Biomedical subjects

Phaedon Avouris

Publications and source records attributed to Phaedon Avouris.

17 recordsLinked to original sources

PubMed2006

An integrated logic circuit assembled on a single carbon nanotube.

Single-walled carbon nanotubes (SWCNTs) have been shown to exhibit excellent electrical properties, such as ballistic transport over several hundred nanometers at room temperature. Field-effect transistors (FETs) made from individual tubes show dc performance specifications rivaling those of state-of-the-art silicon devices. An important next step is the fabrication of integrated circuits on SWCNTs to study the high-frequency ac capabilities of SWCNTs. We built a five-stage ring oscillator that comprises, in total, 12 FETs side by side along the length of an individual carbon nanotube. A complementary metal-oxide semiconductor-type architecture was achieved by adjusting the gate work functions of the individual p-type and n-type FETs used.

Journal Article↗

PubMed2006

Mobility in semiconducting carbon nanotubes at finite carrier density.

Carbon nanotube field-effect transistors operate over a wide range of electron or hole density, controlled by the gate voltage. Here we calculate the mobility in semiconducting nanotubes as a function of carrier density and electric field, for different tube diameters and temperatures. The low-field mobility is a nonmonotonic function of carrier density and varies by as much as a factor of 4 at room temperature. At low density, with increasing field the drift velocity reaches a maximum and then exhibits negative differential mobility, due to the non-parabolicity of the band structure. At a critical density, rho(c) approximately 0.35-0.5 electrons/nm, the drift velocity saturates at around one-third of the Fermi velocity. Above rho(c), the velocity increases with field strength with no apparent saturation.

Electromagnetic Fields↗

PubMed2006

Field-effect transistors assembled from functionalized carbon nanotubes.

We have fabricated field-effect transistors from carbon nanotubes using a novel selective placement scheme. We use carbon nanotubes that are covalently bound to molecules containing a hydroxamic acid functionality. The functionalized nanotubes bind strongly to basic metal oxide surfaces, but not to silicon dioxide. Upon annealing, the functionalization is removed, restoring the electronic properties of the nanotubes. The devices thus fabricated show excellent electrical characteristics.

Electrochemistry↗

PubMed2006

Low-frequency current fluctuations in individual semiconducting single-wall carbon nanotubes.

We present a systematic study on low-frequency current fluctuations of nanodevices consisting of one single semiconducting nanotube, which exhibit significant 1/f-type noise. By examining devices with different switching mechanisms, carrier types (electrons vs holes), and channel lengths, we show that the 1/f fluctuation level in semiconducting nanotubes is correlated to the total number of transport carriers present in the system. However, the 1/f noise level per carrier is not larger than that of most bulk conventional semiconductors, e.g., Si. The pronounced noise level observed in nanotube devices simply reflects on the small number of carriers involved in transport. These results not only provide the basis to quantify the noise behavior in a one-dimensional transport system but also suggest a valuable way to characterize low-dimensional nanostructures based on the 1/f fluctuation phenomenon.

Electrochemistry↗

PubMed2006

Electrically excited, localized infrared emission from single carbon nanotubes.

Carbon nanotube field-effect transistors (CNTFETs) produce band gap derived infrared emission under both ambipolar and unipolar transport conditions. We demonstrate here that heterogeneities/defects in the local environment of a CNTFET perturb the local potentials and, as a result, the characteristic bias dependent motion of the ambipolar light emission. Such defects can also introduce localized infrared emission due to impact excitation by carriers accelerated by a voltage drop at the defect. The correlation of the change in the motion of the ambipolarlight emission and of the stationary electroluminescence with the electrical characteristics of the CNTFETs shows that stationaryelectroluminescence can identify "environmental defects" in carbon nanotubes and help evaluate their influence on electrical transport and device operation. A number of different defects are studied involving local dielectric environment changes (partially polymer-covered nanotubes), nanotube-nanotube contacts in looped nanotubes, and nanotube segments close to the electronic contacts. Random defects due to local charging are also observed.

Electrochemistry↗

PubMed2005

Bright infrared emission from electrically induced excitons in carbon nanotubes.

We used the high local electric fields at the junction between the suspended and supported parts of a single carbon nanotube molecule to produce unusually bright infrared emission under unipolar operation. Carriers were accelerated by band-bending at the suspension interface, and they created excitons that radiatively recombined. This excitation mechanism is approximately 1000 times more efficient than recombination of independently injected electrons and holes, and it results from weak electron-phonon scattering and strong electron-hole binding caused by one-dimensional confinement. The ensuing high excitation density allows us to observe emission from higher excited states not seen by photoexcitation. The excitation mechanism of these states was analyzed.

Journal Article↗

PubMed2005

Tungsten oxide nanowire growth by chemically induced strain.

We have investigated the formation of tungsten oxide nanowires under different chemical vapor deposition (CVD) conditions. We find that exposure of oxidized tungsten films to hydrogen and methane at 900 degrees C leads to the formation of a dense array of typically 10 nm diameter nanowires. Structural and chemical analysis shows that the wires are crystalline WO3. We propose a chemically driven whisker growth mechanism in which interfacial strain associated with the formation of tungsten carbide stimulates nanowire growth. This might be a general concept, applicable also to other nanowire systems.

Journal Article↗

PubMed2005

Electron-phonon interaction and transport in semiconducting carbon nanotubes.

We calculate the electron-phonon scattering and binding in semiconducting carbon nanotubes, within a tight-binding model. The mobility is derived using a multiband Boltzmann treatment. At high fields, the dominant scattering is interband scattering by LO phonons corresponding to the corners K of the graphene Brillouin zone. The drift velocity saturates at approximately half the graphene Fermi velocity. The calculated mobility as a function of temperature, electric field, and nanotube chirality are well reproduced by a simple interpolation formula. Polaronic binding give a band-gap renormalization of approximately 70 meV, an order of magnitude larger than expected. Coherence lengths can be quite long but are strongly energy dependent.

Journal Article↗

PubMed2005

Effect of exciton-phonon coupling in the calculated optical absorption of carbon nanotubes.

We find that the optical properties of carbon nanotubes reflect remarkably strong effects of exciton-phonon coupling. Tight-binding calculations show that a significant fraction of the spectral weight of the absorption peak is transferred to a distinct exciton+phonon sideband, which is peaked at around 200 meV above the main absorption peak. This sideband provides a distinctive signature of the excitonic character of the optical transition. The exciton-phonon coupling is reflected in a dynamical structural distortion, which contributes a binding energy of up to 100 meV. The distortion is surprisingly long ranged, and is strongly dependent on chirality.

Journal Article↗

PubMed2005

Charge transfer induced polarity switching in carbon nanotube transistors.

We probed the charge transfer interaction between the amine-containing molecules hydrazine, polyaniline, and aminobutyl phosphonic acid and carbon nanotube field effect transistors (CNTFETs). We successfully converted p-type CNTFETs to n-type and drastically improved the device performance in both the ON- and OFF-transistor states, utilizing hydrazine as dopant. We effectively switched the transistor polarity between p- and n- type by accessing different oxidation states of polyaniline. We also demonstrated the flexibility of modulating the threshold voltage (Vth) of a CNTFET by engineering various charge-accepting and -donating groups in the same molecule.

Amines↗

PubMed2005

Photoconductivity spectra of single-carbon nanotubes: implications on the nature of their excited States.

We have measured the photoconductivity excitation spectra of individual semiconducting carbon nanotubes incorporated as the channel of field-effect transistors. In addition to the pronounced resonance that correlates with the second van Hove transition (E(22)) in semiconducting carbon nanotubes, a weaker sideband at about 200 meV higher energy is observed. Electronic structure calculations that include electron-phonon coupling indicate that the spectra originate from the simultaneous excitation of an exciton (main resonance) and a C-C bond stretching phonon (sideband). The spectral features are not compatible with an interband interpretation of the excitation involved.

Journal Article↗

PubMed2005

The role of metal-nanotube contact in the performance of carbon nanotube field-effect transistors.

Single-wall carbon nanotube field-effect transistors (CNFETs) have been shown to behave as Schottky barrier (SB) devices. It is not clear, however, what factors control the SB size. Here we present the first statistical analysis of this issue. We show that a large data set of more than 100 devices can be consistently accounted by a model that relates the on-current of a CNFET to a tunneling barrier whose height is determined by the nanotube diameter and the nature of the source/drain metal contacts. Our study permits identification of the desired combination of tube diameter and type of metal that provides the optimum performance of a CNFET.

Computer Simulation↗

PubMed2005

Radiative lifetime of excitons in carbon nanotubes.

We calculate the radiative lifetime and energy bandstructure of excitons in semiconducting carbon nanotubes within a tight-binding approach including the electron-hole correlations via the Bethe-Salpeter equation. In the limit of rapid interband thermalization, the radiative decay rate is maximized at intermediate temperatures and decreases at low temperature because the lowest-energy excitons are optically forbidden. The intrinsic phonons cannot scatter excitons between optically active and forbidden bands, so sample-dependent extrinsic effects that break the symmetries can play a central role. We calculate the diameter-dependent energy splittings between singlet and triplet excitons of different symmetries and the resulting dependence of radiative lifetime on temperature and tube diameter.

Computer Simulation↗

PubMed2004

Mobile ambipolar domain in carbon-nanotube infrared emitters.

We spatially resolve the infrared light emission from ambipolar carbon-nanotube field-effect transistors with long-channel lengths. Electrons and holes are injected from opposite contacts into a single nanotube molecule. The ambipolar domain, where electron and hole currents overlap, forms a microscopic light emitter within the carbon nanotube. We can control its location by varying gate and drain voltages. At high electric fields, additional stationary spots appear due to defect-assisted Zener tunneling or impact ionization. The laterally resolved measurement provides valuable insight into the transistor behavior, complementary to electronic device characteristics.

Journal Article↗

PubMed2004

Scaling of excitons in carbon nanotubes.

Light emission from carbon nanotubes is expected to be dominated by excitonic recombination. Here we calculate the properties of excitons in nanotubes embedded in a dielectric, for a wide range of tube radii and dielectric environments. We find that simple scaling relationships give a good description of the binding energy, exciton size, and oscillator strength.

Journal Article↗

PubMed2004

Molecular interactions in one-dimensional organic nanostructures.

Intermolecular interactions involving pi-pi interaction and hydrogen bonding are used to create one-dimensional molecular nanostructures of hexasubstituted aromatics. Site-selective steady state fluorescence, time-resolved fluorescence, scanning electron microscopy, and atomic force microscopy measurements detail the intermolecular interactions that drive the aromatic molecules to self-assemble in solution to form well-ordered columnar stacks. These nanostructures, formed in solution, vary in their number, size, and structure depending on the solvent used. In addition, our results indicate that the substituents/ side groups and the proper choice of the solvent can be used to tune the intermolecular interactions. The 1D stacks and their aggregates can be easily transferred by solution casting, thus allowing a simple preparation of molecular nanostructures on different surfaces.

Journal Article↗

PubMed2002

Molecular electronics with carbon nanotubes.

Carbon nanotubes have unique properties that make them a most promising system on which to base molecular electronics. We briefly review the electrical characteristics of carbon nanotubes, and then focus on carbon nanotube field-effect transistors (CNTFETs). Procedures by which hole-transport, electron-transport and ambipolar CNTFETs can be fabricated are presented, and their electrical characteristics are discussed and compared with those of Si MOSFETs. Ways to fabricate arrays of CNTFETs are also demonstrated, and electron and hole CNTFETs are integrated to form complementary logic circuits.

Journal Article↗