Patent Application: US-89161504-A

Abstract:
a rotational actuator / motor based on rotation of a carbon nanotube is disclosed . the carbon nanotube is provided with a rotor plate attached to an outer wall , which moves relative to an inner wall of the nanotube . after deposit of a nanotube on a silicon chip substrate , the entire structure may be fabricated by lithography using selected techniques adapted from silicon manufacturing technology . the structures to be fabricated may comprise a multiwall carbon nanotube , two in plane stators s 1 , s 2 and a gate stator s 3 buried beneath the substrate surface . the mwnt is suspended between two anchor pads and comprises a rotator attached to an outer wall and arranged to move in response to electromagnetic inputs . the substrate is etched away to allow the rotor to freely rotate . rotation may be either in a reciprocal or fully rotatable manner .

Description:
the present method and device utilizes a nanotube to which has been affixed a rotor plate that can be rotatably moved about the nanotube axis in a torsional , reciprocating manner ( actuator ) or , alternatively , can be rotated in a 360 ° spinning mode ( motor ). the axial movement is imparted by electrostatic forces between the rotor and at least one stator . these elements are electrically conductive and therefore generate electrical forces and fields that will cause movement of the rotor through attractive or repulsive forces , either electrostatically or magnetically . alternatively , if the rotor is made magnetic ( e . g . if the rotor is made of a ferromagnetic material such as iron ), it can be accessed by magnetic fields . a spinning magnetic material generates an electric current and may be used as a generator . the preferred rotatable element is a multiwalled carbon nanotube ( mwnt ). these nanotubes have a near perfect carbon tubule structure that resembles a sheet of sp2 bonded carbon atoms rolled into a seamless tube . they are generally produced by one of three techniques , namely electric arc discharge , laser ablation and chemical vapor deposition . the arc discharge technique involves the generation of an electric arc between two graphite electrodes , one of which is usually filled with a catalyst metal powder ( e . g . iron , nickel , cobalt ), in a helium atmosphere . the laser ablation method uses a laser to evaporate a graphite target which is usually filled with a catalyst metal powder too . the arc discharge and laser ablation techniques tend to produce an ensemble of carbonaceous material which contains nanotubes ( 30 - 70 %), amorphous carbon and carbon particles ( usually closed - caged ones ). the nanotubes must then be extracted by some form of purification process before being manipulated into place for specific applications . the chemical vapor deposition process utilizes nanoparticles of metal catalyst to react with a hydrocarbon gas at temperatures of 500 - 900 ° c . a variant of this is plasma enhanced chemical vapor deposition in which vertically aligned carbon nanotubes can easily be grown . in these chemical vapor deposition processes , the catalyst decomposes the hydrocarbon gas to produce carbon and hydrogen . the carbon dissolves into the particle and precipitates out from its circumference as the carbon nanotube . thus , the catalyst acts as a ‘ template ’ from which the carbon nanotube is formed , and by controlling the catalyst size and reaction time , one can easily tailor the nanotube diameter and length respectively to suit . carbon tubes , in contrast to a solid carbon filament , will tend to form when the catalyst particle is ˜ 50 nm or less because if a filament of graphitic sheets were to form , it would contain an enormous percentage of ‘ edge ’ atoms in the structure . alternatively , nanotubes may be prepared by catalytic pyrolysis of hydrocarbons as described by endo , et al ., in j . phys . chem . solids , 54 , 1841 ( 1993 ), or as described by terrones , et al ., in nature , 388 , 52 ( 1997 ) or by kyotani , et al ., in chem . mater ., 8 , 2190 ( 1996 ), the contents of all of which are incorporated by reference . other forms of nanotube may be used , so long as they have uniform mechanical properties , have multiple wall layers and are mechanically stable . alternative forms of nanotubes ( e . g . boron nitride ) can be formulated with boron , nitrogen , or other elements ; the key factors in selecting a nanotube are the nanoscale dimensions and the presence of multiple walls that can rotate relative to each other . single walled nanotubes can also be used to provide a rotor support component for embodiments not involving free rotation , i . e . actuators , which have reciprocating radial movement . the actuator / motor is essentially designed like an electric motor which has a plurality of electrically chargeable components in fixed relation to a rotating member on a nanotube axle . the overall size scale of the present actuator / motor is of the order of 300 nm . this will convey a sense of the size of the present device , wherein the diameter of the mwnt is approximately 5 to 100 nanometers and the gap between the anchor pads can be as small as 200 nm . as described in detail below , the components of the actuator / motor are integrated on a silicon chip . that is , the stators are integral with the chip and the rotor plate is formed on the chip and when surrounding material is etched away from the chip . low - level externally applied voltages precisely control the operation speed and position of the rotor plate . repeated oscillations of the rotor plate between positions 180 ° apart , as well as rotations of 360 °, have been demonstrated with no signs of wear or fatigue . unlike existing chemically driven bio - actuators and bio - motors , the present fully synthetic nanometer - scale electromechanical system ( nems ) actuator / motor is designed to operate over a wide range of frequency , temperature , and environmental conditions , including high vacuum and harsh chemical environments . fig1 shows the conceptual design of the present device . the rotational element , rotor plate 10 , a solid rectangular metal plate serving as a rotor plate , is attached transversely to a suspended support shaft 12 . the suspended support shaft 12 is a nanotube , preferably a multiwalled carbon nanotube ( mwnt ) prepared as described above and deposited on a silicon substrate 14 having a silicon oxide layer 16 on top . the support shaft ends 18 , 20 are embedded in electrically conducting anchors ( 22 , 24 ) that rest on the oxidized surface 16 of silicon chip 14 . the rotor plate assembly is surrounded by three fixed stator electrodes : two ‘ in - plane ’ stators ( s 1 , s 2 ), 26 , 28 are horizontally opposed and rest on unetched portions of the silicon oxide surface 16 , and the third ‘ gate ’ stator ( s 3 ) is buried beneath the etched surface ( indicated at 30 ). four independent ( d . c . and / or appropriately phased a . c .) voltage signals , one to the rotor plate and three to the stators ( v 1 , v 2 , v 3 and v 4 ) can be applied to control the position , speed and direction of rotation of the rotor plate . the nanotube 12 serves simultaneously as the rotor plate 10 support shaft and the electrical feed through to the rotor plate ; most importantly it is also the source of rotational freedom . it should be noted that surfaces shown as planar and perpendicular to the top of the substrate are in fact curved , and undercut the top surface . this occurs during the etching step , so that the conductive layer 22 hangs over the substrate . that is , surfaces shown as vertical planar surfaces in fig1 , such as surface 29 , are in fact concave . in the case of the rotor plate 10 , the substrate has been completely undercut . this undercutting can also be used to create differences in height as , e . g . between the stators 26 , 28 and the anchors 22 , 24 , or between the two stators . fig2 shows a scanning electron micrograph illustrating an actuator / motor device prior to etching . while the components are not numbered in the photograph , it is plain that they correspond to the structures illustrated in fig1 . the scale bar in the lower left portion of the scanning electron electron micrograph is 300 nm long and approximates the rotor plate width ( transverse to the support ). typical rotor plate dimensions were 250 - 500 nm on a side . mwnts were synthesized by the standard arc technique as described in ebbesen et al . u . s . pat . no . 5 , 641 , 466 issued jun . 24 , 1997 , hereby incorporated by reference to describe a method for large - scale synthesis of carbon nanotubes . the technique that was used is also reported in the nature publication of reference ( 11 ). in an inert gas at a pressure of 200 - 2500 torr , an arc discharge is made between two carbon rod electrodes by application of a suitable ac or dc voltage ( e . g . about 18 v ) to thereby produce a carbon plasma . the electric current is about 50 - 100 a . as the result a carbon deposit forms on the end of one of the two carbon rods , and a core part of the carbon deposit contains a large amount of carbon nanotubes . this core part can easily be separated from a shell part in which no carbon nanotubes exist . usually carbon nanotubes occupy more than 65 wt % of the core part of the deposit , and the nanotubes coexist with some ( less than 35 wt %) carbon nanoparticles which are nanometer - scale carbon particles with polyhedral cage structures . sometimes a small amount of amorphous carbon also coexists . the present device was formed by the deposition of various layers and components onto a crystralline silicon chip . degenerately doped silicon substrates were covered with 1 μm of thermally grown sio 2 . pre - patterned alignment marks were placed on the substrate by standard lithographic techniques and were located a fixed distance apart for later reference . the substrate is comprised of layers of silicon and silicon oxide . in processing , materials are deposited for the formation of the electrodes and the rotor plate , as described and shown next in connection with fig3 . silicon was chosen because photolithographic , etching , and other techniques for its manipulation are readily available . other inert materials that can be physically shaped could also be used for the present actuator / motor , such as plastic polymer or glass . material ; such as used in the resist could also be used the actuator / motor components ( in - plane rotor plate , in - plane stators , anchors , and electrical leads ) were then patterned in the substrate comprising the sio 2 using electron beam lithography . fig3 a - f represents an end view taken along line 3 - 3 ′ in fig1 . for purposes of illustration , a single mwnt is shown attached to the substrate , and the electrode behind the rotor plate is not shown . referring now to fig3 a , the nanotube ( e . g . an mwnt ) 12 suspended in 1 , 2 - dichlorobenzene was deposited on the above - described substrate , comprising a silicon chip 16 coated with silicon oxide 16 . the mwnt &# 39 ; s were located with respect to the pre - patterned alignment marks on surface 16 using an atomic force microscope ( afm ) or a leo 1550 scanning electron microscope ( sem ). in this way , the subsequently described steps could be accurately positioned around the selected nanotube ( s ). as shown in fig3 b the mwnt 12 on the sio 2 and the sio 2 were coated with a layer of e - beam resist 32 . in adding layer 32 , a single layer of electron beam resist ( polymethyl methacrylate , 950 , 000 relative molecular mass , 5 . 5 % in chlorobenzene ) was spun on the substrate at 4 , 000 r . p . m . for 45 seconds , and subsequently baked in air at 150 ° c . for 2 hours . next , as also shown in fig3 b , the resist was patterned using commercially available electron beam writing software , namely npgs software ( nanometer pattern generating system , which may be obtained from available from joe nabity , ph . d . j c nabity lithography systems p . o . box 5354 bozeman , mont . 59717 usa ), loaded on a jeol 6400 sem ( jeol usa , inc .). the jeol - 6400 with npgs is a high - resolution , electron beam lithography system used for writing complex patterns in resists from the nanometer scale up to 5 mm . the striped regions in fig3 b represent areas of resist where the e beam struck and disrupted the resist so that it could be removed in subsequent steps . the electron beam resist was developed in methyl isobutyl ketone : isopropyl alcohol 1 : 3 for one minute , causing removal of the resist , as shown in fig3 c . next , as shown in fig3 d , chromium ( 10 nm ), then gold ( 90 nm ) was evaporated onto the nanotube and ( incidentally ) the surrounding area . the cr layer improves adhesion of the gold that is used for electrodes and stators . next , as shown in fig3 e , the resist that remained after the mibk step ( fig3 c ), and the au / cr on top of it , were lifted off in acetone . the cr / au was subsequently annealed at 400 ° c . to ensure better electrical and mechanical contact between the cr and the mwnt . then , as shown in fig3 f , an hf etch was used to remove roughly 500 nm of the sio 2 surface 16 to provide clearance sufficient to permit the rotor plate to be rotated by 90 ° ( and more ). note that the area under the rotor r is exposed to the hf from the sides through an undercutting process so that the au / cr attached to the nanotube is free of underlying sio 2 . that is , in fig3 f , the tube and metal are resting on the anchors ( not shown ) that are into and above the plane of the drawing , along the axis of the nanotube . the conducting si substrate ( typically used as the gate electrode in three - terminal nanotube field - effect devices ( refs . 12 , 13 ) here serves as the gate stator , i . e . below the rotor plate . initial actuator characterization was carried out in situ inside the leo sem . applying voltages up to 50 v d . c . between the ( slightly asymmetric ) rotor plate and the gate stator ( s 3 ) generated a net torque sufficient to visibly rotate the rotor plate ( up to 20 ° deflection ). the rotor plate is slightly asymmetric in that one side extends further from the mwnt than the other . it should be noted that only one stator is necessary to create a torsional spring . when the applied voltage was removed , the rotor plate would rapidly return to its original horizontal position . using a finite analysis program ( femlab , a commercially available plug - in for matlab ) and the actuator geometry together with the measured deflection and applied voltages , it was determined that a typical ‘ as produced ’ effective torsional spring constants was 10 15 to 10 − 12 n m . evaluation of the mwnt shear modulus ( assuming a continuum mechanics model [ ref 14 ]) necessitates knowledge of the outer radii of the nanotubes . the outer diameter of the mwnts in the present devices was determined to within 20 %. they ranged from 10 to 40 nm , which was consistent with high - resolution transmission electron microscopy ( tem ) measurements of mwnts from the same preparation batch . tem imaging also showed the mwnts to be of high structural quality , composed of concentrically nested cylindrical tubules with no obvious defects . a 10 - nm - diameter mwnt with an effective length of 2 μm would have a shear modulus of 100 to 300 gpa . these ranges for torsional spring constant and shear modulus overlap those of more direct measurements employing a suspended mwnt subjected to torsional deflection via an atomic force microscope tip ( refs 15 , 16 ). although the actuator / motor devices just described have a number of extremely useful characteristics ( including predicted torsional oscillator mechanical resonance frequencies of the order of tens to hundreds of megahertz ), the strong torsional spring constant effectively prevents large low - frequency angular displacements . the torsional actuator / oscillator has significant applications that rely on the resonance frequency described . for example , it can act as a band pass filter to filter out certain frequencies going through the device . it can also be used to sense changes in fluid flow , when fluid is contacted to the rotor and the displacement of the rotor is measured . because the nanotube is stiff , it has a high resonance frequency and can be useful in a variety of applications . the device will have a high quality factor , making them desirable as filters and for other applications utilizing the resonance frequency . for large - displacement operation , including 360 ° rotation , the mwnt support shaft 12 ( fig1 ) was modified to exploit the intrinsic low - friction bearing behavior afforded by the perfectly nested shells of mwnts ( refs . 9 , 10 , 17 , 18 ). the modification comprises removing or compromising one or more outer mwnt shells in the region between the rotor plate 10 and the anchors 22 , 24 ( fig1 ). several in situ methods were used to achieve the modification while the device was in place in the in the leo sem , including reactive - ion etching , application of current through the nanotube to ‘ blow out ’ outer nanotube shells ( refs . 19 , 20 ), and selective nanotube bond - damage induced by the sem electron beam . a particularly simple yet effective in situ mwnt modification method , and the one used on the devices to be described below , was to mechanically fatigue and eventually shear potions of the outer nanotube shells ( between the rotor and the anchor ) by successive application of very large stator voltages . we found that applied gate stator voltages of order 80 v d . c . would torque the outer nanotube shells past the elastic limit , eventually leading to partial or complete failure of the outer nanotube shells and a resulting dramatic increase in the rotational freedom of the rotor plate . in the ‘ free ’ state , the rotor plate was still held in position axially by the intact nanotube core shells , but could be azimuthally positioned , using an appropriate combination of stator signals , to any arbitrary angle between 0 ° and 360 °. once so positioned , the rotor plate nominally remained in place even with all stator voltages reduced to zero , eventually drifting to a vertical ( 0 ° or 180 °) position only under the charging influence of the sem imaging electron beam . other methods to separate the outer shell and permit free rotation could also be used . for example , the outer shell of the mwnt could be fractured using current passing through the nanotube . or , a reactive ion etch could be used to break away outer walls . in addition , partial breakage of an outer wall could be accomplished using these techniques . this would be useful in the actuator embodiment in that it would result in a weaker torsional spring . it could be used to sense fluid flow or to achieve a lower resonance frequency . more than one nanotube can be used together to support the rotor plate . in this case , the nanotubes can be damaged so that some of the tubes are no longer intact . to verify the operation of the device a series of still sem images were recorded of an actuator / motor device in the free state , being ‘ walked ’ through one complete rotor plate revolution using quasi - static d . c . stator voltages . the stator voltages , never exceeding 5 v , were adjusted sequentially to attract the rotor plate edge to successive stators . by reversing the stator voltage sequence , the rotor plate rotation could be reproducibly reversed . these images may be viewed in the corresponding publication in nature 424 : 408 - 410 ( 24 jul . 2003 ) and accompanying on - line materials . the images verify the rotation of the rotor plate as described . finite frequency operation of the actuator / motor was also performed , using a variety of suitably phased a . c . and d . c . voltage signals to the three stators and rotor plate . in one simple operation mode , out - of - phase common - frequency sinusoidal voltages were applied to stators s 1 , s 2 , and s 3 , and a d . c . offset to the rotor plate r ; that is , s 1 = v 0 sin ( ωt ), s 2 = v 0 sin ( ωt + 240 °), and s 3 = v 0 sin ( ωt + 120 °), where ω is ½ of the frequency of rotation , and r =− v 0 . in this design , the stators are slightly below the plane of the nanotube / rotor / anchors . this dislocation of the stators ( 26 , 28 fig1 ) was accomplished by under etching the stators . this slight mismatch allows the simple voltage scheme described here to work , although other voltage schemes can be used in other configurations . using this drive sequence , one may reliably flip the rotor plate between the extreme horizontal ( 90 ° and 270 °) positions . although in principle very high frequency operation should be possible ( restricted only by the stripline bandwidth of the leads and , ultimately , inertial effects of the rotor plate ), our sem image capture rate limited direct real - time observations of rotor plate oscillations to frequencies of typically several hertz . referring now to fig4 , suitable phased ac signals s 1 , s 2 , s 3 in relation to the position of the rotor 10 are shown . the position of the rotor , as shown in the bottom row , changes relative to the combined effects of the voltages s 1 , s 2 and s 3 , which follow the formulas described above . standard voltage sources are used . other configurations can be designed based on this example . it is simply necessary to place the stators in an arrangement so that they can affect the rotor in a range of rotational positions . in this case , when s 1 is at a peak and s 2 and s 3 are between zero and a negative peak , the rotor is approximately at the default horizontal position ( 90 °). at the peak positive s 3 and near peak negative s 2 and s 3 voltages ( shown at 40 ), the rotor has rotated 90 ° to the 180 ° position . when the s 3 voltage returns to approximately ⅜ negative peak ( shown at 42 ), the rotor , as illustrated , has advanced to the next horizontal position , 270 °. at the peak positive s 3 and positive s 2 voltages , and near - negative s 1 and s 3 ( shown at 44 ) voltages , the rotor has advanced to the 0 ° position and proceed from there to the 90 ° position , etc the transitions as described above ( between the extreme horizontal positions ) were recorded in digital video of the sem images . the recording captured an a . c . voltage driven actuator / motor ‘ flipping ’ between the extreme horizontal positions ( 90 ° and 270 °) in 33 milliseconds . video samples are available in the corresponding on line publication corresponding publication in nature 424 : 408 - 410 ( 24 jul . 2003 ). the transitions between positions could be made faster than the image video capture rate of 33 ms . two images of the actuator / motor , recorded 33 ms apart , showed the rotor plate respectively in the 90 ° and 270 ° positions . actuator / motors were rotationally driven in this fashion for many thousands of cycles , with no apparent wear or degradation in performance . in this configuration , the mwnt clearly serves as a reliable , presumably wear - free , nems element providing rotational freedom . this characterization was performed in a pressure of 10 − 6 - 10 − 5 torr , although reliable operation at higher pressures is anticipated . the present actuator / motor may be characterized as the first true mwnt - based nems device , in that it fully integrates electronic control and mechanical response . this distinguishes it from previous related mwnt - based mechanical devices which require relatively large and complex external control systems ( such as piezo - driven manipulators ) to achieve operation 15 - 18 , 21 . the present disclosure suggests that the present nanotube - based actuator / motors have a number of mems / nems applications . the rotor plate , when covered with metal , could serve as a mirror , with obvious relevance to ultra - high - density optical sweeping and switching devices ( the total actuator / motor size is just at the limit of visible light focusing ). in this case , a light source would be directed onto the rotor r from a position above the substrate . the light source could be any type of optical signal . the rotor plate could also serve as a paddle for inducing and / or detecting fluid motion in microfluidics systems , as a gated catalyst in wet chemistry reactions , as a bio - mechanical element in biological systems , or as a general ( potentially chemically functionalized ) sensor element . in a microfluidics application , the fluid would be channeled between an actuator and an anchor , and such projections would be etched in a way so as to define fluid impermeable channels . it is also possible that the charged oscillating metallic plate could be used as a transmitter of electromagnetic radiation . while the foregoing device and its method of construction and operation has been described in reference to particular embodiments , many variations and embellishments are possible in view of the above teachings . therefore , it is intended that the present invention not be limited to the specific embodiments described above , but rather to the scope of the appended claims . 1 . tour , j . m . et al . recent advances in molecular scale electronics . ann . ny acad . sci . 852 , 197 - 204 ( 1998 ). 2 . judy , j . w . microelectromechanical system ( mems ): fabrication , design and applications . smart mater . struct . 10 , 1115 - 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