arXiv subjects

François Léonard

Publications and source records attributed to François Léonard.

At least 37 records · Page 2Linked to original sources

arXiv2014

Electrical Contacts to Three-Dimensional Arrays of Carbon Nanotubes

We use numerical simulations to investigate the properties of metal contacts to three-dimensional arrays of carbon nanotubes (CNTs). For undoped arrays top-contacted with high or low work function metals, electrostatic screening is very strong, resulting in a small Schottky barrier for current injection in the top layer and large Schottky barriers for current injection in the deeper layers. As a consequence, the majority of the current flows through the top layer of the array. Our simulations show that doping of the CNT array can alleviate this problem, even without direct contact to each tube in the array; however, we find that the charge transfer length is unusually long in arrays and increases with the number of CNT layers under the contact. We also show that a bottom gate can modulate the contact resistance, but only very weakly. These results are important for the design of electronic and optoelectronic devices based on CNT arrays, because they suggest that increasing the thickness of the array does little to improve the device performance unless the film is strongly doped at the contacts and the contact is long, or unless each tube in the array is directly contacted by the metal.

cond-mat.mes-hall↗

arXiv2013

Functionalization of Single-Wall Carbon Nanotubes with Chromophores of Opposite Internal Dipole Orientation

We report the functionalization of carbon nanotubes with two azobenzene-based chromophores with large internal dipole moments and opposite dipole orientations. The molecules are attached to the nanotubes non-covalently via a pyrene tether. A combination of characterization techniques shows uniform molecular coverage on the nanotubes, with minimal aggregation of excess chromophores on the substrate. The large on/off ratios and the sub-threshold swings of the nanotube-based field-effect transistors (FETs) are preserved after functionalization, and different shifts in threshold voltage are observed for each chromophore. Ab initio calculations verify the properties of the synthesized chromophores and indicate very small charge transfer, confirming a strong, non-covalent functionalization.

cond-mat.mes-hall↗

arXiv2013

Spectroscopic Properties of Nanotube-Chromophore Hybrids

Recently, individual single-walled carbon nanotubes (SWNTs) functionalized with azo-benzene chromophores were shown to form a new class of hybrid nanomaterials for optoelectronics applications. Here we use a number of experimental techniques and theory to understand the binding, orientation, and nature of coupling between chromophores and the nanotubes, all of which are of relevance to future optimization of these hybrid materials. We find that the binding energy between chromophores and nanotubes depends strongly on the type of tether that is used to bind the chromophores to the nanotubes, with pyrene tethers resulting in more than 90% of the bound chromophores during processing. DFT calculations show that the binding energy of the chromophores to the nanotubes is maximized for chromophores parallel to the nanotube sidewall, even with the use of tethers; second harmonic generation shows that there is nonetheless a partial radial orientation of the chromophores on the nanotubes. We find weak electronic coupling between the chromophores and the SWNTs, consistent with non-covalent binding. The chromophore-nanotube coupling, while weak, is sufficient to quench the chromophore fluorescence. Stern-Volmer plots are non-linear, which supports a combination of static and dynamic quenching processes. The chromophore orientation is an important variable for chromophore-nanotube phototransistors, and our experiments suggest the possibility for further optimizing this orientational degree of freedom.

cond-mat.mes-hall↗

arXiv2013

Many-body effects on the electronic and optical properties of strained semiconducting carbon nanotubes

We present many-body \textit{ab initio} calculations of the electronic and optical properties of semiconducting zigzag carbon nanotubes under uniaxial strain. The GW approach is utilized to obtain the quasiparticle bandgaps and is combined with the Bethe-Salpeter equation to obtain the optical absorption spectrum. We find that the dependence of the electronic bandgaps on strain is more complex than previously predicted based on tight-binding models or density-functional theory. In addition, we show that the exciton energy and exciton binding energy depend significantly on strain, with variations of tens of meVs per percent strain, but that despite these strong changes the absorbance is found to be nearly independent of strain. Our results provide new guidance for the understanding and design of optomechanical systems based on carbon nanotubes.

cond-mat.mtrl-sci↗

arXiv2013

Physical Removal of Metallic Carbon Nanotubes from Nanotube Network Devices Using a Thermal and Fluidic Process

Electronic and optoelectronic devices based on thin films of carbon nanotubes are currently limited by the presence of metallic nanotubes. Here we present a novel approach based on nanotube alkyl functionalization to physically remove the metallic nanotubes from such network devices. The process relies on preferential thermal desorption of the alkyls from the semiconducting nanotubes and the subsequent dissolution and selective removal of the metallic nanotubes in chloroform. The approach is versatile and is applied to devices post-fabrication.

cond-mat.mtrl-sci↗

arXiv2012

Thermoelectric efficiency in the space-charge-limited transport regime in semiconductors

The thermoelectric efficiency of semiconductors is usually considered in the ohmic electronic transport regime, which is achieved through high doping. Here we consider the opposite regime of low doping where the current-voltage characteristics are nonlinear and dominated by space-charge-limited transport. We show that in this regime, the thermoelectric efficiency can be described by a single figure of merit, in analogy with the ohmic case. Efficiencies for bulk, thin film, and nanowire materials are discussed, and it is proposed that nanowires are the most promising to take advantage of space-charge-limited transport for thermoelectrics.

cond-mat.mtrl-sci↗

arXiv2012

Enhanced Performance of Short-Channel Carbon Nanotube Field-Effect Transistors Due to Gate-Modulated Electrical Contacts

We use numerical simulations to analyze recent experimental measurements of short-channel carbon nanotube field-effect transistors with palladium contacts. We show that the gate strongly modulates the contact properties, an effect that is distinct from that observed in Schottky barrier carbon nanotube transistors. This modulation of the contacts by the gate allows for the realization of superior subthreshold swings for short channels, and improved scaling behavior. These results further elucidate the behavior of carbon nanotube-metal contacts, and should be useful in the optimization of high-performance carbon nanotube electronics.

cond-mat.mes-hall↗

arXiv2011

Efficient terahertz emission from InAs nanowires

We observe intense pulses of far-infrared electromagnetic radiation emitted from arrays of InAs nanowires. The THz radiation power efficiency of these structures is about 15 times higher compared to a planar InAs substrate. This is explained by the preferential orientation of coherent plasma motion to the wire surface, which overcomes radiation trapping by total-internal reflection. We present evidence that this radiation originates from a low-energy acoustic surface plasmon mode of the nanowire. This is supported by independent measurements of electronic transport on individual nanowires, ultrafast THz spectroscopy and theoretical analysis. Our combined experiments and analysis further indicate that these plasmon modes are specific to high aspect ratio geometries.

cond-mat.mes-hall↗

arXiv2011

Nanoscale Effects on Heterojunction Electron Gases in GaN/AlGaN Core/Shell Nanowires

The electronic properties of heterojunction electron gases formed in GaN/AlGaN core/shell nanowires with hexagonal and triangular cross-sections are studied theoretically. We show that at nanoscale dimensions, the non-polar hexagonal system exhibits degenerate quasi-one-dimensional electron gases at the hexagon corners, which transition to a core-centered electron gas at lower doping. In contrast, polar triangular core/shell nanowires show either a non-degenerate electron gas on the polar face or a single quasi-one-dimensional electron gas at the corner opposite the polar face, depending on the termination of the polar face. More generally, our results indicate that electron gases in closed nanoscale systems are qualitatively different from their bulk counterparts.

cond-mat.mtrl-sci↗

arXiv2011

Electrostatic effects on contacts to carbon nanotube transistors

We use numerical simulations to investigate the effect of electrostatics on the source and drain contacts of carbon nanotube field-effect transistors. We find that unscreened charge on the nanotube at the contact-channel interface leads to a potential barrier that can significantly hamper transport through the device. This effect is largest for intermediate gate voltages and for contacts near the ohmic-Schottky crossover, but can be mitigated with a reduction in the gate oxide thickness. These results help to elucidate the important role that contact geometry plays in the performance of carbon nanotube electronic devices.

cond-mat.mes-hall↗

arXiv2011

Reduced Joule heating in nanowires

The temperature distribution in nanowires due to Joule heating is studied analytically using a continuum model and a Green's function approach. We show that the temperatures reached in nanowires can be much lower than that predicted by bulk models of Joule heating, due to heat loss at the nanowire surface that is important at nanoscopic dimensions, even when the thermal conductivity of the environment is relatively low. In addition, we find that the maximum temperature in the nanowire scales weakly with length, in contrast to the bulk system. A simple criterion is presented to assess the importance of these effects. The results have implications for the experimental measurements of nanowire thermal properties, for thermoelectric applications, and for controlling thermal effects in nanowire electronic devices.

cond-mat.mes-hall↗

arXiv2010

Poole-Frenkel Effect and Phonon-Assisted Tunneling in GaAs Nanowires

We present electronic transport measurements of GaAs nanowires grown by catalyst-free metal-organic chemical vapor deposition. Despite the nanowires being doped with a relatively high concentration of substitutional impurities, we find them inordinately resistive. By measuring sufficiently high aspect-ratio nanowires individually in situ, we decouple the role of the contacts and show that this semi-insulating electrical behavior is the result of trap-mediated carrier transport. We observe Poole-Frenkel transport that crosses over to phonon-assisted tunneling at higher fields, with a tunneling time found to depend predominantly on fundamental physical constants as predicted by theory. By using in situ electron beam irradiation of individual nanowires we probe the nanowire electronic transport when free carriers are made available, thus revealing the nature of the contacts.

cond-mat.mes-hall↗

arXiv2010

Observation of Space-Charge-Limited Transport in InAs Nanowires

Recent theory and experiment have suggested that space-charge-limited transport should be prevalent in high aspect-ratio semiconducting nanowires. We report on InAs nanowires exhibiting this mode of transport and utilize the underlying theory to determine the mobility and effective carrier concentration of individual nanowires, both of which are found to be diameter-dependent. Intentionally induced failure by Joule heating supports the notion of space-charge-limited transport and proposes reduced thermal conductivity due to the nanowires polymorphism.

cond-mat.mes-hall↗

arXiv2010

Tunable bandgaps and excitons in doped semiconducting carbon nanotubes made possible by acoustic plasmons

Doping of semiconductors is essential in modern electronic and photonic devices. While doping is well understood in bulk semiconductors, the advent of carbon nanotubes and nanowires for nanoelectronic and nanophotonic applications raises some key questions about the role and impact of doping at low dimensionality. Here we show that for semiconducting carbon nanotubes, bandgaps and exciton binding energies can be dramatically reduced upon experimentally relevant doping, and can be tuned gradually over a broad range of energies in contrast to higher dimensional systems. The later feature is made possible by a novel mechanism involving strong dynamical screening effects mediated by acoustic plasmons.

cond-mat.mtrl-sci↗

arXiv2010

Self-consistent ac quantum transport using nonequilibrium Green functions

We develop an approach for self-consistent ac quantum transport in the presence of time-dependent potentials at non-transport terminals. We apply the approach to calculate the high-frequency characteristics of a nanotube transistor with the ac signal applied at the gate terminal. We show that the self-consistent feedback between the ac charge and potential is essential to properly capture the transport properties of the system. In the on-state, this feedback leads to the excitation of plasmons, which appear as pronounced divergent peaks in the dynamic conductance at terahertz frequencies. In the off-state, these collective features vanish, and the conductance exhibits smooth oscillations, a signature of single-particle excitations. The proposed approach is general and will allow the study of the high-frequency characteristics of many other low-dimensional nanoscale materials such as nanowires and graphene-based systems, which are attractive for terahertz devices, including those that exploit plasmonic excitations.

cond-mat.mes-hall↗

arXiv2009

Terahertz response of carbon nanotube transistors

We present an approach for time-dependent quantum transport based on a self-consistent non-equilibrium Green function formalism. The technique is applied to a ballistic carbon nanotube transistor in the presence of a time harmonic signal at the gate. In the ON state the dynamic conductance exhibits plasmonic resonant peaks at terahertz frequencies. These vanish in the OFF state, and the dynamic conductance displays smooth oscillations, a signature of single particle quantum effects. We show that the nanotube kinetic inductance plays an essential role in the high-frequency behavior.

cond-mat.mes-hall↗

arXiv2009

Diameter-Dependent Electronic Transport Properties of Au-Catalyst/Ge-Nanowire Schottky Diodes

We present electronic transport measurements in individual Au-catalyst/Ge-nanowire interfaces demonstrating the presence of a Schottky barrier. Surprisingly, the small-bias conductance density increases with decreasing diameter. Theoretical calculations suggest that this effect arises because electron-hole recombination in the depletion region is the dominant charge transport mechanism, with a diameter dependence of both the depletion width and the electron-hole recombination time. The recombination time is dominated by surface contributions and depends linearly on the nanowire diameter.

cond-mat.mtrl-sci↗

arXiv2008

Unusually strong space-charge-limited current in thin wires

The current-voltage characteristics of thin wires are often observed to be nonlinear, and this behavior has been ascribed to Schottky barriers at the contacts. We present electronic transport measurements on GaN nanorods and demonstrate that the nonlinear behavior originates instead from space-charge-limited current. A theory of space-charge-limited current in thin wires corroborates the experiments, and shows that poor screening in high aspect ratio materials leads to a dramatic enhancement of space-charge limited current, resulting in new scaling in terms of the aspect ratio.

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