Molecular n-Doping of Large- and Small-Diameter Carbon Nanotube Field-Effect Transistors with Tetrakis(tetramethylguanidino)benzene

The guanidino-functionalized aromatic compound 1,2,4,5-tetrakis(tetramethylguanidino)benzene (ttmgb) has been shown to be an efficient n-dopant for field-effect transistors (FETs) with gold contacts and networks of semiconducting single-walled carbon nanotubes (SWCNTs) with small diameters and large band gaps. Here, we investigate the broader applicability of ttmgb as a molecular n-dopant by fabricating bottom-contact/top-gate FETs with different air-stable, high work function metals as electrodes and with both small- and large-diameter polymer-sorted SWCNTs. Kelvin probe measurements indicate a reduction of the work functions of gold, palladium, and platinum by about 1 eV after ttmgb treatment and, correspondingly, gated four-point probe measurements show orders of magnitude lower contact resistances for electron injection into SWCNT networks. FETs based on networks of (6,5) SWCNTs with large band gaps as well as mixed semiconducting plasma torch SWCNTs with small band gaps can thus be transformed from ambipolar to purely n-type with no hole injection or increased off-currents by applying optimized ttmgb concentrations. Carrier concentration- and temperature-dependent measurements reveal that ttmgb treatment does not impact the electron transport and maximum mobilities in SWCNT networks at high carrier densities, but greatly improves the subthreshold slope of nanotube FETs by removing shallow electron trap states. This effect is found to be particularly pronounced for small-diameter nanotubes with large band gaps.