23 research outputs found
Hot Phonons in an Electrically Biased Graphene Constriction
Phonon carrier interactions can have significant impact on device
performance. They can be probed by measuring the phonon lifetime, which
reflects the interaction strength of a phonon with other quasi-particles in
particular charge carriers as well as its companion phonons. The carrier phonon
and phonon-phonon contributions to the phonon lifetime can be disentangled from
temperature dependent studies. Here, we address the importance of phonon
carrier interactions in Joule-heated graphene constrictions in order to
contribute to the understanding of energy dissipation in graphene based
electronic devices. We demonstrate that gapless graphene grants electron phonon
interactions uncommon significance in particular at low carrier density. In
conventional semiconductors, the bandgap usually prevents the decay of phonons
through electron-hole generation and also in metals or other semimetals the
Fermi temperature is excessively large to enter the regime where electron
phonon coupling plays such a dominant role as in graphene in the investigated
phonon temperature regime from 300 to 1600 K.Comment: Nano Letters (Web publication on 30th Dec. 2009,
DOI:10.1021/nl903167f
Vibrational and electronic heating in nanoscale junctions
Understanding and controlling the flow of heat is a major challenge in
nanoelectronics. When a junction is driven out of equilibrium by light or the
flow of electric charge, the vibrational and electronic degrees of freedom are,
in general, no longer described by a single temperature[1-6]. Moreover,
characterizing the steady-state vibrational and electronic distributions {\it
in situ} is extremely challenging. Here we show that surface-enhanced Raman
emission may be used to determine the effective temperatures for both the
vibrational modes and the flowing electrons in a biased metallic nanoscale
junction decorated with molecules[7]. Molecular vibrations show mode-specific
pumping by both optical excitation[8] and dc current[9], with effective
temperatures exceeding several hundred Kelvin. AntiStokes electronic Raman
emission\cite[10,11] indicates electronic effective temperature also increases
to as much as three times its no-current values at bias voltages of a few
hundred mV. While the precise effective temperatures are model-dependent, the
trends as a function of bias conditions are robust, and allow direct
comparisons with theories of nanoscale heating.Comment: 28 pages, including 4 main figures and 10 supplemental figure
Thermal infrared emission reveals the Dirac point movement in biased graphene
Graphene is a 2-dimensional material with high carrier mobility and thermal
conductivity, suitable for high-speed electronics. Conduction and valence bands
touch at the Dirac point. The absorptivity of single-layer graphene is 2.3%,
nearly independent of wavelength. Here we investigate the thermal radiation
from biased graphene transistors. We find that the emission spectrum of
single-layer graphene follows that of a grey body with constant emissivity (1.6
\pm 0.8)%. Most importantly, we can extract the temperature distribution in the
ambipolar graphene channel, as confirmed by Stokes/anti-Stokes measurements.
The biased graphene exhibits a temperature maximum whose location can be
controlled by the gate voltage. We show that this peak in temperature reveals
the spatial location of the minimum in carrier density, i.e. the Dirac point.Comment: Accepted in principle at Nature Nanotechnolog
Gate Voltage Controllable Non-Equilibrium and Non-Ohmic Behavior in Suspended Carbon Nanotubes
Imaging electronic structure of carbon nanotubes by voltage-contrast scanning electron microscopy
On the Electron−Phonon Coupling of Individual Single-Walled Carbon Nanotubes
We show that the phonon coupling to the electronic system in individual metallic single-walled carbon nanotubes is not due to coupling to low-energy plasmons. The evidence stems from the measured Raman-Stokes G-mode, which for metallic and semiconducting tubes could be fitted well by the superposition of only two Lorentzian lines associated with vibrational modes along the nanotube axis and the nanotube circumference. In the case of metallic tubes the lower-energy G mode is significantly broadened, however maintaining the Lorentzian line shape, in contrast to the theoretically expected asymmetric Breit−Wigner−Fano line shape from phonon-plasmon coupling. The results were obtained by studying 25 individual metallic and semiconducting single-walled carbon nanotubes with atomic force microscopy, electron transport measurements, and resonant Raman spectroscopy
Toward single-chirality carbon nanotube device arrays
The large-scale integration of devices consisting of individual single-walled carbon nanotubes (SWCNT), all of the same chirality, is a critical step toward their electronic, optoelectronic, and electromechanical application. Here, the authors realize two related goals, the first of which is the fabrication of high-density, single-chirality SWCNT device arrays by dielectrophoretic assembly from monodisperse SWCNT solution obtained by polymer-mediated sorting. Such arrays are ideal for correlating measurements using various techniques across multiple identical devices, which is the second goal. The arrays are characterized by voltage-contrast scanning electron microscopy, electron transport, photoluminescence (PL), and Raman spectroscopy and show identical signatures as expected for single-chirality SWCNTs. In the assembled nanotubes, a large D peak in Raman spectra, a large dark-exciton peak in PL spectra as well as lowered conductance and slow switching in electron transport are all shown to be correlated to each other. By comparison to control samples, we conclude that these are the result of scattering from electronic and not structural defects resulting from the polymer wrapping, similar to what has been predicted for DNA wrapping