Sangameshwar Rao Saudari

Thiocyanate-capped PbS nanocubes: ambipolar transport enables quantum dot based circuits on a flexible substrate.

We report the use of thiocyanate as a ligand for lead sulfide (PbS) nanocubes for high-performance, thin-film electronics. PbS nanocubes, self-assembled into thin films and capped with the thiocyanate, exhibit ambipolar characteristics in field-effect transistors. The nearly balanced, high mobilities for electrons and holes enable the fabrication of CMOS-like inverters with promising gains of ∼22 from a single semiconductor material. The mild chemical treatment and low-temperature processing conditions are compatible with plastic substrates, allowing the realization of flexible, nonsintered quantum dot circuits.

Device Configurations for Ambipolar Transport in Flexible, Pentacene Transistors

www.MaterialsViews.com C O M M Device Confi gurations for Ambipolar Transport in Flexible, Pentacene Transistors U N IC By Sangameshwar Rao Saudari, Yu Jen Lin, Yuming Lai, and Cherie R. Kagan* A IO N Organic semiconductors are exciting materials for low-cost, fl exible electronic and optoelectronic applications, [ 1 ] but the device fabrication and confi guration may greatly impact the measured characteristics, masking the intrinsic organic semiconductor’s materials properties. One of the best examples is the typical observation of unipolar transport in organic fi eld-effect transistors (OFETs) and therefore the classifi cation of organic pand n-type materials. It is only recently that theoretical [ 2 ] and experimental [ 3–5 ] reports have shown that the intrinsic properties of organic semiconductors can support both electron and hole transport. Theoretical calculations have predicted that organic semiconductors such as pentacene and tetracene can be as good electron conductors as hole conductors. The experimental realization of ambipolar transport has been attributed to overcoming the extrinsic factors of high injection barriers for one of the charge carrier types (electrons/holes) at the metal-semiconductor interface, carrier trapping at the dielectric-semiconductor interface, and trap generation upon exposure to air. Engineering the interfaces between organic semiconductors and the device electrodes and dielectric has opened up opportunities to fabricate ambipolar devices with balanced electron and hole mobilities and near zero threshold voltage for promising applications of high gain complementary metal-oxide-semiconductor (CMOS)-like inverters [ 3 ] and multifunctional devices such a high effi ciency light emitting [ 6 ] and photosensing FETs. [ 7 , 8 ]

Ambipolar transport in solution-deposited pentacene transistors enhanced by molecular engineering of device contacts

We report ambipolar transport in bottom gold contact, pentacene field-effect transistors (FETs) fabricated by spin-coating and thermally converting its precursor on a benzocyclobutene/SiO2 gate dielectric with chemically modified source and drain electrodes. A wide range of aliphatic and aromatic self-assembled thiolate monolayers were used to derivatize the electrodes and all enhanced electron and hole currents, yet did not affect the observable thin film morphology. Hole and electron mobilities of 0.1–0.5 and 0.05–0.1 cm2/V s are achieved, though the threshold for electron transport was >80 V. These ambipolar FETs are used to demonstrate inverters with gains of up to 94.

Flexible organic electronics for use in neural sensing

Recent research in brain-machine interfaces and devices to treat neurological disease indicate that important network activity exists at temporal and spatial scales beyond the resolution of existing implantable devices. High density, active electrode arrays hold great promise in enabling high-resolution interface with the brain to access and influence this network activity. Integrating flexible electronic devices directly at the neural interface can enable thousands of multiplexed electrodes to be connected using many fewer wires. Active electrode arrays have been demonstrated using flexible, inorganic silicon transistors. However, these approaches may be limited in their ability to be cost-effectively scaled to large array sizes (8×8 cm). Here we show amplifiers built using flexible organic transistors with sufficient performance for neural signal recording. We also demonstrate a pathway for a fully integrated, amplified and multiplexed electrode array built from these devices.

Electron and hole transport in ambipolar, thin film pentacene transistors

Solution-processed, ambipolar, thin-film pentacene field-effect transistors were employed to study both electron and hole transport simultaneously in a single, organic solid-state device. Electron and hole mobilities were extracted from the respective unipolar saturation regimes and show thermally activated behavior and gate voltage dependence. We fit the gate voltage dependent saturation mobility to a power law to extract the characteristic Meyer-Neldel (MN) energy, a measure of the width of the exponential distribution of localized states extending into the energy gap of the organic semiconductor. The MN energy is ∼78 and ∼28 meV for electrons and holes, respectively, which reflects a greater density of localized tail states for electrons than holes. This is consistent with the lower measured electron than hole mobility. For holes, the well-behaved linear regime allows for four-point probe measurement of the contact resistance independent mobility and separate characterization of the width of the localized density of states, yielding a consistent MN energy of 28 meV.