Chapter 7. Carbon Nanotubes. 1. Introduction of CNTs 2. Application of CNTs 3. Growth of CNTs 4. Critical Issues in CNT growth

Transcription

1 Chapter 7. Carbon Nanotubes 1. Introduction of CNTs 2. Application of CNTs 3. Growth of CNTs 4. Critical Issues in CNT growth

2 2018 년 4 월 12 일. 우주엘리베이터가첫운행을시작한다!!

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4 1996 년노벨화학상 크로토 (Kroto), 스몰리 (Smalley), 컬 (Curl) 년 C 60 발견 - C 60 : 내부직경이 0.71nm 인축구공모양의특수한분자구조 (C 60 ). 탄소원자 60 개가오각형모양 12 개와육각형모양 20 개로이루어진것. 이와비슷한모양의돔을설계한건축가풀러 (Buckminster Fuller) 의이름을따서풀러렌 (Fullerlene) 으로명명

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6 Graphite - van-der Waals interaction of interlayers Graphene

7 2010 년노벨물리학상 안드레가임박사 ( 왼쪽 ) 와콘스탄틴노보셀로프박사 탄소원자한층으로이뤄진그래핀은두께가 0.35 nm ( 나노미터 1 nm는 10 억분의 1m) 에불과하지만그강도가강철의 200 배, 다이아몬드의 2 배이상이다. 또구리보다 100 배이상전기가잘통하고휘거나비틀어도부서지지않는다. 그래핀을이용하면종이처럼얇은모니터, 손목에차는휴대전화, 지갑에넣을수있는컴퓨터구현가능 2004 년흑연에서스카치테이프를붙였다떼는방법으로그래핀분리성공

8 성균관대나노과학기술원홍병희교수와삼성전자종합기술원최재영박사팀이그래핀을이용해만든휘어지는전자소자

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10 Chiral vector (n, m) = na 1 + ma 2 electric property : metallic (n - m = 3k) semiconducting (n - m 3k)

11 SWCNT vs. MWCNT 10 walls, 12 nm

12 Properties of CNT thermal conductivity: Diamond electrical conductivity: > Cu mechanical strength: 100 x steel Chemical & mechanical stability High aspect ratio

13 Applications

14 H 2 storage FET

15 Nanotweezer SPM tip Gas sensor

16 Growth of CNTs Arc-discharge Laser vaporization Chemical vapor deposition Fullerene recrystallization

17 Arc discharge Fullerenes deposited as soot SWNT in soot if anode contains metal catalyst (Fe, Co, Ni-Co): deposited everywhere MWNT deposited on cathode under hydrogen gas (0.34 nm layer spacing) Low purity: CNT + carbon particles Constant flow of He or Ar gas

18 (1) Apply a voltage between two graphite electrodes held close together in a chamber filled with an inert gas. (2) Electrical discharge taking place between the electrodes heats up the region to thousands degrees. (3) Then evaporation of carbon. (4) Carbon vapor crystallizes on the end of the negative electrode, forming MWNTs with diameters between 4 and 30 nm. (5) Introduction of small amount of Fe, Co, Ni leads to formation of SWNTs. (6) Highest yield (70-90 %) of SWNTs by positive graphite electrode with 1% Y and 4.2% Ni (7) Crystalline rope of SWNTs with a diameter of 1.4 nm. Ref) Nature 388, 756 (1997).

19 Laser Vaporization Furnace; 1200 o C Water cooled collector Ar CNT laser Graphite target

20 (1) 반응로 : 1200 o C, 레이저로흑연 Target 을기화시킨후타겟에서기화된흑연이차가운 collector 에흡착됨. (2) MWNT+carbon particles (3) Carrier gas: He and Ar, pressure=500 Torr (4) Addition of Co, Ni, Fe gives SWNTs

21 (1) High yield(70 %), large scale production of SWNTs (2) First developed by Smally, 1996(Science 273, 483 (1996). (3) A graphite target containing small amounts of Co and Ni powder is placed in the middle of tube furnace at 1200 o C under a flow of Ar, and hit by a series of laser pulses. (4) A plume of carbon and metal vapors emanates from the target surface and nanotube starts to grow in the gas phase. (5) Continuous growth of nanotubes while flying downstream along the tube until they exit the furnace (6) Collection of nanotubes on a cold finger as a spongy deposit. (7) Each fiber consists of a rope of parallel SWNTs, close-packed as a 2D triangular lattice.

22 Rope 형태의 SWNTs: TEM 이미지 직경이균일하고, 그래파이트면간격이 0.34 nm 인 SWNTs

23 Chemical vapor deposition (CVD)

24 (1) Production of individual SWNTs not ropes of SWNTs. (2) 1998 by Hongjie Dai. (Nature 395, 878 (1998). ) (3) Growth of SWNTs in situ on silicon wafers having lithographically patterned catalytic islands of Al 2 O 3 powders containing Fe and Mo catalytic nanoparticles. (4) Place the substrate in a tube furnace at 1000 o C under a flow of CH 4. (5) Hydrocarbon as a carbon precursor, which decomposed on the catalyst. (CO, ethylene(c 2 H 4 ), benzene (C 6 H 6 ) (6) The carbon crystallized in the form of individual SWNTs emerging from the catalyst islands. (7) Opened up the possibility of producing prototype nanotube chips by growing individual SWNTs in situ on specific locations on a flat substrate.

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26 SWCNTs grown by CVD (a) (b) (c) (d) (a) high D SWNT, ethanol as the feed gas, Fe/Co/Mo catalysts on silica supports (b) moderate D SWNT, methane as feed gas, Fe nanoparticle catalyst (c) partially aligned SWNT film grown on singlecrystalline ST-cut quartz substrate (d) perfectly aligned SWNT arrays grown with Fe catalyst patterned into 10 μm wide strips (bright horizontal lines at top/bottom edges of the image) on a similar quartz substrate

27 Root growth Growth mechanism Tip growth

28 Root growth method (a) Decomposition of hydrocarbon on the nanoparticle and solubilization of carbon therein. (b) Nucleation by formation of a fullerene cap. (c) Elongation of SWNT by incorporation of further carbon into the metal-carbon bonds at the growing end.

29 Fullerene recrystallization -CNTs produced by conventional methods consists of random mixtures of nanotubes with different diameters and chiralities. -Such structural and electronic variety is a serious obstacle toward the application of SWNTs in nanoelectronics : production of homogeneous single crystals of SWNTs (IBM Zurich (2001)) (1) evaporation of alternated layers of Ni and C 60 on Mo or Si substrates through a 300 nm-diam shadow mask formation of an array of C 60 /Ni multilayer pillars (2) heating to 950 o C under a magnetic field of 1.5 T normal to the surface conversion of pillars into micron-long rods of 50 nm diam, emerging from surface (3) TEM and electron diffraction each rod is composed of thousands of physically identical SWNTs, all having the same diameter & chirality Ref) R. R. Schlittler et al., Science 292, 1136 (2001).

30 Issues (1) Separation of semiconducting and metallic nanowires (2) Aligned nanotubes

31 How can we separate s-swnts from m-swnts? -Use the differences in (1) electrical properties (2) chemical properties (3) density - Characterization via Raman/UV-vis spectroscopy or by direct electrical measurements

32 Metal Oxide Semiconductor Field Effect Transistor

33 N-type FET

34 Bottom-gate SWNT Thin film transistor (TFT) Top-gate SWNT Thin film transistor (TFT)

35 N-type nanowire FET p-type CNT FET

36 (1) Electrical breakdown - Selective burning of metallic nanotubes by increasing the bias between S/D electrodes while a gate field is applied to turn the s- SWCNTs off, i.e. high positive gate voltage. - Increase in on/off current ratio by up to 10 5 without significantly decreasing on-state current Ref) Nano Lett. 4, 2031 (2004)

37 Figure. (a) Breakdown voltages used for channels with different length. Electrical breakdown of metallic conducting path is realized by sweeping the source/drain bias voltage with a maximum voltage 1-2 V below the breakdown voltage while applying a positive gate voltage of V. (b) Transfer characteristics of Device 1 (L = 2 μm, W = 200 μm) before and after the electrical breakdown. ON and OFF current difference and the mobility are typically only 10-30% lower after the electrical breakdown. (c) Transfer characteristics in a log format for three different channels. Device 2 (L= 7 μm, W = 100 μm) has a device mobility of 40 cm 2 /Vs which is the highest among nearly 400 channels measured. In Device 3 (L =20 μm, W = 200 μm), the stripes have a length of 20 μm with a width of 3 μm. (d) Output characteristics when the gate voltage is increased from -20 V to -5 V with a step size of 3 V for Device 1. Typical saturation behavior for transistors is clearly observed.

38 (2) Differences in chemical reactivity -m-swcnts are more chemically reactive than s- SWCNTs, since finite DOS near Fermi level can SnOstabilize 2 (b) charge-transfer complexes that SiOform 2 reaction intermediates SnO 2 -Selective reaction with m-swcnts with 50µm chemical 0 µm reagents render them insulating without altering the s-swcnts - increase the on-off ratio SnO 2 NWs 100µm (d) Intrinsic SWCNT Transferred SWCNT G + Intensity (a.u.) Raman Shift (cm -1 ) D D G - G - G Raman Shift (cm -1 ) Raman spectrum

39 Reaction with diazonium (1)The intensity of disorder mode in m-swnts increase in Raman Spectroscopy. (2)Only at high concentration (~10 μm), s-swnts show the similar reaction. (3)Electrical transfer characteristics, on/off ratio increased with reaction at diazonium due to selective removal of m-swnts. * Filled symbol: metallic, open symbol: semiconducting Ref) Science 301, 1519 (2003). J. Am. Chem. Soc. 127, (2005).

40 (3) Density-gradient -Density-gradient ultracentrifugation isolated narrow distribution of SWCNTs in which >97% are within a 0.02 nm diameter range. -Confirm by taking UV-vis-near-infrared absorption spectra m-swcnts s-swcnts ultracentrifugation Ref) Nature Nanotechnology 1, 60 (2006).

41 Alignment of SWCNTs - For nanotubes to be used in nanoelectronics, essential to have an ability to assemble and integrate them in nanocircuitry rather than mass production.

42 Methods for the assembly of CNT architectures (a) Controlled deposition from a polymerstabilized organic solution in a micro-fluidic system with applied electric fields (b) Controlled growth of suspended structures from pillars (c) Lattice-oriented growth on Si(100) (d) Vectorial growth from patterned catalyst nanoparticles under an electric field. (e) Aligned growth along the crystalline direction

43 (1) Controlled deposition from solution - by selective deposition on functionalized nano-lithographic templates - Initially successful, but extension of this wet approach proved to be difficult due to tendency of SWNTs to aggregate due to van der Waals interactions. - If SWNT ropes are good enough for a particular use, micro-fluidics combined with electric fields produced nice crossbar arrays of SWNT ropes. References) (1) J. Liu et al., Chem. Phys. Lett. 303, 125 (1999). (2) M. R. Diehl et al., Angew. Chem. Int. Ed. 41, 353 (2002).

44 Schematics of controlled deposition of SWNT on chemically functionalized lithographic patterns -selective adsorption of SWNT on NH 2 terminated surface -pattern width : nm * TMS: trimethylsilyl Coulomb attraction between positively charged NH2 on surface and negatively charged SWNTs in DMF suspension

45 - Q shaped NH 2 functionalized pattern -bending of CNT: due to strong Coulomb attraction between positively charged NH 2 on surface and negatively charged SWNTs in DMF suspension -NH 2 functionalized pattern is a straight line between the electrodes, and the stiff CNT makes good contact with gold electrode.

46 Atomic-force micrographs showing large-scale selfassembly of single-walled carbon nanotubes (SWCNTs). (a) SWCNTs near the boundary (white arrow, inset) between polar (cysteamine; left arrow) and non-polar (1- octadecanethiol (ODT); right arrow) molecular patterns on gold. (b) Topography of an array of individual SWCNTs covering about 1 cm 2 of gold surface. The friction-force image (inset) shows a single SWCNT (dark line), and the regions containing 2-mercaptoimidazole (bright area) and ODT (dark area). c, Topography of an array of junctions with no SWCNTs (triangles), one SWCNT (circles) or two SWCNTs (squares), covering an area of about 1 cm2. Arrows 1, 2 and 3 indicate octadecyltrichlorosilane (used to passivate the SiO 2 surface), 2-mercaptoimidazole on gold, and ODT on gold, respectively. Ref) Nature 425, 36 (2003).

47 (2) Controlled growth of suspended networks -individual SWNTs can be grown in situ on silicon wafers by CVD method. controlled growth rather than controlled deposition. - In situ approach, it avoids nanotube aggregation. -When SWNTs were grown from catalytic islands deposited on top of microfabricated pillars, nanotube stretched from one pillar to the next one forming suspended networks. -When a nanotube is growing from the top of a pillar, it waves around in every direction, but when it touches the top of another pillar, it gets pinned to it. -Then, the same nanotube can keep growing and jumping from pillar to pillar for more than 100 m. -Directionality of suspended SWNTs can be enhanced by applying an electric field.

48 -Nucleation of SWNTs only on the tower tops since the catalytic stamping method does not place any catalyst materials on the underlying flat surfaces. -As the SWNTs lengthen, the methane flow keeps the nanotubes floating and waving in the wind. -van der Waals interaction between tube-tower catches the nanotubes. -Tubes are directed toward the flow direction. N.R. Franklin and H. Dai, Adv. Mater. 12 (2000) 890.

49 Higher yield and longer CNTs due to conditioning catalyst, which converts CH 4 into reactive benzene. References (1) N. R. Franklin et al., Adv. Mater. 12, 890 (2000). (2) Y. Zang et al., Appl. Phys. Lett. 79, 3155 (2001).

50 Schematic diagram process flow for electric field-directed growth of SWNTs (a) Growth of poly-si on Quartz (b) Patterning by photolithography and plasma etching: 3 parallel trenches. (c) -contact printing of liquid-phase catalyst precursor on top of poly-si (d) CVD growth of SWNTs under electric field ( dc V or ac 30 MHz, 10V peak-topeak) across all of the three trenches.

51 -SEM images of suspended SWNTs grown in various electric fields. -spacing between the edges of the outer poly- Si electrodes is 40 m.

52 (3) Lattice-directed growth -When SWNTs were grown by CVD on etched Si wafers, the nanotubes preferred to grow parallel to the lattice directions of the crystalline surface. -When SWNTs grow on Si(100), the nanotubes are lying with angles of 90 o and 180 o between each other. -When SWNTs grow on Si(111), they are lying with angles of 60 o and 120 o between each other. -Directionality due to specific interactions of SWNTs with aligned rows of Si atoms of the wafer. References) (1) M. Su et al., J. Phys. Chem. B 104, 6505 (2000).

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54 Fe nanoparticles on H-passivated Si surfaces -Energetically more favorable for SWNTs to grow along the directions defined by the underlying Si lattice to maximize the interaction between tubes and substrate. -At the initial stage of the growth, nanotubes have the mobility to rotate on surfaces due to thermal energy until they find the preferred location.

55 (4) Vectorial growth -growth of SWNTs lying on a surface is geometrically defined as a vector, having a particular position, direction and length. -in-situ growth of SWNTs on Si wafers under the action of local electric field parallel to the surface. -define, origin of the vector by position of patterned catalyst nanoparticles, direction by the electric field created by a pair of lithographic microelectrodes, length by reaction time, and the diameter can be controlled by catalyst nanoparticle size. -When nanotubes longer than a critical length (order of m), they are well aligned with the electric field. Ref) E. Joselevich et al., Nano Letters 2, 1137 (2002).

56 (a) Vectorial growth of SWNT on a surface from a welldefined catalytic particle with ideal control over geometry and structure. Geometrical parameters: origin (x,y), direction( ), and length (L). Structural parameters: diameter(d) and chirality ( ). (b) Growth of nanotube catalytically from the ferrihydrite nanoparticles (3-5 nm) deposited on surface between electrodes. (c) CVD growth under electric field E (4x10 6 V/m)

57 (a) vectorial growth of very long SWNTs (L=10-15 m) from ferritin adsorbed on patterned islands of Al 2 O 3. Voltage :50 V (1x10 6 V/m), growth time: 20 min (b) Vectorial growth of SWNTs from ferritin adsorbed on patterned Al 2 O 3 islands of different sizes, showing higher densities of parallel SWNTs. cf) ferritin: upon activation, yields iron oxide cores of 4-5 nm. (required for this growth mechanism)

58 (5) CVD growth on Quartz surface Figure. (a, b) SEM images of aligned SWNT arrays collected at different magnifications. The tube density is 5 SWNT/μm. The bright horizontal lines in (a) correspond to random networks of SWNT that form near the Fe catalyst that exists in these locations. (c) AFM images of selected SWNTs in these arrays. (d) AFM images of iron oxide catalyst particles after 1.5 h annealing process. Distribution of diameters of SWNTs in the arrays (e), and catalyst particles (f) measured by AFM. The average diameter is 1.2 nm for SWNTs and 1.8 nm for the catalyst particles.

59 The Y-cut provides high degree of alignment along [2-1-10] direction

60 Z-cut provides 3-fold symmetric alignment (inset in h) shows a high-magnification SEM image); and X-cut does not provide any alignment.

61 (a) Schematic illustration of the geometry of quartz with a 2 nm thick patterned stripe of amorphous SiO2. (b, c) SEM images of SWNTs grown on this substrate. angle-dependent van der Waals interactions between SWCNT and substrate can account for nearly all aspects of alignment on quartz with X, Y, Z, and ST cuts.

62 Effect of catalyst on aligned growth of CNTs Co Ni Pt Pd Mn Mo Cr Sn Au * The alignment direction is the X direction on ST-cut quartz. ref) Nano Lett. 8, 2576 (2008)

63 Deposition of uniform film of SWCNTs - Mixing methanol and an aqueous suspension of SWNT on a rapidly spinning substrate Ref) Nano Lett. 4, 1643 (2004).

64 - Deposition of films in line geometries by mixing methanol and a suspension of SWNTs in the interdiffusion region of a laminar-flow microfluidic cell. d) Optical image of a SWNT film in the geometry of a line (dark gray in the center of the image) deposited with a microfluidic cell Ref) Angew. Chem. Int. Ed. 45, 581 (2006).

65 e) SEM image of an aligned SWNT film formed by ac dielectrophoresis. Inset: Schematic illustration of the experimental setup. An ac field applied through microelectrodes causes the deposition of aligned SWNTs, often with enhanced content of m-swnts. Ref) Science 301, 344 (2003). Adv. Mater. 18, 1468 (2006). dielectrophoresis" - the net force experienced by a neutral dielectric object in a non-uniform electric field

66 - AFM image of an aligned array of SWNTs assembled with a LB technique. Ref) J. Am. Chem. Soc. 129, 4890 (2007).

67 Langmuir monolayer H 3 C CH 2 H 2 C CH 2 H 2 C CH 2 H 2 C CH 2 H 2 C CH 2 H 2 C CH 2 H 2 C CH 2 H 2 C Langmuir-Blodgett films HO O Stearic acid Langmuir-Shaefer films

68 열전이테이프를이용한전이및패터닝 SWCNTs Au film : 100nm Thermal tape SiO 2 /Si Au film deposition by e-beam evaporator (100nm) SiO 2 /Si attach the thermal tape SiO2/Si Detach the thermal tape Heating ~120 Thermal tape Attach on desired substrate Thermal tape substrate Before transfer After transfer Remove the thermal tape and etching of Au film SWCNTs substrate

69 Electrical properties of SWCNT TFT on off on/off Well aligned Low-coverage, partially aligned high- coverage Partially aligned Nano Lett. 7, 1195 (2007), IEEE Electron Device Lett. 28, 157 (2007).

70 Flexible electronics

71 Figure. TFTs using SWNT random networks as the semiconductor. (a) Transfer curves of a series of devices (VGS: gate-source voltage; IDS: drain-source current; VDS: drain-source voltage = 1 V). The LCs are 5 μm, 10 μm, 25 μm, 50 μm, and 100 μm, respectively, from the top to the bottom. Inset: effective device mobility (μeff) as a function of LC. (b) Width-normalized resistance of the semiconducting responses of TFTs (RsemW) based on SWNT random networks as a function of LC at different VGS (VGS changes from 6 V to 16 V in steps of 2 V from top to bottom.). The solid lines represent linear fi ts. (c) Schematic illustration of the device layout and optical transmittance (d) of an all-tube transparent TFT in which metallic carbon nanotube networks (m-cnns) serve as the electrodes and semiconducting carbon nanotube networks (s-cnns) serve as the semiconductor. Inset: An array of transparent SWNT TFTs on a transparent plastic substrate (PET), resting on top of a piece of paper with printed text. The dashed red line corresponds to transmission through the source/drain (S/D) region of the device.

72 Fabrication of flexible electronics by transfer of CNTs (b) Aligned CNT arrays transferred from a singlecrystal quartz growth substrate to a plastic substrate (c) Triple crossbar arrays of SWCNTs formed by three consecutive transfer processes Ref) Nano Lett. 7, 3343 (2007).

73 Chemical modification of transport Transfer characteristics of a) ambipolar, b) unipolar p-channel, and unipolar n-channel SWNT TFTs achieved with a) dielectric passivation or b) polymer charge-transfer doping

74 Transparent electronics based on CNT thin films a) A transparent, conductive SWNT film on a sapphire substrate. b) An array of all-tube flexible transparent TFTs (TTFTs) on a plastic substrate. The arrow indicates the S/D, visible as grey squares. c) I-V characteristic of a SWNT TTFT. d) Brightness vs voltage for an OLED that uses a SWNT thin film as the anode. Inset: layout of OLED. HTL, hole-transport layer; EML, emission layer. e) Current density (i) vs voltage for organic solar cells that use ITO or SWNT thin films (black square) as the anode. Inset: flexible organic solar cell using SWNT thin film as electrodes on PET substrate.

75 SWNT Thin Films for Sensing - Electronic properties of SWNTs, which consist exclusively of surface atoms are very sensitive to adsorbents. - Electrical evaluation of changes in resistor, transistor, or capacitor

76 Gas sensor: resistor/transistor DMMP (dimethylmethylphosphonate) - SWNT gas sensors respond to the surface coverage of analytes to give high ppb level sensitivity. - Charge transfer between adsorbed molecules and SWNT valence band changes the # of mobile charge carriers, resulting in the change of resistance. - DMMP, nerve agent sarin, high electron-donating property gives ppb level detection - But, slow response due to high desorption energy. -Formation of TFT geometry solves the problem -By application of gate voltage, resistance goes back to initial value due to repulsive Coulomb force between adsorbents and gate-induced charge. -Ref) Appl. Phys. Lett. 83, 4026 (2003).

77 Gas sensor: chem-capacitor -changes in capacitance between the film and a planar electrode in a chem-capacitor structure. -capacitance response comes from 1) quantum capacitance of SWCNTs due to the shift of Fermi level as a result of charge transfer doping associated with adsorbed molecules 2) geometrical capacitance due to the change of dielectric environment closely surrounding SWNTs as a result of electric field alignment of dipole moments and field-induced polarizations of adsorbed molecules. - Different mechanisms for conductance and capacitance responses lead to different responses to molecules with similar structure. Ref) Science 307, 1942 (2005). Nano Lett. 5, 2414 (2005).

78 The magnitude of the capacitance response correlates with the value of its dipole moment. Nonpolar molecules such as hexane and benzene produce a small response, whereas relatively polar molecules like DMMP and DMF produce a large capacitance response. Adsorbates on the SWNTs form a polarizable layer that increases the capacitance. ΔC = C/ ε Δε + C/ Q ΔQ where the first term represents the dielectric effects of the absorbate and the second term arises from the charge-transfer response via C Q.

79 How to solve the lack of chemical specificity of SWNT gas sensors? - Functionalization of SWNTs with specific receptors for targeted analytes. -Decoration of SWNTs with Pd nanoparticles leads to chem-resistor specific for H2 detection. - On exposure to H2, formation of electron-rich Pd hydride hinders hole transport in p-doped s-swnts resulting in the increase of resistance. -but, interfered by O2 due to reaction with Pd. -- So, integrating SWNT gas sensors into microgas chromatography system. Ref) Adv. Mater. 19, 2818 (2007). Angew. Chem. Int. Ed. 47, 5018 (2008).

80 Bio sensors -Incubation with 12-mer DNA probe - incubation with complementary DNA target (hybrid) -SWNTs Function as labels for efficient label-free detection. -DNAs and proteins can nonspecifically bind to SWNT surfaces due to hydrophobic interaction, π-π interaction, and amino-affinity of SWNTs to alter the conductance of SWNT thin films. Ref) Proc. Natl. Acad. Sci. U. S. A. 103, 921 (2006).

81 Direct chemical functionalization of SWNTs -Use of a bifunctional small-molecule linker that binds with SWNTs through π-π stacking interactions and with an antibody through covalent bonding. - Only the introduction of a specific antigen can change the conductance due to electrostatic gating effects. Ref) J. Am. Chem. Soc. 127, (2005).