45 research outputs found
Dimensional cross-over of the charge density wave order parameter in thin exfoliated 1T-VSe
The capability to isolate one to few unit-cell thin layers from the bulk
matrix of layered compounds opens fascinating prospects to engineer novel
electronic phases. However, a comprehensive study of the thickness dependence
and of potential extrinsic effects are paramount to harness the electronic
properties of such atomic foils. One striking example is the charge density
wave (CDW) transition temperature in layered dichalcogenides whose thickness
dependence remains unclear in the ultrathin limit. Here we present a detailed
study of the thickness and temperature dependences of the CDW in VSe by
scanning tunnelling microscopy (STM). We show that mapping the real-space CDW
periodicity over a broad thickness range unique to STM provides essential
insight. We introduce a robust derivation of the local order parameter and
transition temperature based on the real space charge modulation amplitude.
Both quantities exhibit a striking non-monotonic thickness dependence that we
explain in terms of a 3D to 2D dimensional crossover in the FS topology. This
finding highlights thickness as a true tuning parameter of the electronic
ground state and reconciles seemingly contradicting thickness dependencies
determined in independent transport studies
Holographic imaging of the complex charge density wave order parameter
The charge density wave (CDW) in solids is a collective ground state
combining lattice distortions and charge ordering. It is defined by a complex
order parameter with an amplitude and a phase. The amplitude and wavelength of
the charge modulation are readily accessible to experiment. However, accurate
measurements of the corresponding phase are significantly more challenging.
Here we combine reciprocal and real space information to map the full complex
order parameter based on topographic scanning tunneling microscopy (STM)
images. Our technique overcomes limitations of earlier Fourier space based
techniques to achieve distinct amplitude and phase images with high spatial
resolution. Applying this analysis to transition metal dichalcogenides provides
striking evidence that their CDWs consist of three individual charge
modulations whose ordering vectors are connected by the fundamental rotational
symmetry of the crystalline lattice. Spatial variations in the relative phases
of these three modulations account for the different contrasts often observed
in STM topographic images. Phase images further reveal topological defects and
discommensurations, a singularity predicted by theory for a nearly commensurate
CDW. Such precise real space mapping of the complex order parameter provides a
powerful tool for a deeper understanding of the CDW ground state whose
formation mechanisms remain largely unclear
Matériaux céramiques thermoélectriques pour la production d'électricité propre
Ce travail de thÚse porte sur l élaboration et la caractérisation des propriétés physiques et chimiques d une nouvelle famille de composés thermoélectriques, et plus particuliÚrement le composé BiCuSeO. Les composés de cette famille, dite 1111, présentent une structure en couche de type ZrCuSiAs. L une des particularités de cette structure est la nature distincte des couches qui la composent, la couche Bi2O2 étant décrite comme isolante tandis que la couche Cu2Se2 est appelée couche conductrice. L étude approfondie du composé BiCuSeO montre qu en dépit d un facteur de puissance (S ) relativement modéré, ce composé est un matériau thermoélectrique prometteur, notamment à haute température. En effet, BiCuSeO présente une conductivité thermique remarquablement faible, qui permet d atteindre des facteurs de mérite relativement élevés. De plus, BiCuSeO présente de nombreuses voies d améliorations possibles. L une d elle concerne l étude d un dopage aliovalent sur le site du bismuth. L analyse des résultats a montré que l insertion d un élément divalent permet d optimiser la concentration des porteurs de charges, entrainant ainsi une forte augmentation du facteur de mérite du composé. Une autre voie possible d exploration est l étude de l influence de l ion chalcogÚne, au travers notamment de la substitution du sélénium par le tellure, avec l obtention d une solution solide complÚte BiCuSe(1-x)Te(x)O. L étude des propriétés électriques des composés de cette série a permis de mettre en évidence la présence d une transition métal semi-conducteur métal pour les fractions de tellure inférieures à 0.5. Ainsi, bien que l influence du tellure sur le facteur de puissance soit relativement limitée en raison de cette anomalie, des résultats intéressants ont été obtenus pour les fractions de tellure élevées. Par ailleurs, des problématiques autour d une méthode de synthÚse alternative du matériau ainsi que sa stabilité sous air sont également abordées dans ce travail.This thesis addresses the issues of the elaboration and the characterization of the chemical and physical properties of a new family of thermoelectric materials, the oxychalcogenides with the general formula BiCuSeO. This compound, called 1111, cristallises in the ZrCuSiAs structure-type. One feature of this structure lies in the fact that the layers are considered as electronically distinct: the Bi2O2 layers are described as the insulating layers whereas the chalcogenide layers Cu2Se2 are presented as the conductive ones. The study of BiCuSeO exhibits that in spite of a relatively moderate power factor (S ), this compound is very promising as possible thermoelectric material, especially at high temperature. Indeed, BiCuSeO shows a remarkably low thermal conductivity, which can achieve relatively high figures of merit. In addition, BiCuSeO offers many ways for improvement. One of them concerns the study of aliovalent doping on the bismuth site. The results showed that the insertion of a divalent element optimizes the charge carriers concentration, leading to a sharp increase in the figure of merit of the compound. Another possible way of exploration lies the study of the influence of the chalcogen ion, notably through the substitution of selenium and tellurium, with a complete solid-solution BiCuSe(1-x)Te(x)O. The study of the electrical properties of this solid solution has highlighted the presence of a metal - semiconductor - metal transition for tellurium fractions below 0.5. Thus, although the influence of tellurium on the power factor is relatively limited due to this anomaly, interesting results were obtained for the high tellurium fractions. In addition, issues around an alternative method of synthesis of the material and its stability in air are also discussed in this work.PARIS11-SCD-Bib. électronique (914719901) / SudocSudocFranceF
Indirect-to-direct band-gap crossover in few-layer MoTe
We study the evolution of the band-gap structure in few-layer MoTe
crystals, by means of low-temperature micro-reflectance (MR) and
temperature-dependent photoluminescence (PL) measurements. The analysis of the
measurements indicate that, in complete analogy with other semiconducting
transition metal dichalchogenides (TMDs), the dominant PL emission peaks
originate from direct transitions associated to recombination of excitons and
trions. When we follow the evolution of the PL intensity as a function of layer
thickness, however, we observe that MoTe behaves differently from other
semiconducting TMDs investigated earlier. Specifically, the exciton PL yield
(integrated PL intensity) is identical for mono and bilayer and it starts
decreasing for trilayers. A quantitative analysis of this behavior and of all
our experimental observations is fully consistent with mono and bilayer
MoTe being direct band-gap semiconductors, with tetralayer MoTe being
an indirect gap semiconductor, and with trilayers having nearly identical
direct and indirect gaps.This conclusion is different from the one reached for
other recently investigated semiconducting transition metal dichalcogenides,
for which only monolayers are found to be direct band-gap semiconductors, with
thicker layers having indirect band gaps that are significantly smaller, by
hundreds of meV, than the direct gap. We discuss the relevance of our findings
for experiments of fundamental interest and possible future device
applications.Comment: 20 pages, 4 Figure
Reduced phase space of heat-carrying acoustic phonons in single-crystalline InTe
Chalcogenide semiconductors and semimetals are a fertile class of efficient thermoelectric materials, which, in most cases, exhibit very low lattice thermal conductivity Îșph despite lacking a complex crystal structure such as the tetragonal binary compound InTe. Our measurements of Îșph(T) in single-crystalline InTe along the c axis show that Îșph exhibits a smooth temperature dependence upon cooling to about 50 K, the temperature below which a strong rise typical for dielectric compounds is observed. Using a combination of first-principles calculations, inelastic neutron scattering (INS), and low-temperature specific heat and transport properties measurements on single-crystalline InTe, we show that the phonon spectrum exhibits well-defined acoustic modes, the energy dispersions of which are constrained to low energies due to distributions of dispersionless, optical modes, which are responsible for a broad double peak structure in the low-temperature specific heat. The latter are assigned to the dynamics of In+ cations in tunnels formed by edge-sharing (In3+Te42â)â tetrahedra chains, the atomic thermal displacement parameters of which, probed as a function of temperature by means of single-crystal x-ray diffraction, suggest the existence of a complex energy potential. Indeed, the In+-weighted optical modes are not observed by INS, which is ascribed to the anharmonic broadening of their energy profiles. While the low Îșph value of 1.2Wmâ1Kâ1 at 300 K originates from the limited energy range available for acoustic phonons, we show that the underlying mechanism is specific to InTe and argue that it is likely related to the presence of local disorder induced by the In+ sit
MatĂ©riaux cĂ©ramiques thermoĂ©lectriques pour la production dâĂ©lectricitĂ© propre
This thesis addresses the issues of the elaboration and the characterization of the chemical and physical properties of a new family of thermoelectric materials, the oxychalcogenides with the general formula BiCuSeO. This compound, called 1111, cristallises in the ZrCuSiAs structure-type. One feature of this structure lies in the fact that the layers are considered as electronically distinct: the Bi2O2 layers are described as the insulating layers whereas the chalcogenide layers Cu2Se2 are presented as the conductive ones. The study of BiCuSeO exhibits that in spite of a relatively moderate power factor (SÂČÏ), this compound is very promising as possible thermoelectric material, especially at high temperature. Indeed, BiCuSeO shows a remarkably low thermal conductivity, which can achieve relatively high figures of merit. In addition, BiCuSeO offers many ways for improvement. One of them concerns the study of aliovalent doping on the bismuth site. The results showed that the insertion of a divalent element optimizes the charge carriers concentration, leading to a sharp increase in the figure of merit of the compound. Another possible way of exploration lies the study of the influence of the chalcogen ion, notably through the substitution of selenium and tellurium, with a complete solid-solution BiCuSe(1-x)Te(x)O. The study of the electrical properties of this solid solution has highlighted the presence of a metal - semiconductor - metal transition for tellurium fractions below 0.5. Thus, although the influence of tellurium on the power factor is relatively limited due to this anomaly, interesting results were obtained for the high tellurium fractions. In addition, issues around an alternative method of synthesis of the material and its stability in air are also discussed in this work.Ce travail de thĂšse porte sur lâĂ©laboration et la caractĂ©risation des propriĂ©tĂ©s physiques et chimiques dâune nouvelle famille de composĂ©s thermoĂ©lectriques, et plus particuliĂšrement le composĂ© BiCuSeO. Les composĂ©s de cette famille, dite 1111, prĂ©sentent une structure en couche de type ZrCuSiAs. Lâune des particularitĂ©s de cette structure est la nature distincte des couches qui la composent, la couche Bi2O2 Ă©tant dĂ©crite comme isolante tandis que la couche Cu2Se2 est appelĂ©e couche conductrice. LâĂ©tude approfondie du composĂ© BiCuSeO montre quâen dĂ©pit dâun facteur de puissance (SÂČÏ) relativement modĂ©rĂ©, ce composĂ© est un matĂ©riau thermoĂ©lectrique prometteur, notamment Ă haute tempĂ©rature. En effet, BiCuSeO prĂ©sente une conductivitĂ© thermique remarquablement faible, qui permet dâatteindre des facteurs de mĂ©rite relativement Ă©levĂ©s. De plus, BiCuSeO prĂ©sente de nombreuses voies dâamĂ©liorations possibles. Lâune dâelle concerne lâĂ©tude dâun dopage aliovalent sur le site du bismuth. Lâanalyse des rĂ©sultats a montrĂ© que lâinsertion dâun Ă©lĂ©ment divalent permet dâoptimiser la concentration des porteurs de charges, entrainant ainsi une forte augmentation du facteur de mĂ©rite du composĂ©. Une autre voie possible dâexploration est lâĂ©tude de lâinfluence de lâion chalcogĂšne, au travers notamment de la substitution du sĂ©lĂ©nium par le tellure, avec lâobtention dâune solution solide complĂšte BiCuSe(1-x)Te(x)O. LâĂ©tude des propriĂ©tĂ©s Ă©lectriques des composĂ©s de cette sĂ©rie a permis de mettre en Ă©vidence la prĂ©sence dâune transition mĂ©tal â semi-conducteur â mĂ©tal pour les fractions de tellure infĂ©rieures Ă 0.5. Ainsi, bien que lâinfluence du tellure sur le facteur de puissance soit relativement limitĂ©e en raison de cette anomalie, des rĂ©sultats intĂ©ressants ont Ă©tĂ© obtenus pour les fractions de tellure Ă©levĂ©es. Par ailleurs, des problĂ©matiques autour dâune mĂ©thode de synthĂšse alternative du matĂ©riau ainsi que sa stabilitĂ© sous air sont Ă©galement abordĂ©es dans ce travail
Ceramics thermoelectrics materials for âgreenâ power generation
Ce travail de thĂšse porte sur lâĂ©laboration et la caractĂ©risation des propriĂ©tĂ©s physiques et chimiques dâune nouvelle famille de composĂ©s thermoĂ©lectriques, et plus particuliĂšrement le composĂ© BiCuSeO. Les composĂ©s de cette famille, dite 1111, prĂ©sentent une structure en couche de type ZrCuSiAs. Lâune des particularitĂ©s de cette structure est la nature distincte des couches qui la composent, la couche Bi2O2 Ă©tant dĂ©crite comme isolante tandis que la couche Cu2Se2 est appelĂ©e couche conductrice. LâĂ©tude approfondie du composĂ© BiCuSeO montre quâen dĂ©pit dâun facteur de puissance (SÂČÏ) relativement modĂ©rĂ©, ce composĂ© est un matĂ©riau thermoĂ©lectrique prometteur, notamment Ă haute tempĂ©rature. En effet, BiCuSeO prĂ©sente une conductivitĂ© thermique remarquablement faible, qui permet dâatteindre des facteurs de mĂ©rite relativement Ă©levĂ©s. De plus, BiCuSeO prĂ©sente de nombreuses voies dâamĂ©liorations possibles. Lâune dâelle concerne lâĂ©tude dâun dopage aliovalent sur le site du bismuth. Lâanalyse des rĂ©sultats a montrĂ© que lâinsertion dâun Ă©lĂ©ment divalent permet dâoptimiser la concentration des porteurs de charges, entrainant ainsi une forte augmentation du facteur de mĂ©rite du composĂ©. Une autre voie possible dâexploration est lâĂ©tude de lâinfluence de lâion chalcogĂšne, au travers notamment de la substitution du sĂ©lĂ©nium par le tellure, avec lâobtention dâune solution solide complĂšte BiCuSe(1-x)Te(x)O. LâĂ©tude des propriĂ©tĂ©s Ă©lectriques des composĂ©s de cette sĂ©rie a permis de mettre en Ă©vidence la prĂ©sence dâune transition mĂ©tal â semi-conducteur â mĂ©tal pour les fractions de tellure infĂ©rieures Ă 0.5. Ainsi, bien que lâinfluence du tellure sur le facteur de puissance soit relativement limitĂ©e en raison de cette anomalie, des rĂ©sultats intĂ©ressants ont Ă©tĂ© obtenus pour les fractions de tellure Ă©levĂ©es. Par ailleurs, des problĂ©matiques autour dâune mĂ©thode de synthĂšse alternative du matĂ©riau ainsi que sa stabilitĂ© sous air sont Ă©galement abordĂ©es dans ce travail.This thesis addresses the issues of the elaboration and the characterization of the chemical and physical properties of a new family of thermoelectric materials, the oxychalcogenides with the general formula BiCuSeO. This compound, called 1111, cristallises in the ZrCuSiAs structure-type. One feature of this structure lies in the fact that the layers are considered as electronically distinct: the Bi2O2 layers are described as the insulating layers whereas the chalcogenide layers Cu2Se2 are presented as the conductive ones. The study of BiCuSeO exhibits that in spite of a relatively moderate power factor (SÂČÏ), this compound is very promising as possible thermoelectric material, especially at high temperature. Indeed, BiCuSeO shows a remarkably low thermal conductivity, which can achieve relatively high figures of merit. In addition, BiCuSeO offers many ways for improvement. One of them concerns the study of aliovalent doping on the bismuth site. The results showed that the insertion of a divalent element optimizes the charge carriers concentration, leading to a sharp increase in the figure of merit of the compound. Another possible way of exploration lies the study of the influence of the chalcogen ion, notably through the substitution of selenium and tellurium, with a complete solid-solution BiCuSe(1-x)Te(x)O. The study of the electrical properties of this solid solution has highlighted the presence of a metal - semiconductor - metal transition for tellurium fractions below 0.5. Thus, although the influence of tellurium on the power factor is relatively limited due to this anomaly, interesting results were obtained for the high tellurium fractions. In addition, issues around an alternative method of synthesis of the material and its stability in air are also discussed in this work
MatĂ©riaux cĂ©ramiques thermoĂ©lectriques pour la production dâĂ©lectricitĂ© propre
This thesis addresses the issues of the elaboration and the characterization of the chemical and physical properties of a new family of thermoelectric materials, the oxychalcogenides with the general formula BiCuSeO. This compound, called 1111, cristallises in the ZrCuSiAs structure-type. One feature of this structure lies in the fact that the layers are considered as electronically distinct: the Bi2O2 layers are described as the insulating layers whereas the chalcogenide layers Cu2Se2 are presented as the conductive ones. The study of BiCuSeO exhibits that in spite of a relatively moderate power factor (SÂČÏ), this compound is very promising as possible thermoelectric material, especially at high temperature. Indeed, BiCuSeO shows a remarkably low thermal conductivity, which can achieve relatively high figures of merit. In addition, BiCuSeO offers many ways for improvement. One of them concerns the study of aliovalent doping on the bismuth site. The results showed that the insertion of a divalent element optimizes the charge carriers concentration, leading to a sharp increase in the figure of merit of the compound. Another possible way of exploration lies the study of the influence of the chalcogen ion, notably through the substitution of selenium and tellurium, with a complete solid-solution BiCuSe(1-x)Te(x)O. The study of the electrical properties of this solid solution has highlighted the presence of a metal - semiconductor - metal transition for tellurium fractions below 0.5. Thus, although the influence of tellurium on the power factor is relatively limited due to this anomaly, interesting results were obtained for the high tellurium fractions. In addition, issues around an alternative method of synthesis of the material and its stability in air are also discussed in this work.Ce travail de thĂšse porte sur lâĂ©laboration et la caractĂ©risation des propriĂ©tĂ©s physiques et chimiques dâune nouvelle famille de composĂ©s thermoĂ©lectriques, et plus particuliĂšrement le composĂ© BiCuSeO. Les composĂ©s de cette famille, dite 1111, prĂ©sentent une structure en couche de type ZrCuSiAs. Lâune des particularitĂ©s de cette structure est la nature distincte des couches qui la composent, la couche Bi2O2 Ă©tant dĂ©crite comme isolante tandis que la couche Cu2Se2 est appelĂ©e couche conductrice. LâĂ©tude approfondie du composĂ© BiCuSeO montre quâen dĂ©pit dâun facteur de puissance (SÂČÏ) relativement modĂ©rĂ©, ce composĂ© est un matĂ©riau thermoĂ©lectrique prometteur, notamment Ă haute tempĂ©rature. En effet, BiCuSeO prĂ©sente une conductivitĂ© thermique remarquablement faible, qui permet dâatteindre des facteurs de mĂ©rite relativement Ă©levĂ©s. De plus, BiCuSeO prĂ©sente de nombreuses voies dâamĂ©liorations possibles. Lâune dâelle concerne lâĂ©tude dâun dopage aliovalent sur le site du bismuth. Lâanalyse des rĂ©sultats a montrĂ© que lâinsertion dâun Ă©lĂ©ment divalent permet dâoptimiser la concentration des porteurs de charges, entrainant ainsi une forte augmentation du facteur de mĂ©rite du composĂ©. Une autre voie possible dâexploration est lâĂ©tude de lâinfluence de lâion chalcogĂšne, au travers notamment de la substitution du sĂ©lĂ©nium par le tellure, avec lâobtention dâune solution solide complĂšte BiCuSe(1-x)Te(x)O. LâĂ©tude des propriĂ©tĂ©s Ă©lectriques des composĂ©s de cette sĂ©rie a permis de mettre en Ă©vidence la prĂ©sence dâune transition mĂ©tal â semi-conducteur â mĂ©tal pour les fractions de tellure infĂ©rieures Ă 0.5. Ainsi, bien que lâinfluence du tellure sur le facteur de puissance soit relativement limitĂ©e en raison de cette anomalie, des rĂ©sultats intĂ©ressants ont Ă©tĂ© obtenus pour les fractions de tellure Ă©levĂ©es. Par ailleurs, des problĂ©matiques autour dâune mĂ©thode de synthĂšse alternative du matĂ©riau ainsi que sa stabilitĂ© sous air sont Ă©galement abordĂ©es dans ce travail
Screening quaternary Heusler by machine learning for application in thermoelectricity
International audienc
Prédictions de la stabilité et des propriétés électroniques de phases Heusler quaternaires par apprentissage automatique
National audienc