Supporting Information Single-walled carbon nanotubes spontaneous loading into exponentially-grown LBL films** Materials used: Sudhanshu Srivastava, Paul Podsiadlo, Kevin Critchley, Jian Zhu, Ming Qin, Bong Sup Shim, and Nicholas A. Kotov* Departments of Chemical Engineering, Materials Science and Engineering, Biomedical Engineering, University of Michigan, Ann Arbor, Michigan 48109. E-mail: kotov@umich.edu The polymers used to build up LBL films were all purchased from Aldrich and used without further purification: poly(diallyldimethyl ammonium chloride) (PDDA, ref 409014, typical molecular mass : 1x10 5-2x10 5 g.mol -1 ) was dissolved at 0.5 % (w/v) in E-pure water ( = 18.2 M.cm). This solution had a natural ph of 6.8. Poly(acrylic acid) (PAA, ref 416002, typical molecular mass : 2.5 x 10 5 g.mol -1 (Aldrich)) was dissolved at 1.0 % (w/v). The ph of this solution was 2.9 in all of our experiments. Sodium Dodecyl Sulfate (SDS, 99+%). SWNTs were purchased from Unidym Inc. PDDA/PAA LBL films preparation: The (PDDA/PAA) n film build-up was carried out by dipping the glass slides alternately in the PDDA and PAA solutions for 2 min each. Each adsorption step was separated from the next one by a 1 min rinse with E-pure water. The deposition of each PDDA and PAA layer defines one layer pair. The films were blown dry with an air flow every layer pair or after the deposition of n layer pairs in order to check the influence of the drying-rehydration steps on the film build-up. Note that the layer deposition time of 2 min may not correspond to the maximum incorporation of the polymers, but this time was chosen to reduce the duration of the film preparation, because we wanted to prepared films with a high number of layer pairs. Prepration of Single-Walled Carbon Nanotubes (SWNTs) dispersions: The Sodium Dodecyl Sulfate (SDS) coated SWNTs were prepared initially by adding SWNTs to deionized water to make 0.5mg/ml solution. 0.2 wt% of SDS was then mixed with the SWNTs solution. The solution was then sonicated for ~24 hours so as to make complete dispersiton of SDS-coated SWNTs. To avoid any precipitaes the dispersion solution was left standing overnight and the top solution was used for all the experiments. The dispersions in poly(acrylic acid) were prepared following similar procedure. PAA-coated SWNTs were dispersed in the 35 wt% PAA solution and was diluted to 0.8wt% for the studies. Sample prepration: The LBL films of PDDA/PAA on glass slides were introduced in the dispersion solutions of SDS and PAA-coated SWNTs for different time intervals to infuse the CNTs completely in the films. Once the samples were prepared, they were dried under laminar flow of air for 3 hours. The dried films loaded with CNTs were then analyzed under SEM. The swelling in the LBL films in contact with SWNTs was seen anything beyond 10 to 20 bilayers of PDDA/PAA. SEM: For SEM studies the samples were broken into small pieces and then sputtered with gold for 90 sec. The prepared samples were then analyzed under FEI Nova Nanolab Dualbeam Focussed Ion Beam instrument. Conductivity measurements: For the resistance measurements, Agilent 34401A Digital Multimeter was used for all the SWNT loaded LBL slides. I/V Studies: I-V curves were obtained using a Keithley Electrometer 6517A. Copper electrodes were used to make electrical contact with the films. Each point was an average of six measurements.
Figure S1: Scanning Electron Microscopy (SEM) images of the cross-section of the LBL films loaded with sodium dodecyl sulfate (SDS)-coated single-walled carbon nanotubes (SWNT 1): a) Films kept in contact with SWNT 1 for 7 days, b) The top thick layer clearly indicates the presence of SWNTs. d) The bottom portion of the film. Figure S2: a) SEM images of the cross-section of the free-standing LBL films loaded with sodium dodecyl sulfate (SDS)-coated SWNTs from both the ends. b) SEM image from the middle of the film c) Zoomed-in image from the middle of the LBL films.
Figure S3: Image of the free standing films with SWNT 1 infusion from both the sides. Scale bar in the image corresponds to 1cm. Figure S4: A) Top view of LBL film infused with SWNT 1. B) Zoom-in image for SWNT 1. C) Top view of LBL film infused with multi-walled carbon nanotubes. D), E) Top view of LBL film infused with CdTe and Te nanowires. The nanowire samples did not show any swelling and incorporation inside the films.
To further compare the difference in conductivity for SWNTs alone as prepared and loaded in the LBL films I-V measurements were carried out. The I-V curve for a PDDA/PAA film with SWNT 1 (Figure S5-A) showed no deviation from a linear response and demonstrated an Ohmic contact with the copper electrodes. The gradient of the curve gives the resistance of 4.6 k, which corresponds to a surface resistivity of 1.5 k /sq. Similarly, the I-V curve for a film that has been immersed in polyacrylic acid (PAA) coated carbon nanotubes (SWNT 2) showed resistance value of 4.0 M (Figure S5-B) as stated earlier, which was considerably higher than that found for the films with SWNT 1 incorporated. This resistance corresponds to a surface resistivity of 6 M /sq. The I-V curves over a 20 V range (S5-B insert) showed that the curve deviates from the linear curve at higher voltages. To compare the difference SWNT 1 alone were also dried across gold electrodes to compare the I-V response of the SWNTs in the absence of the PDDA/PAA film. The gold electrodes were separated by ~2 m. A droplet of ~5 L of SWNT 1 was placed onto an array of gold electrodes and dried. The results showed the I- V curve of SWNT 1 to be close to a linear curve over the 3 V range and then showed deviation (Figure S5-C, D). These results indicate that loading of SWNT 1 in LBL films does tune their physical response. Similarly, the I-V curve of SWNT 2 samples were also measured in the same manner. The resistance of the PAA coated SWNTs was found considerably higher than the SDS coated SWNT, which is consistent with what was found when incorporated into the PDDA/PAA films. The I-V response of SWNT 2 were also non-linear with slight change in the curve compared to the SWNT 2 loaded films and resembles a Schottky barrier or non-ohmic contact with the gold. These results suggest that the loaded PDDA-PAA films do affect the stability and properties of SWNTs in the LBL films.
Figure S5: The I-V curves of the PDDA/PAA LBL film with SWNT 1 (A) and SWNT 2 (B) incorporated. The insert in (b) shows the IV curve over a 20 V range. C) SDS-coated SWNTs and D) poly(acrylic acid) (PAA)-coated SWNTs that were deposited between Au-electrodes (insert)
Figure S6: Resistance measurement vs. temperature change for the traditional LBL films prepared by alternate absorbtion of (A) SDS- or (B) PAA-coated SWNTs and polyvinyl alcohol (PVA) polymer as a partner LBL polymer. Figure S7: Zeta Potential values for SDS-coated SWNTs (-54 mv) and PAA coated SWNTs (-31 mv) as prepared in solution.
Figure S8: UV-vis data for SDS-coated SWNTs and PAA coated SWNTs in solution presented well-dispersed solution of CNTs.