Charge and Thermoelectric Transport in Polymer-Sorted Semiconducting Single-Walled Carbon Nanotube Networks.

Understanding the charge transport mechanisms in chirality-selected single-walled carbon nanotube (SWCNT) networks and the influence of network parameters is essential for further advances of their optoelectronic and thermoelectric applications. Here, we report on charge density and temperature-dependent field-effect mobility and on-chip field-effect-modulated Seebeck coefficient measurements of polymer-sorted monochiral small-diameter (6,5) (0.76 nm) and mixed large-diameter SWCNT (1.17-1.55 nm) networks (plasma torch nanotubes, RN) with different network densities and length distributions. All untreated networks display balanced ambipolar transport and electron-hole symmetric Seebeck coefficients. We show that charge and thermoelectric transport in SWCNT networks can be modeled by the Boltzmann transport formalism, incorporating transport in heterogeneous media and fluctuation-induced tunneling. Considering the diameter-dependent one-dimensional density of states (DoS) of the SWCNTs composing the network, we can simulate the charge density and temperature-dependent Seebeck coefficients. Our simulations suggest that scattering in these networks cannot be described as simple one-dimensional acoustic and optical phonon scattering as for single SWCNTs. Instead the relaxation time is inversely proportional to energy (τ ∝ (E - EC)s, s = -1, EC being the energy of the first van Hove singularity), presumably pointing toward the more two-dimensional character of scattering events and the necessity to include scattering at the SWCNT junctions. Finally, our observation of higher power factors in trap-free, 1,2,4,5-tetrakis(tetramethylguanidino)benzene-treated (6,5) networks than in the RN networks emphasizes the importance of chirality selection to tune the width of the DoS. To benefit from both higher intrinsic mobilities and a large thermally accessible DoS, we propose trap-free, narrow DoS distribution, large-diameter SWCNT networks for both electronic and thermoelectric applications.

Charge Transport in Mixed Semiconducting Carbon Nanotube Networks with Tailored Mixing Ratios.

The ability to prepare uniform and dense networks of purely semiconducting single-walled carbon nanotubes (SWNTs) has enabled the design of various (opto-)electronic devices, especially field-effect transistors (FETs) with high carrier mobilities. Further optimization of these SWNT networks is desired to surpass established solution-processable semiconductors. The average diameter and diameter distribution of nanotubes in a dense network were found to influence the overall charge carrier mobility; e.g., networks with a broad range of SWNT diameters show inferior transport properties. Here, we investigate charge transport in FETs with nanotube networks comprising polymer-sorted small diameter (6,5) SWNTs (0.76 nm) and large diameter plasma torch SWNTs (1.17-1.55 nm) in defined mixing ratios. All transistors show balanced ambipolar transport with high on/off current ratios and negligible hysteresis. While the range of bandgaps in these networks creates a highly uneven energy landscape for charge carrier hopping, the extracted hole and electron mobilities vary nonlinearly with the network composition from the lowest mobility (15 cm2 V-1 s-1) for only (6,5) SWNT to the highest mobility (30 cm2 V-1 s-1) for only plasma torch SWNTs. A comparison to numerically simulated network mobilities shows that a superposition of thermally activated hopping across SWNT-SWNT junctions and diameter-dependent intratube transport is required to reproduce the experimental data. These results also emphasize the need for monochiral large diameter nanotubes for maximum carrier mobilities in random networks.

Efficient n-Doping and Hole Blocking in Single-Walled Carbon Nanotube Transistors with 1,2,4,5-Tetrakis(tetramethylguanidino)ben-zene.

Efficient, stable, and solution-based n-doping of semiconducting single-walled carbon nanotubes (SWCNTs) is highly desired for complementary circuits but remains a significant challenge. Here, we present 1,2,4,5-tetrakis(tetramethylguanidino)benzene (ttmgb) as a strong two-electron donor that enables the fabrication of purely n-type SWCNT field-effect transistors (FETs). We apply ttmgb to networks of monochiral, semiconducting (6,5) SWCNTs that show intrinsic ambipolar behavior in bottom-contact/top-gate FETs and obtain unipolar n-type transport with 3-5-fold enhancement of electron mobilities (approximately 10 cm2 V-1 s-1), while completely suppressing hole currents, even at high drain voltages. These n-type FETs show excellent on/off current ratios of up to 108, steep subthreshold swings (80-100 mV/dec), and almost no hysteresis. Their excellent device characteristics stem from the reduction of the work function of the gold electrodes via contact doping, blocking of hole injection by ttmgb2+ on the electrode surface, and removal of residual water from the SWCNT network by ttmgb protonation. The ttmgb-treated SWCNT FETs also display excellent environmental stability under bias stress in ambient conditions. Complementary inverters based on n- and p-doped SWCNT FETs exhibit rail-to-rail operation with high gain and low power dissipation. The simple and stable ttmgb molecule thus serves as an example for the larger class of guanidino-functionalized aromatic compounds as promising electron donors for high-performance thin film electronics.

Highly efficient gold-catalyzed synthesis of dibenzocycloheptatrienes.

Dibenzocycloheptatrienes are obtained by a gold-catalyzed 7-exo-dig hydroarylation protocol in a highly efficient manner. The gold-catalyzed reaction usually gives the products in high yields and excellent selectivity. This procedure provides an easy and efficient access to dibenzocycloheptanoids, which are an interesting and unique class of natural products. This was underlined by the first total synthesis of reticuol.