Pierre Levesque - Academia.edu (original) (raw)

Papers by Pierre Levesque

Meeting abstracts, 2016

Thin layers of black phosphorus attract a growing interest in electronics and optoelectronics for... more Thin layers of black phosphorus attract a growing interest in electronics and optoelectronics for their bidimensional properties (2D) giving raised to high mobility and tunable direct band gap. Thin films are however unstable in air and synthesis of high quality samples consisting of pristine few-layered materials remains challenging, as a result. We recently established that a combination of oxygen, light and moisture provides the conditions leading to a photo-oxidation of the layers. In controlled conditions, the kinetic of the reaction is well captured by a redox model, but the evolution is less clear in normal humidity conditions where water can simply accumulate at the surface and forms droplets. In the present study, we investigate the wetting properties of thin black phosphorous films and its evolution during degradation using FastScan atomic force microscopy in different humidity conditions. From a statistical analysis of the bubbles formed, the wettability is found to decrease with layer thickness. This difference in surface tension between thinner and thicker layers is ascribed to an accumulation of phosphoric acid in the bubbles, which reveals increasing degradation rates with decreasing layer thickness. From our analysis and a simple rate equation model, it is found that: i) A threshold humidity is necessary for water condensation; ii) The photo-oxidation occurs on single bubble sites; iii) Black phosphorus layers immersed in water slowly thickens and crumbles anisotropically due to water etching during degradation.

Physical review research, Sep 23, 2020

The dynamics of low energy charge carriers in a graphene quantum dot subjected to a timedependent... more The dynamics of low energy charge carriers in a graphene quantum dot subjected to a timedependent local field is investigated numerically. In particular, we study a configuration where a Coulomb electric field is provided by an ion traversing the graphene sample. A Galerkin-like numerical scheme is introduced to solve the massless Dirac equation describing charge carriers subjected to space-and time-dependent electromagnetic potentials and is used to evaluate the field induced interband transitions. It is demonstrated that as the ion goes through graphene, electronhole pairs are generated dynamically via the adiabatic pair creation mechanism around avoided crossings, similar to electron-positron pair generation in low energy heavy ion collisions.

We study nonlinear THz effects in photoexcited gated graphene, using optical-pump/intense-THz-pro... more We study nonlinear THz effects in photoexcited gated graphene, using optical-pump/intense-THz-probe spectroscopy. A crossover from negative to positive differential THz transmission has been observed at the lowest doping concentration when the THz field is increased. Using a simple Drude model of the graphene photoconductivity, we attributed the observed behavior to a transition from a domain in which the increase in the Drude weight dominates over the increase in the scattering rate after photoexcitation, to a domain with the opposite properties when the THz probe field is increased.

In this work, we study carrier dynamics in gated graphene using tunable-infrared (IR)-pump/terahe... more In this work, we study carrier dynamics in gated graphene using tunable-infrared (IR)-pump/terahertz (THz)-probe spectroscopy. Interband transitions are Pauli blocked in highly doped graphene when pumping at long wavelengths.

Using optical-pump/THz-probe spectroscopy on gated, undoped graphene, we find that as the amplitu... more Using optical-pump/THz-probe spectroscopy on gated, undoped graphene, we find that as the amplitude of the THz probe is increased, we observe a cross-over from optically-induced transmission decrease to increase.

Physical Review B, Dec 28, 2015

We have measured the electronic heat dissipation of hot electrons in highly disordered millimeter... more We have measured the electronic heat dissipation of hot electrons in highly disordered millimeter scale graphene at temperatures T = 0.3-3 K. Disorder was introduced by hydrogenation of graphene, bringing low-temperature electron conduction below the Ioffe-Regel criterion for metallic conduction. Resistive thermometry was employed to determine the dependence of electron temperature on applied electrical power. The relation between heat flow and electron temperature was found to be well described by a power law with an exponent β ∼ 3.7-3.9 and a coupling coefficient ∼ 1 mW/m 2 K β. Our observations are similar to electronic heat dissipation of a two-dimensional electron gas in the hydrodynamic limit of electron-phonon coupling, corresponding to acoustic phonon emission into the substrate.

Applied Physics Letters, Dec 21, 2015

Journal of Chemical Physics, Feb 24, 2005

Low-energy vibrational and electronic electron-energy-loss (EEL) spectra of pyrimidine condensed ... more Low-energy vibrational and electronic electron-energy-loss (EEL) spectra of pyrimidine condensed on a thin film of solid argon held at 18 K are reported for the incident-energy range of 2–12 eV. Sensitivity to symmetry and spin forbidden transitions as well as correlations to the triplet states of benzene make it possible to ascribe the main features, below 7 eV in the electronic part of the EEL spectrum, to triplet transitions. The lowest EEL feature with an energy onset at 3.5 eV is attributed to a transition to the B13(n→π*) valence electronic state and the next triplet n→π* transition to a A23 state located around 4.5 eV. The remaining EEL features at 4.3, 5.2, 5.8, and 6.5 eV are all assigned to π→π* transitions to states of symmetry B23, A13, B23, and B23+A13, respectively. The most intense maximum at 7.6 eV is found to correspond to both B21 and A11 transitions, as in the vacuum ultraviolet spectra. Absolute inelastic cross sections per scatterer are derived from a single collision treatment described herein. Their values are found to lie within the 10−17cm2 range for both the electronic and the vibrational excitations. Features in the energy dependence of the cross sections are discussed, whenever possible, by comparison with data and mechanisms found in the gas phase. A maximum over the 4–5 eV range is attributed to a B̃B12 shape resonance and another one observed in the 6–7 eV range is ascribed to either or both σ* shape resonances of A12 and B22 symmetries.

Nature Communications, Aug 10, 2015

Graphene can be transformed from a semimetal into a semiconductor if it is confined into nanoribb... more Graphene can be transformed from a semimetal into a semiconductor if it is confined into nanoribbons narrower than 10 nm with controlled crystallographic orientation and welldefined armchair edges. However, the scalable synthesis of nanoribbons with this precision directly on insulating or semiconducting substrates has not been possible. Here we demonstrate the synthesis of graphene nanoribbons on Ge(001) via chemical vapour deposition. The nanoribbons are self-aligning 3°from the Geh110i directions, are self-defining with predominantly smooth armchair edges, and have tunable width to o10 nm and aspect ratio to 470. In order to realize highly anisotropic ribbons, it is critical to operate in a regime in which the growth rate in the width direction is especially slow, o5 nm h À 1. This directional and anisotropic growth enables nanoribbon fabrication directly on conventional semiconductor wafer platforms and, therefore, promises to allow the integration of nanoribbons into future hybrid integrated circuits.

Meeting abstracts, May 1, 2020

With increasing interest in thermoelectrics for energy applications, determining the thermoelectr... more With increasing interest in thermoelectrics for energy applications, determining the thermoelectric power (also known as the Seebeck coefficient) is critical for the characterization and optimization of thermoelectric materials. Obtaining the absolute Seebeck coefficient, however, is difficult due to induced response in the probes contributing to the observed Seebeck effect. Current methods for obtaining the absolute Seebeck coefficient require using a reference material as probes and: estimating it via the Thomson effect, or measuring it directly using a superconductor as a zero coefficient reference. These methods are either cumbersome or are limited to low temperatures, respectively. Graphene, like superconductors, has a zero absolute Seebeck coefficient at the dirac point; additionally, it offers many advantages over superconductors as a reference material as it is stable over a wider range of temperatures and conditions. In this work, we use graphene as a zero coefficient reference to obtain direct measurements of the absolute Seebeck coefficient of five different materials (Au, W, Mo, chromel and constantan) from 250 K to 390 K, and compare to known results with good agreement. Here, we show that graphene’s unique characteristics, including its stability, insensitivity to impurities and ease of tunable electrical properties allowing for in-situ calibration, make it an excellent candidate as a reference standard for direct measurements of the absolute Seebeck coefficient. This work minds an important 75 years pending gap in the field of thermoelectricity.

Journal of Physical Chemistry Letters, Apr 20, 2016

We report on real time observations of the sublimation of exfoliated black phosphorus layers thro... more We report on real time observations of the sublimation of exfoliated black phosphorus layers throughout annealing using in situ low energy electron microscopy. We found that sublimation manifests itself above 375±20°C through the nucleation and expansion of asymmetric, faceted holes with the long axis aligned along the [100] direction and sharp tips defined by edges consisting of alternating (10) and (11) steps. This thermally activated process repeats itself via successive sublimation of individual layers. Calculations and simulations using density functional theory and kinetic Monte Carlo allowed to determine the involved atomic pathways. Sublimation is found to occur via detachments of phosphorus dimers rather than single atoms. This behavior and the role of defects is described using an analytical model that captures all essential features. This work establishes an atomistic-level understanding of the thermal stability of exfoliated black phosphorus and defines the temperature window available for material and device processing.

European Microscopy Congress 2016: Proceedings, Dec 20, 2016

HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific re... more HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Journal of Physical Chemistry C, May 15, 2015

Previous work has demonstrated that the growth temperature for the chemical vapor deposition of g... more Previous work has demonstrated that the growth temperature for the chemical vapor deposition of graphene can be decreased by tailoring the precursor composition. Here, we fix the precursor as CH and instead explore the effects of the catalyst facet, synthesis conditions, and growth rate on the quality of graphene grown at reduced temperature on Cu. We find that in order to obtain graphene films with low defect density, it is critical to maintain a slow growth rate, which is achieved using a low CH 4 partial pressure. Furthermore, growth on Cu(111) is more efficient than other low index Cu facets. Using optimized growth conditions, we achieve high-quality continuous monolayer graphene films, with a low nucleation density of 4x10-2 µm-2 and a negligible Raman D:G ratio, at 750 °C, which is 125 °C cooler than previous reports achieving full graphene coverage on Cu. The field effect mobility is 2600 cm 2 V-1 s-1 at a carrier concentration of 10 12 cm-2 and low-energy electron microscopy indicates that there is one predominant graphene crystallographic orientation with respect to the underlying Cu(111). These results demonstrate that by tailoring the catalyst facet and growth rate, the temperature required to achieve high-quality continuous films, using a given precursor, can be reduced without sacrificing the excellent properties of graphene. 1 INTRODUCTION Chemical vapor deposition (CVD) has become a leading candidate for the scalable production of graphene due to its ability to yield continuous monolayer graphene sheets over large areas. Furthermore, graphene films grown on Cu have exhibited electronic properties that are on par with mechanically exfoliated graphene. 1 Cu is the most widely used graphene catalyst, in part

arXiv (Cornell University), Aug 2, 2014

Thin layers of black phosphorus have recently raised interest for their two-dimensional (2D) semi... more Thin layers of black phosphorus have recently raised interest for their two-dimensional (2D) semiconducting properties, such as tunable direct bandgap and high carrier mobilities. This lamellar crystal of P atoms stacked together by weak van der Waals forces can be exfoliated down to the stratophosphane monolayer (also called phosphorene) using procedures similar to those used for graphene. Properties of this 2D material are however challenging to probe due to a fast and ubiquitous degradation upon exposure to ambient conditions. Herein, we investigate the crystal degradation using in-situ Raman and transmission electron spectroscopies and highlight a process involving a photo-induced oxidation reaction with adsorbed oxygen in water. The experimental conditions to prepare and preserve mono-, bi-and multilayers of stratophosphane in their pristine states were determined. Study on these 2D layers provides new insights on the effect of confinement on the chemical reactivity and the vibrational modes of black phosphorus.

Carbon, Apr 1, 2019

Graphene films grown on copper by chemical vapor deposition were exposed to the late afterglow of... more Graphene films grown on copper by chemical vapor deposition were exposed to the late afterglow of a reduced-pressure N 2 plasma sustained by microwave electromagnetic fields. X-ray photoelectron and Raman spectroscopies reveal extremely high incorporation of plasmagenerated N atoms into the graphene film (N/C = 29%) while maintaining an unprecedentedly low-damage generation (D:G = 0.35-0.45) compared to the literature (0.5 to 2.5). The incorporation dynamics between graphene on copper and graphene on copper oxide are also compared and discussed. After transfer on SiO 2 /Si substrate, the N/C content decrease to only 6%. This reveals that a large part of the N atoms are weakly bonded to the graphene surface. Most of the nitrogen incorporation seems linked to the functionalization of weakly bonded hydrocarbons initially adsorbed from air exposure or carbon-nitrogen structures arising from plasma-surface interactions.

Meeting abstracts, 2015

The rational synthesis of graphene nanoribbons that are semiconducting with sub-10 nm width, cont... more The rational synthesis of graphene nanoribbons that are semiconducting with sub-10 nm width, controlled crystallographic orientation, and well-defined edges on non-metallic substrates has been a significant challenge. The growth of nanoribbons on metal substrates precludes their direct use in semiconducting electronics due to the conducting substrate, and the direct synthesis of nanoribbons in solution is complicated by challenges of their post-synthetic assembly. In this talk, we demonstrate the scalable synthesis of graphene nanoribbons from the bottom-up via chemical vapor deposition (CVD) on Ge(001). Low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) show that the ribbons are self-orienting ±2.9° from the Ge[110] directions and are self-defining. The nanoribbons have predominately smooth armchair edges that give rise to electron interference patterns that are indicative of the high quality of the edges. By tuning the precursor flux, growth time, and growth temperature, the ribbon anisotropy and growth kinetics can be tailored to yield ribbons with controlled width < 10 nm and aspect ratio > 60. Compared to previous reports of the growth of low aspect ratio crystals of graphene on Ge, we find that in order to realize high aspect ratio nanoribbons, it is critical to operate in a regime in which the growth rate is especially slow, on the order of 5 nm/h for growth in the width direction. Scanning tunneling spectroscopy shows that the ribbons have electronic structures that are consistent with semiconductors with bandgaps that are > 500 meV and that vary inversely with width. This work is important because unlike continuous two-dimensional graphene, which is semimetallic, one-dimensional graphene nanoribbons can be semiconducting, allowing for the substantial modulation of their conductance and enabling their application in semiconductor logic, optoelectronics, photonics, and sensors. Moreover, the direct synthesis of ultranarrow and smooth graphene nanoribbons on Ge demonstrated here provides a scalable, high throughput pathway for integrating semiconducting graphene directly on conventional large-area semiconductor wafer platforms that are compatible with planar processing.

Journal of Chemical Physics, Jun 8, 2005

The absolute cross sections for electronic excitations of thymine by electron impact between 5 an... more The absolute cross sections for electronic excitations of thymine by electron impact between 5 and 12eV are determined by means of electron-energy loss (EEL) spectroscopy for the molecule deposited at submonolayer coverage on an inert Ar substrate. The lowest EEL features at 3.7 and 4.0eV are attributed to the excitation of the triplet 1A′3(π→π*) and 1A″3(n→π*) valence states of the molecule. The higher EEL features located at 4.9, 6.3, 7.3, and 9eV with a weak shoulder around 6eV are ascribed mostly to triplet valence (π→π*) excitation manifold of the molecule. The energy dependence of the cross section for both the lowest triplet valence excitations shows essentially a peak at about 5eV reaching a value of 2.9×10−17cm2. The cross sections for the higher EEL features are generally characterized by a common broad maximum around 8eV. The latter reaches a value of 1.36×10−16cm2 for the combined 6 and 6.3eV excitation region. The maxima in the present cross sections are found to correspond to the resonances that have been reported at about the same energies in the O− yield from electron impact on thymine in the gas phase.

Nano Letters, Jul 11, 2017

The lack of large-area synthesis processes on substrates compatible with industry requirements ha... more The lack of large-area synthesis processes on substrates compatible with industry requirements has been one of the major hurdles facing the integration of 2D materials in mainstream technologies. This is particularly the case for the recently discovered monoelemental group V 2D materials which can only be produced by exfoliation or growth on exotic substrates. Herein, to overcome this limitation, we demonstrate a scalable method to synthesize antimonene on germanium substrates using solid-source molecular beam epitaxy. This emerging 2D material has been attracting a great deal of attention due to its high environmental stability and its outstanding optical and electronic properties. In situ low energy electron microscopy allowed the real time investigation and optimization of the 2D growth. Theoretical calculations combined with atomic-scale microscopic and spectroscopic measurements demonstrated that the grown antimonene sheets are of high crystalline quality, interact weakly with germanium, exhibit semimetallic characteristics, and remain stable under ambient conditions. This achievement paves the way for the integration of antimonene in innovative nanoscale and quantum technologies compatible with the current semiconductor manufacturing.

Meeting abstracts, 2011

not Available.

2016 41st International Conference on Infrared, Millimeter, and Terahertz waves (IRMMW-THz), 2016

We study nonlinear THz effects in photoexcited gated graphene, using optical-pump/intense-THz-pro... more We study nonlinear THz effects in photoexcited gated graphene, using optical-pump/intense-THz-probe spectroscopy. A crossover from negative to positive differential THz transmission has been observed at the lowest doping concentration when the THz field is increased. Using a simple Drude model of the graphene photoconductivity, we attributed the observed behavior to a transition from a domain in which the increase in the Drude weight dominates over the increase in the scattering rate after photoexcitation, to a domain with the opposite properties when the THz probe field is increased.

Meeting abstracts, 2016

Thin layers of black phosphorus attract a growing interest in electronics and optoelectronics for... more Thin layers of black phosphorus attract a growing interest in electronics and optoelectronics for their bidimensional properties (2D) giving raised to high mobility and tunable direct band gap. Thin films are however unstable in air and synthesis of high quality samples consisting of pristine few-layered materials remains challenging, as a result. We recently established that a combination of oxygen, light and moisture provides the conditions leading to a photo-oxidation of the layers. In controlled conditions, the kinetic of the reaction is well captured by a redox model, but the evolution is less clear in normal humidity conditions where water can simply accumulate at the surface and forms droplets. In the present study, we investigate the wetting properties of thin black phosphorous films and its evolution during degradation using FastScan atomic force microscopy in different humidity conditions. From a statistical analysis of the bubbles formed, the wettability is found to decrease with layer thickness. This difference in surface tension between thinner and thicker layers is ascribed to an accumulation of phosphoric acid in the bubbles, which reveals increasing degradation rates with decreasing layer thickness. From our analysis and a simple rate equation model, it is found that: i) A threshold humidity is necessary for water condensation; ii) The photo-oxidation occurs on single bubble sites; iii) Black phosphorus layers immersed in water slowly thickens and crumbles anisotropically due to water etching during degradation.

Physical review research, Sep 23, 2020

The dynamics of low energy charge carriers in a graphene quantum dot subjected to a timedependent... more The dynamics of low energy charge carriers in a graphene quantum dot subjected to a timedependent local field is investigated numerically. In particular, we study a configuration where a Coulomb electric field is provided by an ion traversing the graphene sample. A Galerkin-like numerical scheme is introduced to solve the massless Dirac equation describing charge carriers subjected to space-and time-dependent electromagnetic potentials and is used to evaluate the field induced interband transitions. It is demonstrated that as the ion goes through graphene, electronhole pairs are generated dynamically via the adiabatic pair creation mechanism around avoided crossings, similar to electron-positron pair generation in low energy heavy ion collisions.

We study nonlinear THz effects in photoexcited gated graphene, using optical-pump/intense-THz-pro... more We study nonlinear THz effects in photoexcited gated graphene, using optical-pump/intense-THz-probe spectroscopy. A crossover from negative to positive differential THz transmission has been observed at the lowest doping concentration when the THz field is increased. Using a simple Drude model of the graphene photoconductivity, we attributed the observed behavior to a transition from a domain in which the increase in the Drude weight dominates over the increase in the scattering rate after photoexcitation, to a domain with the opposite properties when the THz probe field is increased.

In this work, we study carrier dynamics in gated graphene using tunable-infrared (IR)-pump/terahe... more In this work, we study carrier dynamics in gated graphene using tunable-infrared (IR)-pump/terahertz (THz)-probe spectroscopy. Interband transitions are Pauli blocked in highly doped graphene when pumping at long wavelengths.

Using optical-pump/THz-probe spectroscopy on gated, undoped graphene, we find that as the amplitu... more Using optical-pump/THz-probe spectroscopy on gated, undoped graphene, we find that as the amplitude of the THz probe is increased, we observe a cross-over from optically-induced transmission decrease to increase.

Physical Review B, Dec 28, 2015

We have measured the electronic heat dissipation of hot electrons in highly disordered millimeter... more We have measured the electronic heat dissipation of hot electrons in highly disordered millimeter scale graphene at temperatures T = 0.3-3 K. Disorder was introduced by hydrogenation of graphene, bringing low-temperature electron conduction below the Ioffe-Regel criterion for metallic conduction. Resistive thermometry was employed to determine the dependence of electron temperature on applied electrical power. The relation between heat flow and electron temperature was found to be well described by a power law with an exponent β ∼ 3.7-3.9 and a coupling coefficient ∼ 1 mW/m 2 K β. Our observations are similar to electronic heat dissipation of a two-dimensional electron gas in the hydrodynamic limit of electron-phonon coupling, corresponding to acoustic phonon emission into the substrate.

Applied Physics Letters, Dec 21, 2015

Journal of Chemical Physics, Feb 24, 2005

Low-energy vibrational and electronic electron-energy-loss (EEL) spectra of pyrimidine condensed ... more Low-energy vibrational and electronic electron-energy-loss (EEL) spectra of pyrimidine condensed on a thin film of solid argon held at 18 K are reported for the incident-energy range of 2–12 eV. Sensitivity to symmetry and spin forbidden transitions as well as correlations to the triplet states of benzene make it possible to ascribe the main features, below 7 eV in the electronic part of the EEL spectrum, to triplet transitions. The lowest EEL feature with an energy onset at 3.5 eV is attributed to a transition to the B13(n→π*) valence electronic state and the next triplet n→π* transition to a A23 state located around 4.5 eV. The remaining EEL features at 4.3, 5.2, 5.8, and 6.5 eV are all assigned to π→π* transitions to states of symmetry B23, A13, B23, and B23+A13, respectively. The most intense maximum at 7.6 eV is found to correspond to both B21 and A11 transitions, as in the vacuum ultraviolet spectra. Absolute inelastic cross sections per scatterer are derived from a single collision treatment described herein. Their values are found to lie within the 10−17cm2 range for both the electronic and the vibrational excitations. Features in the energy dependence of the cross sections are discussed, whenever possible, by comparison with data and mechanisms found in the gas phase. A maximum over the 4–5 eV range is attributed to a B̃B12 shape resonance and another one observed in the 6–7 eV range is ascribed to either or both σ* shape resonances of A12 and B22 symmetries.

Nature Communications, Aug 10, 2015

Graphene can be transformed from a semimetal into a semiconductor if it is confined into nanoribb... more Graphene can be transformed from a semimetal into a semiconductor if it is confined into nanoribbons narrower than 10 nm with controlled crystallographic orientation and welldefined armchair edges. However, the scalable synthesis of nanoribbons with this precision directly on insulating or semiconducting substrates has not been possible. Here we demonstrate the synthesis of graphene nanoribbons on Ge(001) via chemical vapour deposition. The nanoribbons are self-aligning 3°from the Geh110i directions, are self-defining with predominantly smooth armchair edges, and have tunable width to o10 nm and aspect ratio to 470. In order to realize highly anisotropic ribbons, it is critical to operate in a regime in which the growth rate in the width direction is especially slow, o5 nm h À 1. This directional and anisotropic growth enables nanoribbon fabrication directly on conventional semiconductor wafer platforms and, therefore, promises to allow the integration of nanoribbons into future hybrid integrated circuits.

Meeting abstracts, May 1, 2020

With increasing interest in thermoelectrics for energy applications, determining the thermoelectr... more With increasing interest in thermoelectrics for energy applications, determining the thermoelectric power (also known as the Seebeck coefficient) is critical for the characterization and optimization of thermoelectric materials. Obtaining the absolute Seebeck coefficient, however, is difficult due to induced response in the probes contributing to the observed Seebeck effect. Current methods for obtaining the absolute Seebeck coefficient require using a reference material as probes and: estimating it via the Thomson effect, or measuring it directly using a superconductor as a zero coefficient reference. These methods are either cumbersome or are limited to low temperatures, respectively. Graphene, like superconductors, has a zero absolute Seebeck coefficient at the dirac point; additionally, it offers many advantages over superconductors as a reference material as it is stable over a wider range of temperatures and conditions. In this work, we use graphene as a zero coefficient reference to obtain direct measurements of the absolute Seebeck coefficient of five different materials (Au, W, Mo, chromel and constantan) from 250 K to 390 K, and compare to known results with good agreement. Here, we show that graphene’s unique characteristics, including its stability, insensitivity to impurities and ease of tunable electrical properties allowing for in-situ calibration, make it an excellent candidate as a reference standard for direct measurements of the absolute Seebeck coefficient. This work minds an important 75 years pending gap in the field of thermoelectricity.

Journal of Physical Chemistry Letters, Apr 20, 2016

We report on real time observations of the sublimation of exfoliated black phosphorus layers thro... more We report on real time observations of the sublimation of exfoliated black phosphorus layers throughout annealing using in situ low energy electron microscopy. We found that sublimation manifests itself above 375±20°C through the nucleation and expansion of asymmetric, faceted holes with the long axis aligned along the [100] direction and sharp tips defined by edges consisting of alternating (10) and (11) steps. This thermally activated process repeats itself via successive sublimation of individual layers. Calculations and simulations using density functional theory and kinetic Monte Carlo allowed to determine the involved atomic pathways. Sublimation is found to occur via detachments of phosphorus dimers rather than single atoms. This behavior and the role of defects is described using an analytical model that captures all essential features. This work establishes an atomistic-level understanding of the thermal stability of exfoliated black phosphorus and defines the temperature window available for material and device processing.

European Microscopy Congress 2016: Proceedings, Dec 20, 2016

HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific re... more HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Journal of Physical Chemistry C, May 15, 2015

Previous work has demonstrated that the growth temperature for the chemical vapor deposition of g... more Previous work has demonstrated that the growth temperature for the chemical vapor deposition of graphene can be decreased by tailoring the precursor composition. Here, we fix the precursor as CH and instead explore the effects of the catalyst facet, synthesis conditions, and growth rate on the quality of graphene grown at reduced temperature on Cu. We find that in order to obtain graphene films with low defect density, it is critical to maintain a slow growth rate, which is achieved using a low CH 4 partial pressure. Furthermore, growth on Cu(111) is more efficient than other low index Cu facets. Using optimized growth conditions, we achieve high-quality continuous monolayer graphene films, with a low nucleation density of 4x10-2 µm-2 and a negligible Raman D:G ratio, at 750 °C, which is 125 °C cooler than previous reports achieving full graphene coverage on Cu. The field effect mobility is 2600 cm 2 V-1 s-1 at a carrier concentration of 10 12 cm-2 and low-energy electron microscopy indicates that there is one predominant graphene crystallographic orientation with respect to the underlying Cu(111). These results demonstrate that by tailoring the catalyst facet and growth rate, the temperature required to achieve high-quality continuous films, using a given precursor, can be reduced without sacrificing the excellent properties of graphene. 1 INTRODUCTION Chemical vapor deposition (CVD) has become a leading candidate for the scalable production of graphene due to its ability to yield continuous monolayer graphene sheets over large areas. Furthermore, graphene films grown on Cu have exhibited electronic properties that are on par with mechanically exfoliated graphene. 1 Cu is the most widely used graphene catalyst, in part

arXiv (Cornell University), Aug 2, 2014

Thin layers of black phosphorus have recently raised interest for their two-dimensional (2D) semi... more Thin layers of black phosphorus have recently raised interest for their two-dimensional (2D) semiconducting properties, such as tunable direct bandgap and high carrier mobilities. This lamellar crystal of P atoms stacked together by weak van der Waals forces can be exfoliated down to the stratophosphane monolayer (also called phosphorene) using procedures similar to those used for graphene. Properties of this 2D material are however challenging to probe due to a fast and ubiquitous degradation upon exposure to ambient conditions. Herein, we investigate the crystal degradation using in-situ Raman and transmission electron spectroscopies and highlight a process involving a photo-induced oxidation reaction with adsorbed oxygen in water. The experimental conditions to prepare and preserve mono-, bi-and multilayers of stratophosphane in their pristine states were determined. Study on these 2D layers provides new insights on the effect of confinement on the chemical reactivity and the vibrational modes of black phosphorus.

Carbon, Apr 1, 2019

Graphene films grown on copper by chemical vapor deposition were exposed to the late afterglow of... more Graphene films grown on copper by chemical vapor deposition were exposed to the late afterglow of a reduced-pressure N 2 plasma sustained by microwave electromagnetic fields. X-ray photoelectron and Raman spectroscopies reveal extremely high incorporation of plasmagenerated N atoms into the graphene film (N/C = 29%) while maintaining an unprecedentedly low-damage generation (D:G = 0.35-0.45) compared to the literature (0.5 to 2.5). The incorporation dynamics between graphene on copper and graphene on copper oxide are also compared and discussed. After transfer on SiO 2 /Si substrate, the N/C content decrease to only 6%. This reveals that a large part of the N atoms are weakly bonded to the graphene surface. Most of the nitrogen incorporation seems linked to the functionalization of weakly bonded hydrocarbons initially adsorbed from air exposure or carbon-nitrogen structures arising from plasma-surface interactions.

Meeting abstracts, 2015

The rational synthesis of graphene nanoribbons that are semiconducting with sub-10 nm width, cont... more The rational synthesis of graphene nanoribbons that are semiconducting with sub-10 nm width, controlled crystallographic orientation, and well-defined edges on non-metallic substrates has been a significant challenge. The growth of nanoribbons on metal substrates precludes their direct use in semiconducting electronics due to the conducting substrate, and the direct synthesis of nanoribbons in solution is complicated by challenges of their post-synthetic assembly. In this talk, we demonstrate the scalable synthesis of graphene nanoribbons from the bottom-up via chemical vapor deposition (CVD) on Ge(001). Low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) show that the ribbons are self-orienting ±2.9° from the Ge[110] directions and are self-defining. The nanoribbons have predominately smooth armchair edges that give rise to electron interference patterns that are indicative of the high quality of the edges. By tuning the precursor flux, growth time, and growth temperature, the ribbon anisotropy and growth kinetics can be tailored to yield ribbons with controlled width < 10 nm and aspect ratio > 60. Compared to previous reports of the growth of low aspect ratio crystals of graphene on Ge, we find that in order to realize high aspect ratio nanoribbons, it is critical to operate in a regime in which the growth rate is especially slow, on the order of 5 nm/h for growth in the width direction. Scanning tunneling spectroscopy shows that the ribbons have electronic structures that are consistent with semiconductors with bandgaps that are > 500 meV and that vary inversely with width. This work is important because unlike continuous two-dimensional graphene, which is semimetallic, one-dimensional graphene nanoribbons can be semiconducting, allowing for the substantial modulation of their conductance and enabling their application in semiconductor logic, optoelectronics, photonics, and sensors. Moreover, the direct synthesis of ultranarrow and smooth graphene nanoribbons on Ge demonstrated here provides a scalable, high throughput pathway for integrating semiconducting graphene directly on conventional large-area semiconductor wafer platforms that are compatible with planar processing.

Journal of Chemical Physics, Jun 8, 2005

The absolute cross sections for electronic excitations of thymine by electron impact between 5 an... more The absolute cross sections for electronic excitations of thymine by electron impact between 5 and 12eV are determined by means of electron-energy loss (EEL) spectroscopy for the molecule deposited at submonolayer coverage on an inert Ar substrate. The lowest EEL features at 3.7 and 4.0eV are attributed to the excitation of the triplet 1A′3(π→π*) and 1A″3(n→π*) valence states of the molecule. The higher EEL features located at 4.9, 6.3, 7.3, and 9eV with a weak shoulder around 6eV are ascribed mostly to triplet valence (π→π*) excitation manifold of the molecule. The energy dependence of the cross section for both the lowest triplet valence excitations shows essentially a peak at about 5eV reaching a value of 2.9×10−17cm2. The cross sections for the higher EEL features are generally characterized by a common broad maximum around 8eV. The latter reaches a value of 1.36×10−16cm2 for the combined 6 and 6.3eV excitation region. The maxima in the present cross sections are found to correspond to the resonances that have been reported at about the same energies in the O− yield from electron impact on thymine in the gas phase.

Nano Letters, Jul 11, 2017

The lack of large-area synthesis processes on substrates compatible with industry requirements ha... more The lack of large-area synthesis processes on substrates compatible with industry requirements has been one of the major hurdles facing the integration of 2D materials in mainstream technologies. This is particularly the case for the recently discovered monoelemental group V 2D materials which can only be produced by exfoliation or growth on exotic substrates. Herein, to overcome this limitation, we demonstrate a scalable method to synthesize antimonene on germanium substrates using solid-source molecular beam epitaxy. This emerging 2D material has been attracting a great deal of attention due to its high environmental stability and its outstanding optical and electronic properties. In situ low energy electron microscopy allowed the real time investigation and optimization of the 2D growth. Theoretical calculations combined with atomic-scale microscopic and spectroscopic measurements demonstrated that the grown antimonene sheets are of high crystalline quality, interact weakly with germanium, exhibit semimetallic characteristics, and remain stable under ambient conditions. This achievement paves the way for the integration of antimonene in innovative nanoscale and quantum technologies compatible with the current semiconductor manufacturing.

Meeting abstracts, 2011

not Available.

2016 41st International Conference on Infrared, Millimeter, and Terahertz waves (IRMMW-THz), 2016

We study nonlinear THz effects in photoexcited gated graphene, using optical-pump/intense-THz-pro... more We study nonlinear THz effects in photoexcited gated graphene, using optical-pump/intense-THz-probe spectroscopy. A crossover from negative to positive differential THz transmission has been observed at the lowest doping concentration when the THz field is increased. Using a simple Drude model of the graphene photoconductivity, we attributed the observed behavior to a transition from a domain in which the increase in the Drude weight dominates over the increase in the scattering rate after photoexcitation, to a domain with the opposite properties when the THz probe field is increased.