Carla M. Aguirre

Carbon nanotube sheets as electrodes in organic light-emitting diodes.

High performance organic light-emitting diodes (OLEDs) were implemented on transparent and conductive single-wall carbon nanotube sheets. At the maximum achieved brightness of 2800cdm−2 the luminance efficiency of our carbon nanotube-based OLED is 1.4cdA−1 which is comparable to the 1.9cdA−1 measured for an optimized indium tin oxide anode device made under the same experimental conditions. A thin parylene buffer layer between the carbon nanotube anode and the hole transport layer is required in order to readily achieve the measured performance.

Probing Charge Transfer at Surfaces Using Graphene Transistors

Graphene field effect transistors (FETs) are extremely sensitive to gas exposure. Charge transfer doping of graphene FETs by atmospheric gas is ubiquitous but not yet understood. We have used graphene FETs to probe minute changes in electrochemical potential during high-purity gas exposure experiments. Our study shows quantitatively that electrochemistry involving adsorbed water, graphene, and the substrate is responsible for doping. We not only identify the water/oxygen redox couple as the underlying mechanism but also capture the kinetics of this reaction. The graphene FET is highlighted here as an extremely sensitive potentiometer for probing electrochemical reactions at interfaces, arising from the unique density of states of graphene. This work establishes a fundamental basis on which new electrochemical nanoprobes and gas sensors can be developed with graphene.

Graphene field effect transistors with parylene gate dielectric

We report the fabrication and characterization of graphene field effect transistors with parylene back gate and exposed graphene top surface. A back gate stack of 168 nm parylene on 94 nm thermal silicon oxide permitted optical reflection microscopy to be used for identifying exfoliated graphene flakes. Room temperature mobilities of 10 000 cm2/Vs at 1012/cm2 electron/hole densities were observed in electrically contacted graphene. Parylene gated devices exhibited stable neutrality point gate voltage under ambient conditions and less hysteresis than that observed in graphene flakes directly exfoliated on silicon oxide.

Carbon Nanotubes as Injection Electrodes for Organic Thin Film Transistors

We have investigated the charge injection efficiency of carbon nanotube electrodes for organic semiconducting layers and compared their performance to that of traditional noble metal electrodes. Our results reveal that charge injection from a single carbon nanotube electrode is more than an order of magnitude more efficient than charge injection from metal electrodes. Moreover, organic thin film transistors that use arrays of carbon nanotube electrodes display considerable effective mobilities (0.14 cm(2)/(V.s)) and nearly ideal linear output characteristics. These results indicate that carbon nanotubes should be considered a viable alternative to metal electrodes for next-generation organic field-effect transistors.

Electroluminescence from single-wall carbon nanotube network transistors.

The electroluminescence (EL) properties from single-wall carbon nanotube network field-effect transistors (NNFETs) and small bundle carbon nanotube field effect transistors (CNFETs) are studied using spectroscopy and imaging in the near-infrared (NIR). At room temperature, NNFETs produce broad (approximately 180 meV) and structured NIR spectra, while they are narrower (approximately 80 meV) for CNFETs. EL emission from NNFETs is located in the vicinity of the minority carrier injecting contact (drain) and the spectrum of the emission is red shifted with respect to the corresponding absorption spectrum. A phenomenological model based on a Fermi-Dirac distribution of carriers in the nanotube network reproduces the spectral features observed. This work supports bipolar (electron-hole) current recombination as the main mechanism of emission and highlights the drastic influence of carrier distribution on the optoelectronic properties of carbon nanotube films.

Making Contacts to n-Type Organic Transistors Using Carbon Nanotube Arrays

We investigated the performance of carbon nanotube (CNT) array electrodes applied to n-type and ambipolar phenyl-C61-butyric acid methyl ester (PCBM) thin film transistors on a SiO(2) dielectric substrate. Compared to conventional Au electrodes, CNT arrays provide better injection efficiency, improved switching behavior, higher electron mobility, and lower contact resistance. Experiments on ambipolar PCBM transistors indicate that the injection performance is enhanced by the electrostatics of the CNT contacts, which promotes electron and hole tunneling across Schottky barriers at the PCBM/nanotube interface. The use of CNT arrays is a valid replacement to low workfunction metals, which are often reactive in air and difficult to process. Our work paves the way for a widespread use of carbon nanotube array electrodes in high-performance n-type and p-type organic thin film transistors.

Influence of statistical distributions on the electrical properties of disordered and aligned carbon nanotube networks

We have studied the influence of different statistical distributions of various parameters describing the structural and physical properties of carbon nanotube (CNT) networks on their electrical properties. We observed a significant dependence on the distribution of structural parameters (length, diameter, and angle), but nearly no impact for a statistical distribution of the tube-tube resistance beyond percolation threshold. The variation of the conductance of the CNT network as a function of the CNT density observed experimentally can be accurately reproduced through simulations by considering a tube-tube resistance centered at 0.1–1.0 MΩ, in agreement with the experimental estimation. Since the number of tube-tube contacts is increasing rapidly with the size of the simulated network, a statistical representation of tube-tube resistances for large CNT network models does not introduce a significant variation.