Abstract:
A sensor comprising: at least one sensor probe comprising: a pair of electrodes; a vertically aligned nanotube disposed between the pair of electrodes; optionally a piezoelectric polymer on the nanotube; and optionally, a field source for generating a field, the field source operatively connected to the pair of electrodes; whereby when the sensor probe is contacted, a change in the field occurs or electricity is generated. Methods of using the sensors are also described.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of provisional application Ser. No. 60/811,942 entitled Touch And Auditory Sensors Based On Carbon Nanotube Arrays, filed Jun. 8, 2006, which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates generally to touch and auditory sensors, and in particular, to touch and auditory sensors using nanotube arrays. 
       BACKGROUND OF THE INVENTION 
       [0003]    Nanostructured materials have opened the door to realizing devices with ultra-miniature sizes and ultra low electric consumptions, which conventional materials could not have achieved. Nanostructured materials, which can be defined as materials with crystallite sizes less than 100 nm in dimension, are typically synthesized by either “bottom-up” or “top-down” processes. The bottom-up process starts with atoms, ions or molecules as “building blocks” and assembles nanoscale clusters or bulk material from them. The “top-down” methods for processing of nanostructured materials involve starting with a bulk solid and obtaining a nanostructure by structural decomposition. One such approach involves the lithography/etching of bulk material analogous to the processes used in the semiconductor industry wherein devices are formed out of an electronic substrate by pattern formation (such as electron beam lithography) and pattern transfer processes (such as reactive ion etching) to make structures at the nanoscale. 
         [0004]    Carbon nanotubes (CNTs) are expected to be adopted for many applications because of their superior electrical and mechanical characteristics. Moreover, their unique structures are also attractive for sensor applications. For example, in the case of using semi conducting CNTs as a sensor, it is possible to identify gases based on the selection of a donor or an acceptor by control of electron charity. In contrast, conventional gas sensors only detect the change of the electric resistance by gas absorption. However, as this new type of sensor uses only one CNT as a sensor probe for detection of gases, many difficulties remain in producing such a device. 
         [0005]    For example, in order to produce the gas sensor described above, the CNT must be isolated from the carbon soot that was prepared, and it must be moved and set on a desirable point via a “manipulation” process. Today, the manipulations of CNTs are performed using hand-made nano-tweezers used in a transmitting electron microscope (TEM), and carried out using the “top-down” method in this special and limited environment. However, these operations are not adaptable to make uniform devices or to set a plurality of sensors on one chip. 
         [0006]    On the other hand, a gas sensor containing an anode with a vertically aligned CNT array and a cathode has been reported in the prior art. The sensor works by applying a DC voltage to two electrodes, and flowing gas between those electrodes. Ionized gas produced by the voltage affects a breakdown voltage of the CNT array. By observing the differences in the breakdown voltage, the type of gas can be identified. 
         [0007]    Vertically aligned CNT arrays are currently produced using the “bottom-up” method. In particular, the CNT arrays are generally made using a chemical vapor deposition (CVD) process with catalysts, namely the pyrolysis of compounds containing a carbon source and catalyst elements. Based on these CNT arrays, CNTs with proper alignment are produced easily, such that a plurality of sensors can be set on single chip. 
         [0008]    Other sensors based on CNT arrays have been disclosed in the prior art. For example, U.S. Patent Application Publication No. 2004/0004485 discloses a sound sensor to detect sounds by observing a change in capacitance between two CNTs which face each other. Bias voltages are necessary to detect signals. Because the disclosed CNTs made on the electrodes have uniform lengths, the sensor is only useful for detecting a specific frequency. U.S. Pat. No. 6,737,939 discloses a radio frequency (RF) filter device which uses a CNT array on a substrate. The CNT array also contains CNTs that are uniform in length and cross section. By loading a bias voltage to the RF filter device, electrons on the CNT surface are increased. As a result, a quantum effect is caused which changes the lengths of the CNTs on the device, thereby changing a frequency for detection. U.S. Pat. No. 6,445,006 extends a CNT array disclosed by U.S. Pat. No. 5,872,422 for a Field Emission Display (FED) to detection applications used in micro-devices, such as Micro-Electro-Mechanical System (MEMS) based devices like an accelerometer or a flow meter. The disclosed detection principle is the physical contact originated between two CNTs electrically or the change of capacitance between them. U.S. Pat. No. 6,286,226 discloses a touch sensor using a CNT array to detect electronically a physical contact. However, a bias voltage is necessary to be loaded first. 
         [0009]    Therefore, there remains a need for improved touch and auditory sensors. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention meets this need by providing touch and auditory sensors based on nanotube arrays. The sensor includes at least one sensor probe comprising: a pair of electrodes; a vertically aligned nanotube disposed between the pair of electrodes; optionally a piezoelectric polymer on the nanotube; and optionally, a field source for generating a field, the field source operatively connected to the pair of electrodes; whereby when the sensor probe is contacted, a change in the field occurs or electricity is generated. 
         [0011]    Another aspect of the invention is a method of detecting touch or sound. The method includes providing a sensor comprising: at least one sensor probe comprising: a pair of electrodes; a vertically aligned nanotube disposed between the pair of electrodes; optionally a piezoelectric polymer on the nanotube; and optionally, a field source for generating a field, the field source operatively connected to the pair of electrodes; whereby when the sensor probe is contacted, a change in the field occurs or electricity is generated; contacting the sensor probe causing a change in the field or generating electricity; and detecting the change in the field or the electricity generation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    A better understanding of the present invention will be had upon reference to the following description in conjunction with the accompanying drawings in which like reference numerals represent like parts, and wherein: 
           [0013]      FIG. 1A  is a schematic cross-section of a touch sensor using a helical nanotube array; 
           [0014]      FIG. 1B  is a schematic partial top plan view of the touch sensor of  FIG. 1B ; 
           [0015]      FIG. 2A  is a schematic cross-section of a touch sensor using a helical nanotube array; 
           [0016]      FIG. 2B  is a schematic of the of the touch sensor of  FIG. 2A  in operation; 
           [0017]      FIG. 3A  is a schematic of a touch sensor using a nanotube array; 
           [0018]      FIG. 3B  is an enlarged cross-section of a portion of the touch sensor of  FIG. 3A ; 
           [0019]      FIG. 3C  is an enlarged cross-section of a portion of the touch sensor of  FIG. 3B ; 
           [0020]      FIG. 3D  is an illustration of the operation of the touch sensor shown in  FIG. 3A ; 
           [0021]      FIG. 4  is an illustration of a process for making a touch sensor using nanotube arrays; 
           [0022]      FIG. 5A  is a schematic of an auditory sensor using nanotube arrays; 
           [0023]      FIG. 5B  is an illustration of a detected signal for the auditory sensor of claim  5 A; 
           [0024]      FIG. 5C  is a schematic illustration for signal processing on an auditory sensor (frequency separation); 
           [0025]      FIG. 5D  is a schematic illustration for signal processing on an auditory sensor (sound localization); 
           [0026]      FIG. 6A  is a schematic of an auditory sensor using nanotube arrays; 
           [0027]      FIG. 6B  is an enlarged cross-section of auditory sensor of  FIG. 6A ; 
           [0028]      FIG. 7  is a schematic illustration of a process of making an auditory sensor using nanotube arrays; 
           [0029]      FIG. 8A  is a schematic representation of a touch sensor based on aligned CNT arrays; 
           [0030]      FIG. 8B  is a schematic representation of the touch sensor of  FIG. 8A ; 
           [0031]      FIG. 9  are plots showing sensing data for the individual touch sensor probes numbered in  FIG. 8 ; 
           [0032]      FIG. 10A  is a photograph of the pattern of a touch sensor after metal deposition on a substrate; 
           [0033]      FIG. 10B  is a portion of the sensor of  FIG. 10A ; 
           [0034]      FIG. 10C  is a photograph showing CNTs grown a portion of the sensor of  FIG. 10A ; 
           [0035]      FIG. 11  a schematic of the touch sensor of  FIG. 10A  after formation; 
           [0036]      FIG. 12  is a graph showing the result of the voltage signal detection using the touch sensor of  FIG. 11 ; and 
           [0037]      FIG. 13  is a graph showing a comparison of the signal detection test results for a CNT/PVDF sensor and a PVDF sensor. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]    The following description of the embodiments of the invention directed to touch and auditory sensors based on nanotube arrays are merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. 
         [0039]    For ease of discussion, in the following examples the nanotube arrays will be referred to as carbon nanotube (CNT) arrays. However, those of skill in the art will recognize that the nanotubes are not limited only to carbon nanotubes and that other types of nanotubes can also be used. That is, any material that forms metal or oxide or polymer nanotubes with structures having a vertically aligned array may be used in the sensors of the present invention. Suitable materials include but are not limited to semiconductors, conductive and non-conductive materials known in the field. Suitable materials include but are not limited to oxides, carbides, nitrides, borides, or mixed ceramics. For example, the selection of material in the case of an auditory sensor can be based on the resonance corresponding to a desirable frequency for detection. Accordingly, it is envisioned that silicon nanotubes are promising for this purpose. 
         [0040]    In one embodiment depicted by  FIGS. 1A and 1B , a CNT array is used as a touch sensor  10 . The touch sensor  10  shown in  FIG. 1  uses helical shaped CNTs as the CNT array  12 . It is to be appreciated that the center axes of the helical CNTs in the array  12  are vertically aligned on at least one substrate. As shown in  FIGS. 1A and 1B , the helical CNT array  12  is provided in the structure vertically between two substrates  14 ,  16  each having a plurality of electrodes  18 ,  20 . Permanent magnets  22 ,  24  are provided such that a magnetic field (W) is normal to an axis of the helical CNT array, between the two substrates as shown. In the helical CNT array  12 , each individual helical CNT or a bundled helical CNT pattern, with a size ranging from micro-meters to nano-meters, functions as a touch sensor probe. The electrodes  18 ,  20  on each of the substrates  14 ,  16  correspond in number to the number of helical CNTs or bundled helical CNT patterns, and the corresponding electrodes and CNTs together form sensing units on the two substrates. The substrates  14 ,  16  comprise elastomers, for example, to add softness and resistance to wear and tear to the touch sensor  10 , if desired. 
         [0041]    Although not wishing to be bound by theory, it is believed that the moving theory is as explained as follows. When an external force is applied to a surface portion of one of the elastomer substrates  14  or  16 , electricity is generated from the provided magnetic field (W) and the applied external force according to Fleming&#39;s right hand law. By referencing the distribution of the generated electricity (I) on the whole of array, the location on which the external force was applied can be identified. 
         [0042]    In another embodiment shown in  FIGS. 2A and 2B , a touch sensor  26  in which a helical CNT array  12  is also used. However, in this touch sensor  26 , in place of the permanent magnets, bias voltages are applied between the two electrodes  18 ,  20  provided on elastomer substrates  14 ,  16 . When an external force F is loaded to a surface portion of one of the elastomers, the length of the corresponding helical CNT or a bundled helical CNTs provided by the surface portion is changed from L 0  to L 1 . It is to be appreciated that this change in the length affects an inductance of the CNT or bundled helical CNTs. As the result, the bias voltage is changed from a non-loading voltage V 0  to a loading voltage V 1 . By identifying the location at which the voltage has changed, the place on the surface of the elastomer to which the external force is loaded can be detected. 
         [0043]    In still another embodiment,  FIGS. 3A ,  3 B, and  3 C disclose a touch sensor  28  having a straight CNT array  30  coated by a piezoelectric polymer  32 , which is used as touch sensor probes. As shown, the axes of the CNTs in the array  30  are vertically aligned on a substrate  29 . As an example, the CNT array  30  can be produced by a synthetic method using pyrolysis of iron phthalocyanine (FeC 32 N 8 H 16 ). In one embodiment, the piezoelectric polymer  32  is polyvinylidene fluoride (PVDF), for example. For convenience during grasping and protection from wear during contact in the robotic application, the probes are embedded into elastomer A in the shape of a minute protuberance (“micropoint”). Moreover, a number of micropoints are made on the surface of elastomer B. To provide the robot hands  34 ,  36  with an improved grasping surface, in one embodiment, elastomer A is softer than elastomer B in elastic modulus. As an example of these elastomers, a silicone rubber can be used, wherein the desired elastic modulus, as an example, can be adjusted by adding graphite to the silicone rubber. By the use of this structure, the real surface area of the piezoelectric polymer  32  on the probe increases by about 1000 times, thereby enlarging sensor sensitivity and decreasing cost by minimization of consumption on the coating materials. 
         [0044]    Although not wishing to be bound by theory, it is believed that the moving theory of this sensor is as explained as follows. As is shown in  FIG. 3D , two touch sensors  28   a ,  28   b  are set on each robot hand  34 ,  36  which are arranged face to face, and the process of grasping an object  38  is set up. The robot hands  34 ,  36  are holding the object  38 , such as an empty glass, stably by force (F). Next, water is poured into the glass, changing the weight of the object  38 . As a result, the object  38  falls from the robot hands  34 ,  36  towards the ground with a minute slip. At the moment of the slip in the robot hands  34 ,  36 , the sensor probes  28   a ,  28   b  at the slip point detect a high frequency pulse and transmit a signal to a circuit (not shown) to control the grasping force (F). The circuit responds to the received signal, loading new force (F′), which is an increase (ΔF) that is added to grasping force (F), such that the robot hands  34 ,  36  grasp the slipping glass after the weight change. 
         [0045]    One illustrative process of making the touch sensor shown in  FIGS. 3A ,  3 B, and  3 C is shown in  FIG. 4 . In step  40 , a substrate  202  is masked using a physical mask  204  (e.g., a grid mesh for TEM). In step  42 , a material for an electrode  206  (e.g., Au) is deposited on the substrate. Next, in step  44 , catalytic metals  208 , including, but not limited to, Fe, Ni, Mo and Co, are deposited on the surface of the substrate  202  with the electrode  206  deposited thereon. Then in step  46 , the physical mask  204  is removed, and desirable patterns of the metals are built on the substrate  202 . The substrate  202  is then inserted into a furnace in step  48 , and the CNT arrays  210  grow on the substrate using a carbon source (e.g., acetylene) at high temperature. After the substrate  202  cools down, in step  50 , a functional polymer, including but not limited to, PVDF is coated using a mask with a spin coating or plasma polymerization applied to the surface of patterns of CNT arrays  210 . Then in step  52 , a material for an electrode  212  (e.g., Au) is deposited on the surface of polymer-coated CNT patterns. After this process, the mask is removed and the substrate  202  is set into a mold  214  with many circular cavities  216  provided with a liquid silicone rubber in step  54  to provide protuberances to the surface of the touch sensor. In step  56 , the mold is removed after solidification of the elastomer, thereby providing the micropoints of CNTs  218  on the substrate  202 . 
         [0046]    In another embodiment, an auditory sensor  60  using a CNT array  62  is provided as shown in  FIGS. 5A and 5B . A plurality of auditory sensor probes  64   a ,  64   b ,  64   c , etc., with different forms provided between a pair of optically transparent substrates  66 ,  68  (e.g., SiO 2 ) having holes  70 . By the term “different forms” it is meant that the distinctions between the various auditory sensor probes vary in physical parameters such as diameter, length, and elastic modulus. The different forms contribute to each of the auditory sensor probes having a sensitivity to a particular resonant frequency(ies). Specifically, several frequencies in a sound wave are divided among various nanotubes probes provided between the optical transparent substrates  66 ,  68 . This is similar to the frequency selectivity which depends on the point on a basilar membrane in a cochlea in human. The theory of detection is introduced as follows. 
         [0047]    A method of detecting frequencies is provided by observing the existences of light  72  (e.g., from a light source) passing through the holes  70  of each of the substrates  66 ,  68  using a light detection device  74 . As is shown in  FIG. 5 , pairs of the holes  70  of the substrate  66 ,  68  coincide with respective ones of the auditory sensor probes  64   a ,  64   b ,  64   c ,  64   d , etc. As light is irradiated from a side of one of the substrates, e.g., substrate  66 , frequency(ies) of a sound wave  76  is determined on the basis of a light pattern  78  detected passing through the holes  70  by the light detection device  74 . In one embodiment, the light detection device is a CCD camera; however, any light detection device suitable for the above described purpose may be used. 
         [0048]    In the above described process and sensor  60 , some of the light passing through the holes  70  of the first substrate  66  may be blocked temporarily by an associated auditory sensor probe vibrating due to a resonant frequency being provided by the sound wave  76 . For example, should the sound wave  76  resonate the probe  64   d  provided at the lower right hand corner of the illustrated sensor  60 , light will not pass through hole  70   b  of the substrate  68 , such that the corresponding square or pixel  80  in the light pattern  78  in the lower right hand corner will not be illuminated at that time, as shown in  FIG. 5B . It is to be appreciated that multiple probes  64  in the sensor  60  may resonant at each time under the influence of the specific frequencies of the sound wave  76 , thereby providing a number of holes blocked in the substrate  68  and resulting in a light pattern at that time, such as for example, the illustrated light pattern  78 . 
         [0049]    As in shown in  FIGS. 5C and 5D , in advance, resonances of the probes  64  in the sensor  60  for every frequency are mapped as a distribution of the detected light patterns f 1 , f 2 , f 3 , f 4 , f 5 , etc. over a time period. These light patterns  78  are registered to a computer as a library  79 . Accordingly, a frequency is identifiable by comparing actual light pattern observations  81  to the stored light patterns f 1 , f 2 , etc. in the computer library  79 . Moreover, based on the distribution of the light patterns in chronological order, as shown in  FIG. 5D , sound localization is possible using the sensor  60 . 
         [0050]    In the embodiment shown in  FIGS. 6A and 6B , an auditory sensor  82 , which uses a NT array  84  coated by a piezoelectric polymer  86  and an electrode layer  88  on a substrate  90 , is provided. In this embodiment, electricity is generated when the NTs are resonated by an external sound wave. By identification of the place on which electricity is generated, a frequency is recognized. 
         [0051]      FIG. 7  describes a method for making an auditory sensor  82  shown in  FIG. 6 . After masking a substrate, a catalytic metal like Fe, Ni, Mo, and Co is deposited on the substrate in step  92 . Then, the mask is removed and catalytic patterns are made on the substrate. Next, in step  94 , other catalytic patterns are deposited using a different mask on the substrate. In this process, a place already deposited at the first masking is covered selectively using the different mask. Then, the substrate is inserted into a furnace with a flow of an aforementioned carbon source. NTs depend on various catalysts in order to be grown and therefore, it is possible to control growth parameters of NTs, such as, for example, length and diameter. Accordingly, the result of step  94  is the synthesis of NTs with different parameters provided on the same substrate. Finally, a piezoelectric polymer, such as for example, PVDF, is coated on the surface of the NT array patterns in step  96 . The coating layer can be provided using a method such as, for example, spin coating or plasma polymerization. 
       EXAMPLE 1 
       [0052]    As shown schematically in  FIGS. 8A and 8B , a touch sensor  98  was made using vertically aligned straight CNT arrays. The CNT arrays were prepared by a thermal CVD process at 800° C. using ferrocene as a catalyst and xylene as a carbon source. The CNT arrays were created on a semiconductive substrate (SiO 2 ) and then transferred to an aluminum substrate  100 . A connection between the aluminum substrate  100  and the CNT arrays was made using a carbon based adhesive. In particular, six individual CNT sensor electrodes  102  were supported by the aluminum substrate  100  and were made by cutting a 500 μm-long densely packed CNT array to the size of a 2 mm square individually. The outer walls of the CNT electrodes  102  were then coated by a thin layer of polyvinylidene fluoride (PVDF)  104  as a piezoelectric polymer. A thin gold layer was sputter-coated onto the PVDF-coating  104  to construct another electrode  106 . Finally, the whole device was encapsulated with a thin layer of an elastomer (silicon rubber)  108  to ensure the softness and durability of the sensor  98 . A wire lead  110  was extracted from the top surface of each of the sensor probes individually, whereas a common wire lead  112  to all of the six sensor probes was attached at the bottom. The physical size of this sensor was 40 mm in length, 8 mm in width, and 4 mm in thickness. 
         [0053]    The touch sensor was tested by touching/pushing with a finger, and the sensing results are shown in  FIG. 9 . The sensing signal from each sensor probe was independently recorded.  FIG. 9  clearly shows that for all probes, the appearance and disappearance of the electric voltages upon touching/pushing the sensor with a finger and removing the finger from the sensor, respectively, were detected. As shown, voltage signals with the maximum value of + and − in the range from 40 to 50 mV were generated. The observation of the bipolar signals is probably because the PVDF coating covers the whole surface of aligned CNTs. Namely, the coating causes the expansion at the one side of the PVDF coating and contraction on the other side upon torsion of the CNT by a loading force. 
       EXAMPLE 2 
       [0054]    The Preparation of a Patterned Array of a Sensor 
         [0055]    A patterned array of a sensor was prepared on an Al 2 O 3  substrate, which had  16  individual sensor probes, as shown in  FIG. 10A . A gold padding substrate was pre-laid down underneath of the sensor probes for addressing the sensor units. To support the growth of vertically aligned CNTs within each of the patterned sensor probes, chromium (Cr) was first selectively deposited into the patterned area shown in  FIG. 10B  on a pristine Al 2 O 3  substrate by photolithographic patterning. Then, iron (Fe) catalyst was selectively deposited onto the small squared area  304  within the Cr-covered square patterned regions ( FIG. 10B ). Thereafter, the metal patterned substrate was inserted to a furnace to synthesize the CNT array. The synthesis of the CNT array was done under decreased pressure with flowing of an acetylene gas and a hydrogen gas at 750° C. A vertically aligned CNT array thus prepared is shown in  FIG. 10C . The CNT array was grown only on the Fe-deposited area. Polyvinylidene fluoride (PVDF) dispersed in N,N-dimethylformamide (DMF) was deposited onto the sensor probe area using a physical mask to cover the electrode region. The PVDF coating was dried by heating at 90° C. Finally, a solution mixture containing silver powders and ethylene vinyl acetate copolymer was mounted as a flexible electrode on the PVDF. Finally, the poling treatment for the sensor unit was done under an electrical field up to 50 μm −1  at 90° C. using a DC voltage supply. 
         [0056]    Voltage Signal Detection Test 
         [0057]    The voltage signal detection test was done using the sensor unit of  FIG. 11 , which includes one common electrode  306  with other electrode being selected from the numbered terminals  308  between 1 and 16. One side of the electrode on the sensor unit was covered by an aluminum foil to enhance the output signal by summarizing all of the measured. The touching stimulation was provided by using a small stick loaded manually.  FIG. 12  shows the results from the voltage detection test, which shows the “on” and “off” signals in response to the actions of the “push on” and “pull off”, respectively. Although the voltage signals are not uniform because of the manual loading, the range of the signals is approximately between 0.1 mV to −0.2 mV. 
         [0058]    For comparison, a sensor with only a PVDF coating (PVDF sensor) on the metal patterned substrate without CNTs was also prepared and tested separately. As shown in  FIG. 13 , the voltage signals created by CNT/PVDF sensor are much larger than signals by PVDF sensor. 
         [0059]    The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.