Patent Application: US-201715411464-A

Abstract:
described are carbon nanotube dispersions containing single - walled carbon nanotubes dispersed in a dispersant solution comprising a solvent , and an azo compound . the single - walled carbon nanotubes are not cross - linked with covalent bonds . the dispersions are useful for fabricating transparent conductive thin films on flexible and inflexible substrates . methods for making the transparent conductive thin films are also described .

Description:
cr = congo red , i . e ., 3 , 3 ′-([ 1 , 1 ′- biphenyl ]- 4 , 4 ′- diyl ) bis ( 4 - aminonaphthalene - 1 - sulfonic acid . dls = dynamic light scattering . dmf = dimethylformamide . dmso = dimethylsulfoxide . ft - ir = fourier transform infrared spectroscopy . hmpa = hexamethylphosphoramide . ito = indium tin oxide . pdms = polydimethylsiloxane . pet = polyethylene terephthalate . sc = sodium cholate hydrate . sds = sodium dodecyl sulfate . sem = scanning electron microscopy , swnt = single - walled carbon nanotube . tcf = transparent conductive film . tcm = transparent conducting material . tem = transmission electron microscopy . thf = tetrahydrofuran . triton x - 100 - brand surfactants = polyethylene glycol p -( 1 , 1 , 3 , 3 - tetramethylbutyl )- phenyl ether - type surfactants , cas no . 9002 - 93 - 1 . uv - vis - nir = ultra violet - visible - near infrared spectroscopy . “ ω /□”= ohms per square unit , a measure of sheet resistance ( as contrasted to bulk resistance ). “ polar protic solvent ,” explicitly includes , but is not limited to : methanol , ethanol , isopropanol , n - butanol , nitromethane , formic acid , acetic acid , and the like . “ polar aprotic solvent ,” explicitly includes , but is not limited to : acetone , acetonitrile , dmf , dmso , ethyl acetate , hmpa , thf , and the like . polar aprotic solvents lack an acidic hydrogen group and generally have a dipole moment of about 1 . 8 d or larger . the preferred polar aprotic solvents for use in the present disclosure have a dipole moment greater than about 2 . 8 d . “ substrate ” as used herein is to be interpreted broadly to include any suitably robust , flexible or inflexible panel , sheet , rod , bead , particle , etc . glass substrates ( planar slides or curved surface , bulk or particulate ) are included in the definition , along with polymeric substrates ( flexible or inflexible ). the steps to prepare cr - swnt films as disclosed herein are depicted schematically in fig1 a . as shown in the figure , a swnt film was prepared on a glass substrate by a drop - coating method using an aqueous solution of congo red in which carbon swnts were dispersed . spin - coating may also be used . the film was then dried and washed with dmf to yield a transparent conducting film on the glass substrate . see the examples for details . fig1 b depicts the neutral and ammonium forms of cr . swnt films prepared using swnt dispersions in cr / water , sodium dodecylsulfate ( sds ), and triton x - 100 - brand surfactant are compared in fig2 a , 2b , and 2c , respectively . after oven - drying for one hour at 60 ° c ., the swnts dispersed in cr formed uniform thin films . see the right - hand panel in fig2 a . in contrast , the swnts dispersed in sds ( fig2 b ) and triton x - 100 - brand surfactant ( fig2 c ) were far more aggregated . by adjusting the concentration of swnts in cr solution , it was possible to prepare swnts network films of different tube densities and transparencies . again , see the examples for complete details . an attempt was made to disperse swnts in dmf and cr / dmf solution because cr is soluble in dmf ( as well as in other organic polar protic and polar aprotic solvents ). a stable dispersion of cr - swnt in dmf was obtained with high concentration of swnts . however , swnt were also dispersable in dmf alone with low amount of swnts . an attempt was also made to prepare swnt film on polydimethylsiloxane ( pdms ) film by drop - coating . cr - swnt - dmf dispersion was uniformly dried without any aggregation on the pdms film after the drying process at 60 ° c . see fig9 a . swnt film preparation , however , was not successful from swnt - dmf dispersion alone . in the absence of the cr , the swnt did not disperse into a uniform film . serious aggregation occurred when the substrate was dried at 60 ° c . see fig9 b . the consistencies of the cr solution and cr - swnts dispersion were gel - like compared to the swnts dispersions in sds and triton - x - 100 - brand surfactant , which were liquid - like . this behavior was clearly observed by inverting the centrifuge tubes containing those dispersions . see fig3 a . uv - vis spectra of cr solution and cr - swnts showed a strong red - shift for the cr - swnts dispersion . see fig3 b . the absorption peak shift from 496 nm to 535 nm indicates strong charge - transfer between cr and swnts in aqueous solution . to verify non - covalent interaction between swnts and cr , raman spectra of cr - swnts film , cr powder and pristine swnts were measured using a 633 nm excitation laser source . the results are presented in fig3 c . comparing the spectrum of cr - swnts film with that of pristine swnts , it can be seen that the g + band shifted from 1591 to 1596 cm − 1 and g − band decreased substantially , both of which are related to strong electron charge transfer from cr to swnts . see fig3 c and 10a . ( fig3 c and fig1 a )[ 18 , 19 ]. raman peaks of cr are also present in the cr - swnts film even after repeated washing with dmf and ethanol , which further confirmed strong non - covalent binding between cr and the nanotubes . however , the cr peaks disappeared when the cr - swnts film was thermally annealed at 500 ° c . in a vacuum furnace for 10 min . however , the disorder ( d ) band ( at 1312 cm − 1 ) slightly increased . see fig1 b , curve ii . this is attributed to covalent reaction on the nanotubes by radicals generated from cr at high temperature . to avoid covalent functionalization , the thermal annealing of cr - swnt film was limited to 5 min . this resulted in decreased contact resistance and yielded swnts films with sheet resistivity of 533ω /□ at a transmittance of 87 %. see fig4 a which is a photograph of the resulting film superimposed on its transmission spectrum . the swnts films were further characterized using scanning electron microscopy ( sem ) and atomic force microscopy ( afm ), which shows dense and uniform network of swnts forming 20 - nm thick film on the substrate . representative sem photomicrographs at different resolutions are shown in fig4 b ( scale bar = 1 μm ) and 4 c ( scale bar = 1 nm ). fig4 d depicts an afm image of the swnt thin film on the substrate . the dispersion qualities of cr and sds were determined by measuring the diameters of swnts in cr - swnts and sds - swnts dispersions using transmission electron microscopy ( tem ). representative tem photomicrographs at different resolutions are shown in fig5 a ( scale bar = 100 nm , cr - swnt ), 5 b ( scale bar = 200 nm , supt - cr - swnt ), 5 c ( scale bar = 20 nm , cr - swnt bundle ), and 5 d ( scale bar = 100 nm , swnt dispersed in sds , control ). the average diameter of swnts in cr dispersion ( about 2 to about 7 nm ) is slightly higher than in sds dispersion ( about 1 . 2 to about 1 . 5 nm ). these diameter distributions compare favorably with the reported diameter distributions of swnts fabricated via arc - discharge [ 21 ] ( the commercial swnts used in the photographs shown in fig5 a - 5d were made via using arc - discharge .) to elucidate why cr - swnts dispersions yield a uniform nanotube network film on glass , pet and pdms substrates without any surface treatment , the surface tension of the dispersions was analyzed . the meniscuses of water and cr - swnts were similar to each other ( compare fig6 a , tubes “ a ” and “ c ,” respectively ) and different from those of cr and sds - swnts dispersion ( compare fig6 a , tubes “ b ” and “ d ,” respectively ). these visual observations were supported by the measured surface tension values of cr - swnts , which were found to be similar to that of water ( 72 . 8 dynes / cm ), while the surface tensions of cr solution and sds - swnts dispersion was considerably lower [ 22 ]. cr molecules could react or interact with the nanotubes by means of π - π - stacking interactions between aromatic moieties of cr and graphene . see the resonance structures of cr shown in fig1 b . it is also possible that there are other types of interactions through hydrogen bonds and / or electrostatic coulomb forces [ 23 , 24 ]. it is thought that hydrophobic aromatic benzene and napthalene rings ( π - electrons conjugated systems ) were adsorbed on the nanotubes ( graphene ) by π - π - stacking , and ionized amine (— nh 2 ) and sulfonic acid (— so 3 − ) groups were stabilizing nanotubes in the aqueous medium through the coulombic repulsion [ 25 ]. apparently , as shown in fig1 , at lower concentrations ( 0 . 0029 and 0 . 0286 mm ) cr molecules do not interact sufficiently to form the gel - like structure that is observed at higher concentrations (≧ 0 . 0571 mm ). stopa et al . reported that cr molecules could self - aggregate ( in supramolecular form ) and form a ribbon - like complex or helical aggregates [ 26 ]. it is also known that the supramolecular form of cr adheres strongly to hydrophobic proteins [ 27 ]. the distribution of the hydrodynamic radius ( size ) distribution of nanotubes in different surfactants ( see fig7 a , 7b , and 7c ) were measured using dynamic light scattering ( dls ) method which show that the mean particle size was larger in cr - swnts dispersion ( 104 nm , fig7 a ) than in sc - swnts ( fig7 b ) and sds - swnts dispersions ( fig7 c ) ( 70 nm and 75 nm , respectively ). note that these particle sizes represent the hydrodynamic radius ( diameter ) of nanotubes plus any adhering surfactant film . moreover , these large diameter nanotube sizes ( in cr dispersion ) indicate that nanotubes are still in bundles and are covered with the supramolecular structure of cr molecules . the amine and sulfonic acid groups would interact with the surface of the substrate that could form a uniform of swnts film during the drying process ( without any additional surface treatment ). in addition , cr molecules are adsorbed on the nanotubes by non - covalent interaction , as shown in raman spectra , so that cr molecules were removable by washing with dmf and ethanol ; see fig3 c . the interaction between cr and swnts was also confirmed by ftir . the ir spectra of pristine swnts are depicted in fig1 a and fig1 b . these spectra are similar to those previously reported [ 28 , 29 ]. these spectra did not show any distinct features in the range of 600 to 4000 cm − 1 possibly due to the high quality of the nanotubes . the ft - ir spectrum of cr shows a broad and strong absorption band at 3464 cm − 1 ( n — h bonds ), with several other peaks at 1042 , 1176 ( the stretching vibration of s ═ o due to — so 3 − ), 1597 ( assigned to stretching vibration of — n ═ n — bond ), 1364 and 1227 cm − 1 ( the stretching vibrations of ═ c — n ═ group adjacent to aromatic ring ). other bands located at 904 , 832 , 749 and 697 cm − 1 are assigned to the aromatic rings ( c — h ) of cr . a significant band was observed in the spectrum for cr - swnts film . again , see fig1 a and 12b . new bands provide evidence for the presence of functional groups definitely bound to the nanotubes . the s ═ o stretching vibration observed in cr spectrum is still present in cr - swnts , shifted to a lower wavenumber of 1030 cm − 1 . the red shift might be attributed to the π - π stacking between swnts and cr molecules . swnts films were prepared on pet substrates by spin - coating of cr - swnts dispersion and washing the treated substrates with dmf and ethanol . the films were transparent and conductive with a sheet resistance of 600ω /□ at 82 % transmittance . see fig6 b ( a photograph of the resulting film ) and 6 c ( resistivity of the film versus bending cycle ). to increase their conductivity , the cr - swnts dispersion was ultrasonicated prior to the spin - coating to debundle the nanotubes . the effect of the swnts density on the conductivity of the swnts film was studied by afm . films with low density of nanotubes were also prepared using the same cr - swcnts dispersion for comparison . see the afms in fig4 d and fig1 a and 13b . the sheet resistance of the low density ( 16 nanotubes / μm 2 ) films ( 1 . 56 kω /□ at 90 % transmittance ) is higher than that of the high density (& gt ; 50 nanotubes / μm 2 ) films ( 600ω /□ at 82 % transmittance ). see fig8 for transmittance data . the hydrophobicity of the film was studied after drying under flow of nitrogen . the cr - swnts films prepared as described herein showed highest contact angle values of ˜ 100 . see fig6 d . the superhydrophobicity of cr - swnts film is more suitable for flexible electronic display applications . fig6 c shows the variations of the sheet resistivity versus the bending cycles of the swnts - pet films . the changes in the resistance are not significant up to about 1000 consecutive bending cycles , which demonstrates the high flexibility of the swnts tcf films . the cr - swnts film were immersed in concentrated nitric acid solution for 3 h at room temperature . the uv - vis - nir optical transmittance spectra of the swnt films before and after hno 3 treatments are shown in fig8 . after the treatment , cr molecules are completely removed by washing with ethanol and water ( fig8 , blue curve ). the absorption intensity of the inter - band energetic transition s 22 and m 11 of swnts are almost bleached off by the acid treatment which shows acid doping effect [ 32 ]. the sheet resistivity of the swnts - pet film was 300ω /□ which is better than that of untreated cr - swnts films ( 600 □/□). these results reveal that the sheet resistivity and transparency properties of the swnts films disclosed herein are as good as or better than those obtained via other reported methods . moreover , the disclosed method using cr as surfactant is easily implemented , inexpensive , and the film can be cast on any rigid or flexible substrate without surface treatment by either drop - coating or spin - coating . superhydrophobicity , high flexibility , and low cost of production of the cr - swnts films make them well - suited for electronic and indium - free electrode applications . the following examples are included solely to provide a more complete description of the films and methods disclosed herein . the examples are not intended to limit the scope of the claims in any fashion . p2 - swnts were purchased from carbon solutions , inc ( riverside , calif ., usa ). cr , sodium dodecyl sulfate ( sds ), sodium cholate hydrate ( sc ), triton ™ x - 100 - brand surfactant , ethanol and dimethylformamide ( dmf ) were received from sigma - aldrich ( st . louis , mo ., usa ). all other reagents were of analytical grade and used without further purification . deionized water with resistivity of 18 mωcm was used . p2 - swnts ( 5 mg ) were mixed in cr solution ( 10 ml , 1 mm ) and then bath sonicated for 10 min . subsequently , cr - swnt mixture was ultrasonicated with a 130 - w ultrasonic processor for one hour . finally , cr - swnts dispersion was transferred into centrifuge tubes and centrifuged at 12 , 000 rpm for one hour . after centrifugation , the top supernatant solution was collected and stored for characterization and applications studies . for comparison studies , pristine p2 - swnts were similarly dispersed in sds , sc and triton x - 100 - brand surfactant dispersions and supernatant swnts were collected after centrifugation . swnts films were prepared on glass substrate or polyethylene terephthalate ( pet ) film by spin - coating or drop - coating of cr - swnts dispersion . swnt - coated substrate was dried at 60 ° c . for one hour and then washed with dmf and ethanol multiple times . finally , swnt - coated substrates were dried at 60 ° c . for several hours . for drop - coating , cr - swnts dispersion or sds - swnts or triton x - 100 - brand - swnts dispersion was uniformly spread onto the substrate and dried at 60 ° c . in an air oven . all other steps were similar to that followed for spin - coating method . characterization of cr - swnts : swnts films were characterized using a scanning electron microscope ( sem ) ( leo 1530 field emission sem ), labramaramis horiba jobinyvon confocal raman microscope , and bruker afm microscopes . dynamic light scattering ( dls ) analysis were performed using 90 plus particle size analyzer ( brookhaven instruments ). perkinelmer uv / vis spectrophotometer ( lambda 25 ) and perkinelmer spectrum 100 ft - ir spectrometer ( with universal atr sampling accessory ) were used for characterizing swnts . scs speciality coating systems ( 6800 spin coater series ) was used to spin - coat nanotubes . cr - swnts dispersion was centrifuged using eppendorf centrifuge model no . 5415c . contact angle measurements were done using a dataphysics oca 15 optical contact angle measuring system . the van der pauw method was employed to measure resistivity of the swnts films using four point - probe measurement system . 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