Miniature Organic Transistors with Carbon Nanotubes as Quasi-One-Dimensional Electrodes

Abstract

With a cut metallic SWNT (gap L ~ 5-6 nm) bridged by a pentacene nano-crystallite (Fig.1b&c), we observed clear semiconducting FET characteristics in the current vs. gate (I ds -V gs ) curve (Fig. 2a). The device exhibited a current modulation of I max /I min ~ 10 5 under gating at a fixed bias voltage of V ds = -0.5V. The drastic switching clearly differed from the original metallic SWNT device (lack of gate dependence, Fig. 2a inset). This corresponds to the formation of a pentacene FET with channel length L ~ 5-6 nm and width of w ~ 2 nm (i.e., the diameter of the SWNT) as charge transport via hopping between pentacene molecules should be mainly confined in a width on the order of the tube diameter. Notably, the subthreshold swing of the device is S ~ 400 mV/decade (Fig. 2a).Small organic molecules and conjugated polymers can be easily processed to afford functional electronics such as field effect transistors (FETs), 1a and in principle, scaling 1b to singlemolecule long devices could circumvent the low carrier mobility problem for these materials to afford high performance ballistic FETs 2,3 . For highly scaled molecular transistors with short channels however, it is crucial to develop novel device geometries to optimize gate electrostatics needed for ON/OFF switching. 4,5 It is shown here that single-walled carbon nanotubes (SWNT) can be used as quasi one-dimensional (1D) electrodes to construct organic FETs with molecular scale width (~2 nm) and channel length (down to 1-3 nm). The favorable gate electrostatics associated with the sharp 1D electrode geometry allows for room temperature conductance modulation by orders of magnitude for organic transistors that are only several-molecules in length, with switching characteristics superior to devices with lithographically patterned metal electrodes. We suggest that carbon nanotubes may prove to be novel electrodes for a variety of molecular devices.We first developed a reproducible method of cutting metallic SWNTs to form small gaps within the tubes and with control over the gap size down to L~2 nm. The cutting relied on electrical break-down 6 of individual SWNTs between two metal electrodes (Fig. 1a), and the size of the cut was found to be controllable by varying the lengths of the SWNTs (see Ref.6b and Supp. Info). Organic materials were then deposited to bridge the gap in the vapor (for pentacene) or solution phase (for regio-regular ploy (3-hexylthiophene), P3HT), forming the smallest organic FETs with effective channel length down to L~1-3 nm and width ~2 nm. b cWe varied the channel lengths L of SWNT-contacted pentacene FETs (L ~ 1-3 nm, L ~5-6 nm and L ~10-15 nm respectively) and observed length dependent transport properties at various temperatures. At T=300K, the devices exhibited on-current I max scaling approximately with ~1/L (under V ds =-1 V). This suggests

Electrical and thermal transport in metallic single-wall carbon nanotubes on insulating substrates

“…The data (squares) was obtained in the course of this work and from Refs. [14,23]. The dashed trend line is a least-squares fit using Eq.…”

“…[14]. In separate experiments we have noted that breakdown voltages in an argon ambient were nearly twice as large [23]. Assuming an in-air breakdown temperature T BD = 600 o C = 873 K as previously mentioned, we extract a net thermal conductance to substrate g ≈ 0.17 WK -1 m -1 per tube length by fitting Eq.…”

Nanogap Electrodes

“…By using the electrical breakdown method, organic field-effect transistors (OFETs) with an effective channel length down to 1-3 nm were fabricated by Dai et al, [104] and significantly improved switching characteristics were observed with singlewalled carbon nanotube (SWNT) electrodes over conventional metal electrodes. Tsukagoshi et al [105,106] nicely prepared organic nanotransistors using pentacene nanocrystals and MWNT nanogap electrodes, thus circumventing the problem of film discontinuity between gold electrodes and the SiO 2 substrate in organic thin film transistors.…”

Parallel arrays of individually addressable single-walled carbon nanotube field-effect transistors

“…[5][6][7] The feasibility of single-walled carbon nanotube-͑SWNT͒ based electronic devices, such as interconnects 8,9 in molecular electronics, junction rectifiers, [9][10][11] field-effect transistors ͑FETs͒, 12,13 and logic gates, 14 has been demonstrated in recent experiments. More recently, experiments demonstrating the use of SWNTs as the active channel in metal-oxide-semiconductor ͑MOS͒ FETs, which opens the possibility for a wide range of integrated CNT-complementary ͑C͒ MOS nanoelectronics, have also been reported.…”