better CNT-based field emitters can be achieved if the free ends of the carbon nanotubes can be made to be substantially coplanar. By contrast, in many conventional configurations, the free ends of the CNTs protrude at various distances from the surface of the substrate. Accordingly, many embodiments of the invention implement methods for fabricating CNT-based field emitters characterized by relatively uniform and orthogonal protrusion from an underlying substrate. Moreover, in many embodiments of the invention, methods of fabricating a CNT-based field emitter include coating the grown CNTs and the underlying substrate via atomic layer deposition (ALD). This can allow for improved mechanical stability as well as improved field emission characteristics, thereby further bolstering the viability of the structure. Processes for fabricating such robust CNT-based field emitters are now discussed in greater detail below.

Processes for Developing Robust CNT-based Field Emitters Demonstrating Enhanced Performance Characteristics

[0039] In many embodiments of the invention, CNT-based field emitters are implemented that are characterized by relatively uniform CNT-protrusion from an underlying substrate. Additionally, in many embodiments, the strength of the adhesion between CNTs and the corresponding substrate is sufficient to withstand rigorous operating conditions. In many embodiments, the substrate is heated such that it softens and envelopes at least a portion of at least one CNT, such that upon cooling, the CNT is more rigidly adhered to the substrate. In numerous embodiments, the CNTs are implemented via bundles of individual CNTs, which are specifically patterned, e.g. in a manner that can provide for enhanced field emission characteristics.

[0040] A process for fabricating a structure characterized by carbon nanotubes disposed relatively orthogonally on a substrate and further characterized by the carbon nanotubes having a relatively uniform protrusion from the surface of the substrate is illustrated in FIG. 1 . The process 100 includes growing 102 a plurality of vertically aligned carbon nanotubes on a first flat substrate. The vertically aligned carbon nanotubes can be grown 102 in any suitable way in accordance with embodiments of the invention, and can be grown on any suitable flat substrate. In many embodiments, the substrate is optically flat. For example, in many embodiments, the surface is flat to within approximately 25 nm. In numerous embodiments, the roughness of the substrate is less than approximately 25 nm root mean squared (RMS) - the smooth surface can facilitate lithographic patterning and can also give rise to the referenced homogenous CNT protrusion as can be understood from the discussion below. Importantly, the CNTs can be grown on a substrate comprising any material in accordance with embodiments of the invention. For example, in many embodiments, the CNTs are grown on a substrate comprising titanium - titanium provides a number of advantages including that it does not inhibit the growth of CNTs. In many embodiments, the CNTs are grown on a substrate comprising titanium and copper. In numerous embodiments, the CNTs are grown on a silicon substrate.

[0041] Notably, in many embodiments, the grown 102 CNTs are patterned in a predetermined manner. In other words, in many embodiments, the carbon nanotubes are not simply grown uniformly across the substrate surface so as to simply result in a layer of CNTs; rather, the CNTs are grown 102 so as to conform to a pre-determined pattern of CNTs. In many embodiments, the patterned CNTs are characterized by at least two spaced apart regions of CNTs separated by a gap of at least 1 μηι. In numerous embodiments, the grown 102 CNTs take the form of a patterned array of bundles of CNTs. In many embodiments, the grown 102 CNTs take the form of a patterned array of bundles of CNTs between approximately 1 μηι and 2 μηι in diameter, and spaced apart by at least approximately 5 μηι. As disclosed in the '530 patent, this configuration can give rise to particularly effective field emission characteristics. Of course, it should be appreciated that while several examples of CNT patterning are given, the CNTs can be patterned in any suitable way in accordance with embodiments of the invention - not just those ways explicitly listed above.

[0042] In many embodiments, the CNTs are grown to a height of at least approximately 25 μηι. In many embodiments the height of the CNTs are between approximately 25 μηι and 50 μηι. In many embodiments, the height of the CNTs are greater than approximately 50 μηι. CNTs having this height or taller can be beneficial insofar as they can give rise to improved field emission properties.

[0043] Of course the growing of the carbon nanotubes can be achieved in any suitable way in accordance with embodiments of the invention. For example, in many embodiments, a catalyst is patterned on to the substrate and facilitates the growth of the carbon nanotubes. As can be appreciated, the catalyst can be patterned in a manner that gives rise to the pattern desired for the finally grown CNTs. In some embodiments, a PECVD SiO ₂ layer is disposed on the substrate. In many embodiments, a diffusion barrier layer separates the substrate from the catalyst (note that in the context of this application, where a catalyst is patterned on to a layer that is layered on the substrate, such as a diffusion barrier layer, the catalyst is still considered to be patterned on the substrate). The diffusion barrier layer can help prevent the substrate from contaminating the growth of the CNTs. In many embodiments, the diffusion barrier layer includes an oxide. In a number of embodiments, the diffusion barrier layer includes aluminum oxide. In several embodiments, the aluminum oxide is present in the form of a layer having a thickness of less than approximately 30 μηι. The catalyst can be any suitable catalyst that can promote the growth of carbon nanotubes. For example, in many embodiments, the catalyst is one of: iron, nickel, copper, chromium, aluminum, and mixtures thereof. The patterning of the catalyst onto the substrate can be accomplished in any suitable way and in any suitable pattern. For example, in many embodiments, a dot pattern is created using a standard lift off process. As alluded to above, in many embodiments, the catalyst is patterned onto the substrate in a dot pattern such that the carbon nanotubes that are thereafter grown are in the form of bundles of carbon nanotubes that each have a diameter of between approximately 1 μηι and 2 μηι, and are spaced apart from one another at a distance of approximately 5 μηι.

[0044] Chemical vapor deposition techniques can be used to grow the carbon nanotubes. Any suitable growth gas can be used in conjunction with the chemical vapor deposition techniques. For example in many embodiments, a hydrocarbon is used. In many embodiments the growth gas is one of: ethylene, acetylene, methane, and mixtures thereof. In many embodiments, the carbon nanotubes are grown at a temperature of between approximately 575 °C and 775 °C. The growth time can vary based upon the desired height of the carbon nanotubes. In many embodiments, the growth time is approximately 30 minutes. In a number of embodiments, the carbon nanotubes are grown such that they have a height of greater than approximately 25 micrometers. In many embodiments, the carbon nanotubes are not grown so tall that their vertical alignment is compromised.

[0045] The process 100 further includes affixing (104) a second substrate onto the free ends of at least a plurality of the grown carbon nanotubes. Where the carbon nanotubes are grown (102) on the first flat substrate such that they are relatively orthogonal to it, this circumstance can facilitate the affixing (104) of the second substrate to the free ends of the grown carbon nanotubes. The affixing (104) of the second substrate can occur using any suitable technique. For example, in many embodiments, the second substrate is welded to the free ends of the grown CNTs. Note that the welded second substrate can comprise any suitable material, including for example, copper, silicon, or titanium. The welding can be conducted so as to rigidly adhere the plurality of carbon nanotubes to the second substrate. In many instances, the welding is done in the absence of any growth gases. For instance, argon may be used to purge the welding region of any growth gases. In many instances, the welding is done in a vacuum. In many embodiments, the welding process can melt the substrate such that the free ends of the carbon nanotubes become embedded within the second substrate to at least some extent. In many embodiments, the welding region is kept at an elevated temperature for between approximately 15 minutes and approximately 30 minutes. In many embodiments, where the substrate includes Titanium, the welding region is elevated to a temperature of approximately 1050 °C. It has been determined that this temperature can be sufficient to cause the desired effect. Accordingly, upon cooling, the substrate and any CNTs that are enveloped by the substrate can be substantially adjoined and can thereafter withstand more rigorous operating environments. For example, in many embodiments, the CNTs are sufficiently adjoined to the substrate such that typical electric fields that the CNTs are subject to during operation are not sufficient to detach the adjoined CNTs from the substrate. For example, in many embodiments, the CNTs are sufficiently adjoined to the second substrate such that an electric field of approximately 3 V/^m is insufficient to detach the adjoined CNTs from the second substrate. In many embodiments, CNTs are adjoined to the substrate to an extent that pressures of approximately 60 kPa are insufficient to detach the CNTs from the substrate. In many embodiments, the CNTs are more rigidly adhered to the second substrate than they are to the first flat substrate (e.g. as measured by force needed to detach the carbon nanotubes from the respective substrate).

[0046] Although the welding of the second substrate to the free ends of the grown carbon nanotubes has been discussed, it should be clear that the second substrate can be affixed to the free ends of the grown carbon nanotubes in any suitable way in accordance with embodiments of the invention. For example, in many embodiments, the second substrate is epoxy-based, and the free ends of the CNTs are bonded to the epoxy-based substrate. For instance, the free ends of the CNTs can be rooted in the epoxy-based substrate, and the epoxy based substrate can be subsequently cured. In this way, the epoxy-based substrate can be affixed (104) to the free ends of the grown CNTs. In a number of embodiments, the free ends of the grown carbon nanotubes are affixed to a second substrate via adhesion. For example, in many embodiments, the second substrate is an adhesive copper tape, and the free ends of the carbon nanotubes are made to 'stick' to the adhesive copper tape. As can be appreciated, even with these alternative affixation methods, it can be desirable to ensure that the carbon nanotubes are sufficiently bonded to the second substrate as described previously (e.g. capable of withstanding 60kPa of applied pressure and/or capable of withstanding 3 V//zm).

[0047] Importantly, in many embodiments of the invention where the CNTs are grown in accordance with a predefined pattern, the pattern of the CNTs is maintained, e.g. the pattern is not disrupted by the affixation 104 process. In many instances to accomplish this, particular attention must be paid to the manner in which the second substrate is affixed to the free ends. For example, the second substrate should be affixed to the free ends in a manner that does not disrupt the desired CNT pattern. In many instances, this implicates the pressure that is applied between the second substrate and the free ends of the CNTs during the affixation process. However, as can be appreciated, the particular requirements are context dependent. For example, where the second substrate is affixed to the free ends of the grown CNTs via welding, the pressure and temperature may be modulated so as to enable the welding without disrupting the grown CNT pattern.

[0048] Note that in many embodiments, the second substrate is conductive. In this way, the resulting structure can properly function as a field emitter.

[0049] The process (100) further includes detaching (106) the first flat substrate from carbon nanotubes. The carbon nanotubes can be detached using any suitable technique in accordance with embodiments of the invention. For example, they can be detached mechanically. As can be appreciated, in many embodiments, the first flat substrate is detached (106) in a manner so as not to disrupt any patterning of the carbon nanotubes. Once detached, the resulting structure includes a plurality of vertically aligned carbon nanotubes adhered to a substrate (the second substrate). In effect, the (patterned) carbon nanotubes have been transferred to the second substrate. Notably, the plurality of carbon nanotubes can be characterized insofar as they relatively uniformly protrude from the substrate in a vertically aligned manner. As can be appreciated, the ends of the carbon nanotubes that were previously attached to the first flat substrate have now become the free ends, and thereby extend from the substrate to the relatively same extent. In this way, the resulting structure can provide more stable emission characteristics. The resulting structure can also be characterized by carbon nanotubes that are rigidly adhered to the second substrate.

[0050] In many embodiments, the process (100) further includes coating (108) the resulting structure via atomic layer deposition to bolster the performance characteristics. For example, the coating layer can serve to better adhere the carbon nanotubes to the underlying substrate. In this way, taller CNTs can be more practicably implemented. Additionally, the coating layer can comprise a material that serves to enhance the emission characteristics. For example, the coating material can serve to make better electrical contact between the substrate and the CNT, lower the work function of the emitter, help prevent movement of the CNTs (even without changing the aspect ratio of the CNTs). Moreover, such coatings can facilitate cathode operation at high temperatures. Importantly, the structure can be coated with any suitable coating layer, including but not limited to: titanium nitride, vanadium nitride, and tungsten.

[0051] Note that while atomic layer deposition has been discussed in the context of carbon nanotubes that have been transferred to a second substrate, atomic layer deposition as described can also be applied to carbon nanotubes that have been grown on a first substrate in accordance with embodiments of the invention. The same above- described benefits can similarly apply to carbon nanotubes that have been grown on a first substrate and have not been transferred to a second substrate. Thus, for instance, FIG. 2 illustrates using an ALD process on carbon nanotubes grown a first substrate. In particular, FIG. 2 illustrates that the process 200 includes growing 202 carbon nanotubes on a first substrate, and thereafter coating 204 the aggregate structure using atomic layer deposition. For example, the structure can be coated with titanium nitride, vanadium nitride, or tungsten in accordance with embodiments of the invention. In general, any suitable coating layer can be applied to the structure in accordance with embodiments of the invention.

[0052] Of course it should be understood that although processes for developing robust field emitters has been described, there exist many variations of the processes, and the above description should be considered as illustrative and not comprehensive. For example, the referenced substrates can include any suitable material, and are not restricted to titanium or any of the other above-mentioned materials. For instance, in many embodiments, the substrate includes a material that has favorable welding characteristics and that does not contaminate the growth of the carbon nanotubes.

[0053] FIGS. 3A-3F schematically illustrate some of the processing steps that can be implemented in accordance with embodiments of the invention. In particular, FIG. 3A depicts the growing of CNTs on a first flat substrate in accordance with embodiments of the invention. In particular, the illustrated structure 300 embodies carbon nanotubes 302 that have been grown on a flat substrate 304. In particular, the carbon nanotubes 302 have been grown in accordance with a predefined pattern. In the illustrated embodiment, the predefined pattern is characterized by the inclusion of a gap. As mentioned above, the flat substrate can be optically flat; additionally, the carbon nanotubes can be grown using any suitable technique in accordance with embodiments of the invention. In many embodiments, the carbon nanotubes 302 are not grown so as to be rigidly adhered to the flat substrate 304 since they will later be detached from the substrate 304. In many embodiments, the carbon nanotubes 302 are grown such that they can be detached with pressures as light as between approximately 20 kPa and approximately 60 kPa, or less. FIGS. 3B and 3C depict the welding of a second substrate 306 to the free end of the carbon nanotubes 302. In particular, FIG. 3B depicts the second substrate 306 being disposed proximate the free ends of the grown vertically aligned CNTs 302, while FIG. 3C depicts that the free ends of the CNTs 302 have been welded to the second substrate 306. As can be seen in the illustration, the welding process can melt the surface of the second substrate 306 and result in an uneven surface. As mentioned above, the second substrate can comprise any suitable material and can be welded to the carbon nanotubes using any suitable technique in accordance with embodiments of the invention. FIG. 3D illustrates detaching the first flat substrate 304 from the carbon nanotubes 302. As mentioned previously, the carbon nanotubes 302 can be detached using any suitable technique; for example they can be mechanically detached. FIG. 3D illustrates that the first flat surface is detached from the carbon nanotubes 302. FIG. 3E illustrates the resulting structure characterized by carbon nanotubes rigidly adhered to the second substrate and further characterized by a uniform tip extension. In this way, the carbon nanotubes can demonstrate improved field emission performance relative to conventionally manufactured carbon nanotube- based field emitters characterized by non-uniform tip extensions.

[0054] FIG. 3F illustrates optionally coating the CNT - welded substrate aggregate via atomic layer deposition. In particular, it is illustrated that the coating 308 is applied relatively uniformly across the carbon nanotubes 302 and the substrate 306. As mentioned previously, this can serve to further adhere the carbon nanotubes to the substrate. Additionally, the coating can also enhance emission characteristics.

[0055] FIGS. 4A-4H illustrate various SEM images of carbon nanotube-based field emitters fabricated in accordance with embodiments of the invention. In particular, FIGS. 4A-4D illustrate a fabricated 5mm x 5mm sample, while FIGS. 4E-4H illustrate a fabricated 2mm x 2mm sample. Note that each depicted strand actually represents a bundle of carbon nanotubes. Also note that the carbon nanotubes are relatively orthogonal relative to the substrate and extend from the substrate surface at a relatively uniform extent. Using conventional techniques, it can be difficult to achieve this level of uniformity. As mentioned previously, the described level of uniformity can provide much improved field emission performance. [0056] FIGS. 5A-5E illustrate the testing of CNT-based field emitters fabricated in accordance with embodiments of the invention. In particular, FIG. 5A illustrates a setup for testing the CNT-based field emitters. More specifically, it is illustrated that the setup 500 includes an aluminum base 502, an adhesive surface 504, a CNT-based field emitter to be tested 506 disposed on the adhesive surface 504, and a corresponding anode 508 spaced apart from the carbon nanotube-based field emitter. The testing apparatus can be characterized by a first distance characterizing the gap in the enclosure within the base 512, and can be further characterized by a second distance between the base 502 and the upper surface of the substrate of the field emitter 514. The testing can take place in a vacuum.

[0057] FIGS. 5B-5E represent field emission data for CNT-based field emitters fabricated in accordance with certain embodiments of the invention. The data was obtained using a testing apparatus in accordance with the one depicted in FIG. 5A. More specifically, the testing apparatus was characterized by an enclosure gap 514 of 1 .80mm, the field emitter 406 had a lateral dimension of 5 mm, while the anode 408 had a lateral dimension of 4 mm. Additionally, the field emitter substrate comprised titanium and copper, the adhesive surface was a copper tape, the anode was gold-coated, and the base included alumina. Accordingly, the data depicted in FIGS. 5B-5E depict turn-on voltage normalized for gap distance for CNT-based field emitters fabricated in accordance with embodiments of the invention. Note that turn on voltages as low as 300V have been achieved.

[0058] Similarly FIGS. 6A-6E depict data pertaining to lifetime tests for CNT-based field emitters fabricated in accordance with embodiments of the invention. In particular, the graphs depict how a relatively constant electric field (indicated by the lower line in each of the graphs) can result in a relatively constant current (indicated by the upper line in each of the graphs) over time - in other words, the resulting current does not degrade over time. Note that FIG. 6E illustrates this achievement even for extended periods of time, e.g. 12 hours. Accordingly, the presented data validates the viability of the structures that can be fabricated according to the techniques disclosed in the instant application. [0059] Although the present invention has been described in certain specific aspects, many additional modifications and variations would be apparent to those skilled in the art. It is therefore to be understood that the present invention may be practiced otherwise than specifically described without departing from the scope and spirit of the present invention. Thus, embodiments of the present invention should be considered in all respects as illustrative and not restrictive. Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their equivalents.