(a) Schematic of the measurement apparatus. Three red lasers and photodiodes were used to measure the absorbance of the SWCNT suspension at three different levels. Measured absorbance as a function of time for (b) poorly dispersed and (c) well dispersed suspensions. Three settling regimes for well dispersed suspensions are indicated by Roman numerals (see text). Evolution of the state of the SWCNT suspension is depicted in (a), (d), and (e). Initially (a) well dispersed SWCNTs (d) aggregate into bundles and subsequently (e) precipitate to the bottom with progressing settling time.
(a) Schematic of the model used for the interpretation of optical absorbance with time. The contribution of the small particles in a well dispersed suspension to the absorbance is represented by the blue curve as an exponential decrease in the absorbance due to the depletion of the effective medium. The contribution of the cross-sectional enhanced Mie scattering from bundles is represented by the resonance peaks shown in red. The Mie scattering peaks are labeled on the red curve. (b) Replotted experimental absorbance versus time data with the contribution from the effective medium subtracted for the middle laser to clearly identify the resonance peaks. (c) Enhanced cross-sectional vs bundle size for particles with and (realistic values for HiPCO SWCNTs) at various medium refractive indices. The initial particle size of the bundles was calculated to be and each subsequent peak indicates the growth of the bundles by . The curves for are offset for clarity. The inset emphasizes that no cross-sectional enhancement is present for particle sizes below . (d) Summary of bundle diameters as a function of time extracted from the experimental results.
(a) Average sheet resistances for SWCNT thin films prepared at different settling times. The minimum sheet resistance was found at settling . (b) Typical AFM image of the prepared thin film showing individual SWCNTs and bundles of few tens of nanometers in diameter can be readily observed. (c) Schematic of the SWCNT network showing different degrees of bundle percolation as a function of the settling time. The schematic shows bundles percolating across the network for a settling time of around 10 h, giving rise to minimum .
(a) TFT characteristics (source-drain current as a function of gate voltage, ) of typical devices for thin films deposited at settling times of 0, 10, and 100 h. (b) Variation of on/off ratio and on and off currents for SWCNT TFTs prepared at different settling times. Leakage (off) current increases more than an order of magnitude during the initial 10 h giving rise to a low on/off ratio.
On/off ratio of each TFT device mapped against sheet resistance . The device characteristics mostly fall within the three shaded regions depending on the settling regime.
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