Joshua P Small

IBM, Armonk, New York, United States

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Publications (10)84.83 Total impact

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    ABSTRACT: We demonstrate a light emitting p-i-n diode made of a highly aligned film of separated (99%) semiconducting carbon nanotubes, self-assembled from solution. By using a split gate technique, we create p- and n-doped regions in the nanotube film that are separated by a micron-wide gap. We inject p- and n-type charge carriers into the device channel from opposite contacts and investigate the radiative recombination using optical micro-spectroscopy. We find that the threshold-less light generation efficiency in the intrinsic carbon nanotube film segment can be enhanced by increasing the potential drop across the junction, demonstrating the LED-principle in a carbon nanotube film for the first time. The device emits infrared light that is polarized along the long axes of the carbon nanotubes that form the aligned film.
    Optics Express 12/2010; 18(25):25738-45. · 3.55 Impact Factor
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    ABSTRACT: The photoluminescence of a partially suspended, semiconducting carbon nanotube that forms the active channel of a field-effect transistor is quenched and red-shifted upon application of a longitudinal electrical (source-drain) field. The quenching can be explained by a loss of oscillator strength and an increased Auger-like nonradiative decay of the E(11) exciton. The spectral shifts are due to drain-field-induced doping that leads to enhanced dielectric screening. Electroluminescence due to electron impact excitation of E(11) excitons is red-shifted and broadened with respect to the zero-field photoluminescence. A combination of screening and heating of the carbon nanotube can explain both spectral shift and broadening of the electrically induced light emission.
    ACS Nano 11/2009; 3(11):3744-8. · 12.03 Impact Factor
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    ABSTRACT: We investigate the gate field dependence of light absorption and emission of an individual, suspended semiconducting carbon nanotube using Raman and photoluminescence spectroscopies. We find a strong reduction in the absorption strength and a red shift of the E(33) state of the nanotube with increasing gate field. The photoluminescence from the E(11) state is quenched even stronger. We explain these observations in terms of field-doping and its effects on both the radiative and nonradiative decay rates of the excitons. Thus, gate field-induced doping constitutes an effective means of controlling the optical properties of carbon nanotube devices.
    Nano Letters 08/2009; 9(10):3477-81. · 13.03 Impact Factor
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    ABSTRACT: We present a comprehensive picture of the phonon populations in an electrically-driven carbon nanotube transistor, including the Raman-active G and radial breathing modes (RBM), and also the Raman-inactive zone boundary mode (K), and intermediate-frequency mode (IFP), populated by anharmonic decay. The effective temperature of the RBM is considerably lower than the intermediate- and high-frequency mode temperatures, which we explain by a phonon-decay bottleneck. We include substrate polar phonon scattering to fully account for the device electronic characteristics.
    03/2009;
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    ABSTRACT: Carbon nanotubes and graphene are candidate materials for nanoscale electronic devices1, 2. Both materials show weak acoustic phonon scattering and long mean free paths for low-energy charge carriers. However, high-energy carriers couple strongly to optical phonons1, 3, which leads to current saturation4, 5, 6 and the generation of hot phonons7. A non-equilibrium phonon distribution has been invoked to explain the negative differential conductance observed in suspended metallic nanotubes8, while Raman studies have shown the electrical generation of hot G-phonons in metallic nanotubes9, 10. Here, we present a complete picture of the phonon distribution in a functioning nanotube transistor including the G and the radial breathing modes, the Raman-inactive zone boundary K mode and the intermediate-frequency mode populated by anharmonic decay. The effective temperatures of the high- and intermediate-frequency phonons are considerably higher than those of acoustic phonons, indicating a phonon-decay bottleneck. Most importantly, inclusion of scattering by substrate polar phonons is needed to fully account for the observed electronic transport behaviour.
    Nature Nanotechnology 02/2009; 4(5):320-324. · 31.17 Impact Factor
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    ABSTRACT: Top-gated graphene transistors operating at high frequencies (GHz) have been fabricated and their characteristics analyzed. The measured intrinsic current gain shows an ideal 1/f frequency dependence, indicating an FET-like behavior for graphene transistors. The cutoff frequency fT is found to be proportional to the dc transconductance gm of the device. The peak fT increases with a reduced gate length, and fT as high as 26 GHz is measured for a graphene transistor with a gate length of 150 nm. The work represents a significant step towards the realization of graphene-based electronics for high-frequency applications.
    01/2009;
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    ABSTRACT: Top-gated graphene transistors operating at high frequencies (gigahertz) have been fabricated and their characteristics analyzed. The measured intrinsic current gain shows an ideal 1/f frequency dependence, indicating a FET-like behavior for graphene transistors. The cutoff frequency f(T) is found to be proportional to the dc transconductance g(m) of the device, consistent with the relation f(T) = g(m)/(2piC(G)). The peak f(T) increases with a reduced gate length, and f(T) as high as 26 GHz is measured for a graphene transistor with a gate length of 150 nm. The work represents a significant step toward the realization of graphene-based electronics for high-frequency applications.
    Nano Letters 01/2009; 9(1):422-6. · 13.03 Impact Factor
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    ABSTRACT: Thin film transistors (TFTs) are now poised to revolutionize the display, sensor, and flexible electronics markets. However, there is a limited choice of channel materials compatible with low-temperature processing. This has inhibited the fabrication of high electrical performance TFTs. Single-walled carbon nanotubes (CNTs) have very high mobilities and can be solution-processed, making thin film CNT-based TFTs a natural direction for exploration. The two main challenges facing CNT-TFTs are the difficulty of placing and aligning CNTs over large areas and low on/off current ratios due to admixture of metallic nanotubes. Here, we report the self-assembly and self-alignment of CNTs from solution into micron-wide strips that form regular arrays of dense and highly aligned CNT films covering the entire chip, which is ideally suitable for device fabrication. The films are formed from pre-separated, 99% purely semiconducting CNTs and, as a result, the CNT-TFTs exhibit simultaneously high drive currents and large on/off current ratios. Moreover, they deliver strong photocurrents and are also both photo- and electroluminescent.
    ACS Nano 01/2009; 2(12):2445-52. · 12.03 Impact Factor
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    ABSTRACT: Carbon nanotube field effect transistors possess superb device characteristics for electronic applications. However, the non-selective nature of nanotube synthesis, difficulty in accurate nanotube placement, and the high device impedance of single tube devices pose major challenges in the integration of carbon nanotubes in large-scale electronic devices. Here we present a novel approach to address these issues. Carbon nanotubes used in this study have been purified and separated by their electronic structure, where the semiconducting tube percentage is as high as 99%, confirmed by both transport measurements on individual nanotubes and by optical absorption spectra. Through a simple self-assembly technique, we have produced aligned nanotube arrays. Thin film transistors based on these aligned nanotube arrays are fabricated with both back- and top-gate layouts, showing good switching performance and a high drive current. It is found that top-gated and back-gated devices exhibit distinct switching behaviors due to screening effects. Results on device channel length dependence will also be presented.
    03/2008;
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    ABSTRACT: In this paper, we present a new approach for making active carbon nanotube (CNT) electrical devices and demonstrate the first aligned CNT array field effect transistors (FET) from 99% pure separated semiconducting nanotubes. Through evaporation-driven deposition of predominantly semiconducting nanotubes from the liquid phase, we have fabricated aligned, thin-film CNT devices with high on-state currents. The fabrication scheme presented here provides a versatile production method translatable to other substrates such as flexible plastics.
    01/2008;