Supporting Information For: Strong and stable doping of carbon nanotubes and graphene by MoOx for transparent electrodes Sondra L. Hellstrom,1 Michael Vosgueritchian,1 Randall M. Stoltenberg,1 Irfan Irfan,2 Mallory Hammock,1 Yinchao Bril Wang,1 Chuancheng Jia,3 Xuefeng Guo,3 Yongli Gao,2 Zhenan Bao.1* 1
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Stanford University, Stanford, CA, 94305
Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627
College of Chemistry and Molecular Engineering, Peking University, Beijing, P.R.China 100871
[email protected],
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[email protected] CORRESPONDING AUTHOR:
Dr. Zhenan Bao;
[email protected]; 381 North-South Mall,
Stanford, CA, 94305; Tel: 650-723-2419; Fax: 650-723-9780.
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Equipment and Methods
Substrate preparation: Glass substrates were Corning Eagle display borosilicate glass. Samples were cut to appropriate rectangular dimensions (1 to 1.5 cm wide and 2 to 3 cm long) and cleaned by 10 minutes of bath sonication in chloroform followed by 10 minutes of bath sonication in ethanol. They were then dried under a nitrogen or air gun.
CNT network deposition: Samples spray-coated from dispersions in NMP were made with 3 mg CNT in 20 ml NMP. This mixture was sonicated at 225 W using a Cole-Parmer Ultrasonic cup-horn sonicator. For purification, this dispersion was then centrifuged for 30 minutes at 8 ºC and 8000 RPM. The supernatant was loaded into a commercial airbrush, and repeatedly sprayed onto a glass substrate taped to a 220 ºC hot plate, until the desired transmittance was reached.
F4-TCNQ doping: F4-TCNQ (TCI Chemical Co., used as received) was dissolved at a concentration of 0.4 mmol in chloroform by bath sonication at room temperature for 30-90 minutes, until no solid F4TCNQ was observable. This solution was then filtered through a 0.4 micron syringe filter. Samples doped by soaking were then immersed upside-down in this solution for 1 hour, after which they were removed without rinsing and carefully dried under air or nitrogen gun. Unless stability testing was being performed, post-treatment electrical data was taken within 1 hour of treatment to avoid concerns of instability of the doped films.
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Vacuum deposited MoOx doping: MoO3 (Aldrich, used as received) was evaporated from a Mo boat using a thermal evaporator by Angstrom Engineering, at a rate of 0.2-0.5 Å/s. A single 5-7 nm thick layer, as measured by quartz crystal microbalance and calibrated by atomic force microscopy, was deposited. Since MoO3 loses oxygen after evaporation or heating, the term MoOx is used in this work for any layer evaporated or used in doping.
For annealing, MoOx-CNT bilayers on appropriate substrates were placed inside a quartz tube furnace which was then evacuated and re-filled three times with Ar. Under Ar flow, the samples were then heated to 450-500 ºC, left there for three hours, and then cooled to less than 100 ºC before removal.
Solution deposited MoOx doping: We prepared a solution of peroxy- poly- molybdic acid as described in reference (49) using the highest reported concentration of metallic Mo. After filtration, the viscous orange-yellow solution was diluted with water and spin- or spray-coated within 24 hours as material would precipitate if left to sit.
Ultraviolet photoemission measurements: UPS and IPES studies were performed using a VG ESCA Lab, an ultrahigh vacuum (UHV) system equipped with a He discharge lamp and a Mg Kα X-ray source (1253.6 eV). The UHV system consisted of three interconnecting chambers: a spectrometer chamber, an in-situ oxygen plasma (OP) treatment chamber, and an evaporation chamber. The base pressure of the spectrometer chamber was typically 8 x 10−11 torr. The base pressure of the evaporation chamber was typically 1 x 10−6 torr. The UPS spectra were recorded by using unfiltered He I (21.22 eV) as the excitation source with the sample biased at −5.00 V to observe the low-energy secondary cutoff. The UV light spot size on the sample was about 1 mm in diameter. The typical instrumental
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resolution for UPS measurements ranged from 0.03 to 0.1 eV with photon energy dispersion of less than 20 meV.
Substrates were cut from a borosilicate glass sheet from Corning, and were coated with 250 nm thick conducting ITO, with resistivity 15 Ω per square. The substrates were treated in-situ in 600 milli-torr oxygen at a bias voltage of -500V for 30 sec. 55 Å thick thermally evaporated MoOx films were deposited on OP treated ITO substrates. The thickness of the MoOx was monitored by quartz crystal microbalance. The MoOx film was exposed to air for one hour which corresponds to 2.75 x 1012 Langmuir (L) exposure. One L exposure is equal to 10-6 torr-sec. Annealing temperatures were recorded with a non-contact infra-red pyrometer, Palser from E2T Corp. The distance between the sample and the pyrometer was ~25 cm, with a glass window of the UHV chamber just in front of the pyrometer. Substrates were mounted on a small button heater, one at a time. A small electrical current was passed through the electrical feed-through inside the UHV chamber to heat the sample. The sample temperature was calibrated by recording the temperature simultaneously with the pyrometer and a digital thermometer in another small UHV chamber.
Other Instrumentation: Atomic force microscopy was taken using a Veeco Multimode SPM in Tapping Mode. Transmittances and optical spectra were measured by a Cary 6000i UV-Vis-NIR spectrophotometer (Varian, Inc.). Micro-Raman measurements (LabRam Aramis, Horiba Jobin Yvon) were obtained at 633 nm excitation at 100x magnification and 1 µm spot size, and at least four spectra were obtained per sample.
Sheet resistances were measured using a Keithley 2400 source-meter unit in a standard 4-wire configuration, and contact was made to the sample via four spring-loaded metal pins. Resistances were 4
multiplied by geometrical factors as tabulated in "Geometric Factors in Four Point Resistivity Measurement," Haldor Topsoe Semiconductor Division, 2nd ed., May 1968, to account for the effects of fringe electric fields.
X-ray photoelectron spectroscopy was taken with a SSI S-Probe XPS Spectrometer with Al(ka) radiation (1486 eV). Scanning electron microscopy was taken with a FEI XL30 Sirion SEM.
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