A systematic study on the use of 9,9-dialkylfluorene homopolymers (PFs) for large-diameter semiconducting (sc-) single-walled carbon nanotube (SWCNT) enrichment is the focus of this report. The enrichment is based on a simple three-step extraction process: (1) dispersion of as-produced SWCNTs in a PF solution; (2) centrifugation at a low speed to separate the enriched sc-tubes; (3) filtration to collect the enriched sc-SWCNTs and remove excess polymer. The effect of the extraction conditions on the purity and yield including molecular weight and alkyl side-chain length of the polymers, SWCNT concentration, and polymer/SWCNT ratio have been examined. It was observed that PFs with alkyl chain lengths of C10, C12, C14, and C18, all have an excellent capability to enrich laser-ablation sc-SWCNTs when their molecular weight is larger than ∼10 000 Da. More detailed studies were therefore carried out with the C12 polymer, poly(9,9-di-n-dodecylfluorene), PFDD. It was found that a high polymer/SWCNT ratio leads to an enhanced yield but a reduced sc-purity. A ratio of 0.5-1.0 gives an excellent sc-purity and a yield of 5-10% in a single extraction as assessed by UV-vis-NIR absorption spectra. The yield can also be promoted by multiple extractions while maintaining high sc-purity. Mechanistic experiments involving time-lapse dispersion studies reveal that m-SWCNTs have a lower propensity to be dispersed, yielding a sc-SWCNT enriched material in the supernatant. Dispersion stability studies with partially enriched sc-SWCNT material further reveal that m-SWCNTs : PFDD complexes will re-aggregate faster than sc-SWCNTs : PFDD complexes, providing further sc-SWCNT enrichment. This result confirms that the enrichment was due to the much tighter bundles in raw materials and the more rapid bundling in dispersion of the m-SWCNTs. The sc-purity is also confirmed by Raman spectroscopy and photoluminescence excitation (PLE) mapping. The latter shows that the enriched sc-SWCNT sample has a narrow chirality and diameter distribution dominated by the (10,9) species with d = 1.29 nm. The enriched sc-SWCNTs allow a simple drop-casting method to form a dense nanotube network on SiO2/Si substrates, leading to thin film transistors (TFTs) with an average mobility of 27 cm(2) V(-1) s(-1) and an average on/off current ratio of 1.8 × 10(6) when considering all 25 devices having 25 μm channel length prepared on a single chip. The results presented herein demonstrate how an easily scalable technique provides large-diameter sc-SWCNTs with high purity, further enabling the best TFT performance reported to date for conjugated polymer enriched sc-SWCNTs.

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The sheet resistance of hydrogen terminated silicon-on-insulator substrates increases significantly with time in air due to depletion of free carriers, attributed to the growth of electrically active defects as the surface oxidizes. Surprisingly, physisorbed water ͑via adsorption from ambient or controlled exposure in vacuum͒ causes an increase in the conductivity. This effect is largely reversible when the water layer is displaced by inert gas purging, heating, or pumping. The observed conductivity changes are correlated with Hall voltage changes, indicating that the adsorbed water layer induces accumulation of majority carriers on n-doped substrates.

Chemical functionalization of graphene is achieved by hyperthermal reaction with azopyridine molecular ions. The one-step, room temperature process takes place in high vacuum (10(-7) mbar) using an electrospray ion beam deposition (ES-IBD) setup. For ion surface collisions exceeding a threshold kinetic energy of 165 eV, molecular cation beams of 4,4'-azobis(pyridine) covalently attach to chemical vapor deposited (CVD) graphene. A covalent functionalization degree of 3% of the carbon atoms of graphene is reached after 3-5 h of ion exposure of 2 × 10(14) azopyridinium/cm(2) of which 50% bind covalently. This facile approach for the controlled modification of graphene extends the scope of candidate species that would not otherwise react via existing conventional methods.

Accumulation mode pseudo-MOSFETs formed on hydrogen terminated silicon-on-insulator (SOI-H) were used to probe molecular adsorption and reaction events. Current-voltage characteristics of such n-channel devices are found to be sensitive to the environment, with the accumulation threshold voltage, or flat-band voltage, exhibiting large reversible changes upon cycling between ambient atmosphere, high vacuum (<10 À5 Torr), and exposure to water and pyridine vapor at pressures in the Torr range. The field-effect mobility is found to be comparatively less affected through these transitions. Oxidation of the H-terminated surface in ambient conditions leads to irreversible shifts in both the flat-band voltage and the field-effect mobility. A photochemical gas phase reaction with decene is used to form a decyl monolayer on the SOI(100)-H surface. Formation of this monolayer is found to result in a relatively small shift of the threshold voltage and only a slight degradation of the field effect mobility, suggesting that alkyl monolayer dielectrics formed in this way could function as good passivating dielectrics in field effect sensing applications. V

The adsorption of a range of molecular species (water, pyridine, and ammonia) is found to reversibly modulate the conductivity of hydrogen‐terminated silicon‐on‐insulator (H‐SOI) substrates. Simultaneous sheet‐resistance and Hall‐effect measurements on moderately doped (1015 cm−3) n‐ and p‐type H‐SOI samples mounted in a vacuum system are used to monitor the effect of gas exposure in the Torr range on the electrical‐transport properties of these substrates. Reversible physisorption of “hole‐trapping” species, such as pyridine (C5H5N) and ammonia (NH3) produces highly conductive minority‐carrier channels (inversion) on p‐type substrates, mimicking the action of a metallic gate in a field‐effect transistor. The adsorption of these same molecules on n‐type SOI induces strong electron‐accumulation layers. Minority/majority channels are also formed upon controlled exposure to water vapor. These observations can be explained by a classical band‐bending model, which considers the adsorbates as the source of a uniform surface charge ranging from +1011 to +1012q cm−2. These results demonstrate the utility of DC transport measurements of SOI platforms for studies of molecular adsorption and charge‐transfer effects at semiconductor surfaces.

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Adsorption of tetracyanoethylene (TCNE) onto hydrogen terminated, n-type silicon-on-insulator is shown to cause significant depletion of majority carriers. Employing an ambient pseudo-MOSFET, ppm levels of TCNE vapour rapidly decrease the n-channel saturation current by at least two orders of magnitude. Covalent passivation with a decyl monolayer improves the reversibility of the response while only slightly decreasing the sensitivity.