Abstract:
A single-walled carbon nanotube-based planar photodetector includes a substrate; a first electrode and a second electrode disposed on the substrate and spaced apart from each other; a plurality of single-walled carbon nanotubes, each of the plurality of single-walled carbon nanotubes contacting the first electrode and the second electrode; and an adsorbent attached to a surface of at least one of the plurality of single-walled carbon nanotubes, wherein the adsorbent is capable of doping the at least one of the plurality of single-walled carbon nanotubes by photo-excitation.

Description:
BACKGROUND 
     1. Field 
     The present disclosure relates to a single-walled carbon nanotube-based planar photodetector that uses an adsorbent and has improved light detection speed and light detection sensitivity. 
     2. Description of the Related Art 
     Single-wall carbon nanotubes (SWCNTs) have superior electric characteristics, such as high field-effect mobility, ballistic transport, and high compatibility with high-k dielectric materials, and may be made into an electric field transistor that would enable the production of high speed field-effect transistors superior to the currently existing silicon-based devices. In addition, their unique optical properties, such as band gap tunability, may enable the tunable emission and detection of specific wavelengths of light. 
     A photodetector detects an optical signal and converts it to an electric signal. The photodetector may be a vertical photodetector in which two electrodes are arranged parallel to the light&#39;s incident direction or a horizontal photodetector in which two electrodes are arranged vertically with respect to the light&#39;s incident direction. The horizontal photodetector may be referred to as a planar photodetector. 
     Many investigations have been made regarding the use of a SWCNT as a horizontal photodetector. However, when carbon nanotubes are formed into a network structure between two electrodes, the mobility of carriers may be reduced as the result of the existence of various trap sites, such as junctions, impurities, and the like. between the carbon nanotubes. 
     SUMMARY 
     One or more embodiments provide a single-walled carbon nanotube-based planar photodetector that operates at high speed and has improved sensitivity. 
     According to an aspect of an embodiment, there is provided a single-walled carbon nanotube-based planar photodetector including a substrate; a first electrode and a second electrode disposed on the substrate and spaced apart from each other; a plurality of single-walled carbon nanotubes, each of the plurality of single-walled carbon nanotubes contacting the first electrode and the second electrode; and an adsorbent attached to a surface of at least one of the plurality of single-walled carbon nanotubes, wherein the adsorbent is capable of doping the at least one of the plurality of single-walled carbon nanotubes by photo-excitation. 
     The adsorbent may be an electron acceptor that p-dopes the plurality of single-walled carbon nanotubes or an electron donor that n-dopes the plurality of single-walled carbon nanotubes. 
     The electron acceptor may be a fullerene or a fullerene derivative. 
     The fullerene derivative may be at least one selected from the group consisting of phenyl-C61-butyric acid methyl ester (PCBM), phenyl-C71-butyric acid methyl ester (PC71BM), indene-C60 bisadduct (IC60BA), and indene-C70 bisadduct (IC70BA). 
     The electron donor may be at least one selected from the group consisting of poly-3-hexyl thiophene (P3HT), poly[2,5-bis(3-alkylthiophen-2-yl)thieno(3,2-b)thiophene] (PBTTT), and poly[5,5′-bis(3-alkyl-2-thienyl)-2,2′-bithiophene] (PQT). 
     The adsorbent may be a light absorbent and the light absorbent may be any one of a quantum dot, an organic dye, and an inorganic sensitizer. 
     The first electrode and the second electrode may be formed of materials having different work functions. 
     The first electrode and the second electrode may be formed of the same electrode material, and may further include a unit for applying a predetermined voltage to the first electrode and the second electrode. 
     The first electrode and the second electrode may be formed of a material selected from the group consisting of aluminum (Al), gold (Au), platinum (Pt), chromium (Cr), silver (Ag), and indium tin oxide (ITO). 
     The single-walled carbon nanotube-based planar photodetector may further include a gate that adjusts the movement of carriers on the plurality of single-walled carbon nanotubes. 
     The substrate may be a back gate. 
     Each of the plurality of single-walled carbon nanotubes may be formed contacting the first electrode and the second electrode. 
     The first electrode and the second electrode may be arranged at opposite ends of each of the plurality of single-walled carbon nanotubes and may be electrically connected to the plurality of single-walled carbon nanotubes. 
     A length of the plurality of single-walled carbon nanotubes between the first electrode and the second electrode may be from about 0.1 to 1000 μm. 
     The single-walled carbon nanotube-based planar photodetector may further include a self assembled monolayer (SAM) under the plurality of single-walled carbon nanotubes on the substrate in the form of a plurality of lines, wherein the plurality of single-walled carbon nanotubes are aligned so as to closely contact a surface of the SAM. 
     The SAM may be formed of at least one material selected from the group consisting of octadecyltrichlorosilane (OTS), benzocyclobutene (BCB), 16-mercaptohexadecanoic acid (HMA), and alkylsiloxanes. 
     The plurality of single-walled carbon nanotubes may be p-doped or n-doped via irradiation by a light source. 
     Each of the plurality of single-walled carbon nanotubes may be a semiconductor single-walled carbon nanotube. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and/or other aspects will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings in which: 
         FIGS. 1 and 2  are, respectively, a cross-sectional view and a plan view schematically illustrating the structure of a SWCNT-based planar photodetector according to an embodiment; 
         FIG. 3  is an energy band diagram for describing an operation of p-doping an electron acceptor that is an adsorbent; 
         FIG. 4  is an energy band diagram for describing an operation of n-doping an electron donor that is an adsorbent; 
         FIG. 5  is an energy band diagram for describing an operation of doping CNT using a light absorbent; 
         FIG. 6  is a graph showing a characteristic of a photodetector according to an embodiment when an adsorbent is not applied to the photodetector; 
         FIG. 7  is a graph showing a characteristic of a photodetector according to an embodiment when an adsorbent is applied to the photodetector; and 
         FIG. 8  schematically illustrates a PN junction structure using a photo-excitation doping characteristic. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. In this regard, the present embodiments may have different forms and should not be construed as being limited to the descriptions set forth herein. Accordingly, the embodiments are merely described below, by referring to the figures, to explain aspects of the present description. In the following descriptions, the terms “above” or “on” may include not only one element disposed directly on another element thereby contacting the first element, but also one element disposed above another element without contacting the first element. 
     As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Expressions such as “at least one of,” when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list. 
       FIGS. 1 and 2  are respectively a cross-sectional view and a plan view schematically illustrating the structure of a SWCNT-based planar photodetector  100  according to an embodiment. Referring to  FIGS. 1 and 2 , the photodetector  100  includes a plurality of SWCNTs  140  on a substrate  110  and a first electrode  151  and a second electrode  152  electrically connected to the opposite ends of each of the SWCNTs  140 . In the following description, the SWCNTs  140  are also referred to as the carbon nanotubes (CNTs)  140 . 
     The substrate  110  may be formed of a semiconductor, glass, or plastic. The substrate  110  may be operated as a back gate when the substrate  110  is a semiconductor substrate, for example, a silicon substrate. An insulation layer  120  may also be formed on the substrate  110 . The insulation layer  120  may be formed of silicon oxide or silicon nitride. The insulation layer  120  may be referred to as a gate insulation layer. 
     The substrate  110  may be formed of a conductive metal material. Also, when the substrate  110  is formed of glass or plastic, a separate back gate formed of metal may be formed on the substrate  110  at a position corresponding to the CNTs  140 . A detailed description thereof is omitted herein. 
     The CNTs  140  may correspond to a channel of an electric field effect transistor. The CNTs  140  between the first and second electrodes  151  and  152  preferably have a length of about 0.1 to 1000 μm. 
     When the length of the CNTs  140  between the first and second electrodes  151  and  152  is about 0.1 to 2 μm, each of the CNTs  140  has a semiconductor property. When a metallic SWCNT is included in the CNTs  140 , the first electrode  151  and the second electrode  152  are short-circuited so that the planar photodetector  100  does not function as a photodetector. When the length of the CNTs  140  between the first and second electrodes  151  and  152  is about 2 to 1000 μm, 75% of the CNTs  140  have a semiconductor property. 
     When light is irradiated onto the CNTs  140 , excitons, pairs of an electron and a hole, are generated in the CNTs  140 . The generated excitons are disassembled into electrons and holes as a result of the potential difference between the first and second electrons  151  and  152 . The electrons are moved toward the first electrode  151  having a relatively high electric potential and the holes are moved toward the second electrode  152  having a relatively low electric potential. In the present embodiment, since the CNTs  140  are aligned in parallel, carrier mobility toward each electrode is high and the probability of the electrons and the holes recombining is low. 
     In contrast, in a conventional technology, when CNTs are formed in a network, the trap density is high, and carriers are moved to an electrode through several CNTs, and the mobility of carriers is lowered. 
     An adsorbent  160  may be attached to a surface of the CNTs  140 . The adsorbent  160  improves photo-detection sensitivity. 
     When receiving light, the adsorbent  160  dopes the CNTs  140 . The adsorbent  160  may be an electron acceptor or an electron donor. When the adsorbent  160  is an electron acceptor, the adsorbent  160  p-dopes the CNTs  140 . A fullerene or a fullerene derivative may be used as the electron acceptor. Phenyl-C61-butyric acid methyl ester (PCBM), phenyl-C71-butyric acid methyl ester(PC71 BM), indene-C60 bisadduct (IC60BA), indene-C70 bisadduct (IC70BA), and similar materials. may be used as the fullerene derivative. 
       FIG. 3  is an energy band diagram for describing an operation of p-doping an electron acceptor that is the adsorbent  160 . Referring to  FIG. 3 , when light is irradiated onto the CNTs  140 , excitons are generated in the CNTs  140 . The excitons are disassembled into electrons and holes due to a potential difference between the first and second electrodes  151  and  152 . The disassembled electrons are moved toward an electron acceptor. The holes are left in the CNTs  140  and thus the CNTs  140  become p-doped. 
     When the adsorbent  160  is an electron donor, the adsorbent  160  n-dopes the CNTs  140 . Poly-3-hexyl thiophene (P3HT), poly[2,5-bis(3-alkylthiophen-2-yl)thieno(3,2-b)thiophene] (PBTTT), and poly[5,5′-bis(3-alkyl-2-thienyl)-2,2′-bithiophene] (PQT), and similar materials may be used as the electron donor. 
       FIG. 4  is an energy band diagram for describing an operation of n-doping an electron acceptor that is the adsorbent  160 . Referring to  FIG. 4 , when light is irradiated onto the CNTs  140 , excitons are generated in the CNT  140 . The excitons are disassembled into electrons and holes due to the potential difference between the first and second electrodes  151  and  152 . The disassembled holes are moved toward an electron donor. The electrons are left in the CNTs  140  and thus the CNTs  140  become n-doped. 
     The adsorbent  160  may be a light absorbent. The light absorbent may be a quantum dot, an organic dye, or an inorganic sensitizer. The quantum dot may be formed of CdS, CdTe, Si, or similar materials. The organic dye may be N719 dye, Z917 dye, Zn-porphyrin, Zn-phthalocyanine, or similar materials. The inorganic sensitizer may be PbS, PbSe, CdSe, or similar materials. The light absorbent receives light and provides electrons or holes to the CNTs  140 . 
       FIG. 5  is an energy band diagram for describing an operation of doping the CNTs  140  using a light absorbent. Referring to  FIG. 5 , when light having a wavelength corresponding to the band gap of a light absorbent, for example, a CdS quantum dot, is irradiated onto the adsorbent  160 , the light absorbent absorbs the light and generates excitons. The excitons are disassembled into electrons and holes due to the electrostatic potential between the CNTs  140  and the light absorbent. The electrostatic potential is the difference between the lowest unoccupied molecular orbital (LUMO) level between the CNTs  140  and the light absorbent. Since the electrostatic potential is greater than the bonding energy of the excitons, disassembly of electrons and holes occurs. The disassembled electrons are moved to the CNTs  140  having a relatively low LUMO energy. As electrons are accumulated in the CNTs  140 , the CNTs  140  become n-doped. Thus, the current in the CNTs  140  increases. 
     When a material in which the highest occupied molecular orbital (HOMO) of the light absorbent is greater than the HOMO of the CNTs  140 , the holes of the light absorbent are moved to the CNTs  140 , and thus the CNTs  140  become p-doped. 
     The CNTs  140  are arranged on the substrate  110  substantially parallel to each other. The first electrode  151  and the second electrode  152  are electrically connected to opposite ends of the CNTs  140 . Opposite ends of each of the CNTs  140  contact the first electrode  151  and the second electrode  152 , respectively. 
     A SAM  130  for aligning the CNTs  140  may be further formed on the insulation layer  120 . The SAM  130  may be formed of, for example, octadecyltrichlorosilane (OTS), benzocyclobutene (BCB), 16-mercaptohexadecanoic acid (HMA), alkylsiloxanes, and similar materials. 
     The SAM  130  may be arranged on the insulation layer  120  in a pattern taking the shape of a plurality of lines. When the CNTs  140  are dispersed on the SAM  130 , the CNTs  140  closely contact a functional group of the SAM  130  and thus the CNTs  140  may be aligned. Because the CNTs  140  closely contact a surface of the SAM  130 , the number of carriers trapped in the surface of the insulation layer  120  is reduced. 
     The first and second electrodes  151  and  152  may be formed of aluminum (Al), gold (Au), platinum (Pt), chromium (Cr), silver (Ag), indium tin oxide (ITO), and similar materials. When the first and second electrodes  151  and  152  are formed of the same material, when a predetermined voltage is applied to a power source  170  connected to the first and second electrodes  151  and  152 , a potential difference is generated between the first and second electrodes  151  and  152 . For example, a positive voltage may be applied to the first electrode  151 , and the second electrode  152  may be grounded. 
     The first and second electrodes  151  and  152  may be formed of materials having different work functions. For example, when the first electrode  151  is formed of Al, the second electrode  152  may be formed of Au or ITO. A potential difference may be generated between the first and second electrodes  151  and  152  and the electric potential of the first electrode  151  may be higher than that of the second electrode  152 . In such a case, a built-in voltage is formed. 
     When a gate voltage is applied to a substrate  110  that is a back gate, movement of carriers on the CNTs  140 , which functions in a role of a channel, like a typical electric field effect transistor, may be controlled. The signal to dark current ratio (SDR) may be adjusted by the gate voltage. The SDR is a value obtained by dividing the current, I light-on , measured when light is irradiated, by the current, I light-off , measured when light is off. The SDR increases when the photodetector  100  is turned off by the gate voltage. When the photodetector  100  is turned on, many carriers already exist in a channel (CNT) and thus the effect of light irradiation is relatively low. 
       FIG. 6  is a graph showing a characteristic of the photodetector according to an embodiment where the adsorbent  160  is not applied to the photodetector. A change of current I DS  between the first and second electrodes  151  and  152  is measured while changing the gate voltage. In the graph, line G 1  shows a current characteristic of the photodetector when a light source such as an ultraviolet ray is not irradiated, and line G 2  shows a current characteristic of the photodetector when the light source is irradiated. 
     Referring to  FIG. 6 , the SDR varies according to the gate voltage, based on whether an ultraviolet ray having a wavelength of about 1200 nm is irradiated. When the photodetector is off, the SDR increases. When the photodetector is on, the photodetection effect is low because there are too many carriers. When the photodetector is off, the measured current (I DS ) by light irradiation greatly increases. When the gate voltage is about 15 V, the I DS  is very low, at about a pico-amp level, and thus photodetection may be difficult. 
       FIG. 7  is a graph showing a characteristic of a photodetector according to an embodiment where the adsorbent  160  is applied to the photodetector. Fullerene is used as the adsorbent  160 . In the graph, line G 1  shows a current characteristic of the photodetector when an ultraviolet ray is not irradiated, and line G 2  shows a current characteristic of a photodetector when an ultraviolet ray is irradiated. The change in the I DS  between the first and second electrodes  151  and  152  as a function of the gate voltage is illustrated in  FIG. 7 . 
     Referring to  FIG. 7 , the sensitivity of the photodetector  100  is greatly increased by the use of the adsorbent  160 . As may be seen in the figure, an ultraviolet ray having a wavelength of about 1200 mm is irradiated, the CNTs  140  generate excitons, and the adsorbent  160  accommodate electrons. Accordingly, the CNTs  140  are doped with holes. Thus, the threshold voltage is shifted to the right as shown at  FIG. 7 , which shows that the CNTs  140  are p-doped. When the gate voltage is about 20 V, the I DS  is about a nano-amp level, which shows a remarkably high current value as compared to not using the adsorbent (see  FIG. 6 , for example). 
     According to the present embodiment, light detection sensitivity is improved by the application of the adsorbent. Also, since the application of the gate voltage enables photodetection in a state of a high SDR value, the measurement sensitivity is improved. 
       FIG. 8  schematically illustrates a PN junction structure  200  using doping characterized by photo-excitation. Referring to  FIG. 8 , an insulation layer  220  is formed on a substrate  210 . A plurality of SWCNTs  240  are arranged on the insulation layer  220  parallel to each other. A first electrode  251  and a second electrode  252  are arranged at the opposite ends of each of the CNTs  240 . The surface of each of the CNTs  240  is divided into two regions: a first region A 1  close to the first electrode  251  and a second region A 2  close to the second electrode  252 . An electron donor, or a quantum dot or an organic dye for performing n-doping,  261  is adsorbed in the first region A 1 . An electron acceptor, or a quantum dot or an organic dye for performing p-doping,  262  is adsorbed in the second region A 2 . The first region A 1  is an n-doped region when light is irradiated, and the second region A 2  is a p-doped region when light is irradiated. 
     The PN junction structure  200  may be used in, for example, solar cells. 
     As presently disclosed, a SWCNT arranged so as to directly connect two electrodes may improve the mobility of carriers. 
     Since an adsorbent is arranged on the surface of a SWCNT, light detection sensitivity may be improved. Also, since light detection may be performed by the application of a gate voltage when the SDR value is high, measuring sensitivity may be further improved. 
     It should be understood that the exemplary embodiments described therein should be considered to be descriptive only and not limiting. Descriptions of features or aspects within each embodiment should be understood as being available for other similar features or aspects in other embodiments.