Jérôme Saint-Martin

Optimizing the thermoelectric performance of graphene nano-ribbons without degrading the electronic properties

The enhancement of thermoelectric figure of merit ZT requires to either increase the power factor or reduce the phonon conductance, or even both. In graphene, the high phonon thermal conductivity is the main factor limiting the thermoelectric conversion. The common strategy to enhance ZT is therefore to introduce phonon scatterers to suppress the phonon conductance while retaining high electrical conductance and Seebeck coefficient. Although thermoelectric performance is eventually enhanced, all studies based on this strategy show a significant reduction of the electrical conductance. In this study we demonstrate that appropriate sources of disorder, including isotopes and vacancies at lowest electron density positions, can be used as phonon scatterers to reduce the phonon conductance in graphene ribbons without degrading the electrical conductance, particularly in the low-energy region which is the most important range for device operation. By means of atomistic calculations we show that the natural electronic properties of graphene ribbons can be fully preserved while their thermoelectric efficiency is strongly enhanced. For ribbons of width Mu2009=u20095 dimer lines, room-temperature ZT is enhanced from less than 0.26 to more than 2.5. This study is likely to set the milestones of a new generation of nano-devices with dual electronic/thermoelectric functionalities.

Third nearest neighbor parameterized tight binding model for graphene nano-ribbons

The existing tight binding models can very well reproduce the ab initio band structure of a 2D graphene sheet. For graphene nano-ribbons (GNRs), the current sets of tight binding parameters can successfully describe the semi-conducting behavior of all armchair GNRs. However, they are still failing in reproducing accurately the slope of the bands that is directly associated with the group velocity and the effective mass of electrons. In this work, both density functional theory and tight binding calculations were performed and a new set of tight binding parameters up to the third nearest neighbors including overlap terms is introduced. The results obtained with this model offer excellent agreement with the predictions of the density functional theory in most cases of ribbon structures, even in the high-energy region. Moreover, this set can induce electron-hole asymmetry as manifested in results from density functional theory. Relevant outcomes are also achieved for armchair ribbons of various widths as wel...

Non-linear effects and thermoelectric efficiency of quantum dot-based single-electron transistors

By means of advanced numerical simulation, the thermoelectric properties of a Si-quantum dot-based single-electron transistor operating in sequential tunneling regime are investigated in terms of figure of merit, efficiency and power. By taking into account the phonon-induced collisional broadening of energy levels in the quantum dot, both heat and electrical currents are computed in a voltage range beyond the linear response. Using our homemade code consisting in a 3D Poisson-Schrödinger solver and the resolution of the Master equation, the Seebeck coefficient at low bias voltage appears to be material independent and nearly independent on the level broadening, which makes this device promising for metrology applications as a nanoscale standard of Seebeck coefficient. Besides, at higher voltage bias, the non-linear characteristics of the heat current are shown to be related to the multi-level effects. Finally, when considering only the electronic contribution to the thermal conductance, the single-electron transistor operating in generator regime is shown to exhibit very good efficiency at maximum power.

Phonon transmission at Si/Ge and polytypic Ge interfaces using full-band mismatch based models

This paper presents theoretical investigations on the interfacial thermal conductance (Kapitza conductance) in both monotype Si/Ge (cubic 3C) and polytype (cubic 3C/hexagonal 2H) Ge interfaces by using full band extensions of diffusive and acoustic mismatch models. In that aims, phonon dispersions in the full 3D Brillouin zone have been computed via an atomistic adiabatic bond charge model. The effects of crystal orientation are investigated, and the main phonon modes involved in heat transfer are highlighted. According to our calculations, polytype interfaces without any mass mismatch but with a crystallographic phase mismatch exhibit a thermal conductance very close to that of Si/Ge interfaces with a mass mismatch but without any phase mismatch. Besides, the orientations of Ge polytype interface that have been observed experimentally in nanowires, i.e., along [ 115 ] / [ 50 5 ¯ 1 ], exhibit the lowest interfacial conductance and thus may offer new opportunities for nanoscale thermoelectric applications.

Ab initio based calculations of the thermal conductivity at the micron scale

Heat transport in bulk semiconductors is well understood, and during the last few years, it has been shown that it can be computed accurately from ab initio calculations. However, describing heat transport in micro- and nanodevices used in applications remains challenging. In this paper, we propose a method, based on the propagation of wave packets, for solving the phonon Boltzmann transport equation parametrized with ab initio calculations. It allows computing the thermal conductivity of micro- and nano-sized systems, without adjustable parameters, and for any materials. The accuracy and applicability of the method are demonstrated by computing the cross plane thermal conductivity of cubic and hexagonal silicon thin films as a function of their thickness.

Thermoelectric effects in graphene and graphene-based nanostructures using atomistic simulation

Thermoelectric properties of graphene and graphene-based nanostructures have recently attracted great attention from both physics and engineering communities. However, to make graphene a good thermoelectric material, two important issues must be overcome, i.e. (i) its gapless character, which leads to a poor value Seebeck coefficient in pristine graphene and (ii) its high thermal conductivity that leads to low thermoelectric efficiency in graphene devices. By means of atomistic numerical simulation of electron and phonon transport, we show that different techniques of nanostructuring and bandgap engineering can be used to strongly enhance the thermoelectric properties. It includes in particular graphene nanoribbons appropriate shape, hybrid graphene/boron nitride nanoribbbons and graphene nanomeshes and vertical junctions of multilayer graphene. Figures of merit higher than 1 can be obtained at room temperature for such graphene-based nanostructures.

Hybrid states and bandgap in zigzag graphene/BN heterostructures

We study the properties of edge states in in-plane heterostructures made of adjacent zigzag graphene and BN ribbons. While in pure zigzag graphene nanoribbons, gapless edge states are nearly flat and cannot contribute significantly to the conduction, at BN/Graphene interfaces the properties of these states are significantly modified. They are still strongly localized at the zigzag edges of graphene but they exhibit a high group velocity up to 4.3x10^5 m/s at the B/C interface and even 7.4x10^5 m/s at the N-C interface. For a given wave vector the velocities of N/C and B/C hybrid interface states have opposite signs. Additionally, in the case of asymmetric structure BN/Graphene/BN, a bandgap of about 207 meV is open for sub-ribbon widths of 5 nm. These specific properties suggest new ways to engineer and control the transport properties of graphene nanostructures.

Strain effects on the electronic properties of devices made of twisted graphene layers

The effects of uniaxial strain on the electronic and transport properties of twisted graphene bilayer structures are investigated by means of atomistic simulation. It is shown that the strain-induced modulation of band structure makes it possible to break the degeneracy and to modulate the position van Hove singularities. It is even possible to observe low-energy saddle points for a large range of twist angles. It is shown also that the strain-induced separation of Dirac points of the two lattices may generate a finite transport gap as large as a few hundreds of meV for a small strain of a few percent.

Strong negative differential resistance in graphene devices with local strain

The effects of local uniaxial strain on grapshene devices like single-barrier structure and p-n tunnel diode are investigated. The strain-induced displacement of Dirac points allows us toi suppress and/or control the Klein tunneling and the interband tunneling, which leads to strong effect of negative differential conductance. It is shown that when strain is suitably applied, the peak-to-valley ratio of the current-voltage characteristics can reach of a few hundred at room temperature.

Phonon transport in silicon nanowires using a Full-Band Monte Carlo approach

We show that with a Full-Band dispersion, the specific heat is closer to the experimental value than with an isotropic quadratic dispersion. So we use a Full-Band dispersion in the transport algorithm. A Monte Carlo algorithm has been developed to simulate phonon transport in silicon nanowire. It has been successfully used to simulate the thermal conductivity.