Patterned focal plane arrays of carbon nanotube thin film bolometers with high temperature coefficient of resistance and improved detectivity for infrared imaging

Abstract

A method of preparation of focal plane arrays of infrared bolometers includes processing carbon nanotubes to increase a temperature coefficient of resistance (TCR), followed by patterning to form focal plane arrays for infrared imaging.

Claims

1. A focal plane array for an infrared (IR) microbolometric detector, comprising an infrared-sensitive pixel array comprising of semiconducting single-walled carbon nanotubes (SWNTs) subjected to a physical processing step and a nanotube sidewall chemical functionalization step of covalently bonding an organic functional group to the sidewall, thereby increasing a temperature coefficient of resistance (TCR) for semiconducting SWNTs pixels by from about 1% to about 5% per degree C.

2. The focal plane array of claim 1, wherein the physical processing step comprises two or more of ultrasonication, shear mixing, and thermal annealing in vacuum.

3. The focal plane array of claim 2, wherein the semiconducting SWNTs sidewall chemical functionalization step comprises one or more of: a diazonium reaction; non-covalent or covalent attachment of one or more functional groups selected from octadecylamine groups, poly(m-aminobenzenesulfonic acid) groups, polyethyleneglycol groups, and amino phenyl functional groups; an organometallic complexation; a Diels-Alder reaction; a free radical reaction; a Birch reaction; a gas phase reaction; a carbene reaction; a nitrene reaction; and doping or compensation for a natural p-doped state of the carbon nanotubes.

4. The focal plane array of claim 3, wherein the diazonium chemistry functionalization step comprises reacting the semiconducting SWNTs provided as a thin film or as a dispersion of semiconducting SWNTs with a diazonium salt.

5. The focal plane array of claim 4, wherein the diazonium salt is selected from the group consisting of bromobenzene diazonium salt, nitrophenyl diazonium salt, and methoxy diazonium salt.

6. The focal plane array of claim 1, including semiconducting SWNTs having a degree of separation from metallic SWNTs in a range of from about 80% to about 100%.

7. The focal plane array of claim 4, wherein the chemical functionalization is conducted on the dispersion of semiconducting SWNTs in organic solvents or water.

8. The focal plane array of claim 7, comprising a carbon nanotube semiconducting SWNT thin film having a pattern of carbon nanotube thin film pixels, wherein the pattern is provided by airbrushing carbon nanotube dispersions in organic solvents over precision shadow masks.

9. The focal plane array of claim 4, wherein the chemical functionalization is conducted on the semiconducting SWNT thin film.

10. The focal plane array of claim 4, wherein the chemical functionalization is conducted on a suspended semiconducting SWNT thin film.

11. The focal plane array of claim 4, wherein the chemical functionalization is conducted on a patterned semiconducting SWNT thin film.

12. The focal plane array of claim 11, wherein the patterned carbon nanotube thin film is provided by removal of selected portions of the semiconducting SWNT thin film by a focused laser beam to provide a pattern of semiconducting SWNT thin film pixels.

13. A focal plane array for an infrared (IR) microbolometer detector, comprising an infrared-sensitive pixel array comprising semiconducting single-walled carbon nanotubes (SWNTs) subjected to a physical processing step and a nanotube sidewall chemical functionalization step of covalently bonding an organic functional group to the sidewall, thereby increasing a temperature coefficient of resistance (TCR) for semiconducting SWNTs pixels by from about 1% to about 5% per degree C.; further wherein the semiconducting SWNTs are subjected to: i) ultrasonication; ii) shear mixing; iii) the sidewall chemical functionalization step; and iv) thermal annealing in vacuum.

14. The focal plane array of claim 12, wherein the focused laser beam has a laser emission wavelength selected from the group consisting of: a near-infrared spectral range, a visible spectral range, and an ultra-violet spectral range.

15. The focal plane array of claim 2, wherein the nanotube sidewall chemical functionalization step comprises a gas phase reaction performed in a vacuum, the gas phase reaction generating radical species that attach directly to the semiconducting SWNT sidewall.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The accompanying drawing figures incorporated herein and forming a part of the specification, illustrate several aspects of the disclosed methods and devices, and together with the description serve to explain certain principles thereof. In the drawings:

(2) FIG. 1 shows TCR values at 300K for different types of unsorted functionalized SWNTs;

(3) FIG. 2 shows TCR (300K) values for thin films of sorted semiconducting SWNTs of different degree of semiconducting type purity;

(4) FIG. 3A shows temperature dependence of resistance for films of unsorted and SC-SWNTs after annealing in vacuum at 380 K and 470 K;

(5) FIG. 3B shows TCR values at 300K for films of unsorted and SC-SWNTs after annealing in vacuum at 380 K and 470 K;

(6) FIG. 4A shows effect of prolonged (24 hours) ultrasonic treatment combined with vacuum annealing on temperature dependence of resistance for the films of SC-SWNTs;

(7) FIG. 4B shows effect of prolonged (24 hours) ultrasonic treatment combined with vacuum annealing on TCR values at 300K for the films of SC-SWNTs;

(8) FIG. 5A shows effect of combination of different types of physical treatment sonication, shear mixing and annealing on temperature dependence of resistance for the films of SC-SWNTs;

(9) FIG. 5B shows effect of combination of different types of physical treatment sonication, shear mixing and annealing on TCR values at 300K for the films of SC-SWNTs;

(10) FIG. 6 shows synthesis of octadecylamine functionalized SWNTs (SWNT-ODA);

(11) FIG. 7 shows synthetic procedure for the preparation of SWNT-PABS at 10-g scale;

(12) FIG. 8 shows structure of SWNT-PEG;

(13) FIG. 9 shows structure of SWNT-CONH.sub.2;

(14) FIG. 10 shows structure of SWNT-Ph-NH.sub.2;

(15) FIG. 11 shows chemical structure of nitro-phenyl functionalized SWNTs;

(16) FIG. 12A shows: temperature dependence of resistance of thin films of nitrophenyl functionalized SC-SWNTs (SC-SWNT-NP) annealed at 380 K and 470 K for a SC-SWNT-NP film annealed in vacuum at 380 K and 470 K;

(17) FIG. 12B shows: TCR values at 300 K for a SC-SWNT-NP film annealed in vacuum at 380 K and 470 K;

(18) FIG. 13A shows evaluation of TCR values of SC-SWNT films prepared utilizing physical processing and NP-functionalization at different annealing temperatures;

(19) FIG. 13B shows evaluation of TCR(300K) values of SC-SWNT films prepared utilizing physical processing and NP-functionalization at different annealing temperatures;

(20) FIG. 14 shows chemical structure of bromo-phenyl functionalized SWNTs;

(21) FIG. 15A presents graphical evaluation of TCR at 300K of functionalized S99-SWNT-Br film after annealing at 370. Starting S99-SWNT material was shear-mixed and sonicated before the functionalization;

(22) FIG. 15B presents graphical evaluation of TCR at 300K of functionalized S99-SWNT-Br film after annealing at 470K. Starting S99-SWNT material was shear-mixed and sonicated before the functionalization;

(23) FIG. 16A shows TCR evaluations of SC-SWNT material processed by combined sonication and shear-mixing treatment;

(24) FIG. 16B shows TCR evaluations of SC-SWNT material processed by combined sonication and shear-mixing treatment and SC99-SWNT-MeO-Phenyl functionalized material;

(25) FIG. 17A presents an evaluation of TCR obtained for the shear-mixed SC-SWNT material NP-functionalized in a solid-state film form;

(26) FIG. 17B presents an evaluation of resulting TCR values obtained for the shear-mixed SC-SWNT material NP-functionalized in a solid-state film form;

(27) FIG. 18 shows two complementary high precision masks for air brush based on patterning of 60 pixels arrays of SWNT thin film bolometers;

(28) FIG. 19A shows results of high temperature air brushing: SWNT pixels shifted from gold electrodes due to strong thermal expansion mismatch of mylar and nickel mask;

(29) FIG. 19B shows results of room temperature air brushing: uniform SWNT pixel deposited over gold electrodes;

(30) FIG. 19C shows another example of results of room temperature air brushing: uniform SWNT pixel deposited over gold electrodes;

(31) FIG. 20 shows a microscope image and optical transmittance scan of the SWNT thin film sample patterned by air-brushing according to FIGS. 19A-19C;

(32) FIG. 21A shows SWNT-ODA continuous thin film transferred over mylar film with pre-deposited gold interconnects;

(33) FIG. 21B shows laser patterning of SWNT thin films prepared by thin film transfer;

(34) FIG. 22 shows a microscope image and optical transmittance scan of the SWNT thin film sample patterned by laser cutting according to FIGS. 21A-21B;

(35) FIG. 23 shows visible microscopy of laser patterning with reducing feature size from 100 m (Left) to 50 m (Right) channels using advanced laser technology. Overlaying line traces show results of optical transmittance scans across the patterns;

(36) FIG. 24A shows continuous SWNT thin film transferred on a substrate with rectangular shape trenches of 100 m side size;

(37) FIG. 24B shows effects of laser cutting of the SWNT film of FIG. 24A to form a ten-pixel linear array of SWNT thin film bolometric detectors suspended over trenches and individually addressed by gold interconnects;

(38) FIG. 25A shows a microscope image of individual pixel of SWNT thin film IR bolometric detector of characteristic size of 100 m;

(39) FIG. 25B shows the response for SC-SWNT thin film bolometer before chemical functionalization to IR radiation of incident power 4.7 W in mid-wave infrared (MWIR) spectral range;

(40) FIG. 25C shows the response to the same incident radiation power of similar SWNT bolometric detector after chemical functionalization utilizing diazonium chemistry with bromophenyl group. After chemical functionalization the amplitude of the photoresponse increased 2000 times from 2.5 W to 53 mV and the signal-noise ratio increased 150 times from 16 to 2400;

(41) FIG. 26A shows the testing of the pyroelectric detector DSS-LT020A/BAF2 from HORIBA JOBIN YVON in MWIR spectral range under incident power of 4.7 W;

(42) FIG. 26B shows 3 mV signal;

(43) FIG. 26C shows noise of 5-7 V with resulting signal to noise ratio (S/N) of 500; and

(44) FIG. 27 shows the response to the same MWIR radiation power of the SWNT-based microbolometer prepared from SC-SWNTs after optimized physical processing and diazonium (bromophenyl) functionalization showing S/N=2400, which is more than 3-4 times better than for a commercial pyroelectric detector.

(45) Reference will now be made in detail to embodiments of the disclosed methods and devices, examples of which are illustrated in the accompanying drawing figures.

DETAILED DESCRIPTION

(46) Embodiments of the present disclosure provide for a method of modifying carbon nanotubes (CNTs) by one or both of chemical functionalization and physical processing in order to achieve high temperature coefficient of resistance (TCR) of CNT thin films for their applications as active sensitive elements of infrared microbolometers for focal plane arrays (FPAs) for infrared (or thermal) imaging.

(47) The existing data on thin films of unsorted SWNTs, i.e. SWNT bulk material which consist of a mixture of semiconducting (SC) and metallic (MT) SWNTs in typical statistical ratio SC:MT=2:1 show TCR values in the range 0.05 to 0.3. TCR values on chemically functionalized unsorted SWNTs are presented in FIG. 1 and are in the range 0.2-0.7% thus showing only marginal increase of TCR.

(48) Application of separated SC-SWNTs in place of non-sorted (or unsorted) SWNTs (NS- or UNS-SWNTs) typically containing a mixture of MT- and SC-SWNTs in a ratio 1:2 was expected to result in high TCR values. However, thin films of separated semiconducting SWNTs show TCR value in the range 0.2-0.4% even for very high degree of separation of SC-SWNTs of 99.7% (FIG. 2).

(49) Physical treatment in a form of vacuum annealing of SC-SWNT thin films leads to only small increase of TCR from 0.28 to 0.3% (FIG. 3).

(50) Physical treatment in a form of extended bath sonication does not lead to increasing TCR above 0.3% (FIG. 4). Combination of extended bath sonication with annealing of SC-SWNT film in vacuum leads to a small increase of TCR to 0.4% which is not sufficient for bolometric detector applications (FIG. 4).

(51) Embodiments of the disclosure provide a combination of different types of physical treatment as a method for increasing TCR of films of semiconducting SWNTs. For example, the combination of ultrasonic bath treatment of SWNT dispersions, shear-mixing of SWNTs dispersion, and vacuum annealing of SWNT films made of shear-mixed and ultrasonicated SC-SWNTs may lead to TCR reaching but not limited to a value of 2.4% (FIG. 5) comparable to TCR values of VOx which currently dominates the FPA market.

(52) Other embodiments of the disclosure introduce different chemistries of SC-SWNT walls, such as octadecylamine (ODA) functionalization (FIG. 6), PABS functionalization (FIG. 7), polyethyleneglycol functionalization (FIG. 8), amide functionalization (FIG. 9), and amino phenyl functionalization (FIG. 10) as a method to achieve high TCR.

(53) According to other embodiments, solution phase diazonium reactions may be applied to SC-SWNTs to achieve high TCR. Diazonium salts with different substituents on the aromatic ring exploring the different charge transfer ability of the substituents may be applied. As an example, nitrophenyl (NP) functionalization (FIG. 11) may be applied to SC-SWNTs and TCR of 1.5% or higher may be achieved (FIG. 12). This NP functionalization of semiconducting SWNTs results an improvement in TCR values which become comparable to TCR values of vanadium oxide material currently dominating the FPA market.

(54) According to another embodiment, combination of shear-mixing and sonication may be applied to SC-SWNT material following the nitro-phenyl functionalization procedure. The resulted thin films of this material may be annealed in vacuum and high TCR values of 4% (FIG. 13), higher than typical TCR values of vanadium oxide bolometers currently dominating the FPA market, may be achieved.

(55) According to another embodiment, diazonium functionalization with bromo-phenyl substituent may be applied to SC-SWNT material (FIG. 14), for example, processed by a combination of prolonged sonication and shear mixing. This type of functionalization may allow achieving TCR as high of 4.1% (FIG. 15) exceeding typical TCR values of vanadium oxide bolometers currently dominating the FPA market.

(56) According to another embodiment, diazonium functionalization with methoxy substituent may be applied to SC-SWNT material, for example, processed by a combination of prolonged sonication and shear mixing. This type of functionalization may allow achieving TCR as high as 4.65% (FIG. 16) exceeding typical TCR values of vanadium oxide bolometers currently dominating the FPA market.

(57) According to another embodiment, chemical functionalization may be applied to the films of SC-SWNT material, i.e. to SC-SWNT material in solid state, for example, processed by a combination of prolonged sonication and shear mixing. This solid-state functionalization is an efficient alternative to the solution state functionalization procedure and may allow achieving TCR of 2.8% and higher (FIG. 17) compatible with state of the art FPA technology. Examples of such solid-state functionalization include, but not limited by diazonium chemistry with a number of different substitutes including, but not limited by nitrophenyl, bromine phenyl or methoxy phenyl substitutes.

(58) Patterning of CNT FPAs, according to embodiments of the present disclosure, may be achieved by air-brushing technique in which CNTs are deposited from CNT dispersions in organic solvents onto FPA platform through precision shadow masks (FIG. 18-20).

(59) According to another embodiment, patterning of CNT FPAs may be achieved by laser cutting of CNT thin films utilizing focused laser beam with spatially controlled positioning with lasers operating on infrared, visible or UV emission wavelength (FIG. 21-23).

(60) In one embodiment, the CNT thin films are comprised of random network of CNTs. In another embodiment, the thin films are comprised of aligned CNTs with the nanotubes aligned parallel to the electrodes. According to another embodiment, the thin films are comprised of aligned CNTs with the nanotubes aligned perpendicular to the electrodes.

(61) The following examples illustrate embodiments of the disclosure, but should not be viewed as limiting the scope thereof.

Example 1: TCR Values of Thin Films of Unsorted Functionalized SWNTs and Sorted SC-SWNTs of Different Semiconducting Purities

(62) The existing data on thin films of unsorted SWNTs, i.e. SWNT bulk material which consist of a mixture of semiconducting (SC) and metallic (MT) SWNTs in typical statistical ratio SC:MT=2:1, show TCR values in the range 0.05 to 0.3. TCR values on chemically functionalized unsorted SWNTs are presented in FIG. 1 and are in the range 0.2-0.7% thus showing only marginal increase of TCR.

(63) Room temperature TCR values for sorted semiconducting SWNTs (SC-SWNTs) were evaluated in SWNT materials with varying degree of separation of semiconducting and metallic SWNTs:

(64) 1) NS-SWNTsUnsorted (67% semiconducting/33% metallic) SWNTs;

(65) 2) SC95-SWNTs(95% semiconducting/5% metallic) sorted SWNTs;

(66) 3) SC99-SWNTs(99% semiconducting/1% metallic) sorted SWNTs;

(67) 4) SC99.7-SWNTs(99.7% semiconducting/0.3% metallic) sorted SWNTs; In addition, TCRs of thin films of smaller diameter SWeNT SG 65 (6,5)-SWNTs were evaluated. The SWNTs had an average diameter of 0.8 nm and contain more than 90% of semiconducting SWNTs (less than 10% metallic). These SWNTs have a larger energy gap of 1.25 eV in the electronic density of states which in the case of conventional semiconductors would lead to an increased TCR.

(68) The SWNT films of the above materials were in situ annealed in vacuum at 110 C. and their TCRs were measured without exposure to atmosphere. The results are presented in FIG. 2.

(69) The TCR(300K) is slightly higher in the case of the SG65 film (TCR 0.35%) corresponding to the larger semiconducting energy gap, while no significant dependence was observed for different degrees of separation of semiconducting SWNTs of larger diameter with low TCR values in the range 0.25-0.3%.

Example 2: Effect of Vacuum Annealing on TCR of SWNT Films

(70) Thin films of SWNTs of two different types of SWNT material were explored:

(71) 1) Nonsorted SWNTs (NS-SWNTs);

(72) 2) 99% semiconducting SWNTs (S99).

(73) Effect of annealing in vacuum at 380 K and 470 K was explored. FIG. 3 summarizes the results of this study.

(74) For thin films of unsorted SWNT films the resistance increases by 2 times while the TCR at 300K remains low 0.13% despite the annealing. For thin films of SC-SWNTs one order of magnitude increase of the film resistance was observed, but the increase of TCR was very small: from 0.28 to 0.3% at 300K. Thus, physical treatment in the form of vacuum annealing is not able to materially improve the TCR.

Example 3: Effect of Combination of Ultrasonic Bath Treatment and Vacuum Annealing on TCR of SWNT Films

(75) FIG. 4 shows a set of data for S99%-SWNTs prepared utilizing prolonged ultrasonic bath treatment for 24 hours. The resistance of the film is 3 times higher than for S99%-SWNT film prepared without extended sonication, but the TCR values did not change significantly remaining in the vicinity of 0.3%. Increasing the annealing temperature to 470 K led to an increased resistance and an insignificant increase of TCR to 0.4%. Thus, physical treatment in the form of prolonged ultrasonication even combined with vacuum annealing is not sufficient for improving the TCR.

Example 4: The Effect of a Combination of Ultrasonic Bath Treatment, Shear-Mixing and Vacuum Annealing on TCR of SWNT Films

(76) Sonication of SC-SWNT dispersions in water was combined with shear mixing in the form of four 10 minute shear mixing cycles inserted within the 24 hour ultrasonic bath treatment. The resistance of the films increased by several orders of magnitude, especially, at low temperatures (FIG. 5A). The TCR value at 300K exceeded 0.8% after additional annealing at 380 K (FIG. 5B).

(77) Next, a vacuum annealing at higher temperature 470K was applied to the same SC-SWNT films prepared by a combination of sonication and shear mixing.

(78) A strong increase of resistance was observed; more importantly, TCR at 300K increases from 0.8% to 1.5% as shown in FIG. 5. Further increasing the number of shear-mixing and sonication cycles before the film preparation lead to TCRs as high as 2.4%.

Chemical Functionalization of SWNTs

Example 5: Synthesis of ODA Functionalized SWNT (SWNT-ODA)

(79) Present example (FIG. 6) shows the one-step reaction of SWNT-COOH with ODA in dimethylformamide (DMF) using N,N-dicyclohexylcarbodiimide (DCC), as the coupling agent.

Example 6: Synthesis and Purification of PABS Functionalized Unsorted SWNTs

(80) In the present example, the synthesis of SWNT-PABS material at the 10 g scale is described (FIG. 7). Preparation of polyaminobenzoic acid (PABS): In a 1 L round bottom flask (vessel #1), 13.8 g of the monomer, animobenzene sulfonic acid (ABS) was dispersed at RT in 480 mL of 1M HCl; then 1.6 mL of aniline was added and stirred at RT for about 30 min. In a 250 mL conical flask (#2), 27 g of (NH.sub.4).sub.2S.sub.2O.sub.8 was dissolved in 100 mL of 1M HCl. The mixture in the flask #1 was cooled to 0 C. and the oxidant from flask #2 was slowly added by plastic pipette (45 min). After addition, the resulting mixture was stirred at 0 C. for 6 h, then at room temperature overnight. The following day, 480 mL of solvent was removed on a rotary evaporator (the water bath 35 C.), the residue was cooled in ice and filtered (Fluoropore, 0.22 m). The flask was washed with a small amount of HCl to give the product as a black-brown precipitate, which was washed 3-4 times with acetone and dried in a desiccator. Typical yield is 70-80%.

(81) Preparation of SC-SWNT-PABS. In the next step, 10 mg of the PABS was mixed with 5 mL of SC-99 suspension containing 0.05 mg of SC-SWNTs. This mixture was stirred for 20 hrs at room temperature in order for the PABS to fully access the SWNTs. The resulting mixture was filtered with an alumina filter and washed with water until the filtrate became colorless. The SC-SWNT-PABS film which formed on the filtration membrane was used for the TCR evaluation.

(82) The mid-IR spectra confirmed the formation of a covalent bond between the SWNT and PABS: the spectra show a stretching vibration at 1650 cm.sup.1 due to carbonyl in the amide groups, peaks in the range of 2850-2920 cm.sup.1 corresponding to the aromatic CH stretching vibrations and a peak at 695 cm.sup.1 due to SO stretch. The thermo gravimetric analysis (TGA) showed the weight loss profile and the metal content in the SWNT-PABS materials from which the loading of the PABS was estimated to be 66% and the metal residue was estimated to be 2.0%.

Example 7: Synthesis of Polyethyleneglycol-600 Functionalized SWNTs (SWNT-PEG)

(83) 1 g of SWNT-COOH was dried for 2 h at 100 C. and sonicated in 750 mL anhydrous DMF for 4 h and homogenized for 30 minutes. The nanotubes dispersion was stirred overnight at room temperature under Ar atmosphere, cooled to 0 C. and 20-25 mL of oxalyl chloride was slowly added. The reaction mixture was additionally stirred for 1 h at 0 C. and for 1 h at room temperature and then heated and stirred at 70 C. under a flow of Ar overnight to remove the excess oxalyl chloride. After this step the mixture was cooled to room temperature under Ar and 10.0 g of PEG-600 was added and the mixture heated and stirred at 120 C. for 5 days.

(84) The resulting solution was cooled to room temperature, filtrated on a 90 mm 0.2 m Fluoropore filter membrane and washed by soxhlet extraction with DMF until the filtrate was colorless. Washing continued with DI water until the filtrate was colorless. The resulting material was filtered on a 90 mm 0.2 m Durapore membrane and the black solid was dried at room temperature under vacuum in a desiccator. The dried material was ground and washed additionally by soxhlet with DI water, and dried again in a desiccator under vacuum. The typical yield is 110%. The SWNT-PEG (FIG. 8) was characterized using AFM, mid IR spectroscopy, near-infrared (IR) spectroscopy and TGA analysis.

Example 8: Synthesis of Amide Functionalized SWNTs (SWNT-CONH.SUB.2.)

(85) The synthesis of SWNT-CONH.sub.2 was carried out using the same oxalyl chloride addition procedure as explained for SWNT-PEG but the functionalization was performed by purging the suspension with ammonia gas at 0 C. for 4 h. The resulting solution was warmed to room temperature, filtered and washed by soxhlet extraction with DMF and DI water until the sample was clean. The resulting black solid was dried at room temperature under vacuum in a desiccator. Typical yield is 100%. The final SWNT-CONH.sub.2 material (FIG. 9) was characterized using AFM, mid IR spectroscopy, near-infrared (IR) spectroscopy and TGA analysis.

Example 9: Synthesis of Amino Phenyl Functionalized SWNTs (SWNT-pH-NH.SUB.2.)

(86) 2.75 g of SWNT-COOH was dried at 100 C. and sonicated in 450 mL DMF for 4 h and homogenized for 1 h. This mixture was purged with Ar overnight and protected from light during further reaction. 8.7 g of tetra butyl ammonium fluoroborate and 1.4 g of nitrophenyl diazonium fluoroborate were added to this mixture and stirred for 24 h and sonicated for another 30 min. The nitrophenyl functionalized SWNTs were filtered and soxhlet extracted with DMF and reduced to the aminophenyl derivative by heating with Na.sub.2S at 60 C. for 20 h. The final SWNT-ph-NH.sub.2 material (FIG. 10) was filtered and washed by soxhlet extraction with water and acetone and characterized by spectroscopic methods.

Example 10: Nitro-Phenyl Functionalization of SC-SWNTs

(87) Diazonium reactions can be applied to SC-SWNTs utilizing a variety of diazonium salts with different substituents on the aromatic ring allowing the introduction of substituents with a range of charge transfer properties. Nitro-, Bromo-, and Methoxy-substituted aryl groups were explored. The nitro-phenyl (NP) diazonium salt was obtained from Sigma-Aldrich. The nitro-phenyl diazonium salt (0.007 g) was added to a suspension of SWNTs (0.08 mg) in acetonitrile (8 mL). The mixture was stirred for 30 min at room temperature. The color of the suspension changed from pink to green indicating the chemical modification of SWNTs. After 30 min, 15 mL of acetone was added to precipitate the SWNTs and the dispersion filtered and the resulting solid washed with acetone and water. This NP modified material (FIG. 11) was then re-dispersed in DMF and filtered on an alumina membrane. The resulted SC-SWNT-NP film was used for TCR evaluation.

Example 11: TCR of Films of Nitro-Phenyl Functionalized SC-SWNTs

(88) Nitro-phenyl functionalized SC-SWNT-NP material showed much higher resistance values (1-10 MOhm at 300K) than the SC-SWNT-PABS material and much stronger R(T) dependence with R(80K)/R(300K)>100 as can be seen in FIG. 12. Annealing at 380K resulted in a TCR=1.46% (FIG. 12B), matching the best value achieved for SC-SWNT material by application of shear-mixing and sonication (FIG. 5). In contrast to the case of SC-SWNT-PABS material the high temperature annealing of S-SWNT-NP film at 470K resulted in an overall decrease of resistance by 10 times and a decrease of TCR(300K) to 1.08% (FIG. 12B).

(89) Thus, NP functionalization of semiconducting SWNTs results in an improvement in TCR values as compared to previously explored types of chemistry.

Example 12: Combination of Physical Processing and Nitro-Phenyl Functionalization of SC-SWNTs Resulted in Improving TCR

(90) NP functionalization was applied to the SC-SWNT material processed by a combination of prolonged sonication and shear mixing. SC-SWNTs were shear-mixed 8 times for 10 min each and sonicated for 24 h in total. This shear mixed SC-SWNT material was utilized for NP modification. The SWNT to NP ratio was varied from 0.04 mg SWNT: 3 mg NP to 0.04 mg SWNT: 7 mg NP.

(91) This shear-mixed, sonicated and NP modified material was re-dispersed in DMF and filtered on an alumina membrane. Different annealing temperatures were applied before TCR evaluation. FIG. 13 shows TCR as high as 4.0% achieved utilizing a combination of physical treatment and chemical modification.

(92) Thus, the combination of physical processing and nitro-phenyl functionalization of SC-SWNTs may allow the attainment of TCR values as high as 4.0% exceeding typical TCR values of vanadium oxide materials which currently dominate the market for uncooled FPAs.

Example 13: Bromo-Phenyl Functionalization of SWNTs

(93) The use of diazonium salts with a variety of different substituent on the aromatic ring allows the exploration of effect of the electronic character of the substituent on the device performance. In the present case the bromine atom is -electron donating as opposed to the electron withdrawing nature of the nitro group. Of course other factors such as steric hindrance due to the size of the groups and the electronic state of the reacting nanotubes may play a role in these reactions. The resulting SWNT-bromo-phenyl complex is presented schematically in the FIG. 14.

(94) Extended sonication and shear-mixing were applied to the starting SC-SWNTs material and the chemical procedure was as follows. Briefly, 2.4 mg of 4-bromo benzene diazonium salt was added to a 25 mL acetonitrile suspension of S-99 SWNTs (0.025 mg). The mixture was stirred for 30 min at room temperature. The color of the suspension changed to light yellow indicating the chemical modification of SWNTs. After 30 min, 5-10 mL of acetone was added to precipitate the SWNTs and the solid was separated by filtration and washed with acetone and water. The bromo-phenyl modified SWNT material was then re-dispersed in water and filtered on an alumina membrane. The resulted SC-SWNT-Br-Phenyl film was used for TCR evaluation.

(95) The temperature dependence of the resistance was measured on two different SC99-SWNT-Br-Phenyl channels one of which was pre-annealed in vacuum at 200 C for 7 hours. With increasing annealing temperature, the TCR at 300K increased from 3.0 to 4.1% (FIG. 15).

(96) Thus, the bromine substituted diazonium functionalization of SC-SWNTs combined with physical processing allows the development of a material with a TCR as high of 4.1% exceeding the typical TCR values of vanadium oxide materials which currently dominate the market of uncooled FPAs.

Example 14: Methoxy-Phenyl Functionalization of SWNTs

(97) Methoxy-phenyl functionalization was carried out in a similar manner to that of NP functionalization; the MeO substituent is -electron donating in nature. Briefly, 2.6 mg of 4-methoxy benzene diazonium salt was added to a 25 mL acetonitrile suspension of SC-99 SWNTs (0.025 mg). The mixture was stirred for 30 min at room temperature. The color of the suspension changed from pink to pink-orange indicating the chemical modification of SWNTs. After 30 min, 5-10 mL of acetone was added to precipitate the SWNTs and the solid was filtered and washed with acetone and water. This methoxy-phenyl modified material was then re-dispersed in DMF and filtered through an alumina membrane. The resulting SC-SWNT-MeO-Phenyl film was used for TCR evaluation.

(98) FIG. 16 compares TCR values of SC-SWNT material processed by combined sonication and shear-mixing treatment and SC99-SWNT-MeO-Phenyl functionalized material and shows that TCR significantly increases from 1.95 to 4.65% indicating high potential of this new type of chemical functionalization for FPA applications.

Example 15: Chemical Functionalization Applied to the Films of SC-SWNTs

(99) Nitrophenyl functionalization was applied directly to films of SC-SWNT material processed by a combination of prolonged sonication and shear mixing. After the functionalizations, 3 regimes of vacuum annealing at increasing temperature from 100 C. to 200 C. were applied to different films and the temperature dependence of resistance and TCR of the films were measured after each annealing. FIG. 8 shows that the resistance of the films increases by more than order of magnitude with increasing annealing temperature. FIG. 17 shows an increasing slope of R(T) and TCR values which reach 2.8% with increasing annealing temperature.

(100) Thus, solid state functionalization is an efficient alternative to the solution state functionalization procedure and may allow TCR values of 2.8% and higher.

Example 16: SWNT Thin Film Array Patterning by Air-Brushing

(101) An array of 60 SWNT thin film pixels of dimension 100100 m.sup.2 was fabricated on a mylar substrate by using an air-brushing technique. The complete array consisted of 6 linear ten pixels arrays oriented in two orthogonal directions and distributed over an area of 4040 mm.sup.2. Two different high precision nickel masks with complementary patterns were utilized (FIG. 18). The first mask was utilized for e-beam deposition of gold interconnecting lines addressing individual pixels; it featured 100 m gaps between the gold electrodes. The second matching mask had 300 m100 m apertures for SWNT air-brushing.

(102) For air-brushing the SWNTs were dispersed in an organic solvent using am ultrasonic bath. Typically, the substrate for air-brushing is heated to promote efficient solvent evaporation. In the case of the patterned array, when air brushing was applied to a mylar film heated to 110 C., the mismatch in the thermal expansion coefficients of the nickel mask and the mylar film complicates the procedure. This mismatch may interfere with the registry between the deposition of SWNT pixel and the gold electrodes as shown in FIG. 19A. To avoid this mismatch the air brushing may be applied to mylar film held at room temperature; in this case the SWNT pixels match positions of the addressing gold interconnects as shown in FIG. 19 B, C for two orthogonal orientations of the linear arrays.

(103) The quality and resolution of the air-brush patterning was evaluated by scanning the optical transmittance across the linear array of the SWNT pixels. FIG. 20 shows spatial modulation of optical transmittance which is periodic in shape and depth thus confirming the feasibility of patterning utilizing air-brush techniques.

Example 17: Laser Beam Patterning of Continuous SWNT Thin Film

(104) For laser patterning SWNT thin films were transferred onto a mylar film with pre-deposited gold interconnects as shown in FIG. 21A.

(105) The results of patterning a continuous SWNT thin film using a focused laser beam are presented in FIG. 21B. The laser wavelength may be in the infrared, visible or UV spectral range dependent on substrate type and the feature size of the patterning. The top of the 10 pixels in FIG. 21B have a common electrode and do not need to be separated. The 10 interconnects addressing the bottoms of each pixel are separated by the laser beam patterning and the shape of each pixel is well defined as confirmed by an optical transmittance scan in FIG. 22.

(106) The patterning feature size utilizing the laser cutting technique can be reduced to sub-20 m feature size compatible with state of the art FPAs. As an example, FIG. 23 shows laser patterned stripes in a SWNT film with feature size reduced from 100 m to 50 m size with very uniform and homogeneous pattern of the SWNT coating.

Example 18: Laser Beam Patterning of Suspended Pixels of SWNT Thin Films

(107) A linear pattern of 10 trenches of square shape with 100 m sides and a depth of 20 m can be etched in quartz or Si/SiO.sub.2 substrates. A continuous SWNT thin film was transferred onto the substrates over the trenches area (FIG. 24A). Laser patterning was applied to form 10 pixel arrays of SWNT thin films bolometric detectors suspended across the trenches (FIG. 24B). Individual pixels were addressed by a pattern of gold interconnects deposited by e-beam.

Example 19: Improved Performance of SWNT Thin Film Detector after Chemical Functionalization of SC-SWNTs

(108) The bolometric response of individual pixels of SWNT thin film detectors was compared before and after chemical functionalization of SC-SWNTs. In both cases SWNT thin film sensitive element was suspended across the etched trench of 100 m size as shown in FIG. 25A. The bolometric response of the SC-SWNT thin films before chemical functionalization to IR radiation of incident power 4.7 W in mid-wave infrared (MWIR) spectral range of wavelengths 3-5 m is presented in FIG. 25B. The response to the same incident radiation power of a similar SWNT bolometric detector after chemical functionalization utilizing diazonium chemistry with the bromophenyl group is shown in FIG. 25C. FIG. 25 shows that after chemical functionalization the amplitude of the photoresponse increased by 2000 times from 2.5 W to 53 mV and signal-to-noise ratio increased by 150 times from 16 to 2400.

Example 20: Comparison of the Performances of SWNT Thin Film Detector with Commercial Pyroelectric Detector

(109) The performance of SWNT thin film microbolometer was compared with the performance of a commercial pyroelectric detector DSS-LT020A/BAF2 from HORIBA JOBIN YVON. The specifications of the pyroelectric detector quote a noise equivalent power of NEP <110.sup.9 W/Hz.sup.1/2. For this experiment the area of pyroelectric detector was matched to the area of a 100100 m.sup.2 SWNT-based microbolometer by using a diaphragm to equalize the incident radiation power for the detectors to 4.7 W in the MWIR spectral range. FIG. 26 shows the results of the test of the pyroelectric detector which give S/N=660.

(110) FIG. 27 shows the response to the same MWIR radiation power of SWNT-based microbolometer prepared from SC-SWNTs after optimized physical processing and diazonium (phenylbromo) functionalization showing S/N=2400, which is more than 3-4 times better than the commercial pyroelectric detector. Thus, under closely matched conditions, the SWNT-based detector disclosed herein matches and even exceeds the performance of a commercial pyroelectric detector.

(111) The foregoing has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the embodiments to the precise form disclosed. Obvious modifications and variations are possible in light of the above teachings. All such modifications and variations are within the scope of the appended claims when interpreted in accordance with the breadth to which they are fairly, legally and equitably entitled.

REFERENCES

(112) Delamar, M.; Hitmi, R.; Pinson, J.; Saveant, J. M., Covalent Modification of Carbon Surfaces by Grafting of Functionalized Aryl Radicals Produced from Electrochemical Reduction of Diazonium Salts. J. Am. Chem. Soc. 1992, 114, 5883-5884. Liu, Y. C.; Mccreery, R. L., Reactions of Organic Monolayers on Carbon Surfaces Observed with Unenhanced Raman-Spectroscopy. J. Am. Chem. Soc. 1995, 117, 11254-11259. Bahr, J. L.; Yang, J.; Kosynkin, D. V.; Bronikowski, M. J.; Smalley, R. E.; Tour, J. M., Functionalization of Carbon Nanotubes by Electrochemical Reduction of Aryl Diazonium Salts: A Bucky Paper Electrode. J. Am. Chem. Soc. 2001, 123, 6536-6542. Bahr, J. L.; Tour, J. L., Highly Functionalized Carbon Nanotubes Using in Situ Generated Diazonium Compounds. Chem. Mater. 2001, 13, 3823-3824. Bekyarova, E.; Itkis, M. E.; Ramesh, P.; Berger, C.; Sprinkle, M.; de Heer, W. A.; Haddon, R. C., Chemical Modification of Epitaxial Graphene: Spontaneous Grafting of Aryl Groups. J. Am. Chem. Soc. 2009, 131, 1336-1337. Nikitin, A.; Li, X. L.; Zhang, Z. Y.; Ogasawara, H.; Dai, H. J.; Nilsson, A., Hydrogen Storage in Carbon Nanotubes Through the Formation of Stable CH Bonds. Nano Lett. 2008, 8, 162-167. Guisinger, N. P.; Rutter, G. M.; Crain, J. N.; First, P. N.; Stroscio, J. A., Exposure of Epitaxial Graphene in SiC (0001) to Atomic Hydrogen. Nano Lett. 2009, 9, 1462-1466. Balog, R.; Jorgensen, B.; Wells, J.; Laegsgaard, E.; Hofmann, P.; Besenbacher, F.; Hornekaer, L., Atomic Hydrogen Adsorbate Structures on Graphene. J. Am. Chem. Soc. 2009, 131, 8744-8745. Balog, R.; Jorgensen, B.; Nilsson, L.; Andersen, M.; Rienks, E.; Bianchi, M.; Fanetti, M.; Laegsgaard, E.; Baraldi, A.; Lizzit, S.; Sljivancanin, Z.; Besenbacher, F.; Hammer, B.; Pedersen, T. G.; Hofmann, P.; Hornekaer, L., Bandgap Opening in Graphene Induced by Patterned Hydrogen Adsorption. Nat. Mater. 2010, 9, 315-319. Giesbers, A. J. M.; Uhlirova, K.; Konecny, M.; Burghard, M.; Aarts, J.; Flipse, C. F. J., Interface-Induced Room-Temperature Ferromagnetism in Hydrogenated Epitaxial Graphene. Phys. Rev. Lett. 2013, 111, 166101. Gan, L.; Zhou, J.; Ke, F.; Gu, H.; Li, D.; Hu, Z.; Sun, Q.; Guo, X., Tuning the Properties of Graphene Using a Reversible Gas-Phase Reaction. NPG Asia Mat. 2012, 4, e31.