Patent Application: US-71268607-A

Abstract:
in the apparatus and process of the present invention , it is possible to fabricate cnts with specific diameters and morphologies . the morphology selection can yield samples of pre - selected diameter configurations making it possible to take a sample of swnts produced by any synthesis technique and induce a morphology change that causes the sample to be either all conductive , all narrow band gap semiconductive or wide band gap semiconductive , within a given nanotube rope .

Description:
the interest in the cnts was originally sparked by their physical size . the dimension at which cnts exist is essentially a crossover point between the scale typically seen in consumer electronic devices and the molecular and atomic world . the small size of cnts has attracted a great deal of interest in their electronic properties . it has been shown that the various diameters of swnts behave as both conductors and semiconductors . this fact , coupled with the additional fact that their thermal conductivity is high as compared to many other materials , suggests that if cnts are used in electronic devices the lifespan of the devices could be greatly increased . the semiconductive type of cnt has been shown in some cases to perform in a manner similar to a silicon semiconductor . advantageously , the similarity and behavior of cnts to semiconductor devices , coupled with their much smaller size , suggest an increase in overall processing speed of the associated electronics . this has been demonstrated with a single molecule sized transistor . however , many difficulties have been encountered in connection with cnt device fabrication . one difficulty with cnt device construction is that a single nanotube must be disentangled from a rope of nanotubes . further , the removed nanotube must be of the desired type of semiconductive nanotube . then , this semiconductive nanotube must be placed in the correct location on the device to achieve the desired result . because of the scale of these structures , these steps are difficult and time consuming . much research has been undertaken into the synthesis process of swnts in hope of fabricating a nanotube of just one type , semiconducting or conducting . even if such a synthesis process develops , it may not be commercially viable due to low production yields typical of these processes . however , it is possible to fabricate cnt ropes in patterns and in chosen locations on a substrate . thus , what is desired is a process and apparatus to change the cnt ropes , once grown , to contain cnts of only one type . in such case , a molecular device could be fabricated . the present invention comprises an apparatus and process for achieving this objective . using this invention , a semiconductor device can be fabricated by growing a cnt rope between two leads and then forcing the cnts to be only of one type cnt . this method of devising a semiconductor device is shown in fig1 . fig1 is an illustration of a rope of swnts between two electrical leads in a device . the apparatus for implementing the process of the present invention comprises a vacuum system capable of reaching 10 - 5 torr or lower pressures ( the lower the pressure the more optimal the result as extensive oxidation of the sample is prevented ), and a microwave source capable of generating a frequency of 2 . 45 ghz , at 400 watts power , although a range of frequencies from 2 ghz to 90 ghz with power levels from 1 watt to several thousand watts can be used to achieve the objectives of the present invention . in this apparatus , the cnts are exposed to a controlled amount of time , power and frequency of microwave radiation which causes a dramatic rise in temperature . depending on the exposure time to this microwave source , the diameters of the nanotubes will change to become larger than in the original sample . by adjusting the frequency , power level and length of time , any sample of swnts can be shifted to having semiconductor properties or conductive properties . fig2 provides a graphical description of one embodiment of the apparatus employed to achieve the objectives of the present invention . numerous other embodiments can be used so long as they comprise , in general , a vacuum system and a microwave source . the actual configuration that causes this morphology can vary as described above . as seen in fig2 , the microwave source is depicted external to the vacuum system . it is not a requirement that the microwave source be external to the vacuum system . the microwave source , along with the swnts , may both be internal to the main vacuum chamber . further , the nanotube samples can also be placed in a microwave resonant cavity increasing the efficiency of the process . coalescence of carbon nanotubes in general is not a new phenomena , however , the type of coalescence obtained by the process of the present invention is novel . this effect was observed prior to 1991 . the prior work involved fullerene molecules , which are the building blocks of nanotubes , coalescing into larger molecules . this phenomena was later seen in carbon nanotubes . in 1997 , a mechanism was offered for these previous observations . it was observed that if a nanotube sample is heated in a controlled environment to 1400 ° c . for several hours , a small portion of the sample will exactly double in diameter and an even smaller portion of the sample will triple in diameter . if the experiment is performed in a hydrogen environment , the yield of diameter doubled nanotubes can be increased , indicating that a type of free radical chemistry is the mechanism for the phenomena . nonetheless , the effect of diameter doubling still takes several hours , regardless of whether the heating is performed in a vacuum or in a hydrogen environment . the work performed in 1997 suggests two explanations for the susceptibility of narrow diameter nanotubes to undergo a diameter change . the first is that the reactivity of a curved grapheme sheet increases as the tube diameter becomes smaller . this is because the curvature introduces more of an s - orbital effect into the π orbitals of the carbon atoms . the second is the coalescence of smaller diameter nanotubes is an exothermic reaction due to a release of strain energy . in the process of the present invention , when a sample of carbon nanotubes is exposed to an appropriate frequency and power level of microwave radiation , a diameter increase accompanied ( although not as a diameter doubling ) by a chirality shift is observed . fig3 shows the raman breathing modes of swnt before and after microwave irradiation using laser excitation at a frequency of 514 nanometers and a power of 2 milliwatts . in fig3 the raman spectra breathing modes can be seen for nanotubes not exposed to microwave irradiation and raman breathing modes for nanotubes that have been exposed to only 6 seconds of microwave irradiation . this exposure is much shorter than what was required previously . if these breathing modes are compared with the results of well known techniques , it can be seen that the diameter change is not a doubling effect but rather a diameter change from an average of 1 . 0 nanometer to 1 . 5 nanometer ( in the present case ), although this is not the only diameter and chirality shift observed . this diameter increase is associated with a chirality shift , causing the nanotubes to consist of a much larger number of semiconducting nanotubes than existed prior to the exposure to the microwave field . this can be used to produce samples that are completely semiconductors or purely conductors . fig4 and 5 show further raman evidence for this shift in morphology and electrical properties . in fig4 a plot of the raman spectra of a swnt sample produced by the hipco process in a purified form , known as buckypearl , is shown . this sample has not been exposed to microwave radiation of any form . in fig5 a plot of a raman spectra of a swnt sample produced by the hipco process in purified form , known as buckypearls , is also shown . unlike the results of fig4 , the sample of fig5 has been exposed to 6 seconds of microwave radiation at 2 . 45 ghz and 420 watts of power . frequencies from 2 ghz to 100 ghz can be used to produce this effect . the overall speed and efficiency of diameter changes can be greatly increased with the microwave process . through selection of appropriate frequency and power levels of microwave radiation , in addition to environmental conditions , the resulting morphology of the cnt sample can be selected to whatever state is desired , e . g ., narrow band gap semiconductor , wide band gap semiconductor or conductor . the advantage of the present invention is that it provides overall speed and selection capabilities improvements over other types of heating techniques . the innovative teachings of the present invention are described with particular reference to the apparatus and process of selectively changing the diameter and morphology of a cnt rope using specific microwave frequencies and power settings . it should be understood and appreciated by those skilled in the art that the selective change in diameter and morphology described herein in order to obtain a semiconducting cnt provides only one example of the many advantageous uses and innovative teachings herein . various alterations , modifications and substitutions can be made to the apparatus and process of the disclosed invention without departing in any way from the spirit and scope of the invention . j . m . lambert , et al ., chem . phys . lett ., 215 , 509 , ( 1994 ). t . guo , et al ., chem . phys . lett ., 243 , 49 , ( 1995 ). m . yudasaka , et al ., chem . phys . lett ., 278 , 102 ( 1997 ). h . m . cheng , et al ., appl . phys . lett ., 72 , 3282 , ( 1998 ). k . tanaka , h . kobayashi , m . okada , m . kobashi , and t . yamabe , intern . j . quantum chem ., 42 , 45 , ( 1992 ). a . thess , r . lee , p . nikolaev , h . dai , p . petit , j . robert , c . xu , y . h . lee , s . g . kim , a . g . rinzler , d . t . colbert , g . e . scuseria , d . tomanek , j . e . fischer , and r . e . smalley , science , 273 , 483 , ( 1996 ). t . g . dietz , et al ., journal of chemical physics , 74 , 6511 - 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