Patent Publication Number: US-10784444-B2

Title: Light detector

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to applications entitled, “METHOD FOR MAKING CARBON NANOTUBES”, filed May 14, 2018 (Ser. No. 15/979,423), “CARBON NANOTUBE ARRAY”, filed May 14, 2018 (Ser. No. 15/978,266), “THIN FILM TRANSISTOR”, filed May 14, 2018 (Ser. No. 15/978,264), and “PHOTOELECTRIC CONVERSION DEVICE”, filed May 14, 2018 (Ser. No. 15/978,262). 
     FIELD 
     The subject matter herein generally relates to a light detection element. 
     BACKGROUND 
     Carbon nanotubes have excellent properties such as electrical properties, high Young&#39;s modulus and tensile strength, and high thermal conductivity. 
     In carbon nanotubes prepared by the conventional chemical vapor deposition (CVD) method, the proportion of the metallic carbon nanotubes (m-CNTs) and the semiconducting carbon nanotubes (s-CNTs) is typically about 1:2. 
     It has been impossible to directly grow s-CNTs with higher purity. Meanwhile, the chirality of carbon nanotubes cannot be arbitrarily changed during growth, in the prior art. Semiconducting carbon nanotube segments and metallic carbon nanotube segments cannot be alternately formed during growth according to need. Thus, carbon nanotubes with alternating semiconducting carbon nanotube segments and metallic carbon nanotube segments have not been available for carbon nanotube structures or in the application of carbon nanotube structures, such as thin film transistors, light detectors, and photoelectric conversion modules. 
     Therefore, there is room for improvement within the art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the embodiments can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the embodiments. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. Implementations of the present technology will be described, by way of example only, with reference to the attached figures 
         FIG. 1  is a flow chart of an embodiment of a method for making carbon nanotubes. 
         FIG. 2  is a schematic view of an embodiment of controlling the chirality of carbon nanotubes using an electric field. 
         FIG. 3  is a schematic view of an embodiment of controlling the chirality of carbon nanotubes using an electric field. 
         FIG. 4  is a schematic view of an embodiment of carbon nanotubes with chirality after removing the electric field. 
         FIG. 5  is a Scanning Electron Microscope (SEM) image of carbon nanotubes before and after applying a reverse electric field. 
         FIG. 6  is a schematic view of an embodiment of controlling the formation of carbon nanotubes with chirality using an electric field. 
         FIG. 7  is a schematic view of an embodiment of chirality of carbon nanotubes after removing the electric field. 
         FIG. 8  is a schematic view of an embodiment of a pulsed electric field voltage changing with time. 
         FIG. 9  is a schematic view of an embodiment of a chiral change of carbon nanotubes in a pulsed electric field when the pulse width is less than 500 ms. 
         FIG. 10  is a schematic view of an embodiment of a method of removing the head portion of the carbon nanotube structure. 
         FIG. 11  is a schematic view of an embodiment of the chirality of carbon nanotubes changing with electric field when the pulse width is greater than 500 ms. 
         FIG. 12  is a schematic view of an embodiment of the chirality of new carbon nanotubes changing with a pulsed electric field. 
         FIG. 13  is a schematic view of an embodiment of the chirality of new carbon nanotubes changing after removing the pulsed electric field. 
         FIG. 14  is a schematic view of an embodiment of the chirality of new carbon nanotubes changing after removing the pulsed electric field. 
         FIG. 15  is an SEM image of carbon nanotubes when the pulse width is 100 ms. 
         FIG. 16  is an SEM image of carbon nanotubes when the pulse width is 10 seconds. 
         FIG. 17  is a structural schematic view of an embodiment of a carbon nanotube array. 
         FIG. 18  is a structural schematic view of an embodiment of a thin film transistor. 
         FIG. 19  is a chart showing test results in an embodiment of the thin film transistor. 
         FIG. 20  is a structural schematic view of another embodiment of a thin film transistor. 
         FIG. 21  is a structural schematic view of an embodiment of a light detector. 
         FIG. 22  is a structural schematic view of an embodiment of a photoelectric conversion device. 
         FIG. 23  is a sectional schematic view of another embodiment of a photoelectric conversion device. 
         FIG. 24  is a structural schematic view of an embodiment of a photoelectric conversion device. 
     
    
    
     DETAILED DESCRIPTION 
     It will be appreciated that for simplicity and clarity of illustration, where appropriate, reference numerals have been repeated among the different figures to indicate corresponding or analogous elements. In addition, numerous specific details are set forth to provide a thorough understanding of the embodiments described herein. However, it will be understood by those of ordinary skill in the art that the embodiments described herein can be practiced without these specific details. In other instances, methods, procedures, and components have not been described in detail so as not to obscure the related relevant feature being described. The drawings are not necessarily to scale, and the proportions of certain parts may be exaggerated to better illustrate details and features. The description is not to be considered as limiting the scope of the embodiments described herein. 
     Several definitions that apply throughout this disclosure will now be presented. 
     The connection can be such that the objects are permanently connected or releasably connected. The term “substantially” is defined to be essentially conforming to the particular dimension, shape, or other feature that the term modifies, such that the component need not be exact. The term “comprising” means “including, but not necessarily limited to”; it specifically indicates open-ended inclusion or membership in a so-described combination, group, series, and the like. It should be noted that references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean “at least one.” 
     In  FIG. 1  and  FIG. 2 , an embodiment of a method of making carbon nanotubes comprises:
         S 11 , depositing a catalyst layer  12  on a substrate  11 ;   S 12 , placing the substrate  11  having the deposited catalyst layer  12  in a reaction furnace  13 , then heating the reaction furnace  13  to obtain a predetermined temperature, introducing a carbon source gas  14  and a protective gas  15  into the reaction furnace  13  to grow a first carbon nanotube structure  161 , wherein the first carbon nanotube structure  161  includes a plurality of carbon nanotube segments;   S 13 , applying an electric field to the first carbon nanotube structure  161 , wherein the direction of the electric field is the direction in which the catalyst layer  12  is positively charged; and   S 14 , reversing the direction of the electric field to grow a second carbon nanotube structure  162  from the first carbon nanotube structure  161 , wherein the first carbon nanotube structure  161  and the second carbon nanotube structure  162  form a carbon nanotube structure  16 , and the second carbon nanotube structure  162  includes a plurality of semiconducting carbon nanotube segments.       

     In step S 11 , carbon nanotubes are grown on the substrate  11 . The material of the substrate  11  can be silicon, glass, quartz, or the like. In an embodiment, the substrate  11  is ST-cut single crystal quartz substrate. 
     The catalyst layer  12  can be a catalyst powder layer. The material of the catalyst layer  12  can be any catalyst material sufficient to grow single-walled carbon nanotubes. The catalyst layer  12  can be iron powder layer, iron film, nickel powder layer, nickel mesh, or mixture layer of alumina powder and iron powder. The method of forming the catalyst layer  12  on the substrate  11  can be electron beam deposition, vapor deposition, sputtering, or spraying. In an embodiment, the catalyst layer  12  is an iron powder layer with a particle diameter of 0.2 nanometers, the catalyst layer  12  is deposited on the substrate  11  by electron beam deposition. 
     In step S 12 , the substrate  11  having the deposited catalyst layer  12  is placed in the reaction furnace  13 . The predetermined temperature is in a range of 800 degrees Celsius to 960 degrees Celsius. The predetermined temperature can be in a range of 900 degrees Celsius to 950 degrees Celsius. In an embodiment, the predetermined temperature is 950 degrees Celsius. 
     The carbon source gas  14  can be carbon monoxide, or hydrocarbon such as acetylene, methane, ethane, or ethylene, or vapor containing carbon atoms, like ethanol. The protective gas  15  can be hydrogen gas, nitrogen gas, or another inert gas. The flow ratio of the carbon source gas  14  and the protective gas  15  can be adjusted according to the type and requirement of the carbon source gas. In an embodiment, the carbon source gas  14  is methane, and the protective gas  15  is hydrogen gas. The H 2  with a flow rate of 500 sccm and CH 4  with a flow rate of 200 sccm are introduced into the reaction furnace  13  for ten minutes, then the H 2  with a flow rate of 5 sccm and CH 4  with a flow rate of 2 sccm are introduced into the reaction furnace  13  to grow the first carbon nanotube structure  161 . 
     The method of growing the first carbon nanotube structure  161  is not limited to the growth method in steps S 11 -S 12  described above, the method can be arc discharge method or laser ablation method. The first carbon nanotube structure  161  includes a plurality of carbon nanotube segments. The plurality of carbon nanotube segments are arranged in an array, and plurality of carbon nanotube segments substantially extend along the same direction. The quantity density of the carbon nanotube segments in the first carbon nanotube structure  161  is greater than 3/μm to ensure that the chirality of carbon nanotube segments can be effectively controlled by the electric field in the growth process of the carbon nanotube segments. The term “quantity density” refers to the number of carbon nanotube segments per micron along a direction which is perpendicular to the growth direction of the carbon nanotube segments. In an embodiment, the carbon nanotube segments are arranged in a horizontal array, and the quantity density of the carbon nanotube segments is greater than 6/μm. The growth process of the plurality of carbon nanotube segments can be bottom growth, or top growth. When the growth process of the plurality of carbon nanotube segments is bottom growth, the catalyst layer  12  is adhered to the surface of the substrate  11 , and the carbon nanotube segments start growing from the surface of the catalyst layer  12  and grow away from the catalyst layer  12 . When the growth process of the plurality of carbon nanotube segments is top growth, the carbon nanotube segments start growing from the surface of the catalyst layer  12  and grow with the catalyst layer  12 . The first carbon nanotube structure  161  includes a plurality of metallic carbon nanotube segments. The carbon nanotube segments of the first carbon nanotube structure  161  are single-walled carbon nanotubes. The diameter of a single-walled carbon nanotube is less than 2 nanometers. The diameter of the single-walled carbon nanotubes can be in a range of 1.2 nanometers to 1.5 nanometers. In an embodiment, the first carbon nanotube structure  161  includes both semiconducting carbon nanotube segments and metallic carbon nanotube segments, and the ratio of the semiconducting carbon nanotube segments and the metallic carbon nanotube segments is about or less than 2:1. 
     In step S 13 , the electric field applied to the carbon nanotube segment structure  16  can be a direct current (DC) electric field or a pulsed electric field. A direction of the electric field is set to a positive direction when the catalyst layer is positively charged. A direction of the electric field is set to a negative direction when the catalyst layer is negatively charged. The positive direction can be reversed to the negative direction. In  FIG. 3 , “s” refers to semiconducting, “m” refers to metallic, and “E” refers to the applied electric field. In an embodiment, the electric field is the DC electric field. When a positive electric field is first applied to the first carbon nanotube structure  161 , the growth of the first carbon nanotube structure  161  is not affected and the chirality of the carbon nanotube segments in the first carbon nanotube structure  161  remains unchanged. Thus, the metallic carbon nanotube segments remain as pure metallic carbon nanotube segments before and after applying the positive electric field, and the semiconducting carbon nanotube segments remain as pure semiconducting carbon nanotube segments before and after applying the positive electric field. 
     In step S 14 , the positive electric field is reversed to a negative electric field. Such change from the positive electric field to the negative electric field causes the chirality of the grown carbon nanotube segments to change. When the electric field direction changes from the positive direction to the negative direction, the carbon nanotube segments of the second carbon nanotube structure  162  are mostly semiconducting carbon nanotube segments. The second carbon nanotube structure  162 , grown from the semiconducting carbon nanotube segments of the first carbon nanotube structure  161 , are semiconducting carbon nanotube segments. The carbon nanotube segments of the second carbon nanotube structure  162 , that are grown from the metallic carbon nanotube segments of the first carbon nanotube structure  161 , are also semiconducting carbon nanotube segments. 
     The second carbon nanotube structure  162  can become pure semiconducting carbon nanotube segments by applying an inverted electric field. The change of the electric field direction from positive to negative increases negative charges, which increases the Fermi level of the catalyst layer. The higher the Fermi level, the easier is the transition from metallic carbon nanotube segments to semiconducting carbon nanotube segments. In an embodiment, the raised value of the Fermi level of the catalyst layer is greater than or equal to 0.7 eV. In an embodiment, the raised value of the Fermi level of the catalyst layer is 1 eV. 
     When the positive electric field is originally applied to the first carbon nanotube structure  161 , the application time of the positive electric field must not be too short, the carbon nanotube segments must have enough time inside a positive electric field environment. In an embodiment, the application time of the positive electric field is greater than or equal to 2 seconds, and the electric field intensity of the positive electric field is greater than or equal to 200 volts per millimeter (v/mm). When the positive electric field is reversed, the electric field environment of the carbon nanotube segments changes violently, which directly affects the chirality of the carbon nanotube segments. Gibbs free energy for growing semiconducting carbon nanotubes is less than Gibbs free energy for growing metallic carbon nanotubes after the catalyst layer is negatively charged. When the electric field reverses from positive to negative, the second carbon nanotube structure  162  mostly comprises semiconducting carbon nanotube segments. The proportion of semiconducting carbon nanotube segments in the second carbon nanotube structure  162  is related to the application time of the negative electric field. The application time of the negative electric field depends on the area of electrodes. The larger the area of electrodes, the longer the application time of the negative electric field. For example, when the area of electrodes is very small (4 mm×8 mm), the carbon nanotube segments can become completely semiconducting carbon nanotube segments with the application time of the negative electric field being 100 ms; when the area of electrodes is very large (4 cm×4 cm), the carbon nanotube segments can become completely semiconducting carbon nanotube segments with the application time of the negative electric field being 500 ms, the carbon nanotube segments can become completely semiconducting carbon nanotube segments when the application time of the negative electric field is in a range of 100 ms to 500 ms. In an embodiment, the area of electrodes is 5 mm×2 cm, and the electrodes are used in the following. When the application time of the negative electric field is less than 200 ms, only part of the carbon nanotube segments of the second carbon nanotube structure  162  are semiconducting carbon nanotube segments. If the application time of the negative electric field is too short, other part of the carbon nanotube segments of the second carbon nanotube structure  162  remains metallic carbon nanotube segments. When the application time of the negative electric field is greater than 200 ms, all the carbon nanotube segments of the second carbon nanotube structure  162  become semiconducting carbon nanotube segments. In  FIG. 4 , when all the carbon nanotube segments of the second carbon nanotube structure  162  are semiconducting carbon nanotube segments, the growth of the carbon nanotube structure  16  is not affected by the negative electric field. After removing the negative electric field, the chirality of new carbon nanotube segments grown from the carbon nanotube structure  16  will remain unchanged. In  FIG. 5  is an SEM image of carbon nanotubes before and after applying the reverse electric field. The white strips are metallic carbon nanotube segments, the dark strips are semiconducting carbon nanotube segments. In an embodiment, the application time of the positive electric field is 20 seconds, the electric field intensity of the positive electric field is 200 v/mm, the application time of the negative electric field is 500 milliseconds, and the electric field intensity of the negative electric field is −200 v/mm. 
     Alternatively, in  FIG. 6 , when the electric field direction changes from negative to positive, the chirality of the carbon nanotube segments can also change during growth. When a negative electric field is first applied to the first carbon nanotube structure  161 , the growth of the first carbon nanotube structure  161  is not affected and the chirality of the carbon nanotube segments in the first carbon nanotube structure  161  remains unchanged. When the electric field direction changes from negative to positive, the second carbon nanotube structure  162  are mostly metallic carbon nanotube segments. The second carbon nanotube structure  162 , grown from the metallic carbon nanotube segments of the first carbon nanotube structure  161 , are metallic carbon nanotube segments. The second carbon nanotube structure  162 , grown from the semiconducting carbon nanotube segments of the first carbon nanotube structure  161 , are also metallic carbon nanotube segments. The change of the electric field direction from negative to positive increases positive charges, which can reduce the Fermi level of the catalyst layer. The lower the Fermi level, the easier is the transition from semiconducting carbon nanotube segments to metallic carbon nanotube segments. In an embodiment, the reduced value of the Fermi level of the catalyst layer is less than or equal to −0.1 eV. In an embodiment, the reduced value of the Fermi level of the catalyst layer is less than or equal to −0.2 eV. In  FIG. 7 , when all the carbon nanotube segments of the second carbon nanotube structure  162  are metallic carbon nanotube segments, the growth of the carbon nanotube structure  16  is not affected by the positive electric field. After removing the positive electric field, the chirality of new carbon nanotube segments grown from the carbon nanotube structure  16  remains unchanged. 
     Furthermore, applying a pulsed electric field to the first carbon nanotube structure  161  can change the chirality to an alternating chirality. In  FIG. 8 , the pulsed electric field is a periodic electric field formed by a positive electric field pulse followed by a negative electric field pulse. The period T of the pulsed electric field is the sum of the pulse width of one positive electric field pulse and the pulse width of one negative electric field pulse. The chirality of the carbon nanotube segments in the carbon nanotube segment structure  16  can be controlled by adjusting the pulse width of the negative electric field pulse and the electric field direction. 
     In  FIG. 8 , when the pulse width of the negative electric field pulse is less than 200 milliseconds, and the electric field direction changes from positive to negative, only part of the carbon nanotube segments of the second carbon nanotube structure  162  are semiconducting carbon nanotube segments. The change in the field direction (from negative to positive), does not change the chirality of the carbon nanotube segments of the second carbon nanotube structure  162 . 
     In  FIG. 9 , when the pulse width of the negative electric field pulse is greater than 200 milliseconds and less than or equal to 500 milliseconds, and the electric field direction changes from positive to negative, all the carbon nanotube segments of the second carbon nanotube structure  162  are semiconducting carbon nanotube segments; the change in field (from negative to positive), does not change the chirality of the carbon nanotube segments of the second carbon nanotube structure  162 . 
     Furthermore, in  FIG. 10 , pure semiconducting carbon nanotubes can be obtained by removing the metallic carbon nanotube segments of the carbon nanotube structure  16 , wherein the metallic carbon nanotube segments are in the head portion of the carbon nanotube structure  16 . 
     In  FIG. 11 , when the pulse width of the negative electric field pulse is greater than 500 milliseconds, and the electric field direction changes from positive to negative, all the carbon nanotube segments of the second carbon nanotube structure  162  are semiconducting carbon nanotube segments. When electric field direction changes from negative to positive, the carbon nanotube segments of the second carbon nanotube structure  162  can become metallic carbon nanotube segments. When the pulse width of the negative electric field pulse is too long, the catalyst layer can spontaneously discharge so that the negative charge decreases. Upon the electric field direction changing from negative to positive, the positive electric field pulse applied to the catalyst layer is sufficient to change the chirality from semiconducting to metallic. When the pulse width of the negative electric field pulse is greater than 500 milliseconds, as the application time of the pulsed electric field increases, the chirality of carbon nanotube segments which are growing from the carbon nanotube structure  16  changes alternately. 
     In  FIG. 12 , as period T increases, new carbon nanotube segments grown from the carbon nanotube structure  16  consist of alternating metallic carbon nanotube segments and semiconducting carbon nanotube segments. 
     Furthermore, in  FIG. 13  and  FIG. 14 , after removing the pulsed electric field, the chirality of the new carbon nanotube segments remains unchanged. Pure metallic carbon nanotubes can be obtained by removing the head portion of the carbon nanotube structure. 
     In  FIG. 15 , an SEM image of carbon nanotube structure is shown, wherein the period T of the pulsed electric field is 20 seconds, and the pulse width of the negative electric field pulse is 100 milliseconds. The bright white strips are metallic carbon nanotube segments, and the light white strips are semiconducting carbon nanotube segments. 
     In  FIG. 16 , an SEM image of carbon nanotube structure is shown, wherein the period T of the pulsed electric field is 20 seconds, and the pulse width of the negative electric field pulse is 10 seconds. The carbon nanotube structure is an array formed by a plurality of carbon nanotubes, wherein each carbon nanotube is forming semiconducting carbon nanotube segments and metallic carbon nanotube segments on an alternating basis. 
     The method of making carbon nanotubes includes particular advantages. Firstly, the chirality of the carbon nanotube segments grown from a carbon nanotube structure can be adjusted by applying an electric field which is reversible. Secondly, alternating semiconducting carbon nanotube segments and metallic carbon nanotube segments can be obtained by adjusting the pulse width of the negative electric field pulse and the electric field direction. 
     In  FIG. 17 , an embodiment of a carbon nanotube array  20  made by the above method comprises a plurality of carbon nanotubes. Each carbon nanotube includes at least one semiconducting carbon nanotube segment (S) and at least one metallic carbon nanotube segment (M). The plurality of carbon nanotubes are arranged to form an array. The semiconducting carbon nanotube segment and the adjacent metallic carbon nanotube segment are connected by the Schottky barrier. 
     The plurality of carbon nanotubes comprise semiconducting carbon nanotube segments and metallic carbon nanotube segments. Each carbon nanotube can consist of only one semiconducting carbon nanotube segment and only one metallic carbon nanotube segment, and the semiconducting carbon nanotube segment and the metallic carbon nanotube segment are an integrated structure, and are connected by the Schottky barrier. Each carbon nanotube can also consist of a plurality of semiconducting carbon nanotube segments and a plurality of metallic carbon nanotube segments, and each semiconducting carbon nanotube segment and each metallic carbon nanotube segment are alternately arranged. The semiconducting carbon nanotube segments and the adjacent metallic carbon nanotube segments are connected by the Schottky barrier. 
     In  FIG. 17 , (a)-(e), in the carbon nanotube array  20 , the structure of the carbon nanotubes can be M-S-M type carbon nanotubes, S-M-S type carbon nanotubes, or S-M type carbon nanotubes. In the plurality of carbon nanotubes, the lengths of the semiconducting carbon nanotube segment are substantially the same, and the lengths of the metallic carbon nanotube segment are substantially the same. The term “substantially” means that the lengths of the carbon nanotube segments may be slightly different in each carbon nanotube. In the actual production process, since the growth rate of each carbon nanotube is slightly different, the length of each carbon nanotube may be slightly different at the point when the direction of the electric field changes. 
     The length of each semiconducting carbon nanotube segment or the length of each metallic carbon nanotube segment in the carbon nanotubes can be adjusted by changing the application time period of the electric field. The carbon nanotubes in the carbon nanotube array  20  are single-walled carbon nanotubes, and the diameter of the single-walled carbon nanotubes is less than 2 nanometers. Furthermore, the diameter of the single-walled carbon nanotubes is in a range of 1.2 nanometers to 1.5 nanometers. 
     In an embodiment, each carbon nanotube consists of a plurality of semiconducting carbon nanotube segments and a plurality of metallic carbon nanotube segments. The semiconducting carbon nanotube segments and the metallic carbon nanotube segments are alternately arranged, and the diameter of each carbon nanotube is 1.3 nanometers. 
     The carbon nanotube array  20  has particular advantages. Each carbon nanotube includes metallic carbon nanotube segments and semiconducting carbon nanotube segments. The carbon nanotube array  20  is an integrated structure being partly conductive and partly semiconductive. Thus, the carbon nanotube array  20  has a wide range of potential uses. 
     For example, in  FIG. 18 , an embodiment of a thin film transistor  30  comprises an insulating substrate  31 , a gate electrode  32 , a gate insulating layer  33 , a carbon nanotube structure  34 , a source electrode  35 , and a drain electrode  36 . The carbon nanotube structure  34  includes at least one carbon nanotube. One end of the carbon nanotube is a first metallic carbon nanotube segment  341 , and the other end of the carbon nanotube is a second metallic carbon nanotube segment  342 . There is a semiconducting carbon nanotube segment  343  in the middle of the carbon nanotube. The gate electrode  32  is located on a surface of the insulating substrate  31 . The gate insulating layer  33  is located on a surface away from the insulating substrate  31  of the gate electrode  32 . The carbon nanotube structure  34  is located on a surface away from the gate electrode  32  of the gate insulating layer  33 . The source electrode  35  is located on the first metallic carbon nanotube segment  341 , and the drain electrode  36  is located on the second metallic carbon nanotube segment  342 . The source electrode  35  and the drain electrode  36  are electrically connected to the carbon nanotube structure  34 . The semiconducting carbon nanotube segment  343  is used as a channel. The carbon nanotube structure  34 , the source electrode  35 , and the drain electrode  36  are insulated from the gate electrode  32 . Furthermore, the source electrode  35  and the drain electrode  36  are selectable, thus the first metallic carbon nanotube segment  341  can be used as a source electrode, and the second metallic carbon nanotube segment  342  can be used as a drain electrode. 
     The insulating substrate  31  is a support. Materials of the insulating substrate  31  can be rigid materials (e.g., glass, quartz, ceramics, diamond), or flexible materials (e.g., plastic or resin). The insulating substrate  31  can be polyethylene terephthalate, polyethylene naphthalate, or polyimide. In an embodiment, the material of the insulating substrate  31  is polyethylene naphthalate. 
     The gate electrode  32 , the source electrode  35 , and the drain electrode  36  are conductive materials. The conductive materials can be metal, indium tin oxide, antimony tin oxide, conductive silver paste, conductive polymer, and conductive carbon nanotubes. The material of the metal can be aluminum, copper, tungsten, molybdenum, gold, titanium, or palladium. In an embodiment, the gate electrode  32 , the source electrode  35 , and the drain electrode  36  are metal composite structures formed by gold and titanium, the gold being located on surface of titanium. 
     The material of the gate insulating layer  33  can be rigid materials, such as alumina, yttrium oxide, silicon nitride, or silicon oxide. The material of the gate insulating layer  33  can also be flexible materials, such as benzocyclobutene, polyester, or acrylic resin. A thickness of the gate insulating layer  33  is in a range of 0.5 nanometers to 100 micrometers. In an embodiment, the material of the gate insulating layer  33  is alumina, and the thickness of the gate insulating layer  33  is 40 nanometers. 
     The carbon nanotube structure  34  includes at least one carbon nanotube. When the carbon nanotube structure  34  includes a plurality of carbon nanotubes, the plurality of carbon nanotubes are combined tightly by the Van der Waals force to form a carbon nanotube film. The plurality of carbon nanotubes extend substantially along a same direction. Each carbon nanotube consists of two metallic carbon nanotube segments and a semiconducting carbon nanotube segment in the middle of the carbon nanotube. The lengths of the metallic carbon nanotube segments of adjacent carbon nanotubes are substantially the same, and the lengths of the semiconducting carbon nanotube segments of adjacent carbon nanotubes are substantially the same. The metallic carbon nanotube segments of the plurality of carbon nanotubes are the ends of the carbon nanotubes. The metallic carbon nanotube segments at one end of the carbon nanotube structure  34  are in direct contact with the source electrode  35 , and the metallic carbon nanotube segments at the other end of the carbon nanotube structure  34  are in direct contact with the drain electrode  36 . Thus, the source electrode  35  and the drain electrode  36  have a good electrical connection within the carbon nanotube structure  34 . The M-S-M type of the carbon nanotube structure  34  can be obtained by the above method. The lengths of the channel and the metallic carbon nanotube segments  341 ,  342  can be adjusted by controlling the electric field. 
     In use, the source electrode  35  is grounded. A voltage Vg is applied on the gate electrode  32 . Another voltage Vd is applied to the drain electrode  36 . The voltage Vg forms an electric field in the channel of the carbon nanotube structure  34 . Accordingly, carriers exist in the channel near the gate electrode. As the Vg increases, a current is generated and flows through the channel. Thus, the source electrode  35  and the drain electrode  36  are electrically connected. 
     In  FIG. 19 , a chart of test results of an embodiment of the thin film transistor  30  is shown. The carbon nanotube structure  34  of the thin film transistor  30  consists of forty carbon nanotubes. The switching ratio of the thin film transistor  30  can be 2×10 5 :1. 
     The thin film transistor  30  has following advantages. The metallic carbon nanotube segments  341 ,  342  of the carbon nanotubes structure  34  can be used as source electrodes and drain electrodes. Thus, the structure of the thin film transistor is simple and there is no need to prepare extra electrodes. Since the source electrode, the drain electrode, and the semiconductor layer are integrated, the interface between the semiconductor layer and the source electrode, and the drain electrode is reduced as a barrier, and the switching ratio is increased. When an independent metal source electrode and an independent metal drain electrode are provided, both ends of the carbon nanotubes being metallic carbon nanotube segments, the metallic carbon nanotube segments have excellent electrical connection with the independent metal source electrode and the independent metal drain electrode. 
     In  FIG. 20 , an embodiment of a thin film transistor  40  comprises an insulating substrate  41 , a carbon nanotube structure  42 , a source electrode  43 , a drain electrode  44 , an insulating layer  45 , and a gate electrode  46 . The carbon nanotube structure  42  includes at least one carbon nanotube. One end of the carbon nanotube is a first metallic carbon nanotube segment  421 , and the other end of the carbon nanotube is a second metallic carbon nanotube segment  422 . A semiconducting carbon nanotube segment  423  is in the middle of the carbon nanotube. The semiconducting carbon nanotube segment  423  is used as a channel. The carbon nanotube structure  42  is located on a surface of the insulating substrate  41 . The source electrode  43  is in direct contact with the first metallic carbon nanotube segment  421 , and the drain electrode  44  is in direct contact with the second metallic carbon nanotube segment  422 . The source electrode  43  and the drain electrode  44  are electrically connected to the carbon nanotube structure  42 . The insulating layer  45  is located on a surface of the carbon nanotube structure  42 . The gate electrode  46  is located on a surface of the insulating layer  45 . 
     The thin film transistor  40  is similar to the thin film transistor  30  except that the thin film transistor  40  is a top gate type thin film transistor. The insulating layer  45  insulates the gate electrode  46  from the source electrode  43 , from the drain electrode  44 , and from the carbon nanotube structure  42 . 
     In  FIG. 21 , an embodiment of a light detector  50  comprises a carbon nanotube structure  51 , a first electrode  52 , a second electrode  53 , and a current detection device  54 . The carbon nanotube structure  51  is electrically connected to the first electrode  52  and the second electrode  53 . The current detection device  54 , the first electrode  52 , the second electrode  53 , and the carbon nanotube structure  51  are electrically connected to form a circuit in series. The carbon nanotube structure  51  is the same as the carbon nanotube structure  34 . Both ends of the carbon nanotube structure  51  are metallic carbon nanotube segments, and the first electrode  52 , the second electrode  53  are located on surfaces of the metallic carbon nanotube segments and are in direct contact with the metallic carbon nanotube segments. 
     In the carbon nanotube structure  51 , the M-S-M type of the carbon nanotube includes a heterojunction and is used to detect light. 
     The first electrode  52  and the second electrode  53  are both composed of conductive materials. The conductive materials can be metal, indium tin oxide, antimony tin oxide, conductive silver paste, conductive polymer, and conductive carbon nanotubes. The material of the metal can be aluminum, copper, tungsten, molybdenum, gold, titanium, or palladium. The first electrode  52  and the second electrode  53  can also be conductive films. A thickness of the conductive films is in a range of 2 microns to 100 microns. In an embodiment, the first electrode  52  and the second electrode  53  are metal composite structures formed of gold and titanium. The titanium is located on the carbon nanotube structure  51 , and the gold is located on the titanium. The thickness of titanium is 2 nanometers, and the thickness of gold is 50 nanometers. 
     The current detection device  54  detects the current in the circuit. The current detection device  54  can be an ammeter. Furthermore, the light detector  50  includes a power source  55 , the power source  55  can provide a bias voltage between the first electrode  52  and the second electrode  53 . 
     Furthermore, the light detector  50  includes a substrate  56 , the substrate  56  supporting the carbon nanotube structure  51 . When the carbon nanotube structure  51  is a free-standing structure, the substrate  56  can be omitted. The term “free-standing structure” means that the carbon nanotube structure can sustain the weight of itself when hoisted by a portion thereof without any significant damage to its structural integrity. The material of the substrate  56  can be insulating material, such as glass, ceramic, polymer, or wood material. The material of the substrate  56  can also be conductive metal material coated with an insulating material. In an embodiment, the material of the substrate  56  can be glass. 
     The light detector  50  can detect light. The working process of the light detector  50  comprises turning on the power source  55 , and applying a voltage between the first electrode  52  and the second electrode  53 . In the absence of light, no photogenerated carriers are produced in the carbon nanotube structure  51 , the heterojunction is on off-status, and no current passes through the series circuit. No current change is detected by the current detection device  54 . If light irradiates the carbon nanotube structure  51 , photo-generated carriers are produced in the carbon nanotube structure  51 , the heterojunction is on on-status, a current passes through the series circuit, and the current detection device  54  detects the current change. 
     The light detector  50  can be used quantitatively as well as qualitatively. For quantitative function, the light detector  50  is turned on the power source  55  applies a voltage between the first electrode  52  and the second electrode  53 . As light of different strengths irradiates the carbon nanotube structure  51 , different current values corresponding to lights with different strengths are recorded, and a graph about light strengths and current values can be drawn. If a light with unknown strength irradiates the carbon nanotube structure  51 , a current value corresponding to such light can be detected, and according to the graph about light strengths and current values, the strength of the light can be calculated. 
     The light detector  50  has following advantages. Since the detection point of the light detector  50  is the carbon nanotube structure containing a heterojunction, the heterojunction is formed by the semiconducting carbon nanotube segments and the metallic carbon nanotube segments. The semiconducting carbon nanotube segments and the metallic carbon nanotube segments are integrated as a structure, which increases ease of conduction and sensitivity to current. Therefore, the light detector has a simple structure and high sensitivity. 
     In  FIG. 22  and  FIG. 23 , an embodiment of a photoelectric conversion device  60  comprises a photoelectric conversion module  61 , a cover structure  62 , and the substrate  63 . The photoelectric conversion module  61  is located on the substrate  63 . The photoelectric conversion module  61  includes a carbon nanotube structure  64 . The carbon nanotube structure  64  is the same as the carbon nanotube structure  34 . One end of the carbon nanotube structure  64  is a first metallic carbon nanotube segment  641 , the other end of the carbon nanotube structure  64  is a second metallic carbon nanotube segment  642 , and a semiconducting carbon nanotube segment  643  is in the middle of the carbon nanotube. The semiconducting carbon nanotube segment  643  includes a covered area  643   b  and a non-covered area  643   a.  The cover structure  62  covers the covered area  643   b  of the photoelectric conversion module  61 . 
       FIG. 22  shows a top view of the photoelectric conversion device  60 .  FIG. 23  shows a sectional schematic of the photoelectric conversion device  60 . 
     The substrate  63  supports the photoelectric conversion module  61 . When the photoelectric conversion module  61  is a free-standing structure, the substrate  63  can be omitted. The material of the substrate  63  can be insulating material, such as glass, ceramic, polymer, or wood material. The material of the substrate  63  can also be conductive metal material coated with an insulating material. The material of the substrate  63  should not be infrared-absorbent. A thickness of the substrate  63  is not limited. The thickness of the substrate  63  is in a range of 1 millimeter to 2 centimeters. In an embodiment, the material of the substrate  63  is glass, and the thickness of the substrate  63  is 5 millimeters. 
     The semiconducting carbon nanotube segment  643  is divided into the covered area  643   b  and the non-covered area  643   a.  The areas of the covered area  643   b  and the non-covered area  643   a  is not limited. The area of the covered area  643   b  can be greater than, equal to, or less than the area of the non-covered area  643   a.  In an embodiment, the area of the covered area  643   b  is equal to the area of the non-covered area  643   a.    
     The non-covered area  643   a  receives light energy and converts the light energy into heat energy, raising the temperature of the non-covered area  643   a.  Thus, a temperature difference is generated between the covered area  643   b  and the non-covered area  643   a.  A potential difference is generated in the semiconducting carbon nanotube segment  643  by the thermoelectric effect. The light energy can be from sunlight, visible light, infrared or ultraviolet, or even electromagnetic waves outside the visible spectrum. 
     The photoelectric conversion module  61  comprises a first electrode  65  and a second electrode  66 . The first electrode  65  is electrically connected to the first metallic carbon nanotube segment  641 , and the second electrode  66  is electrically connected to the second metallic carbon nanotube segment  642 . The first electrode  65  and the second electrode  66  are voltage outputs of the photoelectric conversion device  60 . Furthermore, the chirality of the first metallic carbon nanotube segment  641  and the second metallic carbon nanotube segment  642  are metallic. Thus, the first metallic carbon nanotube segment  641  and the second metallic carbon nanotube segment  642  can be independent electrodes, or the first metallic carbon nanotube segment  641  and the second metallic carbon nanotube segment  642  can be used as voltage outputs. 
     The photoelectric conversion device  60  further includes a first lead wire (not shown) and a second lead wire (not shown). The first lead wire is electrically connected to the first electrode  65 , the second lead wire is electrically connected to the second electrode  66 . The first lead wire can facilitate a connection to the first electrode  65  and the second lead wire can facilitate a connection to the second electrode  66 , without a circuit. 
     The cover structure  62  is used to cover the covered area  643   b  of the photoelectric conversion module  61  to prevent light-irradiation of the covered area  643   b.  The cover structure  62  should not cover the non-covered area  643   a.  The material of the cover structure  62  can be conductive or insulating. The conductive material can be metal or alloy, such as stainless steel, carbon steel, copper, nickel, titanium, zinc, or aluminum. The insulation material can be resin or plastic. When the cover structure  62  is insulating material, the cover structure  62  can be in direct contact with the covered area  643   b  and cover the covered area  643   b.  When the cover structure  62  is conductive material, the cover structure  62  should be spaced apart from and insulated from the covered area  643   b.  In an embodiment, the cover structure  62  is a housing with an accommodating space, and the cover structure  62  is fixed on the substrate  63 . The covered area  643   b  is located inside the accommodating space of the cover structure  62 . The covered area  643   b  is spaced from the cover structure  62 . When the material of the substrate  63  and the material of the cover structure  62  are both insulating materials, the cover structure  62  and the substrate  63  can form an integrated structure. 
     The photoelectric conversion device  60  has following advantages. When the covered area  643   b  is exposed to light, a temperature difference is generated between the covered area  643   b  and the non-covered area  643 a, electricity being generated by the thermoelectric effect. The semiconducting carbon nanotube segment  643 , the first metallic carbon nanotube segment  641 , and the second metallic carbon nanotube segment  642  are integrated, reducing the resistance of the interface as a barrier and increasing the output power. 
     In  FIG. 24 , an embodiment of a photoelectric conversion device  70  comprises a photoelectric conversion module  71 , a cover structure  72 , and the substrate  73 . The photoelectric conversion module  71  is located on the substrate  73 . The photoelectric conversion module  71  includes a carbon nanotube structure  74 . The carbon nanotube structure  74  comprises a plurality of metallic carbon nanotube segments and a plurality of semiconducting carbon nanotube segments alternately arranged. 
     The photoelectric conversion device  70  is similar to the photoelectric conversion device  60  except that the photoelectric conversion device  70  comprises a plurality of metallic carbon nanotube segments and a plurality of semiconducting carbon nanotube segments alternately arranged. Since the metallic carbon nanotube segments can be used as electrodes, the photoelectric conversion device  70  includes a plurality of semiconducting carbon nanotube segments connected in series. Thus, the output power of the photoelectric conversion device  70  is increased. The cover structure  72  includes a plurality of spaced covering bodies  721  and a plurality of spaced openings  722 . Each semiconducting carbon nanotube segment has a first portion covered by one spaced covering body  721  and a second portion exposed from one spaced opening  722 . The spaced openings can be windows using transparent materials, such as glass. 
     The embodiments shown and described above are only examples. Even though numerous characteristics and advantages of the present technology have been set forth in the foregoing description, together with details of the structure and function of the present disclosure, the disclosure is illustrative only, and changes may be made in the detail, including in matters of shape, size, and arrangement of the parts within the principles of the present disclosure, up to and including the full extent established by the broad general meaning of the terms used in the claims. 
     Depending on the embodiment, certain of the steps of methods described may be removed, others may be added, and the sequence of steps may be altered. The description and the claims drawn to a method may comprise some indication in reference to certain steps. However, the indication used is only to be viewed for identification purposes and not as a suggestion for ordering the steps.