Electric Field Induced Electron Transfer between a ... - ACS Publications

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Electric Field Induced Electron Transfer between a Single-Walled Carbon Nanotube and a Molecularly Doped Hole Transport Layer Mandakini Kanungo, Kock-Yee Law,* and Markus Silvestri Xerox Corporation, Xerox Research Center Webster, 800 Phillips Road, 147-59B Webster, New York 14580, United States ABSTRACT: In this work, we report a novel, electric field induced hole injection reaction between thin films of single-walled carbon nanotubes (CNT) and a molecularly doped charge (hole) transport layer (CTL) comprising a hole transport molecule (TPD) doped in polycarbonate in the bilayer device configuration. The CNT bilayer device was shown to be able to charge and then discharge in the dark, whereas the controlled bilayer device, constructed by replacing the CNT film with a metal (TiZr) layer, remains charged up after charging. The discharge of the charged CNT bilayer device suggests that hole injection from the CNT film to the CTL (or electron transfer from the CTL to the CNT film) occurs and that the injected holes are neutralized by a series of isoenergetic electron transfer across the CTL. Results show that the rate of the initial discharge increases as the surface conductivity of the CNT film increases. Current
voltage and time-of-flight measurements suggest that the discharge process is limited by the mobility of the CTL when the surface resistivity of the CNT film is e250 ohm/sq. For bilayer devices formulated with more resistive CNT films, the discharge process is limited by the hole injection efficiency. Since conductive CNT films are an essentially dense network of carbon nanotubes on Mylar, the decrease in the hole injection efficiency for bilayer devices fabricated from more resistive CNT films can be attributed to the increase in porosity in the CNT network in these films. The increased porosity limits hole injection between the carbon nanotube and the CTL. Since micrometer size pixels of CNT film can be made on a flexible substrate by inkjet printing and a nano imprinting technique, we suggest to couple this electric field induced hole injection reaction with an active-matrix backplane for a novel digital printing application.

’ INTRODUCTION Carbon nanotubes owing to the unique one-dimensional carbon structure are found to exhibit unparallel mechanical, electrical, thermal, electronic, magnetic, and optical properties, being stronger, faster, and more conductive both electrically and thermally than most known materials.1 As a result, carbon nanotubes have been one of the most investigated materials in both academia and industry in the last two decades. There are three main methods to synthesize carbon nanotubes: arc-discharge, laser ablation, and chemical vapor deposition (CVD).2 The first two methods use a solid-state carbon precursor and involve the vaporization of carbon at several thousand degree Celsius to produce high quality, high purity carbon nanotubes (after purification). These two procedures are well established, but large scale production by these two methods is unlikely. CVD is the most common procedure to produce carbon nanotubes today and there are many variants. In the CVD method, carbon nanotubes are made with a carbon source from either a hydrocarbon or an alcohol in the presence of a metal nanoparticle catalyst at 500
1000 °C. Carbon nanotubes made by CVD are less pure, but the process is scalable. Several manufacturers are known to be capable of producing tons of carbon nanotubes annually.3 As for applications for carbon nanotubes, the sky is the limit. Notable applications range from fillers in composites or coatings to enhance mechanical,4
8 electrical,9
16 and thermal properties;17
21 to transparent electrodes for displays, organic light-emitting diodes, and solar cells;22
25 to field emission r 2011 American Chemical Society

devices;26
30 to transistors;31
34 to sensors;35
38 to supercapacitors;39 and to interconnects.40 In this work, we report a new application for carbon nanotubes in printing.41 The novel printing device is in a bilayer configuration comprising a single-walled carbon nanotube (CNT) thin film and a charge (hole) transport layer (CTL). The CTL is basically a solid solution of the hole transport molecule N,N0 diphenyl-N,N0 -bis(3-methylphenyl)-(1,10 -biphenyl)-4,40 -diamine (TPD) in polycarbonate. Results show that hole injection from the CNT thin film to the TPD CTL (or electron transfer from the TPD CTL to the CNT film) occurs after the bilayer device is charged. The efficiency of the hole injection process was studied by monitoring the charge
discharge behavior of various bilayer devices. The data indicate that the initial discharge rate is a dependence of the surface conductivity of the CNT film and the strength of the electric field across the bilayer device. Steadystate current
voltage (I
V) measurements indicate that when the surface resistivity of the CNT film is e250 ohm/sq, the discharge is hole mobility limited. As for more resistive CNT films, the discharge process becomes hole injection limited. The charge
discharge process of the bilayer device was shown to be very stable electrically. The potential of using the bilayer device for printing application is discussed. Received: July 15, 2011 Revised: October 17, 2011 Published: October 24, 2011 23964

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Scheme 1. Device Configuration and Chemical Structures of TPD and Polycarbonate

’ EXPERIMENTAL SECTION Materials. Single-walled carbon nanotube films of surface resistivity ranging from ∼100 to ∼5000 ohm/sq on a Mylar substrate were purchased from Eikos Inc. under a materials transfer agreement. These films were prepared by spray coating a dispersion of purified single-walled carbon nanotubes onto the Mylar substrate. The conductivity of the film was adjusted via the number of passes; the more the spray passes, the denser the carbon nanotube network and the higher the film conductivity. Descriptions of the purification of the carbon nanotube and the film coating procedure have been described in the literature.42,43 Both the hole transporting molecule, N,N0 -diphenyl-N,N0 -bis(3-methylphenyl)-(1,10 -biphenyl)-4,40 -diamine (TPD; commercially available from Aldrich) and the polycarbonate binder PCZ200 (Mitsubishi) (structures are shown in Scheme 1) were photoreceptor grade materials used in Xerox products. They were obtained from an internal source. All coating solvents (methylene chloride, tetrahydrofuran and toluene) were analyzed reagent grade from Fisher and were used as received. Device Fabrication. The bilayer device studied in this work was fabricated by simply coating a solution containing TPD and PCZ200 in a mixed solvent of tetrahydrofuran and toluene (70:30 in ratio) over the carbon nanotube film (on Mylar) on a lab draw-down coater using a 3
5 mL draw bar. A typical coating solution consisted of ∼14% of solid. The concentration of TPD in the charge transport layer (CTL) was at 40%. The thickness of the CTL was typically ∼18 μm and was controlled by the solid concentration of the coating solution as well as the wet gap of the draw bar. The resulting bilayer device was air-dried for 0.5 h followed by vacuum drying at 100 °C for 2 h before electrical evaluation. Measurements and Techniques. The surface resistivity of the CNT films was measured by a four-probe-point method using a Keithley 237 high voltage source measure unit. The charge
discharge characteristics of the bilayer device were performed on an in-house electrical characterization apparatus as shown in Figure 1. To ensure good electrical contacts, gold dots (area = 0.71 cm2) were sputtered on the bilayer device (both on the CTL as well as on the CNT film) and intimate contacts were made through the pressure contact with the assist of an indium pad. The bias of the high voltage source and the timing of the relay were controlled through a LabView program. During the test, the relay was first closed (position a) to charge the bilayer device under constant voltage condition provided by the HV power supply. After 100 ms, the relay was opened (position b), and the discharge of the bilayer device was monitored by

Figure 1. Schematic of the experimental setup for electrical characterization. Experimentally, the bilayer device is charged under constant bias voltage when the HV relay is in position a. After 100 ms, the HV relay switches to position b, and the surface potential (V(t)) of the device was measured by the ESV.

measuring the surface potential V(t) using an ESV (electrostatic voltmeter). The entire discharge process was recorded on a PC. Current
voltage (I
V) measurements of the bilayer devices were performed in a two-probe measurement system using a Keithley 237 High Voltage Source unit. Gold dots were evaporated on the CTL for electrical contact. Time-of-flight mobility of the CTL was studied on a homemade mobility characterization apparatus. The schematic of the apparatus and the measurement procedures have been given elsewhere.44,45 Open circuit potential measurements and cyclic voltammetry were performed on a CH600 instrument, where the redox potentials of the CNT film and TPD CTL layer were determined. Electrochemical measurements were all done in a methylene chloride solution containing tetrabutyl ammonium perchlorate as the supporting electrolyte in a three compartment electrochemical cell. The working electrode was a freshly mirror polished glassy carbon electrode, and the counter electrode was a Pt wire. Ag/AgCl electrode was used as the reference electrode.

’ RESULTS AND DISCUSSION Device Configuration and Electrical Characterization. Scheme 1 shows the configuration of the bilayer device and the materials used in this work. The device comprises a CTL made of a hole transport molecule TPD in polycarbonate solvent coated onto a CNT thin film on Mylar. The charge
discharge characteristic of the bilayer device was studied on an in-house electrical characterization apparatus (Figure 1). When the HV relay is in position a, the device is charged by the HV power supply under constant voltage. After charging for 100 ms, the relay switches to position b electronically, and the surface potential of the bilayer device was measured by the ESV probe. Figure 2 shows a dark discharge curve obtained from a typical CNT bilayer device. The surface potential curve of the controlled bilayer device (replacing the CNT film with a metal (Ti/Zr) layer) is also shown in the figure for comparison. Our results show that both devices charged up comparably. The controlled device shows no dark discharge, and it remains charged for a long time. However, the CNT bilayer device discharges within 2
3 s. The discharging of the CNT bilayer device suggests that hole injection from the CNT film to the TDP CTL (or electron transfer from the CTL to the CNT film) occurs. This is followed by a series of electric-field driven electron transfers across the CTL to discharge the bilayer 23965

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Figure 2. Plot of charge
discharge curves for (a) a carbon nanotube/ CTL bilayer device and (b) a controlled ZrTi (metal)/CTL bilayer device.

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Figure 4. Plot of steady-state current density as a function of the electric field for a CTL layer consisting of 40% TPD in polycarbonate (blue opened diamonds are data calculated from a drift mobility experiment) and 3 CNT bilayer devices with CNT surface resistivity at (a) 250, (b) 1000, and (c) 2500 ohm/sq.

bilayer devices. Figure 4 shows the plot of the steady-state current density (J) measured for three different CNT bilayer devices as a function of the electric field. Independently, we also measured the time-of-flight drift mobility of the CTL alone. The current density of the CTL by itself (JTOF) can be calculated from the drift mobility from eq 1.44,45 J TOF ¼ 9=8 ∈ ∈0 μE2 =L

Figure 3. Charge
discharge curves for bilayer devices with CNT films of different surface resistivity: (a) 250, (b) 1000, (c) 2500, and (d) 5000 ohm/sq (CTL thickness all at ∼18 μm).

device.44
46 This discharge process is very similar to those reported in a bilayer photoconductive device.47,48 The only difference is that the charge
discharge process in the photoreceptor is photogenerated and that of the CNT bilayer device happens in the dark. A more detail description of the discharge mechanism is given later in this article. Effect of Surface Resistivity of CNT Film on the Discharge Process. The discharge characteristic of the CNT bilayer device was found to be sensitive to the conductivity of the CNT film (for a common CTL with 40% TPD in polycarbonate, ∼18 μm thick). Figure 3 depicts a series of charge
discharge curves for four bilayer devices containing CNT films with surface resistivity ranging from ∼250 to ∼5000 ohm/sq. The charge
discharge curve for the device with CNT surface resistivity at ∼100 ohm/sq is not shown as its discharge curve is practically overlapping with that of the 250 ohm/sq device. The overall results suggest that (1) all devices are charged comparably, and (2) the initial discharge rate of the device is sensitive to the conductivity of the CNT film; the higher the film conductivity, the higher the initial discharge rate. Current
Voltage (I
V) Measurements. Further insight about the discharge process can be gained from I
V measurements of the

ð1Þ

Here, ∈ is the relative dielectric constant, ∈0 is the dielectric constant of the vacuum, μ is the drift mobility, E is the effective field, and L is the film thickness. The calculated current density for the CTL (data labeled as blue open diamonds) as a function of the electric field from the drift mobility data is included in Figure 4. The results show that the field effect on the current density of the CTL coincides with that of the CNT bilayer device prepared from the CNT film with a surface resistivity of 250 ohm/sq. The comparable current density dependence between these two devices suggests that the discharge of this particular bilayer device is limited primarily by the hole mobility of the CTL. In other words, the hole injection efficiency for this particular device is close to unity. For bilayer devices prepared from more resistive CNT films, the field effect on the steady-state current density is lower than that of the CTL alone case. Since the CTL is the same among all four devices, the decrease in current density suggests that the discharge process is injection limited for bilayer devices with more resistive CNT films. The decrease in hole injection efficiency for the more resistive CNT films can be attributed to the decrease in fiber density in the carbon nanotube network in the film. As noted in the Experimental Section, the CNT films studied in this work were prepared by spray coating single-walled carbon nanotube dispersion on a Mylar substrate. The conductivity of the film was adjusted by the number of spray passes; the more the spray passed, the denser the carbon nanotube fiber network and the higher the film conductivity.42,43 The conductivity of the film is controlled by the density of the carbon nanotube network. For more conductive films, such as those with surface resistivity at or lower than 250 ohm/sq, the surface of the Mylar substrate is essentially covered with carbon nanotubes. Under this condition, maximum hole injection efficiency is obtained. As the film becomes more resistive, the carbon nanotube fiber network becomes more porous, resulting in the decrease in hole injection efficiency. 23966

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Figure 5. Charge
discharge curves for different CNT bilayer devices with varying CTL film thickness,; (a) CNT surface resistivity at 100, (b) 250, (c) 1000, and (d) 2500 ohm/sq.

Scheme 2. Proposed Discharge Mechanism for the Bilayer Device

Scheme 3. Energy Level Diagram for the CNT/TPD CTL Bilayer Device

Effect of CTL Thickness on the Discharge Process. Figure 5 summarizes the results of the effect of CTL thickness on the discharge process for bilayer devices with CNT surface resistivity ranging from 100 to 2500 ohm/sq. Since all the bilayer devices are charged to about 600 V, a thinner CTL thickness would imply that the electric field across the bilayer will be higher. Bilayer devices in Figure 5a,b were fabricated from CNT films with surface resistivities at or lower than 250 ohm/sq. Since the hole injection efficiencies in these devices are close to unity, the increase in initial discharge rate is then attributable to the electric field effect on the hole mobility of the CTL. As for bilayer devices in Figure 5c,d, the increase in the initial discharge rate for thinner CTL may be due to increases in both hole mobility as well as the hole injection efficiency as the electric field is increased. Charge
Discharge Mechanism in the CNT Bilayer Device. On the basis of the overall results in Figures 3
5, the discharge of the CNT bilayer device can be summarized in Scheme 2. After charging, an electric field induced electron transfer from the CTL to the CNT film (hole injection) occurs. This is followed by a series of field induced isoenergetic electron transfers across the CTL. From the energetic point of view, the highest occupied molecular orbital (HOMO) for TPD can be estimated from the oxidation potential, which was determined to be 0.38 V vs Ag/AgCl in methylene chloride, in agreement with

earlier reported data.49 In this work, we also determined that the oxidation potential of the CNT film by cyclic voltammetry, which is ∼0.28 V vs the Ag/AgCl electrode. The work functions for the HOMOs of TPD and CNT can be estimated to be ∼5 and ∼4.9 eV, respectively, based on a known conversion factor.50 The work function for the CNT film estimated in this work is consistent with literature data measured by photoelectron spectroscopy,51,52 which are in the range of 4.8 to 5 eV. Like many other CNT films, the material used in this work consists of a mixture of singlewalled carbon nanotubes with one-third of the tubes being metallic and two-thirds being semiconducting.1 Thus, the HOMOs of the CNT film in the bilayer device should have certain bandwidth rather than a discrete energy level (Scheme 3). The energy level for the lowest unoccupied molecular orbital (LUMO) of the CNT film can be estimated based on the known band gaps of these materials. For example, metallic carbon nanotubes are known to have zero band gap.1 Therefore, the LUMOs of the CNT film should also have a band structure. We place the lower limit of the LUMOs of CNT below the highest HOMOs because one would expect that there exist a distribution of the CNT work function, and some metallic tubes should have a work function larger than 5 eV. The energy level diagram for the bilayer device summarizing the relationship between the HOMOs and LUMOs of both CNT and TPD is given in Scheme 3. 23967

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that if pixels of carbon nanotube films are made,54
57 one can overcoat the pixels with a CTL and then couple the electric field induced hole injection reaction with an active-matrix backplane for digital printing application. Indeed, the concept of digitalizing electrostatic printing was advanced by Hass and Kubby.58 As for the concept demonstration of the CNT bilayer device, printing experiments have been successfully performed on an offline electrostatic printing fixture.41

’ AUTHOR INFORMATION Corresponding Author Figure 6. Electrical stability test (100 K) for a CNT bilayer device on the static scanner.

Accordingly, electron transfer from the HOMO of TPD to the LUMOs of CNT or hole injection from the CNT film to the TPD CTL should be energetically favorable, particularly under the influence of an electric field. This is consistent with the discharge process observed in this work. Electrical Stability of the CNT Bilayer Devices. The electrical stability of the bilayer device is paramount for any practical application and it was studied on the electrical characterization apparatus shown in Figure 1. Basically, the device was charged and the surface potential was monitored statically, repeatedly, and continuously by electronically switching the HV relay from a to b, then a, and so on. Figure 6 shows the result of a 100 K electrical cycling test for the bilayer device with CNT surface resistivity at 250 ohm/sq. The results indicate that the CNT bilayer device is electrically stable over 100 K cycling, and there is no sign of any degradation in the charge acceptance and the discharge process as judged from the initial surface potential (VH) and the residual potential (VR) throughout the run. Summary and Technological Implications. This work reports for the first time a novel, electric field induced electron transfer reaction from the hole transport molecule TPD to a thin film of carbon nanotubes. The fundamental characteristic of the electron transfer reaction was studied in bilayer devices, which were prepared by overcoating the CNT film (on PET substrate) with a hole transport layer. Basically, upon charging of the CNT bilayer device, hole injection from the CNT film to the TPD CTL occurs followed by a series of isoenergetic electron transfers across the CTL to discharge the device. The discharge process was shown to be hole mobility limited for CNT films with surface resistivity at or lower than 250 ohm/sq. The discharge process becomes hole injection limited when CNT films are more resistive. In today’s laser printer and multifunction printer
copier, bilayer organic photoreceptor devices are used industry-wide.47,48 The configuration of the bilayer photoreceptor comprises a photogeneration pigment layer and a hole transport CTL and is very similar to the CNT bilayer device described in this work. In the bilayer photoreceptor, the latent electrostatic image is generated first by photoexcitation of the photoconductor, followed by hole injection from the CTL to the photogenerated hole and subsequent hole migration through the CTL to create the electrostatic latent image. After that, the latent image is developed electrostatically by toner, and the toned image is then transferred to paper followed by fusing to produce a print.53 The process of discharging the CNT bilayer device is very similar, except that no light is required. The overall results thus suggest

*Tel: 585-422-5229. Fax: 585-422-3833. E-mail: klaw@xeroxlabs. com.

’ REFERENCES (1) Jorio, A., Dresselhaus, G., Dresselhaus, M., Eds. Carbon Nanotubes, Advanced Topics in the Synthesis, Structure, Properties and Applications; Springer-Verlag: Berlin, 2008. (2) Dai, H. Nanotube Growth and Characterization. Carbon Nanotubes; Springer: Berlin, 2001; pp 29
53. (3) Today, there are many manufacturers who produce carbon nanotubes (single-walled, double-walled, and multi-walled) in tons quantity annually; see www.nanotube-suppliers.com/. (4) Shim, B. S.; Zhu, J.; Jan, E.; Critchley, K.; Ho, S.; Podsiadlo, P.; Sun, K.; Kotov, N. A. ACS Nano 2009, 3, 1711. (5) Ci, L.; Suhr, J.; Pusharaj, V.; Zhang, X.; Ajayan, P. M. Nano Lett. 2008, 8, 2762. (6) Liu, L.; Grunlan, J. C. Adv. Funct. Mater. 2007, 17, 2343. (7) Coleman, J. N.; Khan, U.; Blau, W. J.; Gun’ko, Y. K. Carbon 2006, 44, 1624. (8) Barrera, E. V. JOM 2000, 38. (9) Trionfi, A.; Scrymgeour, D. A.; Hsu, J. W. P.; Arlen, M. J.; Tomlin, D.; Jacobs, J. D.; Wang, D. H.; Tan, L. S.; Vaia, R. A. J. Appl. Phys. 2008, 104, 083708. (10) Zhu, B. K.; Xie, S. H.; Xu, Z. K.; Xu, Y. Y. Compos. Sci. Technol. 2006, 66, 548. (11) Zhang, Q.; Rastogi, S.; Chen, D.; Lippits, D.; Lemstra, P. J. Carbon 2006, 44, 778. (12) McLachlan, D. S.; Chiteme, C.; Park, C.; Wise, K. E.; Lowther, S. E.; Lillehei, P. T.; Siochi, E. J.; Harrison, J. S. J. Polymer Sci., Part B: Polymer Phys. 2005, 43, 3273. (13) Barrau, S.; Demont, P.; Peigney, A.; Laurent, C.; Lacabanne, C. Macromolecules 2003, 36, 5187. (14) Ounaies, Z.; Park, C.; Wise, K. E.; Siochi, E. J.; Harrison, J. S. Compos. Sci. Technol. 2003, 63, 1637. (15) Kilbride, B. E.; Coleman, J. N.; Fraysse, J.; Fournet, P.; Cadek, M.; Drury, A.; Hutzler, S.; Roth, S.; Blau, W. J. J. Appl. Phys. 2002, 92, 4024. (16) Coleman, J. N.; Curran, S.; Dalton, A. B.; Davey, A. P.; McCathy, B.; Blau, W.; Barklie, R. C. Phys. Rev. B 1998, 58, 7492. (17) Sihn, S.; Ganguli, S.; Roy, A. K.; Qu, L.; Dai, L. Compos. Sci. Technol. 2008, 68, 658. (18) Liu, C. H.; Huang, H.; Wu, Y.; Fan, S. S. Appl. Phys. Lett. 2004, 84, 4248. (19) Nan, C. W.; Liu, G.; Lin, Y.; Li, M. Appl. Phys. Lett. 2004, 85, 3549. (20) Biercuk, M. J.; Liaguno, M. C.; Radosavljevic, M.; Hyun, J. K.; Johnson, A. T.; Fischer, J. E. Appl. Phys. Lett. 2002, 80, 2767. (21) Home, J.; Liaguno, M. C.; Biercuk, M. J.; Johnson, A. T.; Batlogg, B.; Benes, Z.; Fischer, J. E. Appl. Phys. A: Mater. Sci. Process. 2002, 74, 339. (22) Ou, E. C. W.; Hu, L.; Raymond, G. C. R.; Soo, O. K.; Pan, J.; Zheng, Z.; Park, Y.; Hecht, D.; Irvin, G.; Drzaic, P.; Gruner, G. ACS Nano 2009, 3, 2258. (23) Fanchini, G.; Miller, S.; Parekh, B. B.; Chhowalla, M. Nano Lett. 2008, 8, 2176. 23968

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(24) Gruner, G. J. Mater. Chem. 2006, 16, 3533. (25) Du Pasquier, A.; Unalan, H. E.; Kanwal, A.; Miller, S.; Chhowalla, M. Appl. Phys. Lett. 2005, 87, 203511. (26) Sohn, J. I.; Lee, S.; Song, Y. H.; Choi, S. Y.; Cho, K. I.; Nam, K. S. Appl. Phys. Lett. 2001, 78, 901. (27) Lee, N. S.; Chung, D. S.; Han, I. T.; Kang, J. H.; Choi, Y. S.; Kim, H. Y.; Park, S. H.; Jin, Y. W.; Yi, W. K.; Yun, M. J.; Jung, J. E.; Lee, C. J.; You, J. H.; Jo, S. H.; Lee, C. G.; Kim, J. M. Diamond Relat. Mater. 2001, 10, 265. (28) Fan, S.; Chapline, M. G.; Franklin, N. R.; Tombler, T. W.; Cassell, A. M.; Dai, H. Science 1999, 283, 512. (29) de Heer, W. A.; Chatelain, A.; Ugarte, D. Science 1995, 270, 1179. (30) Rinzler, A. G.; Hafner, J. H.; Nikolaev, P.; Lou, L.; Kim, S. G.; Tomanek, D.; Nordlander, P.; Colbert, D. T.; Smalley, R. E. Science 1995, 269, 1550. (31) Ishikawa, F. N.; Chang, H. K.; Ryu, K.; Chen, P. C.; Badmaev, A.; De Arco, L. G.; Shen, G.; Zhou, C. ACS Nano 2009, 3, 73. (32) Steiner, M.; Freitag, M.; Perebeinos, V.; Tsang, J. C.; Small, J. P.; Kinoshita, M.; Yuan, D.; Liu, J.; Avouris, P. Nat. Nanotechnol. 2009, 4, 320. (33) Aguirre, C. M.; Teenon, C.; Paillet, M.; Desjarardins, P.; Martel, R. Nano Lett. 2009, 9, 1457. (34) Topinka, M. A.; Rowell, M. W.; Goldhaber-Gordon, D.; McGehee, M. D.; Hecht, D. S.; Gruner, G. Nano Lett. 2009, 9, 1866. (35) Zhang, W.; Suhr, J.; Koratkar, N. J. Nano Sci. Nano Technol. 2006, 6, 960. (36) Merkoci, A.; Pumera, M.; Llopis, X.; Perez, B.; del Valle, M.; Alegret, S. Trends Anal. Chem. 2005, 24, 826. (37) Wang, J.; Musameh, M. Anal. Chem. 2003, 75, 2075. (38) Wang, J.; Liu, G.; Jan, M. R.; Zhu, Q. Electrochem. Commun. 2003, 5, 1000. (39) Xiao, Q.; Zhou, X. Electrochim. Acta 2003, 48, 575. (40) Tawfick, S.; O’Brien, K.; Hart, A. J. Small 2009, 5, 2467. (41) Law, K. Y.; Kanungo, M. NSTI Nanotech 2010, Anaheim, CA, June 21
24, 2010; Vol. 1, p 262. (42) Luo, J.; Arthur, D. J.; Glatkowski, P. J. U.S. Patent 7,378,040, May 27, 2008. (43) http://www.eikos.com/coating-deposition.html. (44) Abkowitz, M. A.; Facci, J. S.; Stolka, M. Appl. Phys. Lett. 1993, 63, 1892. (45) Abkowitz, M. A.; Facci, J. S.; Rehm, J. J. Appl. Phys. 1998, 83, 2670. (46) Pai, D. M.; Yanus, J. F.; Stolka, M. J. Phys. Chem. 1984, 88, 4714. (47) Weiss, D.; Abkowitz, M. A. Chem. Rev. 2010, 110, 479. (48) Law, K. Y. Chem. Rev. 1993, 93, 449. (49) Law, K. Y.; Facci, J. S.; Bailey, F. C.; Yanus, J. F. Imaging Sci. J. 1990, 43, 31. (50) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001. (51) Shiraishi, M.; Ata, M. Mater. Res. Soc. Symp. Proc. 2001, 633, A4.4.1. (52) Suzuki, S.; Bower, C.; Watanabe, Y.; Zhou, O. Appl. Phys. Lett. 2000, 76, 4007. (53) Schein, L. B. Electrophotography and Development Physics; Springer-Verlag: New York, 1987. (54) Song, J. W.; Kim, J.; Yoon, Y. H.; Choi, B. S.; Kim, J. H.; Han, C. S. Nanotechnology 2008, 19, 1. (55) Kordas, K.; Mustonen, T.; Toth, G.; Jantunen, H.; Lajunen, M.; Soldano, C.; Talapatra, S.; Kar, S.; Vajtai, R.; Ajayan, P. M. Small 2006, 2, 1021. (56) Zhang, D.; Ryu, K.; Liu, X.; Polikarpov, E.; Ly, J.; Tompson, M. E.; Zhou, C. Nano Lett. 2006, 6, 1880. (57) Zhou, Y.; Hu, L.; Gruner, G. Appl. Phys. Lett. 2006, 88, 123109. (58) Hass, W. E.; Kubby, J. A. U.S. Patent 6,100,909, 2000.

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